Early Stages of Copper Electrocrystallization: Electrochemical and in

10310

J. Phys. Chem. B 1997, 101, 10310-10322

Early Stages of Copper Electrocrystallization: Electrochemical and in Situ X-ray Absorption Fine Structure Studies of Coadsorption of Copper and Chloride at the Au(111) Electrode Surface S. Wu, Z. Shi, and J. Lipkowski* Department of Chemistry and Biochemistry, UniVersity of Guelph, Guelph, Ontario N1G 2W1, Canada

A. P. Hitchcock and T. Tyliszczak Brockhouse Institute for Materials Research, McMaster UniVersity, Hamilton, Ontario L8S 4M1, Canada ReceiVed: February 24, 1997; In Final Form: September 9, 1997X

Cu K-edge X-ray absorption spectra (XAS) recorded in situ using glancing incidence and fluorescence detection have been employed to study the codeposition of copper and chloride ions on a Au(111) surface under an electrolyte solution. Chronocoulometric measurements provided information about the composition of the overlayer and about the number of electrons flowing to the interface per adsorbed copper ion. The XAS displays a remarkable dichroism consistent with a bilayer in which the Cu adatoms are covered by chloride ions. Previous scanning tunneling microscopy (STM) measurements suggested either a (5 × 5) long-range structure similar to that of the (111) plane of a CuCl crystal or to (4 × 4)-based structures. FEFF 6.01 multiple scattering calculations of the XAS were performed on five different model structures derived from the STM proposals. A unique solution could not be obtained. The results indicate that there is a large degree of static disorder. They rule out high-symmetry structures in which there are narrow distributions of Cu-Cl, Cu-Au, and Cu-Cu distances and bond angles.

1. Introduction The electrodeposition of the first monolayer of a metal onto a foreign substrate frequently proceeds at potentials positive with respect to the equilibrium potential of the deposited metal. Such a reaction is then called underpotential deposition (UPD). The structure of the first monolayer has an impact on the deposition of the further layers and therefore on the morphology of the electrodeposited materials. For this reason, the mechanism of UPD and the structure of the underpotentially deposited monolayers are the subjects of intense investigations.1-4 The UPD of copper at gold and platinum surfaces is the example of the most exhaustive studies that employed numerous in situ and ex situ spectroscopic,5-18 diffraction,19-26 surface-imaging,27-34 radiochemical,35-37 and electrochemical38-43 techniques. These studies have shown that copper adatoms coadsorb at Au or Pt electrodes with the anions of the electrolyte and that the anions influence the amount of deposited metal atoms,3,38,41,42 the ordering of the overlayer,3,18-34,44,45 and the amount of charge transfer to the deposited copper.3,14-17,24,25,42,43 Although we have learned a lot about the copper UPD, many important questions about this reaction remain unanswered. These include the following: (i) what is the composition and structure of the mixed overlayers formed by copper and coadsorbed anions, (ii) what is the amount of charge flowing to the interface per one adsorbed copper ion, and (iii) what is the oxidation state of the adsorbed copper? This point is well illustrated by the example of copper UPD in the presence of chloride, a system recently investigated by scanning tunneling microscopy (STM) in many laboratories.27,31-33 Although all these works consistently reported that coadsorbed copper and chloride form a well-ordered overlayer with a characteristic (5 × 5) long-range structure, interpretation of this result in terms of specific packing of copper atoms and chloride ions within a X

Abstract published in AdVance ACS Abstracts, November 15, 1997.

S1089-5647(97)00664-0 CCC: $14.00

unit cell of this structure was quite contradictory. Because the composition of the mixed overlayer was not determined, it was not known whether the mixed overlayer is a monolayer or bilayer. A bilayer structure was assumed; however, in principle, it was not proved. The nature of the imaged species was also a point of controversy. Are anions “invisible” to the STM tip and the images show adsorbed copper adatoms, as suggested in ref 33, or is the (5 × 5) structure an image of chloride ions adsorbed on top of the monolayer of copper, as postulated in refs 27c and 31? Even the details of the latter case are controversial. Some workers suggest that the copper adatoms form a full monolayer in registry with the underlying gold surface and that the imaged chlorides form a hexagonal closepacked layer that is incommensurate with respect to the monolayer of copper.31a Others propose that the bilayer is made of a 1:1 ratio of copper and chloride which closely resembles the (111) plane of CuCl crystal.27c In this work we employ the chronocoulometric technique and the thermodynamics of the so-called perfectly polarized electrode to determine the composition of the mixed overlayer formed by copper coadsorbed with chloride ions. We also use polarization resolved X-ray absorption spectroscopy (XAS) to investigate the local structure of the deposited copper and coadsorbed chlorides. Preliminary results of this work were reported at a recent conference.46 2. Experimental Section 2.1. Solutions, Reagents, and Electrodes. The supporting electrolyte was 0.1 M KClO4 + 10-3 M HClO4. Perchloric acid (Aldrich Chemical Co., redistilled, 99,999%) and potassium chloride (Alfa/Johnson Matthey, Ltd., 99.999%) were used without further purification. Potassium perchlorate was purified as described previously.46 Cupric perchlorate was made by dissolving cupric oxide (Aldrich Chemicals, 99.999%) in perchloric acid. The X-ray absorption experiments were © 1997 American Chemical Society

Early Stages of Copper Electrocrystallization

J. Phys. Chem. B, Vol. 101, No. 49, 1997 10311

Figure 1. (upper) Schematic diagram of the X-ray absorption cell. (lower) Measurement geometries for E| (E parallel to surface) and E⊥ (E perpendicular to surface).

Figure 2. Cyclic voltammogram recorded at an Au(111) electrode in 0.1 MKClO4 + 10-3 M HClO4 + 10-3 M KCl + 5 × 10-5 M Cu(ClO4)2. Insert: CV recorded in 0.1 M KClO4 + 10-3 M HClO4 + 10-3 M KCl + 10-3 MCu(ClO4)2 solution. The sweep rate was 2 mV s-1.

performed using 0.1 M KClO4 + 10-3 M HClO4 + 10-3 M KCl + 5 × 10-5 M Cu(ClO4)2 solution. The adsorption of copper was investigated employing a series of 0.1 M KClO4 + 10-3 M HClO4 + 10-3 M KCl + x M Cu(ClO4)2 solutions, and the adsorption of Cl- was studied using a series of 0.1 M KClO4 + 10-3 M HClO4 + 5 × 10-5 M Cu(ClO4)2 + x M KCl solutions, where x varied between 5 × 10-6 and 5 × 10-3 in the two series. The solutions were purged with argon to remove oxygen before each experiment. Gold single crystals were grown using the Bridgeman technique. The crystals were cut and polished using standard metallographic procedures. A rod-shaped crystal (5 mm diameter) was used in electrochemical studies. A gold disk (15 mm diameter) produced by cutting a bead-shaped single crystal was employed in X-ray absorption measurements. All measurements were conducted at room temperature (20 ( 2) °C. 2.2. Electrochemical Measurements. Details of electrochemical measurements performed to determine surface concentrations of Cu and Cl- were described in the preceding papers.38,39,41,42,47 A standard electrochemical cell equipped with the working Au(111) electrode, gold coil counter electrode, and a reference electrode was used in these measurements. Before each experiment, both the working electrode and the counter electrode were cleaned by flame annealing. The reference electrode was a saturated calomel electrode (SCE) connected to the supporting electrolyte through a salt bridge. The electrochemical experiments were performed using a PAR model 173 potentiostat controlled by a computer. All data were acquired via a plug-in acquisition board (RC Electronics Model IS-16). Custom software was used to record cyclic voltammograms (CVs) and to perform the chronocoulometric experiments. 2.3. X-ray Absorption Measurements and Data Analysis Procedures. The X-ray absorption measurements were conducted in an all glass, syringe-type cell (a modified IR cell), shown in Figure 1, which was equipped with an X-ray window (polypropylene: 6.3 µm thick prior to stretching, Chemplex Industries, Inc.). The Au(111) single crystal was mounted on the syringe plunger. The crystal was flame annealed and aligned using a He-Ne laser so that its surface was normal to the plunger axis. Before X-ray absorption measurements, the electrode was moved away from the polypropylene window and the crystal surface was cleaned electrochemically by applying slow, repetitive oxidation/reduction cycles until the shape of CV recorded in the X-ray absorption cell matched those shown in Figure 2. Next, the electrode was held at a constant electrode potential, corresponding to the copper UPD region, for a period

