One-step synthesis of OH-TiO2/TiOF2 nanohybrids and their enhanced solar light photocatalytic performance.

TiO2/TiOF2 nanohybrids were quickly synthesized through a hydrothermal process using titanium n-butoxide (TBOT), ethanol (C2H5OH) and hydrofluoric acid as precursors. The prepared nanohybrids underwent additional NaOH treatment (OH-TiO2/TiOF2) to enhance their photocatalytic performance. In this paper, the mechanism of NaOH affecting the pathway of transformation from TBOT (Ti precursor) to TiO2 nanosheets was discussed. The synthesized TiO2/TiOF2 and OH-TiO2/TiOF2 were characterized by field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction pattern (XRD), Fourier infrared spectroscopic analysis (FT-IR), Photoluminescence (PL) emission spectra and UV-visible diffuse reflection spectra (UV-vis DRS). The photocatalytic activity and stability of synthesized samples were evaluated by degradation of methylene blue (MB) under the simulated solar light. The results showed that a larger ratio of TiO2 to TiOF2 in TiO2/TiOF2 and OH-TiO2/TiOF2 nanohybrids could allow for even higher MB conversion compared with only TiO2 nanosheets. NaOH treatment can wash off the F ions from TiOF2 and induce this larger ratio. The highest efficiency of MB removal was just above 90% in 1 h. Lower electron-hole pairs recombination rate is the dominant factor that induces the photocatalytic performance enhancement of TiO2/TiOF2 nanohybrids. The synthesized OH-TiO2/TiOF2 nanohybrids exhibit great potential in the abatement of organic pollutants.

Introduction

As one of the most important materials, TiO2 has been widely used as a promising catalyst due to its lack of toxicity, high stability and easy preparation. However, it has intrinsic faults of a wide energy band gap (3.1–3.2 eV, meaning it only responds to UV light) and high electron–hole recombination, which hinders its use under solar or visible light [1-4]. Many studies examining TiO2 has been devoted to reducing its energy band gap or photoelectron–hole separation [5-11].

Recently, Wen et al. [12] proposed a new visible light-driven TiOF2 photocatalyst for H2 evolution. Furthermore, Wang et al. [13] discovered a type of TiOF2 photocatalyst that possesses proper activity and strong durability in photocatalytic degradation of rhodamine B and 4-chlorophenol under visible light, although its photocatalytic performance is not ideal. Heterostructured photocatalysts have attracted increasing attention during the past few years. The electronic assembling of different nanomaterials possessing dissimilar crystal structure and band edge positions allows the complete utilization of incident photons, while in-built electric fields at the interface assist effective charge carrier separation and induce excellent performance in terms of photocatalytic activity [5-11]. However, only a few investigations on the TiO2/TiOF2 nanocomposites with heterostructures have been performed so far. Zhao et al. [14] reported on a Pd@TiO2/TiOF2 photocatalyst made of TiO2 shell and TiOF2 core (labelled as TiO2/TiOF2) and further improved its performance by loading Pd nanoparticles onto the surfaces of TiO2/TiOF2 heterostructure, although its synthesis process is complex.

Among the typical synthesis methods, titanium (IV) butoxide and hydrofluoric acid (HF) are the most common precursors to provide anatase TiO2 nanosheets with exposed facets [15-22]. In these papers, TiOF2 was sometimes characterized by X-ray diffraction with a peak at 2θ = 23.9°, which occurs independently of the main (101) peak of anatase TiO2 [17-22]. Almost all of these studies noted that TiOF2 is the intermediate compound during the transformation of Ti4+ to titanium nanosheets (TiO2) and aimed to suppress its appearance to obtain pure TiO2 nanosheets [17-22]. A few of these studies paid attention to the photocatalytic activity of TiOF2 or TiO2/TiOF2 systems, which performed more poorly when compared with (001)TiO2 under UV or solar light [20-22]. For example, Lv et al. found that when TiOF2 was calcined at 300°C, anatase TiO2 and TiOF2 were both observed, although the TiOF2 and TiO2/TiOF2 samples showed poor photocatalytic activity compared with (001) TiO2 under UV light for reactive brilliant red X3B [20]. Huang et al. also found that TiOF2 has poor photocatalytic performance, although the performances of TiO2/TiOF2 systems were enhanced with an increased proportion of TiO2 [21]. Yua et al. discussed the appearance of TiOF2 and TiO2 under different F/Ti atomic molar ratios, although they aimed to find the optimal ratios of exposed (101) and (001) facets of TiO2. However, they did not mention the role of TiOF2 in the TiO2/TiOF2 system [22]. Zhang et al. found that the mixture of (001)TiO2 and TiOF2 showed better photocatalytic activity for Rhodamine B (RhB) under simulated sunlight when they studied the reaction products of titanium butoxide and hydrogen fluoride. However, this did not show such good performance on MB for a reaction time as long as 10 h [23].

