- Email: [email protected]
zirconia mixed oxides: powders and thin film characterisation
JOURNAL OF ELECTRON SPECTROSCOPY and Related Phenomena
ELSEVIER
Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 951-956
XPS comparative study of ceria/zirconia mixed oxides: powders and thin film characterisation A. Galtayries a'*, R. Sporken a, J. Riga a, G. Blanchard b, R.
Caudano
a
aLaboratoire lnterdisciplinaire de Spectroscopie Electronique (LISE), Facultgs Universitaires Notre-Dame de la Paix, B-5000 Namur, Belgium bRhOne Poulenc Recherches (RP), 52 rue de la Haie Coq, F-93308 Aubervilliers, France
Abstract Ceria/zirconia mixed oxides are important new materials for automotive exhaust catalysis. The addition of ZrO2 in the solid solution formed by the mixed oxides improves the redox properties of the solid and thus the catalyst's efficiency. The work reported uses X-ray photoelectron spectroscopy to characterise mixed oxide powders with different atomic ratios in zirconia. Two series of solids are used corresponding to high- and low specific area samples, which are compared to ceria/zirconia mixed oxide thin films grown on polycrystalline tantalum substrates in an electron-beam evaporator. In particular, the surface Ce/Zr ratio (obtained from the Ce 4d and Zr 3d core levels) and the distribution of the oxidation states of surface ceria (obtained from the Ce 3d core level spectrum decomposition) are discussed both in the powder samples as well as in the thin films. In the case of powders, no surface segregation is observed, whereas different surface compositions are detected in thin films prepared from the same oxide target. It seems from the first evaporation attempts that zirconia is more difficult to evaporate under our experimental conditions than ceria, especially from the low zirconia loading targets. These results are important in view of the growth of thin mixed oxide epitaxial film onto oriented silicon substrates. The next step is to study such oriented oxide crystals in oxidative or reductive conditions simulating the catalyst use. © 1998 Elsevier Science B.V. Keywords: Ceria; Zirconia; Three-way catalysts; Mixed oxides; Thin films
1. Introduction Cerium oxide is widely employed as a p r o m o t e r for noble-metal/alumina automotive exhaust catalysts. Adding ZrO2 to CeO2 significantly increases its thermal stability and improves its ability to store and release oxygen (Oxygen Storage Capacity) under reaction conditions [1]. A better understanding of these three-way catalysts (TWC) is of great importance to improve their efficiency during cold starts, * Corresponding author. E-mail: [email protected]
the width of the operating air/fuel ratio (high OSC) and their life time. As part of a basic research programme on the chemical properties of ceria/ zirconia mixed oxides, we have studied powders and thin films of CeOz/ZrO2 mixed oxides (CeZrMO) by X-ray photoelectron spectroscopy (XPS). The surface segregation of cerium and zirconium (where catalytic reactions take place) as well as the oxidation states of cerium give important information which will be useful for comparison when the powders are further treated under oxidising or reducing conditions.
0368-2048/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved PII S0368~2048(97)00134-5
952
A. Galtayries et al./Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 951-956
2. Experimental
2.1. Materials CeZrMO, with CeO2 molar content ranging from 15 to 80% (referred as CZ-x/100 - x), as well as the two reference oxides of pure CeO2 and ZrO2 were prepared by RhOne-Poulenc. Two series of samples are provided, one with high specific area (HS), around 100 m2/g and the other with low specific area (LS), around 10 m2/g. For polycrystalline thin films, the substrates used were polycrystalline Ta foils 99.9% purity, 0.25 mm thick from Goodfellow.
2.2. Thin film preparation The Ta substrates had been sonicated in ethanol. The CeZrMO targets were made of HS compounds and pressed into pellets. Then evaporation of CeZrMO was carried out in an ultrahigh vacuum (UHV) chamber (base pressure 7 × 10 -9 Tort) using electron gun bombardment. The substrate to target distance was about 20 cm. After preliminary outgassing of the targets to 10 -6 Torr, the evaporation proceeded under 1 0 4 - 1 0 -6 Torr, or under O2 up to 10 -5 Torr. The samples were then analysed ex situ by XPS.
2.3. Techniques The XPS measurements have been performed with an SSX-100 spectrometer equipped with a focused (spot size 600 A) monochromatised A1 Kc~ anode (hu = 1486.6 eV). The X-ray source power was kept around 150 W. CeZrMO were pressed into a sample holder devoted to powder analyses. As they are insulating materials, surface charging effects are very important. Electrostatic charging can be stabilised by mounting a nickel grid about 1-2 mm above the samples and by flooding the sample with a wide beam of low energy electrons (flood-gun). The thin film samples are sufficiently conducting, making use of a charge-stabilising technique unnecessary. The data accumulation time lasts typically from 120 to 150 min and the energy resolution corresponds to a FWHM (Full Width at Half Maximum) of 1.1 eV for the Au 4f7/2 peak. To analyse the individual contributions
of the Ce 3d and O ls peaks quantitatively, peak decomposition was carried out using a computer program developed in our laboratory. Binding energies (BE) are referred to the C ls peak at 284.6 eV (hydrocarbon form contamination) and given with an accuracy of _+ 0.2 eV.
