Water Interactions at Electrified

Apr 19, 2012 - (1) In this picture, the effective concentration of an electro-active ... not only electrostatic but also weak dispersive interactions ...
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Competitive Anion/Water and Cation/Water Interactions at Electrified Copper/Electrolyte Interfaces Probed by in Situ X-ray Diffraction Hubert Keller* Institute of Theoretical and Applied Physics, University of Stuttgart, Pfaffenwaldring 57, 70569 Stuttgart, Germany

Martino Saracino Institute of Physical and Theoretical Chemistry, University of Bonn, Wegelerstr. 12, 53115 Bonn, Germany

Hai M.T. Nguyen, Thi Mien Trung Huynh, and Peter Broekmann Department of Chemistry and Biochemistry, University of Berne, Freiestrasse 3, 3012 Berne, Switzerland ABSTRACT: The full 3D structure of a copper/electrolyte interface is studied by means of in situ surface X-ray diffraction (SXRD) methods. Chloride anions chemisorb on Cu(100) in 10 mM HCl at high potentials under formation of a p(1 × 1)-Cl adlayer. This anionic chemisorption layer serves as a structural template for the lateral ordering of water molecules and hydronium cations in the near-surface liquid electrolyte. Evidence for this interfacial geometry is mainly derived from the intensity distribution of surface-sensitive X-ray diffraction data along the (10L)-adlayer rod. The characteristic oscillating intensity distribution along the (10L) rod is due to a centered bilayer system consisting of the anionic inner Helmholtz layer (IHL) of chemisorbed chloride and the cationic outer Helmholtz layer (OHL). The latter is constituted in the present case by hydronium cations that preferentially populate 4-fold hollow sites of the underlying chloride lattice. IHL and OHL are separated by an extra interfacial water layer. Anions and cations in the IHL and OHL compete for these water species as part of their solvation shell. The Cl/water/hydronium bilayer can be considered as a prototypical model system where the anions and cations in the coupled bilayer system are sharing the interfacial water as part of their solvation shell. In this respect, the Cl−/water/hydronium bilayer considerably differs from the previously studied Cl−/water/K+ system where the interfacial water was clearly assigned to the solvation shell of the alkali metal cation in the OHL. The absence of strongly solvated alkali metal cations in the OHL leads to an increase in the in-plane and out-of-plane exchange dynamics of water and hydronium cations as manifested by an isotropic atomic displacement parameter that is notably higher for the Cl−/water/hydronium than for the more static Cl−/water/K+ system. A comprehensive comparison of our results with other state-of-the-art SXRD studies strongly suggests that the adsorption of partly solvated cations on-top of an anionmodified metal electrode surface has to be considered as a specif ic cation adsorption phenomenon since the particular structure of the formed bilayer system as well as the involved interfacial dynamics clearly depend on the chemical nature of the anions and cations involved in the structure formation.

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layer where the electrode reaction takes place.4 Consequently, the overall velocity of the charge transfer reaction changes. Thanks to the successful implementation of structuresensitive surface science techniques such as scanning probe microscopy (SPM)5−7 and surface X-ray diffraction (SXRD)8,9 into the field of surface electrochemistry, substantial progress has been made over the last two decades concerning our understanding of the geometric and electronic structure of these electrified interfaces under reactive and nonreactive conditions. Most of the previous experimental work was,

INTRODUCTION Deriving a correlation between the full three-dimensional structure of an electrified interface and its reactivity belongs to the major challenges in surface electrochemistry. It was Frumkin who first recognized in 1933 the influence of the particular interfacial structure on the kinetics of an electrode reaction.1 In this picture, the effective concentration of an electro-active species in the electric double layer might deviate from its respective bulk solution concentration2 even under nonreactive conditions due to specific or nonspecific adsorption3 of these reactants. Furthermore, the coadsorption of nonelectroactive “spectators” (e.g., anions, organic molecules, etc.) alters the geometric structure on both sides of the interface and by this the effective potential within the double © 2012 American Chemical Society

Received: February 21, 2012 Revised: April 13, 2012 Published: April 19, 2012 11068

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A recent in situ X-ray diffraction study by Keller et al.22 could experimentally demonstrate that interfacial water and cationic species order themselves in the interfacial regime on the electrolyte side not only along the surface normal (layering effect) but also parallel to the electrode surface when chloride anions are specifically adsorbed on Cu(100). The IHL of specifically adsorbed chloride anions can be considered as a structural template for the lateral ordering in the outer and diffuse Helmholtz layer in the near surface liquid electrolyte phase. A substantial contribution of adsorbed potassium cations in the OHL to the overall scattering in the XRD experiment was explained by a preferential cation adsorption in 4-fold hollow sites of the underlying chloride lattice that introduces a specific integral reflection condition (extinction) into the surface diffraction data. The observed large anion−cation interlayer separation of dCl−CatL = 3.43 Å22 was rationalized in terms of an adsorption of partially solvated potassium cations that do not lose their solvation shell upon adsorption, giving rise to the formation of an interfacial water layer separating anions and cations in the inner and outer Helmholtz layer, respectively. In their previous study, Keller et al.22 used an acidified electrolyte with hydronium and potassium cations both competing for the same adsorption sites in the OHL causing a complex and mixed potassium/hydronium OHL. Furthermore, those alkali metal cations accumulated in the OHL were considered as competitors of chemisorbed anions for the coordinative interaction with interfacial water. The previously studied Cl−/water/K+ interface can be considered as a prototypical model system where the water/cation interaction overrules the respective anion/water interaction.22 Partly solvated potassium cations electrostatically interact here with a layer of almost desolvated chloride anions in the IHL. A similar centered structure model was recently proposed by Nakamura et al. for adsorption of partly solvated Cs+ ions on bromide-covered Ag(100).23,24 In the present study, we focus on the interfacial structure of Cu(100) formed in 10 mM HCl solution where alkali metal ion adsorption is explicitly excluded. Under these noncompetitive conditions, hydronium cations are, as reactants of the hydrogen evolution reaction (HER), the only cationic species expected to be present in the OHL. We will demonstrate that in the interfacial water layer in the Cl−/water/hydronium system is part of the hydration shell of both the anionic IHL and the cationic OHL. The structure of the OHL and the interfacial water layer is expected to depend not only on the particular composition of the bulk electrolyte but also on the exchange dynamics with the electrolyte phase (out-of-plane exchange) and with vacancies in the OHL/water layer (in-plane exchange). The literature terms such a cation adsorption often as nonspecif ic in contrast to the strong specif ic chemisorption of anions on the metallic electrode surface which crucially depends on the chemical nature of both the substrate and the adsorbent. This contribution will question this “artificial” distinction into specific and nonspecific adsorption by demonstrating that the particular structure characteristics of the OHL also depend on the chemical nature of both the substrate (here the IHL) and the adsorbent (solvated cation in this case). In this sense, strong chemisorption is not a necessary requirement for a specific adsorption.

