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Preparation and Up-Conversion Luminescence of Hollow La2O3:Ln (Ln = Yb/Er, Yb/Ho) Microspheres Fei He,† Piaoping Yang,*,† Dong Wang,† Chunxia Li,‡ Na Niu,† Shili Gai,† and Milin Zhang† †
Key Laboratory of Superlight Materials and Surface Technology, Ministry of Education, Harbin Engineering University, Harbin 150001, P. R. China ‡ State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China ABSTRACT: Hollow La2O3:Ln (Ln = Yb/Er, Yb/Ho) microspheres with upconversion (UC) luminescence properties were successfully synthesized via a facile sacrificial template method by employing carbon spheres as hard templates followed by a subsequent heating process. The structure, morphology, formation process, and fluorescent properties are well investigated by various techniques. The results indicate that the hollow La2O3:Ln microspheres can be well indexed to the hexagonal La2O3 phase. The hollow La2O3:Ln microspheres with uniform diameter of about 270 nm maintain the spherical morphology and good dispersion of the carbon spheres template. The shell of the hollow microspheres consists of numerous nanocrystals with the thickness of approximately 40 nm. Moreover, the possible formation mechanism of evolution from the carbon spheres to the amorphous precursor and to the final hollow La2O3:Ln microspheres has also been proposed. The Yb/Er and Yb/Ho codoped La2O3 hollow spheres exhibit bright up-conversion luminescence with different colors derived from different activators under the 980 nm NIR laser excitation. Furthermore, the doping concentration of the Yb3þ is optimized under fixed concentration of Er3þ/Ho3þ. This material may find potential applications in drug delivery, hydrogen and Li ion storage, and luminescent displays based on the uniform hollow structure, dimension, and UC luminescence properties.
1. INTRODUCTION During the past two decades, the construction of templates to synthesize advanced materials has become the focus in scientific and technological fields.14 Moreover, due to the potential applications in drug storage/release, confined-space catalysis, sensors, active-material encapsulation, photonic crystals, thermal and electrical insulators, the fabrication of functional materials with hollow interiors has attracted special interest.510 Hollow microand nanospheres of hybrid materials have been fabricated through numerous routes by using polymer or silica beads,11,12 gold nanoparticles,13 emulsion drops,14 and multilayers vesicles15 as the interior templates. However, in some cases, the yields of hollow products obtained by these template-assisted approaches are usually low. In addition, some removable expensive polymer bead or Au nanoparticles are too uneconomic to be applied for further applications. Moreover, some organic templates and the etching agents (acid or base) used to remove the template often bring the environmental questions. Therefore, a facile, economic and green method to prepare hollow spheres with defined shape, size, and multiple properties (such as luminescent, magnetic and electrical properties) should be highly promising. Recently, many kinds of methods have been developed for the production of high-quality carbon microspheres due to the emerging need in large quantities.1619 Among these methods, the hydrothermal approach using aqueous glucose solution as raw material can synthesize uniform colloidal carbon spheres r 2011 American Chemical Society
with functional groups and reactive surface in a broad size range, which make it possible to integrate differently sized nanoparticles. Thus, carbon microspheres can be used as the templates for fabricating coreshell structures or hollow materials.2022 Currently, rare-earth (RE) doped up-conversion materials have gained considerable attention due to their unique electronic and optical properties resulting from their 4f electrons.2328 In particular, due to the distinct photophysical properties (such as low effective densities, low phonon energy, transparent to visual light) and structural characteristics (such as hollow interior and high specific surface area), the hollow RE doped up-conversion spheres should be applied in drug and enzymes delivery, lightweight fillers, pigments, adsorption materials, specific phosphor powder, confined-space catalysis, and biomolecule release.2935 Moreover, RE ions doped La2O3 phosphors should have great application potentiality because of their preeminent stability, long luminescence lifetime, high excitation efficiency and low toxicity.3638 Singh et al. synthesized La2O3:Yb/Er phosphors through the solution combustion route by using urea as a reducing agent and discussed the effect of the annealing temperature on the fluorescence intensity.39 However, due to the defect of the morphology, the extensive application of this phosphor Received: February 7, 2011 Revised: March 18, 2011 Published: April 11, 2011 5616
