Low temperature synthesis of water dispersible F-doped TiO₂ nanorods with enhanced photocatalytic activity

Abstract

F-doped TiO₂ nanorods (F-TiO₂ NRs) were prepared through a simple hydrolysis of titanium chloride (TiCl₄) and a refluxing process at 100 °C and atmospheric pressure in the presence of sodium fluoride (NaF). The prepared samples were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV-vis spectrophotometry and N₂ adsorption analysis. It is found that F⁻ ions are incorporated into the lattice of TiO₂ and absorbed on the surface of TiO₂ NRs. The NaF/TiCl₄ molar ratio is found to play an important role in the formation of the anatase F-TiO₂ NRs. When the molar ratio of NaF/TiCl₄ is 4.0/1.0, the prepared F-TiO₂ NRs show the highest photocatalytic activity for the degradation of methylene blue (MB) under UV and visible irradiation, showing that F-TiO₂ NRs are efficient photocatalytic degradation catalysts of MB pollutant in wastewater treatment.

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1 Introduction

It is well known that titanium oxide (TiO₂) is one of the most promising photocatalysts due to its high efficiency, non-toxicity, low cost, chemical inertness and photostability.^1–4 The performance of TiO₂ photocatalysts is strongly affected by their structure, heterogeneous interface, size and exposed faces in solution.^5,6 Many efforts have been made for the generation of high-efficiency TiO₂ photocatalysts, and tailoring the particle shape, the size and the crystallinity of TiO₂ has been intensively investigated to increase its surface area and the number of active sites for adsorption and photocatalytic degradation of pollutants.^3,7–12 TiO₂ crystals, particularly nanostructures with various shapes (i.e., nanowire, nanorod, nanobelt, nanotube, nanosphere, and nanosheet) have been realized in the past decades.¹³ Among the various morphologies, TiO₂ nanorods (TiO₂ NRs) can offer a higher surface-to-volume ratio compared to nanospheres, which can increase the density of active surface sites for photocatalytic reactions or load transfer in composites.^14–16 Moreover, TiO₂ NRs enabled a fast charge collection, thus demonstrating improved solar cell efficiencies.¹⁷ Therefore, TiO₂ NRs are particularly interesting for optoelectronic applications, such as photovoltaic devices and self-powered UV detectors. Superior photoconduction efficiencies and gains have been experimentally demonstrated for individual, single-crystalline TiO₂ NRs.^18,19

Various strategies for the preparation of TiO₂ NRs have been developed, such as metal–organic chemical vapour deposition,²⁰ electrochemical anodisation,²¹ direct oxidation of titanium sheets,²² and solution-phase reactions in hydrolytic^23–25 and non-hydrolytic hydrolytic syntheses.^16,26–28 For example, oleate-capped anatase TiO₂ NRs were obtained by low-temperature trimethylamino-noxide dihydrate catalyzed hydrolysis of titanium tetraisopropoxide in oleic acid as surfactant at a temperature as low as 80 °C.²⁵ A facile hydrothermal method was developed for the first time to grow oriented single-crystalline rutile TiO₂ nanorod films on transparent conductive substrates. A light-to-electricity conversion efficiency of 3% could be achieved by using 4 μm-long TiO₂ nanorod films as the photoanode in a DSSC.²⁹ Highly crystalline TiO₂ NRs have been synthesized by a hydrothermal process in a cetyltrimethylammonium bromide surfactant solution.³⁰ By using a seeded growth technique, the highly uniform TiO₂ NRs could also be generated.³¹

