Two Sharp Phase Change Processes of Diphenyl Viologen at a Au

Dec 22, 2014 - Both processes exhibited sharp spikelike voltammetric responses. ... (HOPG) electrode and to exhibit a sharp faradaic phase transition ...
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Two Sharp Phase Change Processes of Diphenyl Viologen at a Au(111) Electrode Surface: Non-Faradaic Transition with Interplay of Ionic Adsorption of Chloride and Bromide and Faradaic One Tomohiro Higashi,† Teppei Kawamoto,§ Soichiro Yoshimoto,⊥,¶ and Takamasa Sagara*,‡ †

Department of Science and Technology and ‡Division of Chemistry and Materials Science, Graduate School of Engineering, Nagasaki University, Bunkyo 1-14, Nagasaki 852-8521, Japan § Graduate School of Science and Technology and ⊥Priority Organization for Innovation and Excellence, Kumamoto University, 2−39−1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan ¶ Kumamoto Institute for Photo-Electro Organics (Phoenics), 3−11−38 Higashi-machi, Higashi-ku, Kumamoto 862−0901, Japan S Supporting Information *

ABSTRACT: Two phase change processes of diphenyl viologen (dPhV) on a Au(111) electrode in KCl and KBr aqueous solutions were described using the results of voltammetric, electroreflectance (ER), and electrochemical scanning tunneling microscopic (EC-STM) measurements. Both processes exhibited sharp spikelike voltammetric responses. In KCl solution, the phase change at 0.30 V versus Ag/AgCl/saturated KCl was found to be a nonfaradaic order−disorder phase transition, from an ordered adlayer of dPhV dication (dPhV2+) with coadsorbed Cl− at more positive potentials than 0.30 V to a gaslike phase at less positive potentials. The faradaic reaction at −0.09 V was found to be the transition from the gaslike phase to a condensed monolayer of dPhV•+. The EC-STM images of the condensed monolayer showed stripe patterns of rows of π−π stacked dPhV•+. Almost the same set of two processes was observed in KBr solution but not in KF solution. In KF solution, although two voltammetric responses were observed, the peaks were small and broad, indicative of sluggish adsorption state changes of individual dPhV cations. Taken together, specific adsorption of coexistent anions is of critical importance for the occurrence of the sharp nonfaradaic phase transition. monomolecular film with a highly packed 2D crystal-like molecular alignment. It is exemplified by the adlayer of dBV•+ observed by electrochemical scanning tunneling microscopy (EC-STM).6 Our previous study on the phase change of diphenyl viologen (dPhV) adlayer on an HOPG electrode in KCl aqueous solution revealed that the one-electron redox reaction of dPhV•+/2+ is of a thin-layer electrochemistry but not a firstorder phase transition.12 Dicationic dPhV2+ molecules strongly adsorbed on the HOPG surface because of π−π interaction between phenyl rings of dPhV2+ and the HOPG surface. This interaction is inherently enhanced by the platelike dPhV2+ molecular structure with extended intramolecular π-conjugation. The strong adsorption of dPhV2+ molecules hampers their sharp phase transition of the first-order. A delicate balance of intermolecular interactions and interaction with electrode surface determines whether the first-order transition takes place. We found a two-step faradaic phase transition of the firstorder for dBV at an HOPG electrode in the presence of Br−

1. INTRODUCTION Sharp and distinctive structural changes of organic adlayers on electrode surfaces are applicable to switching devices and nanofabrication of molecular assemblies in wet systems, if the changes are finely controlled by electrode potential. Various viologens tend to adsorb on a highly oriented pyrolytic graphite (HOPG) electrode and to exhibit a sharp faradaic phase transition of the first-order.1−6 A deeper understanding of the phase transition mechanism is in demand with device applications in mind. Phase transitions of diheptyl viologen (HV)2−5 and dibenzyl viologen (dBV)6 so far studied at an HOPG electrode in aqueous media have been manifested by one anodic and one cathodic spikelike sharp peaks in cyclic voltammograms (CVs). The sharp response arises from a single-step first-order phase transition between a gaslike adlayer of viologen dication (V2+) and an insoluble two-dimensional (2D) condensed monolayer of viologen radical cation (V•+). In the 2D condensed monolayer, the longitudinal molecular axis of the bipyridinium moiety of V•+ is flat-lying with a side-on configuration of the moiety because of the face-to-face π−π stacking intermolecular interactions with the nearest neighbors on the HOPG electrode.6 Such a phase transition has also been observed at Hg7−9 and Au(111) electrodes10,11 in aqueous media. The condensed monolayer of V•+ on an HOPG is a © 2014 American Chemical Society

