Tridoped TiO 2 Composite Films for Improved Photocatalytic Activities

The Fe/B/F tridoped TiO2-ZnO composite films attached to glass substrates were prepared via a simple sol–gel method. We appraised all samples’ photocatalytic activities by the degradation of methyl green and formaldehyde solutions. The samples were characterized by photoluminescence (PL) spectra, UV-Vis diffraction reflectance absorption spectra (DRS), X-ray diffraction (XRD), differential thermal analysis-thermogravimetry (DTA-TG), field emission scanning electron microscopy (FE-SEM) equipped with energy-dispersive spectroscopy (EDS), and Brunner– Emmet–Teller (BET) measurements. According to the results of DRS and PL spectroscopy, the multi-modification could not only enhance visible light absorption intensity, but also decrease the recombination rate of photo-generated electron-hole pairs. XRD results revealed that the sample was mainly in anatase crystal type. FE-SEM results shown that the sample had fewer particle aggregates and almost no cracks. The specific surface area of the Fe/B/F tridoped TiO2-ZnO was 104.9 m2·g−1, while that of the pure TiO2 was 84.0 m2·g−1. Compared to pure TiO2 and TiO2-ZnO, the Fe/B/F tridoped TiO2-ZnO composite film had the highest photocatalytic activity due to their synergies.


Introduction
In recent decades, people's demand for clean water is growing because of the development of industry and the growth of population [1].In order to ensure that we get enough fresh water and avoid harming our health with waste water, it is essential to treat and reuse the sewage [2].Control and treatment of water contamination has now attracted researchers' attention [3].Because traditional treatment processes of sewage can cause huge energy consumption, high cost, and secondary pollution [4], many researchers seek to develop new technologies for waste water treatment.Since TiO 2 was used to decompose water under ultraviolet light by Fujishima and Honda in 1972, photocatalysts represented by titanium dioxide have been widely used for aqueous organic pollutants treatment due to excellent chemical inertness, strong oxidizing power, low cost and long-term stability to light and chemical corrosion [5,6].Titanium dioxide has three common crystalline forms: anatase, brookite, and rutile.The anatasetitanium TiO 2 shows the best photocatalytic performance among the three crystalline types [7].Meanwhile, anatase/brookite biphase TiO 2 can further enhance the photocatalytic perforence by the heterojunction [8].When irradiated with light of sufficient energy, TiO 2 can generate photo-generated electrons (e − ) and holes (h + ).The electrons and holes can react with water and oxygen in the air to engender hydroxyl radicals and superoxide anions.Organic pollutants can be completely degraded by reacting with hydroxyl radicals to produce carbon dioxide and water [9].However, the stability of electron-hole pairs is extremely poor, and they have an exceedingly apparent recombination tend, thus greatly reducing the photocatalytic degradation efficiency [10].Moreover, TiO 2 can be only irradiated by ultraviolet light due to its high band gap energy (3.2 eV for anatase) [11,12].It is well known that ultraviolet light accounts for a small part (<5.0%) of solar light, and most of the visible light cannot be utilized by TiO 2 .In order to expand the optical response to visible region of the TiO 2 and increase the effective separation of electron-hole pairs, many ways can be used to modify TiO 2 , such as semiconductors composite and ions doping [13,14].
ZnO is also an excellent semiconductor photocatalyst.It has been shown that coupling with ZnO can increase the catalytic activity of TiO 2 [15,16].The increase of photocatalytic activity is due to the doping level in the composites.The electrons on conduction band of ZnO are easily transferred to the conduction band of TiO 2 .On the contrary, the holes on the valence band of TiO 2 are transferred to the valence band of ZnO.This process leads to a decrease in the recombination rate of electron-hole pairs generated by radiation, thus increasing the activity of TiO 2 [17].
Ions doping are divided into metal ions doping and non-metal ions doping.Iron ion doping is confirmed to effectively increase the photocatalytic activity of titanium dioxide.Fe has a relatively low Fermi level, which promotes the migration of photogenerated electrons from TiO 2 to Fe.That is to say, Fe can act as electron trapping agents.This process can decrease the recombination rate of photogenerated electron-hole pairs [18].It is widely accepted that B can be doped into the lattice gap of TiO 2 to capture photogenerated electrons, thus improving the lifetime of photogenerated electrons and holes [19].According to Yu et al., F doping may promote converting Ti 4+ to Ti 3+ to trap the photogenerated electrons and transfer to O 2 2− adsorbed on the surface of TiO 2 , thus enhancing the separation of photogenerated electrons and holes [20].The iron dopant levels are below the conduction band edge of TiO 2 , while boron or fluorine dopant levels are above the valence band of TiO 2 [21], so the co-doping of metal and non-metal may effectively reduce the band gap energy and enhance the absorption of visible light.
In this work, we prepared a Fe/B/F tridoped TiO 2 -ZnO thin film and examined its photocatalytic activity by degradation of methyl green and formaldehyde.

