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Nov 18, 2006 - X-ray Diffraction and STM Study of Reactive Surfaces under Electrochemical Control: Cl and I on Cu(100). Sascha Huemann, Nguyen Thi Min...
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J. Phys. Chem. B 2006, 110, 24955-24963

24955

X-ray Diffraction and STM Study of Reactive Surfaces under Electrochemical Control: Cl and I on Cu(100) Sascha Huemann, Nguyen Thi Minh Hai, Peter Broekmann, and Klaus Wandelt Institut fu¨r Physikalische and Theoretische Chemie, UniVersita¨t Bonn, Wegelerstr. 12, 53115 Bonn, Germany

Hubert Zajonz* and Helmut Dosch Max-Planck-Institut fu¨r Metallforschung, Abteilung Dosch, Heisenbergstr. 3, 70569 Stuttgart, Germany, and Institut fu¨r Theoretische und Angewandte Physik, UniVersita¨t Stuttgart, Pfaffenwaldring 57/VI, 70550 Stuttgart, Germany

Frank Renner European Synchrotron Radiation Facility, BP 220, 38043 Grenoble CEDEX 9, France ReceiVed: July 26, 2006; In Final Form: September 22, 2006

The surface structure of Cu(100) modified by chloride and iodide has been studied in an electrochemical environment by means of in-situ scanning tunneling microscopy in combination with in-situ surface X-ray diffraction with a particular focus on adsorbate and potential dependent surface relaxation phenomena. For positive potentials close to the on-set of the copper dissolution reaction, the X-ray data disclose an extraordinarily large Cu-Cl bond length of 2.61 Å for the c(2 × 2)-Cl phase. This finding points to a largely ionic character of the Cu-Cl interaction at the Cu(100) surface, with chloride particles likely to retain their full charge upon adsorption. Together with the positive surface charging at these high potentials, this ionic Cu-Cl bond drives the observed 2.2% outward relaxation between the first two copper layers. These results indicate that the bond between the first and the second copper layer is significantly weakened which appears as the crucial prerequisite for the high surface mobility of copper-chloride species under electrochemical annealing conditions at these high potentials. With 2.51 Å the Cu-I bond is 4% shorter than the Cu-Cl bond implying that the nature of the Cu-I bond is mainly covalent. Accordingly, we observe a significant inward relaxation of the top Cu layers upon substituting chloride by iodide at the same electrode potential, which suggests that the iodide adsorption involves charge transfer from the halide to the copper substrate.

1. Introduction Copper is one of the key materials of the twenty-first century. A premier application field is the silicon chip technology, where Cu is already applied as interconnect material replacing the traditional vacuum deposited aluminum-based techniques.1 The control of copper-electrolyte interfaces with or without external potential is a particular challenge. Miniaturization toward the nanometer scale requires a more sophisticated understanding of the relevant interface properties of those devices containing such reactive materials as copper. A detailed understanding on the atomic scale of the role of additives on copper plating,2-5 copper corrosion and corrosion inhibition by organics,6,7 oxidation and precursor films for oxidation, anodic dissolution,8,9 and passive films formation10-14 is thus of vital interest. Advanced in-situ X-ray scattering techniques are the ideal analytical tool for the characterization of surfaces in a wet electrochemical environment. Up to now, X-ray diffraction studies of electrochemical solid/liquid interfaces have focused on “inert” electrode materials such as gold,15-18 silver,16,19 or other noble metal surfaces relevant for electrocatalysis such as Pt.20,21 It has been demonstrated that these materials can easily be prepared by flame annealing, resulting in clean and rather * Corresponding author phone: (0711) 68 56 52 64; fax: (0711) 68 56 52 71; e-mail: [email protected].

flat surfaces that exhibit similar qualities as is obtained after ion bombardment and subsequent annealing under UHV conditions. In contrast, in-situ X-ray diffraction experiments on oxidizing surfaces under electrochemical control are still on the verge of feasibility.22,23 A particular scientific goal of this contribution is to achieve a more sophisticated understanding of the interaction of electrosorbed halide monolayers with metal surfaces in general and with copper electrodes in particular. Our efforts are motivated by experiments demonstrating that already trace amounts of halides in combination with particular organic additives are sufficient in order to improve copper plating processes.2-5 An understanding of the atomic scale processes is still lacking. While previous X-ray diffraction studies focused mainly on the potential dependent in-plane structure of halide layers16,17,25 and on the kinetics of order-disorder phase transitions19 we particularly address here the entire out-of plane structure of halide modified copper surfaces by analyzing potential dependent CTR (crystal truncation rod) data. CTR data are most sensitive not only to the in-plane surface structure but also to substrate relaxation effects and adsorbate-substrate interlayer spacings. While relaxation phenomena are experimentally and theoretically well understood for “clean” metal and semiconductor surfaces under UHV conditions, far less is known about electrified solid/liquid interfaces.

