Characterization of Platinum Electrode Surfaces by Electrochemical

Nov 20, 2017 - The surface forces between platinum, Pt, electrodes and those between the Pt electrode and mica in aqueous HClO4 were measured at vario...
2 downloads 10 Views 3MB Size
Download PDF
Article Cite This: J. Phys. Chem. C 2017, 121, 26406-26413

pubs.acs.org/JPCC

Characterization of Platinum Electrode Surfaces by Electrochemical Surface Forces Measurement Sho Fujii,† Motohiro Kasuya,‡ and Kazue Kurihara*,†,‡ †

Advanced Institute for Materials Research (AIMR) and ‡Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan S Supporting Information *

ABSTRACT: The surface forces between platinum, Pt, electrodes and those between the Pt electrode and mica in aqueous HClO4 were measured at various potentials (E) applied to the electrodes using an electrochemical surface forces apparatus (EC-SFA). This apparatus uses the twin-path surface forces apparatus, recently developed for opaque samples. The influence of the proton adsorption on the surface interactions was studied. The Pt electrodes were prepared by the template-stripping procedure using glass templates. The electrode surfaces were smooth (RMS roughness: 0.26 nm for a 5 μm × 5 μm area) and polycrystalline based on the atomic force microscopy and cyclic voltammetry results, respectively. When the applied potential E was decreased from 0.5 to 0.2 V (vs Ag/AgCl), the electric double layer (EDL) repulsion between the Pt electrodes decreased. The absolute values of the surface potentials, |ψ0|, calculated using the EDL theory were 58 and 43 mV at E = 0.5 and 0.2 V, respectively. The EDL force at E = 0.2 V was the local minimum, suggesting that the potential of the zero charge (PZC) of the Pt electrode was around 0.2 V in the 1 mM HClO4 solution. With the further decreasing potential E from 0.2 to −0.2 V, the EDL repulsion remained similar in amplitude, took another minimum, |ψ0| = 40 mV, at E = −0.1 V, and started to increase again at E = −0.1 V. These behaviors could be caused by proton adsorption on the Pt surface (Ptδ−···H+), the electrochemical hydrogen adsorption (Pt−H), and the subsequent hydrogen evolution (H2↑). The possibility for characterizing the hydrogen evolution processes on the Pt electrodes based on the surface forces measurement is discussed for the first time.

■

INTRODUCTION The electrode−electrolyte interface plays an important role in electrochemical processes such as those of batteries, sensors, and electrochemical synthesis.1−4 Therefore, understanding of the electrode−electrolyte interface provides not only fundamental knowledge but should also lead to a new technology for designing electrochemical devices. Surface force measurements using a surface forces apparatus (SFA) have been regarded as a powerful tool for studying the electrical double layer phenomena5,6 because it can directly monitor surface interactions including the electric double layer forces as a function of the surface separation with the high resolution of 0.1 nm. Quantitative analysis of the interactions between surfaces of a well-defined geometry is possible, unlike AFM which generally uses a tip of a less-defined shape. Naturally, there are interests in studying electrochemical processes on the electrode surfaces using the so-called electrochemical SFA (EC-SFA), which employs at least one electrode surface under electrochemical potential control. The pioneering attempt of EC-SFA reported by Fan and Bard investigated electro-redox reactions in a nanospace between platinum, Pt, electrodes but not the forces.7 They observed interesting amplification of the redox current which the authors attributed to suppressed diffusion of the redox species in the © 2017 American Chemical Society

space. More recently, Conner and Horn succeeded in measuring the interactions between mica and a mercury droplet which was connected to a potentiostat.8 Vanderlick introduced an electrochemical setup of a three-electrode system in SFA and measured the interactions between mica and a gold electrode, of which the potential was controlled against a gold quasi-reference electrode.9 However, in these studies8,9 and related works,10−14 one surface was always mica because of the limitation in using the fringes of equal chromatic order (FECO) for the distance determination, which requires at least one transparent substrate, except for the case by Bard, who tried to use thin-Pt coated mica for the FECO. Measurements of the interactions between the electrode surfaces both under potential control had not been achieved, to the best our knowledge, though this measurement should be important for understanding electrochemical processes. The fundamental properties of electrodes, such as the surface potential and/or charges, can be easily obtained by quantitative analysis of the EDL interactions between symmetrical surfaces. Received: September 19, 2017 Revised: November 1, 2017 Published: November 20, 2017 26406

