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This is to certify that the
dissertation entitled

INTERCALATED P( )LYMER-LAYERED INORGANIC
NANOCOMPOSITES

presented by

Lei Wang

has been accepted towards fulfillment
of the requirements for

Ph. D , Chemistry
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INTERCALATED POLYMER-LAYERED INORGANIC
NAN OCOMPOSITES

By

Lei Wang

A DISSERTATION

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY
Department of Chemistry

1999

ABSTRACT

INTERCALATED POLYMER-LAYERED INORGANIC
NAN OCOMPOSITES

By

Lei Wang

The exfoliation-encapsulative precipitation (EEP) method, which was
introduced by Kanatzidis and Bissessur in 1993, is both convenient and
general. It enables the formation of an intercalated polymer-layered
inorganic nanocomposite when a polymer solution and an inorganic
monolayer suspension are mixed. Many MoS2/polymer nanocomposites
were synthesized.

In this dissertation, we expand the investigation of the M082 system,
contribute a better understanding and provide a better manipulation of the
reactions. Moreover, we further develop the EEP method by extending its
application to other layered materials such as TaS2, MoO3, oc-RuClg and
W82, and by modifying the method so that the intercalation of intractable
polymers such as polypyrrole is possible.

In the research, it was found that “LiMoS2” would not be oxidized
completely to M082 upon exposure to water or air, and the exfoliation of
“LiMoS2” in water is a process of solvation of Lix(H2O)yM082 (x ~ 0.18).
Like Ax(H2O)yTiS2 (A = Li, Na, K), Lix(H2O)yMoS2 can be exfoliated by
stirring. Additional polymers such as polyacrylamide, polyvinylalcohol,

polypropylene, polystyrene, polymethylmethacrylate, polybenzimidazole,

polyethyleneterephthalate and polypyrrole were encapsulated in M082. A
convenient method to prepare “LiMoS2” was developed, which uses LiBH4
as a reagent and runs at elevated temperatures (300-350 0C). This method
can also be used to lithiate other layered transition metal dichalcogenides.

The lithium intercalated materials exfoliate well and encapsulate
polymers better than the sodium intercalated materials, at least in the
systems of 2H-Ta82 and M003. At room temperature, LiBH4 was proven a
good lithiation reagent for 2H—TaS2, M003 and oc-RuClg. The amount of
LiBH4 used in the reaction is crucial to the exfoliation and encapsulation
. properties of the products. Water-soluble polymers such as poly(ethylene
oxide) (PEO) and polyvinylpyrrolidone were intercalated in all three
layered materials and polypyrrole was intercalated in RuCl3.

In addition, oc-RuCl3/polyaniline nanocomposites were prepared by
the in situ redox intercalative polymerization method, which up to now was
only applicable to FeOCl, V205 and VOPO4. The reaction is driven by the
reduction of some of the Ru3+ centers to Ru2+ and completed by the
participation of ambient oxygen as an electron acceptor.

The large variety of new polymer-nanocomposites thus prepared
have many interesting physical properties. For example, TaS2
nanocomposites are superconductors at low temperatures; RuC13
nanocomposites have adjustable magnetic properties. The arrangement of
PEO chains in the interlayer galleries was explored and we believe that
M082/PE0, TaS2/PE0 and RuCl3/PE0 nanocomposites contain in each
gallery two sheets of PEO chains in the type II PEO-HgCl2 complex
conformation. The two sheets of PEO chains are believed to have -0-
atoms facing each other at the center of the gallery and provide Li+ ions a

good two-dimensional migration channel.

To My Family, Who Have Believed Faithfully in and Dedicated Heartily to
Science for Generations.

iv

ACKNOWLEDGMENTS

I gratefully acknowledge the people who have helped me in all these
years to fulfill the requirements for the Ph. D. degree. It would have been
impossible to achieve such a high goal without the love and support from
many people who are listed and not listed below.

First of all, I would like to thank my advisor, Prof. Mercouri
Kanatzidis, for his guidance, encouragement and financial support. I have.
benefited substantially from his wise choosing of a productive research
project for me, his requirement of a serious and aggressive research
manner, and his priceless advice in many aspects.

I would like to express my great appreciation for Prof. John Allison
for my admission to the Ph. D. program in the Department of Chemistry,
which provided me the opportunity to pursue this degree. I would like to
thank Prof. Thomas Pinnavaia, Prof. Gregory Baker and Prof. Marcos
Dantus, who serve as my guidance committee members and lead me
successfully to the completion of this degree. I also thank Prof. Pinnavaia
and Prof. Baker for their generous permission for letting me use their
instruments and facilities.

I enjoyed being a member in the Kanatzidis’ group, in which one is
always given assistance and offered friendship. I am in gratitude to the day-
to-day help from every member of the group, especially from Joy Heising,
Rhonda Patschke, Jason Hanko, Jennifer Aitken, Andy Axtell, Chenggang
Wang, Rabin Bisserssur, Dr. Xianzhong Chen and Dr. Francois
Bonhomme. I also thank other people in the department for their help,
especially the people in Dr. Baker’s group, Dr. Pinnavaia’s group and Dr.

Smith’s group.

I thank Dr. Rui Huang for many training and measurements, Kermit
Johnson for help with the solid state NMR measurements, Dr. Thomas
Atkinson for the help with computers, Dr. John Heckman and Dr. Stanley
Flegler in the Center for Electron Optics for their help in using TEM and
SEM, and Dr. Reza Loloee in the Department of Physics and Astronomy
for his help in using the SQUID. I also thank Prof. Kannewurf and his
group at Northwestern University for the measurements of electrical
conductivity and thermopower and Prof. Jin-Ho Choy at Seoul National
University in Korea for the measurements of ion conductivity.

I thank the Department of Chemistry for guaranteeing assistantships
for five years. Thanks also for the research funding from the National
Science Foundation, and the Center for Fundamental Materials Research
and the Center for Sensor Materials at MSU.

Finally, thanks to my wife and family for their deepest love and

reserveless support.

vi

TABLE OF CONTENTS

List of Tables ................................................................................ xiii
List of Figures ................................................................................. xv
List of Abbreviations ........................................................................ xx
Introduction ..................................................................................... 1
l. Intercalation Reactions and Layered Host Materials ...................... 1
2. The Development of Polymer Intercalation ................................. 7

3. Exfoliation-Encapsulative Precipitation: An Effective

Method for Polymer Intercalation ............................................. 11
4. Applications of Intercalation Compounds and Nanocomposites ..... 14
5. Important Contributions of the Present Dissertation Work ........... 18
References .................................................................................. 22

Chapter 1. Further Exploration of Exfoliated M082 and Synthesis of
New M082 Nanocomposites with the Exfoliation-Encapsulative

Precipitation Method ........................................................................ 32
Introduction ................................................................................ 32
Experimental Section ................................................................... 35

1. Reagents ........................................................................... 35
2. Lithiation and Exfoliation of M082 ..................................... 35
3. Investigation of LiMoS2 and Freshly Restacked M082 ........... 38
4. Preparation of M082/Polymer Nanocomposites .................... 39
5. Instrumentation ................................................................. 44

vii

Results and Discussion .................................................................. 46

1. The LiBH4 Method of Producing LiMoS2 ............................ 46
2. Manipulation and Properties of Exfoliated and
Restacked M082 ................................................................ 49
3. (PEO)XMoS2 and (PVP)xMoS2 Nanocomposites .................... 51
4. Investigations in (PA6)XM082 Nanocomposites ..................... 55
5. Synthesis of (PS)XM082 Nanocomposites .............................. 59
6. Synthesis of Other M082/Polymer Nanocomposites ............... 61
7. The Phase Transition in Restacked M082 and M082
Nanocomposites ............................................................... 64
8. Thermal Stability of M082 Nanocomposites ......................... 67
Concluding Remarks .................................................................... 70
Appendix A ................................................................................ 72
References .................................................................................. 75

Chapter 2. Insertion of Polypyrrole and Poly(N-methyl pyrrole)
in M082 and W82 by anin situ Polymerization—Encapsulative

Precipitation Method ........................................................................ 78
Introduction ................................................................................ 78
Experimental Section ................................................................... 80

1. Reagents ........................................................................... 8O
2. Preparation of M082/PPY Nanocomposites .......................... 80
3. Preparation of WS2/PPY Nanocomposites ............................ 83
4. Instrumentation ................................................................. 84

viii

 

Results and Discussion .................................................................. 85
1. M082/PPY Nanocomposites ................................................ 85

2. W82/PPY Nanocomposites ................................................. 92

3. An Analysis of Polymer Arrangements inside
the Interlayer Galleries Based on the Dimensions of

the Polymer Molecules ...................................................... 94

4. Thermal Properties ........................................................... 96

5. Charge Transport Properties ............................................ 100
Concluding Remarks .................................................................. 104
Appendix B ............................................................................... 106
References ................................................................................ l 1 1

Chapter 3. Lamellar TaS2/Polymer Nanocomposites through

Encapsulative Precipitation of Exfoliated Layers ............................... 116
Introduction .............................................................................. l 16
Experimental Section ................................................................. 1 18

1. Reagents ......................................................................... l 18
2. Synthesis of 2H-Ta82 ....................................................... 119
3. Synthesis and Exfoliation of LixTaS2 ................................. 119
4. Encapsulative Precipitation of Polymers ............................ 120
5. Instrumentation and Measurements ................................... 121
Results and Discussion ................................................................ 125
1. Exfoliation Properties of LixTaS2 ..................................... 125
2. Polymer Encapsulation .................................................... 126

ix

3. Characterization of LixTaS2/Polymer Nanocomposites ........ 127

4. Structural Studies: The Conformation of PEO in

Lix(PEO)yTaS2 ............................................................... 132

5. Superconductive State ...................................................... 137

6. Electrical Transport Properties ........................................ 143

7. Solid State NMR Spectroscopy .......................................... 146
Concluding Remarks .................................................................. 153
References ................................................................................ 155

Chapter 4. Lamellar LIxMoO3/Polymer N anocomposites

Via Encapsulative Insertion ............................................................. 159
Introduction .............................................................................. 159
Experimental Section ................................................................. 160

1. Reagents ......................................................................... 160
2. Synthesis of LixMoO3 (0.30 < x < 0.40) ............................ 161
3. Preparation of LixMoO3/Polymer Nanocomposites ............. 162
4. Instrumentation ............................................................... 162
Results and Discussion ................................................................ 163
1. Synthesis and Characterization of LixMoO3 ........................ 163
2. Exfoliation and Polymer Encapsulation Chemistry ............. 165
3. Solid State NMR Spectroscopy .......................................... 176
4. Magnetism ...................................................................... 178
5. Electrical Conductivity .................................................... 181

 

Concluding Remarks .................................................................. 181
References ................................................................................ 183

Chapter 5. oc-RuClg/Polymer Nanocomposites: the First Group of

Intercalative Nanocomposites with Transition-Metal-Halides ............... 186
Introduction .............................................................................. 1 86
Experimental Section ................................................................. 189

1 . Reagents ......................................................................... l 89
2. Reactions and Sample Preparations ................................... 189
3. Instrumentation ............................................................... 194
Results and Discussion ................................................................ 194

1. Preparation of (PANDXRuClg, by in situ Redox Intercalative
Polymerization ............................................................... 194

2. Nanocomposites of oc-RuClg with Water Soluble Polymers .. 202

3. oc-RuClg/Polypyrrole Nanocomposites ............................... 204
4. Charge Transport Properties ............................................ 206
5. Magnetic Susceptibility Studies ......................................... 209

6. One-Dimensional Electron Density Calculation and
Arrangement of Polymer Chains in Lix(PE0)yRuCl3 ......... 217
7. PEO Conformation from IR Spectra ................................. 226
Concluding Remarks .................................................................. 229
References ................................................................................ 23 1

xi

 

LIST OF TABLES

Table 1. Elements forming layered transition metal dichalcogenides ....... 2

Table 1.1. Nylon-6 intercalation reactions with PA6/CF3CH20H
solution ........................................................................................... 59

Table 1.2. Information about the synthesis of M082/PS
nanocomposites ................................................................................ 60

Table 1.3. Characteristics of some new M082 nanocomposites ............... 63

Table 1.4. Evolution rates of restacked M082 and nanocomposites
derived from the change of electrical resistivity .................................. 67

Table 1.5. The effect of high temperature annealing to some
nanocomposites ................................................................................ 68

Table 2.1. Parameters of (PPY)xMoS2 prepared with different

pyrrole ratios .................................................................................. 88
Table 2.2. Structural data of PPY and PMPY nanocomposites .............. 94
Table 2.3. Room temperature electrical conductivities of PPY

nanocomposites and restacked M082 and WS2 .................................... 100
Table 3.1. The annealing procedure to produce 2H-TaS2 .................... 119
Table 3.2. Polymer Intercalation in LixTaS2 ..................................... 127
Table 3.3. Properties of Li02TaS2 and polymer nanocomposites ......... 129

Table 3.4. Properties of the superconductive state for TaS2
intercalates .................................................................................... 138

Table 3.5. Effect of film orientation on the 7Li-NMR spectrum
0f Li0,2(PEO)yTaS2 ........................................................................ 153

Table 4.1. Chemical and structural characteristics of
LixM003/polymer nanocomposites ................................................... 166

xii

 

 

Table 4.2. Composition and physicochemical properties of

the LixM003/polymer nanocomposites .............................................. 175

Table 5.1. Magnetic properties of a-RuCl3 and nanocomposites .......... 213

Table 5.2. A comparison of IR absorptions (cm-1) of PEO ................. 228
xiii

LIST OF FIGURES

Figure 1.1. Experimental set-up for the high temperature LiBH4
lithiation reaction ............................................................................. 37

Figure 1.2. XRD patterns of a (PEO)XMoS2 nanocomposite used to
calculate the one—dimensional electron density map .............................. 53

Figure 1.3. One-dimensional electron density map of a (PEO)xMoS2

nanocomposite ................................................................................. 53
Figure 1.4. Structural model for the (PE0)xMoS2 nanocomposite ........ 54
Figure 1.5 . XRD patterns of the new nanocomposite phases ................. 56

Figure 1.6. Evolution of the electrical resistivity of a restacked M082
sample as a function of time .............................................................. 65

Figure 1.7. XRD patterns of the (PE0)xMoS2 and (PP)XM082
nanocomposites before and after the high temperature annealing ........... 68

Figure 1.8. Intensity of the 001 peak in the XRD patterns of

a (PE0)xMoS2 sample at increasing temperatures ................................ 69
Figure 1.9. Curve fitting for the time dependence of electrical

conductivity of (PE0)xMoS2 ............................................................. 74
Figure 2.1. XRD patterns of M082 nanocomposites .............................. 86

Figure 2.2. IR spectra of PPY and a M082/PPY nanocomposite
prepared by procedure 1 ................................................................... 87

Figure 2.3. IR spectrum of a M082/PPY nanocomposite prepared by
procedure 1 and heating treated twice up to 300 0C under N2
atmosphere ...................................................................................... 87

Figure 2.4. IR spectra of a nanocomposite (PPY)0,51(H20)0,13M082
prepared by procedure 2 ................................................................... 90

Figure 2.5. XRD patterns of WS2/PPY nanocomposites ........................ 93

xiv

Figure 2.6. IR spectrum of (PPY)0,25(H2O)0_53WS2 heating treated

twice up to 320 0C under N2 atmosphere ............................................ 93
Figure 2.7. DSC measurements at different heating rates for
(PPY)0_51(H20)0,18MOSZ .................................................................... 97
Figure 2.8. Plot of the DSC heating rate versus the temperature

of the M082 structural transition for (PPY)0,51(H2O)0,18M082 ............... 97
Figure 2.9. DSC measurement for (PPY)0,25(H20)0_53WS2 .................... 98

Figure 2.10. Plot of the DSC heating rate versus the W82 structural
transition temperature in (PPY)0,25(H20)0,53WS2 ................................. 98

Figure 2.11. Variable temperature electrical conductivity measurements
on pressed pellets for pristine M082 and a M082/PPY nanocomposite
prepared with Procedure 1 .............................................................. 102

Figure 2.12. Thermoelectric power measurement on a pressed pellet
for (PPY)0_25(H2O)0_16M082 prepared with Procedure 1 ..................... 102

Figure 2.13. Variable temperature electrical conductivity measurements
on pressed pellets for restacked W82 and (PPY)0,25(H20)0,53WS2 ........ 103

Figure 2.14. Thermoelectric power measurement on a pressed
(PPY)0.25(H20)0.53WSZ pellet ........................................................... 103

Figure 2.15. Schematic structures and dimensions of PPY and PMPY . 109

Figure 2.16. Views of PMPY ......................................................... 110
Figure 3.1. XRD patterns of LixTaS2/polymer nanocomposites .......... 128
Figure 3.2. DSC of a Li0,2(PEO)yTaS2 nanocomposite ....................... 130
Figure 3.3. SEM photograph of a Li0.2(PEO)yTaS2 film ..................... 131

Figure 3.4. Powder XRD patterns of a folded Li0,2(PEO)yTaS2 film 133

Figure 3.5. One-dimensional electron density maps for
the Lix(PEO)yTaS2 nanocomposite ................................................... 134

XV

 

Figure 3.6. Structural model for the Li0,2(PEO)yTaS2 nanocomposite .. 135

Figure 3.7. Variable temperature magnetic susceptibility for

Li02TaS2 and Li0,2(PE0)yTaS2 powders ........................................... 140
Figure 3.8. Variable temperature magnetic susceptibility for
Lio,2(PE0)yTaS2 films .................................................................... 141
Figure 3.9. Variable temperature electrical conductivity measurements

for pressed pellets of Li0,2(PE0)yTaS2 ............................................. 144
Figure 3.10. Variable temperature thermopower data of pressed pellets

of Li0,2(PE0)yTaS2 ........................................................................ 144
Figure 3.11. Electron diffraction pattern and superstructure of

Li0,2TaS2 ....................................................................................... 145
Figure 3.12. Static solid state 7Li NMR spectra of LiOI2TaS2 and

1410203130) yTaS 2 ............................................................................ 148
Figure 3.13. Temperature dependence of the linewidth of 7Li NMR
resonance peaks for Li0,2TaS2 and Li0,2(PEO)yTaS2 ........................... 149
Figure 3.14. Room temperature 7Li NMR spectra for

a Li0,2(PE0)yTaS2 film ................................................................... 152
Figure 4.1. DSC diagram of LixMoO3 .............................................. 164
Figure 4.2. IR spectra of Lix(H2O)y(PVP)zM003 ............................... 167

Figure 4.3. Solid-state optical absorption spectra of the
LixM003/polymer nanocomposites ................................................... 168

Figure 4.4. Typical XRD patterns of nanocomposites with poly(ethylene
glycol) and poly(ethylene oxide) of different molecular mass ............. 169

Figure 4.5. Typical XRD patterns of the various LiXM003/polymer
nanocomposites .............................................................................. 170

Figure 4.6. XRD patterns of Lix(H2O)y(PEO-2000)ZM003 showing
the effect of annealing on the stacking regularity of the layered
structure of a nanocomposite ........................................................... 174

xvi

Figure 4.7. Static 7Li NMR spectra of LixM003 and
Lix(H2O)y(PE0)zM003 at -80 0C ..................................................... 177

Figure 4.8. Temperature dependence of the linewidth of the
resonance peak in solid-state 7Li MNR spectra of LixMoO3 and

Lix(H20)y(PE0)zM003 ................................................................... 177
Figure 4.9. Temperature dependence of magnetic susceptibility of
LixM003 and Lix(H20)y(PE0)zM003 ............................................... 179
Figure 4.10. Variable-temperature electrical conductivity measurements
for LixM003 and Lix(H2O)y(PE0)ZM003 .......................................... 180
Figure 5.1. The structure of oc-RuClg ............................................... 188
Figure 5.2 Experimental set-up for the synthesis of OL-RuC13 .............. 190

Figure 5.3. X—ray diffraction patterns of oc-RuCl3 and nanocomposites 196

Figure 5.4. Transmission X-ray diffraction patterns of OL—RuC13 and
(PANI)xRuCl3 with indexing ............................................................ 197

Figure 5.5(a). Infrared spectra of (PANDXRuClg, PANI and aniline 198

Figure 5.5(b). Infrared spectra of (PPY)xRuCl3 and PPY .................... 199
Figure 5.5(c). Infrared spectra of Lix(PEO)yRuCl3 and PEO .............. 200
Figure 5.5(d). Infrared spectra of Lix(PVP)yRuC13 and PVP .............. 201

Figure 5.6. Electron diffraction patterns with the electron-beam
perpendicular to the planess for oc-RuC13 and LixRuCl3 ...................... 203

Figure 5.7. Variable temperature electrical conductivity mesurements
for pressed pellets of oc-RuClg and nanocomposites ............................ 207

Figure 5.8. Thermopower measurements for pressed pellets of
LixRuCl3 (x ~ 0.2) and nanocomposites ............................................ 208

Figure 5.9(a). Magnetic susceptibility measurements for a sample of
0L-RuCl3 ........................................................................................ 21 1

xvii

Figure 5 .9(b). Magnetic susceptibility measurements for LixRuC13 ...... 211

Figure 5.9(c). Magnetic susceptibility measurements for

Lix(PE0)yRuC13 ............................................................................. 212
Figure 5.9(d). Magnetic susceptibility measurements for

(PANDXRuClg ................................................................................ 212
Figure 5.10(a). Magnetic susceptibility measurements for “Brz oxidized
LixRuClg” (Le. RuCl3) .................................................................... 214
Figure 5.10(b). Magnetic susceptibility measurements for “Brz oxidized
Lix(PEO)yRuCl3” (Le. “(PEO)xRuCl3”) ............................................ 214
Figure 5.10(c). Magnetic hysteresis measurements for “Brz oxidized
LixRuCl3” (Le. RuC13) .................................................................... 215
Figure 5.10(d). Magnetic hysteresis measurements for “Brz oxidized
Lix(PEO)yRuC13” (Le. “(PEO)XRuCl3”) ............................................ 215
Figure 5.11. Possible PEO conformations ......................................... 219

Figure 5.12. X—ray powder diffraction pattern of oriented
Lix(PEO)yRuCl3 film used for the calculation of one—dimensional
electron density maps ..................................................................... 220

Figure 5.13. One-dimensional electron density maps for

Lix(PEO)yTaS2 and Agx(PE0)yTaS2 .......................................... 220-222
Figure 5.14. Structural model for Lix(PEO)yRuCl3 ........................... 223
xviii

LIST OF ABBREVIATIONS

l-D ED ................................................. one-dimensional electron density
1T-MoS2 ............................................................. octahedral form M082
2H— M082 .............................................................. hexagonal form M082
AF ........................................................................... antiferromagnetism
xdia .................................................................. diamagnetic susceptibility
cm ........................................................................................ centimeter
xmolar ...................................................................... molar susceptibility
xpam ............................................................... paramagnetic susceptibility
xTIP .................... susceptibility of temperature-independent paramagnetism
DSC ...................................................... differential scanning calorimeter
Ea ............................................................................... activation energy
EDS .............................................. energy dispersive X-ray microanalysis
EEP ............................................... exfoliation-encapsulative precipitation
FM ...................................................................................... formamide
FTIR ......................................... Fourier transform infrared spectroscopy
g .................................................................................................. gram
GR ................ Guaranteed Reagent (suitable for uses in analytic chemistry)
h .................................................................................................. hour
i-PrOH ................................................................................ isopropynol
ICP ................................................................ inductively coupled plasma
IR ......................................................................... infrared spectroscopy
kJ ............................................................................................ kilojoule
M ............................................................................................ molarity
1.13 ................................................................................. Bohr magneton
Mcel ............................................................................. methyl cellulose
ueff ................................................................ effective magnetic moment
min ............................................................................................ minute
m1 .......................................................................................... milliliter
mmol ..................................................................................... millimole
mol ............................................................................................... mole
MW ............................................................................ molecular weight
NMF ....................................................................... N-methylformamide
NMR ............................................................ nuclear magnetic resonance
PA6 .......................................................................................... nylon-6
PAM .............................................................................. polyacrylamide
PANI ................................................................................... polyaniline
PBI ............................................................................ polybenzimidazole
PE ..................................................................................... polyethylene
PEG ...................................................................... poly(ethylene glycol)

xix

 

PEI .............................................................................. polyethylenimine

PEO ....................................................................... poly(ethylene oxide)
PETP ........................................................... poly(ethylene terephthalate)
PLD-CDW ......................... periodic lattice distortion-charge density wave
PMMA ........................................................... poly(methyl methacrylate)
PMPY ................................................................ poly(N-methyl pyrrole)
PP ................................................................................... polypropylene
PPG .................................................................... poly(propylene glycol)
PPY .................................................................................... polypyrrole
PS ....................................................................................... polystyrene
PVA ......................................................................... poly(vinyl alcohol)
PVP ............................................................... poly(N-vinyl pyrrolidone)
0 (in magnetism) .............................................................. Weiss constant
0 (in XRD) .......................................................................... Bragg angle
s ................................................................................................. second
SEM .......................................................... scanning electron microscopy
Tc ....................................... critical temperature of superconducting state
TEM ..................................................... transmission electron microscope
TGA ........................................................... thermal gravimetric analysis
UV-VIS-NIR ..................... ultraviolet-visible-(near infrared) spectroscopy
XRD .......................................................................... X-ray diffraction

XX

INTRODUCTION

1. Intercalation Reactions and Layered Host Materials

In chemistry, the term "intercalation" has been used to describe
reactions in which guest atoms, molecules or ions are inserted into layered
host lattices while the host lattices reserve their essential structural features.
Literally, intercalation chemistry dates from 1840 when Schafhautl inserted
sulfate ion in graphite [1]. Nevertheless, the subject had not drawn the
attention of chemists, physicists and scientists in other fields until 1926
when Fredenhagen and Cadenbach discovered the insertion of potassium
atoms from the vapor into graphite [2]. In the 19603, research in this area
grew significantly and extended into many scientific disciplines. Most of
the work has focused on the intercalation phenomenon and the effect of
intercalation on the physical properties of the host materials [3]. An
important aspect of this research is the alkali metal intercalation in
transition metal dichalcogenides, which modifies both the lattice structures
and the electronic properties of the hosts [4]. The discovery in the late
19603 by Gamble et al [5], which showed that the critical temperature (Tc)
for the onset of superconductivity in 2H—Ta82 could shift from 0.8 K to
about 7 K upon amine intercalation, was one of the most exciting events in
intercalation chemistry. An eye-catching research at present is the

intercalation of inorganic or organic compounds in the high—Tc

superconductors to adjust the properties and explore the mechanism of the
superconductivity [6]. In recent years, with the accumulation of a substantial
amount of knowledge about intercalation materials and the improvement of
the synthetic skills, the aim is set to explore the use of intercalation
reactions to synthesize novel and potentially applicable materials [7]. The

investigation of inorganic/polymeric intercalation compounds, or

ceramic/polymeric nanocomposites, is accelerated by this aim, and bursts
into a major branch in the field of nanophase—materials [8’ 9].

The materials which are intensively studied by intercalation are
members of the seven most common groups of layered materials, including
graphite, layered transition metal dichalcogenides, layered transition metal
oxides, layered metal oxyhalides, layered metal phosphorus
trichalcogenides, layered acid salts of tetravalent metals, and sheet
silicatesllOl. Graphite is the simplest host material and it shows diverse
intercalation chemistry due to its stability to both oxidation and reduction.
The known intercalating reagents range from strong oxidants to powerful

reductants [11].

Table 1. Elements forming layered transition metal dichalcogenides

 

 

 

 

 

 

Transition Metal Chalcogen
IV V VI VII VIII VI
8
Ti (V) (Cr) Se
Zr Nb Mo Pd Te
Hf Ta W Re Pt

 

 

 

 

 

 

 

 

Other layered materials have been explored since the beginning of
the sixties. Layered transition metal dichalcogenides form a large group,
since most of the transition metals from group IV, V, VI and some from
group VII and VIII can form dichalcogenides of layered structures (Table
1). The intercalation chemistry of the layered dichalcogenides is mainly
characterized by the reduction of the host lattices. Simple and hydrated
cations, and organic and organometallic ions can be included between the

layers of the lattices through electron donating processes. The layered

dichalcogenides have many unusual physical properties due to their
electronic structures (partial d-orbital filling) as well as their low
dimensionality, and these properties are adjustable by intercalation [4].

Similar to layered dichalcogenides are the layered oxides, which
exhibit quite the same intercalation chemistry. The best known of these are
molybdenum oxide, tungsten oxide and vanadium oxide, which form
various bronzes with alkali metals [12]. Layered oxyhalides MOCl ( M: Fe,
Ti, V and Cr) [13] and InOX ( X: Cl, Br and 1) [10b] exhibit some
interesting intercalation chemistry.

Metal phosphorus trisulfides of the formula MPS3 form a broad and
diverse class of compounds. The metal M includes Mg, Ca, V, Mn, Fe, Co,
Ni, Zn, Pd, Cd, Hg, In, Sn, and Pb. Metal phosphorus triselenides of the
similar layered structure also exist. Examples are FePSe3, MgPSeg,
MnPSe3, CdPSe3, SnPSe3, PbPSe3 and HgPSeg. Like transition metal
dichalcogenides, metal phosphorus trisulfides react with electron donors
and their intercalation chemistry is mostly confined to 3 major classes of

guests: alkali metals, organic amines and organometallic molecules. Ion

exchange reactions with intralayer M2+ ions and non-redox intercalation
reactions have also been developed recently [14].

Layered silicates and layered acid salts are currently of considerable
interest, because of technological applications in heterogeneous catalysis,
and as sorbents and inorganic ion—exchangers. Their intercalation
chemistry is predominantly of reactions with cations by ion-exchange, or
with neutral molecules [15]. Layered silicates include many naturally
existing and synthetic clay minerals [16]. Layered acid salts of tetravalent
metals have the general formula MIV(HXO4)2.nH2O (X=P, As). The most
extensively investigated species is oc-zirconium bis(monohydrogen

orthophosphate) [oc—Zr(HPO4)2.H20] [17]. The protons of these acid salts can

be easily replaced by other cations, so the layered acid salts are considered
attractive inorganic ion-exchangers. Also included in layered acid salts are
hydrogen uranyl phosphate and arsenate, HUO2PO4.4H2O and
HU02ASO4.4H20 [13], while vanadyl and niobyl compounds of the type
V0X04 and NbOX04 (X = S, P, As, Mo) are quite similar compounds [19].
Besides the seven large groups of layered materials, some groups of
layered inorganic compounds and some individuals that are not so common
have also been studied in intercalation. Reduced halides such as ZrCl, ZrBr
and ThI2 possess an important property involving reversible hydrogen
uptake to form ordered, stoichiometric phases. oc—RuCl3, Ni(CN)2 and B-
ZrNCl also undergo intercalation reactions [10b]. HFe(SO4)2.4H2O
intercalates monovalent or divalent ions along with the reduction of Fe(III)

centers [20].
Lamellar double hydroxides of the general formula [MELx

me(OH)2][Xm’x/m.nH2O] (where the X” is a halide anion or an oxo-anion)
are designated as "anionic clays", and are of great interest for their
capability to anion exchange [21]. [Zn1_xAlx(0H)2][Clx.nH2O] and [Cu1-
xCrx(OH)2][Clx.nH20] are examples of them. Basic copper acetate is
another anion exchangeable layered compound [7b]. A large series of
layered niobates, titano-niobates and titanates with perovskite related
structure and with alkali metal cations in interlayer sites have been
developed in recent years [15, 22]. Ion-exchange, intercalation of amines,
exfoliation and even electrochemical alkali metal insertion can be applied to
this group of layered oxides. Recently, intercalation chemistry is also found
in the pillaring of buaerite (Na4Mn14026.xH20) [23], the ion-exchange of
birnessite (Na0.32Mn02.nH20) [24], and the metal atom or simple molecule

insertion in the misfit layered sulfides [25].

The variety of layered inorganic compounds mentioned above
provides a broad choice of layered materials. To make these materials
constituents of nanocomposites, the chemistry of intercalation reaction is
the decisive factor. The common synthetic methods for intercalation
include direct insertion, ion-exchange, exfoliation and flocculation, and
electro-intercalation [10b]. The intercalation of alkali metals and other
metals into graphite, transition metal dichalcogenides, transition metal
oxides and oxyhalides, and metal phosphorus trichalcogenides can be
realized by direct intercalation. The insertion processes are accompanied
by the charge transfer from the intercalants to the host lattices. It is
believed that the tendency for this charge transfer is the driving force for
the intercalation reaction. Organic molecules can also be intercalated into
the same type of hosts through direct intercalation. Only Lewis-base
(electron-donating) organic compounds can be intercalated. The nature of
the bond between an intercalated molecule and the host layer is inquired.
An old model proposed a weak covalent bond involving no charge
transfer126l. More recent studies suggest that the molecule-host “bond” is
substantially ionic. In the new model, part of the intercalated molecules are
ionized by donation of an electron to the layer, whereas the remainder of
the molecules are neutral and solvate the ionized species [27].

Ion-exchange can be used to prepare intercalation compounds with
hosts of charged layers, such as clays, layered acid salts, and lamellar
double hydroxides. When a charged layered compound is immersed in a
concentrated solution containing a replacing ion, the interlayer ion can
often be replaced by the other ion. The exchange reaction is driven by the
great excess of the replacement ion. In the cases of neutral oxidizing
layered hosts such as transition metal chalcogenides, once a metal ion has

been intercalated it can subsequently be ion-exchanged. In many cases, the

approach of pre-intercalating with an alkali metal ion and ion-exchanging
the alkali metal cation with the target guest cation provides a useful
strategy for the intercalation of large guest cations which do not intercalate
directly.

Frequently, ion-exchange cannot be achieved in one step because the
replacing ion is very large whereas the interlayer gallery and the resident
ion are small. Multi-step ion-exchange processes are needed to overcome
this difficulty. By replacing the resident ion with suitable ions of
intermediate sizes, the gallery of the host can be opened gradually so that
finally the target guest ion can be inserted [17a]. The introduction of large
target ions by stepwise ion-exchanges clearly demonstrates the fact that the
high activation energies associated with the deformation of the layered
structures to accommodate the incoming guests can be overcome if the host
lattices can be expanded by pre-intercalation of smaller molecules or ions.
If this strategy is taken to its extreme and the host layers are completely
dispersed or exfoliated, very large molecules or ions can be included. This
is the case of intercalation by exfoliation and flocculation. Smectic clays
can be swelled in water and then intercalated with large cations such as
polyoxometallates [16, 28]. Na0,33TaS2 has been exfoliated in N-

methylformamide/H20 solution and intercalated with a large cluster cation

[F3688(PEt3)6]2+ [29]-

Finally, electro-intercalation provides a convenient way to prepare
metal intercalation complexes. Its advantages over conventional techniques
are its simplicity, facile control of stoichiometry, and fast rate of reaction
at room temperature. In addition, it provides a convenient method to carry
out detailed thermodynamic measurements and study the staging

phenomenon [10b].

These four well developed intercalation methods are very effective
in the simple ion and small molecule intercalation. They also shine light on
the strategies for polymer intercalation. However, because the
macromolecules are much greater in size and are usually chemically inert,
the practice of polymer intercalation is still full of obstacles and advances
slowly. The following section will show that the strategies applied to the
polymer intercalation are often different from the simple ion and small

molecule intercalation.

2. The Development of Polymer Intercalation

The synthesis of inorganic/polymeric intercalative compounds is
aimed at the materials for potential applications. Intercalation provides a
molecular level combination of two extremely different components, which
is expected to produce materials with superior or novel properties. Small-
molecule intercalation complexes are not a preferred choice, because they
intend to de-intercalate so that their compositions are not stable. Stability is
a prerequisite for a material in practical use. The polymer intercalants, in
contrast, stay in the interlayer galleries of inorganic lattices after the
formation of intercalative complexes, which assure steady compositions for
the nanocomposites. In addition, the polymers possess mechanical strength.
The polymer chains dangling from the layered structure hold the layered
materials together, so that the nanocomposites can be produced in various
forms, such as films and bulk materials, without additional supporting
substances. Both polymers and layered hosts are usually air and moisture
stable, so the nanocomposites can be used in ambient conditions without
much concern.

There exist a large number of layered materials and polymers, so the

combinations are numerous. The advance of the intercalative

nanocomposites depends on the development of the synthetic methods for
these materials, which are evolving from the conventional intercalation
methods. In the 1960s and early 19703, various types of ions and small
molecules had been intercalated into layered inorganic compounds, while
the intercalation of large molecules such as organic cations, large clusters
and polymers was still rare, even though the synthesis of
inorganic/polymeric intercalative complexes had been carried on since
1965 [30]. This situation existed because the intercalation of large molecules
is kinetically unfavorable and the reaction is technically harder.

The intercalation of robust cations was realized in later 19703, which
led to the intercalated smectite derivatives with pore sizes larger than those
of faujasitic zeolites. These intercalated smectite derivatives are known as
"pillared clays" [31].

On the other side, the worldwide interest in polymer intercalation
increased substantially in 1987 when Okada et a1 developed clay/polyamide
nanocompositesl32'33] and Kanatzidis et a1 synthesized inorganic/(conductive
polymer) layered complexes [34]. The clay/nylon—6 nanocomposites
exhibited extraordinary mechanical and thermal properties, which were
remarkably superior to its individual components, and immediately found
their use in automobile industry as the materials for timing-belt covers [33»
35]. Other polymers such as rubber [33], poly(e—caprolactone) [36], epoxy
resin [37], polyirnide [38] and polystyrene [39] have also been hybridized with
clays to make nanocomposites of excellent mechanical and thermal
mechanical properties.

Encapsulation of conductive polymers inside layered compounds
leads to anisotropically conductive materials. In addition, it is expected that
the overlapping and interaction of the electronic bands of the guest and the

host will bring up unpredicted novel physical properties. The first complex

of this type was FeOCl/polypyrrole and, since then, this aspect has always
been a hot topic in intercalation. So far, conductive-polymer intercalation
compounds with hosts such as FeOCl [34, 40], V205 [41], M003 [47-1, M082 [43],

metal phosphates [2013,44], halides (CdX42' and MnX42‘) [45],
HFe(SO4)2.4H2O[20bl, as well as clays [33b] have been reported. The
conductive polymers include polypyrrole, polyaniline, polythiophene,
polyfuran, poly(p-phenylenevinylene) and poly(diacetylene). Recently, it
has been shown that some of these electronically active nanocomposites
have better electrochemical properties than either the hosts or the
polymersl46, 47].

The importance of the outcome of the clay/nylon-6 and
FeOCl/polypyrrole nanocomposites to the research of polymer
intercalation was not only the demonstration of the applications of the
nanocomposites, but also the introduction of the new synthetic
methodologies associated with the preparation. The surfactant modified
clays used in the preparation of the clay/nylon-6 nanocomposites are good
starting materials for a lot of clay/polymer nanocomposites, while the in
situ redox intercalative polymerization used to synthesize the
FeOCl/polypyrrole nanocomposite suits for a class of layered hosts and
monomers. The procedures of the two methods are easy to carry out,
producing nanocomposites of well defined morphologies. Before the
establishment of these two methods, polymer intercalation was almost
exclusively accomplished by monomer intercalation and subsequent
intergallery polymerization. This classical method has not contributed
many nanocomposites because neither monomer intercalation nor
intergallery polymerization could be accomplished readily.

The success in preparing many new nanocomposites brought more

researchers into this field and gave rise to more efforts to develop new

methodologies for polymer intercalation. N azar et a1 made
MoO3/poly(phenylene vinylene) nanocomposites through the insertion of
the precursor ionomer by ion-exchange and the subsequent conversion to
the polymer [423’ b]. Ruiz-Hitzky et al incorporated poly(ethylene oxide)
(PEO) in layered silicate [48] and V205 xerogel [49] by treatment of the host
with non—aqueous polymer solution. PEO was also intercalated in MPS3
(M=Mn, Cd) by Lagadic et a1 by stirring the hosts with an aqueous solution
or a methanol solution of PEO [50]. In our group, PEO and other water-
soluble polymers were encapsulated in V205 xerogel through mixing the
aqueous solution of the two [51]. All these accomplishments were reported
in a short period in early 19903. By 1993, the preparation of
M082/polymer nanocomposites by exfoliation-encapsulative precipitation
(EEP) was reported by our groupl52], while the preparation of M082/PEO
and TiS2/PEO was reported by Ruiz-Hitzky's group [53] simultaneously.

The EEP method demonstrated its usefulness by intercalating a large
variety of polymers in M082 [52], a host which otherwise is very difficult to
intercalate with even small molecules [54]. It has been expected since that the
application of the EEP method to other layered materials would bring in
many new nanocomposites. This knowledge provided a promising starting
point for the present dissertation [4313’ 551, as well as the work of other
researchers 156’ 57]. On the other side, Giannelis et a1 developed the melt
intercalation method for organoclays in 1995 [58]. This method has a great
advantage in large-scale preparations, so the development is very important

to the commercialization of the organoclay nanocomposites.

10

3. Exfoliation-Encapsulative Precipitation: An Effective Method
for Polymer Intercalation

The exfoliation-encapsulative precipitation (EEP) method
corresponds to the exfoliation-flocculation method in conventional
intercalation. The latter was developed to intercalate robust cations in
negatively charged layered materials in late 19703 and early 19803 [28’ 291.
In the case of robust cation intercalation, the driving force was well
understood. It is the electrostatic attraction between the positively charged
cations and negatively charged layers. The net result of the reaction is the
ion-exchange of large cluster cations for small interlayer metal cations.

