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X-ray photoelectron spectroscopy investigation on chemical states of oxygen on surfaces of mixed electronic–ionic conducting La0.6Sr0.4Co1−yFeyO3 ceramics
Applied Surface Science 228 (2004) 110–114
X-ray photoelectron spectroscopy investigation on chemical states of oxygen on surfaces of mixed electronic–ionic conducting La0.6Sr0.4Co1yFeyO3 ceramics Qing Xu*, Duan-ping Huang, Wen Chen, Hao Wang, Bi-tao Wang, Run-zhang Yuan State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, School of Materials Science and Engineering, Wuhan University of Technology, 122 Luosi Road, Wuhan 430070, PR China Received 4 November 2003; received in revised form 30 December 2003; accepted 30 December 2003
Abstract The chemical state of oxygen on the surfaces of mixed electronic–ionic conducting La0.6Sr0.4Co1yFeyO3 ceramics was characterized by X-ray photoelectron spectroscopy (XPS). It was ascertained that there are five different kinds of oxygen on the ceramic surfaces, including lattice oxygen (OL), chemisorbed oxygen (OC) in the forms of O2, O, and O2 , and oxygen in hydroxyl environment (OH). The concentration of OC þ OH relative to total detected oxygen enhanced with the increase of Co/ Fe ratio. In order to examine the relation between the chemical states of oxygen on the surfaces and the electrical nature of the ceramics, the mixed electronic–ionic conducting properties were investigated. At an identical measuring temperature, the electronic conductivity and ionic conductivity of La0.6Sr0.4Co1yFeyO3 ceramics tended to rise with the increase of Co/Fe ratio. It was considered that the mixed electronic–ionic conducting properties are responsible for the complex chemical states of oxygen on the ceramic surfaces. # 2004 Elsevier B.V. All rights reserved. PACS: 72.60; 79.60 Keywords: La0.6Sr0.4Co1yFeyO3; Perovskite-type ceramics; XPS; Chemical state; Oxygen; Mixed electronic–ionic conducting properties
1. Introduction In recent years, there is a growing interest of investigating perovskite-type complex oxides of La1xSrxCo1yFeyO3 composition because of their superior mixed electronic–ionic conducting properties. At elevated temperatures (about 800 8C), the La1xSrxCo1yFeyO3 compositions exhibit electronic *
Corresponding author. Tel.: þ86-27-87864033; fax: þ86-27-87642079. E-mail address: [email protected] (Q. Xu).
conductivities exceeding 102 S cm1 and oxygen ionic conductivities on the order of 102 to 1.0 S cm1, making them promising candidate materials for many technical applications, including cathodes for intermediate temperature solid oxide fuel cells, oxygen separation membranes, membrane reactors for syngas production, and catalysts for oxidation of hydrocarbons [1–3]. Among these La1xSrxCo1yFeyO3 compositions, La0.6Sr0.4Co1yFeyO3 (x ¼ 0:4) oxides have attracted considerable attention [4–12]. The mixed electronic–ionic conduction of La1xSrxCo1yFeyO3 oxides is essentially a kind of
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2003.12.030
Q. Xu et al. / Applied Surface Science 228 (2004) 110–114
bulk electrical transport property. However, it has been noticed that the performance of La1xSrxCo1yFeyO3 during practical applications highly depended on the behavior of oxygen on the surface of the material [5– 7]. Therefore, it is necessary to investigate the chemical state of oxygen on the surface to increase the understanding about surface kinetic process of the material during applications. Despite of the extensive investigations on La1xSrxCo1yFeyO3 oxides involving electronic and ionic conducting properties [8,9], bulk diffusion and surface exchange of oxygen [10,11] and oxidation catalytic activity [12], the information about the chemical state of oxygen on the surface of the material is relatively limited. X-ray photoelectron spectroscopy (XPS) is an effective surface analytical technique for solids, providing qualitative and quantitative information about chemical states of constituents. XPS has been successfully applied to characterize the chemical states of oxygen on the surfaces for various perovskite-type ceramics [13,14]. In present work, the chemical state of oxygen on the surfaces of mixed electronic–ionic conducting La0.6Sr0.4Co1yFeyO3 ceramics was analyzed by XPS. Moreover, the relation between the chemical state of oxygen on surfaces and the electrical properties of the ceramics was also investigated.
