6552
J. Phys. Chem. 1994,98, 6552-6558
A b Initio XAFS Calculations and in-SituXAFS Measurements of Copper Underpotential Deposition on Pt( 111): A Comparative Study Howell S. Yee and Hector D.Abruiia' Department of Chemistry, Baker Laboratory, Cornell University, Ithaca, New York 14853- 1301 Received: August 24, 1993; In Final Form: February 24, 1994' This study tests the current state-of-the art development in theoretical calculations of X-ray absorption fine structure (XAFS)against recently collected in-situ XAFS structural measurements of electrochemicallydeposited copper on a pristine platinum( 1 11) surface. From the in-situ measurements, copper+pper and copper-oxygen bond distances were found to be. 2.77 f 0.03 and 2.1 f 0.03 A, respectively. Employing an automated ab-initio code, FEFF, developed by Rehr and co-workers5to calculate theoretical XAFS standards, qualitative agreement was found with our experimental findings. Using the FEFF calculations, we examined copper electrodeposited on 3-fold hollow sites and on bridge sites of the platinum substrate. The FEFF calculations for Cu on 3-fold hollow sites of Pt( 111) closely match the experimental data which indicates that the deposited Cu adatoms reside in 3-fold hollow sites of the P t ( l l 1 ) surface, which represents the most stable arrangement.
Introduction Over the past two decades surface science has developed into one of the dominant fields in chemistry and physics. Fueling the development has been the desire to understand, in greater detail, the various interfaces: solid-solid, vapor-solid, and most recently the liquidsolid interface. Deeper understanding of these interfaces has led to advances in areas ranging from catalysis to electroplating. With the development of more sophisticated surface probes, empirical understanding has given way to atomic realization. Ultrahigh-vacuum(UHV) systems coupled to various electron probe techniques provided the first glimpses, on the atomic scale, of the gas-solid and solid-solid interfaces.! However, probing the liquid-solid interface at the atomic level has proved most challenging because of the liquid phase. Various spectroscopic probes like surface-enhanced Raman, second-harmonicgeneration, sum-frequency generation, infrared reflectance, and others have been used to probe in situ the solid-liquid boundary.2 However, the wavelengths employed by these probes are not capable of atomic resolution. Two recent techniques paved the way toward atomic resolution of the liquid-solid interface. The development of the scanning tunneling and atomic force microscopes (STM and AFM, respectively) provided real-time, atomically resolved images of various interfaces includingthe solid-liquid interface.r3 However, despite the extraordinary resolution of these probes, these techniques are inherently intrusive. Because of this, there is a strong likelihood that non-specifically bonded (weakly adsorbed) species will be disturbed. X-ray-based techniquescan provide structural probes that not only have the necessary resolution but also are nonintrusive. Due to the high X-ray flux from a synchrotronsource, probing through a thin liquid layer is no longer an obstacle. Various X-ray techniques have been used to probe the various constituents of the liquidsolid interface, e.g., adsorbed species and weakly bonded species as well as the distribution of interfacial species.2 Depending on the technique employed, either long- or short-range order information can be obtained. The only major drawback to synchrotron-based techniques is the limited number of facilities. In the present study we chose to employ extended X-ray absorption fine structure (EXAFS) to probe the structure of copper electrodeposited onto a clean well-ordered platinum( 1 11) surface. The strength of EXAFS lies in its ability to probe the local structure around an absorbing atom, yielding such information as local structure, oxidation state, and atomicenvironment.28~4 e Abstract published
in Advunce ACS Abstracts, June 1, 1994.
0022-3654/94/2098-6552%04.50/0
Concurrent with the experimental development and use of EXAFS, there has been a steady evolution in the theory with the hope of eventually being able to calculate, from first principles, X-ray absorption spectra. Some of the seemingly intractable components of the complete theory, e.g., high-order multiple scattering calculations, are feasibly attainable with the power and capability of present day computers. A very sophisticated model (FEFF) developed by Rehr and co-workers5incorporates a number of first principle factors in EXAFS theory that had earlier been simplifiede6 Rehr and co-workers have also written a very elegant algorithm to compute ab initio XAFS spectra.5 Here wecompareresults from our in-situ investigation of copper underpotentially deposted on a clean and well-ordered Pt( 111) surface with the predictions made by the FEFF model developed by Rehr and co-workers.
