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Electrochemical indication of surface reconstruction of (100), (311) and (111) gold faces in alkaline solutions
47
Journal of Electroanalytical Chemistry, 362 (1993) 47-53
JEC 02862
Electrochemical indication of surface reconstruction of ( NO), (311) and ( 111) gold faces in alkaline solutions S. hrbac, A. Hamelin
*aand R.R. Ad%
l
Institute of Electrochemistry, ICTM and Center for M&disciplinary Studies, Universi& of Belgrade, P.O. Bax 815, 11001 Belgrade (Serbia) a Laboratoire d’Ekctrochimie Interfaciale du CNRS, 1 Place A. Briand, 92195 Meudon Principal Ceder (France) (Received 29 October 1992; in revised form 5 January 1993)
Abstract The specific adsorption of OH- an&n and the changes in the surface atomic structures of (lOtI), (311) and (111) go14 faces with charge density are discussed on the basis of cyclic voltammograms and differential capacity-potential curves obtained, in dilute NaOH solutions.
1. Introduction The electrochemical behavior of gold surfaces in alkaline solutions has attracted less attention than that in acid media. Cyclic voltammograms (0%) reported for single-crystal gold faces in 0.1 M NaOH [l-3] have generally been used to characterize the surface through oxide :formation and reduction reactions. Differential capacity measurements for the low Miiler index faces [2,4] were performed to demonstrate the specific adsorption of OH- anions in the “double-layer region” as a structuie-dependent process. New data obtained for the atomib structure of gold faces observed by scanning tunneling microscopy (STM) in 0.1 M HClO, (5-81 c&‘f& additional work oh the chadcterization of the t s&face atomic rearrangements at the different electrode potentials which were already suggested by conventiohd’ electrochemical results [9,101. The purp&e of this paper is to give precise CVs and differential capbcity curves (C(E)) ‘in dilute alkaline solutions (0.02, O.Ol’and O.OOPM NaOH) paying attention to the double.-liiyer structure and to analyze them by taking into account a possible re&nstruction of the faces at negative cfiarge den&ties [S-8] and removal of the reconstruction at positive charge densities. As a first apprclximation it, waS Considered that the faces are the “bulk termination” at posit% charge densities. ,i l
To whom correspondence should be addressed.
..’
Three faces from one zone of the unit projected stereographic triangle which h&e shown extre?e electrochemical behavior [9] were investigated. These were the Au&Xl), Au(311) and Au(lll)‘faces.fiom the, [llOl zone. 2. Experimental Au(lOO), Au(311) and A~(1111 single crystals were grown, cut, oriented, polished and checked electrochemically by cyclic voltammetry in 0.01 M HClO, at LEI-CNRS, Meudon, France. Before each measurement the electrode surfaces were flame annealed, cooled in ultrapure water and protected by a drop of the same water before being transferred to the cell, where contact with the solution was made by the hanging meniscus method. Solutions were prepared from NaOH granules (Merck Suprapur) and ultrapure water thoroughly deaerated with nitrogen. A saturated calomel electrode WE) was used as a reference electrode and potentials were then recalculated versus the standard hydrogen electrode (SHE). All the measurements were carried out at room temperature (26 f 1’C). CVs obtained in O.lM NaOH were used for in-situ surface identification by comparison with results already published [l-31. The measuiement were then performed in 0.02, 0.01 and 0.001 M NaOH with a sweep rate of 50 mV s-l during continuous cycling. C(E) curves were obtained using one frequency of the alternating signal (20 Hz) and a sweep rate of 5 mV
S. itrbac
48
et al. / Surface reconstruction of gold faces
inalkalinesdutiom
s-l, slow enough for the response time of the detection device and for the anions present to achieve adsorption-desorption equilibrium. The modulation potential was AE = 5 mV. Differential capacities were calculated assuming that the roughness factor was unity. 3. Results and discussion CVs recorded in 0.1 M NaOH during continuous cycling were used to ascertain the surface cleanliness and orientation of the faces investigated in situ by comparison with previous results [l-4]. More dilute solutions were used to study the double-layer properties. For example, Fig. 1 shows current-potential profiles for the oxidation of the Au(ll1) face in the chosen solutions. It was assumed that the first positive current peak corresponds to the specific adsorption of OHanions [11,121. Indeed, direct evidence for OH- adsorption has been obtained from the reflectivity measurements [13] and from the surface oxygen stretching frequencies obtained using surface-enhanced Raman spectroscofiy 1141.The second positive current peak is identified with irreversible oxide formation by examining the negative-going j-E profile at various switching potentials [3,12]. The onset of the potential-induced OH- adsorption and oxide formation is shifted in the negative direction by approximately 60 mV per pH unit. A similar effect has already been reported for alkaline 13,131and acid [15-B] solutions. The kinetics of OH- adsorption-desorption and oxide formation-reduction processes differs from one face to the other, reflecting the differences in geometrical and physicochemical properties of the surfaces of various crystallographic orientations. As a consequence, the shape of the voltammetry curve depends
AU(llll
Y
!i
.k
a) QIMMaOH
M Qo2wNaai Cl QDfl*lbow
dl QOOIMWOH
ml l-
51)-
ID-0.5
0.0
0.5
E/V
vs. SHE
Fig. 1. Current-potential profiles for the oxidation of the Au(ll1) face in NaOH soh&ns of various concentrations (dE/dt = 50 mV s-l): (a) 0.1 M; (b) 0.02 M; (cl 0.01 M; (d) 0.001 M.
