Cyclic voltammetry at gold single-crystal surfaces. Part 2. Behaviour of high-index faces

JO~JRNAL OF

ELSEVIER

Joumal of Electroanalytical Chemistry 407 (1996) 13-21

Review

Cyclic voltammetry at gold single-crystal surfaces. Part 2. Behaviour of high-index faces A. H a melin

a, * 9

A.M. Martins b

a Laboratoire d'Electrochimie des Interfaces du CNRS, 1 place A. Briand, 92195 Meudon Cedex, France Departamento de Quimica, Faculdade de CiOncias, 4000 Porto, Portugal

Received 7 August 1995; in revised form 9 November 1995

Abstract

Typical cyclic voltammograms at high-index faces of gold in acidic electrolytes are given and discussed comparatively. These findings may be considered as a starting point for further electrochemical studies on these faces; they provide a basis for comparison of results obtained on different samples of the same nominal crystallographic orientation in the same laboratory or in different laboratories. They can also be compared instructively with cyclic voltammograms for the same processes at low-index faces (Part 1, J. Electroanal. Chem., 407 (1996) 1.). The implications of the results for elucidating potential-induced surface reconstruction are also considered. Keywords: Cyclic voltammetry; Gold single-crystal faces; Gold laqueous solution interfaces; High-index gold faces

1. Introduction

An increasing amount of attention is currently being given to high-index faces of gold. High-index faces may be divided into those at less than 5 ° from a low-index face, which are said to be "vicinal faces", and those further than 5 ° from a low-index face, which are said to be stepped faces and open faces. In this paper our interest is focussed on the latter. Some studies of high-index gold faces in aqueous solutions are available in the literature: the determination of the potential of zero charge pzc, [1,2], the adsorption of halides [1,3], the underpotential deposition (upd) of lead [4-6], the kinetics of some electrochemical reactions [7-9], etc. Only recently was it observed, by in-situ scanning tunnelling microscopy (STM), that the superficial atomic structure of some high-index faces is the ideal one (1 X 1 surface structure); i.e. that little or no reconstruction takes place [2,10]. Reconstruction at negative charges, which has been observed in-situ for low-index faces, was not observed for the (210), (221), (311), (410) and (533) faces, but it was observed for (331) [2,10]. In our previous paper (Part 1 [29]) devoted to low-index gold faces, we described how cyclic voltammograms (CVs)

* Corresponding author. 0022-0728/96/$15.00 @ 1996 Elsevier Science S.A. All fights reserved SSDI 0022-0728(95)04500-7

can be utilized to both check the surface state at the metal Isolution interface and also to ascertain some details of the potential-dependent surface structure, especially in conjunction with STM or X-ray techniques. The aim of the present paper is to extend these notions to some high-index faces. This will enable experimenters to examine these electrochemical interfaces readily and indirectly to inspect in-situ the state of their gold faces. Some of the results given below have been collected from the literature, while others have come from unpublished or new experiments. They all provide starting points for further studies of high-index gold faces.

2. Results

For the faces studied, Table 1 gives the Miller indices, the angle from a low-index face, and the ideal (accepting that there are no defects) surface structure accepting the TLK model and following the notation of Lang et al. [ 11 ]. Surface reconstruction, when observed by in-situ STM at negative charges on the electrode, is noted in the last column of Table 1. In section 2 of Part 1 [29] the experimental conditions necessary to obtain meaningful CVs characteristic of a given gold face were described. The same requirements and procedures apply to the present high-index faces as

14

A. Hamelin, A.M. Martins~Journal of Electroanalytical Chemistry 407 (1996) 13-21

Table 1 Face

Angle from low-index plane (°)

Notation [11]

In-situ reconstruction from STM [2]

(Ill) (755) (533) (211) (311)

0 9.45 14.41 19.46 29.50

(lll) 6(111)-(100) 4(I 11)-(100) 3(111)-(100)

Yes No

,/2(111)-(100) ~ 2(100)-(111)

No No

stepped faces may be based on the TLK model, for which the notation is given in Table 1. The CVs of some faces on the six sections of the three main zones of the unit projected stereographic triangle (fig. 2 of Part 1 [29] and Table 1) are given below. They were obtained in 0.01 M perchloric acid. For this electrolyte the adsorption of the anion is far less than in sulphuric acid and most other electrolytes.

