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SURFACE EXAFS AND XANES STUDIES OF S/Ni (110) AND S/Ni (111)
T. Ohta, Y. Kitajima, P. Stefan, M. Shek Stefan, N. Kosugi, H. Kuroda
To cite this version:
T. Ohta, Y. Kitajima, P. Stefan, M. Shek Stefan, N. Kosugi, et al.. SURFACE EXAFS AND XANES
STUDIES OF S/Ni (110) AND S/Ni (111). Journal de Physique Colloques, 1986, 47 (C8), pp.C8-
503-C8-508. �10.1051/jphyscol:1986894�. �jpa-00226225�
SURFACE EXAFS AND XANES STUDIES O F S / N i ( l l O ) AND S / N i ( 1 1 1 )
T . OHTA, Y . KITAJIMA*, P.M. STEFAN'", M.L. SHEK S T E F A N * * * , N. KOSUGI* and H . KURODA*
Photon Factory, National Laboratory for High Energy Physics, Tsukuba, Ibaraki 305, Japan
"Department of Chemistry, Faculty of Science, the University of Tokyo, Hongo, Tokyo 113, Japan
" " N S L S , Brookhaven National Laboratory, Upton, Long Island,
NY 11973, U.S.A.
* * * D e p a r t m e n t of Physics, Brookhaven National Laboratory, Upton, Long Island, NY 11973, U.S.A.
ABSTRACT
S K-edge SEXAFS were measured on c(2x2)S/Ni(110) and S / N i ( l l l ) by monitoring t h e S KLL Auger i n t e n s i t y a s a f u n c t i o n of photon energy and p o l a r i z a t i o n d i r e c - t i o n . Analysis of t h e d a t a from c(2~2)S/Ni(110) gave r e s u l t s t h a t a s u l f u r atom is l o c a t e d on a holJow s i t e with R(S-top l a y e r Ni) of (2.31k0.02) and R(S-bottom Ni) of (2.1950.03) A. These r e s u l t s a r e i n c l o s e agreement with t h e r e c e n t analy- s i s of LEED. From t h e preliminary SEXAFS work on p ( 2 x z ) S / N i ( l l l ) , S-nearest neigh- bor N i bond d i s t a n c e was determined t o be 2.20k0.04 A.
INTRODUCTION
Now, SEXAFS has been proved t o be a powerful technique f o r s u r f a c e s t r u c t u r e a n a l y s i s . However, t h i s method has been applied t o only l e s s than 30 systems, s i n c e 1978, when t h e pioneering works were performed by P. C i t r i n e t a l . [ l ] and J . Stb;hr e t a1.[2] a t SSRL. This i s mainly due t o t h e l i m i t e d experimental f a c i l i t i e s a v a i l a b l e i n t h e world. Recently, s e v e r a l groups i n Daresbury[3] and a l s o BESSY[4] r e p o r t e d t h e SEXAFS works. Here we p r e s e n t t h e f i r s t SEXAFS experiment c a r r i e d out a t t h e Photon Factory.
~ ( 2 x 2 ) S/Ni(100) was s t u d i e d e x t e n s i v e l y by S. Brennan e t a1.151 u s i n g p o l a r i - z a t i o n dependent SEXAFS, b u t s u l f u r on o t h e r two f a c e s have not been s t u d i e d with SEXAFS y e t . Recently, Baudoing e t a1. [ 6 ] c a r r i e d out t h e p r e c i s e LEED a n a l y s i s of c(2x2)S/Ni(110) and found t h e d r a s t i c r e l a x a t i o n of t h e N i t o p and second l a y e r s . The LEED i s r a t h e r s e n s i t i v e t o t h e i n t e r p l a n a r spacing, while t h e SEXAFS g i v e s t h e l o c a l bond d i s t a n c e i n a t h e o r y independent f a s h i o n . Accordingly, it i s worthwhile t o i n v e s t i g a t e t h e same system with t h e SEXAFS method. In t h i s r e p o r t , S on N i ( l l 0 ) and N i ( l l 1 ) were s t u d i e d u s i n g t h e p o l a r i z a t i o n dependent SEXAFS.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986894
JOURNAL D E PHYSIQUE
EXPERIMENTAL
High p u r i t y (5N) s i n g l e c r y s t a l s with two f a c e s , (110) and (111) were
purchased from Metal C r y s t a l s and Oxides Ltd. i n U . K .
.
