Langmuir 1996,11, 2221-2230
2221
Underpotential Deposition of Lead on Copper(l11): A Study Using a Single-CrystalRotating Ring Disk Electrode and ex Situ Low-Energy Electron Diffraction and Auger Electron Spectroscopy Gessie M. Brisard" and Entissar Zenati Department of Chemistry, Universitd de Sherbrooke, Sherbrooke, Qudbec, Canada J l K 2 R l
Hubert A. Gasteiger, Nenad M. Markovie, and Philip N. Ross, Jr. Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720 Received October 25, 1994. I n Final Form: February 6, 1995@ The underpotential deposition of Pb was studied on Cu(ll1) single crystal surfaces prepared both by
a novel electropolishing procedure and by sputtering/annealing in ultrahigh vacuum. Identical results
were found with both methods. Pb atoms are deposited underpotentially on Cu(ll1) into a compact nonrotated hexagonal overlayer. The measured Pb coverage at saturation is 53% with respect to the Cu(ll1) substrate and is identical to the packing density of the (111)plane of bulk Pb. The presence of C1- in the supporting electrolyte has a strong effect on the potential region where depositiodstripping occurs and on the reversibility of the reaction.
1. Introduction Metal underpotential deposition (UPD), i.e., the formation of metal (sub)monolayers positive of the Nernst potential, has been investigated thoroughly in the past decades.' The more recently acquired facility to study UPD processes on well-characterized single crystal surfaces has afforded the application of new techniques both ex situ and in situ, e.g., low energy electron diffraction (LEED), scanning tunneling microscopy (STM), and surface X-ray scattering (SXS), thereby significantly advancing our understanding of the nature of UPD phenomena (for reviews see, e.g., refs2-9. In general, two methods for the preparation of clean and well-ordered single crystal surfaces prior to their transfer into a n electrochemical cell were applied: flame-annealing' and sputterlanneal cycles in ultrahigh vacuum (UHV).899The advantage of the cumbersome UHV preparation method lies in its ability to utilize Auger electron spectroscopy (AES)and LEED to assess both the cleanliness of the electrode surface and the structural symmetry of its outermost surface layers prior to electrochemical experi-
* To whom correspondence should be addressed: phone, (819)fax,(819)-821-8017; e-mail, GBRISARD@STRUCTURE. 821-7093; CHIMIE.USHERB.CA. Abstract published in Advance A C S Abstracts, June 1, 1995. (1)Kolb, D.M. InAdvances in Electrochemistry and Electrochemical @
Engineering; Gerisher, H., Tobias, C. W., Eds.; Wiley: New York, 1978; Vol. 11, p 125. (2)AdziC, R. R. InAdvances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; Wiley: New York, 1984; Vol. 13,p 159. (3)Hamelin, A. In Modern Aspects ofElectrochemistry; Conway, B. E., White, R. E., Bockris, J . O.'M., Eds.; Plenum Press: New York, 1985;Vol. 16. (4)Ross, P. N. Jr.; Wagner, F. In Advances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; Wiley: New York, 1984;Vol. 13,p 69. (5) Hubbard, A. T. Chem. Rev. 1988, 88, 633. (6)Abnuia, H. D., Ed. Electrochemical Interfaces: Modern Techniques for In-Situ Characterization; VCH Publishers: New York, 1990. (7)Clavilier, J.; Faure, R.; Guinet, G.; Durand, R. J. Electroanul. Chem. 1980, 107, 205. ( 8 ) Stickney,J . L.; Rosasco, S. D; Hubbard, A. T. J.Electrochem. SOC. 1984, 131, 260. (9)Aberdam, D.; Durand, R.; Faure, R.; El Omar, F. Su$ Sci. 1985, 162, 782.
ments. For many metals it has been shown that flameannealing and sputterlanneal cyclesin UHV yield identical surface conditionslOJ such that the more accessible flameannealing technique can be applied in single crystal electrochemistry studies. Copper single crystals, however, as well as many other materials cannot be flame-annealed and the only alternative to UHV preparation is electropolishing, most commonly consisting of the anodization of an electrode in orthophosphoriclsulfuric acid mixtures a t very high current densities12-15effecting, under certain circumstances, the formation of an oxide layer.16 On the contrary, Lecoeur and Bellierl' did not report the formation of a n oxide layer after electropolishing Cu(111)and Cu( 100) surfaces in pure orthophosphoric acid. In a n attempt to verify the proper bulk termination of a Cu(100) surface prepared according to the above procedure, Cruickshank et uZ.l4 recorded AFM (atomic force microscopy) images in 0.01 M HC104 and observed a ( d 2 x d2)R45' adlattice at all potentials positive of the onset of hydrogen evolution, which they interpreted as a n oxygen overlayer; only in the vicinity of the potential for hydrogen evolution did they observe atomic spacings consistent with the Cu(100) bulk termination. The authors, however, state that there might be a "small possibility'' that the imaged structure is due to a chloride overlayer formed by trace impurities of C1-, previously shown to form a ( d 2 x d21R45" adlayer upon emersion from 1mM HC1 a t potentials positive of the onset ofhydrogen evolution.18J9To avoid the possible (lO)MarkoviC, N.; Hanson, M.; Mc Dougall, G.; Yeager, E. J. Electroanal. Chem. 1986,241, 309. (11)MarkoviC, N. M.; Ross, P. N. J. Vac. Sci. Technol. A 1993, 11, 2225. (12)Siegenthaler, H.; Juttner, K. J.Electroanal. Chem. 1984, 163, 327. (13)Vilche, J. R.;Juttner, K. Electrochim. Acta. 1987, 32, 1567. (14)Cruickshank, B. J.; Sneddon, D. D.; Gewirth, A. A. S u r f Sci. 1993,281, L308. (15)Tegart,W. J. McG. The Electrolytic and Chemical Polishing of Metals in Research and Industrv, Press: New York, ~. 2nd ed.:. Pergamon 1959. (16)Kinoshita, K.; Landolt, D.; Muller, R. H.; Tobias, C. W. J. Electrochem. SOC.1970, 117, 1246. (17)Lecoeur, J.; Bellier, J. P. Electrochim. Acta 1985,30, 1027.
