Controlled Synthesis And Properties Of Noble Metal Composite Nanostructures

Metal composite nanomaterials have improved chemical stability against pure metals and possess the chemical properties inherited from two or more metals. As a result, they have wide applications such as catalysis, nonlinear optics and gas adsorption, and thus attract more and more attention. The purpose of this dissertation is to explore the design and controlled synthesis of noble metal composite nanomaterials with well-defined structures and on the relationship between their structures and properties. To this end, we have implemented and extended oil-phase and aqueous-phase synthetic methods to CuPt, PdAg and Pd@Ag/Au binary/ternary metal nanocomposites. Specifically, CuxPt100-x alloy nanocubes with controllable Cu/Pt ratios were designed, synthesized and characterized, and the effects of preparation conditions on the formation of alloy structures were investigated. We further investigated the effects of nanocube compositions on the electrocatalytic reduction of CO2. PdAg nanocages with different Pd/Ag ratios were synthesized and characterized. We further investigated their tunable surface plasmon resonance (SPR) properties and applications in catalytic styrene hydrogenation. Pd@Ag/Au core-shell nanocages with various compositions were synthesized and characterized. We further investigated their SPR properties and performance in catalytic benzene acetylene coupling and hydrogen storage. The specific contents are listed as follows:1. Cu-Pt alloy nanoparticles are ideal candidates for the electrocatalytic reduction of CO2. However, it still remains challenging to develop Cu-Pt alloy nanocrystals with well-defined shapes and controllable compositions. We developed a modified protocol for CuxPt100-x alloy nanocubes with high Cu concentrations, which enabled the investigation of the composition-property relationship for electrocatalytic CO2reduction. By systematically performing control experiments, we found out that the reduction rate in the synthesis was the key parameter to increase Cu constituents in the final products. At a relatively high reduction rate, galvanic replacement impeding Cu reduction was substantially suppressed. The resulted products showed greatly improved chemical stability and well-defined morphology owing to the presence of Pt, and exhibited tunable selectivity towards CO2reduction enabled by controllable Cu concentrations. This work also presents a set of promising catalysts for HER by increasing Pt concentrations in the alloy nanocubes, in which the incorporation of Cu significantly reduces materials cost. It is anticipated that this work provides insights into the controlled synthesis of bimetallic nanocrystals and related electrocatalytic applications.2. The surface plasmon of metals, which converts light into heat through a photothermal effect to sustain the reactions, can offer an alternative approach to solar-to-chemical energy conversion at the intersection with catalysis. To achieve high efficiency in such plasmonic-driven organic reactions, the catalysts should have a large number of active catalytic sites for the reactions and sufficient plasmonic crosssections for light harvesting. Therefore, we developed Pd-Ag alloy nanocages in tunable compositions and structures for light-driven catalytic hydrogenation. In the catalysis, Pd provided active sites for hydrogenation reactions, while Ag offered plasmonic properties to convert light into heat on the nanocrystal surface. With the increase of Pd content, the structures of Pd-Ag alloy nanocrystals turned more hollow and porous, which affected both the number of active sites and plasmonic cross sections. In consideration of Pd usage, one had to minimize the amount of Pd used in catalysts so that the alloy nanocages with25.84%and44.08%Pd were the best candidates. As compared with our previous success in bare Pd nanostructures, the44.08%-Pd nanocages achieved the same catalytic yield with only half illumination intensity (50mW/cm2versus100mW/cm2) and1/5Pd (0.04mg Pd/0.2mmol styrene versus2mg Pd/2mmol styrene) used. This work provides new insights into the design of plasmonic catalytic materials, and paves the way toward lowcost catalysts for light-driven organic reactions.3. We designed a core-shell structure composed of ternary metals:the core of Pd nanocubes, and the porous shell layer of Ag/Au alloys. The product exhibited excellent SPR feature, thus better absorbing visible and near-infrared light. The Pd@Ag/Au hollow core-shell nanostructures showed significantly improved performance in phenyl acetylene production. The yield of coupling product against other products increased from about1:8to1:2by using this unique nanostructure, together with the increase in total quantity of various products. Although the Pd@Ag/Au core-shell nanostructures did not yield a very large quantity of coupling product, the yield was still more than3times that by other catalysts. The Pd@Ag/Au nanostructures also possessed high performance in hydrogen adsorption, storage and release, demonstrating their potential cycling use in hydrogen storage. This work provides a new approach to the design of plasmonic catalytic materials, and offers new insights for designing the catalysts of organic coupling reactions. It also produces the nanocatalysts with good hydrogen storage properties and recycling performance.4. We prepared two kinds of SnO2-based composite oxide semiconductor materials SnO2-La2O3and SnO2-CuO via solid-phase synthesis. We also tried to use a new synthetic method, namely the Pechini, to synthesize CuO-BaTiO3composite oxide semiconductor thin film. Different doping was incorporated into the two CO2-sensitive materials, forming CO2sensors. We further tested the performance of doped and undoped materials. Finally, we made a comparative analysis of the two composite oxide semiconductor materials in terms of synthetic process and sensitivity characteristics. According to the results, we found that both SnO2-based and CuO-BaTiO3composite oxide semiconductor materials had good CO2sensitivity and stability. In the synthetic process, the Pechini method could form a gel film to coat on ceramic tube directly. As compared the solid-phase synthesis, the Pechini method eliminated the adhesive process, and avoided some impurity contaminations. However, the sensitivity and stability of CuO-BaTiO3material is slightly worse than SnO2-based semiconductor materials. It is anticipated that we can increase the density and porosity of CuO-BaTiO3film to improve its sensitivity and expand the detection scope.