See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/231012055 Self-Assembly of Hydroxyapatite Nanostructures by Microwave Irradiation Article in Nanotechnology · December 2004 DOI: 10.1088/0957-4484/16/1/017 CITATIONS READS 105 1,113 6 authors, including: Hao Wang Mankang Zhu Chinese Academy of Sciences Beijing University of Technology 997 PUBLICATIONS 13,478 CITATIONS 133 PUBLICATIONS 2,135 CITATIONS SEE PROFILE SEE PROFILE Haiyan Xu Hui Yan Anhui Jianzhu University University of Bristol 40 PUBLICATIONS 1,283 CITATIONS 341 PUBLICATIONS 6,536 CITATIONS SEE PROFILE SEE PROFILE Some of the authors of this publication are also working on these related projects: Piezoceramics View project The National 973 Project entitled "Evolution Rules of Water Resources in the Yellow River Basin and Dualistic Evolution Model" View project All content following this page was uploaded by Haiyan Xu on 20 September 2015. The user has requested enhancement of the downloaded file. INSTITUTE OF PHYSICS PUBLISHING NANOTECHNOLOGY Nanotechnology 16 (2005) 82–87 doi:10.1088/0957-4484/16/1/017 Self-assembly of hydroxyapatite nanostructures by microwave irradiation Jingbing Liu, Kunwei Li, Hao Wang1 , Mankang Zhu, Haiyan Xu and Hui Yan The College of Materials Science and Engineering, Beijing University of Technology, Beijing 100022, People’s Republic of China E-mail: haowang@bjut.edu.cn Received 7 August 2004, in final form 4 October 2004 Published 2 December 2004 Online at stacks.iop.org/Nano/16/82 Abstract Hydroxyapatite (HAp) nanorods, bowknot-like nanostructures and flower-like architectures have been directly synthesized and assembled under microwave irradiation without the help of any templates. The uniform nanorods present an average diameter of about 40 nm and a length of up to about 400 nm. The as-prepared bowknot-like nanostructures consist of sword-like HAp nanorods with a typical width of 150 nm and lengths up to 1–2 µm. The flower-like architectures are composed of leaf-like flakes with typical diameters of 150–200 nm and lengths up to 1–2 µm. The SAED and HRTEM experiments imply that the sword-like HAp nanorods and leaf-like flakes are single crystalline in nature and preferentially grow along the [001] direction. It is found that the pH value and the complex reagent EDTA play important roles in synthesis of the final HAp nanostructures. The possible mechanism is discussed for the formation of the HAp nanostructures in the presence of EDTA under microwave irradiation. 1. Introduction The self-assembly of inorganic nanoparticles into superlattices and nanostructures offers the potential to fabricate materials with tunable physical and chemical properties [1]. These properties of the superstructures are dependent on the design and control of the shape, size and spatial organization of the building blocks; thus, the control of the anisotropic inorganic materials at the mesoscopic level is one of the most challenging issues currently faced by synthetic chemists and materials scientists. Recently, research on nanostructures has expanded rapidly into the assembly of nanoparticles in two-dimensional (2D) and three-dimensional (3D) ordered superstructures [2, 3]. Much effort has been made in the fabrication of patterns of well arranged nanocrystallites, especially the arrangement of one-dimensional (1D) nanotubes and nanorods, because of their interesting physical properties and potential applications in many areas [4, 5]. A variety of nanofabrication techniques and crystal growth methods have been used to achieve shape control, particularly a template-assisted technique. Besides hard templates such 1 Author to whom any correspondence should be addressed. 0957-4484/05/010082+06$30.00 © 2005 IOP Publishing Ltd as a carbon nanotube [6] and anodized alumina [7], soft templates such as regular or inverse micelles [8, 9], liquid crystal [10], polymer [11] and some biological assemblies [12] were also employed to control the morphology of nanocrystals, mainly via the chemical interaction between the reactants and templates. However, the introduction of templates and substrates introduces heterogeneous impurities and increases the production cost, which may restrict the wide development of research and applications. Thus how to develop facile, mild, easily controlled, and template-free methods to control both nanocrystalline morphology and the crystal size is of great significance. As one of the most important biomaterials, hydroxyapatite (HAp), with compositions of stoichiometric Ca10 (PO4 )6 (OH)2 and Ca/P = 1.67 [13], has attracted much interest for many years, due to its similarity with the mineral constituents of human bones and teeth [14]. It is of importance in many industrial applications and in the biomedical field, such as catalysis, ion exchange, sensors and bioceramics [15, 16]. HAp is a polar hexagonal and highly anisotropic crystal, which naturally grows into 1D nanostructures. HAp nanowhiskers or nanowires can be synthesized by a variety of methods such as precipitation hydrolysis [17], sol–gel [18], and