- Email: [email protected]
Oxide formation on gold single crystal stepped surfaces
291
J. Electroanal. Chem., 249 (1988) 291-310 Blsevier Sequoia S.A., Lausarme - Printed in The Netherlands
OXIDE
FORMATION
ON GOLD SINGLE
CRYSTAL
STEPPED
SURFACES
s. STRBAC and RR. AD&C Institute of Electrochemistry, ZCTM, and Center for Multidisciplinary P. 0. Box 815, Belgrade (Yugoslavia)
Studies, University of Belgrade,
A. HAMELIN Laboratoire d’Electrochimie
Znterfaciale, CNRS,
1 Place A. Briand, 92190 Meudon-Bellevue
(France)
(Received 8th December 1987; in revised form 6th April 1988)
ABSTRACT Oxide formation and anion adsorption have been studied on gold single crystal stepped surfaces. Most of the measurements were performed in sulphuric and some in perchloric acid solutions by cyclic voltammetty and by double layer capacity measurements. A pronounced structural sensitivity of both oxide formation and anion adsorption was found. The effects of the terrace orientation, step orientation and step density are reflected in voltammetry and the double layer capacity measurements. A systematic dependence of both processes on step density was found and their interrelation is discussed. The results contradict some gas-phase data reporting no critical role of steps in oxygen chernisorption on gold. A correlation between the relative surface energy of gold single crystals and the peak potentials of the initial stage of oxide formation was found.
INTRODUCTION
Oxide formation on gold has been the subject of numerous electrochemical studies involving polycrystalline [l] and single crystal surfaces [2]. The process, as depicted by cyclic voltammetry, is often used as a criterion for the cleanliness of the surface, and recently its use as a fingerprint of the crystallographic orientation (c.o.) of single crystal surfaces has been proposed [3]. The work of Dickertmamr et al. [4] showed pronounced dependence of the process on the C.O. of the low-index planes. Sotto [5,6] proposed a reaction mechanism which involves the simultaneous existence of three compounds on the surface, viz. Au,O, - 4 H,O, Au0 or Au(OH), and Au,0 or AuOH. Hamelin and the group at Bellevue made a considerable contribution to the study of double layer properties of gold single crystal surfaces, including the oxide formation (ref. 2 and references therein). From these studies the structural sensitivity of oxide formation on Au could also be inferred.
292
A thorough study of the oxidation of low-index planes of Au and the effects of anion adsorption on that process has been reported recently by AngersteinKoxlowska et al. [7,8]. The differences in the patterns of the early stages of this process have been ascribed to the different strengths of adsorption of anions on the low-index planes. The strength of adsorption and the extent of charge transfer of anions having tetrahedral structure (ClO;, SO,‘-) is higher for the planes on which the arrangement of the surface atoms is trigonal, i.e. compatible with their trigonal symmetry. A systematic study of the oxide formation on Au stepped single crystal surfaces has been undertaken by AdtiC and Strbac [9] and is extended in this work. To follow possible regularities in behaviour with variation of density and orientation of steps and terraces, the following orientations of gold single crystals have been chosen: (a) low-Miller-index faces: Au (loo), Au (111) and Au (110);
Fig. 1. Positions triangle.
of the faces investigated
on the three main zones in the unit projected
stereographic
293
(b) faces with low density of steps, and different orientations of steps and terraces:
Au (755)= 6(111)-(100) titid
to
the
Au (332) = 6(111)-(111) Au (610) = w~)--t1~)
Au (ll,l,l)
= 6(1~)-(111)
&+$d
(111j
face.
,
to &.$ (loo> face. 7
(c) faces with high density of steps: Au (211) = 3(111)-(100) Au (331) = 3(111)-(111) Au (311) = 2(111)-(1~) Au (210) = 2(100)-(100) The positions of the faces investigated in the unit projected stereographic triangle are given in Fig. 1.
The experiments have been done in two laboratories: IEH, ICTM, Belgrade, Yugoslavia and LEI, CNRS, Meudon, France. Electrodes were obtained and prepared in different ways. At IEH, ICTM, Belgrade, electrodes were obtained from Metal Crystal Ltd. Company, Cambridge, England, oriented and cut to better than lo. Initial checking of the orientation of some surfaces was done in Belgrade by LEED. No direct transfer into the electrochemical cell was made. Therefore, these data were not exploited further. The electrodes were polished mechanically using diamond paste. The final surface preparation consisted of an electrochemical polishing, followed by a chemical treatment [lo], or flame treatment, followed by cooling in ultrapure water [ll], prepared from 18 MQ cm Mill&ore water distilled further with alkaline permanganate and kept under UV radiation for at least 24 h. In Meudon, growing, orienting and cutting of the gold single crystals was done at LEI, CNRS. Mechanical polishing with ahunina was used, and after checking the orientation by von Latie X-ray backscattering, electrochemical polishing was done. The final surface preparation was done by flame treatment and cooling in ultrapure water (LEI, CNRS, patent No. 86 5103 00). These two different ways of preparing single crystal faces did not give any significant difference in the electrochemical results. The electrolyte solutions were prepared using concentrated HCIO, and H,SO, (Merck) and ultrapure water. The reference electrode was the reversible hydrogen electrode. Capacitance measurements were performed with a linear potential sweep with a superimposed ac component with an amplitude of 10 mV as described elsewhere [12]. All measurements were carried out at room temperature.
294 RESULTS AND DISCUSSION
Differential capacity measurements (C(E) curves) can be used to follow adsorption-desorption processes as a function of potential and C.O. of the gold electrodes. The influence of the co. on the shape of the C(E) curves can be seen in Fig. 2, obtained with Au (100) and Au (111) in 10 mN IICIO,. The influence of the concentration of HSO; and SOi- anions on the C(E) curves for the (111) and (100) faces can be seen in Fig. 3. For 10 mM HCIO, the difference in pzc for Au (100) and Au (111) is about 100 mV (450 mV and 550 mV vs. RHR respectively). A similar difference is found with dilute H,SO, solutions. Increasing the concentration of sulphate ions shifts the pcz negatively. For both faces, for potentials negative of the pzc, the capacity shows a small dependence on the ~n~ntration of the anion in solution (C!lOi, HSO;, SO,‘-). This is in agreement with observations on the
T
f
AdlOO)
Au(lllJ
L
00
05
10
-+
EN
vs RHE
Fig. 2. C(E) curves for Au (100) and Au (111) in 10 mM HCIO,. dV/dt-8 modulation amplitude: A E = 10 mV.
rnV/s;
freq. = 20 Hz;
295
i
00
E/V “S UtE
Fig. 3. C(E) curves for Au (100) and Au (111) in: (1) 50 mM H,SO.,; (2) 2.0 mM H,SO,; H,SO,. dV/dt = 8 mV/s; frcq. = 20 Hz; AE = 10 mV.
