Showing papers by "Toshiharu Teranishi published in 2010"

Photocatalytic Overall Water Splitting Promoted by Two Different Cocatalysts for Hydrogen and Oxygen Evolution under Visible Light

Abstract: Overall water splitting using a particulate photocatalyst and solar energy has attracted significant attention as a potential means of large-scale H2 production from renewable resources without carbon dioxide emission. 2] The reaction occurs in three steps: 1) the photocatalyst absorbs photon energy greater than the band-gap energy of the material and generates photoexcited electron–hole pairs in the bulk, 2) the photoexcited carriers separate and migrate to the surface without recombination, and 3) adsorbed species are reduced and oxidized by the photogenerated electrons and holes to produce H2 and O2, respectively. The first two steps are strongly dependent on the structural and electronic properties of the photocatalyst, while the third step is promoted by an additional catalyst (called cocatalyst). Therefore, it is important to develop a photocatalyst and a cocatalyst in harmony. Recently, our group has focused on active sites for H2 evolution on the surface of a photocatalyst, because most photocatalysts lack surface H2 evolution sites. [2b] Using a solid solution of GaN and ZnO (abbreviated GaN:ZnO hereafter) that can harvest visible photons up to ca. 500 nm, chromium-containing transition-metal oxides or noble-metal/ chromia (core/shell) nanoparticles (NPs) have been shown to function as H2 evolution cocatalysts, resulting in efficient water splitting under visible light. Meanwhile, also several sulfides were proposed as efficient catalysts for H2 evolution, and the role of H2 evolution cocatalysts has been explored by spectroscopic and electrochemical techniques. It would be natural to expect that loading both H2 and O2 evolution cocatalysts onto the same photocatalyst would improve water-splitting activity, compared to photocatalysts modified with either an H2 or O2 evolution cocatalyst. [8] It is easy to imagine how these two different cocatalysts would separately facilitate H2 and O2 evolution, thereby promoting overall water splitting in harmony. Unfortunately, no successful and reliable example of this has been reported since the initial reports on photocatalytic water splitting in the 1980s. The actual demonstration of the concept remains a major challenge. Herein, we show a proof-of-concept using GaN:ZnO loaded with Rh/Cr2O3 (core/shell) and Mn3O4 NPs as H2 and O2 evolution promoters, respectively, under irradiation with visible light (l> 420 nm). First, Mn oxide was introduced onto GaN:ZnO, prepared by our previous method, as O2 evolution cocatalyst. Some Mn oxides have been reported to act as O2 evolution promoters, and it is well known that a Mn complex is the O2 evolution center in the photosynthesis of green plants. MnO NPs with a mean size of (9.2 0.4) nm (Figure S1 in the Supporting Information) were adsorbed onto GaN:ZnO. It was revealed by UV/vis spectroscopy that the introduced MnO NPs (ca. 1.0 wt %) were almost quantitatively anchored on the GaN:ZnO surface, based on the change in the absorption band of the MnO NPs (Figure S2 in the Supporting Information). The as-prepared MnO/GaN:ZnO sample was then calcined in air at 673 K for 3 h to remove organic residues. Separate experiments with thermogravimetry, differential thermal analysis (TG-DTA), and X-ray diffraction (XRD) showed that the organic ligands stabilizing the MnO NPs were completely burned off by calcination in air at 673 K, and that calcination of dried MnO NP powder under the above conditions resulted in phase transformation of the MnO into Mn3O4 (Figure S3 in the Supporting Information). Transmission electron microscopy (TEM) observation revealed that the particle size of the Mn oxide was maintained, even after calcination (Figure S1 in the Supporting Information). Thus, GaN:ZnO particles were successfully decorated with Mn3O4 NPs which were expected to act as water oxidation cocatalysts. Because GaN:ZnO is an n-type semiconductor, it is possible to monitor the photooxidation reaction occurring on its surface using an electrochemical technique. Under [*] Dr. K. Maeda, A. Xiong, N. Sakamoto, Dr. T. Hisatomi, Prof. Dr. K. Domen Department of Chemical System Engineering The University of Tokyo 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan) Fax: (+ 81)3-5841-8838 E-mail: domen@chemsys.t.u-tokyo.ac.jp Homepage: http://www.domen.t.u-tokyo.ac.jp/

Plasmon Hybridization in Individual Gold Nanocrystal Dimers: Direct Observation of Bright and Dark Modes

Abstract: We apply a nanomanipulation technique to assemble pairs of monodispersed octahedral gold nanocrystals (side length, 150 nm) along their major axes with a varying tip-to-tip separation (25−125 nm). These pairs are immobilized onto indium tin oxide coated silica substrates and studied as plasmonic dimers by polarization-selective total internal reflection (TIR) microscopy and spectroscopy. We confirm that the plasmon coupling modes with the scattering polarization along the incident light direction result from the transverse-magnetic-polarized incident light, which induces two near-field-coupled dipole moments oriented normal to the air−substrate interface. In such cases, both in-phase (antibonding) and antiphase (bonding) plasmon coupling modes can be directly observed with the incident light wave vector perpendicular and parallel to the dimer axis, respectively. The observation of antiphase plasmon coupling modes (“dark” plasmons) is made possible by the unique polarization nature of the TIR-generated eva...