2.1. The (111)-(311) segment of the [017] zone (31 l) (511) (11,1,1) (100)

25.26 15.78 7.32 0

idem 3(100)-(111) 6(100)-(111) (100)

(100) (910) (410) (310) (210)

0 6.34 14.04 18.43 26.56

(100) 9(100)-(110) 4(100)-(110) 3(100)-(110) f 2(100)-(110) ~, 2(110)-(100)

(210) (320) (110)

18.43 11.31 0

idem 3(110)-(100) (110)

No

(110) (771) (551) (331)

0 7.76 8.05 13.26

(110) 4(110)-(111) 3(I 10)-(111) f 2(110)-(111) 1,2(111)-(111)

Yes

(331 ) (221) (332) (554) (111)

22.00 15.80 10.03 5.76 0

idem 4(111)-(111) 6(111)-(111) 10(111)-(111) (111)

Yes Yes No No

Yes

Yes Yes No

On the ideal surfaces, as one proceeds from (111) to (311), the density of (100) monoatomic steps increases and the width of the (111) terraces decreases. The atoms on the (100) steps are close packed [1]. CVs obtained in 1979 [12] (Fig. 1), for faces on this segment of the unit projected stereographic triangle, display different profiles of current in region B, i.e. where a monolayer of oxide is formed and subsequently reduced. These results may be compared with the CVs described for the (111) face (fig. 15 of Part 1 [29]). The systematic increase (or decrease) of the amplitude of a current peak observed for upd of lead with systematic increase of the density of steps (fig. 5 of Ref. [6]) is not observed for the formation (and reduction) of a monolayer of oxide. This is probably due to the complexity of this process, which involves several stages. These profiles of current were observed in perchloric acid: - - for (311), four samples; for (211), two samples; - - for (533), four samples; - - for (755), two samples. (In fig. 8 of Ref. [I 3] the CV of the face described there as (755) (made at LEI-CNRS) -

Yes

concerns the surface preparation, solutions, reference electrode, counter electrode, etc. In particular, we consider the cyclic voltammetric responses in region A, where only double layer charging (non-faradaic) processes are involved, and in region B at more positive potentials where oxidative formation and reductive removal of chemisorbed oxygen species dominates the current-potential behaviour. As in Part 1 [29], we note here the structural consequences of observing, in region A, non-symmetric current-potential responses for positive- and negative-going voltammetric traces, which are indicative of non-reversible potentialinduced surface structural changes, especially reconstruction. All the gold faces used in this work were made and studied at LEI-CNRS. For each face, CVs including regions A, B and C (see fig. 1 of Part 1 [29]) are given below, and CVs with enlarged current scales of regions A and C are given when possible. Comparison of different gold faces should be made for the same solution, at one temperature and at a given sweep-rate, to observe clearly the influence of the superficial structure. The understanding of this influence for

-

.2 / (z.)

0--=..=-p-

iLI~!~

-.

U

[ 0

1"

•

J

_)_

EscE/V 0:5-

f.0

Fig. 1. CVs for (755), (533), (211) and (311) in 0.01 M perchloric acid, 23+ t°C, 20 mV s -l [12].

A. Hamelin, A.M. Martins/Journal of Electroanalytical Chemistry 407 (1996) 13-21 100 j 1 p.A cm "z

j //.tA cm "2

(a)

-i00

1;0

0.5

0

15

0

0.5

1.0

(b,~

~

100

T

400 !

100

Fig. 2. CVs for (755) in 0.01 M perchlofic acid, 23 + I°C, 50 mV s - i [14].

is presumably that of a (533) face (from the CV), there was possibly an error in reading the X-ray pattern.) The CVs of Fig. 1 may be compared with CVs obtained more recently: for (755) [14] (Fig. 2), for (533) [14] (Fig. 3) and for (311) (Figs. 4(a) and 4(b)). For the (533) face made at the ULB (Brussels, Belgium) identical CVs as in Fig. 3 were observed for the same electrochemical conditions. For (111) in dilute perchloric acid, the first stage of oxidation is reversible (fig. 15 of Part 1 [29]), this is also observed for (755) (Fig. 2), to a greater extent for (533) (Fig. 3) and (311) (Fig. 4). For (311), in dilute sulphuric acid solution, this reversible stage is not observed because of sulphate adsorption (Fig. 5(a)). For (111), the first enlarged CV of regions A and C, after having explored region B, is different from the following ones observed with a lower positive potential limit (1.1 V vs. RHE), i.e. without exploring region B (fig. 15 of Part 1 [29]). This phenomenon was explained from STM observations (see Part 1); it is observed for (755) but does not seem to exist for (533) (Fig. 3) and (311). From in-situ STM observations in perchloric acid, it can be seen that the (111) face is reconstructed at negative /

j / ~A cm "z

)

't

j I ~tA cm "2

,/

_

200 100 0

0

ZOO

J/ItA cm.2

010

1:.0;2

.

