RuS2 thin films as oxygen-evolving electrocatalyst: Highly oriented growth on single-crystal FeS2 substrate and their properties compared to polycrystalline layers

The compound semiconductor RuS2, known as mineral laurite, has been investigated as a potential (photo) electrochemically active anode material for the oxygen evolution in the process of (photo) electrolytic water splitting. The contribution describes for the first time the preparation of RuS2 thin films deposited on (100)- and (111)-oriented FeS2 (pyrite) substrates using reactive magnetron sputtering. The epitaxial growth of 60 nm thick films was confirmed by X-ray diffractometry, texture measurements, and the evaluation of cross section transmission electron micrographs. By optical reflectance spectroscopy and Seebeck coefficient measurements a direct band gap of 1.9 eV and p-type conductivity could be determined. Due to the modest electrochemical stability of the epitaxial layers in electrochemical investigations, polycrystalline films of laurite were also deposited on Ti sheets and Si wafers. As a function of grain size, [S]:[Ru] ratio and grain orientation highest activity towards oxygen evolution was found when the conditions were fulfilled that the layer composition was close to stoichiometry and increased particle sizes showed a strong texture in the grains. Some structural and chemical properties argue for the (100) surface as catalytically active and stable layer compared to other surfaces. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim