The core–shell microstructures are attracting much interest, most notably for their superior performance compared with their pure counterparts because of the interfacial effect. Comprehensively understanding the mechanism of the interfacial effect is critical but still elusive. Here, we report real-time dark-field optical microscopy (DFM) imaging of the selective etching in the core region of single cuprous oxide–bismoclite (Cu2O@BiOCl) core–shell microcrystals by I–. In situ DFM observations reveal that the reaction activity of Cu2O is significantly improved after coating the BiOCl shell layer, and the I– diffuses through the BiOCl shell and approaches the interface region, followed by etching the Cu2O core. During the etching process, two distinct reaction pathways, such as interfacial Cu2+-driven redox etching and confinement-governed dissolution, are identified. The interfacial Cu2+ is generated due to the coordination number difference at the core–shell interface. Moreover, according to the in situ DFM single-crystal imaging results, the ensemble adsorption capacity improvement for I– is also demonstrated in Cu2O@BiOCl core–shell microcrystals. These findings provide deep insights into the interfacial effect of core–shell microcrystals and establish a bridge between microscopic imaging and macroscopic practical application.

Real-Time Imaging of Interfacial Copper(II) Ion-Initiated Selective Etching in the Core Region of Single Cuprous Oxide–Bismoclite Core–Shell Microcrystals