Cation Depth-Distribution at Alkali Halide Aqueous Solution Surfaces

Article pubs.acs.org/JPCC

Cation Depth-Distribution at Alkali Halide Aqueous Solution Surfaces Héloise Tissot,†,‡,§ Giorgia Olivieri,†,‡,§ Jean-Jacques Gallet,†,‡,§ Fabrice Bournel,†,‡,§ Mathieu G. Silly,§ Fausto Sirotti,§ and François Rochet*,†,‡,§ †

Sorbonne Universités, UPMC Univ Paris 06, UMR 7614, Laboratoire de Chimie Physique Matière et Rayonnement, 11 rue Pierre et Marie Curie, 75231, Paris Cedex 05, France ‡ CNRS, UMR 7614, LCPMR, 75005, Paris, France § Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin, Gif sur Yvette, France S Supporting Information *

ABSTRACT: Using synchrotron radiation Near-Ambient Pressure X-ray photoemission spectroscopy (NAP-XPS) in surface and bulk sensitive conditions, we specifically explored the depth distribution of Na+ ions near the vapor/liquid interface of saturated solutions of sodium halides (NaCl, NaBr, and NaI) kept at 278 K under a pressure of 8 mbar. By varying the photoelectron kinetic energy, and hence the probing depth, we show that the Na+ depth distribution near the surface is highly sensitive to the size and polarizability of the anion. The large polarizable anions, Br− and I− that segregate at the surface, tend to drag the Na+ ions closer to the liquid−water surface. However, this situation is not observed for the NaCl solution, in line with recent theoretical results.

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INTRODUCTION The conventional view of electrolyte surfaces1 was essentially that of ion-depleted surfaces, similar to the surface of neat water. However, for the last decade, theory and experiments have shown a very different picture, pointing to the segregation of ions at the water surface. Naturally, this observation is highly relevant to biological2,3 and environmental4,5 systems. Molecular simulations indicated that large polarizable anions tend to accumulate at the vapor/liquid interface.6−9 The sodium halide series received particular attention.6 For the NaF solution, both ions are repelled from the vapor/liquid interface, leaving an almost ion-free top layer. In contrast to the former case, the Cl− ions penetrate all the way to the vapor/liquid interface. The more polarizable ions, Br− and I−, even accumulate in the outermost liquid layer, but are depleted in the sublayer. The anions in the topmost layer of liquid water are not completely solvated, although a substantial solvation shell remains. The “big” halide ions drag the sodium cations to the surface, forming an ionic double layer, as required by electroneutrality. These segregation trends can be interpreted in terms of the kosmotropic and chaotropic nature of the ions.9 The chaotropic nature of the “bigger” anions Br− and I− make their insertion into the water network difficult, hence their accumulation at the interface, while the kosmotropic Na+ finds its place into the water molecules network. Experiments (grazing incidence X-ray fluorescence,10 vibrational sum frequency generation (SFG) spectroscopy),11,12 indicated that the anions tend to segregate at the liquid water surface according to their size and polarizability. The SFG study by Liu et al.11 showed that the water network at the interface of © 2015 American Chemical Society

sodium bromide or sodium iodide solutions is distorted more strongly than that of sodium chloride or sodium fluoride solutions. The vapor/liquid interface of liquid layers formed on deliquescent samples of alkali halides was investigated by Hemminger’s group using Near-Ambient Pressure X-ray photoemission spectroscopy (NAP-XPS)13−16 under a water pressure of ∼1 mbar. For saturated KBr and KI solutions, it was shown that the anion/cation atomic ratio increases at the vapor/liquid interface.13,14 The enhancement of anion/cation ratio at the liquid surface is more dramatic for the iodide ion, a larger, more polarizable ion than bromide. The Hemminger’s group also showed that the anion/cation ratio increase, at the surface of a KI solution, can be suppressed by the presence of butanol,15 pointing to the disturbing effect of organic surfactants at the water surface. In a further work, the same group examined the vapor/liquid interface of aqueous NaCl, RbCl, and RbBr solutions formed on the deliquescent salts by NAP-XPS.16 RbCl and NaCl solutions also exhibit a small anion/cation ratio surface enhancement, but no dependence on the cation was seen. More specific information on the depth distribution of anions was obtained via anion/O(water) ratios plotted against calculated inelastic mean free paths. Besides this surface science approach, in which a film of water formed on top of a solid is in equilibrium with the vapor pressure, liquid water microjets containing variable concentrations of salts have been extensively studied by extreme UV Received: December 19, 2014 Revised: April 8, 2015 Published: April 9, 2015 9253

