Evidence of band gap features in Fe3O4 Bbmm filled carbon nano-onions

A key challenge in the fabrication of ferromagnetically filled multilayer fullerenes (carbon nano-onions, CNOs) is the manipulation of the structure, composition and electronic band characteristics of both the carbon layers and encapsulated ferromagnetic material. Interestingly, a recent work has demonstrated that the addition of small quantities of water during the chemical vapour synthesis of Fe3C filled CNOs can allow the local manipulation of the Fe3C crystal-structure and induce the nucleation of a novel high pressure Bbmm Fe3O4 crystal-phase. In this report we propose an advanced study of such structural transition. Particularly, we investigate the morphological, optical (band-gap) characteristics and magnetic properties of the as produced CNO materials by using transmission electron microscopy, vibrating sample magnetometry, x-ray photoelectron spectroscopy and UV–vis spectroscopy.

Although recent literature works have reported the encapsulation of Fe 3 C within CNOs as a single reaction product, the encapsulation of ferromagnetic metal-based oxides within these structures remains a strongchallenge [15][16][17]. Among other metal-oxides, iron oxide as recently attracted strong attention for its unusual memory/resistive switching behavior [15][16][17].
Interestingly recent works have shown that mixed iron/iron-oxides Fe 2 O 3 and Fe 3 O 4 poly-crystals and highpressure oxide Bbmm phases [18,19] can be encapsulated within CNOs, carbon nanopolyhedra and carbon nanotubes by pyrolysis of ferrocene or ferrocene/Fe 3 (CO) 12 mixtures [15]; however the difficulties in controlling/selecting (1) the desired oxide composition and (2) the desired carbon nanostructures morphology have limited up to now the development of such approaches for industrial application. In addition, the dynamics of the iron-oxide encapsulation-mechanism inside CNOs (and CNTs) remain highly controversial and have been not yet understood. However, the recently reported CVS experiments on the controlled nucleation of high pressure Bbmm Fe 3 O 4 single crystals [18] show promise towards controlled reproductible fabrication of iron oxide filled CNOs as a result of an abrupt transition in the pyrolysis experiments [19]. As shown by Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Taallah et al high temperature dependent x-ray diffraction (XRD) and Raman spectroscopy could not trigger such an abrupt transition, which instead was reported as a result of water-assisted pyrolysis of ferrocene [19].
In this work we propose an advanced study of such water-induced structural transition. Particularly, through a systematic transmission electron microscopy (TEM) approach we have investigated the morphological, structural properties of these novel materials. In addition, we investigated the not previously reported electronic (band gap) and magnetic properties of the as produced oxide filled CNO materials by employing magnetometry and UV spectroscopy. Existence of semiconducting band-gap characteristics is demonstrated.

Synthesis
The synthesis experiments were carried out by using a CVS system composed of a quartz tube of length 1.5 m (outer diameter of 22 mm and a wall thickness of 2.5 mm), one zone electric furnace and an Ar flow rate of 10 ml min −1 . The precursor (100 mg of ferrocene) was placed in a quartz boat and sublimated/pyrolysed at the temperature of 990°C. In order to accurately understand the effect of water in the reaction, the following quantities of water were used: 0.05 ml, 0.1 ml, 0.2 ml, 0.25 ml and 0.35 ml in analogy with [19]. In all the sets of experiments the reaction time was 5 min; the samples were cooled down by using a quench method (by sliding the furnace along a rail system).

Characterization
The characterization was carried out with: with a 200 kV American FEI Tecnai G2F20 transmission electron microscope (TEM). Raman Spectroscopy measurements were performed with an Andor SR-500i with a wavelength of 633 nm, acquisition time of approximately 100 s per sample-area at the temperature of 25°C. XPS analyses were performed with an Escalab 250Xi. Vibrating sample magnetometry (VSM) measurements were performed with a VSM instrument Quantum Design. UV absorption measurements with a Hanon i5 UV-vis Spectrophotometer (Jinan Hanon Instruments Co., Ltd, China).

