Electronic structure investigation at a zirconia-nickel interface

Surface and Coatings Technology, 45 (1991) 309-315

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E l e c t r o n i c s t r u c t u r e i n v e s t i g a t i o n at a zirconia n i c k e l i n t e r f a c e S. H a r e l a, J . - M . M a r i o t b, E. B e a u p r e z a and C. F. H a g u e b ~ETCA-Centre de Recherches et d'Etudes d'Arcueil, 16 bis Avenue Prieur de la CSte d'Or, F-94114 Arcueil Cedex (France) bLaboratoire de Chimie Physique (CNRS Associate), Universit6 Pierre et Marie Curie, 11 rue P. et M. Curie, F-75231 Paris Cedex 05 (France)

Abstract X-ray photoelectron spectroscopy (XPS) is used to study electronic structure effects at a zirconia nickel interface. ~; method is described to fabricate ZrO2 coatings on metallic substrates under ultrahigh vacuum conditions with a sufficiently low coverage to allow examination of the interracial region. Charging effects which relate to the size of oxide particles deposited on the metal (and not to accumulated charge on the oxide) have been identified for small ZrO2 particles on nickel, palladium and NiO substrates equivalent to a coating approximately 6 thick. The charging effects show up as a broadening of the XPS Zr 3d core-level spectrum. By examining the XPS Ni 2p3/2 and XPS valence band spectra it is found that an oxidized nickel surface layer is reduced during ZrO2 deposition.

1. I n t r o d u c t i o n M e t a l s deposited on oxides h a v e a wide r a n g e of i n d u s t r i a l a p p l i c a t i o n s in optical, e l e c t r o n i c a n d c a t a l y t i c devices. Their f a b r i c a t i o n often involves atomic-scale b o n d i n g m e c h a n i s m s but the c o m p l e x i t y of the m e t a l oxide interface problem is s u c h t h a t d e v e l o p m e n t relies h e a v i l y on empiricism. E v e n so, u n d e r the impetus of m i c r o e l e c t r o n i c s m u c h effort has gone into f u n d a m e n t a l i n v e s t i g a t i o n s a n d the c h a r a c t e r i z a t i o n of metal oxide interfaces (see e.g. ref. 1). E x a m i n i n g the e l e c t r o n i c s t r u c t u r e at the interface or in the i n t e r f a c i a l r e g i o n requires the use of h i g h - e n e r g y e l e c t r o n spectroscopies s u c h as X-ray p h o t o e l e c t r o n s p e c t r o s c o p y (XPS) [2] or X-ray emission s p e c t r o s c o p y [3], b u t the former c a n only be p e r f o r m e d on model systems and the l a t t e r on systems i n v o l v i n g a c o v e r a g e of a few tens of n a n o m e t r e s only. A r e l a t i v e l y new e x p a n d i n g field in m a t e r i a l s science consists of depositing c e r a m i c oxides on metallic alloys so as to take a d v a n t a g e of t h e i r low t h e r m a l c o n d u c t i v i t y and their e x c e p t i o n a l t e m p e r a t u r e , c o r r o s i o n a n d wear resistance. This is e x p e r i m e n t a l l y an even more difficult problem to deal with, so it is t e m p t i n g to deal with it as the same as the metal-on-oxide case. Some c a u t i o n is required, however, for two reasons. Firstly, the a d h e s i o n depends on the deposition technique. For instance, metal-on-oxide t e c h n o l o g y largely depends on the w e t t i n g of ceramics by liquid metals or diffusion b o n d i n g b e t w e e n solid metals a n d the ceramics. Ceramic oxide c o a t i n g s on the o t h e r hand, are m a i n l y f a b r i c a t e d by s p u t t e r deposition or plasma sprays [4]. Elsevier Sequoia/Printed in The Netherlands

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Secondly, t h e r e is a s t r o n g difference in dielectric c o n s t a n t b e t w e e n a n ionic oxide a n d a m e t a l [5]. In this r e s p e c t we c a n expect to d e t e c t some differences in the e l e c t r o n i c s t r u c t u r e at the b e g i n n i n g of the i n t e r f a c e f o r m a t i o n a c c o r d i n g to w h e t h e r t h e c e r a m i c is deposited on the m e t a l or t h e o t h e r w a y around. In this p a p e r we r e p o r t on the p r e l i m i n a r y stages of e x p e r i m e n t s destined to i n v e s t i g a t e the e l e c t r o n i c - s t r u c t u r e - r e l a t e d m e c h a n i s m s for b o n d i n g of ZrO2 to m e t a l l i c s u b s t r a t e s . As a first step we h a v e c h o s e n XPS to m o n i t o r the c o m p o s i t i o n of the deposit a n d s u b s t r a t e d u r i n g the e a r l y stages of deposition. This h a s m e a n t p r e p a r i n g s a m p l e s u n d e r u l t r a h i g h v a c u u m conditions in s i t u . A d u a l - i o n - b e a m - s p u t t e r i n g t e c h n i q u e to p r o v i d e a c o n t r o l l a b l e d e p o s i t i o n of ZrO2 was adopted. At p r e s e n t o u r e x p e r i m e n t s are p e r f o r m e d a t r o o m t e m p e r a t u r e on polished nickel s u b s t r a t e s . P a l l a d i u m s u b s t r a t e s are also u s e d as a c h e c k on the m e a s u r e m e n t s .