of 15 min, until an equilibrium coverage of Cu at the Au(111) surface was established. The plunger was moved periodically back and forth during this period to enhance the mass transport of Cu2+ from the bulk to the electrode surface. The electrode was then pressed against the stretched polypropylene membrane. Most of the overburdened electrolyte was pushed out of the gap between the membrane and the surface through a small notch in the edge of the crystal which allowed the electrolyte to escape. The relative thickness of the electrolyte film in different preparations was estimated to be 20-30 µm from the relative intensity of the copper and gold fluorescence signal. The exterior of the membrane and the cell was surrounded by a glass jacket with a polypropylene window purged with flowing Ar in order to prevent oxygen migration into the solution through the polypropylene window. The X-ray absorption measurements were conducted at the C-2 station of the Cornell High Energy Synchrotron Source (CHESS). The intensity of the incident beam was monitored with an ionization chamber. The Cu KR fluorescence was measured using an energy dispersive EG&G ORTEC solid-state detector (intrinsic Ge, 6 mm diameter of the active area) placed at 90° to the X-ray beam direction in horizontal plane at a glancing exit angle from the sample. Its exact position was adjusted to provide the best signal-to-background ratio (S:B). Soller slits and, in some cases, a nickel filter were used to reduce the background due to Compton and elastic scattering. Polarization dependent measurements were made by rotating the sample surface about the incident X-ray direction while maintaining the surface at a grazing incidence to the incident radiation (see Figure 1). Small adjustments in the incidence angle were made to optimize the S:B. The incidence angle was 1.5-2° for the E| geometry (E in the plane of the sample) and 6-10° for the E⊥ geometry (E along the surface normal). The X-ray beam width was 8 mm for E| and 1 mm for E⊥. Individual spectra (8800 to 9500 eV) were recorded in about 40 min at a counting rate of about 1000 counts s-1 with 400 eV detector resolution. After 2-3 h of acquisition, the sample was retracted and the surface conditions reestablished electrochemically. Each of the x-ray absorption fine structure (XAFS) spectra reported is the sum of about 16 individual spectra (about 11 h total acquisition). The major inflection point in the edge jump was taken to be the position of the edge. The edge energy was calibrated with respect to the Cu K-edge energy measured simultaneously from a very thin film of copper evaporated onto a Mylar foil and placed between two ionization chambers

10312 J. Phys. Chem. B, Vol. 101, No. 49, 1997

Wu et al.

(estimated Cu thickness of 10 nm, 1% absorption). The spectra of a number of reference compounds (Cu foil, Cu2O, CuSO4, and CuCl) were measured in transmission. A reference spectrum for CuAu3 alloy was kindly provided by Dr. Owen Melroy from Almaden Research Center, IBM, San Jose, California. Initial analysis of the data was carried out using standard Fourier filter XAFS analysis (using BAN and MFIT (Tolmar Instruments)). Typical analysis involved summing a number of individually calibrated spectra recorded for a given potential and polarization. After linear background subtraction, the ordinate of the summed spectra was transferred to k-space with an origin chosen to be the same as that for metallic copper. Next a two-section cubic spline function was fit to the k1-weighted data for the final background subtraction. The isolated XAFS signal was then normalized to the magnitude of the edge jump. The XAFS data (2 to 12 C1-) was then Fourier transformed and the first “shell” (1.0-3.4 Å) isolated by a Hanning apodization window and reverse transformed. Three components (Cu-Cu, Cu-Au, and Cu-Cl) were fit simultaneously to these Fourier-filtered first-shell files and to amplitude and phase functions calculated from FEFF 5.03. As noted below, the results of this analysis were considered unsatisfactory. Further analysis was carried out using k-space comparison of the Fourier filtered experimental XAFS spectra with that calculated for a number of different model structures using FEFF 6.01, a multiple scattering code for predicting polarization resolved XAFS spectra which was developed by Rehr et al.48,49 3. Results and Discussion 3.1. Electrochemical Measurements. 3.1.1. Composition of the Interface. The electrochemical measurements were performed to characterize the properties of the Au(111) electrode surface with the help of cyclic voltammetry and to determine the composition of the interface. The solid line in Figure 2 shows the CV determined for the Au(111) electrode in 0.1 M KClO4 + 1 mM HClO4 + 1 mM KCl + 5 × 10-5 M Cu(ClO4)2 solution. In this solution, potassium perchlorate is the supporting electrolyte, acid is added to prevent Cu2+ hydrolysis, and chloride and cupric ions are the surface active species. The dotted line shows a CV recorded in the solution containing chloride ions, but was copper free. The curves denoted by solid and dotted lines merge at potentials higher than 0.4 V (SCE) and diverge at lower polarizations. Copper is therefore totally desorbed from the electrode surface at E > 0.4 V (SCE). The potential range -0.05 V (SCE) < E < 0.4 V (SCE) may be identified as the Cu UPD region (copper adsorption region). In solutions of such a low copper concentration the CV is to a large extent controlled by the diffusion of Cu2+ ions from the bulk to the electrode surface. The inset to Figure 2 shows the CV recorded in a 1 mM Cu2+ solution containing a copper concentration sufficiently high to eliminate the mass transport limitations. The curve displays two reversible peaks indicating that copper adsorption in the presence of chlorides has a twostate character. The shape of that CV (1 mM Cu2+ ) is distinctly different than the shape of CVs reported previously for Cu UPD in the presence of SO42- and Br-.42 This behavior indicates that Cu coadsorbs with Cl- at the Au(111) electrode surface. The surface concentrations of Cu and Cl- can be determined from the thermodynamics of the perfectly polarized electrode. The electrocapillary equation for the Au(111) electrode in equilibrium with the electrolyte containing copper and chloride ions may be written as

-dγ ) Q dE + ΓCl dµKCl + ΓCu dµCu(ClO4)2

(1)

where γ is the interfacial tension, Q is the measured charge

Figure 3. (a) Total charge density-electrode potential curves and (b) electrocapillary curves for Au(111) electrode in: dotted line 0.1 M KClO4 + 10-3 M HClO4 + 10-3 M KCl solution; solid lines 0.1 M KClO4 + 10-3 M HClO4 + 10-3 M KCl solution with the following concentration of Cu(ClO4)2 in mol dm-3; (O) 5 × 10-6, (b) 10-5, (0) 2.5 × 10-5, (9) 5 × 10-5, (4) 10-4, (2) 2.5 × 10-4, (]) 5 × 10-4, ([) 10-3, (3) 2.5 × 10-3, (1) 5 × 10-3.

