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 oxidation to CO2. BDD itself behaved as an "non-active" electrode. In this case the oxidation of organic compounds was mediated by hydroxyl radicals formed during water discharge, without any participation of the electrode surface. The behaviour of "non-active" diamond in organic oxidation processes occurring in the potential region of water discharge was described in greater detail in the chapter about electrochemical oxidation of ethylenediaminetetraacetic acid (EDTA). The theoretical model developed for organic oxidation at "non-active" anodes was used to predict results for the oxidation of EDTA at boron-doped diamond electrodes. The very good fit between experimental results and theoretical values confirmed that EDTA oxidation at diamond in the potential region of electrolyte decomposition was a fast reaction involving free electrogenerated hydroxyl radical intermediates formed at the anode during water discharge. Nanoparticles of IrO2 were deposited by thermal decomposition on boron-doped diamond electrodes (BDD-IrO2) in order to improve and better understand their electrocatalytic activity toward redox processes and the oxygen evolution reaction (OER). Electrodes with different IrO2 loadings were prepared and their electrochemical properties tested. The reaction rate constants of redox processes increased by several orders of magnitude, the overpotential of the oxygen evolution reaction decreased by about one volt. The IrO2 deposition completely altered the electrochemical properties of diamond from an "non-active" to an "active" material. The high electrocatalytic activity of BDD-IrO2 electrodes toward the OER was also confirmed in an indirect way by the observation that the oxidation of organic substances and the production of peroxodisulphate are inhibited. The high degree of dispersion of the IrO2 particles led to a high electrocatalytic activity, even at extremely low IrO2 loadings. On the basis of experimental results a phenomenological model was proposed. Boron-doped diamond was defined as a degenerated semiconductor material because of the quite high charge density on its surface due to the boron doping (> 1021 boron atoms cm-3), to the non diamond (sp2) carbon and in general to surface species. This distribution of charges on the surface promoted processes of electron transfer across the interface diamond/electrolyte. Deposition of particles on the diamond surface (graphite or IrO2) increased further this surface charge density until reaching a perfectly metallic behaviour. Since charge transfer processes occurred on the deposited particles at lower overpotential than on diamond sites, surface modified diamond electrodes showed the electrochemical properties of the deposited material.