# Fluorination of Defective Titanium Dioxide with an Efficient Photocatalytic Activity

Authors: Yijie Zhao; Xuan Hu; Rongrong Hu; Xingjian Wang; Zhangyu Gu; Xiaojuan Zhang; Yuan Zhao
Journal: Journal of Nanomaterials
 (2022)

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## Abstract

Fluorinated-defective titanium dioxide (F-TiO2−x) was successfully synthesized by fluorination of TiO2 through the hydrothermal method. The microstructure and physicochemical properties of the catalyst were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and UV–vis diffuse reflectance spectroscopy (UV–vis DRS). As expected, F-TiO2−x demonstrates high photocatalytic activity and stability on dye degradation. Results indicated that fluorine element can effective for stabilizing Ti3+ and oxygen vacancies on the surface of F-TiO2−x. With the increase in the amount of hydrofluoric acid (HF) added, the photocatalytic performance of the prepared material is first increased and then decreased, and the catalyst shows the best performance when the amount of HF added is 0.3 mL. For three different dyes, the catalysts all showed a certain photocatalytic degradation performance, and the degradation effect of rhodamine B was the best.


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## Body

## 1. Introduction

With the development of human society, energy crisis, and environmental pollution have become the serious topics in the scientific community. Organic pollutants in water seriously endanger human health [1]. Therefore, more attempts have been implemented to eliminate or reduce harmful compounds that are important for economic and social developments [2, 3]. As a standard technique, semiconductor-based photocatalysis is a predictable approach to play an imperative role in solving this problem [4–10]. Particularly, TiO2 is attracting much attention due to its superior abilities, such as nontoxic, environmentally friendly, and low cost [11–13]. However, only a narrow range of the solar spectrum can be absorbed by TiO2, most of which is ultraviolet, due to its large bandgap (∼3.2 eV for anatase). Overcoming electron–hole pairs through unbalanced photogenerated recombination on the surface of crystallites has become an important prerequisite for the commercial application of TiO2 [14, 15].Special facet exposure on TiO2 samples is an effective way to improve the separation of photogenerated electrons and holes, electrons are enriched in the (101) plane, whereas holes are enriched in the (001) plane, which ultimately contributes to the photocatalysis [16–18]. For TiO2, the exposed crystal plane is generally the (101) crystal plane, the crystal plane energy (0.44 J/cm2) of this crystal plane is low, and it has good reducing ability; therefore, it has good performance in photocatalytic hydrogen production, but its activity in degrading pollutants is lower than (001) [19]. Preliminary research shows that Ti3+ self-doped TiO2−x prepared by the reduction method has good photocatalytic degradation performance [20, 21]. However, the instability of surface Ti3+ seriously hinders the photocatalytic performance of the catalyst. At the same time, different crystal planes of the photocatalytic materials tend to have different surface energies, and the crystal planes with high crystal plane energy tend to have higher oxidizing ability which is beneficial for the degradation of pollutants [22].Currently, the methods of exposing crystal planes mainly include chemical etching and high-pressure treatment [23]. Under these circumstances, in order to obtain surfaces with higher reactivity, developing TiO2 crystals with specific facets has recently been investigated as an effective method to improve their efficiency [24, 25]. Numerous studies have shown that the surface fluorination treatment can stabilize the surface Ti3+ and oxygen vacancies, and expose special crystal planes [26, 27]. Zhang et al. found a significant enhancement on photocatalytic activity of TiO2 powders by doping with F− ions, and F ions not only suppressed the formation of brookite phase but also prevented phase transition of anatase to rutile [28]. Choi’s research shows that the formation of surface fluorides on TiO2 (F-TiO2), which can be easily attained by a simple addition of F− to aqueous TiO2 suspensions, uniquely affects both photocatalytic reactions and photoelectrochemical behaviors [29]. Leung et al. studied the effect of F ion content in F-doped TiO2 nanosheets on the photocatalytic degradation of toluene in vacuum ultraviolet. The results show that F ion-doped TiO2 can enhance the photocatalytic activity mainly due to the improvement of the light absorption and the reduction of recombination efficiency of photogenerated carriers [30]. Despite the above-mentioned progress so far, F-doped TiO2 photocatalytic materials have not been deeply studied as a promising photocatalytic hybrid material. In particular, the photocatalytic efficiency of F-titania is far from practical application.Methyl orange (MO) is the most commonly used acid–base indicator and has good pH sensitivity. Under acidic and alkaline conditions, azo and quinoid structures are the main structures of dye compounds and are often chosen as dye models. Methylene blue (MB) is a common dye in the printing and dyeing industry. It is difficult to be oxidized in the environment and its degradation and decolorization is also an important object in the treatment of wastewater. In addition, MB is often used as a simulated pollutant. It is widely used in experimental research. Rhodamine B (RhB) is also a widely used heterocyclic dye. The wastewater of RhB has the characteristics of high chroma, high concentration, poor biodegradability, and difficult biochemical treatment. The photodegradation comparison of the above three organic dyes is of great significance for the study of the application of photocatalysts.Herein, F-TiO2−x were prepared by fluorination of TiO2 through hydrothermal method. The degradation performance of d rhodamine B for F-TiO2−x is much higher than TiO2−x. The enhanced photocatalytic activity is attributed to the high-energy crystal face (001) and stabilized surface Ti3+, which significantly reduces the recombination rate of photogenerated electrons and holes. The degradation rate of MO and MB for F-TiO2−x is improved, which indicates that it has good applicability.


