Anion Adsorption on Gold Electrodes Studied by Electrochemical

Mar 16, 2016 - This article is part of the Kohei Uosaki Festschrift special issue. ... Electrochemical Surface Forces Apparatus (EC-SFA) can provide a...
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Anion Adsorption on Gold Electrodes Studied by Electrochemical Surface Forces Measurement Motohiro Kasuya,† Tsukasa Sogawa,† Takuya Masuda,‡ Toshio Kamijo,† Kohei Uosaki,‡,§ and Kazue Kurihara*,†,∥ †

Institute of Multidisciplinary Research for Advanced Materials and ∥WPI-AIMR, Tohoku University, Sendai 980-8577, Japan ‡ Global Research Center for Environment and Energy based on Nanomaterials Science (GREEN) National Institute for Materials Science (NIMS), Tsukuba 305-0044, Japan § International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), Tsukuba 305-0044, Japan S Supporting Information *

ABSTRACT: The adsorption of ions on electrodes determines the surface potential and charge density of the electrode, thus, quantitative evaluation of the ion adsorption on an electrode is necessary and has been one of the central questions in electrochemistry. Electrochemical Surface Forces Apparatus (EC-SFA) can provide an efficient characterization method of these properties. The interactions between two gold electrodes in various electrolyte solutions, that is, 1 mM aqueous KClO4, K2SO4, and KCl, were measured by controlling of the electrochemical potential (E). The longrange, double layer repulsion and the jump-in due to the van der Waals attraction at the surface separation of about 20 nm were observed between the electrodes in all the solutions. We evaluated the ψ0 and σ values employing DLVO fitting of these interactions. The signs of ψ0 and σ were determined from the interaction between the electrode and negatively charged mica surfaces. This study demonstrated that the σ values were negative and similar in all the solutions when the applied potential E was lower than the potential of zero charge (pzc). When the potential E was increased to near the pzc, the σ values were negative and low and in the order of KClO4 ≈ K2SO4 > KCl. When the potential E was further increased to the pzc, the σ value was positive in aqueous KClO4 because of less anion adsorption on the gold electrode, while those in the K2SO4 and KCl aqueous solutions were negative due to higher adsorption amount of the anions. The method demonstrated in this study enabled us to quantitatively evaluate the influences of the ion adsorption on the effective surface potential and charge density of the electrodes, which should determine the performances of electrochemical devices.

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apparatus (EC-SFA) has been developed.13−17 However, previous studies using FECO (fringes of equal chromatic order) for the distance determination required at least one surface to be transparent; therefore, only interactions between mica and electrodes were measured.13−15 In these studies, the surface potential was evaluated by fitting the force−distance profiles with the DLVO (Derjaguin−Landau−Verwey−Overbeek) theory. Quantitative analysis of the interaction between dissimilar surfaces is more difficult than the measurement of two identical surfaces. EC-AFM was also employed for the forces measurement to evaluate the surface potential of the electrode.18−20 However, this method was also applied for only the measurement between dissimilar surfaces such as the pair of silica colloids attached to the AFM tip and the gold electrode substrate.

INTRODUCTION The surface potential and surface charge density of electrodes are important properties in electrochemistry because they sensitively influence the interactions between the electrode surface and ions in the electrolyte solution. They are also important for controlling the performance of the electrodes in a wide variety of electrochemical devices such as solar cells, batteries and sensors. Ion adsorption on electrodes has been regarded as a major factor that controls the surface potentials and charge density and has been studied using many sensitive analytical methods such as surface X-ray scattering (SXS),1−3 electrochemical scanning tunneling microscopy (EC-STM),4,5 atomic force microscopy (EC-AFM),6 electrochemical quartz microbalance (EQCM),7,8 infrared spectroscopy,5 and sum frequency generation spectroscopy (SFG).9 However, these methods mostly provide only qualitative information. Surface forces measurements have been regarded as one of the promising tools for evaluating the surface potential and the surface charge density based on an analysis of the electric double layer forces.10−12 In order to perform these measurements on electrodes, an electrochemical surface forces © XXXX American Chemical Society

Special Issue: Kohei Uosaki Festschrift Received: December 29, 2015 Revised: February 13, 2016

