Shape Evolution of Highly Crystalline Anatase TiO2 Nanobipyramids

Oct 27, 2011 - Synopsis. Introduced by NaF, fluorine was employed as a novel morphology control agent. Nanobipyramids enclosed by two {001} and eight ...
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Shape Evolution of Highly Crystalline Anatase TiO2 Nanobipyramids Chao Chen,† Rong Hu,† Kaiguang Mai,† Zhimin Ren,† Hua Wang,‡ Guodong Qian,† and Zhiyu Wang*,† †

State Key Laboratory of Silicon Materials, Department of Materials Science & Engineering, Zhejiang University, Hangzhou 310027, China ‡ Center of Analysis and Measurement, Zhejiang University, Hangzhou 310029, China

bS Supporting Information ABSTRACT: Introduced by sodium fluoride, we employed fluorine ions as novel morphology control agents and synthesized highly crystalline TiO2 truncated tetragonal nanobipyramids enclosed by {001} and {101} facets via a facile microemulsion method. By investigating the influence of halogens ions such as fluorine, chloride, and bromine on the morphology of TiO2 nanocrystals, we demonstrate that the rate of crystal growth could be promoted by fluorine and that the selective absorption of fluorine onto the surface of nanoparticles would alter the relative surface free energies of {001} and {101}, eventually controlling the ration of {001} facets to total surface area. Additionally, chainlike 1-D TiO2 nanostructures aligned from nanobipyramids were generated via the mechanism of oriented attachment at a relatively high temperature. The driving force for shape evolution is reducing the high surface free energy. The photocatalytic properties were performance by the photodegradation of methylene blue. This “greener” and versatile synthetic strategy may provide an attractive and effective route for shape-controlling of inorganic nanoparticles.

1. INTRODUCTION Anatase titanium dioxide (TiO2) is one of the most studied transition metal oxides due to its wide applications in fields such as photocatalysis, solar energy conversion, photonic devices, and sensors.1 4 Specially, the photocatalytic properties, which have potential applications in dye-sensitized solar cells, photodecomposition of organic species for environment cleaning, and photosplitting of water, have attracted tremendous attention. Nanoparticles with different morphologies expose different crystal facets and exhibit different surface physical chemical properties.5,6 Li et al.7 investigated the different photocatalytic properties of TiO2 nanoparticles with tunable shapes. Yu et al.8 found that exposed {001} facets enhanced photoelectric conversion of TiO2 nanocrystals. Due to their high surface free energy, {001} facets of anatase are generally considered to be more reactive than {101} facets. The synthesis of TiO2 nanocrystals with exposed {001} facets is a thermal issue in the study of TiO2. Many efforts have been devoted to obtain nanosized TiO2 with a unitary size and morphology, and a number of approaches have been described.9 12 It is well-known that the essence of shape-controlling is tuning the growing rate in specific directions. A popular approach for controlling the shape is named the “surfactant-assisted method”, in which, organic surfactants would selectively bind to specific crystal facets and control the growing rate of specific facets. However, as to anatase TiO2 crystals, {001} facets have high surface free energy, so they would always be eliminated by {101} facets in the crystal growth stage. It is ineffective to control the growing of {001} facets simply by r 2011 American Chemical Society

organic surfactant.13,14 Recently, in the study of metal nanocrystals, Zhang et al.15 found that Br ions could effectively stabilize the {001} facets of Rh nanocrystals. It showed us a new strategy for morphology control of nanocrystals. Yang et al.16 used hydrofluoric acid as a shape-controlling agent and synthesized TiO2 crystals with a large percentage of {001} facet on the surface. However, the use of hydrofluoric acid is dangerous due to its high toxicity. Therefore, it is still a challenge to develop a facile, green, and effective method for syntheses of TiO2 nanocrystals with exposed {001} facets. In this contribution, we employed fluorine ions as morphology control agents and synthesized highly crystalline anatase TiO2 nanobipyramids with exposed {001} facets. Morphologies like spherical particles, nanorods, and chainlike nanocrystals have also been obtained. Compared with hydrofluoric acid, the use of sodium fluoride (NaF) as fluorine source is “greener” and more convenient. In our study, we demonstrate that introducing fluorine into the reaction system would promote the formation of {101} and {001} facets, and it also induce the generation of truncated nanobipyramids with exposed {001} facets. In analogy with organic surfactants, the selectively absorption of fluorine onto the surface of TiO2 nanocrystals would alter the surface free energies and the crystal growth rates of {001} and {101} facets. Additionally, by virtue of exposed {001} facets, chainlike Received: April 12, 2011 Revised: October 26, 2011 Published: October 27, 2011 5221

