Layering and Ordering in Electrochemical Double Layers

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Layering and Ordering in Electrochemical Double Layers Yihua Liu, Tomoya Kawaguchi, Michael S Pierce, Vladimir Komanicky, and Hoydoo You J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00123 • Publication Date (Web): 26 Feb 2018 Downloaded from http://pubs.acs.org on February 26, 2018

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The Journal of Physical Chemistry Letters

Layering and Ordering in Electrochemical Double Layers Yihua Liu†,¶, Tomoya Kawaguchi†, Michael S. Pierce,§ Vladimir Komanicky, ‡ Hoydoo You*,†

†

Materials Science Division, Argonne National Laboratory, Argonne, Illinois, 60439, United

States. §

Rochester Institute of Technology, School of Physics and Astronomy, Rochester NY, 14623,

United States. ‡

Faculty of Science, Safarik University, Kosice, Slovakia.

AUTHOR INFORMATION Corresponding Author *[email protected]

ACS Paragon Plus Environment

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ABSTRACT Electrochemical double layers (EDL) form at electrified interfaces. While Gouy-Chapman model describes moderately charged EDL, formation of Stern layers was predicted for highly charged EDL. Our results provide structural evidence for a Stern layer of cations, at potentials close to hydrogen evolution in alkali fluoride and chloride electrolytes. Layering was observed by x-ray crystal truncation rods and atomic-scale recoil responses of Pt(111) surface layers. Ordering in the layer is confirmed by glancing-incidence in-plane diffraction measurements.

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Electrochemical interphase, the interfacial region close to an electrode-electrolyte interface, encompasses an electrode surface, solvents, and solutes. All faradaic reactions occur in the interphase and the structure of the interphase is critical to the reaction processes. As the electrode surface is charged by the external potential or chemical modification, cations, anions, and electrolyte molecules all redistribute and rearrange to form electrochemical double layers (EDL) by minimizing the overall potential energy. Gouy and Chapman (GC) model1 is by far most successful in describing the diffuse ion distributions and has been experimentally verified for charged phospholipid interfaces2 and for liquid-liquid interfaces under electrochemical potential control3. However, the GC model breaks down at large potentials, due to its prediction for the unlimited rise in differential capacitance, which is unphysical for the finite sizes of the ions and molecules. Stern has suggested that there should be a layer with a finite ion density, known as ‘Stern layer’4. Numerous theoretical improvements5-6 were proposed and experimental investigations of the Stern layer have continued until today7-8 in electrochemical systems and in mineral interfaces9-10. Crystal truncation rods11-12 (CTR) is a powerful x-ray technique for electrode surface structures13-14. Its high sensitivity was also used to determine the structure of light molecules on metallic electrodes, such as water layering15-16. The sensitivity comes from the anti-Bragg region of CTR, which is weak but extremely sensitive to surface structures. The sensitivity to light elements can be further enhanced by modeling the CTR normalized by a standard CTR17 because the normalization eliminates the insensitive but dominantly strong near Bragg CTR intensities. The standard CTR is the CTR of a standard state, e.g., that of clean Pt(111) surface. When the crystal electrode substrate is weakly disturbed by the formation or redistribution of EDL, the CTR changes are expected to occur mainly near the weak anti-Bragg regions.


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Figure 1. (A) CTR data at various potentials normalized by the CTR at 400 mV are shown with the fits. All potentials are recorded using Ag/AgCl reference electrodes. (B) Density profiles of the four layers (α-δ) obtained from the fits. The distances are measured from the top Pt(111) layer. The water background density is subtracted. (C) The integrated electron densities of the layers (α-δ) are plotted vs. potential. The density of a Pt layer is 10.1 e−/Å2. (D) The ϕ scans are shown for the in-plane peaks, (0.5 0.5 0.33) and (0.5 0 0.33), and an anti-Bragg (0 1 0.5), whose integrated intensities are 4.5(1), 4.7(1), and 170(2) × 103 counts⋅deg, respectively. The upper inset: the real space model is shown with (1×1) and (2×2) unit cells marked by blue and red parallelograms, respectively. The open circles are Pt and the solid circles are (2×2) positions. The large circles represent Cs+ and small (pink) circles available sites for water. The lower inset: the in-plane reciprocal space is shown with the red arrows for unit vectors and the green arrows for the ϕ scan directions. In this report, the structure of Stern layer was investigated for Cs+ ion distributions on Pt(111) surface in 0.1 M CsF aqueous electrolyte. The experimental details are given in supplementary materials18. The CTR data for 5 potentials, normalized by the CTR at 400 mV (vs. Ag/AgCl), are


