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Preparation of Ultrahigh-Aspect-Ratio Hydroxyapatite Nanofibers in Reverse Micelles under Hydrothermal Conditions Minhua Cao,† Yonghui Wang,† Caixin Guo,† Yanjuan Qi,† and Changwen Hu*,†,‡ Institute of Polyoxometalate Chemistry, Northeast Normal University, Changchun, People’s Republic of China 130024, and Department of Chemistry, Beijing Institute of Technology, Beijing, People’s Republic of China 100081
Figure 1. XRD pattern of hydroxyapatite nanofibers.
Received January 20, 2004. In Final Form: March 19, 2004
One-dimensional (1D) nanostructures, such as nanotubes, nanofibers/nanwires, and nanorods, have attracted special interest in recent years because of their great potential for increasing the understanding of fundamental physics concerning the roles of dimensionality and size characteristics and for various nanotechnological applications.1-4 Considerable effort has been devoted to the synthesis of such 1D nanostructures as well as the study of their corresponding properties. Some 1D nanostructures have been shown to exhibit a range of interesting properties superior to their bulk counterparts.5-7 Among the reported studies, however, little work has dealt with the preparation of high-aspect-ratio hydroxyapatite [Ca10(PO4)6(OH)2] nanofibers. The mineral part of calcified tissues (bone, teeth) consists of calcium phosphates with an apatitic structure. Hydroxyapatites are widely applied nowadays to coat artificial joint and tooth roots and hold great promise as a potential biomaterial for bone implantation because of their bioactivity, dissolution range, and resorption properties close to those of natural bones.8 In recent years, with the growing necessity of biomaterials, hydroxyapatites have received extensive attention. Previous work mainly placed emphasis upon controlling the stoichiometry of the products, whereas with the development of nanotechnologies, considerable effort is now focused on controlling the morphology and size9-12 because studies have shown that many clinical capabilities of hydroxyapatites mainly depend on their morphology and size.13 Therefore, synthesis of nanoscale hydroxyapatites will largely improve their clinical applications. Herein we first report the synthesis of ultrahigh-aspect-ratio hydroxyapatite nanofibers in reverse micelles under hydrothermal condi* To whom correspondence should be addressed. E-mail:
[email protected]. Tel. (Fax): +86-10-82571381. † Northeast Normal University. ‡ Beijing Institute of Technology. (1) Morkoc, M.; Mohammad, S. N. Science 1995, 267, 51. (2) Lieber, C. M. Solid State Commun. 1998, 107, 607. (3) Hu, J.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435. (4) Kane, C.; Balents, L.; Fisher, M. P. A. Phys. Rev. Lett. 1997, 79, 5086. (5) Westcott, S. L.; Oldenburg, S. J.; Lee, T. R.; Halas, N. J. Chem. Phys. Lett. 1999, 300, 651. (6) Kim, S. W.; Kim, M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc. 2002, 124, 7642. (7) Sun, Y.; Xia, Y. Anal. Chem. 2002, 74, 5297. (8) Rodrı´guez-Lorenzo, L. M.; Vallet-Regı´, M. Chem. Mater. 2000, 12, 2460. (9) Walsh, D.; Mann, S. Chem. Mater. 1996, 8, 1944. (10) Sarda, S.; Heughebaert, M.; Lebugle, A. Chem. Mater. 1999, 11, 2722. (11) Hirai, T.; Hodono, M.; Komasawa, I. Langmuir 2000, 16, 955. (12) Yuan, Z. Y.; Liu, J. Q.; Peng, L. M. Langmuir 2002, 18, 2450. (13) Elliott, J. C. Recent Studies of Apatites and other Calcium Orthophospates. In Calcium Phosphate Materials, Fundamentals; Bres E., Hardouin P., Eds.; Sauramps Medical: Monpellier, 1998; p 25.
