Synthesis And Characterization Of Ag2S Based Heterostructures And Ag Nanocrystals

Heterostructures and noble metal nanocrystals have attracted much interest ofresearchers owing to their unique optical and electrical properties in applications suchas bio-labeling, photocatalysis and solar cells. Owing to their physical, chemicalproperties which are strongly dependent on their sizes and shapes, the controllablesynthesis of heterostructures and noble metal nanocrystals has become a subject ofintensive research. However, the oxidation etching in the synthesis of noble metalnanocrystals and the heterogeneous nucleation and lattice mismatch in the synthesisof heterostructure nancrystals, always make it hard to achieve single-crystalnanocrystals with controlled size and shape. Besides，precise understanding on themechanism of heterostructures and noble metal nanocrystals can provide guidance forthe particle size and morphology control during synthesis. Based on the above, weproposed the morphology-controlled synthesis of Ag nanocubes, Ag2S-Ag andAg2S-CdS heterostuctures by wet-chemical method in oleylamine and oleic acidsystem, and explored their growth mechanisms and optical properties, the main resultsare listed as follows:Firstly, a new method takes advantage of a two-step process in which CdSnanorods were employed, benefiting the selective nucleation sites of Ag2S. Themorphology-controlled Ag2S-CdS heterostructures with coherent, quasi-epitaxialinterfaces between both components were obtained. By adjusting the amounts of Agprecursor, the heterostructures exhibit Ag2S growth at the tips of the nanorods, or atmultiple locations across the nanorods. The modified optical properties were observedafter Ag2S growth on rods: the band edge emission almost quenched and the trapemission was enhanced, which may be attributed to strained interface between bothcomponents. On the basis of HRTEM analysis, we conclude that the strained epitaxialinterface may play an important role in the enhanced trap emission. Furthermore, theAg2S-ZnS heterostructure nanorods were also successfully synthesized by thismethod.Secondly, monodisperse and well self-assembled Ag nanocrystals were preparedby “one pot” route. The as-prepared Ag nanocrystals with the size of6nm exhibitedfive-fold twinning structure. Based on the HRTEM image, the individual Agnanocrystal consists of five sub-crystals rotated by to each other along a common <011> edge, which is further analyzed from thermodynamics point. Through“hot-injection” method, Ag2S-Ag heterostructure NCs with different morphologywere obtained. By comparing samples with different reaction time, we find the firstlyproduced Ag2S promote the hetero-nucleation of Ag NCs.Finally, we describe the formation of silver cubic single crystals by heat-inducedcoalescence of silver nanospheres drop-casted on silicon wafer. In this method,14nmmonodisperse Ag nanocubes in the form of a colloidal suspension were obtained. Wereveal the mechanism of silver seeds transformation to nanocubes and nanorodsthrough favored aggregation along the crystal facet with weak ability of ligandadsorption. Furthermore, we demonstrate that the preferential adsorption of theco-ligand of OA and OLA on Ag (100) leads to the formation of anisotropic metalnanoparticles including nanocubes and nanorods. The results show that the cappingligand with amino-group could manipulate the size of the nanocubes by changing thelength of the hydrocarbon chains.