Temperature-dependent c-axis lattice-spacing reduction and novel structural recrystallization in carbon nano-onions filled with Fe3C/α-Fe nanocrystals

Carbon nano-onions are approximately spherical nanoscale graphitic shells. When filled with ferromagnetic Fe3C/α-Fe nanocrystals, these structures have several important applications, such as point electron-sources, magnetic data recording, energy storage, and others, that exploit the interaction of either or both the shells and the magnetic moments in the filling. Despite these applications receiving much recent attention, little is known about the structural relationship between the carbon shells and the internal nanocrystal. In this work, the graphitic c-axis lattice-spacing in Fe3C/α-Fe-filled multi-shell structures was determined by XRD in the temperature range from 130 K to 298 K. A significant reduction in the c-axis lattice-spacing was observed in the multi-shell structures. A defect-induced magnetic transition was probed and ascribed to the formation of randomly oriented ferromagnetic clusters in the recrystallized disclination-rich regions of the CNOs-shells, in agreement with the percolative theory of ferromagnetism.

CNOs have been also considered for the encapsulation of nanocrystals [3,8,[25][26][27][44][45][46][47][48]. The possibility of encapsulating magnetic materials, especially ferromagnetic crystals, has attracted great attention owing to possible applications in magnetic data recording and energy storage devices [25][26][27][44][45][46][47][48]. Recent studies on these types of materials have also reported the presence of high-temperature structural memory effects, opening possible new applications of these structures in thermoelectric systems [27]. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
CNOs have been filled with many types of ferromagnetic nanocrystals, including metals, carbides and substitutional alloys. The c-axis spacing in the carbon shells has been reported to be 0.34-0.35 nm (corresponding to the 002 reflections of graphite) for CNOs filled with Fe 3 C [25] and 0.34-0.56 nm for FePd 3 -nanocrystal fillings [26,27]. When filled with metal or alloy materials, the diameter of CNOs is generally much larger (in the order of one hundred nanometres) than that reported for unfilled structures (in the order of 5 nanometres) [4,5,[25][26][27]. This important difference can be attributed to the different growth mechanism of these two types. Unfilled CNOs can be produced in powder-form, mainly through arc-discharge approaches [4,5]. Whereas, filled CNOs are grown by chemical vapour synthesis [25] or by chemical vapour deposition methods, for which the addition of sulphur to the ferrocene precursor has been identified as a means of controlling the diameter [48,49].
Despite these important advances little is still known about the structure of these materials at low temperatures. Previous reports have been concerned with high-temperature structural changes in encapsulated Sn and Pb nanocrystals [3], and FePt 3 , FePd 3 , Fe 3 C and Fe 7 C 3 /Fe 5 C 2 fillings [25,27,48,50]. The properties of ferromagnetically filled CNOs at low temperatures are yet to be investigated.
Interestingly, it was recently shown that temperature-dependent structural memory effects are present in the unit cell of Fe 3 C-filled carbon nanotubes [51]. This effect was attributed to cooperative behaviour of the nanotube walls and the encapsulated Fe 3 C nanowires, an effect of potential significance for spacesuit technology as shape memory alloy actuators [52,53].
In this work we report an x-ray diffraction (XRD) study of the structural properties of Fe 3 C/α-Fe-filled CNOs in the temperature range 130 K to 298 K. Fe 3 C/α-Fe-filled multi-shell CNOs with variable diameter were employed in this study.
As the temperature was decreased, we observed a clear reduction in the graphitic c-axis lattice-spacing. This effect was found to be enhanced by improved inter-shell stacking and an unusual recrystallization of outer shells at low temperature. This interpretation was supported by the appearance of multiple peaks and broadening features in the XRD data in the 2θ range 15°to 18°as the temperature was lowered to 130 K. Complementary electron spin resonance (ESR) measurements revealed the presence of a defect-induced magnetic transition at 77 K in the π-electron signal. This magnetic transition was interpreted on the basis of the percolative theory of ferromagnetism and ascribed to the presence of ferromagnetic correlation (with a T c below 150 K) originating from the recrystallized disclination-rich regions of the CNOs shells [54].

Synthesis
The Fe 3 C/α-Fe-filled multi-shell CNOs were produced with the following synthesis method: 5 g of ferrocene were evaporated at 110°C and pyrolysed at 990°C under an Ar flow rate of 5 ml min −1 . The duration of the growth experiment at the pyrolysis temperature was of 1 h. The vapour deposition reactor consisted of the following dimensions: a quartz tube with a length of 1.5 m, an inner diameter of 44 mm and a wall thickness of 3 mm. A Si/SiO 2 substrate was placed in the reaction zone for the deposition of the growth product, which consisted of 3.5 g of Fe 3 C filled CNOs (yield of approximately 70%). A fast cooling method was used to bring the CNOs to room temperature by removing the furnace along a rail system (quench).

