Highly Uniform Gd2O3 Hollow Microspheres: Template-Directed

Nov 18, 2009 - Highly Uniform Gd2O3 Hollow Microspheres: Template-Directed Synthesis and Luminescence ... E-mail: [email protected]; [email protected]...
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Highly Uniform Gd2O3 Hollow Microspheres: Template-Directed Synthesis and Luminescence Properties Guang Jia, Hongpeng You,* Kai Liu, Yuhua Zheng, Ning Guo, and Hongjie Zhang* State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China, and Graduate University of the Chinese Academy of Sciences, Beijing 100049, PR China Received September 22, 2009. Revised Manuscript Received November 1, 2009 Well-dispersed, uniform Gd2O3 hollow microspheres have been successfully fabricated via a urea-based homogeneous precipitation method in the presence of colloidal melamine formaldehyde (MF) microspheres as templates, followed by subsequent heat treatment. The main process was carried out under aqueous conditions without any organic solvents, surfactants, or etching agents. The as-obtained Gd2O3 microspheres with a spherical shape and hollow structure are uniform in size and distribution, and the thickness of the shell is about 200 nm. The lanthanide ion (Ln3þ)doped Gd2O3 hollow microspheres exhibit bright down- and upconversion luminescence with different colors coming from different activator ions under ultraviolet or 980 nm light excitation, which might find potential applications in fields such as drug delivery or biological labeling because of their excellent dispersing and luminescence properties. Furthermore, this synthesis route may be of great significance in the preparation of other hollow spherical materials.

1. Introduction In modern chemistry and materials science, the precise architectural manipulation of nano/microcrystals with well-defined morphologies and accurately tunable sizes remains a research focus and a challenging issue because it is well known that the properties of the materials closely interrelate with geometrical factors such as shape, dimensionality, and size.1 Remarkably, hollow nano/microspheres currently represent one of the fastest growing areas compared with other structural and geometrical features.2 The possibility of modifying the outer and inner surfaces may enhance the advantageous characteristics of hollow spheres. Because of the distinct low effective density, high specific surface area, and encapsulation ability, hollow spherical materials are exceptionally promising in various fields such as confined catalysis,3 biotechnology,4 photonic devices,5 and electrochemical cells.6 It is worth noting that inorganic functional materials with hollow spherical morphology have potential biomedical applications in the fields of drug delivery, disease diagnosis, and biological labeling.7 Furthermore, microspheres are widely accepted *Corresponding authors. E-mail: [email protected]; [email protected]. (1) (a) Mac Lachlan, M. J.; Manners, I.; Ozin, G. A. Adv. Mater. 2000, 12, 675. (b) Hu, J.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435. (c) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. D. Nature 2005, 437, 121. (2) (a) Zeng, H. J. Mater. Chem. 2006, 16, 649. (b) Hua, H.; Zeng, H. Angew. Chem., Int. Ed. 2004, 43, 5205. (c) Im, H. S.; Jeong, U.; Xia, Y. Nat. Mater. 2005, 4, 671. (3) Ikeda, S.; Ishino, S.; Harada, T.; Okamoto, N.; Sakata, T.; Mori, H.; Kuwabata, S.; Torimoto, T.; Matsumura, M. Angew. Chem., Int. Ed. 2006, 45, 7063. (4) (a) Wei, W.; Ma, G. H.; Hu, G.; Yu, D.; Mcleish, T.; Su, Z. G.; Shen, Z. Y. J. Am. Chem. Soc. 2008, 130, 15808. (b) Zhu, Y.; Shi, J.; Shen, W.; Dong, X.; Feng, J.; Ruan, M.; Li, Y. Angew. Chem., Int. Ed. 2005, 44, 5089. (c) Zelikin, A. N.; Li, O.; Caruso, F. Angew. Chem., Int. Ed. 2006, 45, 7743. (5) Xu, X. L.; Asher, S. A. J. Am. Chem. Soc. 2004, 126, 7940. (6) (a) Lou, X. W.; Wong, Y.; Yuan, C.; Lee, J. Y.; Archer, L. A. Adv. Mater. 2006, 18, 2325. (b) Lou, X. W.; Archer, L. A. Adv. Mater. 2008, 20, 1853. (7) (a) Zhao, W.; Chen, H.; Li, Y.; Li, L.; Lang, M.; Shi, J. Adv. Funct. Mater. 2008, 18, 2780. (b) Yang, J.; Lee, J.; Kang, J.; Lee, K.; Suh, J. S.; Yoon, H. G.; Huh, Y. M.; Haam, S. Langmuir 2008, 24, 3417. (c) Zhu, Y.; Kockrick, E.; Ikoma, T.; Hanagata, N.; Kaskel, S. Chem. Mater. 2009, 21, 2547. (d) Yan, E.; Ding, Y.; Chen, C.; Li, R.; Hu, Y.; Jiang, X. Chem. Commun. 2009, 2718. (8) (a) Kawaguchi, H. Prog. Polym. Sci. 2000, 25, 1171. (b) Freiberg, S.; Zhu, X. X. Int. J. Pharm. 2004, 282, 1.

