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Interaction of Oxygen with Thin Cobalt Films† G. Benitez, J. L. Carelli, J. M. Heras,* and L. Viscido University of La Plata, Institute of Physical Chemistry (INIFTA), C.C. 16, Sucursal 4, (1900) La Plata, Argentina Received September 16, 1994. In Final Form: February 27, 1995X The interaction of oxygen with thin cobalt films supported on oxidized Si(100) substrates was studied using Auger electron spectroscopy (AES) and work function changes (∆φ) at 130 and 300 K. Oxygen uptake curves were derived from the ratio changes of the OKLL and CoLMM Auger signals at 513 and 778 eV, respectively. They showed a constant sticking coefficient (so) up to ≈15 langmuirs, especially at 130 K, possibly indicating adsorption through a molecular precursor that diffuses over the chemisorbed layer. At 300 K, so ) 0.2, while at 130 K, 0.5 g so g 0.3, depending on the dosing pressure. Up to ≈10 langmuirs O2 at 300 K, the work function increased 0.20 eV. At that coverage, the Co MVV Auger transitions indicated an oxide formation. From this coverage onward, the work function decreased, saturating at ≈80 langmuirs with a value ≈-1.2 eV below the clean Co surface, pointing to oxygen diffusion into the bulk.
Introduction Due to the interest in the magnetic and catalytic properties of Co and Co oxides, the interaction of oxygen and oxide formation on cobalt has lately been intensively studied. Some early work reported only the X-ray photoelectron spectra of the oxides and hydroxides.1 Later on, most of the research has been performed on single h 0),5 Co(112 h 0),6,7 and even Cocrystals: Co(0001),2-4 Co(101 (0001) after being repeatedly cycled through the hcp-fcc phase transformation temperature8 (i.e., reconstructed surfaces) or on bulk polycrystalline surfaces.9-12 The development of multimetal catalysts is not far from these interests. Such catalytic structures require a deep and comprehensive study, especially on the relationship between morphology and activity. Heterogeneous metal catalysts need a “support” such as Al2O3 or SiO2, on which the metal is dispersed. Our aim is to study the behavior of Co and Co oxide surfaces formed onto silica in the CO hydrogenation reaction. Various steps are necessary in order to accomplish this aim. Among them, in this paper we present the results of oxygen adsorption at 130 K and at room temperature (≈300 K) on thin Co layers, deposited onto SiO2 formed over Si(100) by O2+ bombardment. To the authors knowledge, only one paper refers to a similar system, Co onto SiO2/Si(111),13 in which the oxidation was conducted in atmospheric conditions and its progress followed through the changes in the magnetic properties. Experimental Details Experiments were performed in an in-house-built system operating at a base pressure of ≈2 × 10-10 mbar (mainly H2 + * To whom all correspondence should be sent. † Presented at the symposium on Advances in the Measurement and Modeling of Surface Phenomena, San Luis, Argentina, August 24-30, 1994. X Abstract published in Advance ACS Abstracts, January 1, 1996. (1) McIntyre, N. S.; Cook, M. G. Anal. Chem. 1975, 47, 2208. (2) Freund, H.-J.; Hohlneicher, G. Ber. Bunsenges. Phys. Chem. 1979, 83, 100. (3) Lee, B. W.; Ignatiev, A.; Taylor, J. A.; Rabalais, J. W. Solid State Commun. 1980, 33, 1205. (4) Bridge M. E.; Lambert R. M. Surf. Sci. 1979, 82, 413. (5) Bogen, A.; Ku¨ppers, J. Surf. Sci. 1983, 134, 223. (6) Klingenberg, B.; Grellner, F.; Borgmann, D.; Wedler, G. Surf. Sci. 1993, 296, 374. (7) Atrei, A.; Bardi, U.; Rovida, G.; Torrini, M.; Zanazzi, E.; Maglietta, M. J. Vac. Sci. Technol. 1987, A5, 1006. (8) Matsuyama, T.; Ignatiev A. Surf. Sci. 1981, 102, 18. (9) Castro, G. R.; Ku¨ppers, J. Surf. Sci. 1982, 123, 456. (10) Moyes, R. B.; Roberts, M. W. J. Catal. 1977, 49, 216. (11) Wang, N-L.; Kaiser, U.; Ganschow, O.; Wiedmann, L.; Benninghoven, A. Surf. Sci. 1983, 124, 51. (12) Lahtinen, J.; Vaari, J.; Talo, A.; Vehanen, A.; Hautoja¨rvi, P. Vacuum 1990, 41, 112. (13) Smardz, L.; Ko¨bler, U.; Zinn, W. J. Appl. Phys. 1992, 71, 5199.
