978
Langmuir 1994,10, 978
Reply to Comment+on “Electrochemical and Ultrahigh Vacuum Characterization of Ultrathin Cu Films on Pt(ll1)” The comments of Ross and Markovic regarding our work described in ref 1 are interesting speculation; however, they are clearly inconsistent with the facts. For example, in the fourth paragraph the authors’ conclusion “that the additional charge found between 0.6 V and the Nernst potential is due to the stripping of Cu” completely contradicts the experimental observations. Although not stated explicitly in ref 1,a latter publication by us2on this system does state clearly (in footnote 4) that “the vacuumdeposited Cu was transferred to the electrochemical cell, the electrode potential scanned to +0.3V....and (the sample) transferred back to the UHV chamber. No loss of Cu was noted by AES”. Therefore, the authors’assertion in the fourth paragraph that “(our) coulometry neglects the anodic charge (or Cu desorbed) in the potential region between the peak a t 0.6 and the Nernst potential” is simply false and unfounded. Regarding the second point in the fourth paragraph about the effects of the sweep rate, our work naturally included the effects of the sweep rate on the charge. Since sweep rates as low as 20 mV/s showed no effect on the apparent charge, no mention of the sweep rate was made in ref 1. It would seem most unusual that lowering the sweep rate to 5 mV/s would alter the results significantly; however, we do note, on the basis of the data presented in Figure 1of ref 2, that a sufficiently slow scan must in the limit yield a charge for the Cu monolayer near the theoretical value. From recent work of Tadjeddine et aL3 on the Cu/Au(lll) system, “sufficiently slow”would likely be scan rates far slower than 5 mV/s. The comment in the fourth paragraph about previous researchers having found a total charge of 460 pC/cm2 we fiid totally irrelevant to the discussion. The authors agree that their results agree with ours;the point of contention is the interpretation of the results. Most surely the authors can devise an arbitrary algorithm to “find” the additional charge. This they demonstrate. That others also have in no way strengthens their argument. t Ross,P. N., Jr.; Markovic, N. Langmuir, preceding paper in this issue. (1) Leung, L.-W. H.; Gregg, T. W.; Goodman,D. W. Langmuir 1991, 7, 3205. (2) Leung, L.-W.; Gregg, T.; Goodman, D. W. Chem. Phys. Lett. 1992,
188.467.
(3) Tadjeddine, A.; G u y , D.; Ladouceur, M.;Tourillon,G. Phys. Rev. Lett. 1991,66, 2235-2238.
0743-7463/94/2410-0978$04.50/0
Regarding point no. 2, the authors apparently do not understand the nature of our arguments in ref 1regarding the origin of the charge discrepancy. In discussing the nature of the interaction of monolayer Cu with the anion, we drew on our extensive work with Cu monolayers in vacuum (for example, see ref 4). Clearly the origin of the deficiency or excess of charge with Cu coverage at the UPD potential is determined by the nature of the differential screening of the anion in the presence and in the absence of the Cu. The magnitude of this differential screening is determined by the nature (strength) of the Cu-anion interaction. This, in turn, is determined by the polarizability of the Cu, which is a function of the Cu coverage. Thus, how the Cu interacts with the anion is determined by its interaction with the substrate, precisely the nature of our discussion of the Cu-Pt bonding in ref 1. A review of ref 4 and the references therein might be helpful to the authors in this regards. Finally, we note that a recent publication by Kolb and co-workers5reports charge discrepancies of Cu UPD on Pt(ll1) that agree very closely with our results, both at low and high Cu coverages. This group, using combined ultrahigh vacuum (UHV)/X-ray photoelectron spectroscopy (XPS)/low-energy electron diffraction (LEED)/ electrochemical methods, conclude that “the deviations of the electrochemically determined copper coveragefrom those derived from LEED are in almost perfect agreement with the number given by Leung et al. [ref. 11.” This agreement includes the observation of excess charge at the UPD potential for low Cu coverages and a deficiency of charge at 1.0 monolayer of Cu that are identical to those reported in refs 1 and 2. With these considerations we suggest that the state of discharge of monolayer UPD Cu onPt(ll1)is a t this point merely speculative, and awaits experimental determination.
L.-W.h u n g $and D. W. Goodman’ Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255 Received June 15,1993 In Final Form: September 3,1993 (4) Rodriguez, J. A.; Goodman,D. W. Science 1992,257,897. (5) Michaelis, R.; Zei, M. S.; Zhai, R. S.; Kolb, D. M. J. Electroaml. Chem. 1992,339, 299-310. New address: DuPont Chemicals, Jackson Laboratory, Deepwater, NJ 08023.
*
0 1994 American Chemical Society