In the case of neutral polymer intercalation, however, no driving
force had been expected in such a situation. The anionic hosts, which can be
exfoliated, are considered extremely polar materials. The neutral
polymers, on the other hand, are usually non-polar or slightly polar
materials. It is hard to see the affinity between the two classes of materials.
Although Morrison et al had encapsulated a list of small organic molecules
in M082 by exfoliation-flocculation in 1986 [54], the event seemed not
impacting enough to persuade people to generalize this strategy to polymer
intercalation. Nazar et al [423, b] chose an ionomer rather than a polymer to
intercalate in an exfoliatable host, NaxMoO3. In the work of Ruiz-Hitzky et
al on the layered silicates [43] and the work of Lagadic et al on MP83 [50],
exfoliation was not applied. On the other hand, in the work of polymer
intercalation in V205 xerogel, which was done by our group [51], an
exfoliation step was not involved in the synthesis. Although all these efforts
enriched the preparation chemistry of inorganic/polymeric
nanocomposites, the peculiarities existing in either the polymers, the hosts,
or the reactions stopped people from extensively generalizing these

methods.

11

The design of the EEP method [52] was based on the expectation that
the monolayers of the inorganic compounds would combine with polymer
molecules in solution to form nanocomposites. The success of the method
relied on the co-precipitation of the monolayers and the polymer from
solution. The design of this reaction was somewhat different from that of
Ruiz-Hitzky et al for PEO intercalation in M082 and Ti82 [531, in which the

exfoliation was utilized to facilitate the polymer insertion. The trial of the
EEP method on M082 nanocomposites was a brilliant success. The direct
intercalation in M082 with a list of polymers, which included water soluble
polymers such as poly(ethylene oxide), poly(propylene oxide),
poly(ethylenimine), poly(vinyl pyrrolidone) and methyl cellulose,
commonly used polymers such as polyethylene and nylon-6, and conducting
polymer polyaniline [52], through a very simple procedure, brought the
attention of other researchers in this area. The successful application to a
common transition metal dichalcogenide using so many polymers showed
the generality of the method. The intercalation of these polymers also
indicated the existing affinity between the polymers and exfoliated layered
materials, which is a critical factor for the on going research in polymer
intercalation. Accordingly, the EEP method was established as a new
promising method for polymer intercalation.

For the EEP method to apply, the most important issue is the
availability of monolayer suspensions of the host materials. Monolayer
suspensions have been explored increasingly in the last 20 years as an
approach to prepare intercalative compounds with very large guest
molecules and high surface areas as catalysts, and to form monolayer
coatings and films. They have been reported in clays, layered transition
metal disulfides (TiS2, 2H-NbS2, 2H-Ta82, M082 and W82), transition metal

oxides (M003), metal oxychlorides (FeOCl 1591), transition metal

12

phosphorus sulfides (MnPS3), metal hydrogen phosphates [Zr(HPO4)2.H2O
and (H3O)U02PO4.3H20 (HUP) [601] and niobium oxides (HTiNb05 and
HCa2Nb3010). Dispersions of clay minerals have been studied most
extensively [61], which led to the synthesis of "pillared clays" and
delaminated clay-polymer nanocomposites. Alkali metal or proton
intercalated Ti82 [62], 2H—Nb82 [62, 63] and 2H-TaS2 [62, 63v 641 exfoliate when
mechanical stirring is applied. Lithiated M082 [54] and M003 [65] form
monolayer dispersions with the assistance of ultrasonic vibration. W82 [66’
67] monolayer suspension has been prepared following M082. Lithium ion
exchanged MnPS3 forms a colloidal dispersion in water spontaneously [68].
Acid zirconium phosphates, Zr(HPO4)2.H2O [69], and proton exchanged
niobium oxides (HTiNb05 [70] and HCa2Nb3010 [711) exfoliate after the

interlayer surface modification with pre-intercalated alkylamines. Further
development of the surface modification on zirconium phosphates leads to
zirconium organophosphates and phosphonates that can exfoliate
spontaneously in water [72]. A review for colloidal dispersion of layered
inorganic compounds was written recently by Jacobson [73]. In addition,
more reports about exfoliation have come out recently in misfit—layered
metal sulfides [74], layered metal phosphates [75] and niobium oxides [76], and
even high-Tc superconductors [77]. Our new success in exfoliation of oc-
RuC13 is described in Chapter 5. With the increasing interest in colloidal
monolayer dispersions, more exfoliated materials will be discovered.
Therefore, many host materials exist which might be amenable to the EEP
method and this promises a bright future for polymer intercalation and
associated nanocomposite materials.

The successful application of the EEP method also depends on the

affinity between polymers and the dispersed monolayers. This factor is

13

more decided by the nature of the dispersed monolayers rather than the
manipulation of the chemistry. Thus, even with the monolayer suspension
of a host material, intercalation of polymer is not guaranteed. However, it
is possible to change the preparative method or condition to adjust the
nature of the monolayer dispersions, so that the monolayers become
attractive to the polymers. The new preparation procedures for LixTaS2
and LixMoO3 described in Chapter 3 and 4 are examples of this
modification. It is also likely to choose some special conditions of
intercalation for some monolayer dispersions so that polymer intercalation
is favored. Examples of these reaction designs can be found in Chapter 1.
As a result of the factors described above, searching for the best conditions

to conduct polymer encapsulation is still inevitable.

4. Applications of Intercalation Compounds and Nanocomposites

The study of chemical intercalation is not only interesting but also
beneficial from a practical viewpoint. Intercalation offers a mild way to
modify the structures and properties of the host lattice through electron
transfer, ion-exchange, and guest molecule insertion. It is a unique
approach to obtain new materials that cannot be prepared by the
conventional techniques of solid state chemistry. Searching for new
materials for potential applications has been a hot topic in the field of
chemical intercalation for many years, and has become an important
driving force for research.

The most important application of intercalation materials is as
reversible electrodes for rechargeable batteries. The utilization of graphite
intercalation compounds as electrode materials in electrochemical
generators was initially proposed by Armand in 1973 [78]. In 1977, the idea

of intercalative chalcogenide batteries was developed by Whittingham [79].

14

Several ambient temperature secondary lithium battery systems such as Li—
TiS2, Li-MoS2, Li—NbSe3, Li-MoO2, Li—V205 etc. have been investigated at
various battery companies and R&D organizations for consumer and
defense purposes [80]. At present, the research on positive electrode
materials for secondary lithium batteries is mainly focused on LiCoO2 and
LiMnO2 [31].

On the other hand, the application or potential application of
intercalation materials as catalysts, ion-exchangers and sorbents is well
known [82, 33]. Pillared and delaminated clays are studied as shape selective,
heterogeneous catalysts for petroleum cracking, and their performance is
competitive with zeolite catalysts [16]. Layered double hydroxides are used
for the selective conversion of propylene oxide to primary and secondary
alcohols, the cross-aldol condensation reactions to form methylvinylketone,
the polymerization of lactones and the decomposition of ketones [34].
Pillared MoS2 can be used in hydrodesufurization [851. As sorbents, smectite
clays are used for decoloring edible oils, clarifying alcoholic beverages,
removing grease from raw wool and treating the radioactive waste
solutions [83]. Layered double hydroxides are used to take up acidic
impurities such as HCl generated from the photo decomposition of
poly(vinyl chloride) [84]. As sorbents, intercalation compounds also have
potential applications in the controlled releasing of perfumes, insecticides,
fertilizers [36], as well as medicines.

With advances in intercalation chemistry and the increase in the
number of investigated host and guest species, intercalation as an approach
for materials design is more and more clearly claimed, and more effort is
directed towards the synthesis of materials with novel structures and
properties. Intercalation readily provides materials with highly anisotropic

dielectric, conductive and optical properties [720. For example, layered

15

vanadyl phosphonates are used to place naphthalene units into ordered
arrays to achieve the control of their photophysical properties [371.
Intercalation can stabilize materials of nanometer dimensions, which
exhibit novel electrical, optical, magnetic, and chemical properties.

Intercalates of various functions have been included [38]. Luminescent

compounds such as Ru(bpy)32+ have been encapsulated in transparent hosts
such as smectite. The luminescent compounds are effectively isolated by the
hosts so that self-quenching is suppressed. A molecularly dispersed system
is also a basic prerequisite for composing efficient photochemical hole
burning (PHB) materials to avoid line broadening due to energy transfer.
The intercalation of PI-IB probes such as 1,4—dihydroxyanthraquinone
(DAQ) into transparent hosts is an effective way to achieve this
prerequisite [891. Electrochromic and photochromic compounds such as
viologen (1,1'-disubstituded-4,4'-bipyridinium salts) have been intercalated
into layered hosts to make electrochromic, photochromic and optical
memory materials [891. Dyes have also been intercalated in order to obtain
pigments with special spectral properties [89].

As a major branch of the class of intercalation compounds and
probably the most applicable group, nanocomposites currently receive
worldwide interest. As mentioned earlier, the organoclay/nylon
nanocomposites have already been used in timing-belt covers in
automobiles [35] due to their high strength, high yielding temperature and
good processibility. In addition, organoclay/polymer nanocomposites have
been produced as high-barrier plastic packaging materials [90], which have
gas barrier improvements as high as 800%. Nanocomposites can also have
better flame retardant properties [91, 9d] and lower dielectric permittivity
than the component polymers [921. The regular void structures, which exist

in specially designed nanocomposites, provide low dielectric constants and

16

may produce good electronic packaging materials. Materials of extremely
low dielectric permittivity can also be generated from the restricted
motions of ions and electrons in the network structures of nanocomposites.
Those materials have potential applications for optoelectronic packagingl7al.

In the field of electrode materials, the preliminary research on V205
nanocomposites shows that V205/polyaniline has a better reversibility and
an increased Li capacity in the electrochemical redox cycles, implying that
it is a better cathode material for Li rechargeable batteries [47]. The
V205/polyaniline also has a Li chemical diffusion coefficient one order of
magnitude higher than that of the host itself, which would provide better
performance at high current densities [47]. The electroactive polymer
intercalated nanocomposites usually have cation diffusion coefficients
higher than those of the unexpanded hosts, which makes it worthwhile to
explore the possibility of using the nanocomposite analog of a layered
electrode material as a substitute.

Polyether intercalation compounds, especially poly(ethylene oxide)
intercalation compounds, are of interest in solid state ionics. PEO acts as a
solid solvent for different salts. The PEO solutions of salts are
polyelectrolytes [931 and are used in solid state batteries [94] and other
electronic devices. The inclusion of PEO in clays and other charged
layered hosts provides fast ion conduction for the incorporated cations,
while the host materials act as inert, often insulating counter ions.
Therefore, the ionic conductivity is exclusively due to cations of the
interlayer region. This makes the transference number of the cation equal
to 1 (14:1), which is of interest in the study of ion-transport phenomena in
polymer electrolyte systems. Finally, the intercalation of PEO in
electronically conductive layered solids opens the way to new

polyelectrolyte materials of mixed ionic/electronic conductivity. PEO

l7

intercalation compounds have been reported with layered silicates [43],
layered metal phosphates [72b], layered metal phosphorus trisulfides [50],
V205 1511, M003 [55, 56b], transition metal dichalcogenides [52. 53, 571 and
carbon oxides [56d].

Using polymer intercalative nanocomposites, as precursors to
produce new ceramics and graphite films, is another direction of research
that carries the polymer intercalation into potential applications. Kato and
coworkers used magadiite-poly(acrylonitrile) complex as a precursor to
prepare silicon carbide and silicon nitride in carbothermal reduction
processes [39]. In an ordinary carbothermal reduction process, a mixture of

carbon and SiO2 is used as the starting material. A carbonized oxide-

polymer nanocomposite, in which the carbon and the oxide are intimately
mixed, may produce ceramics of better quality. At least, this is the case
with silicon nitride. The same group also prepared aluminum nitride by
conversion of a hydrotalcite-polyacrylonitrile intercalative nanocomposite
through a carbothermal reduction process. Kyotani and coworkers
prepared graphite films from montmorillonite-polyacrylonitrile
nanocomposites [95]. The films consist of highly crystallized but small
graphite crystallites. They have a much weaker mechanical strength and a
much poorer electrical conductivity than the graphite films prepared from

polymer films, but they have some flexibility.

5. Important Contributions of the Present Dissertation Work
The research described in the present dissertation started at a time
when a list of polymers had been intercalated in M082 by the EEP
methodl52]. To promote the research of polymer intercalation in M082,
which had not been finished by Bissessur [96], the study in the exfoliation of
M082, intercalation of polymers, and properties of hydrated M082,

18

restacked M082 and nanocomposites was continued. The study provides a
better understanding of this new group of materials, which leads to a better
manipulation of the materials and of the encapsulation reactions. During
this study, many new M082 nanocomposites, such as (polystyrene)xMoS2,
(polyacrylamide)xMoS2, (polyvinylalcohol)xMoS2, (polypropylene)xMoS2,
(polymethylmethacrylate)xMoS2, (polybenzimidazole)xMoS2 and
(polyethyleneterephthalate)xMoS2 have been synthesized. This part of the
research is described in Chapter 1. In order to intercalate insoluble
polymers in M082, a modification was made to the EEP method, which
brought in the in situ polymerization-encapsulative precipitation method.
This method has been successfully applied to the synthesis of
M082/polypyrrole nanocomposites, and is discussed in Chapter 2. The
polypyrrole intercalation in W82 is also reported in Chapter 2.

The most important achievement with respect to M082, in this
dissertation, is the invention and development of the high temperature
LiBH4 lithiation method. In this procedure, LiBH4 is proved to be a very
powerful reducing reagent at high temperatures. Up to now, this method
has been used to prepare other lithium-intercalated materials such as
LiWS2l67l and LiNbSe2 [571, which can exfoliate in water. It is expected to
be applicable to the lithiation of many additional layered materials. The
new lithiation method provides a convenient approach to prepare large
quantities of LiMoS2.

Believing that the EEP method can be generalized, other hosts were

also explored. Polymer intercalation was tried on 2H-TaS2, the exfoliation

of which had been reported [62» 63’ 64]. However, exfoliated TaS2’" prepared
according to the literature did not include polymers except polyimines. In

order to achieve the polymer intercalation, different forms of LixTaS2

19

were prepared with different lithiation reactants and under different
reaction conditions. Although most of the LixTaS2 phases can be hydrated
and dissolve somewhat in water, no polymer other than a polyimine can be
included. All this changed after the preparation of consistently uniform
LixTaS2 phases through a quantitatively controlled lithiation reaction using
a 0.2 equivalent of LiBH4. The exfoliation of and polymer intercalation in
2H-Ta82 is described in Chapter 3.

The two LiBH4 lithiation methods, by high temperature solid state
reaction and room temperature solution reaction, are complementary. The
high temperature lithiation, which has a strong driving force, works with
layered materials hard to reduce, such as M082 and W82. Over-lithiation
should not be a problem in these materials because they oxidize readily
when exposed to water in the exfoliation step. The controlled room-
temperature solution lithiation, using a low equivalent of LiBH4, can deal
with hosts that are readily reduced, such as TaS2 and M003. Since the
reduced forms of these materials are not readily oxidized by water and the
high charge density in the layers favors neither exfoliation nor polymer
intercalation, the charge carried by the layers should be controlled. The
two lithiation methods are expected to cover the exfoliation of a broad
range of layered materials.

Since the availability of stable monolayer suspensions is the
prerequisite to applying the EEP method, the importance of the two
lithiation methods is obvious. The exfoliation and polymer intercalation of
W82 [67, 97] and NbSe2 [571 is based on the first lithiation method. By using
the second lithiation method, M003 and oc-RuCl3 are exfoliated and
encapsulated with polymers. The nanocomposites with the latter two hosts

are described in Chapter 4 and Chapter 5.

20

The polymer insertion in M082, W82, NbSe2 [57], 2H-TaS2, M003 and
oc-RuCl3 using the EEP method has demonstrated its effectiveness. The
diversity of the host materials shows the generality of this method. At this
stage, the EEP method has been firmly established and become one of the
most effective tools in polymer intercalation. The present dissertation work
contributes a substantial part to the generalization of this new polymer

intercalation method and introduces many new lamellar nanocomposites.

21

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Elsevier, New York, 1979, pp 131. (c) M. Armand, Solid State
Ionics 1983, m, 745. (d) M. Armand, Adv. Mater. 1990, 2, 278.

M. Gauthier, D. Fauteux, G. Vassort, A. Bélanger, M. Duval, P.
Ricoux, J. M. Chabagno, D. Muller, P. Rigaud, M. B. Armand and
D. Derco, J. Electrochem. Soc: Electrochem. Sci. Technol. 1985,
Q, 1333. (b) C. C. Hunter, D. C. Sinclair, R. West and A. Hooper,
J. Power Sources 1988, E, 157.

T. Kyotani, T. Mori and A. Tomita, Chem. Mater. 1994, Q, 2138.

R. Bissessur Synthesis and Characterization of Novel Intercalation
Compounds of Molybdenum Trioxide and Molybdenum Disulfide,
Ph. D. Dissertation, Department of Chemistry, Michigan State
University, 1994.

The intercalation of polypyrrole in WS2 is reported in Chapter 2.

The intercalation of other polymers, done by H.—L. Tsai and M. G.
Kanatzidis, is to be published in the future.

31

Chapter 1

FURTHER EXPLORATION OF EXFOLIATED M082 AND SYNTHESIS
OF NEW M082 NANOCOMPOSITES WITH THE EXFOLIATION—
ENCAPSULATIVE PRECIPITATION METHOD

Introduction

The intercalation of polymers in layered inorganic solids is much
more difficult than the intercalation of small molecules because of their
large macromolecular size. The conventional direct intercalation method,
which accounts for the intercalation of most simple organic molecules, is
usually not applicable to polymers. The alternative route, which involves
monomer intercalation followed by intra-gallery polymerization, seems
elegant but is hard to carry out. Prior to our exploration of the
M082/polymer system, through which a new convenient synthesis method
was developed [1], the majority of the inorganic/polymeric nanocomposites
were classified into two categories: (a) nanocomposites of conjugated
polymers in oxidative hosts synthesized by the redox intercalative
polymerization, and (b) nanocomposites of clays fabricated with
organoclay precursors which can be delaminated. Due to the lack of highly
oxidizing layered materials, the application of the redox intercalative
polymerization is limited to certain hosts [2]. On the other hand, versatile
clay nanocomposites had been prepared with clays swelled and exfoliated
by surfactants 13]. This route pointed to a promising approach to new
classes of nanocomposites. As for other layered materials, however, the

methodology had been seldom applied, since the swelling property of clays

is mirrored in only a few layered inorganic compounds. The V205 xerogel,

32

which swells spontaneously in water and encapsulates a variety of water-
soluble polymers [4], is an example.

The new intercalation method, which we name exfoliation-
encapsulative precipitation (EEP) method, was first reported with the
simple organic compound intercalation in M082 [5]. It was then introduced
in the field of polymer intercalation in the development of the
M082/polymer nanocomposites [1] and proved to be convenient and
effective. In this method, the layered inorganic host is exfoliated and forms
a monolayer suspension. The monolayers combine with dissolved
polymeric molecules when a polymer solution is added, and the two types
of components precipitate out as a lamellar nanocomposite. The successful
application of the EEP method to M082, which is chemically inert, and
which cannot be directly intercalated by even simple organic molecules,
demonstrates the potential of the delamination methodology to the synthesis
of nanocomposites with common layered inorganic compounds. Later, the
EEP method was generalized to other layered hosts such as M003 161,
TaS2[7], W82 18], NbSe2 [9] and so on. Nanocomposites prepared with this
method are even superior to the clay nanocomposites in respect of
compositional and structural simplicity, since no surfactant is involved.

Earlier work in this group [1] succeeded in encapsulating many types
of soluble polymers, which included water soluble polymers such as
poly(ethylene oxide) (PEO), poly(propylene glycol) (PPG),
polyvinylpyrrolidone (PVP), polyethylenimine (PEI) and methyl cellulose
(MCel), common plastics such as polyethylene (PE) and nylon-6 (PA6),
and conductive polymers such as polyaniline (PANI). The physical
properties of the restacked M082 and the synthesized M082/polymer

nanocomposites were also examined. It was noticed at that time that the

33

restacked M082 probably had the intralayer structure of 1T-MoS2 [10] in
which the Mo atoms were octahedral coordinated. This form was
metastable converting exothermically to the normal 2H form of M082
which had Mo atoms in the trigonal—prismatic coordination. Later, it was
proved by electron diffraction that the M0 in the exfoliated and freshly
restacked M082 has a distorted octahedral coordination [11]. The conversion
of the M082 causes many interesting changes in the properties of restacked
M082 and nanocomposites. Although extensive, the investigation in
exfoliated/restacked M082 and nanocomposites was not completed in
Bissessur’s dissertation work. Since M082 is a readily available commercial
material and has many important applications such as lubrication, the
catalyzation in hydrodesulfurization [12] and the rechargeable battery
fabrication [13], a more extensive exploration in M082/polymer
nanocomposites was conducted in the present dissertation research. This ‘
research was carried in four directions: (a) searching for more economic
and convenient methods to prepare exfoliated M082, (b) further studies to
better understand the nature and properties of exfoliated M082, restacked
M082, and important M082/polymer nanocomposites such as (PEO)xMoS2,
(c) further development of the EEP method, and (d) exploration of new
strategies to synthesize M082 nanocomposites with insoluble polymers. The
research on the former three parts is reported in this chapter. The last one

will be described in Chapter 2.

34

Experimental Section

1. Reagents

M082 (99%, lum size aver.) was purchased from Cerac Inc. n-Butyl
lithium (2.5 M solution in hexane), LiBH4 (95%), poly(ethylene oxide)
(Molecular Weight 100,000), nylon-6 (MW 10,000), polystyrene (MW
280,000), polyacrylamide (MW 5,000,000), poly(vinyl alcohol) (MW
124,000—186,000), polypropylene (MW 250,000), poly(methyl
methacrylate) (MW 120,000), polybenzimidazole (melting point > 300 OC),
poly(ethylene terephthalate) (MW 18,000) and high density polyethylene
(MW 125,000) were purchased from Aldrich Chemical Company.
Polyvinylpyrrolidone (MW 10,000) was from Sigma Chemical Co. CaH2
(>95%), hexane (GR) and ethyl acetate (GR) were purchased from EM
Science Inc. Formic acid (88%, ACS Grade), decalin (purified), 200 proof
ethanol and phenol (reagent grade) were from Columbus Chemical
Industries Inc., Fisher Chemical Company, Quantuum Chemical Company
and J. T. Baker Chemical Co. respectively. 2,2,2-Trifluoroethanol (99%)
was purchased from both Lancaster Synthesis Inc. and PRG Catalog Inc.

Water for experiments and processing was deionized water.

2. Lithiation and Exfoliation of M082

a. Preparation of LiMoSz by reaction of M082 with n-butyl lithium
In this classical method of preparation of LiMoS2 [14], M082 was

reacted with excess n-butyl lithium (n-LiBu) in solution under an inert

atmosphere. In a typical reaction, 4 g of M082 (0.025 mol) was reacted in

75 m1 of 1 M n-BuLi/hexane solution (0.075 mol of n-BuLi) for 3 days.

The LiMoS2 was collected by filtration, washed with copious hexane and

35

pumped to dryness. The reaction and processing were under nitrogen

atmosphere. The hexane was distilled from CaH2 under nitrogen before

use. The product was stored in a nitrogen glove box after preparation.

b. Preparation of LiMoS2 by reaction of M082 with LiBH4

This convenient new method of LiMoS2 preparation was developed
in the present research. In this method, M082 was combined with LiBH4 in
a solid state reaction at an elevated temperature to produce LiMoS2. In a
typical reaction, 10.00 g of M082 (0.0625 mol) and 2.04 g of LiBH4
(0.0938 mol) were ground and mixed together in a nitrogen glove box.
They were put into a ceramic crucible and placed in a quartz tube (2.5 in.
diameter, 34.5 in. length) which had a balloon connected to it, see Figure
1.1. The quartz tube was taken out of the glove box and inserted into a
programmable furnace so that the crucible was in the middle of the hot
zone while the outlet, connected to the balloon, was out of the furnace. The
furnace was heat up to 350 0C in 3 h, kept at this temperature for 72 h and
then cooled to room temperature in 3 h. The gas produced by the reaction
was accumulated in the balloon which expanded, thus avoiding explosion.
After the reaction, the quartz tube was separated from the balloon and
taken into the nitrogen glove box. The product was collected as a mixture

of LiMoS2, LiBH4 and LiH, and stored under a nitrogen atmosphere.

c. Exfoliation of LiMoS2

The LiMoS2 prepared by the LiBu procedure was pure. An amount
of 0.2 g of this LiMoS2 (1.2 mmol), was typically used in a reaction, and
was exfoliated in 20 ml of water by 1 h of stirring. The exfoliated M082

was collected by centrifugation to separate from the LiOH produced

36

    
  
  
 

Balloon

Rubber tubing

Furnace

Quartz tube

Figure 1.1. Experimental set—up for the high temperature LiBH4 lithiation
reaction.

37

by the reaction. It was re-dispersed in 20 ml of fresh water by 30 min of
stirring for succeeding reactions.

The product from the LiBH4 lithiation method was a mixture of
LiMoS2, LiBH4 and LiH. An amount 0.213 g of this mixture contained
about 0.2 g LiMoS2 (1.2 mmol). This amount of the mixture was stirred in
20 ml of water for 1 h. Centrifugation was used to collect exfoliated M082
from the solution. The restacked M082 was washed again with 20 ml of
water and re-dispersed in 20 ml of fresh water by 30 min of stirring for

subsequent reactions.

3. Investigation of LiMoS2 and Freshly Restacked M082

a. Thermal stability of LiMoS2
LiM082 prepared by the LiBu procedure was checked by the

differential scanning calorimeter (DSC) up to 300 CC. The curve of DSC
did not show exothermic or endothermic peaks, which meant no obvious
phase change in the above temperature range. A second heating cycle gave
almost the same curve as the first one. The sample could exfoliate after this

process.

b. Elemental analysis and 7Li NMR of freshly restacked M082
An amount of 1.07 g of the LiBH4, LiH and LiMoS2 mixture, which

was produced from the LiBH4 lithiation reaction and contained about 1.00
g LiMoS2 (6.0 mmol), was washed 6 times by stirring in 100 m1 portions
of water for 30 min and collecting by contrifugation. The restacked M082
so obtained was pumped for a week to dryness and used in 7Li solid state
nuclear magnetic resonance (NMR) experiments and for inductively

coupled plasma (ICP) spectroscopy analysis.

38

NMR experiments detected strong 7Li signals which assured the

existance of Li inside the sample. The ICP analysis showed 0.686 wt% Li,
51.8 wt% Mo and 35.9 wt% 8, which corresponds to Li0,13MoS2.

4. Preparation of M082/Polymer N anocomposites
a. Polyl ethylene oxide) (PEO) nanocomposites

The ratio of PEO to M082 used typically in this work was ~ 2 to 1.
In a typical reaction, 20 ml of an aqueous solution containing 0.106 g (2.4
mmol) PEO was mixed with 20 ml of aqueous M082 monolayer suspension
containing 1.2 mmol of M082, and stirred for 2 days in a stoppered 125 m1
Erlenmeyer flask. The product nanocomposite was collected by
centrifugation and washed thrice with 20 ml portions of water. The
product was then dried in air and pumped to dryness. To cast a
(PEO)xMoS2 film, the paste of the product was spread on a glass plate in
air before drying. For reactions using other PEO/MoS2 ratios, the amount
of polymer varied accordingly.

In reactions designed to produce delaminated (PEO)xMoS2
nanocomposites, the ratios of PEO to M082 used were 2, 4, 8 and 16, and
the concentrations of the polymer in solution were 3%, 4%, 8% and 13%
respectively. The concentration of the M082 monolayer suspension was
twice as that of the typical one described above. After the two solutions, 10
ml of M082/H2O and a corresponding amount of PEO/H2O, were mixed
and stirred for 2 days, the mixture was pumped to dryness. X-ray
diffraction indicated that the mixture was composed of the known
(PEO),,M082 phase ((1 spacing 16.2-17.7 A) and bulk PEO. Neither

delaminated (PEO)xMoS2 nanocomposites nor nanocomposites with widely

spaced layers were produced.

39

b. Polle-vinyl pyrrolidone) (PVP) nanocomposites

In a reaction, 20 ml of aqueous M082 monolayer suspension, which
contained 0.2 g of M082 (1.2 mmol), was mixed with 20 ml of aqueous
PVP solution which contained 0.533 g of PVP (4.8 mmol VP repeat units),
and was stirred for 2 days. The (PVP)xMoS2 was collected by
centrifugation, washed 3 times with water, dried first in air and then under

vacuum. The (PVP)xMoS2 product had a basal spacing ~ 28 A. Washing

with water did not have a significant effect on the d spacing of the product.

c. N Ion-6 A6 nanocom osites
Procedure 1: As the first of the two procedures, (PA6)xMoS2
nanocomposites were prepared using PA6/CF3CH2OH solution. The method
was a modification from the one used earlier [1]. In a typical reaction, 10
ml of M082 suspension containing 0.20 g M082 (1.2 mmol) was added
dropwise into 20 ml of PA—6/CF3CH2OH solution under stirring. The
CF3CH2OH solution contained 0.49 g of PA6 (4.32 mmol C6H11NO repeat
units). The solution was stirred for 2 days before it was centrifuged to
isolate the nanocomposite from it. The product was washed with 5 ml of
CF3CH2OH/H2O (2:1) solution, and dried first in air and then under
vacuum.

In the reaction above, the ratio of PA6 to M082 was 3.621. Other
ratios such as 0.2, 0.5, 1.0, 2.0, 4.0, or 10 to 1 were also used.

Procedure 2: (PA6)xMoS2 nanocomposites can also be synthesized by
a reaction between an aqueous M082 monolayer suspension and a PA6
formic-acid solution. In a typical reaction, 20 ml of M082 suspension

containing 0.20 g M082 (1.2 mmol) was added dropwise into 100 ml of

40

PA6/HCOOH solution which contained 0.68 g of PA6 (6.0 mmol C6H11NO
repeat units) under vigorous stirring. A large volume of HCOOH was
required to keep PA6 in solution. The solution was stirred for 3 h before it
was centrifuged to separate the nanocomposite. The product was washed
with water 3 times and methanol once, and dried first in air and then under

vacuum.

d. Polystyrene (P8) nanocomposites

(PS)xMoS2 nanocomposites were successfully prepared by mixing
and reacting the aqueous M082 monolayer suspension with very
concentrated PS/(ethyl acetate) solutions. In a typical reaction, 20 ml of
M082 suspension containing 0.20 g M082 (1.2 mmol) was mixed with 20 ml
of ethyl acetate solution which contained 5.0 g of PS. The two solutions
were immiscible, so the mixture was stirred vigorously for 2 days. When
the mixture was centrifuged, it separated in 3 layers: PS/(ethyl acetate)
solution on the top, water solution at the middle and (PS)xMoS2
nanocomposite at the bottom. The (PS)xMoS2 nanocomposite was collected

and dried without washing. Washing with ethyl acetate de-intercalates PS
from M082.

e. Polyacgylamide (PAM) nanocomposites
In a typical reaction to prepare a (PAM)xMoS2, 20 ml of M082

suspension containing 0.20 g M082 (1.2 mmol) was mixed with 40 ml of
aqueous PAM solution which contained 0.43 g of PAM (containing 6.0

mmol AM units). The mixture was stirred for 2 days before it was

centrifuged to collect (PAM)xMoS2, which was washed with water 3 times,

and dried first in air and then under vacuum.

41

f. Polylvinyl alcohol) (PVA) nanocomposites

PVA is only slightly water soluble. In a typical reaction to prepare a
(PVA)xMoS2 nanocomposite, 20 ml of M082 suspension containing 0.20 g
M082 (1.2 mmol) was mixed with 80 ml of aqueous PVA solution which
contained 0.53 g of PVA (12 mmol VA units). The PVA did not totally
dissolve. The mixture was stirred for 9 days before it was filtered through
a coarse filter paper to remove undissolved PVA. The solution through the
filter paper was centrifuged to collect (PVA)xMoS2. The nanocomposite

was washed and dried as in the case of (PAM)xMoS2 above.

g. Polypropylene (PP) nanocomposites
An amount of 0.056 g PP (1.3 mmol C3H6 repeat units) was

dissolved in 40 ml of hot decalin (100 - 140 OC). The hot PP solution was
added into 20 ml of cold aqueous M082 monolayer suspension, which
contained 0.20 g of M082 (1.2 mmol), under vigorous stirring. The
mixture was stirred at 90 0C for 4 h. The product was collected hot by
centrifugation, and washed first with copious hot decalin (100 0C) and then

with ether.

h. Poly(methyl methacrylate) (PMMA) nanocomposites

Procedure 1: An amount of 0.12 g of PMMA (containing 1.2 mmol
MMA units) was dissolved at 50 0C in a mixture of 50 m1 ethanol and 5 ml
water. 10 ml of aqueous M082 monolayer suspension, which contained 0.1
g M082 (0.6 mmol), was added dropwise into the PMMA/C2H50H/H2O
solution while stirring vigorously at 50 0C. The mixture was stirred at 50

0C for 12 h after the addition of M082. The product was collected by

42

centrifugation, washed 3 times with formic acid and 3 times with water,

and dried first in air and then in vacuum.

Procedure 2: An amount of 0.60 g of PMMA (containing 6 mmol
MMA units) was dissolved in 100 ml of formic acid. 10 ml of aqueous
M082 monolayer suspension, which contained 0.1 g M082 (0.6 mmol), was
added dropwise into the PMMA/HCOOH solution, which was stirred
vigorously. The mixture was stirred for 12 h after the addition of M082.
Since PMMA did not precipitate out, the product was collected by
centrifugation. The nanocomposite was washed and dried as in Procedure 1

above.

1. Polybenzimidazole (PBI) nanocomposites

An amount of 10 ml of aqueous M082 monolayer suspension, which
contained 0.1 g of M082 (0.6 mmol), was added dropwise into 75 ml of
saturated PBI formic-acid solution (containing about 0.37 g PBI, or 1.2
mmol BI units), which was stirred vigorously during addition. After the
mixture was stirred for 12 h, the product was isolated and processed the

same as described in Procedure 1 above.

j. Polyl ethylene terephthalate (PETP) nanocomposites
An amount of 0.56 g of PETP (containing 3.0 mmol ETP units) was

dissolved in 25 n11 of phenol at 50 0C. The PETP/phenol solution was
added dropwise into 10 ml of aqueous M082 monolayer suspension which
contained 0.1 g M082 (0.6 mmol) and which was stirred vigorously at 50
0C. The mixture was stirred at 50 0C for 2 days after the addition. Phenol

and water are immiscible, so vigorous stirring was required. The product

43

was collected by centrifugation at an elevated temperature (50 - 75 0C),

washed with 75 0C phenol 3 times, and dried as above.

k. Elevated temperature annealing and variable temperature X-ray powder

diffraction measurements for nanocomposites
Samples of nanocomposites, (PEO),,M082, (PA6)xMoS2, (PS)xMoS2,

(PE)xMoS2 and (PP)xMoS2, were vacuum sealed in Pyrex tubes and heated
in an oven at temperatures between 150 and 175 0C for several days to
check the possible thermal decomposition of the nanocomposites.

X-ray powder diffraction patterns were collected as a function of
rising temperature for a (PEO),,M082 sample (not heat treated) to
investigate the decomposition process. The patterns were collected at room
temperature, 80 OC, and then every 10 0C from 100 0C to 230 OC. The
sample was stabilized at each temperature for 10 min before the pattern
was collected. The patterns were collected from 20 5° to 8° and then from

13° to 16°. Each temperature step took 40 min.

5. Instrumentation

Powder X-ray diffraction (XRD) patterns were obtained on a Rigaku
Ru-200B X-ray diffractometer, at 45 kV and 100 mA with a scintillation
counter detector and a graphite monochromator to produce Cu Koc beam
(wavelength 1.54184 A). Powder samples and a continuous scanning mode
with a scanning speed of 1°/min in 20 and an increment of 001° were
chosen for general purpose spectra. Variable temperature XRD
measurements were done with a ceramic heating mask under the protection

of nitrogen. The scanning speed was 0.2°/min. XRD experiments for one-

44

dimensional electron density calculations were done as described in Chapter
3.

The amount of polymers in the nanocomposites was determined by
TGA measurements with a Shimadzu TGA-50 under a 46 ml/min oxygen
flow, and a heating rate of 10 oC/min. The decomposition temperature of
materials was checked with TGA measurements under a 57 ml/min
nitrogen flow. DSC was carried out on a Shimadzu DSC-50 under nitrogen
flow of a rate of 20 ml/min. The heating and cooling rates were 5 oC/min.
Sample cells were made of aluminum, and were annealed at 450 0C in
vacuum sealed tubes after they were cleaned. Samples were sealed in cells
under a nitrogen atmosphere before measurement.

Infrared spectra were collected with a Nicolet IR/42 FTIR
spectrometer in 2 cm-1 resolution. Generally 64 scans were collected.
Samples were measured as KBr pellets. The amount of Li in restacked
M082 was measured with Inductively Coupled Plasma (ICP) spectroscopy
at Animal Health Diagnostic Laboratory [15] on the MSU campus. Variable
temperature solid state 7Li NMR spectra were taken on a 400 MHz Varian
Nuclear Magnetic Resonance Instrument.

Room temperature electrical conductivity measurements for Figures
1.6 and 1.9 were done on pressed sample pellets with a four-probe detector
connected to a Keithley-236 source measure unit. Room temperature
conductivity measurements for data in Table 1.4 were done on pressed
sample pellets with a four—probe detector connected to a Keithley-580

micro-Ohmmeter.

45

Results and Discussion

1. The LiBH4 Method of Producing LiMoS2

The exfoliated M082, which is the crucial material in the preparation
of M082 nanocomposites using the EEP method, is generated from the
reaction of LiMoS2 with water. In earlier studies of M082/polymer
nanocomposites, LiMoS2 was prepared by the conventional method [14]
which uses LiBu in a room temperature reaction. Despite the strict
anaerobic reaction conditions and the large excess of highly reactive LiBu,
this method is still the best way to prepare pure LiMoS2, especially in small
quantities. Here this route was still used occasionally to prepare LiMoS2 for
the reactions in which pure LiMoS2 was preferred as a starting material.
(By “pure”, we mean free of LiH and LiBH4, as discussed below.) In the
preparation of most of the M082/polymer nanocomposites, however, the
use of pure LiMoS2 is not necessary, which suggests the possibility of a
more convenient and economical route for preparation. The synthesis of
LiMoS2 with LiBH4 at elevated temperature provides such a route.

LiBH4, and its analogs NaBH4 and KBH4, are convenient reducing
reagents in terms of storage and manipulation. Their stability in dry air
and slow hydration in water [16] requires neither sophisticated equipment
nor extreme care in processing. In most of its reducing reactions, the
byproducts are B2H6 and H2. These gases are separated from the solid or
liquid products spontaneously, therefore, the products are usually pure and
no additional purification step is needed. In our LiBH4 route to prepare

LiMoS2, the reaction also generates these two byproducts:

2M0$2 + 2LIBH4 ------ > 2L1M0$2 + B2H6 + H2

46

The reaction probably generates some LiH because of the decomposition of

LiBH4 at high temperature:

2 LIBH4 ------ > 2LIH + B2H6

The LiH and the excess LiBH4 stay in the solid phase with LiMoS2 as a
mixture. Fortunately, in order to generate exfoliated M082, it is not
necessary to purify LiMoS2 from this mixture of products which contains
LiBH4 and probably LiH. When the product is put into water, the LiBI-I4

and LiH dissolve and decompose to form LiOH, and the resulting exfoliated

M082 can be readily purified by washing with water [171.

2LiMoS2+2(1-X)H2O ----- >2LixM082+2(1-X)LiOH+(l-x)H2
2LiBH4 + 2H2O ------ > B2H6 + 2H2 + 2LiOH
L1H + H20 ------ > H2 + LIOH

Therefore, this new route produces pure exfoliated M082 for intercalation
reaction, although the LiMoS2 from the lithiation reaction is in a mixture
with LiH and LiBH4.

A high quality monolayer M082 suspension should be free from the
unexfoliated material. This requires all M082 to be converted to LiMoS2 in
the lithiation reaction. Since both M082 and LiBH4 can be ground to fine
powder and mixed well before the lithiation reaction, complete lithiation of
M082 is achieved. Therefore, the high temperature LiBH4 lithiation
reaction produces high-quality exfoliated M082.

Since the new reaction is carried out in the solid state, a small

reaction vessel can deliver a large quantity of product. The reaction set-up

47

is simple and no strict air-exclusion is necessary. LiBH4 is more expensive
than LiBu ($118/mol vs. $53/mol according to the price of Aldrich
Chemical Co.). However, considering the fact that only 1.5 equivalents of
LiBH4 is used and no solvent purification is required, the LiBH4 approach
is actually more economic.

LiBH4 is generally considered as a mild reducing reagent and has
been used mostly in organic reactions. In the field of inorganic chemistry,
it has been used to reduce M03, V205, FeOCl, TiS2, TaS2 [131. At room
temperature, LiBH4 cannot reduce chemically inert materials such as M082,
while its usage at elevated temperatures had not been seen reported.

The preparation of LiMoS2 with LiBH4 at an elevated temperature is
the most convenient method to obtain LiMoS2 so far. There have existed
several other routes to prepare alkali metal dichalcogenide ternary
compounds, however, all of them have some shortcomings. The
preparation reaction using binary chalcogenides and alkali metals in liquid
NH3 at ca. -30 0C [191 is even harder to deal with than the reaction with
LiBu. The reaction of binary chalcogenides with alkali halide melts in a
H28 gas flow [20] needs solid-gas reaction facilities. In addition, the constant
blowing of toxic H28 is undesirable. The reaction of binary chalcogenides
with alkali metals at ca 800 0C [21] seems convenient. However, when we
used this method to prepare alkali metal intercalated M082 and then
exfoliated M082, we found the existence of un-reacted M082 in the product.

