Keiko Uto

Epitaxial growth of Au@Pd core–shell nanocrystals prepared using a PVP-assisted polyol reduction method

Au@Cu core-shell nanocrystals were prepared using a two-step polyol reduction method. First, mixtures of ochedral, triangular and hexagonal platelike, decahedral, and icosahedral Au core seeds were prepared by reducing HAuCl 4 ·4H 2 O in ethylene glycol (EG) using microwave (MW) heating in the presence of polyvinylpyrrolidone (PVP) as a polymer surfactant. Then Cu shells were overgrown on Au core seeds by reducing Cu 2 (OAc) 4 in EG with PVP using oil-bath heating. Resultant crystal structures were characterized using TEM, high-resolution (HR)-TEM, TEM-EDS, and selected area electron diffraction (SAED) measurements. A large mismatch exists in lattice constants between Au (0.4079 nm) and Cu (0.3615 nm). No monometallic Cu nanocrystals having well-defined facets were prepared by reducing Cu 2 (OAc) 4 in EG. Therefore, the epitaxial growth ofCu shells over Au cores was expected to be difficult. Nevertheless, flat {111} facets of Cu shells were grown epitaxially on {111} facets of Au cores. The SAED patterns and Moire patterns showed Cu layers parallel to Au layers. The Cu shell growth on sharp Au-core corners was slower than that on flat {111} facets and single twin facets. This report is the first describing epitaxial growth of core—shell nanocrystals despite a large lattice mismatch (11.4%). The Au@Cu nanoparticles were more antioxidative than pure Cu particles prepared under identical conditions.

Rapid Transformation from Spherical Nanoparticles, Nanorods, Cubes, or Bipyramids to Triangular Prisms of Silver with PVP, Citrate, and H2O2

Rapid sphere-to-prism (STP) transformation of silver was studied in aqueous AgNO(3)/NaBH(4)/polyvinylpyrrolidone (PVP)/trisodium citrate (Na(3)CA)/H(2)O(2) solutions by monitoring time-dependent surface plasmon resonance (SPR) bands in the UV-vis region, by examining transmission electron microscopic (TEM) images, and by analyzing emitted gases during fast reaction. Roles of PVP, Na(3)CA, and H(2)O(2) were studied without addition of a reagent, with different timing of each reagents addition, and with addition of H(2)O(2) to mixtures of spheres and prisms. Results show that prisms can be prepared without addition of PVP, although it is useful to synthesize smaller monodispersed prisms. A new important role of citrate found in this study, besides a known role as a protecting agent of {111} facets of plates, is an assistive agent for shape-selective oxidative etching of Ag nanoparticles by H(2)O(2). The covering of Ag nanoparticles with carboxylate groups is necessary to initiate rapid STP transformation by premixing citrate before H(2)O(2) addition. Based on our data, rapid prism formation starts from the consumption of spherical Ag particles because of shape-selective oxidative etching by H(2)O(2). Oxidative etching of spherical particles by H(2)O(2) is faster than that of prisms. Therefore, spherical particles are selectively etched and dissolved, leaving only seeds of prisms to grow into triangular prisms. When pentagonal Ag nanorods and a mixture of cubes and bipyramids were used as sources of prisms, rod-to-prism (RTP), cube-to-prism (CTP), and bipyramid-to-prism (BTP) transformations were observed in Ag nanocrystals/NaBH(4)/PVP/Na(3)CA/H(2)O(2) solutions. Shape-selective oxidative etching of rods was confirmed using flag-type Ag nanostructures consisting of a triangular plate and a side rod. These data provide useful information for the size-controlled synthesis of triangular Ag prisms, from various Ag nanostructures and using a chemical reduction method, having surface plasmon resonance (SPR) bands at a desired wavelength.

Synthesis and growth mechanism of Au@Cu core–shell nanorods having excellent antioxidative properties

Au@Cu core–shell nanorods (NRs) were prepared using Au NRs as seeds. The resultant crystal structures were characterized using transmission electron microscopy (TEM), TEM-energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). When Cu shells were grown over Au NRs as seeds by reducing CuCl2·2H2O in an aqueous solution in the presence of hexadecylamine (HDA) and D-(+)-glucose (GLC) at 97 °C, Au@Cu NRs with {110} side facets were grown epitaxially as major side facets through a single island-growth mechanism. In this mechanism, crystal growth of Cu shells over Au core NRs starts from the formation of single semi-spherical nuclei on a wide side facet, followed by growth to neighbouring facets, with eventual full coverage by rectangular Cu shells with {110} side facets. When CuCl2·2H2O was replaced with Cu(OAc)2·H2O, layered growth of Cu shells was observed. Effects of the addition of NaCl to Cu(OAc)2·H2O show that Cl− ions play an important role in the single-island growth of Cu shells. The Au@Cu NRs exhibited much stronger antioxidative properties than spherical Cu particles.

