The effect of heteroatoms on the formation of magnetic ceramic nanocomposites in pyrolysis of organometallic precursors: similar molecular structure, but totally different morphology, composition and properties

Four organometallic compounds were synthesized for solid-state pyrolysis (SSP) to research the structure-property relationship between the precursors and the as-generated magnetic ceramic nanocomposites (MCNs). In which, the only saturated carbon atom in M C was replaced by N or O or S atom, to produce M N , M O and M S , respectively. It was found that, the crystal phase of the cobalt catalyst could be regulated by introducing different heteroatoms during the pyrolysis: fcc-Co for M C /M N , while fcc/hcp-Co hybrid for M O /M S . Metal cobalt with different crystal phases has their special catalytic and magnetic properties. Thus, MCNs with totally different morphology, composition and properties could be prepared by just changing one heteroatom in the precursors upon SSP. Uniform nanotubes were generated from pyrolysis of M C /M N , while nanospheres were generated from M O /M S . The obtained MCNs all show excellent magnetic properties with Ms ranged from 47.6 to 54.2 emu g -1 . Analyzing carefully, due to the magnetic difference between fcc-Co and hcp-Co, the Ms of the MCNs obtained from M O /M S were slightly lower than those of M C /M N , but, their Mr and Hc were 2 to 5 times higher than the latter. 2H, ArH), 7.85 (d, J = 8.1 Hz, 2H, ArH), 7.65 (m, 6H, ArH), 7.36 (m, 6H, ArH). 13 C NMR (75 MHz, CDCl 3 ) δ (ppm): 199.4, 139.8, 138.5, 136.0, 135.5, 129.3, 128.7, 128.2, 128.0, 123.6, 122.6, 92.5, 91.8. FTIR (thin lm), ν (cm − 1 ): 2019.6, 2051.5, 2088.5 (Co 2 (CO) 6 ). Anal. calcd for C 40 H 16 Co 4 O 12 S: C 50.24, H 1.69; found: C 50.86, H 1.51.

In MCNs, the magnetic nanoparticles well wrapped by the carbonaceous or other species, can possess excellent stability, dispersibility and biocompatibility [11,12]. So far, various approaches have been successfully explored for the fabrication of functional MCNs, such as chemical vapor deposition, electric arc discharge, hydrothermal/solvothermal method and pyrolysis procedure etc [13][14][15][16]. And the prepared MCNs with different morphology and composition have their speci c properties and application elds [17][18][19][20]. Although there are many fabrication methods for various MCNs, but controllable preparation of MCNs with desired morphology and properties is always a great challenge. More importantly, the growth mechanism of MCNs is still ambiguous.
Recently, great achievements of Vollhardt and Müllen et al. demonstrated that solid-state pyrolysis (SSP) of organometallic complexes is a promising approach for the preparation of CNPs [35][36][37][38]. In SSP, powders of the organometallic precursors in quartz tubes were sealed under high vacuum, placed into a furnace underwent a stepwise thermolysis without the requirements for feedstock gases, inert atmosphere, extra catalyst, and expensive instruments. Especially, the well-de ned organometallic precursors played a vital role in governing the structure and properties of the corresponding CNPs in SSP Page 3/13 [39][40][41]. In which, the structure of organic precursors could be subtly modi ed to control the morphology and the properties of the obtained CNPs. Thus, it is badly needed to know more about the structureproperty relationship between the precursors and the products. Additionally, fewer experimental parameters in SSP provided favorable conditions for related research. Therefore, to further research the structure-property relationship between the precursors and the asgenerated MCNs, and explore the inherent mechanism, four organometallic compounds (M C~MS ) with precise molecular structure were successfully synthesized for SSP. Surprisingly, just changing one heteroatom in the precursors could lead to totally different morphology, composition and properties of the as-generated MCNs (Fig. 1). After pyrolysis, the Co-containing moieties in M C /M N were transformed into fcc-Co, but it was converted to fcc-Co/hcp-Co hybrid in M O /M S . Subsequently, morphology of the MCNs could be well controlled as nanotubes (for M C /M N ) or nanospheres (for M O /M S ) with adjustable size and magnetic properties. Experimental results indicated that heteroatom in the precursor could regulate the crystal phase of metal Co, and the crystal phase greatly affect the catalyst activities and magnetic properties of Co, thereby controlling the morphology and properties of the resultant MCNs upon SSP. The result once again con rmed the power of the concept of precursor-controlled pyrolysis towards de ned MCNs, and provided valuable rules and guidance for the controllable preparation of MCNs.
Compounds 1-4 were synthesized according to previous literatures. All other reagents were used as received without further puri cation.

