Efficient purification of single-walled carbon nanotube fibers by instantaneous current injection and acid washing

Abstract

Growth of carbon nanotubes is usually accompanied by the trapping of large amount of amorphous impurities and catalyst residue. So far, most purification methods have focused on powder-form products and their dispersions. Here, we present a simple yet highly efficient method to directly purify twist-spun single-walled carbon nanotube (SWNT) fibers by applying an instantaneous high current through the fiber. The generated Joule heat burns away the amorphous carbon species coated on SWNT bundles, and exposes Fe particles that can be easily washed by diluted acid, resulting in very clean fibers. The purified fibers, consisting of only larger-size and clean-surface SWNT bundles, show significantly enhanced electrical and mechanical properties. Our purification method is suitable to clean long continuous single-walled or multi-walled nanotube fibers fabricated in various ways and those containing a large amount of residual catalyst and co-deposited carbonaceous impurity.

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Introduction

Carbon nanotubes (CNTs) are one-dimensional nanostructures that typically grow from catalysts such as metallic nanoparticles, therefore as-synthesized products usually contain a significant amount of catalyst particles.^1–4 For practical applications, it is demanding to remove those residual catalysts and other carbonaceous impurities as their presence is detrimental to the mechanical and electrical properties, and consequently impair the ultimate performance. To this end, there have been extensive efforts in exploring effective techniques to produce high-purity CNT samples.^5–7 Although a number of work can synthesize high purity single-walled carbon nanotubes (SWNTs) by depositing a very thin catalyst layer or by in situ monitoring the role of working catalyst,⁸ most of studies have focused on post-treatment methods which can be low-cost and scalable for industrial production. So far, a variety of purification methods have been reported, such as microwave heating,⁹ high-temperature calcinations,¹⁰ in combination with acid treatment where ultrasonication, filtration and centrifugation processes are frequently involved.^11,12 The majority of above recipes are developed to treat random, powder-form samples dispersed in solvents, and are not suitable for assembled CNT structures at different dimensions, for example, aligned arrays, porous networks, and twist-spun fibers.

Among various macroscopic assemblies, CNT fibers have stimulated tremendous interest owing to their high mechanical strength, electrical conductivity, flexibility and weavability.^13–18 Traditionally, CNT fibers were prepared by extruding surfactant-stabilized suspensions in continuous way.^19,20 After that, twist-spun fibers were obtained by directly spinning as-grown CNT aerogels or spiderwebs, or extracting a continuous fiber/sheet from a substrate-supported aligned array.^21–24 Compared to solution extrusion methods, dry-spinning of CNT fibers holds important advantages as the original lengths and interconnections between as-grown nanotubes/bundles are well protected, a key factor for reaching high strength and conductivity. Twist-spun CNT fibers have demonstrated many intriguing applications such as torsional actuators and artificial muscles,^25,26 energy storage wires such as supercapacitors and batteries,^27,28 and high purity of those fibers is critical for achieving superior performance in various fields. However, so far there have been no efficient methods to purify those long continuous fibers, and the trapped impurities as well as catalyst particles make a potential source for increasing the fiber weight, reducing structure homogeneity, and causing performance degradation in mechanical or electrical applications. To purify a CNT fiber, a major obstacle is how to access the inner regions enclosed by tightly-twisted CNTs, and remove the impurities without dismissing the integral fiber structure.

Here, we present a simple yet highly efficient method to purify twist-spun SWNT fibers, while simultaneously improve their mechanical and electrical properties. We find that an instantaneous large current can burn away the carbonaceous impurities and expose residual Fe catalyst particles that can be easily washed by diluted acid. Our method could obtain very pure SWNT fibers (from surface to inside), and is especially suitable for cleaning very dirty fibers containing a large amount of carbonaceous impurity and residual catalyst. Preliminary tests have revealed 3 to 4-fold improvement of electrical conductivity and enhanced strength as well as cyclic loading behavior of those highly purified fibers.