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shown in Figure 1a. The unnormalized CTR data shown in Figure S218 exhibit only marginal differences between the potentials. However, the normalized CTR data show very clear and systematic differences between potentials. At 400 mV, the Pt(111) surface remains closest to an ideal surface. It is the potential below the place exchange oxidation17, 19. OH− is known to adsorb in acid17 and F− may accumulate near the surface at 400 mV. However, their electron densities are close to that of water and should have little effect on the CTR, even if they adsorb to the substrate. In fact, the cyclic voltammogram in Figure S1c shows little evidence for OH− or F− adsorption and the Pt(111) at 400 mV should be a good standard surface. There were relaxations of Pt(111) surface lattice planes depending on the potentials as shown in Table S118. Our preliminary study indicated that the lattice relaxations do not impede20 our structure determination of Cs+ distributions. This is because the lattice relaxations affect mostly intensities near Bragg peaks, (0 0 3) and (0 0 6), while the Cs+ distributions change the intensities near antiBragg regions between the Bragg peaks. There were steps on the Pt(111) surface with the mosaic distribution of ~0.1°. However, the surface steps do not change within the potential range of this study and do not affect the CTR modeling either. In analyzing the normalized CTR data, a direct inversion method using Hilbert transformation20 was developed and the result was used as the guide in fitting the data. In the direct inversion, three narrow layers and one broad layer were identified. Therefore, a four-layer model was constructed and further refined iteratively by fitting to the normalized CTR shown in Figure 1a. The fits using only three layers, shown in Figure S3, clearly miss the data between L=3 and L=4, validating the importance of weak layers, α and γ. The results of the four layer fits are plotted in Figure 1b and the integrated charge densities of the layers vs. the potentials are shown in Figure 1c. The sharp dense layer at 3.5 Å from the top Pt layer must be the Cs+ layer


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since nothing else in the electrolyte can justify the high density. The thin broad layer, δ, at ~15 Å is also from Cs+ since no other elements can double the water density over a nanometer. When we consider only the Cs+ layers, β and δ, they are qualitatively similar to the result of the molecular dynamics study21-22 and a self-consistent quantum hard-sphere model5. The density of β decreases as the potential increases. At 0 mV, the density drops suddenly while the broad layer, δ, persists to 200 mV. Note that a single diffuse layer based on GC model cannot produce the oscillations seen in the data even at this potential, indicating that there is a considerable discrepancy between this study and GC model. In-plane diffraction scans in Figure 1d show half-integer peaks consistent with a singlesublattice (2×2) structure. To reduce the background scattering of water, the surface was emersed from the electrolyte at −850 mV23. The (0.5 0.5 0.33) peak was measured within ~30 sec after the emersion. The peak was similar to the peak measured in situ in electrolyte23 but better defined due to the lower background. The peak intensity increases over time after emersion because the in-plane structure changes eventually to the two-sublattice (2×2) structure23. Peak (0.5 0 0.33) was more difficult to distinguish from the background in situ, and it was measured slowly over several minutes for better counting statistics because its intensity did not change significantly after the emersion. (0.5 0.5 0.33) and (0.5 0 0.33) intensities are compared to antiBragg (0 1 0.5) intensity. If the (2×2) structure were as well ordered as Pt(111) surface atoms, their integrated intensities would have been ~10% of (0 1 0.5)23. They are at ~3%, or at ~30% of the well-ordered Cs+ layer, indicating the significant disorder in the (2×2) structure. The correlation length estimated from the peak widths is ~30 nm. The density of layer β in Figure 1b is also consistent with the single-sublattice (2×2) structure. The density of a Pt monolayer is 10.1 e−/Å2 and the well-ordered single-sublattice (2×2) Cs+


