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The orientation dependence of zero charge potentials and surface energies of gold crystal faces
Surface Science 0 North-Holland
51 (1976) 771-774 Publishing Company
THE ORIENTATION
DEPENDENCE OF ZERO CHARGE POTENTIALS
AND SURFACE ENERGIES OF GOLD CRYSTAL Received
15 January
1976; manuscript
received
in final form
FACES 15 March
1976
The orientation dependence of the surface energy of face centered cubic metals has been studied not only for simple index faces but also for faces of higher index [I ,2] Determinations of the anisotropy of surface energy [3] of metals has been made either by the step model or by bond models (nearest-neighbour bond [4] and nest-nearest-neighbour approximations [5] ). These are more or less in agreement with experimental plots obtained for copper [6] , for gold [7] . The work function of a crystal face is related to its surface energy [8] . When a metal is immersed as an electrode in a solution, the charges developed on its surface are either positive, negative or absent depending on the potential imposed on it. In the simplest case - a solution of extremely pure water with non-adsorbable ions -- there are only water dipoles in contact with the metal. Then in the potential range where there is no electrochemical reaction, ionic double layer theory [9] may be used to determine the potential at which the charge density on the electrode surface is zero (J!?~=~). This was done for silver successfully for the three simple index faces [lo] and with more difficulty for gold for the three simple index planes [ 11, 121, and also for higher index faces as we will show below. Different surface preparations give similar values for E,=u [10,13] , therefore surface treatment does not interfere greatly with the measurement of this parameter. For gold, as for silver, in the absence of adsorption the value of E4=u for the (110) face (Earn)), is much more cathodic than f$_$), while I$$’ lies between the two [ 1 I] . The orientation dependence of the adsorption of halide ions on gold has already been shown for the [ 1 TO] zone for the chloride ion [ 141 , and for the three main zones for the iodide ion [15]. When there is no adsorption at the electrochemical interface, the zero charge potential is directly related to the electronic work function of the face of the metal used as an electrode [ 161 ; the relation is linear. The relation [ 171 : b&
= 9 - 4.61 - 0.40 OL
fits quite well for our measurements on polycrystalline silver [lo] and for the electronic work function q5published by the Russian school [ 181 ; a is the degree of orientation of water dipoles on the surface which depends on the electronegativity of the metal [ 171. Therefore zero charge potential is related to surface energy and its orientation dependence must follow the same variations as that for surface energy.
172
A. Hamelin, J. Lecoeur/Orientation
dependence
of zero charge potentials
r00d Fig. 1. Stereographic triangle ofLcfc rnitals showing the studied orientations.
We have determined the zero charge potentials of gold surfaces for (554), (332) and(221)on the [l]O] zone,for(Zll)and(4ll)on the[Oil] zone,forf310), (210) and (430) on the [OOl] zone (fig. 1). (Preparation of single crystals, mechanical and electrochemical preparation of the surfaces and the sample holder have already been detailed elsewhere [19] .> All work was conducted in deaerated 0.01 N NaF solutions. Admittance data for the interface, with linear variation of imposed potential with time, was recorded. A minimum admittance value exists for all faces at this concentration of the solution. When diluting the solution, this minimum admittance value decreases without any change of the potential; so this results from the well known contribution of the diffuse part of the electrochemical double layer. Potentials of this minimum, which can be thought near zero charge potentials, are plotted (fig. 2). Although these results on faces of gold are less striking than those obtained on faces of silver - owing probably to interactions between the solvent and the gold [ 111 for positive charge densities - they are precise enough to give reliable dependence of EqZo on crystallographic orientation. Fig. 2 also shows the calculated surface energies, taking as an arbitrary unit the surface energy of the (210) face, and as an approximation using only nearest-neighbour interactions [4]. We have supposed that the relative surface energy value corresponds to that of the potential of zero charge for the (210) face, so that comparison can be made on the simplest basis. For both parameters it is observed that the same characteristic features occur: - cusps for the (111) and (100) planes; - maxima for the [OOI] and [Oil] zones; - a continuously increasing curve for the third zone. It should be pointed out that calculations of surface energy using other approximations give less good agreement. An experimental r-plot for gold surfaces obtained by thermal faceting at 103O’C has been published [7]. Again the characteristic features are similar to those given above, namely: - cusps for the two same planes;
A. Hamelin, J. LecoeurfOrientation dependence of zero charge potentials
113
Eq:o(V/(s.c.e.) - 0,2
t
Fig. 2. ~rystallo~aphic
orientation dependences of the zero charge potential .!+o of relative surface energies calculated for the nearest-neighbor approximation.
