Growth Mechanism Of Pd Nanocube And Its Transformation Into PtPd Bimetallic Nanostructure

Pd nanomaterials have wide applications in organic synthesis,catalysis,hydrogen production,fuel cells and so on.Pd nanocubes have six {100} crystal faces,which is one of the typical model particles for understanding the relationship between their properties and crystal faces.Moreover,based on Pd nanocubes,a variety of polymetallic nanoparticles can be obtained through galvanic displacement reaction including PtPd,AuPd,PtAuPd,etc..However,the formation mechanism of Pd nanocubes has not been clarified yet,especially the functionality of halide ions(Br-and I-).Therefore,this thesis systematically studied the formation mechanism of Pd nanocubes experimentally and theoretically,and their transformation into PtPd bimetals.The main content is as follows:(1)Using PdCl42-as precursor,polyvinylpyrrolidone(PVP)as protective and ethylene glycol(EG)as reducing agent,Pd nanocubes were obtained by adjusting the pH value regardless of whether Br-or I-was used.The results show that Pd nanocubes were formed in the pH range of 2.37 to 11.45 when I-was used whereas,when Br-was used,Pd nanocubes are formed in the pH range of 1.01 to 1.94.Moreover,with I-as the additional ions,Pd nanocube particles were formed over a wide range of pH values and their sizes can be controlled by adjusting the pH value.(2)Finke-Watzky kinetic model fitting study indicates that the reduction rate of Pd2+ is closely linked with the pH value of the solution,and increased with the increase of pH value,independent of I-and Br-.This suggests that the reduction rate of Pd2+ is the decisive factor dictating the growth of Pd nanocubes.The effect of pH on the reduction rate of Pd2+ was further confirmed by density functional theory(DFT)calculations.(3)Pd nanocubes were used as sacrificial templates,I-as the additional ions,ethylene glycol(EG)as the solvent,and K2PtCl6 as etching agent,a series of PtPd bimetallic nanoparticles with different morphologies and compositions were obtained through galvanic displacement reaction.Transmission electron microscopy(TEM),powder X-ray diffraction(XRD)and elemental distribution imaging were employed to characterize as-prepared samples.The results shows that co-existance of galvanic displacement and co-reduction(Pd2+ and Pt4+)greatly supports the formation of PtPd bimetallic hollow-structures.