(3) 0.32 mM
adsorption of other anions on gold (Cl-, Br-, I-) [23. For less negative potentials, the capacity increases, reaching maximum(s) and then decreases till a constant value is attained for m~um anion coverage. The sharp increase of the capacity at very positive potentials E = 1.0 V is due to the beginning of a faradaic process: i.e. the oxidation of gold surfaces. In concentrated solutions, however, the influence of the diffuse part of the double layer cannot be seen and the pzc cannot be obtained directly, but the adsorption-desorption processes and the influence of the C.O.can be followed. The shift of the pzc with the anion concentration is not as large as would be deduced directly from Fig. 3 since the reference potential @HE) shifts in the positive direction with increased anion (acid) concentration. The number, height and shape of the adsorption capacitance peaks and their relative position for a given anion depend on the concentration of the anion and the C.O.of the electrode surface. No full frequency dispersion analysis has been made. Several frequencies have been tried, however, to ascertain the equilibrium conditions for all concentrations of H2S04 used. In Figs. 4-6, C(E) curves for the faces investigated in the three main zones of the unit projected stereographic triangle in 50 mM II,SO, are given, The general conclusions are: (1) The pzc, as a parameter reflecting the electrochemical properties of the surface, is dependent on the C.O.in solutions containing specifically adsorbed HSO; and SOi- anions. The curves for the (ill), (lOO), (311) and (755) faces did not
296
I
0.0
#
0.5
1
t.0
EIY vsRHE *
Fig. 4. C(E) cusves for investigated faces from the zone [(lOa)-(ill)] mV/s; freq. = 20 Hz; AE = 10 mV.
I
0.0
I
05
i
10
ElVvs RtlE
in 50 mM H,SO,;
.
Fig. 5. C(E) curves for investigated faces from the-zone [(ill)-(llO)] mV/s; freq. = 20 Hz; A E = 10 mV.
in 5@ m M
dV/dt = 8
291
I 00
1
05
I
10
E/V vs RHE
l
Fig. 6. C(E) curves for the investigated faces from the zone [(llO)-(lOO)] in 50 mM H,SO,; mV/s; freq. = 20 Hz; AE =lO mV.
dV/dt = 8
show a contribution of the diffuse part of the double layer because the adsorption of sulphate ion is stronger on these faces, but minima were observed in more dilute solutions. (2) On the (210) face, the surface with the highest surface energy [13], adsorption of sulphate ions begins at a more negative potential than on other faces. (3) On the (111) face, the surface with the lowest surface energy, adsorption of sulphate ions begins at a more positive potential than on the other faces investigated. The Au (110) surface is exceptional considering the very small capacitance peak associated with HSO; or SOi- adsorption. The process seems to be completed already at 0.5 V. There is apparently no increase in the coverage of adsorbed ions with further increase of potential. This suggests that only a part of the surface is occupied by anions. Considering this, and the similarity of the C(E) curves for the Au (331) and Au (110) faces, one is tempted to conclude that the adsorption of these anions on Au (110) takes place in the rails, i.e. at the two-atoms wide (111) oriented terraces. (The (110) orientation can be represented as the stepped surface 2(111)-(ill).) The peak at 0.95 V for Au (331), not seen for Au (llO), is probably due to adsorption of anions at the (111) oriented terrace sites. For this surface the terraces are three atoms wide. The notation (331) is equivalent to 3(111)-(111). Tetrahedral anions can adsorb on such a terrace, in addition to the anions adsorbed in the step [14].
298
The energetic heterogeneity of stepped surfaces is reflected in the adsorption of sulphates (Figs. 4-6). It commences at the most active sites-steps, and continues at terraces. This can be deduced from the adsorption capacitance peaks. If the heights of the adsorption peaks are compared, it can be seen that the adsorption coverage on Au (111) and its vicinal faces Au (755) and Au (332) involves a higher charge transfer, which is also indicative of the strength of adsorption 171.This is probably due to the three-fold symmetry of surface atoms of terraces, which is in registry with the tetrahedral structures of the HSO; and SO:anions. On Au (110) and faces with high step densities, i.e. with narrow terraces, high coverages of anions become difficult to achieve, probably due to the unfavourable steric factor and/or lateral repulsion of anions. In the case of high surface coverages of specifically adsorbed anions there is no gradual building of an OH sublattice [l] which might create induced heterogeneity of the surfaces. The heterogeneity of the surfaces is caused primarily by the surface geometry and consequently the process of oxide formation can be followed as a function of the geometrical arrangement and activity of surface atoms, i.e. as a function of C.O.and surface energy. Oxide formation on Au (hkl); the influence of specific adsorption The oxide formation on the low-index planes of Au has been analysed in detail by Angerstein-Kozlowska et al. f7,S). The proposed mechanism for oxide formation and reduction on gold [l] in the presence of specific adsorption explains the appearance of four anodic peaks on the current-potential profiles by induced heterogeneity of the surface and by gradual building up of the OH sublattice. In concentrated solutions, however, for higher coverages of specifically adsorbed anions, some of the stages of adsorption are blocked [7]. The proposed mechanism is simplified, thus making it easier to follow the effects of the co.
According to the mechanism proposed by Angerstein-Kozlowska et al. [l] the first stage of oxidation corresponds to the formation of the first sublattice of OH deposited in between specifically adsorbed anions: [M,A-]M+H,o~~:[M,A-]MOH+H++~-
(1) In CV curves this is seen as a reversible region, OAl, as a stage of preoxidation (Fig. 7a). Compared with the C(E) curve for Au (111) in 10 mM HClO,, it can be seen that this preoxidation commences before a high coverage by specifically adsorbed anions is attained. Sharp peaks, OA2 and OA3, correspond to the continuing formation of OH sublattices followed by the turnover (reconstruction) processes (RTO): [M,A_]
+ H,O --, M,_, +MOH+A-+H++e-
(2)
MOHMOH + MOHOHM M,_, + (X - I) H,O + [MOEI],_,
(3) +
(X -
1)
H+ +(x
-
1)
e-
(4)
299
Fig. 7. Voltammetric
curves for Au (100) and Au (ll,l,l)
in 10 mM HCIO,; dV/dt
= 80 mV/s.
After completion of a monolayer (which corresponds to the exchange of the first electron per site), transfer of a second electron takes place in a broad potential range, OA4, according to the process: MOH-+MO+H++e(5) Our data (Fig. 7a) are not in complete agreement with those of Fig. 2 of ref. 8. A comparison with later published CVs (for instance Fig. 1 of ref. 3) shows a better agreement. Considering the data obtained with stepped faces (to be shown
300
below), one can argue that the first peak (termed 2-1 in ref. 8) could be caused by the presence of a small quantity of steps and kinks at the surface of a non-ideal (100) face.
Fig. 8. Voltammetric curves for Au (111) and Au (7.55) in 10 mM HCIO,; dV/df = SOmy/s.
301
The CV of the UHV prepared Au (100) surface in 30 mM HCQ, reported by D’Agostino and Ross (151, is not in agreement with that in Fig. 7a, but is very similar to those obtained in the presence of strongly adsorbed anions. For the vicinal Au (Il,f,l) face (Fig. 7b) four peaks can be identified, as for the (100) face in 10 mM HClO,. The preoxidation peaks apparently merge with peak OA2. The Au (755) surface, vicinaI to Au (ill), shows a CV with a slight sixty to the CV of the latter surface. Sites with a unique reversibility of OH adsorption are found on this surface (peak 1, Fig. 8). Overlapping of the anodic peaks occurs, which can be a consequence of a small difference in energy among the sites on which a gradual buil~ng of the OH sublattices takes place.
recently
The voltammetry of oxide formation on Au (100) in sulphuric acid of various concentrations is given in Fig. 9. In very dilute solutions the current profile is
r - oA-----l
Fig. 9. Volleys curvesfor Au (100) in (- *-I -) 0.32m&f H&I,; 50 lnM H2S04. t -_)
(- - -)
2.0 mM H,SO, and
302
similar to that obtained for 10 mM HClO,. By increasing the concentration of H2S04, the first reversible stage of oxidation disappears due to the increased coverage of the surface by specifically adsorbed anions. The oxide formation peaks are shifted positively by increasing the concentration of sulphate ions, while the adsorption peaks are shifted negatively (see Fig. 3). Peaks 0A2 and 0A3 become less separated with increased anion concentration. In 50 m.M H,SO, they overlap, forming one sharp peak at 1.42 V. This peak is dominant and characteristic for Au (lOO)_The majority of anions is replaced by OH at these potentials and the RTO process is associated with it [7]. The small prepeak probably corresponds to the formation of an OH monolayer on defects, the presence of which is difficult to avoid completely or to control. This can be inferred from the CVs of another, apparently better prepared sample, where a pre-peak is transformed into a shoulder (see Figs. 10 and 12 below). Similar curves have been reported recently in ref. 3. Some general conclusions are: (1) in sufficiently concentrated solutions of H,SO,, which facilitate the formation
CM
Fig. 10. Profile of current for formation of a monolayer on the investigated faces from the ((lOO)-(ill)] zone in 50 mM H,S04; dV/dt = 80 mV/s.
