SURFACE Ni OXIDE STUDIED BY OXYGEN K-XANES

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SURFACE Ni OXIDE STUDIED BY OXYGEN K-XANES

I. Davoli, M. Tomellini, M. Fanfoni

To cite this version:

I. Davoli, M. Tomellini, M. Fanfoni. SURFACE Ni OXIDE STUDIED BY OXYGEN K-XANES.

Journal de Physique Colloques, 1986, 47 (C8), pp.C8-517-C8-520. �10.1051/jphyscol:1986896�. �jpa- 00226227�

JOURNAL DE PHYSIQUE

Colloque C8, supplkment au n o 12, Tome 47, dkcembre 1986

SURFACE Ni OXIDE STUDIED BY OXYGEN K-XANES

I. D A V O L I * . * * , M. TOMELLINI*' and M. F A N F O N I * *

'Dipartimento di Matematica e Fisica, ~ n i v e r s i t A di Camerino, I-62032 Camerino, Italy

" " P U L S (Istituto Nazionale di Fisica Nucleare / CNR, Laboratori Nazionali di Frascati, I-00044 Frascati, Italy

Abstract.

We have studied the surface Ni oxide by XANES tecniques at the oxygen K-edge. The spectra were detected by partial yield spectroscopy, in the range of 529-570 eV at the "Grasshopper" beam line of the Frascati Syncrotron Radiation Facility. We find that the oxygen react with nickel producing an oxide where the local structure at room temperature is similar to the defective {Ni(l-glO}. Data are interpreted following the recent electronic-configuration interaction theory.

Introduction.

The oxygen K-XANES for stoichiometric NiO has been recently interpreted in the framework of the initial state multielectron configuration interaction(') and a subsequent work(*) made on defective Ni oxide has shown that even a very little deviation from stoichiometric composition (from 1 to 3%) determines a sensible modification of the XANES spectra at the oxygen K-edge in defective nickel oxides. Furthermore the N1-0, phase diagram(3) shows that the thermodinamic conditions for the defective samples are very critical because for a stable NiO phase high temperature (T>1000 K) and low oxygen partial pressure (1 0'6 mbar) are required.

In the present work we have studied the XANES at the oxygen K-edge after the interaction of the dry gas with a clean Ni surface. The

policristalline Ni was kept at room temperature and exposed to the oxygen partial pressure as high as 400 mbar for about 1 hour. In such a condition the mentioned Ni-0, binary phase diagram shows that our sample should not be a stable defective Ni oxide.

Ex~eriment.

The experiment was performed at the Frascati Synchrotron Radiation Facility using the soft X-ray beam line named " ~ r a s s h o p p e r " ( ~ ) . The grazing incidence monochromator is equipped with a 1200 Ilmm holographic grating and variable slits from 15 to 400 pm.We have worked with a 15 p m slits obtaining an energy resolution AE=1.8 eV at the edge of oxygen.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986896

C8-5 18 JOURNAL DE PHYSIQUE

The emitted photoelectrons were detected using a two-stage cylindrical mirror analyzer. The XANES were measured by the partial yield technique with the analyzer operating in a constant final-state mode and collecting electrons at the maximum of the secondary electron energy distribution ( 2 eV ).

The sample used was a slab of policrystalline Ni cleaned by ion sputtering, and the cleanliness of the surface were checked by Auger analysis. To prevent any speculation about the formation of Ni20, we have also checked the ammount of water in the sample chamber by a mass spectrometer . We have found that the peak of water do not change during the oxygen exposure. So that the ammount of water present in the vacuum chamber are only the one due to the residual traces of reserch purity oxygen cilinder.

Results and discussion.

The oxygen K-edge XANES in stoichiometric NiO has been explained in a previous work within the theory of the configuration interaction of the initial state (I). The ground state is given by the:

where TlU5 can be also indicate as L-I: the ligand hole.

a

The comparison between the experimental data and the calculated spectrum, within the one electron full multiple scattering theory, have shown that only part of the experimental features are reproduced (I), other peaks are interpreted using multielectron configuration interaction. In fig. 1 we report the experimental data for the stoichiometric NiO (top pannel) and the defective Ni(,-.,,)O (lower pannel). The first peak, labeled A, is due to a transition from the ground state (*) to the lowest energy-allowed final-state configuration:

I N ~ ( ~ ~ ~ ) O ( ~ S ' , T , ~ ~ ) > . This is the transition to the first unoccupied state which is dipole allowed because of the strong Ni(3d) and O(2p) hybridization.

The intensity of the peak A is a measure of the probability of l d 9 ~ - ' >

configuration in the ground state ( the excited photoelectron neutralizes the ligand hole). In a chemistry language this is a measure of the N i - 0 bond ionicity. In fact an increasing of the ionicity means a reduction in the configuration interaction and a subsequent intensity reduction of peak A. Such a statement is also comfirmed by the comparison in XANES spectra of two samples with different defectivity

.

Finally in figure 2 is reported the XANES of the surface Ni oxide detected at room temperature after a long exposure of a clean Ni surface to dry oxygen. The spectrum was obtained by the ratio between the signal from the oxidized Ni surface and the signal of clean Ni surface.

This spectrum is compared with the stoichiometric NiO in order to evidence that the exposure of a clean Ni surface to dry oxygen does not produce NiO. To have stoichiometric NiO we need high temperature (1250 K) and high partial pressure of oxygen (200 mbar.). XANES data of the surface nickel oxide are very symilar to the difective {Ni(,- ,,O) although this is, also, in a strong contradiction with the Ni-0, binary phase diagram(3). The cited diagram says that is not possible to find a stable defective N i - 0 unless of consider Ni,O, as defective and in this case it is necessary to have a consistent presence of wather during the reaction.

From our data we observe that in the surface Ni oxide the peak A is quenced indicating an augmented Ni-0 bond ionicity and a shift of peak D towards low energy as expected in the defective (Ni(,-b,O), with an expansion of the average NiO distance.

Therefore the surface niche1 oxide is more ionic than the stoichiometric one. We suppose that the surface oxide grown at room temperature can be formed by a distorted clusters of NiO.

Fig. 2. Oxygen K-edge XANES of the oxidized Ni surface (top curve) compared with NiO (lower curve).

Fig.1. Oxygen K-edge XANES for the stoichiometric NiO (top curve) and for defective [Ni(1.,03)0) (lower curve).

Note the strong reduction of the peak A in the defective sample.

PHOTON ENERGY ( e ~ )

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References

1) - I. Davoli, A. Marcelli, A. Bianconi, M. Tomellini, M. Fanfoni.

Phy. Rev. B, 2979 (1 986).

2) - M. Tomellini, D. Gozzi, A. Bianconi and 1. Davoli.

J. of Chem. Soc. Trans. Farad. (1986) in press.

3) - D.P. Bogatski, Zhur. Obshchei Khim., a , 9 , (1 951).

4) - P. Chiaradia, M. Fanfoni, S. Priori, P. De Padova, P. Nataletti, I. Davoli, S.Modesti. VUOTO (1 986) in press.

5)

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Journal de Physique

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