J. Phys. Chem. C 2010, 114, 4895–4900
Photoluminescence and Raman Spectroscopy Studies of Eu(OH)3 Rods at High Pressures Q. G. Zeng,*,† Z. J. Ding,‡ Z. M. Zhang,‡ and Y. Q. Sheng† Department of Mathematics & Physics, Wu Yi UniVersity, Jiangmen, Guangdong 529020, People’s Republic of China, and Hefei National Laboratory for Physical Sciences at Microscale and Department of Physics, UniVersity of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China ReceiVed: December 14, 2009; ReVised Manuscript ReceiVed: February 17, 2010
Eu(OH)3 rods, with diameters of 140 nm and lengths of 100-500 nm were prepared by a hydrothermal method. X-ray diffraction indicated a pure hexagonal phase (space group P63/m) of the rods. The relations between structural and optical properties of Eu(OH)3 rods under high pressures were obtained by photoluminescence (PL) and Raman spectra. Two structural phase-transition points at around 4 and 8 GPa were observed in this work. When a pressure of about 4 GPa was applied to the samples, one new emission peak at 593 nm was observed in the PL spectra, indicating the splitting of the 7F1 Stark level in Eu3+ ions. Such a splitting was attributed to the decrease of site symmetry of Eu3+ ions from C3h to D2 at high pressures. Two new Raman bands appeared under a pressure up to about 8 GPa due to the pressure-induced amorphization. After the pressure was released, the original PL and Raman spectra were recovered. Introduction The optical properties of rare-earth oxides have been extensively studied due to their potential uses in optical devices, such as optical amplifiers and display phosphors in optoelectronic applications.1-4 In particular, considerable attention has been focused on one-dimensional (1D) nanostructured materials, as the reduction in dimensions and particle sizes often result in an improvement of the structural, electronic, and optical properties.4-6 For example, the synthesis and optical properties of Eu(OH)3 and Eu2O3 nanotubes and nanorods at low temperatures were reported by Wu et al.4 and Du et al.6 Recently, we have also reported on the relation between morphological and optical properties of Eu-doped titanium oxide nanotubes and nanorods.7 The emission line width and relative intensities of the Eu3+ ions were found to depend on the morphology of the matrix. However, all of the studies mentioned above were conducted under ambient pressure. When a high pressure is applied to a sample, the interatomic distance is reduced, leading to an overlap between adjacent electronic orbitals. As a result, some new effects (such as structural phase transition and energy transfer) are often observed. Photoluminescence (PL) and Raman spectroscopy are popular techniques to study the electronic structure and phase transition of materials at high pressure. In general, discontinuities in spectral parameters, such as changes in the number of lines, evolution of frequencies, and intensity variations with pressure, can be used for identifying structural modifications. The appearance and/or disappearance of lines can be used as a criterion for assigning a structural phase transition.8 On the other hand, high-pressure Raman spectroscopy has proved to be a convenient method to study the phase transition.9-11 Previously, we reported on the Raman spectra of Eu/TiO2 nanocrystals at high pressures, which showed a phase transition with higher local symmetry.12 * To whom correspondence should be addressed. E-mail: [email protected]
mail.ustc.edu.cn. † Wu Yi University. ‡ University of Science and Technology of China.
It is well-known that PL spectroscopy can carry information about the sites occupied by rare-earth ions. The energy levels of rare-earth centers are established with high precision and are very sensitive to the local environment of the corresponding cations. In recent years, pressure-induced phase transitions and pressure-induced amorphization studied by PL spectroscopy were reported.8,13-17 Jayasankar et al.13 and Babu et al.14 investigated the PL spectra of Eu3+ ions in glasses at high pressures, and they obtained the crystal-field parameters for the Stark splitting of the 7F level. It was found that the crystal-field perturbations experienced by the 4f6 (Eu3+) electrons changed significantly with the applied pressure. In this work, Eu(OH)3 rods were synthesized by a hydrothermal method. The morphology, structure, and optical properties of the Eu(OH)3 rods were characterized by field emission scanning electron microscopy (FE-SEM) and X-ray powder diffractometry (XRD). The structure and optical properties of the Eu(OH)3 rods at high pressures were studied by PL and Raman spectra. Two structural phase-transition points were obtained. A new emission peak at 593 nm was observed at about 4 GPa (the first phase-transition point), which was attributed to a new Stark splitting at the 7F1 energy level of Eu3+ ions. Two new Raman bands at 173 and 534 cm-1 were observed at 8 GPa, which indicated the appearance of another new structural phase. When the pressure was released, the PL and Raman spectra were completely recovered. Experimental Section Eu(OH)3 rods were prepared by a hydrothermal method. Two grams of Eu2O3 powder (purity ) 99.99%) was added to 60 mL of NaOH (10 M) solution, and the mixture was poured into a stainless steel, Teflon-lined autoclave. The autoclave was maintained at 150 °C for 24 h and then cooled to room temperature in air. The resulting white products were collected, washed with water, and then dried in air at 120 °C. The white powder of Eu(OH)3 rods was obtained. Structural studies of Eu(OH)3 rods were performed in an X-ray diffractometer (X’PERT PRO, Panalytical) with a monochromatic Cu KR1 radiation source at the wavelength of 0.15405 nm. The
10.1021/jp911791u 2010 American Chemical Society Published on Web 03/02/2010
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Figure 1. FE-SEM image of Eu(OH)3 rods.