## 2. Experimental Section

### 2.1. Materials

Tetrabutyl titanate (TBOT), hydrochloric acid (HCl), hydrofluoric acid (HF), and ethanol were provided by NanJing chemical Reagent Co., Ltd., China. RhB, MO, MB, and barium sulfate were purchased from Aladdin Industrial Corporation. Sodium hydroxide and hydrazine hydrate were purchased from Sinopharm Chemical Reagent Co., Ltd., China. The deionized water used was produced in lab. All chemical reagents used in this experiment were of analytical grade, procured commercially, and used without further purification.


### 2.2. Preparation of Surface Fluorinated TiO2−x

3 mL TBOT was added into 30 mL HCl solution (0.1 mol/L), stir with a magnetic stirrer, and then 0.6 mL hydrazine hydrate was added during the stirring process. Continue stirring at room temperature for 30 min to obtain a yellow–brown precursor mixed solution, then a certain amount (0, 0.15, 0.3, 0.6, and 0.9 mL) of HF was added to the mixture. Transfer the mixed solution to 50 mL of teflon autoclave and heated at 180°C for 24 hr in an oven. After cooling to room temperature, the samples were washed three times with water and alcohol, respectively, and then dried in vacuum oven at 60°C for 6 hr. The samples were named TiO2−x–n (n is the amount of HF used).


### 2.3. Characterization

The prepared samples were measured by X-ray diffraction (XRD) patterns (Bruker D8 Advance diffractometer) at room temperature with Cu Kα radiation (λ = 0.15418 nm). X-ray photoelectron spectra (XPS) of samples were collected using PHI 5000 VersaProbe Ⅱ. UV–vis diffuse reflectance spectroscopy (DRS) was obtained on a UV-1800 UV–vis spectrophotometer. The surface morphology of composite catalysts was analyzed by scanning electron microscopy (SEM) using Hitachi S-4800 scanning electron microscope, and the accelerating voltage is 10 kV. Transmission electron microscopy (TEM) micrographs were obtained by JEOL JEM-2100F microscope (Japan) that was operated at an accelerating voltage 200 kV. Brunner−Emmet−Teller (BET) surface area measurements were carried out by N2 adsorption at 77 K using an ASAP2020 instrument.


### 2.4. Photocatalytic Experiment

The photocatalytic performance of samples was evaluated by photodegradation of dyes under 300 W xenon lamp with a UV cut-off filter (λ > 420 nm). The photocatalytic experiments were carried out as follows: 50 mg of photocatalyst were dispersed in the breaker containing 50 mL of 10 mg/l dye aqueous solution under continuously stirring at room temperature. After stirring the solution in dark for 30 min to achieve the adsorption–desorption equilibrium, the xenon lamp was turned-on and 5 mL of suspension was taken out in every 5 min. The suspension was immediately centrifuged to obtain supernatant at 8,000 rpm for 5 min. The absorbance of the supernatant was measured at 554 nm through UV–visible spectrophotometer. The photocatalytic performance is evaluated in two ways. One is the degradation efficiency of dye by photocatalyst. The other is to indirectly reflect the degradation rate of the catalyst according to the reaction kinetic constant of photocatalysis.


