1 Introduction

Environmental pollution has aroused global concern and measures to mitigate related problems are extensively investigated. The phenomenon that irradiated TiO2 electrode could decomposed water was first found by Fujishima [1]. Subsequently, the applications of TiO2 to waste water treatment and air pollution control have been extensively investigated [2–4]. TiO2 exhibits the strong redox ability that can convert pollutants into CO2, H2O, and other small molecules [5–7]. Studies on increasing the anatase content, which is the most active phase of TiO2 in photocatalysis, have been performed. However, the low catalytic activity and difficulties associated with the efficient utilization of visible light and recycling have seriously hampered large-scale application of TiO2 as a photocatalyst.

In order to improve the photocatalytic activity under visible light, TiO2 nanoparticles are doped with various elements. The band gap which is responsible for the photo-response can be narrowed by doping with nonmetals such as N, C, F, S, etc. [8–11]. The separation of photo excited electrons and holes which play important roles in the photocatalytic activity can be enhanced by doping with metallic elements such as Cr, V, Fe, and so on [12–14]. However, TiO2 doped by a single element has not been found to meet practical requirements and co-doping with different elements may lead to better synergistic effects. Co-doped TiO2 materials include S–N-co-doped TiO2 [15], F–N-co-doped TiO2 [16–19], B–N-co-doped TiO2 [20], C–N-co-doped TiO2 [21], La-F co-doped TiO2 [22] and C–N-F co-doped TiO2 powders [23]. Not only can the optical absorption efficiency be improved, but also the physical origin of the photocatalytic activity improvement has been clarified. For example, N–F co-doped TiO2 has a smaller number of oxygen defects than the N-doped materials and this is probably the reason for the better photostability and photocatalytic activity.

Co-doping does not, however, guarantee success and the proper choice of dopants is crucial. In this work, a series of TiO2 samples co-doped with F and Fe were prepared through peroxidation at low temperature. The photocatalytic properties of the catalysts were determined by monitoring the degradation of methylene blue (ML) and the results show that compared to the undoped and singly-doped TiO2, the photocatalytic effect of the co-doped TiO2 is the highest due to the synergistic effects rendered by dual element doping.

2 Experimental details

2.1 Materials

Titanium tetrachloride (TiCl4) is industrial grade and ammonia (NH3·H2O), ammonium fluoride (NH4F) and hydrogen peroxide (H2O2) are chemically pure. methylene blue (ML) was analytical and purchased from Beijing co., Ltd.

2.2 Preparation

The F-doped, Fe-doped and F–Fe-co-doped TiO2 materials were prepared by a stepwise sol–gel reaction and hydrogen peroxide oxidation [24]. First of all, 0.3–0.6 mol/L titanium tetrachloride solutions were prepared and then appropriate amounts of the raw material containing the doping elements (NH4F or FeCl3 or NH4F and FeCl3) were added to produce the solutions which had mass ratios of F− to Ti4+, Fe3+ to Ti4+, and (F− + Fe3+) to Ti4+ to be 0.15, 0.15 and 0.30%, respectively. The composition of the samples is summarized in Table 1. The system was kept for 24 h without stirring and then an ammonia solution was added slowly to obtain a pH value of 10. The precipitate formed was washed with distilled water 6 to 8 times to remove impurities. Subsequently, distilled water was used to dilute the slurry to yield a suspension (about 10 g of sediment in 0.1 L of water) and saturated hydrogen peroxide was dropped using a drip funnel until an orange-red transparent solution was formed. The solution was heated to 80–100 °C for 8 h to form the sol. The TiO2 and doped TiO2 were used or freeze dried to produce powders of TiO2 and doped TiO2 for further studies.

Table 1 Composition and adsorption edge of TiO2 powders
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2.3 Characterization

X-ray diffraction (XRD) was performed on a D/max-rA 12kw diffractometer (Beijing) using the Cu Kα (λ = 1.5406A˚) radiation to determine the crystal structure of the TiO2 and doped TiO2 powders. The UV–visible diffuse reflection spectra were acquired on a UV–visible spectrophotometer (UV-λ900LMADDA, USA) with a light path length of 1 cm. The morphology of the TiO2 and doped TiO2 sol was observed by a transmission electron microscope (TEM; JEM-100CX, JEOL) at an acceleration voltage of 200 kV.

