Over the last decade great concern has arisen over the increase in environmental pollution. One of the solutions to this is the use of renewable sources of energy such as solar energy, wind energy etc. Solar energy is seen as the most effective of all the three and can be used to generate electricity as well as can be converted into chemical energy in order to be used as a catalyzing agent in various chemical reactions.1 Photocatalysis is one such field in which the importance of solar energy to purify the air by way of destroying harmful organic air containments has been successfully researched upon. In a chemical reaction usually certain amount of energy is used as a catalyst however in photocatalytic reactions, light plays that role. An intermediate role is played by the catalytic material in the absorption of light energy thus promoting the desired chemical reaction. The efficiency of a photocatalyst depends upon a number of factors such as environment of active site, energy range of the solar spectrum for the excitation of the material etc. Surface acidity is another major factor which effects the efficiency of the catalyst. Most of the successful photocatalyst material have been reported to consist of base material such as titania and silica due to their higher stability under extreme conditions and their ability to obtain a number of physio-chemical properties by simply changing their particle dimensions. They usually enhance the charge carrier separation thus facilitating charge transfer to an adsorbed species.2
Titanium Dioxide or titania based photocatalysis is one of the most studied photocatalytic system. It is a semiconductor and has a band gap of 3.2eV and has proved to promote water splitting, CO2 reduction upon UV exposure and to promote mineralization of organic pollutants. Titania is naturally found in three forms namely anatase, rutile and brookite however a premise mixture of anatase and rutile named as Degussa P25TiO2 is reported to have an ever higher photocatalytic properties due to relatively wider band gap, it absorbs light corresponding to wavelengths shorter than 388nm which is only 3-4% of the total solar energy. Thus the photocatalytic activity shall be enhanced by adjusting the band gap toward visible light energy by the way of doping. Doping with a number of non-metallic compound such as nitrogen has been carried out in order to obtain visible light photo activity of titania photocatalyst. Nitrogen doped titania has been reported to show an intense band to band absorption within the range of 400-500nm of the solar spectrum which in turn brings the modified band gap of titania between 2.46-2.20eV and very high photocatalytic activity towards formic acid mineralization under visible light. Doping with similar non-metals can induce the formation of new energy level in the band gap.
A similar feat can also be achieved by introducing various metals and metal oxides as well. A number of metal ions such as Fe3+, Ru3+ can be used to enhance the photocatalytic activity. It however depends upon the fact that whether the metal ion used serves as recombination centre or a mediator of interfacial charge transfer.3 The ionic radii of the dopant metal also plays a crucial role in the final structure of the photocatalytic material and thus they have a direct impact on the photoactivity. Usually metal ions such as Pt4+ and Cr3+ have similar radii to Ti4+ ions of titania and thus can replace titania ions without causing much distortion. Larger dopant ions donot get incorporated within the titania framework due to larger radii and thus are likely to be found as dispersed metal oxides within or on the surface of Ti034. It has been further proved by Choi and his co-workers that metal ions with similar radii have been successful in incorporating themselves within the matrix of titania thus developing additional bands which further induce the absorption in the visible range of photons.5
The next topic of discussion is the chemistry that photocatalysis induce. The mechanism of photocatalytic activity of titanium based photocatalysis is usually confirmed by studying the hydroxyl radical generation upon exposure to UV light.6 Formation of hydroxyl radicals upon exposure to UV light and visible light is indicated in titanium-based samples. Titania being a semiconducting pigment, the valence electrons can be promoted into the conduction band which results into the formation of an electron-hole pair upon irradiation with suitable light. However the resulted electron-hole pair are required to be spatially separated in order for the electron-hole pair to undergo a chemical reaction.
Although titania and titania with dopants have been successful, there are a number of other material that have also shown high photocatalytic activities. Cheng and his co-workers have proved that the photocatalytic efficiency of titania-based material can be enhanced by the introduction of secondary material such as silica. Mixed metal oxides assist in promoting the photocatalytic activity in numerous ways such as the absorption of reactants can be improved by using binary material as a solid acid to support titania-based system.7
Usually photoactive sites of insulator-based photocatalysis takes place due to the presence of highly dispersed metal oxide species of quantum size. However according to the recent discoveries of Yoshida and co-workers, it is also possible to obtain similar photocatalytic abilities because of the surface quantum defects on the silica surface. When doping of silica with highly dispersed metal oxides takes place, photoexcitaion takes place which are a result of the quantum defects on the surface of silica. Also, the absorption in the visible region by the introduction of suitable dopant elements to silica can be enhanced as it is evident that higher energy UV light is required to generate the photoactive sites in silica. In most of the cases due to the fact that highly dispersed transition metal oxides tend to show better oxidation properties when compared to those of highly concentrated photocatalyst, the amount of metal oxide dopant required is also very small.
Not only is photocatalysis is restricted to semiconducting material but is also possible with pure insulating material and with mixtures of both as well. Thus in order to enhance the photodegradation activities in the visible light range, it is important to design a novel system. The future of photocatalysis will be benefited by both, new doping agents as well as by new methods of preparation.