Electrochemical Potential Control Research Articles

Evaluation of hydrogen evolution activity by bubbles growth rates as descriptor

Evaluation of the electrocatalytic behavior in the high current density region is critical toward the applications in the industrial water electrolysis system. Here, we utilized the bubble growth rate as descriptor for evaluation of hydrogen evolution reaction. Ag electrode was selected as a model electrode. Pressure-controlled cell was used for the evaluation of the bubble behavior under the wider range in the overpotential. Hydrogen bubble evolution behavior was evaluated by video observation under electrochemical potential control. Even in the presence of the complex processes such as nucleation, growth and detachment of the bubbles, the hydrogen evolution behavior is evaluated with respect to the classical Tafel analyses even in high overpotential region. These analyses provide the methods for the rapid screening toward the material discovery of the green hydrogen production in high current density region.

(Invited) Confocal Raman Microscopy in the Study of Ion Insertion Processes and Material Dimensional Change

Confocal Raman microscopy is being applied to quantify chemical composition at electrode-solution interfaces and within electrode-supported films under electrochemical potential control. Experimental strategies (i.e., cell designs and data acquisition and processing techniques) that are enabling sensitive in-situ spectral measurements will be discussed with focus on redox transformations at electrode-solution interfaces and within electrode-supported polymer membranes and thin films. Emphasis will be on the ability to investigate ion insertion/de-insertion processes and measure material dimensional (e.g., swelling/de-swelling) changes as the applied potential is varied (in-situ). The microscope system being utilized employs a high numerical aperture oil immersion objective to achieve (i) diffraction-limited focusing and probe volume characteristics and (ii) high efficiency in collecting Raman scattered radiation from molecules of interest. With the objective mounted to an inverted microscope frame, the electrochemical cell is positioned above the objective on the microscope stage. A glass microscope coverslip attached to the bottom of the electrochemical cell serves as the optical window. One electrochemical cell platform has been constructed around an optically transparent indium tin oxide (ITO) coated coverslip that serves as both the cell working electrode and optical window. The objective brings excitation radiation through the coverslip and to a focus a few micrometers above the ITO surface. Species diffusing into the confocal probe volume region are detected and quantified based on their Raman spectral band frequencies and intensities. A second electrochemical cell platform has a thin layer design and enables use of conventional disk working electrodes. The performance of this cell together with a spectral difference technique that allows detection of transformations within redox polymers supported on carbon electrodes will be discussed. The platform is being applied to detect swelling/de-swelling in electrode-supported redox polymers in response to changes in material redox state. Molecular composition (i.e., the mix of polymer framework, solvent and mobile ions) within the confocal probe volume is measured quantitatively from Raman spectra. Tracking the frequency and intensity of spectral bands as electrode potential is varied enables voltage-induced swelling and de-swelling to be correlated to changes in the composition of specific types of mobile species (solvent and ions). Similar application of the strategy to investigate material dimensional change with variation in atmospheric composition will be highlighted.

Understanding Spatial Distributions of Dye Molecules Coupled to the Surface Lattice Resonance Mode through Electrochemical Reaction Control.

The strong coupling, which is the light-matter interaction, leads to changes in the energy landscape of the chemical dynamics, resulting in the modulation of the reaction pathways. In this study, we achieved strong coupling between dye molecules dispersed in the polymer films and the surface lattice resonance mode, which is excited on plasmonic lattice arrays. In addition, we successfully tuned the coupling strength by introducing the electrochemical potential control method. Reversible decreases and increases in the coupling strength were observed as a result of the reversible electrochemical redox reactions of dye molecules. It is important that the spatial distribution of the molecules coupled to the lattice resonance mode was clarified by using various polymer film thicknesses. Our present electrochemical method for controlling strong coupling states represents a promising method for tuning the light-absorption properties of systems.

Unique Electronic Excitations at Highly Localized Plasmonic Field.

