Epitaxial growth of CeO2(111) film on Ru(0001): Scanning tunneling microscopy (STM) and x-ray photoemission spectroscopy (XPS) study Tomo Hasegawa, Syed Mohammad Fakruddin Shahed, Yasuyuki Sainoo, Atsushi Beniya, Noritake Isomura, Yoshihide Watanabe, and Tadahiro Komeda Citation: The Journal of Chemical Physics 140, 044711 (2014); doi: 10.1063/1.4849595 View online: http://dx.doi.org/10.1063/1.4849595 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/140/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Epitaxial growth of large-area bilayer graphene on Ru(0001) Appl. Phys. Lett. 104, 093110 (2014); 10.1063/1.4868021 Intercalation of metal islands and films at the interface of epitaxially grown graphene and Ru(0001) surfaces Appl. Phys. Lett. 99, 163107 (2011); 10.1063/1.3653241 Epitaxial BaTiO3(100) films on Pt(100): A low-energy electron diffraction, scanning tunneling microscopy, and xray photoelectron spectroscopy study J. Chem. Phys. 135, 104701 (2011); 10.1063/1.3633703 Scanning tunneling microscopy on epitaxial bilayer graphene on ruthenium (0001) Appl. Phys. Lett. 94, 133101 (2009); 10.1063/1.3106057 Interaction of Au with CeO 2 ( 111 ) : A photoemission study J. Chem. Phys. 130, 034703 (2009); 10.1063/1.3046684
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THE JOURNAL OF CHEMICAL PHYSICS 140, 044711 (2014)
Epitaxial growth of CeO2 (111) film on Ru(0001): Scanning tunneling microscopy (STM) and x-ray photoemission spectroscopy (XPS) study Tomo Hasegawa,1 Syed Mohammad Fakruddin Shahed,1 Yasuyuki Sainoo,1 Atsushi Beniya,2 Noritake Isomura,2 Yoshihide Watanabe,2 and Tadahiro Komeda1,3,a) 1
Institute of Multidisciplinary Research for Advanced Materials (IMRAM, Tagen), Tohoku University, 2-1-1, Katahira, Aoba-Ku, Sendai 980-0877, Japan 2 Toyota Central R&D Labs., Inc., 41-1 Yokomichi, Nagakute, Aichi 480-1192, Japan 3 JST, CREST, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
(Received 30 July 2013; accepted 3 December 2013; published online 30 January 2014) We formed an epitaxial film of CeO2 (111) by sublimating Ce atoms on Ru(0001) surface kept at elevated temperature in an oxygen ambient. X-ray photoemission spectroscopy measurement revealed a decrease of Ce4+ /Ce3+ ratio in a small temperature window of the growth temperature between 1070 and 1096 K, which corresponds to the reduction of the CeO2 (111). Scanning tunneling microscope image showed that a film with a wide terrace and a sharp step edge was obtained when the film was grown at the temperatures close to the reduction temperature, and the terrace width observed on the sample grown at 1060 K was more than twice of that grown at 1040 K. On the surface grown above the reduction temperature, the surface with a wide terrace and a sharp step was confirmed, but small dots were also seen in the terrace part, which are considerably Ce atoms adsorbed at the oxygen vacancies on the reduced surface. This experiment demonstrated that it is required to use the substrate temperature close to the reduction temperature to obtain CeO2 (111) with wide terrace width and sharp step edges. © 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.4849595] I. INTRODUCTION
Ceria (cerium dioxide, CeO2 ) is a catalyst used for various applications like as a vehicle exhaust removal and an anode of a fuel cell,1–3 which is due to its unique ability as an oxygen reservoir. The cerium atom possesses two interchangeable oxidation states, Ce3+ and Ce4+ and can uptake or release of oxygen depending on its partial pressure. Intensive theoretical and experimental researches have been executed in recent years for the understanding of the redox behavior of ceria. However, despite such scientific and industrial interests, there was limited number of reports which used well-defined ceria surfaces. This is partially due to difficulties in obtaining a single crystal ceria sample. Alternatively, a formation of a ceria film that is epitaxially grown on metal substrates has been studied extensively. The substrates used in such researches include Pd(111),4 Pt(111),5–7 Rh(111),8 Ni(111),9 Cu(111),10, 11 and Ru(0001).9, 12–15 It has been pointed out that step-edges, kinks, and vacancies are active sites for the catalytic reaction.16, 17 In order to examine the role of surface active sites, it is required to prepare the surface with wider terraces, straight steps, and low vacancy numbers. Although large islands and terraces can be obtained with a higher growth temperature due to an increase of the diffusion length of the deposited species, the probability of the formation of oxygen vacancies also increases. The
a) Author to whom correspondence should be addressed. Electronic mail:
[email protected] 0021-9606/2014/140(4)/044711/6
substrate temperature for the growth of oxide film has been kept at relatively low temperature with this reason. In this paper we report epitaxial film growth of CeO2 (111) on Ru(0001) by using scanning tunneling microscope (STM) and x-ray photoemission spectroscopy (XPS) measurement. The decrease (increase) of Ce4+ (Ce3+ ) component in the XPS spectra was detected in a small temperature window of the growth temperature between 1070 and 1096 K. The decrease of Ce4+ component was originated from the formation of oxygen vacancies, which made the surface reduced. A surface with a wide terrace and a sharp step edge was observed in STM observation when the sample was grown at the temperature close to the reduction temperature. The terrace width of the sample prepared at 1060 K was more than twice of that prepared at 1040 K. We discuss the condition required for the growth of the CeO2 (111) surface with a wide terrace and small oxygen vacancies.
II. EXPERIMENTAL
All experiments were carried out in the ultra-high vacuum (UHV) condition (base pressure below 1 × 10−8 Pa). The Ru(0001) single crystal (8 mm diameter and 2 mm thick, MaTeck) was used as a substrate, which was cleaned by repetitive cycles of Ar+ sputtering (1 KeV) followed by annealing at 1073 K in O2 ambient with a pressure of 5 × 10−5 Pa. The oxygen introduction was intended to remove hydrocarbons and the induced oxide was removed by flashing the sample to 1423 K. Cleanliness of Ru(0001) was confirmed by low-energy electron diffraction (LEED), XPS, and STM.
