Band and fermi-level engineering of Cu₂ZnSnS₄photocathodes for high-efficiency photoelectrochemical water purification

The Fermi-level engineering of semiconductor materials is critical for enhancing photoelectrochemical (PEC) water purification efficiency. In this paper, we report the fabrication of highly crystalline Cu₂ZnSnS₄ (CZTS) photocathodes via a simple spin-coating method combined with optimized annealing to precisely control their band structures and Fermi-level positions. By carefully adjusting the annealing temperature up to 400 °C, we achieved CZTS thin films with narrowed band gaps (∼1.47 eV), Fermi levels aligned closer to the valence band, and significantly enhanced hydrophilicity. These engineered electronic properties facilitated improved charge-carrier separation and accelerated electron transfer at the semiconductor/electrolyte interface. The optimized CZTS photocathode exhibited a high photocurrent density of ∼3.1 mA cm⁻²under 1-sun illumination at −0.3 V vs. Ag/AgCl, enabling the efficient degradation of 99.1 % of methyl orange (MO) within 135 min. This study highlights how the precise control of the band structure and Fermi-level alignment can dramatically enhance PEC water purification, offering a scalable and sustainable pathway for developing next-generation, earth-abundant photocathodes.