In Situ Observations of Shell Growth and Oxidative Etching Behaviors of Pd Nanoparticles in Solutions by Liquid Cell Transmission Electron Microscopy.

It is demonstrated that the redox reaction behaviors of Pd nanoparticles in HAuCl4 solutions can be substantially modified by the introduction of hexadecyl trimethyl ammonium bromide (CTAB) agents through systematic liquid cell transmission electron microscopy (LCTEM) investigations. The gradual dissolution of Pd nanoparticles is observed when HAuCl4 solution is pumped into liquid flow cells, the etching characteristics of which are depended on both HAuCl4 concentrations and incident electron doses. In comparison, with the presence of CTAB agents, the dominated phenomenon appears to be the precipitation of Au species and incorporation onto the surface of Pd seeds. It is also observed that the rapid growth of Au on Pd seeds occurs by loading Pd and HAuCl4 solutions into static liquid cells. The resultant Au shells exhibit rather sparse structural configurations and are formed possibly by homogeneous nucleation/coalescence of Au species as well as monomer attachments. The observed Au-shell growth instead of Pd dissolution is attributed to the presence of the residual regents, which may be also responsible for the initially already existing small Au adsorptions at the corner/edge sites of Pd seeds. The study provides a useful reference for the convenient fabrication of complex nanostructures and functional nanomaterials in a controllable way.