Toshiharu Teranishi

Indium Tin Oxide Nanoparticles with Compositionally Tunable Surface Plasmon Resonance Frequencies in the Near-IR Region

Here we report the synthesis of conducting indium tin oxide (ITO) nanoparticles (NPs) and their surface plasmon resonance (SPR) properties. The SPR peaks of the ITO NPs can be easily tuned by changing the concentration of Sn doping from 3 to 30 mol %. The shortest SPR wavelength of 1618 nm in 10% Sn-doped ITO NPs may reflect the highest electron carrier density in the ITO NPs. The controllable SPR frequencies of metal oxides may offer a novel approach for noble-metal-free SPR applications. Unlike noble-metal nanostructures, ITO has no inter- and intraband transitions in the vis-near-IR region and represents a free-electron conduction, allowing us to systematically study the origin of optical effects arising from the SPRs of conduction electrons.

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

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/

Direct Observation of Ferromagnetic Spin Polarization in Gold Nanoparticles

We report the first direct observation of ferromagnetic spin polarization of Au nanoparticles with a mean diameter of 1.9 nm using x-ray magnetic circular dichroism (XMCD). Owing to the element selectivity of XMCD, only the gold magnetization is explored. Magnetization of gold atoms as estimated by XMCD shows a good agreement with results obtained by conventional magnetometry. This evidences intrinsic spin polarization in nanosized gold.

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

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.

Anomalous magnetic polarization effect of Pd and Au nano-particles

Abstract The magnetization of Pd and Au nano-particles with a diameter of around 3 nm was measured down to 4.2 K. The data of magnetization of both nano-particles show an unexpectedly large magnetic moment of about 20 spins per particle. This result can not be understood by so-called odd/even electron number effect and Stoners enhancement model. The result might suggest the existence of some common and characteristic spin correlation mechanism in the nano-particles. The experimental result of size dependence on Pd nano-particles shows existence of a critical size of about 3 nm from a atomic spin correlation region to a metallic spin correlation region.

Controllable Polyol Synthesis of Uniform Palladium Icosahedra: Effect of Twinned Structure on Deformation of Crystalline Lattices†

Palladium plays a key role in technologies used for hydrogen storage, hydrogen purification, water treatment, and fuel cells. Palladium is also widely used as the primary catalyst for low-temperature reduction of automobile pollutants, organic reactions, hydrogenation, and petroleum cracking. These numerous applications result in palladium drawing considerable interest. Most applications of palladium are related to its remarkable hydrogen-adsorption capacity. A recent study indicated that icosahedral palladium nanoparticles (Pd NPs) can absorb a larger quantity of hydrogen than their cuboctahedral analogues. Moreover, the catalytic activity of a metal NP is commonly enhanced by surface atoms located on the corners and edges. Accordingly, icosahedral Pd NPs, with a high density of twins and corners on their surfaces, are expected to be the most active catalysts, and this has led to an explosion of interest in their synthesis. Much effort has been devoted to synthesizing various metal NPs with specific shapes in aqueous or nonhydrolytic media. Among these strategies, the polyol process is a convenient, versatile, and low-cost route for the synthesis of metal NPs. In recent years, this technique has been further modified through the introduction of polymers, foreign ions, and seeds, as well as careful regulation of reaction temperature, to yield metal NPs with well-defined sizes and geometric shapes. 8,12, 13] Initial nucleation is known to be one of the determining factors for the shape of final products. The metal nuclei can adopt single-crystal, singly twinned, or multiply twinned structures. Compared with gold, multiply twinned Pd nuclei are highly susceptible to oxidation under the reaction conditions. Multiply twinned Pd NPs are depleted in favor of stable single-crystalline NPs during the growth process. Consequently, the shape of the final Pd products of a solution-phase synthesis is restricted to spherical NPs, single-crystalline plates, bars, rods, cubes, and cuboctahedra owing to highly oxidative etching, poorer protection of the twinned structures, and fast reduction and growth rate. 16] In comparison, under slow reaction conditions, nucleation of metal atoms and growth of nuclei can be kinetically controlled through polymers or foreign ions. The icosahedral structure is favored over both decahedral and cuboctahedral structures for Pd at small sizes (number of Pd atoms N< 309). Therefore, it is possible to selectively synthesize high-quality Pd icosahedra by carefully manipulating the growth process of icosahedral seeds generated by slow reactions. Thus, Pd icosahedra were synthesized in 80% yield by a water-based synthetic strategy. To the best of our knowledge, it remains a challenge to control the synthesis of uniform Pd icosahedra in high yield. Here we present a facile and effective polyol route for controllable synthesis of icosahedral Pd NPs with uniform size in ethylene glycol (EG) solution. A high yield of icosahedral Pd NPs was obtained in a one-pot reaction. Furthermore, the dimensions of the icosahedral NPs can be readily tailored from 15 to 42 nm by tuning the experimental parameters. A unique powder X-ray diffraction (PXRD) pattern of Pd owing to the multiply twinned structure was observed for the first time for Pd icosahedra. The as-synthesized Pd icosahedra are stable in air for months. Compared with spherical Pd NPs, Pd icosahedra can maintain their high catalytic activity even after several cycles. For a typical synthesis of Pd icosahedra, an EG solution containing a given amount of sodium chloride (NaCl), polyvinylpyrrolidone (PVP, Mw = 360 000), and sodium tetrachloropalladate (Na2PdCl4) was vigorously stirred and heated in air at an appropriate temperature (see the Supporting Information). Transmission electron microscopy (TEM) images demonstrate that the Pd NPs synthesized in an EG solution containing 5 mm Na2PdCl4, 10 mm NaCl, and 200 mm PVP have a hexagonal projection with a size of 31 2 nm (Figure 1a and b). Energy-dispersive X-ray spectroscopy (EDS, Figure S1, Supporting Information) and field-emission scanning electron microscopy (FESEM, Figure 1c) analyses showed that these Pd NPs consist of only Pd and have an icosahedral shape. Figure 1 d shows a high-resolution TEM [*] Dr. C. Li , R. Sato, Dr. M. Kanehara, Prof. T. Teranishi Department of Chemistry Graduate School of Pure and Applied Sciences University of Tsukuba Tennodai 1-1-1, Tsukuba, Ibaraki 305-8571 (Japan) Fax: (+ 81)29-853-4011 E-mail: teranisi@chem.tsukuba.ac.jp Homepage: http://www.chem.tsukuba.ac.jp/teranisi/index_E.html

