Carbon-Modified SnS2/g-C3N4 S-Scheme Heterostructure with Enhanced Interfacial Charge Transfer for Efficient Photocatalytic Reduction of Cr(VI) and 4-Nitrophenol.

Hexavalent chromium (Cr(VI)) and 4-nitrophenol (4-NP) are persistent and highly toxic water pollutants that require efficient treatment under mild conditions. In this work, L-cysteine-derived carbon-modified SnS2 (SnS2-C) was prepared hydrothermally and combined with urea-derived g-C3N4 to form a 2D/2D heterostructure. CHNS analysis confirmed that SnS2-C contained 0.53 ± 0.01 wt.% carbon, while ToF-SIMS depth profiling showed a stronger carbon signal than pristine SnS2, confirming successful carbon modification. Carbon modification induced lattice distortion and strain without altering the hexagonal SnS2 structure and improved its dispersion on g-C3N4. The SnS2-C/g-C3N4 heterostructure achieved 98.8% Cr(VI) reduction within 10 min (𝑘app = 0.202 min- 1) and 96% 4-NP reduction within 120 min (𝑘app = 0.021 min- 1). Experimental characterization and density functional theory calculations indicate enhanced interfacial charge redistribution (∼1.03 e transfer) and stronger adsorption of Cr(VI) intermediates. Valence-band XPS, optical band-gap measurements, work-function analysis, and DFT calculations consistently support a direct S-scheme charge-transfer pathway, which promotes efficient charge separation and preserves highly reducing electrons for the photocatalytic reduction of both Cr(VI) and 4-NP. These findings show that carbon modification regulates interfacial electronic interactions and, together with S-scheme charge separation, enhances photocatalytic reduction of inorganic and organic pollutants.