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Effect of Anions on the Underpotential Deposition of Cu on iPt(ll1) and Pt(100) Surfaces Nenad Markovic' and P. N. Ross Materials Sciences Division, Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720 Received July 27, 1992. In Final Form: November 16,1992 The effect of chloride and (bi)sulfate anions in the supporting acid electrolyte on the chemistry of underpotential deposition of Cu on Pt(ll1) and Pt(100) single crystal surfaces was studied using a combination of electrochemical and nonelectrochemicaltechniques. The presence of these anions above a threshold concentration caused a splitting in the voltammetry peaks for Cu UPD on both Pt(ll1)and Pt(100), with the second or split-off peak at a lower underpotential. This splitting was attributed to competitionbetween the Cu adatoms and the adsorbed anions and the increase in thermodynamicdriving force needed to displace the anions from the Pt substratein order to form a Cu monolayer. The magnitude of the splitting appeared to be proportional to the relative strengths of the anion-Pt bonding, being larger for chloride than (bi)sulfateon either surface and being larger for the (100) surface than for the (111) surface for either anion. By the use of ex situ AES and LEED, we determined that in nearly "Cl-free" supporting electrolyte Cu appeared to be deposited at underpotential in metallic islands (or 'patches") having the Pt lattice constant (pseudomorphicgrowth). In the presence of C1- the Cu was deposited at underpotentials into a Cu-Cl adlattice. At the Nernst potential, however, in both "Cl-free" and C1containing electrolyte, Cu formed a uniform metallic monolayer having the Pt lattice constant, i.e. a pseudomorphic monolayer.
Introduction The underpotentialdeposition (UPD)of a metal, defined
as the deposition of a metal onto a dissimilar metal substrate at a potential that is anodic of the Nernst potential for bulk deposition,has been a subject of intense theoretical and experimental interest. Breiter et ale1 provide the f i t evidence that copper atoms adsorbed on platinum in the underpotential region interfere with hydrogen adsorption. Since then a number of authors have focused their attention to the interaction between the platinum substrate and copper adatoms. Several electrochemical techniques have been used for studying the equilibrium and dynamic properties of copper deposition. In a rotating ring-disk electrode study, Tindall and Bruckenstein2reported that in a sulfuricacid solution the deposition of two monolayers of copper was necessary before bulk deposition was observed. Using the thin layer method, Schultze3reported that the nature of adsorbed copper on platinum depended on the coverage and that the copper UPD in sulfuricacid obeyed a Temkin isotherm. Important observations on the role of anions on copper UPD phenomena were discussed in the radiotracer work of H0ranyi.~9~ He found that the potential dependence of the adsorption of C1- or HS04- was modified in the presence of an Cu2+ion and concluded that the electrosorption of Cu2+ions, i.e. Cu UPD, induced the adsorption of acid anions. The effect of specificadsorption of anions on UPD has since been discussed by a number of other research g r 0 ~ p s . eHowever, ~~ no consistent interpretation has been
* Permanent address: Inetitut of Electrochemistry, ICTM,University of Belgrade, Belgrade, Yugoslavia. (1) Breitar, M.; Knorr, C.; Volkl, W. J. Elektrochem. 195& 59, 681. (2) Tindall, G.; Bruckenstein, S. Anal. Chem. 1968,40, 1637. (3) Schultze, J. Ber Bumen-Ges. Phy8. Chem. 1970, 74, 705. (4) Horanyi, G.; Vertea, G. J. Electroanal. Chem. 1973,45, 63. (5) Horanyi, G. J. Electroonal. Chem. 1974,55, 45. (6) Schmidt, E.; Wlithrich, N. J. Electroanul. Chem. 1970, 28, 349. (7) Kolb, D.; Przasnyski, M.; Geriecher, H. Elektrokhimiya 1977,13, 700. (8) Schultze, J.; Vettar, K. Electrochim. Acta 1975, 19, 915. (9) Juttner, K.; Lorenz, W.; Staikov, G.; Budevski, E. Electrochim. Acta 1978, 23, 741.