The possibility of using another less-expensive reducing reagent such
as LiAlH4 or LiH to substitute LiBH4 was also explored without success.
Although LiAlH4 can produce LiMoS2 at temperatures higher than 300 0C,
the by-product A1H3 forms Al(OH)3 in water, which is hard to separate

from exfoliated M082 and interferes with the polymer encapsulation

48

process. It causes the production of low quality nanocomposites with short

coherence length. LiH did not lithiate M082 under similar conditions and a
more severe condition (720 0C) caused decomposition. Similar to LiBH4,
NaBH4 also introduces Na” in M082 and the product forms hydrated
NaxMoS2 in an aqueous solution. Nevertheless, hydrated NaXMoS2 does not
readily include polymers, therefore more investigation is needed to develop

it into a starting material for nanocomposites.

2. Manipulation and Properties of Exfoliated and Restacked M082

In the old exfoliation procedure of LiMoS2, the suspension was
sonicated for 30 min after the material had been put in water. The purpose
of this ultrasonic wave treatment was to maximize the possibility of full
exfoliation. The strong perturbation associated with this process broke the
M082 up into small sized layers and reduced the quality of the
nanocomposites. Later we found that this ultrasonic process was not
necessary because completely exfoliated M082 can be obtained when
LiMoS2 is just stirred in water.

The cause of exfoliation of LiMoS2 in water was once suggested to
be the splitting of the M082 layers by the violent reaction of the material
with water and the vigorous generation of the hydrogen gas inside the
interlayer space [53, 5C]. This mechanism predicts that pre-oxidation of
LiMoS2 would severely affect the exfoliation process, because it would
reduce the strength of this explosive reaction. This prediction demanded
the strict exclusion of oxygen and moisture from LiMoS2 before its
exfoliation.

The fact that we were able to insert PEO in LiMoS2, which was

exposed to air for ten hours, showed the ability of exfoliation of freshly

49

oxidized LiMoS2 and demonstrated a negligible effect of the pre-oxidation
to the exfoliation property of LiMoS2. Therefore, the splitting mechanism
of the exfoliation seems questionable. Further investigations show that the
exfoliation is due to the hydration or solvation of the M082 layers. The
above experiments also suggest that storage of the LiMoS2 under inert
atmosphere for the purpose of exfoliation does not have to be extremely
stringent.

It was believed that when LiMoS2 was exposed to water, it was fully
oxidized back to M082 and the exfoliated layers were neutral. Researchers
in this area were excited by the thought of neutral suspended M082
monolayers in water because this phenomenon would be an exception
among the exfoliated layered inorganic compounds. Other layered
inorganic compounds either have charged layers or need assistance from
surfactants to form monolayer suspensions. Our experience suggests that
the concept of neutral M082 layers is incorrect and they actually carry
negative charge. One reason for believing this is that exfoliated M082
encapsulates cations more readily than neutral molecules. Another reason is
that solid state 7Li nuclear magnetic resonance (NMR) of restacked M082,
which was carefully washed with water, proved the existence of Li in this
material. In addition, ICP spectroscopic analysis revealed that the
composition of the freshly restacked M082 was probably LixMoS2 (x =
0.18). More convincing experiments which prove that the exfoliated and
freshly restacked M082 layers possess charges are those done by Heising
and Kanatzidis [11], which include the electron diffraction study revealing
the intralayer structure of the freshly restacked M082 different from 2H-
MoS2, and 1H—NMR experiments which prove that the freshly restacked

M082 evolves hydrogen gas when it is heated. Therefore, restacked M082 is

50

analogous to LixTiS2 and LixTaS2, which exfoliate in water and some polar
solvents because the layers carry charges.

The incomplete oxidation of the M082 layers when LiMoS2 is
exposed to water and the charges that the exfoliated M082 layers carry
suggests a solvation mechanism for the exfoliation. The negative charges of
the M082 layers hold Li+ cations inside the interlayer galleries to balance
the charge, while the Li+ cations are solvated by water molecules. The
solvation of the Li+ cations makes the M082” layers exfoliate. This
mechanism explains why the pre-oxidized LiMoS2, which in fact is not
M082 but LixMoS2, can still exfoliate. It also explains why the water
molecules are hard to remove out the freshly restacked M082 1221 and why
the freshly hydrated M082 can exfoliate again. Dehydration at elevated
temperature such as 45 or 60 0C [51’] is more effective, since heating
accelerates the conversion to 2H-MoS2 [11 and the additional oxidation of
LixMoS2. A similar explanation applies to the fact that the pre-oxidized and

high temperature (150 OC) treated LiMoS2 could not exfoliate.

3. (PEO)xMoS2 and (PVP)xMoS2 Nanocomposites

In previous work [1], the reaction for (PEO),,M082 nanocomposites
was carried out exclusively around the stoichiometrical ratio of 1 mol PEO
to 1 mol M082. In the present research, reactions have been run under
different PEO to M082 ratios to explore other (PEO)xMoS2 nanocomposite
phases. Although the ratio of PEO to M082 has been increased gradually
from 1 to 20 in the reactions, the basal spacing of the resulting
nanocomposite always falls in the range between 15.3 to 18.5 A, as
indicated by XRD patterns. Even in the reactions designed to produce

delaminated (PEO),,M082 nanocomposites, in which the aqueous M082/PEO

51

solution was dried under constant agitation, the reaction would not produce
nanocomposites of higher d spacing, or delaminated (PEO)xMoS2
nanocomposites. (A constant agitation during drying the mixture is
considered the best way to keep the M082 and PEO from phase separation.)
Therefore, basically only one type of (PEO)xMoS2 nanocomposite has been
prepared from the solution-encapsulation reaction.

Contrary to the results obtained with 1 to 1 ratio of PEO to M082,
when the ratio is increased to 2:1 or above, the probability of producing a
well ordered nanocomposite product is largely increased, and the products
are free from the restacked M082 phase. The order in the structure of the
nanocomposite can be judged from the sharpness of the peaks in the X-ray
diffraction patterns. The d spacings of the products usually range between
16 and 17 A, and the nanocomposites usually contain 1.5 equivalents of
PEO instead of 1.0, as revealed by TGA measurements in an oxygen flow.
These experiments indicate that some excess of polymer is needed to fully
occupy the galleries of M082.

It must be mentioned that prolonged excessive washing of the freshly
collected nanocomposites with water will lower the quality of the product.
After this treatment, the nanocomposites frequently have X-ray patterns of
low and broad peaks, which indicates poor stacking orders. From X-ray
diffraction patterns, it is calculated that the coherence lengths of the
nanocomposites in the stacking direction may decrease from about 120 A to
about 43 A [23]. In some cases, excessive washing would also cause the
appearance of the deintercalated phase.

The one-dimensional electron density map of a (PEO),,M082
nanocomposite has been calculated from high quality reflection XRD

patterns carefully collected from an oriented nanocomposite film, see

52

 

 

 

 

 

 

001
002
3‘
a
d
a
C:
1—1
003005 007008 0011
00 008010 0012
d 0013
x5
J _. AAAAA x10
I I I I I I I I I I I I I I I I I I I l I I T] I—I I
0 20 40 6O 80 100 120 140
20 (deg)

Figure 1.2. XRD patterns of a (PEO)xMoS2 nanocomposite used to
calculate the one-dimensional electron density map.

 

 

Electron Density

 

 

 

 

 

c—axis

Figure 1.3. One-dimensional electron density map of a (PEO),,M082
nanocomposite. (Solid line, calculated from experimental data; dash line,
from model.)

53

 

 

I1.51 A
3.27 A

Pacesgssee .0...

PEG ‘3‘ ’3‘ a a a a
,.

A A ' A A A A A A a A ~ A

 

 

 

M082 0.0.0.000...

14.46 A

 

Figure 1.4. Structural model for the (PEO),,M082 nanocomposite.

54

Figures 1.2 and 1.3. The features of the map are almost the same as those
for the Li0,2(PEO)xTaS2 nanocomposite which will be presented in Chapter
3. The model proposed according to the one-dimensional electron density
map is shown in Figure 1.4, which is similar to the one for
Li0_2(PEO)xTaS2. In this model, the PEO chains take a conformation that
was found in type II PEO-HgCl2 complex. Since the one-dimensional
electron density map and the model for the Li0,2(PEO)XTaS2 nanocomposite
will be described in detail in Chapter 3, a similar discussion is not given
here.

Possible existing phases other than the phases reported in the
preceding research have also been searched in (PVP)XM082
nanocomposites. In previous work, the PVP to M082 ratio used in the
encapsulation reaction was 1 to 1 [11. It was suspected that the amount of
PVP might not have been sufficient to produce the nanocomposite with
maximum polymer content. Reactions with excess PVP have verified the
suspicion and produced a (PVP)XM082 of higher basal spacing, 28.8 A (see
Figure 1.5A) instead of 21.1 A. The amount of polymer in the
nanocomposite is 0.96 VP units rather than 0.76 VP units obtained from
the 1:1 ratio. The temperature of the structural transition from the

octahedral to the trigonal-prismatic M082, however, is 174 OC, almost the

same as that of the previously reported (PVP)xMoS2, 177 0C.

4. Investigations in (PA6)xMoS2 Nanocomposites
The reaction to encapsulate PA6 in M082 used earlier [1] was

modified here. The new procedure makes use of the advantage that a

solution composed of 1/3 of water and 2/3 of CF3CH2OH does not cause

PA6 to precipitate, so that no PA6 will come out of solution when the

55

 

 

 

 

 

 

Intensity

(PAM)O~6M082
r k ‘%

16.9 A
-- JPVA)35M082

 

 

 

 

 

 

Figure 1.5A. XRD patterns of the new nanocomposite phases (1).

56

 

35.2 A

 

 

 

 

 

 

 

 

 

(PA6)025M082
- 1. ; ; "':;T
44.4A
(PMMA)O.90M082
g.» 60.1 A
E”
.2 (PMMA) MoS
'5 044 2
28.6A
(PBI)0_65M082
33.4A
(PETP)028M082
[IIIIITIIIIIIIIIIIIIIIllllllfl'—I
0 5 10 15 20 25 3O
20 (deg)

Figure 1.5B. XRD patterns of the new nanocomposite phases (2).

57

aqueous M082 monolayer suspension is added into the PA6/CF3CH2OH
solution. These conditions are considered good for nylon-6 encapsulation,
because they are free from competing side reactions, and the product is
free from excess nylon-6. The basal spacing of the nanocomposite is 14.8
A, smaller than the previously reported value 17.5 A [1], see Figure 1.5A.
The composition determined by TGA is (PA-6)xMoS2 (x=0.26—0.44).

The driving force to form (PA6)xMoS2 is not strong in the
CF3CH2OH/water solution. Although the amount of nylon-6 in the
nanocomposite is only about 0.26-0.44 C6H11NO per M082, more than 2
equivalents of nylon-6 must be used to make the product free from the
restacked M082 phase. When the ratio of nylon-6 to M082 is 1 or less, the
(PA6)xMoS2 nanocomposite of 15.0 A d spacing forms together with
restacked M082. When the ratio decreases from 1 to 0.2, the amount of
restacked M082 increases substantially from 1/3 to 2/3 of the total product.
When the ratio is 2, no restacked M082 is observed. Instead, a phase that
has a basal spacing of 10.6 A appears. This phase will diminish when the
ratio increases, however, it will not be eliminated even when the ratio
reaches 10. At a ratio of 2, the amount of the 10.6 A phase is about 40%; at
a ratio of 3.6, the amount is about 30%; at a ratio of 10, it is about 22%.
The reactions are summarized in Table 1.1.

The encapsulation using M082 dispersed in CF3CH2OH did not
succeed. The product from this reaction in water-free CF3CH2OH solution
was found to be mostly restacked M082. The weak driving force to form
(PA6)xMoS2 nanocomposite might be the reason for the failure of
encapsulation.

CF3CH2OH is not an ideal solvent for reactions. It is volatile, highly

toxic and extremely expensive. An alternative solvent for nylon-6 is formic

58

Table 1.1. Nylon-6 intercalation reactions with PA6/CF3CH2OH solution

 

 

 

 

 

 

 

PA6:MoS2 phase of phase of phase of
(mol:mol) d=15 A d=10.6 A 6.2 A
02:10 33% — 67%
10:10 67% - 33%
2.0: 1.0 60% 40% -
3.6210 70% 30% -
10: 1.0 78% 22% -

 

 

 

 

 

acid, however, it causes the exfoliated M082 to precipitate, which is an
unwanted competing side reaction. Delaminated (PA6)xMoS2
nanocomposites or high d—spacing products have been obtained by mixing
the aqueous M082 monolayer suspension and the nylon-6 formic-acid
solution but some restacked M082 phase often exists. Because the quick
precipitation of exfoliated M082 in formic acid solution, the arrangement

of polymer chains is poor and loose. A sample with a 35 A d spacing (see

Figure 4B) has only a polymer content of 0.25 C6H11NO units per M082.

5. Synthesis of (PS)xMoS2 N anocomposites

Polystyrene (P8) is the most common commercial polymer. It can be
dissolved in many organic solvents, including both polar and non—polar
solvents. Therefore, it is often a model polymer for basic studies in
polymer science. However, a difficulty has been met in the experiments
encapsulating polystyrene in exfoliated M082: polystyrene does not have a
strong tendency to combine with the M082 monolayers. The solvents for

polystyrene, on the other hand, are more or less prone to combine with

59

M082 monolayers and stay in the interlayer galleries of M082. As a result,
in many cases the solvents compete with polystyrene to form solvated M082 '
phases. These solvents include benzene, carbon tetrachloride, chloroform,
acetone, toluene, decalin and THF.

Out from all of the solvents we tried, ethyl acetate turns out to be
suitable for polystyrene encapsulation. To maximize polystyrene's ability to
compete, concentrated polystyrene/(ethyl acetate) solutions were used. The
concentration of the solution was 20 wt% or above, which is very high
compared to the concentrations of other polymer solutions mentioned
above. (PS)xMoS2 nanocomposites prepared are either delaminated
nanocomposites, or nanocomposites with a very large (1 spacing (see Figure
1.5A), depending on the concentration of the polystyrene solution and
other encapsulation conditions. Washing (PS)xMoS2 nanocomposites with
ethyl acetate causes the deintercalation of polystyrene and produces

restacked M082. The information of some (PS)xMoS2 nanocomposites

prepared under different conditions are summarized in Table 1.2.

Table 1.2. Information about the synthesis of M082/PS nanocomposites

 

 

 

 

 

 

 

 

 

PS reaction PS/MoS2 in

reaction concentration temperature d spacing composite

(V%) (A) (mol/mol)
trial 1 20% RT. 38.4~41.4 2.3/1.0
trial 2 23% RT. -* 3.8/1.0
trial 3 20% 65 0C -* 6.8/1.0

 

* The latter two samples are delaminated.

60

 

The reaction at an elevated temperature (65 OC) favors the
completion of intercalation. This phenomenon is reminiscent of the
successful high temperature encapsulation of the other two non-polar
polymers, polyethylene and polypropylene. At elevated temperature, the
exfoliated M082 converts to the 2H form, which probably facilitates the

intercalation of non-polar polymers.

6. Synthesis of Other M082/Polymer N anocomposites

Since many water-soluble polymers were already intercalated in
M082 [1] with no difficulty, the effort in the present research was mainly on
the intercalation of polymers dissolved only in non-aqueous solutions. The
only two new nanocomposites which contain water-soluble polymers are
(PAM)xMoS2 (PAM = polyacrylamide) and (PVA)xMoS2 (PVA =
polyvinylalcohol) (see Figure 1.5A and Table 1.3). Again, these two
polymers are intercalated readily.

It was noticed by Bissessur that polypropylene (PP) can be
intercalated in M082 with the same procedure as that for polyethylene.
However, because no good sample was obtained, he did not mention this
nanocomposite in his dissertation. Satisfactory (PP)xMoS2 samples have
been obtained in the present research. Like (PE)xMoS2 nanocomposites,
(PP)xMoS2 nanocomposites have basal spacings around 10.2-10.3 A, see
Figure 1.5A. The weight percentage of polymer is also similar in these two
nanocomposites. (PE)1,5MoS2 contains 21% polymer while (PP)1.1M082
contains 22% polymer.

Poly(methyl methacrylate) (PMMA) is also a very common polymer,
which is soluble in many solvents. Encapsulation of PMMA is successfully

carried out in formic acid or a 50 OC ethanol/water solution. The

61

encapsulation using PMMA/(formic acid) solution is similar to the
encapsulation of nylon-6 in formic acid. M082 precipitates immediately
when it is added into PMMA/(formic acid) solution, encapsulating some
polymer to form the nanocomposite. The polymer chains are not well
arranged in the interlayer galleries. The product has a large basal spacing,
60 A (see Figure 1.5B), but a low polymer content, 0.44 MMA units per
M082. If the reaction is not controlled well, restacked M082 phase may
appear. In 50 0C ethanol/water solution, neither polymer nor exfoliated
M082 precipitates in short time, so the encapsulation can be carried out
without strongly competing side reactions. The polymer chains can take
time to arrange themselves inside the interlayer galleries and pack more
tightly. Therefore, the product should have a better quality (in terms of
stacking order) and a higher polymer content. The basal spacing was only
44 A (see Figure 1.5B) but the polymer content is 0.9 MMA units per
M082. PMMA can also be dissolved in ethyl acetate, however,
encapsulation with ethyl acetate solution was not successful.
Polybenzimidazole (PBI) is a high-temperature engineering polymer.
The (PBI)xMoS2 nanocomposite synthesized using PBI/(formic acid)
solution has a basal spacing of 28.6 A (see Figure 1.5B) and a polymer
content of 0.65 BI units per M082. Poly(ethylene terephthalate), an
important engineering polymer like nylon, was also encapsulated in M082
using a 50 oC phenol solution. The nanocomposite has a basal spacing of 33
A (see Figure 1.5B) and a polymer content of 0.28 ETP units per M082.

Information on these nanocomposites is summarized in Table 1.3.

62

Table 1.3. Characteristics of some new M082 nanocomposites

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

nanocomposite solvent for Treaction d spacing T0h-->2H
reaction (A) (DC)
(PEO)1,5MoS2 H20 R.T. 17.0 159
(PVP)0,96MoS2 H2O R.T. 28.8 174
(PA6)0,44M082 CF3CH2OH R.T. 15.1 153
(PA6)0,25M082 HCOOH R.T. 35.2 126
(PS)2,3MoS2 CH3COOCH2CH3 R.T. 38.4 undetected
(PS)6_8M082 CH3COOCH2CH3 65 OC delaminated undetected
(PAM)0,60MoS2 H2O R.T. 15.6 200
(PVA)35MoS2 H20 R.T. 16.9 -
(PP)1_1MoS2 decalin 40-90 0C 10.3 184
(PMMA)0,90M082 ethanol 50 0C 44.4 -
(PMMA)0,44M082 HCOOH R.T. 60.1 1 17
(PBI)0.65MoS2 HCOOH R.T. 28.6 -
(PETP)0_28M082 phenol 50 0C 33.4 -

 

63

 

7. The Phase Transition in Restacked M082 and M082 Nanocomposites

The exfoliated M082 and freshly restacked M082 have a metastable
structure in which Mo atoms have a distorted local octahedral coordination.
This structure originates from the lithiation reaction, which converts the
2H form M082 (trigonal-prismatic coordination) to the IT form LiMoS2
(octahedral coordination). The structural conversion takes place to keep a
lower coordination energy in the product LiMoS2 [101. When LiMoS2 is
converted to exfoliated M082 by reacting with water, Mo changes an
oxidation state +3 to an oxidation state close to +4, in which the trigonal
prismatic coordination rather than the octahedral coordination provides
lower coordination energy. However, exfoliated M082 or freshly restacked
M082 does not complete the structural conversion immediately due to a
kinetic detainment. Mo adapts a distorted octahedral coordination [10] and
slowly converts to the trigonal prismatic coordination over time. The
conversion is exothermic and is accelerated at an elevated temperature or
under pressure [11.

It is important to have knowledge of the rate of the structural
conversion, because the two forms of M082 have different physical
properties and the structural conversion of M082 brings about a slow
property evolution in restacked M082 and nanocomposites. For example,
the octahedral M082 is metallic while the trigonal prismatic M082 is a
semiconductor. The conversion of M082 makes the conductivity of
restacked M082 drop gradually at room temperature from 12 S/cm to 0.07
S/cm in 4 months. Although the activation energy of the structural
transition was measured by Bissessur 11], the room-temperature conversion

of the materials was not tracked. Here, the progress of the room-

64

 

    
   

 

 

 

 

 

 

 

(A) 1 month —~>I 1 year——+§ o
8 1" i
E E :
_cz _ _
g _ -
29 _ .
IE 100 :— _,
.‘é E E
3:3 3 curve fit: , . .
101— p = 32.531 1 ~39" 0.000194 (t- 1230);] _
_ I I 1 [III I I I I II:II| I I I l I III] I I :
102 103 104
Time (hr)
1.00
(B) /
A 9’
E 0.80— / —
U /
* /
/
CF) 060— / —
v /
>3 0/
E 0.40— / / —
”ti; / °
E /
m 0.20- / ’6 —
)3
9
000 / I I I I I I
20 40 60 80 100 l 20 140 l 60
Time (hr)

Figure 1.6. Evolution of the electrical resistivity of a restacked M082
sample as a function of time. (The sample was dehydrated by methanol.)

65

temperature conversion was followed by a time dependent measurement of
the materials' electrical conductivity.

Figure 1.6A shows the follow-up of the resistivity of a restacked
M082 sample over more than 3 years. In the 3 years, the resistivity
increased almost 3 orders of magnitude, with the major change happening
in the first four months. DSC experiments showed that the sample after the
3 years of measurement did not have any thermal transition from room
temperature up to 300 °C, which indicates the completion of the structural
conversion. The profile of resistivity can be roughly fitted with an
exponential function as follows [241:

p = 32.5 .k [1 _ e — 0.000194(t-18.0)] (2*cm (1)
Possible relationships between the change of conductivity (or resistivity)
and the conversion of the M082 layers are discussed in Appendix A. Figure
1.6B shows that the resistivity increases linearly along with time in the
early stages. Actually, the short-time-period resistivity-time relationship is
almost linear in all the ranges. The linear relationship of resistivity and
time can be expressed in a first order equation:
P = 90 + b t (2)

This relationship is also observed in other restacked M082 and M082
nanocomposite samples. The coefficient b, which reflects the rate of the
structural conversion of M082, is listed in Table 1.4.

The coefficient b of a nanocomposite is smaller than the one of
restacked M082, which indicates a slower conversion of M082 in
nanocomposites. This agrees well with the previous findings of the higher
transition temperatures of nanocomposites detected by DSC. A new fact
revealed by the comparison of coefficient b is that the additional oxidation

in air accelerates the structural conversion of M082. This conclusion is

66

Table 1.4. Evolution rates of restacked M082 and nanocomposites derived
from the change of electrical resistivity

 

 

 

 

 

 

 

 

 

samples environment b (Q*cm*day'1)
restacked M082 air 3.3, 5.0, 6.7 [25]
(PEO)xMoS2 air 0.066, 0.40
(PA6)xMoS2 air 1.0, 2.7, 3.4
restacked M082 nitrogen 1.0, 1.5

 

* The different b values of the same nanocomposite were obtained from
different samples.

drawn because the coefficients b of the restacked M082 samples in air are

larger than those in nitrogen.

8. Thermal Stability of M082 N anocomposites

Annealing at high temperatures, for example 160-170 0C, causes a
slow decomposition in some of the nanocomposites. Deterioration has been
seen in samples of (PEO)XMoS2 and (PA6)xMoS2 heated at 160-170 0C for
2.5 days. X-ray patterns of the nanocomposites after annealing show a
significant weakening and broadening of the 001 peaks. The 001 peaks also
shift to lower d spacings. After annealing at 160-170 0C for 2.5 days, the
(PS)xMoS2 sample that had a broad 001 peak around 38.4 A lost its peak in
the X-ray pattern and became amorphous. However, annealing at this
temperature seems to have no effect on (PE)xMoS2 and (PP)xMoS2
nanocomposites. The higher stability of these two nanocomposites probably
originates from the PE and PP’s higher heat resistance and stronger
tendency to align and pack in lattices or templates. In all the five
nanocomposites, annealing does not bring up an intense restacked M082

peak in the X—ray patterns. Table 1.5 summarizes the effects of annealing

67

Table 1.5. The effect of high temperature annealing to some
nanocomposites

 

 

 

 

 

 

 

 

 

 

sample dbefore dafter comments on XRD pattern
(A) (A) after the annealing

(PEO)1,5MoS2 17.0 16.4 001 peak intensity decreased

(PA6)0,1M082 16.8 15.2

(PA6)0,25MoS2 35 .2 - 001 peak intensity decreased

(PS)2,3MoS2 38.4 amorphous a very low peak at 6.4 A
like

(PE)4_5MoS2 10.2 10.2 no change in the peak

(PP)1,1M082 10.3 10.3 no change in the peak

 

 

* The samples were annealed at 160-170 0C for 60 hours.

 

 

 

 

 

 

 

 

AM (PEO) M08
,9 AK (PEO) M08 150 °c 72 hours
a L...
3
E.
[I (PP)XMOSZ 160-170 °C 60 hours
IllIl llllllIl
0 10 20 30 40 50 60
20 (deg)

Figure 1.7. XRD patterns of the (PEO)xMoS2 and (PP)xMoS2
nanocomposites before and after the high temperature annealing.

68

 

250_ .
200 _ j
>. - -
9‘5, : _ :
g 150 _ : i
H .. _.
a - _
p—i _ _
eg 100_' j
o - -
Q-t - _
50_§ -
t - 2

0 .

 

 

80 100 120 140 160 180 200 220
Temperature (0C)

Figure 1.8. Intensity of the 001 peak in the XRD patterns of a
(PEO)xMoS2 sample at increasing temperatures.

on the M082 nanocomposites. In Figure 1.7, the X-ray patterns of some
annealed (PEO)xMoS2 and (PP)xMoS2 samples are compared with those of
the original samples.

A variable temperature powder X-ray diffraction experiment was
done on a (PEO),,MoS2 sample to investigate the effect of temperature on
the rate of the deterioration. Figure 1.8 shows the result of the experiment
with a plot which displays the variation of the intensity of the 001 peak at
increasing temperatures. The plot shows that the deterioration was slow
when the temperature was less than 100 0C. In the range of 100 to 140 0C,
the deterioration speeded up. After 140 0C, the intensity change of the 001
peak slowed down, probably due the fact that the order of the lamellar
structure had been so poor that additional deterioration did not bring up a

more significant disorder. In this high temperature XRD experiment, the

6.2 A peak, which belongs to restacked M082, was not observed.

69

Concluding Remarks

This chapter presents extensive results on the exfoliation and
polymer intercalation of M082. A significant achievement was the
development of the high temperature LiBH4 lithiation method, which
provides a convenient way to produce large quantities of exfoliated M082
and which promises to be applicable to many other layered metal
chalcogenides. Further investigation was devoted to the exfoliated M082
and restacked M082 in order to identify the material, explain the
mechanism of the exfoliation and seek better design of intercalation
reactions. We learned that the identity of the exfoliated M082 and restacked
M082 is hydrated LixMoS2, with x about 0.18. This material exfoliates in
water under weak shearing forces, similar to hydrated LixTiS2 and
LixTaS2. Once the hydrated LixMoS2 is dehydrated, it cannot be re-
hydrated or re-exfoliated, probably due to the additional oxidation
involved in those de—intercalation experiments, and the conversion to 2H-
M082.

Better synthesis methods were found for known nanocomposites. For
example, (PE0)xMoS2 and (PPV)xMoS2 of higher polymer content were
produced when sufficient polymer was provided in the encapsulation
reaction. In addition, many new M082 nanocomposites were synthesized
and added to the list, which include (PS)xMoS2, (PAM)xMoS2,
(PVA)xMoS2, (PP)xMoS2, (PMMA)xMoS2, (PBI)xMoS2 and (PETP)xMoS2.
One-dimensional electron density maps obtained for (PEO)xMoS2 based on
XRD powder patterns suggest a (PEO)xMoS2 model in which the PEO

chains take a conformation found in type II PEO-HgCl2 complex, similar to

70

the cases of Li0_2(PEO)xTaS2 and Li0,2(PEO)xRuCl3 which will be
presented in Chapters 3 and 5.

The evolution of the electrical conductivity of the restacked M082
was monitored. The curve of the increasing resistivity can be roughly fit
with an exponential function of time. The electrical conductivity, which is
inversely proportional to the resistivity, mainly changes in the first 4
months, by almost 3 orders of magnitude. The electrical conductivity
studies also demonstrate that the intercalation of polymers slows down the
process of the structural conversion of the M082 layers. In addition, they
show that the conversion is slower in nitrogen than in air. Many
M082/polymer nanocomposites are not stable at temperatures higher than
100 OC. Heat treatment not only causes the structural conversion of the

M082 layers, but also destroys the lamellar structure.

71

Appendix A

The decrease of the conductivity of restacked M082 is caused by the
gradual conversion of the M082 layers, from a metallic form to a
semiconducting form. There must exist some relationship between the
conductivity (or resistivity) and the amount of M082 converted. On the
other hand, the conversion of M082 is a certain function of time. These two
events together decide the evolution of the conductivity (or resistivity).
This section provides a discussion about the possible mathematical functions
in these two events by deducing the equations which express the
conductivity as a time function and which fit the experimental data.

In conductor-insulator mixtures, the conductivity often follows a
simple power law:

6 = C2(¢-¢p)° (3)
where 0 is the conductivity, C2 a constant, 6 the volume fraction of the
conductor, and 0p the percolation threshold. Percolation theory [261 predicts
that the (pp for a three-dimensional network of conducting globular
aggregates in an insulating matrix should be ~0.16. In many conductor-
insulator mixtures, however, this threshold is not seen or is extremely
small [27].

Equation 4, which generally fits the conductivity profile (see Figure
1.9), is deduced supposing that the simple power law suits the mixture of
the restacked M082 and the conversion of M082 precedes as a second order
reaction.

6 = 233*[1 + k(t—18.0)]'°‘ + 0035*{1 + [k(t-18.0)]’l }'B (4)

k=0.06155, OL=1.517, 13:2.701

72

 

The coefficients in the equation are derived by curve fitting to the
experimental data. The unit of time, the moment of t = 0, and the time
adjustment of 18.0 hours, are the same as explained in discussion section

7 [24]. The deduction of equation 4 uses the equations of the second order

reaction
[maul - 060:0)? = la. (5)
¢1T(t=0) = 1, (6)
and the equation of the simple power law
6 = Cushion” + C2H*[¢2H(t)]B. (7)

If the assumption above is correct, the time for half of the lT-MoS2 to
convert is k-l, which is about 16 hours.

In nature, most self conversion reactions are first order reactions,
for example, the decay of the radioactive isotopes. The combination of the
first order reaction of the restacked form of M082 and the simple power
law of conductivity can not produce a function which fits the experimental
conductivity data. As described in the discussion section, the profile of
resistivity can be roughly fit with an exponential function, equation 1:

p = 32.5 * [1 _ e - 0.000194(t-l8.0)] gram (1)
If the conversion of M082 layers from the octahedral to the prismatic
coordination proceeds as a first order reaction

¢1T(t) = 6"“, (8)
to derive an equation of the above type the resistivity need to be

proportional to the amount of the converted M082
P = 90*[1'¢1T(t)]. (9)
Nevertheless, the assumption that the resistivity is proportional to the

amount of converted M082 does not have a solid experimental or

theoretical basis, so this second curve fitting is less likely than the first one.

73

 

100 curve fit:

I IITI
I 1 111111

0' = 28.3*[1+k(t-18.0)]'° + 0.035*[1+1/[k(t-1 6.0)]1"3
=0.0615, 0t=1.52, 0:270

10

I I IIIIIII
I I IlIlIIl

I I IIIIIII
I I IlIIIII

0.1

Conductivity (S/cm)

 

I I 111111]

Tj IIIIIII

 

 

 

0.01 l 11111111 1 11111411 1 11111111 1 1111.“
101 102 103 104 105
Time (hr)

Figure 1.9. Curve fitting for the time dependence of electrical
conductivity of (PEO)XMoS2.

74

References

10

(a) R. Bissessur, M. G. Kanatzidis, J. L. Schindler, C. R. Kannewurf,
J. Chem. Soc., Chem. Commun. 1993, 1582. (b) R. Bissessur,
Synthesis and Characterization of Novel Intercalation Compounds of
Molybdenum Trioxide and Molybdenum Disulfide, Ph. D.

‘ Dissertation , Department of Chemistry, Michigan State University,

1994.

(a) M. G. Kanatzidis, L. M. Tonge, T. J. Marks, H. O. Marcy and C.
R. Kannewurf, J. Am. Chem. Soc. 1987, £12, 3797. (b) M. G.
Kanatzidis, C.-G. Wu, H. O. Marcy and C. R. Kannewurf, J. Am.
Chem. Soc. 1989, m, 4139. (c) G. Matsubayashi and H. Nakajima,
Chem. Lett. 1993, 31.

(a) Y. Kojima, A. Usuki, M. Kawasumi, A. Okada, T. Kurauchi and
O. Kamigaito, J. Polym. Sci. A: Polym Chem, 1993, 31, 983. (b)
H. Shi, T. Lan and T. J. Pinnavaia, Chem. Mater. 1996, 3, 1584. (c)
E. P. Giannelis, Adv. Mater., 1996, 3, 29.

Y. -J. Liu, D. C. DeGroot, J. L. Schindler, C. R. Kannewurf and M.
G. Kanatzidis, Adv. Mater. 1993, 5, 369.

(a) P. Joensen, R. F. Frindt and S. R. Morrison Mater. Res. Bull.
1986, 21_, 457. (b) M. A. Gee, R. F. Frindt, P. Joensen and S. R.
Morrison Mat. Res. Bull, 1986, 21_, 543. (c) W. M. R.
Divigalpitiya, R. F. Frindt and S. R. Morrison, Science 1989, A6,
369.

L. Wang, J. Schindler, C. R. Kannewurf and M. G. Kanatzidis J.
Mater. Chem. 1997, Z, 1277.

(a) Chapter 3. (b) L. Wang, J. Schindler, C. R. Kannewurf and M.
G. Kanatzidis, paper in preparation.

H.-L. Tsai, L. Wang, J. Schindler, C. R. Kannewurf and M. G.
Kanatzidis, paper in preparation.

H.—L. Tsai, J. L. Schindler, C. R. Kannewurf and M. G. Kanatzidis,
Chem. Mater. 1997, 2, 875.

(a) F. Wypych and R. Schbllhorn, J. Chem. Soc., Chem. Commun.

75

11

l2

13

14

15

16

17

18
19

20
21

22

1992, 1386. (b) D. Yang, S. J. Sandoval, W. M. R. Divigalpitiya, J.
C. Irwin and R. F. Frindt, Phys. Rev. B. 1991, $3, 12053. (c) M. A.
Py and R. R Haering, Can. J. Phys. 1983, Q, 76.

(a) J. Heising and M. G. Kanatzidis, J. Am. Chem. Soc. 1999, 1__1,
638. (b) J. Heising, Synthesis and Characterization of Novel
Intercalation Compounds of Molybdenum Trioxide and Molybdenum
Disulfide, Ph. D. Dissertation, Department of Chemistry, Michigan
State University, 1999.

O. Weisser and S. Landa, Sulfided Catalysts, Their Properties and
Applications, Pergamon, New York, 1973.

(a) H. Tributsch, Faraday Discuss. Chem. Soc. 1980, _7_()_, 190. (b) C.
Julien, S. I. Saikh and G. A. Nazri, Mater. Sci. Eng. 1992, B_15, 73.

(a) M. B. Dines, Mat. Res. Bull. 1975, 10, 287. (b) D. W. Murphy,
F. J. Di Salvo, G. W. Hull, Jr., and J. V. Waszczak, Inorg. Chem.
1976, L5, 17.

We thank Dr. W. Emmett Brazelton and Kirk J. Stuart for the
measurements.

 

F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, 5th
Ed., John Wiley & Sons, New York, 1988, pp 190-191.

LiMoS2 itself also generates LiOH when it reacts with water. The

presence of the LiOH will usually not significantly affect the
polymer intercalation reaction.

M. G. Kanatzidis and T. J. Marks, Inorg. Chem. 1987, 2_6, 783.

(a) W. Riidorff, Chimia, 1965, _1_9_, 489. (b) J. Rouxel, M. Danot and
J. Bichon, Bull. Soc. Chim. Fr. 1971, 3930.

R. Schollhorn and A. Lerf, J. Less-Common Met. 1975, :12, 89.

W. P. Omloo and F. Jellinek, J. Less-Common Met. 1970, 20, 121.
A restacked M082 freshly collected from an aqueous monolayer
suspension was a hydrated phase with a basal spacing of 12.1 A. Two

days of pumping the sample, at room temperature, only dehydrated
about 50% of the material giving a basal spacing of 6.2 A.

76

23

24

25

26

27

Washing the freshly restacked M082 with anhydrous methanol
altered the basal spacing of M082 from 12.1 A to 9.4 A. The de-
intercalation of methanol could have been accomplished by one t9
two days of dynamic pumping, to produce a phase with a 6.2-6.3 A
basal spacing. This dehydrated M082 could not be re-exfoliated.

For the calculation of the X-ray scattering coherence length of the

nanocomposites in the restacking direction, see Reference 18 of
Chapter 2.

The unit of time in the equation is hour. The time at which the
LiMoS2 was exposed to water is chosen as t = 0. Since the structural
conversion of M082 probably started at some time after the first
exposure to water, a time adjustment is imposed in the equation. The
time adjustment of 18.0 hours is decided by the best fitting of the
equation to the experimental data. This time adjustment also
compensates the fact that the M082 converted at a slower speed when
it was stirred in a closed reaction flask or dried in a vacuum
chamber than when it was stored and measured in air.

The b value calculated from Figure 5B is 0.16 Q*cm*day-1. Since

the measurements for Figure 5 were done with pressed pellets and a
four-probe detector connected to a Keithley-236 source measure unit
rather than the Keithley-580 micro-Ohmmeter, the values should not
be compared to those in this table.

R. Zallen, The Physics of Amorphous Solids, Wiley, New York,
1983, Chapter 4.

A. J. Heeger and P. Smith, in J. L. Brédas and R. Silbey ed.,
Conjugated Polymers, Kluwer Academic Pub.,
Dordrecht/Boston/London, 1991, pp 141.

77

Chapter 2

INSERTION OF POLYPYRROLE AND POLY(N—METHYL PYRROLE)
IN M082 AND W82 BY AN IN SITU POLYMERIZATION-

ENCAPSULATIVE PRECIPITATION METHOD

Introduction

Great effort has been dedicated to the development of transition—
metal-compound/conductive-polymer nanocomposites [1] under the
expectation that novel electrochemical properties will be obtained by the
molecular level interaction of the two electrochemically active components.
These expectations now begin to materialize based on recent research
which reveals that certain polymer nanocomposites indeed possess better
electrical or electrochemical properties, especially in ion conductivity [2]
and ion mobility 13]. Early transition metal dichalcogenides make a large
class of layered materials which possesses interesting electronic,
photoconductive and other physical properties [4]. It is an important class of
host materials and has been subject to intensive studies [5]. Many papers
have been published on the intercalation of metal atoms and small organic
compounds in these dichalcogenides in the past 25 years [5’ 6]. In addition,
there have been a few reports about polymer intercalation in these
compounds [71, including the recent reports from our group [1d, 16].
However, reports about conductive polymer intercalated nanocomposites of
these compounds are still scarce. Further work to synthesize and
characterize more nanocomposites with conductive polymers and this
group of layered chalcogenides would be useful.

Polypyrrole (PPY) is an important conductive polymer which has

attracted considerable attention due to its high electrical conductivity. It has

78

been investigated as an electrode material for solid-state batteries [8a] and
capacitors [8b], as an anode material for polymer light-emitting diodes [8C],
an electromagnetic interference shielding material [3d] and sensor
materiall8e]. Therefore, PPY intercalative nanocomposites could be
promising materials for applications.

PPY is an intractable polymer, however, which does not dissolve in
any solvent. The exfoliation-encapsulative precipitation (EEP) method,
which was described in Chapter 1, cannot be applied to PPY. By 1994,
when we started the exploration of encapsulating PPY in M082, the
material had already been intercalated in a number of layered inorganic
compounds such as FeOCl [la] V205 [1b] and VOPO4 [1‘]. The intercalation
was achieved by the method of in situ redox intercalative polymerization,
which requires oxidative hosts and is suitable for only a few layered
inorganic compounds. Some non-oxidative hosts can be rendered oxidative

(suitable to the conductive polymer intercalation) by incorporating in them

oxidizing ions such as Cu2+19l. However, this may not be a good method

for restacked M082 and W82, because these materials themselves could

reduce Cu2+. Therefore, we tried a new methodology to intercalate PPY in
this type of hosts.

Besides mixing with polymers in solution, exfoliated layered
compounds can be mixed with monomers or monomer solutions. When
polymerization is initiated in such mixtures, nanocomposites are frequently
formed. The important clay/polyamide nanocomposites were synthesized
with this strategy [10]. This approach also leads to the new polymer-
intercalation method described here, namely the in situ oxidative
polymerization-encapsulative precipitation. This new procedure has given

several new PPY nanocomposites such as M082/PPY, WS2/PPY and

79

RuCl3/PPY nanocomposites. The former two nanocomposites will be
presented in this chapter. The last one will be reported in Chapter 5.