2. Experimental Reagent grade La(NO3)26H2O, Sr(NO3)2, Fe(NO3)39H2O, Co(NO3)26H2O, and glycine were used as starting materials. La0.6Sr0.4Co1yFeyO3 (y ¼ 0–1.0) powders were synthesized by a glycinenitrate process (GNP), which has been reported elsewhere [15]. A single-phase perovskite structure with rhombohedral symmetry was identified for the powders by X-ray diffraction (XRD). Scanning electron microscope (SEM) analysis shows that the powders consist of homogeneous, fine particles with the mean sizes of 200–400 nm depending on their composition. The powders were uniaxially pressed into rectangular bars (30 mm 4 mm 4 mm) and disks (13 mm in diameter and 2 mm in thickness), respectively. Then the compressed powders were sintered at 1200 8C for 4 h in air. XPS measurement was performed at room temperature by a VG Scientific ESCALAB MK II multi-
111
Fig. 1. Schematic diagram of electrochemical cells.
technique electron spectrometer using Al Ka radiation. The instrument was operated at a power of 125 W (12:5 kV 10 mA) with a passing energy of 50 eVand a scanning step of 0.05 eV. The XPS survey spectrum and O 1s spectrum were taken from the ground surfaces of ceramic specimens in an analysis chamber under a pressure below 106 Pa. The binding energy of C 1s (284.6 eV) was used as an internal standard. The ceramic specimens were polished to ensure surface flatness. The rectangular specimens were painted with platinum paste for measuring electronic conductivity. The electronic conductivity was then measured at 20–900 8C by a dc four-terminal method in air. The oxygen ionic conductivity was measured using disk specimens by the two-terminal electron blocking electrode method described by Chen et al. [9]. Fig. 1 shows the configuration of electrochemical cells for measuring ionic conductivity. Y2O3 stabilized ZrO2 (YSZ) disks with a composition of (Zr0.92Y0.08)O2d were used as electron blocking electrodes. The YSZ disks were prepared using a sol–gel method by sintering at 1450 8C for 4 h. As-sintered YSZ disks were polished to about 0.5 mm in thickness and then painted with platinum paste on the outer surfaces. The YSZ disks were mechanically contacted with the both surfaces of La0.6Sr0.4Co1yFeyO3 disks. The ac impedance spectroscopy of the electrochemical cells was measured by a TH2816 precision digital bridge (0.05–150 kHz) at 400–800 8C in air. Taking the geometric factors of La0.6Sr0.4Co1yFeyO3 disks into consideration, the ionic conductivities of the specimens were determined by fitting measured impedance plots using Zview2.1 software.
3. Results and discussion Fig. 2 shows the XPS survey spectrum of La0.6Sr0.4Co0.4Fe0.6O3 (y ¼ 0:6) specimen. Six relatively strong
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Fig. 2. XPS survey spectrum of La0.6Sr0.4Co0.4Fe0.6O3 (y ¼ 0:6) specimen.
Fig. 3. O 1s spectra of La0.6Sr0.4Co1yFeyO3 specimens with different compositions.
