Continuous flow laser-induced unzipping of multiwalled carbon nanotubes

Abstract

Unzipped multiwalled carbon nanotubes (MWCNTs) provide a route to graphene carbon ribbons (GNRs), which have application in electronic devices. Pulsed irradiation using an Nd:YAG laser operating at 1064 nm mediates such unzipping of MWCNTs dispersed in ethanol under shear stress within the vortex fluidic device (VFD). The method is scalable with the thin film device operating under continuous flow, while also avoiding the use of harsh chemicals and auxiliary substances. Unzipped MWCNTs are formed in 90% yield and have been characterized using scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, thermogravimetric analysis, X-ray powder diffraction and X-ray photoelectron spectroscopy. The systematically derived optimal laser power for unzipping was 250 mJ and increasing the laser power results in the fragmentation of the MWCNTs.

KEYWORDS:

• Vortex fluidic device
• scalability
• continuous flow
• unzipping
• MWCNTs
• laser irradiation

1. Introduction

Manipulation of the structure of carbon nanotubes (CNTs) in a controlled way is crucial for numerous nanotechnology applications [1–5]. In this context, unzipping of multiwalled CNTs (MWCNTs) has been extensively studied in recent years due to their attractive electrical and mechanical properties [1,2,6,7]. Indeed, unzipping tubular MWCNTs is an effective strategy for synthesizing graphene nanoribbons (GNRs)/graphene material in large quantities [8,9] with motivation for this work primarily a desire to create flat carbon networks. The initial diameter and length of CNTs can determine the size of the fabricated GNRs, noting that flat semiconducting GNRs with a width of <10 nm are desirable, in particular, in the production of electronic devices [10–12].

Several “top-down and bottom-up” methods for producing GNRs have been reported, including chemical vapour deposition, using plasmas, lithography, chemical solution-based oxidative processes and laser ablation [13–22]. The disadvantages of these technologies include low yield of production, complex processing step, time-consuming operations and the use of toxic auxiliary chemicals [20,23–27]. Rao et al. used a fast and simple method based on laser irradiation to unzip MWCNTs on a glass substrate [28]. However, this method is not scalable, has limited productivity, and it is inherently difficult to collect the unzipped material. Kumar et al. demonstrated that laser irradiation can unzip MWCNTs into GNRs dispersed in dimethylformamide (DMF) [24], and Shim et al. have developed a simple method for fabricating GNRs involving laser irradiation of a solution of MWCNTs in ethanol [29]. However, this method was preformed using batch processing and the product was not uniform.

GNRs and their composite can be used as shielding materials for electromagnetic interference (EMI) [30,31]. Park et al. [32] synthesized graphene nanoribbons (GNRs) by unzipping MWCNTs and incorporated the GNRs into thermoplastic polyurethane (TPU) and fabricated a composite material for EMI shielding. These composite materials (GNRs/TPU) achieved 24.9 dB EMI shielding effectiveness [32]. Sun et al. [33] fabricated graphene on polyimide (PI) substrates using laser scribing. The fabricated materials showed an EMI shielding effectiveness between ∼20 dB and ∼50 dB, depending on the nature of the graphene layers [33].

Recently, we reported the unzipping of MWCNTs, in moderate yield (75%) using a vortex fluidic device (VFD) operating under continuous flow [34]. Here the MWCNTs are suspended in aqueous hydrogen peroxide (H₂O₂) and processed under high shear continuous flow conditions, resulting in partially and fully unzipped MWCNTs. [34]. We note that laser irradiation of an aqueous hydrogen peroxide suspension of MWCNTs in a VFD results in the formation of carbon dots [35]. The VFD itself is an inexpensive thin film microfluidic platform housing a tilted, rapidly rotating quartz tube, typically 18.5 cm in length and 20 mm in outside diameter. VFD processing induces mechanical energy, as well as intense micromixing within the liquid in a controlled way [36–38], with processing using the VFD suited for developing processes high in green chemistry metrics [36,37]. The VFD has a diverse range of applications in general, also including slicing carbon nanotubes (CNTs) [39,40] exfoliating 2D materials [41–43], controlling carbon nanomaterial morphologies [44,45] and others [46].

In this work, we demonstrate a simple and controlled method for unzipping MWCNTs under continuous flow, depending on the operating conditions, in particular the rotational speed of the tube, choice of solvent, concentration of the MWCNTs, and the application of field effects for which the VFD is ideally designed. Herein this involved irradiating the VFD with a pulsed laser, using environmentally acceptable ethanol as the choice of solvent in which the MWCNTs were suspended. Moreover, the processing is under continuous flow, such that the scalability of the process is established as an integral part of optimizing the process from the outset, and the processing results in high yielding unzipping. The mechanism of unzipping the MWCNTs and the nature of the resulting material were examined using transmission electron microscopy (TEM), with the resulting nanocarbon material characterized using TEM, high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), Raman spectroscopy, X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS).

