Revisiting the Mechanism of Oxidative Unzipping ... - ACS Publications

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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*,§ †

Laboratory for Advanced Carbon Nanomaterials, Kazan Federal University, Kremlyovskaya Street 18, Kazan 420008, Russian Federation ‡ Institute of Physics, Kazan Federal University, Kremlyovskaya Street 18, Kazan 420008, Russian Federation § Departments of Chemistry and Materials Science and NanoEngineering, Smalley-Curl Institute and the NanoCarbon Center, Rice University, 6100 Main Street, Houston, Texas 77005, United States S Supporting Information *

ABSTRACT: Unzipping multiwall carbon nanotubes (MWCNTs) attracted great interest as a method for producing graphene nanoribbons (GNRs). However, depending on the 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 nonoxidized through heavily oxidized materials. We answer several questions such as the reason for the innermost walls of the nanotubes remaining zipped. The intercalation-driven reaction mechanism provides a rationale for the difficulty in unzipping single-wall and few-wall CNTs and aids in a reevaluation of the data from the oxidative unzipping process. KEYWORDS: unzipping multiwall carbon nanotubes, graphene nanoribbons, mechanism, intercalation from transparent conductive films14 through electrode materials for lithium-ion batteries.15 These types of GNRs are produced commercially.16 Multiwall carbon nanotubes (MWCNT) unzipping methods can be classified into four major types: the reductiveintercalation-assisted approach,10−12 the oxidative unzipping,13,17−23 the electrochemical 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, resulting in the bursting or longitudinal opening of the tubes. Lithium metal10 and potassium metal11,12 have been used as the intercalants. The resulting GNRs are highly conductive, but

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raphene, a two-dimensional lattice of carbon atoms, has attracted enormous interest from a broad base of the research community for more than a decade.1,2 Graphene nanoribbons (GNRs), narrow strips of graphene, being quasi 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 © 2018 American Chemical Society

Received: March 1, 2018 Accepted: March 26, 2018 Published: March 26, 2018 3985

DOI: 10.1021/acsnano.8b01617 ACS Nano 2018, 12, 3985−3993

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Cite This: ACS Nano 2018, 12, 3985−3993

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

they remain multilayered and foliated. Due to the attraction between the surfaces, they do not exfoliate to single-layer ribbons. The oxidative approach 13,17−21 involves treatment of MWCNTs in acidic oxidative media with a formulation almost identical to that used in the 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 a 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 length of the reaction on the yield and morphology of the as-obtained GONRs was studied.23 Similarly, Cruz-Silva et al. investigated the effect of the above-mentioned factors on unzipping of N-doped MWCNTs.22 Xiao et al. demonstrated that the degree of unzipping depends on the length of the reaction. On the basis of these observations, the authors 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 H2SO4graphite intercalation compound (GIC); (b) conversion of a 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 a 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 the

RESULTS AND DISCUSSION It is known that 3 to 4 wt equiv of KMnO4 is needed to convert 1 wt equiv of graphite to GO.30 The use of the 4 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 3 wt equiv of KMnO4, is less oxidized; sonication is normally required to exfoliate such GO in water. It was well demonstrated 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-GNRs, 0.12GNRs, 0.50-GNRs, and 1.00-GNRs, 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).

Figure 1. Raman spectra from MWCNTs while in the acidic media at different KMnO4/MWCNT ratios. (a) Full spectrum; (b) the xaxis 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. 3986


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Thus, both Raman and XRD point toward intercalated conditions 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 of KMnO4, the Raman spectrum represents the combination of that for a stage1 GIC and for GO.29,38 The intense and broad D- and G-peaks along 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 of KMnO4, the two phases coexist and they are spatially resolved.29 To further confirm the unzipping, we performed scanning electron microscopy (SEM) analyses of the four GNR samples (Figure 2). For 0.06-GNRs (Figure 2a), approximately half of

For the sample prepared with 0.06 wt equiv of KMnO4, the spectrum contains the high-intensity G-band and the lowintensity 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 or more graphene layers. We attribute the 1585 cm−1 component to the inner walls of the nonintercalated 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 cm−1, typical for a stage-2 GIC,34,35 is distinguishable on the spectrum (Figure S1). It likely originates from the outermost walls of the nonunzipped MWCNTs; they are exposed to the intercalant only from the outside. This is similar to the configuration of a stage-2 GIC. The 1585 and 1608 cm−1 components are attributable to nonintercalated, intact MWCNTs. Thus, in 0.06-GNRs 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, 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-GNRs, originates solely by the nonintercalated, intact MWCNTs. For 0.12-GNRs, 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, the stage-1 GIC phase dominates the sample, suggesting that the majority of MWCNTs are fully intercalatedunzipped. Intercalation is further confirmed by X-ray diffraction (Figure S2). The diffraction pattern of 0.12-GNRs in the acid solution contains a strong signal at a 21.6° 2θ angle. We assign it to the 002 signal of a stage-1 GIC with the repeat distance 8.24 Å.29,31,34 The lower intensity signal at a 23.7° 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 stage2 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 nonintercalated MWCNTs: no signals are registered at a ∼26.3° 2θ angle, which is the main graphitic signal for pristine MWCNTs. Note that the 21.6° 2θ peak of intercalated-unzipped MWCNTs is notably narrower than the 26.3° peak for intact MWCNTs, suggesting a more ordered structure, i.e., more uniform interlayer distance in intercalated GNRs compared to the interwall distance in intact MWCNTs.