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DOI: 10.1021/acs.jpcc.5b12683 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Recently, we developed a new surface forces apparatus, called the twin-path SFA, which uses two-beam (twin-path) interferometry for measuring the distance between opaque substrates.21 Based on this apparatus, we have constructed a new EC-SFA that enables us to measure the forces between identical opaque electrodes as a function of the surface separation under the applied potential.17 We used this method for characterizing gold electrodes modified with a ferrocene terminated self-assembled monolayer and obtained the values of the surface charge density and the ion pairing ratio between the adsorbed ferrocene and counteranions.22 A gold electrode is the most extensively studied electrode for ion adsorption because it has a large electrochemical potential window and the flat and homogeneous crystallized surfaces of the electrode can be easily prepared. The gold electrode effectively adsorbs molecules, such as thiol compounds and proteins; therefore, it is frequently used in electrochemical devices such as electrochemical sensors and solar cells. The density and structures of the ion adsorbed on the gold electrode were studied by EQCM,7,8 SXS,1−3 and EC-STM.4 These characterization methods can provide qualitative information about the surface charges of the electrodes. EQCM provides the density of anions adsorbed on the gold electrodes.7,8 For example, ClO4− and SO42− were reported to be adsorbed when the potentials of the electrode are higher than the potential of zero charge (pzc = 0.3−0.4 V vs Ag/ AgCl), and the former is less adsorbed than the latter.7 Cl− is adsorbed on the electrode when the potential is higher than 0.1 V.8 However, the surface potentials and charge densities have not been quantitatively evaluated. Electrochemical impedance spectroscopy was used for evaluating the surface charge density, however, it is difficult to obtain accurate values because the complex model for the equivalent circuit of the electrode− electrolyte interface was necessary for the analysis.23,24 In this study, we performed the forces measurements on the interactions between gold electrodes in various aqueous electrolytes employing EC-SFA. The absolute values of ψ0 and σ of the electrode were evaluated at various electrochemical potentials employing the DLVO fitting to the force−distance profiles between the electrodes. The signs of ψ0 and σ were determined from the interaction between the electrode and negatively charged mica surfaces. Anions differently adsorbed on the gold electrodes, that is, ClO4−, SO42−, and Cl−, were studied to obtain the effective potential and charges of the electrode.

Figure 1. Cyclic voltammogram of the gold electrode in 100 mM aqueous KClO4 in a conventional electrochemical cell. Scanning rate of the applied potential was 50 mV/s.

electrode in 100 mM aqueous KClO4. An oxidation wave at 1.3 V (vs Ag/AgCl) and a reduction wave at 0.8 V were observed. The redox potentials of these peaks were identical as that for Au (111) surfaces, as reported in the litrature,3 indicating that the crystal axis of the electrode surface was (111). Surface Forces Measurement. A schematic illustration of the measurement system17 is shown in Figure 2. The prepared

Figure 2. Schematic illustration of the electrochemical surface forces apparatus (EC-SFA).

gold electrode was used as the working electrode (WE) using a potentiostat (ALS/CH Instruments electrochemical analyzermodel 700C, BAS) for controlling the electrochemical potential (E). The counter electrode (CE) was a Pt wire, and the reference electrode (RE) was a Ag/AgCl (saturated KCl) electrode (BAS). It was reported that a trace amount of Pt dissolved from a Pt counter electrode could adsorb the gold electrode.26 The reported density value of adsorbed Pt is 10−4 monolayer, which is negligibly small compared with the density of surface charges and adsorbed anions on the electrode in our study. Therefore, we think the dissolution could show no effects on the forces measurement even if it could dissolve from the counter electrode. A salt bridge made of agar gel was used for connecting the twin-path SFA chamber and the RE. To avoid any contamination by Cl− ions from the salt bridge, we used salts containing ions the same as in electrolyte solutions for the salt bridges. Electrochemical measurements can be performed in the three-electrode cell arrangement inside the chamber of the twin-path SFA. The SFA chamber was filled with the 1 mM aqueous electrolytes. Before all the measurements, argon (99.9999%) was bubbled through the solutions for more than 30 min for deaeration, and the experiments were done under an argon atmosphere. The interaction force (F) between the gold electrode surfaces was measured as a function of the surface separation (D) in an

