Optical Unzipping of Carbon Nanotubes in Liquid Media - The

Article pubs.acs.org/JPCC

Optical Unzipping of Carbon Nanotubes in Liquid Media Prashant Kumar,*,† Sharma S. R. K. C. Yamijala,‡ and Swapan K. Pati§ †

Department of Physics, Indian Institute of Technology, Patna, Bihta Campus, India 801103 Chemistry and Physics of Materials Unit, JNCASR, Bangalore, India 560064 § Theoretical Sciences Unit, JNCASR, Bangalore, India 560064 ‡

ABSTRACT: Optical unzipping of carbon nanotubes (CNTs) in liquid media is one of the most awaited technologies as it promises instant material transformation from CNTs to graphene nanoribbons (GNRs) and also an easy transfer of GNRs to arbitrary substrates. In the present article, we report the laser-induced optical unzipping of CNTs, dispersed in dimethylformamide (DMF) solvent. In a nutshell, laser unzipping of CNTs dispersed in liquid solvent is a photophysicochemical process where molecular interactions between CNTs and solvent are tuned by the laser irradiation and results in the formation of GNRs in a scalable manner. The proposed mechanism includes the creation of defects together with vacancies upon laser irradiation, followed by their migration toward the energetically favorable axis of the CNTthe longitudinal directionfinally leading to the unzipping/fragmentation of the nanotube. Distinct laser thresholds have been observed for each of the three events, namely, (a) the formation of the first defect, (b) vacancy migration along the longitudinal direction, and (c) fragmentation of CNTs into graphene nanosheets. Our experimental findings of the unzipping process have further been supported by the density functional theory (DFT) and density functional tight binding (DFTB) calculations performed on both single-walled and multiwalled CNTs.

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INTRODUCTION Due to its novel attributes,1−5 graphene has been used in various interesting applications.6−14 However, due to its zero band gap semimetallic behavior, graphene cannot be useful for the semiconductor industry. To circumvent this problem, various strategies such as doping,15,16 electronic tuning in biand trilayer graphene,17,18 sandwiching graphene between BN layers,19and straining,20 etc. have been used to create a bandgap in graphene. Apart from these strategies, recent experiments show that graphene nanoribbons (GNRs) do possess a finite electronic band gap, which can conveniently be tuned by controlling its width and would thus open avenues to fabricate materials with designer physical/chemical behavior.21−23 For example, GNRs’ bandgap can be tuned to make them either semiconducting24 or half-metallic.25 Moreover, due to their sp2carbon lattice and edges states,26,27 GNRs find applications in exhibiting high magnetoresistance,28,29 as electronic devices,30−34 as magnetic field sensors,35 as transparent conducting electrodes,36 and as nanofillers for strengthening polymers.37,38 Various approaches have been developed to date to unzip CNTs to achieve GNRs, and some of the notable ones include: longitudinal unzipping via catalytic oxidation,39 plasma etching of partly embedded CNTs,40 gas-phase oxidation followed by sonication,41 surface-assisted coupling of molecular precursors into linear polyphenylenes and subsequent cyclodehydrogenation,42 self-organized growth on a templated silicon carbide substrate,43 and acid cutting along the folded edges44 via reaction with molecular hydrogen45,46 or by electrochemical unzipping.47 Electric fields have also been proposed to unzip CNTs.48 Most of these techniques have their own limitations © XXXX American Chemical Society

such as (a) production of irregular edges, (b) chemical impurity at the edges, (c) development of pores in ribbons, (d) scalability, (e) reproducibility, and (f) complexity of experimental design. Chemical synthesis, in particular, involving a combination of catalytic oxidation and thermal treatment, has marginal yield (as random defect formation most often triggers fragmentation of graphene sheets), and the obtained GNRs have oxygenated edges. Moreover, chemical unzipping involves several reaction steps which make such strategies cumbersome. Thus, a scalable and reproducible synthetic technology for defect-free GNRs with clean edges is a natural need, and in this article we address this issue using laser fields. Lasers are capable of delivering optical energy in a controllable manner because their direction, beam profile, fluence, pulse width, repetition rate, and wavelength can all be precisely controlled. Thus, they carry tremendous potential for photochemical transformations. Also, the laser-induced unzipping process is expected to deliver relatively pure GNRs because it is devoid of harsh chemicals (like strong acids) and oxidizing catalysts (like KMnO4) (which is a common practice for unzipping CNTs, and it yields relatively impure samples). One of the authors of this article has already demonstrated the use of laser-induced unzipping of CNTs coated on a quartz plate in solid form.49 However, despite being a single-step process, laser-induced unzipping of CNTs in the solid phase is not a scalable process. This is because only those CNTs which Received: March 10, 2016 Revised: July 12, 2016

