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Identifying Defects in Ceria-Based Nanocrystals by UV Resonance Raman Spectroscopy Takaaki Taniguchi,† Tomoaki Watanabe,‡ Naota Sugiyama,† A. K. Subramani,† Hajime Wagata,† Nobuhiro Matsushita,*,† and Masahiro Yoshimura† Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Midori, Nagatsuta, Yokohama 226-8503, Japan, and Department of Applied Chemistry, Meiji UniVersity, 1-1-1 Higashimita, Tama-ku, Kawasaki, 214-8571, Japan ReceiVed: May 27, 2009; ReVised Manuscript ReceiVed: October 6, 2009
Local structures in ceria-based materials were investigated by UV resonance Raman spectroscopy using a 363.8 nm laser line for the first time. In the spectra of a highly crystalline and undoped ceria, overtone LO series are clearly detected due to the multiphonon relaxation by the UV resonance Raman effect. Gd3+ doping in ceria additionally activates the disorder bands that are attributable to lattice spaces with or without an oxygen vacancy. The crystal size dependency of the UV Raman spectra suggests that Ce3+ ions preferably form the MO8-type (M ) Ce3+) complex in the undoped ceria nanoparticles and are eliminated by oxidation while heating in air. Gd3+ ions also tend to form a MO8-type complex in the nanocrystalline ceria matrix. However, they diffuse to form the more thermodynamically stable defect clusters that accompany the grain growth upon heating. 1. Introduction Ongoing efforts seek better electrolyte and electrode materials that lower the operating temperature of solid-oxide fuel cells (SOFCs). Rare earth-doped ceria (cerium(IV) oxide, CeO2) is a promising candidate, owing to its higher ionic conductivity at intermediate temperatures than that of the conventional zirconia-based electrolytes.1 Furthermore, recent studies have revealed that reducing the crystal size to nanometer dimensions beneficially alters the electrical properties.2-6 It has been reported that nanostructured Gd3+-doped ceria thin film shows a total ionic conductivity as large as 10 times that of the microcrystalline films or single crystals.6 Raman spectroscopy is sufficiently powerful to analyze structural properties of nanomaterials at a local level, owing to the strong sensitivity of the phonon characteristics to the crystalline nature of the material. Therefore, this technique has frequently been used to investigate structures that contribute to the unique properties of ceria-based nanomaterials. These studies have demonstrated that a Raman-allowed F2g mode near 464 cm-1 shifts to lower energies, and that the line shape gets progressively asymmetric as the crystal size decreases.7,8 The size dependence has been attributed to the inhomogeneous strain broadening associated with dispersion in particle size and phonon confinement.8 The spatial correlation model used for Raman analysis suggested an increased defect concentration for the rare earth-doped ceria nanocrystals as compared with the undoped ones.9 Although vis (visible) lasers have been used as an excitation source in the referenced Raman studies, UV Raman spectroscopy, employing UV excitation, has advanced recently, owing to the investigation of wide band gap metal oxides10-15 involving CeO2.16 UV Raman spectroscopy enables shallow analytical depth. This is due to the strong absorption of UV light having energy higher than the band gap of these materials, which may * To whom correspondence should be addressed. Fax: +81-45-924-5358. E-mail:
[email protected]. † Tokyo Institute of Technology. ‡ Meiji University.
