J. Phys. Chem. B 2002, 106, 1247-1251
1247
Formation Mechanism of Carbon-Nanocapsules and -Nanoparticles Based on the In-Situ Observation Ayumu Yasuda,*,†,‡,§ Noboru Kawase,| Florian Banhart,⊥ Wataru Mizutani,†,‡ Tetsuo Shimizu,†,‡ and Hiroshi Tokumoto†,‡ Nanotechnology Research Institute (NRI), National Institute of AdVanced Industrial Science and Technology (AIST) Tsukuba Central 4, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8562, Japan, Joint Research Center for Atom Technology (JRCAT), c/o AIST Tsukuba Central 4, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8562, Japan, New Energy and Industrial Technology DeVelopment Organization (NEDO), c/o AIST Tsukuba Central 4, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8562, Japan, Nitto Analytical Techno-Center (NTC), 1-1-2 Shimohozumi, Ibaraki, Osaka 567-8680, Japan, and Zentrale Einrichtung Elektronenmikroskopie, UniVersita¨ t Ulm, Albert-Einstein-Allee 11, 89069 Ulm, Germany ReceiVed: July 24, 2001; In Final Form: October 31, 2001
A new way to form carbon nanotubes (CNTs), carbon nanocapsules (CNCs), and carbon nanoparticles (CNPs), where polyyne-containing carbons are heated and irradiated by an electron beam in a transmission electron microscope (TEM), has been developed. The technique was applied to carry out an in-situ observation of those formation processes. Though the CNPs have been reported to be formed accompanied by the CNTs, the CNPs and the CNCs were found to be formed independently from the CNTs. The CNTs are preferentially inside of the polyyne-containing carbon films, while the CNPs/CNCs are outside of the films. The difference of the inside and the outside are discussed to lead to understanding of the CNC and CNP formation process. From the in-situ observation, the existence of metal particles, the high surface energy, the high wettability, and the high viscosity of the polyyne-containing carbons are assumed to be relevant to the preferential formation of CNCs/CNPs to CNTs.
Introduction CNTs1-3
much attention has been paid to this After finding material, while a variety of carbon nanostructures, including CNCs and CNPs, have been found. After the successful formation of LaC2-filled CNC,4,5 a variety of metal-carbidefilled CNCs (metal: V,6 Cr,6 Zr,6 Cr,7 Mo,7 W,7) have been reported. Besides the metal-carbide-filled CNCs, metal-filled CNCs have been found as well (metal: Ru,8 Rh,8 Pd,8 Os,8 Ir,8 Pt,8 Au9). The CNCs have been studied for prospective applications: magnetic nanoparticles protected from oxidation,10,11 superconductivity of encapsulated YC2,12 and radioactive nanoparticles13 designed for radiology, radiation therapy, and disposal of radioactive substances. A growth model of CNCs has been presented for rare-earthfilling CNCs.14 First, rare-earth/carbon alloys, which are in a liquid or quasi-liquid phase, are formed. Second, graphitization proceeds from the surface of the alloys and the graphite cages are formed. In the case of Fe, Co, and Ni, graphite cages precipitate on the surface of the metal/carbon alloy cores. CNPs, or nanopolyhedrons, which have hollows inside the CNCs, have been first reported in coexistence with CNTs.15 Those are also considered as gigantic multilayered fullerenes or gigantic hyper-fullerenes.16 The carbon-arc process to form the CNPs has been extensively studied,17-21 and the mechanism to form CNPs has been assumed as follows.22 First, carbons * Corresponding author. E-mail:
[email protected]. † NRI/AIST. ‡ JRCAT. § NEDO. | NTC. ⊥ Universita ¨ t Ulm.
deposit, coagulate to form small clusters, and grow further until the size finally observed. Second, the graphitization proceeds from the outside to the inside and the pore is formed through the condensation from low-density amorphous carbons to highdensity graphitized carbons. The formation mechanism of carbon onions (COs) has been well revealed by a series of beautiful photographs showing the transformation of CNPs to COs.23,24 The photographs show that the CNPs partially collapse with elimination of the hollow cavity and evolve into a concentric-structured sphere. The latter step takes place from the outside in, and has been described as “internal epitaxial growth.” On the other hand, another mechanism, where CNPs are transformed by the sputtering of atoms out of their original positions by knock-on collisions with electrons of the beam, followed by a shrinkage of the shells, has been presented.25,26 Though those do not coincide, they show that the COs evolve from the CNPs. A new process for CNT, CNC, and CNP formations, where polyyne-containing carbons are heated and irradiated by an electron beam in a TEM, has been developed.27-30 The polyynecontaining carbons are prepared in the following scheme:
(-CF2-CF2-)n + 4n Mg•+ f (-CtC-)n + 4n Mg2+ + 4n Fwhere the reduction by the magnesium cation radical is tentatively assumed,31 though it is not well understood as a reductant. So far, polyyne-route formation of fullerenes has been assumed theoretically32 and supported by experimental data.33-35 As for CNTs, the polyyne-route formation has been shown to
10.1021/jp012864s CCC: $22.00 © 2002 American Chemical Society Published on Web 01/19/2002
1248 J. Phys. Chem. B, Vol. 106, No. 6, 2002
Figure 1. Schematics of the CNC, CNP, and CNT formations. 1: PTFE film (cross section); 2: carbonized layer of the PTFE film; 3: CNCs and CNPs formed on the outside of the carbonized layer; 4: CNTs formed on the inside of the layer; 5: direction of the electronbeam irradiation.
