"An increasing global energy demand along with detrimental consequences of extensive usage of fossil fuel calls for the development of abundant, cost effective and carbon neutral alternative energy sources. Solar energy is well distributed and is the most abundant energy source available on Earth. This work focuses on using copper tungstate (CuWO4) photoelectrode to harvest sunlight to oxidize water, which constitutes the rate limiting half reaction of photoelectrochemical water splitting to generate clean and carbon neutral energy dense fuel, hydrogen, and the byproduct, oxygen. CuWO4 is a promising photoanode material for water splitting owing to its reasonably small bandgap, suitable band position, stability and abundance, but has received limited attention due to its reported poor bulk properties and low hole collection efficiency at the surface. A lack of fundamental insight into its limitations hinders rational design of strategies to improve its performance. We developed an atomic layer stack deposition-annealing (SDA) method derived from atomic layer deposition (ALD) to synthesize our CuWO4 thin films. While keeping the benefit of precise thickness control of a uniform film as traditional ALD procedures, this new approach of ALD of ternary metal oxides allows for a wider selection of precursors since their ALD temperature windows do not have to overlap, and it allows for an easier control of stoichiometry. The hole collection efficiency at the electrode surface was quantified by comparing water oxidation and the oxidation of a suitable hole scavenger, as well as by intensity modulated photocurrent spectroscopy (IMPS). Our results showed that water oxidation with CuWO4 is limited by surface recombination at low applied bias, but is essentially quantitative at higher potentials. A series of photoelectrochemical and electrochemical impedance spectroscopic measurements were employed to elucidate the role of surface state of CuWO4 during water oxidation. Our results suggested that the surface state of CuWO4 is related to a water oxidation intermediate species rather than intrinsic, as is opposed to previous literature. Relatively thick CuWO4 electrodes were made by spray pyrolysis and the pores were back-filled with Al2O3 to produce compact films for bulk studies. Illumination direction and wavelength dependent photoelectrochemical measurements were employed to investigate the bulk properties of CuWO4 for water oxidation, which suggested that the major problem of CuWO4 is a short electron collection length."--Pages ii-iii.
Read
