Over certain amount of energy is usedOver certain amount of energy is used

Over the last decade
great concern has arisen over the increase in environmental pollution. One of
the solution to this is the use of non-relishing or renewable sources of energy
such as solar or wind energy. Solar energy is seen as the most efficient  of all the three renewable sources of energy 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.

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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.

1 Environmental
Pollution and Impacts on Public Health, United Nations, Environmental

2 Y.T.
Liang, B.K. Vijayan, K.A. Gray, M.C. Hersam, Minimizing graphene defects
enhances titania nanocomposite-based photocatalytic reduction of CO2 for
improved solar fuel production, Nano Lett. 11 (2011) 2865–2870

3 J.
Choi, H. Park, M.R. Hoffmann, Effects of single metal-ion doping on the
visible-light photoreactivity of TiO2, J. Phys. Chem. C 114 (2010) 783–792

4 J.
Choi, H. Park, M.R. Hoffmann, Effects of single metal-ion doping on the
visible-light photoreactivity of TiO2, J. Phys. Chem. C 114 (2010) 783–792

5 V.N.
Kuznetsov, N. Serpone, Visible light absorption by various titanium dioxide
specimens, J. Phys. Chem. B 110 (2006) 25203–25209.

6 T.
Hirakawa, Y. Nosaka, Properties of O?2 and OH formed in TiO2 aqueous
suspensions by photocatalytic reaction and the influence of H2O2 and some ions,
Langmuir 18 (2002) 3247–3254.

7 P.
Cheng, M. Zheng, Y. Jin, Q. Huang, M. Gu, Preparation and characterization of
silica-doped titania photocatalyst through sol–gel method, Mater. Lett. 57
(2003) 2989–2994.