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The surface structure of CeO2(110) single crystals studied by STM and RHEED
Surface Science 433–435 (1999) 127–130 www.elsevier.nl/locate/susc
The surface structure of CeO (110) single crystals studied 2 by STM and RHEED H. No¨renberg *, G.A.D. Briggs University of Oxford, Department of Materials, Parks Road, Oxford OX1 3PH, UK
Abstract We have studied the surface structure of CeO (110) single crystals by elevated temperature STM and electron 2 diffraction. The surface shows line features running along the [11: 0] direction after annealing to 940°C. The lines are ˚ separated by 11 A which corresponds to a (2×1) surface reconstruction. After annealing to 1030°C the surface forms (111) and (111: ) facets with ridges running along the [11: 0] direction over long distances exceeding the maximum scan ˚ . The facetting can be observed in RHEED and STM. The STM contrast originated from tunneling range of 1300 A into unoccupied metal states. © 1999 Elsevier Science B.V. All rights reserved. Keywords: CeO (110); Ceria; RHEED; STM; Surface reconstruction 2
1. Introduction CeO has important applications in catalysis, in 2 particular as washcoat additive to oxidize CO and hydrocarbons and reduce NO in car exhausts [1]. x CeO can readily change its oxidation state, which 2 makes it useful as an oxygen storage material. The surface structure of the oxide seems to play a crucial role during the chemical reactions [2,3]. Although CeO is an electrically insulating oxide 2 with a bandgap of a few electronvolts [2,4] and therefore difficult to investigate by scanning tunneling microscopy (STM ), this oxide can be made conductive to some extent by creating oxygen vacancies after thermal annealing [5,6 ]. On the CeO (111) surface which is the lowest energy 2 surface of fluorite structure metal oxides [7,8] it * Corresponding author. Fax: +44-1865-273783. E-mail address: [email protected] (H. No¨renberg)
was possible to obtain atomic resolution by STM [5]. An interesting question is what structure the higher energy low index surfaces of CeO such as 2 (110) will assume. Experiments on thin CeO films 2 which were grown on Si wafers showed a tendency to form {111} facets at higher film thicknesses [9,10]. Atomic force microscopy (AFM ) experiments on these CeO films showed elongated facet2 ted features [10]. STM studies on UO [11], which has the same 2 crystallographic structure and within 1% the same lattice constant as CeO , showed that the (110) 2 surface forms an (n×1) surface reconstruction (with n=1, 2, 3). Facetting has been observed too [12]. STM investigations on CeO (110) surfaces 2 have not been reported so far. In this paper we want to show what the CeO (110) surface looks like and that flat or 2 facetted surfaces can be obtained by annealing procedures under ultra-high vacuum conditions.
0039-6028/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 99 ) 0 00 7 0 -9
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H. No¨renberg, G.A.D. Briggs / Surface Science 433–435 (1999) 127–130
2. Experimental Epipolished CeO (110) wafers supplied by 2 Commercial Crystal Laboratories Inc. were used as substrates. The greyish appearance of the crystals was probably caused by traces of CaO which might also be responsible for the conductivity of 1×10−3 (V cm)−1 [13]. To clarify this point we have carried out proton induced X-ray emission (PIXE) experiments [14], which revealed a Ca content of 400 ppm in our samples. The amount of Ca on the surface was below 1% as estimated by Auger electron spectroscopy (AES), and therefore the surface of the crystals can be considered clean. The room temperature (RT ) STM experiments were carried out with an OMICRON STM1 working at a base pressure of 3×10−11 mbar. Although the samples still had a conductivity of only 0.02 (V cm)−1 it was possible to study the surface morphology by STM at RT at very low tunneling currents, in the range 10 to 20 pA. The bias voltage was applied between the tip and the grounded sample holder. Another way of overcoming the poor conductivity is imaging at higher temperatures, as the conductivity of ceria increases with temperature [15]. Experiments at elevated temperatures were carried out with a JEOL JSTM 4500-XT in the temperature range up to 250°C. Lateral distance and height measurements were calibrated against a Si(001) standard. Commercially produced, etched Pt–Ir tips were used. Electron diffraction (RHEED and LEED) was used to monitor the surface morphology in situ during surface preparation.
