This lecture describes the synthesis strategies leading to mesocrystals. These mesocrystals result from the assembly or orderly aggregation of nanoparticles that are well calibrated in size and perfectly defined in shape. We began by explaining the key differences between conventional crystallization, based on nucleation of nuclei and their subsequent growth by adsorption of monomers present in solution, and mesocrystal formation. The latter process involves the temporary stabilization of well-defined nanobricks, which aggregate through an oriented bonding process to form a supracrystal. In the final stage, nanocrystallites are gummed or fused together to form a single-crystal or apparently single-crystal edifice. When the nano-particle sub-structure is not apparent, mesocrystallization is sometimes difficult to detect. In such cases, it is necessary to monitor the entire process in situ, analyzing all stages of crystal formation by electron microscopy and X-ray diffraction or scattering.
Assemblies of pre-formed nanobricks, then assembled in a second stage into mesocrystals, represent the model media currently being studied by numerous research groups. Thanks to the control of nanoparticle surface states (charge, hydrophilic or hydrophobic ligands), the nature of the solvent and temperature, the drying method and the possible presence of a support, a veritable "supra-nanoparticle chemistry" is emerging, in which the nano-objects making up the assembly are the analogues of atoms in crystalline edifices.
This mastery of attractive (van der Waals, hydrophobic, solvophobic, hydrogen bonds, capillary forces, dipolar and magnetic forces) and repulsive (electrostatic and steric forces, dipolar and magnetic forces) interactions is part of the chemist's art, and enables the controlled formation of two- and three-dimensional mesoscopic assemblies. This lecture was illustrated with a wide range of examples, from relatively simple structures (assemblies of cubes or spheres) to more complex ones. It was also shown that the strategies developed were very general whatever the composition and shape of the nanobricks. To highlight the parameters governing the success of supracrystal-forming assemblies, we first presented simple edifices formed of manganese oxide octahedra, silver or gold nanoparticles. This was followed by more complex assemblies of latex and silica of different sizes, composite meso-lattices of nanoparticles of different metals, and superb bi-particle pavements of rare-earth halide or metal chalcogenide nanocrystals and metals. The specific and collective physical properties of these supra-nanoparticulate assemblies are still being studied. In a second part, we described even more original mesocrystals consisting of periodic superstructures of entangled cadmium sulfide octopodes or superb ordered assemblies of lead sulfide nanoparticles with star-shaped morphologies. In conclusion, we have shown that the application of an external field (magnetic or electric) can very effectively assist the formation of three-dimensional mesocrystals whose constituent elements are, for example, iron oxide-silica core-crown nanoparticles.
Environments such as those for the formation of biominerals or biomorphs with curved final shapes are even more complex, as in this case, during synthesis, the nanobricks are generated in situ and their assembly into mesocrystals is consecutive or simultaneous. These exciting examples were the subject of the sixth lesson.
