CRYSTAL GROWTH & DESIGN
Synthesis of Mesoporous Eu2O3 Microspindles
2007 VOL. 7, NO. 12 2670–2674
Shufen Wang,†,‡ Feng Gu,‡ Chunzhong Li,*,‡ and Mengkai Lü*,† State Key Laboratory of Crystal Materials, Shandong UniVersity, Jinan 250100, China; Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China UniVersity of Science and Technology, Shanghai 200237, China ReceiVed January 30, 2007; Accepted September 24, 2007; ReVised Manuscript ReceiVed July 23, 2007
ABSTRACT: Mesoporous Eu2O3 microspindles have been prepared by a facile solution process followed by subsequent heat treatment. By adding urea stepwise and varying the reaction time, the dimension of the Eu2O3 microspindles can be easily tuned from 250 × 100 nm to 900 × 400 nm. The products were characterized by X-ray diffraction, small-angle X-ray scattering, (highresolution) transmission electron microscopy, scanning electron microscopy, N2 adsorption, and photoluminescence spectroscopy. The Eu2O3 samples exhibit a relatively broad pore-size distribution, and the wall of the pore is constructed by well-crystalline Eu2O3 nanocrystals with diameters of about 15 nm. A possible formation mechanism of the mesoporous microspindles was also discussed. Introduction Mesoporous materials, obtained by the favorable selfassembly between organic templates and inorganic precursors, have opened many new possibilities for applications in catalysts, separation, and nanoscience because of their special structural characteristic and easy functionlization.1 Since the mesoporous silica was reported,2 many researchers have devoted to preparing mesoporous-structured oxides via surfactant or polymer templated and nanoreplicated pathways, including transition metal oxides such as ZrO2, TiO2, and SnO2 and other oxides such as Nd2O5 and V2O5.3–7 Recently, synthesis of mesoporous products with controlled shapes became a new challenge because the combination of the shape-specific and the mesoporous natures may lead to collective functions. So far, many systems have been reported, such as silica, metal oxides, phosphate mesoporous spheres, and γ-Al2O3 mesostructures.8–10 Rare earth compounds have been extensively utilized as highperformance magnets, luminescent devices, catalysts, and other functional materials based on the electronic, optical, and chemical characteristics arising from their 4f electrons.11 Most of these useful functions depend strongly on their composition and structures. Recently, many studies have been devoted to the synthesis of rare earth compounds with different morphologies, including Eu2O3 nanoparticles, nanotubes, and nanorods, Dy(OH)3 and Dy2O3 nanotubes, and Y2O3:Eu nanowires, nanotubes, and CeO2 nanorods.12–18 If rare earth oxides adopt the mesoporous form, they would find promising applications not only as adsorbing or separating agents but also as a host for the homogeneous dispersion of optically, electrically, or magnetically functional species. Although, there have been few reports concerning the synthesis of mesoporous rare earth oxides, especially with controlled shapes,19 studies on this kind of materials have attracted much attention of the researcher’s. As far as we referenced, Yada et al. reported for the first time the synthesis of mesoporous rare earth oxides with irregular shapes.20 They also reported the synthesis of rare earth (Er, Tm, Yb, Lu) oxides nanotubes by the assemblies of surfactants.21 Lyons et al. reported the preparation of ordered mesoporous
ceria using a neutral surfactant as the template.22 Zhang et al. used polystyrene (PS) spheres as templates to synthesize threedimensionally well-ordered macroporous rare-earth oxides.23 Here, we reported, for the first time, the synthesis of mesoporous Eu2O3 microspindles using a simple solution method followed by heat treatment. By changing the experimental conditions, the samples with different dimensions and aspect ratios were obtained. The optical properties of the samples were also investigated. Experimental Section A simple solution process was used to synthesize the precursor of Eu2O3 using urea as a stepwise-added precipitator. In a typical synthesis, 5 mL of 0.2 M europium acetate was added to 50 mL of aqueous solution containing 0.0002 M polyethylene glycol (PEG, Ms ) 20000). A 1.5 g portion of urea was added subsequently. The europium acetate solution was obtained by dissolving Eu2O3 (99.99%) into acetic acid. After stirring for about 30 min, the transparent solution was transferred to a flask to reflux at 102 °C for 2.0 h. Then, 3.5 g of urea was added to react for another 3.0 h. The white precipitation, produced gradually in the second reflux stage, was collected by centrifugation, washed three times with distilled water and anhydrous ethanol, and dried at 60 °C for 5.0 h. Mesoporous Eu2O3 was obtained by calcining the precursor at 700 °C in air for 2.0 h with a rate of 2.5 °C/min. To investigate the effects of the experimental conditions, parallel experiments were carried out as listed in Table 1. The morphologies of the samples were characterized by transition electron microscopy (TEM, Japan JEM-100CXII transition electron microscope), scanning electron microscopy (SEM, JEOL JSM-6700F field-emission microscope), and high-resolution TEM images (HRTEM, Philips Tecnai 20U-TWIN). The phase composition was characterized by X-ray diffraction (XRD) pattern (Japan Rigaku D/max-γA X-ray diffractometer with graphite monochromatized Cu KR irradiation (λ ) 1.5418 Å). The Fourier-transform infrared (FT-IR) spectrum was recorded by an American Nicolet FTIR 5SXC spectrometer using KBr pellets. Small-angle X-ray scattering (SAXS) patterns were measured by a Holland Philips HMBG-SAX scattering meter (50 kV and 40 mA). Nitrogen (N2) adsorption properties and the pore-size distribution measurements were performed by an American Coulter OMNISORP100CX automatic gas adsorption analyzer. The photoluminescence spectrum was measured by an Edinburgh FLS920 spectrometer.