3. Results and discussion In the case of both HS and LS powder samples, the Ce 3d5/2 BE of ceria is in good agreement with the literature data on ceria (Table 1), while in the case of mixed oxides the BE are 0.7-1 eV lower. The Zr 3d BE values correspond to ZrOy-type compounds. For all samples, carbon contamination is detected, mainly as graphite or hydrocarbons. However, the presence of C - O bonds characteristic of CO, CO2 or CO3 groups cannot be neglected either. The carbon contamination is thought to be due, to a large extent, to the powder synthesis. The O ls BE (529.8 eV) of CZ-100/00 and CZ-00/100 corresponds to the metallic oxides CeO2 and ZrO2 respectively, associated with oxygen anions in metallic oxide powders [2]. For the CeZrMO samples, the O ls BE range (529.2-529.5 eV) is due to the introduction of Zr in the lattice of ceria. A shoulder on the high BE side is also observed which can be attributed to adsorbed oxygen [7] or surface hydroxyl species and/or adsorbed water species present as contaminants at the surface [2]. In the case of thin film, the BEs of the different Ce or Zr core levels show that the cations are present as the oxide form, as in the case of the corresponding powders. The Ta 4f BE value, when visible, corresponds to tantalum oxide. Deeper analysis in the deposited film reveals metallic tantalum after a few minutes of Ar + ion sputtering. Table 2 presents various parameters calculated from the XPS spectra in the case of HS powders. Similar results are obtained on LS samples. The cerium surface enrichment can be estimated from the Ce/Zr ratio calculated with the Ce 4d and Zr 3d core levels--corresponding to comparable kinetic energy and thus comparable analysis depth--as well as with the Ce 3d/Ce 4d ratio. Both ratios indicate that the surface composition of mixed oxides is close to the bulk one, regardless of zirconium content.
A. Galtayries et al./Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 951-956
953
Table 1 Comparison of experimental BE of HS samples with literature data on cerium and zirconium compounds Reference
BE (eV)
CZ-00/100 CZ- 15/85 CZ-50/50 CZ-68/32 CZ-80/20 CZ- 100/00 CeO...J2] C...£eO2 [3] C.~eO~[4] C.~eO~ [5] C..~eO2 [6] CeO2 [7] Z rO z [8] ZrOy 0 < y < 2 [8] Zr-OH [8] Z rOx [81 Zr [8] "Main peak. b Ce 3d5/2 refers
0 ls ~
C Is
Ce 3d~/2
Zr 3d5/2
529.8 529.5 529.4 529.3 529.2 529.8 529.8
284.6 284.6 284.6 284.6 284.6 284.6
-882.1 882.0 882.1 881.8 882.8
181.8 181.8 181.8 181.7 181.7 --
284.7
882.6 882.9 882.6 883.1 881.2
---. ---182.9 182.4 183.4 181.4 178.8
529.2
to C e 4+ component.
The o x i d e s t o i c h i o m e t r y is g i v e n by the O / ( C e + Zr) ratio: it is close to 2, as expected. Values slightly above 2 can be correlated with adsorbed water as w e l l as with the presence o f carbon (not s h o w n here) in the carbonate form, m a i n l y c o m i n g f r o m the p o w d e r synthesis. The c e r i u m is m a i n l y in the Ce 4+ oxidation state, with a certain increase in the Ce 3+ distribution for the m i x e d oxides. This point has already b e e n reported in the literature [9]. A n o t h e r important question c o n c e r n i n g the C e Z r M O p o w d e r s is the p h o t o r e d u c t i o n process taking place under X-rays during analyses [3,5,10]: an X P S t i m e - d e p e n d e n t study o f C Z - 1 0 0 / 0 0 confirms that if
reduction takes place, it occurs during the v e r y first minutes w h i l e sample position and flood gun are adjusted. T h e r e a f t e r a steady state is o b t a i n e d on the surface. T h u s no t i m e d e p e n d e n c e is f o u n d in our X P S spectra. In the case o f thin films h o w e v e r , X P S analyses (Table 3) indicate a great i n h o m o g e n e i t y within the surface. At first sight (Fig. 1), all films present a z i r c o n i u m surface loss (dots) c o m p a r e d to the b u l k values o f the c o r r e s p o n d i n g targets (crosses), e x c e p t in one case ( C Z - 5 0 / 5 0 powder). T h e p r e s e n c e o f tantalum substrate w h e n detected, justifies the O / c a t i o n stoichiometric ratios: the thicker and cleaner
Table 2 Quantitative aspects of the HS analyses of CeZrMO by XPS Reference
Ce/Zr bulk atomic ratio
Ce/Zr XPS atomic ratio"
O/(Ce + Zr) XPS atomic ratio ~
Ce 3d/Ce 4d
%
CZ-00/100 CZ- 15/85 CZ-50/50 CZ-68/32 CZ-80/20 CZ- 100/00
0.00 0.18 1.00 2.12 4.00 --
0.00 0.20 0.93 2.31 4.15 --
3.0 2.2 2.3 2.4 2.3 2.2
-1.17 1.04 1.09 0.99 1.07
-63 62 58 57 70
Calculated from the areas of the Ce 4d and Zr 3d core levels. u Calculated from the areas of the O ls, Ce 4d and Zr 3d core levels.
