Solid State Chemistry and Photocatalysis of Titanium Dioxide

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Authors: Ryuhei Nakamura, Yoshihiro Nakato
Abstract: Recent studies on the molecular mechanism of water photooxidation (or oxygen photoevolution) reaction on TiO2 and related metal oxides or oxynitrides are reviewed. It is shown that a lot of experimental and theoretical studies give definite support to our recently proposed new mechanism, called “nucleophilic attack of H2O” or “Lewis acid-base” mechanism. The new mechanism has the prominent features that it possesses energetic and kinetics different from the conventional electron-transfer mechanism and can explain water photooxidation reaction on visible-light responsive metal oxides or oxynitrides, contrary to the conventional one. The result indicates that the new mechanism is useful for searching for new efficient visible-light responsive materials for solar water splitting.
Authors: Ryo Niishiro, Akihiko Kudo
Abstract: This review paper represents photocatalytic properties of metal cation-doped TiO2 (rutile) and SrTiO3 photocatalysts for O2 evolution from an aqueous silver nitrate solution and H2 evolution from an aqueous methanol solution under visible light irradiation. Photocatalytic activities for the O2 evolution of Cr/Sb and Rh/Sb-codoped TiO2 are strongly dependent on the codoping ratio and the amount of doped chromium and rhodium. The codopant controls the oxidation number of doped chromium and rhodium. Rh-doped SrTiO3 in which the doped Rh species possesses a reversible redox property is active for the H2 evolution reaction under visible light irradiation. Overall water splitting under visible light irradiation proceeds with Z-scheme photocatalyst systems consisting of the Rh-doped SrTiO3 as a H2 evolution photocatalyst combined with BiVO4 as an O2 evolution photocatalyst and an Fe3+/Fe2+ electron mediator.
Authors: Pierre Pichat, Rosario Enriquez, Estelle Mietton
Abstract: Charge recombination is the main drawback of heterogeneous photocatalysis; it considerably limits the applications. Consequently, it is of paramount importance to study this process. Time resolved microwave conductivity (TRMC) is an appropriate technique for that purpose since it allows measurements without electrical contacts. Here are presented and discussed results obtained by TRMC for TiO2 powders. They indicate the importance of the TiO2 particle diameter in determining the conductivity. The attraction of TiO2 photoproduced electrons to deposited Pt particles was also shown by TRMC, as well as the very rapid charge recombination in TiO2 homogeneously doped with 0.85 ion % of Cr3+ cations. TRMC measurements for TiO2 affixed on thin fiberglass tissues using SiO2 as a binder demonstrated that TRMC is also adapted to composite materials; the type of SiO2 had only a secondary influence on the conductivity. Additionally, the effects of alcohols were investigated by TRMC to illustrate the occurrence of interfacial charge transfer. On the other hand, oxygen isotope exchange (OIE) of gaseous 18O2 occurs over photo-excited Ti16O2. It is believed to be facilitated by surface irregularities which can thus be probed. OIE measurements in the presence of compounds whose photocatalytic oxidation primary step requires the incorporation of an O atom suggested that labile O atoms of TiO2 do not play the dominant role. Contrasted effects on OIE and photocatalytic removal of methanol in air showed (i) the influence of thermal treatment on TiO2 surface re-structuring (ii) the preferential adsorption and subsequent UV-induced reduction of Se salts at more labile surface O atoms.
Authors: Dariusz Mitoraj, Horst Kisch
Abstract: Titanium dioxide has received great attention both in fundamental and applied photocatalysis due to its low cost, non-toxicity, and stability against photocorrosion [ - ]. Unfortunately it can utilize only the very small UV part (about 3%) of solar light arriving at the earth surface. However, also the visible part (λ > 400 nm) may induce photocatalysis if titania is modified by transition or main group elements. Accordingly, many publications appeared in the last 20 years dealing with the problem of sensitizing titania for visible light photooxidation reactions. This may be achieved by doping, which means substitution of lattice ions, and by surface modification. In many cases authors not clearly differentiate between these two possibilities and so called doped titania quite often is a surface modified material. The latter is easily prepared by heating titania or its precursor compounds like titanium hydroxide in the presence of a modifier at temperatures in the range of 100 – 500 °C. In the following we summarize our work in this field using inorganic and organic sensitizers.
