Study On Controllable Synthesis Of Metal And Metallic Chalcogen Nanocrystals And Their Electrocatalytic Properties

In view of current synthesis and application research status of metal and metal chalcogenide nanocrystals (NCs), we designed simple solid-liquid as well as the combination of solid-liquid and solid-solid phase chemical routes, and controllably synthesized some inorganic functional nanostructures, including single metal (e.g. Cu), bimetallic (e.g. Cu-Ag, Pd-Ag, Pd-Ru), trimetallic (e.g. Pd-Ni-Ag) and metal chalcogenide (e.g. Cu2Se) NCs. Combined the experimental data with corresponding theoretical analysis, the growth mechanisms of those metal and metal chalcogenide NCs were discussed. Moreover, their applications in electrochemical biosensors and regenerative H2-O2fuel cells were explored. Some primary and interesting research results have been achieved. The detailed achievements are given below.(1) Cu nanowires (NWs) have been synthesized through thermal treatment of solid CuCl in the liquid mixture of oleic acid, dodecylamine, and1-octadecene under temperature programming mode. The diameter of the obtained Cu NWs was about20nm and their length was several tens to hundreds of micrometers. By assembling the Cu NWs on graphene (G), Cu/G can be obtained. Electrochemical tests reveal that the obtained Cu NWs can be used to catalyze oxygen reduction reaction (ORR) in alkaline media, which is even better than that of commercial Pd/C and some reported non-Pt electrocatalysts. In addition, such NWs not only possess good conductivity but also can be served as template and reaction resoures to synthesize other Cu-based compounds nanostructures.(2) The novel octopus-tentacle-like Cu NW-Ag NCs heterostructures have been fabricated in solution phase via heterogeneous nucleation and growth of Ag NCs on pre-synthesized Cu NWs. The growth environment and dynamic factors of Ag NCs play an important role for formation of such heterostructures. Combining the physical constants of Cu and Ag with a series of control experiments, the epitaxial growth means of Ag NCs on Cu NW is found to abide by "layer-plus-island"(Stranski-Krastanow) mode. Due to the presence of multiple junctions and strong synergistic effect of their constituents, the obtained heterostructures exhibit greatly enhanced electrocatalytic performance toward oxygen reduction reaction compared with that of pure Ag NCs, Cu NWs, and mechanically mixed dual components as well as recently reported some non-Pt materials, which can be served as an alternative cathodic electrocatalyst to apply in alkaline fuel cells. Moreover, our method can be extended to fabricate octopus-tentacle-like Cu NW-Au NCs and Cu NW-Pd NCs heterostructures.(3) Pure tetragonal and cubic phases Cu2Se NWs that assembled by small nanocubes have been controllably synthesized via a simple solid-liquid phase chemical transformation method, i.e. thermal treatment of pre-synthesized Cu NWs in Se precursor solution containing proper surfactant and1-octadecene. Besides reaction temperature and ripening time, the functional groups and alkyl chain length of used surfactant greatly affect the kinetics of the transformation reaction and phase structure control of Cu2Se NWs. The trioctylphosphine is found to be the optimal surfactant, which not only accelerates the transformation reaction but also improves the stable temperature of tetragonal phase Cu2Se NWs about80℃compared with that of their bulk counterparts. Electrochemical tests reveal that both the obtained Cu2Se NWs can be used to catalyze oxygen reduction reaction (ORR) in alkaline media. But the catalytic performance of tetragonal phase Cu2Se NWs is much higher than that of cubic phase ones, which is even better than that of commercial Pd/C and some reported non-Pt electrocatalysts. The diverse catalytic performances of those Cu2Se NWs result from their distinct spatial arrangement means of Cu and Se atoms that lead to different adsorption and activation of O2molecules approaches, as evidenced by electrocatalytic dynamic experiments. The ORR on tetragonal phase Cu2Se NWs abides by direct4e-mechanism, whereas that on cubic phase ones complies with dual-path mechanism comprising both2e-+2e" pathways.