Film Preparation
The chemicals involved in the experiment were of analytical grade (Sinopharm, Beijing, China).Pure and modified TiO 2 films were obtained via a simple sol-gel method.Ti(C 4 H 9 O) 4 and Zn(CH 3 COO) 2 •2H 2 O) were used as TiO 2 and ZnO precursor respectively [22].Fe(NO 3 ) 3 •9H 2 O, H 3 BO 3 , and NH 4 F served as Fe, B, and F dopants source separately.We mingled 1 mL Ti(C 4 H 9 O) 4 to 20 mL absolute ethanol, and then added dropwise 20 mL 0.2 mol/L HNO 3 to the mixture with stirring for 30 min.TiO 2 sol could be obtained after aging for two days of the mixed solution.In order to prepare the ZnO sol, we first dissolved 0.0988 g Zn(CH 3 COO) 2 •2H 2 O in 90 mL absolute ethanol to obtain A solution.Then, we prepared 0.02 mol/L NaOH ethanol solution (B solution) by dissolving 0.008 g NaOH in 10 mL absolute ethanol.Lastly, in a volume ratio of 1/9, we added B solution to A solution with stirring for half an hour [23].After three days aging, ZnO sol would be obtained.TiO 2 and ZnO sols were mixed in optimal volume ratio of 2/1 and aging for 12 h to obtain the TiO 2 -ZnO composite sols.The glass substrates (square substrate with a side length of 25 mm and thickness about 1 mm thick), after being rinsed with chromic acid, were cleaned with deionized water and ethanol.Pure TiO 2 and TiO 2 -ZnO sols were deposited onto the above glass substrates via a dip-coating process to prepare pure TiO 2 and TiO 2 -ZnO films.0.05 mL 7 × 10 −3 mol/L Fe(NO 3 ) 3 , 0.05 mL 1.5 × 10 −2 mol/L H 3 BO 3 , and 0.05 mL 7 × 10 −3 mol/L NH 4 F solutions were doped into the surface of the TiO 2 -ZnO films to obtain the ions doping TiO 2 -ZnO films.Finally, we baked the sample in a muffle furnace at 450 • C for 60 min for better crystallization [24].In our experiments, the thickness of the film ranges from 0.5 micron to 1 micron.

Catalyst Characterization
The diffraction reflectance absorbance spectra (DRS) of the samples was measured by a UV-Vis spectrophotometer equipped with an integrating sphere accessory (TU-1901, Beijing general instrument co.LTD, Beijing, China).The X-ray diffraction with a diffractometer was used to characterize the crystalline structure of the samples (AXS, Bruker, Karlsruhe, Germany).The recombination of e − /h + pairs was measured via the photoluminescence (PL) spectrum recorded by a spectrometer (FLS920, Edinburgh Instruments, Edinburgh, UK).The nitrogen adsorption-desorption isotherm of the sample was obtained using the apparatus to analyze the Brunner-Emmet-Teller (BET) surface area (ASAP2020, Microme, Atlanta, GA, USA).The differential thermal analysis-thermogravimetry (DTA-TG) curves of the samples were used to investigate the crystallization behaviors with temperature (HCT-1, Beijing hengjiu experimental equipment co.LTD, Beijing, China).The field emission scanning electron microscopy (FE-SEM) was used to observe the surface morphology of the samples (SUPRA 55, ZEISS, Oberkochen, Germany).

