Solvent-mediated morphology engineering: pseudomorph of fullerene C60 microcrystals

Crystal structure, shape, and size are some of the factors affecting the properties of fullerene crystals. To produce unique forms of fullerene crystals desirable for various applications, several growth methods have been developed. In particular, the liquid-liquid interfacial precipitation method is used in solution methods for the preparation of fullerene micro- and nanocrystals. It uses a mixture of good and poor solvents that exhibit great potential in morphology, shape and size control. The crystal precipitation is generally defined by the difference in the solubility of fullerenes for two solvents used. Thus, solvent selection is of great importance in the growth of novel micro- and nanocrystals.

In a recent paper published in the journal, Chemical Physical Letters: Saori Yamamoto, Yuto Funamori, Yuko Kaneda, Makoto Tanimura and led by Professor Masaru Tachibana from Yokohama City University developed a new method for growing C₆₀ crystals with hexagonal shape using liquid-liquid interfacial precipitation method. Specifically, carbon tetrachloride and C₆₀-saturated toluene solution were used as poor and good solvents respectively. Finally, the morphology transformations of the samples were investigated.

Carbon tetrachloride-solvated C₆₀ crystals with hexagonal shapes were obtained. It was fascinating to note that the morphology of the hexagonal crystals with the growth liquid was significantly changed with drying air at 23^°C. Additionally a lot of fine rod crystals appeared in the original crystal despite the hexagonal outline the original crystal being kept even after transformation. Through the transformation, the hexagonal symmetry of the original crystals was passed down to rod crystal orientation. The resulting morphology exhibited a kind of hierarchical architecture. The unique morphology was attributed to the good agreement in the periodicity for solvated crystals before and after transformation. This explained the reason for arranging the long axes of the rod crystals parallel to the edges of the original hexagonal outline.

It was necessary to prepare additional samples though solvent treatments to gain more insight into the morphological transformation. Considering that the morphology of the solvated crystals of C₆₀ depend on the solvent, the obtained results were used to ascertain results of previous publications about the role of carbon tetrachloride and toluene in the transformation of both rod and hexagonal crystals. For instance, they expected to promote and hinder morphological transformation by introducing or removing toluene respectively.

The authors confirmed that toluene played a significant role in promoting the morphological transformation. The relationship among the three samples: as-grown crystal covered by the mother liquid, hexagonal crystal dried in the air, and rod crystal dried in the air, it was worth noting that the morphology could be related to the solvent-meditated phase transformation reported for various crystals. Furthermore, various methods such as the X-ray diffraction were used to clarify the transformation mechanism of the hexagonal and rod crystal before and after transformation.

In summary, Yokohama City University scientists reported a solvent-based phase transformation of C₆₀ crystals with a well-defined hexagonal shape. The morphology change was attributed to the dissolution of the original crystals by toluene followed by re-precipitation of the rod crystals. Therefore, in a statement to Advances in the Engineering Professor Masaru Tachibana, the lead author mentioned their study will advance the growth of novel nano/microcrystals.