The partially controllable growth trend of carbon nanoparticles in solid-state pyrolysis of organometallic precursor by introducing POSS units, and their magnetic properties

Abstract

On the basis of organometallic compound T (without POSS unit), compound PT containing three POSS units was synthesized for solid-state pyrolysis. It was found that the introduction of POSS units could prevent metal nanoparticles from sintering into larger particles effectively, and change the growth trend of carbon nanoparticles. When the pyrolysis temperature was increased from 700 to 850 °C, as for compound T, the metal particles sintered considerably, and the morphology of the obtained materials were gradually transformed from carbon nanotubes to hollow carbon nanospheres. However, compound PT exhibited good sinter-resistant performance: the size of the metal particles did not obviously increase with the raised temperature. In comparison with T, the growth trend of carbon nanoparticles in solid-state pyrolysis of PT was exactly opposite, showing a trend of producing carbon nanotubes upon the increased pyrolysis temperature. Meanwhile, the obtained nanomaterials all demonstrated good magnetic properties, with the highest saturation magnetization (M_s) as 64.9 emu g⁻¹, and the magnetic properties exhibited a good accordance with their structure.

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Introduction

Carbon-based nanomaterials, due to their novel structure and unique electronic, photonic, thermal, magnetic and mechanical properties, are attractive materials for potential applications in various fields such as nanoelectronics, biosensors, biomedicine and energy storage.¹ Various methods have been developed to produce carbon nanoparticles (CNPs), but the control of the morphology and the properties of CNPs are always a big challenge. Chemical vapor deposition (CVD) is a popular method for the production of CNPs, and a lot of works have focused on studying the influence of the experimental conditions on the growth and the properties of the obtained CNPs, such as the catalyst, temperature, pressure, gas-flow rate and deposition time.² Organometallic complexes contained both metal and carbon source, were useful precursors for the preparation of CNPs.³ Currently, much efforts have been attracted, especially for the synthetic chemists, on the fabrication of CNPs through solid-state pyrolysis (SSP) of organometallic complexes, in which the metal in metal-containing parts could be easily transformed into metal nanoparticles. Then, the formed metal nanoparticles could act as the catalyst to catalyze the transformation of the carbon source from the organometallic precursor to CNPs under pyrolysis. Great achievements of the groups of Vollhardt, Bunz and Müllen had demonstrated that well-defined organic precursors played an important role in governing the structure of the obtained CNPs.⁴

Polyhedral oligosilsesquioxanes (POSS) is a type of organic/inorganic hybrids nanostructured molecule, which contains Si–O cores and organic surface. It is a useful nanometer-scale building block in many polymeric materials and nanocomposites, due to its unique cage-like molecular structure and excellent physicochemical properties.⁵ In our recent works, organometallic compounds/polymers containing POSS groups were used as the precursors for SSP to yield carbon-based magnetic materials.⁶ It was found that the introduction of POSS moieties could hinder cobalt nanoparticles from aggregating into larger particles effectively, even at a high temperature. In general, the morphology and physical properties of carbon/metal nanostructures are highly dependent on the size of the metal nanoparticles. Based on these exciting results described above, maybe we could use this feature to control/change the growth trend of the CNPs through rational introduction of POSS units.

In this work, on the basis of organometallic compound T,⁷ compound PT containing three POSS units were synthesized. Then, we investigated the influence of the POSS units on the morphology and the properties of the obtained CNPs under different pyrolysis conditions. When compounds T and PT underwent the same pyrolysis conditions (when the pyrolysis temperature was 700 °C), the obtained nanoparticles from T and PT were mainly composed of carbon nanotubes (CNTs) and nanospheres, respectively. However, once increasing the pyrolysis temperature from 700 to 850 °C, the obtained materials from T were gradually transformed from CNTs to hollow graphitized carbon nanospheres, while, the result of PT was exactly opposite, with a trend for the production of CNTs. Meanwhile, the obtained nanoparticles from compounds T and PT possessed very good magnetism with the highest saturation magnetization of 64.9 emu g⁻¹, and the magnetic properties exhibited a good accordance with their structure.