C e 4+
A. Galtayries et al./Journal o f Electron Spectroscopy and Related Phenomena 8 8 - 9 1 (1998) 9 5 1 - 9 5 6
954
Table 3 XPS quantitative results of thin films analyses; each line corresponds to a different experiment with a new substrate Source material
Thin films
Target reference
Bulk Ce/Zr ratio
CZ-00/100
0.00 0.00
0.00 0.00
---
CZ- 15/85
0.18 0.18
0.00 2.86
1.1 l 1.14
20.50 7.60
6 8
0.18 1.00 1.00 2.12 2.12 2.12 2.12 4.00 4.00 ----
0.25 1.85 0.72 8.33 14.28 33.33 16.66 16.66 33.33 ----
0.97 0.86 0.79 0.75 0.70 0.85 1.17 0.69 1.04 0.77 0.91 1.13
2.52 3.48 2.78 3.16 3.18 2.96 4.91 3.16 4.14 3.61 5.09 2.37
--46 ---3 -51 -11 --
CZ-50/50 CZ-68/32
CZ-80/20 CZ-100/00
XPS Ce/Zr ratio ~
Ce 3d/Ce 4d
O (Zr + Ce) ~ 2.51 2.33
O/Ta c
% Ce 4+
---
----42 52 49 62 64 66 0 55 54 56 47 71
Films obtained under 0 2 exposure shown in italic. a Calculated from the Ce 4d and Zr 3d core level spectra. b Calculated from the O ls, Ce 4d and Zr 3d core level spectra. c Calculated from the O l s, Ta 4f core level spectra.
the mixed oxide film, the closer to 2 the ratio. On the other hand, XPS spectra recorded during argon sputtering profiles reveal the presence of a thin layer of contamination due to transfer of the film through air, between the evaporator and the spectrometer. This contamination contributes also to the O/cation ratio. Regardless of the initial target composition, O ls core level spectra have been systematically decomposed into up to four components, corresponding to oxygen in the oxide network (BE 530.1 eV) [2], oxygen attributed to hydroxyl surface species or defects at the surface (531.7eV) [11], oxygen ÷ o
~4 • Zr/Ce (XPS) ÷ Zr/Ce (bulk)
e~
~2 b~
0
2 4 Zr/Ce bulk ratio
6
Fig. I. XPS Zr/Ce composition (calculated using the Zr 3d, Ce 4d peaks) of thin films as a function of the bulk Zr/Ce ratio of the initial targets.
attributed to molecular water (533.2 eV) [11] and an oxide component from tantalum oxide at the interface (532.3 eV). Fig. 2 presents the oxygen species distribution of all O Is spectra as a function of the O/cations. The curves obtained correspond logically to the decrease of the oxide network component in favour of defects or water-induced contamination on the film surface with increasing total O/cation stoichiometry. This oxygen decomposition will be used later for comparison with single-crystal thin films when obtained, in view of the OSC of ceria. Putna et al. [12] already focused on the nature of a weakly bound oxygen species on pure and zirconiasupported ceria compounds, and noticed its absence from ceria single-crystal surfaces. The Ce 4+ distribution in the case of thin films depends strongly on their thickness and composition, but for CZ-100/00 (Table 3, under 02 atmosphere), the cerium redox distribution is similar to that obtained with CZ-100/00 powder (70% Ce4+). However for the CZ- 15/85 (Table 3, third try, under UHV), only 42% of the cerium is in the Ce 4+ oxidation State, instead of 63% in the powder itself. For CZ-68/32 and CZ-80/20, where cerium is the main species on the thin film surface, the Ce 4+ distribution is in the same
A. Galtayries et al./Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 951-956
955
80
g g ff~ 60
• 530,1 +/- 0.2 eV A 531,7 +/- 0,2 eV
it
"~ 40
• 533,2 +/- 0,3 eV
A A A
o
20
i
5
10
I
20 15 O/cationsratio
Fig. 2. Oxygen species distribution as obtained by O 1s spectra decomposition as a function of O/cation ratio (calculated using the O I s, Ce 4d and Zr 3d peaks). range as in the mixed oxide powders, and even the introduction of 02 does not seem to interfere much with it. From depth profile analyses of several thin films, we obtained the atomic distribution of C, O, Ce, Zr and Ta in the films. A example of such a depth profile is shown in Fig. 3. These preliminary results show that oxide films with uniform composition over about 1 5 0 - 2 5 0 .A (using calibration carried out with SiO2 thin films) can be obtained. Hence, we manage to obtain pure zirconia and ceria films from the corresponding pure oxide powder, while on the mixed oxides side, real mixed oxide films are only obtained from CZ-50/50 and CZ-15/ 85 powders, for which the initial zirconium content is important. Fig. 3 presents the profile of the CZ-50/ 50-derived thin film where a rate inversion takes place in the Ce/Zr ratio during film deposition: the
cerium evaporates first and gradually the zirconium proportion increases to become the main cationic component in the outermost layers. The mixed oxide region thus obtained corresponds to a Ce/Zr ratio equal to 0.6 (virtual CZ-37.5/62.5 oxide). The zirconium behaviour during evaporation may be correlated with sample reduction before evaporation, as the temperature corresponding to a vapour pressure is about 600 degrees higher for Zr than for Ce [131. To our knowledge, only very few published results deal with oxides co-evaporation. A1-Dhhan et al. [14] have worked on CeO2 thermally co-evaporated with SiO and GeO2 or SnO2 which they studied by XPS. A k i m o v et al. [15] studied the composition and growth structure of Zr, Ce and Z r - C e alloy deposited on Si (100) by laser ablation in an oxygen atmosphere as a function of oxygen pressure and substrate
80 ! 60
--
--O
Is
/ - - - C I s
40
--
-
Zr 3d
Ta 4f
O
20 -
-
-
Ce 4d
0 0
200
400 sputtering time (s)
600
800
Fig. 3. XPS depth profile of a thin film obtained from a CZ-50/50 target powder, polycrystalline Ta substrate, Ar÷ (4 keV).