A

DOI: 10.1021/acs.jpcc.6b02524 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. (a) Schematic depicting the laser-induced unzipping of MWNTs in DMF medium. FESEM images of (b) untreated Sample 1 and those treated by laser at (c) 0.75, (d) 1.00, and (e) 1.25 J/cm2. TEM images (f−i) corresponding to those of (b−e). (j) AFM image of the sample achieved at laser fluence of 1.5 J/cm2. Inset in (j) shows the height profile along one of the graphene sheets (shown in yellow square) achieved due to laserinduced fragmentation. Raman spectra of sample 1 for (k) before and (m) after the laser treatment at the fluence of 1.25 J/cm2. (Scale of FESEM and TEM images: Panel width is marked in each panel separately.) Photographs (see (l) and (n)) show CNT dispersion in DMF and supernatant solution containing laser unzipped graphene nanoribbons, respectively.

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METHODS A. Optical Unzipping of MWNTs to Synthesize GNRs and GNSs. To achieve optical unzipping of CNTs in liquid, the first requirement for the desired solvent is that it should give rise to good dispersion of CNTs and at the same time should not be inflammable under laser irradiation. We have, therefore, investigated various solvents for this purpose. It was observed that CNT dispersion in alcohol, acetone, and alcohol−water mixed solvent is poor, and CNT settles down relatively fast. On the other hand, solvents such as chlorobenzene, dichlorobenzene, and N-methylpyrrolidone (NMP) etc. are inflammable and degrade into amorphous carbon which again is not acceptable as it would deposit on CNTs and will result in impurity. We found that dimethylformamide (DMF) on the one hand gives rise to good dispersion of CNTs and on the other hand is not inflammable with laser irradiation. The relatively higher boiling point of DMF guarantees good dispersion of CNT throughout the laser unzipping process. Moreover, liquid medium like DMF provides a cooling mechanism which helps attain unzipping in a controlled manner. The most important achievement of optical unzipping in liquid media is the relatively broader working window of laser fluence as compared to the unzipping in the solid phase having exhibited a marginal working window, which poses restrictions as far as reproducibility and scalability are concerned. Lambda Physik (model LPX300) KrF excimer laser (λ = 248 nm, τ = 248 nm, beam area 96 mm2) was used for irradiation of CNTs dispersed in liquid media (DMF works best) in a quartz vial, and the dispersion was magnetically stirred while the laser irradiation experiment was carried out. The details of the

are at the surface will attain laser exposure (and, hence, will unzip), and the majority of CNTs underneath them will remain unexposed and therefore will not undergo the unzipping process leading to the problem of scalability. Further, substrate transfer is a challenge and always involves material loss. Thus, a laser unzipping process in the liquid phase for which a laser has 3D access would improve efficiency and is expected to be a scalable process. Since laser unzipping of CNTs in liquid media does not require any additional chemical treatment with or without external heating, it is expected to result in a green synthetic approach. Therefore, it is imperative to understand this emerging procedure and its potential for scalable production. In this article, we demonstrate laser-induced optical unzipping of multiwall CNTs in liquid media (see Figure 1a). Among various liquid media examined, dimethylformamide (DMF) solvent has been found to be the most suitable; therefore, the influence of laser and CNT diameter on the nature of products has been investigated mainly in this solvent. Scalability and reproducibility of the process have thoroughly been examined. Transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), and atomic force microscopy (AFM) are employed for morphological characterization as well as Raman spectroscopy to assess chemical changes. Lastly, we interpret the process of laserinduced unzipping of multiwalled nanotubes (MWNTs) by density functional theory (DFT) and density functional tight binding (DFTB) calculations. B

DOI: 10.1021/acs.jpcc.6b02524 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Table 1. Summary of Formation Energies for Vacancies system

abs. energy (eV)

carbon atom (7, 7) v1_(7, 7) v2a_(7, 7) v2b_(7, 7) v3_(7, 7) (15, 15) v1_(15, 15) v2a_(15, 15) v2b_(15, 15) v3_(15, 15) (26, 0) v1_(26, 0) v2a_(26, 0) v2b_(26, 0) v3_(26, 0) (7, 7), (16, 16) v1_(7, 7), (16, 16) v2_(7,7), (16, 16) v3_(7,7), (16, 16)