Formation of Au@Pd@Cu core–shell nanorods from Au@Pd nanorods through a new stepwise growth mode

Au@Pd@Cu core–shell nanorods (NRs) were prepared using Au@Pd NRs as seeds. The resultant crystal structures were characterized using transmission electron microscopic (TEM), TEM-energy dispersed X-ray spectroscopic (EDS), and X-ray diffraction (XRD) measurements. Au@Pd seeds were prepared by reducing H2PdCl4 with cetyl trimethyl ammonium bromide (CTAB) and ascorbic acid in an aqueous solution. Dumbbell-type Au@Pd particles were formed at low Pd/Au molar ratios of 0.5–2.5, whereas rectangular Au@Pd NRs with {100} facets were produced at high Pd/Au molar ratios of 5 and 10. When Cu shells were further grown over rectangular Au@Pd NRs as seeds, Au@Pd@Cu nanocrystals with {100} facets were grown epitaxially through a new single island-growth mechanism designated as the Tsuji–Ikedo mechanism. In this mechanism, crystal growth of Cu shells over Au@Pd cores starts from the formation of single semispherical nuclei on a wide side facet followed by growth to one rectangular rod shell, further growth of neighboring rectangular rod shells, and eventual full covering by large rectangular Cu shells with {100} facets. In many cases, the growth rates of Cu shells over respective surfaces of Au@Pd NRs differ, so that Au@Pd@Cu particles with different thickness of Cu shells are prepared. Similar Au@Pd@Cu NRs with Au@Pd cores deviated from the center were also grown using dumbbell-type Au@Pd NRs, which indicates that flat Pd surfaces are unnecessary for the formation of rectangular Cu shells over Au@Pd NRs. The optical properties of Au@Pd and Au@Pd@Cu NRs were examined by observing ultraviolet (UV)-visible (Vis)-near infrared (NIR) extinction spectra.

Rapid spontaneous alloying between Pd nanocubes and Ag nanoparticles in aqueous solution at ambient temperature

Rapid spontaneous alloying between Pd nanocubes and spherical or triangular Ag nanoparticles was studied in an aqueous solution at ambient temperature using transmission electron microscopy (TEM), TEM-energy-dispersed X-ray spectroscopy (EDS), XRD, and UV-Vis spectroscopy of product particles. The results show that alloying occurs between Ag particles and Pd cubes and finishes within a few seconds, preserving the cubic shape with maximum Ag content of approximately 22%.

Enhancement of catalytic activity of AgPd@Pd/TiO2 nanoparticles under UV and visible photoirradiation

The effects of photoirradiation for the production of hydrogen from the decomposition of formic acid (FA) were studied using Ag93Pd7@Pd/TiO2 (anatase (A) or P25 (P)) nanocatalysts. The catalytic activity was enhanced by a factor of 1.5–1.6 under UV and visible (vis) photoirradiation at room temperature for both TiO2 supports. It was explained by the formation of an electron-rich Pd shell because of the migration of photogenerated electrons from the TiO2 surface. The catalytic activity of Ag93Pd7@Pd/TiO2 (A) was 1.7–7.3 times higher than that of Ag93Pd7@Pd/TiO2 (P) without and under photoirradiation at 27–90 °C. The catalytic activity of Ag93Pd7@Pd/TiO2 (A) under photoirradiation at 27 °C with 468 mmol H2 g per catalyst per h, is the best value ever reported out of all of the heterogeneous catalysts using TiO2 (A) as a photocatalyst at room temperature.

Syntheses of Carbon Supported Pt-YOx and PtY Nanoparticles with High Catalytic Activity for Oxygen Reduction Reaction Using Microwave-Polyol Method

Carbon‐supported PtY alloy nanoparticles were prepared as oxygen reduction reaction (ORR) catalysts by reducing a mixture of cis‐[Pt(NH3)2(NO2)2] or Pt(C5H7O2)2 and Y(CH3COO)3⋅4 H2O in ethylene glycol (EG) with microwave (MW) heating. Microstructure and composition analyses of products by using TEM, TEM–energy‐dispersive X‐ray spectroscopy (EDS), XRD, X‐ray photoelectron spectroscopy (XPS), and inductively coupled plasma atomic emission spectroscopy (ICP‐AES) data showed that Pt–YOx/C (Y/Pt=0.11–0.75) catalysts involving amorphous yttrium oxide were formed as major products. When the YOx component in the catalysts was removed by using HNO3 treatment, Pt99.1–99.6Y0.4–0.9/C alloy catalysts with low Y contents remained. Higher ORR activity was shown by Pt–YOx/C and PtY/C catalysts than by Pt–Y(OH)3/C, Pt–YOx/C, or PtY/C catalysts prepared by using other conventional chemical reduction methods and thermal treatment methods under a H2/Ar or Ar atmosphere. The mass activity (MA) and surface specific activity (SA) of the best Pt99.5Y0.5/C catalyst, MA=245 A gPt−1 and SA=711 μA cmPt−2, were equal to or higher than those of the commercially used Pt86Co14/C catalyst, MA=245 A gPt−1 and SA=512 μA cmPt−2. The major reasons for the high ORR activity of these Pt–YOx/C and PtY catalysts are discussed. These Pt99.1–99.6Y0.4–0.9/C alloy catalysts prepared by using acid treatment are new and promising catalysts for use in proton exchange membrane fuel cells (PEMFCs).