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α‑Ag2–2xZnxWO4 (0 ≤ x ≤ 0.25) Solid Solutions: Structure, Morphology, and Optical Properties
journal contribution
posted on 2017-06-12, 18:35 authored by Paula
F. S. Pereira, Clayane C. Santos, Amanda F. Gouveia, Mateus M. Ferrer, Ivo M. Pinatti, Gleice Botelho, Julio R. Sambrano, Ieda L. V. Rosa, Juan Andrés, Elson LongoA theoretical
study was elaborated to support the experimental
results of the Zn-doped α-Ag2WO4. Theses
α-Ag2–2xZnxWO4 (0 ≤ x ≤ 0.25)
solid solutions were obtained by coprecipitation method. X-ray diffraction
data indicated that all α-Ag2–2xZnxWO4 (0 ≤ x ≤ 0.25) microcrystals presented an orthorhombic
structure. The experimental values of the micro-Raman frequencies
were in reasonable agreement with both previously reported and calculated
results. Microscopy images showed that the replacement of Ag+ by Zn2+ promoted a reduction in the average crystal size
and modifications in the morphology, from rod-like with hexagonal
shape to roll-like with a curved surface. A theoretical methodology
based on the surfaces calculations and Wulff constructions was applied
to study the particle shapes transformations and the surface energy
variations in α-Ag2–2xZnxWO4 (0 ≤ x ≤ 0.25) system. The decrease in the band gap value (from
3.18 to 3.08 eV) and the red shift in photoluminescence with the Zn2+ addition were associated with intermediary energy levels
between the valence and conduction bands. First-principles calculations
with density functional theory associated with B3LYP hybrid functional
were conducted. The calculated band structures revealed an indirect
band gap for the α-Ag2–2xZnxWO4 models. The electronic
properties of α-Ag2WO4 and α-Ag2–2xZnxWO4 microcrystals were linked to distortion effects and
oxygen vacancies (VOx) present in the clusters,
respectively. Finally, photoluminescence properties of α-Ag2WO4 and α-Ag2–2xZnxWO4 microcrystals
were explained by means of distortional effects and oxygen vacancies
(VOx) in [AgOy] (y = 2, 4, 6, and 7) and [WO6] clusters, respectively,
causing a red shift. Calculations revealed that the substitution for
Ag+ with Zn2+ occurred randomly in the α-Ag2WO4 lattice, and it was more favorable on the Ag4
site, where the local coordination of Ag+ cations was four.