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. 2024 May 10;19(5):e0303359.
doi: 10.1371/journal.pone.0303359. eCollection 2024.

UV-Vis quantification of the iron content in iteratively steam and HCl purified single-walled carbon nanotubes

Markus Martincic  1 , Gerard Tobías-Rossell  1
Affiliations

UV-Vis quantification of the iron content in iteratively steam and HCl purified single-walled carbon nanotubes

Markus Martincic et al. PLoS One. .

Abstract

As-produced carbon nanotubes contain impurities which can dominate the properties of the material and are thus undesired. Herein we present a multi-step purification treatment that combines the use of steam and hydrochloric acid in an iterative manner. This allows the reduction of the iron content down to 0.2 wt. % in samples of single-walled carbon nanotubes (SWCNTs). Remarkably, Raman spectroscopy analysis reveals that this purification strategy does not introduce structural defects into the SWCNTs' backbone. To complete the study, we also report on a simplified approach for the quantitative assessment of iron using UV-Vis spectroscopy. The amount of metal in SWCNTs is assessed by dissolving in HCl the residue obtained after the complete combustion of the sample. This leads to the creation of hexaaquairon(III) chloride which allows the determination of the amount of iron, from the catalyst, by UV-Vis spectroscopy. The main advantage of the proposed strategy is that it does not require the use of additional complexing agents.

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Conflict of interest statement

The authors have declared that no competing interests exist

Figures

Fig 1
Fig 1. Iron content in as-received and purified SWCNTs.
a) Amount of iron in samples of SWCNTs, which underwent a single steam treatment, “direct purification”, as assessed by TGA; all values were recorded after treating the sample with HCl. Both as-received SWCNTs (0 h) and steam (2 h, 3 h, 4 h, 5 h, 6 h) and HCl purified SWCNTs are included. b) Amount of iron determined by SQUID for both as-received SWCNTs (0 h) and iteratively purified SWCNTs. In this case, the steam treatment was conducted either one time for a given duration (“direct purification”) or in an iterative manner (1, 2, or 3 h steps), with a total steam treatment of 6 h. It is worth noting that two distinct batches of SWCNTs were used for the studies reported in (a) and (b).
Fig 2
Fig 2. UV-Vis calibration curves.
a) UV-Vis spectra obtained for the Fe2O3 standards and b) the resulting calibration curves using the recorded intensities of the three main peaks.
Fig 3
Fig 3. Quantitative determination of the iron content in SWCNT samples.
Iron content determined by UV-Vis spectroscopy, SQUID, and TGA in purified SWCNT samples. These analyses were conducted following a cumulative 6-hour steam treatment, involving both direct purification (1 x 6 h) and multiple steam-HCl iterative purifications (2 x 3 h, 3 x 2 h, 6 x 1 h). An HCl wash was conducted after each steam treatment.
Fig 4
Fig 4. Determination of morphology and residual impurities in 6 x 1 h steam and HCl purified SWCNTs.
a) SEM image of purified SWCNTs. b) EDX analysis of purified SWCNTs. c) EDX analysis of the solid residue collected after the oxidation of the purified SWCNTs.
See this image and copyright information in PMC

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Publication types

Grants and funding

Ministerio de Ciencia e Innovación, PID2020-113805, Dr. Gerard Tobías-Rossell Ministerio de Ciencia e Innovación, CEX2019-000917-S European Commission, 290023, Dr. Gerard Tobías-Rossell.
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