Surface Force Measurements at a Copper Electrode/Electrolyte Interface C. Dedeloudis, J. Fransaer,* and J.-P. Celis Department of Metallurgy and Materials Engineering, Katholieke UniVersiteit LeuVen, 3001 LeuVen, Belgium ReceiVed: September 9, 1999; In Final Form: December 13, 1999 The surface force between a silica sphere and a copper electrode was measured in concentrated solutions of MgSO 4 with an atomic force microscope as a function of the electrode potential. The interaction between the two surfaces was compared with the DLVO theory, and the surface potential of the copper electrode was determined as a function of the cathodic overpotential. The potential of zero charge of the copper electrode was identified in this way. This was found to correspond to the potential of zero charge obtained from the differential capacitance minimum. Moreover, the influence of the pH on the surface force between silica and copper was examined and the presence of a chemically adsorbed oxygen layer on copper was deduced from force measurements at high pH. Introduction Surface forces play a very important role in many industrial processes where colloidal particles are employed. One such industrial process is composite plating, where particles are incorporated into a metal deposit during electrodeposition. Fransaer et al. 1 have previously shown the importance of surface forces in the incorporation of such particles into a metal matrix. Moreover, they indicated the role of the potential of zero charge of the electrode in composite plating. The advent of the surface force apparatus (SFA) 2 and atomic force microscope (AFM) 3 allowed the direct measurement of surface forces with high precision. During the past decade, many direct measurements of DLVO forces have been carried out between mica surfaces, 4 between surfactant monolayers 5,6 or bilayers, 7 between silica, 8,9 and between alumina surfaces. 10 More recently, force measurements have been performed in electrochemical systems. A dependence of the double layer force on the electrode potential was reported by Hillier et al. 11 using an AFM. They measured the forces between a silica probe and gold electrodes in a series of aqueous alkali halide electrolyte solutions and found that the double layer forces were a strong function of the electrode potential. Similar results were obtained by Raiteri et al. 12 on measuring the force between a Si 3 N 4 tip and a gold or platinum electrode in 1 mM KCl solution at pH 9-10. Finally, Campbell et al. 13 observed potential-dependent double layer adhesion and friction forces between a silica tip and a glassy carbon electrode or a thin film of sulfonate- derivatized poly(aniline) in 1 mM KCl solution at pH 5.2. This dependence of the surface forces on the electrode potential was attributed to the variation of the surface charge of the electrode with the applied potential. The dependence of the surface charge of an electrode on the electrode potential has been known for a long time. Frumkin 14 pointed out that the surface charge on an electrode depends on the electrode potential. That surface charge evolves from positive to negative and becomes zero at a certain electrode potential known as the potential of zero charge (E pzc ). Many methods are used for the determination of the zero charge potential of an electrode such as the electrocapillarity method, 15 the crossed polarized metallic thread method, 16 the immersion method, 17 and the differential capacity minimum method. 18 Up to now, force measurements in electrochemical systems were limited to dilute electrolytes (10 -3 M or less) and noble metals. In industrial electrochemical processes, however, highly concentrated solutions are used and the electrode surfaces are usually less noble than gold or platinum. The aim of this work is the investigation of the surface force between a silica particle and copper by AFM and its dependence on the electrode potential in concentrated aqueous solutions relevant for the codeposition of particles in electrochemical systems. Experimental Setup The surface force measurements were performed using the Nanoscope III (Digital Instruments, Santa Barbara, CA). Commercial AFM cantilevers (ThermoMicroscopes, Sunnyvale, CA) with a spring constant of 2.1 N/m were modified by gluing a single glass sphere to each. Cantilevers with a high spring constant were used in order to measure surface forces close to the electrode without interference of the cantilever instability. The glass spheres were obtained from Potters-Ballotini (Valley Forge, PA). The diameter of the glass spheres was between 20-30 µm and consisted of soda-lime glass (72.5% SiO 2 , 13.7% NaO, 9.8% CaO). Even though these glass spheres are not made of pure silica, their behavior with respect to the double layer force was found to be similar to that of pure silica particles. 8 Henceforth, the glass will be referred to as silica. Such silica spheres were glued to the cantilevers with the epoxy resin Epikote 1004 (Shell) as described by Ducker et al. 8 The cantilever was placed on a heating stage at a temperature above the melting point of the glue. A thin tungsten wire attached to a three-dimensional translation stage was used to position a drop of molten glue near the tip of the cantilever. With the help of another tungsten wire, a particle was positioned on the cantilever and the glue was solidified by lowering the temperature of the hot plate. After the surface force measurement experiments, the cantilevers were examined by scanning electron microscope (SEM) and the radius of the sphere was measured. * Corresponding author. E-mail: jan.fransaer@mtm.kuleuven.ac.be. Fax: +32-16-32 19 91. 2060 J. Phys. Chem. B 2000, 104, 2060-2066 10.1021/jp9931814 CCC: $19.00 © 2000 American Chemical Society Published on Web 02/11/2000