- Email: [email protected]
The dependence of the oxygen—metal distance for water adsorbed at a metal electrode
Volume 216, number
3,4,5,6
CHEMICAL
PHYSICS
3 1 December
LETTERS
The dependence of the oxygen-metal adsorbed at a metal electrode
1993
distance for water
Wolfgang Schmickler Abteilung Elektrochemie, Universitdt Urn, D-89069 Ulm. Germany
Douglas Henderson
’ and Owen R. Melroy
IBMResearch Division, Almaden Research Center, 650 Harry Road, San Jose, CA 95120, USA Received
2 1 April 1993; in final from 19 October
1993
It has been observed in SEXAFS studies that the oxygen-metal distance for water or an electrolyte adsorbed at an electrode, formed by a monolayer of lead deposited on silver, becomes increasingly dilated as the electrode becomes more negative, whereas this distance is unchanged when the electrode is formed by a monolayer of silver deposited on gold. It is suggested that this difference may be due to the fact that the potential is positive of the point of zero charge in the latter case. As a result, the oxygenmetal distance is determined by the hard core repulsion of the metal ions on the electrode surface which we approximate as a hard wall. In contrast, the potential is negative in the former case, and the conduction electrons in the metal penetrate into the electrolyte and push the water away from the metal. The distance the oxygen atoms are pushed from the metal surface increases as the potential becomes more negative. A simple calculation based on the jellium model of a metal leads to reasonable agreement with experiment.
1. Introduction In recent years, a variety of experimental techniques has been developed which are capable of probing the structure of electrode surfaces, in situ and on an atomic scale. These have been applied to study the structure of well-defined metal electrodes [ 11, potential induced reconstructions [ 21, and the structure of adsorbed metal [ 3-7 ] and halide monolayers [ 81. However, most of these techniques are not well suited to probing the detailed structure of the electrolyte side of the electrochemical interface. Consequently, our knowledge of the structure of an electrolyte at an electrochemical interface rests mainly upon the interpretation of capacity data. In surface extended X-ray adsorption fine structure (SEXAFS) studies of metal monolayers deposited at an underpotential on silver and gold sub’ Present address: Departamento t6noma Metropolitana/Iztapalapa, Mexico DF, Mexico.
424
de Fisica, Universidad AuApdo Postal 55-534, 09340
0009-2614/93/$
strates, backscattering from oxygen has been observed [4-71. From this it has been concluded that either water or the supporting electrolyte is not only adsorbed on the surface but that it is adsorbed at welldefined distances. The dependence of this distance between the adsorbed water or electrolyte and the metal monolayer on the applied potential has been examined, in a limited fashion, in two recent studies [ $71. For a monolayer of lead adsorbed on silver ( Ill), the lead-oxygen distance was found to dilate with decreasing potential from 2.33 8, at -0.53 V (versus Ag/AgCl) to 2.38 8, at - 1 V. A subsequent study of underpotentially deposited silver on gold ( 111) showed no dependence of the observed silver-oxygen distance on applied potential with measurements taken at +0.7, 0.5, and - 0.1 V (versus Ag/AgCl). As the authors stated in the latter paper, the reason for the difference between the two systems is not understood. We believe, and will assume here, that the observed oxygen signal is caused by water, for three reasons: ( 1) The surface excess one would expect for 06.00 0 1993 Elsevier Science Publishers
B.V. All rights reserved.