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The CeO2 (111) film was grown by sublimating Ce atoms on Ru (0001) in an oxygen ambient (pressure 5.0 × 10−5 Pa).12, 18 Ce was evaporated at the rate of 0.23–0.35 Å/min by electron-beam heating of the Ta crucible containing precleaned Ce metal pieces (Ce foil from Alfa Aesar, 99.9%). The amounts of deposited Ce atoms were monitored by a quartz crystal thickness monitor (INFICON, USA), and the read of the thickness monitor was calibrated with using the thickness data of STM and XPS. During the deposition, substrate was kept at an elevated temperature which was varied as a critical parameter of the film growth. The temperature was monitored by using an optical pyrometer which was calibrated by a thermo couple in advance. At the end of the deposition process, the pressure of oxygen gas was increased to 1.0 × 10−4 pa and pumped out after the substrate was cooled down to 427 K. The film was characterized using LEED, XPS, and STM. XPS spectra were obtained using monochromatized Al Kα X-ray radiation (1486.6 eV) and the hemispherical mirror analyzer with a total energy resolution of ∼ 0.5 eV. The background of XPS spectra was subtracted by the Shirley procedure and the peaks were fitted using the Gaussian-Lorentzian function.
III. RESULTS AND DISCUSSION
We first show results of the STM observation. Figures 1(a)–1(c) show a morphology change of the CeO2 (111) film with a variation of the substrate temperature during the deposition. The film shown in Fig. 1(a) was grown at the substrate temperature of ∼1040 K. We see triangle islands with sharp step edges, whose shape was formed by a
(a)
(b)
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(d)
(1x1) (2x2)
(1.4 x 1.4)
FIG. 1. (a)–(c) Variation of CeO2 (111) film with different growth temperatures of 1040 K (a), 1060 K (b), and 1150 K (c). Film thickness of the majority islands is 4 ML for all cases. STM images were obtained with tunneling conditions of Vs = −2.0 V, I = 0.1 nA. The areas are 100 × 100 nm2 and the length marks are 25 nm. (d) LEED pattern observed for the sample of (b).
preferential growth along the crystalographically equivalent step edges.13 In Fig. 1(b), we show the STM image of the film deposited at the substrate temperature of 1060 K, which is only 20 K higher than that used for Fig. 1(a). The height of the majority islands of CeO2 (111) was 4 ML. There appeared a significant change of the size of the island from that of Fig. 1(a). The perimeter of the top islands exceeds 30 nm on the film of Fig. 1(b) and straight step edges can be observed showing much better crystal quality than that of Fig. 1(a). We have to mention about a slightly protruded circle with a size of ∼5 nm in the middle of the flat terrace. Similar circular protrusions were observed on the Ru(0001) surface after the cleaning. Jakob et al. reported a similar protrusion for Ru(0001) surface after the cleaning process of Ar sputtering and annealing, which was explained by the subsurface gas bubbles induced by Ar ion-bombardment/implantation.19 In Fig. 1(c), we show the film grown on the substrate whose temperature was kept at 1150 K. Many small spots are visible on the terrace part. However, below the particles, we can identify straight step edges and flat terraces, which seem to be similar to those observed in Fig. 1(b). In analyzing the size of particles observed by STM, we should note the image is the convolution of the shapes of the particle and the tip apex. Thus, an atomic size protrusion appears much larger in the STM image. The dots in our images were estimated to be less than 1 nm in the actual lateral size. Fledge group reported a growth of CeO2 (111) islands by using the low-energy electron microscopy (LEEM).15, 20 The result for the growth at the substrate temperature 1123 K should be compared with the result of Fig. 1(c) of our experiment. Islands with sharp step edges and flat terraces were observed commonly in both cases. The dots shown in Fig. 1(c) were not detected by the LEEM observation. This is because the size of the dots is ∼1 nm and is probably too small to be observed by LEEM. Figure 1(d) shows the LEED pattern obtained for the sample grown at the substrate temperature of 1060 K and the majority islands were 6 ML. Sharp and strong diffraction patterns corresponding to p(1 × 1) and p(1.4 × 1.4), and a weak pattern of p(2 × 2) were observed. We assign p(1 × 1) and p(1.4 × 1.4) spots to Ru(0001) and CeO2 (111) film, respectively. p(2 × 2) peak is assumingly originated from the oxygen adlayer. The presence of the Ru component suggests that Ru(0001) surface was not fully covered by CeO2 (111) film, even though islands with ∼6 ML height was locally observed. As mentioned in the Introduction, one of the motivations of this work is to establish the recipe of the formation of CeO2 (111) film with a larger terrace width. The terrace width and island size changed drastically with the growth temperatures of 1040 K (Fig. 1(a)) and 1060 K (Fig. 1(b)). The size of the islands can be changed depending on the number of the layers. Thus, even though the sizes of the islands for the three growth temperatures were compared with the same thickness, we have to check how the island size depends on the thickness. We prepared films thicker than that of Fig. 1(b) (thickness of ∼4 ML) with the same substrate temperature of 1060 K: STM images for total thickness of ∼6 ML and ∼ 8 ML are shown in Figs. 2(a) and 2(b), respectively. In both cases we can see almost identical terrace width with that
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Binding energy (eV) 2 nm FIG. 2. (a) and (b) Ceria film grown with the same condition as that of Fig. 1(b) but film thickness of the majority islands is 6 ML (a) and 8 ML (b). (c) Magnified image of the terrace, which is 2 ML high from the metal surface. A moiré pattern with 5 × 5 periodicity with a reference to the CeO2 (111) surface is marked by a white triangle. (d) LEED pattern observed for the surface of (c). A spot corresponds to the superstructure is marked by an arrow.
