Since the scanning tunneling microscope (STM), which is one of the scanning probe microscope (SPM), was invented by Drs. Binnig and Rohrer, more than 30 years have passed. Because SPM can be used not only in a vacuum but also even in a solution, this invention has greatly contributed to the development of electrochemistry research. I was asked to top researchers in each field as a chairman of the electrode surface science research branch, in this time, to describe in this special issue about the progress and current status of the electrode surface science using the SPM.
Surface charges on nanometer-scale structures in liquids play an essentially important role in various physical, chemical and biochemical phenomena such as ionic adsorptions, microscopic electrode reactions and specific ligand bindings. These molecular-scale interactions are mediated through local electric double layers (EDLs) formed by the counter ions in electrolyte solution. Although static-mode atomic force microscopy (AFM) including colloidal probe AFM is a powerful technique for surface charge density measurements and EDL analysis on a submicron scale in liquids, precise surface charge density analysis with molecular resolution has not been realized because of its limitation of the resolution and the electrical force detection sensitivity. Here recent molecular-scale surface charge measurements of self-assembled micellar structures and biomolecules by three-dimensional (3D) force mapping based on frequency modulation AFM are presented.
Design and control of functionalized surfaces play an important role in the formation of two-dimensional highly ordered and stable monomolecular adlayers, and the connection of various organic and/or organometallic components in nanotechnology applications. Electrochemical scanning tunneling microscopy (EC-STM) have been widely recognized as a powerful technique for in situ characterization with atomic and/or molecular resolution under potential control. In this article, several selected topics are focused on, bisterpyridine derivative and metal ion coadsorption, molecular assembly and ligand exchange reaction due to the redox reaction in the diruthenium complex, and a self-assembled monolayer composed of pyridine-terminated thiol. From the standpoint of molecular assembly related to electrochemistry and coordination chemistry, molecular adlayer structures and dynamics such as phase transition are reported.
An electric double layer (EDL) formed at an electrolyte/electrode interface provides a steep potential slope which facilitates electrochemical reactions at the interface. This review paper summarizes recent our study on the structuring of liquid phase molecules at some electrolyte/electrode interfaces investigated using electrochemical frequency modulation AFM (EC-FM-AFM), which we developed to analyze such interfaces under electrode potential control for local information of the liquid side as well as atomic scale imaging of the solid side. It was found that the rigidness of the hydration layer at an aqueous electrolyte/graphite electrode interface was greatly affected by electrochemical potential and kinds of the electrolyte, which also affected the organization of adsorbed molecules at the interface. It was revealed that ionic liquids, promising electrolytes for electrochemical devices, showed different degree of structuring at the interfaces strongly dependent on the electrode materials. Such experimental results were supported by molecular dynamics (MD) calculations.
Scanning electrochemical cell microscopy with a single barrel nanopipette (nanoSECCM), which is one of scanning electrochemical microscope (SECM) families, is a powerful tool for analyzing electrochemical activities (e.g. redox reaction and lithium-ion (de) intercalation processes) at localized area. A glass nano-pipette is used as a probe of nanoSECCM filled with electrolyte and a reference electrode. As the pipette is in proximity of sample surface, a meniscus is created. Through the meniscus, electrochemical activities can be obtained at localized area on the sample. Further, as the pipette scans the sample surface, electrochemical activities can be also visualized. Our experiments demonstrate that nanoSECCM is applicable to investigation of current response related to lithium-ion transport, redox cycling on various types of electrodes.
Current-sensing atomic force microscopy (CS-AFM) was applied to the analysis of surface morphology/conductivity of the membranes used for polymer electrolyte fuel cells (PEFCs). Nano-scale images of surface structures and protonconductive paths were simultaneously obtained under the conditions similar to those for the power generation. The “conditioning” processes enhancing the performances of PEFCs were examined by CS-AFM. Effects of the high protonic currents through the membrane and hot-water pretreatment were investigated in detail on the power generation of PEFCs with hydrocarbon electrolyte membranes.
There are many biogenic mineral deposits of diameter 25–100 μm on the leaf of the Morus of “Ichinose”. Using a chemical mapping, we can determine these deposits as “opal phytoliths” which are composed of biogenic silica. We tried to observe precisely the distribution pattern in a leaf and the change of shape depending on the maturation of the leaf using an optical microscope. From the results of the observation, it was found that many phytolithes were in close proximity to the veins, and while phytolithes change their shapes depending on the maturation of the leaf, other new phytolithes appear continually.