Oxidation-induced segregation at the Pt0.5Ni0.5(111) surface studied by medium-energy ion scattering

121

Applied Surface Science 45 (1990) 121-129 North-Holland

Oxidation-induced segregation at the Pt o.sNi,., ( 111) surface studied by medium-energy ion scattering S. Deckers, F.H.P.M. Departmeni

Habraken, W.F. van der Weg

Atomic and Inierface Physics, Universiv

of Utrecht, P.O. Box 80.000, 3508 TA Utrecht, The Netherlamb

A.W. Denier van der Gon, J.F. van der Veen FOM Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam,

The Nether/an&

and J.W. Geus Anorganic

Chemistry Department,

University of Utrecht, Croesestraat

72A. 3522 AD Utrecht, The NetherIan&

Received 6 April 1990; accepted for publication 5 May 1990

The kinetics of oxygen uptake at 400“ C at low Oz pressures on a Pt,,sNi,,s(lll) surface is investigated using elhpsometry. The oxidation process exhibits a gradually decreasing oxidation rate. It saturates above 1500 langmuir of 0s exposure. Medium-energy ion scattering combined with shadowing and blocking is used to investigate the surface composition before and after exposure to 350 langmuir of 0s at 400 o C. This exposure results in an oxygen coverage of (2.3 * 0.3) X lOI atoms/cm’, about half the coverage of the saturated surface. Whereas the first atomic layer of the ahoy surface before exposure to oxygen contains mainly Pt atoms, after exposure to 0, the top layer is disordered and contains only Ni and 0 atoms. Underneath this disordered top layer a crystalline layer of metal atoms is present containing (65 f 5)X of Pt. The third atomic layer (the second crystalline layer) contains (63 f 8)% of Pt. The total number of Ni atoms present in the disordered toplayer and the first crystalline layer is (2.1 f 0.3) x 10” atoms/cm2. The first two crystalline interlayer distances are found to be expanded by (2.4 f 0.5)s and (2.3 f 0.5)s respectively, both with respect to the bulk value. Upon 0, exposure at 4OO”C, the initial oxidation rate of a bulk Pt e.sNi,,,(lll) surface is equal to that on a Ni(ll1) surface covered with 4.7 monolayers of platinum. The similarity is explained with previously observed surface ahoy formation on a Pt-covered Ni(ll1) upon annealing. The surface region of this ahoy is very similar to the Pt,,,Ni,~,(lll) surface.

1. Introduction In the last few years several studies of the oxidation of alloy surfaces have been published. Here we refer to Pt-Ti [l-3], Pt-Sn [4-61, Pt-Rh [7], Pt-Re [8,9], Pt-Pb [10,6], Pt-Ni [ll], Ni-Ru [12], Ni-Ti [13], Ni-Cr [14], and Ni-Fe [15]. In several of these systems, such as Pt-Ti and Pt-Sn, an oxygen-induced reversal of the segregation is observed. Paul and coworkers [l] observe an almost pure first layer of Pt atoms on the clean Pt,Ti(lll) surface. After 0, exposure TiO, islands are formed on top’ of the Pt-enriched layer, which 0169-4332/90/%03.50

remains unaffected. Dauscher et al. [2] report formation of Ti,O, islands together with some oxidized platinum. Bardi et al. [3] show that on the PtsTi(510) stepped surface, the ordered step array of the clean surface is changed upon oxygen exposure into large facets of TiO,. On polycrystalline Pt-Sn samples Asbury and Hoflund [4,5] have examined segregation at clean and O,-exposed surfaces. The clean surface is found to be enriched in platinum to an extent depending on the annealing temperature. Upon 0, exposure a tin oxide overlayer is formed with a maximum thickness at 200” C. Furthermore,

@ 1990 - Elsevier Science Publishers B.V. (North-Holland)

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S. Deckers et al. / Oxidation-induced segregation at the Pt,,Ni,,(l