Since this procedure of PPY-nanocomposite preparation involves the
reaction between pyrrole, FeC13 and exfoliated host materials, and
unavoidably includes some side reactions such as the restacking of the host
materials and the formation of the bulk polymer, the formation of
nanocomposite products and their morphologies depends not only on the
thermodynamics but also on the kinetics of these reactions. Therefore,
reaction parameters such as the ratio of the reactants, the order of addition
of reactants, reactant concentrations and temperature are all important. It
will be seen in the following sections that the change of the pyrrole/FeCl3
ratio alone causes changes in the structure of M082/PPY nanocomposites,

and affects the formation of WS2/PPY nanocomposites.

Experimental Section

1. Reagents

Pyrrole (98%), N-methyl pyrrole (99%), anhydrous FeCl3 (purified
grade) and W82 (99.8%) were purchased from Mallinckrodt Chemical Inc.,
Aldrich Chemical Co., Fisher Scientific and Alfa Aesar respectively.
Acetone was industry grade purchased from Quantum Chemical Company.

Other chemicals were the same as in Chapter 1. Deionized water was used.

2. Preparation of M082/PPY N anocomposites
a. Procedure 1: preparation with a controlled amount of pyrrole

The M082/PPY nanocomposite was prepared by oxidizing pyrrole in
an aqueous monolayer M082 suspension with FeCl3. In this particular

procedure described here, pyrrole and exfoliated M082 were chosen as the

80

limiting reactants in the reaction. In a typical reaction, an amount of 0.20 g
LiMoS2 (1.20 mmol) was exfoliated in 20 ml of H20. To this solution
0.016 g of pyrrole (0.24 mmol) dissolved in 5 ml of H20 was added. The
mixture was stirred in an ice bath for 15 min and to it an aqueous solution
of 0.156 g FeCl3 (0.96 mmol) was added dropwise. The reaction mixture
was stirred in an ice bath for ~24 h. The product was collected by
centrifugation as a black solid. It was washed with acetone and dried in
vacuum. Elemental analysis, which was done by Oneida Research Services,
Inc., Whitesboro, New York, showed a composition C6.59%, H 0.59%, N
1.31% and Cl 0.00%. The corresponding formula is
(C4H3N)0.25(H20)0,16M082. (Calculated: C 6.70%, H 0.60%, N 1.95%.) 1111
Thermal gravimetric analysis (TGA) experiments in oxygen indicated a
1033 around 20.0 wt% at 650 0C, while the above formula suggests a loss of
19.6 wt% supposing that the residue is M003 1121. Energy dispersive X-ray
microanalysis (EDS) indicated that there was some Fe compound existing
in the product. The ratio of Fe to M0 was about 0.19:1. After having been
washed with 1 M HCl, the ratio changed to 0.02:1. No chlorine was found
by EDS.

The ratio of pyrrole to M082 in the reaction above was 0.2210. In
other reactions, the pyrrole to M082 ratio varied from 0.1 to 0.6 ( 0.1, 0.2,
0.25, 0.3, 0.4 and 0.6, etc.), while the ratio of FeCl3 to pyrrole was always
held at 4:1. A pyrrole/M082 ratio of ~0.2—0.3 provided well ordered
product.

Similarly, corresponding M082/poly(N-methyl pyrrole)
(M082/PMPY) nanocomposites were also prepared. Elemental analysis
showed: C 7.63%, H 0.77%, N 1.38%; C 7.65%, H 0.77%, N 1.38% [131.
This corresponds to a formula (C5H5N)0,23(H2O)0,12MoS2. (Calculated: C
7.65%, H 0.77%, N 1.78%.) A nanocomposite with this formula should

81

lose 20.1% of the weight assuming the residue is M003, which agrees with

the TGA experiments of 21.6 wt% loss at 650 0C in air.

b. Procedure 2: preparation with an excess amount of pyrrole

In this procedure, FeCl3 was kept to be equal to or less than 0.5
equivalent to MoSz. In a typical reaction, an amount of 0.20 g LiMoSz
(1.20 mmol) was exfoliated in 20 ml of H20. To this solution 0.32 g of
pyrrole (4.8 mmol) dissolved in 40 ml of H20 was added. The mixture was
stirred in an ice bath for 15 min and to it an aqueous solution of 0.098 g
FeC13 (0.60 mmol) was added dropwise. The reaction mixture was stirred
in an ice bath for ~24 h. The product was collected by centrifugation as a
black solid, washed with water, dried in air for 1 day, and then dried in
vacuum. Elemental analysis of a sample showed: C 12.64%, H 0.99%, N
3.19%; C 12.40%, H 0.94%, N 3.15%. The corresponding formula is
(C4H3N)0,51(H20)0,13M0S2. (Calculated: C 12.46%, H 0.96%, N 3.64%.) A
nanocomposite with the above formula should lose 26.7% of its weight and
convert to M003. This agrees well with the TGA experiments which
detected a loss around 26.4 wt% at 650 0C in air.

MoSZ/PMPY nanocomposites were also prepared this way. Elemental
analysis showed a composition: C 14.30%, H 1.37%, N 3.01%;
C 14.22%, H 1.26%, N 2.95%. The corresponding formula is
(C5H5N)0,475(H20)0,13MoS2. (Calculated: C 14.26%, H 1.32%, N 3.33%.) A
nanocomposite with the above formula should lose 28.0 wt% on the basis
that the residue is M003. This agrees with the TGA experiments, which
showed a loss of 27.5 wt% at 650 0C in air.

82

3. Preparation of WS2/PPY N anocomposites

LiWSz was prepared by reacting W82 with 3.0 equiv. of LiBH4 at
350 0C for 3 days, similar to LiMoS2 in Chapter 1.

The PPY was encapsulated in WS2 with a procedure similar to the
Procedure 2 used to prepare MoSZ/PPY. In a typical reaction, 0.40 g
mixture of LiWS2/LiBH4/LiH (~1:2: 1) (about 1.34 mmol) was stirred in 20
ml of H20 for 2 h. Hydrated WS2 was collected by centrifugation, washed
with 20 ml of fresh water, and suspended in 20 ml water by 30 min
stirring. The resulting suspension was mixed with 10 ml water solution
which contained 0.225 g pyrrole (3.35 mmol) and stirred in an ice bath. To
this mixture, 10 ml of cold (~ 0—1 0C) aqueous solution of 0.109 g FeC13
(0.67 mmol) was added dropwise. The mixture was stirred for 24 h. The
product was collected by centrifugation as a black solid, washed with
water, dried in air for 1 day, and then dried under vacuum.

Elemental analysis gave: C 4.71%, H 0.88%, N 1.17%; C 4.67%, H
0.87%, N 1.16%, corresponding to (C4H3N)0,27(H20)0,32WSZ [14],
(Calculated: C 4.62%; H 0.87%; N 1.35%.) The elemental analysis of
restacked WS2 which was washed by hydrochloric acid and had a d spacing
of 6.3 A showed a result: C 0.30%, H 0.23%, N 0.07%; C 0.28%, H
0.21%, N 0.07. If the contents of C, H and N from the control experiment
is deducted from the results of the elemental analysis of (PPY)XWS2, the
suggested formula becomes (C4H3N)0,25(H20)0,53WS2. The formula above
should lose 15.3 wt% if the nanocomposite is converted to W03, which
does not match the TGA experiments [15] well. An EDS measurement
detected only a trace of Fe existing in the product. The ratio of Fe to Mo
was about 0.07:1.

83

4. Instrumentation

The instrumentation in the measurements such as powder X-ray
diffraction (XRD), thermal gravimetric analysis (TGA), differential
scanning calorimetry (DSC), infrared (IR) spectroscopy and inductively
coupled plasma (ICP) spectroscopy were the same as described in Chapter
1.

Room temperature conductivity measurements were done on pressed
sample pellets with a four-probe detector connected to a Keithley-236
source measure unit. Variable temperature direct-current electrical
conductivity measurements and thermopower measurements were made on
pressed pellets, by Dr. Kannewurf’s group at Northwestern University,
with 60 and 25 um gold wires used for the current and voltage electrodes
respectively. Measurements of the pellet cross-sectional area and voltage
probe separation were made with a calibrated binocular microscope.
Electrical conductivity data were obtained with a computer-automated
system described elsewhere [16]. Magnetic susceptibility measurements were

done with a Quantum Design MPMS2 SQUID magnetometer at the

Department of Physics and Astronomy at MSU. Samples were sealed in low
density polyethylene (LDPE) bags under a nitrogen atmosphere. The
magnetic moments of the bags were acquired and deducted from the
measurements. To obtain the paramagnetic susceptibility, the diamagnetic

susceptibility and XTIP were subtracted from the total susceptibility. The

former was derived by adding up the diamagnetic susceptibility of each
component, which was obtained from the literature [17]. The latter was
determined by adjusting its value by a try-and-error process so that the

paramagnetism optimally fit Curie-Weiss law.

84

Results and Discussion

1. MoSz/PPY N anocomposites

The synthesis of MoSz/PPY nanocomposites with a controlled
amount of pyrrole and an excess amount of FeCl3 (Procedure 1) produces a
single phase nanocomposite product with a basal spacing around 10.5 A,
based on X-ray diffraction, see Figure 2.1(b). The interlayer expansion of
~4.3 A indicates the successful inclusion in the interlayer gallery of a guest
species, which is obviously PPY, as supported by infrared spectra. The IR
spectrum of the nanocomposite displayed in Figure 2.2 is of a sample from
a reaction with relatively high pyrrole ratio, ~0.4. The nanocomposites
produced from reactions using S 0.25 equivalent of pyrrole have IR
spectra showing almost no peaks of organic species, due to the strong
reflectivity of the samples originated from the conductive MoS2 layers.
Heat treatment of the samples causes the structural transition of MOS; from
the octahedral to the trigonal prismatic coordination (2H-MoS2), which
alters the MoSz from a metallic to a semiconducting state. This reduces the
infrared reflectivity of the MOS; layers, so that the absorbance of the
intercalated species becomes visible. Figure 2.3 shows the IR spectrum of a
heating treated (PPY)0,25(H20)0,16M0S2 sample, which had been produced
with 0.2 equivalent pyrrole and was heated up to 300 0C. In both spectra in
Figures 2.2 and 2.3, the observed peaks are related to PPY.

To explore the possible nanocomposite phases and the optimum
conditions to produce nanocomposites, the ratio of pyrrole to exfoliated
MoS2 in the reaction was altered from 0.1 to 0.6. As summarized in Table
2.1, the products have (I spacings varying from 10.2 to 12.3 A. It seems
that higher pyrrole ratios result in slightly higher expansions. Since the

corresponding interlayer gallery expansion is between 4.0 and 6.1 A,

85

Intensity

 

 

 

 

 

 

 

 

6-2 A (a) restacked M082
-2 A. m A _J
10.5A
(b) (PPY)O.25(H20)0.16MoS2
(by Procedure 1)
11.9A
(C) (PPY)O.51(H20)0.18M 0 82
(by Procedure 2)
10-9 A (d) (PMPY)023(H20)012M082
(by Procedure 1)
12.7A
(e) <PMPY>.....<H.O>...M082
(by Procedure 2)
""l""l"‘FT""I'T"l""
10 20 30 40 50 60

20 (deg)

Figure 2.1. XRD patterns of MoSz nanocomposites.

86

 

 

  

(PPY)n0 4"M032

 
   

/,.._..,...

Irnpurity in KBr

  
  

Transmittance

 

1550

\O
O\
N
'—¢

777

<1-
v—I N
00
v—I

v—I

\D
.4
O\

C
m
C
v—r

 

 

 

I I I I I I I I I I I I I I r I I I I I I I I I I T I

2000 1800 1600 1400 1200 1000 800 600
Wavenumber (cm' 1 )

Figure 2.2. IR spectra of PPY and a MoS2/PPY nanocomposite prepared by
procedure 1.

 

  
   

Impurity in KIBr

Intensity

 

8%
‘22

 

 

 

I I I I I I I 1 I I I I I I I I I I l I I T I I I I I I I I I

2000 1800 1600 1400 1200 1000 800 600 400
Wavenumber (cm'l)

Figure 2.3. IR spectrum of a M082/PPY nanocomposite prepared by
procedure 1 and heating treated twice up to 300 0C under N2 atmosphere at
a heating rate of 5 0C/min.

87

Table 2.1. Parameters of (PPY)xMoS2 prepared with different pyrrole

 

 

 

 

 

 

 

ratios
pyrrole/MoS2 d spacing of coherence length *[18]
ratio nanocomposites (A) (A)
0.1 10.2 40
0.2 10.9, 10.6, 10.5 102, 102, 124
0.25 10.7, 10.5 149, 131
0.3 11.2 89
0.4 12.3 52
0.6 11.8, 11.5 40, 47

 

 

 

 

 

* The X-ray scattering coherence length in a hkl direction is calculated
from the peak broadening of the hkl reflection and is an index of the
structural order in that direction.

the space can only accommodate one molecular layer of PPY. It is likely
that the pyrrole rings of the polymer lie almost parallel to the inorganic
layers when the gallery height is low, while they rise more vertically when
the gallery height is high. According to the molecular dimensions of the
polymer, which will be discussed in detail in Appendix B, the 4.0 A height
is appropriate for the flat orientation of the pyrrole rings. When the
gallery is higher and the pyrrole rings tilt up, more polymer chains can be
packed in the gallery.

Since all pyrrole forms PPY upon oxidation with FeC13, the ratio of
pyrrole to MoS2 in the reaction needs to be controlled. When this ratio is
high, excess polymer forms, which cannot be accommodated as a single
molecular layer in the gallery. As a result, it forms bulk polymer, which
makes the product a mixture of phases. On the other hand, deficiency of
pyrrole makes the gallery partially filled, which is also not an ideal

situation because it gives rise to restacked MoSz. Furthermore, too much or

88

too little PPY hurts the lamellar structure of the nanocomposites. The
structural order (Le. stacking order) in the nanocomposites is indicated by
the coherence length [13] of the materials along the stacking direction. The
coherence lengths of the products clearly show that the stacking of the
layers is poor when the ratio of pyrrole is lower than 0.2 or higher than
0.3. When the ratio is 0.25, the reaction produces nanocomposites of the
highest coherence length (i. e. sharpest diffraction peaks).

The PPY nanocomposites can also be prepared using excess pyrrole
and limited FeCl3 (Procedure 2). Contrary to when excess FeCl3 is used,
adjusting the amount of pyrrole does not affect the coherence length of the
nanocomposites significantly when the FeCl3 is limited or deficient. The
resulting nanocomposites always have fairly high coherence length along
the restacking direction. The basal spacing and the amount of polymer in
the nanocomposites remains unchanged, i.e. ~11.9 A and ~0.5 repeat—unit
per MoSz respectively. Figure 2.1(c) shows the powder X-ray pattern of a
nanocomposite (PPY)O,51(H20)0.18M0S2, prepared with these conditions.

The claim that the nanocomposites from Procedure 2 contain PPY
rather than pyrrole is supported by the IR spectra, see Figure 2.4. In
addition, it is supported by the fact that the nanocomposites cannot be de-
intercalated. Pumping at 60 0C did not cause a shrinkage in the gallery
spacing. In Procedure 2, the amount of PPY present in the product is more
than what the FeCl3 alone could have produced. The greater amount of
PPY is explained by the fact that ambient oxygen takes part in the reaction
as an electron acceptor. It has been proven that the ambient oxygen can
oxidize and polymerize pyrrole in the presence of FeCl3 [19]. The whole
process can be described as follows: the addition of FeC13 causes or initiates

polymerization of pyrrole producing radical cationic oligomers which

89

 

Transmittance

 

 

 

 

 

IIIIIIIIIIIIIIIIIIIIIIIIIITIIII

2000 1800 1600 1400 1200 1000 800 600 400

Wavenumber (cm' 1‘)

Figure 2.4. IR spectra of a nanocomposite (PPY)0.51(H2O)0,18MoS2
prepared by procedure 2. (a) vacuum dried at room temperature only, and
(b) heat treated twice up to 300 0C under N2 atmosphere at a heating rate

of 5 oC/min.

90

themselves are more reducing than the pyrrole monomer.

The IR spectra of the (PPY)xMoS2 from both procedures show
polymer stretching vibrations occurring at energies higher than those
found in bulk PPY, prepared under similar experimental conditions (minus
the MoS2 layers). The average blue-shift for most peaks is ~10 cm-1 and it
reflects the lower molecular weight (MW) of the intercalated polymer.
Similar observations were made in previous work where the MW of in situ
intercalated polymers is always found to be considerably smaller than that
of the corresponding bulk materials [13, 1b, 20]. This is a consequence of
polymer growth under kinetically restricted conditions.

As it has already been mentioned, the interaction among pyrrole,
FeCl3 and exfoliated host materials could potentially give rise to a number
of undesirable side reactions such as the restacking of the host materials
and the formation of bulk polymer. The formation of the single phase
nanocomposites is therefore remarkable. Explanations for this include the
possible formation of a MoS2.pyrrole complex in solution before the

addition of FeCl3, or more likely the strong electrostatic attraction between

the negatively charged [MoS2]"' layers to the positively charged polymer
chains.

Since the polymer is produced in the doped form, an important issue
to be addressed here is: what is the dopant anion in PPY? This has not been
simple to resolve. There are several possible candidates for counter ions in
(PPY)xMoS2, which include [FeC14]', Cl', [MoS2]"' and [OH]’. Elemental
analysis found no chlorine in the nanocomposite sample, which excluded

the possibility of [FeCl4]', and Cl'. It is possible that negatively charged

[MoS2]"‘ acts as the dopant, as is the case with FeOCl and V205. Indirect

91

experiment data to be presented below are consistent with [MoSz]"' being

the most likely counterion.

2. WS2/PPY Nanocomposites

The WS2/PPY nanocomposites were prepared using excess pyrrole
and limited FeCl3. The successful production of a PPY nanocomposite is
indicated by the strong reflections in the XRD pattern, which has seven 001
reflections in the 20 range from 2° to 60°, see Figure 2.5. The basal
spacing of the (PPY)0,25(H2O)0,53WS2, 11.0 A, corresponds to an interlayer
expansion of 4.8 A which is comparable to that of (PPY)0,25(H2O)0,16M0S2
at 4.3 A. The IR spectra of the nanocomposite are shown in Figure 2.6.

Contrary to the case of MoS2, single-phase products of WS2/PPY
nanocomposites cannot be prepared with controlled amount of pyrrole and
excess FeCl3 (Procedure 1). Reactions under those conditions always
produced restacked WS2 mixed with the WS2/PPY products. In addition,
the nanocomposites were much less ordered with very broad X-ray
diffraction peaks. This dissimilarity between the exfoliated MoS2 and WS2
systems could stem from the small differences in the charge density of the
layers and their affinity for the polymer. According to ICP analysis, the
exfoliated layers carry a charge of 0.15 e'lW S2 [21], a little less than that of

M082 layers, 0.18 e'lMoS2. This small charge difference does not seem
enough to explain all the differences in reaction and nanocomposite
composition. The observation in PPY intercalation reactions and the fact
that much higher poly(ethylene oxide) quantities are needed to achieve a
WS2/PEG nanocomposite phase [22], may suggest a lower affinity of WS2,

than exfoliated MoS2, for organic polymers.

92

 

6.3 A
(a) restacked W82

 

 

 

 

 

 

 

E?

m A

g 11.0/R

5 (b) (PPY)0.25(H20)0.53W82

W
""ll"llT"'l""l""|""
0 10 20 30 4O 50 60
20 (deg)

Figure 2.5. XRD patterns of WS2/PPY nanocomposites.

 

     

/

/ /
Impurity in KBr

Transmittance

8
E
/

 

 

I I I I I I I I I I T I I I I If I I I I I I I I I f I I I I
2000 1800 1600 1400 1200 1000 800 600 400
Wavenumber (cm' 1)

 

Figure 2.6. IR spectrum of (PPY)0,25(H2O)0,53WS2 heating treated twice up
t0 320 0C under N2 atmosphere at a heating rate of 5 OC/min.

93

3. An Analysis of Polymer Arrangements inside the Interlayer Galleries
Based on the Dimensions of the Polymer Molecules

The structural parameters of the PPY nanocomposites and the
poly(N-methyl pyrrole) (PMPY) nanocomposites, prepared with the two
procedures are presented in Table 2.2. The temperatures for the octahedral
to trigonal prismatic structural transition, which will be discussed in the
next section, are also listed. The chosen nanocomposites prepared with
Procedure 1 are those of good structural order, which have smaller (I

spacings and lower polymer contents.

Table 2.2. Structural data of PPY and PMPY nanocomposites

 

 

 

 

 

 

sample reaction d spacing coherence Ttrans d spacing
procedure (A) length (0C) after DSC
(PPY)0,25(H2O)0.16M0S2 1 10.5 124 192 9 .6
(PPY)0,51(H2O)0,13M0S2 2 l 1.9 88 208 1 1.1
(PMPY)0,23(H2O)0_12MOSZ 1 10.9 95 198 9 . 9
(PMPY)0,475(H2O)0,13M0S2 2 12.7 155 216 12.3

 

 

 

 

 

 

 

As mentioned above, when the polymer content is low the pyrrole
rings lie almost flat with respect to the inorganic layers, while they rise
more vertically when the polymer content is high. An analysis of the
dimensions of a PPY chain is necessary for a correct insight into the
possible polymer chain arrangement in the galleries. According to the
geometric calculation described in Appendix B, which is based on the bond
lengths and angles, and the van der Waals' radii of the atoms, the PPY
chain looks like a ribbon, which has a width of about 7.8 A. The thickness
of the ribbon is about 3.5 A, which is decided by the van der Waals'
diameter of C atoms. At the edges, the ribbon is a little narrower, 2.8 A,

corresponding to the van der Waals' diameter of the H atoms. If the PPY

94

chains lie totally flat, the nanocomposite would have a d spacing of 9.7 A.
This arrangement can accommodate a maximum of 0.31 equivalent of
PPY. In (PPY)0,25(H2O)0,16MoS2 which has a d spacing of 10.5 A, the

polymer chain can tilt at an angle of ~18° to the MoS2 layers. This tilt
increases the maximum PPY content to 0.32 equivalent. The 0.25
equivalent content of polymer in the nanocomposite is less than this

maximum value, and thus quite reasonable. In the nanocomposite with (I ~

11.9 A, i.e. (PPY)0,51(H2O)0,18MoS2, the polymer chain can tilt ~36°. This
relatively large tilt makes the partial overlap of the polymer chains
possible, so the content of PPY can reach as high as 0.51 equivalent.
MoS2/PMPY nanocomposites have also been synthesized with the two
procedures used to synthesize MoS2/PPY analogs. It is hoped that a
comparison of the PPY and PMPY nanocomposites will lead to a better
understanding of the arrangement of polymers inside the galleries. A
PMPY ribbon is a little thicker than a PPY ribbon due to the bulky methyl
group. The thickness of the ribbon becomes about 4.9 A, a 1.3-1.4 A
increase from PPY. The width of PMPY is about the same as PPY, 7.8 A.
The two PMPY nanocomposites listed in Table 2.2 have d spacings around
10.9 and 12.7 A respectively. The interlayer gallery height of the first
nanocomposite, 4.7 A, is just enough for a monolayer of PMPY to lie with
its pyrrole rings parallel to the MoS2 layers. This arrangement of PMPY
will let the MoS2 nanocomposite accept a maximum of 0.31 equivalent of
polymer. The detected amount of polymer in the nanocomposite, 0.23

equivalent, is therefore a reasonable value. The interlayer gallery height of

the second nanocomposite, 6.5 A, allows the PMPY chains tilt about 22°.
This small tilt angle will not diminish the projection area that an N-methyl

pyrrole ring produces. Therefore, the M082 will not accept, in its galleries,

95

an amount of PMPY as much as 0.475 equivalent, unless the PMPY chains

partially overlap or arrange themselves in an inter-digitated fashion.

4. Thermal Properties

As discussed in Chapter 1, the electronic structure of MoS2 changes
after intercalation of lithium, corresponding to the structural transition
from the trigonal prismatic MoS2 to the octahedral MoS2 [1d]. In the
restacked MoS2, which preserves the octahedral coordination of M0, the
layers are similar to the 1T-MoS2 which is metastable [23]. The MoS2 layers
in (PPY)xMoS2 are, accordingly, also prone to conversion to the 2H-MoS2-
like modification upon standing or with application of temperature or even
pressure. The activation energy of this transition in restacked MoS2 is
relatively small, ~53 kJ/mol [243], so the transition temperature measured by
DSC is only 100 0C at a heating rate of 5 OC/min. The intercalation of
polymers raises the activation energy as judged by the higher transition
temperature. For example, the (PEO)1,0MoS2 has an activation energy 82.2
kJ/mol and a transition temperature 138 OC.

The intercalation of PPY raises the transition temperature higher
than any of the other polymers thus far. The value for
(PPY)0,25(H2O)0,16M0S2 and (PPY)0,51(H2O)0,1gMoS2 are 185 OC and 208
0C respectively, as listed in Table 2.2. To determine the activation energy
for the transition of (PPY)0,51(H2O)0,13M0S2, a series of DSC
measurements was done under different heating rates, see Figure 2.7.
Different heating rates provided different transition temperatures, although
the heat of transition detected by DSC was always around 37 kJ/mol. The
heating rate and the transition temperature are related by the Arrhenius

equation [25],

96

 

 
 
  
  
    

 

   

 

  

 

4o:— 20 °C/min 228.7 °c
30 '—
A : 10°C/min 221.2 °C
3 :
E 20: 5°C/min 205.9 °C
0 3 o . 0
<8 - 2.5 C/mm 199.9 C
10 —
:1°C/min 188.7°C
O - I-
" I I I I l I I I I L I I I I I I

 

1 60 1 80 200 220 240 260
Temperature (°C)

Figure 2.7. DSC measurements at different heating rates for
(PPY)0.51(H20)0.18M082-

3.5
3.0
2.5
2.0
1.5
1.0
0.50
0.0

-0.50
l.

 

  

Ea = 138 kJ/mol

  

ln(Rate)

Ill‘IlIIlIllIlllIIII'IIIIIIle11IT1

O

IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII

 

 

I J I I I I I I I I 41 I I I I I I I I I I I I I

5 2.00 2.05 2.10 2.15 2.20
1000/T (l/K)

 

\o IIII

Figure 2.8. Plot of the DSC heating rate versus the temperature of the
MoS2 structural transition for (PPY)0,51(H2O)0,13M0S2.

97

 

 

 

 

 

 

 

5 _
: 292 °C
4 _-
e 33
5; 2:—
8 first cycle second cycle
Q 1 - N \
C
0 E- ’ ’ ‘7‘— ’ \ /
-1 :I I I I I I I I I I I I I I I J_I I I I I I I I I 141 I I I I I I
O 50 100 150 200 250 300 350

Temperature (0C)

Figure 2.9. DSC measurement for (PPY)0,25(H2O)0_53WS2. The
measurement was done in 2 heating cycles up to 320 0C at a rate of 5
OC/rnin. The curve of the first heating cycle is labeled with the arrows.

 

3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0

_O I I I I I I I I I I I I I I I I 14 l L I I I

1.65 1.70 1.75 1.80 1.85 1.90
1000/T (1/K)

    

Ea = 136 kJ/mol

  

ln(Rate)
IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII
IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIJIIIJLL

 

 

 

Figure 2.10. Plot of the DSC heating rate versus the WS2 structural
transition temperature in (PPY)0,25(H2O)0,53WS2.

98

 

 

Rate = A >1< C'Ea/RT

where A is the pre—exponential coefficient, R the gas constant and E3 the
activation energy. The activation energy can be calculated from the slope
of the ln(Rate)~1/T plot. The plot for (PPY)0,51(H2O)0,13MoS2 gives an
activation energy of 138 kJ/mol (see Figure 2.8), which is much higher
than those of restacked MoS2 or (PEO)1,oMoS2. The large increases in the
transition temperature and activation energy may result from the reducing
nature of PPY, thus stabilizing [MoSz]x'.

The same situation exists in restacked WS2 and (PPY)XWS2
nanocomposite, but the transition temperatures are higher than MoS2
compounds. The values for restacked WS2 and (PPY)0,25(H2O)0,53WS2 are
207 0C [26] and 292 0C respectively. A DSC measurement for
(PPY)0,25(H2O)0_53WS2 is shown in Figure 9. The change of the transition
temperature, by 85 0C, is the same as in the case of
(PPY)0,25(H2O)0,16M0S2. Under heating rates of 1.0, 2.5, 5.0, 10.0 and 20
OC/min, the transition temperatures of (PPY)0,25(H2O)0,53WS2 detected by
DSC were 265.3, 280.9, 292.1, 307.0 and 324.5 0C respectively. The
activation energy of the transition calculated from the ln(Rate)~1/T plot
(see Figure 2.10) is 136 kJ/mol, much higher than the value of restacked
WSZ, 82.4 kJ/mol [26], but close to that of (PPY)0,51(H2O)0,13MoS2. The heat
of transition detected by DSC was around 20 kJ/mol, slightly higher than
the 18 kJ/mol of restacked WS2, but less than 37 kJ/mol found for
(PPY)0.51(H20)0.18M052-

After the transition, samples of these nanocomposites still have well
ordered lamellar structure. The d spacings of the M082 nanocomposites

after the DSC experiments are listed in Table 2.2. The d spacing of the

99

WS2/PPY nanocomposite after the DSC measurement is 10.3 A. The IR

spectra of the heated nanocomposites show only PPY peaks.

5. Charge Transport Properties

Room temperature electrical conductivity values of the materials
described here are listed in Table 2.3 [27]. The conductivities of the
restacked WS2 and the fresh restacked MoS2 are high, about 100 and 12
S/cm respectively, as measured with pressed pellets. The conductivity of
restacked MoS2 diminishes quickly, due to the gradual structural transition
of restacked MoS2 at room temperature. According to experiments, the
transition proceeded almost completely in about 4 months and the
conductivity reached about 0.07 S/cm. The conductivity stayed at about
0.06 S/cm for the subsequent 3 years. Comparing the samples more than 6
months old, the intercalation of PPY raises the conductivity more than 10
times. A higher PPY content gives the material a higher conductivity. In

contrast, the intercalation of PPY reduces the conductivity of restacked

Table 2.3. Room temperature electrical conductivities of PPY
nanocomposites and restacked M082 and WS2

 

 

 

 

 

 

 

 

sample R.T. conductivity (S/cm)
restacked MoS2 (1 day) 12

restacked M082 (3 years) ~0.06
(PPY)0.25(H20)0.16M032 0.8
(PPY)0.51(H20)0.18M032 2.5

restacked WS2 (10 days) 100
(PPY)0.25(H20)0.53W32 27

 

 

 

100

 

WS2, because the conductivity of bulk PPY is only 13 S/cm, smaller than
that of restacked WS2. In addition, (PPY)XWS2 may have more grain
boundaries giving resistance.

Variable-temperature electrical conductivity measurements for
pressed (PPY)0,25(H2O)0,16M0S2 pellets indicate a considerable higher
conductivity with respect to pristine MoS2, as shown in Figure 2.11. In the
temperature range 50-300 K, the material exhibits weak, thermally
activated behavior, which is common in conductive granular
nanocomposites. The plot of conductivity versus reciprocal temperature
does not follow a straight line, which indicates that the thermal activation is
more likely through the boundary areas between the conductive domains.
The conductivity is similar in magnitude to that of (PANI)xMoS2 consistent
with the similar nature of the materials. The corresponding thermoelectric
power measurements show relatively large and positive Seebeck coefficient
values which indicate a p-type conductor, see Figure 2.12. The relatively
large values and the curved thermoelectric power plot indicate a deviation
from an ideal metallic behavior.

Similar to (PPY)0,25(H2O)0_16M0S2, (PPY)0,25(H2O)0,53WS2 has also a
thermally activated electrical conductivity. A variable temperature
conductivity measurement for (PPY)0.25(H2O)0,53WS2 is shown in Figure
2.13 together with that for restacked WS2. The comparison demonstrates
that (PPY)0.25(H2O)0,53WS2 is not as electrically conductive as the restacked
WS2, which has been seen in the room temperature conductivity
measurements. Thermoelectric measurements for (PPY)0,25(H2O)0,53WS2
show typical p—type metallic behavior, with small positive values which
decreases linearly with falling temperatures, see Figure 2.14. The restacked

WS2 has similar thermoelectric behavior [28].

101

 

 

 

  
 

 

 

10"
(E) 10:3 (H2O)0.16M082
@
3*
IE 105 O O
3 o
:3
"O
8
o 10‘7 A,
9 m. 1 1 1 . . . 1 1 1 . . 1 1 1 .A. 1
1° 0 5 10 15 20

1000/T (K'l)

Figure 2.11. Variable temperature electrical conductivity measurements on
pressed pellets for pristine MoS2 and a MoS2/PPY nanocomposite prepared
with Procedure 1.

 

 

 

 

 

100 _
SE 90 '—
5‘ Z
:1. :
V 80 —
I... .—
d.) ..
3 :
8, 70 :-
é _
_g 60 _—
H I.
50 l I_I_I I I I I I I I I I; I l l I I I I I l I
50 100 150 200 250 300

Temperature (K)

Figure 2.12. Thermoelectric power measurement on a pressed pellet for
(PPY)0,25(H2O)0,16M0S2 prepared with Procedure 1.

102

 

 

 

 

101 5 a
\mmmmo restacked WS2
”E? *’ C>00 o o o o o o o o o q
o 0 _ _.
a 10 g g
b F 3
:E C o (PPY)025(H20)0.53WS
-1 _ ° _
g 10 E O o o E
U E o o 3
10-2 J g1 I I I l I I I I I I I I I J I I I I I I I
O 10 20 30 40 50
1000/r (K'l)

Figure 2.13. Variable temperature electrical conductivity measurements on
pressed pellets for restacked WS2 and (PPY)0_25(H2O)0,53WS2.

 

  

 

 

 

5 .-
9 4:—
; -
:1. I ‘
v 3 __
H .—
qg -
g :
cu 2 f
a _

: o

g 1 .— OOWO
E-‘ :

O - I I l I l I l I I l l l I I I I l J I l l I l I

100 150 200 250 300

Temperature (K)

Figure 2.14. Thermoelectric power measurement on a pressed
(PPY)0.25(H20)0.53W32 pellet.

103

 

Concluding Remarks

MoS2/PPY, WS2/PPY and MoS2/PMPY nanocomposites have been
successfully synthesized using the new in situ oxidative polymerization-
encapsulative precipitation reaction. This reaction is kinetically controlled,
so some optimization to find the optimum conditions to produce the
nanocomposites, or conditions to get the most ordered structure, is crucial.
A great effect has been made in this research to optimize the reactions and
the best conditions are reported and discussed. The MoS2/PPY, WS2/PPY
and MoS2/PMPY nanocomposites synthesized have ordered structures and.
large coherence lengths along the stacking direction.

The new nanocomposites were characterized and identified. A
discussion about the arrangement of polymer chains inside the MoS2/PPY
nanocomposites suggests that the polymer chains lie almost flat inside the
galleries when the polymer content is about 0.25 equivalent. The chains tilt
up and overlap when the polymer content becomes about 0.5. The
intercalation of PPY stabilizes the metastable restacked MoS2 and WS2
layers, by raising structural transition temperatures to 208 and 292 0C
respectively. These increases in transition temperature are the largest ones
observed yet in M082 and WS2 nanocomposites. Both MoS2/PPY and
WS2/PPY nanocomposites are good conductors.

The development of a synthetic strategy to intercalate an intractable,
infusible polymer in non-oxidizing hosts is the most important achievement
in this research. This is an advance from prior work where (a) an
oxidizing host was required to proceed polymer intercalation, (b) a soluble
polymer was necessary for insertion into a non-oxidizing host, or (c) a
monomer intercalation was required followed by a second step of

polymerization. The methodology developed here can be applied further to

104

other intractable polymers such as polythiophene. This development
broadens the field of organic/inorganic nanocomposite materials as it
enables interesting new inorganic/polymer combinations for study and

development.

105

 

 

Appendix B

Based on the conjugated structure of PPY, we assume that it is a flat
or nearly flat molecule. As shown in Figure 2.15(a), the distance between
the farthest two atoms in the direction perpendicular to the polymer chain
(two H atoms) is 2.48 A * 2 = 4.96 A. By adding the van der Waals radius

of H atom, 1.4 A, the width of the PPY chain should be 7.8 A. The
thickness of the chain should be about 3.54 A, which is deduced from the
wan der Waals radius of the C atom, 1.77 A.

The length of each pyrrole unit is 3.59 A, so the area that each

pyrrole unit occupies is 3.59 A * 7.8 A = 27.9 A2 [291. The area of each
MoS2 unit is 8.653 A2, according to the a and y of the 2H—MoS2, 3.161 A

and 120° [30]. This indicates that the MoS2 nanocomposite can accept a

maximum of 0.31 equivalent of PPY in the galleries if the PPY chains lie
flat.
In the MoS2 nanocomposite of a d spacing 10.5 A, where the gallery

is 4.3 A high, the polymer chain can tilt at an angle of ~18° to the M082
layers according to the following calculation:
arcsin[(4.3A-2*l.4A)/(7.8A-2*1.4A)] = 17.50
1.4 A is the van der Waals radius of the H atoms which are on the two
sides. When a PPY chain tilts 18°, the area of a projection of a pyrrole unit
is (4.96 A * cos18° + 2 * 1.4 A) * 3.59 A = 27.0 A2. The M082 can accept
a maximum of 0.32 equivalent of PPY arranged this way.
When the expansion is 5.7 A, the polymer chains can tilt ~36°:
arcsin[(5.7A-2*1.4A)/(7.8A-2*1.4A)] = 355°

106

The projection of a pyrrole unit in a such placed chain will have an area of
(4.96 A * cos36° + 2*1.4 A) >t= 3.59 A = 24.4 A2. The PPY content can

reach a maximum of 0.35 equivalent if there is no overlapping of the
chains. The high PPY content in the nanocomposite, 0.51, can be best
explained by the partial overlapping of the PPY chains in the galleries.
Since the edge of the PPY chain is only ~2.8 A, the 5.7 A gallery height is
just enough for the edges of two chains to overlap.

In the case of PMPY, the bulky methyl group makes the PMPY chain
thicker, 4.87 A, as obtained in the following calculation [also see Figures
2.16(b) and (c)]:

2*(C-H * sin110° + 1.4 A): 2*(110 .1. sin110° + 1.4 A) = 4.9 A.
Again, 1.4 A is the van der Waals radius of a H atom. The
(PMPY)0_23(H2O)0_12M0S2 has a d spacing of 10.9 A. Its 4.7 A gallery
spacing is just enough for the PMPY chains to lie flat inside. The width of
the PMPY chains is the same as that of PPY chains, because the added
methyl groups are buried inside the grooves of the PPY chain, see Figure
2.15(b). If the PMPY chains lie flat, each N-methyl pyrrole unit will cover
an area approximately the same as a pyrrole unit. Correspondingly, the
MoS2 nanocomposite will accommodate a maximum of 0.31 equivalent of
PMPY.

The PMPY chains must tilt in (PMPY)0,23(H2O)0_12M0S2, which has a
d spacing of 12.7 A. The tilt of the chains is calculated by the following
approach, which is different from the calculation for PPY. As shown in
Figure 2.16(b), the distance between the diagonal H atoms in the two
methyl groups on the two sides, d6, is 5.27 A. When a PMPY chain lies

flat, the d6 direction of the chain tilts by an angle of arcsin(2.07 A/5.27 A)
= 23.1°, see Figure 2.16(c). In a gallery 6.5 A high, the chain can tilt so

107

that the d6 vector tilts a maximum angle of arcsin[(6.5 A - 2* 1.4 A) / 5.27

A] = 446°. The tilt of the chain is the change of the angle of the diagonal
plane, 21.5°, see Figure 2.16(d).

The 21.5° tilt of the PMPY chain produces a larger projection on the
M082 plane, as shown in Figure 2.16(d). The PMPY chains in the
nanocomposite must partially overlap to accept a 0.475 equivalent of
polymer. Otherwise, according to the calculation, the polymer content
would not exceed 0.29 equivalent. The 6.5 A gallery height does not exceed
the sum of the thicknesses of the methyl group and H atom, 4.9 and 2.8 A
respectively. In order to achieve a partial overlap with nearby polymer
chains, a polymer chain must match its edge H atoms or methyl groups

with those of the neighbor chains in an inter-digitated fashion.

108

(a) polypyrrole

\i

xi
:1:—-z-
\E
\f:
/i

I—Z .
\l

l!
\/:
.. a 5
8f \:
g =.\
: F8
sea—4+

I.
1

Length of covalent bonds:
C-C 1.54 A, C=C 1.34 A, C-N 1.47 A, C=N 1.27 A, N-H 0.98 A, OH 1.10 A.

d 1 = N-H + [(C-N + C=N) =1: sin36°] / 2 - [(C-C + C=C) .1 sin 18°] /4 = 1.56 A
d2 = C-H * sin126° + [(C-C + C=C) * sin108°] / 2 + [(C-C + C=C) >1: sin 18°] 74 = 2.48 A
d3 = 2 ... [(C-N + C=N) * cos36°] / 2 + [(c-c + C=C) .. cos 18°] /2 = 3.59 A

(b) poly(N-methyl pyrrole)

\ / ° \ / 070% ------------ I"
’///:—/—\\/1’\////:\\\\/‘36/18 ...... din
T M i T \n/ ,2
WC\H H \H WC\H / 126\ ___________________

d1 = OH 1. sin20° + C-N + [(C-N + C=N) * sin36°] / 2 — [(C-C + C=C) =t< sin 18°] /4 = 2.43 A
d2 = 2.48 A
d3 = 3.59 A

Figure 2.15. Schematic structures and dimensions of PPY and PMPY.