Results and discussion

Synthesis and structural characterization

Fig. 1 shows the chemical structures of compounds T and PT, the synthetic route of compound PT was illustrated in Scheme S1.† Compound 2 was first synthesized through Pd-catalyzed Sonogashira coupling reaction with high yield. Then, organometallic precursor PT was prepared through the reaction of compound 2 with excess of Co₂(CO)₈. The final products and intermediates were fully characterized by using nuclear magnetic resonance (NMR), mass spectroscopy (Fig. S6–S13†), elemental analysis (EA) and Fourier transform infrared (FTIR) spectra, which well confirmed their explicit molecular structures. For example, in the IR spectra of 2, T and PT (Fig. S1†), three strong absorption bands in the range of 2025 to 2095 cm⁻¹, typical absorptions for [CCCo₂(CO)₆] cluster were observed in the spectra of T and PT, confirming that Co₂(CO)₈ had successfully reacted with the alkynyl groups. Meanwhile, in the spectra of 2 and PT, in comparison with that of T, a broad absorption band appeared at 1110 cm⁻¹, corresponding to the stretching Si–O–Si bonds in the POSS cage.

Fig. 1 Chemical structures of PT and T.

The influence of POSS units on the formation of the obtained CNPs through SSP under different pyrolysis conditions

The thermal properties and pyrolysis program of the organometallic precursors were given in the ESI.† To differentiate the obtained nanoparticles, the CNPs obtained by SSP of T and PT under different pyrolysis conditions were named by combining the name of their organometallic precursor and the pyrolysis conditions. For example, the name of T-700-8h represented the CNPs obtained from compound T, which was heated to 700 °C and held at this temperature for 8 h. The SEM and TEM images of the obtained CNPs from compounds T and PT under different heating programs were shown in Fig. 2–5 (to make it clearer, the same picture as Fig. 4 in a large size was given in the ESI as Fig. S3†).

Fig. 2 SEM and TEM images of the materials obtained through thermolysis of compound T under different conditions. (a and b) For T-700-8h, pyrolysis program: rt → 180 °C (1.2 °C min⁻¹), 180 °C for 2 h, 180 → 700 °C (2.2 °C min⁻¹), 700 °C for 8 h; (c and d) for T-700-24h, pyrolysis program: rt → 180 °C (1.2 °C min⁻¹), 180 °C for 2 h, 180 → 700 °C (2.2 °C min⁻¹), 700 °C for 24 h; (e and f) for T-800-8h, pyrolysis program: rt → 180 °C (1.2 °C min⁻¹), 180 °C for 2 h, 180 → 800 °C (2.6 °C min⁻¹), 800 °C for 8 h; (g and h) for T-850-8h, pyrolysis program: rt → 180 °C (1.2 °C min⁻¹), 180 °C for 2 h, 180 → 850 °C (2.8 °C min⁻¹), 850 °C for 8 h.

Slowly heating compound T to 700 °C and holding at this temperature for 8 h, the yielded CNPs were mainly composed of curved CNTs (Fig. 2a and b). When the heating time was extended to 24 h, it also yielded CNTs primarily (Fig. 2c and d). This result indicated that 8 h was long enough for compound T to produce CNTs. TEM analysis revealed that the average inner and outer diameters of the CNTs were about 30 and 60 nm (Fig. 2c and d), and the tubes were graphitized with interplanar spacings (d₀₀₂) of 0.34 nm (Fig. 4a and b, S3a and b†). However, at the same conditions, compound PT only yielded nanospheres with small size, TEM analysis showed that the diameter of the nanospheres mainly ranged from 20 to 45 nm (Fig. 3a and b and 4e). From the HRTEM (high-resolution TEM) image, the shell of the nanospheres was made up by graphene layers with some amorphous materials (Fig. 4e and S3e†). Under the same pyrolysis conditions, such a huge difference caused by the incorporation of three POSS units could be explained as following: the introduced POSS groups could greatly reduce the concentration of the cobalt catalyst. In Table S1,† in comparison with T, the content of the cobalt in compound PT was reduced from 24.3 to 9.0%. With a low metal loading concentration, the metal catalyst was not enough to catalyze the formation of CNTs.⁸

Fig. 3 SEM and TEM images of the materials obtained through thermolysis of compound PT under different conditions. (a and b) For PT-700-24h, pyrolysis program: rt → 180 °C (1.2 °C min⁻¹), 180 °C for 2 h, 180 → 700 °C (2.2 °C min⁻¹), 700 °C for 24 h; (c and d) for PT-850-8h, pyrolysis program: rt → 180 °C (1.2 °C min⁻¹), 180 °C for 2 h, 180 → 850 °C (2.8 °C min⁻¹), 850 °C for 8 h.

Fig. 4 HRTEM images of the materials obtained through thermolysis of compounds T and PT. (a and b) For T-700-24h; (c and d) for T-850-8h; (e) for PT-700-24h; (f) for PT-850-8h.