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Langmuir should be limited. Park et al. investigated the concentration quenching of the Eu3þ emission on La2O3 phosphors by a simple solgel method.40 Rosa et al. doped Eu3þ ions in La2O3 host coating on the nanometric Al2O3 by a soft chemical method.41 Kong et al. prepared Yb3þ/Er3þ codoped into Y2O3, La2O3, and Gd2O3 via a combustion method and discussed the effect of precursor concentration on the luminescence property.42 Liu and his co-workers studied the Er3þ and Yb3þ codoped La2O3 nanocrystals.43,44 However, the preparation of the up-conversion La2O3 phosphors with well-defined hollow structure has rarely been reported. Herein, hollow Yb3þ/Er3þ and Yb3þ/Ho3þ codoped La2O3 microspheres with up-conversion emission property have been fabricated through a facile hydrothermal method employing colloid carbon spheres as the precursor template and urea as the assisted precipitation, followed by an annealing process. The formation process and the physicochemical properties of the hollow microspheres have been well characterized by various analysis techniques. We also investigated the UC luminescent properties of hollow La2O3:Ln (Ln = Yb/Er, Yb/Ho) spheres by varying Yb3þ doping concentrations. As the preparation method is economical, environmentally friendly, and high-yield mass production, this route may open up new possibilities to synthesize hollow spheres of other oxides and extend their applications.
2. EXPERIMENTAL SECTION 2.1. Materials. All materials were used as received including analytical grade glucose, urea, and dilute HNO3 (Beijing Chemical Corporation, A. R.) without further purification. La(NO3)3, Yb(NO3)3, Er(NO3)3, and Ho(NO3)3 aqueous solution were prepared by dissolving La2O3, Yb2O3, Er2O3, and Ho2O3 (Science and Technology Parent Company of Changchun Institute of applied Chemistry, 99.99%) in dilute HNO3 solution under heating and agitation. The superfluous HNO3 was driven off by heating until the pH value of the solution reached between 2 and 3. 2.2. Preparation of Carbon Microspheres. The carbon microspheres templates were synthesized through the poly condensation reaction of glucose under hydrothermal condition. In a typical procedure, 8 g of glucose was dissolved in 40 mL of deionized water to form a clear solution. The solution was then sealed in a 50 mL Teflon-lined stainless autoclave and maintained at 170 °C for 9 h. After the autoclave was naturally cooled to room temperature, the obtained black-brown precipitates were washed with deionized water six times then dried at 80 °C for 6 h in air.
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Figure 1. XRD patterns of the carbon microspheres (a), uncalcined precursor (b), hollow La2O3:Yb/Er (c), and La2O3:Yb/Ho microspheres (d). assisted precipitation followed by calcination process except that no carbon microspheres were added. Moreover, bulk La2O3:Yb/Er and La2O3:Yb/Ho phosphors were prepared by heating the corresponding rare earth nitrates at 700 °C in air for 2 h. 2.4. Characterization. X-ray diffraction (XRD) was examined on a Rigaku-Dmax 2500 diffractometer using Cu KR radiation (λ = 0.15405 nm). The morphologies and composition of the as-prepared samples were inspected on a field emission electron microscope (FESEM, S4800, Hitachi) equipped with an energy-dispersive X-ray spectrum (EDS, JEOL JXA-840). Transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM) were performed on a FEI Tecnai G2 S-Twin transmission electron microscope with a field emission gun operating at 200 kV to elucidate the dimensions and the structural details of the particles. Fourier transform IR (FT-IR) spectra were measured on a PerkinElmer 580B IR spectrophotometer using KBr pellet technique. Thermogravimetry (TG) and differential thermal analysis (DTA) were carried out on a Netzsch STA 409 thermoanalyzer with a heating rate of 5 °C/min. The UC emission spectra were obtained using a 980 nm laser from an OPO (optical parametric oscillator, Continuum Surelite, USA) as the excitation source and detected by R955 (HAMAMATSU) from 400 to 900 nm. All of the measurements were performed at room temperature.