Alkali modification of catalysts has also been proven to be an effective way to enhance the photoactivity with methane dehydroaromatization, cumene cracking and CO oxidation [23-27]. For example, Han et al. found that alkali modification could form more hydroxyl groups on Au catalysts to enhance their catalytic activity [24]. Alkali-modified ZSM-5 zeolite also showed enhanced catalytic performance due to the formation of additional mesopores and the improvement in mass transfer and reaction kinetics [25,26]. NaOH-modified Pt/TiO2 also showed enhanced performance in terms of the oxidation of formaldehyde under room temperature [27]. These studies have encouraged the modification of TiO2/TiOF2 by NaOH to enhance its catalytic performance.

In this study, TiO2/TiOF2 nanohybrids were synthesized and (001) TiO2 was obtained through NaOH washing to remove the surface fluorine ions. NaOH was also used to modify the TiO2/TiOF2 nanohybrids to enhance their performance. This has not been investigated before. TiO2/TiOF2 nanohybrids even showed superior catalytic photoactivity towards methylene blue (MB) degradation under simulated sunlight for the first time. The possible mechanism of the TiO2/TiOF2 system in enhancing photocatalytic performance was also discussed.

Material and methods

Materials

Tetrabutyl titanite (TBOT) was purchased from Fu Chen Chemical Reagent Factory, Tianjin, China. HF was purchased from Xilong Chemical Industry Co Ltd, Sichuan, China. Sodium hydroxide (NaOH) and ethanol (C2H5OH) were purchased from Fuyu Fine Chemical Co., Ltd. Tianjin, China. Terephthalic acid, potassium iodide and p-benzoquinone were purchased from Shanghai Macklin Biochemical Co., Ltd., Shanghai, China. All reagents were A.R. grade and used without further purification. Ultra-pure water (18.2 MΩ•cm) was used as the water in all experiments.

Synthesis of TiO2/TiOF2 nanohybrids

A total of 15.2 ml of ethanol was added into 17.6 ml of TBOT, which was named solution A. Another 15.2 ml of ethanol and 5 ml of HF were added into 90 ml of ultra-pure water, which was named solution B. Solution A was dropped into solution B under medium-speed magnetic stirring at room temperature for 1.5 h to obtain a faint yellow sol. After this, the sol was transferred into a 200-ml Teflon-lined stainless steel autoclave. The autoclave was placed into an oven, which was maintained at 100°C for 0.5, 1, 1.5 and 2 h before being cooled to room temperature naturally to obtain a white precipitate. Ultra-pure water and C2H5OH were used to wash the precipitates several times to reach a pH of 7, before the precipitates were dried at a temperature of 100°C. The prepared samples were denoted as S0.5, S1, S1.5 and S2. The samples were then dispersed in 80 ml of 5 M NaOH solution for 30 min, before being washed with ultra-pure water and C2H5OH to reach a pH of 7. Finally, these samples were dried at 100°C for 12 h. The prepared samples were denoted as OH-S0.5, OH-S1, OH-S1.5 and OH-S2.

