Journal of Electroanalytical Chemistry 461 (1999) 121–130

X-ray and electrochemical studies of Cu upd on single crystalelectrodes in the presence of bromide: comparison between Au(111)

and Pt(111) electrodes1

Enrique Herrero, Samantha Glazier, Lisa J. Buller, Hector D. Abruna *

Department of Chemistry, Baker Laboratory, Cornell Uni6ersity, Ithaca, NY 14853-1301, USA

Received 10 November 1997; received in revised form 9 February 1998; accepted 10 February 1998

Abstract

Results from electrochemical and in-situ surface X-ray scattering studies of Cu underpotential deposition (upd) on Au(111) andPt(111) surfaces in the presence of bromide anions are compared. On Au(111) two different ordered structures have been foundby GIXD. At the initial stages of Cu deposition, an enhancement of bromide adsorption is observed resulting in the formationof a bromide incommensurate hexagonal structure, which compresses when the potential is scanned in the negative direction. At+0.32 V, there is a phase transition giving rise to the formation of a commensurate (4×4) bromide structure. This phasetransition coincides with the appearance of a very sharp peak in the voltammetric profile. In this structure, copper adatoms areprobably sandwiched between the electrode surface and the bromide adlayer. Unlike Cu upd on Pt(111) electrodes, no orderedCuBr adlayer is observed. The differences in behavior of Cu upd on Au(111) and Pt(111) electrodes is ascribed not to energeticconsiderations, but rather to geometric constraints imposed by the lattice structure of the metal relative to the deposited adlayer.The stability (to rinsing) and kinetics of the adlayers formed on Pt(111) electrodes in the presence of bromide have also beenexamined. © 1999 Elsevier Science S.A. All rights reserved.

Keywords: X-ray and electrochemical studies; Cu upd; Single crystal electrodes

1. Introduction

The underpotential deposition (upd) of metals ontoforeign metal substrates has been widely studied in partbecause it provides an excellent way to investigate theearly stages of metal deposition and the different fac-tors that affect it [1,2]. Most recently the use of single-crystal electrodes in conjunction with in-situ surfacetechniques, including STM [3–6], AFM [7,8] and sur-face X-ray based techniques [8–12] has allowed a de-tailed investigation of upd processes. The main drivingforce for the upd processes is the interaction betweenthe deposited metal and the foreign metal substrate.

For this reason, upd phenomena are generally restrictedto the deposition of one monolayer although, in somecases, up to three layers of the metal can be depositedprior to bulk deposition.

As aforementioned, the main driving force for theupd processes is the interaction between the depositedmetal and the substrate. However, other interactionscan also play important roles in the process. The pres-ence of strongly adsorbing anions in the electrolyte hasparticular importance, since the anion–metal and an-ion–substrate interactions can modify upd processessignificantly. Anions, such as chloride [13], have beenshown to form stable bilayers on electrode surfaces, inwhich the metal adatom is generally sandwiched be-tween the electrode surface below and the anion adlayerabove [9,14]. In these cases, the strong interaction be-tween the adsorbed metal and the anion plays a key

* Corresponding author.1 Dedicated to Professor W. Vielstich on the occasion of his 75th

birthday.

0022-0728/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.PII S0022-0728(98)00066-7

E. Herrero et al. / Journal of Electroanalytical Chemistry 461 (1999) 121–130122

role in bilayer formation. Such strong anion–metalinteractions result in the formation of ordered struc-tures similar to those present in the correspondinghalide salt of the metal being deposited [14].

Substrate–anion interactions can also affect updprocesses, since the relative strength of the metal–an-ion and substrate–anion interactions may result in achange in the mechanism of deposition for the updprocess. In the present paper, we compare the Cu updprocess in the presence of bromide anions on twodifferent single crystal electrode surfaces: Au(111) andPt(111). Bromide adsorbs more strongly than chlorideon both Pt(111) and Au(111) surfaces and, in thisrespect, it can provide useful insights in the under-standing of how substrate–anion interactions affectthe upd process. In addition, the use of two differentsubstrate metals allows for a more detailed analysis ofthe interactions present. For the upd of Cu onAu(111) electrodes, in-situ surface X-ray diffractionexperiments were carried out to obtain detailed struc-tural information about the adlayer. These results arecompared with those previously obtained by Ross etal. and Kolb et al. for Pt(111) surfaces [15–18] withnew electrochemical data regarding the stability of theadlayers. These results will allow for a detailed analy-sis of the effects of the bromide–platinum and bro-mide–gold interactions on the Cu upd process.