proposed for these effects, although several mechanisms have been suggested. Schmidtand Wfithrich6reported that halide ionsformed a compound with the polycrystalline platinum that strongly inhibited the UPD of metal monolayers. On the other hand Kolb et al.' and some other argued that halide interacted with the adsorbate more strongly than with the substrate and thus influencedthe UPD state of the admetal. More recent studies using single crystal surfaces have demonstrated that concurrent anion adsorption at the electrode solution interface plays an important role in the ordering and growth of the UPD metal. In the work by Kolb et a l . l O it was shown that anions can have a pronounced influence on the copper underpotential deposition of Pt(lll),e.g. two adsorption/ desorption peaks hardly separated in Cl- free solution (0.05 M H2S04) become clearlyseparated in the presence (- 106 M) of C1-. Using a UHV surface analysis apparatus for sample preparation and surface characterization, Andricacos and RosslI found that copper UPD on clean wellordered platinum surfacesexhibited multiple adsorption/ desorption peaks from an 0.3 M HF solution containing C1-ion as an impurity. With the same technique Stickney et al.12has demonstrated that UPD of copper on platinum single crystalspretreated with I2 vapor can produce several ordered Cu-I overlayers. Specifically,the Pt(ll1) (3 X 3) structure was observed for partially covered surface by copper adatoms, and a (10 X 10) structure was observed at full coverage. Recently in situ scanning tunneling microscopy (STM) data for UPD of Cu on Pt(ll1) in sulfuric acid demonstrateI3that the structure of the first Cu adlayer is ( 4 3 X 43)R3O0. It was suggested that the (10) Kolb, D.; A1 Jaaf-Golze, K.; Zei, M. Dechema-Monographien; Verlag Chemie: Weinheim, 1986; Vol. 12, p 53. (11) Andricacos, P.; Rosa, P. J. Electroanal. Chem. 1984, 167, 301. (12) Stickney,J.; Roeaaco, S.; Hubbard,A. J. Electrochem. SOC.1984, 131, 260. (13) Sashikata,K.; Furuya, N.; Itaya, K. J. Electroonal. Chem. 1991, 316, 361. (14) Leung, L.-W.; Gregg, T.; Goodman, D. Chem. Phys. Lett. 1992, 188,467. (15) Aberdam,D.; Durand, R.; Faure, R.; El Omar, F.Surf.Sci. 1985, 162, 782.
0743-746319312409-0580~04.00l0 Q 1993 American Chemical Society
Deposition of Cu on Pt appearance of the widely spaced structure of ( 4 3 x d3)R30° is due to the presence of coadsorbed anions like HS04- and sod2-.Although an ordered structure was observed, a discrepancy between the coverage of copper adatoms estimated from STM versus coulometry was observed; e.g. the charge under the f i i t UPD peak was nearly twice that expected for an ordered ( 4 3 X d3)R3O0 structure (160pC/cm2). The authors attributed the charge imbalance to the desorption of bi(sulfate) anions during the UPD process. Leung et al." reported a discrepancy between the coverage of Cu on Pt(ll1) in HC104 determined ex situ by AES and that calculated from coulometry which they attributed to perchlorate anion adsorption. However, Aberdam et al.lS have reported that the Cu coverage from the Cu/Pt AES peak ratio correlated well with the coulometric measurement of the amount of Cu deposited on Pt(ll0) in perchloric acid. An interesting observation about coadsorption of copper with different halide anions on Pt(ll1) was recently discussed by White and Abruna.16 They found that a partial charge transfer from adsorbed copper to the coadsorbate occurs in the following manner S2- > I- > Br- > C1-. Further evidence for charge transfer from the UPD metal to the coadsorbed anion was provided by X-ray adsorption data.16 The variety of different structures observedfor Cu UPD on Pt surfaces, as well as the dramatic differences in voltammetry in different electrolytes, suggests there is a strong effect of the anions present in the electrolyte in which Cu UPD occurs. The purpose of the present work was to understand this effect more completely by adding varying amounts of different anions to either perchloric or hydrofluoric acid supporting electrolyte and observing the systematic changes in the voltammetry. In the case of HF supportingelectrolyte,these chsngesin voltammetry could be correlated to the changes in the surface structure observed on emersed electrodes with LEED.
Experimental Section The preparation and pretreatment of platinum single crystal electrodes were fully described in ref 17. Most of the results reported here were obtained with the flame technique for the Resultswith UHV prepared preparation of large single cry~tala.~S surfaces1sare noted specifically. The emersion procedure for ex situ Surfaceanalysiswas described in detail in ref 19. Calibration factors for estimating the coverages of C1 and Cu from Cl/Pt, CVCu, and Cu/Pt AES peak ratioa were given in ref 17. The surface coverages (in monolayers (ML)) were based on the abeoluteatomicdeneitiesofthePt(ll1)and (100) surfaces (1.505 X 1015and 1.303 x 1015atoms/cm2,respectively) assuming one adatom per Pt atom at 1 ML. The eolutions were prepared wing J.T. Baker reagent grade acids and pyrolitically triply distilled water. All potentiah are given against the reversible hydrogen electrode in the same electrolyte (RHE). The physical surface areas for the flameannealed Pt(ll1) and Pt(100) crystala were 0.568 and 0.572 cmZ, respectively. The contact area of the electrolyte drop on the crystal Surfaces in the thin layer cell was approximately 0.350.45 cm*. The voltammograms were recorded with sweep rates of 50 and 5 mV/s.
Results 1. Voltammetryof Pt(ll1) andPt(100)in Different Acid Electrolytee. Cyclic voltammograms for flame(16) White, J.; Abruna, H. J. Phys. Chem. 1990,94,894. (17) Markovic, N.; Rocur, P.J. Electroanul. Chem. 1992,330,499. (18)Msrkovic,N.;Haneon,M.;McDougal,G.;Yeager, E. J.Electroanu1. Chem. 1966,214,666. (19)Fbs, P.;Wagner, F. In Adoancee in Electrochemistry and ElectrochemicalEngineering; GerLcher, H., Tobias, C. W., Eds.;Wiley: New York, 19W, Vol. 13, p 69.