Printed in the UK 82 20 30 50 322 321 402 004 213 222 40 132 130 131 113 301 202 102 210 200 111 002 300 Intensity (a.u.) 112 211 Self-assembly of hydroxyapatite nanostructures by microwave irradiation 60 2θ (degree) Figure 1. XRD pattern of the sample prepared in the solution with a pH of 9 using EDTA as the complex reagent. hydrothermal methods [19, 20]. The use of homogeneous Ca/citrate/phosphate solutions in synthesis by the microwave method has also been reported [21]. Only nanosized (30– 60 nm) needle-like HAp of poor crystallinity were obtained. Compared with the conventional method, microwave synthesis has the advantages of very short time, small particle size, narrow particle size distribution, and high purity [22]. The microwave heating is a quite fast, simple and efficient method to prepare nano-sized inorganic materials. Although the exact nature of the interaction between the microwaves and the reactants during the synthesis of materials is somewhat unclear and speculative, it is known that the interaction between dielectric materials and the microwaves leads to dielectric heating. Electric dipoles present in such materials respond to the applied electric field. This constant reorientation creates friction and collisions between molecules, which subsequently generates heat. In this paper, we report a general, controllable synthesis and assembly of HAp nanostructures in one step under microwave irradiation without any templates. Calcium nitrate tetrahydrate (Ca(NO3 )2 ·4H2 O) and dibasic anhydrous sodium phosphate (Na2 HPO4 ) have been utilized as starting materials, using ethylenediaminetetracetic acid disodium salt (EDTA) as complex reagent. The shape of the obtained nanocrystals can be easily varied between the bowknot-like and flower-like nanostructures as well as monorods by simply changing the pH value of the solution. 2. Experimental procedure All reagents were analytical grade and used without further purification. In a typical procedure, 50 ml of a mixed solution of Ca(NO3 )2 ·4H2 O (0.1 M) and EDTA (0.1 M) was introduced into 50 ml of Na2 HPO4 (0.06 M) solution. The pH of initial solution without adjusting was 4. The pH of solution was adjusted to 9–13 by adding NaOH solution. After stirring for several minutes, the ready-adjusted clear aqueous solutions with a certain pH value were put into a household type microwave oven of 700 W power with a refluxing system and the reaction was performed under ambient air for 30 min. Figure 2. TEM image of HAp powders synthesized from the solution with a pH of 9. The inset shows the SAED pattern taken from a single nanorod. The microwave oven followed a working cycle of 6 s on and 10 s off (37% power). After cooling to room temperature, the precipitate was centrifuged, washed with deionized water, and dried in vacuum at 70 ◦ C for 2 h. The x-ray diffraction (XRD) analysis was performed in θ – 2θ mode using a Bruker D8 x-ray diffractometer with graphite monochromatized Cu Kα radiation (λ = 0.154 178 nm). Scanning electron microscopy (SEM) images of samples were obtained with a Hitachi model S-3500N scanning electron microscope. Transmission electron microscopy (TEM), selected area electron diffraction (SAED), and highresolution transmission electron microscopy (HRTEM) images were taken on a JEOL-JEM 2010F transmission electron microscope, using an accelerating voltage of 200 kV. 3. Results and discussion XRD analysis was used to examine the crystal structure of the products. Figure 1 shows the XRD pattern of the sample prepared in the solution with a pH of 9 using EDTA as the complex reagent. All the diffraction peaks in the pattern can be indexed as the hexagonal HAp with lattice constants a = 9.430 and c = 6.882 Å, which are consistent with the values in the standard card (JCPDS No. 73-293). Furthermore, it can be seen that the diffraction peaks are high and narrow, implying that the HAp crystallizes well, and for crystallographic orientations in the direction parallel to the major axis sharper peaks will be expected. As for the other samples prepared in solutions with different pH values, the XRD patterns are almost identical to figure 1. Figure 2 shows the TEM micrograph of HAp powders synthesized from the solution with a pH of 9. The uniform nanorods with an average diameter of about 40 nm and a length of up to about 400 nm had been obtained with this condition. 