2. Experimental section

2.1 Materials

MWCNTs were purchased from Sigma-Aldrich. The length and diameter of the as-received material were 3–6 µm and 10–50 nm, respectively. Ethanol with a purity of 99% was purchased from Chem-supply (HA 154-2).

2.2 Preparation of unzipping MWCNTs

The as-received MWCNTs (10 mg) were dispersed in ethanol (100 mL) over 30 min using bath sonication (6 KHz). A suspension of MWCNTs in ethanol was delivered via a stainless-steel jet feed close to the hemispherical bottom of the VFD tube rotating at high speed, while irradiated with a pulsed laser (Nd: YAG 1064 nm, Quanta Ray-LAB190, 8 mm diameter spot size) operating at 250 mJ as the optimal power (Figure 1). The repetition rate and pulse width were 10 Hz and 5 ns, respectively. The beam of the laser was directed to the middle of VFD tube and the setup of the VFD and irradiation laser was according to a previously reported work on slicing CNTs under flow using the VFD [39]. The VFD was tilted at 45° as the optimal angle for most processing in the device [39,40], with the concentration of the MWCNTs then systematically varied along with the rotational speed, which was initially set at 8k rpm, which is often optimal for VFD processing [25,34]. After unzipping MWCNTs, the yield was determined to be 90%, based on the volume and concentration of the starting materials of MWCNTs (0.45 mg mL⁻¹) and the quantity of the isolated and dried unzipped MWCNTs.

Figure 1. Scheme illustrating the preparation of unzipping MWCNTs in a VFD irradiated with a pulsed laser, wavelength 1064 nm.

2.3 Characterization

The morphology, size and properties of the unzipped MWCNTs were determined using SEM (FEI Quanta 450), TEM, HRTEM (Philips CM200), Raman spectroscopy (WiTec Alpha 300R λ_exc = 532 nm), TGA (Perkin Elmer STA8000). Thermogravimetric was carried out under nitrogen atmosphere from 50°C to 800°C at a rate of 10°C min⁻¹, XPS (Phoibos 100 hemispherical analyser SPECS) with an Mg anode and XRD [Bruker Advanced D8 diffractometer (capillary stage)] using Co-Kα radiation.

3. Results and discussion

For laser-induced unzipping of MWCNTs in the VFD, the as-received materials were dispersed in ethanol at a concentration of 0.1 mg mL⁻¹, and the solution then sonicated for 5 min in a bath sonicator prior to VFD processing. Then the solution was injected into the VFD using the continuous flow mode of the device, with the rotational speed at 8k rpm, tilt angle at 45° and injection rate of 0.45 mL min⁻¹ Initially a pulsed laser with a power of 250 mJ and operating at a wavelength of 1064 nm was used to irradiate the VFD tube. A schematic illustrating the unzipping MWCNTs using laser irradiation with the VFD is presented in Figure 1.

Based on our previous findings [34,39], the shear stress within the VFD is effective for slicing CNTs in high yield with control over their length using an immiscible biphasic mixture of o-xylene and water [26]. This was in the absence of laser irradiation, and the slicing is likely to involve contact electrification [47,48]. This is likely to prevail also in the most recent study in the VFD, establishing that shear stress acted as “scissors” to partially and completely unzip tubular structure of MWCNTs suspended in aqueous hydrogen peroxide (30% aqueous solution) which acts as an oxidizing agent. We sort to develop a one-step process for unzipping MWCNTs and producing GNRs in high yield using ethanol as a green solvent. In so doing several parameters, including MWCNT concentration and laser power, must be optimized during the VFD processing. Other operational parameters, such as the rotational speed (8k rpm), the tilt angle (45°) and the flow rate (0.45 mLmin⁻¹), were found to be appropriate for processing carbon nanomaterials within the VFD [39,40,49,50]. Different concentrations of MWCNTs were studied, namely 0.5, 0.25 and 0.1 mg mL⁻¹, as highlighted in the SEM images in Figures S1–S3. The concentration of 0.1 mg mL⁻¹ was effective for complete unzipping of the MWCNTs, with the higher concentrations (0.25 and 0.5 mg mL⁻¹) resulting in partial unzipping. It is important to note that processing materials with a high concentration in the VFD can disturb the high-shear topological fluid flow regimes which are responsible herein for the induced mechanical energy. This contributes to the unzipping on absorption of 1064 nm laser irradiation which serves to increase vibrational energy [49,51]. To optimize the laser power, four different laser powers of 100, 250, 400 and 600 mJ were explored. Pulsed laser irradiated at 250 mJ was the only power effective for completely unzipping the MWCNTs. Other laser powers did not dramatically change the tubular structure of MWCNTs, for both lower power, 100 mJ (Figure S2) and high power, 400 and 600 mJ (Figure S3).

