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The dependence of the potential of zero charge of silver electrodes on the crystallographic orientation of the surface
J. Electroanal. Chem., Elsevier Sequoia S.A.,
200 (1986) Lausanne -
389-396 Printed
389 in The Netherlands
Preliminary note THE DEPENDENCE OF THE POTENTIAL OF ZERO CHARGE OF SILVER ELECTRODES ON THE CRYSTALLOGRAPHIC ORIENTATION OF THE SURFACE *
M. BACHETTA
and S. TRASATTI
Department of Physical 21, 20133 Milan (Italy) L. DOUBOVA Laboratoire (France) (Received
***
Chemistry
and Electrochemistry,
University
of Milan,
Via Venezian
and A. HAMELIN
d ‘Electrochimie 16th
**
December
Znterfaciale, 1985;
CNRS,
in revised form
1 Place A. Briand, 28th January
92190
Meudon-Belleuue
1986)
INTRODUCTION
The importance of investigating single crystal-face electrodes for the understanding of the factors governing the structure of the metal/solution interface can hardly be overemphasized [ 1,2] . In particular, it has been found that the potential of zero charge depends on the crystal orientation in a way which closely parallels the behaviour of other anisotropic properties such as the electron work function [3,4] and the surface energy [5]. However, a fairly complete collection of data of the potential of zero charge (E,= ,,) exists only for Au [6], while the dependence of the electron work function on the crystal orientation has been investigated exhaustively only for Cu [7] . With such inhomogeneity of experimental data there exists no definite proof that all sdmetals (Au, Ag, Cu) behave electrochemically in a comparable way. Although Ag single crystal electrodes give reliable capacitance curves [ 81, the crystal faces thus far investigated are appreciably fewer than for Au [6], which prevents all of the characteristic features of the (E,= 0 orientation) relationship to be brought into evidence. Precise data of E,,=. have been collected for the low-index faces of Ag such as (ill), (100) and (110) [ 8-111. One value is available also for the (210) face [ 121 , whereas no data exist for higherindex faces which would permit the gaps in the [llO] and [Oil] zones to be filled up. In these zones, the (311) face is expected [6] to be a “turning-point”, like the (210) face, wheras the (331) face would be useful to confirm that no minimum is present in the relationship inside the [ 1101 zone [ 51. From a paper presented at the 36th ISE Meeting, 23-27 September 1985, Spain. ** To whom correspondence should be addressed. l ** Supported by the Department of Physical Chemistry and Electrochemistry versity of Milan. l
0022-0728/86/$03.50
0 1986
Elsevier
Sequoia
S.A.
Salamanca,
of the Uni-
390
The present work has been aimed at filling the above gaps. In particular, we report some preliminary results on the behaviour of the (311) and (331) faces in solutions of weakly ‘adsorbed or not-adsorbed electrolytes. In order to be able to appraise the reliability of the data obtained for the above faces which have never been investigated before, the (110) face has been used as a reference system in view of the precise data existing in the literature [9] . EXPERIMENTAL
Ag single crystals were grown starting from a 3 mm diameter bar (Metalli Preziosi 99.999%) using a technique developed at the LEI [ 131. They were oriented to ?2” with the aid of X-rays, cut and mechanically polished with alumina of different grades down to a mirror finish. In the case of the (110) and (311) faces, the bar was enclosed in a Teflon holder and the working surface was isolated by a Scotchcast non-contaminating resin. The final surface preparation before each experimental run was carried out by means of a chemical polishing procedure based on Cr03 * . The electrode was transferred to the cell while protected by a drop of working solution. In the case of the (331) face, after the mechanical polishing, the crystal was annealed in a gas flame and cooled in pure argon [ 141. The electrochemical measurements were then carried out using the hanging electrolyte technique developed by Schultze and co-workers [ 151. Capacitance curves were obtained by recording the faradaic (IF) and capacitive (I,) components of the alternating current with the aid of a Brucker E-310 polarograph equipped with a lock-in amplifier. The differential capacitance of the electrode/solution interface was calculated by means of the formula: C = [(I;
+ I;)&]
(1/2~
AVA)
The frequency v was 17 Hz, the alternating potential difference AV 10 mV, and the rate of electrode potential variation 10 mV s-l . The geometric surface area of the electrode was used for A. Solutions were prepared with “Millipore” water, NaF Suprapure (Merck) and KPF, 98% (Ventron) which was submitted to further purification [ 161, A two-component cell provided with a Luggin capillary from the reference electrode was used for the measurements. The counter-electrode was made of glassy carbon, while a saturated calomel electrode was used as the reference electrode. The temperature of the solution was kept at 25?O.l”C by means of a water thermostat. RESULTS
Figure 1 shows a typical capacitance curve for the Ag (110) face in fluoride solutions as obtained in this work compared to curves taken from the literature [ 8,9] . The positions of the minimum and the two maxima correspond very accurately to the reference data. The general shape of the curve is also in good agreement with the previous measurements. A systematic difference is however * The authors
are grateful
to Prof.