of Fig. 1(b) accompanying atomically flat and well defined step edges. We judged that there was a significant widening of the terrace width with a difference of ∼20 K in the growth temperature. In the inset of Fig. 2(a), we show a magnified image of the terrace part with an atomic resolution. An ordered structure with a threefold symmetry can be observed. In previous reports, the protrusions have been assigned to the positions of the oxygen atoms of CeO2 (111) and the oxygen defects appear as missing protrusions.12, 21 The probability of finding oxygen defects was small, even though we searched many areas of the surface. Figure 2(c) shows a moiré pattern observed on the second layer of the film. The ordered structure can be best described with the unit of the triangle superimposed in the STM image of Fig. 2(c). The unit has the periodicity of five times of the nearest neighbor distance (nnd) of the oxygen atoms of the CeO2 (111) film. A similar superstructure of 5 × 5 with a respect to the CeO2 lattice was reported on the CeO2 (111)/Ru(0001) by using LEEM.15, 20 The appearance of moiré pattern indicates a lattice mismatch between the substrate and the film, and it became weaker when we observed the terrace part of the 5 ML film. The LEED pattern obtained for the CeO2 (111) film, whose growth temperature and film thickness were 1060 K and 2 ML, respectively, is shown in Fig. 2(d). In addition to the features discussed for Fig. 1(d), we observed the spot marked by an arrow in the Fig. 2(d). The spot was located at 7 × 7 position with a reference to the Ru(0001) lattice, which can be converted to 5 × 5 superstructure of CeO2 (111) order-
FIG. 3. Ce 3d components of XPS spectra of ceria film grown with different substrate temperatures: (a) 925 K, (b) 1008 K, (c) 1070 K, (d) 1096 K, (e) 1207 K, and (f) 1306 K.
ing. The spots were clear on the marked position, but not so for other corresponding sites. In order to analyze the chemical conditions of the films grown at different substrate temperatures, we executed in situ XPS measurements. It has been demonstrated that the electronic states of oxide surfaces can be monitored with the use of photoemission spectroscopy.22, 23 In the XPS spectrum for Ce 3d, there appear multiple and complex peaks originated from different final states reflecting a mixed valency.24–29 Figure 3 shows the variation of the Ce 3d peak with the change of the substrate temperature during the Ce deposition. It should be noted that there appear large changes at energies of ∼885 and ∼905 eV when the substrate temperature exceeds 1096 K. A stoichiometric CeO2 (111) surface is oxygenterminated and the surface oxygen atoms have a threefold coordination to cerium atoms. With a formation of oxygen vacancy, two electrons are left and localized on two cerium atoms in the vicinity of the vacancy. The multiplicity of the Ce 3d peaks increases with the appearance of new oxidation states.30, 31 Three final states of Ce4+ , which include Ce 3d9 4f0 O 2p6 , Ce 3d9 4f1 O 2p5 , and Ce 3d9 4f2 O 2p4 , are expressed as u (v ), u (v ), u (v), respectively, for Ce3d3/2 (Ce3d5/2 ). Note there appears a spin-orbit splitting for the Ce 3d peak. Two final states of Ce3+ , which contain 3d9 4f1 O 2p6 and Ce 3d9 4f2 O 2p5 , are expressed as u (v ) and u0 (v0 ) for Ce3d3/2 (Ce3d5/2 ). Such expressions of the final sates configurations have been discussed previously,24–31 and the corresponding peaks deconvoluted from Fig. 3(b) are shown in Fig. 4(a).32 The variation of the peak-shape with the growth temperature can be more clearly seen in Fig. 4(b) that compares the spectra for the samples grown at 1070 K and 1096 K. Though the temperature change is relatively small (∼26 K), we can see a rapid increase of u4 and v4 components when the temperature increased to 1096 K. The proportion of the Ce4+ component is plotted in Fig. 4(c) for several substrate tem-
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FIG. 5. (a) Deconvolution of O 1s peaks into components 1 and 2 that are corresponding to that of CeO2 and Ce2 O3 , respectively. (b) Variation of proportion of the components 1 and 2 out of total O1s signal as a function of growth temperature.
0.8 0.7 0.6 0.5
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Annealing Temperature (K) FIG. 4. (a) Deconvoluted Ce 3d features. Marks are representing different final states and oxidation states of Ce 3d. (b) Comparison of two spectra for the samples with growth temperatures of 1070 K and 1096 K. Note components of u and v were increased for the latter case. (c) Variation of the ratio of Ce4+ to total Ce.
