Publication date: 16th April 2014
With late onset and low photocurrent production observed on CuWO4, co-catalyst loading strategies common to BiVO4 and Fe2O3 have proven to be unfruitful for increasing photocurrent density. Electrochemical impedance spectroscopy (EIS) was used to probe the electrode/electrolyte interface of CuWO4 thin films prepared by sol-gel methods for water oxidation under simulated solar irradiation. The results indicate that the onset of photocurrent is dictated by the presence of a mid-gap state that participates in water oxidation. This mid-gap state was identified experimentally by electrochemical impedance spectroscopy under simulated solar irradiation in borate buffer at pH 7.00. Our results show the evolution of two charge-transfer events in the Nyquist and Bode plots of EIS data, as well as Fermi-level pinning by Mott-Schottky analysis in the potential range of 0.81 – 1.01 V (RHE).[3] The same evolution of two separate charge transfer events was observed upon oxidation in a methanol solution.
In light of these phenomena we have focused our attention on the ability of CuWO4 to oxidize other substrates besides water. Under visible light irradiation, organic amines, such as propylamine, are photoelectrochemically oxidized to their imine on CuWO4. Other visible light active oxide semiconductors, such as WO3, were also investigated. However, CuWO4 proves to be a unique case as propylamine may be selectively oxidized by the valence band whereas alcohol based groups may rely on the mid-gap state energy for oxidation. This would produce the possibility for sustainable and selective synthesis of specific functional groups by CuWO4.
[1] Currao, A. Photoelectrochemical Water Splitting. Chimia 2007, 67, 815-819.
[2] Yourey, J. E.; Bartlett, B. M. Electrochemical Deposition and Photoelectrochemistry of CuWO4, a Promising Photoanode for Water Oxidation J. Mater. Chem. 2011, 21, 7651-7660.
[3] Pyper, K. J.; Yourey, J. E.; Bartlett, B.M. Reactivity of CuWO4 in Photoelectrochemical Water Oxidation Is Dictated by a Midgap Electronic State J. Phys. Chem. C, 2013, 117 (47), 24726–24732.
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