A LEIS study of potassium enriched vanadium oxide surfaces

598

Surface Science 126 (1983) 598-604 North-Holland Publishing Company

A LEIS STUDY OF POTASSIUM SURFACES J.P. LANDUYT,

L. VANDENBROUCKE,

Laboratorium uoor Kristallografie B - 9000 Gent, Belgium Received

31 August

ENRICHED VANADIUM

R. DE GRYSE

en Studie ~)an de Vaste Stoh Rijksunioersiteit

1982; accepted

for publication

22 October

OXIDE

and J. VENNIK Gent, Krijgslaan 281,

1982

An important surface segregation of potassium to the V,O,,(OOl) surface is observed. From combined LEED and angular dependent LEIS experiments, it is found that the most probable K site at the V,O,,(OOl) surface is an interstitial site close to or within the topmost VO layer. It is furthermore shown that on V,O,, the neutralization processes active in He+ LEIS experiments can be described by the Hagstrum formalism. This result allows a semi quantitative estimation of the surface concentration. A model for the surface structure is presented.

1. Introduction During a study of the structure and composition of the V,O,,(OOl) surface at elevated temperatures (k 500 K), a surface segregation of potassium was observed simultaneously by LEIS, SIMS and TDS experiments. As potassium is frequently added to industrial V,O, based catalysts, in order to improve the selectivity and activity of the respective reactions, this surface segregation might be of interest for the behaviour of potassium doped V,O, based catalysts. In this paper the experimental evidence for this surface segregation is presented. A careful analysis of the neutralization mechanisms active during the LEIS experiment allows a semi-qauntitative estimation of the potassium surface concentration to be made. A model for the surface structure with incorporated potassium is presented.

2. Experimental The experiments were carried origins. The first kind is obtained on top of a V,O, crystal [ 11. Before were heated in situ to +750 K experiments, V,O,, single crystals

out using V,O,, crystals of two different by the topotactical growth of a V,O,, layer recording the discussed spectra, the samples for several days. In a second series of were used. These crystals were grown by a

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0 1983 North-Holland

J.P. Lmduyt

et al. / LEIS

study of K enriched V60,, oxide

599

chemical vapour transport method [2]. The surface was cleaned in situ by ion bombardment and annealed at 750 K. Both kinds of samples reveal essentially the same LEIS spectra and the same LEED periodicity. Combined LEIS, SIMS and TDS experiments were performed in the same UHV apparatus and using a mass filtered primary ion beam, avoiding in this way surface contamination by the primary beam. The angular dependent LEIS equipment was described elsewhere [l]. The SIMS spectra were recorded using a quadrupole (Riber SQ 156) mass filter. A series of combined LEIS-AES experiments were performed in order to elucidate some contradiction between the results of the LEIS-SIMS experiments presented in this paper and the results of earlier AES experiments [3,4]. For this purpose a CMA-type LEIS spectrometer (3M Model 525) was used, to which an off-axis electron gun was added. The AES spectra were recorded in puls-counting mode and were differentiated digitally. In this way a resolution of AE/E = 1.5% was obtained.

3. Results and discussion 3. I. Surface segregation Table 1 shows the bulk concentration of some typical contaminants of V,O, and V,O,, samples, similar to those used for the present experiments, illustrating the low bulk contamination. In the LEIS spectra taken at elevated temperatures (T > 600 K) and using both He+ and Ne+ as primary ions, a structure is observed which can be explained by the scattering from potassium atoms. In order to obtain supplementary evidence, SIMS experiments (Ar+ , 2 keV) were performed. The two potassium lines (39 and 41 amu) are the most important features in the spectrum. Alkali lines are frequently observed in SIMS spectra. They often are due to a surface contamination by the primary beam. In the present experiments, however, a mass filtered primary beam was used. To completely rule out all doubts with regard to beam contamination, originating for instance from a lack of mass selectivity in the primary beam (Ar+ against K+), similar

