Architectures of Strontium Hydroxyapatite Microspheres: Solvothermal

Aug 11, 2009 - Furthermore, the surfactants CTAB and trisodium citrate have an obvious impact on the morphologies and the luminescence properties of t...
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Architectures of Strontium Hydroxyapatite Microspheres: Solvothermal Synthesis and Luminescence Properties Cuimiao Zhang, Ziyong Cheng, Piaoping Yang, Zhenhe Xu, Chong Peng, Guogang Li, and Jun Lin* State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China, and Graduate University of the Chinese Academy of Sciences, Beijing 100049, P. R. China Received June 2, 2009. Revised Manuscript Received July 12, 2009 Strontium hydroxyapatite (Sr5(PO4)3OH, SrHAp) microspheres with 3D architectures have been successfully prepared through a efficient and facile solvothermal process. The experimental results indicate that the SrHAP microspheres are composed of a large amount of nanosheets, which are assembled in a radial form from the center to the surface of the microspheres. The as-obtained SrHAp samples show an intense and bright blue emission from 350 to 570 nm centered at 427 nm (CIE coordinates: x = 0.153, y = 0.081; lifetime: 9.2 ns; quantum efficiency: 31%) under long-wavelength UV light excitation (344 nm). This blue emission might result from the CO2•- radical impurities in the crystal lattice. Furthermore, the surfactants CTAB and trisodium citrate have an obvious impact on the morphologies and the luminescence properties of the products, respectively. The possible formation and luminescent mechanisms for Sr5(PO4)3OH microspheres have been presented in detail.

1. Introduction Generally, the chemical and physical properties of function materials consisting of either inorganic compounds or inorganic/organic hybrids are fundamentally related to their size, shape, and dimensionality.1 In recent years, the threedimensional (3D) nano/microstructured architectures have been of great interest in the area of materials science.2 The ability of building blocks (nanorods, nanoplates, nanospheres, etc.) to synthesize uniformly hierarchical superstructures with diameters ranging from nano- to microscale dimensions is especially desirable due to the interesting properties of such superstructures and their potential applications in catalysis,1c optoelectronics,3a lithium-ion batteries,3b drug delivery system,3c supercapacitors,3d,3e and sensors.3f The simplest synthetic route to 3D architectures is probably self-assembly, in which ordered aggregates are formed in a spontaneous process.2b,4 Up to now, the obtained inorganic materials, *Corresponding author. E-mail: [email protected]. (1) (a) Alivisatas, A. P. Science 1996, 271, 933. (b) Ng, H. T.; Li, J.; Smith, M. K.; Nguyen, P.; Cassell, A.; Han, J.; Meyyappan, M. Science 2003, 300, 1249. (c) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. Rev. 2005, 105, 1025. (d) Yang, J.; Li, C. X.; Zhang, X. M.; Quan, Z. W.; Zhang, C. M.; Li, H. Y.; Lin, J. Chem.;Eur. J. 2008, 14, 4336. (e) Chen, A.; Peng, X.; Koczkur, K.; Miller, B. Chem. Commun. 2004, 2004, 1964. (2) (a) Whitesides, G. M.; Grzybowski, B. Science 2002, 296, 106. (b) Zhong, L. S.; Hu, J. S.; Liang, H. P.; Cao, A. M.; Song, W. G.; Wan, L. J. Adv. Mater. 2006, 18, 2426. (c) Liu, J.; Wu, Q.; Ding, Y. Eur. J. Inorg. Chem. 2005, 2005, 4145. (d) Cho, I. S.; Kim, D. W.; Lee, S.; Kwak, C. H.; Bae, S. T.; Noh, J. H.; Yoon, S. H.; Jung, H. S.; Kim, D. W.; Hong, K. S. Adv. Funct. Mater. 2008, 18, 2154. (3) (a) Duan, X.; Huang, Y.; Cui, Y.; Wang, J.; Lieber, C. M. Nature 2001, 409, 66. (b) Cao, A. M.; Hu, J. S.; Liang, H. P.; Wan, L. J. Angew. Chem., Int. Ed. 2005, 44, 4391. (c) Cai, Y.; Pan, H.; Xu, X.; Hu, Q.; Li, L.; Tang, R. Chem. Mater. 2007, 19, 3081. (d) Ghosh, S.; Inganas, O. Adv. Mater. 1999, 11, 1214. (e) Rolison, D.; Dunn, B. J. Mater. Chem. 2001, 11, 963. (f) Favier, F.; Walter, E. C.; Zach, M. P.; Benter, T.; Penner, R. M. Science 2001, 293, 2227. (4) Whitesides, G. M.; Boncheva, M. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 4769. (5) (a) Hou, Y.; Kondoh, H.; Ohta, T. J. Phys. Chem. B 2003, 107, 13583. (b) Love, J. C.; Urbach, A. R.; Prentiss, M. G.; Whitesides, G. M. J. Am. Chem. Soc. 2003, 125, 12696. (6) Yang, J.; Li, C. X.; Quan, Z. W.; Zhang, C. M.; Yang, P. P.; Li, Y. Y.; Yu, C. C.; Lin, J. J. Phys. Chem. C 2008, 112, 12777.

