Structures of Electrode Surfaces Studied by In Situ Surface X-ray Diffraction: The New Structures of Water Adsorbed on Au(111) Electrode Surface

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Figure 1 T h e a d s o r p t i o n o f a w a t e r m o l e c u l e a n d co a d so rp ti o n o f a w a te r mo l e cu l e w i th e l e ct ro l yt e ions on a metal electrode surface is a fundamental i s s u e o f e l e c t r o c h e m i c a l s u r f a c e s c i e n c e . S o f a r , t h e r o l e o f t h e h y d r a t i o n w a t e r m o l e c u l e i n e l e c t r o d e r e a c t i o n s o n a n e l e c t r o d e s u r f a c e remains unclear. The two-dimensional structure of water molecules on a surface gives an insight into i m p o r t a n t m e c h a n i s m s c o n c e r n i n g e l e c t r o n a n d p r o t o n t r a n s f e r o r w e a k i n t e r a c t i o n s o f w a t e r molecules with surfaces. Recent studies of water a d s o r p t i o n o n m e t a l s u r f a c e s a r e a l m o s t e n t i r e l y l i m i t e d t o t h e a d s o r p t i o n o f w a t e r u n d e r U H V conditions. In situ observation of water adsorption on a real electrode surface is quite difficult due to t h e h i n d r a n c e o f electrolyte solution on the e l e c t r o d e s u r f a c e . T o u n d e r s t a n d t h e r o l e o f h y d r a t i o n w a t e r m o l e c u l e s o n e l e c t r o c h e m i c a l d o u b l e l a y e r s o n a n e l e c t r o d e s u r f a c e , i t i s imperative to reveal the structure of hydration water molecules in the double layers under an electrode potential control. It is well known that water forms a tetrahedral network structure not only in ice (solid), but also in l i q u i d ( s o l u t i o n ) p h a s e s . T h e d e n s i t y o f t h e s e p h a s e s i s a p p r o x i m a t e l y o n e u n i t . R e c e n t l y , however, the existence of a high-density water (1.5 - 2.0) has been observed for a water structure on an electrode surface [1]. Thus, it might be possible t o f i n d a n e w t w o - d i m e n s i o n a l l y e x t e n d e d w a t e r p h a s e o n a m e t a l e l e c t r o d e s u r f a c e . R e c e n t p r o g r e s s w i t h s y n c h r o t r o n r a d i a t i o n h a s m a d e i t p o s s i b l e t o o b s e r v e s t r u c t u r e s e x t e n d e d o n surfaces or interfaces by two - or three - dimensional s t r u c t u r e a n a l y s i s , a n d w e h a v e a p p l i e d t h i s s u r f a c e X - r a y d i f f r a c t i o n t e c h n i q u e t o o n e o f t h e m o s t i m p o r t a n t s y s t e m s o f e l e c t r o c h e m i s t r y : t h e u n d e r p o t e n t i a l d e p o s i t i o n ( U P D ) o f c o p p e r o n a n Au(111) electrode surface. shows a cyclic v o l t a m m o g r a m ( C V , c u r r e n t - p o t e n t i a l c u r v e ) as so ci at ed wi th co pp er de po si ti on on an Au (1 11 ) e l e c t r o d e s u r f a c e i n H 2 S O 4 s o l u t i o n . I n g e n e r a l , metal monolayer deposition was observed to occur a t a c e r t a i n “ u n d e r v o l t a g e ” w i t h r e s p e c t t o t h e equilibrium Nernst potential of the reaction M(z+) + ze - → M. He re , M( z+ ) de si gn at es a me ta l io n in Au Cu (H) SO 4 H 2 O 250 A B C I a II b II a 50 μ A 2mm 2mm 500 750 1000 Potential ( mV vs. SHE) Fig. 1. CV of an Au(111) electrode in 0.5 M H 2 SO 4 + 1 mM CuSO 4 at a scan rate of 50 mV /s. In situ S T M i m a g e s a t t h e A r a n g e ( 0 . 3 5 V R H E ) a n d C range (1.20 V, RHE ) of adsorbed molecules on the Au(111) electrode in 0.5 M H 2 SO 4 + 1 mM CuSO 4 solution are shown in the inset. STM (A range): in situ STM of sulfate + UPD copper adsorbed on the A u ( 1 1 1 ) e l e c t r o d e s u r f a c e . I t = 2 . 