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Electronic conductivity of pure ceria
Solid State Ionics 192 (2011) 476–479
Contents lists available at ScienceDirect
Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
Electronic conductivity of pure ceria Yue-Ping Xiong a,1, Haruo Kishimoto a,⁎, Katsuhiko Yamaji a, Masashi Yoshinaga a, Teruhisa Horita a, Manuel E. Brito a, Harumi Yokokawa a,b a b
National Institute of Advanced Industrial Science and Technology (AIST), AIST Central No. 5, Higashi 1-1-1, Tsukuba, Ibaraki, 305-8565 Japan Advanced Research Laboratories, Tokyo City University, Todoroki 8-15-1, Setagaya-ku, Tokyo, 158-0082 Japan
a r t i c l e
i n f o
Article history: Received 31 August 2009 Received in revised form 28 June 2010 Accepted 22 July 2010 Available online 19 August 2010 Keywords: Ceria Electronic conductivity Impurity Ion blocking cell SOFC
a b s t r a c t The electronic conductivity of pure ceria with two different impurity levels is examined by dc polarization technique based on the Hebb–Wagner ion blocking method. The impurity level for the ceria with 99.999% purity (5N-CeO2) is about 1/100 of that with 99.9% purity (3N-CeO2) as confirmed by the fluorescence intensity of impurities obtained by Raman spectroscopy. The electronic conductivity for the 5N-CeO2 was measured at T = 973 K to 1173 K, and the results are essentially the same as those for the 3N-CeO2. The electronic conductivity increases with decreasing of P(O2) following slope values of − 1/4 to − 1/6. The − 1/4 dependent region becomes narrower for the 5N-CeO2 than that for the 3N-CeO2. For both types of ceria, the P(O2) independent region appears in the same region of higher than 10− 2 and 10− 3 MPa at T = 1073 K and 973 K, respectively. Activation energies for the 5N-CeO2 were 2.2 eV, 2.6 eV and 1.9 eV in P(O2) dependent regions of − 1/6, − 1/4 and 0, respectively. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Rare-earth oxide doped ceria (RDC) shows outstanding properties compared with yttria stabilized zirconia (YSZ), which is the conventional oxide ion conductor used as electrolyte material for solid oxide fuel cells (SOFCs). RDC shows one-order of magnitude higher oxide ion conductivity [1], higher electronic conductivity under reducing condition [2–5], higher hole conductivity under oxidation condition [2–5], and higher proton solubility [6,7]. These properties are deeply related to lattice defects, such as oxygen vacancy produced by dopant cations or other types of impurities. To fully understand the properties of ceria base oxides, it is important to clarify the properties of highly pure ceria, e.g., ceria with lower density of lattice defects. In this way, it is possible to classify how impurities do affect the intrinsic properties of ceria. Several reports about impurity effect discuss the total electric conductivity of pure ceria, including both the ionic conductivity and the electronic conductivity [8,9]. Zhou et al. reported differences in the P(O2) dependence of total conductivity for ceria with 99.5% and 99.995% purity [8]. Their log σ − log P(O2) plot, for reducing condition, shows slopes following the −1/4 and −1/6 dependence for ceria with
⁎ Corresponding author. AIST Tsukuba Central 5, Higashi 1-1-1, Tsukuba, 305-8565 Japan. Tel.: +81 29 861 4542; fax: +81 29 861 4540. E-mail address: [email protected] (H. Kishimoto). 1 Present address: School of Chemical Engineering & Technology, Harbin Institute of Technology, Harbin 150001, 411 PR China. 0167-2738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2010.07.017
99.5% and 99.995% purity, respectively. A P(O2) independent region (slope 0, hereinafter) was observed only for the 99.5% purity sample. Panhans and Blumenthal showed the impurity effect of 3N (99.9% purity) and 4N (99.99% purity) of ceria samples, and they also reported deviations from the expected oxygen potential dependence for the electronic conductivity [9]. The phenomenon is not trivial, and to fully understand the overall electric conduction mechanism for pure ceria, it is required to analyze the electronic conductivity independently, avoiding the ionic conduction effect. Our group has precisely measured, and duly reported, the electronic conductivities for ceria base oxides by dc polarization technique based on the Hebb–Wagner ion blocking method [2–5]. Recently, we have found an interesting behavior pertaining to electronic conduction for ceria with a 99.9% purity [10]. Two interesting points arise: One is that electronic conductivity is independent of oxygen potential in a high P(O2) and low temperature region, i.e. in the P(O2) higher than 10− 4 MPa at 973 K region. The other one is that the P(O2) dependence of electronic conductivity changes gradually under reducing condition. It is still unclear if this electronic conduction behavior is intrinsic to pure ceria or if it is still related to impurities in low concentration, namely, an extrinsic (impurity) effect. To further study the phenomenon, electronic conductivity for high purity ceria (99.999% purity) has been precisely measured by using a modified Hebb–Wagner ion blocking cell. Results are compared with those for the ceria with 99.9% purity. We were able to clearly distinguish between the impurity effect and the intrinsic electronic conductivity of pure ceria. The relationship between electronic conductivity and the nature of the defect in the lattice is also briefly discussed.