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EN
Figure 1. 'Cyclic voltammetry of Pt(ll1) and Pt(100) single crystals in different acid electrolytes of the same pH (=l).50 mV/s.
annealed Pt(ll1) and Pt(100) obtained in three different acid electrolyte are shown in Figure 1. The general shapes of these curvesare well but the interpretation of the processes which occur within hydrogen adsorption and oxide formation potential regions is still controversal.2')-22For our purposes here we are focusing on the effecta of acid anions on the voltammetricfeatures in these two potential regions with both Pt(ll1) and Pt(100) surfaces. Figure 1 shom that hydrogen adsorption/ desorption at Pt(ll1) in the potential region -0.05 IE I 0.26 is not dfected by the nature of the anions, indicating that in this potential region on this surface there ia no coupling between hydrogen adsorption and anion desorption. The curves for the Pt(100) surface show, however, that within the hydrogen region -0.05 IE I0.4 V anion admrptiorddesorptionon this surfaceisconcurrent with hydrogen desorption/adsorption. In contrast to relatively symmetrical peaks for Pt(100) obtained in perchloric and sulfuric acid, that in HF exhibits asymmetry, which has been attributed to the relatively high concentration ( 5 X 10-6 M)of C1- impurities present in 0.3 M HF.17J8 The different levels of C1- impurities in HC104 versus HF also have a significanteffecton the shape and the position of both the anomalous features on Pt(111)as well as the "true" oxide formation peak at -0.86 V. The sharp oxide peak at -0.86 V obtained in 0.1 M HC104 is shifted anodically in HF due to the higher
-
(20) Markovic, N.; Marinkovic, N.; Adzic, R. J. Electroanol. Chem. 1988,241,309. (21) Clavilier, J.; Rodea, A.; Achi, K. El. J. Chim. Phys. 1991,88,1291. (22) Wagner, F.;Roas, P. J. ElectroanaI. Chem. 1988,260, 301.
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582 Langmuir, Vol. 9, No. 2, 1993
Pt(111)
- 0.1M HClOr +
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1 X 10% Cu2*
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concentration of C1-.12 The most anodic peak for oxide formation on Pt(ll1) is obtained, however, in the electrolyte containing sulfuric acid anions, probably due to the strong adsorption of 3-fold coordinated (bilsulfate on the (111)sites.18 In contrast to this, the oxide formation peak on Pt(100) in 0.3 M HF appears at the potential which is -0.15 V more anodic than the potential for the oxide peaks in H2S04. The relative peak positions for oxide formation on Pt(100) in chloride versus (bi)sulfate electrolyte suggest that chloride is more strongly bound on this surface than (bi)sulfate, which is the opposite to the relative strengths of bonding to the (111)surface. These anion effects on Pt(ll1) and Pt(100) voltammetry in Cufree electrolyte have a related role in the voltammetry of Cu UPD on these surfaces. 2. Voltammetry of Cu UPD-Low Concentrations (O.l mM). Figures 4 and 5 show the voltammetrycurves for Pt(ll1) and Pt(100) in 0.1 M HClOl with varying amounts of Cu2+ ions above 0.1 mM. At these concentrations, hydrogen adsorption was completely suppressed indicating that Cu formed a uniform monolayer (at least) at UPD potentials on both Pt(ll1)and Pt(100). It should be noted that the surface coverage (by coulometry) by copper was still stronglydependent on the scan rate applied (the surface coverage by copper increased at lower scan rate) even at these higher concentrations, indicating deposition of copper is a relatively slow process. The maximum amount of Cu depoeited at underpotentials was determined by integration of current potential curves (anodic sweep) in Figures 4 and 5 and is summarized in Table I. T h e charge values for Cu on Pt(ll1)and Pt(100) also indicate that for >1 mM Cu2+ essentially a full monolayer of copper was deposited underpotentially from HClO, electrolyte. A comparison of Figures 4 and 5 shows that Cu deposition started at a more (-0.1 V)anodic potential on the (100) versus the (111) surface, but otherwise the voltammetry curves for the two surfaces in HClO, shared severalcharacteristics. First, on the cathodic sweep metal deposition onto the initially bare Pt surface produced
Pt(lll) 0.1M HCIO,
II
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Figure 4. Cu UPD on Pt(ll1) in HClO, supporting electrolyte with different levels of Cuz+in solution. 6 mV/s.
relatively broad deposition peaka at all Cu2+ion concentrations (with the exception of Cu on Pt(100) from 1 X
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584 Langmuir, Vol. 9, No.2, 1993
pt(ll1)
E
8
3
I
I
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0
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EN
Table I. Coulometry of Cu UPD in 0.1 M HClO4 Pt(ll1) Q/& cm-2 35 125 400 46P
Pt(100) Q/& cm-2 30 103 360 413O
a The ideal charge for a monolayer of zero valent copper atoms having the Pt lattice constant is 480(111) and 420(100) pC/cm2.
