Nanoscale Electrochemical Surface Science on Molecular Assembly and Surface Function

Abstract

This study focuses on electrochemical potential control and fabrication methods for various metal complexes and polycyclic aromatic hydrocarbons at solid–liquid interfaces, particularly using electrochemical scanning tunneling microscopy (EC-STM) and atomic force microscopy (AFM). The in situ observation of molecular assemblies and understanding of the phase transition dynamics and ligand exchange reactions at the molecular and/or submolecular levels provide information on functional molecular design and surface engineering. In addition, ionic liquids electrochemistry is summarized from the viewpoint of single-crystal electrochemistry.

1. Introduction

Electrochemical surface science at solid–liquid interfaces has significantly progressed since the first report on the adsorption–desorption of hydrogen on a platinum single-crystal electrode in 1980,¹ with the establishment of in situ observation methods for various solid/liquid interfaces.² In particular, scanning tunneling microscopy (STM), developed by Binnig and Rohrer in 1981,³ has been recognized as a powerful technique that enables direct observation of electrode surfaces at the atomic level, as well as atomic force microscopy (AFM), which was developed after STM. Understanding the nature of the solid–liquid interface in detail elucidates the electrode reaction mechanism, such as the dissolution and underpotential deposition of metal ions and organic molecules, leading to useful information for designing new interfaces.^4–8 Several research groups have discussed the importance of studying electrode–electrolyte interfaces in the field of electrochemistry. In early 1988, Itaya et al. proposed a new concept for in situ STM with a four-electrode configuration, where the electrochemical potentials of the tunneling tip and substrate can be independently controlled with respect to a common reference electrode.⁹ Other research groups independently developed a similar potentiostatic STM apparatus in 1988.^10–12 The organic molecules observed by STM are limited to molecular layers with a thickness of a few nanometers owing to their low conductivity, which allows the monitoring of tunneling currents by applying a bias voltage between the substrate and STM tip. By contrast, AFM is versatile, with almost no restrictions on the observations. However, STM is widely accepted as a powerful tool for understanding the structure of adsorbed layers of molecules on metal surfaces at the atomic scale, not only in UHV^13–16 but also in solution.^17–22 This is because STM imaging is still prioritized for identifying individual atoms and molecules and visualization of the intramolecular structure at electrode–electrolyte interfaces.

Controlling the molecular assembly of metal complexes and π-conjugated compounds is essential for developing new materials science and devices. In particular, the adlayers of thin films and the domain growth process are extremely important for understanding their fundamental physical properties, which are expected to develop into nanoarchitectonics, as recently proposed by Ariga.^23,24 The epitaxial growth of these functional molecules can be controlled in both the gas phase (in vacuum) and the liquid phase (in liquid). The driving forces for growth are changes in the substrate temperature and/or the equilibrium between the molecules and substrate in solution.

In this study, we focus on the molecular assemblies of metal complexes and polycyclic aromatic hydrocarbons (PAHs) at electrochemical interfaces using electrochemical STM and AFM. In addition, ionic liquid electrochemistry is described from the viewpoint of the electrochemical surface science of gold single-crystal electrodes.

2. Preparation of Well-defined Electrode Surfaces

Clean electrode surfaces were prepared using single-crystal electrochemistry to prepare the molecular adlayers. Studies on Pt single-crystal electrodes in sulfuric and perchloric acids were first reported in 1980 to prepare well-defined clean Pt surfaces in aqueous solutions, in which mechanically prepared Pt single-crystal electrodes were annealed in an oxygen flame and quenched in ultrapure water.¹ This report is recognized as a pioneering work in electrode surface science using STM and spectroscopic techniques. Figure 1 shows the typical voltammograms of clean Pt(111) and Au(111) surfaces fabricated by our group. The voltammograms show characteristic peaks, symmetric spikes, and so-called “butterfly peaks,” indicating that a clean Pt(111)¹ and/or Au(111) surface was exposed to the electrolyte solution.^25,26 It is now recognized that the observed spikes are associated with an order–disorder phase transition of the bisulfate/sulfate adlayer. The magnitude of the current spike reflected the cleanliness of the electrode surface. Detailed investigations on the adsorption of bisulfate/sulfate on Au(111) surfaces were performed using in situ STM,^27–29 radioactive labeling,³⁰ second harmonic generation,³¹ electrochemical quartz microbalance,³² in situ infrared reflection absorption spectroscopy (IRAS),³³ and surface-enhanced infrared reflection absorption spectroscopy (SEIRAS).³⁴ Several methods have been used to clarify the adlayer structure and dynamics of sulfate/bisulfate ions. Among these techniques, the reconstructed rows, the so-called $\sqrt{3} $ × 22 structure on terraces at potentials lower than the potential of zero charge, are lifted to the (1 × 1) structure because of the adsorption of bisulfate/sulfate ions and the ordered ($\sqrt{3} $ × $\sqrt{7} $) structure of bisulfate/sulfate on Au(111) by changing the electrode potential. In the 5.0 M sulfuric acid solution, an order–disorder phase transition occurs further at the positive potential side, and a new reversible spike appears as the oxidation potential window becomes wider owing to the increased concentration (Fig. 1c).³⁵ At such high potentials, the $\sqrt{3} $ × $\sqrt{7} $ structure disappears and a random adsorption state is observed. In any case, the degree of cleanliness, as evidenced by the voltammograms, may affect the molecular resolution for the observation of molecular adlayers, and sufficient attention should be paid not only to the quality of the electrode surface but also to the purity of the solution and cleaning of the electrochemical cell.

