Photoinduced reactivity of titanium dioxide

Molecular self-assembly is the spontaneous association of molecules under equilibrium conditions into stable, structurally well-defined aggregates joined by noncovalent bonds. Molecular self-assembly is ubiquitous in biological systems and underlies the formation of a wide variety of complex biological structures. Understanding self-assembly and the associated noncovalent interactions that connect complementary interacting molecular surfaces in biological aggregates is a central concern in structural biochemistry. Self-assembly is also emerging as a new strategy in chemical synthesis, with the potential of generating nonbiological structures with dimensions of 1 to 10(2) nanometers (with molecular weights of 10(4) to 10(10) daltons). Structures in the upper part of this range of sizes are presently inaccessible through chemical synthesis, and the ability to prepare them would open a route to structures comparable in size (and perhaps complementary in function) to those that can be prepared by microlithography and other techniques of microfabrication.

Molecular self-assembly and nanochemistry: A chemical strategy for the synthesis of nanostructures

With the increased presence of nanomaterials in commercial products, a growing public debate is emerging on whether the environmental and social costs of nanotechnology outweigh its many benefits. To date, few studies have investigated the toxicological and environmental effects of direct and indirect exposure to nanomaterials and no clear guidelines exist to quantify these effects.

The potential environmental impact of engineered nanomaterials

Semiconductor heterogeneous photocatalysis, the subject of this review, is a versatile, low-cost and environmentally benign treatment technology for a host of pollutants. These may be of biological, organic and inorganic in origin within water and air. The efficient and successful application of photocatalysis demands that the pollutant, the catalyst and source of illumination are in close proximity or contact with each other. The ability of advanced oxidation technology to remove low levels of persistent organic pollutants as well as microorganisms in water has been widely demonstrated and, progressively, the technology is now being commercialized in many areas of the world including developing nations. This review considers recent developments in the research and application of heterogeneous semiconductor photocatalysis for the treatment of low-level concentrations of pollutants in water and air using titanium dioxide as a “model” semiconductor. The review considers charge transport characteristics on the semiconductor surface, photocatalyst reactor design and organic degradation mechanistic pathways. The effects of photoreactor operating parameters on the photocatalytic process are discussed in addition to mineralization and disinfection kinetics.

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Heterogeneous Photocatalysis: Recent Advances and Applications

Preceding work on photoelectrochemistry at semiconductor single-crystal electrodes has formed the basis for the tremendous growth in the three last decades in the field of photocatalysis at semiconductor powders. The reason for this is the unique ability of inorganic semiconductor surfaces to photocatalyze concerted reduction and oxidation reactions of a large variety of electron-donor and -acceptor substrates. Whereas great attention was paid to water splitting and the exhaustive aerobic degradation of pollutants, only a small amount of research also explored synthetic aspects. After introducing the basic mechanistic principles, standard experiments for the preparation and characterization of visible light active photocatalysts as well as the investigation of reaction mechanisms are discussed. Novel atom-economic C-C and C-N coupling reactions illustrate the relevance of semiconductor photocatalysis for organic synthesis, and demonstrate that the multidisciplinary field combines classical photochemistry with electrochemistry, solid-state chemistry, and heterogeneous catalysis.

Semiconductor Photocatalysis—Mechanistic and Synthetic Aspects

The photocatalytic activity of TiO2 nanoparticles of different size has been studied by following the photodegradation of dichloroacetic acid (DCA), and was tested by measuring the initial rate of the H+ and Cl− ion formation. The catalyst efficiency was found to be in correlation with the specific surface area of the semiconductor particles. The efficiency of the nanocrystalline TiO2 photocatalyst could be increased by doping the particles with copper ions. The structure and the partitioning of different surface copper complexes has been studied using ESR technique. The influence of the pH value of the solutions, as well as of the adsorbed copper ion content, on the efficiency of the DCA photodegradation is discussed in the paper.

Photodestruction of dichloroacetic acid catalyzed by nano-sized TiO2 particles

Deep and detailed discussions on chemistry, chemical physics, photoelectrochemistry, photophysics, photocatalysis and possible applications of nanostructured semiconductor materials have shown increasing interest in the matter by scientists representing various research areas as well as industrial enterprises Indeed, solar energy conversion and ch

Chemical Physics of Nanostructured Semiconductors

It is shown that the E.S.R. spectra of long-chain nitroxide biradicals represent the superposition of the spectrum of the non-reacting radical moieties in the elongated conformation and of that of the cage where the radical moieties are close together and can interact with each other. Inside the cage a fast intramolecular motion is observed. Some thermodynamic parameters of the cage as well as thermodynamic parameters of the motion inside the cage are calculated from the experimental E.S.R. spectra of four biradicals.

On the mechanism of spin exchange in long-chain nitroxide biradicals

X-band EPR studies of a series of undoped and carbon-doped titania powders reveal the presence of a carbon-centered radical species. The concentration of spins is five to six orders of magnitude smaller than the total carbon content. Photoexcitation of the carbon-doped samples increases the corresponding EPR signal intensities both under ultraviolet and visible light irradiation. After illumination, the original intensity is restored within a few hours. Samples with higher carbon radical concentration reveal higher photocatalytic activity in degradation of 4-chlorophenol under visible light irradiation.

Carbon-Doped Titanium Dioxide : Visible Light Photocatalysis and EPR Investigation

Sol−gel-derived nanostructured TiO2 thin films and powders treated with Cu2+-containing solutions have been studied using electrochemical methods, ESR, XPS, and electroreflection spectroscopies. Analysis of the voltammograms in combination with ESR data has allowed us to reveal at least four types of copper species formed on the TiO2 surface after the adsorption of aqueous Cu2+ ions. These are monovalent copper ions, magnetically isolated Cu(II) ions, Cu(II) ions forming specific areas (“associates”) with a high local concentration and strong interaction between the ions, and formally diamagnetic copper hydroxide species. The last type dominates when adsorbing Cu(II) ions from solutions with pH > 5 and its fraction increase with increasing the Cu2+ ion concentration in solution. The presence of Cu(I) ions at the TiO2 surface was proved independently by XPS measurements. The appearance of them can be associated with the partial reduction of adsorbed Cu2+ ions by electrons of the TiO2 matrix. The Cu(II) ion...

Alexander I. Kokorin

Papers

Photodestruction of dichloroacetic acid catalyzed by nano-sized TiO2 particles

Chemical Physics of Nanostructured Semiconductors

On the mechanism of spin exchange in long-chain nitroxide biradicals

Carbon-Doped Titanium Dioxide : Visible Light Photocatalysis and EPR Investigation

Structure and Electrochemical Properties of Species Formed as a Result of Cu(II) Ion Adsorption onto TiO2 Nanoparticles