ELECTROCHEMISTRY OF NANOSTRUCTURED CARBON
MATERIALS IN AQUEOUS ELECTROLYTES AND
ROOM TEMPERATURE IONIC LIQUIDS
By
A DISSERTATION
Submitted to
Michigan
State University
in
partial fulfillment of the
requirements
for the degree of
Chemistry
-
Doctor of Philosophy
201
9
ABSTRACT
ELECTROCHEMISTRY OF NANOSTRUCTURED CARBON MATERIALS IN
AQUEOUS ELECTROLYTES AND ROOM TEMPERATURE IONIC LIQUIDS
By
Romana Jaro
Carbon is
one of the most plentiful elements on the planet.
From a materials perspective,
carbon
is unique because of the microstructurally distinct allotropes it forms. These include
single and polycrystalline diamond,
diamond
-
like carbon,
glassy carbon, and grap
hite.
All of
these carbon materials are commonly used in electroanalysis, energy storage and conversion,
separation, and chemical analysis, due to numbers of reasons, including low cost
, high
mechanical strength, wide usable potential range, rich surface c
hemistry, chemical inertness,
and compatibility with a variety of solvents and electrolytes.
For the optimal usage of carbon
electrodes in electrochemistry,
it is critical to fully understand and control the variables that
impact
background voltammetric cu
rrent, capacitance, and
heterogeneous electron
-
transfer
kinetics
at these materials. Over the years
of carbon electrode usage in electrochemistry
, much
knowledge has been gained about the
structure
-
function relationship at sp
2
-
and sp
3
-
bonded
carbon elect
rodes. Nevertheless, as most of this knowledge pertains to aqueous electrolyte
solution, there is still a significant gap about the properties of the electric double layer and the
transport processes near the electrode interface in room temperature ionic l
iquids.
The room temperature ionic liquids are solvent
-
free medium, composed purely of ions,
with a melting point near or below room temperature. In electrochemistry, they are appreciated
for several of their excellent properties, such as wide working pot
ential window, moderate
electrical conductivity, high thermal and chemical stability, negligible vapor pressure etc.
As
the RTILs does not contain any solvent, their interfacial structure at an electrified interface
is significantly distinguished from the
conventional Gouy
-
Chapman
-
Stern model describing the
double layer in aqueous solutions. Additionally, also the redox analyte environment in RTILs
is expected to different compared to those in aqueous solutions.
The work in this dissertation thesis is focu
sed on the electrochemical performance
of
microstructurally
different carbon electrodes in aqueous electrolytes and room temperature
ionic liquids. The physical, chemical and electronic properties of glassy carbon, boron
-
doped
diamond, and tetrahedral amor
phous carbon electrodes are discussed. Furthermore, the
microstructure of carbon electrodes
is
correlated to the heterogeneous electron transfer rate
constants of soluble inorganic and organic redox couples in aqueous electrolytes and room
temperature ioni
c liquids.
The attention was primarily focused on tetrahedral amorphous carbon electrode that can
be doped with nitrogen, resulting in significant physical, chemical an electrochemical
properties. Moreover, the electrode surface chemistry was alternated b
y an oxygen plasma
modification and its effect on the electrochemical performance (background voltammetric
current, capacitance and electron transfer kinetics), as well as potential surface damage was
studied.
Lastly, as it is believed that the nitrogen i
ncorporated tetrahedral amorphous material
possesses an equally superb properties compared to the boron doped diamond, both electrode
materials were used for determination of endocrine disruption compounds, specifically estriol,
estradiol and estrone, usin
g high
-
pressure liquid chromatography with electrochemical
detection. The detection figures of merit for both boron doped diamond and nitrogen
-
incorporated tetrahedral amorphous carbon thin
-
film electrodes were determined.
iv
To my grandma
I wish more than anything you could be
here and see that I made it!
v
ACKNOWLEDGEMENTS
Where would I be, and what would I be without you?
I remember my first day at MSU very clearly. I entered the chemistry building and saw
a big poster
an invitation to a seminar called
. I kept thinking about it for the rest of the day. Leaving the ch
emistry building later
that evening, being happy with my new project, I
was sure that
hard work with a little luck
always leads into success. And I thought that if I keep working hard, I can
achieve
whatever I
want. Now, 5 years later, when finishing this
thesis, I know how wrong I was. What really drove
my success was an amazing combination of several factors, that I have found in
P
rofessor
I would never reach this point without Professor Greg Swain. There are many chemistry
professors
all a
round the world, but ther
e is only one who has done so many things for me.
Sometimes, he pushed me so hard until I felt that I want to pack my baggage and give up. But
he was always there, watching me and ready to offer his hand when he thought I was falling.
He was there when my research went well and
so he could say
He was also there when my research did not go anywhere; ready to speak with me, to cheer me
up, and to give me
a new point of view and some ideas
. He inspired me and motivated me every
time when I felt that I am not good enough to become a
scientist. Thank you,
Eye
, for
everything
I would never make it without YOU.
People do not always live a
sunny and shiny
dream, as there are some darker moments in
eve
de everything better. Thank you for being there! Coming to
vi
the lab every day, knowing that I can share my happiness, problems, or just to speak with you,
made the grad school so much better. You have taught me so many things an
d I will always
remember our
coffee breaks,
little lab dramas,
wine parties,
making Christmas decorations, or
just our normal lab days... I truly hope to stay in touch with you
and please do not forget the
There are so many people that
des
erve an acknowledgement, as they made my research
possible! Some of them gave me many valuable ideas, provided electrodes, trained me, or
simply gave me an opportunity to collaborate with them and to learn something new. Thank
you, Prof. Gary Blanchard, Pr
of. Tim Grotjohn, Dr. Andreas Lesch, Dr. Lars Haubold, and
many others.
Dr. Kathy Severin, thank you for choosing me as your TA. Working with you gave me
a completely new perspective and I have found a hidden passion inside of me. You are an
amazing mentor
and your guidance was one of the most important experiences I have got here
at MSU.
Thank you, Bob, Heidi, LuAnn
you were
always
so
nice to me
and willing to help
with all my
little problems! I will miss seeing you around!
And thank you, Lucas, for
keeping up with me during this
difficult process!
A
nd this is what dr
ove
a success in
research.
THANK YOU!
vii
TABLE
OF CONTENT
S
LIST OF TABLES
................................
................................
................................
.....................
x
LIST OF FIGURES
................................
................................
................................
..................
xii
CHAPTER 1. INTRODUCT
ION
................................
................................
..............................
1
1.1. Carbon Electrodes
................................
................................
................................
...............
1
1.1.1. Glassy Carbon
................................
................................
................................
......
1
1.1.2. Boron
-
Doped Diamond
................................
................................
........................
3
1.1.3. Tetrahedral Amorphous Carbon
................................
................................
...........
5
1.1.3.1. Nitrogen Incorporation into the Tetrahedral Amorphous Carbon
.................
7
1.2. Characterization of Car
bon Electrodes
................................
................................
...............
8
1.2.1. Raman Spectroscopy
................................
................................
............................
8
1.2.2. X
-
ray
Photoelectron Spectroscopy
................................
................................
.....
11
1.3. Room Temperature Ionic Liquids (RTILs)
................................
................................
.......
13
1.4. Research Objectives and Specific Aims
................................
................................
............
17
REFERENCES
................................
................................
................................
.........................
20
CHAPTER
2. EXPERIMENTAL METH
ODS
................................
................................
........
31
2.1. Electrode Preparation
................................
................................
................................
........
31
2.1.1. Glassy Carbon Electrode
................................
................................
....................
31
2.1.2. Boron
-
Doped Nanocrystalline Diamond Thin Films
................................
.........
31
2.1.3. Nitrogen
-
Incorporated Tetrahedral Amorphous Carbon Thin Films
.................
32
2.2. Physi
cal and Chemical Characterization of Electrodes and RTILs
................................
..
33
2.2.1. Visible Raman Spectroscopy
................................
................................
.............
33
2.2.2. Atomic Force Microscopy
................................
................................
..................
34
2.2.3. Scanning Electron Microscopy
................................
................................
..........
34
2.2.4. X
-
ray Photoelectron Spectroscopy
................................
................................
.....
35
2.2.5. Contact Angle Measurement
................................
................................
..............
35
2.2.6. Fourier Transform Infrared Spectroscopy
................................
..........................
35
2.3. Electrochemical Characterization
................................
................................
.....................
36
2.3.1. Cyclic Voltammetry
................................
................................
...........................
36
2.3.2. High
-
Pressure Liquid Chromatography
................................
.............................