(4) The small-sized monodisperse Pd-Ag alloy NCs have been synthesized via a solid-liquid and solid-solid phase chemical route,i. e., sequential reduction of Pd(NO3)2and AgNO3solid precursors in the liquid mixture of dodecylamine and1-octadecene, and then fusion of formed Pd and Ag NCs at250℃. By controlling the adding sequence and molar ratio of metallic precursors, a series of Pd-Ag alloy NCs including PdsAg, Pd2Ag, PdAg, PdAg2, and PdAgs are obtained. Those alloy NCs are highly crystallized and exhibit strong atomic ensemble as well as component-dependent electronic effects. Among them, Pd2Ag NCs possess unique structure and electronic properties, showing much faster electron transfer process at modified glassy carbon electrode interface compared with those of other alloy ones. Thus, the Pd2Ag NCs are chosen as the electrocatalyst to evaluate the performance of Pd-Ag nanoalloy. Compared with those of pure Pd and Ag NCs, greatly enhanced electrocatalytic performance toward oxidation of glucose is observed in Pd2Ag alloy NCs. Based on this result, a novel non-enzymatic glucose biosensor is fabricated. Such biosensor shows a linear response range of0.04to46mM and the detection limit is 0.02mM at a signal-to-noise ratio of3. Furthermore, it exhibits an acceptable reproducibility, a good stability and low interferences, which can be used to examine glucose in clinic blood serum samples.(5) A series of Pd-Ni-Ag trimetallic NCs with different componential ratios, such as Pd2NiAg, PdNiAg, PdNi2Ag2and PdNi2Ag, have been synthesized via thermal treatment of Pd(NO3)2, Ni(acac)2and AgNO3solid precursors in the liquid mixture of DDA and ODE. By assembling those trimetallic NCs on graphene (G), a series of desired hydrid materials can be obtained. Compared with that of Pd-Ag/G, Pd-Ni/G and Pd-Ni-Ag/G, the Pd2NiAg/G exhibits the best electrocatalytic ORR performance in alkaline media, which is comparable to conventional Pt/C catalyst and better than that those of recently reported some non-Pt electrocatalytic materials. In contrast to Pd-Ag/G, the greatly enhanced electrocatalytic ORR performance observed in Pd2NiAg/G may be attributed to the introduction of magnetic Ni element, which not only reduces the interfacial electron transfer resistance as evidenced by EIS analysis but also changes the spin status of the catalyst, beneficial to activate the O2(supermagnetic) and enhance catalytic ORR activity. Corresponding magnetic measuremnts and theoretical calculation results give the strong evidences for this. This work not only screens out a promising electrocatalyst for substitution of Pt/C catalyst to apply in fuel cells but also confirms that the introduction of magnetic element can enhance ORR activity from the experiments for the first time, which paves the way for designing highly active and long-durability non-Pt electrocatalysts to apply in H2-O2fuel cells in the future.(6) Novel2D-0D type metallic hetero-nanostructures, Pd-Ru alloy nanodisk (ND)-Pd NCs, have been synthesized by co-reduction of Pd(NO3)2and RuCl3·H2O precursors in the presence of cetyltrimethylammonium bromide (CTAB), polyvinyl-pyrrolidone (PVP) and ethylene glycol. The reaction temperature is found to trigger the formation Pd-Ru ND-Pd NCs heterostructures. By analyzing the XRD data of Pd-Ru alloy NDs (obtained at110℃) and Pd-Ru ND-Pd NCs heterostructures as well as the supernatant of Pd-Ru alloy NDs sample, we conclude that the observed Pd NCs in the heterostructures (obtained at185℃) mainly result from further reduction of residual Pd precursor species and subsequent epitaxial growth on formed Pd-Ru NDs triggered by reaction temperature, not from the segregation of in-situ formed Pd-Ru alloy NDs. Using hydrogen evolution reaction (HER) as a probe reaction, the electrocatalytic performance of Pd-Ru ND-Pd NCs heterostructures is evaluated. The results demonstrate that the obtained heterostructures exhibit unprecedented electrocatalytic activity toward HER and their onset resuction potential is about0.057V(vs. RHE). Their exchange current density (j。) is calculated to be1.95mA cm’2, which is much higher than that of commercial Pt/C catalyst (jo=0.95mA cm-2). Based on the calculated slop of Tafel plot (-77mV/dec), the HER mechanism on obtained heterostructural catalyst abides by Volmer-Tafel mechanism. Compared with pure Pd-Ru alloy NDs, some reported non-Pt materials and Pt/C catalyst, the obtained heterostructure possess much higher electro-catalytic activity for HER. Moreover, the durability of the heterostgructures is also better than that of pure alloy NDs and comparable to Pt/C catalyst, showing great promise for substitution of Pt-based catalysts to apply in H2production or regenerative H2-O2fuel cells.