DOI: 10.1021/acs.jpcc.7b09301 J. Phys. Chem. C 2017, 121, 26406−26413

The Journal of Physical Chemistry C

■

Article

EXPERIMENTAL SECTION Preparation of Platinum Electrodes. The Pt electrode surfaces were prepared by the template-stripping method21,22 using glass and mica substrates as the templates. A flat cover glass (Matsunami, AF 45; 50 μm thickness; 0.19 nm RMS roughness, for 5 μm × 5 μm area) used as a template was cleaned by an ultrasonic cleaner in acetone for 5 min. After rinsing with ethanol and pure water, the cover glass was further cleaned in a piranha solution (3:7 v/v 30% H2O2/conc H2SO4) and then thoroughly rinsed with pure water. Pt film of ca. 100 nm in thickness was then deposited on the glass or freshly cleaved mica by a sputtering deposition system in an argon atmosphere as previously reported.23 The as-deposited Pt films were glued on cylindrical quartz disks (curvature radius, R = 20 mm) by a hot-melt Epikote epoxy resin.17 The glass template was spontaneously removed when the disk was dipped in pure water (Figure 2), while the mica template was mechanically

We recently developed a new type of surface forces apparatus, i.e., the twin-path SFA, using the modified twobeam (twin-path) interferometry for measuring the interactions between nontransparent substrates.15,16 The twin-path SFA uses a laser light reflected on the back of the bottom surface for the distance determination. Therefore, the twin-path SFA is well suited for the EC-SFA which allows us to measure the surface forces between two electrode surfaces both under potential control. Actually, this system was used to measure the interactions between opaque gold electrodes which selectively adsorb various anions.17,18 Furthermore, the gold surfaces can be modified using thiol derivatives. The surface force and interface properties for the gold electrodes modified with a ferrocene thiol compound were investigated by the EC-SFA technique.19 Platinum is one of the most active metal catalysts for electrochemical reactions and has attracted much attention in the field of electrochemistry. Applications of Pt electrodes include the catalyst of hydrogen generation, which proceeds by proton adsorption on the electrodes. It is important to determine how the surface properties associated with the hydrogen adsorption change because redox reactions on the electrodes should modify the surface properties. In spite of this importance, surface force measurements on Pt surfaces have been attempted by only one group.20 White et al. measured the surface forces between the Pt surfaces in pure water, but the absence of the jump-in was attributed to a possible surface contamination; moreover, the measurements were conducted without applying an electric potential. In this study, we measured the surface forces between the Pt surface in an HClO4 solution using the EC-twin-path SFA (Figure 1). Smooth Pt surfaces were prepared by the template-

Figure 2. Schematic illustration for preparation of a smooth Pt surface for EC-SFA: (i) deposition of Pt on a glass surface by sputtering; (ii) the Pt/glass substrate is glued on the SFA disk using epoxy resin; (iii) conductive wire is connected to Pt surface; (iv) the glass template is removed by water immersion.

removed. For the EC-SFA measurements, a conductive wire was connected to the Pt surface using a conductive epoxy (ITW Chemtronics, CW2400), which was covered by an epoxy resin before immersion in the water. After removal of the glass template, a hydrophilic Pt surface was obtained. The tappingmode AFM measurements (Digital Instruments, Nanoscope IV) were conducted to monitor the surface roughness of the template-stripped Pt surfaces using a silicon cantilever (Olympus, OMCL-AC240TS-C2). The surface analysis of the Pt film was performed by XPS (ULVAC, PHI 5600). EC-SFA Measurement. A schematic illustration of EC-SFA is shown in Figure 1. After the removal of the glass template, we set the surface in the SFA chamber and immersed the surfaces in the electrolyte solution quickly (in a few minutes). We confirmed no contamination on the Pt surface due to the following results: (1) The CV showed only same peaks as the previously reported peaks for the flame-annealed Pt electrodes (Pt oxidation and reduction, hydrogen adsorption, and spillover: see Figure 3 and refs 27 and 30). (2) The value of the potential of zero charge determined in this study for Pt agreed with previously reported ones (ca. 0.19 V vs Ag/AgCl) (see Figure 5c and ref 32). (3) We observed the jump-in when the distance between the Pt surfaces was shorter than several nanometers (see Figures 4 and 5a,b). This can be observed only when sample surfaces are sufficiently clean. The electrode potential, E (V), of the Pt was controlled by a potentiostat

Figure 1. Schematic illustration of the EC-SFA. The reference electrode (RE) is Ag/AgCl (saturated KCl solution), which is connected to the SFA chamber by a salt bridge (SB) of agar gel containing the 0.1 M KClO4 solution. The counter electrode (CE) is Pt wire, and the working electrodes (WE1, WE2) are smooth Pt electrodes, held at the same potential. In the case of using a mica surface, WE2 is replaced by mica.

stripping procedure using a thin glass template which is bendable and suitable for the SFA measurements to make a curved surface. The surface forces between the Pt electrode surfaces were measured, for the first time, under various applied potentials. The obtained results were discussed in terms of ion and proton (hydrogen) adsorptions and hydrogen generation. 26407

