Composites Based on Electrodeposited WO and TiO Nanoparticles for Photoelectrochemical Water Splitting.

Photoelectrochemically active WO films were fabricated by electrodeposition from an acidic (pH 2), hydrogen-peroxide-containing electrolyte at -0.5 V vs. SCE. WO-TiO composites were then synthesized under the same conditions, but with 0.2 g/L of anatase TiO nanoparticles (⌀ 36 nm), mechanically suspended in the solution by stirring. After synthesis, the films were annealed at 400 °C. Structural characterization by XRD showed that the WO films exhibit the crystalline structure of a non-stoichiometric hydrate, whereas, in WO-TiO, the WO phase was monoclinic. The oxidation of tungsten, as revealed by XPS, was W for both materials. Ti was found to exist mainly as Ti in the composite, with a weak Ti signal. The efficiency of the WO films and composites as an oxygen evolution reaction (OER) photo-electrocatalyst was examined. The composite would generate approximately three times larger steady-state photocurrents at 1.2 V vs. SCE in a neutral 0.5 M NaSO electrolyte compared to WO alone. The surface recombination of photogenerated electron-hole pairs was characterized by intensity-modulated photocurrent spectroscopy (IMPS). Photogenerated charge transfer efficiencies were calculated from the spectra, and at 1.2 V vs. SCE, were 86.6% for WO and 62% for WO-TiO. Therefore, the composite films suffered from relatively more surface recombination but generated larger photocurrents, which resulted in overall improved photoactivity.