G. Piazza for the details of the procedure.
391
observable in the depth of the diffuse layer minimum. Whether this is a real effect related to the degree of crystallinity of the surface [ 81 or is an effect of the experimental technique used in this work is still to be clarified. According to the data for Au [ 3,5], the potential of zero charge around the (110) face is sensitive to the crystal orientation (although not as much as in other regions) and its accurate values can be used in principle as a proof of surface perfection.
-10
f /
-05 V(SCE)
Fig. 1. Typical capacitance (referred to the geometric surface area)-potential curves of the (110) face of Ag in 0.02 mol dmm3 NaF solution. (1) Ref. 8; (2) ref. 9; (3) this work.
The capacitance curves in KPF6 solutions showed the same kind of agreement with the literature data [ 91. The potential of the diffuse layer minimum was found to shift towards more negative values in NaF but not in KPF6 solution in close agreement with Valette’s observations [ 91. The shift is attributable to the specific adsorption of F-ions at more positive charges, this being responsible for the anodic peak. The depth and the position of the minimum is thus essentially dependent on the height of the positive maximum. A detailed analysis of the specific adsorption of F- ions on the (110) face of silver will be reported in a forthcoming paper. In Fig. 2 the potential at the minimum in the capacitance curves is plotted vs. the electrolyte concentration on a linear scale. As shown for Hg by Antropov et al. [ 171, the potential varies linearly with concentration so that an accurate value of E,=, in the absence of specific adsorption can be obtained by extrap-
392
olation to c + 0. The figure shows the close agreement between our values and those reported previously [ 91. Both sets of data in NaF and KPF6 give the same extrapolated value of Eozo, i.e. -0.975+0.005 V (SCE), in excellent agreement with the value recommended in the literature [6]. It is interesting that if EazO is estimated by back-integration of capacitance curves in F- + PF; mixed electrolyte solutions at constant ionic strength, the shift with F- concentration is almost not apparent, which suggests that specific adsorption is essentially suppressed at u = 0 under these conditions. Figure 3 emphasises the effect of fluoride ions on the capacitance curve for the (311) face. The general features are essentially the same as those for the
NaF
0.06 c/m01
L._
009 dmm3
Fig. 2. Dependence of the potential of the capacitance minimum electrolyte concentration. (a,~) Ref. 9; (A,=) this work.
for the (110)
face on the
NaF
Fig. 3. Typical tions.
capacitance
curves
of the (311)
face of Ag in 0.02
mol dmm3 electrolyte
solu-
393
(110) face in Fig. 1. More specifically, the height of the two peaks is the same in KPF6 solution. The increase of the positive peak in NaF solution causes the position of the minimum to shift towards more negative potentials. The variation of the potential of the diffuse layer minimum with the electrolyte concentration is shown in Fig. 4. Linear extrapolation gives, in both electrolytes, E, =o = -0.905+0.005 V (SCE) for the (311) face. Comparison of Fig. 4 with Fig. 2 leads to the qualitative consideration that the rate of change of E,,,,with electrolyte concentration is higher for the (311) than for the (110) face. The real significance of this experimental fact needs further study to be assessed. Preliminary measurements have been carried out with the (331) face in one dilute NaF solution. The potential of the capacitance minimum can be regarded as the Eo=,, only to a first approximation (cf. Figs. 2 and 4). Its value is -0.910 +O.OlO V (SCE) but the true value of Eoco for the (331) face could be 5 to 10 mV more positive. The values of Eo=,,have been summarized in Table 1. The values of E,h at c = 0.010 mol dme3 NaF for the (110) and (311) faces have also been reported for comparison.
