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ISSN 0018-1439, High Energy Chemistry, 2015, Vol. 49, No. 1, pp. 48–52. © Pleiades Publishing, Ltd., 2015. Original Russian Text © A.G. Ryabenko, D.P. Kiryukhin, G.A. Kichigina, O.M. Zhigalina, E.N. Nikolaev, A.N. Krasnovskii, 2015, published in Khimiya Vysokikh Energii, 2015, Vol. 49, No. 1, pp. 51–55. NANOSTRUCTURED SYSTEMS AND MATERIALS Reactions on Single-Walled Nanotubes: 1. Radiation-Stimulated Reactions in Aqueous Suspensions of Single-Walled Carbon Nanotubes in Surfactant Solutions A. G. Ryabenkoa, D. P. Kiryukhina, G. A. Kichiginaa, O. M. Zhigalinab, E. N. Nikolaevc, and A. N. Krasnovskiid a Institute of Problems of Chemical Physics, Russian Academy of Sciences, pr. Akademika Semenova 1, Chernogolovka, Moscow oblast, 142432 Russia e-mail: aryabenk@icp.ac.ru b Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskii pr. 59, Moscow, 119333 Russia c Talrose Institute of Energy Problems of Chemical Physics, Leninskii pr. 38, Moscow, 119334 Russia d Moscow State Technological Institute STANKIN, Vadkovskii per. 1, Moscow, 127994 Russia Received May 30, 2014; in final form, August 1, 2014 Abstract—Chical reactions initiated by 60Co γ-radiation in the suspensions of single-wall carbon nanotubes (SWCNTs) in the aqueous solutions of a surfactant, cetyltrimethylammonium bromide (CTAB), have been studied. In a pure aqueous solution of CTAB, the radiolysis of water leads to the appearance of OH radicals, which induce the oxidation of CTAB molecules. In the suspensions, CTAB molecules are concentrated on SWCNTs and oriented in one direction and the OH radicals induce the crosslinking of surfactant molecules. A moss-like coating is formed on the nanotubes, and the suspension is converted into jelly. The coating does not have covalent bonds with the nanotube walls. The formation of the polymer is due to the packing of molecules around the nanotube, rather than their chemical properties. DOI: 10.1134/S0018143915010129 The γ-irradiation of nanotubes leads to a number of specific features on radiation grafting. For example, the irradiation of SWCNTs nanopaper increases its Young’s modulus and conductivity because of the formation of defects, which can lead to the crosslinking of nanotubes in bundles [8]. Guo et al. [9] concluded that irradiation gives a new approach to different forms of the modification of carbon nanotubes because γ-ray photons generate a large amount of new defects on the walls of multiwall nanotubes. This was repeatedly confirmed in works with multiwall nanotubes [10–14]. It was established that γ-irradiation changes the surface and structure of multiwall nanotubes [15, 16]. Jovanovic et al. [17] described a new method for highly efficient functionalization of SWCNTs by DNA wrapping. Because SWCNTs are tightly packed in the bundles, the surface of the nanotubes is inaccessible to modifying reagents. For the uniform modification of the walls of nanotubes, it is necessary to split a nanotube from the bundle by preliminary treatment. Jovanovic et al. [17] used γ-irradiation as this pretreatment, whereas Castell et al. [18] stimulated the formation of crosslinks between nanotubes and a polymer using γ-irradiation. XRD analysis showed the presence of only an α-monoclinic crystal; this fact unambiguously demonstrated that nanotubes create a preferred orientation of polymer molecules [18]. The γ-irradia- Single-wall carbon nanotubes (SWCNTs) possess unique mechanical and electronic properties. However, they spontaneously stick together in the course of synthesis and purification to form thick bundles, which prevent the practical applications of their unique properties. The self-assembly of nanotubes into dense bundles occurs due to the strong van der Waals interaction of nanotubes over a large contact surface. This interaction can be decreased either by the use of a surfactant (noncovalent modification) or by the grafting of polar functional