Bridging Electrochemistry and Ultrahigh Vacuum: “Unburying” the Electrode–Electrolyte Interface

ConspectusElectrochemistry has a central role in addressing the societal issues of our time, including the United Nations’ Sustainable Development Goals (SDGs) and beyond. At a more basic level, however, elucidating the nature of electrode–electrolyte interfaces is an ongoing challenge due to many reasons, but one obvious reason is the fact that the electrode–electrolyte interface is buried by a thick liquid electrolyte layer. This fact would seem to preclude, by default, the use of many traditional characterization techniques in ultrahigh vacuum surface science due to their incompatibility with liquids. However, combined UHV-EC (ultrahigh vacuum-electrochemistry) approaches are an active area of research and provide a means of bridging the liquid environment of electrochemistry to UHV-based techniques. In short, UHV-EC approaches are able to remove the bulk electrolyte layer by performing electrochemistry in the liquid environment of electrochemistry followed by sample removal (referred to as emersion), evacuation, and then transfer into vacuum for analysis.Through this Account, we highlight our group’s activities using UHV-EC to bridge electrochemistry with UHV-based X-ray and ultraviolet photoelectron spectroscopy (XPS/UPS) and scanning tunneling microscopy (STM). We provide a background and overview of the UHV-EC setup, and through illustrative examples, we convey what sorts of insights and information can be obtained. One notable advance is the use of ferrocene-terminated self-assembled monolayers as a spectroscopic molecular probe, allowing the electrochemical response to be correlated with the potential-dependent electronic and chemical state of the electrode–monolayer–electrolyte interfacial region. With XPS/UPS, we have been able to probe changes in the oxidation state, valence structure, and also the so-called potential drop across the interfacial region. In related work, we have also spectroscopically probed changes in the surface composition and screening of the surface charge of oxygen-terminated boron-doped diamond electrodes emersed from high-pH solutions. Finally, we will give readers a glimpse into our recent progress regarding real-space visualizations of electrodes following electrochemistry and emersion using UHV-based STM. We begin by demonstrating the ability to visualize large-scale morphology changes, including electrochemically induced graphite exfoliation and the surface reconstruction of Au surfaces. Taking this further, we show that in certain instances atomically resolved specifically adsorbed anions on metal electrodes can be imaged. In all, we anticipate that this Account will stimulate readers to advance UHV-EC approaches further, as there is a need to improve our understanding concerning the guidelines that determine applicable electrochemical systems and how to exploit promising extensions to other UHV methods

Fabrication of Sharp Gold Tips by Three-Electrode Electrochemical Etching with High Controllability and Reproducibility

Gold (Au) tips have wide application in local spectroscopies not only because of their high chemical stability but also their strong localized surface plasmon resonance (LSPR) in the visible and near-infrared regions. The energy and intensity of LSPR strongly depend on the tip shape. However, the conventional fabrication method of Au tips using electrochemical etching with two electrodes has problems regarding both the controllability and reproducibility of the tip shape. Here, we demonstrate a novel three-electrode electrochemical etching method to fabricate the Au tips by precisely tuning the applied electrochemical potential. The sharpness of the tip is well controlled by the applied potential, with high reproducibility

Fabrication of Sharp Gold Tips by Three-Electrode Electrochemical Etching with High Controllability and Reproducibility

Gold (Au) tips have wide application in local spectroscopies not only because of their high chemical stability but also their strong localized surface plasmon resonance (LSPR) in the visible and near-infrared regions. The energy and intensity of LSPR strongly depend on the tip shape. However, the conventional fabrication method of Au tips using electrochemical etching with two electrodes has problems regarding both the controllability and reproducibility of the tip shape. Here, we demonstrate a novel three-electrode electrochemical etching method to fabricate the Au tips by precisely tuning the applied electrochemical potential. The sharpness of the tip is well controlled by the applied potential, with high reproducibility

Origin of Current Enhancement through a Ferrocenylundecanethiol Island Embedded in Alkanethiol SAMs by Using Electrochemical Potential Control

Electroactive ferrocenylundecanethiol islands embedded in an n-decanethiol SAM matrix were studied under potential control using in situ scanning tunneling microscopy (STM). Contrary to previous reports, the positive charges on ferrocene moieties are not a prerequisite to enhancing the current through ferrocenylundecanethiol on gold. Rather, conduction paths are opened at determined potentials for both neutral and monocationic ferrocene moieties. In addition, although stable and reproducible images were obtained under potential control, conventional STM measurements under N2 atmosphere were sometimes unstable and irreversibly changed the sample, indicating that current measurements of ferrocenylundecanethiol are much easier under an electrochemical environment

Fabrication of Sharp Gold Tips by Three-Electrode Electrochemical Etching with High Controllability and Reproducibility