further enable selective detection of the crystal structure in the surface region.10,13,14 In addition, the resonance Raman effect is often observed when the excitation energy is comparable to the band gap energy. Resonance enhancement is normally desirable, as it increases detection sensitivity in the molecular system. This effect may also activate second-order Raman modes that are sensitive to atomic scale disorder in solids.15 Recently, we have developed a hydrothermal process that allows the synthesis of undoped and Gd3+-doped ceria nanocrystals with a uniform size distribution.17 High-quality nanocrystals are attractive for the facile fabrication of functional nanostructured films and coatings. In addition, well-defined nanocrystals are suitable for fundamental research that attempts to reveal the origin of nanodimensional effects on physical and chemical properties. In the present study, UV Raman spectroscopy was used to investigate the local structure of the nanocrystals. To obtain the resonance Raman spectra, a UV laser line (363.8 nm) with photon energy (3.41 eV) close to the direct band gap energy of ceria (ca. 3.5 eV) was selected as an excitation source for the first time. On the basis of the analysis of Gd3+-doping, heat-treatment, and grain-size effects on the spectra, the defect formation in the ceria-based nanocrystals is discussed herein. 2. Experimental Section 2.1. Sample Preparation. An oleate-modified precipitation route was used to prepare the nearly monodispersed, undoped, and Gd3+-doped ceria nanoparticle. The experimental procedure is presented in detail in ref 17. Briefly, cerium-oleate complexes were condensed by a base at room temperature to produce oleate-passivated nuclei, which were then subjected to hydrothermal treatment at 200 °C for 6 h. To investigate the crystalsize and heat-treatment effects, part of the product obtained by hydrothermal synthesis was subsequently annealed at 600 and 1000 °C for 5 h. 2.2. Characterization. The products were characterized by powder XRD using a MAC Science MX 3 VA diffractometer with Cu KR radiation (λ ) 1.54056 Å), operating at 40 mA
10.1021/jp9049457 CCC: $40.75 2009 American Chemical Society Published on Web 10/26/2009
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Figure 1. (a) Absorption spectra and (b) UV/vis Raman spectra of an undoped ceria sample annealed at 1000 °C for 5 h.
and 40 kV. The average crystallite size was determined by means of the Debye-Scherrer formula from the peak position and line broadening of the (220) XRD reflection. The lattice constants were calculated by means of a least-squares method using all the available refection peaks in the measured 2θ range of 20-80°. Visible and UV Raman spectroscopy were carried out on a Jobin Yvon T64000 spectrometer (resolution of 1 cm-1) using a visible laser (λ ) 514.5 nm) and UV laser (λ ) 363.8 nm) with an output laser power of 50 mW at room temperature. The scattered light was collected in backscattering geometry using a liquid nitrogen cooled charge-coupled device (CCD) detector. The measurement system for UV Raman spectra, which was designed by our research group, is described in detail in the ref 18. UV-vis diffuse reflectance spectroscopy was performed using a PerkinElmer lambda 35 UV-vis spectrometer to obtain the absorption spectra. 3. Results and Discussion First, an undoped ceria sample annealed at 1000 °C (average crystal size ) 82.2 nm) was investigated. Figure 1a shows the absorption spectra obtained by UV-vis diffuse reflectance spectroscopy, where the sample strongly absorbs UV light due to band gap absorption, while there is much weaker absorption in the visible range. This demonstrates that the UV and vis Raman spectra were measured on the resonant and nonresonant (or weakly resonant) conditions, respectively. Note that, given the absorption coefficient of undoped ceria,19 the penetration depth of light with a wavelength of 368 nm is estimated to be ca. 200 nm, which indicates that the UV laser passed through more than one grain layer in the samples. This shows that an overall structure of the crystallite involving grain core, interface, and surface contributes to the UV Raman scattering (as well as to the vis Raman scattering). Raman spectra (Figure 1b) showed distinctly different features when vis (514.5 nm) and UV (363.8 nm) excitation were employed. In the vis Raman spectra, an F2g band, a triply degenerate Raman-active phonon of the cubic CeO2 fluorite phase, is observed at 464 cm-1, whereas the UV Raman spectroscopy gave overtone bands of longitudinal optic (LO) mode, 2 LO (1070 cm-1), and 3 LO (1500 cm-1),20 along with the F2g band. Activation of the LO series is a characteristic of multiphonon relaxation by the resonance Raman effect. Notably, Livneh et al. found that UO2, which exhibits a cubic fluorite structure like that of CeO2, shows an LO band activation at a resonance Raman condition.21 Thus, we concluded that the LO band activation detected here is due to the multiphonon
Taniguchi et al.
Figure 2. (a) XRD patterns of Ce1-xGdxO2-x/2 samples annealed at 1000 °C for 5 h. (b) Calculated lattice constants and grain sizes.
