Magnetism for understanding catalyst analysis of purified carbon nanotubes
Introduction
Carbon nanotubes (CNTs) and especially single walled carbon nanotubes (SWNTs) have remarkable intrinsic physical and chemical properties that make them as excellent candidates for fundamental studies and applications in many fields [1], [2]. However, whatever the synthesis methods, as-produced CNT samples contain both secondary carbon products and metallic-based impurities involved in the growth mechanism as catalyst. They are often used in high content (10–30 wt%) in order to improve the nanotube formation and limit that of carbon byproducts. These metal-based impurities are reported to dramatically modify both chemical and physical properties of CNT [3], [4], [5], [6], [7]. Without any post-synthesis purification treatment, SWNTs become unusable because their intrinsic properties are lost or hidden due to the presence of these metallic particles. For this very reason, the development of purification treatments is a long-standing and active research area. Standard processes are often multi-step and generally combine (i) a reflux in concentrated nitric acid solution in order to weaken the carbon shells and oxidize the metal-based impurities (ii) a gas-phase or wet oxidation and (iii) a final annealing treatment to restore the defects created through previous steps [8], [9], [10], [11], [12], [13], [14], [15], [16]. More recently, high temperature treatment under halogens were evidenced as an interesting alternative technique to selectively remove metal-based impurities [17], [18]. The assessment of the efficiency of such treatment indeed depends on the reliability of quantitative analysis of the remaining impurities. On one hand, local or surface analysis such as X-ray photoelectron spectroscopy or transmission electron microscopy (TEM) are used to determine their size and/or their chemical nature, however they are inefficient for a quantitative analysis [19]. On the other hand, macroscopic techniques such as thermogravimetry analysis (TGA) provide a quantitative analysis but without any discrimination regarding the chemical nature of the impurities in the purified samples. Among them, magnetic analysis (MAG) is a less commonly used technique [20], [21], [22]. It has been however shown to be very helpful to quantify magnetic residues from CNT samples [23], [24], [25]; especially because it is an ultrasensitive and non destructive method which can even detect very low amount of isolated paramagnetic impurities [19].
In this work, we have applied a Cl-based purification treatment on arc-discharged as-produced SWNTs under different temperature conditions. For comparison, a more standard purification method, based on an acid treatment has also been applied. Analysis and quantification of Ni-based impurities have been investigated by TEM, X-rays diffraction (XRD), TGA and MAG. A good agreement between TGA and MAG is found for the acid treatment whereas a strong deviation is observed between the results obtained from the different used techniques in the case of the Cl-based treatment. Thanks to magnetic measurements, these discrepancies are attributed to oxidation of Ni0 into Ni2+ by the halogen gas. This result proves that chemical treatment used for purification can modify the impurity.
Section snippets
Sample preparation
The SWNTs used in this study have been synthesized in an arc-discharge homemade reactor described elsewhere [16]. The catalyst mixture is prepared with a graphite powder SFG6 (synthesized flake graphite 6 µm) from Timcal, nickel particles of around 3 μm purchased from Sigma Aldrich and yttrium powder (40 mesh) from Acros Organics. Ni/Y/C is fixed at 4.2/1/94.8 at% (2.8/1/96.2 wt%). The as-produced SWNT powder (referred as “raw”) was purified using 2 different treatments: (i) a standard purification
Transmission electron microscopy: CNT and impurity observation
The as-produced SWNT powder (referred as “raw”) was purified using 2 different treatments: (i) a standard purification process that involves three main steps: air oxidation, acid reflux and annealing. The corresponding purified SWNT sample is referred as “sp”. (ii) An alternative halogen-based treatment that mainly consists in heating the SWNT powder under high chlorine partial pressure for 2 h at 800 °C, 950 °C, or 1100 °C [17], respectively assigned to “hp-800”, “hp-950”, “hp-1100”.
Fig. 1 shows
Conclusion
Quantitative analysis of metal-based residues in SWNT samples after chemical purification is an open and tricky question. In this work, we have investigated the commonly used characterization techniques combined with magnetization measurements. This latter allows to determine the exact state of the impurities that is of importance to know the chemical mechanisms involved in the purification treatments. Based on two different purification procedures, the results are discussed regarding the
Acknowledgment
The authors thank Jean-François Marêché for his technical help for the chemical treatments. The authors would like to thank Lionel Aranda for his help for TGA experiments and Pascal Franchetti for his valuable assistance for Raman spectroscopy.
References (37)
- et al.
Chem. Phys. Lett.
(2008) - et al.
J. Magn. Magn. Mater.
(2006) - et al.
Carbon
(2009) - et al.
Carbon
(2010) - et al.
Carbon
(2010) - et al.
Carbon
(2009) - et al.
Chem. Phys. Lett.
(2005) - et al.
Carbon
(2015) - et al.
Carbon
(2006) - et al.
Synth. Met.
(1999)
Chem. Rev.
Acc. Chem. Res.
J. Phys. Chem. B
Phys. Rev. B
Nanoscale
New J. Chem.
Adv. Mater.
Carbon
Cited by (6)
Purification of carbon nanotubes produced by the electric arc-discharge method
2021, Surfaces and InterfacesCitation Excerpt :A large number of purification methods have been reported in the literature [28, 50–53]. In general, these methods involve several processing steps that can combine: oxidation in gas or liquid phase (reflux in a solution containing oxidizing agents), physical separation processes such as filtration, centrifugation, chromatography, or selective functionalization methods [40, 50, 54–59]. Oxidative steps have been widely used in CNTs purification protocols for eliminating both unliked metal particles and carbons.
Transition metal impurities in carbon-based materials: Pitfalls, artifacts and deleterious effects
2020, CarbonCitation Excerpt :This distribution shows directly which fraction of the metallic nanoparticles has been removed [200]. Vigolo et al. studied the Ni catalyst content in arc-discharge SWCNTs purified by two techniques, a Cl2 treatment at 800–1100 °C and a standard method including air oxidation at 350 °C followed by HCl reflux and an annealing under vacuum at 1100 °C using MAG techniques [201]. Thanks to magnetic analysis, they proved that a particular purification process can significantly alter the chemical state of a metallic impurity.
Dramatic enhancement of double-walled carbon nanotube quality through a one-pot tunable purification method
2016, CarbonCitation Excerpt :Pure gas phase treatments can be preferred but nevertheless they have been poorly studied. They are either based on high temperature annealing [22,23], or assisted by halogen gases [24–29]. The advantages of these gas-phase methods are several, they are simple (one-step), they do not introduce defects in the CNT structure but they fail to remove carbon impurities.
Magnetism as indirect tool for carbon content assessment in nickel nanoparticles
2017, Journal of Applied Physics