Carbon-Quantum-Dot-Mediated Z-Scheme Charge Transfer in Vertically Aligned CuBi2O4/CuO Photocathodes for Enhanced Hydrogen Evolution

Photoelectrochemical water splitting represents a pivotal technology for sustainable hydrogen generation; however, its practical application remains constrained by limitations in visible-light utilization efficiency and suboptimal charge carrier management. Here, we rationally designed a Z-scheme heterostructure photocathode featuring hierarchical CuBi₂O₄@carbon quantum dots (CQDs)/CuO core-shell nanosheet arrays. The vertically aligned CuBi₂O₄/CuO nanoarchitecture provides a high surface area and directional charge transfer. Strategic integration of CQDs as electron mediators shifts charge transfer from type-II to Z-scheme mechanisms, preserving strong reduction potential for hydrogen evolution while enhancing spatial charge separation. This configuration achieves broad-spectrum light-harvesting capability through complementary bandgap alignment and dramatically enhanced interfacial charge transfer kinetics. The optimized photocathode demonstrates a photocurrent density of −0.95 mA cm⁻² at 0.4 V versus RHE under AM 1.5 G irradiation in neutral electrolyte, representing a 6.3-fold and 38-fold increase over pristine CuBi₂O₄ arrays and disordered CuBi₂O₄ thin-film counterparts. Notably, the heterojunction system exhibits remarkable charge transfer kinetics due to the CQDs-mediated charge transport pathways. This work establishes an interfacial engineering strategy for developing effective Z-scheme photocathodes, offering critical insights into the rational design of solar-driven hydrogen evolution from water splitting architectures.