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EXPERIMENTAL SECTION Materials. KClO4 (99.99%, Aldrich), K2SO4 (99.99%, Aldrich), and KCl (99.99%, Aldrich) were used as received. Ultrapure water (Barnstead, NANOpure DIamond) was used after double distillation. The gold (99.99%) and Pt wire (99.9999, ϕ = 0.2 mm) were from Tanaka Kikinzoku. Preparation of Gold Electrode Surface. The gold electrode surfaces were prepared on cylindrical silica disks (curvature radius, R = 20 mm) following a previously reported procedure.25 The gold was vapor-deposited (VPC-410A, Ulvac) on a freshly cleaved mica template, which was subsequently glued on the disk with the gold side down using an epoxy resin (Epikote 1004, Shell). The mica template was then removed just prior to use. Teflon-coated Cu wire was connected to the gold surface using a conductive epoxy (CW2400, Chemtronics), and then the connected area was covered with epoxy resin. Figure 1 shows a cyclic voltammogram of the Au B

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The Journal of Physical Chemistry C aqueous solution following a previously reported procedure.17 The spring constant of the cantilever was in the range of 160− 220 N/m. We continuously changed the surface separation between the gold surfaces at a constant approaching rate (20 nm/s) in order to obtain the force−distance profiles. The obtained force F was normalized by the radius R of the surface curvature using the Derjaguin approximation,10 F/R = 2πGf, where Gf is the interaction free energy per unit area between two flat surfaces. We used 20 ± 2 mm for R, which is a typical value in our laboratory. When the two gold surfaces were separated after the first contact, the gold surfaces were broken because the adhesion forces were very high (>1000 mN/m). This strong adhesion could be caused by cold welding reported in ref 27. It was reported that the cold welding was observed in the time scale of seconds.27 We attempted to separate gold surfaces as quick as possible (a few seconds) after the jump into contact for avoiding the damage of the surfaces; nevertheless, we observed damaging of the surfaces. Thus, we could only obtain force− distance profiles before the first contact between the surfaces. This limited number of the forces measurements between the gold−gold surfaces. The potential of zero charge (pzc) of the (111) gold electrode was 0.3−0.4 V;28 therefore, we measured the profiles at three potentials, E ≃ pzc (0.3 V vs Ag/AgCl), E < pzc (−0.1 V), and E > pzc (0.7 V). We performed the measurements on at least three different samples under each condition. There was no such restriction for the measurement between the gold and the mica surfaces, so the potentials were changed in a more stepwise fashion. The measurements were carried out on three different samples under each condition and more than three times for each sample. The averaged value and the error were obtained from these measurements. All measurements were done at room temperature (21 ± 1 °C).

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RESULTS AND DISCUSSION Interaction between Two Gold Electrodes in Aqueous Electrolyte. Figure 3a shows the force−distance profiles for the gold−gold electrode surfaces upon approach in the 1 mM aqueous KClO4 at the various applied potentials (E). The forces consisting of a long-range repulsion, which followed the exponential function, and a jump-in due to the van der Waals attraction at D < 25 nm were observed at E = −0.1 V vs Ag/ AgCl. The decay lengths of the repulsion was 9.7 ± 0.5 nm and in good agreement with the theoretical Debye length for the current concentration of the electrolyte (1/κ = 9.6 nm), indicating that the observed repulsion was attributed to the double layer repulsion. At E = 0.3 and 0.7 V, similar long-range repulsion and shorter-range attraction were observed. The decay lengths of the repulsion (10.5 ± 0.6 (0.3 V) and 10.3 ± 1.2 nm (0.7 V)) were similar to that at E = −0.1 V and were in good agreement with 1/κ, indicating that the observed repulsion was attributed to the double layer repulsion. The amplitude of the repulsion decreased with the increasing E, indicating that the surface charge density decreased with the increasing E. Considering the nature of the pzc, the surface charge density at 0.7 V should be higher than that at the pzc (0.3−0.4 V). However, the result showed the reduction of the charge density when E was increased from 0.3 to 0.7 V. It was reported that a small amount of ClO4− was adsorbed on the electrodes when E = 0.7 V,7 which could be the reason for the reduction of the charge density. The jump-in distance values were similar for all the E values. When the gold surfaces were separated after the contact, they were broken at any E values

Figure 3. Force−distance profiles for two gold electrode surfaces in 1 mM aqueous electrolyte solutions of (a) KClO4, (b) K2SO4, and (c) KCl at various applied potentials. The arrows indicate the distances where the jump-in to the contact occurred.