Received: March 3, 2014 Revised: December 18, 2014 Published: December 22, 2014 1320

DOI: 10.1021/jp5099238 J. Phys. Chem. C 2015, 119, 1320−1329

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The Journal of Physical Chemistry C

compartment after flame heating/water quenching. The quartz electrochemical cell was boiled in a sulfuric acid + nitric acid mixture and was rinsed with a copious amount of purified water before use. All the voltammetric and ER measurements were made in a hanging-meniscus (H-M) configuration of the Au(111) electrode in an Ar atmosphere (>99.998%) in a Faraday cage at room temperature (24 ± 2 °C). This configuration was set by horizontal touching of the electrode to the surface of dPhV solution from the Ar gas phase as follows. First, a Au(111) single crystal electrode was flameannealed and was quenched by water. Second, the anneal/ quench procedure was repeated. The electrode while hot after final annealing was carried in the main compartment and was cooled to room temperature in the Ar gas flow over the dPhV solution. Then, the electrode was horizontally touched to the gas/dPhV solution interface under setting the potential at 0.50 or 0.70 V. It was vertically lifted to set in the H-M configuration. C-E curves were obtained from the ac voltammograms assuming an equivalent series circuit of a capacitance (C) and a resistance. For the ac measurements including ER, a lock-in amplifier (EG&G, model 5210) was employed. 2.3. Electroreflectance (ER) Measurements. Normal incidence of monochromatic light was used for the Au(111) electrode in an H-M configuration. The details of the instrumentation and spectroelectrochemical cell are given elsewhere.19 The potential modulation is described as

over 75 mM.13 The first step of the transition at a less negative potential than the second-step potential is due to dBV•+ Br− mesophase formation upon reduction of dBV 2+ . The mesophase consists of dBV•+ monomer and Br− both directly adsorbed on the HOPG surface. The second step is the phase transition of the mesophase to a 2D condensed phase of dBV•+. Two-step faradaic phase transition is uncommon for viologens on an HOPG electrode surface, whereas examples of multistep transition are known at a few metal electrodes. They include HV on a Hg electrode in (water + DMSO) mixed solution of KBr,8 HV on a Au(111) in KBr aqueous solution,11 and dBV on a Cl− adlayer modified Cu(100) electrode in acidic KCl aqueous solution.14−17 These multistep transitions involve concerted assembling processes of viologen cations and counteranions. Recently, we have found two sharp voltammetric responses at a Au(111) electrode/dPhV aqueous solution interface in the presence of KCl or KBr. Surprisingly, one of the two responses emerges at 0.30 V (vs Ag/AgCl/saturated (sat’d) KCl), being ca. 0.40 V more positive than the reduction potential of dPhV2+. We herein report the results of in-depth studies of the transition mechanisms, especially the roles played by coexistent Cl− and Br−. We use the results of voltammetric, electroreflectance (ER), and EC-STM measurements to clarify the mechanisms of the surface processes and adlayer structures. Intriguingly, we have found that the sharp response at positive potentials is nonfaradaic. We further discuss anion effect using Cl− and Br− as specific-adsorption active anions at a Au(111) surface as well as F− as an inactive one. The results of this study may provide us with the new physical insight into the 2D assembling of cationic organic molecules at electrified interfaces with the interplay of the counteranions.

E = Edc + Eac = Edc + ΔEacexp(jωt )

(1)

where E is the electrode potential, Edc is the dc potential, Eac is the ac potential, ΔEac is the zero-to-peak ac amplitude, ω = 2πf is the angular frequency (f is the frequency of the potential modulation), t is the time, and j = √−1. Phase-sensitive detection of the ac intensity of the reflected light Iac was made by the aforementioned lock-in amplifier. When the response Iac to Eac is linear, Iac = ΔIac exp[j(ωt − ϕ)] where ΔIac is the zeroto-peak amplitude of Iac and ϕ is the phase of Iac with respect to Eac. The ER signal (ΔR/R)ER is defined as

