Crystallization of complex inorganic systems within the confinement of miniemulsion droplets
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Abstract
Nanometric confinement strongly affects crystallization processes. In the small volume of miniemulsion droplets, precursor concentrations are limited and, consequently, the crystal growth is slowed down and smaller crystals are obtained. In an ideal case, each miniemulsion droplet can act as an independent “nanoreactor” because it is stable against molecular diffusion and, therefore, without exchange of material among the droplets, the particles grow only from the initial quantity of precursors. Furthermore, confinement controls the particle size by delimiting the maximum space where the crystallization can occur. This dissertation demonstrates this control of growth and size through the crystallization of ferrites, ammonium phosphomolybdates (APM), and manganites within miniemulsion droplets. All the materials were obtained as nanoparticles with dimensions below 100 nm. Especially, APM nanostructures were three orders of magnitude smaller than analogous samples produced in bulk solution synthesis. Ferrites and manganites were achieved at much lower temperature (i.e., 80 °C and 100 °C, respectively) than generally required and without post synthesis thermal treatment, most likely because of the combination of nucleation–growth control and of the high Laplace pressure within droplets, consequences of the miniemulsion confinement. To achieve highly crystalline spinel ferrites, the miniemulsion technique was performed under solvothermal conditions. In this case, a synergy between colloidal confinement and higher pressures yielded products with higher crystallinity when compared to the corresponding one synthesized in miniemulsion at ambient pressure and in bulk. The high crystallinity and the controlled particle size influenced positively the functional properties of the materials. Ferrites are well-known ferrimagnetic materials and, in the present work, display superparamagnetic behavior as a result of their nanometric size. Ferrites and ammonium phosphomolybdates were used as active catalysts for epoxidation reactions in organic media because of their enhanced dispersibility in organic solvents and their high specific surface area and porosity. APM catalysts produced in miniemulsion were compared to analogous synthesized from bulk solution, showing much higher values of precursor conversion after only 1 h and better recyclability. Ferrites were also magnetically recoverable from the reaction medium and reusable for several cycles without losing activity.