Simultaneous co-doping of RuO2 and IrO2 into anodic TiO2 nanotubes: A binary catalyst for electrochemical water splitting

Abstract

The simultaneous doping of RuO2 and IrO2 catalysts into anodic TiO2 nanotubes (NTs) was successfully achieved by single-step anodization. KRuO4 was used as the precursor for the RuO2 dopant. However, for IrO2 doping, IrOx nanoparticles (NPs) were synthesized from IrCl3 as an intermediate species to avoid damage to the NTs by chloride ions during doping. IrO4− generated from the IrOx NPs through selective dissolution in the electrolyte was simultaneously doped into the positively biased TiO2 NTs along with RuO2. The structural features, NT length, and amount of catalyst doping were controlled by the concentration of HF in the electrolyte and the anodizing time. The binary-catalyst-doped TiO2 NTs exhibited an outstanding onset potential of 0.84 V for the oxygen evolution reaction (OER). In addition, the amount of O2 gas evolved during the OER at 2.0 V was measured to be 230 μmol cm−2 min−1 by gas chromatography, which corresponds to a faradaic efficiency of 99%. The major oxidation states of the metals in the catalysts were found to be Ru4+ and Ir4+ by X-ray photoelectron spectroscopy and transmission electron microscopy selected area electron diffraction analysis, indicating the presence of RuO2 and IrO2 in the TiO2 NTs.