2.3. Synthesis of Hollow La2O3:Ln (Ln = Yb/Er, Yb/Ho) Microspheres. In a typical process for the synthesis of La2O3: 3 mol %
3. RESULTS AND DISCUSSION
Yb/1 mol % Er with doping concentration of 3 mol % Yb and 1 mol % Er, 3.2 mL La(NO3)3 (0.3 M), 1.5 mL Yb(NO3)3 (0.02 M), 0.5 mL Er(NO3)3 (0.02 M), and some deionized water are mixed together to form a 30 mL solution. Then 3.0 g of urea was dissolved in the solution under vigorous stirring to form a clear solution. A total of 100 mg of asprepared carbon microspheres was then added into above solution under ultrasonication for 15 min. Finally, the mixture was transferred into a 100 mL flask and heated at 90 °C for 4 h with vigorous stirring before the product was collected by centrifugation. The precursors were washed by deionized water and alcohol for three times respectively and then dried at 60 °C in air. The final products were obtained through a heat treatment at 700 °C in air for 2 h with a heating rate of 2 °C min1. Hollow La2O3:Yb/Ho microspheres were prepared via the similar process, except that Er3þ ions have been replaced by Ho3þ ions. For comparison, nanosolid La2O3:Yb/Er and La2O3:Yb/Ho particles with the same doping compositions were obtained by a similar urea-
3.1. Phase, Structure, and Morphology. Figure 1 shows the XRD patterns of the as-prepared carbon microspheres, uncalcined precursor, hollow La2O3:Yb/Er and La2O3:Yb/Ho microspheres, respectively. In Figure 1a for the carbon microspheres, a broad peak at about 2θ = 22° suggests the amorphous carbon nature. As for the uncalcined precursor (Figure 1b), the main diffractions at 2θ = 20.56°, 23.81°, 29.99°, 38.10°, 43.36°, and 44.78° can be directly indexed to orthorhombic La(OH)CO3 phase (JCPDS No. 49-0981). Besides, an obvious peak at 2θ = 22° assigned to the amorphous carbon microspheres suggests that the sample is composed of La(OH)CO3 and carbon spheres. From panels c and d of Figure 1, all of the diffraction peaks of hollow La2O3:Yb/Er and La2O3:Yb/Ho microspheres can be readily indexed to the hexagonal La2O3 phase in P-3m space group (JCPDS No. 05-0602), and no diffractions from carbon 5617
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Table 1. Unit Cell Lattice Constants and Crystallite Sizes for Hexagonal Hollow La2O3:Yb/Er and La2O3:Yb/Ho Microspheres samples
a,b/deviation
c/deviation
crystallite size
(Å)
(Å)
(nm)
JCPDS No. 05-0602
3.937
6.1299
La2O3:Yb/Er
3.941/0.004
6.1133/0.0166
24.7
La2O3:Yb/Ho
3.923/0.014
6.1150/0.0149
23.5
Figure 3. FE-SEM images of hollow La2O3:Yb/Er microspheres with low magnification (inset is corresponding particle size distribution histograms counted from this FE-SEM images) (a), high magnification (b), TEM image (c), EDS (d), SAED (e), and HRTEM image (g) of hollow La2O3:Yb/Er microspheres. Figure 2. FE-SEM image (inset is corresponding Particle size distribution histograms counted from this FE-SEM images) (a) and TEM image (b) of as-prepared carbon spheres; TEM image (c), EDS (d), and HRTEM image (e) of uncalcined precursor.
