Electrochimica Acta 55 (2010) 5210–5222 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta Numerical simulation of probing the electric double layer by scanning electrochemical potential microscopy R.F. Hamou ∗ , P.U. Biedermann, A. Erbe, M. Rohwerder 1 Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Straße 1, 40237 Düsseldorf, Germany article info Article history: Received 20 January 2010 Received in revised form 8 April 2010 Accepted 10 April 2010 Available online 18 April 2010 Keywords: Electric double layer SECPM Debye screening EDL overlap Scanning probe microscopy abstract Computational modeling of probing the electric double layer (EDL) by scanning electrochemical potential microscopy (SECPM) was carried out in order to evaluate the applicability of this method. The modeling is based on a continuum approach for the electric double layer in dilute solution using the modified Poisson–Boltzmann equation. This model takes into account the finite ion size and prevents steric effects near the charged surface. The approach of the SECPM probe towards a charged electrode, immersed in electrolyte solution, is simulated obtaining the potential profile in the direction normal to the electrode. The effects of the shape and the size of the metallic protrusion at the apex of the probe were studied. A clear dependence of the probe potential on the apex geometry was observed, and correlated to the apex surface charge density distribution. The overlap of the EDLs located at the probe and the electrode is studied by setting different initial open circuit potentials, i.e., different charging, to the probe. We have found that the obtained potential profiles are consequences of the EDL overlap, characterized by an ionic distribution in the gap separating the probe and electrode. Depending on the strength of both EDLs and their polarity, the Debye screening effect severely influences the probe potential. Crown Copyright © 2010 Published by Elsevier Ltd. All rights reserved. 1. Introduction Scanning probe microscopy (SPM) covers a wide range of experimental tools used to investigate surface properties. Several sophisticated techniques appeared in the last decades, using dif- ferent concepts to generate surface images with high-resolution. Considerable knowledge was gained with the advance of SPM techniques. A more recent application field is the high-resolution measurement of electrode potential distributions. High-resolution measurements of the distribution of the lateral electrode potentials or of the potential profile normal to the surface are of interest in many research fields, from galvanic elements in corrosion to electrochemically active sites in biological cells. The Scanning Kelvin Probe can measure potential distributions with high-resolution on dry surfaces and surfaces covered by thin layers of electrolyte layers [1–3]. However the potentials measured on the dry surface are not directly correlated with the ones during immer- sion [3]. Such measurements in electrolytes were planned to be possible with the scanning electrochemical potential microscopy (SECPM). The SECPM has been introduced by Li and Kjoller [4]. The idea was to investigate the electric double layer (EDL) by approaching ∗ Corresponding author. Tel.: +49 211 6792 454; fax: +49 211 6792 218. E-mail address: hamou@mpie.de (R.F. Hamou). 1 ISE member. a metallic probe towards a charged surface immersed in an elec- trolyte solution. The potential drop between tip and sample surface can be measured at different steps during the approach, potentially allowing the identification of the charge distribution and the poten- tial profile in the normal direction of the EDL. This is potentially a promising technique, which allows direct access to the spatially resolved EDL properties as opposed to impedance spectroscopy measuring the capacitance [5], or atomic force microscopy mea- suring surface forces [6]. In addition, SECPM offers the possibility to map the surface potential of an electrode/electrolyte interface. In particular, it can image non-conductive samples, e.g., biological species adsorbed on an electrode surface [7], which is an interesting advantage com- pared to electrochemical scanning tunneling microscopy (EC-STM). However, SECPM is not extensively used by electrochemists. Only few scientific articles exist in the literature that use this technique for electrochemical investigations. The first published work apply- ing the SECPM concept was done by Woo et al., using a modified EC-STM setup to probe the EDL potential [8]. The authors showed the possibility to measure the potential on the nanometer scale for an electrode/electrolyte interface. Based on the linear superpo- sition approximation (LSA), a simplified interpretation was given for the measurement mechanism and for the EDL overlap that can occur between the tip and the sample. Later, Hurth et al. [9] reported EDL potential profile measure- ments with the SECPM, and concluded that for several situations the data did not fit the classical Gouy–Chapman–Stern (GCS) model. 0013-4686/$ – see front matter. Crown Copyright © 2010 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.04.042