pubs.acs.org/Langmuir © 2010 American Chemical Society
Facile Fabrication of Hierarchical Hollow Microspheres Assembled by Titanate Nanotubes Yufeng Tang, Li Yang,* Jizhang Chen, and Zheng Qiu School of Chemistry and Chemical Technology, Shanghai Jiao Tong University, Shanghai 200240, PR China Received January 18, 2010. Revised Manuscript Received April 18, 2010 Hierarchical hollow microspheres assembled by titanate nanotubes were fabricated via a hydrothermal process. During the entire process, the hydrous titanium oxide microspheres served as a template and source of titanate ions and H2O2 was used to facilitate the conversion of titanate sheets into nanotubes in low-concentration NaOH (0.1 M). Furthermore, this synthesis route is more friendly than the previous hydrothermal synthesis of TiO2-derived nanotubes in a highly alkaline (10-15 M) medium.
Introduction Nanotubular materials have been demonstrated to possess superior electrical, optical, and mechanical properties since the discovery of carbon nanotubes.1,2 In this regard, remarkable progress has been made in the synthesis and application of nanotubes of metals, semiconductors, copolymers, biomaterials, and organic-inorganic hybrid materials.3-8 Moreover, controlling nanotubes assembled into hierarchical structures represents another challenge for their effective utilization,9-12 whereas the excellent work that has been reported mainly focused on nanotubes in separated form. Considering many novel characteristics owing to their unique properties, the hierarchical self-assembly of nanotube is often more wishful for novel technologies derived on nanoscale devices. In a previous report, hollow spherical aggregates composed of titanate and titania 1D nanostructures were produced using a general redox strategy combined with a hydrothermal reaction involving a titanium source (Ti foil or Ti powder), a basic NaOH or KOH solution, and an oxidizing H2O2 solution,12 but the controlled synthesis of uniform TiO2-derived nanotubes selfassembled into hollow spheres has still not been achieved. In this article, we report a facile hydrothermal route to preparing hierarchical hollow microspheres assembled by titanate nanotubes. During the entire process, hydrous titanium oxide (TiO2 3 nH2O)13 microspheres served as a template and a source of *Corresponding author. E-mail:
[email protected].
(1) Iijima, S. Nature 1991, 354, 56. (2) Wong, E. W.; Sheehan, P. E.; Lieber, C. M. Science 1997, 277, 1971. (3) Chen, Z.; Waje, M.; Li, W.; Yan, Y. Angew. Chem., Int. Ed. 2007, 46, 4060. (4) Liu, Y.; Chu, Y.; Zhuo, Y.; Dong, L.; Li, L.; Li, M. Adv. Funct. Mater. 2007, 17, 933. (5) Ishai, B. M.; Patolsky, F. J. Am. Chem. Soc. 2009, 131, 3679. (6) Huang, K.; Rzayev, J. J. Am. Chem. Soc. 2009, 131, 6880. (7) Lu, F.; Gu, L.; Meziani, J. M.; Wang, X.; Luo, P.; Veca, L. M.; Cao, L.; Sun, Y. Adv. Mater. 2009, 21, 139. (8) Wang, Z. J.; Qu, S. C.; Zeng, X. B.; Liu, J. P.; Zhang, C. S.; Tan, F. R.; Jin, L.; Wang, Z. G. Appl. Surf. Sci. 2008, 255, 1916. (9) Wang, Y.; Wang, G.; Wang, H.; Cai, W.; Zhang, L. Chem. Commun. 2008, 6555. (10) Piao, Y.; An, K.; Kim, J.; Yu, T.; Hyeon, T. J. Mater. Chem. 2006, 16, 2984. (11) Yada, M.; Taniguchi, C.; Torikai, T.; Watari, T.; Furuta, S.; Katsuki, H. Adv. Mater. 2004, 16, 1448. (12) Mao, Y.; Kanungo, M.; Hemraj-Benny, T.; Wong, S. S. J. Phys. Chem. B 2006, 110, 702. (13) Wang, Y. W.; Xu, H.; Wang, X. B.; Zhang, X.; Jia, H. M.; Zhang, L. Z.; Qiu, J. R. J. Phys. Chem. B 2006, 110, 13835.