2. Experimental

X-ray diffraction experiments were performed at theX20-C beamline at the National Synchrotron LightSource using a four circle diffractometer. On thisbeamline, radiation from a bending magnet of theelectron storage ring was focused with a Pt-coatedbent cylindrical mirror and monochromatized with adouble Si(111) monochromator. X-ray photons of1.4067 A wavelength were employed. A reflection ge-ometry sample cell similar to those used in previousstudies [19,20] was employed. X-ray photons penetratethrough a 2.5 mm Mylar film (Chemplex) as well as athin film of electrolyte (estimated to be about 30 mm)covering the Au(111) crystal. To prevent the diffusionof oxygen from air through the Mylar film, an outershield with a Kapton window (Chemplex) was placedon top of the sample cell and continuously flushedwith ultra pure N2. Cyclic voltammograms in the X-ray cell were carried out with the Mylar film inflatedby filling with additional electrolyte solution. For X-ray measurements, part of the electrolyte was with-drawn so that the Mylar film was tightly pressedagainst the crystal surface achieving a thin layerconfiguration. Each time the electrode potential waschanged to a new value, the Mylar film was againinflated by adding more deoxygenated electrolyte and

held in this condition for about 5 min to ensure elec-trochemical equilibration.

The X-ray reflections are referred to the hexagonalcoordinates of the Au(111) substrate with as and bs

along the nearest-neighbor direction in the surfaceplane (as=bs=2.885 A) and cs (cs=2.3556 A) normalto the surface. Grazing incidence X-ray diffraction(GIXD) measurements were carried out in the azimuthfixed mode where the incident and outgoing angleswere kept small (a=b:3.5°, L=0.1 reciprocal latticeunits (rlu)) so as to reduce the background and theabsorption by the Mylar film and water solution.

In order to ascertain the stability of the underpoten-tially deposited copper layer, a series of rinsing experi-ments was performed. Two different types of rinsingprocedures were employed, referred to as open-circuitrinsing and controlled potential rinsing. In the so-called open-circuit rinsing, the potential sweep washalted at +0.1 V, after copper underpotential deposi-tion, and the circuit broken by draining the solutionaway from the electrode. The cell and the electrodewere rinsed with fresh supporting electrolyte solution(0.1 M sulfuric acid alone). Subsequent electrochem-istry was performed in supporting electrolyte alone. Incontrast to the open-circuit rinsing, the controlled-po-tential rinsing experiments were performed withoutbreaking electrical contact between the electrode andthe solution. The initial sweep was again halted at+0.1 V, after the deposition process. While fresh sup-porting electrolyte solution was added to the top ofthe cell, the initial copper (and bromide, in somecases) containing solutions were drained out throughthe bottom of the cell in such a way that the solutionlevel was never allowed to drop below that of theelectrode. Although the cell was rinsed with at leasttwo cell-volume equivalents of supporting electrolyte,trace amounts of copper and/or bromide might re-main. However, it will be shown that these traceamounts, if present, affect neither subsequent copperstripping nor deposition.

Electrochemical and X-ray measurements were car-ried out with Au(111) and Pt(111) electrodes with amiscut of less than 0.3°. Prior to any measurement,electrodes were flame annealed, quenched with ultra-pure water [21] and transferred to the cell for theelectrochemical or the X-ray measurements.

All potentials were measured versus a Ag�AgCl elec-trode in 3 M NaCl. A large area coiled gold or plat-inum wire was used as a counterelectrode both in theelectrochemical and X-ray cells. All experiments werecarried out at room temperature. Solutions were pre-pared using ultrapure water (18 MV cm Millipore®

Milli-Q® water), H2SO4 (Ultrex J.T. Baker), HClO4

(Ultrex J.T. Baker), CuO (99.9999%, Aldrich) andNaBr (99.99%, Aldrich).

