Enhanced Photoanodic Activity and Outermost Surface Crystallinity of Tungsten Oxide via High-temperature Sintering

Tungsten oxide (WO3) is a promising photoanode material capable of water oxidation under visible-light irradiation. Although WO3 is usually prepared via sintering at 500 °C–550˚C, this work shows that high-temperature sintering (i.e., at 600 °C) can lead to efficient output at the WO3 photoanode. The material characteristics such as the crystal system, surface structure, film thickness, and optical properties were essentially independent of the sintering temperatures employed. However, the high-temperature-sintered WO3 showed low charge transfer resistance at the electrode–electrolyte interface, resulting in improved charge injection efficiency for water oxidation at the WO3 photoanode. WO3 sintered at 550 °C and 600 °C showed the similar visible Raman spectra with strong band intensities, indicative of improved crystallinity in WO3 bulk particularly in the comparison with WO3 sintered at 450 °C. However, the ultraviolet Raman spectrum exhibited intense bands for only the WO3 prepared at 600 °C, indicating the enhanced crystallization of the WO3 outermost surface. Thus, the high crystallinity in the WO3 bulk and at its surface results in efficient photoanodic output owing to the suppression of electron–hole recombination.