Publication de l'Axe NNS

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"TiO₂ nanotubes have emerged as potential building blocks in nanostructured solar cells and photocatalytic devices, but the improvement of their crystalline quality and morphological control still requires further development. We report an original fabrication process to form open-ended TiO₂ nanotubes in the framework of the positive template-assisted growth using ZnO nanowires. The process involves the combination of atomic layer deposition with a postdeposition thermal treatment in air at a moderate temperature to crystallize an amorphous TiO_x layer into a soluble, cubic-Zn₂TiO₄ shell on the top face of ZnO nanowires and an insoluble, anatase-TiO₂ shell with large-sized domains on their sidewalls. The differential crystallization process originates from the differential reactivity on the polar c-plane and nonpolar m-plane surfaces of ZnO nanowires, further enhanced by the different thickness of the amorphous TiO_x layer. Following chemical etching, open-ended TiO₂ nanotubes exhibit a hexagonal shape and high crystalline quality with sharp, well-oriented walls. These findings open the way for integrating high-quality open-ended TiO₂ nanotubes with tunable length and wall thickness on a wide range of substrates. The present approach can further be developed to form other types of nanotubes and original nanowire heterostructures with highly localized, distinct phases for the shell using one simple thermal treatment."