The photoactivity of TiO₂ NRs depends on their crystal phase composition, geometric morphology, surface area, thermal treatment.³² Compared to P25–TiO₂ catalyst, the activity of TiO₂ NRs in some photocatalytic reactions need to be further improved.³³ It has been reported that F-doped TiO₂ (F-TiO₂) is demonstrated an effective approach to promote the photocatalytic activity of TiO₂.³⁴ Fluorine has been used as a structure directing agent to generate TiO₂ with exposed high-energy {0 0 1} facets with large surface area or as a capping agent and/or dopant in TiO₂ to modify their surface and bulk properties, and consequently, to enhance their photocatalytic performance.^35–37 It was demonstrated that F-doped TiO₂ could not only slow down the radiative recombination process of photo-generated electrons and holes in TiO₂ but also facilitate the generation of more ˙OH radicals which are responsible for the oxidation of organic molecules in photocatalytic reaction.^38,39 More recent research has found that fluoride doping could tune the morphology of TiO₂.^6,40 For example, the flower-like TiO₂ nanostructures with exposed {0 0 1} facets were synthesized by a low-temperature hydrothermal process from Ti powders in hydrofluoric acid (HF) solution, and they exhibited enhanced photocatalytic degradation of MB under UV light.^41–43 Using TiCl₄ as precursor, a series of TiO₂ hierarchical nanostructures, such as pompon-like and football-like microspheres composed of aligned rutile and anatase nanoparticles, were synthesized by a facile template-free hydrothermal method in the presence of NaF.⁴⁴ Hierarchical porous F-doped TiO₂ microspheres exhibiting high visible light photocatalytic activity have been fabricated by a one-step low-temperature hydrothermal approach without using any templates.⁴⁵ Fluorine-doped hierarchical porous single-crystal rutile TiO₂ NRs have been synthesized through a silica template method, in which F⁻ ions acts as both n-type dopants and capping agents to make the anisotropic growth of the nanorods.⁴⁶ Usually a hydrothermal method with HF as a morphology controlling agent is exploited to prepare TiO₂ nanomaterials. However, the hydrothermal method need high-pressure equipment such as Teflon-lined stain-less steel. Moreover, the HF is corrosive.

Although great progress has been made in TiO₂ NRs, the synthesis of TiO₂-based nanomaterials at low temperature and atmospheric pressure still remains a challenge. Moreover, the effect of NaF in the formation of the nanostructure of F-TiO₂ NRs has not been investigated so far. In this work, a facile synthesis route has been developed for the preparation of anatase F-TiO₂ NRs at low temperature by simple hydrolysis of TiCl₄ and then a refluxing process at 100 °C and atmospheric pressure in the presence of NaF. In the process of synthesis, no other organic surfactant was used. By simply adjusting the reaction conditions such as the molar ratio of NaF/TiCl₄ the morphology of TiO₂ nanoparticles can be tailored. Finally, photocatalytic activities of the synthesized F-TiO₂ NRs for the degradation of pollutant MB under UV and visible light irradiation were evaluated.

2 Experimental section

2.1 Materials

2.1.1 Materials. TiCl₄ (99%) was purchased from J&K Scientific Ltd., China. Methylene blue (MB) purchased from Sigma-Aldrich was used as a standard material to estimate adsorption and photocatalytic degradation. NH₃·H₂O (25–28%), H₂O₂ (30%) and NaF was purchased from Sinopharm Chemical Reagent Co., Ltd, China. All the chemicals were used without further purification.

2.2 Synthesis of TiO₂ NRs

Anatase TiO₂ NRs was synthesized through a facile and low temperature hydrothermal method at normal pressure. In a typical procedure, TiCl₄ aqueous solution was prepared by dissolving 100 μL of TiCl₄ into 1.0 mL of water under strong stirring in an ice water bath, after mixing uniformly another 9.0 mL of water was added. The pH value of the TiCl₄ aqueous solution was adjusted to 8–9 by NH₃·H₂O solution (2.5 M) to obtain titanium hydrate precipitates. The precipitates was washed with pure water for several times to remove Cl⁻ completely and then redispersed into 100 mL of water. Peroxotitanium acid solution was obtained by addition of 110.0 μL of H₂O₂ (30%) solution to the above titanium hydrate dispersion drop by drop under stirring.⁴⁷ After peroxotitanium acid the solution was held at 50 °C for 2 hours, given amount of NaF (NaF/TiCl₄ = 0, 2.0, 4.0 and 6.0, molar ratio) was added and then the solution was refluxed at 100 °C for 2–6 hours in a thermostat water bath. One can see from the photocatalytic activity of the F-TiO₂ NRs obtained at different refluxing temperature that 100 °C is the optimum synthesis temperature when the refluxing was carried out at atmospheric pressure (Fig. S1†). After the reaction finished, the products were collected by centrifugal separation and washed with deionized water for 5 times. All the products were dried in a vacuum oven at 50 °C overnight. To remove the F⁻ ions absorbed on the surface of F-TiO₂ NRs, the samples were first washed with 0.1 M NaOH aqueous solution, and then with pure water till its pH value was neutral.

