Surfactant-free Synthesis of Anatase TiO2 Nanorods in an Aqueous Peroxotitanate Solution Yanfeng Gao,*,† Hongjie Luo,† S. Mizusugi,‡ and Masayuki Nagai‡ Shanghai Institute of Ceramics (SIC), Chinese Academy of Sciences (CAS), Dingxi 1295, Shanghai 200050, China, and Musashi Institute of Technology, AdVanced Research Laboratories, Tokyo 158-0082, Japan
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 6 1804–1807
ReceiVed NoVember 23, 2007; ReVised Manuscript ReceiVed March 23, 2008
ABSTRACT: We report for the first time the formation of spindle-type, monodispersed anatase nanorods from a peroxotitanate solution without the use of organics (such as surfactants). The formation process is highly dependent upon soaking time, solution pH, and concentration, undergoing both morphology and phase transformation during growth. We found that the mechanism of formation is similar to that of a gel-sol process: the initial amorphous gel is converted to anatase titania rods using ammonia as a shape controller. Both previous and current studies on the synthesis of TiO2 from peroxotitanate solution show that, under appropriate conditions, the peroxotitanate solution can be used to produce large quantities of TiO2 nanoparticles of controlled polymorph type, morphology, and size. Low-dimensional TiO2 nanostructures with controllable crystalline phases show unique optical, electronic, and bioactive properties.1,2 Because of their high surface-to-volume ratios, these types of materials are especially attractive for application as photocatalysts,1 supports for heterogeneous catalysts,3 electrodes for dye-sensitized solar cells,4 and candidate materials for H2-storage.5 Soft solution approaches to synthesizing these well-defined TiO2 nanostructures mainly comprise surfactant-directed methods,2,6 the hydrothermal treatment of anatase powders or their precursors,3a,7 and templating8 using porous aluminum,8a–e polymer fibers,8f organic gelators,8g or inorganic rods such as ZnO.8h Here, we report the synthesis of nanorod-shaped anatase TiO2 particles from an aqueous peroxotitanate solution at a low temperature (95 °C), without the use of surfactants or organics. The process was initiated with the preparation of a peroxotitanium complex solution using H2TiO3 (Mitsuwa Chem.), H2O2 (Kishida, 30% in water), and ammonia (NH3, Kishida, 28% in water) as raw materials. The detailed procedure for the preparation of the stock solutions has been reported previously.9 The reaction solution contained 0.4-6 mM Ti4+ in a total volume of 100 mL. The solution pH (1-10) was regulated by adding either acid (HNO3) or base (NaOH) of an appropriate amount. All solutions were reagent grade and purchased from the Wako Chemical Corporation. Crystal growth was conducted by incubating the solution at 95 °C for 168-504 h (1-3 weeks). After these reactions, powders were obtained by centrifugation with distilled water. The operation was repeated five times so that careful rinsing was achieved. The transparent solution was subsequently removed, and the residual suspension was dried at 60 °C. The crystalline phases of the powders were measured by X-ray diffraction (XRD, Rigaku 2500, 40 kV, 300 mA). The formation of TiO2 polymorphors (anatase and/or rutile) depends closely on the solution pH. At pH 1 and pH 10, only rutile9e or anatase (this study, Figure 1) monophases were present in the precipitates, respectively. At pH 1.2 and 2, a mixture phase containing both anatase and rutile appeared (Figure 1), but the molar ratio of anatase to rutile (evaluated by comparing the integrated intensities of diffraction peaks, 101 for both) noticeably decreased with an increase in pH, indicating the transformation of anatase to rutile (Figure 1). Under ambient conditions macrocrystalline rutile is thermodynamically stable relative to macrocrystalline anatase or brookite;10 however, anatase has been found to be a major product * Corrensponding author. Fax: 86-21-5241-5270; e-mail: yfgao@ mail.sic.ac.cn. † Shanghai Institute of Ceramics (SIC), Chinese Academy of Sciences (CAS). ‡ Musashi Institute of Technology, Advanced Research Laboratories.
