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

Rohringer P, Shi L, Liu X, Yanagi K, Pichler T - Carbon N Y (2014)

Bottom Line: We found that by using a low sonication intensity before applying density gradient ultracentrifugation (DGU), only inner tube species with a diameter [Formula: see text]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.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.

View Article: PubMed Central - PubMed

Affiliation: Faculty of Physics, University of Vienna, Strudlhofgasse 4, 1090 Wien, Austria.

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 [Formula: see text]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.

No MeSH data available.


Absorption spectra of the pristine DWCNT sample not treated by DGU and using a low centrifugation speed (Procedure A) and the HiPco SWCNT control. Spectra normalized to their optical density at 900 nm, with the green graph multiplied by 2 for better differentiation. (A colour version of this figure can be viewed online.)
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f0005: Absorption spectra of the pristine DWCNT sample not treated by DGU and using a low centrifugation speed (Procedure A) and the HiPco SWCNT control. Spectra normalized to their optical density at 900 nm, with the green graph multiplied by 2 for better differentiation. (A colour version of this figure can be viewed online.)

Mentions: Before analyzing the samples after the DGU treatment, we compare the absorption spectra of the DWCNT sample not treated by DGU and using a low centrifugation speed (Procedure A) with the HiPco-sample that mainly contains SWCNT (for ease of comparison, the spectra were normalized to their optical density at 900 nm): In Fig. 1 we can see that the DWCNT absorption shows only weak features of the excitonic transitions between corresponding van Hove singularities in the density of states of inner tubes with diameters between 0.6 and 1 nm. The reason for this lies within the DWCNT structure: For DWCNT, the space between inner and outer tubes is given by the van der Waals radius which can vary depending on the synthesis procedure [28]. DWCNT produced by similar CVD processes with comparable diameter distributions as in this work (e.g. [29,30]) showed values in between 0.33 and 0.41 nm, this means that the outer tube diameters in our samples are between 1.3 and 1.8 nm. Early works assigned the absorption signal in the interval between 900 and 1200 nm to an overlap of the inner tube E11 and outer tube E22 transition for such a diameter distribution [31,32]. In the work of Iakoubovskii et al. [10], where a similar diameter distribution is used like in this work, the outer walls of DWCNT have been exposed to ozone etching to decompose the absorption spectra of DWCNT to their inner and outer shell contributions. After applying this method it can be seen that the E11 transition wavelengths of the inner tubes are in the same interval (between 900 and 1200 nm) as the E22 transition wavelengths of the outer tubes. Therefore we can safely assume that the small size of the peaks in the absorption spectrum of our samples is caused by the same mentioned overlap of inner tube and outer tube contributions.


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

Rohringer P, Shi L, Liu X, Yanagi K, Pichler T - Carbon N Y (2014)

Absorption spectra of the pristine DWCNT sample not treated by DGU and using a low centrifugation speed (Procedure A) and the HiPco SWCNT control. Spectra normalized to their optical density at 900 nm, with the green graph multiplied by 2 for better differentiation. (A colour version of this figure can be viewed online.)
© Copyright Policy - CC BY-NC-ND
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4375560&req=5

f0005: Absorption spectra of the pristine DWCNT sample not treated by DGU and using a low centrifugation speed (Procedure A) and the HiPco SWCNT control. Spectra normalized to their optical density at 900 nm, with the green graph multiplied by 2 for better differentiation. (A colour version of this figure can be viewed online.)
Mentions: Before analyzing the samples after the DGU treatment, we compare the absorption spectra of the DWCNT sample not treated by DGU and using a low centrifugation speed (Procedure A) with the HiPco-sample that mainly contains SWCNT (for ease of comparison, the spectra were normalized to their optical density at 900 nm): In Fig. 1 we can see that the DWCNT absorption shows only weak features of the excitonic transitions between corresponding van Hove singularities in the density of states of inner tubes with diameters between 0.6 and 1 nm. The reason for this lies within the DWCNT structure: For DWCNT, the space between inner and outer tubes is given by the van der Waals radius which can vary depending on the synthesis procedure [28]. DWCNT produced by similar CVD processes with comparable diameter distributions as in this work (e.g. [29,30]) showed values in between 0.33 and 0.41 nm, this means that the outer tube diameters in our samples are between 1.3 and 1.8 nm. Early works assigned the absorption signal in the interval between 900 and 1200 nm to an overlap of the inner tube E11 and outer tube E22 transition for such a diameter distribution [31,32]. In the work of Iakoubovskii et al. [10], where a similar diameter distribution is used like in this work, the outer walls of DWCNT have been exposed to ozone etching to decompose the absorption spectra of DWCNT to their inner and outer shell contributions. After applying this method it can be seen that the E11 transition wavelengths of the inner tubes are in the same interval (between 900 and 1200 nm) as the E22 transition wavelengths of the outer tubes. Therefore we can safely assume that the small size of the peaks in the absorption spectrum of our samples is caused by the same mentioned overlap of inner tube and outer tube contributions.

Bottom Line: We found that by using a low sonication intensity before applying density gradient ultracentrifugation (DGU), only inner tube species with a diameter [Formula: see text]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.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.

View Article: PubMed Central - PubMed

Affiliation: Faculty of Physics, University of Vienna, Strudlhofgasse 4, 1090 Wien, Austria.

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 [Formula: see text]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.

No MeSH data available.