TiO2 photocatalysis and related surface phenomena

Jenny Schneider,*,† Masaya Matsuoka,‡ Masato Takeuchi,‡ Jinlong Zhang, Yu Horiuchi,‡ Masakazu Anpo,‡ and Detlef W. Bahnemann*,† †Institut fur Technische Chemie, Leibniz Universitaẗ Hannover, Callinstrasse 3, D-30167 Hannover, Germany ‡Faculty of Engineering, Osaka Prefecture University, 1 Gakuen-cho, Sakai Osaka 599-8531, Japan Key Lab for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, Shanghai 200237, China

Understanding TiO2 photocatalysis: mechanisms and materials.

The detection methods and generation mechanisms of the intrinsic reactive oxygen species (ROS), i.e., superoxide anion radical (•O2–), hydrogen peroxide (H2O2), singlet oxygen (1O2), and hydroxyl radical (•OH) in photocatalysis, were surveyed comprehensively. Consequently, the major photocatalyst used in heterogeneous photocatalytic systems was found to be TiO2. However, besides TiO2 some representative photocatalysts were also involved in the discussion. Among the various issues we focused on the detection methods and generation reactions of ROS in the aqueous suspensions of photocatalysts. On the careful account of the experimental results presented so far, we proposed the following apprehension: adsorbed •OH could be regarded as trapped holes, which are involved in a rapid adsorption–desorption equilibrium at the TiO2–solution interface. Because the equilibrium shifts to the adsorption side, trapped holes must be actually the dominant oxidation species whereas •OH in solution would exert the reactivity...

Generation and Detection of Reactive Oxygen Species in Photocatalysis

Photocatalytic degradation of organic water contaminants: Mechanisms involving hydroxyl radical attack

http://nathan.instras.com/documentDB/paper-12.pdf

Applications of functionalized transition metal complexes in photonic and optoelectronic devices

Spin trapping and ESR detection (with the traps ..cap alpha..-phenyl N-tert-butyl nitrone and ..cap alpha..-(4-pyridyl N-oxide) N-tert-butyl nitrone) were employed to detect free-radical intermediates formed during in situ irradiation of TiO/sub 2/ and platinized TiO/sub 2/ powders in aqueous solutions. Evidence for the production of hydroxyl radical (.OH) (formed in the oxidation of water) and perhydroxyl radical (HO/sub 2/.) (probably formed in a reduction step) is presented. Other spin adducts, attributed to decomposition reactions of the spin traps, were also found. The results suggest an important role for photogenerated .OH in many photocatalytic and photosynthetic processes at TiO/sub 2/ powders.

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Spin trapping and electron spin resonance detection of radical intermediates in the photodecomposition of water at titanium dioxide particulate systems

Journal of the American Chemical Society is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Direct observation of radical intermediates in the photo-Kolbe reaction heterogeneous photocatalytic radical formation by electron spin resonance Bernhard Kraeutler, Calvin D. Jaeger, and Allen J. Bard J. Am. Chem. Soc., 1978, 100 (15), 4903-4905• DOI: 10.1021/ja00483a052 • Publication Date (Web): 01 May 2002 Downloaded from http://pubs.acs.org on February 16, 2009

Direct Observation of Radical Intermediates in the Photo Kolbe Reaction—HeterogeneousPhotocatalytic Radical Formation by Electron Spin Resonance

Compacted pellets of tetrathiafulvalene-tetracyanoquinodimethane (TTF-PCNQ) were prepared and the proper- ties of this material as an electrode in aqueous media were investigated. The electrode was stable over a potential range of about 0.9 V; within this region the electrode exhibited residual faradaic currents of less than 1-2 pA/cm2 and the oxidation and reduction of soluble redox couples at this electrode could be carried out. Lpon exceeding the potential limits of stability of the electrode, the subsequent voltammograms showed symmetrical peaks attributable to insoluble compounds of TTF or TCNQ and supporting electrolyte ions (e.g.. TTFBr, KTCNQ). The redox reaction of these species could also be observed in the cyclic voltammograms

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Electrochemical behavior of tetrathiafulvalene-tetracyanoquinodimethane electrodes in aqueous media

: Spectral sensitization by metal-free phthalocyanine (H2Pc) films was observed on various semiconductor electrodes (single crystal n-TiO2, n-SrTiO3, n-WO3, n-ZnO, n-CdS, n-CdSe, n0Si, and n-GaP; SnO2 conducting glass). The spectral response of the sensitized photocurrent was generally the same as the absorption spectrum of the phthalocyanine. The rather thick (400 A to 1 micron H2Pc films showed both anodic and cathodic photocurrents depending upon the applied potential. The anodic photocurrents represented the usual sensitization of the n-type semiconductor, while the cathodic photocurrents were attributed to p-type behavior of the phthalocyanine itself. The current-potential curves of the semiconductor electrodes depended on the nature of the H2Pc film, the presence of a redox couple (i.e., p-hydroquinone/p-benzoquinone) in solution and the wavelength of the irradiating light. The magnitude of the steady state sensitized photocurrent was linear with light intensity and was strongly affected by the addition of the supersensitizer. (Author)

