Simon Cotton takes a look at those compounds that find themselves in the news or relate to our everyday lives.
Why is titanium dioxide so important?
Titanium dioxide, or titanium(IV) oxide (TiO2), is the best white pigment available. The compound is bright white and doesn't dull on exposure to polluted air. Titanium dioxide is found in ores such as rutile and anatase, and some four million tonnes of TiO2 are used each year in a range of applications, from a colourant (E171) in the food industry, in toothpaste and in paper, through medicines, to sunscreens. Its opaque white colour means TiO2 strongly absorbs uv light, which helps it to be a photocatalyst.
How does it act as a photocatalyst?
Ultraviolet light (just below 400 nm wavelength) promotes an electron into the conduction band of TiO2, leaving a vacancy in the valence band. At the surface of the compound these promoted electrons and vacancies react with molecules, such as H2O and O2, forming reactive free radicals such as O2- and •OH. These radicals oxidise organic substances and cause the breakdown of cell walls, thus killing bacteria.
Coatings of TiO2 can be annealed onto glass. The treated glass is transparent, but has self-cleaning properties. When the TiO2-coated glass is exposed to uv light its surface also becomes hydrophilic, so that water spreads across the glass. This has two benefits: the glass does not fog up (especially useful for bathroom mirrors and car wing mirrors); and the spreading water washes away dirt. This technology is the basis of Pilkington's Activ self-cleaning glass.
So you won't need window cleaners?
No. But it makes their job easier, and saves on detergent too.
Does the coating work on indoor surfaces?
No. You need uv light to keep generating new radicals. However, scientists, led by Shinri Sato of Hokkaido University in Japan, have discovered that by doping the TiO2 layer with chemicals such nitrogen or silver, longer-wavelength, visible light from indoor lights will initiate radical formation. These treated surfaces could find self-cleaning applications in areas such as kitchens and bathrooms.
This technology might help reduce the risk of infections in hospitals too, so TiO2-coated tiles
are being tested.
Any other self-cleaning applications?
Titanium dioxide built into the surfaces of buildings, pavements and roads has been trialled in London and Rome. The ability of TiO2 to oxidise organic matter could lead to pavements that don't get marked with chewing gum. A street in the Dutch town of Hengelo is to be paved with stones with a top layer of TiO2. The plan is that this photocatalytic surface will catalyse the oxidation of NOx emissions from car exhaust fumes to nitrates, which will condense on the road and then be washed away by rain. Removing the pollutants will improve air quality in the town.
The paint, Ecopaint, developed by Lincolnshire-based Millenium Chemicals, is based on a mixture of TiO2 and CaCO3 embedded in a porous polysiloxane base. Nitrogen oxides diffuse into the paint layer, react on the TiO2 forming nitric acid which is then neutralised by CaCO3.