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nanostructures were aligned via oriented attachment. This “greener” synthetic strategy provides an attractive and effective strategy for fabricating morphology-controlled nanocrystals.

2. EXPERIMENTAL SECTION 2.1. Synthesis. All the regents were purchased from Shanghai Chemical Regent Co. and used as received. In a typical synthesis, 4 mL of titanium butoxide (TNBT, 12 mmol), 38 mL of oleic acid (OLA, 120 mmol), and 0.20 g of sodium fluoride (NaF, 4.8 mmol) were mixed together in a conical flask under vigorous stirring at room temperature for 2 h. Then deionized water (1.5 mL, 84 mmol) was injected into the flask, and the mixture was magnetically stirred for another 48 h to ensure homogeneity. The molar ratio of TNBT to NaF was 5:2. Then, the raw material solution was transferred into a stainless steel autoclave and heated at 250 °C for 24 h. After the reaction, the autoclave was cooled to room temperature naturally, and the water-soluble white precipitate was collected by centrifugation and washed several times with ethanol, acetone, dilute nitric acid, and deionized water. For further investigating the role of NaF, we replaced the NaF by sodium chloride (NaCl, 0.28 g), sodium bromine (NaBr, 0.49 g), and sodium oleate (Na-oleate, 1.57 g), respectively. 2.2. Characterization. Powder X-ray diffraction (XRD) patterns were collected on a Philips PW1050 X-ray powder diffractometer with Cu Kα radiation (λ = 1.5406 Å) and a graphite monochromator from 20° to 80°. The particle size and morphology of the resulting nanocrystals were characterized on a transmission electron microscope (TEM, JEM-1230) at 80 kV. The structure of the nanocrystals was observed by high-resolution transmission electron microscope (HRTEM, Philips CM200) at 160 kV. UV visible spectra were recorded on a Hitachi U-4100 spectrometer in the spectral range of 250 800 nm at room temperature.

Figure 1. (a) TEM image of anatase TiO2 nanobipyramids prepared at 250 °C for 24 h with a molar ratio of TNBT to NaF of 5:2. (b) XRD pattern of the as-prepared TiO2 nanobipyramids. (c, d) HRTEM images of nanobipyramids; the inset shows a schematic model of a typical nanobipyramid.

3. RESULTS AND DISCUSSION 3.1. Controllable Synthesis of TiO2 Nanobipyramids. The as-prepared TiO2 nanocrystals were first characterized by transmission electron microscopy (TEM). As shown in Figures 1a and S1 (Supporting Information), the product consisted of rhombic particles with an average apex-to-apex diameter of ∼40 nm. XRD in Figure 1b shows that all the peaks are in good agreement with the anatase TiO2 (JCPDS No. 211272), indicating that the sample was highly crystalline with few impurities. To better visualize the morphology and the crystal structure of the nanocrystals, we characterized the sample by HRTEM. In Figure 1c, the lattice fringe with the spacing of 0.34 nm corresponds to the {001} planes of anatase, and the apex angle was measured to be 43.5°, corresponding to the angle between {101} and {101} facets. In Figure 1d, the lattice fringe of 0.45 nm corresponds to the (001) planes, indicating that the long axis of the particle is along the [001] crystallographic orientation. The apex angle was also 43.5°, and the rhomb is slightly truncated in apex. On the basis of the Wulff construction17 and our HRTEM observations, we propose that the morphology of rhombic particles in (HR)TEM images are truncated tetragonal nanobipyramids, each of which is enclosed by two {001} facets and eight {101} facets. In the TEM images, most of the particles were observed as oblique-angled parallelograms with only the opposite side equal simply because the nanobipyramid preferred to lie flat on the substrate using of a {101} plane. When the particle was hindered by neighboring particles, it was able to settle on the substrate by only one side of the equatorial plane, as is shown in Figure 1d. Overall, the three-dimensional structure of the nanocrystals was a

