Bi2S3 for sunlight-based Cr(VI) photoreduction: investigating the effect of sulfur precursor on its structural and photocatalytic properties

This study investigated the suitability of multiple bismuth sulfide (Bi₂S₃) samples for the photoreduction of Cr(VI) under simulated sunlight, aiming to elucidate the effect of different sulfide sources (thiourea, thioacetamide, sodium sulfide, potassium sulfide, and ammonium sulfide) on the final structural and photocatalytic properties of this semiconductor. The sulfides were produced through simple precipitation methods, without the necessity of complex methodologies or equipment. Additionally, the effect of thermal treatment on the properties of the Bi₂S₃ samples was also evaluated. The choice of the sulfide precursor imparted distinct characteristics onto the synthesized Bi₂S₃, such as distinct morphologies, specific surface areas (SSA), and crystalline structures. Notably, the efficiency of Cr(VI) photoreduction was found to be intricately linked to the adsorption capacity of Bi₂S₃. In this context, the calcination process emerged as a significant impediment, as it substantially diminished both the SSA and adsorption capacity of the materials. Among the sulfide sources investigated, Bi₂S₃ synthesized using K₂S exhibited superior photoreduction efficiency, attributed primarily to its remarkable adsorption capacity and rod-like morphology. The photoreduction mechanism was determined to be carried out by the direct reaction between Cr(VI) and photogenerated electrons. Regarding operational parameters, initial concentration, pH and temperature had major effects on the photoreduction efficiency; high initial concentrations led to the saturation of the active sites and lower reaction rate constants, whereas lower pHs and higher temperatures favored the photoreduction process. As for the recycle tests of the best photocatalyst, it was discovered a significant efficiency loss between cycles, which was linked to the occlusion of active sites through the formation of chrome-based species on the surface of the photocatalyst.

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