The photocatalytic activity of the samples was evaluated by monitoring the degradation of ML. A 10 mg/L standard solution of ML was kept in the dark for about 12 h. While magnetically stirred, an appropriate amount of the catalyst was added to produce a suspension. The concentration of TiO2 in the suspension was about 3 wt%. The suspension was irradiated with sun light (intensity of about 25 mW/cm2) for 4 h and then centrifuged at a speed of 3,000 r/min for 15 min to yield a clear solution. The concentration of methylene blue in the solution was determined by the spectrophotometry at 665 nm in order to evaluate the optical catalytic effect. The decolorization rate η was determined in accordance with the photocatalytic activity of TiO2 as shown in the following:

$$ \eta = \left( {A - A_{o} } \right)/A_{o} \times 100\% , $$

where A o is the absorbance of initial methylene blue solution and A is the the absorbance of the methylene blue solution after photocatalytic degradation by TiO2 and doped TiO2.

3 Results and discussion

3.1 XRD

Figure 1 shows the XRD patterns acquired from the pure TiO2, F-TiO2, Fe-TiO2, and F0.15-Fe0.15 co-doped TiO2. The diffraction peak at 25.28° observed from the XRD pattern of the pure TiO2 shows that the main crystal phase is anatase, and the peak at 27.4° indicates the presence of the rutile phase. All the peaks in the XRD patterns of the F and Fe singly-doped TiO2 can be designated to the anatase phase (most active phase) without any indication of other crystalline phases such as rutile or dopant related ones. As a variant valence metal cation, Fe ions can react with Ti4+ on the surface of TiO2, and Ti4+ is reduced to Ti3+ which inhibits the transformation of anatase to rutile [25]. Fluorine ions also enhance the crystallization of the anatase phase [26]. It leads to the reduction in the oxygen vacancies on the TiO2 surface and suppresses the crystallization of other phases by adsorbing onto the surface of the TiO2 particles [27]. Fe and F lead to the formation of Fe-TiO2 and F-TiO2 resulting in the formation of the complete anatase phase, but in the XRD pattern of F–Fe co-doped TiO2, the (110) diffraction peak at 27.4 o of the rutile phase is detected again. It may be because the Fe–F co-doped TiO2 reduces the effects of phase transformation inhibition and the rutile phase observed from the Fe–F co-doped TiO2.

Fig. 1
figure 1

XRD patterns of doped TiO2

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The average unit dimension of anatase in all of the samples is almost unchanged. Incorporation of F does not give rise to a significant change in the average unit cell dimension since the bond lengths of Ti–F (2.022 and 2.253A˚) are similar to those of Ti–O bonds (1.940 and 2.000A˚), even though a F atom has a smaller radius (0.57A˚) than oxygen atom (0.60A˚) [28]. Fe3+ (0.64 A˚) has a similar radius as Ti4+ (0.62 A˚) and can penetrate easily into the crystal lattice of the TiO2 [29]. The small doping concentration may also be reason for the insignificant changes in the crystalline structure.

The XRD patterns show that all of the peaks are widened. It may be because the crystalline degree of TiO2 is not very high or the long–term order has been destroyed since the particle size is in nanometer region as determined by TEM.

3.2 TEM

The morphology of the pure TiO2 sol and doped TiO2 sols diluted by distilled water to the detection limit of TEM is depicted in Fig. 2. The particle size of the singly doped TiO2 is about a few nanometer and decreases compared to pure TiO2 (50–70 nm). In comparison, the co-doped TiO2 has almost the same diameter as pure TiO2. The results indicate that the single doping by F or Fe can influence the condensation reaction of hydroxide in the presence of hydrogen peroxide and inhibit the particle size growth. Meanwhile, since the doping reaction takes place in the solution in which the reaction can proceed more easily than in a solid reaction, the presence of fluorine or Fe has a significant effect on the particle morphology. However, the combined effect of F and Fe diminishes the influence of F or Fe on the particle size. These results and the XRD data show that co-doping with Fe and F has no obvious influence on the particle size and crystalline structure compared to doping by Fe or F. In the TEM micrographs, agglomeration is observed due to the high surface energy of TiO2 [23].