109

(a) Front view of poly(N-methyl pyrrole)

(c) Contour of a flatly lay
poly(N-methyl pyrrole)

 

Figure 2.16. Views of PMPY.

c/H

(b) Side view of poly(N-methyl pyrrole)

 

:‘ d5 '1

d4=2* dl =2*2.43A=4.86A
d5 = 2 *1.10A *sin110°=2.07 A
d6 = (d42 + d52)”2 A = 5.27 A

(d) Tilted poly(N-methyl pyrrole)
in a 6.7 A gallery

o‘ ”’ 4
6.5 A ‘ 3.7 A
/' I" x.

 

110

 

 

References

1 (a) M. G. Kanatzidis, L. M. Tonge, T. J. Marks, H. O. Marcy and C.
R. Kannewurf, J. Am. Chem. Soc. 1987, &, 3797. (b) M. G.
Kanatzidis, C.-G. Wu, H. O. Marcy and C. R. Kannewurf, J. Am.
Chem. Soc. 1989, 111, 4139. (c) L. F. Nazar, X. T. Yin, D.
Zinkweg, Z. Zhang and S. Liblong, Mat. Res. Soc. Symp. Proc.
1991, 2_10_, 417. (d) R. Bissessur, D. C. DeGroot, J. L. Schindler, C.
R. Kannewurf and M. G. Kanatzidis, J. Chem. Soc., Chem.
Commun. 1993, 687. (e) M. G. Kanatzidis, R. Bissessur, D. C.
DeGroot, J. L. Schindler and C. R. Kannewurf, Chem. Mater. 1993,
_5_, 595. (f) G. Matsubayashi and H. Nakajima, Chem. Lett. 1993, 31.

2 F. Croce, G. B. Appetecchi, L. Persi and B. Scrosati, Nature 1998,
fl, 456.

3 (a) L. F. Nazar, H. Wu and W. P. Power, J. Mater. Chem. 1995, _5_,
1985. (b) F. Leroux, B. E. Koene and L. F. Nazar, J. Electrochem.
Soc., 1996, m, L181. (c) T. A. Kerr, H. Wu and L. F. Nazar,
Chem. Mater. 1996, 8, 2005. (d) B. E. Koene and L. F. Nazar,
Solid State Ionics 1996, 8_9_, 147. (e) K. Ramachandran and M. M.
Lerner J. Electrochem. Soc. 1997, L44, 3739. (f) F. Leroux, G.
Goward, W. P. Power and F. Nazar J. Electrochem. Soc., 1997 ,
144, 3886.

4 (a) J. A. Wilson and A. D. Yoffe, Adv. Phys. 1969, 18, 193. (b) L.
F. Mattheiss, Phys. Rev. B 1973, 8, 3719.

 

5 F. R. Gamble and T. H. Geballe, in N. B. Hannay ed., Treatise on
Solid State Chemistry, V013, Plenum Press 1976, pp 89.

6 (a) R. H. Friend and A. D. Yoffe, Adv. Phys. 1987, x, 1. (b) W. R.
McKinnon, in A. P. Legrand and S. Flandrois ed., Chemical Physics
of Intercalation (NATO ASI Ser., B: Phys, Vol. 1_72, Plenum Press,
New York 1987, pp 181.

7 (a) C.—H. Hsu, M. M. Labes, J. T. Breslin, D. J. Edmiston, J. J.
Winter. H. A. Leupold and F. Rothwarf, Nature, Phys. Sci., 197 3,
246 (155), 122. (b) V. M. Chapela and G. S. Parry, Nature, 1979,
2_8_1, 134. (c) W. M. R. Divigalpitiya, R. F. Frindt and S. R.
Morrison, J. Mater. Res. 1991, _6_, 1103. (d) E. Ruiz-Hitzky, R.
Jimenez, B. Casal, V. Manriquez, A. S. Ana and G. Gonzalez, Adv.

111

10

11

12

13

14

15

Mater. 1993, 5, 738. (c) J. P. Lemmon, J. Wu, C. Oriakhi and M.
M. Lerner, Electrochim. Acta 1995, Q , 2245. (f) C. O. Oriakhi,
R. L. Nafshun and M. M. Lerner, Mater. Res. Bull. 1996, Q, 1513.

(a) L. W. Shacklette, R. R. Chance, D. M. Ivory, G. G. Miller and
R. H. Baughman, Synth. Met. 1979, 1, 101. (b) M. Satoh, H. Yageta,
K. Amano and E. Hasegawa Synth. Met. 1997 , 8:4, 167. (c) J. Cao,
A. J. Heeger, J. Y. Lee and C. Y. Kim Synth. Met. 1996, 82, 221.
(d) H. Kuhn, R. Gregory and W. Kimbrell, Int. SAMPE Electron.
Conf. 1989, 3, 570. (e) M. D. Imisides, R. John, P. J. Reiley and G.
G. Wallace, Electroanalysis 1991, 3, 879.

E. P. Giannelis, V. Mehrotra, O. Tse, R. A. Vaia and T. -C. Sung,
Mat. Res. Soc. Symp. Proc. 1992, E, 969.

(a) Y. Fukushima and S. Inagaki, J. Inclusion Phenom. 1987, _5_, 473.
(b) Y. Fukushima, A. Okada, M. Kawasumi, T. Kurauchi and O.
Kamigaito, Clay Minerals 1988, Q, 27.

Because the polymer exists inside the nanocomposite, it could
experience somewhat incomplete oxidation in the process of
elemental analysis experiments. This may lead to lower N amount
detected. The formula is derived based on the amounts of C and H.

An elemental analysis of another sample done in our department at
MSU showed: C 7.28%, H 0.85%, N 1.40%; C 7.19%, H 0.82%, N
1.36%. The corresponding formula is (C4H3N)0,23(H2O)0,35MoS2.
Calculated: C 7.28%; H 0.83%; N 2.12%. A nanocomposite with the
formula above should lose 22.0% of the weight if the final residue is
MoO3. This agrees well with the result of the TGA experiment in
oxygen which had a loss around 21.0 wt% at 650 0C.

This measurement and elemental analysis measurements hereafter
were done by Dr. R. Huang in this department.

Another analysis showed C 4.11%, H 0.71% and N 1.19%, which
suggests a formula (C4H3N)0,235(H2O)0,62WS2: C 4.11%; H 0.71%; N

1.20%.

Two T GA experiments in oxygen had losses around 11.5 wt% and
11.7 wt% up to 650 0C.

112

16

17

18

19

20

(a) B. N. Diel, T. Inabe, J. W. Lyding, K. F. Schock, Jr., C. R.
Kannewurf and T. J. Marks, J. Am. Chem. Soc., 1983, 1Q, 1551.
(b) J. W. Lyding, H. O. Marcy, T. J. Mark and C. R. Kannewurf,
IEEE. Trans. Instrum. Meas. 1988, if, 76.

(a) P. W. Selwood, Magnetochemistry (2nd Ed.), Interscience
Publishers, New York, 1956, pp 78. (b) R. S. Drago, Physical
Methods for Chemists (2nd Ed.), Saunders College Publishing,
Philadelphia/San Diego/New York, 1992, Chapter 11. (c) For
polyaniline: A. J. Epstein, J. M. Ginder, A. F. Richter and A. G.
MacDiarmid, in L. Alcacer ed., Conducting Polymers, D. Reidel
Publishing Company, Dordrecht, 1986, pp 121.

X-ray scattering coherence lengths were calculated from the
Scherrer formula Lhk1=KMBcosez (a) D. M. Moore and R. C.

Reynolds, Jr., X-Ray Diffraction and the Identification and Analysis
of Clay Minerals, Oxford University Press: Oxford/Newyork, 1989,
pp 83. (b) H. P. Klug and L. E. Alexander, X-ray Diffraction
Procedures for Polycrystalline and Amorphous Materials, John
Wiley & Sons, New York, 1962, pp 491. In the formula, K is a
constant close to 1. It is assigned 0.9 in the calculation. A is the X-ray
wavelength, which is 1.5419A. B is the broadening of the reflection
peak (in 20) in the XRD pattern, and is calculated according to the
following equation:

9 = [b2 _ 1302
b is the width of the peak at a half height. b0 is the instrumental

broadening, which is 014° according to the width of pristine 2H-
MoS2. The unit of [3 is in are rather than degrees, so there is a factor
of 7t/180.

N. Toshima and O. Ihata, Synth. Met. 1996, B, 165.

(a) M. G. Kanatzidis, C.-G. Wu, H. O. Marcy and C. R. Kannewurf,
Adv. Mater.1990, 2, 364. (b) M. G. Kanatzidis, H. O. Marcy, W. J.
McCarthy, C. R. Kannewurf and T. J. Marks, Solid State Ionics,
1989, 32-33, 594. (c) C.-G. Wu, M. G. Kanatzidis, H. O. Marcy, D.
C. DeGroot and C. R. Kannewurf, Polym. Mat. Sci. Eng. 1989, 61,
969. (d) M. G. Kanatzidis, C.-G. Wu, H. O. Marcy, D. C. DeGroot,
C. R. Kannewurf, Chem. Mater. 1990, _2_, 222.

113

21

22

23

24

25

26

27

28

29

30

Freshly restacked WS2 was carefully prepared for the inductively
coupled plasma (ICP) analysis by stirring and washing the LiBH4,
LiH and LiWS2 mixture in water 6 times, similar to the procedure
used for the LixMoSZ ICP sample described in Chapter 1. ICP
showed Li 0.383 wt% and S 23.0 wt%, which suggest a formula
L101 5MoS2. The amount of W was not analyzed because of the lack
of W standard.

H.-L. Tsai, L. Wang and M. G. Kanatzidis, paper in preparation.

F. Wypych and R. Schollhom J. Chem. Soc., Chem. Commun.
1992, Q, 1386.

(a) R. Bissessur, Synthesis and Characterization of Novel
Intercalation Compounds of Molybdenum Trioxide and Molybdenum
Disulfide, Ph. D. Dissertation, Department of Chemistry, Michigan
State University, 1994, pp 186. (b) idib, pp 183 and 193.

Annual Book of ASTM Standards 1979, E-698, pp 601-608.

H.-L. T sai, J. Heising, J. L. Schindler, C. R. Kannewurf and M. G.
Kanatzidis, Chem. Mater. 1997, 9, 879.

The room temperature electrical conductivity data listed in Table 3
were measured at Michigan State University. Although the data are a
little different from the room temperature values of the variable
temperature conductivity measurements, because of the differences
in sample, instrument and manipulation, the set of data in this table
should be comparable among themselves.

J. Heising, Synthesis and Characterization of Novel Intercalation
Compounds of Molybdenum Trioxide and Molybdenum Disulfide,
Ph. D. Dissertation, Department of Chemistry, Michigan State
University, 1999.

In the calculation to obtain the value 27.9 A2, 7.76 A rather than 7.8
A was used to avoid too much approximation and too few significant
figures.

2H-MoS2 has a hexagonal structure (P63/mmc): a=3. 161 A, c=12.295
A; a MoS2 unit occupies 8.653 A2: P. Villars and L. D. Calvert,

114

Pearson 's Handbook of Crystallographic Data for Intermetallic
Phases, 2nd Ed., Vol. _4, 1991, ASM International, Materials Park,
OH 44073, pp 4445. (cited from: B. Schonfeld, J. J. Huang and S. C.
Moss, Acta Crystallogr., Sec. B, 1983, _3_9__B, 404.)

115

Chapter 3

LAMELLAR TaSz/POLYMER NAN OCOMPOSITES THROUGH
ENCAPSULATIVE PRECIPITATION OF EXFOLIATED LAYERS

Introduction

Two-dimensional inorganic/polymeric nanocomposites represent an
important and growing class of hybrid materials with interesting physical
properties. The research and development activity is concentrated mainly
in two major fields, nanocomposites as structural materials [11 and as
electro-active materials [2]. To synthesize two-dimensional nanocomposites,
the insertion of polymers inside the layered hosts is the critical step.
Polymers can be sandwiched between the galleries of the layered materials
through one of four currently known approaches: (a) by monomer
intercalation and subsequent polymerization in the galleries, (b) by in situ
intercalative polymerization, (c) by direct polymer insertion and (d) by
encapsulative precipitation of polymers from solutions of exfoliated
lamellar solids. The latter two methods produce nanocomposites with
preformed and well characterized polymers, and are capable of producing
large varieties. The direct polymer insertion approach works well with
organically modified aluminosilicates, or clays 11]. On the other hand, the
encapsulative precipitation method, which has been developed in our
laboratory, has been successful in making nanocomposites with electro-
active layered hosts [31. In this method, the layered host exfoliates to give
monolayers in solution, which interact with dissolved polymer chains and
encapsulate them during a restacking process. This approach has led to the

synthesis of MoS2 [3a], M003 [3b] and NbSe2 [3C] nanocomposites with various

116

polymers in our group, M003/poly(ethylene oxide) (MoO3/PEO)
nanocomposites in Nazar’s group [4], and MoS2, MoSe2, TiS2, TaS2, M003
and MPS3 nanocomposites with PEO [53] and/or polyethylenimine (PEI) [5b]
in Lerner’s group.

Here we report the synthesis of TaS2/polymer nanocomposites
through the encapsulative precipitation method [6’ 7]. The encapsulation of
PEO, PEI and poly(vinylpyrrolidinone) (PVP) into TaS2 were examined in
detail and the nanocomposites were characterized by a large variety of
techniques. Most of all, our discovery about the different characters of
various forms of LixTaS2 in exfoliation and polymer intercalation is
extremely valuable to the synthesis of intercalative TaS2 complexes. The
difference between PEI and other polymers, and the subtleties of the
exfoliation and encapsulation of exfoliated TaS2 will be elucidated in this
chapter.

The exfoliation of TaS2 has long been known [8» 9» 101 and cations of
many sizes have been encapsulated between the layers of this material.
Early work in this area showed that electrolytically prepared HxTaS2 could
be monodispersed in aqueous surfactant solutions and formed adsorption
complexes with cationic dyes [8]. Later, chemically prepared NaxTaS2 was
dispersed in water or NMF/water mixtures and cluster cations were
included inside to form ordered intercalation compounds [10b].

Cation intercalation is driven more or less by cation exchange, which
does not require an exfoliated solid [11]. Neutral polymer intercalation,
however, needs a different driving force, for example, polymer-LP
interactions and/or polymer-TaS2 van der Waals interaction. The

intercalation of polymers most probably takes advantage of the affinity of

117

the polymer chains for the interlayer surface of the host and for the cations
(i.e. LP“) in the interlayer galleries.

Given that some metal dichalcogenides become superconductors at
low temperatures, it is intriguing to consider polymer nanocomposites
containing superconductive components. Such materials would combine the
superconducting properties of inorganic solids with the processible
properties of polymers giving rise to new forms of superconductors such
as polymer matrix-based wires and free standing films, thus enabling new
kinds of applications. We have made a first step in this direction by
inserting polymers into NbSe2 and TaS2 to produce lamellar
inorganic/polymer superconducting solids with plastic-like characteristics.
This work is an outgrowth of our studies of intercalative polymer
nanocomposites of MoS2 [38] using the exfoliation procedure [12].
Remarkably, the flexible metallic TaS2/polymer nanocomposites display

bulk superconductivity.

Experimental Section

1. Reagents

PEO (5,000,000), PEO (100,000), PEI (25,000) and PVP (10,000)
were purchased from Aldrich Chemical Company, Inc. After the polymers
were dissolved, the polymer solutions were filtered to purify from
insoluble polymer residues. LiBH4 (95%), LiOH-H2O (98%) and Ta
(99.9%, 325 mesh) were also purchased from Aldrich. Sublimed sulfur
was from Spectrum Chemical Mfg. Co. Anhydrous ether (99.0%),
acetonitrile (99.5%) isopropanol (99.9%) and carbon disulfide (100%)

were from Columbus Chemical Industries Inc., EM Science Inc.,

118

Mallinckrodt Chemical Inc. and J. T. Baker Inc. respectively. No further
purification was applied to the chemicals above. The water was distilled

and degassed by bubbling nitrogen through it for 30 min before use.

2. Synthesis of 2H-TaS2

2H-TaS2 was synthesized with a modified literature procedure [131.
An amount of 3.619 g of tantalum (20 mmol) and 1.300 g of sulfur (40.5
mmol) sealed in a quartz tube was heated at 450 0C for 12 h and then 950
0C for 36 h. The quartz tube was 13 mm in diameter and ~13 cm long with
a volume of about 12-14 ml. The tube was quenched from 950 CC in cold
water and excess sulfur was deposited on the tube walls. The lT-TaS2
formed was ground and washed with CS2 to remove any excess sulfur. The
purified lT-TaS2 was transformed to 2H-TaS2 by an annealing procedure
of slow cooling from 910 to 450 0C in two weeks, according to the

protocol shown in Table 3.1.

Table 3.1. The annealing procedure to produce 2H-TaS2

 

 

 

 

 

 

 

 

 

 

temperature (0C) 910 750 650 550 450
time to reach (h) 12 16 20 20 20
time to hold (h) 12 24 48 72 72

 

3. Synthesis and Exfoliation of LixTaS2

An amount of 4.00 g 2H-TaS2 was reacted with n equivalents of
LiBH4 (n = 0.1, 0.2, 0.3, 0.4, 0.5, and 1.0) in 100 ml of anhydrous ether

for 3 days under a nitrogen atmosphere to obtain LixTaS2. LixTaS2 was

119

black when n = 0.1, 0.2 and 0.3, while it was reddish brown when n 2 0.4
[14],

LixTaS2 was exfoliated in degassed water, in a concentration of 1.0
g/L, by 30 min of sonication in an ultrasonic cleaner under a nitrogen
atmosphere. When n = 0.1, not much of LixTaS2 was exfoliated in water.
The suspension was slightly yellowish black. When n = 0.2 or 0.3, most of
the LixTaS2 went into water. Especially in the case of n = 0.2, the amount
of unexfoliated LixTaS2 was very small (~2.6%) and the colloidal
suspension had an intense greenish yellow color. When n 2 0.4, the LixTaS2
became increasely difficult to exfoliate. The color of the resulting
suspension ranged from brownish yellow to reddish brown. The higher the
value of n, the lighter the color of the suspension. Therefore, LixTaS2 from
reactions with n=0.2 was chosen as the host material for polymer
intercalation. Lio2TaS2 is used to present this form of LixTaS2 for

convenience [15].

4. Encapsulative Precipitation of Polymers

In a typical reaction, 0.40 g of Li02TaS2 was exfoliated in 400 ml of
degassed H2O by 30 min of sonication. The resulting suspension was mixed
with 100 Hi] of polymer solution (5 or 10 times in excess by equivalents of
repeat units) and stirred for 2 days under a nitrogen atmosphere. The
supernatant of the reaction mixture was centrifuged. A half part of the
polymer intercalated TaS2 was separated from the solution by
centrifugation and is referred to as batch-I nanocomposite. The centrifuged
supernatant was pumped to remove most of the water. A corresponding

solvent (acetonitrile for PEO and isopropanol for PVP and PEI) was added

to the concentrated supernatant to precipitate the intercalated TaS2

120

remaining in the supernatant. This precipitate is referred to as batch-II
nanocomposite. The products were pumped to dryness and vacuum-sealed

in glass ampules.

5. Instrumentation and Measurements
a. Instrumentation

The instrumentation in the measurements such as thermal
gravimetric analysis (TGA), differential scanning calorimetry (DSC) and
infrared (IR) spectroscopy were the same as described in Chapter 1. Room
temperature conductivity measurements, variable temperature direct—
current electrical conductivity measurements and thermopower
measurements, and magnetic susceptibility measurements were conducted
as described in Chapter 2.

Generally, X-ray powder diffraction (XRD) patterns were collected
as described in Chapter 1. For one-dimensional electron density
calculations, we obtained XRD data (in the region 2° S 20 S 135°) from
highly oriented samples and a stepwise scanning mode with 0.1° per step.
Variable temperature solid state 7Li NMR spectra were taken on a 400
MHz Varian Nuclear Magnetic Resonance Instrument. Samples were loaded
in a glove box under a nitrogen atmosphere.

Scanning electron microscopy (SEM) and energy dispersive X-ray
microanalysis (EDS) were done with JEOL-JSM 35 CF microscope at an
accelerating voltage of 15 kV and 20 kV respectively, at the Center for
Electron Optics at MSU. Samples were mounted on the sample stab with a
conductive tape. Electron diffraction was done on a transmission electron
microscope (TEM) JEOL-100CX at 120 kV, at the Center for Electron

Optics. Samples were ground and suspended in water or acetone before

121

deposition on copper grids. Gold film doposited on a copper grid was used

as standard in the calibration of camera length.

b. Composition of nanocomposites

The amount of polymer in the nanocomposites was estimated by
TGA in oxygen flow. For comparison, the weight loss of Li0,2TaS2 in
oxygen flow was checked by TGA up to 800 OC and was found to be about
8.2% [16]. The amount of polymer and water in the nanocomposites was
calculated assuming that the Lio,2TaS2 in the nanocomposites lost the same
amount of weight. The final product from the TGA in an oxygen flow was
mainly Ta2Os, which was confirmed by XRD and IR spectra, however, it
probably also contains some Li2SO4 [17].

In TGA measurements under either oxygen or nitrogen flow, the
nanocomposites barely lost weight at a temperature lower than 190 0C (<
0.5% for batch-I nanocomposites and < 2.5% for batch-II nanocomposites),
suggesting they contain almost no water. The major weight loss step for all
the nanocomposites occurred between 200 0C to 350 OC and corresponds to

polymer decomposition.

c. One-dimensional electron density measurements

One-dimensional electron density (l-D ED) maps were calculated
according to XRD data collected from highly oriented film samples in a
stepwise scanning mode. These film samples were made by casting aqueous
nanocomposite solutions under a nitrogen flow, so that the basal planes of
the TaS2 layers restacked parallel to the substrate. Several layers of a film
were loaded in the sample plate so as to obtain maximum diffraction

intensity. The XRD experiments were carried out under a nitrogen

122

atmosphere. In the measurements, slits for different beam width, as well as
different data collection times, were used for different 20 ranges, in order
to achieve a compromise between peak broadening, peak intensity and
experiment time. Specifically, 0.5° slits and a data collection time of 3 s per
step were chosen for measurements in the range from 2° to about 40°; 10°
slits and 3 8 per step between 26° and 70°; 20° slits and 60 s per step
between 59° and 120°; 4.0° slits and 60 s per step from 101° to 135°. The
step width was kept the same (01°) in the entire 20 range.

A full range XRD pattern (2° S 20 S 135°) was obtained by merging
the data from the different 20 ranges and normalizing (scaling) them based
on the overlapped regions. The overlapped data regions had at least two
peaks in common. The full data set was put into an XRD analysis program,
PEAKOC [131, to calculate the integrated peak area of each 001 reflection.
In pattern analysis, 001 peaks were fit with the split-pseudo-Voigt function
with a linear background subtraction for each peak. In the case that an 001
peak overlapped a little with another peak, or was cut off at the end of the
pattern, it was fit with the pseudo-Voigt function (which is symmetric).

The integrated peak area of the 001 peaks was used as the intensity of
the peaks, and put in a FORTRAN program to calculate the 1-D ED map.
The absolute value of the structure factor F of the 001 reflections was
calculated from the intensities and the Lorentz-polarization factor (LP)

according to (1).
|F(l)| = (1/L,,)“2 (1)

The Lorentz-polarization factor (LP) is defined as:

123

= (1 + cosz20)/(sin20cos(9) (2)

The signs (phases) of the structure factors were assigned based on the signs

of the corresponding structure factors of the TaS2 framework alone. This

reasonably assumes that the scattering contribution from the intercalated

polymer is relatively small compared to that of TaS2 component. The 1-D

ED map was calculated from the structure factors of the 00l reflections

according to (3).

p(z) = (l/L)[ F0 + 22 F(|)cos(27c|z) ] (3)
I

In (3), z is the fractional coordinate on the c axis, L is the basal spacing of

the layered structure, and F0 is the zeroth-order structure factor.
To obtain their signs, the structure factors of the TaS2 framework

were calculated from:

N
Flcal =212£11coslj2ttlz ) (4)

The scattering factor fj(l) for atom j was calculated according to (5) with

ai, bi and c for the element obtained from literature [19], and 0

corresponding to the reflection position.

2 ' 2
bléa-epr b—i—xineh elem-35%) (51

After the model for the structure was established, the sign of the F(l) was

checked by recalculating F0310) from the scattering of the atoms including

those of the polymer.

124

Results and Discussion

1. Exfoliation Properties of LixTaS2

N axTaS2, as reported previously, did not form ideal suspensions in
water, so formamide (FM), N-methylformamide (NMF) or NMF/H2O 1:1
mixture were used to produce concentrated suspensions [9. 10b]. FM and
NMF are not preferred for the intercalation of polymers, because not only
they are expensive and toxic, they also compete with the polymers and co-
intercalate in the host. In order to be able to use TaS2 as a polymer host we
needed a material which exfoliates well in water. Therefore, we explored
the swelling and exfoliation properties of LixTaS2 as a function of x.
LixTaS2 with different degrees of lithiation was prepared under different
conditions, through (a) reaction with butyllithium in hexane, (b) reaction
with sodium dithionate in water followed by ion exchange with Li+ ions,
and (0) reaction with LiBH4 in ether in various stoichiometric ratios and
conditions. The LiBH4 method [20] was most successful. We found that the
best LixTaS2 suspension was obtained with x ~ 0.2. Namely, Lio2TaS2
formed concentrated and stable colloidal single layer solutions in water,
superior to those formed with N axTaS2 in NMF/H2O solution.

In all cases, there was always a fraction of LixTaS2 which did not
exfoliate, regardless of how dilute the solution was. Usually, the more
readily the LixTaS2 exfoliates, the more concentrated suspension it forms
and the less residue it leaves behind. To demonstrate how the amount of the
LiBH4, in the lithiation reaction, affected the exfoliation of LixTaS2, a
semi-quantitative experiment was performed with 0.050 g LixTaS2 with n =

0.1, 0.2, 0.3 and 0.4 [14]. After LixTaS2 was sonicated in 200 ml degassed

water for 30 min, the precipitates were collected, dried and weighed. Based

125

on the LixTaS2 used, the unexfoliated fraction was 53%, 2.6%, 21% and
21% respectively for LixTaS2 with n = 0.1, 0.2, 0.3 and 0.4. These results
showed that 0.1 equivalent of LiBH4 was not enough, while 0.3 or 0.4
equivalent of LiBH4 were over the optimum ratio. It needs to be pointed
out that the charge density of Li02TaS2, ~ 48 AZ/e' [21] , is close to the up
limit of the 40 - 120 AZ/e' range [22], in which stable colloidal dispersions
are most likely to form.

LixTaS2 used by Oriakhi et al [5b] to prepare the PEI/TaS2
nanocomposite was lithiated by reacting with LiOH [23]. Our investigation
into that lithiation reaction reveals that control of the amount of LiOH is
equally important and the optimum stoichiometric ratio is approximately

the same.

2. Polymer Encapsulation

The success of polymer encapsulation depends on the nature of both
LixTaS2 and polymer. Table 3.2 shows which attempts succeeded or failed
in the preparation of nanocomposites. PEO was intercalated only in
“Li0,2TaS2”. PVP was intercalated in LixTaS2 prepared with 0.2 < x < 0.4.
PEI has the most affinity for LixTaS2 and intercalated in all LixTaS2
samples [24]. This must due to the fact that PEI is a strong organic base
which could be protonated in solution. The protonated PEI has positive
charges which facilitate (through electrostatic attraction) the interaction
with the [TaS2]"' layers. In this sense, the insertion of PEI in [TaS2]"' is an
ion-exchange process where Li+ ions are replaced with positively charged
PEI molecules. Other polymers such as polyacrylamide (PAM) and methyl
cellulose (MCel) were also tried, with partial intercalation achieved with

PAM and no intercalation with MCel.

126

Table 3.2. Polymer Intercalation in LixTaS2

 

 

 

 

 

 

 

 

 

intercalation of polymers

PEI PVP PEO
n=0.1 Yes No
n=0.2 Yes Yes Yes
n=0.3 Yes Yes
n=0.4 Yes Yes No
n=0.5 Yes No
n=1.0 Yes No

 

 

 

 

 

* "Yes" indicates that the intercalation was successful, while "No" shows
that the intercalation failed.

3. Characterization of LixTaS2/Polymer N anocomposites

The formation of lamellar LixTaS2/polymer nanocomposites was
evident in the XRD patterns of the products. The presence and position of
00l reflections indicated the separation of the TaS2 slabs, which is evidence
for the insertion of polymer chains. The pristine TaS2 and the hydrated
Lix(H2O)yTaS2 show interlayer spacings of 6.0 A and 7.4 A respectively,
which can be readily distinguished from a polymer intercalated phase. The
d-spacings of batch-I and batch-II for both PEO and PVP intercalated
compounds were comparable within :1 A. The XRD patterns of the two
types of products were similar, except that in the former type (batch-I) we
could observe some weak diffraction peaks from residual un—intercalated
solid. In the case of PEI, the d-spacing of batch-I product was about 10.0
A, while that of batch-II product was about 37 A indicating the

encapsulation of multiple layers of polymer. Some typical XRD patterns of

127

 

(a)

 

 

 

 

 

 

 

 

E?
.5 L
5 100A
:3 ' (C)
.fiL_—4— - _SL_
36.8A
(d)
1L L1
IlIIIIIIIIIIIIIIIIIIIIIIIIIfi
0 10 20 30 40 50 60
20(deg)

Figure 3.1. XRD patterns of LixTaS2/polymer nanocomposites.
(a) Li0,2(PEO)yTaS2 (batch-II), (b) Li0,2(PVP)yTaS2 (batch-II),
(c) Lix(PEI)yTaS2 (batch-I) and (d) Lix(PEI)yTaS2 (batch-II).

128

 

 

 

Table 3.3. Properties of Li02TaS2 and polymer nanocomposites

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

room
sample d- coherence composition temperature
spacing length conductivity

(A) (A) (SIcm)
LiO,2TaS2 6.0 >103
Hydrated Lio,2Ta82 7 .4
Lio_2(PEO)yTaS2 15 .6 93 Li0.2(PE0)1.36(H20)0.00Ta32 19
(100K) (batch I)
Li0.2(PE0)yTa32 15.2 83 Li0.2(PEO)1.51(H20)0.23Ta32 2.2
(100K) (batch H)
Li0,2(PEO)yTaS2 15.1 106 Li0.2(PEO)0.75(H20)0.06TaS2 17
(5M) (batch I)
Li0.2(PEO)yTaS2 15 .5 83 Li0.2(PEO)l.68(H2O)0.00T352 3 .0
(5M) (batch H)
Li0.2(PVP)yTa82 3 1 . 1 256 Li0.2(PVP)0.95(H20)0.08T332 3 1
(10K) (batch I)
Lio,2(PVP)yTaS2 30.7 186 Li0.2(PVP)1.28(H20)0.35T332 0.5
(10K) (batch II)
Lix(PEI)yTaS2 10.0 93 Lix(PEI)0.82(H20)o.07Ta82 125
(25K) (batch I)
Lix(PEI)yTaS2 36.8 - Lix(PEI)3.2(H2O)0.56TaSZ 0.18
(25K) (batch H)

 

 

* The X-ray scattering coherence length is determined from the half-width
of the peak using Scherrer formula Lhk, = K A/Bcose, see Reference 18 of

Chapter 2.

129

the nanocomposites are shown in Figure 3.1. More information about d-
spacings and coherence lengths in the direction of layer stacking is listed in
Table 3.3. From these, one can estimate the ordered domains in particles of
the nanocomposites to be between 5 and 10 TaS2 layers thick. As the
number of layers in the nanocomposite particles is considered,
Lix(PEI)y(H2O)zTaS2 (batch I) has the largest value (9.3),
Li0.2(PVP)y(H2O)zTaS2 has medium values (6. 1-8 .2), and
Lio,2(PEO)y(H2O)zTaS2 has the smallest values (5.4-7.0). Table 3.3 also
lists the compositions of the nanocomposites. It is obvious that a batch-H
nanocomposite contains a little more polymer than its batch-I analog. This
is understandable because batch-II product was well dissolved in water and
reacted more completely with the dissolved polymer.

DSC measurements on a Li0,2(PEO)1,o4TaS2 showed no phase

transition between room temperature and 300 0C, see Figure 3.2.

 

 

      

 

 

 

 

 

 

3 I 330 °c
" .9
6 ‘ E
Z <1)
A . 5
_ O
E 4 a:
if I 1st cycle 2nd cycle 0 .9
8 2 _ 308 c g
a: b / 2
‘6'
0 - 8
_ (D
_2 .- I I J I I I I I I I I I I I I I I I L I I I
0 100 200 300 400

Temperature (°C)

Figure 3.2. DSC of a Li0,2(PEO)yTaS2 nanocomposite.

130

 

Figure 3.3. SEM photograph of a Lio_2(PEO)yTaS2 film (batch-II; M.W. =
SM).

131

The melting peak of PEO at 66 0C is not observed in the nanocomposite. A
small exothermic peak at 308 0C (2.21 J/g) and a large one at 330 0C
(46.18 J/g) correspond to the decomposition of the PEO in the
nanocomposite and are associated with massive weight loss, detected by
TGA under nitrogen flow. Pure PEO shows an exothermic decomposition
peak around 370 0C.

Mass spectra of the Lio,2(PEO)1,04TaS2 in a temperature increase
ramp showed that the major decomposition fragments around 350 0C had
molecular masses of 45, 58, 73, 87, 88, 89, 103, 120, 133 and so on, which
were all PEO related fragments. There was no obvious evidence that the
decomposition fragments combined with sulfur from the TaS2 layers [25].

The LixTaS2/polymer nanocomposites described here can be cast into
free-standing films readily. The films of batch-II nanocomposites are
stronger than that of batch-I products and can be folded without breaking.
This is attributed to the higher polymer content of the batch-II materials.
Figure 3.3 shows a photo of a folded Li0.2(PEO)1,6gTaS2 film ( batch-II,
Mw = 5,000,000) taken on SEM. The film was folded before it was pressed

onto the conductive tape. The folding edge did not crack under pressure.

4. Structural Studies: The Conformation of PEO in Lix(PEO)yTaS2

There are many known PEO chain conformations. The most
common is the helical conformation [26] which exists in PEO spherulites
[271. Planar zigzag conformation was obtained in stretched PEO samples [281.
Two kinds of conformations were found in PEO-HgCl2 complexes [29’ 301.
More PEO conformations have been found in other PEO complexes in
literature and they will be described in Chapter 5. Models for the most

important PEO conformations will be shown in Figure 5.11.

132

The conformation and orientation of polymer chains in the interlayer
galleries of the layered nanocomposites has always been an important issue.
We calculated 1-D ED maps for (PEO)XV2Os.nH2O [31], and determined
that the helical conformation of PEO in that nanocomposite was not
possible. A bilayer planar zigzag structure was proposed. In the present
case, fihns of batch-II Li0,2(PEO)yTaS2 nanocomposite had well defined
sharp XRD patterns with seventeen 001 reflections corresponding to a
resolution of 0.85 A, see Figure 3.4. This intense XRD pattern offered a
good starting point for 1-D ED calculation for Lio,2(PEO)yTaS2
nanocomposite, which provides information for the internal structure of
the intercalated species projected on the c axis. As mentioned in the
experimental section, the phases for calculating this l-D ED map were
obtained from the positions of the Ta and S atoms which are taken to be

known. A 1-D ED map is shown in Figure 3.5d.

 

Intensity

0012

0013 0015 0017
0010 J\ 0014 0016 ,
0011

IIIIIITIIIIII

0 20 40 60 80 100 120 140
20 (deg)

 

 

 

 

 

 

 

Figure 3.4. Powder XRD patterns of a folded Li0,2(PEO)yTaS2 film (batch-
II; M.W. = 100K).

133

 

 

 

 

 

——:—-—'
A
E'-

$

 

 

Electron density
5

 

 

 

 

I I l I I I I L I J I I I

0 0.2 0.4 0.6 0.8 l

 

c-axis

Figure 3.5. One-dimensional electron density maps for the Lix(PEO)yTaS2
nanocomposite. (a) Model with two layers of planar zigzag PEO chains
while the zigzag planes are perpendicular to the TaS2 layers, (b) Model
with two layers of planar zigzag PEO chains while the zigzag planes are
parallel to the TaS2 layers, (c) Model with two layers of PEO in the type II
PEO-HgCl2 complex conformation, and (d) Computed from experimental
data.

134

 

 

888%¢8888&EW.A
8888888é%84 '

 

 

 

 

~3V%®%?Wwv:mfi
fikai-fo-Jkfi sh}- ‘91-‘84?»

«— S atoms
o——Ta atoms

 

Figure 3.6. Structural model for the Li0,2(PEO)yTaS2 nanocomposite. The

oxygen atom region in the middle of the gallery accommodates the Li+
ions.

135

The profile of 1-D ED map of Li0,2(PEO)yTaS2 shows clearly the
presence of substantial electron density between the TaS2 peaks. This
density is due to the organic polymer and it is distributed away from the
center of the gallery, peaking in two separate locations symmetrically
above and below the gallery central plane. The peak shape in each location
is asymmetric. This immediately excludes the helical conformation of PEO,
which must have one symmetric envelop of electron density in the central
region of the gallery.

The observed profile in the Li0,2(PEO)yTaS2 is also different from
that of the (PEO)XV2Os.nH2O system which displays two symmetric bumps
with two maxima on each of them, so the planar zag-zig model does not fit
here. Figures 3.5a and 3.5b present the profiles of the 1-D ED map
calculated for two layers of planar zigzag PEO chains arranged parallel
and perpendicular inside the gallery space of TaS2 [321. As expected, they
have two symmetric bumps and their maxima are too far apart to match the
profile calculated from XRD data of Li0,2(PEO)yTaS2.

The structural model best matching the experimental data is set up
with two layers of PEO with the conformation found in the type H PEO-
HgCl2 complex. This conformation has oxygen atoms on one side and
carbon and hydrogen atoms on the other side of the molecule. By putting
two layers of PEO inside the gallery so that the oxygen atoms face each
other in the middle of the gallery, as shown in the model in Figure 3.6, two
asymmetric bumps appear in the electron density map and the positions of
the atoms can be decided by fitting maxima of the bumps (see Figure 3.5c).
In this model, the atoms of PEO occupy reasonable positions in the gallery.
In addition, the orientation of the PEO chains is chemically plausible

because the hydrophobic part of the PEO (—CH2-CH2- groups) forms van

136

der Waals contacts with the sulfur atoms in the TaS2 layers and the oxygen
atoms form a more polar, hydrophilic environment in which, presumably,
the small Li+ cations reside. This model not only matches the experimental
data very well, it also makes good chemical sense. The type II PEO-HgCl2
complex conformation proposed here is probably brought about by
coordinating interactions of PEO with the Li+ ions, just as for the HgCl2
complex. That the PEO conformation in Lix(PEO)yTaS2 is different from
that of (PEO)XV2Os-nH2O [29] is attributed to the lack of coordinating ions
(e.g. Li+) in the latter.

5. Superconductive State

The magnetic properties of LixTaS2 and nanocomposites were
measured with a SQUID. As observed earlier with the NbSe2 system [3°],
the lamellar nanocomposites reported here undergo superconducting
transitions at temperatures higher than that of the pristine 2H-TaS2, 0.6 K
(or 0.8 K according to different publications). It is known that the To of
TaS2 is pushed down by the periodic lattice distortion-charge density wave
(PLD-CDW) of TaS2. If the PLD-CDW is suppressed, the Tc could be
raised to 4.1-4.5 K. In fact 0.08 equivalents of electrons are enough to
suppress the PLD-CDW in TaS2 [33, 341. Therefore in the LixTaS2 we expect
a higher To. The Cooper pairs in the superconductive state have a
coherence length in the magnitude of a micron. This spatial correlation
length is much longer than the gallery space in the LixTaS2/polymer
nanocomposites which is occupied by the insulating polymer chains. The
Cooper pairs can penetrate the barrier of the polymer layers and move

around in the nanocomposites, so the intercalation of polymers (at least

up to a certain expansion of the TaS2 layers) does not destroy the

137

Table 3.4. Properties of the superconductive state for TaS2 intercalates

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

sample xmolar percentage of
sample State To (K) (2K,5G) superififliuaa
LixTaS2 powder 3.7, 4.3 -2.64 82
(n=0.2)
LixTaS2 powder 4.1, 4.6 -0.128 4.0
. (n=0.4)

LixTaS2 powder 2.7, 4.3 _1.1*10-3 3.41.10:2
(n=0.5)
Lix(PEI)yTaS2 powder 2.9 -0.60 14
(batch I)
Li0,2(PVP)yTaS2 powder 2.5 -0.82 5.8
(batch I)
Lio,2(PEO)yTaS2 film 2.7 -0. 17
(batch 1) parallel

film 2.6 -2.7 42

perpendicular

Li0,2(PEO)yTaS2 film 2.9 -5 .6* 10'3
(batch 11) parallel

film 2.9 .53410-2 0.9

perpendicular

* Densities of the materials are needed in the calculation. The density

of 2H-TaS2, 6.075 g/cm3 is used in the calculation of LixTaS2 (n=0.2, 0.4
and 0.5). The densities deduced from the compositions and structural
parameters of the nanocomposites are used in the calculation of the
percentage of superconductive state in the nanocomposites.

138

superconductive state.

Measurements of Tc and Meissner effect are summarized in Table
3.4. A pure superconductor has the magnetic susceptibility of x=-1/471:. The
percentage of the superconductive state in the samples is calculated by
comparing the susceptibility of the samples at 2 K with -1/41t. The high
percentages found in many of the nanocomposites indicate that we are
observing bulk superconductivity. The TC values were determined by the
point of intersection of the extrapolations from the linear magnetization of
the superconductive state and the normal-state magnetization.