Disk-shaped (2 mmtx10 mm @) c r y s t a l s were p o l i s h e d down mechanically t o 1 pm with a diamond p a s t e and then e l e c t r o - p o l i s h e d . Each sample was cleaned i n a UHV chamber by t h e successive treatment o f ~ r + s p u t t e r i n g and annealing (-800 OC). Surface cleanness and c r y s t a l p e r f e c t n e s s were monitored with AES (Auger E l e c t r o n Spectroscopy) and LEED. A f t e r 5 t o 10 c y c l e s of cleaning, we could o b t a i n c l e a n N i c r y s t a l s u r f a c e s , on which S and C were nondetectable with AES.S u l f u r overlayer was produced by dosing H2S on each c l e a n c r y s t a l . About 2 Langmuir of H2S ( 1 0 ~ ~ torrx20 see) was dosed on t h e N i ( l l 0 ) c r y s t a l , heated a t about 200 O C . Bright ~ ( 2 x 2 ) LEED p a t t e r n was observed and t h i s s u r f a c e s t r u c t u r e was s t a b l e a t l e a s t f o r 24 hours under t h e base p r e s s u r e lower than 10-lo t o r r .
For t h e case of N i ( l l l ) , t h e dosage of 1 L H2S on t h e sample a t room temperature y i e l d e d ~ ( 2 x 2 ) p a t t e r n , while more than 3 L dosage y e i l d e d v e r y complicated ( 5 6 x 2 ) p a t t e r n . These s u r f a c e s t r u c t u r e s were comparatively un- s t a b l e and t h e i r LEED s p o t s became dim a f t e r 6 hours.
Surface EXAFS experiments were performed using synchrotron r a d i a t i o n from t h e s o f t X-ray double c r y s t a l monochromator a t t h e Photon Factory[7].
By use of a s e t of InSb(ll1) c r y s t a l p a i r , t h e photon f l u x of 4 x 1 0 10 photons/sec i s a v a i l a b l e i n t h e 1650-3800 eV energy range with 1 . 5 eV r e s o l u t i o n . The SEXAFS s i g n a l was obtained by monitoring t h e S KLL Auger e l e c t r o n y i e l d , whose energy i s -2100 eV. Spectra were recorded u s i n g a c y l i n d r i c a l m i r r o r a n a l y s e r (CMA) i n t h e nonretarding mode, and were f l u x normalized by a monitor c o n s i s t i n g o f a Cu g r i d and a s p i r a l t r o n e l e c t r o n m u l t i p l i e r . I t took about 1 . 5 hours t o o b t a i n each EXAFS spectrum i n t h e range 2400-2900 eV, whose upper energy
i s l i m i t e d by t h e appearance of t h e N i 2p photoelectrons i n t h e CMA window.
N i S was chosen a s a model compound, i n which t h e n e a r e s t neighbor bond
3 2 0
length o f S-Ni i s 2.28?0.01 A, and t h e coordination number i s 6[8]. T o t a l e l e c t r o n y i e l d spe&trum of a s i n t e r e d d i s k of Ni3S2 was measured i n t h e same energy range a s t h a t f o r SEXAFS s p e c t r a of above-described systems.
