Copper(ii) tungstate nanoflake array films: sacrificial template synthesis, hydrogen treatment, and their application as photoanodes in solar water splitting

Abstract

We report the preparation of CuWO₄ nanoflake (NF) array films by using a solid phase reaction method in which WO₃ NFs were employed as sacrificial templates. The SEM, TEM and XRD results demonstrated that the obtained CuWO₄ films possessed a network structure that was composed of single crystalline NFs intersected with each other. The CuWO₄ NF films showed superior photoelectrochemical (PEC) activity to other CuWO₄ photoanodes reported recently for the oxygen evolution reaction (OER). We attributed the high activity to the unique morphological and crystalline structure of the CuWO₄ film, which enhanced the photoactivity by providing a large specific area, a short hole transport distance from the inside of CuWO₄ to the CuWO₄/solution interface, and a low grain boundary density. Hydrogen treatment by annealing the CuWO₄ NF film in mixed gases of H₂ and Ar could further enhance the photoactivity, as hydrogen treatment significantly increased the electron density of CuWO₄ by generating oxygen vacancy in the lattice. The photocurrent density for OER obtained on the hydrogen-treated (H-treated) CuWO₄ NF film is the largest ever reported on CuWO₄ photoanodes in the literature. Moreover, the CuWO₄ photoanodes exhibit good stability in weak alkaline solution, while the H-treated CuWO₄ photoanodes exhibit acceptable stability. This work not only reveals the potential of CuWO₄ as a photoanode material for solar water splitting but also shows that the construction of nanostructured CuWO₄ photoanodes is a promising method to achieve high PEC activity toward OER.