Synchotron radiation photoemission study of adsorbate-induced 3d core level shifts of Pd(110)

Progress in Surface Science, Vol. 35, pp. 71-74 Pdnted in the U.S.A. All rights reserved.

0079-6816/90 $0.00 + .50 Copyright © 1991 Pergamon Press plc

SYNCHOTRON RADIATION PHOTOEMISSION STUDY OF ADSORBATE-INDUCED 3d CORE LEVEL SHIFTS OF Pd(110) G. COMELLI, M. SASTRY, G. PAOLUCCI, K.C. PRINCE and L. OLIVI* Sincrotrone Trieste, Padriciano 99, 34012 Trieste, Italy, and *Area per la Ricerca Scientifica e Tecnologica nella Provincia di Trieste, Padriciano 99, 34012 Trieste, Italy Abstract. The surface core level shifts of Pd(llO) and oxygen and CO covered Pd(110) have been measured using synchrotron radiation. The results are consistent with values predicted using a BornHaber cycle, confirming the usefulness of this approach. The largest shift occurs for the CO covered surface, indicating that initial state charge transfer does not dominate the shifts. L Introduction. The difference in core level binding energies of bulk and surface atoms, known as the Surface Core Level Shift (SCLS), is a sensitive probe of the bonding at clean and adsorbate-covered surfaces (1-9). To measure them, sharp core levels are necessary and for 4d metals, the 3d levels are suitable. They lie at 335.2 eV (3d5/2) and 340.7 eV (3d3/2) for Pd, and have theoretical widths of 0.3 eV and 0.4 eV respectively (10) but very few measuren~nts of these levels have been reported (2, 12). For best surface sensitivity, measurement with synchrotron radiation is dictated. The required photon energy lies in the trsditionally difficult soft x-ray region where only recently has it been possible to achieve high resolution and flux (11). . . . . . . . h " 2. Experimental. The measurements were pertormm at me ueruner t~lemronenspmc errmgGesellschaft for Synchrotronstrahlung mbH (BESSY) on the High Energy Toroidal Grating Monochromator 2 beam line. The measurement chamber was equipped with the usual facilities for surface preparation, such as sputtering, LEED and Auger spectroscopy. Photoemission spectra were measured with a VSW Ltd HA100 analyser in normal emission, and at ~ angle of emission, using a photon energy of 415 eV. The sample was cleaned by cycles of sputtenng, anneanng ano oxygen treatment. The oxygen covered surface was prepared according to the method of Jo et al (13), namely exposure to 20 L of 02, followed by heating to 150 oc. This treatment produced a sharp c(2 x 4) ~ pattern. Adsorption of I L of CO at 110 K gave a sharp p_2n~.g.LEED.s~. cmre (14)" __ . To extract quantitative information, it was necessary to m ~ oat& m me presem case, omy me spectra fixxn the CO covered surface show clearly resolv.ed peaks. However careful .c~..anson.of.the clean and oxygen covered surface shows that on adsorpuon the envelope of the peak shifts to mgner binding energy, without appreciable change in the width, but wl;~ a small peak shape change. Thi's indicates the presence of..un.r~.~,lved.peaks due to.surt.ace ~ ouu~ corn.Ponents, ano ..mrm.er,mat m~ clean and oxygen induced shifts are m opposxte mrecuons, tt mey were m me same mrecuon, out o different magnitude, the peak width would change. We examined ~ e CO data m obtain the .intensity ratio and peak form (sum of a Gaussian and Doniach-Sunjic tinesnape, wnicn approxnnates a convolution) and intensity ratios and then fitted the clean surface and oxygen covered surfaces_The fLrStfit was done allowing only the peak energies and linear, backgro.u.n.,d ~ variable p ~ . ~ . llnen these parameters were fixed and the widths and intensity rauo were attowexl to vary to obtain a best ttt. A criterion for a good fit was that the hulk/surface intensity ratio did not vary too much from the clean to the CO-covered surface. An Ad41tional complication was considered in the case of the oxygen-covered surface. Here there are two types of surface atoms, those adjacent to oxygen and those without oxygen atom nearest neighbours. Their concentration ratio is 1:1. The peaks are not sufficiently resolved to be able to say whether they have different shifts, hut we hypothesise that they are. The data were therefore fitted with 71