DOI: 10.1021/jp512695c J. Phys. Chem. C 2015, 119, 9253−9259

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The Journal of Physical Chemistry C

Figure 1. (a) Cl 2p spectra of a saturated NaCl (6M) solution surface and (b) I 4d spectra of a saturated NaI (11M) solution surface measured with two photon energies at RH = 95% (P = 8 mbar, T = 278 K). The BE is referenced with respect to the Fermi level of the gold substrate. Inelastic mean free path (λ) are theoretically determined for pure liquid water.31,32

before using them. A saturated salt solution was prepared with ultrapure water (>18 MΩ×cm). Samples were prepared by evaporation of the saturated salt water solution on a gold coated silicon surface. The carbon contamination was estimated on the water surface to be less than 10% relative to the oxygen concentration (C/O ratio). XPS measurements were performed at 278 K and at a pressure of 8 mbar, corresponding to RH close to 100%. Lowering the temperature of the sample was achieved by using a circulating chiller and a Peltier block attached to the sample holder. The gold substrate was grounded, and therefore the binding energies are related to the gold Fermi level. It was also possible to bias the substrate negatively with respect to the spectrometer, to eliminate the contribution of the gas phase (O 2s spectrum) when necessary. We examined the issue of beam damage with great care, displacing the beam spot repeatedly on the surface to minimize the effects.

(EUV) photoemission spectroscopy and X-ray photoemission spectroscopy (XPS).17−22 Contrary to the “film” or “droplets” studies, the “micro-jet” study of the NaI solution could not explicitly identify the enrichment of anions over cations at the surface.17 The microjet study of the KF solutions did not point to any increase of the anion/cation ratio close to the surface, but this was expected from theoretical predictions.20 While depth distribution and water coordination of the anion near the surface has been the object of several NAP-XPS studies (see e.g. ref 16), much less work has been devoted to cations.22 In the present work, we prepared saturated solutions of NaCl, NaBr, and NaI by exposing alkali halide salts to water (under 8 mbar) at 5 °C. Once the saturated solution was formed, we characterized the vapor/liquid interface by NAPXPS of the anion/sodium core-levels. Following a procedure similar to that outlined in ref 16 for the anion, we determined the Na/O ratio using the Na 2p and O 2s core-levels to examine the influence of the anion size on the cation depth distribution in the various sodium halide solutions.

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RESULTS AND DISCUSSION We analyzed the surfaces of saturated solutions of NaCl (∼6 M), NaBr (∼8 M), and NaI (∼11 M) formed at 278 K under a water pressure of 8 mbar. Vapor/liquid interfaces studied at these high concentrations may be notably different from those addressed theoretically by molecular dynamics in the diluted case (below 1 M). Indeed, at saturation concentration, in the bulk, the hydration spheres of the ions overlap, and sodium and halide ions mainly form contact ion pairs,24 while the concentration of such pairs is much lower at 1 M. In Figure 1, we present the Cl 2p and I 4d XPS spectra of sodium chloride and sodium iodide saturated solutions. Each core-level is actually a doublet. The spin−orbit splitting is 1.6 eV between Cl 2p3/2 and Cl 2p1/2 and 1.7 eV between I 4d5/2 and I 4d3/2.25 For both solutions, two doublets are necessary to fit the spectra properly. The I 4d5/2 (Cl 2p3/2) binding energy (BE) of the two chemically distinguishable components are found at 197.6 and 199.8 eV (49 and 50.6 eV). Note that the “high BE”-to-“low BE” intensity ratio diminishes when the kinetic energy (KE) of the photoelectrons increases, proving that the high BE species are located at the water surface. The BE energy shifts between the surface and bulk anion