Spontaneous Formation of Wurzite-CdS/Zinc Blende-CdTe Heterodimers through a Partial Anion Exchange Reaction

Ion exchange of ionic semiconductor nanoparticles (NPs) is a facile method for the synthesis of type-II semiconductor heterostructured NPs with staggered alignment of band edges for photoelectric applications. Through consideration of the crystallographic orientation and strain at the heterointerface, well-designed heterostructures can be constructed through ion exchange reactions. Here we report the selective synthesis of anisotropically phase-segregated cadmium sulfide (CdS)/ cadmium telluride (CdTe) heterodimers via a novel anion exchange reaction of CdS NPs with an organic telluride precursor. The wurtzite-CdS/zinc blende-CdTe heterodimers in this study resulted from spontaneous phase segregation induced by the differences in the crystal structures of the two phases, accompanying a centrosymmetry breaking of the spherical CdS NPs. The CdS/CdTe heterodimers exhibited photoinduced spatial charge separation because of their staggered band-edge alignment.

Simple reductant concentration-dependent shape control of polyhedral gold nanoparticles and their plasmonic properties.

We report a facile seed-mediated method for the synthesis of monodisperse polyhedral gold nanoparticles, with systematic shape evolution from octahedral to trisoctahedral structures. The control over the particle growth process was achieved simply by changing the concentration of the reductant in the growth solution, in the presence of small spherical seed nanoparticles. By progressively increasing the concentration of the reductant used in the growth solution (ascorbic acid), while keeping the amount and type of added surfactant constant, the morphology of the gold nanoparticles was varied from octahedral to truncated octahedral, cuboctahedral, truncated cubic, cubic, and finally trisoctahedral structures. These nanoparticles were monodisperse in size, possessed similar volumes, and were naturally oriented so that their larger crystal planes were face down on quartz substrates when deposited from the solution. By adjusting the volume of gold seed nanoparticle solution added to a growth solution, the size of the simplest gold nanoparticles (with a highly symmetric cubic morphology) could be tuned from 50 ± 2.1 to 112 ± 11 nm. When other seed nanoparticles were used, the size of the cubic Au nanoparticles reached 169 ± 7.0 nm. The nanoparticle growth mechanism and the plasmonic properties of the resulting polyhedral nanoparticles are discussed in this paper.

Shape Control of Pt Nanoparticles

A preparative method to control the shape of platinum (Pt) nanoparticles in the presence of sodium polyacrylate (PAA) or poly(N-vinyl-2-pyrrolidone) (PVP) is described. Regardless of the kind of protective polymer used, the dominant shape of Pt nanoparticles was controlled by changing the reduction rate of Pt4+ ions. Tetrahedral particles predominated by using H2 reduction of H2[PtCl6]. Methanol reduction generated mainly truncated octahedral particles. It seems that the slow H2 reduction of Pt4+ ions favorably leads to the formation of tetrahedral Pt nuclei enclosed with four {111} planes that have the lowest surface energy. The truncated octahedral nuclei are formed by the faster reduction with methanol. The selective growth of the {111} planes takes place at a lower polymer concentration and results in the generation of cubic nanoparticles.

Drastic Structural Transformation of Cadmium Chalcogenide Nanoparticles Using Chloride Ions and Surfactants

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.