LaAlO3 Hollow Spheres: Synthesis and Luminescence Properties

Aug 17, 2011 - The UV–vis absorption spectra show energy absorption at 211, 223, and 313 nm corresponding to the host lattice absorption and ...
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LaAlO3 Hollow Spheres: Synthesis and Luminescence Properties Biaohua Chen, Jianfei Yu, and Xin Liang* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China

bS Supporting Information ABSTRACT: Nearly monodisperse LaAlO3 hollow spheres are synthesized by a novel precursor thermal decomposition method. Spherical colloids of capsulelike precursors with uniform diameters of 273 ( 35 nm have been synthesized by a solvothermal method. These spherical colloids could convert to LaAlO3 hollow spheres with diameters of 166 ( 26 nm by a thermal decomposition process. The thermal transformation process from the precursors to LaAlO3 was characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD), and the Fourier transform infrared spectroscopy (FT-IR). By the doping of various lanthanide ions (Sm3+, Eu3+, and Tb3+), the emission luminescence of lanthanide-doped LaAlO3 hollow microspheres can be tuned from red to green. In particular, these luminescent LaAlO3 hollow spheres can be well dispersed in polar solvents such as the ethanol and water, which broadens the range of potential applications of these hollow spheres. The UV vis absorption spectra show energy absorption at 211, 223, and 313 nm corresponding to the host lattice absorption and charge-transfer transitions. The results are in good agreement with the peaks observed in the excitation spectra.

’ INTRODUCTION Hollow colloidal nanoparticles have recently attracted much attention because of their distinct low densities, large surface areas, and promising applications in multitudinous fields ranging from artificial cells and drug-delayed release to acoustic insulators, photonic crystals, and lightweight filler materials and chemical reactors.1 5 Luminescent hollow sphere colloids have great potential as labels and containers for biological labeling and drug release. Although much progress has been made in the fabrication of hollow spheres,6 8 it is still a challenge to seek a rational protocol for synthesizing hollow, spherical colloids. LaAlO3 is one kind of perovskite-type mixed oxide and possesses a rhombohedral crystal structure. LaAlO3 has intensive applications as substrates for superconductors, magnetic and ferromagnetic thin films, and luminescent host materials and has a high thermal stability and good dielectric character.9 11 Recently, some interesting work focused on the luminescent properties of lanthanide ion-doped LaAlO3 and proved the excellent properties of LaAlO3 as luminescent host materials.12 14 For example, singlehost full color emission has been realized in LaAlO3/Eu phosphor by the codoping of Eu2+and Eu3+. Thus, LaAlO3 hollow spheres with luminescence might serve as a multifunctional material and find great applications in the fields of biological labeling, drug delayed release, and white-light-emitting devices. Various synthesis approaches have been developed for the fabrication of LaAlO3, such as a solid-state method,15,16 coprecipitation,17,18 an aerosol furnace method,19 a sol gel technique,20,21 and a precursor decomposition approach.22 However, the size and shape control of LaAlO3 materials remains a large challenge. The formation of perovskite-type LaAlO3 usually requires relatively high temperatures. The particles easily aggregate with each other r 2011 American Chemical Society

at high temperature during the fabrication process. Irregular particles were obtained through most synthetic methods. To the best of our knowledge, LaAlO3 hollow spheres have not been prepared until now. Herein, a novel precursor thermal decomposition approach based on a soft-chemical process has been proposed for preparing nearly monodisperse LaAlO3 hollow microspheres. Spherical hybrid capsules were synthesized through a HNO3-assisted solvothermal method and used as precursors. The precursors favor the phase formation and morphology control of LaAlO3 hollow spheres and were converted to nearly uniform LaAlO3 hollow spheres through thermal treatment. LaAlO3 hollow spheres show strong luminescence emission under UV irradiation through controllable doping with lanthanide ions.