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EXPERIMENTAL SECTION The experiments were performed at TEMPO beamline of synchrotron SOLEIL on the NAP-XPS end station. Details on the TEMPO beamline can be found elsewhere.23 NAP-XPS experiments are performed in a NAP (20 mbar) chamber hosting a SPECS Phoibos 150-NAP electron analyzer. The beamline and the analyzer are protected from the high pressures in the NAP chamber by differential pumping. To maximize the photoelectron signal intensity, photoelectrons are focused using an electrostatic lens system and the analyzer aperture (of diameter 0.3 mm) is brought close to the sample surface at a distance of ∼1 mm. The acceptance angle is ±22°. The sample holder is mounted on a vertical manipulator, and the analyzer axis is perpendicular to the sample surface. The windowless beam entrance axis makes an angle of 54° with the analyzer axis. As the radiation polarization is horizontal, the angle ϑc between the electric field (the polarization vector) and the axis of the analyzer lens is 36°. Salts of high purity (>99.9−99.999%) purchased from SigmaAldrich were used. NaCl and NaBr salts were roasted overnight 9254

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The Journal of Physical Chemistry C

Figure 2. Na 2s XPS spectra of saturated sodium chloride (6M), sodium bromide and sodium iodide (11M) solutions surfaces measured at photon energies of 1200 and 360 eV. The RH is 95% (P = 8 mbar, T = 278 K). The BE is referenced with respect to the Fermi level of the gold substrate. Inelastic mean free paths (λ) are theoretically determined for pure liquid water.31,32

limits damage. Oxidized halogen species could be created at the vapor/liquid interface by reaction with water fragments (ions, radicals) produced by the beam. On dry KI, IO3− (I5+) is produced by exposure to ozone, inducing a component 4.9 eV higher than the I− component BE,30 a shift much larger than the chemical shift observed here (2.2 eV) between the surface and bulk components (naturally intermediate halogen oxides like XO− (X+) or XO2− (X3+) could also be formed). However, the halogen oxides could decompose in the presence of water (see also ref 30). Among the three solutions, the NaBr one is the more prone to be damaged by the beam. The high BE component of the Br 3d core-level tends to be destroyed under the beam (see SI, section A). This observation contradicts the hypothesis of halogen oxide species produced by beam damage at the water surface. In Figure 2, we present the Na 2s X-ray photoelectron spectra of the three sodium halide saturated solutions measured at a relative humidity (RH) of 95% (P = 8 mbar, T = 278 K). We use a photon energy of 1200 eV, i.e., a kinetic energy (KE) of ∼1140 eV, to reach bulk sensitive probing conditions. In neat water, this corresponds to an estimated inelastic mean free path λ of ∼40 Å (to our knowledge, no λ calculations for saturated salt solutions exist).21,31,33,32 With a photon energy of 360 eV, i.e., KE ≈ 300 eV, we are operating in surface sensitive conditions (the calculated λ is ∼12 Å in neat water31,32). In contrast to liquid microjet experiments that refer the zero energy to the gas phase vacuum level,34 the BE is referenced to the gold substrate Fermi level (the Fermi edge, or alternately, the Au 4f7/2 BE measured at 84.0 eV on dewetted areas). Due to the low resistance of the saturated solutions (a saturated NaCl solution has a resistivity of ∼5 Ω × cm),35 no charge effect is measured, and the Fermi level of the gold substrate and of the water droplet are aligned. The Na 2s BE is found at 63.2 eV, for all sodium halide solutions and for both photon energies (bulk and surface sensitive conditions). The spectra are constituted by a single component, whose Gaussian width (GW = 1.4 eV) is not affected by the photon energy change. Therefore, the Na 2s BE is independent of the nature of the anion. In fact, at saturation cation−anion pairs form in the solution, and thus the presence of anions of various size could possibly affect the cation binding energy, which is apparently not the case. This observation is in line with previously published liquid jet experiments on iodide,

