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Structural manipulation of the photocatalytic activity of TiO2 nanotube arrays

Highly ordered TiO2 nanotube arrays on Ti substrates were prepared by an electrochemical anodization process in a mixture of glycerol and water containing 0.5 wt% NH4F and then thermally annealed from 400°C to 600°C. The sample structures were characterized by scanning electron microscopy and X-ray diffraction measurements, where the nanotube pore diameter and length were found to increase with increasing anodization voltage and reaction time. Thermal annealing at 450°C was found to lead to the generation of the largest anatase crystalline domains, and at higher annealing temperature, the anatase components started to convert into the rutile ones. The corresponding photocatalytic activity was then evaluated by the photodegradation of Alizarin Red S under ultraviolet light irradiation. The results showed that the nanotubes prepared by anodization at 25V for 10 h and subsequently annealed at 450°C exhibited the optimal photocatalytic activity with a rate constant of 2.70 × 10−3min−1 and 39.2% of Alizarin Red S degraded after 180 min of photoirradiation. These results indicated that the photocatalytic activity was primarily determined by the crystalline properties of the TiO2 nanotubes, and additional improvement might be achieved by a deliberate manipulation of the nanotube dimensions as a combined contribution of light penetration and mass transport within the nanotube arrays.