2. Experimental details We h a v e c o n s t r u c t e d two b r o a d b e a m ion guns b a s e d on the K a u f m a n design [6]. T y p i c a l l y a 150 m A p l a s m a d i s c h a r g e is c r e a t e d inside a cylindrical c h a m b e r 25 m m in d i a m e t e r u s i n g a low p o t e n t i a l difference ( 4 0 - 7 0 V ) b e t w e e n a h e a t e d t u n g s t e n f i l a m e n t c a t h o d e a n d stainless steel anodes. Ions are e x t r a c t e d t h r o u g h a p a i r of g r a p h i t e grids designed to p r e v e n t the ion b e a m f r o m diverging. E a c h g u n fits t h r o u g h a s t a n d a r d 38 m m Conflat flange port. T h e A E I ES 200 B p h o t o e l e c t r o n s p e c t r o m e t e r is fitted w i t h a c u s t o m - b u i l t p r e p a r a t i o n c h a m b e r . T h e r e s i d u a l p r e s s u r e in the s p e c t r o m e t e r is a b o u t 2 × 10 s P a a n d the b a s e p r e s s u r e in the p r e p a r a t i o n c h a m b e r is t y p i c a l l y a r o u n d 2 × 10 7 Pa. S u b s t r a t e s a m p l e s are m o u n t e d on a r o t a t a b l e t r a n s f e r r o d w h i c h serves to a d j u s t the s u b s t r a t e a n g l e r e l a t i v e to the guns a n d to c a r r y the s a m p l e t h r o u g h a v a c u u m lock into the s p e c t r o m e t e r . T h e guns o p e r a t e at a p a r t i a l p r e s s u r e of 4 × 10 .2 P a w i t h a r g o n or a n Q - A r gas m i x t u r e . D e p o s i t i o n f r o m the s p u t t e r e d t a r g e t c a n t a k e p l a c e in the p r e s e n c e of ion b e a m s p u t t e r i n g of the s u b s t r a t e in the w a y d e s c r i b e d by W e i s s m a n t e l [7] so as to o b t a i n low e n e r g y i m p l a n t a t i o n , or, as in t h e s e e x p e r i m e n t s , the t a r g e t c a n simply be p o l a r i z e d n e g a t i v e l y r e l a t i v e to b o t h t h e ion g u n a n d the s u b s t r a t e ; the s p u t t e r e d t a r g e t m a t e r i a l simply c o n d e n s e s on the s u b s t r a t e . A surface-oxidized z i r c o n i u m foil w a s used in t h e s e e x p e r i m e n t s . W h e n an a p p r o x i m a t e l y 5 0 0 p A , 2 k e V A r ÷ ion b e a m s p u t t e r s a 2 cm × 2 cm z i r c o n i u m t a r g e t , a d e t e c t a b l e Z r Q signal c a n be o b t a i n e d in a b o u t 10 min. A t h i c k e n o u g h c o a t i n g to m a s k the s u b s t r a t e c o m p l e t e l y as far as XPS is c o n c e r n e d t a k e s 300 min. S u r f a c e c o n t a m i n a t i o n f r o m h i g h l y p o l i s h e d 0.6 cm × 2 cm n i c k e l or pall a d i u m s u b s t r a t e s w a s r e m o v e d by 500 eV A r ÷ ion b e a m e t c h i n g at 30 ° to the s u r f a c e u n t i l the O l s a n d C l s signals w e r e n e g l i g i b l y small before deposition. Oxidized n i c k e l s u b s t r a t e s w e r e p r e p a r e d by feeding a n Ar 02 m i x t u r e (02 p a r t i a l p r e s s u r e ca. 1 0 - 3 P a ) to one of the guns a n d s p u t t e r i n g the

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substrate normally to its surface with 400 eV ions. Characteristic XPS core level spectra of NiO were obtained by this technique within about 30 min.

3. R e s u l t s and d i s c u s s i o n The binding energies of the zirconium core levels vary considerably as a function of the state of oxidation. For instance, the Zr 3d peaks are subject to a chemical shift of about 4 eV in Zr02 relative to the metal. Moreover, the spin-orbit splitting of the Zr 3d core levels is 2.3 eV so that the XPS spectrum consists of two well-resolved peaks. It follows that it is very easy to detect the presence of even small quantities of another chemical state by observing the shape of the 3d doublet [8-11]. We illustrate this by an extreme case in Fig. l(a) which shows the XPS Zr 3d spectrum for a lightly etched zirconium foil. The spectrum is composed of a signal from the surface ZrO2 and Zr20 [11]. The Zr 3d5/2 peak for the latter is shifted to 180.0 eV binding energy, while it lies at 183.0 eV in ZrO2. We also show the XPS spectra obtained from a 99.997% pure ZrO2 powder (Fig. l(b)) and from a nickel substrate on which ZrO2 particles have been deposited by sputtering for 120 min as described in the previous section (Fig. l(c)).