however, restricted to the solid electrode (= metal surface including the inner Helmholtz layer of chemisorbed ions as the outermost part of the solid next to the liquid electrolyte), thereby neglecting the liquid electrolyte-side of the electric double layer (= outer and diffuse Helmholtz layer as the outermost part of the liquid electrolyte next to the solid electrode) where solvated counterions accumulate and counterbalance charges of opposite sign at the solid electrode surface. In particular, halide anions (Cl−, Br−, I−) reveal a strong tendency toward specific chemisorption on metallic electrode surfaces.10 The impact of the specifically adsorbed anions on the reaction kinetics is diverse. Their presence at the electrode surface can either (passively) block reaction sites at the metallic electrode surface or they can (actively) attract electro-active cations (e.g., hydronium cations as an important precursor reactant for the hydrogen evolution reaction), thus locally increasing the effective concentration of an electro-active reactant. Vital for our understanding of the particular role of these “spectators” is a profound knowledge about their adsorption behavior in the submonolayer and monolayer regime and how they interact with coadsorbed electro-active species on the electrolyte side of the interface. In this respect, anion−cation pairing effects might be important for the understanding of the double layer structure under reactive conditions.11 A recent study by Lucas et al. demonstrated that not only electrostatic but also weak dispersive interactions (van der Waals type) have to be taken into account to rationalize the adsorption of hydrated cations on electrode surfaces.12 Comparing the out-of-plane structure of metal/halide systems in an electrochemical environment with the structural characteristics of the respective solid/vacuum interface gives important insights into the particular interaction between anionic spectators adsorbed in the inner Helmholtz layer (in the following abbreviated as IHL) and cationic reactants in the outer Helmholtz layer (in the following abbreviated as OHL). Prototypical model systems to study these structural phenomena in both experimental environments are fcc(100) surfaces of Cu on which most halides chemisorb under the formation of simple low-order commensurate p(1 × 1) adlayers.9,13−18 Note that the p(1 × 1) notation of the halide adlayer refers to the conventional fcc unit cell of the copper bulk material. It describes the smallest surface cell and allows the simplest indexing of CTRs and ADRs in the diffraction experiment. Instead of p(1 × 1), also c(2 × 2) or (√2 × √2)R45° notations are often used throughout the literature but relate the adlayer structure to an unconventional body-centered tetragonal Cu-substrate cell. Previous XRD studies by Huemann et al.19 and Gründer et 20 al. consistently report a metal/chloride interlayer spacing of dCu−Cl = 1.880 Å and dCu−Cl = 1.856 Å for Cu(100)−p(1 × 1)Cl that reveals an enormous outward relaxation. It is about 17% larger than the respective halide−copper spacing of the corresponding p(1 × 1)-Cl adsorption layer under UHV conditions.21 These notable differences in the out-of-plane structure are rationalized by specifically adsorbed anions that find their binding partners not solely in the atoms of the underlying metal surface but are additionally coordinated by water dipoles and solvated cations residing in the OHL, thereby sandwiching the specifically adsorbed anions in the IHL. These additional binding partners are missing at the respective solid/ vacuum interface, thus explaining the smaller outward relaxation of the halide adlayer under UHV (ultrahigh vacuum) conditions.21 11069