M Cu2+),but only a single stripping peak appeared on the anodic sweep (provided no bulk Cu is deposited). Second, when scanning into the region negative of the Cu/Cu2+Nernst potentials bulk Cu was deposited only when the Cu2+concentration was higher than -1 X M. For these concentrations, the shape of the voltammogram in the overpotential region was exactly that expected for diffusion limited reversible deposition of a metal,Z5 Third, the sharp anodic stripping peak at 0.60.7 V shifted anodically with an increase of Cu2+concentration. Finally a very sharp peak at -0.65 V (111)or 0.67 V (100) was superimposed on the narrow stripping maxima at 0.65 (111)or 0.69 V (100)when the potential was swept through the overpotential region. We hypothesized that this splitting was caused by desorption of specifically adsorbed impurity anions (e.g. C1-) concurrent with Cu deposition. To confiimthat multiple voltammetricpeaks arise from an effect of anion (C1-) on the underpotential deposition, small amounts of HCl were added to the HC104solution, as shown in Figure 6. The addition of HCl had a significant (25) Andricacos, P.;Rose, P. J . Electrochem. SOC.1984, 131, 1353,
1531.
C
, 0.
Figure 6. Cu UPD on Pt(100) in same solutions as in Figure 4. concentration of Cu2+/M 1 x 10" 1 x 10-5 1 x 10-4 1 x 10-3
R# 0.4
0.6
k
,,
,
I
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0.8
I
0.6
I
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Figure 6, Effect of additions of C1- (asHCI) to 0.1 M HClO, on Cu UPD on P t ( l l 1 ) and (100). 5 mV/s.
effect on both the position and the shape of copper UPD peaks. For Pt(ll1) in the presence of ca. 1X MC1-, the depositionpeaks at 0.6 and 0.47 V were changedslightly and a small shoulder appeared at ca. 0.58 V on the cathodic side of the stripping peak for the UPD monolayer. However, with higher concentrations of C1- (>5 X 10-5 M) deposition (designatedA' and B') as well as stripping peaks (A and B)became sharp and well resolved. On Pt(ll1) two desorption processes at 0.7 and 0.58 V which were not clearlyresolvedinthesolutionwith90%) of the Cu was stripped
-P t ( l l 1 )
- - - - Pt(100) 0.1 M HCIO, 1 x 10' M Cu2*+ 5 x 1 0 ' ~M CI'
e
g c 1 I I
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I
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I
0.2
0.6
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Figure 7. Comparison of Cu UPD on Pt(ll1) and Pt(100) in HC104 and HF (which contains C1- as an impurity) of the same pH showing the similarity when C1- is added (as HC1) to the HC104. 5 mV/s.
from the Pt surface, while in HF nearly 0.5 ML remained on the surface. These comparisons indicate that there are qualitative similarities to the effects of these two adsorbing anions on Cu UPD but also some quantitative differences that have both kinetic and thermodynamic consequences. 3. LEED/AESAnalysis of Emersed Electrodes. As we discussed in our previous paper,17emersion experiments from electrolytes containing nonvolatile solutes are complicated by the residue left from the adhering film of bulk electrolyte. In our apparatus, the concentration of nonvolatile solute must be below 1mM in order for the residue to have a minimal effect on ex situ surface analysis. Emersion from supporting electrolytes like 0.05 M H2SO4 or 0.1 M HClOI produces the undesirable complication of acid hydrate residues26 which we preferred to avoid for the purposes of this study. Thus our emersion experiments were done from HF electrolyte, containing either HC1 as an impurity or with HC1 added, since both are volatile solutes. In previous studies of C1adsorption on Pt in this laboratoryF4AES analyses of electrodesemersed from 0.3 M HF, containingHCl as an impurity, were used to obtain C1adsorption isotherms. To extend that study and make use of that data in a meaningful way in this study, we conducted a series of AES analyses of electrodes emersed from Cu containing HF electrolyte at various concentrations of Cu2+ and HF and at various emersion potentials. The objectives of these analyses were (i) to establish that the presence of Cu2+in solution does not effect the coverage of C1 on Pt at potentials above ca. 0.7 V (where there is no UPD Cu), (ii) to measure the maximum concentration ~
~~~~~
(26) Zei,
M.;Weick, D.Surf. Sci. 1989,208, 426.
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586 hangmuir, Vol. 9, No.2,1993
.
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5
I
5 x 10’M
Cu”
CI
0
08
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-I
200
I
400
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I 800
I 1000
Electron Energy (eV)
Figure 9. Voltammogram for Cu UPD on Pt(ll1) in “nearly C1-free” electrolyte showing the potential of emersion (insert) and the corresponding LEED pattern (134 eV) AES spectrum for emersed electrode.
