Passivation of Surface States in CdIn₂S₄ via Type II Heterostructure for Boosting Photoelectrochemical Water Splitting Reaction

The visible light active semiconductors are considered as promising materials for achieving high efficiency in producing green hydrogen (H2) via the photoelectrochemical (PEC) water splitting reaction. Here CdIn2S4 (CIS) is developed as a highly visible-light-absorbing semiconductor for PEC water splitting reactions. However, CIS suffers from severe recombination of charge carriers and the photocurrent density is found to be 0.35 mA/cm2 at 1.0 V vs RHE in 0.5 M Na2SO4, despite having high visible light absorbance. The presence of surface trap states causes the Fermi level pinning effect of CIS, resulting in low surface photovoltage and PEC activity. To remove the surface trap states present in CIS, the in situ heterostructures of CdS/CIS nanosheets are developed, which induces the formation of both bulk and surface sulfur vacancies in the heterostructures. As a result, the photocurrent density is enhanced to 1.0 mA/cm2 at 1.0 V vs RHE. Further, the photocurrent density and photostability of the heterostructure are enhanced by developing the CdS/CIS/In2S3 (n-n-n) heterojunction which passivates the surface sulfur vacancies and creates the type II heterojunction. The photocurrent density is increased to 1.69 mA/cm2 at 1.0 V vs RHE. The carrier density and charge carrier conductivity are enhanced as observed from the Mott–Schottky (MS) analysis and the photoelectrochemical impedance spectroscopy (PEIS), respectively. The charge carrier density in the CdS/CIS/In2S3 heterostructure is almost 9.3 times enhanced over that of the CdS/CIS nanosheets. The charge injection and charge transportation efficiency of the heterojunction is also increased. The incident photon to current conversion efficiency (IPCE) of the CdS/CIS/In2S3 heterostructure is increased 2.21 times compared to CdS/CIS. A type II staggered heterojunction is developed between semiconductors, which enhances the overall PEC performance of the CdS/CIS/In2S3 heterostructure.