| Graphene is considered as one of the most promising two-dimensional materials in the 21st century,and arouses continuous research interests of scientists from various fields due to its excellent optical,electrical,thermal and mechanical properties.Graphene supported metal nanocatalysts,known as its large specific surface area,high activity and high stability,are studied extensively.However,the ambiguous structures of surface and interface,such as the co-mixing synthesis and assembly of electrocatalysts,hinder the deep understandings of electrochemical nanocatalysis.To this,this thesis aims to construct the model catalytic system of single-layer graphene supported metal nanocrystals,and to discover the correlation between its structure and performance.First,heat annealing was employed to tune the sp3-defect density of single-layer graphene and,thus,to improve its capability of heterogeneous electron transfer(HET).Then,a potential modulation method was developed to synthesize hexagonal Ag nanocrystals on single-layer graphene to construct the Ag nanocrystal/single-layer graphene model catalytic system,and the structureperformance correlation on its HET behavior was investigated.The details results are as followed:(1)Thermal annealing method to optimize the HET of single-layer graphene.HET kinetics of pristine single-layer graphene is reported to be relatively slow with the outsphere reactions,and introduce of appropriate sp3-defects can improve its HET capability.Here,Si/SiO2 wafer supported single-layer graphene was annealed in Ar atmosphere.The results show that,varying annealing temperature from 400℃ to 800℃,HET rate(k0)of single-layer graphene presented a Marcus turnover with Fe(CN)63-/Fe(CN)64-as redox mediator.Increasing temperature to 600℃ in a rate of 10℃/min,and helding for 15 min before natural cooling,the annealing effect is optimized and k0 of the annealing single-layer graphene was increased by two orders of magnitude comparing with the untreated one.Raman imaging shows that thermal annealing can regulate and homogenize the distribution of sp3-defects on the plane of single-layer graphene.XPS analysis further shows that the sp3-defects are mainly C-O single bonds.Thus,Annealing is demonstrated to be a simple and easy way to regulate the sp3-defect degree and to improve the crystallinity and cleanliness of single-layer graphene,and thus,to improve k0 of single-layer graphene.(2)Architecture and HET performance of single silver nanocrystal/single-layer graphene model system.In acidic electrolyte containing 2 mM Ag2SO4 and 0.5 M H2SO4,single-layer graphene was anodic polarized in advance.Then,the potentials were set to be OCP+400 mV,OCP,OCP-175 mV,OCP+400 mV,OCP-75 mV respectively,the silver nanocrystal would nucleate,dissolute and regrow on the pretreated surface of single-layer graphene.Hexagonal Ag nanocrystals were obtained due to the orientational adsorption of SO42-on Ag(111)facet,the size of which can be up to 40 microns and the thickness ranges from several hundred nanometers to several microns.By scanning Kelvin probe microscopy(SKPM),a contact electric field was found across the boundary between the single Ag nanocrystal and the single-layer graphene supporter with a space charge layer on each side.Because the density of electronic states near Fermi Level of graphene is much lower than than that of silver,electrons transfer from Ag to graphene resulting in the shifts of local Fermi level of both with an electron-doping N-type graphene and a positive charged Ag at the boundary.At equilibrium,the Fermi levels of both are equal.This local surface potential field across the boundary significantly altered the current feedback of Scanning Electrochemical Microscope(SECM)across the boundaries,revealing the correlation between the surface potential aacorss the boundary between the metal catalysts and its supporters towards the electrochemical activity for the first time.Further results demonstrate that surface plasmons effect enhanced SECM current feedback,i.e.,the improved photoelectrochemical activity of the system.In summary,the construction of model electrocatalytic systems is very important for the investigation on the structure-performance correlation of nanocatalysis.This thesis proposed a thermal annealing method to tune the sp3-defect degree of single-layer graphene and to improve its HET kinetic rate,constructed the single Ag nanocrystalline/single-layer graphene model system,demonstrated the influence of electric field across the Ag/graphene boundary on the local HET behaviors and the enhancement of surface plasmon effect on the local electrode process,and provided a new approach to study the structure-performance corelations of nanocatalysis. |
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