Immobilization of TiO2 and ZnO by facile surface engineering methods to improve semiconductor performance in photocatalytic wastewater treatment: A review

Photocatalysis is widely researched in water and wastewater treatment processes owing to its unique capacity in the potential mineralization of organic pollutants. Suspended nanoparticles provide high specific surface area, but their practical application has been very limited due to drawbacks such as catalyst agglomeration during treatment and difficulty of catalyst reuse after treatment. It is therefore of paramount importance to immobilize catalysts to realize continuous photocatalysis operations towards commercial scale, and surface engineering provides an ideal strategy to overcome the problems associated with using suspended nanoparticles. Of different semiconductors used for the photocatalytic degradation of organic pollutants, TiO2 is considered a benchmark photocatalyst with ZnO as a potential alternative. Hence, the scope of this research is to review the application of several surface engineering methods including physical vapor deposition, dip coating, spin coating, spray coating, and electrophoretic deposition in the immobilization of TiO2 and ZnO. Overall, electrophoretic deposition is considered very promising for the successful immobilization of photocatalysts, and sintering particularly is recommended to improve the adhesion strength of the as-deposited films, expediting the practical applications of photocatalysis through electrophoretic deposition.