SSDZ OOOOS-2614(93)El306-2
Volume 216, number
3,4,5,6
CHEMICAL
PHYSICS
an adsorbed anion is too small to account for the magnitude of backscattering observed from oxygen. (2) This oxygen is also seen at potentials negative of the point of zero charge (PZC), where the adsorption of anions is unlikely. (3) Surface water was clearly seen in a recent X-ray diffraction and reflectivity study of the Au ( 111 )/aqueous electrolyte interface [ 21. The data could be fitted to a gold-water distance of 2.9rt0.3 A, which did not change with potential within the experimental error. The change in the metal-oxygen distance observed in the SEXAFS studies can then be understood in terms of the jellium model of a metal. At potentials negative to the PZC, the conduction electrons penetrate the solution, pushing the water away from the surface w’ h decreasing potential. At po4t tentials positive to Ithe PZC, the conduction electrodes recede into t# metal and the distance of closest approach of thei water or ion is limited by the surface formed by the metal ions in the electrode which we treat as a~hard wall. The same reasoning can be used if the observed oxygen belongs to a physisorbed anion, whiles for chemisorption the situation is more complicated. We note that this penetration of the electrons into he solution, or their retreat into the metal, has been, Itused to provide a quantitative explanation of the capacitance of electrochemical interfaces (for recent reviews of this subject, see ref.
LETTERS
3 1 December
1993
profile n(x) of the electrons must be known. In this Letter, we use the parameterization introduced by Smith [ 10 ] and Partenskii and Smorodinskii [ 111, n(x)=n+-(tn++aq/2eo)eM, n(~)=(fn+-aq/2e~)e-~“,
forxt0, forx>O,
(1)
where x is the perpendicular distance from the jellium edge, located at x=0, n, is the electronic density in the bulk, q is the charge density on the metal surface and e, is the unit charge. The parameter a! is the inverse of the decay length, and is usually determined by minimization of the surface energy. However, in this calculation we find that our result for the distance of closest approach of the adsorbed water is independent of (Y. As a result, we need not determine CY.When there is no charge on the electrode, the density profile is symmetric and the value x0 for which n ( x0) = 4n + is zero. As the electrode is negatively charged, the electrons penetrate further into the solution and the value of x0 increases. On the other hand, as the electrode is positively charged, the electrons recede into the metal and the distance of closest approach is determined not by the conduction electrons but by the jellium edge. Let us calculate x0; substituting its definition into eq. ( 1) gives
(2)
[91).
If q is not too large, i.e. if aq/eo n, K 1, this becomes 2. Theory x0
The jellium model of a metal treats the metal as a combination of a smeared out continuum of positive charge representing the metal ions and an inhomogeneous electron gas representing the conduction electrons. At an interface the continuum of positive charge is assumed to terminate abruptly, forming a step function. We refer to this abrupt termination or edge as the jellium edge. We treat the interface formed by ~this jellium edge as a hard wall. The density of the free conduction electrons is assumed to change continuously from its bulk value to zero. This model oif the metal interface has been widely applied to metal/vacuum and to metal/electrolyte interfaces. ~ To apply this model of the interface, the density
=
-
4
(3)
eon+
which is independent of cr. For a negative charge density we take x0, calculated from (3), as the dilation A of the water-surface distance. As we have already mentioned, for a positive charge the distance of closest approach of the water is determined by the jellium edge and there is no change in the water-surface distance.
3. Results Since the surface charge density q is the natural variable for theoretical calculations, we have converted the potentials at which the oxygen-metal dis425
Volume 216, number 3,4,5,6
CHEMICAL PHYSICS LETTERS