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as useful drug-delivery systems because they can be ingested or injected and have homogeneous morphology.8 Recently, many efforts have been made in the development of different methods for the design and preparation of nano- or microsized hollow spheres. Template-directed synthesis, with hard templates (including silica, carbon spheres, and metal colloids)9-12 and soft templates (consisting of vesicles, emulsion droplets, micelles, and gas bubbles),13-16 have been demonstrated to be an effective approach to preparing inorganic hollow spheres. Among various hard templates, polymer latex particles, especially polystyrene (PS) beads, have been reported to be effective templates for the preparation of hollow spherical inorganic materials such as TiO2,17,18 SiO2,19 Ta2O5,20 and ZnS.21 Recently, melamine formaldehyde (MF) colloidal particles that act as templates have been steadily employed in fabricating carbon spheres,22 functionalized polyelectrolyte-coated particles or hollow polymer capsules,23 and core-shell-structured composite particles.24 However, to the best of our knowledge, no report (9) (a) Caruso, F.; Caruso, R. A.; Mohwald, H. Science 1998, 282, 1111. (b) Kim, S. W.; Kim, M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc. 2002, 124, 7642. (10) Liang, H.; Zhang, H.; Hu, J.; Guo, Y.; Wan, L.; Bai, C. Angew. Chem., Int. Ed. 2004, 43, 1540. (11) (a) Sun, X.; Li, Y. Angw. Chem., Int. Ed. 2004, 43, 3827. (b) Sun, X.; Liu, J.; Li, Y. Chem.;Eur. J. 2006, 12, 2039. (12) Sun, Y. G.; Mayers, B. T.; Xia, Y. N. Nano Lett. 2002, 2, 481. (13) Bao, J.; Liang, Y.; Xu, Z.; Si, L. Adv. Mater. 2003, 15, 1832. (14) Putlitz, B. Z.; Landfester, K.; Fischer, H.; Antonietti, M. Adv. Mater. 2001, 13, 500. (15) Zhang, D.; Qi, L.; Ma, J.; Cheng, H. Adv. Mater. 2002, 14, 1499. (16) Peng, Q.; Dong, Y.; Li, Y. Angew. Chem., Int. Ed. 2003, 42, 3027. (17) Yang, Z. Z.; Niu, Z. W.; Lu, Y. F.; Hu, Z. B.; Han, C. C. Angew. Chem., Int. Ed. 2003, 42, 1943. (18) Wang, P.; Chen, D.; Tang, F. Q. Langmuir 2006, 22, 4832. (19) Chen, M.; Wu, L. M.; Zhou, S. X.; You, B. Adv. Mater. 2006, 18, 801. (20) Agrawal, M.; Pich, A.; Gupta, S.; Zafeiropoulos, N. E.; Simon, P.; Stamm, M. Langmuir 2008, 24, 1013. (21) Hosein, I. D.; Liddell, C. M. Langmuir 2007, 23, 2892. (22) (a) Friedel, B.; Greulich-Weber, S. Small 2006, 2, 859. (b) Li, W. R.; Chen, D. H.; Li, Z.; Shi, Y. F.; Wan, Y.; Wang, G.; Jiang, Z. Y.; Zhao, D. Y. Carbon 2007, 45, 1757. (23) (a) Choi, W. S.; Koo, H. Y.; Huck, W. T. S. J. Mater. Chem. 2007, 17, 4943. (b) Zheng, S. P.; Tao, C.; He, Q.; Zhu, H. F.; Li, J. L. Chem. Mater. 2004, 16, 3677. (c) Kato, N.; Schuetz, P.; Fery, A.; Caruso, F. Macromolecules 2002, 35, 9780. (24) Jia, G.; Liu, K.; Zheng, Y. H.; Song, Y. H.; You, H. P. Cryst. Growth Des. 2009, 9, 3702.