Figure 2. SEM images of GNRs obtained with (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 from the unzipped MWCNTs, lying flat on the Si/SiO2 surface.

the MWCNTs have longitudinal cuts. For 0.12-GNRs (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-GNRs (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 of KMnO4 (Figure 2d), the GNRs are peeled 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 from, but are still attached to, the nonoxidized stems. In general, the SEM images (Figure 2) confirm the degree of unzipping and oxidation suggested by the Raman spectra (Figure 1). 3987


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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 °C region, suggesting that this is rather decomposition of residual GIC than decomposition of the oxygen functional groups, which are normally complete by 300 °C.39 The nonoxidized character of 0.06-GNRs and 0.12-GNRs is further confirmed by XPS. The C 1s XPS spectra for the two samples consist of one single peak centered at 284.8 eV, originated from the nonoxidized 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 Dband 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-GNRs and 0.19/1 for 0.12GNRs. The D/G ratio for the 0.12-GNRs 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 nonoxidized GNRs using strongly oxidative media. The 0.50-GNRs and 1.00-GNRs show clear evidence of oxidation. The TGA curves demonstrate the 7% and 15% weight loss, respectively, in the 160−220 °C region, associated with decomposition of the oxygen functional groups (Figure 4a). Their C 1s XPS spectra (Figure 4b) contain the wellpronounced second component at 286.8 eV, afforded by the oxygen-bonded C−O carbon atoms. The D-band further grows

Additional details can be observed on the TEM images (Figure 3). While intact MWCNTs appear smooth and uniform

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

(Figure 3a), 0.12-GNRs have contrast longitudinal lines (cuts) and expose clear openings (Figure 3b). 1.00-GNRs exhibit broader openings and layers of sheets that are peeling from the nanotube stems (Figure 3c). Figure 3d shows an unzipped MWCNT, where the outer layers are heavily unwrapped, forming the 200 nm broad GNR. 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 show that 0.06-GNRs and

Figure 4. Characteristics of isolated GNRs obtained at different KMnO4/MWCNT ratios. (a) TGA; (b) C 1s XPS spectra; (c) Raman spectra; (d) the x-axis expansion of the Raman spectra from (c) in the D-band and G-band areas. 3988


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Figure 5. Characteristics of GNRs obtained at varying reaction times. (a) TGA; (b) C 1s XPS spectra; (c) Raman spectra; (d) the x-axis expansion of the Raman spectra from (c) in the D-band and G-band area. The KMnO4/MWCNT ratio is 3/1.

Both TGA and XPS indicate that even the 5s-GNR sample is notably oxidized. The sample loses ∼6% weight in the 160−220 °C temperature region (Figure 5a). The C 1s XPS spectrum (Figure 5b) contains a well-pronounced shoulder at 286.6 eV, associated with carbon atoms of tertiary alcohols and epoxides. Based on TGA and XPS data, 5s-GNRs have an oxidation level similar to that of 0.50-GNRs (Figure 4a,b). At the same time, one can clearly see that the oxidation degree gradually increases with the length of the reaction. Raman spectroscopy (Figure 5c,d) shows that the spectra for 5s-GNRs, 10s-GNRs, and 60sGNRs consist of both GO-like and the nonoxidized graphene phases, the same as for the 0.50-GNRs and 1.00-GNRs (Figure 4c,d). This situation is different from that when the density of scattering defects gradually increases on the initially intact graphene. In such cases the D-band first increases in height, not in width, the D′-band manifests to the right of the G-band, and both G- and D-bands remain narrow until the D/G ratio reaches a value of 2.9.40,41 In our case, the GO phase grows at the expense of the graphenic phase; this is different from the situation discussed above, where one phase is slowly converted into another phase. Again, this observation is similar to the oxidation of graphite.29 Interestingly, even 720s-GNRs lose only 14% weight in the 160−220 °C region. For comparison, GO prepared with 3 wt equiv of KMnO4 and an exposure time of 1 h loses ∼25% of its original weight in this region.29 On the C 1s XPS spectrum of 720s-GNRs, the 286.8 eV component is only ∼2/3 of the 284.8 eV component. This is again in sharp contrast with GO, where the former is normally higher than the latter. Even for GONR prepared with 5 wt equiv of KMnO4 for 12 h, the oxidation level is always notably lower than that for GO prepared under similar conditions.13,23,42 As will be shown below, we explain this observation by the zipped nonoxidized condition of the stems of the MWCNT inner walls.