Figure 2. (a c) TEM images of the samples prepared at a series of reaction durations: (a) 2 h, (b) 10 h, and (c) 20 h. (d) Schematic illustration of the model for the growth of TiO2 nanobipyramids.

truncated tetragonal nanobipyramid, as is shown in the inset of Figure 1d, and the ratio of edge lengths labeled by A and B is 1:5. The ratio of A to B can be used to calculate the ration of {001} facets to total surface area. 3.2. Time-Dependent Evolution of TiO2 Nanobipyramids. The evolution of nanobipyramids has been monitored by TEM 5222

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Figure 3. (a c) TEM images of nanobipyramids synthesized with the addition of different sodium salts: (a) NaCl, (b) NaBr, and (c) Na-oleate. All of the syntheses were carried out under the same condition as in Figure 1a, except the variation of the sodium salts. (d) XRD patterns taken from the samples in parts a c demonstrating that the additions of those salts had no effect on the composition of the final products.

characterization over the products formed at 250 °C for different reaction durations (2, 10, 20 h). Figure 2 compares the morphologies of the nanoparticles formed at different reaction stages, clearly showing the evolution of nanobipyramids as the elongation of the reaction time. In Figure 2a, quasi-dot-shaped nanoparticles with the size about 10 nm were formed at the early stage (2 h). In the sample of 10 h (Figure 2b), some truncated nanobipyramids (15 30 nm) with exposed {001} facets occurred, and the rest of the particles remained the size of ∼10 nm (Figure 2b). A HRTEM image shows a typical truncated nanobipyramid, which is similar to the Wulff model. While extending the reaction to 20 h, as is seen in Figure 2c, two distinctive morphologies were generated: dots (about 5 nm) and nanobipyramids (30 40 nm). This feature of morphology distribution could be explained by the Ostwald ripening (OR) crystal growth mechanism. Figure 2d schematically depicted the shape evolution of nanobipyramids. The formation of the nanocrystals involves two steps: nucleation from original solution and the subsequent crystal growth from these preformed nuclei. Reducing the total surface free energy under the constraint of fixed volume is the driving force for shape evolution. As to anatase TiO2, the evolution is mainly determined by the growth rate ratio between {101} and {001} facets,18,19 and the surface free energy of the {001} facet is higher than that of the {101} facet.14,20 As the reaction evolved, {001} facets would be gradually eliminated by {101} facets, as is shown in Figure 2d. According to the OR mechanism, in the later stages of crystal growth, the precursor concentration in solution became low; thus, the growth of larger nanocrystals is at the expense of smaller ones by the ion diffusion.21 This can well explain the coexistence of two distinctive morphologies in Figure 3c. Eventually, the minimization of the overall surface energy is achieved by the OR process.

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Figure 4. (a c) TEM images of TiO2 nanoparticles synthesized in the presence of NaF at different concentrations, demonstrating the role fluorine in the formation of exposed {001} facets. The molar ratios of TNBT to NaF were (a) 5:0, (b) 5:1, and (c) 5:4. All of the syntheses were carried out under the same condition as in Figure 1a, except the variation of NaF concentration. (d, e) HRTEM images of nanocrystals taken from the samples in part c and their schematic models.