,:0

/

j

Fig. 4. CVs for (311) in 0.01 M perchloric acid, 23 5: I°C, 50 mV s- t : (a) for different positive potential limits; (b) for different negative potential limits [ 14].

charges (Part 1 [29]), while for (533) and (311) only a relaxation is observed at negative charges [10]. These findings are in agreement with the results in Figs. 3 and 4(b), i.e. for these high-index faces there is no influence of the potential of the negative limit on the pzc value and the general shape of the CVs in region A. In other words, there is no evidence for non-reversible induced changes of the surface structure (see above). The shape of these CVs is not altered for sweep-rates from 25 to 80 mV s-1 (Fig. 5(b)) and temperatures from 288 to 307 K (fig. 1 of Ref. [15]). CVs obtained in sodium hydroxide solutions for (311) are given in Refs. [16] and [17].

2.2. The (311)-(100) segment of the [017] zone

0

0.5

1.0

1.5

Fig. 3. CVs for (533) in 0.01 M perchloric acid, 23 5: l°C, 80 mV s - l ; enlarged CVs for different positive and negative potential limits [14].

On the ideal surfaces, from (311) to (100), the density of densely packed (111) monoatomic steps decreases, and the width of the (100) terraces increases. The (3 t 1) face was discussed in the above section and the (100) face in Part 1 [29].

16

A. Hamelin, A.M. Martins/Journal of Electroanalytical Chemistry 407 (1996) 13-21

j / p-A cm"2

(a)

100 0

0.5

1.0 ~

Em-IE/ V

-I00 -200

C / ]ti.Fcm "2

(b)

(100) terraces decreases. These (110) steps are not densely packed; they are corrugated on the atomic scale. The CVs for the (100) face have been discussed in Part 1 [29]. The CVs for the (210), (310), (410) and (910) faces obtained in 0.01 M perchloric acid are given in Fig. 7. For (910), the results are reminiscent of those of (100), both with regard to the profile of current in region B and the influence of the negative and positive potential limits. On the enlarged CV, the current peak C is less important than that for (100) (fig. 18 of Pan 1 [29]) but more visible than that for (410) (Fig. 8). Peak C disappears when the lower potential limit is set at 0.3 V or when the positive potential limit is shifted to progressively smaller positive values, as it does for (100) (fig. 18(b) of Pan 1 [29]). On

2000

j / ,A cm-~

0

0.5

1.0

200 0,!

(a) /

.....

-Jt/ Fig. 5. CVs for (311): (a) in 0.01 M sulphuric acid, 23:t: I°C, 50 mV s 1, for different positive potential limits; (b) in 0.01 M perchloric acid, 235:t°C ( . . . . . ) 25 mV s - l , ( ~ ~ ~ ) 50 mV s 1, ( ) 80 mV s -~ .

For (511), two samples were prepared at LEI-CNRS. Some recent results [14] are given in Fig. 6(a); for this face on the enlarged CV, there is no change of the pzc value or modification of the CV, with a change of the lower potential limit or without exploration of region B. No reversible first stage of oxidation is observed for this face. For (11,1,1), two samples were prepared at LEI-CNRS. Some recent results are given in Fig. 6(b), other results are shown in fig. 7 of Ref. [13]. From the CVs observed in 0.01 M perchloric acid there is no reversible first stage of oxidation for this face, and no change of the pzc value when the negative limit is shifted positively, in contrast to the behaviour of (100) (see Pan 1 [29]). Therefore there appears to be no reconstruction for the (11,1,1) face. 2.3. The (100)-(210) segment o f the [001] zone

On the ideal surfaces, from (100) to (210), the density of (110) monoatomic steps increases and the width of the

J / P'A cm'~

o

o.s

t l.O .

/

'i 200 0 ~

l

Fig. 6. CVs in 0.01 M perchloric acid, 23+ I°C: (a) for (511), 80 mV s- 1, enlarged CVs for two negative potential limits and after four short cycles from -0.1 to 1.0 V vs. RHE [14]; (b) for (11,1,1), 50 mV s-l, for different positive potential limits, enlarged CVs for different negative and positive potential limits.

