ChemicalPhysics 141 (1990) 131-141 North-Holland zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA ELECTRON TRANSFER REACTIONS ON PASSIVE IRON ELECTRODES P. MEISTERJAHN, J.W. SCHULTZE, B. SIEMENSMEYER Institut $3 Physikalische Chemie und Elektrochemie, Universitiit Dtlsseldorf; Universitiitstrasse I. D 4000 DiisseldorJ FRC U. STIMMING and M.H. DEAN Department of Chemical Engineering and Applied Chemistry, Columbia University, New York, NY 10027, USA Received 19 July 1989 The mechanism of electron transfer reactions ( zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGF ETRs) on passive iron electrodes is studied using the redox system [ Fe( CN)6]4-/ [ Fe( CN ), ] s- in a borate buffer at pH 8.4. Oxide films are prepared by potentiostatic polarisation at constant time, tax= 300 s in a potential range, CJa=0.45- 1.4 V (SHE). The tilm stoichiometry is evaluated from XPS measurements. The current densities of ETRs are measured at constant film thickness in the potential range from 0 to 1 V with sweep rates up to 5 V/s. The constancy of the film properties is verified by cyclic voltammetry in redox free electrolyte before and after the ETR measurements. During all measurements the electrode capacity is recorded simultaneously. Current potential curves show various regions which are discussed separately. On the cathodic side a small influence of the film thickness is observed and the potential dependence of the ETR can be explained by a rate determining tunnel process from the lowest part of the conduction band of the film to the redox system in the electrolyte. On the anodic side the strong decrease of the current density with increasing film thickness in an inter- mediate potential range can be explained by a rate-determining Poole-Frenkel effect involving localized states within the film. For thick films at high potentials a contribution of low-lying states is indicated. 1. Introduction Electron transfer reactions (ETRs) can be inter- preted semi-quantitatively using the band structure model of the passive film [ 1,2]. In addition to elec- tronic states of the conduction band, CB, and the va- lence band, VB, localized states near the Fermi level were taken into account for resonance tunnelling [ 3 1. Such states within the band gap can also contribute to photoelectrochemical processes, as has been shown for various amorphous or non-stoichiometric passive films [ 41. The contribution of low-lying localised states has been proved for oxide films which were disordered by ion implantation [ 51. As a conse- quence of all these considerations, single and multi- ple tunnelling processes, resonance tunnelling, and the Poole-Frenkel effect may contribute to the rate of ETRs in addition to CB and VB processes, the latter being well known for crystalline semiconductors [ 6 1. Recent measurements of ETRs on passive metal electrodes, e.g., iron, involved current measurements as a function of electrode potential U and concentra- 0039-6028/90/$03.50 0 Elsevier Science Publishers B.V. (North-Holland) tion [ 7,8] and film thickness d [9]. In addition, a careful monitoring of film properties, e.g., as ex- pressed in the capacity, would be important. This de- termines the thickness of the space charge layer and is, itself, influenced by the distribution of localized states in the band gap. In addition, the film compo- sition plays an important role. It is, therefore, the aim of this paper to re-investi- gate the rate of ETRs on passive iron in neutral solu- tions using the redox system [Fe(CN),14-/ [ Fe ( CN ) 6 ] 3- [ 7-9 1. Due to recent developments of experimental techniques, the measurements have now been extended and improved with respect to the fol- lowing points: (I) Anodic and cathodic current densities i, and i_ were measured for the same values of film thick- ness ranging from 1.5 to 2.7 nm. (ii) The state of the oxide film was carefully checked by simultaneous measurements of the elec- trode capacity and a subsequent coulometric deter- mination of the film thickness and an XPS analysis. (iii) The current density was measured in a wide