Fluorine ions in photocatalysts’ synthesis: an obstacle or an ally? The investigation of photocatalysts in Ti-O-F system

Titanium(IV) oxide (TiO₂) is one of the most promising photocatalysts, which is expected to be applied in the photocatalytic degradation of xenobiotics. For sustainable development and green chemistry approach recently, much effort has been paid to enhance TiO₂ photocatalytic activity in UV-vis light by modifying its structure. The photocatalytic process’s complexity cannot be simply described as electron-hole pairs generations and •OH formation; therefore, the interaction between the surface structure of photocatalyst and reactant molecules needs further investigation. The surface chemistry and morphology of TiO₂ are crucial factors that can affect its photocatalytic performance by influencing the selective adsorption and photocatalytic oxidation of substrates.

In this regard, considering the strong complexation between F^- and Ti and the high electronegativity of fluorine, the present study focused on Ti-O-F photocatalytic system. An advantage of introducing fluorine compounds inside the reaction system is the stabilization of thermodynamically unfavoured {0 0 1} facets, which are described as more active than other TiO₂ facets with defined morphology.

The obtained results for anatase nanosheets with exposed {0 0 1} facets have shown that a higher amount of surface fluorine enhances the photocatalytic degradation of phenol and its mineralization. This observation can be explained by F atoms influencing the surface charge distribution and raising charge separation in the 2D structure of TiO₂. Meanwhile, samples with a lower F/Ti ratio on the surface and the presence of Ti³⁺ exhibited lower photocatalytic activity.

On the other hand, titanium oxyfluoride (TiOF₂) used as a precursor for the synthesis of 2D TiO₂ had a negative effect on phenol photocatalytic degradation and TOC mineralization. In this case, fluorine ions could induce the surface formation of nonstoichiometric TiO_2−x, which decreased photocatalytic activity of TiO₂ particles with exposed {0 0 1} facets.

Acknowledgments

The research was financially supported by the Polish National Science Centre (grant no. NCN 2018/30/E/ST5/00845).