303
of a high coverage of specifically adsorbed anions before the initial stage of oxidation, the first reversible peak, UAl, is blocked. (2) peaks 0A2 and 0A3 are very close and eventually overlap. One sharp peak is observed for the oxidation of “flat” Au (100) and Au (111) surfaces. It will be seen below that for stepped surfaces this peak is split into several peaks due to the heterogeneity of the surfaces caused by the presence of sites with different energies (steps and terraces). A comparison of the CVs for vicinal stepped surfaces with those for the low-index planes suggests strongly that the first peak corresponds to the formation of an OH monolayer on steps (more active sites), while the second corresponds to the fo~ation of OH on terraces (less active sites). The peak potenti~s appear ch~~te~stic for a given Au (j&I) face. A broad potential range, OA4, on the CV curves is associated with the exchange of a second electron. Oxide fo~at~~n on Au (hkl); the ~n~u~n~~of orientation and density of steps Zone ~l~~-~~~~~ The profiles of the anodic currents for the faces of this zone are given in Fig. 10.
02
OL
06
06
10
t2
IL
16
E/V VSRHE
Fig. 11. Profile of current for formation of a monolayer on the investigated faces from the [(Ill)-(llO)] zone in SO mM H,SO,; dV/dr = 80 mV/s.
304
A simple analysis of the CVs indicates that, by decreasing the fraction of the surface with (100) orientation, the first peak decreases till its disappearance at Au (111). From Au (311) towards the (111) orientation, by increasing the fraction of the surface with the (111) orientation, the second peak increases and shifts to a more positive potential, becoming a single peak for Au (111). The first peak corresponds to the oxidation of the steps of (100) o~entation, which occurs at potentials somewhat more negative than the oxidation of the “flat” Au (100). The second peak corresponds to the oxidation of terraces of the (111) orientation. For the vi&al (755) surface this peak coincides with the one for the “flat” Au (111) surface (cf. Fig. 13 below). Gradual changes in the current profiles have also been found with surfaces from the other two zones. Zone (Ill)-(110) The anodic parts of the CVs are given in Fig. 11 for all faces from this zone investigated. From the (111) orientation, the fraction with (110) orientation decreases and the fraction of the first peak decreases till its disappearance at Au (111). From Au (331), the fraction of the second peak increases with increasing width of the (111) oriented terraces and finally one single peak for “flat” Au (111) is obtained. The separation of the peaks for oxide formation on the (100) or (110) oriented steps and (111) oriented terraces is large, in agreement with the large difference in
‘:
200
400
(100)
-1
02
I
I
0.4
06
06
10
12
lb
16
E/V
YS WE
Fig. 12. Profile of current for formation of a monolayer on the investigated faces from the [(llO)-(lOO)] ulne in 50 mM H,SO,; dV/dt = 80 mV/s.
305
the oxidation potential of Au (100) and Au (110) on one side and Au (111) on the other. It reflects the large difference in energy of these sites. Zone (IO@-(110)
The profiles of the anodic current for aII faces from this zone investigated are given in Fig. 12. As expected, because of the smah difference in the peak potentials for the oxidation of the Au (100) and Au (110) faces, the separation of the peaks for stepped surfaces between these two faces is small, so the second peak can be seen only as a shoulder of the first peak. GENERAL DISCUSSION AND CONCLUSIONS
Oxide formation on low-Miller-index faces In 50 mM stdphuric acid solution, because of the considerable adsorption of sulphates, there is no graduaj. building of the OH lattice. ~gerstein-Ko~owska et al. [7] found that adsorbed sulphates block the preoxidation peak at Au (lOO), the one resulting from the incomplete charge transfer on OH adsorption. However, their curves show a smaII peak, prior to the major peak, as discussed above. The adsorption of OH on flat (100) and (111) faces gives rise to a single peak at 1.42 V and 1.58 V, respectively. These surfaces, covered by adsorbed sulphate ions under these conditions, seem to be energetically homogeneous. This could be expected since they consist crystahographically of atoms of the same coordination number (eight and nine, respectively), i.e. ah atoms should exhibit the same reactivity. The less positive potential for oxide formation on Au (100) than on Au (111) can be explained by a lower coordination and consequently higher activity of the surface atoms of that plane. The symmetry of the surface atoms on Au (ill), compatible with the tetrahedral, trigonal-face symmetry of sulphate ions [7], causes a shift of the potential for oxide formation to a more positive value, thus increasing the potential difference for oxide formation on Au (100) and Au (111). However, the effect of the symmetry of surface atoms seems smaller than the effect of the surface energy difference between Au (100) and Au (ill), although these two effects are interrelated. Oxide fo~tion
on Au (hkl;l with a low densi~ of steps
For the following faces investigated in this work: Au (610) = 6(100)-(lOO), Au (11,lJ) = 6(100)-(ill), Au (755) = 6(111)-(MO), and Au (332) = 6(111)-(ill), the existence of more than one peak on the curves for OH adsorption in 50 mM H,SO, is clearly a consequence of the heterogeneity of the surfaces. The coordination number of the step atoms is lower than that of the atoms in wide terraces, so the steps are more active and stericahy more convenient sites for OH adsorption. The OH adsorption on steps is always associated with the peak at more negative potentiaI, while the peak of OH adsorption on terraces of the (100) or (111)
306
$ i _==__--A-------.-a-_
--__ *. rl: .I_
.
307
orientation is at the same potential as the peak for OH adsorption at the flat Au (100) and Au (111) faces. This is illustrated well in Fig. 13 where the CV curve for the Au (111) face is compared with the CV curves for its vicinal faces, Au (755) and Au (332).
