Surface fluorinated TiO₂ material can be easily obtained by simple ligand exchange between F- and the surface hydroxyl group (OH⁻) through exposing TiO₂ photocatalyst to a mild aqueous solution containing F⁻ (NaF, NH₄F, ILs-F) (Park and Choi, 2004; Wang et al., 2008; Lin et al., 2020). After being immersed in NaF aqueous solution, the coordination unsaturated surface Ti⁴⁺ ions in TiO₂ will combine with water to form various hydrates, and the chemisorbed water molecules will dissociate to ≡Ti-OH to produce surface hydroxyl. Ligand exchange occurs between F⁻ and ≡Ti-OH to complete the adsorption of fluorine on the TiO₂ surface to form Ti₁-F (Park and Choi, 2004). Compared with simple exchange ≡Ti-OH, etching TiO₂ surface with HF can change the surface properties more strongly (Wang et al., 2008). When low concentration HF is etched, F⁻ not only replaces the end hydroxy-group on the surface but also the lattice oxygen. However, F⁻ does not penetrate into the interior of the TiO₂ lattice, and the substitution of lattice oxygen only occurs on the surface (Wang et al., 2008). Some studies also believe that during HF etching, HF dissociates and adsorbates on the clean TiO₂ surface during surface fluoridation. When the adsorption site on the surface is completely occupied by fluorine, the exposed hydroxyl group on the surface will be replaced by fluorine, and then a completely fluorinated surface covered by -TiOF₂ will be formed. Under the action of high concentration HF, These -TiOF₂ will further react with HF to produce oxygen vacancies, as shown in Figures 1a–d (Wang et al., 2011). Surface fluorinated TiO₂ prepared by post-treatment in liquid phase method often contains both surface adsorbing and inner surface phase doping fluorine. Researchers should comprehensively consider the fluorination effect and better understand the influence of inner surface phase doping fluorine in photocatalyst and distinguish it from the influence of surface adsorbing fluorine.

The radius of F⁻ (0.133 nm) is close to that of O^2- (0.132 nm), and F⁻ has a strong bonding ability with titanium atoms, so it is easier for F⁻ to stably dope TiO₂ than other elements (Wardman, 1989). As mentioned in the previous section, when TiO₂ is corroded by HF, lattice fluorine doping can be introduced while surface fluorine adsorption is achieved, but such lattice fluorine doping only exists in a few atomic layers on the surface and cannot enter the material phase. The realization of fluorine doping in the internal phase lattice of materials often requires the introduction of fluorine in the preparation process of TiO₂ for in-situ synthesis. As the commonly used synthesis method, sol-gel method usually involves the nucleophilic reaction of fluorine ions in the hydrolysis process of titanium salts, and then is included in the material phase.

As shown in Figures 1e,f, according to the different number of titanium atoms coordinated with fluorine, Wang et al. proposed that fluorine exists in F-TiO₂ in three forms: surface Ti₁-F bond formed through replacing OH⁻ by F⁻; 2-bridged fluorine F2c (Ti₂-F) and 3-coordinated fluorine F3c(Ti₃-F) by substituting F atoms for O atoms (Wang et al., 2013). Due to possessing large number of lattice F atoms which could be converted into the lattice F3c atoms in the bulk TiO₂ phase during the preparation processes, TiOF₂ and HTiOF₃ are reported to be the promising intermediates to synthesis anatase TiO₂ (Liu et al., 2012). Hu et al. also reported the characterization of fluorine species such as Ti₁-F, Ti₂-F and Ti₃-F in TiOF₂/TiO₂ composites by solid-state nuclear magnetism (Hu et al., 2020a).

