Michio Matsumura

Crystal faces of rutile and anatase TiO2 particles and their roles in photocatalytic reactions

A titanium dioxide powder consisting of 1 μm size rutile and anatase particles was obtained, on which developed crystal faces were observed by a scanning electron microscope. From electron diffraction analyses, it was found that the rutile particles exposed {011} and {110} crystal faces, and the anatase particles exposed {001} and {011} faces. This powder showed high activity for some photocatalytic reactions, including oxidation of water. After photocatalytic oxidation of water on the powder using hexachloroplatinate(IV) ions as the electron acceptors, Pt deposits were observed mostly on the rutile particles, especially on the {110} face. When 2-propanol was added to the solution, Pt was deposited on both the anatase and rutile particles. Using the thus prepared Pt-deposited TiO2 powder, Pb2+ ions were photocatalytically oxidized into PbO2. After this reaction, PbO2 deposits were seen on the {011} face of the rutile particles. On the anatase particles, PbO2 deposits were observed in a larger amount on the {001} face than on the {011} face. These results indicate that the crystal faces help in the separation of electrons and holes, and that this effect is stronger for the rutile particles than for the anatase particles.

Synergism between rutile and anatase TiO2 particles in photocatalytic oxidation of naphthalene

Abstract Photocatalytic oxidation of naphthalene was investigated in a mixed solution of acetonitrile and water using various kinds of titanium dioxide (TiO 2 ) powders as the photocatalysts and molecular oxygen as the electron acceptor. The main product from naphthalene is 2-formylcinnamaldehyde. For this reaction, anatase small TiO 2 particles, which are commonly used as photocatalyst, are inactive, probably because band bending is necessary for the oxidation of naphthalene. If the particles are not extremely small, pure rutile and pure anatase powders show fairly high activity, and those containing both anatase and rutile phases show the highest activity. When a pure anatase powder is partly (about 90%) converted to the rutile form by heat treatment, the activity is largely enhanced. The activity of pure rutile particles is also enhanced by physically mixing them with a small amount of small-sized anatase particles, which are inactive for this reaction. These results can be explained by the synergism between rutile and anatase particles. We consider that electrons are transferred from rutile particles to anatase particles, i.e. naphthalene is mainly oxidized on rutile particles and oxygen is mainly reduced on anatase particles. This electron transfer process is supported by electrochemical properties of TiO 2 electrodes for reduction of oxygen.

Splitting of water by electrochemical combination of two photocatalytic reactions on TiO2 particles

Photochemical splitting of water was achieved by combining two photocatalytic reactions on suspended titanium dioxide particles, namely, the reduction of water to hydrogen using bromide ions, which were oxidized to bromine and the oxidation of water to oxygen using FeIII ions, which were reduced to FeII ions. These two reactions were carried out in separate compartments and combined via platinum electrodes and cation-exchange membranes. At the electrodes, FeII ions were oxidized by bromine, and protons were transported through the membranes to maintain the electric neutrality and pH of the solutions in the two compartments. As a result, water was continuously split into hydrogen and oxygen under photoirradiation. Reversible reactions on photocatalysts often suffer from the effects of back reactions, unless the products are removed. In the present system the problem is largely prevented, because the concentrations of the products in solution are automatically maintained at a low level.

Determination of Oxygen Sources for Oxidation of Benzene on TiO2 Photocatalysts in Aqueous Solutions Containing Molecular Oxygen

Photocatalytic oxidation of benzene to CO(2) was studied in aqueous solutions using different kinds of TiO(2) powders, and isotopic oxygen tracers (H(2)(18)O and (18)O(2)) were used to investigate the oxidation process. Phenol was produced as a main intermediate in solution. When anatase powders, which showed high activity for oxidation of benzene, were used, 70-90% of oxygen introduced into phenol was from water. On the other hand, when rutile powders were used, only 20-40% of the oxygen was from water. The rest was from molecular oxygen in both cases. The rate of phenol production by using molecular oxygen was nearly the same between anatase and rutile powders. Hence, the high activity of anatase powders for oxidation of benzene to CO(2) is attributed to their high activity for oxidation of benzene to phenol, which is considered to be the rate-determining step, using water as the oxygen source. The processes using water and molecular oxygen as the oxygen sources are ascribed, respectively, to oxygen transfer and hole transfer processes in the initial step of benzene oxidation.

Pore formation in silicon by wet etching using micrometre-sized metal particles as catalysts

Au, Pt, or Ag particles with particle sizes of ca. 1 μm were used as catalysts for boring pores in p-type Si(100) wafers by wet etching in aqueous solutions containing hydrofluoric acid and hydrogen peroxide. Boring speed was fastest when Pt particles were used as the catalyst. However, the sidewalls of the pores and the surface of the wafer were covered with a nanoporous silicon layer of ca. 500 nm in thickness, and the pore showed a tapered structure. When micrometre-sized Ag particles were used, no deep pores were formed because the particles were unstable in the solution. In contrast to Pt and Ag particles, Au particles bored straight pores under some conditions. However, the morphology of pores depended on the shape of the Au particles. Spherical Au particles formed straight pores, whereas non-spherical Au particles formed pores with spiral sidewalls. When Au particles formed aggregates consisting of a small number of particles (<10 particles), crooked pores tended to be formed. In contrast, when the aggregates were composed of a larger number of particles, straight pores were formed and the boring speed was faster than the pores formed with isolated Au particles.