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EXPERIMENT STM data have been obtained using a “Bonn-type” in situ STM that has been described elsewhere.25 The X-ray diffraction experiment reported here has been performed in an electrochemical cell utilizing a hanging droplet configuration that allows a stable potential control during the SXRD experiment.26 The voltammetric data were measured directly in the in situ SXRD cell that was used for the X-ray diffraction experiments.26 The surface area of the copper substrate exposed to the electrolyte was 19.6 mm2. In the present paper, we focus our analysis on the interfacial structure at a potential of Ework = +150 mV. Under these conditions, the ordering in the OHL is developed most since the p(1 × 1)-Cl reveals a chloride surface coverage close to the saturation limit of Θ = 0.5 monolayers (= ML, defined by number of atoms with reference to the number of copper atoms in the (100) plane) so that there is no disturbance in the ordering of cations and interfacial water by vacancies as structural defects within the anionic IHL. These point defects appear in the chloride layer at more negative potentials close to the potential-induced order−disorder transition within the p(1 × 1)-Cl phase. A synchrotron beam of 20 keV was provided by beamline X04SA at the Swiss Light Source (SLS). Our Cu crystal was 5 mm in diameter and polished to within 0.2° of the crystallographic (100) plane. The bulk mosaic was 0.07° at the (202) reflection. For Ework = +150 mV, we measured the specular (00L) rod, the (20L) CTR (Crystal Truncation Rod),27 and the (10L) ADR (Adlayer Rod) up to a maximum normal momentum transfer of qz = 6.61 Å−1, which is equivalent to 3.8 reciprocal lattice units. The results are presented with reference to the crystallographic bulk notation of copper with a cubic face-centered unit cell and a lattice constant of aCu = 3.61 Å at room temperature. In our former study on the p(1 × 1)-Cl phase,19 only CTRs could be measured mostly due to a different electrochemical cell design which limited the accessibility to ADR data. The intensity distribution along the CTRs presented here and in ref 19 is, however, comparable to each other within the error margins. The absence of ADR data in ref 19 was caused by a large inhomogeneous scattering background close to its location in reciprocal space due to the use of a thin-layer conf iguration. The lack of this data resulted in a less detailed model than the one presented here since this kind of data is considerably more sensitive to surface relaxations than offspecular CTR data. In other words: it is this sensitivity which enables a more sophisticated analysis due to a better distinction between models and smaller errors of the model parameters. Our X-ray diffraction analysis is based on the measurement of 96 symmetrically independent reflections along the (00L), (10L), and (20L) rods that were measured at room temperature in z-axis geometry.28 The data set was collected using a PILATUS II pixel detector29 with a dynamic range of 106 for each of its 487 × 195 pixels. The 2D PILATUS detector technology appears advantageous in particular in the combination with an electrochemical setup since we expect significantly lower signal-to-noise ratios compared to respective solid/vacuum interfaces.30,31 A careful background correction and a fast data collection appear therefore essential for the success of the experiment. The data were collected by rotating the sample around its surface normal, hereby measuring a series of 2D areas of diffracted intensity around the region of a surface

rod. This series was then integrated along the scan direction of the rotation and represents a volume of reciprocal space that contains typically about a quarter of a surface rod (Δqz = 0.75), together with its scattering background. Slices of that volume are then used to retrieve the intensity and background for a certain qz value along a rod of scattering. In contrast to the data collection with 2D detectors, conventional point detectors produce only integrated intensities for one specific qz value per scan. Integrated intensities were corrected for background scattering, polarization, and Lorentz factor. The background was mainly caused by the electrolyte droplet and by the Kapton foil that was used to seal the electrochemical cell from air and to establish a protective Ar-gas atmosphere.22 The experimental error of the diffraction intensities includes their standard deviation, the error of the background correction, and the error resulting from the reproducibility of symmetry-equivalent reflections that produced an internal Rvalue of 16% based on |F|2 (F = scattering amplitude) for all reflections. The sample preparation follows a standard procedure for copper single crystals based on an electrochemical etching route in highly concentrated phosphoric acid as described in more detail in refs 19, 26, and 32.

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RESULTS AND DISCUSSION A representative cyclic voltammogram (CV) of Cu(100) in 10 mM HCl is presented in Figure 1a. It was measured in the in situ X-ray diffraction cell22 and agrees well with published CV data.10,16,17,33 This CV defines the accessible potential window for Cu(100) in 10 mM HCl that is limited on the anodic side by the onset of the copper dissolution reaction and on the cathodic side by the starting hydrogen evolution reaction. The stability range of the chloride p(1 × 1) adlayer on Cu(100) can be best visualized by means of voltammetric measurements through varying the potential scan rate (Figure 1b). STM images of the p(1 × 1)-Cl adlayer on Cu(100) are presented in Figures 1c,d. This surface phase can be observed in the STM experiment in the potential regime between Ework = +200 mV and Ework = −120 mV as indicated in Figure 1b. Chloride desorption starting at Ework = −120 mV leads to the structural breakdown of the p(1 × 1)-Cl adlayer,15−17 leaving a disordered 2D liquid-like chloride phase on the copper surface behind.20 Tunneling tips easily remove laterally mobile chloride anions from the copper surface upon scanning, giving rise to the imaging of the Cu(1 × 1) substrate in the STM experiment.34 In full agreement with these findings, we observe a double layer regime between copper dissolution and hydrogen evolution that can be subdivided into two parts: the low capacity regime represents the stability regime of the p(1 × 1)Cl adlayer phase, while in the high capacity regime a disordered chloride phase is observed.19,20 In the presence of chloride, the copper substrate step edges are preferentially aligned parallel to the close-packed chloride rows along the ⟨100⟩ substrate directions16,17 that do not correspond to the energetically most favorable step orientation along the ⟨110⟩ substrate directions in the absence of any strongly adsorbing halide.35,36 This morphological feature is characteristic for any adsorbing halide on metal surfaces independently of whether the formed adlayer is commensurate11−17 or incommensurate37,38 with respect to the underlying metal substrate. Accordingly, we observe the disappearance of 11070

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Figure 1. (a) Cyclic voltammogram (CV) indicating the stability range of Cu(100) in 10 mM HCl. The CV was measured in the in situ XRD cell utilizing a hanging droplet configuration26 (scan rate: dEwork/dt =10 mV/s). b) Scan rate dependence of the CV in the double layer regime indicating the stability regime of the p(1 × 1)-Cl adlayer on Cu(100). CV1: dEwork/dt = 5 mV/s. CV2: dEwork/dt = 20 mV/s. CV3: dEwork/dt = 40 mV/s. CV4: dEwork/dt = 60 mV/s. CV5: dEwork/dt = 80 mV/s. CV6: dEwork/dt = 100 mV/s. (c) Representative STM image of the p(1 × 1)-Cl adlayer on Cu(100), 4.8 nm × 4.8 nm, dEwork = +150 mV, It = 1 nA, Ubias = 27 mV. The box represents the p(1 × 1) unit cell of Cl/Cu(100). (d) Step edge orientation along the substrate [001] direction, 9.4 nm × 9.4 nm, dEwork = +150 mV, It = 1 nA, Ubias = 47 mV.

