Trace-Level Fluorination of Mesoporous TiO₂ Improves Photocatalytic and Pb(II) Adsorbent Performances

Fluorination is an effective way of tuning the physicochemical property and activity of TiO₂ nanocrystallites, which usually requires a considerable amount of hydrofluoric acid (or NH₄F) for a typical F/Ti molar ratio, R_F, of 0.5-69.0 during synthesis. This has consequential environmental issues due to the high toxicity and hazard of the reactants. In the present work, an environmentally benign fluorination approach is demonstrated that uses only a trace amount of sodium fluoride with an R_F of 10^-6 during synthesis. While it maintained the desirable high surface area (102.4 m²/g), the trace-level fluorination enabled significant enhancements on photocatalytic activities (e.g., a 56% increase on hydrogen evolution rate) and heavy metal Pb(II) removal (31%) of the mesoporous TiO₂. This can be attributed to enriched Ti³⁺ and localized spatial charge separation due to fluorination as proved by X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance spectroscopy (EPR), and density functional theory (DFT) analyses.