Characterization

The crystal structure was analysed by a XD-2 X-ray diffractometer (Beijing Purkinje, China) with Cu-Kα radiation. The morphology was examined by FE-SEM (JEOL JSM6700, Japan) equipped with EDS to probe elemental analysis and high-resolution transmission electron microscopy (HRTEM; Tecnai G2 F20, FEI, USA) using an accelerating voltage of 200 kV. Specific surface area and porosimetry were measured using Micromeritic TriStar II 3020 micrometrics (Micromeritics, USA), and the Brunauer–Emmett–Teller (BET) method was used to calculate the surface area (SBET). Fourier transform infrared (FT-IR) spectra were recorded using a TENSOR27 (Bruker, Germany). The optical properties were determined by UV–vis diffuse reflectance spectroscopy (UV–vis DRS; Shimadzu 2600, Japan). Photoluminescence (PL) emission spectra were measured at room temperature with a fluorescence spectrophotometer (Hitachi F-2700, Japan) using a 325-nm line with an Xe lamp. X-ray photoelectron spectroscopy (XPS) took place under an ultra-high vacuum (10 Pa) at a pass energy of 100 eV on a Escalab 250 Xi system (ThermoFisher, USA) equipped with a dual X-ray source by using a Al K Alpha anode and a hemispherical energy analyser. All binding energies were calibrated with contaminant carbon (C1 s = 284.6 eV) as a reference.

Photocatalytic experiments

Photocatalytic activity was measured by degradation of MB. A total of 0.015 g of the catalyst was dispersed in a 150-ml double-layered quartz reactor containing 100 ml of a 10.0 mg l−1 MB solution. Cooling water was introduced into the interlayer of the quartz reactor to maintain the solution at room temperature. A Jiguang-300 W Xe lamp (simulating solar light) was located 30 cm away from the MB solution. A JB-420 cutoff filter was chosen to filter off light less than 420 nm to simulate visible light. The solution was magnetically stirred for 0.5 h in the dark to obtain the adsorption–desorption equilibrium, before the Xe lamp was turned on to start the degradation. At time intervals of 0.5 h, about 4.0 ml of the solution was extracted and centrifuged at a speed of 10 000 r.p.m. to remove catalysts. After this, the MB concentration was analysed with a Purkinje UV1901 UV–vis spectrophotometer at 665 nm. The photocatalyst was separated from the MB solution, before another run was started to investigate the durability of catalysts.

Radical-scavenging experiments

Radical-scavenging experiments were performed to ascertain the main active species in the photocatalytic process. Terephthalic acid (3 mmol l−1), potassium iodide (3 mmol l−1) and p-benzoquinone (3 mmol l−1) were added to a mixed solution containing 15 mg of OH-TiO2 and 100 ml of 10 mg l−1 of MB solution, respectively, while MB was degraded as a control.

Results

The formation mechanism of TiOF2, TiO2/TiOF2 and TiO2

The XRD patterns of the prepared samples were obtained under different experimental conditions (figure 1). All S0.5, S1, S1.5 and S2 samples showed diffraction peaks at 23.42° for TiOF2 (100) facet and at 25.3° for TiO2 (101) facet, indicating that the TiOF2 (JCPDS No. 01–0049) and anatase TiO2 (JCPDS No. 21–1272) phases coexist. Therefore, the S0.5, S1 and S1.5 samples are TiO2/TiOF2 nanohybrids. It also can be seen that with a longer reaction time, the ratio of peak height of (100) for TiOF2 and TiO2 for (101) changed from 0.44 to 4.4–4.5 and then to 0.55. This indicates that TiO2 existed at the beginning, before the level decreased and was dominated by other compounds, then appeared and dominated again. This result is consistent with previous research [22]. After being washed with NaOH, the peak at 23.42° for (100) facet of TiOF2 decreased and the peak at 25.3° for TiO2 (101) facet increased, indicating an increase in the TiO2/TiOF2 ratio with a longer reaction time. For OH-S1.5 and OH-S2, the peak at 25.3° for TiO2 (101) facet increased, indicating an increase in TiO2/TiOF2 ratio with a longer reaction time. For OH-S1.5 and OH-S2, the TiOF2 complex disappeared. The (004) facet of TiO2 even emerged, which was named the (001)-faceted TiO2 existence. It seems that OH in NaOH can exchange with F ions in the crystal lattice of TiOF2, converting it into (001)-faceted TiO2 nanosheets [14,18,22].

Figure 1.

XRD patterns of prepared samples of S0.5, S1, S1.5, S2, OH-S0.5, OH-S1, OH-S1.5 and OH-S2.