I
-0.5
1
1
0.0
0.5
E/Vvs.SiiE
Fig. 2. CVs for A&111), Au0ll) and A&00) faces in 0.01 M rJaOH over the “doubl&layer” and oxide formation-reduction region during repetitive cycling (dE/dt = 50 mV s-l).
on the crystallographic orientation of the surfac& The, voltammetric curves obtained in 0.01 M NaOH for the faces investigated are given in Fig. 2. The corre@onding C(E) curves for 0.02 M NaOH, showing clearly’the effect of crystallographic orientation on OH’ ‘adsorption, are shown in Fig. 3. Although for 0.02 M HClO, solutions [15,16] the contribution of the diffuse part of the double layer is clearly visible on the C(E)_curves, it was not observed in NaOH solutions of the same concentration. This is probably becat& ‘OH- adsorbs 1121such that the contributionofthe diffuse part of the double layer could not be observed until extremely low concentrations. The specific &orption of ,OH- anions is much strongerthan the ‘s@e&ic adsorption of ClO; anions, which is confirmed’ in the work of Rorkowska and Stimming 1191for polyc@stalline gold as well as in the earlier work of Kolb and Schneider on single-crystal gold electrodesjl7,18]. Since the OH- adsorption differential capacity peaks in dilute solutions overlap with oxide formation peaks, the separation of these two processes was achieved by a progressive opening of the positive potential limit. Other processes assumed to occur at
S.
hbac et al / Surface reconstruction of gold facesin alkalinesolutions
AUlhkll 0.02M NoOH
Y
6
IL i *
101
5
-
I
I
0.5
0.0
I
0.s E/V vs. SH E
Fig. 3. C(E) curves for Au(lOO), Au(311) and Au(ll1) faces in 0.02 M NaOH. Posit&e sweeps recorded during permanent cycling: dE/dt = 5 mV s-‘;:LW = 5 mV, frequency, 20 Hz.
.I
: ‘8 .\,
negative chtiges, ’ such as surface reconstruction, can be detected by shifting of the negative potential limit. 3.1. The AdlOO) surface CVs for Au(100) in the “double-layer” potential region obtained by gradually decreasing the positive potential limit are given in Fig. 4(a). The most positive
49
potentials do not reach the oxide fmation region. Positive scans of curve 1 are not affected by decrease of the positive potential limit (curves 2-5).‘Some differences at positive charge densities, caused by extending the potential to the oxide formation region, will be discussed later in the paper. A broad “pre-oxidation wave” commencing at 0.05 V, i.e. the specific adsorption of OH- anions, which seems to occur with a partial charge transfer D-41, is clearly seen in these curves. The peak labeled C has no counterpart in the negative-going sweeps and probably arises from removal of the potential-induced surface reconstruction. A similar peak appears in a number of diffeient electrolytes [3,9,17,18,20,211. Kolb and Schneider [17,18] studied the reconstructed Au(lOO)-(5 x 201, Au(lll)-(1 X 23) and A&10)-(1 x 2) surfaces using ultrahigh vacuum WHY) technibues, cyclic voltammetry, double-layer capacity and electroreflectance measurements. They found that the reconst&cted surfaces obtained by flame treatment were stable only in the potential region where ho chejmisorption takes place. A single potential excursion into the region where specific adsorption of anions (HSO;, ClO;, Cl-) occurs removed the reconstruction completely, which is shown by a pronounced anodic~current peak. Another smaller anodic peak tias ‘observed on the subsequent positive sweeps at potential slightly more negative; it corresponds to change d &ucture in the surface layer of Au atoms [3,17,2O]. In our measurements the charge associated with peak ‘C was 12-15 PC an-’ in 0.1 M NaOH [3], ca; 5 WC
,t
,I
rE 2
Au(lO0) 10.02MNaOH
3 ._
C
C
20-
-20 -
(a) I -0.5
(b) ‘.
I N I 0.5 “-0.5
I 0.0 E/V vs. SHE
I
I
0.0
0.5
_
E/V vs. SHE
Fig. 4. Curr~nt~potential p&iles for the “double-layer” potential region for the isucl%b) face in 0.02 h4 NaOH recorded during continuous 1’ cycling with a gradual decrease of (a) the. positive potential limit and (b) the negative potenkal limit (dE/dt = 50 mY s-l).
