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Development of a Method for High Purity Separation of SWCNT by AIST

Carbon nanotubes (CNTs) come in two main forms,one being the single-walled carbon nanotubes (SWCNT) and the other is the multi-walled nanotubes (MWCNT). The structure of a SWCNT is a one-atom-thick layer of graphite which is called graphene, wrapped into a seamless cylinder with open or closed ends. Depending on the arrangement of the carbon atoms, SWCNT exhibits metallic or semiconducting properties. Normally, the synthesis of SWCNTs yields a mixture of these two types. However, if the these two types can be separated in high purity, they can be utilized in many potential applications. For instance, metallic SWCNTs can be used as transparent electrodes in production of liquid crystal displays or solar cells instead of the transparent conductive materials which is prepared using rare metals, also the metallic SWCNTs can be used in wiring ,in development of high-integration high-speed SWCNT computers. On the other hand semiconducting SWCNT can be employed as transparent and flexible transistors.
Up to now, there has not been a cost effective method for independently synthesizing the two materials which have different electrical properties. other issues that hampered the isolation of individual materials from mixture have been associated with yield, purity. Based on this challenge, AIST (Advance Industrial Science and Technology) has developed a cost effective method in separating metallic and semiconductor SWCNT, through an automated, continuous and repeatable process. The method employs electrophoresis using SWCNT-contained agarose gel capable of efficiently separating metallic and semiconducting SWCNTs with a high yield. Additionally, the two types of SWCNTs could be separated from the SWCNT-containing agarose gel without application of an electric field. In achieving the high yield, AIST has employed column chromatography in the gel-based separation method. The column chromatography is a widely used separation technique in the industrial processes for production of chemical products as well as pharmaceuticals. Through use of this method, when a SWCNT dispersed solution was passed through the column, the metallic SWCNTs were collected with the solution for separation, while the semiconducting SWCNTs were adsorbed onto the gel. After washing out the remaining metallic SWCNTs in the column, the semiconducting SWCNTs which was adsorbed onto the gel were eluted with a solution containing suitable surfactants.The advantage of this method is that the column packed with agarose gel can be used again after equilibration. Furthermore, the separation performance of the column was not deteriorated by repeated use. The method has resulted in obtaining semiconducting and metallic SWCNTs with purity of 95% and 90% respectively. The method still have room for improvement both in terms of cost and separation efficiency.

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