3. Results and discussion Fig. 1a shows an elevated temperature STM image of a CeO (110) surface which was annealed 2 to about 940°C for a few minutes. The main features are lines running along the [11: 0] direction. A close-up shows that these lines are separated by ˚ , which is twice the lattice constant of CeO 11 A 2 (Fig. 1b). Occasionally this separation is three times the lattice constant (Fig. 1b). So far, atomic resolution along the [001] direction could not be achieved. Some of the lines visible on the surface
Fig. 1. STM image of CeO (110), imaging conditions 250°C, 2 ˚ ×1300 A ˚ ; (b) U =+3 V, I =50 pA: (a) image size 1300 A BIAS T˚ ˚. image size 530 A×185 A
( Fig. 1b) are on different height levels, which might indicate an early stage of facetting. RHEED shows half-order streaks in the [11: 0] azimuth (Fig. 2a) confirming the surface reconstruction visible in the STM image ( Fig. 1b). Halforder spots were visible in LEED in the [11: 0] direction, but no superstructure features could be observed in the [001] azimuth. Based on these three investigation methods, the surface structure of CeO (110) at this stage of annealing can be 2 described as (2×1) reconstructed. Similar to the UO (110) surface [11], some disorder in the [001] 2 direction occasionally leads to small (3×1) reconstructed regions. According to theoretical calculations, the (110) surface of CeO undergoes 2 substantial relaxation in order to reduce the surface energy [16 ]. After further annealing the surface started to form facets. We have monitored the facetting process by RHEED during annealing. Fig. 2b shows a RHEED pattern taken in the [11: 0] azimuth after annealing to 1030°C. The prominent features in this RHEED pattern are the streaks which are tilted by 35°. This is exactly the angle between the (110) plane and the (111) and (111: ) planes in CeO . This result is in agreement with 2
H. No¨renberg, G.A.D. Briggs / Surface Science 433–435 (1999) 127–130
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Fig. 3. STM image of CeO (110), at RT: (a) image size 2 ˚ ×390 A ˚ , imaging conditions: 390 A U =3 V, I =50 pA; (b) BIAS T ˚ ˚ image size 200 A×70 A, imaging conditions: U =2 V, BIAS I =20 pA. T
Fig. 2. RHEED patterns of CeO (110): (a) after annealing to 2 940°C, [11: 0] azimuth, 12 keV, half-order diffraction features are indicated by arrows; (b) after annealing to 1030°C, [11: 0] azimuth, 20 keV; (c) after annealing to 1030°C, [001] azimuth, 20 keV.