There are usually two F_1s peaks in the XPS spectrum of fluorinated TiO₂ materials (Figure 1g), respectively in the range of 684.4–685.3eV (attributed to the physical adsorption of Ti₁-F or the presence of TiOF₂-like F (Ti₂-F) in the material). And in the 687.8–688.6eV range (attributed to F⁻, which is substituted for O²⁻ into the lattice by either alone or co-doped with other elements (Yu et al., 2002). As shown in Figure 1h, Li et al. observed in the F_1s XPS spectra of typical FT powder and pure TiOF₂ prepared by treating TiO₂ with HF, that pure TiOF₂ had a symmetric peak at 685.3eV, attributed to the Ti₂-F atoms in TiOF₂, and the peak at 687.8eV was attributed to the doped fluorine atoms in TiO₂ (Li et al., 2005). Yang et al. also observed a symmetry peak at 684.5eV on F_1s XPS of anatase single crystal synthesized by TiF₄ and HF, which could not be accurately attributed to either TiOF₂ (Ti₂-F) or surface adsorbed F (Ti₁-F) (Yang et al., 2008). Wang et al. believe that the binding energy of F_1s is related to the coordination state of F-Ti, and the peak near 687.6eV on the XPS spectrum of F_1s can be attributed to the 3-coordination F (Ti₃-F). However, since the fluorinated surface of Ti₂-F is more stable, and the test depth of XPS is generally about 5–10 nm, the binding energy of F_1s can be classified into 3 Ti₃-F. The surface of TiO₂ fluoride synthesized by hydrothermal or sol-gel method is often unable to detect the peak near 687.6eV, but after Ti₃-F is exposed to the sample surface by NaOH treatment, the signal of Ti₃-F near 687.6eV can be detected by XPS. Therefore, considering the fuzzy allocation of F_1s signals in XPS and the detection limit of XPS in the bulk phase, additional characterization techniques are needed to clearly distinguish fluorine species (Wang et al., 2013).

Because of its high natural abundance, high sensitivity and wide chemical shift range, ¹⁹F NMR is suitable for qualitative analysis of fluorine-containing compounds. Reyes-Garcia et al. studied the Ti-F coordination through solid-state ¹⁹F NMR testing, and they found TiO₅F species in fluorine and boron co-doped TiO₂ (Reyes-Garcia et al., 2007). After this, ¹⁹F NMR was used to study fluorine in F-doped TiO₂(Hu et al., 2020a; Wang et al., 2022) and TiOF₂/TiO₂ mixtures (Hu et al., 2020a). Koketsu et al. tested solid ¹⁹F NMR to show that in sample Ti_0.78□0.22O_1.12F_0.4(OH)0.48, fluoride ions near the vacancy were in three different chemical environments according to the coordination relationship between titanium atoms and vacancy (□): Ti₃-F, Ti_2□1-F and Ti_1□2-F (Koketsu et al., 2017). The coordination environment of fluorine in the bulk phase can significantly affect the photocatalytic performance of TiO₂. Wang et al. reported that Ti₃-F with high 1s binding energy contribute to the enhancement of visible light activity of TiO₂ fluoride. The introduction of such F leads to the formation of Ti³⁺, shrinks the band gap, and the presence of Ti₃-F enhances the adsorption of hydroxyl. The photocatalytic activity was further improved (Wang et al., 2013). Subsequently, Hu et al. used NMR to study the Ti-F coordination of the sample TiOF₂/TiO₂ (Hu et al., 2020a). As shown in Figure 1i, multiple resonance signals at ∼ 15ppm can be attributed to the Ti₂-F environment in the TiOF₂ lattice, and the resonance at −84ppm can be attributed to the bulk phase Ti₃-F. It was further confirmed that F was successfully incorporated into TiO₂. After light treatment, the formation of a new signal at −151 ppm was attributed to the Ti₁-F environment, indicating that the doped fluorine transformed from Ti₂-F to Ti₁-F and generated Ti³⁺ at the interface of TiOF₂ and TiO₂, which significantly enhanced the charge transfer efficiency in TiOF₂/TiO₂, thereby improving the photocatalytic performance. Therefore, according to the solid ¹⁹F NMR test results, fluorine atoms coordinate with different numbers of titanium atoms can be distinguished, but this research needs further exploration.

Furthermore, more comprehensive sample information can be provided by the combination of other technologies, such as electron paramagnetic resonance spectroscopy (Hu et al., 2020b) and electron energy loss spectroscopy (Wang et al., 2022).

Fluorinated TiO₂ photocatalysts show stronger UV-visible light adsorption with a red shift (Figure 2a) were developed by Yu et al. through hydrolysis of titanium tetraisopropoxide in a mixed NH₄F-H₂O solution (Yu et al., 2002; Chen et al., 2022; Hou et al., 2024). The reduction of Ti³⁺ from Ti⁴⁺ by charge compensation of F doping form a donor level between the band gaps of TiO₂ may benefit to the enhanced light absorption (Figure 2b). In addition, surface fluoridation also produces some oxygen vacancies, resulting in visible-induced photocatalytic activity. Le et al. used the thermal shock method to fluoridate TiO₂ P25 powder at different temperatures, and the fluoridated sample produced oxygen vacancy at 400°C–600°C, which was confirmed by XPS spectroscopy as the formation of TiO₂ surface fluoridation (Khoa Le et al., 2012).