−38.0549 −14374.083 −14319.082 −14267.436 −14266.623 −14217.437 −30846.978 −30788.167 −30736.183 −30734.214 −30685.726 −29163.451 −29099.805 −29047.375 −29047.147 −28994.849 −47272.573 −47218.904 −47166.822 −47116.296

EForm (eV) = Evn + (nEcarbon) − Epristine

Ecum (eV) = Evn − (nEv1)

Evn − Epristine (eV)

16.947 30.537 31.350 42.481

−3.356 −2.543 −8.359

55.002 106.647 107.460 156.646

20.756 34.685 36.655 47.087

−6.827 −4.857 −15.181

58.811 110.795 112.784 161.252

25.592 39.966 40.194 54.438

−11.218 −10.990 −22.339

63.646 116.076 116.304 168.602

15.614 29.641 42.112

−1.586 −4.729

53.669 105.751 156.277

instrument fitted with Gatan CCD camera operating at an accelerating voltage of 300 kV, and atomic force microscopy (AFM) was carried out in an Innova atomic force microscope. Raman spectroscopy was achieved in a JobinYvonLabRam HR spectrometer with 633 nm Ar laser. D. Modeling. Even though the experiment has been carried out with MWNTs of larger diameters, to avoid computational difficulties, we have considered thinner nanotubes. Also, to understand various results, we modeled SWNTs instead of MWNTs as the latter ones are computationally highly demanding. However, we have also performed calculations on MWNTs to verify our findings on SWNTs. Since it is well-known that the properties of SWNTs depend on both their diameters and chiralities,51 we have modeled our SWNTs considering these parameters. Accordingly, three NTs with chiralities (7, 7), (15, 15), and (21, 0) have been considered. Here (7, 7) and (15, 15) are armchair nanotubes (and, hence, metallic) with diameters of approximately 1 and 2 nm, respectively. On the other hand, (21, 0) is a semiconducting zigzag NT with diameter similar to that of (15, 15). For modeling MWNTs, we have considered a double-walled nanotube, where the inner and outer tubes have the chirality (7, 7) and (16, 16), respectively, and the intertube spacing has been kept as 6 Å (this is the typical spacing between NTs observed in our experiments as evidenced in HRTEM images). To model defects created in SWNTs by a laser pulse, we have created vacancies (only along the direction of the tube axis51) in SWNTs by directly removing the carbon atoms from the hexagonal lattice. To know whether there is any cumulative effect during the formation of vacancies, we have gradually increased the vacancy number from 1 to 3. Finally, to know the favorable positions of vacancies with respect to each other, in a multivacant SWNT, we have varied the position between the vacancies. E. Computational Methods. To understand how vacancy defects affect the single-walled (SW) and multiwalled (MW) carbon nanotubes (NTs), we have performed our calculations using both density functional tight-binding (DFTB) theory and density functional theory (DFT). A majority of the calculations

experimental setup have been depicted in a schematic diagram in Figure 1a. Typically, 1 mg of CNT sample was dispersed in 5 mL of DMF solvent. Laser fluence was varied for different samples, and the maximum laser fluence used for the purpose was 2.5 J/cm2. For each experiment at different laser fluence values, separate quartz vials with the same physical dimensions were used, and the same volumes of the dispersion with the same concentration of CNTs were used. Each sample was laserirradiated for 30 min at the laser pulse rate of 5 Hz. Two samples of CVD grown multiwalled CNTs (NanoLab Inc., USA) of average diameters 7 nm (sample 1) and 18 nm (sample 2) were used for the purpose. The number of layers in two samples used by us was 6 and 13, respectively, as was observed in TEM images. Laser fluence was varied for each experiment to explore exactly at what laser fluence one achieves unzipping and at what laser fluence one would achieve graphene nanosheets instead of nanoribbons. Magnetic stirring helps in homogenizing the solute, while laser irradiation is carried out and thus makes the process reproducible and scalable as well. B. Sample Preparation for Microscopy. After optical unzipping, we find the formation of GNRs with a varied number of layers. Upon centrifuging at appropriate speed and time, one can isolate sheets with a lower number of layers. The GNR and GNS samples dispersed in DMF were centrifuged at 2500 rpm for 5 min, and the supernatant was then used for spin coating on thoroughly cleaned silicon substrates and TEM grids. Highly polished silicon (100) substrates purchased from MTI Corporation were cleaned by a routine cleaning procedure. In brief, the substrates were rinsed in acetone first and then boiled in IPA at 80 °C for 20 min and washed in DI water followed by flushing with N2 gas. Spin-coated samples were adequately dried (first vacuum-dried for a few hours and then dried with a hair dryer at 150−200 °C) on cleaned silicon substrates and stored in vacuum desiccators. C. Microscopy and Raman Spectroscopy. Field emission scanning electron microscopy (FESEM) was carried out in FEI NOVANANOSEM 600. Transmission electron microscopy (TEM) was carried out in a JEOL JEM3010 C

