A review on alkaline earth metal titanates for applications in photocatalytic water purification

The search of novel photocatalysts active under solar radiation has been object of extensive work over the last decades. Alkaline earth titanates, i.e. CaTiO3, SrTiO3 and BaTiO3, are perovskites oxides with promising photocatalytic properties due to their band gaps of respectively 3.65, 3.20 and 3.00 eV. SrTiO3 and BaTiO3 present adequate band gaps and band alignment for activation with solar radiation and produce radicals for the oxidation of aqueous pollutants or electrons with enough redox potential to reduce aqueous hexavalent chromium. This review provides a comprehensive overview of the state‐of‐the‐art on the applications of alkaline earth metal titanate-based photocatalysts focused on the abatement of water pollutants. Some strategies have been extensively explored with the aim of extending radiation absorption to the visible range. Doping, metal deposition onto the surface, and the combination in double, or sometimes ternary, hybrid heterostructures have improved the photocatalytic performance of titanates by decreasing the band gap energy and enabling the formation of defects that minimize electron-hole recombination. The presence of carbonaceous materials also contributes to the delocalization of the photogenerated electrons. This review summarizes the commonly used methods for titanates preparation and their characterization. The main attention of this review is focused on the discussion of the relevant achievements of pristine and titanates-based photocatalysts on the removal of dyes, contaminants of emerging concern and reduction of hexavalent chromium. Also, some critical points that might compromise their use in a future real application are discussed; in particular, those dealing with problems of expensiveness or leaching of toxic metals. Finally, the techniques commonly used for the assessment of the contribution of different reactive oxidation species, mainly hydroxyl and superoxide radials, are overviewed.