Visualizing charges accumulated in an electric double layer by three-dimensional open-loop electric potential microscopy.

Charges accumulated in an electric double layer (EDL) play key roles in various interfacial phenomena and electronic devices. However, direct imaging of their spatial distribution has been a great challenge, which has hindered our nano-level understanding of the mechanisms of such interfacial phenomena and functions. In this study, we present direct imaging of charges accumulated at an electrode-electrolyte interface using three-dimensional open-loop electric potential microscopy (3D-OL-EPM). Conventional OL-EPM allows us to visualize two-dimensional potential distributions in liquid yet the zero of the measured potential is not well defined due to the influence of the long-range (LR) interaction between the cantilever and the sample. Here, we present practical ways to reduce such an influence by improving the equation for the potential calculation and subtracting the LR contribution estimated from a Z potential profile. These improvements enabled the calibration of the measured potential values with respect to the bulk solution potential. With these improvements, we visualized opposite charge accumulation behaviors on a polarizable and non-polarizable electrode with a varying electrode potential. Combining OL-EPM with a 3D tip scanning method, we also performed a 3D-OL-EPM measurement on a Cu fine wire and visualized the nanoscale distribution of the charges accumulated at the interface. Such real-space information on the charge distributions in an EDL should provide valuable insights into the mechanisms of interfacial phenomena and functions that are important in various academic and industrial research on electronic devices, electrochemistry, tribology and life sciences.

Visualizing Nanoscale Distribution of Corrosion Cells by Open-Loop Electric Potential Microscopy.

Corrosion is a traditional problem but still one of the most serious problems in industry. To reduce the huge economic loss caused by corrosion, tremendous effort has been made to understand, predict and prevent it. Corrosion phenomena are generally explained by the formation of corrosion cells at a metal-electrolyte interface. However, experimental verification of their nanoscale distribution has been a major challenge owing to the lack of a method able to visualize the local potential distribution in an electrolytic solution. In this study, we have investigated the nanoscale corrosion behavior of Cu fine wires and a duplex stainless steel by in situ imaging of local corrosion cells by open-loop electric potential microscopy (OL-EPM). For both materials, potential images obtained by OL-EPM show nanoscale contrasts, where areas of higher and lower potential correspond to anodic areas (i.e., corrosion sites) and cathodic areas, respectively. This imaging capability allows us to investigate the real-time transition of local corrosion sites even when surface structures show little change. This is particularly useful for investigating reactions under surface oxide layers or highly corrosion-resistant materials as demonstrated here. The proposed technique should be applicable to the study of other redox reactions on a battery electrode or a catalytic material. The results presented here open up such future applications of OL-EPM in nanoscale electrochemistry.

Biological treatment of estrogenic substances.

The fate and behavior of estrogenic substances in various biological wastewater treatment processes and several advanced sewage treatment processes were examined. The removal of 17beta-estradiol (E2), estrone (E1) and estriol (E3) was investigated by using a pilot-scale activated sludge plant supplied with domestic sewage. Several sewage treatment plants adopting the activated sludge process were evaluated for the removal of estrogenic substances using an in vitro recombinant yeast assay and chemical analysis. The results indicated that E2 significantly contributed to estrogen-like activity particularly in secondary treated effluents. The removal rate of E2 was found to be sufficiently high throughout a one-year study on estrogens in domestic sewage, whereas E1 often remained in the effluent. The optimization of operational conditions based on E1 removal is important for reducing estrogenic activity in treated water.