density, ΓCl is the Gibbs excess of adsorbed Cl-, and ΓCu is the Gibbs excess of adsorbed copper. According to eq 1, the Gibbs excesses of adsorbed copper and coadsorbed chloride can be determined by differentiation of γ with respect to µCu(ClO4)2:

(

∂γ ∂µCu(ClO4)2

ΓCu ) -

)

(2)

E,µKCl,T,p

or with respect to µKCl,

( )

ΓCl ) -

∂γ ∂µKCl

(3)

E,µCu(ClO ) ,T,p 42

respectively. The direct measurement of the interfacial tension for a solid electrode is difficult; however, we have already shown that the relative interfacial tension can be determined from the total charge density mesurements using the back integration procedure.38,39,41,42 To determine the surface concentrations of Cu and coadsorbed Cl-, two series of charge density measurements had to be carried out. The first series involved measurements of solutions of a constant KCl concentration (10-3 M) and a Cu(ClO4)2 concentration variable between 5 × 10-6 and 5 × 10-3 M. The charge densities determined for this series of measurements are shown in Figure 3a. They were acquired by holding the electrode at a potential E, where adsorption of Cland Cu2+ takes place for a period of time long enough to establish the adsorption equilibrium and then stepping to E ) 0.6 V (SCE). The current transients corresponding to the recharging of the interface were recorded and integrated to give the charge difference between potential E and E ) 0.6 V (SCE), (∆Q). The absolute charge densities Q were then calculated

Early Stages of Copper Electrocrystallization from ∆Q with the help of the value of Q at E ) 0.6 V (SCE) determined in the preceding paper.42b Next, the charge density data were integrated. The integration started at E ) 0.6 V (SCE) and ran in the negative direction. It gave a difference between the interfacial tension at a potential E and at E ) 0.6 V (SCE), ∆γ ) γ - γE)0.6V. The value of γE)0.6V(SCE) is not known; however, it is independent of Cu2+ concentration, and hence it is identical for the whole series of solutions. In principle we can set this value to be equal to zero. However, for consistency with our previous papers42 we shall report the values of the interfacial tension using convention that γ ) 0 at E ) -0.75 V. The value of γE)0.6V at this scale was determined in ref 42b. The integration gave the family of electrocapillary curves shown in Figure 3b. Next the interfacial tensions at a constant E were plotted against RT ln CCu(ClO4)2 and differentiated. Since we used supporting electrolyte in a large excess then (a) dµCu(ClO4)2 ) RT d ln CCu(ClO4)2 and (b) the activity coefficient of KCl did not change with the concentration of Cu(ClO4)2. (At constant KCl concentration, the chemical potential of KCl is constant; in fact, the equilibrium potential of a Ag/AgCl electrode does not change after addition of Cu2+ to the electrolyte if the copper concentration is smaller than 5 × 10-3 M). Under these conditions, the derivative of γ with respect to RT ln cCu(ClO4)2 gives the Gibbs excess of adsorbed copper (eq 2). The first series of measurements gives the surface concentration of copper but no information about the amount of coadsorbed chloride. To determine the surface concentration of Cl-, the charge densities for the second series of solutions were measured. This time the concentration of Cu(ClO4)2 was kept constant at 5 × 10-5 M, while the concentration of KCl varied between 5 × 10-6 M and 5 × 10-3 M. The measured quantity was again the charge difference between the potential E at which copper and chloride coadsorb and E ) 0.6 V (SCE) where copper is totally desorbed from the surface. In solutions of variable Clconcentration, the charge at E ) 0.6 V (SCE) depends on the bulk chloride concentration. The values of QE)0.6V for copper free KCl solutions of the same supporting electrolyte were determined earlier.42b These data were used to convert the measured ∆Q’s into the absolute Q values shown in Figure 4a. The charge density plots were integrated (using values of γE)0.6V taken from ref 42b as the lower integration constants) to give the family of electrocapillary curves presented in Figure 4b. Finally, the interfacial tensions at a constant E were plotted against RT ln cKCl and differentiated to give the Gibbs excess of adsorbed chloride (eq 3). This series of measurements gave surface concentrations of adsorbed chloride without knowledge about the amount of coadsorbed copper. However, for one electrolyte composition, the 0.1 M KClO4 + 10-3 M HClO4 + 10-3 M KCl + 5 × 10-5 M Cu(ClO4)2 solution, we determined the Gibbs excesses of copper in series one and the Gibbs excesses of chloride in series two. For this electrolyte we were able to determine the absolute composition of the interface over the whole range of investigated potentials. These results are shown in Figure 5. The Gibbs excesses of copper (open circles) and chloride (closed circles) are plotted against potential in Figure 5a. The dotted line in Figure 5a shows changes of ΓCl with E in the copper-free electrolyte. Starting inspection of Figure 5a from the positive limit of E and moving in the direction of negative potentials, ΓCu rises steeply until a concentration of 1.05 × 1015 atoms cm2 is reached. This corresponds to the coverage of 0.75 ML where the monolayer coverage (ML, 1.39 × 1015 atoms cm2) corresponds to the surface concentration of gold atoms at the ideal (111) surface. Moving further in the negative direction


Figure 4. (a) Total charge density versus potential curves and (b) electrocapillary curves for the Au(111) electrode. Dashed lines 0.1 M KClO4 + 10-3 M HClO4 solution; solid lines 0.1 M KClO4 + 10-3 M HClO4 + 5 × 10-5 M Cu(ClO4)2 solution with the following concentration of KCl in mol dm-3: (O) 5 × 10-5, (b) 1 × 10-4, (0) 2.5 × 10-4, (9) 5 × 10-4, (4) 10-3, (2) 2.5 × 10-3, (]) 5 × 10-3.

Figure 5. (a) Plot of the Gibbs excess of adsorbed copper (open circles) and coadsorbed Cl- (closed circles). (b) Plot of the ratio of adsorbed copper to adsorbed chloride versus the electrode potential in 0.1 M KClO4 + 10-3 M HClO4 + 10-3 M KCl + 5 × 10-5 M Cu(ClO4)2 solution. Arrows indicate potentials at which XAS were acquired.

the coverage of copper changes weakly with potential so that a quasi-plateau is seen in the ΓCu verus E plot. The Gibbs excess of copper rises again at E = 0 V (SCE) so that the full monolayer coverage of copper is reached. In general the shape

