Hydrothermal synthesis of the novel rutile-mixed anatase TiO₂ nanosheets with dominant {001} facets for high photocatalytic activity

Abstract

In this paper, we introduce a process to synthesize a novel photocatalyst called rutile-mixed anatase nanosheets with dominant {001} facets from a hydrothermal route by using a solution mixed with Ti(OC₄H₉)₄, HF, NH₄F and H₂O. The crystal structure, micrographs, chemical compositions, and photocatalytic property are characterized and evaluated by using an X-ray diffractometer (XRD), high-resolution transmission electron microscope (HRTEM), X-ray photoelectron spectroscopy (XPS), UV-vis spectrophotometer, and fluorescence spectrophotometer. The experimental results reveal that (1) the addition of NH₄F played a crucial role, because it not only changes the chemical environment but also make the phase transformation from anatase to rutile. (2) Compared with the regular anatase TiO₂ nanosheets with dominant {001} facets, the present novel photocatalyst exhibits a greatly enhanced photocatalyst activity, i.e., its highest level of photocatalytic activity was about four times higher than that of commercial P25 and the regular anatase TiO₂ nanosheets with dominant {001} facets. The mechanism is because the formation of the rutile/anatase heterostructure enhances the separation of the photo-generated electrons and holes. It is expected that this novel photocatalyst will provide wide applications in areas of solar cells, hydrogen generation, photocatalytic environmental pollution treatment, etc.

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1. Introduction

TiO₂ has become an essential semiconducting material applied in photocatalysis, photovoltaics and water photocatalysis.^1–4 In general, the photocatalytic activity of TiO₂ can be further enhanced by using variant methods, such as non-metal doping, metal doping, dye sensitization, etc., for increasing the absorption of visible light and reducing the recombination efficiency of the photo-induced electrons and holes.^5–7 Research also reveals that the morphology, phase structures, crystallinity, and surface energy of TiO₂ also play a key role in its photocatalytic activity.^8–10 The requirement for a high-efficient photocatalytic reaction is that the redox potential for the generation of oxygen and hydrogen from water and for the generation of reactive radicals should lie within the band gap of TiO₂.¹¹ However, in the actual situation the large band gap of TiO₂ and the recombination of the photo-generated electrons and holes limit its photocatalytic efficiency.¹² Recently, various strategies have been devoted for enhancing the photocatalytic activity of TiO₂ by semiconductor coupling,¹³ doping,¹⁴ exposure of reactive facets¹⁵ and fabrication of photonic crystal.¹⁶

Exposure of the reactive facets is frequently used to promote the photocatalytic activity of TiO₂. In general, different phase of TiO₂ exhibits different dominant faces. For example, the dominant faces for rutile TiO₂ are {110}, {100}, and {101}, while anatase TiO₂ are {011} and {001}.¹⁷ The {110} facet of rutile TiO₂ is the most intensely studied facets because of its lowest energy. For the facets of anatase TiO₂, it has been demonstrated that the {001} exhibits the remarkable photocatalytic activity, which is based on the fact that {001} facets consist of high densities of under-coordinated Ti atoms and very large Ti–O–Ti bond angles at the surface.¹⁸ It is well-known that the average (001) surface energy of anatase TiO₂ is higher than that of the (100) and (101) surfaces,¹⁹ and if regular processes are used, the most exposed facets of anatase TiO₂ are the low-energy {101} facets.²⁰ Recently, increasing efforts have focused on exploring different routes to improve the percentage of {001} facets, such as hydrofluoric acid (HF) etching,²¹ shape control,²² doping,²³ quantum dots sensitization,²⁴ etc. In 2008, H. G. Yang et al.²¹ found that if hydrofluoric acid (HF) was applied as a capping solvent under hydrothermal conditions, the micrometer-sized anatase TiO₂ single crystals with dominant high energetic (001) exposed facets could be synthesized, and this kind of TiO₂ exhibits high photocatalysis and efficiency of splitting water for hydrogen. Hereafter, variant processes have been reported for synthesizing a high percentage of exposed {001} facets or doped with non-metal in anatase TiO₂.^25–30 J. S. Chen et al.³¹ demonstrated an unusual formation of large 2D nanosheets from nanomosaic building blocks of anatase TiO₂ nanosheets with exposed {001} facets. In addition, the nanometer-sized TiO₂ sheets with a high percentage of exposed {001} facets were also prepared by a hydrothermal route in a Ti(OC₄H₉)₄ and HF mixed solution,^32,33 and showed a high photocatalytic activity.^32–35

