Ultraviolet inverse photoemission study of the oxidation of Fe, Co and Ni

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Surface Science North-Holland

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(1993) 264-268

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Ultraviolet inverse photoemission study of the oxidation of Fe, Co and Ni M. Finazzi, L. Du6, G. Bacchin, F. Ciccacci and E. Puppin Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, I-20133 Milano, Italy Received

14 August

1992; accepted

for publication

24 November

lY92

We present a room temperature inverse photoemission isochromat (IPES) spectroscopy study of the oxidation of polycrystalline Fe, Co and Ni in the lo-25 eV photon energy range. In the case of cobalt and nickel we find that each isochromat spectrum can be decomposed as the sum of the signal from the metal substrate superimposed on the contribution from the oxide layer. For the case of cobalt, in the exposure range from 30 to lo4 L the oxide lineshapes do not vary and is comparable to the signal at saturation, suggesting that the chemical composition of the oxide is independent of the film thickness. We discuss the features of these two contributions with respect to exposure and isochromat energy. In the case of oxidized Fe, saturation occurs at very low exposure (about 5 L). Comparison to IPES results from the full family of Fe bulk oxides shows that the lineshape of chemisorbed 0, on poly-Fe is similar to FesO, rather than FeO.

1. Introduction The study of oxidation of metal surfaces has great importance in heterogeneous catalysis and corrosion and is a widely studied topic in surface science [l]. Moerover, oxidation of transition metals leads to the formation of compounds in which correlation effects are relevant [2]. The electronic properties of these materials have been the subject of controversial debate in the last few years because of their wide variety of physical properties and their relevance in understanding high T, superconductors. Among them the transition metal oxides have been playing a major role since the discovery of their insulating nature while single particle band models predict them to be metallic [3]. Due to their high electrical resistivity, the investigation of these transition metal compounds with a spectroscopic technique involving electrons is very difficult since charging effects can actually hinder interpretation of electron affinity spectra. A way to obtain qualitative information is to investigate, with a surface sensitive probe, thin oxide films grown on top of a metal substrate. 0039-6028/93/$06.00

0 1993 - Elsevier

Science

Publishers

The electronic structure of oxidized 3d metal single crystals or polycrystals has been widely studied with spectroscopy techniques sensitive to occupied electron states, while electron states above E, have received less attention. In particular inverse photoemission spectroscopy has been performed only on oxidized Ni polycrystals [4] and monocrystals [5-91 or Fe polycrystals [lO,lll and no data on empty electronic states of oxidized Co are available in the literature. Results concerning the growth of a thin film of oxide on late 3d metals show that room temperature oxidation of poly- or monocrystalline Ni or Co samples at saturation leads to the formation of stoichiometric NiO [12] and Co0 [13,14], respectively, and that oxidation of Fe single crystals produces Fe0 [15,16]. We present an angle integrated inverse photoemission spectroscopy (IPES) study of room temperature oxidation of polycrystalline Fe, Co and Ni. The spectra have been taken in the ultraviolet region with tunable hv in the range between 10 and 25 eV. The possibility of varying the photon energy gives a powerful experimental tool, based on photoemission cross-section con-

B.V. All rights reserved

M. Finazi et al. / Study of the oxidation of Fe, Co and Ni

siderations, for an assignment of the orbital nature of the features shown by the spectra. Photon energy tunability allows us to demonstrate the d-nature of the features produced in the IPES spectra of Fe, Co, Ni after exposure to 0,.

265

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IPES (hv = 14.5 eV) EDOS

2. Experimental the IPES spectra are acquired in the isochromat mode, i.e. the energy of the electrons impinging on the sample is varied continuously while the photons emitted, dispersed by a grating on a flat focal field, are detected at fixed hv [17]. Twelve isochromat spectra can thus be collected simultaneously by a position sensitive detector in the lo-25 eV photon energy range. Integration in reciprocal space is a fortiori verified by the polycrystalline nature of the samples and by the characteristics of the electron source which is an activated filament of current heated thoriated tungsten. In order not to damage the sample surface the current impinging on it was kept below 100 PA/cm’. The experimental broadening increases with photon energy and is mostly due to the aberrations of the grating. It ranges from 0.4 eV for the isochromat at 12 eV to 0.9 eV for the one at 24.5 eV. The samples are cut from commercially available high purity polycrystal rods and experiments are performed scraping the metal surface with a diamond file at a base pressure of about 4 X lo-” Torr before exposure to oxygen. Oxidations are made at room temperature in the l-lo4 langmuir exposure range [l langmuir (L) = lop6 Torr * s].