tances were measured to charge densities. For this purpose we require the values of the PZC and the capacities of the systems investigated. Lead on Ag( 111) forms a monolayer with a Pb ( 111) structure which is slightly compressed with respect to the surface of a bulk lead crystal. Consequently, the PZC of Pb/Ag( 111) should be close to that of Pb ( 111), which is at - 0.62 versus NHE [ 121, and we took this value. Capacity data for single crystal lead are not available, so we took data for polycrystalline electrodes from ref. [ 131, which we had to extrapolate to higher charge densities. This procedure introduces some uncertainty in the experimental values for q, but this does not affect our conclusions. Fig. 1 shows the results for Pb/Ag( 111); there are two experimental data points, one on each side of the PZC. We have assumed that the water-metal distance does not change at positive charges - which is true for Ag/Au ( 111) - and taken this value as our reference, i.e. A is the oxygen-metal distance at negative charges minus that at positive charges. Our theoretical curve gives a good estimate of the experimental value at negative charges, even though it is based on a simple, approximate calculation. Silver forms a dense commensurate monolayer on Au ( 111) , since the lattice constants of silver and gold are close, and has the Ag( 111) structure. We took the corresponding value for the ZPC ( - 0.5 1 versus NHE [ 12 ] ) . Capacity data for Ag ( 111) are given in
31 December 1993
ref. [ 14 ] ; they cover a smaller range than we require,
so again we had to extrapolate the experimental data in order to convert potentials to charge densities. Since the oxygen-metal distance is constant for this system, the uncertainties in the experimental charge densities do not matter. There are three experimental points, all at positive charge densities, and the shift A is zero in each case (see fig. 2 ) . The variation A of the water-metal distance with the charge density q plays an important role in several contemporary models of the metal-solution interface. Kornyshev and Urbakh [ 15 ] have calculated values for a polycrystalline lead electrode from electroreflectance data. They assume that only the water-metal distance changes with the surface charge density, while the optical constants of the interface remain constant. We have included their calculations in fig. 1, taking the PZC as the reference point. The various single crystal faces of lead have similar electrochemical properties [ 121, so it should not matter that their calculations are for polycrystalline lead rather than for Pb( 111). Evidently their model overestimates the magnitude of the shift A considerably. Amokrane and Badiali [ 16,171 and Halley et al. [ 18 ] have performed explicit model calculations for Ag( 111). Both groups try to calculate the water-
Ag
on
Au(ll1)
0.20
Pb on
0.60
Ag(ll1)
I
0.10 2
0.00
? z
-.lO
F Q -20 \ a
-.30 -.40 -.50
_____------*_
-20
-10
0
10
q / 7
-15.0
-10.0
-50
q /
0.0
50
10.0
1.5.c
,&cm-z
Fig. 1. The shift d of the oxygen-metal distance for a Pb/Ag( 111) electrode; ( A ) experimental data. The curve labelled KU was adapted from ref. [ I 51.
426
20
30
40
I
50
pCcmw2
Fig. 2. The shift d of the oxygen-metal distance for a Ag/Au( 111) electrode; (A ) experimental data. The curves labelled ABl and AB2 were adapted from ref. [ 16 1,fig. 2b; AB 1 corresponds to a value of 0.9 au of their parameter Vu, AB2 to V, =0.6 au. The curves labelled HJPSl and HJPS2 were adapted from ref. [ 181, figs. 4 and 5, respectively.
Volume 216, number
3,4,5,6
CHEMICAL
PHYSICS
metal distance self-consistently using model potentials for the water-metal interaction. The difficulty with this approach is that little is known about the short-range repulsive part of the interaction. Both groups include the interaction of the water dipole with its own image, and different models for the repulsive part. The details of these calculations can be found in the cited papers. We have included their results in fig. 2. Though these groups predict different behavior at negative charge densities, they both agree that for positive charge densities the water distance should vary substantially with the surface charge. If their predictions were correct, a change of the order of 0.5 8, should be observed if the potential is varied over the range of about 1 V. Such a large change would be surprising; for comparison we note that in aquo-complexes of transition metal ions the metalligand bond distances change typically by about 0.1 8, when the charge on the central ion is changed by one unit [ 191. Judging from fig. 2 the calculations of these authors considerably overestimate the variation of A with q, particularly at positive charge densities. While our own calculations presented above have the virtue of being analytical, they are based on a rather simplified treatment of the jellium model. For comparison we have performed accurate calculations for a model of jellium with a lattice of pseudopotentials; details of the method are given elsewhere [ 20,2 11. The variation of the electronic density pro-
LETTERS
31 December
1993
tile with the charge density was found to be similar for Pb(ll1) and a monolayer of Pb(ll1) on Ag( 111 ), and both agree well with the simple results from eq. (2) (see fig. 3).
