An XPS study of the UV reduction and photochromism of MoO3 and WO3

Crystalline powder pellets of MoO3 and WO3 were irradiated with 253.7 nm radiation at room temperature in vacuum, in the presence of H2 and in the presence of Hg vapor and H2. Electronic changes induced in the surface of the solids were studied by x-ray photoelectron spectroscopy. In vacuo both MoO3 and WO3 turned blue upon irradiation and the XPS spectra supported the reversible formation of +5 oxidation states which reverted upon exposure to air. When MoO3 was irradiated in the presence of H2, a hydrogen/molybdenum bronze containing Mo in the +6, +5, and +4 oxidation states and hydroxyl groups were irreversibly formed. Exposure to UV radiation in the presence of Hg vapor and H2 enhanced the formation of Mo+4 and hydroxyl groups. Irradiation of WO3 in the presence of H2 and Hg vapor/H2 mixtures produced no further reduction of tungsten compared to irradiation alone. The conclusions based upon core electron emissions from molybdenum, tungsten, and oxygen were supported by changes in the valence band spectra.