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including metals,5 metal oxides,1e,3b,6 sulfides,7 borates,1d tungstates,8 etc.,2c,9 have been successfully prepared with complex 3D hierarchical superstructures by a variety of methods. However, it is still a big challenge to develop simple and reliable synthetic methods for hierarchically self-assembled architectures with designed chemical components and controlled morphologies, which strongly affect the properties of nano/micromaterials.1,6 Hydroxyapatite (HAp) has attracted great interest in modern materials chemistry because of its outstanding biocompatibility, which has the potential to be used as bone substitutes.10 The hydroxyapatite has also been widely used in many other fields, such as catalyst for dehydration or dehydrogenation of some organic compounds,11 liquid chromatographic columns for separation of protein and nucleic acid,12 powder carriers for removing heavy metal ions or drugs,13 chemical sensors for various gases or humidity, photoluminescence materials,13c,14 etc.15 However, the bioactivity, biocompatibility, stability, and other properties of hydroxyapatite are determined by their morphology, crystallite size, composition, and structure. For instance, biological hydroxyapatite is nonstoichiometric and (7) Liu, Z. P.; Peng, S.; Xie, Q.; Hu, Z. K.; Yang, Y.; Zhang, S. Y.; Qian, Y. T. Adv. Mater. 2003, 15, 936. (8) Li, Y. Y.; Liu, J. P.; Huang, X. T.; Li, G. Y. Cryst. Growth Des. 2007, 7, 1350. (9) Yu, S. H.; C€olfen, H.; Xu, A. W.; Dong, W. F. Cryst. Growth Des. 2004, 4, 33. (10) Weiss, P.; Obadia, L.; Magne, D.; Bourges, X.; Rau, C.; Weitkamp, T.; Khairoun, I.; Bouler, J. M.; Chappard, D.; Gauthier, O.; Daculsi, G. Biomaterials 2003, 24, 4591. (11) (a) Matsumura, Y.; Sugiyama, S.; Hayashi, H.; Shigemota, N.; Saitoh, K.; Moffat, J. B. J. Mol. Catal. 1994, 92, 81. (b) Sugiyama, S.; Minami, T.; Hayashi, H.; Tanaka, M.; Moffat, J. B. J. Solid State Chem. 1996, 126, 242. (12) Kawasaki, T. J. Chromatogr. 1991, 544, 147. (13) (a) Kim, T. G.; Park, B. Inorg. Chem. 2005, 44, 9895. (b) Takeda, H.; Seki, Y.; Nakamura, S.; Yamashita, K. J. Mater. Chem. 2002, 12, 2490. (c) Yang, P. P.; Quan, Z. W.; Li, C. X.; Kang, X. J.; Lian, H. Z.; Lin, J. Biomaterials 2008, 29, 4341. (d) Schachschal, S.; Pich, A.; Adler, H. J. Langmuir 2008, 24, 5129. (14) Zeng, Q.; Liang, H. B.; Zhang, G. B.; Birowosuto, M. D.; Tian, Z. F.; Lin, H. H.; Fu, Y. B.; Dorenbos, P.; Su, Q. J. Phys.: Condens. Matter 2006, 18, 9549. (15) (a) Adachi, G.; Imanaka, N.; Tamura, S. Chem. Rev. 2002, 102, 2405. (b) Misra, D. N. Langmuir 1988, 4, 953.

Published on Web 08/11/2009

DOI: 10.1021/la9019684

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shows nanometric size in bone and dentin and micrometric size in enamel.16 Hence, the hydroxyapatite crystals with specific morphological and structural properties have attracted much attention. Recently, luminescent materials have been found a wide variety of applications, including information displays, X-ray-intensifying and scintillation, and so on.17 Most of the commercially available lamp phosphors require excitation by short-wavelength ultraviolet (UV) light of mercury vapor plasma, which can causes environmental contamination. Recent investigation on new materials that convert long-wavelength UV light (340-400 nm) into visible light could help to replace the mercury currently used in fluorescent lights with a less toxic alternative.18 On the other hand, rare-earth ions, such as Eu3þ, Eu2þ, Ce3þ, Dy3þ, and Tb3þ, which are required for use as luminescent centers, tend to be very expensive.19,20 Therefore, much efforts have been devoted to exploring novel luminescent materials that do not contain expensive elements as activators or do not need mercury vapor plasma as the excitation source.18,21 Herein, in this paper, we demonstrate a general strategy for the synthesis of strontium hydroxyapatite (Sr5(PO4)3OH, SrHAp) via an efficient and facile hydrothermal route. More importantly, we can easily control the morphology and luminescence of the products through the elaborate choice of reaction time and the content of organic additives. In addition, the as-obtained SrHAp samples show a strong luminescence ranging from 350 to 570 nm with a maximum at about 427 nm with a quantum efficiency of 31%, which are potentially used as a kind of environmentally friendly luminescent materials without expensive or toxic metal elements as activator. The structure, formation mechanism, and photoluminescence (PL) properties of the products were also investigated in detail.