peratures during the deposition. The ratio of Ce4+ and Ce3+ in CeO2 (111) film was calculated by comparing the integrated intensity of the peaks of u, u and u (ICe 4+ ), and ICe 3+ integrating u and u0 .5 We should notice that a rapid decrease of the Ce4+ component, which corresponds to the reduction of the film, occurs in a small temperature window between 1070 and 1096 K. Here after we call this temperature of 1096 K as reduction temperature. A similar behavior was also observed for the O1s feature. Figure 5(a) shows an O1s peak which is deconvoluted into two peaks. Components 1 and 2 have the peak energies of 529.7 eV and 530.7 eV. An XPS study of two forms of oxides of CeO2 and Ce2 O3 was reported previously, in which O 1s peaks were detected at 529.2 and 530.3 eV for CeO2 and Ce2 O3 , respectively.33, 34 The energy separation for the two components agreed well with our deconvolution, and we attribute components 1 and 2 to the oxygen atoms in a nonreduced and a reduced environment, respectively. The proportions of the two components in the total O 1s signal were
plotted in Fig. 5(b) as a function of the growth temperature. Again we see a large change for the sample treated at 1096 K. It should be noted that XPS spectra from Figs. 3 to 5 were obtained after the surface was cooled to room temperature in oxygen ambient. While it is possible that the oxygen vacancies were filled in the cooling process, the surface morphologies formation were almost completed near the growth temperature due to the rapid decrease of the migration rate with the temperature. So far we examined the effect of the growth temperature on the morphology and oxidation state by using STM and XPS, respectively. Large islands and terrace width appeared only when the growth temperature was close to the reduction temperature. In addition, a large terrace-width change occurred with a small difference of the growth temperature, which was shown in Figs. 1(a) and 1(b) corresponding to 1040 K and 1060 K, respectively. For the samples grown above the reduction temperature, characteristic dots were observed as shown in Fig. 1(c), even though wide terrace and clear step edges were observed. It has been considered that the topmost layer of the ceria film is terminated with an oxygen layer. Thus the overlayer growth for the non-reduced surface proceeds in a manner that adatoms hop on the oxygen covered top-most layer. However in the presence of oxygen vacancies, the adatom adsorption on the Ce layer in the defect sites should also be considered. Atomic processes that govern the thin-film growth have been investigated for variety of systems.35–38 In the initial stage, impinging atoms are trapped on a surface and move
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across the surface. They migrate on available sites of the surface before adsorbed as a stable island. The migration rate is one of important factors that determine the size of the islands, which is a function of the barrier height of the migration and the substrate temperature. In an analysis of a metal film growth on metal surfaces, the following formula have been examined for various cases, which simulates the density of the islands that is a reciprocal of the area of the islands:39–41 n ∝ exp(−εi /(i + 2)kB T ),
(1)
in which n is the density of islands, εi is the migration barrier and kB is the Boltzmann constant, T is the substrate temperature, and i is the critical cluster size. Here we make a rough examination whether the island-size variation of the surface of Figs. 1(a) and 1(b) can be scaled with this formula. The dentistry of the islands is roughly two times larger for the surface of Fig. 1(a) than that of Fig. 1(b). Assuming simple i = 1 corresponding to a monomer diffusion, we can deduce the energy barrier for the migration from Eq. (1), which is close to 10 eV. It is often referred that the barrier is of the order of one tenth of the adsorption energy. This leads to the adsorption energy of 100 eV, which is unrealistic. We consider there must be a mechanism other than the thermal activation that accelerates the diffusion at this temperature range. We consider a hypothesis that the oxygen vacancy creation is related to this mechanism. It has been considered that metal particles are likely to nucleate at oxygen vacancy sites which are argued for TiO2 surface42–44 and MgO45 the number of cluster is proportional to the number of oxygen defect sites. At the same time Galhenage et al. reported a larger diffusion coefficient on the reduced TiO2 surface than that on non-reduced one.46 They measured a size distribution of the metal clusters formed on vacuum-annealed (reduced) and defect-free TiO2 (110) surface. On the reduced TiO2 surface, they found a decrease of the number of clusters and an increased averaged-size of the clusters than on a defect-free TiO2 surface, which suggests the enhanced diffusion on reduced TiO2 (110) surface. These are a few examples how the oxygen vacancies affects the metal cluster formation on oxide surfaces through the variation of the bonding strength and diffusion length of the impinging metal atoms and the substrate. We speculate a similar effect should appear in our experiment case and be correlated with the enhanced terrace widening for the cases where the growth temperature was close to the reduction temperature. However, much experimental and theoretical study is required to prove such hypothesis. For the sample grown at 1150 K, the island size continued to increase. However, as we can see the reduced feature in the XPS spectra, oxygen vacancies remained on the surface. It is likely that deposited Ce metal remained at these sites as a small cluster. Those species are considerably observed as bright dots in the image of Fig. 1(c). In the last section, we consider the growth condition of the CeO2 (111) surface planned in the Introduction, which has a wide terrace width and is free of oxygen vacancies. In the XPS measurement, we observed that the reduction of the surface occurred in an abrupt manner and the intensity of the Ce4+ component decreased rapidly in a small temperature
range. The surface grown at the temperature at 1060 K, ∼40 K below the reduction temperature, showed a large terrace width and the atomic image of the surface showed no oxygen vacancies. The island size was smaller when it was grown at 1040 K. The surface grown at the temperature over the reduction temperature, the terrace contains dots even though the terrace width was large. These results suggest that the growth temperature should be set just below the reduction temperature, and the surface should have wide terrace width and oxygen vacancy free. IV. CONCLUSION
We studied an epitaxial film growth of CeO2 (111) on Ru(0001) by using STM and XPS. CeO2 (111) films were prepared by sublimating Ce atoms in an oxygen ambient where Ru(0001) substrate were kept at elevated temperature, which was cooled in oxygen ambient to room temperature to execute STM and XPS measurements. The oxidation state was examined by comparing the intensities of Ce3+ and Ce4+ components in XPS spectra, the former component should be originated from oxygen vacancies in the reduced state. The reduced state was identified when the growth temperature exceeded 1096 K. The decrease of Ce4+ component occurred in a small temperature window between 1070 and 1096 K. A CeO2 (111) surface with a wide terrace width and a sharp step edge was obtained when the film was grown at the temperatures close to the reduction temperature. The terrace width measured on the sample surface grown at 1060 K was more than twice of that grown at 1040 K. The morphology change between 1040 K and 1060 K was too abrupt to be explained by a simple thermal activation. On the surface grown above the reduction temperature, a surface with a wide terrace and a sharp step was also confirmed. However, small dots were seen in the terrace part, which are considerably Ce atoms adsorbed at the oxygen vacancies on the reduced surface. This experiment demonstrated that it is required to use the substrate temperature close to the reduction temperature to obtain CeO2 (111) with a wide terrace width, where oxygen vacancies were hardly found by STM. 1 S.