components are in agreement with previous works on saturated solution films formed on deliquescent NaCl.16 Our NaBr solutions present the same features in the Br 3d spectra, i.e., a high BE surface component and a low BE bulk component, exactly as in the case solutions formed on deliquescent KBr.14 However, we have observed that the surface high BE component is unstable under synchrotron beam irradiation, as shown in the Supporting Information (SI). Cheng et al.16 suggest that the majority species at low BE correspond to anions in bulk water with a symmetric solvation shell, while the minority species, found at higher BE, are assigned to anions close to the surface in an asymmetric solvation shell. There are further indications that the anion core-level BE are sensitive to changes in the solvation shell in the case of ice (at 90 K): the Cl 2p BE of the Cl− ion (resulting from HCl dissociation) shifts down in BE by −0.7 eV, from low to high HCl exposures.26 In stark contrast with the preceding, the high BE surface component of the anion core-level (this work and refs 14,16) is not observed in liquid-jet experiments of solutions containing fluoride (F 1s), 20 bromide (Br 3d) 22 and iodide (I 4d).17−19,21,22 The discrepancy can found several explanations. First, the surface peak could be related to a specific property of saturated solutions. Indeed liquid jet experiments are performed far from saturation (for instance a 6 M LiBr solution is examined in ref22 while the saturation concentration is ∼19M). The hypothesis of microrafts of solid alkali halide floating on the solution surface must be excluded, as no surface component is observed for the cation (see below). Second, the different experimental systems may impact the measurements. In the present NAP-XPS experiment, the liquid droplet is in equilibrium with its vapor pressure (8 mbar), while in most liquid jet experiments17−19,21,22 the background pressure is in the 10−5 mbar range,27 with the exception of the setup described in refs28,20 where NAP can be reached. Preissler et al.29 provide evidence for charge evaporation in liquid jets of NaI solutions under the form of I−(H2O)n clusters due to water ablation when the jet enters into vacuum. Could nonequilibrium conditions affect the anion concentration at the vapor/liquid interface, especially if the diffusion velocity of anions in water is smaller than the ablation velocity? Third, the surface component observed on saturated solutions could be related to beam damage. Indeed, in liquid jets experiments, the sample is continuously renewed, which 9255

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The Journal of Physical Chemistry C bromide, and chloride sodium solutions.19 One could argue that the low molalities (2m to 3m) of ref 19 lead to nonoverlapping solvation spheres; however, it is worth noticing that the Na 2p BE in NaI solutions remains constant up to high molality (12m).17 A Na 2s core-level width constant with changing probing depth also points to the absence of a surface contribution, unlike for the anions (Figure 1). In fact, sodium atoms close to the surface could exhibit a BE different from that of sodium in the bulk considering the ab initio Green’s function calculation of the Na 1s−1 core ionization spectra of Na+(H2O)n clusters.36 The authors show that the Na 1s BE decreases with the number of water molecules surrounding it. From a Na+ ion with a full solvation shell (n = 6) to a Na+ ion with a half solvation shell (n = 3) the BE increases by about 3 eV. Therefore, Na+ ions sitting at the vapor/liquid interface could be seen by photoemission at a substantially higher BE than the bulk component, considering that the Na 2s and Na 1s core-levels (localized on the nucleus) should present the same BE shifts. Therefore, the absence of any surface component in the Na 2s spectra suggests that the cation does not sit at the liquid water surface. To quantify the amount of Na+ relative to water molecules (Na/O ratio) in the three solutions as a function of the probing depth, it is more advantageous to use the Na 2p and O 2s corelevels, rather than the Na 2s and O 1s core-levels as in ref 16. Indeed, as the Na 2p core-level BE is close to that of O 2s one, the photoelectrons emitted from these levels have practically the same KE (distant only by ∼7 eV). Consequently, the O 2s and Na 2p ejected photoelectrons have practically the same inelastic mean free path, and the same liquid depth is probed (see also ref 37, where the same procedure is used, and the discussion below). The signal attenuation due to inelastic collision within the ∼1 mm of vapor (under 8 mbar) is also identical for both core-levels. Naturally, as the same photon energy is used to acquire both spectra, there is no need for a calibration of the photon flux arriving on the surface, especially for photon energies above the O 1s edge. Under a pressure of 8 mbar, the O 2s peak contains two contributions, one from the gas phase and one from the liquid phase. In order to separate these two peaks, the gold substrate was polarized by applying a negative bias equal to −40 V (as illustrated for the NaI sample in SI, section B). All photoelectrons KEs (gold core-levels and liquid phase corelevels) are pushed up by 40 eV (which incidentally confirms the absence of charge effect in the solution). With this procedure, the O 2s peak of the vapor is eliminated from the acquisition energy window. The Na 2p and O 2s spectra of the NaI saturated solution measured with the gold substrate biased at −40 V are presented in Figure 3 for various photon energies (hν was varied between 200 and 1000 eV). The Na 2p position, measured at a BE of 30.7 eV (referenced to the Fermi level), remains constant with increasing KE (increasing probing depth), similarly to the Na 2s. The O 2s peak corresponding only to liquid water has a BE of 25.8 eV. The atomic ratios Na/O are obtained from the raw Na 2p and O 2s peak areas, corrected by the calculated atomic photoionization cross sections σnl(hv) and asymmetry factors βnl(hv) of the atoms in the gas phase.38 The use of gas phase calculated values for σnl(hv) and βnl(hv) to correct the photoemission intensity is a commonly used procedure, see ref 16, where the Cl/O ratio is deduced from Cl 2p and O 1s

