Controlled Synthesis Of Noble Metal Nanosheets And Their Oxygen Reduction And Oxygen Evolution Performance

Noble metal nanomaterials have been widely used in the field of catalysis,especially in the electrocatalytic reactions related to hydrogen energy technology.Considering the high price and scarce resources of noble metals,how to effectively reduce the actual amount of noble metals while maintaining or further improving their catalytic activity and stability has always been a research hotspot.The number of active sites and intrinsic activity of noble metal catalysts are not only related to their composition,morphology and size,but also related to their atomic arrangement and coordination states.In recent years,thanks to the help of synchrotron radiation spectroscopy and aberration-corrected HAADF-STEM,noble metal nanomaterials have developed from the traditional control on the size,morphology,crystal planes to the precise control at atomic level,such as the construction of ultrafine nanostructures,accurate control of the number of atomic layers,crystalline phase regulation and amorphization of nanocrystals and so on.Atomic-scale controllable synthesis can achieve effective control of the surface and interface atoms,atomic distances and interactions of noble metal materials,thereby significantly improving the atomic utilization of noble metals and their intrinsic catalytic activity.In this dissertation,we developed a set of controlled synthesis methods to precisely regulate the platinum and ruthenium-based nanosheets at the atomic level.We mainly focus on controlling the atomic layers and surface atomic arrangement of twodimensional ultrathin nanosheets to regulate the number of active sites and intrinsic catalytic activity of the catalyst.Moreover,the formation mechanism,catalytic application and structure-activity relationship of the unique atomic structure of noble metals are deeply investigated.The main content of the paper is summarized as follows:1.The recent research progress of atomic scale controllable synthesis of noble metal nanosheets is briefly summarized.2.An active and stable oxygen evolution reaction(OER)catalyst has been successfully prepared by controlling the number of atomic layers of RuO2 grown epitaxially on PdO nanosheets.The epitaxial growth of face-centered cubic Ru on Pd nanosheets was realized by a simple solution synthesis method.By adjusting the proportion of Ru precursor to Pd seed properly,the number of atomic layers of Ru coating can be precisely controlled at the atomic level.Pd@Ru nanosheets are converted into PdO@RuO2 nanosheets by thermal oxidation treatment,which exhibits excellent catalytic activity and stability in acidic OER.It was found that the OER activity of PdO@RuO2 nanosheets was related to the atomic layer number of RuO2.Among them,the RuO2 nanosheet has the best catalytic activity when it is about 4 atomic layers and the overpotential is only 257 mV.Density functional theory calculations also reproduce this thickness dependence of OER activity well,as the weak binding of O*on RuO2 with four atomic layers is conducive to the subsequent formation of HOO*intermediates,thus ensuring the best OER performance.3.By using inorganic salt as a template,two-dimensional coplanar Pt/C nanomesh fuel cell catalysts composed of Pt nanomesh and carbon was prepared by one-step pyrolysis.The thickness of coplanar Pt/C nanomeshes is 10 nm.The coplanar Pt/C nanomeshes are composed of interconnected ultrafine distorted Pt networks(2.05±0.72 nm)and coplanar carbon.This unique coplanar Pt/C nanomesh structure utilizes coplanar carbon to effectively separate the Pt networks to prevent agglomeration.At the same time,the two-dimensional nanomesh structure is conducive to the exposure of more active sites,thus ensuring high mass activity and stability of the catalyst.In fuel cell test,the mass activity of the coplanar Pt/C nanomeshse at 0.9 V is 0.57 A mgPt1,which only loses 12.4%after 30,000 cycles.In contrast,the commercial Pt/C catalyst had an initial mass activity of 0.12 A mgPt-1 and lost 62.8%of its mass activity after 30,000 cycles.4.Through effective control of the diffusion and amorphization of Pt nanoparticles on the surface of ruthenium oxide,an efficient and stable acidic OER catalyst is screened.The amorphous Ru nanosheets were loaded with 1.83 nm Pt nanoparticles to form the heterogeneous structure of Pt/amorphous Ru nanosheets.By heating the heterostructures of Pt/amorphous Ru nanosheets in situ in a vacuum environment with aberration-corrected HAADF-STEM,the atomic structure transformation of Pt/amorphous Ru nanosheets was directly revealed at the atomic scale.During heating treatment,amorphous Ru crystallized to form RuO2,and Pt atoms diffused into RuO2.At 300℃,the diffusion of Pt atoms on the surface of RuO2 leads to the amorphization of Pt nanoparticles.At 500℃,the further diffusion of Pt in RuO2 eventually forms a solid solution of Pt and RuO2.Among them,the heterostructure of amorphous Pt/RuO2 nanosheets showed the best catalytic activity and stability in the acidic OER reaction.