10314 J. Phys. Chem. B, Vol. 101, No. 49, 1997 of the ΓCu versus E plot is consistent with a two state adsorption of Cu proposed earlier from the analysis of the CV. We can identify state one as that corresponding to the formation of 0.75 ML coverage and state two as that corresponding to the completion of the monolayer coverage by Cu adatoms. The adsorption of copper apparently promotes a coadsorption of chloride ions. The surface concentration of Cl- rises simultaneously with increasing copper coverage until the Gibbs excess of 7 × 1014 ions cm2 (0.5 ML coverage) is reached at the copper coverage 0.75 ML. Further increase of the surface concentration of copper causes a weak decrease of ΓCl. The Gibbs excess of Cl- displays a further drop when the coverage of copper attains one monolayer. Overall, the shape of the ΓCl versus E plot is in good qualitative agreement with the curve that was determined by Horanyi et al.35 for Cl- adsorption in the presence of Cu2+ at a polycrystalline Au electrode using radiotracers. The sum of the surface concentration of copper and the surface concentration of chloride exceeds the coverage of one monolayer and the overall coverage of the surface by the two species varies between 1.25 and 1.4 ML at potentials lower than 0.2 V (SCE). This result indicates that coadsorbed copper and chloride form a bilayer. The ratio of the surface copper atoms to the surface chloride ions is shown in Figure 5b. In the bilayer region the ratio varies between 1.5 and 2. It should be mentioned that a somewhat higher surface concentration of Cl-, equal to 8 × 1014 ions cm-2,42 was determined for solutions of much higher copper concentration (10-3 M), comparable to the copper concentrations used in STM imaging experiments. The surface concentration of 8 × 1014 ions cm-2 corresponds to the coverage of 0.58 ML. The packing density of adsorbate for the (5 × 5) overlayer structure, seen by STM under similar experimental conditions, is in between 0.64 and 0.56 ML. This value is closer to the coverage of Cl- than to the coverage of Cu. Therefore, the surface concentration data suggest that the species imaged by STM is the adsorbed Clrather than the copper adatom, in agreement with.27c,31 Finally, we would like to mention that we reported somewhat higher coverages of Cl- than that shown in Figure 5b in proceedings of a recent conference.46 In fact we have run two series of measurements. After completion of the first series we discovered that, with solutions of low Cl- concentration, we did not wait long enough at the adsorption potential for the adsorption equilibrium to be established. We repeated the measurements controlling the state of the adsorption equilibrium. The data presented here are taken from the second series of measurements. Due to an oversight we included data from the wrong series in ref 46. 3.1.2. Number of Electrons per Adsorbed Copper Ion. Almost all previous estimates of the copper coverage at the Au (or Pt) electrodes were based on the integration of CVs for Cu UPD with the contribution of coadsorbed anion to the measured charge being ignored. In those cases where an independent estimate of the coverage was made using STM,3,27-30 Auger emission spectroscopy (AES),16,17,24,26 or XAS,14,15 there is a significant difference between coverages determined by the coulometry and the non-electrochemical techniques. It has therefore been suggested that copper UPD may involve a partial charge transfer14-16 and that the adsorbed Cu species has an oxidation state higher than zero. In the present case, we measure the charge and we know the surface concentrations of both Cu and Cl-. Hence, we can provide a precise estimate of the number of electrons flowing to the interface per one adsorbed Cu ion. The measured charge density is a function of three variables E, ΓCl, and ΓCu, hence its total differential is given by:

Wu et al.

dQ )

( ) ∂Q ∂E

dE +

ΓClΓCu

( ) ∂Q ∂ΓCl

E,ΓCu

dΓCl +

( ) ∂Q ∂ΓCu

E,ΓCl

dΓCu (4)

where (∂Q/∂E)ΓClΓCu ) C∞ is the infinite frequency capacity, (1/ F)(∂Q/∂ΓC∞)E,ΓC ) nCl is the number of electrons flowing to ∞ the interface per one adsorbed Cl- ion and - 1/F(∂Q/∂ΓCu)E,ΓCl ) nCu is the number of electrons flowing to the interface per one adsorbed copper ion. In principle the numbers nCl and nCu can be determined exactly, provided one has a sufficiently large set of experimental data. Acquisition of that amount of data would require about a year of intense experiments, and the cost is for the moment prohibitive. However, we can give quite a good estimate of nCu using reliable approximations. We assume that (i) nCl is independent of ΓCl and is equal to one, (ii) nCu is independent of ΓCl and ΓCu. In that case, integration of eq 4 gives the following expression for the charge:

Q ) σM + FΓCl - nCuFΓCu

(5)

where σM is the charge-corresponding to the charging of the infinite frequency capacitor, FΓCl ) QCl is the charge resulting from the adsorption of chloride ion, and -nFΓCu ) QCu is the charge resulting from the adsorption of copper. The value QCl may be calculated using the measured Gibbs excess for Cl-. The value of σM cannot be determined; however, it should have the same order of magnitude as the charge density measured in a solution free from copper and Cl- ions. The absolute value of σM should be less than 10% of the absolute value of QCu and hence, with an accuracy of 10%, we can use the value Q′Cu ) Q - FΓCl ) QCu + σM as a measure of the charge resulting from adsorbed copper. Figure 6a shows the dependence of QCl, Q′Cu, and Q on the electrode potential for 0.1 M KClO4 + 1 mM HClO4 + 1 mM KCl + 5 × 10-5 M Cu(ClO4)2 solution. The charges due to adsorption of chloride and copper have opposite signs and their contributions to the total charge cancel each other. For this reason, the absolute values of the charge due to adsorbed copper |Q′Cu| are higher than the absolute values of the measured charge. This fact has significant implications for the estimate of the copper coverage. If the charge Q is taken as an estimate of the copper coverage and the contribution from the coadsorbed anion is ignored, the estimate gives too low a result. Therefore most of the published estimates for Cu coverage at gold (or Pt) electrode based on integration of CVs are too low, with the error being larger the stronger the extent of the anion coadsorption. The ratio of |Q′Cu| to FΓCu gives the number of electrons flowing to the interface per one adsorbed copper ion. The open circles in Figure 6b show the dependence of nCu on the electrode potential. The number nCu is essentially equal to 2 in the whole range of E corresponding to Cu UPD. The oscillations on the nCu plot seen at E > 0.2V (SCE) reflect higher experimental errors in the regions where copper coverage changes rapidly or is very low. This result indicates that, in contrast to what was suggested in the literature, the adsorption of Cu does not involve a partial charge transfer. We emphasize, however, that the fact that two electrons are flowing to the interface per one adsorbed copper ion does not necessarily mean that adsorbed copper is at the zero oxidation state. The nCu was determined using Gibbs thermodynamics and, therefore, the Gibbs model of the interface. This model assumes that the interface is a two-dimensional plane located somewhere between two neighboring phases. The location of the interface is not exactly known, and hence, we do not know the location of the two electrons. Therefore the electrochemical experiments cannot distinguish between two



Figure 6. (a) Plot of the total charge density [Q] and the charge densities corresponding to the adsorption of Cu (Q′Cu(a)) and Cl- (QCl-) and (b) plot of the number of electrons flowing to the interface per one adsorbed Cu atom versus electrode potential for 0.1 M KClO4 + 10-3 M HClO4 + 10-3 M KCl + 5 × 10-5 M Cu(ClO4)2 solution

limiting cases: (i) adsorption of a Cu2+ (or Cu1+) ion with the two (one) electrons forming an image on the metal side of the interface, (ii) adsorption with charge transfer leading to the formation of a neutral copper adatom (zero oxidation state). Below we try to determine complementary information about the polarity of the chemisorption bond and the oxidation state of the Cu from the X-ray absorption results. 3.2. X-Ray Absorption Measurements. 3.2.1. OVerView of Structural Questions. The electrochemical studies indicate that the coadsorbed copper and chloride form a bilayer. Independent diffraction26 and STM imaging27,28,31-33 experiments indicate that the bilayer is well ordered and has a longrange structure, which has been described as having a (5 × 5) periodicity over a broad range of electrode potentials. STM images recorded at 50-250 mV/SCE, between the two peaks in the CV (see inset to Figure 2), show very good atomic resolution. Images taken at potentials more negative than the second (small) peak in the CV display a loss of atomic resolution and a significant roughening of the surface. The (5 × 5) is a nominal notation for the bilayer structure. Careful analysis of STM images by Hotlos et al.27c revealed that the long-range modulation seen by STM is not exactly a small integer multiple of the Au lattice constant and that, on average, the periodicity of the surface corresponds to about 4.4 Au lattice spacing. This result suggests that the real structure is in fact something intermediate between a (4 × 4), which gives a packing density of 0.56 ML, and a (5 × 5) geometry with a packing density of 0.64 ML. These values are close to the packing density of chloride ions determined in this work. It suggests that the bilayer consists of a top layer of chloride ions and a middle layer of copper adatoms. Given that several untested assumptions were made in our analysis of the coulometric data, it is desirable to use X-ray absorption as an independent measurement to confirm the result. Also X-ray absorption can potentially determine how the copper atoms and coadsorbed