50
S. &bac et al. / Surface reconstmtion ofgoldfaces in alkalinesolutions
cme2 in 0.01 M NaOH and ca. 3 PC cmb2 in 0.001 M NaQH. Some changes in current-potential profiles are seen in negative sweeps. Since the positive potential limit in all cases (curves l-5 from Fig, 4(a)) was out of oxide formation, and assuming that OH- adsorption-desorption are reversible processes, these changes have some other explanation. Curves 3-5 from Fig. 4(a) show almost symmetrical current profiles at positive and negative sweeps, indicating the reversibility of OHadsorption-desorption processes. The absence of the reversibility when the positive potential limit encompasses peak C ( curves 1 and 2 from Fig. 4(a)) indicates that some other processes different from OH- adsorption (or oxide formation) take place. It appears to be related to the change in the surface charge distribution, probably caused by the surface reconstruction process, which is further corroborated by the effect. of changing the negative potential limit (Fig. 4(b)). A decrease in the negative potential limit causes peak C to diminish (curves 1, 2 and 3 from Fig. 4(b)). Finally it disappears when the potential is reversed at values positive of -0.05 V when the reversibility of OHadsorption-desorption is established. This is explained by a gradual surface reconstruction during negative sweeps when the potential encompasses the value of -0.05 V for 0.02 M NaOH and when the process of OH- desorption is finished. This potential depends on the concentration of the solution because the kinetics of reconstruction depend on it. Removal of the reconstruction of gold surfaces at positive charges was found for a number of anions [9,17,18,20,22,23]. In the positive sweeps, at the potential of peak C, there is compensation of the difference between the positive and negative sweeps as a consequence of surface relaxation processes. The amplitude of peak C depends on whether or not the surface was oxidized (Fig. 5). Curve 1 was recorded with the positive potential limit in the oxide formation region (see CV for AuWJO) from Fig. 2). It appears that the oxide reduction reaction does not leave the surface in the same order as existed before oxidation. This is clearly indicated by peak C’s being smaller than in curve 2 recorded for the surface not subjected to oxidation with the positive potential limit of +0.5 V. The sharper peak C of this curve indicates a greater &tent of reconstruction than in the previous case. On reversing the sweep at a less negative potential (-0.05 V) in the next cycle, ‘curve 3 is obtained which shows a reduced peak C and almost full reversibility of OH- adsorption-desorption. This indicates a very small amount of reconstruction. Differential capacity measurements with different potential limits confirm these results (Fig. 6). As differ-
AullOO) O.OIMNdH
I
I
I
-0.5
0.5
0.0 E/V vs. SHE
Fig. 5. Current-potenti& pmfiles for tb “do&k-layer” potential region for the Au(100) face in 0.01 M NaOH. See text for explanation.
1
-as
I
1
O.O E/V “I.SHE OS
Fig. 6. C(E) amcs for A&O) in 0.02 M NaOH: positive and ntgative sweepa rrwMdtd dwiag aoetinuous cycling in a range of potentials (1) -0.7 to +0.5 V, (2) -0.1 to 0.5 V.
S. hbac et al. / Surface reconstruction of goki faces in alkaline solutions
51
7
E Y * ._
Au~311)/0.02MNaOH
I
I
-0.5
I
III
I
I
0.5” -0.5
0.0
I
0.5
0.0 E/V vs. SHE
E/V vs. SHE Fig. 7. As Fig. 4 but for Au(311).
ential capacities were measured at equilibrium, hysteresis between positive and negative is probably due to changes in atomic structure with potential, as long as the range of potential imposed does not encompass the oxidation of the surface. Curve 1 in Fig. 6 was recorded with a lower limit (-0.7 VI, which is sufficiently negative for the assumed reconstruction to occur, and a positive limit short of oxide formation ( + 0.5 VI. Significant hysteresis is noticed. The curve for the negative sweep is completely different from that for the positive sweep. At the potential corresponding to peak C (see Fig. 51, a process different from simple OH- adsorption occurs, which is certainly related to changes in the surface charge distribution due to the changes in the surface structure. A decrease in the negative potential limit to -0.1 V (curve 2, Fig. 61, which may cause a decrease in the amount of surface reconstruction, or prevent it completely, does not eliminate hysteresis. However, it seems to be larger, but shows no loop as in the previous case. This suggests that a small amount of reconstruction still occurs in these potential limits, but the major cause of the hysteresis is probably the kinetics of OH- chemisorption. STM examination showed that there are several structures on the reconstructed Au(100) surface [5]. The amount of reconstruction is dependent on both time and potential.
positive potential limit (Fig. 7(a)). This means that identical surface structures are created at a particular potential in positive and negative sweeps as long as the negative potential limit is -0.7 V. When the negative potential limit is decreased and the positive limit is fixed ‘at +0.4 V (Fig. 7(b)), symmetric changes in current densities can be seen in both positive and negative sweeps. This is probably caused by potentialdependent anion adsorption-desorption. A small amount of surface reconstruction should require a di-
:
A~(3111 ROZM NoOH E
IL tu 100
50
a
3.2. The Au(311) surface OH- adsorption-desorption for the A~(3111 surface is reversible and is not affected by changing the
-
,
1
.
0.0
I
0.5 EN a.
SHE
Fig. 8. C(E) curves for A~(3111 in 0.02 M NaQH. Positive and negative Scveeps recorded during cmtinuous cycling in a range of poteatiak (1) -0.7 to +0.45 V, (2) -0.1 to +0.15 V.