## 2.1. Materials

Tetrabutyl titanate (TBOT), hydrochloric acid (HCl), hydrofluoric acid (HF), and ethanol were provided by NanJing chemical Reagent Co., Ltd., China. RhB, MO, MB, and barium sulfate were purchased from Aladdin Industrial Corporation. Sodium hydroxide and hydrazine hydrate were purchased from Sinopharm Chemical Reagent Co., Ltd., China. The deionized water used was produced in lab. All chemical reagents used in this experiment were of analytical grade, procured commercially, and used without further purification.


## 2.2. Preparation of Surface Fluorinated TiO2−x

3 mL TBOT was added into 30 mL HCl solution (0.1 mol/L), stir with a magnetic stirrer, and then 0.6 mL hydrazine hydrate was added during the stirring process. Continue stirring at room temperature for 30 min to obtain a yellow–brown precursor mixed solution, then a certain amount (0, 0.15, 0.3, 0.6, and 0.9 mL) of HF was added to the mixture. Transfer the mixed solution to 50 mL of teflon autoclave and heated at 180°C for 24 hr in an oven. After cooling to room temperature, the samples were washed three times with water and alcohol, respectively, and then dried in vacuum oven at 60°C for 6 hr. The samples were named TiO2−x–n (n is the amount of HF used).


## 2.3. Characterization

The prepared samples were measured by X-ray diffraction (XRD) patterns (Bruker D8 Advance diffractometer) at room temperature with Cu Kα radiation (λ = 0.15418 nm). X-ray photoelectron spectra (XPS) of samples were collected using PHI 5000 VersaProbe Ⅱ. UV–vis diffuse reflectance spectroscopy (DRS) was obtained on a UV-1800 UV–vis spectrophotometer. The surface morphology of composite catalysts was analyzed by scanning electron microscopy (SEM) using Hitachi S-4800 scanning electron microscope, and the accelerating voltage is 10 kV. Transmission electron microscopy (TEM) micrographs were obtained by JEOL JEM-2100F microscope (Japan) that was operated at an accelerating voltage 200 kV. Brunner−Emmet−Teller (BET) surface area measurements were carried out by N2 adsorption at 77 K using an ASAP2020 instrument.


## 2.4. Photocatalytic Experiment

The photocatalytic performance of samples was evaluated by photodegradation of dyes under 300 W xenon lamp with a UV cut-off filter (λ > 420 nm). The photocatalytic experiments were carried out as follows: 50 mg of photocatalyst were dispersed in the breaker containing 50 mL of 10 mg/l dye aqueous solution under continuously stirring at room temperature. After stirring the solution in dark for 30 min to achieve the adsorption–desorption equilibrium, the xenon lamp was turned-on and 5 mL of suspension was taken out in every 5 min. The suspension was immediately centrifuged to obtain supernatant at 8,000 rpm for 5 min. The absorbance of the supernatant was measured at 554 nm through UV–visible spectrophotometer. The photocatalytic performance is evaluated in two ways. One is the degradation efficiency of dye by photocatalyst. The other is to indirectly reflect the degradation rate of the catalyst according to the reaction kinetic constant of photocatalysis.


