Oxygen species ionosorbed on powder photocatalyst oxides from room-temperature photoconductivity as a function of oxygen pressure

Abstract

Using a cell designed to study the electronic interactions of gases with illuminated powders, the photoconductance, σ, of a series of oxides (TiO₂, ZrO₂, V₂O₅, ZnO, SnO₂, Sb₂O₄, CeO₂ and WO₃) has been measured at equilibrium under various oxygen pressures P_O2(from 7 × 10² down to 0.7 Pa) at room temperature in an attempt to obtain information on the nature of negative oxygen species adsorbed on these illuminated surfaces. For the majority of the samples, the conductance of the non-illuminated part was negligible compared with that of the illuminated layer. In a log–log plot, the slopes of σ against P_O2 were very close to –1, –½ or 0. A slope of –1 (found only for TiO₂ on the higher-pressure side) and a slope of –½(observed for ZrO₂, ZnO and, for part of the pressure range examined, for TiO₂, Sb₂O₄ and CeO₂) were interpreted as indicating that O^–₂ and O^– species, respectively, controlled the equilibrium between the free electrons and the gas. SnO₂ and WO₃ were characterized by a poor relative response to illumination, so that σ variations with P_O2 were masked; consequently the measurements failed in determining with certainty the type of adsorbed oxygen species, although the σ sensitivity to initial O₂ admission showed the occurrence of ionosorption. The V₂O₅ sample did not behave as a photosemiconductor. This is consistent with the photocatalytic activities of these samples in propene oxidation (very small for SnO₂ and WO₃, nil for V₂O₅). However, no simple correlation exists between the photoconductance variations thus measured and the photocatalytic properties, which shows that the surface densities of electrons and of ionosorbed oxygen species are not the only factors to consider for oxidation reactions.