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Semiconductor electrodes. 26. Spectral sensitization of semiconductors with phthalocyanine

Electrodes prepared as compacted pellets from conductive donor-acceptor complexes of tetracyanoquinodimethane (TCNQ) with several donors (D) (tetrathiotetracene, acridine, quinoline, N-methylphenazine, 2,2'and 4,4'-bipyridine) were investigated in aqueous solutions. The results were compared to those of a previous investigation of TTF-TCNQ pellets (TTF = tetrathiafulvalene) and to results on single crystals of TTF-TCNQ reported here. The electrodes are stable over a potential region in which they could be employed as inert electrodes. The potential limits depended upon the stabilization gained upon complexation and the relative electrode potentials of the constituent compounds. Upon exceeding the potential limits of stability of the electrode, the electrode was reduced or oxidized. Subsequent voltammograms showed peaks attributable to insoluble compounds formed on the electrode surface. The potentials observed for the redox processes could be correlated to the degree of charge transfer, p, as well as gas-phase ionization potentials and electron affinities. Introduction Hundreds of different molecular complexes containing tetracyancquincdimethane (TCNQ) as the acceptor have been made.14 TCNQ is a very good electron acceptor; because of the presence of the four cyano groups on the molecule and the relatively large a conjugation system, reduction to the radical anion occurs very easily. As a result, TCNQ forms stable solid complexes with a wide variety of electron-donating compounds. Because of their special structural features, some of these complexes exhibit the highest conductivity observed for organic compounds. The structure of the donor, the stoichiometry, and the electrical conductivity for some of these highly conducting donor-TCNQ complexes are shown in Figure 1. Donor-acceptor complexes of intermediate conductivity also exist. T C N Q forms three types of complexes with donor mo1ecules.l The first can be generally characterized as a molecular complexes with relatively low electrical conductivities. The second group includes the simple radical ion salts, M\"+TCNQ--,, where Mnf can be either a metal cation or an organic cation. The third type includes the complex (1:2) radical ion salts, M+(TCNQ--)(TCNQO). In these complexes the electron is generally considered to be delocalized over both of the TCNQ molecules. Although the properties and synthesis of these materials have been investigated quite extensively, few reports on the application (1) Melby, L. R.; Harder, R. J.; Hertler, W. R.; Mahler, W.; Benson, R. (2) Garito, A. F.; Heeger, A. J. Acc. Chem. Res. 1974, 7 , 232, and ref(3) Perlstein, J. H. Angew. Chem., Inr. Ed. Engl. 1977, 16, 519, and (4) Wheland, R. C.; Gillson, J . L. J . Am. Chem. Soc. 1976, 98, 3916. n-' cm-' ) and very low Q-] cm-I ) E.; Mochel, W. E. J . Am. Chem. SOC. 1962, 84, 3374. erences cited therein. references cited therein. of such donor-acceptor complexes have appeared. We recently discussed the behavior of sintered pellets of TTF-TCNQ as electrode^.^,^ These were shown to have a stable region where they behaved as inert electrodes capable of carrying out electrochemical reactions of dissolved solution species without oxidation or reduction of the electrode material itself. Outside of this region different reactions of the electrode occurred that depended upon the nature of the supporting electrolyte in the solution. In this paper these studies are extended to single-crystal TTF-TCNQ and a number of other donor-TCNQ complexes. Such studies are of interest because they provide information about the redox behavior, stabilization energies, and degree of charge transfer in these complexes. These materials are of potential interest as electrodes for electroanalytical studies or in electrochemical cells and may have application to electrochromic devices. Experimental Section Preparation of Complexes and Electrodes. In general most of the complexes cannot be recrystallized without some degradation. Thus they were synthesized with materials of the highest purity. TCNQ and the donor molecules were generally sublimed several times and then recrystallized from a spectroscopic grade solvent (benzene, MeCN). The inorganic salts were similarly recrystallized before they were used in a synthesis. All solvents were spectroscopic grade and the water was triply distilled starting with alkaline potassium permanganate. TCNQ forms several different types of compounds. They can be simple 1:l complexes, represented by the formula D\"+(TCNQ-),, or complex salts represented by the formula D\"+(TCNQ-),(TCNQo).117 In both cases complete and incomplete charge transfer from the donor to ( 5 ) Jaeger, C. D.; Bard, A. J. J . Am. Chem. Soc. 1979, 101, 1690. (6) Wallace, W. L; Jaeger, C. D.; Bard, A. J. J . Am. Chem. Soc. 1979, (7) Torrance, J. B. Ann. N.Y . Acad. Sci. 1978, 313, 210. 101, 4840.

Calvin D. Jaeger

Papers

Spin trapping and electron spin resonance detection of radical intermediates in the photodecomposition of water at titanium dioxide particulate systems

Direct Observation of Radical Intermediates in the Photo Kolbe Reaction—HeterogeneousPhotocatalytic Radical Formation by Electron Spin Resonance

Electrochemical behavior of tetrathiafulvalene-tetracyanoquinodimethane electrodes in aqueous media

Semiconductor electrodes. 26. Spectral sensitization of semiconductors with phthalocyanine

Electrochemical behavior of donor-tetracyanoquinodimethane electrodes in aqueous media