Catalyst Test
The photocatalytic activity of pure TiO 2 or modified TiO 2 film was evaluated by the decomposition of methyl green or formaldehyde in solutions with initial concentration of 64 and 5 mg/L respectively.Each photocatalyst was added to 5 mL of methyl green or formaldehyde solution in an uncovered weighing bottle.In order to reach an adsorption/desorption equilibrium, the samples were settled in the dark for 30 min before irradiation.A lamp centered at 365 nm and a UV lamp equipped with UV cut-off filters (λ > 420 nm) were employed as UV light and visible light sources, separately.The degradation rate of the above solutions was measured by a UV-Vis spectrophotometer.Because of its colorlessness, it is difficult to determine formaldehyde directly.Therefore, acetyacetone spectrophotometry can be used in our experiments.

Photocatalytic Activity
Methyl green (MG) is a common organic dye.The industrial wastewater containing MG is discharged into the natural environment in most cases.In view of its carcinogenicity, the removal of methyl green from wastewater as much as possible is a prerequisite.The UV-Vis absorption spectra (a) and the degradation percentage (b) of the methyl green solutions employing pure TiO 2 , TiO 2 -ZnO, Fe doped TiO 2 -ZnO, Fe/B didoped TiO 2 -ZnO, and Fe/B/F tridoped TiO 2 -ZnO films under the irradiation of UV-lamp centered at 365 nm for 45 min are shown in Figure 1.From Figure 1b, the order of the degradation percentage using a pure TiO 2 or a modified TiO 2 was as follows: Fe/B/F tridoped TiO 2 -ZnO (47.9%) > Fe/B didoped TiO 2 -ZnO (43.5%) > Fe doped TiO 2 (39.1%) > TiO 2 -ZnO (33.6%) > pure TiO 2 (30.3%).Obviously, the degradation rate of the Fe/B/F tridoped TiO 2 -ZnO film was the highest among all the samples.In other words, the Fe/B/F tridoped TiO 2 -ZnO film has the best photocatalytic activity in these samples.
The photocatalytic performance of the catalyst was further evaluated by the degradation rate of the methyl green solution and formaldehyde solution under visible light irradiation.The decomposition kinetics of the methyl green and formaldehyde solutions using pure TiO 2 , TiO 2 -ZnO, and Fe/B/F tridoped TiO 2 -ZnO films under visible irradiation for 120 min were illustrated in Figure 2a,b, respectively.As depicted in Figure 2a, the degradation percentage of the methyl green solutions utilizing the Fe/B/F tridoped TiO 2 -ZnO film was 62.4, exceeding that of pure TiO 2 film (27.1) and TiO 2 -ZnO film (38.9). Figure 2b demonstrated that the degradation percentages of the formaldehyde solutions using pure TiO 2 , TiO 2 -ZnO, and Fe/B/F tridoped TiO 2 -ZnO films were 27.3, 31.8, and 52.9, separately.That is to say, organic dyes and volatile organic pollutants can be effectively degraded by Fe/B/F tridoped TiO 2 -ZnO photocatalyst.
irradiation of UV-lamp centered at 365 nm for 45 min are shown in Figure 1.From Figure 1b, the order of the degradation percentage using a pure TiO2 or a modified TiO2 was as follows: Fe/B/F tridoped TiO2-ZnO (47.9%) > Fe/B didoped TiO2-ZnO (43.5%) > Fe doped TiO2 (39.1%) > TiO2-ZnO (33.6%) > pure TiO2 (30.3%).Obviously, the degradation rate of the Fe/B/F tridoped TiO2-ZnO film was the highest among all the samples.In other words, the Fe/B/F tridoped TiO2-ZnO film has the best photocatalytic activity in these samples.The photocatalytic performance of the catalyst was further evaluated by the degradation rate of the methyl green solution and formaldehyde solution under visible light irradiation.The decomposition kinetics of the methyl green and formaldehyde solutions using pure TiO2, TiO2-ZnO, and Fe/B/F tridoped TiO2-ZnO films under visible irradiation for 120 min were illustrated in Figure 2a,b, respectively.As depicted in Figure 2a, the degradation percentage of the methyl green solutions utilizing the Fe/B/F tridoped TiO2-ZnO film was 62.4, exceeding that of pure TiO2 film (27.1) and TiO2-ZnO film (38.9). Figure 2b demonstrated that the degradation percentages of the formaldehyde solutions using pure TiO2, TiO2-ZnO, and Fe/B/F tridoped TiO2-ZnO films were 27.3, 31.8, and 52.9, separately.That is to say, organic dyes and volatile organic pollutants can be effectively degraded by Fe/B/F tridoped TiO2-ZnO photocatalyst.
The composition of the Fe/B/F tridoped TiO2-ZnO film by EDS is shown in Figure 3.The atomic weight concentrations of Fe, B, F, Zn, C, O, and Ti in the film were 1.29, 1.54, 0.29, 0.98, 11.45, 47.16, and 37.29, separately.C atoms may come from residual organic matter.

