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In a narrow region of an electrode/electrolyte interface, an electrical double layer (EDL) exists in which ions with an opposite charge to the electrode line up in an excess concentration. The EDL is believed to be extremely thin, and the internal electric field strength can exceed 10 V/m in routine electrochemical environments. Since its presence was first predicted by theory in the early 20th century, understanding of the EDL has been a key to the subjects of practical and fundamental importance such as electrochemistry, corrosion, and the stability of colloidal particles. Numerous experimental approaches have been made to uncover the nature of the EDL. Careful electrochemical studies have measured the interfacial capacitance and the excess charge on electrode surfaces. Spectroscopic studies of the vibrational frequency shift of molecules adsorbed on electrode surfaces (vibrational Stark effect) have revealed existence of strong electric fields inside the EDL. Realspace investigation of the EDL was first made with a surface force apparatus, and then with an atomic force microscope by attaching a silica colloidal particle to the tip. These studies measured the electrostatic forces present between EDLs formed at the charged surface and the probe in close distances, from which the thickness of the EDL was deduced. While these studies have greatly contributed to the current understanding of EDLs, many physical aspects still remain in veil for this nanoscopic environment. One of the key features that defines the electrode/electrolyte interface is the electrical potential inside the EDL, but it has not yet been directly measured by experiments and thus far been theoretically predicted or deduced from other related physical parameters. In this work, we developed a miniaturized probe that can measure the local potential of the solution with subnanometer spatial resolution, and we applied this technique to investigation of the inner potential of electrode/ electrolyte interfaces. The concept of the experimental setup is illustrated in a diagram shown in Figure 1. The electrochemical cell consisted of four electrodes, for which the working electrode (WE) was a flame-annealed Au(111) film deposited on glass, and the counter and reference electrodes (CE and RE) were also gold. If necessary, an Ag/AgCl electrode (in saturated KCl) was used as an RE for comparison of the observed potentials with the reported values. The electrode potentials were controlled by a bipotentiostat (Pine model AFRDE5). The fourth electrode was a metal probe at its open-circuit potential (ocp). The probe was an electrochemically etched gold wire, coated with a nail polish material except its apex. Aqueous electrolyte solutions were prepared to contain NaBF4 at various concentrations, which are known to have negligible specific adsorption on a gold surface. The choice of the electrolytes, the gold electrodes, and the probe simplified the electrochemical interface such that it consisted only of the EDL in the absence of specific adsorption of electrolytes or electrochemically active species. To investigate the interfacial potential the instrument has to meet three requirements: (i) the positioning of a probe inside the EDL formed on an electrode surface with subnanometer spatial precision, (ii) a negligible leakage current through a probe such that it monitors the solution potential without disturbing the local electrolyte concentration, and (iii) a miniaturized probe that can probe into a narrow region of interest with the intended spatial resolution. The first requirement was met by controlling the probe position with a piezo actuator and the control circuit of a scanning tunneling microscope (STM; RHK Technology Inc.). The probe, initially located at the tunneling distance in an STM mode |