0743-746319512411-2221$09.0010 0 1995 American Chemical Society
2222 Langmuir, Vol. 11, No. 6, 1995
formation of an oxide film during the electropolishing step, . ~a postpolishing ~ step consisting of Wong et ~ 1 added cathodic annealing of a Cu(ll1) crystal in NaF, but the difference between SHG (second harmonic generation) signals from in situ experiments and the SHG response from Cu(ll1) in UHV was interpreted as the formation of an oxygen adlayer on the electropolished surface. Taking advantage of a n electrochemical cell coupled with a UHV chamber Villegas et a1.21 reported the restructuring of a disordered Cu(100)surface upon contact with a dilute HC1 etching solution, a process analogous to electropolishing. LEED and STM indicated the restoration of the (100)terraces on a nanometer scale, even though the more macroscopic (on a micrometer scale) morphology of the etched surface, revealed by SEM (scanning electron microscopy), was characterized by numerous large pits. The extensive pitting in the presence of C1- ions is not surprising owing to the strong chemical interaction of chloride and copper and electropolishing in a phosphoric acid bath as described above should alleviate this effect. There is, therefore, no established procedure for preparing a clean, well-ordered metallic copper single crystal surface for electrochemistry, without using a UHV chamber and sample transfer. So far, the only literature reports on metal UPD phenomena on copper single crystals are by Bewick et a1.,22Siegenthaleret a1.,12and Vilche et al.13 In the latter two reports, the electropolished (anodization in orthophosphoridsulfuric acid mixtures a t ~3 A/cm2) Cu( 111) surfaces exhibited cyclic voltammetric features in perchlorate electrolytes with a pH ranging from 0.5 to 3.5 which they attributed to the adsorptioddesorption of oxygen-containing species, a n interpretation which later was adopted by the authors of refs 14 and 20. These postulated oxygen-containing species were suggested to interfere with the UPD of Pb, the kinetics of which were observed to increase significantly upon the addition of acetate andor chloride anions. Evaluating the charges in the cyclic voltammetry of Pb UPD on C u ( l l l ) , a n apparent electrosorption valency larger than 2 was rationalized by the concomitant desorptiodadsorption of negatively charged, e.g., hydroxyl, chloride or acetate anions, during the depositiodstripping of UPD Pb. From the above review it is evident that several questions regarding the ex-vacuo preparation of copper single crystals and the nature of Pb UPD in terms of lead coverage and its structure are still unanswered. An important issue to be resolved is the verification of the equivalency of a well-characterized UHV prepared copper single crystal surface with a n electropolished Cu(hk1) surface, a test which may be carried out in a similar fashion as in the case of flame-annealed versus UHV prepared platinum single crystals.lOJ1 In the following we will compare the voltammetry of Pb UPD on Cu(ll1) prepared in a UHV system coupled with an electrochemical cell with the voltammetry of an electropolished Cu(ll1) surface. As a n electropolishing procedure, we adopted a three-step methodz3which utilizes a leveling agent in the anodization step, thereby elimi(18)Stickney, J. L.;Villegas, I.; Ehlers, C. B. J . Am. Chem. SOC. 1989,111,6473. (19)Ehlers, C.B.; Villegas, I.; Stickney, J. L. J. Electroanal. Chem. 1990,284,403. (20)Wong, E. K. L.; Friedrich, K. A.; Robinson, J. M.; Bradley, R. A.; Richmond, G.L. J. Vac. Sci. Technol. A 1992.10. 2985. (21)VilIegas, I.; Ehlers, C. B.; Stickney, J.'L. J. Electrochem. SOC. 1990,137,3143. (22)Bewick, A,;JoviCeviC,J.;Thomas, B. Faraday Symp. Chem. SOC. 1977,12,24. (23)Kellar, S. Lawrence Berkeley Laboratory, mail-stop 2-300, Berkeley, CA 94720.Private communications.
Brisard et al. nating the primary current distribution during copper dissolution and minimizing the microscopic pitting of the electrode surface. In addition, the 2 orders of magnitude lower current density in the anodization step of this procedure followed by a rinsing and a cathodic annealing step is expected to avoid the possible formation of an oxide film a t high current densities. Subsequently, we will report the effects of chloride on the cyclic voltammetry of Cu(ll1) in 0.01 M HC104 a s well as Pb UPD with and without chloride in solution. A newly developed method to assemble flame-annealed,24s25UHV prepared,26 or electropolishedsingle crystal electrodes into a true rotating ring disk assembly was used for the unambiguous determination of the lead adsorption isotherm on Cu(ll1). Pb flux measurements using the current shielding properties of the rotating ring disk electrode g e ~ m e t r yin~ ~ , ~ ~ this case employed with a rotating ring Cu-single-crystaldisk electrode (RRCuSCDE), make possible the unambiguous assessment ofthe UPD lead coverage as a function of potential, Le., its adsorption isotherm. The structure of the Pb UPD overlayers was examined in ex situ experiments (LEED, AES) and thus the determined Pb coverage is compared with the adsorption isotherm established by RRCu(111)SCDE measurements. Finally we discuss the effect of chloride upon the energetics and kineticsofPbUPDonCu(lll),contrastingitto the related Pt(lll)/Cu/Cl UPD system. 2. Experimental Procedures 2.1. Electropolishing Method. The Cu(ll1) single crystal (Monocrystals Company) was mechanically polished with emery paper and mirror-finished with 0.25pm diamond paste (Buehler). With the perimeter being wrapped with Teflon tape, the 6 mm 0.d. crystal (0.283 cm2)was mounted in a Kel-F collet for the three-step electropolishing procedure, the first of which is the anodization of copper at 0.8 V vs SCE (=40 mA/cmz) in a solution of 930 mL of orthophosphoric acid (99.999% Aldrich), 45 g of mannitol (Baker Analyzed), and 270 mL of HzO (triply pyrolytically distilled water) for 1min. After thorough rinsing of the electrode with water it was transferred into a second vessel containing 2.5 M HzS04 (EM Science, Tracepur Plus) purged with argon, where it was immersed at open circuit for 20 min. The third step consists of cathodic annealing at -0.45 V vs SCE in 0.5 M NaF for 15 min. After a thorough, final rinse with pure water the crystal was protected by a drop of water from airborne contamination while it was being mounted for subsequent electrochemical measurements into different collets for either rotating hanging meniscus (RHME)l0 or RRCu(ll1)SCDE experiments. The insertion of the single crystal disk electrode into a true RRSCDE configuration is described in detail in ref 25. All experiments on electropolished samples were carried out in a standard three-compartment electrochemical cell, with a normal calomel reference electrode (NCE) separated by a bridge in order to prevent C1- contamination of the electrolyte. The 0.01 M HC104 electrolyte (Baker, Ultrex) solution was prepared with triply pyrolytically distilled water. Lead was added as PbC104-3Hz0(EM Science)to the desired concentration and the addition of C1- was in form of HCl (Baker, Ultrex) or NaCl (Aldrich, 99.99%). The electropolished Cu(ll1) surface was immersed at a potential of x-0.4 V. A PAR EG&G Model 273 potentiostat with an electrochemical analysis system for data acquisition (Model 270 software) and a Pine Instruments bipotentiostat (Model AF'RDE4) were used; data from the latter were acquired digitally on an IBM PC using LabView for Windows. (24)MarkoviC, N.M.; Gasteiger, H. A.; Lucas, C.; Tidswell, I. M.; Ross, P. N., Jr. Surf Sci., in press. (25)MarkoviC, N. M.; Gasteiger, H. A.; Ross, P. N. J . Phys. Chem. 1996,99,3411. (26)Gasteiger, H.A,;MarkoviC, N. M.; Ross,P. N., Jr. J. Phys. Chem., in press. (27)Cadle, S . H.; Bruckenstein, S. Anal. Chem. 1971,43,932. (28)Swathirajan, S.;Bruckenstein, S. Electrochim. Acta 1983,28, 865.