956
A. Galtayries et aL/Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 951-956
temperature. They reported a Zr/Ce ratio in the film equal to that of the CZ-12/88 target alloy. In the case of formation of ceria/zirconia films on indium tin oxide substrates, Veszelei et al. [16] have observed that depending on the experimental conditions, different mixed oxides may be obtained from the same targets. Ingo [17] presents zirconia-based plasmasprayed coatings which are mixed 2 5 . 5 C E O 2 2.5 Y 2 0 3 - 7 2 ZrO2 samples deposited on AISI 306 stainless steel substrates for which he reported an XPS Ce/Zr ratio of 0.26 compared to the bulk value of 0.35. In all these cases, the proportion of zirconium is important compared to cerium in the initial target, as in the case of our CZ-50/50 or CZ-15/85 with which we also succeeded to obtain mixed oxide films. These initial results show the importance of choosing the proper experimental conditions, such as composition and preparation of the mixed oxide targets (including the outgassing conditions), the UHV conditions, nature of substrate, substrate cleaning procedure (to avoid many compositional variations at the interface), and substrate temperature. A systematic process for gas introduction during deposition will also be chosen (after 3 0 - 5 0 A of film deposited).
4. Conclusions The XPS measurements performed on HS and LS samples show no significant surface segregation of cerium. As far as thin films syntheses are concerned, the following main conclusions can be drawn from preliminary experiments: (1) it is possible to obtain polycrystalline pure ceria and zirconia films with definite compositions over at least 200 .~; (2) in the case of mixed oxide films, the CZ-50/50 and CZ-15/85 powders seem to present the most suitable starting materials for thin film synthesis. With CZ-80/20 and CZ-68/32, we face difficulties in obtaining a mixed oxide composition.
Acknowledgements This work has been partly supported by the TMR Programme of the EU under contract ERB FMRXCT96-0060 (CEZIRENCAT Project) and by the Belgian Federal Office for Scientific Technical and Cultural Affairs. RS acknowledges support from the Belgian Fund for Scientific Research (F.N.R.S.).
References [1] P. Fornasiero, R. Di Monte, G. Raga Rao, J. Kaspar, S. Meriani, A. Trovarelli and M. Graziani, J. Catal. 151 (1995) 168. [2] M. Romeo, K. Bak, J. El Fallah, F. Le Normand and L. Hilaire, Surf. Interf. Anal. 20 (1993) 508. [3] A.E. Hughes, J.D. Gorman, P.J.K. Patterson and R. Carter, Surf. Interf. Anal. 24 (1996) 634. [4] C.E. Guillaume, M. Vermeersch, R. Sporken, JJ. Verbist, S. Mathot and G. Demortier, Surf. Interf. Anal. 22 (1994) 186. [5] P. Worn Park and J.S. Ledford, Langmuir 12 (1996) 1794. [6] S.J. Schmieg and D.N. Belton, North American Operations Research and De,~elopmentCenter, General Motors Corporation, R&D-8235, 1994. [7] K.B. Sundaram, P.F. Wahid and O. Melendez, J. Vac. Sci. Technol. A 15 (1) (1997) 52. [8] P.C. Wong, Y.S. Li and K.A.R. Mitchell, Surf. Rev. Lett. 2 (3) (1995) 297. [9] L.G. Appel, A. Frydman,C.A. Perez, J.G. Eon, D.G. Castner, C.T. Campbelland M. Schmal, Phys. Stat. Sol. (b) 192 (1995) 477. [10] K. Bak and L. Hilaire, Appl. Surf. Sci. 70/71 (1995) 191. [11] A. Galtayries, S. Wisniewski and J. Grimblot, J. Electron Spectrosc. Relat. Phenom. 87 (1997) 31. [12] E.S. Putna, J.M. Vohs and R.J. Gorte, Phys. Chem. 100 (1996) 17862, 1997. [13] Kurt J. Lesker Company, Vacuum Products (1992) p. 25-16 and 25-25. [14] Z.T. AI-Dhhan,T. Hashemi and C.A. Hogarth, Spectrochim. Acta 44B (2) (1989) 205. [15] A.G. Akimov, D.B. Bogomolov, A.E. Gorodetskii, L.P. Kazanskii, A.N. Khodan, I.L. Krylov, J.-P. Langeron, N.A. Melnikova, D. Michel, J.-L. Vignes and J. Perriere, Thin Solid Films 238 (1994) 15. [16] M. Veszelei, L. Kullman, A. Azens, C.G. Granqvist and B. Hj6rvarsson,J. Appl. Phys. 81 (4) (1997) 2024. [17] G.M. Ingo, Appl. Surf. Sci. 70/71 (1993) 235.
