Surfactant free fractions of metallic and semiconducting single-walled carbon nanotubes via optimised gel chromatography

Abstract

We report the procedure of sorting/purification of carbon nanotubes by electronic type using chromatographic column with sodium dodecylsulfate (SDS) and sodium deoxycholate (DOC) solutions as the eluents. The non-commercial agarose gel in different concentrations has been tested in the process. It was found that in optimal gel concentration the fractionation resulted in ∼96.2% yield of semiconducting species. Importantly, to get surfactant-free fractions the post-separation purification procedure has been carried out. The UV–vis–NIR and Raman spectroscopy have been utilised for the samples analysis. High resolution transmission microscopy and thermogravimetric analysis allowed to study the sample morphology and purity, respectively.

Introduction

The as-produced single-walled carbon nanotubes (SWCNTs) are featured by a heterogeneous composition, which precludes their application in electronics components. Both types (metallic and semiconducting) of SWCNTs occur as a mixture in raw material [1]. However, as yet, none of the fully selective synthetic methods used to prepare SWCNTs were developed. This is a major issue in regard to commercial applications, where SWCNT electronic homogeneity is essential. Metallic SWCNTs which are nanometre-sized conductors can find application as a print on flexible electronic circuits [2], while semiconducting SWCNTs which are field effect transistors [3] can be utilised as a nanometer-sized processors. In fact, towards obtaining either pure metallic or pure semiconducting SWCNTs many of separation methods were proposed. The first approach on M/S (metallic/semiconducting) separation reported by Krupke et al. [4], was alternating current dielectrophoresis of SWCNT. The process resulted in 100 pg deposit containing 80% of metallic SWCNTs. This experiment showed that separating the electric types of SWCNTs is possible. This initiated a worldwide research, which result in many different separation methods. In the current state of the art there is a variety of techniques available for M/S fractionation such as: electrophoresis [4], density gradient ultracentrifugation [5], polymer wrapping [6], laser irradiation [7], electric discharge [8], selective destruction [9], selective functionalisation [10], selective dispersion [11], freeze–squeeze [12], HPLC [13] and gel chromatography [14]. However, most of these methods led to the introduction of additional contamination and suffer from shortcoming, which make them insufficient for a large scale separation of SWCNTs. One of the most promising techniques in regard to scaling up is chromatography. This is one of the most effective techniques for high-purity solvents used for organic synthesis, and in drug industry for the separation of the active components [15]. Therefore, this method after modification can be utilised for SWCNT M/S separation in a large scale what was reported by Tanaka et al. [14]. However, this process still requires improvement to increase the sample purity and to lower its cost. To lower the cost of the process instead of using the commercially available sepharose^®, the self made agarose gel beads with different concentrations as an active adsorbent have been here studied. During the permeation, semiconducting nanotubes are adsorbed on the agarose bead while the metallic nanotubes pass through the filling. In the next step, the column is flushed with DOC (sodium deoxycholate) solution causing the desorption of material. Therefore, during this process: (i) metallic SWCNTs called SDS-fraction (sodium dodecylsulphate) and (ii) semiconducting SWCNTs (DOC-fraction) fractions were obtained. In this study we examine the effect of concentration of agarose in gel beads on the efficiency of M/S separation. To get surfactant-free fractions the post-separation purification procedure has been also optimised. This was realised via multistep filtration/sonication procedure.