NANO LETTERS
Direct Solvent-Free Amination of Closed-Cap Carbon Nanotubes: A Link to Fullerene Chemistry
2004 Vol. 4, No. 5 863-866
Elena V. Basiuk,† Marel Monroy-Pela´ez,† Ivan Puente-Lee,‡ and Vladimir A. Basiuk*,§ Centro de Ciencias Aplicadas y Desarrollo Tecnolo´ gico, Facultad de Quı´mica, and Instituto de Ciencias Nucleares, UniVersidad Nacional Auto´ noma de Me´ xico, Circuito Exterior C.U., 04510 Me´ xico, D.F., Mexico Received February 13, 2004; Revised Manuscript Received March 22, 2004
ABSTRACT We attempted the direct solvent-free amination of closed caps of multiwalled carbon nanotubes (MWNTs) with octadecylamine (ODA), which is essentially similar to the amination of spherical fullerenes. Thermogravimetric analysis revealed a relatively high content of organics in the product of derivatization (ODA-MWNTs), suggesting that a large ODA fraction is distributed over MWNT sidewalls through chemical attachment. This was confirmed by high-resolution transmission electron microscopy observations. Quantum chemical calculations showed that the presence of pyracylene units in the closed caps is not crucial for the amine addition, although the site specificity of the reaction does depend on the mutual position of five-membered rings. If the caps contain pyracylene units, then the addition preferentially takes place on their 6,6 bonds; if they do not, then the preferential reaction sites are C−C bonds of the pentagons. Whereas ideal nanotube sidewalls composed of solely benzene rings were found to be inert with respect to amines, the real nanotube sidewalls must contain numerous reactive five-membered rings as defects. ODA-MWNTs exhibited enhanced dispersibility/solubility in propanol. The proposed amination reaction is the most direct link between carbon nanotube and fullerene chemistry, contrary to all derivatization methods designed previously.
The very low solubility of carbon nanotubes (CNTs) hampers many of their potential applications. This is especially true for multiwalled carbon nanotubes (MWNTs). One of the most efficient ways to increase CNT solubility and dispersibility is by covalent derivatization. (See recent reviews1-4 and references therein.) All known types of chemical reactions previously employed for this purpose can be divided into two main groups: (a) defect-group derivatization and (b) covalent sidewall derivatization. The first group of reactions involves oxygenated functionalities (mainly carboxylic groups) formed on CNT tips, as well as on their sidewalls to some extent, as a result of oxidative treatment with strong mineral acids.1-7 As a matter of fact, these reactions (mainly amidation) have to do with neither graphene-sheet nor fullerene-cap chemistry but instead with the chemistry of carboxylic acids. On the contrary, covalent sidewall derivatization reactions employ the chemistry of graphene sheets,1,2,8-11 although fullerene caps of pristine CNTs are reactive sites as well. Surprisingly, despite the wide use of the term “tubular fullerenes” for CNTs, no method for their derivatization has * Corresponding author. E-mail:
[email protected]. † Centro de Ciencias Aplicadas y Desarrollo Tecnolo ´ gico. ‡ Facultad de Quı´mica. § Instituto de Ciencias Nucleares. 10.1021/nl049746b CCC: $27.50 Published on Web 04/14/2004
© 2004 American Chemical Society
been developed that would rely precisely upon the chemistry of closed fullerene caps and not graphene-sheet and carboxylic acid chemistry. Nevertheless, within the rich fullerene chemistry there is a very appropriate reaction employing simple and common reagents: organic amines. It was discovered more than a decade ago.12,13 Both primary and secondary amines (which are all neutral nucleophiles) add onto C60 at room temperature by reacting with fullerenes dissolved in liquid amines or in their solutions in dimethylformamide, dimethyl sulfoxide, chlorobenzene, and so forth.12-16 The reaction stoichiometry varies significantly depending on the size of the amine reactant molecule. For smaller amine molecules such as 