Heterojunction Formation of Cu 2− x S/CdS Nanoplates via Cation Exchange for Effective Photocatalytic Hydrogen Evolution

The development of high‐performance photocatalysts is essential for sustainable, carbon‐free hydrogen production. To address the limitations of rapid charge recombination in single component systems, heterojunction architectures combining distinct semiconductors have emerged as a promising strategy. In this study, 2D Cu 2− x S/CdS heterostructures are constructed by partial cation exchange of single‐crystalline Cu 2− x S nanoplates, yielding a series of heterojunctions with tunable Cu‐to‐Cd ratios and well‐aligned epitaxial interfaces with minimal lattice mismatch. Comprehensive structural and spectroscopic analyses confirm the formation of type‐II band alignment, in which Cu 2− x S serves as a hole acceptor and CdS acts as a primary light absorber, hence a site for hydrogen evolution reaction (HER). The optimized Cu 2− x S/CdS‐65 composition, with a 65% Cd molar fraction, achieves a hydrogen production rate of 5.75 mmol g −1 h −1 , 7.6 times higher than that of pure CdS and superior to Cu/Cd sulfide‐based photocatalysts. This enhanced performance is attributed to the synergistic combination of rapid interfacial charge transfer facilitated by epitaxial p–n junctions, as well as sufficient CdS domains ensuring efficient light absorption and surface reactivity for HER. These results highlight the crucial role of interfacial engineering and composition control in guiding the rational design of high‐performance 2D photocatalytic systems for solar‐to‐hydrogen energy conversion.