Sachie Hikino

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.

Syntheses of Ag/Cu alloy and Ag/Cu alloy core Cu shell nanoparticles using a polyol method

Ag–Cu bimetallic nanoparticles were prepared by reducing mixtures of AgNO3 and Cu(OAc)2·H2O in ethylene glycol (EG) in the presence of poly(vinylpyrrolidone) (PVP) at 175 °C for 5–60 min. At high [Ag]/[Cu] molar ratios of 1 and 2 or at a short reaction time below 5 min, Ag rich Ag/Cu alloy particles were formed. On the other hand, at low [Ag]/[Cu] molar ratios of 0.25 and 0.5 or a long reaction time above ≈15 min, Cu shells were overgrown on Au/Cu cores and new Ag/Cu alloy core Cu shell nanoparticles, denoted as Ag/Cu@Cu, were produced. The formation of Ag/Cu and Ag/Cu@Cu particles was examined using energy dispersed X-ray spectroscopic (EDS) measurements. The growth mechanisms of Ag/Cu and Ag/Cu@Cu particles are discussed on the basis of TEM-EDS and ultraviolet (UV)-visible (Vis)-near infrared (NIR) extinction spectral data. The time dependence of UV-Vis spectra indicated that the Cu component of Ag/Cu@Cu particles has higher antioxidized property than that of Cu and Cu@Ag particles.

Synthesis of Au@Ag@Cu trimetallic nanocrystals using three-step reduction

Au@Ag@Cu trimetallic nanocrystals were prepared using a three-step reduction method. In the first step, decahedral Au core seeds were prepared by reducing HAuCl4·4H2O in diethylene glycol (DEG) under oil-bath heating in the presence of polyvinylpyrrolidone (PVP) as a polymer surfactant. In the second step, Ag shells were overgrown on these Au seeds in N,N-dimethylformamide (DMF) in the presence of PVP under oil-bath heating to prepare decahedral Au@Ag nanocrystals. In the third step, Cu shells were overgrown further on Au@Ag core–shell nanocrystals in ethylene glycol (EG) in the presence of PVP under oil-bath heating. The resultant crystal shapes were characterized using transmission electron microscopic (TEM), TEM-energy dispersed X-ray spectroscopic (EDS), and X-ray diffraction (XRD) measurements. Results show that Cu shells of two kinds are grown over Au@Ag core seeds: a phase-separated major Cu component attached to one or two side edges of decahedral Au@Ag cores, and a minor Cu component that appears as thin Cu shells over decahedral Au@Ag cores. Partial reservation of pentagonal shape and appearance of Moire patterns in Au@Ag@Cu particles suggest that epitaxial growth occurs on some parts of the Au@Ag cores despite a large lattice mismatch between Ag and Cu (11.5%). The growth mechanism of Au@Ag@Cu nanocrystals was discussed in terms of lattice mismatch, decahedral particle defects, and the favorable shape of metallic shells. Optical properties of Au@Ag@Cu nanocrystals were determined by measuring extinction spectra.