spheres and other phases coupled with the doped component can be detected, indicating that the two samples are of high purity. Thus, it can be concluded that the calcination process has a dual function: the formation of hollow crystalline structures from the La(OH)CO3 precursor layer and elimination of carbon spheres cores. Table 1 gives the unit cell lattice constants and the crystallite sizes of the two samples, the standard data of hexagonal La2O3 (JCPDS No. 05-0602) are listed for comparison. It can be seen that the calculated unit cell lattice constants for the hollow La2O3:Yb/Er and La2O3:Yb/Ho microspheres are well consistent with the standard data. Furthermore, the diffraction peaks of the two samples show some broadening, which indicates the fine nature. The average crystal sizes can be calculated from the Scherrer formula: Dhkl = Kλ/(β cos θ), where θ and β is the diffraction angle and full-width at half-maximum (fwhm), respectively. K is a constant (0.89), and Dhkl means the size along the (hkl) direction. The strongest three peaks (101) at 2θ = 29.960°, (110) at 2θ = 46.084°, and (103) at 2θ = 52.132° are used to calculate the average crystal size (D) of the samples. The estimated crystallite sizes of La2O3:Yb/Er and La2O3:Yb/Ho are calculated to be 24.7 and 23.5 nm, respectively. The typical FE-SEM and TEM images of as-prepared carbon spheres, TEM image, EDS and HRTEM of uncalcined precursor are shown in Figure 2. In Figure 2a for carbon spheres, the sample consists of well-dispersed microspheres with average size of
250 nm. From the size distribution histograms of the as-prepared carbon microspheres (inset in Figure 2a), which were counted from the FE-SEM image from Figure 2a, we can see that the carbon microspheres are uniform in diameter with a relatively narrow distribution. The diameter of carbon microspheres can be tuned by altering the reaction conditions such as reaction temperature, reaction time and concentration of starting materials. Therefore, the diameter of final hollow spheres should also be regulated by employing carbon microspheres as templates. The TEM image (Figure 2b) shows that the carbon microspheres prepared from glucose exhibit a uniform size distribution and an apparent coreshell structure, comprised of a relatively dense hydrophobic core and a hydrophilic shell (inset in Figure 2b). The uncalcined precursor still shows a coreshell structure with a rougher surface and a larger particle size (400450 nm) than the carbon microspheres, as shown in Figure 2c. Due to the urea coprecipitation step, the incremental part at exterior can be attributed to the coated La(OH)CO3 layer, which can be confirmed by the XRD results and the following FT-IR results. The EDS analysis (Figure 2d) confirms the presence of carbon (C), oxygen (O), and lanthanum (La) in the uncalcined precursor (Si from the Si substrate). The corresponding HRTEM image in Figure 2e reveals the presence of clear crystal lattices with various directions, which suggests the polycrystalline nature of the sample. The morphology, microstructure and the elemental composition of the hollow La2O3:Yb/Er microspheres are displayed in Figure 3. From the SEM images (Figure 3a,b), we can see that the sample consists of relatively uniform microspheres with an mean diameter of about 270 nm, suggesting the shape and size of the final hollow phosphors are related with the carbon spheres templates. 5618
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Figure 4. TG curves of as-prepared carbon spheres (a) and uncalcined precursor (b); insets are the corresponding DTA curves.
Furthermore, from the size distribution histogram of the hollow La2O3:Yb/Er microspheres (inset in Figure 3a counted from this image), a relatively size distribution of the as-obtained hollow microspheres can also be obtained. The shrinkage of the particle size should be due to dehydration of the cross-linked structure on the carbon spheres and the further densification of the loose La(OH)CO3 precursor layer converting into closely compact oxides. Additionally, a hollow spherical structure can be apparently observed. The magnified SEM image (Figure 3b) exhibits the detailed morphology of the sample. Some broken hollow spheres (inset in Figure 3b) can be ascribed to the carbon dioxide escape from the shell during the calcination process. The thickness of the shells for the hollow spheres is estimated to be about 40 nm. From the TEM image (Figure 3c), the intense contrast between the black margins and the bright centers of the spheres confirms the existence of hollow structures, which is consistent with the SEM result. The EDS analysis in Figure 3d confirms the presence of the oxygen, lanthanum. It should be noted that the very weak carbon signal suggests that the carbon core is nearly burned off by the heat treatment. There is no signal of the Yb, Er, or Ho in the EDS pattern because the doping concentration of these ions is so low that signals of these ions cannot be detected. In the selected-area electron diffraction (SAED) pattern (Figure 3e), the strong concentric ring patterns can be indexed to the (110), (102), (100), (002), and (101) planes of hexagonal La2O3 phase, respectively. The obvious lattice fringes in the HRTEM image (Figure 3f) confirm the high crystallinity, which is in agreement with the XRD results. The distances of 0.30 nm between the adjacent lattice fringes agree well with the hexagonal La2O3 phase (JCPDS No. 05-0602). TG-DTA curves of the as-prepared carbon spheres and uncalcined precursor are shown in Figure 4 (the insets are the corresponding DTA), respectively. Two steps of weigh loss can be discovered in TG-DTA curves of carbon spheres (Figure 4a). The first slow weight loss can be associated with the dehydration and densification process of carbon spheres. The second sharp weight loss may be attributed to the splitting burning of carbon spheres. For the coreshell structured La(OH)CO3 precursor (Figure 4b), the weight loss behavior is similar to that of carbon spheres before 520 °C. Beyond 520 °C, almost no weight loss can be observed for carbon spheres while the mass of the La(OH)CO3
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Figure 5. FT-IR of as-prepared carbon spheres (a), uncalcined precursor (b), and hollow La2O3:Yb/Er microspheres (c).
