A review on transition metal oxides based photocatalysts for degradation of synthetic organic pollutants

•Fundamentals and research advancements in transition metal oxides (TMOs)-based photocatalytic dye degradation are comprehensively reviewed.•TMOs-based catalysts are carefully classified into four generations and compared.•Advantages and disadvantages of suspended and supported catalysts are detailed.•Systematic and schematic illustrations of catalysis mechanisms are provided.•Challenges hindering commercialization and their possible solutions are presented. This review provides insight into the current research trend in transition metal oxides (TMOs)-based photocatalysis in removing the organic colouring matters from water. For easy understanding, the research progress has been presented in four generations according to the catalyst composition and mode of application, viz: single component TMOs (the first-generation), doped TMOs/binary TMOs/doped binary TMOs (the second-generation), inactive/active support-immobilized TMOs (the third-generation), and ternary/quaternary compositions (the fourth-generation). The first two generations represent suspended catalysts, the third generation is supported catalysts, and the fourth generation can be suspended or supported. The review provides an elaborated comparison between suspended and supported catalysts, their general/specific requirements, key factors controlling degradation, and the methodologies for performance evaluation. All the plausible fundamental and advanced dye degradation mechanisms involved in each generation of catalysts were demonstrated. The existing challenges in TMOs-based photocatalysis and how the researchers approach the hitch to resolve it effectively are discussed. Future research trends are also presented. [Display omitted]