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J. Nelin, P. S. Bagus, E. S. Ilton, S. A. Chambers, H. Kuhlenbeck, and H. J. Freund, Int. J. Quantum Chem. 110, 2752 (2010). 31 P. S. Bagus, C. J. Nelin, E. S. Ilton, M. Baron, H. Abbott, E. Primorac, H. Kuhlenbeck, S. Shaikhutdinov, and H. J. Freund, Chem. Phys. Lett. 487, 237 (2010). 32 See supplementary material at http://dx.doi.org/10.1063/1.4849595 for a descritption of fitting method of the deconvolution and obtained parameters. 33 J. Moulder, W. F. Stickle, P. E. Sobol, and K. D. Bomben, Handbook of X-ray Photoelectron Spectroscopy (Perkin-Elmer, Eden Prairie, MN, USA, 1992). 34 K. B. Sundaram, P. F. Wahid, and O. Melendez, J. Vac. Sci. Technol. A 15, 52 (1997). 35 H. Poppa, Catal. Rev. - Sci. Eng. 35, 359 (1993). 36 M. Zinkeallmang, L. C. Feldman, and M. H. Grabow, Surf. Sci. Rep. 16, 377 (1992). 37 J. A. Venables, G. D. T. Spiller, and M. Hanbucken, Rep. Prog. Phys. 47, 399 (1984). 38 M. Baumer and H. J. Freund, Prog. Surf. Sci. 61, 127 (1999). 39 J. A. Venables, Philos. Mag. 27, 697 (1973). 40 J. A. Venables, Phys. Rev. B 36, 4153 (1987). 41 H. Brune, Surf. Sci. Rep. 31, 125 (1998). 42 H. Iddir, S. Ogut, N. D. Browning, and M. M. Disko, Phys. Rev. B 72, 081407 (2005). 43 H. Iddir, V. Skavysh, S. Ogut, N. D. Browning, and M. M. Disko, Phys. Rev. B 73, 041403 (2006). 44 D. Pillay, Y. Wang, and G. S. Hwang, Catal. Today 105, 78 (2005). 45 A. V. Matveev, K. M. Neyman, I. V. Yudanov, and N. Rösch, Surf. Sci. 426, 123 (1999). 46 R. P. Galhenage, H. Yan, S. A. Tenney, N. Park, G. Henkelman, P. Albrecht, D. R. Mullins, and D. A. Chen, J. Phys. Chem. C 117, 7191 (2013).
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THE JOURNAL OF CHEMICAL PHYSICS 140, 044711 (2014)
Epitaxial growth of CeO2 (111) film on Ru(0001): Scanning tunneling microscopy (STM) and x-ray photoemission spectroscopy (XPS) study Tomo Hasegawa,1 Syed Mohammad Fakruddin Shahed,1 Yasuyuki Sainoo,1 Atsushi Beniya,2 Noritake Isomura,2 Yoshihide Watanabe,2 and Tadahiro Komeda1,3,a) 1
Institute of Multidisciplinary Research for Advanced Materials (IMRAM, Tagen), Tohoku University, 2-1-1, Katahira, Aoba-Ku, Sendai 980-0877, Japan 2 Toyota Central R&D Labs., Inc., 41-1 Yokomichi, Nagakute, Aichi 480-1192, Japan 3 JST, CREST, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
(Received 30 July 2013; accepted 3 December 2013; published online 30 January 2014) We formed an epitaxial film of CeO2 (111) by sublimating Ce atoms on Ru(0001) surface kept at elevated temperature in an oxygen ambient. X-ray photoemission spectroscopy measurement revealed a decrease of Ce4+ /Ce3+ ratio in a small temperature window of the growth temperature between 1070 and 1096 K, which corresponds to the reduction of the CeO2 (111). Scanning tunneling microscope image showed that a film with a wide terrace and a sharp step edge was obtained when the film was grown at the temperatures close to the reduction temperature, and the terrace width observed on the sample grown at 1060 K was more than twice of that grown at 1040 K. On the surface grown above the reduction temperature, the surface with a wide terrace and a sharp step was confirmed, but small dots were also seen in the terrace part, which are considerably Ce atoms adsorbed at the oxygen vacancies on the reduced surface. This experiment demonstrated that it is required to use the substrate temperature close to the reduction temperature to obtain CeO2 (111) with wide terrace width and sharp step edges. © 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.4849595] I. INTRODUCTION
Ceria (cerium dioxide, CeO2 ) is a catalyst used for various applications like as a vehicle exhaust removal and an anode of a fuel cell,1–3 which is due to its unique ability as an oxygen reservoir. The cerium atom possesses two interchangeable oxidation states, Ce3+ and Ce4+ and can uptake or release of oxygen depending on its partial pressure. Intensive theoretical and experimental researches have been executed in recent years for the understanding of the redox behavior of ceria. However, despite such scientific and industrial interests, there was limited number of reports which used well-defined ceria surfaces. This is partially due to difficulties in obtaining a single crystal ceria sample. Alternatively, a formation of a ceria film that is epitaxially grown on metal substrates has been studied extensively. The substrates used in such researches include Pd(111),4 Pt(111),5–7 Rh(111),8 Ni(111),9 Cu(111),10, 11 and Ru(0001).9, 12–15 It has been pointed out that step-edges, kinks, and vacancies are active sites for the catalytic reaction.16, 17 In order to examine the role of surface active sites, it is required to prepare the surface with wider terraces, straight steps, and low vacancy numbers. Although large islands and terraces can be obtained with a higher growth temperature due to an increase of the diffusion length of the deposited species, the probability of the formation of oxygen vacancies also increases. The
a) Author to whom correspondence should be addressed. Electronic mail:
[email protected] 0021-9606/2014/140(4)/044711/6
substrate temperature for the growth of oxide film has been kept at relatively low temperature with this reason. In this paper we report epitaxial film growth of CeO2 (111) on Ru(0001) by using scanning tunneling microscope (STM) and x-ray photoemission spectroscopy (XPS) measurement. The decrease (increase) of Ce4+ (Ce3+ ) component in the XPS spectra was detected in a small temperature window of the growth temperature between 1070 and 1096 K. The decrease of Ce4+ component was originated from the formation of oxygen vacancies, which made the surface reduced. A surface with a wide terrace and a sharp step edge was observed in STM observation when the sample was grown at the temperature close to the reduction temperature. The terrace width of the sample prepared at 1060 K was more than twice of that prepared at 1040 K. We discuss the condition required for the growth of the CeO2 (111) surface with a wide terrace and small oxygen vacancies.