DOI: 10.1021/acs.jpcc.6b02524 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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and does not get carbonized upon laser exposure. Therefore, detailed experimentation with laser fluence and CNT diameter was carried out only with DMF solvent. Laser-irradiated multiwall CNTs dispersed in DMF medium were observed to be stable for 3−4 days. Interactions of carbon nanotubes with solvents play a crucial role in preventing CNTs from bundling. Favorable CNT−solvent interactions decrease CNT−CNT coupling and effectively minimize the enthalpy of mixing.50−52 Nonirradiated CNT dispersions in DMF are black (see Figure 1(n)). The supernatant liquid achieved after centrifugation of the laser-irradiated dispersion is yellow in color (see Figure 1(l)) and stable even after a month. The FESEM image in Figure 1(b) shows uniform diameter CNTs in Sample 1 (MWNTs of 7 nm diameter) before laser irradiation. With small variations in laser fluence, significant changes in morphology of the product are observed. When the dispersion was laser-irradiated at 0.75 J/cm2 at a 5 Hz pulse rate for 30 min, some cross-sectional modifications were observed as shown in Figure 1(c). In another laser irradiation experiment, at 1 J/cm2 fluence, even more prominent effects were apparent in the resultant sample, and most locations were observed to have already been longitudinally unzipped (see FESEM image in Figure 1(d)). Similarly, in a separate experiment with 1.25 J/ cm2 laser fluence, complete unzipping occurred (see FESEM image in Figure 1(e)). Figures 1(f)−1(i) display TEM images corresponding to Figures 1(b)−1(e) and reveal more details of the walls and cross-section variations of laser-irradiated samples. Wall thinning can clearly be seen in Figure 1(h). At 1.5 J/cm2 laser fluence, laser irradiation of the dispersion containing sample 1 becomes fragmented into graphene nanosheets as shown in the AFM image of Figure 1(j). Graphene nanosheets so obtained have average lateral dimension of 25 nm and are typically as thin as 2−3 atomic layers of carbon. Thus, in this particular case of 1.5 J/cm2 fluence, unzipping and exfoliation both occur, and it demonstrates that the present technique can be used even for achieving a single-layer graphene nanoribbon starting from multiwalled CNTs. Raman spectra in Figure 1(k) and 1(m) correspond to sample 1 before and after laserinduced unzipping. The radial breathing modes that were present earlier in the nonirradiated sample disappear after laser unzipping. The absence of radial breathing modes in Raman signatures of CNTs provides clear evidence of unzipping and supplements physical characterization in terms of the multifold increase in the cross-sectional profile of the product as revealed by FESEM/TEM images. Also an increase in ID/IG ratio occurs with unzipping. When sample 2 (MWNTs of 18 nm diameter) dispersed in DMF medium was laser-irradiated, we observed similar findings (distinct laser fluence thresholds for unzipping and fragmentation) as observed for sample 1. The major difference was in the laser fluence values needed for unzipping. FESEM (top-left 3 images) and TEM (bottom-left 3 images) images for original nonirradiated sample 2 and those laser processed at laser fluences 1.5 J/cm2 and 2.00 J/cm2 are shown in Figure 2. The image shown in Figure 2(a) and (d) exhibits diameter homogeneity of CNTs across the length. The nonuniformity of diameters of CNTs upon laser processing suggests optical unzipping. The TEM image in Figure 2(g) and AFM image in Figure 2(h) show tiny graphene sheets achieved when sample 2 was laser-irradiated at 2.5 J/cm2. A large lateral size distribution for graphene nanosheets is observed. One typical graphene sheet was found to be 40 nm laterally wide and have thickness of 1.2 nm (i.e., 3 atomic layers of carbon). As compared to

