Elucidation of CuWO4 Surface States During Photoelectrochemical Water Oxidation.

Electrochemical, photoelectrochemical, and impedance spectroscopy measurements were performed to investigate the role of CuWO4 surface states during water oxidation. We found that a capacitive feature related to a surface state is clearly observable under water oxidation conditions. The magnitude of the surface state capacitance is light intensity-dependent, with a peak potential that coincides with the water oxidation onset potential region. The surface state is not observed in the dark nor in contact with nonaqueous solvents. These results strongly support our assignment of this surface state as the buildup of water oxidation intermediate species at the surface of CuWO4 photoanodes, not a permanent or intrinsic state as previously reported. We suggest this is a general feature that controls the behavior and efficiency of solar water splitting reactions.