investigations on thin CeO films, where in depen2 dence on the preparation conditions elongated (111) and (111: ) facets have been observed by AFM [10] and RHEED [9]. The RHEED pattern taken in the [001] azimuth (Fig. 2c) shows a rather flat surface without reconstruction. We have investigated these facetted surfaces by STM. Fig. 3a shows a RT STM image of CeO (110) which was annealed to 1030°C for a 2 few minutes. Line features are still running along [11: 0], but their separation is much bigger compared with Fig. 1a. Fig. 3b shows these lines to be ˚ high and ridges which are approximately 25 A ˚ . The ridges are considerseparated by 80–100 A ably longer than the maximum scan range of this ˚ ). Because these images were taken STM (1300 A at RT, the conductivities of the sample even at the lowest possible tunneling currents (20 pA) are not sufficient to get atomic resolution. The smudged features in Fig. 3a are most likely caused by this effect. The facet angle calculated from the STM images shown in Fig. 3b is approximately 30°. This value, though less accurate than the value determined by
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H. No¨renberg, G.A.D. Briggs / Surface Science 433–435 (1999) 127–130
RHHED, is in reasonable agreement with the expected angle and demonstrates the complementarity between STM and RHEED. The facetting appears to be dependent on annealing temperature and duration. While a brief anneal (a few minutes) to 940°C produced rather flat surfaces ( Fig. 1a), annealing close to 900°C for 1 h already showed facetting in RHEED. The annealing procedure promotes the formation of the low energy facets (111) and (111: ). It appears that, although some oxygen might be lost during this transformation of the surface structure, the overall surface stoichiometry must be close to the single crystalline value as RHEED ( Fig. 2b and c) shows a rather flat surface structure. Compared with the CeO (111) surface where clustering of 2 oxygen vacancies was theoretically predicted [7,16 ] and experimentally verified [5], clustering of oxygen vacancies on the CeO (110) surface is 2 energetically not favored [16 ]. The STM images showing atomic resolution at elevated temperatures (Fig. 1b) and those acquired at RT ( Fig. 3a and b) were obtained at rather high positive bias voltages (2–3 V ). This implies that unoccupied metal states are imaged. For UO (110) it was argued that U 5f states are 2 responsible for the STM contrast obtained under similar conditions [11]. It is suggested that in the case of CeO (110) the cations are imaged too, 2 tunneling might occur into unoccupied Ce 4f states [4].
4. Conclusion We have investigated (110) surfaces of CeO 2 single crystals. After annealing to 940°C the surface shows a disordered (2×1) reconstruction. Further annealing to 1030°C leads to a facetted surface. RHEED and STM show these facets to be (111)
and (111: ) planes. They run along the [11: 0] direction and the ridges have a length in excess of ˚ . No surface reconstruction could be 1300 A detected on these facetted surfaces. The STM contrast originates from tunneling into the unoccupied Ce-related states.
Acknowledgements The authors wish to thank G.W. Grime for performing the PIXE experiments, and C. Muggelberg for helpful discussions. This work was supported by EPSRC Grant GR/K08161.
References [1] G. Ertl, H. Kno¨zinger, J. Weitkamp ( Eds.), Handbook of Heterogeneous Catalysis vol. 4, VCH, Weinheim, 1997, Chapter Environmental Problems. [2] A. Pfau, K.D. Schierbaum, Surf. Sci. 321 (1994) 71. [3] J. Stubenrauch, J.M. Vohs, J. Catal. 159 (1996) 50. [4] F. Marabelli, P. Wachter, Phys. Rev. B 36 (1987) 1238. [5] H. No¨renberg, G.A.D. Briggs, Phys. Rev. Lett. 78 (1997) 4222. [6 ] H. No¨renberg, G.A.D. Briggs, Surf. Sci. 402–404 (1998) 738. [7] J.C. Conesa, Surf. Sci. 339 (1995) 337. [8] P.W. Tasker, Surf. Sci. 87 (1979) 315. [9] T. Inoue, Y. Yamamoto, M. Satoh, T. Ohsuna, Fractal aspects of materials, Mater. Res. Soc. Symp. Proc. 367 (1995) 323. [10] T. Inoue, Y. Yamamoto, M. Satoh, A. Ide, S. Katsumata, Thin Solid Films 281/282 (1996) 24. [11] C. Muggelberg, M.R. Castell, G.A.D. Briggs, D.T. Goddard, Surf. Sci. 402–404 (1998) 673. [12] W.P. Ellis, J. Chem. Phys. 48 (1968) 5695. [13] J.W. Dawicke, R.N. Blumenthal, J. Electrochem. Soc. 133 (1986) 904. [14] S.A.E. Johansson, J.L. Campbell, PIXE: A Novel Technique for Elemental Analysis, Wiley, New York, 1988. [15] M.A. Panhans, R.N. Blumenthal, Solid State Ion. 60 (1993) 279. [16 ] T.X.T. Sayle, S.C. Parker, C.R.A. Catlow, Surf. Sci. 316 (1994) 329.
