Langmuir 1994,10, 976-977
976
Comments Comment on "Electrochemical and Ultrahigh Vacuum Characterization of Ultrathin Cu Films on Pt(111)"2 In our recent paper in this journal, "Effect of Anions on the Underpotential Deposition of Cu on Pt(ll1) and (loo)",' we reported a value for the charge passed during the stripping of a monolayer of Cu on Pt(ll1) that was significantlydifferent from that reported by Leung et al.,2 but did not comment on this difference. One reviewer of our paper was already critical of its "many details", and the explanation of the difference between our result and that of Leung et al. involved details the reviewer thought were unnecessary. However, we feel it is important to comment on the difference and provide the details necessary for the explanation of the different results. The experimentaldata in the two reports appear to agree very closely, e.g., the voltammetry curves in Figure 2 of Leung et al. and the one in our Figure 4. However, there appearsto be a differencein the way the coulometriccharge for stripping the Cu from the Pt surface was calculated from the voltammetry curves. Unfortunately, there is no established procedure, at present, for how this calculation is made, nor is it a "detail" that one finds presented in papers on underpotential deposition (UPD). It is, therefore, not surprising that two groups would get different numbers from similar data. The method we used was as follows. Figure 1shows the voltammetry curves from Figure 4 in our paper. The charge associated with Cu deposition/ dissolution is most easily distinguished from other electrode processes by following the changes in the voltammetry as Cu is added progressively to the solution. At all concentrations of Cu, the deposition process is distributed over a wide potential region between ca. 0.6 V and the Nernst potential for Cu/Cu2+ (0.396 + 0.0295 log Ccuz+ versus a normal hydrogen electrode (NHE) reference electrodein 0.1 M HClO43). At concentrationsof Cu below ca. 106 M, the amount of Cu deposited is quite small, even at potentials below the Nernst potential. At these low coverages, the stripping occurs in a single peak at ca. 0.6 V. At higher concentrations, however, additional anodic charge (versus the curve for Cu-free solution) appears not only in the peak at 0.6 V but also in the potential region between this peak and the Nernst potential. A critical issue in the differencesin coulometry between refs 1 and 2 is whether or not this additional anodic charge is the stripping of Cu,or whether it is another process. As we discuss in detail below, we argue that the most reasonableconclusionis that the chargeis the anodic stripping of Cu, not onlyby the exclusion of other processes but also from the fact that the deposition of Cu occurs in the same potential region; Le., if the UPD were a quasireversible electrode reaction, the anodic process would take place at the same potential as the cathodic process. To calculate the charge, either anodic or cathodic, we corrected the total integrated charge between the Nernst potential and 0.65 V by the "double-layer" charge calcu-
......1 x lo4 M Cu2' ( .
_ - _ _1 x - 1 x IO"
I 0
I
0.4
I 0.t
EN
Figure 1. Cyclicvolta"etryforPt(lll)in0.1 MHClO~solution containing different concentrationsof Cu2+(5mV/s, area 0.568 cm2). The potential scale is NHE. See ref 1for details.
lated from the flat current region in Cu-free solution between 0.25 and 0.5 V. This correctionis shown in Figure 2 for the voltammetry curve for a Cu2+concentration of 0.1 mM. The area under both the anodic and cathodic branches is 400 (f5 5% ) pC/cm2,with ca. 350 pC/cm2of the anodic charge under just the peak at 0.6 V. At 1 mM Cu2+,the total charge is 460 pC/cm2, with most of the additional 60 pC/cm2of charge occurring in the potential region between the Nernst potential and 0.6 V. The theoretical charge for a monolayer of zerovalent Cu atoms having the Pt lattice constant is 480 pC/cm2. Our lowenergy electron diffraction (LEED) observations of electrodes emersed at the Nernst potential indicated Cu was deposited pseudomorphically on Pt, in the same way as Leung et al. found for the vapor deposition of Cu in ultrahigh vacuum (UHV). Leung et al. calculated the Cu stripping charge as being only that under the peak at 0.6 V, and for a monolayer of UHV predeposited Cu reported essentiallythe same charge under this peak as we found, 340 pC/cm2. Sincethey knew from their UHV measurement that their electrode had a full monolayer of Cu on the surface when it was immersed in electrolyte, they concluded that the Cu layer is only partially discharged. We assert that this conclusion is unwarranted, since their coulometry neglects the anodic charge in the potential region between the peak at 0.6 V and the Nemst potential. Another problem they may have in their coulometry is the relatively high sweep rate used (50 mV/s versus 5 mV/s in our experiments). We found that the charge in the potential region between 0.6 V and the Nemst potential was particularly sensitiveto the sweep
(1) Markovic, N.;Rose, P.N.Langmuir 1993,9,580. (2) Leung, L.-W. H.; Gregg, T. W.; Goodman, D. W. Langmuir 1991, 7, 3205. (3) Pourbaix, M. Atlaa of Electrochemical Equilibria in Aqueoua Solutione; J. W. Arrowsmith: Briatol, Great Britain,1966; pp 384-392.
0743-7463/94/2410-0976$04.50/0
M Cu2' M CU'+
(b
1994 American Chemical Society
Comments
Langmuir, Vol. 10, No. 3, 1994 977
so that overall the process can be written as
Cu2++ 8A- + [2 - e(l - @]e-- BCu.-Ab
1 0
I
I 0.4
I
I
0.8
EN Figure 2. Voltammetry curve for 0.1 mM Cu2+showing shaded areas used to calculatethe anodicand cathodicchargesassociated with UPD Cu (5 mV/s, area 0.568 cm2). The arrow indicates the Nernst potential for the Cu/Cu2+couple in this solution.
rate, probably due to kinetic limitations. We note that in recent papers another research group4y5also commented on the need to use low sweep rates (