Figure 2. (a) Experimental data and best fit of the models A, B, and C to the (00L) specular rod and the (20L) crystal truncation rod. The fits of the models cannot be distinguished within the width of the solid line. Ework = +150 mV. (b) Experimental data and best fit of the models A, B, and C to the (10L) adlayer rod in comparison to the model (dashed line) proposed by Gründer et al.,20 Ework = +150 mV. The fits of the models cannot be distinguished within the width of the solid line. The unit 2π/c is the length of the momentum transfer vector perpendicular to the surface, qz, with c = aCu = 3.61 Å as the lattice constant of Cu. The inset indexes the reciprocal lattice points in the (HK0) plane with reference to the conventional fcc unit cell of copper.

developed for the previous system hold also for the respective hydronium adsorption on the same halide lattice in the pure HCl electrolyte. However, notable differences between both structure models are expected for the interlayer spacings and the in-plane and out-of-plane exchange dynamics in the OHL and the interfacial water layer. We found three models that fitted equally well to the data. Distinctions between these models with reference to their lateral geometry cannot be achieved by measuring additional scattering data due to symmetry reasons. Simulations of data even far beyond our measurable range with reference to their vertical geometry showed that the difference in the structure factor for these three models is marginal in comparison to the experimental error. Depending on a particular model, they are based on 6 to 8 parameters in the structure factor (see eq 1: Goodness of Fit) that include in all cases the topmost Cu layer spacing dCu2−Cu1 (see Figure 3a), the separation of the chloride chemisorption layer from the Cu(100) substrate dCu1−Cl, the separation dCl‑SL between the IHL and the so-called “interfacial water layer” SL, and the spacing dSL‑CatL between the interfacial water layer SL and the so-called “cation” layer CatL as the outermost part of the interface (see Figure 3a). In Model C in Figure 3b, the CatL consists of split positions of interfacial water molecules in different adsorption geometries which leads

this morphological feature at potentials where the order− disorder transition sets in.34 Under these experimental conditions, the in situ STM experiment is only sensitive to the lateral ordering of specifically adsorbed anions in the IHL, while the in situ X-ray diffraction experiment provides comprehensive information on the full 3D structure of the entire interface including the near-surface layers in the solid electrode and the near-surface liquid electrolyte as well. Figure 2 contains the complete set of CTR, specular, and ADR data points represented by symbols with error bars. Our best fit to the measured structure factors is given by the solid lines. The underlying 2D surface diffraction pattern is depicted in the inset of Figure 2b. Each symbol indicates a cut through a rod parallel to the surface normal along which the scattered Xray intensity is distributed. The full circles and those with a cross represent CTRs, and the open circles indicate ADRs that originate solely from scattering contributions of the IHL and OHL without any additional contribution from the copper bulk material. Qualitatively, the presented data (Figure 2) are similar to those data reported by Keller et al.22 for the potassium cation adsorption on the same chloride layer. This similarity justifies the assumption that the basic features of the structure model 11071

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interlayer spacing reported by Gründer et al. (dCu1−Cl = 1.856 Å)20 and Huemann et al. (dCu1−Cl = 1.88 Å)19 who both did not include the OHL and interfacial water in their model, we observe an increase by 7%. Compared to the results reported by Tolentino et al.21 for the same lateral structure of Cl on Cu(100) under UHV (ultrahigh vacuum) conditions, the separation found for dCu1−Cl is even by 25.9% smaller than our result. The general trend of an expanded copper−halide interlayer separation consistently reported by numerous studies19,20,22 clearly indicates the tremendous impact of the OHL on the entire out-of-plane structure of the interface. The observed large copper−chloride interlayer spacings are in full agreement with the assumption that chloride anions retain almost their full charge in the electrochemical environment upon adsorption on copper. Saracino et al. used the potentialdependent copper−halide spacing as an indirect measure for the change of the charge state of the adsorbed halide that varies according to changes of the electrode potential.26 While polarizable bromide anions reveal a strong potential dependence in their bonding to the copper substrate, Saracino et al. further describe the charge state of the more electronegative chloride as being almost unaffected by changes of the electrode potential.26 Such a conservation of the chloride charge state is achieved in particular by attractive interactions between adsorbed chloride anions in the IHL and water dipoles together with solvated cations that accumulate in the OHL. It is this interaction pathway that prevents a significant charge transfer from the chloride into the metallic copper upon adsorption. The interfacial out-of-plane structure remains decisive by the metal−substrate interaction in conjunction with potential effects when bromide is involved, while anion/ cation pairing effects between ionic species in IHL and OHL are dominant when chloride is chemisorbed on the copper. A distinct difference to the potassium cation adsorption on the p(1 × 1)-Cl lattice concerns the interfacial water layer SL that has been introduced into the previous model as an intermediate layer that fits well between the chloride and cation layer which are separated by dCl−CatL = 3.43 Å.22 Keller et al. considered the interfacial water SL as an inherent part of the potassium solvation shell that remains partially intact upon adsorption on the chloride lattice on Cu(100). This has been concluded from the small spacing between the potassium cations and this interfacial water layer SL (dSL‑CatL = 1.30(7) Å) (see dashed line in Figure 3a) in conjunction with the accordingly larger distance of this interfacial water layer SL to the underlying chemisorbed halides in the IHL (dCl−SL = 2.13(11) Å). In contrast to this, we determine for the 10 mM HCl a smaller halide/water spacing of dCl−SL = 1.70(5) Å and a slightly larger water/cation layer distance of dSL−CatL = 1.81(5) Å (Figure 3). These findings point to chloride anions in the IHL that undergo a stronger solvation with interfacial water in the pure hydrochloric acid than in the potassium containing electrolyte where the interfacial water obviously coordinates more strongly with the metal cations accumulated in the OHL. The similar values of the interlayer separations dCl‑SL and dSL‑CatL point to interfacial water species that are commonly shared as coordinating ligands by the anionic IHL and the cationic OHL. Note: In this simple picture, we correlate the experimentally found bond length or layer separation with the apparent bond strength.