Optical Absorption
The UV-Vis absorption spectra of TiO2, TiO2-ZnO, and Fe/B/F tridoped TiO2-ZnO films are illustrated in Figure 4.The absorption edge of the Fe/B/F tri-doped TiO2-ZnO film had a significant red shift, namely, the photons with lower energy can also participate in the photocatalytic reaction, thus increasing the photocatalytic activity of the catalyst.The photocatalytic performance of the catalyst was further evaluated by the degradation rate of the methyl green solution and formaldehyde solution under visible light irradiation.The decomposition kinetics of the methyl green and formaldehyde solutions using pure TiO2, TiO2-ZnO, and Fe/B/F tridoped TiO2-ZnO films under visible irradiation for 120 min were illustrated in Figure 2a,b, respectively.As depicted in Figure 2a, the degradation percentage of the methyl green solutions utilizing the Fe/B/F tridoped TiO2-ZnO film was 62.4, exceeding that of pure TiO2 film (27.1) and TiO2-ZnO film (38.9). Figure 2b demonstrated that the degradation percentages of the formaldehyde solutions using pure TiO2, TiO2-ZnO, and Fe/B/F tridoped TiO2-ZnO films were 27.3, 31.8, and 52.9, separately.That is to say, organic dyes and volatile organic pollutants can be effectively degraded by Fe/B/F tridoped TiO2-ZnO photocatalyst.
The composition of the Fe/B/F tridoped TiO2-ZnO film by EDS is shown in Figure 3.The atomic weight concentrations of Fe, B, F, Zn, C, O, and Ti in the film were 1.29, 1.54, 0.29, 0.98, 11.45, 47.16, and 37.29, separately.C atoms may come from residual organic matter.

Optical Absorption
The UV-Vis absorption spectra of TiO2, TiO2-ZnO, and Fe/B/F tridoped TiO2-ZnO films are illustrated in Figure 4.The absorption edge of the Fe/B/F tri-doped TiO2-ZnO film had a significant red shift, namely, the photons with lower energy can also participate in the photocatalytic reaction, thus increasing the photocatalytic activity of the catalyst.Coatings 2019, 9, 127 5 of 10

Optical Absorption
The UV-Vis absorption spectra of TiO 2 , TiO 2 -ZnO, and Fe/B/F tridoped TiO 2 -ZnO films are illustrated in Figure 4.The absorption edge of the Fe/B/F tri-doped TiO 2 -ZnO film had a significant red shift, namely, the photons with lower energy can also participate in the photocatalytic reaction, thus increasing the photocatalytic activity of the catalyst.