relaxation by the resonance Raman effect. Apart from these bands, the broad band around 600 cm-1 was also activated. The origin of that band is discussed in the ensuing sections. Subsequently, Gd3+-defect contributions to the resonance Raman spectra were studied. Substitution of every two Ce4+ ions with two Gd3+ ions produces an oxygen vacancy to balance the charge (eq 1). Therefore, oxygen vacancies are introduced in the CeO2 lattice by doping with Gd3+ cations: CeO2
′ + VO•• + 3OOx Gd2O3 98 + 2GdCe
(1)
Figure 2a shows the X-ray diffraction (XRD) patterns of a series of Ce1-xGdxO2-x/2 samples (x ) 0, 0.05, 0.1, 0.15, and 0.2) calcined at 1000 °C. All patterns correspond to the cubic CeO2 phase without any reflections from impurity phases, and they exhibit well-defined reflection peaks due to the high crystalline nature after high-temperature calcination. The lattice constant progressively increased with increasing Gd3+ concentrations (Figure 2b). An increase in the unit cell volume indicated that the Ce4+ sites were partially substituted by Gd3+ to form a solid solution, given that the ionic radius of Gd3+ (1.08 Å)22 is larger than that of Ce4+ in an octahedral environment (0.97 Å). The slope of the fitted lines (8.4 × 10-2), which is comparable with the reported values, indicates that Gd3+ concentrations were well-controlled.23 Simultaneously, the grain size decreased from 84 to 59 nm with an increase of Gd3+ concentration (Figure 2b), while the Gd3+ doping gave no noticeable shift in the optical absorption edge of ceria (see Figure S1, Supporting Information). The latter result confirms that UV Raman spectroscopy was performed at the resonant condition for Gd3+-doped ceria. Figure 3a shows the UV Raman spectra of 0-20 mol % Gd3+doped ceria samples annealed at 1000 °C. As observed in this figure, Gd3+ doping remarkably altered the shape of the UV Raman spectra, where the F2g and 2 LO bands got progressively broader, and the disorder band around 600 cm-1 (D band)24-26 became pronounced with the increase in the dopant concentration. Note that similar trends are seen in the vis Raman spectra (Figure S2, Supporting Information); however, they are significanly weaker. This indicates that the resonance Raman effect provides a better sensitivity for investigating the defect structure relating to Gd3+ doping. Figure 3b shows plots of ID/IF2g and I2 LO/IF2g versus the x value of Ce1-xGdxO2-x/2, where IF2g, ID, and I2 LO correspond to the maximum intensity of F2g, D, and 2 LO bands, respectively. This figure reveals that the relative intensity of the D band to F2g band systematically increases with an
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Figure 4. (a) Normalized UV Raman spectra of Ce1-xGdxO2-x/2 samples annealed at 1000 °C with F2g band intensity. (b) Defect complexes corresponding to D1 and D2 bands.
Figure 3. (a) As-measured UV Raman spectra of Ce1-xGdxO2-x/2 samples annealed at 1000 °C. (b) Plots of ID/IF2g and I2 LO/IF2g with an x value of Ce1-xGdxO2-x/2.
increase in the Gd3+-doping concentration, whereas the relative intensity of 2 LO is comparable among the samples. This indicates that the D band is more defect-sensitive in comparison with the 2 LO band. Thus, we next focused on the D band to investigate the defect contributions to UV Raman spectra. Figure 4a displays the disorder band normalized by F2g band intensity. The D band is split into two bands, D1 and D2, which are centered on ca. 550 and 600 cm-1, respectively. The relative band intensity from the D1 band to D2 band increased with increasing Gd3+ concentration. In addition, the D1 peak position shifts slightly to a higher-energy level with increasing Gd3+ concentration. The disorder bands are usefull to invesigate defect association formed to decrease the total lattice energy in ceria-based materials. Defect contributions to vis Raman spectra for Y2O3-, La2O3-, and ZrO2-doped CeO2 have been studied extensively by Nakajima et al.27 Following their experimental/theoretical work, the D2 band is assigned to defect spaces with Oh symmetry that include a dopant cation in 8-fold coordination of O2- but does not contain any O2- vacancy, GdO8-type complex. The D1 band at ca. 560 cm-1 is attributed to defect spaces that include an O2- vacancy with symmetry different from that of the Oh point group, (2Gd′Ce:V••O)x, and (Gd′Ce:V••O)• complexes (see Figure 4b). Furthermore, the high-energy shift of the D1 band position with increased Gd3+ concentration was attributed to the increase in the number of (2Gd′Ce:V••O)x complexes compared ′ :VO••)• complex. Their study also suggested to that of the(GdCe
Figure 5. UV Raman spectra of (a) undoped and (b) 20 mol % Gd3+doped ceria samples as-prepared by the hydrothermal method at 200 °C and calcined at 600 and 1000 °C. Average grain size calculated from XRD patterns and hydrothermal or annealing temperatures are inserted in the figures.
