Local Immiscibility Control On Shape And Size Of Nano-Objects In The Solvo-Thermal Process: Implications For Ferroelectric Nanoparticles

Solvo-thermal synthesis is an effective method to obtain ferroelectric nanoparticles. The morphology and size of these nanoparticles dramatically impact the dielectric, ferroelectric, piezoelectric, and optical properties of these materials at the nanoscale. This is of primary importance for miniaturized electroactive devices based on these materials. Herein, we propose to harness the full potential of water-ethanol mixture solvents to control the shape and size of nanoparticles of a model ferroelectric, barium titanate. To take advantage of the local inhomogeneities that exist even in fully miscible liquids such as water and ethanol, we use a barium precursor that is only soluble in water together with a titanium precursor that is present in both components. Indeed, ethanol having hydrophilic and hydrophobic parts makes the two liquids remain distinct at the local scale. A threshold molar fraction exists at X-E(tOH) = 0.3 in water-ethanol mixtures; below this threshold, the water H-bond network percolates throughout the sample, whereas above, the water forms clusters that act as "nanoreactors". Below the concentration threshold, scanning electron microscopy images reveal only elongated shapes (at best mixed with nano-spheres); just above the threshold, nanotori and nanocubes are observed; and even further above the threshold, the reaction is limited to the aggregation-nucleation stage and only nanocubes are obtained. This illustrates the fact that in solvothermal syntheses, when the solvent is a mixture and one reactant is confined in one of the constituents of the solvent, it is possible to control the morphology and size of the synthesized nano-objects.