A. Hamelin, A.M. Martins~Journal of Electroanalytical Chemistry 407 (1996) 13-21

(910)

~

•Y-- " " - I

r<~/~.~ /

(41o) O-

•11 "" .'I .J., \ , ~ , ...

(310)

O-

-

(210)

\-1

l y-~" ""

0~

i

0

i

110 ~

0.5

1.5

Fig. 7. CVs in 0.01 M perchloric acid, 23 + I°C, 50 mV s- ~, for (910), (410) [14], (310) [25], (210).

an ideal (910) face the terraces are 9 atoms wide. From the above electrochemical observations, a partial surface reconstruction seems to occur on these (100) terraces for negative charges at the gold surface. For (410), the CV and enlarged CV are given in Fig. 8. A positive shift of the lower potential limit does not change the pzc value significantly. A very small peak C

j / ptA em -2

/.~,,

/

o

C...~..

v

: 0:s

1.0

17

(see Part 1 [29]) disappears when the potential exploration is restricted to about 0.3 V vs. RHE, as it does for (100) and (910). For an ideal (410) face, the terraces are 4 atoms wide according to the TLK model. A slight reconstruction seems to occur for this face, although it was not observed by STM [2] or on C ( E ) curves obtained at Purdue University [2] for this face. Both (410) oriented samples used were prepared at LEI-CNRS. From the enlarged CVs observed during continuous cycling, it is discerned that the value of the potential at the capacity minimum, corresponding to the pzc value, is more positive on the negative-going sweep than on the positivegoing one, by 60 mV for (100), 40 mV for (410) and (910). This is probably due to the non-reversible reconstruction-deconstruction of the (100) terraces. For (310), with different positive potential limits (Fig. 9(a)) and different negative potential limits (Fig. 9(b)), there are marked differences compared with the behaviour of the (100) face. On the ideal structure, the terraces are only 3 atoms wide. This presumably means that no reconstruction is possible. Six (210) faces were prepared at LEI-CNRS and one at IQSC-USP [18]. They were studied at LEI-CNRS, IQSCUSP (Brazil), Purdue University (USA), the University of Porto (Portugal) and the ULB (Belgium). The CVs obtained are closely similar. CVs given in Fig. 10 are a selection of results obtained at LEI-CNRS in 1995. The influence of the temperature on the CVs is shown in fig. l(b) of Ref. [20]. Already in the 1970s, (210) was recognized from electrochemical observations as not being reconstructed even at negative charges [19]. This stability enabled it to be used to study the influence of changes of different parameters, for instance: the temperature [20], the concentration and pH of the solution [21,22]; and to investigate electrochemical reactions, for instance: the oxidation of CO [8,23]. Recently this face was studied as a model for impedance spectroscopy at solid electrodes [24]. It was confirmed from STM observations [2] that, in perchloric acid, there is no reconstruction of the (210) face at negative charges. CVs in sodium hydroxide solutions for (100) and (210) are given in Ref. [16].

1;s / 2.4. The (210)-(110) segment o f the [001] zone

.200

I

Fig. 8. CVs for (410) in 0.01 M perchloric acid, 23 5: I°C, 50 mV s- ~, for two positive potential limits, enlarged CV for two negative potential limits [14].

On the ideal surfaces from (210) to (110), the density of (100) monoatomic steps decreases and the width of the (110) terraces increases. The ideal (210) and (320) faces are rough on an atomic scale, as is (310) (Fig, 11) [26]. These models ignore the multilayer relaxation which exists on open metal surfaces [2]. The CVs of the (210) face were discussed in the above section and those of the (110) face in Part 1 [29]. Two (320) oriented gold faces, prepared at LEI-CNRS, gave similar results. The electrochemical behaviour of

18

A. Hamelin, A.M. Martins/Journal of Electroanalytical Chemistry 407 (1996) 13-21 j //.tA em-2

"j / ~tA cm -=

(a)

(a)

100 .100 0 0.5

0 0

1;0 ~

0.5

j

i

"i

I7

-100

ni~ I V

100 200

-200

j / ~ . cm-z

(hl

(b)

/

A

200

Fig. 10. CVs for (210) in 0.01 M perchloric acid, 23+ I°C, 50 mV s - l : (a) influence of the positive potential limit; (b) enlarged CV, influence of the negative and positive potential limits.