Gold (Au) tips have wide application in local spectroscopies not only because of their high chemical stability but also their strong localized surface plasmon resonance (LSPR) in the visible and near-infrared regions. The energy and intensity of LSPR strongly depend on the tip shape. However, the conventional fabrication method of Au tips using electrochemical etching with two electrodes has problems regarding both the controllability and reproducibility of the tip shape. Here, we demonstrate a novel three-electrode electrochemical etching method to fabricate the Au tips by precisely tuning the applied electrochemical potential. The sharpness of the tip is well controlled by the applied potential, with high reproducibility

Electrodeposited Gold Probe for Electrochemical Scanning Tunneling Microscopy

Direct measurements at the solid–liquid interface on the nanometer scale provide information about various physical and chemical processes from both fundamental and applied aspects. Electrochemical scanning tunneling microscopy (EC-STM) is a powerful tool that makes it possible to perform STM measurements at the solid–liquid interfaces. However, EC-STM remains challenging due to the difficulty of fabricating the probe for tunneling current measurements in solutions. In this study, we established a novel and versatile method to easily fabricate the EC-STM probe with high reproducibility. By electrochemically depositing Au on a pyrolytic carbon formed within the glass nanopipette which has an aperture of several hundred nanometers, we achieved a sufficient insulation with almost 100% probability, which overcomes the 30-year challenge of previous EC-STM studies. High-quality EC-STM images could be stably obtained with a yield of >80% using the probe. It will pave the way for future advanced measurements at a variety of solid–liquid interfaces, such as single molecule spectroscopy and electronic structure characterizations

Electrodeposited Gold Probe for Electrochemical Scanning Tunneling Microscopy

Direct measurements at the solid–liquid interface on the nanometer scale provide information about various physical and chemical processes from both fundamental and applied aspects. Electrochemical scanning tunneling microscopy (EC-STM) is a powerful tool that makes it possible to perform STM measurements at the solid–liquid interfaces. However, EC-STM remains challenging due to the difficulty of fabricating the probe for tunneling current measurements in solutions. In this study, we established a novel and versatile method to easily fabricate the EC-STM probe with high reproducibility. By electrochemically depositing Au on a pyrolytic carbon formed within the glass nanopipette which has an aperture of several hundred nanometers, we achieved a sufficient insulation with almost 100% probability, which overcomes the 30-year challenge of previous EC-STM studies. High-quality EC-STM images could be stably obtained with a yield of >80% using the probe. It will pave the way for future advanced measurements at a variety of solid–liquid interfaces, such as single molecule spectroscopy and electronic structure characterizations

Unusual Electrochemical Properties of Low-Doped Boron-Doped Diamond Electrodes Containing sp² Carbon

Unexpected phenomena displayed by low-boron-doped diamond (BDD) electrodes are disclosed in the present work. Generally, the presence of sp2 nondiamond carbon impurities in BDD electrodes causes undesirable electrochemical properties, such as a reduced potential window and increased background current, etc. However, we found that the potential window and redox reaction in normally doped (1%) BDD and low-doped (0.1%) BDD exhibited opposite tendencies depending on the extent of sp2 carbon. Moreover, we found that contrary to the usual expectations, low-doped BDD containing sp2 carbon hinders electron transfer, whereas in line with expectations, normally doped BDD containing sp2 exhibits enhanced electron transfer. Surface analyses by X-ray/ultraviolet photoelectron spectroscopy (XPS/UPS) and electrochemical methods are utilized to explain these unusual phenomena. This work indicates that the electrochemical properties of low-doped BDD containing sp2 might be due partially to the high level of surface oxygen, the large work function, the low carrier density, and the existence of different types of sp2 carbon

Monatomic Iodine Dielectric Layer for Multimodal Optical Spectroscopy of Dye Molecules on Metal Surfaces

Fluorescence and Raman scattering spectroscopies have been used in various research fields such as chemistry, electrochemistry, and biochemistry because they can easily obtain detailed information about molecules at interfaces with visible light. In particular, multimodal fluorescence and Raman scattering spectroscopy have recently attracted significant attention, which enables us to distinguish chemical species and their electronic states that are important for expressing various functions. However, a special strategy is required to perform simultaneous measurements because the cross sections of fluorescence and Raman scattering differ by as much as ∼1014. In this study, we propose a method for the simultaneous measurement of dye molecules on a metal surface using a monatomic layer of iodine as the dielectric layer. The method is based on adequately quenching the photoexcited state of the molecules near the metal surface to weaken the fluorescence intensity and using the resonance effect to increase the Raman signal. We have validated this concept by experiments with insulating layers of different thicknesses and dye molecules of different chemical structures. The proposed multimodal strategy paves the way for various applications such as catalytic chemistry and electrochemistry, where the adsorption structure and electronic states of molecular species near the metal surface determine functionalities