Recrystallization and coalescence kinetics of TiO2 and ZnO nano-catalysts towards enhanced photocatalytic activity and colloidal stability within slurry reactors

Abstract

Advanced oxidation processes rely on the development of stable photocatalytic materials, offering specific band-gap attained upon reaching appropriate crystalline phases. A key research gap is related to the recrystallization of nano-catalysts and their impact on performance. This paper describes the systematic preparation of TiO2 and ZnO nanoparticles by hydrothermal-assisted sol-gel method followed by systematic calcination steps with the aim to shed light on the recrystallization process to engineer higher photocatalytic activity. Spherical anatase-TiO2 nanoparticles with a 20 nm diameter size present higher photocatalytic activity than 70 nm of ZnO NPs calcined at 500 °C. A photocatalytic yield of 84% within 30 min of irradiation for the degradation of model dyes was observed for the titania particles, which were also less sensitive to a gglomeration, a key challenge when designing slurry reactors. The variation in zeta potential of TiO2 and ZnO with pH exhibited isoelectric points (IEP) in aqueous media at 5.1 and 6.5, respectively, suggesting amphoteric behaviors while X ray photo-electron spectroscopy and diffusive reflectance spectroscopy data were used to characterize the changes in surface vacancies and the band gap of the materials. These data open the door to the development of advanced oxidation processes in complex industrial environments.