Controlled Synthesis Of Cubic Palladium Nanocrystals With Mechanism Study

The nanoscale world is a field concerning about Chemistry,Physics,Material Science,and so on.The approaches to prepare palladium nanoparticles have been greatly developed,and large quantities of functional nanomaterials with controlled morphologies,structures,and compositions have been synthesized.However,theoretical descriptions have always been demanded,especially for those nanoparticles with well-defined shapes and manipulated sizes.In this work,combining chemical synthetic methods with reaction kinetics,experiments on well-defined cubic Pd nanocrystals have been designed and taken to supplement the current theoretical system,and the reduction kinetics of Pd precursors has been determined with relative theories discussed in detail.Pd nanocubes with uniform morphologies and controlled sizes have been prepared with K2PdCl4 as precursor,PVP as stabilizer,KBr as capping agent,and AA as reductant,where a typical synthesis procedure has been defined.Both KBr and AA can significantly influence the reduction of Pd precursors,and thus impact the growth path of Pd nanocubes.Besides,over dosed KBr can lead to the formation of twinned crystals,and excess AA causes concave nanocubes.Through adjusting the dosages of KBr and AA,the generation pathway of Pd nanocrystals can be maneuvered,leading to the selective product of Pd nanoubes.On the basis of typical synthesis procedure,by adding acetate species(KOAc,NH4OAc,and HOAc)into the reaction system,the reduction rate of Pd precursors can be changeable with the change in concentrations of acetate species,and thus Pd nanocubes with comparatively better morphology and smaller size distribution have been obtained.Owing to the unfitness of Finke and Watzky two-step nucleation theory in desicribing the current system,probable generation mechanism of Pd nanocubes has been proposed based on the change in concentration of Pd precursors against reaction time.Interestingly,the reduction of Pd precursors has been determined as a first-order reaction,where the apparent reduction rate constant can be utilized as an indicator to reveal the inside relationship for macroscale phenomena(changes in precursor concentration)and nanoscale transformations(evolvement of nanostructures).Some important equations have been built for the process of crystal growth.With the help of classical nucleation theory,information of the nucleation process for Pd nanocubes has been deduced from the growth period.To this point,theoretical description for the complete generation process of Pd nanocubes has been achieved.With seed-mediated growth method,concave Pd nanocubes have been prepared from Pd nanocubes in different sizes.By regulating the concentration of KOAc in reaction system,the reduction kinetics of Pd precursors has been controlled,and the growth behavior of Pd atoms over cubic seeds can be manipulated.The concavity of concave nanocubes is related with the reduction rate of Pd precursors on seed surfaces,and surface catalytic reduction model has been put forward to describe the reduction process.Furthermore,the growth pathway of concave nanocubes has been investigated.It has been found that the reduction activation energy for precursors during seed-mediated growth is lower than that in the procedure of preparation nanocubes.The smaller the sizes of Pd seeds,the lower the concavities of the products,and the lower the growth priority of Pd atoms at the corners of the seeds.Based on 18-nm cubic seed,5 kinds of concave nanocubes with different concavities have been prepared and applied to catalyze the electro-oxidation of formic acid.The performance difference for the nanoparticles can be tuned to at most 2.17 times.