Solid state NMR characterization of NaNbO3 and LiNbO3 nanoparticles obtained by microwave-assisted combustion

Abstract

Nanoparticles of LiNbO₃ and NaNbO₃ were obtained for the first time by microwave-assisted combustion. Preliminary experiments reveal that the synthetic conditions influence their microstructure and optoelectronic features. Therefore, there is a need for performing the structural characterization of these materials, obtained by this new route. In the case of NaNbO₃, there are two polimorphs which are stable at room temperature, space groups P2₁ma and Pbma. Powder x-Ray diffraction experiments were not capable to identify the crystalline phases present in the nanoparticles. Therefore, we have performed a detailed structural characterization of the nanoparticles by 1D and 2D solid state ²³Na and ⁹³Nb Nuclear Magnetic Resonance (NMR) techniques. ²³Na results reveal the presence of both phases, Pbma and P2₁ma, for samples prepared using NaNO₃ precursor in a 1:1 Na:Nb ratio or NaCl in excess. On the other hand, the P2₁ma polymorph could be isolated in the synthesis using NaCl salt in 1:1 Na:Nb ratio. On the other hand, LiNbO₃ nanoparticles display the usual rhombohedral structure R3c. ⁷Li MAS NMR results reveal the presence of two types of Li species, with distinct dynamics. Highly mobile Li⁺ ions are found at the surface of the nanoparticles, while bulk Li⁺ show restricted movement. Finally, as a proof of principle, the photocatalytic activity of these niobates was tested for the degradation of methylene blue dye, a common organic-water contaminant.