10.1021/jp064764y CCC: $33.50 © 2006 American Chemical Society Published on Web 11/18/2006

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Figure 1. Electrochemical cell for surface X-ray scattering experiments. ki and kf represent the scattering geometry for incoming and scattered X-rays, respectively. A and B represent the electrolyte in- and output.

In previous studies which are dealing with potential dependent relaxation effects in an electrochemical environment,18,20,21 an outward surface relaxation between the first two substrate layers is observed at any potential, quite in contrast to what is commonly observed for “clean” surfaces in UHV. This characteristic outward relaxation for Pt(100) and Pt(111) in alkaline or acidic electrolytes has been attributed to strong hydrogen adsorption within the potential regime just above the onset of the hydrogen evolution reaction (HER). For Au(111) in 0.1 M H2SO4, Nichols et al.18 report a maximum of the outward relaxation (+1.5% with respect to the bulk interlayer spacing) close to the point of zero charge (pzc) with a decrease of the interlayer spacing upon decreasing the potential (negative surface charging). Rather unexpectedly, the outward relaxation decreased upon positive surface charging at higher electrode potentials. This anomalous trend with respect to a simple “bonding model”26 has been explained in terms of additional “band energy effects” upon charging.18 A small inward relaxation was reported by Lucas et al. for a bromide terminated Pt(111) electrode surface 27 with a covalent Pt-Br bond. Very recently, Wang et al.28 could even demonstrate “local” and nonuniform surface relaxation effects of a CO covered Pt(111) electrode surface. Their X-ray data clearly indicate an outward surface relaxation which strongly depends on the specific CO adsorption sites within the large high-order commensurate (x19 × x19)-13CO unit cell.30 It is this very delicate interplay between “pure” surface charging effects on one hand and related electrosorption phenomena on the other hand which needs to be clarified in order to obtain a full understanding of relaxation effects at the electrified solid/liquid interface. As the substrate of choice, we have used a single crystalline copper surface with (100) orientation whose surface electrochemistry in halide containing acidic electrolytes is well documented in the literature.8,9,29-32 2. Experimental Details STM experiments presented in this paper were carried out using a home-built scanning tunneling microscope described in detail elsewhere.33 The X-ray scattering experiments have been performed at the ID32 beamline of the European Synchrotron Radiation Facility (ESRF) in Grenoble using a special electrochemical cell34 (see Figure 1) which allowed potential control through a potentiostat during the experiment. The X-ray energy has been set to 17 keV.

Huemann et al. The Cu crystal was 10 mm in diameter and polished to within 0.2° of the crystallographic (100) plane. The bulk mosaic was 0.07° at the (1,0,1) reflection. Generally, it appears to be quite difficult to prepare laterally well-ordered and clean surfaces of reactive materials like Cu since the conventional flame anneal techniques often fail. Instead, electrochemical etching procedures are commonly used to remove the native oxide films from these reactive surfaces. This, however, appears problematic for the application of diffraction techniques due to the destructive character of the etching that leads to substrates with a rather high defect density. While this is still sufficient for local observations with scanning probe microscopy techniques, we need to significantly improve the surface quality for in-situ diffraction experiments characterizing the whole macroscopic sample on a microscopic scale. Therefore, we take advantage from so-called “electrochemical annealing”35 after an initial electropolishing procedure. The success of this treatment is based on the strong enhancement of the surface mobility by the presence of adsorbed anions which are excellent complexing agents such as chloride.36 By this we obtain surfaces revealing similar low-defect densities as achieved by conventional flame annealing. Our experimental sample environment and preparation was qualitatively identical to that of the STM experiments.29-32 The Cu crystal was electropolished in 50% H3PO4 and subsequently treated in a 10 mM HCl solution. Under these conditions a full monolayer of chloride ions adsorbs at the Cu(100) surfaceseven under “open circuit conditions”sand prevents reoxidation. The crystal was then mounted in the electrochemical cell that contained a 10 mM HCl solution. All potentials were measured with reference to an Ag/AgCl or Ag/ AgI electrode, respectively. For the sake of convenience all potentials are referred to a RHE. We have performed three experiments at room temperature which differ in the chemical nature of the adatoms and in their surface concentration. Different adatoms were introduced to the Cu surface by exchanging the 10 mM HCl electrolyte with a mixture of 5 mM H2SO4 and 1 mM KI under potential control. Changing the potential of the sample allows further control over the density of the halide ad-layer. The results presented below include a full three-dimensional structure analysis of the Cu(100) surface in contact with the chloride containing electrolyte. The structural data have been taken for potentials ranging between Ework ) +260 mV and Ework ) +95 mV and for Ework ) -330 mV. For Ework ) +95 mV, the data on chloride adsorption have been compared with the in-plane and out-ofplane surface structure of Cu(100) after iodide adsorption. In the X-ray diffraction study, a total of 342 integrated outof plane intensities have been measured in z-axis geometry by rotating the sample around its surface normal and subtracting the diffuse background which is mainly caused by a polyethylene foil used to seal the electrochemical cell. In the data analysis, the set of 342 reflections, which included symmetryequivalents, was reduced to 171 nonequivalent reflections. The error of the diffraction intensities was estimated from the measured reproducibility of symmetry equivalent reflections and produced an internal R-value of 21% based on |F|2 for all reflections. All data were corrected for polarization, Lorentz factor, active sample area, and the resolution function of the instrument. For the chloride covered surfaces, we measured the (2,0,L) and (2,2,L) CTRs and obtained a set of 67 CTR reflections for reach rod at Ework ) +95 mV and Ework ) +260 mV. For Ework ) -330 mV we measured the (2,0,L) CTR of Cu(100) in 10 mM HCl. CTR data were taken up to a maximum normal