Figure 6. NIR-to-visible UC emission spectra of hollow La2O3:Yb/Er (a) and La2O3:Yb/Ho (b) spheres upon 980 nm excitation. 5619
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Figure 7. Proposed energy transfer mechanisms under 980 nm diode laser excitation in hollow La2O3:Yb/Er and La2O3:Yb/Ho microspheres, respectively.
precursor still slowly decreases until 700 °C, which may be caused by the decomposition of the precursor to crystalline La2O3. In the inset of Figure 4b, different from the second exothermic peak (524 °C) of carbon spheres, the exothermic peak of the uncalcined precursors shifts to lower temperature (462 °C). It is worth noting that the weight loss of pure carbon template is nearly 100%, while the residual weight percentage of the assynthesized La(OH)CO3 precursor is 46.6%, suggesting the considerable high yield of the hollow phosphors prepared by this method. The functional groups related with the surface information of the carbon spheres, the uncalcined precursors, and hollow La2O3:Ln microspheres were examined by FT-IR, as shown in Figure 5. In Figure 5a for carbon spheres, the peaks corresponding to hydroxyl (3425 cm1), carbonyl (1705 cm1), unsaturated CdC (1621, 1513 cm1), COH (1216 cm1), and glycosidic COC linkage (1028 cm1) are obvious. The glycosidic linkage absorption confirms the polymerization reaction of glucose, and unsaturated CdC groups indicate that a carbonization process has occurred. The hydroxyl groups improve the stability of the carbon spheres in aqueous solution, which can be confirmed by the SEM results. In Figure 5b for the uncalcined precursor, the respective IR band at 3425, 1422, 1497, 1076, 859, 725, and 696 cm1 can be assigned to OH (υ), CO (υas), CO (υas), CO (υs), CO (δ), OH (δ), and CO (δ) (υ = stretch; υs = symmetric stretch; υas = asymmetric stretch; δ = deformation), indicating that the composition of the precursor should be La(OH)CO3. In the FTIR spectrum of hollow La2O3:Ln microspheres (Figure 5c), the band at about 560 cm1 can be assigned to the LaO stretching adsorption of the La2O3, which also confirms the formation of crystalline hollow La2O3:Ln microspheres. 3.2. Luminescent Properties. The up-conversion emission spectra of hollow La2O3:Yb/Er and La2O3:Yb/Ho microspheres have been recorded in the 400700 nm region, as shown in Figure 6a and Figure 6b, respectively. In Figure 6a for hollow La2O3:Yb/Er microspheres, the three emission bands at around 528 nm, 548 and 667 nm can be assigned to the thermalized
Figure 8. Emission spectra of La2O3: Yb/Er (a) and La2O3: Yb/Ho (b): hollow La2O3:Ln microspheres (green line), nanosolid La2O3:Ln particles (red line), and bulk La2O3:Ln phosphors (black line). 2
H11/2 f 4I15/2, 4S3/2 f 4I15/2, and 4F9/2 f 4I15/2 transitions of Er3þ, respectively.39,42 It is notable that the emission intensity of the sample show fairly large Stark splitting due to large crystal field of La2O3 matrix in bulk samples.45 For hollow La2O3:Yb/Ho microspheres in Figure 6b, a predominant emission bands at 548 nm and a weak band at 661 nm can also be found, which can be ascribed to the 5F4/5S2 f 5I8 and 5F5 f 5I8 transitions of Ho3þ, respectively.46 Obviously, the emission intensity of La2O3: Yb/Ho also show obvious Stark splitting similar to La2O3:Yb/Er, which also indicate that both samples are of high crystallization after the calcination process. Figure 7 presents the possible energy transfer mechanism of hollow La2O3:Yb/Er and La2O3: Yb/Ho microsphere. When excited at 980 nm, the Yb3þ absorbs the photon and promotes to its excited 2F5/2 state. Then the energy transfer (energy transfer UC process) occurs from the Yb3þ ion 2F5/2 state to the nearby lying Er3þ ion 4I13/2 and Ho3þ ion 5I6 state. Subsequently, some of the exited ions relax to the low-lying level of Er3þ4I13/2 and Ho3þ5I7 states. All of above excited states can absorb the energy from an incident photon or a second energy transfer from a neighboring Yb3þ ion, leading to the further population of the 4F7/2, 4F9/2, and 5F5 of these ions (excited state UC process). Meanwhile, these population states 5620
