Photocatalysts for degradation of dyes in industrial effluents: Opportunities and challenges

Discharging dye contaminants into water is a major concern around the world. Among a variety of methods to treat dye-contaminated water, photocatalytic degradation has gained attention as a tool for treating the colored water. Herein, we review the recent advancements in photocatalysis for dye degradation in industrial effluents by categorizing photocatalyst materials into three generations. First generation photocatalysts are composed of single-component materials (e.g., TiO 2 , ZnO, and CdS), while second generation photocatalysts are composed of multiple components in a suspension (e.g., WO 3 /NiWO 4 , BiOI/ZnTiO 3 , and C 3 N4/Ag 3 VO 4 ). Photocatalysts immobilized on solid substrates are regarded as third generation materials (e.g., FTO/WO 3 -ZnO, Steel/TiO 2 -WO 3 , and Glass/P-TiO 2 ). Photocatalytic degradation mechanisms, factors affecting the dye degradation, and the lesser-debated uncertainties related to the photocatalysis are also discussed to offer better insights into environmental applications. Furthermore, quantum yields of different photocatalysts are calculated, and a performance evaluation method is proposed to compare photocatalyst systems for dye degradation. Finally, we discuss the present limitations of photocatalytic dye degradation for field applications and the future of the technology.