/

Fig. 9. CVs for (310) in 0.01 M perchloric acid: (a) for different positive potential limits; (b) enlarged CVs, influence of the negative and positive potential limits.

(320) is very close to that of (210): no reversible first stage of oxidation, no modification of the enlarged CV when the lower potential limit is shifted or when region B is not

,10~o] .I05ot

..° -3 i

i

i

j

.2

[oo,1 (210)

_Lx BoO

[oo0 (310)

(320)

Fig. 11. The (210), (310) and (320) ideal atomic structures looking normally at the model [26]. The centre of each surface atom which lies on, or inside, the unit cell boundary ( - - - ) is marked with a dot. 0, 1, 2, 3, 4 are the number of layer atoms.

A. Hamelin, A.M. Martins/Journal of Electroanalytical Chemistry 407 (1996) 13-21 j / p.A em-2

j I BA cm .I00

19

-2

0~

Earm/V

V

o's

1'o

ERrm / V

1:5

Fig. 14. CVs for (551) in 0.01 M perchloric acid, 235= l°C, 50 mV s -I , for different positive potential limits. "200

Fig. 12. CVs for (320) in 0.01 M perchloric acid, 23 + l°C, 50 mV s- 1, for different positive potential limits.

explored (Fig. 12); this could be anticipated from their ideal atomic structures (Fig. 11). 2.5. The (110)-(331) section of the [170] zone On the ideal surfaces from (110) to (331) the density of (111) monoatomic steps increases and the width of the (110) terraces decreases. The (111) steps are densely packed and the (110) terraces are corrugated on the atomic scale•

From in-situ STM observations in perchloric acid [2], the (110) face and the (331) face are reconstructed at negative charges in perchloric acid. The CVs obtained with (110) faces were discussed in Part 1 [29]. The CVs of the (110), (771), (551) and (331) faces are given in Fig. 13. For (551) and (331) the first reversible stage of oxidation in perchloric acid is superimposed on the non-reversible stages of oxidation (Figs. 14 and 15). For (331), there is no clear indication of reconstruction on the enlarged CVs (Fig. 15); however, reconstruction at negative charges was observed by STM [2,10]. 2.6. The (331)-(111) segment of the [170] zone On the ideal surfaces, from (331) to (111), the density of densely packed (1 l l) monoatomic steps decreases and

J j I p.A cm-2 (110)

0-,

--~ °.

°.

(771) . ..._-.._

(SSl) 0.

. . .

:-

'

I (-

-~/~

I

1.o j '

" -:,

-

031) O-r

V 0

0'.5

1'•0

Emm/V

l'.s

Fig. 13. CVs for (110), (771) [14], (551) and (331) in 0.01 M perchloric acid, 2 3 + I°C, 50 mV s - l .

200

\

Fig. 15. CVs for (331) in 0.01 M pcrchloric acid, 235: I°C, 50 mV s - t , for different positive potential limits; enlarged CV for different negative potential limits.

A. Hamelin, A.M. Martins~Journal of Electroanalytical Chemistry 407 (1996) 13-21

20

I"I t\ (221)

.At

33:)

.l'

0..~=

(554)

/

"." I

3. Discussion and conclusions

4 Epa~E / V 0

0.5

1.0

1.5

Fig. 16. CVs for (221), (332) and (554) in 0.01 M perchloric acid, 23-t- I°C, 50 mV s - l .

the width of the (111) oriented terraces increases. These faces may be considered either as n ( l l l ) - ( l l l ) or as (n - 1)(111) - (110), i.e. with non-kinked (110) steps (see fig. 2 of Part 1 [29]). The current profiles for region B for (221), (332) and (554) are given in Fig. 16. The first reversible stage of

/ rtAem2

_

~

o

Ep.HE / V 0

0.5

1.0

oxidation is more clearly observed for (332) (Fig. 16) than for (221) and (554) (Fig. 17). From the enlarged CV, the behavior of (554) (Fig. 17) is slightly reminiscent of that of the (111) face; not exploring region B lowers the capacitive currents slightly at negative charges.

1.5

Fig. 17. CVs for (554) in 0.01 M perchloric acid, 2 3 + I°C, 50 mV s -I for different positive potential limits; enlarged CVs for different negative potential limits; ( - - - ) enlarged CV recorded afl~r four " s h o r t " cycles from - 0 . 0 4 to 1.02 V vs. RHE.