I
0.02
‘-
KPF 6
0.04
006
c/m01
dme3
Fig. 4. Dependence of the potential electrolyte concentration.
TABLE
of the capacitance
minimum
for the (311)
face on the
1
Potential of zero charge of Ag single crystal faces. Eb for the (110) and -0.915 V for the (311) face Face
E,=,IV
(110) (311) (331)
-0.975+0.005 -0.905+0.005 (-0.910+0.010)
(SCE)
Electrolyte KPF,, NaF (c + 0) KPF,, NaF (c -t 0) 0.010 M NaF
in 0.010
M NaF solutions:
-0.980
V
394 DISCUSSION
The data presented here for the (110) face show that the chemical polishing procedure used in this work results in surface conditions which are quite comparable to those obtained by means of electropolishing in NaCN solutions [ 8,9] . Since chemical polishing in NaCN solutions produces Ag surfaces which give the characteristic LEED pattern [ 181, a well ordered surface is expected to result also from the chemical polishing in Cr03 solutions, but no direct experimental proof of this exists at present. The dependence of Eo=,, on the crystal orientation can be plotted in a way that gives rise to characteristic graphical features [3,5,6]. This is shown in Fig. 5, where, as expected from the characteristic dependence observed [3] in the case of E,=, for Au and of Cpfor Cu, E,=, of Ag can be seen to vary with the crystallographic orientation in such a way that cusps appear for the (100) and (111) faces, minima for the (311) and (210) facesin the [Oil] and [OOl] zones, respectively, while the (331) face does not show any specific feature in the [ 1101 zone. Therefore, the qualitative effect of the crystal orientation on EozO is now also well documented for Ag over all three main crystallographic zones. It may be of interest to try to argue about the “correct” quantitative dependence of E,=, of Ag on the crystal orientation. Since Au and Ag have the same crystal structure and almost the same atomic size, a correlation between the potentials of zero charge of the two metals may reveal interesting features. This is shown in Fig. 6a. Eozo for Au have been taken from Lecoeur et al. [3].
07 5 co > \ E 08 ul
/
09
/
I
! IO
1 1
110)
Fig. 5. The potential lographic orientation.
of zero charge
of Ag single crystals
plotted
as a function
of the crystal-
395
,
(4
iii -O78 5 >-09-2 -09 -
/ (331)*13:‘: ’ / /
-01
0
/
/
&O,
01
02
f,:U, / i’(.SCE)
/
/
/*;11,
b)
mo), ’ z (311) / .a/ / (331) /
/
/
09 J’(hkl)/
/
(111) ,* /
08 J’(210)
Fig. 6. (a) Correlation between the potentials of zero charge of Ag and Au single crystal-face electrodes. (b) Correlation between the potential of zero charge of Ag single crystal-faces and the relative surface energy calculated on the basis of the sole nearest-neighbour interactions [5,6].