0
0.4
0.8
1.2
W Figure 8. Comparison of Cu UPD on Pt(ll1) at high Cu2+ion concentration in different supporting electrolytes a t the same pH (=l).
of Cu2+in solutionthat produced a Cu-freesurfaceemersed at potentials above ca. 0.7 V, and (iii) to determine the maximum concentrationof HF supportingelectrolytesuch that emersion of Pt at 0.7 V produced no C1 AES signal. The first objective was easily confirmed,as expected.The maximum Cu2+ concentration that would produce the correct “blank” result (emersion above 0.7 V without an AES signal for Cu) was ca. 1mM, just high enough to do emersions from the electrolyte in Figure 7. However, the dilution of HF needed to produce a truly C1-free emersed electrode was not practical, as it would require a reexamination of Cu UPD in the other supporting electrolytes in the unusual pH range of 3-4. A dilution to 0.1 M HF appeared to be a practical compromise, producing “nearly C1-free” emersed surfaces. The LEED/AES analysis of Pt(ll1) emersed from 0.1 M HF containing 5 X M Cu2+is shown in Figure 9, along with the correspondingvoltammetry prior to emersion at 0.2 V. Coulometry indicates a totalcharge of UPD Cu of ~ 2 0 pC/cm2, 0 or a coverage of ca. 0.4ML, while the AES Cu/Pt peak ratio suggests a coverage of 0.2-0.3ML. While a C1 AES peak at 181eV is clearly seen in Figure 9, the C1 coverage was actually quite small, less than 0.1 ML (C1 has the highest AES sensitivity factor of any element!). The LEED pattern was (1X l),indicating that from nearly C1-free solution Cu was deposited as metallic islands (or “patches”) having the Pt lattice constant (pseudomorphic growth), as occurs in the vapor-phase deposition of Cu on Pt(lll).27 Other emersion experiments were done in 0.3 M HF + mM Cu2+in order to examine the coverages of Cu and C1 that may be associatedwith the multiple peaks seen in the voltammetry of Figure 7. AES measurements were made (27) Yeatea, R.;Somorjai, G. Surf. Sci. 1983, 134, 729.
Table 111. Summary of AES Peak Ratios and Corresponding Coverages for Emersed Pt Single Crystal Electrodes, 0.3 M HF + 1 mM Cu2+ coverage (ML)* potentido AES ratio (V) Cl/Pt Cu/Pt Cl/Cu c1 cu Pt(ll1) 0.35 3.8 1.4 2.7 0.25 (i0.l) 0.50(*0.1) 0.1 7.3 2.6 2.8 0.45 1.0 Pt(100) 0.35 4.5 1.6 2.9 0.30 0.55 0 7.5 2.5 3.0 0.50 1.o a Versus Cu/Cu2+Nernst potential. Calculated as if only C1 or Cu were on the Pt surface (see ref 17).
at potentials near the Nernst potential for Cu/Cu2+and at a potential between the two voltammetry peaks. The coverages were calculated from the AES ratios using calibration factors determined independently17 by adsorption of Cl2 and by the vapor-phase deposition of Cu. No attempt was made to correct the AES signals for the effect one adsorbate had on the Auger electron emission from the other. The results are summarized in Table 111. In the case of Cu, the AES coverages indicated the formation of a monolayer at the Nernst potential and approximately a half-monolayer at the potential in the middle of the two UPD peaks in Figure 7. The coverage by C1increased monotonically with increasing Cu coverage. The simplest interpretation of the coverages for the electrodes emersed near the Nernst potential is that C1 is on top of the Cu monolayer. It that were the case, the C1 coverages are in excellent agreement with the saturation coverages of C1 adsorbed on Cu(111)28aand Cu(100).28b To determine more about the structural arrangement of the Cu and C1 atoms on the surface as a function of potential, LEED analyses were done on these same emersed electrodes. The results for the AES/LEED experiments of the Pt(111)electrode related to the voltammetry in Figure 7 are shown in Figures 10 and 11. The electrode was emersed on the cathodic sweep about 0.1 V positive to the Nernst potential for bulk deposition,where according to the AES peak ratio there was the formation of a metallic Cu monolayer. The LEED pattern is shown in detail in Figure 11. The pattern has three sets of spots, the fundamental (28)(a)Goddard, P.; Lambert,R.Surf.Sci. 1977,67,180. (b) Westphal, D.; Goldman, A. Surf. Sci. 1983, 131, 92.
