Study On Controllable Synthesis And Properties Of Precious Metals Nanocrystals

Metal nanocrystals (NCs) exhibit unique physicochemical properties which are different from their bulk counterparts and have potential applications in optical imaging or labels, electrical and mechanical engineering, chemical sensing, and catalysis. Usually, three main factors including size, shape and composition greatly affect the surface area, expression degree of specific active facets and electronic structure of the metal NCs. One of the noble metal, palladium, has attracted great interests because of its extraordinary properties. It can be used as the primary catalyst owing to its remarkable capacity for hydrogen absorption. Efforts have been made in the past decades on the fabrication of palladium nanostructures with monodispersive sizes and well-defined morphologies to improve its catalytical activity. Palladium nanoparticles with various morphologies have been prepared by using surfactants, polymers and others. However, the heterogeneous system, especially solid/liquid-phase system, is rarely used to synthesize metal NCs with controlled size, shape and composition. On the other hand, preparing high catalytic activity Pd NCs (small size, three-dimensional structure) is necessary and stills to be a key challenge. According to these two aspects, this paper carries out a series of researchs.1.Utilizing static self-assembly of hexadecylpyridinium chloride and [PdCl4]2- and subsequent chemical reduction strategy, our group have successfully fabricated porous Pd nanospheres with the diameter of about 30-50 nm. It was found that hexadecylpyridinium chloride had significant influence on the formation of the porous Pd nano-structure because we could only get the solid spherical structure without this surfactant. Meanwhile, acetone significantly contribute to the morphology of the micelle. The electrocatalytic activity of Porous Pd nanostructures electrode for the formic acid oxidation in 0.5 mol/L HCOOH+0.5 mol/L H2SO4 was investigated. Results showed that the electrochemical characterization of porous Pd nanostructures exhibit better electrocatalytic activity and stability towards formic acid oxidation over that_of Pd solid spherical structure. 2. Palladium nano-flowers with an average size of 30-50 nrn were also successfully synthesized using ascorbic acid as the reducing agent and cetyltrimethylammoniumbromide (CTAB) and L-arginine as the surfactants in aqueous solution. The commercial solid particles and the morphology of our system are also used for direct formic acid fuel cells, we found that the electro-catalytic activity of Palladium nano-flowers were much better.3. A simple and facile one-pot solid/liquid-phase chemical route was developed for the synthesis of small-sized monodisperse and well-crystallined Ag, Cu, Pd and Pd-Ag alloy nanocrystals (NCs) on gram-scale. The typical synthesis is based on thermal treatment of corresponding solid nitrate or formate in dodecylamine and 1-octadecene mixed solvent from room temperature to 320℃under ambient pressure. It is found that the obtained metal (Ag, Cu, Pd) NCs exhibit different shapes which may result from their different electronegativity or redox potential and the different capping/complexing ability of the DDA on these metals. The size of the metal NCs can be controlled by changing the amount of solid precursor and reaction temperature. Based on the temperature- and time-dependent experimental results, a possible high temperature "digestive ripening" of initial formed large multiply twinned or polyhedron-like nanostructures mechanism is suggested to explain the formation of these small-sized monodisperse metal nanocrystals. The preliminary study shows these small-sized metal NCs have interesting optical and catalytic properties.4. Tadpole-like Palladium nanoparticles were obtained via a simple redox reaction of selenium dioxide with dodecylamine in the octadecene solution using oleic acid as surfactant. We found that the dodecylamine has a significant influence on the formation of the tadpole-like Palladium nanoparticle. According to the intermediate procedure, we suggest that high temperature "digestive ripening" of initial polyhedron-like nanostructures mechanism are contributing to the formation of these tadpole-like Palladium nanoparticle. The polyhedron Pd nanocrystal and tadpole-like Palladium nanoparticle are both used as catalysts in oxidizing the formic acid. The results show that the catalytic activity of tadpole-like palladium nanoparticles perform much better than the polyhedron Pd nanocrystals.