2. EXPERIMENTAL SECTION 2.1. Materials. Diphenyl viologen (1,1′-diphenyl-4,4′bipyridinium dichloride: dPhV2+ 2Cl−) purchased from Tokyo Chemical Industry Co. was recrystallized from ethanol + acetone and was dried in vacuo. Water was purified through a Milli-Q integral (Millipore) to a resistivity over 18 MΩ cm. All other chemicals were of the highest reagent grade commercially available and were used as received. A single crystal Au disk electrode with a (111) surface at its bottom (surface area A = 0.502 cm2 as a photographically obtained geometrical area, facet precision −0.15 V. The occurrence of the potential dependent adsorption−desorption and its finite kinetics resulted in the greater |tan ϕ|. The amplitude of the change of Cl− adsorption amount by potential change took a maximum at 0.03 V. These potential dependencies are in harmony with CE curves. In the presence of 0.1 mM dPhV2+, the value of |tan ϕ| was ca. 0.5 between 0.00 and 0.15 V (Figure 3d). This much lower value than that in Figure 3b in the same potential region indicates that the coadsorption of dPhV2+ suppressed the 1324

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The Journal of Physical Chemistry C Scheme 1. Mechanism of Two-Phase Transition Processes of dPhV at a Au(111) Electrode Surface

phase species. The ordered monomolecular film of dPhV•+ is frequently referred to as the condensed state in which adsorbates are fixed to the surface and are close-packed, but sometimes their orientation is aligned with an allowed tilt and the packing may leave inter-row spacing. 3.3. EC-STM Images. To understand the adlayer structure of dPhV at the nanoscale, EC-STM measurements were carried out in the electrolyte solution. Figure 4 shows a series of ECSTM images of a Au(111) electrode surface obtained in 0.30 M KCl solution in the presence of 0.1 mM dPhV2+ 2Cl−. Figure 4a and b represents the STM images at Ework = 0.53 V, a more positive potential than Em1. In Figure 4a, several dark areas were observed on the terraces. The height differences between the dark and bright areas were found to be about 2.5 Å, which is equivalent to the monatomic step height of the Au(111) surface.36−38 In the higher-resolution STM image of the domains (Figure 4b), the rows consisting of dPhV2+ molecules with flat-lying orientation were straightly formed on the Au surface. Although we could not obtain more clear STM images of both dPhV2+ and Cl−, the obtained STM images suggested that the adlayer structure was consistent with the model proposed in Scheme 1. When Ework was stepped to 0.13 V (Em2 < Ework < Em1) from 0.53 V (Ework > Em1) during the scan of the terrace (Figure 4c), the ordered dPhV2+ adlayer was immediately changed into a totally disordered phase (upper part of the image over the horizontal arrow position). Although various combinations of Itip and Ework were examined in the potential region between the two transition potentials in Scheme 1, we could not resolve any adsorption species on the terrace as the image. On the basis of the results of CV and ER measurements, however, there are dPhV2+ and Cl− on the electrode surface in the potential region. Therefore, dPhV2+ forms a gaslike adlayer on a Au(111) surface in coexistence with Cl−, whereas the amount of Cl− is not enough to induce an ordered phase of dPhV2+. Most likely, high mobility of dPhV2+ and Cl− prevents the direct visualization by our conventional STM. Suto and Magnussen demonstrated a video STM image of the disordered adlayer of (bi)sulfate adsorbates moving on the millisecond time scale at Au(111) surface,39 while corresponding STM images of the disordered adlayer captured by a conventional STM mode40 are similar to the upper part of Figure 4c. When setting the Ework at more positive than Em1 after scanning the gaslike phase at Ework = 0.13 V, the ordered adlayer of dPhV2+ reappeared on the terrace, indicating that the order−disorder phase transition process is rapid and reversible.