E. Herrero et al. / Journal of Electroanalytical Chemistry 461 (1999) 121–130 123

3. Results and discussion

3.1. Cu upd on Au(111) electrodes in presence ofbromide anions

Fig. 1 shows the voltammetric profile of a Au(111)electrode in 0.1 M HClO4 +1.0 mM NaBr + 1.0 mMCu2+. Four different Cu upd peaks are present in thismedium. It is worth noting the sharpness of the twopeaks appearing at the onset (peak I at +0.324 V) andat the final stages (peak IV at +0.147 V) of Cu upddeposition. These two peaks are very sensitive to thepresence of contaminants or the presence of defects onthe electrode surface and both reduce the amplitudeand sharpness of the peaks. Such sharp peaks aretypical of phase transitions of adlayers on electrodes,and therefore, ordered structures are anticipated forthis system. Bulk Cu deposition starts at potentialsbelow +0.10 V. Peaks I, II and III partially overlap inthe voltammetric profile even at very low scans rates,making the separation (and the distinction) between theprocesses giving rise to the different peaks difficult. Anadditional shoulder at potentials positive to peak I isalso found (see inset of Fig. 1). This shoulder is similarto that found for Cu upd on Au(111) electrodes insulfuric acid [22].

Fig. 2. In-plane (A) radial and (B) azimuthal scans of the Au(111)surface diffraction, fitted to a Lorentzian.

The sharpness of the peaks in the voltammetricprofile indicates strong attractive near-neighbor interac-tions and the likely presence of ordered structures. Inorder to explore such a possibility, GIXD measure-ments were carried out for this system. Prior to anyX-ray measurement, the voltammetric profile in theX-ray cell was recorded, to ensure the cleanliness of thesystem. The voltammetric profile obtained in the X-raycell is qualitatively the same as that depicted in Fig. 1,with the four peaks, characteristic of Cu upd onAu(111) electrodes in a bromide containing medium,being present. However, due to ohmic drops and thenon-ideal current distribution, the peaks were less sharpthan those found in Fig. 1.

GIXD peaks for the Au(111) surface are shown inFig. 2. The presence of well-defined Au(111) surfacepeaks ensures that the surface has not been disorderedduring the electrode treatment nor during the Cu updprocess. From the azimuthal scan in Fig. 2A a correla-tion length of 1270 A was obtained which, since it waslimited by the detector slits, represents a lower limit.

Previous work by Ocko et al. has shown that onAu(111) surfaces, bromide anions (at the same solutionconcentration) adsorb at potentials above +0.55 Vforming an incommensurate hexagonal structure[23,24]. This structure is rotated relative to the Au(111)surface at an angle that depends on the electrodepotential. Moreover, the Br–Br distance compresseswhen the potential is made more positive. The poten-tials where the ordered bromide structure is found arepositive of those for Cu upd on Au(111) in the presenceof bromide anions. Therefore, the same ordered struc-ture should be observable under the present conditions.Fig. 3 presents a radial scan for the peak found at+0.70 V. This peak is rotated 26.8° with respect to the(10) direction, as shown in Fig. 4 (closed squares). Dueto the hexagonal symmetry of the surface, another peakshould be found at 33.2°. The position of these surfacepeaks and the potential dependence are in good agree-ment with the previously mentioned work of Ocko etal. in the absence of Cu [24]. Therefore, the presence of

Fig. 1. Voltammetric profile of an Au(111) electrode in contact witha 0.1 M HClO4+1.0 mM NaBr+1.0 mM Cu2+ solution. Scan rate:1 mV s−1.

E. Herrero et al. / Journal of Electroanalytical Chemistry 461 (1999) 121–130124

Fig. 3. In-plane radial scan of the diffraction peak of the bromideoverlayer found at +0.70 V, fitted to a Lorentzian. Fig. 5. In-plane radial scan of the diffraction peak of the overlayer

found at (A) +0.36 V, (B) +0.34 V and (C) between +0.32 and+0.10 V.Cu in solution does not appear to affect the adsorption

of bromide at potentials positive of the Cu upd pro-cesses. At a potential of +0.71 V, the Br–Br distanceobtained from the structure was 4.102 A, which gives abromide coverage of 0.495.

