Direct Observation of Double Layer Interactions between the Potential-controlled GoldElectrode Surfaces Using the Electrochemical Surface Forces Apparatus Toshio Kamijo, 1,³ Motohiro Kasuya, 1 Masashi Mizukami, 1 and Kazue Kurihara* 1,2 1 Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai,Miyagi 980-8577 2 WPI-Advanced Institute of Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai,Miyagi 980-8577 (Received March 9, 2011; CL-110203; E-mail: kurihara@tagen.tohoku.ac.jp) We have designed a new apparatus, an electrochemical SFA, for measuring the forces between symmetric goldelectrode surfaces under electrochemical potential control. The surface separation was determined by two-beam (twin-path) interferom- etry. The potential was applied to the gold surfaces (working electrode) in 1 mM aqueous KClO 4 using Ag/AgCl as the reference and Pt as the counter electrode. We observed the van der Waals attraction and the double layer repulsion which decreased with the increasing potentialfrom ¹0.1 to 0.2 V (vs. Ag/AgCl). The electric double layer at the electrodeelectrolyte inter- face plays an important role inelectrochemical processes. 1 Surface forces measurements have been regarded as a promising toolfor understanding the double layer phenomena. 24 Toward thisaim, attempts have been made for preparing metal electrode surfaces, i.e., platinum 5 and gold 6 on mica, for the forces measurements. Some groups 2,3,7 had developed an electrochem- ical surface forces apparatus (EC-SFA) to study the potential dependence of the electric double layer. Their systems employed nonidentical surfaces such as mica/mercury electrodes 2 and mica/goldelectrodes. 3,7 Therefore, quantitative analysisof any interactions is more difficult compared to the symmetric cases. This limitation comes from the multiple beam interferometry of white light employing fringes of equal chromatic order (FECO) used for the distance determination. For FECO, one of the substrates needs to be transparent. Recently, we designed a new surface forces apparatus using two-beam (twin-path) interferometry, i.e., twin-path SFA, for measuring the interactions between nontransparent substrates. 8 In this study, we developed a new EC-SFA which enabled us to perform the forces measurements on symmetric goldelectrode surfaces as a function of the surface separation under potential control. A schematic illustration of the measurement system is shown inFigure 1a. The goldelectrode surface was prepared by the template stripping method 9 on cylindrical silica disks (curvature radius, R = 20 mm). The gold (99.99% pure, Tanaka Kikinzoku Kougyo) was vapor-deposited on a mica template, which was glued on the disk with the goldside down and subsequently removed just prior to use. These prepared gold surfaces were molecularly smooth. Their RMS roughness evaluated by atomic force microscopy (AFM, Seiko II, SPI3800-SPA400) was 0.19 nm for a 1 ¯m © 1 ¯m area, and 0.22 nm for a 10 ¯m © 10 ¯m area. A wire was connected to the gold surfaces using conductive epoxy (ITW Chemicals, CW2400), then the connected area was covered with epoxy resin (Shell, Epikote1004). This goldelectrode was used as the working electrode (WE) along with a potentiostat (BAS, ALS/ CH Instruments electrochemical analyzer model 600C) for controlling the WE potential. The counter electrode (CE) was a Pt wire (99.9999%, Tanaka Kikinzoku Kougyo, 0.2º, 200 cm), and the reference electrode (RE) was a Ag/AgCl (saturated KCl) electrode (BAS). A salt bridge made of agar gel was used for connecting the twin-path SFA and the RE. This arrangement allowed us to run electrochemical measurements in the three- electrode cell arrangement inside the twin-path SFA. The KClO 4 solution was prepared from KClO 4 of the highest available purity (99.99%,Aldrich, Miliwaukee, WI) which was used without further purification and dissolved in pure water (NANOpureII, Barnstead, 18 M³ cm ¹1 resistance). Before all the measurements, argon (99.9999%) was bubbled through the solution for more than 1 h for deaeration, and the experiment was done under an argon atmosphere. The interaction force (F) between the gold electrode surfaces was measured as a function of the surface separation (D) in a 1 mM KClO 4 solution (unbuffered, pH 5.6) following a previously reported procedure. 8 We continuously changed the surface separation between gold surfaces at a constant approach- ing rate (25 nm s ¹1 ) when we obtained force curves. The obtained force was normalized by the radius R of the surface curvature using the Derjaguin approximation, 10 F=R ¼ 2³G f ð1Þ where G f is the interaction free energy per unit area between two flat surfaces. We used 20 « 2 mm for R which is a typical value when we measured R of surfaces using the FECO SFA in our laboratory. Figure 1b is a cyclic voltammogram obtained in the EC-SFA. No specific redox peak appeared during the potential sweep between ¹0.3 to +0.6 V vs. Ag/AgCl. This result indicates that there was onlyaslight faradic current, and this potential region is called the potential window, where no redox reaction occurs. The current was also monitored throughout the -120 -80 -40 0 40 80 120 -0.4 -0.2 0.0 0.2 0.4 0.6 Current / μA Potential / V vs. Ag/AgCl (a) (b) Figure 1. Schematic drawing of the EC-SFA (a). A cyclic voltammogram obtained for gold working electrodes in 1 mM aqueous KClO 4 in the EC-SFA, at the scan rate of 0.1 V s ¹1 (b). Published on the web May 28, 2011 674 doi:10.1246/cl.2011.674 © 2011 The Chemical Society of Japan Chem. Lett. 2011, 40, 674675 www.csj.jp/journals/chem-lett/