The surface structure of CeO (110) single crystals studied 2 by STM and RHEED H. No¨renberg *, G.A.D. Briggs University of Oxford, Department of Materials, Parks Road, Oxford OX1 3PH, UK
Abstract We have studied the surface structure of CeO (110) single crystals by elevated temperature STM and electron 2 diffraction. The surface shows line features running along the [11: 0] direction after annealing to 940°C. The lines are ˚ separated by 11 A which corresponds to a (2×1) surface reconstruction. After annealing to 1030°C the surface forms (111) and (111: ) facets with ridges running along the [11: 0] direction over long distances exceeding the maximum scan ˚ . The facetting can be observed in RHEED and STM. The STM contrast originated from tunneling range of 1300 A into unoccupied metal states. © 1999 Elsevier Science B.V. All rights reserved. Keywords: CeO (110); Ceria; RHEED; STM; Surface reconstruction 2
1. Introduction CeO has important applications in catalysis, in 2 particular as washcoat additive to oxidize CO and hydrocarbons and reduce NO in car exhausts [1]. x CeO can readily change its oxidation state, which 2 makes it useful as an oxygen storage material. The surface structure of the oxide seems to play a crucial role during the chemical reactions [2,3]. Although CeO is an electrically insulating oxide 2 with a bandgap of a few electronvolts [2,4] and therefore difficult to investigate by scanning tunneling microscopy (STM ), this oxide can be made conductive to some extent by creating oxygen vacancies after thermal annealing [5,6 ]. On the CeO (111) surface which is the lowest energy 2 surface of fluorite structure metal oxides [7,8] it * Corresponding author. Fax: +44-1865-273783. E-mail address: [email protected] (H. No¨renberg)
was possible to obtain atomic resolution by STM [5]. An interesting question is what structure the higher energy low index surfaces of CeO such as 2 (110) will assume. Experiments on thin CeO films 2 which were grown on Si wafers showed a tendency to form {111} facets at higher film thicknesses [9,10]. Atomic force microscopy (AFM ) experiments on these CeO films showed elongated facet2 ted features [10]. STM studies on UO [11], which has the same 2 crystallographic structure and within 1% the same lattice constant as CeO , showed that the (110) 2 surface forms an (n×1) surface reconstruction (with n=1, 2, 3). Facetting has been observed too [12]. STM investigations on CeO (110) surfaces 2 have not been reported so far. In this paper we want to show what the CeO (110) surface looks like and that flat or 2 facetted surfaces can be obtained by annealing procedures under ultra-high vacuum conditions.
0039-6028/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0 0 39 - 6 0 28 ( 99 ) 0 00 7 0 -9
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H. No¨renberg, G.A.D. Briggs / Surface Science 433–435 (1999) 127–130
2. Experimental Epipolished CeO (110) wafers supplied by 2 Commercial Crystal Laboratories Inc. were used as substrates. The greyish appearance of the crystals was probably caused by traces of CaO which might also be responsible for the conductivity of 1×10−3 (V cm)−1 [13]. To clarify this point we have carried out proton induced X-ray emission (PIXE) experiments [14], which revealed a Ca content of 400 ppm in our samples. The amount of Ca on the surface was below 1% as estimated by Auger electron spectroscopy (AES), and therefore the surface of the crystals can be considered clean. The room temperature (RT ) STM experiments were carried out with an OMICRON STM1 working at a base pressure of 3×10−11 mbar. Although the samples still had a conductivity of only 0.02 (V cm)−1 it was possible to study the surface morphology by STM at RT at very low tunneling currents, in the range 10 to 20 pA. The bias voltage was applied between the tip and the grounded sample holder. Another way of overcoming the poor conductivity is imaging at higher temperatures, as the conductivity of ceria increases with temperature [15]. Experiments at elevated temperatures were carried out with a JEOL JSTM 4500-XT in the temperature range up to 250°C. Lateral distance and height measurements were calibrated against a Si(001) standard. Commercially produced, etched Pt–Ir tips were used. Electron diffraction (RHEED and LEED) was used to monitor the surface morphology in situ during surface preparation.