Figure 3. Full 3D structure model of the metal/electrolyte interface of Cu(100) in (a) out-of plane structure (side view) (the dashed line indicates the position of the screening layer SL in 10 mM KCl)22 and (b) models A, B, and C of the in-plane-structure (top view). The dashed rectangles indicate the size and position of the Cu bulk unit cell. The numbers on the oxygen atoms indicate their occupancy.

for this model only to a seventh fit parameter. An additional parameter is introduced to fit the x-component of the SL2 part of the interfacial water layer (see Figure 3b) for Models B and C. x- and y-components for the SL2 oxygen atoms depend on each other due to symmetry conditions. A further fit parameter is the layer occupancy ΘCl. The occupancies Θ SL1 and Θ SL2 of the two symmetrically independent water species SL1 and SL2 that form the interfacial water layer SL and the occupancy of the “cation” layer ΘCatL are directly dependent on ΘCl and constitute no extra fit parameters (ΘCatL = ΘCl and ΘSL1+2 = 2ΘCl). The dependency is derived from the assumption that only the presence of Cl induces a notable ordering in the electrolyte above. Keller et al. identified the coupled system of the interfacial water layer and cation layer in the Cl−/water/K+ system as the outer Helmholtz layer (OHL) and the interfacial water belonging to the K+ solvation shell.22 Finally, a scaling factor and an atomic displacement parameter (ADP) are needed to describe the data sufficiently. The meaning of the latter will be discussed further below. The copper−halide interlayer separation at a given electrode potential of Ework = +150 mV remains largely unaffected by slight compositional changes in the OHL by going from potassium-containing to potassium-free electrolytes.22 In the present study, we found a spacing of dCu1−Cl = 2.00(4) Å (Table 1) that is identical within the experimental error to the one reported in the previous work for the p(1 × 1)-Cl phase in the presence of potassium cations.22 Compared to the dCu1−Cl 11072

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Table 1. Fitting Results: Structural Parametersa model A dSL−CatL dCl−SL,1 dCl−SL,2 dCu1−Cl dCu2−Cu1 dCu−Cu

a

1.00 (4) 0.94 (3) 1.11 (2) 1.012 (2) 1.000

model B

model C

layer spacings dx‑y: in relative coord. of a/2 (=1.805 Å), in [Å] 1.81 (7) 1.00 (4) 1.81 (7) 1.70 (5) 0.88 (3) 1.56 (5) 2.00 (4) 1.11 (2) 2.00 (4) 1.830 (4) 1.012 (2) 1.830 (4) 1.805 1.000 1.805 relative coordinates of atoms within unit cell in units of a (=3.61 Å)

x

y

z

Cu1,1 Cu1,2 Cu2,1 Cu2,2 Cl SL1,1 SL1,2 SL2,1 SL2,2 SL2,3 SL2,4 CatL

0 0.5 0.5 0 0 0.5 0 0.5

0 0.5 0 0.5 0 0 0.5 0.5

0 0 0.506 (1) 0.506 (1) 2.06 (1) 2.53 (1) 2.53 (1) 3.03 (2)

ΘCl = ΘH3O+ ΘSL1,ΘSL2

0.45 (5) 0.9 (1), 0.0

ADP (H3O+)

19 (3)

x

y

0 0.5 0.5 0 0 0.25 0.75 0.25 0.75 0.5

z

0 0 0.5 0 0 0.506 (1) 0.5 0.506 (1) 0 2.06 (1) (3) 0.25 (3) 2.50 (1) (3) 0.25 (3) 2.50 (1) (3) 0.75 (3) 2.50 (1) (3) 0.75 (3) 2.50 (1) 0.5 3.00 (2) layer occupancy Θ [ML] 0.45 (5) 0.0, 0.9 (1) atomic displacement parameter ADP [Å2] 19 (3)

1.00 (4) 0.94 (3) 0.88 (3) 1.11 (2) 1.012 (2) 1.000 x

0 0.5 0.5 0 0 0 0.5 0.25 0.75 0.25 0.75 0.5

(3) (3) (3) (3)

1.81 (7) 1.70 (5) 1.56 (5) 2.00 (4) 1.830 (4) 1.805 y

z

0 0.5 0 0.5 0 0.5 0 0.25 (3) 0.25 (3) 0.075 (3) 0.75 (3) 0.5

0 0 0.506 (1) 0.506 (1) 2.06 (1) 2.53 (1) 2.53 (1) 2.50 (1) 2.50 (1) 2.50 (1) 2.50 (1) 3.00 (2)

0.45 (5) 0.45 (5), 0.45 (5) 19 (3)

The number in brackets indicates the error to the value next to it with reference to its last digit.

