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 this study we examine photostimulated reactions taking place on the surface of wide band-gap metal oxides such as ZrO2 and TiO2. In particular, we explore the photostimulated adsorption (PhA) of oxygen on ZrO2 upon illumination with UV light (λ < 260 nm) and its postphotostimulated photoadsorption (PPSPhA) on irradiation with visible light (λ > 400 nm). The emphasis is on an examination of the decay channels of PhA-active surface centers to rationalize the interdependencies of the rate r(ρ ,p) on pressure and light intensity The distinguishing feature of O2 PhA kinetics, dp/dt ∝ ρ m p n , resides in the dependence of the order of the reaction on the order m with respect to photon flow (ρ m ) and on the order n with respect to pressure (p n ). In other words, m → 1 if n → 0 whereas n → 1 if m → 0. The reaction orders m and n are interdependent in the case of PhA of O2 on ZrO2. A general mechanism is described for the several stages implicated in both PhA and PPSPhA surface processes. A preliminary consideration is also given to expand the PhA pathway for O2 on ZrO2 to processes that take place in the photocatalyzed oxidation of phenol in irradiated TiO2 aqueous dispersions and the interdependence of the rate of the photoreaction on the concentration of phenol and on the incident light intensity.

This article reports on two novel visible-light-active strontium bismuthate photocatalysts (Sr 6 Bi 2 O 11 and Sr 3 Bi 2 O 6) prepared by solid-state synthesis for which the number of strontium atoms exceeded the number of bismuth atoms in the cation sublattice; for comparison, the bismuthate Sr 2 Bi 2 O 5 was also reexamined. All three bismuthates were characterized by a variety of spectroscopic techniques (XRD, XPS, EDX, DR, Raman, SEM, and EIS). Both newly as-synthesized bismuthates displayed photocatalytic activity toward the photodegradation of substrates in the gas phase (acetaldehyde) and in aqueous media (phenol), with the Sr 6 Bi 2 O 11 phase exhibiting a significantly greater photoactivity than the Sr 3 Bi 2 O 6 phase; by comparison, the bismuthate Sr 2 Bi 2 O 5 was photocatalytically inactive. Detailed photocatalytic mechanisms have been proposed to explain how composition and structure of the three bismuthates affect their photocatalytic activity. The role of point defects in their crystal lattice is described for processes in which the photocatalytic activity was inhibited. Inferences made were aided by examining the UV-induced coloration of these ternary metal oxides that provided information regarding defect levels within their respective bandgaps; such defects acted as electron traps and thus affected the photocatalytic performance of the bismuthates.

This Article reports on the thermo-and photostimulated effects on the optical properties of rutile titania ceramic layers fabricated in an air atmosphere by hightemperature calcination of (technical grade) titanium substrates. The so-formed layers peeled off spontaneously during the cooling phase back to ambient temperature to reveal a yellow-colored upper surface and a cream-colored bottom surface that was in contact with the titanium plate. The two surfaces of the layers and a powdered specimen (formed from grinding the peeled-off layers) were characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, electron dispersive spectroscopy, and diffuse reflectance spectroscopy. The upper surface demonstrates a strong photochromic effect. A pronounced increase of the amplitude of the absorption bands at 2.06 eV (AB3) and 1.56 eV (AB4) seen under irradiation in the UV or visible spectral region and a strong decrease of these bands during the heating of irradiated samples to 200−230 °C were characteristics of the upper layer's surface. A wide set of spectra resulting from the reversible absorption changes made possible the disclosure of higher-energy absorption bands at 2.91 eV (AB1) and 2.54 eV (AB2); the latter were not affected by irradiation and heating. An electronic mechanism based on known properties of intrinsic point defects of TiO 2 , F-type centers (two electrons trapped in oxygen vacancies) and Ti 3+ centers, is proposed to account for the optical changes that occurred through the photoinduced formation, photobleaching, and thermal bleaching of the absorption bands.

The set of Al-doped titania x-Al-TiO 2 (x = 0.0, 0.1, 0.3, 0.5, 0.7, 0.9, 1.1 wt% Al) has been synthetized by sol-gel method and tested in the reaction of photocatalytic degradation of phenol in aqueous suspension. XRD and Raman studies show that TiO 2 samples have mixedphase rutile-anatase crystalline structure with linear increase of anatase fraction from 0.0 wt% for 0.0-Al-TiO 2 up to 18 wt% for 1.1-Al-TiO 2 . The decrease of the particle size from ∼ 800 nm to 50 nm and the increase of specific surface area from 1.7 m 2 /g up to 28 m 2 /g with increased content of Al have been observed. The data of XRD, XPS, Raman spectroscopy and EDS