Experimental Section A detailed description of the electrochemical and X-ray experimental arrangement can be found elsewhere? We will mention some key points. Measurements were conducted at the B-2 station of the Cornell High Energy Synchrotron Source (CHESS). All measurements were taken in-plane (a-polarization) at grazing incidence with the incident beam impinging on the sample at approximately 5 mrad. Typically 20 scans were collected and averaged toobtain a reasonable signal-to-noise ratio. Standard methods of EXAFS analysis4were followed employing the commercially available programs BAN and MFIT (Tolmar Instruments). Background removal was achieved by using (i) Victoreen background subtraction, (ii) linear background, and (iii) cubic spline. The specific choice of background removal produced no significant variation in the analyses. The reference compounds were treated in exactly the same manner as the experimental data. The electrochemical cell, contained inside an aluminum housing, consisted of a cylindrical Teflon body with feedthroughs for electrolytes and electrode connections. The electrode was covered with a thin polypropylene film serving as a window. Both faces and the edges of the Pt( 111) crystal were exposed to the electrolyte; however, the X-ray interacted only with the top face. The aluminum housing was continuously flushed with prepurified nitrogen gas, thus ensuring that the effects of oxygen diffusion through the polypropylene would be minimal. A new protocol was developed that ensured the integrity (cleanliness and order) of the platinum(ll1) surface during the course of the experiment.' Cyclic voltammetric scans of the Pt(111) crystal in 0.1 M HzS04 were taken every 2 h to verify the presence of a clean and well-ordered Pt( 1 11) surface. 0 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, No. 26, 1994 6553
Cu Underpotential Deposition on Pt( 111) Underpotential deposition (UPD) of copper was carried out from a dilute (50 X 1od M) aqueous solution of Cu2+in 0.1 M H2SO4 (CuSOcSH20, Aldrich Gold Label; Baker Ultrex, highpurity H2SO4; high-purity 18-Mohm Millipore Milli-Q water) by sweeping the applied potential negatively from the rest potential, typically about +0.7 V, at a rate of 2 mV/s. For XAS measurements the potential was held at 0.0 V. The copper UPD layer was stripped and redeposited after collectingtwo XAS scans (1-h period). The X-ray/electrochemical cell was rinsed thoroughly with 0.1 M H2S04 after each deposition and before the next deposition of copper. This procedure ensured that timedependent changes of the adlayer would not be measured and, if present, would be detected electrochemically?
Theoretical Calculations We will briefly highlight key features of single scattering XAFS theory. A number of assumptions were made in the development of EXAFS theories including the small-atom approximation, single-scattering events, independent particle model, mean free path, and passive electron effects? These assumptionswere made in order to simplify calculations; however, they are not without consequences. The small-atom approximation, also known as the plane-wave approximation, allowed the outgoing wave front to be treated by a plane-wave formalism. The standard equation is presented below
Here Nj is the number of neighboring atoms of type j , F,(k) is the backscattering amplitude, and u,is the DebyeWaller factor, which takes into ,account thermal vibration and possible static disorder (Le., distributions of distances). 4, = $12 4 b - la represents the total phase shift experienced by the photoelectron scattered between the absorber (4a) and backscatter (&). The term e - W arises from inelastic scattering losses where A corresponds to the mean free path of the electron. The validity of the plane-wave approximation holds well when the effective size of the backscattering atom is small relative to the distance between the backscattering atom and the absorbing atom. In the plane-wave approximation F d a ) (backscattering amplitude through an angle of 180O) is independent of ri. The single-scattering approximation assumes that the outgoing wave front is backscattered only once before combining with the formed (unscattered) wave. As long as the scattering is small, the singlescattering framework remains valid. At low k (k = wave vector) the approximation is no longer reliable, because the effective atomic size is comparable to the interatomic distance. At these lower energies, calculations must be made using the curved-wave formalism, where the curvatures of the outgoing and backscattered photoelectron waves are taken into account. When this approximation cannot be made, F(a) is a function of ri. Many-body effects have also been neglected in early EXAFS formulations. The EXAFS formalism is built around the independent particle model, in which, each electron is influenced by an external potential that is dependent on only the average motion of the other electrons. However, when electrons interact with one another, their interactions depend on the instantaneous position of the other electrons which is not considered in the averaging by the independent particle model. Many-body effects become apparent when one considers the differences in potential seen by the outer electrons between the neutral atom and its excited state. These field effects arise as the absorbing atom is ionized by the X-ray, and distortions develop in both the excited electron state and neighboring atoms as a result of the Coulomb potential of the absorbing atom. In summary, limitations of past theories are largely due to (i) the plane-wave approximation, (ii)
+
an inadequate account of multiple scattering effects, and (iii) an incorrect account of the fiels induced by the absorbing atom. For theoretical calculations, an automated code (FEFF) developed by Rehr and co-workers5 was used to generate the XAFS spectra. FEFFcalculates a spectrum from ab-initio highorder multiple scattering XAFS. Some key points to note from Rehr and co-workers’ FEFF development and implementation include (i) an exact treatment of CUrved-WaVe effects, (ii) a complex, energy-dependent self-energy (an excited-stateexchange correlation potential -characteristic “energies” of the “localized” orbitals), (iii) an approximate molecular potential derived from relativistic atoms, (iv) a fixed energy reference for the photoelectron wavenumber, and (v) distance-dependent “scattering matrices”. The backscattering amplitude F(a),used in the plane-wave approximation, was modified to take into account curved-wave effects. For a K-shell the curved-wave backscattering amplitude is given by” F(T,P,R) = P - l ~ ( - 1 ) + / [ ( 1 + 1) (c/+l(PR))2+ I
4c/-l(PR))21 (2) Here p is the complex photoelectron momentum, c~(pR)is the polynomial coefficient of the momentum Hankel function, and t / is the diagonal t-matrix element which is the multiple scattering component. By thereplacement ofFby (p/k) exp[2i(k-p)]Fe~, the complete equation can be rewritten in standard form5*f
+(k))e-2R/Xe-22k2(3)
where A(k) = So2@) exp[-Im((r$,(k)/2))] is a factor that combines intrinsic losses, final-state interference effects, and central-atom losses. By consideringthe above, FEFF overcomes some of the earlier drawbacks in the theoretical developments. For example, there is a reduction in the use of nonphysical fitting parameters as well as extending the range of the theory to lower values of k. The design of the computer code itself allows the calculations to be performed quite efficiently on a desktop personal computer. The output file consists of x as a function of k. Our primary interest was in the k region spanning 2.5-12 A-l.
