Designing novel morphologies of l-cysteine surface capped 2D covellite (CuS) nanoplates to study the effect of CuS morphologies on dye degradation rate under visible light

Pure self-assembled l-cysteine stabilized covellite nanoplates (CuS@l-Cys NPs) with the best control over size, phase purity structure, morphology, and electrochemical properties were synthesized using a facile, template-free hydrothermal route. The effects of temperature (100-180 °C), reaction time (8-24 h), pH of reaction medium (7-14), concentration of base (0.1-2.4 mL NH₃) and concentration of thiourea (1-4 mmol) on the morphologies of CuS@l-Cys nanoplates were studied. The photocatalytic performance of CuS@l-Cys NPs under visible light was studied using methyl orange (MO) as a model dye. CuS@l-Cys NPs were reused successfully for the photodegradation of dye due to the recycling ability of CuS@l-Cys NPs. The hydroxyl radicals (OH) generated by CuS@l-Cys NPs were detected using terephthalic acid (TA) as a probe molecule through the photoluminescence (PL) technique. The successful capping of l-Cys on the surface of CuS NPs was confirmed by FTIR spectroscopy. The BET surface areas of the hexagonal CuS@l-Cys NPs and oblong CuS@l-Cys NPs were measured to be 11.87 and 5.66 m² g^-1, respectively. The optical band gaps of the hexagonal CuS@l-Cys NPs (2.0 eV) and oblong CuS@l-Cys NPs (2.04 eV) were determined according to direct bandgap calculations. These results support the presence of hexagonal CuS@l-Cys NP catalysts as efficient accelerators of the photodegradation of MO. Furthermore, the l-cysteine (l-Cys) protective layer could efficiently alleviate the photocorrosion of CuS, giving rise to excellent stability. The high photocatalytic activity of the hexagonal CuS@l-Cys NPs can be ascribed to the reduction of photoinduced electron-hole pair recombination and high specific surface area, as confirmed by BET for other representative samples.