Since discovering the photocatalytic properties of titanium dioxide, great expectations have been placed on this semiconducting material. A very promising application, besides effluent water purification, is air quality improvement by photocatalytic degradation of atmospheric pollutants. Initial investigations with titanium dioxide in architectural finishes, road coatings and interior paints have, however, not shown convincing results regarding air quality improvements. It was therefore necessary to seek methods of increasing the photocatalytic efficiency of the incorporated titanium dioxide. By appropriate doping of ultrafine titanium dioxide in the anatase modification, the effectiveness can be enlarged significantly, which leads to a considerable increase in the rate of pollutant decomposition.

From Pigment to Cleaner (Purifier)

Titanium dioxide itself has inherent cleansing properties. Since its first commercial manufacture in 1916, it is by far the most important white pigment in terms of production volume. Due to its extraordinary light scattering power and high brightness, it is incorporated in, for example, coatings, plastics, textiles and paper, where it ensures a white or brightened color with good coverage. It is used as a whitener in pharmaceuticals such as powders and tablets. Titanium dioxide is also used in cosmetics, in ointments, as a UV blocker in sun creams, and in common toothpaste. As the additive “E171”, it is even added to foodstuffs such as chocolates or cough sweets.

Innovative processes to manufacture thin titanium dioxide films have in recent decades substantially widened the application spectrum. Such thin films, with their high refractive indices, are of major interest for optical applications or as functional films in gas sensors.

Apart from interesting optical and mechanical properties, crystalline titanium dioxide also exhibits photocatalytic activity. This property first attracted attention in 1972 when the Japanese workers Honda and Fujishima identified the photocatalytic fission of water into hydrogen and oxygen on the surface of titanium dioxide particles. Since then, intensive scientific research has been devoted to photocatalytic effects of titanium dioxide on the decomposition of diverse, usually organic, compounds.

In 1995, researchers discovered another interesting effect closely linked with the photocatalytic property: photoinduction, which could be used to render titanium dioxide hydrophilic. UV irradiation of crystalline titanium dioxide-coated surfaces lowers the water contact angle of these surface to less than 10°, so that droplets lying on the surface coalesce to form thin water films. This photoinduction effect is exploited in numerous applications and still has an enormous technical and commercial potential.

Possible application areas for photocatalytic titanium dioxide include air and water purification plants; pollutant-adsorbing, self-cleaning or easy-to-clean films on window panes, facade and road coatings; and anti-fogging films on mirrors and glasses.

Titanium Dioxide Properties and Structure

Titanium Dioxide Properties and Structure
Titanium dioxide, or titanium (IV)-oxide, is, industrially, by far the most important titanium compound. It was first discovered in 1791 by the Englishman, William Gregor. Four years later, the German chemist, Heinrich Klapproth, named the chemical element after the gigantic figures in Greek mythology, the Titans. Titanium is a transition metal that makes up 0.4% w/w of the earth’s crust. It is the tenth most abundant element ahead of carbon, phosphorus or sulphur, for example. In nature, it is always found as a compound. The most important titanium minerals include titanium iron ore (ilmenite) and the mineral derivatives of titanium dioxide, rutile, anatase and brookite. Perovskite and sphene (titanite) are also worth mentioning.

Under normal conditions, titanium dioxide is a non-toxic, colorless solid. It is insoluble in water, in organic solvents, in all alkalis and acids, with the exception of concentrated sulphuric and hydrofluoric acids.

The thermodynamic stability of the three modifications of titanium dioxide
- rutile (tetragonal crystals),
- anatase (tetragonal crystals), and
- brookite (orthorhombic crystals),
decreases in the order listed above. In all three structures, each titanium atom is surrounded octahedrally by six oxygen atoms, forming a TiO6 octahedron. The three modifications differ in the way the octahedra are arranged to each other. In rutile they are linked via two, in brookite via three and in anatase via four edges with neighboring octahedra (Figure 1). Rutile is the most stabile modification of titanium dioxide. Anatase and brookite change monotropically (i.e., irreversibly) at temperatures above ~700 °C into rutile.