## 3. Results and Discussion

Figure1(a) shows the comparison of the photocatalytic degradation performance of RhB by catalysts prepared with different amounts of HF. The photocatalytic degradation performance does not show a linear relationship with the amount of HF used. When the amount of HF used was 0.3 mL, the prepared photocatalyst had the best photocatalytic degradation performance of RhB, the degradation rate of RhB reached 96.8% in 50 min of illumination. Figure 1(b) is the first-order kinetic constant curve calculated according to the fitting; it can be seen that with the increased amount of HF used, the reaction rate first decreases, then increases and then decreases; the specific kinetic constants are shown in Table 1; the first-order kinetic constant Kapp first decreased, then increased and then decreased. The reason for this may be that with the increase of HF, the high-energy (001) crystal face is exposed on the one hand, which increases the surface activity of the catalyst. On the other hand, with the increase in the amount of HF, HF inhibits the crystal transformation, promotes the grain growth of the material, reduces the specific surface area, and then reduces the active center of the reaction, the photocatalytic degradation efficiency is reduced. The degradation rate of TiO2−x–0.3 was 27% higher than that of TiO2−x.Figure 1
(a) Degradation performance diagram of F-TiO2−x catalyst prepared with different HF usage; (b) quasi-first-order kinetic fitting.
(a)(b)Table 1
Pseudo-first-order kinetic constants of rhodamine B degradation by F-TiO2−x prepared with different amounts of HF.
SampleTiO2−xTiO2−x–0.15TiO2−x–0.3TiO2−x–0.6TiO2−x–0.9Kinetic constant (min−1)0.0510.0450.0680.0350.025In order to obtain the substance type and grain size of F-TiO2−x prepared by different fluorination treatments, XRD analysis was performed on the prepared materials, and the analysis results are shown in Figure 2. The XRD patterns of TiO2−x, TiO2−x–0.15, TiO2−x–0.3, TiO2−x–0.6, and TiO2−x–0.9 show the obvious peaks at 25.3°, 36.9°, 37.8°, 38.6°, 48.1°, 53.9°, 55.1°, 62.1°, 62.7°, and 68.8° are correspond to the crystal plane peaks of (101), (103), (004), (112), (200), (105), (211), (213), and (204) of anatase type TiO2 (PDF No. 99-0008) [31, 32]. TiO2−x without fluorination showed obvious peaks at 27.4°, 36.1°, and 56.6° corresponding to the (110), (101), and (220) crystal planes of rutile TiO2, respectively, according to the PDF No. 99-0090 standard card [33, 34]. The analysis results show that fluorine atoms inhibit the anatase–rutile transition. In addition, with the increase of the degree of fluorination, the sharpness of the characteristic peak of the (101) crystal plane at 25.3° increased continuously, indicating that the fluorination treatment improved the crystallinity of the material. Table 2 shows that the grain size and phase ratio of the material were calculated according to the Scherrer formula and the empirical formulas. According to the calculation results in Table 2, it can be seen that the presence of fluorine promotes the grain growth.Figure 2
XRD patterns of F-TiO2−x catalysts prepared with different amounts of HF.Table 2
Crystal forms and grain sizes of F-TiO2−x prepared with different amounts of HF.
SampleCrystal ratio (%)Grain size (nm)AnataseRutileTiO2−x811920TiO2−x–0.15100026TiO2−x–0.3100031TiO2−x–0.6100046TiO2−x–0.91000122XPS was used to analyze and test the elemental composition and valence state of the TiO2−x–0.3 surface. Figure 3 shows the full spectrum of TiO2−x–0.3 and the high-resolution XPS images of C1s, N1s, O1s, F1s, and Ti2p. The results of the fit to the test data are also shown in Figure 3. Figure 3(a) shows the full spectrum of TiO2−x–0.3; it can be seen from the figure that there are elements such as C, N, O, F, and Ti on the surface of the material, indicating that F was successfully introduced into the surface of TiO2. Figure 3(b) is the high-resolution XPS image of C1s, which only has a peak at 284.8 eV, which is caused by the surface adventitious carbon. Figure 3(c) is the high-resolution XPS image of N1s. Through fitting analysis, there are binding energy peaks at 401 and 399.5 eV. The former is because N replaces O to form O–Ti–N, and the latter is because N atoms enter lattice gap. Figure 3(d) is the high-resolution XPS image of O1s. It is obtained by fitting that the binding energy peaks appear at 529.5 and 531.6 eV, which are due to the existence of lattice oxygen and surface hydroxyl and adsorbed oxygen, respectively [35]. Figure 3(e) shows that the central peak of F1s located at 684.3 eV, which is a typical characteristic peak of fluorinated TiO2, and F plays a pivotal role in the formation of Ti3+ or oxygen vacancies [36, 37]. Figure 3(f) is the high-resolution XPS image of Ti2p, the peaks are located at binding energies 464.3, 458.5, 463.5, and 458.0 eV, corresponding to Ti4+ 2p1/2, Ti4+ 2p3/2, Ti3+ 2p1/2, and Ti3+ 2p3/2, respectively. The fitting results show that Ti3+ exists on the surface of TiO2−x–0.3 [38].Figure 3