A linear correlation whose slope is lower than unity because E, =,, extends over a larger range for Au, does indeed exist. However, it is difficult to decide where the straight line should be located. If it is drawn so as to go through the points for the (210), (100) and (111) faces, those for the (llO), (311) and (331) faces can be seen to fall away from the line by more than expected, in terms of experimental accuracy. It is in principle hard to say if the deviations are due to the values for Au or to those for Ag. In order to be able to discriminate among various possibilities, it may be helpful to resort to a non-electrochemical surface parameter viz. the surface energy, for an additional correlation. In view of the correspondence between the variation with crystal orientation of the calculated relative surface energy and of Ea=,, for Au [ 5,6] an attempt is made in Fig. 6b to correlate these two parameters for Ag also. If the straight line is drawn so as to go through the points of the (210) and (111) faces as in Fig. 6a, the (331) face is seen to come closer to the correlation. Therefore, at least for this face, the point in Fig. 6a is far from the line, seemingly because the E,=. of Au is too negative. This is confirmed by a close inspection of the yrel vs. EozO correlation for the [ 1101 zone of Au [6] showing that the points of the potential of zero charge for the various faces including the (331) one lie well below the curve of the relative surface energy. Since no strict quantitative significance can be attached to the correlation in Fig. 6b, surprisingly, it appears to be as good as that observed for Au. However, there remains the ambiguous position of the (311) face which needs further clarification. One might wonder whether the deviation is real and so, what factor could be responsible for this. CONCLUSIONS
The data reported in this paper show that, while the predicted relationship between E,=, and surface orientation is correctly observed in the case of Ag also, a more quantitative analysis of the situation requires further study with
396
crystal faces of higher indices and a specific investigation with different samples of the same faces used in this investigation in order to verify the stability and reproducibility of the observed interfacial parameters. ACKNOWLEDGEMENT
Financial support of this work by the Ministry fully acknowledged.
of Education
of Italy is grate-
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
A. Hamelin in B.E. Conway and J.O’M. Bockris (Eds.), Modern Aspects of Electrochemistry, Vol. 16, Plenum Press, New York, 1985, p. 1. S. Trasatti, Mater. Chem. Phys., 12 (1985) 507. J. Lecoeur, J. Andro and R. Parsons, Surf. Sci., 114 (1982) 320. S. Trasatti, NATO Advanced Study Institute Trends in Interfacial Electrochemistry, Reidel, Dordrecht, in press. A. Hamelin and J. Lecoeur, Surf. Sci., 57 (1976) 771. A. Hamelin, T. Vitanov, E. Sevastyanov and A. Popov, J. Electroanal. Chem., 145 (1983) 225. L. Peralta, Y. Berthier and J. Oudar, Le Vide, (1978) 83. G. Valette and A. Hamelin, J. Electroanal. Chem., 45 (1973) 301. G. Valette, J. Electroanal. Chem., 122 (1981) 285. G. Valette, J. Electroanal. Chem., 138 (1982) 37. T. Vitanov, A. Popov and E.S. Sevastyanov, J. Electroanal. Chem., 142 (1982) 289. G. Valette, A. Hamelin and R. Parsons, Z. Phys. Chem., 113 (1978) 71. J. Clavilier, A. Hamelin and G. Valette, C. R. Acad. Sci., Ser. C, 265 (1967) 221. A. Hamelin, Z. Borkowska and J. Stafiej, J. Electronanal. Chem., 189 (1985) 85. D. Dickertmann, F.D. Koppitz and J.W. Schultze, Electrochim. Acta, 21 (1976) 967. W.R. Fawcett and M.D. Mackey, J. Chem. Sot. Faraday Trans. I, 69 (1973) 634. L.I. Antropov, M.A. Gerasimenko and Yu.S. Gerasimenko, Elektrokhimiya, 7 (1971) 1524. R.R. Adzic, M.E. Hanson and E.B. Yeager, J. Electrochem. Sot., 131 (1984) 1730.