8 n A , V bi as = –65.6 mV. STM (C range): in situ STM of HSO 4 – + H 5 O 2 + adsorbed on the Au(111) electrode surface. I t = 4.2 nA, V bias = –967.2 mV . S t r u c t u r e s o f E l e c t r o d e S u r f a c e s S t u d i e d b y I n S i t u S u r f a c e X - r a y D i f f r a c t i o n : N e w S t r u c t u r e s o f W a t e r A d s o r b e d o n Au ( 1 1 1 ) E l e c t r o d e S u r f a c e I b 61 Fig. 2. ORTEP drawing of the structure based o n t h e f i n a l p o s i t i o n a l p a r a m e t e r s a n d temperature factors for Cu UPD on the Au(111) e l e c t r o d e i n 0 . 5 M H 2 S O 4 + 1 m M C u S O 4 solution. Electrode potential was 0.32 V ( RHE ). F i g u r e s 2 3 Fig. 1 F i g . 1 solution, and M, a metal atom in a lattice position o n a s u r f a c e . I t w a s r e c o g n i z e d t h a t t h e u n d e r v o l t a g e r e s u l t s e n e r g e t i c a l l y f r o m t h e contribution of the heat of adsorption of the metal a t o m s o n t h e i n e r t s u b s t r a t e m e t a l e l e c t r o d e surface on the energy of activation of the discharge r e a c t i o n . T h e c u r r e n t p e a k s , I I a a n d I I b , a r e a t t r i b u t e d t o a d s o r p t i o n o r d e p o s i t i o n o f C u a n d d e s o r p t i o n o r s t r i p p i n g o f C u 2 + i n h o n e y c o m b c o p p e r a t o m s ( 2 / 3 c o v e r a g e ) , r e s p e c t i v e l y . T h e pe ak s, I a an d I b, ap pe ar to be th os e of ho ne yc om b- c e n t e r e d c o p p e r a t o m s f o r 1 / 3 c o v e r a g e , r e s p e c t i v e l y . We have determined the structures in those three p o t e n t i a l r a n g e s a t A ( m o r e n e g a t i v e t h a n I a ) , B (between I a and II a ) and C (more positive than II a ). T h e i n s i t u s p e c u l a r X - r a y r e f l e c t i v i t y measurements were carried out using a multi-axis diffractometer installed on beamlines BL09XU and BL 13 XU . Th e wa ve le ng th of X- ra y ra di at io n wa s 0.130 nm. The total number of unique reflections obs erv ed for the sym met ry ( P31 m ) was 53 at the p o t e n t i a l o f 0 . 3 2 V ( R H E : R e v e r s i b l e H y d r o g e n Electrode) where UPD copper forms a honeycomb s t r u c t u r e . T h e f i n a l R - f a c t o r c o n v e r g e d t o 8 . 8 % . a n d s h o w t h e C u U P D h o n e y c o m b structure formed on the Au(111) electrode in 0.5 M H 2 S O 4 + 1 m M C u S O 4 s o l u t i o n a t 0 . 3 2 V i n t h e po te nt ia l ra ng e B. We su cc ee de d in l oc at in g th e h y d r a t i o n w a t e r m o l e c u l e s o n t o p o f e a c h U P D co pp er at om , so th at a 1 × 1 cl os es t - pa ck ox yg en a d l a y e r d o m i n a t e s o n t h e U P D c o p p e r A u ( 1 1 1 ) surface. The honeycomb copper layer on Au(111) does n o t s h o w a m e t a l l i c s t a t e . T h e d i f f e r e n c e i n d i a m e t e r b e t w e e n c o p p e r ( 0 . 2 5 6 n m ) a n d g o l d ( 0 . 2 8 8 n m ) c a u s e s a g a p i n t h e c o p p e r - c o p p e r d i s t a n c e o f t h e h o n e y c o m b s t r u c t u r e . T h e hydration water that is formed on copper is a new phase of water with a high density. The hydration water phase consists of a 1 × 1 -O structure on the Au(111) surface, 1/3 of which is an oxygen atom of s u l f a t e O a n d 2 / 3 o f w h i c h i s h y d r a t i o n w a t e r m o l e c u l e o n e a c h c o p p e r a t o m . T h e d i s t a n c e ( O H … O ) , 0 . 2 8 8 n m , i s t y p i c a l o f t h e h y d r o g e n b o n d i n g v a l u e o f O … O s e e n i n a n i c e s t r u c t u r e under high pressure. The important point is that all o f t h e o x y g e n a t o m s f o r m a c o p l a n a r s t r u c t u r e , which is in remarkable contrast to an ice structure. T h e i c e s t r u c t u r e s h a v e t h e c o m m o n f e a t u r e o f t e t r a h e d r a l b o n d i n g o f w a t e r o x y g e n a t o m s . I n c o n t r a s t , t h e n e w w a t e r p h a s e i s f o r m e d b y a c l o s e s t - p a c k e d o x y g e n s t r u c t u r e w i t h a c o p l a n a r layer. In region C , with a more positive potential than t h e s m a l l d i p i n C V a t 9 4 0 m V , t h e i n s i t u S T M image shows a 3 × 7 structure as seen in the inset in . Intensities in a non-integer reflection ( 1 / 5 , 1 / 5 , 0 . 