Surface modification or doping of WO_3 for enhancing the photocatalytic degradation of organic pollutant containing wastewaters: A Review

摘要

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.