Hybrid Overlayer\nof Defective Ni-MOF and NiO Nanoparticles\ntoward Efficient TiO<sub>2</sub>/CdS Type-II Heterojunction

The\nsevere photocorrosion of cadmium sulfide (CdS) restricts its\npractical application for solar hydrogen production. Although remarkable\nprogress has been achieved with an overlayer strategy for isolating\nthe CdS surface, the lifetime of CdS-based photoanodes is still far\nfrom the actual requirements. Herein, a hybrid overlayer of defective\nNi-MOF and NiO nanoparticles has been developed through the chemical\nbath deposition method with postannealing. This hybrid overlayer of\nNi-MOF-d is coated on the surface of the TiO<sub>2</sub>/CdS type-II\nheterojunction. The composite photoanode exhibits a photocurrent density\nof 4.41 mA cm<sup>–2</sup> at 1.23 V<sub>RHE</sub>, which is\n3.47- and 1.32-fold that of CdS and TiO<sub>2</sub>/CdS, respectively.\nThe Ni-MOF-d overlayer gives rise to a negative shift of the onset\npotential by 59.51 mV. After a long-term stability test of 11 h, a\nphotocurrent retention of 70% is observed, which is among the most\nrobust CdS-based photoanodes. The kinetics studies reveal that the\nperformance improvements can be attributed to the multiple functions\nof the Ni-MOF-d hybrid overlayer, including isolating the CdS surface\nfrom the electrolyte, cocatalyzing the electrode oxidation processes,\npassivating the surface defect states of CdS, and facilitating the\ncharge injection from the photoanode to the electrolyte.