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layer would have been 1.78 e−/Å2. Integrated electron density of layer β at −850 mV is 0.60(1) e−/Å2 (Figure 1c). This 30% of the ideal value agrees well with the estimation made with the in plane peaks. This means, on average, 30% of the layer is ordered at a given time. This indicates that Stern layer is highly fluctuating, short-range ordered, and dynamically forming above the solid crystal substrate. This further indicates that the energy gain by the lateral ordering, albeit short-ranged, is essential and possibly even necessary to the formation of Stern layer. Additional evidence for the cation monolayers comes from dynamic responses of Pt lattice, measured at a point of CTR sensitive to the lattice expansion, to potential jumps. The experiments were carried out with several alkali cations to show the monolayer formation is not just limited to Cs+ ion. Alkali chlorides were used because high purity chemicals were readily available for the chlorides than fluorides18. The (2×2) in plane scans were reproduced in CsCl solution demonstrating that the Cs+ monolayer structure discussed earlier is unaffected by the chloride electrolytes23. The normalized CTR’s were not studied in CsCl because Cl chemisorbs at high potentials and Cl layer can significantly alter the Pt(111) CTR. The Cl monolayer formation is evident in Figure S418. When a potential is set to −850 mV from an open circuit condition, the time response is dominated by double layer formation. The electrons rush to the interface as cations diffuse toward the interface and anions away from the interface until the charges at both sides of the interface balance. The current transient response of Pt(111) in 0.5 M NaCl electrolyte to the potential jumps, shown in Figure 2a, was fit to the sum of two simple exponential functions. A single exponential function did not fit the curve. This indicates at least two, one fast and one slow, relaxation processes are involved in the current transient: a1exp(−t/τ1)+a2exp(−t/τ2), where a1=2.53 mA/cm2, τ1=0.026 sec, a2=0.95 mA/cm2, and τ2=0.88 sec. The fast component results


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from the rearrangement of the cations, anions, and water dipoles near the interface and the slow component describes the formation of ion distributions requiring long distance diffusion. It can also involve faradaic reactions such as H underpotential deposition (H-UPD) (Figure S1). As shown in the inset, this process is completely repeatable. The two-exponential fit cannot fully capture the dynamic response of the double layer formation. Rather, it simply provides


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timescales of the ionic responses to the external potential applied. When a potential is jumped to a positive potential from −850 mV, the reversal of the ionic redistributions must occur. The dense cation monolayer formed and trapped by the large electric field will suddenly be freed. Since the whole monolayer departs simultaneously with a

Figure 2. Data and fits for the potential jump experiments. (A) The current spike was measured for the potential jump from −850 mV to 0 mV in 0.5 M NaCl. The red line is a fit to the sum of two exponential curves. Repeated potential jumps from 0 to −850 mV and back to 0 mV are also shown as the inset. (B) The intensity at (0 0 3.3) is monitored while the potential is cycled from −850 mV repeatedly to 0 mV with 50 mV increment. The potential was held at −850 mV for 10 sec before it was jumped to the potentials. (C) The steady-state intensities at the potentials to which the potential was jumped (black solid line) and at −850 mV when it was returned from each jump (the red solid line). (D) The time constants estimated from the x-ray intensity spikes at (0 0 3.3)hex measured in 0.1 M CsCl, 0.5 M KCl, 0.5 M NaCl, and 0.5 M LiCl (E-H) and the fits with the masses of fully hydrated cations (red) and to partially hydrated cations (blue).