Y.-P. Xiong et al. / Solid State Ionics 192 (2011) 476–479
2. Experimental Ceria (CeO2) powders with 99.9% purity (3N-CeO2, Wako Pure Chemicals Industries, Ltd.) and 99.999% purity (5N-CeO2, Acros Organics) were used as starting material. These powders were used in the “asreceived” condition to avoid contamination by handling and milling. Powders were pressed into disks by uni-axial press and later by cold isostatic press (CIP) under 390 MPa. The disks were sintered at 1873 K for 20 h in air in the closed platinum (Pt) crucible to avoid contamination during sintering. Surfaces of sintered disks were polished with diamond suspension (about 1 μm). Samples were identified by X-ray diffractometry (XRD, Cu–Kα, 40 kV–40 mA, RINT-UltimaIII, Rigaku Co., Japan), scanning electron microscopy (SEM, VE-7800, Keyence Co., Japan) and Raman spectroscopy (NRS-3100, JASCO Co., Japan). Platinum paste and Pt mesh were attached to the polished surface as electrodes for conductivity measurements. The electronic conductivity was measured by a dc polarization method using a modified Hebb–Wagner ion blocking cell. Detailed cell configuration and operation have been reported elsewhere [2–5]. The electronic conductivity was obtained by dc polarization between one closed compartment, where oxygen was indirectly blocked by an alumina spacer and glass seals, and a reversible electrode located in the opposite side of the sample disk. On this side, the oxygen partial pressure is controlled at 10− 3 MPa by flowing a mixture of 1% O2 and balanced Ar gas. The dc polarization was applied by a potentiostat (1285, Solartron, England). When the voltage, Eapp, was applied to the samples, the current changed to approach a steady state. The electronic conductivity, σe, was determined from the steady current, Ie, by using the following equation:
σe =
L ∂Ie A ∂Eapp
! ð1Þ
477
where A and L are the electrode area and the thickness of the sample disk, respectively. The oxygen partial pressure at the blocking electrode, P(O2), was calculated by using the following equation: Eapp =
RT Prev ðO2 Þ ln 4F P ðO2 Þ
ð2Þ
where R, T, F and Prev(O2) are the gas constant, the sample temperature, the Faraday constant and the oxygen partial pressure around the reversible electrode, respectively. In the present case, the oxygen vacancy concentration is extremely small in pure ceria, so that the oxide ion diffusion is extremely limited especially in the oxidative side. It took a long time (more than 20 h in most cases) to reach the steady state. It is necessary that oxide ion flows through the sample disk to reach the equilibrium oxygen potential that corresponds to the applied voltage, which is established after a necessary amount of oxygen is transported through the sample disk as oxide ions and holes. The electronic conductivity was evaluated as a function of temperature, T, and oxygen partial pressure, P(O2), using Eqs. (1) and (2) in the analysis. 3. Results and discussion From the XRD analysis, both the 5N-CeO2 and the 3N-CeO2 were identified as cubic CaF2 type single phase with well crystallized particles. From the SEM images, particle sizes of the 5N-CeO2 and the 3N-CeO2 were determined to be about 500 nm and 20 μm, respectively. Fig. 1 shows the Raman spectra for the 5N-CeO2 and the 3N-CeO2. The intensity is normalized for the main peak at 470 cm− 1. The observed spectra with a 532 nm excitation laser (Fig. 1(a-1)) were the same for both samples: A sharp and very intense peak was observed at 470 cm− 1, the typical one (the F2g mode) for CeO2 [11]. In the range of 900–
Fig. 1. Raman spectra for the 5N-CeO2 and the 3N-CeO2. (a) Exited by 532 nm (green) laser and (b) exited by 785 nm (red) laser. The intensity is normalized for the main peak at 470 cm− 1 between the 5N-CeO2 and the 3N-CeO2. Note: In (b-2), intensity of the 5N-CeO2 is ×100.
