Template-Assisted Growth of Open-Ended TiO2 Nanotubes with Hexagonal Shape Using Atomic Layer Deposition

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 TiOx 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 TiOx 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.

Automated summary

An original fabrication process was reported to form open-ended TiO2 nanotubes using ZnO nanowires as positive templates. The differential crystallization process resulted in hexagonal-shaped TiO2 nanotubes with high crystalline quality. This approach allows for the integration of high-quality, open-ended TiO2 nanotubes with tunable length and wall thickness on a wide range of substrates.