Enhanced azo dye decolorization through charge transmission by σ-Sb3+-azo complexes on amorphous Sb2S3 under visible light irradiation

Abstract

Three types of Sb2S3 with different degrees of crystallinity were synthesized via a hydrothermal procedure by adjusting the concentration of hydrochloric acid. The amorphous Sb2S3 sample (A-Sb2S3) exhibited nearly 13 times higher photocatalytic activity for the decolorization and biodegradability improvement of MO solution compared with both crystalline samples under visible light (λ > 420 nm). It was verified that MO and Congo Red azo dyes were adsorbed onto A-Sb2S3 by displacing hydroxyl groups (OH−) to complex with Sb3+ on A-Sb2S3, which occurred via σ bonding to the lone pairs of azo nitrogen atoms with Sb3+ (σ-Sb-(NN)). Moreover, the decolorization rate of MO was positively related to its adsorption onto A-Sb2S3, and no significant degradation was observed for non-azo intermediates. The predominant photodegradation pathways of MO were reduction and oxidization to sulfanilic acid, NO3− and small-molecule acids. On the basis of photoelectron spectra, ESR (Electron spin resonance) measurements and other experimental information, it was proposed that the complexes (σ-Sb-(NN)) could transmit photogenerated electrons and holes to the azo group, breaking the NN bond, causing azo dye decolorization to improve their biodegradability; while on the surface of bare A-Sb2S3, the strong coordination interaction of OH− with Sb3+ made OH− groups become the recombination centers for photogenerated electrons and holes during the photocatalytic reaction.