XPS and UPS study of oxygen adsorption on Re(0001) at low temperatures

Surface Science 134 (1983) LSOS-L512 North-Holland Publishing Company

SURFACE

SCIENCE

LEITERS

XPS AND UPS STUDY LOW TEMPERATURES S. TATARENKO, J.J. EHRHARDT

L505

OF OXYGEN

ADSORPTION

P. DOLLE, R. MORANCHO, and R. DUCROS

ON Re(OOO1)AT

M. ALNOT,

Laboratoire Maurice Retort, CNRS, Laboratoire assock! ri I’UniuersitP de Nancy I, BP 104, F-54600 Villers - les Nancy, France Received

15 March

1983; accepted

for publication

Oxygen adsorption was studied in the found: (i) an atomic state existing in (physisorbed oxygen molecule) between between the two oxygen atoms is around

1 July 1983

temperature range 50-300 K. Three adsorbed states were the whole temperature range, (ii) a molecular state 50 and 80 K, (iii) a peroxide state whose bond order one and which exists between 80 and 300 K.

Oxygen adsorption on Re(OOO1) has already been studied near room temperature and at high temperature [l-4]. It has been found that oxygen is dissociated into atoms as on transition metals like tungsten, nickel and platinum. However on the latter material at room temperature Norton [5] and Campbell et al. [6] found atomic and molecular oxygen which was identified by Gland et al. [7] and Steininger et al. [8] as a peroxide species in which the two neighbouring oxygen atoms of this peroxide are supposed to be singly bonded with the O-O bond axis parallel to the surface. A similar state would exist on Ag(ll0) [9,10] too, where the O-O bond order would be lower than one, and on Cu(lOO), Cu(ll0) and Cu(ll1) [11,12]. Another molecular state was observed in the low temperature range 20-30 K by Opila and Gomer [13,14] on W(llO), by Hofmann et al. [15] on Al(lll), by Ya-po Hsu et al. [16] and by Benndorf et al. [17] on Ni(ll0). On Al(111) and Ni(ll0) it has been shown that the molecular orbital levels of this oxygen are very close to the molecular orbital levels of the oxygen in the gas phase leading to the conclusion that this state is an oxygen molecule weakly bound to the surface. Finally three different states have been found depending on the metal and on the temperature range: an atomic and two molecular states, a “peroxide” species and a physisorbed molecule. We have studied the oxygen adsorption on Re(OOO1) between 50 and 300 K by photoelectron spectroscopy (UPS and XPS) to obtain information on the oxygen adsorbed states and their temperature dependence. 0039-6028/83/0000-0000/$03.00

0 1983 North-Holland

S. Tutarenko et al. / Oxygen adsorptron on Re(0001)

L506

The experimental apparatus has been described previously in detail [18]. Briefly, a rhenium single crystal is fixed on a sample manipulator. It can be placed in front of the electron analyzer (Auger and photoelectron spectroscopy). The sample can be heated by electron bombardment or cooled by liquid helium circulation. The heat transfer between the sample and the copper reservoir being ensured by a 12 cm long copper tress. The temperature is measured by a W-6%Re/W-26%Re thermocouple spot welded to the rear face of the crystal. The sensitivity of this thermocouple is very poor at low temperature but good in the high temperature range used for the cleaning procedure of the surface (heating during several hours in 1O-6 Torr of oxygen at 1500°C followed by annealing under vacuum at 1800°C to remove oxygen). We have studied xenon adsorption on the crystal to obtain more accuracy on the lower temperature reached by the sample. To carry out this measurement, we tried to correlate the pressure of the appearance of the second xenon monolayer with the substrate temperature. This pressure is usually very close to P,/2 where P, is the saturated pressure at the temperature T of the substrate and is given by the classical Clausius-Clapeyron relation In P,, = -1893/T

+ 23.10.

From these measurements, the corresponding surface temperature is easily deduced and is found to be close to 50 K. There are some inaccuracies for the pressure measurements but a variation of a factor of two or three in this value gives a temperature variation of only a few K. The UPS (He I, He II) spectra of oxygen adsorbed at 50 K on the crystal is 4 N(E)

(ctds)

9.13 18.54

4.65

12.79

105.

15

10

Fig. 1. UPS (He I and He II) spectra

5 of adsorbed

Ed

oxygen on Re(0001)

at T = 50 K.

