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.

A nonphotocatalytic reaction occurring on the surface of an irradiated wide band gap metal oxide, such as ZrO2, can affect the process of photoinduced formation of Zr3+, F- and V-type color centers. The effect of such reactions is seen as the influence of photostimulated adsorption on the photocoloration of the metal oxide specimen. In particular, photoadsorption of electron donor molecules leads to an increase of electron color centers, whereas photoadsorption of electron acceptor molecules leads to an increase of hole color centers. Monitoring the photocoloration of a metal oxide during a surface photochemical reaction probes whether the reaction is photocatalytic: accordingly, the influence of simple photoreactions on the photocoloration of ZrO2, reactions that involved the photoreduction of molecular oxygen, the photooxidation of molecular hydrogen, the photooxidation of hydrogen by adsorbed oxygen, and the photoinduced transformation of ammonia and carbon dioxide. Kinetics of the photoprocesses are reported, as well as the photoinduced chesorluminscence (PhICL effect) of ammonia. Thermoprogrammed desorption and mass spectral monitoring of the photoreaction involving NH3 identified hydrazine as an intermediate and molecular nitrogen as the final product. The photoreactions involving NH3 and CO2 are nonphotocatalytic processes, in contrast to the photooxidation of hydrogen which is photocatalytic. Carbon dioxide and carbonate radical anions are formed by interaction of CO2 with Zr3+ centers and hole states (OS-*), respectively. Mechanistic implications are discussed.

The interconnection between photostimulated formation and destruction of defect centers (F-and V-type color centers) and surface photochemical reactions taking place on the surface has been examined for a series of 21 wide band gap metal-oxide specimens, which comprise insulators and semiconductors whose band-gap energies span the range from ca. 3 eV (ZnO, TiO 2 ) to ca. 11 eV (BeO). Photostimulated post-adsorption of O 2 was seen for 9 of the 21 metal oxides tested. Three of these specimens, namely scandia (Sc 2 O 3 ), zirconia (ZrO 2 ), and normal spinel (MgAl 2 O 4 ), were chosen for detailed study to establish that spectral sensitization of metal oxides by UV illumination is a generally occurring phenomenon which is carried over to photostimulated adsorption of molecules and to surface photochemical reactions. Results demonstrate that irradiation into the absorption bands of the color centers of the photocolored metal oxides leads to a red shift of the spectral limit of surface photoreactions. A simple mechanism is described quantitatively for the photocoloration and surface reactions. Quantum yields of photobleaching of color centers in vacuo and in the presence of H 2 and O 2 are reported for scandia and zirconia, together with the quantum yields of photoadsorption of H 2 and O 2 molecules.

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