For compound T, when the heating temperature was elevated from 700 to 800 °C, the yield of CNTs began to decline (Fig. 2e and f). Further improving the temperature from 800 to 850 °C, the yield of CNTs was much less, but a large number of intact or chipped hollow carbon nanospheres accompanied with many big cobalt particles were produced (Fig. 2g and h and 5a and b). HRTEM analysis revealed that the nanospheres were well graphitized. In Fig. 4c (and Fig. S3c†), the hollow carbon nanosphere was made up of about 30 graphene layers, and the interplanar spacings of graphite was 0.35 nm, in consistent with the range reported for the interplanar spacings of graphite (d₀₀₂ = 0.34–0.39 nm).⁹ Co particles were also encapsulated by the well graphitized carbon nanospheres (about 60 graphene layers with d₀₀₂ = 0.35 nm) (Fig. 4d and S3d†). As to PT, when elevated the pyrolysis temperature from 700 to 850 °C, the obtained CNPs were still mainly composed by nanospheres. To our surprise, CNTs were also produced, although the content was relatively low (Fig. 3c and d and 5d). In comparison with the CNPs (PT-700-8h) obtained from pyrolysis of compound PT at 700 °C, the diameter of the nanospheres in PT-850-8h did not have a noticeable increase (Fig. 3d and 5c). Also the shell of the nanospheres was made up by graphene layers with some amorphous materials (Fig. 4f and S3f†).

Fig. 5 TEM images of the materials obtained through thermolysis of compounds: (a and b) T-850-8h and (c and d) PT-850-8h. Inset: electron diffraction pattern of Co nanocrystal in T-850-8h.

It was interesting, when changed the heating temperature from 700 to 800 °C and then to 850 °C, the obtained CNPs from compound T were gradually transformed from CNTs to hollow graphitized carbon nanospheres. However, as to compound PT, the result was exactly opposite, showing a trend of producing CNTs. Taking into account of the large numbers of the incorporated POSS units in PT, it was reasonably to explain this phenomenon as following. For compound T, the improved heating temperature would facilitate cobalt nanoparticles gathering into larger particles, thus making them inactive for the nanotube nucleation. Meanwhile, when the catalytic particles became larger, the formation of graphitic overcoat was favored.¹⁰ So, as shown in the TEM images, when the heating temperature elevated to 850 °C, many large Co nanoparticles and intact/chipped hollow nanospheres were formed (Fig. 2g and h and 5a and b). It should be the violent movement of the cobalt in agglomeration that caused the formation of the chipped hollow nanospheres. Simultaneously, the aggregation of Co NPs to large particles reduced its catalytic activity and leaded to the reduction of CNTs. However, as to compound PT, the large numbers of the incorporated POSS units could effectively prevent the sinter of cobalt nanoparticles, even under a higher temperature. It was proposed that at the initial stage of the heating process, when [CCCo₂(CO)₆] moieties began to transform into cobalt nanoparticles (about 180 °C), the unique cage structure of POSS units could efficiently limit the aggregation of cobalt by dispersing cobalt nanoparticles. At a higher temperature, the formed SiO_x derived from the silicon-oxygen cage of POSS units, that covered on the surface of the cobalt nanoparticles, could limit the sintering of cobalt nanoparticles due to its high stability, thus, the diameter of the obtained nanospheres has not increased with the increased temperature (Fig. 5c). At this time, the major effect of the increased heating temperature was promoted the diffusion of carbon, which would facilitate the growth of CNTs.¹¹

As known, carbon/metal and carbon/metal oxide nanocomposites are very important functional materials for a wide range of applications, such as information storage, magnetic resonance imaging, microwave absorption, catalysis, electrode and capacitance materials. The aggregation of the metal or metal oxide nanoparticles of these materials in preparation or application will observably damage their performance. For example, as to catalysts, which were important for industrial applications, the sintering was an important reason for the loss of catalyst activity, especially at high temperature.¹² Here, the introduced POSS groups could prevent metal nanoparticles sintering into larger particles, even under a higher temperature. This property could be used to develop sinter-resistant catalysts and other nanocomposites. On the other hand, the introduction of POSS groups could also change the growth trend of carbon nanoparticles, this strategy of utilizing POSS unit to adjust the morphology of the obtained nanoparticles should be also applied to produce other functional nanomaterials with different morphology.

Composition of the CNPs

Through powder X-ray diffraction (XRD), energy-dispersion X-ray (EDX), electron diffraction pattern combined with TEM, we studied the crystal phase and chemical compositions of the organometallic precursors and the obtained CNPs.