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S. Tatarenko et al. / Oxygen adsorption on Re(OOO1)

given in fig. 1. Four peaks below the Fermi level are visible at 4.65, 9.13, 10.54 and 12.79 eV. By adding the work function of the surface, we obtain the energy level of these states relative to the vacuum level. The work function for the surface is the result of two components: the work function of the clean rhenium surface, + = 5.4 eV, and the work function change due to the atomic oxygen layer, A+ = 0.8 eV. The total work function is $J = 6.2 eV. The corrected values of the UV energy levels are given in table 1 for comparison with the corresponding molecular orbital levels for gaseous oxygen. The energy shift between gaseous and adsorbed 0, is 1.28, 1.27, 1.76 and 1.61 eV for rs 2p, rr,, 2p, us 2p and ug 2p respectively, taking the mean shift from the three references. A very similar value found for the shifts allows us to presume that the adsorbed species has an electronic structure very close to the structure of gaseous 0,. This layer must be composed of oxygen molecules very weakly bound (physisorbed) to the surface. However in fig. 1 another peak at 6.25 eV below the Fermi level can be seen on the He II spectra. This peak can be identified as the 2p level of the oxygen atom, as it has already been shown for adsorption of 0, at room temperature on the same surface [2]. This result shows that even at T = 50 K dissociative adsorption takes place on the surface, probably before the build up of the molecular layer. However as it is impossible to perform experiments with different coverages, it is difficult to be sure that the atomic layer builds up and saturates before molecular adsorption takes place. In order to study the temperature range where this molecular layer exists, the crystal has been annealed under vacuum at several temperatures after saturation at 50 K. The corresponding UPS (He I) spectra are given in fig. 2. At T = 100 K the four mean peaks of the oxygen molecular orbitals have completely disappeared. A small peak at 10.6 eV, on which we will comment later, still remains, and a broad peak at 6.25 eV develops with increasing temperature. At the same time the electronic state density of the metal near the

Table 1 Binding energy of the molecular orbitals for gaseous oxygen and for oxygen adsorbed at 50 K on Re(OO01) Molecular orbital

5 2P T” 2P 0s 2P 0s 2P 0” 2s 0” 2s

EbV (W

EbY WI

(physisorbed 0,)

(gaseous 0,) Ref. (191

Ref. [ZO]

Ref. [21]

This work

13.1 17 18.8 21.1 25.3 27.9

12.33 16.70 18.17 20.43 24.58 27.4

12.07 16.12 18.7 20.29

4.65 + 6.2 9.13+6.2 10.54+ 6.2 12.19+6.2

= 10.85 =15.33 = 16.77 =18.99

S. Tatarenko

L5OX

et al. / Oxygen

adsorption on Re(OOO1)

N(E)

(ctds) t

105

Fig. 2. UPS spectra (He I) of oxygen adsorbed (...~.~)T=5OK;(---)T=100K;(~-~-~)T=300K;(-

at T = 50 K and annealed

at several temperatures: ) clean surface.

Fermi level increases due to the decreasing coverage. It is quite clear that heating the crystal up to 100 K removes the molecular layer and increases the atomic oxygen coverage. In fig. 3 the XPS spectra of the 0 Is level after oxygen adsorption at several temperatures are given. On all curves a peak at 530.2 eV (A) is present without any shift with adsorption temperature. This value is characteristic of atomic oxygen which is present on the surface whatever the adsorption temperature. At T = 50 K the peak at 535.6 eV (B) must be the fingerprint of the molecular species already shown by UPS (i.e. fig. 1). At 65 K this peak (curve 2) has been strongly attenuated and another peak at 533.2 eV (C) develops. It is still present at 100 K but has been completely removed at 300 K. Annealing at several temperatures a layer saturated at T = 100 K or T = 50 K yields interesting information about these states and their temperature dependence. The results are given in figs. 4 and 5. For each temperature the spectrum has been decomposed into two or three peaks. For states (A) and (B), which are well separated for adsorption at 300 and 50 K, the peaks have been assumed to have a Gaussian shape, For the state (C) which is never found alone between 50 and 300 K, we assume the shape to be Gaussian and the full width at half maximum (FWHM) to be linearly dependent on the maximum position between 530.2 and 535.6 eV.

S. Tatarenko et al. / Oxygen adsorption on Re(OOO1) N(E)
L509

0 $

t

546

535

530

525

Fig. 3. XPS spectra of 0 Is level for oxygen T=50K;(2)T=65K;(3)T=lOOK;(4)T=300K. t

NCE)

0

adsorbed

on the surface

at several temperatures:

(1)

0

Fig. 4. XPS spectra of 0 Is level for oxygen adsorbed on the surface at T = 50 K and after annealing at several temperatures: (1) T = 50 K; (2) T = 63 K; (3) T = 73 K.

Fig, 5. XPS spectra of 0 Is Ieve{ for oqrgen adsorbed at T=100 K and anneafinlr, at several temperatures: (I) T=lOO K; (2) T=lSO K; (3) T- 200 K; (4) T= 248 K.

The following parameters have been determined and are given in table 2. Annealing the saturated layer from 50 to 63 K reduces the molecular state (9) and develops the state (C). The spectrum does not change very much for an annealing temperature of 73 K. The total area under these XPS peaks remains roughly constant in this temperature range. no important desorption in the gaseous phase or dissohxtion into the buIk takes place and the main process under these conditions is the transfer from one state to another, At 100 K the (B) state has been completely removed and only states (A) and Table 2 Binding energy and full width at half maximum (FWHM) for the three adsorbed states of oxygen

11111

state

Binding energy (eV)

FWHM (eV) .---Y ---~1--~

?A)

530.2kO.3

2.4

(B)

535.6kO.3

3.2

533210.3 533.6 i 0.3

2.8 2.8

G) T==50K T*l(fUK

-p..