morphology of the sample was observed with an FE-SEM (NanoSEM430, FEI). A confocal laser micro-Raman spectrometer (LABRM-HR) was used to measure the high-pressure PL and Raman spectra of Eu(OH)3 rods in the reflection mode. The excitation source was a focused Ar+ laser with a 488 nm line with a spot size of 5 µm. The sample was filled into a hole with a diameter of 0.15 mm in a stainless steel gasket in a diamond anvil cell (DAC). The gasket with a thickness of 0.28 mm was preindented to 0.15 mm by the diamond anvils. For hydrostatic pressure measurements, a small amount of sample was fitted in the sample chamber with the mixture of methanol/ethanol/water (16: 3:1) as the pressure-transmitting medium. A few grains of ruby powder were introduced in the chamber for in situ pressure measurements, using the well-known fluorescence technique. From the shift of the R1 line of the ruby fluorescence, the pressure can be determined.18 Results and Discussion Morphology and Structure of Eu(OH)3 Rods at Ambient Pressure. Figure 1 shows the FE-SEM image of the Eu(OH)3 rods. Short, rodlike structures have been achieved, with a diameter around 140 nm and lengths between 100 and 500 nm. Figure 2 shows the XRD pattern for the powder of Eu(OH)3 rods. The diffraction pattern can be indexed to the hexagonal phase (space group ) P63/m, no.176) with lattice constants a ) 6.352 Å and c ) 3.653 Å, which are consistent with the values in the standard cards (JCPDS card no. 83-2305).6 No diffraction peaks from other phases can be observed. Detailed information about the crystal structures of Eu(OH)3 rods were described in ref 19. The Eu(OH)3 structure consists of ninecoordinated Eu3+ ions in approximately D3h symmetry, with two different Eu-O distances and three independent O-O distances. The coordination polyhedron (as shown in Figure 2 in ref 19) approximates a trigonal prism, with the Eu3+ ion located in the center of the prism. Six of the nine coordinating oxygen (O2) atoms are found at each of the corners of the trigonal prism: three above the midhorizontal plane and three below. The other three coordinating oxygen (O1) atoms are equatorial, with each of them located on a normal to a rectangular face of the trigonal prism. PL Spectra of Eu(OH)3 Rods at Ambient Pressure. The PL spectra of Eu(OH)3 rods at ambient pressure are given in
Figure 2. XRD pattern of Eu(OH)3 rods. The dashed line is the reference XRD pattern (JCPDS card no. 83-2305) of hexagonal phase Eu(OH)3.
Figure 3. Comparison of photoluminescence spectra of Eu(OH)3 rods at different conditions, from bottom to top: 300 and 83K at ambient pressure and released pressure at room temperature.
Figure 3. Six emission peaks (located at 592, 595, 616.6, 651, 690, and 696 nm) can be fully resolved; these peaks correspond to the Stark components of the 5D0-7FJ (J ) 1-4) transition of the Eu3+ ions. The intensity of the emission peak at 616.6 nm (5D0-7F2 ) electric dipole transition), which is very sensitive to the local environment of the Eu3+ ions, is weaker than that of the 5D0-7F1 magnetic dipole transition (592 nm). This indicates the high site symmetry of the Eu3+ ions in Eu(OH)3 rods. No emission from 5D1 and 5D2 of the Eu3+ ions is detected in the Eu(OH)3 rods because of the large vibrational energy (about 3600 cm-1) of the -OH radicals, which causes these transitions to be nonradiative.20 PL Spectra of Eu(OH)3 Rods at High Pressures. Figure 4 shows the pressure dependence of the PL spectra for Eu(OH)3 rods. In this figure, only the 5D0-7FJ (J ) 1, 2) transition lines of Eu3+ ions are shown. The 5D0-7FJ (J ) 3, 4) transition lines are neglected because they are disturbed by the ruby fluorescence. The three emission bands at 592, 595, and 616.6 nm can be fully resolved at all measured pressures. Contradicting results concerning the behavior of the Stark levels 7FJ under pressure have been reported in studies, with both upward and
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Figure 5. Emission peak position of the 5D0-7FJ (J ) 1, 2) transition in Eu(OH)3 rods as a function of pressure. The 5D0-7F1 transition: before the broken point. The 5D0-7F2 transition: after the broken point.