Results and Discussion * To whom correspondence should be addressed. E-mail address: mengkailu@ icm.sdu.edu.cn (M. Lu),
[email protected] (C. Li). † Shandong University. ‡ East China University of Science and Technology.
The XRD pattern of the precursor is shown in Figure 1a. The peaks can be well indexed to europium oxide carbonate hydrate (Eu2O(CO3)2 · H2O), in agreement with the standard
10.1021/cg070111a CCC: $37.00 2007 American Chemical Society Published on Web 11/08/2007
Crystal Growth & Design, Vol. 7, No. 12, 2007 2671 Table 1. Effects of the experimental conditions on the morphology of the precursors
a
samplea
A
B
C
D
E
U2/g T2/h dimension/nm × nm
1.5 3.0 450 × 300
2.5 3.0 600 × 300
3.5 3.0 900 × 400
3.5 0.5 250 × 100
3.5 1.0 850 × 400
U2 and T2 stand for the amount of urea and the reflux time used in the second reaction stage, respectively.
Figure 1. (a) XRD pattern, (b-d) TEM and (e and f) SEM images of the Eu2O(CO3)2 · H2O precursor (Sample B).
value (JCPDS no. 43–0603). For further insight into the chemical composition of the as-prepared products, the FTIR spectrum was also given (see the Supporting Information, Figure S1). The band in the range of 3000–3700 cm-1 corresponds to the O-H stretching frequency. The sharp peaks at 1510 and 1440 cm-1 are assigned to the bending vibration of the C-H bond.24 The peaks at 1080, 850, and 723 cm-1 are attributed to νC-O, δCO32- and νasCO2, respectively.25 Figure 1, panels b-f, show SEM and TEM images of the precursors obtained by a stepwise reaction process. The SEM image in Figure 1e shows that relatively uniform microspindles with a mean length of 600 nm and width of 300 nm (aspect ratio of ∼2:1) are obtained. An enlarged image in Figure 1f clearly shows that the microspindle is composed of substructures and grows oriented along a certain direction. The electron diffraction (ED) pattern (Figure 1d) measured with an incident electron beam at the edge of the spindles further confirms the single-crystalline and the oriented growth natures of the substructures. TEM results of the precursor prepared by adding 4.0 g urea in only one step were also investigated (see the Supporting Information, Figure S2). Spindle-like structures can also be observed after refluxing for 5.0 h, but the samples congregate greatly, and the integrity of the spindle shape is poor, indicating that the addition step of urea has a great influence on the formation of uniform Eu2O(CO3)2 · H2O microspindles. The advantage of the stepwise-addition process lies in the fact that the first-added urea can neutralize the acidic environment of the initial solution and increase the pH value of the solution to a proper level (from 3.40 to 6.98), which provides a favorable environment for the subsequently added urea to decompose more homogeneously. The other advantage of the stepwise-addition process is that the dimension and the aspect ratio of the Eu2O(CO3)2 · H2O precursors can be adjusted by changing the experimental conditions in the second reaction stage. Figure 2 shows the TEM images of the Eu2O(CO3)2 · H2O precursors obtained under different conditions. When the concentration of urea is low (U2 ) 1.5 g), spindle-like structures, together with some nanoparticles, were obtained (Figure 2a). In comparison with Figure 1b, the microspindles are not so compact, and the aspect ratio is reduced (∼1.5:1). If more urea was added (U2 ) 3.5 g), Eu2O(CO3)2 · H2O microspindles with average dimensions of 900 × 400 nm were obtained (Figure 2b). The ordered diffraction spots observed in the ED pattern (inset of Figure
Figure 2. TEM and ED results of the Eu2O(CO3)2 · H2O precursors (a) Sample A, (b) Sample C, (c) Sample D, and (d) Sample E.