Chueng [6] mentioned that platinum-based alloys of Sn and Pb are less prone to oxidation than pure tin and lead. Lad and Blakeley [15] observed that after 0, exposure at 100°C the Ni,,,Fe,,,(lOO) surface consists only of Fe0 and Fe,O,. At the Ni-enriched clean Ni-lS%Cr(lll) surface, different stages of oxide formation are found by Jeng et al. [14]. At 200-300 o C, initially a chromium oxide is formed and only at higher 0, exposures is NiO also observed. Whereas at 200-300 o C the formation of NiO is delayed, at 400 o C almost no NiO is detected. On Ni-Ru [12] XPS and UPS unambiguously indicated the formation of an oxide overlayer. Segregation induced by adsorbed gases other than oxygen has also been observed. On NiCu(ll1) alloys, which have a Cu-rich first layer on the clean surface, a CO-induced Ni segregation has been reported [16]. The Pt-Ni system has drawn the attention of both experimentalists and theoreticians owing to the interesting segregation effects which occur on the clean surface. Chemisorptive and catalytic properties of Pt-Ni alloys have been investigated on small particles and on poly- and monocrystalline samples [11,17-191. The deposition of thin platinum films on nickel substrates has also been studied, see refs. [20,21]. According to SedlaEek et al. [ll], Pt-enriched polycrystahine Pt-Ni surfaces are covered with NiO after 0, exposure at 300 o C. On Pt-rich samples they additionally observe formation of some PtO. The Ni atoms in the top layer remain after reduction of the oxide layer with H,. A similar oxidation behaviour is observed on Pt-covered Fe(100) [22] or Ni(ll1) [23] surfaces. In both cases the substrate material forms an oxide layer on top of the platinum overlayer, for iron upon adsorption of oxygen at room temperature and for nickel at or above 250°C. Medium-energy ion scattering combined with shadowing and blocking (MEIS-SB) has proved to be a valuable technique for assessing surface structures and relaxations. It has been used very rarely in studies of the composition of alloy surfaces (see for example ref. [24]). Here we show that MEIS-SB is very useful for studying adsorp-

I I) surface

tion-induced segregation effects. A full account of the MEIS-SB method is given in ref. [25]. In a previous MEIS-SB experiment, the composition of the first three layers of the clean Pt,,Ni,,(lll) surface was determined [26]. An oscillatory composition profile was observed, with Pt enrichment in the first and Ni enrichment in the second layer. The first two interlayer distances were found to have an equal inward relaxation and the thermal vibration amplitudes of the first layers were enhanced compared to the bulk values. In the present study, the 0, exposure of a Pt,.,N&(lll) surface is monitored using ellipsometry. Furthermore, MEIS-SB is used to determine the atomic composition in the first layers of the O,-exposed surface and to examine the growth of the observed oxide overlayer. A disordered NiO layer is found at the surface and the absolute amounts of nickel and oxygen present in this top layer are determined. Furthermore, unlike many other surface-sensitive techniques MEIS-SB enables us to also investigate the chemical composition and the crystallinity of the atom layers below this disordered layer. The structure and composition of the first and second crystalline layer underneath the NiO top layer have been measured. The results show a close similarity to the oxidation behaviour of Pt-covered Ni(ll1) at 400°C.

2. Experimental 2.1. Crystal preparation The sample preparation procedures are identical to those described in refs. [27,28]. The Pt,,,Ni,,,(lll) crystal was cleaned by repeated sputtering and annealing cycles. Thereafter, no contaminants could be detected using AES (approximately 2% of a monolayer detection limit) while the LEED diagram showed a sharp (1 X 1) pattern. 2.2. Ellipsometry The ellipsometry measurements were performed in a multi-chamber UHV system attached