2. Experimental Section All chemicals were of analytical grade reagents and purchased from Beijing Chemical Corp. and used without further purification. Preparation. In the procedure for the preparation of SrHAp, 3 mmol of strontium nitrate [Sr(NO3)2] and 0.3 g of hexadecyltrimethylammonium bromide (CTAB) were dissolved in 40 mL of deionized water to form solution 1. Then, 6 mmol of trisodium citrate (labeled as Cit3-) and 2 mmol of NH4H2PO4 were added into 25 mL of H2O to form solution 2. After vigorously stirring for 30 min, solution 2 was introduced into solution 1. After additional agitation for 20 min, the mixed solution was transferred into an 80 mL Teflon bottle held in a stainless steel autoclave, sealed, and (16) Gomez-Morales, J.; Torrent-Burgues, J.; Boix, T.; Fralle, J.; Rodrı´ guez-Clemente, R. Cryst. Res. Technol. 2001, 36, 15. (17) (a) Blasse, G.; Grabmaier, B. C. Luminescent Materials; Springer-Verlag: Berlin, 1994. (b) Feldmann, C.; J€ustel, T.; Ronda, C. R.; Schmidt, P. J. Adv. Funct. Mater. 2003, 131, 511. (18) (a) Green, W. H.; Le, K. P.; Grey, J.; Au, T. T.; Sailor, M. J. Science 1997, 276, 1826. (b) Lin, J.; Baerner, K. Mater. Lett. 2000, 46, 86. (c) Ohashi, N.; Ebisawa, N.; Sekiguchi, T.; Sakaguchi, I.; Wada, Y.; Takenaka, T.; Hanedad, H. Appl. Phys. Lett. 2005, 86, 091902. (d) Wang, L.; Estevez, M. C.; O0 Donoghue, M.; Tan, W. H. Langmuir 2008, 24, 1635. (e) Hao, Y.; Meng, G.; Ye, C.; Zhang, L. Appl. Phys. Lett. 2005, 87, 033106. (f) Chen, Z.; Wang, Y. X.; He, H. P.; Zou, Y. M.; Wang, J. W.; Li, Y. Solid State Commun. 2005, 135, 247. (g) Ogi, T.; Kaihatsu, Y.; Iskandar, F.; Wang, W. N.; Okuyama, K. Adv. Mater. 2008, 9999, 1. (19) (a) Takahashi, Y. Appl. Phys. Lett. 2006, 88, 151903. (b) An, X.; Meng, G.; Wei, Q.; Zhang, X.; Hao, Y.; Zhang, L. Adv. Mater. 2005, 17, 1781. (c) Gu, F.; Wang, S. F.; Lu, M. K.; Zhou, G. J.; Xu, D.; Yuan, D. R. J. Phys. Chem. B 2004, 108, 8119. (d) Peng, H. S.; Wu, C. F.; Jiang, Y. F.; Huang, S. H.; McNeill, J. Langmuir 2007, 23, 1591. (20) (a) Li, C. X.; Quan, Z. W.; Yang, P. P.; Yang, J.; Lian, H. Z.; Jun, L. J. Mater. Chem. 2008, 18, 1353. (b) Li, C. X.; Quan, Z. W.; Yang, J.; Yang, P. P.; Lin, J. Inorg. Chem. 2007, 46, 6329. (21) (a) Lin, C. K.; Luo, Y.; You, H.; Quan, Z. W.; Fang, J.; Lin, J. Chem. Mater. 2006, 18, 458. (b) Lin, C. K.; Zhang, C. M.; Lin, J. J. Phys. Chem. C 2007, 111, 3300. (c) Lin, C. K.; Yu, M.; Cheng, Z. Y.; Zhang, C. M.; Meng, Q. G.; Lin, J. Inorg. Chem. 2008, 47, 49. (d) Zhang, C. M.; Lin, C. K.; Li, C. X.; Quan, Z. W.; Liu, X. M.; Lin, J. J. Phys. Chem. C 2008, 112, 2183.

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Figure 1. XRD patterns of as-prepared SrHAp sample by solvothermal process at 180 °C for 24 h and the standard data of strontium hydroxyapatite (JCPDS No. 33-1348) as a reference. maintained at 180 °C for different time. As the autoclave cooled to room temperature naturally, the precipitates were separated by centrifugation, washed by deionized water and ethanol in sequence, and then dried in air at 80 °C for 12 h to obtain the final samples. Additionally, different hydrothermal treatments in the absence of CTAB or trisodium citrate at 180 °C were selected to investigate the morphological evolution process and luminescence properties of SrHAp samples. Characterization. X-ray powder diffraction (XRD) measurements were performed on a D8 Focus diffractometer (Bruker) at a scanning rate of 12°/min in the 2θ range from 10° to 80°, with graphite monochromatized Cu KR radiation (λ = 0.154 05 nm). The morphology and composition of the samples were inspected using a field emission scanning electron microscope (FESEM, S-4800, Hitachi) equipped with an energy-dispersive X-ray spectrum (EDX, JEOL JXA-840). The Fourier transform infrared (FT-IR) spectrum was measured with a Perkin-Elmer 580B infrared spectrophotometer with the KBr pellet technique. Transmission electron microscopy (TEM) was performed using a FEI Tecnai G2 S-Twin with a field emission gun operating at 200 kV. Images were acquired digitally on a Gatan multiople CCD camera. The photoluminescence (PL) excitation and emission spectra were recorded with a Hitachi F-4500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. Luminescence lifetimes were measured with a Lecroy Wave Runner 6100 digital oscilloscope (1 GHz) using a tunable laser (pulse width = 4 ns, gate = 50 ns) as the excitation (Continuum Sunlite OPO). Electron paramagnetic resonance (EPR) spectra were taken on the JESFE3AX electronic spin resonance spectrophotometer. Elemental analyses of Sr in the solid samples were carried out by inductively coupled plasma optical emission spectrometry analysis (ICP-OES, ICAP 6300, Thermo Scientific). The elemental analyses of C, H, N, and O were performed with an Elementar Analysensysteme GmbH VarioEL CHN Mode. The quantum efficiency of the phosphor was performed by the quantum yield measurement system (C9920-02, Hamamatsu Photonics K. K., Japan). All the measurements were performed at room temperature.

