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XPS studies of nanometer CeO2 thin films deposited by pulse ultrasonic spray pyrolysis
Journal of Alloys and Compounds 305 (2000) 121–124
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XPS studies of nanometer CeO 2 thin films deposited by pulse ultrasonic spray pyrolysis a, a b a Shengyue Wang *, Zhengping Qiao , Wei Wang , Yitai Qian a
Department of Chemistry, University of Science and Technology of China, Hefei 230026, PR China b Department of Physics, Huaibei Coal Industry Teachers’ College, Anhui 235000, PR China Received 26 October 1999; received in revised form 27 January 2000; accepted 27 January 2000
Abstract Cerium(IV) oxide nanometer (4.2–20 nm) thin films are deposited on Si(100) substrates by nebulization of a 0.01 M solution of cerium acetylacetonate in a 50% ethanol / water mixture followed by pyrolysis in flowing air. Influence of X-ray irradiation ( 60 Co) and the proportion of pulse and interval time between pulses on the microstructure of CeO 2 films are determined by XPS. The results reveal that the film can be reduced from Ce 41 to Ce 31 by X-ray irradiation for about 10 h. Ce 31 content increases with the increase of irradiation dose and there is loss of lattice oxygen greatly. Peak of lattice oxygen of O 1s increases with the decrease of the proportion, but decreases obviously with the increase of irradiation dose. Appropriate proportion is necessary for obtain pure CeO 2 nanometer thin films. 2000 Elsevier Science S.A. All rights reserved. Keywords: A. nanostructures; A. thin films; B. nanofabrications; D. irradiation effect; E. photoelectron spectroscopies
1. Introduction Rare earth oxides are potentially useful materials for various optical and electronic applications such as optical waveguides, optical filters, and capacitors. One such material is cerium dioxide and cerium dioxide thin films have been investigated by many researchers [1–4]. Recently, a novel spray pyrolysis process [5–7] has been developed for the preparation of nanometer CeO 2 thin films of high quality. This novel process has been found to be very effective in eliminating the columnar microstructure of CeO 2 nanometer thin films by using appropriate proportion of pulse and interval time between pulses. It has been proved that during XPS depth profile measurement of CeO 2 / Si films, the reduction of CeO 2 occurred [8], and Yang et al. [9] has ruled out the possibility of the X-ray inducing the reaction by their experimentation. Their research indicates that the main mechanism responsible for inducing the reaction is Ar 1 etching, not X-ray irradiation. However, in this paper we report on an unusual method where there is an obvious *Corresponding author. E-mail address: [email protected] (S. Wang)
increase of the Ce 31 peaks, and a decrease of lattice oxygen peak in the film of CeO 2 (10.3 nm) on Si(100) with exposure to the X-ray source.
2. Experimental In this study, thin films of cerium dioxide in the fluorite structure were deposited on Si(100) substrates by pulse ultrasonic spray pyrolysis under the same conditions, the only difference being the proportion of pulse and time interval between pulses. Cerium acetylacetonate, Ce(CH 3 COCHCOCH 3 ) 3 (Ce(acac) 3 ), was chosen as a precursor for the preparation of CeO 2 films. The process of preparation of Ce(acac) 3 was similar to that in the literature [7]. Ce(NO 3 ) 3 ?6H 2 O (0.01 M) was dissolved in 15 ml of distilled water and 15 ml of ethanol. This solution was immersed in an ice bath with constant stirring and 3 ml of acetylacetonate and 4 ml of propylene oxide were added to the solution. Concentrated ammonium hydroxide was added to the solution dropwise until the pH was approximately 7 and a yellow precipitate of Ce(acac) 3 was formed. The precipitate and solution were refrigerated overnight, filtered and dried.
0925-8388 / 00 / $ – see front matter 2000 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 00 )00748-9
S. Wang et al. / Journal of Alloys and Compounds 305 (2000) 121 – 124
122
Fig. 1. Typical XPS spectrum of CeO 2 film on Si(100).