The proposed mechanism for unzipping MWCNTs via laser irradiation with the VFD is illustrated as a scheme in Figure 2. The high-shear stress imparted in this film of liquid in the device and the effect of laser irradiation on the MWCNTs are two distinct processes, but they can act in concert. At high rotating speeds (8k rpm), the shear stress is dominated by the double-helical topological fluid flow in the VFD [36]. At the double-helical flow, the VFD-generated mechanical energy can be used to slice CNTs and unzip MWCNTs depending on the choice of solvent(s). Interestingly, SWCNTs processed in the VFD can be coiled into regular toroidal structures which happen where the double helical flow strikes the surface of the tube, noting that the centrifugal force will accelerate the dense carbon material to the inner surface of the tube [52]. The bending of the SWCNTs here provides stress on the C–C bonds. If at least bending of the MWCNTs used in the present study occurs, then this is possibly the starting point for the unzipping process in the presence of the induced vibrational energy from the pulsed laser. This is indeed the established mechanism for sling SWCNTs [37]. Then why isn't there slicing of the MWCNTs in the present study? Presumably this is a trade-off between the ease of unzipping and exfoliating single shells of the MWCNTs versus slicing involving simultaneous C–C bond cleavage within all the centric rings in the material.

Figure 2. Proposed mechanism of laser-induced unzipping MWCNTs in the VFD in ethanol media using different laser powers: (a) 100 mJ, (b) 250 mJ (optimized conditions) and (c) 400 mJ.

Low laser power (100 mJ) has little effect on the tubular structure of MWCNTs, as determined by TEM (Figure 2a). Increasing the pulsed laser irradiation to 250 mJ provides sufficient vibrational energy to result in C–C bond cleavage and unzipping. The heat generated by the irradiated laser plays a crucial role in unzipping MWCNTs, and when the VFD tube rapidly rotates, the aforementioned defects generated by the laser will expand and the unzipping process will be accelerated (Figure 2b). Increasing the laser power to 400 and 600 mJ results in more defects of MWCNT walls and the formation of irregular shapes, which can be attributed to the damaging of the graphitic structure and the formation of amorphous carbon, as shown in Figure 2(c). These results are in good agreement with our SEM and TEM results.

The unzipped MWCNTs were characterized using SEM, TEM, and HRTEM. The as-received MWCNTs exhibit entangled long tubes ranging in length from 3 to 6 µm and in diameter from 5 to 50 nm (Figure 3a–d). The tubular structure of as-received MWCNTs is evident from TEM and HRTEM images, and their outer diameter is ∼20 nm, as shown in Figure 4(c and d). The morphology of MWCNTs exposed to laser irradiation during VFD processing is dramatically changed, resulting in the formation of unzipped MMWCNTs. Figure 3(c–f) shows SEM images of the stated materials following VFD processing at 8k rpm using 250 mJ of laser irradiation. Low-magnification SEM images display unzipped MWCNTs with some GNRs and some small flat graphene sheets (Figure 3e and f). The low-magnification TEM image (Figure 4c) validates the unzipping process and displays MWCNTs that are completely unzipped and entangled. Figure 4(d and e) depict HRTEM images demonstrating that the width of unzipped MWCNTs is ∼30 nm. Figure 4(f) shows the HRTEM image of multitubes opened and formed unzipping MWCNTs.

Figure 3. SEM images with various magnifications of (a,b) as-received MWCNTs (before processing) and (c–f) GNRs (after processing) using the VFD at optimized conditions with laser power at 250 mJ.

Figure 4. TEM and HRTEM images of (a,b) as-received MWCNTs (before processing) and (c–f) unzipped MWCNTs (after processing) using laser irradiation (at 250 mJ) with the VFD operating at other optimized conditions.

The thermal stability of both materials was examined using TGA (Figure 5a). For the as received MWCNTs (black line), there is no weight loss until 600°C, followed by a significant weight loss between 600°C and 650°C, which is attributed to the decomposition of the MWCNTs [53]. After processing in the VFD with laser irradiation and generating unzipped MWCNTs (red line), the weight loss begins at ∼200°C and decreases significantly due to the loss of oxygen functional groups [54]. The unzipped MWCNTs should become oxidized as a result of the high laser irradiation during the VFD processing, as determined using Raman spectroscopy and XPS, as detailed below.

Figure 5. (a) TGA curves were carried out under nitrogen atmosphere from 50°C to 800°C at a rate of 10°C min⁻¹, (b) Raman spectra and (c) XRD spectra of as-received MWCNTs (before processing) and unzipped MWCNTs (after processing) using laser irradiation (at 250 mJ) with the VFD operating at other optimized conditions.