52
S. bat
et al. / Surface reconstructian of gold faces in alkaline solutions N
Y E Y a .b
Au(111)/0.02MNaOH
20 -
l/2
1 IO-
O-
(a)
(b)
I 0.0
I 0.5
I 0.0 E/V
vs. SHE
E/V
vs. SHE
Fig. 9. As Fig. 4 but for Au(ll1).
rect method of identification. STM observations in acid solutions show that Au(3.11) reconstructs in a less dramatic way but faster than Au(100) [8]. Hysteresis on the c(E) curves with wide potential limits (curve 1, Fig. 8) is probably caused by irreversible desorption of a larger coverage of OH-, while in the narrower potential range (curve 2, Fig. 8) “true reversibility” of the OH- adsorption-desorption is seen. 3.3. The Au(ll1) surface CVs for the “double-layer” potential region with a gradual decrease in the positive potential limit show the asymmetry between current densities for positive and negative sweeps (Fig. 9(a)). The situation is complicated for higher potential limits (curves 1 and 2) because irreversible oxide formation begins at 0.4 V [3]. If the positive potential limit is below 0.4 V (curve 31, the asymmetry is due to the irreversible OHadsorption-desorption processes. This indicates different surface structures for the same potential but different sweep directions, which are probably caused by surface reconstruction at negative potentials. A decrease in the negative potential limit affects only the current profiles for positive sweeps (Fig. 9(b)). It seems that the amount of surface reconstruction depends on the value of the negative potential limit,
Au (111I I0.02MNaOH
I
-0.5
I
1
0.0
0.5
E/V
vs.SHE
Fig. 10. c(E) curves for A&111) in 0.02 M NaOH. Positive and negative sweeps recordeLiduring cycling in a range of potential from: (1) -0.7 to 0.55 v; (2) 0.0 to 0.55 v.
S. &ac
et al. / Surface reconstwctionof gokdfaces in alkalinesolutions
which affects the current at positive charge densities corresponding to removal of reconstruction and OHadsorption. Reversible OH- adsorption-desorption is achieved when the negative potential limit is not sufficiently negative for reconstruction to occur (curve 6, Fig. s(b)). Differential capacity curves for Au(ll1) with wide potential limits (from -0.07 to 0.55 V) and the hysteresis which appears with a complete disappearance of a counterpart of a second peak in the negative sweeps indicate, as in the case of Au(lOO), that surface reconstruction occurs at negative potentials (curve 1, Fig. 10). This hysteresis decreases on changing the negative potential limit to 0.0 V (curve 2, Fig. 10). Further work with direct observation of these reconstructions could provide important additional data. In summary, electrochemical results at Au(lOO), Au(311) and Au(ll1) faces, where the specific adsorption of OH- anions occurs, indicate changes in the surface structure with charge density. The similarity of the results in alkaline solutions to those observed in acid and neutral media [3,17,18,20] leads to the assumption that surface reconstruction occurs in the presence of negative charges and it is removed at positive charges. The interesting catalytic properties of gold in alkaline media suggest that similar studies should be undertaken for other gold faces. References 1 R.R. A&if, A.V. Tripkovib and N.M. Markovi& J. Electroanal. C&em., 150 (1983) 79.
53
2 S. Strbac, Ph.D. Thesis, University of Belgrade, 1990. 3 A. Hamelin, M.J. Sottomayor, F. Silva, Si-Chung Chang and ‘M.J. Weaver, J. Electroanal. Chem., 295 (1990) 291. 4 S. Strbac and R.R. Ad%& in preparation 5 X. Gao, A. Hamelin and M.J. Weaver, Phys. Rev. Lett., 67 (1991) 618; J. Electroanal. Chem., 323 (1992) 361. 6 X. Gao, A. Hamelin and M.J. Weaver, J. Chem. Phys., 95 (1991) 6993. 7 X. Gao, A. Hamelin and M.J. Weaver, Phys. Rev. B, 44 (1991) 10983. 8 X. Gao, A. Hamelin and M.J. Weaver, Surf. Sci. Lett., 274 (1992) L588. 9 A. Hamelin, J. Electroanal. Chem., 142 (1982) 299. 10 S. Strbac and R.R. Ad%, J. Electroanal. Chem., 337 (1992) 355. 11 D.D. Bode, T.N. Anderson and H. Eyring, J. Phys. Chem., 71 (1962) 792. 12 H. Angerstein-Kozlowska, B.E. Conway, B. Bamett and J. Mozota, J. Electroanal. Chem., 100 (1979) 185. 13 C. Nguyen Van Huong, C. Hinnen and J. Lecoeur, J. Electroanal. Chem., 106 (1980) 185. 14 J. Desilvestro and M.J. Weaver, J. Electroanal. Chem., 209 (1986) 377. 15 H. Angerstein-Kozlowska, B.E. Conway, A. Hamelin and L. Stoicoviciu, Electrochim. Acta., 31 (8) (1986) 108. 16 H. Angerstein-Kozlowska, B.E. Conway, A. Hamelin and L. Stoicoviciu, J. Electroanal. Chem., 228 (1987) 429. 17 D. Kolb and J. Schneider, Surf. Sci., 162 (1985) 764. 18 D. Kolb and J. Schneider, Electrochim. Acta, 31 (1986) 929. 19 Z. Borkowska and U. Stimming, J. Electroanal. Chem., 312 (1991) 237. 20 A. Hamelin in J. O’M. Bockris, B.E. Conway and R.E. White (Eds.), Modem Aspects of Electrochemistry, Vol. 16, Plenum Press, New York, 1985, Ch. 1. 21 A. Hamelin, J. Electroanal. Chem., 255 (1988) 281. 22 B. Ckko, J. Wang, A Davenport and H. Isaacs, Phys. Rev. L&t., 65 (1990) 1466. 23 P.N. Ross and A.T. D’Agostino, Electrochim. Acta, 37 (1992) 615.
