A simple, yet effective, method to increase the yield of gel chromatography separation of single-chirality carbon nanotubes

Preparing high-concentration individualized carbon nanotubes for industrial separation of multiple single-chirality species-Nature Communications Large-scale production of single-chirality carbon nanotubes has long been a major challenge. Liu et al. report a simple, yet effective method to incre....

Single-wall carbon nanotubes (SWCNTs) are considered ideal electronic and photoelectronic materials in the post-Moore era due to their extremely high carrier mobility, structure-tunable bandgap and nanoscale body [1-2]. However, a slight difference in atomic arrangement between different SWCNTs induces large changes in their optical and electrical properties. Industrial production of single-chirality carbon nanotubes is critical for their applications in high-speed and low-power nanoelectronic devices [2]. In the past, intense efforts have been made to obtain chirality-enriched SWCNTs on a large scale [3, 4], but both their growth and separation have been major challenges.

Most recently, Prof. Huaping Liu's research group from the Institute of Physics (IOP) of the Chinese Academy of Sciences (CAS) reported a simple, yet effective, method to increase the yield of gel chromatography separation of single-chirality carbon nanotubes by enabling significantly higher concentrations of raw nanotubes solution. With this technique, milligram-scale separation of multiple single-chirality SWCNTs have been achieved. This study was mostly recently published in Nature Communications (Preparing high-concentration individualized carbon nanotubes for industrial separation of multiple single-chirality species, Nature Communications, 2023, 14:2491).

Figure 1 The preparation of a high-concentration individualized SWCNT solution. (a) Schematic diagram of the preparation of high-concentration individualized SWCNT solution; (b) The relationship between the shear viscosity and initial concentration of SWCNT solution before and after the first ultracentrifugation; (c) Optical absorption spectra of the as-prepared individualized SWCNTs with different initial concentrations; (d) The relationship between the initial SWCNT concentration and the concentration of the as-prepared SWCNT solution.

The preparation of monodisperse raw SWCNT solution is a critical step in the separation of single-chirality SWCNTs that determines the structural purity of the separated SWCNTs [5]. However, high concentration and high dispersity are difficult to reconcile, which limits the separation efficiency of single-chirality SWCNTs. Huaping Liu and his colleagues developed a strategy for dispersing a highly concentrated individualized SWCNT solution by redispersion, in which the SWCNT solution was first ultrasonically dispersed, followed by ultracentrifugation and reultrasonic dispersion. With this technique, the dispersible initial concentration of SWCNTs increased from 1 to 8 mg/mL, and the corresponding concentration of the resulting individualized SWCNT solution increased from 0.19 to ~1.02 mg/mL. And the separation yields of multiple single-chirality species, including (6, 4), (6, 5), (11, 1), (7, 5), (7, 6), (8, 3), (8, 4) and (9, 1), were increased by approximately six times to the milligram scale in one separation run with gel chromatography.

Figure 2. Milligram-scale separation of single-chirality SWCNTs from a high-concentration individualized SWCNT solution. (a) Schematic diagram of the separation of single-chirality SWCNTs.  (b) Optical absorption spectra of single-chirality (6, 4) SWCNTs from SWCNT solutions with different concentrations using a 40-mL gel.  (c) The yield of single-chirality (6, 4) that separated by large (40 mL) and small (10 mL) gel columns as a function of the concentration of individualized SWCNT solution. (d) Optical absorption spectra of separated (n, m) species on the milligram scale. (e) Photograph of the solution of single-chirality species separated from high-concentration SWCNT solution. 

This dispersion technique was shown to be applicable for the low-cost and commercial hybrid of graphene and SWCNTs (G-SWCNTs) with a wide diameter range of 0.8-2.0 nm. By increasing the initial dispersible concentration of the G-SWCNT raw materials from the typical 1 to 4 mg/mL, the separation yield of single-chirality SWCNTs was increased by more than one order of magnitude. In particular, nine types of single-chirality SWCNTs, namely, (6, 4), (6, 5), (7, 3), (7, 5), (7, 6), (8, 4), (9, 1), (9, 4) and (10, 3), were prepared on the submilligram scale.

Figure 3 | Dispersion and structure separation of high-concentration G-SWCNTs. (a) Optical absorption spectra of as-prepared G-SWCNTs with different concentrations. (b) Raman spectra of raw G-SWCNTs at excitation wavelengths of 514 nm, 488 nm, 633 nm and 785 nm. (c) TEM image of raw G-SWCNTs containing graphene sheets and SWCNTs with different diameters. (d) Plot of the SWCNT concentration in the as-prepared dispersion (blue) and the corresponding yields of (6, 4) SWCNTs (orange) as a function of the initial concentration of G-SWCNTs.  (e) Optical absorption spectra of various single-chirality species separated from G-SWCNTs with different initial concentrations. 

The distinct improvement in the separation yield of SWCNTs by increasing the concentration of SWCNT solution is mainly ascribed to the enhanced transfer of SWCNTs from bulk solution to the gel surface and thus their adsorption onto gel, which reduces the proportions of unadsorbed and irreversibly adsorbed SWCNTs. By life techno-economic and life cycle assessments, the mass separation of single-chirality species showed distinct advantages in efficiency, energy consumption and cost compared with the previous methods by increasing the concentration of SWCNTs. This dispersion and separation strategy provide a method for the industrial separation of single-chirality SWCNTs over a wide diameter range.

Figure 4 | Potential and challenge analysis for industrialization and commercialization of the current technique. a) Life cycle greenhouse gas emissions, b) cumulative energy demand and c) cost of producing 1-mg single-chirality SWCNTs from HiPco- and G-SWCNTs under different concentrations. 


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