- Email: [email protected]
The behaviour of water at stepped surfaces of single crystal gold electrodes
Surface Science 114 (1982) 320-330 North-Holland Publishing Company
320
THE BEHAVIOUR OF WATER AT STEPPED SINGLE CRYSTAL GOLD ELECTRODES
Received
29 April
19X1: accepted
for pubhcation
2X Scptembcr
SURFACES
OF
1981
New information concerning the structure of the metal-electrolyte interface 1s obtained b\ comparing the electronic work function of metallic single crystals in vacuum and the potential of zero charge (pzc) at the metal-electrolyte interface. The close parallelism of these two properties is notable in comparing our results for the pzc of gold single crystals of a number of orientations located along the sides of the stereographic triangle and those for the work function of copper by Peralta et al. [24]. This comparison shows that the presence of an adsorbed water layer at the metalLelectrolyte interface does not perturb to any large extent the atomic arrangement of the outermost metallic surface with respect to the structure assumed to be displayed in vacuum. In the case of stepped gold surfaces near the (1I I) face, a comparison between the work function measured by Besocke et al. (231 and our results for pzc allows us to conclude that the metal-solvent interaction depends on the nature of the monoatomic step whether it corresponds to a quaternary site in the case of the (100) step structure or to a ternary site for the (I I I) step structure
1. Introduction The study of the metal-electrolyte interface in the simplest case when the inner layer consists only of solvent molecules has been extensively performed especially on mercury electrodes. This has resulted in a series of theoretical models which have been comprehensively summarized by Trasatti [I]. These models are based on the assumption of a uniform structureless metal surface; consequently they are applicable principally to mercury but less satisfactorily to solid metals. For the part of the interface far from the metal where long-range interactions dominate (diffuse layer), the Gouy-Chapman theory seems to work well in many cases (Ag, Au, Bi, Cd, Cu, Pb, etc.). However, in the region of the interface close to the surface (inner layer) where short-range interactions between the metal and solvent are important, the present models are unable to account for the observed behaviour. These models clearly do not take sufficient account of the specific nature of the metal, even though the experiments on, for example, Hg, liquid Ga [2] and Pb [3], have shown a long time ago that the behaviour of the inner layer depends on the nature of the metal. This led to a first classification of the metals in terms of their 0039-6028/82/0000-0000/$02.75
0 1982 North-Holland
hydrophilic character [4], thus indicating that the short-range interactions (physisorption or chemisorption) are not negligible and must be considered in the development of a model of the interface. Further evidence can be obtained from the study of single crystal electrodes with a particular crystallographic orientation which permit the study of the effect of crystal structure on the properties of the adsorbed solvent and will lead to a better understanding of the metal-solvent bond. Experimental work carried out on Au [5], Ag [6,7] and Cu [8] has already shown that useful evidence can be obtained in this way. One of the principal problems in such experiments arises from the fact that in situ characterization of the interfacial structure cannot be done by the now well-known methods used for metal surfaces in vacuum. It is thus necessary to develop indirect methods using electrochemistry to study the energetic parameters most suitable for characterizing the interface and then to correlate these with the properties of the clean metal surface in vacuum. These correlations can then lead to the establishment of the different types of interactions which govern the structure of the two types of interface and to the proposal of structural models. For metal-electrolyte interfaces the potential of zero charge (pzc) is one of the most significant energetic parameters, as is shown by the considerable effort already made to determine this quantity. Both, the surface energy (y) and the work of extraction of electrons, are equally widely used for characterizing a metal surface in vacuum. This is why in a previous study [9] of single crystal electrodes we have compared the variations of pzc and y with the crystallographic orientation. However, experimental values of y are sparse [lO,l l] and difficult to use for a comparative study, and we had to use calculated values [12]. This attempt to obtain a correlation has not been pursued, because at the present time the calculation of the surface energy is still not very well understood. This may explain the lack of a direct relation between y and the pzc. On the other hand, several relations have been suggested between the pzc and the electronic work of extraction Cp [13-191. Parsons [16] followed by Trasatti [19] has established relations inspired by the first suggestions of Frumkin [14]. These include terms arising from the change of the surface potential of the two phases when they are put in contact. In the form proposed by Trasatti the relation becomes E,=,
= Q”/e
+ 6xM + gFM,(dip) + K,
where E,=, is the potential of the metal M at its pzc with respect to a reference electrode, 6x M is the change of the surface potential of M when it comes in contact with the solvent, g&(dip) is the contribution to the potential at the metal/solution interface due to the orientation of solvent dipoles, and K is the potential drop at the reference electrode (it remains constant if the same type of reference electrode is used).
Measurements of QM and E,,,, for different crystallographic orientations of the same metal can thus lead, according to eq. (1). to a way of studying the metal-solvent interactions from the changes of the surface potentials. The work function @ is notoriously sensitive to contamination as well as to perturbations of the surface structure of the crystal [20]. Hence, it was only recently, that, thanks to the modern UHV technique, reproducible and significant values were obtained. Precise studies of the effects of surface structure on O”, for example the effect of step density, have been achieved [22-241. On the basis of the Smoluchowski model [21] of surface electronic distribution, these authors have shown that steps on high density surfaces can modify the surface dipole moment. So far, only a few metals have been studied: W [22,23], Pt [23], Au [23] and Cu [24], and except for Cu only a limited number of orientations has been used. In order to compare with work function values in refs. [23,24], we have used a series of Au electrodes whose orientations correspond in the TLK model to surfaces having (111) (100) and (110) terraces and monoatomic steps of (111) and (100) type. To simplify the situation we have chosen, for each zone of the
LlllliXillll
1221) _
3(lll~x(llli 13311 *-
:
2~llo)xlllli
\ h5511-
10~1001x11111-1191.11
/ (1001
3~110)x~111)
IOOll
L 0 0 L t 0 zz x s 5 cl-
Fig. I. Stereographic stcp/terracc structure.
triangle
indicating
the
crystallographic
orientationa
studied
and
thut-
stereographic triangle easily comparable.