Figure 1.

Typical cyclic voltammetry (CV) profiles of a well-defined (a) Pt(111) and (b) Au(111) electrode in 0.5 M H₂SO₄ and (c) Au(111) in 5.0 M H₂SO₄, recorded at a scan rate of 0.05 V s⁻¹. (d) STM images of a well-defined Au(111) electrode obtained in 0.5 M H₂SO₄ with illustrations of reconstructed and unreconstructed Au surfaces. High-resolution STM image and illustration of a structural model of the ($\sqrt{3} $ × $\sqrt{7} $) adlayer on Au(111)–(1 × 1). Reprinted with permission from Ref. 35, Copyright Elsevier Science (2006).

Potential control is important for controlling the adlayer structure and molecular assembly, particularly for monolayers of various metal complexes. Our group has investigated the 2D self-assembly of porphyrins, phthalocyanines, porphycenes, triple-decker complexes, and terpyridine derivatives on electrode surfaces. These highly hydrophobic complexes usually maintain stable adlayer structures near the potential region of hydrogen evolution because of the low solubility to the aqueous solution phase. However, in some cases, the adlayer structure changes (phase transition) when the surface excess and/or molecular symmetry is low.

3. Water-soluble Cationic Organic Molecules

The adsorption of anions, such as chloride, bromide, iodide, cyanide, and sulfate/bisulfate, on electrode surfaces was one of the most important subjects in electrochemistry in the early 1980. Various electrochemical processes, such as the underpotential deposition of hydrogen and metal ions, are strongly affected by coadsorbed anions.^5,8 These adlayers contribute to forming highly ordered arrays of water-soluble organic molecules. The highly ordered 2D formation of 5,10,15,20-tetrakis(N-methylpyridinium-4-yl)-porphyrin (TMPyP) molecules on I-modified Au(111) was one of the first successful attempts.³⁶ In general, the adlayer structures of organic molecules on a substrate are controlled by the balance between adsorbate and substrate (epitaxial) interactions and intermolecular interactions. Relatively weak van der Waals interactions between the hydrophobic iodine adlayer and organic molecules are key factors in promoting self-ordering processes on the I-Au(111) substrate. Subsequently, the ordering of a free-base porphyrin array directly attached to a clean Au(111) surface at the electrochemical interface was reported by Borguet et al. Potential manipulation plays a significant role in controlling the surface mobility of tetrakis(4-pyridyl)porphyrin (H₂TPyP) molecules.³⁷ We reported that an electrochemically-produced sulfate/bisulfate adlayer on Au(111) serves as a template for a cationic tetraphenyl porphyrin with two carboxyphenyl moieties in the cis- and trans-positions (cis- and trans-H₄DCPP²⁺), as shown in Fig. 2.³⁸ In this system, the formation of supramolecularly organized nanostructures of cis-H₄DCPP²⁺ such as dimers, trimers, and tetramers on the ($\sqrt{3} $ × $\sqrt{7} $) bisulfate/sulfate adlayer was formed. The high-resolution STM image suggests the importance of both electrostatic interactions between the cationic porphyrin core and bisulfate/sulfate adlayer and formation of hydrogen bonds between the carboxyl groups of the nearest-neighbor cationic porphyrins. By contrast, when the potential was changed to potentials more negative than 1.1 V, H₄DCPP²⁺ disappeared from the terrace. The electrode potential changes revealed that H₄DCPP²⁺ was mobile on the terrace owing to the phase transition of the bisulfate/sulfate adlayer. The formation of the ($\sqrt{3} $ × $\sqrt{7} $) bisulfate/sulfate adlayer plays an important role in controlling cis-H₄DCPP²⁺ adlayer formation. A similar structural change by potential manipulation was found in diphenyl viologen (dPhV) on an Au(111) electrode in KCl and KBr aqueous solutions.³⁹ The EC-STM images revealed two phase change processes assigned to a non-faradaic order–disorder phase transition, from an ordered adlayer of dPhV dication (dPhV²⁺) with coadsorbed Cl⁻ and the gas-like phase to a condensed monolayer of dPhV^•+. Thus, 2D organization and order–disorder phase transitions of water-soluble (cationic) organic molecules are achieved at electrochemical interfaces.

