Xinling Tang

Poly(N-vinyl-2-pyrrolidone) (PVP)-capped dendritic gold nanoparticles by a one-step hydrothermal route and their high SERS effect

Dendritic gold (Au) nanoparticles have been successfully synthesized by the one-step hydrothermal reduction of HAuCl4.4H2O using ammonium formate (AF) as a reducing agent in the presence of PVP. Effects of different reactant concentrations on the morphologies of obtained products have been systematically investigated. On the basis of the morphologies of the products observed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM), it has been found that an excessive number of AF molecules are the origin of the dendritic Au particles besides PVP as a stabilizer. AF molecules serve not only as a reductant but probably also as a capping reagent. The study implies that the use of two or more capping reagents with different adsorption abilities will be beneficial to the formation of hyperbranched Au nanoparticles. The new finding will have the potential to be extended to the construction of other highly branched noble metal nanoparticles only by one-step synthesis. In addition, as an example, application of the dendritic particles as an active material in surface-enhanced Raman scattering has been investigated by employing 4-aminothiophenol molecules as a probe.

Syntheses of Silver Nanowires in Liquid Phase

Nanowires which were defined as having at least two dimensions between 1 and 100 nm, have received a great interest due to their unique optical, electrical, magnetic, and thermal properties with dimensionality and size confinement [1-10]. The intrinsic properties of nanowires are mainly determined by its size and composition. In order to study the size dependent properties, it is the crucial task to synthesize size-controlled nanowires. Among all these nanowires, the synthesis of silver (Ag) nanowires has been and continues to be an area of active research due to the high electrical and thermal conductivities of bulk silver, which is an important material in many fields. Moreover, silver nanowires have also been used as sacrificial templates to generate other nanostructures such as gold nanotubes, which are difficult to be fabricated [9,10]. The research progress in synthesis strategy is mandated by advancements in all areas of industry and technology. In the past ten years, owing to the efforts from many research groups, splendid strategies were developed for the synthesis of Ag nanowires with various levels of control over the growth parameters. These synthetics strategies have been conveniently categorized into vapor phase approaches and liquid phase growth approaches. Vapor phase approaches mainly utilize physical methods such as an electronic-beam. Limited by the space of the chapter, we lay a strong emphasis on the introduction of liquid phase synthesis of Ag nanowires. Liquid phase syntheses were most widely used because these approaches have the advantages of nature of homogeneous reaction, wide range of solvents, simple monitoring technology, and low cost. In this article, we review some current research activities that center on Ag nanowires. Representative techniques are discussed and supply a basic understanding of the methods and mechanisms for preparing Ag nanowires.

Synthesis of Au core Au/Ag alloy shell nanoparticles using branched Au nanoparticles as seeds

Novel Au core Au/Ag alloy shell particles denoted as Au@Au/Ag were prepared through the following two-step reduction method using branched Au nanoparticles as seeds. In the first step, branched Au seeds were prepared from an aqueous solution of an HAuCl4/ammonium formate (AF)/poly(vinylpyrrolidone) (PVP) mixture. In the second step, AgNO3 was reduced in N,N-dimethylformamide (DMF) in the presence of branched Au seeds and PVP. Then larger branched particles with smaller numbers of short branches were formed. On the basis of transmission electron microscopic (TEM), high-resolution (HR)-TEM, energy dispersed X-ray spectroscopic (EDS) and XRD data, crystal structures and their growth mechanisms were discussed. It was found that Au/Ag alloy shells were formed by simultaneous occurrence of melt of fine long branches in Au seeds and slow reduction of Ag+ to Ag0 during heating the DMF solution at 140 °C for 3 h. The fusion of fine Au branches was confirmed by observing shape change from fine branches to spherical particles on Au core particles in DMF without the presence of AgNO3.