Although anatase TiO₂ nanosheets with dominant exposed {001} facets exhibit a higher level of photocatalytic activity, however, its recombination efficiency of the photo-generated electrons and holes is as high as general TiO₂ nanoparticles, which limits its further applications. Coupling TiO₂ with other wide band gap semiconductors are reported to promote the carrier separation efficiency and thus improve the photocatalytic activity.^36,37 In addition, recent researches showed that mixtures of anatase/rutile (Degussa P25, for example) are more active than pure rutile or anatase in photocatalytic reactions.^38,39 It has been known that compared with simple anatase TiO₂, the rutile-mixed anatase TiO₂ is beneficial for high photocatalytic activity due to electron–hole pair transformation between a rutile/anatase heterostructure.⁴⁰ The anatase/rutile phase junction is supposed to improve charge separation and then prolong charge lifetime for photocatalytic reactions. T. Ohno et al.³⁸ found synergism between anatase and rutile particles for the photocatalytic oxidation of naphthalene. By simply mixing these particles, they attained higher activity than those of the original powders. Furthermore, the activity was higher than those of many kinds of TiO₂ powders obtained from different sources. S. Shen et al.³⁹ found that the electron transfer percentage highly depends on phase composition and anatase–rutile interface in mixed phase TiO₂. The charge transfer process improved charge separation of photo-generated carriers and then enhanced the photocatalytic activity of the mixed phase TiO₂.

Based on the above analysis, it would be a great advancement to prepare rutile-mixed anatase with dominant {001} facets to further promote its photocatalytic activity. However, reports about the rutile-mixed anatase with dominant {001} facets are rare. H. Tian et al.⁴¹ prepared the rutile-mixed anatase photo-anode through surfactant-assisted anchoring ultrathin anatase nanosheets on vertically ordered rutile nanorod arrays. This cactaceae-like TiO₂ possesses high-exposed {001} facets outer layer, large specific surface area, efficient photo-to-current conversion and excellent photocatalytic ability to degrade bisphenol A due to the synergetic effects of the architecture design of high-active {001} facets and hierarchical heterojunctions. C. Wang et al.⁴² reported a novel nonaqueous solvothermal approach to fabricate hierarchical TiO₂ microspheres assembled by ultrathin nanoribbons where an anatase/TiO₂(B) heterojunction and {001} facets coexist. The photocatalytic test for acetaldehyde decomposition showed that this photocatalyst exhibited superior photocatalytic efficiency compared with the relevant commercial product P25.

In this work, a kind of the rutile-mixed anatase nanosheets with dominant {001} facets was synthesized by a hydrothermal method in a solution mixed with Ti(OC₄H₉)₄, HF, NH₄F and H₂O. The impacts of NH₄F content on the phase structures, crystallinity and photocatalytic activity of TiO₂ nanosheets were examined. With an optimum NH₄F content, a highly active rutile-mixed anatase nanosheets photocatalyst was obtained, and achieved the photocatalytic activity 4 times higher than commercial TiO₂ (P25) and regular anatase TiO₂ nanosheets with dominant exposed {001} facets.

2. Experimental section

Synthesis of samples

Anatase TiO₂ nanosheets with dominant {001} facets were synthesized by a hydrothermal route, and the details have been reported elsewhere.^18,19 The precipitation steps are as follows: (1) 15 mL Ti(OC₄H₉)₄ was mixed with an appropriate amount of HF in a Teflon-lined autoclave with a capacity of 60 mL then kept at 180 °C for 24 hours; (2) the precipitates were then separated from the suspension by centrifugation (4000 rpm, 15 min); (3) in order to wash the powder, the products were further suspended and centrifuged in absolute ethanol for three times, and dried under an infrared lamp. (4) The samples obtained from different HF contents 0.6, 1.2, 1.8 to 2.4 mL were correspondingly labeled as TF1, TF2, TF3 and TF4. (5) For comparison, the anatase TiO₂ nanoparticles were prepared by using 15 mL Ti(OC₄H₉)₄ mixing with 0.6 mL H₂O, and this sample was labeled as TF0; (6) the rutile-mixed anatase TiO₂ nanosheets were synthesized in the same way, but with the addition of NH₄F, i.e., 15 mL Ti(OC₄H₉)₄ and an appropriate amount of NH₄F were mixed with 1.2 mL HF in a Teflon-lined autoclave with a capacity of 60 mL, and then kept at 180 °C for 24 hours. (7) The precipitates were separated, washed and dried. The samples obtained from different NH₄F content with 0.1, 0.2, 0.3 to 0.4 g were correspondingly labeled as TN1, TN2, TN3 and TN4.

