Electrochimica Acta 47 (2002) 1641 – 1649 www.elsevier.com/locate/electacta Electron transfer reactions at conductive diamond electrodes Sergio Ferro 1 , Achille De Battisti 1 * Department of Chemistry, Uniersity of Ferrara, Via L. Borsari 46, I -44100 Ferrara, Italy Received 30 October 2001; received in revised form 12 December 2001; accepted 19 December 2001 Abstract The electrochemical behavior of the iron(III)/iron(II) and the ferri/ferro – cyanide redox couples in aqueous media has been investigated at conductive diamond, to obtain information on the properties of this electrode material. The investigation has been carried out at as prepared and oxidized diamond surfaces, under conditions of different concentration and temperature. The electron transfer kinetics has been followed by cyclic voltammetry, quasi-steady polarization (low-field conditions) and AC impedance, and the results have been interpreted on the basis of the Hush model, attempting its extension to the ferri/ferro–cy- anide redox couple as well. © 2002 Elsevier Science Ltd. All rights reserved. Keywords: Electron transfer; Highly boron-doped diamond electrodes; Iron(III)/iron(II); Ferri/ferro – cyanide; Redox couples 1. Introduction The electrochemical response of a redox couple in solution depends on the specific properties of the ion or molecule under investigation, and on the electrode ma- terial. The first aspect is related with changes in the hydration shell (if simple electron transfer is involved) or with molecule reorganization, when a more complex behavior can be expected. On the other hand, the electrode material can be only a source/sink of elec- trons, or can interact with the redox couple, when contact of electroactive species takes place. Dealing with well-known redox couples, their electro- chemical behavior can be used to characterize a new electrode material, with the aim of elucidating its prop- erties [1–23]; vice versa, a new redox couple should be studied at an electrode material, whose characteristics are established and, more important, well reproducible [24–42]. The main redox couples of iron, in aqueous solution, are the iron(III)/iron(II) and the ferri/ferro – cyanide redox equilibria, both studied at different electrode materials and in different conditions of reactant con- centrations, supporting electrolyte and temperature. Both couples have been deeply investigated in the past, but recent works have underlined some details that cast doubt on previous findings. In particular, the ferri/ ferro – cyanide couple has been for long considered an outer-sphere reaction [28], while, on the contrary, it is significantly dependent on the presence of chemical functionalities on the electrode surface [6,9,32,34 – 36,40,43–45]. Also the kinetics for Fe 3 + /Fe 2 + is highly sensitive to the electrode surface [2,5,31,41]; in particu- lar, at carbon electrodes, it seems stimulated by the presence of oxides [7,43 – 45]. Taking into account these indications and also those recently reported on conducting diamond electrodes [10–13,17–19,21,22], in this paper the results of investi- gations are described on both iron couples, carried out at highly boron-doped diamond (BDD) thin film elec- trodes, with the aim of elucidating the electrochemical behavior of the films and to obtain information related with the physical properties of this material. 2. Experimental 2.1. Electrodes Highly BDD thin film electrodes were prepared on p -silicon (100) wafers (1–3 m cm, 1 mm thick, 100 mm diameter), as described elsewhere [46]; the p -dia- mond coating was conformal, pinhole-free and had a thickness of 1 m( 5%) and a resistivity of 15 (  * Corresponding author. Fax: +39-0532-240709. E-mail address: dba@dns.unife.it (A. De Battisti). 1 ISE member. 0013-4686/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved. PII:S0013-4686(01)00898-2