Experimental section

Preparation of as-spun SWNT fibers

We synthesized thin films consisting of interconnected single-walled nanotubes by chemical vapor deposition as reported in our previous work. Precursors are ferrocene (catalyst) and xylene (carbon source). Reaction temperature was set as 1160 °C; at this temperature single-walled nanotubes will form in the vapor phase and be blown out to low-temperature zone by the carrier gas (mixed Ar/H₂). The freestanding CNT films were picked up from the downstream side of the CVD reaction quartz tube, and then used for spinning. One end of the CNT film was fixed to the shaft tip of an electric motor that was controlled by a power supply (voltage controller, set as 1.5 V for spinning). The other end of the film was fixed to the edge of a metal block that could move freely across a smooth glass surface. The entire film was suspended between the motor and the block. The motor was operated at a speed of about 3000 revolutions per minute (rpm), and the CNT film was spun into a straight yarn.

Purification of as-spun SWNT fiber by current injection and acid washing

An as-spun SWNT fiber was connected to electrical wires by conductive adhesive and silver paste applied at its two ends on glass slides, and the fiber between two ends was suspended between glass slides to avoid touching other substrates and non-uniform heating. The resistance of sample was measured with a multimeter. Typically, we applied a voltage of 45 V on a SWNT fiber with a resistance of 25 Ω (length = 8 cm, diameter = 138 μm). Such an appropriate voltage value depending on the fiber resistance was loaded for a short period of about 1 second. In a word, the voltage was selected to ensure that the current density through the fiber is in a suitable range to avoid inadequate or excessive oxidation. After loading the voltage, the fiber was soaked in dilute hydrochloric acid (HCl, 0.1 mol L⁻¹) for 12 hours, and then washed in deionized water for several times. At last the fiber was dried in a vacuum drying oven at 80 °C for 4 hours. We also prepared purified fibers without current-induced heating as a reference sample. In this case, as-spun fibers were directly purified through hydrogen peroxide (H₂O₂, 1 mol L⁻¹, 12 hours) and dilute hydrochloric acid (HCl, 0.1 mol L⁻¹, 12 hours) treatment. At last the fibers were dried in a vacuum drying oven at 80 °C for 4 hours.

Characterization and calculation

The surface and internal morphology of SWNT fibers before and after purification were characterized by scanning electron microscopy (SEM, JEOL JSM-6700F, Japan) and transmission electron microscopy (TEM JEOL JEM-2100). Chemical composition of the purified fibers is detected by X-ray photoelectron spectroscopy (XPS). The resistance of fiber were recorded by four-electrode method with a digital source-meter (Keithley2636 and 2400). Raman spectrometer (Renishaw-in Via Reflex) was used to authenticate the degree of purification while the excitation wavelength was 514 nm. The mass ratio of the original SWNTs fiber and purified fiber were evaluated by thermal gravimetric analysis (TGA LINSEIS STA PT1600) in air at 1000 °C (10 °C min⁻¹).

Mechanical tests were carried out in a single-column testing instrument (Instron 5843) equipped with a load cell of 10 N. The two ends of a CNT fiber were fixed on a small piece of paper cutting centrally with a rectangular window by polyvinyl alcohol as adhesive paint. Both ends of the paper were clamped with the grips with the fiber dangling vertically and aligned along the grip axis. The paper was cut from the side to free the CNT fiber. For tension tests, the upper grip was moved away at a constant speed of 1.0 mm min⁻¹. For 1000 loading–unloading cycles, the strain rate was 125% per minute for all the cycles.