DOI: 10.1021/acs.jpcc.7b09301 J. Phys. Chem. C 2017, 121, 26406−26413

Article

The Journal of Physical Chemistry C

deaerated by Ar bubbling for more than 30 min. The potential of the Pt electrode was controlled between −0.2 and 0.5 V (vs Ag/AgCl). At a potential more positive than 0.6 V, it has been reported that the Pt electrode surface becomes rougher due to oxidation.24 For negative potential range, we avoided setting the potential more negative than −0.3 V because hydrogen bubbles were generated in this negative potential range, which disturbed the EC-SFA measurements. All the experiments were carried out at room temperature. Force measurements were carried out by a twin-path SFA (homemade) which determines the distance between two surfaces by the modified two-laser beam interferometry technique.15 The force was calculated by the deflection of the double-cantilever spring (spring constant, k = 220 N/m). The observed force, F (N), was normalized by the radius of the surface curvature, R(m), using the Derjaguin approximation,6 F/R = 2πGf, where Gf is the interaction free energy per unit area between two flat surfaces. The forces between the charged metal surfaces are usually expressed by the Derjaguin− Landau−Verwey−Overbeek (DLVO) force, which is the sum of the electric double layer force, FEDL, and the van der Waals force, FVDW.6 FEDL for a 1:1 electrolyte assuming a constant surface potential was calculated using the nonlinear Poisson− Boltzmann theory, as described by Chan.25 FVDW for the interaction across the crossed cylinders was calculated using the equation FVDW/R = −AH/6D2, where AH is the Hamaker constant which is 20 × 10−20 J for two Pt surfaces in water and an aqueous electrolyte.26 D is the surface separation distance. A surface potential, ψ0, of the Pt surface was determined by fitting of the force profiles with the DLVO theory. The data shown in Figure 5c are the average and the standard deviation of results of the measurements of at least more than three measurements on three samples for each condition. Adhesion forces (Fad) were evaluated by the product of jump-out distance ΔD of pulloff process (see Figures 5d and 7b) and the spring constant k, Fad = kΔD.

Figure 3. (a) AFM image of the template-stripped Pt surface. The RMS roughness of the surface was 0.26 nm. (b) Cyclic voltammogram of Pt electrode on SFA disk in 1 mM HClO4 solution. The voltammogram was measured by scanning the potential from 0.1 V to negative. Scan rate: 10 mV/s.

■

RESULTS AND DISCUSSION Characterization of Pt Electrode. Figure 3a shows an AFM image of the template-stripped Pt electrode surface. The image showed a smooth surface, and the RMS roughness was 0.26 nm for the 5 μm × 5 μm area. The diameter of contact area for EC-SFA is ca. 20 μm, which was in similar scale to the AFM images. These results well agreed with the previous report on the preparation of flat Pt surfaces using a silicon wafer template.23 XPS measurements were performed to check for possible impurities from the glass on the Pt surface after removing the glass template. The XPS spectrum showed the absence of silicon atoms from the template glass on the Pt surface (see Supporting Information, Figure S1). Therefore, the glass template can be used to prepare a smooth and clean Pt surface. The thin glass substrate is bendable, making it convenient to glue the substrate on the cylindrical SFA disk. In the case of using a mica substrate as a template, a fragment of the mica often remained on the Pt surface after the mica was stripped. Accordingly, we used the glass template for this study. A cyclic voltammogram of the thus-prepared Pt electrode in a 1 mM HClO4 solution is shown in Figure 3b. The electrochemical behavior was identical to the reported responses of a polycrystalline Pt electrode, indicating that the obtained Pt electrode is polycrystalline.27 The cathodic and anodic current peaks in the potential range of −0.2 V < E < 0 V (vs Ag/AgCl) have been assigned to the adsorption and desorption of

Figure 4. Surface force curves for Pt surfaces in pure water (red) and 0.1 and 1 mM HClO4 solutions (blue and green, respectively). Symbols (△, □, ○) indicate duplicate measurements for each experimental condition.

(BAS, ALS/CH 660) using a three-electrode system. The working electrode was the prepared Pt electrode, the counter electrode was a Pt wire, and the reference electrode was Ag/ AgCl (saturated KCl solution). The reference electrode was connected to an electrochemical cell (SFA chamber) by a salt bridge of agar gel prepared in a 0.1 M KClO4 solution. The measurements were performed in a 1 mM HClO4 solution 26408

