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Articles Control of TiO2 Structures from Robust Hollow Microspheres to Highly Dispersible Nanoparticles in a Tetrabutylammonium Hydroxide Solution Yong Joo Kim, Seung Yong Chai, and Wan In Lee* Department of Chemistry, Inha UniVersity, Incheon, 402-751 Korea ReceiVed March 19, 2007. In Final Form: July 4, 2007 A new process for controlling the structure of TiO2 from hollow microspheres to highly dispersible nanoparticles has been developed by altering the concentration of tetrabutylammonium hydroxide (TBAH) in the solvothermal reaction of titanium isopropoxide. Robust and size-controllable hollow TiO2 microspheres, constructed by the assembly of 18 nm TiO2 nanoparticles, were synthesized at relatively high TBAH concentration. The diameters of hollow spheres, with a shell thickness of ∼250 nm, were controlled to 1.5-4 µm by varying the concentration of TBAH in the range of 0.1-0.5 M. After calcination at 450 °C, the hollow microspheres were not appreciably deformed and were still floating on the surface of the water. However, highly dispersible TiO2 nanoparticles with an average diameter of 13 nm were obtained at a low TBAH concentration such as 9.2 mM. The colloidal particle size of TiO2 in an aqueous suspension at pH 2 was 12.5-13.5 nm, which indicates that the each nanoparticle is completely separated. The overall procedure is simple and highly reproducible, and large-scale synthesis is available at low cost.
* To whom correspondence should be addressed. E-mail: wanin@ inha.ac.kr.
oil droplets,18-20 microemulsion droplets,21,22 and surfactants,23-25 have been developed. Also, there have been a few reports on the preparation of hollow TiO2 microspheres without introducing templates.26-28 In this synthetic method, however, issues regarding the control of shape and size and problems with the crystallinity and structural stability of hollow TiO2 microspheres need to be further investigated. In our new process, highly crystallized, robust, hollow TiO2 microspheres were synthesized in a rich bulky-alkyl ammonium hydroxide environment during a solvothermal reaction without the addition of any templates or surfactants. The prepared hollow TiO2 microspheres were made up of highly crystallized anatase TiO2 nanoparticles without the contamination of inorganic ions. Furthermore, their structures were physically strong and demonstrated extended thermal stability during calcination without pinholes or cracks on their surfaces. Extensive efforts have been made so far to synthesize TiO2 nanoparticles.29-36 However, controlling the size and shape is
(1) Honda, K.; Hujishima, A. Nature 1972, 238, 38. (2) Turchi, C. S.; Ollis, D. F. J. Catal. 1990, 122, 178. (3) Nozik, A. J. Ann. ReV. Phys. Chem. 1978, 29, 189. (4) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (5) Pelizzetti, E. Sol. Energy Mater. Sol. Cells 1995, 38, 453. (6) O’Regan, B.; Gra¨tzel, M. Nature 1991, 335, 737. (7) Gra¨tzel, M. Nature 2001, 414, 338. (8) Gra¨tzel, M. Nature 2001, 409, 575. (9) Jokanovic, V.; Jokanovic, B.; Nedeljkovic, J.; Milosevic, O. Colloids Surf., A 2004, 249, 111. (10) Jokanovic, V.; Spasic, A. M.; Uskokovic, D. J. Colloid Interface Sci. 2004, 278, 342. (11) Iida, M.; Sasaki, T.; Watanabe, M. Chem. Mater. 1998, 10, 3780. (12) Strohm, H.; Loebmann, P. J. Mater. Chem. 2004, 14, 2667. (13) Yin, J.; Chen, H.; Li, Z.; Qian, X.; Yin, J.; Shi, M.; Zhou, G. J. Mater. Sci. 2003, 38, 4911. (14) Lu, Y.; Yin, Y.; Xia, Y. AdV. Mater. 2001, 13, 271. (15) Zhang, K.; Zhang, X.; Chen, H.; Chen, X.; Zheng, L.; Zhang, J.; Yang, B. Langmuir 2004, 20, 11312. (16) Caruso, R. A.; Susha, A.; Caruso, F. Chem. Mater. 2001, 13, 400. (17) Yang, Z.; Niu, Z.; Lu, Y.; Hu, Z.; Han, C. C. Angew. Chem., Int. Ed. 2003, 42, 1943.