3. Results and discussion In fig. 1 the spectra of clean Fe, Co and Ni at hv = 14.5 eV are shown in comparison with the corresponding calculated single particle empty density of states (EDOS) [18] convoluted with a Gaussian to simulate experimental broadening. The spectra clearly resemble the density of empty states in accordance with the simplest models of IPES. This agreement is confirmed for the whole

EF

2

4

Energy above

EF

Fig. 1. Isochromat spectra at the same value of the photon energy (14.5 eV) from Fe, Co and Ni (dotted lines) in comparison with the corresponding calculated EDOS (solid lines

1181).

range covered by the spectrometer provided the broadening is changed in accordance to the experimental resolution. Theory [18] predicts that the IPES spectra of the late 3d metals are dominated in the region extending up to 2-3 eV above E, by the emission from d-derived empty states. In fig. 2 is presented a selection of IPES spectra taken at different values of hv for clean and oxidized Fe, Co and Ni at saturation exposures. The signals from clean metal have all been normalized to the amplitude of the d-derived peak a few eV above E,. Spectra from oxygen exposed metal surfaces are plotted without changing their relative intensity with respect to the corresponding isochromat from the clean sample. Fig. 2 enlightens the same hv dependence of the features present in the spectra from both the clean and oxidized samples at an energy extending up to 6 eV above the Fermi level. This fact demonstrates that these features have the same orbital nature, otherwise their dependence on the isochromat energy would be quite different, since the ratio between 4sp and 3d cross-sections are strongly dependent on the isochromat energy in the energy range between 10 and 25 eV (for direct photoemission it decreases by a factor of 4 in Cu [19]>.

M. Finazzi et al. / Study of the oxidation of Fe, Co and Ni

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I

For space reasons and since oxidation of nickel is a well studied topic, only the oxidation of iron and cobalt will be discussed in detail, exploiting their dependence on hv and on exposure.

hv=152eV

3.1. Iron The evolution of the IPES spectra upon oxidation stops at a very low exposure (- 5 L) above which no change in the data is observed. Two clearly visible effects occur: (9 the shoulder just above E, characteristic of clean Fe disappears, and (ii) the onset of the signal moves towards higher kinetic energies. The first behavior depends on the change of the electron density of states while the second is a consequence of the insulating nature of the oxide film resulting in the formation of a gap. A comparison of the spectrum from oxidized Fe at the exposure of lo3 L and a set of IPES data obtained from bulk Fe oxides [20,21] is presented in fig. 3 for an isochromat energy of 15 eV. Although it is well known that room temperature oxidation of a monocrystalline iron surface

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Co clean co+104Lo~

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Ni clean NI + 103L 0,

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4

8

12

Energy above

E, (eV)

Fig. 3. Comparison of the isochromat IPES spectra taken at hv = 15 eV from an oxidized iron surface and bulk Fe oxides. From the bottom: clean Fe, Fe exposed to 10’ L of OZ. FeO, Fe&?,

1211,Fe,O, MU.

gives rise to stoichiometric Fe-monoxide [ 15,161, in the present case the spectrum from the oxidized polycrystal sample shows a clear difference with respect to the spectrum of FeO. On the contrary the lineshape of Fe + 10” L 0, shows a more pronounced similarity with the signal from Fe,O, suggesting that this compound is formed. 3.2. Cobalt

0

4

8

12

Energy above E, (eV) Fig. 2. Comparison of IPES spectra for clean (solid lines) and oxidized (dotted lines) Fe, Co and Ni at different hv. The intensities of the spectra from the clean metals have been normalized to the intensity of the peak near E, while the spectra of oxidized samples have been resealed consequently.