Figure 7. Models of the bilayer explored in this work. Models A and B are consistent with a (5 × 5), and models C, D, and E are consistent with a (4 × 4) overlayer structure. (white ball ) Au, black ball ) Cu, gray ball ) Cl-. The true atomic dimensions are not preserved in these models). In model A copper is adsorbed in registry with the Au(111) substrate; in models B-E, the Cu is in registry with Cl. See Table 2 for the coverage of Cu and Cl atoms in each structure. The Cu coverage increases progressively from C to D to E. The numbered sites in model B are those for which site-specific calculated XAFS spectra are shown in Figure 14.

chloride ions are organized, information which cannot be obtained from either surface coverage data or STM images. Two models of the bilayer have been proposed in the STM literature. The first, shown in Figure 7a (model A), assumes that the bilayer consists of 1 ML of Cu atoms and a top closed packed hexagonal monolayer of Cl-, which has a coverage of 0.64 ML.26,27c,32 The second, shown in Figure 7b (model B), proposes that the bilayer resembles the (111) plane of CuCl crystal with the Cu-Cu and Cl-Cl spacings equal to 3.67 Å, which gives 0.64 ML coverage for both Cu and Cl.27c,31b Neither of the two models agrees with the composition of the

10316 J. Phys. Chem. B, Vol. 101, No. 49, 1997 bilayer determined by our chronocoulometric measurements. Figure 5 shows that, within the potential range where the (5 × 5) structure was observed by STM, the copper coverage changes between 0.75 and 1 ML, which is always smaller than that predicted by model A and always larger than that expected for model B. Apparently, a 25% change of the copper coverage has a small effect on the long-range structure of the bilayer. To account for the variable copper coverage, as well as for the fact that the actual chloride coverage is 0.5 ML rather than 0.64 ML, we have considered three additional models of the bilayer. These models, which are C, D, and E in Figure 7, will be discussed in detail later. How are the copper atoms packed within the unit cell of this bilayer? Are they adsorbed in registry with the underlying gold surface or are they in registry with the top layer of chloride ions? Why does not the long-range structure of the bilayer change with the copper coverage? We have used X-ray absorption spectroscopy to investigate these questions. We expected that differences between the Cu K-edge spectra determined using X-ray photons with E⊥ (polarized normal to the surface) and E| (polarized parallel to the surface) would provide information about the location of copper atoms in the bilayer. In particular, we anticipated that the polarization dependent XAFS would distinguish whether the copper atoms are sandwiched, as shown in Figure 7, or are adsorbed on top of a chloride layer, as suggested in ref 33. In addition we expected that XAFS would determine whether the copper is in registry with the gold layer or with the chloride layer. 3.2.2. X-ray Adsorption Near Edge Spectra (XANES). Figure 8 compares X-ray absorption spectra for various Cu coverages, determined using both E| and E⊥. The electrode potentials and estimated coverages are indicated (see also arrows in Figure 5). We note that the chloride coverage changes very little within this range, and hence, the copper coverage and the copper-tochloride ratio in the bilayer are the major compositional changes. The insert to Figure 8b shows that, for the spectra acquired using E⊥, the absorption edge amplitude changes in proportion to the copper coverage. In this series of experiments, the collection geometry could be reproduced precisely. The small spread of the experimental points seen in the inset to Figure 8 is caused chiefly by the variation of the thickness of the film of electrolyte trapped between the electrode surface and the polypropylene window. This proportionality was not observed using E|, due mainly to difficulties in reproducing the optimal collection geometry for this position of the electrode. In this case the solid angle and thus the attenuation of the emitted photons changes significantly when the detector is moved slightly with respect to the electrode surface. With regard to the spectral features, at any given coverage, there are very significant differences in both near edge and XAFS signals for the two photon polarizations. In addition, there is a small but detectable evolution of the shape of the spectrum with the copper coverage. The X-ray absorption spectra are discussed in several parts; first we discuss the near edge region which corresponds roughly to the first 8-10 eV; then we qualitatively examine the extended fine structure, followed by an analysis of the extended XANES region 1050 eV beyond the edge, using FEFF 6.01 calculations. There is a general consensus that the shape of the near edge region is best described in terms of electronic transitions to bound and quasi-bound states, while the XANES structure is best described as arising from multiple scattering of the photoelectron by neighboring atoms, in some cases out to distances of more than 5 Å.50-52 In our discussion of the near edge features we propose a qualitative interpretation, based on comparison to spectra of

Wu et al.

Figure 8. In situ Cu K-edge X-ray absorption spectra for Cu submonolayer or monolayer deposited at the Au(111) electrode surface from 0.1 M KClO4 + 10-3 M HClO4 + 10-3 M KCl + 5 × 10-5 M Cu(ClO4) solution. Each curve is marked with the potential and copper coverage for which the spectrum was measured. The potentials at which Cu was deposited at the Au(111) electrode surface are indicated by arrows in Figure 5. Part A shows spectra acquired using E|. Part B shows spectra acquired with E⊥. The spectra are plotted with a constant edge jump to facilitate spectral comparison. The insert to part B shows that there was a good relationship between the edge jump intensity and the Cu coverage as estimated from the electrochemical measurements.

model systems. After our qualitative discussion of the extended fine structure (section 3.2.2), we report in section 3.2.3 our attempts to find a quantitative interpretation of both XANES and extended X-ray absorption fine structure (EXAFS) by comparison to spectra calculated for a number of trial geometries using the FEFF 6.01 polarization dependent multiple scattering theory of Rehr et al.48,49 In a few recent papers, the near edge spectra of Cu electrodeposited on gold8,10 or Pt14 electrode surfaces were compared to the spectra of model copper compounds and the similarity between the edge position and shape for the electrodeposited copper and a model compound was used to estimate the oxidation state of copper adatoms. Therefore, we first review the Cu K-edge features for selected copper(0) and copper(I) compounds. Figure 9 shows normal (part A) and differential (part B) Cu K near edge spectra for these models. The spectra for the model compounds agree well with the spectra published in the literature (Cu ,53,54 Cu2O,55 and CuCl56). There is general agreement that the edge signal for metallic copper arises from 4p r 1s dipole-coupled transitions.51-53 The intensity of this transition is described by

I(E) ∝ |〈Ψf|e‚r|Ψi〉|2N(E) ∝ cos2 θ |〈Ψf|r|Ψi〉|2N(E)

(6)

where Ψf and Ψi are the final and initial state wave functions, e is the vector which represents the direction of the polarized radiation, r is the transition dipole operator, Θ is the angle


Figure 9. Cu K-edge X-ray absorption spectra of selected model compounds; part A as-recorded; part B differential spectra. The zero of the energy scale in part B is set at the inflection point of the edge in Cu metal. These spectra were measured in transmission.

