Precipitation of solvent-free C₆₀(CO₂)_0.95 from Conventional solvents: A new antisolvent approach to controlled crystal growth using supercritical carbon dioxide

C₆₀(CO₂)x, where x = 0.2 or 0.95, has been synthesized from solutions of C₆₀ in conventional organic solvents using antisolvent precipitation with supercritical CO₂. The technique requires much lower pressures and temperatures than current routes to C₆₀(CO₂)x and, for small quantities, is quicker. Products were characterized by SEM, IR, ¹³C solid-state NMR, and powder XRD. Reitveld refinement of the powder XRD shows CO₂ to be located in the octahedral interstitial sites of the C₆₀ lattice. The cell lattice parameter is observed to increase for higher occupancies of CO₂. Experimental conditions can be varied to generate radically different morphologies of C₆₀(CO₂)x. Using rapid antisolvent precipitation, irregular aggregates of ca. 200 nm particles are formed, where x = 0.2. With slower, diffusion-controlled precipitation, regular, highly crystalline; octahedral-shaped particles (1-70 μm) can be formed, where x = 0.95. All products were precipitated completely free from the original organic solvent, and we conclude that CO₂ has entered the lattice during crystallization.