Self-assembled synthesis of carbon-coated Fe₃O₄ composites with firecracker-like structures from catalytic pyrolysis of polyamide

Abstract

Carbon-coated Fe₃O₄ composites with firecracker-like structures have been fabricated by catalytic pyrolysis of polyamide (PA) in a sealed reaction system. As revealed in field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) analysis, the diameter of the one-dimensional firecracker-like structures is about 4 μm and the diameter of the secondary nanorods is about 620 nm. The diameter of Fe₃O₄ nanorods inside the carbon shells is about 82 nm, and the thickness of the carbon shells is about 265 nm. Some important preparative parameters related to the synthesis have been identified and investigated by some designed experiments. The magnetic measurement at room temperature indicates that the values of saturation magnetization (19.2 emu g⁻¹) and coercivity (270.1 Oe) of the carbon-coated Fe₃O₄ composites with firecracker-like structures are different from those of Fe₃O₄ nanoparticles and bulk Fe₃O₄ due to the different carbon content, dipolar interaction, size and morphology of the products. The results also indicate that the one-dimensional Fe₃O₄@C core–shell structures possess good acid resistance.

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1. Introduction

Magnetite (Fe₃O₄) is a kind of important magnetic material and has attracted enormous research interests in lithium storage,¹ targeted drug delivery,² and water treatment.³ Due to the unique magnetic nature of Fe₃O₄, it has also been applied in magnetism,⁴ as well as magnetic resonance imaging.⁵ However, there are some limitations in their practical application. One is that naked Fe₃O₄ particles can't be used in strong acidic solution. The other important limitation is that Fe₃O₄ microspheres have few functional groups on their surface. As is well-known, the support for drug or catalyst must have abundant functional groups on its surface to adsorb the drug or catalyst. However, Fe₃O₄ nanoparticles can hardly adsorb the drugs or catalysts. Additionally, the highly active ferromagnetic nanoparticles are easily oxidized in air and tend to agglomerate for reducing the energy associated with high surface/volume ratio, which results in the loss of magnetism and dispersibility of naked ferromagnetic nanoparticles. By coating ferromagnetic nanoparticles with other materials, Fe₃O₄ particles can be protected from being dissolved in acid environment⁶ and their properties can also be tailored with different desirable functional properties such as biocompatibility or electrical conductivity or chemical and physical stability which are necessary for particular applications.⁷ Among them, silica has being mostly used as the coating material because it can efficiently inhibit the hydrogen ions to get through.^8,9 However, inorganic silica shells also have few functional groups on their surface and must be chemically modified before being used as support for catalyst. The modification process is usually inconvenient for industrial application and the modifiers such as N-hydroxysuccinimidyl ester are usually toxic, which limit their application in drug delivery, catalyzing the degradation of water pollutants and other processes related to life science.¹⁰ For the reason, various polymers, such as polypyrrole and polyaniline, have been used as the coating material for Fe₃O₄ nanoparticles.^11,12 Though there are always a plenty of functional groups on their surface, the gaps in the network of polymers may allow hydrogen ions to get through and dissolve the Fe₃O₄ cores. Consequently, it is not very appropriate to use such polymers as the coating material for Fe₃O₄ spheres. Compared to polymer and silica shells which were studied most, carbon shells exhibit much higher stability in various chemical and physical environments.^13,14 Encapsulating ferromagnetic nanoparticles inside carbon shells is of considerable significance, because the encapsulated species can be immunized against environmental degradation effects, but also it offers an opportunity to investigate dimensionally confined systems.^15,16 Such carbon coated ferromagnetic nanoparticles are expected to play a significant role in a wide range of applications. For example, serving as adsorbents,¹⁷ catalytic systems supports,¹⁸ enzyme and protein immobilization,¹⁹ and so on.

Until now, some techniques have been developed to synthesize carbon-encapsulated magnetic nanoparticles.^20,21 Recently, 3D hierarchical nanostructures have attracted much attention because of their unique chemical and physical properties and great potential applications.^22–26 The most efficient route to 3D nanostructures is probably self-assembly through a spontaneous process.²⁷ However, the practical use of carbon-encapsulated magnetic nanoparticles is significantly hampered by the intrinsically high energy consumption, environmental load and economic factors of these mentioned techniques. The growing need for carbon-encapsulated magnetic nanoparticles has prompted numerous research efforts for developing effective and economic synthesis techniques. Recently, decomposition of polymer in closed systems to prepare carbon materials aroused many interests.^28,29 The preparative process belongs to “green chemistry” because of the use of safe reactants and the preparative process doesn't produce pollution into environment. To the best of our knowledge, there have been few reports about the synthesis of complex Fe₃O₄@C hierarchical structures.