The LixTaS2 (n=0.2, 0.4 and 0.5) samples exhibit two Tc’s, so they
are mixed phase superconductors. This can be compared to the multiphase
samples prepared from reaction of 2H-TaS2 with less than 0.5 equivalent of
NaOH reported by Biberacher et a1 [23]. After Li02TaS2 is intercalated with
polymers, the second phase disappears or is not detectable because Tc
moves to lower temperature. From Table 3.4, it can be seen that the more
the TaS2 is reduced, the lower the percentage of superconductive state in
the sample. From LixTaS2 (n=0.2) to LixTaS2 (n=0.5), the value decreases
three orders of magnitude. This shows the enormous effect of reduction to
the electronic structure of TaS2. It explains why the control of the amount
of LiBH4 in the lithiation reaction is so important to the exfoliation and
intercalation properties of the LixTaS2. The intercalation of polymer causes
the percentage of superconductive state to drop, but not by orders of
magnitude. This is shown by the powder samples of Lix(PEI)yTaS2 and
Li0,2(PVP)yTaS2, and the film sample of batch-I Li0,2(PEO)yTaS2, see
Table 3.4. The low percentage values of batch-II Lix(PEO)yTaS2 films
could result from the smaller dimensions of TaS2 slabs in the

nanocomposite.

139

 

(a) 0.5[IIIIIIIIIIIIIIIIIIIIIIIIIIlfir

1

Zero field cooling

IIIIII_IIIIIIIIIIIIIIIIIIIIIIII

2.5 3.0 3.5 4.0 4.5 5.0
Temperature (K)

 

-0.5

-1.0

X molar

-1.5

-2.5

 

 

JJIIIIJIIIIIJIIIJIJIIILIIIIII III

-3.0

 

N llllllIITIIIIIIIIIIIIIIIIIIIIIIIII

'o

 

(b) 0.5-rIIIIlllllIIIIIIIIIIIIIIIFIIr]
O 3 Field cooling

 

x molar

 

 

 

I
p—I
LII
IIIIIIIIIIIIIIIIIIIIIIIIIIIIII

Zero field cooling

IIIIIIIIIIIIIIIIIIIIIIIIIIIIII

2.0 2.5 3.0 3.5 4.0 4.5

 

mIIIIIIIIIIIIIIIIIIIIIIIIIIIIIJ_LIJ

Temperature (K)

Figure 3.7. Variable temperature magnetic susceptibility (at 5 Gauss) for
(a) Li02TaS2 (powder), and (b) Li0,2(PEO)yTaS2 (batch-I; M.W. = 100K;
films perpendicular to the applied magnetic field).

140

(a)

(b)

X molar

X molar

0.5

-0.5
-1.0

-1.5

p—I

1.5

 

 

IIII lllI IIII I'll IIII IIIT
I I I I I

Field cooling

 

 

 

 

Zero ield coolin

IIIIIIIII IIII IIIIII

 

IIIJIIII

 

in IIIIIIIITII1IIIIIIIIIIIIIIIIIIIIII

P IIIIIIIIIIIIIIIIIIIIIIIIIIlII III

2.0 2.5 3.0 3.5
Temperature (K)

4.0

 

IIIIIITIIIIIIIIIIIIIIIIIIIIII1IIII

 

IIITIIIIIIIIIIIIIIIIIIIIIIITI

IILII ILIILIIJJILLI III1

 

JIIIIJIIIIIIIIIIIIIIIIIIIJIIL

 

P IIII
L1]

2.0 2.5 3.0 3.5 4.0
Temperature (K)

Figure 3.8. Variable temperature magnetic susceptibility (at 5 Gauss) for
Li0,2(PEO)yTaS2 films (batch-I; M.W. = 100K). (a) films perpendicular to
the applied magnetic field, (b) films parallel to the applied magnetic field.

141

 

 

Field dependent magnetic measurements on LixTaS2 and the
nanocomposites show that the Meissner effect decreases with increasing
magnetic field. Variable temperature magnetic susceptibility measurements
show that the Meissner effect decreases gradually with increasing
temperature. Furthermore, the field cooling curve diverges from the zero-
field cooling curve in these materials, which is consistent with a type—II
superconductor [35].

In the variable temperature magnetic susceptibility measurements,

displayed in Figure 3.7, the field cooling curve was very different from the

zero-field cooling curve in Li0_2TaS2 [Figure 3.7(a)], while the two curves
were much closer together in batch-I Li0_2(PEO)yTaS2 [Figure 3.7(b)] .
The two curves of batch-H Li0,2(PEO)yTaS2 were also close together. The
drift of the field cooling curve toward the zero-field cooling curve, which
was also found in the PEI and PVP intercalated nanocomposites, indicates
that these materials are less able to pin the electromagnetic vortices present
in the material. This is typical of granular superconductivity which appears
in materials composed of tiny grains of superconductor particles
surrounded by insulating layers [35]. In contrast, NbSe2 nanocomposites do
not show this effect [30].

A dramatic difference is observed with sample orientation in the

magnetization curves below To, see Figure 3.8. The Meissner effect is

much stronger when the film is placed perpendicular to the magnetic field.
In both batch-I and batch-II LixTaS2/PEO samples, the xmolar was > 10

times larger when the film was perpendicular to the field (i. e. TaS2 layers

perpendicular to field) than when it was parallel. This observation suggests

that the Cooper pairs are moving predominantly in a circular way within

the TaS2 layers without passing through the polymer layers. This is a

142

 

 

beautiful demonstration of the two-dimensional nature of these
nanocomposites.

The magnetic properties of LixTaS2 and LixTaS2/polymer
nanocomposites were also measured at temperatures above TC. In this
temperature range the magnetic susceptibility of LixTaS2 (n=0.2 and 0.4)

was Pauli-like and temperature independent with molar XTIP around

1.0*10'4 cm3/mol.

6. Electrical Transport Properties

Room temperature electrical conductivity measurements show that
Lio_2TaS2 and its nanocomposites are good conductors, see Table 3.3.
Li0_2TaS2 itself is highly conductive, with a conductivity higher than 1000
S/cm. The intercalation of polymers brings the conductivity down by
several orders of magnitude. The batch-I products of nanocomposites are l
to 2 orders of magnitude less conductive, while the batch-II products are 3
to 4 orders of magnitude less conductive. This is probably because the
batch-II products contain more polymer, and have TaS2 slabs of smaller
dimensions. Variable temperature measurements for batch-I and batch-II of
Lio,2(PEO)yTaS2 showed that the conductivity of both increases with falling
temperature, consistent with metallic charge transport in these materials,
see Figure 3.9. The conductivity reached a broad maximum at 75 K in
batch-I Li0,2(PEO)yTaS2 and at about 130 K in batch-II Li0,2(PEO)yTaS2
before it started to drop. The superconductive behavior was not detected
because our experimental set up could not reach the low temperature at
which the metallic to superconductive transition occurs. The metallic
character is further confirmed by thermopower data shown in Figure 3.10.

The Seebeck coefficient is very small and negative suggesting n-type

143

 

_7 —' 7' _”_*%Fi T, 7—;

2O

 

 

 

 

 

L -
t -
L -
”a“ 15 — 5
2 ' .
re : 3
b u ..
IE 10 T i
g _ _
_d - .
g ' 1
c.) 5 T 7.
0 - I I I I I I I I I I I I I I I I LL I I I l I I I I I I I -

0 50 100 150 200 250 300

Temperature (K)

Figure 3.9. Variable temperature electrical conductivity measurements for
pressed pellets of Li0,2(PEO)yTaS2 (batch-I product).

 

 

 

 

 

9 -:
E. -1;- —;
8 ~25- 5
g 3 :
8‘ '35‘ r:
E -4; _-
J: : ' :
I" _-_ (b I
-5 r __
C
_6bl I I I I I I I I I I I I I I I I I I I IJ I I I I I I I
0 50 100 150 200 250 300

Temperature (K)

Figure 3.10. Variable temperature thermopower data of pressed pellets of
Li0,2(PEO)yTaS2 (batch-I product).

144

(a) .

 

 

Figure 3.11. Electron diffraction pattern and superstructure of L102TaS2.
The sample was exfoliated in water before deposition on carbon-coated
copper grids. The / 3 x,/ 3 superlattice reflections are shown with an

arrow.

145

metallic behavior. The n-type transport is somewhat surprising since one
might expect them to be hole conductors, given that 2H-TaS2 has a half-
filled conduction band and injection of a small amount of electrons should
result in a band which is more than half-filled. The failure of this
prediction is due to the formation of a 3/5 x\/3 superlattice in the TaS2
layers, which was detected by electron diffraction, see Figure 3.11. This
superlattice is believed to be due to Ta-Ta bonding interactions within the
TaS2 layer and leads to a modification of the electronic band structure
which changes the charge transport properties.

There is no doubt from the above data that the charge transport
within the layers is metallic. However, when the carriers go through the
boundary areas and polymer layers, activation energy is needed. This
contributes a thermally activated contribution to the conductivity,
especially when the TaS2 slabs become more separated as the polymer

content increases.

7. Solid State N MR Spectroscopy

In order to probe the behavior of lithium ions in the two different
gallery environments, (i. e. gallery with and without polymer) we measured
the variable temperature static solid state 7Li NMR spectra for Li02TaS2
and Li0_2(PEO)yTaS2 (batch-II). Examination of the temperature
dependence of the 7Li N MR line-width provides an independent perspective
on the cation mobility in inhomogeneous solid, especially in (Li
salt)/polymer complexes [36, 371 and Li/polymer/inorganic nanocomposites
[381. Both Li02TaS2 and Lio,2(PEO)yTaS2 showed just one resonance peak
due to a first order quadrupolar transition with no significant satellite

peaks.

146

The spectra of Li02TaS2 at -80 0C and 100 0C are presented in
Figures 3.12(a) and 3.12(b). At -80 0C, Li02TaS2 exhibits a broad peak
with the maximum situated at a chemical shift of 5.5 ppm higher than the
solid LiCl reference. The peak was somewhat asymmetric with a barely
noticeable shoulder on the low field side. With rising temperature the
position of the peak maximum did not change, but the shoulder protruded
more and moved towards lower field. At 100 0C, the shoulder became a
second peak at a chemical shift of 15 ppm higher than that of LiCl. The
shape and position of the peak changed reversibly over many cooling and
heating cycles. The change of the linewidth (the width at half-height) of the
whole peak, versus temperature, is presented in Figure 3.13A(a). The
broadening of the 7Li resonance peek is mostly due to many overlapping
signals corresponding to slightly different lithium ion sites. The slow
decrease of the linewidth with increasing temperature suggests that there is
a large activation barrier for Li+ to hop from site to site. The presence of
the shoulder in the spectrum indicates a different chemical environment for
Li+ ions, which might be related to the multiphase observed in the
superconductive state discussed above.

The low temperature (-80 0C) static solid state 7Li NMR spectra of
Li0,2(PEO)yTaS2 (batch-II) showed a broad symmetric resonance peak with
a chemical shift almost identical to that of solid LiCl, see Figure 3.12(c).
With increasing temperature, the position of the peak did not change, but
its shape became asymmetric, see Figure 3.12(d). The peak base on the low
field side extended a little farther than on the opposite side. The linewidth
did not change appreciably in the temperature range from -80 0C to -40 0C
but it narrowed dramatically from -40 0C to 60 0C and then continued to

narrow albeit at a lower rate. At 100 0C, the linewidth of the peak was only

147

 

 

 

   
 

 

 

 

 

120 I. I I I I I I I I I I I I .-
100 '— fio (a) 4'
I O I
.- . O -1
1' . 00 '2
b t g 0 :
.o-mq _— O. 0 1
11:): I 0 C22) I
E r ’o ':
H : .‘~ :
(b) 1
_20 I I I I I L I I I I I I I‘

60 20 0.0 -20 -40

Field (ppm)

120 '- I l .I l I I I I I I I I ‘-
Z ’ I
100 — o _
: (d) :
80 3- ‘ 0 —‘
3;. I o I
a 60 :' 1
8 : 9 ’ :
E 4": s . '5

 

 

 

0.0 -20 —40
Field (ppm)

Figure 3.12. Static solid state 7Li NMR spectra of (a) Lio,2TaS2 at -80 0C,
(b) at 100 0C, (c) Li0,2(PEO)yTaS2 (batch-H; M.W. = 100K) at -80 0C and

(d) at 100 0C.

148

 

8000 . .44 T , . . . . , . . . . , . . . . , .
7000 (A)
6000
5000
4000

3000

Linewidth (Hz)

2000
1000

IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII

 

O
O
O
IlllIlllLIlILLIIIlIIIIlLLIIIIIIIILIIIII

 

 

O IIIIIIIIJIIAIIIIIIIII

-100 —50 O 50 100
Temperature (0C)

 

8000
7000

6.

L110
CO
CO
CO

80.)
o8
CO

1000

Linewidth (Hz)
.h
8
O
IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII

 

 

I I I I I I I I I I I I I I I I I I I I I I I I I I I I I

%L5 31) 215 41) 4L5 51)
1000/T (K'l)

 

A
0'
v
0
Sn IIJIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII

Figure 3.13. Temperature dependence of the linewidth of 7Li NMR
resonance peaks for (a) Lio,2TaS2 and (b) Li0,2(PEO)yTaS2 (batch-II; M.W.

= 100K).

149

28% of that at -80 0C, see Figure 3.13A(b). This behavior is called
dynamical motional narrowing and has been intensively studied in many
solid polymer electrolytes [36. 37] and some PEO [or poly(ethylene glycol)
(PEG)] nanocomposites [381. The change of line—width is caused by the
averaging of magnetic couplings over the local magnetic field associated
with other spins such as other Li nuclei, proton nuclei and unpaired
electrons, due to dynamical motion. In most solid polymer electrolytes and
PEO [or (PEG)] nanocomposites, dynamical motional narrowing is caused
by the dissociation of a dipole-dipole coupling between pairs of nuclear
spins, specifically 7Li and proton of the PEO [36,37, 38] and reflects the onset
of the dynamical motion of either the polymer chains or the Li+ ions. In
some materials with unpaired electrons, which produce much larger spin
fields than nuclei, the line narrowing is caused by the averaging over the
dipole coupling in the electron spin field [38].

Lio_2(PEO)yTaS2 is close to Li/PEO/fluorohectorite [38b], a PEO
intercalated nanocomposite without paramagnetic centers in the inorganic
layers. One might expect that the broadening of the 7Li resonance peak
mainly results from the dipole-dipole coupling between 7Li and 1H of the
PEO, as in the case of Li/PEO/fluorohectorite [38b]. This assignment would
be supported by the similar linewidths before narrowing in these two
nanocomposites, ~5 kHz, and the appearance of the plateau on the low-
temperature as well as high-temperature side. If this is the case, the
dynamical motional narrowing should correspond to the onset of the
segmental motion of the polymer chains. However, the temperature range
of the narrowing, -40 0C ~ 60°C, is more close to the onset temperature of
the Li+ ion motion obtained in Li/PEO/montmorillonite, -20 0C ~ 400C [3321’

bl, rather than that of the polymer segmental motion observed in

150

28% of that at -80 0C, see Figure 3.13A(b). This behavior is called
dynamical motional narrowing and has been intensively studied in many
solid polymer electrolytes [36, 37] and some PEO [or poly(ethylene glycol)
(PEG)] nanocomposites [381. The change of line-width is caused by the
averaging of magnetic couplings over the local magnetic field associated
with other spins such as other Li nuclei, proton nuclei and unpaired
electrons, due to dynamical motion. In most solid polymer electrolytes and
PEO [or (PEG)] nanocomposites, dynamical motional narrowing is caused
by the dissociation of a dipole-dipole coupling between pairs of nuclear
spins, specifically 7Li and proton of the PEO [36,3738] and reflects the onset
of the dynamical motion of either the polymer chains or the Li+ ions. In
some materials with unpaired electrons, which produce much larger spin
fields than nuclei, the line narrowing is caused by the averaging over the
dipole coupling in the electron spin field [33].

Lio,2(PEO)yTaS2 is close to Li/PEO/fluorohectorite [38b1, a PEO
intercalated nanocomposite without paramagnetic centers in the inorganic
layers. One might expect that the broadening of the 7Li resonance peak
mainly results from the dipole-dipole coupling between 7Li and 1H of the
PEO, as in the case of Li/PEO/fluorohectorite [381?]. This assignment would
be supported by the similar linewidths before narrowing in these two
nanocomposites, ~5 kHz, and the appearance of the plateau on the low-
temperature as well as high-temperature side. If this is the case, the
dynamical motional narrowing should correspond to the onset of the
segmental motion of the polymer chains. However, the temperature range
of the narrowing, -40 0C ~ 600C, is more close to the onset temperature of
the Li+ ion motion obtained in Li/PEO/montmorillonite, -20 0C ~ 40°C [33%

bl, rather than that of the polymer segmental motion observed in

150

Li/PEO/fluorohectorite, -100 0C ~ 40°C [38b]. In LiClO4(PEG)9,
LiClO4(PEG)25 and LiBF4(PEG)9, where the motion of Li‘r ions is
essentially governed by the segmental motion of the polymer chains so that
the line narrowing corresponds to both the onset of the correlated motions
of Li+ ions and polymer segments, the narrowing occurs in the range —50
OC ~ 500C [3631. Considering the line narrowing of Lio2TaS2, which happens
in the same temperature range as that of Li0.2(PEO)yTaS2, we believe that
this dynamical motional narrowing observed in Li0,2(PEO)yTaS2 is caused
by the onset of the Li+ ion hopping in the gallery. This is because the
[TaS2]"' anions are massive and only the motion of Li+ ions can cause the
line narrowing in Li02TaS2. On the other hand, the linewidth after the
narrowing is much wider in Li02(PEO)yTaS2 (~1500 Hz) than in
LiClO4(PEG)9, LiClO4(PEG)25 and LiBF4(PEG)9 (60 - 90 Hz), which also
suggests that the broadening of the 7Li resonance peak in Lio,2(PEO)yTaS2
is not caused only by the dipole-dipole coupling between 7Li and the
polymer protons. For comparison with literature data, the 7Li NMR
linewidths of Li02TaS2 and Li0.2(PEO)yTaS2 are ploted against 1/T in
Figure 3.13B.

The considerable narrowing of the 7Li resonance peak of
Li0,2(PEO)yTaS2 at high temperatures suggests that the lithium ions begin
to undergo facile site hopping. The mobility of lithium ions is affected by
temperature more readily in the polymer intercalated galleries than in the
un-intercalated galleries. This is attributed to the more disordered state of
the Li lattice sites in the nanocomposites which lowers the activation
barrier for hopping, relative to the more ordered, crystallographically well

defined sites in Li02TaS2. The narrowing of linewidth in solid state 7Li

151

 

100
: I
E (a) Parallel to field [I

linewidth = 2893 Hz —)H
- chemical shift = 3.14 ppm I I

: (b) Perpendicular to field I
- linewidth = 4618 Hz I

00
O

O’\
O

    

Intensity
N A
O O

O

I-

_20IIJIIIIIIIIIIIIIIIIIJIIIIIIIIIIIIIILIII
200 150 100 50 0 -50 -100 -150 -200

Field (ppm)

 

 

 

Figure 3.14. Room temperature 7Li NMR spectra for a Li0,2(PEO)yTaS2
film (batch-I; M.W. of PEO, 100K). (a) film parallel to the magnetic field
and (b) film perpendicular to the field.

NMR has also been observed in LixV2O5 [31] and LixMoOg [3b], as well as

their PEO nanocomposites.

A high degree of anisotropy was observed in the solid state 7Li NMR
spectra of Li0,2(PEO)yTaS2 films (both batch-I and batch-II). As shown in
Figure 3.14, both the resonance peak position and peak—width vary with the
change in the orientation of the film of batch-I relative to the direction of
applied magnetic field. When the film is perpendicular to the magnetic
field, the peak width is broadened considerably more than when it is
parallel. The mechanism of the change of the linewidth corresponding to
the orientation is not yet clear. However, the high electron mobility in the

two-dimensional TaS2 slab probably couples to the external applied

magnetic field and causes local magnetic field distortions which affect the

152

Li nuclei and consequently the 7Li NMR signals. This orientation effect is
less pronounced in films of batch-II Li0,2(PEO)yTaS2 probably due to the
fact that the dimensions of TaS2 slabs are smaller and the slabs are less well
stacked and more separated by PEO. The results of these experiments are

summarized in Table 3.5.

Table 3.5. Effect of film orientation on the 7Li-N MR spectrum of
L10,2(PEO)yT382

 

L10.2(PEO)yT382 (batch-I) L10,2(PEO)yTaSZ (batch-II)

 

 

parallel perpendicular parallel perpendicular

 

 

 

chemicalshift of 3.14 12.57 10.99 9.42
peak p081t10n(ppm)
lineWidth 2893 4618 2924 2902
(H2)

 

* "parallel" and "perpendicular" refer to the film with respect to the
instrument's applied magnetic field.

Concluding Remarks

The exfoliation properties of LixTaS2 were systematically explored
and it was found that LixTaS2, prepared from controlled lithiation with 0.2
equivalent LiBH4, exfoliates well in water and has high affinity for various
polymers. Lamellar nanocomposites of PEO, PEI and PVP were thus
obtained through the encapsulative precipitation method. The
nanocomposites dissolve in water and are easily cast into free-standing
films. These plastic like films convert into superconductors at temperatures

below their Tc’s, which raises the possibility of developing flexible

superconductors with these or other materials in the future. Solid state 7Li

153

NlVIR measurements indicate that Lix(PEO)yTaS2 provides a more facile

hopping environment for Li ions. According to the 1-D EM calculation for

Lix(PEO)yTaS2, the nanocomposites are probably constructed with two
sheets of PEO chains inserted in each gallery. The PEO chains adapt a

conformation similar to that found in type II PEO-HgCl2 complex and are

arranged with -CH2- groups facing the TaS2 layers and -O- atoms towards

the center of the gallery where the Li+ ions seem to be located.

154

 

 

References

10

(a) E. P. Giannelis, Adv. Mater. 1996, 8, 29. (b) R. Krishnamoorti,
R. A. Vaia. and E. P. Giannelis, Chem. Mater. 1996, 8, 1728.

(a) E. Ruiz-Hitzky, Adv. Mater. 1993, 8, 334. (b) E. Ruiz-Hitzky,
P. Aranda, B. Casal and J. C. Galvan, Adv. Mater. 1995, Z, 180.

(a) R. Bissessur, M. G. Kanatzidis, J. L. Schindler and C. R.
Kannewurf, J. Chem. Soc., Chem. Commun., 1993, 1582. (b) L.
Wang, J. Schindler, C. R. Kannewurf and M. G. Kanatzidis, J.
Mater. Chem. 1997, Z, 1277. (Also Chapter 4 of this dissertion.) (c)
H.-L. Tsai, J. L. Schindler, C. R. Kannewurf and M. G. Kanatzidis,
Chem. Mater. 1997, 9, 875.

L. F. Nazar, H. Wu and W. P. Power, J. Mater. Chem. 1995, 5,
1985.

(a) J. P. Lemmon, J. Wu, C. Oriakhi and M. M. Lerner,
Electrochim. Acta 1995, Q , 2245. (b) C. O. Oriakhi, R. L.
Nafshun and M. M. Lerner, Mater. Res. Bull. 1996, 3_1, 1513.

A TaS2/PEI nanocomposite by almost the same preparation approach
has been reported by Oriakhi et a1 [5b].

TaS2/Polymer(oligomer) nanocomposites were also synthesized by
other methods. (a) TaS2/Poly(4-vinylpyridine) nanocomposite was
synthesized by in situ monomer intercalation and interlayer gallery
polymerization: C.-H. Hsu, M. M. Labes, J. T. Breslin, D. J.
Edmiston, J. J. Winter, H. A. Leupold and F. Rothwarf, Nature,
Phys. Sci., 197 3, _2_4_6_(155), 122. (b) Polypeptides were intercalated
into 2H-TaS2 by direct insertion: V. M. Chapela and G. S. Parry,
Nature 1979, £1, 134.

D. W. Murphy and G. W. Hull Jr., J. Chem. Phys. 1975, 6_2, 973.
A. Lerf and R. Schdllhorn, Inorg. Chem. 1977, m, 2950.
(a) A. J. Jacobson, Mater. Sci. Forum 1994, 152-153 (Soft

Chemistry Routes to New Materials), 1. (b) L. F. Nazar and A. J.
Jacobson, J. Mater. Chem. 1994, 4, 1419. (c) A. J. Jacobson, in G.

155

 

 

ll

12

13

14

15

l6

17

18

Alberti and T. Bein ed., Comprehensive Supramolecular Chemistry,
Vol. 1, Elsevier Science Ltd., 1996, pp 315.

A. Lerf, E. Lalik, W. Kolodziejski and J. Klinowski, J. Phys. Chem.
1992, %, 7389.

(a) P. Joensen, R. F. Frindt and S. R. Morrison, Mater. Res. Bull.
1986, 2_1, 457. (b) M. A. Gee, R. F. Frindt, P. Joensen and S. R.
Morrison, Mater. Res. Bull. 1986, Q, 543. (c) D. W. Murphy and
G. W. Hull, Chem. Phys. 1975, Q, 973.

J. F. Lomax, Intercalation Chemistry of Layered Transition Metal
Dichalcogenides with Organic Bases, Ph. D. Dissertation,
Department of Chemistry, Northwestern University, 1986.

The equivalent of LiBH4 used in the lithiation reaction, 11, directly
affects the amount of Li, x, in LixTaS2. Since no experiments were
arranged to determine the x values for different forms of LixTaS2, n
is used here and in the discussion.

When n is small (< 0.5), x, the amount of Li in LixTaS2, is expected
close to the value of 11. However, this expectation is not checked with
experiments. Therefore, “Li02TaS2” is used to present the form of
LixTaS2 prepared under the stoichiometry n=0.2 only for
convenience.

Under oxygen flow, Li02TaS2 gained weight at first and then lost
weight at about 600 0C. The net loss up to 750 0C was 9.18%,
8.55%, 7.63% and 7.37% respectively in 4 measurements due to the
transformation to Ta205 and Li2SO4.

For Li02TaS2 to become Li2SO4 and Ta2Os, the weight loss should
be 5.89%. Because Li compounds can be volatile at high
temperatures, the weight loss of the sample in an oxygen flow could
be somewhat higher than the theoretical value. In addition, a trace of
small molecules such as water and diethyl ether in the Li0,2TaS2 can
also cause a slightly higher weight loss than the theoretical value.

PEAKOC is an XRD powder pattern analysis program provided by

Inel Inc. (Mail Adress in USA: P. O. Box 147, Stratham, NH
03885.)

156

19

20

21

22

23

24

25

26

27

28

International Tables for X-ray Crystallography , Kynoch Press,
1974.

M. G. Kanatzidis and T. J. Marks, Inorg. Chem. 1987, 26, 783.

While we expect the n values to be close to the x values in LixTaS2
when n is < 0.5, no attempt was made to determine the exact x in the
samples.

A. Jacobson, Mater. Sci. Forum 1994, I, 152.

W. Biberacher, A. Lerf, F. Buheitel, T. Butz and A. Hfibler, Mat.
Res. Bull. 1982, 1_7_, 633.

PEI was also intercalated in the LixTaS2 prepared from either 2H-
TaS2 or 1T-TaS2 through a solid state reaction with 3 equivalent of
LiBH4 in the temperature range from 300 0C to 525 OC.

The Lio2TaS2 and Li0,2(PEO)1_04TaS2 were heated to 350 0C under

nitrogen atmosphere and kept for 10 minutes at this temperature to
check if there was any loss of sulfur in the TaS2 layers. The sulfur

content in these two samples as well as the untreated samples was
checked with EDS, using synthesized 2H-TaS2 as a standard. The
stoichiometric number for sulfur, m (as TaSm), was 1.96, 2.01, 1.98
and 1.96 for Li02TaS2, L10,2(PEO)1.04T332, treated Lio2TaS2 and
treated Lio,2(PEO)1,o4TaS2 respectively. This indicates that almost no
sulfur is lost in the lithiation, intercalation and heating treatment of
Li02TaS2 and Li0_2(PEO)1,04TaS2 up to 350 0C under nitrogen. This
is consistent with TGA which showed that Li02TaS2 had no weight
loss in nitrogen up to 530 0C.

(a) Y. Takahashi and H. Tadokoro, Macromolecules 197 3, _6, 672.
(b) J. Brandrup and E. H. Immergut, Polymer Handbook, 3rd Ed.,
John Wiley & Sons, New York, 1989, pp VI/72.

F. P. Price and R. W. Kilb, J. Polym. Sci. 1962, 5_7, 395.

Y. Takahashi, I. Sumita and H. Tadokoro, J. Polym. Sci., Polym.
Phys. Ed., 1973, fl, 2113.

157

29

30

31

32

33

34

35

36

37

38

R. Iwamoto, Y. Saito, H. Ishihara and H. Tadokoro, J. Polym. Sci.,
A-Z, 1968, _6, 1509.

M. Yokoyama, H. Ishihara, R. Iwamoto and H. Tadokoro,
Macromolecules 1969, 2, 184.

Y.-J. Liu, J. L. Schindler, D. C. DeGroot, C. R. Kannewurf, W.
Hirpo and M. G. Kanatzidis, Chem. Mater. 1996, 8, 525.

The positions of the atoms of PEO were decided according to the
structural data available from the PEO crystal structure with planar
zigzag conformation [28]. When the ac plane of the unit cell is parallel
to the layers, the PEO zigzag plane is almost parallel to the TaS2
layers; when the bc plane of the unit cell is parallel to the TaS2
layers, the PEO zigzag plane is almost perpendicular to the layers.

P. Garoche, P. Manuel, J. J. Veyssie and P. J. Molinié, Low. Temp.
Phys. 1978, E, 323.

R. H. Friend and A. D. Yoffe, Adv. Phys. 1978, E, l.

V. Z. Kresin and S. A. Wolf, Fundamentals of Superconductivity,
Plenum Press, New York/London, 1990, pp 95.

(a) S. Panero, B. Scrosati and S. G. Greenbaum, Electrochim. Acta
1992, 40, 1533. (b) W. Gorecki, E. Belorizky, C. Berthier, P.
Donoso and M. Armand, Electrochim. Acta 1992, 3_7, 1685.

(a) S. H. Chung, K. R. Jeffrey and J. R. Stevens, J. Chem. Phys.
1991, 94, 1803. (b) J. P. Donoso, T. J. Bonagamba, P. L. Frare, N.
C. Mello, C. J. Magon, H. Panepucci, Electrochim. Acta 1995, J),
2361.

(a) S. Wong, S. Vasudevan, R. A. Vaia, E. P. Giannelis and D. B.
Zax, J. Am. Chem. Soc. 1995, 1_l7_, 7568. (b) S. Wong and D. B.
Zax, Electrochim. Acta 1997, $2, 3513. (c) D.-K. Yang and D. B.
Zax, J. Chem. Phys. 1999, m, 5325.

158

Chapter 4

LAMELLAR LixMoO3/POLYMER NAN OCOMPOSITES VIA
ENCAPSULATIVE INSERTION

Introduction

The investigation of inorganic/polymer nanophase composites is
motivated by many reasons, including the need for novel electronic
anisotropic materials, better performing battery cathode materials,
functionalized structural materials with superior mechanical properties,
hierarchical materials, and systems in which to study polymer orientation,
epi- and endo-taxy and polymer/inorganic surface interactions [“4].
Polymer-based nanocomposites have been reported with layered silicates
(e.g. montmorillonite, hectorite, etc.) [11, FeOCl 12], V205 [3, 4], M003 [5. 6t
71, layer metal phosphates [3], MS2 (M=Mo [9’ 10, 11, 73], Ti [1131), NbSe2 [12],
layered metal phosphorus chalcogenides (MPS3) [13, 7] and layered double
hydroxides [14]. The most common methods of preparing
inorganic/polymer nanocomposites are (a) by monomer intercalation
followed by polymerization, (b) by in situ intercalative polymerization, (c)
by direct insertion and (d) by encapsulative precipitation from solutions of
exfoliated lamellar solids. The last two methods give inorganic/polymer
nanocomposites in which the molecular masses and nature of the polymers
can be decided before intercalation. Encapsulative precipitation has been
applied with V205 133-Ct 4], MoO3 153-0’73], MoS2 [10131 C, 11, 7a], TiS2 [11a] and
NbSe2 [12], in combination with various polymers.

M003 is one of the layered metal oxides which shows reversible Li

ion insertion properties which are relevant to rechargeable Li batteries [15].

159

 

 

In intercalative electrodes of rechargeable batteries, ion conductivity is
very important. Other solid-state ionic applications such as electrochromic
devices also need good ion conductivity. Inorganic/Polymer
nanocomposites should exhibit fast ion conduction 11%] and introduction of a
polymer with affinity for Li ions between the sheets of MoO3 could
improve its performance as an intercalative electrode.

Polymer insertion into MoO3 has been reported previously, namely,
with a polymeric ionomer [53. b], with PEO [56, 7a] and with polyaniline l5d~ 6’
7b]. This research develops further the polymer intercalation chemistry
associated with M003 and introduces a new family of LixMoOglpolymer
nanocomposites. Nanocomposites with poly(ethyleneoxide) (PEO),
poly(ethyleneglycol) (PEG), poly(propyleneglycol) (PPG),
poly(vinylpyrrolidinone) (PVP), methylcelulose (MCel), polyacrylamide
(PAM) and nylon-6 (PA-6) are reported here. These nanocomposites were
characterized by thermal gravimetric analysis (TGA), differential scanning
calorimetry (DSC), powder X-ray diffraction, FTIR and solid-state UV-
VIS spectroscopy, variable-temperature solid-state 7Li and 13C NMR
spectroscopy, magnetic susceptibility measurements and electrical

conductivity measurements.

Experimental Section

1. Reagents

PEO (5,000,000), PEO (100,000), PEG (10,000), PEG (2,000),
PEG (400), PPG (1,000), PVP (10,000), MCel (63,000), PAM
(5,000,000), PA-6 (10,000) and LiBH4 (95%) were purchased from

Aldrich Chemical Company, Inc. After the polymers were dissolved, the

160

   

polymer solutions were filtered to remove insoluble polymer residues.
MoO3 (99.95%) was purchased from Johnson Matthey Catalog Company.
Anhydrous diethyl ether (99.0%), 2,2,2-trifluoroethanol (99%),
acetonitrile (99.5%), isopropynol (99.9%) and 200 proof ethanol were
from Columbus Chemical industries Inc., Lancaster Synthesis Inc., EM
Science Inc., Mallinckrodt Chemical Inc. and Quantuum Chemical
Company respectively. No further purification was applied to the chemicals
above. Water used in the reactions was distilled water provided by the
Department of Chemistry, and was degassed by bubbling nitrogen for 30

min before use.

2. Synthesis of LixMoOg (0.30 < x < 0.40)

Commercial MoO3, used as the starting material, was fired in an
open quartz vial in the air at 600 0C for 36 h during which the crystal size
of MoO3 remarkably increased. This M003 was used to react with LiBH4
to prepare the lithium molybdenum bronze [16]. In a typical reaction, 0.1
mol of MoO3 was reacted with 0.04 mol of LiBH4 in 80 ml diethyl ether
for 24 h, under a nitrogen atmosphere. The product was collected by
filtration, washed with ether and dried in vacuo. The yield was >98%. The
lithium molybdenum bronze was thereafter stored in a nitrogen dry-box.
The X-ray powder diffraction pattern can be indexed on the basis of an
orthorhombic cell similar to that of MoO3 but expanded along the stacking
(a-axis) direction, with a: 16.528 A, b=3.775 A and c=3.969 A. The dw-
spacings (A) are: 8.26200 (vs), 4.132400 (s), 3.578201 (In), 3.434210 (m),
2.755600 (w), 2.453311 (m) and 2.372411 (m).

The amount of lithium in the bronze was analyzed by TGA under

oxygen flow, heating up to 650 0C, and by elemental analysis which was

161

 

 

accomplished by Oneida Research Services, Inc., Whitesboro, New York.
Elemental analysis of Li in the molybdenum bronze was done by ion

chromatography, while Mo was measured by X-ray fluorescence.

3. Preparation of LixMoOglPolymer Nanocomposites

The LixMoOg was exfoliated in degassed water by 5 minutes of
sonication, to form a suspension with a concentration of 5 g/L. This
suspension was added dropwise into a stirred polymer solution of the same
volume, which contained five times of excess polymer (per repeat unit) to
M003 and the mixture was stirred for 2 days under a nitrogen atmosphere.
The nanocomposites formed were isolated in different ways according to
their behavior in solution. Nanocomposites of methylcelulose,
polyacrylamide and nylon-6 precipitated and were collected by filtration.
Those containing MCel and PAM were washed with water, while the
nanocomposites of PA-6 were washed with trifluoroethanol.
Nanocomposites with poly(ethylene oxide) (PEO), poly(ethylene glycol)
(PEG), poly(propylene glycol) (PPG) and poly(vinylpyrrolidinone) (PVP)
remained in colloidal form and were isolated by pumping off the water to
dryness. The dried material was stirred in an appropriate solvent for
several hours to dissolve the extraneous polymer. The solid product was
filtered and was washed again with the solvent. The solvents used to
process the products are list in Table 4.1. The products were pumped to

dryness and stored in a nitrogen atmosphere.

4. Instrumentation
The instrumentation in the measurements such as X-ray powder

diffraction (XRD) patterns, thermal gravimetric analysis (TGA),

162

 

differential scanning calorimetry (DSC) and infrared (IR) spectrosc0py
were the same as described in Chapter 1. Room temperature conductivity
measurements, variable temperature direct-current electrical conductivity
measurements and thermopower measurements, and magnetic susceptibility
measurements were conducted as described in Chapter 2.

Variable-temperature solid-state 7Li and 13C NMR measurements
were taken on a 400 MHz Varian Nuclear Magnetic Resonance Instrument.
Samples were loaded in a glove-box under a nitrogen atmosphere.
Electronic transmission spectra were recorded with a Shimadzu UV-
3101PC UV-VIS-NIR scanning spectrophotometer. Samples were dried as
thin films on quartz slides.

Results and Discussion

1. Synthesis and Characterization of LixMoO3

The lithium molybdenum bronze exfoliates readily in water to form
stable colloidal solutions, and this makes it an appealing candidate for
polymer intercalation. The LiBH4 route to LixMoO3 is the best one to date
in providing the material conveniently and in high yield. The previously
reported method for Lix(H2O)yMoOg [17] involves one step to prepare
N ax(H2O)yMoOg and two steps to accomplish ion-exchange and gives only
26% yield [18]. An additional advantage of the LiBH4 method is that it is
conducted in diethyl ether and so provides the anhydrous form of the
molybdenum bronze. The LixMoOg product prepared in this fashion,
though still crystalline, shows broader diffraction maxima than the

precursor M003. The Li insertion into M003 is topotactic as evidenced by

163

our ability to fully index the X-ray powder diffraction pattern of the
product.

The amount of Li in LixMoOg was determined both by TGA and by
direct elemental analysis. When the material is heated in an oxygen
atmosphere to 650 0C, it gains 1.71 mass%, owing to a change from
LixMoOg to Li2O and MoO3. This gain of mass is reproducible and
corresponds to an x value of 031(3). On the other hand, the elemental
analysis showed that the molybdenum bronze consisted of 1.64% Li and
56.39% Mo. This gives a ratio of Li to M0 of 0.4:1 corresponding to the
molar ratio of LiBH4 used in the lithiation reaction. It is of course possible
that x varies slightly from sample to sample in the range 0.3 < x < 0.4.

The lithium molybdenum bronze is metastable and undergoes an
intense, irreversible and exothermic phase change at 356 0C, as detected by

DSC (Figure 4.1) and X-ray powder diffraction. The product appears to be

 

 

 

 

 

 

 

 

 

5 >—
: 356°C
4 :- .9.
3 ~ g
A : o
3 2 :. ><
E 2nd cycle 0
V E_lst cycle
8 1 .9.
Q :/ g _4
0 Q g
j e
-1 _ ~ i £3
_ 0 I
-2 L- 1 1 1 l l 1 1 1 1 l 1 1 1 1 l 1 m 1 l l l I
0 100 200 300 400

Temperature (0C)

Figure 4.1. DSC diagram of LIXMOO3.

164

 

 

 

a mixture of at least two unknown phases. As this mixture is heated to
higher temperatures it undergoes additional phase changes yielding other

new phases.

2. Exfoliation and Polymer Encapsulation Chemistry

The lithium molybdenum bronze described above can be readily
exfoliated in water after several minutes of sonication. The exfoliated
product forms supramolecular complexes with most water-soluble
polymers. The complexes form solutions or precipitates in water,
depending on the type and molecular mass of the polymers. We have
encapsulated PEO, PEG, PPG, PAM, MCel and PVP inside the lithium
molybdenum bronze to obtain lamellar nanocomposite materials.

Poly(ethylenimine) (PEI) could not be successfully intercalated.
Instead, the blue Lix(H2O)yMoO3 monolayer suspension decolorized and
totally dissolved in the aqueous PEI solution. The same phenomenon
occurred when ammonia was introduced in the Lix(H2O)yMoO3 suspension,
suggesting that the PEI solution is too basic and attacks the M003 lattice.
Water-insoluble polymers were also tried, however of these, only PA6 was
successfully intercalated. Details of the reactions are given in Table 4.1.

