Progress in the application of surface engineering methods in immobilizing TiO₂ and ZnO coatings for environmental photocatalysis

Photocatalysis is widely used for the degradation of organic pollutants, with TiO₂ and ZnO as the best candidates with unique properties. However, agglomeration and recycling are major challenges in practical photocatalysis applications. Advanced deposition processes can provide nanotubular or hierarchical structures that are more promising than suspended particles. More importantly, higher efficiency of photoelectrocatalysis than photocatalysis for the degradation of persistent organic pollutants including perfluorooctanoic acid (PFOA) necessitates catalyst immobilization. Photoelectrocatalysis exhibited remarkably higher efficiency (56.1%) than direct photolysis (15.1%), electrocatalysis (5.0%) and photocatalysis (18.1%) for PFOA degradation. This paper aims to review the progress in the application of anodizing and thermal spraying as two major industrial surface engineering processes to bridge the gap between laboratorial and practical photocatalysis technology. Overall, thermal spraying is considered as one of the most efficient methods for the deposition of TiO₂ and ZnO photocatalytic films.