Revisiting the Mechanism of Oxidative Unzipping of Multiwall Carbon

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Revisiting the Mechanism of Oxidative Unzipping of Multiwall Carbon Nanotubes to Graphene Nanoribbons Ayrat M. Dimiev, Artur Khannanov, Iskander Vakhitov, Airat Kiiamov, Ksenia Shukhina, and James M. Tour ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b01617 • Publication Date (Web): 26 Mar 2018 Downloaded from http://pubs.acs.org on March 27, 2018

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Revisiting the Mechanism of Oxidative Unzipping of Multiwall Carbon Nanotubes to Graphene Nanoribbons Ayrat M. Dimiev,a* Artur Khannanov,a Iskander Vakhitov,b Airat Kiiamov,a,b Ksenia Shukhina,a and James M. Tourc* a

Laboratory for Advanced Carbon Nanomaterials, Kazan Federal University, and bInstitute of

Physics, Kazan Federal University, Kremlyovskaya str. 18, Kazan 420008, Russian Federation; c

Departments of Chemistry and Materials Science and NanoEngineering, Smalley-Curl Institute and the NanoCarbon Center, Rice University, 6100 Main Street, Houston, Texas, 77005, USA

Corresponding authors: e-mail: [email protected] (A. M. Dimiev); [email protected] (J. M. Tour)

Abstract Unzipping multiwall carbon nanotubes (MWCNT) attracted great interest as a method for producing graphene nanoribbons (GNR). However, depending on production method, the GNRs have been proposed to form by different mechanisms. Here, we demonstrate that the oxidative unzipping of MWCNTs is intercalation-driven, not oxidative chemical-bond cleavage as was formerly proposed. The unzipping mechanism involves three consecutive steps: intercalation-unzipping, oxidation, and exfoliation. The reaction can be terminated at any of these three steps. We demonstrate that even in highly oxidative media one can obtain nonoxidized GNR products. The understanding of the actual unzipping mechanism lets us produce GNRs with hybrid properties varying from non-oxidized through heavily oxidized materials. We answer several questions such as the reason for the inner-most walls of the nanotubes remaining zipped. The intercalation-driven reaction mechanism provides a rationale for the difficulty in

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unzipping singlewall and fewwall CNTs, and aids in a reevaluation of the data from the oxidative unzipping process.

Keywords unzipping multiwall carbon nanotubes, graphene nanoribbons, mechanism, intercalation

Graphene, a two-dimensional lattice of carbon atoms, has attracted enormous interest from a broad base of the research community for more than one decade.1,2 Graphene nanoribbons (GNRs), narrow strips of graphene, being quazi one-dimensional, possess complementary features relative to their two-dimensional counterpart of graphene sheets. Based on theoretical calculations, GNRs’ electrical properties can be controlled by the width and edge configuration and they can vary from being metallic to semiconducting.3,4 The physical properties of the GNRs depend significantly on the size and number of layers, which in turn depend on their synthesis method. There are three major approaches for synthesis of GNRs: cutting graphene by different lithographic techniques;5-7 bottom-up synthesis from polycyclic molecules;8,9 and unzipping of carbon nanotubes (CNTs).10-13 While the bottom-up method provides a route to precise edge control, and the lithographic method can afford GNRs with precise placement, the unzipping method has the advantage of mass-production on a large scale. The GNRs from unzipping have been successfully used in numerous applications from transparent conductive films14 through electrode materials for lithium-ion batteries.15 These types of GNRs are produced commercially.16 MWCNT unzipping methods can be classified into four major types: the reductiveintercalation-assisted

