Tunnelling Processes at Highly Doped ZnO Electrodes in Contact with Aqueous Electrolytes. I. Electron Exchange with the Conduction Band

Abstract

AbstractThe anodic behaviour of highly doped ZnO(In) electrodes (ND = 1019  2020 cm−3) was studied in aqueous electrolytes. Addition of the reduced species of inorganic redox couples (I−, Fe2+) to the electrolyte causes oxidation currents at a bandbending above 1 eV in the otherwise blocking region of the electrode. The reverse reaction is also possible where the oxidized species of a suitable redox couple (e.g. Ir(Cl)62‐) is reduced by conduction electrons. The corresponding bandbending precludes the overcoming of the potential barrier by thermal activation. Electrons can, accordingly, tunnel through the space charge barrier of such highly doped ZnO electrodes in both directions. Sensitizing dyes (rhodamine B, crystal violet) are also oxidized at such electrodes in the absence of illumination by tunnelling electrons. The oxidation of organic reducing agents (e.g. hydroquinone) which can act as “supersensitizers” can, however, only be achieved after addition of traces of dyes.