The photo-electrokinetics of the O2 evolution reaction on ZnO nanorods

Abstract

In the present study, based on potentiodynamic and potentiostatic measurements as well on specific surface area analysis data we present a detailed study on the photoelectrochemical properties of ZnO nanorod (NR) arrays aiming to shed light on the photo-electrokinetics of oxygen evolution reaction (OER) on the ZnO surface interfaced with 0.1 M NaOH aqueous solution. The faradaic selectivity for the oxygen evolution is around 70% and takes place on the side-wall (e.g. 110) planes of the NRs, while H2O2 is produced on the (001) polar planes, which appear to be 3–4 times more reactive than the side-wall (110) planes. By the use and the kinetic analysis of potentiodynamic experiments it has been inferred that the OER mechanism involves two active sites on the (110) surface: (I) the surface Zn atoms where OH− are being discharged and adsorbed electrochemically OHadZn and (II) the photo-induced oxygen vacancies where the former species migrate and are adsorbed as OHadV and evolved as O2 under the effect of incident radiation. The transient kinetic analysis of the potentiodynamic measurements resulted in the determination of the electrokinetic parameters (Tafel slope (b), symmetry factors (β) and exchange current densities (Io)) of the OHadZn and OHadV species, thus providing significant insight as of their binding energies and their reactivity on the ZnO surface. The turnover frequency at the maximum of the applied bias photoconversion efficiency (ABPE) for the production of one molecule of O2 is 0.35 s−1.