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1900 cm− 1 (Fig. 2(a-2)), a small peak was observed at 1177 cm− 1. When excitation laser for Raman spectroscopy was changed from 532 nm (green) to 785 nm (red), Raman spectra (Fig. 1(b1–b2)) were essentially the same as those obtained with 532 nm excitation. A small peak was also observed at 1177 cm− 1 with a similar feature although intensity was much smaller. This peak is identified as Raman scattering for CeO2. In addition, especially in the range of 1000–1900 cm− 1, a number of small and sharp peaks were observed only for the spectrum obtained with 785 nm excitation. These peaks were common for both the 5N-CeO2 and the 3N-CeO2. These small and sharp additional peaks (observed only with 785 nm excitation laser) are not due to Raman scattering but, probably due to fluorescence from remnant impurities in the samples. Considering the purification process for CeO2 powder, impurity elements, which could cause fluorescence, are lanthanide ions, such as Eu3+. The relative intensity of these fluorescence lines for the 5N-CeO2 was about 1/100 compared with those for the 3N-CeO2. It is plausible to conclude that the concentration of those impurity elements which promote fluorescence for the 5N-CeO2 is about 1/100 lower than that for the 3N-CeO2. This result is consistent with the difference in nominal impurity level, reported by the powder suppliers. The electronic conductivity observed for the 5N-CeO2 is compared with that for the 3N-CeO2 [10] in Fig. 2. P(O2) dependence of electronic conductivity is essentially the same for the 5N-CeO2 and the 3N-CeO2. Two regions can be identified in Fig. 2. One is the P(O2)
dependent region where electronic conductivity decreases with increasing of P(O2) and the other is the P(O2) independent region which appears in a higher P(O2) region at a temperature lower than 1073 K. For the 5N-CeO2, σe decreased with increasing P(O2) following a slope of −1/6 at T = 1173 K. As for the 3N-CeO2, the dependence changed gradually from −1/6 slope to − 1/4 slope with increasing of P(O2) at the same temperature. With decreasing temperature, the slope for the 5N-CeO2 gradually changed from −1/6 to −1/4 with increasing of P(O2); basically, this is the same behavior as the 3N-CeO2. As seen in Fig. 2(b), however, the −1/4 dominant region for the 5N-CeO2 is narrower than that for the 3NCeO2. This region is difficult to distinguish from the −1/6 dependent region. In this region, carrier electrons for pure CeO2 are formed by the following nonstoichiometric reaction: x x ′ + 3OOx + VO + 1 O2 2CeCe + 4OO = 2CeCe
ð3Þ
2
and the conduction mechanism for electron is reported as small polaron hopping [12,13]. Carrier electron concentration, [Ce′Ce], is calculated by the following equation: 1 1 ′ = 2 VO = 2KV 3 P ðO2 Þ−6 CeCe
ð4Þ
O
where KVO is the equilibrium constant for oxygen vacancy formation, as described in Eq. (3), and ½VO is the concentration of oxygen vacancy. When mobility of hopping electron is assumed to be a constant, the −1 electronic conductivity changes with P ðO2 Þ 6 . On the other hand, when ½V O formed by impurity cations, such as di- and/or tri-valent lanthanide ions (extrinsic ½VO ), is much larger than that formed by reaction (3), ½VO can be assumed to be constant. In this case, the carrier concentration is determined by the following equation: 1 −1 2 ′ = KV = V CeCe P ðO2 Þ 4 : O O
ð5Þ
In other words, electronic conductivity changes with P ðO2 Þ−4 . These equations suggest that the P(O2) dependence changes from −1/6 to −1/4 around the P(O2) where VO formed by impurity cations (extrinsic VO ), is equal to that formed by reaction (4) (intrinsic VO ). Moreover, it also suggests that the critical P(O2) value, where dominant P(O2) dependence changes between −1/4 and − 1/6, becomes lower with increasing of the extrinsic V O . In the present results, the critical P(O2) for the 5N-CeO2 is lower than that for the 3NCeO2. From the Raman spectra, the impurity level for the 5N-CeO2 is certainly lower than that for the 3N-CeO2. The slight, but nevertheless, clear difference in electronic conductivity of the nonstoichiometric region between the 5N-CeO2 and the 3N-CeO2 is related to the extrinsic oxygen vacancy concentration, which, as mentioned before, is produced by di- and/or tri-valent lanthanide ion impurities in the lattice. These critical P(O2) values should correspond to the oxygen potentials at which the oxygen vacancy concentration becomes 10− 3 and 10− 5 mol per 1 mol of CeO2. Furthermore, the result is consistent with the thermodynamic study of nonstoichiometric CeO2 by Panlener et al. [14]. The Arrhenius plot at P(O2) = 10− 7 MPa is shown in Fig. 3(a). Electronic conductivity for the − 1/6 dependent region and the −1/4 dependent region was obtained by linear extrapolation of data for each region. The apparent activation energy of each region for the 5NCeO2 is calculated as 2.2 eV and 2.6 eV for the −1/6 and the −1/4 dependent region, respectively. These values were about the same as for the 3N-CeO2 [10], although the difference in activation energy for −1/4 dependent region is rather large. The explanation for this is simple. There are only two data points owing to that the identification of the − 1/4 dependent region is difficult for the 5N-CeO2. Nevertheless, it is clear that the activation energy, associated to electron hopping associated to reaction (3) [8,10], cannot be directly related to the impurity level. 1
Fig. 2. P(O2) dependence of the electronic conductivity for the 5N-CeO2 compared with that for the 3N-CeO2. (a) In the P(O2) range of 10 to 10− 26 MPa and in the σ range of 1 to 10− 7 Scm− 1, and (b) expanded in the P(O2) range of 1 to 10− 12 MPa and in the σ range of 10− 2 to 10− 5 Scm− 1.