Journal of Electroanalytical Chemistry, 362 (1993) 47-53
JEC 02862
Electrochemical indication of surface reconstruction of ( NO), (311) and ( 111) gold faces in alkaline solutions S. hrbac, A. Hamelin
*aand R.R. Ad%
l
Institute of Electrochemistry, ICTM and Center for M&disciplinary Studies, Universi& of Belgrade, P.O. Bax 815, 11001 Belgrade (Serbia) a Laboratoire d’Ekctrochimie Interfaciale du CNRS, 1 Place A. Briand, 92195 Meudon Principal Ceder (France) (Received 29 October 1992; in revised form 5 January 1993)
Abstract The specific adsorption of OH- an&n and the changes in the surface atomic structures of (lOtI), (311) and (111) go14 faces with charge density are discussed on the basis of cyclic voltammograms and differential capacity-potential curves obtained, in dilute NaOH solutions.
1. Introduction The electrochemical behavior of gold surfaces in alkaline solutions has attracted less attention than that in acid media. Cyclic voltammograms (0%) reported for single-crystal gold faces in 0.1 M NaOH [l-3] have generally been used to characterize the surface through oxide :formation and reduction reactions. Differential capacity measurements for the low Miiler index faces [2,4] were performed to demonstrate the specific adsorption of OH- anions in the “double-layer region” as a structuie-dependent process. New data obtained for the atomib structure of gold faces observed by scanning tunneling microscopy (STM) in 0.1 M HClO, (5-81 c&‘f& additional work oh the chadcterization of the t s&face atomic rearrangements at the different electrode potentials which were already suggested by conventiohd’ electrochemical results [9,101. The purp&e of this paper is to give precise CVs and differential capbcity curves (C(E)) ‘in dilute alkaline solutions (0.02, O.Ol’and O.OOPM NaOH) paying attention to the double.-liiyer structure and to analyze them by taking into account a possible re&nstruction of the faces at negative cfiarge den&ties [S-8] and removal of the reconstruction at positive charge densities. As a first apprclximation it, waS Considered that the faces are the “bulk termination” at posit% charge densities. ,i l
To whom correspondence should be addressed.
..’
Three faces from one zone of the unit projected stereographic triangle which h&e shown extre?e electrochemical behavior [9] were investigated. These were the Au&Xl), Au(311) and Au(lll)‘faces.fiom the, [llOl zone. 2. Experimental Au(lOO), Au(311) and A~(1111 single crystals were grown, cut, oriented, polished and checked electrochemically by cyclic voltammetry in 0.01 M HClO, at LEI-CNRS, Meudon, France. Before each measurement the electrode surfaces were flame annealed, cooled in ultrapure water and protected by a drop of the same water before being transferred to the cell, where contact with the solution was made by the hanging meniscus method. Solutions were prepared from NaOH granules (Merck Suprapur) and ultrapure water thoroughly deaerated with nitrogen. A saturated calomel electrode WE) was used as a reference electrode and potentials were then recalculated versus the standard hydrogen electrode (SHE). All the measurements were carried out at room temperature (26 f 1’C). CVs obtained in O.lM NaOH were used for in-situ surface identification by comparison with results already published [l-31. The measuiement were then performed in 0.02, 0.01 and 0.001 M NaOH with a sweep rate of 50 mV s-l during continuous cycling. C(E) curves were obtained using one frequency of the alternating signal (20 Hz) and a sweep rate of 5 mV
S. itrbac
48
et al. / Surface reconstruction of gold faces
inalkalinesdutiom
s-l, slow enough for the response time of the detection device and for the anions present to achieve adsorption-desorption equilibrium. The modulation potential was AE = 5 mV. Differential capacities were calculated assuming that the roughness factor was unity. 3. Results and discussion CVs recorded in 0.1 M NaOH during continuous cycling were used to ascertain the surface cleanliness and orientation of the faces investigated in situ by comparison with previous results [l-4]. More dilute solutions were used to study the double-layer properties. For example, Fig. 1 shows current-potential profiles for the oxidation of the Au(ll1) face in the chosen solutions. It was assumed that the first positive current peak corresponds to the specific adsorption of OHanions [11,121. Indeed, direct evidence for OH- adsorption has been obtained from the reflectivity measurements [13] and from the surface oxygen stretching frequencies obtained using surface-enhanced Raman spectroscofiy 1141.The second positive current peak is identified with irreversible oxide formation by examining the negative-going j-E profile at various switching potentials [3,12]. The onset of the potential-induced OH- adsorption and oxide formation is shifted in the negative direction by approximately 60 mV per pH unit. A similar effect has already been reported for alkaline 13,131and acid [15-B] solutions. The kinetics of OH- adsorption-desorption and oxide formation-reduction processes differs from one face to the other, reflecting the differences in geometrical and physicochemical properties of the surfaces of various crystallographic orientations. As a consequence, the shape of the voltammetry curve depends
AU(llll
Y
!i
.k
a) QIMMaOH
M Qo2wNaai Cl QDfl*lbow
dl QOOIMWOH
ml l-
51)-
ID-0.5
0.0
0.5
E/V
vs. SHE
Fig. 1. Current-potential profiles for the oxidation of the Au(ll1) face in NaOH soh&ns of various concentrations (dE/dt = 50 mV s-l): (a) 0.1 M; (b) 0.02 M; (cl 0.01 M; (d) 0.001 M.