peaks can be observed, attributed to Sr 3d, C 1s, O 1s, Fe 2p, Co 2p, and La 3d photoelectrons, respectively. The C 1s peak was assigned to the adventitious carbon for calibrating binding energy as a reference. The XPS survey spectra of the specimens with other compositions are very similar to that of La0.6Sr0.4Co0.4Fe0.6O3 (y ¼ 0:6) specimen. The result of XPS survey spectrum analysis is in agreement with the elementary composition of the specimens. Fig. 3 shows the O 1s spectra of La0.6Sr0.4Co1yFeyO3 specimens with different compositions. The O 1s spectra are identical in shape for the specimens with different compositions, showing two slightly asymmetric peaks. This implies that the spectra originated from the contribution of oxygen in different chemical environments on the ceramic surfaces. After a deconvolution of measured photoelectron signals, a peak fitting was performed for the O 1s spectra. During the peak fitting, the full width at half maximum (FWHM) and Gaussian/Lorentzian ratio of the O 1s peaks corresponding to different kinds of oxygen were kept as constant values of 1.8 eV and 0.3, respectively. Fig. 4 shows the fitting pattern of O 1s spectrum for La0.6Sr0.4Co0.8Fe0.2O3 (y ¼ 0:2) specimen. It can be seen that the O 1s spectrum comprises five independent peaks with very small chemical shifts, corresponding to five different kinds of oxygen on the ceramic surfaces. The O 1s peak at 528.70 eV is attributed to the lattice oxygen (OL) at the normal sites of the perovskite structure, while the O 1s peaks at 530.50, 531.45, and 532.35 eV are assigned to the chemisorbed oxygen (OC) in the forms of O2, O, and O2 , respectively. In addition, the O 1s peak at
533.50 eV is ascribed to the oxygen in hydroxyl environment (OH). Similar peak fitting results were obtained for the specimens with other compositions. The atomic percentages of oxygen in different chemical states on the surfaces of La0.6Sr0.4Co1yFeyO3 ceramics are shown in Table 1. It was found that the concentration of OC þ OH relative to total detected oxygen enhances with the increase of Co/Fe ratio from 63.20% for La0.6Sr0.4FeO3 (y ¼ 1:0) specimen to 70.02% for La0.6Sr0.4CoO3 (y ¼ 0) specimen. In order to examine the relation of the chemical states of oxygen on the ceramic surfaces to the electrical characteristics of La0.6Sr0.4Co1yFeyO3 ceramics, the mixed electronic–ionic conducting properties were investigated. Fig. 5 shows the electronic conductivity (se) of La0.6Sr0.4Co1yFeyO3 specimens as a function of measuring temperature. The electronic conductivity of La0.6Sr0.4CoO3 (y ¼ 0) spe-
Fig. 4. Fitting pattern of O 1s spectrum for La0.6Sr0.4Co0.8Fe0.2O3 (y ¼ 0:2) specimen.
Q. Xu et al. / Applied Surface Science 228 (2004) 110–114
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Table 1 Atomic percentages of oxygen in different chemical states on the surfaces of La0.6Sr0.4Co1yFeyO3 specimens y
Atomic percentage (%) OL
OC 2
O 0 0.2 0.4 0.6 0.8 1.0
29.98 31.47 33.53 34.19 35.94 36.80
26.73 20.78 22.21 24.15 19.67 21.14
OH O
O2
(OC þ OH)/ P Oi (%)a
27.42 11.03 30.88 11.52 27.00 11.93 26.09 11.40 29.84 10.28 24.30 13.72 P a The term ðOC þ OH Þ/ Oi represents OC þ OH relative to total detected oxygen.
4.84 5.35 5.33 4.17 4.27 4.04
70.02 68.53 66.47 65.81 64.06 63.20
the concentration of
cimen decreases with measuring temperature in the range of 20–900 8C, while those of the specimens with other compositions present a rather similar variation, increasing with measuring temperature through a maximum value near 600 8C and then decreasing. Comparing the electronic conductivities of the specimens measured at an identical temperature, it was found that the electronic conductivity enhances with the increase of Co/Fe ratio. Fig. 6 shows the ionic conductivity (sion) of La0.6Sr0.4Co1yFeyO3 specimens as a function of measuring temperature. The Arrhenius plots over the whole measuring temperature range yielded straight lines. For a given composition, the ionic conductivity increases with the elevation of measuring temperature. In the case of an identical measuring temperature, the variation of ionic conductivity with Co/Fe ratio is generally similar to that of electronic conductivity, tending to rise with the
Fig. 5. The electronic conductivity (se) of La0.6Sr0.4Co1yFeyO3 specimens as a function of measuring temperature.
Fig. 6. The ionic conductivity (sion) of La0.6Sr0.4Co1yFeyO3 specimens as a function of measuring temperature.