2-methylaziridine, the average amine/C60 ratio can reach 10:1.16 Recently, we studied a solvent-free reaction of silica-supported C60 with vaporous nonylamine at 150 °C, which also produces a mixture of addition products.17 Quantum chemical calculations showed that the addition reaction most likely takes place across the 6,6 bonds of C60 pyracylene units (that is, those connecting two five-membered rings) and not across the 5,6 bonds. According to elemental analysis data (C/N ratio), the number of nonylamine molecules attached to C60 is three on average, although a field-desorption mass spectrometric study revealed a number of molecular and fragment ions
corresponding to the adducts with up to six nonylamine moieties attached to C60. In the present work, we tested the possibility of performing a direct amination reaction on closed caps of MWNTs to improve their dispersibility/solubility in this way. MWNTs from the CVD process (ILJIN Nanotech Co., Inc., Korea; 97%+ purity, diameter of 10-20 nm and length of 10-50 µm) were used as received. Such nanotubes have closed fullerene caps. 1-Octadecylamine (ODA) was used as an organic amine agent. It has a long linear hydrocarbon radical and is commonly used in the end amidation of CNTs to enhance their solubility. (See reviews1,2,4 and references therein.) To perform the amine derivatization with ODA, we employed the gas-phase solvent-free procedure designed recently by us.18 MWNTs (100 mg) and ODA (20 mg) were placed together in the reactor, and the reaction was performed at 150-170 °C for 2 h. During this procedure, ODA melted and evaporated, reacting with MWNTs, and its excess condensed a few centimeters above the heated zone. The high derivatization temperature not only facilitates the reaction but also helps to minimize the amount of ODA physically adsorbed on the nanotube material. Before extracting the reaction product (referred to as ODA-MWNTs), to avoid contaminating the nanotubes, the upper reactor part with unreacted ODA was wiped with cotton wool wet with ethanol. In principle, the reaction can be equally performed by baking an MWNTs/ODA mixture in a sealed vial (a solvent-free process as well);19 however, excess ODA should afterward be removed in some way (washing or evacuating/ heating). Besides two low-intensity bands at about 750 and 900 cm-1 due to ODA, the IR spectra of ODA-MWNTs (recorded in KBr pellets on a Nicolet 5SX FTIR spectrometer) did not show pronounced changes as compared to the starting nanotube material. This is commonly a result of the very poor quality of infrared spectra of CNTs, on one hand, and the low concentration and consequently negligible spectral contribution of the organic moieties, on the other hand. We also conducted thermogravimetric analysis (TGA; DuPont Thermal Analyzer 951 with a heating ramp of 10 °C min-1 until 1000 °C and under air flow of 100 mL min-1). As seen from Figure 1b, the steepest weight loss due to organics decomposition for ODA-MWNTs (shown with a rectangle) is observed in a temperature interval of 250-400 °C. If we take into account high aspect ratios of 103 that are typical for MWNTs, this weight loss (ca. 5%) cannot correspond to ODA molecules attached to the fullerene caps only. An overwhelming ODA fraction seems to be distributed over MWNT sidewalls. These ODA molecules might be either (a) simply physisorbed on the sidewalls or (b) chemically attached there, in a way similar to their attachment to the fullerene caps. The first explanation seems less likely because we expect physisorbed ODA molecules to be removed at temperatures lower than 250-400 °C. Indeed, Chattopadhyay et al.20 found that the TGA decomposition of ODA physically adsorbed on SWNTs is observed in a temperature interval of 150-300 °C. The second explanation is hardly compatible with the chemistry of ideal graphene 864
Figure 1. Thermogravimetric curves for (a) starting MWNTs and (b) ODA-MWNTs. The temperature interval (250-400 °C) of the steepest weight loss due to organics decomposition for ODAMWNTs is shown with a rectangle.