Figure 4. Photoluminescence spectra for 5D0-7FJ (J ) 1, 2) transitions of Eu3+ ions in Eu(OH)3 rods as a function of pressure, categorized into different pressure ranges.
downward shift of energy of the 7FJ sublevels observed with increasing applied pressure. On the other hand, the red shift of the 5D0 f 7FJ (J ) 1, 2) transition with increasing pressure is agreed by most reports.8,21-23 Moreover, it is also found that the red shifts of the emission peak positions are discontinuous at around 4 and 8 GPa (Figure 5). In addition, it is observed from Figure 4 that the emission intensities of all peaks decrease with pressure smaller than 4.0 GPa, become almost invariant with the increasing pressure up to 8 GPa, and dramatically drop again with pressure above 8 GPa. The most striking feature of Figure 4 is the appearance of a new emission band at 593 nm, corresponding to a new Stark component of the 5D0-7F1 transitions of the Eu3+ ion when the pressure reaches about 4 GPa. The result suggests that one new Stark level of 7F1 of Eu3+ ions is split at such a high pressure. To verify that the new peak is indeed due to the pressure effect, Figure 3 shows the comparison of PL spectra of Eu(OH)3 rods measured at room temperature (300 K) and 83 K under ambient pressure. The PL spectra at the two temperatures are identical, except for a slight difference in the emission intensities. At 83 K, the new emission band at 593 nm cannot be observed, and we conclude that the Stark splitting is due to the application of pressures. It is well-known that some factors, such as the nonhydrostatic pressure effect, variations of strength, and symmetry of the crystal field in a free rare-earth ion, can also lead to the splitting of the Stark levels (of the same LSJ manifold).8,22 In our experiments, the ruby pressure technique (shift and broadening of R1 and R2 lines) used to measure the
pressure also confirms the hydrostatic pressure condition on the sample, which can rule out the nonhydrostatic effect first. Therefore, in the present work, we suggest that the variation of the crystal field in site symmetry may be the main factor for the Stark splitting.22 Generally, the number of energy states and their energy values are determined by the symmetry of the sites where the Eu3+ ions are located. The energy levels are split by the ligand field surrounding the Eu3+ ion. At room temperature, the 7F1 multiplet only shows two well-resolved Stark components, corresponding to transitions at 592 and 595 nm, as shown in Figure 3. This indicates that the site symmetry of Eu3+ ions is very high (C3h) at low pressure,21 which is consistent with the XRD result shown in Figure 2. Above 4 GPa, a new Stark component of the 5 D0-7F1 transition (emission peak at 593 nm) appears and, thus, the Stark components of the 5D0-7F1 transitions attain the maximum value of 2J + 1 ) 3 for J ) 1. The complete removal of degeneracy for the 5D0-7F1 transitions suggests a lower local site symmetry of the Eu3+ ions in Eu(OH)3 rods. In addition, a new small emission peak at 621 nm (shown in Figure 4) that corresponds to the 5D0-7F2 transition is also observed above 4 GPa. Therefore, it can be deduced that the new site symmetry of Eu3+ ions is lower (such as in orthorhombic (D2, C2V), monoclinic (C2, CS), or triclinic (C1) structures) at pressures higher than 4 GPa.8,24,25 To determine the new site symmetry of Eu3+ ions at 4-8 GPa, the 5D0 f 7F0 transition of Eu(OH)3 rods at different pressures has been checked and summarized in Figure 6. Although the intensity of this transition is very weak as compared with that of the5D0 f 7F1,2 transitions at the same pressure, some information can still be obtained. The 5D0 f 7 F0 transition (at 580 nm) cannot be observed when the pressure is lower than 8 GPa, so we can attribute the new site symmetry of the Eu3+ ions to D2 for the pressure range of 4-8 GPa. For pressures higher than 8 GPa, the appearance of the new 5D0 f 7 F0 transition at 580 nm indicates that the point symmetry of the site of the Eu3+ ions in Eu(OH)3 is shifted toward the noncentrosymmetric point group.14 The luminescence intensity ratio of 5D0 f 7F2 to 5D0 f 7F1 (I2/1), known as the asymmetric ratio, provides valuable information about changes of the local structure around Eu3+ ions and the Eu-O covalence.8,14,25,26 Figure 7 shows the variation of I2/1 of Eu(OH)3 rods with pressures. It is found that the value
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Figure 6. Photoluminescence spectra for the 5D0-7F0 transition of Eu3+ ions in Eu(OH)3 rods as a function of pressure.