2.3 Characterization

X-ray Diffraction (XRD) was performed using a D8 FOCUS diffractometer (Bruker-AXS, Germany) with Cu Kα radiation (λ = 1.5418 Å). Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) were performed on a JEM-2100 electron microscope (JEOL Ltd., Japan) operating at 200 kV. The samples were prepared by mounting a drop of the dispersion on a carbon-coated Cu grid and allowing it to dry in air. Nitrogen adsorption/desorption experiments were performed on a Micromeritics ASAP 2020 surface area analyzer after outgassing at 200 °C for 4 h prior to analysis. The standard multipoint Brunauer–Emmett–Teller (BET) method was utilized to calculate the specific surface area. The pore size distributions of the materials were derived from the adsorption branches of the isotherms on the basis of the Barrett–Joyner–Halenda (BJH) model. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo ESCALAB 250 with Al Kα excitation.

2.4 Photocatalytic activity measurements

The photocatalytic activity was measured by decomposing MB dye. In a typical process, 40.0 mL of MB aqueous solution with a concentration of 10.0 mg L⁻¹ and 2.0–10.0 mg of TiO₂ NRs were placed in a quartz conical flask and kept in the dark with stirring for 30 min to achieve adsorption equilibrium. Then the conical flask was irradiated with UV light (λ = 254 nm) produced by 2 × 15 W UV lamps (Spectronics Co., USA) or visible light irradiation (λ > 400 nm) produced by a 400 W metal halide lamp (HPI-T Plus 400 W, Philips) with cut off filter. At given intervals, 3 mL of suspension was extracted and then centrifuged at 10 000 rpm for 5 min to get rid of the catalysts from the supernatant. Then, the solution was analyzed by recording the UV-vis spectrum of MB at the maximum absorbance of 665 nm.

3 Results and discussion

3.1 Synthesis and characterization of TiO₂ NRs

The anatase TiO₂ NRs were synthesized through a hydrolysis TiCl₄ at pH 8–9 to form titanium hydrate precipitates, and then the titanium hydrate precipitates were subjected to a peptizing, complexing and refluxing process at 100 °C and atmospheric pressure using NaF as shape control agent.^47,48

Without addition of NaF in the synthesis process, there are only irregular or elongated TiO₂ nanoparticles of about 9.5 nm are obtained (Fig. 1a). The lattice fringes can be observed clearly (Fig. 1b), the selected-area electron diffraction (SAED) pattern in Fig. 1c displayed the diffraction rings corresponding to lattice planes ca. (101), (004), (200) and (211) of anatase crystal structure of TiO₂ is in accordance with the XRD pattern of TiO₂ nanoparticles (Fig. 3).

Fig. 1 TEM image, HRTEM image, SAED pattern and size distributions of TiO₂ nanoparticles obtained through a hydrolysis TiCl₄ and a refluxing process at 100 °C for 6 hours. No NaF was added in the synthesis process.

When NaF was added during the refluxing process, a significant change in the morphology of TiO₂ nanoparticles was observed. One can see from the TEM images (Fig. 2a and b) that the product consisted of rod-like structures. The average length and diameter determined by TEM image analysis are about 32.7 nm and 9.2 when the molar ratio of NaF/TiCl₄ is 4.0/1.0. The lattice fringes of TiO₂ NRs were also observed clearly (Fig. 2b). The SAED pattern in Fig. 2c displayed the diffraction rings is similar to that in Fig. 1c. Christy et al.⁴⁹ and Nian et al.⁵⁰ prepared TiO₂ NRs having diameter 6–10 nm. When the molar ratio of NaF/TiCl₄ was changed to 2.0 or 6.0, similar results were obtained. The shape of the TiO₂ NRs was maintained, however, the average length and diameter are changed into 26.5 nm and 8.6 for the NaF/TiCl₄ molar ratio of 2.0/1.0, 34.7 nm and 8.2 for the NaF/TiCl₄ molar ratio of 6.0/1.0 (Fig. S2 and S3†).

Fig. 2 TEM image, HRTEM image, SAED pattern, length and diameter distributions of F-TiO₂ NRs obtained through a hydrolysis TiCl₄ and a refluxing process at 100 °C for 6 hours. The molar ratio of NaF/TiCl₄ was 4.0/1.0.