DOI: 10.1021/acs.jpcc.7b09301 J. Phys. Chem. C 2017, 121, 26406−26413

Article

The Journal of Physical Chemistry C proton/hydrogen, respectively, on the Pt electrode surface.28 The electrode was labeled Pt−H under this condition. The non-Faradaic (flat) current region from 0 to 0.5 V is attributed to charging of the double-layer capacitance. In this range, E more negative than the potential of the zero charge (PZC)29 would provide charging current by proton adsorption on the Pt surface (Ptδ−···H+). The small irreversible peak around 0.3 V was observed for the Pt/glass interface and attributed to the spillover of hydrogen from Pt nanoelectrode (radius 0.5 V). Surface Forces between Pt Surfaces in Aqueous Solution. First, the interactions between the Pt surfaces were measured without any applied potential. Figure 4 shows the force curves for the Pt surfaces in pure water and in the HClO4 aqueous solutions (0.1 and 1 mM). The force curves are described by an exponential function with the decay length of 107 ± 3, 30.7 ± 0.4, and 10.9 ± 0.5 nm for the pure water and 0.1 and 1 mM HClO4 solutions, respectively. These decay lengths well agreed with the theoretical Debye length of the double layer force for the corresponding salt concentrations; i.e., 30.4 nm for 0.1 mM HClO4 and 9.6 nm for 1 mM HClO4.6 The decay length in pure water was in reasonable agreement with the reported values for the double layer force in pure water (129 and 137 nm).15,17 A jump-in contact was observed in all the curves, and the distance when the jump-in occurred was in the range 8−10 nm. When attraction increases with decreasing distance D and the slope dF/dD of a force curve becomes larger than the spring constant k of the cantilever, the surface separation D spontaneously jumped into the contact, which is usually called “jump-in”.6 This jump-in can be ascribed to the van der Waals attraction. This interaction implies the Pt surface prepared is clean and suitable for the SFA measurement. By fitting the data using the DLVO theory,6 the surface potential, ψ0, of the Pt surface under each experimental condition was calculated (Table 1). The surface potential decreased with the increasing HClO4 concentration, i.e., 139 mV for pure water, 69 mV for 0.1 mM HClO4, and 52 mV for 1 mM HClO4. The adsorption property of the perchlorate anions on the Pt polycrystalline surfaces is known to be weak or

negligible at least for the 1 mM and lower concentrations.31 Thus, the reduction of the surface potential in the HClO4 solutions was likely to be due to the proton adsorption at a lower pH of the higher HClO4 concentration. The sign of the Pt surface charge in the 1 mM HClO4 solution was negative, which was determined from the repulsive interaction observed between Pt and the negatively charged mica (see the section Interactions between Platinum Electrode and Mica Surfaces). This well agrees with the presence of the proton adsorption on the Pt electrode. For the fitting analysis, the Hamaker constant, temperature, and relative permittivity were 20 × 10−20 J, 298 K, and 78.6, respectively. EC-SFA Measurements between Pt and Pt. Second, the forces between Pt electrode surfaces were measured at various applied potentials as shown in Figure 5a,b. The potentials applied to both Pt electrodes were identical. Basically, the electric double layer repulsion was observed at all electrode potential conditions studied. The repulsion observed at E = 0.5 V vs Ag/AgCl was nearly identical to the one observed at 0.4 V. Using the DLVO theory, the surface potential was calculated from the repulsion to be 58 mV at E = 0.5 V and 61 mV at E = 0.4 V (see Supporting Information, Figure S2). The absolute values of the obtained surface potentials, |ψ0|, are plotted in Figure 5c versus the applied potential together with the values calculated for the other applied potentials. The surface potential decreased to 46 mV at E = 0.3 V and to 43 mV at 0.2 V with the decreasing applied potential. The surface potential exhibited a minimum at E = 0.2 V, which was close to the reported potential of the zero charge (PZC) of the Pt electrode, ca. 0.19 V (vs Ag/AgCl) in 1 mM HClO4.32 Therefore, the PZC of the studied Pt electrodes should be around 0.2 V.35 With the further decreasing applied potential E from 0.2 to −0.2 V, the repulsion was still observed. An interesting observation was in the amplitude of the repulsion which remained nearly the same between 0.1 and 0.2 mN/m at the maximum (Figure 5b), instead of sharp increase expected below the PZC. The plot of the surface potential, |ψ0|, vs the applied potential shows another minimum in the surface potential (40 mV) at −0.1 V (Figure 5c). The values of |ψ0| should increase with the decreasing E from the PZC because of charging the electrode. However, the data in Figure 5c showed a slight increase in |ψ0| and the obvious drop of |ψ0| at E = −0.1 V, which corresponds to the cancellation of the negative charges on the electrode. These successive changes in the surface potential should reflect the process of the electrochemical reaction due to the proton adsorption on the Pt electrode (Ptδ−···H+) to form the Pt−H state. The surface potential again increased at E = −0.2 V at which hydrogen started to evolve. There was a jump-in contact at D = 8−10 nm in each case, even in the applied potential range of the hydrogen adsorption (Pt−H). Figure 5d describes the interaction between the Pt and Pt surface on the retraction trace at the applied potential E from 0.3 to −0.1 V. The values of the adhesion force, Fad, were 24 ± 2, 30 ± 1, and 22 ± 2 mN/m at E = 0.3, 0.1, and −0.1 V, respectively. The adhesion forces at the various applied potentials are summarized in Figure 6. The adhesion between the Pt electrodes was similar over the entire studied applied potential range. The slightly lower adhesion at or above 0.3 V should correspond to the higher surface charges, and the values at the applied potential below −0.1 V should correspond to the hydrogen adsorption. Interactions between Platinum Electrode and Mica Surfaces. In order to determine the positive or negative sign of the Pt surface potential, we performed EC-SFA measurements