Figure 2.

Cyclic voltammogram (central) and potential-dependent STM images (upper part) of Au(111) electrodes in 0.05 M H₂SO₄ nearly saturated with cis-H₄DCPP²⁺ (blue solid line). The scan rate was 10 mV s⁻¹. The dotted line indicates a CV of a bare Au(111) electrode. UV–Visible spectrum of 0.5 M H₂SO₄ nearly saturated with cis-H₄DCPP²⁺ (left part) and STM images of nanostructured cis-H₄DCPP arrays of tetramer, trimer, and dimer are shown with structural models (right part). Reprinted with permission from Ref. 38, Copyright American Chemical Society (2008).

4. Molecular Assembly of Organic Molecules

4.1 Porphyrin and phthalocyanine assemblies

The molecular assemblies and nanostructures of metalloporphyrins and metallophthalocyanines (MPcs) have been extensively investigated. In particular, Hipps et al. reported various MPcs (M = Cu, Co, Ni, Fe, and VO)^40–43 and metallotetraphenylporphyrins (MTPP) on Au(111).^44,45 The brightness of the center spot of Pc or TPP depends on the central metal. The difference in contrast between the metal ions in the STM images can be explained in terms of the occupation of the $\text{d}_{z^{2}}$ orbital. These studies encouraged us to investigate these adlayers at electrochemical interfaces. We succeeded in spontaneously forming highly ordered molecular arrays of these molecules on Au(111) surfaces by immersing Au(111) in benzene solutions containing the molecules.^46–48 The adlayers of CoTPP and CuTPP thus formed were observed in 0.1 M HClO₄, and the adlayer structures were identical to those obtained in UHV⁴⁶ and independent from the central metal ions. The CoTPP or CuTPP molecules were easily identified by the strong tunneling current resulting from orbital-mediated tunneling through the half-filled $\text{d}_{z^{2}}$ orbital of the Co^II ion (d⁷); a bright spot appeared at the center of each molecule, whereas the Ni^II (d⁸), Cu^II (d⁹), and Zn^II (d¹⁰) ions had a fully filled $\text{d}_{z^{2}}$ orbital.^40–43 For the CoPc, CuPc, and ZnPc adlayers, identical phenomena were observed for the brightness in the central spot.^47,48

In addition to the molecular-level identification of porphyrins and phthalocyanines depending on the central metal ion, several issues need to be addressed at the electrochemical interface, such as the differences in their interactions with the substrate (atomic arrangement), effects of different chemical frameworks and functional groups on the packing arrangement, and effect of the electrode potential on the mixed two-component structure in an electrolyte solution. Several interesting aspects need to be elucidated at the electrochemical interfaces. In the mixed system of copper octaethylporphyrin and cobalt phthalocyanine (see Fig. 3a), molecular-level identification of the two molecules, the effects of regular alternation formation and atomic arrangement differences on structure formation, and the phase separation of the two components by controlling the potential were revealed.⁴⁹ In the former system, the center spot of each CuOEP appeared dark, whereas that of CoPc was the brightest. The two-component adlayer consisting of CuOEP and CoPc, which has p(9 × 3$\sqrt{7} $R-40.9°) or p(9 × 3$\sqrt{7} $R-19.1°) structures with the constituent molecules arranged alternately, each involving two molecules on the Au(111) surface. In this system, the surface mobility and molecular reorganization of CuOEP and CoPc were accelerated by manipulating the electrode potential.^49,50 Investigation of the difference in the chemical frameworks of ZnOEP and ZnPc also revealed that the difference in the frameworks contributed to the formation of the electrochemical phase-separated adlayer.⁵¹ The mixing state of the two components changed continuously with precise potential control (Fig. 3b). For bi-component adlayers consisting of hydrophobic metalloporphyrin and metallophthalocyanine, the adsorption equilibrium of the ‘mild’ physisorption also contributes to precise and unique 2D molecular assembly control at electrochemical interfaces.

Figure 3.

