Visible light driven photooxidation of imazapyr herbicide over highly efficient mesoporous Ag/Ag2O–TiO2 p-n heterojunction photocatalysts

Abstract

This work explores the photoefficiency of mesoporous TiO2 photocatalysts modified with Ag0 and Ag2O in the photooxidation of imazapyr herbicide using visible light radiation. Ag/Ag2O–TiO2 photocatalysts with a variable Ag content (0.5–2.0 wt%) were synthesized through a sol-gel process. Their physicochemical properties were investigated by different techniques: N2 physisorption, XRD, HRTEM, FTIR, DRS UV–Vis, Raman, XPS, PL and photocurrent measurements. XRD and XPS showed the existence of both Ag2O and metallic Ag on TiO2 surfaces. UV–Vis spectra obtained for all Ag/Ag2O–TiO2 samples showed a shift in the absorption edge to the visible region and a decrease in the band gap energy (Eg) with respect to the pure TiO2. Photocatalysts with 1.5% and 2 wt % Ag loading showed total imazapyr destruction within 180 min upon visible light illumination. Both photocatalysts were found to degrade imazapyr herbicide 20.5 fold and 12.5 fold faster than the parent TiO2 and commercial Degussa P25, respectively. The high photoefficiency of the optimized Ag/Ag2O–TiO2 photocatalysts is explained by the combined effect of the presence of mesoporous anatase in the TiO2 matrix, creation of a p-n junction amongst the Ag2O nanoparticles and mesoporous TiO2 matrix, a low Eg and low recombination between photoinduced electrons and holes derived from the Ag2O–TiO2 p-n heterojunctions, and the presence of metallic Ag in the Ag2O particles, which promotes electron transfer in the p-n heterojunction.