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Figure 9. UC emission spectra (left) and CIE chromaticity diagram (right) of hollow La2O3:Yb/Er spheres with doping concentrations of 1% Yb3þ/1% Er3þ (a), 2% Yb3þ/1% Er3þ (b), 3% Yb3þ/1% Er3þ (c), and 4% Yb3þ/1% Er3þ (d).
Figure 10. UC emission spectra (left) and CIE chromaticity diagram (right) of La2O3:Yb/Ho with doping concentrations of 1% Yb3þ/1% Ho3þ (a), 2% Yb3þ/1% Ho3þ (b), 3% Yb3þ/1% Ho3þ (c), and 4% Yb3þ/1% Ho3þ (d).
may also occur through a nonradiative decay to the low-lying 2 H11/2, 4S3/2, of the Er3þ and 5I4 of the Ho3þ. Therefore, the emissions from these populated states to the ground states take place. The emission spectra of hollow La2O3:Ln spheres, nanosolid La2O3:Ln particles, and the bulk La2O3:Ln phosphors are shown in Figure 8, respectively. In Figure 8a for La2O3:Yb/Er, the emission spectrum of hollow La2O3:Yb/Er microspheres is very similar to those of nanosolid La2O3:Yb/Er particles and bulk La2O3:Yb/Er phosphors due to their same f-f transitions which are strongly shielded by the outside 5s and 5p electrons. The spectrum intensity of the hollow La2O3:Yb/Er microspheres is slightly higher than those of the bulk La2O3:Yb/Er phosphors and the nanosolid La2O3:Yb/Er particles at around 528548 nm while lower at around 667 nm, which is consistent with the previous report.47 As for La2O3:Yb/Ho (Figure 8b), the emission spectrum of hollow La2O3:Yb/Ho spheres is also much similar to
those of corresponding nanosolid particles and bulk phosphors, except for a slight decrease of the intensity. In order to further investigate the UC emission properties of hollow La2O3:Yb/Er and La2O3:Yb/Ho microspheres, hollow samples with different Yb3þ doping concentrations have been prepared by using the same template under same conditions. Figure 9 (left) shows the UC emission spectra of La2O3:Yb/Er with four different Yb3þ concentrations of 1% Yb3þ/1% Er3þ, 2% Yb3þ/1% Er3þ, 3% Yb3þ/1% Er3þ, and 4% Yb3þ/1% Er3þ, respectively. It can be seen that when the doping concentration of Er3þ is fixed at 1%, the emission intensity increases with the increased Yb3þ concentration from 1% to 3%, and decreases with further raise of the Yb3þ concentration, indicating that the hollow La2O3:3% Yb3þ/1% Er3þ microspheres have the highest emission intensity. A possible interpretation is proposed as follows. When the Yb3þ concentration increases from 1% to 3%, more Yb ions become available to furnish and transfer the energy to the Er3þ, 5621
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Langmuir Scheme 1. Schematic Illustration for the Formation of Carbon Spheres, Core-Shell Structured Precursor, and Final Hollow La2O3:Ln Microspheres
resulting in the higher emission intensity. When the Yb3þ concentration is further increased, the emission intensity decreases gradually, which may be ascribed to the cross-relaxation process for superfluous Yb ions. With the additional increase of the Yb3þ concentration, the distance between neighboring Yb ions becomes shorter, which can enhance the interaction of the neighboring Yb ions and intensify the cross-relaxation process of Er ions, thus resulting in the concentration-dependent quenching. In the emission spectra for hollow La2O3:Yb/Ho microspheres with different Yb ions concentration (Figure 10 (left)), similar results can be observed, 