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Depo8ition of Cu on Pt
I
Pt (I I I)
R R
CI
0.1
1 - 0 I
200
I
I
I
I
400 600 800 Electron energy (eV)
I
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IO00
Figure 10. AES spectrum for Pt(100) emersed on the cathodic sweep(insert). Conditions for Cu UPD were the same aa in Figure 7.
beams for (1111-1X 1, superlattice beams as triplets near X fill330 positions, and a set of weaker triplets the (fi which are double diffraction spots. We interpret this pattern based on the physical model of a pseudomorphic Cu monolayer having the Pt(ll1)-1X 1lattice. On the top of this Cu monolayer there is an adsorbed layer of C1 which is associated with the LEED spots near the fi positions. The C1superlattice is more easily visualized by X f i ) R 3 0 unit mesh, as done referencing it to the (fi in Figure l l b in reciprocal space and l l c in real space. The C1 superlattice is compressed by ca. 20% relative to the fistructure and is rhombic. As shown in Figure llc, X 15) the nearest coincident cell with the substrate is (fi rect, and there are three domains of these coincident cells each rotated by 120° with respect to the other. The Cl-Cl near-neighbor distance is 3.83 A, and the theoretical C1 coverage is 7/15 or 0.47. The AES peak ratio is consistent with this theoretical coverage. The C1 superlattice primitive unit mesh is actually nearly identical to that reported by Goddard and Lambert% for the high coverage state of C1 on Cu(ll1) in vacuum. The principal difference between the two structures is the relationship to the substrate, where in the case of the pseudomorphic Cu monolayer the substrate unit mesh is expanded by ca. 8% relative to the bulk Cu unit mesh, producing a different coincidence relation with the C1 adlayer. In both cases, the Cl-Cl bond distance is nearly the same as it is in the CuCl zincblend structure.29 Experiments to determine the structure of the Cu adlayer after just the fmt stage of deposition,by emersion at 0.35 V positive of the Nernst potential, were not as successful. The LEED patterns were poorly ordered with a high diffuse background intensity and somepoorly structured intensity around the (dX f i ) R 3 0 positions of reciprocal space. However, the patterns did appear to be more indicative of two superstructures corresponding to coadsorbed Cu and C1, as opposed to the Cu island growth observed with nearly Cl-free electrolyte (Figure 9). T h e results of LEED/AES analysis of Pt(100) emersed electrodes are shown in Figures 12 and 13. For emersion at 0.35 V positive of the Nernst potential, where the total charge indicates the coverage of Cu was 0.67 ML, the Cu/ Pt AES peak ratio suggesta the coverage was somewhat lower, 0.54.6 ML. The LEED pattern, shown in Figure (29) Wyckoff, R. Crystal Structures,2nd ed.;Interscience: New York and London, 1963.
13a, was a reasonably sharp c(2 X 21, but with a relatively high background intensity. We attribute this background intensity to disordered coadsorbed C1, and the c(2 X 2) to the UPD Cu. An ideal c(2 X 2)-Cu lattice would have a coverageof 0.5. Flashing the crystal in UHV to 900 K and repeating the AES analysis after cooling showed (Figure 12b) that the C1 was completely desorbed, and the Cu/Pt AES ratio decreased from 1.65 to 1.25, indicating the Cu coverage decreased to 0.3-0.4 ML. The LEED pattern after flashing, shown in Figure 13b, was p(2 X 21, which corresponds to an ideal coverage of 0.25 ML. It appears that in flashing the crystal to 900 K,not only is the C1 desorbed, but some Cu is also lost from the surface, either by diffusion into the or evaporation (possibly as CuC1). The p(2 X 2) probably represents the formation of a surface alloy, as observed by Yeates and Somorjai.27 Emersion at the Nernst potential also produced a c(2 X 2) LEED pattern, and a Cu/Pt AES ratio of 2.5, consistent with the coulometric indication of a full monolayer of Cu (Table 111). The Cl/Cu AES peak ratio was essentially the same as for the partial monolayer. In this case,flashing the emersed electrode to 900 K desorbed the C1 and reduced the Cu/Pt AES peak ratio only slightly (5%1, but changed the LEED pattern from c(2 X 2) to (1X 1). Thus, we interpret the original (100)-c(2 X 2) LEED pattern from the electrode emersed at the Nernst potential as a (100)-(1 X 1) pseudomorphic monolayer of Cu with an adsorbed layer of C1 in a (100)-c(2 X 2)-C1 adlattice, i.e. the analogous structure to that observed on the Pt(ll1) surface emersed at the Nernst potential. It is not possible to determine definitively the real space structure of an adlayer based solely on the symmetry of the LEED pattern. There are other (than the ones presented above) real space structures that could have the same symmetries as the LEED patterns in Figures 11 and 13,but we were able to eliminatethese other structures based on additional experiments which are presented in detail elsewhereem Briefly summarizing, these experiments used a combination of AES chemical shifts and thermal desorption spectroscopy to eliminate adlayer structures derived from the CuCl (zinc blend) lattice in favor of the C1-adsorbed on Cu monolayersstructuresjust described.