of gold and
- a maximum is observed, this time 6” from the (100) plane, for the [OOl] zone; - a continuously increasing function occurs between the (111) and (100) planes; - a maximum is observed for the [Oil] zone. While the correspondence between this and the electrochemic~ data is not perfect, we should point out that Winterbottom and Gjostein [7] had access to very few data points around the (210) reference pole and around the (1 IO) plane. In addition neither the above authors nor ourselves have taken into account possible surface rearrangement which may occur for monocrystalline faces; such effects have already been shown to exist for the (100) face of gold. According to the above experimental results, the choice of three simple index faces to show the influence of crystallographic orientation, cannot give a complete account of the phenomenon, though often only these have been chosen for study. In previous work, a comparison of the zero charge potential of polycrystalline silver to the zero charge potentials for silver single crystals [IO] , suggested that these three simple index faces should display the extremes of electrochemical behaviour. However, the present results show that (1 I 1) and (2 IO) have the most anodic and cathodic zero charge potentials in the case of fee metals. If we take into account the correspondence between zero charge potentials and surface energies as given above, the usually accepted assumption that the packing of superficial atoms corresponds to that in a plane parallel to the surface in the bulk of the metal seems to be confirmed by experiment. In addition, the agreement observed for our electrochemical data and calculated or experimental surface energies seems to open a further promising experimental technique for surface physics.
A. Ham&n,
714
J. Lecoeur/Orientation
dependence
of zero charge potentials
The authors wish to thank J.-P. Bellier who initiated the high index planes of gold.
the experimental
A. HAMELIN
Laboratoire
d’Electro@se
92190 Meudon,
studies on
and J. LECOEUR
du CNRS,
France
References [l] P.G. Shewmon and W.M. Robertson, [ 21 (31 [4] [5] [6] [7] [ 81 [9]
[lo] [ll] [12] [13] 1141 [ 151 [16] [17] [ 181 [19]
Metal Surfaces: Structure, Energetics and Kinetics (ASM, 1962) p. 67. S.N. Zadumkin and I.G. Shebzukhova, Phys. Metals Metallog. 28(3) (1969) 50. W.L. Winterbottom, Structure and Properties of Metal Surfaces, Vol. I (Maruzen, Tokyo, 1973) p. 36. J.K. Mackenzie, A.J.W. Moore and J.F. Nicholas, J. Phys. Chem. Solids 23 (1962) 185. J.F. Nicholas, Australian J. Phys. 21 (1968) 21. M. MacLean and B. Gale, Phil. Mag. 20 (1969) 1033. W.L. Winterbottom and N.A. Gjostein, Acta Met. 14 (1966) 1041. S.N. Zadumkin, I.G. Shebzukhova and B.B. Alchagirov, Phys. Metals Metallog. 30 (6) (1972) 195. B.E. Conway, Theory and Principles of Electrode Processes (Ronald Press, New York, 1965). G. Valette and A. Hamelin, J. Electroanal. Chem. 45 (1973) 301. A. Hamelin and J. Lecoeur, Collect. Czech. Chem. Commun. 36 (1971) 714. J. Lecoeur and A. Hamelin, Compt. Rend. (Paris) C 279 (1974) 1081. J. Lecoeur, C. Sella, L. Tertian and A. Hamelin, Compt. Rend. (Paris) C 280 (1975) 247. A. Hamelin and J.P. Bellier, J. Electroanal. Chem. 41 (1973) 179. A. Hamelin, Winter College on Surface Science, Trieste, 1974. A.N. Frumkin, Sven. Kern. Tidskr. 77 (1965) 300. S. Trasatti, J. Electroanal. Chem. 33 (1971) 351. M.R. Tarasevich, N.A. Shumilova and R.Kh. Burshtein, Izv. Akad. Nauk SSSR, Otd. Khim. Nauk l(l964) 17. J. Clavillier, A. Hamelin and G. Valette, Compt. Rend. (Paris) C265 (1967) 221.