that the ID1/ID2 ratio represents the relative concentration between the two types of defect complexes; namely, an increase in the ratio implies an increase in concentration of the O2- vacancyassociated defects in comparison with that of the GdO8-type complex. Local structures in ceria-based nanocrystals were investigated using defect-sensitive resonance Raman spectroscopy. Figure 5 shows the UV Raman spectra of undoped and 20 mol % Gd3+doped ceria samples as-prepared by the hydrothermal method at 200 °C and calcined at 600 and 1000 °C (the grain sizes determined from the XRD patterns are inserted in the figures). In the spectra of undoped ceria samples, development of the disorder bands and broadening of the F2g band were clearly observed with the decrease in the crystal size from 82.2 to 5.2 nm. In contrast, the overall band shapes are rather similar among
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the 20 mol % Gd3+-doped samples before and after the annealing treatment at 1000 °C; the crystal size of both the undoped and the Gd3+-doped samples increased by a factor of around 20 as a result of the annealing. These trends indicate that the grain size had weak influence on the spectra. Indeed, the nanocrystals as-prepared by the oleate precipitation-based method, were free of the surface amorphous layer and were nearly monodisperse.17 Therefore, the contributions from amorphous phase and crystalsize inhomogeneities are not significant. Hence, local disorders in the lattice seem to have an influence on the size-dependent spectral feature of undoped ceria nanocrystals. Most likely, the deficiency of Ce3+ species produced the size dependency and has been assumed to be responsible for lattice expansion and blue shift of the band gap energy in ceria nanocrystals.28,29 UV Raman spectroscopy could resolve the Ce3+ location. The D2 band is predominantly detectable within the two defect bands in the spectra of undoped ceria crystals. Such a low ID1/ID2 ratio can be interpreted to mean that Ce3+ ions preferably locate in a MO8-type (here, M ) Ce3+) complex in the ceria nanocrystals, assuming that the Ce3+-based deficiencies produce D1 and D2 bands corresponding to the local defect structures. Indeed, UV Raman spectroscopy is potentially useful for detecting Ce3+ concentration in ceria nanocrystals. If the intensity ratio of IF2g/ ID depends on the concentration of trivalent rare earth ions in ceria, the Ce3+ concentration in the total Ce ions could be indirectly estimated to be around 5-10 mol % for 5.2 nm undoped ceria crystals. The estimation is on the basis of the intermediate IF2g/ID ratio between 5 and 10 mol % for Gd3+doped ceria samples annealed at 1000 °C (Figure 4a). This range, in fact, accurately reproduces Ce3+ concentrations for ceria nanocrystals of similar sizes, as reported in other studies. For example, Ce3+ concentrations were estimated to be 6.5% and 9.4% for ca. 6 nm undoped ceria nanocrystals using XAFS (X-ray absorption fine structure) analysis30 and the point defect model based on the size-dependent lattice expansion,31 respectively. Thus, UV Raman spectroscopy is potentially useful for detecting Ce3+ concentration in ceria nanocrystals. Note that the much higher Ce3+ concentrations in ceria nanocrystals are suggested by XPS (X-ray photoelectron spectroscopy)32 and EELS (electron energy loss spectroscopy)33 studies. For example, EELS analysis combined with TEM shows that ca. 6 nm ceria nanocrystals contain a 50% Ce3+ concentration.33 However, these methods might overestimate the Ce3+ concentration due to the reduction of Ce4+ to Ce3+ during measurements preformed under high-vacuum (UHV) conditions. According to the detailed XPS study,30 Ce3+/(Ce4+ + Ce3+) values in ceria microcrystals were determined to be 11.6% before and 29.3% after keeping the sample in a UVH chamber (-5 × 10-9 Torr) overnight. Finally, the defect structures in Gd3+-doped ceria nanocrystals will be discussed. As mentioned previously, ID/IF2g ratios observed in the UV Raman spectra of Gd3+-doped samples show weak dependency on the grain size. In contrast and of interest, it was found that the ID1/ID2 ratio noticeably increased with increase in the crystal size (Figure 5b). Following the Gd3+doping study, the trend of the ID1/ID2 ratio to increase with an increase of heating temperature can be interpreted to mean that the number of GdO8-type complexes decrease compared with that of the vacancy-associated defect complexes during the annealing process. So far, we have assumed that this observation relates to the Gd3+ location, depending on the history of the synthetic procedure. The possible mechanism is described as follows. In the nanoparticles as-prepared hydrothermally, Gd3+ ions were distributed homogeneously on an atomic scale, owing