(a) Full spectrum of TiO2−x–0.3; (b) C1s; (c) N1s; (d) O1s; (e) F1s; (f) Ti2p high-resolution XPS image.
(a)(b)(c)(d)(e)(f)Figure4(a) shows the morphological characteristics of the sample TiO2−x without fluorination treatment. It can be seen from the figure that the particles are relatively uniform and block-like, and the particle size is about 20 nm. Figure 4(b) is the topography image of the sample TiO2−x–0.15. It can be seen from the figure that compared with TiO2−x, the sharp edges of TiO2−x–0.15 are gradually etched, and the particle shapes are diversified and growing trend. As shown in Figures 4(c) and 4(d), the HF usage continues to increase, the TiO2 particles are continuously etched, and these particles are self-assembled to form large particles. As shown in Figure 4(d) that when the amount of HF used is 0.6 mL, large particles with a particle size larger than 200 nm have been formed. Figures 4(e) and 4(f) are the TEM and SEM images of the sample TiO2−x–0.9, respectively. From the TEM and SEM morphology images, it can be found that the small bulk particles have disappeared, and the grains are all larger than 100 nm. With the addition of HF, the edges of the TiO2 particles were gradually etched, and these particles are self-assembled eventually to form large particles [30].Figure 4
TEM images of F-TiO2−x prepared with different HF usage: (a) TiO2−x; (b) TiO2−x–0.15; (c) TiO2−x–0.3; (d) TiO2−x–0.6; (e) TiO2−x–0.9; (f) SEM images of TiO2−x–0.9.
(a)(b)(c)(d)(e)(f)Figure5 is the TEM image, particle size distribution image, and high-resolution transmission electron microscope (HRTEM) image of TiO2−x–0.3. Figure 5(a) shows that the particles have good dispersion and are mainly polygonal. The particle size distribution statistics are carried out, and the statistical results are shown in Figure 5(b). It can be seen from the figure that the particle size distribution is between 20 and 60 nm, and the average particle size is 35 nm, which is basically consistent with the XRD analysis and calculation results. Figure 5(c) is the HRTEM image of TiO2−x–0.3, and Figure 5(d) is its partial enlarged image. The lattice fringe spacings are 0.352 and 0.237 nm, respectively, and the angle between the crystal planes is 68.3°, which exactly matches the theoretical value of the angle between the (101) and (001) crystal planes [39, 40]. Studies have shown that holes tend to migrate to the high-energy (001) crystal plane, whereas electrons tend to aggregate at the (101) crystal plane, thereby separating electrons and holes [41, 42].Figure 5
(a) TEM image of TiO2−x–0.3; (b) particle size distribution image; (c) HRTEM image; (d) HRTEM enlarged image.
(a)(b)(c)(d)It was observed that the particle size of TiO2 gradually increased with the increase of HF usage in the SEM and TEM images. In general, the increase in particle size of nanomaterials leads to a decrease in specific surface area. To further confirm the effect of HF usage on the specific surface area of TiO2, BET tests were performed on the samples. Figure 6 is the nitrogen adsorption/desorption isotherms of samples prepared with different HF usages, and Table 3 shows the specific surface area values of the samples. The specific surface area of the sample TiO2−x without fluorination treatment reached 81 m2/g. After the fluorination treatment, the specific surface area of the sample decreased, and with the increase in HF usage, the specific surface area of the sample continued to decrease. The specific surface area of the sample TiO2−x–0.9 is only 28.5 m2/g, which is only one-third of TiO2−x without fluorination treatment. The result was consistent with the SEM and TEM.Figure 6
Nitrogen adsorption/desorption isotherms of F-TiO2−x prepared with different HF usage.Table 3
The specific surface area of F-TiO2−x prepared with different amounts of HF.