An ethanol / water solution of 0.01 M Ce(acac) 3 was ultrasonically nebulized, sprayed on substrates, and thermally decomposed at about 723 K in a reactor, as described previously [7]. The carrier gas was compressed air with a flow rate of 5 l / min. The nebulized solution was delivered to the substrates in pulses lasting a few seconds with a few seconds interval between pulses. The deposition time was 30 min and the nozzle–substrate distance was 7.5 cm. CeO 2 films had different crystallite sizes in the range 4.2–20 nm and thicknesses in the range 340–790 nm, estimated from XRD (Rigaku D/ MAX-gA ) patterns and SEM (Hitachi X-650) observation, respectively. X-ray photoelectron spectroscopy (XPS) measurements were carried out in a ESCALAB MKh system. X-ray fluorescence spectrometry (XRFS) demonstrated that the quantity of impurities in the sample was less than 0.01%. IR measurements showed no peaks for any organic compound appear in the spectra. This suggested that the as-prepared thin films are of high purity, which is in agreement with the results of the XRD analysis.
3. Results and discussion Fig. 1 shows the surface XPS spectrum of a typical CeO 2 film on Si(100) as deposited. Peaks due to Ce3d, Ce4p, Ce4d, O1s and C1s are observed. The correlation between the experimental conditions and the ratio of lattice
oxygen and absorption oxygen, the ratio of O / Ce is listed in Table 1. In general, with a decrease in proportion, the relative intensity ratio O / Ce displays a monotonic decrease. But after being irradiated by the X-ray source, the relative intensity ratio O / Ce displays a rapid increase with an increase of radiation dose. With a decrease in proportion, Ce 31 from Ce(acac) 3 have enough time to react with oxygen, and the films can be well oxidized by increasing the interval time. So, as the proportion decreases from 30 / 5 to 5 / 15, the relative intensity ratio of lattice oxygen and absorbed oxygen correspondingly increases from 2:1 to 13.5:1, as shown in Table 1. Fig. 2 displays a series of high resolution Ce3d spectra measured at decreasing proportions of pulse and interval time between pulses for ceria films and increasing irradiation dose of the X-ray source for sample 3 (10.3 nm). As shown in Fig. 2a–f, the high-resolution Ce3d spectra of samples 1–2 show the co-existence of Ce 31 (at 885.2 and 903.8 eV) and Ce 41 , and the signal from Ce 31 disappears as the proportion decreases to less than 10 / 5, which suggests only the presence of CeO 2 in sample 3–6. Therefore, an appropriate proportion is necessary for obtaining pure CeO 2 nanometer thin films. Fig. 2g–h shows the Ce3d spectra of sample 3 irradiated in the field of a 2.59310 15 Bq 60 Co X-ray source for about 10 h with a dose rate of 25.9 and 110 Gy / min, respectively. The results show that two additional peaks appear at 885.2 and 903.8 eV, implying the formation of Ce 31 again; and the relative intensity of the Ce 31 signal increases with the irradiation dose. The reduction reaction of Ce 41 to Ce 31 in our case is induced by the X-ray irradiation, because the possibility of X-ray effects have been ruled out here [9]. According to the literature [10–12], the higher binding-energy component is likely to be a combination of a number of species, including surface hydroxyl groups, adsorbed water and molecular oxygen species (peroxide, superoxide), which have been observed on ceria surfaces by Fourier transform infrared spectroscopy [13] and electron spin resonance [11]. In our case, the generation of Ce 31 by the X-ray source can be explained by follows: X-ray
2 H 2 O → OH, H 2 O 2 , H, H 3 O 1 , H 2 , e aq
Table 1 Relative intensity ratio of lattice oxygen (O L ) and absorbed oxygen (OA ) of O 1s and content ratio of O / Ce of surface in various samples Sample 1
2
3
4
5
6
Proportion (s / s) or Irradiation dose (Gy / min)
30 / 5
15 / 5
10 / 5
5/5
5 / 10
5 / 15
O L / OA ratio
2:1
2.2:1
3.4:1
5.3:1
10:1
13.5:1
O / Ce ratio
4.59
2.87
2.85
2.60
2.29
2.05
7
8
25.9
110
0.6:1
0.2:1
5.76
7.42
S. Wang et al. / Journal of Alloys and Compounds 305 (2000) 121 – 124
Fig. 2. High-resolution Ce3d spectra for CeO 2 / Si(100) as function of the proportion of pulse and interval time between pulses (a–f) and irradiation dose of g-ray source (g–h). (a) Sample 1, 30 / 5; (b) sample 2, 15 / 5; (c) sample 3, 10 / 5; (d) sample 4, 5 / 5; (e) sample 5, 5 / 10; (f) sample 6, 5 / 15; (g) sample 3, 25.9 Gy / min for 10 h; (h) sample 3, 110 Gy / min for 10 h. 41 e2 → Ce 31 aq 1 Ce
4 CeO 2 → 2Ce 2 O 3 1 O 2 This is just the reduction reaction of Ce 41 to Ce 31 . Fig. 3 shows the corresponding O 1s spectra for the Ce3d spectra presented in Fig. 2. The binding energy of the O 1s around 529.2 eV is characteristic of metallic oxides. Simultaneously, as the X-ray radiation dose increases from 25.9 to 110 Gy / min, the relative intensity ratio of lattice oxygen and absorbed oxygen of sample 3 which is 3.4:1 correspondingly decreases from 0.6:1 to 0.2:1, as shown in Table 1 and Fig. 3g–h. These results may be explained by the amount of lattice oxygen that escaped from the CeO 2 and became absorbed oxygen on the surface. Besides, the peaks of O 1s on the high binding energy side for all the samples are due to chemisorbed oxygen [14], while the peaks on the low binding energy side are due to lattice oxygen. So, it can be shown that the shoulder peak due to lattice oxygen is very dominant for as-deposited films as shown in Fig. 3a–f, while the shoulder peak due to chemisorbed oxygen is very dominant for the films irradiated by the X-ray source as shown in Fig. 3g–h.
