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Water adsorption on metal surfaces: an electrochemical viewpoint
Vacuum/volume 31/numbers 10-12/pages 483 to 486/1981
0 0 4 2 - 2 0 7 X / 8 1 / 1 0 0 4 8 3 - 0 4 5 0 2 00/0 (~ 1981 Pergamon Press Ltd
Printed in Great Britain
W a t e r a d s o r p t i o n on m e t a l surfaces: an electrochemical viewpoint J K Sass
and
K Kretzschmar,
Fr/tz-Haber-lnst/tut der Max-Planck-Gesellschaft, Faradayweg 4-6,
D- 1000 Berlin 33, West Germany Inst/tut fur Theoret/sche Phys/k der Freten Umvers/tat Berlin, Armmallee 3, D- 1000 Berlin 33,
S Holloway,
West Germany
Recent results of water adsorption/n ultra-high vacuum, both on clean metal surfaces and/n the presence of a coadsorbate, are compared with classical models of the electric double layer at a metal-electrolyte interface It/s concluded from this comparmon that the electrochemical concepts regarding the mmroscop/c structure of this tmportant boundary requ/re malor rewston, although further adsorption expenments are necessary to construct an improved, alternative model
U~
1. I n t r o d u c t i o n
In the light of the rapid advances which have been m a d e in surface science in the last decade, it has been necessary for electrochemlsts to re-examine their basic ideas r e g a r d m g the solid electrolyte Interface ~ F o r example, the o r i e n t a t i o n of water molecules in the electrolyte layer lmmedtately adjacent to the electrode surface c a n n o t possibly be determined from the m e a s u r e m e n t of such macroscopic quantities as potential a n d current a n d resort to complex models has been necessary 2 In contrast, spectroscop~es developed for the charactertzatton of the s o h d - v a c u u m interface provide the capabfltty for directly determining the c o m p o s i t i o n a n d geometry of species a d s o r b e d on a surface In thts paper, we discuss recent results of water adsorption on metal surfaces in uhv, with particular emphasis being placed on their significance to electrochemistry 2. l n t e r f a c l a l
models
in e l e c t r o c h e m i s t r y
Electrochemical studtes of metal-electrolyte interfaces s are usually performed in a galvantc cell As schematically represented in Figure 1, the working electrode is immersed together with a non-polarizable reference electrode into an electrolyte solution In this configuration the external voltage U appears as a potential d r o p A ~ only at the mterface adjacent to the working electrode The resulting electric double layer consists of excess charge on the metal surface a n d a n a c c u m u l a t i o n of oppositely charged ions in the electrolyte The extension of this layer depends on the electrolyte c o n c e n t r a t i o n a n d can be restricted to wtthm 2 3 A of the electrode in c o n c e n t r a t e d s o l u u o n s ( > 0 5 mole l ~) The enlarged insert in Figure 1 is a sketch of the microscopic structure in th~s region for an aqueous electrolyte, with b o t h the concentration of ions and the o r i e n t a t i o n of water molecules &ffering from
.]
a:
,-+1o/
o"f II~
0.~.
~+
--
0#'
"~
I
w o r k i n g - " ." . b'--° I electrode_ - double ] . . . . Ioyer _
/
.
.
.
.
(~) reference electrode_ electrolyte solution
Figure l Schematic representation of an electrochemical cell. the use of the non-polarizable reference electrode ensures that the apphed cell voltage U drops at the Interface of the working electrode and the electrolyte The enlarged region shows an oversmlphfied picture of the molecular compostuon m the electric double layer where both the concentration of ions and the orlentauon of water molecules differ from the bulk of the electrolyte.