Figure 3. XRD pattern of Eu2O3 obtained by calcining the precursor (Sample C) at 700 °C for 2 h.
2b) clearly indicate the well-crystalline of the sample. Keeping the other experimental conditions fixed (U2 ) 3.5 g), the effect of the reaction time was also investigated. Figure 2, panels c and d, show the TEM images of the precursors obtained when T2 is 0.5 and 1.0 h. From Figure 2c, it can be found that the spindle-shaped Eu2O(CO3)2 · H2O microstructures have been formed in this stage, although the spindles became thinner and the dimension was obviously smaller (∼250 × 100 nm). As the reaction time was increased to 1.0 h, the dimensions as well as the crystallinity of the sample were increased greatly (Figure 2d), which can also be observed from the corresponding ED pattern. Eu2O3 microstructures are expected to be obtained by calcining the precursors at a proper temperature. Figure 3 shows the XRD pattern of the samples obtained by calcining the Eu2O(CO3)2 · H2O precursor at 700 °C for 2 h. The diffraction lines are in agreement with the standard value (JCPDS No. 34–0392) of Eu2O3 with cubic structure, indicating the complete conversion of the Eu2O(CO3)2 · H2O precursor to Eu2O3. TEM and SEM images of the Eu2O3 microstructures are shown in Figure 4. After heat-treatment, the spindle-like shapes of the precursors were well maintained, no obviously sintered or
2672 Crystal Growth & Design, Vol. 7, No. 12, 2007
Figure 4. TEM (a and b), SEM (c and d), and HRTEM (e and f) images of Eu2O3 mesoporous structures obtained by calcining the precursor (Sample C) at 700 °C for 2 h.
compressed phenomena were observed (Figure 4a). From Figure 4b, it can clearly be found that the Eu2O3 sample exhibits mesoporous structure and that the distribution of the pore structure is irregular. The well-crystalline nature of the pore walls can be confirmed by the corresponding ED pattern (inset of Figure 4b). SEM images in Figure 4, panels c and d, clearly display the three-dimensional spindle-like morphology and the mesoporous nature of Eu2O3 microspindles. To further verify the microstructure of the mesoporous Eu2O3 microspindles, HRTEM images have been taken. Figure 4e shows that the nanocrystals are interconnected closely, forming a random network, and the distribution of the pore is irregular. The average pore size is estimated to be 15 nm. The interplanar spacing of 0.32 nm corresponds to the (222) plane of cubic Eu2O3, further indicating the walls of the mesoporous structures are constructed by Eu2O3 nanocrystals. SAXS patterns and N2 adsorption/desorption isotherms were used to characterize the mesoporous samples. From the SAXS pattern (see the Supporting Information, Figure S3a) a lowangle scattering maximum can be observed, which is characteristic of mesostructured compounds. It was reported that a much lower short-range structural order resulting from irregular pore sizes only led to a broad XRD shoulder at low angles.5 The relatively broad band indicates that the distribution of the pore structure is irregular, which is consistent with the TEM observations. It is noteworthy that the samples still exhibit mesoporous structures after calcining at 700 °C for 5 h (see the Supporting Information, Figure S3b), confirming that the framework is stable. Figure 5 displays the N2 adsorption/ desorption isotherms of the mesoporous Eu2O3microspindles. It shows an IV-like isotherm with H1-type hysteresis according to the IUPAC classification, indicating the mesoporous structure of the Eu2O3 microspindles. The Barrett–Joyner–Halenda (BJH) pore-size distribution obtained from the adsorption branch of the isotherm is shown in the inset of Figure 5. A relatively broad band was observed. The centered peak at about 13 nm is consistent with the HRTEM observations. A shoulder peak at about 23 nm may arise from the overlap of the Eu2O3 nanocrystals. Figure 6 shows TEM images of the mesoporous Eu2O3 microspindles obtained by calcining different precursors. When the calcining time was increased to 5 h, no collapse phenomenon was observed. The intact spindle structures are reserved (Figure
Wang et al.