around −0.10 V. These ERV responses were also clearly observed at a lower frequency, 14 Hz (Figure 3e, f). The value of Γ = (1.9 ± 0.2) × 10−10 mol cm−2 is the adsorption amount of dPhV•+ formed at Pc2. The same value of Γ was obtained from CVs measured with various dPhV2+ concentrations in the range of 0.03−0.3 mM. Therefore, this Γ value is the saturated amount corresponding to a monolayer with the area occupied by one dPhV•+ of 0.87 nm2. The projection area for one flat-lying dPhV•+ was estimated to be 1.2 nm2 for the face-on configuration of the bipyridiniumdiphenyl platelike moiety and 0.67 nm2 for the side-on configuration.12 We inferred from the experimental value of 0.87 nm2, much smaller than a close-packed flat-lying face-on dPhV•+, that dPhV•+ assumes almost side-on orientation. Meanwhile, a slightly greater value than 0.67 nm2 should be due to the face-to-face stacking distance between dPhV•+ or should be due to the inter-row spacing provided that dPhV•+ stays in rows. This should be clarified by EC-STM observations (vide infra). 3.2. A Model for Two Phase Changes. Using the voltammetric and ER data, we propose a model for the two phase transition processes of dPhV at a Au(111) electrode surface as shown in Scheme 1. A nonfaradaic sharp phase change gives rise to the Pc1-Pa1 peak couple at 0.30 V. An order−disorder phase transition of the adlayer coupled with adsorption−desorption of both dPhV2+ and Cl− takes place. Most likely, the Cl− underlayer is covered with dPhV2+ to form an ordered coadsorption layer. Electrostatic attractive interaction between the two ions on the Au(111) surface drives the new adlayer formation. A faradaic phase transition gives rise to the Pc2-Pa2 peak couple at −0.09 V. It takes place between a gaslike adlayer of dPhV2+ and an ordered monomolecular film of dPhV•+, although the transition is not of the first order. The face-toface π-stacking of dPhV•+ effects its side-on configuration on the electrode surface. In the intermediate potential region between the two main responses, dPhV2+ and Cl− coadsorb onto a Au(111) surface in a gaslike state, and their superficial concentrations are mildly potential dependent. In this potential region, dPhV2+ cannot form an ordered adlayer because of insufficient electrostatic attractive interaction between dPhV2+ and Cl−. The lower amount of adsorbed Cl− than the monolayer amount prevents an ordered adlayer formation. In the above model, the gaslike state signifies an adsorption layer in which the adsorbates are highly mobile on the surface and in which they are in dynamic equilibrium with the solution1325

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phase (lower part of the image below the horizontal arrow position), again indicating that the potential-driven phase transition process is reversible. The models in Scheme 1 are now fully supported by ECSTM imaging. Although detailed molecular packing structures remained to be clarified in future work, it is likely that, at more positive potentials than Pa1, dPhV2+ and Cl− form a bilayer in which dPhV2+ assumes an ordered layer over the specifically adsorbed Cl− underlayer on the Au(111) surface. Without coadsorptive anions, water-soluble V2+ exhibits sparse or no adsorptivity at the positively charged Au surface. Specifically adsorptive anion-assisted adsorption of a water-soluble cationic surfactant on a Au electrode surface at positive potentials has also been reported by Vivek and Burgess,42,43 Brosseau et al.,44 and Sek et al.45 With changing the electrode potential to more positive from ca. 0.0 V, the Cl− adsorption amount was increased in the disordered coadsorption layer with dPhV2+, and when Pa1 was reached, the Cl− adsorption amount exceeded a threshold, and new ordered phase formation suddenly occurred with additional ingress of dPhV2+ and Cl−. In the negative potential scan from ca. 0.4 V, this ordered coadsorption layer was disordered at Pc1 accompanied by at least partial desorption of dPhV2+ and Cl−. The remaining dPhV2+ and Cl− reverted back to the random and dilute adsorption state, namely, a gaslike adlayer. Formation of an upper layer of viologens on the Cl− underlayer is known at a Cu(100) electrode.14−17,46 Those bilayers, however, do not exhibit a sharp nonfaradaic phase transition, in contrast to the present Au(111)/dPhV. The bilayer structures of V2+ and Cl− on a Cu(100) electrode surface were previously observed by in situ STM by Safarowsky and co-workers, who found a square-shaped assembly unit consisting of four dBV2+ on a Cl− adsorbed Cu(100) electrode surface.16,17 Jiang et al. observed an ordered 2D dot-array structure of adsorbed HV2+ on a Cl− underlayer at a Cu(100) electrode surface.46 They proposed a model in which HV2+ assumes a flat-lying orientation of the longitudinal axis of the bipyridinium moiety with a face-on configuration of the moiety. Both of these bilayers of dBV2+−Cl− and HV2+−Cl− exhibit faradaic phase transitions to 2D condensed monolayers of V•+ in their previous works. However, the transitions are less sharp than the faradaic phase transition of Au(111)/dPhV under the present study. At more negative potentials than Pc2, Cl− completely desorbs from the Au(111) electrode surface when the surface reduction of dPhV2+ to dPhV•+ takes place, as indicated by the CV and C-E curves. The formation of the condensed monolayer of dPhV•+ is not a first-order transition process. The ordered adlayer structures of viologen should be determined by the molecular structures of dPhV2+ and dPhV•+. The results of DFT calculation in our previous report for dPhV2+ in vacuum showed a twisted molecular structure with a dihedral angle of 48.1° between bipyridinium moiety and its side phenyl rings; in contrast, the structure of dPhV•+ is platelike.12 These structures allow us to predict both a low molecular density alignment of sterically bulky dPhV2+ with leaving some intermolecular spacing in the rows at Ework > Em1 and a close-packed, π-stacking alignment of dPhV•+ in their rows at Ework < Em2. The obtained images look in line with the prediction so that the monomolecular film of dPhV•+ has a higher packing density than the ordered layer of dPhV2+. To confirm it using EC-STM, we are planning to acquire highresolution images.