3% Yb3þ/1% Ho3þ codoped La2O3:Yb/Ho microspheres have the optimal emission intensity and the results can be a witness to the explanation presented previously. The respective CIE (Commission Internationale de I’Eclairage 1931 chromaticity) coordinates for the UC emission spectra of La2O3: Yb/Er and La2O3:Yb/Ho products with different Yb3þ doping concentrations are given in the Figure 9 (right) and Figure 10 (right), respectively. In both diagrams, the different CIE chromaticity values are due to different green to red intensity ratios for the products. Furthermore, there is a simple regularity to each phosphor, the CIE chromaticity values of different doping level samples are arranged approximately in line, which would be a
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date base for the further study in the color parameters of this kind of phosphors. 3.3. Formation Process. According to the above XRD, FESEM, TEM, TG/DTA, and FTIR analysis, the formation mechanism of hollow La2O3:Ln (Ln = Yb/Er, Yb/Ho) microspheres depends on a series of chemical and structural transformations, as shown in Scheme 1. In the initial stage, uniform carbon spheres templates were prepared by the polymerization of D-glucose molecules and further dehydration of cross-linked polymer by high temperature. Then, the coreshell structured precursors were fabricated by the homogeneous precipitation of rare earth ions on the surface of spherical carbon templates using urea as precipitating agent. In this stage, the urea dissolved in water decomposes into CO2 and OH, coupled with a large number of OH bonds on the surface of the carbon spheres which is confirmed by FT-IR and the easy dispersion of the carbon spheres in water, so the La3þ, Yb3þ, and Er3þ/Ho3þ are easy to adsorb on the surface of the carbon spheres to form into amorphous La(OH)CO3 nucleus and further grow into La(OH)CO3 nanoparticles by the reaction of La3þ þ 3OH þ CO2 f La(OH)CO3 þ H2O, which is confirmed by the XRD results and coreshell structure shown in the SEM and TEM images. Finally, the precursors were calcined in the muffle furnace in the air where the reaction of 2La(OH)CO3 f La2O3 þ H2O þ 2CO2 and C þ O2 f CO2 take place, which is confirmed by the TG-DTA results.
4. CONCLUSIONS Hollow La2O3:Ln (Ln = Yb/Er, Yb/Ho) microspheres with a narrow distribution and average diameter of about 270 nm are successfully prepared via a sacrificial template method by employing carbon spheres as hard template. The hydrothermal treatment and the presence of urea established the effective and homogeneous precipitation of La3þ and the codoped rare-earth irons on the surface of carbon microspheres. The subsequent calcination ensure the highly crystallization of the La2O3 hollow spheres and the remove of carbon templates. Furthermore, no organic template and corrosive leaching agents were involved in the whole fabrication process, suggesting that the approach is facile and completely green. The current method is suitable for inexpensive, mass production of hollow La2O3:Ln phosphors. The as-prepared hollow phosphors demonstrate strong emissions under the 980 nm NIR laser excitation. In particular, due to their uniform hollow structure, dimension and UC luminescence properties, the as-prepared hollow La2O3:Ln microspheres should have the potential applications in drug delivery, hydrogen and Li ion storage, display fields. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This project is financially supported by the National Natural Science Foundation of China (NSFC 20871035), China Postdoctoral Special Science Foundation (200808281), and Harbin Sci.-Tech. Innovation Foundation (No. 2009RFQXG045). 5622
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