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Figure 2. Cyclic voltammograms of Cu(100) in 10 mM HCl (black dotted curve) and in 5 mM H2SO4/1 mM KI (grey curve), sweep rate: dE/dt ) 10 mV/s

momentum transfer of Qz ) 6.6 Å-1, which is equivalent to 3.8 reciprocal lattice units. These results are presented in a crystallographic surface notation suitable and often used in other references for the Cu(100) substrate with rectangular lattice system and lattice constants of a ) 2.55 Å and c ) 3.61 Å at room temperature. 3. Results and Discussion 3.1. Electrochemical Characterization. Figure 2 summarizes representative steady-state cyclic voltammograms (CV) as obtained in the electrochemical cell of the in-situ STM setup for Cu(100) in 10 mM HCl (chloride source) and 5 mM H2SO4 + 1 mM KI (iodide source). It demonstrates the strong dependence of the copper electrode reactivity on the chemical nature of the halide anion being present in the bulk electrolyte. The potential window accessible for our experiments in hydrochloric acid is limited by two reactions, the copper dissolution reaction at the anodic limit and the hydrogen evolution reaction (HER) at the cathodic limit. Compared to the CV obtained in chloride containing electrolyte the entire shape of the CV changes when trace amounts of iodide anions are present in the acidic electrolyte. Reasons for these differences are mainly related to the intermediate and final products of the copper dissolution reaction. While soluble cupric [CuCl2]- chloro-complexes are formed in hydrochloric acid9,39 several highly insoluble 2D- and 3D-CuI phases emerge at the electrode in the presence of iodide,31,32 thus leading to a surface passivation against further dissolution. By comparison of both CVs in Figure 2 it becomes obvious that the chloride anion acts as dissolution promoter while iodide behaves more like a dissolution inhibitor due to the binary compound formation at the surface. Note that the onset of the exponential increase of the anodic current due to copper dissolution is significantly shifted toward higher potentials in the iodide containing electrolyte (Figure 2). Recent in-situ STM31,32 and ex-situ synchrotron X-ray photoelectron spectroscopy studies38 allowed the clear assignment of the broad anodic current wave (peak maximum centered at Ework ) +210 mV) to the copper oxidation followed by the CuI passive film formation and the asymmetric cathodic current

Figure 3. (a) Step stabilization on Cu(100) in 10 mM HCl at positive electrode potentials in the presence of the c(2 × 2)-Cl ad-layer, 8.7 nm × 8.7 nm, I t ) 2 nA, Ubias ) 35 mV, Ework ) +260 mV; the in-set shows an atomically resolved image of the c(2 ×b0 2)-Cl ad-layer (b) schematic ontop view of the steps formed by Cl on Cu(100) under the conditions in (a).

wave (peak maximum at Ework ) -140 mV) in the reverse potential scan to the corresponding reduction of the CuI passive film. A more detailed analysis of the electrochemical data will be given in a separate paper.38 3.2. STM Results. Figure 3 displays the atomic structure and surface morphology of the Cu(100) electrode in the presence of specifically adsorbed chloride anions at potentials within the double layer regime. The nearest neighbor distance of 3.6(2) Å and the 4-fold symmetry of the ad-lattice is pointing toward the presence of an ordered c(2 × 2)-Cl phase on Cu(100).7-9,39-41 The presence of chloride anions affect both the surface structure on a atomic scale and also the entire surface morphology, provided an adequate surface mobility has been induced at high electrode potentials close to the on-set of the dissolution reaction.36 Chloride stabilized substrate steps are aligned parallel to the 〈110〉 directions which coincide with the close packed chloride rows as can be seen in Figure 3a and b. Furthermore, these 〈110〉 steps reveal an extreme low kink density after applying an “electrochemical annealing” procedure. These “morphological effects” result from tremendous changes of the step and kink energies induced by the chloride anion ad-layer. A direct correlation of the covering chloride lattice to the underlying substrate can be achieved by a systematic variation of the tunneling conditions in the STM experiment. Figure 4a represents the c(2 × 2)-Cl ad-layer with the characteristic nearest neighbor distance of 3.6(2) Å imaged under “moderate” tunneling conditions (e.g., low tunneling currents). An atomic lattice with significant smaller lattice constants becomes visible in Figure 4b after increasing the tunneling current from 8.9 to 50 nA. Lattice constants of 2.5(2) Å and main symmetry axis that are rotated by 45° with respect to those of the chloride ad-layer (Figure 4b) let us conclude that the copper substrate is imaged under these drastic tunneling conditions (Figure 4c). Similar “spectroscopic” STM experiments involving tunneling processes through adsorbate layers were reported several times