Discussion The cyclic voltammetry of Pt single crystals we have reported here clearly indicate the presence of specifically adsorbed anions like C1- and HSOI- (SO& in the supporting electrolyte has a dramatic effect on the underpotential deposition of Cu. The sensitivity is especially great for C1-, where concentrations as small as M have a pronounced effect. The complexity of the voltammetry for Cu UPD in the presence of these anions points to complex interactions between Cu and the adsorbed anions which are difficult to unravel without additional information, e.g. coverages of the different species as a function of potential. There were two significant pieces of information provided by the AES/LEED results that help to resolve the complexity of the voltammetry. The fmt is that in a supporting electrolyte in which the concentration of specifically adsorbing anions is below a critical level, the growth of the Cu monolayer on Pt by electrodepositionis very similar to the growth during vapor-phase deposition (30) (a) Markovic,N.; Ross, P. Submittadto d. Vac. Sci. Technol. A. (b) Ross, P. In Structure of the Electrified Interface, Volume II of Frontiers in Electrochemistry; Lipkowski, J., R m , P., Eda.; VCH Publishers, Inc.: New York and Wienheim, in preen.
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588 Langmuir, VoZ.9, No. 2, 1993
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Figure 11. (a) LEED pattern (68 eV) for the emersed Pt(ll1) electrode of Figure 10. (b) Schematic of the LEED pattern showing the relation of hypothetical ( 4 3 X d3)R30 unit mesh (dashed) to the observed “rhombic” unit mesh. (c) Superposition of the real-space rhombic unit cell onto the (1X 1)substrate lattice showing the ( d 3 X 15) rect. coincidence unit cell; a ( d 3 X d3)R30 unit cell (dashed) is shown for comparison.
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Figure 12. AES spectrum for Pt(100) emersed on the anodic sweep (insert). Cu UPD conditions were the same as those given in Figure 7.
on Pt at room temperature in UHV.27 In this process, Cu adatoms sit in the bulk lattice positions that Pt atoms would occupy in the next atomic layer, a growth process termed pseudomorphic. In the submonolayer state, the Cu atoms are nucleated into islands, and the monolayer forms by expansion and coalescence of the islands. The second is that for deposition in the presence of C1- in solution above a critical level, there is an attractive interaction between Cu and C1such that deposition of Cu draws C1 to the surface. This interaction also leads to a submonolayer structure in which Cu ‘and adsorbed C1 coexist on the Pt surface (most probably in a superlattice). Then at the Nernst potential, an epitaxial Cu monolayer
is formed with Cu atoms having the Pt lattice constant and with C1 adsorbed on the Cu layer to saturation. The structures we found for the C1 layers on these Cu monolayers are also closely related to structures reported for C1adsorbed on Cu(ll1) and Cu(100)surfaces in vacuum at saturation coverage. In the case of the Cu monolayer (100) surface, the C1 adlayer has the same (100)-c(2 X 2) symmetry as C1on Cu(100)at saturation in the gas-phase, but because the Cu monolayer is ca. 8%expanded versus the bulk Cu crystal lattice, the Cl-Cl distance is slightly larger. Ehlers et al.32reported a structure for C1 on a Cu(100) electrode emersed from 0.1 mM HC1 that is essentially identical to the (100)-c(2 X 2)-C1 structure observed in vacuum. They also reported that the C1 adlayer structure was insensitive to the emersion potential. From these observationscombined with our own, it appears that at least for the (100) surface the pseudomorphic Cu monolayer interacts with C1 in the same way as the bulk Cu(100) surface. With respect to the (111)surface, that conclusion cannot be reached as simply. The structure we observed here is very different from that reported by Stickney and for Cu(ll1) emersed from 0.1 mM HC1 but is essentially the same structure as that reported by Goddard and Lambert% for C1on Cu(111)at saturation in vacuum. Stickney and Ehlers reported the formation of a Cu-C1 bilayer having a structure closely related to the (111)plane of the CuCl zincblend lattice. Although both papers came from the same laboratory, there was no discussion in the later paper on Cu(100) of the difference in the nature of the structure between the (111)and (100) faces, e.g. a CuCl-like layer formed on Cu(100) only when (31) Durand, R.; Faure, R.; Aberdam, D.; Traore, S. Electrochim. Acta 1989,34,1653. Stuve, E.; Borup, R.; Sauer, D. In h o c . Symp. on Appl. Surf. Anal. to Enu.lMat. Inter.; Baer, D., Clayton, C., Davis, G., Eds.; The Electrochemical Society: Pennington, NJ, 1991; PV-91-7. (32) Ehlers, C.; Villegas, I.; Stickney, J. J. Electroanal. Chem. 1990, 284, 403. (33) Stickney, J.; Ehlers, C. J. Vac. Sci. Technol. 1989, A7, 1801.
