Photocatalytic and degradation mechanisms of anatase TiO2: a HRTEM study

Abstract

The photocatalytic process for degradation of methylene blue with P25 TiO₂ was directly observed using a high-resolution transmission electron microscopy (HRTEM). It was found that: (1) the pristine anatase TiO₂ nanoparticles exhibited a perfect crystal lattice with a clear HRTEM image; (2) after adsorption and degradation, there were many methylene blue crystals as 1 nm molecular adsorbed at the surface of TiO₂ nanoparticles, which therefore resulted in a fuzzy HRTEM image; (3) when the TiO₂ was exposed in air for a period of time, the methylene blue molecules disappeared and the TiO₂ lattice image again became integrated as the pristine one; (4) however, if the TiO₂ nanoparticles became deactivated after degradation of methylene blue for more than 20 cycles, the HRTEM lattice image was fuzzy fully and could not recover even it was exposed in air for a long time. The results reveal that the lattice distortion on the anatase TiO₂ (101) surface induced by chemical adsorption of methylene blue molecules is a crucial intermediate step during photocatalyzing. The distorted lattice atoms absorb photons under illumination of light and tend to recover and cut the molecule bond which causes the degradation of methylene blue. Furthermore, we propose that there exists a surface lattice driving force forming on the surface distortion leads to the TiO₂ lattice HRTEM image from fuzzy to integrate, and it decides the photocatalytic ability.