The conductivity of the SWNT fibers was calculated using follow formula:

where k represents the electrical conductivity, L is the diameter of the fibers measuring centimeter scale; R is the resistance of the fibers; S is the sectional area of the fibers and it was calculated by formulawhere d is diameter of fiber. Four-electrode method was used to measure the resistance. Neighboring distance of four electrodes which were marked with A, B, C and D in sequence were about L_AB = 2.6 cm, L_BC = 1.7 cm and L_CD = 2.3 cm as an example, while homologous sectional area were labeled as S_AB, S_BC, S_CD. We loaded constant current 0.002 A between electrode A and D (I_AD) and measured simultaneously the voltage between electrode A and B (V_AB), B and C (V_BC), C and D (V_CD) by turn. Consequently, the resistances of three sections wereand

Finally the conductivity is average value of these three conductivities

Results and discussion

Our purification method involves two sequential steps including (1) an instantaneous high current injection through the fiber length and, (2) washing by diluted hydrochloride acid (illustrated in Fig. 1a). The first step removes amorphous carbon and exposes Fe particles, whereas the second step cleans residual catalyst completely by mild acid dissolution. To do this, an as-spun SWNT fiber suspended between two metal electrodes was subjected to a current injection by applying a high voltage instantaneously (40 to 110 V, depending on the fiber length and diameter). Given a fiber resistance of 25 Ω at a span of 80 mm (shown in Fig. 1b), the current flow through the fiber is on the order of hundreds of mA. Upon the current injection, the fiber exhibited very bright incandescent light with a slight yellow smoke coming out, indicating that carbonaceous species and some Fe particles are burnt away. We estimate that the instantaneous temperature of the fiber might reach more than 1000 °C to give such bright emission. After subsequent acid washing, rinsing and drying, the fiber maintained the original morphology without obvious change in the fiber length and diameter as seen by eyes. In principle, this method is applicable to fibers consisting of SWNTs or multi-walled nanotubes made from various methods, and fibers of different diameters and lengths.

Fig. 1 Mechanism of purification of SWNT fiber by instantaneous current injection and acid washing.

We have carried out scanning electron microscopy (SEM) and transmission electron microscopy (TEM) characterization on the structural evolution of the SWNT fibers through the above process. The as-spun fiber shows a fluffy surface containing twisted SWNTs, among which a lot of small particles and short curved nanotubes are observed (Fig. 2a). We have adopted a high ferrocene concentration and precursor injection rate to grow fibers with a considerable amount of impurities. The micro-morphology has changed noticeably after current injection. There are numerous polyhedral nanocrystals appearing at the gaps between SWNT bundles, indicating the formation of iron oxide crystals due to the current-induced Joule heating in air (Fig. 2b). These crystals distribute uniformly at both the surface part and inside of the fiber, a consequence of the in situ current injection through the cross-section. After acid washing and drying, the fiber diameter has decreased from originally 100 μm to now 64 μm, due to the removal of trapped oxide crystals (Fig. 2c). SEM images on both the fiber surface and inside do not reveal any trace of catalyst particles (Fig. 2c, and ESI, Fig. S1†), suggesting that our purification method is highly efficient for tightly twisted fibers. We also notice that initially the lateral sizes of SWNT bundles are relatively small (20–50 nm or less), but the bundle sizes in the purified fiber have increased to 50–100 nm or more (Fig. 2i). This is a clear evidence that the interaction between SWNTs has been enhanced owing to the removal of amorphous carbon coating and residual catalyst, and adjacent small bundles tend to aggregate under van der Waals force between SWNT shells.

Fig. 2 Structural evolution of as-spun (a), current injected (b), and current injected/acid washed (c) SWNT fibers during purification process.

TEM images reveal more useful information. In the as-spun fiber, a thick amorphous carbon layer is coated on virtually all as-grown SWNT bundles (Fig. 3a). Co-deposition of such amorphous coating is a common phenomenon that has been observed in many SWNT products synthesized from different methods.^29–31 Furthermore, it also covers on the residual Fe particles, forming carbon-encapsulated metal particles that are tough to remove. The main function of our current-injection step is to remove all the carbonaceous species, from both the SWNTs and Fe particles. As a result, we observe SWNT bundles with very clean surface and those exposed Fe oxide crystals where the carbon coating has disappeared (Fig. 3b). This has paved the way for subsequent acid dissolution of Fe particles. After HCl washing and drying, the fiber became very dense and we have to split it in order to characterize its inner portion by TEM. Along the split fiber edge there are close-packed SWNT bundles with few catalyst residue (Fig. 3c), consistent with SEM image. Although most of bundles are aligned along the fiber axis, the circular cross-sections of SWNTs arranged in triangular lattice are frequently observed in those bending or kinked bundles. TEM results reveal that the catalyst residue trapped inside the fiber has been removed completely. To further confirm that Fe atoms have been removed, the as-spun, current injected and purified fibers were characterized by X-ray photoelectron spectroscopy measurements (XPS). As shown in Fig. 3d, the wide XPS spectrum of as-spun SWNT fiber indicates the presence of C, O, and Fe elements in the sample. After purification by instantaneous current injection and hydrochloric acid, signals of C and O1s remain there but Fe2p peak has disappeared, indicating that the iron oxide particles have been removed completely.

