To Design Synthesis Of Au Or Ag Noble Metal Nanostructures With High-index Facets And To Modulate Their Catalytic Activity

The increasingly serious energy crisis and environmental problems are the biggest obstacles for us to protecting our homes.Electrocatalytic energy-conversion processes are expected to be the most promising clean energy in the development of sustainable technologies that mitigate global warming and lower our dependence on fossil fuels.But,the highly efficient catalysts are very important for electrocatalytic energy-conversion processes.Therefore,it is critical to improve catalytic activity and generate cost-savings.That is to say,we need to optimize the compositions,nanostructures,shapes,lattice and sizes of the catalysts.Noble metal nanocrystals,including gold(Au)and silver(Ag),are the most efficient and stable catalysts owing to its relatively low cost compared with Pt,unique biocompatibility and high stability.They are commonly used to degrade contaminants,purify automotive exhaust and electrocatalytically reduce CO2.It is confirmed by experimental measurements and theoretical simulations that the reaction rate and activation energy depends on the crystal facet exposed on its surface.Compared with low-index facets,there are a large amount of steps,crystal defects and low-coordinated atoms on the high-index facets.Therefore,nanocrystals with high-index facets have better catalytic performance.In this thesis,we focus on the controlled synthesis of noble metal nanocrystals with high-index facets(including Au and Ag)by a straightforward,environmental and economical means and further optimizing the catalytic degration of p-nitrophenol,glucose electrooxidation reaction(GOR)and hydrogen evolution reaction(HER).During the synthetic process,we select different reductant,modulate the solution p H environment,and design the reaction kinetics.And,we revealed structure-function relationship between catalytic performance and the shapes,sizes and nanostructures of nanocatalysts.The related research works are summarized as follows.1.Design of porous Ag platelet structures with tunable porosity and high catalytic activity.To optimize the catalytic performance for Ag nanocatalysts, we design shape-,size-,porosity-,and crystallinity-controlled and surface defect tailored porous Ag nanostructures.And,we implemented the following parameters to tune their catalytic activity: the surface-to-volume ratio,and the number of atomic steps,ledges and kinks on the porous Ag.In this work,a low cost and facile synthetic route is presented to produce regular porous Ag platelet structures with controlled size,porosity,crystallinity and surface defects by a two-step process.Initially,size-tailored regular Ag platelet precursors from 2.5 mm to 36 mm are obtained by merely adjusting the solution p H value;Simply by optimizing the annealing time,temperature and heating rate,porous Ag platelets with effectively tunable porosity,crystallinity and surface defects can be achieved.Besides,the reduction of p-nitrophenol to p-aminophenol using Na BH4 was chosen as a representative catalysis to evaluate the catalytic activity of the tunable porous Ag platelet structures.And,we study the relationship between catalytic performance and their shapes.The porous Ag platelet structures could catalyze the reduction of p-nitrophenol and dyes quite effectively at room temperature.It is noted that the QPP Ag(I)structure is the best one among the four porous Ag platelet structures,which is attributed to their regular morphologies,porosity,high surface-to-volume ratio,short diffusion length and good permeability.Moreover,these porous Ag platelet structures because of their unique morphology and network characteristics will exhibit excellent electrochemical catalytic activity,or act as outstanding electrodes,sensors,actuators,etc.2.Kinetically Controlled Synthesis of Nanoporous Au and their Enhanced Electrocatalytic Activity for Glucose-based Biofuel Cells: we design surface-clean nanoporous gold(NPG)structures with high-index facets by a kinetically controlled self-assembly manner.This strategy breaks through the traditional trisodium citrate thermal-reducing chloroauric acid approach,which solutions are necessarily heated to a certain temperature for the reaction to initiate.Meanwhile,water-ice bath leads to citrate ad-layer physically absorbing on Au,instead of chemisorbing by O-H bond cleavage.And,citrate ad-layer could be completely removed by a cryogenic washing technique.Finally,the Au nanocatalysts were evaluated as the anodes in glucose electrooxidation (GEO)to demonstrate their highly promising application.And,we investigated surface structure-activity relationship.As a result,NPG exhibits excellent GEO catalytic performance with current densities of 9 A cm-2 mg-1,which is 20 times higher than those of Turkevich-Au NPs(0.45 A cm-2 mg-1)at 1.2 V(vs.RHE).The current density,j,of 9 A cm-2 mg-1 is 3.4 larger than the latest and best value(2.65 A cm-2 mg-1).The remarkable catalytic performance of NPG can be ascribed to the following aspects: i)the clean surfaces facilitate free access,charge transfer of glucose and maximize the exposure of active sites;ii)the porous structure possesses large electrochemically active surface areas,better permeability and more accessible high-index facets and active site.These results illustrate the promising prospect of NPG nanocatalyst in biofuel cell application.3.Icosahedral Au nanoparticle catalysts with Pt-like activity for hydrogen evolution reaction: we demonstrate a kinetic controlled means to prepare icosahedral multiply twinned Au nanoparticles with clean surface by reducing the concentration of reactant.At the same time,we design the truncated icosahedral Au by increasing the reaction time.There are a lot of crystal defects and steps etc.active sites,which are beneficial to catalyze hydrogen evolution reaction(HER),on Au icosahedrons.Icosahedral Au serves as electrocatalysts for hydrogen generation and exhibits enhanced catalysis for HER with potential cycling increasing.The data showed a clear monotonic trend,the HER over-potentials,required to obtain a specified current density,shifts positively with the increased number of cyclic voltammogram(CV)cycles.After 30 000 CV cycles,icosahedral Au nanocatalysts exhibit excellent catalysis for HER with Pt-like activity and much robust durability.Preliminary studies demonstrate that: i)Lattice strain,either compressive or tensile,can alter the surface electronic structure by modifying the distances between surface atoms and in turn catalytic activity;ii)Multiple twins in icodahedral Au nanocrystals can offer the necessary space for the change in the lattice;iii)Aqueous solution electrochemical means can compress the lattice in icosahedral Au nanocrystals,strengthen Au-H bonds and improve their catalytic performance for HER.