Oxide formation on Au (hkl) with a high density of steps The experiments were performed with the following surfaces with high density of steps: Au (311) = 2(111)-(lOO), Au (210) = 2(100)-(loo), and Au (331) = 3(111)-(111). In 50 mM H,SO, it appears that at the beginning of oxidation the surface exhibits a state of average energy, with OH adsorption starting at the energetically more convenient sites (atoms with lower coordination number). This causes the appearance of the first peak. The reaction at less convenient sites (atoms with higher coordination number) gives rise to the shoulder or to the second peak at more positive potentials. The separation of the peaks is much lower than for surfaces with a low step density. Au (110) may be regarded as a highly stepped surface 2(111)-(111). The presence of a high density of (111) oriented steps gives rise to a small peak before the main oxidation peak (Fig. 11). Table 1 gives some parameters of the oxidation of gold single crystal electrodes. These include: E, (potentials of the first peak); E, (potentials of the second peak); E, (average values of the potentials) (see next paragraph); E(hkl) - E(210) (normahzed peak potentials); N (number of atoms per cm2, taking into account all surface atoms associated with the unit cell whose coordination is less than 12); Q, (calculated charge associated with the oxidation of Au (hkl) taking into account one
TABLE 1 Some parameters for the oxide formation on Au (hkl) in 50 mM H,SO,. see text
Au 0-W
El/V
E2/V
&v/V
mw
-
1O’5
N
Q,/PC~-~
(311) (211) (755) 011) (332) (331) (110) (210) (610)
1.42 1.39 1.38 1.39 1.39 1.43 1.42 1.38 1.39
1.51 1.50 1.58 1.58 1.58 1.55 1.45
1.38 1.47 1.54 1.55 1.48
0.97 0.99 1.00 0.93 0.90 0.86 0.89 0.93 0.95 1.00 0.99
Q,x,/
n
PC cmP2
WW/V (1W (11,lJ)
For explanation of the symbols,
1.30 1.41 1.56 1.54 1.57 1.50 1.66 1.79 1.84 1.74 1.50
19 225 228 235 231 222 246 264 272 258 221
370 460 511 500 470 411 505 510 4.56 525 456
1.90 2.05 2.25 2.10 2.10 1.85 2.05 1.90 1.70 2.03 2.06
308
electron per site), Q,_, ( ex p erimental charge of the oxidation of Au (l&l); and n (number of electrons exchanged, obtained from the ratio of Q&Q,). It is noteworthy that the gas-phase measurements indicate that the steps do not play a critical role in oxygen chemisorption on gold [16]. This certainly contrasts with the behaviour of stepped surfaces of Au in electrolyte solutions. In calculating Qexp,the minimum on the CV curves at = 1.6 V has been taken as the potential of the completion of a monolayer of oxide (MO). Table 1 shows that for low-index planes n is less than 2, while for the stepped surfaces it is very close to 2 or slightly higher. The explanation of the values below 2, especially in the case of Pt (110) may be sought in incomplete transfer of the second electron (reaction 5), steric hindrance or some repulsion within the layer. Further work is needed to explain these differences.
Correlation between y(hkl)/ y(210) and E(hk1) - E(210) as a function of C.O. In order to explain the strong structural dependence of the oxide formation on Au (l&l), an effort has been made to correlate some parameters of the process with some surface properties. Several correlations of the physical properties of metal surfaces as a function of C.O. have been tried, viz., surface energy vs. work function [13,17], work function vs. pzc [18], and surface energyvs. pzc [19]. By virtue of this fact it was allowed to consider these values as energetic, electronic and electrochemical surface parameters by which the individual physico-chemical behaviour of the surface in dependence of the C.O. is determined. As, for any one metal, these parameters are related to the geometrical arrangement of the surface atoms, any particular plane behaves as a particular physico-chemical entity, indicating that it keeps its identity even in some electrochemical processes. One parameter which can be used for characterization of the energy state of the surface is the relative surface energy. According to the nearest neighbour approximation [13], the calculated value of the relative surface energy, y(hkl)/y(210), takes into account the coordination of surface atoms and the fraction of particular types of atoms. This gives an average value of the surface energy, its discrete distribution over the surface being difficult to assess. The peak potential for oxide formation reflects the discrete structure of the surface. A larger separation for the oxidation of steps and terraces is seen only for vicinals of Au (111). “Flat” surfaces are homogeneous in energy, while the distribution of energy over the faces with high densities of steps is almost uniform and equal to the value of the average surface energy. Consequently, for the surfaces with high densities of steps and vicinals of Au (100) and Au (llO), no such peak separation has been found. A correlation between the dependence on C.O. of y(hkl)/y(210) and E(hkl) - E(210) has been found (Fig. 14), where E(hkl) is the potential of the first peak. The fact that a correlation exists for vicinals of the (111) orientation, despite the large peak separations, can be explained by the large energy of the steps compared to the low energy of the (111) oriented terraces.
309
Fig. 14. Correlation between the dependence on C.O. of @kl)/y(210) E,, - E(210) (see text).
and E(hkl)-
E(210)
and
In addition, we propose a way to compensate for the discreteness of the oxidation process, i.e., to make the oxidation potentials of such surfaces more appropriate for comparison with the surface energy, which is an average parameter. Let the fraction of steps at the surface be l/m, and the fraction of terraces (m - 1)/m, where m is the number of atoms in a terrace according to Lang et al’s notation [20]. This ratio should be taken into account when taking the oxidation potential for correlation with the average surface energy. The weighted average values of the peak potentials were calculated in the following way: K,=(I/m)Ei+
{(m--1)/m)&
(6)
where: (l/m)E, is the fraction of the potential of the first peak (oxide formation on steps) and {(m - 1)/m} E, is the fraction of the potential of the second peak (oxide formation on terraces). A comparison of the variations of y(hkl)/y(210) and E(hkl) - E(210), where EWl) = E,, calculated according to eqn. (6) for the faces with a low density of steps, is given as a function of the C.O. in Fig. 14. This approach gives a somewhat better parallelism of the curves for vicinals of the (111) face. The very good congruence of both curves suggests strongly that this correlation exists and that the surface energy of gold faces determines their oxidation behaviour. REFERENCES 1 H. Angerstein-Kozlowska, B.E. Conway, B. Bamett and J. Mozota, J. Electroanal. Chem., 100 (1979) 417.
310 2 A. Hamelin in B.E. Conway, J.O.‘M. Bockris and R.E. White (Eds.), Modem Aspects of Electrochemistry, Vol. 16, Plenum Press, New York, 1985, Ch. 1. 3 A. Hamelin and S. Rottgermamr, Electrochim. Acta, 32 (1987) 723. 4 D. Dickertmann, J.W. Schultze and K.D. Vetter, J. Electroanal. Chem., 55 (1974) 429. 5 M. Sotto, J. Electroanal. Chem., 69 (1976) 229. 6 M. Sotto, J. Electroanal. Chem., 70 (1976) 291; 72 (1976) 287. 7 H. Angerstein-Kozlowska, B.E. Conway, A. Hamelin and L. Stoicoviciu, Electrochim. Acta, 31(1986) 1051. 8 H. Angerstein-Kozlowska, B.E. Conway, A. Hamelin and L. Stoicoviciu, J. Electroanal. Chem., 228 (1987) 429. 9 R. AdZit and S. Strbac, J. Serb. Chem. Sot., 52 (1987) 587. 10 K. Engelsmann, W.J. Lorenz and E. Schmidt, J. Electroanal. Chem., 114 (1980) 1. 11 J. Clavilier, R. Faure, G. Guinet and R. Durand, J. Electroanal. Chem., 107 (1980) 205. 12 J. Clavilier, C.R. Acad. Sci. Ser. C, 263 (1966) 191. 13 J.K. Mackenzie, A.J.W. Moore and J.F. Nicholas, J. Phys. Chem. Solids, 23 (1962) 185. 14 N.M. Markovic, N.S. Marinkovic and R.R. Ad%%, J. Electroanal. Chem., 241 (1988) 309. 15 A.T.D’Agostino and P.N. Ross, Jr., Surf. Sci., 185 (1987) 88. 16 M.A. Chesters and G.A. Somojai, Surf. Sci., 52 (1975) 21. 17 L. Peralta, Y. Berthier and J. Oudar, “Le Vide”, (1978) 83. 18 J. Lecoeur, J. Andre and R. Parsons, Surf. Sci., 114 (1982) 320. 19 A. Hamelin and J. Lecoeur, Surf. Sci., 57 (1976) 771. 20 B. Lang, R.W. Joyner and G.A. Somojai, Surf. Sci., 30 (1972) 440.
