Passivation\nof Surface\nStates in CdIn<sub>2</sub>S<sub>4</sub> via Type II Heterostructure\nfor Boosting Photoelectrochemical\nWater Splitting Reaction

The visible light active semiconductors\nare considered\nas promising\nmaterials for achieving high efficiency in producing green hydrogen\n(H<sub>2</sub>) via the photoelectrochemical (PEC) water splitting\nreaction. Here CdIn<sub>2</sub>S<sub>4</sub> (CIS) is developed as\na highly visible-light-absorbing semiconductor for PEC water splitting\nreactions. However, CIS suffers from severe recombination of charge\ncarriers and the photocurrent density is found to be 0.35 mA/cm<sup>2</sup> at 1.0 V vs RHE in 0.5 M Na<sub>2</sub>SO<sub>4</sub>, despite\nhaving high visible light absorbance. The presence of surface trap\nstates causes the Fermi level pinning effect of CIS, resulting in\nlow surface photovoltage and PEC activity. To remove the surface trap\nstates present in CIS, the in situ heterostructures of CdS/CIS nanosheets\nare developed, which induces the formation of both bulk and surface\nsulfur vacancies in the heterostructures. As a result, the photocurrent\ndensity is enhanced to 1.0 mA/cm<sup>2</sup> at 1.0 V vs RHE. Further,\nthe photocurrent density and photostability of the heterostructure\nare enhanced by developing the CdS/CIS/In<sub>2</sub>S<sub>3</sub> (n-n-n) heterojunction which passivates the surface sulfur vacancies\nand creates the type II heterojunction. The photocurrent density is\nincreased to 1.69 mA/cm<sup>2</sup> at 1.0 V vs RHE. The carrier density\nand charge carrier conductivity are enhanced as observed from the\nMott–Schottky (MS) analysis and the photoelectrochemical impedance\nspectroscopy (PEIS), respectively. The charge carrier density in the\nCdS/CIS/In<sub>2</sub>S<sub>3</sub> heterostructure is almost 9.3\ntimes enhanced over that of the CdS/CIS nanosheets. The charge injection\nand charge transportation efficiency of the heterojunction is also\nincreased. The incident photon to current conversion efficiency (IPCE)\nof the CdS/CIS/In<sub>2</sub>S<sub>3</sub> heterostructure is increased\n2.21 times compared to CdS/CIS. A type II staggered heterojunction\nis developed between semiconductors, which enhances the overall PEC\nperformance of the CdS/CIS/In<sub>2</sub>S<sub>3</sub> heterostructure.