Defect-assisted symmetric cleavage of naphthalene sulphonic acid group in azo dyes using β-In2S3 quantum dots as visible light photocatalyst

Abstract

Naphthalene sulfonic acid, which is used in the synthesis of azo dye pigments, rubber processing chemicals and pharmaceuticals, is a carcinogenic substance that pollutes water bodies. In this work, we report on the visible light-driven symmetric cleavage of naphthalene sulfonic acid group, present in methyl orange dye and its conversion into the intermediate compounds. These compounds further degrade to inorganic ions such as carbon-di-oxide, nitrates, sulfates, water, and chlorine. Complete degradation of the dye, under visible light irradiation, is attained using β-In2S3 quantum dots as photocatalyst. During homogenous precipitation process, the β-In2S3 quantum dots (~ 9 nm) self-assemble to form microflowers (~ 50 nm) with high surface-to-volume ratio. These quantum dots exhibit size-dependent, active F2g, Eg and Ag1 Raman modes with peaks at ~ 150 cm−1 (In–In stretching mode), 219 cm−1 (In–S bending mode) and 300 cm−1, corresponding to the vibrational modes of cubic phase β-In2S3. The cubic phase β-In2S3 quantum dots are photoactive under visible light exposure and releases highly oxidizing OH· radicals. They have strong band-to-band emission in ultraviolet region (~ 380 nm) and exhibit broad band defect emission with maxima at blue (~ 484 nm), green (~ 580 nm) and red (~ 600 nm) region of electromagnetic spectrum. The emission intensity from these defect energy bands, which are due to sulfur vacancy, indium interstitials and oxygen incorporation, are tuned by varying the In-to-S ratio in the sample. These defects enhance their visible light absorption coefficient and assist in improving the photocatalytic efficiency of the cubic phase β-In2S3 quantum dots. Thus, defect-assisted complete (100%) photodegradation of the azo dye is achieved using cubic β-In2S3 quantum dots with low In-to-S ratio (1:1), low mass of 20 mg and minimum irradiation time (30 min). These photocatalysts can be reused 4 times under 30-min visible light irradiation. Cubic β-In2S3 quantum dots-microflowers is a highly efficient, ecofriendly photocatalyst, which even in very low concentration can remove toxicity from the dye-contaminated water, by exposure to direct sunlight for 30 min.