II. EXPERIMENTAL
All experiments were carried out in the ultra-high vacuum (UHV) condition (base pressure below 1 × 10−8 Pa). The Ru(0001) single crystal (8 mm diameter and 2 mm thick, MaTeck) was used as a substrate, which was cleaned by repetitive cycles of Ar+ sputtering (1 KeV) followed by annealing at 1073 K in O2 ambient with a pressure of 5 × 10−5 Pa. The oxygen introduction was intended to remove hydrocarbons and the induced oxide was removed by flashing the sample to 1423 K. Cleanliness of Ru(0001) was confirmed by low-energy electron diffraction (LEED), XPS, and STM.
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© Author(s) 2014
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The CeO2 (111) film was grown by sublimating Ce atoms on Ru (0001) in an oxygen ambient (pressure 5.0 × 10−5 Pa).12, 18 Ce was evaporated at the rate of 0.23–0.35 Å/min by electron-beam heating of the Ta crucible containing precleaned Ce metal pieces (Ce foil from Alfa Aesar, 99.9%). The amounts of deposited Ce atoms were monitored by a quartz crystal thickness monitor (INFICON, USA), and the read of the thickness monitor was calibrated with using the thickness data of STM and XPS. During the deposition, substrate was kept at an elevated temperature which was varied as a critical parameter of the film growth. The temperature was monitored by using an optical pyrometer which was calibrated by a thermo couple in advance. At the end of the deposition process, the pressure of oxygen gas was increased to 1.0 × 10−4 pa and pumped out after the substrate was cooled down to 427 K. The film was characterized using LEED, XPS, and STM. XPS spectra were obtained using monochromatized Al Kα X-ray radiation (1486.6 eV) and the hemispherical mirror analyzer with a total energy resolution of ∼ 0.5 eV. The background of XPS spectra was subtracted by the Shirley procedure and the peaks were fitted using the Gaussian-Lorentzian function.
III. RESULTS AND DISCUSSION
We first show results of the STM observation. Figures 1(a)–1(c) show a morphology change of the CeO2 (111) film with a variation of the substrate temperature during the deposition. The film shown in Fig. 1(a) was grown at the substrate temperature of ∼1040 K. We see triangle islands with sharp step edges, whose shape was formed by a
(a)
(b)
(c)
(d)
(1x1) (2x2)
(1.4 x 1.4)
FIG. 1. (a)–(c) Variation of CeO2 (111) film with different growth temperatures of 1040 K (a), 1060 K (b), and 1150 K (c). Film thickness of the majority islands is 4 ML for all cases. STM images were obtained with tunneling conditions of Vs = −2.0 V, I = 0.1 nA. The areas are 100 × 100 nm2 and the length marks are 25 nm. (d) LEED pattern observed for the sample of (b).
preferential growth along the crystalographically equivalent step edges.13 In Fig. 1(b), we show the STM image of the film deposited at the substrate temperature of 1060 K, which is only 20 K higher than that used for Fig. 1(a). The height of the majority islands of CeO2 (111) was 4 ML. There appeared a significant change of the size of the island from that of Fig. 1(a). The perimeter of the top islands exceeds 30 nm on the film of Fig. 1(b) and straight step edges can be observed showing much better crystal quality than that of Fig. 1(a). We have to mention about a slightly protruded circle with a size of ∼5 nm in the middle of the flat terrace. Similar circular protrusions were observed on the Ru(0001) surface after the cleaning. Jakob et al. reported a similar protrusion for Ru(0001) surface after the cleaning process of Ar sputtering and annealing, which was explained by the subsurface gas bubbles induced by Ar ion-bombardment/implantation.19 In Fig. 1(c), we show the film grown on the substrate whose temperature was kept at 1150 K. Many small spots are visible on the terrace part. However, below the particles, we can identify straight step edges and flat terraces, which seem to be similar to those observed in Fig. 1(b). In analyzing the size of particles observed by STM, we should note the image is the convolution of the shapes of the particle and the tip apex. Thus, an atomic size protrusion appears much larger in the STM image. The dots in our images were estimated to be less than 1 nm in the actual lateral size. Fledge group reported a growth of CeO2 (111) islands by using the low-energy electron microscopy (LEEM).15, 20 The result for the growth at the substrate temperature 1123 K should be compared with the result of Fig. 1(c) of our experiment. Islands with sharp step edges and flat terraces were observed commonly in both cases. The dots shown in Fig. 1(c) were not detected by the LEEM observation. This is because the size of the dots is ∼1 nm and is probably too small to be observed by LEEM. Figure 1(d) shows the LEED pattern obtained for the sample grown at the substrate temperature of 1060 K and the majority islands were 6 ML. Sharp and strong diffraction patterns corresponding to p(1 × 1) and p(1.4 × 1.4), and a weak pattern of p(2 × 2) were observed. We assign p(1 × 1) and p(1.4 × 1.4) spots to Ru(0001) and CeO2 (111) film, respectively. p(2 × 2) peak is assumingly originated from the oxygen adlayer. The presence of the Ru component suggests that Ru(0001) surface was not fully covered by CeO2 (111) film, even though islands with ∼6 ML height was locally observed. As mentioned in the Introduction, one of the motivations of this work is to establish the recipe of the formation of CeO2 (111) film with a larger terrace width. The terrace width and island size changed drastically with the growth temperatures of 1040 K (Fig. 1(a)) and 1060 K (Fig. 1(b)). The size of the islands can be changed depending on the number of the layers. Thus, even though the sizes of the islands for the three growth temperatures were compared with the same thickness, we have to check how the island size depends on the thickness. We prepared films thicker than that of Fig. 1(b) (thickness of ∼4 ML) with the same substrate temperature of 1060 K: STM images for total thickness of ∼6 ML and ∼ 8 ML are shown in Figs. 2(a) and 2(b), respectively. In both cases we can see almost identical terrace width with that
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(b)
50 nm
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Binding energy (eV) 2 nm FIG. 2. (a) and (b) Ceria film grown with the same condition as that of Fig. 1(b) but film thickness of the majority islands is 6 ML (a) and 8 ML (b). (c) Magnified image of the terrace, which is 2 ML high from the metal surface. A moiré pattern with 5 × 5 periodicity with a reference to the CeO2 (111) surface is marked by a white triangle. (d) LEED pattern observed for the surface of (c). A spot corresponds to the superstructure is marked by an arrow.
