COVERAGE DEPENDENCE OF ADSORBATE-SUBSTRATE BOND LENGTHS ; CHLORINE AND CAESIUM ADSORBED ON SILVER SINGLE CRYSTALS

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COVERAGE DEPENDENCE OF

ADSORBATE-SUBSTRATE BOND LENGTHS ; CHLORINE AND CAESIUM ADSORBED ON

SILVER SINGLE CRYSTALS

G. Lamble, D.J . Holmes, D. King, D. Norman

To cite this version:

G. Lamble, D.J . Holmes, D. King, D. Norman. COVERAGE DEPENDENCE OF ADSORBATE-SUBSTRATE BOND LENGTHS ; CHLORINE AND CAESIUM ADSORBED ON SILVER SINGLE CRYSTALS. Journal de Physique Colloques, 1986, 47 (C8), pp.C8-509-C8-515.

�10.1051/jphyscol:1986895�. �jpa-00226226�

JOURNAL D E P H Y S I Q U E

C o l l o q u e C8, s u p p l g m e n t a u

n o

1 2 , Tome 47,

decembre

1 9 8 6

COVERAGE DEPENDENCE OF ADSORBATE-SUBSTRATE BOND LENGTHS : CHLORINE AND CAESIUM ADSORBED ON SILVER SINGLE CRYSTALS

G.M. L A M B L E ( ~ ) , D.J. HOLMES, D . A . K I N G and D . NORMAN'

Department of

I.P.I.

Chemistry, Donnan Laboratories, University of Liverpool, GB-Liverpool L69

3 B X ,

Great-Britain

"SERC

Daresbury Laboratory, GB-Warrington W A 4 4AD, Great-Britain

A b s t r a c t

SEXAFS h a s been employed to i n v e s t i g a t e s e v e r a l submonolayer phases of C1 and Cs adsorbed o n v a r i o u s s i n g l e c r y s t a l s i l v e r p l a n e s . For c h l o r i n e a d s o r p t i o n , t h e nearest-neighbour bond d i s t a n c e is found to be i n v a r i a n t with coverage. I n complete c o n t r a s t , a dramatic i n c r e a s e in t h e s u b s t r a t e - a d s o r b a t e bond l e n g t h i s observed w i t h i n c r e a s e in coverage of Cs. T h i s d i f f e r e n t behaviour of C1 and C s a d s o r b a t e s i s d i s c u s s e d in t e n s of t h e e l e c t r o n t r a n s f e r p r o c e s s e s between e l e c t r o n e g a t i v e and e l e c t r o p o s i t i v e a d s o r b a t e s and t h e s u b s t r a t e .

INTRODUCTION

The aim of t h e s u r f a c e EXAFS work p r e s e n t e d here was t o i n v e s t i g a t e v a r i o u s submonolayer phases of b o t h a s t r o n g l y e l e c t r o n e g a t i v e s p e c i e s , c h l o r i n e , and a s t r o n g l y e l e c t r o p o s i t i v e s p e c i e s , caesium, adsorbed on s i l v e r . Evidence f o r a fundamental d i f f e r e n c e in t h e e l e c t r o n i c behaviour of t h e two s p e c i e s i s given by t h e way i n which t h e y modify t h e s u b s t r a t e work f u n c t i o n ; w i t h i n c r e a s e i n coverage of t h e a l k a l i , t h e s u b s t r a t e work f u n c t i o n i n i t i a l l y e x h i b i t s a r a p i d non-linear decrease. T h i s r e a c h e s a minimum a t l e s s t h a n a monolayer, t h e n i n c r e a s e s s l i g h t l y

( t o a p l a t e a u ) beyond a monolayer. Adsorption of c h l o r i n e , on t h e o t h e r hand, merely c a u s e s a l i n e a r i n c r e a s e i n t h e s u b s t r a t e work f u n c t i o n .

From a macroscopic p o i n t of view, t h e systems i n v e s t i g a t e d a r e of p a r t i c u l a r i n t e r e s t with r e g a r d to t h e commercial o x i d a t i o n of e t h y l e n e to e t h y l e n e oxide.

S i l v e r i s a very e f f e c t i v e c a t a l y s t i n this r e a c t i o n , w h i l s t adsorbed c h l o r i n e i s used a s a m o d i f i e r , i n c r e a s i n g t h e s e l e c t i v i t y of t h e c a t a l y s t t o t h e e p o x i d a t i o n r o u t e . Addition of small amounts of a l k a l i , however, c a u s e s a v a s t i n c r e a s e i n t h e t o t a l a c t i v i t y of t h e c a t a l y s t . Thus, due to t h e d i f f e r e n c e i n n a t u r e of t h e two s p e c i e s ( C s i s a 'donor' a t a n w h i l s t C 1 i s an

'

a c c e p t o r ' s p e c i e s ) , t h e way in which t h e s u r f a c e p r o p e r t i e s a r e modified i s by very d i f f e r e n t mechanisms.

