Highly stable and efficient photoelectrochemical water oxidation at an anisotropically crystallized monoclinic WO3 film with predominant growth of the (202) plane

Abstract

An anisotropically crystallized monoclinic WO₃ film was synthesized on a fluorine-doped tin oxide (FTO) electrode by a one-step hydrothermal method using oxalic acid (Oxa) as a structure directing agent. The WO₃ film (denoted as WO₃(w-Oxa)) prepared with Oxa was composed of relatively large layered-sheets with 2–5 μm length with predominant growth of the (202) plane (parallel to the substrate), in contrast to small layered-sheets with 1–2 μm length with predominant growth of the (020) or (200) planes for the WO₃(w/o-Oxa) film prepared without Oxa. Although the photocurrents were generated above 0.6 V vs. RHE for WO₃(w-Oxa) and WO₃(w/o-Oxa) electrodes, the incident photon to electron conversion efficiency (IPCE₄₂₀ = 38%) at 420 nm and 1.23 V vs. RHE for the former was 2.7 times higher than that (14%) of the latter. The higher IPCE₄₂₀ for WO₃(w-Oxa) is ascribed to higher charge separation (η_sep = 50%) and catalytic (η_cat = 95%) efficiencies compared to those (η_sep = 19% and η_cat = 82%) for the WO₃(w/o-Oxa) electrode. Photoelectrochemical impedance spectroscopy (PEIS) measurement suggested effective bulk charge transport in the WO₃(w-Oxa) electrode compared to the WO₃(w/o-Oxa) electrode, with advantages suppression of charge recombination, being responsible for the higher η_sep, eventually causing a higher IPCE for the WO₃(w-Oxa) electrode. The WO₃(w-Oxa) electrode showed high photocurrent stability (95% maintained for 7 h) and a Faraday efficiency (FE_O2) of 95% for water oxidation. The high stability and FE_O2 for the WO₃(w-Oxa) electrode result from the suppression of the competed photo-oxidation of the surface by the attenuated hole accumulation due to efficient water oxidation at the (202) facet surface, which is supported by surface energy calculation of the exposed (202), (020) and (200) facets.