Effect of Composition on the Optical and Photocatalytic Properties of Visible Light Responsive Materials Bi_26-MgO₄₀

We report the synthesis and the crystal and electronic structure as well as the optical and photocatalytic properties of novel photoactive materials of the general formula Bi_26-MgO₄₀. Two compounds with compositions of Bi_24.28(3)Mg_1.72(3)O₄₀ and Bi_24.05(3)Mg_1.95(3)O₄₀ are synthesized using the pyrolytic method. According to X-ray diffraction analysis, the materials are monocrystalline species. Their electronic bandgaps determined from Tauc plots are 2.41 eV [Bi_24.28(3)Mg_1.72(3)O₄₀] and 2.69 eV [Bi_24.05(3)Mg_1.95(3)O₄₀]. Keeping in mind that optical bandgaps are typically larger than their electronic counterparts, we find that the bismuthate bandgaps match well that of Bi₂₄Mg₂O₄₀ (2.26 eV) predicted by density functional theory. Apparently, the synthesized bismuthates are indirect bandgap semiconductors just like Bi₂₄Mg₂O₄₀. Both materials demonstrate nearly identical luminescence spectra. Their luminescence emission at 620 nm is most efficiently excited by 365 nm light. The materials' photocatalytic properties are evaluated in a visible light-induced photocatalytic phenol degradation reaction. Rather low activity of both compounds is detected. However, Bi_24.05(3)Mg_1.95(3)O₄₀ is ∼2 times more photocatalytically active than Bi_24.28(3)Mg_1.72(3)O₄₀, which is associated with a higher Bi⁵⁺ content in the former.