Characterization
Low-temperature XRD measurements were performed with a Rigaku Smartlab powder X-ray diffractometer (Cu K-α , 〈λ〉 = 0.154 18 nm) under vacuum of less than 10 −7 Pa in the temperature range from 130 K to 298 K. A 200 kV American FEI Tecnai G2F20 was employed to obtain TEM/HRTEM images. ESR measurements were performed at 77 K, 150 K and 300 K using a JEOL JES-FA200 spectrometer instrument. Raman Spectra were collected in a custom-built instrument using a triple grating monochromator (Andor Shamrock SR-303i-B, EU) with an attached EMCCD (ANDOR Newton DU970P-UVB, EU), excitation by a solid-state laser at 532 nm (RGB lasersystem, NovaPro 300 mW, Germany) and collection by a 100×, 0.90 NA objective (Olympus, Japan). A variation of the lattice-spacing in the inner, middle and outer shells of the CNO was also found, as shown in figure 2. In particular, variation in the disorientation angle between the CNOs-shells was measured when moving from the inner toward the outer layers of the CNOs. Such a variable level of structural ordering is attributable to the presence of disoriented graphitic stacks with clear disclinations in between. This interpretation can explain the observed increasing interlayer spacing and defect density in the spherical carbon shells, as a result of the disclinations, in figure 2 [55][56][57][58][59][60][61][62][63][64][65]. As previously indicated also by Siklitskaya et al [55] and Akatyeva et a. [56], defects can play also a significant role in altering the carbon shell curvature in this type of structures. Structural defects in CNOs have been also reported by Balaban et al [61], Blank et al [62] and Hiura et al [65].