4. Conclusion The SEXAFS experiments indicate that the oxygen-metal distance does not change with potential when the electrode charge is positive, but it is slightly dilated with increasing negative charge density. While there are at present only few data points, and caution must be exercised in forming conclusions on such a limited data set, the measurements are estimated to have an accuracy of f 0.0 1 8, or better. These findings are also in accord with the data for water on Au( 11 1 ), which show no variation in the distance within an experimental accuracy of k 0.3 A [ 21. So it is unlikely that this conclusion will be overturned by future experiments. These experimental findings can be explained by a simple argument based on the jellium model. If the observed peaks are due to water, as seems likely at present, theories which predict a substantial variation of the water-metal distance with charge do not agree with the experimental data and may have to be revised.
Acknowledgement 0.050
Financial support by the IBM Corporation and by the NATO Science Committee (Collaborative Research Grant No. RG-86/0068) is gratefully acknowledged.
0.040
F + 0.030 & 2 \
/ 0.020
References [ 1] H. Honbo, S. Suguwara and K. Itaya, Anal. Chem. 62 ( 1990)
a 0.010
Fig. 3. The shift A for lead calculated from eq. (3) (solid curve), foramonolayerofPb(111)onAg(111),andforPb(111),calculated from jellium with pseudopotentials.
24524. [2] J. Wang, E. Ocko, A. Davenport and H. Isaacs, Phys. Rev. B46(1992)10321. [ 31 H. Honbo and K. Itawa, J. Electroanal. Chem. 3 15 ( 1991) 215. [4] N. Samant, M. Toney, G. Borgas, L. Blum and 0. Melroy, J. Phys. Chem. 92 (1988) 220. [ 5 ] M. Samant, G. Borgas, J. Gordon II, 0. Melroy and L. Blum, J. Am. Chem. Sot. 109 (1987) 5970.
427
Volume 2 16, number 3,4,5,6
CHEMICAL PHYSICS LETTERS
[ 61 M. Samant, G. Borgas and 0. Melroy, J. Electrochem. Sot. 140 (1993) 421. [ 7 ] 0. Melroy, M. Samant, G. Borgas, J. Gordon II, L. Blum, J. White, M. Albarelli, M. McMillan and H. Abruha, Langmuir 4 (1988) 728. 18] B.‘Schardt, S.L. Yau and F. Rinaldi, Science 243 (1989) 1050; R. Vogel, I. Kamphausen and H. Baltruschat, Ber. Bunsenges. Physik. Chem. 96 ( 1992) 525. 19] S. Amokrane and J.P. Badiali, in: Modem aspects of electrochemistry, Vol. 22 (Plenum Press, New York, 1992); W. Schmickler, in: Structure of electrofield interfaces, eds. J. Lipkowski and P.N. Ross (VCH Publishers, Weinheim, 1993) p. 201. [ 101 J.R. Smith, Phys. Rev. 181 (1969) 522. [ 111 M. Partenskii and Ya. Smorodinskii, Sov. Phys. Solid State 16 (1974) 423.
428
3 1 December I993
[ 121 A. Hamelin, T. Vitanov, E. Sevastyanov and A. Popov, J. Electroanal. Chem. 145 (1983) 225.
[ 131 K.V. Rybalkov and D.J. Leikis, Elektrokhimiya 3 (1967) 383.
[ 141 G. Valette and A. Hamelin, J. Electroanal. Chem. 45 ( 1973) 301.
[ 151 A. Komyshev and M. Urbakh, J. Electoanal. Chem. 235 (1987) 11;257 (1988) 305.
[ 161 S. Amokrane and J.P. Badiali, J. Electroanal. Chem. 266 (1989) 21; Electrochim. Acta 34 (1989) 39.
[ 171 S. Amokrane and J.P. Badiali, Electrochim. Acta 34 ( 1989) 30 a,.
[ 181 J.W. Halley, B. Johnson, D. Price and M. Schwalm, Phys. Rev. B 31 (1985) 7695.
[ 191 N. Sutin, Progr. Inorg. Chem. 30 (1983) 441. [20] E. Leiva, Chem. Phys. Letters 187 (1991) 143. [ 2 1 ] E. Leiva and W. Schmickler, Electrochim. Acta, submitted for publication.