No other structure was found between +0.55 and+0.38 V. However, at +0.36 V a new structure with asurface diffraction peak at (0.708, 0) was found (Fig.5A and Fig. 6A). The complementary peak at (0, 0.708)as well as the second order diffraction peak were alsofound. This diffraction feature corresponds to an in-commensurate hexagonal structure. For this incommen-

surate structure, the interatomic distance was 4.08 Awith a coverage of 0.501. Moreover, this structurecompresses when the potential is shifted negatively. At+0.34 V, an interatomic distance of 3.93 A is obtainedwhich, in turn, gives a coverage of 0.538 (Fig. 5B andFig. 6B).

A significant change in the structure is observed at+0.32 V, when the surface structure becomes commen-surate with a diffraction peak at (3/4, 0) (Fig. 5C andFig. 6C). This structure, which can also referred to as(4×4), has the diffraction peaks in reciprocal spacedepicted in Fig. 4. For this structure, a 9/16 (0.563)coverage is obtained with an interatomic distance of3.85 A. This structure is stable between +0.32 and+0.10 V, and no change in diffraction peak position orshape was observed over this range of potentials. Thephase transition between the commensurate and theincommensurate structure is completely reversible, thatis, it always takes place at the same potential (+0.32V) regardless of the starting structure. The potential forthe phase transition coincides with the potential of peakI. Therefore we conclude that peak I is associated witha phase transition from the incommensurate structureto the commensurate (4×4) structure. No other struc-tures were found in the potential range between +0.10and +0.75 V.

The diffraction peaks of the commensurate and in-commensurate structures exhibit one main difference.In the azimuthal scans, the peaks always have the sameFWHM, indicating that the correlation length of thedifferent structures is the same. In this case, a value of

Fig. 4. Reciprocal space map of the in-plane diffraction peaks fromthe Au(111) surface (closed circles), the incommensurate bromideoverlayer at +0.70 V (closed squares) and the commensurate over-layer found between +0.32 and +0.10 V (open circles).

E. Herrero et al. / Journal of Electroanalytical Chemistry 461 (1999) 121–130 125

1000940 A is obtained independent of the appliedpotential, implying that the domain size remains con-stant throughout the potential range where the over-layer structures are present. However, the radial scansshow a clear change between the commensurate andincommensurate structures. The commensurate struc-ture presents a much sharper and better-defined radialpeak. On the other hand, the incommensurate structureexhibits a broad radial peak that can be deconvolutedin at least three peaks. While the azimuthal scans in thiscase provide information about the domain size, theradial scan gives information about the interatomicdistances. This means that at the potentials where theincommensurate structure is found, a wide distributionof interatomic distances can be found in the differentsurface domains.

In principle, the structures found between +0.36and +0.10 V can have three different compositions:either the ordered overlayer is composed entirely ofbromide, of copper or of copper bromide species. Inorder to assign the composition of the ordered struc-ture, we will compare the coverages derived from thestructures with the results obtained by chronocoulome-try by Lipkowski et al. [25–27]. From an analysis of theGIXD results, it is clear that even at the onset of Cuupd there is already a relatively high coverage (around0.50), it increases up to 0.563 at +0.32 V and after-wards remains constant. Of the three possible species,only bromide can exhibit such behavior. The chrono-coulometric data has shown that the surface coverageof bromide (in the absence of copper ions) remainedhigh at potentials above +0.0 V [26]. Moreover, varia-tions in the bromide surface concentration in the pres-ence of copper cations obtained by that technique[25–27] are very similar to those obtained here. From+0.35 to +0.28 V, the bromide coverage increasedfrom 0.45 to 0.58, and remained constant for potentialsbelow +0.28 V [27]. These data allow the assignmentof the ordered structures obtained to a bromide layer.

Previous STM work for this system has identifiedtwo different structures: a (4×4) structure (coverageU=0.56) at potentials below voltammetric peak IVand a (7×7) R19.1° structure (coverage U=0.43)between voltammetric peaks I and IV [28,29]. However,these results are at odds with those presented here andthose obtained by chronocoulometry [25–27], since theincrease in bromide concentration takes place in thepotential region around peak IV.

