Flame annealing effects on self-organized TiO2 nanotubes

In the present work we study the modification of ordered TiO2 nanotube layers by flame annealing. We investigate different type of nanotubes (with variation in tube length, diameter, and grown in different electrolytes). We vary flame annealing parameters and analyze the morphology, phase composition and the resulting photoelectrochemical properties of the modified TiO2 nanotubes. The results show that flame annealing allows the crystallization of TiO2 nanotubes to anatase or rutile within seconds. The crystallization times strongly depend on the nanotube length and flame parameters. Samples from the outer cone show typically carbon burn off, while in the inner cone carbon accumulation occurs. The resulting photo electrochemical properties are to a large extent determined by the phase composition of the nanotubes (and not by the carbon content). Additionally, it may be noted that although the flame annealing process is fast and cheap, we could not achieve a maximum UV photoelectrochemical response comparable to classic furnace annealing.