have been performed using self-consistent charge DFTB (SCCDFTB) 68 within third-order expansion of the energy (DFTB3)69 and with 3ob parameter set, as implemented in the DFTB+ package.70 The DFTB level of theory is used mainly due to the large system sizes (>1000 atoms) considered here. Geometry optimizations have been performed using the conjugate gradient method, and systems are considered to be optimized only when forces on all the atoms are less than 0.0001 Ha/Bohr. A 1 × 1 × 15 k-point grid has been considered within the Monkhorst−Pack scheme, for all the SWNTs. For MWNTs, we were able to afford only the gamma point calculations. An electronic temperature of 100 K is kept for all the calculations, to avoid convergence issues. For the case of (7, 7) SWNTs, we have compared the results from the DFT with that of DFTB3, and the results are comparable (exact) in the trends and almost the same when the relative energies are considered (which are the relevant ones for the present study) as can be seen from Table 1 and 2. VMD71 and CoNTub72 are used to generate the coordinates for the single-walled and multiwalled nanotubes, respectively. VMD is also used to create a few images. Table 2. Seista Results system

abs. energy (eV)

EForm (eV) = Evn + nEc − Eprist

Ecum (eV) = Evn − nEv1

Evn − Eprist (eV)

carbon atom (7, 7) v1_(7, 7) v2a_(7, 7) v2b_(7, 7) v3_(7, 7)

−151.588 −47649.222 −47485.915 −47326.054 −47325.398 −47168.143

11.715 19.992 20.649 26.316

−3.438 −2.781 −8.829

163.303 323.168 323.824 481.079

All the DFT calculations have been performed within the generalized gradient approximation (GGA), considering the Perdew−Burke−Enzerhof (PBE) exchange and correlation functional and using the double-ζ polarized basis set (DZP) for all atoms as implemented in the SIESTA package. Normconserving pseudopotentials in the fully nonlocal Kleinman− Bylander form have been considered for all the atoms. A real space mesh cutoff of 300 Ry and a Monkhorst k-point grid of 1 × 1 × 5 have been used. Systems are considered to be relaxed only when the forces acting on all the atoms are less than 0.04 eV Å−1.

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RESULTS AND DISCUSSION Electronic properties of GNRs are determined by the smoothness of edges (physical aspect) and attached functionalities (chemical aspect). The laser unzipping process features experimental control parameters including (a) CNT parameters (tube diameter, tube length, number of atomic layers in a tube, its chirality which determines metallic/semiconducting nature, and defect density), (b) solvent parameters (density, transparency, polar/nonpolar nature, dispersibility, and inflammability), and (c) laser parameters (wavelength, fluence values, and polarization). The unzipping procedure studied in this manuscript is primarily for multiwalled nanotubes. As the solubility of CNT depends on the kind of interaction it has with the solvent, we have considered various solvents. Among them, colorless solvents, namely, chlorobenzene and dichlorobenzene, are observed to give black carbon upon laser exposure, while yellowish NMP solvent was observed to turn into red. DMF has been found to be the best solvent which disperses CNTs D

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NTs seems to be a feasible reason. Table 1 summarizes absolute and relative energies of SWNTs and MWNTs with and without defects as obtained by our calculations. Symbols Epristine and Evn denote the energy of a nanotube in its pristine state and when it has “n” vacancies, respectively. Ecarbon is the energy of the single carbon atom. EForm and Ecum are the formation and cumulative energies. As observed in the present investigation, there are thresholds of laser energies where the first defect forms and where defect migration (crack propagation) starts which eventually leads to unzipping. At even higher laser fluence thresholds, CNTs fragment into pieces. We note that laser processing is very delicate, providing a very small window of laser fluence for achieving GNRs; yet, if one carries out laser irradiation with the correct laser fluence inside the favorable laser fluence window, one achieves a clean way of unzipping, producing relatively pure GNR samples having soft edges. Also, graphene nanosheets (GNS) achieved by this technique are significant as one can in principle start with multiwalled CNTs and end up with GNS having 1−3 atomic layers. Such GNS would find applications in catalysis, light emission, and many other applications. GNSs have a finite electronic band gap due to its size, and the band gap can further be tuned by edge functionalization. It should be noted that the metallic/ semiconducting nature of CNTs would also affect the coupling of laser to the CNTs. In fact, our calculations also show different formation energies for armchair and zigzag nanotubes of similar diameters (see (15, 15) and (26, 0) NTs) with the same number of vacancies. It should be noted that metallic CNTs respond differently to oxidation under laser irradiation, which should also affect the unzipping process.53 CNT−laser interaction has earlier been exploited,54,55 and the role of defects and surface functionalities in laser−CNT interactions has also been studied.56 The diameter dependence of laser-induced damage for SWNTs has been studied by atomistic simulations earlier,57 although not specifically for unzipping. Also, the band gap of CNTs is diameter-dependent,58and therefore CNT’s response to laser light with a particular wavelength is diameter-dependent. The chirality of CNTs can affect the intermediate stages of unzipping.59 Also, the location for triggering the unzipping process has been an issue for quite some time. As of now, it is established that such trigger sites may be located anywhere, either at the tip or midway, and such sites are most probably determined by the defect profile which ignites hot spots. Potassium vapor has been used for unzipping, where random site unzipping was reported by Kosynkin et al.62 This technique has now been established to attain graphene nanoribbons free from oxygen functionalities which is needed for clean device fabrication. Qualities of GNR edges have also been studied earlier.60 Thus, apart from laser fluence, which of course is a major parameter, there are several other factors determining the laser−CNT interactions which in turn affect the laser-induced unzipping phenomenon. In the present work, sample 1 after unzipping gives GNRs of approximately 22 nm width when the laser fluence was 1.25 J/ cm2 at 5 Hz pulse rate for 30 min. Sample 2 after unzipping gives rise to GNRs approximately 47 nm wide at a fluence of 2 J/cm2 at 5 Hz pulse rate for 30 min. Our observation of a 3-fold increase in the sample width for sample 1 and nearly the same for sample 2 clearly represents unzipping and not flattening.61 In our experiments, 9000 total laser pulses have been delivered to samples. Laser fluence values were 1.25 J/cm2 and 2 J/cm2 for unzipping of CNTs in sample 1 and sample 2, respectively.