123
Fig. 3. The corresponding high-resolution O 1s spectra for CeO 2 / Si(100) for the same conditions as in Fig. 2.
4. Conclusion CeO 2 films are deposited by a pulse ultrasonic spray pyrolysis method and characterized using XPS for different spray proportions of pulse and interval time between pulses and X-ray irradiation dose, respectively. The XPS analysis indicates that the films deposited in proportions less than 10 / 5 have no mixed oxides of cerium. The reduction reaction of the surface of CeO 2 that occurs during X-ray irradiation measured by XPS has been identified. The reduction reaction is induced mainly by the effect of X-ray irradiation. The intensity of the reduction reaction increases with the increase of irradiation dose. Appropriate proportions of pulse and interval time between pulses can promote the oxidization of CeO 2 . Exposure of CeO 2 / Si(100) films to the X-ray source for more than 25.9 Gy / min irradiation dose can result in lattice oxygen loss from lattice to surface.
Acknowledgements Financial support from the Chinese National Foundation of Natural Science Research, and Anhui provincial Foundation of Nature Science Research are gratefully acknowledged.
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S. Wang et al. / Journal of Alloys and Compounds 305 (2000) 121 – 124
References [1] T. Masui, K.-I. Machida, T. Sakata, H. Mori, G.-Y. Adachi, Chem. Lett. (1996) 75. [2] A.H. Morshed, M.E. Moussa, S.M. Bedair, Appl. Phys. Lett. 70 (13) (1997) 1647. [3] Dading Huang, Fuguang Qin, Zhengyu Yao, Zhizhang Ren, Lanying Lin, Appl. Phys. Lett. 67 (25) (1995) 3724. [4] M. Pan, G.Y. Meng, H.W. Xin, C.S. Chen, D.K. Peng, Y.S. Lin, Thin Solid Films 324 (1998) 89. [5] Y. Xie, W. Wang, Y. Qian, Y. Li, Z. Chen, J. Crystal Growth 167 (1996) 656. [6] Y. Xie, W. Wang, Y. Qian, Y. Li, Z. Chen, Surf. Coat. Technol. 82 (1996) 291.
[7] W.J. Desisto, Yitai Qian, Mater. Res. Bull. 25 (1990) 183. [8] Z. Wu, Vacuum 49 (1998) 133. [9] X. Yang, Z. Wu, J. Zhao, H. Wang, D. Huang, F. Qin, Vacuum 49 (1998) 139. [10] B.E. Koel, G. Praline, H. Lee, J.M. White, J. Electron Spectrosc. Relat. Phenom. 21 (1980) 31. [11] E. Abi-aad, R. Bechara, J. Grimbolt, A. Aboukais, Chem. Mater. 5 (1993) 793. [12] P. Dolle, S. Drissi, M. Besancon, J. Jupille, Surf. Sci. 269–270 (1992) 687. [13] C. Li, K. Domeu, K. Maruya, T. Onishi, J. Am. Ceram. Soc. 111 (1989) 7683. [14] T.L. Barr, C.G. Fries, F. Cariati, J.C. Bart, N. Giordano, J. Chem. Soc. Dalton Trans. (1983) 1825.