Results and Discussion ImSifu Measurement of Cu UPD on Pt(ll1). We begin by presenting both electrochemical and XAFS findings from our in-situ measurements. Figurer, a and b, shows the voltammetries (inside the XAS cell) of the clean and well-ordered Pt(ll1) electrode and of copper UPD on a Pt( 111) surface, respectively. Thevoltammetric features in Figure 1a have been shown by UHV techniques to correspond to a clean and well-ordered platinum (1 11) surface.8a.bThe underpotential deposition of copper on the Pt(ll1) electrode, shown in Figure lb, shows a broad deposition peak which is expected because of the low Cu2+ concentration, 50 pM. However, the sharpness of the stripping peak provides evidence that the Pt(ll1) surface is clean and well-ordered. The charge (Q) associated with the stripping of the electrodeposited copper was found to be Q = 346 mC/cm2, which corresponds to a calculated coverage of = 0.83 monolayer (ML). In these calculations it was assumed that the copper had an electrosorption valency of two, which corresponds to complete discharge of copper. No correction was made for the difference in the potential of zero charge since this has a negligible effect on the calculated charge. It should be noted that several studies have shown that the copper may not be completely discharged upon deposition and that nonelectroactive copper exists in close
Yee and Abrufia
6554 The Journal of Physical Chemistry, Vol. 98, No. 26,1994
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Rgwe 2. In-situ, X-ray absorption spectrum around the copper K-edge for underpotentially deposited copper on a Pt( 111) electrode in contact with a 50 mM Cu2++ 0.1 M HSO4 at EIPp= 0.0 V, Q = 346 mC/cm2, and 8h = 0.83 (Q = 420 mC/cm2, = 1 ML).
v
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Figure 1. (a, top) Voltammogram of the clean well-ordered Pt( 111) electrode in the XAS cell. The electrolyte was 0.1 M H2SO4, scan rate Y = 50 mV/s, and Ag/AgCI was the reference electrode. (b, bottom) Cyclic voltammogram of Cu UPD on Pt( 111). The electrolyte was 50 pM Cu2++ 0.1 M H2SO4 solution, scan rate Y = 2 mV/s, and Ag/AgCl was the reference electrode.
proximity to the electrodesurface? There have been speculations that the charge of the anodic peak is not due solely to copper stripping. The in-situ XAFS spectrum is shown in Figure 2. The nearedge features are similar to a Cu+ (CuzO) reference compound, which would be consistent with a partially charged Cud+species. In the EXAFS region, severalwell-defined oscillationsareevident. The data were treated and analyzed in the standard fashion. The normal procedure in EXAFS for dealing with spikes or seemingly noisy data is called “deglitching”. We consider this technique rather irreproducible; thus, we chose to use a reproducible smoothing function. The data were smoothed in order to facilitate the possibility of analyzing second- and third-shell contributions. However,we found it was not necessary to examine contributionsbeyond the first shell in order to describeour system. Figure 3a,b compares the unsmoothed (raw) and smoothed (five-point boxcar smoothing) data. The five-point averaging helped decreased the scatter in the data at large values of k. Smoothing had no effect on the first shell. Even without smoothing, EXAFS oscillations can be clearly seen up to at least 8 A-1. We believe that the distinct peak in the smoothed data at 9.5 A-1 is an artifact in the measurement. Further analyses
2
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li (Ang ) Figure 3. klx(k)of the in-situ XAFS measurement for the (a, top) raw data and (b, bottom) data smoothed usinga five-pointsmoothingscheme. were performed on the smootheddata to extract such information as interatomic distances, number, and identity of nearest neighbors. Fourier transform of the smoothed data produced the pseudo radial distribution function shown in Figure 4a. In the region 2-3 A, where we expect to find the first copper-copper nearestneighbor distances, the peak form is complex. Within this region there could be several different atomic species comprising this particular coordination shell. Beyond the first shell (3 A) the spectrum becomes increasingly more complicated. No attempt was made to extract structural information beyond 3 A. Our primary interest lies in the unresolved peaks (not corrected for atom phase shifts) at 1.5, 1.8, and 2.5 A in the pseudo radial distribution (Figure 4a). The windows used for the forward and reverse Fourier transform are shown in Table 1. Specific results from analyzing the experimental data have been tabulated in Table 2. A specific assignment of the peak at 1.5 A is uncertain at this time. We believe that this peak is not an artifact of the analysis.