The XRD patterns of the organometallic precursor T before and after pyrolysis were shown in Fig. 6. The broad diffraction peak appeared near 20° was due to the amorphous silica matrix. There was no obvious diffraction peak could be found in precursor T, and all the obtained nanoparticles exhibited Bragg reflections at 2θ angles of 44.2, 51.6 and 76.0°, which could be identified to the (111), (200) and (220) planes of Co with a face centered cubic (fcc) structure. Electron diffraction pattern was also used to detect Co nanocrystal in T-850-8h, and the result demonstrated that the Co nanocrystal was in its fcc phase (Fig. 5b). Obvious diffraction peak could be observed at about 26° in T-800-8h and T-850-8h, which could be assigned to the graphite structure. As the heating time was extended or the pyrolysis temperature was increased (from Fig. 6a–e), the intensity of diffraction peaks of fcc-Co and graphite became narrower and sharper, indicating an improvement in crystallinity and an increase of size of cobalt nanocrystals.¹³ However, there were no obvious diffraction peak observed at about 26° in T-700-8h and T-700-24h, since the impurities covered on the graphene layers of the as-synthesized samples and the broad diffraction peak of the amorphous silica matrix interfered the observation of the graphitic peak with weak intensity.¹⁴ The XRD patterns of the organometallic precursor PT before and after pyrolysis were shown in Fig. 7. The diffraction peak of PT shown at 2θ = 8.4° was associated with the hexagonal crystalline structure of the POSS cage, also the Co nanoparticles in the obtained CNPs was in its fcc phase. Unlike the case of T, due to its good sinter-resistant properties, the intensity of diffraction peaks of Co in the CNPs obtained from PT did not have an obvious change.

Fig. 6 XRD diffractograms of (a) T, (b) T-700-8h, (c) T-700-24h, (d) T-800-8h and (e) T-850-8h.

Fig. 7 XRD diffractograms of (a) PT, (b) PT-700-24h and (c) PT-850-8h.

EDX was used to analyze the chemical compositions and element distribution of the obtained CNPs. From the SEM-EDX, there were almost no oxygen existed in the obtained nanoparticles from compound T (Fig. S4 and Table S1†), it meant that the oxygen existed in compound T (oxygen from the ester groups and the [CCCo₂(CO)₆] units) were transformed into some oxygen-containing gas compounds. However, CNPs obtained from PT had a high content of silicon and oxygen, in comparison with T, demonstrating that the silicon–oxygen cages of POSS units existed in PT were converted into SiO_x after pyrolysis. TEM-EDX was used to further research the element distribution of the obtained CNPs, especially to find the distribution of the formed SiO_x in the core–shell architecture of the obtained CNPs. When the electron beam passed through the center (label 1-1 in Fig. 8a) and the shell (composed by graphene fragment) (label 1-2) of PT-700-24h, Co and C were the main element being detected, respectively. When the electron beam passed through the shells composed by amorphous materials (label 1-3), Si and O were detected with relative high proportion, indicating that the core–shell nanospheres had a Co@C-SiO_x architecture. And from the HRTEM analysis, we could find that the accumulation of C and SiO_x in the shell was interpenetrated. Taking into account of their good sinter-resistant properties, it should be the special Co@C-SiO_x structure that hindered the aggregation of cobalt nanoparticles. Similarly, the TEM-EDS analysis results in Fig. 8b and S5† also clearly demonstrated the chemical compositions and element distribution of PT-850-8h, T-700-24h and T-850-8h.

Fig. 8 TEM-EDX spectra of (a) PT-700-24h and (b) PT-850-8h.

The influence of POSS units and pyrolysis conditions on the magnetism of the CNPs

The magnetic properties of the obtained CNPs had been well investigated, all the CNPs were magnetizable and could be readily attracted to a magnet at room temperature. By vibrating sample magnetometer, their magnetic behaviors were quantitatively analyzed. Fig. 9 showed the magnetization curves of the magnetic CNPs, all the obtained nanomaterials exhibited soft-magnetic behavior with high magnetizability and low coercivity, with the magnetic data summarized in Table 1. As to the nanomaterials obtained from T (excepted T-850-8h), the saturation magnetization (M_s) ranged from 52.8 to 64.9 emu g⁻¹, considering its metal content, these value were high enough. And due to the big size and better crystallinity of cobalt, the M_s increased with increasing temperature accompanied with an obvious decrease of the coercivity. However, as to T-850-8h, the cobalt had the best crystallinity, while its M_s was the lowest (about 30.0 emu g⁻¹). This might be due to the effect of graphite on the magnetization behavior of the obtained CNPs as following: graphite is diamagnetic, with −21.5 × 10⁻⁶ emu g⁻¹ parallel to the magnetic field and −0.5 × 10⁻⁶ emu g⁻¹ perpendicular to the magnetic filed.¹⁵ Because the value of susceptibility of cobalt was much higher than that of graphite, usually the effect of graphite was very small. In this work, when the temperature increased to 850 °C, due to the production of large numbers of graphite with better graphitization (Fig. 4c and d and 6e), the effect of graphite would become larger. Then, the graphite could obviously decrease the saturation magnetization.¹⁶ As to PT-700-24h and PT-850-8h, their M_s were 16.6 and 24.5 emu g⁻¹, respectively. Due to the similar size and crystallinity of Co, their coercivity had a slight change. And in comparison with the CNPs obtained from T, the relative low M_s was mainly due to the appeared SiO_x transformed from the POSS groups, which decreased their cobalt content in the obtained CNPs.