Synthesis of the organometallic compounds
General procedure for the synthesis of organometallic compounds M C , M N , M O and M S : Co 2 (CO) 8 (6.00 equiv) and compound 1 or 2 or 3 or 4 (1.00 equiv) were dissolved in THF under an argon atmosphere. The mixture was stirred overnight at room temperature and the solvent was removed under vacuum. The residue was puri ed by using column chromatography on neutral Al 2 O 3 with DCM-petroleum ether mixture as the eluent.
Compound M C : Black solid was obtained in 68% yield. 1

Instrumentation
1 H and 13 C NMR spectroscopy study was conducted with a Varian Mercury 300 spectrometer and 400 MHz Bruker Avance NMR spectrometer using tetramethylsilane (TMS; δ = 0 ppm) as internal standard. EI-MS spectra were recorded with a Finnigan PRACE mass spectrometer. Elemental analyses were performed by a CARLOERBA-1106 by a micro-elemental analyzer. The Fourier transform infrared (FTIR) spectra were recorded on a PerkinElmer-2 spectrometer in the region of 4000 − 400 cm − 1 . Thermogravimetric analysis (TGA) was performed on Netzsch STA 449C thermal analyzer at a heating rate of 10 o C/min in nitrogen at a ow rate of 20 cm 3 /min. Scanning electron microscopy (SEM) was performed on a ZEISS-SIGMA scanning electron microscope. Transmission electron microscopy (TEM) was performed on a JEM-2010HT or JEM-2010FEF microscope at an accelerating voltage of 200 kV. TEM samples were prepared by drying a droplet of the suspension on a TEM copper grid with a carbon lm. Energy-dispersive X-ray spectroscopy (EDX) was taken on the SEM or TEM. The X-ray diffraction (XRD) analyses were performed on a Bruker D8 Advanced X-ray diffractometer with CuKα radiation (λ = 1.5418 Å). Magnetization curves were recorded on a physical property measurement system (PPMS-9T) with a vibrating sample magnetometer (VSM) option at room temperature.

Synthesis and characterization
M C was synthesized with uorene as the basic unit. Then, replacing the only saturated carbon atom in M C with nitrogen (or oxygen or sulfur) atom, M N , M O and M S were obtained, respectively. Scheme S1 illustrates the synthetic routes to the intermediates and nal products. Firstly, alkynyl-functionalized compounds 1 ~ 4 were synthesized by the palladium-catalyzed Sonogashira coupling reaction. Then, organometallic compounds M C , M N , M O and M S were all prepared via the reaction between the alkynylfunctionalized intermediates (1 ~ 4) and an excess of Co 2 (CO) 8 , affording [CCCo 2 (CO) 6 ] moieties with a satisfactory yield. All the compounds were well characterized by nuclear magnetic resonance (NMR), mass spectroscopy, elemental analysis (EA), and Fourier transform infrared (FTIR) spectroscopy, which well con rmed their explicit molecular structures (Fig. S7-S22).  6 ], con rming that alkynyl group has been completely reacted with the Co 2 (CO) 8 [42]. Meanwhile, due to the presence of the N-H bond in 2 and M N , they also showed an absorption peak at about 3400 cm − 1 . The IR spectra of all the organometallic compounds and the intermediates are given in Fig. S1, and similar results were obtained. Figure 3 shows the 1 6 ] was appeared at about 199.4 ppm [43]. Moreover, for both 1 H NMR and 13 C NMR spectra, great changes had also taken place in the aromatic areas that close to the [CCCo 2 (CO) 6 ] unit.

