Morphology and structure of heat-treated titania nanotubes

Titania-based nanostructures have gained considerable attention in the past decade due to their potential for enabling energy-related applications, such as catalysis and photovoltaics. Successful development of these materials relies on optimization of their band structure, which is a sensitive function of their local crystal structure and morphology. In this study, a dependence of resultant nanoscale morphology and crystallinity on the postfabrication processing conditions of electrochemically synthesized arrays of titania nanotubes is demonstrated. The as-synthesized nanotubes are amorphous; onset temperature of crystallization to anatase occurs at TC ~280ºC when annealed in oxygen, and at a lower temperature of TC ~250ºC when annealed in an oxygen-deficient atmosphere (N₂). However, it is found that annealing in the oxygen-deficient atmosphere causes a transformation of the nanotubular morphology to nanocubes with average dimension of up to 0·02 μm3, based on scanning electron microscopy data, leaving a porous tubular morphology behind. However, annealing in pure oxygen maintains the original tubular architecture. This study reveals the importance of postfabrication processing design to develop nanomaterials with tailored crystal structure and morphology, which in turn is critical for the optimization of the optical and transport properties for various energy-related applications.