Solid state crystallization of amorphous calcium carbonate nanoparticles leads to polymorph selectivity

While amorphous calcium carbonate (ACC) has been widely exploited as a precursor to crystalline calcium carbonate in solution, the synthetic potential of crystallising ACC in the solid state has received little attention. Building on our recent results which demonstrate that additives occluded within ACC can facilitate its crystallisation in the solid state, we here demonstrate that annealing of ACC nanoparticles precipitated within Na-AOT (sodium bis(2-ethylhexyl)sulfosuccinate) reverse microemulsions not only leads to remarkable control over the size and morphology of the particles, but that polymorph selectivity can also be achieved through selection of the thermal profile. The ACC precipitated within the microemulsions crystallised at just 70 degrees C on incubation at this temperature for 12 hours and non-aggregated, sub-micron crystals of either calcite or vaterite - which are often difficult to access using standard synthetic protocols - were produced according to the heating rate employed. The observed low-temperature crystallisation was attributed to surfactant molecules entrapped with the ACC particles during their formation, where these impurities act to facilitate nucleation of a crystalline phase within the ACC. These results therefore highlight the potential of using thermal treatment of dry ACC as a route to achieving control over calcium carbonate formation that is sometimes difficult to achieve in solution.