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Figure 4. Series of STM of high-resolution STM images relating the c(2 × 2)-Cl ad-layer to the Cu(100) substrate, (a) 4.2 nm × 4.2 nm, I t ) 8.9 nA, Ubias ) -3 mV mV, Ework ) -290 mV; (b) 4.2 nm × 4.2 nm, The tunneling current is changed from I t ) 8.9 nA to I t ) 50 nA, Ubias ) -3 mV, Ework ) -290 mV; (c) 4.2 nm × 4.2 nm, I t ) 50 nA, Ubias ) -3 mV, Ework ) -290 mV.

in the literature.36 Provided each intensity maximum in the STM image corresponds to an atomic position within the particular lattice one can even derive absolute adsorption sites by this kind of experiments,36 e.g., 4-fold hollows for the Cu(100)-c(2 × 2)-Cl phase (Figure 4b). An electrolyte exchange procedure as described in the Experimental Section leads instantaneously to the substitution of the chloride by iodide anions. The resulting iodide structure on Cu(100) is presented in Figure 5. Clearly visible is a onedimensional long-range height modulation that is superimposed on the atomic corrugation suggesting that the iodide phase is not on full registry with the underlying copper substrate. The stripe distance in Figure 5a amounts to d ) 13(1) Å. Obviously, adsorbed iodide anions do not fit into the c(2 × 2) arrangement on Cu(100) due to their size. Instead, a uniaxially expanded ad-layer with a reduced symmetry is obtained.29,30 Our preliminary structure model was based on an electro-compressible/ electro-decompressible c(p × 2)-I unit-cell with a p-vector decreasing with increasing potentials.31,32 Such a c(p × 2)-I unitcell is labeled in Figure 5b as I. At potentials above Ework ) +80 mV we obtain an iodide saturation coverage with Θ ) 0.4 ML and a corresponding p-value of p ) 2.5. However, this preliminary structure model was based on a uniformly expanded unit-cell neglecting slight distortions within the iodide ad-layer. The X-ray experiments presented in this paper are also used

Huemann et al.

Figure 5. STM results showing the iodide saturation structure that can be described by a c(2 × 5)-I unit-mesh; (a) 11 nm × 11 nm, I t ) 5 nA, Ubias ) 24 mV, Ework ) +95 mV; (b) 5.1 nm × 5.1 nm, I t ) 5 nA, Ubias ) 24 mV, Ework ) + 95

for a further refinement of the in-plane structure model of the iodide structure under saturation conditions. 3.3. X-ray Diffraction Analysis. 3.3.1. Chloride on Cu(100). The analysis of two symmetrically inequivalent copper CTRs (Figure 6) obtained at Ework ) +260 mV and Ework ) +95 mV confirms the presence of a Cu(100)-c(2 × 2)-Cl ad-layer and leads to a detailed description of its geometry with a P4mm symmetry (Figure 7). The fit parameters of the c(2 × 2)-Cl structure model include a surface roughness due to a statistical distribution of uncorrelated up and down steps,42 an ad- and surface layer occupancy and the first two interlayer spacings. Due to symmetry reasons, the lateral positional parameters remained fixed during the refinement. Since Bragg reflections along the CTRs were also included in the data collection, we have been able to achieve an absolute calibration of the model with the data, which substantially improved the overall reliability of the fitted details for data between Bragg peaks and for different surface preparations. The parameters have been determined using an adapted fast-simulated reannealing algorithm,42,44 which is able to describe a wide variety of surface configurations and calculating the associated diffraction intensities (to avoid the dominance of strong CTR reflections during the refinement, we minimized a weighted residual with respect to the logarithm of the measured intensity45). Table 1a contains the fit results. Numbers in brackets indicate the error with respect to the last given digit of the fit parameter. Numbers without errors were not changed in the refinement. The agreement between model and data (Figure 6) given by a weighted residual is 5.0%.45

Study of Reactive Surfaces under Electrochemical Control

Figure 6. Plot of the intensity distribution of the (2,0,L) and (2,2,L) CTRs as function of momentum transfer perpendicular to the surface for Cl on Cu(100). The filled circles represent the experimental values; the solid red line represents the best fit based on the model shown in Figure 7. The solid black line shows the calculated intensity based on an uncovered, relaxed Cu(100) surface under UHV conditions. Inset: Plot of the width of the (2,0,0.3) reflection (see black circle) versus time. The decrease in intensity represents a growing domain size. t ) 0 represents the start of the electrochemical annealing process and the opening of the beam shutter. The CTR data was collected for t > 40 min.