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In this reaction, gaseous oxygen is not involved, and, therefore, the electronic conductivity shows no dependence on the P(O2) when mobility of the carrier is constant. The activation energy of this region for the 5N-CeO2 is the same as that for the 3N-CeO2, as shown in the Arrhenius plot of Fig. 3(b). The activation energy for this region is described by the following relationship [10]: 0 E = Eg = 2 + Eμ
ð7Þ
where Eg0 is activation energy for reaction (6), and it corresponds to the band gap energy for CeO2. The band gap energy for present pure CeO2 is calculated as 3.2 eV when activation energy for mobility is assumed to be 0.3 eV [9,12,13]. This value corresponds well to the literature reported band gap energy for pure ceria [16,17]. Summarizing, we conclude that the P(O2) independent region of electronic conductivity observed for pure ceria corresponds to its intrinsic behavior and it can be explained in similar form to the “autoionization” mechanism, described by reaction (6). 4. Conclusion The electronic conductivity of the pure ceria with 99.999% purity (5N-CeO2) was measured by using of a modified Hebb–Wagner ion blocking cell. The result is compared with that for the CeO2 with 99.9% purity (3N-CeO2). Fluorescence induced by impurities was useful to confirm the impurity level, which for the 5N-CeO2 was two orders of magnitude lower than that for the 3N-CeO2. The electronic conductivity for the 5N-CeO2 showed essentially two regions in the log σe − log P(O2) plot. One is the region where σe decreases with increasing of P(O2). Here, the P(O2) dependence changed gradually from −1/6 to −1/4. The −1/4 dependent region became narrow with the decrease of the extrinsic oxygen vacancy concentration. The other region is the P(O2) independent one, and it is not related to the impurity level. This behavior is explained by an intrinsic reaction similar to the “autoionization” mechanism. Acknowledgement A part of this study was financially supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References
Fig. 3. Arrhenius plots of electronic conductivity for the 5N-CeO2 compared with those for the 3N-CeO2 (a) at P(O2) = 10− 7 MPa and (b) at P(O2) = 10− 2 MPa, respectively.
The P(O2) independent region for the 5N-CeO2 is observed for P(O2) higher than 10− 2 and 10− 3 MPa at T = 1073 K and 973 K, respectively. This result is the same as for the 3N-CeO2, although the electronic conductivity for the 5N-CeO2 is slightly higher. Comparison shows that the origin of this P(O2) independent region is not associated to extrinsic impurities. On the contrary, it must be considered as the intrinsic electronic conduction behavior for pure CeO2. In our previous report on the 3N-CeO2 [10], we proposed a carrier formation mechanism similar to the “auto-ionization” reaction described by Mimuro et al. [15]: ′ + OO + OO· CeCe + 2OO = CeCe x
x
x
ð6Þ
[1] H. Yahiro, Y. Eguchi, K. Eguchi, H. Arai, J. Appl. Electrochem. 18 (1988) 527. [2] Y. Xiong, K. Yamaji, T. Horita, N. Sakai, H. Yokokawa, J. Electrochem. Soc. 149 (2002) E450. [3] Y. Xiong, K. Yamaji, T. Horita, N. Sakai, H. Yokokawa, J. Electrochem. Soc. 151 (2004) A407. [4] Y. Xiong, K. Yamaji, N. Sakai, H. Kishimoto, T. Horita, M.E. Brito, H. Yokokawa, J. Electrochem. Soc. 153 (2006) A2198. [5] Y. Xiong, K. Yamaji, H. Kishimoto, M.E. Brito, T. Horita, H. Yokokawa, J. Electrochem. Soc. 155 (2008) B1300. [6] N. Sakai, K. Yamaji, T. Horita, H. Yokokawa, Y. Hirata, S. Sameshima, Y. Nigara, J. Mizusaki, Solid State Ionics 125 (1999) 325. [7] H. Yokokawa, T. Horita, N. Sakai, K. Yamaji, M.E. Brito, Y.-P. Xiong, H. Kishimoto, Solid State Ionics 174 (2004) 205. [8] X.-D. Zhou, W. Huebner, H.U. Anderson, Defect Diffus. Forum 277 (2005) 242–244. [9] M.A. Panhans, R.N. Blumenthal, Solid State Ionics 60 (1993) 279. [10] Y.-P. Xiong, H. Kishimoto, K. Yamaji, M. Yoshinaga, T. Horita, M.E. Brito, H. Yokokawa, Electrochem. Solid State Lett. 13 (2010) B21. [11] I. Kosacki, V. Petrovski, H.U. Anderson, J. Am. Ceram. Soc. 85 (2002) 2646. [12] I.K. Naik, T.Y. Tien, J. Phys. Chem. Solids 39 (1978) 311. [13] H.L. Tuller, A.S. Nowick, J. Phys. Chem. Solids 38 (1977) 859. [14] R.J. Panlener, R.N. Blumenthal, J.E. Garnier, J. Phys. Chem. Solids 36 (1975) 1213. [15] S. Mimuro, S. Shibako, Y. Oyama, K. Kobayashi, T. Higuchi, S. Shin, S. Yamaguchi, Solid State Ionics 178 (2007) 641. [16] D.D. Koelling, A.M. Boring, J.H. Wood, Solid State Commun. 47 (1983) 227. [17] E. Wuilloud, B. Delley, W.-D. Schneider, Y. Bear, Phys. Rev. Lett. 53 (1984) 202.