Figure 2. FTIR spectra of hydroxyapatite nanofibers.
tions. The process is thought to be composed of two steps. The hydroxyapatite nanoparticle building blocks were initially formed and then assembled into high-aspect-ratio 1D nanostructures. A quaternary reverse micelle system, cetyltrimethylammonium bromide (CTAB)/cyclohexane/n-pentanol/ water, was used for the synthesis. In a typical procedure, the reverse micelles with a surfactant-to-oil concentration of 0.1 M and [H2O]/[surfactant] molar ratio w ) 10 were prepared by adding an aqueous solution of CaCl2 (1 M) or H3PO4 (0.7 M) to a solution of CTAB in cyclohexane and n-pentanol. Two reverse micelle solutions of the same volume with CaCl2 in one solution and H3PO4 in the other were mixed and stirred for 30 min. The resulting microemulsion solution was then transferred into a stainless Teflon lined autoclave and heated at 130 °C for 12 h. The resulting suspension was naturally cooled to room temperature. Samples were collected and washed several times with absolute ethanol and distilled water. Finally, the hydroxyapatite nanofibers were obtained after the samples were centrifuged and dried in a vacuum at room temperature. Under these conditions, the [H2O]/[CTAB] molar ratio was changed by modifying the content of water in the microemulsion water droplets. A powder X-ray diffraction (XRD) pattern of the resulting product is shown in Figure 1. All of the diffraction peaks can be readily indexed to a pure hexagonal phase [space group: P63/m(176)] with lattice constants a ) 9.424 Å and c ) 6.879 Å, which is in accordance with that of bulk hydroxyapatite crystals (JCPDS 75-0566). In addition, it was found that peak (002) is the only peak that gains a substantial increase in relative intensity and other peaks’ relative intensities relatively decrease, indicating that the nanofiber growth may occur along the [001] direction, that is, the c-axis direction. The nanofiber product was further investigated using Fourier transform infrared (FTIR) spectroscopy as shown in Figure 2. The FTIR spectra of hydroxyapatite nanofibers matched well with the standard spectra observed in the corresponding bulk hydroxyapatites. The absorption bands at 600 and 1000-1100 cm-1 can be ascribed to PO43- ions,
10.1021/la0498197 CCC: $27.50 © 2004 American Chemical Society Published on Web 04/24/2004
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Figure 3. Hydroxyapatite nanofibers synthesized in CTAB/ cyclohexane/n-pentanol/water reverse micelles, [Ca2+] ) 1 M, w ) 10. (a) SEM image; (b) TEM image; (c) SAED pattern; (d) HRTEM image.
at 870 cm-1 to HPO42- ions, and at 630 and 3560 cm-1 to OH- ions. Weak bands at 893 and 1472 cm-1 are ascribed to CO32- ions, which are different from those of carbonates. The CO32- band positions indicate that these groups may have incorporated into the hydroxyapatite crystal structure and replaced PO43-. The FTIR spectrum shows, therefore, that the nanofibers obtained in microemulsions are predominantly hydroxyapatites with a minor carbonate impurity. A representative scanning electron microscopy (SEM) image (Figure 3a) of hydroxyapatite nanofibers shows that nanofibers were deposited in the form of bundles of tightly aggregated fibers with highly uniform widths and straight edges, rather than disordered arrangements that many studies have reported. Similar structures have been reported for many materials in microemulsions.14-17 From the SEM image, it can be seen that, although the samples were dealt with via repeated washing, the bundles of fibers have not been destroyed. A transmission electron microscopy (TEM) of the samples (Figure 3b) shows that hydroxyapatite nanofibers are also in the form of bundles of aggregated fibers, only the bundles are looser than those of the SEM image. This may be because that the samples for TEM must be treated via sonication, resulting in the bundles of loosely aggregated fibers. Both the SEM and the TEM images are only a very short segment of the hydroxyapatite nanofibers. In fact, the length of the nanofibers can be up to several hundreds of micrometers. The nanofibers are 50-120 nm in diameter, with an aspect ratio of above 1000. The selected area electron diffraction (SAED) taken from a single nanofiber is shown in Figure 3c with an electron beam along its [100] zone axis. Diffraction patterns taken from different parts of the nanofiber show exactly the same pattern without further tilting the nanofiber, which indicates excellent single crystallinity of the whole nanofiber. A representative high-resolution TEM (HRTEM) image of hydroxyapatite nanofibers shown in Figure 3d clearly displays the lattice of a single nanofiber with an interlayer spacing of 0.344 nm, which corresponds to an interplanar distance of the (002) planes of hexagonal hydroxyapatites. The HRTEM image also shows that the [002] direction is aligned with the nanofiber axis, indicat-
Figure 4. (a) SEM image of hydroxyapatite aggregates prepared at w ) 20. (b) A high-magnification TEM image of an aggregated hydroxyapatite bundle.