the bulk of the solution The purpose of this oversimplified Illustration is merely to draw a t t e n t i o n to the multitude of competing lnteracttons which occur between the ions, the water molecules and the metal surface The traditional electrochemical a p p r o a c h to the study of double layer properties is based on the analysis of differential capacity vs excess charge relationships 2 The classical experiments by G r a h a m e 4 on mercury electrodes in sodium fluoride solutions 483
J K Sass, K Kretzschmar and S Holloway Water adsorption on metal surfaces an electrochemical wewpomt I
i
E
I
I
cluster IS reduced The total potential d r o p Aq) across the b o u n d a r y is then given by
I
35
=L
4
A@=a/Ko+ ~
g dlp
(2)
i=l
j30 >, o O
25 -6 c
20 c5 15 I
10
I
]
I
0 -10 -20 Excess Chorge o [ g C c m -21
Figure 2 Inner-layer d~fferenUal capacity as a function of excess charge for
mercury electrodes 4 m contact wuh aqueous sodmm fluoride solution at 0°C Note the pronounced asymmetry for posHwe and negatwe charge on the electrode surface
were performed more t h a n two decades ago Figure 2 shows a typical curve with strong asymmetric b e h a v l o u r of the differential capacity with respect to the polarity of the excess charge a on the metal surface The differential capacity is defined as 1 Cd,rr
-
ct(A£b)
(1)
~a
Both A@ a n d a are macroscopic quantities and characterize only some average dielectric b e h a v l o u r of the strongly heterogeneous assembly of ions a n d water molecules in the d o u b l e layer F o r this reason, one c a n n o t be expected to u n d e r s t a n d the microscopic structure of the metal~electrolyte interface using m e a s u r e m e n t s of the differential capacity Despite these limitations, various models for the o r i e n t a t i o n of water molecules at electrode surfaces have been proposed in an a t t e m p t to explain the experimental capacitance data The 'fourstate model' suggested by P a r s o n s 5 may be used to illustrate the principle features of such a p p r o a c h e s The sodium ion a n d the fluoride are assumed to be separated from the metal surface by a m o n o l a y e r of water molecules Further, in concentrated solutions, the interracial potential d r o p is taken to occur across this one layer 6 T h e thickness of the sandwiched water layer then defines the dimensions of a parallel-plate capacitor, with one plate being the metal surface a n d the other being located at the plane where the excess ionic charge is accumulated This capacity is assumed to be a c o n s t a n t Ko i n d e p e n d e n t of charge In addition to the potential d r o p across this model capacitor, c o n t r i b u t i o n s from the variable, c h a r g e - d e p e n d e n t orientations of the water molecules are i n c o r p o r a t e d into the model F o u r distinct states were chosen by P a r s o n s s two m o n o m e r configurations have either the oxygen or h y d r o g e n oriented towards the metal surface a n d the dipole m o m e n t s parallel to the surface n o r m a l , the r e m a i n i n g two states deal with water clusters having dipole orientations similar to the two m o n o m e r states However, the m a g n i t u d e of the dipole m o m e n t per water molecule in the 484
where the subscript t denotes one of the four states and g d,p lS the c o n t r i b u t i o n to A@ from the dipolar o r i e n t a t i o n of water molecules in t h a t state The relative a b u n d a n c e s of clusters and m o n o m e r s are calculated using B o l t z m a n n statistics, with the assumption that the opposing charges on the metal and on the Ions give rise to a h o m o g e n e o u s electric field acting on the dipole m o m e n t s in the water layer In addition, it is assumed that, in order to create the m o n o m e r states from clusters, energy must be invested, the m a g n i t u d e of which depends u p o n the orientation of the m o n o m e r The capacity curve calculated from this model reproduces in considerable detail the shape of the experimental curve in Figure 2 Due to the large n u m b e r of adjustable parameters in the model, however, its usefulness as a microscopic description of the double layer is doubted As recently pointed out by Trasattl-', the differential capacity is a very complex q u a n t i t y and ~ts b e h a v l o u r provides very little direct physical insight into the properties of the double layer at a molecular level The remainder of the paper is devoted therefore to the e x a m i n a t i o n of results obtained in ultrahigh v a c u u m where experimental techmques with great structural sensitivity have been employed The ultimate goal underlying this a p p r o a c h is to o b t a i n ingredients for a model of the double layer which have b o t h stronger experimental a n d theoretical foundations
3. Results
from adsorption
studies
The study of water a d s o r p t i o n u n d e r uhv conditions o n t o well characterized single-crystal metal surfaces has attracted the a t t e n t i o n o f e x p e r l m e n t a h s t s only during the last few years 7 12 In this section, rather t h a n describing the details of the experimental results the emphasis is placed u p o n s u m m a r i z i n g the main conclusions a r n v e d at in these studies, in particular those which may be relevant to electrochemistry The most interesting information on the adsorption b e h a v l o u r of water on clean metal surfaces has come from several recent studies in which vibrational spectroscoples were employed 8 ~0 13 For b o t h r u t h e n i u m 9 13 and p l a t i n u m s lo surfaces it was found that formation of h y d r o g e n - b o n d e d clusters l~ occurs for all coverages in the t e m p e r a t u r e range investigated ( ~ 9 0 150 K) Evidence that these clusters consist of hexagonal rings of water molecules has been provided by the observation of a ( x / 3 × x / 3 ) R 3@ superstructure L E E D pattern over a wide range of coverages 9 The observation of the desorptlon of water at relatively low temperatures ( ~ 150-180 K), indicates the presence of a weak b o n d to the metal surface The a d s o r p t i o n geometry suggested by surface electronic calculations~S 16, namely interaction t h r o u g h the oxygen lone-pair orbitals with the molecular axis perpendicular to the surface, has not been confirmed in any of these investigations There exists the possibility that m o n o m e r a d s o r p t i o n m this configuration occurs only at m u c h lower temperatures where surface diffusion is significantly reduced F o r this reason, experiments where the substrate is cooled with liquid helium would be of great interest