Figure 7. Side and top view of the Cu(100)-c(2 × 2)-Cl model. The side view indicates a series of layers and their labels as used in the text. The solid rectangle in both views represent the choice of unit cell with reference to the Cu(100) surface. The dashed rectangle represents the c(2 × 2) superstructure cell. The arrows define the crystallographic main directions.

In contrast to clean Cu(100) surfaces under UHV conditions, where the topmost layers are slightly compressed by 1% with respect to the Cu bulk interlayer spacing, we find a 2.2% interlayer expansion.46,47 The adjacent Cu layers adopt the bulk spacing. Such a reversed relaxation of the topmost layers is not unexpected for Cu(100) in a 10 mM HCl electrolyte under electrochemical conditions. At Ework ) +95 mV the copper sample is positively polarized leading to a depletion of electrons between the first Cu layers. Hence the positive charge of the Cu cores is less shielded and supports an increased separation through electrostatic repulsion. The observed extraordinary large Cu interlayer spacing is clearly pointing toward a decreased bond strength of the topmost copper layer with respect to the underlying bulklike copper. This potential induced weakening of the Cu-Cu bond strength in combination with the presence of chloride as a complexing agent at the surface seems to be a

J. Phys. Chem. B, Vol. 110, No. 49, 2006 24959 crucial prerequisite for the “electrochemical annealing” effect observed at these positive potentials. It should be noted that the maximum of the Cu outward relaxation has already reached its maximum at Ework ) +95 mV and does not further change upon increasing the electrode potential from Ework ) +95 mV to Ework ) +260 mV (the associated CTR sets are identical). Our analysis further reveals a Cu-Cl layer spacing of 1.88 Å that is 6.0% larger with respect to the Cu bulk interlayer separation. Since the inplane structure is known, this information suffices to calculate all interatomic distances at the surface. The obtained structure of Cu(100)-c(2 × 2)-Cl must be compared with bulk phases formed by CuCl or CuCl2. The most common stable bulk phase of CuCl is Nantokite with a zinc blende like structure and a Cu-Cl-bond length of 2.48 Å.48 With a length of 2.61 Å the Cu-Cl bond on Cu(100) is 5% larger. In both cases Cl has 4 next Cu neighbors, however, in different coordination geometries. On Cu(100), Cl resides on 4-foldhollow sites, in Nantokite, Cl is tetragonally surrounded by Cu. High-pressure modifications of CuCl with NaCl like structures have interatomic Cu-Cl distances of 2.46 Å which is only 0.02 Å smaller than those in Nantokite.49 A drastically smaller CuCl distance is realized in Cu(II) salts such as Tolbachite with a Cu-Cl separation of 2.26 Å.50 In turn, we conclude that there is no known CuCl or CuCl2 bulk phase that shows a such a large Cu-Cl separation like the one found for Cl on Cu(100) under electrochemical conditions and at applied potentials ranging from Ework ) +95 mV to Ework ) +260 mV. To get more insight, whether this observation is related to the surface charge density or to the details of the Cl-Cu interaction, we compare our findings to results from vacuum experiments, where chlorine molecules are adsorbed in a dissociative process that leads to the same in-plane structure 51,52 as obtained by a specific anion adsorption from solutions. In a vacuum, the Cu-Cl distance is not larger than 2.41 Å. This value is 3% smaller than in Nantokite and even 8% smaller than for electrolytically deposited Cl on Cu(100) at positive potentials.53-55 From this comparison it becomes obvious that the nature of the chemical bond between Cu and Cl is different in both environments although the corresponding in-plane structures are identical. Apparently, the ionic character of the Cu-Cl bond is much more pronounced in the electrochemical environment than for the Cu(100)-c(2 × 2)-Cl phase in a vacuum. Chloride anions on Cu(100) practically retain their full negative charge upon adsorption from solution. As reviewed by Koper in ref. 56 main contributions to the adsorption energy originate from “depolarization effects” of the adsorbed halide ion on the metal electrons. Based on this “simple bonding model” one can further speculate that it is the nature of this particular ionic Cu-Cl bond which leads to a further depletion of metal electrons at the surface. This kind of image charge effect56 should reveal the same trend of outward relaxation as induced by the potential induced surface charging. The observed occupancy of the c(2 × 2)-Cl layer on Cu(100) at Ework up to +260 mV turns out to be 20% lower than the expected value of θ ) 0.5 ML for a fully covered sample surface. We attribute this to the aforementioned electrochemical annealing effect which has been observed by in-situ scanning probe techniques.36 However, it must be stressed that besides the “natural” annealing effect there is an additional photon induced enhancement of this etching process probably based on a radiation mediated Cl-radical formation and subsequent creation of mobile linear chloro-copper complexes. Consequently, the equilibrium between chloride adsorption and chloro-