Figure 8. Raman spectra of Eu(OH)3 rods under high pressure.
Figure 7. Luminescence intensity ratio I2/1 of the I(5D0 f 7F2) to I(5D0 f 7F1) transitions as a function of pressure.
of I2/1 decreased with increasing pressure before 8 GPa. This could be due to either a decrease in the covalence of the Eu-O bonds or an increase in the average Eu-O bond distance with pressure. After 8 GPa, as obtained from Figure 6, the site symmetry of Eu3+ ions is shifted toward the noncentrosymmetric point group with increasing pressures. Therefore, this observation of the I2/1 value increase with pressure can be attributed to an increase of the local deformation around the Eu3+ ion sites. From the above discussions, two transformation points at about 4.0 and 8 GPa are clearly found. The former point can be characterized by a decrease in the site symmetry of Eu3+ ions from C3h to D2, as evidenced by the appearance of one new Stark component of the 5D0-7F1 transitions for the Eu3+ ions in high pressure PL spectra. For pressures higher than 8 GPa, one new weak emission peak at about 580 nm is observed in PL spectra, corresponding to the 5D0-7F0 transition. This new emission peak means that the site symmetry of Eu3+ ions is lower. However, the degeneracy for the 5D0-7F1 transitions has been removed completely for pressures higher than 8 GPa. It is difficult to assign a phase-transition point at 8 GPa solely by
Figure 9. Raman shift of Eu(OH)3 rods as a function of pressure.
high-pressure PL spectra; Raman spectra of the Eu(OH)3 rods can provide necessary information to address this issue, as demonstrated as follows. Raman Spectra of Eu(OH)3 Rods at High Pressures. According to the factor group analysis based on the space group P63/m, the hexagonal Eu(OH)3 structure yields the following Raman active vibrations: Γ ) 4A1g + 2E1g + 5E2g.27 Thus, 11 Raman vibrational modes are predicted in total. However, as
PL and Raman Spectroscopy Studies of Eu(OH)3 Rods TABLE 1: Pressure Derivatives of Frequencies (dωi)/(dp) for Various Raman Modes of Eu(OH)3 Rods in Three Pressure Ranges (below 4 GPa, between 4 and 8 GPa, and above 8 GPa) (dωi)/(dp) (cm-1/GPa) Raman mode
E2g Ag E2g E1g
137 304 379 485 172*a
1.4 3.2 4.8 -0.7
1.2 4.3 5.8 -1.2
1.3 5.9 8.1 -2.2 4.8
a (*): the wavenumber of this Raman mode was obtained at 8.2 GPa.
shown in Figure 8, the number of observed Raman modes is fewer than the predicted. This is due to the weak intensities of the other modes. Only four fully resolved Raman active vibrations in all pressures can be attributed to the external modes: Two bands at around 137 and 379 cm-1 are assigned as E2g translatory modes. The bands located at 304 and 485 cm-1 are assigned to Ag translatory and E1g libration modes, respectively. The frequencies of all observed Raman modes are plotted as a function of pressure in Figure 9. Two transition points at about 4 and 8 GPa are also observed. (dωi)/(dp) at the three pressure ranges are listed in Table 1. The (dωi)/(dp)of the E1g Raman mode (485 cm-1) in Eu(OH)3 is assigned to the vibrational mode of the OH dipoles, which is negative at all pressure ranges. This indicates that the distance of the OH dipoles becomes larger at high pressure. Thus, we can speculate that the OH dipoles are extended along the a and b planes under pressures.27 Considering the complete pressure dependence of the Raman spectra, the other three Raman modes (137, 304, and 379 cm-1) are found shifting to higher wavenumbers under pressure, indicating a decrease in bond length. In one unit cell, the E2g mode (137 cm-1) arises due to the Eu3+ cation translation behavior, whereas the other two Raman modes Ag and E2g (304 and 379 cm-1) occur due to the anion translation behaviors. Among these three Raman modes, the (dωi)/(dp)of the Raman mode E2g (137 cm-1) is about 1.2-1.4 cm-1/GPa in the whole measured pressure range, which is lower than the values (about 3.2-5.9 and 4.8-8.1 cm-1/GPa) of the other two Raman modes Ag and E2g (304 and 379 cm-1). This indicates that the pressure effects on Eu3+ cations are weaker than those on the anions.