ELSEVIER
Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 951-956
XPS comparative study of ceria/zirconia mixed oxides: powders and thin film characterisation A. Galtayries a'*, R. Sporken a, J. Riga a, G. Blanchard b, R.
Caudano
a
aLaboratoire lnterdisciplinaire de Spectroscopie Electronique (LISE), Facultgs Universitaires Notre-Dame de la Paix, B-5000 Namur, Belgium bRhOne Poulenc Recherches (RP), 52 rue de la Haie Coq, F-93308 Aubervilliers, France
Abstract Ceria/zirconia mixed oxides are important new materials for automotive exhaust catalysis. The addition of ZrO2 in the solid solution formed by the mixed oxides improves the redox properties of the solid and thus the catalyst's efficiency. The work reported uses X-ray photoelectron spectroscopy to characterise mixed oxide powders with different atomic ratios in zirconia. Two series of solids are used corresponding to high- and low specific area samples, which are compared to ceria/zirconia mixed oxide thin films grown on polycrystalline tantalum substrates in an electron-beam evaporator. In particular, the surface Ce/Zr ratio (obtained from the Ce 4d and Zr 3d core levels) and the distribution of the oxidation states of surface ceria (obtained from the Ce 3d core level spectrum decomposition) are discussed both in the powder samples as well as in the thin films. In the case of powders, no surface segregation is observed, whereas different surface compositions are detected in thin films prepared from the same oxide target. It seems from the first evaporation attempts that zirconia is more difficult to evaporate under our experimental conditions than ceria, especially from the low zirconia loading targets. These results are important in view of the growth of thin mixed oxide epitaxial film onto oriented silicon substrates. The next step is to study such oriented oxide crystals in oxidative or reductive conditions simulating the catalyst use. © 1998 Elsevier Science B.V. Keywords: Ceria; Zirconia; Three-way catalysts; Mixed oxides; Thin films
1. Introduction Cerium oxide is widely employed as a p r o m o t e r for noble-metal/alumina automotive exhaust catalysts. Adding ZrO2 to CeO2 significantly increases its thermal stability and improves its ability to store and release oxygen (Oxygen Storage Capacity) under reaction conditions [1]. A better understanding of these three-way catalysts (TWC) is of great importance to improve their efficiency during cold starts, * Corresponding author. E-mail: [email protected]
the width of the operating air/fuel ratio (high OSC) and their life time. As part of a basic research programme on the chemical properties of ceria/ zirconia mixed oxides, we have studied powders and thin films of CeOz/ZrO2 mixed oxides (CeZrMO) by X-ray photoelectron spectroscopy (XPS). The surface segregation of cerium and zirconium (where catalytic reactions take place) as well as the oxidation states of cerium give important information which will be useful for comparison when the powders are further treated under oxidising or reducing conditions.
0368-2048/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved PII S0368~2048(97)00134-5
952
A. Galtayries et al./Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 951-956
2. Experimental
2.1. Materials CeZrMO, with CeO2 molar content ranging from 15 to 80% (referred as CZ-x/100 - x), as well as the two reference oxides of pure CeO2 and ZrO2 were prepared by RhOne-Poulenc. Two series of samples are provided, one with high specific area (HS), around 100 m2/g and the other with low specific area (LS), around 10 m2/g. For polycrystalline thin films, the substrates used were polycrystalline Ta foils 99.9% purity, 0.25 mm thick from Goodfellow.
2.2. Thin film preparation The Ta substrates had been sonicated in ethanol. The CeZrMO targets were made of HS compounds and pressed into pellets. Then evaporation of CeZrMO was carried out in an ultrahigh vacuum (UHV) chamber (base pressure 7 × 10 -9 Tort) using electron gun bombardment. The substrate to target distance was about 20 cm. After preliminary outgassing of the targets to 10 -6 Torr, the evaporation proceeded under 1 0 4 - 1 0 -6 Torr, or under O2 up to 10 -5 Torr. The samples were then analysed ex situ by XPS.
2.3. Techniques The XPS measurements have been performed with an SSX-100 spectrometer equipped with a focused (spot size 600 A) monochromatised A1 Kc~ anode (hu = 1486.6 eV). The X-ray source power was kept around 150 W. CeZrMO were pressed into a sample holder devoted to powder analyses. As they are insulating materials, surface charging effects are very important. Electrostatic charging can be stabilised by mounting a nickel grid about 1-2 mm above the samples and by flooding the sample with a wide beam of low energy electrons (flood-gun). The thin film samples are sufficiently conducting, making use of a charge-stabilising technique unnecessary. The data accumulation time lasts typically from 120 to 150 min and the energy resolution corresponds to a FWHM (Full Width at Half Maximum) of 1.1 eV for the Au 4f7/2 peak. To analyse the individual contributions
of the Ce 3d and O ls peaks quantitatively, peak decomposition was carried out using a computer program developed in our laboratory. Binding energies (BE) are referred to the C ls peak at 284.6 eV (hydrocarbon form contamination) and given with an accuracy of _+ 0.2 eV.