Catalytic activity and regeneration property of a Pd nanoparticle encapsulated in a hollow porous carbon sphere for aerobic alcohol oxidation.

A core-shell composite consisting of a palladium (Pd) nanoparticle and a hollow carbon shell (Pd@hmC) was employed as a catalyst for aerobic oxidation of various alcohols. The core-shell structure was synthesized by consecutive coatings of Pd nanoparticles with siliceous and carbon layers followed by removal of the intermediate siliceous layer. Structural characterizations using TEM and N(2) adsorption-desorption measurements revealed that Pd@hmC thus-obtained was composed of a Pd nanoparticle core of 3-6 nm in diameter and a hollow carbon shell with well-developed mesopore (ca. 2.5 nm in diameter) and micropore (ca. 0.4-0.8 nm in diameter) systems. When compared to some Pd-supported carbons, Pd@hmC showed a high level of catalytic activity for oxidation of benzyl alcohol into benzaldehyde using atmospheric pressure of O(2) as an oxidant. The Pd@hmC composite also exhibited a high level of catalytic activity for aerobic oxidations of other primary benzylic and allylic alcohols into corresponding aldehydes. The presence of a well-developed pore system in the lateral carbon shell enabled efficient diffusion of both substrates and products to reach the central Pd nanoparticles, leading to such high catalytic activities. This core-shell structure also provided high thermal stability of Pd nanoparticles toward coalescence and/or aggregation due to the physical isolation of each Pd nanoparticle from neighboring particles by the carbon shell: this specific property of Pd@hmC resulted in possible regeneration of catalytic activity for these aerobic oxidations by a high-temperature heat treatment of the sample recovered after catalytic reactions.

Photocatalytic oxidation of water on TiO2-coated WO3 particles by visible light using Iron(III) ions as electron acceptor

Photocatalytic oxidation of water on TiO2-coated WO3 particles was studied using iron(III) ions as the electron acceptor with the aim of constructing a photochemical energy conversion system. Although WO3 photocatalysts can utilize part of visible light, the reaction was decelerated as the concentration of iron(II) ions in solution increased. This was a marked contrast with the reaction using TiO2 photocatalysts, whose photocatalytic activity is scarcely affected by iron(II) ions in solution. In order to modify the surface of WO3 particles, they were coated with a thin TiO2 layer. Using such photocatalysts, the harmful effect by iron(II) ions on the WO3 photocatalyst was restrained to some extent, and the efficiency of photooxidation of water by visible light was improved.

Effect of a plasma treatment of ITO on the performance of organic electroluminescent devices

Abstract We investigated the effect of plasma treatments of indium-tin oxide (ITO) surface on the performance of electroluminescent (EL) devices using different gases. In the case of air or argon, an intense EL emission was observed at low applied voltages. On the other hand, when hydrogen was used, very high voltages were needed to obtain the EL emission. The change in voltage was attributed to the removal of contaminants and to the change in work function of ITO. For the devices with the ITO treated with hydrogen plasma, the EL efficiency was very low. This result suggested that electrons accumulated at the organic/organic interface exert a harmful effect on the light emission.

Lowering of operational voltage of organic electroluminescent devices by coating indium-tin-oxide electrodes with a thin CuOx layer

We devised a method of modifying indium-tin-oxide (ITO) electrodes for organic electroluminescent devices. It consists of deposition of a nanometer-thick Cu layer on the ITO electrode and an oxygen plasma treatment. By this modification, the surface of the ITO substrate is covered with a partly oxidized Cu layer (CuOx). The CuOx-coated ITO electrode possesses strong hole-injection ability, which leads to lowered operational voltage and high luminance from the devices consisting of tris(8-quinolinato)aluminum and diamine hole-transport layers. The hole-injection ability of the CuOx-coated ITO electrode is better than that of the ITO electrode modified by conventional methods, such as insertion of a Cu-phthalocyanine buffer layer. Moreover, the CuOx layer is effective to improve the durability of the devices.

Formation of new crystal faces on TiO2 particles by treatment with aqueous HF solution or hot sulfuric acid

We have demonstrated that new crystal faces are generated on anatase and rutile TiO2 particles by means of chemical etching in aqueous hydrofluoric acid or hot sulfuric acid. In the treatment with aqueous hydrofluoric acid, the {112} face of anatase particles and the {021} face of rutile particles are newly formed. When treated with hot sulfuric acid, anatase particles exposed the {122} face and rutile particles exposed the {001}, {010}, {021} and {121} faces. In both cases, anatase particles are etched at a higher rate than rutile particles. The etched particles are expected to show photocatalytic properties unique to the crystal faces. For example, the {112} face of anatase particles is demonstrated to be active in the oxidation of Pb2+ ions.