Published on Web 11/18/2009

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has been devoted to the preparation of hollow spherical lanthanide compounds by using MF colloidal particles as templates. It is well known that the lanthanide oxides are excellent host lattices for the luminescence of various optically active lanthanide ions.25 Gadolinium oxide (Gd2O3) is a promising host matrix for down- and upconversion luminescence because of its good chemical durability, thermal stability, and low phonon energy.26 Until now, various morphologies of Gd2O3 have been synthesized via different methods such as nanorods and microrods,26a nanotubes,26b nanorings and nanoplates,27 and rectangular microcrystals.28 Yeh et al. have introduced biological gelatin particles as central cores to synthesize uniform nanosized superparamagnetic Gd2O3 hollow spheres of about 200 nm.29 However, to the best of our knowledge, there have been few reports on the synthesis of uniform, well-dispersed micrometer-scaled rare-earth-doped Gd2O3 hollow spheres and their corresponding luminescence properties. The hollow spheres of rare-earth-ion-doped phosphors would achieve a reduction in the amount of expensive rare earth. Moreover, because of the low density of hollow spherical materials, when coating a screen in display applications, the phosphors can be well dispersed, enhancing the uniformity and giving high packing densities of the coating.25b Recently, Li et al. have successfully prepared monodisperse (Y1 - xGdx)2O3 (x = 0-1) colloidal spheres via a urea-based homogeneous precipitation method and have investigated their precipitation and growth behaviors.30 In this article, the synthesis technique was used to fabricate very uniform Gd2O3 hollow microspheres with melamine formaldehyde (MF) colloidal microspheres as hard templates, followed by a subsequent calcination process. The structure, formation process, and luminescence properties (down- and upconversion emissions) of the as-obtained hollow microspheres were investigated in detail. Our work may open new possibilities to synthesize hollow spheres of other oxides and extend their applications.

2. Experimental Section Ln2O3 (Ln = Gd, Eu, Yb, Er, and Tm; 99.99%) was dissolved in dilute HNO3 solution (1:1 v/v) by heating to 100 °C with agitation to form clear Ln(NO3)3 aqueous solutions. Superfluous HNO3 was driven off by continuous heating until the pH value of the solution reached between 3 and 4. All other chemicals were analytical-grade reagents and were purchased from the Beijing Chemical Corporation and used without further purification. 2.1. Synthesis of Monodisperse MF Microspheres. Monodisperse MF colloidal microspheres were prepared according to the literature with a slight modification.22a,24 In a typical synthesis, a solution of formaldehyde (9.8 g, 37%) and deionized water (200 mL) was prepared and heated to 80 °C. Subsequently, 2.5 g of melamine was added with stirring. When the melamine dissolved completely, formic acid was introduced into the vigorously stirred solution until reaching pH 5. The transparent solution was stirred (25) (a) Mao, Y. B.; Tran, T.; Guo, X.; Huang, J. Y.; Shih, C. K.; Wang, K. L.; Chang, J. P. Adv. Funct. Mater. 2009, 19, 748. (b) Jia, G.; Yang, M.; Song, Y. H.; You, H. P.; Zhang, H. J. Cryst. Growth Des. 2009, 9, 301. (c) Jia, G.; Zheng, Y. H.; Liu, K.; Song, Y. H.; You, H. P.; Zhang, H. J. J. Phys. Chem. C 2009, 113, 153. (26) (a) Yang, J.; Li, C. X.; Cheng, Z. Y.; Zhang, X. M.; Quan, Z. W.; Zhang, C. M.; Lin, J. J. Phys. Chem. C 2007, 111, 18148. (b) Jia, G.; Liu, K.; Zheng, Y. H.; Song, Y. H.; Yang, M.; You, H. P. J. Phys. Chem. C 2009, 113, 6050. (c) Guo, H.; Dong, N.; Yin, M.; Zhang, W. P.; Lou, L. R.; Xia, S. D. J. Phys. Chem. B 2004, 108, 19205. (27) Paek, J.; Lee, C. H.; Choi, J.; Choi, S. Y.; Kim, A.; Lee, J. W.; Lee, K. Cryst. Growth Des. 2007, 7, 1378. (28) Hu, L.; Ma, R.; Ozawa, T. C.; Sasaki, T. Angew. Chem., Int. Ed. 2009, 48, 3846. (29) Huang, C. C.; Liu, T. Y.; Su, C. H.; Lo, Y. W.; Chen, J. H.; Yeh, C. S. Chem. Mater. 2008, 20, 3840. (30) (a) Li, J. G.; Li, X. D.; Sun, X. D.; Ikegami, T.; Ishigaki, T. Chem. Mater. 2008, 20, 2274. (b) Li, J. G.; Li, X. D.; Sun, X. D.; Ishigaki, T. J. Phys. Chem. C 2008, 112, 11707.

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for several minutes and became turbid. After additional agitation for 20 min, the obtained white colloidal particles were washed with deionized water and ethanol and dried in air at 60 °C. 2.2. Preparation of Gd2O3 Hollow Microspheres. In the preparation procedure, 1 mmol of Gd(NO3)3 aqueous solution was added to 30 mL of distilled water. Then 3.0 g of urea was dissolved in the solution after vigorous stirring to form a clear solution. The as-prepared MF microspheres (0.2 g) were added and well dispersed into the above solution with the assistance of ultrasonication for 15 min. Finally, the mixture was transferred to a round-bottomed flask and heated to 85 °C for 3 h with vigorous stirring before the product was collected by centrifugation. The precursor was washed with deionized water and ethanol several times and dried in air at 60 °C. The final Gd2O3 hollow microspheres were obtained through heat treatment at 800 °C for 2 h in air at a heating rate of 2 °C min-1.