The Journal of Physical Chemistry, Vol. 98, No. 26, 1994 6555
Cu Underpotential Deposition on Pt( 111)
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k (Ang-l) Figure 4. (a, top) Pseudo radial distribution function (klweighted) and back-transform window function from approximately 1.8to 2.8 A. (b, bottom) Raw EXAFS(opencircles)Fourier filtered and back-transformed (filled circles) data, backscattering envelope (inverted open triangles), and recalculated curve (inverted filled triangles) at an applied potential of 0.0 v.
TABLE 1: Fourier Transform Parameters of Experimental Data and FEFF Simulations Hanning back-transform window k space (A-1) R space (A) R(1 1 1)/Cu (0.0V) 3.1-12 1.65-2.83 2.6-13 1.27-2.86 FEFF 3-fold sites FEFF bridge sites 2.6-13 1.74-3.18 2.6-13 1.84-2.84 FEFF oxygen model TABLE 2
EXAFS Parameters'
.,
. ,
Cu-0 2.14 4.1 0.0094 15.00 Cu-Cu 2.77 6.3 0.0055 -0.82 a Bond lengths R,effective coordination number NN, Debyc-Waller factor Au, and energy variation AE (accuracy: R 0.03 A;NN f 20%). Electrochemical values. Applied potential Eappl,calculated charge Q (1 ML = 420 pC/cm2). 0.0
allows a distinction between a high and a low atomic number backscattering), would allow an unambiguous assignment of this peak. However, at this time we can only speculate as to the possible assignment of this peak. Fourier filtering of the composite peaks at 1.8 and 2.5 A (not corrected for phase shift) was considered next. A symmetric back transform window was used with a range of 1.8-2.8 A. Two factors could give rise to the unresolved peaks. One possibility is that several different atomic species constitute a coordination shell or, alternatively, that there are two coordination shells that are not completely resolved. Two backscatterers, oxygen and copper, were needed to fit the data. Copper-oxygen and coppersopper bond distances of 2.17 f 0.03 A (NN = 4.1 f 20%) and 2.77 0.03 A (NN = 6.3 f 20%) were found. The copper-oxygen distance agrees with the various copper-oxygen bonds of C U S O ~ S H Z OA. ~distance ~ of 2.16 A would place the oxygen in a location where the oxygen would be shared. Having oxygen as a backscatterer implies that it occupies either bridge or 3-fold hollow (but not atop sites) sites with respect to the copper adlayer. Another, although less likely, possibility would place the oxygen directly in contact with the platinum substrate. The possible Cu-0 species that could give rise to the observed copper-oxygen interaction are Cu-(HzO), Cu-HS04b, or Cu-SO4&. These experimental findings suggest that the oxygen does not occupy atop sites as postulated in earlier surface EXAFS measurements of other UPD systems.lla,b The copper-copper bond length of 2.77 f 0.03 A is very close to the platinum-platinum lattice spacing in the (1 11) direction, suggesting that the deposited copper forms a commensurate adlayer on the platinum (1 11) surface. The number of copper nearest neighbors was found to be 6.3 f 20%, which is consistent with the formationof hexagonalsurfacestructures. Theformation of a commensurate copper layer on Au( 111) surfaces has been reported.11 Those studies also suggest that the Cu atoms occupy 3-fold hollow sites on the Au(1 ll).ll Figure 4b graphically depicts the above analysis. The filled circles represent the smoothed data as mentioned above. The open circlesrepresent the back Fourier transform of the windowed peaks in Figure 4a, with the open triangles as the corresponding amplitude envelope. The filled triangles are the recalculated points based on the bond lengths, number of nearest neighbors, DebyeWaller factor, and EO shift obtained from an analysis based on the standard EXAFS equation. The complex features in the radial distributions could be understood in light of recent findings by Abruila et a1.lZ Using X-ray standing waves to study copper UPD on an iodine treated platinum surface (from a Pt/C multilayer), they observed an accumulation of additional copper near the adsorbed Cu adlayer. The amount of accumulated copper (up to about a monolayer) is potential dependent and in all cases is present in close proximity to the surface (less than 40 %I). This layer of copper is likely present in a disordered manner, and in addition, its interaction with the surface is relatively weak since it does not survive rinsing with supporting electrolyte. If we assume that a comparable amount of accumulated copper is also present in our system, then it may give rise to a "smearingmeffect in the radial distribution because of its disordered nature. This could, in turn, cause a shift in the peaks in the radial distribution. More recently Abruila and co-workers have completed SEXAFS measurements on the above system.l3 Their study examined the local interaction of copper in both the u- and *-polarization. A simple rinsing experiment of the electrode surface at controlled potential was performed. Upon rinsing the electrode, all the loosely bonded copper species were removed. A striking difference in the radial distribution function was seen between the unrinsed and rinsed measurements. Peaks that were shoulders in the unrinsed system gave rise to distinct peaks after rinsing. Likewise, the fwhm of the peaks obtained after rinsing were narrower than the corresponding peaks without rinsing. Further discussion on the SEXAFS measurements of