200 (1986) Lausanne -
389-396 Printed
389 in The Netherlands
Preliminary note THE DEPENDENCE OF THE POTENTIAL OF ZERO CHARGE OF SILVER ELECTRODES ON THE CRYSTALLOGRAPHIC ORIENTATION OF THE SURFACE *
M. BACHETTA
and S. TRASATTI
Department of Physical 21, 20133 Milan (Italy) L. DOUBOVA Laboratoire (France) (Received
***
Chemistry
and Electrochemistry,
University
of Milan,
Via Venezian
and A. HAMELIN
d ‘Electrochimie 16th
**
December
Znterfaciale, 1985;
CNRS,
in revised form
1 Place A. Briand, 28th January
92190
Meudon-Belleuue
1986)
INTRODUCTION
The importance of investigating single crystal-face electrodes for the understanding of the factors governing the structure of the metal/solution interface can hardly be overemphasized [ 1,2] . In particular, it has been found that the potential of zero charge depends on the crystal orientation in a way which closely parallels the behaviour of other anisotropic properties such as the electron work function [3,4] and the surface energy [5]. However, a fairly complete collection of data of the potential of zero charge (E,= ,,) exists only for Au [6], while the dependence of the electron work function on the crystal orientation has been investigated exhaustively only for Cu [7] . With such inhomogeneity of experimental data there exists no definite proof that all sdmetals (Au, Ag, Cu) behave electrochemically in a comparable way. Although Ag single crystal electrodes give reliable capacitance curves [ 81, the crystal faces thus far investigated are appreciably fewer than for Au [6], which prevents all of the characteristic features of the (E,= 0 orientation) relationship to be brought into evidence. Precise data of E,,=. have been collected for the low-index faces of Ag such as (ill), (100) and (110) [ 8-111. One value is available also for the (210) face [ 121 , whereas no data exist for higherindex faces which would permit the gaps in the [llO] and [Oil] zones to be filled up. In these zones, the (311) face is expected [6] to be a “turning-point”, like the (210) face, wheras the (331) face would be useful to confirm that no minimum is present in the relationship inside the [ 1101 zone [ 51. From a paper presented at the 36th ISE Meeting, 23-27 September 1985, Spain. ** To whom correspondence should be addressed. l ** Supported by the Department of Physical Chemistry and Electrochemistry versity of Milan. l
0022-0728/86/$03.50
0 1986
Elsevier
Sequoia
S.A.
Salamanca,
of the Uni-
390
The present work has been aimed at filling the above gaps. In particular, we report some preliminary results on the behaviour of the (311) and (331) faces in solutions of weakly ‘adsorbed or not-adsorbed electrolytes. In order to be able to appraise the reliability of the data obtained for the above faces which have never been investigated before, the (110) face has been used as a reference system in view of the precise data existing in the literature [9] . EXPERIMENTAL
Ag single crystals were grown starting from a 3 mm diameter bar (Metalli Preziosi 99.999%) using a technique developed at the LEI [ 131. They were oriented to ?2” with the aid of X-rays, cut and mechanically polished with alumina of different grades down to a mirror finish. In the case of the (110) and (311) faces, the bar was enclosed in a Teflon holder and the working surface was isolated by a Scotchcast non-contaminating resin. The final surface preparation before each experimental run was carried out by means of a chemical polishing procedure based on Cr03 * . The electrode was transferred to the cell while protected by a drop of working solution. In the case of the (331) face, after the mechanical polishing, the crystal was annealed in a gas flame and cooled in pure argon [ 141. The electrochemical measurements were then carried out using the hanging electrolyte technique developed by Schultze and co-workers [ 151. Capacitance curves were obtained by recording the faradaic (IF) and capacitive (I,) components of the alternating current with the aid of a Brucker E-310 polarograph equipped with a lock-in amplifier. The differential capacitance of the electrode/solution interface was calculated by means of the formula: C = [(I;
+ I;)&]
(1/2~
AVA)
The frequency v was 17 Hz, the alternating potential difference AV 10 mV, and the rate of electrode potential variation 10 mV s-l . The geometric surface area of the electrode was used for A. Solutions were prepared with “Millipore” water, NaF Suprapure (Merck) and KPF, 98% (Ventron) which was submitted to further purification [ 161, A two-component cell provided with a Luggin capillary from the reference electrode was used for the measurements. The counter-electrode was made of glassy carbon, while a saturated calomel electrode was used as the reference electrode. The temperature of the solution was kept at 25?O.l”C by means of a water thermostat. RESULTS
Figure 1 shows a typical capacitance curve for the Ag (110) face in fluoride solutions as obtained in this work compared to curves taken from the literature [ 8,9] . The positions of the minimum and the two maxima correspond very accurately to the reference data. The general shape of the curve is also in good agreement with the previous measurements. A systematic difference is however * The authors
are grateful
to Prof.