Cobalt oxidation effects on IPES spectra saturate at lo4 L, at which exposure a gap is formed. At lower exposures, as already pointed out by Scheidt et al. for Ni/O, in ref. [5], spectra can be deconvoluted into two contributions: the first, near the Fermi level, derived from the metal substrate, and the second, extending from 2-5 eV, representing the total effect of the oxidation reactions. For exposures higher than or equal to 30 L the lineshape of the latter is equal to the saturation spectrum while for 0 < 30 L the contribution from the oxide is dependent upon exposure. In order to study the film growth the ratio R(0) = S(e)/& Iw h ere S(0) is the intensity of the oxide contribution at exposure 0 and S, is the intensity of the spectrum at saturation] has been plotted versus exposure in fig. 4a. The experi-

M. Finazzi et al. / Study of the oxidation of Fe, Co and Ni

2 0.8 -0 3

0.6

g

0.4

10’

lo2

lo3

lo4

Exposure 13(L)

Energy (eV) Fig. 4. (a) Experimental results (dots with error bars) of the ratio R(0) = s@)/5(104) between the oxide signal intensity. The fitting curve (full line), is based on the points with 0 2 30 L. In the right part of the figure the asymptotic R = 1 value is also shown. (b) Comparison between the oxide contributions to IPES spectra (dots) (hv = 15.2 eV) at 0 = 7 L (upper panel) and at 0 = lo4 L (lower panel) after background subtraction and bulk Co oxide spectra (line) from ref. [23] (hv = 1486.6 eV) relative to the conduction band minimum.

mental points have been fitted with curves of the following analytical expression: R(6) = 1 - (1 + e/e,)+, where B0 and p are parameters depending upon oxidation conditions. This model has already been applied successfully in the case of nickel oxidation [5] and is based on a logarithmic growth law [22]. If all experimental points are taken into account for the fitting, the obtained result is very poor, summarized in the value of x2 = 1.97 X lo-*. If the points representing exposures less

261

than 30 L are excluded a value of x2 = 2.46 x 10e3 is obtained and the corresponding curve (solid line) fits very well the points at 8 2 30 L, suggesting that in this range a compound independent of film thickness is formed. The strong dependence of x2 on the number of points used in the fitting shows that for 8 < 30 L the model fails and that oxidation products are different from the ones at higher exposures, in accordance with the fact that the lineshape of the oxide contribution is different from the saturation spectrum in this exposure range. In the upper and lower panel of fig. 4b a comparison of BIS spectra at hv = 1486.6 eV of bulk Co0 and CYo,O, [23] with the oxide contribution for UV IPES spectra of Co + 7 L and lo4 L 0, is shown. Our spectra have been broadened with a Gaussian with FWHM = 0.5 eV to simulate the same experimental resolution of the data in ref. [23]. Because of cross-section reasons, 3d peak-to-background ratio is different in BIS and UV IPES spectra. To compensate for this fact a linear background has been subtracted from our spectra. The shape of the background is not important for the present discussion. Because of the insulating nature of the samples a Fermi level is not defined and the spectra have been aligned to the conduction band minimum (CBM) as defined in ref. [23]. In the lower panel of fig. 4b a close resemblance between the spectra from the oxide on top of Co and from Co0 is evident, in accordance with previous results on cobalt oxidation 113,141 in which formation of cobalt monoxide after exposure of cobalt to oxygen is pointed out. The discussion summarized in fig. 4a shows how formation of stoichiometric Co0 starts already at 30 L. On the other hand, at an exposure of 7 L the contribution from the oxide is very similar to the BIS spectrum of Co,O,. Though it is very difficult to demonstrate the formation of stoichiometric cO,O, simply by this comparison, this hypothesis is consistent with the fact that R(0) for 8 < 30 L is lower than the value expected from the model working at high exposures, as shown in fig. 4a, implying an oxygen enriched surface despite the low total amount of chemisorbed oxygen atoms.

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Acknowledgements

The authors thank L. Braicovich for fruitful discussions. This work has been supported by the Minister0 dell ‘UniversitA e della Ricerca Scientifica e Technologica through the Consorzio INFM.

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