Figure 3. Na 2p and O 2s XPS spectra of a saturated sodium iodide solution surface measured at several photon energies with the gold substrate biased at −40 V at RH = 95% (P = 8 mbar, T = 278 K). The spectra are normalized to the O 2s peak intensity and a vertical offset is added to make the figure clear. The BE is referenced with respect to the Fermi level measured on the gold substrate. (1) hv = 200 eV, λ ≈ 13 Å; (2) hv = 400 eV, λ ≈ 17 Å; (3) hv = 600 eV, λ ≈ 20 Å; (4) hv = 800 eV, λ ≈ 23 Å; (5) hv = 1000 eV, λ ≈ 33 Å; and (6) hv = 1200 eV, λ ≈ 50 Å are theoretically determined for pure liquid water.31,32

peak intensities. The photoemission differential cross section of atom X is written as follows: ⎛ ⎞ βnX, l dσ X (ϑ) = σnX, l × ⎜⎜1 + (3cos2 ϑ − 1)⎟⎟ dΩ 2 ⎝ ⎠

(1)

where ϑ is the angle between the polarization vector of the radiation and the emission direction of the photoelectrons. In SI section C, we explain how the corrections are made, considering that the analyzer axis makes an angle ϑc of 36° with respect to the polarization vector, and the acceptance angle of ±22°. We plot in Figure 4, the corrected Na/O ratio against the KE (that determines the inelastic mean free path and hence the probing depth). At high photoelectron kinetic energy, the bulk of the solution is probed, and the corrected Na/O ratio is ∼0.28 for the three solutions. A strong deviation from this ratio is observed at low KE for the NaI solution (that increases by a factor of 2). A molecular dynamics calculation6 of sodium and oxygen abundancies, although performed for a diluted solution (1 M), suggests that, as iodide accumulates at the vapor/liquid surface, a counterion plane (Na+) forms. Another molecular dynamics calculation22 shows that the anion−cation plane distance decreases with increasing salt concentration (calculations are made up to 4M) much more effectively for NaI than for NaBr or NaCl. In other words, the Na plane should be closer to the surface for the NaI solution than for the two other ones. This explains why the enhancement of the corrected Na/ O ratio is less strong for the NaBr solution. For the NaCl solution, no Na/O ratio increase is observed at low KE. Note that the Na/O ratios of ∼0.28 obtained at high KE are higher than expected (see below) as the Na/O ratio at saturation should be 0.22 for NaI (11.3 mol/L at 283 K), 0.17 for NaBr (8.2 mol/L at 283 K) and 0.11 for NaCl (6.1 mol/L at 283 K). 9256

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CONCLUSIONS While a preceding NAP-XPS study of saturated solutions formed on deliquescent alkali chloride focused on the effect of the cation size (Na+, Rb+) on the anion depth distribution,16 the present work specifically examines the effect of the anion size (Cl−, Br−, I−) on the depth distribution of the sodium ion. First we confirm a previous observation,16 i.e., the anions at the liquid/vapor interface exhibit a core-level shifted to higher binding energies with respect to the bulk liquid species. Considering that this halogen surface species is not observed in liquid jet experiments, we discuss its possible origins. Then, the Na/O atomic ratios versus photoelectron kinetic energy (deduced from Na 2p and O 2s core-level intensities) show that the Na+ depth distribution near the surface is highly sensitive to the size and polarizability of the anion. Unlike the smaller Cl− ions, the I− and Br− ions induce the formation of an electrical double layer by dragging the Na+ ions close to the vapor/liquid interface where they accumulate. This observation agrees with molecular dynamics calculations showing that the bigger the anion the smaller the distance between the anion plane and the cation one.22

