Purification, separation and extraction of inner tubes from double-walled carbon nanotubes by tailoring density gradient ultracentrifugation using optical probes

Abstract

We studied the effect of varying sonication and centrifugation parameters on double-walled carbon nanotubes (DWCNT) by measuring optical absorption and photoluminescence (PL) of the samples. We found that by using a low sonication intensity before applying density gradient ultracentrifugation (DGU), only inner tube species with a diameter ⩽0.8 nm can be identified in absorption measurements. This is in stark contrast to the result after sonicating at higher intensities, where also bigger inner tubes can be found. Furthermore, by comparing PL properties of samples centrifugated either with or without a gradient medium, we found that applying DGU greatly enhances the PL intensity, whereas centrifugation at even higher speeds but without a gradient medium results in lower intensities. This can be explained by extraction of inner tubes from their host outer tubes in a two-stage process: the different shearing forces from the sonication treatments result in some DWCNT to be opened, whereas others stay uncut. A subsequent application of DGU leads to the extraction of the inner tubes or not if the host nanotube stayed uncut or no gradient medium was used. This work shows a pathway to avoid this phenomenon to unravel the intrinsic PL from inner tubes of DWCNT.

Photoluminescence from an individual double-walled carbon nanotube

“…In such samples, it is unclear whether the observed PL signal originates from the ISCT [14,18,19,28] or from SWNT impurities present in the starting materials or originating from inner tubes extracted from outer tubes during the suspension preparation process [11,20,21,29]. This highlights the difficulties of studies conducted on ensembles and emphasizes the call for investigations at the single-DWNT level.…”

Selective Enhancement of Inner Tube Photoluminescence in Filled Double‐Walled Carbon Nanotubes

“…Individualization of Double-Walled Carbon Nanotubes : To obtain wellisolated DWCNT without extracting their inner tubes, a method similar to Procedure A given by Rohringer et al [ 14 ] was used. The DWCNT sample was individualized via ultrasonication in deionized water with 2% w/v sodium deoxycholate (DOC, Sigma-Aldrich), using a steel tip with a diameter of 1/4 in, 60 W of power, for 4 h (Branson Sonifi er 450).…”

“…The parameters of the procedure on how homogeneous DWCNT suspensions are received are of uttermost importance for the absorption and the photoluminescence of DWCNT. The applied technique follows a method which has proven that inner tube extraction can be avoided; [ 14 ] the exact details can be found in the Experimental Section. Figure 2 a shows the absorption spectra of a pristine, not-fi lled DWCNT sample in comparison with HiPco SWCNT before background subtraction as reported earlier, [ 14 ] (n,m) assignment was performed based on the work of Weisman and Bachilo.…”

“…[ 32 ] Features of the excitonic transitions between corresponding van Hove singularities of DWCNT are strongly attenuated in comparison to SWCNT. [ 12,14,33,34 ] On the one hand, a decrease in absorption features is often a result of carrier/hole doping to the carbon nanotube as observed for SWCNT. [35][36][37][38] As mentioned earlier, Zólyomi et al found that a charge transfer from outer to inner tube is taking place.…”

“…a) Absorption spectra of the pristine DWCNT sample as compared to a HiPco SWCNT reference sample. [ 14 ] Assignment based on the work of Weisman and Bachilo. [ 32 ] b) Comparison of the normalized E 11 absorption spectra from pristine DWCNT and the samples annealed at 1400 and 1460 °C.…”