increase of Co/Fe ratio. The ionic conductivity of the specimens varied in the ranges of 2:5 103 to 5:0 103 S cm1 and 4:0 102 to 6:2 102 S cm1 at 600 and 800 8C, respectively. A previous research indicates that the ionic conductivities of La0.6Sr0.4Co1yFeyO3 ceramics measured by a dc four-terminal method using electron blocking electrodes were in the range of 102 to 101 S cm1 at 800 8C, showing a decrease of ionic conductivity with Co/Fe ratio by almost one order of magnitude [1]. The difference in ionic conductivity values for La0.6Sr0.4Co1yFeyO3 ceramics between present work and the previous research is presumably attributed to different preparation processes and methods for measuring ionic conductivity. It is noteworthy that the variation trend of the concentration of OC þ OH relative to total detected oxygen with Co/Fe ratio is rather consistent with those of electronic conductivity and ionic conductivity. It infers an essential relation between the chemical states of oxygen on the surfaces and the electrical nature of the ceramics. This can be qualitatively interpreted with respect to the formation process of OC and OH on ceramic surfaces [13,16]. On one hand, oxygen vacancies as a kind of defect on the ceramic surfaces provided suitable adsorption sites for oxygen molecules. On the other hand, the trapping of mobile electrons of La0.6Sr0.4Co1yFeyO3 ceramics by the adsorbed oxygen molecules and dissociated oxygen atoms resulted in different kinds of OC such as O2, O, and O2 . Furthermore, the reaction between OC and adsorbed gaseous H2O as a surface contaminant generated hydroxyls. It indicates that the formation of
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OH is closely associated with the appearance of OC on the ceramic surfaces. The electronic conductivity and ionic conductivity of La0.6Sr0.4Co1yFeyO3 ceramics depend on the concentration and mobility of electrical carriers. It has been well established that small polaron hopping and oxygen vacancy diffusion are responsible for the electronic conduction and ionic conduction of La1xSrxCo1yFeyO3 compositions, respectively [8]. For small polaron hopping, it is well known that the mobility of small polarons is mainly determined by temperature. For Sr-substituted lanthanum-transitional metal perovskite-type oxides, the mobility of oxygen vacancies is also highly dependent on temperature [8] and is relatively insensitive to composition when temperature is unchanged [17]. Therefore, it can be deduced that the concentration of electrical carries appears to be the main contributing factor to the electrical transport properties of La0.6Sr0.4Co1yFeyO3 ceramics at an identical temperature. Hence, the rising of electronic conductivity and ionic conductivity with Co/Fe ratio can be attributed to an increase in the concentrations of mobile electrons and oxygen vacancies. It benefits the formation of OC and OH on the ceramic surfaces. As a result, the concentration of OC þ OH relative to total detected oxygen enhanced with the increase of Co/Fe ratio.
4. Conclusion The chemical state of oxygen on the surfaces of mixed electronic–ionic conducting La0.6Sr0.4Co1yFeyO3 ceramics was characterized by XPS. It was detected that there are five different kinds of oxygen on the ceramic surfaces, including lattice oxygen, chemisorbed oxygen such as O2, O, and O2 , and oxygen in hydroxyl environment. An essential relation between the chemical states of oxygen on the surfaces and the electrical nature of the ceramics was certified. The concentration of OC þ OH relative to total detected oxygen enhanced with the increase of Co/Fe ratio.
Acknowledgements This work was financially supported by the Special Research Found for the Doctoral Program of High Education (grant no. 20330497008), the Natural Science Foundation of Hubei Province of China (grant no. 2001ABB075), and the Foundation for Excellent Youths of Wuhan City of China (grant no. 20015005031). The State Key Laboratory of Advanced Technology for Materials Synthesis and Processing also provided partial financial support for this work.