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momentum gained by the electric field change, an atomic scale recoil force pressures the top Pt(111) surface layers to contract. The x-ray intensity at Pt(0 0 3.3), sensitive to the Pt(111) surface layer compression, is shown in Figure 2b. The transient behavior of the intensity was measured in a series of potential jump from −850 mV to 0 mV in 0.5 M NaCl. The potential was held at −850 mV for 10 sec and jumped to the indicated potentials in 50 mV incremental intervals, then returned to −850 for 10 sec and jumped again. In Figure 2c, the steady-state intensity is shown without the spikes as a function of the applied potential (black) and the intensity measured as the potential is returned to −850 mV (red). The steady-state intensity is mainly of the Pt(111) layer relaxations due to the H adsorption at low potentials and the Cl adsorption at high potentials

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. Qualitatively similar results were obtained in LiCl and KCl

electrolytes. The spikes present at all potentials positive to −550 mV. The spikes also present in an equivalent off-specular (1 0 4.3) shown in Figure S5, indicating that the intensity spikes occur due to the sudden contraction of the Pt top layer. The fits to the ‘spikes only’ CTR for Cs+ and Li+ in Figure S6 show that the spikes are indeed from the 1.6% contraction of the Pt top layer. In Figure 2e-h, the spikes measured at −400 mV for the four electrolytes are presented. The spikes appear in all electrolytes studied and the spike heights were all similarly ~10% of the steadystate intensities at (0 0 3.3). However, the widths of the spikes depend systematically on the masses of the cations as shown in Figure 2d. The recoil force occurs when the impulse given to the cations by the potential jumps is sufficient for the cation layer to escape from the surface. The average escape velocity, ν, of the cations in an impulse approximation is e∆E·δt / M where e and M are the charge and mass of the cation or cation complex, respectively, ∆E is the electric field change, and δt is the transient


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time. The impulse, e∆E·δt is independent of cations because the cations are all single-charged and under the identical transient field. Although the whole layer starts escaping simultaneously, the individual cations will eventually disperse stochastically. It results in a slow increase of the width of the layer as the layer moves away from the surface, which can be described by the Einstein equation24. Since the cations diffusing toward the Pt surface will reflect, the equation is modified to:

ρ ( x, t ) =

−

N 2π (σ 2 + 2 Dt )

e

( x −νt )2 2 (σ 2 + 2 Dt )

Eq. (1)

[1 + sgn (x )]

where σ is the initial width of the layer, D is a diffusion coefficient, and x is the distance at time t from the original position of the layer at t = 0. The width of the initial layer is small since it is trapped under a large electric field. Then the density at x=0, where the cations deliver the recoil pressure, decreases in time with a modified exponential decay, ρ (0, t ) ≈ time constant

τ=

4D

ν

2

=

4 DM 2 (e∆E ⋅ δt )2

by neglecting the slowly varying term,

ν2

− t N e 4D 4πDt

1 / 4πDt

with the relaxation

. This shows that τ is

roughly proportional to DM2. This M2 dependency is a unique result of the diffusive recoil process outlined above. The time constants vs. DM2 is plotted in a log-log scale in Figure 2d. In the log-log scale, the slope should be 1 (blue solid line) to satisfy the relationship. The literature values25 for the freesolution diffusion constants of the alkali ions were used. The values were similar among the alkali ions and do not change the overall behavior in Figure 2d. The masses of fully hydrated ions26 are used for the red open circles. The slope is much larger than the blue line. To make the data agree with the blue line, a partial hydration (30%) was assumed, which suggests the ions leave some water molecules behind. The choice of 30% based on the Cs+ model may not be accurate uniformly for different cations. Nonetheless, the blue line with the partial hydration


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works reasonably well and the recoil compression of the Pt(111) surface layer is a powerful additional evidence for a dense layer formation at the interface. The recoil compression is a general behavior for cations and anions, even though some are too fast to measure with the time resolution (~0.1 sec) of the detectors currently available. An example with ClO4− anion is shown in Figure S7 where only single high point observed in the spikes.

Figure 3. (a) A 3D view of the proposed (2×2) ball model at −850 mV. Pt top-layer atoms are shown by the large grey balls. Cs+ ions, layer β in Figure 1b, are shown by the blue balls. Layers α and γ in Figure 1b are the hydration layers. The red and pink balls represent the water molecules. The water molecules, especially the top (pink) ones, are likely disordered. (b) Top view. The arrows indicate the directions of the side views: (c) vertical (d) horizontal.

