Recent developments in metal oxide semiconductor photocatalysts for solar-driven environmental applications

Due to the upgrowing global industrialization, energy shortages and environmental pollution have become common challenges. Researchers keep developing simple and efficient techniques to mitigate these challenges. A major focus has been placed on photocatalysts based on semiconductor metal oxides as an important key for alleviating such challenges. The ability of metal oxides semiconductors photocatalysts (MOSP) to absorb visible (VIS) and/or ultraviolet (UV) light as well as their large abundance, chemical and biological inertness, low-cost and nontoxic characteristics are the key factors for successful implementation of these materials in solar photocatalytic application. Upon irradiation of MOSP, electrons and holes are generated. Electrons (e-) present in the conduction band and they undergo reduction reaction while holes (h+) in the valence band (VB) go through a reduction process. Advances in controlling the crystal structures, morphology, band gap and electronic structure have been attained through proper designing, and optimization of the synthesis techniques. Directing toward potential applications provokes researchers to investigate efficient ways to increase the solar-absorption capability of MOSP. Much success has been achieved regarding (1) engineering of the band gap energy to widen the range of wavelength response (2) inhibition of the action of (e-/ h+) recombination process via increasing the charge separation and (3) changing the selectivity or yield of a particular product. This review comprehensively covers a wide range of metal oxide semiconductor photocatalysts, including binary oxides (such as TiO2, ZnO, CuO, and WO3), spinel ferrites, perovskite oxides (e.g., BaTiO3, BiFeO3, and BaSnO3), as well as their doped, co-doped, and heterostructured derivatives. Emphasis is placed on how compositional tuning and structural engineering of these materials influence light absorption, charge carrier dynamics, and photocatalytic efficiency under solar irradiation. The review is focusing light on the current advancement and recent researchers’ contributions regarding enhancing the photocatalytic activity of metal oxides semiconductors via some effective techniques including: metal and non-metal doping, heterojunction construction, dye sensitization and surface modification. Overview on some synthesis routes as well as characterization of MOSP have been also reported. Furthermore, the implementation of MOSP in some potential applications such as: degradation of pollutants (dyes and pharmaceuticals) has been discussed. Highlights have been focused on the factors affecting the performance of these materials in such applications.