Figure 4. EC-STM images of a Au(111) electrode surface in contact with 0.1 mM dPhV2+ 2Cl− + 0.30 M KCl solution. Tunneling conditions: (a) Ework = 0.53 V, Etip = 0.18 V, Ebias = −0.35 V, and tunneling current (Itip) of 0.40 nA. (b) Ework = 0.53 V, Etip = −0.17 V, Ebias = −0.70 V, Itip = 0.50 nA. (c) Ework = 0.13 V stepped from 0.53 V, Etip = −0.17 V, Ebias = −0.30 V stepped from −0.70 V, Itip = 0.80 nA. The image-capturing direction is from bottom to top. The arrow indicates the potential step upon passing Pc1-Pa1. (d) Ework = −0.15 V, Etip = 0.18 V, Ebias = 0.33 V, Itip = 0.80 nA. (e) Ework = −0.25 V, Etip = −0.12 V, Ebias = 0.13 V, Itip = 0.80 nA. (f) Ework = 0.13 V stepped from −0.15 V, Etip = −0.28 V, Ebias = −0.41 V stepped from −0.13 V, Itip = 1.5 nA. The image-capturing direction is from top to bottom. The arrow indicates the potential step upon passing Pc2-Pa2.

Figure 4d and e represents the STM images at Ework more negative than the potential of Pc2. The obtained stripe pattern phase of the ordered dPhV•+ provided clear evidence of the formation of dPhV•+ adlayer on the Au(111) electrode surface. Breuer et al. found similar stripe phases of dPhV•+ at Cu(100)/ dPhV•+ solution interface.41 They concluded that a monolayer of close-packed dPhV•+ is formed with flat-lying longitudinal axis of the bipyridinium moiety in a side-on configuration. The remarkable resemblance between the STM images in Figure 4d and e and those in ref 41 indicates the same alignment of dPhV•+ on Cu(100) and Au(111). The π−π stacking interactions both between bipyridinium moieties and between phenyl rings resulted in a face-to-face configuration. The platelike shape of dPhV•+ and the extended π-conjugation in individual dPhV•+ may originate the π-stacked adlayer formation. In Figure 4e, the stacked dPhV•+ form parallel rows, and the row-to-row distance is ca. 2.1 nm. This distance is longer than the dPhV•+ molecular length, 1.8 nm, from our density functional theory (DFT) calculation,12 indicating the presence of the inter-row spacing. This may explain the smaller Γ of dPhV•+ condensed layer than the calculated value for maximum packing. When Ework was stepped from −0.15 to 0.13 V during the scan (Figure 4f), the condensed monolayer of dPhV•+ was immediately transformed into a disordered gaslike 1326