of Fig. 1(b) accompanying atomically flat and well defined step edges. We judged that there was a significant widening of the terrace width with a difference of ∼20 K in the growth temperature. In the inset of Fig. 2(a), we show a magnified image of the terrace part with an atomic resolution. An ordered structure with a threefold symmetry can be observed. In previous reports, the protrusions have been assigned to the positions of the oxygen atoms of CeO2 (111) and the oxygen defects appear as missing protrusions.12, 21 The probability of finding oxygen defects was small, even though we searched many areas of the surface. Figure 2(c) shows a moiré pattern observed on the second layer of the film. The ordered structure can be best described with the unit of the triangle superimposed in the STM image of Fig. 2(c). The unit has the periodicity of five times of the nearest neighbor distance (nnd) of the oxygen atoms of the CeO2 (111) film. A similar superstructure of 5 × 5 with a respect to the CeO2 lattice was reported on the CeO2 (111)/Ru(0001) by using LEEM.15, 20 The appearance of moiré pattern indicates a lattice mismatch between the substrate and the film, and it became weaker when we observed the terrace part of the 5 ML film. The LEED pattern obtained for the CeO2 (111) film, whose growth temperature and film thickness were 1060 K and 2 ML, respectively, is shown in Fig. 2(d). In addition to the features discussed for Fig. 1(d), we observed the spot marked by an arrow in the Fig. 2(d). The spot was located at 7 × 7 position with a reference to the Ru(0001) lattice, which can be converted to 5 × 5 superstructure of CeO2 (111) order-
FIG. 3. Ce 3d components of XPS spectra of ceria film grown with different substrate temperatures: (a) 925 K, (b) 1008 K, (c) 1070 K, (d) 1096 K, (e) 1207 K, and (f) 1306 K.
ing. The spots were clear on the marked position, but not so for other corresponding sites. In order to analyze the chemical conditions of the films grown at different substrate temperatures, we executed in situ XPS measurements. It has been demonstrated that the electronic states of oxide surfaces can be monitored with the use of photoemission spectroscopy.22, 23 In the XPS spectrum for Ce 3d, there appear multiple and complex peaks originated from different final states reflecting a mixed valency.24–29 Figure 3 shows the variation of the Ce 3d peak with the change of the substrate temperature during the Ce deposition. It should be noted that there appear large changes at energies of ∼885 and ∼905 eV when the substrate temperature exceeds 1096 K. A stoichiometric CeO2 (111) surface is oxygenterminated and the surface oxygen atoms have a threefold coordination to cerium atoms. With a formation of oxygen vacancy, two electrons are left and localized on two cerium atoms in the vicinity of the vacancy. The multiplicity of the Ce 3d peaks increases with the appearance of new oxidation states.30, 31 Three final states of Ce4+ , which include Ce 3d9 4f0 O 2p6 , Ce 3d9 4f1 O 2p5 , and Ce 3d9 4f2 O 2p4 , are expressed as u (v ), u (v ), u (v), respectively, for Ce3d3/2 (Ce3d5/2 ). Note there appears a spin-orbit splitting for the Ce 3d peak. Two final states of Ce3+ , which contain 3d9 4f1 O 2p6 and Ce 3d9 4f2 O 2p5 , are expressed as u (v ) and u0 (v0 ) for Ce3d3/2 (Ce3d5/2 ). Such expressions of the final sates configurations have been discussed previously,24–31 and the corresponding peaks deconvoluted from Fig. 3(b) are shown in Fig. 4(a).32 The variation of the peak-shape with the growth temperature can be more clearly seen in Fig. 4(b) that compares the spectra for the samples grown at 1070 K and 1096 K. Though the temperature change is relatively small (∼26 K), we can see a rapid increase of u4 and v4 components when the temperature increased to 1096 K. The proportion of the Ce4+ component is plotted in Fig. 4(c) for several substrate tem-
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(a)
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Ce4+/ total Ce
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O 1S comp1
0.6 0.4
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400
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1000 1200 1400
Annealing Temperature (K)
0.9
FIG. 5. (a) Deconvolution of O 1s peaks into components 1 and 2 that are corresponding to that of CeO2 and Ce2 O3 , respectively. (b) Variation of proportion of the components 1 and 2 out of total O1s signal as a function of growth temperature.
0.8 0.7 0.6 0.5
200
400
600
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1000 1200 1400
Annealing Temperature (K) FIG. 4. (a) Deconvoluted Ce 3d features. Marks are representing different final states and oxidation states of Ce 3d. (b) Comparison of two spectra for the samples with growth temperatures of 1070 K and 1096 K. Note components of u and v were increased for the latter case. (c) Variation of the ratio of Ce4+ to total Ce.