J K Sass, K Kretzschmar and S Holloway Water adsorption on metal surfaces an electrochemmal wewpomt
F r o m an electrochemical point of view, studies of the coadsorptlon of water and ionic species on metal surfaces are of particular significance It has recently been suggested lÈ that in this way, model systems of the electric double layer may be synthesized in uhv Support for this novel concept m a y be found in the emerslon experiments of H a n s e n ~8 which show that in m a n y instances a double layer may be removed from an electrochemical cell without loss of either charge or change in potential drop Relatively few investigations of water a d s o r p t i o n in the presence of other surface species have been reported In these studies it has frequently been observed that water is more tightly b o u n d to metal surfaces in the presence of c o a d s o r b a t e s such as b r o m i n e ~', o x y g e n l l 13 ~9 and sulphur 2° These species are believed to form a surface b o n d with considerable transfer of charge to the metal and may provide strong electrostatic attraction to the water dipoles thus increasing the desorptlon energy In a recent infrared spectroscopic Investigation of water adsorption on Ru(001) in the presence of p r e a d s o r b e d oxygen, the m e c h a n i s m for the increase in the binding energy has been further studied 13 Using the fact that the intensity in the O H - s t r e t c h mode at a frequency of ~ 3 4 0 0 cm 1 is directly p r o p o r t i o n a l to the a m o u n t of h y d r o g e n - b o n d e d water clusters on the surface, the b e h a v l o u r of this a b s o r p t i o n b a n d was studied as a function of the pre-exposure of oxygen In Figure 3 an exposure of 0 5 L 0 2 has >"
1
I
Ru (001)
2~oo
3,.oo ,.ooo wovenumber v(cen-'~
c
////
_/ 0
_
/cleon
/ / /
/ / 0 5 L 02
molecules, thereby preventing their participation in the formation of h y d r o g e n - b o n d e d water clusters At higher coverages this effect becomes less p r o n o u n c e d and disappears completely at i L O2 exposure where one essentially regains the clean surface behavlour S u m m a r i z i n g gas phase adsorption experiments on water so far performed, it may be concluded that water tends to aggregate on clean metal surfaces whereas in the presence of a c o a d s o r b a t e surface solvatlon complexes are formed The orientation of the water molecules in these complexes is as yet u n k n o w n and further studies are required
4. Discussion The orlentatlonal models of the double layer used by electrochemists suffer from two i m p o r t a n t drawbacks There are no physical grounds for assuming the ionic countercharge In the electrolyte to be smeared out as in a parallel plate capacitor Further, the restriction on the allowed orientations of dipole m o m e n t s in the water layer immediately adjacent to the metal surface (up a n d d o w n ) seems u n r e a s o n a b l e I m p r o v e m e n t s on these models must start from the recognition of the actual molecular composition of the layer For example, even at an excess charge of 20 FtC c m - 2 (cf Figure 1) the m e a n separation between the counter-ions is still approximately 10/~ Simple electrostatic considerations imply t h a t at such large separations the local e n v i r o n m e n t in the immediate vicinity of an ion will contrast sharply with those regions in between the ions To a first a p p r o x i m a t i o n , the structure of the water in these latter areas would be expected to resemble that as adsorbed on a clean uncharged metal surface In the n e l g h b o u r h o o d of the counterions, however, strong local electric fields must occur which will result in a modification of the water structure O n the basis of data collected from the gas phase adsorption studies (Section 3) we propose that the structure of water in the regions between the ions will be as shown in Figure 4 Thl~
_.,-/I 1
2
H20 exposure (L) Figure 3 Infrared absorption intensity of the OH-stretch wbrauon m the water adlayer at 85 K on a clean Ru(001 ) surface and with preadsorptlon of 0 5 L O2 for water exposures up to 2 L The insert shows the spectral dependence of absorpnon intensity which is characteristic of hydrogenbonded water clusters In the presence of oxygen on the surface the formauon of these clusters Is delayed (see textl
been chosen to d e m o n s t r a t e that the e n h a n c e d binding of water, as evidenced by thermal desorptlon spectroscopy, is correlated with the f o r m a t i o n of a new water species on the surface The loss of infrared activity at 3 4 0 0 c m l indicates that there is no significant lntermolecular b o n d i n g between water molecules which form a surface complex with the pre-adsorbed oxygen S o m e w h a t surprisingly, we did not observe a new a b s o r p t i o n b a n d at a different frequency for this water species, but this m a y be attributed to the strong variations in a b s o r p t i o n intensity of the OH-stretch m o d e in different e n v i r o n m e n t s 14 21 The specific influence of the c o a d s o r b e d oxygen is most strongly felt at low coverages where it IS estimated from the exposure that one oxygen a t o m is capable of trapping six water
/ Figure 4. Model for the orientation of water on clean metal surfaces In the hexagonal hydrogen-bonded rings three lone-pair orbltals are directed towards the metal surface The structure may be viewed as a twodimensional representauon of the lh-phase of ice configuration is based u p o n a single "sheet' taken from the bulk of the /h-phase of ice, with the additional requirement that three lone-pair orbltals are oriented parallel to the surface n o r m a l O f course, this picture is s o m e w h a t idealized as there will be some degree of disorder and c o n t i n u o u s r e a r r a n g e m e n t within the layer 14 The structure of the water in the vicinity of the ions ~s still a matter of speculation as it is only recently that experiments in the gas phase have addressed this problem Following the nomenclature used in electrochemistry 3, three principal surface solvatlon configurations may be envisaged Figure 5a shows two possible
485
J K Sass, K Kretzschmar and S Holloway Water adsorption on metal surfaces an electrochemical viewpoint
a)
~,
H~O'"
I H/O~H ////////////////
b)
In conclusion, the aim of future studies will be to select from this wide range of possible surface solvatton complexes those which actually occur Clearly, adsorption from the gas phase is the most promaslng technique with which to achieve thas goal
.o.