(18) Nakashima, T.; Kimizuka, N. J. Am. Chem. Soc. 2003, 125, 6386. (19) Hu, J.-S.; Guo, Y.-G.; Liang, H.-P.; Wan, L.-J.; Bai, C.-L.; Wang, Y.-G. J. Phys. Chem. B 2004, 108, 9734. (20) Kimura, I.; Isono, Y.; Tanaka, M. Mater. Res. Bull. 2005, 40, 1202. (21) Imhof, A.; Pine, D. J. AdV. Mater. 1999, 11, 311. (22) Schacht, S.; Huo, Q.; Voigt-Martin, I. G.; Stucky, G. D.; Schuth, F. Science 1996, 273, 768. (23) Wong, M. S.; Cha, J. N.; Choi, K.-S.; Deming, T. J.; Stucky, G. D. Nano Lett. 2002, 2, 583. (24) Ren, T.-Z.; Yuan, Z.-Y.; Su, B.-L. Chem. Phys. Lett. 2003, 374, 170. (25) Eiden-Assmann, S.; Widoniak, J.; Maret, G. Chem. Mater. 2004, 16, 6. (26) (a) Yang, H. G.; Zeng, H. C. J. Phys. Chem. B 2004, 108, 3492. (b) Li, J.; Zeng, H. C. Angew. Chem., Int. Ed. 2005, 44, 4342. (27) Wang, X. M.; Xiao, P. J. Mater. Res. 2005, 20, 796. (28) Li, Y.; Song, C.; Hu, Y.; Wei, Y.; Wei, Y. Chem. Lett. 2006, 35, 1344. (29) Martin, S. T.; Herrmann, H.; Choi, W.; Hoffmann, M. R. J. Chem. Soc., Faraday Trans. 1994, 90, 3315. (30) Anpo, M.; Aikawa, N.; Kubokawa, Y. J. Chem. Soc., Chem. Commun. 1984, 644. (31) Stathatos, E.; Lianos, P.; Del Monte, F.; Levy, D.; Tsiourvas, D. Langmuir 1997, 13, 4295.
Introduction With its unique characteristics in band position and surface structure, TiO2 has promising application as photocatalysts,1-5 photovoltaics,6,7 electrochromic devices,8 and others. Tailoring the TiO2 nanostructure to large surface area, high crystallinity, and controlled shape and pore structure would be a crucial subject in the realization of the above-mentioned applications. Hollow TiO2 microspheres, which have unique pore structures in the wall and inner space, have drawn increasing scientific attention as a promising candidate. Thus far, hollow TiO2 microspheres have been prepared by a spray pyrolysis technique9-11 or by a sol-gel reaction utilizing polymer or silica microspheres as templates.12-17 Recently, several other preparation methods using soft templates, such as synthesis by an interfacial reaction using
10.1021/la700797v CCC: $37.00 © 2007 American Chemical Society Published on Web 08/14/2007
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still very difficult and is quite different from that of other metal oxides. The preparation of stable suspensions free from mutual agglomeration is another important issue in the preparation of TiO2 nanoparticles. Previously, Chemseddine et al. synthesized highly monodisperse TiO2 nanoparticles under basic conditions with tetramethylammonium hydroxide (N(CH3)4OH, TMAH).32 The prepared TiO2 nanoparticles were highly monodisperse and stably suspended. However, a weak point in this process was that the concentration of titanium alkoxide used for the formation of nanoparticles was as low as 7 mM, which makes large-scale synthesis difficult. In our preparation route, the concentration of titanium isopropoxide was more than 10 000 times higher, and the prepared TiO2 nanoparticles were moderately monodisperse and completely separated from each other in aqueous suspension. Experimental Section Preparation of Hollow TiO2 Microspheres and Nanoparticles. Typically, for the preparation of TiO2 hollow spheres, 2 mmol of titanium isopropoxide (TTIP, Aldrich Chemical Co., 97%) was dissolved in 20 mL of ethanol (99%, Aldrich). Tetrabutylammonium