CVD-grown copper tungstate thin films for solar water splitting

Abstract

In this paper, a direct chemical vapor deposition (CVD) approach is applied for the first time to synthesize high quality copper oxide (CuO), copper tungstate (CuWO₄) and tungsten oxide (WO₃) on F:SnO₂ (FTO) substrates for photocatalytic water splitting. Variation of process parameters enables us to tune the stoichiometry of the deposits to obtain stoichiometric, W-rich, and Cu-rich deposits. It is found that the presence of Cu in WO₃ thin films reduces the bandgap and enhances the absorption properties of the material in the visible range. The photoelectrocatalytic performance of stoichiometric CuWO₄ was found to be superior to that of WO₃ oxide under frontside illumination when thin films were used. However, detailed photoelectrochemical investigations of both thin and thicker CuWO₄ films reveal that the incorporation of copper also decreases the mobility of both electrons and holes, the latter being the performance-limiting factor. These results are in line with our first-principles calculations of the electronic structure of CuWO₄. A charge carrier mobility and diffusion length of ∼6× 10⁻³ cm² V⁻¹ s⁻¹ and 30 nm were determined by time-resolved microwave conductivity measurements, values comparable to those of undoped bismuth vanadate (BiVO₄). Our findings establish new insights into the advantages and limits of CuWO₄-based photoanodes, and suggest a possibility of using very thin CuWO₄ films on top of highly absorbing semiconductors with optimal electronic properties.