Photocatalysis for water oxidation by Fe₂O₃ nanoparticles embedded in clay compound: correlation between its polymorphs and their photocatalytic activities

Iron oxide (Fe₂O₃) nanoparticles of distinct polymorphs (i.e., α and amorphous phases) were prepared in montmorillonite (MT) particularly with three types of methods (i.e., calcining, hydrothermal reaction, and hydrolysis) and used as photocatalysts for the oxidation of water to O₂. Although α-Fe₂O₃ has usually been applied to a photocatalysis system [employing the saturated loading of α-Fe₂O₃ (25 wt%) in MT], this study first showed that amorphous Fe₂O₃ has similar photocatalysis ability to α-Fe₂O₃ in terms of water oxidation. Moreover, irrespective of the types of polymorphs, their photocatalyses were dependent on the loading amount of Fe₂O₃ in MT; that is, the O₂ amount increased with the amount of the photocatalyst employed until the loading amount reached ca. 3 wt%, and decreased in the larger loading amount. In this article, the reason why such an optimal photocatalysis condition appears was considered in terms of kinetic aspects.