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titanate ions, and H2O2 was used to facilitate the conversion of titanate sheets into nanotubes in low-concentration NaOH (0.1 M). It also should be noted that TiO2-derived nanotubes have attracted much attention in recent years.14-18 Various methods, such as a replication process, template technique, and hydrothermal process, have been used to prepare TiO2-devrived nanotubes.16-18 Among them, the hydrothermal process has been widely applied to prepare nanotubes. However, the previous hydrothermal reaction usually needed a highly alkaline (typically 10-15 M) medium.16-18 In this article, hydrothermal synthesis under a low-concentration alkaline condition provides a more friendly route than the previous methods.
Experimental Section Synthesis. The starting material was titanium tetraisopropoxide (TTIP, Ti-(OC3H7)4, 97%, Aldrich). All chemicals were used as received. In a typical synthesis, ca. 0.1 g of TiO2 3 nH2O with a spherical shape, which was prepared in ref 13 (Supporting Information), was added to 20 mL of a 0.1 M NaOH aqueous solution, after which 0.3 mL of H2O2 (3 wt %) was injected into the solution and stirred for 5 min. Afterwards, the solution was transferred to a 30 mL Teflon-lined stainless steel autoclave and kept inside an oven at 180 °C for 10 h. After synthesis, the sodium titanate precipitate was collected and washed several times with deionized water and pure ethanol and then vacuum dried at 80 °C for 4 h. For the preparation of protonated titanate, the collected sodium titanate precipitate was immersed in 15 mL of HCl (0.1 M) for 10 min and subsequently washed with deionized water to pH 7. Corresponding anatase TiO2 was synthesized by annealing protonated titanate in air at 400 °C for 4 h. Characterization Techniques. The structure and morphology of the products were characterized by X-ray diffraction (XRD, Rigaku, D/max-RBusing Cu Ka radiation), transmission electron microscopy (TEM, JEOL JEM-2010 equipped with an energy-dispersive X-ray detector), and field-emission scanning electron microscopy (FESEM, JEOL JSM-7401F). High-resolution TEM was performed on JEOL JEM-2100F with an acceleration voltage of 200 kV. The N2 sorption studies were carried out at 77 K using a Micromeritics ASAP 2000 instrument. (14) Bavykin, D. V.; Friedrich, J. M.; Walsh, F. C. Adv. Mater. 2006, 18, 2807. (15) Chen, X.; Mao, S. S. Chem. Rev. 2007, 107, 2891. (16) Kasuga, T.; Hiramatsu, M.; Hoson, A.; Sekino, T.; Niihara, K. Langmuir 1998, 14, 3160. (17) Ou, H. H.; Lo, S. L. Sep. Purif. Technol. 2007, 58, 179. (18) Chen, Q.; Zhou, W.; Du, G.; Peng, L. M. Adv. Mater. 2002, 14, 1208.
Published on Web 04/29/2010
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Figure 1. FESEM images of hierarchical sodium titanate hollow spheres. (a) Overall product morphology, (b) a single sphere assembled by nanotubes, and (c) a cracked hollow sphere.
Figure 2. XRD patterns of (a) sodium titanate, (b) protonated titanate, and (c) anatase TiO2. Vertical bars below the patterns represent the standard diffraction data from the JCPDS file for anatase (no. 21-1272).
Results and Discussion Figure 1a shows the FESEM image of sodium titanate, which is composed of spherical particles with a diameter of ca. 500 nm. The magnified image in Figure 1b indicates that spheres are composed of 1D nanostructures, and the following TEM images further certify that the 1D nanostructures are nanotubes. Moreover, a hollow interior can be clearly seen from the cracked opening in Figure 1c. The XRD patterns of sodium titanate (Figure 2a) and protonated titanate (Figure 2b) are basically the same and can be indexed to the lepidocrocite-type titanate phase (e.g., HxTi2 - (x/4)0x/4O4 3 H2O).19-21 The strongest peak at 9.5° (d = 0.93 nm) is assigned to the 020 refraction, and the lattice constants of protonated titanate a, b, and c are 0.378, 1.873, and (19) Ma, R.; Bando, Y.; Sasaki, T. Chem. Phys. Lett. 2003, 380, 577. (20) Ma, R.; Fukuda, K.; Sasaki, T.; Osada, M.; Bando, Y. J. Phys. Chem. B 2005, 109, 6210. (21) Tsai, C. C.; Teng, H. Langmuir 2008, 24, 3434.