Contents lists available at ScienceDirect
Solid State Ionics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s s i
Electronic conductivity of pure ceria Yue-Ping Xiong a,1, Haruo Kishimoto a,⁎, Katsuhiko Yamaji a, Masashi Yoshinaga a, Teruhisa Horita a, Manuel E. Brito a, Harumi Yokokawa a,b a b
National Institute of Advanced Industrial Science and Technology (AIST), AIST Central No. 5, Higashi 1-1-1, Tsukuba, Ibaraki, 305-8565 Japan Advanced Research Laboratories, Tokyo City University, Todoroki 8-15-1, Setagaya-ku, Tokyo, 158-0082 Japan
a r t i c l e
i n f o
Article history: Received 31 August 2009 Received in revised form 28 June 2010 Accepted 22 July 2010 Available online 19 August 2010 Keywords: Ceria Electronic conductivity Impurity Ion blocking cell SOFC
a b s t r a c t The electronic conductivity of pure ceria with two different impurity levels is examined by dc polarization technique based on the Hebb–Wagner ion blocking method. The impurity level for the ceria with 99.999% purity (5N-CeO2) is about 1/100 of that with 99.9% purity (3N-CeO2) as confirmed by the fluorescence intensity of impurities obtained by Raman spectroscopy. The electronic conductivity for the 5N-CeO2 was measured at T = 973 K to 1173 K, and the results are essentially the same as those for the 3N-CeO2. The electronic conductivity increases with decreasing of P(O2) following slope values of − 1/4 to − 1/6. The − 1/4 dependent region becomes narrower for the 5N-CeO2 than that for the 3N-CeO2. For both types of ceria, the P(O2) independent region appears in the same region of higher than 10− 2 and 10− 3 MPa at T = 1073 K and 973 K, respectively. Activation energies for the 5N-CeO2 were 2.2 eV, 2.6 eV and 1.9 eV in P(O2) dependent regions of − 1/6, − 1/4 and 0, respectively. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Rare-earth oxide doped ceria (RDC) shows outstanding properties compared with yttria stabilized zirconia (YSZ), which is the conventional oxide ion conductor used as electrolyte material for solid oxide fuel cells (SOFCs). RDC shows one-order of magnitude higher oxide ion conductivity [1], higher electronic conductivity under reducing condition [2–5], higher hole conductivity under oxidation condition [2–5], and higher proton solubility [6,7]. These properties are deeply related to lattice defects, such as oxygen vacancy produced by dopant cations or other types of impurities. To fully understand the properties of ceria base oxides, it is important to clarify the properties of highly pure ceria, e.g., ceria with lower density of lattice defects. In this way, it is possible to classify how impurities do affect the intrinsic properties of ceria. Several reports about impurity effect discuss the total electric conductivity of pure ceria, including both the ionic conductivity and the electronic conductivity [8,9]. Zhou et al. reported differences in the P(O2) dependence of total conductivity for ceria with 99.5% and 99.995% purity [8]. Their log σ − log P(O2) plot, for reducing condition, shows slopes following the −1/4 and −1/6 dependence for ceria with
⁎ Corresponding author. AIST Tsukuba Central 5, Higashi 1-1-1, Tsukuba, 305-8565 Japan. Tel.: +81 29 861 4542; fax: +81 29 861 4540. E-mail address: [email protected] (H. Kishimoto). 1 Present address: School of Chemical Engineering & Technology, Harbin Institute of Technology, Harbin 150001, 411 PR China. 0167-2738/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2010.07.017
99.5% and 99.995% purity, respectively. A P(O2) independent region (slope 0, hereinafter) was observed only for the 99.5% purity sample. Panhans and Blumenthal showed the impurity effect of 3N (99.9% purity) and 4N (99.99% purity) of ceria samples, and they also reported deviations from the expected oxygen potential dependence for the electronic conductivity [9]. The phenomenon is not trivial, and to fully understand the overall electric conduction mechanism for pure ceria, it is required to analyze the electronic conductivity independently, avoiding the ionic conduction effect. Our group has precisely measured, and duly reported, the electronic conductivities for ceria base oxides by dc polarization technique based on the Hebb–Wagner ion blocking method [2–5]. Recently, we have found an interesting behavior pertaining to electronic conduction for ceria with a 99.9% purity [10]. Two interesting points arise: One is that electronic conductivity is independent of oxygen potential in a high P(O2) and low temperature region, i.e. in the P(O2) higher than 10− 4 MPa at 973 K region. The other one is that the