Table 1. Experimental and Theoretical Values of Debye Length, κ−1, and the Surface Potential, ψ0, Obtained by Fitting the Force Curves with the DLVO Theory Debye length, κ−1 (nm) solution

exptl

theor

surface potential, ψ0 (mV)

H2O 0.1 mM HClO4 1 mM HClO4

107 ± 3 30.7 ± 0.4 10.9 ± 0.5

30.4 9.6

139 69 52 26409

DOI: 10.1021/acs.jpcc.7b09301 J. Phys. Chem. C 2017, 121, 26406−26413

Article

The Journal of Physical Chemistry C

Figure 5. Surface force curves between Pt electrode surfaces of which the electrochemical potential (E) was controlled at (a) 0.5 (△), 0.4 (□), 0.3 (×), 0.2 (○), and 0.1 (●) V vs Ag/AgCl; (b) 0 (△), −0.1 (○), and −0.2 (×) V vs Ag/AgCl. (c) Surface potential, ψ0, of Pt electrode plotted as a function of applied potential. (d) Adhesion (pull-off) force−distance profile at 0.3 (△), 0.1 (□), and −0.1 V (○).

potential as shown in Figure 7a. At 0.3 V ≤ E ≤ 0.5 V (vs Ag/AgCl), a long-range attraction appeared at D = ca. 40 nm, which was longer D range than the range for the van der Waals attraction as shown in the inset of Figure 7a. Thus, this attraction was ascribed to the electrostatic attraction between the negatively charged mica and a positively charged Pt, and the short-ranged attraction was the van der Waals attraction. The attraction decreased with the decreasing potential and changed to a repulsion at the potential E < 0.3 V. This interaction change indicated that the surface charge of the Pt became negative. The potential at which the interaction changed from the attraction to the repulsion was between 0.2 and 0.3 V and should correspond to the PZC of Pt−mica. This PZC value was slightly more positive than the reported value and the measured value for Pt−Pt surfaces, ca. 0.19 V (vs Ag/AgCl) in 1 mM HClO4.32 This shift is possibly due to the interaction, repulsion, between mica surface with the double layer of countercations and the Pt of zero charge. With the further decreasing applied potential from 0.2 to 0 V, the repulsion between the Pt and negatively charged mica surfaces increased. The short-range attraction (jump-in, D < 5 nm) was constantly observed at the applied potential above 0 V, regardless of the sign of the surface charges. When the applied potential was decreased to a value below 0 V (E < 0 V), the repulsion slightly increased (Figure S3) and the jump-in contact disappeared. It is known that the adsorption and desorption of proton/hydrogen occur in this potential range as described by the scheme Pt + H+ + e− ⇄ Pt− H. The repulsive interactions between the Pt and mica surfaces had almost the same profiles at −0.2 V ≤ E < 0 V, indicating that the Pt−H surface maintained a negative charge. When the

Figure 6. Dependence of the adhesion force on the potential applied to the Pt electrode. The blue symbols (○) represent the force between Pt electrodes, and the red (△) is between the Pt and mica surface in 1 mM HClO4 solution.

of the forces between the Pt electrode and the mica surface. Because the mica surface is negatively charged in the 1 mM HClO4,33 it is possible to determine the sign of the surface charge from the observed interactions. First, the force curves were measured without any applied potential (open circuit potential), and repulsive interactions were observed. This result indicated that the Pt surface was negatively charged in the 1 mM HClO4. The forces between the Pt electrode and the mica surface were measured by varying the applied electrode 26410

DOI: 10.1021/acs.jpcc.7b09301 J. Phys. Chem. C 2017, 121, 26406−26413

Article

The Journal of Physical Chemistry C

Figure 7b describes the jump-out behaviors between the Pt and mica surface on retraction trace at the potential from E = 0.3 to −0.1 V. The adhesion force Fad decreased with the decreasing applied potential (Fad = 29 ± 3, 19 ± 1, and 0 mN/ m at E = 0.3, 0.1, and −0.1 V, respectively), which are summarized in Figure 6. Adhesion Forces between Pt−Pt and Pt−Mica. Adhesions between the Pt−Pt and Pt−mica are summarized in terms of the applied potential as shown in Figure 6. The adhesion force between the Pt−Pt was in the range 20−40 mN/m at all the applied potential values even at E < 0 V where the electrode adsorbed hydrogen (Pt−H). In these cases, the van der Waals interaction must determine the adhesion. A slightly less adhesion at above E = 0.3 V should correspond to the larger surface charges and the values at the applied potential lower than −0.1 V to the hydrogen adsorption (Pt−H). The adhesion force between the Pt and mica behaved differently from those between the Pt−Pt. The value of the adhesion force decreased from 72 to 0 mN/m when the applied potential changed. At 0.4 V > E > 0 V, the value of the adhesion force gradually decreased with the decreasing applied potential. When E was above 0.3 V, the adhesion force was significantly higher than the values between the Pt−Pt. Not only the van der Waals force but also the electrostatic interaction between the negatively charged mica and positively charged Pt electrode contributed to the higher adhesion values between the Pt and mica. At E < 0 V, no adhesion force was observed. The adsorbed hydrogen layer on Pt surface would block the adhesion between mica and Pt electrode. Electrochemical quartz-crystal nanobalance analysis has estimated that the Pt− H reduces the interaction between the electrode and electrolyte or water molecule,34 and our result would support the interface structure reported. The results of these interactions are summarized in Table 2, and the surface states on the Pt electrode are illustrated in Chart 1.

