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Highlights: [•] The microwave non-thermal effect(s) in photocatalytic TiO2 reactions is inferred. [•] Novel wastewater treatment method by microwave photocatalytic system is proposed. [•] The non-thermal effect(s) in microwaved metal-oxide photocatalyst systems are described. [Copyright &y& Elsevier]

The concept Photocatalysis and, of greater import here, Heterogeneous Photocatalysis were first introduced in the second decade (1910–1920) of the 20th century according to the CAPLUS and MEDLINE databases (SciFinder). This review reports a brief historical perspective on the origins of the two concepts, whether implied or explicitly stated, in some detail up to about the mid-1980s when heterogeneous photocatalysis witnessed the beginning of an exponential growth, with particular emphasis on the use of nanosized TiO2 particles in powdered form as the (so-called) photocatalyst of choice in environmental applications because of its inherent properties of abundance and chemical stability in acidic and alkaline aqueous media (in the dark), in contrast to ZnO that had been the metal oxide of choice in the early days. The early workers in this area often used the term photosensitization rather than the current popular term photocatalysis, used since the early 1980s. The term Photocatalysis appeared in the literature as early as 1910 in a book by Plotnikow (Russia) and a few years later it was introduced in France by Landau. The review also reports on contributions during the early years by Terenin at the University of St. Petersburg (previously Leningrad, Soviet Union), and in the decade spanning 1975–1985 contributions by Bard's group at the University of Texas at Austin (USA) as well as those of other groups. Some activities into the conversion of light energy to chemical fuels (e.g. H2) during the 1975–1985 decade are also considered. [ABSTRACT FROM AUTHOR]

This review article revisits the band model often used to describe semiconductors following recent findings from our laboratory that such a model is inadequate to explain the generally occurring spectral dependence of quantum yields (Φ) of surface photocatalyzed reactions by irradiated metal-oxide nanomaterials in several gas/solid and liquid/solid heterogeneous systems. A one-dimensional model is described to account for experimentally observed spectral variations of Φ. From the solution to the continuity equation and given boundary conditions, certain predictions are made that are consistent with and can explain currently available experimental data. Some key factors that affect the activity and selectivity of photocatalysts in heterogeneous photocatalysis are identified and their relevance discussed; for example, surface charge (i.e.surface potential) and the subsurface electric field in the space charge region are two such key factors. [ABSTRACT FROM AUTHOR]

Electron transfer quenching of the fluorescence of rhodamine-6G in polyvinyl acetate and poly-α-methylstyrene films by aromatic amine quenchers has been examined, using both steady-state and time-resolved methods. Observed quencher concentration dependences do not conform to expectations from either the simple Perrin or Stern–Volmer models. The steady-state quenching results are consistent with long-range electron (tunnel) transfer, with collective quenching at higher quencher concentration, but existing models of this process do not quantitatively rationalize our results. Some potential difficulties with these models are accordingly discussed. Time-resolved (streak camera) experiments confirm that all the quenching events are of dynamic origin, to the exclusion of the involvement of a fractional population of fluorescence-inactive absorbers owing to ground-state complex formation. [ABSTRACT FROM AUTHOR]

Presents information on a study which observed optical limiting behavior in silver bromide nanosols. Experimental procedure; Results; Discussion.

Abstract: The wavelength-dependence of photostimulated adsorption of O2 and H2 on visible-light-active N-doped TiO2 has been examined in the intrinsic (λ<387nm) and in the N-doping induced extrinsic absorption range (λ>387nm). Rates of photoadsorption scaled linearly with photon flow ρ at 4Pa at four selected wavelengths from 300 to 550nm, but were independent of gas pressure and correspond to rates of photogeneration of surface-active centers. The spectral dependence of quantum yields Φ of surface photoprocesses displayed multi-band-like structural features in both the intrinsic and extrinsic regions ascribed to the existence of three (and perhaps four) different types of defect states, confirming the absorption edge red-shift in N-doped TiO2 to be due to electronic excitation of intragap localized states that leads to the photogeneration of electrons and/or holes. [Copyright &y& Elsevier]