Langmuir, VoZ.9, No. 2,1993 589
Deposition of Cu on Pt Pt (100)
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Electron energy (eV) Figure 13. (a) LEED pattern (62 eV) and AES spectrum for the emersed Pt(100) electrode of Figure 12. (b) LEED pattern and AES spectrum after flashing (15 K/s) to 900 K.
polarized into the potential region for Cu dissolution, whereas a CuC1-like layer formed on (111)at all potentials. The structure we observed for C1 on the emersed (111)-1 X 1Cu monolayer was essentially the same as that observed for the saturation coverage of C1 on Cu(ll1) in the gasphase, having the same C1 adlattice primitive unit mesh (rhombic)and Cl-C1 near neighbor distances (3.86 A), but because of the different lattice spacing of the monolayer there is a different coincidence lattice versus the bulk crystal. The independentlydetermined coveragesand structures of Cu and C1 as a function of potential can be used as the
basis of an explanation or rationalization for most of the features of the Cu UPD voltammetry. Let us consider each of the characteristic features in the voltammetry associated with the presence of adsorbing anions and considerhow that feature may be related to the coverages and structures just described. (1) The deposition of a small amount of Cu from supportingelectrolyte-containingchloride or sulfateanions caused a redistribution in the statesof adsorbed hydrogen similar to the redistribution caused by increasing the concentration of these anions to Cu-free electrolyte. The explanation for this characteristic was presented in detail previously17 and is summarized here for completeness. As revealed by the AES coverages, the initial deposition of Cu onto Pt from these electrolytes induces anion adsorption onto the Pt surface in the vicinity of the Cu adatom. This enhanced anion adsorption around Cu adatoms we have attributed to a lowering of the local work function18at Pt atoms neighboring the Cu. In the case of HC104supporting electrolyte, the similarity of the effect of Cu in all three electrolytes in this work (Figure 2) suggeststhat the anionsadsorbing from the HC104 solution were the (bi)sulfate and chloride anions present as impurities in the concentrated perchloric acid used to prepare the solution. (2) For the Pt(ll1) surface, the addition of C1- to perchloric acid supporting electrolyte enhanced the reversibility of the UPD process and split the UPD process into two distinct stages, separated in potential by ca. 0.12 V. The catalytic effect of chloride can be rationalized by the attractive interaction between Cu and adsorbed C1 indicated by the AES data. The first (more anodic) UPD peak appears to correspond to the formation of a Cu-Cl adlayer having a more favorablethermodynamic potential than Cu adatoms alone and thus lowering the activation energy for the initial deposition. A t the same time, this stabilization by coadsorbed C1 would require an increase in the thermodynamic driving force to decompose this adlayer and form the Cu monolayer. The second (more cathodic) UPD peak thus would correspond to displacement of C1 from the Pt sites and replacement by Cu to form the pseudomorphic Cu monolayer. C1 is adsorbed onto the Cu monolayer to saturation. (3)For the Pt(100) surface,the addition of C1- also split the UPD process into (at least) two stages, but the second (more cathodic stage)was distributed over a 0.2-V potential region and the monolayer was not formed until the potential was very close to the Nernst potential. The reversibilitywas not enhanced significantly, and there was a distinct cathodic shift in the potential for the onset of Cu deposition. The AES derived C1 adsorption isotherms for the (100) and (111)Pt surfaces are very different and indicative of a much stronger Pt-C1 bonding on the (100) versus the (111)surface. As on the (111)surface, the first deposition peak for Pt(100) appears to correspond to the formation of a Cu-Cl adlayer, perhaps with the Cu adatoms in a c(2 X 2) adlattice and C1 randomly occupying the vacant Pt sites. The thermodynamic stability of a Cu-C1 adlayer is likely to depend on a delicate balance of chemical interactions between the Pt surface, Cu adatoms, and adsorbed C1. The stronger binding of C1 to Pt(100) may reduce the stability of the Cu-C1 adlayer on this surface relative to the comparable layer on the (111) surface, producing the observed cathodic shift for the first stage of deposition. A t the same time, the stronger Pt-C1 interaction on the (100) surface would require a larger
Markovic and Rosa
690 Langmuir, Vol. 9, No.2, 1993
shift in thermodynamic driving force to displace C1 from the Pt s i b , accounting for the much larger splitting in potential between the two stageson the (100) surfacethan on the (111)surface. The random structure of the adsorbed C1followingthe first stage of deposition may explain why the final stage of deposition to form the monolayer was distributed over a relatively broad potential region. This study has shown dramatic effecta of the supporting electrolyteanion on the chemistry of Cu UPD on Pt, with the effectabeing largest for C1-. It is of interest to compare the present results for Cu UPD on Pt single crystals with earlier studies by Hubbard and co-workers12on the effect of other halide anions, e.g. I- and Br,on Cu and Ag UPD on Pt(ll1). In the case of I-, the iodine was not present as an anion in the electrolyte12but was pre-adsorbed from the gas-phase prior to immersion. It is known from both ex situ LEED12and in situ STMMthat upon immersion at a potential anodic to Cu UPD the Pt(ll1) surface is covered b a compactiodineadlayer having the coincidence lattice ( 7 X d7)R19.1. In our case with trace quantities of C1- in HF, exsitu LEED showsthat at the samepotential the Pt surfacehas only a very low coverageby halide atoms, e.g.
