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 reagent's 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.
Au@Pd core-shell nanocrystals were prepared using a two-step polyol reduction method. First, decahedral Au core seeds with small amounts of octahedral, triangular-platelike, and icosahedral Au particles were prepared by reducing HAuCl 4 $4H 2 O in diethylene glycol (DEG) under oil-bath heating in the presence of polyvinylpyrrolidone (PVP) as a polymer surfactant. Then Pd shells were overgrown epitaxially on Au core seeds by reducing Na 2 PdCl 4 in EG with PVP, KBr, and H 2 O. The resultant crystal shapes were characterized using transmission electron microscopic (TEM), TEM-energy dispersed X-ray spectroscopic (EDS), and selected area electron diffraction (SAED) measurements. Effects of addition of KBr and H 2 O in the second step and reaction temperature for the yield of coreshell particles and their shape, size, and composition distributions were examined. Results show that both {111} and {100} facets of Pd shells were formed in the presence of KBr, depending on the shapes of Au core seeds, whereas only {111} facets were produced in the absence of KBr. New triangularplatelike, five-twin rod, decahedral, and icosahedral shapes of Au@Pd nanocrystals were prepared using triangular-platelike, decahedral, and icosahedral Au cores. When decahedral Au particles were used as seeds, monodispersed decahedral Au@Pd nanocrystals were prepared in high yield at low temperatures of 20-100 C in PVP/EG solution. Au acts as a catalyst for Pd 2+ reduction in the presence of PVP at low temperatures. This work demonstrates a simple two-step technique for the epitaxial growth of various shapes of Au@Pd nanocrystals.
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 H 2 PdCl 4 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.
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 CuCl 2 •2H 2 O 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 CuCl 2 •2H 2 O was replaced with Cu(OAc) 2 •H 2 O, layered growth of Cu shells was observed. Effects of the addition of NaCl to Cu(OAc) 2 •H 2 O 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.
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).
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