Impact of Average, Local, and Electronic Structure on Visible Light Photocatalysis in Novel BiREWO₆ (RE = Eu and Tb) Nanomaterials

Abstract

Crystal structures of hydrothermally synthesized BiEuWO6 and BiTbWO6 nanomaterials are deduced for the first time by combined Rietveld refinement of neutron and synchrotron data using the ordered and disordered models available in literature. The ordered model is validated for the average structure of these nanomaterials, and it is further supported by the local structure analysis using neutron pair distribution function. Nanomaterials are characterized by field-emission scanning electron microscopy, transmission electron microscopy, Brunauer–Emmett–Teller surface area, diffused reflectance spectroscopy, and Raman Spectroscopy. Rare-earth-substituted nanomaterials are found to be efficient photocatalysts over the parent Bi2WO6 under visible light irradiation for Congo-red dye degradation. Particularly, BiTbWO6 shows an enhanced photocatalytic (PC) activity compared to BiEuWO6, as evidenced from the photoelectrochemical and time-resolved fluorescence studies. The difference in the observed PC activity of these nanomaterials is also explored through a detailed comparison of crystal structure and electronic structure calculated through the density functional theory method.

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

“…From the refinement, it is evident that the Eu ion is present in both orthorhombic and monoclinic phase of mixed-phase materials, and the respective compositions were established by the occupancy refinement. Control and 15 kGy single-phase compositions obtained from the occupancy refinement are in good agreement with the reported literature, while in the mixed-phase material (50 kGy sample), the compositions of monoclinic and orthorhombic phases are calculated as Bi 0.78 Eu 1.33 WO 6 and Bi 1.79 Eu 0.21 WO 6 , respectively. The phase fraction obtained from the Rietveld refinement indicates nearly 90% of monoclinic phase and 10% of orthorhombic phase in the mixed-phase materials.…”

“…Single-phase Rietveld refinement (Figure ) on the PXRD data of control and 15 kGy samples was carried out using the GSAS-EXPGUI suite . The BiEuWO 6 monoclinic phase in the space A 2/ m obtained by the combined neutron and synchrotron data was used as a crystallographic model for the refinement . A shifted Chebyshev polynomial consisting of 12 coefficients was used to refine the background, since the isotropic displacement parameters are more sensitive than the atomic positions to the background modeling , and profile was refined by fitting to the pseudo-Voigt function.…”

“…Similar to single-phase refinement, multiphase Rietveld refinement was also carried out on samples irradiated with EB of 50, 100, and 200 kGy doses (Figure ). Here, the monoclinic phase of BiEuWO 6 in the A 2/ m space group and Bi 1.8 La 0.2 WO 6 in the P 2 1 ab space group were used as the crystallographic model for the multiphase refinement. Similar to the monoclinic phase Rietveld refinement, the background for the biphasic material was also refined with a shifted Chebyshev polynomial consisting of 12 coefficients.…”

“…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. , …”

“…In the literature, it has been shown that defects on the TiO 2 surface are produced by the sputtering method, by annealing the photocatalyst at higher temperatures, or by electron bombardment . However, the photogenerated electron mobility can be improved by tuning the M–O bond lengths and M–O–M bond angles of the photocatalyst . Moreover, EBI is found to affect the crystal structure of the metal oxides. , Thus, controlled and uniform energy deposition on the materials can be achieved within shorter time intervals through the pulsed EBI process on par with other physical methods.…”