hydroxide (TBAH, 4 mmol, 40% aqueous solution, Aldrich) was then added slowly to this solution. The molar ratio in the resulting solution was 1.0:2.0:43.5:170 TTIP/TBAH/H2O/ethanol. The mixture was stirred for 30 min to obtain a clear solution and transferred to an autoclave made of titanium. The temperature of the autoclave was raised to 240 °C at a rate of 5 °C/min and held at this temperature for 6 h. The white precipitate obtained was washed several times with ethanol. For the preparation of TiO2 nanoparticles, 2 mmol of TTIP was dissolved in 20 mL of ethanol, and 0.184 mmol of TBAH was slowly added to this solution. The molar ratio in the solution was 1.0:0.092:2.0:170 TTIP/TBAH/H2O/ethanol. The other reaction conditions were the same as those used to prepare the TiO2 microspheres. Characterization. X-ray diffraction (XRD) patterns for the prepared TiO2 samples were obtained by using a Rigaku Multiflex diffractometer with monochromated high-intensity Cu KR radiation. The structure and shape of the hollow TiO2 microspheres and nanoparticles were analyzed by a transmission electron microscope (TEM, Philips CM30) operating at 250 kV. For this measurement, 3 mg of hollow TiO2 microspheres or 1 mg of TiO2 nanoparticles was dispersed in 50 mL of methanol, and a drop of the suspension was then spread on a holey amorphous carbon film deposited on a Ni grid (JEOL Ltd.). The overall shape of hollow TiO2 microspheres was also observed by a field-emission scanning electron microscope (FESEM, Hitachi S-4500). Colloidal particle sizes of TiO2 nanoparticles suspended in aqueous solution were measured by a zeta potential and particle size analyzer (Otsuka Electronics, ELS-Z).
Figure 1. SEM images of the as-prepared hollow TiO2 microspheres (a) and a broken microsphere (b). Low-magnification TEM image of the as-prepared hollow TiO2 microspheres (c). TEM images of the sliced hollow TiO2 microspheres (d, e) and the broken edge of a microsphere (f). All of the hollow TiO2 microspheres were formed with 0.2 M TBAH during a solvothermal reaction at 240 °C for 6 h.
Results and Discussion Figure 1a shows an SEM image of the as-prepared TiO2 microspheres under the condition of 0.2 M TBAH. The diameter of the spheres was in the distribution of 1-3 µm, and their surfaces were smooth, without cracks or pinholes. The rarely found broken microsphere shown in the SEM image of Figure 1b indicates that the prepared microsphere has a hollow structure with a thick shell. The shell thickness was estimated to be about (32) (a) Mortiz, T.; Reiss, J.; Diesner, K.; Su, D.; Chemseddine, A. J. Phys. Chem. B 1997, 101, 8052. (b) Chemseddine, A.; Moritz, T. Eur. J. Inorg. Chem. 1999, 235. (33) Trentler, T. J.; Denler, T. E.; Bertone, J. F.; Agrawal, A.; Colvin, V. L. J. Am. Chem. Soc. 1999, 121, 1613. (34) Cozzoli, P. D.; Kornowski, A.; Weller, H. J. Am. Chem. Soc. 2003, 125, 14539. (35) (a) Chae, S. Y.; Park, M. K.; Lee, S. K.; Kim, T. Y.; Kim, S. K.; Lee, W. I. Chem. Mater. 2003, 15, 3326. (b) Pan J. H.; Lee, W. I. Bull. Korean Chem. Soc. 2005, 26, 418. (36) Han, S.; Choi, S.-H.; Kim, S.-S.; Cho, M.; Jang, B.; Kim, D.-Y.; Yoon, J.; Hyeon, T. Small 2005, 1, 812.
Figure 2. XRD patterns of the hollow TiO2 microspheres (a) and the TiO2 nanoparticles (b). All of the indexed peaks were assigned to the anatase phase.