Abstract: Studies involving the TiO2-assisted photodegradation of organic substances report that the processes are photocatalytic in nature. Yet, no evidence exists confirming such assertions. Previously, we examined the usage of relative photonic efficiencies [N. Serpone, G. Sauve, R. Koch, H. Tahiri, P. Pichat, P. Piccinini, E. Pelizzetti, H. Hidaka, J. Photochem. Photobiol. A: Chem. 94 (1996) 191; N. Serpone, J. Photochem. Photobiol. A: Chem. 104 (1997) 1] and quantum yields Φ [N. Serpone, R. Terzian, D. Lawless, P. Kennepohl, G. Sauve, J. Photochem. Photobiol. A: Chem. 73 (1993) 11] to systematize discrepant claims about process efficiencies. An experimental protocol is now available [N. Serpone, A. Salinaro, Pure Appl. Chem. 71 (1999) 303] to measure true Φ in heterogeneous media. Photoinduced reduction of O2 and photooxidation of H2 occurring on oxidized and reduced surfaces of ZrO2 were recently examined [A.V. Emeline, G.N. Kuzmin, L.L. Basov, N. Serpone, J. Photochem. Photobiol. A: Chem. 174 (2005) 214] to probe the spectral variations of the photoactivity and photo-selectivity of ZrO2 by determining Φ for the two redox reactions at various wavelengths of photoexcitation (200< λ <400nm). Irradiation of ZrO2 in the intrinsic absorption region (λ <260nm) led predominantly to photoreduction of O2, whereas photooxidation of H2 predominated on irradiation in the extrinsic spectral region (260< λ <360nm). A difficult task in heterogeneous catalysis and photocatalysis is determination of the actual number of active sites, an issue that has heretofore been elusive to assess the (photo)catalytic activity of a given material in heterogeneous solid–liquid and solid–gas (photo)catalysis. A kinetic description of the three turnover quantities, viz., turnover number (TON), turnover rate (TOR) and turnover frequency (TOF) has been described [N. Serpone, A. Salinaro, A.V. Emeline, V.K. Ryabchuk, J. Photochem. Photobiol. A: Chem. 130 (2000) 83], concluding that turnover quantities are conceptually distinct, with TON and TOR requiring knowledge of the number of active sites on the (photo)catalyst''s surface. Apparently, turnovers depend on the nature of the active state of the catalyst and how it is described. The number of surface-active sites on the ZrO2 particle surface have been determined quantitatively (∼1016 active centers) through thermoprogrammed desorption spectroscopy, affording an estimate of TONs for the photooxidation of H2 (TON>14.5) and photoreduction of O2 (TON>6.6) on ZrO2 and demonstrating for the first time that a photoreaction occurring on the surface of a metal oxide is truly photocatalytic [A.V. Emeline, A.V. Panasuk, N. Sheremetyeva, N. Serpone, J. Phys. Chem. B 109 (2005) 2785]. Photocoloration of a metal oxide such as ZrO2 (process of photoinduced formation of Zr3+, F- and V-type color centers) during a surface photochemical reaction was also used to assess whether a reaction is photocatalytic. Our study on the influence of simple photoreactions involving the photoreduction of O2, photooxidation of H2, photooxidation of H2 by adsorbed O2, and photoinduced transformation of NH3 and CO2 on the photocoloration of ZrO2 concluded that photoreactions involving NH3 and CO2 are non-photocatalytic processes, in contrast to the photooxidation of H2 which is photocatalytic [A.V. Emeline, G.V. Kataeva, A.V. Panasuk, V.K. Ryabchuk, N.V. Sheremetyeva, N. Serpone, J. Phys. Chem. B 109 (2005) 5175]. In this article, we describe the criteria and conditions by which a photoreaction taking place on the surface of a solid can be said to be photocatalytic by considering both a chemical approach and a physical approach. [Copyright &y& Elsevier]

A number of calcium bismuthates were synthesized (25 to 50 mol% in Ca) and characterized by XRD, SEM, EDX, XPS and DRS techniques; the latter provided an estimate of the bandgap energies (Ebg=2.41 to 3.29 eV) via Tauc plots for indirect transitions, whereas XPS established the potentials (vs NHE) of their respective valence bands (and thus the conduction bands). Linear correlations existed between EVB/ECB and Ebg that when compared with those of strontium bismuthates (reported earlier) showed that differences in energies at Ebg=0 eV are related to the difference in the absolute electronegativities of Ca and Sr, from which the following empirical phenomenological rule is postulated: replacing one alkaline earth metal in bismuthates by another causes the points of intersection of the linear correlations ECB(Ebg) and EVB(Ebg) to be displaced by an amount equal to twice the difference in absolute electronegativities of these metals. [ABSTRACT FROM AUTHOR]

This article reports an in situ UV–Vis–NIR diffuse reflectance (DR) spectroscopic and kinetic study of the photoformation and thermal annealing of light absorbing electronic point defects (color centers) in photochromic TiO2 in the temperature range 90–720 K using a simple laboratory-made cryostat-type accessory (for a Cary 5000 spectrophotometer equipped with an integrating sphere). The accessory also allowed for UV–Vis–NIR DR studies to be undertaken either in vacuum or in an oxygen atmosphere at significantly high temperatures (to 720 K) to assess dark chemical events occurring in photochromic titania with the participation of color centers. The DR spectral and kinetic measurements provided the opportunity to examine the separation of photoinduced charge carriers at traps and thermally stimulated carrier detrapping and recombination, as well as the response of color centers to oxidative/reductive treatments of photochromic TiO2. Kinetic results also demonstrate the applicability of the fabricated DR accessory as a high-temperature reaction cell in the systematic study of the principal regularities in the formation and destruction of color centers in titania at various temperatures and gaseous atmospheres. [ABSTRACT FROM AUTHOR]