Results and discussion
The morphology of the CNOs obtained by pyrolysis of 0.1 ml of water with ferrocene is shown in figure 1. As shown in figures 1 and 2(A)-(C), the encapsulated crystals were found to be arranged into two main areas characterized by different lattice spacings. The area 1 with lattice spacings compatible with those of Fe 3 C with space group Pnma (unit cell a: 0.51 nm, b: 0.67 nm and c: 0.45 nm) and the area 2 with lattice spacings   partial-detachment of the CNOs layers, as indicated by the yellow and green arrows. Additionally, the presence of single walled CNOs (blue arrow in figure 3(A)) and locally damaged multi-walled CNOs (see figure 3(B)) was found. Such an unusual detachment and local-damage of the CNOs-layers was observed also in the experiments performed with 0.25 ml of water as shown in figures 4(A)-(B). Also in this case the formation of iron oxide single crystals in some of the few-walled CNOs was found, as shown in figures 4(C)-(D). However, it is important to underline that the oxygen-species obtained by the water pyrolysis do not seem to act uniformly within all the CNOs in the sample. Indeed, presence of thick graphitic layers could be observed (figures 4(A), (B)).
The morphological investigation of the CNOs for experiments involving pyrolysis of 0.35 ml of water is further shown in figure 5. In agreement with the report of Taallah et al [19], the majority of the analysed CNOs were found to exhibit an unusual distorted-like morphology (which can be seen in figures 5(A), (B)) together with a variable diameter. Note also the CNOs-layers, which appear to be partially damaged and distorted by the exposure to the oxygen species deriving from the water pyrolysis. The effect of water concentration on these experiments is further summarized in table 1.
Further verification of the oxide nucleation dynamics was then sought by the use of XPS. As shown in figure 6, a large quantity of oxygen was found together with Fe and C confirming the presence of oxide species inside the onion structures. These measurements clearly confirm the formation of the oxide-phase within the CNOs and agree with the recent observation of Taallah et al [19]. Particularly, these results suggest that when a critical concentration of water is used in the reaction, the oxidation occurs directly within the CNO core; this mechanism implies a consequent strong distortion of the graphitic-like layers, as indicated by the analyses of the C1s XPS spectrum in figure 7 where presence of a large quantity of sp 3 carbon is indicated in the deconvolution (green curve), which results from the structural transition of Fe 3 C into a Fe 3 O 4 Bbmm phase. Presence of C-O and C=O signals imply further the presence of partial oxidation of graphitic layers of the CNO during the aforementioned structural transition.
The attention was then turned on the magnetic characteristics of this latter type of CNOs. A typical hysteresis loop measurement performed at 300 K with VSM is shown in figure 8. Interestingly, a weak ferromagnetic-like behaviour of the Fe 3 O 4 /Fe-filled CNOs was found with a relatively low coercivity in the order of 100-150 Oe and low saturation magnetization of 10.4 emu g −1 . In the attempt to better visualize the magnetic arrangement of the sample and its possible dependence on the chosen temperature, the use of zero field cooled ZFC measurements at the field of 300 Oe was considered. The result of these measurements is shown in figure 9.
Curiously a magnetic transition is observed in the range from 100 K to 50 K. While at higher temperature, an increase of the magnetization is found. The origin of such intense magnetization peak in the range from 100K to   50K is currently under investigation and further studies are required to clearly attribute its magnetic origin. However possible superparamagnetic origin of such contribution can not be excluded at this stage. Additional investigations were further performed by UV-vis spectroscopy, as shown in figure 10. These investigations revealed the presence of two main absorption features, namely F1 and F2. While the first feature F1 could be attributable to the π-plasmon contribution, the second absorption feature is very unusual and appears to possibly indicate the presence of a wide energy band gap for these types of onions, attributable to the encapsulated oxide phase. Note also that the observed broad energy gap feature is mostly larger than that typically expected for iron oxide nanoparticles, (2-3 eV [20]) and may depend on the wide size distribution of the encapsulated oxide nanocrystals.

Conclusion
In conclusion we have reported an advanced study on the morphological, optical (band-gap) characteristics and magnetic properties of CNO materials filled with Bbmm Fe 3 O 4 crystals by using transmission electron microscopy, vibrating sample magnetometry, x-ray photoelectron spectroscopy and UV-vis spectroscopy. Our Figure 9. Zero Field Cooled measurement of the CNOs grown in presence of 0.35 ml of water. Note the presence of a peak in the range between 100 K to 50 K. No lower temperature could be investigated due to instrument limitation. Figure 10. UVvis measurements of the Bbmm Fe 3 O 4 filled CNOs obtained by pyrolysis of ferrocene with water (0.35 ml). Note the presence of two features, namely F1 and F2. While the first feature F1 could be attributable to the π-plasmon contribution, the second absorption feature is very unusual and appears to possibly indicate the presence of a wide energy band gap for these types of onions. Notice that the observed energy gap is mostly larger than that typically expected for Fe 3 O 4 nanoparticles, 2-3 eV [20].
results demonstrate the existence of important band gap characteristics, and imply the suitability of these type of CNO-hybrid materials for possible application in semiconducting technology and devices.