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TABLE 1: a-c: Fit Parameters, Amount of Data, and Residuals for Each Model and Experiment [RHE]

a Cl/Cu(100)-c-(2 × 2) in 10 mM HCl U ) 260mV/95mV dad-layer

Cu-d12

Cu-d23

d-spacing [c/2]

1.08(2)

1.022(4)

1.0

lateral shift [a] coverage [ML] σRMS [Å] data Rw(Log10(I))

0.4(1) 3.0(1) 67 5.0%

1.0

1.0

b Cu(100) in 10 mM HCl U ) -330mV dad-layer

0.0(1) 3.8(2) 37 4.8%

complex formation may locally shift toward the latter where the synchrotron beam hits the sample. After several hours of radiation exposure a visible hole was etched into the sample where the beam hit its surface. We propose an inverse step flow mechanism as transport phenomenon as previously observed by in-situ techniques.36,57,58 The inset in Figure 6 shows the width of the surface sensitive Cu(2,0,0.3) reflection as a function of time of exposure to the synchrotron radiation. A 20% decrease of the width can be observed within 40 min at a surface potential of Ework ) +260 mV. This process reflects a “photon assisted” electrochemical annealing procedure of the surface leading to a 20% increase of the average domain size. The roughness of the surface is accordingly small with an σRMS of only 3.0 Å. The high mobility of Cu and Cl at the Cu surface also supports the result of a relatively large Cu-Cl distance and relatively weak bonding in comparison to all other known CuCl phases. Generally, the fact of the high mobility of copperchloro complexes on Cu(100) leads not only to a smoother surfaces but also plays a crucial role in a template guided growth of Cu as interconnecting material for semiconductor devices.57 At more negative potentials up to Ework ) -330 mV chloride desorbs from the Cu(100) surface.9 The analysis of the (2,0,L)

Figure 8. Plot of the intensity distribution of the (2,0,L) CTR as function of momentum transfer perpendicular to the surface and after Cl-desorption at Ework ) -330 mV. The filled circles represent the experimental values; the solid red line represents the best fit based on the model shown in Figure 6. The solid black line shows the calculated intensity based on an uncovered, relaxed Cu(100) surface under UHV conditions.

c I/Cu(100)-c-(2 × 5) in 1 mM KI U ) 95mV

Cu-d12

Cu-d23

dadatom-Cu

Cu-d12

Cu-d23

1.018(9)

1.0

I(3): 1.19(6) I(2): 1.04(6) I(1): 0.94(6)

0.97(4)

1.02(2)

0.8(2)

1.0

1.0

1.0

0.036(4) 0.5(1) 4.3(2) 67 9.4%

CTR confirms the absence of chloride with a fitted occupancy of 0 ML for this species (Table 1b). However, along with hydrogen formation at the Cu surface the observed surface roughness increases by 26% to an σRMS of 3.8 Å in comparison to the electrochemically annealed surface at Ework ) +95 mV. The data and fit are shown in Figure 8. It is mainly the increase in surface roughness which explains the deviation of the experimental data in comparison to a calculated data of a clean and perfectly smooth Cu(100) surface (black straight line in Figure 8). The fit results in Table 1b show still a 1.8% expansion of the outmost layer spacing of Cu despite the applied negative potential. The electron-transfer necessary to reduce hydronium cations to molecular hydrogen at the Cu(100) surface may, in fact, be an important aspect to understand the behavior of the Cu substrate during HER. Hydronium cations might act as electron acceptors at the Cu surface leading to a local increase of the positive charge at the underlying interface and consequently to a small but noticeable expansion of the top two Cu layers. The fit of subsequent Cu layer spacings did not improve the result and were, therefore, kept fixed at the bulk value in the refinement. 3.3.2. Iodide on Cu(100). In contrast to chloride, iodide forms several well ordered monolayer phases on Cu(100) differing in their potential dependent coverage.29-32 Our X-ray diffraction

Figure 9. Plot of the intensity distribution of the (2,0,L) and (2,2,L) CTRs as A function of momentum transfer perpendicular to the surface for iodine on Cu(100). The filled circles represent the experimental values; the solid red line represents the best fit based on the model shown in Figure 10. The solid black line shows the calculated intensity based on an uncovered, relaxed Cu(100) surface under UHV conditions.