(a) Potential-dependent STM images of the (a) CuOEP and CoPc bi-component adlayer and (b) ZnOEP and ZnPc bi-component adlayer. Both adlayers were prepared by immersing the Au(111) substrate into benzene solutions consisting of the two compounds and studied on Au(111) in 0.1 M HClO₄. Reprinted with permission from Refs. 49–51, Copyright American Chemical Society (2004, 2006 & 2008).

4.2 O₂ in the CoT_pivPP arrays

Understanding the dynamics of ligand coordination, such as the binding of O₂ to related species, provides a model of the dioxygen storage process. Because Co porphyrins exhibit electrocatalytic activity for O₂, understanding their role and electrochemical properties is essential in relation to phenomena such as dioxygen storage and electrocatalytic mechanisms for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The binding affinity of O₂ was controlled by the coordination of the axial ligand to the vacant sixth coordination site. On Au single-crystal surfaces, interesting phenomena have been observed in the basket consisting of four tert-butyl terminal groups, the so-called ‘picket-fence’ porphyrin adlayers. As shown in Fig. 4, individual CoT_pivPP molecules are square-shaped with four bright spots at the corners of the characteristic nanobelt array on Au(111), whereas the adlattice is approximately identical to that of CoTPP. For CoTPP, a slight difference in the height of the central brightest spot was found between STM images recorded at different potentials in an air-contained 0.1 M HClO₄, i.e., the difference in the corrugation heights observed at 0.70 and 0.20 V vs. RHE, which was ∼0.1 nm.⁵² The result indicates that the dioxygen species coordinated to the central Co(III) ions are reduced at 0.20 V, and the oxygen species are released. Subsequently, the central Co(III) ion is reduced to the Co(II) ion. At potentials more positive than 0.70 V vs. RHE, the difference in the height of the central Co ion implied the possibility of Co(III)–O₂⁻ complex. This difference was observed even in the CoTPP adlayer (0.85 V, as shown in Fig. 4b). The height of the central spot in the CoTPP molecule (dashed circle) is lower than that of the other CoTPP molecules. The height difference is ∼0.1 nm. Subsequently, Wang et al. carefully investigated the potential dependence of the CoTPP-modified Au(111) electrode surface under O₂-saturated 0.1 M HClO₄.^53,54 EC-STM imaging provides coordination of O₂ species at the molecular scale as an electrocatalyst for ORR. Details of the electrocatalysis of metalloporphyrin-modified electrode surfaces were summarized in a recent review by Facchin and Durante.⁵⁵ In the CoT_pivPP adlayer, the framework of a CoT_pivPP molecule was brighter, indicating the coordination of O₂ or related species to the central Co ion. An interesting phenomenon regarding the state of molecular oxygen trapped in the cavity of CoT_pivPP was distinctly observed in the STM images as a bright spot in the nanobelt array formed when a reconstructed Au(100)–(hex) surface was used as the substrate for the CoT_pivPP adlayer. These results suggest that the arrangement of the underlying Au atoms plays an important role as the sixth ligand coordination site, assisting in forming O₂-adducted CoT_pivPP.

Figure 4.

(a) Typical CV profiles of ORR. Typical high-resolution EC-STM images of the highly ordered (b) CoTPP and (c) CoTpivPP array on Au(111) were observed in 0.1 M HClO₄. (d) O₂-coordinated CoTPP and CoT_pivPP models. Reprinted with permission from Ref. 52, Copyright American Chemical Society (2007).

We extended this approach to more complex metal complexes, as shown in Fig. 5. The use of complexes other than metalloporphyrins and phthalocyanines expanded. In the direct coordination synthesis of bis-terpyridine and cobalt ions on an Au(111) substrate, we identified the coordination sites between the cobalt ions and terpyridine groups. In general, the structure of a metal ion surrounded by six pyridine groups between two bis-terpyridine molecules is often drawn in a schematic.⁵⁶ However, on the Au(111) substrate, terpyridine moieties are adsorbed parallel to the Au(111) substrate, making the formation of an intermeshed coordination structure between the two molecules difficult, as shown in Fig. 5a. In the EC-STM image, a bright spot, possibly a cobalt ion, is observed in the gap between the long axes of the terpyridine moieties adsorbed in a parallel orientation. The monolayer also showed the presence of cobalt in the XPS measurements, and the ORR behavior of the monolayer in perchloric acid was similar to that of the cobalt porphyrins, indicating that the cobalt ions were coordinated. Porphycene is an isomer of porphyrin with lower symmetry (D_2h), and its derivatives have been studied for their potential role in understanding biological functions such as oxygen binding affinity, as well as their use in nanoscience owing to the cis-trans tautomerization of the inner protons in their free base form.⁵⁷ We investigated the adlayer of a Ni porphycene derivative with n-propyl functional groups.⁵⁸ The molecular assembly of the low symmetry of the framework reveals a characteristic packing arrangement and undergoes electrochemically reversible phase transitions by the conformational change in the n-propyl groups. Individual NiTPrPc molecules are identified in alternating bright and dark rows, assigning different configurations to the n-propyl groups on the porphyrin ring. Density functional theory (DFT) calculation supports several possible configurations, such as the “flat” orientation and “arms-spread” and “arms-up” types of orientations for the four n-propyl groups on the porphycene ring. An extended porphyrinoid complex in 30-electron systems with n = 7 in 4n + 2 aromaticity is one of the challenging targets for a larger metalate complex.⁵⁹ The direct STM observation of the cobalt- and nickel-complexed adlayers on Au(111) revealed that these three ions were arranged specifically in the shape of a triangle (see Fig. 5c).