because the adhesion forces were so high (>1000 mN/m). Thus, it was not possible to measure the adhesion forces between the two gold electrodes. Figure 3b shows the force−distance profiles for the gold− gold electrode surfaces upon approach in the 1 mM aqueous K2SO4 at the various E values. The forces consisting of a longrange repulsion, which followed the exponential function, and the jump-in due to the van der Waals attraction at D < 25 nm were observed at E = −0.1, 0.3, and 0.7 V. The decay lengths of the repulsion (7.3 ± 0.6 (−0.1 V), 6.9 ± 0.2 (0.3 V), and 7.5 ± 0.5 nm (0.7 V)) were similar to 1/κ (5.6 nm), indicating that the observed repulsion should be attributed to the double layer repulsion. The amplitude of the repulsion decreased with the increasing E from −0.1 to 0.3 V, indicating that the negative surface charge decreased with the increasing E. The amplitude of the repulsion showed no change with the increasing E from 0.3 to 0.7 V. It was reported that SO42− was adsorbed on the electrode at only E ≥ pzc.7 This large amount of adsorbed SO42− could keep the σ values negative and constant when E was increased from 0.3 to 0.7 V. The jump-in distances at E = −0.1 and 0.3 V were similar and much longer than that at E = 0.7 V. This suggested that the large amount of adsorbed SO42− at E = 0.7 V could reduce the observed attraction. C

DOI: 10.1021/acs.jpcc.5b12683 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Figure 3c shows the force−distance profiles for the gold− gold electrode surfaces upon approach in the 1 mM aqueous KCl at various E. The forces consisting of a long-range repulsion, which followed the exponential function, and the jump-in due to the van der Waals attraction at D < 25 nm were observed at E = −0.1, 0.3, and 0.7 V. The decay lengths of the repulsions (10.4 ± 1.8 (−0.1 V), 10.5 ± 0.4 (0.3 V), and 9.7 ± 1.0 nm (0.7 V)) were in good agreement with 1/κ (9.6 nm), indicating that the observed repulsion was attributed to the double layer repulsion. The amplitude of the repulsion decreased with the increasing E from −0.1 to 0.7 V, indicating that the surface charge density decreased with the increasing E. The jump-in distances decreased when E was increased. It was reported that Cl− was adsorbed on the electrodes when E > 0.1 V,8 suggesting that the adsorbed anions could reduce the attraction and the jump-in distance. The absolute values of ψ0 and σ could be evaluated by fitting these force−distance profiles to the conventional DLVO theory expressed by the electric double layer force30 and the van der Waals force using the Hamarker constant of the gold−gold surfaces (4.4 × 10−19 J).31,32 The inward shift of the zero distance, which was due to the deformation of the surfaces needed to be considered. The distance for the surfaces in contact (zero distance) was shifted negative from the value for the long separation range due to the deformation of the surfaces because the twin-path SFA monitored the laser beam reflected on the back of the sample surface. This shift was corrected by about 10 nm for estimating the double layer.33 All profiles well agreed with the fitting curves based on the DLVO theory, as shown in Figure 3. As a result of fitting of the force profiles at E = −0.1 V in the aqueous KClO4, the obtained ψ0 and σ were 170 ± 21 mV and 7.1 ± 1.8 μC/cm2, respectively. All other results and parameters for the fittings are summarized in Table S1 in the Supporting Information. Interaction between Gold Electrode and Mica Surfaces in Aqueous Electrolyte. The analysis of the force−distance profiles for the two gold electrode surfaces provides the absolute values of ψ0 and σ, but not the sign of the surface potential and charge of the electrode. To determine them, we performed the measurement of forces between the gold electrode and a negatively charged mica surface in the aqueous electrolytes. Figure 4a shows the profiles of the forces between the gold electrode and mica surface in the 1 mM aqueous KClO4 at various E values. At E = −0.1 V vs Ag/AgCl, the forces consisted of a long-range repulsion, which followed the exponential function, and a shorter-range attraction. The decay length of the repulsion was 9.7 ± 0.5 nm and in good agreement with the Debye length, 1/κ (9.6 nm), indicating that the repulsion was due to the double layer forces. Considering that the mica was negatively charged, the gold surface should also be negatively charged at this potential, E = −0.1 V. The shorter-range attraction could be attributed to the van der Waals attraction. When the gold and mica surfaces were retracted, the jump-out was found. The adhesion force of 35.0 ± 4.3 mN/m at E = −0.1 V was obtained from the jump-out distance calculated from the pulse motor displacements for separating two surfaces and the spring constant. When E was increased from −0.1 to 0.3 V (E < pzc), a long-range repulsion and shorter-range attraction were observed as the case for E = −0.1 V. The decay lengths of the repulsion (8.8 ± 0.9 nm) were in good agreement with 1/κ, indicating that the repulsion was ascribed to the double layer force. The amplitude of the

