In-situ Study The Oxidative Etching Of Palladium Nanorods By Liquid Cell Transmission Electron Microscopy

The structure,size and shape of noble metal nanocrystals determining their physical and chemical properties.Oxidative etching has become a versatile tool to precisely tailor these parameters of noble metal nanocrystals,especially for anisotropic nanocrystals.Some progress has been made in tuning the anisotropy of nanocrystal,but many complicated behaviors and mechanisms involved have not been well explained.The main reason is that it's harder to directly observe the dynamic etching of nanocrystals in solution due to the size of noble metal nanocrystals is about nanometer.Liquid cell electron microscopy is a rapidly developing and powerful technique that is capable of real-time tracking the dynamics of the physical and chemical processes,and it provides sub-nanometer resolution of structure and components.Liquid cell electron microscopy can provide great help to the research on oxidative etching behavior of noble metal nanocrystals.We took an in situ study of a microscopic process of anisotropic oxidative etching of palladium nanorods in ferric chloride solution using liquid cell transmission electron microscopy.By recording the changes in morphology and quantitatively analyzing variations in dimensions,we found that shortening of the Pd nanorods and decreasing of the aspect ratio which results in etching preferentially on the tips.The five-fold twinned structure may cause the anisotropic oxidative etching behavior.By changing the solution properties,the oxidative etching behaviors of the palladium nanorods could be effectively controlled.Because of the higher chemical reactivity of the apex of the palladium nanorods,oxidative etching preferentially proceeded along the longitudinal direction,showing strong anisotropic behavior.An isotropic-like etching was observed when the reaction was performed in an ultrathin solution,where the diffusion of reactants and dissolved agents were inhibited due to the confinement effect.Furthermore,we demonstrate that the tip-deposition of gold can effectively protect the tips of the palladium nanorods from oxidative etching.And oxidative etching selectively took place on the lateral sides.The results presented here show successful in situ monitoring of the controlled oxidative etching process of anisotropic noble metal nanocrystals.The presented results should be important for the precise structural tuning of noble metal nanocrystals for various practical applications.