(fig. 1) orientations
whose surface
atomic
structures
are
2. Experimental 2. I. Preparation
of single crystals
Single crystals of Au (Johnson-Matthey 99.999%) 3 mm diameter were obtained by growing from the melt in vacuum (5 X 10 -’ TOW) in a graphite crucible (spectrographic quality). The crystals were oriented to 2 1” with the aid of X-rays, cut and mechanically polished with 0.5 pm diamond paste. The damaged surface layer was removed by anodic dissolution in aqueous KCN
P61. To examine the effect of the preparation technique, other procedures were tried. Thin layers of (111) and (100) were grown epitaxially in vacuum on different substrates [27,28]. These electrodes gave the same pzc as those prepared by the method described above. 2.2. Measurement
of the potential
of zero charge
The pzc was obtained by measuring the differential capacity C as a function of the electrode potential E when the electrode was in contact with a dilute solution of a non-adsorbing electrolyte (NaF). Following the Gouy-Chapman model of the diffuse part of the double layer, we could locate the pzc at the sharp minimum of the C(E) curve. However, it must be noted that when single crystal surfaces with steps are being studied, the presence of macroscopic defects can complicate the interpretation of any type of measurement. The advantage of the determination of the pzc from C(E) curves is that the analysis of the shape of the C(E) curves permits the effects of macroscopic defects to be separated [25]. Indeed, the existence of surface defects, which have an area large enough to give rise to a capacity in parallel to the capacity of the main part of the interface, cause only a modification of the general form of the C(E) curve but not in the position of the minimum corresponding to the pzc [25]. In contrast, the presence of monoatomic steps, kinks, etc., which are too small compared with the thickness of the interface to behave as separate capacitors, causes only a shift of the pzc. Hence, this method of measurement of the pzc is particularly suitable for the study of single crystal surfaces with steps, because it permits the effects of macroscopic perturbations to be avoided, while being very sensitive to the atomic relief of the surface, such as the presence of monoatomic steps. It should also be recalled that the method is at the same time very sensitive to any contamination of the electrode surface. The effect of this has been minimized by using a cyclic voltametric technique which enables one to verify
324
J. LAWXU~ ef ul. / Behaoiour of wctrer ut
srepped.surfotrs
the state of cleanliness of the surface and, at the same time, to clean the surface in situ. In this way the pzc can be determined within 2 10 mV.
3. Results and discussion 3.1. Stability
of surface structure
Measurements of C(E) curves were made with all crystals in aqueous 0.01 M NaF with pH 5.6 at 25°C. Fig. 2 shows the corresponding values of the pzc with respect to the saturated calomel electrode for 18 orientations of Au crystals as a function of the angle (Ywhich each surface makes with the plane of the same zone having the highest surface atomic density. From the results for the [Oli] and [OOl] zones (fig. 2) it can be seen that the pzc becomes more negative as the surface density of monoatomic steps increases. Thus the E,=,(a) curve has a minimum for the planes (3 11) and (210), respectively, in the two zones where these two planes have the highest step density. In the [ 110)
[ii0
[oii]
~
+0.3.
Fig. 2. The potential of zero charge for Au, obtained experimentally from the minimum of the capacity-potential curve, plotted as a function of the crystallographic orientation along the directions [IiO], [Oli] and [OOl].
zone the E&LY) curve has a different form, the ~nimum occurring at the (110) plane which is at the corner of the stereographic triangle. This is due to the fact that in the TLK model the (1 IO) plane is considered as a terrace, nevertheless it does not correspond to a high density plane. The (1 IO) plane could in fact be equally well written as 2( 111) X (111); that is, the highest density of (111) steps which can exist on a (111) plane. This remarkable structure of the (110) explains how the atomic roughness can be further increased by forming (100) steps (fig. 3a), which causes a negative shift of the pzc ([OOl] zone, fig. 2), or alternatively, the atomic roughness can be decreased by forming (111) steps (fig. 3b) and so shifting the pzc positively ([ 1101 zone, fig. 2). This inte~retation, which explains the general form of the E,=,( LY)curve in terms of the atomic roughness of the surface, is confirmed by noting that the (210) surface-which is the roughest in the TLK model of the surfaces-on the boundaries of the stereographic triangle has the most negative pzc ( - 0.13 V (SCE)), while the (111) plane, which is close packed, has the most positive pzc (+0.33 V (SCE)). This relationship suggests that the dipolar model proposed by Smoluchowski [21] to represent the surface electronic structure of a metal remains valid when the metal is brought into contact with aqueous electrolyte. On the other hand, it is interesting to note that according to the TLK model the (311) plane has an atomic roughness greater than that of the (110) plane, so that one would expect from the above considerations that the pzc of the (31 I) should be more negative than that of the (110) plane. It is clear from fig. 2 that
Fig. 3. Representations step; (b) (1 I i) step.
