Distinct Phase Formation of BiREWO₆ (RE = La–Yb) Nanoparticles by a One Step Hydrothermal Synthesis and Their Photocatalytic Applications

Abstract

Under identical hydrothermal condition, remarkably, Aurivillius BiREWO 6 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 BiLaWO 6 and BiCeWO 6 has been observed for the first time. Calcination of as-synthesized BiLaWO 6 and BiCeWO 6 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 RE 3+ and Bi 3+ ions in solution. Rietveld refinement confirms the BiCeWO 6 and BiGdWO 6 are isostrutural to the low temperature (LT) orthorhombic phase and monclinic high temperature (HT) phase of Bi 2 WO 6 , respectively. Additionally phase dependent distinct morphology and photocatalytic activity is observed. As a representative example, it is observed that needle/ plate-shaped monoclinic BiGdWO 6 nanoparticles show superior visible light driven photocatalytic activity for the Congo-red dye degradation over the spherically agglomerated orthorhombic BiCeWO 6 nanomaterial.

High-Energy Electron-Beam-Induced Evolution of Secondary Phase and Enhanced Photocatalytic Activity in Monoclinic BiEuWO6 Nanoparticles

“…From our earlier work, we know that both hydrothermal and solid-state methods yield a monoclinic BiEuWO 6 phase and that on further calcination (1000 °C for 8 h) the phase is retained with no secondary phase formation. 33 Thus, EB irradiation with an optimum dose would favor the material to form the secondary phase.…”

“…58,64 Similar to the monoclinic phase, a negligible but a constant increase in the unit cell volume is also observed in the secondary orthorhombic phase as a function of dose (inset image Figure 5). The formation of orthorhombic phase upon EBI could be a self-organization phenomenon, where a small fraction of monoclinic phase evolves into an energetically less favored orthorhombic phase, 33 since nanoparticles are subjected to sudden intense electron beam irradiation and thus are far from thermal equilibrium. 39 This is well observed in the work of Zaiser et.al., wherein the thermodynamically unfavored transformation of graphite to diamond happens under EBI.…”

“…31,32 However, the low-temperature solution-based approach yields distinct crystallographic phases for different rare earths, i.e., La and Ce substitution yields an orthorhombic phase (Pca2 1 ) and Nd−Yb substitution yields a monoclinic phase (A2/m). 33 From our previous work, we have observed improvement in the visible-light-induced photocatalytic activity of monoclinic phase BiREWO 6 (RE = Eu, Gd, and Tb) nanomaterials compared to orthorhombic Bi 2 WO 6 , for the degradation of Congo red dye. This is attributed to the increased electron−hole pair recombination in the monoclinic phases.…”

“…This is attributed to the increased electron−hole pair recombination in the monoclinic phases. 33,34 Apart from chemical modification, the morphology, 35 crystal structure, 36 chemical composition, 37 and electronic band structure 38 of inorganic materials can be altered by bombarding solids with high-energy particles such as protons, electrons, and γ-radiation. 39,40 On irradiation, the energetic particles penetrate through the solid and the projectile energy of the energized particles is transferred to the atoms of the target material through their interaction with the subatomic particles.…”

3D Porous Bi2‐xGdxWO6/rGO Composite for Efficient Adsorption‐Enrichment and Photocatalytic Degradation of Organic Dyes

“…We selected the Gd element because the ionic radius of Gd 3 + (0.938 Å) is smaller than that of Bi 3 + (1.03 Å). [16] The substitution may decrease the lattice parameter of Bi 2 WO 6 , resulting in a decrease in the lattice spacing d. [15] Besides, the poor electronic stability of Gd is very beneficial for capturing the overflowing electrons from photocatalyst surfaces. [4a,16] Therefore, Bi 2 WO 6 doped with Gd 3 + would appreciably improve its photocatalytic performance.…”