.....
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./
Acknowledgement
.G. H\^/H "
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References
0jH
I
__-H~o/H /
HII
-/~ .//////////,
I
o\ H( xx
.///
,H
@ /////"/" "///////
/
H /
//////
//////z///.
~ Zl///
Figure 5. Possible geometries for water onentauon m surface solvauon complexes (a) non-specific adsorption of cations, (b) non-specific adsorption of amons, (ct speofic adsorpuon of amons
geometraes for the case of non-specifically a d s o r b e d cations where water resides between the ton and the metal surface The basas for these configurations are models for solvatlon in bulk electrolyte solutions, with the electrode surface provldmg an additional b o u n d a r y condition Similar ideas have been used in Figure 5b to construct possable models for non-specifically adsorbed antonsZ2 23 Finally, in Fagure 5c the case of specific adsorption of anions is considered In this situation the ions have stripped away part of thear solvatton shell m order to permat the formation of a chemlsorptlon b o n d wath the metal surface In conjuncUon w~th Figure 5 tt should be stressed that for the purposes of clarity a planar approx~mahon to the more complex three-dimensional situation has been used
486
The authors would like to thank K H Bennemann and H Gerlscher for useful discussions during the cause of this work In addition, it is a pleasure to acknowledge the helpful co-operaUon of A M Bradshaw
l Proc Int Confon Non-Traditional Approaches to the Solid Electrolyte Interface, Surface Scl 101 (1980) 2S Trasattl, in Modern Aspect~ of Electtodwmtstr). (Edited by B E Conway and J O'M Bockns), vol 13, p 81 Plenum Press, New York (1979) 3 j O'M Bockrls and A K N Reddy, Modern Electrochemtstr> Plenum Press New York (1970) 4 D C Grahame, J Am Chem Soc, 79, 2093 (1957) s R Parsons, J Eledroanal Chem, 59, 229 (1975) 6 D C Grahame, Anal Chem, 30, 1736 (1958) 7 T E Madey and J T Yates, Chem Phy~ Lett, 51, 77 (1977) s H Ibach and S Lehwald, Surjace Sct, 91, 187 (1980) 9 p A Thlel, F M Hoffman and W H Welnberg, Pro¢ ECOSS 3, Suppl Le Vide et Couches Minces, 201,307 (1980) lO B A Sexton, Surface Sct, 94, 435 (1980) 11 G B Fischer and B A Sexton, Ph>s Ret Lett, 44, 683 (1980) 12 C Au, J Breza and M W Roberts, Chem Phys Lett, 66, 340 (1979) 3 K Kretzschmar, J K Sass, P Hofmann, A Ortega, A M Bradshaw and S Holloway, Chem Phys Lett, 78, 410 (1981) 1,* D Elsenberg and W Kauzmann, The Structure and Propertws o/Water, Oxford University Press, London (1969) 15 M A Leban and A T Hubbard, J Electroanal Chem, 74, 253 (1976) 16S Holloway and K H Bennemann, m Proc Int Conf on NonTradlUonal Approaches to the Solid Electrolyte Interface, Sulfate S~t, 101,327 0980) i7 j K Sass, N V Richardson, H Neff and D K Roe, Chem Pllys Lett, 73, 209 (1980) 18 W N Hansen, in Proc Int Confon Non-Traditional Approaches to the Sohd-Electrolyte Interface, Surface Scl, 101, 109 (1980) 19 M Bowler, M A Barteau and R J MadJx, Surface SCl, 92, 528 (1980) 2o N V Richardson, Prwate commumcatlon 21 p A Kollman and L C Allen, J Chern Phvs, 51, 3286 (1969) 22 j E Enderby and G W Nedson, in Water a Comprehensive Treatise (EdJted by F Franks), vol 6, p 1 Plenum Press, New York (1979) 23 D W Wood, m Watel a Comprehenslre Treatise (Edited by F Franks), vol 6, p 279 Plenum Press, New York (1979)
0 0 4 2 - 2 0 7 X / 8 1 / 1 0 0 4 8 3 - 0 4 5 0 2 00/0 (~ 1981 Pergamon Press Ltd