3,4,5,6
CHEMICAL
PHYSICS
3 1 December
LETTERS
The dependence of the oxygen-metal adsorbed at a metal electrode
1993
distance for water
Wolfgang Schmickler Abteilung Elektrochemie, Universitdt Urn, D-89069 Ulm. Germany
Douglas Henderson
’ and Owen R. Melroy
IBMResearch Division, Almaden Research Center, 650 Harry Road, San Jose, CA 95120, USA Received
2 1 April 1993; in final from 19 October
1993
It has been observed in SEXAFS studies that the oxygen-metal distance for water or an electrolyte adsorbed at an electrode, formed by a monolayer of lead deposited on silver, becomes increasingly dilated as the electrode becomes more negative, whereas this distance is unchanged when the electrode is formed by a monolayer of silver deposited on gold. It is suggested that this difference may be due to the fact that the potential is positive of the point of zero charge in the latter case. As a result, the oxygenmetal distance is determined by the hard core repulsion of the metal ions on the electrode surface which we approximate as a hard wall. In contrast, the potential is negative in the former case, and the conduction electrons in the metal penetrate into the electrolyte and push the water away from the metal. The distance the oxygen atoms are pushed from the metal surface increases as the potential becomes more negative. A simple calculation based on the jellium model of a metal leads to reasonable agreement with experiment.
1. Introduction In recent years, a variety of experimental techniques has been developed which are capable of probing the structure of electrode surfaces, in situ and on an atomic scale. These have been applied to study the structure of well-defined metal electrodes [ 11, potential induced reconstructions [ 21, and the structure of adsorbed metal [ 3-7 ] and halide monolayers [ 81. However, most of these techniques are not well suited to probing the detailed structure of the electrolyte side of the electrochemical interface. Consequently, our knowledge of the structure of an electrolyte at an electrochemical interface rests mainly upon the interpretation of capacity data. In surface extended X-ray adsorption fine structure (SEXAFS) studies of metal monolayers deposited at an underpotential on silver and gold sub’ Present address: Departamento t6noma Metropolitana/Iztapalapa, Mexico DF, Mexico.
424
de Fisica, Universidad AuApdo Postal 55-534, 09340
0009-2614/93/$
strates, backscattering from oxygen has been observed [4-71. From this it has been concluded that either water or the supporting electrolyte is not only adsorbed on the surface but that it is adsorbed at welldefined distances. The dependence of this distance between the adsorbed water or electrolyte and the metal monolayer on the applied potential has been examined, in a limited fashion, in two recent studies [ $71. For a monolayer of lead adsorbed on silver ( Ill), the lead-oxygen distance was found to dilate with decreasing potential from 2.33 8, at -0.53 V (versus Ag/AgCl) to 2.38 8, at - 1 V. A subsequent study of underpotentially deposited silver on gold ( 111) showed no dependence of the observed silver-oxygen distance on applied potential with measurements taken at +0.7, 0.5, and - 0.1 V (versus Ag/AgCl). As the authors stated in the latter paper, the reason for the difference between the two systems is not understood. We believe, and will assume here, that the observed oxygen signal is caused by water, for three reasons: ( 1) The surface excess one would expect for 06.00 0 1993 Elsevier Science Publishers
B.V. All rights reserved.
SSDZ OOOOS-2614(93)El306-2
Volume 216, number
3,4,5,6
CHEMICAL
PHYSICS
an adsorbed anion is too small to account for the magnitude of backscattering observed from oxygen. (2) This oxygen is also seen at potentials negative of the point of zero charge (PZC), where the adsorption of anions is unlikely. (3) Surface water was clearly seen in a recent X-ray diffraction and reflectivity study of the Au ( 111 )/aqueous electrolyte interface [ 21. The data could be fitted to a gold-water distance of 2.9rt0.3 A, which did not change with potential within the experimental error. The change in the metal-oxygen distance observed in the SEXAFS studies can then be understood in terms of the jellium model of a metal. At potentials negative to the PZC, the conduction electrons penetrate the solution, pushing the water away from the surface w’ h decreasing potential. At po4t tentials positive to Ithe PZC, the conduction electrodes recede into t# metal and the distance of closest approach of thei water or ion is limited by the surface formed by the metal ions in the electrode which we treat as a~hard wall. The same reasoning can be used if the observed oxygen belongs to a physisorbed anion, whiles for chemisorption the situation is more complicated. We note that this penetration of the electrons into he solution, or their retreat into the metal, has been, Itused to provide a quantitative explanation of the capacitance of electrochemical interfaces (for recent reviews of this subject, see ref.