Fig. 2
figure 2

TEM images of pure TiO2 and doped TiO2: a pure TiO2, b Fe-TiO2, c F-TiO2, and d F0.15-Fe0.15-TiO2

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3.3 UV–visible diffuse reflectance spectra

The light absorption characteristics of TiO2 is changed after doping and the UV–vis diffuse reflectance spectra are usually used to investigate the band structure of TiO2 [30]. As shown in Fig. 3, compared to pure TiO2, the absorption spectra of the doped TiO2 expand into the visible region. The adsorption edge and red shifts are summarized in Table 1.

Fig. 3
figure 3

UV-visible diffuse reflectance spectra of TiO2 powders: a pure TiO2, b F-TiO2, c Fe-TiO2, d F0.15-Fe0.15- TiO2

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The absorption edge of the F doped TiO2 does not display an appreciable shift to the visible region, indicating that F doping does not cause an absorption edge red shift which is consistent with the report by Huaming Yang [24]. When the concentration of Fe is 0.15%, the adsorption edge of the Fe-TiO2 red shifts to about 478 nm. With F and Fe co-doping, further red shift can be observed from the spectra. According to quantum effect, the absorption edge should blue shift with the particle size, but co-doping with F and Fe results in a red shift in the adsorption edge [31]. The UV–vis diffuse reflectance data show that the influence of Fe on the adsorption edge shift is larger than that rendered by F. As a result of the combined effects of F and Fe, the adsorption edge of the co-doped TiO2 shifts from 386 nm to 510 nm and the co-doped TiO2 shows the largest magnitude.

3.4 Photocatalytic properties

Figure 4 shows the photocatalytic degradation of MB in the presence of the pure TiO2, F-TiO2, Fe-TiO2, and F–Fe co-doped TiO2 samples. After irradiating with visible light for 4 h, the degradation rate of MB catalyzed by pure TiO2 is about 43%, whereas that catalyzed by TiO2 doped by Fe or F increases to 68 and 60%, respectively. The enhanced photocatalytic activity of the F-doped TiO2 can be mainly attributed to the larger area on the active site of the TiO2 [24]. The enhanced photocatalytic activity results from the crystal distortion when Fe3+ partly substitutes for Ti4+ in the TiO2 lattice, and the absorption edge of TiO2 red shifts to a longer wavelength. Hence, more carriers are generated under visible light irradiation thereby increasing the photocatalytic efficiency [14]. A 76% degradation rate, the highest of all samples, is observed from the F–Fe co-doped-TiO2. The phenomena can be attributed to the incorporation of metal ions which not only inhibit complexing of the non-metallic photo-carriers but also improve the mutual effects of the photocatalytic properties.

Fig. 4
figure 4

Photoactivity of TiO2 sols

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When the content of F and Fe in the system is constant, a decreased degradation rate is observed with different ratios of F to Fe. Fe as an electron donor plays an important role in the doped materials. With increasing Fe concentration, the absorption edge of the TiO2 red shifts to a longer wavelength and more TiO2 are thus active under visible light irradiation naturally enhancing the photocatalytic efficiency [32]. However, Fe may also act as the recombination centers for the photogenerated carriers and so the Fe exceeds a certain concentration (0.15%), the photocatalytic efficacy is reduced. Based on our experiments, the best doping ratio is F:Fe:Ti = 0.15:0.15:100.

4 Conclusion

F–Fe co-doped TiO2 sols are prepared by peroxidation using FeCl4 and NH4F as the doping materials at low temperature. The structure, morphology, light adsorption, and catalytic efficiency (monitored by ML degradation) are determined. The results show that the crystal structure of F–Fe co-doped TiO2 contains both the anatase and rutile phases and are different to the phases observed from singly-doped TiO2. F or Fe doping inhibits size growth of TiO2 but this phenomenon is not observed from the co-doped TiO2. The photocatalytic efficiency of the F–Fe co-doped TiO2 increases to 76% (the highest of all samples) since the light adsorption edge red shifts to 510 nm. The best doping ratio is found to be F:Fe:Ti = 0.15:0.15:100.