Fig. 9 Plots of magnetization (M) versus applied magnetic field (H) at 300 K for the samples obtained from pyrolysis of T and PT under different conditions.

Table 1 Magnetization data for various samples measured at 300 K

Sample	Ms (emu g^−1)	Mr (emu g^−1)	Mr/Ms	Hc (Oe)
T-700-8h	52.8	8.3	0.16	315
T-700-24h	54.1	6.2	0.11	235
T-800-8h	64.9	3.4	0.05	120
T-850-8h	30.0	2.2	0.07	165
PT-700-24h	16.6	3.1	0.19	205
PT-850-8h	24.5	3.4	0.14	230

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Conclusions

Organometallic precursor PT containing three POSS units was synthesized, combined with precursor T that without POSS unit, we well researched the influence of POSS units on the morphology and the properties of the obtained CNPs under different pyrolysis conditions through SSP. It was found that the introduction of POSS groups could prevent metal nanoparticles from sintering into larger particles effectively, and change the growth trend of carbon nanoparticles in SSP. Meanwhile, all the obtained nanoparticles possessed good magnetic properties, and the magnetic properties exhibited a good accordance with their structure. The strategy of utilizing POSS unit to adjust the morphology and properties of the obtained nanoparticles should be also applied to produce other functional nanomaterials.

Experimental

Materials and instrumentation

PSS-(3-hydroxypropyl)-heptaisobutyl substituted (POSS) was purchased from Sigma-Aldrich, and octacarbonyldicobalt (Co₂(CO)₈) was purchased from Alfa Aesar. All other reagents were used as received without further purification. Organometallic compound T was synthesized in our previous work.⁷ The synthesis and characterization of organometallic compound PT and its precursors (compounds 1 and 2) were presented in the ESI.† Tetrahydrofuran (THF) was dried over and distilled from K–Na alloy under an atmosphere of dry nitrogen. Dichloromethane (DCM) was dried over and distilled from CaH₂. ¹H and ¹³C NMR spectroscopy study was conducted with a Varian Mercury 300 spectrometer using tetramethylsilane (TMS; δ = 0 ppm) as internal standard. ²⁹Si NMR spectra were measured on a 600 MHz Bruker Avance III NMR Spectrometer. EI-MS spectra were recorded with a Finnigan PRACE mass spectrometer. MALDI-TOF mass spectra were measured on a GCT premier CAB048 mass spectrometer using a 337 nm nitrogen laser and DCTB as matrix. The Fourier transform infrared (FTIR) spectra were recorded on a PerkinElmer-2 spectrometer in the region of 4000–400 cm⁻¹. Thermal analysis was performed on Netzsch STA 449C thermal analyzer at a heating rate of 10 °C min⁻¹ in nitrogen at a flow rate of 20 cm³ min⁻¹ for thermogravimetric analysis (TGA). Elemental analyses were performed by a CARLOERBA-1106 by a micro-elemental analyzer. 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 film. SEM was performed on a QUANTA scanning electron microscope. Energy-dispersive X-ray spectroscopy (EDS) was taken on the SEM or that attached to the TEM. The 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 Lake Shore 7037/9509-P vibrating sample magnetometer at room temperature.