of monoatomic
steps on a (I 10) face following
the TLK model:
(a) (too)
this is not observed. Hence we conclude that there are exceptions to the general correlation in particular cases. At present, we have no explanation for this; but the discussion in the next section suggests that it is an intrinsic structural property. 3.2. Compurison vacuum
of E,=,( cx) curves for Au in solution with a( a) curves for Cu in
The correlation of pzc with Q for Au is limited to a rather narrow range of orientations because only these have been studied in vacuum [23]. On the other hand, a much wider range of orientations of Cu single crystals has been studied by Peralta et al. [24]. Since both Cu and Au crystallize in the fee system, it is reasonable to suppose that their A@( (Y) curves will be similar. Hence. we shall compare the A@(a) curve for Cu (fig. 4) obtained by Peralta et al. with our results for Au in the same zones (fig. 2). Comparison of the two diagrams shows that there is a close similarity between the genera1 form of the two curves, the extrema occurring at the same orientations. This similarity suggests that immersion of the gold electrodes in aqueous solution has not caused the formation of any surface compounds such as oxides. It may be noted that when Peralta et al. [24] adsorbed S on their electrodes, the whole shape of the curve changed. Such large modifications of the A@(a) curve are usually attributed to a change of surface structure as a result of the reaction of the adsorbate with it (striation, facetting) [29]. Hence, the similarity of figs. 2 and 4 supports the assumption that the Au surfaces are not perturbed either structurally or chemically by contact with water. The stability of the atomic structure of these surfaces in the presence of aqueous electrolyte is further confirmed by electroreflectance measurements
o*7_h
0.6 _
0.8 50
0
Fig. 4. Results function
100
of Peralta
of crystallographic
150
a/o
et al. [24] for the change orientation.
of the electronic
work
function
AQ of <‘u as I
[31]. For example, the symmetry properties of the low index surfaces of gold are retained in solution. Since the electroreflectance is sensitive only to the surface atomic layer, this suggests that the average structure of this layer remains unchanged when they are put into contact with aqueous electrolyte. Finally, it should be noted that Peralta [30] found the same anomaly in his measurement of the work function of Cu in vacuum as we found for the (311) and (110) planes. This supports the view that this anomaly is not a result of the perturbation of the Au surface by the immersion in aqueous electrolyte, but is related to the surface structure of the metal. 3.3. Stepped surfaces close to the (1 II) plane A more detailed study of the interaction between Au and Hz0 was made using the measurements of Q obtained for Au by Besocke et al. [23]; these were confined to orientations close to the (111) plane and lying in the [110] and [Oli] zones, and showed that @ varied linearly with step density (n) (fig. 5). Although for practical reasons they could not use orientations more than 7” from the (111) plane; the A@(a) diagram obtained by Peralta et al. suggests that the relation is linear at least up to 10”. It may also be noted that the slope of the A@(n) plot is 2.5 to 3 times greater for Au than for Cu. This was interpreted by Peralta et al. [24] following Besocke et al. [23] as due to the different dipole moments of Cu and Au atoms in the steps. Whatever the explanation, it seems clear that the difference in the amplitude of the variation in E,=, in the case of Au in fig. 2 and that of A@ in the case of Cu in fig. 4 is largely due to differences arising from the different properties of Au and Cu and not from the presence of the electrolyte solution. Examination of the E,=,(a) relation in fig. 2 shows that in the neighbourhood of the (111) plane there is a similar linear variation of pzc up to the orientations (332) and (755), which lie at 10.02” and 9.44”, respectively, from the (111) plane in the [ 1101 and [Oli] zones. These linear regions of Q(n) and Ear,(n) show that, both in vacuum and in solution, the interaction between steps is negligible so long as the (111) terraces separating them are bigger than 6 atoms wide. The larger slope of Q(n) in the [Oli] zone (fig. 5) led Besocke et al. [23] to suggest that the dipole moment of a (110) step on a (111) plane is larger than that of a (111) step. This was confirmed for Pt by the same authors and for Cu by Peralta et al. [24]. However, for the pzc of Au this difference is reversed; that is, the E,=,(n) slope is larger than the A@(n) slope in the [1101 zone, while the opposite occurs for the [Oli] zone. This results in the E,=,(n) slope being larger in the [ITO] zone than in the [Oli] zone. Thus for the (332) and (755) orientations, which have almost the same step density: AE’!!2) - A@(332’/e = + (70 _t 20) mV, 0-O
(2)
A,!?‘?’ 0-O
(3)
- A@(755)/e = - (70 * 20) mV.
8 (332)
6
4
2
(55&l
0 1"')
t t 6(111)x(1111 10(111)x(111)
2
L U',W
6
8
n1106.steps.cni' -w
(755) I
t t 10~111)xi100) 6U")xi100)
Fig. 5. Comparison of the change of the electronic work function A0 of Au measured by Besocke et al. [23] and the change of pzc of Au (E,,,) obtained in the present work for stepped surfaces close to the (I I I) face, Open circles with error bars denote A@ values (note inversion of the abscissa axis by comparison with the original [23]): solid circles with error bars denote .!I II,__,, values.
From eq. (1) it is evident that these differences in the surface potentials the monoatomic steps must arise from changes in the surface potential metal produced by the contact with water and in the dipole potential water itself, that is: A[8xM + g&,(dip)](332’ Ajax”
+ g&~(dip)](“”
due to of the of the
= +70 mV.
(4)
= -70
(5)
mV.
