Sulfur-source-regulated BiVO4/SnS2/Bi2S3 ternary heterojunctions for synergistic TC-Cr(VI) removal: Experimental and DFT mechanistic insights

Abstract

The coexistence of tetracycline (TC) and hexavalent chromium (Cr(VI)) in water creates a typical refractory combined pollution, whereas single photocatalysts are limited by severe carrier recombination, narrow spectral response and poor simultaneous removal capability. Herein, a series of BiVO4/SnS2/Bi2S3 ternary heterojunction photocatalysts were synthesized via a two-step hydrothermal method using four sulfur sources (Na2S·9H2O, L-cysteine, thiourea and thioacetamide) to regulate Bi2S3 in-situ growth. The effects of sulfur source on morphology, structure, optical properties and photoelectrochemical performance were systematically studied, alongside the photocatalytic activity for single pollutant and mixed TC-Cr(VI) pollutants. XRD, SEM, TEM and XPS confirmed successful heterojunction formation, with sulfur source significantly affecting Bi2S3 nucleation morphology and interfacial contact. The morphologies of Bi2S3 varied significantly with the sulfur source, from a complete film coating (BSS-Na2S with Na2S·9H2O), small nanoparticles (BSS-C3H7NO2S with L-cysteine) and aggregated nanoparticles (BSS-CH4N2S with thiourea), to uniformly dispersed elongated whiskers (BSS-C2H5NS with thioacetamide), with the former exhibiting the smallest specific surface area. BSS-C2H5NS extended the visible-light absorption to the near-infrared region with a narrowed bandgap of 2.04 eV. Photoelectrochemical tests showed that BSS-C2H5NS possessed the highest capacitance (15.34 mF/cm2), longest carrier lifetime, smallest Nyquist arc and lowest Mott-Schottky slope, indicating optimal carrier separation and transport. In mixed TC-Cr(VI) system, BSS-C2H5NS achieved 100 % Cr(VI) reduction (60 min) and 80.9 % TC degradation (100 min), with good stability over four cycles. DFT calculations combined with EPR analysis, confirm that the work function gradient (BiVO4 < SnS2 < Bi2S3) drives the formation of a built-in electric field at the heterojunction interface, establishing a Type-II charge transfer pathway that enables stepwise directional migration of photogenerated electrons and spatial charge separation. The accumulated electrons reduce Cr(VI), while the holes oxidize TC, and this synergistic effect substantially suppresses electron-hole recombination. This work establishes the “sulfur source-microstructure-performance” relationship, offering a rational strategy for designing multi-sulfide heterojunctions toward combined wastewater treatment.