0.25 µm. The dark color over the whole microsphere in the TEM image of Figure 1c also suggests that the shell of the hollow structure is quite thick. Figure 1d,e shows TEM images of the TiO2 microspheres sliced with a thickness of 50 nm by the technique of ultamicrotomy. Each microsphere was a hollow sphere structure with a wall thickness of ∼0.25 µm. It is found from the high-magnification TEM image shown in Figure 1f that the hollow spheres are made up of TiO2 nanoparticles with an average diameter of 18 nm. Figure 2a shows the XRD patterns for the hollow TiO2 microsphere. It is indicated that the hollow TiO2 microspheres consist of highly crystallized anatase without
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Figure 3. SEM images of the hollow TiO2 microspheres prepared at several TBAH concentrations. The scale bar is 3 µm in each panel. (a) 0.1, (b) 0.2, (c) 0.4, and (d) 0.5 M TBAH.
any impurity phase. The crystallite size, as determined from the anatase (101) peak using the Scherrer equation, was 19.0 nm, which is comparable to the particle size obtained from TEM image. In the preparation of hollow TiO2 microspheres, the TBAH concentration of the precursor solution for the solvothermal reaction was varied from 0.1 to 0.5 M. As shown in Figure 3, the average size of the synthesized TiO2 microspheres gradually increased with increasing TBAH concentration, even though their sizes were not highly monodisperse. The average diameter of the hollow spheres derived from the 0.5 M TBAH solution was 4 µm. However, the smallest hollow sphere with an average diameter of 1.5 µm was obtained from the 0.1 M TBAH solution, and the structure of the hollow spheres was no longer formed at concentrations lower than this critical concentration. The influence of water in the Ti-precursor solution was also investigated. It turned out that the water content was not appreciably relevant to the formation of hollow microspheres.37 A notable observation was that ultrahighly dispersible TiO2 nanoparticles were formed at TBAH concentrations lower than 0.02 M. Typically, in a solvothermal reaction at 240 °C for 6 h in 9.2 mM TBAH, TiO2 nanoparticles were collected as a white precipitate in the ethanol-based solution. The as-prepared particles were not well dispersed in water or ethanol. However, this opaque suspension containing 0.10 g of TiO2 nanoparticles in a 150 mL aqueous solution became dramatically transparent as soon as the pH of the solution was adjusted to 1.4-2.3 with 1.0 M HCl. Even after several months, the suspension still remained transparent. Figure 4a shows photographs of the TiO2 suspensions at various pH values. At pH higher than 12, controlled by the addition of tetramethylammonium hydroxide (TMAH), the suspension also became transparent. The zeta potential of the TiO2 suspension was measured as a function of pH, as shown in Figure 4b. At pH 2, the zeta potential turned out to be +28 mV, which is a sufficiently high potential for the formation of a stable suspension resulting from electrostatic repulsion among the nanoparticles. Counterions also influence the stability of colloids in aqueous solution38 and can reduce the thickness of the diffuse layer and decrease the zeta potential. With the decrease in pH to values lower than 1.2, the concentration of Cl- becomes sufficiently high, and this leads to an abrupt decrease in the zeta potential, as shown in Figure 4b. Thus, the electrolytic coagulation of TiO2 nanoparticles occurs at very low pH. An aliquot of the TiO2 suspension (5 mg of TiO2 nanoparticles in 100 mL of water), adjusted to pH 2, was dropped on a holey (37) See Supporting Information. (38) Reed, J. S. Principle of Ceramics Processing, 2nd ed.; John Wiley & Sons; New York, 1995; Chapter 10.
Figure 4. Photographs (a) and zeta potential (b) for aqueous suspensions of TiO2 nanoparticles at various pH values. TiO2 nanoparticles (0.10 g) with an average size of 13 nm were suspended in 150 mL of water, and 1.0 M HCl or 1.0 M TMAH was added for pH control. A suspension of Degussa P25 under the same conditions was also included for comparison.