Photoluminescence (PL) Analysis
The photogenerated electron-hole pair's recombination rate of pure TiO2, TiO2-ZnO, and Fe/B/F tridoped TiO2-ZnO films can be evaluated via PL spectrum.As shown in Figure 5, although the samples had different fluorescence luminescence intensities, they had the same peak position in 370 nm.Among the three samples, the Fe/B/F tridoped TiO2-ZnO film had the lowest emission peak intensity, namely, it had the lowest recombination rate of the photogenerated electron-hole pairs.The decreasing recombination rate of electron-hole pairs can enhance the photocatalytic activity of the sample.

Crystal Structure
The XRD patterns of pure TiO2, TiO2-ZnO, and Fe/B/F tridoped TiO2-ZnO films heated at 450 °C for 60 min are demonstrated in Figure 6.The samples exhibited a mixture of anatase (25.32, 37.52, 48.36), brookite (30.24), and rutile (27.48, 36.11), but the anatase phase is dominant.Compared to pure TiO2 and TiO2-ZnO, the diffraction peak of the Fe/B/F tridoped TiO2-ZnO sample was wider and weaker, indicating that the crystal size was smaller.According to the Scherrer formula: d = 0.9 λ/β cos θ, the average size of Fe/B/F tridoped TiO2-ZnO (5.6 nm) was smaller than that of pure TiO2 (9.2 nm) and TiO2-ZnO (6.8 nm).On account of the small doping amount and the even distributing of Fe, B, and F ions, there were no obvious compound peaks associated with them.In addition, the content of ZnO in the TiO2-ZnO composite film was extremely small or the peak intensity of TiO2 was too strong to cover the weak peak of ZnO, so the diffraction peak of ZnO was not also observed.Catalysts

Photoluminescence (PL) Analysis
The photogenerated electron-hole pair's recombination rate of pure TiO 2 , TiO 2 -ZnO, and Fe/B/F tridoped TiO 2 -ZnO films can be evaluated via PL spectrum.As shown in Figure 5, although the samples had different fluorescence luminescence intensities, they had the same peak position in 370 nm.Among the three samples, the Fe/B/F tridoped TiO 2 -ZnO film had the lowest emission peak intensity, namely, it had the lowest recombination rate of the photogenerated electron-hole pairs.The decreasing recombination rate of electron-hole pairs can enhance the photocatalytic activity of the sample.

Photoluminescence (PL) Analysis
The photogenerated electron-hole pair's recombination rate of pure TiO2, TiO2-ZnO, and Fe/B/F tridoped TiO2-ZnO films can be evaluated via PL spectrum.As shown in Figure 5, although the samples had different fluorescence luminescence intensities, they had the same peak position in 370 nm.Among the three samples, the Fe/B/F tridoped TiO2-ZnO film had the lowest emission peak intensity, namely, it had the lowest recombination rate of the photogenerated electron-hole pairs.The decreasing recombination rate of electron-hole pairs can enhance the photocatalytic activity of the sample.

Crystal Structure
The XRD patterns of pure TiO2, TiO2-ZnO, and Fe/B/F tridoped TiO2-ZnO films heated at 450 °C for 60 min are demonstrated in Figure 6.The samples exhibited a mixture of anatase (25.32, 37.52, 48.36), brookite (30.24), and rutile (27.48, 36.11), but the anatase phase is dominant.Compared to pure TiO2 and TiO2-ZnO, the diffraction peak of the Fe/B/F tridoped TiO2-ZnO sample was wider and weaker, indicating that the crystal size was smaller.According to the Scherrer formula: d = 0.9 λ/β cos θ, the average size of Fe/B/F tridoped TiO2-ZnO (5.6 nm) was smaller than that of pure TiO2 (9.2 nm) and TiO2-ZnO (6.8 nm).On account of the small doping amount and the even distributing of Fe, B, and F ions, there were no obvious compound peaks associated with them.In addition, the content of ZnO in the TiO2-ZnO composite film was extremely small or the peak intensity of TiO2 was too strong

