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:
- Annealing / crystal structure. Annealing above 300°C results in significant improvements of catalytic performance over amorphous (as-formed) TiO2 nanotubes. Catalytic activity generally increases with annealing temperature, as a result of forming a mixed anatase / rutile phase. However, nanotubes begin to collapse at around 650°C, negating the effect of annealing.
- Nanotube length. In general, longer tubes are reported to have stronger catalytic performance, though the literature disagrees on when this effect ceases. Some researchers found an increase in catalytic activity up to 16µm in ATO nanotube length, while others reported no increase in catalytic activity beyond 3-7µm in titania nanotube length.
- Nanotube diameter. The literature is mixed on the impact of tube diameter on catalytic properties, though it should be noted that for many of the studies it appeared difficult to separate the tube length effect from the diameter effect. (The high voltages used to grow nanotubes with larger diameters tend to concurrently result in longer nanotubes.) Some researchers find an optimal tube diameter to be around 100nm.
- Nanotube decoration. Common techniques reported to significantly enhance photocatalytic properties of ATO include electrodeposition, photocatalytically reduced silver, and UHV evaporation of silver and gold.
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.