Hisayoshi Kobayashi

An oxynitride, TaON, as an efficient water oxidation photocatalyst under visible light irradiation (λ≤ 500 nm)

Under visible light irradiation (λ = 420–500 nm), a tantalum oxynitride, TaON, functions as a stable and very efficient photocatalyst for oxidation of water into O2 with a sacrificial electron acceptor (Ag+).

Photophysical properties and photocatalytic activities under visible light irradiation of silver vanadates

α-AgVO3, β-AgVO3, Ag4V2O7 and Ag3VO4 prepared by precipitation and solid-state reactions showed intense absorption bands in the visible light region due to band gap transitions. Comparison of a diffuse reflectance spectrum of α-NaVO3 with that of α-AgVO3 with a diopside-type structure revealed that a band gap (2.5 eV) of α-AgVO3 was 0.6 eV smaller than that (3.1 eV) of α-NaVO3. The electronic band structure study using a plane-wave based density functional method indicated that the decrease in the band gap of α-AgVO3 was due to Ag 4d orbitals which formed a valence band at a more negative level than O 2p orbitals. Among α-AgVO3, β-AgVO3, Ag4V2O7 and Ag3VO4, only Ag3VO4 showed a photocatalytic activity for O2 evolution from an aqueous silver nitrate solution under visible light irradiation. Holes photogenerated in Ag3VO4 can migrate to the reaction sites on the surface more easily than those of other silver vanadates, because the content of silver forming a valence band is large. It resulted in that holes photogenerated in Ag3VO4 are able to oxidize H2O to form O2.

Photocatalytic activities for water decomposition of RuO2-loaded AInO2 (A=Li, Na) with d10 configuration

The photocatalytic activities for water decomposition of RuO2-loaded alkaline metal indates, AInO2 (A=Li, Na), with d10 configuration were studied. Under Hg–Xe lamp illumination, RuO2-loaded LiInO2 had little activity, but RuO2-loaded NaInO2 showed the ability to photocatalytically decompose water to hydrogen and oxygen. The photocatalytic activity strongly depended on the calcination temperature of NaInO2: with increasing calcination temperature, the activity increased, passed through a maximum, and sharply decreased. In case that a part of Na ion in NaInO2 was replaced by Li or K ion, the photocatalytic activity decreased sharply with a small amount of Li and gradually with increasing content of K. The calculation based on a plane wave density function theory (DFT) was employed to elucidate the band structure of AInO2 (A=Li, Na). The photocatalytic activities were compared with those of previously reported RuO2-loaded MIn2O4 (M=Ca, Sr), and the activity differences among them are discussed.

New aspects of heterogeneous photocatalysts for water decomposition

Several new photocatalysts for overall water splitting are described. Under UV light irradiation (270 nm), La-doped NaTaO3 modified with NiO decomposed water into H2 and O2 with extremely high quantum efficiency. Under an optimized condition, the apparent quantum efficiency, which was estimated with numbers of irradiated photons and evolved H2 molecules, reached 56%. New stable photocatalytic materials containing elements with d10 electronic configuration such as In3+ Sn4+ and Sb5+ were developed for overall water splitting. Some mesoporous oxides were proved to be effective photocatalysts. (Oxy)nitrides of some early transition metals, i.e., Ta, Nb and Ti, were found to be stable materials having potentials for H2 and O2 evolutions under visible light irradiation (⪯600 nm). The electronic structures of these photocatalysts are also discussed based on DFT calculation.

Theoretical approach to the design of supramolecular conjugated porphyrin polymers

The design of novel conjugated polymers mainly consisting of porphyrin and/or phthalocyanine rings is studied by the use of the tight-binding crystal orbital method based on the extended Huckel approximation. This paper focuses on the tuning of the electronic structures of these polymers by ethynyl bridging as well as various linkage modes. It has been clarified that the bridging sites (meso-to-meso, meso-to-β and β-to-β) and the bridging patterns (linear or zigzag) in the ethynyl-bridged porphyrin polymers are crucial for tuning the π-conjugation throughout the main chain. Furthermore, comparison of two-dimensional (2D) porphyrin polymer sheets and stacked porphyrin polymers demonstrates quite different electronic properties.

Electronic structure of two-dimensional porphyrin polymers

Abstract The electronic structure of the acetylenyl-linked two-dimensional (2D) porphyrin polymer was studied with the use of the tight-binding crystal orbital (TBCO) method based on the extended Huckel approximation. Effects of the different bridging modes (bridging site and linking species) were also examined. It has been shown that meso -to- meso -bridged pophyrin polymerin linked with acetylenyl groups has the largest π-conjugation among the present polymer models. Moreover, analysis of the electronic properties has suggested that n-type doping will be promising toward electrically conductive 2D polymers.

Band structure of orthorhombic Rb1C60

Abstract The band structure of orthorhombic Rb 1 C 60 ( o -Rb 1 C 60 ) using the crystal structure obtained by X-ray diffraction measurements was studied based on the tight-binding calculation including both σ and π electrons. The Fermi surface of this substance shows a certain nesting property in the plane perpendicular to the Γ-H direction but is three-dimensional due to the interchain interaction. The bandwidth is still smaller by one order of magnitude than the on-ball Coulomb repulsion U , in spite of some increase due to the interchain interaction.

CHANGE IN BAND STRUCTURE OF ORTHORHOMBIC RB1C60 WITH THE CHAIN ROTATION MODELING MEROHEDRAL DISORDER

Abstract Change in the band structure of orthorhombic Rb1C60 (o-Rb1C60)) caused by the rotation of one of the two kinds of C60 polymer chains has been examined on the basis of the tight-binding calculation including both σ and π electrons. This rotation models merohedral disorder in this substance and the band-structure analysis has indicated that the chain rotation causes drastic variation of the highest occupied (HO) bandwidth. It has been suggested that such change influences on the dimensionality of the electronic properties of o-Rb1 C60.

30 Ta3N5 and TaON as novel photocatalysts responding to visible light

Abstract TaON and Ta 3 N 5 , Ta 5+ -based (oxy)nitrides, were studied as visible light driven photocatalysts. Under visible light irradiation (λ≥420 nm), the (oxy)nitrides oxidize water to O 2 and reduce H + to H 2 in the presence of sacrificial reagents (Ag + and methanol). TaON oxidizes water into O 2 efficiently, with a maximum quantum yield of 34%. The photocatalytic reactions proceed via the bandgap transitions (Eg, TaON: 2.5 eV, Ta 3 N 5 : 2.1 eV) without any noticeable degradation of the catalysts.

Electronic structures of transition metal-C60 coordination polymers (η6-C60)nMn(M = Sc, Ti, V, or Cr)

Abstract The electronic structures of a series of transition metal (M)–C 60 dumbbell polymers (η 6 -C 60 ) n M n have been examined with the one-dimensional (1D) tight-binding crystal orbital (TBCO) method at the extended Huckel (EH) approximation level. The band structures for M=Sc, Ti, and V are metallic but that for M=Cr is semiconductive with a narrow bandgap, E G ≈8.15×10 −3 eV. For all the polymers, there are five specific bands near the Fermi level, four of which show the interaction between the d atomic orbitals (AO) (d xy and d x 2 − y 2 ; the coordinate axes are given in the text) of M and the π orbitals on C 60 . The structural instability of the 1D polymer for M=Cr is discussed based on the very narrow bandgap.