Yasuda et al.
Figure 2. Beginning stage of the CNC formation. From this stage, graphitization proceeds further.
work experimentally, as well.36,37 The technique employed here features an advantage: in-situ observation of the formation. The in-situ observation of the CNT formation has been already partially presented.30 In this paper, the formation mechanism of the CNPs and the CNCs is delineated and discussed, based on the in-situ observation. Experimental Section The CNC and CNP formation is comprised of two processes: preparation of polyyne-containing carbons, and irradiation of an electron beam at 600 °C under low pressure. The details have been presented in other publications.27-30 For the preparation of the polyyne-containing carbons, poly(tetrafluoroethylene) (PTFE) films were reduced electrochemically by a two-electrode method (anode: magnesium; cathode: stainless steel) under argon at 0 °C, associated with a sacrificial anode technique. The PTFE films (10 mm × 10 mm × 60 µm) were charged with the solvent containing supporting salts (tetrahydrofuran (THF): 30 mL; LiCl: 0.8 g; FeCl2: 0.48 g) in a flask. The DC voltage (40 V) was applied between the anode and the cathode for 10 h. After the reduction, the films were washed in THF and dried in a vacuum. The films were analyzed by an IR spectrograph (MagnaIR, Nicolet), Raman spectrograph (HoloLab 5000, Kaiser), and XPS (ESCA 750, Shimazu). The prepared polyyne-containing carbon film was embedded in an epoxy resin (Araldite CY211, Ciba) and cross sections were cut by a microtome (UltraCut, Leica). The specimen was heated from a room temperature to 600 °C and irradiated by an electron beam in a TEM. The in-situ observation was carried out on two TEMs (ARM1250, JEOL; H7100, Hitachi) equipped with heating stages, parallel to the irradiation. Results and Discussion The formation of CNTs by the technique described here has been presented,30 and its mechanism has been analyzed.38 Together with the CNT formation, the CNCs and the CNPs have been found to be formed by this technique as well, whereas the places of these formations are contrasting: inside and outside of the carbonized layer (Figure 1). The PTFE film (A in Figure 1) is reduced electrochemically to form the carbonized layers on both sides (B). The reduced PTFE film is heated to 600 °C to decompose thermally and evaporate the untouched PTFE layer (C). By the irradiation of the electron beam, CNTs, CNCs,
Figure 3. The developing CNC incorporating the metal core (magnesium) at its center.
and CNPs are formed. The CNTs are formed preferentially inside of the carbonized layer, while the CNCs and CNPs are formed outside of the layer. The morphology and the chemical structure are almost the same between the both sides, and this may suggest clues on the preferential formation mechanism between CNTs and CNCs/CNPs. Polyyne-containing carbons, which are essential to the CNT formation,30 have been synthesized by a chemical reduction of PTFE.39-41 The bands specific to carbon-triple bonds are observed by a Raman-scattering measurement, while the triple bonds are not stable and disappear immediately in air. The polyyne-containing carbons prepared here are much more stable, though not absolutely. The anomalous stability is still not well understood. The Raman bands are still observed after the sixmonth exposure to air, whereas the band intensity decreases by half. After one year, the bands disappear completely. In addition, the bands disappear after dipping the prepared polyynecontaining carbons in water. So, the polyyne-containing carbons prepared here are able to be handled in air with much care and are damaged by water, possibly by light and oxygen in air. The in-situ observation of CNC and CNP formations was carried out. Graphene layers are formed around the metal particles, which are assumed magnesium (Figure 2). At this
Formation of Carbon-Nanocapsules and -Nanoparticles
J. Phys. Chem. B, Vol. 106, No. 6, 2002 1249
Figure 4. Evaporation process of the metal cores. Some CNCs encapsulate the metal cores and some metal cores have melted and evaporated.
Figure 6. Magnified photograph of the B in Figure 4. The evaporation of the metal core is completed to form a CNP.