P(O2) dependence of electronic conductivity changes gradually under reducing condition. It is still unclear if this electronic conduction behavior is intrinsic to pure ceria or if it is still related to impurities in low concentration, namely, an extrinsic (impurity) effect. To further study the phenomenon, electronic conductivity for high purity ceria (99.999% purity) has been precisely measured by using a modified Hebb–Wagner ion blocking cell. Results are compared with those for the ceria with 99.9% purity. We were able to clearly distinguish between the impurity effect and the intrinsic electronic conductivity of pure ceria. The relationship between electronic conductivity and the nature of the defect in the lattice is also briefly discussed.
Y.-P. Xiong et al. / Solid State Ionics 192 (2011) 476–479
2. Experimental Ceria (CeO2) powders with 99.9% purity (3N-CeO2, Wako Pure Chemicals Industries, Ltd.) and 99.999% purity (5N-CeO2, Acros Organics) were used as starting material. These powders were used in the “asreceived” condition to avoid contamination by handling and milling. Powders were pressed into disks by uni-axial press and later by cold isostatic press (CIP) under 390 MPa. The disks were sintered at 1873 K for 20 h in air in the closed platinum (Pt) crucible to avoid contamination during sintering. Surfaces of sintered disks were polished with diamond suspension (about 1 μm). Samples were identified by X-ray diffractometry (XRD, Cu–Kα, 40 kV–40 mA, RINT-UltimaIII, Rigaku Co., Japan), scanning electron microscopy (SEM, VE-7800, Keyence Co., Japan) and Raman spectroscopy (NRS-3100, JASCO Co., Japan). Platinum paste and Pt mesh were attached to the polished surface as electrodes for conductivity measurements. The electronic conductivity was measured by a dc polarization method using a modified Hebb–Wagner ion blocking cell. Detailed cell configuration and operation have been reported elsewhere [2–5]. The electronic conductivity was obtained by dc polarization between one closed compartment, where oxygen was indirectly blocked by an alumina spacer and glass seals, and a reversible electrode located in the opposite side of the sample disk. On this side, the oxygen partial pressure is controlled at 10− 3 MPa by flowing a mixture of 1% O2 and balanced Ar gas. The dc polarization was applied by a potentiostat (1285, Solartron, England). When the voltage, Eapp, was applied to the samples, the current changed to approach a steady state. The electronic conductivity, σe, was determined from the steady current, Ie, by using the following equation:
σe =
L ∂Ie A ∂Eapp
! ð1Þ
477
where A and L are the electrode area and the thickness of the sample disk, respectively. The oxygen partial pressure at the blocking electrode, P(O2), was calculated by using the following equation: Eapp =
RT Prev ðO2 Þ ln 4F P ðO2 Þ
ð2Þ
where R, T, F and Prev(O2) are the gas constant, the sample temperature, the Faraday constant and the oxygen partial pressure around the reversible electrode, respectively. In the present case, the oxygen vacancy concentration is extremely small in pure ceria, so that the oxide ion diffusion is extremely limited especially in the oxidative side. It took a long time (more than 20 h in most cases) to reach the steady state. It is necessary that oxide ion flows through the sample disk to reach the equilibrium oxygen potential that corresponds to the applied voltage, which is established after a necessary amount of oxygen is transported through the sample disk as oxide ions and holes. The electronic conductivity was evaluated as a function of temperature, T, and oxygen partial pressure, P(O2), using Eqs. (1) and (2) in the analysis. 3. Results and discussion From the XRD analysis, both the 5N-CeO2 and the 3N-CeO2 were identified as cubic CaF2 type single phase with well crystallized particles. From the SEM images, particle sizes of the 5N-CeO2 and the 3N-CeO2 were determined to be about 500 nm and 20 μm, respectively. Fig. 1 shows the Raman spectra for the 5N-CeO2 and the 3N-CeO2. The intensity is normalized for the main peak at 470 cm− 1. The observed spectra with a 532 nm excitation laser (Fig. 1(a-1)) were the same for both samples: A sharp and very intense peak was observed at 470 cm− 1, the typical one (the F2g mode) for CeO2 [11]. In the range of 900–
Fig. 1. Raman spectra for the 5N-CeO2 and the 3N-CeO2. (a) Exited by 532 nm (green) laser and (b) exited by 785 nm (red) laser. The intensity is normalized for the main peak at 470 cm− 1 between the 5N-CeO2 and the 3N-CeO2. Note: In (b-2), intensity of the 5N-CeO2 is ×100.
