Microwave-Assisted Hydrothermal Synthesis of Co-Doped ZnO Nanoparticles for Water Oxidation Electrocatalysis

Abstract

Microwave-assisted hydrothermal reaction has successfully achieved synthesis of Co-doped ZnO nanocrystals at a low reaction temperature of 160ºC and in a short reaction time of 30 min. Partial replacement of 4-coordinated Zn(II) ions of ZnO with Co(II) ions up to about 10at% has been suggested from evaluation of the products by UV-Vis absorption spectra, morphological observations and X-ray diffraction. The higher addition of Co resulted in its phase-separated precipitation as Co(OH)2. While mesoporous electrode processed from ZnO nanocrystals was totally inactive for water oxidation, that of Co-doped ZnO exhibited a good catalytic activity to achieve about 1 mA cm-2 current of oxygen evolution reaction (OER) with an overvoltage of 0.545 V in a neutral aqueous KCl solution, which in fact was far superior to a Co3O4 electrode known to suffer from its limited conductivity. The doped Co ions are only expected to act as reaction centers for charge transfer on the very surface in contact with the electrolyte, while ZnO acts as a highly conductive and chemically stable host matrix to support the catalyst. Doping of transition metals into ZnO by the present method thus provides a new strategy to obtain sustainable OER catalysts from earth-abundant elements for conversion and storage of renewable electricity.