Titanium dioxide (TiO2) has become part of our everyday lives. It is found in various consumer goods and products of daily use such as cosmetics, paints, dyes and varnishes, textiles, paper and plastics, food and drugs, and even paving stones. 4.68 million tons of titanium dioxide were produced worldwide in 2009 ; 1,5 million tons/year are produced in the European Union . Production was even higher before the financial crisis in 2007 and 2008.
The great versatility of titanium dioxide is owing to its various forms and sizes. Titanium dioxides may be used in the form of microscale pigments or as nano-objects. Their crystal structures may vary: Depending on the arrangement of TiO2 atoms, one differentiates between rutile and anatase modifications.
Due to its high diffraction index and strong light scattering and incident-light reflection capability, TiO2 is mostly used as white pigment. It is these properties and a high UV resistance that make TiO2 the standard pigment found in white dispersion paints with high hiding power. Since light scattering does not occur anymore in nanoscale particles, the white titanium dioxide pigments used are almost exclusively rutile modification particles with grain sizes in the micrometer range. These white pigments are not only found in paints and dyes but also in varnishes, plastics, paper, and textiles. Having E number E171, they are used as food additives and occur in toothpastes, several other cosmetics, and drugs. TiO2 pigments for use in plastics constitute the fastest growing market. It is in particular due to the packaging industry’s strong demand that the consumption of titanium dioxide pigments is on the increase.
Nanoscale titanium dioxide that is manufactured for specific applications is by approximately a factor of 100 finer than the TiO2 pigments and has other physical properties. The production volume of nanoscale TiO2 amounts to less than 1 percent that of TiO2 pigments . Unlike TiO2 pigments, nanoscale titanium dioxides are not used as food additives. Currently, they are mainly found in high-factor sun protection creams, textile fibers or wood preservatives. For a long time, suncreams have been manufactured adding titanium oxide microparticles that gave the products a pasty, sticky consistency. Leaving a visible film, application of such suncreams was not easy and not pleasing to the skin. Suncreams that contain the transparent nanoscale titanium dioxides can be applied much more easily. In addition, their protective effect against harmful UV radiation is much better. At present, high sun protection factors can only be achieved using nanoscale titanium dioxides .
The German Association for Cosmetic, Toiletry, Perfumery and Detergent (Industrieverband Körperpflege und Waschmittel e.V. - IKW) has been reporting that only nanoscale titanium dioxides are used in sunscreens presently .
To achieve better dispersion properties and ensure photostability, these TiO2, moreover, are coated with further materials . The photocatalytic activity, which is another property of TiO2, is increased considerably through the high surface-to-volume ratio of the nanoparticles as compared to that of microparticles. However, not each of the above modifications can be used for photocatalytic purposes. While, as has been shown above, rutile TiO2 are applied mainly in suncreams, paints, and dyes, anatase modifications are rather suited for photocatalysis. In the presence of UV radiation, anatase TiO2 can form radicals from air or water which can degrade oxidatively organic pollutants. In the German town of Fulda, Franz Carl Nüdling Basaltwerke has developed paving stones which by means of titanium dioxide can “free” the air from exhaust emissions. Similar paving stones and tiles are used already in Japan along the traffic routes. Researchers at Universität Kassel have found a method of interlocking nanoscale TiO2 with dye molecules in such a way that the photocatalytic process can be triggered also by visible light and not exclusively by UV radiation.
Due to the hydrophilic character of titanium dioxide, water forms a closed film on the surface in which pollutants and degradation products can be easily carried away. House paints or tiles containing TiO2 particles thus are self-cleaning and pollutant-degrading. Besides, so-called anti-fog coatings benefit from the hydrophilic properties of nanoscale titanium dioxide. The ultra-thin water film on a glass pane coated with a transparent layer of nanoscale TiO2 impedes the formation of water droplets and, thus, avoids fogging. Nanoscale titanium dioxides are also suited for use in dye-sensitized solar cells (Graetzel cells).
Titanium dioxide mostly occurs together with other types of rock, thus must be separated from these. Ilmenite (FeTiO3) is one of the most well-known minerals. Different methods are used for refinement.
In the European Union, 70 percent of all titanium dioxide are extracted from natural minerals using the sulfate method while the remaining 30 percent are obtained by means of the chloride method. In Germany, both methods are used equally. The sulfate method came under criticism some decades ago for producing dilute sulfuric acid (referred to as dilute acid) which through to the eighties was dumped in the North Sea. North Sea dumping has been forbidden in Germany since 1990. Today, dilute acid is being treated or fed into manufacturing processes. During the chloride method, TiO2 ores react with chloride gas while forming hydrochloric acid. Being much more significant to industry than dilute acid, hydrochloric acid can be recycled into production or be sold.
Further processes are necessary for production of nanoscale TiO2. The so-called titanium alkoxylates can be hydrolysed and subsequently be treated thermally. The particles’ crystal modification depends on the temperature applied during the process. Moreover, nanoscale titanium oxide particles can be obtained by reacting titanium chloride compounds with ammonia. Under the influence of heat, the titanium oxide hydrate forming during that reaction turns into rutile TiO2. The aerosol method that was developed by Degussa in the forties for silicon dioxide was applied to titanium dioxide in the fifties. It enables production of nanoscale titanium dioxide from titanium chloride compounds through reaction of the latter with water vapor.