Bromide coverages obtained in the presence of Cuupd are significantly higher than those obtained in itsabsence. Also, the bromide coverage changes from0.495 at +0.71 V to 0.563 at +0.32 V. The change inbromide coverage takes place at potentials close to theshoulder of peak I (inset to Fig. 1). In this region, thecopper coverage changes from essentially 0 to about0.10 [27], implying that small amounts of copper on theelectrode surface can change the coverage of the anionsignificantly, enhancing its adsorption. Such enhance-ment in the adsorption of anions has been previouslyobserved for copper deposited on platinum electrodes.On Pt(111) electrodes, small amounts of upd copperenhance the adsorption of chloride and bromide [30]and copper deposited on stepped platinum surfaces canenhance the adsorption of (bi)sulfate [31]. In these twoprevious cases, the enhancement in anion adsorptionostensibly took place in the vicinity of the depositedcopper atoms. In this case, the presence of smallamounts of upd copper induces an increase in theadsorption of bromide throughout the surface. This isprobably a consequence of an increase in stability ofthe adlayer associated with the formation of an orderedlayer. The driving force for this enhanced adsorption is

Fig. 6. In-plane azimuthal scan of the diffraction peak of the over-layer found at (A) +0.36 V, (B) +0.34 V and (C) between +0.32and +0.10 V.

E. Herrero et al. / Journal of Electroanalytical Chemistry 461 (1999) 121–130126

Fig. 7. Hard ball model of the overlayers found by GIXD at different potential values: (A) above +0.55 V; (B) between +0.38 and +0.55 V;(C) between +0.32 and +0.36 V; (D) between +0.14 and 0.32 V; and (E) below +0.14 V.

likely to be the diminution in the work function thatalways accompanies the upd process. Copper has alower work function than platinum or gold [32], andsmall amounts of deposited copper lower the workfunction of the surface and hence the pzc, thus enhanc-ing the anion adsorption at a given potential. Thismechanism is in good agreement with the data pre-sented in this section. The initial stages of copperdeposition enhance the adsorption of bromide, thusincreasing its coverage. When additional copper is de-posited on the electrode surface, the surface work func-tion is lowered further, causing a further increase in thebromide coverage which, in turn, gives rise to a com-pression of the adlayer, until a point is reached wherefurther compression of the adlayer is not possible. Thiscoincides with the transformation from the incommen-surate bromide overlayer to the commensurate layer.For that structure, the Br–Br distance found (3.85 A) isin good agreement with the Van der Waals diameter ofbromide (3.70–4.00 A) [33].

As mentioned earlier, the cyclic voltammogram ex-hibits two sharp peaks (I and IV). We believe that peakI arises from a phase transition as described above.However, for peak IV, no additional phase transitionwas observed. If the bromide coverage does not changeover the potential range where peak IV appears [27],then copper is likely to be responsible for this peak.Since peak IV is close to the onset of bulk copperdeposition, it could be due to a transition from a

disordered copper layer to the formation of a (1×1)layer. It would be quite difficult to detect a (1×1)copper layer from GIXD measurements since its dif-fraction peak would overlap with that of the surface.However, confirmation of a (1×1) copper structurecan be done with a technique that probes the layerstructure normal to the surface, i.e. crystal truncationrod measurements [34].

Having established the nature of the ordered struc-tures, the process of Cu upd on Au(111) electrodes inthe presence of bromide anions can be described indetail. At potentials above +0.55 V, a hexagonal or-dered bromide adlayer, rotated with respect to thesubstrate, is formed on the electrode surface (Fig. 7A).This ordered structure disappears at +0.55 V, resultingin a disordered bromide adlayer between +0.34 and0.55 V (Fig. 7B). At +0.36 V, the initial stages ofcopper deposition enhance the adsorption of bromideand an incommensurate hexagonal structure is formed(Fig. 7C). This incommensurate structure undergoescompression as the potential is made progressivelymore negative and more copper atoms are deposited.The potential region where this incommensurate struc-ture is found corresponds to the shoulder region in thevoltammetric profile (Fig. 1 inset). At +0.32 V, theincommensurate bromide overlayer undergoes a phasetransition to a (4×4) commensurate layer (Fig. 7D).As the potential is scanned to even more negativevalues, copper deposition continues, likely sandwiched

E. Herrero et al. / Journal of Electroanalytical Chemistry 461 (1999) 121–130 127

between the gold surface and the bromide overlayer, ashas been previously observed for Cu upd in the pres-ence of chloride [14]. Voltammetric peaks II and IIIappear in this potential region and correspond to thecharge transferred during copper deposition. At+0.146 V, the copper adlayer transforms to probably a(1×1) layer, giving rise to voltammetric peak IV (Fig.7E). Scanning the potential further negative results inbulk copper deposition.

