Synthesis Of Palladium–based Alloy Nanosheets

The discovery of graphene and its novel properties has motivated scientists to prepare more different kinds of two-dimensional nanomaterial.Different from graphene,transition metal dichalcogenides and most of the two-dimensional nanomaterials with lamellar structure,two-dimensional nanomaterials of metal,especially those of transition metal,are difficult to be synthesized and stabilized.This is mainly due to their close-packed crystal structure.How to make nanocrystals grow anisotropically in plane direction,synthesize ultrathin metal nanosheets in control and further stabilize them is a challenging issue.In the recent years,ultrathin nanosheets of different metals have been synthesized under the effort of scientists.These nanosheets are mainly noble metals like palladium(Pd),rhodium(Rh),gold(Au),ruthenium(Ru)and their core-shell nanostructures.The synthesis of alloy ultrathin nanosheets is still limited to Pt-Cu,Pd-Ag and Pd-Pt-Ag.On one hand,metal atoms in alloy have interaction between each other,and it can change the geometric,electronic and chemical properties of the alloy.The catalytic activity,selectivity and stability of alloy catalysts can thus been enhanced.On the other hand,ultrathin metal nanosheets have high specific surface area and large amount of unsaturated surface atom.Thus they can provide large amount of catalytic active sites,and further enhance the catalytic performce.In conclusion,controllable synthesis of different alloy ultrathin nanosheets is critical to the exploration of novel catalysts.Among the reported metal ultrathin nanosheets,the study about Pd is one of the most developed.It was reported the synthesis of 1.8-nm thick Pd hexagonal nanosheets under carbon monoxide(CO)atmosphere.CO can interact strongly with Pd(111)facets,thus it can limit the growth of Pd nanocrsytals along [111] direction effectively.However,such CO-confinement effect has yet been applied to the synthesis of Pd-based alloy nanosheets.In this thesis,based on the reported synthesis of ultrathin Pd nanosheets by CO-confinement effect,we have successfully synthesized ultrathin Pd–Ni alloy nanosheets by increasing atmosphere of CO during synthesis.Such method canextend to the synthesis of other palladium-based alloy systems like Pd–Fe,Pd–Cu and even trimetallic Pd–Fe–Ni.The alloyed nature of as-synthesized ultrathin nanosheets is confirmed by X-ray powder diffraction(XRD),measurement of lattice fringe,selected-area electron diffraction(SAED)and mapping.By studying products in different stage of the synthesis,formation mechanism of ultrathin Pd–Ni alloy nanosheets was proposed.Ultrathin Pd nanosheets with ridges were first formed in the solution.Ni atoms that were being reduced at higher temperature were deposited on the ridges of the nanosheets,and further diffuse into the nanosheets.Finally,ultrathin Pd–Ni alloy nanosheets formed.