Cm-size free-standing self-organized buckypaper of bucky-onions filled with ferromagnetic Fe₃C

Abstract

Novel cm-size free-standing buckypapers of bucky-onions filled with a single-phase of ferromagnetic Fe₃C single crystals were serendipitously discovered. These buckypapers are obtained directly in situ as the dominant product of the pyrolysis of ferrocene. Vibrating sample magnetometry also revealed an extremely large coercivity of 0.120 tesla and a saturation magnetization of 41 emu g⁻¹.

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Introduction

Since 1985, novel carbon nanostructures such as Buckminster fullerene (C₆₀),^1,2 carbon nanotubes (CNTs),^3,4 bucky-onions^5,6 and recently graphene⁷ have attracted the attention of numerous research groups owing to their exceptional physical and chemical properties. In particular, the high chemical stability of C₆₀,^1,2 bucky-onions^5,6 and CNTs^3,4,8–14 make these structures ideal nano-capsules that can be filled with a specific material of interest.^8–17 However, despite the numerous reports in the literature, the nucleation mechanism of these nanostructures is not well understood and remains strongly controversial.^18–21

In 1997 Elliot et al.¹⁹ suggested that two main parameters are responsible for the nucleation and growth of carbon nanostructures: the local-path carbon-to-metal ratio (LCM) and the global carbon-to-metal ratio (GCM). LCM refers to metal particles having the same local conditions along the same spatial pathway, while the GCM determines the weighting between different LCMs and is generally defined by the stoichiometry of the molecule used as precursor. Chemical vapour deposition (CVD) techniques have been generally designed to avoid local pathway fluctuations.^8–17

Interestingly the research works reported by Raty et al.²⁰ and Anna Moisala et al.²¹ proposed two mechanism of nanotube nucleation from heterogeneously nucleated particles. The first mechanism²⁰ involves the formation of a floating graphene-cap and can be divided into three step: (i) diffusion of single carbon atoms on the surface of the catalyst, (ii) formation of a graphene sheet floating on the catalyst surface with edge atoms covalently bonded to the metal, (iii) root incorporation of diffusing single C atoms.

Differently, a nucleation process involving the formation of islands of C atoms was proposed by Moisala et al.²¹ (second mechanism). In this process, the competition between two fluxes of carbon was described. These fluxes were identified as (1) the segregation flux of dissolved carbon atoms towards the particle surface and (2) the diffusion flux of carbon atoms toward the particle surface seeking their lowest energy states. Due to the competition between the segregation and diffusion fluxes, two situations would then occur in the system. (i) First situation: if the segregation flux is higher than the diffusion flux, initiation of CNT growth can occur; (ii) second situation: if the segregation flux is lower than the diffusion flux, carbon will form the thermodynamically most stable system consisting of metal particles surrounded by graphitic layers (bucky-onions).

These synthesis products have attracted the attention of numerous researchers for applications in magnetic data storage system, exchange bias systems,^8–17 supercapacitors and power-storage systems^22–27 (owing to their resistance–inductance–capacitance properties). However, despite the nucleation and growth models described above, the control of these processes through CVD approaches remains challenging and depends strongly on the CVD parameters (i.e. reactor design, vapour flow-rate, pyrolyzed vapour concentration). Numerous approaches have been developed for the creation of magnetic and non-magnetic films^10,22–25 and gels²⁶ suitable for the applications listed above. However, the in situ synthesis of large scale buckypapers comprising bucky-onions remains challenging. Interestingly recent works from Boi et al.²⁸ have shown that CNTs urchin-structures with a core composed of Fe₃C, γ-Fe and α-Fe filled bucky-onions can be obtained in conditions of locally perturbed vapour-flow, in the viscous boundary layer created at the boundary between a rough substrate-surface and the main Ar gas-flow.²⁸

