# Photoelectrochemical Stability Enhancement of (311)-Oriented Indium Sulfide Thin Films via In-Cystine Complex Formation under Hydrothermal Synthesis

## TL;DR

This paper shows how to make more stable indium sulfide thin films for solar water splitting by using a specific hydrothermal synthesis method.

## Contribution

The study introduces In-cystine bonding via hydrothermal synthesis to enhance the stability of indium sulfide thin films for photoelectrochemical applications.

## Key findings

- Slow-heated thin films showed high and stable photocurrent densities (1.0 and 0.93 mA cm–2) with improved resistance to photocorrosion.
- Fast-heated thin films had lower photocurrent density (0.35 mA cm–2) and contained mixed In-cystine and In-cysteine bonding.
- Stability testing showed that slow-heated films retained significant photocurrent after 2 hours of illumination.

## Abstract

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.