The use of deaerated 10 mM perchloric acid solution for observation of typical CVs at gold faces in a range of potential including the double layer region A, the formation of a monolayer of oxide and its subsequent reduction B, and the beginning of the reduction of protons C, is fully justified. In sulphuric acid solution the adsorption of sulphate ions would mask the details in B. In non-buffered neutral solutions the clear observation of B is difficult, and in buffered neutral solutions there are ions adsorbing more strongly than perchlorate. In very dilute perchloric acid, the ohmic potential drop has to be compensated (see fig. 7 of Part 1 [29]); this can cause problems. One interesting result of the present study is the deduction from the presence (or absence) of hysteresis in the voltammetric response, within the (double layer) region A, whether or not potential-dependent reconstruction is taking place. In particular, it is evident on this basis that while the low-index (111) and (100) planes clearly undergo reconstruction at negative electrode charges, the stepped faces (755) and (11,1,1) [i.e. 6(111) - (100) and 6(100) - (111)] remain unreconstructed throughout region A (i.e. at negative and positive charges). The former finding is consistent with the observed stability of the unreconstructed (755) face in ultrahigh vacuum [27], as well as with in-situ STM data for stepped faces within this crystallographic zone [2,10]. This stability of the (1 X 1) stepped faces can be rationalized on the basis of the relatively large unit cells (v'-3 X 23) and (5 X 27) for the reconstructed (111) and (100) faces respectively; such structures cannot readily form given the narrowness of the (111) and (100) terraces present on these surfaces. More generally, the present results point to the usefulness of the CV technique for signalling the occurrence of potential-induced surface reconstruction, at least if it is non-reversible in nature. This is especially true in the wake of the in-situ microscopic (STM and X-ray) techniques that turn out to be in good accord with the conventional electrochemical results in this regard. In contrast to this double layer region A, no clearcut consistent connection could be established here between the crystallographic orientation of the electrode and the CV response within the surface oxidation/reduction region B. This situation, however, is understandable given the complexity of these redox processes. The first re-

A. Hamelin, A.M. Martins~Journal of Electroanalytical Chemistry 407 (1996) 13-21 Table 2 Density of broken bonds per area a of unit cell for different gold faces dbt~/ a 2

Face

dr,t, / a 2

(111) (755) (533) (211) (311) (511) (11,1,1)

Face

6.92 7.63 7.93 8.16 8.44 8.46 8.29

(310) (210) (320) (110) (771) (551) (331)

8.88 8.94 8.87 8.48 8.44 8.40 8.25

(1oo)

8.00

(221)

8.0o

(910 (410)

8.39 8.73

(332) (554) (111)

7.67 7.38 6.92

a Area in units of a 2. a, lattice parameter.

versible stage of oxidation described for (111) (see Part 1 [29]) is observed along the whole (111)-(311) section, and particularly for (533) which is made of (111) terraces 4 atoms wide. Is it that such terraces are too small for reconstruction and allow a reversible first stage of oxidation more easily? This first reversible stage of oxidation is also observed from (111) to (511); therefore it is related to the three-fold symmetry sites at the gold surface. Nonetheless, it is worth mentioning that the variations of the pzc values, extracted from the diffuse-layer minimum situated within region A, can be correlated with the crystallographic orientation on the basis of the "terraceledge-kink" (TLK) model. Indeed, this provides an additional justification to the deployment of the segments of crystallographic zones to organize the experimental data summarized above. A closely related comparison with the pzc values involves the density dbb of broken bonds per unit cell at the surface [28]. This parameter is given in Table 2 for the faces studied above. It should be noted that the ideal (310), (210) and (320) faces have the highest densities of broken bonds: 8.88, 8.94 and 8.87 respectively. It is therefore not surprising that their CVs are nearly similar in region B (Figs. 7, 10, 12). These models (Fig. 11) ignore the multilayer relaxation which exists on such open surfaces [2]. The TLK model was also used to rationalize the systematic dependance of the voltammetry of upd of lead on the gold faces [4], although clear agreement with the TLK model was found only for the (111)-(311) and (111)-(110) zone segments. Although the foregoing is only an incomplete (and subjectively selective) survey of experimental CVs at gold faces, it is evident that much may be learned concerning ordered gold [aqueous solution interfaces by such a simple technique.

21

Acknowledgements

A.H. thanks all her coworkers over the past 27 years. The authors are indebted to Roger Parsons and Michael Weaver for great improvements of the text.

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