38
2.4. Reagents
................................
................................
................................
............................
40
2.4.1. Electrochemical Measurements in Aqueous Electrolytes
................................
..
40
2.4.2. Electrochemical Measurements in
Ionic Liquids
................................
...............
40
2.4.3. Water Samples for the HPLC Estrogen Compound Analysis
............................
42
REFERENCES
................................
................................
................................
.........................
43
viii
CHAPTER 3. ASSESSMEN
T OF HETEROGENEOUS E
LECTRON
-
TRANSFER RAT
E
CONSTANT FOR
SOLUBLE REDOX ANALYT
ES AT TETRAHEDRAL AM
ORPHOUS
CARBON, BORON
-
DOPED
DIAMOND AND GLASSY C
ARBON ELECTRODES
........
46
3.1. Abstract
................................
................................
................................
.............................
46
3.2. Introduction
................................
................................
................................
.......................
46
3.3. Results
................................
................................
................................
...............................
50
3.3.1. Material Characterization
................................
................................
...................
50
3.3.2. Background Cyclic Voltammetric Curves
................................
.........................
53
3.3.3. Cyclic Voltammetric Curves for Different Redox Analytes
..............................
57
3.3.4. Heterogeneous Electron
-
Tran
sfer Rate Constants
................................
.............
61
3.3.5. Electrolyte Effects on the Heterogeneous Electron
-
Transfer Rate Constant
.....
64
3.4. Discussion
................................
................................
................................
.........................
67
3.5. Conclusions
................................
................................
................................
.......................
69
REFERENCES
................................
................................
................................
.........................
71
CHAPTER 4. RAPID PRE
PARATION OF ROOM TEM
PERATURE IONIC LIQUI
DS WITH
LOW WATER CONTENT AS
CHARACTERIZED WITH
A
t
a
-
C:N
ELECTRODE
..........
78
4.1. Abstract
................................
................................
................................
.............................
78
4.2. Introduction
................................
................................
................................
.......................
78
4.3. Results and Discussion
................................
................................
................................
......
81
4.3.1. Background Current and Potential Window
................................
......................
81
4.3.2. Effect of Water on the Voltammetric Respons
e for FCA
................................
..
84
4.3.3. Effect of Water on the Background Voltammetric Response
............................
86
4.4. Conclusion
................................
................................
................................
.........................
88
REFERENCES
................................
................................
................................
.........................
90
CHAPTER 5. TEMPERATU
RE DEPENDENCE OF THE
HETEROGENEOUS
ELECTRON
-
TRANSFER RA
TE CONSTANT AND DIFF
USION COEFFICIENT FO
R
FERROCENE CARBOXYLIC
ACID
................................
................................
.....................
94
5.1. Abstract
................................
................................
................................
.............................
94
5.2. Introduction
................................
................................
................................
.......................
94
5.3. Results and Discussion
................................
................................
................................
....
100
5.3.1. Background Current Capacitance
................................
................................
.....
100
5.3.2. Cyclic Voltammetric Studies of FCA as a
Function of Temperature
..............
104
5.3.3. Rate Constants and Activation Energies for Electron Transfer as a Function of
Temperature
................................
................................
................................
...............
111
5.4. Discussion
................................
................................
................................
.......................
115
5.5. Conclusions
................................
................................
................................
.....................
119
REFERENCES
................................
................................
................................
.......................
121
CHAPTER 6. EFFECT OF
THE NITROGEN CONTEN
T AND OXYGEN PLASMA
TREATMENT ON THE HET
EROGENEOUS ELECTRON
-
TRANSFER KINETICS AT
THE TETRAHEDRAL AMOR
PHOUS CARBON ELECTRO
DES IN AQUEOUS
ELECTROLYTES AND ROO
M TEMPERATURE IONIC
LIQUIDS
...............................
129
6.1. Introduction
................................
................................
................................
.....................
129
ix
6.2. Results and Discussion
................................
................................
................................
....
1
33
6.2.1. The Nitrogen Content Effect
................................
................................
............
133
6.2.1.1. Characterization of t
a
-
C(N)
x
Electrode Surfaces
................................
......
133
6.2.1.2. Electrochemical Properties of t
a
-
C(N)
x
Thin Films
................................
..
144
6.2.2 Surface Modification by RF Oxyge
n Plasma Treatment
................................
..
158
6.3. Conclusions
................................
................................
................................
.....................
170
REFERENCES
................................
................................
................................
.......................
173
CHAPTER 7. HPLC
-
ED A
NALYSIS OF ESTROGENI
C COMPOUNDS: A
COMPA
RISON
OF AN ANALYTICAL PER
FORMANCE OF DIAMOND
AND TETRAHEDRAL
AMORPHOUS CARBON ELE
CTRODE PERFORMANCE
................................
..............
178
7.1. Introduction
................................
................................
................................
.....................
178
7.2. Results and Discussion
................................
................................
................................
....
181
7.3. Conclusions
................................
................................
................................
.....................
190
REFERENCES
................................
................................
................................
.......................
191
CHAPTER 8. CONCLUSIO
NS AND FUTURE DIRECT
IONS
................................
..........
196
x
LIST OF TABLES
Table 1.1.
Molecular structure and viscosity of RTILs used in this study.
.............................
15
Table 3.1.
Slopes of background current (0.2 V)
-
scan rate plots and the calculated electrode
capacitance for GC, BDD, and ta
-
CN electrodes in 1 mol L
1
KCl.
................................
.......
56
Table 3.2.
H
eterogeneous electron
-
transfer rate constants for different redox systems at t
a
-
C:N,
BDD, and GC electrodes.
................................
................................
................................
.........
63
Table 5.1.
Physical property data for the ionic liqu
ids at 298 K (25
o
C).
.............................
100
Table 5.2.
Experimental cyclic voltammetric data and calculated diffusion coefficients for
0.5
mmol L
-
1
FCA in [BMIM][BF
4
] at a t
a
-
C:N ele
ctrode as a function of temperature.
.....
108
Table 5.3.
Diffusion coefficients and heterogeneous electron
-
transfer rate constants for FCA
in [EMIM][BF
4
] and [BMIM][BF
4
] at different temperatures for the three distinct carbon
electrodes.
................................
................................
................................
...............................
113
Table 6.1.
Summary of XPS data for the t
a
-
C(:N)
x
films deposited with a flow of 0, 10, 30 and
50 sccm of N
2
gas flow.
................................
................................
................................
.........
135
Table
6.2.
Raman spe
ctral parameters for t
a
-
C and t
a
-
C:(N)
x
electrodes.
............................
139
Table
6.3.
Physical properties of t
a
-
C and t
a
-
C(:N)
x
electrodes.
................................
..........
141
Table 6.4.
Calculated capacitance values for t
a
-
C and t
a
-
C:N thin
-
films deposited with various
N
2
gas flows (10, 30 and 50 sccm). GC and BDD electrode data incl
uded for comparison. (n=3
electrodes each)
................................
................................
................................
......................
145
Table 6.5.
Summary of cyclic voltammetric data for Fe(CN)
6
-
3/
-
4
, Ru(NH
3
)
6
+3/+2
, Ferrocene,
FCA, and FeMeOH
in1M KCl me
asured at t
a
-
C:(N)
x
electrodes (n=3).
...............................
150
Table 6.6.
Summary of cyclic voltammetric data for FCA, FeMeOH and ferrocene in KCl,
[BMIM][OTF], [BMIM][BF
4
], and [BMIM][
PF
6
] measured at t
a
-
C:N 30 sccm electrodes
(n=3).
................................
................................
................................
................................
......
155
Table
6.7.
Raman spectral parameters for as grown and after oxygen plasma modified t
a
-
C:N
(30 sccm)
electrodes (n = 10 000).
................................
................................
........................
160
Table 6.8.
Calculated capacitance (cyclic voltammetric current at 0.2 V) for t
a
-
C and t
a
-
C:N
thin
-
film electrodes before and after O
2
plasma treatment. The films used were grown with 0,
10, 30 and 50 sccm N
2
added. (n = 3).
................................
................................
...................
164
xi
Table 6.9.
Summary of cyclic voltammetric data for Fe(CN)
6
-
3/
-
4
and Ru(NH
3
)
6
+3/+2
in 1 M KCl
measured at t
a
-
C:(N)
x
electrodes before (grey background) and after (
white background) O
2
plasma modification (n = 3 electrodes of each type).
................................
............................
166
Table 6.10.