approach,10-12 the oxidative unzipping,13,17-23 the electrochemical


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unzipping,24,25 and the group of methods that we denote here as "miscellaneous".26-28 The first approach is based on the well-known ability of alkali metals to intercalate graphite with expansion in the Z-axis direction. Being applied toward MWCNTs, such lattice expansion induces extreme stress within the concentric walls, resulted in the bursting, or longitudinal opening, of the tubes. Lithium metal10 and potassium metal11,12 have been used as the intercalants. The resulted GNRs are highly conductive, but they remain multi-layered and foliated. Due to the attraction between the surfaces, they do not exfoliate to single-layer ribbons. The oxidative approach13,17-21 involves treatment of MWCNTs in acidic oxidative media with the formulation almost identical to that used in production of graphene oxide (GO) from graphite by the Hummers method. The resulting product is graphene oxide nanoribbons (GONRs). Unlike GNRs obtained by the reductive-intercalation method, GONRs easily exfoliate in aqueous solution, and they can be obtained as single-layered structures.13 GONRs have a chemical composition and structure similar to that of GO, for example they are not electrically conductive in their as-prepared state. A reaction mechanism for oxidative unzipping was proposed by Kosynkin et al.13 Invoking the classical oxidation of the alkenes by permanganate in acids, the first step is the formation of manganate ester on a C-C bond, and the second step is the rupture of the C-C bond with formation of ketones at the newly formed edges. This mechanism was further developed in the theoretical work by Rangel et al.21 The original synthesis13 spawned numerous studies on oxidative unzipping of MWCNTs.17-19,22,23 In particular, the role of the oxidizer/CNT ratio, temperature and time of reaction on the yield and morphology of the asobtained GONRs was studied.23 Similarly, Cruz-Silva et al. investigated the effect of the abovementioned factors on unzipping of N-doped MWCNTs.22 Xiao et al. demonstrated that the degree of unzipping depends on the time of reaction. Based on these observations, the authors


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specified four steps for the reaction: surface etching, partial unzipping, full unzipping, and finally fragmentation and aggregation.17 In many reports, the unzipping process was denoted as "chemical" as opposed to the "intercalation-exfoliation",22 indicating that the permanganateinduced oxidative mechanism has been commonly accepted, and was even suggested toward unzipping SWCNTs.20,21 Besides intercalation-driven and oxidative unzipping, there were several studies reporting successful unzipping of MWCNTs by different approaches. Among them are laser and microwave irradiation methods in ethanol and ionic liquids,26,27 electrochemical unzipping,24,28 and even thermolysis in aqueous solutions of simple inorganic salts.25 The peculiarity of these reports is the absence of an apparent driving force for unzipping from the perspectives of the reaction mechanisms discussed in this work. None of them used chemicals that have oxidative character, and none of them used conditions that were reported to intercalate graphite. We recently demonstrated29 that formation of GO from graphite involves three consecutive steps: (a) intercalation of graphite by sulfuric acid with formation of a stage-1 H2SO4-graphite intercalation compound (GIC); (b) conversion of stage-1 H2SO4-GIC into pristine GO, i.e. formation of covalent C-O bonds on graphene's basal planes, and (c) exfoliation of GO to single-layer sheets upon exposure to water. Thus, under given conditions, formation of stage-1 H2SO4-GIC is unavoidable for any graphitic material. Subsequently, the mechanism of the oxidative unzipping of MWCNTs might be also intercalation-driven. If this is correct, we might be able to stop the reaction after the first intercalation-unzipping step before the second oxidation step proceeds. If attained, this will afford unzipped but not oxidized or minimally oxidized products possessing properties similar to reductively unzipped GNRs obtained by potassium or sodium-potassium metal intercalation.11,12 In this work we investigate the impact of


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the two key parameters, the KMnO4/MWCNT ratio, and the time of reaction on the structure and composition of as-obtained GNR products, and we derive a revised and more complete understanding of the unzipping process.

