Engineering Charge Separation in the S-TiO ₂ /BiSbS ₃ /ZnS Heterostructure for Efficient Photoelectrochemical Water Splitting with Visible Light

Photoelectrochemical (PEC) water splitting has attracted significant interest owing to its ability to produce hydrogen (H2) as a clean fuel using semiconductors, thereby reducing CO2 emissions, climate change, and fossil fuel consumption. In this work, we fabricated an efficient heterostructure S-TiO2/BiSbS3/ZnS photoanode using sol–gel, chemical bath deposition (CBD), and successive ionic layer adsorption and reaction (SILAR) methods. In a neutral aqueous electrolyte, the photoanode exhibited a photocurrent density of 3.5 mA.cm−2 at 1.23 V vs RHE and a 1.01% solar to hydrogen (STH) efficiency at 0.15 V vs RHE. Under alkaline conditions, an increased photocurrent density of 6.4 mA.cm−2 at 1.23 V vs RHE and an STH of 2.8% at 0.28 V vs RHE was observed, significantly outperforming the corresponding binary and single-component systems, and this was attributed to hole scavenging from sacrificial (S22− and SO32– ions) reagents in alkaline media. The ternary photoanode also demonstrated the best current density and photostability for over 3600 s compared to the binary photoelectrodes. Density functional theory (DFT) was employed to investigate the charge-transfer behavior in the heterojunctions of n–n-type S-TiO2/ZnS and n–p-type S-TiO2/BiSbS3 and their suitability as photoanodes for the oxygen evolution reaction (OER). Work function calculations and charge density difference investigations showed a positive vacuum-level shift of 5.57 eV in S-TiO2/BiSbS3/ZnS, an n–p–n-type Z‑scheme heterojunction developed by introducing ZnS into S-TiO2/BiSbS3. The ternary photocatalyst was found to show Z-scheme electron transfer, wherein photogenerated electrons from ZnS recombine with photogenerated holes of S-TiO2/BiSbS3, and the oxidation potential of ZnS is preserved as the photogenerated holes stay in the valence band of ZnS, leading to higher photoelectrochemical activity. Mechanistic investigation on the OER coordinates validated that the higher photoelectrochemical activity of the S-TiO2/BiSbS3/ZnS heterostructure is due to the stabilization of OER intermediates and a thermodynamic OER overpotential of 1.06 V.