Thermal properties and pyrolysis process
In SSP, the samples would undergo a stepwise thermolysis program: they were rst heated to their decomposition temperature of the metal carbonyl groups and held at this temperature for several hours, then heated to a higher temperature and held there for another several hours. Thus, the thermal properties of the organometallic compounds were investigated by using TGA. The thermal-decomposition temperatures (T d , corresponding to 5% weight loss) of the precursors were at about 170 o C (Fig. S2).
Thus, the pyrolysis program was determined as following: powders of the organometallic precursors in quartz tubes were sealed under high vacuum, and then placed into one furnace, and underwent exactly the same heating programs. The samples were rst heated to 170 °C with a rate of 2 o C/min, held for 2 h, and then heated to a higher temperature (700 o C) with a rate of 5 o C/min, held for 8 h. After cooling to room temperature, the obtained products were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive spectroscopy (EDX), powder X-ray diffraction (XRD) and vibrating sample magnetometer (VSM).

Morphological characterization of the MCNs
As discussed above, it is a powerful approach to yield de ned CNPs through the precursor-controlled pyrolysis process. Actually, the prepared organometallic compounds could be well converted to two types of MCNs, nanotubes and nanospheres, under the same pyrolysis program with high yield. Upon the SSP process, M C and M N produced uniform multiwalled CNTs in high yields (Fig. 4a-f). TEM analysis revealed that the average inner and outer diameters of the CNTs were about 20 and 40 nm for M C , respectively (Fig. 4b). And the length of these CNTs was above 10 µm. From the high-resolution TEM (HRTEM) image, the nanotubes were well graphitized, made up of ~ 25 layers, and the space between the layers was 3.51 Å (Fig. 4c and S3a), in consistency with the range (d 002 = 0.34-0.39 nm) reported for the interplanar spacings of graphite [44]. Co NPs having a diameter of 15-50 nm, were located mainly at the tip of the CNTs and wrapped in orderly graphene layers ( Fig. 5a and S3b) or encapsulated by well graphitized carbon nanospheres. As to the precursor M N , CNTs with reduced length (< 1 µm) but increased diameter were obtained. The as-prepared CNTs had a uniform size with an inner diameter about 30 nm and an outer diameter about 60 nm (Fig. 4d, (Fig. 4g-i). And the Co NPs were well embedded in amorphous carbon and graphitized carbon with a thickness of 5-10 nm. As showed in Fig. 5c and S4, Co NPs was well embedded in graphitized carbon (d 002 = 3.43 Å), and its lattice structure was very clear.
The spacing between the lattice planes was, respectively, 0.174 nm and 0.216 nm corresponding to the (111) planes of face centered cubic (fcc) and (100) planes of hexagonal-closed-packed (hcp) phases of cobalt [45,46], illustrating the coexistence of the fcc (bule area in Fig. 5c) and the hcp (gree area) structures. And, the fcc phase locating at the center was surrounded by the hcp phase.
As to precursor M S , SEM and TEM analysis showed that the diameter of the nanospheres mainly ranged from 35 to 55 nm, and Co NPs were encapsulated very well in carbonaceous materials (Fig. 4j-l). And the lattice fringes of Co could be clearly observed through the HRTEM image (Fig. 4l). Interestingly, a special phenomenon was observed in the case of precursor M S : cobalt core/carbon sheath nanocable with diameter of about 200 nm and length of 7 µm was obtained ( Fig. 5d and S5). TEM-EDX was used to analyze the element distribution of the nanocable (Fig. 5e). When the electron beam passed through the core (label 1-1 in Fig. 5d) and the sheath (label 1-2) of the nanocable, Co and C were the main elements detected (Cu was come from the copper grid), con rming its cobalt core/carbon sheath architecture.
It is amazing how does the changing of just one atom leads to the formation of MCNs with totally different morphology? Generally, fcc and hcp are the two main different crystalline structures of cobalt.
Among them, the hcp-Co is stable at room temperature, while the fcc-Co is a metastable phase formed at temperature above 450 °C [47]. In pyrolysis, the heating temperature was generally higher than 500 °C, so cobalt mainly existed in the fcc phase, especially for the production of CNTs [48,49]. Although, cobalt is a very common catalyst for the synthesis of CNTs, and the morphology, density and size effect of Co have been extensively studied [50][51][52][53]. However, the impact of the crystal phase was rarely considered, mainly because of the absence of the room temperature stable hcp phase (transformation to high temperature stable fcc phase) during the pyrolysis under high temperature. On the other hand, even though hcp-Co could exist under some special conditions in pyrolysis, but their main pyrolysis products were carbonencapsulated cobalt NPs without CNTs [54][55][56]. These results indicated that, fcc-Co was a better active catalyst phase for the fabrication of CNTs, while hcp-Co was conducive to the generation of carbonencapsulated cobalt NPs.
Very importantly, in the classic cobalt-catalyzed Fischer-Tropsch reaction, great achievements have demonstrated that hcp-Co has better catalytic activity than fcc-Co [57,58]. This is mainly due to the special surface structure of the hcp crystal phase, which has higher intrinsic activity and density of active sites. So it is more bene cial to the decomposition, diffusion and recombination of carbon source on its surface [59]. Similarly, during the catalytic pyrolysis, the hydrocarbons carbon source was rst absorbed and decomposed by the catalyst, and then diffused and restructured into shaped CNPs [60,61]. Thus, based on the experimental results and inspired by the above mechanism, it was speculated that the excessively fast diffusion and deposition rate of the carbon source on hcp-Co would cause the completely encapsulation of cobalt NPs by carbon (formation of cobalt core-carbon shell NPs), which was not favoring to the nucleation and growth of the CNTs. On the contrary, the moderate catalytic activity of fcc-Co could contribute to the gradual nucleation and growth of CNTs. The exact mechanism was unclear, further research was still needed to better understand this interesting phenomenon. However, in this work, for the rst time, we found that heteroatom (even just changing one atom) in the organometallic precursor could regulate the crystal phase of metal Co, thereby controlling the morphology the resultant MCNs upon SSP, providing an effective approach to prepare MCNs controllably.  [66]. SEM-EDX was used to analyze the chemical compositions of the obtained nanocomposites. From the EDX results, except a small amount of sulfur existing in M S -S, the main compositions of all the MCNs that could be detected were Co and C elements, and the cobalt contents were about 30% (Fig. S6).

