Electric field-tuned band engineering and photocatalytic H2 evolution mechanism in ZnS/SnS2 and ZnSe/SnSe2 heterojunctions

Abstract

The rational design of heterojunction photocatalysts with tunable electronic properties is critical for efficient solar-driven water splitting. Herein, using first-principles calculations, the role of external electric field (E-field) in modulating the electronic structure, charge carrier dynamics, and photocatalytic mechanism of ZnS/SnS₂ and ZnSe/SnSe₂ heterojunctions is systematically investigated. Results reveal that the bandgaps widen monotonically within distinct stability windows (ZnS/SnS₂: −0.9 to 0.8 V Å⁻¹; ZnSe/SnSe₂: −0.8 to 0.5 V Å⁻¹), beyond which the systems undergo a metal transition at critical field strengths. Remarkably, the direction and magnitude of interlayer charge transfer can be precisely controlled by the polarity and intensity of the applied E-field: positive fields drive electrons from SnS₂ (SnSe₂) to ZnS (ZnSe), while negative fields reverse the flow, with transferred charge increasing progressively with field strength. Optical absorption is found to decrease monotonically across the −0.8 to 0.8 V Å⁻¹ range. More importantly, negative E-fields preserve the favorable Z-scheme configuration within specific ranges (ZnS/SnS₂: −0.6 to 0 V Å⁻¹; ZnSe/SnSe₂: −0.4 to 0 V Å⁻¹), leading to enhanced redox capability for water splitting. In contrast, positive fields induce an undesirable transition from Z-scheme to Type-II or Type-I configurations, degrading photocatalytic performance. Quantitative solar-to-hydrogen (STH) efficiency estimates indicate that optimal negative fields could improve theoretical efficiency by approximately 1.8-fold for ZnS/SnS₂ and 3-fold for ZnSe/SnSe₂. This work establishes a comprehensive E-field-dependent phase diagram for these heterojunctions and highlights the potential of external electric fields as a powerful strategy for dynamically controlling photocatalytic functionality.