Figure 5.

Graphical images of the (a) direct synthesis of Co²⁺ and the bis-terpyridine derivative, (b) phase transition of the Ni porphyrin derivative on Au(111), and (c) direct observation of triangular expanded hemiporphyrazines on Au(111). Reprinted with permission from Refs. 56, 58, 59, Copyrights the Royal Society of Chemistry (2010), American Chemical Society (2016), and the Chemical Society of Japan (2021).

We also investigated the organization of ruthenium multinuclear clusters.^60,61 Multinuclear metal complexes exhibit mixed-valence states, which differ from mononuclear porphyrin complexes in terms of their multiredox properties and the development of molecular computing systems, including quantum-dot cellular automata (QCA) cells. We found that the organization of paddlewheel-type mixed-valence complexes with two perpendicularly stacked ruthenium centers provided by Prof. Kimizuka (Kyushu University) depended on the type of functional group, and the phenyl, naphthyl, and anthracenyl groups via vinyl groups were effective for the organization on Au(111). As shown in Fig. 6a, the characteristic redox couple of complex 1 was observed in the potential region between 0.75 and 0.10 V vs. RHE in 0.1 M HClO₄. The amount of transferred electronic charge can be estimated by integrating the peak area, and the average value was 1.12 × 10⁻⁵ C cm⁻², indicating that one-electron reaction from Ru^II/Ru^III to Ru^II/Ru^II. Interestingly, these derivatives were oriented by the electrochemical reduction of Ru ions. Furthermore, in situ, observation of the redox process of the complexes allowed direct visualization of the exchange reaction of the axial ligands. Figure 6a shows an overview of the potential-dependent in situ STM images of the same area of the paddlewheel diruthenium complex on Au(111). The molecules of complex 1 appear as bright and dark spots in the ordered domain because the Ru^II/Ru^III and Ru^II/Ru^II redox states can coexist in this potential region. The average height amplitude difference was 0.4 nm. At the potentials where the Ru^II/Ru^III and Ru^II/Ru^II redox states were in equilibrium, two distinct redox states were identified at the single-molecule level. Axial Cl⁻ ligand exchange took place in the 2D-ordered adlayer 1 during the scan, suggesting that Cl⁻ has a potentially controlled weak interaction with the diruthenium complex, as suggested in the models shown in Fig. 6a. We also conducted research on Ru trinuclear clusters in collaboration with Prof. Abe (University of Hyogo), who provided complexes with various functional groups coordinated to the ruthenium trinuclear core and is conducting electrochemical and surface science studies on the effects of functional groups on the organization and redox potentials. The EC-AFM images of a trinuclear cluster thin film of dichloroacetic acid-type bipyridine-coordinated complex-cast HOPG are shown in Fig. 6b. Although the resolution of AFM was not as high as that of STM, structural changes in the complex molecules were observed on a submicron scale. Complex 2 exhibited oxidation states of Ru^II/Ru^III/Ru^III for the three Ru ions in the reduced state but Ru^III/Ru^III/Ru^III in the oxidized state, indicating a one-electron redox reaction. The 2-adsorbed HOPG electrode exhibited a clear redox couple at 0.72 V vs. RHE. The cyclic voltammograms indicated a fast electron transfer reaction, even at a sweep rate of 5 V/s. By contrast, the reduced state showed disordered AFM images; however, island-like structures appeared on the terrace in the oxidized state. Stripe-like structures are observed on these islands. The island structure disappeared immediately upon re-reduction, but the structural changes were reversible. These domains disappeared immediately when the potential was stepped from 1.0 to 0.5 V. When the potential was switched from 0.5 to 1.0 V, several domains were formed again. A clear change is also observed near the step region between the dotted lines in the upper portion of AFM images in Fig. 6b. Similar rapid formation and deformation of domains occurred during scanning, and a comparison of the two AFM images in Fig. 6b shows that the position and size of the domains changed, particularly in the lower half of the scan area. These results indicate that the structural changes were reversible and occurred rapidly, verifying the importance of metal complex design not only for redox chemistry but also for molecular assembly and nanoarchitecture construction.