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peak height at −0.11 V was approximately equal to that at −0.02 V (Pc1-Pa1 peak couple). In contrast, the ERV signal of 600 nm at −0.11 V was stronger than that at −0.02 V (Figure S5b of the Supporting Information). The stronger adsorptivity of Br− than Cl− causes the greater ER signal because of greater amplitude of the Au surface charge change. The gaslike dPhV2+ adsorbed on the Au surface should have been displaced from the Au surface by adsorption of Br− with the potential change to positive, resulting in emergence of higher voltammetric peaks and ER signal at −0.11 V than the responses in the presence of Cl−. When the displacement of dPhV2+ upon adsorption of Br− occurred at −0.11 V, ER bands due to absorption of gaslike adsorbed dPhV2+ should appear positivegoing (case II, vide supra). However, the UV−vis absorption spectra of gaslike adsorbed dPhV2+ and of dPhV2+ in solution are almost the same. Therefore, the positive-going real part ER signal at 390 nm was not observed (Figure S5a of the Supporting Information). Figure 6 shows the CV of 0.1 mM dPhV2+ 2Cl− + 0.30 M KF solution (solid line). We found that F− gives rise to no spike

3.4. Anion Effects for Phase Transitions of dPhV. Potential dependent adsorptivity of Cl− is a prerequisite for the occurrence of order−disorder phase transition in Scheme 1. We examined the use of Br− and F− as coexistent anions to clarify how the adsorptivity of anion affects the phase transition behavior of dPhV. Figure 5 shows the CV and the C-E curve of 0.1 mM dPhV2+ 2Cl− + 0.30 M KBr solution. Both nonfaradaic voltammetric

Figure 6. CV for a Au(111) electrode in an H-M configuration with 0.1 mM dPhV2+ 2Cl− + 0.30 M KF solution. The dotted line represents a background voltammogram obtained in 0.30 M KF solution free of dPhV. v = 50 mV s−1. Figure 5. (a) CV and (b) C-E curve for a Au(111) electrode in an HM configuration with 0.1 mM dPhV2+ 2Cl− + 0.30 M KBr solution. For CVs, v = 50 mV s−1. For C-E curves, v = −2 mV s−1, f = 14 Hz, and ΔEac = 7 mV.

response. The CV shows only small broad peaks at 0.50 and −0.11 V. A holding of the electrode potential at 0.70 V over 10 min did not change the CV curve. When the potential was swept more negative to −0.40 V, a diffusion-controlled bulk redox reaction of dPhV•+/2+ was recorded. The adsorption amount of dPhV2+ on the electrode surface in KF solution is lower than that of dPhV2+ in KCl and KBr solution. Because F− is a strongly hydrated hard anion, it does not stay near the surface of the dPhV2+ adlayer, in which electrostatic repulsion is predominant. Specifically adsorptive Cl− and Br− serve to concentrate dPhV2+ onto the surface because of the attractive electrostatic interaction. The gaslike adlayer of dPhV2+ with a sufficient amount to be a precursor state for the sharp faradaic transition does not form with 0.3 M of F−. We concluded that the origin of order−disorder nonfaradaic phase transition of dPhV2+ is specific adsorptivity of Cl− and Br− on the Au electrode surface. We are planning an extensive future work on the anion-type dependence. Note that negligibly weak binding of F− to a surface-exposed viologen moiety of viologen-thiol self-assembled monolayers was established, whereas Cl− and Br− exhibit strong binding.47,48 Finally, it is worthwhile to note the high symmetry of the coupled cathodic and anodic CV peaks and the absence of considerable scan-direction dependent hysteresis of C-E curves for the nonfaradaic processes of dPhV2+ when Cl− or Br− is the counteranion. In the absence of dPhV2+, the low symmetry and the hysteresis are obvious because of reconstruction and its lifting of the Au(111) electrode surface, and they are rather scan-direction dependent as exemplified by the dotted line in Figure 1a. Most likely in the presence of dPhV2+, the Au surface

Pc1-Pa1 peak couple at −0.02 V and faradaic Pc2-Pa2 peak couple at −0.22 V were uncovered by the measurements of ERS (see Figure S4 of the Supporting Information). In 0.30 M potassium salt solution, CV peaks in KBr solution shifted to more negative potentials compared with the peaks in KCl solution. The peak shifts may result from stronger adsorptivity of Br− than Cl− to the Au(111) surface. In fact, significant Br− adsorption onto a Au electrode takes place at less positive potential than Cl− adsorption potential.21 The Pc1-Pa1 peak couple exhibited smaller peak widths in KBr solution than in KCl solution. The strong attractive electrostatic interaction between dPhV2+ and Br− may cause the smaller width; relatively stronger adsorption of Br− on the Au surface promoted coadsorption of dPhV2+ and thus resulted in stable adlayer formation. The Pc2-Pa2 peak couple exhibited nearly the same peak width in KBr solution as that in KCl solution. A minor peak at −0.11 V was more obviously observed in KBr solution (Figure 5) than the 0.02 V minor peak in KCl solution (Figure 1). The process at −0.11 V represents no indication of faradaic process in ERS (see Figure S5a of the Supporting Information); the ERS at −0.11 V did not exhibit the 390 nm band, which represents adsorption−desorption process of dPhV2+. The ERS exhibited the same waveform as Figure 2b, indicating that the adsorption−desorption process of Br− takes place at −0.11 V. As shown in Figure 5b, the capacity 1327