G. Piazza for the details of the procedure.
391
observable in the depth of the diffuse layer minimum. Whether this is a real effect related to the degree of crystallinity of the surface [ 81 or is an effect of the experimental technique used in this work is still to be clarified. According to the data for Au [ 3,5], the potential of zero charge around the (110) face is sensitive to the crystal orientation (although not as much as in other regions) and its accurate values can be used in principle as a proof of surface perfection.
-10
f /
-05 V(SCE)
Fig. 1. Typical capacitance (referred to the geometric surface area)-potential curves of the (110) face of Ag in 0.02 mol dmm3 NaF solution. (1) Ref. 8; (2) ref. 9; (3) this work.
The capacitance curves in KPF6 solutions showed the same kind of agreement with the literature data [ 91. The potential of the diffuse layer minimum was found to shift towards more negative values in NaF but not in KPF6 solution in close agreement with Valette’s observations [ 91. The shift is attributable to the specific adsorption of F-ions at more positive charges, this being responsible for the anodic peak. The depth and the position of the minimum is thus essentially dependent on the height of the positive maximum. A detailed analysis of the specific adsorption of F- ions on the (110) face of silver will be reported in a forthcoming paper. In Fig. 2 the potential at the minimum in the capacitance curves is plotted vs. the electrolyte concentration on a linear scale. As shown for Hg by Antropov et al. [ 171, the potential varies linearly with concentration so that an accurate value of E,=, in the absence of specific adsorption can be obtained by extrap-
392
olation to c + 0. The figure shows the close agreement between our values and those reported previously [ 91. Both sets of data in NaF and KPF6 give the same extrapolated value of Eozo, i.e. -0.975+0.005 V (SCE), in excellent agreement with the value recommended in the literature [6]. It is interesting that if EazO is estimated by back-integration of capacitance curves in F- + PF; mixed electrolyte solutions at constant ionic strength, the shift with F- concentration is almost not apparent, which suggests that specific adsorption is essentially suppressed at u = 0 under these conditions. Figure 3 emphasises the effect of fluoride ions on the capacitance curve for the (311) face. The general features are essentially the same as those for the
NaF
0.06 c/m01
L._
009 dmm3
Fig. 2. Dependence of the potential of the capacitance minimum electrolyte concentration. (a,~) Ref. 9; (A,=) this work.
for the (110)
face on the
NaF
Fig. 3. Typical tions.
capacitance
curves
of the (311)
face of Ag in 0.02
mol dmm3 electrolyte
solu-
393
(110) face in Fig. 1. More specifically, the height of the two peaks is the same in KPF6 solution. The increase of the positive peak in NaF solution causes the position of the minimum to shift towards more negative potentials. The variation of the potential of the diffuse layer minimum with the electrolyte concentration is shown in Fig. 4. Linear extrapolation gives, in both electrolytes, E, =o = -0.905+0.005 V (SCE) for the (311) face. Comparison of Fig. 4 with Fig. 2 leads to the qualitative consideration that the rate of change of E,,,,with electrolyte concentration is higher for the (311) than for the (110) face. The real significance of this experimental fact needs further study to be assessed. Preliminary measurements have been carried out with the (331) face in one dilute NaF solution. The potential of the capacitance minimum can be regarded as the Eo=,, only to a first approximation (cf. Figs. 2 and 4). Its value is -0.910 +O.OlO V (SCE) but the true value of Eoco for the (331) face could be 5 to 10 mV more positive. The values of Eo=,,have been summarized in Table 1. The values of E,h at c = 0.010 mol dme3 NaF for the (110) and (311) faces have also been reported for comparison.
I
0.02
‘-
KPF 6
0.04
006
c/m01
dme3
Fig. 4. Dependence of the potential electrolyte concentration.