Table 1 Concentration

“2% “,%

of impurities

in weight ppm as determined

by neutron

activation

Pt

Mn

Na

K

65.6 16.3

0.13

9.77 7.38

9.64 97.0

1.06

analysis

600

J.P.Lmduyt et al. / LEIS study of K enriched V6013 oxide

SIMS experiments were performed with He+ (2 keV), giving rise to identical results. Potassium is thus unambiguously originating from the sample and not from the primary beam. Other external sources of surface contamination should be ruled out, because the potassium contribution is present even in the case of samples cleaved under UHV conditions or after cleaning the sample by ion bombardment. The presence of potassium is also ascertained in another way. At a sufficiently high temperature (T > 600 K) a spontaneous emission of potassium ions is observed. The potassium surface concentration is temperature dependent, as can be illustrated by LEIS and TDS experiments. It is believed that the concentration is determined by the competition between the transport of potassium to the surface and the spontaneous thermal emission from the surface. At f700 K the surface concentration saturates and remains approximately constant up to 800 K. At this temperature the potassium signal suddenly drops to zero in the LEIS as well as in the TDS spectra. This effect is probably due to the sublimation of the V,O,, surface. 3.2. Electron stimulated

desorption of potassium

The V,O,-V,O,, system has been studied intensively, using e.g. Auger spectroscopy [3,4]. The above mentioned surface segregation, however, was never observed. A similar contradiction between AES and SIMS-LEIS results has been found for different kinds of glasses. This is generally understood by a diffusion of the alkali ions away from the surface, as a consequence of the electrical charging of the sample by the incoming electron beam. However, as V,O,, is an electrical conductive material, this model is rather unlikely to explain the observed discrepancy. A possible explanation is the decrease of the surface concentration by electron stimulated desorption of potassium or by the local heating of the sample due to the electron bombardment. In order to verify this hypothesis, Auger experiments were performed using a very low intensity primary electron beam. As explained before a 3M LEIS spectrometer was used to compare the results of LEIS and AES is a straightforward way. For Iprim < 10 nA cmm2, it was possible to detect traces of potassium in the Auger spectrum. Moreover it was also possible to decrease the amplitude of the potassium contribution in a LEIS spectrum by simultaneously bombarding the sample with an electron beam. Consequently it can be concluded that the potassium atoms leave the surface under the influence of the electron bombardment.

J.P. Landuyt et al. / LEIS

3.3. Ion survival probability

study of K enriched V,O,,

P in LEIS

oxide

601

experiments

Elsewhere [5] it was shown that the presence of potassium at the V,O,,(OOl) surface influences the neutralization probability of the reflected ions in a global way. Indeed a variation of the K surface concentration, for instance due to e-bombardment, results in a scaling of all features of the LEIS spectrum. Consequently it was concluded that the neutralization behaviour has to be regarded as a “surface” property characteristic of the metallic V,O,, phase, rather than as a property of a localized ion-atom interaction during the collision. The neutralization mechanism thus can be described [6-91 by the Hagstrum formalism (PA: ion survival probability; vi: velocity of incident ion; vOA: ion velocity after collision with atom A; 0: scattering angle; v~: characteristic velocity): P,=exp

cos Bi c1. I VOA

‘+ v,

[

i

l

(1)

v

Additional evidence for this model can be obtained by measuring the intensities as a function of primary beam energy and plotting ln(aTZ,,/Z,,) as a function of v;’ (a: differential cross section; T: analyser transmission; I,: primary ion current; I,,: measured fraction of backscattered current). This also allows the determination of the characteristic velocity v, to be made. The cross section u can be estimated by the Moliere approximation of the Thomas-Fermi potential [lo]. Fig. 1 represents these results for the interaction of He+ with the oxygen and the vanadium atoms. When measuring the LEIS intensities,

, G 2 \ 2 c

t)

a

-32

c

-35 2.10-6

4.10+

6.10-6

6.10-6

10.10-6

v;' ( s m“)

Fig. 1. Determination of characteristic with vanadium and oxygen.

velocities,

corresponding

to the scattering

of He+ (8 = 138”)

602

J.P. Landuyi et al. / LEIS study

of K enriched V,O,, oxide

two main problems arise. At the lower energies an anomalous shift and splitting of the V signal is observed for energies between 250 to 800 eV. A detailed investigation showed that this shift is similar in nature to the one observed in the case of He+ scattering from Ta metal surfaces, with an adsorbed oxygen layer [ 11,121. For the present case, however, the inelastic loss of the vanadium peak seems to be proportional to the primary energy. This fact suggests that this loss cannot be ascribed to a well defined electronic excitation. Furthermore, as the beam energy increases, an important background develops which, as is known from the literature, can be ascribed to the backscattering from deeper layers. In this way the measured intensities are no longer representative for the surface and eq. (1) may probably fail to represent the expected results. This may explain the anomalous behaviour of the oxygen signal at higher energies, i.e. the lower 0; ’ values. From fig. 1 it follows that the interaction with oxygen and vanadium yields nearly the same characteristic velocity: u, = 2.1 X lo5 m s- * for oxygen and 2.6 X lo5 m s-’ for vanadium. This result supports the hypothesis of the formalism of eq. (1). Moreover, these values fit very well within the range obtained for other metallic surfaces [7-91.

