Improved Charge Separation in WO₃/CuWO₄ Composite Photoanodes for Photoelectrochemical Water Oxidation

Porous tungsten oxide/copper tungstate (WO₃/CuWO₄) composite thin films were fabricated via a facile in situ conversion method, with a polymer templating strategy. Copper nitrate (Cu(NO₃)₂) solution with the copolymer surfactant Pluronic^®F-127 (Sigma-Aldrich, St. Louis, MO, USA, generic name, poloxamer 407) was loaded onto WO₃ substrates by programmed dip coating, followed by heat treatment in air at 550 °C. The Cu²⁺ reacted with the WO₃ substrate to form the CuWO₄ compound. The composite WO₃/CuWO₄ thin films demonstrated improved photoelectrochemical (PEC) performance over WO₃ and CuWO₄ single phase photoanodes. The factors of light absorption and charge separation efficiency of the composite and two single phase films were investigated to understand the reasons for the PEC enhancement of WO₃/CuWO₄ composite thin films. The photocurrent was generated from water splitting as confirmed by hydrogen and oxygen gas evolution, and Faradic efficiency was calculated based on the amount of H₂ produced. This work provides a low-cost and controllable method to prepare WO₃-metal tungstate composite thin films, and also helps to deepen the understanding of charge transfer in WO₃/CuWO₄ heterojunction.