Comparison of modification strategies towards enhanced charge carrier separation and photocatalytic degradation activity of metal oxide semiconductors (TiO₂, WO₃ and ZnO)

Graphical abstract: Semiconductor metal oxides: Modifications, charge carrier dynamics and photocatalysis. - Highlights: • TiO₂, WO₃ and ZnO based photocatalysis is reviewed. • Advances to improve the efficiency are emphasized. • Differences and similarities in the modifications are highlighted. • Charge carrier dynamics for each strategy are discussed. - Abstract: Metal oxide semiconductors (TiO₂, WO₃ and ZnO) finds unparalleled opportunity in wastewater purification under UV/visible light, largely encouraged by their divergent admirable features like stability, non-toxicity, ease of preparation, suitable band edge positions and facile generation of active oxygen species in the aqueous medium. However, the perennial failings of these photocatalysts emanates from the stumbling blocks like rapid charge carrier recombination and meager visible light response. In this review, tailoring the surface-bulk electronic structure through the calibrated and veritable approaches such as impurity doping, deposition with noble metals, sensitizing with other compounds (dyes, polymers, inorganic complexes and simple chelating ligands), hydrogenation process (annealing under hydrogen atmosphere), electronic integration with other semiconductors, modifying with carbon nanostructures, designing with exposed facets and tailoring with hierarchical morphologies to overcome their critical drawbacks are summarized. Taking into account the materials intrinsic properties, the pros and cons together with similarities and striking differences for each strategy in specific to TiO₂, WO₃ & ZnO are highlighted. These subtlety enunciates the primacy for improving the structure-electronic properties of metal oxides and credence to its fore in the practical applications. Future research must focus on comparing the performances of ZnO, TiO₂ and WO₃ in parallel to get insight into their photocatalytic behaviors. Such comparisons not only reveal the changed surface-electronic structure upon various modifications, but also shed light on charge carrier dynamics, free radical generation, structural stability and compatibility for photocatalytic reactions. It is envisioned that these cardinal tactics have profound implications and can be replicated to other semiconductor photocatalysts like CeO₂, In₂O₃, Bi₂O₃, Fe₂O₃, BiVO₄, AgX, BiOX (X = Cl, Br & I), Bi₂WO₆, Bi₂MoO₆, etc., to improve their competence for various environmental applications.