peratures during the deposition. The ratio of Ce4+ and Ce3+ in CeO2 (111) film was calculated by comparing the integrated intensity of the peaks of u, u and u (ICe 4+ ), and ICe 3+ integrating u and u0 .5 We should notice that a rapid decrease of the Ce4+ component, which corresponds to the reduction of the film, occurs in a small temperature window between 1070 and 1096 K. Here after we call this temperature of 1096 K as reduction temperature. A similar behavior was also observed for the O1s feature. Figure 5(a) shows an O1s peak which is deconvoluted into two peaks. Components 1 and 2 have the peak energies of 529.7 eV and 530.7 eV. An XPS study of two forms of oxides of CeO2 and Ce2 O3 was reported previously, in which O 1s peaks were detected at 529.2 and 530.3 eV for CeO2 and Ce2 O3 , respectively.33, 34 The energy separation for the two components agreed well with our deconvolution, and we attribute components 1 and 2 to the oxygen atoms in a nonreduced and a reduced environment, respectively. The proportions of the two components in the total O 1s signal were
plotted in Fig. 5(b) as a function of the growth temperature. Again we see a large change for the sample treated at 1096 K. It should be noted that XPS spectra from Figs. 3 to 5 were obtained after the surface was cooled to room temperature in oxygen ambient. While it is possible that the oxygen vacancies were filled in the cooling process, the surface morphologies formation were almost completed near the growth temperature due to the rapid decrease of the migration rate with the temperature. So far we examined the effect of the growth temperature on the morphology and oxidation state by using STM and XPS, respectively. Large islands and terrace width appeared only when the growth temperature was close to the reduction temperature. In addition, a large terrace-width change occurred with a small difference of the growth temperature, which was shown in Figs. 1(a) and 1(b) corresponding to 1040 K and 1060 K, respectively. For the samples grown above the reduction temperature, characteristic dots were observed as shown in Fig. 1(c), even though wide terrace and clear step edges were observed. It has been considered that the topmost layer of the ceria film is terminated with an oxygen layer. Thus the overlayer growth for the non-reduced surface proceeds in a manner that adatoms hop on the oxygen covered top-most layer. However in the presence of oxygen vacancies, the adatom adsorption on the Ce layer in the defect sites should also be considered. Atomic processes that govern the thin-film growth have been investigated for variety of systems.35–38 In the initial stage, impinging atoms are trapped on a surface and move
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across the surface. They migrate on available sites of the surface before adsorbed as a stable island. The migration rate is one of important factors that determine the size of the islands, which is a function of the barrier height of the migration and the substrate temperature. In an analysis of a metal film growth on metal surfaces, the following formula have been examined for various cases, which simulates the density of the islands that is a reciprocal of the area of the islands:39–41 n ∝ exp(−εi /(i + 2)kB T ),
(1)
in which n is the density of islands, εi is the migration barrier and kB is the Boltzmann constant, T is the substrate temperature, and i is the critical cluster size. Here we make a rough examination whether the island-size variation of the surface of Figs. 1(a) and 1(b) can be scaled with this formula. The dentistry of the islands is roughly two times larger for the surface of Fig. 1(a) than that of Fig. 1(b). Assuming simple i = 1 corresponding to a monomer diffusion, we can deduce the energy barrier for the migration from Eq. (1), which is close to 10 eV. It is often referred that the barrier is of the order of one tenth of the adsorption energy. This leads to the adsorption energy of 100 eV, which is unrealistic. We consider there must be a mechanism other than the thermal activation that accelerates the diffusion at this temperature range. We consider a hypothesis that the oxygen vacancy creation is related to this mechanism. It has been considered that metal particles are likely to nucleate at oxygen vacancy sites which are argued for TiO2 surface42–44 and MgO45 the number of cluster is proportional to the number of oxygen defect sites. At the same time Galhenage et al. reported a larger diffusion coefficient on the reduced TiO2 surface than that on non-reduced one.46 They measured a size distribution of the metal clusters formed on vacuum-annealed (reduced) and defect-free TiO2 (110) surface. On the reduced TiO2 surface, they found a decrease of the number of clusters and an increased averaged-size of the clusters than on a defect-free TiO2 surface, which suggests the enhanced diffusion on reduced TiO2 (110) surface. These are a few examples how the oxygen vacancies affects the metal cluster formation on oxide surfaces through the variation of the bonding strength and diffusion length of the impinging metal atoms and the substrate. We speculate a similar effect should appear in our experiment case and be correlated with the enhanced terrace widening for the cases where the growth temperature was close to the reduction temperature. However, much experimental and theoretical study is required to prove such hypothesis. For the sample grown at 1150 K, the island size continued to increase. However, as we can see the reduced feature in the XPS spectra, oxygen vacancies remained on the surface. It is likely that deposited Ce metal remained at these sites as a small cluster. Those species are considerably observed as bright dots in the image of Fig. 1(c). In the last section, we consider the growth condition of the CeO2 (111) surface planned in the Introduction, which has a wide terrace width and is free of oxygen vacancies. In the XPS measurement, we observed that the reduction of the surface occurred in an abrupt manner and the intensity of the Ce4+ component decreased rapidly in a small temperature
range. The surface grown at the temperature at 1060 K, ∼40 K below the reduction temperature, showed a large terrace width and the atomic image of the surface showed no oxygen vacancies. The island size was smaller when it was grown at 1040 K. The surface grown at the temperature over the reduction temperature, the terrace contains dots even though the terrace width was large. These results suggest that the growth temperature should be set just below the reduction temperature, and the surface should have wide terrace width and oxygen vacancy free. IV. CONCLUSION
We studied an epitaxial film growth of CeO2 (111) on Ru(0001) by using STM and XPS. CeO2 (111) films were prepared by sublimating Ce atoms in an oxygen ambient where Ru(0001) substrate were kept at elevated temperature, which was cooled in oxygen ambient to room temperature to execute STM and XPS measurements. The oxidation state was examined by comparing the intensities of Ce3+ and Ce4+ components in XPS spectra, the former component should be originated from oxygen vacancies in the reduced state. The reduced state was identified when the growth temperature exceeded 1096 K. The decrease of Ce4+ component occurred in a small temperature window between 1070 and 1096 K. A CeO2 (111) surface with a wide terrace width and a sharp step edge was obtained when the film was grown at the temperatures close to the reduction temperature. The terrace width measured on the sample surface grown at 1060 K was more than twice of that grown at 1040 K. The morphology change between 1040 K and 1060 K was too abrupt to be explained by a simple thermal activation. On the surface grown above the reduction temperature, a surface with a wide terrace and a sharp step was also confirmed. However, small dots were seen in the terrace part, which are considerably Ce atoms adsorbed at the oxygen vacancies on the reduced surface. This experiment demonstrated that it is required to use the substrate temperature close to the reduction temperature to obtain CeO2 (111) with a wide terrace width, where oxygen vacancies were hardly found by STM. 1 S.
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