EXPERIMENT AND ANALYSIS

The SEXAFS measurements were made on s t a t i o n 3, beamline 6 a t t h e Daresbury Synchrotron Radiation Source (SRS). The beamline and monochromator a r e d e s c r i b e d i n d e t a i l by MacDowell e t a l . [ I ] .

(''present address : Department of Pure and Applied Chemistry. University of Strathclyde.

GB-Glasgow G1 1XL. Great-Britain

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986895

C8-5

10 JOURNAL DE PHYSIQUE

The experiments were c a r r i e d o u t i n a c o n v e n t i o n a l , s t a i n l e s s s t e e l u l t r a - h i g h - vacuum chamber, w i t h a base p r e s s u r e of 5 x 10-l1 mbar. The c r y s t a l s were c l e a n e d by argon i o n bombardment and anneal c y c l e s u n t i l no contaminants were e v i d e n t i n t h e Auger s p e c t r a and s h a r p (1x1 ) Low Energy E l e c t r o n D i f f r a c t i o n (LEED) p a t t e r n s were observed. The c r y s t a l was dosed with c h l o r i n e u s i n g an e l e c t r o l y t i c source, and with caesium u s i n g a g e t t e r source.

The measurements were made by c o l l e c t i n g t h e t o t a l secondary e l e c t r o n y i e l d by monitoring t h e d r a i n c u r r e n t l e a v i n g a tungsten mesh, p l a c e d a t a d i s t a n c e of 3 an

from t h e sample. The b u l k d a t a used t o o b t a i n c o r r e c t phase s h i f t s were o b t a i n e d b y mounting a s e r i e s of model compounds i n t h e same vacuum system, and measuring t h e bulk EXAFS by t h e same method. The s e r i e s of model compounds included: AgC1, AgBr, CsC1, C s I , CsBr, and Ag2S. A l l d a t a were taken a t 110 K, where d e v i a t i o n from t h e simple harmonic approximation i s n e g l i g i b l e [ 2 ] .

The d a t a were analysed using t h e Daresbury-based EXCURVE [31 program, which u s e s t h e

'

curved wave' s c a t t e r i n g d e s c r i p t i o n [4

I .

RESULTS

Chlorine adsorbed on Ag( 1 1 1 )

SEXAFS d a t a were taken f r a n C 1 adsorbed on A ~ { 11 1

]

a t coverages of 0.4 and 0.7 monolayers ^' (where a monolayer i s d e f i n e d a s t h e same s u r f a c e c o n c e n t r a t i o n a s a close-packed ~ g { f l l } p l a n e ) . A complete s t r u c t u r a l d e t e r m i n a t i o n f o r t h e h i g h e r

Fig.l ( a ) The f u l l l i n e shows t h e k3 weighted, back transformed experimental f r e - quency spectrum ( t o i n c l u d e r e a l space components up t o 5.0 8 ) t o g e t h e r with t h e t h e o r e t i c a l m u l t i s h e l l f i t (broken l i n e ) , of t h e phase observed a t a coverage of 0.7 monolayers of ~ l / ~ g { l l l } .

( b ) Experimental and t h e o r e t i c a l F o u r i e r t r a n s f o r m s of ( a ) .

coverage phase i s published elsewhere [ 5 ] . A LEED I ( V ) a n a l y s i s i s n o t a v i a b l e technique f o r e i t h e r of t h e s e phases, s i n c e no long-range o r d e r e x i s t s f o r t h e low coverage phase, w h i l s t t h e r e i s o n l y weak o r d e r [7,81 a t t h e higher coverage.

The d a t a f o r t h e high and low coverages a r e p r e s e n t e d in f i g s . 1 and 2, respec- t i v e l y , where t h e k 3 weighted, experimental frequency s p e c t r a ( F o u r i e r f i l t e r e d t o i n c l u d e components o u t t o 5.00 8 ) and t h e corresponding r a d i a l d i s t r i b u t i o n func- t i o n s , a r e compared t o t h e t h e o r e t i c a l f i t s . The n e a r e s t neighbour adsorbate- s u b s t r a t e bond d i s t a n c e d e t e r m i n a t i o n , i n both c a s e s , was 2.70 f 0.01

8.

Fig.2. ( a ) The f u l l l i n e shows t h e k 3 weighted, back transformed experimental frequency spectrum ( t o i n c l u d e r e a l space components up t o 5.0 8 ) t o g e t h e r with t h e t h e o r e t i c a l m u l t i s h e l l f i t ( b r o k e n l i n e ) , of t h e phase observed a t a coverage of 0.4 monolayers of ~ l / ~ g { l l l } . ( b ) Experimental and t h e o r e t i c a l Fourier t r a n s f o r m s of ( a ) .