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1900 cm− 1 (Fig. 2(a-2)), a small peak was observed at 1177 cm− 1. When excitation laser for Raman spectroscopy was changed from 532 nm (green) to 785 nm (red), Raman spectra (Fig. 1(b1–b2)) were essentially the same as those obtained with 532 nm excitation. A small peak was also observed at 1177 cm− 1 with a similar feature although intensity was much smaller. This peak is identified as Raman scattering for CeO2. In addition, especially in the range of 1000–1900 cm− 1, a number of small and sharp peaks were observed only for the spectrum obtained with 785 nm excitation. These peaks were common for both the 5N-CeO2 and the 3N-CeO2. These small and sharp additional peaks (observed only with 785 nm excitation laser) are not due to Raman scattering but, probably due to fluorescence from remnant impurities in the samples. Considering the purification process for CeO2 powder, impurity elements, which could cause fluorescence, are lanthanide ions, such as Eu3+. The relative intensity of these fluorescence lines for the 5N-CeO2 was about 1/100 compared with those for the 3N-CeO2. It is plausible to conclude that the concentration of those impurity elements which promote fluorescence for the 5N-CeO2 is about 1/100 lower than that for the 3N-CeO2. This result is consistent with the difference in nominal impurity level, reported by the powder suppliers. The electronic conductivity observed for the 5N-CeO2 is compared with that for the 3N-CeO2 [10] in Fig. 2. P(O2) dependence of electronic conductivity is essentially the same for the 5N-CeO2 and the 3N-CeO2. Two regions can be identified in Fig. 2. One is the P(O2)
dependent region where electronic conductivity decreases with increasing of P(O2) and the other is the P(O2) independent region which appears in a higher P(O2) region at a temperature lower than 1073 K. For the 5N-CeO2, σe decreased with increasing P(O2) following a slope of −1/6 at T = 1173 K. As for the 3N-CeO2, the dependence changed gradually from −1/6 slope to − 1/4 slope with increasing of P(O2) at the same temperature. With decreasing temperature, the slope for the 5N-CeO2 gradually changed from −1/6 to −1/4 with increasing of P(O2); basically, this is the same behavior as the 3N-CeO2. As seen in Fig. 2(b), however, the −1/4 dominant region for the 5N-CeO2 is narrower than that for the 3NCeO2. This region is difficult to distinguish from the −1/6 dependent region. In this region, carrier electrons for pure CeO2 are formed by the following nonstoichiometric reaction: x x ′ + 3OOx + VO + 1 O2 2CeCe + 4OO = 2CeCe
ð3Þ
2
and the conduction mechanism for electron is reported as small polaron hopping [12,13]. Carrier electron concentration, [Ce′Ce], is calculated by the following equation: 1 1 ′ = 2 VO = 2KV 3 P ðO2 Þ−6 CeCe
ð4Þ
O
where KVO is the equilibrium constant for oxygen vacancy formation, as described in Eq. (3), and ½VO is the concentration of oxygen vacancy. When mobility of hopping electron is assumed to be a constant, the −1 electronic conductivity changes with P ðO2 Þ 6 . On the other hand, when ½V O formed by impurity cations, such as di- and/or tri-valent lanthanide ions (extrinsic ½VO ), is much larger than that formed by reaction (3), ½VO can be assumed to be constant. In this case, the carrier concentration is determined by the following equation: 1 −1 2 ′ = KV = V CeCe P ðO2 Þ 4 : O O
ð5Þ
In other words, electronic conductivity changes with P ðO2 Þ−4 . These equations suggest that the P(O2) dependence changes from −1/6 to −1/4 around the P(O2) where VO formed by impurity cations (extrinsic VO ), is equal to that formed by reaction (4) (intrinsic VO ). Moreover, it also suggests that the critical P(O2) value, where dominant P(O2) dependence changes between −1/4 and − 1/6, becomes lower with increasing of the extrinsic V O . In the present results, the critical P(O2) for the 5N-CeO2 is lower than that for the 3NCeO2. From the Raman spectra, the impurity level for the 5N-CeO2 is certainly lower than that for the 3N-CeO2. The slight, but nevertheless, clear difference in electronic conductivity of the nonstoichiometric region between the 5N-CeO2 and the 3N-CeO2 is related to the extrinsic oxygen vacancy concentration, which, as mentioned before, is produced by di- and/or tri-valent lanthanide ion impurities in the lattice. These