3.3. The Function of Sodium Fluoride (NaF). In the present synthesis, fluorine played a prominent role in the formation of truncated nanobipyramids. To decipher the function of fluorine, we varied the additive to be NaCl, NaBr, and Na-oleate, keeping the molar ratio of TNBT to sodium salt as 5:2. When NaCl was added, the products were elongated and spherical (seen in Figure 3a), and the average size along the long axis was about 16 nm. In the cases of NaBr and Na-oleate (Figure 3b, c), the morphologies of the products were mixed: spherical and rodlike. The nanorods grew along the [001] direction. The XRD patterns in Figure 3d indicated that the products were pure anatase TiO2, and the addition of those sodium salts had no effect on the composition of the final products. It should be noticed that the size of particles synthesized in the presence of different sodium salts could be sequenced as SNaF > SNaCl ≈ SNaBr > SNa‑oleate. What is the driving force for the difference of size in the current work? The dependence of size on the sort of sodium salt can be attributed to the difference in the binding nature of halide ions and OLA molecules onto the surface of nanocrystals. Due to their high oxophilicity, OLA ligands always form strong bond to the surface of nanocrystal,22 and long hydrocarbon chains absorbed on nanocrystal result in a steric hindrance layer, which hinders the monomers transfer of Ti and O atoms from solution phase to nanoparticles to support the growth of crystals. Halide ions are able to chemisorb onto the surface of crystal via coordination with metal atoms.15,23,24 However, the adsorption of halogen produces a nonsignificant stereohindrance effect. In a defined colloidal system, the more halide ions that are bound to the surface of nanocrystals, the less OLA molecules that would be adsorbed. Thus, in the present of halide ions, the effect of stereohindrance originated from OLA would be 5223

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Crystal Growth & Design weakened, and the crystal growth would be promoted. Furthermore, according to the difference of elemental electronegativity, fluorine ions have stronger bonding ability than chloride or bromine ions. So when NaF was added into the original solution, the rate of crystal growth would be faster than that of NaCl and NaBr. Furthermore, we performed a series of syntheses by altering the concentration of NaF in raw material solution, and the results are summarized in Figure 4. As expected, synthesis in the absence of fluorine only produced elongated spherical nanoparticles (Figure 4a). Both morphology and size were similar to the sample synthesized by NaCl (Figure 3a). This is yet another demonstration that the fluorine can promote the rate of crystal growth in the current system and suggests that Na+ ions have no effect on the morphology of the products. At low concentration of fluorine (0.1 g of NaF, TNBT/NaF = 5:1), the products showed mixed morphologies from elongated spheres to bipyramids (Figure 4b). Further increase the addition of NaF to 0.4 g (TNBT/NaF = 5:4) resulted in an increase of exposed {001} facets, as is shown in Figure 4c. The HRTEM images in parts d and e of Figure 4 showed the ratio of A to B as 1:3 and 1:2, implying that with the increasing of NaF concentration the ration of {001} facets to total surface area increased. The dependence of morphology on the concentration of fluorine strongly suggested that the presence of fluorine would effectively stabilize the {001} facets, and the selective absorption of fluorine would alter the relative surface free energy of {001} and {101} facets. This is in good agreement with the study of Yang et al.,16 in which they demonstrated that the surface free energy of the fluorineterminated (001) surfaces is lower than fluorine-terminated (101). In analogy with organic surfactants, we demonstrate that the selective adsorption of fluorine onto surface of anatase nanocrystals would lead to an adjustment of the growth rate ratio between {101} and {001} facets; thus, fluorine ions could be regarded as a new morphology control agent in the preparation of TiO2 nanocrystals. On the basis of the above results, we believe that the final shapes of anatase nanoparticles could be tuned by the addition of different sodium salts, and this technique may be developed into an alternative synthesis route for shape controlling, which is analogous to the organic surfactant assisted route. 3.4. Effect of Reaction Temperature. A proper temperature is important for the shape evolution of TiO2 nanocrystals. TEM images in parts a, b, and c of Figure 5 present the samples synthesized at 150, 200, and 300 °C, respectively. In Figure 5a, the products grown at 150 °C are mainly quasibipyramids, and the average size is about 10 nm. When the reaction temperature increased to 200 °C, the morphologies of the sample are not quite uniform and the average size is about 18 nm, as is seen in Figure 5b. Comparison with the products synthesized at 250 °C (Figure 1a) indicates that with the increasing of the temperature the crystal growth rate increased. It is noteworthy that at an elevated temperature of 300 °C, some particles self-assembled into chainlike nanostructures (highlighted by straight lines in Figure 5c). A detailed crystal structure analysis was performed by HRTEM, as shown in Figure 5d. According to the HRTEM images, the building particles are single crystal, and a highmagnification image of the interfacial junction shows that the fringe spacing is 0.47 nm, indicating that the primary particles fused with each other via common {001} facets, and the nanochains were coalesced in well-defined [001] crystalline orientation. A reasonable model for such a chainlike TiO2 nanostructure is shown in the inset of Figure 5d. This novel structure is formed by