Herein, we demonstrate the design and synthesis of carbon-coated Fe₃O₄ composites with firecracker-like structures from PA as carbon source without solvents via a simple and economical catalytic pyrolysis process at lower temperature (500 °C). Thus, 1D magnetic structure with novel morphology was obtained. Importantly, the carbon-coated Fe₃O₄ architecture exhibits well acid resistance and have a higher coercivity (Hc) value. Furthermore, potential applications of the complex firecracker-like architectures have also been addressed on the basis of the unique physicochemical properties.

2. Experimental section

In a typical procedure, PA (1.0 g), ferrocene (0.5 g) and ammonium acid carbonate (1.0 g) were loaded into a stainless steel autoclave of 20 mL capacity. The autoclave was sealed and put into an electronic furnace, and the temperature of the furnace was increased to 500 °C in 50 minutes and maintained at 500 °C for 12 h. Then, the autoclave was cooled to room temperature naturally. It was found that the final products in the autoclave were dark precipitates and residual gases. The black precipitates were collected and divided into two parts: one part was washed with distilled water and ethanol for several times, and the other part was heated in HCl solution of about 3 mol L⁻¹ at 80 °C for 2 h and washed with distilled water and ethanol for several times. Afterwards, the products were dried in a vacuum box at 50 °C for 4 h, and were collected for characterization.

The X-ray powder diffraction (XRD) pattern of the products without acid treatment was recorded on a Rigaku (Japan) D/max-γA X-ray diffractometer equipped with graphite-monochromatized Cu Kα radiation (λ = 1.54178 Å). The Raman spectra were recorded at ambient temperature on a SPEX 1403 spectrometer with an argon-ion laser at an excitation wavelength of 514.5 nm. X-ray photoelectron spectroscopy (XPS) of the products was performed using a VGESCALABMK X-ray photoelectron spectrometer and non-monochromated Mg Kα radiation as the excitation source. The field-emission scanning electron microscopy (FESEM) images of the products were examined by a field-emission scanning electron microscope (JEOL-6300F). The transmission electron microscope (TEM) images, high resolution transmission electron microscope (HRTEM) image were taken on a JEOL 2010 high-resolution transmission electron microscope at an acceleration voltage of 200 kV. Element dispersive spectrometric (EDS) analysis was carried out on an X-ray energy spectrum instrument equipped with INCA300 (Oxford). The magnetic properties (M–H curve) were measured at room temperature on an MPMS XL magnetometer made in Quantum Design Corporation.

3. Results and discussion

To ascertain the component and structure of the products, XRD was used to investigate the crystal phases of the products before acid treatment. Fig. 1a shows a XRD pattern of the products before acid treatment. The sharp diffraction peaks with relative high peak intensity were indexed as crystalline face-centered cubic (fcc) Fe₃O₄ (JCPDS Card no. 85-1436), while there is a broad peak with low peak intensity at 2θ = 22–26°, corresponding to the noncrystalline carbon phase. No other noticeable peaks induced by impurities can be observed in the XRD pattern. Raman spectrum is a standard nondestructive tool for the characterization of carbon materials, particularly to determine the graphitization degree.³⁰ Fig. 1b gives the corresponding Raman spectra, showing two similar Raman bands at 1341 cm⁻¹ (D band) and 1593 cm⁻¹ (G band). More specifically, the two peaks exhibit an E_2g mode of graphite related to the vibration of sp²-bonded carbon atoms in a 2D hexagonal lattice (such as in a graphene layer) and A_1g mode of graphite related to the disorder features due to the finite particle size effect or lattice distortion of the graphite crystals. The relative intensity ratio of the D-G band, I_D/I_G, can give reliable information for the graphitization degree.³⁰ It is found that the carbon-coated Fe₃O₄ composites have a higher I_D/I_G, implying a highly disordered graphitic structure for carbon shells, and these results are consistent with the XRD pattern.

Fig. 1 (a) XRD pattern of carbon-coated Fe₃O₄ composites with firecracker-like structures; (b) Raman spectrum of carbon-coated Fe₃O₄ composites with firecracker-like structures.

In order to analyze the surface component of the carbon-coated Fe₃O₄ composites, X-ray photoelectron spectra of the products was measured. The XPS of carbon-coated Fe₃O₄ composites with firecracker-like structures are shown in Fig. 2. It can be found that the peaks on the full pattern are mainly attributed to C1s (284.5 eV), O1s (533.2 eV) and their corresponding Auger peaks. However, there are almost no peaks for the Fe2p binding energy detected, suggesting that Fe₃O₄ nanorods are completely encapsulated in thicker carbon layers. Quantitative analysis reveals that the molar ratio of C/O/Fe is 95.45/4.26/0.28. The high percent of element O may come from the oxygen containing groups on the carbon shells.