3. Results and discussion Fig. 1a shows an elevated temperature STM image of a CeO (110) surface which was annealed 2 to about 940°C for a few minutes. The main features are lines running along the [11: 0] direction. A close-up shows that these lines are separated by ˚ , which is twice the lattice constant of CeO 11 A 2 (Fig. 1b). Occasionally this separation is three times the lattice constant (Fig. 1b). So far, atomic resolution along the [001] direction could not be achieved. Some of the lines visible on the surface
Fig. 1. STM image of CeO (110), imaging conditions 250°C, 2 ˚ ×1300 A ˚ ; (b) U =+3 V, I =50 pA: (a) image size 1300 A BIAS T˚ ˚. image size 530 A×185 A
( Fig. 1b) are on different height levels, which might indicate an early stage of facetting. RHEED shows half-order streaks in the [11: 0] azimuth (Fig. 2a) confirming the surface reconstruction visible in the STM image ( Fig. 1b). Halforder spots were visible in LEED in the [11: 0] direction, but no superstructure features could be observed in the [001] azimuth. Based on these three investigation methods, the surface structure of CeO (110) at this stage of annealing can be 2 described as (2×1) reconstructed. Similar to the UO (110) surface [11], some disorder in the [001] 2 direction occasionally leads to small (3×1) reconstructed regions. According to theoretical calculations, the (110) surface of CeO undergoes 2 substantial relaxation in order to reduce the surface energy [16 ]. After further annealing the surface started to form facets. We have monitored the facetting process by RHEED during annealing. Fig. 2b shows a RHEED pattern taken in the [11: 0] azimuth after annealing to 1030°C. The prominent features in this RHEED pattern are the streaks which are tilted by 35°. This is exactly the angle between the (110) plane and the (111) and (111: ) planes in CeO . This result is in agreement with 2
H. No¨renberg, G.A.D. Briggs / Surface Science 433–435 (1999) 127–130
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Fig. 3. STM image of CeO (110), at RT: (a) image size 2 ˚ ×390 A ˚ , imaging conditions: 390 A U =3 V, I =50 pA; (b) BIAS T ˚ ˚ image size 200 A×70 A, imaging conditions: U =2 V, BIAS I =20 pA. T
Fig. 2. RHEED patterns of CeO (110): (a) after annealing to 2 940°C, [11: 0] azimuth, 12 keV, half-order diffraction features are indicated by arrows; (b) after annealing to 1030°C, [11: 0] azimuth, 20 keV; (c) after annealing to 1030°C, [001] azimuth, 20 keV.