Crystal Structure
The XRD patterns of pure TiO 2 , TiO 2 -ZnO, and Fe/B/F tridoped TiO 2 -ZnO films heated at 450 • C for 60 min are demonstrated in Figure 6.The samples exhibited a mixture of anatase (25.32, 37.52, 48.36), brookite (30.24), and rutile (27.48, 36.11), but the anatase phase is dominant.Compared to pure TiO 2 and TiO 2 -ZnO, the diffraction peak of the Fe/B/F tridoped TiO 2 -ZnO sample was wider and weaker, indicating that the crystal size was smaller.According to the Scherrer formula: d = 0.9 λ/β cos θ, the average size of Fe/B/F tridoped TiO 2 -ZnO (5.6 nm) was smaller than that of pure TiO 2 (9.2 nm) and TiO 2 -ZnO (6.8 nm).On account of the small doping amount and the even distributing of Fe, B, and F ions, there were no obvious compound peaks associated with them.In addition, the content of ZnO in the TiO 2 -ZnO composite film was extremely small or the peak intensity of TiO 2 was too strong to cover the weak peak of ZnO, so the diffraction peak of ZnO was not also observed.Catalysts composed of smaller particles or crystal sizes are more effective in degrading organic pollutants.

Thermal Analysis
The DTA-TG curves of TiO2 (a), TiO2-ZnO (b), and Fe/B/F tridoped TiO2-ZnO powders are shown in Figure 7. From TG curves in Figure 7, the curves tended to be straight lines after 450 °C and the weight of the sample had almost no change, indicating that the organic matter in the sample had been completely burned [25].According to DTA curves in Figure 7a, there was an absorption peak at a temperature of 106.2 °C due to the loss of water.An exothermic peak at 281.7 °C was owing to the burning of organic matter [26].At the temperature of 351.4 °C, an absorption peak was due to form anatase and a small amount of brookite.The exothermic peak at 431.8 °C was caused by the conversion of brookite to anatase.The exothermic peak at the temperature of 523.3 °C, the anatase of TiO2 converted to rutile because of anatase's unstable at high temperatures.By comparing the DTA curves in Figure 7a-c, we found that they were similar in shape, but the temperature of crystal transition was slightly different, which was caused by ZnO compositing and Fe/B/F tridoping.

Thermal Analysis
The DTA-TG curves of TiO 2 (a), TiO 2 -ZnO (b), and Fe/B/F tridoped TiO 2 -ZnO powders are shown in Figure 7. From TG curves in Figure 7, the curves tended to be straight lines after 450 • C and the weight of the sample had almost no change, indicating that the organic matter in the sample had been completely burned [25].According to DTA curves in Figure 7a, there was an absorption peak at a temperature of 106.2 • C due to the loss of water.An exothermic peak at 281.7 • C was owing to the burning of organic matter [26].At the temperature of 351.4 • C, an absorption peak was due to form anatase and a small amount of brookite.The exothermic peak at 431.8 • C was caused by the conversion of brookite to anatase.The exothermic peak at the temperature of 523.3 • C, the anatase of TiO 2 converted to rutile because of anatase's unstable at high temperatures.By comparing the DTA curves in Figure 7a-c, we found that they were similar in shape, but the temperature of crystal transition was slightly different, which was caused by ZnO compositing and Fe/B/F tridoping.