A considerable extension of our existing structure model22 consists of the potential existence of two further structure motifs of the interfacial water layer SL that fit both equally well to the experimental data (Table 1, Figure 3). Independently of the particular structure motif, we determine a total occupancy in the interfacial water layer of ΘSL1+2 = 0.9 ML that doubles the one of the anions in the IHL (Table 1). Normalized to a single surface unit cell (Figure 3b) we expect two water molecules being present in the SL layer per the unit cell in all models (blue dots in Figure 3). The in-plane structure of Model A is identical with the model recently introduced by Keller et al. for the potassium cation adsorption on p(1 × 1)-Cl where water populates exclusively bridge sites of the chloride lattice (Table 1).22 Model A includes also the occupancy of two water species per unit cell that reside in all four adsorption sites. The probability P equals 1 to observe a water molecule at the bridge site and equals 0 to observe a water species at any other interstitial site shown in structure Models B and C. Hence, there is only one possible configuration of water molecules allowed for this structure motif. By this, the p4mm plane symmetry of the underlying chloride lattice is inherently conserved by the interfacial water layer. The coordination number of the central cation in the 4fold hollow site would be 4. In structure Model B, two of four possible interstitial adsorption sites are populated to fulfill the boundary condition of two water species per unit cell. Accordingly, the probability P equals 0.5 to observe a water molecule in one of the four interstitial sites at (x, x, z), (x,̅ x,̅ z), (x,̅ x, z), and (x, x,̅ z), with x = 0.25(3) and z = 2.50(1) in relative units of aCu (Table 1). The probability to observe a water species at a bridge site equals zero. In total, six different configurations of water

molecules can be realized on the basis of this structure motif. Conservation of the p4mm symmetry requires the averaging over all six possible configurations with time. Surface dynamics in terms of direct lateral site exchanges or exchange processes via the OHL or the bulk solution phase are the crucial prerequisite for such symmetry conservation as observed by the SXRD experiment. The coordination number of the central cation in the 4-fold hollow site would be 2. Structure Model C represents a combination of A and B where two bridge sites (Model A) and just one of the four interstitial sites (Model B) are populated. In total, 24 different configurations of structure motif C are equally possible. An individual configuration of structure motif C breaks the p4mm symmetry similar to structure motif B. The p4mm symmetry is again conserved due to fast interchange processes between all different configurations of this structure motif with time. The coordination number of the central cation in the 4-fold hollow site would be 3. Key for the development of our structure models is the (10L) ADR. The fitting of the intensity distribution along the (10L) ADR succeeds in all cases only by considering an additional scattering center above the 4-fold hollow sites of the chloride lattice (Figure 3). It is the absence of this extra scattering center in the structure model proposed by Gründer et al.20 that explains the dissatisfying agreement between fit and experimental data of the (10L) ADR in particular for qz-values up to a momentum transfer of qz = 2.0.20 The observed oscillatory modulation of the intensity along the (10L) ADR and the low intensity at small qz-values cannot be rationalized solely on the basis of subsurface buckling effects as proposed by Gründer et al. for this Cu(100)/Cl/electrolyte system.20 It turns out that the characteristic intensity distribution along the 11073

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ADRs is typical for centered double adlayer systems (here IHL and OHL) where the position of their broad peaks determines the layer separation, while the peak height reflects their chemical nature (scattering power) and occupancy. In our analysis, no subsurface buckling could be detected since the involved displacements of Cu are too small to adjust or to improve the fit of our model significantly. The “subsurface buckling effect” is clearly overruled in the present case by the dominating scattering contribution originating from the OHL. It is this unique feature of the electrochemical environment that actually explains the pronounced differences to the ADR intensity distribution reported by Tolentino et al. for the respective Cu(100)/Cl/vacuum interface21 where the oscillatory behavior is indeed governed by slight subsurface buckling effects. In full agreement with the above argumentation is our experimental observation that the measured (10L) ADR of the corresponding p(1 × 1)-Br at the Cu(100)/Br/electrolyte interface26 resembles more the characteristics of the Cu(100)/ Cl/vacuum21 than those of the Cu(100)/Cl/electrolyte interface.20,22 Bromide is assumed to be largely discharged in the adsorbed state at the Cu(100)/Br/electrolyte interface, at least under saturation conditions at high potentials. Accumulation and lateral ordering of water and counter cations in the OHL are therefore less pronounced for the bromide- compared to the chloride-modified Cu(100) electrode surface,26 at least if we consider potassium and hydronium adsorption. A recent in situ XRD study on the Ag(100)/Br/electrolyte interface by Nakamura et al.23 could indeed prove cation adsorption in the OHL in the case of weakly solvated Cs+, while no cation adsorption was seen in the case of strongly solvated Li+. Both the strong oscillation in the ADR and the resulting structure model for the Cs+ adsorption on the p(1 × 1)-Br phase on Ag(100) are qualitalitely similar to our previous and present XRD work on Cu(100)/Cl with solvated cations residing in 4-fold hollows of the anion lattice. Slight oscillations visible in the bromide ADR when Li+ was present were assigned by Nakamura et al. to subsurface buckling effects in the second substrate layer similar to the results in the XRD study by Saracino et al.26 Therefore also the XRD work by Nakamura et al.23 clearly confirms the existence of two extreme cases where the oscillatory behavior in the ADRs is dominated by either the ordering of solvated cations in the OHL (large amplitude) or the subsurface buckling effect in the absence of cation adsoption in the OHL (small amplitude). This comparison of the state-of-the-art in situ XRD work on the halide adsorption on Cu(100) and Ag(100) clearly demonstrates that the in-plane and out-of-plane ordering of solvated cations next to an anion-modified metal electrode has to be considered as a specific adsorption phenomenon and clearly not as a nonspecific one. The resulting interfacial structures depend on both the symmetry characteristics of the solid substrate (anionic IHL) and the chemical nature of the cationic species that accumulates in the OHL. In the pure hydrochloric acid solution, only the oxygens from water or hydronium cations can serve as additional scattering centers in the OHL. Therefore, the mixed hydronium/ potassium cation layer in the model by Keller et al.22 has to be replaced in the present case by a water/hydronium layer (Figure 3). The cation layer occupancy ΘCatL matches then the one of the anion layer ΘCl. The separation between the chloride and this cation layer CatL is slightly higher (dCl−CatL = 3.51 Å, Table 1) compared to the one reported for the interface