Figure 2. FESEM images [(a), (b), and (c)] and corresponding TEM images [(d), (e), and (f)] for multiwall CNTs (sample 2) before laser processing and for those laser treated at 1.50 and 2.00 J/cm2. FESEM image (g) and AFM image (h) for multiwall CNT laser treated at 2.50 J/cm2. Inset shows the line profile of the tiny graphene sheet shown in the yellow square.

CNT unzipping by laser irradiation in solid medium,49 more laser fluence is required to unzip it in liquid medium. This result is expected as the liquid medium absorbs laser energy and also cools the CNT surface to quench the local hot spots developing at adsorbed sites due to laser irradiation. Therefore, to achieve laser-induced unzipping, more laser fluence is required to cross the energy barrier for achieving local hot spots that in turn trigger the longitudinal unzipping of CNTs. DMF, being a polar solvent, interacts with the CNTs and therefore enables excellent dispersion of CNTs. DMF−CNT interactions are expected to help in the unzipping process. We have observed that thinner CNTs require less laser fluence to become unzipped. Such an observation can intuitively be attributed to fewer walls, which in turn require less laser fluence for propagation of laser energy to the interiors of CNTs. Also, it is expected that higher in-built strain would be present in thinner CNTs as compared to the thicker CNTs. To gain an in-depth understanding of the unzipping process, we have performed both DFT and DFTB calculations for the energetics of vacancy formation and its migration in SWNTs and MWNTs. It should be noted that the nanotubes considered for calculations are thinner than those of the actual experimental sample. However, as the vacancy formation and accumulation and hence crack propagation occur at the nanotube surface only, we expect that thicker nanotubes would exhibit similar trends. Relative energies of all nanotubes considered in this study, with and without vacancies, by DFTB, are given in Table 1, which reveals that increased nanotube diameter (for example, see (7, 7) ∼ 1 nm and (15, 15) ∼ 2 nm) produces a clear increment in the energy required to create a defect (EForm), corroborating the experimental results. These wider nanotubes have less curvature and greater overlap between the pz-orbitals on the adjacent carbon atoms (maximum overlap occurs when the orbitals are parallel to each other, as in the graphene sheet). Less curvature leads to the formation of stronger bonds locally, compared to a narrower nanotube. Compromised orbital overlap in thinner E