Chlorine adsorbed on ~ g { 110

1

The measurements were made a t two d i f f e r e n t c h l o r i n e coverages: 0.5 monolayers and 0.75 mono1 a y e r s . Long-range o r d e r i s observed f o r both of t h e s e phases [6,81.

A t a coverage of 0.5 monolayers a simple ~ ( 2 x 2 ) X E D p a t t e r n e x i s t s , w h i l s t a t t h e h i g h e r coverage an incommensurate s t r u c t u r e is observed. ^A model f o r t h e l a t t e r phase i s p r e s e n t e d elsewhere [91.

The experimental and t h e o r e t i c a l frequency s p e c t r a and r a d i a l d i s t r i b u t i o n f u n c t i o n s a r e shown in f i g s . 3 and 4 f o r t h e high and low coverages, r e s p e c t i v e l y . The nearest-neighbour d i s t a n c e was determined t o be 2.59 f 0.04

8

f o r both phases, thus, t h e i n v a r i a n c e with coverage of t h e a d s o r b a t e - s u b s t r a t e d i s t a n c e is v e r i f i e d f o r t h e c h l o r i n e s p e c i e s . A l a r g e e r r o r b a r i s a s s o c i a t e d with t h e s e measurements, compared to t h o s e from c ~ / A ~ { ~ I I } , due to a g l i t c h o c c u r r i n g i n t h e raw s p e c t r a .

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PHYSIQUE

Fig. 3 . ( a ) The f u l l l i n e shows t h e k3 weighted, back transformed experimental frequency spectrum ( t o i n c l u d e r e a l space components up t o 6 . 0

8 )

t o g e t h e r with t h e t h e o r e t i c a l m u l t i s h e l l f i t (broken l i n e ) , of t h e phase observed a t a coverage of 0.75 monolayers of C ~ / A ~ { I I 0 ) . ( b ) Experimental and t h e o r e t i c a l F o u r i e r t r a n s f o r m s o f ( a ) .

Fig.4. ( a ) The f u l l l i n e shows t h e k 3 weighted, back transformed experimental frequency spectrum ( t o i n c l u d e r e a l space components up t o 6 . 0

a )

t o g e t h e r with t h e t h e o r e t i c a l m u l t i s h e l l f i t (broken l i n e ) , of t h e phase observed a t a coverage of 0.5 monolayers, of ~ 1 / ~ ( 1 { 1 1 0 } . ( b ) Experimental and t h e o r e t i c a l F o u r i e r t r a n s f o r m s of ( a ) .

Caesium adsorbed on A g { l l l }

C s on ~ g { l l l } was i n v e s t i g a t e d a t coverages of 0.15 and 0.30 monolayers. The l a t t e r corresponds t o an almost close-packed l a y e r of Cs atoms and t h e raw d a t a from t h i s a r e shown i n f i g . 5.

Photon energy (eV)

Fig.5. A normalised raw s u r f a c e EXAFS spectrum, of t h e caesium Lg edge, from t h e 0.3 monolayer phase on Ag{ll l } .

The f i r s t nearest-neighbour d i s t a n c e f o r t h e h i g h e r coverage phase i s determined t o b e 3.50 i 0.03

A;

w h i l s t t h a t f o r t h e lower coverage phase i f found t o b e

3.20 i 0.3

a.

The v a r i a t i o n i n bond d i s t a n c e i s i n t e r p r e t e d a s t h e t r a n s i t i o n from an

'

i o n i c

'

t o a

'

c o v a l e n t ' n a t u r e of t h e adsorbate- s u b s t r a t e bond with i n c r e a s e i n coverage. Fig. 6 p r o v i d e s a v i s u a l a p p r e c i a t i o n , i n frequency t e r m s , of t h e observed

k ( & - ' I

Fig. 6. Superimposition of t h e f i r s t s h e l l c o n t r i b u t i o n s from both t h e h i g h coverage ( c o v a l e n t ) and low coverage ( i o n i c ) phases of Cs/Ag {I 11

} .

The obvious d i f f e r e n c e in frequency p r o v i d e s a v i s u a l demonstration of t h e change i n bond d i s t a n c e with i n c r e a s i n g coverage.

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bond d i s t a n c e i n c r e a s e . It shows t h e s u p e r i m p o s i t i o n of t h e f i r s t s h e l l contribu- t i o n s from b o t h t h e h i g h and low coverage phases. The overlayed frequency c u r v e s a r e e x t r a c t e d from t h e t h e o r e t i c a l s p e c t r a ( a back-transform of t h e f i r s t peak i n t h e experimental F o u r i e r t r a n s form would a l s o i n c l u d e t h e i n t e r f e r i n g back- s c a t t e r i n g resonance from t h e second s h e l l [ l o ] ) .