References [1] Y. Teraoka, H.M. Zhang, K. Okamoto, N. Yamazoe, Mater. Res. Bull. 23 (1988) 51. [2] S.J. Beson, D. Waller, J.A. Kilner, J. Electrochem. Soc. 146 (1999) 1305. [3] C.Y. Tsai, A.G. Dixon, Y.H. Ma, W.R. Moser, M.R. Pascucci, J. Am. Ceram. Soc. 81 (1998) 1437. [4] L.W. Tai, M.M. Nasrallah, H.U. Anderson, D.M. Sparlin, S.R. Sehlin, Solid State Ionics 76 (1995) 273. [5] B.C.H. Steele, J.M. Bae, Solid State Ionics 106 (1998) 255. [6] S.P. Jiang, Solid State Ionics 146 (2002) 1. [7] Y. Teraoka, Y. Honbe, J. Ishii, H. Furukawa, I. Moriguchi, Solid State Ionics 152–153 (2002) 681. [8] J.W. Stevenson, T.R. Armstrong, R.D. Carneim, L.P. Pederson, W.J. Weber, J. Electrochem. Soc. 143 (1996) 2722. [9] C.C. Chen, M.M. Nasrallah, H.U. Anderson, M.A. Alim, J. Electrohem. Soc. 142 (1995) 491. [10] J.E. ten Elshof, M.M.R. Lankhorst, H.J.M. Bouwmeester, Solid State Ionics 99 (1997) 15. [11] M. Katsuki, S.R. Wang, M. Dokiya, T. Hashimoto, Solid State Ionics 156 (2003) 453. [12] M. Weston, I.S. Metcalf, Solid State Ionics 113–115 (1998) 247. [13] Y. Nakano, N. Ichinose, J. Mater. Res. 5 (1990) 2910. [14] S. Kumar, V.S. Raju, T.R.N. Kutty, Appl. Surf. Sci. 206 (2003) 250. [15] Q. Xu, D.P. Huang, H. Chen, W. Wang, R.Z. Yuan, Chem. J. Chin. Univ. 24 (2003) 2271. [16] J.A. Kilner, R.A. de Souza, I.C. Fullarton, Solid State Ionics 86–88 (1996) 703. [17] R.A. de Souza, J.A. Kilner, Solid State Ionics 106 (1998) 175.
X-ray photoelectron spectroscopy investigation on chemical states of oxygen on surfaces of mixed electronic–ionic conducting La0.6Sr0.4Co1yFeyO3 ceramics Qing Xu*, Duan-ping Huang, Wen Chen, Hao Wang, Bi-tao Wang, Run-zhang Yuan State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, School of Materials Science and Engineering, Wuhan University of Technology, 122 Luosi Road, Wuhan 430070, PR China Received 4 November 2003; received in revised form 30 December 2003; accepted 30 December 2003
Abstract The chemical state of oxygen on the surfaces of mixed electronic–ionic conducting La0.6Sr0.4Co1yFeyO3 ceramics was characterized by X-ray photoelectron spectroscopy (XPS). It was ascertained that there are five different kinds of oxygen on the ceramic surfaces, including lattice oxygen (OL), chemisorbed oxygen (OC) in the forms of O2, O, and O2 , and oxygen in hydroxyl environment (OH). The concentration of OC þ OH relative to total detected oxygen enhanced with the increase of Co/ Fe ratio. In order to examine the relation between the chemical states of oxygen on the surfaces and the electrical nature of the ceramics, the mixed electronic–ionic conducting properties were investigated. At an identical measuring temperature, the electronic conductivity and ionic conductivity of La0.6Sr0.4Co1yFeyO3 ceramics tended to rise with the increase of Co/Fe ratio. It was considered that the mixed electronic–ionic conducting properties are responsible for the complex chemical states of oxygen on the ceramic surfaces. # 2004 Elsevier B.V. All rights reserved. PACS: 72.60; 79.60 Keywords: La0.6Sr0.4Co1yFeyO3; Perovskite-type ceramics; XPS; Chemical state; Oxygen; Mixed electronic–ionic conducting properties
1. Introduction In recent years, there is a growing interest of investigating perovskite-type complex oxides of La1xSrxCo1yFeyO3 composition because of their superior mixed electronic–ionic conducting properties. At elevated temperatures (about 800 8C), the La1xSrxCo1yFeyO3 compositions exhibit electronic *
Corresponding author. Tel.: þ86-27-87864033; fax: þ86-27-87642079. E-mail address: [email protected] (Q. Xu).
