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.