between e and r, and N(E) is the projected density of states. The shape of the copper edge is essentially determined by the empty 4p band structure, that is, by the term N(E) in eq 6.53 There is compelling evidence that the edge features for Cu (I) compounds can also be assigned to the 4p r 1s transition.54,57-59 In the case of chlorides and oxides the 4p band of copper has a significant admixture of 3p and 2p states of the chlorine and the oxygen atoms, respectively.53,60 Both the oxidation state of copper and the coordination geometry change in these compounds. Therefore, the edge shape and the edge position are different for metallic copper and copper(I) compounds. These differences are displayed particularly well by the differential spectra shown in Figure 9b. The maxima of these curves correspond to the inflection points of the edge structures. The energy of the major inflection point is usually taken as a measure of the edge position and hence the edge shift may easily be determined from the relative position of the first peak in the dµ(E)/dE curves. The edges for Cu(I) compounds are shifted toward higher energies; this shift may partially be explained by the dependence of the Cu 1s energy level on the oxidation state. However, we also note that differences between the edge position and shape for CuCl and Cu2O (both Cu(I)) are larger than those between the Cu K-edges of metallic copper (Cu(0)) and Cu2O (Cu(I). This indicates that changes in the inorganic ligand and the coordination geometry of copper may have a much more significant impact on the Cu K-edge shape and position than a change of the oxidation state of copper. Consequently, in the present case, the use of Cu K-edge features of model compounds to estimate the oxidation state of electrodeposited copper adatoms must be viewed with some skepticism. That procedure may lead to erroneous identification of the oxidation state unless it can be determined that the coordination geometry and the nature of nearest neighbors for the copper atom in the model and in the adsorbed state are similar. Since we cannot find such a model (see below), we

J. Phys. Chem. B, Vol. 101, No. 49, 1997 10317 conclude we cannot unambiguously determine the oxidation state of Cu coadsorbed with Cl- at the Au(111) electrode surface. The near edge spectra for Cu deposited at the Au(111) electrode surface are shown in Figures 10a (normal spectra) and 10b (differential spectra). E| spectra are presented as solid lines while E⊥ spectra are denoted by dotted lines. The near edge spectra for metallic Cu and CuCl are also included in Figures 10a and 10b. Remarkable differences are observed between the shape and position of the spectra determined for the two photon polarizations. This dichroism suggests that the Cu 4p band is different in the direction parallel and normal with respect to the electrode surface. Similar dichroism was observed in polarization resolved NEXAFS (NEXAFS: near extended X-ray absorption fine structure) studies of single crystal complexes of Cu(I) and Cu(II) with mixed ligands, when only ligands of one kind were distributed along certain crystallographic directions.55,58 The dichroism observed in the present case indicates that the structure of the mixed copper-chloride bilayer is anisotropic. At all coverages, when the spectra for electrodeposited copper are compared to the spectra of metallic copper and copper chloride, the spectra for E⊥ appear more copper chloride-like, whereas the spectra for E| are more similar to the spectrum of metallic copper. One may also note that the edge features for the spectra acquired with E⊥ change little with increasing copper coverage. In contrast the edges determined with E| change in shape and the threshold energy (as measured by the first peak in the differential spectrum) shifts toward lower energy with increasing copper coverage. Significantly, the edge shape and position become more similar to those of metallic copper when the copper coverage becomes equal to a full monolayer. However, when the copper coverage is as low as 0.4 ML, the edge shape and position has some similarity to those of CuCl. This behavior is consistent with the laminar structure of the bilayer represented by the hard ball models in Figure 7. 3.2.3 Extended X-ray Absorption Fine Structure. Figure 11 plots the k1-weighted, normalized XAFS for all copper coverages and the two photon polarizations investigated. Overall, the amplitude and frequency of the oscillations change quite profoundly with the photon polarization but vary weakly as a function of the copper coverage. These changes can be seen better in the Fourier transforms of the χ(k) spectra, which are shown in Figure 12a (E|) and Figure 12b (E⊥). There are significant differences between the RDFs determined for the two photon polarizations. The peaks in the RDF measured using E| are less intense than the peaks in the radial distribution functions (RDFs) measured using E⊥. To facilitate further discussion, the RDFs for 0.4 ML (220 mV) and 1.0 ML (0 mV) Cu coverage and E⊥ polarization are compared to those determined for selected model compounds in Figure 13. The RDFs for sub-monolayer and monolayer amounts of Cu deposited at the Au(111) electrode surface display multiple overlapping peaks between 0.5 and 3.5 Å corresponding to the first coordination shell. The positions of these peaks correspond well to the positions of the peaks seen in the RDFs derived from the XAFS of model compounds such as Cu, Cu2O, CuCl, and CuAu3. This behavior indicates that up to four elements (Cu, O, Cl, and Au) may be present in the first coordination shell of the copper adatoms and that their peaks in the RDFs are more or less overlapping in the 0.8 to 2.6 Å region. At the highest copper coverage at E ) 0 mV, the E| data is consistent not only with relatively weak copper-chlorine contributions, but also with quite strong copper-copper and copper-gold contributions. For E|, the contribution from copper-chloride


Figure 10. In situ Cu K-near edge X-ray absorption spectra for Cu sub-monolayer or monolayer deposited at the Au(111) electrode surface from 0.1 M KClO4 + 10-3 M HClO4 + 10-3 M KCl + 5 × 10-4 M Cu(ClO4) solution. Part A shows as-recorded spectra while Part B plots the derivative. The dotted lines show spectra acquired using E⊥; the solid line shows spectra acquired with E|. The K-edge spectra for Cu and CuCl are included for comparison. The thick solid line shows the spectra for Cu and the dash-dots line the spectra for CuCl. Each curve is marked with the potential (part A) or the corresponding copper coverage (part B) for which the spectrum was measured.

contacts becomes significant when the copper coverage drops to 0.7 ML and the structure of the bilayer become more open. The strength of the copper-chloride interaction then becomes comparable to that measured using E⊥. In contrast, the RDFs determined using E⊥ are dominated by the copper-chloride interaction over the whole range of potentials. The coppergold bonds are also visible for all coverages. In addition, copper-oxygen signal is seen and becomes particularly strong at lower Cu coverages. The polarization dependence of the RDFs confirms the laminar structure of the bilayer and in addition clearly indicates that copper adatoms are sandwiched between the gold surface and that the chloride layer rather than being adsorbed on the top of a monolayer of Cl-. We note that RDFs for the two photon polarizations display an increasing metallic character (increasing contribution from Cu-Au and Cu-Cu interaction when moving from E ) 220 mV to E ) 0 mV. This trend is consistent with the change of the copper coverage determined from electrochemical measurements. It has been reported in STM studies of Cu UPD in the presence of chlorides that the deposited Cu may form clusters at E ) 0 mV (SCE).27c We do not see a dramatic change in the shape of the RDFs and near edge portions of XAS which would indicate massive cluster formation at this potential. However, we cannot rule out the possibility that the more metallic character of the spectra at E ) 0 mV may partially be due to the formation of small quantities of clusters. We would like to emphasize that the STM experiments27c were performed in solutions with about 2 orders of magnitude higher Cu2+ concentration than that employed for our XAS studies. A

Wu et al.

Figure 11. kχ(k) versus k plots of the in situ XAFS determined from the spectra shown in Figure 8. Part A shows spectra acquired using E|. Part B shows spectra acquired with E⊥.

