Electrochemical reactivity of TiO2 nanoparticles adsorbed onto boron-doped diamond

TiO2 (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, TiO2 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 TiO2 particles are studied voltammetrically in different electrolyte media. Boron-doped diamond as an inert substrate material allows the reduction of TiO2 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 NaClO4 with variable amounts of HClO4 suggest that the reduction of TiO2 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 TiO2 surface such as the dihydrogen evolution process and the two-electron–two-proton reduction of maleic acid to succinic acid are discussed.