Summary of cyclic voltammetric data for Fe(CN)
6
-
3/
-
4
and Ru(NH
3
)
6
+3/+2
in 1 M
KCl and different RTILs measured at t
a
-
C:(N)
x
electrodes before (grey background) and after
O
2
plasma
(white background) treatment (n = 3 electrodes of each type).
............................
169
Table 7.1
. Short term response reproducibility over 30 consecutive injections of the estrogen
meta
bolite solution recorded at t
a
-
C:N and BDD thin
-
film electrode.
................................
..
185
Table 7.2
. Summary of the analytical detection figures of merit for the estrogenic compounds
at
t
a
-
C:N and BDD thin
-
film electrodes.
................................
................................
...............
188
Table 7.3.
Recovery studies of estriol, estradiol and estrone in three different water samples
a
-
C:N.
................................
................................
.............
189
xii
LIST OF FIGURES
Figure 1.1
. Structure of various carbon materials: (A) graphite, (B) glassy carbon (C) diamond
and (D)
tetrahedral amorphous carbon. (Reprinted with a permission obtained from The
Springer Nature).
19
................................
................................
................................
.....................
2
Figure 1.2
. Amorphous carbon ternary phase diagram. The corners represent graphite, diamond
and hydrocarbons. (Reprinted with a permission obtained from The Royal Society)
...............
5
Figure 1.3.
Double layer in aqueous solutions (
A
) and room temperature ionic liquid (
B
).
..
17
Figure 2.1
.
The three
-
electrode single compartment glass cell.
................................
..............
36
Figure 2.2.
Diagram of the thin
-
layer, cross
-
flow electrochemical detection cell.
.................
39
Figure 3.1.
SEM images of (
A
) a boron
-
doped nanocrystalline diamond (BDD) film and (
B
) a
nitrogen
-
incorporated tetrahedral amorphous carbon (t
a
-
C:N) thin film. Notice the difference
in scale bars on the two images: 1 µm for B
DD and 0.1 µm for t
a
-
C:N.
................................
50
Figure 3.2.
Visible micro
-
Raman spectra for a BDD thin
-
fi
lm (black) and a t
a
-
C:N thin
fi
lm
(red). Integration time
=
10 s. Excitation wavelength
=
5
32 nm.
................................
.............
51
Figure 3.3.
Background cyclic voltammetric
i
-
E
curves for GC, BDD and t
a
-
C:N electrodes in
1
mol L
-
1
KCl. Scan rate = 0.1 V s
-
1
.
................................
................................
........................
53
Figure 3.4
. Background cyclic voltammetric
i
E
curves for (A) BDD, (B) ta
-
C:N, and (C) GC
electrodes as a function of scan rate in 1 mol L
1
KCl. Scan rates of 0.1, 0.2, 0.3, 0.4, and
0.5
V
s
1
are shown.
................................
................................
................................
.................
54
Figure 3.5.
Cyclic voltammetric background current (positive or anodic) at 0.2 V as a function
of the scan rate for BDD, t
a
-
C:N, and GC electrodes i
n 1 mol L
-
1
KCl.
................................
55
Figure 3.6.
Cyclic voltammetric
i
E
curves for 0.1 mmol L
-
1
Fe(CN)
6
in 1 mol L
-
1
KCl at
(
A
) t
a
-
C:N, (
B
) BDD, and (
C
) GC electrodes at varying scan rates (0.1, 0.2, 0.3, 0.4,
and
0.5
V
s
-
1
).
(
D
)
Corresponding Randles
Sevcik plots of
i
p
ox
and
i
p
red
vs.
1/2
calculated from
i
E
curves for each of the three electrodes. R
2
D.
................................
................................
................................
................................
..............
57
Figure 3.7.
Cyclic voltammetric
i
E
curves for 0.1 mmol L
1
Ru(NH
3
)
6
3+/2+
in 1 mol L
1
KCl
at (
A
) t
a
-
C:N, (
B
) BDD, and (
C
) GC electrodes at varying scan rates (0.1, 0.2, 0.3, 0.4,
and
0.5
V
s
1
). (
D
) Corresponding Randles
Sevcik plots of
i
p
(ox) and
i
p
(red) vs.
1/2
calculated
from current
potential curves for each of the three electrodes. R
2
shown with the data in D.
................................
................................
................................
.........
59
xiii
Figure 3.8.
Cyclic voltammetric
i
E
curves for 0.1 mM methyl viologen, Ru(NH
3
)
6
3+/2+
,
Fe(CN)
6
3
-
/4
-
, FCA and IrCl
6
2
-
/3
-
, all in 1 mol L
1
KCl, at a t
a
-
C:N thin
-
film electrode. Scan rate
= 0.1 V s
1
.
................................
................................
................................
...............................
60
Figure 3.9.
Cyclic voltammetric
i
E
curves for (
A
) Fe(CN)
6
and (
C
) Ru(NH
3
)
6
3+/2+
,
in 1
mol L
1
KCl at a t
a
-
C:N thin
-
film electrode as a function of the analyte concentration from 0.1
to 1 mmol
L
1
. (
B
)
Plots of
i
p
(ox) and
i
p
(red) as a function of the Fe(CN)
6
solution
concentration.
R
2
1
.
................................
................................
......
61
Figure 3.10.
Experimental and digitally simulated cyclic voltammetric
i
E
curves for
(
A
)
0.1
mmol
L
1
Fe(CN)
6
and (
B
) 0.1 mmol L
1
Ru(NH
3
)
6
3+/2
+
, both in 1 mol L
1
KCl.
Scan rate = 0.1 V s
1
redox molecules.
................................
................................
................................
.......................
64
Figure 3.12.
Cyclic voltammetric
i
E
curves for (
A
)
0.1 mmol L
-
1
Ru(NH
3
)
6
3+
, and (
B
)
0.1
mmol
L
-
1
Fe(CN)
6
4
-
in 0.1 mol L
-
1
LiCl, NaCl, KCl, and CsCl at a t
a
-
C:N thin
-
film
electrode. Scan rate = 0.1 V s
-
1
. Geometric area = 0.2 cm
2
.
................................
....................
65
Figure 3.11.
Background cyclic voltammetric
i
E
curves for (
A
)
BDD, (
B
) t
a
-
C:N, (
C
) GC
electrodes in 0.1 mol L
-
1
LiCl, NaCl, KCl, and CsCl. Scan rate = 0.1 V s
-
1
. Geometric area =
0.2
cm
2
.
................................
................................
................................
................................
.....
65
Figure 3.13.
Cyclic voltammetric
i
E
curves for (
A
)
0.1 mmol L
-
1
Ru(NH
3
)
6
3+
, and (
B
)
0.1
mmol
L
-
1
Fe(CN)
6
4
-
in 0.1 mol L
-
1
LiCl, NaCl, KCl, and CsCl at a BDD thin
-
film electrode.
Scan rate = 0.1 V s
-
1
. Geometric area = 0.2 cm
2
.
................................
................................
.....
66
Figure 3.14.
Cyclic voltammetric
i
E
curves for (
A
)
0.1 mmol L
-
1
Ru(NH
3
)
6
3+,
and
(
B
)
0.1
mmol
L
-
1
Fe(CN)
6
4
-
in 0.1 mol L
-
1
LiCl, NaCl, KCl, and CsCl at a GC electrode. Scan rate
= 0.1 V s
-
1
. Geometric area = 0.2 cm
2
.
................................
................................
.....................
67
Figure 4.1.
Background cyclic voltammetric
curves for a t
a
-
C:N electrode in [E
MIM][BF
4
]
(A)
of intentional water contamination (10 wt. % = 10 ppt added) is also shown (dotted line). The
background voltammetric curves shown in
(B)
are for [EMIM
][BF
4
method with scans recorded over a narrow (solid line) and wide potential range (dashed line).
Scan rate = 0.1 V s
-
1
. Geometric area = 0.2 cm
2
.
................................
................................
.....
81
Figure 4.2.
Cyclic voltammetric
curves for a 0.5 mM ferrocene carboxylic acid (FCA) in
[EMIM][BF
4
] at a t
a
-
C:N electrode after (
A
) employing the vacuum drying (dotted) and
sweeping (solid line) purification methods. (
B
) demonstrating the effect of air humidity on the
electrochemical be
havior.
................................
................................
................................
.........
84
Figure 4.3.
Background cyclic voltammetric
i
E
curves for a t
a
-
C:N electrode in [EMIM][BF
4
]
rate = 0.1 V s
-
1
. Geometric area = 0.2 cm
2
.
................................
................................
..............
86
xiv
Figure
4.4.
(
A
) TGA mass change versus temperature profile during the heating of
[
EMIM][BF
4
B
) FTIR transmission spectrum for the
purified ionic liquid.
