SYNTHESIS OF NANOCOMPOSITE PARTICLES BY INTRAVESICULAR COPRECIPITATION

Aqueous solutions of aluminum magnesium, aluminum calcium, or aluminum barium chloride and aluminum magnesium nitrate were encapsulated inside unilamellar vesicles and made to react with hydroxyl ions diffusing into the intravesicular space through the vesicle walls. The normal intravesicular coprecipitation product was nanometer-sized, nearly spherical crystalline particles, with one particle in each vesicle. The particle sizes are controllable and their size distribution remained unchanged over several days. Both the cationic species were incorporated, and, with one exception, the divalent/trivalent component ratio was higher in the intravesicular product than that in the original solution. In contrast, when the same reactant solutions were brought together for free precipitation, two distinct phases and particle morphologies resulted-each corresponding to the expected single cation hydroxide, and both radically different from the intravesicular product. Thus coprecipitation inside vesicles opens up the possibility of combining two normally "incompatible" components into one particle with the maximum scale of heterogeneity equal to the vesicle inner diameter-a low-temperature homogenization technique. It was possible to precipitate barium hydroxide in the intravesicular space when the initial solution had a barium ion concentration below its solubility, while a free reaction at the same concentration did not yield any precipitate. The ability to overcome traditional solubility limitations can be exploited for the production of unique phases. Except in a few cases, it was difficult to match electron diffraction patterns for the intravesicular product with those of known compounds, further confirming the possibility of making novel phases by this technique. By increasing the outflux of the chloride ions and hence enhancing the influx of the hydroxyl ions, it was possible to nucleate more than one particle in each vesicle-a potentially useful feature for forming polycrystalline or amorphous nanocomposites. © 1990.