Reactivity of CuWO₄ in Photoelectrochemical Water Oxidation Is Dictated by a Midgap Electronic State

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 presented results indicate that the onset of photocurrent is dictated by the presence of a midgap state that participates in water oxidation. The state is likely composed of Cu­(3d) orbitals because of both experimental and theoretical evidence of Cu-based orbitals comprising the top of the valence band and the bottom of the conduction band in the bulk. This midgap 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 the Fermi level pinning by Mott–Schottky analysis in the potential range of 0.81–1.01 V (reversible hydrogen electrode, RHE). The Mott–Schottky analysis at low frequencies in the dark suggests that it is not a photogenerated state but rather a permanent state in the electronic structure of CuWO₄. The same results are observed in pH 9.24 borate buffer, and the midgap state shows a Nernstian pH response.