2.2.1 Materials
The following materials were used:
• SWNT: P2, HiPco, CoMoCat (Table 2.1, as received, without further purification)
• Solvents: toluene (VWR , 99.8+%, redistilled), tetrahydrofuran (VWR, 99.7+%, redistilled). Anhydrous solvents must be used during the syntheses, because both the alkali metals and the carbanions are air and water sensitive. Special care has to be taken all along the reactions. Air sensitive steps have to be performed in inert atmosphere. In our case we used an Ar-filled dry box. Toluene was cryo-distilled from Na-K alloy in a vacuum line. THF was redistilled from K-benzophenone.
• Reducing agents: K, Rb (Sigma-Aldrich, 98+%), naphthalene (Sigma-Aldrich, 99+%, as received)
• Reactants: methanol (VWR, 99.8+%, as received), 1-iodobutane (Sigma-Aldrich, 99%, as received)
• Filtering and washing: tetrahydrofuran, ethanol (VWR, 99.7+%, as received), 1:3 HCl:H2O, distilled water, acetone (VWR, 99.8+%, as received)
2.2.2 Synthetic routes
The functionalization method, which was inspired by exfoliated graphite intercala-tion compounds, is described below. The reacintercala-tion scheme is shown in Figure 2.1.
Figure 2.1: Reaction scheme of reduction of nanotubes by alkali metal and subsequent addition of hydrogen (n-butyl) groups.
About 100 mg of as-received SWNT was first annealed in dynamic vacuum (10−6 mbar) at 250 ◦C for 12 hours, followed by transfer into an Ar dry box. In the dry box, alkali metal (potassium in case of P2 and HiPco and rubidium in case of CoMoCat) was added in a glass vial, keeping the carbon:alkali metal molar ratio 4:1.
The glass vial was sealed on a vacuum line. Annealing at 200 ◦C for 12 hours was enough for the alkali metal to intercalate into the nanotube bundles. Intercalation was indicated by the copper/gold color of the sample [80].
Subsequently, the intercalated sample was taken back to the dry box. The vial was opened and the intercalated nanotubes were put into a Schlenk-type flask with a funnel (Figure 2.2). 40 ml anhydrous toluene was added to the flask and 20 ml to the funnel. Toluene was used as an aprotic solvent to avoid side reactions with any other H source. Outside of the dry box, sonication was applied for 15 minutes to enhance the intercalation process. Next, 5 ml methanol was filled fast and carefully to the funnel. Methanol/toluene was added dropwise into the flask during sonication.
Sonication was continued for 2 more hours, and the mixture was left overnight. The sample was filtered on a Millipore nylon membrane filter (0.1 µm pore size), washed with ethanol, 1:3 HCl:H2O, distilled water, ethanol and acetone. Finally, it was dried in dynamic vacuum at 200◦C for 12 hours.
The product obtained this way was transferred back into the dry box. The whole process described above, except the initial annealing, was repeated two more times in order to investigate whether it is possible to improve the degree of hydrogenation by applying successive steps.
The main products of reactions with methanol are hydrogenated nanotubes, but there are side reactions, such as hydrogen evolution, when attachment of H to the
Figure 2.2: The Schlenk-type reaction flask. In the flask, there are K-intercalated nano-tubes. It is clearly seen that they are not dispersed in the toluene, even after sonication, since they are negatively charged in a totally apolar solvent.
nanotube is kinetically hindered, or when the unreacted alkali metal reduces methanol directly. In case of reactions with 1-iodobutane, n-butylated nanotubes are the product.
Reference samples were made of pristine nanotubes by performing the same steps as at the hydrogenation reactions (initial annealing, annealing in sealed glass tube, addition of methanol, washing, annealing in dynamic vacuum),except for the addition of alkali metal.
The preparation using modified Birch reduction (the reaction scheme is shown in Figure 2.3) was started by a prior annealing of 100 mg of the as-received HiPco for 12 hours at 250 ◦C in dynamic vacuum (at 10−6 mbar). Subsequently, the nanotubes were transferred into the dry box. The same Schlenk-type flask as for alkali metal in-tercalation was used for the reaction. Naphthalene and potassium were dosed in excess.
Then THF was added both to the flask and to the funnel (100 ml and 20 ml, respec-tively). The sample was left for 15 min. Meanwhile potassium reacted with naphthalene and composed a dark green complex. This complex reduced the nanotubes to carban-ions in an equilibrium process. Then the flask was sonicated for 15 min to loosen the nanotube bundles and to promote the carbanion formation. 5 ml methanol was added to the funnel. The reactant was added slowly, dropwise during continuous sonication.
The reaction is quite fast, indicated by the almost instant disappearance of the green
Figure 2.3: Reaction scheme of hydrogenation (n-butylation) of nanotubes by modified Birch reduction.
color of the complex. Small bubbles were also observed, indicating the hydrogen evo-lution from the side reaction of potassium and methanol, and their excess. Sonication was kept going for 1 more hour. The sample was left overnight. The same filtering and washing procedure was applied as after the alkali metal intercalated reaction, but completed by a step of washing with THF.
Using HiPco, two more series of samples were prepared. By potassium intercala-tion and by modified Birch reducintercala-tion, following the processes described above, n-butyl groups were attached to the HiPco nanotubes. The reagent was 1-iodobutane instead of methanol. Three successive steps were also performed.