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H2O + N2 in the ratio 20:1:1), with Auger capabilities (a Phi single-pass CMA) and mass spectrometry. A Si(100) single crystal was used as support, over which a thin SiO2 film was grown by O2+ bombardment (details have been given elsewhere14). This thin SiO2 layer not only mimics a catalyst support but also is so thin that it avoids charging problems during AES. The actual sample was prepared by depositing Co (Goodfellow Metals 99.998%) at a rate of 7.16 × 10-7 kg m-2 min-1 from a coiled filament heated by Joule effect and mounted inside a watercooled shroud. Before each experiment the Co filament was thoroughly outgassed at ≈1300 K. During evaporation, the base pressure raised to 10-9 mbar, the gases consisting mainly of H2 with a very low contamination of CO2. The amount of Co deposited at 300 K was monitored by a quartz crystal microbalance (QCM), mounted in a position geometrically equivalent to that of the substrate and at the same temperature. Such depositions were performed at 300 K as at this temperature the QCM shows an acceptable frequency temperature coefficient. In this way, the Co LMM Auger signal was calibrated in equivalent thickness against the QCM response. The thickness of the Co films always was ≈10 monolayers (ML). Oxygen (Messer Griesheim 4.8) was dosed through a leak valve at pressures ranging from 4 × 10-9 to 2 × 10-8 mbar. The dose was measured with an uncalibrated ion gauge placed right in front of the leak valve, at ≈10 cm of the sample. Accordingly, and taking into account the correlation between mass spectrometer and ion gauge signals, the exposure measured in langmuirs was affected by a 10% uncertainty. The oxygen uptake was monitored by following both, the Auger signal ratio IO513/ICo778 and the work function (WF) changes. The changes in WF were determined by the threshold shifts of the secondary electron energy distribution of the sample biased to -27 V, upon excitation with the Auger electron gun, as measured by the CMA.15-18 The sample was cooled down to 130 K by bringing it into contact with the head of a cryopump; sample temperature was measured with a type “E” thermocouple. Auger spectra were obtained in the derivative mode using a 2-keV, 1-mA primary beam incident normal to the sample surface, and either 2 or 4 eVp-p modulation amplitude. Data acquisition in a PC-486 was performed with 12 bits resolution and 30 kHz sampling frequency. Thus, a complete Auger spectrum between 20 and 850 eV kinetic energy was acquired in 25 s.
(14) Benitez, G.; Carelli, J. L.; Heras, J. M.; Viscido, L. Surf. Interface Anal. 1994, 22, 214. (15) Janssen, A. P.; Akhter, P. Harland, C. J.; Venables, J. A. Surf. Sci. 1980, 93, 453. (16) Eng, G.; Kan, H. K. A. Appl. Surf. Sci. 1981, 8, 81. (17) Eyink, K. G.; Lamartine, B. C.; Haas, T. W. Appl. Surf. Sci. 1985, 21, 29. (18) Bachman, G.; Oechsner, H.; Scholtes, J. Fresenius Z. Anal. Chem. 1987, 329, 195.
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Figure 2. Oxygen uptake curve on a 10 ML Co film at 300 K: filled circles, adsorption on the film as deposited; blank squares, adsorption onto an overlayer ≈3 ML thick, redeposited after sputtering off the oxide formed in the previous experiment. The signal ratio has been corrected by the Auger sensitivity factors derived from a Co3O4 sample. More details in the text.
Figure 1. Changes of the high energy part of the Auger spectrum of a thin Co film (≈3 nm) upon oxygen exposure. More details in the text.
