Mechanism underlying visible-light photocatalytic activity of Ag/AgBr: Experimental and theoretical approaches

Abstract

Silver/silver halides (Ag/AgX) are potential visible-light (VL) photocatalysts due to synergistic effects of surface plasmon resonance (SPR) and semiconductor photocatalysis. However, many contradictory opinions are found in published literature about the mechanism underlying photocatalytic oxidation (PCO) by Ag/AgX. In this study, Ag/AgBr particles were prepared by a simple one-pot synthesis method and used for degradation of Reactive Black 5 (RB5) as a model pollutant aiming to understand the effect of concentration of excess silver ions on SPR phenomenon, and to employ experimental and theoretical approaches to pinpoint the reactive oxidative species (ROS) responsible for RB5 degradation. 16.2% of initial 50 mg/L RB5 was degraded using 0.5 g/L AgBr (Ag+:AgBr = 0, i.e. no excess Ag+) under 60 min VL irradiation. Under identical experimental conditions, RB5 degradation increased to 86.5% using Ag/AgBr synthesized as Ag+:AgBr = 0.4, due to improved SPR effect. Experimental evidence supported by theoretical calculations revealed that superoxide radicals (•O2) played a prime role in PCO followed by photo-generated holes, whereas the contribution of •OH was negligible. The presence of Ag0 on AgBr greatly influenced band energies and regulated the formation of ROS. With an increase in ratio Ag+:AgBr, the conductance and valence bands increasingly became more electro-negative and electro-positive, respectively, as compared to that at Ag+:AgBr = 0. The presence of Cl ̅ and SO42− ions adversely affected RB5 degradation, whereas CO32− did not cause any adverse impact. Ag/AgBr could be successfully immobilized over bacterial cellulose which, under identical conditions, provided 79.1% RB5 degradation in 60 min. The Ag/AgBr-BC could be reused five times.