Figure 12. Magnitude of the Fourier transforms of the XAFS plotted in Figure 11. Part A shows spectra acquired using E|. Part B shows spectra acquired with E⊥.

smaller tendency toward cluster formation is therefore expected under the experimental conditions used in our work. 3.2.4. Multiple Scattering XAFS Calculations. The identification of the positions of the copper atoms trapped between the gold and chloride layers is a considerable challenge,



Figure 13. Fourier transforms of the E⊥ XAFS for the lowest and the highest copper coverages compared to those for Cu model compounds. The dashed lines are meant to guide the eye and are not meant to imply a 1:1 correspondence between peaks in the Cu/Au(111) RDFs and those of the model compounds.

TABLE 1: Structural Parameters (R (Å), N) Derived from a Three-Component, Single-Scattering Analysis of the Fourier-Filtered First-Shell Signala,b Cu coverage

polarization

RCu-Cl

RCu-Au

RCu-Cu

NCl

NAu

NCu

0 mV 1.0 ML 100 mV 0.8 ML 200 mV 0.7 ML 220 mV 0.4 ML

E| E⊥ E| E⊥ E| E⊥ E| E⊥

2.41 2.41 2.43 2.43 2.43 2.43 2.41 2.41

2.63 2.63 2.69 2.69 2.66 2.66 2.67 2.67

2.59 2.59 2.59 2.59 2.69 2.69 2.59 2.59

1.5 1.7 1.3 1.7 1.7 1.8 1.9 1.5

4.8 2.5 2.6 3.9 3.0 3.5 3.3 3.7

1.8 0.2 1.4 1.7 2.4 0.9 1.3 0.4

a The Debye-Waller factors (σ2) were 0.005-0.02 Å2. b See text for discussion of why we do not believe these results adequately represent the averaged local structure.

particularly given the limited precision of the data and the large extent of static disorder. Initially, we attempted to determine structural information (average first shell distances and coordination numbers) without constructing specific structural models. Since the peaks in the RDFs overlap strongly, we had to perform a multiple backscatterer analysis. Using conventional Fourier-filtering techniques and single-scattering FEFF model amplitudes and phases, the analysis gave absorber-scatterer bond lengths which were independent of potential and photon polarization within statistical precision (see Table 1). The average first shell distances of 2.33(3) Å, 2.60(2) Å, and 2.58(2) Å for the Cu-Cl, Cu-Au, and Cu-Cu respectively are quite reasonable. The presence of significant Cu-Au signal is direct evidence that the Cu atoms are sandwiched between the Au substrate and a chloride outer layer, not in the outermost layer, as had been suggested earlier.33 While the qualitative result of this simplistic analysis is reasonable, it is clearly quantitatively inadequate. In particular, the sum of the coordination numbers

was consistently too small. The number of nearest neighbors varied between 0.7 and 1.1 for Cu-Cl and between 3 and 5 for Cu-Au, and it was essentially zero for Cu-Cu, irrespective of the photon polarization. The sum of nearest neighbor numbers is far too low since one would expect a total coordination of 10-12 around Cu. We interpret the low first-shell coordination number to indicate there is very large static disorder such that the signal from slightly different local geometries undergoes destructive interference thereby reducing the observed amplitude. This strongly suggests that the Cu layer is incommensurate and that copper atoms occupy adsorption sites of nonequivalent coordination geometry. To obtain further structural insight, we have performed theoretical XAFS calculations for different models and compared the results to the experimental spectra. We have used FEFF 6.0148,49 multiple scattering calculations of both the near edge (XANES) and extended (XAFS) fine structure in our analysis. FEFF 6.01 calculates the polarization-resolved X-ray absorption spectrum of an atom in a user-defined environment. We have represented our system as a cluster consisting of a top hexagonal layer of 33-55 chloride ions, a middle layer with a variable number of copper atoms, and two to five hexagonal layers of gold with a total of 80-150 gold atoms. The total number of atoms in any cluster was limited to less than 250. We considered the five models shown in Figure 7. The first two are based on structures proposed in the STM literature. The top Cl- layer was assumed to form an incommensurate (5 × 5) structure with respect to the gold surface with a Cl--packing density of equal to 0.64 ML. In model A, we assume that the 1 ML of copper atoms in the middle layer are adsorbed in registry with the bottom gold layer and out of registry with the top Cl-layer. In model B, we place 0.64 ML of Cu atoms in registry with the top chloride layer and out of registry with the bottom gold layer. The copper-to-chloride ratio in model B is one. We performed FEFF 6.01 calculations for 2 variations of model B. In one approach, the Cu and Cl atoms were placed in high-symmetry sites. In the other approach (model B′), the positions of the copper and chlorine atoms were shifted slightly from their initial higher symmetry positions to remove short contacts and generate a more evenly spaced local environment for Cu in individual sites. Each site is slightly different because the Cu layer is incommensurate with the gold substrate. In the next three models C-E, we assume that the top layer of Cl- forms a (4 × 4) structure, incommensurate with respect to the gold surface. It has a chloride coverage of 0.56 ML, close to the value of 0.50-0.57, determined for potentials of 0-180 mV (SCE). The copper concentration varies in models C-E, in part as an attempt to simulate the copper coverage dependence of XAFS. In model C we incorporate 0.56 ML of Cu atoms into the middle layer and assume the Cu atoms are adsorbed in registry with the top layer of Cl- and out of registry with the gold surface. Effectively, the Cu atoms are placed in hollow sites of the Cl- layer and the angle between the surface normal and the Cu-Cl bond is roughly 45°. For this structure, chlorine should be seen as a first neighbor of Cu in both photon polarizations. The copper-to-chloride ratio in this model is close to the Cu:Cl ratio observed for potentials between 220 and 200 mV. Models D and E are modifications of model C which incorporate an additional 1.0 (model D) or 1.5 rows of copper atoms (model E) per one unit cell of the (4 × 4) structure. The additional copper atoms are placed in bridge sites of the top Cl- layer. The composition of model C is close to the measured composition of the bilayer at E ) 220 mV while the composition for model E is close to that measured at E ) 100 mV. The


Wu et al.

TABLE 2: Coverage, Symmetries, and Average Local Geometry of the Five Structural Models for Which FEFF 6.01 Calculations Were Performed interatom distance (Å) model

Cl:Au registry

Cu:Au registry

A Ba C D E

5×5 5×5 4×4 4×4 4×4

1×1

Cl:Cu registry

Cu:Au ratio

Cl:Au ratio

Cu:Cl

Cu:Cu

Cu:Au

1×1 1×1 4 /3 × 1 9/ × 1 6

1.00 0.64 0.56 0.75 0.84

0.64 0.64 0.56 0.56 0.56

2.5-3.2 2.5-2.8 2.6-2.8 2.6-2.8 2.44-2.8

2.88 2.55-2.85 3.85 2.53-2.94 2.50-2.82

2.72 2.7-3.0 2.72-3.2 2.72-3.2 2.72-3.2

a In contrast to the treatment of the other four models, in-plane reorganization was carried out on one version of model B (labeled B′) in order to give each Cu atom a more regular local environment.

