X-ray photoelectron diffraction study of CO- and NO-saturated Ni(111)

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Surface Science 282 (1993) 62-66 North-Holland

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X-ray photoelectron diffraction study of COand NO-saturated Ni( 111) L.S. Caputi, R.G. Agostino, A. Amoddeo,

E. Colavita

INFM and Dipartimento di Fisica, Universitci della Calabria, 87036 Arcavacata di Rende, Cosema, Italy

and A. Santaniello Sincrotrom

Trieste, Padriciano 99, 34012 Trieste, Italy

Received 24 June 1992; accepted for publication 12 October 1992

We studied the molecular axis orientation of CO and NO adsorbed on Ni(lll), at saturation and at room temperature, by X-ray photoelectron diffraction of the C, N and 0 1s core level peaks. Although previous studies are indicative of strong interactions between adsorbed molecules, we do not observe any induced tilt of the molecular axis with respect to the surface normal.

The interaction between molecules adsorbed on metal surfaces is a topic of great interest because of its relevance in heterogeneous catalysis [l]. The investigations of molecular catalytic reactions are generally subsequent to specific studies of each partner adsorbed on the same metal surface, in order to learn about each adsorption mechanism versus substrate temperature and coverage. The adsorption of CO on Ni(ll1) is an example of multiple CO coordination possibilities to surface Ni atoms, in which a continuous conversion from three-fold to two-fold CO bridge bonding has been observed for increasing coverage [2-51. Moreover, infrared reflection-absorption spectroscopy (IRAS) [6] and high resolution electron energy loss spectroscopy (HREELS) [2l measurements showed that the interaction between CO molecules at high coverages is strongly repulsive. A strong repulsive interaction was in fact suggested to explain the results obtained for CO on Ni(ll0) by the X-ray photoeletron diffrac0039-6028/93/$06.00

tion (XPD) technique. Because of such interaction, the adsorbed molecules are forced to a tilt of 21” in the [lOO] direction to lower the total energy of the system [71. The interaction of NO with Ni(ll1) has been studied with several techniques and some controversy exists in the literature about the orientation of adsorbed NO molecules. Breitschafter et al. [8] by XPS measurements found that the adsorption in the 300-400 K temperature range is partially dissociative. They also observed perpendicularly oriented molecules up to 330 K, while Netzer and Madey [9] found perpendicular NO molecules at 85 K, which change in a bent configuration at 250 K. Steinruck et al. [lo] demonstrated by synchrotron radiation photoemission that in a c(4 x 2) layer at 120 K NO molecules are perpendicular to the surface. Metastable quenching spectroscopy measurements [ll] show no changes for a c(4 X 2) NO layer between 90 and 300 K. Very recently, Asensio et al. [12] by NEXAFS measurements on both N and 0 K edges have

0 1993 - Elsevier Science Publishers B.V. Ah rights reserved

63

L.S. Caputi et al. / XPS study of CO- and NO-saturated Ni(lll)

W

NitIll) Y t

a)

X-RAY

ENERGY ANALYZER

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Fig. 1. (a) Sketch of the experimental apparatus; (b) Ni(ll1) surface with [lOi] and [llz] crystallographic directions.

shown that NO molecules adsorbed in the c(4 x 2) phase on Ni(ll1) are within 10” of the surface normal. The goal of the present work was to determine the orientation of CO and NO molecules on Ni(ll1) at saturation and at room temperature

Polar scan d-I..______

[13]. Although a wealth of results exists for CO and NO adsorbed on Ni(lll), to our knowledge the XPD technique was never used to study the CO-Ni(ll1) nor the NO-Ni(ll1) systems. This technique is a powerful tool to assess the orientation of diatomic molecules on surfaces by a simple direct inspection of the raw data. We performed the XPD measurements moving the analyzer and keeping fixed the X-ray incidence angle as well as the sample. Our results show that the CO-CO interaction does not cause any tilt of the molecular axis with respect to the surface normal, even at high coverages. For NO, we found molecular adsorption still at 300 K, although we cannot rule out the possibility of a partial dissociation. Also in this case, the molecular orientation is perpendicular to the surface. The measurements were performed at the Surface Physics Laboratory of the University of Calabria, in an ultra-high vacuum system in the lo-*’ Torr range. The Ni(ll1) sample was cleaned by the standard ion etching and annealing procedure, and was oriented by LEED. The surface cleanliness was checked by X-ray photoelectron spectroscopy (XPS) at 1486.4 eV photon energy (AlKa source) using a hemispherical analyzer (LHS EA lO/lOO). XPD measurements were performed by a 50 mm hemispherical analyzer with 3” angular resolution (LHS GEA) mounted on a goniometer, which allowed us to span over a wide angular range of photoemitted electrons. Fig. 1

Clean Ni(ll1) Y = 0” t

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Polar angle (“) Fig. 2. Ni 2p photoelectron

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80

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30

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diffraction results obtained by scanning over the polar angle (a) and the azimuthal angle (b).