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The Journal of Physical Chemistry C The total energies delivered to a 1 cm2 area would thus amount to 11 250 and 18 000 J for sample 1 and sample 2, respectively. However, since the experiment is carried out in liquid medium, CNTs can move macroscopically due to magnetic stirring and are expected to experience recurrent laser exposure due to this motion. In comparison to laser-induced unzipping of CNTs in solids, unzipping in liquid thus requires more laser energy to be delivered. However, it has the advantage of heat being dissipated via liquid. Also each CNT when irradiated by a laser beam while moving in the liquid has marginal chance of destruction (fragmentation) at moderate laser fluence. Thus, liquid media provides a comfortably larger working window of laser fluence. Also, material delaminations from the substrate and ignition/oxidation are effectively prevented. The GNR edges achieved by laser unzipping of CNTs in liquid medium are significantly smooth as shown in Figure 2(d). After unzipping of MWNTs, solvent contains multilayered graphene nanoribbons, which upon further laser exposure can exfoliate. It should be noted that a laser itself has recently been reported to exfoliate the graphite sheet.63,64 To our surprise, we indeed observed unzipping as well as exfoliation both occurring simultaneously, resulting in fewer atomic layers in GNRs as compared to the original CNTs. The laser-induced unzipping technique thus provides a way to achieve graphene nanosheets having 1−3 atomic layers. Liquidphase optical unzipping can thus be helpful in device fabrication on substrates of arbitrary choice, as the transfer process is facile for nanoribbons achieved by the present technique. GNSs of 5−10 nm diameters, which have been achieved in the present experiment, may also be useful for biosensing applications.65 To gain a better understanding of the interaction of the laser field (we apply static electric field) with CNT with or without DMF, we have optimized energetically favorable relative configurations of DMF molecules when adsorbed on CNTs. Laser would strongly interact at locations where DMF is adsorbed as it would provide enhanced local charge density (Eext = −e*E*r). Interestingly, with even up to 105 V/m electric field value, no significant changes in energetics were found for either CNT or for CNT+DMF. The calculated gains in energies are −1.3195, −2.0489, and −2.4099 eV for a CNT, CNT+DMF, and CNT+V1+DMF, respectively, when these systems are placed in extreme electric field considered in our study, i.e., 109 V/m. It should be noted that even though the average electric field in the laser beam is in the order of 106 V/ m the local electric field at DMF adsorbed sites would be 3−4 orders of magnitude higher as charge clusters at the interface would not allow swift passage of laser; instead these charge clusters would rather strongly distort the local energy landscape. Carbon atoms close to DMF therefore would become unstable. Charge distribution themselves changes under application of strong electric field (due to laser exposure). Charge density difference calculated for CNT+DMF upon application of electric fields is shown in Figure 3(a) and (b) for 105 and 109 V/m, respectively. It is apparent that the influence of electric field is minimal up to 105 V/m, and one can visually observe clear changes in the charge distribution at field as high as 109 V/m. Thus, under laser exposure, DMF and CNT interact more strongly. This strong interaction with laser (electric field) can create defects in the nanotubes at solvent adsorbed sites. We further calculated charge density distribution for CNT+DMF+V1 at 105 V/m and 109 V/m fields (as shown in Figure 3(c) and (d), respectively). It seems that there

Figure 3. Charge density difference calculated for the CNT+DMF system at (a) 105 V/m and (b) 109 V/m and for the CNT+V1+DMF system at (c) 105 V/m and (d) 109 V/m.

is a dramatic influence of just a single vacancy on charge distribution when the CNT+DMF system is placed in a 109 V/ m field. This dramatically enhanced charge density caused by the formation of a single vacancy visually demonstrates that formation of vacancies becomes more and more favorable as its number increases as the process of unzipping progresses. This proves that DMF solvent and laser field together act as an excellent catalyst, which works quite well in such a laser-based green chemistry approach to synthesize graphene nanoribbons. Avoiding the use of chemical catalyst and other reagents guarantees enhanced purity of the product. Further propagation of the defect is more favored along the longitudinal direction, as longitudinal unzipping will not change the alignment of the induced dipole in the CNT (see Figure 3(e) and (f)). Among the local defects in CNTs generated by laser irradiation, Stone−Wales defects (5−8−5, 5−7, and 5−7−5− 7 rings instead of 6−6 rings) and vacancies are the primary ones. A carbon atom vacancy in graphene for example breaks three strong bonds in the 2D network and requires 7.8 eV.66 The likelihood of formation of such vacancies while CNT growth takes place in chemical vapor deposition is quite low but cannot be ruled out a priori. Divacancy energy is ∼9 eV. Thus, formation of a divacancy is much easier than isolated monovacancies. Similarly, trivacancies would cost even less energy as compared to that required to form three isolated monovacancies. Our calculations on SWNTs and MWNTs establish that the energy required to create a multivacancy (Evn) in a nanotube is not just equal to the energy required to create the same number (n) of isolated single vacancies (Ev1) times the number of vacancies (n); rather, it is always less (i.e., Evn < n × Ev1). This is true even when the position of the defect changes; for example, see v1 versus v2a and v2b for any F