Results and Discussion It is known that three to four wt equiv of KMnO4 are needed to convert one wt equiv of graphite to GO.30 The use of the four wt equiv of KMnO4 results in fully oxidized GO with a density of the oxygen functionalities sufficient to afford spontaneous exfoliation to single-layer GO sheets upon gentle stirring in water.30 GO, prepared with three wt equiv of KMnO4, is less oxidized; sonication is normally required to exfoliate such GO in water. It was welldemonstrated by electrochemical oxidation of graphite in sulfuric acid that at relatively low potentials, several stages of GICs consecutively form without chemical oxidation of graphite; substantially higher potentials are required to form covalent C-O bonds.31-33 The small amount of KMnO4 should be sufficient to change the redox potential of the acid solution to the level needed for intercalation, but not sufficient to covalently oxidize the GNRs. To check this hypothesis we used as little as 0.06, 0.12, 0.50, and 1.00 wt equiv of KMnO4. The as-obtained GNRs will be referred to as 0.06-GNR, 0.12-GNR, 0.50-GNR, and 1.00-GNR, respectively. Importantly, we investigated not only purified products, but even the MWCNTs while they were still in the acidic mixture. This was needed to confirm intercalation. The Raman spectra were acquired from MWCNTs through the acid layer using a technique we had developed earlier for similar applications.29,34 The spectra, acquired from MWCNTs, while in acidic mixtures, are completely different from those acquired from intact MWCNTs as dry powders (Figure 1).


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For the sample prepared with 0.06 wt equiv KMnO4, the spectrum contains the highintensity G-band, and the low-intensity 2D-band (Figure 1a). The G-band consists of the two apparent components: the minor at 1585 cm-1, and the major at 1630 cm-1 (Figure 1b). The latter is the signature of the stage-1 H2SO4-GIC,34,35 where every graphene layer alternates with a layer of the intercalant. The peak is blue-shifted due to charging of the graphene by the intercalant. That there is such intercalation suggests the unzipping since such high intercalation cannot occur without MWCNT opening. This further permits expansion of the newly formed GNR stacks in the radial direction.11,12 Thus, the 1630 cm-1 component is indicative of unzipped-intercalated MWCNTs. The 1585 cm-1 component is only slightly blue-shifted compared to the G-band position for intact MWCNTs as a dry powder. This position is typical for the G-band in the high stage GICs, i.e. for graphene layers isolated from the intercalant by one and more graphene layers. We attribute the 1585 cm-1 component to the inner walls of the non-intercalated nanotubes; it is slightly shifted due to the partial polarization of the inner walls through the outermost wall that is in contact with the acidic mixture. The third minor component at 1608 cm1

, typical for stage-2 GIC,34,35 is distinguishable on the spectrum (Figure S1). It likely originates

from the outermost walls of the non-unzipped MWCNTs; they are exposed to the intercalant only from the outside. This is similar to the configuration of stage-2 GIC. The 1585 cm-1 and 1608 cm-1 components are attributable to non-intercalated, intact MWCNTs. Thus, in 0.06-GNR we have two types of MWCNTs: intact forms and fully intercalated and unzipped MWCNTs that are now GNRs. Unfortunately, the relative intensities of the two G-band components cannot be used to quantitatively assess the actual ratio of unzipped/zipped MWCNTs since the 1630 cm-1 component is selectively enhanced due to elimination of destructive interference at laser energies in the vicinity of the double Fermi energy.35,36 In the Raman spectrum of a stage-1 H2SO4-GIC,


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the 2D-band is fully suppressed due to the Pauli-blocking principle.34-35,37 Therefore, the low intensity 2D peak, observed on the spectrum of 0.06-GNR, originates solely by the nonintercalated, intact MWCNTs.

Figure 1. Raman spectra from MWCNTs while in the acidic media at different KMnO4/MWCNT ratios. (a) Full spectrum; (b) the X-axis expansion of the G-band area. The spectrum for parent MWCNTs in dry powder is given for reference purposes. The arrow represents the blue-shift of the G-band due to the charging of graphene by the intercalant.