Fig. 3 TEM images of as-spun (a), current injected (b), and purified by current injected/acid washed (c) SWNT fibers. XPS (d), Raman (e) and TGA (f) curves of as-spun, current injected and purified by current injected/acid washed SWNT fibers.

We also used Raman spectroscopy to check the structural evolution during our purification process and the presence of impurities or defects, structural integrity and distribution of different diameters of SWNTs. The as-spun fiber, fiber after current injection, and purified fiber (after washing away catalyst) all show typical G-band (1590 cm⁻¹), D-band (1340 cm⁻¹), and radial breathing modes (RBM) related to SWNTs (Fig. 3e). Specifically, the D-band has diminished after current injection, due to the burning of amorphous carbon which is highly defective. Correspondingly, the intensity ratio of G/D increases from 16 in as-spun fiber to 42 after purification. In addition, some RBM peaks (e.g. peaks at 168 cm⁻¹, under a laser excitation wavelength of 514 nm) seen from as-spun fibers have disappeared in purified fibers. These SWNTs are categorized as metallic species.³² As metallic SWNTs carry out most of the current flow, they tend to be burned early (than semiconducting tubes) during large current injection, as described in a previous report on SWNT micro-bundles.³³ It might be possible to fabricate semiconductor-dominating SWNT fibers by controlling the current injection and selectively removing metallic species. Nonetheless, TEM and Raman study confirms that the integrity of bundle structure has been maintained and the defect degree has been reduced substantially.

To determine the remaining impurities in the SWNT fibers in a quantitative way, we performed thermogravimetry analysis (TGA) on as-spun, current injected and purified samples (Fig. 3f). Raising the temperature from ambient to 1000 °C in air, the SWNTs are burned away (at around 600 °C) leaving catalyst residue behind (in the form of iron oxide). The as-spun fibers show a remaining mass of nearly 40%, due to the trapping of significant amount of Fe particles introduced during chemical vapor deposition (CVD). In contrast, the mass retention of purified fibers has decreased to less than 6% after combustion, indicating a high purity of 94%. Our method is thus amenable to directly purify twist-spun CNT fibers. Compared with the TGA result tested in air, CNTs in purified fibers are very stable in N₂ environment, with a mass loss of less than 5% after rising temperature to 1000 °C (Fig. S2†). In contrast, CNTs are burnt in air at a combustion temperature of 500–600 °C, resulting in a sharp mass decrease.

Efficient purification of SWNT fibers lead to considerable improvement in both electrical and mechanical properties. The resulting purified fiber shows a wrinkled surface morphology due to the volume shrinkage after removal of internal impurities (Fig. 4a and S3†), which is distinct from other twist-spun fibers without purification. These surface wrinkles indicate a strong condensation of SWNTs inward, and also might be useful as fatigue resistant fibers if the wrinkles can be stretched reversibly. As a result, the measured 4-probe electrical conductivity has increased from below 1600 S cm⁻¹ in as-spun fibers to about 4000 S cm⁻¹ after purification, mostly owing to the formation of bigger bundles and shrinkage of cross-sectional area (Fig. 4b). We also tested a control sample by the same purification process but without the current injection step. It shows that many impurities are still trapped within the bundle since the amorphous carbon-encapsulated Fe particles could resist acid attacking (Fig. S4†). The electrical conductivity increases to about 2000 S cm⁻¹, which is not prominent.

