Photocatalytic and degradation mechanisms of anatase TiO2: a HRTEM study

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.