Well-faceted Concave Noble Metal Nanomaterials:Controllable Synthesis And Properties Study

Noble-metal nanomaterials have been shown to play an irreplaceable role in an extensive range of applications,especially in heterogeneous catalysis.Considering their low abundance and high cost,how to minimize the usage and increase utilization efficiency of precious metals have long been important goals pursued by both practical and fundamental research.To fulfill these objects,there are two main approaches that have received substantial research interest over the past decades from the view of nanocrystals(NCs)growth.An impressive route is regulating the structures of noble-metal NCs with small sizes to obtain large specific areas.The other is controlling the structures to expose high-energy facets to realize superior properties.However,it seems that these two methods are incompatible with each other as it is difficult to synthesize small NCs with well-defined specific facets while well faceted NCs always possess low surface area.From the structural point of view,NCs with concave polyhedral structures could combine features of large surface areas and specific exposed crystal facets.Furthermore,their highly concave three-dimensional morphology could prevent agglomeration.Therefore,NCs with concave polyhedral structures have an ideal morphology for increasing utilization efficiency in various applications.However,the formation of concave polyhedral NCs,especially the nanosheets assembly excavated structures,are thermodynamically not favored during crystal growth owing to the highly concave features and large surface area/volume ratios.Thus far,only very few examples of NCs with excavated polyhedral structures have been reported.The exploration of such fascinating concave polyhedral nanostructures and the corresponding growth mechanisms should be a fundamental research project of great interest.In this dissertation,noble metal nanomaterials,ranging from single Au NCs to Au-Pd alloy and Pt-based alloy NCs,were chosen as the objects to explore the mechanism and method of concave polyhedral nanostructures.The dissertation is arranged by focusing on the above questions as follows:(I)Excavated cubic Pt-Sn alloy nanocrystals(NCs)constructed from twelve nano-sheets with {110} facets were synthesized by a simple co-reduction method with the assistance of the surface regulator polyvinylpyrrolidone.The specific surface area of the excavated cubic Pt-Sn NCs is comparable to that of commercial Pt black despite their larger particle size.By regulating the amount of PVP,Pt-Sn NCs could achieve shape evolution from {110} exposed concave cubes to {100} exposed cubes.PVP acts as a surface aborbents and controls the reaction kinetic as well.Because of the structure characteristics and alloy nature,the excavated cubic Pt-Sn NCs exhibited superior electro-catalytic activity in terms of both the specific area current density and the mass current density towards methanol oxidation.(II)A facile synthetic strategy for preparing unique excavated octahedral PtCo alloy nanocrystals constructed of 12 ultrathin nanosheets with {100} facets by co-reduction of the metal precursors Pt(acac)2 and Co(Ac)2 · 2H2O with hexadecyltrimethyl ammonium bromide(CTAB)as a surface regulator was introduced.The selective capping of Br-ions and the manipulating of reduction kinetics caused by competive coordination of Br-ions with Co2+ were considered indispensable for constructing this special morphology.With the unique morphology and alloying feature,the as-prepared excavated octahedral PtCo NCs displayed extraordinary catalytic activity towards HER in alkaline environment,MOR and ORR.Besides,this PtCo catalyst exhibited superior stability in structure and performance towards HER.(III)The overgrowth of Pd atoms onto the surface of trioctahedral(TOH)Au NCs as a model system was taken to understand the growth mechanism of excavated structures via tracing the anisotropic growth sites resulting from the protection of a surfactant.To obtain excavated structures,atoms should deposit on the edges of seeds.It is found a moderate amount of surfactant at relatively low temperature facilitates the selective growth of Pd atoms on edges of TOH Au NCs.Then,based on the deep understanding of the growth mechanism for an excavated structure,we successfully design a simple and effective one-step strategy for the successful synthesis of Au NCs and Au-Pd alloy NCs with an excavated TOH structure.Due to the novel feature of the excavated structure and exposed high energy {110} facets,excavated TOH Au NCs exhibited optical extinction at the near-infrared region and showed high catalytic activity towards the reduction of p-nitrophenol.(IV)The unique shape evolution from trisoctahedron to excavated rhombic dodecahedron and multipod structure for Au-Pd alloy nanocrystals by simply adjusting the concentration of hexadecyltrimethyl ammonium chloride(CTAC)was reported.In wet chemical synthesis of noble NCs,surfactants play key roles in shape control.People pay more attention to the chemical adsorption of surfactants on specific surface,but neglect its physical adsorption effect on the growth kinetics.It is found different concentration of surfactant(CTAC)may cause the different packing density of CTA+ bilayers on different sites(face,edge or vertex)of crystallite surface,which change the crystal growth kinetics and result in preferential crystal growth along the edge or vertex of crystallites.Besides,it is also found alloying process changes the growth rate of NCs,which leads that the Au-Pd alloy NCs could only keep the excavated morphology with the compositon of Pd in the range of 0-25%.(V)A new synthesis strategy for concave cubic intermetallic Pt3Zn nanocrystals with {hk0} facets by reduction of noble metal precursors via a slow reduction process and reduction of transition metal ions via an underpotential deposition(UPD)process in wet chemical synthesis was proposed.The UPD of Zn on Pt decreasing the reduction potencial difference between Zn(II)and Pt(II)facilitates the formation of alloy.Besides,the slow reduction and growth rate are favorable for the formation of intermetallic compounds.The as-prepared intermetallic Pt3Zn nanocrystals exhibited superior CO poisoning tolerance and high electro-catalytic activity in both methanol and formic acid oxidation reactions in comparison with solid solution Pt3Zn nanocrystals and Pt/C.(VI)The relationship between the size of Pt tips and length of CdS nanorods with charge separation rate in CdS-Pt nanorods was studied by transient absorption spectra.It has been found charge separation rate increases with the Pt domain increases in a size range,after which the charge separation rate shows no relationship with Pt sizes.As for the length of CdS nanorods,charge separation rate shows a linear relationship.