modified by potassium cation adsorption on Cu(100) (dCl−CatL = 3.43 Å).22 It should be explicitly noted at this point that the XRD experiment cannot discriminate between water molecules and hydronium cations. Our fit procedure considers only oxygen as a scattering center, while hydrogen is not taken into account. Therefore, we cannot unambiguously assign the hydronium cations to adsorption sites in the so-called “interfacial water layer” SL or in the so-called “cation layer” CatL (Figure 3). The interfacial structure in the potassium-free electrolyte is therefore better described by a mixed water/hydronium bilayer system (Figure 3). In contrast to other interfacial water bilayer systems reported in the literature,39 the structural properties of this bilayer are in the present case clearly controlled by the templating chloride lattice whose p4mm symmetry is transferred into the OHL as the outermost part of the near surface liquid electrolyte. For various transition metals, hexagonal bilayers of water are described that adopt icelike structure motifs.39 Water bilayers adsorbed on metallic substrates are mainly governed by interwater H-bonding, while the substrate− water interactions are commonly described just as weak, with binding energies ranging from −0.1 to −0.4 eV.40 The interaction of interfacial water with the anionic IHL and the cationic OHL, however, can be considered as much stronger compared to those water−metal interactions. This explains the profound disturbance of the common hexagonal water bilayer motif by the anionic and cationic adsorbents at the interface and the resulting p4mm symmetry of the water layer that adopt the symmetry properties of the coupled anion/cation system. We should not consider the particular structure of the OHL and the interafcial water layer as a static one with water and hydronium cations being fixed on their adsorption sites but as a highly dynamic one. A crucial prerequisite for the experimentally observed symmetry transfer from the chloride lattice into the near surface electrolyte as observed in the scattering experiment is the huge exchange dynamics of hydronium cations and interfacial water. These exchange dynamics include fast adsorption/desorption processes with the bulk solution phase along the surface normal as well as direct in-plane site exchange processes. Therefore, we assume that the apparent symmetry transfer from the halide lattice into the near surface electrolyte is the result of an averaging over huge numbers of individual configurations which intrinsically do not reveal p4mm symmetry (with the exception of structure motif A in Figure 3b). The quality of our model and the statistical significance of the results are expressed by the Goodness of Fit (GoF) that equals 0.9 for all three models. It is defined by GoF =

1 N−P

exp calc 2 2 − Ihkl ) /σhkl ∑ (Ihkl hkl

(1: Goodness of Fit) calc with Iexp hkl and Ihkl as the observed and calculated intensity of each reflection (hkl). σhkl is the standard deviation of Iexp hkl ; N is the number of data; and P is the number of fitted parameters. A GoF of 0.9 implies that the extent of the match between experimental observations and estimates is in accord with the error variance. Within the range of the errors and for the given set of fit parameters, there is then barely any more information contained in the data which would change a particular model significantly, for instance by introducing a subsurface buckling. As a consequence, a distinction between the three models is not possible since differences in their agreement of the fit to the

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cations are introduced into the system. Prime examples hereof are divalent viologen cations (1,1′-dibenzyl-4,4′-bipyridinium cations) which adsorb on the same p(1 × 1)-Cl lattice also with a partly intact solvation shell.41,42 The lateral order in the OHL is in any case connected to the structural integrity of the IHL. The breakdown of the anion structure is commonly associated with the disappearance of positional and translational order in the OHL.43,34

data are statistically not meaningful and, therefore, negligible. Each model was fitted to all data simultaneously by using the following kinematic approximation, calculating its intensity contribution for all experimental (hkl) reflections according to I(q) = S Cu bulk

