Surface and Bulk Distribution of Fluorides and Ti3+ Species in TiO2 Nanosheets: Implications on Charge Carrier Dynamics and Photocatalysis

The effect of the reactivity and kinetics of photogenerated e--h+ pairs induced by postsynthesis treatments on shape-controlled TiO2 anatase nanosheets with dominant {001} facets is reported. Anatase nanosheets were prepared under hydrothermal conditions in the presence of fluoride ions as shape controllers. Fluorides were removed from the surface by washing with NaOH 0.1 M or by thermal treatment at 873 K. The effect of the progressive removal of fluorides on the electronic properties of the samples was monitored by electron paramagnetic resonance (EPR) and ultraviolet-visible, near-infrared, and midinfrared spectroscopy techniques, clearly separating the contributions of surface and bulk Ti3+ sites. The charge carrier dynamics was studied by EPR and photoelectrochemical measurements. These two complementary techniques provide insight into the recombination kinetics of photogenerated carriers, highlighting the role of both surface chemistry (F- vs OH- coverage) and morphology. The results indicate that the photocatalytic activity of TiO2 anatase nanosheets could be controlled using different postsynthesis treatments. Electrochemical measurements revealed that the removal of surface fluorides leads to an increase in the total depletion rate of photogenerated charge carriers, whereas EPR experiments showed that the interfacial charge transfer to O2 and H2 is favored. As a result, the increase in the carrier total depletion rate is mainly due to the increase in the interfacial charge transfer rate, in turn leading to an increase in the photoreactivity, as witnessed by NO photo-oxidation.