Jie Hu
Xianhao Shan2
Zhengyuan Gao- 1School of Materials and Science, Chongqing Jiaotong University, Chongqing, China
- 2School of Mechatronics and Vehicle Engineering, Chongqing Jiaotong University, Chongqing, China
- 3Institute of Aerospace Intelligence and Innovation, Academy of Aerospace System and Innovation, Beijing, China
As an efficient and environmentally friendly photocatalyst, TiO2 has garnered significant interest among researchers. However, the rapid recombination of photogenerated carriers leads to the inhibition of its photocatalytic activity. Fluorine modification has been proven to be an effective method to improve the photocatalytic performance of TiO2, leading to a multitude of research reports on this subject. Surface fluorine adsorption or lattice fluorine doping can deftly modulate the surface chemical attributes and electronic configuration of the TiO2 photocatalyst, thereby amplifying its functional performance. The role of fluorine atoms coordinated with different number titanium atoms (terminal Ti1-F, bridging Ti2-F and Ti3-F) are also discussed. This paper provides a minireview of various aspects of fluorine-modified TiO2, including its classification (surface-adsorbed fluorination, lattice-doped fluorination and Tix-F) and characterization techniques (X-ray photoelectron spectroscopy and solid-state nuclear magnetic resonance). Finally, this treatise elucidates the mechanistic impact of fluorine modification on the photocatalytic hydrogen production performance of TiO2.
1 Introduction
Titanium dioxide (TiO2) is one of the most extensively utilized photocatalysts due to its excellent stability, cost-effectiveness and eco-friendliness (Nishiyama et al., 2021; Bhom and Isa, 2024; Wen et al., 2024). The low efficiency of photogenerated carrier separation and transport limits the wide application of TiO2 (Cheng et al., 2024; Zhao et al., 2025). To enhance the photocatalytic activity of TiO2, numerous modification studies have been undertaken, including noble metal deposition (Li et al., 2021), doping (Zhang et al., 2019) and heterojunction construction (Ma et al., 2019). A pivotal discovery made by Lai et al., in 1993 revealed that adjusting the ratio of hydrofluoric acid (HF) to fluoride ion in fluorinated reaction solutions could alter the band edge potential of TiO2 (Lai et al., 1993). This finding established fluorine modification as an effective strategy to improve the photocatalytic performance of TiO2 due to enhanced surface acidity, stronger adsorption of reactant molecules, additional Ti3+ self-doping, and stabilized {001} facets (Yang et al., 2008). After substituting O atoms or surface hydroxyl groups, fluorine introduced into TiO2 is usually classified as surface-adsorbed fluorine or lattice-doped fluorine (Wu and Schmuki, 2023). Moreover, according to the different number of titanium atoms coordinated with fluorine, Wang et al. and Hu et al. proposed that the F atoms doped in fluorinated TiO2 system by a variety of chemical bonds: terminal Ti1-F bond (F1s), bridging fluorine F2c (Ti2-F) and 3-coordinated fluorine F3c (Ti3-F), where the x in Tix-F represents the number of titanium atoms bonded to this fluorine atom (Wang et al., 2013). However, due to the complex types of Ti-F bonds in fluorinated TiO2, the mechanism of carrier separation and transport is unclear, the precise control of Ti-F bonds is difficult, the mechanism of fluorination reaction is not clear, and the understanding of the structure-property relationship is insufficient. There is an urgent need for follow-up and cooperation in related theoretical research. This minireview aims to summarize the impact of fluorine modification on the photocatalytic hydrogen production performance of TiO2 through a comprehensive review of relevant literature. The discussion will encompass aspects such as classification of TiO2 fluorination, characterization of fluorine species, and effect of fluorine on the photocatalytic hydrogen generation performance of TiO2.
2 Classification of TiO2 fluorination
The fluorination route influences the physicochemical properties and photocatalytic performance of F-TiO2. Generally, the modification of TiO2 with fluorine encompasses both surface-adsorbed fluorination and lattice-doped fluorination (Zulfiqar et al., 2021). Surface-adsorbed fluorine is typically achieved through post-treatment fluorination via a ligand exchange between F− ions and the surface functional groups of TiO2. 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. Herein, we will briefly describe the fluorination principle and fluorine species of these fluorination method, as well as the detailed structures and descriptions of corresponding examples.
2.1 Surface-adsorbed fluorination
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.
Figure 1. DFT (density functional theory) calculated reaction energies and structures for different stages of HF interaction with single crystal anatase TiO2(101) (left) and (001) (right) surfaces: (a) Clean surfaces; (b) full HF-covered surfaces; (c) complete fluorinated surfaces; (d) etched surfaces; (e) fluorinated surface with lattice F2c atoms; (f) fluorinated surface with lattice F3c atoms. All structures are optimized structures (Wang et al., 2011; Wang et al., 2013). XPS spectra of (g) F1s spectra of F-doped TiO2 (Yu et al., 2002) and (h) F1s spectra of FT powder and pure TiOF2 (Li et al., 2005); (i) 19F NMR spectra of TiOF2/TiO2 (Hu et al., 2020a).
