Direct anodic growth of thick WO3 mesosponge layers and characterization of their photoelectrochemical response

Thick mesoporous tungsten oxide (WO3) layers can be formed by anodization of tungsten in a 10 wt% K2HPO4/glycerol electrolyte, if the electrolyte temperature is around 80–100 °C. At 90 °C, a regular mesoporous WO3 layer was grown up to a thickness of approximately 9 μm. This WO3 mesosponge layer consists of typical feature sizes of 20–30 nm and pore widths of 10–30 nm. The photoresponse of different layer thicknesses and different annealing treatments was characterized in a photoelectrochemical cell. The highest photocurrents were observed with a 2.5 μm thick WO3 layer annealed at 550 °C consisting of a mixture of orthorhombic, triclinic and monoclinic phases. Incident photon to current efficiencies (IPCEs) of the samples were 73.4% in a 1 M HClO4 electrolyte and 167.5% for methanol photo-oxidation in 0.1 M CH3OH/1 M HClO4 electrolyte, at 1 V vs. Ag/AgCl under illumination at a wavelength of 420 nm.