83 J B Liu et al (c) (a) (b) (d) Figure 3. (a) Low magnification SEM image showing bowknot-like HAp nanostructures. (b) Higher magnification SEM image of a typical bundle of the HAp nanostructures. (c) TEM image of a typical bowknot-like bundle of HAp. Inset: SAED pattern taken from an individual HAp nanorod from the bowknot-like HAp nanostructures. (d) HRTEM image recorded from the tip of the individual sword-like HAp nanorod. The SAED pattern taken from a single nanorod shows that the nanorod was a single crystal of hexagonal HAp (inset in figure 2). The SEM and TEM images of as-prepared HAp nanostructures with a pH of 11 are shown in figure 3. The overall morphology of the samples which is shown in figure 3(a) indicates that there exist a great many uniform bowknot-like bundles with their two ends fanning out while the middle part is tied together. Typical bundles of the HAp nanocrystals (figure 3(b)) indicate that the bundles consist of HAp nanorods with a typical width of 150 nm and lengths up to 1–2 µm. After long-period ultrasonic treatment, the bowknotlike HAp nanostructures were not destroyed, indicating the nanostructures were not due to aggregation. Figure 3(c) shows the TEM image of a typical bowknot-like bundle of HAp. It can be seen from this image that the end of the HAp nanorod is a cusp like a sword. More detail about the structure of a selected nanorod from the bowknot-like HAp was investigated by the SAED pattern (inset in figure 3(c)). The highly arrayed diffraction spots in the pattern indicate the single-crystalline property of the HAp nanorod. Figure 3(d) shows the HRTEM image recorded from the sword-like tip of the individual HAp nanorod shown in figure 3(c). The regular spacing of the observed lattice planes was about 0.82 nm, which is consistent 84 with the (100) lattice spacing of HAp, showing that the nanorod was of uniform crystal structure. Further studies of both the HRTEM image and SAED pattern demonstrate that the growth axis is the [001] direction of HAp. Figure 4 shows the SEM and TEM images of the typical flower-like HAp nanostructures prepared in the solution of pH 13. The morphology of the obtained HAp is in the form of leaf-like flakes of 150–200 nm width and of 1–2 µm length extending radially from the centre. The corresponding SAED pattern (inset in figure 4(c)) taken from individual leaflike flake confirms that the flakes are well crystallized single crystals. The typical HRTEM image (figure 4(d)), recorded from an individual leaf-like flake, reveals a perfect crystal structure, and unambiguously distinguishes the (100) and (002) atomic planes of hexagonally structured HAp with interplanar spacings of about 0.82 and 0.34 nm, respectively. The (002) crystal planes are approximately vertical to the long axis of the HAp flakes, which shows that the HAp flakes predominantly grow along the [001] direction. In order to reveal the growth mechanism and the effect of EDTA, comparative experiments in the absence of EDTA under the same synthesis conditions were studied. When CaNO3 solution was added to Na2 HPO4 solution, a mass of white precipitate of amorphous calcium phosphate (ACP) Self-assembly of hydroxyapatite nanostructures by microwave irradiation (c) (a) (b) (d) Figure 4. (a) Low-magnification SEM image showing flower-like HAp nanostructures. (b) Higher magnification SEM image of typical flower-like HAp nanostructures. (c) TEM image of a typical flower-like HAp. Inset: SAED pattern taken from an individual HAp leaf-like flake from the flower-like HAp nanostructures. (d) HRTEM image recorded from the edge of the individual leaf-like HAp flake. immediately appeared at room temperature. An analogous phenomenon was also reported in the literature [23]. After 30 min microwave irradiation, ACP can be transformed to the HAp nanocrystals. Distinct different morphology could be seen compared with the sample prepared in the presence of EDTA. The TEM image of sample prepared from solution with a pH of 11 in the absence of EDTA under microwave irradiation for 30 min is shown in figure 5. The morphology is not regular; it has a diameter of 5–50 nm and length of 100–200 nm. As for the other samples prepared in solution with pH values of 9 and 13 in the absence of EDTA, only 1D HAp nanocrystals similar to the sample prepared with a pH of 11 could be seen. The main reason for the formation of smaller nanosized crystals without EDTA is likely to be that the reaction rate of Ca2+ and PO3− 4 is too fast to form ACP, which served as the nucleus for HAp crystals, under this reaction condition. If the fraction of ions consumed in the nucleation step is high, the increase in particle size due to the diffusion-controlled growth that immediately follows the nucleation burst is drastically limited, and only slightly larger particles than the nuclei can be obtained. It can be confirmed that EDTA plays an important role in controlling the nucleation and growth of the product. Based on the above experiments and results, we suggest the formation mechanism of the HAp in the presence of EDTA as follows: Figure 5. TEM image of sample prepared from solution with a pH of 11 in the absence of EDTA under microwave irradiation for 30 min. EDTA is a strong complex reagent with Ca2+ , which leads to the formation of Ca–EDTA complexes. The formation of the 85 J B Liu et al Figure 6. Molecular structure of Ca–EDTA complex. 