Influence of nitrogen species on the porous-alumina-assisted growth of TiO2 nanocolumn arrays

Abstract

Porous-anodic-alumina (PAA)-assisted anodizing in oxalic acid electrolytes combined with re-anodizing to a more anodic potential of a titanium layer on substrate, followed by chemical dissolution of the PAA overlayer result in TiO2 nanocolumn arrays, which may however be chemically unstable and destroy during the PAA dissolution. Here we show that this is because the TiO2 nanocolumns have easy-to-dissolve alumina-titania mixed-oxide ‘nanoroots’ penetrating the alumina barrier layer, where gas nano-bubbles and voids form respectively inside and between the titania roots, owing to the field-assisted crystallization of the bottom titanium oxide. The problem is solved by alloying nitrogen with titanium in the precursor film, with a compositional spread of nitrogen from 2 to 50 at%, to grow differently N-doped TiO2 nanocolumn arrays via the PAA-assisted anodizing in oxalic acid at 40 V followed by re-anodizing up to 240 V. The stability of such arrays increases with increasing nitrogen content, reaching an ideal 100% level for the Ti–50 at%N alloy films. The effect is proved to be due to the incorporation of nitrogen into the columns and roots with formation of oxynitride, which increases their chemical resistance and effectively suppresses the field-assisted crystallization of titania, leading to the obstructed O2 evolution, smaller bubbles and voids, less mixing of the two oxides in the roots, and thickening and merging the roots. A model is developed of the oxide growth and dissolution, explaining the instability of the pure TiO2 nanocolumn arrays and defining the stabilizing effect of the nitrogen species. The doping with nitrogen may make the TiO2 nanocolumn arrays highly appropriate for applications to photocatalysis and energy conversion.