In this paper we report another effect of the locally perturbed vapour flow induced by the flow-facing leading edge of the used substrate. We find that cm-size free-standing buckypapers of bucky-onions filled with a single-phase of ferromagnetic Fe₃C single crystals (Fe₃C-filled CPBO) can be grown on the leading edge of a silicon-substrate and on the top of CNTs films and directly removed with a permanent magnet. These structures are obtained directly in situ in cm scale through the pyrolysis of ferrocene with very low flow-rates. Differently, if no silicon substrate is used, CNTs-films were obtained in the inner wall of the reactor. The morphology of these structures is investigated through scanning electron microscopy (SEM). The cross-section, single crystal arrangement and phase-composition of the encapsulated crystals is demonstrated through transmission electron microscopy (TEM) and high resolution TEM (HRTEM). The single-phase Fe₃C filling is demonstrated through X-ray diffraction (XRD) analyses while the magnetic properties are investigated through vibrating sample magnetometry (VSM). An extremely large coercivity of 0.120 tesla and a saturation magnetization of 41 emu g⁻¹ were measured.

Experimental

Cm-size Fe₃C-filled CPBO were obtained by sublimation and pyrolysis of 180–200 mg of ferrocene in an Ar flow of 11 ml minute⁻¹ on the top of Si/SiO₂ substrates positioned inside a quartz tube reactor of length 1.5 metres. A one-zone electrical furnace was used for reaching the desired temperature of pyrolysis (temperature of 990 °C). The sublimation temperature was 500 °C. The used reaction time was 25 min. A fast cooling method was used to bring the Fe₃C-filled CPBO to room temperature. In this method, cooling times of approximately 20 min were achieved by removing the furnace along a rail system (quench).

The Fe₃C-filled CPBO was removed through the use of a permanent magnet from the upper-surface of aligned CNTs grown on the Si/SiO₂ substrates and from the first edge of the Si/SiO₂ substrates (edge facing the Ar flow). SEM investigations of the morphology of the obtained Fe₃C-filled CPBO were performed with a JSM-7500F at 15 kV. TEM and HRTEM investigations were performed using a 200 kV American FEI Tecnai G²F20 fitted with field emission gun. XRD analyses were performed with a Philips Xpert pro MPD (Cu Kα with λ = 0.154 nm). In order to identify the phases composing the Fe₃C-filled CPBO and extract the unit cell values, the Rietveld refinement method was used. The magnetic measurements were performed at room temperature by employing a VSM 2.5 tesla electromagnet East Changing 9060 at the magnetic field of 1.3 tesla.

Results and discussion

SEM micrographs revealed the morphology of the Fe₃C-filled CPBO. A typical example of the morphology is shown in Fig. 1A with a top view and in Fig. 1B with a closer view.

Fig. 1 Scanning electron micrographs (A and B) showing the morphology of a typical cm size free standing Fe₃C-filled CPBO. See ESI† for SEM images with higher detail and example of energy dispersive X-rays analyses.

Note that the morphology of the Fe₃C-filled CPBO was slightly damaged due to the pressing procedure necessary for the SEM sample preparation. The crystal-structure and crystal symmetry of the Fe₃C-filled CPBO was investigated through XRD analyses. A typical XRD diffractogram of the Fe₃C-filled CPBO is shown in Fig. 2A.

Fig. 2 X-ray diffractogram (A) and Rietveld refinement (B) of a typical cm size free standing Fe₃C-filled CPBO. Each peak is labelled with the corresponding crystal-plane reflection.

The CPBO structure was identified by the 002 peak of graphitic carbon with space group P6₃/mmc. The Fe₃C filling was instead identified by the 111, 200, 210, 002, 201, 211, 102, 220, 031, 112, 131, 221 and 122 reflections. Two reflections of Fe₂O₃ were also observed (104 and 110 reflections). These observations suggest that each bucky-onion composing the CPBO is filled with a single phase of Fe₃C. Instead the small Fe₂O₃ reflections can be associated to particles not completely covered with graphitic layers that oxidize after sample extraction, due to interaction with oxygen.