................................
................................
................................
.................
87
Figure 5.1.
Background cyclic voltammetric
i
E
curves for GC (red), BDD (black) and t
a
-
C:N (blue) electrodes in (
A
) [BMIM][BF
4
] and (
B
) [EMIM][BF
4
]. Scan rate 0.1 V s
-
1
.
Geometric area for all
three electrodes = 0.2 cm
2
.
................................
................................
.
101
Figure 5.2
. Background
cyclic voltammetric
i
E
curves for GC, BDD and t
a
-
C:N electrodes
in (
A
) [BMIM][BF
4
] and (
B
) [EMIM][BF
4
].
i
meas
at 0.4 V. Scan rate = 0.1 V s
-
1
. Geometric
area = 0.2 cm
2
.
................................
................................
................................
........................
103
Figure 5.3.
(
A
) Cyclic voltammetric
i
-
E
curves for a ta
-
C:N electrode showing the
effect of
scan rate on the response of 0.5 mmol L
-
1
FCA in [BMIM][BF4] at 338 K. Curves were recorded
at scan rates from 0.1 to 0.5 V s
-
1
. (
B
P
1/2
for 0.5 mmol L
-
1
FCA
in [BMIM][BF
4
] at the t
a
-
C:N electrode for various temp
), 323
................................
................................
................................
.............
104
Figure 5.4.
(
A
) Cyclic voltammetric
i
-
E
curves for a t
a
-
C:N electrode for different
concentrations of FCA in [BMIM][BF
4
] at 298 K. Curves are shown
for concentrations from
0.2 to 1.0 mmol L
-
1
. Curves were recorded at a scan rate of 0.1 V s
-
1
. (
B
) Plots of
i
P
a
and
i
p
c
vs.
concentration
for FCA in [BMIM][BF
4
]
.
................................
................................
..............
105
Figure 5.5.
Cyclic voltammetric
i
-
E
curves 0.5 mmol L
-
1
for FCA in (
A
) [EMIM][BF
4
]
and (
B
)
[BMIM][BF
4
] at a t
a
-
C:N electrode as a function of temperature. Scan rate = 0.3 V s
-
1
.
.....
107
Figure 5.6.
Cyclic voltammetric
i
-
E
curves for 0.5 mmol L
-
1
FCA in (
A
) [EMIM][BF
4
] and
(
B
) [BMIM][BF
4
] at GC and (
C
) [EMIM][BF
4
] and (
D
) [BMIM][BF
4
] at a BDD thin
-
film
electrode as a function of temperature. T
he scan rate was 0.3 V s
-
1
.
................................
....
109
Figure 5.7.
Cyclic voltammetric
i
-
E
curves for 0.5 mmol L
-
1
FCA in (
A
) [EMIM][BF
4
]
and (
B
)
[BMIM][BF
4
] at GC, BDD and t
a
-
C:N electrodes. Scan rate = 0.3 V s
-
1
. T = 298 K.
..........
110
Figure 5.8.
Cyclic voltammetric
curves for 0.5 mmol L
-
1
FCA in (
A
) [BMIM][BF
4
] and
(
B
) [EMIM][BF
4
] at a t
a
-
C:N thin
-
film electrode.
Experimental (solid line) and simulated
(circles) curves are presented.
Scan rate = 0.5 V s
-
1
, temperature = 338.15 K and area = 0.2 cm
2
.
Simulation parameters used were
D
red
= 2.3 x 10
-
7
cm
2
s
-
1
,
C
dl
= 2.2
µ
........
111
Figure 5.9.
Arrhenius plots of ln
k
0
app
versus 1000/
T
in (
A
) [BMIM][BF
4
] and (
B
)
[EMIM][BF
4
] in the range of 298.15
nd t
a
-
electrodes.
................................
................................
................................
...............................
114
Figure 6.1.
XPS spectra for t
a
-
C(:N)
x
thin films deposited with 0, 10, 30 and 50 sccm of N
2
added to the reaction chamber.
................................
................................
...............................
133
xv
Figure 6.2.
The C 1s core peak and its sub
-
peaks (sp
2
, sp
3
, C
-
OH, C=O, O=C
-
C) for t
a
-
C thin
-
film electrode grown with no added N
2
. The red line represents the measured raw data.
.....
134
Figure 6.3
. Atomic concentration of nitrogen in the t
a
-
C(:N)
x
thin films as a function of
nitrogen flow used during the deposition.
................................
................................
..............
136
Figure 6.4.
Raman spectra of t
a
-
C and t
a
-
C:(N)
x
electrodes with various nitrogen level.
....
137
Figure. 6.5
. Raman spectra deconvolution example for t
a
-
C:N 30 sccm. Red line represents the
measured raw data, blue line the D band, pink line the G band.
................................
............
138
Figure 6.6.
Variation of D to G peak intensity ratio with the nitrogen content for
t
a
-
C:(N)
x
thin
film electrodes (n = 10 000).
................................
................................
................................
..
138
Figu
re 6.7.
The variation of G band width with the stress (
A
) and the nitrogen flow rate (
B
) for
t
a
-
C:(N)
x
thin film electrodes (n = 10 000).
................................
................................
...........
140
Figure 6.8.
AFM images of t
a
-
C and t
a
-
C:(N)
x
thin
-
film electrodes deposited with (
A
) 50, (
B
)
30, (
C
) 10 and (
D
) 0 sccm of N
2
. Images were recorded
in the contact mode.
.....................
142
Figure 6.9.
Photographs of the
water droplet on t
a
-
C and t
a
-
C:(N)
x
thin films deposited with
(
A
) 0, (
B
) 10, (
C
) 30 and (
D
) 50 sccm of N
2
. Water drop volume = 1µL.
............................
143
Figure 6.10.
Background cyclic voltammetric
i
-
E
curves for a t
a
-
C electrode (black) and t
a
-
C:N electrodes deposited with 10 sccm (red), 30 sccm (gre
en) and 50 sccm N
2
(blue) in 1 M
KCl. Scan rate = 0.1 V s
-
1
. Geometric area = 0.2 cm
2
.
................................
..........................
144
Figure 6.11.
Capacitance
potential profiles for
t
a
-
a
-
C:N (
), 30 sccm t
a
-
C:N
(
), and 50 sccm t
a
-
C:N (
) in
1 M KCl in range from
-
1 to 1 V. Data normalized to the
geometric area = 0.2 cm
2
.
................................
................................
................................
.......
147
Figure 6.12.
Cyclic voltammetric
i
-
E
curves for a
(
A
) t
a
-
C, (
B
) 10 sccm t
a
-
C:N, (
C
) 30 sccm
t
a
-
C:N, and (
D
) 50 sccm t
a
-
C:N (
D
) films in 0.1 mM Fe(CN)
6
3
-
/4
-
+ 1 M KCl at increasing scan
rate from 0.1 to 0.5 V s
-
1
. Geometric area = 0.2 cm
2
.
................................
............................
148
Figure 6.13.
Comparison of cyclic voltammetric
i
-
E
curves recorded at all used ele
ctrodes in
0.1 mM Fe(CN)
6
3
-
/4
-
in 1 M KCl, measured at t
a
-
C:N
x
thin
-
film electrodes prepared with
different N content. Scan rate = 0.1 V s
-
1
. Geometric area = 0.2 cm
2
.
................................
...
149
Figure 6.14.
Comparison of cyclic voltammetric
i
-
E
curves for Ru(NH
3
)
6
+3/+2
in 1 M
KCl at the
different t
a
-
C(N)
x
thin film electrodes. Scan rate = 0.1 V s
-
1
. Geometric area = 0.2 cm
2
.
....
151
Figure 6.15.
Cyclic voltammetric
i
-
E
curves for 1 mM ferrocene in [BMIM][OTF] (
A
),
[BMIM][BF
4
] (
B
), and [BMIM][PF
6
] (
C
), measured using a 30 sccm ta
-
C:N thin
-
film electrode.
Scan rates = 0.1 to 0.5 V s
-
1
. Geometric area = 0.2 cm
2
.
................................
.......................
153
xvi
Figure 6.16.
Cyclic voltammetric
i
-
E
curves for 1 mM ferrocene in [BMIM][OTF] (black),
[BMIM][BF
4
] (red), and [BMIM][PF
6
] (blue). Measured at t
a
-
C:N (30 sccm of nitrogen) thin
-
film electrode, at scan rate 0.1 V s
-
1
.
Geometric area = 0.2 cm
2
.
................................
...........
154
Figure 6.17.