3. Results and discussion In the case of both HS and LS powder samples, the Ce 3d5/2 BE of ceria is in good agreement with the literature data on ceria (Table 1), while in the case of mixed oxides the BE are 0.7-1 eV lower. The Zr 3d BE values correspond to ZrOy-type compounds. For all samples, carbon contamination is detected, mainly as graphite or hydrocarbons. However, the presence of C - O bonds characteristic of CO, CO2 or CO3 groups cannot be neglected either. The carbon contamination is thought to be due, to a large extent, to the powder synthesis. The O ls BE (529.8 eV) of CZ-100/00 and CZ-00/100 corresponds to the metallic oxides CeO2 and ZrO2 respectively, associated with oxygen anions in metallic oxide powders [2]. For the CeZrMO samples, the O ls BE range (529.2-529.5 eV) is due to the introduction of Zr in the lattice of ceria. A shoulder on the high BE side is also observed which can be attributed to adsorbed oxygen [7] or surface hydroxyl species and/or adsorbed water species present as contaminants at the surface [2]. In the case of thin film, the BEs of the different Ce or Zr core levels show that the cations are present as the oxide form, as in the case of the corresponding powders. The Ta 4f BE value, when visible, corresponds to tantalum oxide. Deeper analysis in the deposited film reveals metallic tantalum after a few minutes of Ar + ion sputtering. Table 2 presents various parameters calculated from the XPS spectra in the case of HS powders. Similar results are obtained on LS samples. The cerium surface enrichment can be estimated from the Ce/Zr ratio calculated with the Ce 4d and Zr 3d core levels--corresponding to comparable kinetic energy and thus comparable analysis depth--as well as with the Ce 3d/Ce 4d ratio. Both ratios indicate that the surface composition of mixed oxides is close to the bulk one, regardless of zirconium content.
A. Galtayries et al./Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 951-956
953
Table 1 Comparison of experimental BE of HS samples with literature data on cerium and zirconium compounds Reference
BE (eV)
CZ-00/100 CZ- 15/85 CZ-50/50 CZ-68/32 CZ-80/20 CZ- 100/00 CeO...J2] C...£eO2 [3] C.~eO~[4] C.~eO~ [5] C..~eO2 [6] CeO2 [7] Z rO z [8] ZrOy 0 < y < 2 [8] Zr-OH [8] Z rOx [81 Zr [8] "Main peak. b Ce 3d5/2 refers
0 ls ~
C Is
Ce 3d~/2
Zr 3d5/2
529.8 529.5 529.4 529.3 529.2 529.8 529.8
284.6 284.6 284.6 284.6 284.6 284.6
-882.1 882.0 882.1 881.8 882.8
181.8 181.8 181.8 181.7 181.7 --
284.7
882.6 882.9 882.6 883.1 881.2
---. ---182.9 182.4 183.4 181.4 178.8
529.2
to C e 4+ component.
The o x i d e s t o i c h i o m e t r y is g i v e n by the O / ( C e + Zr) ratio: it is close to 2, as expected. Values slightly above 2 can be correlated with adsorbed water as w e l l as with the presence o f carbon (not s h o w n here) in the carbonate form, m a i n l y c o m i n g f r o m the p o w d e r synthesis. The c e r i u m is m a i n l y in the Ce 4+ oxidation state, with a certain increase in the Ce 3+ distribution for the m i x e d oxides. This point has already b e e n reported in the literature [9]. A n o t h e r important question c o n c e r n i n g the C e Z r M O p o w d e r s is the p h o t o r e d u c t i o n process taking place under X-rays during analyses [3,5,10]: an X P S t i m e - d e p e n d e n t study o f C Z - 1 0 0 / 0 0 confirms that if
reduction takes place, it occurs during the v e r y first minutes w h i l e sample position and flood gun are adjusted. T h e r e a f t e r a steady state is o b t a i n e d on the surface. T h u s no t i m e d e p e n d e n c e is f o u n d in our X P S spectra. In the case o f thin films h o w e v e r , X P S analyses (Table 3) indicate a great i n h o m o g e n e i t y within the surface. At first sight (Fig. 1), all films present a z i r c o n i u m surface loss (dots) c o m p a r e d to the b u l k values o f the c o r r e s p o n d i n g targets (crosses), e x c e p t in one case ( C Z - 5 0 / 5 0 powder). T h e p r e s e n c e o f tantalum substrate w h e n detected, justifies the O / c a t i o n stoichiometric ratios: the thicker and cleaner
Table 2 Quantitative aspects of the HS analyses of CeZrMO by XPS Reference
Ce/Zr bulk atomic ratio
Ce/Zr XPS atomic ratio"
O/(Ce + Zr) XPS atomic ratio ~
Ce 3d/Ce 4d
%
CZ-00/100 CZ- 15/85 CZ-50/50 CZ-68/32 CZ-80/20 CZ- 100/00
0.00 0.18 1.00 2.12 4.00 --
0.00 0.20 0.93 2.31 4.15 --
3.0 2.2 2.3 2.4 2.3 2.2
-1.17 1.04 1.09 0.99 1.07
-63 62 58 57 70
Calculated from the areas of the Ce 4d and Zr 3d core levels. u Calculated from the areas of the O ls, Ce 4d and Zr 3d core levels.