ConspectusUnder visible light illuminations, noble metal nanostructures can condense photon energy into the nanoscale region. By precisely tuning the metal nanostructures, the ultimate confinement of photoenergy at the molecular scale can be obtained. At such a confined photon energy field, various unique photoresponses of molecules, such as efficient visible light energy conversion processes or efficient multielectron transfer reactions, can be observed. Light-matter interactions also increase with the condensation of photons with nanoscale regions, leading to efficient light energy utilizations. Moreover, the strong field confinement can often modulate electronic excitations beyond normal selection rules. Such unique electronic excitations could realize innovative photoenergy conversion systems. On the other hand, such interactions lead to changes in the optical absorption property of the system via the formation of hybridized electronic energy states. This hybridized state is expected to have the potential to modulate the chemical reaction pathways. Taking these facts into consideration, a probe for the molecular absorption process with high sensitivity allows us to find novel ways for further precise tuning of light-matter interactions. In this Account, we review phenomena of unique electronic excitations from the perspective of our previous investigations using surface-enhanced Raman scattering (SERS) spectroscopy at electrified interfaces. Because the enhancement mechanism of Raman scattering at interfaces is deeply correlated with the photon absorption process accompanied by the electronic excitations between molecules and electrode surfaces, the detailed SERS investigations of the well-defined system can provide information on the electronic excitation processes. Through SERS observations of single-molecule junctions at electrodes or well-defined low-dimensional carbon materials, we have observed the characteristic Raman bands containing additional polarization tensors, indicating the occurrence of electronic polarization induced by electronic excitations based on a distinct selection rule. The origins for the observed facts were attributed to the highly condensed electric field producing the huge intensity gradient at the nano scale. The electrochemical potential control of the system would be valuable for the control of the excitation process. Additionally, from Raman spectra of dye molecules coupled to the plasmonic field, the changes in the Raman scattering intensity depending on the strength of interactions suggested the modulation of the absorption characteristics of the system. In addition, we have proved that the electrochemical potential control method can be a powerful tool for the active tuning of the light-matter interaction, leading to the change in the light absorption property. The molecular behaviors of dyes in the strong-coupling regime were reversibly tuned to show intense SERS. The current descriptions provide novel insights for these unique electronic excitations, realized by the plasmon excitation, that lead to advanced photoenergy conversions beyond the limits of present systems.

Room-Temperature Molecular Manipulation via Plasmonic Trapping at Electrified Interfaces.

For the motion control of individual molecules at room temperature, optical tweezers could be one of the best approaches to realize desirable selectivity with high resolution in time and space. Because of physical limitations due to the thermal fluctuation, optical manipulation of small molecules at room temperature is still a challenging subject. The difficulty of the manipulation also emerged from the variation of molecular polarizability depending on the choice of molecules as well as the molecular orientation to the optical field. In this article, we have demonstrated plasmonic optical trapping of small size molecules with less than 1 nm at the gap of a single metal nanodimer immersed in an electrolyte solution. In situ electrochemical surface-enhanced Raman scattering measurements prove that a plasmonic structure under electrochemical potential control realizes not only the selective molecular condensation but also the formation of unique mixed molecular phases which is distinct from those under a thermodynamic equilibrium. Through detailed analyses of optical trapping behavior, we established the methodology of plasmonic optical trapping to create the novel adsorption isotherm under applying an optical force at electrified interfaces.

Surface‐enhanced Raman scattering probe for molecules strongly coupled with localized surface plasmon under electrochemical potential control

AbstractThe light–matter interaction is the crucial tool for the efficient light energy usage. Electrochemical potential control method can tune the coupling strength between the molecules and the light field. In this study, electrochemical surface‐enhanced Raman scattering measurements have been carried out to clarify the detailed molecular behavior in the strong coupling regime. The Raman and fluorescence intensities from dye molecules in the strong coupling regime provided us the information about the change in the distance between the molecules and the metal surface in angstrom level. The detailed investigation of spectra revealed the origin for the potential dependence on the coupling strength. The present insight obtained in the current study would be valuable to understand the electrochemically controlled molecule–light interaction for photochemistry research.

Preparing DNA SAM Electrochemical Sensors Using Potential Assisted Deposition Methods. Controlling the Coverage and Local Organization of the DNA in the SAM.

Typically, DNA SAMs used for electrochemical based sensing are prepared without controlling the gold electrode potential[1]. Applying a potential during DNA deposition enables control over the coverage and DNA organization[2-4]. These characteristics are determined by the composition of the immobilization buffer, the immersion time, the DNA concentration, the extent of backfilling with a small alkylthiol and whether deposition occurs on a bare or alkythiol coverage gold surface[3-4]. Few studies have explored the influence of the underlying surface crystallography. Here, we show the possibilities of using electrochemical potential control to manipulate the characteristics of the DNA SAM. Moreover, the use of a single crystal gold bead electrode, coupled with the use of a fluorophore tagged DNA and fluorescence microscopy, enables examination of the influence of the underlying surface crystallography on the DNA SAM prepared. Various constant potentials were used as well as using a square-wave potential perturbation during deposition. There are significant differences in the DNA SAM produced under these conditions. In some cases, the DNA coverage was low on the <111> surfaces, higher on more open or atomically rough regions. In other cases, the coverage was higher on the <100> surfaces as compared to the other regions. The use of constant positive or negative potentials (vs SCE) resulted in significant differences in the relative coverages among regions of different atomic arrangements (Fig 1). In addition, a significant difference was observed between immobilization buffers that contain Cl and those that are Cl free. Also, the use of constant or square-wave potential perturbations resulted in significantly different local organizations of DNA in the SAM. Examples of these types of DNA SAM surfaces will be presented with some conclusions on the implications for use on polycrystalline planar gold surfaces.