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Figure 3. TEM images of hierarchical sodium titanate hollow spheres. (a) Panoramic TEM image, (b) magnified TEM image (many holes on the surfaces of spheres), (c) individual hollow spheres assembled by nanotubes, (d) TEM image of nanotubes growing on the surfaces of hollow spheres, (e) SAED pattern taken from a hollow sphere composed of nanotubes, and (f) HRTEM image of a nanotube with an interlayer spacing of 0.8 nm.
0.297 nm, respectively. The thermogravimetry curve demonstrates that the protonated titanate has a weight loss of ∼12% after being heated to 600 °C (Supporting Information Figure S1). When the protonated titanate is annealed at 400 °C for 4 h, it is converted to anatase TiO2 (Figure 2c). The detailed morphology and structure of the samples were investigated by TEM. Figure 3a indicates that sodium titanate hollow microspheres consist of a large quantity of nanotubes with lengths in the range of 100-200 nm. The spherical shell is holey because of the open-ended nanotubes, and many loops (holes) can be seen on the shell surface because of the open ends of some nanotubes parallel to the electron beam (Figure 3b). An individual hollow microsphere composed of nanotubes can be clearly observed in Figure 3c. Most of the nanotubes align vertically on the surface of the sphere, and the outer and inner diameters are 10-20 and 3-10 nm, respectively. Figure 3d-f reveals that the nanotubes are crystalline and composed of multilayer walls with an interlayer distance of about 0.8 nm, which is smaller than d020 (0.93 nm) in XRD. An acceptable explanation is that the titanate may be dehydrated during microscopic observations because of the high-vacuum environment, leading to some shrinkage in the interlayer spacing. After acid washing and further annealing, the hierarchical assemblies of nanotubes can be transformed into their protonated titanate analogues as well as into their corresponding anatase TiO2 nanostructured counterparts without destroying the hierarchical hollow structure (Supporting Information Figure S2-S4). To certify the effect of H2O2 on nanotube growth, comparison experiments without adding H2O2 were carried out. Figure 4 demonstrates that the main products are hollow microspheres assembled by nanosheets (Supporting Information Figure S5). The sodium titanate nanosheets are several nanometers thick and have the same structure as the nanotubes (Supporting Information Figure S6). This result reveals that H2O2 can facilitate the formation of titanate nanotubes. Langmuir 2010, 26(12), 10111–10114
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Figure 4. TEM images of the samples prepared without H2O2. (a) Hollow sphere assembled by nanosheets and (b) nanosheets growing on the surface of a hollow sphere.
Figure 6. TEM images of the products obtained at 180 °C for different reaction times: (a) 0, (b) 1, (c) 2, and (d) 10 h. (e) Schematic illustration of the formation of nanotube-based hollow spheres. Figure 5. N2 absorption of hierarchical sodium titanate hollow microspheres.
The pore structure of the nanotube-based hierarchical hollow microsphere was analyzed by nitrogen adsorption and desorption (Figure 5). The Barrett-Joyner-Halenda (BJH) pore-size distribution curve indicates that the average pore diameter is 14.5 nm consistent with the TEM observation. The Brunauer-EmmettTeller (BET) surface area is 218 m2g-1, and the total pore volume is 0.93 cm3g-1. The large specific surface area and mesoporous structure will enhance the potential application of the TiO2derived hollow microspheres. For example, the hollow microspheres composed of TiO2 nanotubes exhibited a high reversible capacity of 90 mA h g-1 at 8 C (Supporting Information Figure S7), showing better results than most TiO2 nanoparticles, nanotubes, nanowires, nanorods, and commercial TiO2.22,23 For a complete view of the formation process of hierarchical hollow microspheres, detailed time-dependent evolutions of morphology and structure were studied by TEM and XRD. It can be seen in Figure 6a that the initial TiO2 3 nH2O consists mainly of smooth spheres with diameters of ca. 300 nm. After 1 h of reaction, the surfaces of the spheres became rough and strewn with sheetlike structures with a curling edge (Figure 6b). For 2 h of the reaction, the clearly blank boundary indicates the formation of a core-shell structure, and a complete thin shell composed of scrolling nanosheets was observed around the spheres (Figure 6c). The XRD patterns show that the characteristic peaks of sodium titanate become strong with the process of the reaction (Supporting Information Figure S8), implying that the formed shell is sodium titanate. When the reaction time was prolonged to (22) Armstrong, A. R.; Armstrong, G.; Canales, J.; Garcia, R.; Bruce, P. G. Adv. Mater. 2005, 17, 862. (23) Guo, Y. G.; Hu, Y. S.; Maier, J. Chem. Commun. 2006, 2783.