SamplesTiO2−xTiO2−x−0.15TiO2−x−0.3TiO2−x−0.6TiO2−x−0.9Specific surface area (m2/g)81.271.560.149.328.5In order to understand the effect of fluorination treatment on the light absorption properties of the materials and to analyze and calculate the bandgap width of the materials, UV–vis DRS tests were performed on the prepared materials, the test results are shown in Figure7(a). Compared with the TiO2−x without HF added in the preparation process, the solar light absorption properties of the samples with HF added during the preparation were enhanced. The forbidden band width of each sample calculated by K–M function and Tauc equation is shown in Figure 7(b). The bandgap of TiO2 decreases gradually with the increase of the amount of HF added. When the amount of HF added is 0.9 mL, the bandgap of TiO2−x–0.9 is 2.76 eV. It is generally believed that the fluorine atom itself cannot reduce the bandgap width, but the fluorine element can stabilize the surface Ti3+ and oxygen vacancies and promote their formation [35, 43]. Therefore, the fluorination treatment can affect the energy band structure of the material and improves the absorption range of sunlight. Although TiO2−x–0.9 has the largest light absorption range, TiO2−x–0.3 has the best photocatalytic performance in the photocatalytic degradation test. Based on the results of XRD and TEM analyses, with the increase of HF addition, the grain size of TiO2 gradually became larger, resulting in a decrease in specific surface area, which, in turn, reduced the photocatalytic performance.Figure 7
(a) DRS diagrams of F-TiO2−x catalysts prepared with different HF usage; (b) calculation and analysis diagram of forbidden band width.
(a)(b)In order to further investigate the applicability of F-TiO2−x photocatalyst, TiO2−x–0.3 with the best photocatalytic performance was selected as the photocatalyst to investigate its degradation of different reaction substrates. In the experiment, the same concentrations of MO and MB were selected for comparison with the RhB used in the previous experiments. The obtained experimental results are shown in Figure 8. The results show that TiO2−x–0.3 has different photocatalytic degradation performance for different reaction substrates and also the best degradation performance for RhB, with a degradation rate of 96.8%, followed by MO with a degradation rate of 89%, and the worst with MB with a degradation rate of 85%. Figure 8(b) shows the kinetic constants of TiO2−x–0.3 for different reaction substrates. From the kinetic constants, it can be inferred that the degradation rates of different substrates are RhB > MO > MB in order from fast to slow. This shows that the degradation performance of TiO2−x–0.3 for different reaction substrates is still very different. As shown in Table 4, compared with TiO2−x, the photocatalytic performance of F-TiO2−x is improved [44].Figure 8
(a) Comparison of degradation performance of TiO2−x–0.3 for different substrates; (b) first-order kinetic constant diagram.
(a)(b)Table 4
Comparison of photocatalytic performance of TiO2−x and F-TiO2−x.
RhBMOMBTiO2−xKinetic constant (min−1)0.05180.03830.0321Degradation rate (%)94.890.383.9F-TiO2−xKinetic constant (min−1)0.06580.04210.0397Degradation rate (%)96.88985We also investigate the stability of F-TiO2−x photocatalyst, TiO2−x–0.3 with the best photocatalytic performance was selected as the photocatalyst and RhB as the degradation substrate. The test method of the cycle stability of the experiment in this chapter is the same as that of the previous chapter, and the experiment tests the cycle performance of the material. After the first degradation process, the photocatalytic reaction material was obtained by centrifugation and directly freeze-dried without cleaning. After the material was dried, the next photocatalytic degradation experiment was performed, which was repeated five times. The experimental results are shown in Figure 9. The experimental results show that the initial degradation rate of TiO2−x–0.3 photocatalyst is 96.8%, and after five cycles of use, the degradation rate still reaches 95%, and TiO2−x–0.3 still maintains a blue appearance. The stability test of the sample TiO2−x shows that with the increase in the number of cycles, the photocatalytic degradation performance decreases, and the sample gradually turns white after being exposed to the air for a period of time. For samples treated with HF, the blue color remained stable. The comparison shows that the stability of TiO2−x–0.3 is greatly improved compared with that of TiO2−x. This indicates that the surface fluorination treatment is beneficial to the formation and stable existence of Ti3+ and oxygen vacancies on the surface of TiO2−x [35, 45].Figure 9