This interpretation is confirmed by the Rietveld refinement shown in Fig. 2B. The relative abundances of 95% of Fe₃C and 5% of Fe₂O₃ were extracted (in order to compare the relative abundance of Fe₃C and Fe₂O₃ the graphitic carbon contribution was excluded from the refinement). The extracted unit-cell parameters from the Rietveld refinement are as follow:

(1) Fe₃C a = 0.51 nm, b = 0.68 nm, c = 0.45 nm orthorhombic with space group Pnma,

(2) Fe₂O₃ a = 0.51 nm b = 0.51 nm c = 1.38 nm rhombohedral with space group R3̄ch.

The cross section and the single-crystal arrangement of the Fe₃C-filled CPBO phase were then investigated through TEM and HRTEM. The homogeneity of the CPBO was revealed by the TEM micrographs of Fig. 3A and B.

Fig. 3 TEM micrographs showing in A and B the cross-section of a typical Fe₃C-filled CPBO.

Note that the black regions represent thick-areas in which the CPBO structure appears to be bent.

A higher detail of the cross section of the Fe₃C-filled CPBO structure is shown in Fig. 4 where large quantities of Fe₃C-filled bucky-onions can be clearly observed.

Fig. 4 TEM micrograph showing with higher detail the Fe₃C-filled bucky-onions comprised in the CPBO. A typical example of a Fe₃C particle is indicated with the red arrow.

The bucky-onions composing the CPBO appear to interact between each other with van der Waals forces. We will show later that the origin of such close contact in the long range has to be associated to the nucleation mechanism of these structures. Such close-contact between the onions-structures was also confirmed by the HRTEM analyses (see Fig. 5A and B) where a high detail of the connection between two bucky-onions is shown. Note that the lattice fringes observed in the onion structure in Fig. 5A correspond to the graphitic interatomic distance between the 002 planes. The observation of this interatomic distance is in agreement with the XRD analyses in Fig. 2. The single-crystal arrangement of the encapsulated Fe₃C phase was confirmed by the observation of a preferred lattice orientation in the HRTEM analyses of the crystals encapsulated in single bucky-onions. This interpretation is also confirmed by the FFT of the area shown in Fig. 5B were a single pattern of reciprocal lattice spots is found. The yellow circles indicate the 211 reflections of orthorhombic Fe₃C with space group Pnma corresponding to a lattice spacing of 0.211 nm. The red circles indicate the 101 reflection of orthorhombic Fe₃C with space group Pnma corresponding to a lattice spacing of 0.338 nm.

Fig. 5 HRTEM micrographs (A and B) showing with high detail the cross section of a typical bucky-onion (comprised in the CPBO structure) filled with a Fe₃C single crystal (indicated by the red arrow). In B the FFT of the crystal lattice shows (in the inset) a single set of reciprocal lattice spots which proves the single crystal arrangement of Fe₃C.

The presence of a single phase of Fe₃C within the CPBO can be considered a consequence of the fast cooling rate imposed by the furnace removal. Also, the presence of numerous graphitic layers surrounding the Fe₃C particles suggest that the CPBO is formed directly in the pyrolyzed ferrocene-vapour during the reaction time of 25 min and not during the cooling process. These observations suggest therefore a mechanism of formation different with respect to what described in previous reports by Elliot et al. and Moisala et al.^19,21 where the formation of bucky-onions is considered favourable only with slow cooling rates and slow carbon supply or in conditions where the diffusion carbon flux is dominant.