346
Various background removal functions were employed which had no effect on the peak. One possibility could be a copper-oxygen nearest neighbor with a calculated distance of 1.73 A. This value is considerably shorter than known copper-xygen distances in various inorganic compounds.10 The distance of 1.73 A would have to represent a projected distance. For the Cu-0 distance to have a reasonable value (e.g., 2.1-2.2 A) would imply that the oxygen rests at an angle of approximately 20° relative to the substrate. This suggests that the oxygen could be shared by neighboring copper atoms. Another possibility for 1.5-A peak could be copper-platinum interactions. A distance of 1.78 A was calculated (after phase correction) for a copper-platinum pair. From geometric calculations of Cu residing in a Pt( 111) 3-fold hollow site, the projected Cu-Pt distance would be 1.62 A. Measurements in the *-polarization would allow a more precise determination of the number of platinum nearest neighbors. This, along with the backscattering envelope (which
I
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k (Ang-‘) F i e r e 6. FEFF simulation of copper adatoms on bridge sites and 3-fold
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Representation of the copper atoms in the (A) true 3-fold hollow sites, (B)pseudo-3-foldhollow sites,(c)platinum(l il)+pperFigure 5.
oxygen system, and (D)
bridge sites.
copper adatoms in 3-fold hollow sites and on
Pt/C-I-CusystemhasbeenprescntedeIsewhe~e.’~Thechanging featuresin their radial distributioniinsing/nonrinsing-would suowrt the notion of “smearine” in the radial distribution. FEW Computations. We proceed now to describe the results from the FEFF simulation of the Cu monolayer on a platinum (1 1I ) surface. The following simulations were performed (i) Cu adlayer placed in 1 X I , 3-fold hollow sites of Pt(lll); (ii) Cu on bridge sites on the Pt surface; (iii) Cu adlayer with a slight displacement from the 3-fold hollow sites of R(l1I); and (iv) an oxygen layer in wntact with the Cu adlayer (see Figure 5 ) . The model input file involved designating various positions for the absorber atom and the respective backscattering atoms. The platinum substrate was placed in a { I l l ) arrangement, three atomic layers in depth with a Pt-Pt distance of 2.77 A. The Cu adatoms were placed in either bridge sites or 3-fold hollow sites withrespect tothe Pt adlatticewitba Cu-Cubondlengthof2.77 A. The temperature for the DebycWaller term was taken to be 300 K. Bridge sites and 3-fold hollow sites were chosen because they rcprcscnt the most energetically favorable positions for
..
hollow sites compared with in-situ XAFS measurements. adsorption. The FEFF x(k) files were scaled by a factor of 3 to match the amplitude in the experimental data. The copper adlayer with a slight displacement from the 3-fold hollow site of platinum(l1 I ) (Figure 56). which in the present context has been named the “pseudo-3-fold hollow site model”, was also modeled. The backscattering atoms are 2.77 A from the central atom in the hexagonal arrangement, but the distance from backscatter to backscatter is slightly different as depicted in Figure 5B. This arrangement was made in order to test the sensitivity of the calculations to small displacements. TheBridgeSites. Webegin by examining resultsofcuplaced on bridge sites of the Pt(l I I ) surface. Although 3-fold hollow sites are the most energetically favored, we cannot completely rule out bridge sites as possible sites for adsorption. The same number of wpper adatoms was used in the bridge site simulation as in the 3-fold hollow site. The results are shown in Figure 6. Although similarities exist between the two models, differences are noticable at low k values. At low values of k, multiplescattering events are more prevalent. Thus, it is not surprising to see differences at low k values for the two models. However, asnoted byRehrandco-workersintbeirpaperslargeuncertainties exist at low k values. The3-FoldHollow Sites. Theamplitudeof 3-fold-hollowsite simulation matches the data more closely than the bridge site calculations. The 3-fold hollow site simulation does not Provide an exact match with the experimental data at low k values; there i s a slight shift in phase. From the FEFF calculations the most likely Psition of the WPpcr adatom would be in 3-fold hollow sites and not,on bridge sites on the Pt(lll) surface. To explorethe sensitivityof the FEFFcalculations, weexamined a copper adlayer with three Cu-Cu distances, 2.67, 2.77, and 2.87 A (Figure 5). Figure 7a shows the simulated XAS spectra i n k soace for the two different 3-fold hollow site models. As can be seen from Figure 7a, the two calculations are not particularly sensitive to small changes in the position of the atoms. Figure 7bwmpares both 3-fold hollow models to the experimental data. Despite