Taniguchi et al. to the coprecipitation reaction and low reaction temperature (200 °C). In this case, the Gd3+ ion pair necessary for the formation ′ :VO••)x complex was scarcely present. Therefore, the of a (2GdCe GdO8-type complex should be dominant, rather than the (2Gd′Ce: VO••)x complex, in the nanoparticles. However, such a Gd3+ distribution was metastable, and Gd3+ ions (as well as oxygen ions) migrated to the stable positions with subsequent annealing due to the increasing rate of diffusion. Namely, the treatment led to the formation of the thermodynamically more stable (2Gd′Ce:V••O)x complex and simultaneously decreased concentration of the GdO8-type complex. Note that an XAFS analysis ′ :VO••)x complex is a dominant has also suggested that the (2GdCe 3+ defect complex in 20 mol % Gd -doped CeO2 bulk material.34 The metastable Gd3+ location suggested by UV Raman spectroscopy may be significant for understanding the electrical properties of nanocrystalline ceria-based electrolytes. A theoreti′ : cal study has suggested that the oxygen vacancy in the (2GdCe V••O)× complex is relatively hard to dissociate, as binding energy of the complex is high among defect complexes in Gd3+-doped CeO2.35 In addition, nanostructured materials are potentially metastable due to the low temperature and short duration processes used to suppress excessive grain growth. Therefore, we suggest that the low concentration of the stable defect complex plays a central role in enhancing the ionic transport properties of ceria-based nanocrystalline electrolytes with a high Re3+-dopant concentration. 4. Conclusion We have demonstrated for the first time that the UV resonance Raman spectroscopy using 363.8 nm excitation is very effective for the investigation of ceria-based materials. The UV resonance effect activates the overtone LO series due to the multiphonon relaxation in the undoped ceria sample annealed at 1000 °C. A Gd3+-doping study showed that the effect also greatly activates the defect-induced bands, providing higher sensitivity in identifying the local structures. The crystal size dependency indicated that Ce3+ and Gd3+ ions metastably form the MO8-type complex in ceria nanoparticles of atomic scale origin, which can impact the physical and chemical properties of ceria-based nanomaterials. Acknowledgment. Dr. Y. V. Kolen’ko (Max Planck Institute) is thanked for the fruitful discussions. Supporting Information Available: Absorption spectra, vis Raman spectra, and XRD patterns. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Steele, B. C. H. Solid State Ionics 2000, 129, 95. (2) Anselmi-Tamburini, U.; Maglia, F.; Chiodelli, G.; Tacca, A.; Spinolo, G.; Riello, P.; Bucella, S.; Munir, Z. A. AdV. Funct. Mater. 2006, 16, 2363. (3) Huang, H.; Gur, T. M.; Saito, Y.; Prinz, F. Appl. Phys. Lett. 2006, 89, 143107. (4) Laberty-Robert, C.; Long, J. W.; Pettigrew, K. A.; Stroud, R. M.; Rolison, D. R. AdV. Mater. 2007, 19, 1734. (5) Bellino, M. G.; Lamas, D. G.; de Reca, N. E. W. AdV. Funct. Mater. 2006, 16, 107. (6) Suzuki, T.; Kosacki, I.; Anderson, H. U. Solid State Ionics 2002, 151, 111. (7) Kosacki, I.; Suzuki, T.; Anderson, H. U.; Colomban, P. Solid State Ionics 2002, 149, 99. (8) Spanier, J. E.; Robinson, R. D.; Zheng, F.; Chan, S. W.; Herman, I. P. Phys. ReV. B 2001, 64, 245407. (9) Patil, S.; Seal, S.; Guo, Y.; Schulte, A.; Norwood, J. Appl. Phys. Lett. 2006, 88, 243110. (10) Zhang, J.; Li, M. J.; Feng, Z. C.; Chen, J.; Li, C. J. Phys. Chem. B 2006, 110, 927.
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