3. Results and Discussion 3.1. Phase Identification and Morphologies of SrHAp. The 3D architectures of strontium hydroxyapatite (SrHAp) microspheres were synthesized directly by hydrothermal treatment at 180 °C for 24 h. Figure 1 shows the XRD pattern of SrHAp powder sample prepared at 180 °C for 24 h together with the standard data for SrHAp. The diffraction peaks of the sample can be indexed as pure hexagonal phase, which coincide well with the standard data of SrHAp (JCPDS No. 33-1348, space group: P63/m, No. 176). No peak shift and other phases can be detected in the XRD patterns, indicating that the pure SrHAp crystals have been obtained by this method. Langmuir 2009, 25(23), 13591–13598

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Figure 2. SEM and TEM images of as-prepared sample at 180 °C for 24 h: (a, b) low- and high-magnification SEM images, (c, d) low- and high-magnification TEM images (inset in part d) SAED, (e) HRTEM image, and (f ) EDX spectrum.

Figure 2a shows a general SEM image of a typical sample. It can be seen that the sample is composed of abundant 3D microspheres with diameter of about 3-4.5 μm. The spherical structure of the as-obtained SrHAp sample could not be destroyed and completely broken into discrete individual nanosheets even by ultrasonically treating their aqueous suspension for 0.5 h. This indicates that the microspheres are not a random aggregate but the ordered self-assembly of the nanosheets. The magnified SEM image shown in Figure 2b reveals that the microsphere is composed of a great deal of nanosheets with a width of 200300 nm and a thickness of about 50 nm. Low- and highmagnification TEM images are shown in Figure 2c,d, which indicates that the nanosheets are assembled in a radial form from the center to the surface of microsphere. The inset in Figure 2d shows the SAED pattern recorded from a nanosheet in the SrHAp microsphere. The rings in the SAED pattern can be indexed as the (002), (102), (211), (112), and (300) reflections of the hexagonal SrHAp, in agreement with the XRD result. Figure 2e shows the corresponding HRTEM image, in which the lattice fringes with d spacing of 0.36 nm can be observed clearly, just corresponding to the distance of the (002) planes of hexagonal SrHAp crystal. The EDX spectrum (Figure 2f ) of the SrHAp product shows the presence of Sr, P, O, and C Langmuir 2009, 25(23), 13591–13598

(Au from the coating for SEM measurement). The detected carbon (C, also observed in the XPS and FI-IR spectra) from the sample prepared by the hydrothermal process, in which some organic additives were employed, indicates that some carbon impurities have been induced into the SrHAp host lattices and/or the surface. The similar situation holds for other samples. Figure 3 shows the survey XPS spectrum of as-prepared SrHAp microspheres in binding energy range of 0-1200 eV, and the XPS narrow scan spectra of O 1s, P 2p, Sr 3d, and C 1s core level peaks are shown in Figure S1 in the Supporting Information. In Figure 3, the XPS spectrum indicates that the main peaks at 134.60, 134.95, 531.40, and 284.55 eV can be assigned readily to the binding energy of Sr 3d, P 2p, O 1s, and C 1s, respectively. Since the positions of core level peaks for Sr 3d and P 2p are very close, we cannot distinguish them in Figure 3. However, the two peaks can be discerned from the XPS narrow scan spectra (Figure S1 in the Supporting Information). The detected carbon from the SrHAp sample can further indicate that the carbon-related impurities exist on the surface of the asprepared SrHAp sample. 3.2. Formation Mechanisms. To understand the growth mechanism of the SrHAp architectures, we performed a series of experiments to explore the formation process. DOI: 10.1021/la9019684

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Figure 3. XPS spectrum for as-synthesized SrHAp sample prepared at 180 °C for 24 h.

Figure 5. FT-IR spectra for trisodium citrate (a) and the as-prepared samples for different reaction time of 0.5 (b), 1.0 (c), and 24 h (d).

Figure 4. XRD patterns of the as-prepared samples at 180 °C for different reaction time: (a) 0.5, (b) 1, (c) 6, and (d) 48 h. These samples were prepared under the similar conditions to those for the sample prepared at 180 °C for 24 h.

