Design of a simple solution-processed universal shell for synthesizing reverse type-I core–shell structures toward high-efficiency water-splitting photocathodes

Core-shell colloidal nanocrystals (CNCs) are promising candidates for photoelectrochemical (PEC) photocathodes due to their strong light absorption, tunable bandgaps, and efficient charge separation. In this study, we developed a simple and versatile strategy for fabricating narrow-bandgap shells compatible with various core materials. Among the configurations tested, the matrix-type MoS _<i>x</i> shell demonstrated the most effective performance, significantly enhancing photocurrent generation and operational stability through improved surface defect passivation and charge carrier separation. Band-level engineering further enabled the formation of reverse type-I heterojunctions in both CdSe and CIS₂ CNCs. Although type-II systems are traditionally favored for charge separation, our results show that the reverse type-I architecture not only enhances photocarrier separation under standard illumination but also effectively suppresses dark current. This is attributed to the dual physical and electronic passivation provided by the reverse type-I structure, which stabilizes the core-shell interface and reduces nonradiative recombination. Notably, the Cu₂O/CuO/red CIS₂ CNCs with a high indium ratio achieved the highest photocurrent density and retained over 86% of their initial performance after 24 hours of continuous operation at -0.1 V <i>vs.</i> RHE, demonstrating excellent long-term stability. These results highlight the strong potential of matrix-type reverse type-I core-shell CNCs as efficient and durable photocathode materials for PEC applications.