S. Deckers et a[. / Oxidation-induced

to the 3 MV Van de Graaff generator of the University of Utrecht. This system and details of the ellipsometry measurements are described elsewhere [29]. The sample was exposed to 1500 langmuir (L) of 0, at an Oz partial pressure of 5 X lo-’ Torr at 4OO“C. During 0, exposure the change in the ellipsometric parameter A, i.e. the value of M, was monitored at a fixed wavelength of 554 nm with off-null intensity measurements. The parameter 6A is proportional to the number of oxygen atoms and has been previously calibrated using the “O(p, a) “N nuclear reaction v31. 2.3. Ion scattering The MEIS-SB experiments are performed in a UHV system connected to the 200 kV accelerator of the FOM Institute for Atomic and Molecular Physics (AMOLF). This system consists of a main scattering chamber (base pressure of about 5 X lo-” Torr), equipp ed with LEED and Auger facilities, a preparation chamber with an ion gun for sputter-cleaning and a load lock for introducing samples into the vacuum. A detailed description of this system is given elsewhere [30,31]. A 100 keV H+ ion beam is used in all MEIS-SB experiments. The clean sample was exposed to a total of 350 L of 0, at an Oz partial pressure of 1 X lo-’ Torr at 400 o C. After exposure, the sample was kept at 400°C until the 0, partial pressure dropped to a (a) Geometry

I

segregation at the Pt,,Ni0,5(lII)

surface

123

value of less than 1 x lo-” Torr. Earlier exposures of 50 L at room temperature showed no significant peak in the Auger spectra. After the 0, exposure a diffuse background without observable spots was observed with LEED. During ion scattering data acquisition care was taken not to damage the crystal or the oxide layer by using different spots on the sample after every 25 PC/cm2 of beam dose. With this dose the change in the Pt surface peak content was less than 5% and no change was observed in the minimum yield at detection energies below that of the Ni peak. Measurable damage occurred after 30 @Z/cm’, which is a 10 times lower beam dose than the damaging dose for the clean sample. Data analysis involves extraction of the surface peak area from the energy spectra at the different exit angles and conversion of the surface peak areas into the number of visible monolayers of Pt and Ni [31], one layer containing 1.61 X 1015 atoms/cm2. From energy spectra taken around the oxygen elastic backscattering energy the absolute oxygen coverage was obtained. The composition of the first few layers was determined using the two different scattering geometries shown in figs. la and lb. Note that a possible disordered overlayer at the surface is left out from these figures, since it will not give rise to additional shadowing and blocking effects. Instead it contributes uniformly to the blocking patterns. In geometry I all [OOl] strings terminate in the first crystalline atomic layer. All other layers (b) Geometry

II

[1101 \ \

Fig. 1. Two different scattering geometries used, labeled I and II and both in the (001) plane perpendicular to the Pt,,Ni,,,(lll) surface. In geometry I, all [MU] strings terminate in the first atomic plane (a). In geometry II, the [112] strings terminate in the first and second atomic layer (b).

S. Deckers et al. / Oxidation-induced

124

segregation at the Pto,,Nio,,(lIl)

are shadowed from the incoming beam. In geometry II, all [112] strings terminate alternatively in the first and second atomic plane, while third and deeper layers are shadowed.

80

-

4 ____----

3 s

m-

__-_ _/- __-r __-z _A _,-----____z’_-----_-------

E 4 % 2

-2 I

2’

2

,*’ & iI E ”

zII;/_ 0

0 -0

tA....t....n,.,.l,,I

0

1000

500

Oxygen

exposure

1500

(Langmuir)

Fig. 2. Ellipsometric oxygen uptake curves for the Pt,,sNi,,,(lll) surface (dashed line) and from Ni(ll1) covered with 4.7 MLE of platinum (solid line). Both curves were taken at a sample temperature of 400 o C with a pressure of 7 X 10W5 Torr. The dashed horizontal line represents the oxygen coverage at which the MEIS-SB measurements are performed.

