Surfactant-assisted assembly of fullerene (C60) nanorods and nanotubes formed at a liquid-liquid interface

Herein we report the surfactant-triggered assembly of fullerene (C60) into 3D flowerlike microcrystals at the liquid-liquid interface. C60 crystals were grown using a liquid-liquid interfacial precipitation (LLIP) method by layering surfactant solution in butanol with a saturated solution of C60 in benzene. In the LLIP method, it is suggested that the crystal formation mechanism is driven by supersaturation related to the low C60 solubility in alcohol. We found that the dimensions of the synthesized C60 flowers depend on the concentration and surfactant type. In the absence of surfactant (i.e., in the butanol/benzene system), 1D C60 nanowhiskers (nanorods) and C60 nanotubes (diameter 400 nm-2 μm and length 5-20 μm) are obtained. However, when surfactants are incorporated into the system flowerlike microcrystals consisting of C60 nanotubes are observed. For instance, crystals grown at the interface of a 0.01% diglycerol monolaurate (C12G2) nonionic surfactant in butanol with benzene lead to the formation of flower-shaped microcrystals of average sizes in the range of 10-35 μm. To the best of our knowledge, this is the first example of the surfactant-assisted assembly of C60 crystals. X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements have shown that fullerene flowers have a hexagonal structure with cell dimensions of a = 2.539 nm and c = 1.021 nm, which differ from that of pristine C60. Mixtures of flower-shaped C60 crystals and free-standing C60 nanotubes are found in the 0.1% C12G2/butanol system. However, clusters or giant aggregates of nanowhiskers lacking any specific shape are observed in the 1% C12G2/butanol system although these crystals exhibit hexagonal close-packed structures. Flower-shaped C60 microcrystals are also observed with anionic surfactants cetyltrimethylammonium bromide (CTAB) and cetyltrimethylammonium chloride (CTAC). C60 flowers obtained from 0.01% CTAB and 0.01% CTAC also exhibit hexagonal structures with cell dimensions of a = 2.329 nm and c = 1.273 nm, a = 2.459 nm and c = 0.938 nm, respectively. Our C60 flowers exhibit intense photoluminescence (PL) and a blue-shifted PL intensity maximum compared to the same parameters for pristine C60, demonstrating the potential to control the optoelectronic properties of fullerene-based nanostructures.