Figure 7. Curves of surface forces between the Pt and mica surfaces at various potentials applied to the Pt electrode: (a) Curves at the applied potential of 0.5 (▲), 0.4 (■), 0.3 (●), 0.2 (□), 0.1 (○), and −0.2 (+) V vs Ag/AgCl. Inset: enlarged graph of an attraction side of the force curves. A solid line is the van der Waals attraction between mica and Pt surfaces calculated by the theoretical equation.6 Hamaker constant, AH, between Pt and mica (6.6 × 10−20 J) was calculated from the geometric average of the values in the case of Pt−Pt (20 × 10−20 J)26 and mica−mica (2.2 × 10−20 J).6 (b) Adhesion (pull-off) force vs distance profiles at 0.3 (△), 0.1 (□), −0.1 (○) V.

■

CONCLUSIONS We measured the surface forces between the Pt electrode surfaces in 1 mM HClO4 at various applied potentials using the EC-twin-path SFA for the first time. The hydrogen adsorption on the Pt electrodes significantly modified the interactions. The major observations include the following: (1) The potential-dependent repulsive forces were observed when the interactions between the platinum electrodes were measured using the EC-SFA. With the decreasing potential E

applied potential decreased below −0.2 V, hydrogen gas was generated, which disturbed the measurement.

Table 2. Interactions between Pt−Pt and Pt−Mica Surfaces at Various Applied Potentials and Surface Properties of the Pt Electrode Surface

a

Mica surface is charged negative in the 1 mM HClO4 solution.33 26411

DOI: 10.1021/acs.jpcc.7b09301 J. Phys. Chem. C 2017, 121, 26406−26413

Article

The Journal of Physical Chemistry C

Therefore, the EC-twin-path SFA we developed will become a powerful tool for not only obtaining fundamental knowledge about electrode interfaces but also developing electrochemical devices such as sensors, batteries, and fuel cells.

Chart 1. Surface Charge and State of Pt Electrode Surface at Applied Potentials

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b09301. XPS data of Pt surface and force curves between mica and Pt electrode (−0.2, −0.1, and 0 V); DLVO fitting of force curves for Pt−Pt at various applied potentials (PDF)

from 0.5 V (vs Ag/AgCl), the repulsion decreased, and the minimum value of the repulsion was observed at E = 0.2 V, suggesting that the PZC of the Pt electrode in the solution was around 0.2 V. (2) The EDL interactions between the Pt electrode surfaces in the potential E range below 0.2 V, in which the hydrogen adsorption occurs, were sensitively modified by the hydrogen adsorption on the Pt electrodes. The repulsion decreased when the hydrogen adsorption increased and reached the minimum at E = −0.1 V. The repulsion again increased when hydrogen started to evolve at E = −0.2 V. The jump-in attractions were observed at each potential (−0.2 V ≤ E ≤ 0.5 V). In contrast, the EDL interaction between the Pt electrode and the negatively charged mica changed from attractive (E > PZC) to repulsive (E < PZC) due to the electrostatic force. At the applied potential at which the Pt−H surface (E < 0 V) was formed, the force curve remained similar and did not show either an increase or a decrease and no jump-in contact behavior. (3) The electrochemically adsorbed hydrogen suppressed any further increase in the surface force, which means little charging on the Pt surface by the applied potentials below PZC, but the Pt−H surface was still negatively charged. The hydrogen adsorption also influenced the surface property of adhesion for the Pt/mica. The adhesion forces of the Pt/mica decreased with the decreasing applied potential, and the force disappeared for the Pt−H/mica. The hydrogen on the Pt electrode acted as a blocking layer for the dissimilar surface system. In contrast, the adhesion force between the similar surfaces (Pt/Pt, Pt−H/Pt−H) was observed in the range 20− 40 mN/m at all the studied applied potential values, which indicated that the van der Waals force dominated for similar metal surfaces. The force measurement between similar electrode surfaces, which was difficult to be measure by the conventional SFA, provided new information. One of the advantages is that the SFA can evaluate the hydrogen evolution reaction on Pt electrode by monitoring EDL changes accompanied by the electrochemical reaction. The EC-SFA measurement can clearly demonstrate when (at which applied potential) the hydrogen (proton) adsorption starts, saturates, and hydrogen evolves. Such information should be important to develop an efficient electrochemical catalysis. Quantitative information on the effective electrochemical potential and surface charges as well as the adhesion between two electrode surfaces in the reaction involved in complex electrochemical devices and batteries can be obtained by the SFA. Such information should determine the sensitivity and the efficiency of the electrochemical sensors and batteries. The adhesion can be used as an additional parameter. Such information is essential for developing effective electrode materials for respective electrochemical processes.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (K.K.). ORCID