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Figure 5. (a c) TEM images of nanostructures synthesized at different temperatures: (a) 150, (b) 200, and (c) 300. (d) HRTEM image of the chainlike nanostructures corresponding to the sample in part c. The inset in the lower left corner shows a high-magnification image of a typical interfacial junction, and the inset in the top right corner shows a schematic model of chainlike nanostructures.

a newly proposed crystal growth mechanism named “oriented attachment” (OA), where in the individual crystals could aggregate and combine to form a large “single crystal”.25,26 At the beginning of the reaction, the nucleation and crystal growth were accelerated at a relative high reaction temperature. In this stage, primary nanoparticles were generated. Then the precursor concentration in solution gradually decreases, and the ordinary crystal growth process terminated. In the later of crystal growth stage, the OA mechanism became dominant, and the minimization of the overall surface energy was achieved by elimination of high energy surfaces. The {001} facets have the largest number of dangling bonds and higher surface energy than {101} facets. In colloidal solution, when separated nanoparticles approached each other closely, they would rearrange along the [001] direction and fuse high-energy facets to form a low-energy configuration. On the basis of the temperature-dependent study, we propose that the OA mechanism may occur at a relatively high temperature. When a sufficient thermal energy was supplied, the fusion of high energy faces is thermodynamically favorable. It is expected that this strategy could be further extended to prepare longer TiO2 nanowires via modification for the experiment, and these TiO2 nanowires will find promising application in the fabrication of dye-sensitized solar cells and electronic devices. 3.5. Photocatalytic Properties of the Anatase TiO2 Nanobipyramids. The photocatalytic activity of the nanobipyramids (taken from the products as is shown in Figure 1a) was investigated by photodegradation of methylene blue (MB) aqueous solutions under full spectrum light irradiation at room temperature. A 300 W (optical output power: 50 W) xenon lamp with the wavelength range of 300 1100 nm was used as light source to trigger the photocatalytic reaction. Typically, 40 mg of TiO2 nanocrystals was dispersed in 200 mL of 20 μmol/L MB 5224

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Figure 6. (a) The absorption spectra of the MB aqueous solution under exposure of full spectrum light irradiation in the presence of the anatase TiO2 nanobipyramids. (b) Photocatalytic properties in decomposition of MB dye in comparison with different catalyst: (A) TiO2 nanobipyramids, (B) Degussa P25, (C) and no catalyst. The inset in part b shows the dependence of ln(C/C0) on irradiation time.

aqueous solution in an ultrasonic treatment to form a suspension, and then the suspension was magnetically stirred for 1 h to allow the mixture to reach the adsorption desorption equilibrium before light irradiation. At regular irradiation time intervals (10 min) the reaction solution was sampled and centrifuged to separate the TiO2 nanocrystals. The concentration of MB was determined by a UV visible spectrometer. For comparison, commercial TiO2 P25 (Degussa P25, Degussa Co.) was used as the reference sample for the photocatalytic activity. Figure 6a shows the variation of absorption spectra of the MB aqueous solution under full spectrum light irradiation for a series of different reaction durations in the presence of TiO2 nanobipyramids. The drop of the absorption peaks (λ = 655 and 292 nm) of MB indicates that the MB dye is decomposed by photocatalysis. A comparison of photocataltic activities of the TiO2 nanobipyramids and P25 is shown in Figure 6b. The nanobipyramids exhibit a photocatalytic activity superior to that of P25. The linear relationship of ln(C/C0) vs time (inset in Figure 6b) shows that the photocatalytic degradation of MB dye follows pseudo-first-order kinetics. We propose that the excellent photocatalytic activity of nanobipyramids may be attributed to their high crystallinity and few defects on the surfaces. The TiO2 nanobipyramids show enhanced photocatalytic efficiency and could possibly be further used in photovoltaic cells.