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that if we combine all these results we conclude that under favorable conditions longitudinal unzipping of nanotubes occurs (Figure 4i), and for even higher laser intensity, CNTs would fragment into tiny graphene nanoflakes (Figure 4j). It is not that such unzipping has occurred at one place or in one trial. We have carried out series of experiments on how different conditions impact the unzipping process. Every time we have observed a similar outcome for the same set of conditions. For example, at 1.00 J/cm2, when we attempted to reproduce unzipping for the 7 nm diameter nanotube, we achieved similar nanostructural features in generated material. We have earlier attempted an experiment at lower pulse rate (3 Hz) and found that lower pulse rate needs longer duration for unzipping, and the yield is less in that case. In our attempt to unzip at higher pulse rate at 7 Hz, we found that rupture starts to develop at the same laser fluence. This was the reason that we did more detailed experiments at the 5 Hz pulse rate.

nanotube (see Table 1). This suggests cooperative effect while creating defects in a CNT. We believe that this cooperative effect is manifested in the form of crack propagation under laser exposure. At this point of discussion, it should be noted that a monovacancy or bivacancy results in chemically active edges inside the sp2 network. Such vacancy defects are vulnerable for possible reconfiguration to Stone−Wales defects, if annealed in equilibrium condition. Similar results have also been observed in our calculations, where we found that the annealing of vacancies forms 5−9 rings, though not planar, as shown in Figure 4c, d, and e, for single, double, and triple vacancies, respectively.

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SUMMARY AND CONCLUSION In conclusion, optical unzipping of CNTs in liquid media to achieve GNRs has been demonstrated comprehensively for the first time. Such optical unzipping is facile and scalable. It is relatively easy to transfer the produced GNRs to functional substrates. Thinner CNTs require less laser fluence for unzipping. Laser irradiation above some higher threshold yields tiny graphene nanosheets. Such a single-step optical technique for the green synthesis of graphene nanoribbons in scalable manner may open new avenues for materials design and inspire novel applications. DFT and DFTB calculations on vacancy formation and the influence of laser on charge density distribution at the CNT/DMF interface strongly support experimental findings. Thus, the present research provides a novel strategy toward the green synthesis of GNRs, and we strongly believe that variants of this technique can be employed to unzip nanotubes of inorganic analogues of graphene as well.

Figure 4. (a), (b) Front, side views of MWNT; (c), (d), and (e) top views of MWNT with a single, double, and triple vacancy, respectively. 5- and 9-membered rings have been highlighted with pink and light green, respectively. Localized defect formation (f), defect migration and alignment along longitudinal direction (g), and complete unzipping (h). Above a laser intensity threshold tiny graphene nanosheets form (i).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Phone: +91 0612 302 8141. Mobile: +91-9102830953.

Extremely intense laser irradiation would give rise to nonequilibrium processes. In CNTs, the vacancy migration barrier is only 1 eV, suggesting the mobility of vacancy at relatively low temperatures.67 Periodic local energy bursts (after every 1/5 s, as repetition rate in our laser is 5 Hz) would locally accumulate heat and would catalyze vacancy migration. Empirically, therefore, single defects (Figure 4g) would migrate longitudinally and merge (Figure 4h) and grow into larger voids, which would eventually align longitudinally, giving rise to crack propagation in this direction and hence unzipping (Figure 4i), as these are the locations with equivalent sets of physical and chemical conditions. To gain deeper insights, we went ahead with calculations for energies. Even if the vacancies reconfigure to form 5−9 defects (though it may not occur due to the nonequilibrium conditions), as observed in our equilibrium calculations on MWNTs as well as on SWNTs, they all will align linearly (see DFTB results in Table 1 and Siesta results in Table 2). In Figure 4a and b, the formation of a crest is obvious, which forms to release in-built longitudinal strain developed upon laser exposure. Required energy for the formation of vacancies is minimal, when vacancies are longitudinally aligned, which is supported by MD study on the unzipping of MWNTs under stress.51 The main point is

Author Contributions

PK conceived the idea and performed all the experiments. SSRKCY carried out DFT and DFTB calculations under the supervision of SKP. PK wrote the experimental part of the paper. SSRKCY and SKP wrote the theoretical part. All the authors revised the combined version of the paper. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors would like to acknowledge financial support from Science and Engineering Research Board, Dept. of Sci. and Tech., Govt. of India in the form of The Ramanujan Fellowship (sanction no. SB/S2/RJN-205/2014). SSRKCY thanks TUECMS for computational facilities.

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