Figure 1. XRD profiles of TiO2 powders obtained under different pH values.
of nanoscale natural and synthetic samples.11–13 Although bulk anatase is higher in enthalpy than rutile and brookite, the surface enthalpy of anatase is the smallest among the three polymorphors.15 This property dominates the phase transformation of TiO2 polymorphors. In a solution process such as sol-gel, the pH value is a decisive factor for controlling the final particle phase6e and shape.6f These reports agree with simulation results, which show that the surface chemistry, both the surface free energy and surface tension, affects the production of TiO2 polymorphors in solution.14 According to these results, anatase and rutile nanoparticles are found to be stabilized by surface-adsorbates containing a large fraction of hydrogen and oxygen, respectively.13 Therefore, anatase is usually formed under acidic conditions.14 This prediction contradicts what is seen in our experiments, as our anatase particles were grown in a strongly basic medium. Moreover, the transformation sequence among the three titania polymorphs, mainly anatase, brookite, and rutile, is also size dependent, because the energies of the three polymorphs are sufficiently close so that the transformation can be reversed by small differences in the surface energy.13 The occurrence of the anatase-rutile transformation depends on impurities, initial grain sizes, reaction atmospheres of calcination, and synthesis conditions.14,15 Even with the same peroxotitanate solutions, various polymorphors of TiO2 have been found to grow under different conditions, such as pH9 and temperature.16,17 A suspension containing anatase powders was spread and coated on a glass substrate and further characterized by an ultraviolet-visible
10.1021/cg701157j CCC: $40.75 2008 American Chemical Society Published on Web 04/29/2008
Communications
Crystal Growth & Design, Vol. 8, No. 6, 2008 1805
Figure 2. UV-vis spectrum (inset) and optical bandgap of TiO2 nanorods.
Figure 3. TG/DTA curves of TiO2 nanorod powders.
(UV-vis) spectrum (see Figure 2). The sample showed a relatively bad transparency, probably due to aligned nanorods that strongly scattered incident light. The adsorption edge, evaluated according to the data of the UV-vis spectrum, gave an optical bandgap at about 3.20 eV, comparable to the values for anatase TiO2 (3.20 eV,9a 3.239a). This finding suggests that the powders are crystalline, which was also confirmed by thermogrametric and differential thermal analysis (TG-DTA, 6300, served with EXSTAR6000, SEIKO, accuracy: TG 0.2 µg, DTA 0.06 µV, in air flow: 200 mL min-1). The weight loss at above 100 °C was only 5.5%, which remained unchanged at about 400 °C (see Figure 3). In the DTA curve, a strong exothermal peak appeared at about 250 °C, which can be assigned to the crystallization from a trace amount of amorphous residue to anatase. At about 500-600 °C, an endothermic valley was seen. This effect should be caused by the phase transformation from anatase to rutile.14 The morphology was characterized with a scanning electron microscope (FE-SEM, JSM-6700F, JEOL). As shown in Figure 4, homogeneous, monodispersive, spindle-shaped particles with a smooth surface were obtained. The mean diameter in the middle of the particle was found to be 40-70 nm. The length ranged from 300-600 nm, but was mostly in the 350-500 nm range. The highresolution transmission electron microscopy (HRTEM, 100 kV, JEM2100F, JEOL) data suggest the development of a well-defined crystalline structure (Figure 5). The fringes agree well with those of anatase. No obvious boundaries were observed, suggesting a single-crystalline characteristic. This conclusion was also confirmed by the selected area electron diffraction (SAED) pattern, which is sharp and consists of a group of isolated spots (Figure 5 inset). In the TEM image, polyline-shaped edges were also shown as indicated by the arrows, which may be due to the growth steps. The appearance of these edges can also be seen, however, as the result of the attachment of small adjacent bipyramid-shaped anatase particles into the large one via appropriate crystallographic planes, suggesting the possible growth mechanism: orientation attachment of small crystals.18
Figure 4. (A) SEM image and (B, C) size analysis of TiO2 nanorod powders.
Figure 5. HRTEM image and SADP (inset).
