Electrochemical deposition and photoelectrochemistry of CuWO4, a promising photoanode for wateroxidation

Abstract

Polycrystalline thin film photoanodes composed of CuWO₄ have been prepared by electrodeposition from an acidic aqueous solution. This 2–3 μm thick n-type material has an indirect band gap of 2.25 eV, 0.45 V smaller than the gap of WO₃. The flat band potential and donor density determined by Mott–Schottky analysis of the electrochemical impedance spectra are +0.4 V (vs. NHE) and 2.7 × 10²¹ cm⁻³ respectively, similar to those observed in crystalline WO₃. This implies that the lower band gap is due to a raised valence band maximum, desirable for water oxidation photoanodes. Under neutral conditions afforded by a pH 7 potassium phosphate buffer, electrodeposited CuWO₄ generates a photocurrent density of 0.16 mA cm⁻² at an applied bias of +0.5 V (vs Ag/AgCl) under simulated solar irradiation. Electrodeposited CuWO₄ films demonstrate higher chemical stability than polycrystalline WO₃, both under continuous illumination and in the dark. CuWO₄ oxidizes methanol and maintains a stable steady-state photocurrent density of 0.11 mA cm⁻² after 12 h of continuous illumination in a 10% methanol solution in aqueous electrolyte. This is twice the current density observed without methanol added.