Figure 4. Force−distance profiles for the mica and gold electrode surfaces in 1 mM aqueous electrolyte solutions of (a) KClO4, (b) K2SO4, and (c) KCl at various applied potentials. The arrows indicate the distances where the jump-in to the contact occurred.

repulsion decreased with the increasing E. This indicated that the negative surface charge on the electrode decreased with the increasing E. When we further increased the potential E beyond 0.4 V (to 0.7 V), the interaction changed to attraction from repulsion, though their intensities were weak. This indicated that the sign of the surface potential as well as the charge of the gold electrode changed to positive. The amplitude of the attraction was similar for the range of E values from 0.4 to 0.7 V, indicating that the positive surface charges on the electrode did not significantly change with the increasing E in the case of the 1 mM aqueous KClO4. When the gold and mica surfaces were retracted, the jumpout was found at all the applied potential E. The adhesion forces were nearly constant, about 30 mN/m, until 0.3 V, then sharply increased to 184 ± 8 mN/m at 0.5 V, as shown in Figure 5. This dependence of the adhesion force on the applied D

DOI: 10.1021/acs.jpcc.5b12683 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The adhesion forces in the case of the aqueous K2SO4 and KCl were plotted in Figure 5. They were lower than the value in the case of the aqueous KClO4 and almost constant at −0.1 V ≤ E ≤ 0.7 V. For example, the adhesion forces in the aqueous KCl were 12.6 ± 1.2 mN/m at E = −0.1 V and 15.4 ± 2.0 mN/ m at E = 0.6 V. On the other hand, the adhesion forces in the aqueous KClO4 were 35.0 ± 4.3 mN/m at E = −0.1 V and 188.3 ± 7.6 mN/m at E = 0.6 V. This suggested that the surface charge of the electrode remained negative due to the adsorption of the anions when E > pzc. These observations about the surface charges from adhesion forces at various E were consistent with that obtained from the force curves upon approach. Effects of Ion Adsorption on Surface Potential and Charge Density of Gold Electrodes. Evaluation of ψ0 and σ provides a quantitative understanding about how the ion adsorption influenced the effective surface potentials and charges. Figure 6a plots the ψ0 values versus the potential

potential agreed well with the dependence of the surface potential and charge. This will be discussed in the later sections.

Figure 5. Adhesion forces between the mica and gold electrode surfaces in 1 mM aqueous electrolyte solutions of KClO4 (●), K2SO4 (■), and KCl (▲) at various applied potentials.

The force−distance profiles for the gold electrode and mica surface upon approach in the 1 mM aqueous solutions of the divalent anion, K2SO4, were measured at the various E values (Figure 4b). For E from −0.1 to 0.5 V, the interactions also consisted of a long-range repulsion, which followed the exponential function, and the shorter-range attraction. The decay lengths of the repulsion (6.4 ± 0.4 nm) were in good agreement with the 1/κ (5.6 nm), indicating that the repulsion was due to the double layer forces between the negatively charged gold electrode and mica surfaces. When E was increased to higher than 0.6 V, only the double layer repulsion was observed. The amplitude of the repulsion decreased with the increasing E from −0.1 to 0.3 V (E < pzc). This indicated that the surface charge on the electrode was negative and decreased with the increasing E. On the other hand, the double layer repulsion did not change with the increasing E beyond 0.4 V (E > pzc). It was reported by a QCM study that SO42− was adsorbed on the gold electrode when E > pzc.7 The repulsion independent of the potential E indicated that the surface charge of the electrode remained negative and constant even at E > pzc due to the adsorbed SO42− anion. The absence of attraction at E ≥ 0.6 V could be due to a large amount of the adsorbed anions, which shielded the attraction. A similar phenomenon was reported in the case of interactions between mica−mica surfaces in various aqueous electrolytes; the van der Waals attraction is shielded by hydration of adsorbed cations on the mica.29 Figure 4c shows the force−distance profiles (approach) for the gold electrode and mica surfaces in the 1 mM aqueous KCl at the various E values. When E was in the region from −0.1 to 0.3 V, the interactions consisted of a long-range double layer repulsion, which followed the exponential function of the decay length of 10.5 ± 0.5 nm, and the shorter-range van der Waals attraction. The repulsion decreased with the increasing E from −0.1 to 0.3 V. This indicated that the surface charge on the electrode was negative and decreased with the increasing E values. On the other hand, the constant double layer repulsion without attraction was observed with the increasing E values higher than 0.4 V (E > pzc). The adsorption of Cl− on the gold electrode was reported for E > 0.1 V.12 The surface charge of the electrode remained negative and constant even at E > pzc due to adsorbed Cl−. The large amount of the adsorbed anion at E > 0.4 V could indicate a short-range of repulsion, which might have shielded the attraction.