Clearly, the two terms in (4) and (5) cannot be separated experimentally, but a simple model of the interface in the region of a step may help us to understand the way in which the change in the surface potential is produced. To represent the structure of water on the (111) and (100) monoatomic steps, it is necessary to consider not just a single molecule but an ensemble of molecules in the region of the step. The diagram in fig. 6 takes account of the size of the water molecule (close to that of gold) on the one hand, and of the surface structure of the crystal given by the TLK model on the other hand, but it is simplified by focussing attention on a section perpendicular to the step
J. L~orur
et ul. / Behcrwour
329
of wuter ut stepped surJuces
containing three molecules A, B and C. For the two types of step the geometric positions of these molecules are similar but their interactions with the metal surface are different. Those interactions depend on the one hand on the
Fig. 6. Schematic
representation
of water dipoles adsorbed
on a monoatomic
step
electrostatic field created by the electronic “smoothing” proposed by Smoluchowski [21] and on the other hand by the bonding characteristic of the type of atomic site on which the molecule is adsorbed. This schematic picture of molecular and metallic dipoles at a step (fig. 6) shows that adsorption of polar molecules tends to neutralize the excesses and deficiencies of electronic charge in the metal and leads to reduction of the dipole moment of the steps (Ax”) which is larger if the dipole penetrates more deeply into the adsorption site. This can occur particularly for the dipoles C which adsorb with their positive pole directed toward the metal, so that one of the hydrogens can penetrate into the metal. This can occur more easily in the quaternary site of a (100) step than for the ternary site of the (111) step, resulting in a larger reduction of the metallic dipole moment (Ax”) for the (100) step than for the (111) step. The net reduction of the surface potential due to water adsorption on (111) steps (eq. (4)) can then be attributed predominantly to the effect of the dipoles of molecules A and B, whereas on the (100) steps the increase of the surface potential (eq. (5)) would be due to a marked reduction of the metal dipole moment and a larger contribution from molecule C. This strong adsorption of water molecules with their hydrogen atoms in quaternary sites on Au is supported by experimental results obtained on (100) single crystal surfaces. An unexpected adsorption of atomic hydrogen due to reduction of water is observed only on this plane [32].
Acknowledgment We would like to express our thanks to Melle A. Hamelin work on single crystal electrodes in this laboratory.
who initiated
References [I] S. Trasatti. in: Modern Aspects of Electrochemistry, Vol. 13, Eds. BE. Conway and J.O’M. Bockris (Plenum. New York, 1979) p. 8 I. [2] A. Frumkin. N. Polianovskaya, N. Grigoryev and I. Bagotskaya. Elcctrochim. Acta IO (1965) 793. [3] K. Rybalka and D. Leikis, Elektrokhimiya 3 (1967) 3X3. [4] A. Frumkin, B. Damaskin, N. Grigoryev and 1. Bagotskaya, Elcctrochim. Acta 19 (1974) 69. [5] A. Hamelin and J. Lecoeur, Collection Czech. Chcm. Commun. 36 (1971) 7 14. [6] G. Valette and A. Hamelin, J. Electroanal. Chem. 45 (1973) 301. [7] T. Vitanov. A. Popov and E. Sevastyanov. Elektrokhimiqa IO (1974) 346. [X] I.M. Novosel’skii, N.I. Maksimyuk and L. Egorov. Elektrokhimiya 9 (1973) I5 IX. [9] A. Hamelin and J. Lecoeur, Surface Sci. 57 (1976) 774 [IO] M. McLean and B. Gale, Phil. Mag. 20 (1969) 1033. [I I] W.L. Winterbottom and N.A. GJostein, Acta Met. I4 (1966) 1041. [ 121 J.K. Mackenzie, A.S.W. Moore and J.F. Nicholas, J. Phys. Chcm. Solids 23 (1962) 1X5. [ 131 A. Frumkin and A. Gorodetskaya. Z. Physik. Chem. 136 ( 1928) 45 I. [ 141 A. Frumkin, J. Colloid Sci. I( 1946) 290. [ 151A.N. Frumkin, Svensk. Kern. Tidskr. 77 (1965) 314. [ 161 R. Parsons, in: Modern Aspects of Electrochemistry, Ed. J.O’M. Bockris (1954) p. 170. [ 171 R. Vasenin. Zh. Fiz. Khim. 27 (1953) 878; 2X (1954) 1672. [IX] S.D. Argade and E. Gileadi, in: Electrosorption, Ed. E. Gileadi (Plenum, New York, 1967). [I91 S. Trasatti, J. Electroanal. Chem. 33 (1971) 351. [20] G.E. Rhead. Surface Sci. 68 (1977) 20. [21] R. Smoluchowski, Phys. Rev. 60 (1941) 661. [22] B. Krahl-Urban, E.A. Niekisch and H. Wagner. Surface Sci. 64 (1977) 52. [23] K. Besocke, B. K&l-Urban and H. Wagner, Surface Sci. 6X (1977) 39. [24] L. Peralta, Y. Berthier and J. Oudar. in: 4e Colloq. de Physique et Chimie dea Surfaces Solidcs (Le Vide (1978) X3). [25] J. Lecoeur, Compt. Rend. (Paris) C2X3 (1974) 651. (261 W.J. Tegart, The Electrolytic and Chemical Polishing of Metals (Pergamon, 1956) p. 57. [27] J. Lecoeur, C. Sella. L. Tertian and A. Hamelin, Compt. Rend. (Paris) C280 (1975) 247. [2X] J. Lecoeur. C. Sella and J.C. Martin, Compt. Rend. (Paris) C2X7 (197X) 447. [29] J. Oudar, in: Colloq. CNRS, Processus de Nucl&ations dans les Reactions Grtr-Metaux. 1963. [30] L. Peralta, Thesis, Paris (1979) p. 35. [31] C. Nguyen Van Huong, C. Hinnen. J. Lecoeur and R. Parsons, J. Electroanal. Chem. 92 (1978) 239. [32] J. Lecoeur, Thesis, Paris (1979); J. Lecoeur. to be published.
