WO3 photocatalysts: Influence of structure and composition

Hexagonal (h-) and monoclinic (m-) WO3 nanoparticles with controlled composition (oxidized/yellow color or partially reduced/blue color) were prepared through annealing (NH4)xWO3−y. The formation, structure, composition, morphology, and optical properties of the samples were analyzed by powder X-ray diffraction, scanning and transmission electron microscopy combined with electron diffraction, and Raman, X-ray photoelectron, 1H magic angle spinning nuclear magnetic resonance, diffuse reflectance ultraviolet–visual, and photoluminescence spectroscopy. Their photocatalytic properties were tested by decomposing methyl orange in the aqueous phase and acetone in the gas phase. Oxidized m-WO3 (m-WO3 ox) was the most active photocatalyst both in the aqueous and in the gas phase, followed by the oxidized h-WO3 (h-WO3 ox) sample. Reduced h-WO3 (h-WO3 red) and m-WO3 (m-WO3 red) exhibited much lower activity. Thus, in contrast to TiO2, where crystalline structure (rutile or anatase) plays a key effect in photocatalysis, for WO3, it is the composition that is of greatest importance: the more oxidized the WO3 sample, the better a photocatalyst it is. The crystal structure of WO3 has only an indirect effect, in that it influences the composition of WO3 samples. While oxidized m-WO3 is completely oxidized, oxidized h-WO3 is always in a partially reduced state due to the presence of stabilizing positive ions in its hexagonal channels. Consequently, an oxidized monoclinic WO3 material will always provide better photocatalytic activity than an oxidized hexagonal one.