Raman spectroscopy was used to determine the defect level of the as received and the transformed unzipped MWCNTs. The Raman spectrum displays two bands (D band) at 1330 and (G band) 1576 cm⁻¹, which are associated with disorders in the sp² carbon network and C–C inside the graphite network, respectively [55,56]. To determine the fraction of graphite structure crystallites, we calculated the ratio between the D and G bands. After processing and unzipping the MWCNTs, the average ratio (I_D/I_G) increased from 1.12 in the as-received material (black line) to 1.50 on processing in the VFD. Results demonstrate that laser irradiation with high-shear stress from the device induces additional defects on the surface of MWCNTs, which results in completely unzipped MWCNTs [53].

To further investigate the structure of unzipped MWCNTs, X-ray diffraction (XRD) patterns of the as received MWCNTs and unzipped MWCNTs at 250 mJ were conducted, as shown in Figure 5(c). For the as received MWCNTs, two main strong diffraction peaks at 30° and 51° can be assigned to the (002) and (100) planes, which reflects of the hexagonal graphite lattice [27,57]. Similar peaks can be seen after processing MWCNTs in the VFD at low laser power 100 mJ, indicating no changes to the MWCNTs structure (see Figure S7). However, irradiating at high laser power (400 mJ), Figure S7, results in significant changes to the structure, with no crystalline peaks from the XRD pattern and all peaks for MWCNTs disappeared and the presence of a broad peak at 25.3°, which is clearly confirmed that MWCNTs derived material is amorphous [58], which is in agreement with TEM results. After the MWCNTs were unzipped using 250 mJ laser power “optimum power”, Figure 5c, two main sharp diffraction peaks are observed at ∼22° (001) and ∼34.8° (220), which indicate unzipping of the MWCNTs and the presence of defects [27,59–61]. Other peaks at 27.2° (331), 42.5° (111) and 47.2° (200) can be seen, which may be due to either more oxygen functional groups on the exfoliated layers or the presence of the catalyst on the unzipped MWCNT surface [62]. Note that the graphitic (002) (100) peaks in the as received MWCNTs remains, albeit very weak, which indicates unzipping of the MWCNTs and the fabrication of GNRs [3,18]”.

The surface composition of both the as-received MWCNTs and the unzipped MWCNTs was analysed using XPS. XPS spectra of C 1s are shown in Figure 6. The C 1s spectrum was fitted with four major peaks, as follows: the peaks at ∼284.7 and ∼285.3 eV correspond to carbon C–C and C=C, respectively. The other two peaks C–O at ∼287 eV and C=O at ∼289.2 eV correspond to functional groups of oxygen [63,64]. After unzipping the MWCNTs involving laser-mediated VFD processing, the percentage of C–C decreases from 42.69% to 39.20%, while the percentage of C=C increased from 34.60% to 39.69%. This shows that more functional groups of oxygen and defects are formed on the unzipped MWCNTs during laser irradiation in the VFD (inserted table in Figure 6). Additionally, the intensity of the C–O peak decreases slightly from 12.12 to 10.12, while the percentage of the C=C peak increases slightly from 5.91 to 6.73 after unzipping the MWCNTs. This can be attributed to an increase in oxygen functional groups during oxidation and unzipping when laser irradiated during the VFD processing.

Figure 6. XPS survey spectra and C 1s spectra for (a) as-received MWCNTs (before processing) and (b) unzipped MWCNTs (after processing) using the VFD at optimized conditions with the laser irradiation power at 250 mJ.

4. Conclusion

We have established that pulsed laser irradiation at 1064 nm on the rapidly rotating quartz tube in a vortex fluidic device (VFD) can convert the tubule structure of MWCNTs in an ethanolic suspension into unzipped MWCNTs under continuous flow conditions. This process is convenient using low capital outlay and small footprint device technology, and it is scalable, achieving a high yield of 90%. Additionally, ethanol is used as a dispersion medium for MWCNTs, which can be considered as a green solvent as opposed to harsh chemicals that would contribute to a waste stream.

Acknowledgements

T.M.D.A extends his appreciation to Prof. Colin L. Raston from Flinders University in Australia for his valuable discussions and proofreading this work. T. M. D. A thanks Amjad Alotaibi for assistance with XPS measurements. T.M.D.A. thanks Dr. Ashley Slattery from Adelaide Microscopy for assistance with TEM and HRTEM imaging. The author also thanks Taibah University (Ministry of Education, Saudi Arabia) for help and support and Australian Research Council (DP200101105) and The Government of South Australia for support of this work.

Disclosure statement

No potential conflict of interest was reported by the author(s).