Shigeo Arai

Atomic origins of the high catalytic activity of nanoporous gold

Distinct from inert bulk gold, nanoparticulate gold has been found to possess remarkable catalytic activity towards oxidation reactions. The catalytic performance of nanoparticulate gold strongly depends on size and support, and catalytic activity usually cannot be observed at characteristic sizes larger than 5 nm. Interestingly, significant catalytic activity can be retained in dealloyed nanoporous gold (NPG) even when its feature lengths are larger than 30 nm. Here we report atomic insights of the NPG catalysis, characterized by spherical-aberration-corrected transmission electron microscopy (TEM) and environmental TEM. A high density of atomic steps and kinks is observed on the curved surfaces of NPG, comparable to 3-5 nm nanoparticles, which are stabilized by hyperboloid-like gold ligaments. In situ TEM observations provide compelling evidence that the surface defects are active sites for the catalytic oxidation of CO and residual Ag stabilizes the atomic steps by suppressing {111} faceting kinetics.

Anomalous production of vacancy clusters and the possibility of plastic deformation of crystalline metals without dislocations

High-speed heavy plastic deformation of thin foils of fcc metals, including aluminium, is found to produce a high density of small vacancy clusters, in the form of stacking-fault tetrahedra. The dependences of the density of the clusters on the deformation temperature and deformation rate indicate the production of vacancy clusters from deformation-induced dispersed vacancies. Neither dislocations nor any indication of the reaction of dislocations are present in the regions containing a high density of vacancy clusters. A possible model is proposed that describes, at extremely high strain rates where dislocation generation is difficult, how a high concentration of point defects is produced by a large number of parallel shifts of atomic planes without dislocations.

Production of Colored Pigments with Amorphous Arrays of Black and White Colloidal Particles

There are many technical and industrial applications for colored pigments with nonfading properties. The development of a low-cost, high-volume production method for nonfading pigments with low toxicity and minimal environmental impact may promote their widespread use. To accomplish this goal, pigments need to be prepared using abundant and environmentally friendly compounds. Here, we report on the variously colored aggregates formed by spraying fine, submicrometer-sized spherical silica particles. The microstructure of the aggregate is isotropic with a shortrange order on a length scale comparable to optical wavelengths, and exhibits an angle-independent structural color as a result of wavelength-specific constructive interference. Interestingly, the color saturation of these aggregates can be controlled by the incorporation of a small amount of conventional black particles, such as carbon black (CB). We demonstrate that a Japanese-style painting can be successfully drawn with this method. Silicon dioxide, which is a major component of silica particles, is chemically stable and used in scientific glassware suitable for chemical experiments. It is also a primary component of soil and found in abundant supply in nature. Furthermore, in vivo toxicity of silica particles that are greater than 300 nm in diameter has not been detected. Therefore, submicrometer-sized silica particles are one of the best candidates for fabricating environmentally friendly materials. Fine submicrometer-sized spherical silica particles usually appear white to the human eye when they are in powdered form. However, assemblies of these particles can appear colored because of wavelength-specific optical interference, 5, 7] despite the absence of light-absorbing pigments and dyes. Such color is generally referred to as structural color, because it is essentially caused by the microstructure through optical phenomena, such as interference, diffraction, and scattering. 9] Crystalline arrays of fine submicrometersized spherical silica particles (colloidal crystals) are well known examples of assembled particles that have structural colors as a result of a very high reflectance at a certain wavelength of light. However, the structural colors produced by colloidal crystals show distinct variations, which depend on viewing and light illumination angles. Such iridescence makes the use of colloidal crystals as pigments difficult, because typical pigments generally require a constant color at different viewing angles. The iridescences of the colloidal crystals originate from Bragg reflection, which is the reflection mechanism that occurs as a result of the long-range order in the particle arrangement. Thus, if the arrangement is changed from the crystalline structure to the amorphous state, which has only a short-range order, iridescence is expected to be suppressed. In fact, amorphous aggregates of colloidal particles have been reported to exhibit angle-independent structural colors. 4, 5,12] However, amorphous colloidal arrays are difficult to fabricate because submicrometer-sized particles have a strong tendency to crystallize. Previously, amorphous colloidal arrays have been prepared by mixing two different kinds of submicrometer-sized silica particles. 4, 5, 13] These mixtures exhibit structural colors, but the colors are very pale. 4,5] Therefore, such amorphous colloidal arrays are unsuitable for use as brightly colored pigments. A simple synthetic method for the preparation of assemblies of submicrometer-sized particles with angle-independent brilliant structural colors for use as pigments has not yet been reported. Herein, we report a simple and reproducible synthetic procedure for the preparation of pigments that exhibit angleindependent, bright structural colors from amorphous colloidal arrays by spraying fine submicrometer-sized spherical silica particles of uniform size. We added a small amount of black particles to the colloidal amorphous array to enhance the saturation of the structural color by reducing incoherentlight scattering across the entire visible spectrum. Variously [*] Prof. Y. Takeoka, Prof. A. Takano, M. Teshima, Y. Ohtsuka, Prof. T. Seki Graduate School of Engineering, Nagoya University Furo-cho, Chikusa-ku, Nagoya, 464-8603 (Japan) E-mail: ytakeoka@apchem.nagoya-u.ac.jp

Atomic observation of catalysis-induced nanopore coarsening of nanoporous gold.