51 (1976) 771-774 Publishing Company
THE ORIENTATION
DEPENDENCE OF ZERO CHARGE POTENTIALS
AND SURFACE ENERGIES OF GOLD CRYSTAL Received
15 January
1976; manuscript
received
in final form
FACES 15 March
1976
The orientation dependence of the surface energy of face centered cubic metals has been studied not only for simple index faces but also for faces of higher index [I ,2] Determinations of the anisotropy of surface energy [3] of metals has been made either by the step model or by bond models (nearest-neighbour bond [4] and nest-nearest-neighbour approximations [5] ). These are more or less in agreement with experimental plots obtained for copper [6] , for gold [7] . The work function of a crystal face is related to its surface energy [8] . When a metal is immersed as an electrode in a solution, the charges developed on its surface are either positive, negative or absent depending on the potential imposed on it. In the simplest case - a solution of extremely pure water with non-adsorbable ions -- there are only water dipoles in contact with the metal. Then in the potential range where there is no electrochemical reaction, ionic double layer theory [9] may be used to determine the potential at which the charge density on the electrode surface is zero (J!?~=~). This was done for silver successfully for the three simple index faces [lo] and with more difficulty for gold for the three simple index planes [ 11, 121, and also for higher index faces as we will show below. Different surface preparations give similar values for E,=u [10,13] , therefore surface treatment does not interfere greatly with the measurement of this parameter. For gold, as for silver, in the absence of adsorption the value of E4=u for the (110) face (Earn)), is much more cathodic than f$_$), while I$$’ lies between the two [ 1 I] . The orientation dependence of the adsorption of halide ions on gold has already been shown for the [ 1 TO] zone for the chloride ion [ 141 , and for the three main zones for the iodide ion [15]. When there is no adsorption at the electrochemical interface, the zero charge potential is directly related to the electronic work function of the face of the metal used as an electrode [ 161 ; the relation is linear. The relation [ 171 : b&
= 9 - 4.61 - 0.40 OL
fits quite well for our measurements on polycrystalline silver [lo] and for the electronic work function q5published by the Russian school [ 181 ; a is the degree of orientation of water dipoles on the surface which depends on the electronegativity of the metal [ 171. Therefore zero charge potential is related to surface energy and its orientation dependence must follow the same variations as that for surface energy.
172
A. Hamelin, J. Lecoeur/Orientation
dependence
of zero charge potentials
r00d Fig. 1. Stereographic triangle ofLcfc rnitals showing the studied orientations.
We have determined the zero charge potentials of gold surfaces for (554), (332) and(221)on the [l]O] zone,for(Zll)and(4ll)on the[Oil] zone,forf310), (210) and (430) on the [OOl] zone (fig. 1). (Preparation of single crystals, mechanical and electrochemical preparation of the surfaces and the sample holder have already been detailed elsewhere [19] .> All work was conducted in deaerated 0.01 N NaF solutions. Admittance data for the interface, with linear variation of imposed potential with time, was recorded. A minimum admittance value exists for all faces at this concentration of the solution. When diluting the solution, this minimum admittance value decreases without any change of the potential; so this results from the well known contribution of the diffuse part of the electrochemical double layer. Potentials of this minimum, which can be thought near zero charge potentials, are plotted (fig. 2). Although these results on faces of gold are less striking than those obtained on faces of silver - owing probably to interactions between the solvent and the gold [ 111 for positive charge densities - they are precise enough to give reliable dependence of EqZo on crystallographic orientation. Fig. 2 also shows the calculated surface energies, taking as an arbitrary unit the surface energy of the (210) face, and as an approximation using only nearest-neighbour interactions [4]. We have supposed that the relative surface energy value corresponds to that of the potential of zero charge for the (210) face, so that comparison can be made on the simplest basis. For both parameters it is observed that the same characteristic features occur: - cusps for the (111) and (100) planes; - maxima for the [OOI] and [Oil] zones; - a continuously increasing curve for the third zone. It should be pointed out that calculations of surface energy using other approximations give less good agreement. An experimental r-plot for gold surfaces obtained by thermal faceting at 103O’C has been published [7]. Again the characteristic features are similar to those given above, namely: - cusps for the two same planes;
A. Hamelin, J. LecoeurfOrientation dependence of zero charge potentials
113
Eq:o(V/(s.c.e.) - 0,2
t
Fig. 2. ~rystallo~aphic
orientation dependences of the zero charge potential .!+o of relative surface energies calculated for the nearest-neighbor approximation.