Effect of Reaction Time-Formation Process. To reveal the growth process of SrHAp microspheres, time-dependent experiments were carried out, keeping other reaction parameters constant. Figure 4 shows the XRD patterns of the samples prepared at different reaction time. The samples obtained at 180 °C for 0.5 and 1.0 h shows a unique XRD pattern (Figure 4a,b), and no standard card can be indexed. We speculate that the sample is the complexes formed by the Sr2þ cations and the Cit3- anions, which can be confirmed by the FT-IR spectra (see next section). When the reaction time increased to 6.0 h, an obvious peak at 31° and some weak peaks at 22-28° and 40-51° are present (Figure 4c). These peaks are assigned to the hexagonal phase of Sr5(PO4)3OH. With increasing the reaction time to 48 h, the diffraction peaks of the as-prepared sample (Figure 4d) can be indexed as a pure hexagonal phase with a space group of P63/m (176), which coincides well with the standard data of SrHAp (JCPDS No. 33-1348). This result is similar to the sample prepared for 24 h, indicating that the crystal SrHAp sample can be obtained when the reaction time exceeds 24 h. Figure 5 shows the FT-IR spectra of the commercial trisodium citrate (a) and the as-obtained products prepared by hydrothermal process at 180 °C for 0.5 h (b), 1 h (c), and 24 h (d). At early growth stage, the FT-IR spectra of the products prepared for 0.5 h (Figure 5b) and 1 h (Figure 5c) are very similar to that of the commercial trisodium citrate (Figure 5a). This indicates that Sr2þ-citrate complex intermediate was formed at the prophase 13594 DOI: 10.1021/la9019684

(0-1 h) during the hydrothermal process. The elementary analysis of the sample prepared for 0.5 h was performed to detect the exact composition of the intermediate. The result shows that the intermediate contains 36.0 wt % of Sr, 19.8 wt % of C, 2.92 wt % of H, 41.2 wt % of O, and 0.08 wt % of other elements (such as N and Na). The tiny amount of nitrogen and sodium elements may be caused by the addition of the surfactants. The corresponding molar ratio of Sr/C/H/O is calculated to be 1.00/4.01/7.05/ 6.27. On the basis of the elementary analysis and FT-IR results, it can be deduced that the Sr2þ-citrate intermediate has the structure formula of Sr3(C6H5O7)2 5H2O (the slight deviation of the H and O elements might be caused by the absorbed water on the surface of the sample). In addition, in the FT-IR spectrum of the sample prepared for 1 h (Figure 5c), we can find a weak broad peak (mark with an red asterisk) at 1018 cm-1 for PO43-, which cannot be observed in Figure 5a,b. Therefore, we can confirm that the conversion from Sr2þ-citrate complex to crystal SrHAp began after about 1 h. In Figure 5d, the FT-IR result indicates the presence of OH-, PO43-, HPO42-, H2O, and CO32-. A set of characteristic peaks representing PO43- groups are observed. The bands centered at 1080 and 1018 cm-1 are ascribed to the asymmetric stretching vibrations of the P-O, and the band at 949 cm-1 is assigned to the symmetric stretching mode of the P-O in PO43- groups. In addition, two groups of bands in the low wavenumber region from 490 to 630 cm-1 (center: 560, 595 cm-1) are due to the bending vibrations of the O-P-O in PO43- groups. The weak peak at 878 cm-1 is caused by HPO42- groups.22,23 The peak at 3593 cm-1 is due to OH- ions, and the broad band centered at 3431 cm-1 is ascribed to O-H vibration of H2O absorbed in the SrHAp sample. The range of 1300-1600 cm-1 clearly shows the existence of the carbon-related impurities, (22) Kikuchi, M.; Yamazaki, A.; Otsuka, R.; Akao, M.; Aoki, H. J. Solid State Chem. 1994, 113, 373. (23) (a) Doat, A.; Fanjul, M.; Pelle, F.; Hollande, E.; Lebugle, A. Biomaterials 2003, 24, 3365. (b) Li, Y. B. J. Mater. Sci.: Mater. Med. 1994, 5, 326.

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Figure 6. SEM images of the as-prepared samples at 180 °C for different reaction time: (a) 0.5 h; (b) 1.0 h; (c, d) low and high magnification, 6 h; and (e, f ) low and high magnification, 48 h.

indicating their incorporation into the crystal structure of SrHAp sample. The SEM images of the corresponding intermediates and timeprolonged products prepared at the same hydrothermal temperature (180 °C) are shown in Figure 6. Interestingly, the Sr2þ-citrate complex has formed microrods and microsheets with smooth surface after hydrothermal treatment for 0.5 h (Figure 6a). By increasing the reaction time to 1 h, the size of the intermediate formed at the first stage decreased and some spherical particles appeared, as shown in Figure 6b. The result can also be confirmed by the FT-IR spectra (Figure 5b,c). After 6 h of hydrothermal treatment, the microrods and microsheets disappeared, while inhomogeneous hollow structure microspheres with different sizes became the exclusive products (Figure 6c). More careful examination of the magnified SEM image (inset of Figure 6c) clearly shows the hollow structure. The high-magnification SEM image (Figure 6d) shows that these hollow particles are similar to that of the solid microspheres prepared for 24 h (Figure 2), which are also composed of abundant radial nanosheets with a width of about 200 nm and a thickness of 20-30 nm. Further increasing the reaction time to 48 h, the size of the microspheres increased slightly, and the solid structure formed instead of hollow structure, as shown in inset of Figure 6e. Furthermore, from the higher magnification SEM image (Figure 6f), we can see that the solid microspheres were also composed by abundant nanosheets except the increase in thickness (70-90 nm) comparing with the hollow structures. It indicates that the size and crystallinity of the products increased with the increase of the reaction time. Langmuir 2009, 25(23), 13591–13598

Figure 7. SEM image of the sample prepared at 180 °C for 24 h without CTAB.