The crystallographic structure of TiO₂ NRs synthesized at different conditions was characterized by powder XRD (Fig. 3). The diffraction peaks could be indexed to anatase-phase TiO₂ (JCPDS no. 21-1272), indicating that all the as-prepared products with and without addition of NaF were anatase TiO₂. The increased peak intensity and narrow width for the (004) reflex and a comparatively lower diffraction intensity for the (200), (211) reflexes, also indicate the formation of rod-like nanostructures with a preferred growth orientation along the c-axis of the anatase lattice.²⁵ One can also see from Fig. 3 that the crystallinity of the samples increase with the increase of the molar ratio of NaF/TiCl₄. Therefore, it is reasonable to conclude that the concentration of NaF plays an important role in the synthesis of the anatase TiO₂ NRs.

Fig. 3 XRD patterns of TiO₂ nanoparticles and F-TiO₂ NRs obtained with different NaF/TiCl₄ molar ratio, all were refluxed at 100 °C for 6 hours.

The surface information of the F-TiO₂ NRs prepared with NaF/TiCl₄ molar ratio of 4.0/1.0 was detected by XPS. Characteristic peaks of Ti, O, F and C were observed (Fig. 4). The Ti 2p spectrum displays peaks at 458.5 and 464.3 eV corresponding to the 2p_3/2 and 2p_1/2 spin–orbital components, respectively,⁵¹ showing the presence of Ti⁴⁺ ions.⁵² The binding energy of O 1s at 529.8 eV is attributed to the O²⁻ in TiO₂. A broadening on the higher bonding energy side at 531.6 eV indicates the presence of another type of oxygen, which was assigned to Ti–OH bonds.⁵³ The binding energy of F 1s core electrons as ∼684.6 eV correspond to the surface ≡Ti–F species formed by ligand exchange between F⁻ and surface hydroxyl groups, which is consistent with the reported results.^54–57 Another F 1s core electron peak at 689.8 eV is ascribed to the contribution by surface lattice F (doped F⁻ in the lattice of TiO₂).^58–61 Therefore, one can conclude that F⁻ ions are incorporated to the lattice of TiO₂ and absorbed on the surface of TiO₂. From above results, it can be deduced that F atoms play important roles in the formation of rod-like morphology and structure of TiO₂. After removal of the F⁻ ions absorbed on the surface by washing with 0.1 M NaOH solution, the binding energy of F 1s at 684.3 eV is attributed to Ti–F species (Fig. 4e). The peak at 689.8 eV becomes very weak, indicating the low concentration of the F⁻ in the lattice of TiO₂. The chemical states of elemental Ti and O are not changed (Fig. S4†).

Fig. 4 XPS spectra of F-TiO₂ NRs synthesized at 100 °C for 6 hours, the molar ratio of NaF/TiCl₄ = 4.0/1.0. (a) Survey, (b) Ti 2p, (c) O 1s and (d) F 1s. In (c), black trace shows the experimental data, blue and orange traces show the curves fitted to the experimental data, and the red traces show the envelope, i.e. the sum of the blue and orange traces. (e) F 1s spectrum of F-TiO₂ NRs after removal of the F⁻ ions absorbed on the surface by washing with 0.1 M NaOH solution.

Light absorptions of commercial P25 nanoparticles, pure TiO₂ nanoparticles and the TiO₂ NRs prepared with different NaF/TiCl₄ molar ratio were measured and the diffuse reflectance spectra are shown in Fig. 5a. The absorption spectrum of P25 only exhibits a stronger absorption in the UV range. However, pure TiO₂ nanoparticles and all F-TiO₂ NRs show UV and visible light absorption. To determine the value of energy gap of TiO₂, the relation of (αhν)^1/2 with the incident photon energy (hν) is shown in Fig. 5b. The band gap energies of these materials are about 2.94 (P25), 2.23 (pure TiO₂ nanoparticles), 2.20 (NaF/TiCl₄ = 2/1), 2.16 (NaF/TiCl₄ = 4/1), 2.26 (NaF/TiCl₄ = 6/1) and 2.30 eV (NaF/TiCl₄ = 4/1, washing with 0.1 M NaOH solution). A smaller band gap of pure TiO₂ is attributed to surface disorders, oxygen vacancies, etc.⁶⁰ caused by the use of H₂O₂ during preparation process. It has been reported that the band gap TiO₂/fly-ash cenospheres treated with H₂O₂ was reduced to 2.6 eV.⁶² In F-TiO₂ NRs, the band gap energy of TiO₂ NRs is further decreased, which is attributed to surface disorders, oxygen vacancies and Ti³⁺. It has been reported that the F doping can convert some Ti⁴⁺ to Ti³⁺. The Ti³⁺ surface states in TiO₂ form a donor level between the bandgaps of TiO₂, which would enhance the visible light absorption.⁶⁰ In addition, the F doping can promote the creation of oxygen vacancies, which induces the enhancement of the visible light absorption.⁴⁵ Such an enhancement in absorbance can increase the number of photo-generated electrons and holes to be involved in the photocatalytic reaction.⁶³ If the F⁻ ions absorbed on the surface were removed by washing with 0.1 M NaOH solution, F-TiO₂ NRs also exhibited UV and visible light absorption, the band gap energy of the F-TiO₂ NRs (NaF/TiCl₄ = 4/1) was increased slightly. That is, and the F-doping in the lattice could improve the light absorption of TiO₂ according to the UV-vis result (Fig. 5a).