In t h e c a s e of S on N i ( l l O ) , t h e r e a r e two angular dependences: ( i ) on t h e p o l a r angle 13 between t h e
8
v e c t o r and t h e s u r f a c e normal n and ( i i ) on t h e -+azimuthal angle @ i n t h e s u r f a c e plane. Thus, we measured two p o l a r - a n g l e dependent SEXAFS s p e c t r a ; one f o r
8
p a r a l l e l t o t h e [loo](a
= 0 ° ) and t h e o t h e r f o r2
p a r a l l e l t o [JlO] (@ = 9 0 ° ) . On t h e o t h e r hand, t h e r e a r e no azimuthal angular dependence of SEXAFS f o r S / N i ( l l l ) and we measured p o l a r i z a t i o n dependent SEXAFS s p e c t r a f o r one azimuthal angle.Figure 1 shows t h e raw S K-edge SEXAFS s p e c t r a o f c(Zx2)S/Ni(110) a t photon i n c i d e n t a n g l e s O of 90°, 4 S 0 , and 10" a t Q = O O . The o s c i l l a t i o n s and t h e i r p o l a r i z a t i o n dependency a r e v i s i b l e with good s i g n a l t o n o i s e r a t i o . The background was s u b t r a c t e d from each spectrum assuming t h a t it f o l l o w s t h e s u l f u r atomic a b s o r p t i o n above t h e edge. The EXAFS s i g n a l x(k) was normalized t o t h e edge h e i g h t , which was determined by e x t r a p o l a t i n g t h e atomic a b s o r p t i o n l i n e t o
2 0
t h e edge. F o u r i e r t r a n s f o r m of x(k)xk (Fig. 2) shows a s t r o n g peak around 2 A, which can be assigned t o S - n e a r e s t neighbor N i , although i t s d i s t a n c e i s a p p a r e n t l y s h o r t e r due t o t h e phase s h i f t . A f t e r f i l t e r i n g , t h e s t r o n g peak i n t h e R-space was . ~ o u r i e r transformed back i n t o k-space, which gave t h e amplitude of t h e n e a r e s t neighbor bond. By comparing t h e amplitude f u n c t i o n with t h a t from t h e model compound, Ni3SZ, we could o b t a i n t h e e f f e c t i v e c o o r d i n a t i o n number N* a t each angle. A s shown i n Fig. 3, t h e r e a r e f o u r kinds of p o s s i b l e a d s o r p t i o n s i t e s f o r S on N i ( l l 0 ) . The e f f e c t i v e c o o r d i n a t i o n number N* was c a l c u l a t e d f o r each
0
p o s s i b l e a d s o r p t i o n s i t e , assuming t h e n e a r e s t S-Ni bond l e n g t h t o b e 2.31 A.
C a l c u l a t e d and experimental N*s a r e compared i n Table 1. I t i s p o s s i b l e t o determine t h e a d s o r p t i o n s i t e uniquely a s a hollow s i t e from t h e d a t a f o r two azimuthal a n g l e s , Q = 0" and Q = 90".
PHOTON ENERGY ( e V 1
Fig.1 S K-edge SEXAFS spectra of c(Zx21SINi
(1101 a t with three polar angles, Fig 2 Fourier t r a n s f o r m of X I k lx k2 for ( 0 1 8 = 9 0 ' ( b l B = L s ' l r l ~ , . l n ' SEXAFS spectra i n Fig. 1
JOURNAL DE PHYSIQUE
Back t o t h e Fourier transformed s p e c t r a (Fig. 2)
,
i t should be noted t h a t t h e peak p o s i t i o n is s h i f t e d t o t h e s h o r t e r d i s t a n c e by chang- ing t h e angle O from normal t o grazing incidence. This r e s u l t i n d i c a t e s t h a t t h e peak contains two components ; one
from t h e t o p l a y e r f o u r n i c k e l F1g. a d s o r p t i o n s i t e of S on N i ( 1 1 0 ) atoms and another from t h e
bottom n i c k e l atom. A t thenormal incidence, t h e r e is no c o n t r i b u t i o n t o EXAFS from t h e bottom n i c k e l atom. Thus, we can e a s i l y determine t h e bond length
0
between S and f o u r t o p l a y e r N i atoms, Rtop, a s 2.3120.02 A. Next, we determined t h e bond length of S-bottom N i , F$ottom, through t h e l e a s t square f i t , where R
t o p and 0 and N*top/N*bottom
t o p were f i x e d , and RbotSom, Obottom, and N*bottom were parametrized. The v a l u e obtained was 2.19+-0.02 A. By using t h e determined bond lengths, we can g e t t h e i n t e r - s p a c i n g between t h e t o p l a y e r and t h e second l a y e r , which is 1.5720.02
i.
This i s longer than t h e bulk i n t e r l a y e r spacing, 1.246 by (10+2)%. It i n d i c a t e s t h a t t h e s t r o n g bond between t h e S atom and t h e bottom N i (second layer) pushes t h e t o p l a y e r N i upwards. These r e s u l t s a r e i n c l o s e agreement with t h o s e by Baudoing e t a1.[6] who analysed t h e LEED d a t a , t a k i n g t h e m u l t i l a y e r r e l a x a t i o n i n t o account.Table 1 Comparison of Experimental and Calculated N*
Curve F i t t i n g of Obsd. Spectra Calcd. Coordination Number N*
P o l o r i z a t i o n R N* Atop Short Long Hollow Hollow**
0
1
Bridge BridgeHollow**; r e s u l t s i n c l u d i n g t h e bottom N i atom,
i n Fig. 4, which i n d i c a t e s d i s t i n c t p o l a r i z a t i o n de- pendence of t h e s p e c t r a . Here, we denote t h e coordi- n a t e of t h e S/Ni(llO) system a s t h e x a x i s along <loo>, y a x i s along <110> and z a x i s along t h e s u r f a c e normal.
