Photoelectrochemical Stability Enhancement of (311)-Oriented Indium Sulfide Thin Films via In-Cystine Complex...

Indium sulfide is a promising photoactive material for light-induced applications, particularly photoelectrochemical (PEC) water splitting. However, its practical application is limited by photocorrosion, which hinders its long-term efficiency. In this study, we report a hydrothermal synthesis of In-cystine bonded (311)-oriented indium sulfide thin films using a mixed sulfur source of l-cysteine hydrochloride and l-cystine, the latter generated in situ via Fe³⁺-induced oxidation of l-cysteine. Synthesis parameters such as temperature and ramp rate are found to affect the indium-organic complex's physical and chemical properties such as composition, morphology, thickness, crystal structure, and thereby the PEC performance of the resulting films. The results indicated that thin films synthesized under slow heating conditions (e.g., 160-3 at 160 °C with a ramp rate of 3 °C/min for 6 h; 180-3 at 180 °C with a ramp rate of 3 °C/min for 6 h) exhibited a dominant indium sulfide phase bonded with In-cystine and demonstrated high and stable photocurrent densities of 1.0 and 0.93 mA cm^-2 at -0.2 V vs Ag/AgCl, respectively. In contrast, the fast-heated thin film 160-10 (prepared at 160 °C with a ramp rate of 10 °C/min for 6 h) primarily contained indium-organic complexes with mixed In-cystine and In-cysteine bonding and exhibited a photocurrent density of 0.35 mA cm^-2 at -0.2 V vs Ag/AgCl. Stability testing further revealed that after 2 h of continuous illumination at -0.2 V vs Ag/AgCl, the thin film 160-3 retained 0.75 mA cm^-2, while 180-3 maintained 1.1 mA cm^-2, demonstrating improved resistance to photocorrosion. This work presents an effective strategy for improving the long-term PEC performance of metal sulfide photoelectrodes by introducing In-cystine bonding at their surface, offering a pathway toward more stable and efficient solar-driven water-splitting devices.