Authors: Tadeusz Bak, Truls Norby, Janusz Nowotny, Maria K. Nowotny, Nikolaus Sucher
Abstract: The present work considers the performance of TiO2-based photosensitive oxide semiconductors as photocatalysts for water purification. This paper brings together the concepts of solid state chemistry for nonstoichiometric compounds and the concepts of photocatalysis in order to discuss the reactivity between TiO2 and water including microorganisms (bacteria and viruses). The performance of TiO2 photocatalysts are considered in terms of a model of photoelectrochemical cell. The experimental data on photocatalytic removal of microorganisms from water are considered in terms of the effect of several properties, including pH, dispersion, light intensity, and temperature. It is argued that correct understanding of the performance of TiO2 photocatalysts requires recognition that properties of TiO2, which is a nonstoichiometric compound, are determined by defect disorder and the related ability to donate or accept electrons. The photocatalytic properties of TiO2 are considered in terms of the reactivity of both anodic and cathodic sites with water and the related charge transfer at the TiO2/H2O interface. It is shown that the formation of well defined photocatalysts requires knowledge of mass and charge transfer during processing and performance, respectively. The main hurdles in the development of high-performance photocatalysts are discussed.
Authors: Bernhard Neumann, Thorsten Brezesinsky, Bernd Smarsly, Helmut Tributsch
Abstract: Mesoporous titanium dioxide (m-TiO2) thin film electrodes were synthesized by evaporation-induced self-assembly (EISA), utilizing a novel type of amphiphilic block copolymer as template. The ordered network of pores shows an accessible inner volume that results in a huge BET-surface and a distinct transparency. According to X-ray diffraction analyses the mesoporous films are highly crystalline after calcination at 550°C. 1D and 2D small-angle X-ray scattering and transmission electron microscopy investigations prove the high quality of the mesopore texture over micrometer-sized areas. These well-defined, crystalline m-TiO2 films show an increased photoactivity for overall water splitting and oxidation of formic acid as compared to porous films prepared in the same manner without a template. The performance of the electrodes was analyzed by measuring the photocurrent and the mass signal of liberated gas by electrochemical mass spectroscopy (EMS). These experiments reveal that film morphology have a great influence to the I-V characteristic of photoelectrodes. An appropriate crystallization temperature is indispensable to obtain an optimum between crystallinity, morphology and photoactivity and to prevent collapse of the mesopore architecture.
Authors: Akira Sasahara, Hiroshi Onishi
Abstract: Surface science studies of photochemistry on titanium dioxide (TiO2) were reviewed. In the studies, photochemical processes were investigated in relation to atomic-scale surface structures by applying surface-sensitive analytical methods to single crystal TiO2 surfaces with well-defined structures. It is demonstrated that a surface science approach is promising for full description of the photochemical processes on TiO2.
Authors: Yaron Paz
Abstract: Composite photocatalysts, made of titanium dioxide and high surface area adsorbents become more and more common. To large extent, this is due to the phenomenon of "Adsorb & Shuttle", i.e. the adsorption of molecules on the inert, adsorptive, domains, followed by diffusion to the photocatalytic domains. This manuscript reviews the published literature on composite photocatalysts, and analyzes the various aspects affecting their performance. One of these aspects is the enhancement of the degradation rate of pollutants, which is governed by a variety of factors including surface area, adsorpticity, strength of interaction, the loading of photocatalyst in the composite, and operation parameters (such as temperature, humidity and pH). Other aspects include a reduction in the emission of intermediate products and a different distribution of end-products. Care was taken to describe the possibility of using the inert adsorptive domains to enhance the degradation of specific species, as well as to discuss the effect of composite photocatalysts on deactivation phenomena, and the interrelation between "Adsorb & Shuttle" phenomena and out- diffusion of oxidizing species from the TiO2 domains, known as the "remote degradation" phenomenon.
Authors: Pyrgiotakis Georgios, Sigmund M. Wolfgang
Abstract: A high efficiency nanocomposite photocatalyst is reported. Multiwall carbon nanotubes (MWNTs) were coated with titania. Two distinct types of nanocomposites were synthesized with variation in the synthetic procedure. One of the nanocomposites is based on a core of arc-discharge synthesized multiwall carbon nanotubes; whereas the other is based on a core of chemical vapor deposition (CVD) grown MWNTs. The nanocomposites have a similar appearance in transmission electron microscopy pictures in that they have a core of MWNTs that are chemically bonded to a few nanometer thick layer of pure anatase phase of titania. Yet they show a dramatically different photocatalytic behavior when they are compared on the basis of rates of photocatalytic decomposition of an organic dye in aqueous media following the Langmuir-Hinshelwood mechanism. This article focuses on the analysis of the differences in the nanocomposites using X-Ray Photoelectron Spectroscopy (XPS). XPS spectra of these materials indicate that there are distinct differences. However, they also show that both share C-O-Ti bonds that transform the carbon in the carbon nanotubes into a dopant for the anatase titania. This doping seems to be essential for long wavelength, i.e. visible light induced photocatalysis.

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