Langmuir, Vol. 11, No. 6, 1995 2223
Underpotential Deposition of Lead on Copper(ll1)
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E/V [vs. SCE] Figure 1. Comparison of lead UPD on Cu(ll1): (a) crystal prepared in UHV by sputtedanneal cycles, with a final characterization by AES (3keV incident beam) and LEED (60 eV) indicating a clean and well-ordered surface; (b) crystal prepared by the electropolishing procedure outlined in the Experimental Section. Conditions were 0.3 M HF and 10 mV/s. The dotted line marks the peak potentials in the cyclic voltammograms. 2.2. Ex Situ Experiments. The Cu(111)single crystal was prepared by cycles of sputtering (0.5 keV Ar+)and annealing (550 “C in UHV) until AES and LEED indicated a clean and well-ordered surface, respectively. Subsequently,the crystal was transferred into a thin-layer electrochemical cell with a Pd/H reference electrode and immersed under potential control at z-0.4 V vs SCE in 0.3 M HF (Baker, Ultrex) with a pH of -2. After electrochemical characterization in the presence of 5 x M Pb2+,the crystal was emersed from the electrolyte and returned to the UHV environment for postelectrochemical analysis. All electrode potentials are referenced to the saturated calomel electrode.
3. Results 3.1. UHV Prepared vs Electropolished Cu(ll1). In order to assess the cleanliness of the three-step electropolishingmethod as well as to assure the restoration of the surface structure after the removal of the selvedge layer duringthe anodic dissolution of copper, we conducted a comparative study of Pb UPD on electropolished and vacuum prepared Cu(111).Figure l a shows the finalAES spectrum of a clean Cu(ll1) surface after a series of sputtedanneal cycles; all AES signals are due to Cu. The
sharp LEED pattern recorded for this surface indicates the well-ordered bulk termination of the (111)plane of the Cu crystal. After surface characterization by AES and LEED the crystal was transferred under inert atmosphere into an electrochemical cell and immersed a t =-0.4 V into 0.3 M HF with 5 x M PbC104; HF was chosen as supporting electrolyte for these measurements as it affords emersion experiments without the interference from nonvolatile molecules (e.g., HC104). cyclic voltammetry in this solution a t 10 mV/s exhibits a UPD lead deposition peak a t -0.36 V and a Pb strippingpeak a t %-0.32 V, resembling the observations by Vilche et aZ.13for Pb UPD on Cu(ll1) in “chloride-free”HC104 a t pH 3.5. The voltammogram recorded on an electropolished Cu(1ll)surface under otherwise identical conditions, Figure lb, is essentially identical with the voltammetric response of the vacuum prepared sample, especially in terms of the peak potentials for the voltammetric peaks of UPD lead stripping/depositionand the bulk Pb stripping peak near the negative potential limit. The slightly increased width of the UPD peaks exhibited by the electropolished surface may be rationalized by a reduced terrace size.29 In summary, however, it is evident that the less elaborate electropolishing procedure does produce a clean and wellordered Cu( 111)surface comparable in quality to a UHV prepared sample, a proof which so far was missing in the literature on copper single crystals. In the following we will focus our attention on the electrochemicalinteraction of Cu(111)in supporting electrolyte without and with C1-, using a RHME configuration with an electropolished Cu(111)crystal. 3.2. Interaction of C1- with Cu(ll1). The cyclic voltammogram of electropolished Cu(111)in 0.01 M HClO4 electrolyte (pH 2) in Figure 2 exhibits a relatively sharp (29) Ross, P. N.J.
Vac.Sci. Technol.A 1987,5, 948.
2224 Langmuir, Vol. 11, No. 6, 1995
Brisard et al.
anodic peak a t e-0.15 V and a corresponding broad cathodic peak at e-0.3 V. These voltammetric features could be attributed to the adsorptioddesorption of oxygencontaining species, e g . , OH, so the Coulombic charge associated with the anodic peak in chloride-free electrolyte is on the order of 40 pC/cm2. According to refs 12 and 13 the voltammetric curve of Cu( 111)in acid electrolyte (pH = 2) as well as the overall Coulumbic charge inferred under the voltammetric peak do not agree with the ones in Figure 2. In particular, the position of the anodic 6' I I 1 voltammetric peak which they report a t a potential of e-0.3 V vs SCE, associated with a much larger coulombic ( b ) 5 4 O - ~ M CIcharge of e60 ,uC/cm2. Similarly, the study by Wong et aL20 shows a n anodic voltammetric peak at e-0.35 V vs SCE and larger Coulombic charges under otherwise identical conditions. Seeking to resolve these discrepancies, we investigated the behavior of Cu(ll1) in the presence of small amounts of C1-. At a chloride trace level of a10-6 M the anodic voltammetric peak began to shift negatively from its value in chloride-free electrolyte, concomitant with an increase in the overall Coulombic charge. Figure 2 illustrates this effect, displaying the cyclic voltammetry of Cu(ll1) in M C1-, a 0.01 M HC104 in the presence of 2 x concentration for which the increase in the overall cathodic charge has already reached its maximum value of e210 ,uC/cm2, corresponding to a n average anodic/cathodic coulombic charge of e 9 5 pC/cm2, i.e., 8 e 0.34 for a oneelectron reaction (after double layer correction). These voltammetric features are now very similar to the ones in refs 12, 13, and 20, which would suggest that trace impurities of chloride (on the order of M) were present 1 I I I I I I I I I in their studies; we will return to this point in section 3.3. -0.5 -0.4 -0.3 -0.2 -0.1 Further increasing the chloride level did not, as mentioned, affect the Coulombic charge associated with chloride E/V [vs. NCE] adsorptioddesorption. Figure 3. Cyclic voltammetry of electropolished Cu(ll1) in The evidently strong interaction of chloride with CuM Pb2+at various sweep rates: 0.01 M HClO4 with 5 x (111)has already been observed in previous emersion M C1- added (a) in chloride-free electrolyte;(b) with 5 x experiments from 1 mM HC1 by Stickney et ~ l .who , ~ ~ to the electrolyte. reported the formation of a (43 x ~3)R30"-split-chloride Table 1. Anodic (EP,Jand Cathodic (Ep,=) Voltammetric LEED pattern with a theoretical coverage of 8 e 0.55 ML. Peak Potentials as a Function of Sweep Rate for Pb UPD It should be noted that the voltammetry in 1 mM HC1 on Cu(ll1) in 0.01 M HC104" shown in their study appears to be different from what 