Characterizations

The crystal structures of the samples were measured by using an X-ray diffractometer (XRD) (D8 Advance, Bruker AXS, Germany) with Cu-K radiation. Micrographs of the samples were characterized by a high-resolution transmission electron microscope (HRTEM) (JEM2010, JEOL, Japan). The visible absorption spectra of methylene blue (MB) in the photocatalytic experiment were performed by an UV-vis spectrophotometer (UV-2550, Shimadzu, Japan). The chemical binding energy of the photocatalysts was evaluated by using a X-ray photoelectron spectroscopy (XPS) (PHI 5600, Physical Electronics, USA), and the C 1s peak at 284.8 eV of the adventitious carbon was referenced to rectify the binding energies. The photoluminescence (PL) emission spectra of the samples were detected by a fluorescence spectrophotometer (F-4600, Hitachi, Japan) using a 300 nm line from a xenon lamp.

Photocatalytic experiments

Photocatalytic properties of the samples were tested by measuring the decomposition rate of methylene blue (MB) with the presence of the photocatalyst. In the experiment, a 250 W high pressure mercury lamp which generated light in the 350–450 nm range with a maximum intensity at 365 nm was used as a light source. The lamp was placed 8 cm above the liquid surface. 50 mg of photocatalyst was added into a 100 mL of 12 mg L⁻¹ methylene blue (MB) solution. The mixed solution was stirred incessantly, and after every 5 minutes, 3 mL solution was extracted to test the residual concentration of methylene blue (MB), which was evaluated by measuring the change of maximum absorbance in the UV-vis spectrometry.

3. Results and discussion

Currently, XRD is the most used approach for characterizing the percentage of exposed {001} facets in anatase TiO₂. It is based upon the following principles:¹⁵ (1) the thickness of the TiO₂ nanosheets in the [001] direction and the length in the [100] direction can be calculated from the full width at half-maximum of (004) and (200) diffraction peaks, respectively. (2) Theoretically, the shape of the TiO₂ nanosheets is supposed to be a standard cuboid. (3) According to the geometrical relationships between thickness and side length, the surface area of the exposed {001} facets and the total area of the TiO₂ nanosheets can be calculated. (4) At last, the percentage of the exposed {001} facets in anatase TiO₂ is roughly calculated from the ratio of these two areas. Fig. 1 illustrates the XRD patterns of the TiO₂ nanosheets without NH₄F adding. Obviously, only anatase phase was obtained, which was similar to the other works.^32,33 However, with increasing HF content, the peak intensity of anatase (200) plan steadily increased and the width of the peak became narrowed, which indicated an increase in the average length. Conversely, the peak intensity of anatase (004) plan steadily decreased and the width of peak came broadened, which indicated a decrease in the average thickness. These variations revealed that the HF content had a direct proportion with the percentage of dominant {001} facets.^32,33 Therefore, according to the XRD results, the average length, thickness and percentage of dominant {001} facets could be calculated, as listed in Table 1. That was to say, with increasing the HF content from 0.6 to 2.4 mL, the percentage of dominant {001} facets increased from 43% to 77%. When HF was absent, only regular anatase TiO₂ nanoparticles were prepared.

Fig. 1 XRD patterns of the anatase TiO₂ nanosheets.