C e 4+
A. Galtayries et al./Journal o f Electron Spectroscopy and Related Phenomena 8 8 - 9 1 (1998) 9 5 1 - 9 5 6
954
Table 3 XPS quantitative results of thin films analyses; each line corresponds to a different experiment with a new substrate Source material
Thin films
Target reference
Bulk Ce/Zr ratio
CZ-00/100
0.00 0.00
0.00 0.00
---
CZ- 15/85
0.18 0.18
0.00 2.86
1.1 l 1.14
20.50 7.60
6 8
0.18 1.00 1.00 2.12 2.12 2.12 2.12 4.00 4.00 ----
0.25 1.85 0.72 8.33 14.28 33.33 16.66 16.66 33.33 ----
0.97 0.86 0.79 0.75 0.70 0.85 1.17 0.69 1.04 0.77 0.91 1.13
2.52 3.48 2.78 3.16 3.18 2.96 4.91 3.16 4.14 3.61 5.09 2.37
--46 ---3 -51 -11 --
CZ-50/50 CZ-68/32
CZ-80/20 CZ-100/00
XPS Ce/Zr ratio ~
Ce 3d/Ce 4d
O (Zr + Ce) ~ 2.51 2.33
O/Ta c
% Ce 4+
---
----42 52 49 62 64 66 0 55 54 56 47 71
Films obtained under 0 2 exposure shown in italic. a Calculated from the Ce 4d and Zr 3d core level spectra. b Calculated from the O ls, Ce 4d and Zr 3d core level spectra. c Calculated from the O l s, Ta 4f core level spectra.
the mixed oxide film, the closer to 2 the ratio. On the other hand, XPS spectra recorded during argon sputtering profiles reveal the presence of a thin layer of contamination due to transfer of the film through air, between the evaporator and the spectrometer. This contamination contributes also to the O/cation ratio. Regardless of the initial target composition, O ls core level spectra have been systematically decomposed into up to four components, corresponding to oxygen in the oxide network (BE 530.1 eV) [2], oxygen attributed to hydroxyl surface species or defects at the surface (531.7eV) [11], oxygen ÷ o
~4 • Zr/Ce (XPS) ÷ Zr/Ce (bulk)
e~
~2 b~
0
2 4 Zr/Ce bulk ratio
6
Fig. I. XPS Zr/Ce composition (calculated using the Zr 3d, Ce 4d peaks) of thin films as a function of the bulk Zr/Ce ratio of the initial targets.
attributed to molecular water (533.2 eV) [11] and an oxide component from tantalum oxide at the interface (532.3 eV). Fig. 2 presents the oxygen species distribution of all O Is spectra as a function of the O/cations. The curves obtained correspond logically to the decrease of the oxide network component in favour of defects or water-induced contamination on the film surface with increasing total O/cation stoichiometry. This oxygen decomposition will be used later for comparison with single-crystal thin films when obtained, in view of the OSC of ceria. Putna et al. [12] already focused on the nature of a weakly bound oxygen species on pure and zirconiasupported ceria compounds, and noticed its absence from ceria single-crystal surfaces. The Ce 4+ distribution in the case of thin films depends strongly on their thickness and composition, but for CZ-100/00 (Table 3, under 02 atmosphere), the cerium redox distribution is similar to that obtained with CZ-100/00 powder (70% Ce4+). However for the CZ- 15/85 (Table 3, third try, under UHV), only 42% of the cerium is in the Ce 4+ oxidation State, instead of 63% in the powder itself. For CZ-68/32 and CZ-80/20, where cerium is the main species on the thin film surface, the Ce 4+ distribution is in the same
A. Galtayries et al./Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 951-956
955
80
g g ff~ 60
• 530,1 +/- 0.2 eV A 531,7 +/- 0,2 eV
it
"~ 40
• 533,2 +/- 0,3 eV
A A A
o
20
i
5
10
I
20 15 O/cationsratio
Fig. 2. Oxygen species distribution as obtained by O 1s spectra decomposition as a function of O/cation ratio (calculated using the O I s, Ce 4d and Zr 3d peaks). range as in the mixed oxide powders, and even the introduction of 02 does not seem to interfere much with it. From depth profile analyses of several thin films, we obtained the atomic distribution of C, O, Ce, Zr and Ta in the films. A example of such a depth profile is shown in Fig. 3. These preliminary results show that oxide films with uniform composition over about 1 5 0 - 2 5 0 .A (using calibration carried out with SiO2 thin films) can be obtained. Hence, we manage to obtain pure zirconia and ceria films from the corresponding pure oxide powder, while on the mixed oxides side, real mixed oxide films are only obtained from CZ-50/50 and CZ-15/ 85 powders, for which the initial zirconium content is important. Fig. 3 presents the profile of the CZ-50/ 50-derived thin film where a rate inversion takes place in the Ce/Zr ratio during film deposition: the
cerium evaporates first and gradually the zirconium proportion increases to become the main cationic component in the outermost layers. The mixed oxide region thus obtained corresponds to a Ce/Zr ratio equal to 0.6 (virtual CZ-37.5/62.5 oxide). The zirconium behaviour during evaporation may be correlated with sample reduction before evaporation, as the temperature corresponding to a vapour pressure is about 600 degrees higher for Zr than for Ce [131. To our knowledge, only very few published results deal with oxides co-evaporation. A1-Dhhan et al. [14] have worked on CeO2 thermally co-evaporated with SiO and GeO2 or SnO2 which they studied by XPS. A k i m o v et al. [15] studied the composition and growth structure of Zr, Ce and Z r - C e alloy deposited on Si (100) by laser ablation in an oxygen atmosphere as a function of oxygen pressure and substrate
80 ! 60
--
--O
Is
/ - - - C I s
40
--
-
Zr 3d
Ta 4f
O
20 -
-
-
Ce 4d
0 0
200
400 sputtering time (s)
600
800
Fig. 3. XPS depth profile of a thin film obtained from a CZ-50/50 target powder, polycrystalline Ta substrate, Ar÷ (4 keV).