3.2. Comparison between Cu upd on Au(111) electrodesand Pt(111) electrodes in the presence of bromideanions

Cu upd on Pt(111) electrodes in the presence ofbromide anions exhibits a voltammetric profile withtwo well-defined peaks (Fig. 8). At potentials positiveto the first upd peak, bromide anions are adsorbed onthe Pt(111) surface and present an incommensuratestructure aligned along the (1,0) surface direction [35–37], in which the Br–Br distance changes from 4.00 Aat +0.30 V to 3.90 A at +0.60 V [36]. After the firstupd peak, an incommensurate hexagonal structurealigned along the (1,0) surface direction is observed,corresponding to a CuBr layer with an interatomicdistance of 3.74 A [15–17]. At potentials negative of thesecond upd peak, it has been proposed that copperforms a (1×1) structure which is, in turn, covered by adisordered bromide layer. It is also known that anadsorbed copper atom enhances the adsorption of bro-mide in its vicinity [30].

Although on both electrode surfaces similar hexago-nal structures are observed, the formation of and thetransition between the structures is different. OnPt(111) electrodes copper is deposited to form a CuBrbilayer, with a hexagonal structure. In the case ofAu(111) electrodes, the presence of small amounts ofcopper triggers an increase in the bromide concentra-tion that leads to the formation of a bromide overlayer.This behavior is not observed on Pt(111) electrodes,and is probably the result of the different Pt–Pt andAu–Au distances and their influence in the formationof the bromide adlayer. The Au–Au distance in the(111) direction is slightly larger than that in Pt–Pt(2.885 A for Au, 2.77 A for Pt) and can accommodatea commensurate (4×4) structure with a Br–Br dis-tance (3.85 A) that is in the range of the proposed Vander Waals diameter of bromide [33]. To accommodatethis very same structure on Pt(111) surfaces, the Br–Brdistance would need to be compressed below the Vander Waals diameter (3.69 A). Thus, the formation of acommensurate structure on Au(111) electrodes wouldincrease the stability of the adlayer over cases whereincommensurate structures are formed (i.e. on Pt(111)surfaces). This enhanced stability of the bromide layeron Au(111) electrodes aids in the formation of thecommensurate (4×4) layer prior to the formation of aCuBr layer. On the other hand, on Pt(111) electrodes,ordered structures would be present only when a stoi-chiometric CuBr is formed.

However, the presence of a stoichiometric CuBr ad-layer on Au(111) cannot be conclusively discarded,particularly between peaks II and III where the ratio ofcopper atoms to bromide atoms on the surface isapproximately 1 [27]. The lower resolution (higheroverlap) of peaks II and III on Au(111) electrodes whencompared to the voltammetric profile obtained forPt(111) electrodes, allows for only a very narrow regionof stability for this adlayer, making its identificationdifficult.

After completion of the Cu upd and the formation ofa (1×1) epitaxial Cu layer on the electrode surface,only ordered bromide overlayers are found on Au(111)electrodes. The reason again is the formation of acommensurate structure on this electrode surface,which provides additional stability to the bromide over-layer. On the other hand, the similar Br–Br distancesobtained for both electrodes suggests that the maindriving force for the structure is Br–Br interactions,modified by the other interactions specific to eachsystem.

3.3. Stability and dynamics of the copper-bromideadlayers on Pt(111) electrodes

Since high bromide and/or copper concentrations canresult in faster upd kinetics, in the present study low

Fig. 8. Voltammetric profile of a Pt(111) electrode in contact with a0.1 M HClO4+1.0 mM NaBr+1.0 mM Cu2+solution. Scan rate: 1mV s−1.