Figure 5. TEM images of TiO2 nanoparticles spread on a holey amorphous carbon film (a, b). High-resolution TEM image of an as-prepared TiO2 nanoparticle (c). Distribution of colloidal particles in an aqueous suspension of TiO2 nanoparticles (d). TiO2 nanoparticles (10 mg) with an average size of 13 nm were suspended in 100 mL of water.
amorphous carbon film deposited on a Ni grid (JEOL Ltd.). The TEM images in Figure 5a,b indicate that the TiO2 nanoparticles are ultrauniformly spread in the form of a monolayer over the whole TEM grid. The size and shape of individual TiO2 nanoparticles were not highly uniform, but the each nanoparticle
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with an average size of 13 nm was completely separated without agglomeration. As shown in Figure 2b, the XRD patterns of the TiO2 nanoparticles are similar to those of the hollow spheres, and the crystallite size was calculated to be 14 nm, which is close to the size determined by TEM. Figure 5c shows a high-resolution TEM image of a TiO2 nanoparticle. Uniform fringes with an interval of 0.35 nm, corresponding to the (101) lattice spacing of the anatase phase, were observed over the entire particle. This indicates that each nanoparticle consists of single anatase grain. The colloidal particle size in the TiO2 aqueous suspension adjusted to pH 2 was also measured by the light scattering method. Figure 5d shows the distribution of colloidal particles as a function of particle size. Most of the colloids in the suspension were 12.313.4 nm in size, which indicates that all of the individual particles were completely separated without agglomeration in the aqueous solution. Compared to the report by Chemseddine et al.,32 who synthesized highly monodisperse TiO2 nanoparticles in very dilute titanium alkoxide with tetramethyl ammonium hydroxide (TMAH), the TiO2 nanoparticles obtained in this work were less uniform in shape and size, but their dispersibility in aqueous solution was comparable. Considering that these nanoparticles were synthesized at more than 10 000 times higher Ti concentration, this process has great a advantage in large-scale synthesis and practical applications. The sizes of the TiO2 nanoparticles were not greatly dependent on the TBAH concentration during the hydrothermal reaction, but its concentration strongly influences the interparticle assembly. That is, the TiO2 nanoparticles prepared at 9.2 mM TBAH were completely separated from each other, whereas those prepared at concentrations higher than 0.10 M were fully aggregated to form large hollow spheres. Why, then, are hollow microspheres formed at relatively higher TBAH concentration? With the increase in TBAH concentration, the ionic strength of the solution will increase. Therefore, there is a higher aggregation tendency for the TiO2 nanoparticles during the solvothermal reaction. Furthermore, a noticeable property of TBAH is that it has an unusually low flash point (∼110 °C). From mass spectrometry, the evolution of gaseous C4H8 and the formation of N(C4H9)3 were detected, and a relatively small amount of TBAH remained after the solvothermal reaction at 240 °C. Therefore, it is deduced that the majority of TBAH in ethanol is thermally decomposed and evolves gaseous C4H8 during the solvothermal reaction, as described in the following equation
N(C4H9)4OH(aq) f C4H8(g) + N(C4H9)3(aq) + H2O
(1)
. It was reported by Li et al. that the evolution of microbubbles during the solvothermal reaction can provide the aggregation center.39 In particular, at high TBAH concentration, the nanoparticles will be heavily aggregated because of the high ionic strength. Hence, the individual TiO2 nanoparticles formed in the early stage of solvothermal reaction could be assembled at the gas-liquid interface (between C4H8 microbubbles and ethanolbased solution) and finally induces the formation of hollow TiO2 microspheres, as described in Figure 6. Also, the size increase of the hollow microspheres according to the increase in TBAH concentration, as shown in Figure 3, is presumably due to the size increase of microbubbles caused by the high concentration of TBAH. However, at low TBAH concentration, the formation of microbubbles will be suppressed because of the small amount of evolved gas. Also, the tendency toward interparticle aggregation (39) Peng, Q.; Dong, Y.; Li, Y. Angew. Chem., Int. Ed. 2003, 42, 3027.
Figure 6. Schematic diagram of the formation process of hollow TiO2 microspheres during the solvothermal reaction.