Figure 5. Magnified photograph of the A in Figure 4. The metal (magnesium) core is evaporating through the graphene layers and reducing its size.
stage, the graphenes include defects and are not well crystallized (Figure 3). The metal particles evaporate gradually parallel to the further graphitization (Figures 4-6). Finally, the metal cores evaporate completely to form CNPs. In the case that the process is stopped before the magnesium evaporates, the CNCs incorporating magnesium are obtained. The formation process was observed on two TEMs. One is of low accelerating voltage (H7100, Hitachi, accelerating voltage: 100 kV, current intensity: ca. 1A/cm2) and another of high accelerating voltage (ARM1250, JEOL, accelerating voltage: 1.25 MV, current density: ca. 50 A/cm2). The current intensities are estimated from the beam currents and the spot areas. Though the voltage and the current are greatly different, the formation of CNCs/CNPs is almost the same between them, in contrast to the CNT formation, where the CNTs move greatly like spring-rods at 1.25 MV.38 The photographs in Figure 2 and Figures 4 to 7 were taken on H7100, and Figure 3 on ARM1250. The polyyne-containing carbons contain iron, magnesium, and lithium in the form of metal or cation. Those were added to the
Figure 7. Final stage of the CNP formation. All metal cores have evaporated.
solvent for the electrochemical reduction, and were incorporated in the carbons. The metal observed in this study is mainly magnesium metal. The metal melts and evaporates at 600 °C in the TEM, which corresponds to magnesium (melting point: 650 °C; boiling point: 1107 °C). Lithium metal (melting: 179 °C; boiling: 1317 °C) is not observed and is supposed to melt and evaporate at the early stage of heating in the TEM. Iron metal (melting: 1535 °C; boiling: 2730 °C) is observed occasionally, much less than magnesium, and exists until the end of the formation process. The small amount of iron metal and ion might work as catalysts, whereas the details of the effects are not sure in this study. The new findings in this study are in the preferential formation of CNTs and CNCs/CNPs: the CNT formation inside of the carbonized layer and the CNCs/CNPs outside. So far, CNPs have been reported to be formed accompanied by the CNTs,42 where CNPs were formed by the arc-discharge at ca. 3000 °C. In this paper, the formation was carried out at much lower temperature (600 °C), and the selectivity between CNTs and CNCs/CNPs emanates more clearly dependent on the preferential formation conditions.
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Yasuda et al. of the long rods of the CNTs. As far as we observed, the rodlike carbons are formed first, and the formation of hollows and graphitization are followed.30 If the rodlike carbons were not formed by the high viscosity, the long CNTs would not be formed and tiny CNCs/CNPs would be formed instead. The observed formation mechanism of the CNCs/CNPs are shown schematically in Figure 8. The micro-graphenes in the polyyne-containing carbons form a sphere shape around the core metal (A in Figure 8). The graphitization proceeds as a whole (B). The graphitization of the shell specifically from the outside is not observed in this study and proceeds as a whole. The core metal evaporates through the graphene layers, whose graphitization is not perfectly completed (C). Finally, the CNP is formed (D). As for core metals of the CNCs, the encapsulated elements have been presented to belong to the group of nonvolatile metals.43 The volatile magnesium is shown, however, to be incorporated into the CNCs by applying the low-temperature process described here. Further study on the CNC-formation at a low temperature may expand the metal species to be encapsulated into the CNCs. Conclusion
Figure 8. Schematics of the CNC and CNP formation process: 1, metal core (magnesium); 2, carbon micro-graphenes. The process proceeds from the A to the D, where each time-interval is assumed a few minutes.
Besides the formation temperature, the composition of the polyyne-containing carbons is supposed to be different. The metal particles, mainly magnesium, are incorporated in the carbons and are observed mainly outside of the carbon layer. From the incorporation mechanism during the electrochemical reduction, the rich incorporation at the outside is reasonable. The metal particle may work as a core to form the CNCs/CNPs. The existence of the metal core is the second requirement for the preferential formation of the CNCs/CNPs. In addition, the chemical structure of the polyyne-containing carbons may play an important role for the selective formation of CNTs and CNCs/CNPs. As we mentioned, the prepared polyyne-containing carbons are not stable and partially damaged during the specimen preparation for the TEM: embedding by the epoxy-resin and cutting by the microtome. The damage may include an oxidation and a cross-linking, and takes place from the outside. The outside, where the CNCs/CNPs are formed, is more oxidized and cross-linked than the inside, where the CNTs are formed. The oxidized polyyne-containing carbons are more wettable on the metal particles that are supposed partially oxidized on their surfaces. Further, the oxidized polyynecontaining carbons have higher surface energy, which leads to a tendency to form a spherical shape to reduce the surface area. The wettability on the metal and the high surface energy are related to the preferential formation of CNCs/CNPs to CNTs. The damage is relevant to the cross-linking in the polyynecontaining carbons, as well. The cross-linking increases the viscosity of the melted carbons and does not allow the formation
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