This Letter reports a comparative study of the luminescence from pristine and Cr-doped MgAl 2 O 4 spinel induced by different excitation mechanisms: photoluminescence (PhL), thermoluminescence (ThL) and Photo-Induced Chemisorption Luminescence (the PhICL phenomenon) to understand the mechanism of PhICL emission. Cr-doping alters the major pathway of physical relaxation through a luminescence pathway: quenching of the luminescence associated with intrinsic defects and appearance of the luminescence from Cr 3+ -states. The similarity between ThL and PhICL spectra suggest the mechanism of the PhICL phenomenon is due to electron transfer from the surface to the emission centers of luminescence; an energy transfer pathway is not precluded. [ABSTRACT FROM AUTHOR]

This Article reports on the thermo- and photo-stimulatedeffectson the optical properties of rutile titania ceramic layers fabricatedin an air atmosphere by high-temperature calcination of (technicalgrade) titanium substrates. The so-formed layers peeled off spontaneouslyduring the cooling phase back to ambient temperature to reveal a yellow-coloredupper surface and a cream-colored bottom surface that was in contactwith the titanium plate. The two surfaces of the layers and a powderedspecimen (formed from grinding the peeled-off layers) were characterizedby X-ray diffraction, Raman spectroscopy, scanning electron microscopy,electron dispersive spectroscopy, and diffuse reflectance spectroscopy.The upper surface demonstrates a strong photochromic effect. A pronouncedincrease 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 spectralregion and a strong decrease of these bands during the heating ofirradiated samples to 200–230 °C were characteristicsof the upper layer’s surface. A wide set of spectra resultingfrom the reversible absorption changes made possible the disclosureof higher-energy absorption bands at 2.91 eV (AB1) and 2.54 eV (AB2);the latter were not affected by irradiation and heating. An electronicmechanism based on known properties of intrinsic point defects ofTiO2, F-type centers (two electrons trapped in oxygen vacancies)and Ti3+centers, is proposed to account for the opticalchanges that occurred through the photoinduced formation, photobleaching,and thermal bleaching of the absorption bands. [ABSTRACT FROM AUTHOR]

The present study explored photoinduced radical processescausedby interaction of CH4and NH3with a photoexcitedsurface of a complex metal oxide: magnesium–aluminum spinel(MgAl2O4; MAS). UV irradiation of MAS in vacuoyielded V-type colorcenters as evidenced by the 360 nm band in difference diffuse reflectancespectra. Interaction of these H-bearing molecules with photogeneratedsurface-active hole states (OS–•)yielded radical species which on recombination produced more complexmolecules (including heteroatomic species) relative to the initialmolecules. For the MAS/CH4system, photoinduced dissociativeadsorption of CH4on surface-active hole centers produced•CH3radicals that recombined to yield CH3CH3. For MAS/NH3, a similar dissociative adsorptionprocess led to formation of •NH2radicals with formationof NH2NH2as an intermediate product; continuedUV irradiation ultimately yielded N2. For the mixed MAS/CH4/NH3system, however, interaction of adsorbed NH3and CH4on the UV-activated surface of MAS yielded•NH2and •CH3radicals, respectively,which produced CH3–NH2followed by lossof the remaining hydrogens to form a surface-adsorbed cyanide, CNS, species. Recombination of photochemically produced radicalsreleased sufficient energy to re-excite the solid spinel, generatingnew surface-active sites and a flash luminescence (emission decaytime at 520 nm, τ ∼ 6 s for the MAS/NH3case)referred to as the PhICL effect. [ABSTRACT FROM AUTHOR]

Abstract: The photoinduced reduction of oxygen and photooxidation of hydrogen occurring on oxidized and reduced surfaces of the metal-oxide dielectric material ZrO2 (in monoclinic form) have been examined to probe the spectral variations of the photoactivity and photoselectivity of this metal oxide by determining (true) photochemically defined quantum yields for the two redox reactions at various wavelengths of photoexcitation (200nm&#60;λ &#60;400nm). Irradiation of zirconia in the fundamental absorption (intrinsic; λ &#60;260nm) region leads predominantly to the photoreduction of oxygen, whereas the photooxidation of hydrogen predominates on irradiation in the extrinsic spectral region (260nm&#60; λ &#60;360nm). A pathway for the formation of electron-trapped states (Zr3+) and hole-trapped states (O −) in the extrinsic spectral region is described since both oxidation and reduction take place in this extrinsic region with oxidation predominating. The reported experimental results provide the long sought-after spectral variations into the photoactivity and photoselectivity of metal-oxide photocatalysts in gas/solid heterogeneous systems. [Copyright &y& Elsevier]