72

G. COMELLI et al.

two surface peaks whose concentration is each half of that on the CO-covered surface. We emphasize that this is a reasonable hypothesis rather than a direct conclusion from the data. 3. Results. Photoemission spectra from the Pd 3d5/2 level for the clean, oxygen-covered and CO-covered surfaces are shown in figs. 1 and 2. Fig. 1 shows the shift of the peak on oxygen adsorption, while fig. 2 shows that, as expected, increasing the angle of emission changes the relative bulk and surface sensitivities, so that the surface peak grows relative to the bulk peak. While the COinduced shift is sufficient to produce two clearly resolved peaks, the shifts of the clean and oxygencovered surfaces are not resolved. However as fig. 1 shows, their existence is unambiguous because on adsorption, intensity is removed from one side of the peak and appears on the other. The results of fits are shown in figs. 2 and 3, and the numerical results are summarised in Table 1. I

I

I

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//

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Fig. 1 Normal emission speca'a of the clean

I

cD

Pd(110) surface (full line) and the oxygencovered surface (dashed line). Photon energy = 415 eV. The zero of energy is the binding energy of the bulk peak.

¢o 0

3

I

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2

1

0

-I

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-2

-3

Binding energy Es(eV) (1) Es(eV) (2) Ib/I s (0 o) Ib/Is(60°)

Clean surface

c(2x4)-O/Pd(lI0)

-0.24

-0.28 0.58 0.48

1.19 0.94

0.48 0.58 0.48

p2mg-CO/Pd(l I0) 0.98 1.15 0.98

Table 1 4. Discussion. It has been argued elsewhere (5) that initial state effects, such as charge transfer, do not adequately explain SCLSs. This is particularly true for materials with a nearly full d shell such as Pd. The alternative approach is to use the Born-Haber cycle (8, 16, 17) which relates the SCLS to heats of adsorption. Under appropriate assumptions (14, 8), the adsorbate induced SCLS referred to the clean surface is equal to the difference in the heat of adsorption of the adsorbate on Pd and its (Z+I) neighbour Ag, multiplied by the adsorbate-subswate stoichiometric ratio: SCLS = Eb(clean)-Eb(adsorbate-covered) = o[AH(Ag)-AH(Pd)] where o = ratio of number of surface atoms to adsorbates and AH = heat of adsorption. Using data from the literature (16-19), we can predict the SCLS for the present case. The factor o is 0.5 for oxygen and 1 for CO. Data for a comparison of predicted and measured values are given in table 2.

ADSORBATE-INDUCED

SHIFTS

-

Pd(110)

i

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a

73 I

E 3 @ Fig. 2 (a) Normal emission spectrum of the I

CO-covered surface (fullline),and fit

4

(dashed line).The residualis shown below

3

Photon energy = 415 cV.

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2 1 0 Binding energy

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[

-1

-2

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(b) As (a), but at 60 ° emission angle.

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3

2 1 0 Binding energy

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[

-1

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-2

m

c

Fig. 3 Normal emission spectrum of the 3 clean s~faee (full line), and fit (dashed line), -~£ and residual.

<~ G~ -4..

E 3 0

3

2

,

1 0 -1 Binding energy

-2

I

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-3

74

G. COMELLI et al.

Adsorbate Pd(110) AFI Ag(110) calc. SCLS meas. SCLS

O 2.90 eV/atom (ref. 22) 1.71 eV/atom (ref. 23) 0.59 eV 0.48 eV

CO 1.27 eV/atom (ref. 22) 0.14 eV/atom (ref. 24,21) 1.13 eV 0.98 eV

Table 2

The agreement between the measured SCLS and the value expected from the Born-Haber cycle is good. 5. Summary. The good agreement between the SCLS predicted by the Born-Haber cycle and the measured values shows that this is a reliable formalism for discussing and predicting SCLS. Oxygen is an adsorbate which is certainly more electronegative than CO and may be expected to cause a larger charge transfer, but the shift is smaller. The analysis shows that this has two origins. Firstly, the stoichiometry is different so that the effect of one oxygen atom is "diluted" by being shared by two Pd atoms. Secondly, the heat of adsorption on the Z+I atom is related to the total energy in the final state, and there are differences between the final states of oxygen and CO. The oxygen atom is still fairly strongly bound to a core ionised Pd atom, whereas the CO molecule is not bound. These final state effects, combined with the stoichiometric effect, explain the differences between CO and O.

Acknowledgements. We thank Prof. K. Baberschke for the use of his experimental chamber. M.S. acknowledges financial support of the International Centre for Theoretical Physics. This work was supported by the European Community under the Large Scale Installations program.

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