Electrochemical reactivity of TiO ₂ nanoparticles adsorbed onto boron-doped diamond surfaces

TiO ₂ (anatase) nanoparticles of ca. 6-10 nm diameter are adsorbed from acidic aqueous solution onto polycrystalline industrially polished boron-doped diamond electrode surfaces. After immobilisation at the electrode surface, TiO ₂ nanoparticles are imaged in vacuum by electron microscopy (FEGSEM) and when immersed in a liquid film of aqueous 12 M LiCl by in situ scanning tunnelling microscopy (STM). Mono-layer films of TiO ₂ particles are studied voltammetrically in different electrolyte media. Boron-doped diamond as an inert substrate material allows the reduction of TiO ₂ particles in phosphate buffer solution to be studied and two distinct steps in the reduction-protonation process are identified: (i) a broad reduction signal associated with the binding of an outer layer of protons and (ii) a sharper second reduction signal associated with the binding of an inner (or deeper) layer of protons. Voltammetric experiments in aqueous 0.1 M NaClO ₄ with variable amounts of HClO ₄ suggest that the reduction of TiO ₂ particles is consistent with the formation of Ti(III) surface sites and accompanied by the adsorption of protons. Saturation occurs and the total amount of surface sites can be determined. Preliminary data for electron transfer processes at the reduced TiO ₂ surface such as the dihydrogen evolution process and the two-electron-two-proton reduction of maleic acid to succinic acid are discussed.