Single-Particle Imaging Reveals Charge Redistribution on TiO₂: Engineering Facets and Oxygen Vacancies for Photocatalytic Toluene Oxidation.

Abstract

The manipulation of exposed anisotropic facets and surface microstructures facilitates the directional migration of photoinduced charge carriers, offering a promising strategy for enhancing the photocatalytic activity. However, the charge dynamics behavior associated with facet engineering remains unclear and elusive. Herein, the critical role of the {001}/{101} facet ratio on carrier lifetime was determined by precisely monitoring the charge redistribution in the micronano regions of individual TiO₂ microcrystals via single-particle fluorescence spectroscopy <i>in situ</i>. The results demonstrate that TiO₂ with a medium ratio of oxidative and reductive sites exhibits the longest PL lifetimes on both {001} and {101} facets due to the synergistic effect of the facets promoting efficient charge carrier migration. Correspondingly, the optimal utilization of electron-hole pairs was also demonstrated in the further toluene oxidation process. Furthermore, <i>in situ</i> monitoring of the fluorine (F) removal process via annealing of a single TiO₂ particle confirms that the formation of oxygen vacancy (O_V) after Ti-F bond breakage, rather than the removal of F, is the key factor in further promoting charge separation. In addition, real-time observation of toluene oxidation on a single TiO₂ microcrystal was conducted at the single-particle level, indicating that the effective charge transfer between TiO₂ and toluene is beneficial for the preferential formation of benzaldehyde on the TiO₂-{101} facet. This work provides feasible monitoring methods and strong evidence for facet engineering to promote charge separation, which is beneficial for guiding the rational design of photocatalysts.