160

Pt

1203

3. Results

3.,....,....,....,.,

6000

lo

Eergy

Fig. 2 shows 8A during the exposure of 0, to the bulk alloy surface at 400 o C (dashed curve). In the same figure, the result of a similar 0, exposure to a Ni(ll1) surface covered with 4.7 monolayer equivalent (MLE) Pt is also shown [23]. A MLE is defined as the number of atoms in a bulk Ni(ll1) layer, 1.86 x 10” atoms/cm’. A gradually decreasing oxidation rate is observed in both cases and the initial stages of oxygen uptake are found to be remarkably similar. Only at 0, exposures above 500 L is the oxidation proceeding somewhat faster on the thin film structure than on the bulk alloy. The oxidation of the bulk alloy tends to saturate after an exposure of about 1700 L at an oxygen coverage of about 4 x lOi atoms/cm’. After this exposure, no platinum could be detected with Auger electron spectroscopy on the oxidized alloy surface. The comparison of the two 0, exposures in the two UHV systems is most reliably made using the oxygen coverages determined by

surface

%e”,

B

- 8000

s

90

A

3 i 4000 8g

Oxygen-covered Clean

L 08 E:rrgy

0 100

(keV)

Fig. 3. Energy spectra measured on the clean (0) and the oxygen-covered (X) sample in geometry I at an exit angle of 54.7 o with respect to the surface plane. The spectra are scaled to equal beam doses. The inset shows the oxygen peak that is present at lower energies in the spectra.

means of the ‘*O(p, a) “N nuclear reaction (for the ellipsometric measurements) and by MEIS-SB. Using this procedure, possible differences in the exposure scale are avoided. The oxygen coverage at which the MEIS-SB experiment has been carried out is indicated in fig. 2 by the horizontal dashed line. That coverage is roughly half the saturation coverage of the considered process. Fig. 3 shows the energy spectra for the clean and oxygen-covered sample as measured in geometry I. The Pt peak contents are the same in both spectra, but in the spectrum from the oxygencovered surface the peak maximum is shifted to lower energies by 270 &-25 eV. The Pt peak width in the latter spectrum is 1.6 times the peak width of the clean spectrum. The Ni peak area for the oxygen-covered sample is 6.1 times larger than in that for the clean surface but the widths of the Ni peaks in the two spectra are equal. In the inset of fig. 3 the oxygen peak is shown as it appears in the spectra of geometry I at lower energies. From the peak area an oxygen coverage of (2.3 1_ 0.3) X lOi atoms/cm’ is deduced. The blocking patterns for geometry I are shown in fig. 4, together with the blocking patterns for the clean crystal. The Pt curve is identical to that for the clean surface, except for a small increase in the off-blocking-directions. The Ni signal in the spectrum of the oxygen-covered surface has increased by 0.98 + 0.05 monolayer of visible atoms

S. Deckers et al. / Oxidation-induced

segregation at the Pto~,NiO,,(lll)

surface

125

surface pattern is only 0.4 monolayer, while in the [lli] direction the increase is comparable to that in geometry I.

4. Composition + 5

analysis

1.5 1 0.5 A Y

65

60

50

45

Exit

a~$:

(deg)

Fig. 4. Number of visible monolayers of Pt and Ni atoms versus exit angle determined in geometry I for the clean (0) and the oxygen-covered ( X) sample. The lines represent computer simulations for the clean () and the oxygencovered (- - -) sample, see text and table 1.

and the blocking minimum in the [llO] direction has disappeared. The blocking patterns of the Pt and Ni peaks in geometry II are shown in fig. 5. In single alignment the platinum signal has increased by roughly 0.4 monolayer. In the [ill] direction the yield is almost identical to that for the clean surface, as in geometq I. There is also a clear shift of the [lli] and [332] blocking minima of 0.55” f 0.1” towards larger exit angles compared to the bulk blocking minima. The Ni blocking pattern exhibits almost no structure. In single alignment (shadowing only) the increase with respect to the clean

ot”“““““““““‘,’ 15

20 Exit

25 angle

30

35

(deg)

Fig. 5. Number of visible monolayers of Pt and Ni atoms versus exit angle determined in geometry II for the clean (0) and the oxygen-covered ( X) sample. The lines represent computer simulations for the clean () and the oxygencovered (- - -) sample, see text and table 1.