320
THE BEHAVIOUR OF WATER AT STEPPED SINGLE CRYSTAL GOLD ELECTRODES
Received
29 April
19X1: accepted
for pubhcation
2X Scptembcr
SURFACES
OF
1981
New information concerning the structure of the metal-electrolyte interface 1s obtained b\ comparing the electronic work function of metallic single crystals in vacuum and the potential of zero charge (pzc) at the metal-electrolyte interface. The close parallelism of these two properties is notable in comparing our results for the pzc of gold single crystals of a number of orientations located along the sides of the stereographic triangle and those for the work function of copper by Peralta et al. [24]. This comparison shows that the presence of an adsorbed water layer at the metalLelectrolyte interface does not perturb to any large extent the atomic arrangement of the outermost metallic surface with respect to the structure assumed to be displayed in vacuum. In the case of stepped gold surfaces near the (1I I) face, a comparison between the work function measured by Besocke et al. (231 and our results for pzc allows us to conclude that the metal-solvent interaction depends on the nature of the monoatomic step whether it corresponds to a quaternary site in the case of the (100) step structure or to a ternary site for the (I I I) step structure
1. Introduction The study of the metal-electrolyte interface in the simplest case when the inner layer consists only of solvent molecules has been extensively performed especially on mercury electrodes. This has resulted in a series of theoretical models which have been comprehensively summarized by Trasatti [I]. These models are based on the assumption of a uniform structureless metal surface; consequently they are applicable principally to mercury but less satisfactorily to solid metals. For the part of the interface far from the metal where long-range interactions dominate (diffuse layer), the Gouy-Chapman theory seems to work well in many cases (Ag, Au, Bi, Cd, Cu, Pb, etc.). However, in the region of the interface close to the surface (inner layer) where short-range interactions between the metal and solvent are important, the present models are unable to account for the observed behaviour. These models clearly do not take sufficient account of the specific nature of the metal, even though the experiments on, for example, Hg, liquid Ga [2] and Pb [3], have shown a long time ago that the behaviour of the inner layer depends on the nature of the metal. This led to a first classification of the metals in terms of their 0039-6028/82/0000-0000/$02.75
0 1982 North-Holland
hydrophilic character [4], thus indicating that the short-range interactions (physisorption or chemisorption) are not negligible and must be considered in the development of a model of the interface. Further evidence can be obtained from the study of single crystal electrodes with a particular crystallographic orientation which permit the study of the effect of crystal structure on the properties of the adsorbed solvent and will lead to a better understanding of the metal-solvent bond. Experimental work carried out on Au [5], Ag [6,7] and Cu [8] has already shown that useful evidence can be obtained in this way. One of the principal problems in such experiments arises from the fact that in situ characterization of the interfacial structure cannot be done by the now well-known methods used for metal surfaces in vacuum. It is thus necessary to develop indirect methods using electrochemistry to study the energetic parameters most suitable for characterizing the interface and then to correlate these with the properties of the clean metal surface in vacuum. These correlations can then lead to the establishment of the different types of interactions which govern the structure of the two types of interface and to the proposal of structural models. For metal-electrolyte interfaces the potential of zero charge (pzc) is one of the most significant energetic parameters, as is shown by the considerable effort already made to determine this quantity. Both, the surface energy (y) and the work of extraction of electrons, are equally widely used for characterizing a metal surface in vacuum. This is why in a previous study [9] of single crystal electrodes we have compared the variations of pzc and y with the crystallographic orientation. However, experimental values of y are sparse [lO,l l] and difficult to use for a comparative study, and we had to use calculated values [12]. This attempt to obtain a correlation has not been pursued, because at the present time the calculation of the surface energy is still not very well understood. This may explain the lack of a direct relation between y and the pzc. On the other hand, several relations have been suggested between the pzc and the electronic work of extraction Cp [13-191. Parsons [16] followed by Trasatti [19] has established relations inspired by the first suggestions of Frumkin [14]. These include terms arising from the change of the surface potential of the two phases when they are put in contact. In the form proposed by Trasatti the relation becomes E,=,
= Q”/e
+ 6xM + gFM,(dip) + K,
where E,=, is the potential of the metal M at its pzc with respect to a reference electrode, 6x M is the change of the surface potential of M when it comes in contact with the solvent, g&(dip) is the contribution to the potential at the metal/solution interface due to the orientation of solvent dipoles, and K is the potential drop at the reference electrode (it remains constant if the same type of reference electrode is used).
Measurements of QM and E,,,, for different crystallographic orientations of the same metal can thus lead, according to eq. (1). to a way of studying the metal-solvent interactions from the changes of the surface potentials. The work function @ is notoriously sensitive to contamination as well as to perturbations of the surface structure of the crystal [20]. Hence, it was only recently, that, thanks to the modern UHV technique, reproducible and significant values were obtained. Precise studies of the effects of surface structure on O”, for example the effect of step density, have been achieved [22-241. On the basis of the Smoluchowski model [21] of surface electronic distribution, these authors have shown that steps on high density surfaces can modify the surface dipole moment. So far, only a few metals have been studied: W [22,23], Pt [23], Au [23] and Cu [24], and except for Cu only a limited number of orientations has been used. In order to compare with work function values in refs. [23,24], we have used a series of Au electrodes whose orientations correspond in the TLK model to surfaces having (111) (100) and (110) terraces and monoatomic steps of (111) and (100) type. To simplify the situation we have chosen, for each zone of the
LlllliXillll
1221) _
3(lll~x(llli 13311 *-
:
2~llo)xlllli
\ h5511-
10~1001x11111-1191.11
/ (1001
3~110)x~111)
IOOll
L 0 0 L t 0 zz x s 5 cl-
Fig. I. Stereographic stcp/terracc structure.
triangle
indicating
the
crystallographic
orientationa
studied
and
thut-
stereographic triangle easily comparable.