conductivities exceeding 102 S cm1 and oxygen ionic conductivities on the order of 102 to 1.0 S cm1, making them promising candidate materials for many technical applications, including cathodes for intermediate temperature solid oxide fuel cells, oxygen separation membranes, membrane reactors for syngas production, and catalysts for oxidation of hydrocarbons [1–3]. Among these La1xSrxCo1yFeyO3 compositions, La0.6Sr0.4Co1yFeyO3 (x ¼ 0:4) oxides have attracted considerable attention [4–12]. The mixed electronic–ionic conduction of La1xSrxCo1yFeyO3 oxides is essentially a kind of
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2003.12.030
Q. Xu et al. / Applied Surface Science 228 (2004) 110–114
bulk electrical transport property. However, it has been noticed that the performance of La1xSrxCo1yFeyO3 during practical applications highly depended on the behavior of oxygen on the surface of the material [5– 7]. Therefore, it is necessary to investigate the chemical state of oxygen on the surface to increase the understanding about surface kinetic process of the material during applications. Despite of the extensive investigations on La1xSrxCo1yFeyO3 oxides involving electronic and ionic conducting properties [8,9], bulk diffusion and surface exchange of oxygen [10,11] and oxidation catalytic activity [12], the information about the chemical state of oxygen on the surface of the material is relatively limited. X-ray photoelectron spectroscopy (XPS) is an effective surface analytical technique for solids, providing qualitative and quantitative information about chemical states of constituents. XPS has been successfully applied to characterize the chemical states of oxygen on the surfaces for various perovskite-type ceramics [13,14]. In present work, the chemical state of oxygen on the surfaces of mixed electronic–ionic conducting La0.6Sr0.4Co1yFeyO3 ceramics was analyzed by XPS. Moreover, the relation between the chemical state of oxygen on surfaces and the electrical properties of the ceramics was also investigated.
2. Experimental Reagent grade La(NO3)26H2O, Sr(NO3)2, Fe(NO3)39H2O, Co(NO3)26H2O, and glycine were used as starting materials. La0.6Sr0.4Co1yFeyO3 (y ¼ 0–1.0) powders were synthesized by a glycinenitrate process (GNP), which has been reported elsewhere [15]. A single-phase perovskite structure with rhombohedral symmetry was identified for the powders by X-ray diffraction (XRD). Scanning electron microscope (SEM) analysis shows that the powders consist of homogeneous, fine particles with the mean sizes of 200–400 nm depending on their composition. The powders were uniaxially pressed into rectangular bars (30 mm 4 mm 4 mm) and disks (13 mm in diameter and 2 mm in thickness), respectively. Then the compressed powders were sintered at 1200 8C for 4 h in air. XPS measurement was performed at room temperature by a VG Scientific ESCALAB MK II multi-
111
Fig. 1. Schematic diagram of electrochemical cells.
technique electron spectrometer using Al Ka radiation. The instrument was operated at a power of 125 W (12:5 kV 10 mA) with a passing energy of 50 eVand a scanning step of 0.05 eV. The XPS survey spectrum and O 1s spectrum were taken from the ground surfaces of ceramic specimens in an analysis chamber under a pressure below 106 Pa. The binding energy of C 1s (284.6 eV) was used as an internal standard. The ceramic specimens were polished to ensure surface flatness. The rectangular specimens were painted with platinum paste for measuring electronic conductivity. The electronic conductivity was then measured at 20–900 8C by a dc four-terminal method in air. The oxygen ionic conductivity was measured using disk specimens by the two-terminal electron blocking electrode method described by Chen et al. [9]. Fig. 1 shows the configuration of electrochemical cells for measuring ionic conductivity. Y2O3 stabilized ZrO2 (YSZ) disks with a composition of (Zr0.92Y0.08)O2d were used as electron blocking electrodes. The YSZ disks were prepared using a sol–gel method by sintering at 1450 8C for 4 h. As-sintered YSZ disks were polished to about 0.5 mm in thickness and then painted with platinum paste on the outer surfaces. The YSZ disks were mechanically contacted with the both surfaces of La0.6Sr0.4Co1yFeyO3 disks. The ac impedance spectroscopy of the electrochemical cells was measured by a TH2816 precision digital bridge (0.05–150 kHz) at 400–800 8C in air. Taking the geometric factors of La0.6Sr0.4Co1yFeyO3 disks into consideration, the ionic conductivities of the specimens were determined by fitting measured impedance plots using Zview2.1 software.
3. Results and discussion Fig. 2 shows the XPS survey spectrum of La0.6Sr0.4Co0.4Fe0.6O3 (y ¼ 0:6) specimen. Six relatively strong
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Q. Xu et al. / Applied Surface Science 228 (2004) 110–114
Fig. 2. XPS survey spectrum of La0.6Sr0.4Co0.4Fe0.6O3 (y ¼ 0:6) specimen.
