Formation of hierarchical TiO₂ nanoporous structure from free-standing TiO₂ nanotubes layers

Hierarchical TiO₂ nanoporous structures with tailored porosity have been fabricated by simply annealing of free-standing TiO₂ nanotubes (TNTs) layers in the presence of in situ trapped F-species. The hierarchical nanostructures consist of concatenated anatase TiO₂ nanoparticles (NPs). TiO₂ with different hierarchical nanostructure are obtained by changing the annealing rate of the free-standing TNTs layers. The hierarchical structure with nanopores formation in the walls of TNTs is achieved when the annealing rate is 3 °C/min. Increase in the annealing rate of the free-standing TNTs layers leads to the formation of hierarchical nanostructure of anatase TiO₂ NPs morphologically resembling beads threaded into linear strings. The TiO₂ hierarchical nanostructure undergoes two evolution steps, which include formation of nanopores in the walls of nanotubes and finally to concatenated anatase TiO₂ NPs when the annealing rate is increased. Results show that hierarchical nanoporous structure of anatase TiO₂ consist of nanopores whose diameter varies from 20 to 100 nm. EDX analysis confirms the presence of C and F-species in the as-prepared free-standing TNTs layer in significant amount. It is found that the hierarchical TiO₂ nanostructure formation mainly depends upon the annealing rate and presence of F-species. GAXRD, Raman analysis and SAED pattern exhibits enhancement in the crystallinity of the hierarchically TiO₂ nanostructure. This study provides an easy method for the synthesis of the hierarchical anatase TiO₂ nanostructure on a large scale.