RuO2-Doped Anodic TiO2 Nanotubes for Water Oxidation: Single-Step Anodization vs Potential Shock Method

Simultaneous single-step anodization (SSA) or potential shock (PS) were performed for ruthenium/ruthenium oxide doping into anodic TiO2 nanotubes, which can be used in an oxygen evolution reaction (OER) in KOH. Regardless of the electrolyte type, SSA allows for the formation of nanotube walls more than twice as thick as other methods, leading to increased stability in the electrolyte. As a result, formation of nanograss is suppressed even after prolonged anodization. However, the SSA method exhibits a high onset potential for OER, whereas a newly grown barrier oxide is formed in PS. As a higher PS voltage is applied, a greater amount of ruthenium/ruthenium oxide is doped into TiO2 and a thicker barrier oxide is formed. Thus, optimization of PS voltage is required for each sample. The amount of ruthenium in TiO2 depends on the electrolyte used rather than the doping method: a higher amount of Ru in TiO2 is generally obtained in aqueous electrolytes compared to non-aqueous electrolytes. Empirical equation for maximum OER current density was deduced in terms of the total surface area of nanotubes, concentration of ruthenium, and barrier oxide thickness.