LETTERS
3 1 December
1993
profile n(x) of the electrons must be known. In this Letter, we use the parameterization introduced by Smith [ 10 ] and Partenskii and Smorodinskii [ 111, n(x)=n+-(tn++aq/2eo)eM, n(~)=(fn+-aq/2e~)e-~“,
forxt0, forx>O,
(1)
where x is the perpendicular distance from the jellium edge, located at x=0, n, is the electronic density in the bulk, q is the charge density on the metal surface and e, is the unit charge. The parameter a! is the inverse of the decay length, and is usually determined by minimization of the surface energy. However, in this calculation we find that our result for the distance of closest approach of the adsorbed water is independent of (Y. As a result, we need not determine CY.When there is no charge on the electrode, the density profile is symmetric and the value x0 for which n ( x0) = 4n + is zero. As the electrode is negatively charged, the electrons penetrate further into the solution and the value of x0 increases. On the other hand, as the electrode is positively charged, the electrons recede into the metal and the distance of closest approach is determined not by the conduction electrons but by the jellium edge. Let us calculate x0; substituting its definition into eq. ( 1) gives
(2)
[91).
If q is not too large, i.e. if aq/eo n, K 1, this becomes 2. Theory x0
The jellium model of a metal treats the metal as a combination of a smeared out continuum of positive charge representing the metal ions and an inhomogeneous electron gas representing the conduction electrons. At an interface the continuum of positive charge is assumed to terminate abruptly, forming a step function. We refer to this abrupt termination or edge as the jellium edge. We treat the interface formed by ~this jellium edge as a hard wall. The density of the free conduction electrons is assumed to change continuously from its bulk value to zero. This model oif the metal interface has been widely applied to metal/vacuum and to metal/electrolyte interfaces. ~ To apply this model of the interface, the density
=
-
4
(3)
eon+
which is independent of cr. For a negative charge density we take x0, calculated from (3), as the dilation A of the water-surface distance. As we have already mentioned, for a positive charge the distance of closest approach of the water is determined by the jellium edge and there is no change in the water-surface distance.
3. Results Since the surface charge density q is the natural variable for theoretical calculations, we have converted the potentials at which the oxygen-metal dis425
Volume 216, number 3,4,5,6
CHEMICAL PHYSICS LETTERS
tances were measured to charge densities. For this purpose we require the values of the PZC and the capacities of the systems investigated. Lead on Ag( 111) forms a monolayer with a Pb ( 111) structure which is slightly compressed with respect to the surface of a bulk lead crystal. Consequently, the PZC of Pb/Ag( 111) should be close to that of Pb ( 111), which is at - 0.62 versus NHE [ 121, and we took this value. Capacity data for single crystal lead are not available, so we took data for polycrystalline electrodes from ref. [ 131, which we had to extrapolate to higher charge densities. This procedure introduces some uncertainty in the experimental values for q, but this does not affect our conclusions. Fig. 1 shows the results for Pb/Ag( 111); there are two experimental data points, one on each side of the PZC. We have assumed that the water-metal distance does not change at positive charges - which is true for Ag/Au ( 111) - and taken this value as our reference, i.e. A is the oxygen-metal distance at negative charges minus that at positive charges. Our theoretical curve gives a good estimate of the experimental value at negative charges, even though it is based on a simple, approximate calculation. Silver forms a dense commensurate monolayer on Au ( 111) , since the lattice constants of silver and gold are close, and has the Ag( 111) structure. We took the corresponding value for the ZPC ( - 0.5 1 versus NHE [ 12 ] ) . Capacity data for Ag ( 111) are given in