Figure 6.

(a) Potential- and time-dependent STM images of complex 1 adlayer on Au(111). Model of Cl coordination and noncoordination during the redox process of complex 1 in the highly ordered Au(111) adlayer in 0.1 M HClO₄. The CV profile was recorded at a scan rate of 20 mV s⁻¹. (b) Typical cyclic voltammograms of 2-deposited HOPG electrode in 0.1 M HClO₄ recorded at various scan rates from 5 to 100 mV s⁻¹. The potential dependent EC-AFM images of 2-deposited HOPG in 0.1 M HClO₄. The electrode potential was kept at 0.50 V and/or 1.00 V by stepping potential during the scan. Reprinted with permission from Refs. 60 and 61, Copyrights American Chemical Society (2012) and the Royal Society of Chemistry (2022).

5. Coadsorption of PAHs with Halides and Thiols

Over the last five years, we have actively researched the interfacial organization of water-insoluble nanographenes. For example, coronene is often one of the target molecules for 2D molecular assemblies on surfaces because of its planar and symmetrical structure.⁶² Because coronene is also a hydrophobic molecule, the adlayer is stable on Au surfaces at aqueous electrolyte solution interfaces. The potential-dependent STM of a coronene adlayer was investigated on an Au(111) electrode surface and revealed that a phase transition occurred at negative potentials near hydrogen evolution.⁶³ By immersing a coronene and/or circobiphenyl adlayer on a Au(111) substrate in a 0.1 mM aqueous KAuBr₄ solution, a characteristic mixed adlayer consisting of coronene and/or circobiphenyl molecules and Br⁻ ions with monatomic Au islands was produced on the Au(111) surface by the spontaneous reduction of AuBr₄⁻ (see Fig. 7a). When PtBr₄²⁻ was used on such PAH-coated Au(111) surfaces, Pt nanoclusters with diameters of 2–3 nm are uniformly formed on the PAH adlayer.⁶⁴ Despite spontaneous electrodeposition, the amount and shape of the electrodeposition are controlled. The circobiphenyl adlayer plays a role in forming characteristic Pt nanoclusters, whereas low-symmetry aromatic hydrocarbons such as ovalene are more oriented than coronene owing to their low symmetry. Conversely, the long-axis directions were random. However, their low symmetry leads to the formation of regular and highly symmetric nanoscale vacancies via electrochemical phase transitions. In the case of ovalene, the nanoscaled vacancies obtained at lower potentials are adequately large to adsorb a single S atom of a thiol, and when thiol molecules were adsorbed while maintaining the pore structure by controlling the potential, 4-pridinethiol and MPA were adsorbed (see Fig. 7b).⁶⁵ The thiol concentration and adsorption time are also relevant, but under the right conditions, only one molecule of thiol can be adsorbed in the pore, and “isolated” adsorption without “self-assembly” can be achieved. We call this method “molecular planting” and propose it as a way to fabricate molecular adlayers in which thiol molecules are arranged at equal intervals. Therefore, monolayers of aromatic hydrocarbons with low-symmetry structures show promising possibilities in terms of structural control (vacancy control) from both surface science and electrochemical viewpoints, and the search for PAH monolayers with larger chemical structures is expected.

Figure 7.

(a) High-resolution STM images of coronene and circobiphenyl adlayers on Au(111) (upper) and coadsorption systems of coronene-Br and circobiphenyl-Br (lower). (b) An illustration of the “molecular container” concept, high-resolution STM image, and model structure of dicoronylene-Cl. (c) Cyclic voltammograms of ovalene-adsorbed Au(111) electrode (black line) with potential-dependent STM images observed at (b) 0.78 V and (c) 0.25 V vs. RHE in 0.1 M HClO₄. Typical high-resolution STM images of isolated 3-MPA, 4-pyridinethiol (PyS), and 2-pyrazinethiol (PyzS) molecules in the precisely arranged ovalene adlayer on the Au(111) electrode surface acquired at 0.25 V versus RHE, respectively. Reprinted with permission from Refs. 64–66, Copyrights Wiley-VCH (2018), American Chemical Society (2019), the Royal Society of Chemistry (2013 & 2022).