Nature of interfacial water molecules surrounding cations have been the subject of discussion in recent studies27-28. Therefore, one can postulate that the weak layers, α and γ, in Figure 1b are the interfacial water layers. With the in-plane scans alone, it is difficult to determine whether water molecules are ordered and, if ordered, where they are. It is similarly difficult only with the


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CTR scans that are sensitive only to the out-of-plane density distribution. However, when the inplane and out-out-plane results are combined, the structure becomes clear. The position of layer β in Figure 1b indicates that there is no room for the intervening water molecules between the Cs+ layer and the top Pt layer. The fit value of 3.5 Å is even smaller than the previously suggested value of 4.1 Å28, making it impossible for any water or hydroxyl species to fit between the Cs+ and Pt. Instead, we propose that the water molecules occupy the intra-layer positions between Cs+ cations as shown in the inset of Figure 1d. This is reasonable because there are ample spaces between Cs+ cations and there is no reason for the space is left empty in electrolytes. However, the water molecules may not be coplanar with the Cs layer. Considering the size of water molecules, they are likely closer to the Pt surface than the Cs+, forming layer α. Additional water molecules can form a hydration shell at the position of layer γ. Remarkably, the Cs+ to water distances are 3.1 Å for both layers, α and γ, and close to the known Cs+-H2O distances in literature, 3.07 Å26. H-UPD layer may suppress Cs+ chemisorption, as it inhibits PtCl42− reduction aiding the self-terminating layer deposition 29. A ball model satisfying the water configurations for the (2×2) structure is proposed in Figure 3. The Cs+ cations are indicated by the blue balls on Pt(111) surface modeled by grey balls. The red balls indicate the water molecules at the position of layer α. Since layer α (water) and layer β (Cs+) are almost coplanar, 3 water molecules and 1 Cs+ ion form a stable (2×2) lattice (Figure 3b). By adding layer γ (pink balls), each Cs+ is surrounded by 6 water molecules. For clarity, the three water molecules of layer γ are grouped by a triangle. Note that the orientations of the triangles are likely disordered. The side views of the ball model are shown in Figs. 2C and 2D. This configuration explains naturally the partial hydration observed in Figure 2d. Only water molecules in layer γ depart with Cs+ while the water molecules in layer α do not.


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In summary, ordering in Stern layer was observed in the normalized CTR and in-plane diffraction. The analyses show that the surface normal density profiles at negative potentials are composed of a Stern layer and a diffuse layer. At the lowest potential measured, the Stern layer is formed with one Cs+ layer and two water layers. The Cs+ layer forms a short-range singlesublattice (2×2) structure. One water layer interconnects Cs+ ions in plane and the other forms a hydrating layer between the Cs+ plane and the diffuse layer. The top Pt(111) surface layer to Cs+ distance is 3.5 Å and there is no water between them. Even at this short distance, the Cs+ does not chemisorb and reversibly respond to the potential jumps. Experimental Methods Platinum single crystal was cut and polished to expose (111) surface. The bulk mosaic of the crystal was ~0.1º after annealing at ~2000 K close to the bulk melting temperature. The average surface miscut was < 0.02º and the miscuts of mosaic blocks thus were essentially limited by the bulk mosaic. A transmission cell23 was used. The crystal was heated in the cell to ~1500 K to anneal the surface in dry Ar-3%H2 inert gas flow before each experiment. The Pt(111) crystal is cooled to room temperature and the gas flow is switched to bubbled humid N2 prior to x-ray measurements. The electrolytes were prepared with solid salts of 99.99% in metals-basis purity from Puratronic® dissolved in 18 MΩ·cm water. The concentration was 0.1 M unless noted otherwise. Synchrotron x-ray measurements were performed at 11ID-D beamline, equipped with a ‘4S+2D’ geometry six-circle diffractometer30, Advanced Photon Source (APS). The hexagonal (hex) index (a* = 4π√2 / √3a and c* = 2π / √3a where a=3.9242Å) of face-centered cubic (fcc) structure was used in the experiments31 where (111)fcc, (11ത1)fcc, and (200)fcc are indexed to (003)hex, (101)hex, and (012)hex, respectively. The open circuit potential can drift during x-ray


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exposure32. Therefore, the exposure of x-rays to the electrolyte was minimized, precautions were exercised, and experiments were repeated with different intensities of incoming x-rays to ensure that the results presented here is in any way affected by the x-ray exposure.