(fig. 1) orientations
whose surface
atomic
structures
are
2. Experimental 2. I. Preparation
of single crystals
Single crystals of Au (Johnson-Matthey 99.999%) 3 mm diameter were obtained by growing from the melt in vacuum (5 X 10 -’ TOW) in a graphite crucible (spectrographic quality). The crystals were oriented to 2 1” with the aid of X-rays, cut and mechanically polished with 0.5 pm diamond paste. The damaged surface layer was removed by anodic dissolution in aqueous KCN
P61. To examine the effect of the preparation technique, other procedures were tried. Thin layers of (111) and (100) were grown epitaxially in vacuum on different substrates [27,28]. These electrodes gave the same pzc as those prepared by the method described above. 2.2. Measurement
of the potential
of zero charge
The pzc was obtained by measuring the differential capacity C as a function of the electrode potential E when the electrode was in contact with a dilute solution of a non-adsorbing electrolyte (NaF). Following the Gouy-Chapman model of the diffuse part of the double layer, we could locate the pzc at the sharp minimum of the C(E) curve. However, it must be noted that when single crystal surfaces with steps are being studied, the presence of macroscopic defects can complicate the interpretation of any type of measurement. The advantage of the determination of the pzc from C(E) curves is that the analysis of the shape of the C(E) curves permits the effects of macroscopic defects to be separated [25]. Indeed, the existence of surface defects, which have an area large enough to give rise to a capacity in parallel to the capacity of the main part of the interface, cause only a modification of the general form of the C(E) curve but not in the position of the minimum corresponding to the pzc [25]. In contrast, the presence of monoatomic steps, kinks, etc., which are too small compared with the thickness of the interface to behave as separate capacitors, causes only a shift of the pzc. Hence, this method of measurement of the pzc is particularly suitable for the study of single crystal surfaces with steps, because it permits the effects of macroscopic perturbations to be avoided, while being very sensitive to the atomic relief of the surface, such as the presence of monoatomic steps. It should also be recalled that the method is at the same time very sensitive to any contamination of the electrode surface. The effect of this has been minimized by using a cyclic voltametric technique which enables one to verify
324
J. LAWXU~ ef ul. / Behaoiour of wctrer ut
srepped.surfotrs
the state of cleanliness of the surface and, at the same time, to clean the surface in situ. In this way the pzc can be determined within 2 10 mV.
3. Results and discussion 3.1. Stability
of surface structure
Measurements of C(E) curves were made with all crystals in aqueous 0.01 M NaF with pH 5.6 at 25°C. Fig. 2 shows the corresponding values of the pzc with respect to the saturated calomel electrode for 18 orientations of Au crystals as a function of the angle (Ywhich each surface makes with the plane of the same zone having the highest surface atomic density. From the results for the [Oli] and [OOl] zones (fig. 2) it can be seen that the pzc becomes more negative as the surface density of monoatomic steps increases. Thus the E,=,(a) curve has a minimum for the planes (3 11) and (210), respectively, in the two zones where these two planes have the highest step density. In the [ 110)
[ii0
[oii]
~
+0.3.
Fig. 2. The potential of zero charge for Au, obtained experimentally from the minimum of the capacity-potential curve, plotted as a function of the crystallographic orientation along the directions [IiO], [Oli] and [OOl].
zone the E&LY) curve has a different form, the ~nimum occurring at the (110) plane which is at the corner of the stereographic triangle. This is due to the fact that in the TLK model the (1 IO) plane is considered as a terrace, nevertheless it does not correspond to a high density plane. The (1 IO) plane could in fact be equally well written as 2( 111) X (111); that is, the highest density of (111) steps which can exist on a (111) plane. This remarkable structure of the (110) explains how the atomic roughness can be further increased by forming (100) steps (fig. 3a), which causes a negative shift of the pzc ([OOl] zone, fig. 2), or alternatively, the atomic roughness can be decreased by forming (111) steps (fig. 3b) and so shifting the pzc positively ([ 1101 zone, fig. 2). This inte~retation, which explains the general form of the E,=,( LY)curve in terms of the atomic roughness of the surface, is confirmed by noting that the (210) surface-which is the roughest in the TLK model of the surfaces-on the boundaries of the stereographic triangle has the most negative pzc ( - 0.13 V (SCE)), while the (111) plane, which is close packed, has the most positive pzc (+0.33 V (SCE)). This relationship suggests that the dipolar model proposed by Smoluchowski [21] to represent the surface electronic structure of a metal remains valid when the metal is brought into contact with aqueous electrolyte. On the other hand, it is interesting to note that according to the TLK model the (311) plane has an atomic roughness greater than that of the (110) plane, so that one would expect from the above considerations that the pzc of the (31 I) should be more negative than that of the (110) plane. It is clear from fig. 2 that
Fig. 3. Representations step; (b) (1 I i) step.