S. Tatarenko Ed al. / Oxygen adsorption on Re(0001)

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I

(C) are present. As the temperature increases, the population of state (C) decreases to the benefit of state (A) (oxygen atom). From 100 to 250 K the area under the XPS peak does not change very much. This result leads also to the conclusion that, in this temperature range, the annealing of the adsorbed layer mainly brings about a transfer from one state to another. However, molecular oxygen desorption has been detected under these conditions, but the amount was very low and was not determined quantitatively. The problem remains to identify the (C) state whose XPS spectrum shows a peak maximum at 533.2 eV; Norton [5] found the same peak on the XPS spectrum during oxygen adsorption on Pt(ll1). He assumed that the corresponding state is molecular. On the same substrate Steininger et al. [8] found by EELS that this oxygen species would be a peroxide bonded to one or two metal atoms and the bond order between the two oxygen atoms would be roughly one. More experiments with EELS would be very useful to describe this state more precisely, but it seems reasonable to assume the existence of a peroxide species which gives the (C) peak on the XPS spectra. This is also consistent with the UPS results of fig. 2 where at 100 K the small peak at 10.6 eV below the Fermi level could be identified as a ua 2p orbital remaining on the surface, the 7ru 2p and ~a 2p orbitals being completely removed, show&g the presence of single u bond between two oxygen atoms. To conclude we have observed three adsorbed states of oxygen on Re(OOO1): an atomic state at any temperature, a molecular state very weakly bound to the surface - its molecular structure being very close to the molecular structure of gaseous oxygen (it is present on the surface for a temperature not higher then 70 K), and a third state which could reasonably be a peroxide as already identified on Pt(ll1) which exists between 60 and 250 K. When the temperature increases, the molecular state is transferred mainly to the peroxide state which is also transferred to the atomic state between 100 and 300 K. No important desorption into the gas phase and dissolution into the bulk takes place in this whole temperature range.

References [l] J. Fusy, B. Bigeard and A. Cassuto, Surface Sci. 46 (1974) 177. [2] R. Ducros, M. Alnot, J.J. Ehrhardt, M. Housley, G. Piquard and A. Cassuto, Surface Sci. 94 (1980) 154. [3] M. Alnot and J.J. Ehrhardt, J. Chim. Physique 79 (1982) 735. [4] R. Ducros, M. Housley and C. Piquard, Phys. Status Solidi 56 (1979) 187. [5] P.R. Norton, Surface Sci. 47 (1975) 98. [6] C.T. Campbell, G. Ertl, H. Kuipers and J. Segner, Surface Sci. 107 (1981) 220. [7] J.L. Gland, B.A. Sexton and G.B. Fisher, Surface Sci. 95 (1980) 587. [8] H. Steininger, S. Lehwald and H. Ibach, Surface Sci. 123 (1982) 1. [9] B.A. Sexton and R.J. Madix, Chem. Phys. Letters 76 (1980) 294.

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[lo] [ll] [12] [13] [14] [15] [16] [17] (181 [19]

S. Tatarenko

et al. / Oxygen adsorption on Re(0001)

C. Backx, C.P.M. de Groot and P. Biloen, Surface Sci. 104 (1981) 300. A. Spitzer and H. Ltith, Surface Sci. 118 (1982) 121. A. Spitzer and H. Liith, Surface Sci. 118 (1982) 136. R. Opila and R. Gomer, Surface Sci. 105 (1981) 41. H. Michel, R. Opila and R. Gomer, Surface Sci. 105 (1981) 48. P. Hofmann, K. Horn, A.M. Bradshaw and K. Jacobi, Surface Sci. 82 (1979) L610. Ya-po Hsu, K. Jacobi and H.H. Rotermund, Surface Sci. 117 (1982) 581. C. Benndorf, B. Egert, C. Nobl, H. Seidel and F. Thieme, Surface Sci. 92 (1980) 636. M. Alnot, B. Weber, J.J. Ehrhardt and A. Cassuto, Appl. Surface Sci. 2 (1979) 578. K. Siegbahn, C. Nordling, G. Johansson, J. Heddan, P.F. Heden, K. Hamrin, U. Gelius. T. Bergmark, L.O. Werme, R. Manne and Y. Bayer, ESCA Applied to Free Molecules (NorthHolland, Amsterdam, 1969) p. 69. [20] M.S. Banna and D.A. Shirley, J. Electron Spectrosc. Related Phenomena 8 (1976) 255. [21] D.W. Turner, C. Baker, A.D. Baker and C.R. Brundle, Molecular Photoelectron Spectroscopy (Wiley-Interscience, New York, 1970) p. 33.