Cyclic volt
ammetric
i
-
E
curves of 1 mM FCA (
A
) and FeMeOH (
B
) in
[BMIM][OTF] measured at 30 sccm t
a
-
C:N thin
-
film electrode at increasing scan rate from 0.1
to 0.5 V s
-
1
. Geometric area = 0.2 cm
2
.
................................
................................
..................
157
Figure 6.18.
Raman spectra maps of the G
-
band peak position recorded over 100 × 10
0 µm on
the t
a
-
C:N films (30 sccm N
2
)
(A)
before and
(B)
after 15 min O
2
plasma treatment. Laser
exc
= 532 nm, integration time = 3 sec.
................................
.......................
159
Figure 6.19.
Static contact angle values for water on the t
a
-
C and t
a
-
C:(N)
x
electrodes
deposited with 0, 10, 30 and 50 sccm of N
2
before and after a 15
min O
2
plasma treatment. Data
obtained on as grown (black squares) and O
2
plasma modified (red circles) electrodes are shown
(n=3).
................................
................................
................................
................................
......
160
Figure 6.20.
AFM images of the (
A
) as grown and (
B
) O
2
plasma modified t
a
-
C:N (30
sccm)
electrodes. The plasma treatment conditions were 300 mtorr O
2
, 18 W, 15 min.
.................
162
Figure 6.21.
Background cyclic voltammetric
i
-
E
curves for t
a
-
C:N thin film electrode (30
sccm N
2
) as grown (black) and after oxygen plasma treatment (red curve) t
a
-
C:N 30 sccm
electrode in 1 M KCl. Scan rate = 0.1 V s
-
1
.
Geometric area = 0.2 cm
2
. The O
2
plasma treatment
conditions: 300 mtorr O
2
, 18 W, 15 min.
................................
................................
...............
163
Figure 6.22.
Cyclic voltammetric
i
-
E
curves for (
A
) 0.1 mM Fe(CN)
6
3
-
/4
-
and (
B
) Ru(NH
3
)
6
3+/2+
both in 1 M KCl, at an as deposited (black) and O
2
plasma treated (red) t
a
-
C:N thin film
electrode (30 sccm N2).
Plasma treatment conditions: 15 min, 300 mtorr, and 18 W. Scan rate
= 0.1 V s
-
1
. Geometric area = 0.2 cm
2
.
................................
................................
...................
165
Figure 6.23.
The variation of heterogeneous electron transfer rate constant for Fe(CN)
6
-
3/
-
4
in
1 M KCl at t
a
-
C(:N)
x
thin
-
film electrodes depositited with different gas flow rates of N
2
.
..
167
Figure 6.24.
Comparison of
cyclic voltammetric
i
-
E
curves of 1mM FeMeOH in 1M KCl (
A
),
[BMIM][OTF] (
B
), [BMIM][BF
4
] (
C
), and [BMIM][PF
6
] (
D
)
recorded before (black) and after
(red) oxygen plasma modification at t
a
-
C:N 30 sccm. Scan rate = 0.1 V s
-
1
. Geometric area =
0.2 cm
2
.
................................
................................
................................
................................
...
168
Figure 7.1
. Chemical structures of steroidal estrogens: estradiol (
A
), estriol (
B
), and estrone
(
C
).
................................
................................
................................
................................
.........
179
Figure 7.2.
Plots of the (
A
) mean background current and
(B
) noise for
ta
-
C:N and
BDD thin
-
film electrodes as function of the applied potential from 0.8 to 1.5 V. Measurements were made
in the mobile phase 10 mmol L
-
1
phosphate buffer (pH 3)/acetonitrile (60:40 v/v%). Flow
rate
=
2
mL
min
-
1
.
................................
................................
................................
...................
181
xvii
Figure 7.3.
-
1
estriol, estrone and
estradiol
in 10 mmol L
-
1
phosphate buffer (pH 3)/acetonitrile (60:40 v/v%) recorded at (
A
)
ta
-
C:N and (
B
) BDD thin
-
film electrodes. Flow rate = 2 mL min
-
1
.
................................
..........
183
Figure 7.4.
Plots of the signal
-
to
-
background ratio vs. potential for all three es
trogenic
compounds at (
A
) t
a
-
C:N and (
B
100
L
-
1
estriol, estrone and estradiol in 10 mmol L
-
1
phosphate buffer (pH 3)/acetonitrile
(60:40
v/v%). Flow rate = 2 mL min
-
1
.
................................
................................
..................
184
Figure 7.5.
Reversed
-
1
of estriol, estradiol
and estrone in 10
mmol
L
1
phosphate buffer (pH 3)/acetonitrile (60:40%, v/v) recorded at t
a
-
C:N thin
-
film electrode. Detection potential = 1.4 V. Flow rate = 2 ml min
1
.
.....................
185
Figure 7.6.
Ten repeated injections of 100
L
1
estriol, estradiol and estrone in 10
L
1
phosphate buf
fer (pH 3)/acetonitrile (60:40 v/v%)
recorded at (
A
)
t
a
-
C:N and (
B
) BDD thin
-
film electrodes. Flow rate = 2 mL min
1
. Detection potential = 1.4 V.
................................
..
186
Figure 7.7.
Logarithmic
c
alibration curves for
e and estradiol
in 10
mmol L
1
phosphate buffer (pH 3)/Acetonitrile (60/40 v/v%). Injected concentrations from
1
, recorded at
(
A
)
ta
-
C:N and (
B
) BDD thin film electrodes. Flow rate = 2
mL min
1
.
................................
................................
................................
................................
187
1
CHAPTER 1. INTRODUCT
ION
1.1. Carbon Electrodes
Carbon is one of the most plentiful elements on the planet.
From a materials perspective,
carbon
is unique because of the microstructurally distinct allotropes it forms. These include single
and polycrystalline diamond,
diamond
-
like carbon,
glassy carbon, and graphite.
Carbon materials are some of the most comm
only used electrodes in electroanalysis,
electrochemical storage and conversion, and electrosynthesis.
1
5
The
re are numbers of reasons for
this including low cost, high mechanical strength, wide usable potential range, rich surface
chemistry, chemical inertness, and compatibility with a variety of solvents and electrolytes.
In
order to optimally use carbon electr
odes, it is critical to fully understand and control the
variables that impact heterogeneous electron
-
transfer kinetics
, capacitance and adsorption
at these
materials. It has
been previously established
that the variations in the
sp
2
carbon
electrode
microstructure and surface chemistry can significant
ly
affect the heterogeneous electron
-
transfer
rate
constant for some,
(
e.g.
Fe(CN)
6
-
3/
-
4
)
but not all
(
e.g.
Ru(NH
3
)
6
+3
/
+2
)
, soluble redox systems,
often by multiple orders of magnitude.
6
11
1.1.1. Glassy Carbon
The
commonly used
sp
2
-
bonded carbons,
graphite
and
glassy
carbon
, represent one
extreme
and
-
ordered carbon, graphite, consists of layers of planar
polycyclic aromatic sheets with the carbon atoms
or
ganized
into a
hexagonal
lattice (Figure 1.1A).
The edges of the individual
sheets
that are separated by 3.35 Å
form
very reactive sites for
These carbon atoms react with
oxygen
and
water to
form a
variety
of surface oxygen functional groups, such as carboxylic acid,
2
carbonyls, phenols, ethers,
etc
. In contrast, the layer plane surface is the so
-
Due to a lack of functional groups and a lower density of electronic states,
the basal plane sites are
unreactive with oxygen and water, and support weak molecular adsorption and sluggish electron
-
transfer.
12
14
The edge plane sites are well known to exhibit significantly faster electron
-
transfer
kinetics
for many redox molecules
in comparison with the basal plane sites.
15,16
The capacitance
of the edge plane sites is also greater than the basal plane sites.
Glassy carbon is a more microstructurally disordered graphitic carbon has a structure
consisting of nanometer dimensioned graphitic domains that are randomly intertwined in a
ribbon
-
like
structure (Figure 1.1B). Glassy carbon has a relatively high ratio o
f edge plane sites exposed
at the surface
because of the microstructural disorder
.
17
19
Graphite has a hexagonal structu
re with
the carbon atoms in 3
-
fold bonding configuration. There is a strong intraplanar bonding within the
3
extended polycyclic aromatic rings and weak intraplanar bonding that allows the hexagonal sheet
to slide across one another. Glassy carbon has a glas
sy
-
like appearance, hence its name. It is hard
and brittle unlike graphitic forms of carbon. The material consists of a ribbon
-
like network of
entangled nm
-
dimension graphitic carbon domains. The entanglement leads to the nano porosity
of the material.