Table 1 Structural information of the anatase TiO₂ nanosheets

Samples	Average thickness (nm)	Average length (nm)	Percentage of (001) facet
TF1	9	20	43%
TF2	7	30	63%
TF3	6	40	74%
TF4	6	48	77%

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Fig. 2 shows the TEM micrograph and HRTEM lattice image of the sample TF2. The TiO₂ nanosheets were in an average length of 30 nm and an average thickness of 7 nm. The lattice spacing parallel to the top and bottom facets was 0.235 nm, which indicated that the dominant surface was the TiO₂ {001} facets.

Fig. 2 (a) TEM micrograph and (b) HRTEM lattice image of sample TF2.

Fig. 3 gives the photocatalytic activities of the samples by decomposition of methylene blue (MB). It was found that with increase of the percentage of dominant {001} facets from 43% to 77%, the corresponding photocatalytic activity of the anatase TiO₂ nanosheets only exhibited a small variations. This phenomenon revealed that a higher percentage of dominant {001} facets did not make more difference for creating higher photocatalyst activity.^20,33 Comparatively, the anatase TiO₂ nanosheets with 63% percentage of dominant {001} facets (sample TF2) showed the highest level of photocatalytic activity, which was different from other report,³³ because of the decomposition of different organic compounds.

Fig. 3 The photocatalytic activity of the anatase TiO₂: (a) methylene blue decomposition by the anatase TiO₂ nanosheets upon irradiation and (b) dependence of ln(C/C₀) on time.

Based upon the above results, we then chose the sample TF2 as the basic solution (15 mL Ti(OC₄H₉)₄ and 1.2 mL HF), and further added different NH₄F contents. Fig. 4 illustrates the XRD patterns of the novel obtained samples. It could be seen that after adding NH₄F, a (110) rutile TiO₂ peak appeared, which demonstrated the formation of the rutile-mixed anatase TiO₂. In addition, with increasing NH₄F contents, the peak intensity of the (200) anatase plan steadily decreased and the width of the peak broadened indicating an increase in average length. Conversely, the peak intensity of (004) anatase steadily increased and the width of peak narrowed indicating a decrease in average thickness. These variations implied that the percentage of dominant {001} facets decreased when added more NH₄F.

Fig. 4 XRD patterns of the rutile-mixed anatase TiO₂ nanosheets.

Table 2 listed the calculation data of the average length, thickness, percentage of dominant {001} facets and the percentage of rutile.⁴³ Obviously, with increasing NH₄F contents from 0.1 to 0.4 g, the percentage of dominant {001} facets decreased from 63% (sample TF2) to 34% and the percentage of rutile increased to 21.4%. These changes revealed that NH₄F played a key role to influence the exposure of {001} facets and also made a phase transformation from anatase to rutile. As the investigation pointed out, F⁻ ions absorbed on the surface of anatase TiO₂ could reduce the surface energies. That is to say, after absorption of F⁻ ions, the surface energy of the {001} facets will be smaller than that of the {101} facets.²¹ Therefore, by adding NH₄F, the NH₄⁺ ions absorbed on the surface of TiO₂ would increase the surface energy of the anatase {001} facets and reduced the percentage of dominant {001} facets.

Table 2 Structural information of the rutile-mixed anatase TiO₂ nanosheets

Samples	Average thickness (nm)	Average length (nm)	Percentage of (001) facet	Percentage of rutile
TN1	7	25	58%	14.1%
TN2	7	21	53%	16.7%
TN3	8	18	44%	19.6%
TN4	9	15	34%	21.4%

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Fig. 5 shows the HRTEM lattice images of the sample TN3. Comparing with Fig. 2 of the sample TF2, the samples TN3 is of a longer average length but thinner average thickness. The average length and thickness of the sample TN3 are about 18 and 8 nm, respectively, based on the XRD results.

Fig. 5 TEM micrograph and HRTEM lattice images: (a and b) anatase and (c) rutile (R)/anatase (A) heterostructure of sample TN3.