ing that the nanofibers grow along the c-axis direction, which is consistent with the observations from the XRD image. Further study indicated that ultrahigh-aspect-ratio nanofibers of crystalline hydroxyapatites were formed specifically at a Ca2+ aqueous solution concentration of [Ca2+] ) 1 M and w ) 10. Under identical conditions but with a high w value (w ) 15, 20, and 25), hydroxyapatite aggregates were observed as shown in Figure 4a. Figure 4b is a high-magnification TEM image of one of the aggregates. The aggregate looks like an enlaced straw. It seemed that the aggregate also consists of fibers. However, the fibers aggregate so tightly that they form a whole. The products are primarily composed of such enlaced strawlike aggregates. When experiments were undertaken at other concentrations of Ca2+ but with the water-tosurfactant molar ratio reestablished at w ) 10, similar results were obtained. This confirmed that the water-tosurfactant molar ratio and Ca2+ concentration both were responsible for the ultrahigh-aspect-ratio nanofiber structure of crystalline hydroxyapatites. A few studies for the preparation of calcium phosphate microskeletons have been reported also in microemulsions.9-12 However, the products in stoichiometry, morphology, crystallinity, and so forth were not satisfactory. All these aspects have been remarkably improved by the present synthesis method, that is, the hydrothermal microemulsion, which leads to a substantial increase in the aspect ratio, a fairly satisfying uniformity in diameter, a well-crystallized product, and a shorter reaction time. Most existing microemulsion reactions, from which many 1D nanostructures were obtained, are carried out under routine conditions, which usually require longer reaction
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times.14-16 Studies have shown that the hydrothermal method has been proved to be an effective method to synthesize nanomaterials, which has prominent advantages over other methods for the preparation of 1D nanostructures. However, among various microemulsions of synthesizing 1D nanostructures, few are undertaken under hydrothermal conditions.17,18 Therefore, the present synthesis method opens up the possibility of the welldefined synthesis of 1D nanostructures. In addition, Liu et al.19 and Yan et al.20 recently reported hydroxyapatite 1D nanostructures with lower aspect ratios. They both used CTAB as templates. For the formation mechanism of our developed experimental method, a directional aggregation process, which has been established for the formation of some 1D nanostructures in microemulsions,16,21 should be responsible for the formation of ultrahigh-aspect-ratio hy(14) Li, M.; Schnablegger, H.; Mann, S. Nature 1997, 402, 393. (15) Hopwood, J. D.; Mann, S. Chem. Mater. 1997, 9, 1819. (16) Qi, L. M.; Ma, J. M.; Cheng, H. M.; Zhao, Z. G. J. Phys. Chem. B 1997, 101, 3460. (17) Cao, M. H.; Hu, C. W.; Wang, E. B. J. Am. Chem. Soc. 2003, 125, 11196. (18) Zhang, P.; Gao, L. Langmuir 2003, 19, 208. (19) Liu, Y. K.; Wang, W. Z.; Zhan, Y. J.; Zheng, C. L.; Wang, G. H. Mater. Lett. 2002, 56, 496. (20) Yan, L.; Li, Y. D.; Deng, Z. X.; Zhuang, J.; Sun, X. M. Int. J. Inorg. Mater. 2001, 3, 633. (21) Ocana, M.; Rodriguez-Clemente, R.; Serna, C. J. Adv. Mater. 1995, 7, 212.
Notes
droxyapatite nanofibers. In the present case, it is assumed that, after Ca10(PO4)6(OH)2 nucleation in CTAB reverse micelles, the surfactant headgroups preferentially adsorb on surface planes parallel to the c axes of the nuclei, resulting in the formation of the growth of anisometric particles. These tiny particles then aggregate in a directional manner through their c axes onto ultrahighaspect-ratio hydroxyapatite nanofibers. Although the proposed mechanism needs to be further confirmed by more detailed experimental data, it is clear that the nanofibers are formed by the aggregation growth rather than in the water pools of the CTAB reverse micelles. This is because it is not likely lead to the formation of such a long water nanochannel in reverse micelles in such a low-water-content environment. In summary, an effective method was developed for the formation of ultrahigh-aspect-ratio hydroxyapatite nanofibers. The nanofibers are highly crystalline and uniformly structured. These high-quality hydroxyapatite nanofibers represent well-defined nanoscale structures needed for both fundamental studies and clinical applications. Acknowledgment. This work was supported by NSFC (No. 20331010 and 20271007) and SRFDP (No. 20030007014). LA0498197