The different spatial linking of the octahedra results in differing physical properties (Table 1). Since the extraction of pure brookite is difficult, only anatase and rutile are considered in the following text.

Titanium Dioxide as a Photocatalyst

The band gap is of particular relevance for photocatalytic applications. In pure anatase, the band gap is 3.2 eV; in rutile it is somewhat smaller at 3.0 eV. Due to the size of this band gap, titanium dioxide is considered to be a semiconductor (Figure 2).

Semiconductors are characterized by the fact that light absorption can cause electrons from the so-called valence band to be excited through the band gap into the conduction band. The energy absorbed must correspond at least to the energy difference between the valence and conduction band. Vacant electron positions, known as holes, then remain in the valence band forming electron hole pairs (also called excitons). There are as many negative as there are positive charges so that the semiconductor crystal as a whole is neutral.

If light of a suitable wavelength falls on the surface of the semiconducting titanium dioxide, the electrons (e-) are excited into the conduction band (Figure 3-A). This results in holes or defect electrons (h+) in the valence band. These can also migrate as a result of the movement of electrons (Figure 3-B). Some of the electrons and defect electrons recombine immediately; some migrate to the surface of the titanium dioxide particles where redox reactions can result. The oxygen for this originates mainly from water adsorbed on the semiconductor surface and molecular oxygen (Figure 3-C).

In this step, the electrons in the conduction band reduce oxygen to perhydroxyl radicals (HO2·); the holes in the valence band, on the other hand, oxidize water to hydroxyl radicals (HO·) (Figure 3-D). Subsequently, the highly reactive hydroxyl and perhydroxyl radicals can react with and oxidize organic and many inorganic compounds into innocuous carbon dioxide and water or water-soluble salts (Figure 3-E).

Smaller Band Gap by Doping

Between anatase and rutile, the former exhibits the higher photocatalytic characteristics, which are due to its larger band gap. But this is not so much due to a higher oxidation potential (valence band positions of anatase and rutile are almost the same) but rather due to the position of the conduction bands of anatase relative to those of atmospheric oxygen where, in reduction processes, reactive radicals are formed. The rutile conduction band lies below that of anatase, so that its position, for an interaction with atmospheric oxygen, is less favorable.

To excite the electrons of titanium dioxide in the anatase modification, the necessary wavelength due to the band gap lies in the UV region at 390 nm and for rutile at 415 nm. When the sun is used as an irradiation source, less than 10% of the emitted radiation spectrum can effectively be used for photocatalysis. Consequently, titanium dioxide use as a photocatalyst has been, up until now, limited.

One possible way to narrow the anatase band gap in order to use the photocatalytic effect over a significantly larger range of wavelengths is by doping it, which involves introducing specific defects in the crystal lattice. Theoretical options include exchanging either cations or anions to produce defects in the lattice structure. The defects obtained lead to modifications in the band structure, which can result in the formation of intermediary levels or broadening of the original band (Figure 4).

Carbon is a suitable doping element for titanium dioxide. By introducing carbon atoms into the crystal lattice, the band gap of 3.2 eV (pure anatase) is reduced to 2.32 eV (carbon-doped version) and the required activation wavelength for the carbon-doped version shifted to 535 nm. As a result, modified titanium dioxide has a much greater effective spectral window (UV and visible light > 400 nm) than current anatase photocatalysts.

Photocatalytic Decomposition of Nitrogen Oxides and Aldehydes

Air pollution in large cities and industrial areas is becoming a matter of international concern. In particular, the atmospheric pollution caused by nitrogen oxides, benzenes and aldehydes is steadily increasing. According to the latest climate reports, nitrogen oxides have a greater influence on the greenhouse effect than equal amounts of carbon dioxide. In addition, the related health hazards caused by nitrogen oxides and aldehydes are well known. The urgent need to reduce these environmentally impairing compounds is beyond doubt.