2.3. Synthesis of Ln3þ-Doped Gd2O3 Hollow Microspheres. Ln3þ-doped Gd2O3 hollow microspheres were prepared

by the same synthesis procedure for the Gd2O3 sample except that a stoichiometric amount of Ln(NO3)3 (Ln=Eu, Yb, Er, and Tm) aqueous solutions was added to Gd(NO3)3 for the precursors in the initial stage as described above [0.95 mol of Gd(NO3)3 and 0.05 mol of Eu(NO3)3 for Gd2O3:5% Eu3þ; 0.99 mol of Gd(NO3)3 and 0.01 mol of Er(NO3)3 for Gd2O3:1% Er3þ; 0.94 mol of Gd(NO3)3, 0.05 mol of Yb(NO3)3, and 0.01 mol of Er(NO3)3 for Gd2O3:5% Yb3þ/1% Er3þ; 0.94 mol of Gd(NO3)3, 0.05 mol of Yb(NO3)3, and 0.01 mol of Tm(NO3)3 for Gd2O3:5% Yb3þ/1% Tm3þ]. 2.4. Characterization. The samples were characterized by powder X-ray diffraction (XRD) performed on a D8 Focus diffractometer (Bruker). Fourier transform infrared spectroscopy (FTIR) spectra were recorded with a Perkin-Elmer 580B infrared spectrophotometer using the KBr pellet technique. Thermogravimetric analysis and differential scanning calorimetry (TGA-DSC) data were recorded with a thermal analysis instrument (SDT 2960, TA Instruments, New Castle, DE) at a heating rate of 10 °C min-1. The morphology and composition of the samples were inspected using a scanning electron microscope (SEM; S-4800, Hitachi) equipped with an energy-dispersive X-ray spectrum spectrometer (EDX; XFlash-Detector 4010, Bruker). Transmission electron microscopy (TEM) images and selected-area electron diffraction (SAED) patterns were obtained using a JEOL 2010 transmission electron microscope operating at 200 kV. 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. The luminescence decay curve was obtained from a Lecroy Wave Runner 6100 digital oscilloscope (1 GHz) using a tunable laser (pulse width=4 ns, gate = 50 ns) as the excitation source (Continuum Sunlite OPO). The upconversion (UC) emission spectra were obtained using a 980 nm laser from OPO as the excitation source and were detected by an R955 (Hamamatsu) from 400 to 730 nm. All measurements were performed at room temperature.

3. Results and Discussion 3.1. Phase Identification and Morphology. Figure 1 shows the X-ray diffraction results of the precursor and the final product after calcination at 800 °C for 2 h. No obvious diffraction peak appears in Figure 1a, which indicates that the as-formed coreshell-structured precursor is amorphous before calcination. The component of the amorphous precursor before calcination has been confirmed to be Gd(OH)CO3 on the basis of previous reports.25b,30 When the precursor was calcined at 800 °C for 2 h, all of the diffraction peaks can be well indexed to the cubic phase of Gd2O3 (JCPDS no. 88-2165). No additional peaks of other phases have been found, indicating the formation of a purely cubic Gd2O3 phase (Figure 1b). The lattice constant a of the calcined product was calculated according to the equation DOI: 10.1021/la903584j

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Figure 1. XRD patterns of the samples for (a) the uncalcined precursor and (b) after calcination at 800 °C for 2 h. The standard data for cubic-phase Gd2O3 (JCPDS no. 88-2165) is presented for comparison.