the anomalous p k in the expcrimental data at 9.5 A-1 (uidesupra), thecalculated XAFS spectra appear nearly identical in terms of phase, to the experimental data from k = 3.5 to 12 A-1. Taking into amount the uncertainties in the theoretical model, both 3-fold hollow site models are identical. It is interesting to compare the pseudo radial distribution functionof the in-situmeasurements, thesimulated 3-fold hollow site model, and the bridge site model (Figure 8). Both model simulations are rather featureless at R values below 1.8 A. Noticeable differences can be seen in the main peak at a p proximately 2.5 A, with the 3-fold hollow site model matching moreclosely tbeexpcrimental data. The peak in theexperimental pseudo radial distribution at about 1.7 A. as discussed above,
The Journal of Physical Chemistry, Vol. 98, No. 26, 1994 6557
Cu Underpotential Deposition on Pt( 111)
5
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k (Ang-l) Figure 7. FEFF simulation of copper adatoms on a platinum( 1 1 1) surface: (a, top) true 3-fold hollow sites and pseudo-3-foldhollow sites; (b, bottom) FEFF simulations and experimental data of Pt(1 lI)/Cu at
- Pt(Lll)/C"
.- -
0.ov
FEFF 3 Fold Sites FEFF Brldge S i t e s
0
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Figure 8. Pseudo radial distribution function (k' weighted) of experimental data of Pt(1 1 1)/Cu at 0.0 V, FEFF simulated Cu on true 3-fold hollow sites of Pt(l1 l), and FEFF simulated Cu on bridge sites of Pt(111).
could be either real or an artifact of the system. Since we have not simulated a system with 3-D copper+oppe.r clusters, assignment of the peak at 1.7 A cannot be made unambiguously based on these simulations. Though no peaks beyond 3 A were analyzed from the experimentaldata set, we can see from Figure 8 that the complex feature centered around 4.3 Ais not an artifact in theexperimental measurement as it appears in the simulation as well. The broad peak centered around 4.3 A represents a higher order shell(s). It should be noted that multiple-scatteringcontributions such as shadowing become more evident in a Fourier transform at larger distances. One of the stengths of the simulation would appear to be the multiple-scatteringmatrices employed, since the model correctly calculated the higher order shells. At large R values,
4
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k (Ang-l) Figure 9. (a, top) Pseudo radial distribution plots: experimental data compared with FEFF simulation of Pt( 11 l)-Cu( 1 X 1)-0. (b, bottom) Comparison of k l x ( k ) : experimental data and the FEFF simulationof the Pt( 11 l)-Cu( 1 X1)-0 model.
the magnitudeof the 3-fold hollow site model matches more closely to the in-situ data than the bridge site model. One qualitative feature that needs to be mentioned is the fwhm of the main peak at 2.5 A. Both of the simulated peaks are narrower than the peak from experimentalmeasurements. There is also a shoulder present at 1.9 A in the experimentaldata which is not present in the simulations. As we discussed earlier, we believe that the broadness in the experimental data is due to the presence of loosely bound copper species in close proximity to the electrodeposited copper. This loosely bound copper contributes to the overall multiple scattering. In addition, since we limited the simulation to just a copper adlayer and a platinum substrate, we have completely neglected the electrolyte which can add to the overall complexity of the experimental measurement. Lastly, we simulated the platinum(l1 l)-copper system with an oxygen overlayer. For this simulation we have assumed that the source of oxygen comes from either bisulfate or sulfate. The choice of these anions over the solvent molecule, Hz0,arises from ab initio SCF calculations and FTIR measurements by Philpott and co-workers.14 The above experimental findings suggest that the oxygen does not occupy atop sites as postulated in earlier surface EXAFS measurements on other UPD systems.lla.b Bearing these two points in mind, the oxygen atoms were selectively arranged so that three oxygen atoms would surround a copper atom. The Cu-0 distance used as 2.13 A. This orientation represents the '3-down" adsorption geometry for the anions which would place the oxygens in the scattering plane. The orientation of the anions (Figure 5c) is debatable, but for the purpose of this work we propose that this is but one of many plausible arrangements at the copper adlayer/solution interface. Figure 9 depicts the Pt( 11l)-Cu( 1X l)-oxygen simulation in comparison to the experimental data. It should be noted that the Cu adlayer in the Pt( 11l)-Cu( 1X 1 ) - 0 has the same coordinates as the Pt( 11l)-Cu( 1X 1) 3-fold hollow model. The presence of oxygen has a striking effect on the simulated XAFS spectra. In
6558 The Journal of Physical Chemistry, Vol. 98, No. 26,1994
XAFS spectra as well. Based on the simulations,the most likely position of the Cu adatoms would be in 3-fold hollow sites. Experimental measurement in the r-polarization will unambiguously address the question of the copper location.