Printed in Great Britain
W a t e r a d s o r p t i o n on m e t a l surfaces: an electrochemical viewpoint J K Sass
and
K Kretzschmar,
Fr/tz-Haber-lnst/tut der Max-Planck-Gesellschaft, Faradayweg 4-6,
D- 1000 Berlin 33, West Germany Inst/tut fur Theoret/sche Phys/k der Freten Umvers/tat Berlin, Armmallee 3, D- 1000 Berlin 33,
S Holloway,
West Germany
Recent results of water adsorption/n ultra-high vacuum, both on clean metal surfaces and/n the presence of a coadsorbate, are compared with classical models of the electric double layer at a metal-electrolyte interface It/s concluded from this comparmon that the electrochemical concepts regarding the mmroscop/c structure of this tmportant boundary requ/re malor rewston, although further adsorption expenments are necessary to construct an improved, alternative model
U~
1. I n t r o d u c t i o n
In the light of the rapid advances which have been m a d e in surface science in the last decade, it has been necessary for electrochemlsts to re-examine their basic ideas r e g a r d m g the solid electrolyte Interface ~ F o r example, the o r i e n t a t i o n of water molecules in the electrolyte layer lmmedtately adjacent to the electrode surface c a n n o t possibly be determined from the m e a s u r e m e n t of such macroscopic quantities as potential a n d current a n d resort to complex models has been necessary 2 In contrast, spectroscop~es developed for the charactertzatton of the s o h d - v a c u u m interface provide the capabfltty for directly determining the c o m p o s i t i o n a n d geometry of species a d s o r b e d on a surface In thts paper, we discuss recent results of water adsorption on metal surfaces in uhv, with particular emphasis being placed on their significance to electrochemistry 2. l n t e r f a c l a l
models
in e l e c t r o c h e m i s t r y
Electrochemical studtes of metal-electrolyte interfaces s are usually performed in a galvantc cell As schematically represented in Figure 1, the working electrode is immersed together with a non-polarizable reference electrode into an electrolyte solution In this configuration the external voltage U appears as a potential d r o p A ~ only at the mterface adjacent to the working electrode The resulting electric double layer consists of excess charge on the metal surface a n d a n a c c u m u l a t i o n of oppositely charged ions in the electrolyte The extension of this layer depends on the electrolyte c o n c e n t r a t i o n a n d can be restricted to wtthm 2 3 A of the electrode in c o n c e n t r a t e d s o l u u o n s ( > 0 5 mole l ~) The enlarged insert in Figure 1 is a sketch of the microscopic structure in th~s region for an aqueous electrolyte, with b o t h the concentration of ions and the o r i e n t a t i o n of water molecules &ffering from
.]
a:
,-+1o/
o"f II~
0.~.
~+
--
0#'
"~
I
w o r k i n g - " ." . b'--° I electrode_ - double ] . . . . Ioyer _
/
.
.
.
.
(~) reference electrode_ electrolyte solution
Figure l Schematic representation of an electrochemical cell. the use of the non-polarizable reference electrode ensures that the apphed cell voltage U drops at the Interface of the working electrode and the electrolyte The enlarged region shows an oversmlphfied picture of the molecular compostuon m the electric double layer where both the concentration of ions and the orlentauon of water molecules differ from the bulk of the electrolyte.