Efforts to understand the growth underlying mechanism have been made in several ways. By changing the initial concentration of Ti(IV), at concentrations above 2 mM, a light green-yellowish precipitate was observed suspended in the solution. After centrifugation, gelatin-like solids were obtained. The XRD pattern did not reveal any peaks in the range of 5-70°, suggesting the appearance of an amorphous phase. The HRTEM measurement revealed the
1806 Crystal Growth & Design, Vol. 8, No. 6, 2008
Communications as a reservoir of metal and hydroxide ions. This reaction environment affords the possibility of controlling the degree of supersaturation in regard to TiO2 nucleation and growth. Mass transfer in this system is relatively slow and allows for the development of well-crystallized, small anatase powders that can attach to large particles along specific crystalline planes. Such a system appears to be promising for the production of uniform anatase TiO2 nanoparticles, which can be systematically controlled in size and shape, in large quantities. In summary, we report for the first time the formation of anatase nanorods from a peroxotitanate solution without the use of organics (such as surfactants). The formation process is highly dependent on soaking time, solution pH, and concentration, undergoing both morphology and phase transformation during growth. We found that the initial formation of amorphous gel was converted to anatase rods. Both previous and current studies on the synthesis of TiO2 from peroxotitanate solution show that, under appropriate conditions, the solution can be used to control the polymorph, morphology, and size of titania.9,16,17
Acknowledgment. Y.F.G. thanks the Century Program (OneHundred-Talent Program) of the Chinese Academy of Sciences for special funding support. This study was also supported in part by a fund from the National Natural Science Foundation of China (NSFC, Contract No. 50772126).
References
Figure 6. (A) Plate-shaped amorphous deposits (dark areas). (B) The crystalline spots (indicated by arrows) in an amorphous matrix. Both scale bars are 5 nm.
formation of extremely thin, plate-shaped amorphous solids (see Figure 6A). On some spots, tiny crystals were also observable (see Figure 6B). A time-resolved experiment was conducted by observing the powder formation process using similar conditions for the nanorod growth. The gelatin-like solid, found to be synthesized first, had the appearance of suspended algae, and transformed both in color and shape as the treatment progressed. Finally, a white suspension was obtained. This process may share a similar mechanism with that which Sugimoto et al. employed in their synthesis of uniform spindle-type anatase TiO2 particles by a gel-sol process.6c Although they used a complex solution system containing titanium(IV) alkoxide, triethanolamine, and ammonia, and a two-step treatment to obtain the gel precursor and anatase, respectively, the basic consideration employed is similar to ours. We deduce that the nanorods of the current study were also grown from a titanium hydroxide gel, which has a network structure that inhibits the coagulation of existing TiO2 nanoparticles and serves