E. Herrero et al. / Journal of Electroanalytical Chemistry 461 (1999) 121–130128

Fig. 9. Voltammetric profile of a Pt(111) electrode in contact with a0.1 M H2SO4+50 mM Cu2+ solution (—) and a 0.1 M H2SO4+50mM NaBr+50 mM Cu2+ solution (- - -). Scan rate: 2 mV s−1.

change in shape when compared to the response priorto rinsing. Controlled potential rinsing when bromidewas present during the deposition also affects the re-sulting voltammetric profile as seen in Fig. 10, dottedline. In this case, both stripping peaks are clearlypresent although they are smaller in amplitude as wellas broader in shape.

In addition to the presence of copper inducing theadsorption of halides, as shown in the previously de-scribed examples, these data also suggest that the pres-ence of bromide in solution affects the stability of thecopper ad-layer on the surface. Under open-circuitrinsing conditions, the presence of bromide appears tohelp retain that part of the copper ad-layer associatedwith the most positive stripping peak and thus ostensi-bly the most stable structure. On the other hand, in itsabsence, the copper ad-layer is completely rinsed away.Under the less stringent conditions of controlled-poten-tial rinsing, only the most negative peak is affected(ostensibly less stable), while the most positive copperdeposition peak is retained on the surface of the plat-inum regardless of the presence or absence of bromidein solution.

As mentioned earlier, copper adatoms at sub-mono-layer coverages induce the co-adsorption of halides(chloride, bromide) onto the platinum surface and inthe vicinity of the deposited copper adatoms forming aCuX (X=Cl−, Br−) overlayer [30]. At copper cover-

concentrations for both species Cu2+and Br− in 0.1 MH2SO4 were employed. Fig. 9 shows the voltammetricprofiles for Pt(111) in 0.1 M H2SO4 containing 50.0 mMCu2+in the absence (dashed line) and presence (solidline) of 50 mM NaBr. Due to the low copper concentra-tion, the characteristic splitting of the upd peaks is onlyapparent in the positive going (stripping) scan.

If the potential sweep is arrested at +0.1 V in 0.1 MH2SO4+50 mM Cu+2, the electrode is rinsed (solutionexchanged) at open circuit and the scan is subsequentlycontinued, the copper stripping peak is absent from thevoltammetric profile, providing clear evidence that thecopper adlayer does not survive the rinsing procedure.When identical rinsing experiments were performedwith bromide containing copper solutions, an unex-pected behavior was observed, as shown in Fig. 10 (fullline). After the open-circuit rinsing was performed at+0.1 V and the potential was swept in the positivedirection through the stripping region, there was onepeak present at the potential normally ascribed to themore positive stripping peak in bromide containingsolutions.

In contrast to the open-circuit rinsing describedabove, the controlled-potential rinsing gave signifi-cantly different results. Fig. 10, dashed line, shows thestripping profile for copper upd in a bromide-free solu-tion after controlled-potential rinsing at +0.1 V. Incontrast to results for the open-circuit rinsing where nostripping peak was observed, the copper stripping peakis very well defined with no diminution in size nor

Fig. 10. Three different stripping profiles of the Cu overlayer: the fullline was obtained in 0.1 M H2SO4+50 mM NaBr+50 mM Cu2+

after rinsing at open circuit, the dashed line was obtained in 0.1 MH2SO4+50 mM Cu2+ after rinsing at controlled potential (+0.1 V)and the dotted line was obtained in 0.1 M H2SO4+50 mM NaBr+50mM Cu2+ after rinsing at controlled potential (+0.1 V).

E. Herrero et al. / Journal of Electroanalytical Chemistry 461 (1999) 121–130 129

ages above one-half a monolayer, there is a gradualdisplacement of the bromide. At a full monolayer cov-erage of copper, the bromide is completely displacedfrom the platinum surface, but forms an adsorbed layeron top of the electrodeposited copper. The data pre-sented here support these assertions. In essence, uponcopper deposition, the bromide anions are initiallypresent in an arrangement where they are interspersedwithin the copper layer forming a copper-bromide lat-tice structure. Upon additional copper deposition(above one half of a monolayer), the bromide anionsrearrange so that some sit on top of the copper layerwhile the others remain in the copper lattice (ostensiblyas CuBr) to stabilize the copper layer. When there is nobromide present in solution to stabilize the copperlayer, the open circuit rinsing results in the completeremoval of the layer. When the crystal is rinsed atopen-circuit with bromide present, part of the copperlayer is displaced with bromide anions occupying theplatinum surface sites released by the copper thus stabi-lizing the copper layer so that it is not rinsed off thesurface as in the bromide free case.

