α-NiS/Bi₂O₃ Nanocomposites for Enhanced Photocatalytic Degradation of Tramadol

In this study, a series of α-NiS/Bi2O3 composite nanomaterials were prepared and evaluated as efficient photocatalyst for degradation of tramadol under visible light. Two polymorphs of Bi2O3, namely, α- (monoclinic) and β- (tetragonal), were prepared by using combustion method and bismuth subcarbonate decomposition route, respectively. A facile method was also developed for synthesis of ultrathin α-NiS nanosheets under mild conditions using hexamethylenetetramine as hydrolyzing agent and Na2S2O3 as sulfur source. The NiS/α-Bi2O3 and NiS/β-Bi2O3 composite materials were thoroughly characterized using a variety of techniques to understand their structural, optical, electrochemical, microstructural, and morphological attributes. The α-NiS/Bi2O3 materials exhibited improved visible light absorption, enhanced charge carrier separation, and photoelectrochemical properties. The microscopic close contact between the two semiconductor phases is established from the morphological studies. The α-NiS/Bi2O3 materials exhibited excellent photocatalytic activity for aqueous phase degradation of tramadol under visible light irradiation. Particularly, the α-NiS/β-Bi2O3 system is highly active achieving 94% degradation in 3 h. The mechanism of photocatalytic action involves a z-scheme electron transfer from the CB of Bi2O3 to the VB of NiS which is responsible for efficient space separation of charge carriers and their increased reactivity. The •OH radicals and h+ species have been identified as the major transient species responsible for tramadol oxidation. The photocatalytic method developed in this study can be a viable alternative for TiO2 based UV active photocatalyst studied so far for tramadol degradation.