Thermal annealing of ordered TiO2 nanotube arrays with water vapor-assisted crystallization under a continuous gas flow for superior photocatalytic performance

Ordered titanium dioxide nanotube arrays fabricated via electrochemical route are considered as a promising material for photocatalytic applications. Due to their amorphous character, a subsequent crystallization step is usually required. In this work, well-aligned nanotube arrays were annealed under flowing gas streams of different compositions (air/steam, air, N-2/steam, N-2, and O-2/steam) in a temperature range from 140 to 543 degrees C. Under continuous gas flow, the morphology of the crystallized titanium dioxide is strongly affected by the gas atmosphere. When using a dry gas for annealing, the tube structure was almost destroyed after 1 h treatment at 473 degrees C. In contrast, when annealing the titania nanotube array in a water vapor/gas stream, the tube morphology can be maintained up to 543 degrees C and 10 h annealing time. Moreover, nanotube arrays crystallized in a flowing gas atmosphere containing water vapor showed a significantly higher photocatalytic activity for phenol degradation under UV irradiation than nanotube arrays crystallized in a dry gas stream. Reasons for this behavior might be the preservation of the tubular structure as well as a lower concentration or density of structural defects in presence of water vapor which leads to an enhanced generation of reactive oxygen species when the arrays are exposed to UV-light.

Automated summary

Ordered titanium dioxide nanotube arrays fabricated via electrochemical route require a subsequent crystallization step for their amorphous nature. The morphology of crystallized titania under different gas atmospheres was affected, with better preservation of tube structure and enhanced photocatalytic activity observed when annealed in a water vapor/gas stream.