20
The material is made from the pyrolysis of polymer precursors. The material is
classified by its post formation heat treatment temperature (e.g., 1000, 2000, 3000
°C
). The higher
the heat treatment temperature the more microstructural ordered the material becomes presumably
die to a flattering and straightening of the ribbons. The microstructure order (nm dimensions) is
characterized by the XRD
-
determined height of th
e graphite
-
like ribbons,
L
e
, of
40 nm and length
of the ribbons,
L
a
, of
100
Å.
20
Importantly, glassy carbon has
electrons
in both
and
orbitals
that leads into a
relatively
small
band
gap between the valence and conduction gap, as w
ell as high
possesses several unwanted features, such as specific adsorption, high background current, and
microstructural changes at high positive pote
ntials.
15
1.1.2.
Boron
-
Doped
Diamond
Diamond
represents the other extreme of carbon allotropes.
(Figure 1.1C).
Diamond has
four
-
fold, sp
3
hybridized carbon boundary consisting purely of
Diamond is one of Mother
-
known
large
band gap
of
5.5 eV an
d
a
low density of
electronic states.
The low electrical conductivity arises from a low number of free charge carrier
as these it
little thermal activation of electron in the conduction band at room temperature
.
21
To
increase t
he
electrical
conductivity of diamond
,
the material can be doped using
p
-
or
n
-
type
dopants. Boron is one of the most commonly used dopants
that is incorporated from the gas phase
during the growth process.
for diamond material,
commonly implemented
into its growing process.
4
Boron atoms serve as electron acceptors and form a
n impurity
band located ~0.35 eV
, or less,
above the valence band edg
e
. Typical doping levels for diamond films used in electrochemistry
are 10
20
10
21
cm
-
3
.
Boron
-
doped
diamond
(BDD)
is
an increasing
used
carbon
electrode material.
22
24
BDD
electrode ex
hibits excellent electrochemical properties, such as a
wide
working potential window,
low background current
and
noise, and rapid electron transfer kinetics for many redox species.
The
basic
electrochemical properties of BDD have been reviewed
extensively in publications
dating back to early 1990s.
25
30
Compare
d
to the
glassy carbon
, BDD is microstructurally and
morphologically stable even at high positive potentials and high cur
rents and has
weak
molecular
optically transparent material, that can be advantageously used in for
transmission
spectroelectrochemical
measurements in the UV/Vis
31
35
and mid
-
IR
36
regions of the
electromagne
tic spectrum.
5
1.1.3. Tetrahedral Amorphous Carbon
Another carbon electrode that is receiving increase study is
tetrahedral amorphous
carbon
(t
a
-
C). t
a
-
C is a type of
diamond
-
like
carbon
(
Figure 1.1D
). Microstructurally,
it consists of
a
mixture of randomly arranged sp
2
,
and
sp
3
bonded carbon. The materi
al can be generally tailored
based on the required properties by controlling the ratio of
sp
2
and sp
3
content, as well as the
hydrogen content. The ternary phase diagram conveniently displaying possible
diamond
-
like
carbon composition is shown in Figure 1.
2.
37
The
diamond
-
like
material with a random
arrangement of sp
2
and sp
3
bonded carbon
is defined
as amorphous carbon. In this research, the
focus is lying on the amorphous material, that can
be deposited
with a high portion of tetrahedrally
bonded sp
3
carbon, generally from 40 up to 85 %.
38
Films with such as high content of sp
3
bonded
carbon
are denoted
as
t
a
-
C.
39
The sp
3
hybridized atoms form a random network with neighboring
6
atoms connected by strong
bonds. Inters
persed are sp
2
carbon domains that link the sp
3
carbon
regions. These sp
2
sites form
states which control the optical and electrical properties.
40
The structural and electronic properties of the t
a
-
C are mainly a
function
of the
fraction
of
the sp
3
-
bonded carbon sites, ordering of the sp
2
site
s, and
hydrogen
content.
The atomic structure
was described by Robertson et al
4
1
.
A
carbon atom
has
four sp
3
orbitals that provides four strong
atom
s
. In contrast, in the sp
2
configuration
, a carbon atom creates three
trigonal sp
2
2
atoms lies in a p
orbital
41
The
amount of
incorporated
hydrogen
is
generally low
in t
a
-
C
materials.
Due to the high
percentage
of sp
3
bonded
carbon,
t
a
-
C exhibits
several
properties akin to
diamond
, such as high chemical inertness, hardness, elastic modulus, optical
transparency
(band
gap
~3.5 eV), and exceptional electrochemical behavior.
42
47
Importantly, the t
a
-
C electrode
possesses
a key
benefit over the BDD
, and t
hat is the
possibility
of deposition at
low temperatures
(25
-
100 °C)
, as
compare to the BDD (>600 °C)
. This makes possible deposition of t
a
-
C
on a
variety of sub
strate materials
.
Moreover, the deposition of the t
a
-
C electrode is significantly
cheaper, an
d the
deposition
of
continuous
films takes in
order
of tens of minutes,
compared
to
hours needed for a BDD film growth.
t
a
-
C film generally have an sp
2
-
rich layer on the surface because the material grows by a
sub plantation process. In this
process, incident carbon cations enter subsurface sites, densifying
these and converting the bonding to sp
3
.
38,48,49
This leaves behind an outer sp
2
carbon layer that
could significantly impact the electrochemical properties. This outer sp
2
layer is a few nanometers
thick.
7
1.1.3.1. Nit
rogen Incorporation into the Tetrahedral Amorphous Carbon
Similarly to diamond , the t
a
-
C material can
be doped
by
incorporation
of impurities.
One
of the most commonly used n
-
type dopants is nitrogen. The
incorporation
of the nitrogen into
the film can
be easily performed
by adding the nitrogen gas flow into the chamber during the film
deposition.
50
Electronic
and
optical properties are significantly affected by the nitrogen
incorporation. While the
pure
t
a
-
C
film
is considered to be a weak p
-
type semiconductor,
it
has
been found
that the controllable doping of t
a
-
C with nitrogen increase the elec
trical
conductivity.
51
54
Two main types of nitrogen incorporation have
been described previously by Robertson at
el.
55
and confirmed by other research groups.
56
For the
implementation
of nitrogen into the t
a
-
C
network at a
partial
pressure of nitrogen lower than 3.5 Pa, the nitrogen is incorporated mainly
. Consequently
, the bonds aroun
d nitrogen atoms
undergo graphitization, accompanied by a significant narrowing of the band gap and an increase
of electrical conductivity.
Only
a
small
fraction of all incorporated nitrogen atoms attributes to the
conductivity
. Nevertheless
, for the nitrogen doping at partial pressure higher than 3.5 Pa, the
optical gap gets wider and electrical conductivity of the resulting t
a
-
C
:N
material begins to
dec
rease.
For example, for samples deposited at partial pressure of 7 Pa, the actual fraction of
incorporated nitrogen atoms reaches up to 47%, while the optical gap rises to 4.05 eV and the
electrical conductivity drops
.
56
Such as reduction of conductivity is caused by a transformation of
the initially
bonded sp
3
-
CN matrix into a new sp
2
configuration containing a polymeric carbon
nitride (>C=N
-
) film.
43,56,57
8
1.2.
Characterization of Carbon Electrodes
If one wants to understand structure
-
function relationships at the nanostructured carbon
electrodes, detailed electrode material characterization is necessary to know what the electrode
morphology, microstructure, and
bulk and surface chemical composition area. The microstructure,
ratio of the sp
2
and sp
3
carbon bonding
and the chemical composition of the carbon
at the surface
and in the bulk
play
a
key
role in
its
properties. The most useful techniques for characterizi
ng the
material properties of carbon electrodes include
electron
microscopic and spectroscopic methods,
such as transmission electron microscopy,
58
scanning electron microscopy,
59
x
-
ray photoelectron
spectroscopy,
60
electron
energy
-
loss spectroscopy,
39,61
and Auger electron spectroscopy.
60
Optical
spectroscopic methods ar
e also useful including infrared spectroscopy,
62
Visible and UV Raman
spectroscopy.
63,64
Lastly, other tec
hniques such as
13
C nuclear magnetic resonance,
65
inelastic
neutron scattering spectroscopy,
66
and scanning probe microscopies (
e.g.,
AFM)
67
are also us
eful
.
For determination of the sp
2
and sp
3
carbon bonding ratio
in
t
a
-
C and t
a
-
C:N
electrodes
v
isible and UV Raman spectroscopy,
and
x
-
ray photoelectron spectroscopy
were used.