Fig. 4 Electrical and mechanical properties of purified SWNT fibers. (a) SEM image of the purified fiber with a wrinkled surface morphology. (b) Statistical data of electrical conductivities of as-spun, purified and purified without current injection fibers. (c) Recorded current flow through an illuminated purified fiber. Insets show the photos of the fiber at point A and B. (d) Tensile stress–strain curves. (e) Cyclic stress–strain curves of a purified fiber.

As a result of high purity and enhanced conductivity, the purified SWNT fibers can be illuminated more stably in air. As-spun fibers usually break very quickly (within several seconds) after applying a moderate voltage because of air oxidation at elevated temperature. However, when we applied a constant voltage of 7.5 V on a purified fiber (2 cm segment), it produced a rather stable emission in the middle part, which lasted for nearly 2 minutes before breaking (Fig. 4c). During this stage, the electric current passing through the fiber maintains a stable value (90 mA) without fluctuation. Based on a fiber diameter of 250 μm, the current density sustained by the fiber is estimated to be 1.8 × 10⁵ mA cm⁻². The removal of impurities especially amorphous carbon enhances the fiber resistance to air oxidation at high temperature, leading to improved structural stability in air environment. Our high-purity SWNT fibers might have applications as stable filament-like light-emission sources.

Mechanical properties have been evaluated by uniaxial tension tests and stress–strain (σ–ε) curves. As-spun SWNT fibers with a length of 10 mm exhibit tensile stresses of about 200 MPa with failure strains in the range of 15% to 20% (Fig. 4d). Purified fibers show higher tensile stresses (300 to 400 MPa) but reduced strains (<10%). To study the structural homogeneity along the fiber axis, we adopt a larger gauge length (40 mm) to test our purified SWNT fibers, and find that both the tensile strength (300 MPa) and strain (8%) decrease compared with the results from shorter fibers (10 mm) (Fig. S5†). The enhancement of fiber strength is mainly due to the decrease of cross-sectional diameter (thus densification of SWNT bundles as shown in Fig. 3c) after purification. SEM images of the fractured section indicate that the failure mechanism of the SWNT fiber in tension is dominated by the sliding between aligned SWNT bundles (Fig. S6†). On the other hand, burning of metallic SWNTs upon current injection might have negative effect on the mechanical strength. Purified fibers are also less porous than as-spun sample, resulting in lower tensile strains upon breaking.

We also performed cyclic tests on the purified fibers with a wrinkled morphology. The fiber can be repeated stretched to a modest strain (ε = 2%, producing a stress of ∼20 MPa) and recover to its initial state, as the unloading curve returns to origin enclosing a hysteresis loop (Fig. 4e). Over 1000 loading–unloading cycles, the slope of loading curve remains constant although the maximum stress at ε = 2% drops slightly. A small permanent deformation (less than 0.5% residual strain) has been developed after 1000 cycles. These results suggest potential applications of purified SWNT fibers under various external conditions such as cyclic stresses.

Conclusion

We demonstrated a simple and efficient method to directly purify long carbon nanotube fibers by introducing an instantaneous high current injection. This step burns away amorphous carbon coated on SWNT bundles and residual catalyst, and the exposed catalyst particles can be washed thoroughly by mild acid dissolution. Our high-purity SWNT fibers show enhanced electrical and mechanical properties, and have many potential applications in developing high performance artificial muscles as well as flexible fiber-shaped energy conversion and storage devices. Our method described here is suitable for purifying various carbon nanotube fibers, for example, those made from multi-walled or single-walled nanotubes, and those fabricated by dry or wet-spinning techniques.

Acknowledgements

The authors greatly acknowledge financial support from the National Natural Science Foundation under grants of NSFC 51325202, 51502267, and the Outstanding Young Talent Research Fund of Zhengzhou University (1521317003), Startup Research Fund of Zhengzhou University (1512317001), Henan Province Science and Technology Research Project (162102410069). Certificate of Postdoctoral Research Sponsorship Henan Province (2015009).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra20967j

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