Larger PAHs, such as dicoronylene, are also attractive from the viewpoint of the effect of the low symmetry of the chemical framework on 2D assembly. However, dissolving dicoronylene in organic solvents is difficult. In 2017, we discovered a combination of nanospecies based on supramolecular chemistry and electrochemical surface science. We successfully fabricated a highly ordered 2D adlayer of dicoronylene from the solid–liquid interface to the Au(111) surface by transporting hydrophobic nanographene using water-soluble micelle-type capsules (Fig. 7c).⁶⁶ Amphiphilic molecules have three hydrophilic ammonium cations and two hydrophobic anthracene moieties; therefore, micellar capsules are formed by self-assembly. This molecule was synthesized by Prof. Yoshizawa earlier.^67,68 Their group has demonstrated the possibility of the V-shaped amphiphilic molecule by using several hydrophobic π-conjugated molecules such as fullerenes, carbon nanotubes, and planar molecules (porphyrins and phthalocyanines) as guest molecules.^68,69 The key point is the discovery of the concept of the “molecular container” method, which enabled the controlled release of encapsulated nanographenes by pH adjustment, i.e., recombination of V-shaped amphiphilic molecules and/or structural change of micellar capsules occurs in an electrolyte solution. The release rate depends on the concentration and pH of the electrolyte solution. Thus, this method overcomes the solubility limitation of hydrophobic nanographene and opens a new door for nanographene science. This method is effective for much larger PAHs, such as C₉₆H₄₂, which has a triangular shape.⁷⁰ We succeeded in forming a C₉₆H₄₂ adlayer on Au(111). Furthermore, we extended this to iron(II) phthalocyanine (FePc), which is a water-insoluble molecule.⁷¹ In alkaline solutions, FePc molecules in the micelle capsules accelerated the ORR, indicating that O₂ molecules can easily pass through the gaps of the capsule. This molecular capsule extended the electrochemistry of water-insoluble metal complexes in aqueous electrolyte solutions and acted as an electrochemical nanoreactor.

6. Single Crystal Electrochemistry for Ionic Liquids

Focusing on the wide potential window of ionic liquids (ILs) and flexibility and combination of cation and anion designs, we conducted the study from the viewpoint of interfacial electrochemistry, particularly using single-crystal gold electrodes. We found that the electric double-layer region and potential window of 1-alkyl-3-methylimidazolium-based ILs depend on the crystallographic orientation (atomic arrangement) of gold.^72,73 We also reported that the activation energy of cobaltocenium/cobaltocenium depends on the alkyl chain length of the cations in the ILs.⁷⁴ Using IL electrochemical interfaces with a wide potential window, fullerene adlayers formed on Au single-crystal electrodes exhibited six-electron redox responses to both C₆₀ and C₇₀ radical anions at room temperature. It is known that fullerene can be successively reduced to its hexaanion (e.g., C₆₀⁶⁻).⁷⁵ The redox potential depends on not only the combination of cations and anions^76,77 but also temperature of the ILs.^78,79 Furthermore, the specific adsorption of iodide ions and adsorption of imidazolium cations on the Au(111) electrode surface were resolved at the nanoscale for the first time.⁸⁰ In addition, the specificity of the surface oxidation reaction of iodide ions in ionic liquids was clarified in detail.⁸¹ Particularly, the iodine adlayer exerts a significant impact on the oxidation process at the Au(111)|[Tf₂N]⁻-based IL interfaces, as shown in Fig. 8a. Among the halogen adlayers, only iodine adlayers initiate Au complexation at room temperature (25 ± 1 °C). This is because the iodine adsorption on Au is the strongest among the halides (Cl⁻, Br⁻, and I⁻). The chlorine and bromine adlayers on the Au(111) were partially or completely disrupted by the adsorption of the IL, resulting in featureless voltammograms nearly identical to those of the bare Au(111) surfaces. An oxidation peak was observed for the I–Au(111)|[Tf₂N]⁻-based IL interfaces; however, continuous CV indicated a gradual decrease in the oxidation peak current density in subsequent cycles. The loss of the catalytic ability of the iodine adlayer in this oxidation step is due to the eventual dissolution of the iodine adlayer into the IL. This suggests that iodine adatoms were involved in the reaction, with some iodine adatoms diffusing away from the electrode surface. Furthermore, anion dependence was observed, as shown in Fig. 8b. The oxidative peak was observed in the TFSA anion, not PF₆⁻ and methide anions, indicating the complexation ability of the anion with gold. The mechanism of the characteristic oxidation of I–Au(111) in [Tf₂N]⁻-based ILs was proposed based on the results of CV, STM (Fig. 8c), and UV–Visible spectroscopy of the colored [C₃mpyrr][Tf₂N], and X-ray photoelectron spectroscopy of the colored Pt counter electrode after multiple oxidation measurements of I–Au(111) (Fig. 8d).⁸¹ We found that Au can be oxidized in [Tf₂N]⁻-based ILs at high temperatures (≥70 °C).⁸² The temperature change of ILs promotes the elution of gold complexes even in the absence of specific adsorption of halide ions. These are fundamental studies, but we could elucidate only one aspect of the surface electrochemistry of ILs. Some of these studies are summarized as an account in Chem. Rec.⁸³ The details are also summarized in a chapter in the Encyclopedia of Solid–Liquid Interfaces to be soon published by Elsevier.⁸⁴

Figure 8.