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The Journal of Physical Chemistry C exhibits entirely the (1 × 1) structure at more positive potentials than the peak at 0.00 V in KCl (Figure 1) and −0.11 V in KBr (Figure 5). In other words, C-E curve measurements at positive potential are performed entirely on the Au(111) (1 × 1) surface. This is supported by the experimental fact that Au island formation, which is frequently reported as an indication of surface reconstruction and its lifting,49 was never observed as far as positive potentials were kept in our EC-STM measurements.

Notes

The authors declare no competing financial interest.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Plot of peak currents (ip) in cyclic voltammograms for a Au(111) electrode in an H-M configuration with 0.1 mM dPhV2+ 2Cl− + 0.30 M KCl solution as a function of potential sweep rate (v). UV−vis absorption spectrum of 0.1 mM dPhV2+ 2Cl− + 0.30 M KCl aqueous solution. UV−vis absorption spectrum of powder dPhV2+ 2Cl− analyzed by means of Kubelka−Munk function using a UV−vis diffuse absorption reflectance spectrum. ER spectra for a Au(111) electrode in an H-M configuration with 0.1 mM dPhV2+ 2Cl− + 0.30 M KBr solution. ER spectrum and ER voltammogram for a Au(111) electrode in an H-M configuration with 0.1 mM dPhV2+ 2Cl− + 0.30 M KBr solution. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

This work was financially supported in part by Grant-in-aid for Scientific Research from MEXT, Japan (No. 24550158) to T. S., by Tokyo Ohka Foundation to T. S., by Kato Foundation for Promotion of Science to T. H., and by Japan Society for the Promotion of Science to T. H.

4. CONCLUSION Two phase transition processes of dPhV on a Au(111) electrode were investigated in KCl and KBr aqueous solution. In 0.30 M KCl solution with 0.1 mM dPhV2+, a nonfaradaic order−disorder phase transition, from an ordered coadsorption layer of dPhV2+ and Cl− at positive side to a gaslike phase at negative side, takes place at 0.30 V. The ordered coadsorption adlayer formation is driven by electrostatic attractive interaction between dPhV2+ and Cl− on a Au(111) surface. The EC-STM image at more positive potential than 0.30 V represented a stripe pattern of the rows of adsorbed dPhV2+ with flat-lying orientation. We proposed a model that this ordered adlayer of dPhV2+ formed on a Cl− underlayer on the Au surface. The redox reaction of thin-layer electrochemistry of dPhV•+/2+ at −0.09 V involves phase transition between a gaslike adlayer of dPhV2+ and a 2D condensed monolayer of dPhV•+. The 2D condensed monolayer formation is due to face-to-face π−π stacking interaction between dPhV•+ originated from their platelike molecular structure and extended π-conjugation. The condensed layer in EC-STM images showed stripe pattern phase of close-packed dPhV•+ with a flat-lying orientation in a side-on configuration. Almost the same set of two phase transition processes was observed in KBr solution but not in KF solution. The specifically adsorptive anions effect the adsorption of dPhV2+ onto the surface because of the attractive electrostatic interaction. We conclude that the order−disorder nonfaradaic phase transition of dPhV2+ is originated from interplay of specific adsorption of Cl− and Br− with dPhV2+ adsorption. Such a nonfaradaic phase change process has been uncommon so far among the potential-driven phase transitions of viologen adlayers.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 1328

DOI: 10.1021/jp5099238 J. Phys. Chem. C 2015, 119, 1320−1329

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The Journal of Physical Chemistry C

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DOI: 10.1021/jp5099238 J. Phys. Chem. C 2015, 119, 1320−1329