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10 h, the titanate spherical structure with a hollow interior appeared (Figure 6d). The formation of hierarchical hollow spheres of sodium titanate could be explained as follows. After TiO2 3 nH2O spheres were added to the NaOH solution, OH- would interact with TiO2 3 nH2O to form titanate ions. Then titanate ions would react with the sodium ions around the TiO2 3 nH2O spheres. As a result, sodium titanate formed and preferentially deposited on the surfaces of spheres. As the reaction proceeded, TiO2 3 nH2O was consumed and more sodium titanate was generated, and then the shell gradually became complete. The boundary shown in Figure 6c could be ascribed to the consumption of TiO2 3 nH2O cores during the reaction. The titanate shells continued to growing thicker until the cores was exhausted according to the above reaction process. Finally, the hierarchical hollow spheres were formed. The whole synthetic flowchart of this work is shown in Figure 6e. In the process, TiO2 3 nH2O spheres served as a template and a source of titanate ions, and hollow spheres were obtained after TiO2 3 nH2O cores were dissolved in NaOH solution. There is currently much dispute over the formation mechanism of the TiO2-derived nanotubes, although many groups have investigated the structure and formation mechanism for the TiO2-derived nanotubes obtained via an alkali hydrothermal process. A general opinion is that the nanotubes are formed by the scrolling of titanate sheets.14 The driving force for the curvature of these nanotubes may be the excess surface energy that is derived from the imbalance of Hþ or Naþ ion concentration on two different sides of a nanosheet. Previous studies indicated that a high concentration of NaOH (10-15 M) was essential for preparing nanotubes in this reaction system.17,18 However, in the present work, it was discovered that titanate sheets could scroll into tubes in a low-concentration NaOH solution after adding a certain amount of H2O2 to the reaction DOI: 10.1021/la1002379
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system. This might be due to the side-on peroxo complex that was probably generated by the reversible rupture of one Ti-O-Ti bridge,24 which would lead the titanate layer to scroll into tube. Moreover, the titanate nanotubes that formed in H2O2 have larger outer diameters (10-20 nm) than do nanotubes that had outer diameters of 6-10 nm in previous reports.16-18 An acceptable explanation might be due to their different formation mechanism in alkaline solution. Although a detailed formation mechanism needs to be further studied, the present work provides a new approach to the synthesis of TiO2-derived nanotubes under mild reaction condition
Conclusions Hierarchical hollow microspheres assembled by titanate nanotubes can be constructed via a hydrothermal process in a mildly alkaline solution, where the titanate nanotubes were (24) Francesca, B.; Alessandro, D.; Gabriele, R. J. Phys. Chem. B 2004, 108, 3573.
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formed by rolling sheets with the assistance of H2O2. This synthesis route may also extend to the preparation of hollow structures of other metal oxides. Because of the large specific surface area, porous structure, and good penetration, the hierarchical TiO2-derived hollow microspheres may find great applications in catalysis, photovoltaic cells, and high-surface-area electrodes. Acknowledgment. We are indebted to the National Key Project of China for Basic Research under grant no. 2006CB202600 and the National High Technology Research and Development Program of China under grant no. 2007AA03Z222. We thank the Instrumental Analysis Center of Shanghai Jiaotong University for materials characterization. Supporting Information Available: SEM, TEM, EDX, TG, XRD, and charging-discharging measurements. This material is available free of charge via the Internet at http:// pubs.acs.org.
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