In summary, the chemical reactions of the formation of titanium nanosheets can be proposed. The first step is the hydrolysis reaction. The reaction of TBOT to form Ti(OH)4 is shown in equation (3.1). Secondly, in equation (3.2), Ti(OH)4 can react with HF to produce TiOF2 through water condensation [28,29]. Finally, TiOF2 can react with NaOH to form TiO2, as shown in equation (3.3) [30].

This study provides a simple conversion method of TiOF2 to TiO2 in addition to the calcination of TiOF2 [20,21].

Morphology analysis

The morphology of the prepared samples was characterized by FE-SEM and high-resolution transmission electron microscope (HRTEM) in figures 2 and 3. The SEM and HRTEM images shown in figure 2 and figure 3a,b, respectively, were used to obtain the morphology of photocatalysts. The S0.5, S1 and S1.5 samples exhibit mixtures of two types of crystals connected together, which correspond to TiOF2 and TiO2 according to the XRD results in figure 1. By contrast, S2 shows a nano-network look, corresponding to TiO2. The OH-S0.5, OH-S1 and OH-S1.5 samples exhibited a nano-network look after reaction with NaOH, permitting light scattering inside the catalyst and enhancing its absorption in light.

Figure 2.

SEM of (a) S0.5, (b) OH-S0.5, (c) S1, (d) OH-S1, (e) S1.5, (f) OH-S1.5, (g) S2 and (h) OH-S2.

Figure 3.

HRTEM and EDS spectra of S0.5 and OH-S0.5 samples: (a) TEM of S0.5, (b) TEM of OH-S0.5, (c) HRTEM of S0.5, (d) HRTEM of OH-S0.5, (e) EDS of OH-S0.5, and (f–h) EDS elemental mapping of OH-S0.5.

As also can be seen in figure 3c,d, the lattice fringes of 0.19 nm, 0.235 nm and 0.352 nm were assigned to the (200) (001) and (101) planes of TiO2, respectively [18,22,30]. The lattice fringes of 0.38 nm were assigned to the (100) planes of TiOF2 [12,13]. This also indicates that the S0.5 and OH-S0.5 samples are nanohybrids of TiO2 and TiOF2, which is consistent with the XRD and SEM results. The EDS spectrum (figure 3e) shows that OH-S0.5 consists of Ti, O and F elements. The uniform distribution of Ti, O and F can be seen from the EDS elemental mapping (figure 3f–h), which suggests the successful formation of a heterojunction.

SBET and pore-size distribution

Figure 4 shows the nitrogen adsorption–desorption isotherms and the corresponding pore-size distribution curves of prepared samples. Considering the BET data in electronic supplementary material, table S1, the SBET and pore volume of S2 can reach as high as 59.3 m2·g−1 with a pore size of 62.5 nm, while the SBET of S0.5 and OH-S0.5 are significantly lower with values of 23.6 and 27.21 m2·g−1, respectively, and pore sizes of 122.6 and 14.49 nm, respectively. After NaOH washing, the SBET of OH-S0.5 was a bit lower than that of S0.5, but the pore size was dramatically decreased.

Figure 4.

(a) Nitrogen adsorption–desorption isotherms and (b) the corresponding pore-size distribution curves of S0.5, OH-S0.5 and OH-S2.

FT-IR analysis

As shown in figure 5, broad absorptions centered around 3418 and 2920 cm−1 as well as the weak sharp absorption band centred around 1624 cm−1 were attributed to the hydroxyl free radicals, associated hydrogen bonds and absorption water. The OH and water in all NaOH-reacted samples are all stronger than those modified before, with the exception of OH-S2. For the OH-S2 sample, the OH from the broad absorptions centred around 3418, 2920 and 1624 cm−1 sharply decreased. According to XRD results, the S2 and NaOH-reacted samples are all TiO2/TiOF2 nanohybrids, except OH-S2. It was concluded that TiO2/TiOF2 nanohybrids can adsorb more OH to enhance photocatalytic performance [31-37].

Figure 5.

FT-IR spectra of S0.5, S1, S1.5, S2, OH-S0.5, OH-S1, OH-S1.5 and OH-S2.

