Defective phase engineering of S-scheme TiO2–SnS/SnS2 core–shell photocatalytic nanofibers for elevated visible light responsive H2 generation and nitrogen fixation

Abstract

To promote the fast separation of photogenerated charge carriers and promote stability, we designed core–shell TiO₂–SnS/SnS₂ heterostructures with a mixed-phase shell wall of the SnS/SnS₂ composition with enriched oxygen-related defect states without compromising their morphology. By engineering the shell wall composition, narrow-band-gap SnS_x was tagged on TiO₂ nanofibers to form core–shell TiO₂–SnS/SnS₂ heterostructures through co-axial electrospinning followed by sulfidation. The enriched oxygen vacancies have prolonged the visible adsorption by creating a mid-energy level on TiO₂, which narrowed the bandgap and made the wide bandgap TiO₂ visible light active. At the intimate interface, the build-in electric field at the heterostructure interface favors the S-scheme heterostructure pathway that promotes the photogenerated electrons and holes for the redox reactions to produce radical species. Compared to the core–shell TiO₂–SnS₂ nanofiber photocatalyst, the phase-engineered TiO₂–SnS/SnS₂ (2 : 2) heterostructure nanofibers exhibit the highest catalytic efficiency through the defect-mediated interface with an effective photocarrier separation rate in a S-scheme pathway. The core–shell TiO₂–SnS/SnS₂ (2 : 2) heterostructure had the fastest H₂ evolution rate of 337 μmol g⁻¹ h⁻¹ and a photocatalytic nitrogen fixation rate of 517 μmol g⁻¹ h⁻¹. The H₂ evolution rate of the TiO₂–SnS/SnS₂ (2 : 2) heterostructure is 1.47 and 2.27 times faster than that of the TiO₂–SnS₂ (2 : 0.5) and TiO₂–SnS (0.5 : 2) core–shell nanofibers, and its structure and catalytic activity stayed stable over time. The energy band analysis, radical trapping, and density functional theory (DFT) calculations proved that the SnS₂-based interface with enriched oxygen vacancies has improved light absorption and increased photocatalytic effectiveness with the S-scheme heterojunction pathway. This study comprehensively analyzes heterostructure interfaces for engineering a high-quality charge carrier transportation pathway to enhance photocatalytic performances in heterostructure compounds.