Visualization of molecular trapping at plasmonic metal nanostructure by surface-enhanced Raman scattering imaging

Optical manipulation has been used for the trapping of micrometer-scaled objects, but it is still difficult to control the motion of small molecules on the nanometer scale at room temperature. Plasmonic metal nanostructures are expected to be useful for the optical manipulation of nanoscale molecules using a highly localized electric field. We use the plasmonic Ag nanostructure for a demonstration of optical trapping through the observation using surface-enhanced Raman scattering (SERS) imaging. The optical measurements were conducted under electrochemical potential control to stabilize the nanostructure with target molecules, 4,4′-bipyridyl (44 bpy). Upon increasing the concentration of 44 bpy molecules in an electrolyte solution at room temperature, the blinking frequency of the SERS signal was different in both the spectra and imaging. The dwell time of the SERS signals was increased from several tens of milliseconds to a few seconds, which suggested the successful observation of plasmonic trapping of small molecules through the surface diffusion control. The observed results prove the importance for the control of the surface coverage of the molecules and its influence on surface diffusion under plasmonic molecular trapping.

Generalized Model for the Electrical Conductance of a One-Atom Metallic Constriction with Electrochemical Potential Control

Starting from the Breit–Wigner model, we present a generalized approach for calculating the electrical conductance of a one-atom metallic constriction with electrochemical-potential control. In addition, the corresponding current intensity is determined. On the one hand, the nanoconstriction is conceived as a dilute (degenerate) Fermi gas (relatively low electron concentrations are considered) and, on the other hand, non-dilute gas is considered. Under these conditions, the Fermi energy-level into the Breit–Wigner formula is replaced by the corresponding chemical-potential energy. We emphasize the role of the optical potential as significant ingredient of the chemical potential.

Observing the Reversible Single Molecule Electrochemistry of Alexa Fluor 647 Dyes by Total Internal Reflection Fluorescence Microscopy.

Alexa Fluor 647 is a widely used fluorescent probe for cell bioimaging and super-resolution microscopy. Herein, the reversible fluorescence switching of Alexa Fluor 647 conjugated to bovine serum albumin (BSA) and adsorbed onto indium tin oxide (ITO) electrodes under electrochemical potential control at the level of single protein molecules is reported. The modulation of the fluorescence as a function of potential was observed using total internal reflectance fluorescence (TIRF) microscopy. The fluorescence intensity of the Alexa Fluor 647 decreased, or reached background levels, at reducing potentials but returned to normal levels at oxidizing potentials. These electrochemically induced changes in fluorescence were sensitive to pH despite that BSA-Alexa Fluor 647 fluorescence without applied potential is insensitive to pH between values of 4-10. The observed pH dependence indicated the involvement of electron and proton transfer in the fluorescence switching mechanism.

Observing the Reversible Single Molecule Electrochemistry of Alexa Fluor 647 Dyes by Total Internal Reflection Fluorescence Microscopy

AbstractAlexa Fluor 647 is a widely used fluorescent probe for cell bioimaging and super‐resolution microscopy. Herein, the reversible fluorescence switching of Alexa Fluor 647 conjugated to bovine serum albumin (BSA) and adsorbed onto indium tin oxide (ITO) electrodes under electrochemical potential control at the level of single protein molecules is reported. The modulation of the fluorescence as a function of potential was observed using total internal reflectance fluorescence (TIRF) microscopy. The fluorescence intensity of the Alexa Fluor 647 decreased, or reached background levels, at reducing potentials but returned to normal levels at oxidizing potentials. These electrochemically induced changes in fluorescence were sensitive to pH despite that BSA‐Alexa Fluor 647 fluorescence without applied potential is insensitive to pH between values of 4–10. The observed pH dependence indicated the involvement of electron and proton transfer in the fluorescence switching mechanism.

Determination of Molecular Orientation in Bi-analyte Mono-molecule Layer through Electrochemical Surface-enhanced Raman Scattering Measurements

Surface-enhanced Raman scattering measurements have been conducted using plasmonic dimer structures under electrochemical potential control. We have confirmed the potential dependence of the adsorp...

