Nanostructural evolution: From one-dimensional tungsten oxide nanowires to three-dimensional ferberite flowers

Based on a W₁₈O₄₉ nanowire template, iron-substituted W₁₈O₄₉ nanowires and FeWO₄ ferberite flowers have been generated by using mixed FeCl₃ and WCl₆ 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 FeCl₃ concentration during solvo-thermal synthesis promotes a morphological evolution from the long one-dimensional nanowires of the precursor through short Fe-containing W₁₈O 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 W₁₈O₄₉ template at low Fe concentrations, which introduces internal stresses to the W ₁₈O₄₉ nanowires. At high Fe concentrations, close to the stoichiometric composition of FeWO₄, 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 MnNb₂O₆ bronze flowers, which exhibit structures with a morphology resembling a purposely carved microgear. Photoluminescent and magnetic properties of the products are also investigated.