Fig. 5 Diffuse reflectance UV-vis spectra of P25, pure TiO₂ nanoparticles and F-TiO₂ NRs prepared with different NaF/TiCl₄ molar ratio.

3.2 Formation mechanism of F-TiO₂ NRs

To understand the formation process of the F-TiO₂ NRs, time-dependent experiments were carried out. After the system was heated to 100 °C for 0 min, 20 min and 60 min, the morphologies of the products were observed with TEM. When the reaction time was 0 min, Fig. 6a and a′ indicate that the as-obtained product are a large quantity of aggregates of nanoparticles, and no lattice fringes were observed, which might be assigned as amorphous titania. When the reaction went on at 100 °C for 20 to 60 min, the very short rod-like structures with an average length of up to 2–3 nm formed (Fig. 6b and c′). Obvious lattice fringes were observed (Fig. 6b′ and c′), indicating the appearance of crystal structure of TiO₂. When the reaction time was prolonged to 2 h, willow-leaf-like TiO₂ NRs with length of ∼30 nm were observed (Fig. 6d). Further increasing the reaction time (4 h, 6 h and 24 h), there was no significant changes of the F-TiO₂ nanorod morphology (Fig. 6e–g), indicating prolonged heating did not further influence the particle growth.

Fig. 6 TEM images of F-TiO₂ nanoparticles synthesized under a refluxing process at 100 °C for 0 min (a), 20 min (b), 1 h (c), 2 h (d), 4 h (e), 6 h (f) and 24 h (g), the NaF/TiCl₄ molar ratio is 2.0/1.0. (a′, b′ and c′ are high magnification images).

In terms of the above results, the nanorod formation may be understood as follows: after addition of H₂O₂ to the titanium hydrate dispersion, peroxotitanium acid is produced. Upon refluxing at 100 °C, dehydration reaction of the peroxotitanium acid to form TiO₆ octahedra along with the condensation of TiO₆ octahedra to TiO₂ takes place. The nucleus of crystalline nanocrystals are formed first in the solution when the temperature is increased from 50 °C to 100 °C. With the increase of the reaction time, amorphous phase of TiO₂ gradually transforms to crystalline phase, collision of nanoparticles along with the generation of one-dimensional metastable aggregates may occur. The adsorption of F⁻ ions might lead to the growth of the metastable aggregates along a certain direction to create nanorod-like TiO₂ crystals, which may be based on the Ostwald ripening mechanism. When the reaction time is longer, the anatase TiO₂ NRs will not change due to the low supersaturation degree in this case (<9 mmol L⁻¹). It has been reported that F⁻ ions adsorbed on the surface of TiO₆ octahedra can catalyze the bridging of adjacent TiO₆ octahedra, facilitating the face-sharing rearrangement of the octahedra and leading to the spiral chain growth,^64,65 as a result of condensation reaction anatase TiO₂ is formed. In addition, it has been reported that the average crystallite size and degree of crystallinity of anatase nanoparticles increased with increasing concentration of fluoride.⁶⁶ It is also proposed that the mediating role of F⁻ ions in the dissolution and re-crystallization processes of metastable TiO₂ intermediates results in the facilitation of the crystal growth and crystallization, accounting for the greater crystal size and higher degree of crystallinity.³⁶