20mV/s 10mV/s 5 mV/s 2 mV/s is shown in Figure 2 only because the negative potential limit in Stickney's investigation was chosen too positive (a)without C1-175 -190 -165 Ep,a (mV) -150 to effect the desorption of chloride. Similar saturation -345 -300 -280 Ep,c(mV) -380 coverages for chloride on Cu(ll1) have also been reported -238 -235 -265 -255 EWPD (mV) in gas phase e ~ p e r i m e n t s . ~ lAn B ~interesting parallel to (b) with C1the phenomenon of chloride adsorption on Cu(111)can be -320 -307 -315 Ep,a (mV) -295 discerned in the UPD of Cu on Pt(ll1) in the presence of Ep,c(mV) -365 -352 -345 -337 chloride, where the coadsorption of chloride on a (1 x 1) -330 -330 -330 EUPD(mV) -330 Cu UPD monolayer results in a uniaxially compressed a The mean of anodic and cathodic peak potentials, EUPD , are (43x 43)R3Oochloride overlayer at negative potentials, also shown below. Data are extracted from Figure 3a and 3b. corresponding to a theoretical coverage of 8 = 0.47.33I t is also due to this strong chemical interaction of chloride 3.3. P b UPD on C u ( l l 1 ) with and without C1-. In and copper that the adsorption of oxygen-containing the absence of chloride in the supporting electrolyte, the species in the absence of chloride, Figure 2, is completely UPD process ofPb on Cu( l l l ) , Figure 3a, is characterized suppressed by the addition of chloride to the electrolyte, by rather slow kinetics, evidenced by both the large consistent with the observations by Stickney et aL30that potential separation of the voltammetric Pb stripping and traces of initially present oxide on Cu(111)vanished upon deposition peaks and its strong-sweep rate dependence. contact with C1-. The average UPD potential, EWD,i.e., the averaged Having addressed the behavior of a Cu( 111)electrode potential of Pb stripping and deposition, is summarized in the presence of chloride, we will now address its effects in Table 1 and approaches a value of e-0.23 V as the upon the underpotential deposition of lead on Cu(111). sweep rate is reduced. Referenced to the lead Nernst as inferred from the bulk lead stripping potential (Epmp+), (30) Stickney, J. L.; Ehlers, C. B. J. Vac. Sci. Technol. A 1989, 7, , to appeak, the UPD potential shift, ~ E w Damounts 1801. V (according to AEWD = Epbpp+ - EUPD), proximately 0.26 (31)Goddard, P. J.; Lambert, R. M. Surf: Sci. 1977,67, 180. indicating a strongly bonded state. These data are in (32) Westphal, D.; Goldmann, A. Su$ Sci. 1983,131, 113. (33)MarkoviC, N.; Ross, P. N.Langmuir 1993,9,580. disagreement with refs 12 and 13, who report a AEWDof
Underpotential Deposition of Lead on Copper(ll1)
only x0.2 V in "chloride-free" HC104 (pH 0.5-3.5) and a peak splitting of ~ 5 mV 0 a t a sweep rate of 10 mV/s compared to the x180 mV splittingobserved in Figure 3a. This discrepancy is resolved upon the addition of chloride, Figure 3b, which causes the UPD potential shift to decrease to xO.16 V, with a peak splitting of only 30 mV a t a sweep rate of 5 mV/s (see Table 1).Concomitantly, the kinetics of the UPD process are increased if compared to Pb UPD in chloride-free solution, effecting significantly higher current densities and a more reversible stripping and deposition of UPD lead which now, however, is less strongly bonded to the substrate (i.e.,AEWD is reduced by ~ 0 .VI. 1 The fact that our electrochemical measurements of the Pb UPD process on Cu(ll1) in the presence of chloride are in excellent agreement with the voltammetry shown in refs 12 and 13, together with our observations in lead-free electrolyte (section 3.2) leads us to conclude that the "chloride-free" electrolyte in these two studies were not truly chloride-free. The analysis of Figure 3 seems to indicate that the adsorption of chloride in the positive-going sweep effects the desorption of UPD lead at a significantly reduced AEWD,even though the kinetics of the UPD process are increased. Similarly, during the negative-going sweep, the deposition oflead, negative ofEUpD x -0.33 V, appears to force the desorption of chloride a t a potential significantly positive of C1- desorption in lead-free electrolyte (Figure 2). This action of chloride has a n interesting parallel in Cu UPD on Pt(111),33734 where C1- effects the splitting of the Cu stripping peak into one component with a reduced AEwD, forcing the partial stripping of copper (the analog to Pb on Cu(ll1)) and into a more cathodic stripping peak which is stabilized by the formation of a CuC1-like2D Therefore, the absence of a second UPD stripping peak in the Cu(lll)/pb/Clsystem suggests that lead a t these negative potentials does not exhibit a similarly strong interaction with chloride as does copper, a point to which we will return in the discussion sectioQ. If it is true that chloride desorptiodadsorption contributes to the Coulombiccharge under the lead deposition/ stripping peak, a n evaluation of the lead coverage as a function of potential from simple coulometry is not meaningful, resulting in a n apparent UPD lead electrosorption valency in excess of 2, as was mentioned in refs 12 and 13. The UPD lead coverage can, however, be assessed accurately with a rotating ring disk assembly and thus we will now present experimental data collected with a RRCu(ll1)SCDE. 3.4. Rotating Ring Disk Measurements. 3.4.1. Potentiodynamic RRCu(ll1)SCDE Experiments. Experiments with the RRCu(ll1)SCDE were conducted at a lower lead concentration than in the previous experiments in order to avoid excessively large currents on the ring electrode when it is potentiostated a t -0.65 V where the diffusion limited bulk deposition of lead takes place. Figure 4 shows the Cu(ll1) disk electrode voltammetry in 0.01 M HC104 in the presence of 5 x loe5M Pb2+and C1-. Without rotation, the deposition of Pb a t 10 mV/s is diffusion limited to such a n extent that only a partial monolayer of Pb is formed on the Cu(ll1) disk electrode, evident from the reduced coulombic charge under the lead stripping peak in the positive-going sweep. With the ring disk assembly rotating a t 900 rpm, the diffusional transport of lead to the electrode during the negativegoing sweep is sufficient to allow multilayer deposition as witnessed by the bulk stripping peak at the sweep reversal (34) Michaelis, R.; Zei, M. S.; Zhai, R. S.; Kolb, D. M. J.Electroanal. Chem. 1992,339, 299. (35) Tidswell, I. M.; Lucas, C. A.; MarkoviC, N. M.; Ross, P. N. Phys. Reu. B , 1995, 51, 10205.