XPS analysis

In figure 6, XPS survey spectra of the prepared samples of S0.5 and OH-S0.5 are presented. As observed in figure 6a, all the samples have sharp photoelectron peaks at binding energies of Ti 2p, O 1s and F 1s. Another sharp photoelectron peak appears in all samples at the binding energy (BE) of 285 eV (C 1s) due to the contamination of the XPS instrument itself [23,38-41]. The F 1s BE of 684.8 eV in this spectrum (figure 6b) corresponds to that of F adsorbed on TiO2, while there was no sign of F ions in the lattice (BE = 688.5 eV) [38,39]. The measured binding energies of Ti 2p3/2 and Ti 2p1/2 (figure 6c) for S0.5 were 459.3 and 465.2 eV, respectively. The main sharp peaks of Ti 2p3/2 were assigned to Ti4+ in TiO2 [23,39-41]. As compared to S0.5, the binding energies of OH-S0.5 shifted to 459.16 and 464.9 eV, respectively. One possible explanation is that TiO2 is partially reduced into Ti3+ [23,41]. As Ti3+is more hydrophilic than Ti4+, oxygen and water molecules are easily adsorbed on the surface of TiO2, which consequently facilitates the formation of surface OH groups. The measured BE of O 1s (figure 6d) was 530.42 eV, while this shifted to 530.36 eV in OH-S0.5. This can be explained by OH-S0.5 possibly having many oxygen vacancies that are generated at the interface of TiO2/TiOF2. Oxygen from the gas phase could dissociate and become adsorbed on such defects, thus resulting in a decrease in binding energies of O 1s to TiO2 lattice oxygen (Ti−O−Ti) [23]. So it can be concluded that NaOH treatment can induce more surface OH groups and oxygen vacancies at the interface of TiO2/TiOF2, which would enhance the photocatalytic performance of TiO2/TiOF2 nanohybrids.

Figure 6

High-resolution XPS survey spectra for (a) all samples, (b) F 1s, (c) Ti 2p and (d) O 1s of the S0.5 and OH-S0.5 samples.

UV–vis DRS analysis

Figure 7 shows the UV–vis absorption spectroscopy and band gap of the prepared TiO2/TiOF2 nanohybrids and TiO2 samples. It can be seen that S2 and OH-S0.5 have a greater increase in adsorption in both the range of UV and visible light compared with other samples. By contrast, they contain more TiO2 compared to S0.5, which corresponds to the XRD results. While the OH-S2 sample has weaker adsorption than S2, it becomes TiO2 after NaOH washing. Therefore, a larger ratio of TiO2/TiOF2 causes the stronger light adsorption. The band gaps of S2, OH-S2 and OH-S0.5 are 2.77, 3.05 and 3.12 eV, respectively, which are lower than that of P25 (3.2 eV). This indicates that they are more easily excited by visible light.

Figure 7.

(a) UV–vis absorption spectroscopy and (b) band gap of prepared TiO2 /TiOF2 nanohybrids and TiO2 samples.

PL analysis

PL emission spectra were used to investigate the efficiency of charge carrier trapping, immigration and transfer as well as to understand the fate of electron hole pairs in catalysts (figure 8). Six peaks were observed in the spectra. The broad emission bands centred at 385.3 nm (peak 1), 399.0 nm (peak 2) and 412.4 nm (peak 3) were ascribed to form the boundaries of exciton emission due to the trapping of free excitons by titanite groups near defects [42]. The long wavelength range of 452.5–468.7 nm (peaks 4 and 5) is attributed to the oxygen vacancy with two trapped electrons. Oxygen vacancy sites are important for the formation of superoxide (O2•−) and hydroxyl (•OH) radicals for photocatalytic degradation. A lower PL intensity also indicates a lower recombination rate of electron–hole pairs and higher separation efficiency, thus representing higher photocatalytic activity [43].

Figure 8.

PL spectra of S0.5, OH-S0.5 and OH-S2 samples.

Catalytic activities of TiO2/TiOF2 and TiO2 photocatalysts

Figure 9a shows the solar light photocatalytic properties of the prepared samples and P25. Figures 9a and S1 showed that with 0.015 g/100 ml of catalysts, the decrease in MB was very small in the first 0.5 h in dark and in light without a catalyst, indicating that this decrease in MB was a photocatalytic process with all samples being activated in solar or visible light. It also shows that the S2 and OH-S0.5 samples can cause almost complete decomposition of MB in about 1.5 h with better photocatalytic performance than all of the other samples. By contrast, P25 performed poorly compared to TiO2/TiOF2 and (001) TiO2. All NaOH-treated nanohybrids showed higher performance than untreated ones except the pure (001)-facet TiO2 from S2.