’ EXPERIMENTAL SECTION Preparation of LaAlO3 Hollow Spheres. . In the preparation procedure, 1 mL of 0.5 M La(NO3)3, 1 mL of 0.5 M Al(NO3)3, and 2 mL of a 5 M HNO3 solution were added to ethylene glycol. After being stirred for 10 min, the mixture was transferred to a 150 mL Teflon-lined vessel, which was sealed in an autoclave and then treated at 180 °C for about 10 h. As the autoclave cooled to room temperature naturally, the supernatant solution was poured out and the precursors could be directly collected from the bottom of the vessels. The precursor was washed with ethanol several times and dried in air at 100 °C. The final LaAlO3 hollow microspheres were obtained through heat treatment at Received: May 21, 2011 Revised: August 13, 2011 Published: August 17, 2011 11654

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Figure 1. (a) TEM image of spherical precursors. (b) HRTEM image of a single precursor sphere and elemental analysis maps of Al (blue) and La (red) over the sphere. (c) EDXA spectrum of the precursors. 800 °C for 2 h in air at a heating rate of 2 °C min 1. The obtained LaAlO3 was dispersed in ethanol to form colloids by ultrasound. Synthesis of Ln3+-Doped LaAlO3 Hollow Microspheres. Ln3+doped LaAlO3 hollow spheres were prepared by the same synthesis procedure for the LaAlO3 sample except that a stoichiometric amount of Ln(NO3)3 (Ln = Sm, Eu, and Tb) aqueous solutions was added to La(NO3)3 for the precursors in the initial stage as described above (0.99 mol of La(NO3)3 and 0.01 mol of Eu(NO3)3 for LaAlO3/1%Eu3+; 0.98 mol of La(NO3)3 and 0.02 mol of Sm(NO3)3 for LaAlO3/2% Sm 3+; and 0.98 mol of La(NO3)3 and 0.02 mol of Tb(NO3)3 for LaAlO3/2% Tb 3+). Characterization. The samples were characterized by powder X-ray diffraction (XRD) performed on a D8 Focus diffractometer (Bruker). Fourier transform infrared (FT-IR) 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 (SDT2960, 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). Transmission electron microscopy (TEM) images were obtained using a JEOL 2010 transmission electron microscope operating at 200 kV. Element analysis mapping and EDAX analysis were carried out on a JEOL JEM-2100 TEM. 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. UV vis spectra were obtained on a Hitachi U3100 UV vis spectrophotometer. Fluorescent spectra were recorded with a Hitachi F-4500 fluorescence spectrophotometer. All measurements were performed at room temperature.

’ RESULTS AND DISCUSSION Capsulelike precursors formed through a soft chemical process in which La3+ ions and Al3+ ions were homogeneously mixed in molecular levels. Transmission electron microscope (TEM) images of an as-prepared precursor sample are shown in Figure 1a. The precursors exhibit capsulelike properties with a narrow size distribution (273 ( 35 nm). No crystal fringes were observed by high-resolution TEM (HRTEM) analysis, indicating that the precursors are noncrystalline. Energy-dispersive X-ray analysis (EDXA) was adopted to determine the chemical

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Figure 2. XRD patterns of LaAlO3 precursors and the samples annealed at 600, 700, and 800 °C for 2 h.

Figure 3. TGA curve of LaAlO3 precursors recorded from room temperature to 800 °C in air.

composition of the precursors. The results (Figure 1c) confirmed the presence of La, Al, C, and O in the precursors. The quantitative analysis based on the EDXA profile (Figure S2) shows that the Al/La molar ratio in the precursor is nearly 1:1. Element analysis mapping technology was used to analyze the spatial distribution of the compositions in the precursors. The element analysis maps of La and Al in the precursor in Figure 1b show that La and Al atoms are distributed evenly in the capsulelike sphere, which favors the formation of LaAlO3 by following a thermal treatment. The capsules could convert to the pure phase of LaAlO3 by being annealed in air at the designated temperature. Figure 2 shows X-ray diffraction (XRD) patterns taken from the capsules and after the sample had been annealed at various temperatures for 2 h. The capsules were noncrystalline in structure with essentially no diffraction peaks. After being annealed at 600 °C, the product is still noncrystalline with no diffraction peaks observed in the XRD pattern. LaAlO3 with a pseudocubic phase (JCPDS 31-0022) formed when the annealing temperature was elevated to 700 °C. The crystallization temperature of LaAlO3 through the current precursor thermal decomposition process is lower than in the conventional solid-state method.15 The X-ray diffraction peaks become narrow and the strength of the peaks 11655

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Figure 4. FT-IR spectra taken from the LaAlO3 precursors and the samples annealed at various temperatures.