J. Phys. Chem. C, Vol. 114, No. 11, 2010 4899 As shown in Table 1, the behaviors of (dωi)/(dp) at every Raman mode are various at different pressure ranges (before 4 GPa, between 4 and 8 GPa, and after 8 GPa). The first transition (4 GPa) can be attributed to an obvious structural transition due to breaking of the crystal symmetry, as mentioned in the previous section of high-pressure PL spectra. The second transition at 8 GPa can also be identified from the high-pressure Raman spectra in Figure 8. Two new Raman bands located at 173 and 534 cm-1 are observed at 8.2 GPa, which means that there is yet another structural transition at that pressure. As previously mentioned in high-pressure PL spectra, the degeneracy for the 5D0-7F1 transitions is completely removed. The site symmetry of Eu3+ ions in Eu(OH)3 rods is the noncentrosymmetric point group for the pressure after 8 GPa. Chen et al.17 and Lavin et al.28 have also reported the similar phenomenon of pressure-induced amorphization in amorphous Eu(OH)3 and EuZrF7 compounds. Moreover, in Figures 4 and 8, it is observed that the intensities of PL and Raman bands decrease dramatically with pressures only when the pressure is higher than 8 GPa. This means that the crystal symmetry may be driven to lower symmetry or toward an amorphous state by high pressures. Therefore, we speculate the appearance of pressureinduced amorphization in Eu(OH)3 rods for pressures higher than 8 GPa. Here, we summarize the above discussions as follows: At high pressure, the distance of OH dipoles becomes longer, while the Eu-Eu and O-O distances decrease. These indicate the appearance of the polyhedron tilts induced by high pressures.29 Two phase-transition points at about 4 and 8 GPa are observed from high-pressure PL and Raman spectra. The first transition point (4 GPa) is due to the decrease in crystal symmetry, while the second transition point (8 GPa) can be attributed to pressureinduced amorphization. The two phase transitions bring about the discontinuous changes in PL and Raman spectra at high pressures. After releasing pressure, the PL and Raman spectra can be recovered, as shown in Figures 3, 8, and 10, which indicate that the two new phases only exist at high pressures. Moreover, after carefully checking the PL spectra of Eu(OH)3 rods at different releasing pressures, as shown in Figure 10, it is found that three new PL lines at 580, 593 and 616.6 nm undergo two processes before they disappear. All the three mentioned emissions peaks are observable when the pressure decreases to 5.6 GPa, which is consistent with the process of applying pressure. However, the later two emission peaks disappear, while
Figure 10. Photoluminescence (a) and Raman (b) spectra of the Eu(OH)3 rods as a function of releasing pressure.
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the former one can still be observed when the pressure is reduced to 1.5 GPa. After completely releasing the pressure, the emission peaks at 580 nm also vanishes. This implies that the recovered spectra undergo two transitions. To obtain detailed structure information about the two new structure phases, high-pressure XRD has to be performed in further work. Conclusion The pressure-dependent PL and Raman spectra of hexagonal phase Eu(OH)3 rods, synthesized by a hydrothermal method, were investigated in this work. Two structural transition points are observed due to the polyhedron tilts induced by high pressure. One emission peak at 593 nm, corresponding to the new Stark splitting component of 5D0-7F1 transitions of Eu3+ ions due to reduced on-site symmetry from C3h to D2, was observed at the first transition point (about 4 GPa). At about 8 GPa, two new Raman bands at 173 and 534 cm-1 were observed, which indicated another structural phase transition. This new phase transition was attributed to the pressure-induced amorphization. When the pressure was released, the new PL and Raman bands disappeared and the symmetry of the Eu3+ ion can be recovered to that before the application of pressure by two processes. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant No. 10874160), the Chinese Academy of Sciences, the Science and Technology Project of Guangdong in 2009 (No. 2009B030803015), the Science and Technology Project of Jiangmen in 2008, and the Key Research Project of Wu Yi University in 2008. References and Notes (1) Dantelle, G.; Mortier, M.; Vivien, D.; Patriarche, G. Chem. Mater. 2005, 17, 2216. (2) Sivakumar, S.; Veggel, J.; Raudsepp, M. M. J. Am. Chem. Soc. 2005, 127, 12464. (3) Patra, A.; Sominska, E.; Ramesh, S.; Koltypin, Y.; Zhong, Z. J. Phys. Chem. B 1999, 103, 3361. (4) Wu, G. S.; Zhang, L.; Cheng, B. C.; Xie, T.; Yuan, X. Y. J. Am. Chem. Soc. 2004, 126, 5976.
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