Supporting Information. The following files are available free of charge. X-ray methods, Figures S1-S7, and a Table S1 (PDF) AUTHOR INFORMATION Notes The authors declare no competing financial interests. ACKNOWLEDGMENT The work was supported by the U.S. Department of Energy (DOE), Office of Basic Energy Science (BES), Materials Sciences and Engineering Division and use of the APS by DOE BES Scientific User Facilities Division, under Contract No. DE-AC02-06CH11357. One of the authors (TK) thanks the Japanese Society for the Promotion of Science (JSPS) for JSPS Postdoctoral Fellowships for Research Abroad. The work at RIT was supported by the Research Corporation for Science Advancement (RCSA) through a Cottrell College Science.

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20. Kawaguchi, T.; Liu, Y.; Pierce, M. S.; Komanicky, V.; You, H., Direct determination of the one-dimensional interphase structure by crystal truncation rod analysis. submitted to J. Appl. Crystallography, 2017. 21. Spohr, E., Computer simulation of the structure of the electrochemical double layer. Journal of Electroanalytical Chemistry 1998, 450 (2), 327-334. 22. Spohr, E., Molecular simulation of the electrochemical double layer. Electrochimica Acta 1999, 44 (11), 1697-1705. 23. Liu, Y.; Kawaguchi, T.; Pierce, M. S.; Komanicky, V.; You, H., In situ and non situ surface x-ray scattering studies of ordering in Helmholtz planes. unpublished 2017. 24. Einstein, A., Investigations on the theory of the brownian movement. Ann. d. Phys. 1905, 17, 549. 25. Reiser, S.; Horsch, M.; Hasse, H., Temperature Dependence of the Density of Aqueous Alkali Halide Salt Solutions by Experiment and Molecular Simulation. Journal of Chemical and Engineering Data 2014, 59 (11), 3434-3448. 26. Mahler, J.; Persson, I., A Study of the Hydration of the Alkali Metal Ions in Aqueous Solution. Inorganic Chemistry 2012, 51 (1), 425-438. 27. Strmcnik, D.; van der Vliet, D. F.; Chang, K. C.; Komanicky, V.; Kodama, K.; You, H.; Stamenkovic, V. R.; Markovic, N. M., Effects of Li+, K+, and Ba2+ Cations on the ORR at Model and High Surface Area Pt and Au Surfaces in Alkaline Solutions. Journal of Physical Chemistry Letters 2011, 2 (21), 2733-2736. 28. Lucas, C. A.; Thompson, P.; Gruender, Y.; Markovic, N. M., The structure of the electrochemical double layer: Ag(111) in alkaline electrolyte. Electrochemistry Communications 2011, 13 (11), 1205-1208. 29. Liu, Y. H.; Gokcen, D.; Bertocci, U.; Moffat, T. P., Self-Terminating Growth of Platinum Films by Electrochemical Deposition. Science 2012, 338 (6112), 1327-1330. 30. You, H., Angle calculations for a '4S+2D' six-circle diffractometer. Journal of Applied Crystallography 1999, 32, 614-623. 31. Huang, K. G.; Gibbs, D.; Zehner, D. M.; Sandy, A. R.; Mochrie, S. G. J., Phase-behavior of the au(111) surface - discommensurations and kinks. Physical Review Letters 1990, 65 (26), 3313-3316. 32. Nagy, Z.; You, H., Radiolytic Effects on the in-Situ Investigation of Buried Interfaces with Synchrotron X-Ray Techniques. Journal of Electroanalytical Chemistry 1995, 381 (1-2), 275-279.


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a 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

b

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Fig. 1

c

d


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a

1.7

e

b

1.6

1.5

1.4 1.6

c

1.5

INTENSITY / MONITOR

INTENSITY (ARB. UNIT)

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f

g

d h


Fig. 2

The Journal of Physical Chemistry Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

b

a

c

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d Layer γ Layer ͏β Layer α


Fig. 3

Layering and Ordering in Electrochemical Double Layers

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