L
www.elsevier.com / locate / jallcom
XPS studies of nanometer CeO 2 thin films deposited by pulse ultrasonic spray pyrolysis a, a b a Shengyue Wang *, Zhengping Qiao , Wei Wang , Yitai Qian a
Department of Chemistry, University of Science and Technology of China, Hefei 230026, PR China b Department of Physics, Huaibei Coal Industry Teachers’ College, Anhui 235000, PR China Received 26 October 1999; received in revised form 27 January 2000; accepted 27 January 2000
Abstract Cerium(IV) oxide nanometer (4.2–20 nm) thin films are deposited on Si(100) substrates by nebulization of a 0.01 M solution of cerium acetylacetonate in a 50% ethanol / water mixture followed by pyrolysis in flowing air. Influence of X-ray irradiation ( 60 Co) and the proportion of pulse and interval time between pulses on the microstructure of CeO 2 films are determined by XPS. The results reveal that the film can be reduced from Ce 41 to Ce 31 by X-ray irradiation for about 10 h. Ce 31 content increases with the increase of irradiation dose and there is loss of lattice oxygen greatly. Peak of lattice oxygen of O 1s increases with the decrease of the proportion, but decreases obviously with the increase of irradiation dose. Appropriate proportion is necessary for obtain pure CeO 2 nanometer thin films. 2000 Elsevier Science S.A. All rights reserved. Keywords: A. nanostructures; A. thin films; B. nanofabrications; D. irradiation effect; E. photoelectron spectroscopies
1. Introduction Rare earth oxides are potentially useful materials for various optical and electronic applications such as optical waveguides, optical filters, and capacitors. One such material is cerium dioxide and cerium dioxide thin films have been investigated by many researchers [1–4]. Recently, a novel spray pyrolysis process [5–7] has been developed for the preparation of nanometer CeO 2 thin films of high quality. This novel process has been found to be very effective in eliminating the columnar microstructure of CeO 2 nanometer thin films by using appropriate proportion of pulse and interval time between pulses. It has been proved that during XPS depth profile measurement of CeO 2 / Si films, the reduction of CeO 2 occurred [8], and Yang et al. [9] has ruled out the possibility of the X-ray inducing the reaction by their experimentation. Their research indicates that the main mechanism responsible for inducing the reaction is Ar 1 etching, not X-ray irradiation. However, in this paper we report on an unusual method where there is an obvious *Corresponding author. E-mail address: [email protected] (S. Wang)
increase of the Ce 31 peaks, and a decrease of lattice oxygen peak in the film of CeO 2 (10.3 nm) on Si(100) with exposure to the X-ray source.
2. Experimental In this study, thin films of cerium dioxide in the fluorite structure were deposited on Si(100) substrates by pulse ultrasonic spray pyrolysis under the same conditions, the only difference being the proportion of pulse and time interval between pulses. Cerium acetylacetonate, Ce(CH 3 COCHCOCH 3 ) 3 (Ce(acac) 3 ), was chosen as a precursor for the preparation of CeO 2 films. The process of preparation of Ce(acac) 3 was similar to that in the literature [7]. Ce(NO 3 ) 3 ?6H 2 O (0.01 M) was dissolved in 15 ml of distilled water and 15 ml of ethanol. This solution was immersed in an ice bath with constant stirring and 3 ml of acetylacetonate and 4 ml of propylene oxide were added to the solution. Concentrated ammonium hydroxide was added to the solution dropwise until the pH was approximately 7 and a yellow precipitate of Ce(acac) 3 was formed. The precipitate and solution were refrigerated overnight, filtered and dried.
0925-8388 / 00 / $ – see front matter 2000 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 00 )00748-9
S. Wang et al. / Journal of Alloys and Compounds 305 (2000) 121 – 124
122
Fig. 1. Typical XPS spectrum of CeO 2 film on Si(100).