956
A. Galtayries et aL/Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 951-956
temperature. They reported a Zr/Ce ratio in the film equal to that of the CZ-12/88 target alloy. In the case of formation of ceria/zirconia films on indium tin oxide substrates, Veszelei et al. [16] have observed that depending on the experimental conditions, different mixed oxides may be obtained from the same targets. Ingo [17] presents zirconia-based plasmasprayed coatings which are mixed 2 5 . 5 C E O 2 2.5 Y 2 0 3 - 7 2 ZrO2 samples deposited on AISI 306 stainless steel substrates for which he reported an XPS Ce/Zr ratio of 0.26 compared to the bulk value of 0.35. In all these cases, the proportion of zirconium is important compared to cerium in the initial target, as in the case of our CZ-50/50 or CZ-15/85 with which we also succeeded to obtain mixed oxide films. These initial results show the importance of choosing the proper experimental conditions, such as composition and preparation of the mixed oxide targets (including the outgassing conditions), the UHV conditions, nature of substrate, substrate cleaning procedure (to avoid many compositional variations at the interface), and substrate temperature. A systematic process for gas introduction during deposition will also be chosen (after 3 0 - 5 0 A of film deposited).
4. Conclusions The XPS measurements performed on HS and LS samples show no significant surface segregation of cerium. As far as thin films syntheses are concerned, the following main conclusions can be drawn from preliminary experiments: (1) it is possible to obtain polycrystalline pure ceria and zirconia films with definite compositions over at least 200 .~; (2) in the case of mixed oxide films, the CZ-50/50 and CZ-15/85 powders seem to present the most suitable starting materials for thin film synthesis. With CZ-80/20 and CZ-68/32, we face difficulties in obtaining a mixed oxide composition.
Acknowledgements This work has been partly supported by the TMR Programme of the EU under contract ERB FMRXCT96-0060 (CEZIRENCAT Project) and by the Belgian Federal Office for Scientific Technical and Cultural Affairs. RS acknowledges support from the Belgian Fund for Scientific Research (F.N.R.S.).
References [1] P. Fornasiero, R. Di Monte, G. Raga Rao, J. Kaspar, S. Meriani, A. Trovarelli and M. Graziani, J. Catal. 151 (1995) 168. [2] M. Romeo, K. Bak, J. El Fallah, F. Le Normand and L. Hilaire, Surf. Interf. Anal. 20 (1993) 508. [3] A.E. Hughes, J.D. Gorman, P.J.K. Patterson and R. Carter, Surf. Interf. Anal. 24 (1996) 634. [4] C.E. Guillaume, M. Vermeersch, R. Sporken, JJ. Verbist, S. Mathot and G. Demortier, Surf. Interf. Anal. 22 (1994) 186. [5] P. Worn Park and J.S. Ledford, Langmuir 12 (1996) 1794. [6] S.J. Schmieg and D.N. Belton, North American Operations Research and De,~elopmentCenter, General Motors Corporation, R&D-8235, 1994. [7] K.B. Sundaram, P.F. Wahid and O. Melendez, J. Vac. Sci. Technol. A 15 (1) (1997) 52. [8] P.C. Wong, Y.S. Li and K.A.R. Mitchell, Surf. Rev. Lett. 2 (3) (1995) 297. [9] L.G. Appel, A. Frydman,C.A. Perez, J.G. Eon, D.G. Castner, C.T. Campbelland M. Schmal, Phys. Stat. Sol. (b) 192 (1995) 477. [10] K. Bak and L. Hilaire, Appl. Surf. Sci. 70/71 (1995) 191. [11] A. Galtayries, S. Wisniewski and J. Grimblot, J. Electron Spectrosc. Relat. Phenom. 87 (1997) 31. [12] E.S. Putna, J.M. Vohs and R.J. Gorte, Phys. Chem. 100 (1996) 17862, 1997. [13] Kurt J. Lesker Company, Vacuum Products (1992) p. 25-16 and 25-25. [14] Z.T. AI-Dhhan,T. Hashemi and C.A. Hogarth, Spectrochim. Acta 44B (2) (1989) 205. [15] A.G. Akimov, D.B. Bogomolov, A.E. Gorodetskii, L.P. Kazanskii, A.N. Khodan, I.L. Krylov, J.-P. Langeron, N.A. Melnikova, D. Michel, J.-L. Vignes and J. Perriere, Thin Solid Films 238 (1994) 15. [16] M. Veszelei, L. Kullman, A. Azens, C.G. Granqvist and B. Hj6rvarsson,J. Appl. Phys. 81 (4) (1997) 2024. [17] G.M. Ingo, Appl. Surf. Sci. 70/71 (1993) 235.