3-D MnNb2O6 nanogears from 1-D Nb2O5 nanorods

Abstract

MnNb2O6 nanogears have been generated by using mixed NbCl5 and MnCl2 at an optimized ratio of 1:1 in a cyclohexanol solvent in a simple solvothermal process. It has shown that the Mn:Nb ratio determines the shape of the products. Detailed characterization by electron microscopy has shown that increasing the Mn+2 concentration during the solvo-thermal synthesis promotes a morphological evolution from relatively long Nb2O5 nanorods to a mixture of short Nb2O5 nanorods and MnNb2O6 6-teeth nanogears, then to a mixture of short Nb2O5 nanorods and more MnNb2O6 6-teeth nanogears, then to more and more MnNb2O6 6-teeth nanogears that are occasionally accompanied with under-developed MnNb2O6 5-teeth nanogears, and eventually to fully developed pure 3-D nanogears. The driving force for such interesting geometry transformations is attributed to the inclusion of Mn2+ into the Nb2O5 template at low Mn2+ concentrations, which introduces internal stresses to the Nb2O5 nanorods. At high Mn2+ concentrations, close to the stoichiometric composition of MnNb2O6, the formation of the gears is triggered by the intrinsic sixfold symmetry of crystalline columbite, via a combination of initial nanorod nucleation and the competing self-assembly of neighboring parallel nanorods.