Zhao et al. concluded that the surface lattice F3c atoms (Ti₃-F) with higher 1s binding energy are identified to be the origin of visible light activity by analyzing the 1s CLSs of various types of F atoms in the fluorinated TiO₂ (Wang et al., 2013). Further analyzing the electronic structures of the fluorinated TiO₂ using semi-local density functional theory and non-local hybrid density functional theory calculations demonstrates that the introduction of the 3-coordinated surface F atoms leads to the formation of Ti³⁺ ions in the sub-surface, which is the cause for the bandgap shrinking, increasing the visible-light activity. However, the photocatalytic efficiency of fluorinated TiO₂ for water splitting is limited due to the limited absorption under visible light irradiation and the high recombination rate of photogenerated electron-hole pairs (Yu et al., 2010; Li et al., 2020). Developing a method to synthesize F-TiO₂ materials that with considerable visible-light photocatalytic activity is still a challenge.

Several investigations have been reported for increasing the efficiency of carriers separation/transport in TiO₂ based materials through fluorine modified. Surface fluorination of TiO₂ can significantly change the physicochemical properties and structure of the material surface: increasing the surface electronegativity, promoting the separation and transfer of surface charge, and inhibiting the recombination of electron hole pairs; promoting the formation of hydroxyl free radical and other active reactive substances (Yuan et al., 2025). The oxygen vacancy defects and Ti³⁺ centers formed on the surface of TiO₂ during fluorination process also favor the separation of charge carriers (electrons and holes) and can trap the holes (Wang et al., 2021).

The surface charge separation can be further enhanced by loading Pt, Ag, Pd and other precious metals as cocatalyst on the fluorinated TiO₂ (Vaiano et al., 2018; Díaz-Sánchez et al., 2021). Yu et al. reported that the F ions on the surface of TiO₂ can greatly decrease the recombination rate of photogenerated carriers by acting as an electron-trapping sites to trap the photogenerated electrons due to its strong electronegativity and then transfer electrons to the Pt loaded (Yu et al., 2010). As shown in Figure 2c, our previous work further proved that the surface F anions with negative electric will attract the holes to migrate to the surface of TiO₂ and inhibit the migration of photogenerated electrons, which further prevents electron-hole recombination (Hu et al., 2020b). Besides, the introduction of surface fluorine provides anchoring sites for Pt nanoparticles and strengthens the interaction between Pt nanoparticles and the TiO₂ substrate resulting in significantly improved catalytic performance (Ji et al., 2019). Many recent works focus on the loading of metal single atoms (SAs) on TiO₂ as cocatalyst for photocatalytic reactions (Hejazi et al., 2020; Cha et al., 2022). For example, Wu et al. reported that both surface and lattice Ti³⁺ suitable for Pt anchoring and charge compensation can be generated in pristine TiO₂-F nanosheets with surface terminal F species. After surface F species are removed by NaOH treatment, Pt single atoms (SAs) were stabilized by lattice F (Figure 2d), and shows much higher photocatalytic hydrogen generation efficiency than Pt SAs on TiO₂-F (Figure 2e (Wu et al., 2023; Wu and Schmuki, 2023). Recently, combined with the surface stabilizing effect of the as-formed F-C/F-Ti bonds, single-atom catalysts (Pd, Ir, Pt) on TiO_xN_y nanorods surface via in situ fluoride ion etching for hydrogen evolution could be obtained (Zeng et al., 2025).

The crystallinity of fluorine-doped TiO₂ could be improved upon F⁻ doping and then benefit to the higher bulk electronic conductivity, which is responsible for enhanced water splitting (Fang et al., 2014). Next, Hu et al. simulated the geometric structures and calculated the deformation density of the Ti₂-F, Ti₃-F, and Ti₁-F sites, respectively. The neighboring Ti atoms of Ti₁-F sites got more electrons, compared with those on theTi₂-F or Ti₃-F sites. The generation of terminal Ti₁-F in TiOF₂/TiO₂ moved more electrons toward the terminal F atom resulting in the acceleration of the interfacial charge transfer (Hu et al., 2020b).