The use of photocatalytic substances to help improve air quality has been investigated for several years. In 2002, for example, 7000 square meters of road surface in Milan were covered with a photocatalytic, cement-like material, which led to a 60% reduction in concentration of nitrogen oxides. In Japan, similar investigations using photo-catalytic cements and paving stones led to a marked decrease in air pollution.

The development of photocatalytic titanium dioxide for use in building materials and coatings is a further important step in eliminating pollutants more effectively. With the carbon-doped catalysts, the photocatalytic processes can also be initiated by artificial or diffused light sources so that these photoactive substrates also permit the desired decomposition reactions in shady areas, such as the north side of buildings, in tunnels and underpasses and in multi-story car parks. Studies on the decomposition of nitrogen oxides and aldehydes show that carbon-doped titanium dioxide can eliminate a major proportion of these and related pollutants. An example is the decomposition of 1 ppm nitric oxide in a gas stream of three liters per minute over a photocatalytic surface irradiated with UV(A) or visible light. The nitric oxide concentration after the photocatalytic reaction was measured using a fluorescence detector with a sensitivity of 1 ppb. Results for the nitric oxide concentrations are shown in Table 2.

Table 2 and Figure 5 show that the efficiency of decomposing nitric oxide, using carbon-doped titanium dioxide, is markedly increased due to the strengthened effect coming from the visible region of the spectrum.

A further example of possible pollutant reduction by photocatalysis is shown by investigation of the decomposition of acetaldehyde using carbon-modified titanium dioxide (Figure 6). The oxidation of acetaldehyde to carbon dioxide is followed by monitoring the IR spectra of acetaldehyde and carbon dioxide during photocatalysis. Two moles of carbon dioxide are generated for each mole of acetaldehyde, which is eliminated; this is shown in the doubling of the carbon dioxide signal and simultaneous halving of the acetaldehyde signal.

Titanium Dioxide

Commercial manufacture of titanium dioxide began in the second decade of the 20th century. Worldwide, ~5.5 million tonnes/annum of titanium dioxide are produced (2007), of which three quarters originates from DuPont, Lyondell (Millennium), Tronox, Huntsman Tioxide and Kronos.

The raw material for titanium dioxide production is mainly iron titanate, a black, shiny solid with the chemical formula FeTiO3, which is usually mixed with the magnetic iron magnetite (iron oxide) and other minerals as gangue. The rarer titanium ore rutile, which contains less iron, is also used. The ore is obtained by open-cast mining. The most important deposits in Europe are in Norway (Ekersund-Soggendal), Finland and in the Ilmen Mountains (South Urals). There are also deposits in Canada, Australia and the United States. To obtain titanium, the ore or titanium-containing slag is first crushed, finely ground and then the magnetic fraction separated off using strong electromagnets. It is further purified by preparing aqueous suspensions, where separation occurs by settling and flotation. This titanium ore is processed further either via the sulphuric acid or chloride processes. For ecological reasons (formation of dilute acid) the sulphuric acid process is less used

Summary

The photochemical properties of the semiconductor titanium dioxide have already led to its use in numerous applications such as purification of air, water and effluent. Photocatalysts doped with carbon or other impurity atoms can also be used in closed spaces or areas with diffused light. Applied in coatings for buildings, paving stones, concrete walls or roof tiles, they can markedly increase the degradation of atmospheric pollutants such as nitrogen oxides, aromatic hydrocarbons and aldehydes. Combined with the superb hydrophilic properties of the titanium dioxide surface (spreading water droplets to extremely thin films) this offers many further exciting areas of application for the future. Appropriately coated walls, tiled floors, plastic products, glass or textile surfaces will be much easier to clean and may even be self-cleaning. This makes titanium dioxide, as a photocatalyst, not only an environmentally safe cleaning agent but also one that, by saving energy and resources, can make a significant contribution to climate protection.