Langmuir, Vol. 11, No. 6, 1995 2225
.-cn C
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a t X-0.5 V. A further increase in the rotation rate conveys that Pb deposition, i.e., the nature of the cathodic peak, is still determined by the diffusion limited transport ofPb to the Cu(ll1) disk electrode, whereas the Pb stripping peak remains essentially unchanged as the rotation rate is increased above 900 rpm. In potential-step experiments in section 3.4.2, we will make use of the diffusion limited rate of Pb deposition in order to evaluate the adsorption isotherm of Pb on Cu(ll1). In the top section of Figure 5 we show again the cyclic voltammetry for Pb UPD on a C u ( l l 1 j RRDE a t 900 rpm under the same conditions as in Figure 4. While this voltammogram was recorded on the Cu(111)disk electrode, the ring electrode was potentiostated at -0.65 V, a potential a t which solution phase Pb2+is deposited onto the ring electrode at a diffusion controlled rate, according to = 0 . 6 2 p 2 ' 3 n F A ~ ~ 2 ' 3 y - 1 ' 6 0 ~ 2 (1)
where p is a constant based on the ring and disk radii (p = 0.89),36 A denotes the disk area (0.283 cm2),D is the diffusivity of Pb2+ ( D = 0.925 x cm2/s3'), v is the C ~ ~ / and S ~ ~ ) , viscosity of the electrolyte (v = 8.86 x o is the rotation rate in radians per second. As lead is plated out on the Pt ring electrode, which after a short time is essentially transformed into a Pb ring electrode, the reduction of Pb2+on the ring electrode is described accurately by a two-electron process ( n = 2)
+
-
Pb2+ 2e- Pbo (2) In order to minimize the magnitude of the concentration of Pb2+was chosen to be relatively small (cg = 5 x M) such that the theoretical diffusion limited current on the ring electrode at 900 rpm is evaluated as = -14.7 p A (eq l ) , thus ~ 9 % larger than the unshielded ring current of - 13.4pA(Ip, = when no lead deposition/ stripping is occurring on the disk electrode) measured (36) Albery, W. J.; Hitchman, M. L. RingDisc Electrodes; Oxford University Press: New York, 1971. (37) Mills, R.; Lobo, V. M. M. Self-diffusionin Electrolyte Solutions; Elsevier: New York, 1989. (38) CRC Handbook of Chemistry and Physics; 65th ed.; Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1985.
2226 Langmuir, Vol. 11, No. 6, 1995 1
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I
-0.2
E/V [ v s . N C E ]
Figure 5. Cyclic voltammogram on an electropolished Cu(111)disk electrode in a RRDE assembly at 900 rpm in 0.01 M HClOd in the presence of 5 x low5M Pb2+and C1-: (top) Cu UPD on Cu(ll1) at 5 mV/s; (bottom) ring electrode currents recorded with the ring being potentiostated at -0.65 V; (insert) Coulombic charging currents on the disk, &disk, and the ring, Qri?g (&ring is the Coulombic charge on the ring electrode, QR, divlded by N), electrode during the positive-goingsweep.
above -0.3 V (lower part of Figure 5). Besides the error associated with the measurement of diffusivities (estimated to be %5%),the discrepancy between theoretical and measured diffusion limiting currents derives from the high overpotential for hydrogen evolution a t the ring electrode at a potential of -0.65 V. In order to grow a compact lead deposit (on which hydrogen evolution is suppressed) on the Pt ring electrode (on which hydrogen evolution is very facile), the ring disk assembly has to be rotated at a high rate ( ~ 5 0 0 0rpm) to detach bubbles of evolved hydrogen while slowly reducing the ring potential from potentials above the onset of hydrogen evolution on Pt (2-0.28 V) to the potential a t which the deposition of lead is diffusion limited (-0.65 V). During this process the concentration of lead, CO, is reduced by =:5-10% as lead is plated out on the ring, effecting a relatively thick deposit on the ring electrode which in turn does affect its collection efficiency, N , in a detrimental way and necessitates an internal calibration on which we will comment later. As the electrode potential is scanned negatively from -0.2 V, the cathodic voltammetric peak on the Cu(ll1) disk electrode (top part of Figure 5 ) is followed by a reduction in the ring shielding current (bottom of Figure 51, indicating the deposition of Pb on the disk. At X-0.45 V the formation of a Pb UPD layer is completed and the ring current returns to its unshielded value of -13.4 PA. The onset of multilayer Pb deposition near the negative
potential limit again brings about a reduction of the ring current from its unshielded value until, in the reverse sweep, the onset of multilayer Pb stripping a t the Nernst potential is reached, a t which point the disk current increases above zero and the ring current grows above its unshielded value due to the increase of the lead concentration in the vicinity of the ring electrode. After completion of bulk lead stripping a t e-0.5 V, no Pb2+is released by the disk electrode until the UPD Pb stripping peak commences a t X-0.35 V; consequently, the ring electrode current in this potential range is a t its unshielded value, above which it rises as UPD Pb is stripped off the disk (between X-0.35 and -0.3 V). Therefore, the voltammetry of Pb UPD on the Cu(ll1) disk electrode is mirrored perfectly by the currents on the ring electrode, establishing qualitatively that the voltammetric features on the disk electrode are essentially due to the deposition/ stripping of Pb. To attain a quantitative measure of the UPD Pb coverage, &b, as a function of potential, without the possible interference of anion discharge with the coulometry on the Cu(111)disk electrode, one may utilize the lead deposition currents on the ring electrode once its collection efficiency, N , is known. As we outlined in the beginning of this section, the thick lead deposit on the ring electrode necessary to suppress parasitic currents from hydrogen evolution effects a deviation of N (generally a decrease) from its value measured on the lead-free Ptring electrode with a ferrolferricyanide redox couple, N = O.XLZ5 A means of internal calibration of the collection efficiency of the Pb-ring electrode is the Pb multilayer stripping peak near the Nernst potential (x-0.55 V) for which the reaction on the disk as well as on the ring electrode is known to be a n exactly two-electron process, i.e., the reverse reaction of eq 2. Therefore, the ratio of the integrated currents passed through the ring electrode (in excess of the shielding current, g)and the disk electrode during multilayer stripping corresponds to the true collection efficiency of the ring electrode and is evaluated from Figure 5 to be N = 0.18. Knowing the collection efficiency it is now possible to evaluate 6 P b by integrating the ring current in excess of its unshielded value during the stripping of UPD Pb in the positive-going sweep according to
(3) where u is the sweep rate, QRis the Coulombic charge on the ring electrode, and 284pClcm2corresponds to the ideal one-electron surface charge density of C u ( l l l 1(based on the atomic density, e, of the Cu(ll1) plane, e = 1.77 x 1015atoms/cm2 39). The Pb2+reduction occurring on the ring electrode is given by eq 2 with the number of electrons being n = 2. To evaluate the UPD lead coverage on the Cu(ll1) disk electrode, the positive-going sweep was chosen because it is essentially free of diffusion limitations (see Figure 4) a t the chosen set of sweep rate and rotation rate. The insert of Figure 5 (dashed line) gives the numerical evaluation of eq 3 in terms of the lead stripping charge, &ring = QrJN (i.e., integration from -0.4 to -0.2 V during the positive going sweep). Hence, stripping of the entire UPD Pb overlayer indicates a saturation coverage of e0.50 ML ( ~ 2 8pC/cm2), 5 slightly below its theoretical limit of 0.53 ML referenced to the maximum (39)Kittel, CIntroduction t o Solid StatePhysics; 6th ed.;John Wiley & Sons: New York, 1986.
