Studies Of Structure Evolution Of Palladium Nanocrystals By Liquid Phase In-situ Transmission Electron Microscopy

Nanocatalyst is a key component of catalysis,which is one of the foundation of modern industry.The structure of nanocatalyst matters for its catalytic perfoemance.Meanwhile,catalytical reactions would make an impact on the sctructure at the same time.Therefore,exploration of the structure of catalyst requires not only the basic understanding of its intrinsic structure,but more attentions on the in-situ behaviors during the reaction.Transmission electron microscopy(TEM)proves to be one of the most powerful tool to understand the microstructure of nanocrystals.With the development of in-situ TEM,gas and liquid phase,electronic,thermal and flow flieds are subsequently coupled within the microscope.And in-situ TEM,especially liquid phase,functions more and more importantly in the field of crystallization,crystal growth and dynamic behaviors of catalysts under reaction condition.Palladium(Pd),as a member of noble metals,has a wide range of applications,such as,industrial catalysis and electrocatalysis.Besides,fundamental research about its synthetic methodology has always been a hotspot in inorganic chemistry.However,understanding about formation mechanism and structure transformation of Pd nanocrystals in liquid phase remains insufficient and requires to be further explored.Hence,liquid phase in-situ TEM experiemtns were carried out in this thesis to understand structure of Pd nanocrystals during crystallization and phase transformation.The key points are as follows:1.Liquid phase in-situ TEM study of dense liquid phase(DLP)and its performance during crystallization of Pd nanocrystals.In this study,the role of the DLP during the crystallization of Pd nanocrystals was examined.The newly-formed nanocrystals with higher energy and metastable structure,could relax into their stable structures inside DLP.The DLP mediates mass transportation between particles and induces the orientation attachment and coalescence process.2.Explorations about the structure and formation mechanism of Pd nanosheets.HRTEM simulations were performed to identify the relationship between stacking faults and the appearance of forbidden reflection 1/3 {422}.It turns out that stacking faults are indeed responsible for 1/3 {422},but the type of stacking faults has nothing to do with the intensity of 1/3 {422}.In-situ TEM study of the growth of Pd nanosheets exhibits that its formation goes through a hcp intermediate and Pd nanosheets nucleate based on the intermediate through heterogeneous nucleation mechanism.The intermediate nanoparticle would gradually get consumed by the nanosheets through Ostwald ripening.3.Structural study of phase transformation of PdHx nanocrystals produced under electron beam.PdHx nanocrystals with high H/Pd ratio were produced under electron beam.Atomic resolution HAADF-STEM studies show that the hydrogen distribution in the PdHx nanocubes are unevenly distribued and present a core-shell structure.HRTEM of the transformation intermediate along[110]suggests that uneven hydrogen distribution could cause lattice distortion in the PdHx nanocubes.4.Phase transformation of Pd nanocubes during electrochemical hydrogen evolution reaction(HER).At last,study of structure evolution of Pd nanocubes as HER electrocatalysts was conducted using liquid phase in-situ electrochemical TEM.The phase transformation of Pd nanocubes from α-PdHx to β-PdHx was observed.Volume expansion of Pd nanoparticles happened at the same time.Detail studies and DFT calculations prove that phase transformation from α-PdHx to β-PdHx commenced from the surface of cube and continued along<100>direction.To sum up,researches about the structure of Pd nanocrystals were carried out usingliquid phase in-situ TEM,including the change of structure of Pd nanocrystals in DLP,,structure evolution of Pd nanosheets,phase transformation of PdHx nanocrystals under electron beam flux and under HER condition。This thesis fulfills the research of structure and formation mechanism of Pd nanocrystals and could procide guidance for synthesis of nanocrystals and Pd electrocatalysis.