1/d2 = (h2 þ k2 þ l2)/a2, where d is the interplanar distance, h, k, and l are the crystal indices (Miller indices), and a is the lattice constant. On the basis of the (222) crystal plane (d = 0.311 nm), the lattice constant a is calculated to be 1.076 nm, which is very compatible with the literature value of a=1.079 nm.31 It can also be seen that the diffraction peaks of the Gd2O3 samples are very sharp and strong, revealing that the Gd2O3 product with high crystallinity can be synthesized by this method. This is important for phosphors because high crystallinity generally means fewer traps and stronger luminescence. The reaction of melamine with formaldehyde leads to hydroxymethylation, whereas hydrogen atoms in the NH2 groups of melamine are substituted by methylol groups (CH2OH), followed by cross linking of the resulting methylolmelamines. The FTIR spectra were used to identify the functional groups of the asobtained MF template, the core-shell-structured precursor, and the Gd2O3 product. The FTIR spectrum of the MF template (Figure 2a) shows the characteristic absorption bands of MF resins, which are similar to some in the literature.22a,24,32 The absorption bands at about 3377, 1557 (1494, 1352), 1166, 1006, and 813 cm-1 are assigned to the vibrations of hydroxy/amino (-OH/-NH2), amino (-NH2), amine (C-N), ether (C-O-C), and C-N-C groups, respectively. This result indicates the formation of MF resins. The FTIR spectrum of the core-shellstructured precursor (Figure 2b) is similar to that of the MF templates. However, we can clearly see that the IR band of ether groups (1006 cm-1) disappears, which may be caused by coating the precursor nanoparticles onto the surfaces of MF microspheres (Figure 5a,b). Figure 2c shows the FTIR spectrum of the assynthesized Gd2O3 hollow microspheres. It can be seen that all of the functional groups of the MF template nearly disappear, revealing that the MF template can be effectively removed during the calcination process. Moreover, a new IR band centered at about 540 cm-1 appears and can be assigned to the Gd-O stretching frequencies of Gd2O3.33 The result is consistent with that of the XRD pattern and confirms the formation of crystalline (31) Bartos, A.; Lieb, K. P.; Uhrmacher, M.; Wiarda, D. Acta Crystallogr., B 1993, 49, 165. (32) (a) Choi, W. S.; Koo, H. Y.; Kim, D. Y. Langmuir 2008, 24, 4633. (b) Gao, C. Y.; Moya, S.; Lichtenfeld, H.; Casoli, A.; Fiedler, H.; Donath, E.; M€ohwald, H. Macromol. Mater. Eng. 2001, 286, 355. (33) (a) Li, G. Z.; Wang, Z. L.; Yu, M.; Quana, Z. W.; Lin, J. J. Solid State Chem. 2006, 179, 2698. (b) García-Murillo, A.; Luyer, C. L.; Dujardin, C.; Pedrini, C.; Mugnier, J. Opt. Mater. 2001, 16, 39.

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Figure 2. FTIR spectra of (a) pure MF microspheres, (b) the core-shell-structured precursor, and (c) Gd2O3 hollow microspheres.

Figure 3. TGA curves of (a) the as-dried core-shell-structured precursor and (b) pure MF templates. The inset is the corresponding DSC of curve a.

Gd2O3 via urea-based homogeneous precipitation and the annealing process. Figure 3 shows the TGA curves of the as-prepared core-shellstructured precursor and the pure MF templates. Weight loss can be observed from the TGA curves before 100 °C, which may be caused by the absorbed water on the surfaces of the samples. In addition, there are two stages of weight loss for the MF templatecoated gadolinium compound precursor (Figure 3a): One is a slow weight loss peak at 310.5 °C that is due to the dehydration and densification of the MF resin. The other sharp weight loss can be attributed to the combination of the burning of MF templates and the decomposition of the amorphous gadolinium compound precursor. The corresponding DSC result (inset in Figure 3) of the precursor agrees well with the TGA curve. From Figure 3b, we can see that the weight loss behavior of the pure MF templates is similar to that of the as-formed core-shell-structured precursor. It can also be seen from Figure 3b that the weight loss of the pure MF template is nearly 100%, indicating that the MF template can be removed completely after calcination. The residual weight percentage of as-prepared precursors is about 35.0%, which accounts for the final Gd2O3 product. One of the main advantages of this method is the high-yield synthesis, which is strongly supported by the TGA result. Figure 4a shows the SEM image of the bare MF microspheres. One can see that the as-obtained MF templates consist of very Langmuir 2010, 26(7), 5122–5128

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Figure 4. (a, b) SEM images of pure MF colloidal microspheres. SEM images of the (c) gadolinium precursor and (d) Gd2O3 samples prepared without MF templates.

uniform microspheres with diameters of about 2.3 μm. It can also be observed in the high-magnification SEM image that the surfaces of monodisperse MF microspheres are very smooth (Figure 4b). Figure 4c,d shows the SEM images of the gadolinium precursor and the corresponding Gd2O3 sample annealed at 800 °C without MF microspheres as templates. It can be seen that the asformed precursor is composed of well-dispersed colloidal spheres with a narrow size distribution (Figure 4c). The calcined Gd2O3 sample retains the spherical shape and the excellent dispersion of the precursor particles, except for the shrinkage in particle size (Figure 4d). The result agrees well with previous reports.30 Figure 5a shows the SEM image of the precursor with MF templates before calcination. It is noted that the uniform coreshell-structured microspheres inherit the spherical morphology and good dispersion of the MF templates but the surfaces of core-shell-structured particles are much rougher than those of bare MF cores because of the precipitation of a large number of uniform nanoparticles, which can be clearly observed from the enlarged SEM image (Figure 5b). Naturally, the size of the core-shell-structured particles (ca. 2.8 μm) is larger than that of the bare MF template (2.3 μm) because of the amorphous Gd(OH)CO3 shell. Figure 5c,d shows the morphology of the Gd2O3 sample calcined at 800 °C for 2 h. A panoramic SEM image in Figure 5c indicates that the sample consists of a large scale of uniform, well-dispersed hollow microspheres with diameters of about 2 μm, which implies that the MF templates essentially determine the shape and structure of the final products. During the calcination process, the as-synthesized oxide hollow microspheres only decreased in size but still retained the spherical profile of the MF templates. In this case, oxide hollow microspheres could be prepared from metal salts by using the MF microspheres as templates. It can also be seen that the average diameters of these oxide microspheres (ca. 2 μm) with rough surfaces obviously decrease in comparison with the core-shellstructured precursor microspheres (2.8 μm). It is believed that the relative shrinkage is caused by the dehydration of the cross-linked structure of the MF templates and the conversion from the loosely arranged gadolinium compound precursor to very compact oxides in the surface layer. Consequently, cross linking and crystallization of Gd2O3 hollow spheres occurred when the MF cores were removed during the calcination process in air. A small quantity of ruptured hollow microspheres (Figure 5c) imply that the microspheres have hollow structures, and the rupture of the microspheres may be attributed to the release of gaseous Langmuir 2010, 26(7), 5122–5128