31 December 1993
ref. [ 14 ] ; they cover a smaller range than we require,
so again we had to extrapolate the experimental data in order to convert potentials to charge densities. Since the oxygen-metal distance is constant for this system, the uncertainties in the experimental charge densities do not matter. There are three experimental points, all at positive charge densities, and the shift A is zero in each case (see fig. 2 ) . The variation A of the water-metal distance with the charge density q plays an important role in several contemporary models of the metal-solution interface. Kornyshev and Urbakh [ 15 ] have calculated values for a polycrystalline lead electrode from electroreflectance data. They assume that only the water-metal distance changes with the surface charge density, while the optical constants of the interface remain constant. We have included their calculations in fig. 1, taking the PZC as the reference point. The various single crystal faces of lead have similar electrochemical properties [ 121, so it should not matter that their calculations are for polycrystalline lead rather than for Pb( 111). Evidently their model overestimates the magnitude of the shift A considerably. Amokrane and Badiali [ 16,171 and Halley et al. [ 18 ] have performed explicit model calculations for Ag( 111). Both groups try to calculate the water-
Ag
on
Au(ll1)
0.20
Pb on
0.60
Ag(ll1)
I
0.10 2
0.00
? z
-.lO
F Q -20 \ a
-.30 -.40 -.50
_____------*_
-20
-10
0
10
q / 7
-15.0
-10.0
-50
q /
0.0
50
10.0
1.5.c
,&cm-z
Fig. 1. The shift d of the oxygen-metal distance for a Pb/Ag( 111) electrode; ( A ) experimental data. The curve labelled KU was adapted from ref. [ I 51.
426
20
30
40
I
50
pCcmw2
Fig. 2. The shift d of the oxygen-metal distance for a Ag/Au( 111) electrode; (A ) experimental data. The curves labelled ABl and AB2 were adapted from ref. [ 16 1,fig. 2b; AB 1 corresponds to a value of 0.9 au of their parameter Vu, AB2 to V, =0.6 au. The curves labelled HJPSl and HJPS2 were adapted from ref. [ 181, figs. 4 and 5, respectively.
Volume 216, number
3,4,5,6
CHEMICAL
PHYSICS
metal distance self-consistently using model potentials for the water-metal interaction. The difficulty with this approach is that little is known about the short-range repulsive part of the interaction. Both groups include the interaction of the water dipole with its own image, and different models for the repulsive part. The details of these calculations can be found in the cited papers. We have included their results in fig. 2. Though these groups predict different behavior at negative charge densities, they both agree that for positive charge densities the water distance should vary substantially with the surface charge. If their predictions were correct, a change of the order of 0.5 8, should be observed if the potential is varied over the range of about 1 V. Such a large change would be surprising; for comparison we note that in aquo-complexes of transition metal ions the metalligand bond distances change typically by about 0.1 8, when the charge on the central ion is changed by one unit [ 191. Judging from fig. 2 the calculations of these authors considerably overestimate the variation of A with q, particularly at positive charge densities. While our own calculations presented above have the virtue of being analytical, they are based on a rather simplified treatment of the jellium model. For comparison we have performed accurate calculations for a model of jellium with a lattice of pseudopotentials; details of the method are given elsewhere [ 20,2 11. The variation of the electronic density pro-
LETTERS
31 December
1993
tile with the charge density was found to be similar for Pb(ll1) and a monolayer of Pb(ll1) on Ag( 111 ), and both agree well with the simple results from eq. (2) (see fig. 3).
