Control of electron transfer kinetics at boron-doped diamond electrodes by specific surface modification

Diamond films with different levels of doping have been the subject of applications and fundamental research in electrochemistry, opening up a new branch known as the electrochemistry of diamond. The electrochemical properties of diamond mostly depend on the operating conditions during the deposition of the film and on the treatment of the surface. The absence so far of a standard procedure in the production and treatment of diamond films has created a wide range of diamond quality and properties. This work would try to clarify some aspects of the electrochemical activity of boron-doped diamond electrodes in light of the results obtained in our and other laboratories. Boron-doped diamond (BDD) electrodes, both as-grown and polarised anodically under different conditions, were prepared in order to study the chemical and electrochemical changes of diamond and clarify the rule played by the surface-state density. Many different treatments were employed, but only the most relevant results will be presented in this work, viz., those obtained with four kinds of surface treatment: as-grown (BDDag), mildly polarised (BDDmild), strongly polarised in perchloric acid (BDDsevere), and strongly polarised in a sulphuric acid-acetic acid mixture (BDDAcOH). Charge transfer processes at the electrode surface were studied by steady-state (polarisation) and dynamic (cyclic voltammetry) potential step methods. Simple electron transfer processes such as the outer-sphere redox system ferri/ferrocyanide (Fe(CN)6III/II) and complex charge transfer reactions such as the inner-sphere 1,4-benzoquinone/hydroquinone (Q/H2Q) redox reaction were chosen to test the electrochemical properties of the electrodes. The properties of the diamond electrodes were found to undergo strong modification as a function of surface treatment. The reaction rate constants decreased significantly upon anodic polarisation. Important drops in the carrier surface concentration and in true surface area led to hindrance of electron transfer at the electrode surface. The concept of non-diamond (sp2) impurities as charge transfer mediators on a diamond surface was suggested as an explanation for the electrochemical behaviour of diamond electrodes after surface oxidation treatment. To verify this concept, graphite particles were deposited on mildly polarised diamond electrodes (BDDmild) so as to prepare diamond-graphite composite electrodes (BDD-g), and their properties were compared with those of as-grown (BDDag) diamond and carbon electrodes. The sp2 coverage on BDD was estimated by cyclic voltammetry. A strong analogy existed between as-grown diamond electrodes and diamond-graphite composite electrodes. In fact, by depositing graphite particles on diamond after its deactivation by anodic polarisation is was possible to restore the initial properties of the as-grown diamond electrode. In the potential region of electrolyte decomposition, sp2 was eliminated from the BDD-graphite composite surface by oxi