Powdered zirconia and colloidal zirconia aqueous sols have been examined by diffuse reflectance and absorption spectroscopies and by photoluminescence methods in solid/gas and solid/liquid systems. The former system was examined following high-temperature treatment in vacuo and under reducing and oxidizing atmospheres. Studies of the influence of H2 and O2 on the photophysics of microparticles (powder) and nanoparticles (colloidal sols) of zirconia at solid/gas interfaces and the effects of free carrier scavengers (CH3OH and O2) on the photophysics at solid/liquid interfaces were undertaken to explore the correlation between surface chemistry and the nature of preexisting or photogenerated defect centers (e.g., F-type and V-type color centers). ZrO2 is an insulating, direct wide-gap metal oxide with an optical band gap of ∼5.0 eV; another optical transition occurs at 5.85 eV. The optical behavior depends on whether zirconia is preirradiated in the intrinsic (hν > 5.0 eV) or extrinsic (hν < 5.0 eV) absorption regions. The red limits of the effects are 3.0 and 3.2 eV for microparticles and nanoparticles, respectively. New defects are formed by the photoionization of, and/or by free carrier trapping by, existing defects. New defects formed by tunneling electron transfer from donor to acceptor defect states in zirconia nanoparticles are not precluded. Regardless of the type of mechanism, the influence of surface chemical reactions on the formation of defect centers is typical of both systems which luminesce under irradiation. Powdered ZrO2 shows a decrease in luminescence the longer it is irradiated. Emission decay in ZrO2 sols depends on whether the sols were preirradiated in the intrinsic or extrinsic regions; luminescence intensity was affected by the type of carrier scavengers present (methanol or oxygen). Different origins have been identified for the decay of emission: (i) for powdered ZrO2 samples, nonradiative recombination of free electrons with photogenerated hole centers after preirradiation with UV light; (ii) for preirradiated colloidal ZrO2 sols, photoionization of, and recombination of free carriers with, emissive defect centers.

In a recent article, Ollis analyzed heretofore reported photocatalyst kinetics of surface photochemical reactions that take place in heterogeneous systems and that rely heavily on the Langmuir-Hinshelwood (LH) kinetic model to interpret the experimental observations. This model assumes a fast adsorption/desorption equilibrium step and a subsequent slow surface step. His interesting analysis of the experimental results reported in 2000 by Emeline and co-workers, Xu and Langford, and Martyanov and Savinov prompted our reexamination of the LH kinetic model along with several other dogmas that continue to propagate in the heterogeneous photocatalytic landscape. This short article discusses some of these issues and reexamines certain misinterpretations. Specifically, we reexamine (1) the a priori assumed validity of the LH kinetic model in heterogeneous photocatalysis, (2) the recombination of photogenerated free charge carriers on the solid (metal oxide) photocatalyst by the band-to-band recombination pathway, and (3) the mistaken assertion that the kinetics of a heterogeneous photoreaction are either only first-order dependent or half-order dependent on photon flow (i.e., light irradiance).

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In this study, we examine theoretically some of the factors that influence the spectral dependencies of the quantum yield, Φ, of photochemical processes on the surface of nano-/microparticulates of wide-band-gap metal-oxide photocatalysts. The approach taken is based on solving the steady-state ((dn(t)/dt) = 0) continuity equation (eq 7) for a one-dimensional plate with dimensions x = 0−d irradiated on both sides and considering the spatial nonuniformity of photogeneration of charge carriers in the bulk of the solids and their limited probability of diffusion toward the particulate surface. This spatial nonuniformity of charge carrier photogeneration depends on whether the solid photocatalyst is photoexcited in the intrinsic or extrinsic regions or whether only the surface is photoexcited. Essential conditions required to observe similarities between the spectral dependencies of the quantum yield of surface processes and the absorption spectrum are (i) the relative values of the absorption coefficient α (cm-1) and (ii) the complexity of the absorption spectrum. Spectral selectivity of photocatalysts appears to depend on the different probabilities that electrons and holes reach the photocatalyst surface which are the result of differences in their mobilities and lifetimes (i.e., different diffusion lengths of the carriers).