2.2 Lattice-doped fluorination
The radius of F− (0.133 nm) is close to that of O2- (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).
3 Characterization of fluorine in TiO2
In general, X-ray photoelectron spectroscopy (XPS) and solid-state nuclear magnetic resonance (NMR) are used to characterize fluorine-modified TiO2 to determine the presence of fluorine species.
3.1 Analysis of fluorine species by X-ray photoelectron spectroscopy (XPS)
There are usually two F1s 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 F1s 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 F1s 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 F1s is related to the coordination state of F-Ti, and the peak near 687.6eV on the XPS spectrum of F1s 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 F1s 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 F1s 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).
3.2 Nuclear magnetic resonance (NMR) to study the Ti-F coordination
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 Ti0.78□0.22O1.12F0.4(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, Ti2□1-F and Ti1□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).
4 Effect of fluorine on the photocatalytic hydrogen generation performance of TiO2
In the past years, fluorine-modified TiO2 has attracted attention in the field of photocatalytic hydrogen production (Wang et al., 2019; Bhom and Isa, 2024), which consists of the following steps: light absorption, charge separation and transport, and redox reactions at the photocatalyst’s surface.
4.1 Light absorption
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).
Figure 2. (a) UV-visible absorption spectra of Degussa P25 and the F-doped TiO2; (b) schematic energy level diagram for Ti3+ and charge-carrier dynamics in F-doped TiO2 (Yu et al., 2002); (c) proposed mechanism for the photocatalytic properties of TM-S (Hu et al., 2020b); (d) structure models and (e) normalized photocatalytic H2 evolution rate of Pt SAs/TiO2-F and Pt SAs/TiO2-OH (Wu and Schmuki, 2023).
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.
4.2 Carriers separation and transport
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 TiOxNy 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).
5 Conclusion
The current minireview focuses on the investigation of the surface-adsorbed fluorination and lattice-doped fluorination for F-TiO2 nanomaterials, and the role of fluorine in photocatalytic water splitting. According to the different number of titanium atoms coordinated with fluorine, the F atoms introduced to fluorinated TiO2 system are classified into terminal Ti1-F, bridging Ti2-F and Ti3-F. In conclusion, both surface-adsorbed fluorination and lattice-doped fluorination are effective measures to improve the photocatalytic performance. Fluorine ions on the surface 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. Especially, the surface Ti3-F is identified to be the origin of visible light activity. The surface lattice Ti2-F are beneficial to stabilize Pt SAs and then bring high photocatalytic efficiency. Defects such as surface Ti3+ and oxygen vacancy defects formed during fluorination process could change the local electronic structure and improve the photocatalytic performance. Ti3+ defects introduced by lattice-doped fluorination can regulate the band structure of TiO2 and inhibit photogenerated carrier recombination. The generation of terminal Ti1–F moved more electrons toward the terminal F atom resulting in the acceleration of the interfacial charge transfer.
Although great progress has been made in the role of fluorine in photocatalysis, there are still many problems that need to be fully studied further. For example, due to the varied fluorine species in fluorine-modified TiO2, there are challenges in the precise regulation of doped fluorine species, and the mechanism of action of various doped fluorine species on the improvement of photocatalytic performance at the atomic scale also needs to be improved.
Author contributions
JH: Writing – original draft, Writing – review and editing. XS: Writing – review and editing. SWu: Writing – review and editing. PS: Writing – review and editing. ZG: Writing – review and editing. ZR: Writing – review and editing. XF: Writing – review and editing. SWa: Writing – review and editing.
Funding
The author(s) declare that financial support was received for the research and/or publication of this article. This work was supported by the National Natural Science Foundation of China (22272013), the Science and Technology Research Program of Chongqing Municipal Education Commission (KJQN202300739), Undergraduate Innovation and Entrepreneurship Training Program (S202410618006), Research Start-up Fund Project of Chongqing Jiaotong University (F1230081, 20JDKJC-B047) and Construction Project for Chongqing Graduate Tutor Team (JDDSTD2022006).
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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The author(s) declare that no Generative AI was used in the creation of this manuscript.
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Keywords: surface-adsorbed fluorination, lattice-doped fluorination, TiO2, Ti-F bonds, photocatalytic hydrogen production
Citation: Hu J, Shan X, Wu S, Sun P, Gao Z, Ren Z, Feng X and Wang S (2025) Effects of fluorine modification on the photocatalytic hydrogen production performance of TiO2. Front. Chem. 13:1621188. doi: 10.3389/fchem.2025.1621188
Received: 30 April 2025; Accepted: 19 May 2025;
Published: 05 June 2025.
Edited by:
Heng Zhao, Eastern Institute of Technology, ChinaReviewed by:
Jie Ying, Sun Yat-sen University, ChinaYuxuan Xiao, Sun Yat-sen University, China
Yi Lu, Wuhan University of Technology, China
Copyright © 2025 Hu, Shan, Wu, Sun, Gao, Ren, Feng and Wang. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Zhengyuan Gao, emhlbmd5dWFuZ2FvQGNxanR1LmVkdS5jbg==; Zhong Ren, enJlbjEyOEAxNjMuY29t