Figure 14. FEFF 6.01 calculated Cu-K spectra for the six nonequivalent copper sites of model B′, along with weighted sum of all 16 Cu sites in unit cell. The number indicates the position in the unit cell of model B′ shown in Figure 14. Part A shows spectra calculated for E|. Part B shows spectra calculated for E⊥

Cu:Au and Cu:Cl ratios and the ranges of distances between the copper-gold and copper-chloride layers in each of these models are given in Table 2. In generating the cluster structure used as FEFF 6.01 input, the Cu and Cl atoms in models C-E were positioned in close-packed contact with the Au substrate by moving atoms perpendicular to the surface to fill any empty space under that atom. In contrast to model B′, the Cu atom positions in models C - E were not rearranged in the Cu plane (i.e., vertical but no lateral relaxation). In constructing model D we applied a simple bonding energy minimization routine to obtain further relaxed positions of the Cu and Cl atoms. This minimization assumed harmonic diatomic potentials with force constants proportional to the heats of formation of each pair of atoms. The unit cell of each of these models contains many copper atoms with nonequivalent coordination geometries. FEFF calculations were performed for each unique copper atom site and summed over all of these sites. As an example, Figure 14 shows X-ray absorption spectra calculated with E| and E⊥ for six nonequivalent copper atoms in a unit cell of model B′, as well as the weighted sum of 16 spectra for all Cu atoms in the unit cell (see Figure 7b for identification of the individual sites in model B′). There are significant differences between the

Figure 15. FEFF 6.01 calculated Cu-K near edge spectra for the models shown in Figure 7 in comparison with experimental spectra recorded at a potential 100 mV versus SCE (0.8 ML coverage of Cu adatoms). The labels indicate the structural model or experimental data (expt.). Part A shows spectra for E|. Part B shows spectra for E⊥.

shape of the spectra calculated for Cu atoms residing in different coordination sites. Comparing the average spectrum to the spectra for individual Cu atoms, we note that the average spectrum has a different shape, it is less structured and the amplitude of oscillations is smaller. Very clearly, in cases where there is large static disorder, first-shell EXAFS may not be interpreted in terms of single values of the absorber-scatterer bond lengths and single values of the coordination numbers. This explains why our attempts to determine bond lengths and number of nearest neighbors using standard XAFS analysis did not succeed. This also indicates that peaks in the Fourier transforms (Figure 12) cannot be interpreted in a simple manner. The Fourier transform of the experimental EXAFS for an absorber in an incommensurate overlayer results from interference between signals for multiple neighbor backscattering for a number of distinct copper atom sites each with a different coordination geometry. This leads to significant mixing of the peaks corresponding to different scatterers with the result that a peak in such an RDF may no longer be assigned to a single scatterer. The results of the FEFF calculations for models A-E are compared to the experimental XANES in Figure 15 and to the k-space XAFS in Figure 16. In these figures, parts a and b show spectra for E| and E⊥, respectively. For each model the calculated spectrum is an average over many copper atoms of nonequivalent coordination geometry. Because of the rather weak dependence on copper coverage only one experimental


Figure 16. FEFF 6.01 calculated Cu-K EXAFS for all of the models shown in Figure 7, compared with that derived from the experimental spectra recorded at a potential of 100 mV/SCE (0.8 ML coverage of Cu adatoms). The labels indicate the structural model or expt. Part A shows spectra acquired using E|. Part B shows spectra acquired with E⊥.

spectrum, which is corresponding to the 0.8 ML copper coverage (100 mV/SCE) and a Cu:Cl ratio of 1.5, is included in Figures 15 and 16. The agreement between the experimental spectrum and the spectra calculated for the models varies considerably from model to model. Clearly, A and B, the two (5 × 5) models of the bilayer proposed in the STM literature, are not supported by the XAFS data. Models C-E, based on the (4 × 4) structure, reproduce some qualitative features of the experimental spectrum and correctly predict a weak copper coverage dependence of XAFS. However, the agreement with the experimental result is not satisfactory. The discrepancies between the experimental spectrum and the spectrum calculated for model A, which assumes that copper adatoms are in registry with the gold surface, are much larger than for the spectra calculated for the other four models which place copper atoms in registry with the top layer of chlorine. This result suggests that in the bilayer, copper adatoms prefer to occupy high-symmetry sites of the top layer of chlorine rather than those of the gold substrate. We note that the amplitude of the oscillations calculated for all models A and B is systematically larger than that observed experimentally. However, model B; which includes some static disorder is better. Similarly, in model D, the optimization of copper atoms positions in both vertical and lateral directions improved the agreement with the experiment. This suggests that the copper atoms are less ordered than in the idealized structures represented by these models. The trends depicted in Figures 15 and 16, as well as the inadequacy of the conventional analysis (Table 1) both strongly indicate that there is significant static disorder in the Cu positions. We suggest that this static disorder may be related to shifts from higher symmetry positions to accommodate different amounts of local free space in the region between the Cl- and Au layers. This point is illustrated by the near edge spectra in Figure 15, which show that, of the models investigated, the spectra calculated for model B′ are the closest to the experimental result. In this model we moved Cu and Cl atoms in both in-plane and out-of-plane directions to simulate a possible relaxation of the (5 × 5) structure.

J. Phys. Chem. B, Vol. 101, No. 49, 1997 10321 Unfortunately, there are an extremely large number of possible reorganizations and relaxations for which there is no a priori basis to differentiate. Thus it is not practical to search for a unique solution based on this type of “hit-or-miss” approach. It would be much better to use first principles energy minimization or molecular dynamic techniques to generate structural models which could then be tested against our XAFS results, using FEFF 6.01. Such studies are beyond the scope of this work. Overall, while our XAFS data and analysis do not give a definitive answer to the structure of this system, they do answer many of the qualitative questions raised in section 3.2.0. Our XAFS results place specific constraints on models to be considered in the future and eliminate certain high-symmetry structures postulated on the basis of STM. In particular, they rule out structures with Cu as the outermost layer33 and strongly suggest the Cu atom positions will be in closer registry to the chloride overlayer than the Au surface. It would appear that the ordered regions observed in STM are dominated by regular positions of the chlorides relative to the Au substrate and that the Cu positions, which are subject to a very large degree of static disorder, do not contribute to the STM images. The conclusion of large static disorder in the Cu positions is consistent with LEED measurements made on this system.19,20 The absence of any dramatic evolution of the XAFS with coverage indicates there are no significant local structure changes with the range of coverage explored in this work. 4. Summary We have performed a combined electrochemical and polarization-resolved XAS investigation of mixed overlayers formed by underpotential deposited Cu and Cl- coadsorbed on Au(111). The copper coverages determined electrochemically correlated well with the amplitude of the Cu K absorption edge. This is the first accurate surface composition measurements over a wide range of coverages/potentials. The electrochemical and XAS experiments clearly indicate the surface structure consists of a bilayer in which the copper atoms are sandwiched between the top layer of chloride ions and the gold electrode surface. The species imaged by STM have been shown to be chloride ions. FEFF 6.048,49 multiple scattering calculations of the XAS spectra were carried out on five models, two (A and B) with each having a “(5 × 5)” long-range structure and three (C-E) with each having an incommensurate (4 × 4) structure with respect to the underlying gold surface, with the electrodeposited copper atoms packed between the outer layer of gold and the top chloride layer. None of the spectra computed for these models were in satisfactory agreement with the experimental EXAFS. We interpret the lack of agreement to indicate that copper atom positions are affected by significant static disorder. The detailed nature of this disorder is not known. Based on improvements generated by empirical modifications to one of the models, it may involve shifts of Cu atoms from highsymmetry positions to accommodate different amounts of local free space between the chlorine and gold layers. Acknowledgment. This research was supported financially by NSERC (Canada). The X-ray absorption measurements were carried out at CHESS which is operated with funding from the National Science Foundation (USA) under Grant DMR9311772. We thank Dr. Ken Finkelstein and the rest of the CHESS staff for their assistance. We thank Dr. O. R. Melroy for the XAS spectra of CuAu3 used in this paper and for helpful discussions.

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