Publication date: 16th April 2014
With late onset and low photocurrent production observed on CuWO4, common co-catalyst loading strategies have proven to be unfruitful for increasing the photocurrent density. Electrochemical impedance spectroscopy (EIS) was used to probe the electrode/electrolyte interface of CuWO4 thin films 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. The state is likely composed of Cu(3d) orbitals due to both experimental and theoretical evidence of Cu-based orbitals comprising the top of the valence band and bottom of the conduction band in the bulk. This mid-gap state was identified experimentally by EIS at 100 mW/cm2 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).[iii] 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 films 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 were also investigated. However, CuWO4 proves to be unique as propylamine may be selectively oxidized based on applied potential compared to alcohol based groups. This would produce the possibility for sustainable and selective synthesis of specific functional groups by CuWO4.
[i] Currao, A. Photoelectrochemical Water Splitting. Chimia 2007, 67, 815-819.
[ii] 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.
[iii] 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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