groups to the walls of nanotubes (covalent modification). Much attention has been devoted to the latter of these modifications [1–7]. It is believed that, on the covalent modification, functional groups are joined to defects on the walls of the nanotubes because the intact walls are chemically inert. Modification enlarges these defects to unavoidably decrease the strength of nanotubes and change their electronic properties. Radiation-induced grafting has a number of advantages over usual chemical modification. The products of radiolysis do not contain initiator catalyst residues. The process can be performed in gas, liquid, and solid phases. It is likely that functional groups can be radiation grafted not only onto defects on nanotube walls but also to intact walls. 48 REACTIONS ON SINGLE-WALLED NANOTUBES As noted above, the surface of SWCNTs in bundles is inaccessible to modifying agents. The ultrasonic treatment of SWCNTs in the solutions of surfactants is a standard method for splitting the bundles. In such suspensions, the average thickness of the bundles noticeably decreases; a portion of the tubes occurs in an individual state, and the surfactant molecules are concentrated on the surface of the nanotubes. In our opinion, γ-irradiation will lead to the grafting of these molecules to the walls of the nanotubes. In this work, we studied the products of the radiolysis of the aqueous suspensions of SWCNTs in the solutions of CTAB in comparison with the radiolysis of the pure solutions and the radiolysis of SWCNTs in water. EXPERIMENTAL The nanotubes were prepared by an electric arc method with the use of a Ni/Y catalyst. The tubes were purified to a 95% concentration by gas-phase oxidation [22]. The suspensions were prepared by the 20-min ultrasonication of a 0.25% solution of CTAB, which contained 1 mg of SWCNTs in 6 mL of the solution [22]. Nanopaper, which was used for determining the radiation stability of the nanotubes, was prepared by the filtration of this solution through a filter with 0.1-µm pores. All of the samples were irradiated with 60Co γ-rays on a Gammatok-100 source (dose rate, 0.15 Gy/s). Electron microscopy was performed on a HIGH ENERGY CHEMISTRY Vol. 49 No. 1 2015 284.331 CTAB 3500 N+ (CH3)3 = C19H42N + O 290 310 342.269 344.284 346.301 500 C19H42N + 3O–6H C19H42N + 2O–4H + O C19H42N + O–2H + 2O C19H42N + 3O 326.274 328.288 330.304 332.321 1000 115 kGy 312.292 314.308 316.324 1500 H C H C19H42N + 2O–4H C19H42N + O–2H + O C19H42N + 2O 2500 2000 CH3 H C N+ CH3 = C19H42N H CH3 H + C N (CH3)3 = C19H42N + O–2H H H H H O C C H H 75 kGy 3000 H HC H + O H C C H 35 kGy 4000 298.312 300.328 301.333 However, the reactions of the covalent modification of nanotubes can also be accompanied by reactions between the molecules of a grafted reagent. It is reasonable to assume that some molecules, especially long ones, can temporarily adhere and thus become fixed on the walls of nanotubes due to van der Waals interaction. Reactions between the adhered molecules are limited by a specific orientation to the axis of a thin nanotube on a molecular scale. This can produce products that are strongly different from those formed upon free reactions in a liquid and accelerate reactions between the adhered molecules. This can be of special importance for understanding the processes of the production of a SWCNT/polymer composite by in situ polymerization. As far as we know, this problem was not studied in detail. Let us note two publications where it was noted that the presence of nanotubes in a reaction medium changes the structure of the resulting polymer [18, 20]. Rance et al. [21] found an effect of SWCNTs on chiral chemical reactions. That is, a chiral nanosized carbon surface can control the chirality of the products of organic reactions. C16 285.335 tion of threads woven from carbon nanotubes in an atmosphere of air increases their tensile strength and Young’s modulus, probably, because of an increase in interactions between individual nanotubes [19]. All of the cited publications were dedicated to reactions between nanotubes or between a grafted reagent and a nanotube. 