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Titanium Dioxide Photocatalysis – one approach to self cleaning ceramic surfaces

Titanium dioxide has recently gained a lot of interest in the development of the self-cleaning surfaces through photocatalysis. The oxidation of hydrocarbons, which are common contaminants on several surfaces, is a slow process, but can remarkably be accelerated with the help of photocatalyst, such as titanium dioxide, creating strongly oxidative conditions on the surface. Such semiconducting particles in contact with a liquid or gaseous medium generate under exposure of light exited states which are able to inititate redox reactions and molecular transformations.

In 1972 Fujishima and Honda [1] discovered the photocatalytic splitting of water on TiO2 electrodes. This was a beginning of strong development of heterogeneous photocatalysis. Titanium dioxide has three polymorphs: rutile, brookite and anatase. Anatase shows the highest photoactivity. TiO2 is a semiconductor with the band gap energy of 3.2 eV. The photons having the energy > 3.2 eV ( wavelength < 388 nm, UV light, about 5% of the solar light reaching the surface of the earth) can promote an electron from the valence to the conduction band which results in chemical reactions on the surface creating either highly reactive hydroxyl radicals or superoxide ions which both are very powerful oxidants capable to oxidize most organic contaminants.

The heterogeneous photocatalytic oxidation with TiO2:

  • utilizes a low-cost photocatalyst
  • has quite fast reaction rates at mild operating conditions (RT and ambient pressure)
  • converts a wide spectrum of organic contaminants to water and CO2
  • works without additional chemical reactants and does not produce additional side reactions

Figure 1 describes the major areas of activity in titanium dioxide photosynthesis.


Fig. 1. Major areas of activity in titanium dioxide photocatalysis

Recently a lot of research is done to extend the range of photocatalysis wider to the range of wave lengths of visible light [2,3]. This is also one aim in SHINE PRO -project. In addition to that further techniques, ALD (Atomic Layer Deposition), Sputtering, Thermal Spraying and Sol-Gel synthesis, are studied to produce such photocatalytically active surfaces on different materials.

References:

  1. A. Fujishima, K. Honda, Electrochemical Photocatalysis of Water at a Semiconductor Electrode, Nature 238 (1972) 37.
  2. Z. Zou, J. Ye, K. Sayama and H. Arakawa, Direct splitting of water under visible light irradiation with an oxide semiconductor photocatalyst, Nature 414 (2001) 625-27.
  3. R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga, Visible-Light Photocatalysis in Nitrogen-Doped Titanium Oxides, Science 293 (2001) 269-271.

     
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