An ethanol / water solution of 0.01 M Ce(acac) 3 was ultrasonically nebulized, sprayed on substrates, and thermally decomposed at about 723 K in a reactor, as described previously [7]. The carrier gas was compressed air with a flow rate of 5 l / min. The nebulized solution was delivered to the substrates in pulses lasting a few seconds with a few seconds interval between pulses. The deposition time was 30 min and the nozzle–substrate distance was 7.5 cm. CeO 2 films had different crystallite sizes in the range 4.2–20 nm and thicknesses in the range 340–790 nm, estimated from XRD (Rigaku D/ MAX-gA ) patterns and SEM (Hitachi X-650) observation, respectively. X-ray photoelectron spectroscopy (XPS) measurements were carried out in a ESCALAB MKh system. X-ray fluorescence spectrometry (XRFS) demonstrated that the quantity of impurities in the sample was less than 0.01%. IR measurements showed no peaks for any organic compound appear in the spectra. This suggested that the as-prepared thin films are of high purity, which is in agreement with the results of the XRD analysis.
3. Results and discussion Fig. 1 shows the surface XPS spectrum of a typical CeO 2 film on Si(100) as deposited. Peaks due to Ce3d, Ce4p, Ce4d, O1s and C1s are observed. The correlation between the experimental conditions and the ratio of lattice
oxygen and absorption oxygen, the ratio of O / Ce is listed in Table 1. In general, with a decrease in proportion, the relative intensity ratio O / Ce displays a monotonic decrease. But after being irradiated by the X-ray source, the relative intensity ratio O / Ce displays a rapid increase with an increase of radiation dose. With a decrease in proportion, Ce 31 from Ce(acac) 3 have enough time to react with oxygen, and the films can be well oxidized by increasing the interval time. So, as the proportion decreases from 30 / 5 to 5 / 15, the relative intensity ratio of lattice oxygen and absorbed oxygen correspondingly increases from 2:1 to 13.5:1, as shown in Table 1. Fig. 2 displays a series of high resolution Ce3d spectra measured at decreasing proportions of pulse and interval time between pulses for ceria films and increasing irradiation dose of the X-ray source for sample 3 (10.3 nm). As shown in Fig. 2a–f, the high-resolution Ce3d spectra of samples 1–2 show the co-existence of Ce 31 (at 885.2 and 903.8 eV) and Ce 41 , and the signal from Ce 31 disappears as the proportion decreases to less than 10 / 5, which suggests only the presence of CeO 2 in sample 3–6. Therefore, an appropriate proportion is necessary for obtaining pure CeO 2 nanometer thin films. Fig. 2g–h shows the Ce3d spectra of sample 3 irradiated in the field of a 2.59310 15 Bq 60 Co X-ray source for about 10 h with a dose rate of 25.9 and 110 Gy / min, respectively. The results show that two additional peaks appear at 885.2 and 903.8 eV, implying the formation of Ce 31 again; and the relative intensity of the Ce 31 signal increases with the irradiation dose. The reduction reaction of Ce 41 to Ce 31 in our case is induced by the X-ray irradiation, because the possibility of X-ray effects have been ruled out here [9]. According to the literature [10–12], the higher binding-energy component is likely to be a combination of a number of species, including surface hydroxyl groups, adsorbed water and molecular oxygen species (peroxide, superoxide), which have been observed on ceria surfaces by Fourier transform infrared spectroscopy [13] and electron spin resonance [11]. In our case, the generation of Ce 31 by the X-ray source can be explained by follows: X-ray
2 H 2 O → OH, H 2 O 2 , H, H 3 O 1 , H 2 , e aq
Table 1 Relative intensity ratio of lattice oxygen (O L ) and absorbed oxygen (OA ) of O 1s and content ratio of O / Ce of surface in various samples Sample 1
2
3
4
5
6
Proportion (s / s) or Irradiation dose (Gy / min)
30 / 5
15 / 5
10 / 5
5/5
5 / 10
5 / 15
O L / OA ratio
2:1
2.2:1
3.4:1
5.3:1
10:1
13.5:1
O / Ce ratio
4.59
2.87
2.85
2.60
2.29
2.05
7
8
25.9
110
0.6:1
0.2:1
5.76
7.42
S. Wang et al. / Journal of Alloys and Compounds 305 (2000) 121 – 124
Fig. 2. High-resolution Ce3d spectra for CeO 2 / Si(100) as function of the proportion of pulse and interval time between pulses (a–f) and irradiation dose of g-ray source (g–h). (a) Sample 1, 30 / 5; (b) sample 2, 15 / 5; (c) sample 3, 10 / 5; (d) sample 4, 5 / 5; (e) sample 5, 5 / 10; (f) sample 6, 5 / 15; (g) sample 3, 25.9 Gy / min for 10 h; (h) sample 3, 110 Gy / min for 10 h. 41 e2 → Ce 31 aq 1 Ce
4 CeO 2 → 2Ce 2 O 3 1 O 2 This is just the reduction reaction of Ce 41 to Ce 31 . Fig. 3 shows the corresponding O 1s spectra for the Ce3d spectra presented in Fig. 2. The binding energy of the O 1s around 529.2 eV is characteristic of metallic oxides. Simultaneously, as the X-ray radiation dose increases from 25.9 to 110 Gy / min, the relative intensity ratio of lattice oxygen and absorbed oxygen of sample 3 which is 3.4:1 correspondingly decreases from 0.6:1 to 0.2:1, as shown in Table 1 and Fig. 3g–h. These results may be explained by the amount of lattice oxygen that escaped from the CeO 2 and became absorbed oxygen on the surface. Besides, the peaks of O 1s on the high binding energy side for all the samples are due to chemisorbed oxygen [14], while the peaks on the low binding energy side are due to lattice oxygen. So, it can be shown that the shoulder peak due to lattice oxygen is very dominant for as-deposited films as shown in Fig. 3a–f, while the shoulder peak due to chemisorbed oxygen is very dominant for the films irradiated by the X-ray source as shown in Fig. 3g–h.