Results and Discussion Figure 1 shows the high energy part of the Auger spectra of a Co sample at 300 K during O2 exposure. On the basis of such spectra and after smoothing with splines procedures,19 the oxygen uptake curves shown in Figures 2 and 3 were calculated taking the ratio of the oxygen KLL and the cobalt LMM Auger signals at 513 and 778 eV, respectively. In Figure 2 two experiments are depicted, the adsorption of O2 on a virgin Co film deposited onto the SiO2 (filled circles) and the adsorption onto a Co overlayer ≈3 ML thick (blank squares). Previous to overlayer deposition, which was also performed at 300 K, the surface was cleaned with Ar+ bombardment until the OKLL signal was negligibly small. Some difference in surface morphology is expected to become apparent with this procedure, which should manifest in the uptake curve. However, such an effect cannot be seen in Figure 2, where the initial oxygen uptake kinetics from both experiments is identical. Evidently, a more dramatic change in surface morphology is needed in order to modify adsorption kinetics. From the initial slope, a sticking coefficient so was calculated assuming that: (i) the sensitivity factors for oxygen and Co are in this case similar to those we found in a Co sheet oxidized at 700 K in an O2 atmosphere, in which according to our X-ray diffraction analysis Co3O4 is mainly formed, and (ii) the surface density is that of Co(0001): 1.85 × 1019 m-2 Co atoms. With a relative sensitivity factor O513/Co778 ) 0.48, and correcting the Co778 signal in order to take into account only the first layer,
the calculated sticking coefficient was estimated to be so ) 0.2. This value, though affected by an uncertainty of ≈50% (resulting from the assumptions made and the error in the exposure measurement), is smaller than those calculated for Co single crystals (so ≈ 0.6).9 Nevertheless, reported so values for O2 adsorption on Fe(110) at 300 K, which are highly reliable because of the careful calibration through XPS and LEED measurements, are within 0.2 g so g 0.13,20,21 although Fe surfaces are more reactive toward O2 than Co surfaces. Figure 2 also shows that O2 saturation at 300 K is not reached even after 100 langmuirs exposure. Figure 3 shows the uptake curves at two different O2 pressures, with the sample at 130 K. The sticking
(19) Ramirez Cuesta, A., University of San Luis, C.C. 290, (5700) San Luis Argentina, private communication.
(20) Dorfeld, W. G.; Hudson, J. B.; Zuhr, R. Surf. Sci. 1976, 57, 460. (21) Pirug, G.; Brode´n, G.; Bonzel, H. P. Surf. Sci. 1980, 94, 323.
Figure 3. Oxygen uptake curve on a 10 ML Co film at 130 K: blank circles, exposure at P ) 1.2 × 10-8 mbar; inverted triangles, exposure at P ) 4 × 10-9 mbar. More details in the text.
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Figure 4. Work function changes upon O2 adsorption at 300 K: curve 1, data obtained with 2 keV beam energy, -0.40 µA sample current (0.15 mA emission) and 4 eV analyzer modulation; curve 3 with same energy and modulation, but -2.1 µA sample current (1.2 mA emission). Curve 3 and the dashes on the right margin are various saturation values obtained with decreasing sample current. The saturation value marked E was obtained with 1 keV energy and +0.47 µA sample current.
probability is higher than the one at 300 K (0.3 and 0.5) and more noticeable at low pressure exposures (inverted triangles). Saturation is reached above 50 langmuirs. Up to ≈10 langmuirs the uptake curve behaves linearly. This extended constant adsorption rate is interpreted by several authors4,6,9,22 as a dissociative adsorption through a precursor state of weakly bound molecular oxygen, which diffuses over the occupied sites. No specific experiments were performed to confirm this adsorption model, which also explains why so is higher when the exposure is performed at low pressures. The changes in WF (∆φ) upon adsorption are shown in Figure 4. Following an initial increase after a ≈10 langmuirs exposure, a steep decrease is observed up to ≈50 langmuirs, then a slow change continues up to saturation above 80 langmuirs. While the maximum ∆φ, 0.20 eV, is highly reproducible, the saturation value shows a strong dependence on the primary beam intensity and/ or energy. Curve 1 in Figure 4 refers to data obtained with 2-keV beam energy, -0.40 µA sample current (0.15 mA emission) and 4 eV analyzer modulation, while curve 3 was obtained with the same energy and modulation, but a -2.1 µA sample current (1.2 mA emission). In between there are various saturation values obtained with decreasing sample currents (curve 3 and the dashes on the right margin). The saturation value indicated by the dash marked E was obtained with 1 keV energy and +0.47 µA sample current and shows that the beam energy has no effect on the WF, at least at these values. Accordingly, the scattering of the saturation ∆φ value cannot be ascribed to a beam effect but to an artifact of the measuring method, possibly because the distribution curves of the secondary electrons are not strictly parallel, and the inflection point as a measure of the displacement does not give accurate results (actually, the method is strictly valid for the onset of the secondaries distribution). Nevertheless, it is known that the WF of clean Co films thoroughly annealed at 373 K is 5.12 ( 0.03 eV,23 while that of Co2O3 (22) Brundle, C. R.; Broughton, J. Q. In The chemical physics of solid surfaces and heterogeneous catalysis; King, D. A.; Woodruff, D. P., Eds.; Elsevier: Amsterdam, 1990; Vol. 3A, Chapter 3.
