Electrochemical behavior of fluorinated diamond electrodes

The rapid development of different techniques for the synthesis of diamond has attracted the interest of many researchers, more and more applications being suggested for this outstanding material. In most cases, the synthesis is focused on the formation of a thin film, to supply the supporting material with otherwise unattainable physical properties. It is well know that diamond is the hardest material and has a great chemical inertness; nevertheless, the modification of the outer surface of the diamond film may open new possibilities, also in terms of an increased stability. In this context, fluorine appears as a promising modifier, the stability and the inertness of some perfluorinated compounds (Teflon®, Kalrez®) having already found industrial applications. To form such a monolayer, different approaches can be followed, as summarized in the Smentkowski and Yates’ patent, although with different results (in terms of fluorine coverage). The use of different redox couples is common practice for the characterization of an electrode material: in a previous work, we used both the iron(III)/iron(II) and the ferri/ferro-cyanide redox equilibria to describe the electrochemical properties of highly boron-doped (BDD) electrodes. The obtained results showed that high quality BDD specimens behave like the usual (oxide-free) noble-metal electrodes, if care is devoted to the preparation of solutions, in order to avoid the presence of species which adsorb onto the electrode surface or are able to catalyze the electron transfer reaction. In the present work, the electrochemical properties of fluorinated, conducting diamond thin films have been investigated. Experiments have been carried out making use of F-BDD electrodes, synthesized at CSEM (Neuchâtel, Switzerland) by plasma-treating the pristine BDD samples in a CF4 atmosphere. XPS analysis showed that the above preparation most probably leads to the fluorination of the near-surface region of the diamond film. The electrochemical characterization showed that the fluorination of BDD shifts the cathodic water decomposition by about 2 V in the negative direction; this behavior could not be ascribed to problems of film conductivity and/or to a decrease in electronic density of states. The electron transfer kinetics for model redox couples like the Eu3+/Eu2+, Fe3+/Fe2+ and ferri/ferro-cyanide redox systems has been found to be quite significantly affected by the surface modification, while the electrochemical behavior of methyl viologen remains practically unchanged. The exchange current densities for the aquo-complexes are decreased about five times, while for the cyano-complex of more than three orders of magnitude. A detailed investigation of the anodic limit of the window of ideal polarizability of BDD has been carried out by Comninellis and coworkers; in the case of the fluorinated surface, no significant variations can be found in the anodic part of this window, while the cathodic one results to be strongly affected by the F-termination of surface dangling bonds. As a result, a significant current increase, due to the hydrogen evolution reaction, is only observed at potentials below –2 VSHE, which means an extension of the already wide window of ideal polarizability of BDD (~ 3 V) to an extraordinary value of about 5 V. In aqueous solutions, the limits are reasonably represented by the formation of free hydrogen [E°(H•/H2) = –2.3 VSHE] and hydroxyl [E°(•OH,H+/H2O) = 2.74 VSHE] radicals.