Underpotential Deposition of Lead on Copper(ll1) I
I
Langmuir, Vol. 11, No. 6, 1995 2227 I
currents and the lead coverage on the disk may be assessed from the decrease of the ring shielding current (due to reaction 2) below its unshielded value:
(4) where again is -13.4 pA. The numerical result of eq 4, epb, is shown in the insert of Figure 6 (solid circles). As the potential is stepped more negatively (potential steps 3 and 41, the establishment of diffusion-limited Pb deposition on the disk (itwas already apparent from Figure 4 that lead deposition under our conditions would be diffusion limited) is evident from the initial plateau of the ring currents, followed by kinetically controlled Pb deposition as the ring current approaches its unshielded value. The diffusion-limited disk current, G, for reaction 2 (i.e., for the complete two-electron discharge of Pb) may be evaluated from either eq 5 or 6
I"D = y
3
q
I"D = (q- Y , / N
.-8 I-
0
10
20
30
time [SI Figure 6. RRDE stepping experiments on an electropolished Cu(ll1l disk electrode under the same conditions as in Figure 5: (top) disk currents; (botton) ring currents. The voltammogram (identical with the top of Figure 5) is meant to help visualize the sequence of the potential steps, originating from Ei (-0.2 V): 1= -0.35 V, 2 = -0.375 V, 3 = -0.4V;4 = -0.45 V. Insert: Ring (solid circles) and disk (circles)charges during lead deposition on the disk electrode (&ring = QP/"
atomic density ofthe fcc (111)lead plane (e = 0.943 x 1015 atoms/cm2 39). Integration ofthe disk currents in the same potential window (insert of Figure 5, solid line) now explains the observation of a lead electrosorption valency of larger than 2 which was derived from coulometry in previous cyclic voltammetry experiment^:^^^^^ The excess of ~ 9 pC/cm2 5 assessed for the Cu(ll1) disk electrode is due to the adsorption of C1- and, to a small extent, to double layer charging. Negative of the lead stripping peak, the coverage of Cu(ll1) with UPD lead is essentially identical with its theoretical packing density. 3.4.2. Potentiostatic RRCu(ll1)SCDE Experiments. In order to assess the equilibrium UPD Pb coverage (Pb adsorption isotherm) as a function of potential, which, due to kinetic limitations, may differ from the coverages shown in the insert of Figure 5, we conducted potentialstep experiments using the RRCu(1ll)SCDE method with a time resolution of 25 ms. We chose a n initial potential of -0.2 V, where the Cu(ll1) surface is free of lead as is indicated by cyclicvoltammetry. The initial potential (EJ and the final potential for each stepping experiment (numbers 1through 4) are marked in the voltammogram in Figure 6 in order to aid the comparison between voltammetric features and the resulting potential-step responses. As the potential is stepped to the onset of the cathodic peak (potential step 1= -0.35 V) only a small transient current is observed on both the disk (top of Figure 6) and the ring (bottom of Figure 6) electrode, vanishing to zero after a few seconds. Decreasing the value of the final potential into the potential region of the cathodic peak (potential step 2) yields a n increase in the transient
(5)
(6)
with being the minimum ring current for the diffusion-limited two-electron UPD Pb deposition on the ring electrode. The numerical values of G according to eqs 5 and 6 are -13.9 and -14.5 pA, respectively, such that G %-14.2 p A f 2 % . Quite clearly, the measured disk currents of %-18 p A (3 and 4) are significantly larger than the theoretical current value for the two-electron Pb deposition such that the excess current must be due to the desorption of chloride concomitant with the deposition of the lead UPD layer. The charge contribution for C1desorption as a complete Pb monolayer is being formed may be quantified by the differencebetween the Coulombic charges of the disk and the ring electrode (insert of Figure 6) a t the most negative stepping potential of -0.45 V, amounting to ~ 8 pC/cm2 0 (note: this experiment does not contain contributions from double layer charging in contrast to the potentiodynamic experiment, Figure 5). This is in excellent agreement with the charge/discharge of C1- on the Cu(ll1) surface assessed by cyclic voltammetry in the absence of Pb (see section 3.1). It should be noted that without the aid of RRCu(ll1)SCDEmeasurements the coverage of lead could not be assessed, as the Coulombic charge on the disk electrode contains a large component which is not due to the discharge of Pb2+ions and it would thus appear that the lead electrosorption valency is larger than 2, which, ofcourse, is not possible.12J3 The solid line in the insert of Figure 6 represents the true adsorption isotherm of Pb UPD on Cu(ll1) (in contrast to the non-steady-state relation derived from the potentiodynamic experiment shown in Figure 5) and the equilibrium coverage of the complete monolayer, reached a t %-0.45V, corresponds to %0.53ML (%300pC/cm2)and is identical with the theoretical maximum packing density of Pb on Cu(ll1). An independent confirmation of these results as well a s structural information on the Pb UPD layer can be attained from ex situ experiments and will be presented in the following. 3.5. Ex-SituLEEDandAES Measurements. Figure 7 shows the cyclic voltammetry of UHVprepared Cu(ll1) in 0.3 M HF in the presence of 5 x M Pb2+a t 2 mV/s. Before the transfer into the thin-layer electrochemical cell, the AES spectrum and the LEED pattern of the Cu(111)surface were identical with the ones in Figure 1. The voltammetric peak splitting of ~ 3 mV 0 as well as the
2228 Langmuir, Vol. 11, No. 6, 1995 (a) LEED schematic
L-7-T m
Brisard et al.
I
Cu(l11) 0.3M HF 5xlO"M Pb+*
(b) real-space representation
0
= Cu substrate
0
= Pb overlayer
yr\15pA
- _ cu
100
300
500
700
kinetic energy [eV]
900
0
c
-
!A 0
-0.5
-0.4
-0.3
EIV
-0.2
Figure 7. Emersionof UHV preparedCu(111) at the indicated potential duringthe negative-goingvoltammetric sweep (2 mV/ s) in 0.3 M HF. AES and LEED were recorded after transfer from the electrochemical cell into UHV.
average Pb UPD potential, = -335 mV, are in close agreement with the values observed in 0.01 M HC104with 5x C1- (see Figure 3b and Table l), indicating the well-known presence of trace impurities of chloride in 0.3 M HF.10p33940After emersion a t -0.45 V, the potential at which according to our RRCu(111)SCDEmeasurements a saturated monolayer of Pb is deposited on the Cu(ll1) surface, the sample was returned into UHV for postelectrochemical analysis. AES of the emersed electrode, Figure 7, indicates a surface nearly free of oxygen and chloride with an estimated coverage of (3% and ~ 1 of% a monolayer, respectively (based on AES sensitivity factors41),the major features being the Auger transitions of Cu form the substrate and of Pb in the overlayer. In contrast, the emersion of Pt(ll1) with a saturated UPD layer of Cu in the same electrolyte conveys a strong coadsorption of chloride with the copper, forming a uniaxially compressed (2/3 x 2/3)R3Oochloride overlayer (0 = 0.47).33 This further supports our hypothesis, developed in the above discussion of our in situ experiments, that the deposition of Pb on Cu(111)is concomitant with the desorption of chloride from the copper surface and vice versa. The only diffraction spots visible in the LEED pattern, Figure 7, are the principal spots (i.e., from the Cu substrate),the first-order diffraction pattern from the Pb overlayer and double diffraction spots. It should be mentioned, that the same LEED pattern with, however, slightly fuzzier spots, was observed upon emersion at the cathodic voltammetric peak potential (at a sweep rate of 1mV/s), indicating a somewhat less ordered Pb overlayer. ~~
(40)Markovii.,N.; Ross, P. N.J. Electroanal. Chem. 1992,330,499. (41) Davis, L. E.; MacDonald, N. C.; Palmberg, P. W.; Riach, G. E. Handbook of Auger Electron Spectroscopy, 2nd ed.; Physical Electronics: Eden Prairie, MN, 1976.
