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Simultaneous Separation of High-Purity Semiconducting and Metallic Carbon Nanotubes by Surfactant Concentration-Controlled Gel Chromatography

Simultaneous Separation of High-Purity Semiconducting and Metallic Carbon Nanotubes by Surfactant Concentration-Controlled Gel Chromatography
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대학원 화학신소재공학과
이화여자대학교 대학원
Single-walled carbon nanotubes (SWNTs) represent a cylindrically rolled graphene sheets on a nanometer scale, and can exhibit either metallic or semiconducting electronic properties depending on how they are rolled. Furthermore, the band-gap of semiconducting SWNT varies with its diameter when it is undoped. Due to these unique behaviors, SWNTs have been applied as conductive transparent electrodes in display devices, active layers in solar cells, electrode materials in secondary batteries, high-strength/ultra-light composite materials, and biological sensors. Their successful application requires specific SWNTs having typical electronic properties that satisfy the characteristics required of the device. However, current technologies synthesizing SWNTs only produce mixtures of metallic (M-) and semiconducting (S-) SWNTs; therefore, post-processing methods for separating S- and M-SWNTs from their initial mixtures have been proposed using techniques such as ultracentrifugation, electrophoresis, gel chromatography, polymer wrapping, and DNA wrapping. Among these, gel chromatography methods utilizing a relative difference in the binding force between SWNT–surfactant assemblies and gels have been intensively investigated because of the ease of separation and scalability they offer. Liu et al. first demonstrated dextran-based single-surfactant multicolumn gel chromatography and showed that S- and M-SWNTs can be separated when 2 wt% sodium dodecyl sulfate (SDS) is used as a surfactant. However, for high-purity separation of M- and S-SWNTs, more than 30 columns are needed and 2nd gel separations are necessary for samples already separated first. To improve the separation efficiency and purity, various gel-based separation methods using surfactant mixtures rather than single surfactants and specific gel materials for specific SWNTs have been reported. Recent studies on the binding kinetics of the SWNT on gel revealed that S-SWNTs strongly interact with the gel and adsorb onto it, while M-SWNTs weakly interact with the gel and do not bind to it: this can help separate the M-SWNTs from S-SWNTs. Blanch et al. showed that small-diameter S-SWNTs adsorb onto gels at high surfactant concentrations, while large-diameter S-SWNTs adsorb onto gels at low surfactant concentrations. Hirano et al. calculated the adsorption equilibrium constants of various SWNT–SDS assemblies and showed that M-SWNT–SDS assemblies have higher adsorption rates to sephacryl gel than S-SWNT–SDS assemblies at all SDS concentrations. Strano et al. also compared the adsorption rate constants of various S-SWNTs on gels and showed that small-diameter S-SWNTs have higher adsorption rate constants than large-diameter S-SWNTs, suggesting that it is easier to separate small-diameter S-SWNTs than large-diameter S-SWNTs using gel chromatography. All these results indicate that simultaneous high-purity separation of M- and S-SWNTs using multicolumn gel chromatography is difficult when a single surfactant concentration is utilized and suggest that surfactant concentration adjustment during gel chromatography may enable simultaneous separation of both M- and S-SWNTs with high purity. In this work, we developed a new method for high-purity separation of both S- and M-SWNTs simultaneously, by adding steps to optimize the surfactant concentrations of SWNT dispersions prior to the separation of each SWNT, i.e., S- or M-SWNTs. Recently, Flavel et al. also controlled surfactant concentrations of SWNT solutions to reduce (n,m) species in the initial SWNT mixtures for high purity S-SWNT separation. Even though their method was successful in separation of highly pure 6~8 S-SWNTs, but couldn't separate M-SWNT. We intended to first separate small-diameter S-SWNTs at high SDS concentrations and then lower the SDS concentration of the solution, which passes through the first columns, to separate mid-diameter S-SWNTs using a second gel chromatography step. The SDS concentration of the solution that passes through the second columns was further lowered to separate large-diameter S-SWNTs, leaving high-purity M-SWNTs to be collected at the last stage. The surfactant concentration was lowered using simple dilution or dialysis, and the difference in the purity of separated SWNTs was also investigated. In addition, we calculated the adsorption equilibrium constant of representative SWNTs–SDS on gels and explain why the surfactant concentrations we utilized in this study are appropriate for high-purity separation of both S- and M-SWNTs.;탄소나노튜브(Carbon Nanotube, CNT)는 흑연(graphite)을 이루고 있는 2차원 구조인 그래핀(graphene)이 나노크기의 직경으로 둥글게 말린 나선의 형태이며, 이때 그래핀의 나선성의 정도에 따라 전기적 특성이 금속(metal) 또는 반도체(semiconductor)의 성질을 띠고, 직경에 따라 밴드갭(band-gap)이 달라진다. 탄소나노튜브는 LCD, OLED 등 디스플레이 소자의 필수 요소인 전도성 투명전극 및 고집적 메모리소자, 에너지 소재인 태양 전지 및 2차 전지, 그리고, 고강도/초경량 복합재료 및 화학 sensor 등에 응용된다. 하지만, 이들의 성공적인 응용을 위해서는 구현하고자 하는 소자의 특성에 맞는 전기 특성을 지닌 탄소나노튜브들의 선택적 제조가 필요하다. 현재의 기술로는 어떠한 방법으로 합성하여도 금속성 탄소나노튜브와 반도체성 탄소나노튜브가 혼합된 형태로 제조되며, 그에 따라 반도체성 탄소나노튜브 및 금속성 탄소나노튜브를 분리하는 방법에 대한 연구가 대두되고 있다. 분리 방법으로 ultracentrifugation, dielectrophoresis, gel electrophoresis, selective oxidation, amine extraction, aromatics extraction, polymer wrapping, DNA wrapping, protein wrapping 등이 연구되었지만 젤을 이용한 멀티 컬럼 크로마토그래피가 가장 간단하며 대량생산이 가능한 장점이 있어 많은 연구 성과가 보여져 왔다. 본 연구에서는 Allyl dextran-based gel(sephacryl gel)을 이용하여 계면활성제(Sodium dodecyl sulfate, SDS) 농도를 조절한 멀티 컬럼 크로마토그래피를 통해 금속성 탄소나노튜브와 반도체성 탄소나노튜브를 분리하는데 성공했다. 계면활성제 수용액에 분산한 탄소나노튜브를 젤을 담지한 컬럼에 흘려 젤과의 상호작용이 강한 반도체성 탄소나노튜브를 흡착시키고 젤을 통과한 금속성 탄소나노튜브는 컬럼의 맨 마지막 단계에서 수집하는 방식으로 분리하였다. 멀티 컬럼 젤 크로마토그래피를 이용한 기존의 연구는 너무 많은 컬럼을 사용해야 하고, 또한 하나의 농도의 계면활성제로만 실험하여 금속성 탄소나노튜브 또는 반도체성 탄소나노튜브 중 하나만을 수득했기 때문에 순도가 떨어진다는 문제점이 있었다. 우리는 계면활성제가 탄소나노튜브를 둘러싼 양이 흡착 정도에 영향을 준다는 점을 이용하여 계면활성제 농도가 높은 탄소나노튜브 분산용액을 컬럼에 주입하여 반도체성 탄소나노튜브를 젤에 흡착시킨 후, 컬럼을 통과한 용액의 계면활성제의 농도를 낮춰서 컬럼 크로마토그래피를 반복 수행함으로써 금속성 탄소나노튜브를 분리하였다.
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