49 330 350 m/z Fig. 1. Electrospray mass spectra of a 0.1% CTAB aqueous solution after γ-irradiation at doses of 35, 75, and 115 kGy. Philips EM430ST instrument with an operating voltage of 200 kV. RESULTS AND DISCUSSION γ-Irradiation of SWCNT in Water The irradiation of the nanopaper in water to a dose of 7000 kGy did not change the UV–Vis–NIR spectrum of the nanotubes. The Raman spectrum did not reveal changes with reference to the initial sample. The IR spectrum of the nanopaper after irradiation exhibited a considerable decrease in the absorption of CTAB residues without changes in the spectrum of the nanotubes. All the above suggests the high radiation stability of the nanotubes in water; the OH radicals resulting from the radiolysis of water did not interact with the nanotubes. Consequently, unlike published data [8, 10–17, 20, 21], radiation had no effect on the structure of nanotube walls in water under the conditions of our experiments. At the same time, the irradiation of the nanopaper in air impaired the UV–Vis– NIR spectrum; this is consistent with conclusions made by Skakalova et al. [8]. We assume that this was caused by ozone formed upon irradiation. γ-Irradiation of CTAB Solutions Figure 1 shows the electrospray mass spectra of a 0.1% solution of CTAB after γ-irradiation at doses of 35, 75, and 115 kGy. The degree of oxidation of CTAB molecules increased with dose. From Fig. 1, it is evident that double bonds were formed on irradiation to cause the appearance of additional absorption in the UV range (Fig. 2a). Figure 2b shows the dependence of this absorption on radiation dose and on the initial 50 RYABENKO et al. (а) 500 kGy 3.0 320 kGy 250 kGy 2.5 15 2.0 1.5 Total absorption, arb. units Absorption, arb. units (b) 120 kGy 1.0 40 kGy 0.5 0 0 237 262 Wavelength, nm 212 287 312 0.3% 0.2% 10 0.1% 5 0 100 200 300 400 500 Dose, kGy 600 700 Fig. 2. (a) Changes in the spectrum of a 0.3% CTAB solution with γ-irradiation dose. (b) The dose dependence of the accumulation of products with double bonds in solutions with different CTAB concentrations. concentration of CTAB. It is evident that, at the initial stage, the product was linearly accumulated with dose regardless of the initial concentration. Consequently, possible micelle formation (critical micelle concentration (CMC) = 0.25%) does not influence the rate of oxidation. The CTAB content of the solution rapidly decreased with γ-irradiation dose (see Fig. 1), and the concentration of products also began to fall after the initial linear increase (Fig. 2b). At high doses, CTAB was completely oxidized. 7.5 Mrad CTAB + SWCNTs 250 615.304 500 750 996.524 800.450 687.363 284.331 7.5 Mrad 11.5 Mrad 543.244 504.370 11.5 Mrad 127.123 45 40 35 30 25 20 15 10 5 C20H39O6NH2 ? 391.285 1000 m/z Fig. 3. Electrospray mass spectra of the soluble fraction of suspensions after γ-irradiation at doses of 75 and 115 kGy and settling for eight months. γ-Irradiation of SWCNT Suspensions Other results were obtained upon the irradiation of the suspensions of SWCNTs. Doses of 75 and 115 kGy led to the loss of the long-time stability of the suspensions, and the nanotubes settled down after several months. Figure 3 shows the mass spectrum of transparent liquid above these sediments. A comparison between Figs. 1 and 3 indicates that, in the presence of the nanotubes, free CTAB in solution was consumed more rapidly, and the peaks of products in the solution with the nanotubes were several times higher than the peak of the parent substance (a peak at 284.331); the products were very different, and their molecular weights were noticeably higher; the fraction of heavy masses increased with dose. Several peaks with distances of ~110 dalton between them stand out; that is, a molecule of ~110 