50

64

L.S. Caputi et al. / XPS study of CO- and NO-saturated Ni(lll)

shows the Ni(ll1) surface with the crystallographic directions [lOi] and [1121, along which the polar scans were performed (a), and a schematics of the experimental apparatus (b). XPD measurements were performed by counting simultaneously at three fixed energies for each photoelectron peak, i.e. at the peak maximum and at two further energies, one on either side of the peak. The latter were used to subtract the background signal by a simple linear extrapolation [7]. The Ni 2p intensity was normalized to the background. Fig. 2a shows an XPD polar scan from the clean Ni(ll1) surface. The polar angle was measured with respect to the surface normal, and the scans were recorded along the [ll?] direction which was taken to correspond to the azimuthal angle 9 = 0”. Analogous results were obtained along equivalent directions determined by * = 60”. The peak at 35” in fig. 2a is due to first neighbour alignment. Fig. 2b shows an azimuthal scan at a polar angle of 55”, in which the peak centered at 60“ is due to second neighbour alignment. These data were used as a further check of the sample alignment. High purity CO and NO gases were admitted into the system at room temperature in the 10e8 Torr range. Saturation of the surface was checked by the XPS technique, following the C, N and 0 1s peak intensities versus exposure. The LEED pattern of the NO-Ni(ll1) system confirmed the saturation phase at room temperature, because a c(4 X 2) pattern, showing the three equivalent domains, was clearly visible and stable. XPD data relative to CO and NO molecules were taken by scanning over the polar angle 8 along two non-equivalent directions, the [lOi] and [ll?] directions, which correspond to the azimuthal angles V = -30 and q = O”, respectively. The N 1s and C 1s peak intensities were normalized to the 0 1s peak intensity. Fig. 3 shows the XPD measurements for the CO-saturated Ni(ll1) surface. The C 1s photoelectron intensity is shown versus the polar angle 13at 2” steps from - 20” to 40” with respect to the surface normal, for the [lOi] azimuth. The measurements in the [1121 azimuth give the same results, within the experimental uncertainty. The fact that the C 1s photoelectron intensity is clearly

I

4 -30

-20

-10

7

0

I0

20

30

40

so

Polar angle (“)

Fig. 3. Cls intensity obtained in the X-ray photoelectron diffraction experiment on a Ni(ll1) surface saturated with CO molecules, for polar angles in the range -20” to 40” with respect to the surface normal. The emission azimuth was in the [lOi] direction of the Ni crystal.

enhanced in the direction perpendicular to the surface, indicates that the CO molecules are adsorbed perpendicularly on Ni(ll1). We estimate the FWHM of the peak to be 23” f 2”. This value can be compared with the results of a single scattering calculation for oriented CO molecules parallel to each other [14]. Our result is in good agreement with this calculation when it is performed for CO molecules within a root-mean square angle displacement of lo”. A similar FWHM was obtained by other authors for CO and Ni(ll0) at low coverages [7]. At saturation, however, they observed a clear tilt of about 21” in the [OOl] and [OOi] directions, due to strong interactions between adsorbed molecules. CO molecules are also expected to interact strongly at high coverages on Ni(ll1). In a recent IRAS investigation [6], for example, the CO stretching band was observed to shift from 1815 cm-i at the lowest coverages, up to 1915 cm-’ upon saturation because of an increased C-O bond strength at higher coverages, caused by a reduced backdonation into the 2~* antibonding CO orbitals. This is due to indirect chemical and electrostatic interactions between adsorbed molecules [15], and, at the highest coverages, to strong intermolecular repulsive forces, which arise because of the overlapping of wavefunctions of the adjacent CO molecules 1161.

L.S. Caputi et al. / XPS study of CO- and NO-saturated Ni(lll)

0.6

/ -10

I 0

10

20 30 40 Polar angle (“1

50

60

Fig. 4. XPD results obtained for a NO-saturated Ni(lll1 surface. The emission azimuth was in the [loll direction of the Ni crystal. The LEED pattern was a c(4 x 2).