In the case of the controlled-potential rinsing, (whichis more benign) even in the absence of bromide, thecopper layer remains on the surface after the rinsingprocedure. In addition, when bromide is present, boththe topmost bromide layer and the Cu layer survive thecontrolled potential rinsing procedure so that bothstripping peaks are present in the subsequent scan.From the sweep rate dependence of copper upd in thepresence of bromide, the kinetics of the bromide rear-rangement can be qualitatively ascertained. At sweep-rate values above 7 mV s−1, the most negative peak isabsent and at 7 mV s−1 it is barely discernible. Atfaster sweep-rates, however, the total amount of chargein the remaining peak remains constant within experi-mental error. It would appear that the kinetics of therearrangement into the copper-halide lattice structuretake place on time scales that are longer than theamount of time necessary to complete a cycle at 7 mVs−1. At lower sweep-rates (below 7 mV s−1), the morenegative peak begins to grow suggesting that the bro-mide and copper ions have time to rearrange into thelattice structure. The amplitude of this peak increases asthe sweep rate is decreased. We thus propose that theoverall process initially involves anions that are ad-sorbed on the surface at positive potentials, prior toany copper deposition and once copper depositioncommences, some bromide is displaced. When partialcopper coverage is attained within the first depositionpeak, there is a lattice structure of bromide and copperpresent on the platinum surface. Only if a stable latticestructure is formed will more copper deposition beallowed at the more negative potentials. If the voltam-metric sweep-rate is too fast for the rearrangement tooccur, then there is no further copper deposition cou-

Fig. 11. Sweep rate normalized voltammetric profiles of a Pt(111)electrode in 0.1 M H2SO4+50 mM NaBr+50 mM Cu2+. Scan rates:5(—), 6 (--- -) and 7 (– – – –) mV s−1.

pled with bromide displacement. At the low concentra-tions employed here, this threshold appears to be at asweep-rate of 7 mV s−1. This suggests that it is arelatively slow process.

If sweep-rate normalized voltammetric profiles areoverlaid, as shown in Fig. 11, the presence of a sharplydefined isopotential point between the two strippingpeaks can be easily ascertained. Isopotential points areobserved when there is an equilibrium of species at aconstant total coverage [38]. This indicates that regard-less of whether the rearrangement of bromide ions hasoccurred, there is an equilibrium between the copperand bromide at this coverage.

These observations demonstrate that data obtainedwhen the electrode is rinsed as well as from ex-situmethods must be analyzed carefully since breakingcontact between the electrode and the solution can alterthe delicate balance between the copper and halide ionson the surface. It has also been shown that, by usinglow concentrations, one can observe a sweep-rate de-pendence of the voltammetric profile and establish,through the presence of an isopotential point, that thereis an equilibrium of the copper and bromide species onthe surface.

4. Conclusions

These studies demonstrate once again that upd pro-cesses are quite sensitive to the presence of adsorbing

E. Herrero et al. / Journal of Electroanalytical Chemistry 461 (1999) 121–130130

anions and that the stability of the deposited adlayer isstrongly influenced by their presence. We have alsoshown that the identity of the substrate can play asignificant role as well. In the present case, we believethat the origin of the differences in the behavior of Cuupd on Au(111) and Pt(111) electrodes originates notfrom energetic considerations, but rather from geomet-ric constrains imposed by the lattice structure of themetal relative to the deposited adlayer. In essence, thisis a manifestation of the consequences of a latticemismatch. We have also shown that great care must beexercised in the interpretation of results whenever elec-trochemical contact with the solution is broken, such asin open circuit rinsing, especially when delicate adlayersare present on the electrode surface.

Acknowledgements

This work was supported by the Office of NavalResearch and the National Science Foundation. Syn-chrotron X-ray experiments were performed at beam-line X20-C at the National Synchrotron Light Source,Brookhaven National Laboratory, which is supportedby the US Department of Energy, Division of MaterialsScience and Division of Chemical Sciences (DOE con-tract number DE-AC02-76CH0016). E. Herrero ac-knowledges support by a fellowship from the Ministryof Education and Science of Spain.

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