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

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.