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

The simultaneous doping of RuO 2 and IrO 2 catalysts into anodic TiO 2 nanotubes (NTs) was successfully achieved by single-step anodization. KRuO 4 was used as the precursor for the RuO 2 dopant. However, for IrO 2 doping, IrO x nanoparticles (NPs) were synthesized from IrCl 3 as an intermediate species to avoid damage to the NTs by chloride ions during doping. IrO 4 − generated from the IrO x NPs through selective dissolution in the electrolyte was simultaneously doped into the positively biased TiO 2 NTs along with RuO 2 . 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 TiO 2 NTs exhibited an outstanding onset potential of 0.84 V for the oxygen evolution reaction (OER). In addition, the amount of O 2 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 Ru 4+ and Ir 4+ by X-ray photoelectron spectroscopy and transmission electron microscopy selected area electron diffraction analysis, indicating the presence of RuO 2 and IrO 2 in the TiO 2 NTs.