3.3 Nitrogen adsorption/desorption experiments

The specific surface areas and the pore volumes of pure TiO₂ the F-TiO₂ NRs with different NaF/TiCl₄ molar ratio were determined by nitrogen gas adsorption–desorption isotherms measurement. Fig. 7 shows the typical isotherm and the pore size distribution. According to the IUPAC classification, the nitrogen adsorption–desorption isotherm is attributed to the Langmuir IV type. The formation of the hysteresis loops in the plot of Fig. 7a reveal the presence of mesopores, which may be ascribed to agglomerates of TiO₂ nanoparticles. The BET specific surface area, the average pore size and pore volume of the samples are listed in Table 1. Compared to spherical nanoparticles, the larger BET specific surface area of F-TiO₂ NRs may be due to the high surface-to-volume ratio and the reduced aggregation of the nanoparticles.

Fig. 7 (a) Nitrogen adsorption/desorption isotherm. (b) Corresponding BJH pore size distribution curve of pure TiO₂ nanoparticles and F-TiO₂ NRs with different NaF/TiCl₄ molar ratios.

Table 1 Summary in characterizations of F-TiO₂ samples prepared with different NaF/TiCl₄ molar ratios

NaF/TiCl4(molar ratio)	0.0/1.0	2.0/1.0	4.0/1.0	6.0/1.0
BET surface area (m^2 g^−1)	117.4	172.0	181.2	165.9
Pore volume (cm^3 g^−1)	0.20	0.29	0.21	0.22
Average pore size (nm)	4.7	5.6	4.1	4.2

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3.4 Photocatalytic activity of F-TiO₂ NRs

In this experiment, the photocatalytic activities of F-TiO₂ NRs were tested by the photocatalytic degradation of MB in aqueous solution under UV and visible light. Fig. 8a shows the evolution of MB absorption spectra in the presence of TiO₂ NRs catalyst, from which it can be see that the absorbance peaks of MB decreased quickly with irradiation time. Under UV light, one can see from Fig. 8b that the photocatalytic activity is greatly improved by fluorine doping, which is in agreement with previous report.⁴⁴ NaF/TiCl₄ molar ratio has a great influence on the photocatalytic activity of the F-TiO₂ NRs samples. Before and after removal of the F⁻ ions absorbed on the surface, the optimal NaF/TiCl₄ molar ratio always is 4.0/1.0. When the NaF/TiCl₄ molar ratio is 4.0/1.0, 80.5% MB was eliminated within 100 min (Fig. 8b, curve 4/1). It should be noted that the photocatalytic activities of F-TiO₂ NRs still lower than that of P25 under UV light though F-TiO₂ NRs have higher adsorption capability. However, the photocatalytic activity of F-TiO₂ NRs is improved when the F⁻ ions absorbed on the surface are removed by washing with 0.1 M NaOH solution, in which 96.8% MB was eliminated within 100 min (Fig. 8b, curve 4/1 (r)). This can be attributed to the reduced recombination of electrons and holes at the surface of F-TiO₂ NRs. Under visible light, similar results were obtained when NaF/TiCl₄ molar ratio was varied (Fig. 8c). It was found that the degradation rate of MB for pure TiO₂ and P25 was very low (Fig. 8c). However, 72% of MB was degraded in the presence of F-TiO₂ NRs (NaF/TiCl₄ molar ratio of 4.0/1.0) after 260 min (Fig. 8c, curve 4/1). After removal of the F⁻ ions absorbed on the surface by washing with NaOH solution, 90.0% MB was eliminated within 130 min (Fig. 8c, curve 4/1 (r)). These results indicated that F-TiO₂ NRs show much higher visible light catalytic activity than that of P25 or pure TiO₂ nanoparticles.

Fig. 8 (a) UV-vis spectra of MB solution under UV irradiation for different times using F-TiO₂ NRs as photocatalyst prepared with NaF/TiCl₄ molar ratio of 4.0/1.0. (b) The change in the concentration of MB as a function of UV light irradiation time using P25 and the F-TiO₂ photocatalysts prepared with different NaF/TiCl₄ molar ratios. For the curves of 2/1 (r), 4/1 (r) and 6/1 (r), the F⁻ ions absorbed on the surface were removed by washing with 0.1 M NaOH aqueous solution. (c) The change in the concentration of MB as a function of visible light irradiation time using P25 and the F-TiO₂ photocatalysts prepared with different NaF/TiCl₄ molar ratios. For the curve s of 2/1 (r), 4/1 (r) and 6/1 (r), the F⁻ ions absorbed on the surface were removed by washing with 0.1 M NaOH aqueous solution. C₀ = 10 mg L⁻¹, mass of F-TiO₂ NRs = 2.0 mg, solution volume = 40 mL.