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Figure 10. Top and side view of the I/Cu(100)-c(2 × 5) model. The solid rectangle represents the choice of unit cell with reference to the c(2 × 5) superstructure. The dashed circles indicate the position of iodine prior to fitting this model. The labels 1, 2, and 3 help to distinguish iodine atoms in 4-fold, quasi bridge and bridge sites, respectively.

data as obtained at Ework ) +95 mV is shown in Figure 9. The corresponding fit results are listed in Table 1c. To avoid 2Dand 3D-CuI formation, no potentials higher than Ework ) +95 mV were applied in our surface X-ray diffraction experiment.31,32 In this study we focus onto the characterization of the iodide saturation structure at electrode potentials just before reaching the anodic current wave (displayed in Figure 2). Our surface X-ray diffraction results give clear evidence for the formation of a high-order commensurate c(2 × 5)-I structure under saturation conditions which is in qualitative agreement with the proposed c(p × 2)-I unit cell (with p ) 2.5)29,32 (see also Figure 5). The iodide occupancy was found to be θ ) 0.5 ML in excellent agreement with the STM (Figure 5). However, other competing structures based on less densely packed p(2 × 2), c(2 × 6) or c(2 × 8) phases that have been reported from STM studies in a vacuum and under electrochemical conditions 29-32,59,60 cannot be confirmed by our X-ray data. The associated least squares-fits lead to residuals that are 14-25% larger than the best fit with a c(2 × 5) model. The following structure refinement focuses onto the internal structure of the c(2 × 5)-I unit-cell with a P2mm symmetry. Our structural fit parameters include the Cu-I bond lengths, the Cu-Cu interlayer spacings, roughness and occupancy parameters as well as a lateral shift

of iodide particles along the [100] direction. Note that the reduced overlayer symmetry from P4mm to P2mm implies the emergence of two rotational domains one one terrace. In our refinement procedure we applied an incoherent average over the associated intensities. Figure 10 shows a top and a side view of the “X-ray”-model. The STM results suggest a lateral displacement of iodine. A free fit of this parameter moves iodide from a quasi bridge site toward a 4-fold-hollow site as indicated by the dotted and solid circles in Figure 10 (improving the residual of the fitting process by 6 to a value of 9.4%). The shift of two iodine particles within the unit cell maximizes the local coordination and, thereby, minimizes the repulsive nearest-neighbor interactions. Our starting model assumed an even distribution of iodine with maximum symmetry that requires it to be at (x,y) ) (0.25, 0.50), which is a quasi bridge site (see dotted circle in Figure 10). On the other hand, our STM and X-ray data suggest that iodine lies closer to the 4-fold hollow site with an x-component of 0.20. The fitting of this parameter leads finally to a value for x of 0.216(4), which lies between the initial value and that required for the 4-fold site. Our observation that iodine (labeled 2 in Figure 10) does not reach the 4-fold hollow site at x ) 0.2 is due to the fact that the distance to the next iodide particle (labeled 1 in Figure 10) would otherwise amount to 3.61 Å which is 19% smaller than in Marshite, the most common CuI phase with zinc blende like structure under normal conditions.61 The fit value keeps the I(1)-I(2) distance with 3.81 Å clearly larger but leaves it still 11% smaller than observed in Marshite. Even though this separation seems still significantly compressed with respect to all known CuI phases it is within the range of I-I distances in solid I2 (exhibiting a I-I bond distance of only 2.72 Å).62 In order obtain a full 3-dimensional model, the interlayer spacings of the first three layers including the iodide ad-layer have been included in the fitting procedure. However, since iodine resides in different heights on Cu(100) due to adsorption in hollow, bridge and quasi bridge sites, we could not simply adjust a single layer spacing between Cu and this species. To keep to the number of fitting parameters low and still work with a physically meaningful model we made the approximation that iodide behaves like hard spheres on Cu(100) which would be equivalent to just one average bond length between the latter. With this model we fitted the bond length between Cu and I to be 2.51(7) Å. Figure 10 (lower part) displays a side view of the resulting structural model. Our analysis shows that the topmost Cu layers experience a compression of 3% with respect to the ideal bulk layer spacing. This behavior is reminiscent of the inward relaxation of Pt(111)-layers after bromide adsorption.27

TABLE 2: Positional Parameters of Cl and I on Cu(100) in the Asymmetric Unit in Relative Units of the Superstructure Metrics or Cu Lattice Constants [RHE][Å] Cl/Cu(100)-c-(2 × 2) in 10 mM HCl U ) 95mV/260mV a ) 7.22, b ) 7.22, c ) 3.61