Preparation of core-shell-structured nanoparticles (with a noble-metal or metal oxide core and a chromia shell) and their application in water splitting by means of visible light.

Abstract: Core-shell-structured nanoparticles, consisting of a noble metal or metal oxide core and a chromia (Cr(2)O(3)) shell, were studied as promoters for photocatalytic water splitting under visible light. Core nanoparticles were loaded by impregnation, adsorption or photodeposition onto a solid solution of gallium nitride and zinc oxide (abbreviated GaN:ZnO), which is a particulate semiconductor photocatalyst with a band gap of approximately 2.7 eV, and a Cr(2)O(3) shell was formed by photodeposition using a K(2)CrO(4) precursor. Photodeposition of Cr(2)O(3) on GaN:ZnO modified with a noble metal (Rh, Pd and Pt) or metal oxide (NiO(x), RuO(2) and Rh(2)O(3)) co-catalyst resulted in enhanced photocatalytic activity for overall water splitting under visible light (lambda>400 nm). This enhancement in activity was primarily due to the suppression of undesirable reverse reactions (H(2)-O(2) recombination and/or O(2) photoreduction) and/or protection of the core component from chemical corrosion, depending on the core type. Among the core materials examined, Rh species exhibited relatively high performance for this application. The activity for visible-light water splitting on GaN:ZnO modified with an Rh/Cr(2)O(3) core-shell configuration was dependent on both the dispersion of Rh nanoparticles and the valence state. In addition, the morphology of the Cr(2)O(3) photodeposits was significantly affected by the valence state of Rh and the pH at which the photoreduction of K(2)CrO(4) was conducted. When a sufficient amount of K(2)CrO(4) was used as the precursor and the solution pH ranged from 3 to 7.5, Cr(2)O(3) was successfully formed with a constant shell thickness (approximately 2 nm) on metallic Rh nanoparticles, which resulted in an effective promoter for overall water splitting.

Drastic structural transformation of cadmium chalcogenide nanoparticles using chloride ions and surfactants.

Abstract: In the present work, we studied a unique and facile method for the drastic structural transformation of hydrophobic small CdE (E = S, Se, Te) nanoparticles into large, high-quality pencil-shaped nanoparticles through an Ostwald ripening process induced by Cl− and surfactants (oleic acid and oleylamine). This study revealed that Cl− is the effective anion for the controlled structural transformation of CdE nanoparticles. This transformation reaction can be readily extended to the formation of various functional materials.

Single-Electron Transistor Fabricated by Two Bottom-Up Processes of Electroless Au Plating and Chemisorption of Au Nanoparticle

Abstract: Coulomb diamonds were clearly observed on single-electron transistors (SETs) fabricated by bottom-up processes of electroless plating of Au nanogap electrodes and chemisorption of a Au nanoparticle at 80 K. In the drain current–drain voltage characteristics, Coulomb staircases were modulated by the side gate voltage. Tunneling resistances and source/drain/gate capacitances of the SET were evaluated by fitting the theoretical Coulomb staircase determined on the basis of the full orthodox theory in a double-barrier tunneling junction to the experimental results of Coulomb blockade under the application of side gate voltages. The theoretical results for the Coulomb diamond are in good agreement with the experimental results.

Room-Temperature Coulomb Blockade from Chemically Synthesized Au Nanoparticles Stabilized by Acid--Base Interaction

Abstract: Sub-2-nm-size basic ligand Au nanoparticles were chemically synthesized and chemisorbed on an acidic self-assembled monolayer/Au(111) substrate by acid–base interaction. Coulomb blockade behaviors with clear Coulomb gaps were observed in current–voltage (I–V) and log I–V curves of the chemisorbed Au nanoparticles by scanning tunneling spectroscopy at room temperature. By fitting the measured I(V) and log I(V) to a Coulomb blockade model, we estimated the charging energy of one electron on the Au nanoparticles to be 10 times greater than the thermal energy k T; the tunneling resistance of the Au core–Au(111) surface was evaluated to be 3.5 GΩ ±15%.