The effect of the content of catalysts on the photocatalytic activities of TiO₂ NRs was also performed. As shown in Fig. 9, the photocatalytic degradation rate of MB increases with the amount of catalysts. The MB was removed almost completely (Fig. 9c) when the solution was irradiated under UV-light for 20 min. In addition, it is noted that prior to UV irradiation, obvious colour change was observed for the MB solution when the content of TiO₂ NRs was increased to 10.0 mg in 40 mL MB solution, and the C/C₀ value of MB solution after equilibrium in the dark was 0.06 (Fig. 9c), indicating that the adsorptive capacity of TiO₂ NRs for MB is very large. The adsorption rate and the amount of adsorbed dye with contact time for the solution of MB (10 mg L⁻¹) is illustrated in Fig. 10. As can be seen from Fig. 10 that the adsorption process is very rapid. When the amount of TiO₂ NRs is 10.0 mg, the equilibrium was achieved and eliminated almost 95% of MB from the solution after 10 min. If the amount of TiO₂ NRs is increased 20.0 mg, the equilibrium was achieved very quickly and eliminated almost 98% of MB from the solution after 5 min. We observed previously that N-doped TiO₂ nanosheets with larger adsorption capability exhibited enhanced photocatalytic activity for the decomposition of MB.⁵²

Fig. 9 Photocatalytic degradation (UV light) of MB solution (40 mL, C₀ = 10 mg L⁻¹) with different amount of F-TiO₂ NRs. (a) 2.0 mg, (b) 6.0 mg and (c) 10.0 mg. F-TiO₂ NRs photocatalysts were prepared with NaF/TiCl₄ molar ratio of 4.0/1.0.

Fig. 10 Adsorption ratio for the solutions of MB (10 mg L⁻¹, 40 mL) in the presence of F-TiO₂ NRs prepared with NaF/TiCl₄ molar ratio of 4.0/1.0. (a) 2.0 mg, (b) 6.0 mg, (c) 10.0 mg and (d) 20.0 mg.

It has been reported that fluorine doping can increase the specific surface area of TiO₂ and enhance the adsorption of MB, increase the visible light absorption and decrease the recombination of the photoinduced electrons and holes, which is beneficial to the photocatalytic performance.^66,67 The synergistic effect of fluorine doping and hierarchical nanostructure are responsible for the improved photocatalytic activity under visible light illumination.⁶⁸ Therefore, the enhanced photocatalytic activity of F-doped TiO₂ NRs in this case is mainly ascribed to the increase in the visible light absorption and the reduction in electron–hole pair recombination (Fig. 5). Furthermore, F-TiO₂ NRs with the high adsorption capacity to MB (Fig. 10) and mesostructure (Fig. 7) can exhibit more effective photocatalytic performance.^52,69,70 The photocatalytic activity of F-TiO₂ NRs depends on the F doping concentration, the optimal NaF/TiCl₄ molar ratio is 4.0/1.0, which is in agreement with previous report that, when the F doping concentration is more than a special value, the degradation rate decreases due to the promotion of the recombination of electron–hole pairs.^53,57

4 Conclusions

Anatase F-TiO₂ NRs were synthesized through a hydrolysis TiCl₄ and a refluxing process at 100 °C and atmospheric pressure in the presence of NaF. During the synthesis procedure no surfactant or capping agents were added. F⁻ ions are incorporated to the lattice of TiO₂ and absorbed on the surface of TiO₂ NRs. The NaF/TiCl₄ molar ratio is found to plays an important role in the formation of the anatase F-TiO₂ NRs. The prepared F-doped TiO₂ NRs show the high photocatalytic activity for the degradation of MB under UV and visible light irradiation, showing that F-TiO₂ NRs are efficient photocatalytic degradation catalysts of pollutant MB in wastewater treatment.

Acknowledgements

This work was funded by the National Natural Science Foundation of China (51173069, 51473068).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ra00850c

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