4. CONCLUSIONS In summary, we have successfully prepared highly crystalline TiO2 truncated nanobipyramids enclosed by two {001} facets and eight {101} facets via a facile microemulsion method. In this system, induced by the addition of NaF, fluoride acted as novel morphology control agent. The absorption of fluoride not only promoted the crystal growth rate but also altered the surface energies of different facets and influenced the final morphology of TiO2 nanocrystals. Two distinctive crystal growth mechanisms, named Ostwald ripening and oriented attachment, were employed to simulate the process of shape evolution, which would lead us to a further understanding of formation mechanisms of nanostructures. Certainly, a more direct contribution of this work is the demonstration of a “green” and versatile strategy for shape-controlling of nanoparticles, in which specific anions (such as F , Cl , Br , oleate, and so on) induced by the addition of special salts could act as shape control agents in the synthesis of nanoparticles.

’ ASSOCIATED CONTENT

bS

Supporting Information. Low-magnification TEM and SAED images of nanobipyramids, photo of the photodegradation of MB aqueous solution, and the variation of absorption spectra of MB aqueous solution. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Fax: (+86) 571-879-52334. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by NSFC (50928201, 50972127, 51010002), the Zhejinag Provincial Natural Science Foundation of China (Z4080021 and Y4090067), the Fundamental Research Funds for the Central Universities, and the Doctoral Fund of the Ministry of Education of China (20100101110039). ’ REFERENCES (1) Fujishima, A.; Honda, K. Nature 1972, 238, 37–38. (2) Regan, B. O.; Gr€atzel, M. Nature 1991, 353, 737–740. (3) Li, Y.; Somorjai, G. A. Nano Lett. 2010, 10, 2289–2295. (4) Gr€atzel, M. Nature 2001, 414, 338–344. (5) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. Rev. 2005, 105, 1025–1102. (6) Zhou, Z. Z.; Tian, N.; Li, J. T.; Broadwell, I.; Sun, S. G. Chem. Soc. Rev. 2011, 40, 4167–4185. (7) Li, J. M.; Yu, Y. X.; Chen, Q. W.; Li, J. J.; Xu, D. S. Cryst. Growth Des. 2010, 10, 2111–2115. (8) Yu, J. G.; Fan, J. J.; Lv, K. L. Nanoscale 2010, 2, 2144–2149. (9) Yu, J. C.; Yu, J.; Ho, W.; Zhang, L. Chem. Commun. 2001, 19, 1942–1943. (10) Dinh, C. T.; Nguyen, T. D.; Kleita, F.; Do, T. O. ACS Nano 2009, 3, 3737–3743. (11) Adachi, M.; Murata, Y.; Takao, J.; Jiu, J.; Sakamoto, M.; Wang, F. J. Am. Chem. Soc. 2004, 126, 14943–14949. (12) Deng, Q.; We, M.; Ding, X.; Jiang, L.; Wei, K.; Zhou, H. J. Cryst. Growth 2010, 312, 213–219. (13) Wu, B.; Guo, C.; Zheng, N.; Xie, Z.; Stucky, G. D. J. Am. Chem. Soc. 2008, 130, 17563–17567. (14) Jun, Y.; Casula, M. F.; Sim, J. H.; Kim, S. Y.; Cheon, J.; Alivisatos, A. P. J. Am. Chem. Soc. 2003, 125, 15981–15985. 5225

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