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Figure 4. Na/O ratio (solid lines, left vertical axis) as a function of photoelectron KE after correction by calculated cross sections and asymmetry factors38 (see section C of the SI). Raw Na 2p/O 2s intensity ratios of the NaI and NaBr solutions divided by that of the NaCl one (dotted lines, right vertical axis). Photoelectron KE of ∼170, 370, 570, 770, 970, and 1170 eV correspond to calculated31,32 (neat water) λ of ∼13, 17, 20, 23, 33, and 50 Å, respectively.

ASSOCIATED CONTENT

S Supporting Information *

Details on the beam damaging of NaBr solutions (section A), on the measurement of Na 2p and liquid phase O 2s core-level intensities (section B), and on the Na/O atomic ratio determination using subshell cross sections and asymmetry parameters (section C). This material is available free of charge via the Internet at http://pubs.acs.org.

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The use of σnl(hv) values calculated for the isolated atom may raise some issues when applied to the liquid phase. Indeed EXAFS-like structures due to the backscattering of the photoelectron by neighboring atoms can appear as “resonances” in the cross section. For instance, “resonances” are seen up to 70 eV above the O 1s ionization energy of liquid water.39 In the present case, the photon energies of 200 and 400 eV for which the NaI solution exhibits a strong increase in the Na/O ratio corresponds to photoelectron KE of ∼170 and ∼370 eV, respectively, for which the EXAFS oscillations should be damped.40 Also, the conservation of photoemission asymmetry (eq 1) in liquid water is based on the assumption that elastic scattering is unimportant. According to Ottosson et al.,41 this is a valid assumption in the bulk, although the situation might be more complicated at the surface. Therefore, we need to evaluate the possible effect of a strong elastic scattering in the liquid phase, that would lead to an effective asymmetry parameter equal to zero (photoemission isotropy).41 In fact, in section C of the SI, we show that the major correction comes from the subshell cross sections σn,l. Neglecting the βn,l correction, the Na/O ratios (Figure S4 of the SI) present the same general trends as observed in Figure 4. Then the Na/O ratios tend to ∼0.2 at high KE, in better agreement with the ratios calculated from the concentrations. To circumvent the issues concerning the subshell cross sections and asymmetry parameters, we have divided the raw Na 2p/O 2s intensity ratios of the NaI and NaBr solutions by that of the NaCl one. In fact, it is expected that the Na/O is practically constant with depth.6 The corresponding curves are given as dotted lines in Figure 4, showing that in conditions sensitive to the surface the Na/O ratio of the NaI (NaBr) solution is ∼2.5 (∼1.5) greater than that of the NaCl solution.

AUTHOR INFORMATION

Corresponding Author

*Tel: +33 (0)1 44 27 66 23; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors express their thanks to Dr. Jean Daillant, director of Synchrotron SOLEIL, for enlightening discussions concerning the surface of electrolytes. They also much appreciated the very efficient technical support from Christian Chauvet (TEMPO beamline, SOLEIL). This NAP-XPS experiment was funded by the Ile-de-France Region (Photoémission Environnementale en Ile-de-France, SESAME n°090003524), by the Agence Nationale de la Recherche (Surfaces under Ambient Pressure with Electron Spectroscopies, ANR- 08BLAN-0096), and by the Université Pierre et Marie Curie. Synchrotron SOLEIL supported the integration of the setup to TEMPO beamline. LABEX MiChem (UPMC) also partially funded the experiment. H.T. received a PhD scholarship from Synchrotron SOLEIL.

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DOI: 10.1021/jp512695c J. Phys. Chem. C 2015, 119, 9253−9259

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The Journal of Physical Chemistry C (41) Ottosson, N.; Faubel, M.; Bradforth, S. E.; Jungwirth, P.; Winter, B. Photoelectron Spectroscopy of Liquid Water and Aqueous Solution: Electron Effective Attenuation Lengths and EmissionAngle Anisotropy. J. Electron Spectrosc. Relat. Phenom. 2010, 177 (2− 3), 60−70.

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DOI: 10.1021/jp512695c J. Phys. Chem. C 2015, 119, 9253−9259