J. Electroanal. Chem., 249 (1988) 291-310 Blsevier Sequoia S.A., Lausarme - Printed in The Netherlands
OXIDE
FORMATION
ON GOLD SINGLE
CRYSTAL
STEPPED
SURFACES
s. STRBAC and RR. AD&C Institute of Electrochemistry, ZCTM, and Center for Multidisciplinary P. 0. Box 815, Belgrade (Yugoslavia)
Studies, University of Belgrade,
A. HAMELIN Laboratoire d’Electrochimie
Znterfaciale, CNRS,
1 Place A. Briand, 92190 Meudon-Bellevue
(France)
(Received 8th December 1987; in revised form 6th April 1988)
ABSTRACT Oxide formation and anion adsorption have been studied on gold single crystal stepped surfaces. Most of the measurements were performed in sulphuric and some in perchloric acid solutions by cyclic voltammetty and by double layer capacity measurements. A pronounced structural sensitivity of both oxide formation and anion adsorption was found. The effects of the terrace orientation, step orientation and step density are reflected in voltammetry and the double layer capacity measurements. A systematic dependence of both processes on step density was found and their interrelation is discussed. The results contradict some gas-phase data reporting no critical role of steps in oxygen chernisorption on gold. A correlation between the relative surface energy of gold single crystals and the peak potentials of the initial stage of oxide formation was found.
INTRODUCTION
Oxide formation on gold has been the subject of numerous electrochemical studies involving polycrystalline [l] and single crystal surfaces [2]. The process, as depicted by cyclic voltammetry, is often used as a criterion for the cleanliness of the surface, and recently its use as a fingerprint of the crystallographic orientation (c.o.) of single crystal surfaces has been proposed [3]. The work of Dickertmamr et al. [4] showed pronounced dependence of the process on the C.O. of the low-index planes. Sotto [5,6] proposed a reaction mechanism which involves the simultaneous existence of three compounds on the surface, viz. Au,O, - 4 H,O, Au0 or Au(OH), and Au,0 or AuOH. Hamelin and the group at Bellevue made a considerable contribution to the study of double layer properties of gold single crystal surfaces, including the oxide formation (ref. 2 and references therein). From these studies the structural sensitivity of oxide formation on Au could also be inferred.
292
A thorough study of the oxidation of low-index planes of Au and the effects of anion adsorption on that process has been reported recently by AngersteinKoxlowska et al. [7,8]. The differences in the patterns of the early stages of this process have been ascribed to the different strengths of adsorption of anions on the low-index planes. The strength of adsorption and the extent of charge transfer of anions having tetrahedral structure (ClO;, SO,‘-) is higher for the planes on which the arrangement of the surface atoms is trigonal, i.e. compatible with their trigonal symmetry. A systematic study of the oxide formation on Au stepped single crystal surfaces has been undertaken by AdtiC and Strbac [9] and is extended in this work. To follow possible regularities in behaviour with variation of density and orientation of steps and terraces, the following orientations of gold single crystals have been chosen: (a) low-Miller-index faces: Au (loo), Au (111) and Au (110);
Fig. 1. Positions triangle.
of the faces investigated
on the three main zones in the unit projected
stereographic
293
(b) faces with low density of steps, and different orientations of steps and terraces:
Au (755)= 6(111)-(100) titid
to
the
Au (332) = 6(111)-(111) Au (610) = w~)--t1~)
Au (ll,l,l)
= 6(1~)-(111)
&+$d
(111j
face.
,
to &.$ (loo> face. 7
(c) faces with high density of steps: Au (211) = 3(111)-(100) Au (331) = 3(111)-(111) Au (311) = 2(111)-(1~) Au (210) = 2(100)-(100) The positions of the faces investigated in the unit projected stereographic triangle are given in Fig. 1.
The experiments have been done in two laboratories: IEH, ICTM, Belgrade, Yugoslavia and LEI, CNRS, Meudon, France. Electrodes were obtained and prepared in different ways. At IEH, ICTM, Belgrade, electrodes were obtained from Metal Crystal Ltd. Company, Cambridge, England, oriented and cut to better than lo. Initial checking of the orientation of some surfaces was done in Belgrade by LEED. No direct transfer into the electrochemical cell was made. Therefore, these data were not exploited further. The electrodes were polished mechanically using diamond paste. The final surface preparation consisted of an electrochemical polishing, followed by a chemical treatment [lo], or flame treatment, followed by cooling in ultrapure water [ll], prepared from 18 MQ cm Mill&ore water distilled further with alkaline permanganate and kept under UV radiation for at least 24 h. In Meudon, growing, orienting and cutting of the gold single crystals was done at LEI, CNRS. Mechanical polishing with ahunina was used, and after checking the orientation by von Latie X-ray backscattering, electrochemical polishing was done. The final surface preparation was done by flame treatment and cooling in ultrapure water (LEI, CNRS, patent No. 86 5103 00). These two different ways of preparing single crystal faces did not give any significant difference in the electrochemical results. The electrolyte solutions were prepared using concentrated HCIO, and H,SO, (Merck) and ultrapure water. The reference electrode was the reversible hydrogen electrode. Capacitance measurements were performed with a linear potential sweep with a superimposed ac component with an amplitude of 10 mV as described elsewhere [12]. All measurements were carried out at room temperature.
294 RESULTS AND DISCUSSION
Differential capacity measurements (C(E) curves) can be used to follow adsorption-desorption processes as a function of potential and C.O. of the gold electrodes. The influence of the co. on the shape of the C(E) curves can be seen in Fig. 2, obtained with Au (100) and Au (111) in 10 mN IICIO,. The influence of the concentration of HSO; and SOi- anions on the C(E) curves for the (111) and (100) faces can be seen in Fig. 3. For 10 mM HCIO, the difference in pzc for Au (100) and Au (111) is about 100 mV (450 mV and 550 mV vs. RHR respectively). A similar difference is found with dilute H,SO, solutions. Increasing the concentration of sulphate ions shifts the pcz negatively. For both faces, for potentials negative of the pzc, the capacity shows a small dependence on the ~n~ntration of the anion in solution (C!lOi, HSO;, SO,‘-). This is in agreement with observations on the
T
f
AdlOO)
Au(lllJ
L
00
05
10
-+
EN
vs RHE
Fig. 2. C(E) curves for Au (100) and Au (111) in 10 mM HCIO,. dV/dt-8 modulation amplitude: A E = 10 mV.
rnV/s;
freq. = 20 Hz;
295
i
00
E/V “S UtE
Fig. 3. C(E) curves for Au (100) and Au (111) in: (1) 50 mM H,SO.,; (2) 2.0 mM H,SO,; H,SO,. dV/dt = 8 mV/s; frcq. = 20 Hz; AE = 10 mV.