Figure 7. SEM image of hollow TiO2 microspheres calcined at 450 °C for 2 h (a) and photograph of calcined TiO2 microspheres floating on water (b).
would not be high at low ionic strength. As a result, the TiO2 nanoparticles do not form the self-assembled structure. The chain length of the alkyl group in the tetraalkylammonium hydroxide used in the solvothermal synthesis of TiO2 was also varied from methyl to pentyl. Of these, only the TBAH-containing butyl group induced a hollow microsphere structure. From the other tetraalkylammonium hydroxides, only the nanoparticles were formed without the formation of any self-assembled hollow microsphere structure.37 This is closely related to the formation of microbubbles by the decomposition of tetraalkylammonium hydroxide. It is reported that the tetraalkylammonium hydroxides with methyl-to-propyl groups are thermally stable during reflux.37 Furthermore, after the solvothermal reaction the pressure of the hydrothermal bomb was not unusually increased, which indicates no appreciable evolution of gaseous chemical species. In the case of tetrapentylammonium hydroxide, there is no clear information about the thermal stability,37 but at least it does not provide gas molecules. This observation indicates that the formation of microbubbles is essential for the formation of hollow TiO2 microspheres. The hollow TiO2 microspheres prepared in 0.2 M TBAH were calcined at 450 °C in air for 2 h. As shown in Figure 7a, the spherical structures were not appreciably deformed or damaged, and there were no apparent pores or cracks on their surfaces. Figure 7b shows the heat-treated TiO2 microspheres floating on the surface of the water. Among all of the samples, approximately 70% of the hollow microspheres were floating, and there was no additional precipitation for more than 1 month. This is an unexpected result, considering that nanopores are expected to adhere to the shell of TiO2 hollow spheres, which were constructed from 18 nm TiO2 nanoparticles. The high thermal stability and long-term buoyancy of the TiO2 microspheres were presumably due to the tight aggregation among the TiO2 nanoparticles and the large shell thickness (∼0.25 µm), which protects the framework of TiO2 microspheres from physical damage and prevents water from penetrating the vacant core of the hollow sphere. It is expected that the prepared TiO2 microspheres have promising applications as photocatalysts in the treatment of oil spills and other waterborne pollutants in the sea or in lakes40-42 (40) Jackson, N. B.; Wang, C. M.; Luo, Z.; Schwitzgebel, J.; Ekerdt, J. G.; Brock, J. R.; Heller, A. J. Electrochem. Soc. 1991, 138, 3660.
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because they can float on water surfaces for a long time. However, the ultrahighly dispersible TiO2 nanoparticles prepared in this study will also be promising candidates for the fabrication of transparent photocatalytic and superhydrophilic films35,43 as well as for the formation of electrode layers in dye-sensitized solar cells.6,7
Conclusions The concentration of TBAH during the solvothermal reaction was critical to the formation of hollow TiO2 microspheres. Hollow microspheres were formed in 0.1-0.5 M TBAH, and the average diameter was controlled to range from 1.5 to 4 µm, which suggests that the size of a hollow microsphere was proportional to the concentration of TBAH. However, ultrahighly dispersible TiO2 nanoparticles were formed at low TBAH concentration. Typically, (41) Rosenberg, I.; Brock, J. R.; Heller, A. J. Phys. Chem. 1992, 96, 3423. (42) (a) Li, X. Z.; Liu, H.; Cheng, L. F.; Tong. H. J. EnViron. Sci. Technol. 2003, 37, 3989. (b) Li, X. Z.; Liu, H.; Cheng, L. F.; Tong. H. J. J. Chem. Technol. Biotechnol. 2004, 79, 774. (43) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature 1997, 388, 431.
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TiO2 nanoparticles, synthesized by a solvothermal reaction at 240 °C in 9.2 mM TBAH, were very soluble at pH 1.4-2.3 and >12. The corresponding suspensions were transparent, and the colloidal particle size was only 12.5-13.5 nm. The formation of hollow microsphere is ascribed to the evolution of gaseous C4H8 induced by the decomposition of TBAH during the solvothermal reaction. The evolved microbubbles in ethanolbased solution can provide the aggregation center for the TiO2 nanoparticles, and they are tightly assembled at their gas-liquid interfaces. Acknowledgment. This work was financially supported by the Korean Science and Engineering Foundation (KOSEF R012006-000-10956-0) and the support project of the University ITRC Program (IITA-2006-C109006030030). Supporting Information Available: Experimental results for the effect of water content and for the dependence of tetraalkyl ammonium hydroxide in the synthesis of TiO2 hollow microspheres and/or nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org. LA700797V