in intensity (Figure 4c,d). Both the D-band and G-band are notably broadened, 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 Dband suggest that the nonoxidized 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 nonoxidized phase is composed of the MWCNT inner walls, which might remain zipped. To conclude this section, by varying the amount of the oxidizing agent, one can obtain a range of 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 of KMnO4, which is sufficient to heavily oxidize GNRs to GO-like 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 of 500−1000 μm, intercalation occurs in 20 nm. The small number of walls makes the entire structure flexible, and the large central channel most likely accommodates the expansion of the few walls upon intercalation,

CONCLUSIONS We demonstrate that the mechanism of the oxidative unzipping of MWCNTs is most likely intercalation-driven. The overall unzipping process involves the same three steps as in the course of GO production from graphite by the Hummers and modified Hummers methods: intercalation, oxidation, and exfoliation. With MWCNTs, the intercalation is associated with simultaneous unzipping. At low KMnO4/MWCNT ratios, one can obtain GNRs with characteristics similar to those produced by reductive unzipping. A 0.12 wt equiv amount of KMnO4 is the threshold ratio sufficient for almost complete unzipping, with only small amounts of covalent oxidation. Controlling the KMnO4/MWCNT ratio and length of reaction allows one to 3991


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ACS Nano produce GNRs with properties varying in a broad continuous range from multilayered graphenic GNRs through singlelayered GONRs.

Analytical Microscopy, KFU. The work at Rice University was sponsored by the Air Force Office of Scientific Research (FA9550-14-1-0111).

METHODS

REFERENCES

To investigate the effect of the KMnO4/MWCNT ratio, four different weight ratios have been chosen: 0.06, 0.12, 0.50, and 1.00. MWCNTs (100 mg) were immersed into sulfuric acid (20 mL), containing a certain amount of predissolved potassium permanganate. The mixture was agitated with a magnetic stirrer until all the KMnO4 was consumed, as noted by the disappearance of the red tint upon diluting a sample of the reaction mixture with water. This took from 10 min to 1 h depending on the molar ratio used. The mixture was slowly poured onto 100 mL of ice−water. The reacted MWCNTs were separated from diluted acid by filtration through the cyclopore track etched membrane (pore size 0.4 μm) under vacuum and washed on filter with copious amounts of deionized (DI) water. The washed samples were dried in air. To investigate the role of the length of the reaction, the KMnO4/MWCNT ratio was 3.0 in all the time experiments. The reaction time was from 5 s through 720 s. The reaction was terminated immediately upon reaching the designated exposure time by quenching with an ice−water mixture. The reacted MWCNTs were separated from the diluted acid and purified as above. The MWCNTs were from Hodogaya Co., Japan. These are the same type of MWCNTs as were used in the former study13 although the distributor name had changed. These nanotubes are 4−12 μm long and 40−80 nm in diameter and composed of 30−60 walls. The walls constitute almost all the MWCNT’s body: the hollow innermost channel is 4−8 nm in diameter (Figure S1). Sulfuric acid (96%) was from Shchekinoazot Trading House, LLC, Russia; potassium permanganate (98% purity) was from CJSC TatKhimProduct, Russia. The SEM images were acquired with a Merlin field-emission highresolution scanning electron microscope from Carl Zeiss at an accelerating voltage of incident electrons of 5 kV and current probe of 300 pA. The TGA data were collected with a STA 449 F5 Jupiter analyzer from Netzsch in both Ar and synthetic air atmospheres (21% O2, 79% N2). The XPS spectra were acquired in the UHV chamber of the Phoibos 100/150 multitechnique surface analysis system from SPECS. A Mg Kα X-ray source operating at 12.5 kV and 250 W was used. A pass energy of 30 eV (step size of 0.5 eV) was used for widerange scans; a pass energy of 20 eV (step size of 0.1 eV) was used for high-resolution measurements. All spectra were analyzed by using the CasaXPS software. The Raman spectra were acquired with a Senterra Bruker Raman microscope with a 532 nm excitation laser source.

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b01617. Raman and XRD spectra and TEM images (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected] (A. M. Dimiev). *E-mail: [email protected] (J. M. Tour). ORCID

James M. Tour: 0000-0002-8479-9328 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The work is performed with support of the Russian Government Program for Competitive Growth of Kazan Federal University. The TEM and SEM images were acquired using the equipment of the Interdisciplinary Center for 3992


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