Gradient Built‐In Electric Field Engineering in Hierarchical Co 0.85 Se Quantum Dots‐Modified Cadmium Sulfide Nanorods for Boosted Photocatalytic H 2 Evolution

Efficient charge separation and durable cocatalyst interfaces are critical for advancing visible‐light photocatalytic hydrogen production. Here, we develop an in situ selenization strategy that anchors defect‐rich Co 0.85 Se quantum dots onto CdS nanorods while simultaneously forming a Se‐doped CdS surface layer, yielding a tightly coupled QDs–shell–core heterostructure. This construction establishes a spatial dual built‐in electric field arising from Se‐doping‐induced band modulation and the Co 0.85 Se/CdS interfacial junction, which strongly promotes directional electron transfer. Experimental characterizations and DFT calculations jointly reveal that the defect‐rich Co 0.85 Se QDs act as efficient electron sinks, accelerating H 2 evolution. Consequently, the optimized photocatalyst delivers a hydrogen evolution rate of 61.66 mmol g −1 h −1 with only 0.7 wt% Co 0.85 Se loading, which is far exceeding pristine CdS and outperforming Pt/CdS and presents markedly enhanced photostability. This study offers a practical pathway for designing low‐loading, non‐noble‐metal cocatalyst systems for high‐performance solar hydrogen production.