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Figure 5. SEM images of the (a, b) core-shell-structured precursor before calciantion and (c, d) Gd2O3 hollow microspheres. The inset in c is an enlarged SEM image of a ruptured microsphere.

Figure 6. TEM images of (a) the core-shell-structured precursor and (b) the Gd2O3 hollow microspheres.

carbon/nitrogen oxides when the oxidation process of MF microspheres occurred during the calcination process. From the enlarged SEM image of a ruptured microsphere (inset in Figure 5c), we can see that the wall thickness of the hollow microspheres is about 200 nm. As can be seen from the magnified SEM image of the Gd2O3 sample (Figure 5d), the rough shells of the welldispersed hollow microspheres are made of uniform nanoparticles, which are similar to the surface of the core-shell-structured precursor (Figure 5b). The result is in good agreement with the SEM images of the gadolinium precursor and the Gd2O3 sample prepared without an MF templates. To provide further insight into the Gd2O3 hollow microstructures, a TEM investigation was also performed. Figure 6a shows the TEM image of the precursor before calcination. It can be seen that the precursor microspheres with a diameter of 2.8 μm have rough surfaces and a solid structure, which agrees well with the SEM images (Figure 5a,b). The TEM image of the Gd2O3 sample (Figure 6b) exhibits spherical morphology with good uniformity. The strong contrast between the dark edge and the pale center is direct evidence of the hollow nature of the microspheres. The average size of the hollow spheres and the thickness of the shells are estimated to be about 2 μm and 200 nm, respectively, which is in good agreement with the SEM observations (Figure 5c,d). 3.2. Formation Process. On the basis of the experimental results and analysis, the whole process for the formation of uniform Gd2O3 hollow microspheres can be mainly divided into three steps: First, the monodisperse micrometer-sized MF colloidal particles were fabricated by a condensation and cross-linking process in aqueous solution.22a,24 Second, the core-shell-structured DOI: 10.1021/la903584j

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Scheme 1. Schematic Illustration of the Overall Formation Process of Gd2O3 Hollow Microspheres

precursors were obtained by a homogeneous precipitation method using MF microspheres as templates and urea as the precipitating agent. Because the MF colloidal microspheres were prepared in aqueous solution and consist of a large number of hydrophilic functional groups, which can be confirmed by the FTIR spectrum (Figure 2a), the MF templates are hydrophilic and may have a good affinity for Gd3þ, OH-, and CO32- (OHand CO32- result from the decomposition of urea) in aqueous solution. It is believed that the amorphous Gd(OH)CO3 nuclei that formed in the reaction media may be easily absorbed onto the surfaces of hydrophilic MF beads during the formation process of the composite particles. This nucleation process was followed by the growth of the formed nuclei until the supersaturation state was achieved. With the reaction proceeding, the amorphous precursor nuclei continued to grow and served as the seeds for the growth of Gd(OH)CO3 nanoparticles on the surfaces of the MF templates, resulting in the core-shell-structured composite particles. During this process, the precipitating agent urea, which serves as a reservoir for precipitating anions (mainly OH- and CO32-), plays a crucial role in forming precursor shells on the surfaces of the MF templates. The decomposition of urea releases precipitating anions slowly and homogeneously into the reaction system, avoiding the localized distribution of the reactants, and thus results in the uniform nanoparticle coating on the surfaces of the MF microspheres.25b Finally, the MF templates were burned out and the amorphous Gd(OH)CO3 shell was converted into crystalline Gd2O3 during the calcination process, resulting in the formation of uniform oxide hollow microspheres. A schematic illustration for the overall formation process of the uniform Gd2O3 hollow microspheres is presented in Scheme 1. 3.3. Luminescence Properties. As we know, Gd2O3 is an excellent host matrix for downconversion26a,b,34 and upconversion26c,35 luminescence because of its chemical durability, thermal stability, and lower phonon energy. As reported in many previous papers, the rare earth activator ions can be effectively doped into the lanthanide oxides prepared by the urea-based homogeneous precipitation method.30,36 In our case, a stoichiometric number of lanthanide ions (Eu3þ, Yb3þ, Er3þ, and Tm3þ) were doped into the Gd2O3 host lattice to investigate the luminescence properties. (34) Goldys, E. M.; Drozdowicz-Tomsia, K.; Jinjun, S.; Dosev, D.; Kennedy, I. M.; Yatsunenko, S; Godlewski, M. J. Am. Chem. Soc. 2006, 128, 14498. (35) Chen, X. Y.; Ma, E.; Liu, G. K.; Yin, M. J. Phys. Chem. C 2007, 111, 9638. (36) (a) Sardar, D. K.; Nash, K. L.; Yow, R. M.; Gorski, W.; Zhang, M. J. Appl. Phys. 2007, 101, 113116. (b) Li, Y. H.; Hong, G. Y.; Zhang, Y. M.; Yu, Y. N. J. Alloys Compd. 2008, 456, 247.

