Kinetics and mechanism of hydroxyl radical formation studied via electron spin resonance for photocatalytic nanocrystalline titania: Effect of particle size distribution, concentration, and agglomeration

Abstract

A photocatalytic process was carried out with two types of TiO2: commercial (C-TiO2) and in-house synthesized (S-TiO2). Parameters, such as, initial particle concentration and the nanoparticle (NPs) agglomerations effect on hydroxyl radical (•OH) concentration were investigated using electron spin resonance (ESR) spectroscopy with a spin trapping technique. The experimental results demonstrate that generation of •OH and DMPO/•OH (5,5-dimethyl-1-pyrolline-N-oxide/hydroxyl radical) adduct formation is controlled by a shorter time-scale of the chemical reaction on particle surfaces and longer time-scale particle agglomerations in the bulk dynamics. It was found that S-TiO2 has a smaller particle size than C-TiO2 NPs. As a consequence, S-TiO2 NPs yield a higher concentration of •OH compared to that of C-TiO2 NPs of the same concentration. These findings reveal an agreement between the ESR signals, agglomeration size analysis, and transmission electron microscopy (TEM) data. Detail explanations are presented mainly on the drive of dynamic time scales and the limitation of the number of NPs governed by their associated distributions. With the kinetic studies, we propose the mechanism for the generation of •OH via a study of ESR DMPO/•OH spin trap technique. The mechanism accounts for the active surface area as the agglomeration process occurred throughout the suspension and the possibility of DMPO/•OH recombination as the surface of TiO2 became dense with DMPO/•OH adduct.