the bulk of the solution The purpose of this oversimplified Illustration is merely to draw a t t e n t i o n to the multitude of competing lnteracttons which occur between the ions, the water molecules and the metal surface The traditional electrochemical a p p r o a c h to the study of double layer properties is based on the analysis of differential capacity vs excess charge relationships 2 The classical experiments by G r a h a m e 4 on mercury electrodes in sodium fluoride solutions 483
J K Sass, K Kretzschmar and S Holloway Water adsorption on metal surfaces an electrochemical wewpomt I
i
E
I
I
cluster IS reduced The total potential d r o p Aq) across the b o u n d a r y is then given by
I
35
=L
4
A@=a/Ko+ ~
g dlp
(2)
i=l
j30 >, o O
25 -6 c
20 c5 15 I
10
I
]
I
0 -10 -20 Excess Chorge o [ g C c m -21
Figure 2 Inner-layer d~fferenUal capacity as a function of excess charge for
mercury electrodes 4 m contact wuh aqueous sodmm fluoride solution at 0°C Note the pronounced asymmetry for posHwe and negatwe charge on the electrode surface
were performed more t h a n two decades ago Figure 2 shows a typical curve with strong asymmetric b e h a v l o u r of the differential capacity with respect to the polarity of the excess charge a on the metal surface The differential capacity is defined as 1 Cd,rr
-
ct(A£b)
(1)
~a
Both A@ a n d a are macroscopic quantities and characterize only some average dielectric b e h a v l o u r of the strongly heterogeneous assembly of ions a n d water molecules in the d o u b l e layer F o r this reason, one c a n n o t be expected to u n d e r s t a n d the microscopic structure of the metal~electrolyte interface using m e a s u r e m e n t s of the differential capacity Despite these limitations, various models for the o r i e n t a t i o n of water molecules at electrode surfaces have been proposed in an a t t e m p t to explain the experimental capacitance data The 'fourstate model' suggested by P a r s o n s 5 may be used to illustrate the principle features of such a p p r o a c h e s The sodium ion a n d the fluoride are assumed to be separated from the metal surface by a m o n o l a y e r of water molecules Further, in concentrated solutions, the interracial potential d r o p is taken to occur across this one layer 6 T h e thickness of the sandwiched water layer then defines the dimensions of a parallel-plate capacitor, with one plate being the metal surface a n d the other being located at the plane where the excess ionic charge is accumulated This capacity is assumed to be a c o n s t a n t Ko i n d e p e n d e n t of charge In addition to the potential d r o p across this model capacitor, c o n t r i b u t i o n s from the variable, c h a r g e - d e p e n d e n t orientations of the water molecules are i n c o r p o r a t e d into the model F o u r distinct states were chosen by P a r s o n s s two m o n o m e r configurations have either the oxygen or h y d r o g e n oriented towards the metal surface a n d the dipole m o m e n t s parallel to the surface n o r m a l , the r e m a i n i n g two states deal with water clusters having dipole orientations similar to the two m o n o m e r states However, the m a g n i t u d e of the dipole m o m e n t per water molecule in the 484
where the subscript t denotes one of the four states and g d,p lS the c o n t r i b u t i o n to A@ from the dipolar o r i e n t a t i o n of water molecules in t h a t state The relative a b u n d a n c e s of clusters and m o n o m e r s are calculated using B o l t z m a n n statistics, with the assumption that the opposing charges on the metal and on the Ions give rise to a h o m o g e n e o u s electric field acting on the dipole m o m e n t s in the water layer In addition, it is assumed that, in order to create the m o n o m e r states from clusters, energy must be invested, the m a g n i t u d e of which depends u p o n the orientation of the m o n o m e r The capacity curve calculated from this model reproduces in considerable detail the shape of the experimental curve in Figure 2 Due to the large n u m b e r of adjustable parameters in the model, however, its usefulness as a microscopic description of the double layer is doubted As recently pointed out by Trasattl-', the differential capacity is a very complex q u a n t i t y and ~ts b e h a v l o u r provides very little direct physical insight into the properties of the double layer at a molecular level The remainder of the paper is devoted therefore to the e x a m i n a t i o n of results obtained in ultrahigh v a c u u m where experimental techmques with great structural sensitivity have been employed The ultimate goal underlying this a p p r o a c h is to o b t a i n ingredients for a model of the double layer which have b o t h stronger experimental a n d theoretical foundations
3. Results
from adsorption
studies