investigations on thin CeO films, where in depen2 dence on the preparation conditions elongated (111) and (111: ) facets have been observed by AFM [10] and RHEED [9]. The RHEED pattern taken in the [001] azimuth (Fig. 2c) shows a rather flat surface without reconstruction. We have investigated these facetted surfaces by STM. Fig. 3a shows a RT STM image of CeO (110) which was annealed to 1030°C for a 2 few minutes. Line features are still running along [11: 0], but their separation is much bigger compared with Fig. 1a. Fig. 3b shows these lines to be ˚ high and ridges which are approximately 25 A ˚ . The ridges are considerseparated by 80–100 A ably longer than the maximum scan range of this ˚ ). Because these images were taken STM (1300 A at RT, the conductivities of the sample even at the lowest possible tunneling currents (20 pA) are not sufficient to get atomic resolution. The smudged features in Fig. 3a are most likely caused by this effect. The facet angle calculated from the STM images shown in Fig. 3b is approximately 30°. This value, though less accurate than the value determined by
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H. No¨renberg, G.A.D. Briggs / Surface Science 433–435 (1999) 127–130
RHHED, is in reasonable agreement with the expected angle and demonstrates the complementarity between STM and RHEED. The facetting appears to be dependent on annealing temperature and duration. While a brief anneal (a few minutes) to 940°C produced rather flat surfaces ( Fig. 1a), annealing close to 900°C for 1 h already showed facetting in RHEED. The annealing procedure promotes the formation of the low energy facets (111) and (111: ). It appears that, although some oxygen might be lost during this transformation of the surface structure, the overall surface stoichiometry must be close to the single crystalline value as RHEED ( Fig. 2b and c) shows a rather flat surface structure. Compared with the CeO (111) surface where clustering of 2 oxygen vacancies was theoretically predicted [7,16 ] and experimentally verified [5], clustering of oxygen vacancies on the CeO (110) surface is 2 energetically not favored [16 ]. The STM images showing atomic resolution at elevated temperatures (Fig. 1b) and those acquired at RT ( Fig. 3a and b) were obtained at rather high positive bias voltages (2–3 V ). This implies that unoccupied metal states are imaged. For UO (110) it was argued that U 5f states are 2 responsible for the STM contrast obtained under similar conditions [11]. It is suggested that in the case of CeO (110) the cations are imaged too, 2 tunneling might occur into unoccupied Ce 4f states [4].
4. Conclusion We have investigated (110) surfaces of CeO 2 single crystals. After annealing to 940°C the surface shows a disordered (2×1) reconstruction. Further annealing to 1030°C leads to a facetted surface. RHEED and STM show these facets to be (111)
and (111: ) planes. They run along the [11: 0] direction and the ridges have a length in excess of ˚ . No surface reconstruction could be 1300 A detected on these facetted surfaces. The STM contrast originates from tunneling into the unoccupied Ce-related states.
Acknowledgements The authors wish to thank G.W. Grime for performing the PIXE experiments, and C. Muggelberg for helpful discussions. This work was supported by EPSRC Grant GR/K08161.
References [1] G. Ertl, H. Kno¨zinger, J. Weitkamp ( Eds.), Handbook of Heterogeneous Catalysis vol. 4, VCH, Weinheim, 1997, Chapter Environmental Problems. [2] A. Pfau, K.D. Schierbaum, Surf. Sci. 321 (1994) 71. [3] J. Stubenrauch, J.M. Vohs, J. Catal. 159 (1996) 50. [4] F. Marabelli, P. Wachter, Phys. Rev. B 36 (1987) 1238. [5] H. No¨renberg, G.A.D. Briggs, Phys. Rev. Lett. 78 (1997) 4222. [6 ] H. No¨renberg, G.A.D. Briggs, Surf. Sci. 402–404 (1998) 738. [7] J.C. Conesa, Surf. Sci. 339 (1995) 337. [8] P.W. Tasker, Surf. Sci. 87 (1979) 315. [9] T. Inoue, Y. Yamamoto, M. Satoh, T. Ohsuna, Fractal aspects of materials, Mater. Res. Soc. Symp. Proc. 367 (1995) 323. [10] T. Inoue, Y. Yamamoto, M. Satoh, A. Ide, S. Katsumata, Thin Solid Films 281/282 (1996) 24. [11] C. Muggelberg, M.R. Castell, G.A.D. Briggs, D.T. Goddard, Surf. Sci. 402–404 (1998) 673. [12] W.P. Ellis, J. Chem. Phys. 48 (1968) 5695. [13] J.W. Dawicke, R.N. Blumenthal, J. Electrochem. Soc. 133 (1986) 904. [14] S.A.E. Johansson, J.L. Campbell, PIXE: A Novel Technique for Elemental Analysis, Wiley, New York, 1988. [15] M.A. Panhans, R.N. Blumenthal, Solid State Ion. 60 (1993) 279. [16 ] T.X.T. Sayle, S.C. Parker, C.R.A. Catlow, Surf. Sci. 316 (1994) 329.