Direct in Situ Observation and Analysis of the Formation of Palladium Nanocrystals with High-Index Facets

Wenpei Gao, Yusheng Hou, Zachary D. Hood, Xue Wang, Karren More, Ruqian Wu, Younan Xia, Xiaoging Pan, and Miaofang Chi, 2018

Image courtesy of Nano Letters

Abstract

Synthesizing concave-structured nanoparticles (NP) with high-index surfaces offers a viable method to significantly enhance the catalytic activity of NPs. Current approaches for fabricating concave NPs, however, are limited. Exploring novel synthesis methods requires a thorough understanding of the competing mechanisms that contribute to the evolution of surface structures during NP growth. Here, by tracking the evolution of Pd nanocubes into concave NPs at atomic scale using in situ liquid cell transmission electron microscopy, our study reveals that concave-structured Pd NPs can be formed by the cointroduction of surface capping agents and halogen ions. These two chemicals jointly create a new surface energy landscape of Pd NPs, leading to the morphological transformation. In particular, Pd atoms dissociate from the {100} surfaces with the aid of Cl ions and preferentially redeposit to the corners and edges of the nanocubes when the capping agent polyvinylpyrrolidone is introduced, resulting in the formation of concave Pd nanocubes with distinctive high-index facets. Our work not only demonstrates a potential route for synthesizing NPs with well-defined high-index facets but also reveals the detailed atomic-scale kinetics during their formation, providing insight for future predictive synthesis.

Impact Statement

Morphological changes in the structure of palladium nanocubes were simultaneously induced (by the electron beam) and imaged using liquid cell transmission electron microscopy. Electron beam interactions induced the dissolution of pallladium ions from the sides of the nanocubes and resulted in the reduction and re-deposition of the palladium ions on the corners leading to a concave structure with high-index surfaces.