of monoatomic
steps on a (I 10) face following
the TLK model:
(a) (too)
this is not observed. Hence we conclude that there are exceptions to the general correlation in particular cases. At present, we have no explanation for this; but the discussion in the next section suggests that it is an intrinsic structural property. 3.2. Compurison vacuum
of E,=,( cx) curves for Au in solution with a( a) curves for Cu in
The correlation of pzc with Q for Au is limited to a rather narrow range of orientations because only these have been studied in vacuum [23]. On the other hand, a much wider range of orientations of Cu single crystals has been studied by Peralta et al. [24]. Since both Cu and Au crystallize in the fee system, it is reasonable to suppose that their A@( (Y) curves will be similar. Hence. we shall compare the A@(a) curve for Cu (fig. 4) obtained by Peralta et al. with our results for Au in the same zones (fig. 2). Comparison of the two diagrams shows that there is a close similarity between the genera1 form of the two curves, the extrema occurring at the same orientations. This similarity suggests that immersion of the gold electrodes in aqueous solution has not caused the formation of any surface compounds such as oxides. It may be noted that when Peralta et al. [24] adsorbed S on their electrodes, the whole shape of the curve changed. Such large modifications of the A@(a) curve are usually attributed to a change of surface structure as a result of the reaction of the adsorbate with it (striation, facetting) [29]. Hence, the similarity of figs. 2 and 4 supports the assumption that the Au surfaces are not perturbed either structurally or chemically by contact with water. The stability of the atomic structure of these surfaces in the presence of aqueous electrolyte is further confirmed by electroreflectance measurements
o*7_h
0.6 _
0.8 50
0
Fig. 4. Results function
100
of Peralta
of crystallographic
150
a/o
et al. [24] for the change orientation.
of the electronic
work
function
AQ of <‘u as I
[31]. For example, the symmetry properties of the low index surfaces of gold are retained in solution. Since the electroreflectance is sensitive only to the surface atomic layer, this suggests that the average structure of this layer remains unchanged when they are put into contact with aqueous electrolyte. Finally, it should be noted that Peralta [30] found the same anomaly in his measurement of the work function of Cu in vacuum as we found for the (311) and (110) planes. This supports the view that this anomaly is not a result of the perturbation of the Au surface by the immersion in aqueous electrolyte, but is related to the surface structure of the metal. 3.3. Stepped surfaces close to the (1 II) plane A more detailed study of the interaction between Au and Hz0 was made using the measurements of Q obtained for Au by Besocke et al. [23]; these were confined to orientations close to the (111) plane and lying in the [110] and [Oli] zones, and showed that @ varied linearly with step density (n) (fig. 5). Although for practical reasons they could not use orientations more than 7” from the (111) plane; the A@(a) diagram obtained by Peralta et al. suggests that the relation is linear at least up to 10”. It may also be noted that the slope of the A@(n) plot is 2.5 to 3 times greater for Au than for Cu. This was interpreted by Peralta et al. [24] following Besocke et al. [23] as due to the different dipole moments of Cu and Au atoms in the steps. Whatever the explanation, it seems clear that the difference in the amplitude of the variation in E,=, in the case of Au in fig. 2 and that of A@ in the case of Cu in fig. 4 is largely due to differences arising from the different properties of Au and Cu and not from the presence of the electrolyte solution. Examination of the E,=,(a) relation in fig. 2 shows that in the neighbourhood of the (111) plane there is a similar linear variation of pzc up to the orientations (332) and (755), which lie at 10.02” and 9.44”, respectively, from the (111) plane in the [ 1101 and [Oli] zones. These linear regions of Q(n) and Ear,(n) show that, both in vacuum and in solution, the interaction between steps is negligible so long as the (111) terraces separating them are bigger than 6 atoms wide. The larger slope of Q(n) in the [Oli] zone (fig. 5) led Besocke et al. [23] to suggest that the dipole moment of a (110) step on a (111) plane is larger than that of a (111) step. This was confirmed for Pt by the same authors and for Cu by Peralta et al. [24]. However, for the pzc of Au this difference is reversed; that is, the E,=,(n) slope is larger than the A@(n) slope in the [1101 zone, while the opposite occurs for the [Oli] zone. This results in the E,=,(n) slope being larger in the [ITO] zone than in the [Oli] zone. Thus for the (332) and (755) orientations, which have almost the same step density: AE’!!2) - A@(332’/e = + (70 _t 20) mV, 0-O
(2)
A,!?‘?’ 0-O
(3)
- A@(755)/e = - (70 * 20) mV.
8 (332)
6
4
2
(55&l
0 1"')
t t 6(111)x(1111 10(111)x(111)
2
L U',W
6
8
n1106.steps.cni' -w
(755) I
t t 10~111)xi100) 6U")xi100)
Fig. 5. Comparison of the change of the electronic work function A0 of Au measured by Besocke et al. [23] and the change of pzc of Au (E,,,) obtained in the present work for stepped surfaces close to the (I I I) face, Open circles with error bars denote A@ values (note inversion of the abscissa axis by comparison with the original [23]): solid circles with error bars denote .!I II,__,, values.
From eq. (1) it is evident that these differences in the surface potentials the monoatomic steps must arise from changes in the surface potential metal produced by the contact with water and in the dipole potential water itself, that is: A[8xM + g&,(dip)](332’ Ajax”
+ g&~(dip)](“”
due to of the of the
= +70 mV.
(4)
= -70
(5)
mV.
