Probing the formation and optical properties of Ti3+–TiO2 with (001) exposed crystal facet by ethanol-assisted fluorination

Abstract

In TiO₂ based photocatalysis, it is crucial to conceal the catalytically active Ti³⁺ defect sites inside the crystal, since the Ti³⁺ defects on the surface can be rapidly lost after capturing O₂ molecules in the air. In this work, we report a novel method to synthesize TiO₂ with Ti³⁺ defects that are exclusively embedded in the bulk lattice near the surface. We show that the binary synthetic system consisting of ethanol and HF is indispensable for the precise location control of the formed Ti³⁺ defects, and the near-surface bulk Ti³⁺ defects can only be formed under the coexistence of both these reagents. The underlying mechanism of achieving such remarkable control of the position of the Ti³⁺ defect sites was clarified by extensive analysis through X-ray diffraction, electron microscopy, electron spin resonance, X-ray photoelectron spectroscopy, as well as theoretical simulation using the DFT+U method. We show that ethanol plays a crucial role in assisting fluorine atoms adsorbing on the exposed (001) crystal facets, penetrating the near-surface bulk lattice and inducing Ti³⁺ defect formation via substitution of oxygen atoms.