increase for the samples derived at 800 °C, revealing the good crystallinity and pure phase of as-obtained LaAlO3. Thermogravimetric analysis (TGA) and FT-IR were carried out to characterize and understand the composition and structural transformation of the precursors under thermal treatment. Figure 3 shows the TGA curve of the precursors recorded from room temperature to 800 °C in air. Three weight-loss steps were observed in the TGA curve. The first weight loss between 30 250 °C in the TGA curve is due to the desorption of physically absorbed water, glycol molecules, glycol oligomers, and some other organic molecules. The second sharp weight loss step between 300 to 450 °C corresponds to the decomposition and oxidation of chemically bonded organic species to carbonate species. The third weight loss step between 500 to 800 °C corresponds to the decomposition of the carbonate species. Figure 4 shows the FT-IR spectra of LaAlO3 precursors and the samples annealed at various temperatures. As shown in the FT-IR spectrum of the samples without annealing, typical bands (the intermolecular hydrogen-bonded O H stretching vibrations at 3000 3600 cm 1, the asymmetric and symmetric methylene stretching modes at ∼2921 cm 1, and the C O stretching vibrations at∼1100 cm 1) confirmed the presence of H2O, glycol, and other organic species contained in capsule precursors. These peaks almost disappeared for the samples annealed at 600 °C in air, and the peaks corresponding to carbonate species (the bands at ∼1496 and ∼1385 cm 1)23 appeared, indicating that the organic species were oxidized to carbonate species. The bands corresponding to carboxylate ions disappear as the annealing temperature increases to 800 °C. New bands corresponding to AlO6 octahedra in LaAlO3 formed at 556 and 658 cm 1, showing the formation of rhombohedral-phase LaAlO3.24 The morphology and microstructural details of as-prepared LaAlO3 nanoparticles were obtained from TEM and HRTEM observations. Typical TEM images with various magnifications of LaAlO3 obtained by annealing at 800 °C for 2 h are shown in Figure 5a c. The LaAlO3 nanoparticles are nearly monodisperse hollow spheres with an average size 166 ( 26 nm. Compared with the precursors, the hollow spheres condensed and the size decreased after being annealed. The hollow structure is well maintained under thermal treatment at 800 °C. It can be found that some hollow spheres in the TEM images contain a small solid core inside the shells. This interesting phenomenon shows that the precursors have a crucial influence on the morphology of as-prepared LaAlO3 materials. Because some of the capsulelike precursor particles have a core inside the shells, as-prepared

Figure 5. (a c) TEM images of LaAlO3 hollow spheres at various magnifications. (d) HRTEM image of a LaAlO3 hollow sphere, where the inset is the Fourier transformation pattern. (e) Schematic illustration of the overall formation process of LaAlO3 hollow microspheres.

LaAlO3 maintains its core/shell morphology during the annealing process as well. Clear and continuous crystal fringes were observed in the HRTEM image (Figure 5d). The interplanar lattice spacing is measured to be around 0.379 nm, corresponding to the (012) crystal plane of pseudo-cubic-phase LaAlO3. The FTT-transfer pattern of the HRTEM image exhibits a typical single-crystal diffraction pattern, confirming the good crystallinity of the hollow sphere. These results show the good crystallinity of LaAlO3 hollow spheres and confirm that noncrystalline capsules converted to crystalline LaAlO3 hollow spheres via thermal treatment, which is in good agreement with the XRD results. On the basis of experimental results and analysis, the whole process for the formation of uniform LaAlO3 hollow microspheres can be mainly divided into two steps (Figure 5e). First, the spherical precursors were obtained through a soft-chemical process. In the reaction process, glycol acted as both a solvent and ligand.25 A series of organic ligands, such as ethane diacid, ethanoic acid, and poly(ethylene glycol), form from the reaction of glycol and HNO3 under the solvothermal condition. Because lanthanum and aluminum are both oxyphilic elements, lanthanide and aluminum coordinate with these organic ligands to form hybrid precursors during the initial period. Because of the principle of minimum surface energy, the precursors take on a spherical morphology. In the second step, the noncrystalline precursors convert to the crystalline pseudocubic phase of LaAlO3 by annealing in air at a designated temperature. The organic ligands decomposed and combusted during the annealing process. The hollow morphology is well maintained during the calcination process. As we know, LaAlO3 is an excellent luminescent host matrix because of its thermal stability and lower phonon energy. 11656

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Figure 6. (Left) Excitation spectrum and (right) emission spectrum of (a) LaAlO3/Tb3+, (b) LaAlO3/Sm3+, and (c) LaAlO3/Eu3+ phosphors, respectively (λex = 254 nm), where the insets show corresponding photographs of LaAlO3/Ln (Ln = Tb, Sm, Eu) under a UV lamp, and (d) a digital photograph of the ethanol-dispersed colloids of LaAlO3 and LaAlO3/Ln3+ (Ln = Eu, Sm, Tb) hollow spheres.