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Figure 7. (a) XRD patterns of Gd2O3:5% Eu3þ, Gd2O3:1% Er3þ, Gd2O3:5% Yb3þ/1% Er3þ, and Gd2O3:5% Yb3þ/1% Tm3þ samples. (b) EDX spectrum of the Gd2O3:5% Eu3þ sample.

The XRD patterns of Gd2O3:Eu3þ, Gd2O3:Er3þ, Gd2O3:Yb3þ/ Er3þ, and Gd2O3:Yb3þ/Tm3þ samples (Figure 7a) exhibit the diffraction peaks of purely cubic Gd2O3 (JCPDS no. 88-2165). According to the equation 1/d2 = (h2 þ k2 þ l2)/a2, the lattice constants of a were calculated to be 1.081, 1.075, 1.072, and 1.071 nm for the Gd2O3:5 %Eu3þ, Gd2O3:1% Er3þ, Gd2O3:5% Yb3þ/1% Er3þ, and Gd2O3:5% Yb3þ/1% Tm3þ samples, respectively. Because the ionic radius of Gd3þ is smaller than that of Eu3þ and larger than that of Yb3þ, Er3þ, and Tm3þ, the variation of the lattice constant of the Ln3þ-doped samples agrees well with the ionic radii of Gd3þ and the activator ions. (The lattice constant of undoped Gd2O3 is 1.076 nm.) This result indicates that the activator ions have been doped into the Gd2O3 host lattice. The Gd2O3:Eu3þ sample was taken as an example to investigate the element composition by energy-dispersive X-ray (EDX) spectroscopy (Figure 7b). The EDX result confirms the presence of gadolinium (Gd), oxygen (O), and europium (Eu) elements in Gd2O3:Eu3þ hollow microspheres. The peaks of carbon and nitrogen elements were not detected, indicating that the MF templates can be removed completely and the amorphous Gd(OH)CO3:Eu3þ precursor has been converted to Gd2O3:Eu3þ during the calcination process, which is consistent with the XRD and FTIR results. Under ultraviolet (UV) light excitation, the as-obtained Gd2O3:5% Eu3þ hollow microspheres exhibit strong red emission, which can be confirmed by the luminescence photograph upon excitation at 254 nm with a UV lamp (inset in Figure 8a). The excitation spectrum consists of a strong, broad band at about 254 nm and some weak lines in the longer-wavelength region that are due to the charge-transfer band (CTB) between O2- and Eu3þ and the f-f transitions of the Gd3þ and Eu3þ ions, respectively. The UV-vis absorption spectrum of the Gd2O3:Eu3þ sample is also shown in Figure 8a for comparison (dotted red line). A broad Langmuir 2010, 26(7), 5122–5128

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Figure 9. Upconversion PL emission spectra of lanthanide-iondoped Gd2O3 hollow microspheres under 980 nm light excitation: (a) Gd2O3:1% Er3þ, (b) Gd2O3:5% Yb3þ/1% Er3þ, and (c) Gd2O3:5% Yb3þ/1% Tm3þ. The insets are the corresponding luminescence photographs under 980 nm light excitation.

Figure 8. (a) Excitation, UV-vis absorption (dottted red line), and emission spectra and (b) decay curve of as-synthesized Gd2O3: Eu3þ hollow microspheres. The inset in a is the corresponding luminescence photograph of Gd2O3:Eu3þ under UV excitation (254 nm).

