Studies On The Controlled Synthesis Of Pt-and Rh-based Metastable Nanocrystals And Their Electrocatalytic Properties

Noble metal nanocrystals have great applications in many fields because of their specific physical and chemical properties.The diversity of their bulk and surface structures gives us significant chances for exploring new functional materials.Meanwhile,the very diversity also makes the synthesis of noble metal nanocrystals as a new but more complicated discipline compared with the traditional molecular chemistry,but the absence of theory makes the synthesis more like a "trial and error"experimental science.Fortunately,great improvement has been achieved for the fabrication of surface structures of noble metal nanocrystals since the term"nanotechnology" was put forward in 1959.However,only a few studies about the regulation of bulk especially for the crystal phases,were reported up to now.The energy difference between two typical crystal phases(face-centered cubic and hexagonal closepacked)adapted by the noble metals is very tiny,even much smaller than the bond energy between the surfactant molecule and noble metal atoms.This makes the regulation of crystal phases very difficult.In addition,it is also a great challenge to build dynamic models about the growth of noble metal nanocrystals due to the complexity of the reaction solution and process.Therefore,studies about the fabrication of crystal phases of noble metal nanocrystals are rare but very necessary.In this dissertation,with the study of Pt and Rh,we aim to explore many nanocrystals with uncommon crystal phases and try to summarize intrinsic growth mechanisms.Besides,the relationship between the crystal phases and properties is also investigated.The main research results are summarized as follows:1)The unique Pt-Ni alloy nano-multipods adopted in hcp phase were successfully synthesized.The building blocks of the multipods are highly concaved hexagonal prisms assembled by six nanosheets of 2.5 nm in thickness and with exposed high energy {1120} facets.Control studies showed that formaldehyde in the growth solution plays the pivotal role in formation of the unique hcp structure.In addition,the formation of excavated polyhedral morphology can be attributed to the preferential deposition of Pt atom on the edge of the solid branch of hcp Pt-Ni alloy,followed by the diffusion of face-sited Ni atoms to the edges.Benefitted from the unique crystal structure and excavated polyhedral morphology,the Pt-Ni multipods exhibited a superior catalytic activity compared to their fcc counterpart and commercial Pt/C.2)Based on the understanding that the alloying process between Pt and Ni is more preferential than the solely reduction of Pt in the presence of oleylamine,we successfully synthesize the well-epitaxial Pt-Ni alloy core-shell structure with Ni-rich alloy as core and Pt-rich alloy as shell.The monocrystalline Pt-Ni branched hexagonal nanocages was also acquired through etching the core.The wall thickness of the nanocages is only 2.8 nm,and the active surface area reaches 32.5 m2·gPt-1.Benefiting from unique monocrystalline nature and large surface area,the unique Pt-Ni branched nanocages exhibit great catalytic performance for hydrogen evolution reaction in alkaline solution,whose mass current density is 2.5 times as large as the commercial Pt/C.3)The unprecedented orthorhombic Rh nanocrystals were successfully synthesized through a mild and facile solvothermal method.The new crystal structure was confirmed by both X-ray diffraction and aberration corrected TEM characterizations.The imine formed from formaldehyde and amine seemed to play a vital role in the formation of the orthorhombic phase.The unique crystal phase and well-defined surface structure endow the orthorhombic Rh nanocrystals superior catalytic activity and selectivity towards ethanol oxidation reaction.The discovery of new allotropies of noble metals will not only enrich the chemistry and physics but also shed light on the design and synthesis of new functional materials.4)A new catalytic system with balanced activity and durability toward oxygen reduction reaction was developed by partially covering uniform,carbon-supported Pt nanocrystals with a polyacrylonitrile derived film.The uniformity of the Pt nanocrystals and the partial coverage by polyacrylonitrile both helped mitigate the adverse impacts caused by Ostwald ripening,migration,and detachment of Pt nanocrystals from the support.Moreover,the pyridinic N atoms on the thermally treated polyacrylonitrile could facilitate the reduction of oxygen,enhancing the catalytic activity.