CV profiles of (a) bare Au(111), Cl–Au(111), Br–Au(111), and I–Au(111) electrodes in [C₃mpyrr][Tf₂N] and (b) I-Au(111) electrodes in [C₄mim][PF₆], [C₄mim][Tf₂N], and [C₄mim][Tf₃C] recorded at the scan rate of 50 mV s⁻¹, respectively. The dotted line in (b) indicates the typical CV profile of a clean Au(111) electrode in [C₄mim][Tf₂N] and (c) STM images (200 × 200 nm²) of an I–Au(111) electrode surface obtained under ambient conditions after holding in [C₃mpyrr][Tf₂N] at 0.45, 1.25, 1.45, and 1.65 V vs. Fc/Fc⁺ for 5 min. (d) Proposed dissolution mechanism of Au in [Tf₂N]⁻-based ILs through the electrochemical oxidation catalyzed by an iodine adlayer. Reprinted with permission from Ref. 81. Copyright Elsevier (2021).

7. Conclusion

This study focuses on the 2D molecular assembly of porphyrins, phthalocyanines, their related complexes, and PAHs at electrochemical interfaces using in situ STM with molecular resolution. In situ, STM allows us to not only determine the interfacial structures but also follow the EC reactions. Electrochemical phase separation provides the control and design of characteristic molecular assemblies for constructing nanoarchitectures based on a bottom-up strategy. Moreover, vacancies can be controlled at the single-molecule level, leading to the construction of various electrocatalytic reaction sites.

Understanding the structural changes in molecular adlayers at the submicron scale is important. This can be achieved by overcoming the problems of molecular solubility or electrode potential windows. Still, many challenges need to be addressed at the solid–liquid interface, including understanding the functions and processes that emerge through the molecular assembly.

Acknowledgments

We thank the following collaborators: Prof. Nagao Kobayashi (Shinshu University), Prof. Yoshio Hisaeda (Kyushu University), Prof. Nobuo Kimizuka (Kyushu University), Prof. Takamasa Sagara (Nagasaki University), and Prof. Masaaki Abe (Univ. Hyogo), Prof. Michito Yoshizawa (Tokyo Tech), Prof. Masashi Kunitake (Kumamoto University), Dr. Takahiro Sawaguchi (AIST), Dr. Hiroyuki Ueda (Deakin University), and many students and collaborators. This work was supported by JSPS KAKENHI [grant number 19H02560] and by the IINa Interdisciplinary Research Project of the Institute of Industrial Nanomaterials, Kumamoto University.

CRediT Authorship Contribution Statement

Soichiro Yoshimoto: Conceptualization (Lead), Investigation (Lead)

Conflict of Interest

The authors declare no conflict of interest in the manuscript.

Funding

Japan Society for the Promotion of Science: 19H02560

Japan Society for the Promotion of Science: 19K22115

Kato Foundation for Promotion of Science: KJ-2746

Sumitomo Foundation

Murata Science Foundation

Footnotes

S. Yoshimoto: ECSJ Active Member

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Biographies

Soichiro Yoshimoto (Associate Professor at the Institute of Industrial Nanomaterials at Kumamoto University)

Soichiro Yoshimoto was born in 1973. He received his Ph.D. from Kumamoto University (KU) in 2000. He joined Prof. Itaya’s group at Tohoku University as an assistant professor in 2000. In 2005, he became a researcher at the National Institute of Advanced Industrial Science and Technology. In 2008, he received a tenure-track post at the KU. In 2012, he was promoted to associate professor at the Priority Organization for Innovation and Excellence (KU). Since 2020, he has been an Associate Professor at the Institute of Industrial Nanomaterials at KU. He received The Chemical Society of Japan Award for Young Chemists in 2005 and the Young Researcher Award of the Electrochemical Society of Japan (Sano Award) in 2006. His current research focuses on 2D and thin-film construction of functional molecular nanoarchitectures based on coordination chemistry, interfacial electrochemistry, and ionic liquid electrochemistry.