amu was formed in the process of radiolysis in water, and these molecules were consecutively added to the CTAB molecule. At a dose of 200– 300 kGy, the suspension was converted into jelly, which was insoluble in water and organic solvents. Figure 4 shows a micrograph of a film of this jelly from a transmission electron microscope (TEM). The insert shows that the nanotubes were covered with a moss-like coating, which was nonuniform in thickness but no more than 1.5 nm thick (the thickness of the coating was comparable with or smaller than the thickness of a nanotube). In the suspension, the concentrations of water, CTAB, and SWCNTs were 99.73, 0.25, and 0.02 wt %, respectively. The only difference of the suspension from pure solution was that CTAB molecules in the suspension were concentrated at the HIGH ENERGY CHEMISTRY Vol. 49 No. 1 2015 REACTIONS ON SINGLE-WALLED NANOTUBES 51 the latter version because the CTAB molecule is too long to be arranged in the visible outgrowths according to the former two versions. Figure 5 shows a hypothetical diagram of the formation of a coating. Note that, in the course of irradiation (at the stages of 10, 20, 30, and 50 kGy when the suspension remained stable), the UV–Vis–NIR spectrum of the suspension did not deteriorate; therefore, the SWCNTs did not undergo chemical alteration in the course of the growth of the coating. Nanotubes served as a catalyst for the formation of a polymer. Our preliminary experiments with other surfactants (such as Triton-100 and sodium dodecylbenzenesulfonate) showed that jelly was also formed in the suspensions prepared based on these surfactants under the action of γ-radiation. This fact suggests that the formation of polymers was caused by the packing of molecules around the nanotube rather than their chemical properties. 25 nm 150 nm CONCLUSIONS Fig. 4. TEM image of a film of the jelly formed after the irradiation of a suspension of SWCNTs at a dose of 250 kGy. Insert: a thin bundles and a separate nanotube (marked with an arrow). walls of SWCNTs. It is likely that, for this reason, the OH radicals crosslinked the molecules; thus, a polymer was formed. There is no general agreement on the packing of surfactant molecules around a nanotube. O’Connell et al. [23] and Shin et al. [24] assumed surfactant organization as a cylindrical micelle or micellar hemispheres, respectively, whereas Wallace and Sansom [25] proposed the possibility of the stacking of surfactant molecules along the nanotube. Our data confirm CTAB molecule The presence of SWCNTs radically changes the rate and direction of the reactions of OH radicals with CTAB molecules; however, the addition of these latter to the nanotube walls does not occur. In the presence of the nanotubes, the OH radicals crosslink the molecules of CTAB with the formation of a polymer coating around the nanotubes and their bundles. The rate of CTAB consumption increases. ACKNOWLEDGMENTS This work was supported by the Ministry of Education and Science of the Russian Federation within the framework of a scientific state contract. E.N. Nikolaev acknowledges the support of the Russian Foundation for Basic Research, project nos. 13-04-40110-N and 13-08-01445, and the Rus- 2.5 nm 2.5 nm N O N C C N O Carbon nanotube Fig. 5. Illustration of polymer formation around a nanotube. Insert: TEM image of polymer outgrowths on the nanotube, which shows that CTAB molecules concentrated at the nanotubes are arranged along the nanotube. HIGH ENERGY CHEMISTRY Vol. 49 No. 1 2015 52 RYABENKO et al. sian Science Foundation, grant no. 14-24-00114) for the development of mass spectrometry methods. REFERENCES 1. Mickelson, E.T., Chiang, I.W., Zimmerman, J.L., Boul, P.J., Lozano, J., Liu, J., Smalley, R.E., Hauge, R.H., and Margrave, J.L., J. Phys. Chem. B, 1999, vol. 103, no. 24, p. 4318. 2. Dyke, C.A. and Toiur, J.M., Nano-Lett., 2003, vol. 3, no. 9, p. 1215. 3. 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