Sonoelectrochemistry at tungsten-supported boron-doped CVD diamond electrodes

Highly boron-doped (atomic concentration 10²⁰-10²¹ cm^-3) conductive diamond films were deposited on tungsten substrates by hot-filament assisted chemical vapour deposition from a gaseous feed of methane and diborane in hydrogen. The boron-doped diamond film electrodes were characterised by Raman spectroscopy, scanning electron microscopy (SEM), and both conventional and sonoelectrochemical methods. The one-electron reduction of Ru(NH₃)³⁺₆ was investigated in aqueous solutions under normal and power ultrasound conditions. Well-defined voltammetric responses were observed from which the standard rate constant for electron transfer, k⁰=3×10^-3 cm s^-1, was estimated ignoring the effect of surface roughness. The electrode was used for electrochemical processes in the presence of 90 W cm^-2 ultrasound without any significant deterioration of the properties. Although diamond is known to be exceptionally chemically inert, it was found that anodic polarisation of the boron-doped diamond electrode gave rise to changes in the surface properties. In order to rejuvenate the diamond electrode surface, a hydrogen plasma treatment was used. The two-electron reduction of dioxygen to give H₂O₂ was studied in an aqueous 0.1 M phosphate buffer solution (pH 2). This process was found to be strongly affected by the state of the electrode surface with an increase in the observed current after negative polarisation. Voltammograms obtained under ultrasound conditions suggest that a potential pretreatment can switch the process from being nearly mass-transport controlled to one where mass transport effects are virtually absent.