Figure 9.

Solar light photocatalytic properties of the prepared samples, P25 and UV–vis spectra of MB with irradiation time: (a) comparison of solar light-sensitized degradation of MB in the suspension of samples; (b) the reaction rate of all samples; (c) the effect of different amounts of catalysts on the solar light photocatalytic properties of the prepared OH-S0.5 samples; and (d) UV–vis absorption spectral changes of MB with solar light irradiation time by OH-S0.5.

The activity arrangement order was consistent with the TiO2/TiOF2 ratio. A larger ratio of TiO2/TiOF2 resulted in better photocatalytic activity. This result is consistent with Yua et al.'s research [22]. It can be explained by the better charge separation capability of the TiOF2–TiO2 mixed phase, which reduces the recombination rate of electron–hole pairs. However, a high amount of TiOF2 phase in the TiO2 nanosheets would decrease the photoactivity due to poor photoactivity of TiOF2.

The reaction rate of all of the samples is shown in figure 9b. The data were fitted with the first-order reaction equation as follows: where t is the reaction time, is the concentration of RhB at time 0, C is the concentration of RhB at time t, and k is the reaction rate constant. Figure 9b shows that P25 had a rate constant of only 0.07 h−1, indicating poor photocatalytic performance. The calculated rate constants are 1.4 and 2.3 h−1 for S2 and OH-S0.5, respectively. The OH-S0.5 sample shows the best performance among all the photocatalysts, with a degradation rate that is much higher than that of the P25, TiO2/TiOF2 and TiO2 samples. This excellent performance could be mainly attributed to its stronger light adsorption and TiO2/TiOF2 combination.

The catalytic abilities of 0.006 g/100 ml, 0.030 g/100 ml and 0.050 g/100 ml (OH-S0.5) are also shown in figure 9c. For the catalytic amount of 0.015 g/100 ml, the photocatalytic degradation rate of MB was the strongest and had the best degradation effect (figure 9c). When the content of OH-S0.5 was increased, its adsorptive capacity increased but photocatalytic degradation performance decreased (figure 9c).

Figure 9d shows the UV–vis absorption spectral changes of the MB solution. According to previous research, the discoloration of MB can be caused in two ways: the oxidative degradation and the two-electron reduction to leuco-MB, which can be detected by the UV–vis absorption at 256 nm [44,45]. Figure 9d shows that there is a blue-shift from 665 to 625 nm with an absorbent peak emerging at 256 nm after the spectral change of MB with the irradiation time by OH-S0.5. This means that a reductive conversion to leuco-MB exists in the degradation path of MB.

Radical-scavenging experiments were performed to complete an in-depth study of the photocatalytic degradation mechanism. The reactive species were detected through trapping experiments of hydroxyl radicals (•OH), holes (h+) and superoxide radical anions (O2•−) by introducing terephthalic acid [46], potassium iodide [46] and p-benzoquinone [47]. Terephthalic acid can combine with the hydroxyl radicals (•OH), potassium iodide can combine with holes (h+), and p-benzoquinone can combine with the superoxide radical (O2•−), to decrease the activity of the catalyst [46,47]. The effects of a series of scavengers on the photocatalytic oxidation towards the MB dye over the photocatalysts are shown in figure 10. Under simulated sunlight illumination, the percentage of MB loss decreased most rapidly after the addition of potassium iodide, indicating h+ was the main active species in the photocatalytic process. When terephthalic acid and p-benzoquinone were added, this also reduced activity, which implied that •OH and O2•− radicals also played a role in the photooxidation of MB. The order of importance of the active species is h+, •OH and O2•−.

Figure 10.

Photocatalytic degradation of MB in the presence of different scavengers over OH-S0.5 under simulated solar light irradiation.