(Invited) Exotic Electronic Excitation of a Single-Layer Graphene By Surface Localized Plasmons Under Electrochemical Potential Control

Localized surface plasmons (LSPs) of novel metal nanoparticles has distinct electromagnetic interactions with matter compared with the case using far-filed light. The high field gradient near metal nanostructures opens up the possibility for additional transitions beyond those allowed by normal selection rules, enhancing light-matter interactions. Moreover, plasmon-induced ‘hot carriers’ can accelerate chemical reactions. The energy distribution, lifetime, and transfer of ‘hot carriers’ are key factors affecting electrochemical processes. We find that the LSPs of the Au nanodimer structures prepared on defect-free graphene provide highly-confined electromagnetic field to generate holes and electrons with the electrochemical potentials beyond the limit of those under normal light illumination.

Nano-scale oxide formation inside electrochemically-formed Pt blisters at a solid electrolyte interface

We report on platinum oxide formation during electrochemical anodic polarization of a platinum film on yttria-stabilized zirconia (YSZ) under electrochemical oxygen potential control. The electrochemical potential drives oxygen through the YSZ electrolyte towards a nominally 175 nm thin Pt film, which we found to locally delaminate from the substrate by forming nano-scale blisters. High resolution scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDX) mapping of focused-ion beam (FIB)-prepared cross-sections of single bubbles of a few micrometers in diameter reveal them to be hollow and enclosed by a Pt outer and a few tens of nanometers thick PtOx inner shell. The oxide shell presumably formed due to the increase of local oxygen chemical activity under the applied process conditions (723 K, 500 mbar O2, bias voltage +100 mV). Interface X-ray diffraction indicates that the solid electrolyte surface morphology is largely unaffected by the process suggesting that the YSZ surface is stable on the atomic scale under application relevant oxygen transport conditions. Platinum is known to be rather stable towards oxidation, even at elevated oxygen pressure, leading to oxide-scale thicknesses of the order of 1 nm. Our results however indicate that many of the kinetic barriers for oxidation during the nano-confined blistering process are lowered. This may have implications in general for the mechanism how oxygen is stored in an electrode at such an internal metal - oxide/metal - gas interface, which is important for the functionality of many solid-state electrochemical and memresistive devices.

Thermal Effect on Plasmon-induced Electron Transfer System under Intense Pulsed Laser Illumination

The effect of plasmon excitation on metal nanostructures was investigated under electrochemical potential control with intense pulsed laser illumination. The well-defined Au nano-rod structures showed a distinct thermal effect in the form of shape transformation, in the absence of the plasmon-induced electron transfer reaction. It was found that the shape transformation only occurred under the resonance condition of the localized surface plasmon mode excitation.

Electrochemical Fine Tuning of the Plasmonic Properties of Au Lattice Structures

We tuned the plasmonic properties of the Au lattice structure by electrochemical potential control. Au lattice structures with different values of the spacing, diameter, and height show characteristic optical properties determined by the surface lattice resonance of the localized surface plasmon mode. Electrochemical potential control can change the metal structures through metal dissolution, as well as the energy of the electrons in metals. In situ real time observation of the optical properties of Au lattice structures by electrochemical dark-field scattering microscopy shows the fine-tuning of the plasmonic properties with characteristic resonance energy and controlled spectral width. By controlling surface dissolution of the Au lattice structure at a rate of a few nanometers per minute, we tuned the plasmonic properties and achieved a spectral width of 0.145 eV at a maximum resonance of 1.74 eV (714 nm).

Sensitive Raman Probe of Electronic Interactions between Monolayer Graphene and Substrate under Electrochemical Potential Control

In situ electrochemical Raman spectroscopic measurements of defect-free monolayer graphene on various substrates were performed under electrochemical potential control. The G and 2D Raman band wavenumbers (ωG, ω2D) of graphene were found to depend upon the electrochemical potential, i.e., the charge density of graphene. The values of ωG and ω2D also varied depending on the choice of substrates. On metal substrates where graphene was synthesized by chemical vapor deposition, a strong blue shift of ω2D was induced, which could not account for the strain and charge doping. We attributed the blue shift of ω2D to a change in the electronic properties of graphene induced by distinct electronic interactions with the metal substrates. To explain the unique characteristics in the Raman spectrum of graphene on various substrates, a novel mechanism is proposed considering reduction of the Fermi velocity in graphene owing to dielectric screening from the metal substrates.