(1 − e−2πiq3 − μ)−1 ∑ fCu (q)e 2πiqr m

2

+

m

∑ fCu (q)e

surface 2πiqr Cu n

Cl

+ 2ΘClfCl (q)e 2πiqr +

■

CONCLUSIONS

■

AUTHOR INFORMATION

This study provides a significant extension of a new finestructure model of the electric double layer by considering the lateral ordering of interfacial water and coadsorbed cations that are attracted in the outer Helmholtz layer (OHL) by chemisorbed anions in the inner Helmholtz layer (IHL). Such a centered bilayer system (structurally coupled IHL and OHL) was necessary to reproduce all surface sensitive diffraction data, especially the adlayer and specular data which are most sensitive to the present interfacial structure. Stronger solvation of chloride anions in the IHL by interfacial water takes place in particular in the absence of metal cations in the OHL as exemplarily demonstrated for the 10 mM HCl solution. Layer separations between the chemisorbed chloride lattice and the interfacial water layers are by 20.2% smaller compared to the respective chloride water spacing in the presence of the potassium cation in the OHL. The interfacial water layer is shared by the anions in the IHL and the cations in the OHL. Interfacial water serves as complexing ligands for the anion and cation solvation shells at the interface. This is the major and most important difference from the previously studied Cl/water/K+ system where the interfacial water was interpreted as an inherent part of the alkali metal cation solvation shell. The accumulation of nonchemisorbing species in the OHL has to be considered as a specific adsorption although the binding energy of solvated cations and solvent molecules is presumably smaller than the one of the specifically adsorbed anions that are in direct contact to the metallic substrate. The particular structure of the OHL and the interfacial water layer clearly depends on the chemical nature of the chemisorbed anions, their lateral structure on the metallic substrate, and chemical nature of the adsorbing cations. In this sense, we denote the observed phenomenon as specif ic cation adsorption on the layer of specifically adsorbed anions.

n 1 ⎡ ⎤ 2πiqr SL p + ⎥ ⎢∑ ΘSL1fH2O (q)e ⎢ p ⎥ ⎢ ⎥ 2 2 Θ SL e−ADP·q /(4π ) 2ΘCl⎢∑ SL 2 f (q)e 2πiqr o 2+⎥ ⎢ ⎥ 2 H 2O ⎢ o ⎥ + ⎢ ⎥ 2πiqr H3O ⎢⎣ fH3O+ (q) ·e ⎥⎦

(2: Structure Factor)

where I(q) is intensity, S scaling factor, q momentum transfer, qz momentum transfer perpendicular to the sample surface, μ attenuation factor, f x(q) atomic form factor of atom x, rx vector in relative coordinates pointing from the origin of the unit cell to atom x, ADP atomic displacement parameter in Å2, Θx occupancy of atom x in the ML,25 and ΘSLx occupancy of oxygen SL1 or SL2 in ML.25 As a physically meaningful parameter, we introduced an isotropic “atomic displacement parameter” (ADP) (eq 2: Structure Factor and Table 1) into our structure model as a generic term that not only includes the thermal motion of the adsorbed species in the OHL and the interfacial water layer but rather comprises the huge in-plane and out-of-plane exchange dynamics of those nonchemisorbed particles as a dominant contribution to this fit parameter. Similar to the thermal motion, these reversible adsorption/ desorption and lateral site-exchange processes represent also distinct displacements of scattering centers (oxygens in the present case) as function of time, thus leading to an attenuation of the scattered X-rays and by this to an increase of the diffuse background intensity under a sharp Bragg peak. It was sufficient to fit only one isotropic ADP for the oxygens of the hydronium and the interfacial water layer together to achieve a good agreement between experimental data and fit. In the present case, this atomic displacement parameter reveals a value of ADP = 19 (3) Å2 (Table 1) which is notably higher than the one reported for the potassium containing OHL with ADP = 8.4(9) Å2 for similar experimental conditions.22 Obviously, the activation barriers for siteexchange processes are much higher when potassium metal cations are present in the OHL. From this comparison, it becomes evident that not only the “integrated” interfacial structure on the electrolyte side is strongly affected by the chemical nature of the anionic and cationic species involved into the bilayer formation but also the in-plane and out-of-plane dynamics. However, both the potassium-containing and the potassiumfree interfaces represent a particular kind of double-layer structure where the cationic part (OHL) still remains liquidlike. A 2D condensation of solvated cations in the OHL is, in contrast to that, observed when larger and more hydrophobic

Corresponding Author

*E-mail: [email protected]. Phone: ++49 711 685 65264. Fax: ++49 711 685 55264. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS

The authors gratefully acknowledge the excellent support by Steven Leake, Michael Lange, Dominik Meister, and the beamline manager Philip Willmott at the beamline X04SA of the SLS. The support by Peter Königshoven, Rolf Backhausen, and Martin Böhmer who contributed to the build-up of the SXRD cell is gratefully acknowledged. We also thank the Swiss National Foundation and in particular the copper plating team at BASF SE (Electronic Materials/CAE) for financial support. 11075

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(34) Pham, D. T.; Tsay, S.-L.; Gentz, K.; Zoerlein, C.; Kossmann, S.; Tsay, J.-S.; Kirchner, B.; Wandelt, K.; Broekmann, P. J. Phys. Chem. C 2007, 111, 16428. (35) Giesen, M. Prog. Surf. Sci. 2001, 68, 1. (36) Steimer, C.; Giesen, M.; Verheij, L.; Ibach, H. Phys. Rev. B 2001, 64, 085416. (37) Cuesta, A.; Kolb, D. M. Surf. Sci. 2000, 465, 310. (38) Broekmann, P.; Spaenig, A.; Hommes, A.; Wandelt, K. Surf. Sci. 2002, 517, 123. (39) Schnur, S.; Gross, A. New J. Phys. 2009, 11, 125003. (40) Michaelides, A. Appl. Phys. A: Mater. Sci. Process. 2006, 85, 415. (41) Safarowsky, C.; Wandelt, K.; Broekmann, P. Langmuir 2004, 20, 8261. (42) Breuer, S.; Pham, T. D.; Huemann, S.; Gentz, K.; Zoerlein, C.; Hunger, R.; Wandelt, K.; Broekmann, P. New J. Phys. 2008, 10, 125033. (43) Pham, D. T.; Keller, H.; Breuer, S.; Huemann, S.; Hai, N. T. N.; Zoerlein, C.; Wandelt, K.; Broekmann, P. Chimia 2009, 63, 115.

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