TABLE
of the capacitance
minimum
for the (311)
face on the
1
Potential of zero charge of Ag single crystal faces. Eb for the (110) and -0.915 V for the (311) face Face
E,=,IV
(110) (311) (331)
-0.975+0.005 -0.905+0.005 (-0.910+0.010)
(SCE)
Electrolyte KPF,, NaF (c + 0) KPF,, NaF (c -t 0) 0.010 M NaF
in 0.010
M NaF solutions:
-0.980
V
394 DISCUSSION
The data presented here for the (110) face show that the chemical polishing procedure used in this work results in surface conditions which are quite comparable to those obtained by means of electropolishing in NaCN solutions [ 8,9] . Since chemical polishing in NaCN solutions produces Ag surfaces which give the characteristic LEED pattern [ 181, a well ordered surface is expected to result also from the chemical polishing in Cr03 solutions, but no direct experimental proof of this exists at present. The dependence of Eo=,, on the crystal orientation can be plotted in a way that gives rise to characteristic graphical features [3,5,6]. This is shown in Fig. 5, where, as expected from the characteristic dependence observed [3] in the case of E,=, for Au and of Cpfor Cu, E,=, of Ag can be seen to vary with the crystallographic orientation in such a way that cusps appear for the (100) and (111) faces, minima for the (311) and (210) facesin the [Oil] and [OOl] zones, respectively, while the (331) face does not show any specific feature in the [ 1101 zone. Therefore, the qualitative effect of the crystal orientation on EozO is now also well documented for Ag over all three main crystallographic zones. It may be of interest to try to argue about the “correct” quantitative dependence of E,=, of Ag on the crystal orientation. Since Au and Ag have the same crystal structure and almost the same atomic size, a correlation between the potentials of zero charge of the two metals may reveal interesting features. This is shown in Fig. 6a. Eozo for Au have been taken from Lecoeur et al. [3].
07 5 co > \ E 08 ul
/
09
/
I
! IO
1 1
110)
Fig. 5. The potential lographic orientation.
of zero charge
of Ag single crystals
plotted
as a function
of the crystal-
395
,
(4
iii -O78 5 >-09-2 -09 -
/ (331)*13:‘: ’ / /
-01
0
/
/
&O,
01
02
f,:U, / i’(.SCE)
/
/
/*;11,
b)
mo), ’ z (311) / .a/ / (331) /
/
/
09 J’(hkl)/
/
(111) ,* /
08 J’(210)
Fig. 6. (a) Correlation between the potentials of zero charge of Ag and Au single crystal-face electrodes. (b) Correlation between the potential of zero charge of Ag single crystal-faces and the relative surface energy calculated on the basis of the sole nearest-neighbour interactions [5,6].
A linear correlation whose slope is lower than unity because E, =,, extends over a larger range for Au, does indeed exist. However, it is difficult to decide where the straight line should be located. If it is drawn so as to go through the points for the (210), (100) and (111) faces, those for the (llO), (311) and (331) faces can be seen to fall away from the line by more than expected, in terms of experimental accuracy. It is in principle hard to say if the deviations are due to the values for Au or to those for Ag. In order to be able to discriminate among various possibilities, it may be helpful to resort to a non-electrochemical surface parameter viz. the surface energy, for an additional correlation. In view of the correspondence between the variation with crystal orientation of the calculated relative surface energy and of Ea=,, for Au [ 5,6] an attempt is made in Fig. 6b to correlate these two parameters for Ag also. If the straight line is drawn so as to go through the points of the (210) and (111) faces as in Fig. 6a, the (331) face is seen to come closer to the correlation. Therefore, at least for this face, the point in Fig. 6a is far from the line, seemingly because the E,=. of Au is too negative. This is confirmed by a close inspection of the yrel vs. EozO correlation for the [ 1101 zone of Au [6] showing that the points of the potential of zero charge for the various faces including the (331) one lie well below the curve of the relative surface energy. Since no strict quantitative significance can be attached to the correlation in Fig. 6b, surprisingly, it appears to be as good as that observed for Au. However, there remains the ambiguous position of the (311) face which needs further clarification. One might wonder whether the deviation is real and so, what factor could be responsible for this. CONCLUSIONS
The data reported in this paper show that, while the predicted relationship between E,=, and surface orientation is correctly observed in the case of Ag also, a more quantitative analysis of the situation requires further study with
396
crystal faces of higher indices and a specific investigation with different samples of the same faces used in this investigation in order to verify the stability and reproducibility of the observed interfacial parameters. ACKNOWLEDGEMENT
Financial support of this work by the Ministry fully acknowledged.
of Education
of Italy is grate-
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
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