(3) 0.32 mM
adsorption of other anions on gold (Cl-, Br-, I-) [23. For less negative potentials, the capacity increases, reaching maximum(s) and then decreases till a constant value is attained for m~um anion coverage. The sharp increase of the capacity at very positive potentials E = 1.0 V is due to the beginning of a faradaic process: i.e. the oxidation of gold surfaces. In concentrated solutions, however, the influence of the diffuse part of the double layer cannot be seen and the pzc cannot be obtained directly, but the adsorption-desorption processes and the influence of the C.O.can be followed. The shift of the pzc with the anion concentration is not as large as would be deduced directly from Fig. 3 since the reference potential @HE) shifts in the positive direction with increased anion (acid) concentration. The number, height and shape of the adsorption capacitance peaks and their relative position for a given anion depend on the concentration of the anion and the C.O.of the electrode surface. No full frequency dispersion analysis has been made. Several frequencies have been tried, however, to ascertain the equilibrium conditions for all concentrations of H2S04 used. In Figs. 4-6, C(E) curves for the faces investigated in the three main zones of the unit projected stereographic triangle in 50 mM II,SO, are given, The general conclusions are: (1) The pzc, as a parameter reflecting the electrochemical properties of the surface, is dependent on the C.O.in solutions containing specifically adsorbed HSO; and SOi- anions. The curves for the (ill), (lOO), (311) and (755) faces did not
296
I
0.0
#
0.5
1
t.0
EIY vsRHE *
Fig. 4. C(E) cusves for investigated faces from the zone [(lOa)-(ill)] mV/s; freq. = 20 Hz; AE = 10 mV.
I
0.0
I
05
i
10
ElVvs RtlE
in 50 mM H,SO,;
.
Fig. 5. C(E) curves for investigated faces from the-zone [(ill)-(llO)] mV/s; freq. = 20 Hz; A E = 10 mV.
in 5@ m M
dV/dt = 8
291
I 00
1
05
I
10
E/V vs RHE
l
Fig. 6. C(E) curves for the investigated faces from the zone [(llO)-(lOO)] in 50 mM H,SO,; mV/s; freq. = 20 Hz; AE =lO mV.
dV/dt = 8
show a contribution of the diffuse part of the double layer because the adsorption of sulphate ion is stronger on these faces, but minima were observed in more dilute solutions. (2) On the (210) face, the surface with the highest surface energy [13], adsorption of sulphate ions begins at a more negative potential than on other faces. (3) On the (111) face, the surface with the lowest surface energy, adsorption of sulphate ions begins at a more positive potential than on the other faces investigated. The Au (110) surface is exceptional considering the very small capacitance peak associated with HSO; or SOi- adsorption. The process seems to be completed already at 0.5 V. There is apparently no increase in the coverage of adsorbed ions with further increase of potential. This suggests that only a part of the surface is occupied by anions. Considering this, and the similarity of the C(E) curves for the Au (331) and Au (110) faces, one is tempted to conclude that the adsorption of these anions on Au (110) takes place in the rails, i.e. at the two-atoms wide (111) oriented terraces. (The (110) orientation can be represented as the stepped surface 2(111)-(ill).) The peak at 0.95 V for Au (331), not seen for Au (llO), is probably due to adsorption of anions at the (111) oriented terrace sites. For this surface the terraces are three atoms wide. The notation (331) is equivalent to 3(111)-(111). Tetrahedral anions can adsorb on such a terrace, in addition to the anions adsorbed in the step [14].
298
The energetic heterogeneity of stepped surfaces is reflected in the adsorption of sulphates (Figs. 4-6). It commences at the most active sites-steps, and continues at terraces. This can be deduced from the adsorption capacitance peaks. If the heights of the adsorption peaks are compared, it can be seen that the adsorption coverage on Au (111) and its vicinal faces Au (755) and Au (332) involves a higher charge transfer, which is also indicative of the strength of adsorption 171.This is probably due to the three-fold symmetry of surface atoms of terraces, which is in registry with the tetrahedral structures of the HSO; and SO:anions. On Au (110) and faces with high step densities, i.e. with narrow terraces, high coverages of anions become difficult to achieve, probably due to the unfavourable steric factor and/or lateral repulsion of anions. In the case of high surface coverages of specifically adsorbed anions there is no gradual building of an OH sublattice [l] which might create induced heterogeneity of the surfaces. The heterogeneity of the surfaces is caused primarily by the surface geometry and consequently the process of oxide formation can be followed as a function of the geometrical arrangement and activity of surface atoms, i.e. as a function of C.O.and surface energy. Oxide formation on Au (hkl); the influence of specific adsorption The oxide formation on the low-index planes of Au has been analysed in detail by Angerstein-Kozlowska et al. f7,S). The proposed mechanism for oxide formation and reduction on gold [l] in the presence of specific adsorption explains the appearance of four anodic peaks on the current-potential profiles by induced heterogeneity of the surface and by gradual building up of the OH sublattice. In concentrated solutions, however, for higher coverages of specifically adsorbed anions, some of the stages of adsorption are blocked [7]. The proposed mechanism is simplified, thus making it easier to follow the effects of the co.
According to the mechanism proposed by Angerstein-Kozlowska et al. [l] the first stage of oxidation corresponds to the formation of the first sublattice of OH deposited in between specifically adsorbed anions: [M,A-]M+H,o~~:[M,A-]MOH+H++~-
(1) In CV curves this is seen as a reversible region, OAl, as a stage of preoxidation (Fig. 7a). Compared with the C(E) curve for Au (111) in 10 mM HClO,, it can be seen that this preoxidation commences before a high coverage by specifically adsorbed anions is attained. Sharp peaks, OA2 and OA3, correspond to the continuing formation of OH sublattices followed by the turnover (reconstruction) processes (RTO): [M,A_]
+ H,O --, M,_, +MOH+A-+H++e-
(2)
MOHMOH + MOHOHM M,_, + (X - I) H,O + [MOEI],_,
(3) +
(X -
1)
H+ +(x
-
1)
e-
(4)
299
Fig. 7. Voltammetric
curves for Au (100) and Au (ll,l,l)
in 10 mM HCIO,; dV/dt
= 80 mV/s.
After completion of a monolayer (which corresponds to the exchange of the first electron per site), transfer of a second electron takes place in a broad potential range, OA4, according to the process: MOH-+MO+H++e(5) Our data (Fig. 7a) are not in complete agreement with those of Fig. 2 of ref. 8. A comparison with later published CVs (for instance Fig. 1 of ref. 3) shows a better agreement. Considering the data obtained with stepped faces (to be shown
300
below), one can argue that the first peak (termed 2-1 in ref. 8) could be caused by the presence of a small quantity of steps and kinks at the surface of a non-ideal (100) face.
Fig. 8. Voltammetric curves for Au (111) and Au (7.55) in 10 mM HCIO,; dV/df = SOmy/s.
301
The CV of the UHV prepared Au (100) surface in 30 mM HCQ, reported by D’Agostino and Ross (151, is not in agreement with that in Fig. 7a, but is very similar to those obtained in the presence of strongly adsorbed anions. For the vicinal Au (Il,f,l) face (Fig. 7b) four peaks can be identified, as for the (100) face in 10 mM HClO,. The preoxidation peaks apparently merge with peak OA2. The Au (755) surface, vicinaI to Au (ill), shows a CV with a slight sixty to the CV of the latter surface. Sites with a unique reversibility of OH adsorption are found on this surface (peak 1, Fig. 8). Overlapping of the anodic peaks occurs, which can be a consequence of a small difference in energy among the sites on which a gradual buil~ng of the OH sublattices takes place.
recently
The voltammetry of oxide formation on Au (100) in sulphuric acid of various concentrations is given in Fig. 9. In very dilute solutions the current profile is
r - oA-----l
Fig. 9. Volleys curvesfor Au (100) in (- *-I -) 0.32m&f H&I,; 50 lnM H2S04. t -_)
(- - -)
2.0 mM H,SO, and
302
similar to that obtained for 10 mM HClO,. By increasing the concentration of H2S04, the first reversible stage of oxidation disappears due to the increased coverage of the surface by specifically adsorbed anions. The oxide formation peaks are shifted positively by increasing the concentration of sulphate ions, while the adsorption peaks are shifted negatively (see Fig. 3). Peaks 0A2 and 0A3 become less separated with increased anion concentration. In 50 m.M H,SO, they overlap, forming one sharp peak at 1.42 V. This peak is dominant and characteristic for Au (lOO)_The majority of anions is replaced by OH at these potentials and the RTO process is associated with it [7]. The small prepeak probably corresponds to the formation of an OH monolayer on defects, the presence of which is difficult to avoid completely or to control. This can be inferred from the CVs of another, apparently better prepared sample, where a pre-peak is transformed into a shoulder (see Figs. 10 and 12 below). Similar curves have been reported recently in ref. 3. Some general conclusions are: (1) in sufficiently concentrated solutions of H,SO,, which facilitate the formation
CM
Fig. 10. Profile of current for formation of a monolayer on the investigated faces from the ((lOO)-(ill)] zone in 50 mM H,S04; dV/dt = 80 mV/s.