Figure 6. Surface potentials and charge densities of the gold electrodes in 1 mM aqueous electrolyte solutions of KClO4 (●), K2SO4 (■), and KCl (▲) at various applied potentials.

applied to the electrode E. The ψ0 value was obtained by fitting the force−distance profiles for two gold electrode surfaces, and the sign of the surface potential and charge was evaluated from the interactions between the gold electrode and mica surfaces. At E = −0.1 V, ψ0 was negative and similar in its amplitude, about −160 mV, for all of three aqueous electrolytes. This is because all three anions were not adsorbed on the electrode at this potential, as reported in refs 7 and 8. When E was increased from −0.1 to 0.3 V (nearly pzc), ψ0 increased to about −90 mV in the cases of the aqueous KClO4 and K2SO4, and the increase was less, −136.7 mV, in the case of the aqueous KCl. It was reported that Cl− was adsorbed on the gold electrodes at 0.3 V; however, ClO4− and SO42− did not adsorb.7,8 These differences E

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observed. We evaluated the magnitude of ψ0 and σ on the electrode by DLVO fitting to the force−distance profiles for two gold electrode surfaces. Their signs were evaluated from the interaction between the gold electrode and negatively charged mica surfaces. In all the studied potassium salts, KClO4, K2SO4, and KCl, the evaluated ψ0 and σ values were negative and similar in magnitude when the potential E was well below the pzc (point of zero charge). These values decreased and started to differ in the order of ClO4− ≈ SO42− > Cl− when E was increased close to the pzc. When E was significantly higher than the pzc, the ψ0 and σ values were reversed to positive in solutions containing ClO4−, which is known as a weakly adsorbed anion. On the other hand, in the solutions containing SO42− or Cl−, which are strongly adsorbed anions, the ψ0 and σ values remained negative. It was possible to quantitatively evaluate the effective potential and charge density of the electrode in the different electrolyte solutions under various applied potentials. These results revealed how the anion adsorption influences the effective ψ0 and σ values of the electrodes at various E values. We believe that this approach is useful for obtaining fundamental insight into the electrodes used in electrochemical devices such as a battery.

in the ion adsorption behavior could bring about the different ψ0 values at E = 0.3 V. When E was increased from 0.3 (near pzc) to 0.7 V, ψ0 became positive in the aqueous KClO4, however, remained negative in both the aqueous K2SO4 and KCl. It was reported that ClO4− ions are adsorbed on the gold electrode only in a small amount at an E positive to the pzc,8 thus charge inversion from positive to negative was observed when E increased across the pzc, which well agreed with a previous report. On the other hand, the two other anions known to be adsorbed much more when they compared to ClO4−, could maintain negative charges even at an E higher than the pzc. Figure 6b plots the E dependence of the σ value, evaluated using the same procedure as that for ψ0. At E = −0.1 V ( pzc in the aqueous KClO4. On the other hand, the σ value slowly increased and remained negative even at E > pzc in the aqueous K2SO4 and KCl. These results can be explained by the differences in ion adsorption behavior, which was explained by the changes in ψ0 at the various E values. The change in the σ values in the case of the aqueous KClO4 was 2.3 μC/cm2 when E was increased from 0.3 to 0.7 V (ca. 0.4 V above pzc). This change was less than half of the change, 5.8 μC/cm2, when E was decreased in the negative direction by 0.4 V from 0.3 to −0.1 V. This difference in the density change could be due to the different anion adsorption behavior above and below the pzc; ClO4− anions were adsorbed on the electrode at E = 0.7 V, while not adsorbed at E = −0.1 and 0.3 V.8 The ion adsorption behavior on the gold electrode of various anions and the applied potentials is schematically summarized in Figure 7.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b12683. Table of parameters for DLVO fitting of force profiles for two gold electrode surfaces in 1 mM aqueous electrolyte at various applied potentials (PDF).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the CREST program of the Japan Science and Technology Agency (JST) and JSPS KAKENHI Grant Numbers 26248002 and 15K17801. This work was performed under the Cooperative Research Program of “Network Joint Research Center for Materials and Devices”.

