Micro- and nanostructured layered-kagome zinc orthovanadate BaZn3(VO4)2(OH)2

Abstract

Pure micro- and nanocrystalline powders of the layered-kagome zinc orthovanadate BaZn₃(VO₄)₂(OH)₂ have been successfully prepared and thoroughly characterised. Microstructured samples (BaZn₃-MPs) have been produced by hydrothermal reaction using synthetic martyite Zn₃V₂O₇(OH)₂·2H₂O as the starting reagent. Nanoparticles (NPs) with an average size of ≈ 60 nm (BaZn₃-NPs-7h) or ≈ 50 nm (BaZn₃-NPs-25min) have been obtained by using a coprecipitation method at ambient pressure, and by varying the stirring time. Rietveld refinements of X-ray diffraction data indicate that micro- and nanostructured BaZn₃(VO₄)₂(OH)₂ both crystallize in a Rm structure very similar to that of the known layered-kagome compound BaCo₃(VO₄)₂(OH)₂. Transmission electron microscopy observation of BaZn₃-NPs-7h and BaZn₃-NPs-25min reveals crystallized NPs with homogenous distributions of Ba, Zn, and V elements. FT-IR and Raman spectra show subtle differences between micro- and nanostructured samples which cannot be linked to any differences in the average crystal structures. The high resolution ⁵¹V MAS NMR spectrum of BaZn₃-MPs shows a single isotropic line attributed to VO₄³⁻ groups with C_3v point group. The spectra of the nanostructured samples reveal the presence of a weak additional signal which decreases in intensity with increasing the NPs size, and which has been tentatively assigned to the presence at the surface of the NPs of a small amount of V⁵⁺ ions in a different chemical environment. Nanostructuring also impacts the optical properties of BaZn₃(VO₄)₂(OH)₂. The UV-vis absorption spectra of NPs exhibit an additional weak transition in the visible domain which is not observed for the microstructured sample.