The XPS survey spectrum revealed that the sample TN3 contained four elements Ti, O, F and N, in which the chemical binding energies were Ti 2p, O 1s, F 1s and N 1s are 458.6 eV, 532.0 eV, 684.3 eV and 401.6 eV, respectively, as shown in Fig. 6a. Fig. 6b shows Ti 2p scan. It can be seen that a plenty of Ti atoms are in Ti⁴⁺ state, while some Ti atoms are in Ti³⁺ state because of the hydrothermal treatment. The C 1s peak at 284.7 eV was a signal of adventitious elemental carbon. The F 1s peak at 684.3 eV indicated fluorinated TiO₂ systems as Ti–F on the surface due to the F absorption (Fig. 6c).³³ The N 1s core levels were measured and the results were presented in Fig. 6d. The N 1s peak at 401.6 eV demonstrated that N atoms were presented in the clearance. The atom N content was estimated to be 1.91% by comparing the product of the 401.6 eV peak are a multiplied by the N element sensitive factor to the product of all peaks multiplied by respective element sensitive factor. The XPS results demonstrated that that the adding of NH₄F cannot lead to the doping of N element. Therefore, the origin of N was due to the adsorption effect.

Fig. 6 XPS spectra of the sample TN3: (a) survey spectrum; (b) Ti 2p scan; (c) F 1s scan; (d) N 1s scan.

The photocatalytic activity of the rutile-mixed anatase TiO₂ nanosheets were evaluated by the decomposition of methylene blue (MB), as shown in Fig. 7a and b. The photocatalytic activity was in the order of the samples TN3 > TN4 > TN2 > TN1 > TF2. Obviously, comparing with the regular anatase TiO₂ nanosheets with dominant {001} facets, the present novel rutile-mixed anatase TiO₂ nanosheets with dominant {001} facets exhibited a greatly enhanced photocatalyst activity. And the sample TN3 showed the highest level of photocatalytic activity, which was about four times higher than that of P25 and the sample TF2. The degradation of MB for three cycles was also conducted (Fig. 7c). Obviously, the photocatalytic activity of the TN3 remained almost unchanged, which means the excellent reusability of the rutile-mixed anatase TiO₂ nanosheets. To examine the universality of the catalytic activity of our TiO₂ photocatalyst, the catalytic performance for the degradation of rhodamine B (RhB) and methylene orange (MO) were evaluated. Fig. 7d showed degradation curves of RhB, MO and MB by using TN3 as photocatalyst. It showed that TN3 exhibited superior photocatalytic activity in all the three typical organic pollutants. The slight difference of degradation rate was due to the differences of molecular structures and adsorption properties of these three dyes.

Fig. 7 The photocatalytic activity of the rutile-mixed anatase TiO₂ nanosheets. (a) Methylene blue decomposition by the rutile-mixed anatase TiO₂ sheets upon irradiation; (b) dependence of ln(C/C₀) on time; (c) the degradation curves of MB for three cycles by using TN3 as photocatalyst; (d) the degradation curves of MB, MO, RhB by using TN3 as photocatalyst.

The separation efficiency of the photo-electrons and holes in different rutile-mixed anatase TiO₂ nanosheets were measured by PL emission spectra, as shown in Fig. 8. It can be seen that the emission intensity of the rutile-mixed anatase TiO₂ nanosheets gradually reduced along with increasing percentage of the rutile contents, which suggested an increase in separation efficiency of the photo-generated electrons and holes, and then resulted in the improvement of the photocatalytic activity. The reason was because of the heterostructure that can reduce the recombination efficiency of the photo-generated electron–holes and improve the photocatalytic performance.^44,45

Fig. 8 PL emission spectra of the rutile-mixed anatase TiO₂ nanosheets.

4. Conclusions

A kind of novel rutile-mixed anatase nanosheets with exposed {001} facets were synthesized by using a hydrothermal process in a solution mixed with Ti(OC₄H₉)₄, HF, NH₄F and H₂O. The addition of NH₄F played a crucial rule, because it changed the chemical environment and made the phase transformation from anatase to rutile. Comparing with the regular anatase TiO₂ nanosheets with dominant {001} facets, the present novel photocatalyst exhibited a greatly enhanced photocatalyst activity. This was because the formation of the rutile/anatase heterostructure enhanced the separation of the photo-generated electrons and holes. It is expect this novel photocatalyst will provide a wide applications in areas of solar cells, hydrogen generation, photocatalytic environmental pollution treatment, etc.

Acknowledgements

This work was supported by the National Basic Research Program of China (973 Program) (No. 2009CB939705), National Nature Science Foundation of China (No. 11174227), and Chinese Universities Scientific Fund.

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