Nanocomposite Materials of Alternately Stacked C 60 Monolayer and Graphene

1 Department of Physics, Aichi University of Education, Hirosawa 1, Igayacho, Kariya 448-8542, Japan 2 Research Department, Taiho Kogyo Co., Ltd., 2-47 Hosoya-Cho, Toyota 471-8502, Japan 3 Aichi Industrial Technology Institute, 1-157-1, Ondacho, Kariya 448-0013, Japan 4 Department of Applied Physics and Chemistry, University of Electro-Communications, 1-5-1 Chohugaoka, Chofu, Tokyo 182-8585, Japan 5 Department of Materials and Life Science, Faculty of Science and Technology, Seikei University, 3-3-1 Kichijoji-Kitamachi, Musashino, Tokyo 180-8633, Japan


Introduction
C 60 intercalated graphite has previously been designed, and the electronic properties and stabilities have been studied theoretically [1].It is also reported that the C 60 intercalated graphite may possess potential applications in semiconductor [2] and hydrogen storage device [3].On the other hand, fullerene molecules, especially, C 60 , also have unique lubrication effects, and they have been treated as a lubricant due to their spherical shape [4][5][6].Up to now, we have studied C 60 monolayers confined by graphite flakes [7] and C 60 monolayers included among graphite [8][9][10], which, interestingly, exhibit ultralow friction, because quasifreely rotating C 60 molecules act as molecular bearings.However, it is quite difficult to perform a mass production for intercalating C 60 molecules into graphite and to provide nanomaterials with a perfectly alternate stacking of C 60 monolayer and graphene in previous studies.
Here we show nanocomposite materials consisting alternately of a stacked single graphene sheet, by the graphite intercalation technique in which alkylamine molecules help intercalate large C 60 molecules into the graphite.

Experimental
First, graphite oxide (GO) was synthesized from graphite powder with an average size of 500 μm (Nippon Carbon Co., Ltd.) in accordance with the Hummers method [12] with some modifications, and the intercalation of octylamine into graphite (GO-OA) was performed as described previously in [13,14].Next, octylamine-intercalated graphite oxide (GO-OA) was added to fullerene solution (100 mg of C 60 dissolved in 100 mL toluene), and after that, the toluene used was evaporated at room temperature, leaving behind in the products (GO-OA-C 60 ).The C 60 fullerene used in this experiment was purchased from Frontier Carbon Co., Ltd, in  Japan.The products of GO-OA-C 60 were treated with 0.1 N hydrochloric acid solution at room temperature for at least 30 minutes and dried in air at 80 • C overnight to remove the octylamine, resulting in the C 60 -intercalated graphite oxide (GO-C 60 ).Finally, in order to remove C 60 powders that are not intercalated into the graphite, and moreover, to remove octylamines that are intercalated into the graphite, GO-C 60 was heated for at least 80 minutes at 600 • C under a high  7 mm probe head), and high-resolution electron microscopy (JEM3100FEF: a lattice resolution of 0.1 nm when it is operated at an acceleration voltage of 300 kV).

Results and Discussion
Figure 1(a) shows the X-ray diffraction (XRD) intensity from graphite oxide (GO) and alkylamine-intercalated graphite oxide (GO-amine) with different alkyl chain lengths (C n H 2n+1 NH 2 ) (n = 3 to 8).The appearance of a peak in the GO of Figure 1(a) shows that the spacing between graphite oxide sheets is approximately 8 Å, which is identical to the published data [12].It is found that the spacing between graphite oxide sheets in the case of the GO-amine (n = 3 to 8) increases with the increase in the length of the alkyl chain incorporated in the interlayer space of the GO. the GO-C 60 , which do not include C 60 molecules, also revert to the graphite layers when alkylamines go out from it after heating.It is expected that the peaks of A and B (n = 6 to 8) in Figure 2 are due to the d-spacing between the graphenes intercalating the C 60 monolayer and their stacking faults (the disorder of the stacking) [15], respectively.However, since peak A is also widely distributed, the d-spacings between the graphenes intercalating the C 60 monolayer are considered to be widely distributed.
Figure 4 shows the FT-IR spectra from the specimens of Figure 2.This result is consistent with the conclusion based on Figure 2 that C 60 molecules can be intercalated into the GO with alkylamine chains longer than that of n = 5 because the FT-IR spectra from n = 6, n = 7, and n = 8 in Figure 4 exhibit the C 60 intermolecular IR-active (F 1u ) modes [11] although those with the shorter alkylamine chains do not exhibit these modes.However, one mode of 526 cm −1 among IR-active (F 1u ) modes only appears in these FT-IR spectra because the number of C 60 molecules included in the specimens is rather small.
The rotational dynamics of C 60 molecules between graphenes have been investigated by 13 C NMR in the temperature range from room temperature to −80 • C. We prepared C 60 materials 20-30% enriched in 13 C in order to increase the 13 C NMR signal.The present specimen was mixed with Na 2 SO 4 in a weight ratio of 1 : 50 to avoid arcing in a NMR probe.The NMR experiments were performed at 75.4 MHz for 13 C in an external field of 7.1 T by the pulse inversion recovery method. 13C NMR spectra were taken by Fourier transforming the signal following the π/2 pulse.The typical π/2 pulse width was 5.4 us.It is well known that for C 60 molecules in solid C 60 at room temperature, large rotational motion averages out the chemical-shift anisotropy (CSA) and the 13 C NMR spectra show motional narrowing of 2.5 ppm in width.In contrast, spectra broaden at low temperature and develop the CSA power pattern with a CSA tensor with the principle values δ 11 = 213 ppm, δ 22 = 182 ppm, and δ 33 = 33 ppm [11].
Figure 5 shows the 13 C NMR spectra for the specimens of Figure 2 at room temperature, where the 13 C NMR spectrum at room temperature is the same as that at the temperature of −80 • C.Only one sharp line with a peak position of 144 ppm was observed, and its line shape is a Gaussian-like function with an FWHM value of 5 ppm.The positions are in good agreement with the average principle values for C 60 molecules in solid C 60 , and the linewidth is about one-twentieth narrower than that of the powder pattern [11].These observations clearly demonstrate the lack of the polymerization [16] of C 60 molecules in the present material.Furthermore, the observed Gaussianlike line shape means a motional narrowing and that C 60 molecules rotate quasi-freely with a correlation time on the order of 10 ps.This correction time is similar to that of the same case [17].This means that no strong bonding such as chemical bonding between the graphenes and C 60 molecules is made, and C 60 molecules easily rotate for outer force.
It is possible that the specimens in the case of Figure 2 are broken into nanoribbons of nanometer thickness by ultrasonic vibration, which is analogous to the preparation method of the graphene nanoribbon.A high-resolution electron microscope (HR-TEM) image of the nanoribbon is shown in Figure 6(a).First, two parts of graphene layers (areas A) are shown in Figure 6(a).A missing rows of zigzag chains clearly appear in the graphene network, which indicates that the thickness of the specimen is on an atomic scale [18], and moreover, moire patterns formed by graphenes also appear in area A. It is expected that the contrast of area B originates from the C 60 molecules confined between graphene sheets, although we will discuss this contrast later in detail.Now we illustrate the structure model of C 60 -intercalated graphite in Figure 6(b), that is, the nanocomposite consisting of alternately stacked graphene sheets and C 60 monolayers in which C 60 molecules can form a monolayer and rotate between graphene sheets.

