Effect of the oxygen diffusion in the anatase-rutile transformation in a TiO2 nanotubes array obtained by electrochemical anodization.

Abstract

Titanium dioxide is a key material in the development of alternative energy sources like water splitting and dye-sensitized solar cells. Its photochemical and photoelectrochemical properties are affected by the specific crystal structure. Titanium dioxide nanotubes array obtained by electrochemical anodization shows an amorphous structure and a thermal treatment is needed before its application in solar energy systems. In this work, we evaluated the influence of the oxygen diffusion on the grain boundaries of the substrate on the anatase-rutile transformation of a TiO2 nanotubes array supported in a titanium foil. The nanotubes array was obtained by electrochemical anodization applying 25 V for 5 hours using an electrolyte of 0,5 % wt NH4F, 10 % Vol deionized water in glycol. The anodized samples were treated at 450 ºC for 12 hours to obtain a uniform anatase structure. After that, a second heat treatment was performed at 650 °C different times between 2 to 8 hours to analyze the anatase-rutile transformation. The treated nanotubes array was analyzed by SEM and the results were compared with the theoretical models of transformation in the wall of the tube and the oxygen diffusion in the grain boundaries of the substrate obtaining an agreement between the theoretical model and experimental results.