6, panels c and d), indicating the present route is feasible to synthesize mesoporous rare earth oxides with high stability. On the basis of the experimental results, the formation of the mesoporous Eu2O3 microspindles is considered to arise from the oriented growth of the Eu2O3 nanocrystals by the aid of the surfactant. In the absence of PEG, only small fragments were obtained for the Eu2O(CO3)2 · H2O precursor (see the Supporting Information, Figure S4). Generally, the selective adsorption of the surfactant PEG on the particles plays a critical role in the formation of different microstructures.26 It can act as a reaction moderator, a structure-direct agent, as well as a soft template. In accordance with the observations in literature,27 the formation of the spindle-shaped precursors involves a two-stage growth process: nucleation/growth of primary particles, and the subsequent formation of the spindles by primary particle aggregation. During these processes, PEG adsorbed on the nanoparticles can moderate the reaction rate and direct the as-formed nuclei to grow and aggregate into spindle-shaped structures. These functions of PEG are considered to be closely related to its molecular structure. As a nonionic surfactant, PEG can form rugged long chains when dissolved in water, and the chains have a high degree of flexibility because the C-O bond is easy to rotate.28 In addition, there exists a large amount of activated oxygen in PEG molecular chains because of the -O- of the ether group,29 which results in strong interactions between PEG and europium ions. Because of the existence of the polar hydrophilic heads (-O-) and hydrophobic hydrocarbon (-CH2-CH2-) chains of PEG, the full extension trend and the curl effect coexist in aqueous solution. Thus, spindle-shaped Eu2O(CO3)2 · H2O precursors were obtained. In the mesoporous structure formation stage, PEG acts as a soft template. It was removed by the subsequent heat treatment process. The optical properties of the mesoporous Eu2O3 microspindles were also investigated. Previously, many papers reported the photoluminescence (PL) properties of different Eu2O3 nano- or microstructures. Wu et al. used a sol–gel template method to synthesize Eu2O3 nanotubes and investigated their PL properties. In their emission spectrum, a much stronger emission peak at 612 nm was exhibited.13 Similar results have been found in the emission spectra of Eu2O3 nanoparticles and nanorods.12b,14b Figure 7 shows the PL spectrum of mesoporous Eu2O3 microspindles measured under photoexcitation at 395 nm. Emission peaks observed in the orange and red regions can be attributed to the characteristic fff transitions of Eu3+.30 The peaks from 572 to 687 nm correspond to the 5D0f7FJ, J ) 0–3, transitions of Eu3+, respectively. Compared with the PL spectrum of its precursor (see the Supporting Information, Figure S5) and the previously reported results, the intensity ratio of the emissions arising from 5D0f7F1 and 5D0f7F2 varied. The origin of this phenomenon is not yet clear. This difference may be attributed to the contribution of the special mesoporous structure. The body-centered cubic (bcc) Eu2O3 crystalline structure possesses Ia3 symmetry (Th7). There are two Eu3+ sites in the bulk structure, and these sites possess C2 and S6 (C3i, possessing inversion symmetry) symmetry. The sites occur in a 3:1 ratio in the unit cell, with the C2 site the more abundant site.31 Thus, for bulk Eu2O3, the 5D0f7F2 transition is prominent. It is known that the luminescent properties of inorganic materials can be greatly affected by tailoring their morphology, size, or crystallinity.32 In this case, during the transformation of the microstructure from a microspindle-shaped assembly to a mesoporous Eu2O3 microspindle, the location of some Eu3+ ions at C2 and S6 sites may be changed, leading to the variation
Crystal Growth & Design, Vol. 7, No. 12, 2007 2673
Figure 5. N2 adsorption/desorption isotherms of mesoporous Eu2O3microstructures obtained by calcining the precursor (Sample C) at 700 °C for 2 h. The inset shows the pore-size distribution.
and the dimension and morphology of the Eu2O(CO3)2 · H2O precursors can be adjusted. Mesoporous Eu2O3 microspindles obtained by calcining the precursors are considered to arise from the self-assembly of the Eu2O3 nanocrystals. The samples exhibit high thermal stability, which makes them appealing for practical applications. Acknowledgment. This work was supported by the Funds to Excellent State Key Laboratory by Chinese Ministry of Education (No. 50323006), Shanghai Rising-Star Program (06QA14013), the National Natural Science Foundation of China (20236020, 20176009), the Major Basic Research Project of Shanghai (04DZ14002), the China Postdoctoral Science Foundation (20070410167), and the Special Project for Nanotechnology of Shanghai (0752nm010).
Figure 6. TEM images of mesoporous Eu2O3 microspindles obtained by calcining precursors: (a) Sample A, 700 °C, 2 h; (b) Sample D, 700 °C, 2 h; (c and d) Sample C, 700 °C, 5 h.
Supporting Information Available: FT-IR spectrum of Eu2O(CO3)2 · H2O precursor, TEM image of Eu2O(CO3)2 · H2O precursor obtained by adding urea in one step or without adding any surfactant, SAXS patterns of Eu2O3 microstructures and PL spectrum of Eu2O(CO3)2 · H2O precursor. This material is available free of charge via the Internet at http://pubs.acs.org.
References
Figure 7. PL spectrum of mesoporous Eu2O3 microspindles obtained by calcining the precursor (Sample C) at 700 °C for 2 h.
of the emission spectrum. Systematic investigation on the optical properties of the mesoporous Eu2O3 structures is in progress. Conclusions By adopting a stepwise reaction process, Eu2O(CO3)2 · H2O microspindles have been prepared via a facile solution process,
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CG070111A
