Enhanced Photoassisted Water Electrolysis Using Vertically Oriented Anodically Fabricated Ti−Nb−Zr−O Mixed Oxide Nanotube Arrays

Self-ordered, highly oriented arrays of titanium−niobium−zirconium mixed oxide nanotube films were fabricated by the anodization of Ti35Nb5Zr alloy in aqueous and formamide electrolytes containing NH4F at room temperature. The nanostructure topology was found to depend on the nature of the electrolyte and the applied voltage. Our results demonstrate the possibility to grow mixed oxide nanotube array films possessing several-micrometer-thick layers by a simple and straightforward electrochemical route. The fabricated Ti−Nb−Zr−O nanotubes showed a ∼17.5% increase in the photoelectrochemical water oxidation efficiency as compared to that measured for pure TiO2 nanotubes under UV illumination (100 mW/cm2, 320−400 nm, 1 M KOH). This enhancement could be related to a combination of the effect of the thin wall of the fabricated Ti−Nb−Zr−O nanotubes (10 ± 2 nm) and the formation of Zr oxide and Nb oxide layers on the nanotube surface, which seems to slow down the electron−hole recombination in a way similar to that reported for Grätzel solar cells.