The energy spectrum for the oxygen-exposed sample (displayed in fig. 3) shows some remarkable differences with respect to the analogous spectrum for the clean surface. The Pt peak is shifted and broadened towards lower energies. Since the scattering geometry is identical to the geometry used on the clean surface, this energy shift can only occur if Pt atoms are not present in the first atomic layer. The calculated energy loss of a 100 keV proton beam in a monolayer of NiO is lOO250 eV, where the large uncertainty represents the difference between stopping powers measured under random incidence [32] and channeling [33] conditions. The broadening can be caused by disorder in the surface region or by an overlayer of non-uniform thickness. The shifted Pt peak is accompanied by a 6 times enlarged Ni peak. This large Ni surface peak reflects the presence of nickel at the surface. However, since the Pt peak area is not reduced, this nickel must be present in a disordered layer. This is consistent with the disappearance of the spot pattern in LEED. For a comparison of the energy spectra alone it is clear that the Pt-enriched first layer of the clean sample is covered with a Ni-rich overlayer upon 0, exposure. The Pt blocking patterns in the two geometries confirm this observation and give additional insight in the composition and structure of the atom layers underneath. In a double alignment direction of geometry I, only the first crystalline layer of the sample can be detected (apart from the disordered overlayer). In single alignment, a small fraction of the second crystalline layer contributes also to the blocking pattern, and therefore the off-blocking increase of the platinum signal in geometry I indicates a larger platinum concentration in the second layer that in the corresponding layer of the clean surface. The Ni blocking pattern has increased by roughly one monolayer. This increase without a

126

S. Deckers et al. / Oxidation-induced

concomitant decrease in the platinum signal and the lack of a clear blocking minimum indicate that about one monolayer of disordered nickel has emerged in the top layer of the sample. This is in agreement with the shift of the Pt peak to lower energies (see fig. 3) and the disappearance of the LEED pattern. The Pt blocking pattern in geometry II reveals an increase of the platinum signal in all directions except for the [lli] blocking direction. This points to an oxygen-induced Pt enrichment of the second layer by about 0.4 monolayer. The shift in the positions of the blocking minima evidences an outward relaxation of the first layers with respect to the bulk interlayer distance. The conclusions from the energy spectra and the blocking patterns can be made more precise with computer simulations. For this purpose, the measured blocking patterns were simulated with a Monte Carlo algorithm [34], modified to include scattering from two types of randomly distributed atoms in the crystal. The simulated patterns were compared with the data in a x2 analysis to find the best-fit values for the Pt concentrations in the first two layers below the disordered Ni (Cp’ and C,“), the values of the first and second interlayer distances (D,, and Dz3), and the thermal vibration amplitudes of first and second layer (U, and U,). It is assumed that the Pt and Ni concentrations in a given layer do add up to 100%. From the blocking patterns the concentrations, relaxations and thermal vibrations were determined. The amount of disordered nickel present in the top layer was deduced from the uniform height of the Ni blocking patterns. Apart from this disordered nickel, the simulations show that there is about 35% nickel present on lattice positions in the first crystalline layer. The best-fit parameters are listed in column 2 of table 1, together with their estimated errors. With the values given in this table the dashed curves of figs. 4 and 5 are obtained. The solid curves represent clean-surface blocking patterns simulated using the best-fit parameters listed in column 4 of table 1. Table 1 allows for a detailed comparison between the clean and oxygen-covered sample. The main effect of 0, exposure is the formation of a

segregation at the Pt,,Ni,,(II

I) surface

Table 1 The results of Monte Carlo simulations for the oxygen-covered and clean [26] Pt,,sNi,,,(lll) surface Oxygen covered surface

Clean surface

Parameter

Result

Parameter

Result

Disordered 0 Disordered

(2.3 f 0.3) x 1Or5 atoms/cm’ (1.6 k 0.3) X 10” atoms/cm2 75 27 58 -2.o* -2.o+ 148 121 108

CT Cp

65 63

&5(g) zt7

C,p’ c,p P,

+ 2.4 + 2.3 155 127

f 0.5 + 0.5 *15 k15

AD,, ADa3

c3

ADI, A43 ul/“b u,/ub

ul/“b

u,j"b Yl”b

* 3(S) + 5 + 8 0.5 0.5 +lO *lo *lO

Listed are the best-fit values for the Pt and Ni concentrations of the first and second crystalline layers, C,” and CNi, the enhancement of the thermal vibration amplitudes of the first two crystalline layers, q, with respect to the bulk value, U,, of 0.066 A, and the changes in the first two interlayer distances, D,, and Dz3 relative to the bulk interlayer spacing of 2.164 A. Note that one set of values yields a best-fit to all experimental blocking patterns simulfaneourly, as is shown by the solid and dashed curves in figs. 4 and 5.