Effect of CTAB. In addition, CTAB is another key factor in modifying the morphology of SrHAp particles. To investigate the influence of CTAB on the shape evolution in our current synthesis, a control experiment was carried out in the absence of CTAB, while other parameters remained the same. The XRD pattern of the product in the absence of CTAB (Figure S2 in the Supporting Information) can be indexed as pure, well crystalline, and hexagonal structure of SrHAp (JCPDS No. 33-1348), which agrees well with the pattern of SrHAp microspheres (Figure 1). However, the corresponding morphology shows an obvious change comparing with the SrHAp sample obtained in the presence of CTAB (Figure 2a,b). The typical SEM image of SrHAp sample prepared without CTAB is presented in Figure 7, DOI: 10.1021/la9019684

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which indicates that the product is composed of aggregated nanorod bundles. This demonstrates that CTAB plays an important role in controlling the morphology of the final products. The exact mechanism for the change of the morphology of SrHAp grown with and without CTAB is not very clear. However, it is well-known that CTAB is a cationic surfactant and ionizes completely in water. So far, CTAB has been studied in the synthesis of mesostructured materials and proved to be a versatile “soft template” by self-assembling to form different conformations and lead to the formation of different structures and morphologies in functional materials.24 From the time-dependent experiment results (Figure 6), we can see that the growth of SrHAp microspheres might be an Ostwald ripening process in aqueous solution. The similar formation mechanisms have been reported in the synthesis of 3D flowerlike ZnO and Cu2O nanoarchitectures, InVO4 and MnWO4 microspheres, and WO3 hierarchical structures.24,25 In our synthesis process, CTAB may play the same role as a “soft template”. The formation mechanism of SrHAp samples might involve the following steps: (a) Sr2þcitrate intermediate [Sr3(C6H5O7)2 3 5H2O] was quickly formed at the initial stage of reaction (Figure 6a). (b) Under the hydrothermal conditions (high temperature and pressure), the Sr2þcitrate intermediate would dissolve and release Sr2þ ions gradually. This process can slow down the nucleation and subsequent crystal growth of the SrHAp particles.20 Then, the PO43- and OH- anions in the solution reacted with Sr2þ cations nucleation to form SrHAp nuclei. (c) In the surface-modification process, CTAB could adhere to the formed SrHAp nanoparticles because of the electrostatic attraction between C16H33(CH3)3Nþ (CTAþ) and the negative crystal surfaces (OH- or PO43-) of SrHAp. It is proposed that CTAB adsorbed on the capped SrHAp nuclei tended to form a bilayer structure with adjacent neighbors via hydrophobic interactions of cetyl tails.25c As time goes on and under the influences of CTAB, more and more nanoparticles would attach to each other to lower their surface energy and form nearly spherical structure by the hydrophobic interactions and van der Waals attraction (Figure 6b).24a,24b,25 (d) Finally, with the increase of reaction time, each nucleus would grow into nanosheet, and these aggregates grew and became spherelike hollow intermediates (Figure 6c).The further development of these intermediates results in the formation of solid hierarchical SrHAp microspheres (Figures 2c and 6e). On the basis of the above results, a schematic illustration for the formation of SrHAp microspheres is presented in Scheme 1. 3.3. Photoluminescence Properties. All the as-prepared SrHAp samples exhibit a white color under sunlight (the SrHAp sample obtained at 180 °C for 24 h as a representative instance is shown in Figure S3 in the Supporting Information). Under UVlight irradiation, the as-prepared SrHAp samples exhibit a strong blue luminescence. As a representative, excitation (black line) and emission (blue line) spectra of the SrHAp microspheres obtained at 180 °C for 24 h are shown in Figure 8. The inset in Figure 8 shows the luminescence photograph of SrHAp microspheres dispersing in the ethanol solution under UV lamp (365 nm) in the dark. From Figure 8, we can see that the emission spectrum (24) (a) Zhou, Y. X.; Zhang, Q.; Gong, J. Y.; Yu, S. H. J. Phys. Chem. C 2008, 112, 13383. (b) Zhang, Q.; Yao, W. T.; Chen, X. Y.; Zhu, L. W.; Fu, Y. B.; Sheng, L. S.; Yu, S. H. Cryst. Growth Des. 2007, 7, 1423. (c) Li, Y.; Cao, M. H.; Feng, L. Y. Langmuir 2009, 25, 1705. (d) Cao, M. H.; Wang, Y. H.; Guo, C. X.; Qi, Y. J.; Hu, C. W. Langmuir 2004, 20, 4784. (e) Deng, Y. H.; Qi, D. W.; Deng, C. H.; Zhang, X. M.; Zhao, D. Y. J. Am. Chem. Soc. 2008, 130, 28. (25) (a) Luo, Y.; Li, S.; Ren, Q.; Liu, J.; Xing, L.; Wang, Y.; Yu, Y.; Jia, Z.; Li, J. Cryst. Growth Des. 2007, 7, 87. (b) Zhang, H.; Yang, D.; Ji, Y.; Ma, X.; Xu, J.; Que, D. J. Phys. Chem. B 2004, 108, 3955. (c) Wang, S.; Zhang, Y.; Ma, X.; Wang, W.; Li, X.; Zhang, Z.; Qian, Y. Solid State Commun. 2005, 136, 283.

