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Real-Space Investigation of Electrical Double Layers. Potential Gradient Measurement with a Nanometer Potential Probe

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journal contribution
posted on 08.09.2011, 00:00 by Young-Hwan Yoon, Dae-Ha Woo, Taeho Shin, Taek Dong Chung, Heon Kang
We report measurement of the potential gradient in a nanometer gap between a Au(111) electrode and a gold tip immersed in a NaClO4 solution. A miniaturized potential probe, constructed with a gold tip and an insulator film, oscillates perpendicular to the electrode surface with a small amplitudes and measures the potential gradient in the local solution environment. The potential gradient is measured as a function of gap distance and then integrated over the distance to obtain the interfacial potential profile. When the probe is located far from the electrode surface, the interfacial potential profile decays exponentially with distance, similarly to the potential distribution of an isolated electrical double layer (EDL). At close gap distances, the profile exhibits an inflection point and a sigmoidal bending toward the electrode potential, which might differ somewhat from that of the isolated electrode surface. These features can be attributed to the overlap of the EDLs on the probe and electrode surfaces during the measurement and also to the electron tunneling between the two surfaces at very short distances. It is demonstrated that the potential gradient method detects signals originating exclusively from the interfacial potential component, whereas it effectively filters out time-dependent noises and potential drifts that might be associated with the complicated microscopic structure of the probe apex. This method can measure the potential profile width more accurately than a direct potential reading method, although it cannot quantitatively measure the absolute potential height.