absorption band from 220 to 310 nm can be observed, which is also attributed to the charge-transfer absorption between the O2and Eu3þ. The result agrees well with the excitation spectrum. Upon excitation into the CTB of the Eu3þ ions at 254 nm, the emission spectrum exhibits five groups of emission lines at about 580, 588, 610 (628), 649, and 706 nm, which are ascribed to the 5 D0-7FJ (J = 0, 1, 2, 3, 4) transitions of Eu3þ (Figure 8a). The emission spectrum is dominated by the red 5D0-7F2 (610 nm) transition of the Eu3þ, which is an electric-dipole-allowed transition and is hypersensitive to the environment. The decay curve for the luminescence of Eu3þ (5D0-7F2) in the Gd2O3 host lattice can be well fit to a single-exponential function, and the lifetime of Eu3þ is determined to be 1.341 ms (Figure 8b). The result is in excellent agreement with other Gd2O3:Eu3þ phosphors in previous reports.26a,26b,34 Figure 9 shows the upconversion (UC) luminescence spectra of various Ln3þ-doped Gd2O3 hollow microspheres under 980 nm light excitation. The Gd2O3:1% Er3þ, Gd2O3:5% Yb3þ/1% Er3þ, and Gd2O3:5% Yb3þ/1% Tm3þ samples exhibit bright green, red, and blue emissions, respectively, which evidently can be confirmed by the luminescence photographs under 980 nm light excitation (insets in Figure 9). Figure 9a shows the bright-green emission of the Gd2O3:1% Er3þ sample excited at 980 nm. Two primary bands in the green emission region maximized at 539 and 562 nm are assigned to the 2H11/2-4I15/2 and 4S3/2-4I15/2 transitions of Er3þ, respectively, and a weak band that appears at about 661 nm is attributed to the 4F9/2-4I15/2 transition of Er3þ. It is known that codoping not only increases the efficiency of luminescence Langmuir 2010, 26(7), 5122–5128

but also induces UC luminescence between the donor and acceptor ions.37 In our present work, Yb3þ was chosen as the codopant with Er3þ or Tm3þ because it possesses a large absorption cross section at 980 nm and energy transfer occurs as a result of the large spectral overlap between the Yb3þ emission 2F5/22 F7/2 and the Er3þ absorption 4I11/2-4I15/2 bands or the Tm3þ absorption 3H5-3H6 bands. The UC emission color changes greatly from green to red when Yb3þ was codoped with Er3þ into the Gd2O3 host lattice (insets in Figure 9a,b). Figure 9b shows the UC emission spectrum of the Gd2O3:5% Yb3þ/1% Er3þ sample, including mainly bright-red emission near 661 nm corresponding to the 4F9/2-4I15/2 transition of Er3þ, together with the very weak emissions near 539 and 562 nm assigned to the 2 H11/2-4I15/2 and 4S3/2-4I15/2 transitions of Er3þ, respectively. Under 980 nm light excitation, the Gd2O3:5% Yb3þ/1% Tm3þ sample shows strong blue luminescence (inset in Figure 9c). In the UC emission spectrum (Figure 9c), one can see the intense blue emission at 476 and 486 nm and a much weaker red emission at about 653 nm. The two groups of emission lines are assigned to the 1G4-3H6 and 1G4-3F4 transitions of Tm3þ, respectively. On the basis of the above results, it can be concluded that the UC emission colors of the uniform Gd2O3 hollow microspheres can be tuned from red to green to blue by doping different lanthanide ions.

4. Conclusions In summary, a general, facile method has been successfully developed to fabricate uniform, well-dispersed Gd2O3 hollow microspheres. The core-shell-structured precursors were first prepared via a urea-based homogeneous precipitation technique in the presence of monodisperse MF templates. Subsequently, the MF templates were removed and the amorphous shells were converted to crystalline Gd2O3 during the calcination process, resulting in the formation of the uniform oxide hollow microspheres. The morphology, crystal structure, elemental analysis, and luminescence properties of the as-obtained hollow microspheres were characterized by XRD, FTIR, EDX, TGA-DSC, SEM, TEM, PL, and kinetic decay, respectively. The as-obtained (37) (a) Das, G. K.; Tan, T. Y. J. Phys. Chem. C 2008, 112, 11211. (b) Matsuura, D. Appl. Phys. Lett. 2002, 81, 4526. (c) Yang, J.; Zhang, C. M.; Peng, C.; Li, C. X.; Wang, L. L.; Chai, R. T.; Lin, J. Chem.;Eur. J. 2009, 15, 4649.

DOI: 10.1021/la903584j

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Gd2O3:Eu3þ hollow microspheres show strong red emission under UV light excitation, and the Gd2O3:Er3þ, Gd2O3:Yb3þ/ Er3þ, and Gd2O3:Yb3þ/Tm3þ samples exhibit bright green, red, and blue colors under 980 nm light excitation, respectively. The Ln3þ-doped Gd2O3 hollow microspheres might find potential applications in the fields of drug delivery and biological labeling. Moreover, this general, facile synthesis strategy may be extended

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to the preparation of other inorganic functional materials with hollow spherical morphology. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (grant no. 20771098) and the National Basic Research Program of China (973 Program, grant nos. 2007CB935502 and 2006CB601103).

Langmuir 2010, 26(7), 5122–5128