Novel and Practical Separation Processes for Fullerenes, Carbon Nanotubes and Nanodiamonds

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Nanocarbons, defined as nanometer-sized carbon clusters including fullerenes, carbon nanotubes and nano-diamonds, are one of the most fascinating materials known today. Applications for electronics and optics require controlled size and/or structure of these materials, because the physical properties are well known to closely correlate with the structures. Therefore, separation methods of nanocarbons have been investigated, in particular practical processes applicable to large-scale separation. This review describes three novel and practical methods for separating fullerenes, carbon nanotubes and nanodiamonds, which have been developed in our group. In fullerene separation, a solution of fullerene mixture including C60, C70 and C>70 was filtered through a thin layer of activated carbon to provide pure C60 and C70. Since this process is very useful, many patents were filed, and this technology was successfully transferred to a company. Circular single-walled carbon nanotubes were obtained in high purity by use of ultrasonic atomization. While this sonochemical process has been employed only for homogeneous systems such as concentration of aqueous ethanol, this is the first example to apply the sonoprocess to solid/liquid biphasic system. Separation of nanodiamond by centrifugation was effective to obtain single-digit nanometer size diamonds as small as 4 nm from nanodiamond powder with median size of 30 nm. This process was successfully applied to large-scale separation. Filtration, sonication and centrifugation are very fundamental experimental techniques which can be used for the separation of these three types of nanocarbons. Therefore, these processes are expected to be applied to the industrial scale production of nano-carbons, which should reduce cost and increase availability.