Cu (principal spots) Pb superlattice double diffraction
Figure 8. (a) Schematic representation of the LEED pattern after emersion at the saturated Pb overlayer (Figure 7), with assignment of all the observed LEED spots. (b) Real-space representation of the nonrotated (111)lead overlayer deduced $om the evaluation of Figure 7a. The atomic radii of lead (2.48 A) and copper (1.81 A) were scaled according to the values measured in their respective fcc metal lattices reported in ref 39; the measured lead coverage on the copper substrate (1.77 x 1015 atoms/cm2)is identical with the packing density of a (111) plane in a lead crystal (0.943 x 1015atoms/cm2),ie., 53% with respect to the Cu(ll1) surface.
For the ease of analysis a schematic of the observed LEED pattern is given in Figure 8a. Clearly, the lead overlayer is an expanded, nonrotated hexagonal structure with respect to the Cu substrate. The measured ratio of k vectors, h u / k p b , is 1.37 with the ratio of real-space lattice vectors being the inverse of 1.37, namely, 0.730. Consequently, the LEED-derived coverage of Pb based on the Cu(ll1) substrate is evaluated to be 0.53 ML, according to
in excellent agreement with the value assessed independently by RRCu(111)SCDE measurements. The realspace representation consistent with the LEED pattern of Figure 8a is showp in Figure 8b, for which the atomic radii of lead (2.48 A) and copper (1.81 A) were scaled according to the values measured in their respective fcc metal lattices.39 Hence, the saturated Pb UPD layer on Cu(111)forms a structure identical with the (111)fcc plane of bulk lead metal, without the coadsorption of chloride. The observation of the same LEED pattern at submonolayer coverages of Pb indicates the deposition by nucleation and growth of the hexagonal structure. 4. Discussion Before entering in a more detailed discussion on the interaction of Cu(ll1) with C1-, Pb or a combination of the two species, we would like to briefly summarize the above results. After the validity of the three-step electropolishing procedure was established by a comparison of Pb UPD on a well-characterized UHV prepared Cu(1ll)surface with an electropolished Cu(ll 1)electrode, the investigation of the effect of chloride on the cyclic voltammetry of Cu(ll1) has shown that it is mainly influenced by the adsorptioddesorptionof chloride rather than by the adsorptioddesorption of oxygen-containing species, the coverage of which was estimated to be %O. 14 ML. Pb UPD on Cu(ll1) in chloride-free electrolyte resulted in a strongly bonded UPD structure of Pb characterized by, however, rather slow kinetics. Upon
Underpotential Deposition of Lead on Copper(ll1)
Langmuir, Vol. 11, No. 6, 1995 2229
the addition of small amounts of chloride, the bonding voltammetric wave near -0.6 Vin Figure 2, as was indeed strength ofthe Pb UPD layer decreased significantly while reported by Ehlers et a1.19using AES analysis of Cu(100) the kinetics of Pb strippingldeposition increased dramatiemersed from HC1. Having discussed the interaction of cally. RRCu(ll1)SCDE measurements afforded the unCu(ll1)with C1-, we will now address the Pb UPD process ambiguous assessment ofthe UPD Pb adsorption isotherm and explore the ramifications of the above treatment. and established that the saturated monolayer of Pb on 4.2. Energetics and Kinetics of Pb UPD on CuCu(111)corresponded to the maximum Pb packing density (111). Pb UPD in the absence of C1- (Figure 3a) is inits(lll)fccplane,viz., 0.53ML. Inaddition, the excess characterized by the formation of a very strongly bonded charge contribution to Pb stripping/deposition could be Pb overlayer and rather slow kinetics as deduced from correlated with the adsorptioddesorption of C1-. The Pb the large UPD potential (see Table 1)and the significant coverages and structure determined by ex situ LEED and peak separation between Pb deposition and stripping AES analyses were self-consistent with the RRDE meapeaks, respectively. One can formulate a phenomenologisurements, i.e., with a Pb coverage of 0.53 ML in a densely cal half cell reaction for the UPD process43 packed, nonrotated (111)fcc bulk lead structure, without the presence of coadsorbed chloride. CU Pb2+-k 2e(CUPb),,; E,, (9) In the following discussion we will focus on the nature of the interaction of C1- with Cu(ll1) and its effect upon where the reversible half cell UPD potential, & O b , may the energetics and kinetics of the Pb UPD process. be approximated from the scan rate dependence of the Subsequently, we will address the observed differences anodic and cathodic voltammetric waves (Figure 3a and between two connected UPD processes, namely, C u ( l l l ) / Table la). It is clear that the kinetics of the anodic process Pb/Cl and Pt(lll)/Cu/Cl. (Pb stripping) are faster than the UPD deposition of Pb, 4.1. Interaction of C u ( l l 1 ) and C1-. The cyclic revealed by the stronger sweep rate dependency of the voltammetry of Cu(ll1) in chloride-free 0.01 M HC104 cathodic process compared to the anodic process _(see (Figure 2) exhibits anodidcathodic voltammetric peaks Figure 3a), so that the mean of the peak potentials (EVD which might be correlated with the adsorptioddesorption in Table la) at even 2 mV/s does not precisely coincide of oxygen-containing species as proposed by Vilche et al.,I3 must lie between the mean with & o b . Therefore, but in contrast to their finding the Coulombic charge of the peak potentials and the anodic peak potential, i.e., associated with this process in our experiments would between -235 mV and - 190mV (evaluated a t the slowest only amount to less than 0.14 ML. The strong effect of experimental sweep rate, see Table la). chloride on the voltammetry of Cu(ll1) (Figure 2) a t even On the basis of the above considerations we will seek trace impurity levels of chloride, known to be present in to understand the two major changes in the Pb UPD HF (see section 3.5),10r33840 results in a significant increase process upon the addition of chloride: a negative shift of the Coulombiccharge for chloride adsorptioddesorption, (e-0.1 V,-Table 1 and Figure 3b) of the mean UPD corresponding to a n approximate coverage of 0.3 ML potential, E U ~=D-0.33 V, and the seemingly increased (assuming a complete one-electron discharge) and prokinetics conveyed by the reduced peak separation and duces a negative shift of the anodic voltammetric peak. increased peak currents. Addressing the first question, Both the more negative anodic peak potential in the concerned with the energetics of Pb UPD, one may in the presence of chloride and the increased Coulombic charge way of a Gedankenexperiment add eqs 8 and 9 is then in good agreement with the voltammetry reported ~J~*~~ in the literature for “chloride-free” e l e c t ~ - o l y t e . ~ Ac~ ( C u c l ) , ~(1- x) C u Pb2+ (2 x) ecording to Stickney et u Z . , ~ O the emersion of Cu(ll1) from 1 mM HC1 conveys the formation of a (43 x d3)R3Oo(CUPb),, + XCl-; Ec,b/c, (10) split-C1 overlayer with a theoretical coverage of 0.55 ML and a n arrangement of copper and chloride in the surface where the stoichiometric factor x accounts for the fact layer in close analogy to the bulk (111)plane of CuC1. that the deposition of one Pb atom is not necessarily Similarly, SXS measurements by Tidswell et ~ 1on .a ~ ~accompanied by the desorption of exactly one C1- anion. saturated pseudomorphic (1 x 1)monolayer of UPD Cu Equation 10 may be paraphrased as the substitution of on Pt(111)in the presence of C1- indicate a bulk CuC1-like a (chloride-free) Pb UPD layer with a CuCl2D-layer and structure of the UPD layer. Thus, it seems very probable vice versa. The reversible potential of this process, ECowC1, that the interaction of Cu(ll1) and chloride can be may then be calculated from the potentials associated with formulated in terms of a two-dimensional (2D) CuC1(111) eqs 8 and 9 (ne = ZnBi): bilayer:
+
+
+
ECuFblCl
where Ecda symbolizes the reversible potential of the above2D-process, different from the equilibrium potential of the half cell reaction for bulk CuCl which is a t approximately -0.13 V on the SCE reference potential scale.42Guided by eq 8 the anodidcathodic wave in Figure 2 (in the presence of C1-) would correspond to the adsorptioddesorption of C1- and Ecdcl could be approximated by the mean of the anodidcathodic voltammetric peak potentials, yielding a value of =-0.5 V. At very negative potentials chloride would be expected to desorb from the surface, i.e., giving rise to the cathodic (42)Bertocci. U.:Waeman. D. D. In Standard Potentials in Aaueous Solution; Bard,’ A.‘J., Parsons, R., Jordan, J., Eds.; Marcel Dkkker: New York,1985;p 292.