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Figure 8. (a) PL excitation and (b) emission spectra of as-synthesized SrHAp sample at 180 °C for 24 h. Inset in part b is the corresponding luminescence photograph for sample under UV lamp (365 nm) irradiation in the dark. Scheme 1. Schematic for the Formation Mechanism of SrHAp Microspheres

(blue line) consists of a strong broad band ranging from 350 to 570 nm with a maximum at 427 nm. The corresponding excitation spectrum (black line) includes two broad bands: a weak band 200-280 nm and a strong broad band 280-420 nm with a maximum at 344 nm. The chromaticity coordinates (CIE) of the SrHAp sample are x = 0.153, y = 0.081, located in the blue region (point a in Figure S4 in the Supporting Information), which agrees well with the luminescence photograph in the inset of Figure 8. The PL quantum efficiency was investigated for the SrHAp microspheres. It is found that the PL quantum efficiency for the as-obtained SrHAp sample is 31% under the excitation of 344 nm. In addition, the decay curve (Figure 9) for the luminescence of the SrHAp sample (monitored at 427 nm) can be well fitted into single-exponential function as I = I0 exp(-t/τ) (τ is lifetime), from which the lifetime τ is determined to be 9.2 ns.21 As can be seen in Figure S5 of the Supporting Information, the SrHAp samples obtained at 180 °C for different reaction time shows different emission intensities. It is found that the shape and profile of emission spectra vary little with the change of the reaction time, but the emission intensity changes greatly. The SrHAp sample obtained at 180 °C for 1 h gives nearly no emission. Subsequently, the PL intensity first increases with Langmuir 2009, 25(23), 13591–13598

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Figure 10. EPR spectrum of the SrHAp sample prepared at 180 °C for 24 h.

Figure 9. Decay curve for the luminescence of the SrHAp sample

synthesized at 180 °C for 24 h.

increasing reaction time, reaches a maximum at 24 h, and then nearly keeps the maximum with further increasing the reaction time. This might be due to the improvement of crystallinity, as can be seen from XRD patterns (Figure 4).26 At a certain reaction time (24 h), the reaction is completed and nearly unchanged with further increasing the reaction time. 3.4. Possible Luminescent Mechanisms. Since few literatures concerning the self-activated luminescence of SrHAp (without rare earth or transition metal ions as activators) materials can be found, the exact luminescent mechanisms for the Sr5(PO4)3OH samples are not very clear at present. Herein, it might be explained the above luminescent phenomena in terms of the existing models. In the past decade, many researches have been done on the luminescent materials caused by defects and/or impurities.9,19,27 In previous works, luminescence in amorphous SiO2 materials (including glass,28a,28b gels,18a,18b molecular sieves,27d spheres,18d nanocords,18e,18f etc.), BCNO,18g SnO2,19b,19c BPO4,21a ZrO2,21b and other materials, all of which are wellknown to show luminescence from the blue to the red spectral region, has been generally attributed to defect centers in the hosts including the E0 -type luminescent centers ( 3 Si, an unpaired electron on a silicon-dangling bond),18b,28b,28c nonbridging oxygen hole center (usually denoted by 3 O-SitO3),27d the peroxyradical hole trap ( 3 O-O-SitO3),18b,28c carbon impurities,18a,18g and oxygen defects.19 For organic/inorganic hybrid silicones containing -NH2 (or -NH) groups, Carlos et al. proposed a mechanism based on NH3þ/NH- (or NH2/N-) donor-acceptor pairs to explain the blue luminescence from the materials.27e,27f In addition, Hayakawa et al. ascribed the white luminescence properties of Al2O3-SiO2 glasses to the radical carbonyl terminations on the surface of the pores.28a,28b Angelov et al. reported that the CO2•- radicals in interstitials of the aragonite lattice of SrCO3 were most probably responsible for self-activated luminescence.27c It is well-known that different preparation processes can produce different kinds of defects.26a For the SrHAp sample, (26) (a) Yu, M.; Lin, J.; Wang, Z.; Fu, J.; Wang, S.; Zhang, H. J.; Han, Y. C. Chem. Mater. 2002, 14, 2224. (b) Pang, M. L.; Lin, J.; Cheng, Z. Y.; Fu, J.; Xing, R. B.; Wang, S. B. Mater. Sci. Eng., B 2003, 100, 124. (c) Yu, M.; Lin, J.; Fang, J. Chem. Mater. 2005, 17, 1783. (27) (a) Brankova, T.; Bekiari, V.; Lianos, P. Chem. Mater. 2003, 15, 1855. (b) Li, Y.; Cao, M.; Feng, L. Langmuir 2009, 25, 1705. (c) Angelov, S.; Stoyanova, R.; Dafinova, R.; Kabasanov, K. J. Phys. Chem. Solids 1986, 47, 409. (d) Gimon-Kinsel, M. E.; Groothuis, K.; Balkus, K. J., Jr. Microporous Mesoporous Mater. 1998, 20, 67. (e) Carlos, L. D.; Sa Ferreira, R. A.; Pereira, R. N.; Assunca~o, M.; Bermudez, V. de Z. J. Phys. Chem. B 2004, 108, 14924. (f) Fu, L.; Sa Ferreira, R. A.; Silva, N. J. O.; Carlos, L. D.; Bermudez, V. de Z.; Rocha, J. Chem. Mater. 2004, 16, 1507.

