Nitrogen plasma-induced crystallization of anodic TiO2 nanotubes for solar photoelectrochemistry

Anodic TiO2 nanotubes have gained tremendous interest due to the high specific surface area and facile growth with potential in large scale production. Their geometric features such as length, diameter, wall thickness or spacing between adjacent nanotubes can be tailored through the processing parameters maintained during the anodization. However, the as-anodized nanotubes are amorphous and for practical applications they require phase transformation. Usually, it is realized via furnace annealing. Nonetheless, taking into account the heating and cooling stages the whole process can take several hours. Moreover, furnaces usually use resistive heating, hence crystallization in furnace can be recognized as a highly time- and energy-consuming process, and the size of the sample is limited by the dimensions of uniformly heated area in the furnace. Here we show fast 30-second nitrogen plasma-induced crystallization of TiO2 nanotubes depending on the position in the plasma flame. We found a comparable photoresponse to the furnace annealed TiO2 nanotubes without significant change in their ordered morphology. The proposed method shows promising results regarding the reproducibility and stability of the fabricated photoanodes. We anticipate that the present work will contribute to wider applications of plasma for modifications of functional materials, especially in terms of the phase transitions.