Thermal Analysis
The DTA-TG curves of TiO2 (a), TiO2-ZnO (b), and Fe/B/F tridoped TiO2-ZnO powders are shown in Figure 7. From TG curves in Figure 7, the curves tended to be straight lines after 450 °C and the weight of the sample had almost no change, indicating that the organic matter in the sample had been completely burned [25].According to DTA curves in Figure 7a, there was an absorption peak at a temperature of 106.2 °C due to the loss of water.An exothermic peak at 281.7 °C was owing to the burning of organic matter [26].At the temperature of 351.4 °C, an absorption peak was due to form anatase and a small amount of brookite.The exothermic peak at 431.8 °C was caused by the conversion of brookite to anatase.The exothermic peak at the temperature of 523.3 °C, the anatase of TiO2 converted to rutile because of anatase's unstable at high temperatures.By comparing the DTA curves in Figure 7a-c, we found that they were similar in shape, but the temperature of crystal transition was slightly different, which was caused by ZnO compositing and Fe/B/F tridoping.

Surface Areas
Figure 8 illustrates the N2 adsorption/desorption isotherms (a) and pore size distribution (b) of pure TiO2 and Fe/B/F tridoped TiO2-ZnO powders heated at 450 °C for 60 min.As demonstrated in Figure 8a, the specific surface area of Fe/B/F tridoped TiO2-ZnO was 105.0 m 2 /g, while that of pure TiO2 was 84.0 m 2 /g at the same pressure.Adding Fe/B/F ions and ZnO to TiO2 can reduce particle size, thereby enhancing the specific surface area of TiO2.From Figure 8b, the mean pore size of pure TiO2 was 6.0 nm, while that of Fe/B/F tridoped TiO2-ZnO was 5.2 nm, which was caused by Fe/B/F or ZnO inserted into the pores of TiO2.Smaller pore size and larger specific surface area can facilitate the adsorption of O2, H2O and contaminants, thereby enhancing the photocatalytic performance of TiO2.

Surface Morphology
The FE-SEM micrographs of pure TiO2 and Fe/B/F tridoped TiO2-ZnO films are shown in Figure 9.As can be observed in Figure 9a, more particle aggregates with a non-uniform distribution and cracks appeared on the surface of pure TiO2.However, as shown in Figure 9b, by ZnO compositing and Fe/B/F tridoping, less particle aggregates and almost no cracks on the surface of TiO2 were observed.The uniform dispersion of the particles can increase the contact area of the active reactants, thereby increasing the photocatalytic activity of the catalyst.

Surface Areas
Figure 8 illustrates the N 2 adsorption/desorption isotherms (a) and pore size distribution (b) of pure TiO 2 and Fe/B/F tridoped TiO 2 -ZnO powders heated at 450 • C for 60 min.As demonstrated in Figure 8a, the specific surface area of Fe/B/F tridoped TiO 2 -ZnO was 105.0 m 2 /g, while that of pure TiO 2 was 84.0 m 2 /g at the same pressure.Adding Fe/B/F ions and ZnO to TiO 2 can reduce particle size, thereby enhancing the specific surface area of TiO 2 .From Figure 8b, the mean pore size of pure TiO 2 was 6.0 nm, while that of Fe/B/F tridoped TiO 2 -ZnO was 5.2 nm, which was caused by Fe/B/F or ZnO inserted into the pores of TiO 2 .Smaller pore size and larger specific surface area can facilitate the adsorption of O 2 , H 2 O and contaminants, thereby enhancing the photocatalytic performance of TiO 2 .

Surface Areas
Figure 8 illustrates the N2 adsorption/desorption isotherms (a) and pore size distribution (b) of pure TiO2 and Fe/B/F tridoped TiO2-ZnO powders heated at 450 °C for 60 min.As demonstrated in Figure 8a, the specific surface area of Fe/B/F tridoped TiO2-ZnO was 105.0 m 2 /g, while that of pure TiO2 was 84.0 m 2 /g at the same pressure.Adding Fe/B/F ions and ZnO to TiO2 can reduce particle size, thereby enhancing the specific surface area of TiO2.From Figure 8b, the mean pore size of pure TiO2 was 6.0 nm, while that of Fe/B/F tridoped TiO2-ZnO was 5.2 nm, which was caused by Fe/B/F or ZnO inserted into the pores of TiO2.Smaller pore size and larger specific surface area can facilitate the adsorption of O2, H2O and contaminants, thereby enhancing the photocatalytic performance of TiO2.