The study of water a d s o r p t i o n u n d e r uhv conditions o n t o well characterized single-crystal metal surfaces has attracted the a t t e n t i o n o f e x p e r l m e n t a h s t s only during the last few years 7 12 In this section, rather t h a n describing the details of the experimental results the emphasis is placed u p o n s u m m a r i z i n g the main conclusions a r n v e d at in these studies, in particular those which may be relevant to electrochemistry The most interesting information on the adsorption b e h a v l o u r of water on clean metal surfaces has come from several recent studies in which vibrational spectroscoples were employed 8 ~0 13 For b o t h r u t h e n i u m 9 13 and p l a t i n u m s lo surfaces it was found that formation of h y d r o g e n - b o n d e d clusters l~ occurs for all coverages in the t e m p e r a t u r e range investigated ( ~ 9 0 150 K) Evidence that these clusters consist of hexagonal rings of water molecules has been provided by the observation of a ( x / 3 × x / 3 ) R 3@ superstructure L E E D pattern over a wide range of coverages 9 The observation of the desorptlon of water at relatively low temperatures ( ~ 150-180 K), indicates the presence of a weak b o n d to the metal surface The a d s o r p t i o n geometry suggested by surface electronic calculations~S 16, namely interaction t h r o u g h the oxygen lone-pair orbitals with the molecular axis perpendicular to the surface, has not been confirmed in any of these investigations There exists the possibility that m o n o m e r a d s o r p t i o n m this configuration occurs only at m u c h lower temperatures where surface diffusion is significantly reduced F o r this reason, experiments where the substrate is cooled with liquid helium would be of great interest
J K Sass, K Kretzschmar and S Holloway Water adsorption on metal surfaces an electrochemmal wewpomt
F r o m an electrochemical point of view, studies of the coadsorptlon of water and ionic species on metal surfaces are of particular significance It has recently been suggested lÈ that in this way, model systems of the electric double layer may be synthesized in uhv Support for this novel concept m a y be found in the emerslon experiments of H a n s e n ~8 which show that in m a n y instances a double layer may be removed from an electrochemical cell without loss of either charge or change in potential drop Relatively few investigations of water a d s o r p t i o n in the presence of other surface species have been reported In these studies it has frequently been observed that water is more tightly b o u n d to metal surfaces in the presence of c o a d s o r b a t e s such as b r o m i n e ~', o x y g e n l l 13 ~9 and sulphur 2° These species are believed to form a surface b o n d with considerable transfer of charge to the metal and may provide strong electrostatic attraction to the water dipoles thus increasing the desorptlon energy In a recent infrared spectroscopic Investigation of water adsorption on Ru(001) in the presence of p r e a d s o r b e d oxygen, the m e c h a n i s m for the increase in the binding energy has been further studied 13 Using the fact that the intensity in the O H - s t r e t c h mode at a frequency of ~ 3 4 0 0 cm 1 is directly p r o p o r t i o n a l to the a m o u n t of h y d r o g e n - b o n d e d water clusters on the surface, the b e h a v l o u r of this a b s o r p t i o n b a n d was studied as a function of the pre-exposure of oxygen In Figure 3 an exposure of 0 5 L 0 2 has >"
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molecules, thereby preventing their participation in the formation of h y d r o g e n - b o n d e d water clusters At higher coverages this effect becomes less p r o n o u n c e d and disappears completely at i L O2 exposure where one essentially regains the clean surface behavlour S u m m a r i z i n g gas phase adsorption experiments on water so far performed, it may be concluded that water tends to aggregate on clean metal surfaces whereas in the presence of a c o a d s o r b a t e surface solvatlon complexes are formed The orientation of the water molecules in these complexes is as yet u n k n o w n and further studies are required
4. Discussion The orlentatlonal models of the double layer used by electrochemists suffer from two i m p o r t a n t drawbacks There are no physical grounds for assuming the ionic countercharge In the electrolyte to be smeared out as in a parallel plate capacitor Further, the restriction on the allowed orientations of dipole m o m e n t s in the water layer immediately adjacent to the metal surface (up a n d d o w n ) seems u n r e a s o n a b l e I m p r o v e m e n t s on these models must start from the recognition of the actual molecular composition of the layer For example, even at an excess charge of 20 FtC c m - 2 (cf Figure 1) the m e a n separation between the counter-ions is still approximately 10/~ Simple electrostatic considerations imply t h a t at such large separations the local e n v i r o n m e n t in the immediate vicinity of an ion will contrast sharply with those regions in between the ions To a first a p p r o x i m a t i o n , the structure of the water in these latter areas would be expected to resemble that as adsorbed on a clean uncharged metal surface In the n e l g h b o u r h o o d of the counterions, however, strong local electric fields must occur which will result in a modification of the water structure O n the basis of data collected from the gas phase adsorption studies (Section 3) we propose that the structure of water in the regions between the ions will be as shown in Figure 4 Thl~
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2
H20 exposure (L) Figure 3 Infrared absorption intensity of the OH-stretch wbrauon m the water adlayer at 85 K on a clean Ru(001 ) surface and with preadsorptlon of 0 5 L O2 for water exposures up to 2 L The insert shows the spectral dependence of absorpnon intensity which is characteristic of hydrogenbonded water clusters In the presence of oxygen on the surface the formauon of these clusters Is delayed (see textl