I
-0.5
1
1
0.0
0.5
E/Vvs.SiiE
Fig. 2. CVs for A&111), Au0ll) and A&00) faces in 0.01 M rJaOH over the “doubl&layer” and oxide formation-reduction region during repetitive cycling (dE/dt = 50 mV s-l).
on the crystallographic orientation of the surfac& The, voltammetric curves obtained in 0.01 M NaOH for the faces investigated are given in Fig. 2. The corre@onding C(E) curves for 0.02 M NaOH, showing clearly’the effect of crystallographic orientation on OH’ ‘adsorption, are shown in Fig. 3. Although for 0.02 M HClO, solutions [15,16] the contribution of the diffuse part of the double layer is clearly visible on the C(E)_curves, it was not observed in NaOH solutions of the same concentration. This is probably becat& ‘OH- adsorbs 1121such that the contributionofthe diffuse part of the double layer could not be observed until extremely low concentrations. The specific &orption of ,OH- anions is much strongerthan the ‘s@e&ic adsorption of ClO; anions, which is confirmed’ in the work of Rorkowska and Stimming 1191for polyc@stalline gold as well as in the earlier work of Kolb and Schneider on single-crystal gold electrodesjl7,18]. Since the OH- adsorption differential capacity peaks in dilute solutions overlap with oxide formation peaks, the separation of these two processes was achieved by a progressive opening of the positive potential limit. Other processes assumed to occur at
S.
hbac et al / Surface reconstruction of gold facesin alkalinesolutions
AUlhkll 0.02M NoOH
Y
6
IL i *
101
5
-
I
I
0.5
0.0
I
0.s E/V vs. SH E
Fig. 3. C(E) curves for Au(lOO), Au(311) and Au(ll1) faces in 0.02 M NaOH. Posit&e sweeps recorded during permanent cycling: dE/dt = 5 mV s-‘;:LW = 5 mV, frequency, 20 Hz.
.I
: ‘8 .\,
negative chtiges, ’ such as surface reconstruction, can be detected by shifting of the negative potential limit. 3.1. The AdlOO) surface CVs for Au(100) in the “double-layer” potential region obtained by gradually decreasing the positive potential limit are given in Fig. 4(a). The most positive
49
potentials do not reach the oxide fmation region. Positive scans of curve 1 are not affected by decrease of the positive potential limit (curves 2-5).‘Some differences at positive charge densities, caused by extending the potential to the oxide formation region, will be discussed later in the paper. A broad “pre-oxidation wave” commencing at 0.05 V, i.e. the specific adsorption of OH- anions, which seems to occur with a partial charge transfer D-41, is clearly seen in these curves. The peak labeled C has no counterpart in the negative-going sweeps and probably arises from removal of the potential-induced surface reconstruction. A similar peak appears in a number of diffeient electrolytes [3,9,17,18,20,211. Kolb and Schneider [17,18] studied the reconstructed Au(lOO)-(5 x 201, Au(lll)-(1 X 23) and A&10)-(1 x 2) surfaces using ultrahigh vacuum WHY) technibues, cyclic voltammetry, double-layer capacity and electroreflectance measurements. They found that the reconst&cted surfaces obtained by flame treatment were stable only in the potential region where ho chejmisorption takes place. A single potential excursion into the region where specific adsorption of anions (HSO;, ClO;, Cl-) occurs removed the reconstruction completely, which is shown by a pronounced anodic~current peak. Another smaller anodic peak tias ‘observed on the subsequent positive sweeps at potential slightly more negative; it corresponds to change d &ucture in the surface layer of Au atoms [3,17,2O]. In our measurements the charge associated with peak ‘C was 12-15 PC an-’ in 0.1 M NaOH [3], ca; 5 WC
,t
,I
rE 2
Au(lO0) 10.02MNaOH
3 ._
C
C
20-
-20 -
(a) I -0.5
(b) ‘.
I N I 0.5 “-0.5
I 0.0 E/V vs. SHE
I
I
0.0
0.5
_
E/V vs. SHE
Fig. 4. Curr~nt~potential p&iles for the “double-layer” potential region for the isucl%b) face in 0.02 h4 NaOH recorded during continuous 1’ cycling with a gradual decrease of (a) the. positive potential limit and (b) the negative potenkal limit (dE/dt = 50 mY s-l).