Dealloyed nanoporous metals have attracted much attention because of their excellent catalytic activities toward various chemical reactions. Nevertheless, coarsening mechanisms in these catalysts have not been experimentally studied. Here, we report in situ atomic-scale observations of the structural evolution of nanoporous gold during catalytic CO oxidation. The catalysis-induced nanopore coarsening is associated with the rapid diffusion of gold atoms at chemically active surface steps and the surface segregation of residual Ag atoms, both of which are stimulated by the chemical reaction. Our observations provide the first direct evidence that planar defects hinder nanopore coarsening, suggesting a new strategy for developing structurally stable and highly active heterogeneous catalysts.

Selective hydrogenation of 2-hydroxymethyl-5-furfural to 2,5-bis(hydroxymethyl)furan over gold sub-nano clusters

We report a gold sub-nano cluster supported on Al2O3 catalyzed hydrogenation of 2-hydroxymethyl-5-furfural without furan ring hydrogenation and its opening reaction, which resulted in excellent conversion to 2,5-bis(hydroxymethyl)furan (>96% yield), Fig. 1(a).

Size Specifically High Activity of Ru Nanoparticles for Hydrogen Oxidation Reaction in Alkaline Electrolyte

The hydrogen oxidation reaction (HOR) in alkaline electrolyte was conducted on carbon-supported Ru nanoparticles (Ru/C) of which size was controlled in the range from approximately 2 to 7 nm. The HOR activity of Ru/C normalized by the metal surface area showed volcano shaped dependence on the particle size with a maximum activity at approximately 3 nm. The HOR activity of approximately 3 nm Ru/C was higher than commercially available Pt nanoparticles (ca. 2 nm) supported on carbon. The structural analysis of Ru/C using Cs-corrected scanning transmission electron microscopy with atomic resolution revealed the unique structural change of Ru/C different from Pt/C: Ru nanoparticle structure changed from amorphous-like structure below 3 nm to metal nanocrystallite with roughened surface at approximately 3 nm and then to that with well-defined facets above 3 nm, although Pt/C kept well-defined facets even at approximately 2 nm. It is proposed that the generation of unique structure observed on approximately 3 nm Ru nanoparticles, that is, long bridged coordinatively unsaturated Ru metal surface atoms on its nanocrystallite, is a key to achieve high HOR activity.

Thin single-wall BN-nanotubes formed inside carbon nanotubes

We report a high yield synthesis of single-wall boron nitride nanotubes (SWBNNTs) inside single-wall carbon nanotubes (SWCNTs), a nano-templated reaction, using ammonia borane complexes (ABC) as a precursor. Transmission electron microscope (TEM), high angle annular dark field (HAADF)-scanning TEM (STEM), electron energy loss spectra (EELS) and high resolution EELS mapping using aberration-corrected TEM system clearly show the formation of thin SWBNNTs inside SWCNTs. We have found that the yield of the SWBNNT formation is high and that the most of ABC molecules decompose and fuse to form the thin BNNTs at a temperature of 1,673 K having a narrow diameter distribution of 0.7 ± 0.1 nm. Optical absorption measurements suggest that the band gap of the thin SWBNNTs is about 6.0 eV, which provide the ideal insulator nanotubes with very small diameters.

Damage process in electron-irradiated graphite studied by transmission electron microscopy. II. Analysis of extended energy-loss fine structure of highly oriented pyrolytic graphite

Abstract The change in local atomic configurations during electron irradiation was studied by the analysis of the extended energy-loss fine structure of a highly oriented pyrolytic graphite crystal. The spectral change obtained was interpreted in terms of the local distortion of the basal planes and the formation of alternative types of chemical bonding. The π bonding in the basal planes was retained even after the complete halo pattern was observed in the electron diffraction. This indicated that the disordering within the basal planes first occurs without destroying the layered structure, consistent with the preceding report. A model for the damage process in graphite is proposed.

Quantitative characterization of nanoscale polycrystalline magnets with electron magnetic circular dichroism

Electron magnetic circular dichroism (EMCD) allows the quantitative, element-selective determination of spin and orbital magnetic moments, similar to its well-established X-ray counterpart, X-ray magnetic circular dichroism (XMCD). As an advantage over XMCD, EMCD measurements are made using transmission electron microscopes, which are routinely operated at sub-nanometre resolution, thereby potentially allowing nanometre magnetic characterization. However, because of the low intensity of the EMCD signal, it has not yet been possible to obtain quantitative information from EMCD signals at the nanoscale. Here we demonstrate a new approach to EMCD measurements that considerably enhances the outreach of the technique. The statistical analysis introduced here yields robust quantitative EMCD signals. Moreover, we demonstrate that quantitative magnetic information can be routinely obtained using electron beams of only a few nanometres in diameter without imposing any restriction regarding the crystalline order of the specimen.

Development of an environmental high-voltage electron microscope for reaction science

Environmental transmission electron microscopy and ultra-high resolution electron microscopic observation using aberration correctors have recently emerged as topics of great interest. The former method is an extension of the so-called in situ electron microscopy that has been performed since the 1970s. Current research in this area has been focusing on dynamic observation with atomic resolution under gaseous atmospheres and in liquids. Since 2007, Nagoya University has been developing a new 1-MV high voltage (scanning) transmission electron microscope that can be used to observe nanomaterials under conditions that include the presence of gases, liquids and illuminating lights, and it can be also used to perform mechanical operations to nanometre-sized areas as well as electron tomography and elemental analysis by electron energy loss spectroscopy. The new instrument has been used to image and analyse various types of samples including biological ones.