303
of a high coverage of specifically adsorbed anions before the initial stage of oxidation, the first reversible peak, UAl, is blocked. (2) peaks 0A2 and 0A3 are very close and eventually overlap. One sharp peak is observed for the oxidation of “flat” Au (100) and Au (111) surfaces. It will be seen below that for stepped surfaces this peak is split into several peaks due to the heterogeneity of the surfaces caused by the presence of sites with different energies (steps and terraces). A comparison of the CVs for vicinal stepped surfaces with those for the low-index planes suggests strongly that the first peak corresponds to the formation of an OH monolayer on steps (more active sites), while the second corresponds to the fo~ation of OH on terraces (less active sites). The peak potenti~s appear ch~~te~stic for a given Au (j&I) face. A broad potential range, OA4, on the CV curves is associated with the exchange of a second electron. Oxide fo~at~~n on Au (hkl); the ~n~u~n~~of orientation and density of steps Zone ~l~~-~~~~~ The profiles of the anodic currents for the faces of this zone are given in Fig. 10.
02
OL
06
06
10
t2
IL
16
E/V VSRHE
Fig. 11. Profile of current for formation of a monolayer on the investigated faces from the [(Ill)-(llO)] zone in SO mM H,SO,; dV/dr = 80 mV/s.
304
A simple analysis of the CVs indicates that, by decreasing the fraction of the surface with (100) orientation, the first peak decreases till its disappearance at Au (111). From Au (311) towards the (111) orientation, by increasing the fraction of the surface with the (111) orientation, the second peak increases and shifts to a more positive potential, becoming a single peak for Au (111). The first peak corresponds to the oxidation of the steps of (100) o~entation, which occurs at potentials somewhat more negative than the oxidation of the “flat” Au (100). The second peak corresponds to the oxidation of terraces of the (111) orientation. For the vi&al (755) surface this peak coincides with the one for the “flat” Au (111) surface (cf. Fig. 13 below). Gradual changes in the current profiles have also been found with surfaces from the other two zones. Zone (Ill)-(110) The anodic parts of the CVs are given in Fig. 11 for all faces from this zone investigated. From the (111) orientation, the fraction with (110) orientation decreases and the fraction of the first peak decreases till its disappearance at Au (111). From Au (331), the fraction of the second peak increases with increasing width of the (111) oriented terraces and finally one single peak for “flat” Au (111) is obtained. The separation of the peaks for oxide formation on the (100) or (110) oriented steps and (111) oriented terraces is large, in agreement with the large difference in
‘:
200
400
(100)
-1
02
I
I
0.4
06
06
10
12
lb
16
E/V
YS WE
Fig. 12. Profile of current for formation of a monolayer on the investigated faces from the [(llO)-(lOO)] ulne in 50 mM H,SO,; dV/dt = 80 mV/s.
305
the oxidation potential of Au (100) and Au (110) on one side and Au (111) on the other. It reflects the large difference in energy of these sites. Zone (IO@-(110)
The profiles of the anodic current for aII faces from this zone investigated are given in Fig. 12. As expected, because of the smah difference in the peak potentials for the oxidation of the Au (100) and Au (110) faces, the separation of the peaks for stepped surfaces between these two faces is small, so the second peak can be seen only as a shoulder of the first peak. GENERAL DISCUSSION AND CONCLUSIONS
Oxide formation on low-Miller-index faces In 50 mM stdphuric acid solution, because of the considerable adsorption of sulphates, there is no graduaj. building of the OH lattice. ~gerstein-Ko~owska et al. [7] found that adsorbed sulphates block the preoxidation peak at Au (lOO), the one resulting from the incomplete charge transfer on OH adsorption. However, their curves show a smaII peak, prior to the major peak, as discussed above. The adsorption of OH on flat (100) and (111) faces gives rise to a single peak at 1.42 V and 1.58 V, respectively. These surfaces, covered by adsorbed sulphate ions under these conditions, seem to be energetically homogeneous. This could be expected since they consist crystahographically of atoms of the same coordination number (eight and nine, respectively), i.e. ah atoms should exhibit the same reactivity. The less positive potential for oxide formation on Au (100) than on Au (111) can be explained by a lower coordination and consequently higher activity of the surface atoms of that plane. The symmetry of the surface atoms on Au (ill), compatible with the tetrahedral, trigonal-face symmetry of sulphate ions [7], causes a shift of the potential for oxide formation to a more positive value, thus increasing the potential difference for oxide formation on Au (100) and Au (111). However, the effect of the symmetry of surface atoms seems smaller than the effect of the surface energy difference between Au (100) and Au (ill), although these two effects are interrelated. Oxide fo~tion
on Au (hkl;l with a low densi~ of steps
For the following faces investigated in this work: Au (610) = 6(100)-(lOO), Au (11,lJ) = 6(100)-(ill), Au (755) = 6(111)-(MO), and Au (332) = 6(111)-(ill), the existence of more than one peak on the curves for OH adsorption in 50 mM H,SO, is clearly a consequence of the heterogeneity of the surfaces. The coordination number of the step atoms is lower than that of the atoms in wide terraces, so the steps are more active and stericahy more convenient sites for OH adsorption. The OH adsorption on steps is always associated with the peak at more negative potentiaI, while the peak of OH adsorption on terraces of the (100) or (111)
306
$ i _==__--A-------.-a-_
--__ *. rl: .I_
.
307
orientation is at the same potential as the peak for OH adsorption at the flat Au (100) and Au (111) faces. This is illustrated well in Fig. 13 where the CV curve for the Au (111) face is compared with the CV curves for its vicinal faces, Au (755) and Au (332).