I/Cu(100)-c(2 × 5) in 1 mM KI U ) 95mV a ) 12.76, b ) 5.11, c ) 3.61

Cu(100) in 10 mM HCl U ) -330mV a ) 2.55, b ) 2.55, c ) 3.61

atom

x

y

z

atom

x

Cl Cl Cu Cu Cu Cu Cu

0.500 0.000 0.250 0.000 0.500 0.000 0.500

0.500 0.000 0.250 0.000 0.000 0.500 0.500

-0.53(1) -0.53(1) 0.011(2) 0.500 0.500 0.500 0.500

Cu Cu Cu Cu Cu above coord. in c(2 × 2) notation

0.250 0.000 0.500 0.000 0.500

y

z

0.250 -0.009(5) 0.000 0.500 0.000 0.500 0.500 0.500 0.500 0.500

atom

x

I(3) I(2) I(1) Cu Cu Cu Cu Cu Cu Cu Cu Cu

0.500 0.216(4) 0.000 0.100 0.300 0.500 0.000 0.000 0.200 0.200 0.400 0.400

y

z

0.000 -0.59(3) 0.500 -0.51(3) 0.000 -0.48(3) 0.250 0.008(2) 0.250 0.008(2) 0.250 0.008(2) 0.000 0.492(1) 0.500 0.492(1) 0.000 0.492(1) 0.500 0.492(1) 0.000 0.492(1) 0.500 0.492(1)

24962 J. Phys. Chem. B, Vol. 110, No. 49, 2006 In contrast to the contraction of the Cu surfaces layers, layers 2 and 3 experience a 2% expansion. Further free fitting of subsequent interlayer spacings did not improve the result, in turn, they were kept fixed at the bulk value. Here we would like to stress that the dependence of the surface relaxation on the nature of the anion is enormous: at Ework ) +95 mV adsorbed Cl induces a 2.2% expansion of the Cu top layers and I on Cu(100) a 3% compression of the latter (Table 1). The observed trend can be qualitatively explained in terms of differences in the chemical bonding of both halides to the copper substrate. This is corroborated by the comparison with Cu-halide bond lengths. The Cu-I bond is 4% smaller in comparison to the Cu-Cl distance of 2.61 Å. The above results are indicative for a stronger, but less ionic Cu-I bond with a significant covalent component involving charge transfer form the iodide to the substrate upon adsorption. Both, the reduced ionic nature of the Cu-I bond and a significant charge transfer from the iodide to the copper surface are hold responsible for the decreased outward relaxation with respect to the chloride modified surface at Ework ) +95 mV. Following this argumentation it is also understandable that the iodide modified copper surface does not reveal such a pronounced surface mobility and electrochemical annealing effect as observed for the chloride termination. This is not only due to the stronger Cu-I interaction but also to the enhanced interaction between the two topmost copper layers (inward relaxation!) with respect to the chloride termination that prevents a rapid detachment of copper material from step edges. In full agreement with these results we find a fitted value of the σRMS of 4.3 Å reflecting an increased surface roughness in the absence of pronounced electrochemical annealing effects (Table 1). While the ionic Cu-Cl bond of 2.61 Å appears to be unique due to its large value the more covalent Cu-I bond length of 2.51 Å (Figure 10) in the Cu(100)-c(2 × 5)-I phase at Ework ) +95 mV corresponds in the literature to high-pressure polymorphisms of CuI that show indeed Cu-I distances of 2.52 Å at 10 GPa.60 Finally, Marshite has a Cu-I bond length of 2.62 Å at atmospheric pressures which is 4.4% larger than for Cu(100)-c(2 × 5)-I. Out-of-plane data for iodide modified Cu(100) surfaces in a vacuum is available only for the low coverage Cu(100)-p(2 × 2)-I phase (Θ ) 0.25 ML) with iodide residing in 4-fold-hollow sites and a Cu-I separation of 2.68 Å which is also 7% larger than the one found in our study.63 4. Conclusions The most important result of our in-situ X-ray analysis is the observation of a Cu-Cl distance of 2.61 Å which exceeds that of all known CuCl bulk or surface phases by 8%. This finding suggests a predominantly ionic nature of Cl-Cu bond under the given electrochemical conditions. Our results suggest further that the electrochemical annealing effect is not only mediated by the ability of chloride ions to form mobile and soluble copper complexes but also by a significantly weakened Cu-Cu bond at the surface as reflected in the pronounced outward relaxation of the topmost copper layer by 2.2% with respect to the bulk interlayer spacing. This weakened copper bond facilitates the detachment of copper material from step edges. For specifically adsorbed iodide we determine a mainly covalent character of its bond to the Cu substrate that involves an efficient electron transfer from the adsorbate to the substrate. This result is based on the observation of an unusually short Cu-I bond length of 2.51 Å only found in high-pressure polymorphs of CuI. In contrast to the outward relaxation of top

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Rw(Log[I]) )

2 CALC 2 |Log10[|FOBS ∑ hkl | ] - Log10[|Fhkl | ]| hkl 2 Log10[|FOBS ∑ hkl | ] hkl

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