Electrochemical surface-enhanced Raman scattering measurement on ligand capped PbS quantum dots at gap of Au nanodimer

The vibrational characteristics of ligand-capped lead sulfide (PbS) quantum dots (QDs) were clarified via electrochemical surface-enhanced Raman spectroscopy (EC-SERS) using a hybridized system of gold (Au) nanodimers and PbS QDs under electrochemical potential control. Enhanced electromagnetic field caused by the coupling of QDs with plasmonic Au nanodimers allowed the characteristic behavior of the ligand oleic acid (OA) on the PbS QD surface to be detected under electrochemical potential control. Binding modes between the QDs and OA molecules were characterized using synchronous two-dimensional correlation spectra at distinct electrochemical potentials, confirming that the bidentate bridging mode was probably the most stable mode even under relatively negative potential polarization. Changes in binding modes and molecular orientations resulted in fluctuations in EC-SERS spectra. The present observations strongly recommend the validity of the QD–plasmonic nanostructure coupled system for sensitive molecular detection via EC-SERS.

Active Tuning of Strong Coupling States between Dye Excitons and Localized Surface Plasmons via Electrochemical Potential Control

Here we report the tuning of a number of excited dye molecules that were strongly coupled with the localized surface plasmons (LSPs) of Au nanostructures by electrochemical potential control. Using the redox-state-tuned dye molecules and several types of metal nanostructures with distinct LSP energies, active control of the high coupling strength was achieved via an electrochemical potential-based control method. One interesting finding of the present work is that the parabolic behavior of the coupling strength in the range between 0.10 and 0.27 eV is dependent on the electrochemical potential; this has not been observed previously. Anticrossing plots showing the energies of the upper and lower states of the coupling to the LSP energy suggest that the number of dye molecules contained in the cavity-confined LSP field is controlled not only by the redox states of the dye molecules but also by the interactions between the dyes and the metal surfaces. The present finding provides a novel route to control lig...

Cobalt Catalysts for Oxygen Evolution Studied By Operando Soft X-Ray Absorption Spectroscopy

Recently, hydrogen production system by electrochemical water splitting has received attractive attention to realize a sustainable society that does not depend on fossil fuels. For commercial applications, the improvement of overall water splitting efficiency has been required by the development of highly active oxygen evolution electrocatalyst. In this situation, Nocera and colleagues reported that a cobalt oxide electrodeposited from a dilute Co2+ solution in a potassium phosphate buffered electrolyte (Co-KPi) is efficient electrocatalyst for oxygen evolution reaction (OER) [1]. The local structure of Co species in the Co-KPi catalyst was investigated by Co K-edge X-ray absorption fine structure (XAFS) spectroscopy and X-ray pair distribution function (PDF) analysis, and it was revealed that the Co-KPi is composed of nano-sized clusters with an edge-sharing CoO6 octahedral structure [2]. However, the detailed information about oxygen species in this electrocatalyst under working condition is not yet obtained enough. Thus, we have investigated the reaction mechanism by measuring operando O K-edge XAFS spectra for Co-KPiand similar catalysts under electrochemical potential control. Electrochemical O K-edge XAFS measurements using soft X-rays were performed with transmission mode at BL3U in the UVSOR Synchrotron, according to the previous works [3]. A home-made electrochemical cell was used with Au/Cr/SiC thin film substrates as working electrodes, a Pt counter electrode, and a Ag/AgCl reference electrode. Co-KPi electrocatalyst was electrodeposited on the Au/Cr/SiC working electrode in potassium phosphate buffered electrolyte containing Co(NO3)2. The O K-edge XAFS spectra for Co-KPi catalyst were taken under electrode potential control in potassium phosphate buffered electrolyte (Figure 1). At 0.0 V, an absorption peak attributed to oxygen species of CoOOH was detected around 531 eV. When the electrode potential was changed from 0.0 to 1.1 V, a new absorption peak assigned to oxygen species of CoO2 was observed around 529 eV, indicating that the part of CoOOH cluster was oxidized to CoO2 with the high-valent cobalt species (Co4+). Next, to discuss the relationship between spectroscopic result and catalytic activity, the O K-edge XAFS spectra were taken for the low active cobalt oxide catalysts with CoOOH local structures electrodeposited in a potassium acetate (Co-KOAc) or a potassium chloride (Co-KCl) aqueous solution. Then, the intensities of CoO2 species for Co-KOAc and Co-KCl were much lower than that for Co-KPi. Therefore, we found that Co-KPi can function as highly active OER electrocatalysts due to the presence of CoO2species. [1] D. G. Nocera et al, J. Am. Chem. Soc. 131, 2615 (2009). [2] (a) D. G. Nocera et al, J. Am. Chem. Soc. 132, 13692 (2010). (b) J. Am. Chem. Soc. 135, 6403 (2013). [3] (a) M. Nagasaka et al, J. Phys. Chem. C 117 16343 (2013). (b) M. Yoshida et al, J. Phys. Chem. C 119, 19279 (2015). Figure 1