Teruhisa Ohno

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.

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.

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.

Splitting of Water by Combining Two Photocatalytic Reactions through a Quinone Compound Dissolved in an Oil Phase

Two photocatalytic reactions producing oxygen and hydrogen, respectively, were combined using a quinone compound dissolved in an oil phase. As quinone compound, 2,3dichloro-5,6-dicyano-l,4-benzoquinone (DDQ) was utilized, which was dissolved in n-butyronitrile. When an oil phase containing the reduced form of DDQ (DDHQ) was placed on an aqueous phase containing Pt-loaded TiO, particles and bromide ions, hydrogen and bromine were produced in the aqueous phase by photoirradiation of the Ptloaded TiO, particles. The bromine then oxidized DDHQ in the oil phase. Similarly, by photoirradiation of an aqueous solution containing TiO, particles and iron(III) ions, oxygen and iron(II) ions were produced. When the reaction was carried out in the double phase system consisting of the aqueous phase and the oil phase containing DDQ, DDQ was reduced to DDHQ by the iron(II) ions. The results indicate the feasibility of water splitting by combining two photocatalytic reactions through redox reactions of a quinone compound.

Chiral recognition by cyclic oligosaccharides. Enantioselective complexation of binaphthyl derivatives with cyclodextrins

Abstract Chiral recognition of binaphthyl derivatives, such as 1,1′-bi-2-naphthol (1), 1,1′-binaphthyl-2,2′-diyl hydrogen phosphate (2), and 2,2′-dihydroxy-1,1′-binaphthyl-3,3′-dicarboxylic acid (3), by cyclodextrins (CDxs) has been studied. The S enantiomers of 1 and 2 are bound to heptakis(2,3,6-tri-O-methyl)-β-CDx (TMe-β-CDx) as well as β-CDx more strongly than the R enantiomers. The molecular mechanics and molecular dynamics calculations for the 1:1 complex of 1 and β-CDx suggest that more effective van der Waals contacts and intermolecular hydrogen bonding stabilize the complex of S-1 compared with that of R-1. Meanwhile the R enantiomer of 3 is the preferable guest for β- and TMe-β-CDxs. Circular dichroism spectroscopy suggests that the complex of S-3 is more unstable than that of R-3 because the dihedral angle of the naphthalene planes of S-3 needs to be reduced for forming the inclusion complex. The enantiomers of the guest binaphthyls are completely separated by means of capillary zone electropho...

Photocatalytic Hydrogen or Oxygen Evolution from Water over S- or N-Doped TiO2 under Visible Light

S- or N-doping of powder having an anatase or rutile phase extended the photocatalytic activity for water oxidation and reduction under UV light and visible light irradiation. For the reduction of water, anatase-doped showed higher level of activity than that of doped having a rutile phase using ethanol as an electron donor. Furthermore, the activity level of S-doped for hydrogen evolution was higher than that of N-doped photocatalysts under visible light. Photocatalytic oxidation of water on doped having a rutile phase proceeded with fairly high efficiency when ions were used as electron acceptors compared to that on doped having an anatase phase. In addition, water splitting under visible light irradiation was achieved by construction of a Z-scheme photocatalysis system employing the doped having anatase and rutile phases for and evolution and the redox couple as an electron relay.

TiO2-photocatalyzed oxidation of adamantane in solutions containing oxygen or hydrogen peroxide

Abstract Photocatalyzed oxidation of adamantane has been investigated using several kinds of TiO 2 powders in a mixed solvent of acetonitrile and butyronitrile under aerated conditions. 1-Adamantanol, 2-adamantanol, and 2-adamantanone are obtained as the main products, in which 1-adamantanol is produced at the highest yield. The quantum efficiencies for the production of 1-adamantanol, 2-adamantanol, and 2-adamantanone reach 6.4, 1.0 and 2.1%, respectively. Generally, anatase powders show higher activity than rutile powders. However, by addition of hydrogen peroxide to the solution, the activity of rutile powders is remarkably enhanced and becomes much higher than that of anatase powders. The rate for the production of 1-adamantanol is increased by more than 10 times, and the quantum efficiency for the production of 1-adamantanol reached as high as 25%.

Forwarding reversible photocatalytic reactions on semiconductor particles using an oil/water boundary

Abstract Photocatalytic oxidation of iodide ions and hydrogen evolution proceeded on Pt-loaded titanium dioxide particles in aqueous solutions. The reaction rate levelled off as the concentration of tri-iodide ions increased. This was due to the reduction of tri-iodide ion into iodide ion on the photocatalyst. This process is the back reaction of the energy storing photocatalytic reaction. By introducing an oil phase on the aqueous solution the back reaction was retarded, because part of the tri-iodide ions were transported to the oil phase. The back reaction was much eliminated when durohydroquinone was added to the oil phase. In this case, tri-iodide ions in aqueous solution was scavenged as the result of the redox reaction between tri-iodide ion and durohydroquinone, leading to the prevention of the back reaction. This system is comparable to the functions of photosystem I and plastoquinone in the thylakoid membrane of green plants.