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neither the Sr2þ nor the PO43- group can be able to show luminescence, so the observed luminescence from SrHAp samples might be related to some impurities and/or defects in the host lattice. As a typical value for the luminescence caused by the impurities and/or defects, the short lifetime (9.2 ns) confirms this assumption.21,29 In addition, the EPR spectrum for SrHAp sample was measured (Figure 10). From Figure 10, the SrHAp sample prepared at 24 h exhibits three EPR bands at g = 2.0121, g = 2.0046, and g = 1.9961. This indicates that there indeed exist paramagnetic defects in SrHAp sample. Since the EPR signals cannot be caused by Sr2þ, P5þ, and O2- (no single electron in these ions), it must induced by some radical-related defects.27c,27e Comparing with previous reports, we can see that the g-tensor values of the EPR signal for SrHAp sample are very similar to the SrCO3 reported by Angelov et al.27c In this literature, the CO2•radicals as emission centers resulted from the decomposition of an oxalate anions (-OOCCOO-). As many -COO- anions also present in our synthesis process, we can suppose that the EPR signals are arisen by CO2•- radicals in the SrHAp host lattice. In the experiment process, the CO2•- radicals were formed from the -COO- and/or -COOH groups, which were induced by the additive of trisodium citrate in the reaction solution, not the CTAB. To confirm the influence of trisodium citrate and CTAB on the luminescence properties, the experiment in the absence of trisodium citrate was performed. We can clearly see that the product prepared without trisodium citrate shows no luminescence (Figure S6 in the Supporting Information), indicating that the trisodium citrate is the key factor for the luminescence properties. In addition, the PL excitation and emission spectra were measured for the SrHAp sample prepared in the absence of CTAB (Figure S7 in the Supporting Information). It can be seen that the excitation and emission spectra of the samples with and without CTAB are very similar. The variation of the PL intensity may be attributed to morphology and size of the products (with CTAB: microspheres; without CTAB: nanorod bundles). This result can further demonstrate that the emission centers of the SrHAp microspheres were introduced by trisodium citrate, not CTAB. Because of the complexity and difficulty in accurate analysis on the defect-related emission, there is no verdict on this point by far. So we can only roughly deduce that luminescence is cause by the CO2•- radicals by the experimental results and analysis (EDX, XPS, FT-IR, EPR, PL spectra, and decay curve).18b,27c,28 (28) (a) Hayakawa, T.; Hiramitsu, A.; Nogami, M. Appl. Phys. Lett. 2003, 82, 2975. (b) Yold, B. E. J. Non-Cryst. Solids 1992, 147, 614. (c) Lee, Y. C.; Liu, Y. L.; Shen, J. L.; Hsu, I. J.; Cheng, P. W.; Cheng, C. F.; Ko, C. H. J. Non-Cryst. Solids 2004, 341, 16. (29) (a) Fujimaki, M.; Ohki, Y.; Nishikawa, H. J. Appl. Phys. 1997, 81, 1042. (b) Pifferi, A.; Taroni, P.; Torricelli, A.; Valentini, G.; Mutti, P.; Ghislotti, G.; Zanghieri, L. Appl. Phys. Lett. 1997, 70, 348.

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4. Conclusions In summary, via a simple solvothermal route, strontium hydroxyapatite microspheres with 3D architecture were successfully prepared in aqueous reaction medium. The as-prepared SrHAp microspheres exhibited a strong blue emission peaking at about 427 nm with 31% quantum efficiency under the long-wavelength UV excitation at 344 nm. The CO2•- radicals in the SrHAp lattice might be responsible for the self-activated luminescence. This kind of phosphor does not contain rare earth ions (very expensive) as activators and no toxic elements, so it may be potentially used as a new efficient and environmentally friendly blue luminescent material. Acknowledgment. This project is financially supported by National Basic Research Program of China (2007CB935502, 2010CB327704) and the National Natural Science Foundation of China (NSFC 50702057, 50872131, 00610227).

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Supporting Information Available: XPS narrow scan spectra of Sr 3d (a), P 2p (b), O 1s (c), and C 1s (d) for SrHAp sample prepared at 180 °C for 24 h (Figure S1); XRD pattern of as-prepared SrHAp sample in the absence of CTAB at 180 °C for 24 h (Figure S2); photograph of SrHAp microspheres obtained at 180 °C for 24 h in daylight (Figure S3); CIE chromaticity diagram showing the emission colors of SrHAp sample (Figure S4); emission spectra of SrHAp samples obtained at 180 °C for 1, 6, 24, and 48 h (Figure S5); emission spectrum for the SrHAp sample obtained in absence of trisodium citrate in the prepare process (Figure S6); PL excitation and emission spectra for the SrHAp sample prepared in absence of CTAB in the prepare process (Figure S7). This material is available free of charge via the Internet at http://pubs. acs.org.

Langmuir 2009, 25(23), 13591–13598