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