Nanostructural Evolution: From One-Dimensional Tungsten Oxide Nanowires to Three-Dimensional Ferberite Flowers

Based on a W 18 O 49 nanowire template, iron-substituted W 18 O 49 nanowires and FeWO 4 ferberite flowers have been generated by using mixed FeCl 3 and WCl 6 at different ratios in a simple solvo-thermal process employing cyclohexanol as the solvent. Detailed characterization by electron microscopy and spectroscopy has shown that increasing the FeCl 3 concentration during solvo-thermal synthesis promotes a morphological evolution from the long one-dimensional nanowires of the precursor through short Fe-containing W 18 O 49 nanowires and two-dimensional platelets to three-dimensional flowers with sixfold symmetry. The driving force for these transformations is attributed to Fe inclusion in the W 18 O 49 template at low Fe concentrations, which introduces internal stresses to the W 18 O 49 nanowires. At high Fe concentrations, close to the stoichiometric composition of FeWO 4, the formation of the flower is triggered by the intrinsic sixfold symmetry of crystalline ferberite, via a combination of initial nanoblade nucleation and the competing self-assembly of neighboring parallel nanoblades. A similar solvo-thermal process has also been successful in the synthesis of MnNb 2 O 6 bronze flowers, which exhibit structures with a morphology resembling a purposely carved microgear. Photoluminescent and magnetic properties of the products are also investigated.