Preparation of the compounds

The compounds 1, 2 and organometallic precursor PT were synthesized according to a procedure described here. Organometallic precursor T was synthesized in our previous work.⁷

1. PSS-(3-hydroxypropyl)-heptaisobutyl substituted (POSS) (0.44 g, 0.5 mmol, 1.00 equiv.) and 4-iodobenzoic acid (2.00 equiv.) were dissolved in anhydrous CH₂Cl₂ under argon atmosphere. DMAP (5 mol%) and EDCI (3.00 equiv.) were added, and the mixture was stirred at room temperature overnight. Then, the reaction mixture was poured into saturated citric acid aqueous solution and extracted with dichloromethane. The organic layer was washed with water and dried over anhydrous MgSO₄. Evaporation of the solvent and purification by column chromatography on silica gel using CH₂Cl₂/PE (1/4) as eluent to afford white powder (0.44 g, 80%). ¹H NMR (300 MHz, CDCl₃) δ (ppm): 7.80 (d, J = 7.2 Hz, 2H, ArH), 7.75 (d, J = 8.7 Hz, 2H, ArH), 4.27 (t, J = 6.6 Hz, 2H, –O–CH₂–), 1.86 (m, 9H), 0.96 (m, 42H, –CH₃), 0.72 (t, J = 8.7 Hz, 2H, Si–CH₂(CH₂)₂–), 0.60 (d, J = 7.2, 14H, Si–CH₂CH(CH₃)₂). ¹³C NMR (75 MHz, CDCl₃) δ (ppm): 166.35, 137.95, 131.31, 130.24, 100.86, 67.41, 25.98, 24.15, 22.76, 8.71. MS (MALDI-TOF), m/z [M + Na]⁺: 1127.3, calcd: 1127.2. Anal. calcd for C₃₈H₇₃IO₁₄Si₈: C 41.28, H 6.66; found: C 41.55, H 6.81.

2. A mixture of 1,3,5-triethynylbenzene (0.12 g, 0.8 mmol, 1.00 equiv.), compound 1 (6.00 equiv.), CuI (20% equiv.), triphenylphosphine (PPh₃) (10% equiv.), tetrakis (triphenylphosphine) palladium (Pd(PPh₃)₄) (5 mol%) and THF/triethylamine (2 : 1 in volume), was charged with argon. The reaction was stirred at 40 °C for 12 h. After cooled to room temperature, the mixture was filtered. The filtrate was evaporated to remove the solvent. The crude product was purified by column chromatography on silica gel using CH₂Cl₂/PE (1/4) as eluent to afford white powder (2.10 g, 85%). ¹H NMR (300 MHz, CDCl₃) δ (ppm): 8.05 (d, J = 8.1 Hz, 6H, ArH), 7.72 (s, 3H, ArH), 7.60 (d, J = 8.1 Hz, 6H, ArH), 4.31 (t, J = 6.3 Hz, 6H, –O–CH₂–), 1.86 (m, 27H), 0.96 (d, J = 6.3 Hz, 126H, –CH₃), 0.74 (t, J = 8.4 Hz, 6H, Si–CH₂(CH₂)₂–), 0.62 (m, 42H, Si–CH₂CH(CH₃)₂). ¹³C NMR (75 MHz, CDCl₃) δ (ppm): 166.19, 134.95, 131.86, 130.58, 129.90, 127.42, 124.02, 90.49, 90.33, 67.34, 25.98, 24.14, 22.74, 8.74. ²⁹Si NMR (119 MHz, CDCl₃) δ (ppm): −61.43, −61.72. MS (MALDI-TOF), m/z [M + Na + 3H]⁺: 3105.1, calcd: 3105.0. Anal. calcd for C₁₂₆H₂₂₂O₄₂Si₂₄: C 49.08, H 7.26; found: C 49.34, H 7.42.

PT. Compound 2 (0.25 g, 0.08 mmol, 1.00 equiv.) and Co₂(CO)₈ (6.00 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 purified by using column chromatography on neutral Al₂O₃ using CH₂Cl₂/PE (1/10) as eluent to afford brown solid (0.16 g, 51%). ¹H NMR (300 MHz, CDCl₃) δ (ppm): 7.96 (d, J = 8.1 Hz, 6H, ArH), 7.73 (s, 3H, ArH), 7.63 (d, J = 8.1 Hz, 6H, ArH), 4.29 (br, 6H, –O–CH₂–), 1.85 (m, 27H), 0.96 (m, 126H, –CH₃), 0.73 (t, J = 7.5 Hz, 6H, Si–CH₂(CH₂)₂–), 0.61 (d, J = 4.8 Hz, 42H, Si–CH₂CH(CH₃)₂). ¹³C NMR (75 MHz, CDCl₃) δ (ppm): 198.76, 166.24, 143.39, 140.69, 130.56, 130.11, 129.12, 90.27, 67.28, 26.01, 24.18, 22.80, 8.73. Anal. calcd for C₁₄₄H₂₂₂Co₆O₆₀Si₂₄: C 43.89, H 5.68; found: C 43.96, H 5.59. ²⁹Si NMR (119 MHz, CDCl₃) δ (ppm): −61.50, −61.81. FTIR (thin film), ν (cm⁻¹): 2028, 2060, 2093 (Co₂(CO)₆); 1107 (Si–O–Si).

