It has been known at least since 1972 that TiO2 has photocatalytic properties, when Fujishima and Honda demonstrated that TiO2 catalyzed the photolysis of water, releasing hydrogen gas. It was not until the mid-1990’s that the accompanying UV induced superhydrophilicity phenomenon was reported by Fujishima. The transfer of light energy into reactive chemical species that occurs with the help of TiO2 has generated large interest for various applications, including converting sunlight into energy, catalyzing useful chemical reactions, degrading / destroying unwanted environmental pollutants, and creating self-cleaning / anti-bacterial surfaces. As larger surface area is beneficial for photocatalysis, initial studies focused on using TiO2 nanopowders, which required working with suspensions. Recently discovered TiO2 nanotubes, being a high surface area practical nanoengineered material that could be integrated into functional components, have overcome this downside of powders and became a popular choice for TiO2 -based photocatalysis.
Photocatalytic power of TiO2 nanotubes is commonly demonstrated by measuring the decay kinetics of oxidation of common dyes, such as Methylene Blue, Methyl Orange, or Acid Orange 7 (AO7). A number of key variables have been identified to influence the ATO catalytic behavior:
For an excellent recent review, see Schmuki et al’s “A review of photocatalysis using self-organized TiO2 nanotubes and other ordered oxide nanostructures”, Small, 8: 3073–3103.