Fig. 3. O 1s spectra of La0.6Sr0.4Co1yFeyO3 specimens with different compositions.
peaks can be observed, attributed to Sr 3d, C 1s, O 1s, Fe 2p, Co 2p, and La 3d photoelectrons, respectively. The C 1s peak was assigned to the adventitious carbon for calibrating binding energy as a reference. The XPS survey spectra of the specimens with other compositions are very similar to that of La0.6Sr0.4Co0.4Fe0.6O3 (y ¼ 0:6) specimen. The result of XPS survey spectrum analysis is in agreement with the elementary composition of the specimens. Fig. 3 shows the O 1s spectra of La0.6Sr0.4Co1yFeyO3 specimens with different compositions. The O 1s spectra are identical in shape for the specimens with different compositions, showing two slightly asymmetric peaks. This implies that the spectra originated from the contribution of oxygen in different chemical environments on the ceramic surfaces. After a deconvolution of measured photoelectron signals, a peak fitting was performed for the O 1s spectra. During the peak fitting, the full width at half maximum (FWHM) and Gaussian/Lorentzian ratio of the O 1s peaks corresponding to different kinds of oxygen were kept as constant values of 1.8 eV and 0.3, respectively. Fig. 4 shows the fitting pattern of O 1s spectrum for La0.6Sr0.4Co0.8Fe0.2O3 (y ¼ 0:2) specimen. It can be seen that the O 1s spectrum comprises five independent peaks with very small chemical shifts, corresponding to five different kinds of oxygen on the ceramic surfaces. The O 1s peak at 528.70 eV is attributed to the lattice oxygen (OL) at the normal sites of the perovskite structure, while the O 1s peaks at 530.50, 531.45, and 532.35 eV are assigned to the chemisorbed oxygen (OC) in the forms of O2, O, and O2 , respectively. In addition, the O 1s peak at
533.50 eV is ascribed to the oxygen in hydroxyl environment (OH). Similar peak fitting results were obtained for the specimens with other compositions. The atomic percentages of oxygen in different chemical states on the surfaces of La0.6Sr0.4Co1yFeyO3 ceramics are shown in Table 1. It was found that the concentration of OC þ OH relative to total detected oxygen enhances with the increase of Co/Fe ratio from 63.20% for La0.6Sr0.4FeO3 (y ¼ 1:0) specimen to 70.02% for La0.6Sr0.4CoO3 (y ¼ 0) specimen. In order to examine the relation of the chemical states of oxygen on the ceramic surfaces to the electrical characteristics of La0.6Sr0.4Co1yFeyO3 ceramics, the mixed electronic–ionic conducting properties were investigated. Fig. 5 shows the electronic conductivity (se) of La0.6Sr0.4Co1yFeyO3 specimens as a function of measuring temperature. The electronic conductivity of La0.6Sr0.4CoO3 (y ¼ 0) spe-
Fig. 4. Fitting pattern of O 1s spectrum for La0.6Sr0.4Co0.8Fe0.2O3 (y ¼ 0:2) specimen.
Q. Xu et al. / Applied Surface Science 228 (2004) 110–114
113
Table 1 Atomic percentages of oxygen in different chemical states on the surfaces of La0.6Sr0.4Co1yFeyO3 specimens y
Atomic percentage (%) OL
OC 2
O 0 0.2 0.4 0.6 0.8 1.0
29.98 31.47 33.53 34.19 35.94 36.80
26.73 20.78 22.21 24.15 19.67 21.14
OH O
O2
(OC þ OH)/ P Oi (%)a
27.42 11.03 30.88 11.52 27.00 11.93 26.09 11.40 29.84 10.28 24.30 13.72 P a The term ðOC þ OH Þ/ Oi represents OC þ OH relative to total detected oxygen.
4.84 5.35 5.33 4.17 4.27 4.04
70.02 68.53 66.47 65.81 64.06 63.20
the concentration of
cimen decreases with measuring temperature in the range of 20–900 8C, while those of the specimens with other compositions present a rather similar variation, increasing with measuring temperature through a maximum value near 600 8C and then decreasing. Comparing the electronic conductivities of the specimens measured at an identical temperature, it was found that the electronic conductivity enhances with the increase of Co/Fe ratio. Fig. 6 shows the ionic conductivity (sion) of La0.6Sr0.4Co1yFeyO3 specimens as a function of measuring temperature. The Arrhenius plots over the whole measuring temperature range yielded straight lines. For a given composition, the ionic conductivity increases with the elevation of measuring temperature. In the case of an identical measuring temperature, the variation of ionic conductivity with Co/Fe ratio is generally similar to that of electronic conductivity, tending to rise with the
Fig. 5. The electronic conductivity (se) of La0.6Sr0.4Co1yFeyO3 specimens as a function of measuring temperature.