1.2.1. Raman Spectroscopy
Raman spectroscopy is one of the most commonly use
d techniques for carbon material
microstructure characterization.
68
The technique can be
used
to distinguish the type of carbon
bonding, the domain size, the doping level in boron
-
doped diamond films and the intrinsic
internal
stress
of diamond
.
58
,69
The shape of the Raman spectr
um
for nanocrystalline BDD electrode is significantly
affected by the boron content.
Single crystal
diamond is characterized by a sharp diamond line at
1332
cm
-
1
.
68,70
The Raman spectra for low boron doped diamond film is characterized by a sharp
band at 1329 cm
-
1
, attributed to the sp
3
bonded carbon
, and another peak presented at 519 cm
-
1
,
9
that is assigned to the phonon mode of the crystalline Si substrate on which the BDD film is
deposited.
At the low boron content (~500 ppm), the Si peak possess a Fano shape, given by the
presence of boron in the s
ubstrate. With an increasing concentration of boron, the relatively
symmetric Lorentzian band at 1332 cm
-
1
changes towards an asymmetric Fano
-
like line shape and
moves down to the 1290
cm
-
1
.
71
Moreover
, th
is shift
is usually coupled
with an increasing
scattering intensity at ca. 500 and 1220 cm
-
1
. It has
been previously reported
that the 500
cm
-
1
band
is due to the vibrational modes of boron dimers and increases with an increasing level of boron
doping. The
peak positioned at 1220 cm
-
1
is assigned
to defects in the diamond lattice brought
about by the high of boron doping or the boron
-
carbon complexes. The last peak presented at
spectra of heavily doped BDD film
is presented
at around 1496 cm
-
1
. It
is assign
ed
to a presence
of amorphous sp
2
carbon. The band is typical characteristics for BDD,
and
the intensity of the
1496
cm
-
1
peak increases with
an
increasing boron level.
22,71
74
The Raman spectra for tetrahedral amorphous carbons
are characterized
by
an
asymmetrical band centered between 1000 and 1800 cm
1
. This peak
is attributed
to the vibrational
modes of sp
2
-
bonded carbon clusters. The Raman scattering cross section for sp
2
bonded carbon
(
e.g
,.graphite) is larger than the
cross section
for
diamond
with visible excitation
so
the Raman
scattering
is dominated
by the sp
2
domains i
n the t
a
-
C
:N
films.
75,76
Two peaks of interest
can be
identified after
deconvolution of the asymmetrical band
.
peak centered at 1550 cm
1
. The G band
is attributed
to the first
-
order phonon mode for pairs of
sp
2
approximately at 1358 cm
1
.
This scattering
is attributed
to
the
breathing vibration modes of sp
2
carbon atoms in rings.
62,77
79
In perfect graphite, this mode is forbidden, and therefore becomes
active only for disordered
materials.
75
Ferrari et al.
75
reported that the reason is the domination of
10
the sp
2
sites scattering.
The
because of their lower energy.
Therefore, the sp
2
sites
have
about 50
-
230 times larger Raman cross
section
compared
to the sp
3
sites.
80,81
Consequently, the sp
2
sites dominate the Raman spectra of
t
a
-
C, even for those with low sp
2
content (10
-
15% of sp
2
). Ferrari and Robertson
75
also reported
been several repor
ts on the nitrogen incorporation and its effect on Raman spectra.
50,62,82
As the
increasing nitrogen content causes an increasing fraction of sp
2
bonded carbon,
intensity ratio of
the D peak to
G peak increase. The
full
-
width
-
half
-
maximum of the G peak decreases with an
increasing content, also due to the in
creasing content of sp
2
bonds.
50,78
Finally, the G band position
was reported to shift to the lower wavenumbers with inc
reasing nitrogen content. The observed
trend is due to the increasing sp
2
bonded carbon content and an increasing number
and/or
size of
clusters.
50,62,82
The most commonly used
fitting
technique uses two Gaussians peak
s centered around the
theoretical D and G band positions. The focus for
an characterization
of
tetrahedral
amorphous
carbon electrodes by visible Raman spectroscopy can
be pointed
at following spectra features: the
G band position, the G
band width
and
the
intensity ratio of the D peak to
G peak
(
I
D
/
I
G
)
.
62
,
that
are
related to the structures, size, and density of the sp
2
clusters. Cui et al
83
found
an unique
cor
relation between the full
-
width
-
half
-
maximum of the G peak (FWHM) and the sp
3
content for
the hydrogen free
diamond like
carbon material. The following equation
was reported
83
:
where
W is the FWHM(G) of Raman spectra measured at a wavelength of 514 nm
.
Ferrari et al.
84
reported a quantitative
relation
for Raman spectra measured at
any
excitation wavelength.
(1.1)
11
Combining both equations 1 and 2
, a calculation of sp
3
content in a
hydrogen
-
free
diamond
-
like
carbon at
any
excitation wavelength can
be performed
.
1.2.2. X
-
ray Photoelectron Spectroscopy
Another
method that can be used to estimate the sp
2
and sp
3
carbon bonding in diamond
and tetrahedral amorphous carbon thin films is
x
-
ray photoelectron spectroscopy (XPS). XPS is a
powerful and surface
-
sensitive
method
that measures the uses the peaks position r
eflecting the
electron binding energies for specific levels of atoms, further used for
identification
of the
chemical states.
XPS is a benefitial measurement technique because it not only reveal what
elements are within a film but also what other elements
they are bonded to. XPS spectra are
received by irradiating a material with a beam of x
-
rays while simultaneously measuring the
kinetic energy and number of electrons that escape from the top
~10 nm of the material being
analyzed. For bulk chemical information, depth profiling XPS is used.
For boron
-
doped diamond and nitrogen
-
incorporated tetrahedral amorphous carbon, key
elemental signals are the binding energies for B1s (185 eV), C1s (284 eV), N1s (402 nm) and O1s
(532 nm). There are chemical shifts in these core peaks depending on what a
toms the B, C, N and
O are bonded to. In other words, shifts in the binding energies can be used to determine the atomic
percentages
of carbon atoms bonded to other carbon atoms,
or to other boron, nitrogen or oxygen
atoms. The chemical shift is also depen
dent on the lattice field of the material.
85,86
Therefore, this
can
be used
for an investigation of whether a carbon material is sp
2
or sp
3
bonded. Moreover, the
XPS can
be used
to
investigate a surface composition, valence band density of states, etc.
60
(1.2
)
12
Compared to some other methods, XPS can be used without any enormous damage of the material,
and does not require a reference sample.
The core C1s position is sensitive to the sp
2
and sp
3
bonded carbon in the material, so the
techn
que can be used to estimate the ratio of sp
2
and sp
3
carbon in t
a
-
C and t
a
-
C:N thin
films.
58,82,87,88
In order to analyze the spectra quentitatively, the deconvolution and fitting with a
Gaussian peaks is neccesarry.
60,89,90
The C1s position for a pure sp
2
material like graphite is
284.5
eV and the peak position for a pure sp
3
material like diamond is 285.3 eV. Mizokawa et al.
91
reported that the binding
energy
of the sp
3
sites
is shifted
by 0.8 eV compared to the sp
2
sites.
Another typically presented component found as
a t
a
-
C
surface contamina
tion is
an oxygen
.
Panwar et al.
60
reported on diminishing of the oxygen peak after a cleaning process, proving that
the oxygen
is not presented
in the film bulk, but only on the film surface.
For the
nitrogen incorporated
tetrahedral amorphous carbon, the
characteristic
N 1s line
is
also presented
.
It has been found that the nitrogen content
affects also
the C 1s peak. An increasing
nitrogen content causes a shift of the C 1s line towards higher binding energy, as well as an
increased
broadening of the line. Moreover, the
increasing
intensity of the
higher
binding energy
shoulder with an
increased
nitroge
n content
was reported
.
56
According to Rodil
et al,
88
the N 1s
peak can be deconvoluted and fit to three components, suggesting that the N in the film exists in
three different chemical environments. The peaks are assigned to N b
onded to sp
3
carbon
(398.56
eV), CN triple bonds (399.91 eV) and N
-
sp
2
carbon (401.41 eV).
88,92
The XPS depth
profiles has revealed that the nitrogen content is relatively constant with depth in the t
a
-
C:N films
(6 % for 30 sccm t
a
-
C:N film ).
92
Similarly to Raman spectroscopy, the XPS spectra must
be deconvoluted
to
obtain a
detailed information of the carbon material microstructure. The in
tensity of the core
-
level lines is
13
then directly proportional to the
atomic density. For a calculation of the nitrogen concentration in
the films, the
ratio of the integrated intensities of the N 1s and C 1s lines can be used, with an
implementation of sensitivity factors (S
nitrogen
= 0.42 and S
carbon
= 0.25).