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REFERENCES

(1) Magnussse, O. M. Ordered anion adlayers on metal electrode surfaces. Chem. Rev. 2002, 102, 679−725. (2) Wang, J.; Ocko, B. M.; Davenport, A. J.; Isaacs, H. S. In situ x-raydiffraction and -reflectivity studies of the Au(111)/electrolyte interface: Reconstruction and anion adsorption. Phys. Rev. B: Condens. Matter Mater. Phys. 1992, 46, 10321−10338. (3) Kondo, T.; Morita, J.; Hanaoka, K.; Takakusagi, S.; Tamura, K.; Takahasi, M.; Mizuki, J.; Uosaki, K. Structure of Au(111) and Au(100) Single-Crystal Electrode Surfaces at Various Potentials in Sulfuric Acid Solution Determined by In Situ Surface X-ray Scattering. J. Phys. Chem. C 2007, 111, 13197−13204. (4) Itaya, K. In situ scanning tunneling microscopy in electrolyte solutions. Prog. Surf. Sci. 1998, 58, 121−248. (5) Edens, G. J.; Gao, X.; Weaver, M. J. The adsorption of sulfate on gold(111) in acidic aqueous media: adlayer structural inferences from infrared spectroscopy and scanning tunneling microscope. J. Electroanal. Chem. 1994, 375, 357.

Figure 7. Schematic illustration of the ion adsorption and surface charges on the gold electrode.

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CONCLUSION The electrochemical surface forces apparatus (EC-SFA) based on the twin-path surface forces apparatus can determine the interaction forces between two opaque samples, which was previously difficult. This EC-SFA was employed to measure the interactions between two gold electrodes in 1 mM aqueous electrolyte solutions under electrochemical potential control for studying the effect of adsorbed anions on the ψ0 and σ of the electrodes. The long-range double layer repulsion and the jump-in due to the van der Waals attraction at D < 25 nm were F

DOI: 10.1021/acs.jpcc.5b12683 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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(27) Alcantar, N. A.; Park, C.; Pan, J. M.; Israelachvili, J. N. Adhesion and coalescence of ductile metal surfaces and nanoparticles. Acta Mater. 2003, 51, 31−47. (28) Hamelin, A.; Lecoeur, J. The orientation dependence of zero charge potentials and surface energies of gold crystal faces. Surf. Sci. 1976, 57, 771−774. (29) Pashley, R. M.; Israelachivili, J. N. Molecular layering of water in thin films between mica surfaces and its relation to hydration forces. J. Colloid Interface Sci. 1984, 101, 511−523. (30) Chan, D. Y. C.; Pashley, R. M.; White, L. R. A simple algorithm for the calculation of the electrostatic repulsion between identical charged surfaces in electrolyte. J. Colloid Interface Sci. 1980, 77, 283− 285. (31) Ederth, T. Substrate and Solution Effects on the Long-Range “Hydrophobic” Interactions between Hydrophobized Gold Surfaces. J. Phys. Chem. B 2000, 104, 9704−9712. (32) Butt, H. J.; Kappl, M. Surface and Interfacial Forces; Wiley-VCH, 2010; p 5. (33) The 10 nm inward shift of the zero distance due to deformation can be justified as follows. The reported value of inward shift of the zero distance in the case of jump-in between mica surfaces is ca. 7 nm, which we studied by both FECO and twin-path SFA (see refs 17 and 21). The estimated value of inward shift of the zero distance in the case of gold was ca. 10 nm (a typical value in our DLVO analysis), which was similar but slightly larger than that for the case between mica. This could be appropriate since gold surfaces showed much larger adhesion forces than that of mica surfaces. In addition, we estimated the radius of the surface contact area by surface deformation and compared with the case between mica surfaces. We calculated the radius of the contact area from 10 nm of the shift of the zero distance and the curvature of the surface (20 mm) using the Pythagorean theorem. The value of the obtained radius of the contact area was ca. 30 μm and larger than that of the mica case (ca. 15 μm), again due to the larger adhesion. These support that the estimation of the inward shift of the zero distance in DLVO analysis in this study is appropriate.

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DOI: 10.1021/acs.jpcc.5b12683 J. Phys. Chem. C XXXX, XXX, XXX−XXX