Figure 2. HRTEM microphotographs showing closed caps of both (a) starting MWNTs and (b) ODA-MWNTs and (c) sidewalls of ODA-MWNTs. Black arrows point to amorphous material originating from ODA molecules bound to the defect sites; the white arrow points to almost ideal sidewalls where no similar material can be observed.
sheets but might be possible if we admit the presence of such defects, as, for example, five-membered rings typical of spherical fullerenes. We performed a direct observation of the functionalized MWNTs (along with the starting material, for comparison) by means of high-resolution transmission electron microscopy (HRTEM; JEOL 4000EX instrument, 200 kV). From Figure 2a, one can see the starting MWNTs to be composed of ca. 10 coaxial tubes. Their closed caps have an irregular shape; however, graphene sheet fragments of the outer shell are relatively ordered. Although similar closed caps can be clearly distinguished in ODA-MWNTs, they turn out to be covered with a ca. 2-nm layer of some amorphous material (Figure 2b) whose origin can be explained by the electron beam destruction of ODA molecules bound to the defect sites (pentagons). Furthermore, the HRTEM observation of the sidewalls of ODA-MWNTs revealed similar amorphous formations (Figure 2c), but they were concentrated at the sites with well-pronounced curvature, where the number of Nano Lett., Vol. 4, No. 5, 2004
Table 1: Formation Energies for Isomeric Monoadducts in the Reaction of Methylamine with Model Armchair (5, 5) and Zigzag (10, 10) SWNTsa Calculated with the AM1 Semiempirical Methodb armchair (5, 5)
Figure 3. Structures optimized with the AM1 semiempirical method and atom numbering schemes for (a) armchair (5,5) and (b) zigzag (10,10) SWNT models used to study the reaction of methylamine monoaddition theoretically. Left, side views; right, tip views.
defects must be relatively high. On the contrary, nearly ideal sidewalls were found to be free of any additional material. To suggest an explanation of the results obtained, we employed theoretical calculations. We tested the energetic feasibility of ODA addition to different types of carbon atoms. Although ab initio and DFT methods are known to be more accurate, the nanotube size limited us to the semiempirical level of theory, which is suitable for semiquantitative energy estimates of relatively small SWNT models. The calculations were performed with the AM1 semiempirical method implemented in the HyperChem version 5.1 package (by HyperCube Inc., Canada) and in the Gaussian 98 suite of programs.21 In all HyperChem calculations, full geometry optimization was performed with a Polak-Ribiere conjugate gradient algorithm, convergence limit of 0.001 kcal mol-1 and a root-mean-square gradient of 0.001 kcal Å-1 mol-1. In the Gaussian calculations, preset default criteria were used. Two SWNT structures were used as simplified nanotube models (Figure 3): the armchair (5,5) with both ends closed (Figure 3a) and the zigzag (10,10) SWNT with one closed and one open end (dangling bonds filled with hydrogen atoms, Figure 3b). Formation energies for different isomeric methylamine monoadducts were calculated (Table 1). Addition positions are denoted nNmH, where “N” and “H” denote the sites of CSWNT-NHCH3 and CSWNT-H bond formation, respectively; n and m correspond to the numbering schemes in Figure 3. Both models have five-membered ring right on the tip (including atoms 1 and 2 and three remaining equivalent atoms), but the distribution of other pentagons is very different. The closed cap of the armchair (5,5) model is the C60 fullerene hemisphere. Thus, it contains pyracylene units, and methylamine must preferentially add across their 6,6 bonds (connecting two pentagons) and not across the 5,6 bonds.17 Our calculations corroborated this: 1N-3H and 3N-1H additions are the most exothermic and are equally possible (-22.9 kcal mol-1 relative to the level of reactants), with 4N-5H slightly less exothermic (-19.7 kcal mol-1). Nano Lett., Vol. 4, No. 5, 2004
position
∆E (kcal mol-1)
1N-2H 1N-3H 3N-1H 3N-4H 4N-3H 4N-5H 4N-6H 6N-4H 6N-7H 6N-8H 8N-6H 8N-9H 8N-10H 10N-8H 10N-11H 10N-12H 12N-10H 12N-13H
-4.2 -22.9 -22.9 -4.0 -3.5 -19.7 -5.2 -3.9 0.4 -6.5 -2.8 26.4 22.4 25.4 17.3 21.2 21.5 30.7
zigzag (10, 0) position
∆E (kcal mol-1)
1N-2H 1N-3H 3N-1H 3N-4H 4N-3H 4N-5H 4N-6H 6N-4H 6N-7H 7N-6H 7N-8H 8N-7H 8N-9H 9N-8H
-30.4 -13.6 -11.4 -7.6 -8.5 -26.5 -26.7 -26.5 -8.0 -6.5 18.5 19.2 11.3 13.3
a Structures and atom numbering are shown in Figure 3. b Addition positions are denoted nN-mH, where N and H denote the sites of CSWNTNHCH3 and CSWNT-H bond formation, respectively.