93
1.3. Room Temperature Ionic Liquids
(RTILs)
Room temperature ionic liquids (RTILs) are salts composed purely of ions
with
no solvent
.
These sa
lts are liquids at room temperature due to their near room temperature melting point.
94
96
RTILs
are composed
of a large organic cation, such as imidazolium, pyrid
inium, and pyrrolidinium
derivatives, and a small inorganic anion, such as [BF
4
], [PF
6
], [CF
3
SO
3
], and [(CF
3
SO
3
)
2
N].
Due to the numerous possible combinations of cationic and anionic structures, the physicochemical
properties of the RTILs can
be easily manipulated
by changing the chemical composition.
97
From an electrochemical viewpoint, key properties of the RTILs are the viscosity, electrical
conductivity and working potential window.
RTILs have attracted the attention of scientific word due to their many unique properties,
such as negligible vapor pre
ssure associated
with
high
thermal stability, extraordinary chemical
stability, moderate electrical conductivity, non
-
flammability,
etc.
96
RTILs possess advantageous
properties when us
ed in electrochemistry. When pure, many RTILs are stable over a wide potential
range of approximately 5 to 6 V, which is in
significant
contrast with more aqueous electrolyte
solutions (
~2V).
98,99
The
wide
electrochemical
window enables the study of redox reactions at
extreme positive and negative potentials, something that is no possible in in aqueous electrolytes.
Two
limitations with their use, particularly in electrochemical studies of electron
-
transfer and
mass
-
transport, are their high viscosity and the presence of impurities, such as water or reagents
used in their synthesis.
14
The viscosity of RTILs is caused prima
rily by the strong van der Waals interactions
between the cations and anions.
99
101
It has
been found
that the
viscosity
is more affected by the
structure of the anion compared to the
cation
. The
viscosity
increases with increasing size of the
cation due to the strengthen cation
-
anion interactions. Typically, the
viscosity
of RTILs is
significantly higher than water (
(H
2
0) = 0.89
mPa s
at 25
°C
) ranging from
25
to
312
mPa s
(Table 1.1).
The variability of
viscosity values in the table for a RTIL are likely due to variable
levels of water contaminations.
The
high
viscosity
of RTILs suppresses diffusional mass transport
and slows heterogeneous electron
-
transfer with an electrode.
102
104
The presence of impurities in RTILs can have a significant effect on their physicochemical
properties and on electrochemical processes using them.
One
of the most abundant contaminants
is water. Both hydrophilic and hydrophobic RTILs can adsorb water.
105
107
It has
been reported
that the water contamination in RTILs has a significant
effect on their electrochemical properties
by decreasing viscosity and density,
95,107,108
while increasing the electrical conductivity.
109,110
Water contamination causes a
significant
increase of voltammetric background current, reduces
electrochemical potential window and affec
ts the solubility and solvent environment around a
redox molecule. The presence of water can also affect the structure of the electric double
-
layer
formed at electrified interfaces
.
95,111
As the water removal process is
somewhat
time
-
consuming
,
the
researchers
often neglect trace level of such as contamination, as can be illustrat
ed by the
variable viscosity values reported for the same RTIL. For example, for [BMIM] [PF
6
],
values
of
173, 207 and 312 mPa
-
s
have
been reported. Therefore, not only the experiments with RTILs
require a
fast
and convenient water removal process.
15
RTILs
Structure
Temperature,
°C
Viscosity,
mPa s
Reference
[EMIM][BF
4
]
25
25
112
25
37
113
25
43
114
[BMIM][BF
4
]
25
92
115
25
219
110
25
233
116
20
112
115
20
154
117
[BMIM][PF
6
]
25
173
115
25
207
118
25
312
116
20
201
115
20
286
118
[BMIM][OTF]
25
75
119
25
90
112
20
90
120
20
110
121
Table 1.1.
Molecular structure and viscosity of RTILs used in this study.
16
RTILs consist purely of cations and anions with no solvent
present
. For the aqueous or
organic electrolytes, the electrode
electrolyte interface, the so
-
has
been intensively studied and can be described by Gouy
-
Chapman
-
Stern model (Figure 1.3). This
model is most appropriate for dilute aqueous electrolyte solutions .
It is questionable if this model
is descriptive of the interfacial organization of a RTIL
at an electrode. RTILs possess
high total
ion
concentrations (3
-
6 M)
. The concentration of free ions is less due to ion pairing
. Furthermore,
at the electrified interface, the RTILs can form several layers of oscillatory charge density, which
makes the ty
pical uniform double layer model misleading.
96,102
Over the last decade, several models of the electrical double layers in RTILs have been
proposed and ex
perimentally tested. In 1997, Borukhov et al.
122
described a theory involving a
binary electrolyte with a high ionic concentration. The model considers a
Columbic
interaction
between the charged surface and the ions but there is a missing theory on the ion
-
ion correlation
arising from the short range Coulombic interactions. The ionic
correlations
in concentrated
electrolytes were later heavily investigated by Korny
shev
et al.
102
theory
for the purposes of
that suggests tightly
packed multiple layers of ions (Figure 1.3B
)
is so far
the most trustable model of the electrified
layer in RTILs.
123
The absence of any solvent in the RTILs is important in several respects. First, in contrast
with aqueous electrolyte solutions, the solution environment around a redox analyte in an RTIL is
quite different. There is n
o solvation layer around the redox analyte but rather an organization of
cations and anions. This means that there is likely a larger reorganizational barrier to overcome in
an RTIL when a redox molecule undergoes electron transfer. For example, research
in this project
showed that when trying to measure electron transfer for the Fe(CN)
6
-
3/
-
4
redox system in RTILs,
17
no current was measured voltammetrically in several RTILs even though the analyte was
dissolved. However, when water was added to the RTIL, voltammetric current was observed for
the redox reaction. This likely has to do with a very large reorganizational b
arrier in the pure RTIL,
a barrier that is reduced when a dielectric solvent is added to the system. Second, the solubility of
a redox molecule is affected by the presence of water. This is particularly true for highly charged
redox molecules.
1.4. Rese
arch Objectives and Specific Aims
There is considerable
knowledge about
electron
-
transfer reactions for redox systems in
aqueous and organic electrolyte solutions at carbon electrode
s
. In contrast, there is
a
limited
information about electron
-
transfer re
actions for redox systems in RTILs. The research presented
in this dissertation focused on addressing this knowledge gap.
The overall goal of the research
was to
advance the understanding of the material properties of sp
3
(boron
-
doped diamond films)
and h
ybrid sp
3
/sp
2
(nitrogen
-
incorporated tetrahedral amorphous carbon) nanostructured carbon
electrodes and how these properties affect electrochemical processes
(capacitance and
18
heterogeneous electron transfer)
in RTILs. Comparison studies were performed usin
g soluble
redox systems in aqueous electrolyte solutions.
The research
was
conducted around three specific aims:
Specific Aim 1:
To
fully characterize the physical, chemical and electronic properties of the
different carbon electrode materials used in the
research: glassy carbon (GC), nitrogen
-
incorporated tetrahedral amorphous carbon (t
a
-
C
:N
) and boron
-
doped diamond (BDD).
Specific Aim 2:
To
correlate the
heterogeneous
electron
-
transfer rate constants of soluble
inorganic and organic redox couples with th
carbon electrodes: boron
-
doped nanocrystalline diamond and nitrogen
-
incorporated tetrahedral
amorphous carbon thin films. The goal was to determine if the electrode microstructure
has a
strong influence
on
heterogeneous
electron
-
transfer rate constants in RTILs of different
composition as is the case in aqueous electrolytes.
Specific Aim 3:
To determine how various levels of nitrogen incorporation into the tetrahedral
amorphous carbon thin films affect the physical, chemical and basic electrochemical properties of
the materials.
The electron
-
transfer kinetics for soluble inorganic and orga
nic redox systems were
investigated in both aqueous and RTILs solutions. Moreover, research
was conducted to determine
how changes in the surface chemistry of the t
a
-
C:(N)
x
electrodes affect the electron
-
transfer
kinetics.
Specific Aim 4:
To evaluate and
compare the analytical detection figures of merit achievable with
BDD and t
a
-
CN electrodes for detection of various
bioanalytes
using
high
-
pressure
liquid
19
chromatography coupled with amperometric detection. The target analytes included estrogen and
estro
gen compound metabolites, and isatin.
20
REFERENCES
21
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