To examine the feasibility of as-prepared hollow spheres as efficient and versatile host materials, a stoichiometric number of lanthanide ions (Sm3+, Eu3+, and Tb3+) were doped into the LaAlO3 host lattice to investigate the luminescence properties. These lanthanide ions can crystallize in the homogeneous phase of pseudocubic LaAlO3, and their ionic radii are similar, thus making it easy to dope LaAlO3 hollow spheres with these lanthanide ions. Lanthanide-doped samples were prepared by the same synthesis procedure, except for adding a certain proportion of Ln(NO3)3 to La(NO3)3 in the first stage (2% Tb(NO3)3, 2% Sm(NO3)3, and 1% Eu(NO3)3 in molar fractions). The morphology of LaAlO3 samples doped with lanthanide ions in small proportions has no obvious differences compared with that of the undoped LaAlO3 samples. Figure 6 shows the room-temperature excitation and emission fluorescence spectra of Ln3+-doped LaAlO3 (Ln = Sm, Eu, Tb) hollow spheres. LaAlO3/Tb3+, LaAlO3/Sm3+, and LaAlO3/Eu3+ hollow spheres exhibit strong green, orange, and red emission, respectively. As shown in Figure 6a right, the emission spectrum of LaAlO3/Tb3+ hollow spheres consists of three emission peaks located at 544, 589, and 625 nm, which could be assigned to the 5 D4 f 7F5, 5D4 f 7F4, and 5D4 f 7F3 transitions,26 respectively. A dominant green emission peak can be observed at 544 nm. The inset in Figure 6a shows a photograph of LaAlO3/Tb3+ powders emitting bright-green light excited by a UV lamp. The excitation spectrum shown in Figure 6a left was monitored at the strongest green emission (λ = 544 nm). The excitation spectrum of LaAlO3/Tb3+ mainly consists of two peaks at 210 and 230 nm, which could be assigned to the LaAlO3 host aborption and terbium oxygen charge-transfer band, respectively. Figure 6b shows the fluorescence spectra of LaAlO3/Sm3+ hollow spheres. The emission spectrum in Figure 6b right consists of a series of lines at 565, 598, and 645 nm due to 4G5/2 f 6H5/2, 6H7/2, and 6 H9/2 transitions,27 respectively. The orange fluorescence is visible under a UV lamp by the naked eye. The excitation spectra

of LaAlO3/Sm3+ monitored at 598 nm as shown in Figure 6b left consists of two peaks located at 211 and 244 nm, which could be assigned to the LaAlO3 host aborption and charge-transfer energy between O2 and Sm3+, respectively. The spectra of LaAlO3/Eu3+ hollow spheres is shown in Figure 6c. The emission spectrum (Figure 6c right) of LaAlO3/Eu3+ hollow spheres consists of lines mainly located in the red spectral area. A digital photograph of fluorescence from LaAlO3/Eu3+ excited by a UV lamp is shown in the inset of Figure 6c. Peaks at 579, 592, 618, and 693 nm can be assigned to the 5D0 f 7F0, 5D0 f 7F1, 5D0 f 7F2, and 5D0 f 7F4 transitions, respectively.28 The peak at 592 nm (5D0 f 7F1 transition) arises from the magnetic dipole transitions, and the peak at 618 nm (5D0 f 7F2) arises from the transition originating from electric dipole transitions. According to Judd Ofelt theory, the magnetic dipole transition is allowed but the electric dipole transition is forbidden unless the Eu3+ ions are located at a site without an inversion center. The relative strength of these two typical peaks of Eu3+ was sensitive to the local symmetry. For LaAlO3/Eu3+ hollow spheres, the strength of the peak located at 618 nm is stronger than that of the peak at 598 nm, indicating that most of the Eu3+ ions are located at sites without inversion symmetry. However, for LaAlO3/Eu3+, Eu3+ ions mainly replace La3+ ions in the host crystal structure, which adopts a high Oh symmetry. This result might be due to two reasons. First, Eu3+ ions may partially occupy the Al3+ sites because Eu and Al have similar electronegativities. Second, LaAlO3 shows strong covalency. The enhancement of the (5D0 f 7F2) transition might be caused by the covalent bonding and strong spin orbit coupling with f electrons.29 The excitation spectra (Figure 6c left) of LaAlO3/ Eu3+ monitored at the main red emission (λ = 618 nm) consist of two peaks located at 210 and 254 nm. The band at 210 nm originates from the host adsorption, and the peak at 254 nm arises from the europium oxygen charge-transfer band. It can be concluded that the excitation spectra of these three samples have 11657