(1) (a) Manna, L.; Scher, E. C.; Li, L. S.; Alivisatos, A. P. J. Am. Chem. Soc. 2002, 124, 7136. (b) Ma, R.; Sasaki, T.; Bando, Y. J. Am. Chem. Soc. 2004, 126, 10382. (c) Patzke, G. R.; Krumeich, F.; Nesper, R. Angew. Chem. Int. Ed. 2002, 41, 2446. (2) Wen, P.; Itoh, H.; Tang, W.; Feng, Q. Langmuir 2007, 23, 11782. (3) (a) Wen, B.; Liu, C.; Liu, Y. Inorg. Chem. 2005, 44, 6503. (b) Guo, Y. G.; Wan, L.-J.; Bai, C.-L. J. Phys. Chem. 2003, 107, 5441. (c) Cozzoli, P. D.; Curri, M. L.; Agostiano, A. Chem. Comm. 2005, 3186. (4) Chou, T. P.; Fryxell, G. E.; Li, X.; Cao, G. Proc. SPIE-Inter. Soc. Opt. Eng. 2004, 5510, 129. (5) Lim, S. H.; Luo, J.; Zhong, Z. Y.; Ji, W.; Lin, J. Inorg. Chem. 2005, 44, 12–4124. (6) (a) Chemseddine, A.; Moritz, T. Eur. J. Inorg. Chem. 1999, 235. (b) Kanic, K.; Sugimoto, T. Chem. Comm. 2004, 1584. (c) Sugimoto, T.; Okada, K.; Itoh, H. J. Colloid Interface Sci. 1997, 193. (d) Sugimoto, T.; Zhou, X.-P.; Muramatsu, A. J. Colloid Interface Sci. 2002, 252, 339. (e) Sugimoto, T.; Zhou, X.-P.; Muramatsu, A. J. Colloid Interface Sci. 2003, 259, 43. (f) Sugimoto, T.; Zhou, X.-P.; Muramatsu, A. J. Colloid Interface Sci. 2003, 259, 53. (7) (a) Kasuga, T.; Hiramatsu, M.; Hoson, A.; Sekino, T.; Niihara, K. Langmuir 1998, 14, 3160. (b) Kasuga, T.; Hiramatsu, M.; Hoson, A.; Sekino, T.; Niihara, K. AdV. Mater. 1999, 11 (15), 1307. (c) Zhu, Y.; Li, H.; Koltypin, Y.; Hacoben, Y. R.; Gedanken, A. Chem. Commun. 2001, 2616. (d) Du, G. H.; Chen, R. C.; Che, R. C.; Yuan, Z. Y.; Peng, L. M. Appl. Phys. Lett. 2001, 79, 32. (e) Tian, Z. R.; Voigt, J. A.; Liu, J.; Mckenzie, B.; Xu, H.-F. J. Am. Chem. Soc. 2003, 125, 12384. (8) (a) Imai, H.; Takei, Y.; Simizu, K.; Matsuda, M.; Hirashima, H. J. Mater. Chem. 1999, 9, 2917. (b) Miao, Z.; Xu, D.; Ouyang, J.; Guo, G.; Zhao, X.; Tang, Y. Nano Lett. 2002, 2, 717. (c) Chu, S. Z.; Wada, K.; Inoue, S.; Todoroki, S. Electrochim. Acta 2003, 48, 3147. (d) Lei, Y.; Zhang, L. D. J. Mater. Res. 2001, 16, 1138. (e) Shyue, J.-J.; Padture, N. P. Mater. Lett. 2007, 61, 182. (f) Caruso, R. A.; Schattka, J. H.; Greiner, A. AdV. Mater. 2001, 13, 1577. (g) Kobayashi, S.; Hanabusa, K.; Hamasaki, N.; Kimura, M.; Shirai, H. Chem. Mater. 2000, 12, 1523. (h) Lee, J.-H.; Leu, I.-C.; Hsu, M.-C.; Chung, Y.-W.; Hon, M.-H. J. Phys. Chem. B 2005, 109, 13056. (9) (a) Gao, Y.-F.; Masuda, Y.; Peng, Z.-F.; Yonezawa, T.; Koumoto, K. J. Mater. Chem. 2003, 13, 608. (b) Gao, Y.-F.; Masuda, Y.; Koumoto, K. Chem. Mater. 2004, 16, 1062. (c) Gao, Y.; Masuda, Y.; Koumoto, K. Langmuir 2004, 20, 3188. (d) Gao, Y. F.; Nagai, M.; Seo, W.; Koumoto, K. Langmuir 2007, 23, 4712. (e) Gao, Y. F.; Nagai, M.; Seo, W.; Koumoto, K. J. Am. Ceram. Soc. 2007, 90, 831. (10) Muscat, J.; Swamy, V.; Harrison, N. M. Phys. ReV. B 2002, 65, 224112. (11) Navrotsky, A.; Kleppa, O. J. J. Am. Ceram. Soc. 1967, 50, 626. (12) Mitsuhashi, T.; Kleppa, O. J. J. Am. Ceram. Soc. 1979, 62, 356. (13) Zhang, H.; Banfield, J. F. J. Phys. Chem. B 2000, 104, 3491. (14) Barnard, A. S.; Zapol, P. Nano Lett. 2005, 5, 1261.
Communications (15) Ranade, M. R.; Navrotsky, A.; Zhang, H. Z.; Banfield, J. F.; Elder, S. H.; Zaban, A.; Borse, P. H.; Kulkarni, S. K.; Doran, G. S.; Whitfield, H. J. Proc. Nat. Acad. Sci. U. S. A. 2002, 99, 6476. (16) Fuchs, T. M.; Hoffmann, R. C.; Niesen, T. P.; Tew, H.; Bill, J.; Aldinger, F. J. Mater. Chem. 2002, 12, 1597.
Crystal Growth & Design, Vol. 8, No. 6, 2008 1807 (17) Hoffmann, R. C.; Jeurgens, L. P. H.; Wildhack, S.; Bill, J.; Aldinger, F. Chem. Mater. 2006, 18, 446. (18) Penn, R. L.; Banfield, J. F. Science 1998, 281, 969.
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