Surface Morphology
The FE-SEM micrographs of pure TiO2 and Fe/B/F tridoped TiO2-ZnO films are shown in Figure 9.As can be observed in Figure 9a, more particle aggregates with a non-uniform distribution and cracks appeared on the surface of pure TiO2.However, as shown in Figure 9b, by ZnO compositing and Fe/B/F tridoping, less particle aggregates and almost no cracks on the surface of TiO2 were observed.The uniform dispersion of the particles can increase the contact area of the active reactants, thereby increasing the photocatalytic activity of the catalyst.

Surface Morphology
The FE-SEM micrographs of pure TiO 2 and Fe/B/F tridoped TiO 2 -ZnO films are shown in Figure 9.As can be observed in Figure 9a, more particle aggregates with a non-uniform distribution and cracks appeared on the surface of pure TiO 2 .However, as shown in Figure 9b, by ZnO compositing and Fe/B/F tridoping, less particle aggregates and almost no cracks on the surface of TiO 2 were observed.The uniform dispersion of the particles can increase the contact area of the active reactants, thereby increasing the photocatalytic activity of the catalyst.In summary, the Fe/B/F tridoped TiO 2 -ZnO film has excellent photocatalytic activity due to its strong visible light absorption, reduced recombination of photogenerated electrons and holes, large specific surface area, and unique microstructure.

Conclusions
Water pollution is an urgent problem to be solved under the current environment.In order to treat organic pollutants in sewage with high efficiency, cleanness, and low energy consumption, we prepared Fe/B/F tridoped TiO 2 -ZnO composite films via a simple sol-gel method.The experimental results showed that the catalysts could significantly increase the degradation rate of organic pollutants under both visible light and ultraviolet irradiation.A small amount of ZnO compositing and a low concentration of Fe/B/F tridoping (wt %: Zn 0.98, Fe 1.29, B 1.54, F 0.29) could greatly improve the photocatalytic activity of TiO 2 .The enhancing in photocatalytic activity is mainly due to the reducing recombination of electron-hole pairs and increasing visible light absorption.In addition, doping defects in the TiO 2 lattice might reduce the band gap energy of the semiconductor.The catalyst may have potential application value in wastewater treatment with organics as the main pollutant.

Figure 1 .Figure 1 .
Figure 1.UV-Vis absorption spectra (a) and degradation percentage (b) of Methyl green solutions using pure TiO2 and modified TiO2 films under UV-lamp irradiation with wavelength of 365 nm for 45 min.

Figure 2 .
Figure 2. Decomposition kinetics of Methyl green (a) and formaldehyde (b) solutions using modified TiO2 films under visible light irradiation for 120 min.

Figure 2 .
Figure 2. Decomposition kinetics of Methyl green (a) and formaldehyde (b) solutions using modified TiO 2 films under visible light irradiation for 120 min.The composition of the Fe/B/F tridoped TiO 2 -ZnO film by EDS is shown in Figure 3.The atomic weight concentrations of Fe, B, F, Zn, C, O, and Ti in the film were 1.29, 1.54, 0.29, 0.98, 11.45, 47.16, and 37.29, separately.C atoms may come from residual organic matter.

Figure 2 .
Figure 2. Decomposition kinetics of Methyl green (a) and formaldehyde (b) solutions using modified TiO2 films under visible light irradiation for 120 min.

Figure 8 .
Figure 8. N 2 adsorption/desorption isotherms (a) and pore size distribution (b) of pure TiO 2 and Fe/B/F tridoped TiO 2 -ZnO powders heated at 450 • C in air for 60 min.