AbstractControlling the architectures and crystal phases of metal@semiconductor heterostructures is very important for modulating their physicochemical properties and enhancing their application performances. Here, a facile one‐pot wet‐chemical method to synthesize three types of amorphous SnO₂‐encapsulated crystalline Cu heterostructures, i.e., hemicapsule, yolk–shell, and core–shell nanostructures, in which unconventional crystal phases (e.g., 2H, 4H, and 6H) and defects (e.g., stacking faults and twin boundaries) are observed in the crystalline Cu cores, is reported. The hemicapsule Cu@SnO₂ heterostructures, with voids that not only expose the Cu core with unconventional phases but also retain the interface between Cu and SnO₂, show an excellent electrocatalytic CO₂ reduction reaction (CO₂RR) selectivity toward the production of CO and formate with high Faradaic efficiency (FE) above 90% in a wide potential window from −1.05 to −1.55 V (vs reversible hydrogen electrode (RHE)), and the highest FE of CO₂RR (95.3%) is obtained at −1.45 V (vs RHE). This work opens up a new way for the synthesis of new heterostructured nanomaterials with promising catalytic application.