FEFF 3 Fold S i t e s FEFF Bridge S i t e s I''ElPF Pt-Cu-0 '3Fold
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Acknowledgment. The authors gratefully acknowledge the assistance of D. L. Taylor, J. Hudson, and S.Chen in collecting the data. We thank Keith Bowers of the LAASP machine shop in aiding in the design and construction of the new Teflon electrochemical cell. Bud Addis of the Cornell University Materials Preparation Facility prepared the Pt( 1 11) crystal disk used in the XAS experiments. Our work was generously supported by the Office of Naval Research, the Army Research Office, and the National Science Foundation.
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References and Notes
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(1). (a) Zangwill, A. Physics at Surfaces; Cambridge University Press: Cambndge, UK, 1988. (b) Adamson, A. W. Physical ChemistryofSurfaces; John Wiley: New York, 1982. (c) Somorjai, 0. A. Chemistry in Two Dimensions: Surfaces;Cornell University Press: Ithaca, NY, 1981. (d) Hall, R. B., Ellis, A. B., Eds. Chemistry and Structure at Interfaces; VCH Publishers: Deerfield Beach, FL, 1986. (2) (a) Guticrrez. C., Melendres, C., Eds.Spectroscopicand Dif/roction Techniques in nterfociol Electrochemistry; Kluwer Academic Publishers: Norwell, MA, 1990 and references therein. (b) Abruh, H. D., Ed. Elec?rochemicol Interfaces: Modern Techniques for In-Situ Interface Characterization;VCH: New York, 1991,and references therein. (c) Bard, A. J.; Abrufla, H. D.; Chidsey, C. E.; Faulkner, L. R.; Feldberg,S. W.; Itaya, K.; Majda, M.; Melroy, 0.;Murray, R. W.; Porter, M. D.; Soriaga, M. P.; White, H. S.J. Phys. Chem. 1993,97,7147. (3) (a) Manne, S.;Hansma, P. K.; Maaie, J.; Elings, V. B.; Gerwirth, A. A. Science 1991,251, 183. (b) Sashikata, K.; Furuya, N.; Itaya, K. J. Electroanal. Chem. 1991,316,361. (c) Hachiya, T.; Honbo, H.; Itaya, K. J. Elecrroanal. Chem. 1991,315,275.(d) Nichols, R. J.; Kolb, D. M.; Bchm, R. J. J. Electroanal. Chem. 1991,313, 109. (e) Nichols, R. J.; Beckmann, W.; Meyer, H.; Batina, N.; Kolb, D. M. J. Electroanal. Chem. 1992,330,381. (f) Chen, C.-H.; Vesecky, S.M.; Gewirth, A. A. J. Am. Chem. Soc. 1992, 114, 451. (g) Lorenz, W. J.; Gassa, L. M.; Schmidt, U.;Obrctenov, W.; Staikov, G.; Bostanov, V. Electrochim. Acta 1992,37,2173. (4) (a) Teo, B. K. EXAFS Basic PrinciplesandDoto Anolysis, SpringerVerlag: New York, 1986. (b) Koningsberger, D. C., Prins, R., Eds. X-ray Absorption: Principles, Applications, Techniques of EXAFS, SEXAFS and XANES; John Wiley & Sons: New York, 1988. ( 5 ) (a) Rehr, J. J.; Albers, R. C.; Natoli, C. R.; Stern, E. A. Phys. Rev. E 1986,34,4350.(b) Mustre de Leon, J., Rehr, J. J. Physica E 1989,158, 543. (c) Mustre de Leon, J.; Rehr, J. J.; Natoli, C. R.; Fadley, C. S.; Osterwalder, J. Phys. Rev. E 1989,39,5632. (d) Rehr, J. J.; Albers, R. C. Phys. Rev. E 1990,41,8139.(e) Rehr, J. J.; Mustre de Leon, J.; Zabinsky, S.I.;Albers,R. C.J. Am. Chem.Soc. 1991,113,5135.(0 MustredeLeon, J.; Rehr, J. J.; Zabinsky, S. I.; Albers, R. C. Phys. Rev. E 1991,41, 4146. (g) Schaich, W. L. Phys. R N . E 1984,29,6513. (6) (a) Stern, E.A. Phys. Rev. E 1974.10, 3027. (b) Teo, B. K.; Lee, P. A. J . Am. Chem. Soc. 1979,101,2815.(c) McKale, A. G.; Veal, B. W.; Paulikas, A. P.; Chan, S.-K.; Knapp, G. S.J. Am. Chem. Soc. 1988. 110, 3763. (7) (a) Yee, H. S.; AbruH, H. D. J. Phys. Chem. 1993,97,6278. (b) Yee, H.S.;Abrufla, H. D. fungmuir 1993,9,2460. (8) (a) Clavilier, J. J. Electroanal. Chem. 1980,'107,211. (b) Aberdam, D.;Durand, R.; Faure, R.; El-Omar, F. SurJ Sci. 1986, 171,303. ( 9 ) (a) Leung, L.-W. H.; Gregg, T. W.; Goodman, D. W. fungmuir 1991,7,3205.(b) Leung, L.