The role of adsorption species in the formation of Ag nanostructures by a microwave-polyol route

The role of adsorption species in reaction solutions has been studied in microwave-polyol synthesis of silver (Ag) nanostructures. When Ag+ ions from AgNO3 were reduced in ethylene glycol by the addition of H2[PtCl6] and poly-(vinylpyrrolidone) (PVP), a mixture of one-dimensional (1-D) nanorods and nanowires and 3-D spherical, cubic, and triangular-bipyramidal nanoparticles was obtained within three minutes. It has been previously believed that PVP acts as a surfactant and its selective adsorption on {100} facets results in pentagonal 1-D nanorods or nanowires and cubic and triangular-bipyramidal nanocrystals. We found here that these Ag products could also be formed without the addition of PVP in the presence of Cl- ions, though their yields were lower than those in the presence of PVP by a factor of ˜300. These results indicate that Cl- anions can also act as adsorption species to assist the formation of these Ag nanostructures. It is concluded that the difference in the adsorption ability of different species in solution on Ag nanostructures determines final shapes, sizes, and yields of formed Ag nanostructures

Shape-dependent evolution of Au@Ag core-shell nanocrystals by PVP-assisted N,N-dimethylformamide reduction

Shape-dependent Au@Ag core-shell nanocrystals have been successfully synthesized by using a two-step method. First, Au nanocrystal seeds with various shapes including single-crystal octahedron, single-twinned triangle or hexagon plate, and multiple-twinned decahedron were prepared by reducing HAuCl 4 in ethylene glycol (EG) in the presence of polyvinylpyrrolidone (PVP) as a polymer surfactant under the condition of microwave heating. Subsequently, thus-obtained Au seeds were added into N,N-dimethylformamide (DMF) solution containing Ag+ ions for overgrowth of Ag shells by an oil-bath heating. Transmission electron microscope (TEM) observation demonstrates that shapes of formed Ag shells strongly depend on shapes of initiated Au seeds. These newly produced triangular or hexagonal platelike, octahedral, and multiple-twinned decahedral Au@Ag nanocrystals are mainly dominated by the Ag shells having (111) facets. This case is completely different from that previously investigated by us in EG system where formed triangular-bipyramidal, cubic, and pentagonal rod/wire Au@Ag core-shell nanostructures are surrounded by the Ag shells with {100} facets. Our studies reveal that it is possible to controllably prepare the Au@Ag core/shell nanocrystal structures with optional uniform crystalline planes by the same Au seed source and different reaction solvent

Toward to branched platinum nanoparticles by polyol reduction: a role of poly(vinylpyrrolidone) molecules

Branched Pt nanoparticles with nanometer sizes have been successfully synthesized by reduction of H2PtCl6 center dot 6H(2)O precursor in ethylene glycol (EG) in the presence of small amounts of NaNO3 and PVP Morphologies of the Pt nanoparticles can be systematically evolved from regular octahedron, and triangular plate via tri-pod, penta-pod, and octa-pod to multi-pod needle-like shapes only by decreasing concentrations of H2PtCl6 center dot 6H(2)O and NaNO3 at a constant NaNO3/H2PtCl6 center dot 6H(2)O molar ratio and the same PVP concentration. To the best of knowledge, this is the first report for the synthesis of Pt penta-pod. High resolution transmission electron microscope (TEM) observation of the Pt nanoparticles demonstrates that the Pt branches actually extendedly grow out from certain angles of triangular plates, octahedrons, and decahedrons, respectively. Multi-branched needle-like Pt nanocrystals are believed probably to originate from further anisotropic growth of the Pt octa-pods or overlap of small branched Pt nanoparticles. PVP molecules have been found to play an important role in controlling morphologies of the branch-like Pt nanoparticles besides NaNO3. It probably is the cooperated kinetic adsorption and desorption of PVP molecules and various anions on particle surfaces that influence the growth of the Pt nanoparticles. A reasonable growth mechanism has been suggested to explain the evolution of the Pt branches, in which the difference among growth rates along various crystallographic directions of face-centered cubic Pt crystal probably determines final morphologies of the Pt nanocrystals

マイクロ波加熱を用いた金属ナノ材料の迅速合成と形態制御

Microwave (MW) heating has received a considerable attention as a new promising method for the synthesis of metallic nanostructures in solutions. In this review, advantageous application of this method has been demonstrated by using typical examples for the preparation of Au, Ag, Au@Ag, and Pt/C nanostructures. No only spherical nanoparticles, but also single crystalline polygonal sheets, plates, rods, wires, and core-shell structures were prepared within a few minutes under MW heating. Morphologies and sizes of nanostructures could be controlled by changing various experimental parameters, such as concentrations of metallic salt and surfactant polymer, chain length of the surfactant polymer, and solvent. In general, nanostructures with smaller sizes, narrower size distributions, and higher degree of crystallization were obtained under MW heating than those in conventional oil-bath heating