Even though around
3/4th of the earth’s surface is covered by water only a small fraction of this
is fresh water and thus in turn less than 1% in total is left for human
consumption. Thus water pollution continues to be a pervasive threat.  A large bulk of inorganic material gets
dumped into or enters rivers and other fresh water sources in the form of
leachate from mines or in the form of fertilizers used in a field. However
organic pollutants pose an even graver threat as a large quantity of synthetic
organic pollutants is being dumped into the streams and rivers by various
chemical industries.1 Thus
methods of controlling water pollution has over the years caught the attention
of the majority and is today recognized as a major threat to the human society
and the environment as a whole.

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A lot of heads have
turned because of the use of heterogenous photocatalysis for the destruction of
organic pollutants. The reduction and oxidation ability of a semiconductor are
determined by the band gap. When seen from the principles of thermodynamics, a
redox reaction can only take place if the valence band of the semiconductor is
more positive when compared to electron donors potential and the potential of
the acceptor is less negative than the conduction band. 2

Holes are generated in
a valence band when the electron get excited as a result of the semiconductor
being irradiated with light which has a wavelength where the energy is not less
than the band gap energy of the same semiconductor.


Several pathways can be
followed by the seperated holes and electrons


In order to reduce the acceptor
the semiconductor can donate electron at the surface. In case the solution is
aerated, oxygen is usually the electron acceptor as it can then in turn react
with the electron in order to create superoxide radical anions. Further OH is
produced as a result of further reactions. Series of reactions takes place
which in turn results in the formation of  hydroxyl radicals.

The hole can then rise
to the surface where it in turn reacts with an electron from a donor in order
to yield H+ and OH- radicals.


These are very reactive
elements and can further oxidize organic molecules in order to further initiate
degradation processes.



WO3 or better known as
tungsten oxide has a band gap of 2.7eV which is a visible light reponsive and
thus can absorb light with wavelength upto 480nm.


Howsoever it has a low
photoactivity with repect to the degradation of the organic pollutants. This is
because of the fact that it has a high recombination rate of the electron hole
pair or rather can be explained by its low rate of electron transfer to O2.3
Platinum loading and doping were a few attempts done in order to overcome these
minor defects.4
However these attempts went futile as platinum loaded tungsten oxide was far
less active than TiO2 and the mineralization also proved to be rather low. It
has also been numerous times tested for the photocatalytic reduction of chromium
when added to methylene blue oxidation in a blue aqueous solution.5

There are in total 16
known iron oxides and oxyhydroxides6
and out of them the majority show semiconductor properties and can also be used
as photo catalysts. Several iron oxides that are frequently used are hematite,
maghemite, geothite and lepidocrocite etc. Even though the band gap of the
above mentoned iron oxides are similar, when investigated under UV light it was
observed that they all showed different photocatalytic properties and the rte
of photo degradation thus differed. Niobium oxide or Nb2O5 is another element
which can be used as photocatalyst and has a band gap slightly larger than that
of TiO2, however it is still hardly used for the degradation of contaminants.

Bismuth Metallates:

Bismuth vandate or
BiVO4 exists in 3 forms forms of crystal.7 It
has a band gap of 2.0 and 2.4eV respectively for two of its crystals i.e.
zircon and scheelite. monoclinic scheelite has over studies conducted proved to
have a higher activity for photocatalytic oxygen evolution. This very property
of this crystal can be very well attributed to the distortion that exists in
the Bi-O polyhedron in its monoclynic structured when compared with the
tetragonal forms. Bismuth tungstante is another member of the same family and
has a higher band gap of 2.7eV. It has also received much attention as it can
be simply activated by visible light and optical absorption has been noted to
happen. However there are a number of studies that need to be conducted before
correlating the paricle size, structure and composition of this element.

There are a number of
potential photocatalytic material that can substitute TiO2 and even offer a far
better band structure. Most of these material have been only applied to a few
testing molecules and the majority of these studies have been conducted upon
dye molecules. However even after much research and studies that have been
conducted over the years, much remains to be investigated upon in order to
correlate the quantum yeilds with the photoactivity.

1 E.
Brillas, I. Sire´s, M.A. Oturan, Chem. Rev. 109 (2009) 6570.

2 V.
Augugliaro, M. Pagliaro, V. Loddo, G. Palmisano, L. Palmisano, Clean by Light
Irradiation: Practical

Applications of Supported TiO2, Royal Society of
Chemistry, 2010 (Chapter 1).

3 A.
Sclafani, L. Palmisano, G. Marci, A.M. Venezia, Sol. Energ. Mater. Sol. C. 51 (1998) 203.

4 C. Lettmann, H. Hinrichs, W.F.
Maier, Angew. Chem. Int. Ed. 40 (2001) 3160.

5 A.
Watcharenwong, W. Chanmanee, N.R. de Tacconi, C.R. Chenthamarakshan, P.

K. Rajeshwar, J. Electroanal. Chem. 612 (2008) 112.

6 R.M.
Cornell, U. Schwertmann, The Iron Oxides: Structure, Properties, Reactions,
Occurrences, and Uses, second

ed., VCH
Verlagsgesellschaft, Weinheim, 2003.

7 J. Bierlein, A. Sleight, Solid
State Commun. 16 (1975) 69.