-W. H.; Gregg, T. W.; Goodman, D. W. Chem. Phys. Let?. 1992,188,467.(c) McBrcen, J.; OGrady, W. E.; Tourillon, G.; Dartygc, E.; Fontaine, A. J. Electroanal. Chem. 1991,301,229. (d) Durand, R.; Faure, R.; Aberdam, D.; Salem, C.; Tourillon, G.; Guay, D.; Ladoceur, M. Electrochim. Acta 1992,37, 1977. (10) Martens, G.; Rabe, P.; Schwentner, N.; Werner, A. Phys. Rev. E 1978,17, 1481. (1 1) (a) Blum, L.; Abrufla, H. D.; White, J. H.; Gordon 11, J. G.; Borges, G. L.; Samant, M.G.; Melroy, 0. R. J. Chem. Phys. 1986,85,6732. (b) Melroy, 0.R.; Samant, M. G.; Borges, 0. L.; Gordon 11, J. G.; Blum, L.; White, J. H.; Albarolli, M. J.; McMillan, M.; Abruh, H. D. Lungmuir 1988, 4, 728. (c) Tadjeddine, A.; Guay, D.; Ladouceur, M.;Tourillon, G. Phys. Rev. Let?. 1991, 66, 2235. (d) Tadjeddine, A.; Tourillon, G.; Guay, D. Electrochim. Acta 1991,36, 1859. (12) Bommarito, G. M.; Acevedo, D.; Rodriguez, J. R.; AbruFia, H. D. In X-Roys in Material Analysis I I Now1 Applications and Recent Developments; Mills,D.M., Ed.;SPIE Prof. 1991,1550, 156. (13) Bommarito, G. M.; Acevedo, D.; Rodriguez, J. R.; AbruFia, H. D. J. Electroanal. Chem., in press. (14) (a) Barnes, L. A.; Philpott, M. R.; Liu, B. In ElectrochemicalSociety Symposium on ?he Double fuyer, 1993. (b) Parry, D. B.; Samant, M. G.; Seki, H.; Philpott, M. R. fungmuir 1993,9, 1878.
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k (Ang-l) Figure 10. Comparison of experimental data with all FEFF simulations. comparing the radial distribution function, the presence of oxygen gives rise to a new feature at low R values. The low R value features might be indicative of solid/liquid interface systems. In Figure 10 we compare all three models to the experimental data. It appears that each model by itself would be inadequate to explain the experimental data. Each model has its strengths. The true 3-fold hollow site model matches well from 3.8 to 5.8 A-1. The Pt( 11l)-Cu( 1X 1 ) 4 model correlates fairly well from R values of 3.5 to approximately 8 A.
Conclusions SurfaceEXAFS wasused tostudy the underpotential deposition of copper onto a clean and well-ordered Pt( 111) surface. The experimental measurementswere complemented by qualitatively comparing the findings to an ab-inito XAFS model developed by Rehr and co-workers.3 The charge derived from electrochemical measurements implied that the electrodeposited copper was not completely discharged. Near-edge analysis revealed similarities between thecopper adlayer and the Cu+ (CuzO) reference, lending support to earlier findingsof a partially discharged copper adlayer. In the a-polarization, we observed intense Cu-Cu scattering at the potential (0.0 V) studied. We find the Cu-Cu distance to be 2.77 f 0.03 A with a coordination number of 6.3 f 20% corresponding to the formationof well-ordered hexagonal surface structures. At this potential the copper adlattice exhibits an epitaxial arrangement with the Pt( 111) surface. Oxygen is a persistant backscatterer, with a calculated Cu-0 distance of 2.1 f 0.03 A. The performance of the FEFF calculations to modeling the solid/liquid interface was encouraging. As noted earlier, the FEFF calculated x ( k ) had to be scaled by a factor of 3 in order for the amplitude to match the experimental data. The FEFF simulationspredicted the major peaks in the radial distribution. The theoretical calculations were used to determine the position of the copper with respect to the Pt( 111) s u b s t r a t d u adlayer on bridge sites or 3-fold hollow sites. Simulations of the copper positions suggest that the copper adlayer sits on the 3-fold hollow sites of the Pt( 111) surface. The presence of oxygen was also added to the simulation, and this was found to contribute to the
