Publication date: 31st March 2013
Photoelectrochemical (PEC) water splitting is a promising method to convert the immense energy from the sun into storable chemical fuels. One of the key challenges in advancing the efficiency and performance of solar water splitting devices is to improve the water oxidation reaction carried out by semiconductor photoanodes. Photoanode development has been dominated by large band gap materials such as TiO2 and WO3, which are only able to absorb a small portion of the solar spectrum, making them non-practical for large scale implementation. Therefore, new materials must be developed and investigated that meet strict requirements such as a suitable band gap energy, conduction/valence band positions that straddle the water redox potentials, show high stability, and are made from cheap earth abundant materials.
Copper tungstate (CuWO4) is a an attractive candidate for PEC water splitting due to its ideal band gap energy (2.25 eV), n-type conductivity, and stability in aqueous solutions.1,2 However, there have only been limited studies on this material for PEC applications, and thus the overall performance for solar to hydrogen conversion is still very low. Therefore, there is an urgent need to identify the performance limiting factors of CuWO4 in order to determine the fundamental materials deficiencies so that they can be overcome, allowing this material to achieve high water splitting efficiencies.
Here, we report the use a spray pyrolysis technique to deposit nanostructured thin films of CuWO4 using both basic and acidic precursor solutions. The structural, optical, and photoelectrochemical properties of these films were investigated and optimized in order to fabricate highly crystalline films of CuWO4 which show promising performance.
We successfully doped CuWO4 for the first reported time, which shows drastic improvements in the incident photon conversion efficiency (IPCE) and significantly higher photocurrent values at the water splitting potential of 1.23 V vs. RHE. These improvements are explained by the increase in charge carrier density due to the substitutional doping of higher vacancy atoms, as well as due improved charge carrier mobility. In addition, we have also applied phosphate and borate based oxygen evolution catalysts (OECs) to the CuWO4 surface, which show improvements in the catalytic efficiency of the overall photoanode. This is the first time OECs have been reported to be combined with CuWO4.
In this study we have used photoelectrochemical techniques to determine the charge carrier generation, separation, and catalytic efficiencies of CuWO4. By identifying the materials deficiencies, we were able to significantly improve the performance of CuWO4 photoanodes for use in solar water splitting devices.
[1] J. E. Yourey and B. M. Barlett, J. Mater. Chem., 2011, 21, 7651–7660. [2] Y. Chang, A. Braun, A. Deangelis, J. Kaneshiro and N. Gaillard, J. Phys. Chem. C, 2011, 115, 25490–25495.
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