NiO layer at the surface of the crystal and a change in the Pt and Ni concentrations. The Pt concentration in the first layer of the clean surface decreases by 10% and the Ni-rich second layer changes into a Pt-rich layer. Furthermore, the contractions of the first two interlayer distances change into expansions. This expansion can be explained from the lattice mismatch between platinum and nickel. Therefore, the Ni-enriched second layer of the clean surface has a smaller average thickness than the corresponding Pt-enriched second layer of the oxygen-exposed sample. However, this relaxation reversal is also observed on many other metals [31,35]. Finally, the thermal vibrations are larger than the corresponding vibrations of the clean surface. This however seems unlikely, given the fact that the layers are covered with an overlayer. It is more probably that this enhancement reflects the presence of random static displacements of atoms of the first layers induced by the disordered top layer.

S. Deckers et al. / Oxidation-induced segregation at the Pt,, Ni,,(l I I) surface

5. Discussion On Ni(lll), the dissociative chemisorption of oxygen at room temperature is followed by nucleation and lateral growth of NiO islands at the surface [36], while on platinum surfaces no oxidation takes place. The clean Pt,,Ni,,,(lll) surface is enriched in platinum and no oxidation of these platinum regions is expected. However, upon 0, exposure Ni atoms move from deeper layers towards the surface and interact with 0. The mechanism of this NiO overlayer formation is elucidated by a comparison with a similar oxidation study on Pt-covered Ni(ll1) performed by ellipsometry. In these experiments, a Ni(ll1) surface covered with 4.7 MLE of Pt was exposed to 0, at 400°C up to 800 L. It is known from earlier experiments that a surface alloy is then formed, see refs. [21,29]. After the 0, exposure, a NiO overlayer has formed on top of the Pt-Ni alloy layer, with a small fraction of oxygen penetrating through the Pt layer into the Ni bulk. This oxygen distribution is in sharp contrast to the profile obtained after oxidation at the same temperature on the clean Ni(ll1) surface, where most oxygen immediately starts to dissolve into the bulk. The initial oxidation behaviour of the bulk alloy surface and the oxidation of the Pt-Ni film on Ni(lll), as presented in fig. 2, show a remarkable similarity. From previous work [21] there are indications that the concentration profile of the annealed Ni(lll)/Pt system resembles the profile of the clean Pt,,Ni,,(lll) surface. The resemblance between the initial oxidations supports these observations. At higher 0, exposures the oxidation of the bulk alloy tends to saturate, while the oxidation of the thin film continues. This can be explained by the sample supply of Ni atoms in the latter case and by oxygen diffusion into the bulk of the Ni(lll) crystal. The 0, exposure in the MEIS-SB study amounts to roughly 50% of the saturation coverage of the bulk alloy surface, fig. 2. For this oxygen coverage, the MEIS-SB data clearly demonstrate that Ni atoms have moved towards the surface, probably due to the strong interaction with the adsorbed oxygen atoms. According to classic theo-