Recycling degradation performance of TiO2−x–0.3.Understanding the active species during the degradation of RhB by F-TiO2−x is essential for analyzing the photocatalytic reaction mechanism. In order to explore the active species in the process of F-TiO2−x degradation of RhB, the corresponding active species of different radical scavengers were added, and then the changes of the degradation effects were compared to obtain the role of different active species in the reaction process. The transfer of electrons and holes is very important in the photocatalytic process. The catalytic ability of a catalyst depends on the electron–hole pair, the separation of photogenerated electrons and holes is considered to be the initial step in the photocatalytic degradation process. For these reasons, electrons and holes were first trapped, holes (h+) were removed by ammonium oxalate, and electrons were captured by AgNO3. As shown in Figure 10, it is the effect of adding different radical scavengers on the degradation of RhB by F-TiO2−x. When ammonium oxalate was added, the degradation rate became 70% of the original, and after adding AgNO3, the degradation rate became 58% of the original. Tests show that electrons and holes play an important role in the photocatalytic degradation process. The photogenerated holes react with OH− or H2O adsorbed on the surface to generate hydroxyl radicals (·OH), and the photogenerated electrons can react with O2 to generate superoxide radicals (·O2−). In this experiment, isopropanol was used to remove hydroxyl radicals (·OH). After adding isopropanol, the degradation rate of RhB became 86% of the original. The superoxide radical (·O2−) was removed by p-benzoquinone, and the degradation rate of RhB after the addition of p-benzoquinone became 30% of the original. Photogenerated electrons have excellent reducibility and cannot oxidize organic pollutants themselves, but the test results show that photogenerated electrons have a great influence on the photocatalytic degradation rate. This is because photogenerated electrons affect the formation of superoxide radicals (·O2−), whereas photogenerated holes have a greater effect on the photocatalytic degradation rate than hydroxyl radicals (·OH), indicating that photogenerated holes not only affect hydroxyl radicals (·OH) but also oxidize organic pollutants by itself. By analyzing the effect of different active species scavengers on the photocatalytic reaction, it is seen that superoxide radical (·O2−) is the main active species in the process of photocatalytic degradation of RhB, and photogenerated holes can also directly oxidize organic pollutants, and photogenerated electrons affect the formation of superoxide radicals (·O2−).Figure 10
Effect of active species scavenger on the degradation performance of TiO2−x–0.3.Based on the above information, the band structure diagram and possible charge separation process of TiO2−x–0.3 are shown in Figure 11. New secondary states appear near the conduction band (CB) edge of TiO2 due to Ti3+ doping or oxygen vacancies, and the doped N atoms generate new impurity levels above the valence band (VB) of TiO2. Narrowing the bandgap of TiO2 can enhance the visible light absorption. Under visible light irradiation, electrons in the CB of anatase, TiO2 are transferred to the CB of TiO2. Electrons in the CB can react with dissolved oxygen and generate superoxide radicals (·O2−), and holes in the VB can oxidize surface OH− or adsorb water to generate hydroxyl radicals (·OH), which can also RhB adsorbed on the oxidized surface. ·OH, ·O2−, and h+ of highly active substances have strong oxidative ability for the degradation of organic pollutants.Figure 11
Photocatalytic degradation mechanism of rhodamine B by F-TiO2−x.


## 4. Conclusions

In this paper, F-TiO2−x photocatalytic material was prepared by hydrothermal method. We have investigated the effect of the degree of fluorination on the photocatalytic properties of the material, and the recycling stability and applicability of the prepared materials under the optimal degree of fluorination. We also explored the types of active species in the process of photocatalytic degradation of materials, and studied the mechanism of photocatalytic degradation of RhB. The results showed that the fluorine element etched the edge part of TiO2−x, which promoted the self-assembly of small TiO2−x particles and gradually formed large particles. At the same time, fluorine element can be effective for stabilizing Ti3+ and oxygen vacancies on the surface of TiO2−x. With the increase in the amount of HF added, the photocatalytic performance of the prepared material is first increased and then decreased. The high-energy surface (001) was successfully exposed when the material prepared with the addition of 0.3 mL of HF. The F-TiO2−x catalysts still showed great differences in the degradation performance of different organic dyes. Among them, the degradation effect of RhB is the best, followed by MO and MB, which indicates that the catalyst has a relatively wide range of applications.



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