2 ) a n d i t s e q u i v a l e n t r e f l e c t i o n s , d e v e l o p e d i n r e g i o n C , m e a s u r e t h e g r o w t h o f 3 × 7 s t r u c t u r e d o m a i n s o f b i s u l f a t e a n i o n o n t h e s u r f a c e . T h e b a l l m o d e l i n s h o w s t h e b e s t f i t o f t h e o b s e r v a t i o n . T h e b r i g h t a n d t h e dimmer spots in the STM image correspond to the H S O 4 – a n i o n a n d p r o t o n d i m e r H 5 O 2 + m o l e c u l e s , r e s p e c t i v e l y . T h e r e e x i s t a n u m b e r o f h y d r o g e n b o n d i n g p a i r s b e t w e e n a n i o n s a n d w a t e r m o l e c u l e s . T h e i d e a l a n d m o s t s t a b l e O H … O h y d r o g e n b o n d i n g d i s t a n c e o f 0 . 2 8 0 n m m a t c h e s 62 A u C u S O O H o r H 2 O Fig. 3. Schematic model (top and side view) o f U P D c o p p e r , h y d r a t i o n w a t e r m o l e c u l e an d su lf at e an io n ad so rb ed on an Au (1 11 ) electrode at the potential, 0.32 V ( RHE ). Fig. 2 t h e s u b s t r a t e A u l a t t i c e u n i t , y i e l d i n g a h i g h d e g r e e o f s t a b i l i z a t i o n d u e t o t h e h y d r o g e n b o n d n e t w o r k f o r m a t i o n . I n t h e b a l l m o d e l , ox yg en at om s oc cu py th e to p su rf ac e of go ld a t o m s w h i c h e x h i b i t c l o s e s t p a c k i n g a c c o m m o d a t i o n o f o x y g e n , a l t h o u g h t h e oxygen is not coplanar in this case. I n r e g i o n A , w h i c h h a s a m o r e n e g a t i v e potential than the current peak I a in CV, where 1 × × × 1 f u l l m o n o l a y e r c o v e r a g e o f c o p p e r o r f u r t h e r o v e r l a y e r s ( O P D ) o f c o p p e r a r e d e p o s i t e d o n t h e A u ( 1 1 1 ) e l e c t r o d e s u r f a c e , h i g h - i n t e n s i t y a t ( 1 / 3 , 1 / 3 , 1 . 5 ) r e f l e c t i o n continues to evolve even at the potential more n e g a t i v e t h a n 1 0 0 m V , w h e r e m u l t i l a y e r s o f c o p p e r d e p o s i t e p i t a x i a l l y o n A u ( 1 1 1 ) . T h e f u r t h e r - e n r i c h e d i n t e n s i t y o f t h e r e f l e c t i o n a t p otential s more negative than 100 mV indicates t h a t h y d r a t i o n w a t e r a n d s u l f a t e a n i o n s s t i l l f o r m t h e s i m i l a r 3 7 st ru ct ur e sh ow n in on 1 1 epitaxially grown copper layers on the Au(111) electrode. We ha ve re ce nt ly ob se rv ed zi nc UP D on Au(111) in 0.1 M KH 2 PO 4 and 1 mM Zn ( ClO 4 ) 2 s o l u t i o n . W h i l e t h e s u r f a c e s t r u c t u r e o f t h e A u ( 1 1 1 ) e l e c t r o d e i n t h e t o p m o s t l a y e r o f copper UPD is the same as a bulk structure(1 × 1), and lifting of a surface reconstruction takes place at a surface, s urface reconstruction of the A u ( 1 1 1 ) e l e c t r o d e c o n t i n u e s t o o c c u r i n t h e c a s e o f z i n c U P D a s s o c i a t e d w i t h a z i n c p h o s p h a t e f o r m a t i o n ( Z n - U P D ) , e v e n i n v e r y n e g a t i v e e l e c t r o d e p o t e n t i a l r a n g e s [ 3 ] . T h i s i n d i c a t e s t h a t c o p p e r s u l f a t e w i t h a c l o s e s t - packed hydration water sheet causes a strong interaction with the Au(111) electrode, whereas t h e Z n - p h o s p h a t e s u r f a c e c o m p l e x i s o n l y w e a k l y a d s o r b e d o n t h e A u ( 1 1 1 ) e l e c t r o d e . Consequently, the top layer gold surface is only weakly affected by the overlayer Zn-phosphate complex, resulting in the occurrence of surface reconstruction. References [1] M.F. Toney et al. , Nature 368 (1994) 444. [ 2 ] M . N a k a m u r a e t a l . , S u r f a c e S c i e n c e 5 1 4 (2002) 227. [3] M. Naka mura , K. Mats unag a, K. Kita hara , M. Ito and O. Sakata, to be published in J. Electroanal. Chem. Masashi Nakamura and Masatoki Ito Keio University E-mail: masatoki @ chem.keio.ac.jp 63 Au Cu HSO H O 4 2 1.92(10)Å 2.41(3)Å 3.24(3)Å 2.40(1)Å 2.30(1)Å