Sho Fujii: 0000-0002-6774-1482 Motohiro Kasuya: 0000-0002-2324-6121 Present Address

S.F.: Department of Chemistry, Faculty of Science, Hokkaido University, Kita-10, Nishi-8, Kita-ku, Sapporo 060-0810, Japan. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the Core Research for Evolutional Science and Technology (CREST) program of the Japan Science and Technology Agency (JST) and JSPS KAKENHI Grants 26248002 and 17K05740.

■

REFERENCES

(1) Bard, A. J.; Abruña, H. D.; Chidsey, C. E.; Faulkner, L. R.; Feldberg, S. W.; Itaya, K.; Majda, M.; Melroy, O.; Murray, R. W.; Porter, M. D.; Soriaga, M. P.; White, H. S. The Electrode/Electrolyte Interface−A Status Report. J. Phys. Chem. 1993, 97, 7147−7173. (2) Schmickler, W. Electronic Effects in the Electric Double Layer. Chem. Rev. 1996, 96, 3177−3200. (3) Choi, K.-S.; Lichtenegger, H. C.; Stucky, G. D.; McFarland, E. W. Electrochemical Synthesis of Nanostructured ZnO Films Utilizing SelfAssembly of Surfactant Molecules at Solid−Liquid Interfaces. J. Am. Chem. Soc. 2002, 124, 12402−12403. (4) McCreery, R. L. Advanced Carbon Electrode Materials for Molecular Electrochemistry. Chem. Rev. 2008, 108, 2646−2687. (5) Horn, R. G.; Israelachvili, J. N. Direct Measurement of Structural Forces between Two Surfaces in a Nonpolar Liquid. J. Chem. Phys. 1981, 75, 1400−1411. (6) Israelachvili, J. N. Intermolecular and Surface Forces, 3rd ed.; Academic Press, Ltd.: Cambridge, MA, 2011. (7) Fan, F.-R. F.; Bard, A. J. Ultrathin Layer Cell for Electrochemical and Electron Transfer Measurements. J. Am. Chem. Soc. 1987, 109, 6262−6268. (8) Connor, J. N.; Horn, R. G. Measurement of Aqueous Film Thickness between Charged Mercury and Mica Surfaces: A Direct Experimental Probe of the Poisson−Boltzmann Distribution. Langmuir 2001, 17, 7194−7197. (9) Fréchette, J.; Vanderlick, T. K. Double Layer Forces over Large Potential Ranges as Measured in an Electrochemical Surface Forces Apparatus. Langmuir 2001, 17, 7620−7627. (10) Fréchette, J.; Vanderlick, T. K. Electrocapillary at Contact: Potential-Dependent Adhesion between a Gold Electrode and a Mica Surface. Langmuir 2005, 21, 985−991.