123
Fig. 3. The corresponding high-resolution O 1s spectra for CeO 2 / Si(100) for the same conditions as in Fig. 2.
4. Conclusion CeO 2 films are deposited by a pulse ultrasonic spray pyrolysis method and characterized using XPS for different spray proportions of pulse and interval time between pulses and X-ray irradiation dose, respectively. The XPS analysis indicates that the films deposited in proportions less than 10 / 5 have no mixed oxides of cerium. The reduction reaction of the surface of CeO 2 that occurs during X-ray irradiation measured by XPS has been identified. The reduction reaction is induced mainly by the effect of X-ray irradiation. The intensity of the reduction reaction increases with the increase of irradiation dose. Appropriate proportions of pulse and interval time between pulses can promote the oxidization of CeO 2 . Exposure of CeO 2 / Si(100) films to the X-ray source for more than 25.9 Gy / min irradiation dose can result in lattice oxygen loss from lattice to surface.
Acknowledgements Financial support from the Chinese National Foundation of Natural Science Research, and Anhui provincial Foundation of Nature Science Research are gratefully acknowledged.
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S. Wang et al. / Journal of Alloys and Compounds 305 (2000) 121 – 124
References [1] T. Masui, K.-I. Machida, T. Sakata, H. Mori, G.-Y. Adachi, Chem. Lett. (1996) 75. [2] A.H. Morshed, M.E. Moussa, S.M. Bedair, Appl. Phys. Lett. 70 (13) (1997) 1647. [3] Dading Huang, Fuguang Qin, Zhengyu Yao, Zhizhang Ren, Lanying Lin, Appl. Phys. Lett. 67 (25) (1995) 3724. [4] M. Pan, G.Y. Meng, H.W. Xin, C.S. Chen, D.K. Peng, Y.S. Lin, Thin Solid Films 324 (1998) 89. [5] Y. Xie, W. Wang, Y. Qian, Y. Li, Z. Chen, J. Crystal Growth 167 (1996) 656. [6] Y. Xie, W. Wang, Y. Qian, Y. Li, Z. Chen, Surf. Coat. Technol. 82 (1996) 291.
[7] W.J. Desisto, Yitai Qian, Mater. Res. Bull. 25 (1990) 183. [8] Z. Wu, Vacuum 49 (1998) 133. [9] X. Yang, Z. Wu, J. Zhao, H. Wang, D. Huang, F. Qin, Vacuum 49 (1998) 139. [10] B.E. Koel, G. Praline, H. Lee, J.M. White, J. Electron Spectrosc. Relat. Phenom. 21 (1980) 31. [11] E. Abi-aad, R. Bechara, J. Grimbolt, A. Aboukais, Chem. Mater. 5 (1993) 793. [12] P. Dolle, S. Drissi, M. Besancon, J. Jupille, Surf. Sci. 269–270 (1992) 687. [13] C. Li, K. Domeu, K. Maruya, T. Onishi, J. Am. Ceram. Soc. 111 (1989) 7683. [14] T.L. Barr, C.G. Fries, F. Cariati, J.C. Bart, N. Giordano, J. Chem. Soc. Dalton Trans. (1983) 1825.