=
*COb
+ +
-
+ xECdCl 2+x
Using the above estimated values ( E C ~ /-.0.2 P ~ V, Ecdcl = -0.5 V), eq 10 predicts that the equilibrium potential for the Pb UPD process in the presence of a CuCl 2D-layer ( E c o b / C l ) must be negative of the reversible potential in the chloride-free Pb UPD system ( & o b ) , Le., negative of x-0.2 V. Qualitatively, this is consistent with the observed more negative equilibrium potential of e-0.33 V (Table l b ) for the Cu(lll)/Pb/Cl system. Further support for eq 10, viz., the substitution of a CuCl2D-layer wjth a Pb UPD layer, is provided by the absence of chloride on a n emersed Cu(111)surface with a full monolayer of UPD Pb (Figure 7), quite contrary to the strong chloride (43) Ross,P.N. In Chemistry and Physics ofSolid Surfaces; Vanselow, R., Howe, R., Eds.; Springer Verlag: New York, 1982,p 174ff.
Brisard et al.
2230 Langmuir, Vol. 11, No. 6, 1995
signal observed upon the emersion of a Cu UPD layer on Pt(ll1) from the same e l e ~ t r o l y t e . ~ ~ Regarding the kinetics of Pb UPD on Cu(ll1) in the presence of C1-,Vilche et al.l3proposed that solution-phase C1- in its function as a n inhibitor for the adsorption of oxygen-containingspecies (suggested to retard the Pb UPD process) would be responsible for the increased kinetics of Pb UPD compared to its kinetics in chloride-free electrolyte. Owing to the fact that we did not observe a large coverage of oxygen-containing species in truly chloride-free electrolyte ( ~ 0 .ML) 1 this hypothesis should be viewed with critical reservation. Rather, we believe, that the desorption of chloride a t %-0.3 V (in the vicinity of ECflb/C1) produces a bare Cu(ll1) surface with a high overpotential for the formation of a Pb UPD layer (7 = Ecflb/cl - E C f l b -0.1 v) resulting in high deposition currents. Similarly, during the positive-goingsweep, upon Pb stripping (atECflb/C1) the formation of a CuCl2D-layer occurs a t a large overpotential for chloride adsorption on bareCu(lll)(q =ECOWC~ - E C ~ C+0.2V)againeffecting ~ high stripping currents. 4.3. Pb UPD on Cu(ll1) versus Cu UPD on Pt(111). Analogies and differences between Pb UPD on Cu(111)and Cu UPD on Pt(111)have surfaced many times in the above discussion and do deserve a brief summary a t the closure of this discussion. The addition of chloride causes the splitting of the voltammetric peak of Cu UPD on Pt(111)into a slightly more negative peak and a second peak with significantly reduced bonding strength of UPD Cu with the substrate (i.e.,a large negative shift), which is explained by the formation of a stable CuC1-like structure in between these two voltammetric features as shown by LEED and SXSZ4In contrast, the Pb UPD peak on Cu(111)in the presence of chloride undergoes a negative shift (reduced bonding strength to the substrate) without splittinginto two voltammetric peaks. This indicates the absence of a strong interaction of the Pb UPD layer with chloride, which otherwise could provide a stable intermediate Pb/C1 structure effecting a peak splitting in the voltammetry. In analogy, the emersion of a Cu UPD layer on Pt(111)does convey a strong coadsorption of chloride,33 whereas emersion of a Pb UPD layer on Cu(ll1) in the
same electrolyte (0.3 M HF, Figure 7) is free of coadsorbed chloride. In the case of Cu UPD on Pt(ll1) chloride coadsorption was found to be favored due to the close structural match of the cubic Cu UPD layer with the geometry of the bulk CuCl lattice, a cubic, ZnSe-type structure. PbClZ, however, crystallizes in a rhombic structure and the mismatch between the cubic Pb UPD layer and the rhombic PbClz geometry may be the driving force for chloride desorption as Pb is deposited onto the Cu(ll1) substrate. 6. Conclusions (1)An electropolishing procedure was developed which produces electrochemical results on Cu( 111)that are the same as those on a UHV-prepared surface. (2) Pb flux measurements using a ring-disk geometry with a Cu(ll1) disk electrode establish definitively the adsorption isotherm for Pb independently from the effect of chloride adsorptioddesorption. (3) The measured Pb coverage at saturation is 53% with respect to the Cu(ll1) substrate and is identical to the packing density of the (111)plane of bulk Pb. (4) The presence of C l in the supporting electrolyte has a strong effect on the potential region where deposition/ stripping occurs and on the reversibility of the reaction.
Acknowledgment. We thank Lee Johnson and Frank Zucca for their invaluable help in polishing the single crystals and in building many parts for the experimental setup. Without their support most of this work would have been impossible. Also we acknowledge the work of our summer student Andrew Martin who wrote the programs for the digital data acquisition. E. Zenati acknowledges her fellowship from the Canadian International Development Agency, CIDA Marocco. The financial support from Fonds pour la Formation de Chercheurs et I’Aide 6 la Recherche, FCAR Equipe, is gratefully acknowledged. This work was also supported by the Office of Energy Research, Basic Energy Sciences, Materials Science Division of the U S . Department of Energy under Contract No. DE-AC03-76SF00098. LA94084OP