Magnetism of the MCNs
All the Co enriched nanocomposites were magnetizable and could be quickly attracted to a magnet at room temperature. Thus, to further research the magnetism of the obtained MCNs, especially highlight the impact of hcp-Co on the magnetic anisotropy, VSM was utilized to quanti cationally investigate their magnetic properties at a temperature of 300 K. Figure 7 shows the magnetization curves of the MCNs, all the samples exhibited soft ferromagnetic behavior with high magnetizability. The magnetization rapidly increased with an increase in the strength of the applied eld before 5 kOe, with relatively low magnetic remanence (Mr) and coercivity (Hc). The magnetization data were summarized in Table 1 Considering the presence of hcp-Co, this result is understandable: comparing with fcc-Co, hcp-Co has a slightly lower Ms, but with much stronger Ms and Hc [67,68]. Therefore, the hcp-Co generated in M O -S/M S -S, leaded to a signi cant magnetic difference of the prepared MCNs.

Conclusion
In conclusion, through rational molecular design, four organometallic precursors were synthesized successfully for solid-state pyrolysis (SSP) to research the structure-property relationship between the precursors and the as-generated MCNs. It was found that, the crystal phase of the cobalt catalyst could be regulated by introducing different heteroatoms, and the crystal phase greatly affect the catalyst activities and magnetic properties of Co, thereby controlling the morphology, composition and magnetic properties of the resultant MCNs upon SSP. By simply changing one heteroatom in the precursors, uniform nanotubes and nanospheres with adjustable size and magnetic properties could be controllably prepared e ciently with high selectivity. The obtained MCNs all show excellent and differential magnetic properties. Coupled with their high oxidation resistance and good stability, they could be promising candidates in practical magnetic and catalytic applications. Thus, the preliminary results con rmed the power of the precursor-controlled SSP, and also provided valuable guidance for the controllable preparation and the mechanism research of MCNs.

Declarations
Declaration of competing interest The authors declared that they have no con icts of interest to this work.