The XANES spectrum a t @ = O O and 0 = 9 0 ° comes from x- p o l a r i z e d t r a n s i t i o n s , and t h a t a t @ = 90" and 8 = 90"
from y-polarized t r a n s i - t i o n s , while t h e spectrum a t grazing incidence i s domi- n a t e d by z-polarized t r a n s i - t i o n s . According t o t h e preliminary ab i n i t i o SCF-MO c a l c u l a t i o n f o r t h e Ni5S c l u s t e r , t h e r e i s a s t r o n g bond between t h e S atom and t h e bottom N i atom, and 4
t o p l a y e r N i atoms a r e Flg.L S
K
X A N E S o f ~ ( 2 x Z ) S l N i (110) r a t h e r nonbonding.Accordingly, t h e f i r s t peak of z-polarized spectrum can be assigned t o be t h e t r a n s i t i o n t o t h e antibonding (S3pa-Ni4sa)* o r b i t a l . x- and y-polarized s p e c t r a show prominent peaks a t t h e n e a r l y same energy. They a r e t e n t a t i v e l y assigned a s t h e t r a n s i t i o n s t o Rydberg(4P) type o r b i t a l s , where 4P i s s l i g h t l y
Y
lower than 4px, due t o t h e l e s s r e p u l s i v e f o r c e from t h e surrounding N i atoms.
There a r e s e v e r a l d i s t i n c t peaks i n t h e h i g h e r energy region, p o l a r i z e d e i t h e r t o x o r y. They a r e p o s s i b l y due t o t h e shape resonance type t r a n s i t i o n s . F u r t h e r d e t a i l e d c a l c u l a t i o n s a r e now i n progress.
(b) S / N i ( l l l )
The s u r f a c e s t r u c t u r e of t h i s system h a s been s t u d i e d l i t t l e . Rather o l d LEED a n a l y s i s by Demuth e t a l . [ 9 ] showed t h a t a S atom i s located a t t h r e e - f o l d hollow s i t e and t h e n e a r e s t S-Ni bond d i s t a n c e i s 2.02k0.06 A. 0 But, it is worth n o t i n g t h a t they used only t h r e e beam s p e c t r a . We performed t h e SEXAFS measure- ment both f o r ~ ( 2 x 2 ) and ( 5 ~ % 2 ) S / N i ( l l l ) . X.(k)xk and Fourier transform of t h e 2 s p e c t r a i s shown i n Fig. 5 . By t h e phase s h i f t c o r r e c t i o n , t h e n e a r e s t S-Ni bond
C8-508 JOURNAL DE PHYSIQUE
d i s t a n c e w a s d e - p 1 2 x 2 ) S / N ~ ( 1 1 1 ) termined t o b e
0
2.20t0.04 A f o r R I S - N I I = 2 1 8 f a o ~ i \
~ ( 2 x 2 ) and 2 . 1 8 t 0.03 f o r
( 5 8 x 2 ) ~ / ~ i ( l l l ) . These v a l u e s a r e more probable
t h a n t h a t d e t e r - 2 L 6 8 1 0
K ( A - ' I 0 1 2 3 4 5 6
mined with LEED. R I A I
There a r e t h r e e p o s s i b l e adsorp-
R ~ s - E l o = 2 2 0 1 0 . 0 3 A
t i o n s i t e s ; a t o p , b r i d g e and hollow s i t e . Since t h e S K XANES s p e c t r a f o r p ( 2 ~ 2 ) S / N i ( l l 1 ) do n o t
show any d i s t i n c t L 6 k I ~ - ' l 8 10 0 1 2 R I A 1 3 1 5 6
p o l a r i z a t i o n de-
Fig.5
x
( k ) x k 2 and Fourier transform of S E X A F S spectra pendence, we canfrom ~ ( 2 x 2 ) S / ~ i ( l l l ) a n d ( 5 J 3 x 2 ) S I N i ( 1 1 1 ) exclude t h e a t o p
s i t e . F u r t h e r d e t a i l e d experiment i s n e c e s s a r y t o determine t h e a d s o r p t i o n s i t e i n c o n c l u s i v e way.
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