Synergy between Structure Characteristics and the Solution Chemistry in a Near/Non-Equilibrium Oxidative Etching of Penta-Twinned Palladium Nanorods

Oxidative etching possesses an enriched flexibility and atomic-level accuracy in tailoring the size, morphology, surface structure, and composition of colloidal nanocrystals toward their catalytic and sensing applications. In this work, we choose one-dimensional penta-twinned Pd nanorods as a model system and apply liquid cell transmission electron microscopy (LCTEM) to investigate the detailed nanoscale etching process under different solution environments, which are DI water/e-beam (weakly oxidative), sodium chloride (NaCl) solution/e-beam (moderately oxidative), and ferric chloride (FeCl3) solution/e-beam (strongly oxidative), respectively. Increasing the oxidative strength of solution alone accelerates the dissolution kinetics and drives the system from the (near)-equilibrium etching, where nanorods appear in flat and low-indexed ends (for DI water/e-beam) to nonequilibrium etching with slender tips formed as intermediate products (for FeCl3/e-beam). Under a nonequilibrium etching condition, both symmetric and asymmetric etching occurred, that is, the penta-twinned structure either retained (for symmetric etching) or dissolved to a fourfold-, threefold-, and twofold-twinned and single-crystalline structure (for asymmetric etching), depending on the structural defects-induced strain distribution. The formation of slender tips of different structures can be further understood via energetic analysis and strain relaxation. In particular, in situ LCTEM results were successfully reproduced by flask system-based etching experiments; hence, direct correlations were built between LCTEM and conventional colloidal etching reactions. This work comprehends the present understanding on the synergetic roles of structure characteristics and solution chemistry for the etching of multiply twinned nanoparticles toward their precise controlled synthesis and practical applications.