Conclusion
At present, preliminary experiment indicates it is possible to intercalate huge fullerene molecules, such as C 70 and La@C 82 larger than a C 60 molecule into graphite.Hence it is expected that the unique structure comprising huge fullerene molecules intercalated into graphite will give rise to attractive novel applications such as superlubrication materials, variable nanocapacitance, nanoswitches, and hydrogen storage in the future.Moreover, since the tribological test of grease and oils with an additive of this nanocomposite powder exhibits excellent lubricating performance with an ultralow friction previously unattained, the structure and material properties are expected to be more attractive also from scientific and material points of view.These are examples of the application that shows the novel mechanical property, and the intercalated materials will lead to promising materials with novel mechanical, physical, and electrical properties.

Figure 1 :
Figure 1: XRD intensity from as-prepared samples.(a) XRD intensity from graphite oxide (GO) and alkylamine-intercalated graphite oxides (GO-amine) with different chain lengths (C n H 2n+1 NH 2 ) (n = 3-8).The peaks of GO and GO-amine are indicated by the arrows in the figure.(b) XRD intensity from GO-amine with different chain lengths in C 60 solution (n = 3-8).

Figure 2 :
Figure 2: XRD intensity from the specimens heated for 80 minutes at 600 • C under a high vacuum of 10 −6 Torr for the GO-C 60 specimens of Figure 1(b).

Figure 5 :
Figure 5: The 13 C NMR spectra for the specimens of Figure 2 at room temperature.Only one sharp line with a peak position of 144 ppm [11] was observed.

Figure 1 (Figure 6 :
Figure1(a) shows the X-ray diffraction (XRD) intensity from graphite oxide (GO) and alkylamine-intercalated graphite oxide (GO-amine) with different alkyl chain lengths (C n H 2n+1 NH 2 ) (n = 3 to 8).The appearance of a peak in the GO of Figure1(a) shows that the spacing between graphite oxide sheets is approximately 8 Å, which is identical to the published data[12].It is found that the spacing between graphite oxide sheets in the case of the GO-amine (n = 3 to 8) increases with the increase in the length of the alkyl chain incorporated in the interlayer space of the GO.Figure1(b)shows the XRD intensity from the GOamine (n = 3 to 8) in C 60 solution, which we call the C 60intercalated graphite oxide (GO-C 60 ).It should be noted that there appear drastic changes in the XRD intensity between n = 5 and n = 6 in Figure1(b), which indicates that C 60 molecules are intercalated in the interlayer space of the GO by the driving force of alkylamine situated at between the graphene oxide sheets when the interlayer space is sufficiently larger than the C 60 molecule.However, there exist many C 60 powders which are not intercalated into the GO in these GO-C 60 specimens because the stronger peaks, (111), (220), (311), (222), (331), (420), (422), and (511), of XRD intensity from C 60 powders (JCPDS file No.44-0558) appear in the spectra of Figure1(b).In order to remove C 60 powders that are not intercalated into the GO, and moreover, to expel the alkylamines which are intercalated into the graphite, the GO-C 60 specimens of Figure1(b) were heated for 80 minutes at 600 • C under a high vacuum of 10 −6 Torr, as shown in Figure2.It should be noted that n = 6, n = 7, and n = 8 have broad peaks of A and B corresponding to d-spacings of 9 Å and 4.6 Å, respectively, in addition to a broad peak of d = 3.3 Å corresponding to the spacing of graphite layers, although n = 3, n = 4, and n = 5 have only a single broad peak of d = 3.3 Å.Since the GO-amine reverts to the graphite layers when alkylamines leaves the GO-amine host after heating at up to 600 • C, as shown in the XRD intensity of Figure3, the graphene oxide layers in