Oxide formation on Au (hkl) with a high density of steps The experiments were performed with the following surfaces with high density of steps: Au (311) = 2(111)-(lOO), Au (210) = 2(100)-(loo), and Au (331) = 3(111)-(111). In 50 mM H,SO, it appears that at the beginning of oxidation the surface exhibits a state of average energy, with OH adsorption starting at the energetically more convenient sites (atoms with lower coordination number). This causes the appearance of the first peak. The reaction at less convenient sites (atoms with higher coordination number) gives rise to the shoulder or to the second peak at more positive potentials. The separation of the peaks is much lower than for surfaces with a low step density. Au (110) may be regarded as a highly stepped surface 2(111)-(111). The presence of a high density of (111) oriented steps gives rise to a small peak before the main oxidation peak (Fig. 11). Table 1 gives some parameters of the oxidation of gold single crystal electrodes. These include: E, (potentials of the first peak); E, (potentials of the second peak); E, (average values of the potentials) (see next paragraph); E(hkl) - E(210) (normahzed peak potentials); N (number of atoms per cm2, taking into account all surface atoms associated with the unit cell whose coordination is less than 12); Q, (calculated charge associated with the oxidation of Au (hkl) taking into account one
TABLE 1 Some parameters for the oxide formation on Au (hkl) in 50 mM H,SO,. see text
Au 0-W
El/V
E2/V
&v/V
mw
-
1O’5
N
Q,/PC~-~
(311) (211) (755) 011) (332) (331) (110) (210) (610)
1.42 1.39 1.38 1.39 1.39 1.43 1.42 1.38 1.39
1.51 1.50 1.58 1.58 1.58 1.55 1.45
1.38 1.47 1.54 1.55 1.48
0.97 0.99 1.00 0.93 0.90 0.86 0.89 0.93 0.95 1.00 0.99
Q,x,/
n
PC cmP2
WW/V (1W (11,lJ)
For explanation of the symbols,
1.30 1.41 1.56 1.54 1.57 1.50 1.66 1.79 1.84 1.74 1.50
19 225 228 235 231 222 246 264 272 258 221
370 460 511 500 470 411 505 510 4.56 525 456
1.90 2.05 2.25 2.10 2.10 1.85 2.05 1.90 1.70 2.03 2.06
308
electron per site), Q,_, ( ex p erimental charge of the oxidation of Au (l&l); and n (number of electrons exchanged, obtained from the ratio of Q&Q,). It is noteworthy that the gas-phase measurements indicate that the steps do not play a critical role in oxygen chemisorption on gold [16]. This certainly contrasts with the behaviour of stepped surfaces of Au in electrolyte solutions. In calculating Qexp,the minimum on the CV curves at = 1.6 V has been taken as the potential of the completion of a monolayer of oxide (MO). Table 1 shows that for low-index planes n is less than 2, while for the stepped surfaces it is very close to 2 or slightly higher. The explanation of the values below 2, especially in the case of Pt (110) may be sought in incomplete transfer of the second electron (reaction 5), steric hindrance or some repulsion within the layer. Further work is needed to explain these differences.
Correlation between y(hkl)/ y(210) and E(hk1) - E(210) as a function of C.O. In order to explain the strong structural dependence of the oxide formation on Au (l&l), an effort has been made to correlate some parameters of the process with some surface properties. Several correlations of the physical properties of metal surfaces as a function of C.O. have been tried, viz., surface energy vs. work function [13,17], work function vs. pzc [18], and surface energyvs. pzc [19]. By virtue of this fact it was allowed to consider these values as energetic, electronic and electrochemical surface parameters by which the individual physico-chemical behaviour of the surface in dependence of the C.O. is determined. As, for any one metal, these parameters are related to the geometrical arrangement of the surface atoms, any particular plane behaves as a particular physico-chemical entity, indicating that it keeps its identity even in some electrochemical processes. One parameter which can be used for characterization of the energy state of the surface is the relative surface energy. According to the nearest neighbour approximation [13], the calculated value of the relative surface energy, y(hkl)/y(210), takes into account the coordination of surface atoms and the fraction of particular types of atoms. This gives an average value of the surface energy, its discrete distribution over the surface being difficult to assess. The peak potential for oxide formation reflects the discrete structure of the surface. A larger separation for the oxidation of steps and terraces is seen only for vicinals of Au (111). “Flat” surfaces are homogeneous in energy, while the distribution of energy over the faces with high densities of steps is almost uniform and equal to the value of the average surface energy. Consequently, for the surfaces with high densities of steps and vicinals of Au (100) and Au (llO), no such peak separation has been found. A correlation between the dependence on C.O. of y(hkl)/y(210) and E(hkl) - E(210) has been found (Fig. 14), where E(hkl) is the potential of the first peak. The fact that a correlation exists for vicinals of the (111) orientation, despite the large peak separations, can be explained by the large energy of the steps compared to the low energy of the (111) oriented terraces.
309
Fig. 14. Correlation between the dependence on C.O. of @kl)/y(210) E,, - E(210) (see text).
and E(hkl)-
E(210)
and
In addition, we propose a way to compensate for the discreteness of the oxidation process, i.e., to make the oxidation potentials of such surfaces more appropriate for comparison with the surface energy, which is an average parameter. Let the fraction of steps at the surface be l/m, and the fraction of terraces (m - 1)/m, where m is the number of atoms in a terrace according to Lang et al’s notation [20]. This ratio should be taken into account when taking the oxidation potential for correlation with the average surface energy. The weighted average values of the peak potentials were calculated in the following way: K,=(I/m)Ei+
{(m--1)/m)&
(6)
where: (l/m)E, is the fraction of the potential of the first peak (oxide formation on steps) and {(m - 1)/m} E, is the fraction of the potential of the second peak (oxide formation on terraces). A comparison of the variations of y(hkl)/y(210) and E(hkl) - E(210), where EWl) = E,, calculated according to eqn. (6) for the faces with a low density of steps, is given as a function of the C.O. in Fig. 14. This approach gives a somewhat better parallelism of the curves for vicinals of the (111) face. The very good congruence of both curves suggests strongly that this correlation exists and that the surface energy of gold faces determines their oxidation behaviour. REFERENCES 1 H. Angerstein-Kozlowska, B.E. Conway, B. Bamett and J. Mozota, J. Electroanal. Chem., 100 (1979) 417.
310 2 A. Hamelin in B.E. Conway, J.O.‘M. Bockris and R.E. White (Eds.), Modem Aspects of Electrochemistry, Vol. 16, Plenum Press, New York, 1985, Ch. 1. 3 A. Hamelin and S. Rottgermamr, Electrochim. Acta, 32 (1987) 723. 4 D. Dickertmann, J.W. Schultze and K.D. Vetter, J. Electroanal. Chem., 55 (1974) 429. 5 M. Sotto, J. Electroanal. Chem., 69 (1976) 229. 6 M. Sotto, J. Electroanal. Chem., 70 (1976) 291; 72 (1976) 287. 7 H. Angerstein-Kozlowska, B.E. Conway, A. Hamelin and L. Stoicoviciu, Electrochim. Acta, 31(1986) 1051. 8 H. Angerstein-Kozlowska, B.E. Conway, A. Hamelin and L. Stoicoviciu, J. Electroanal. Chem., 228 (1987) 429. 9 R. AdZit and S. Strbac, J. Serb. Chem. Sot., 52 (1987) 587. 10 K. Engelsmann, W.J. Lorenz and E. Schmidt, J. Electroanal. Chem., 114 (1980) 1. 11 J. Clavilier, R. Faure, G. Guinet and R. Durand, J. Electroanal. Chem., 107 (1980) 205. 12 J. Clavilier, C.R. Acad. Sci. Ser. C, 263 (1966) 191. 13 J.K. Mackenzie, A.J.W. Moore and J.F. Nicholas, J. Phys. Chem. Solids, 23 (1962) 185. 14 N.M. Markovic, N.S. Marinkovic and R.R. Ad%%, J. Electroanal. Chem., 241 (1988) 309. 15 A.T.D’Agostino and P.N. Ross, Jr., Surf. Sci., 185 (1987) 88. 16 M.A. Chesters and G.A. Somojai, Surf. Sci., 52 (1975) 21. 17 L. Peralta, Y. Berthier and J. Oudar, “Le Vide”, (1978) 83. 18 J. Lecoeur, J. Andre and R. Parsons, Surf. Sci., 114 (1982) 320. 19 A. Hamelin and J. Lecoeur, Surf. Sci., 57 (1976) 771. 20 B. Lang, R.W. Joyner and G.A. Somojai, Surf. Sci., 30 (1972) 440.