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

Abstract

TiO 2 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 lm. X-ray diffraction measurements showed that the as-prepared nanotubes were largely amorphous, whereas thermal annealing led to the formation of welldefined 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 TiO 2 nanotube arrays prepared by anodization. The impacts of the TiO 2 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-firstorder rate constant was estimated, and the sample thermally annealed at 500°C was found to exhibit the highest activity. The strong correlation between the TiO 2 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.

Fabrication and characterization of nanostructured anatase TiO 2 films prepared by electrochemical anodization and their photocatalytic properties

“…anodized samples were annealed at 500 °C for 2 hours in air to improve the oxide crystallinity and transformation to anatase as reported in some similar studies [33].…”

A novel strategy to enhance ultraviolet light driven photocatalysis from graphene quantum dots infilled TiO2 nanotube arrays

“…5,14,15 In addition to this, the TiO 2 photocatalyst has also been extensively studied with novel heterostructure functional materials, such as narrow band-gap semiconductors, metal and non-metal elements and dye molecules to increase the efficiency and range of photon absorption. [16][17][18] Carbon based materials, such as graphene oxide, graphene sheets and graphene quantum dots anchored with the TiO 2 photocatalyst are highlighted as cost-effective UV driven photocatalysts in the visible region with enhanced activity due to their retarded charge recombination, increased reaction sites and expanded light absorption range properties.…”

Formation of TiO2 photoanodes by simultaneous electrophoretic deposition of anatase and rutile particles for photoassisted electrolytic copper ions removal

“…16 Recently, Kwan and Chen 17 have shown the possibility of improving TiO 2 films reactivity throughout the interaction of two different crystalline phases, in this case Brookite and Anatase, whose mechanism is similar to that outlined previously for Anatase and Rutile.…”