Thus, although CO molecules on Ni(ll1) seem to interact strongly as on Ni(llO), our results show that such interaction does not cause a tilt in any particular direction. This effect can be explained as due to homogeneous CO coverage, in which the total repulsive force acting on each molecule, due to the nearest neighbour molecules, is nearly zero, as is expected for CO-adsorption sites which are symmetrically displaced on the Ni(ll1) surface. A tilt at random angles also seems to be unlikely, because it should give rise to a broadening of the detected diffraction peak, and thus to a higher value of the FWHM. The observation of a c(4 x 2) LEED pattern for the NO-saturated Ni(ll1) surface clearly demonstrates that molecular adsorption takes place at room temperature. The pattern was stable over several days, disappearing only by heating the surface to temperatures higher than 400 K. Fig. 4 shows our XPD result for the c(4 X 2) NO layer on Ni(ll1) at 300 K. A peak centered along the surface normal, with a FWHM of less than 20”, is present, similar to that of the CO case. Thus, NO molecules at saturation stand vertically on Ni(ll1) at room temperature. The observation of molecular adsorption of NO on Ni(ll1) at room temperature has to be compared with the suggestions of Erley and Persson 1171. They found an irreversible increase of the NO stretching linewidth when a c(4 x 2) NO

65

layer, obtained on Ni(ll1) by exposing at 88 K, is heated to above 270 K. The authors attribute this effect to the dissociation of NO. The reason why a c(4 x 2)NO layer obtained at 88 K dissociates above 270 K, while a layer of molecules with the same structure can be obtained at about 300 K is at the moment puzzling. As in the CO case, NO molecules seem to interact strongly on Ni(ll1) at high coverages. Such interaction causes a shift from 1568 to 1581 cm-’ in the infrared adsorption spectra, on increasing the NO exposure at 85 K from 1.6 to 2.4 L [18]. Nevertheless, according to the present results, such an interaction does not cause any tilt of the molecular axis from normal to the surface. In conclusion, we have shown that c(4 x 2) layers of CO and NO on Ni(ll1) are formed of molecules perpendicular to the surface, although in both cases there are strong intermolecular interactions. We are grateful to Professor R. Rosei for a critical reading of the manuscript. Particular thanks are due to V. Fabio and E. Li Preti for their invaluable technical support. This work was partially supported by CNR under contract of Progetto Finalizzato Materiali Speciali per Tecnologie Avanzate.

References [l] H.P. Bonzel, J. Vat. Sci. Technol. A 2 (1984) 866. [2] W. Erley, H. Wagner and H. Ibach, Surf. Sci. 80 (1979) 612. [3] M. Trenary, K.J. Uram and J.T. Yates, Jr., Surf. Sci. 157 (1985) 512. [4] B.N.J. Persson and R. Ryberg, Phys. Rev. Lett. 54 (1985) 2119. [5] S.L. Tang, M.B. Lee, O.Y. Yang, Y.D. Beckerle and S.T. Ceyer, J. Chem. Phys. 84 (1986) 1876. 161 L. Surnev, Z. Xu and J.T. Yates, Jr., Surf. Sci. 2010988) 1. [7] D.A. Wesner, F.P. Coenen and H.P. Bonzel, Phys. Rev. B 39 (1989) 10770. [8] M.J. Breitschafter, E. Umbach and D. Menzel, Surf. Sci. 109 (1981) 493. [9] F.P. Netzer and T.E. Madey, Surf. Sci. 110 (1981) 251. HOI H.-P. Steinriick, C. Schneider, P.A. Heimann, T. Pache, E. Umbach and D. Menzel, Surf. Sci. 208 (1989) 136.

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L.S. Cap&i et al. / XPS study of CO- and NO-saturated Ni(ll1)

[ll] F. Bozso, J. Arias, C.P. Hanrahan, J.T. Yates, Jr., R.M. Martin and H. Metiu, Surf. Sci. 141 (1984) 591. [12] M.C. Asensio, D.P. Woodruff, A.W. Robinson, K-M. Schindler, P. Gardner, D. Ricken, A.M. Bradshaw, J.C. Conesa and A.R. Gonzalez-Elipe, Chem. Phys. Lett. 192 (1992) 259. [13] A more extensive investigation in which we study also the interaction of CO and NO molecules on the same Ni surface is reported in: L.S. Caputi, R.G. Agostino, A.

Amoddeo, S. Molinaro, E. Colavita and A. Santaniello, to be published. [14] K.A. Thompson and C.S. Fadley, J. Electron Spectrosc. Relat. Phenom. 33 (1984) 29. [15] G. Blyholder, J. Phys. Chem. 68 (1964) 2772. 1161A.M. Bradshaw and F.M. Hoffmann, Surf. Sci. 72 (1978) 513. [171 W. Erley and B.N.J. Persson, Surf. Sci. 218 (1989) 494. [18] W. Erley, Surf. Sci. 205 (1988) L771.