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

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 in situ. 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, in situ monitoring of the fluorine (F) removal process via annealing of a single TiO2 particle confirms that the formation of oxygen vacancy (OV) 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.