26412

DOI: 10.1021/acs.jpcc.7b09301 J. Phys. Chem. C 2017, 121, 26406−26413

Article

The Journal of Physical Chemistry C (11) Valtiner, M.; Kristiansen, K.; Greene, G. W.; Israelachvili, J. N. Effect of Surface Roughness and Electrostatic Surface Potentials on Forces Between Dissimilar Surfaces in Aqueous Solution. Adv. Mater. 2011, 23, 2294−2299. (12) Valtiner, M.; Banquy, X.; Kristiansen, K.; Greene, G. W.; Israelachvili, J. N. The Electrochemical Surface Forces Apparatus: The Effect of Surface Roughness, Electrostatic Surface Potentials, and Anodic Oxide Growth on Interaction Forces, and Friction between Dissimilar Surfaces in Aqueous Solutions. Langmuir 2012, 28, 13080− 13093. (13) Tivony, R.; Yaakov, D. B.; Silbert, G.; Klein, J. Direct Observation of Confinement-Induced Charge Inversion at a Metal Surface. Langmuir 2015, 31, 12845−12849. (14) Tivony, R.; Klein, J. Probing the Surface Properties of Gold at Low Electrolyte Concentration. Langmuir 2016, 32, 7346−7355. (15) Kawai, H.; Sakuma, H.; Mizukami, M.; Abe, T.; Fukao, Y.; Tajima, H.; Kurihara, K. New Surface Forces Apparatus Using TwoBeam Interferometry. Rev. Sci. Instrum. 2008, 79, 043701. (16) Kurihara, K. Molecular Architecture Studied by the Surface Forces Measurement. Langmuir 2016, 32, 12290−12303. (17) Kamijo, T.; Kasuya, M.; Mizukami, M.; Kurihara, K. Direct Observation of Double Layer Interactions between the Potentialcontrolled Gold Electrode Surfaces Using the Electrochemical Surface Forces Apparatus. Chem. Lett. 2011, 40, 674−675. (18) Kasuya, M.; Sogawa, T.; Masuda, T.; Kamijo, T.; Uosaki, K.; Kurihara, K. Anion Adsorption on Gold Electrodes Studied by Electrochemical Surface Forces Measurement. J. Phys. Chem. C 2016, 120, 15986−15992. (19) Kasuya, M.; Kurihara, K. Characterization of FerroceneModified Electrode Using Electrochemical Surface Forces Apparatus. Langmuir 2014, 30, 7093−7097. (20) Smith, C. P.; Maeda, M.; Atanasoska, L.; White, H. S.; McClure, D. J. Ultrathin Platinum Films on Mica and the Measurement of Forces at the Platinum/Water Interface. J. Phys. Chem. 1988, 92, 199− 205. (21) Hegner, M.; Wagner, P.; Semenza, G. Ultralarge Atomically Flat Template-Stripped Au Surfaces for Scanning Probe Microscopy. Surf. Sci. 1993, 291, 39−46. (22) Vogel, N.; Zieleniecki, J.; Köper, I. As flat as it gets: Ultrasmooth Surfaces from Template-Stripping Procedures. Nanoscale 2012, 4, 3820−3832. (23) Blackstock, J. J.; Li, Z.; Freeman, M. R.; Stewart, D. R. Ultra-flat Platinum Surfaces from Template-Stripping of Sputter Deposited Flms. Surf. Sci. 2003, 546, 87−96. (24) Biegler, T. An Electrochemical and Electron Microscopic Study of Activation and Roughening of Platinum Electrodes. J. Electrochem. Soc. 1969, 116, 1131−1137. (25) Chan, D. Y. C.; Pashley, R. M.; White, L. R. A Simple Algorithm for the Calculation of the Electrostatic Repul-sion between Identical Charged Surfaces in Electrolyte. J. Colloid Interface Sci. 1980, 77, 283− 285. (26) Derjaguin, B. V.; Rabinovich, Y. I.; Churaev, N. V. Direct Measurement of Molecular Forces. Nature 1978, 272, 313−318. (27) Gamboa-Aldeco, M. E.; Herrero, E.; Zelenay, P. S.; Wieckowski, A. Adsorption of Bisulfate Anion on a Pt(100) Electrode: a Comparison with Pt(111) and Pt(poly). J. Electroanal. Chem. 1993, 348, 451−457. (28) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; John Wiley and Sons: New York, 2001. (29) In this study, we controlled the surface charge (potential) by varying the applied potential, so PZC is to abbreviate the potential of zero charge. One may note that PZC is used to abbreviate the point of zero charge when the surface charge (potential) is controlled by varying pH. (30) Zhan, D.; Velmurugan, J.; Mirkin, M. V. Adsorption/Desorption of Hydrogen on Pt Nanoelectrodes: Evidence of Surface Diffusion and Spillover. J. Am. Chem. Soc. 2009, 131, 14756−14760.

(31) Attard, G. A.; Brew, A.; Hunter, K.; Sharman, J.; Wright, E. Specific Adsorption of Perchlorate Anions on Pt {hkl} Single Crystal Electrodes. Phys. Chem. Chem. Phys. 2014, 16, 13689−13698. (32) Gileadi, E.; Argade, S. D.; Bockris, J. O. M. The Potential of Zero Charge of Platinum and Its pH Dependence. J. Phys. Chem. 1966, 70, 2044−2046. (33) Horiuchi, H.; Nikolov, A.; Wasan, D. T. Calculation of the Surface Potential and Surface Charge Density by Measurement of the Three-Phase Contact Angle. J. Colloid Interface Sci. 2012, 385, 218− 224. (34) Jerkiewicz, G.; Vatankhah, G.; Tanaka, S.-i.; Lessard, J. Discovery of the Potential of Minimum Mass for Platinum Electrodes. Langmuir 2011, 27, 4220−4226. (35) Considering the force data in Figures 4 and 5 and the cyclic voltammogram in Figure 3b, we judged that the surface potential ψ0 took the minimum at the applied potential E = 0.2 V. The sharp drop in the potential ψ0 stopped at 0.2 V with the decreasing E value, which is the local minimum, and slowly increased probably due to the adsorption of small amount of proton, which starts around 0.1 V according to the cyclic voltammogram in Figure 3b. We may add that 0.2 V is very close to the reported PZC value, 0.19 V, of the Pt electrode (see ref 31). When the more adsorption occurred at E below 0 V, the potential decreased again and took the minimum because the hydrogen evolution began at around E = −0.2 V.

26413

DOI: 10.1021/acs.jpcc.7b09301 J. Phys. Chem. C 2017, 121, 26406−26413