Ultrathin Layer Cell for Electrochemical and Electron Transfer Measurements

An electrochemical cell based on electrodes of near-atomically-smooth platinum on mica surfaces with an interelectrode gap width (ro) adjustable with piezoelectric drive elements with a resolution of ca. 1 8, for distances of 0 to >500 nm is described. Measurements of the steady-state current for solutions of Fe(CN),3-/4show good agreement with electrochemical thin layer cell equations derived for physical diffusion of species between the electrodes, corrected for the geometry of this cell (crossed hemicylindrical electrodes) for ro 2 200 8,. For ca. 10 8, < ro < 200 8, at higher concentrations (>2 mM) of electroactive species with small interelectrode potential differences (I 10 mV), the current is larger than predicted; several possibilities for this current enhancement are suggested. Electron transfer from an electroactive polymer film of either reduced N,N‘-bis[3-(trimethoxysilyl)propyl]-4,4’-bipyridinum or oxidized poly(viny1ferrocene acrylonitrile) to a similar film on the second electrode is also demonstrated. We describe an electrochemical cell with near-atomicallysmooth electrode surfaces and an apparatus to produce adjustable and variable interelectrode spacings of 0 to >500 nm; results of electrochemical measurements with dissolved species and polymer layers are reported. This apparatus was designed to probe homogeneous and heterogeneous electron transfer reactions. Thin layer electrochemical cells, with solution layer thicknesses of 10-100 pm, have been widely used for a variety of electrochemical and spectroelectrochemical experiments.’ However, cells with much closer electrode spacing (rO), which we will call ultrathin layer cells (UTLC), encompass a number of new features. The diffusional transit times between electrodes, ca. ro2/D, where D is the diffusion coefficient (ca. IO4 cmz s-l) and r, ca. 10 nm will be <lo4 s, so that very rapid chemical reactions can be studied. Moreover, at spacings 51 nm direct electron transfer between electrodes (e.g., tunneling or intermolecular transfer between species on the electrode surfaces) becomes possible. In the construction of an UTLC and associated apparatus, one must consider a number of factors: (1) electrodes must be fabricated that are atomically smooth and parallel over the interelectrode contact region; (2) these must be moved with respect to one another with A-scale resolution; (3) the interelectrode spacing must be determined with similar resolution; (4) thermal drifts and vibration that can affect the spacing must be minimized; ( 5 ) the electrode surfaces and solutions must be maintained at a high degree of cleanliness to avoid dust particles and other impurities. In designing this apparatus we used the principles and methodology of the surface forces apparatusz4 that has been used to measure the forces between two thin mica sheets attached to crossed hemicylindrical pieces as these are brought together. The details of construction are shown in Figure 1 and are contained in the Experimental Section. Each electrode consists of platinum sputtered on a freshly cleaved mica substrate, which in turn is glued to a hemicylindrical lens. Two such lenses are arranged in a crossed configuration to minimize problems of maintaining the surfaces parallel. They are moved with respect to each other with piezoelectric drives whose movements are calibrated by multiple beam in te r fe r~metry .~-~ In the experiments described here, the current that flows between the electrodes as they are brought together is a direct measure of the rate of charge propagation via solution species, of charge transfer between species, on the electrode surfaces, or of electron tunneling between the (1) Hubbard, A. T.; Anson, F. C. Electroanal. Chem. 1970, 4, 129 and (2) Israelachvili, J. N.; Adams, G. E. J. Chem. SOC., Faraday Trans. 1 (3) Klein, J. J . Chem. SOC., Faraday Trans. 1 1983, 79, 99. (4) Israelachvili, J. N. J . Colloid Interface Sci. 1973, 44, 259. (5) Tolansky, S. In Multiple Beam Interferometry of Surfaces and Films; references therein.