50
S. &bac et al. / Surface reconstmtion ofgoldfaces in alkalinesolutions
cme2 in 0.01 M NaOH and ca. 3 PC cmb2 in 0.001 M NaQH. Some changes in current-potential profiles are seen in negative sweeps. Since the positive potential limit in all cases (curves l-5 from Fig, 4(a)) was out of oxide formation, and assuming that OH- adsorption-desorption are reversible processes, these changes have some other explanation. Curves 3-5 from Fig. 4(a) show almost symmetrical current profiles at positive and negative sweeps, indicating the reversibility of OHadsorption-desorption processes. The absence of the reversibility when the positive potential limit encompasses peak C ( curves 1 and 2 from Fig. 4(a)) indicates that some other processes different from OH- adsorption (or oxide formation) take place. It appears to be related to the change in the surface charge distribution, probably caused by the surface reconstruction process, which is further corroborated by the effect. of changing the negative potential limit (Fig. 4(b)). A decrease in the negative potential limit causes peak C to diminish (curves 1, 2 and 3 from Fig. 4(b)). Finally it disappears when the potential is reversed at values positive of -0.05 V when the reversibility of OHadsorption-desorption is established. This is explained by a gradual surface reconstruction during negative sweeps when the potential encompasses the value of -0.05 V for 0.02 M NaOH and when the process of OH- desorption is finished. This potential depends on the concentration of the solution because the kinetics of reconstruction depend on it. Removal of the reconstruction of gold surfaces at positive charges was found for a number of anions [9,17,18,20,22,23]. In the positive sweeps, at the potential of peak C, there is compensation of the difference between the positive and negative sweeps as a consequence of surface relaxation processes. The amplitude of peak C depends on whether or not the surface was oxidized (Fig. 5). Curve 1 was recorded with the positive potential limit in the oxide formation region (see CV for AuWJO) from Fig. 2). It appears that the oxide reduction reaction does not leave the surface in the same order as existed before oxidation. This is clearly indicated by peak C’s being smaller than in curve 2 recorded for the surface not subjected to oxidation with the positive potential limit of +0.5 V. The sharper peak C of this curve indicates a greater &tent of reconstruction than in the previous case. On reversing the sweep at a less negative potential (-0.05 V) in the next cycle, ‘curve 3 is obtained which shows a reduced peak C and almost full reversibility of OH- adsorption-desorption. This indicates a very small amount of reconstruction. Differential capacity measurements with different potential limits confirm these results (Fig. 6). As differ-
AullOO) O.OIMNdH
I
I
I
-0.5
0.5
0.0 E/V vs. SHE
Fig. 5. Current-potenti& pmfiles for tb “do&k-layer” potential region for the Au(100) face in 0.01 M NaOH. See text for explanation.
1
-as
I
1
O.O E/V “I.SHE OS
Fig. 6. C(E) amcs for A&O) in 0.02 M NaOH: positive and ntgative sweepa rrwMdtd dwiag aoetinuous cycling in a range of potentials (1) -0.7 to +0.5 V, (2) -0.1 to 0.5 V.
S. hbac et al. / Surface reconstruction of goki faces in alkaline solutions
51
7
E Y * ._
Au~311)/0.02MNaOH
I
I
-0.5
I
III
I
I
0.5” -0.5
0.0
I
0.5
0.0 E/V vs. SHE
E/V vs. SHE Fig. 7. As Fig. 4 but for Au(311).
ential capacities were measured at equilibrium, hysteresis between positive and negative is probably due to changes in atomic structure with potential, as long as the range of potential imposed does not encompass the oxidation of the surface. Curve 1 in Fig. 6 was recorded with a lower limit (-0.7 VI, which is sufficiently negative for the assumed reconstruction to occur, and a positive limit short of oxide formation ( + 0.5 VI. Significant hysteresis is noticed. The curve for the negative sweep is completely different from that for the positive sweep. At the potential corresponding to peak C (see Fig. 51, a process different from simple OH- adsorption occurs, which is certainly related to changes in the surface charge distribution due to the changes in the surface structure. A decrease in the negative potential limit to -0.1 V (curve 2, Fig. 61, which may cause a decrease in the amount of surface reconstruction, or prevent it completely, does not eliminate hysteresis. However, it seems to be larger, but shows no loop as in the previous case. This suggests that a small amount of reconstruction still occurs in these potential limits, but the major cause of the hysteresis is probably the kinetics of OH- chemisorption. STM examination showed that there are several structures on the reconstructed Au(100) surface [5]. The amount of reconstruction is dependent on both time and potential.
positive potential limit (Fig. 7(a)). This means that identical surface structures are created at a particular potential in positive and negative sweeps as long as the negative potential limit is -0.7 V. When the negative potential limit is decreased and the positive limit is fixed ‘at +0.4 V (Fig. 7(b)), symmetric changes in current densities can be seen in both positive and negative sweeps. This is probably caused by potentialdependent anion adsorption-desorption. A small amount of surface reconstruction should require a di-
:
A~(3111 ROZM NoOH E
IL tu 100
50
a
3.2. The Au(311) surface OH- adsorption-desorption for the A~(3111 surface is reversible and is not affected by changing the
-
,
1
.
0.0
I
0.5 EN a.
SHE
Fig. 8. C(E) curves for A~(3111 in 0.02 M NaQH. Positive and negative Scveeps recorded during cmtinuous cycling in a range of poteatiak (1) -0.7 to +0.45 V, (2) -0.1 to +0.15 V.
