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

Abstract 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 CuBi 2 O 4 @carbon quantum dots (CQDs)/CuO core‐shell nanosheet arrays. The vertically aligned CuBi 2 O 4 /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 CuBi 2 O 4 arrays and disordered CuBi 2 O 4 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.