Surface Modification or Doping of WO<sub>3</sub> for Enhancing the Photocatalytic Degradation of Organic Pollutant Containing Wastewaters: A Review

Tungsten trioxide (WO 3 ) is an oxygen deficient metal oxide and well known semiconductor with a small band gap of between 2.4 and 2.8 eV. It is also used as a photo-catalyst for degradation of organic pollutants present in aqueous environment. It has stable physico-chemical properties and shows strong absorption of solar spectrum and thus can be used in visible-light driven photocatalysis. WO 3 has a conduction band ( E CB ) of +0.4 V versus NHE (normal hydrogen electrode) at pH = 0. Therefore, pure WO 3 has lower light energy conversion efficiency as compared to other widely used photocatalysts such as zinc oxide (ZnO) and titanium oxide (TiO 2 ). This is because the reduction potential of the electrons in WO 3 is low due to its low conduction band level. O 2 cannot be efficiently trapped in the conduction band electrons to yield superoxide radicals and fast recombination of charge carriers takes place resulting in lesser photocatalytic activity of WO 3 . However, holes in the valence band ( E VB = +3.1 V) are energetically favorably situated to oxidize water to hydrogen. To modify the energy band position and reduce the charge carrier recombination, doping or surface modification of WO 3 is necessary. This review article demonstrates the effect of dopants (low band semiconductor catalyst) on the surface modification of WO 3 to enhance the photo catalytic activity which helps in degradation of the organic pollutants present in the wastewater.