been chosen to d e m o n s t r a t e that the e n h a n c e d binding of water, as evidenced by thermal desorptlon spectroscopy, is correlated with the f o r m a t i o n of a new water species on the surface The loss of infrared activity at 3 4 0 0 c m l indicates that there is no significant lntermolecular b o n d i n g between water molecules which form a surface complex with the pre-adsorbed oxygen S o m e w h a t surprisingly, we did not observe a new a b s o r p t i o n b a n d at a different frequency for this water species, but this m a y be attributed to the strong variations in a b s o r p t i o n intensity of the OH-stretch m o d e in different e n v i r o n m e n t s 14 21 The specific influence of the c o a d s o r b e d oxygen is most strongly felt at low coverages where it IS estimated from the exposure that one oxygen a t o m is capable of trapping six water
/ Figure 4. Model for the orientation of water on clean metal surfaces In the hexagonal hydrogen-bonded rings three lone-pair orbltals are directed towards the metal surface The structure may be viewed as a twodimensional representauon of the lh-phase of ice configuration is based u p o n a single "sheet' taken from the bulk of the /h-phase of ice, with the additional requirement that three lone-pair orbltals are oriented parallel to the surface n o r m a l O f course, this picture is s o m e w h a t idealized as there will be some degree of disorder and c o n t i n u o u s r e a r r a n g e m e n t within the layer 14 The structure of the water in the vicinity of the ions ~s still a matter of speculation as it is only recently that experiments in the gas phase have addressed this problem Following the nomenclature used in electrochemistry 3, three principal surface solvatlon configurations may be envisaged Figure 5a shows two possible
485
J K Sass, K Kretzschmar and S Holloway Water adsorption on metal surfaces an electrochemical viewpoint
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In conclusion, the aim of future studies will be to select from this wide range of possible surface solvatton complexes those which actually occur Clearly, adsorption from the gas phase is the most promaslng technique with which to achieve thas goal
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Acknowledgement
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References
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Figure 5. Possible geometries for water onentauon m surface solvauon complexes (a) non-specific adsorption of cations, (b) non-specific adsorption of amons, (ct speofic adsorpuon of amons
geometraes for the case of non-specifically a d s o r b e d cations where water resides between the ton and the metal surface The basas for these configurations are models for solvatlon in bulk electrolyte solutions, with the electrode surface provldmg an additional b o u n d a r y condition Similar ideas have been used in Figure 5b to construct possable models for non-specifically adsorbed antonsZ2 23 Finally, in Fagure 5c the case of specific adsorption of anions is considered In this situation the ions have stripped away part of thear solvatton shell m order to permat the formation of a chemlsorptlon b o n d wath the metal surface In conjuncUon w~th Figure 5 tt should be stressed that for the purposes of clarity a planar approx~mahon to the more complex three-dimensional situation has been used
486
The authors would like to thank K H Bennemann and H Gerlscher for useful discussions during the cause of this work In addition, it is a pleasure to acknowledge the helpful co-operaUon of A M Bradshaw
l Proc Int Confon Non-Traditional Approaches to the Solid Electrolyte Interface, Surface Scl 101 (1980) 2S Trasattl, in Modern Aspect~ of Electtodwmtstr). (Edited by B E Conway and J O'M Bockns), vol 13, p 81 Plenum Press, New York (1979) 3 j O'M Bockrls and A K N Reddy, Modern Electrochemtstr> Plenum Press New York (1970) 4 D C Grahame, J Am Chem Soc, 79, 2093 (1957) s R Parsons, J Eledroanal Chem, 59, 229 (1975) 6 D C Grahame, Anal Chem, 30, 1736 (1958) 7 T E Madey and J T Yates, Chem Phy~ Lett, 51, 77 (1977) s H Ibach and S Lehwald, Surjace Sct, 91, 187 (1980) 9 p A Thlel, F M Hoffman and W H Welnberg, Pro¢ ECOSS 3, Suppl Le Vide et Couches Minces, 201,307 (1980) lO B A Sexton, Surface Sct, 94, 435 (1980) 11 G B Fischer and B A Sexton, Ph>s Ret Lett, 44, 683 (1980) 12 C Au, J Breza and M W Roberts, Chem Phys Lett, 66, 340 (1979) 3 K Kretzschmar, J K Sass, P Hofmann, A Ortega, A M Bradshaw and S Holloway, Chem Phys Lett, 78, 410 (1981) 1,* D Elsenberg and W Kauzmann, The Structure and Propertws o/Water, Oxford University Press, London (1969) 15 M A Leban and A T Hubbard, J Electroanal Chem, 74, 253 (1976) 16S Holloway and K H Bennemann, m Proc Int Conf on NonTradlUonal Approaches to the Solid Electrolyte Interface, Sulfate S~t, 101,327 0980) i7 j K Sass, N V Richardson, H Neff and D K Roe, Chem Pllys Lett, 73, 209 (1980) 18 W N Hansen, in Proc Int Confon Non-Traditional Approaches to the Sohd-Electrolyte Interface, Surface Scl, 101, 109 (1980) 19 M Bowler, M A Barteau and R J MadJx, Surface SCl, 92, 528 (1980) 2o N V Richardson, Prwate commumcatlon 21 p A Kollman and L C Allen, J Chern Phvs, 51, 3286 (1969) 22 j E Enderby and G W Nedson, in Water a Comprehensive Treatise (EdJted by F Franks), vol 6, p 1 Plenum Press, New York (1979) 23 D W Wood, m Watel a Comprehenslre Treatise (Edited by F Franks), vol 6, p 279 Plenum Press, New York (1979)