Acknowledgements

We are grateful to the National Fundamental Key Research Program (2011CB932702) and the National Natural Science Foundation of China (No. 21325416) for financial support.

Notes and references

1. (a) L. Zhang, S. Zaric, X. Tu, X. Wang, W. Zhao and H. Dai, J. Am. Chem. Soc., 2008, 130, 2686; (b) S. Park, M. Vosguerichian and Z. Bao, Nanoscale, 2013, 5, 1727; (c) K. Balasubramanian and K. Kern, Adv. Mater., 2014, 26, 1154; (d) F. Balavoine, P. Schultz, C. Richard, V. Mallouh, T. W. Ebbesen and C. Mioskowski, Angew. Chem., Int. Ed., 1999, 38, 1912; (e) W. Wu, S. Wieckowski, G. Pastorin, M. Benincasa, C. Klumpp, J. Briand, R. Gennaro, M. Prato and A. Bianco, Angew. Chem., Int. Ed., 2005, 44, 6358; (f) T. R. Fadel, N. Li, S. Shah, M. Look, L. D. Pfefferle, G. L. Haller, S. Justesen, C. J. Wilson and T. M. Fahmy, Small, 2013, 9, 666; (g) R. G. Mendes, A. Bachmatiuk, B. Büchner, G. Cuniberti and M. H. Rümmeli, J. Mater. Chem. B, 2013, 1, 401; (h) M. F. L. de Volder, S. H. Tawfick, R. H. Baughman and A. J. Hart, Science, 2013, 339, 535; (i) Q. Zhang, E. Uchaker, S. L. Candelaria and G. Cao, Chem. Soc. Rev., 2013, 42, 3127.
2. (a) M. Bedewy, E. R. Meshot and A. J. Hart, Carbon, 2012, 50, 5106; (b) G. Y. Xiong, Y. Suda, D. Z. Wang, J. Y. Huang and Z. F. Ren, Nanotechnology, 2005, 16, 532; (c) T. Cui, R. Lv, Z.-H. Huang, M. Wang, F. Kang, K. Wang and D. Wu, Mater. Lett., 2011, 65, 587; (d) X. Qi, C. Qin, W. Zhong, C. Au, X. Ye and Y. Du, Materials, 2010, 3, 4142; (e) J. Kang, J. Li, N. Zhao, P. Nash, C. Shi and R. Sun, Mater. Chem. Phys., 2011, 125, 386; (f) D. Venegoni, P. Serp, R. Feurer, Y. Kihn, C. Vahlas and P. Kalck, Carbon, 2002, 40, 1799; (g) V. Jourdain and C. Bichara, Carbon, 2013, 58, 2.
3. (a) C. N. R. Rao and A. Govindaraj, Acc. Chem. Res., 2002, 35, 998; (b) H. Hou, A. K. Schaper, F. Weller and A. Greiner, Chem. Mater., 2002, 14, 3990; (c) M. Sevilla, C. S. Martínez-de Lecea, T. Valdés-Solís, E. Morallón and A. B. Fuertes, Phys. Chem. Chem. Phys., 2008, 10, 1433; (d) E. Muñoz, M. L. Ruiz-González, A. Seral-Ascaso, M. L. Sanjuán, J. M. González-Calbet, M. Laguna and G. F. de la Fuente, Carbon, 2010, 48, 1807; (e) T. N. Hoheisel, S. Schrettl, R. Szilluweit and H. Frauenrath, Angew. Chem., Int. Ed., 2010, 49, 6496; (f) S. Rondeau-Gagné and J.-F. Morin, Chem. Soc. Rev., 2014, 43, 85.
4. (a) P. I. Dosa, C. Erben, V. S. Iyer, K. P. C. Vollhardt and I. M. Wasser, J. Am. Chem. Soc., 1999, 121, 10430; (b) V. S. Iyer, K. P. C. Vollhardt and R. Wilhelm, Angew. Chem., Int. Ed., 2003, 42, 4379; (c) M. Laskoski, W. Steffen, J. G. M. Morton, M. D. Smith and U. H. F. Bunz, J. Am. Chem. Soc., 2002, 124, 13814; (d) L. Zhi, T. Gorelik, R. Friedlein, J. Wu, U. Kolb, R. Salaneck and K. Müllen, Small, 2005, 1, 798; (e) J. Wu, B. E. Hamaoui, J. Li, L. Zhi, U. Kolb and K. Müllen, Small, 2005, 1, 210; (f) B. E. Hamaoui, L. Zhi, J. Wu, J. Li, N. T. Lucas, Z. Tomovic, U. Kolb and K. Müllen, Adv. Funct. Mater., 2007, 17, 1179; (g) L. Zhi, Y. S. Hu, B. E. Hamaoui, X. Wang, I. Lieberwirth, U. Kolb, J. Maier and K. Müllen, Adv. Mater., 2008, 20, 1727.