The existence of polymer chains between the layers of the lithium
molybdenum bronze was verified by IR spectroscopy and X-ray powder
diffraction. Figure 4.2 shows a comparison of the IR spectra of a
nanocomposite Lix(H2O)y(PVP)zMoOg and its components; from this, it is
obvious that the vibrational peaks of Lix(H2O)y(PVP)zMoO3 is a
combination of the vibrational peaks of PVP and that of Lix(H2O)yMoO3.
The positions of the vibrational peaks arising from the encapsulated PVP

are close to those of pure PVP, while the positions of the peak due to the

165

Table 4.1. Chemical and structural characteristics of LiXMoO3/polymer

 

 

 

 

 

 

 

 

 

 

 

 

nanocomposites
nanocom- polymer solu- washing d- expansion coherence
posite Mw bfljty a solvent spacing of gallery len
(A) (A) < )
LixMoO3 5x 106 yes MeCN 1 6.6 9 .7 64
/PEO
LixMoO3 100,000 yes MeCN l 6 .6 9 .7 121
/PEO
LixMoO3 10,000 yes MeCN 1 6.8 9 .9 108
/PEO L
LixMoO3 2,000 yes MeCN l 3 .8 6.9 72
/PEG 126-14.7 5.7-7.8 - b
LixMoO3 400 yes MeCN 1 3 .5 6 . 6 1 1 1
/PEG 1 2.6 5 .7 97
LixMoO3 1,000 yes 17 .2 10.3 1 13
/PEG EtOH l 1 .8 4 9 108
18.0&1 1.5 - _ b
LixMoO3 63,000 no H20 27 .6 20.7 92
lMCel
LixMoO3 10,000 yes PriOH 3 8 .6 3 1 .7 154
/PVP
LixMoO3 5x 106 no H20 3 8 .0 3 1 . 1 69
/PAM 33.8 26.9 80
4 1 .2 34. 3 - b
LixMoO3 10,000 no CF3CH2OH 22.1 15 .2 39
/PA6 l 6. 8 9 .9 26

 

 

 

 

 

 

 

a. Nanocomposites dissolved in water, except for LixMoOglPA6, dissolved

in 222-trifluoroethanol.
b. Peaks too broad to obtain estimate.

166

 

 

 

 

 

 

% Transmittance

 

(d) MO'O

 

 

 

IIIIIIIIIIIIIIIIIII I II

II I I I I I l I
2000 1800 1600 1400 1200 1000 800 600 400

Wavenumbers (cm'l)

Figure 4.2. IR spectra of (a) poly(vinylpyrrolidinone), (b) Lix(H2O)y(PVP-
10,OOO)ZMOO3, (c) hydrated lithium molybdenum bronze, and (d) M003.

167

 

 

 

 

 

    
 

(b) Lix(I-IZO)yMoO3

Absorbance

(c) Lix(HzO)y(PEO-100,000)zMoO3

 

286 nm

((1) LixG-IZO)y(PAM—5,000,000)zMoO3

 

 

 

 

200 700 1200 1700 2200
Wavelength (nm)

Figure 4.3. Solid—state optical absorption spectra of the LixMoO3/polymer

nanocomposites. The polymer and its molecular mass are indicated on each
spectrum.

168

 

 

 

 

 

 

13.5 A
PEG(400)
' "Turf ' ‘ “—-‘3‘*+M....
13.8 A
PEG(2,000)
54. 16.8 A
'FB
:1
3 PEG(10,000)
.5
16.6A
PEO(100,000)
16.6A
PEO(5,000,000)
T I r I I I ' l I IfT II I I I I Ivl‘ri ffitIAIIifi-IJA-ggh

 

 

 

0 10 20 30 40 50 60
20 (deg)

Figure 4.4. Typical XRD patterns of nanocomposites with poly(ethylene
glycol) and poly(ethylene oxide) of different molecular mass. The polymer
and its molecular mass are indicated on each spectrum.

169

 

17.2 A

 

 

 

PPG(1,000)
27.6 A
MCel(63,000)
38.6 A
>~.
H
a
C:
8
3 PVP(10,000)
38.0 A

PAM(5,000,000)

 

PA6( 10,000)

 

 

 

 

I I f I T I I I I I I I T I r I I I I I I I

O 10 20 3O 40 50 60
20 (deg)

Figure 4.5. Typical XRD patterns of the various LixM003/polymer
nanocomposites.

170

Mo=O stretch [for Lix(HgO)yMoO3] is shifted to higher wavenumbers,
suggesting that the M003 layers in the nanocomposite are slightly more
oxidized.

The optical transmission absorption spectra of these macromolecular
intercalates were examined. The dark-blue color of these systems arises
from the intense intervalence transitions associated with the MOM/Mo6+
couple. These electronic transitions are broad and range from the IR
region to the visible (Figure 4.3) and are responsible for the electrical
conductivity of these materials. The absorption at 286 nm arises from
excitations across the band-gap from the 02’ p-band to the Mo6+ d-band and
is present in all compounds including pristine M003 and Lix(H20)yMoO3.
This is consistent with the expectation that host metal oxide structure is not
disturbed upon intercalation.

The encapsulation of polymers inside the interlayer galleries of
M003 is also demonstrated by X-ray powder diffraction, which shows an
expansion of the gallery space. Figures 4.4 and 4.5 show typical XDR
patterns of some of the nanocomposites. The sharp and intense (001)
reflections indicate that the M003 layers are well stacked. X-Ray scattering
coherence lengths, which are calculated from the Scherrer formula Lhkl =
KNBcose (see Reference 18 of Chapter 2), and the gallery spacings are
given in Table 4.1.

The basal spacing of some nanocomposites depends on the
preparation procedure. For example, Lix(H20)y(PVP)zMoO3, which is
water soluble, showed a d-spacing of 59.0 A before washing with isopropyl
alcohol and 38.3 A after washing. Washing these materials may not only
remove extra-lamellar polymer, but could lead to polymer loss from the

galleries changing their composition. This behavior makes it difficult to

171

 

 

determine at what polymer loading we begin to saturate the intralamellar
space. Similar phenomena were described in polymer-V205 xerogel
systems [33]. Nevertheless, the observed d-spacings were consistent to within
:1 A, as long as the preparation procedures were not altered significantly
from batch to batch. The d-spacings and the degrees of lamellar stacking of
products which contained polymers at the very extremes of the molecular
mass range (very high or very low) were hard to control, as for example
in PEO(5,000,000), PEG(2,000) and PPG(1,000). The Lix(H20)y(PEO-
5,000,000)zMoO3 had a consistent d-spacing ca. 16.6 A, but the peaks were
broad. Lix(H20)y(PEG-2,OOO)ZM003 showed broad peaks in the range
126-147 A. Lix(H20)y(PPG-l,OOO)zMoO3 sometimes showed a peak in the
range 11.8 131-17.2 A, while other samples showed two peaks in this range
suggesting a mixture of phases. Evidently, for low molecular masses the
polymers are mobile enough in the galleries to form several different
arrangements leading to multiple phases.

The effect of polymer molecular mass on product formation was
examined, particularly with PEO and PEG, and was found to be
significant. The high molecular mass PEO(5,000,000) immediately formed
a precipitate with lithium molybdenum bronze in water upon mixing while
this phenomenon did not occur with PEO of lower molecular mass. The
molecular mass also affects the structure of the nanocomposites. Table 4.1
shows that the Lix(H20)y(PEO-5,OOO,OOO)zMoO3 sample has a lower
coherence scattering length than its lower molecular mass analogues, which
is attributed to the fact that it is kinetically unfavorable for extremely long
polymer chains to align in an ordered structure. When the molecular mass
is extremely low, 1'. e. in oligomer range, the gallery expansion of the

intercalate is lower, almost one half of that of the long polymers. For PEO

172

and PEG, a molecular mass of 2,000 is about the upper limit of this
situation. The data listed in Table 4.1 show that the Lix(H20)y(PEG—
2,OOO)zMoO3 sample has a much shorter coherence length than its
analogues with higher or lower molecular masses. An analogous
Lix(H20)y(PEG-2,OOO)ZM003 sample, prepared under similar conditions,
had a broad X-ray basal peak which corresponded to a d-spacing varying
from 12.6 to 14.7 A, suggesting that the local conformation of the polymer
is important.

Annealing the Lix(H20)y(PEG)zMoO3 and the Lix(H20)y(PEO)zMoO3
samples at 150 OC and then gradually cooling them to room temperature
tends to improve their lamellar order, especially when the starting
coherence length is short. For example, after annealing, the
Lix(H20)y(PEG-2,OOO)zMoO3 sample whose XRD pattern had a broad peak
centered at 13.4 A gave a pattern with a sharp peak at 12.7 A, (Figure 4.6).

The water—insoluble nanocomposite Lix(H20)y(PAM-
5,000,000)zMoO3 gave samples with d-spacings of 38.0, 33.8 and 41.2 131.
The Lix(H20)y(PA6-10,000)zMoO3 showed d-spacings of 22.1 and 16.8 A.
Occasionally, in the intercalation of PA-6 a mixed-phase material with
basal spacings of 12.8 and 9.8 A was obtained. This shows the difficulty to
control the reaction when quick precipitation is used to obtain a specific
phase. To prepare nanocomposites of this type, a very dilute
Lix(H20)yMoO3 suspension of < 0.5 mass% is recommended.

The polymer composition of the nanocomposites were determined by
TGA in an oxygen atmosphere and are listed in Table 4.2. The water in the
nanocomposites was estimated by the mass loss step observed below 230 oC,
and the amount of polymer was determined by the mass loss steps observed

at higher temperatures. Despite drying under vacuum, the nanocomposites

173

 

 

 

13.4 A

before annealing

12.7 A

Intensity

after annealing

 

 

 

I I I I I I I I I I I r I I I T I T I

12 16 20 24 28
29 (deg)

—1

-I>—l
00

Figure 4.6. XRD patterns of Lix(H20)y(PEO-2000)ZM003 showing the
effect of annealing on the stacking regularity of the layered structure of a
nanocomposite.

contain some water in the galleries. The water-soluble nanocomposites,
Lix(HzO)y(PEO)ZMoO3, Lix(H20)y(PEG)zMoO3, Lix(H20)y(PPG)zM003,
and Lix(H20)y(PVP)zMoO3, usually contain 2-4 mass% water which is very
difficult to remove. This water is thought to be coordinated to Li+ ions.
The water-insoluble nanocomposites Lix(H20)y(MCel)zMoOg and
Lix(H20)y(PAM)zMoO3, however, contain much less water.

Compared to the corresponding M082/polymer intercalates [10b], most
LixMoOg/polymer intercalates have much higher polymer contents and
larger gallery spacings. As in the case of M082/polymer intercalates, PVP
and MCel give the largest expansions. A marked difference is found in
PAM which gives LixMoO3/PAM an expansion as large as 27-35 A, while
M082/PAM has only an expansion of 9.4 A (see Chapter 1). This confirms

174

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table 4.2. Composition and physicochemical properties of the
LixMoO3/polymer nanocomposites

limit of limit of electronic
nanocom- polymer d— composition thermal thermal conducti-
posite Mw spacing (according to TGA) stability stability ‘ vity

(A) in m /Scm‘1

N 2/ 0C (Dz/QC
pure 8.27 1.3 10'2
LixMOO3 X
pure 6.93 3.3 10'5
M003 X
LixMOO3 5X106 16.6 LiX(H20)0.20PEOO.33MOO3 260 220 2.4X10-5
/PEO
LixMoO3 100,000 16.6 Lix(H20)0_32PE01.04MoO3 260 220 2.9x 1 0-5
/PEO
LixMoO3 10,000 16.8 Lix(H20)0,29PEG0,75M003 260 220 5.2x 1 0-5
/PEG
LixMoOg 2,000 13 .8 Lix(H20)0,28PEG0.57MoO3 260 220 2.2x 10-4
/PEG
LixMOO3 400 13 . 5 Lix(H20)0.38PEGO.33MOO3 260 220 3 . 1x10-4
/PEG
LixMOO3 1,000 17 .2 Li X(H20)0. 18PPGQ99MOO3 220 200 -
/PPG 1 1~8 Lix(H20)o.52PPGo.14M003 5.3x 10'4
LixMoO3 63 ,000 27 .6 LixMCCIOJOMOO3 180 170 2.0,(10-6
/MCel
LixMoO3 10,000 3 8.6 Lix(H20)0,43PVP1.17MoO3 220 220 1.8x10'7
/PVP
LixMoO3 5X106 3 8 .0 LixPAM32MOO3 150 150 6.3x 10'7
/PAM
LixMoOg 10,000 22. 1 Lix(H20)0_44PA60.32MoO3 280 270 -
/PA6 16.8 Lix(H20)0.48PA60.23M003 2.11110”4

 

 

 

 

 

 

 

175

 

that multiple layers of this polymer can enter the accessible space of
LixMoO3.

The water-soluble LixMoO3/polymer nanocomposites can be cast into
films and other shapes, which may provide opportunities for applications.
The nanocomposites with high molecular mass polymers are strong, though
their mechanical properties depend on the polymer. For example, the
nanocomposite of PEO(5,000,000) is tough, while that of PAM(5,000,000)
is hard. Lix(H20)y(PEO-5,000,000)zMoO3 can be swollen by acetonitrile
and becomes resilient and plastic with the consistency of unsulfurized
rubber. When the Lix(H20)y(PAM—5,000,000)ZM003 is swollen by water, it
is not as plastic, but is stronger and tougher. Precise mechanical

measurements have not been taken.

3. Solid State NMR Spectroscopy

In order to probe the effect of the polymer on the behavior of the
lithium ions in the gallery, variable-temperature solid-state 7Li NMR static
spectra were measured for LixM003 and Lix(H20)y(PEO—100,000)zMoO3.
In both cases a broad peak was observed with a chemical shift very similar
to that of the solid LiCl standard. At —80 0C the lineshape in the two spectra
differs, with that of the Lix(H20)y(PEO—100,000)ZMoO3 spectrum being
slightly more asymmetric, Figure 4.7. This suggests that the presence of
PEO causes a distribution of Li ions over several, slightly different sites in
the gallery. Some of the sites may involve coordination of water molecules
while other site are associated with the ether—like oxygen atoms in PEO or
even those in the M003 layers. The linewidth (width at half maximum) of
the resonance peak is greater in the PEO intercalated sample than in the

host LixMoOg material and this too is consistent with a distribution of the

176

 

80

Lix(H20)y(PEO-1OO,OOO)ZM003

 

60 -

E? 40“—

273 i
I:

B -

a 201—

r—4 1-

I

..

 

 

IlllllllllLllllllLlllllllllllllllllllll

0
200 150 100 50 0 -50 -100 -150 -200
5(ppm)

Figure 4.7. Static 7Li NMR spectra of LixMoO3 and Lix(H20)y(PEO-
100,000)ZM003 at -80 0C. The broader assymetric line in the spectrum of
the nanocomposite is evident in the upfield region.

 

 

7000

DO

6000 8 9 O Lix(H20)y(PEO-100,000)ZM003

OD

5000 '

4000

3000

Linewidth (Hz)

2000

1000

 

 

jI—IIIIIIIIIIIIIIIIIIIIIIIIIIIIIII
O

 

0Llllllllllllllllllllllll

-100 -50 O 50 100 150
Temperature (C)

Figure 4.8. Temperature dependence of the linewidth of the resonance peak

in solid-state 7Li MNR spectra of LixMoO3 and Lix(H20)y(PEO-
100,000)ZM003.

177

 

 

Li+ ions over several sites in the former. The data in the low-temperature
region suggest a more well defined coordination environment for the Li+
ion in LixMoO3 as would be expected in this crystalline solid.

The linewidth of 2300 Hz in LixMoO3 at 23 0C is much narrower
than that of ~12000 Hz observed for Li2M0204 indicating substantial
degree of ion motion in the lattice of LixMoOg relative to that of Li2M0204
[19]. This is rationalized by the fact that in the latter the Li+ ions fully
occupy well defined crystallographic positions in the lattice [20] while the
non-stoichiometric nature of LixMoO3 gives rise to Li mobility via vacant
crystallographic sites.

The resonance peak in both samples narrows dramatically as the
temperature is increased from -80 0C to 100 0C, Figure 4.8. This is
attributed to rapid motion of Li+ ions between the M003 layers. At 100 0C
the peak linewidth in the spectra of Lix(H20)y(PEO-100,000)zMoO3 is
comparable to that of Lio,5(H20)1,3Mon4 [19]. The onset temperature of the
transition from a wide to a narrow peak in Lix(HzO)y(PEO-100,000)zMoO3
and LixMoO3 is similar. The spectra of both materials end up with an
equally narrow peak at 100 OC indicating comparable rates of hopping of

Li ions between different positions in the interior of the two materials.

4. Magnetism

Because the materials are formally mixed-valence compounds and
exhibit intense intervalence Mos‘i-Mo6+ optical transitions, we expect
unpaired electrons to be delocalized over the d-orbitals of the Mo atoms.
Magnetic susceptibility measurements were carried out as a function of

temperature for LixMoO3 and Lix(H20)y(PEO-100,000)ZMoO3 and the data
are displayed in Figure 4.9. Surprisingly, the susceptibility of both

178

compounds is rather small with substantial contributions from temperature

independent paramagnetism (XTIP)- Correcting for the latter 1/(Xmolar‘XT1P)
vs. T plots for LixMoOg and Lix(H20)y(PEO-100,000)ZM003 are linear in
the temperature ranges 5-120 K and 2-300 K, respectively. The Curie
constants estimated from the slope of the plots yield [Jeff of 0.15 and 0.09
respectively, however, the weak susceptibilities and the large diamagnetic
and XTIP corrections make Ileff values unreliable. It is interesting that the
magnetic properties of LixMOO3 are more similar to those of Li0,9Mo6017
[211 than of other molybdenum bronzes such as the blue bronze A03MoO3
(A=K,Tl) and purple bronzes A0,9Mo6017 (A=K, Na, T1) [22]. In the latter
bronzes the temperature dependence of the magnetic susceptibility shows
transitions at low temperature associated with charge density waves. Such

phenomena were not observed in the LixMoO3 samples reported here.

 

Xmolar

   
 

 

 

IITWTI

I I I I I I I I I I I I I IW r I I I T I I
100 150 200 250 300 350

Temperature (K)

Figure 4.9. Temperature dependence of magnetic susceptibility of LixMoOg
(circles) and Lix(H20)y(PEO-100,000)ZM003 (triangles). The
measurements were conducted at 1000 G with powder samples.

179

 

 

 

10'4

8

l IIIIIIII
l Illlllll

I I IIIIIII
l I lllllll

Conductivity (S/cm)

 

I I IIIIIII
I Llllllll

 

 

 

10'7llllllllgllrllliilllilll

50 100 150 200 250 300
Temperature (K)

 

(b)

Conductivity (S/cm)
S

 

 

Temperature (K)

Figure 4.10. Variable-temperature electrical conductivity measurements
for (a) LixMOO3 and (b) Lix(H20)y(PEO-100,000)ZM003.

180

5. Electrical Conductivity

Electrical conductivity values for all the nanocomposites are listed in
Table 4.2. The room-temperature electrical conductivity of the LixMoO3
was 0.013 S/cm, which is slightly lower than values reported in related
materials [23]. As shown in Table 4.2, the conductivity of the
nanocomposites is significantly lower than that of LixMoOg and decreases
with increasing layer expansion. For materials with extremely large gallery
spacing, the electrical conductivity is lower than that of M003 itself. When
the measurements are performed under vacuum the conductivity decreases
concomitantly with the loss of water from the galleries. This suggests that
water is contributing to charge transport in these materials probably via
proton mobility. Figure 4.10 shows the temperature dependence of the
electrical conductivity of LixMoOg and Lix(HzO)y(PEO-100,000)zMoO3,
which are thermally activated. Mixed ionic/electronic conducting

nanocomposites are of current interest [43].
Concluding Remarks

A new family of polymer-molybdenum bronze nanocomposites have
been synthesized. The host material, LixMoO3, was synthesized via a LiBH4
route which is different from the conventional approach. This material
exfoliates in water and has affinity for a large variety of polymers.
Polymers such as PEG, PEO, PPG, PVP, MCel, PAM and PA6 give well
stacked lamellar nanocomposites. Depending on the nature of the polymer
and its molecular mass, some of the nanocomposites are soluble and can be
processed into films. Electronic transmission spectra show the broad

transition associated with the MoS+-Mo6+ couple in these nanocomposites

181

ranging from the IR to the visible region. Solid-state 7Li NMR
spectroscopy indicates a more versatile chemical environment in the
nanocomposites than in the host which may lead to high ionic conductivities
in these lamellar systems. The electrical conductivity of these materials is

thermally activated and ranges from 10'2 to 10'7 S/cm.

182

 

 

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Schindler, C. R. Kannewurf and M. G. Kanatzidis, J. Chem. Soc.,
Chem. Commun., 1993, 593. (f) C.-G. Wu, D. C. DeGroot, H. O.
Marcy, J. L. Schindler, C. R. Kannewurf, Y.-J. Liu, W. Hirpo and
M. G. Kanatzidis, Chem. Mater. 1996, 8, 1992.

4 (a) G. M. Kloster, J. A. Thomas, P. W. Brazis, C. R. Kannewurf and
D. F. Shriver, Chem. Mater. 1996, 8, 2418. (b) F. Leroux, B. E.
Koene and L. F. Nazar, J. Electrochem. Soc. 1996, $9), L181.

5 (a) L. F. Nazar, X. T. Yin, D. Zinkweg, Z. Zhang and S. Liblong,
Mat. Res. Soc. Symp. Proc. 1991, 2_10_, 417. (b) L. F. Nazar, Z.
Zhang and D. Zinkweg, J. Am. Chem. Soc. 1992, _11_4, 6239. (c) L.
F. Nazar, H. Wu and W. P. Power, J. Mater. Chem. 1995, _5_, 1985.

 

183

10

ll

12

13

14
15

16

(d) T. A. Kerr, H. Wu and L. F. Nazar, Chem. Mater. 1996, 8,
2005.

R. Bissessur, D. C. DeGroot, J. L. Schindler, C. R. Kannewurf and
M. G. Kanatzidis, J. Chem. Soc., Chem. Commun., 1993, 687.

(a) C. O. Oriakhi and M. M. Lerner, Chem. Mater. 1996, 8, 2016.
(b) P. G. Hill, P. J. S. Foot and R. Davis, Synth. Met. 1996, _7_6, 289.

(a) G. Cao and T. E. Mallouk, J. Solid State Chem. 1991, 9_4, 59. (b)
J. E. Pillion and M. E. Thompson, Chem. Mater. 1991, 8, 777. (c)
A. Clearfield and C. Y. Ortiz-Avila, in T. Bein ed., Supramolecular
Architecture ( ACS Symposium £9), American Chemical Society,
1992, pp 178. (d) Y.-J. Liu and M. G. Kanatzidis, Inorg. Chem.
1993, 8;, 2989. (e) Y. Ding, D. J. Jones, P Maireles-Torres and J.
Roziere, Chem. Mater. 1995, _7_, 562.

W. M. R. Divigalpitiya, R. F. Frindt and S. R. Morrison, J. Mater.
Res. 1991, Q, 1103.

(a) M. G. Kanatzidis, R. Bissessur, D. C. DeGroot, J. L. Schindler
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M. G. Kanatzidis, J. L. Schindler and C. R. Kannewurf, J. Chem.
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(a) E. Ruiz-Hitzky, R. Jimenez, B. Casal, V. Manriquez, A. S. Ana
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Hui—Lien Tsai, Jon L. Schindler, Carl R. Kannewurf and Mercouri
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I. Lagadic, A. Léaustic and R. Clément, J. Chem. Soc., Chem.
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P. B. Messersmith and S. I. Stupp, Chem. Mater. 1995, 1, 454.

(a) N. Margalit, J. Electrochem. Soc. 1974, 1_28, 1460. (b) J.
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M. G. Kanatzidis and T. J. Marks, Inorg. Chem. 1987, 86, 783.

184

17

18
19

20

21

22

23

D. M. Thomas and E. M. McCarron III, Mat. Res. Bull. 1986, 2_1,
945.

Yield obtained by reproducing the experiment in Reference 17.

S. Colson, J. M. Tarascon, S. Szu and L. C. Klein, Mat. Res. Soc.
Symp. Proc. 1991, _2_18, 405.

J. M. Tarascon and S. Colson, Mat. Res. Soc. Symp. Proc. 1989,
188, 421.

Y. Matsuda, M. Sato, M. Onoda and K. Nakao, J. Phys. C: Solid
State Phys. 1986, _l_9, 6039.

(a) M. Greenblatt, in Low-Dimensional Electronic Properties of
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Academic Publishers, 1989. (b) L. F. Schneemeyer, F. J. DiSalvo,
R. M. Fleming and J. V. Waszczak, J. Solid Staste Chem. 1984, g,
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88, 2049.

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Ionics 1983, 8, 61.

185

   

Chapter 5

OL-RuCl3/POLYMER NANOCOMPOSITES: THE FIRST GROUP OF

INTERCALATIVE NANOCOMPOSITES WITH TRANSITION-METAL-
HALIDES

Introduction

Intercalation compounds with polymers as the guest species are a
rapid growing class of nanocomposite materials [11. The combination of the
two extremely different components, at the molecular level, provides an
excellent opportunity to modulate the properties of one or more of the
component materials [2]. In some circumstances, it is also a unique way to
generate materials which have special properties that are unknown in the
individual components [3]. Intercalative polymer hybrids have advantages
over their small molecular analogs in compositional stability and
mechanical strength, which make them more suitable for applications. As a
result of recent efforts, a large variety of intercalative nanocomposites
have been prepared using hosts from most major classes of layered
inorganic compounds, i.e., clays, layered transition metal chalcogenides,
metal oxides, metal phosphates and metal thiophosphates [41.

Layered transition metal compounds such as oxides, chalcogenides,
halides and thiophosphates have interesting electronic, magnetic, optical
and catalytic properties [5]. These properties arise from the close-packed
intralayer atomic arrangement and the special atomic interactions which
results in partially filled d—electron bands. For example, oc-RuClg shows

intralayer ferromagnetism, interlayer antiferromagnetism [6a] and

photoconductivity [6b]. On the other hand TaSz and NbSe2 exhibit charge

186

 

 

density waves and superconductivity [71. One can expect that the physical
properties of these materials can be tuned or changed by means of
intercalation.

Layered transition metal halides, in contrast to layered transition
metal chalcogenides [3], oxides [9] and thiophosphates [10], have not received
much attention in the study of intercalation, because most of them are not
very stable under even mild conditions for soft chemical reactions.
Hydrolysis, dissolution and other decomposition reactions are often
common properties of this group of materials. a-RuClg is an exception,
and is very stable under these conditions.

oc-RuCl3 has a lamellar structure of defect Cdlz structure type [63],
see Figure 5.1. The intercalation chemistry of oc-RuC13 has been reported
with the insertion of simple cations and neutral polar molecules [11]. Cations
can be intercalated through a reduction reaction of oc-RuCl3 or through an
ion-exchange reaction, while neutral polar molecules can be incorporated
through solvent exchange. These properties are similar to those of layered
chalcogenides and oxides such as TiSz, 2H-TaSz and M003, suggesting the
possibility of intercalating polymers in this material. In fact, oc-RuCl3
behaves as an excellent polymer-intercalation host. It has affinity for
various polymers and is suitable for several of the existing polymer
intercalation methods. For example, oc-RuCl3/polyaniline (Ct-RuClg/PANI)
nanocomposites were synthesized with in situ redox intercalative
polymerization [12], while soluble polymers such as poly(ethylene oxide)
(PEO), poly(vinyl pyrrolidone) (PVP) and polyethylenimine (PEI) are
intercalated through encapsulative co-precipitation [13] of polymers and on-
RuC13 from solutions of exfoliated Ot-RuCl3. A modification of the second

method, in situ polymerization coupled with encapsulative precipitation [14]

187

 

 

View down the 001 plane

3! ‘3 ‘5 1! ‘3 3' S 3! S
:‘$‘$‘$‘¥‘$‘z
:‘a.g‘$‘¥;‘3“£‘z
. m. 07- .4- .4.

View down the 100 plane

‘95;§9‘g§

O 1‘

 
 
  
   

   
   
   

'5 O

99~~§29~9~

«i ‘1 C

Figure 5.1. The structure of oc-RuClg.

188

 

from suspensions of exfoliated (X-RUC13, can be used to intercalate

intractable polymers such as polypyrrole (PPY). This chapter describes the

insertion of polymers into (X-RUC13 producing a new group of lamellar

nanocomposites with various physicochemical properties.

Experimental Section

1. Reagents

Ru (99.95%, 325 mesh) was purchased from Cerac Inc. C12 (99.5%)
and CO (99.5) gases were from AGA Specialty Gases. LiBH4 (95%), PEO
(100,000), PVP (10,000) and PEI (25,000) were purchased from Aldrich
Chemical Company, Inc. After the polymers were dissolved, the polymer
solutions were filtered to remove insoluble residues. Aniline and pyrrole
were purchased from Aldrich and Mallinckrodt Inc. respectively, and were
distilled before use. Br2 (reagent grade) and anhydrous FeCl3 (purified)
were from Fisher Scientific. Anhydrous ether (99.0%), acetonitrile
(99.5%) and isopropanol (99.9%) were from Columbus Chemical
Industries Inc., EM Science Inc. and Mallinckrodt Chemical Inc.

respectively.

2. Reactions and Sample Preparations

a. Preparation of oc-RuCl;
oc-RuCl3 was synthesized as described in the literature [15]. The set-up
for the reaction, as shown in Figure 5.2, was first purged with nitrogen

overnight. An amount of 10 g Ru spread in an alumuna boat was then

reacted with mixed C12 and CO gases at 800 0C for 8 h. The ratio of C12 to

CO gas was adjusted to about 1:2 or 1:3 and the flow was set at

189

   

 

C12 regulator

\
Er—

Fumace f

 

 

 

 

 

 

Clz

 

Alumina boat

 

 

To hood

 

   

N aOH Concentrated
solution H2304

Figure 5.2 Experimental set-up for the synthesis of (x-RuCl3.

190

 

 

approximately 60—120 ml/min. The gas was set to flow before heating and
was turned off after the reaction tube cooled to about 50 0C. It is important
to keep the gas under constant flow, otherwise a negative presure could

build up and the H2SO4 (aq.) could be pushed into the reaction tube.

b. Synthesis of (PANI )1R_uC_31_

An amount of 0.1 g oc—RuCl3 in 10 ml of 4% aniline/acetonitrile
solution was stirred in open air for 1 week. The product was washed with
copious acetonitrile and dried in vacuum. Elemental analysis, done by
Quantitative Technologies Inc., gave: 15.33% C, 1.43% H, 3.11% N, which
suggests the formula (PANI)0,57(H2O)0,50RuC13 (calculated: 15.3% C,
1.43% H, 2.97% N). The above formula contains 22.7 wt% intercalated
species, which agrees with the thermogravimetric analysis (TGA)
measurement indicating 22.8 wt% organics and water inside this

nanocomposite [16].

c. Synthesis of LilRu_3Cl_ and preparation of RuCl3 monolayer suspensions
In a typical reaction, 18 mmol oc-RuCl3 was mixed with 3.6 mmol
LiBH4 in 40 ml anhydrous ether under N2 atmosphere for 3 days. The
product was collected by filtration, washed with anhydrous ether and
pumped to dryness. It was stored in a glove box under N2 atmosphere. The
product is designated as LixRuClg, where x z 0.2. An aqueous 0.5 wt%
LixRuCl3 monolayer suspension was prepared simply by stirring LixRuCl3

in water for half an hour.

191

 

d. S nthesis of Li PEO l(RuCli, LiXIPEIIXRuClg and LiXIPVPIXRuCla

 

In the preparation of PEO and PVP nanocomposites, an amount of
40 ml 0.5 wt% LixRuCl3 monolayer suspension (containing 0.96 mmol
LixRuCl3) was mixed with 40 ml of polymer solution containing 3 mmol
repeat-units of polymer. The mixture was stirred for 2 days in ambient
atmosphere. The nanocomposites of PEO and PVP are water-soluble, and
have to be collected from very concentrated solutions. Therefore, the
reaction mixture was condensed to less than 20 ml under vacuum before it
was poured in an organic solvent to precipitate the product. Acetonitrile
and isopropanol were used as solvents for PEO and PVP respectively. In
the preparation of PEI nanocomposite, the amount of 40 ml 0.5 wt%
LixRuCl3 monolayer suspension was mixed with 40 ml of PEI solution
containing 7 mmol repeat-units of polymer. The PEI nanocomposite
precipitated out from water immediately upon mixing. It was collected
with centrifugation after having been stirred as a suspension for 2 days.

The nanocomposites were dried under vacuum. The compositions of the
three nanocomposites were Lix(PEO)1,5RuCl3 (x~0.2), Lix(PVP)2.2RuCl3
(x~0.2) and Lix(PEI)4,6RuC13 according to T GA measurements under

oxygen flow [161.

e. Synthesis of (PPYARuClg
An amount of 0.45 g pyrrole (6.7 mmol) was dissolved in 40 ml of

water and mixed with 40 ml of aqueous LixRuCl3 suspension which

contained 0.20 g LixRuClg (0.96 mmol). The mixture was cooled in an ice

bath before the dropwise addition of 10 ml aqueous solution of 0.11 g

FeC13 (0.67 mmol) which was also cooled in an ice bath. The reaction was

carried out by stirring in a stoppered, ice-cooled flask for 24 h. The

192

 

 

product was collected by centrifugation, washed with copious water, and
dried first in air and then under vacuum.

Elemental analysis showed that a (PPY)xRuC13 sample contained
13.88% C, 1.32% H and 3.95% N. This corresponds to a formula
(PPY)0,77(H2O)0,60RuC13 (calculated: 13.78% C, 1.31% H and 4.01% N).
TGA measurements showed that the material lost 50.2% (theoretical
50.4%) of weight in oxygen flow up to temperatures higher than 450 0C.
X-ray powder patterns of the final residue indicated RuO2.

f. Oxidation of Li_xRuCl_3 and nanocomposites with Br2
The oxidized RuCl3 was obtained by reacting exfoliated LixRuCl3 (x
~ 0.2) with 10 equiv. of Br2 in water for one day. The RuCl3, which

flocculated, was collected by centrifugation, washed with copious water and

dried under vacuum. The oxidized “(PEO)yRuCl3” was prepared by

reacting directly the nanocomposite solutions, produced by the intercalation

reaction, with 10 equiv. of Br2 in water for one day.

g. Preparation of oriented Li1(PEO)¥RuCl_3 sample for one-dimensional

electron density map calculations
One-dimensional electron density (l-D ED) map calculations need

 

highly oriented samples. Lix(PEO)yRuC13 samples for this purpose were
cast films. Immediately before the X-ray experiments, the samples were
pumped at 75 0C for at least 3 days to remove excess water from the
nanocomposite. This procedure ensures that the whole sample has only one
phase with a single basal spacing. The X-ray patterns were collected under
a nitrogen atmosphere to prevent absorption of moisture from the air,

which causes peak broadening and drifting.

193

 

3. Instrumentation

The instrumentation in the measurements such as thermal
gravimetric analysis (TGA) and infrared (IR) spectroscopy were the same
as described in Chapter 1. X-ray diffraction (XRD) powder patterns were
obtained as described in Chapter 1, except the increment for the continuous
scanning mode was 005° for general purpose spectra.

Room temperature conductivity measurements, variable temperature
direct-current electrical conductivity measurements and thermopower
measurements, and magnetic susceptibility measurements were conducted
as described in Chapter 2. Scanning electron microscopy (SEM) and energy
dispersive X-ray microanalysis (EDS) and electron diffraction were done
as described in Chapter 3.

XRD patterns for one-dimensional electron-density calculations were
done as described in Chapter 3. Slits for beam width control and time of
data collection were as follows: 0.5° slits and a data collection time of 12 8
per step were chosen for measurements in the range from 2° to about 29°;
10° slits and 60 s per step between 16° and 71°; 20° slits and 60 s per step
between 57° and 835°; 4.0° slits and 90 8 per step from 72° to 136°. The

step width was kept the same (01°) in the entire 20 range.

Results and Discussion

1. Preparation of (PANDXRuClg by in situ Redox Intercalative
Polymerization

The in situ redox intercalative polymerization reaction is the most
direct method to intercalate conductive polymers. Its topotactic character

least disturbs the crystalline structure of the host. In the case of

194

 

 

FeOCl/polyaniline, even single crystals of the nanocomposite could be
obtained [17]. This type of reaction requires a strongly oxidizing host to
provide a driving force to pull electrons from the monomers and oxidize
them into polymers. In addition, the host should be able to distribute
efficiently those electrons through out their structure. Because of the

scarcity of such highly oxidizing and conducting hosts, the reaction has
been limited to FeOCl [128], V205 [12b] and VOPO4 [120]. We discovered that
oc-RuCl3 also happens to be a suitable such host, and can form intercalative
nanocomposites with polyaniline.

The reaction of an aniline CH3CN solution with (X-RUCI3 in air
results in the formation of polyaniline (PANI) within the gallery space of
RuCl3. The reflection-mode powder X-ray diffraction (XRD) patterns of
the product show a 6.2 A increase in the separation of the RuC13 layers, see
Figure 5.3 [18]. This expansion is reasonable for insertion of a monolayer
of PANI molecules, and comparable to the 5.94 A observed in
(PANI)xFeOCl [171, and 5.2 A in (PANI)XV2OS [12b]. The transmission-mode
powder XRD patterns show that the hk0 reflections of (PANI)xRuCl3
remain the same as those of OL-RuClg, indicating that the structure of oz-
RuC13 is preserved, see Figure 5.4. This result is supported by electron
diffraction experiments which provide the same hk0 diffraction patterns
for (PANI)xRuCl3 and oc-RuCl3.

The formation of polyaniline between the RuCl3 layers is supported
by IR spectroscopy. Almost all peaks in the IR spectra of (PANI)xRuC13
are associated with polyaniline (emeraldine salt) and only a few with
anilinium, see Figure 5.5(a). The peaks corresponding to the polymer are
much more intense than those of anilinium. Heating the product at 120 0C

in air for 5 days removes the peaks corresponding to anilinium at 744 cm-1

195

 

 

 

 

 

 

 

(X-RUCI3
3L -
1L89A.
I (PANI)xRuCl3
E? r422A1
g -—I uyPEokhuog
E L JLA
28.78 A
UJPVPkRUCk

A

 

    
 

(PPYhRucg

 

 

 

IIIIIIIIIII'IIIIIIIIIIerI

20 (deg)

Figure 5.3. X-ray diffraction patterns of oc-RuClg and nanocomposites.

196

 

 

 

 

 

 

 

 

 

 

O
O O
1- O
O 0')
0t-FluCI3 -
(O
O
O
O
O O
N 1-
N
.29
u-4
m
8
E 8
1—4 '-
(PANI)XF1uC|3
S
O
l I I 1
0 10

 

20 (deg)

Figure 5.4. Transmission X-ray diffraction patterns of OL-RuCl3 and
(PANI)xRuC13 with indexing.

197

 

 

 

(PAN|)xRuC|3

PAWJI

Transmittance

ArHHne

 

 

 

 

 

 

1 I I I I I I I I I I I fiI I I r I I I I I r I I I I

1800 1600 1400 1200 1000 800 600 400

Wavenumbers (cm’ 1)

Figure 5.5(a). Infrared spectra of (PANI)xRuC13, PANI and aniline. (The
peak at 1385 cm-1 is due to an impurity in KBr.)

198

 

(PPYkRUCQ

WW

 

Transmittance

PPY

 

 

 

j—IIIIIIIITIIIIIIIIIIITIIIIII

1800 1600 1400 1200 1000 800 600 400

Wavenumbers (cm'l)

Figure 5.5(b). Infrared spectra of (PPY)xRuCl3 and PPY. (The peak at
1385 cm-1 is due to an impurity in KBr.)

199

 

 
 

Lix(PEO)yRuCI3

 
 

Transmittance

PEI)

 

 

 

 

 

 

III—IjIfiIIIIIIIIIIIIIIIIIIII

1800 1600 1400 1200 1000 800 600 400

Wavenumber (cm' 1)

Figure 5.5(c). Infrared spectra of Lix(PEO)yRuCl3 and PEO. (The peak at
1385 cm-1 is due to an impurity in KBr.)

200

 

 

 

  

Lix(PVP)yRuCI3

   

Transmittance

PVP

 

 

 

ITIIIIIIIIIITIIIIIIIIIIIIII

1800 1600 1400 1200 1000 800 600 400

Wavenumber (cm' 1)

Figure 5.5(d). Infrared spectra of Lix(PVP)yRuC13 and PVP. (The peak at
1385 cm-1 is due to an impurity in KBr.)

201

and 687 cm'l. The appearance of such monomer peaks indicates that the
polyaniline formed in the galleries does not have very high molecular
weight. This is reasonable since both the concentration and mobility of the
aniline in the gallery are limited and the polymer is expected to be similar
to the ~5,000 daltons found in the (PAN I)xFeOCl system.

In the process of intercalation, a fraction of Ru3+ is reduced to Ru2+a
giving a mixed valence compound. The Ru2+ centers are very stable
because they are low spin diamagnetic (16 systems. This probably acts as a
powerful driving force for the oxidation of aniline by virtue of the
maximized Ligand Field Stabilization Energy (LFSE) in low spin (16 metal
centers. Similar to the intercalative polymerization of aniline in FeOCl [123]
and V2O5 [12b], the presence of oxygen is key to a successful outcome of the
reaction. This was verified by control experiments where in the absence of

air or oxygen no intercalation reaction occurred in 23 days.

2. N anocomposites of oc-RuCl3 with Water Soluble Polymers

The method of encapsulative precipitation from solutions of
exfoliated lamellar solids provides a most convenient way to prepare
nanocomposites with water-soluble polymers. With a stable and
concentrated aqueous LixRuCl3 monolayer suspension, the intercalation is
performed with water-soluble polymers: PEO, PVP and PEI. The
reflection XRD patterns of the products show that the interlayer spacings
of the Lix(PEO)yRuC13, Lix(PVP)yRuC13 and Lix(PEI)yRuC13
nanocomposites increase by 8.5, 23.0 and 3.6 A respectively, see Figure
5.3. The existence of PEO, PVP and PEI in the nanocomposites is proven
by IR spectroscopy. (See Figures 5.5(c) and (d) for IR spectra of
Lix(PEO)yRuCl3 and Lix(PVP)yRuC13.)

