Microporous aluminum oxide films at electrodes part II. studies of electron transport in the Al 2O 3 matrix derivatized by adsorption of poly(4-vinylpyridine)

Abstract

Porous aluminum oxide films are used as matrices for immobilization of electroactive reagents at electrodes. Aluminum oxide of highly regular porosity is produced by electrooxidation of aluminum substrates in polyprotic acid electrolytes. Its porosity consists of densely packed cylindrical pores perpendicular to the oxide film and penetrating its entire thickness. Depending on the applied voltage of the preparatory electrolysis, the average pore diameter of the oxide film can be varied in the range of ca. 10 nm to 200 nm. After separation of the Al 2O 3 film from the aluminum substrate and the appropriate chemical tratment, the oxide films are overcoated with gold in vacuum in order to prepare porous aluminum oxide coated gold electrodes. The porosity of the Al 2O 3 films ranges from 38% to 55% as assessed by rotating disk voltametry and transmission electron microscopy. Adsorption fo poly(4-vinylpyridine) along the pores of the oxide film results in binding of iron hexacyanide ions from acid media. The rate of electron transport along the pores of the oxide film was studied by a transient method, chronocoulometry and by a steady state technique, rotating disk voltammetry. Due to the heterogeneous nature of the electrode films in which electroactive ions are simultaneously present in the polymer phase and the aqueous phase of the oxide pores, the electron transport involves charge diffusion in both phases coupled by rapid electron and mass exchange equilibria. A new and generally applicable method of chronocoulometric data analysis was developed which allowed calculation of the diffusion coefficient of electron transport in the polymer phase, D p. This data analysis method parallels that developed by Anson et al. for steady-state conditions (J. Phys. Chem., 87 (1983) 214). Both techniques gave a D p value of ca. 1.0 × 10 −8 cm 2/s.