Photoelectrochemical stability enhancement of (311)-oriented indium sulfide thin films via In-cystine complex formation under hydrothermal synthesis Xiuru Yang , Hong Chang, Graf Arthur, Yongde Xia, Asif Ali Tahir, Yanqiu Zhu University of Exeter, UK Indium sulfide are promising photoactive materials for light-induced applications, particularly photoelectrochemical (PEC) water splitting. However, its practical application is limited by photocorrosion, which hinders long-term efficiency. In this study, we report the synthesis of In-cystine bonded (311)-oriented indium sulfide thin films, formed using a mixed sulfur source of L-cysteine hydrochloride and L-cystine, the latter generated in situ via Fe 3+ -induced oxidation of L-cysteine. By adjusting the hydrothermal temperature and ramp rate, we tuned the surface indium-organic complex composition, morphology, thickness, and PEC performance of the resulting films. Thin films 160-3 and 180-3, synthesized under slow heating conditions, exhibit a dominant indium sulfide phase bonded with In-cystine. In contrast, the fast-heated film 160-10 primarily contains indium-organic complexes, with mixed bonding In-cystine and In-cysteine bonding. Consequently, 160-3 and 180-3 demonstrate significantly higher and more stable photocurrent densities—1.0 and 0.93 mA/cm 2 at -0.2 V vs. Ag/AgCl, respectively— compared with 160-10 (0.35 mA/cm 2 at -0.2 V vs. Ag/AgCl). Stability testing further reveals that after two hours of continuous illumination at -0.2 V vs. Ag/AgCl, thin film 160-3 retains 0.75 mA/cm 2 , while 180-3 maintains 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. References 1. S.R. Mishra, V. Gadore, M. Ahmaruzzaman, Shining light on sustainable and clean hydrogen production: Recent developments with In 2 S 3 photocatalysts, Nano Energy, 128 (2024) 109820, https://doi.org/10.1016/j.nanoen.2024.109820. 2. J. Lu, M. Zhang, J. Yao, Z. Zheng, L. Tao, Y. Zhao, J. Li, Nonlayered In 2 S 3 /Al 2 O 3 /CsPbBr 3 Quantum Dot Heterojunctions for Sensitive and Stable Photodetectors, ACS Applied Nano Materials, 4 (2021) 5106-5114, https://doi.org/10.1021/ acsanm.1c00573. 3. J. Lu, Z. Zheng, J. Yao, W. Gao, Y. Zhao, Y. Xiao, J. Li, 2D In 2 S 3 Nanoflake Coupled with Graphene toward High- Sensitivity and Fast-Response Bulk-Silicon Schottky Photodetector, Small, 15 (2019) e1904912, https://doi.org/10.1002/ smll.201904912. 4. P. Wang, W. Xue, W. Ci, R. Yang, X. Xu, Intrinsic vacancy in 2D defective semiconductor In 2 S 3 for artificial photonic nociceptor, Materials Futures, 2 (2023) 035301, https://doi.org/10.1088/2752-5724/acdd87. 5. Z. Qiu, Z. Luo, M. Chen, W. Gao, M. Yang, Y. Xiao, L. Huang, Z. Zheng, J. Yao, Y. Zhao, J. Li, Dual-Electrically Configurable MoTe 2 /In 2 S 3 Phototransistor toward Multifunctional Applications, ACS Nano, 18 (2024) 27055-27064, https://doi.org/10.1021/ acsnano.4c10168. 6. S. B, P. Divyashree, S. Sharma, P. Dwivedi, S. Das, Scalable In 2 S 3 based optical memristor devices as artificial synapse for logic realization and neuromorphic computing, Materials Science in Semiconductor Processing, 185 (2025) 108985, https:// doi.org/10.1016/j.mssp.2024.108985. 7. B, P. Dwivedi, Wafer-Scale, Efficient In 2 S 3 -Based Optical Memory Devices for Neuromorphic Computing, IEEE Transactions on Electron Devices, 71 (2024) 2760-2765, https://doi.org/10.1109/ted.2024.3368399. 8. X. Li, Y. Han, Z. Shi, M. An, E. Chen, J. Feng, Q.J. Wang, β-In 2 S 3 Nanoplates for Ultrafast Photonics, ACS Applied Nano Materials, 5 (2022) 3229-3236, https://doi.org/10.1021/acsanm.1c03542. 9. B. Yang, M. Wang, X. Hu, T. Zhou, Z. Zang, Highly efficient semitransparent CsPbIBr 2 perovskite solar cells via low- temperature processed In 2 S 3 as electron-transport-layer, Nano Energy, 57 (2019) 718-727, https://doi.org/10.1016/j. nanoen.2018.12.097.
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