3. Surface structure In order to obtain some information about the V,O,,(OOl) surface structure, in particular about the position of the potassium sites at the surface, LEED and angular dependent LEIS experiments are performed. It was shown elsewhere [l] that the V,O,, structure is terminated in the c-direction by the mixed VO layer, represented in fig. 2. Angular dependent LEIS experiments did not reveal blocking or shadowing effects involving the potassium atoms. This indicates that the observed potassium should be in or near the outermost VO layer. In LEED experiments the periodicity of the LEED picture remains unaltered as the K concentration increases and no superstructures are observed. However, it is questionable whether classical LEED techniques are appropriate for studying the discussed phenomena due to the perturbing effect of the e- beam. On the other hand, monovalent elements such as Li, Na, K, Cu and Ag form with V,O, a-bronzes with the general formula M,V,O, [ 131, which consist of a V,O, matrix with M interstitials. It is well established from EPR measurements [ 14,151 that the impurity is situated in the C,, interstitial site. As V,O,, has similar interstitial sites, and keeping in mind that the potassium is rather weakly bound in the V,O,, lattice, it can be assumed that also in this lattice, potassium is probably at an interstitial site near the terminating V-O layer. As was mentioned, the potassium concentration is temperature dependent, increasing with temperature and saturating at 700 K. A semi-quantitative estima-

J.P. Landuyt ef al. / LEIS study of K enriched V60,3 oxide

b

603

1

Fig. 2. Projection on the c-plane (001) of the outermost VO layer of V,O,,. The two-dimensional unit mesh of V,O,, is shown. Open circles represent the oxygen atoms, dark spheres the vanadium atoms and cross hatched spheres the proposed potassium sites.

tion of the surface K concentration at saturation, based on the proposed surface neutralization mechanism, is presented elsewhere [5]. It is found that for each two vanadium atoms one potassium atom is present at the V,O,,(OOl) surface at the point of saturation. As can be seen from fig. 2 each surface unit cell contains two vanadium sites. Consequently, it is concluded that the unit cell can accommodate one K atom. For geometrical reasons the most probable site should be as indicated in fig. 2.

Acknowledgements This work is part of a research Instituut voor Kernwetenschappen,

project supported by IIKW Dee1 Vaste Stof).

(Interuniversitair

References [l] R. De Gryse, J.P. Landuyt, A. Vermeire and J. Vennik, Appl. Surface Sci. 6 (1980) 430. [2] M. Saeki, N. Kimizuka, M. Ishii, I. Kawada, M. Nakane, A. Ichionose and M. Nakahira, Crystal Growth 18 (1973) 101. [3] L. Fiermans and J. Vennik, Surface Sci. 24 (1971) 541. [4] L. Fiermans and J. Vennik, Surface Sci. 35 (1973) 42. [5] R. De Gryse, J.P. Landuyt, L. Vandenbroucke and J. Vennik, Surface Interface Analysis (1982) 168.

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[6] H.D. Hagstrum, in: Studies of Adsorbate Electronic Structure Using Ion Neutralization and Photoemission Spectroscopies in Electron and Ion Spectroscopy of Solids, Eds. L. Fiermans, J. Vennik and W. Dekeyser (Plenum, New York, 1978). [7] H.H. Brongersma and T.M. Buck, Nucl. Instr. Methods 132 (1976) 559. [8] L.K. Verhey, B. Poelsema and A.L. Boers, Nucl. Instr. Methods 132 (1976) 565. [9] L.K. Verhey, B. Poelsema and A.L. Boers, Radiation Effects 34 (1977) 163. [IO] M.T. Robinson, Tables of Classical Scattering Integrals, ORNL 4556. [ 1 l] R.C. McCune, J.E. Chelgren and M.A.Z. Wheeler, Surface Sci. 84 (1979) L515. (121 W.L. Baun, Phys. Rev. A17 (1978) 849. [13] P. Hagenmuller, J. Galy, M. Pouchard and A. Casalot, Mater. Res. Bull. 1 (1966) 45. [14] G. Sperlich, Z. Physik 250 (1972) 335. [15] P. Clauws, Verhand. Kon. Acad. Belgili 42 (1980) No. 159.