of gold and
- a maximum is observed, this time 6” from the (100) plane, for the [OOl] zone; - a continuously increasing function occurs between the (111) and (100) planes; - a maximum is observed for the [Oil] zone. While the correspondence between this and the electrochemic~ data is not perfect, we should point out that Winterbottom and Gjostein [7] had access to very few data points around the (210) reference pole and around the (1 IO) plane. In addition neither the above authors nor ourselves have taken into account possible surface rearrangement which may occur for monocrystalline faces; such effects have already been shown to exist for the (100) face of gold. According to the above experimental results, the choice of three simple index faces to show the influence of crystallographic orientation, cannot give a complete account of the phenomenon, though often only these have been chosen for study. In previous work, a comparison of the zero charge potential of polycrystalline silver to the zero charge potentials for silver single crystals [IO] , suggested that these three simple index faces should display the extremes of electrochemical behaviour. However, the present results show that (1 I 1) and (2 IO) have the most anodic and cathodic zero charge potentials in the case of fee metals. If we take into account the correspondence between zero charge potentials and surface energies as given above, the usually accepted assumption that the packing of superficial atoms corresponds to that in a plane parallel to the surface in the bulk of the metal seems to be confirmed by experiment. In addition, the agreement observed for our electrochemical data and calculated or experimental surface energies seems to open a further promising experimental technique for surface physics.
A. Ham&n,
714
J. Lecoeur/Orientation
dependence
of zero charge potentials
The authors wish to thank J.-P. Bellier who initiated the high index planes of gold.
the experimental
A. HAMELIN
Laboratoire
d’Electro@se
92190 Meudon,
studies on
and J. LECOEUR
du CNRS,
France
References [l] P.G. Shewmon and W.M. Robertson, [ 21 (31 [4] [5] [6] [7] [ 81 [9]
[lo] [ll] [12] [13] 1141 [ 151 [16] [17] [ 181 [19]
Metal Surfaces: Structure, Energetics and Kinetics (ASM, 1962) p. 67. S.N. Zadumkin and I.G. Shebzukhova, Phys. Metals Metallog. 28(3) (1969) 50. W.L. Winterbottom, Structure and Properties of Metal Surfaces, Vol. I (Maruzen, Tokyo, 1973) p. 36. J.K. Mackenzie, A.J.W. Moore and J.F. Nicholas, J. Phys. Chem. Solids 23 (1962) 185. J.F. Nicholas, Australian J. Phys. 21 (1968) 21. M. MacLean and B. Gale, Phil. Mag. 20 (1969) 1033. W.L. Winterbottom and N.A. Gjostein, Acta Met. 14 (1966) 1041. S.N. Zadumkin, I.G. Shebzukhova and B.B. Alchagirov, Phys. Metals Metallog. 30 (6) (1972) 195. B.E. Conway, Theory and Principles of Electrode Processes (Ronald Press, New York, 1965). G. Valette and A. Hamelin, J. Electroanal. Chem. 45 (1973) 301. A. Hamelin and J. Lecoeur, Collect. Czech. Chem. Commun. 36 (1971) 714. J. Lecoeur and A. Hamelin, Compt. Rend. (Paris) C 279 (1974) 1081. J. Lecoeur, C. Sella, L. Tertian and A. Hamelin, Compt. Rend. (Paris) C 280 (1975) 247. A. Hamelin and J.P. Bellier, J. Electroanal. Chem. 41 (1973) 179. A. Hamelin, Winter College on Surface Science, Trieste, 1974. A.N. Frumkin, Sven. Kern. Tidskr. 77 (1965) 300. S. Trasatti, J. Electroanal. Chem. 33 (1971) 351. M.R. Tarasevich, N.A. Shumilova and R.Kh. Burshtein, Izv. Akad. Nauk SSSR, Otd. Khim. Nauk l(l964) 17. J. Clavillier, A. Hamelin and G. Valette, Compt. Rend. (Paris) C265 (1967) 221.