For 0.12-GNR, the 1585 cm-1 component decreases in intensity and remains only as a low shoulder at the 1630 cm-1 component. Simultaneously, the 2D-band decreases in intensity. Thus


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the stage-1 GIC phase dominates the sample, suggesting that the majority of MWCNTs are fully intercalated-unzipped. Intercalation is further confirmed by the X-ray diffraction (Figure S2). The diffraction pattern of 0.12-GNR in the acid solution contains a strong signal at 21.6º 2θ angle. We assign it to the 002 signal of stage-1 GIC with the repeat distance 8.24 Å.29,31,34 The lower intensity signal at 23.7º 2θ angle corresponds to the 003 signal of the stage-2 GIC with a repeat distance of 11.3 Å. In this experiment, the presence of the stage-2 GIC phase is explained by the partial decomposition of the stage-1 GIC by its conversion to a stage-2 GIC in air during the sampling. Most importantly, the diffraction pattern confirms the absence of non-intercalated MWCNTs: no signals are registered at ~26.3º 2θ angle which is the main graphitic signal for pristine MWCNTs. Note, the 21.6º 2θ peak of intercalated-unzipped MWCNTs is notably narrower than the 26.3º peak for intact MWCNTs, suggesting more ordered structure, i.e. more uniform interlayer distance in intercalated GNRs compared to the interwall distance in intact MWCNTs. Thus, both Raman and XRD points toward intercalated condition of MWCNTs while in acid solutuion. The D-band on the Raman spectra is the measure of the lateral density of scattering defects in graphene. With respect to our system, the D-band is indicative of the degree of covalent functionalization of graphene by oxygen groups. Importantly, the D-band intensity for 0.12-KMnO4 remains almost the same as that for 0.06-KMnO4 and for intact MWCNTs (Figure 1a). This observation suggests that oxidation of the as-formed GNRs is significantly slower than intercalation and unzipping. For the sample obtained with 1.00 wt equiv KMnO4, the Raman spectrum represents the combination of that for stage-1 GIC and for GO.29,38 The intense and broad D and G peaks along


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with the low intensity 2D and D+G bands are indicative of GO. At the same time, the sharp 1630 cm-1 component of the G-band suggests that the stage-1 GIC is still present. Hence, the GO phase and the stage-1 GIC phase are spatially resolved. The latter observation is in accord with our earlier findings for the oxidation of graphite: at 1.00 wt equiv KMnO4, the two phases coexist and they are spatially resolved.29

Figure 2. SEM images of GNRs obtained with the (a) 0.06, (b) 0.12, (c) 0.50, and (d) 1.00 wt equiv of KMnO4. The yellow arrows on panels (c) and (d) point at the GNRs peeled-out from the unzipped MWCNTs, lying flat on the Si/SiO2 surface.


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To further confirm the unzipping, we performed SEM analyses of the four GNR samples (Figure 2). For 0.06-GNR (Figure 2a), approximately half of the MWCNTs have longitudinal cuts. For 0.12-GNR (Figure 2b), the vast majority of the nanotubes have well-pronounced openings. This is in accordance with the Raman data (Figure 1). For 0.50-GNR (Figure 2c), in addition to opened nanotubes, one can register rare GNRs lying flat on the substrate surface. In order to be flattened, the former nanotube walls have to slide along each other. This is possible only when GNRs are sufficiently oxidized,13 otherwise the as-unzipped MWCNT remains foliated.11,12 With 1.00 wt equiv KMnO4 (Figure 2d), the GNRs are peeled out from almost every unzipped MWCNT. This observation indicates that the MWCNTs’ outer layers are sufficiently oxidized. These are the hybrid nanostructures wherein the oxidized ribbons are peeling out from, but are still attached to, the non-oxidized stems. In general, the SEM images (Figure 2) confirm the degree of unzipping and oxidation suggested by the Raman spectra (Figure 1).


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Figure 3. TEM images of (a) starting MWCNTs, (b) 0.12-GNRs and (c-d) 1.00-GNRs.