Heterogeneous reactions occur on solid photocatalyst particles of semiconductors and dielectric materials. When irradiated with suitable UV/visible light energy these particles generate electrons and holes, which on the surface are poised to undergo reductive and oxidative chemistry with a variety of organics and light gases. Various such particles have been identified in Interstellar Space, specifically in molecular–dust clouds, comets and meteorites. In this article, we examine briefly the nature of these dust clouds and then describe some basic aspects of heterogeneous photocatalysis, a methodology that has been shown useful in transforming organic substrates into smaller molecules and in the synthesis of potential biomolecules. Various types of gas/solid heterogeneous reactions involving mostly small molecules in gas/solid systems find a relationship to abiogenesis. For example, the decomposition of H2O and CO2 in the presence of CH4 yields H2CO; methane is photoconverted into ethane, propane, ethylene and other hydrocarbons and is photooxidized to alcohols and carbon dioxide; photofixation of CO2 occurs to yield formaldehyde, formic acid, methanol and methane; and finally photofixation of molecular nitrogen N2 can take place to produce NH3 and N2H2. Not least is the synthesis of glycine, alanine, aspartic acid and serine from CH4 and NH3 over platinized titania. The relevance of heterogeneous photocatalysis to abiogenesis is discussed. It is argued that the physical conditions available in the interstellar medium are propitious to generate such biomolecules as amino acids and others, albeit this assertion necessitates laboratory simulations. Recent laboratory experiments involving very simple photoinduced processes are encouraging. [Copyright &y& Elsevier]

Examines the photodegradation of p-nonylphenol olyethoxylate (NPE-9) in a cylindrical photoreactor with titanium dioxide (TiO[sub 2]) immobilized fiberglass cloth. Evaluation of various preparative procedures in the fabrication of TiO[sub 2]-fiberglass specimens; Comparison of the photodegradation of NPE-9 with processes occurring in TiO[sub 2] aqueous dispersions.

In this study we explored the photostimulated adsorption of dioxygen and dihydrogen gases on the surface of Degussa P25 titania particles subsequent to a thermal pre-treatment at 600 and 850 K, which of consequence varied the typical anatase/rutile ratio (83–17%, respectively). The focus was to probe how the anatase/rutile content modified the spectral dependence of the quantum yields of photoadsorption of O2 and H2. Dioxygen is photoadsorbed preferentially on the anatase component of the P25 titania specimen, inasmuch as no photoadsorption occurs on an ultrapure rutile particle surface. By contrast, dihydrogen photoadsorbs on both polymorphic structures on the particle surface. A model is proposed wherein the Degussa titania is taken to be composed of rutile domains (surface and bulk) in an otherwise sea of anatase. These represent nothing more than surface defects, which when excited with appropriate radiation generate photoelectrons that spillover onto the anatase phase and photostimulate the adsorption of the acceptor molecules (oxygen). [Copyright &y& Elsevier]

The photodegradation of p-nonylphenol polyethoxylate (NPE-9) has been examined in a large-scale circulating photoreactor to assess a set of different procedures to construct hybrid TiO2 films immobilized on a flexible fiberglass cloth (TiO2–FGC). The method consisted of pasting active TiO2 particles onto the fiberglass cloth support (PA method; 0.24 nm thick; weight, 210 g m−2), followed by a number of dip-coatings in a titania sol–gel solution (SG method). Various TiO2 particulates from different sources have been screened. The Degussa P-25 titania exhibited the greater photocatalytic activity. The hybrid TiO2–FGC specimens retained a greater quantity of the TiO2 catalyst on the FGC support than did the corresponding PA and SG samples. Comparison of the photodegradation of NPE-9 taking place in the various hybrid TiO2–FGC specimens with processes occurring in TiO2 aqueous dispersions, on PA TiO2–FGC and SG TiO2–FGC specimens reveals that the hybrid TiO2–FGC specimens were the more photocatalytically active. Formic and acetic acids are two of the photo-oxidation intermediates identified in the degradation of NPE-9. Experimental conditions used in the present study to assess the cylindrical photoreactor containing the hybrid TiO2–FGC specimens show that such a reactor can be useful in a large-scale treatment of wastewaters. [Copyright &y& Elsevier]

Review: 45 refs. [ABSTRACT FROM AUTHOR]