Study On The Shape-controlled Synthesis Of Pd And Pd-based Alloy Nanoparticles

The properties of nanomaterials are strongly dependent upon their shapes,sizes and indices of lattice planes.Considerable attention has been paid to noble metal nanomaterials owing to their potential applications in many fields such as catalysis,optics,biological information storage.Pd and Pd-based alloy can be widely used as the primary catalysts.To improve their catalytical activities,a lot of efforts have been devoted into the shape-controlled syntheses of Pd nanostructures and Pd-based alloy nanoparticles.Up to now,Pd and Pd-based alloy nanoparticles with various morphologies have been obtained such as Pd hexagonal plates,Pd icosahedra,Pd tetrapods as well as Pd-Pt alloy hypercubes,Pd-Pt alloy tetrahexahedra.In this paper,the syntheses of Pd and Pd-based alloy nanocrystals with novel morphologies as well as their electrocatalytic activities were investigated.By oil-bath heating,the self-assembling ultrathin Pd nanosheets with an average length of 200 nm and an average cross diameter of 25 nm were successfully synthesized with Pd(acac)2 as a precursor,DMF and H2 O as a mixed solvent,PVP as a stabilizer,CO as a reducing agent in the presence of appropriate amount of KI and NaAc at 100 for 3h.The ?optimum molar ratio of Pd(acac)2?PVP?KI?NaAc was 1?9?4?4 and the optimum CO flow rate was 0.033mL·sec-1 in the reaction system.The products were characterized by transmission electron microscopy(TEM),X-ray diffraction(XRD),SEM and X-ray photoelectron spectrometry(XPS).The results demonstrated that the CO flow as well as the use of I-and Acwas crucial to the formation of self-assembling ultrathin Pd nanosheets.One hand,an appropriate flow of CO controlled the reducing rate of the precursor,on the other hand,the growth of Pd nanoparticles was confined due to the selective adsorption of CO on the {111} surfaces,resulting in the formation of ultra Pd sheets,while the selective adsorption of I-ions on the {100} surfaces promoted the growth of Pd sheets along the round edges.Simultaneously,the self-assembling of Pd nanosheets can be ascribed to the Van Der Waals forces between alky groups of Ac-ions adsorbed on the surfaces of Pd nanosheets which resulted in the formation of the self-assemblies of ultra round Pd nanosheets.The electrocatalytic activity of the as-prepared Pd nanosheets for electro-oxidation of formic acid was about 2.1 times greater than that of the commercial Pd black.By oil-bath heating,the hierarchical Pd nanodendrites were successfully synthesized with Pd(acac)2 as a precursor,DMF as a solvent,PVP as a stabilizer,CO as a reducing agent in the presence of appropriate amounts of NaAc and oleylamide(OAm)at 100°C for 3 h under atmospheric pressure.The as-prepared Pd nanocrystals were characterized by UV-Vis,TEM,SEM,XRD etc.The results demonstrated that hierarchical Pd nanodendrites were consisted of {111} planes.At the initial stage of the reaction,tetrapod Pd seeds were formed due to the co-adsorption of CO and acetate anions on Pd nucleus crystallite.With the reaction proceeding,preferential adsorption of OAm on the(111)surfaces of Pd seeds promoted outgrowth from the tips of tetrapod Pd particles,producing primary multipods followed by newly growing along <111> axis directions.As a result,hierarchical branches were generated.The electrocatalytic activity of the hierarchical Pd nanodendrites for electro-oxidation of formic acid was about 2.5 times greater than that of the commercial Pd black.Pd-Rh hollow nanocubes were prepared with a high yield in 100 s under microwave irradiation with Na3RhCl6 and H2PdCl4 as precursors,PVP as a stabilizer,tetraethylene glycol as both a reducing agent and a solvent,and KI as a additive.The resulting products were characterized by SEM,XRD,TEM,high-angle annular dark-field scanning TEM(HAADF-STEM),etc.The results show that I-ions was crucial to the formation of the Pd-Rh hollow nanocubes.The formation of Pd-Rh nanocubes was ascribed to the selective adsorption of I-ions on {100} facets.With the reaction proceeding,galvanic replacement reaction between Pd and Rh took place in the solution,as a result,Pd-Rh hollow nanocubes were formed eventually.In addition,the Pd-Rh hollow nanocubes exhibited enhanced electrocatalytic activity for the electro-oxidation of formic acid compared with Rh nanopraticles.