Ag-AgBr/CaWO4 composite microsphere as an efficient photocatalyst for degradation of Acid Red 18 under visible light irradiation: Affecting factors, kinetics and mechanism

A novel visible-light-driven photocatalyst Ag-AgBr/CaWO4 composite microsphere was fabricated by depositing AgBr onto the self-organized CaWO4 microspheres, then partially reducing Ag+ ions in AgBr to Ag-0 species via photoreduction method. The catalyst was characterized by the surface analytical methods such as X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, diffuse reflectance spectroscopy. The photocatalytic degradation of azo dye Acid Red 18 was studied using Ag-AgBr/CaWO4 heterostructure composite under visible light irradiation. The effect of pH value, catalyst loading, and initial dye concentration on degradation kinetics of AR18 was systematically investigated. The results showed that the prepared catalyst had high photocatalytic activity and stability due to the efficient charge separation deriving from the well-matched band energies between CaWO4 and AgBr, as well as the surface plasmon resonance of Ag nanoparticles. The photodegradation of AR18 was found to follow apparent first-order kinetics and could also be described by an empirical rate expression r = 0.142 exp (-0.0354c(0))[catalyst](0.576) c. The near-neutral pH values favored the photocatalytic degradation of AR18. The apparent rate constant k(app) decreased exponentially with increasing dye initial concentration. In addition, the quenching experiments and fluorescence probe analysis were carried out to identify the active species in the photocatalytic process. The possible mechanisms of photodegradation of AR18 over Ag-AgBr/CaWO4 under visible light were discussed as well. (C) 2014 Elsevier B.V. All rights reserved.