Nanostructured films of tungsten oxide and metal tungstates and their use for photoelectrochemical water splitting

Metal oxide semiconductor materials such as tungsten oxide and metal tungstates are promising candidates for use as photoanodes in solar water splitting. In this context nanostructured tungsten oxide, copper tungstate and nickel tungstate thin films have been prepared, the factors influencing their growth studied, and their PEC properties investigated. Different morphology WO3 films, including one-dimensional (1-D) nanorods, two-dimensional (2-D) nanosheets, and three-dimensional (3-D) nanotrees and nanoflowers, were synthesized using spray chemical vapour deposition (spray CVD), chemical bath deposition (CBD) and aerosol-assisted chemical vapour deposition (AACVD). The spray CVD and AACVD methods provide simple, single-step and industrially applicable ways to fabricate WO3 films with excellent stability, which results from the combined effects of sub stoichiometric WO3-x and enriched (0 0 2) crystal facets. The effect on film growth of deposition temperature, deposition time, and precursor solution concentration and volume were systematically studied and the materials PEC performance optimized. The optimal WO3 nanorod film achieved a photocurrent density of 0.73 mA/cm2, with a thickness of 4.7 μm and an average nanorod diameter of 330 nm. Nanostructure CuWO4 films were successfully prepared using a facile and single-step spray CVD method. The film prepared at 450 ℃ had a bandgap of ~2.3 eV and a photocurrent density of ~0.21 mA/cm2 at 1.23 V (vs. RHE), which is the highest photocurrent reported for CVD deposited CuWO4, and it had significantly enhanced stability in neutral solution compared to WO3. Then, CuWO4/WO3 shell/core films were prepared by a single-step spray CVD method. The films had much better photocurrent density than either WO3 or CuWO4 film, which was ascribed to the heterojunction structure which facilitates photogenerated charge separation and transportation. Nanostructured NiWO4 films were prepared by AACVD using two different precursors. The film prepared using Ni(thd)2 was not uniform and contained both WO3 and NiO impurities, whilst the NiWO4 film prepared using Ni(acac)2 contained no impurities. The influence of deposition temperature and the Ni/W precursor ratio on phase purity was investigated. The PEC performance of the CVD deposited NiWO4 films were studied, and showed a comparable photocurrent density to NiWO4 films prepared in literature by other methods.