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Fig. 1. XPS Zr 3d core level s p e c t r a for (a) l i g h t l y A r + - i o n - e t c h e d surface-oxidized z i r c o n i u m foil, (b) ZrO2 p o w d e r s a m p l e a n d (c) ZrO 2 on n i c k e l (120 m i n d e p o s i t i o n time).

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A detailed quantitative evaluation of the particle size at low "coverage" has yet to be performed. An analysis by secondary ion mass spectroscopy (SIMS) shows that for large depositions (ca. 300min deposition time) the composition of the coating is constant throughout and there is a sharp interface with the substrate. As a rough guide to how the surface is covered by the deposit we show the valence band spectra for various deposition times (Fig. 2). For the lowest deposition time the equivalent thickness is about 6 ,~ from an analysis of the substrate XPS core level intensities. A characteristic peak at 6 eV binding energy grows as the ZrO2 coverage increases, while there is a concomitant decrease in the nickel valence band signal closer to E F (curve A represents the signal from a ZrO2-free nickel substrate; in this case the structure at 6 eV is a satellite). For a coverage corresponding to curves C, D and E in the valence band data the XPS Zr 3d core level spectrum (Fig. 3(a)) is identical to that from bulk powdered Z r Q (Fig. l(b)) both as concerns binding energies and shape. For very low coverages corresponding to sputtering times of 15-30min (curve B is an example of the valence band spectrum after 30 min deposition) the Zr 3d doublet is subject to a marked change in appearance (Fig. 3(a)). There is an apparent increase in the 3d3/2:3ds/2 peak intensity ratio and the depth of the dip between the two peaks is modified. This goes with a 0.2 eV increase in the overall width of the spectrum towards high binding energies. The first explanation which comes to mind is a possible experimental artefact due to some variation in the background signal as a function of coverage. We have eliminated this possibility by performing exactly the same

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experiment on palladium. We observe identical spectral changes as a function of coverage (Fig. 3(b)). By comparing nickel and palladium we also eliminate possible alloying effects. The electronic structure of N i - Z r and P d - Z r alloys has been intensively studied [12-14] because they form metallic glasses. On alloying, the core level shifts are characteristically different because nickel, palladium and zirconium cohesive energies are widely different [14]. As regards the Zr 3d peak binding energies, they remain practically unchanged in Ni Zr relative to zirconium and are small (ca. 0.2 eV) in P d - Z r [15]. However, the XPS Pd 3d core level shifts are very large in the alloy (ca. 1.5 eV). No shifts are observed in the Ni 2p3/2 and Pd 3d core lines here. We can also eliminate charging effects due to accumulated charge because these should increase as the ceramic coating thickens rather than as the coating thickness is reduced. Furthermore, we can eliminate a chemical shift since we know that the coating is fully oxidized from our comparison

314

with bulk ZrO2 (a shift to higher binding energies indicates increased oxidation). A low coverage experiment on NiO was also performed. The NiO substrate was prepared as described in the previous section and Fig. 4 shows the XPS Ni 2p3/2 spectrum for nickel metal, the NiO substrate surface and the substrate after 15 min ZrO2 deposition. A reduction of the NiO layer is clearly identified by a small shoulder in the XPS spectrum after Z r Q deposition. The XPS Zr 3d spectrum is given in Fig. 3(c) and the corresponding valence band data are given in Fig. 5. The modification in the appearance of the Zr 3d spectrum is here due to the weak signal superimposed on a sloping background. When corrected for this effect, the 3d3/2:3ds/2 peak intensity ratio and the dip between the peaks are the same as for the other low coverage spectra. The Zr 3d spectrum also broadens to the high binding energy side. The valence band data confirm the appearance of metallic states at EF. Two conclusions can be drawn from these experiments. Firstly, there is a size effect which sets in for small particles which, from an analysis of the XPS peak intensities, we estimate to cover roughly 10% of the surface. We associate the change in width of the spectrum with a polarization effect. Our attempt to ascertain whether the effect can be attributed directly to size or rather to the oxide-metal interface by performing the same experiment on NiO requires further careful investigation. This follows from our second

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c o n c l u s i o n t h a t the NiO c o a t i n g ( w h i c h has to be k e p t t h i n to a v o i d s p u r i o u s c h a r g i n g e f f e c t s ) is r e d u c e d d u r i n g t h e ZrO2 d e p o s i t i o n p r o c e s s . T h i s s t u d y is b e i n g p u r s u e d f o r o t h e r s u b s t r a t e s as a f u n c t i o n o f substrate temperature and deposition rate.

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