Distinct Phase Formation of Bi RE WO₆ (RE = La-Yb) Nanoparticles by a One Step Hydrothermal Synthesis and Their Photocatalytic Applications

Under identical hydrothermal condition, remarkably, Aurivillius BiREWO₆ nanoparticles crystallize in an orthorhombic phase for RE = Ce and La, while the RE = Nd-Yb materials crystallize in the monoclinic phase. This kind of distinct phase formation in this series is not observed in solid state synthesis, where for all RE substitution, only the monoclinic phase is formed. Moreover, formation of the orthorhombic phase for BiLaWO₆ and BiCeWO₆ has been observed for the first time. Calcination of as-synthesized BiLaWO₆ and BiCeWO₆ nanomaterials results in the monoclinic phase and this indicates that the formation of the orthorhombic phase is favored only under mild reaction conditions. This could be attributed to the crucial role of the ionic radii of RE³⁺ and Bi³⁺ ions in solution. Rietveld refinement confirms the BiCeWO₆ and BiGdWO₆ are isostrutural to the low temperature (LT) orthorhombic phase and monclinic high temperature (HT) phase of Bi₂WO₆, respectively. Additionally phase dependent distinct morphology and photocatalytic activity is observed. As a representative example, it is observed that needle/plate-shaped monoclinic BiGdWO₆ nanoparticles show superior visible light driven photocatalytic activity for the Congo-red dye degradation over the spherically agglomerated orthorhombic BiCeWO₆ nanomaterial.