5. (a) D. B. Cordes, P. D. Lickiss and F. Rataboul, Chem. Rev., 2010, 110, 2081; (b) X. Su, S. Guang, H. Xu, X. Liu, S. Li, X. Wang, Y. Deng and P. Wang, Macromolecules, 2009, 42, 8969; (c) X. Huang, C. Zhi, P. Jiang, D. Golberg, Y. Bando and T. Tanaka, Adv. Funct. Mater., 2013, 23, 1824; (d) B. H. Augustine, W. C. Hughes, K. J. Zimmermann, A. J. Figueiredo, X. Guo, C. C. Chusuei and J. S. Maidment, Langmuir, 2007, 23, 4346; (e) Z. Geng, M. Huo, J. Mu, S. Zhang, Y. Lu, J. Luan, P. Huo, Y. Du and G. Wang, J. Mater. Chem. C, 2014, 2, 1094; (f) S.-D. Jiang, G. Tang, Z.-M. Bai, Y.-Y. Wang, Y. Hu and L. Song, RSC Adv., 2014, 4, 3253.
6. (a) Z. Ruan, C. Y. K. Chan, J. W. Y. Lam, Q. Wu, Q. Li, J. Qin, B. Z. Tang and Z. Li, J. Mater. Chem. C, 2014, 2, 633; (b) Z. Ruan, W. Rong, X. Zhan, Q. Li and Z. Li, Polym. Chem., 2014, 5, 5994; (c) Z. Ruan, W. Rong, Q. Li and Z. Li, J. Inorg. Organomet. Polym., 2015, 25, 98.
7. Z. Ruan, W. Rong, Q. Li and Z. Li, Carbon, 2015, 87, 338.
8. (a) I. Stamatina, A. Morozana, A. Dumitrua, V. Ciupinab, G. Prodanb, J. Niewolskic and H. Figiel, Phys. E, 2007, 37, 44; (b) M. Kumar and Y. Ando, J. Nanosci. Nanotechnol., 2010, 10, 3739.
9. C. H. Kiang, M. Endo, P. M. Ajayan, G. Dresselhaus and M. S. Dresselhaus, Phys. Rev. Lett., 1998, 81, 1869.
10. (a) R. K. Rana, X. N. Xu, Y. Yeshurun and A. Gedanken, J. Phys. Chem. B, 2002, 106, 4079; (b) V. O. Nyamori and N. J. Coville, Organometallics, 2007, 26, 4083; (c) P. Nikolaev, M. J. Bronikowski, R. K. Bradley, F. Rohmund, D. T. Colbert, K. A. Smith and R. E. Smalley, Chem. Phys. Lett., 1999, 313, 91; (d) Y. Chen, D. Ciuparu, S. Lim, G. L. Haller and L. D. Pfefferle, Carbon, 2006, 44, 67.
11. N. S. Kim, Y. T. Lee, J. Park, J. B. Han, Y. S. Choi, S. Y. Choi, J. Choo and G. H. Lee, J. Phys. Chem. B, 2003, 107, 9249.
12. T. W. Hansen, A. T. Delariva, S. R. Challa and A. K. Datye, Acc. Chem. Res., 2013, 46, 1720.
13. (a) G. Ennas, A. Falqui, S. Marras, C. Sangregorio and G. Marongiu, Chem. Mater., 2004, 16, 5659; (b) D. Wang, H. L. Xin, H. Wang, Y. Yu, E. Rus, D. A. Muller, F. J. DiSalvo and H. D. Abruña, Chem. Mater., 2012, 24, 2274.
14. S. Liu, J. Zhu, Y. Mastai, I. Felner and A. Gedanken, Chem. Mater., 2000, 12, 2205.
15. Z. H. Wang, Z. D. Zhang, C. J. Choi and B. K. Kim, J. Alloys Compd., 2003, 361, 289.
16. A. Wu, X. Yang and H. Yang, J. Alloys Compd., 2012, 513, 193.

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Footnote

† Electronic supplementary information (ESI) available: Synthesis and the characterization, IR spectra, TGA thermograms and pyrolysis program of the organometallic compounds. HRTEM images and EDX spectra of the obtained CNPs. See DOI: 10.1039/c5ra05375g

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