Figure 5. Low-energy part of the Auger spectrum of a Co film at 300 K during exposure to oxygen. The shoulder developing at 58 eV points to oxidation. More details are in the text.
is 4.01 eV.24 Hence, upon oxidation of the Co film a WF decrease of ≈1 eV is expected. On polycrystalline Co foils at 300 K and with a similar excitation method as used in our experiments (though measuring the displacement of the characteristic curve of the sample current), enhanced WF changes have been reported:11 An increase of 0.5 eV at ≈5 langmuirs and -2.5 eV at saturation (≈90 langmuirs). In a recrystallized Co sample with a high (0001) texture,9 only the WF changes below 6 langmuirs are reported: at 300 K an increase of 0.55 eV, while at 130 K relative maximum and minimum of 0.55 and 0.25 eV at 1 and 2 langmuirs, respectively. A quantitative calibration of the oxygen penetration depth by ion bombardment could not be established in our system. It is known that in compounds with low and high mass constituents like metal oxides, the lighter component is preferentially sputtered.25 However, the sputtering projectile itself may cause recoil implantation and cascade mixing, which are enhanced with ion energies in the keV region as our Penning type ion gun works. Consequently, not only the shape of depth profiles may be strongly influenced by the sputtering but also deeper Co layers are oxygen enriched through a knock-on mechanism.26 In fact, from the whole film thickness of ≈10 ML, 3 ML are the amount of equivalent hcp layers that generally were sputtered until a reasonable clean Co surface was reached. Nevertheless, there always remained ≈8 atom % oxygen in the volume sampled by AES, (23) Heras, J. M. Acta Cient. Venez. 1980, 31, 308. (24) International Critical Tables; MacGraw-Hill: New York, 1929, Vol. VI, p 54. (25) Taglauer, E. Appl. Surf. Sci. 1982, 13, 80. (26) Sigmund, P. In Sputtering by particle bombardment; Behrischer, R., Ed.; Springer: Berlin, 1981; Chapter 2.
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possibly indicating that the oxygen diffusion front is located at ≈3 Co ML depth. Remarkably, this oxygen contamination persists until the SiO2 layer is reached. Even after redeposition of fresh Co overlayers with thicknesses of the same order of those sputtered (3 ML), it was impossible to get rid of a negligible small oxygen contamination. An oxide formation is clearly demonstrated by AES. It is known that in the MVV spectra of Co oxides, the peak at 54 eV of the clean surface develops a shoulder at 58 eV and two other cross transition peaks at ≈36 eV and ≈20 eV.11,27 Figure 5 shows the low-energy part of the Auger spectrum of a Co film during oxygen exposure at 300 K. Clearly, from 9 langmuirs O2 onward, a shoulder at 58 eV develops, pointing to surface oxidation. Remarkably, at this coverage ∆φ reached its maximum. According to reported XPS and UPS experiments conducted on Co(0001) at 300 K,2 the oxygen molecule is dissociatively adsorbed, followed by diffusion of the O-atoms into the first Co ML in order to establish a quasioctahedral ligand field with formation of the CoO. On the contrary, at ≈120 K, Co3O4 is formed.5,6,9 However, it has been suggested that the actual Co valence state of the oxide formed depends on the surface oxygen concentration obtained under the exposure conditions.9 The different saturation value observed between the uptake curves and the WF changes is a consequence of the different information depth of both techniques. While ∆φ senses only the surface oxygen, the Auger signal also detects the oxygen which has diffused into the bulk. Hence, a saturation value will be reached only after the diffusion front surpasses the escape depth of the oxygen (27) Weissmann, R.; Mu¨ller, K. Surf. Sci. Rep. 1981, 1, 251.
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Auger electrons, ≈4.5 ML. Apparently, with the exposure time used in the experiments, this diffusion depth was not reached. Conclusions Thin polycrystalline Co films are oxidized upon exposure to ≈9 langmuirs oxygen, even at a low temperature. From the uptake curves it follows that the initial sticking coefficient is lower at 300 K than at 130 K but in both cases is constant at the beginning of the exposure, indicating chemisorption controlled by a molecular precursor. The changes in work function also point to precursor mediated chemisorption. The values of the sticking coefficient (0.2 at 300 K and ≈0.4 at 130 K), though smaller than the published for Co single crystals, are in perfect agreement with those reported in Fe(110),21 where a careful calibration procedure has been performed. For a straightforward comparison of the results obtained by different authors, a better description of the dosing system is highly desirable, as the uncertainty in the measurement of the oxygen dose affects all calculations concerning the kinetics of the process. Acknowledgment. The authors acknowledge the financial support of the CONICET (Argentine Research Council) as well as the donation of equipment by the A. von Humboldt- and the Volkswagen Werk-Foundations (Germany) and the International Program for Physical Sciences (Sweden). LA940743W