It should b e emphasised t h a t a v a r i a t i o n in t h e s u b s t r a t e - a d s o r b a t e bond d i s t a n c e , a s a f u n c t i o n of coverage, h a s never been t r e a t e d i n e i t h e r c l a s s i c a l o r quantum mechanical approaches. This work i n d i c a t e s t h a t t h e r e is, indeed, a change i n this parameter and t h a t f u t u r e t h e o r e t i c a l c a l c u l a t i o n s should i n c l u d e t h i s a s a v a r i a b l e i n a q u a n t i t a t i v e way.

DISCUSSION AND CONCLUSIONS

R e s u l t s f o r

c1

and Cs adsorbed on ~ g { l l l } demonstrate t h e unique c a p a c i t y of SEXAFS t o e l u c i d a t e s u r f a c e s t r u c t u r e s without long-range o r d w . I n p a r t i c u l a r , t h e s e n s i t i v i t y of t h e technique i s i l l u s t r a t e d by t h e d e t e c t i o n of t h e bond d i s t a n c e v a r i a t i o n ( o f 0.30

8 )

when Cs i s adsorbed on ~ g { l l l }

.

Most i m p o r t a n t l y , t h e s t u d i e s h e r e r e v e a l a fundamental d i f f e r e n c e i n behaviour of t h e e l e c t r o p o s i t i v e and e l e c t r o n e g a t i v e s p e c i e s ; t h e adsorbate- s u b s t r a t e d i s t a n c e i s independent of coverage f o r C 1 on both ~ g { l 11) and ~ g { i l o } , w h i l s t t h e r e i s a marked dependence w i t h a d s o r p t i o n of Cs

.

Lang and Williams [12,131 have s t u d i e d t h e a d s o r p t i o n of a s t r o n g l y e l e c t r o p o s i - t i v e , a s t r o n g l y e l e c t r o n e g a t i v e and a c o v a l e n t s p e c i e s ( L i , C1 and S i , r e s p e c t i v e - l y ) adsorbed on a j e l l i u m s u b s t r a t e . The c a l c u l a t e d l o c a l d e n s i t y of s t a t e s f o r t h e t h r e e systems a r e shown i n f i g . 7. For l i t h i u m , t h e a d s o r b a t e resonance i s

i n i t i a l l y mostly unoccupied, s i n c e it l i e s mainly above t h e F e m i l e v e l ; whereas t h e c h l o r i n e resonance is t o t a l l y f i l l e d a s i t l i e s completely below t h e Fermi l e v e l . The s i l i c o n resonance l i e s almost e x a c t l y halfway a c r o s s t h e Fermi l e v e l , e x h i b i t i n g c o v a l e n t c h a r a c t e r , w i t h e q u a l amounts of bonding and anti-bonding c h a r a c t e r .

Considering t h e a l k a l i m e t a l c a s e , a s t h e coverage i n c r e a s e s t h e resonance moves down over t h e F e r m i l e v e l and t h e occupation of i t s resonance i n c r e a s e s ; i . e . , t h e e l e c t r o n t r a n s f e r i s back t o t h e a d s o r b a t e atom. The a l k a l i - s u b s t r a t e bond t h e n becomes more c o v a l e n t , resembling t h a t i l l u s t r a t e d f o r S i i n f i g . 7. However, t h e occupation of t h e c h l o r i n e resonance cannot vary w i t h i n c r e a s e d coverage a s it i s f u l l y occupied from t h e i n i t i a l s t a g e s of a d s o r p t i o n . S i n c e t h e occupation of t h e C s resonance i s v a r y i n g with coverage, t h e e q u i l i b r i u m d i s t a n c e ( f o r m h i m u m energy

Energy relative to vacuum ( e V )

Fig.7 The l o c a l d e n s i t y of s t a t e s a t t h e a d s o r p t i o n s i t e f o r an e l e c t r o n e g a t i v e , e l e c t r o p o s i t i v e and c o v a l e n t s p e c i e s (from r e f . [ 121 )

.

o f t h e a d s o r b a t e ) w i l l a l s o vary. However, s i n c e t h e occupation of t h e C 1 resonance i s i n v a r i a n t with coverage, nothing i s t o b e gained (in terms of energy) by a l t e r i n g i t s d i s t a n c e from t h e s u r f a c e .

ACKNOWLEDGEMENTS

We would l i k e t o thank D r . R.S. Brooks, D r . J.C. Campuzano and D r . S. F e r r e r f o r c o n t r i b u t i n g to t h i s m r k . E x c e l l e n t t e c h n i c a l Support from D r . A.A. ~ c D o w e l l is much a p p r e c i a t e d . The SERC a r e g r a t e f u l l y acknowledged f o r f u n d i n g t h e p r o j e c t and

f o r p o v i d i n g s t u d e n t s h i p s f o r CZL and DJH.

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