Clearly, the two terms in (4) and (5) cannot be separated experimentally, but a simple model of the interface in the region of a step may help us to understand the way in which the change in the surface potential is produced. To represent the structure of water on the (111) and (100) monoatomic steps, it is necessary to consider not just a single molecule but an ensemble of molecules in the region of the step. The diagram in fig. 6 takes account of the size of the water molecule (close to that of gold) on the one hand, and of the surface structure of the crystal given by the TLK model on the other hand, but it is simplified by focussing attention on a section perpendicular to the step
J. L~orur
et ul. / Behcrwour
329
of wuter ut stepped surJuces
containing three molecules A, B and C. For the two types of step the geometric positions of these molecules are similar but their interactions with the metal surface are different. Those interactions depend on the one hand on the
Fig. 6. Schematic
representation
of water dipoles adsorbed
on a monoatomic
step
electrostatic field created by the electronic “smoothing” proposed by Smoluchowski [21] and on the other hand by the bonding characteristic of the type of atomic site on which the molecule is adsorbed. This schematic picture of molecular and metallic dipoles at a step (fig. 6) shows that adsorption of polar molecules tends to neutralize the excesses and deficiencies of electronic charge in the metal and leads to reduction of the dipole moment of the steps (Ax”) which is larger if the dipole penetrates more deeply into the adsorption site. This can occur particularly for the dipoles C which adsorb with their positive pole directed toward the metal, so that one of the hydrogens can penetrate into the metal. This can occur more easily in the quaternary site of a (100) step than for the ternary site of the (111) step, resulting in a larger reduction of the metallic dipole moment (Ax”) for the (100) step than for the (111) step. The net reduction of the surface potential due to water adsorption on (111) steps (eq. (4)) can then be attributed predominantly to the effect of the dipoles of molecules A and B, whereas on the (100) steps the increase of the surface potential (eq. (5)) would be due to a marked reduction of the metal dipole moment and a larger contribution from molecule C. This strong adsorption of water molecules with their hydrogen atoms in quaternary sites on Au is supported by experimental results obtained on (100) single crystal surfaces. An unexpected adsorption of atomic hydrogen due to reduction of water is observed only on this plane [32].
Acknowledgment We would like to express our thanks to Melle A. Hamelin work on single crystal electrodes in this laboratory.
who initiated
References [I] S. Trasatti. in: Modern Aspects of Electrochemistry, Vol. 13, Eds. BE. Conway and J.O’M. Bockris (Plenum. New York, 1979) p. 8 I. [2] A. Frumkin. N. Polianovskaya, N. Grigoryev and I. Bagotskaya. Elcctrochim. Acta IO (1965) 793. [3] K. Rybalka and D. Leikis, Elektrokhimiya 3 (1967) 3X3. [4] A. Frumkin, B. Damaskin, N. Grigoryev and 1. Bagotskaya, Elcctrochim. Acta 19 (1974) 69. [5] A. Hamelin and J. Lecoeur, Collection Czech. Chcm. Commun. 36 (1971) 7 14. [6] G. Valette and A. Hamelin, J. Electroanal. Chem. 45 (1973) 301. [7] T. Vitanov. A. Popov and E. Sevastyanov. Elektrokhimiqa IO (1974) 346. [X] I.M. Novosel’skii, N.I. Maksimyuk and L. Egorov. Elektrokhimiya 9 (1973) I5 IX. [9] A. Hamelin and J. Lecoeur, Surface Sci. 57 (1976) 774 [IO] M. McLean and B. Gale, Phil. Mag. 20 (1969) 1033. [I I] W.L. Winterbottom and N.A. GJostein, Acta Met. I4 (1966) 1041. [ 121 J.K. Mackenzie, A.S.W. Moore and J.F. Nicholas, J. Phys. Chcm. Solids 23 (1962) 1X5. [ 131 A. Frumkin and A. Gorodetskaya. Z. Physik. Chem. 136 ( 1928) 45 I. [ 141 A. Frumkin, J. Colloid Sci. I( 1946) 290. [ 151A.N. Frumkin, Svensk. Kern. Tidskr. 77 (1965) 314. [ 161 R. Parsons, in: Modern Aspects of Electrochemistry, Ed. J.O’M. Bockris (1954) p. 170. [ 171 R. Vasenin. Zh. Fiz. Khim. 27 (1953) 878; 2X (1954) 1672. [IX] S.D. Argade and E. Gileadi, in: Electrosorption, Ed. E. Gileadi (Plenum, New York, 1967). [I91 S. Trasatti, J. Electroanal. Chem. 33 (1971) 351. [20] G.E. Rhead. Surface Sci. 68 (1977) 20. [21] R. Smoluchowski, Phys. Rev. 60 (1941) 661. [22] B. Krahl-Urban, E.A. Niekisch and H. Wagner. Surface Sci. 64 (1977) 52. [23] K. Besocke, B. K&l-Urban and H. Wagner, Surface Sci. 6X (1977) 39. [24] L. Peralta, Y. Berthier and J. Oudar. in: 4e Colloq. de Physique et Chimie dea Surfaces Solidcs (Le Vide (1978) X3). [25] J. Lecoeur, Compt. Rend. (Paris) C2X3 (1974) 651. (261 W.J. Tegart, The Electrolytic and Chemical Polishing of Metals (Pergamon, 1956) p. 57. [27] J. Lecoeur, C. Sella. L. Tertian and A. Hamelin, Compt. Rend. (Paris) C280 (1975) 247. [2X] J. Lecoeur. C. Sella and J.C. Martin, Compt. Rend. (Paris) C2X7 (197X) 447. [29] J. Oudar, in: Colloq. CNRS, Processus de Nucl&ations dans les Reactions Grtr-Metaux. 1963. [30] L. Peralta, Thesis, Paris (1979) p. 35. [31] C. Nguyen Van Huong, C. Hinnen. J. Lecoeur and R. Parsons, J. Electroanal. Chem. 92 (1978) 239. [32] J. Lecoeur, Thesis, Paris (1979); J. Lecoeur. to be published.