Aromatic amines — Electron transfer agents which increase the effectiveness of photocatalytic redox reactions

The characteristics are examined of redox photocatalytic systems based on semiconductors containing both electron accepting and electron donating additives which are able to separate the photogenerated charges. The use of aromatic amines as electron transfer agents has been demonstrated experimentally.Translated from Teoreticheskaya Éksperimental'naya Khimiya, Vol. 28, Nos. 5–6, pp. 411–415, September–December, 1992.The authors thank the Ukrainian State Committee on Science and Technology for financial help from the Fund for Fundamental Research.     

Photolysis of titanium (III) complexes in alcoholic matrices at 77‡k

Using the electron spectroscopy and EPR methods, it was found that during UV irradiation of frozen alcoholic solutions of distorted octahedral complexes [Ti(ROH)₄Cl₂]⁺ and [Ti(ROH)₆]³⁺, distorted tetrahedral hydroxide complexes of titanium (III) are obtained. The mechanism of the process, including successive redox reactions of the titanium compounds has been discussed. As the result of a phototransfer of an electron from the central atom orbitals to the vacant orbitals of the ligands (alcohol molecules) a titanium (IV) compound is obtained containing an anion radical of the alcohol as the ligand. The compound is unstable and undergoes ultraspheric transformations. Rupture of the C-O bond occurs in the anionradical of the alcohol; ethyl radicals and hydroxide complexes of titanium (IV) are thus formed, which by the action of light quanta transform into hydroxide complexes of titanium (III).Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 4, pp. 480–484, July–August, 1985.     

Photocatalytic Oxidation of Hydrosulfide Ions by Molecular Oxygen Over Cadmium Sulfide Nanoparticles

Photocatalytic activity of CdS nanoparticles in hydrosulfide-ions air oxidation was revealed and thoroughly investigated. HS⁻ photooxidation in the presence of CdS nanoparticles results predominantly in the formation of SO₃ ²⁻ and SO₄ ²⁻ ions. Photocatalytic activity of ultrasmall CdS crystallites in HS^– photooxidation is much more prononced as compared to bulk CdS crystals due to high surface area of nanoparticles, their negligible light scattering, improved separation of photogenerated charge carriers etc. It was shown that hydrosulfide ions can be oxidized in two ways. The first is HS⁻ oxidation by the CdS valence band holes. This process rate depends on the rate of comparatively slow reaction between molecular oxygen and CdS conduction band electrons. The second reaction route is the chain-radical HS⁻ oxidation induced by photoexcited CdS nanoparticles and propagating in the bulk of a solution. In conditions favourable to chain-radical oxidation of HS⁻(i.e. at low light intensities and CdS concentration and high oxygen and Na₂S concentrations) quantum yields of the photoreaction reach 2.5.     

Photocatalytic production of hydrogen in systems based on Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Zn<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>S/Ni<Superscript>0</Superscript> nanostructures

A relation was established between the composition of Cd _x Zn_1–x S nanoparticles and their ability to accumulate excess negative charge during irradiation. The rate of expenditure of the accumulated charge depends on the composition of the nanoparticles and is determined by their electric capacitance. A correlation was found between the photocatalytic activity of the Cd _x Zn_1–x S nanoparticles in the release of hydrogen from solutions of Na₂SO₃, their composition, and their capacity for photoinduced accumulation of excess charge. It was shown that Ni⁰ nanoparticles photodeposited on the surface of Cd _x Zn_1–x S are effective cocatalysts for the release of hydrogen. It was found that Zn^II additions in photocatalytic systems based on Cd _x Zn_1–x S/Ni⁰ nanostructures have a promoting action on the release of hydrogen from water–ethanol mixtures. Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 45, No. 1, pp. 8–16, January-February, 2009.     

Effect of the method of production of TiO<Subscript>2</Subscript>/CdS nanohetero film structures on the effectiveness of photoinduced charge separation

The properties of TiO₂/CdS nanohetero structures produced by the chemical and photocatalytic deposition of CdS nanoparticles on the surface of nanocrystalline films of TiO₂ under the conditions of laser pulse photolysis were studied. It was shown that in the case of photocatalytically formed TiO₂/CdS nanocomposites the spatial separation of the photogenerated charges between the components of the hetero structure, leading to the formation of intermediates of the photochemical transformations (Ti^III centers in the TiO₂ nanoparticles and S^– radical-anions in the CdS nanoparticles), is an order of magnitude more effective.     

Appearance of autocatalysis in the production of molecular hydrogen by a photoreaction on semiconductor heterojunctions

The photocatalytic evolution of H₂ from ethanol and aqueous solutions of Na₂SO₃ and Na₂S by suspensions of CdS/Cu²⁺ and CdS/Bi³⁺ microheterojunctions has been investigated. The appearance of autocatalysis has been obtained, attributed to a lowering of the energy barrier of the reaction; it is caused by a change in the work function of an electron from the palladium co-catalyst, due to its saturation by the product (hydrogen).Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 29, No. 5, pp. 435–440, September–October, 1993.     

“Hybrid” photocatalytic systems for production of molecular hydrogen

It has been shown on the example of semiconductor (CdS, TiO₂) and metal complex (solvate complexes (V(III), Eu(III), Zr(IV)) photocatalysts that they can form a single photocatalytic system that is characterized by higher efficiency in the direction of the formation of hydrogen from alcohol-water solutions than the individual substances. Successful functioning of such a hybrid photocatalytic system is possible if the principle of energy correspondence is satisfied: the potential of the conduction zone of the semiconductor must be more negative than the redox potential of the Meⁿ⁺/Me^(n–1)+ pair. In this case a photogenerated electron of the conduction zone is accepted by a metal ion Meⁿ⁺ and, thus, the electron-hole recombination process is suppressed. The resulting reduced form of the metal Me^(n–1)+ in the presence of metallic palladium decomposes the protonated molecules of water with the formation of H₂.Translated from Teoreticheskaya i Éksperimentalnaya Khimiya, Vol. 25, No. 4, pp. 452–459, July–August, 1989.     

Effect of Metal Inclusions on the Porous Structure and Photocatalytic Activity of Titanium Dioxide

A study was carried out on the effect of the inclusion of a series of transition metals (V, Cr, Mn, Fe, Ni, Cu, Zr, Mo) and also Al, Si, Zn, and Sn into titanium dioxide on its porous structure, anatase formation, and photocatalytic activity in the generation of molecular hydrogen from aqueous ethanol. An increase in the content of these metals leads to a decrease in photocatalytic activity, while an increase in the anatase content in titanium dioxide leads to an increase in such activity.     

Catalysis of the Sodium Sulfide Reduction of Methylviologene by CuS Nanoparticles

Feasibility was demonstrated for the catalysis of the sodium sulfide reduction of methylviologene (MV²⁺) in aqueous solution using cupric sulfide nanoparticles. The catalytic activity of the nanoparticles depends on their size. The basic features were found for the formation of the MV^+· radical–cation in the reduction of MV²⁺ by HS^– anions in the presence of CuS nanoparticles as the catalyst. This is an equilibrium reaction.