Discussion

TiOF2 photoactivity was quite low, nearly 150 times lower than P25 in the X3B dye degradation experiment [36]. Lv et al. reported that surface fluorination can greatly enhance the photocatalytic activity of TiO2 due to the formation of free •OH radicals, which are highly reactive [37]. In this research, a combination of TiO2 and TiOF2 demonstrated even higher photoactivity performance in MB degradation compared with the pure TiO2 nanosheets. In order to further understand the reason for the enhanced photocatalytic performance, a possible mechanism of charge separation and transfer on the surface of TiO2/TiOF2 nanohybrids is proposed. The conduction band (Ec) and valence band (Ev) potentials of TiO2 and TiOF2 at the point of zero charge can be calculated by the following empirical equation [11,14,44]: Ec = χ − E0 − 0.5 Eg, where Ec is the energy of the conduction band, X is the bulk Mulliken electronegativity of the compound, E0 is the energy of free electrons on the hydrogen scale (about 4.5 eV), and Eg is the band gap energy of the semiconductor. The x-values for TiO2 and TiOF2 are approximately 5.8 and 7.3 eV, respectively. The energy gap (Eg) was estimated from the intercept of the tangent in the plots of versus photon energy , which is shown in figure 7b. The Eg of TiO2 and TiOF2 were evaluated to be 3.2 eV and 2.94 eV, respectively.

The position of the valence band edge (Ev) is determined by the following equation: The calculated Ec and Ev of TiO2 and TiOF2 are shown in the electronic supplementary material, table S2. The Ec edge potential of TiO2 (–0.3 eV) is more active than that of TiOF2 (1.5 eV). Hence, the photogenerated electrons on the TiO2 surface have a strong capability for moving onto the surfaces of TiOF2 via the interface transfer pathway. Similarly, photogenerated holes on the TiOF2 surface migrate to TiO2 under the driving force of the Ev edge potentials (figure 11). As a result, the electron–hole recombination is reduced, which is consistent with PL analysis. Thus, under simulated solar light irradiation, more h+ radicals can react with H2O or OH− to produce •OH, while more electrons can react with the dissolved oxygen molecules to yield superoxide radical anions (O2•−) [48,49]. MB can also be decomposed by the holes directly [50]. Because the photocatalytic activity of TiO2 is better than that of TiOF2, a larger ratio of TiO2 /TiOF2 (S2 and OH-S0.5) results in better photocatalytic activity. NaOH treatment can induce more F ions being disconnected from the surface of TiOF2, which means a larger ratio of TiO2/TiOF2 (XRD results). Furthermore, the presence of OH bonds on the surface of the photocatalyst (FT-IR analysis) and more oxygen vacancies (XPS and PL analysis) also induce the production of more •OH with enhancement of the photocatalytic performance. This can explain why NaOH-treated nanohybrids showed greater performance than untreated ones. OH-S0.5 showed the best photocatalytic activity, even better than S2; this may be attributed to its lower recombination rate of electron–hole pairs introducing more h+, and more Ti4+ being reduced into Ti3+, because its OH, SBET and absorption of light are not the largest according to FT-IR and UV–vis DRS analysis. So a lower electron–hole pairs recombination rate is the dominant factor that induces the photocatalytic performance enhancement of TiO2/TiOF2 nanohybrids. This was in accord with the radical-scavenging experiments.

Figure 11.

Schematic diagram for the conduction and valence bands of TiO2 and TiOF2, and the degradation route of MB under solar light irradiation.

Conclusion

In summary, an easy one-step hydrothermal route to synthesize TiO2/TiOF2 and OH-TiO2/TiOF2 hybrids has been demonstrated. The introduction of NaOH facilitated the conversion from TiOF2/TiO2 and induced a network structure. The prepared TiO2/TiOF2, especially the OH-TiO2/TiOF2 nanocomposite, exhibited excellent activity towards the degradation of MB under simulated sunlight irradiation. A larger ratio of TiO2/TiOF2 in TiO2/TiOF2 and OH-TiO2/TiOF2 nanohybrids could enable better performance. NaOH treatment can wash off the F ions from TiOF2 and induce this larger ratio. The highest efficiency of MB removal was just above 90% in 1 h. A lower electron–hole pairs recombination rate is the dominant factor that induces the photocatalytic performance enhancement of TiO2/TiOF2 nanohybrids.

Therefore, this work opens an avenue to efficiently synthesize TiO2/TiOF2 through a one-step nanocomposite for the removal of organic pollutants.