critical P(O2) values should correspond to the oxygen potentials at which the oxygen vacancy concentration becomes 10− 3 and 10− 5 mol per 1 mol of CeO2. Furthermore, the result is consistent with the thermodynamic study of nonstoichiometric CeO2 by Panlener et al. [14]. The Arrhenius plot at P(O2) = 10− 7 MPa is shown in Fig. 3(a). Electronic conductivity for the − 1/6 dependent region and the −1/4 dependent region was obtained by linear extrapolation of data for each region. The apparent activation energy of each region for the 5NCeO2 is calculated as 2.2 eV and 2.6 eV for the −1/6 and the −1/4 dependent region, respectively. These values were about the same as for the 3N-CeO2 [10], although the difference in activation energy for −1/4 dependent region is rather large. The explanation for this is simple. There are only two data points owing to that the identification of the − 1/4 dependent region is difficult for the 5N-CeO2. Nevertheless, it is clear that the activation energy, associated to electron hopping associated to reaction (3) [8,10], cannot be directly related to the impurity level. 1
Fig. 2. P(O2) dependence of the electronic conductivity for the 5N-CeO2 compared with that for the 3N-CeO2. (a) In the P(O2) range of 10 to 10− 26 MPa and in the σ range of 1 to 10− 7 Scm− 1, and (b) expanded in the P(O2) range of 1 to 10− 12 MPa and in the σ range of 10− 2 to 10− 5 Scm− 1.
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In this reaction, gaseous oxygen is not involved, and, therefore, the electronic conductivity shows no dependence on the P(O2) when mobility of the carrier is constant. The activation energy of this region for the 5N-CeO2 is the same as that for the 3N-CeO2, as shown in the Arrhenius plot of Fig. 3(b). The activation energy for this region is described by the following relationship [10]: 0 E = Eg = 2 + Eμ
ð7Þ
where Eg0 is activation energy for reaction (6), and it corresponds to the band gap energy for CeO2. The band gap energy for present pure CeO2 is calculated as 3.2 eV when activation energy for mobility is assumed to be 0.3 eV [9,12,13]. This value corresponds well to the literature reported band gap energy for pure ceria [16,17]. Summarizing, we conclude that the P(O2) independent region of electronic conductivity observed for pure ceria corresponds to its intrinsic behavior and it can be explained in similar form to the “autoionization” mechanism, described by reaction (6). 4. Conclusion The electronic conductivity of the pure ceria with 99.999% purity (5N-CeO2) was measured by using of a modified Hebb–Wagner ion blocking cell. The result is compared with that for the CeO2 with 99.9% purity (3N-CeO2). Fluorescence induced by impurities was useful to confirm the impurity level, which for the 5N-CeO2 was two orders of magnitude lower than that for the 3N-CeO2. The electronic conductivity for the 5N-CeO2 showed essentially two regions in the log σe − log P(O2) plot. One is the region where σe decreases with increasing of P(O2). Here, the P(O2) dependence changed gradually from −1/6 to −1/4. The −1/4 dependent region became narrow with the decrease of the extrinsic oxygen vacancy concentration. The other region is the P(O2) independent one, and it is not related to the impurity level. This behavior is explained by an intrinsic reaction similar to the “autoionization” mechanism. Acknowledgement A part of this study was financially supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References
Fig. 3. Arrhenius plots of electronic conductivity for the 5N-CeO2 compared with those for the 3N-CeO2 (a) at P(O2) = 10− 7 MPa and (b) at P(O2) = 10− 2 MPa, respectively.
The P(O2) independent region for the 5N-CeO2 is observed for P(O2) higher than 10− 2 and 10− 3 MPa at T = 1073 K and 973 K, respectively. This result is the same as for the 3N-CeO2, although the electronic conductivity for the 5N-CeO2 is slightly higher. Comparison shows that the origin of this P(O2) independent region is not associated to extrinsic impurities. On the contrary, it must be considered as the intrinsic electronic conduction behavior for pure CeO2. In our previous report on the 3N-CeO2 [10], we proposed a carrier formation mechanism similar to the “auto-ionization” reaction described by Mimuro et al. [15]: ′ + OO + OO· CeCe + 2OO = CeCe x
x
x
ð6Þ
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