202

 

 

(é)

 

 

 

 

 

 

 

 

Figure 5.6. Electron diffraction patterns for (a) oc-RuC13 and (b) LixRuClg.

203

Electron diffraction experiments show that the exfoliated RuCl3 has
the same hk0 diffraction pattern as OL-RuCl3, see Figure 5.6. The same
electron diffraction pattern was seen in (PANI)xRuCl3, suggesting no

intralayer structure change during intercalation.

3. oc-RuC13/Polypyrrole Nanocomposites

The exfoliation property of oc-RuCl3 offers the possibility to
synthesize nanocomposites with insoluble polymers, by in situ
polymerization-encapsulative precipitation [14]. In this method, a solution of
a monomer (such as pyrrole) is mixed with an exfoliated inorganic host.
When an initiator is added, the polymerization causes the co-precipitation
of the polymer and the host monolayers, forming a nanocomposite with a
certain lamellar thickness. As described in the Experimental section,
(PPY)xRuCl3 was produced this way.

The production of (PPY)xRuCl3 nanocomposites is indicated by XRD
patterns of the layered products with basal spacings around 11.7 A, see
Figure 5.3. The existence of the conductive form polypyrrole inside the
galleries is confirmed by the characteristic IR spectrum, which exhibits
peaks at 1541, 1314, 1150 1043 and 963 cm-1 [191, see Figure 5.5(b).

The expansion between the RuC13 layers, 6.0 A, corresponds to a
layer of polypyrrole chains arranged almost 40 degrees to the RuCl3
layers. This behavior is similar to that of (PPY)xMoS2 when pyrrole is
used in excess in the reaction [14C]. The packing density of polypyrrole in
(PPY)xRuCl3, 0.77 pyrrole-unit per RuCl3, is comparable to that in
(PPY)xMoS2 prepared under similar conditions, 0.50 pyrrole—unit per
MoS2 [201.

204

 

 

 

In the reaction to form (PPY)xRuCl3, only 0.7 equivalent FeC13 was
used, which could oxidize at maximum 0.35 equivalent pyrrole to
polypyrrole. Therefore, the amount of polypyrrole produced is more than
the amount of pyrrole that FeC13 could have oxidized. This is explained by
the fact that the ambient oxygen takes part in the reaction as an electron
acceptor. It has been proven that ambient oxygen can oxidize and
polymerize pyrrole and its oligomers when FeC13 is present as a catalystm].

A control experiment, which was conducted under the same
experimental conditions but without the addition of FeC13, produced a
lamellar phase with a basal spacing of 10.1 A (expansion of 4.4 A). This

phase cannot be de-intercalated by dynamic pumping. It must correspond to
a RuC13 intercalation compound with one layer of pyrrole or oligomers
lying with the pyrrole rings parallel to the RuC13 layers. This compound
lost 48.0 wt% in a TGA experiment in oxygen flow up to 650 0C, which is
comparable with that of (PPY)0,77(H2O)0,60RuC13, 50.2 wt%. Another
control experiment showed that no polymerization occurred when both
FeC13 and RuCl3 were absent.

As an alternative method, we also tried the in situ redox intercalative
polymerization in order to prepare a RuC13/PPY nanocomposite. When on-
RuCl3 was stirred in an aqueous pyrrole solution [22] in open air,
intercalation occured in two weeks to form a product with 11.2 A basal
spacing. However, IR spectra showed that the polymer formed inside the
gallery spacing was not ordinary polypyrrole and further investigation was

not performed.

205

 

4. Charge Transport Properties

The intercalation of polymers causes large changes in the properties
of oc-RuC13, as expected. Because of the formation of Ru2+ centers which
provide free hopping electrons, the electrical conductivity of LixRuCl3 (x ~
0.2) at room temperature is ~ 0.3 S/cm, about three orders of magnitude
higher than that of oc-RuCl3, 5x10"4 S/cm [23]. Intercalation of insulating
polymers reduces the conductivity, with those of PEO and PVP
nanocomposites being 4.5x10'3 and 1.7x10'3 S/cm respectively.
Nanocomposites with conductive polymers have conductivities higher than

that of LixRuCl3. (PANI)xRuCl3 has a room temperature conductivity of ~
1 S/cm, while (PPY)xRuC13 has one of 23 S/cm. It is obvious that the
conductivities of these two materials are substantially enhanced by the
presence of conductive polymers.

Variable temperature measurements on pressed pellets reveal that the
electrical conductivities of all oc-RuCl3, LixRuCl3 (x~0.2), Lix(PEO)yRuC13,
(PANI)xRuC13 and (PPY)xRuC13 are thermally activated, see Figure 5 .7. In
the case of oc-RuCl3, LixRuCl3 and Lix(PEO)yRuC13, the log(o) versus 1/T
plots are almost linear. The activation energies, which are calculated
according to the formula 0 = (50 e-AE/Zkt, are 0.36, 0.30 and 0.35 eV
respectively. Since the pellets used in measurements have inter-particle
boundaries, the activation energies do not necessarily correspond to the
intrinsic band gaps. In (PANI)xRuC13 and (PPY)xRuCl3, the data do not
form straight lines in the log(o) versus l/I‘ plots, which suggests that
several kinds of electrical barriers exist in these materials, including
electron hopping barriers associated with transport through and across the

chains of the conjugated polymer. At temperatures higher than 100 K, the
data of (PANI)xRuCl3 and (PPY)xRuC13 almost fall in straight lines. The

206

 

  
 
 
   
  

(PPYkRuCg

00°00:

 

 

 

10'1
V ., (PANl)xRuC|3
10'3 V v
v E
A V:
a E DC] 3
3:“ 10-52— DB
> E U
8 : 0.
E :E Du LIXRUC|3
o I -
U _7- LIX(PEO)yRuCI3 .. k
10 _E 151%
@q%
:1
10‘9 0”
oc-RuCl D
3
O
OO
0
10‘1111:11|11|111111I11111111111

2 4 6 8 10 12 14 16

1000frtkrh

Figure 5.7. Variable temperature electrical conductivity mesurements for
pressed pellets of oc-RuCl3 and nanocomposites.

207

 

 

   

    
 
 
 

  

 

 

 

150 I I I I I I I I I I I I I I I T I I l I I I I I
100 _
I.
E. :
H .-
0.)
g - -
Q 50 — —
é ' :
D . -
F5 - (PANl)xRuC|3 -
0: " (PPY)xRuCI3 L
I. L I I I I I l l l I I L l l I l I 1_L I l l I Id
50 100 150 200 250 300

Temperature (K)

Figure 5.8. Thermopower measurements for pressed pellets of LixRuCl3 (x
~ 0.2) and nanocomposites.

208

.. —_——

 

apparent activation energies are 0.18 ev for (PANI)xRuCl3 (> 125 K) and
0.10 ev for (PPY)xRuC13 (> 160 K).

The thermopower, which is less affected by grain boundary effects,
was measured for LixRuCl3 (x ~ 0.2), (PANI)xRuCl3 and (PPY)xRuC13, see
Figure 5.8. All three materials have positive Seebeck coefficients
suggesting that the dominant carriers are holes. The p—type charge-
transport behavior is consistent with the fact that the reduced RuC13 layers,

polyaniline (emeraldine salt) and polypyrrole are all p-type conductors.
Both the high Seebeck coefficient value of ~ 60 ItV/K for LixRuClg and its
decreasing trend with rising temperature indicate that LixRuClg is a
semiconductor. The hole type transport in the [RuCl3]"' layer arises from
the partially empty band composed of t2g type orbitals while electron
configuration is somewhat between t2g5 and t2g6. (PANI)xRuCl3 and
(PPY)xRuCl3 have lower thermopower values which increase as the
temperature increases. This trend is usually seen in metallic conductors.
Considering that the reduced RuC13 layers are semiconductors, the bulk
metallic-like conductivity as suggested by the small thermopower indicates

that charge transport in (PPY)xRuC13 and (PANI)XRuCl3 is controlled by

the conductive polymers.

5. Magnetic Susceptibility Studies

In a-RuCl3 the intralayer Ru3+ ions are ferromagnetically coupled,
however, in adjacent layers the Ru3+ ions are antiferromagnetically (AF)
coupled 163’ 24]. The interlayer AF coupling becomes strong at low
temperatures (2-20K), and causes a remarkable AF ordering at about 15 .6
K [68’ 241, see Figure 5 .9(a). At high temperatures (50-300K), the interlayer

AF coupling is so weak that it has no significant effects on the magnetic

209

 

 

susceptibilities. The magnetic susceptibilities of OL-RuCl3 in this range
follow Curie-Weiss law with a positive Weiss constant 0, which reflects the
weak effect of the intralayer ferromagnetic coupling.

In LixRuCl3 (x ~ 0.2), the conspicuous AF ordering disappears and
the magnetic susceptibility follows Curie-Weiss law to temperatures as low
as 2 K, see Figure 5.9(b). The Weiss constant 0 becomes negative,
indicating a change from the weak overall ferromagnetic coupling to a
weak overall AF coupling. These changes in magnetism are explained by
the introduction of diamagnetic Ru2+ centers in the Ru sublattice, which

alters the magnetic couplings both in the layers and between the layers. The
magnetic susceptibility data of Lix(PEO)yRuCl3, (PANI)xRuCl3 and
(PPY)xRuC13 are similar to those of LixRuCl3, see Figures 5.9(c) and
5.9(d). Since the reduction of the RuCl3 layers has already disrupted the
original interlayer AF coupling, the subsequent insertion of the polymers
does not have any additional effect on the magnetic susceptibility.

The ”eff for (X-RUC13 is 2.32 It}; [251, while those for LixRuCl3 and
nanocomposites Lix(PEO)yRuCl3, (PANI)xRuCl3 and (PPY)xRuC13 range
from 1.60-1.74 I113. The drop in paramagnetic moment is due to the
decrease in the number of unpaired electrons in RuC13 layers because of the
presence of diamagnetic low spin Ru2+ centers. The derived paramagnetic

moments Ileff and Weiss constants 0 for these compounds are listed in Table

5.1. The xdia and xm, used in the manipulation of the magnetic data are also

listed in Table 5.1.
As mentioned above, the intercalation of polymers does not bring

much change in the magnetic susceptibility of the reduced RuCl3 layers,

because the interlayer magnetic coupling has already been broken down by

the generation of Ru2+ centers. If the RuCl3 layers were not reduced, i. e.,

210

Xmolar

 

 

 

 

 

0.020_ _ 500
: ,DE
. —400
0.015 :
'_' 4
- -300 :
' I x
0.010_— - 8‘
: {200 ‘3"
- 1
0.005- -
- :100
@0000 "
".,.’:.1...11.11.11.11111..1...11'
O'°°°0 50 100 150 200 250 300 °

Temperature (K)

Figure 5.9(a). Magnetic susceptibility measurements for a sample of oc-

RUCI3.

Xmolar

 

 

 

 

 

 

0.020 , ,cJ
.0

. 6 ’ I

/O’ .—

0015 , 9’ —> -

9 H

- 28 ’ L

0.010 — 7

0.005 - _‘

f) l l l I l l I l I I I l I I l l l l l l l l l I l l l T: -

0'0000 50 100 150 200 250 300

Temperature (K)

1 000

Figure 5.9(b). Magnetic susceptibility measurements for LixRuCl3 (x~0.2).

211

 

 

Xmolar

 

 

 

 

 

0.035 . 1000
0.030 ,j
j 800
0.025 a
0.020 1‘ 600 I
X
I 8
0.015 _ 400 93.
0.010 j
‘ 200
0.005 .
0.000 "I I 1 l I l I l l I l l I l I L L l I I 1 I l I I 1 l l l I - O
0 50 100 150 200 250 300

Temperature (K)

Figure 5.9(c). Magnetic susceptibility measurements for Lix(PEO)yRuCl3.

xmolar

 

0.020

0.015

0.010

0.005

 

0.000

0

   

IIIIIIIrIIIII

I I l l l I l I l l l I l l l I I l l

 

 

l I I l l l I I I I I I l I I I l l l I l l 1 l I I l l l l

50 100 150 200 250 300
Temperature (K)

1000

0

Figure 5.9(d). Magnetic susceptibility measurements for (PANI)xRuCl3.

212

Table 5.1. Magnetic properties of oc-RuCl3 and nanocomposites

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

“eff 0 Xdia*106 XTIP*106
sample (uB/mol Ru) (K) (l/mol Ru) (1/mol Ru)
0L-RuCl3 2.32 11 -101 0
LixRuC13 (x~0.2) 1.60 -18 -101 176
(PAN I)xRuCl3 1.63 -20 -133, 250
Lix(PEO)yRuC13 1.73 -10 -167 667
(PPY)xRuC13 1 .74 - 15 -142 247
RuC13 (0Xid.) 2.33 6.2 -101 450
(PEO)XRUCI3 1.82 0.2 -143 450
(0Xid.)

 

the magnetic coupling among the Ru3+ ions existed, the weakening or
elimination of the interlayer Ru3+ ion magnetic coupling by the polymer
intercalation should be observed. Such a phenomenon is expected in the
“Br2 oxidized Lix(PEO)yRuC13” (i.e. “(PEO)xRuC13”), in which the Ru2+
centers in the reduced RuC13 layers are oxidized back to Ru3+. In this
material the intralayer Ru3+ ferromagnetic coupling should be restored,
while the interlayer Ru3+ AF coupling should still be retarded by the

increased interlayer separation.

An investigation of the magnetic properties of “Br2 oxidized
LixRuCl3” (i.e. RuC13) was first carried out, because this material is
structurally simpler than the oxidized nanocomposites yet intimately related
to them. The oxidized product indeed reestablished the weak overall

ferromagnetism at temperatures higher than 50 K. However, the significant

AF ordering present in pristine a-RuClg did not recover. Instead, a gradual
change from overall ferromagnetism to overall antiferromagnetism at low

temperatures was observed, see Figure 5.10(a). It is obvious that

213

 

 

 

 

 

 

 

 

 

0.025 _ 500
0.020:- 0’ ’0 j 400
: I O I ..
0.015:- — 300 ,...
é : 3 Ex
X 0.010 3- — 200 3
0.0050:— —_ 100
f I I I I I I I I I I I I I I I I I J I I I I I I I I I I I
0’00 50 100 150 200 250 3000

Temperature (K)

Figure 5.10(a). Magnetic susceptibility measurements for “Br2 oxidized
LixRuCl3” (i. e. RUCI3).

 

 

 

 

 

 

 

0.25 800
I ’0’ "j 700
D -
0.20 ’0. ’ _: 600
’0’ :
, 0 I -_ 500
h 0.15 [(7 : I—l
g D’ 1 400 P?
E o ’ . E
x 0.10 ,0, ’ _j 300 m
o’ -
’0 I 'E 200
0.050 09’ 2
o '1 100
o I
0.0 I I I I I I I I I I I I I I I I I I I I I I; I I I I I I + O
0 50 100 150 200 250 300

Temperature (K)

Figure 5.10(b). Magnetic susceptibility measurements for “Br2 oxidized
Lix(PEO)yRuC13” (i.e. “(PEO)xRuC13”).

214

 

’;,¢; y-ynnzylunu

 

 

 

 

100 IIIIIIIIITIIIIIIITIIIIIIIIIII

50

('11
o

IIIIIIIIIIIIIIIII

 

 

IIJIIIIIIIIIIIIIIII

 

Magnetization (Gauss-cm3/mol)

 

_100 7 I I I I I I I I I I I I I L J l 1 1 1 I r I l I I I l I I
-3000 -2000 —1000 0 1000 2000 3000
Magnetic Field (Gauss)

Figure 5.10(c). Magnetic hysteresis measurements for “Br2 oxidized
LIXRUC13” (i.e. R11C13).

 

 

 

 

 

_ I I I I I I I I I I I I I I I I I I I I I I I I I I I I I
:3 400 _r j
E _ -
5 200 r -'
a) I— _
m - -
a - .
g 0:- --
s: _ -
.9 t .
‘5 -200 - -
d.) " -
c: , -
g0 -400 — —
2 I I I I LI I I I I I I I I I I I I I I I I I I I I I l I -
-3000 -2000 -1000 0 1000 2000 3000
Magnetic Field (Gauss)

Figure 5.10(d). Magnetic hysteresis measurements for “Br2 oxidized
Lix(PEO)yRuC13” (i.e. “(PEO)xRuCl3”).

215

 

 

‘j;g‘;§ run-v

 

exfoliation and restacking causes stacking disorder ( e.g. turbostratic) of
the RuC13 layers, so that optimum interlayer AF coupling in oxidized
RuCl3 is not possible.

The overall ferromagnetic coupling for the oxidation derived
“(PEO)xRuCl3” at temperatures higher than 5 K is slightly weaker than that
of corresponding RuC13 (> 50 K), as is suggested by the less positive 0
value. The intercalation of polymer causes the AF coupling to almost
disappear, as demonstrated by the persistence of the overall
ferromagnetism to as low as 5 K, see Figure 5.10(b). The magnetic
hysteresis measurement, which is characterized by a hysteresis loop,
confirms that the “(PEO)xRuC13” produced by Br2 oxidation of
Lix(PEO)yRuCl3 is ferromagnetic at 5 K, see Figure 5.10(d). In contrast,
RuCl3 produced by Br2 oxidation of LixRuCl3 does not show a hysteresis
loop, see Figure 5.10(c).

The latter material has peg and 0 values close to that of oc-RuClg,
which means that almost all Ru2+ centers are converted to the Ru3+ state.
The [Jeff and 0 of “(PEO)xRuCl3” produced by Br2 oxidation of
Lix(PEO)yRuCl3 are between those of un-oxidized nanocomposites and oc-
RuC13, which might suggest that some Ru2+ centers remain. The incomplete
conversion of Ru2+ to Ru3+ in the nanocomposite could be attributed to the
presence of polymers in the interlayer galleries, which stabilize the reduced
layers by offering coordination to Li+ ions. Other reasons could include
slower oxidation kinetics in the polymer intercalated systems. The
paramagnetic moment ueff and Weiss constant 0 of the oxidation derived
RuCl3 and “(PEO)xRuC13” are shown in Table 5.1.

216

6. One-Dimensional Electron Density Calculations and Arrangement of
Polymer Chains in Lix(PEO)yRuCl3

In inorganic/polymeric nanocomposites, the arrangement of the
atoms in the inorganic layers is well defined. However, the arrangement of
polymer chains in the interlayer galleries is not so clear, yet the structural
issue is critical to a full understanding of the properties. Because the
interlayer gallery provides a two-dimensional space for polymer chains,
their arrangement in the interlayer gallery also stimulates a lot of interest
in fundamental polymer physics. Many researchers have explored this type
of situation with different approaches [26].

Because of the simple repeat structure and flexible chain of PEO, its
intercalative nanocomposites have been the subject of most investigations
[26th 0, d, 6’ H. In addition, many structures for PEO and PEO-complexes are
known.

The most common conformation of bulk PEO is the helical one [27],
which exists in pure PEO spherulites [23]. The planar zigzag conformation
has been observed in stretched PEO samples [29]. Two more types of
conformations have been found in PEO-HgClz complexes [30’ 31]. There are
many PEO—(alkali metal salt) complexes with versatile morphologies and
the determination of their structures has been attempted. The conformation
of PEO in these alkali metal salt complexes is thought to be either a double
helix [32], a waving single helix, which accommodates itself around the
alkali ion lattice [33], or one similar to that in the type II PEO-HgClz
complex [34]. In addition, many PEO complexes form with organic
molecules: PEO-urea complex [35], PEO-thiourea complex [36], PEO-para-
dibromobenzene complex [37] and PEO-para-nitrophenol complex [38].

Several common PEO conformations are shown in Figure 5.11.

217

 

 

Films of oriented Lix(PEO)yRuCl3 have well defined sharp XRD
patterns with up to fourteen 001 reflections corresponding to a resolution
of 0.98 A, see Figure 5.12. The l-D ED maps for Lix(PEO)yRuC13, which
can provide information about the projected structure on the c axis, were
calculated from the X-ray powder pattern of oriented films (see Figure
5.13 [39]), with formulas described in Chapter 3. In the calculation for 1-D
ED map, the phases of structure factors are needed. The phase information
for the calculation was obtained from the positions of the Ru and C1 atoms.
Due to the minor contribution of the diffracted intensity from the polymer,
the inorganic layer plays a critical role here in phasing the structure factors
and consequently in the computation of the electron density map.

The 1-D ED map profile for Lix(PEO)yRuCl3 shows clearly that the
electron density due to the intercalates forms two asymmetric peaks placed
symmetrically away from the center of the gallery, similar to that of
Li0.2(PEO)xTaSQ [1301. As in the case of Lio,2(PEO)xTa82, the 1-D ED map
favors best the structural model with two layers of PEO in the gallery in
the conformation of type II PEO-HgClz complex [40]. (The model is shown
in Figure 5.14.) The wide valley in the l-D ED map in the center of the
gallery immediately excludes the possibility of a layer of helically formed
PEO, which must have appreciable electron density in the central region of
the gallery. Considering that the basal spacing of the Lix(PEO)yRuC13 film
used is 13.76 A [411, which gives an interlayer expansion of 8.0 A, the
accommodation of two layers of helical PEO in such a gallery is physically
impossible, because the Van der Waals diameter of one PEO helix is ~8.0

A [27]. Furthermore, neither the arrangement of PEO in the zigzag

conformation (which was proposed in the (PEO)XV205 [26d]) nor that of the

218

(a) % ”Ev/“M"
(b) 8 op\’b’\p‘o’b"op‘o"o’.

w L?"

(d) ’3’ ‘of‘ohbh’oh'nhtf‘rf‘o’

 

Figure 5.11. Possible PEO conformations: (a) single helix (7/2) [27], (b)
zigzag [29], (c) type I PEO-HgC12 [30b], ((1) type II PEO-HgC12 [31], (e)

double helix [32].

219

 

4‘ “
O;

 

 

 

001

 

002

Intensity
§

 
  
  

0013

005 0011
003 0012 0014 x1000
xlm

A

 

 

 

[IrrlfiTIIIIP['1-fi11lll[1ll

0 20 40 6O 80 100 120 140
20 (deg)

Figure 5 .12. X-ray powder diffraction pattern of oriented Lix(PEO)yRuCl3
film used for the calculation of one-dimensional electron density maps.

 

 

 

 

 

 

 

 

<——Ru atoms
' <—— Cl atoms ,
b ‘. .'
'2‘ '- PEO I
:3 t :
I: | I / \ |' l
8 : : .' '.
E I' ' I, ‘\ : 'l
L“ : ". : '1
I I I I 1
V I’\ , I \__‘l I ‘ ’II' V
. . . . . . l . . . I . I . . . .
0 02 0.4 . 06 08 1
c-ax1s

Figure 5.13(a). One-dimensional electron density map for Lix(PEO)yRuC13
using the model shown in Figure 5.14. (Solid line, calculated from
experimental data; dash line, from model.)

220

 

 

 

 

 

 

 

 

<—-———Ru atoms
' <—— Cl atoms .
>‘ I
.5.) I. PEO I
I: | I
<0 I I I
Q I ‘ / \ II I:
s: I |
g l' I :\| "‘I II I
o I . , I . I I l
2 ' ' |I ' I I. l
LL] ‘| " I I I I
| "\ ' I I s I
v ‘ ' ' ‘ II ‘ I 'I U
s \ _ I ‘ l \
I 41 I I I I I I I I I I I I I
O 0.2 0.4 0.6 0.8 1
c-axis

Figure 5.13(b). One-dimensional electron density map for Lix(PEO)yRuC13
using a model with intragallery double helical PEO that takes an
orientation as shown in Figure 5.11(e). (Solid line, calculated from

experimental data; dash line, from model.)

 

<—— Ru atoms

<——— Cl atoms

 

 

 

 

 

 

 

>5
3:: PEO
m
G
0)
Q / \
C: I
O I
*5 I I‘ \ '
i3 ' .' .
LU 1“ I. ’1' ‘0 L ‘\ : U,
I
I ‘ "' \I II I
I I I I I I I I I I I I I I I
0 0.2 0.4 . 0.6 0.8
C-aXlS

1

Figure 5.13(c). One-dimensional electron density map for Lix(PEO)yRuC13
using an intragallery double helical PEO model that assumes an orientation

of 450 rotation relative to the one shown in Figure 5.11(e). (Solid line,
calculated from experimental data; dash line, from model.)

221

 

 

 

 

 

 

 

 

<——Ru atoms
I I
I <—— Cl atoms
2."
'5
C:
a)
Q
CI
0
b
o
2
LL]
0 0.2 0 8 1

 

° c-axis

Figure 5.13(d). One-dimensional electron density map for Lix(PEO)yRuCl3
using a model with randomly rotated intragallery double helical PEO.
(Solid line, calculated from experimental data; dash line, from model.)

 

 

 

 

 

 

I <——Ru atoms
3*
g I <——— Cl atoms PEO "
Q I / I
I I
c:
e ‘. \ .'
IE I I I‘ ’ “ \ : I
|I It ’ ‘ I, ‘I
g "’ I \ ~ I- \
I I I I I I I I P I _;I I I I I I I l
0 0.2 0.4 0.6 0.8 l
c-axis

Figure 5.13(e). One-dimensional electron density map for
Agx(PEO)yRuCl3 using a model similar to that shown in Figure 5.14.
(Solid line, calculated from experimental data; dash line, from model.)

222

 

‘- 1.43A
3.17A
:108A

:2. 40 A
13.76 A

is:
.)®

 

 

 

 

 

O ; ,2 -. 0 a ‘1
5 4-— Cl atoms
a— Ru atoms

Figure 5.14. Structural model for Lix(PEO)yRuCl3. For clarity, in the two
galleries the directions of the PEO are drawn orthogonal to each other.

223

type I PEO-HgClz complex (which was proposed for the Na0_3(PEO)xMoO3
[260]) is consistent with the observed l-D ED map profile.

In the model of Figure 5.14, the PEO chains are placed at a Van der
Waals distance from the RuC13 layers, i.e., it is in touch with RUC13. The
Van der Waals thickness of a layer of OL-RuCl3 is 5.73(3) A [6a]. According
to the crystal structure of zigzag PEO [291, the distance of the outmost C
atom in PEO to the Van der Waals boundary of the molecule should be
about 1.73 A. Therefore, the distance of the C atoms closer to RuCl3
should be 4.60 A from the Ru layer, and the distance of the O atoms to the
middle of the gallery is 1.20 A. As shown in Figure 5.13(a), the 1-D ED
map profile of the model matches well the profile calculated from
experimental data in all regions except the center of the gallery.

In the center of the gallery, the electron density of the model is
obviously lower than that from the experiment due to the fact that
disordered water molecules could accumulate around the lithium ions in the
free space between the PEO layers. The fact that all the O atoms in the
PEO face towards the center of the gallery makes this region hydrophilic,
suggesting that the Li+ ions may reside there. The width of this hydrophilic
region is different in Lix(PEO)yRuCl3 and Lio,2(PEO)xTaSz models, and
depends on the amount of water molecules present in it.

Electron density due to the Li+ ions themselves is probably not
observable in these ED maps due to insufficient quantities of this very light
metal in the material. The validity of the argument that the lithium ions
could be in the center of the gallery was examined using a control
experiment in which the Li+ ions were replaced with Ag+ ions. We
reasoned that because both ions are single charged the heavier Ag+ may

behave similarly to Li+ in the nanocomposite and at the same time

224

 

 

 

substantially contribute to the diffraction pattern. The 1-D ED map of the
Agx(PEO)yRuC13 sample indicated substantially higher electron density in
the central region, see Figure 13(e). (In the model for Figure 13(e), 0.2
equivalent of Ag+ ions are put in the center of the gallery. An electron
density map calculated from the experimental data using a model without
the Ag+ ions also shows the central peak.) This supports the argument that
the ions occupy the center of the gallery.

There exists some similarity between the present model and the
structure of PEO-NaSCN complex, in which PEO also has a conformation
similar to that found in type II PEO-HgClz complex [34]. In PEO-NaSCN,
the chains are arranged with the CH2CH2 units in proximity with the S
atoms of N aSCN, which are less hydrophilic, whereas 0 atoms coordinate
with the N a”r ions, which are much more hydrophilic. This environment of
PEO is reminiscent of the present Lix(PEO)yRuC13 model. In addition, this
particular form of PEO-NaSCN complex exists under high tension and
converts to another form with a helical PEO conformation if the tension is
released. In the Lix(PEO)yRuC13 case, the polymer chains extend
themselves over a 2-D environment, which of course represents a form of
tension as well.

The proposed Lix(PEO)yRuC13 model matches well the observed l-D
ED map and is geometrically and chemically reasonable. This model is
different from the one proposed for Li/PEO/montmorillonite
nanocomposite in which the Li+ ions are thought to be located near the
surface oxides of the silicate layer [421. Presumably the lower affinity of Li+
for C1' causes its migration in the middle of the gallery which presents an
oxygen rich coordination environment. Such a PEO arrangement should

provide a good diffusion environment for ions in directions parallel to the

225

 

 

layers, which could make Lix(PEO)yRuCl3 a good ion conductor, perhaps
better than the amorphous PEO-(alkali-metal) complexes that have

segments of helical PEO chains in the structure [43].

7. PEO Conformation from IR Spectra

IR spectroscopy may not be used as a decisive technique to probe the
PEO conformation in a nanocomposite. The weakness of IR spectroscopy
can be seen from the uncertainty of the predictions. The region from 800-
1000 cm-1 of the IR spectra is supposed to be sensitive to the conformation
of PEO chains. A trans O-C-C-O would have absorption peaks around 773
and 992 cm-1, while a gauche O-C-C-O would have absorption around 880
and 944 cm-1 [44]. However, all PEO conformations known have only
absorptions around 850 and 950 cm-1, even those in type I PEO-HgC12
complex [30b] and PEO-p-nitrophenol complex [33b], in which the trans O-C-
C-O exists.

Some controversy in the PEO IR peak assignment arises from the
disagreement among the type I PEO-HgC12 IR data reported [303, b]. The
peaks observed at 1324, 1309, 1014 and 815 cm-1 [303] were assigned to the
trans O-C-C-O conformation [44, 260], because there were no corresponding
peaks in the IR spectra of bulk PEO. The more recent and dependable IR
spectra of type I PEO—HgClz [30b] did not have peaks at 1309, 1014 and 815
cm-1, while the spectra of type II PEO-HgC12 had peaks at 1312 and 1015
cm-1 [30b]. Recalling the fact that the type I PEO-HgC12 converts to type II
PEO-HgClz spontaneously [30b], one might suspect that the type I PEO-
HgClz sample used by Blumberg and Pollack [3031 contained type II PEO-
HgClz. If this is so, 1309 and 1014 cm-1 do not belong to trans O-C-C-O

conformation. Therefore, 1324 cm-1 may be the only peak that can be

226

 

 

assigned to the trans O-C-C-O conformation, as used in some references
[26h 6’ H. The spectrum of Lix(PEO)yRuCl3 does not have a peak near 1324
cm-1, see Figure 5.5(c), which suggests that no trans O-C-C-O
conformation exists. This agrees with the choice of the type II PEO-HgClz
conformation proposed in the previous section.

Absorption peaks of Lix(PEO)yRuC13 are compared in Table 5.2
with those of helical PEO, type I PEO-HgClz and type II PEO-HgClz read
from the spectra in Reference 30b [45]. Because of the poor resolution of
the IR spectrum of Lix(PEO)yRuCl3, some nearby peaks may merge and
some weak peaks may not show. In regions 11 and VII, the peaks of
Lix(PEO)yRuCl3 match those of type II PEO-HgClz. In region IV, the
broad peak at 1094 cm-1 matches the peak 1100 cm-1 of the type II PEO-
HgClz. The peak is assigned to C-0 stretching and its slight shift to the
lower frequency is caused by the coordination of O atoms to Li+ ions. The
peak at 1154 cm-1 in type II PEO-HgClz is weak, so it does not appear in
the Lix(PEO)yRuCl3 spectrum. Type II PEO-HgClz has a pair of peaks in
both regions IH and V, while Lix(PEO)yRuCl3 has only one peak in each of
them. In type II PEO-HgClz spectra, the two peaks in each pair have a
difference of 31 cm'1 or less. Each pair might merge to give a single peak
in the nanocomposite spectrum.

The absorption of Lix(PEO)yRuCl3 in regions I and V1 is more
different from that of type II PEO-HgClz than in other regions. The peaks
of Lix(PEO)yRuC13 at 1465, 1455 and 950 cm-1, which are close to those of
the helical PEO, may come from PEO chains dangling outside the layers.
(Other absorption peaks of free PEO chains may be buried in the
absorption of Lix(PEO)yRuC13.) This explanation satisfies region I. In
region VI, the absence of the 892 and 944 cm-1 peak in the Lix(PEO)yRuCl3

227

 

 

 

Table 5.2. A comparison of IR absorptions (cm-1) of PEO

 

 

 

 

 

 

 

 

 

 

 

 

 

Lix(PEO)1_5RuC13 pure PEO PEO.HgC12(I) PEO.HgClz(II) region
(x~0.5)
1465 (w) 1462 (s) 1468 (s)
1455 (w) 1456 (w) 1451 (s) I
1445 (w) 1445 (s)
1352 (w) 1355 (w) 1350 (s) 1356 (m) 11
1303 (w) 1338 (s) 1328 (m) 1312 (s)
1278 (s) 1278 (s)
1262 (m) 111
1248 (w) 1246 (s)
1237(m) 1239 (s)
1143 (vs) [46] 1154 (m) IV
1094 (vs) 1110 (vs) [46] 1110 (vs) 1100 (vs)
1060 (sh) 1064 (s)
1057 (s) 1044 (s) 1046 (vs) V
1031 (m) 1032 (s) 1015 (vs)
950 (m) 950 (s) 945 (s) 944 (w)
920 (sh) 923 (5) VI
892 (m)
876 (s)
846 (m) 859 (m) VII
837 (s) [46] 831 (In) 835 (s)

 

228

 

spectrum might be explained by its low intensity. The absorption in this
region is contributed mostly by the 950 cm-1 peak of the free PEO chains
and the 923 cm-1 peak of the type H PEO-HgClz conformation. In any case,
the IR absorption profile of Lix(PEO)yRuC13 matches the spectrum derived
from a superposition of the absorption peaks of type II PEO-HgClz

conformation and free PEO chains.
Concluding Remarks

The stability of oc-RuCl3 in acidic or basic aqueous solutions as well
as reducing and oxidizing conditions makes it a suitable compound for
intercalation reactions. This is rare among transition-metal halides. The
intercalation of oc—RuCl3 with the conducting polymers PANI and PPY and
the water soluble polymers PEO, PVP and PEI gives a new class of
lamellar (metal halide)/polymer intercalative nanocomposites. The
synthesis of these nanocomposites is enabled mainly by the successful
exfoliation of oc-RuClg after controlled lithiation, which allows the use of
the exfoliation-encapsulative precipitation method. On the other hand, 0L-
RuC13 is also one of the few layered hosts that are suitable for in situ redox
intercalative polymerization, which provides an additional approach for
new nanocomposites.

oc-RuClg nanocomposites contain reduced layers in which free
electron hopping, associated with Ru2+/Ru3+ couples, raise the electrical
conductivity by 2—3 orders of magnitude. The intercalation of conducting
polymers can further increase the conductivity. The dominant carriers in
the reduced RuCl3x’ layers are holes residing in a narrow tzg type band.

The reduction of oc-RuC13 and its polymer intercalation profoundly affect

229

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the intralayer and interlayer Ru3+ magnetic couplings, so that new magnetic
properties appear in the nanocomposites. These magnetic properties can be
adjusted by oxidation or other chemical manipulations.

Based on X-ray scattering and IR spectroscopy, a structural model is
proposed for Lix(PEO)yRuCl3, in which each gallery contains two layers of
PEO chains in a conformation found in type II PEO—HgClz. The Li+ ions

seem to reside exactly in the middle of the interlayer space sandwiched

between two monolayer of PEO. The model suggests that Lix(PEO)yRuC13

could have good two-dimensional ion conductivity.

230

References

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231

10

ll

l2

l3

14

15

16

I. Lagadic, A. Léaustic and R. Clement, J. Chem. Soc., Chem.
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(a) R. Schollhom, R. Steffen and K. Wagner, Angew. Chem. Int. Ed.
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In situ redox intercalative polymerization has been successfully
applied to FeOCl, V205 and VOPO4: (a) M. G. Kanatzidis, L. M.

Tonge, T. J. Marks, H. O. Marcy and C. R. Kannewurf, J. Am.
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The method of encapsulative precipitation from solutions of
exfoliated lamellar solid has be applied to M082, M003, TaSZ and

NbSe2: (a) R. Bissessur, M. G. Kanatzidis, J. L. Schindler and C. R.

Kannewurf, J. Chem. Soc., Chem. Commun., 1993, 1582. (b) L.
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The method of in situ polymerization coupled with encapsulative
precipitation has been applied to MoS2, M003 and W82: (a) L. Wang,
J. L. Schindler, J. A. Thomas, C. R. Kannewurf, and M. G.
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L. F. Nazar Chem. Mater. 1996, 8, 2005. (c) Chapter 2.

G. Grauer, Handbook of Preparative Inorganic Chemistry, Vol. 2,
Academic Press Inc., New York, 1965, pp 1597.

TGA indicated a total loss of 48.2% in oxygen flow at temperatures
up to 650 0C. By comparing with the TGA results of oc-RuCl3 under

the same condition which had losses of 32.85 and 33.07% in two
trials, the amount of organics and water inside the nanocomposite
was determined. The amount of intercalates inside LiX(PEO)yRuCl3,

Lix(PVP)yRuCl3 and Lix(PEI)yRuC13 was also determined this way.

232

 

 

 

 

17

18

19

20

21

22

23

24

25

26

C.-G. Wu, D. C. DeGroot, H. O. Marcy, J. L. Schindler, C. R.
Kannewurf, T. Bakas, V. Papaefthymiou, W. Hirpo, J. P.
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Due to the preferential orientation of the layers on the sample
holder, the reflection-mode powder XRD patterns show
predominantly the 001 reflections. The transmission-mode powder
XRD patterns show mainly hkO reflections.

Polypyrrole exhibits peaks at 1540, 1300, 1150, 1040 and 900 cm-1:
E. T. Kang, K. G. Neoh, T. C. Tan and Y. K. Ong, J. Macromol.
Sci. -Chem. 1987, A2_4(6), 631.

The (PPY)xMoS2, prepared under similar conditions, has an x about
0.5. Considering that the unit area of RuCl3 is 1.75 times that of
MoS2, the amounts of polypyrrole in the galleries are quite
comparable. (A M082 unit cell has one MoS2; its a dimension is
3.159 A. A RuCl3 unit cell has 2 RuC13; its a dimension is 5.87 A.)

N. Toshima and O. Ihata, Synth. Met. 1996, B, 165.

The choice of solvent is important, because a pyrrole/CH3CN

solution significantly reduces the rate of the reaction and does not
produce single-phase (PPY)xRuCl3 even after one month.

L. Binotto, I. Pollini and G. Spinolo, Phys. Stat. Sol. (B), 1971, g,
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Y. Kobayashi, T. Okada, K. Asai, M. Katada, H. Sano and F. Ambe,
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The literature value was 2.25 LLB, see Reference 24.

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1992, M, 6239. (b) P. Aranda and E. Ruiz-Hitzky, Chem. Mater.
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233

 

 

 

27

28

29

30

31

32

33

34

35

36

37

38

113. (g) T. A. Kerr, H. Wu and L. F. Nazar, Chem. Mater. 1996, 8,
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M. Yokoyama, H. Ishihara, R. Iwamoto and H. Tadokoro,
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234

 

 

 

39

40

41

42

43

44

45

46

Since the calculated l-D ED maps for the models with zigzag PEG
and type I PEO-HgC12 conformations are similar to those for their
Li0.2(PEO)xTaS2 anologs, Figure 5.13 shows only those for the
models with type II PEO-HgC12 and double helical PEO

conformations.

This conformation was also suggested for poly(ethylene glycol)
(PEG) in kaolinite/PEG nanocomposites [263], because its hydrophilic
side has higher affinity for the highly polar gibbsite Al(OH)3 side

and its hydrophobic side suits the not so polar tetrahedral Si02 side.

This value is estimated from a (cos20/sin0 + cos20/0) correction: H.
P. Klug and L. E. Alexander, X-ray Diflraction Procedures (for
polycrystalline and amorphous materials), 2nd ed., John Wiley &
Sons, New York, 1974, pp 594.

D.-K. Yang and D. B. Zax, J. Chem. Phys. 1999, 110, 5325.

Preliminary measurements done by Dr. Jin-Ho Choi’s group at Seoul
National University, Korea, demonstrate that the ion conductivity of

Lix(PEO)yRuCl3 at 20 0C is 6.3x10'5 S/cm, equal to or better than the

best (lithium salt)/polymer electrolytes: F. M. Gray, Solid Polymer
Electrolytes, VCH Publisher, New York, 1991, Chapter 5, pp 83.

B. L. Papke, M. A. Ratner and D. F. Shriver, J. Phys Chem. Solids
1981, Q, 493.

Reference 30b provides the spectra of two orientations with the
electric vector perpendicular and parallel to the fiber axes. An un-
oriented spectrum should be close to the superposition of the two
spectra.

The peaks around 1148, 1112 and 851 cm“1 are characteristic of the

helical PEO conformation: H. Matsuura and T. Miyazawa,
Spectrochim. Acta 1967, 23_A, 2433.

235 '

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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