4. Conclusion The SEXAFS experiments indicate that the oxygen-metal distance does not change with potential when the electrode charge is positive, but it is slightly dilated with increasing negative charge density. While there are at present only few data points, and caution must be exercised in forming conclusions on such a limited data set, the measurements are estimated to have an accuracy of f 0.0 1 8, or better. These findings are also in accord with the data for water on Au( 11 1 ), which show no variation in the distance within an experimental accuracy of k 0.3 A [ 21. So it is unlikely that this conclusion will be overturned by future experiments. These experimental findings can be explained by a simple argument based on the jellium model. If the observed peaks are due to water, as seems likely at present, theories which predict a substantial variation of the water-metal distance with charge do not agree with the experimental data and may have to be revised.
Acknowledgement 0.050
Financial support by the IBM Corporation and by the NATO Science Committee (Collaborative Research Grant No. RG-86/0068) is gratefully acknowledged.
0.040
F + 0.030 & 2 \
/ 0.020
References [ 1] H. Honbo, S. Suguwara and K. Itaya, Anal. Chem. 62 ( 1990)
a 0.010
Fig. 3. The shift A for lead calculated from eq. (3) (solid curve), foramonolayerofPb(111)onAg(111),andforPb(111),calculated from jellium with pseudopotentials.
24524. [2] J. Wang, E. Ocko, A. Davenport and H. Isaacs, Phys. Rev. B46(1992)10321. [ 31 H. Honbo and K. Itawa, J. Electroanal. Chem. 3 15 ( 1991) 215. [4] N. Samant, M. Toney, G. Borgas, L. Blum and 0. Melroy, J. Phys. Chem. 92 (1988) 220. [ 5 ] M. Samant, G. Borgas, J. Gordon II, 0. Melroy and L. Blum, J. Am. Chem. Sot. 109 (1987) 5970.
427
Volume 2 16, number 3,4,5,6
CHEMICAL PHYSICS LETTERS
[ 61 M. Samant, G. Borgas and 0. Melroy, J. Electrochem. Sot. 140 (1993) 421. [ 7 ] 0. Melroy, M. Samant, G. Borgas, J. Gordon II, L. Blum, J. White, M. Albarelli, M. McMillan and H. Abruha, Langmuir 4 (1988) 728. 18] B.‘Schardt, S.L. Yau and F. Rinaldi, Science 243 (1989) 1050; R. Vogel, I. Kamphausen and H. Baltruschat, Ber. Bunsenges. Physik. Chem. 96 ( 1992) 525. 19] S. Amokrane and J.P. Badiali, in: Modem aspects of electrochemistry, Vol. 22 (Plenum Press, New York, 1992); W. Schmickler, in: Structure of electrofield interfaces, eds. J. Lipkowski and P.N. Ross (VCH Publishers, Weinheim, 1993) p. 201. [ 101 J.R. Smith, Phys. Rev. 181 (1969) 522. [ 111 M. Partenskii and Ya. Smorodinskii, Sov. Phys. Solid State 16 (1974) 423.
428
3 1 December I993
[ 121 A. Hamelin, T. Vitanov, E. Sevastyanov and A. Popov, J. Electroanal. Chem. 145 (1983) 225.
[ 131 K.V. Rybalkov and D.J. Leikis, Elektrokhimiya 3 (1967) 383.
[ 141 G. Valette and A. Hamelin, J. Electroanal. Chem. 45 ( 1973) 301.
[ 151 A. Komyshev and M. Urbakh, J. Electoanal. Chem. 235 (1987) 11;257 (1988) 305.
[ 161 S. Amokrane and J.P. Badiali, J. Electroanal. Chem. 266 (1989) 21; Electrochim. Acta 34 (1989) 39.
[ 171 S. Amokrane and J.P. Badiali, Electrochim. Acta 34 ( 1989) 30 a,.
[ 181 J.W. Halley, B. Johnson, D. Price and M. Schwalm, Phys. Rev. B 31 (1985) 7695.
[ 191 N. Sutin, Progr. Inorg. Chem. 30 (1983) 441. [20] E. Leiva, Chem. Phys. Letters 187 (1991) 143. [ 2 1 ] E. Leiva and W. Schmickler, Electrochim. Acta, submitted for publication.