121

ries of oxidation [37,38], the Ni atoms migrated to the surface will be oxidized to NiO, since nickel can only form one stable oxide [39,40]. In the introduction several experiments on nickel-based alloys have been mentioned that indicate the formation of NiO upon 0, exposure [11,12,14]. The amount of disordered Ni is lower than the amount of oxygen present, see table 1. If all oxygen atoms, 2.3 x 10” atoms/cm*, were only bonded to the disordered Ni atoms present at the surface, 1.6 X 10” atoms/cm*, the stoichiometry Though NiO is known to exwould be NiO,,,. hibit a slight excess of oxygen, an excess of more than 40% seems to be unreasonably high. However, 0.6 X 1015 atoms/cm* Ni atoms are present in the first crystalline layer, so the total number of Ni atoms in the disordered and the first crystalline layer amounts to (2.1 f 0.5) X 1015 atoms/cm*, which is almost equal to the number of oxygen atoms present. Besides, from oxidation experiments on the pure Pt(ll1) surface it is known that platinum can dissociatively chemisorb one monolayer of oxygen [41,42]. The oxygen surplus may bond to the Ni atoms in the first layer of the lattice and may form a chemisorption bond with the Pt atoms. The present study puts the following mechanism of surface oxidation into evidence. Initially, oxygen is chemisorbed on the surface. This oxygen can react with Ni atoms in the first crystalline layer to form NiO. Subsequently, the concentration of NiO on the surface increases by Ni atoms supplied from deeper layers of the crystal. These atoms partly cover the platinum part of the surface. This mechanism is supported by our observation of a broadened Pt peak. This indicates a non-uniform overlayer. The supply of nickel from deeper layers is reflected in the observation that the second layer, which for the clean surface was enriched in nickel, is now enriched in platinum. The ellipsometry data point to a saturation at higher coverages, and this suggests that the inhomogeneous NiO overlayer possibly will continue to grow until a closed uniform overlayer is reached. This can also be concluded from the absence of a Pt AES signal at 237 eV at saturation coverage. Using an attenuation length for this Pt Auger peak of 0.5 nm (from ref. [21]), we find that this

128

S. Deckers et al. / Oxiabtion-induced

disappearance of the Auger signal corresponds to a closed NiO overlayer having a thickness of at least 1.4 nm. The observed number of oxygen atoms in the saturated oxide layer, 4 X 1015 atoms/cm’, corresponds to an oxide thickness of 1.46 nm (using a NiO lattice constant of 0.417 nm [43]). This figure is in good agreement with the minimal thickness as deduced from the Auger results.

segregation at the Pt,,Nio,,(llI)

surface

Carlo computer code for use for alloy systems and J. Kuiper for polishing the Pt-Ni sample. Dr. Y. Gauthier, Dr. R. Baudoing and Prof. Dr. J.C. Bertolini are acknowledged for helpful discussions. This work is part of the research program of the “Stichting Scheikundig Onderzoek Nederland”. It is made possible by financial support from the “Nederlandse Organisatie voor Wetenschappelij k Onderzoek” (The Netherlands Organisation for Scientific Research).

6. Conclusions A combination of ellipsometric measurements of the kinetics of oxygen uptake on a Pt,,Ni,, (111) surface and a detailed MEIS-SB study of the structure of this surface after 350 L 0, exposure gives valuable insight into the oxidation process. It is found that after exposure to 0, a disordered top layer has formed containing only Ni and 0 atoms. Underneath this disordered top layer an ordered, crystalline layer of metal atoms is present containing (65 k 5)% of Pt. The second crystalline layer contains (63 f 8)% of Pt. The sum of the Ni atoms present in the disordered top layer and in the first crystalline layer is (2.1 f 0.3) x 10” atoms/cm’. The number of 0 atoms in this top layer amounts (2.3 Ifr0.3) X 1015 atoms/cm2. This number would indicate a slight excess of oxygen, assuming that stoichiometric NiO is formed. It is argued that this excess of oxygen might be bonded to Ni atoms present in the first crystalline layer or chemisorbed on platinum regions of the surface. The observed expansion of the interlayer distances compared to the clean surface is likely related to the composition difference in the first two crystalline layers when the difference in lattice constants between platinum and nickel is taken into account. The oxidation of this alloy surface shows a remarkable similarity to the oxidation behaviour of Ni(ll1) covered with 4.7 monolayers of Pt. This can be explained by the formation of a surface alloy region, as has been observed in annealing experiments of Pt layers on Ni(ll1). Acknowledgements The authors like to thank Dr. J.W.M. Frenken for assistance with the adaption of the Monte

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