Additional details can be observed on the TEM images (Figure 3). While intact MWCNTs appear smooth and uniform (Figure 3a), 0.12-GNRs have contrast longitudinal lines (cuts) and expose clear openings (Figure 3b). 1.00-GNRs exhibit broader openings and the layers of sheets that peeling out from the nanotube stems (Figure 3c). Figure 3d shows unzipped MWCNT, where the outer layers are heavily unwrapped, forming the 200 nm broad GNR.


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Figure 4. Characteristics of isolated GNRs obtained at different KMnO4/MWCNT ratios. (a) TGA; (b) C1s XPS spectra; (c) Raman spectra; (d) the X-axis expansion of the Raman spectra from (c) in the D-band and G-band areas.

To further explore the degree of oxidation, we characterized the purified dry GNR products by thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy (Figure 4). The data shows that 0.06-GNRs and 0.12-GNRs are not oxidized. According to the TGA data, they both lose only ~6% of their original weight in the entire temperature interval (Figure 4a). The main weight loss occurs in the 400-600 ºC region, suggesting that this is rather decomposition of residual GIC than decomposition of the oxygen 12 ACS Paragon Plus Environment

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functional groups which are normally complete by 300 ºC.39 The non-oxidized character of 0.06GNRs and 0.12-GNRs is further confirmed by the XPS. The C1s XPS spectra for the two samples consist of one single peak centered at 284.8 eV, originated by the non-oxidized carbon atoms (Figure 4b). There is only a slight peak broadening toward higher binding energies when compared to the parent MWCNTs. The Raman spectra of the two samples are also similar to that for intact MWCNTs (Figure 4c,d). The slightly higher intensity of the D-band is the only difference from the spectrum of the parent MWCNTs. The D/G ratio increases from 0.084/1 for parent MWCNTs through 0.14/1 for 0.06-GNR and 0.19/1 for 0.12-GNR. The D/G ratio for the 0.12-GNR is only slightly higher than that for K-GNRs, obtained by the reductive-intercalationdriven unzipping.11,12 The increase in the D-band intensity is associated with the newly formed edges, rather than with formation of the C-O bonds on the GNRs basal planes. Thus, 0.06-GNRs and 0.12-GNRs are very similar to K-GNRs, obtained by reductive-intercalation-driven unzipping. This demonstrates formation of non-oxidized GNRs using strongly oxidative media. The 0.50-GNR and 1.00-GNR show clear evidence of oxidation. The TGA curves demonstrate the 7% and 15% weight loss, respectively, in the 160-220 ºC region, associated with decomposition of the oxygen functional groups (Figure 4a). Their C1s XPS spectra (Figure 4b) contain the well-pronounced second component at 286.8 eV, afforded by the oxygen-bonded CO carbon atoms. The D-band further grows in intensity (Figure 4c,d). Both the D-band and Gband are notably broaden, and the D+G band manifests at ~2800 cm-1. These are all indicative of the GO phase. The still intense 2D-band, and the higher intensity of the G-band over the D-band, suggest that the non-oxidized graphenic phase is still present. The oxidized phase is most likely composed of GONRs formed from the outer MWCNT walls. They can easily peel away from the nanotubes (Figure 2d). The non-oxidized phase is composed of the MWCNT inner walls, which


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might remain zipped. To conclude this section, by varying the amount of the oxidizing agent, one can obtain a range of the products that signify the transition from intercalation with unzipping to unzipping with high levels of oxidation. In the course of the GO formation from graphite, the intercalation step is fast relative to oxidation. Here we use 3 wt equiv KMnO4 which is sufficient to heavily oxidize GNRs to GOlike material, but we control the time of reaction. The correctly chosen time should be sufficient to intercalate-unzip MWCNTs, but not enough to convert them to oxidized GONRs. The oxidation step is controlled by slow diffusion of the oxidizing agent through the interlayer graphite galleries, which are already occupied by H2SO4. Hence, larger particles take longer to oxidize compared to smaller particles.29 For flake graphite with particle sizes 500-1000 µm, intercalation occurs in

Revisiting the Mechanism of Oxidative Unzipping of Multiwall Carbon

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