1.0 Y HY H6Y 0.8 H 2Y H 5Y 0.6 δ 0.4 H3Y 0.2 0.0 0 2 4 6 8 10 12 14 pH Figure 7. Fraction of EDTA species as a function of pH value. complex sharply decreased the free Ca2+ concentration in the solution, and effectively prevented the formation of ACP upon mixing sources of Ca and P in the presence of EDTA [24]. Figure 6 presents the molecular structure of the Ca–EDTA complex. In the complex, EDTA acts as a hexadentate unit by wrapping itself around the metal ion with four oxygen atoms and two nitrogen atoms and forms several five-member chelate rings. In solutions with different pH values, EDTA (abbreviated to H4 Y, where Y is the EDTA residue) can present in seven forms: H6 Y2+ , H5 Y+ , H4 Y, H3 Y− , H2 Y2− , HY3− , and Y4− . Figure 7 gives the fraction of each form of EDTA as a function of pH value [25]. Since the anion Y4− is the ligand species with the strongest complexability, the larger the fraction of Y4− , the more stable is the complex. When the pH value of solution is higher than 10, the species Y4− will predominate. Therefore, with increasing pH value, the stability of the complex improves. On the other hand, the shape of a crystal is determined by the relative specific surface energies associated with the facets of this crystal. Before growth units are incorporated into the crystal lattice, some facets of the cluster have a preference to absorb OH− due to the different surface energy of the crystallite facets [26]. Thus, the shielding effect of OH− ions on the interface will control the growth rate of the OH-absorbed facets. The quantity of the OH− and its hindrance effect are varied on different facets. The larger the quantity of the OH− present at the interface, the stronger is the hindrance effect of OH− ions on the facet, which consequently reduces the difference of growth rates in various crystal facets. Usually, in the initial stage of the experiment, a threedimensional cluster of critical size would be formed, which could act as a nucleus for HAp crystals and develop into a crystallite under microwave irradiation. The further development of the crystallite will be controlled by the complex stability of Ca–EDTA and the hindrance of OH− on the facets. Figure 8. Schematic illustration of major steps involved in the microwave approach to the synthesis of HAp nanostructures. 86 Self-assembly of hydroxyapatite nanostructures by microwave irradiation − At lower pH value (lower OH concentration), the crystal growth habit is mainly affected by the interior structure rather than the exterior condition. In this condition, free Ca ions can be easily released from Ca–EDTA complexes due to the poor stability of Ca–EDTA. The free Ca ions will incorporate into the crystal lattice site of the obtained crystallite and the crystallite will grow according to the anisotropic structure of HAp. Consequently, HAp nanorods are obtained at a pH of 9. However, with increasing pH value, the concentration of OH− and the stability of the complex increase. At a pH of 13, each facet of the crystallite has almost the same probability to generate active sites due to the strong absorption of OH− ions. In this case, Ca will be incorporated in the Ca–EDTA form, due to the higher stability of the complex, into the active sites of the obtained crystallite. The distance between two neighbouring Ca–EDTA causes the Ca atoms not to arrange according to the crystal lattice of HAp. Thus, a new nucleus is formed at the limited active sites with absorption of Ca–EDTA. The absorbed Ca–EDTA complexes undergo decomposition at an appropriate temperature under microwave irradiation. Subsequently, HAp crystals can present spontaneously preferential growth from the limited active sites due to the anisotropic growth habit of HAp. So, the morphology of particles prepared in solution with a pH of 13 gives the flower-like form. For the condition of pH equal to 11, certain facets with higher free energies will preferentially form the active sites and show higher growth rate; thus, the bowknotlike nanostructures are obtained. A schematic illustration of the major steps involved in the microwave approach to the synthesis of HAp nanostructures is shown in figure 8. As is well known, the plane with the fastest growth rate will disappear quickly. The (001) plane of HAp, the plane with the most rapid growth rate, will disappear in the crystal growth. Therefore, sword-like HAp nanorods (figure 3(c)) and leaf-like HAp flakes (figure 4(c)) are achieved in our process. 4. Conclusion In summary, uniform 1D and 3D (bowknot-like and flowerlike) HAp nanostructures have been successfully prepared by microwave irradiation without any help of templates. A mechanism was proposed to elucidate their formation. The shape of the obtained crystals can be easily controlled by changing the complex stability of Ca–EDTA and the hindrance effect of OH− on the crystallite facets. These bowknot-like and flower-like nanostructures, which have a highly specific area on the surface of the particles, may bring some novel and unexpected properties. We can foresee the scaling-up of the process to form large quantities of this kind of nanomaterial. Further work is under way to study the properties of these novel 3D nanostructures and the possibility of synthesizing other 3D nanostructures. References [1] Shipway N, Katz E and Willner I 2000 Chem. Phys. 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