52
S. bat
et al. / Surface reconstructian of gold faces in alkaline solutions N
Y E Y a .b
Au(111)/0.02MNaOH
20 -
l/2
1 IO-
O-
(a)
(b)
I 0.0
I 0.5
I 0.0 E/V
vs. SHE
E/V
vs. SHE
Fig. 9. As Fig. 4 but for Au(ll1).
rect method of identification. STM observations in acid solutions show that Au(3.11) reconstructs in a less dramatic way but faster than Au(100) [8]. Hysteresis on the c(E) curves with wide potential limits (curve 1, Fig. 8) is probably caused by irreversible desorption of a larger coverage of OH-, while in the narrower potential range (curve 2, Fig. 8) “true reversibility” of the OH- adsorption-desorption is seen. 3.3. The Au(ll1) surface CVs for the “double-layer” potential region with a gradual decrease in the positive potential limit show the asymmetry between current densities for positive and negative sweeps (Fig. 9(a)). The situation is complicated for higher potential limits (curves 1 and 2) because irreversible oxide formation begins at 0.4 V [3]. If the positive potential limit is below 0.4 V (curve 31, the asymmetry is due to the irreversible OHadsorption-desorption processes. This indicates different surface structures for the same potential but different sweep directions, which are probably caused by surface reconstruction at negative potentials. A decrease in the negative potential limit affects only the current profiles for positive sweeps (Fig. 9(b)). It seems that the amount of surface reconstruction depends on the value of the negative potential limit,
Au (111I I0.02MNaOH
I
-0.5
I
1
0.0
0.5
E/V
vs.SHE
Fig. 10. c(E) curves for A&111) in 0.02 M NaOH. Positive and negative sweeps recordeLiduring cycling in a range of potential from: (1) -0.7 to 0.55 v; (2) 0.0 to 0.55 v.
S. &ac
et al. / Surface reconstwctionof gokdfaces in alkalinesolutions
which affects the current at positive charge densities corresponding to removal of reconstruction and OHadsorption. Reversible OH- adsorption-desorption is achieved when the negative potential limit is not sufficiently negative for reconstruction to occur (curve 6, Fig. s(b)). Differential capacity curves for Au(ll1) with wide potential limits (from -0.07 to 0.55 V) and the hysteresis which appears with a complete disappearance of a counterpart of a second peak in the negative sweeps indicate, as in the case of Au(lOO), that surface reconstruction occurs at negative potentials (curve 1, Fig. 10). This hysteresis decreases on changing the negative potential limit to 0.0 V (curve 2, Fig. 10). Further work with direct observation of these reconstructions could provide important additional data. In summary, electrochemical results at Au(lOO), Au(311) and Au(ll1) faces, where the specific adsorption of OH- anions occurs, indicate changes in the surface structure with charge density. The similarity of the results in alkaline solutions to those observed in acid and neutral media [3,17,18,20] leads to the assumption that surface reconstruction occurs in the presence of negative charges and it is removed at positive charges. The interesting catalytic properties of gold in alkaline media suggest that similar studies should be undertaken for other gold faces. References 1 R.R. A&if, A.V. Tripkovib and N.M. Markovi& J. Electroanal. C&em., 150 (1983) 79.
53
2 S. Strbac, Ph.D. Thesis, University of Belgrade, 1990. 3 A. Hamelin, M.J. Sottomayor, F. Silva, Si-Chung Chang and ‘M.J. Weaver, J. Electroanal. Chem., 295 (1990) 291. 4 S. Strbac and R.R. Ad%& in preparation 5 X. Gao, A. Hamelin and M.J. Weaver, Phys. Rev. Lett., 67 (1991) 618; J. Electroanal. Chem., 323 (1992) 361. 6 X. Gao, A. Hamelin and M.J. Weaver, J. Chem. Phys., 95 (1991) 6993. 7 X. Gao, A. Hamelin and M.J. Weaver, Phys. Rev. B, 44 (1991) 10983. 8 X. Gao, A. Hamelin and M.J. Weaver, Surf. Sci. Lett., 274 (1992) L588. 9 A. Hamelin, J. Electroanal. Chem., 142 (1982) 299. 10 S. Strbac and R.R. Ad%, J. Electroanal. Chem., 337 (1992) 355. 11 D.D. Bode, T.N. Anderson and H. Eyring, J. Phys. Chem., 71 (1962) 792. 12 H. Angerstein-Kozlowska, B.E. Conway, B. Bamett and J. Mozota, J. Electroanal. Chem., 100 (1979) 185. 13 C. Nguyen Van Huong, C. Hinnen and J. Lecoeur, J. Electroanal. Chem., 106 (1980) 185. 14 J. Desilvestro and M.J. Weaver, J. Electroanal. Chem., 209 (1986) 377. 15 H. Angerstein-Kozlowska, B.E. Conway, A. Hamelin and L. Stoicoviciu, Electrochim. Acta., 31 (8) (1986) 108. 16 H. Angerstein-Kozlowska, B.E. Conway, A. Hamelin and L. Stoicoviciu, J. Electroanal. Chem., 228 (1987) 429. 17 D. Kolb and J. Schneider, Surf. Sci., 162 (1985) 764. 18 D. Kolb and J. Schneider, Electrochim. Acta, 31 (1986) 929. 19 Z. Borkowska and U. Stimming, J. Electroanal. Chem., 312 (1991) 237. 20 A. Hamelin in J. O’M. Bockris, B.E. Conway and R.E. White (Eds.), Modem Aspects of Electrochemistry, Vol. 16, Plenum Press, New York, 1985, Ch. 1. 21 A. Hamelin, J. Electroanal. Chem., 255 (1988) 281. 22 B. Ckko, J. Wang, A Davenport and H. Isaacs, Phys. Rev. L&t., 65 (1990) 1466. 23 P.N. Ross and A.T. D’Agostino, Electrochim. Acta, 37 (1992) 615.