Several positions were found where the reaction is still slightly exothermic, 1N-2H, 3N-4H, 4N-3H, 4N-6H, 6N-4H (all additions across the 5,6 bonds) as well as 6N-8H and 8N-6H (additions across the 6,6 bonds not belonging to pyracylene units). The exothermicity of the latter series is probably associated with the spherical curvature of the cap. However, the spherical curvature disappears beyond carbon atoms 6 and 7. The remaining combinations (8N-9H, 9N-8H, and so on) imply amine additions to a graphene sheet of cylindrical curvature and naturally turn out to be highly endothermic (about 20-30 kcal mol-1) and therefore unlikely. The closed cap of the zigzag (10,0) nanotube ends with a five-membered ring as well (atoms 1 and 2 and three remaining equivalent atoms); however, it does not contain pyracylene units. All pentagons are separated by two C-C bonds belonging to benzene rings. Nevertheless, several positions were found where the addition is highly exothermic (>25 kcal mol-1); these are 1N-2H, 4N-5H, 4N-6H, and 6N-4H. All of them belong to pentagons. There are other combinations, concluding with 7N-6H (Table 1) with gradually increasing energy, that still belong to the region of spherical curvature, and starting with 7N-8H, the formation energies are positive by >10 kcal mol-1. The main implications of the above results are as follows. (1) The presence of pyracylene units in the closed caps is not mandatory for the amine addition to be possible. (2) The site specificity of the reaction depends on the mutual position of the pentagons. If the caps contain pyracylene units, then the addition preferentially takes place 865
the simple solubilization/dispersion of CNTs but also for the synthesis of chemical linkers for the immobilization of other compounds on the nanotubes (by using bi- and polyfunctional amines). Acknowledgment. Financial support from the National Council of Science and Technology of Mexico (grants CONACYT-36317-E and -40399-Y) and from the National Autonomous University of Mexico (grants DGAPAIN100402-3 and -IN100303) is greatly appreciated. References
Figure 4. Dispersions of (A) starting MWNTs and (B) ODAMWNTs in 2-propanol 24 h after ultrasonication. The ODA-MWNT sample did not show visible changes until after about 45 days.
on their 6,6 bonds; if they do not, then the preferential reaction sites are C-C bonds of the pentagons. (3) In any event, ideal CNT sidewalls composed only of benzene rings are inert with respect to amines. (4) Because introducing pentagons into the ideal sidewalls causes a distortion of the cylindrical curvature, one can expect these defects to be reactive toward amines as well. The high organic content of ODA-MWNTs observed in TGA apparently results from the enhanced reactivity of the real sidewalls containing numerous defects. The solubility/dispersibility of ODA-MWNTs was tested by ultrasonicating them in 2-propanol for 20 min and comparing their behavior to that of the starting MWNTs. While the latter began to precipitate almost immediately after ultrasonication (Figure 4), ODA-MWNT solutions/dispersions did not exhibit visible changes for more than 1 month. In conclusion, solvent-free amination is a simple one-step method for the derivatization and solubilization of CNTs, which were not previously subjected to oxidative treatment with strong acids. This reaction involving nanotube closed caps and five-membered rings of the wall defects is the most direct link between CNT and fullerene chemistry, contrary to all previously-designed derivatization methods. The amination procedure requires no additional chemical activation (unlike usual methods of the carboxylic group amidation) and is relatively fast (ca. 2 h), and excess amine is spontaneously removed from the product. There is no need to use an (organic) solvent medium; this feature not only is attractive from an ecological point of view but also helps to avoid the undesirable aggregation of the nanotube material. A variety of amines can be employed, with the condition of their sufficient thermal stability and volatility under reduced pressure. The described method can be useful not only for
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NL049746B
Nano Lett., Vol. 4, No. 5, 2004