Indeed it was shown that the mechanism of formation of bucky-onions in W-arc processes depends mainly on the kinetics of carbon supply. Similarly to Moisala et al.,²¹ two types of carbon supply were identified: expulsion (from the particle core to the external surface) and deposition (from the external surface to the particle core). For slow growth processes the final product was found to depend on the difference between the particle-graphite interface energy and the vacuum surface energies γ, and the energy to bend the graphite sheets.¹⁹ The formation of bucky-onions was then associated to growth-conditions with low γ interaction.¹⁹ However considering our CVD system, the vacuum surface energies can not be considered since an Ar carrier-gas is used to deliver the ferrocene-vapour and clean the system from oxygen impurities. We suggest that the mechanism of formation of the Fe₃C-filled CPBO depends strongly on the boundary-layer mechanism; note that the boundary layer is created between the substrate-edge (facing the Ar flow) and the Ar flow as shown in Fig. 6A. The bucky-onions can be considered a consequence of a spontaneous homogeneous nucleation process of Fe-based particles induced by the local boundary layer created at the edge of the Si/SiO₂ substrate. The CPBO was indeed observed in the edge of the Si/SiO₂ substrate and on the top of aligned multiwall CNTs grown on the substrate surface from heterogeneously nucleated particles. A schematic of the substrate before and after the CPBO growth is shown in Fig. 6A and B. The formation of such ordered buckypaper can be therefore attributed to the very high concentration of homogeneously nucleated particles produced in the boundary layer vapour and their physical interactions after nucleation and encapsulation. The magnetic properties of the Fe₃C-filled CPBO were then investigated by VSM. A typical hysteresis loop is shown in Fig. 7. Interestingly a very large coercivity of 0.120 tesla and a saturation magnetization of 41 emu g⁻¹ were measured. The measured saturation magnetization is much lower with respect to what expected for a bulk sample of Fe₃C (M_s = 169 emu g⁻¹, T = 483 K).²⁹ This difference could be attributed to the nano-crystalline nature of the sample. However, the observed coercivity is much larger with respect to what expected for nanocrystalline Fe (23 Oe) and for polycrystalline Fe (1 Oe).²⁹ These observations suggest that a fundamental role can be attributed to the strong magnetocrystalline anisotropy (spin–orbit) contribution in the Fe₃C crystals that controls the coercivity of the Fe₃C nanoparticles. The observed coercivity is indeed much larger with respect to that measured at room temperature in the case of the ferromagnetic bucky-paper produced by Ruitao Lv et al. (500 Oe at 300 K)¹⁰ suggesting therefore that the magnetocrystalline anisotropy has a fundamental role in the magnetic properties of the CPBO. Instead the saturation magnetization is comparable with that measured by Ruitao Lv et al.¹⁰ The observation of small Fe₂O₃ reflections suggests that a very small fraction of nanoparticles are not covered completely with graphitic layers. These particles may be formed in the last stages of the CPBO growth when the carbon concentration in the pyrolyzed ferrocene vapour is lower. Therefore, considering the presence of 5% of Fe₂O₃ in the sample (observed with XRD), a magnetic contribution to the saturation magnetization (i.e. exchange coupling) from this phase can not be excluded.

Fig. 6 Schematic of the boundary layer and the deposition area (A and B) of the CPBO inside the quartz tube reactor. The quartz tube reactor is indicated with cyan colour, the Si/SiO₂ substrate is indicated with black colour. The Fe₃C crystals are indicated with blue colour, the γ-Fe and the α-Fe phases are indicated with yellow and light orange colours respectively.

Fig. 7 Hysteresis loop measured from 3 CPBO of cm-size comprising bucky-onions filled with Fe₃C single crystals with an average diameter of 30 nm.

Conclusion

In conclusion we reported the serendipitous discovery of novel cm-size free-standing papers of bucky onions filled with a single-phase of ferromagnetic Fe₃C single crystals. These structures are obtained directly in situ from the pyrolysis of ferrocene on the leading edge of a Si/SiO₂ substrate and on the top of aligned partially filled CNTs. An extremely large coercivity of 0.120 tesla and a saturation magnetization of 41 emu g⁻¹ were measured at room temperature. The magnetic properties and the large scale of these buckypapers can open new avenues toward the development of magnetic data storage, magnetic refrigeration systems and thermoelectrics. In addition these materials could be considered for applications in catalysis,³⁰ supercapacitors,³⁰ energy-storage,³¹ miniaturised fuel cells,³² biomedicine³³ and many others.

Acknowledgements

We acknowledge Prof. Gong Min for his continuous support in this research work. We are also grateful for the NSFC 11404227.

References

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra24983c

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