Self‐Hollowing Synthesis of Hollow Nanosphere‐Stacked CuO Electrodes for Enhanced Photoelectrochemical and Electrochemical Applications

Hollow nanoarchitectures offer unique optical, electrical, and catalytic advantages for energy conversion and environmental applications; however, their scalable fabrication as uniform thin‐film electrodes remains challenging. Herein, we report a template‐free, sol–gel self‐hollowing method for the one‐step deposition of hollow nanosphere‐stacked CuO (HNS‐CuO) films. In this process, soft templates are spontaneously generated through polymer–Cu 2+ complexation, followed by Cu–O nucleation at the template surface, resulting in crystalline hollow CuO nanospheres without sacrificial templates or multistep procedures. The method enables direct film deposition on various substrates via simple precursor solution coating (spin‐, dip‐, or brush‐coating). Structural and mechanistic analyses using SEM, TG/DTA, and FTIR reveal the formation of well‐defined hollow nanospheres with uniform shell thickness and robust crystallinity. Owing to enhanced internal light scattering, reduced charge transport/transfer resistance, and increased acceptor density, HNS‐CuO exhibits superior photoelectrochemical water reduction performance compared to nanoparticle CuO, achieving a high photocurrent density of 2.6 mA cm −2 . In addition, HNS‐CuO enables efficient electrochemical nitrate reduction to ammonia with a Faradaic efficiency of 81.7% and stable operation. Notably, this self‐hollowing strategy demonstrates strong compositional versatility, enabling the synthesis of hollow metal oxide nanospheres, including NiO, ZnO, MgO, and CoO. This work provides fundamental insight into gel‐network‐guided self‐hollowing mechanisms and establishes a scalable platform for hollow nanoarchitectured electrodes.