Photocatalytic reduction of methylene blue by TiO2 nanotube arrays: effects of TiO2 crystalline phase

TiO2 nanotube arrays were synthesized by anodization of Ti metal sheets followed by thermal annealing at elevated temperatures from 400 to 600 °C. Scanning electron microscopic measurements showed that dense arrays of nanotubes were produced with the inner diameter about 100 nm, wall thickness 35 nm, and length about 10 μm. X-ray diffraction measurements showed that the as-prepared nanotubes were largely amorphous, whereas thermal annealing led to the formation of well-defined anatase crystalline phase. More interestingly, at 470 °C, the brookite crystalline phase also started to emerge, which became better defined at 500 °C and disappeared eventually at higher temperatures, a phenomenon that has not been observed previously in TiO2 nanotube arrays prepared by anodization. The impacts of the TiO2 nanocrystalline structure on the photocatalytic activity were then examined by using the reduction of methylene blue in water as an illustrating example. Upon exposure to UV lights, the visible absorption profiles of methylene blue exhibited apparent diminishment. Based on these spectrophotometric measurements, the corresponding pseudo-first-order rate constant was estimated, and the sample thermally annealed at 500 °C was found to exhibit the highest activity. The strong correlation between the TiO2 crystalline characteristics and photocatalytic performance suggests that the synergistic coupling of the anatase and brookite crystalline domains led to effective charge separation upon photoirradiation and hence improved photocatalytic activity, most probably as a consequence of the vectorial displacement at the nanoscale junctions between these crystalline grains that impeded the dynamics of electron–hole recombination. These results demonstrate the significance of nanoscale engineering in the manipulation of oxide photocatalytic performance.