Enhanced photocatalytic degradation of dyes by TiO2 nanobelts with hierarchical structures

TiO2 nanobelt membranes with hierarchical structure were successfully synthesized by sequentially autoclaving Ti at 190 °C for 3 d in the presence of 10 M NaOH and 10 M KOH solutions. Microstructural characterization revealed TiO2 nanoparticles joining together and on the surface of nanobelts. These hierarchical structures form a three dimensional porous membrane which significantly enhances both surface specific area and light absorption, resulting in improved chemical adsorption capacity and photocatalytic degradation efficiency relative to nanobelts with smooth surfaces using methylene blue as a model molecule. The adsorption of methylene blue to these structures follows a pseudo-second order kinetics chemisorption mechanism with rate-limited diffusion correlated to pore structure and size. The dominant reactive oxygen species are identified as hydroxyl radicals and valence band holes through the scavenging reaction. The synergistic enhancement of filtration through surface adsorption and photocatalytic degradation is also demonstrated in a prototype photocatalytic membrane reactor with UV excitation at 365 nm. The reuse of nanobelt membranes after annealing shows the excellent recovery of TiO2 catalyst. These studies may contribute to additional applications of hierarchical TiO2 nanobelt membranes, including those harnessing sunlight for water treatment.