Tailoring the crystallographic orientation and thickness of indium sulfide thin films for enhanced photoelectrochemical water splitting

Abstract

Indium sulfide thin films play a crucial role in photoelectrochemical (PEC) water splitting, offering promising strategies to mitigate energy shortages and global warming. In this study, indium sulfide thin films were synthesized via a hydrothermal method, and the effects of sulfur precursors—L-cysteine (LC) and L-cysteine hydrochloride (LCHCl)—along with hydrothermal temperature ramp rates (3 and 10 °C min⁻¹) on their crystallographic orientation, morphology, and thickness were investigated. The findings revealed that films synthesized with LC predominantly exhibited the (440) facet, while those synthesized with LCHCl had the (311) facet. Additionally, films produced at 3 °C min⁻¹ were thicker than those synthesized at 10 °C min⁻¹. The film (LC-IS-10) synthesized at 10 °C min⁻¹ using LC achieved a photocurrent density of 3.7 mA cm⁻² at −0.2 V vs. Ag/AgCl, which outperformed that of LCHCl-synthesized film (LCHCl-IS-10) at the same heating rate (2.6 mA cm⁻²) and those of the films synthesized at 3 °C min⁻¹ (LC-IS-3: 0.7 mA cm⁻² and LCHCl-IS-3: 2 mA cm⁻²). 2-Hour photocurrent stability assessments indicated that LC-synthesized films (LC-IS-3: 1 mA cm⁻², LC-IS-10: 670 μA cm⁻²) exhibited superior stability to the LCHCl-synthesized films (LCHCl-IS-10: 90 μA cm⁻², LCHCl-IS-3: 33 μA cm⁻²). This improved stability was attributed to their (440) facet, which was structurally more compact and symmetric and exhibited reduced sulfur exposure than the less ordered (311) facet. Although the LC-IS-3 film had the lowest photocurrent density, it showed enhanced stability, owing to the thickness alteration caused by the oxidation of surface S²⁻ species. This research provides insights for optimizing material design in PEC water-splitting applications, advancing sustainable energy solutions.