Fig. 6. The ionic conductivity (sion) of La0.6Sr0.4Co1yFeyO3 specimens as a function of measuring temperature.
increase of Co/Fe ratio. The ionic conductivity of the specimens varied in the ranges of 2:5 103 to 5:0 103 S cm1 and 4:0 102 to 6:2 102 S cm1 at 600 and 800 8C, respectively. A previous research indicates that the ionic conductivities of La0.6Sr0.4Co1yFeyO3 ceramics measured by a dc four-terminal method using electron blocking electrodes were in the range of 102 to 101 S cm1 at 800 8C, showing a decrease of ionic conductivity with Co/Fe ratio by almost one order of magnitude [1]. The difference in ionic conductivity values for La0.6Sr0.4Co1yFeyO3 ceramics between present work and the previous research is presumably attributed to different preparation processes and methods for measuring ionic conductivity. It is noteworthy that the variation trend of the concentration of OC þ OH relative to total detected oxygen with Co/Fe ratio is rather consistent with those of electronic conductivity and ionic conductivity. It infers an essential relation between the chemical states of oxygen on the surfaces and the electrical nature of the ceramics. This can be qualitatively interpreted with respect to the formation process of OC and OH on ceramic surfaces [13,16]. On one hand, oxygen vacancies as a kind of defect on the ceramic surfaces provided suitable adsorption sites for oxygen molecules. On the other hand, the trapping of mobile electrons of La0.6Sr0.4Co1yFeyO3 ceramics by the adsorbed oxygen molecules and dissociated oxygen atoms resulted in different kinds of OC such as O2, O, and O2 . Furthermore, the reaction between OC and adsorbed gaseous H2O as a surface contaminant generated hydroxyls. It indicates that the formation of
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OH is closely associated with the appearance of OC on the ceramic surfaces. The electronic conductivity and ionic conductivity of La0.6Sr0.4Co1yFeyO3 ceramics depend on the concentration and mobility of electrical carriers. It has been well established that small polaron hopping and oxygen vacancy diffusion are responsible for the electronic conduction and ionic conduction of La1xSrxCo1yFeyO3 compositions, respectively [8]. For small polaron hopping, it is well known that the mobility of small polarons is mainly determined by temperature. For Sr-substituted lanthanum-transitional metal perovskite-type oxides, the mobility of oxygen vacancies is also highly dependent on temperature [8] and is relatively insensitive to composition when temperature is unchanged [17]. Therefore, it can be deduced that the concentration of electrical carries appears to be the main contributing factor to the electrical transport properties of La0.6Sr0.4Co1yFeyO3 ceramics at an identical temperature. Hence, the rising of electronic conductivity and ionic conductivity with Co/Fe ratio can be attributed to an increase in the concentrations of mobile electrons and oxygen vacancies. It benefits the formation of OC and OH on the ceramic surfaces. As a result, the concentration of OC þ OH relative to total detected oxygen enhanced with the increase of Co/Fe ratio.
4. Conclusion The chemical state of oxygen on the surfaces of mixed electronic–ionic conducting La0.6Sr0.4Co1yFeyO3 ceramics was characterized by XPS. It was detected that there are five different kinds of oxygen on the ceramic surfaces, including lattice oxygen, chemisorbed oxygen such as O2, O, and O2 , and oxygen in hydroxyl environment. An essential relation between the chemical states of oxygen on the surfaces and the electrical nature of the ceramics was certified. The concentration of OC þ OH relative to total detected oxygen enhanced with the increase of Co/Fe ratio.
Acknowledgements This work was financially supported by the Special Research Found for the Doctoral Program of High Education (grant no. 20330497008), the Natural Science Foundation of Hubei Province of China (grant no. 2001ABB075), and the Foundation for Excellent Youths of Wuhan City of China (grant no. 20015005031). The State Key Laboratory of Advanced Technology for Materials Synthesis and Processing also provided partial financial support for this work.
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