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Figure 7. UV vis spectra of LaAlO3, LaAlO3/Sm3+, LaAlO3/Eu3+, and LaAlO3/Tb3+ colloids.

similar peak patterns, which consist of a host adsorption peak and a charge-transfer band. These hollow spheres can be easily dispersed in ethanol solvent to form colloids, with a characteristic ivory color that corresponds to the color of the respective bulk materials (Figure 6d). The good dispersion of LaAlO3 hollow spheres in water and ethanol may be caused by their unique hollow structures. The UV vis absorption spectra of ethanol-dispersed colloids of LaAlO3 and LaAlO3/Ln3+ (Ln = Eu, Sm, Tb) hollow spheres are displayed in Figure.7. For different lanthanide ion-doped LaAlO3 hollow spheres, the main bands in the absorption spectrum are similar. In the UV region from 200 to 400 nm, there are strong, broad absorption bands at ∼211 and ∼223 nm. The peaks at 211 nm can be assigned to the host lattice absorption, which is in agreement with the position of the excitation band observed at ∼210 nm in the excitation spectrum (Figure 6a c).30 The absorption at ∼223 nm can be attributed to charge transfer from the oxygen ligands to the central aluminum atom inside the AlO33 ion.31 The small, narrow peak at ∼313 nm could be assigned to the charge-transfer transition from oxygen to lanthanide ions, which agrees well with the reported charge-transfer energy of 4.0 eV.32 In the visible region from 400 to 700 nm, the absorbance of the samples is relative low, indicating the good luminosity in the visible region. By doping with other lanthanide ions, the energy absorption edge of doped LaAlO3 hollow spheres is slightly red shifted.

’ CONCLUSIONS A facile method has been successfully developed to fabricate well-dispersed LaAlO3 hollow microspheres with photoluminescence performance. The capsulelike precursors were converted to crystalline LaAlO3 during the calcination process, resulting in the formation of uniform hollow microspheres. The Ln3+-doped LaAlO3 hollow spheres might find potential applications in the fields of drug delivery and biological labeling. Moreover, we believe that this novel precursor decomposition method could be expanded to the synthesis of other hollow nanostructures, especially the perovskite-type ABO3 categories, and bring about new opportunities in these quickly expanding research fields.

’ ASSOCIATED CONTENT

bS

Supporting Information. HRTEM image of a single LaAlO3 hollow sphere and SAED pattern of the whole LaAlO3 hollow sphere. SEM image of the precursors. TEM and SEM images of LaAlO3 hollow spheres. Quantitative elemental analysis of the precursor calculated from the EDXA profile. This material is available free of charge via the Internet at http://pubs. acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was financially supported by the NSFC (grant nos. 21001015 and 20821004) and the RFDP (grant no. 20100010120003) ’ REFERENCES (1) Caruso, F.; Caruso, R. A.; Mohwald, H. Science 1998, 282, 1111. (2) Jiang, P.; Bertone, J. F.; Colvin, V. L. Science 2001, 291, 453. (3) (a) Zeng, H. C. J. Mater. Chem. 2006, 16, 649. (b) Zeng, H. C. Curr. Nanosci. 2007, 3, 177. (4) Zhang, D. B.; Qi, L. M.; Ma, J. M.; Cheng, H. M. Adv. Mater. 2002, 14, 1499. (5) Wang, W. Z.; Poudel, B.; Wang, D. Z.; Ren, Z. F. Adv. Mater. 2005, 17, 2110. (6) Yin, Y. D.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Science 2004, 304, 711. (7) Liang, X.; Wang, X.; Zhuang, Y.; Xu, B; Kuang, S. M.; Li, Y. D. J. Am. Chem. Soc. 2008, 130, 2736. (8) Sun, X. M.; Li, Y. D. Angew. Chem., Int. Ed. 2004, 43, 3827. (9) Kharton, V. V.; Marques, F. M. B.; Atkinson, A. Solid State Ionics 2004, 174, 135. (10) Ohtomo, A.; Hwang, H. Y. Nature 2004, 427, 423. (11) Simon, R. W.; Platt, C. E.; Lee, A. E.; Lee, G. S.; Daly, K. P.; Wire, M. S.; Luine, J. A.; Urbanik, M. Appl. Phys. Lett. 1988, 54, 2677. (12) Liu, X. M.; Yan, L. S.; Lin, J. J. Phys. Chem. C 2009, 113, 8478. 11658

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