Results and discussion
Presence of a temperature dependent variation of the CNO inter-shells distances was revealed by temperature dependent XRD analyses. These investigations were performed in the range from 298 K to 130 K. Figure 3 shows typical examples of the XRD patterns measured in the temperature range from 298 K to 130 K. XRD data collected at 298 K revealed a diffraction peak in the range of 14°-28°2θ which could be ascribed to the 002 reflection of graphitic carbon (corresponding to an average lattice spacing of 0.339 nm) together with a very weak broad feature in the region of 17°2θ corresponding to an inter-shell distance of approximately 0.5-0.6 nm (i.e. outer CNOs shells). Appearance of additional 002 reflections was further found at 243 K, with two diffraction peaks arising in the region of 15°and 17°2θ respectively (inter-shell distances of 0.513 nm and 0.592 nm respectively). The appearance of these three peaks seems to indicate the presence of a partial recrystallization of the CNOs-layers at this temperature. It is important to notice also that a variable intensity and broadening in such peak features could be found in measurements performed at lower temperatures of 200 K, 150 K and 130 K. Note that complete disappearance of such peak features was observed when the sample was taken back to the temperature of 298 K, implying the presence of a reversible transition. Additional calculations performed by using the Scherrer equation [25] revealed the number of CNOs shells contributing to each of the observed diffraction peaks. The results of these calculations are shown in table 1. Interestingly, only a fraction of CNOs shells (average number of 38 per each CNOs in the sample) was found to contribute to the peak in the region of 26°2θ. This result suggests that only the CNOs shells with relatively high level of graphitic ordering contribute to the pattern shown in figure 3 at 298 K, instead the shells with higher number of defects do not appear to contribute to the observed peak. Furthermore, considering the Scherrer thickness associated to the diffraction peaks in the regions of 15°and 17°2θ, we find that a larger number of 74 and 84 CNOs layers can be at the origin of such contribution. However, the lower intensities of those peaks (lower if compared to that of the peak at 26°2θ) may imply that a lower number of CNOs contributes to this diffraction signal in the sample. These analyses appear therefore to confirm the possible presence of a recrystallization effect as the temperature is cooled down to 243 K which could possibly be triggered by a significant depletion in the unit-cell c-axis parameter associated to the inner CNOs-shells, as shown in figures 4(A), (B). This appears to be confirmed by the observation of a shift in the inner-shells-diffraction-peak (peak at 26°2θ) towards higher values of 2θ degrees when the temperature is lowered down below 298 K. Such a temperature-induced depletion in the values of the c-axis has analogy with previous observations reported in multiwalled carbon nanotubes [51] and graphite [57,58]. In particular in figure 4(B) the temperature dependent variation of the c-axis (as extracted from the Rietveld refinement method) reveals an approximately linear trend which is typically observed in turbostratic graphitic materials [51], [57,58].
The additional diffraction peak features observed in this work and assigned to recrystallization appear to be most likely due to defect formation upon cooling, in agreement with previous observations from Siklitskaya et al [55], where the role of defects in altering the CNOs shell curvature and inter-shell distances was predicted through density functional theory analysis of the radial distribution of atoms.
Being interested in investigating possible existence of magnetic transitions induced by the aforementioned caxis depletion observed in the graphitic CNO-shells, complementary ESR measurements were considered at the temperatures of 300 K, 150 K and 77 K. As shown in figure 5 a differential absorption peak (corresponding to the π-electron signal) could be detected at g-values of approximately 1.987-1.989 (at 300 K and 150 K respectively). A significant transition involving a differential absorption peak-shift towards larger g-values was then observed at 77 K together with an additional intense peak-feature which appears to be an indicator of intrinsic ferromagnetic ordering. A possible intepretation of such a transition may refer to existence of an α-Fe to γ-Fe phase conversion [66] together with a possible anomalous thermal expansion effect of the latter (γ-Fe) phase, which was recently reported by Cambedouzou et al [67]. This observation indicates that volume enhancement may create an additional interfacial transition (and consequent sp 3 -type and or vacancy-related stress-induced defects) in the carbon shells leading to the observed π-electron signal-shift.
An alternative interpretation of the intense differential absorption feature observed in the ESR spectrum at 150 K-77 K may refer to the formation of intrinsic randomly oriented ferromagnetic clusters into the (recrystallized) disclination-rich regions of the CNOs as a consequence of the recrystallization effect. This second interpretation is supported by the percolative theory of magnetism previously outlined by Kopelevich et al [54] and agrees with the Rietveld refinement analyses in ESI, where no significant changes in the intensities of the acquired Fe 3 C and α-Fe peak-reflections was found (See ESI figure Supp. 4).
According to the percolation-theory [54], uncorrelated ferromagnetic clusters are formed below a certain temperature indicated as T * , leading to finite values of M s (T, H), M rem (T, H), and ΔM(T, H) in zero field cooled (ZFC) and field cooled (FC) magnetization versus temperature signals [54]. As the temperature decreases, ferromagnetic correlations develop on a larger scale, and a long-range ferromagnetic ordering emerges. In figure 5 it is possible to identify the T∼150 K as a transition temperature below which long range ferromagnetic cluster ordering appears to take place [24]. The origin of this phenomenon can be explained with the presence of recrystallized disclination-rich regions in the CNOs-shell [54].
Further investigation of the structural properties of the CNOs was then sought by Raman spectroscopy. Presence of defective sites in the CNOs in the form of mixed vacancy and sp 3 type (at 300 K) could be deduced by calculating the AD/AD' ratio (yielded value of 9.77) as shown in figure 6. This observation is in agreement with the HRTEM analyses shown in figures 1, 2. . XRD patterns and Rietveld refinements obtained for the multishell graphitic CNOs in the temperature range from 298 K to 130 K. The analysed 2θ range extends from approx. 10°to 35°2θ. In figure 4(B) the temperature dependent variation of the c-axis is shown, as extracted from the Rietveld refinement method. The observed approximately linear trend appears to agree with previous observations on turbostratic graphitic based materials, including carbon nanotubes [51,57,58].

Conclusion
In conclusion we have reported the novel observation of low-temperature-driven recrystallization effects in multi-shells graphitic CNOs filled with Fe 3 C/α-Fe crystals from 298 K to 130 K. Appearance of additional 002 reflections as the temperature was decreased was reported, with two diffraction peaks arising in the region of 15°a nd 17°2θ respectively (inter-shell distances of 0.513 nm and 0.592 nm respectively) and a broad feature in the region of 17°2θ. The appearance of these three peaks was interpreted as a result of a partial recrystallization of the CNOs-layers at this temperature. This effect may possibly be related to a rearrangement of the disclinationrich regions of the CNO graphitic layers. A depletion in the graphitic c-axis of the CNOs was also extracted as the temperature was decreased to 130 K. Complementary ESR measurements revealed a transition in the π-electron signal from 150 K to 77 K. This transition was interpreted as an indicator of the possible existence of localized ferromagnetic clusters, as suggested by the percolative theory of ferromagnetism, in the recrystallized disclination-rich regions of the sample [54].