Template-based chemical synthesis of ferroelectric nanowires

The need to understand how crystal structure, properties and domain structure evolve by changing the system dimensionality and size has stimulated in recent years a lot of interest on the synthesis of ferroelectric nanowires and nanotubes. A variety of solution-based chemical synthesis methodologies has been developed to achieve a good control over the morphology and the crystallographic order of these nanostructures as well as to modify the length to diameter ratio. Many of these methods make use of different types of templates for better controlling the final morphology. Physical templates such as porous membranes with ordered cylindrical pores can be impregnated with a suitable precursor and after conversion to the final compound the template is removed. Reactive single crystal templates can be converted in ferroelectrics by means of liquid phase reactions and topochemical transformations retaining a high crystallographic order. The aim of this presentation is to illustrate the use of reactive templates for the synthesis of nanowires of BaTiO3, SrTiO3 and Bi4Ti3O12. The templates are prepared starting from Na2Ti3O7 nanowires obtained by hydrothermal reaction and converting them in hydrogen titanate, anatase and rutile by means of topochemical transformations retaining the original morphology. Bismuth oxide nanowires can be simply obtained by precipitation. The templates can be directly reacted in hydrothermal conditions with a strongly alkaline solution containing the alkaline-earth cations. The morphology and crystallographic order of the final product is strongly dependent of the chemical nature of the template. Some examples will be illustrated for the synthesis of SrTiO3. Solid-state topochemical transformations can be exploited for the fabrication of nanowires and nanotubes using a two-step process. The selected templates (TiO2 and Bi2O3) are first coated with a shell of a second compound (BaCO3 and TiO2) using colloidal chemistry methods. The solid-state reaction between core and shell then determines the formation of the desired compound. The morphology of the template will be retained provided that the reaction temperature is carefully optimized. Application of this method to the synthesis of BaTiO3 and Bi4Ti3O12 will be discussed.