Fig. 2 X-ray photoelectron spectra of Fe₃O₄@C core–shell structures: (a) a wide scan spectrum; (b) high-resolution XPS spectrum of C 1s region; (c) high-resolution XPS spectrum of Fe 2p region; (d) high-resolution XPS spectrum of O 1s region.

Iron oxides encapsulated in carbon based systems have attracted increasing attention due to their electric and magnetic properties and their variety of potential technological application. In recent years, there have been a number of studies of iron oxides and carbon composites. Among them, nanoparticles, nanowires, nanoplates and hexapod-like microstructures have been synthesized. In Fig. 3a, another type of Fe₃O₄ and carbon composites is presented in high morphological yield. The composites are one-dimensional firecracker-like structures, which were assembled by nanorods. The diameter of one-dimensional firecracker-like structures is about 4 μm and the diameter of secondary nanorods is about 620 nm. The structures were further characterized by TEM. Fig. 3b and c are TEM images before acid treatment, which indicates that the nanorods are core–shell structures, which made of two parts: the outer shell (carbon) and the inner core (Fe₃O₄). Careful observation shows that the diameter of Fe₃O₄ nanorods inside carbon shell is about 82 nm, and the thickness of carbon shells is about 265 nm. From the results of the HRTEM image of carbon shells in Fig. 3d, no obvious lattice images are observed on the shells, indicating their low crystallization. The chemical composition of carbon-coated Fe₃O₄ composites was analyzed using an element dispersive spectrometer attached to a high-resolution electron microscope. The results of EDS in Fig. 3e confirm that Fe, O and C elements are presented in the composites. The signals of Cu element come from the supporting TEM grid during measurements. It is noteworthy mentioning that the Fe₃O₄ nanorods are still in the carbon shells after acid treatment, shown in Fig. 3f, which indicates that the Fe₃O₄@C core–shell structures possess well acid resistance.

Fig. 3 (a) FESEM image of carbon-coated Fe₃O₄ composites with firecracker-like structures; (b) low magnification TEM image of carbon-coated Fe₃O₄ composites with firecracker-like structures before acid treatment; (c) high magnification TEM image of carbon-coated Fe₃O₄ composites with firecracker-like structures before acid treatment; (d) HRTEM image of carbon shell; (e) EDS of spectrum of carbon-coated Fe₃O₄ composites with firecracker-like structures before acid treatment; (f) TEM image of carbon-coated Fe₃O₄ composites with firecracker-like structures after acid treatment.

To understand the possible formation process of the carbon-coated Fe₃O₄ composites, a series of relevant experiments were carried out by altering the experimental parameters. Several factors were found to influence the morphology of the composites: reaction temperature, dose of NH₄HCO₃, and reaction atmosphere. According to the previous study, carbonization of polymer is a pyrolysis, dehydrogenation and cross-linking process, which is greatly affected by reaction temperature. When the reaction temperature was 500 °C, the reaction rate was lower, and the obtained Fe₃O₄ and carbon atoms were self-assembled to carbon-coated Fe₃O₄ composites with firecracker-like structures, shown in Fig. 3. Fig. 4 shows FESEM images of the products obtained at 600 °C and 700 °C. It is found that there are two types of structures at 600 °C. Fig. 4a shows that carbon-coated Fe₃O₄ microparticles are the main products, which are stacked together. Besides them, carbon microspheres are also the products, as shown in Fig. 4b. By increasing the reaction temperature to 700 °C, the products also include two kinds of structures. Fig. 4c shows that the carbon-coated Fe₃O₄ particles are the products, and the main size of particles is about 500 nm. Fig. 4d reveals that the products are also composed of mats-like structures self-assembled by flat carbon nanotubes. It is generally known that the nucleation rate of Fe₃O₄ was accelerated, so large Fe₃O₄ particles were formed, and part of carbon atoms were deposited on the surface of Fe₃O₄ particles and other carbon atoms formed carbon structures such as carbon microspheres and mats-like structures.

Fig. 4 (a) and (b) FESEM images of the products obtained at 600 °C for 12 h; (c) and (d) FESEM images of the products obtained at 700 °C for 12 h.

Fig. 5 shows the FESEM images of carbon and Fe₃O₄ composites prepared at 500 °C for 12 h with different doses of NH₄HCO₃. In the absence of NH₄HCO₃, Fig. 5a and b show that the products have one-dimensional and worm-like morphology with smooth surface, and have a length about 60 μm and a diameter about 4 μm. As 0.5 g of NH₄HCO₃ is added, the main products are one-dimensional and firecracker-like structures, and nanorods have the length about 1.2 μm and the diameter about 750 nm, as shown in Fig. 5c and d. When the amount of NH₄HCO₃ is 2 g, the FESEM images in Fig. 5e and f for the products show abundant one-dimension structures, being tens of micrometers in length. In fact, it is hard for us to discern where the chains' ends are, because they almost attach together.

Fig. 5 (a) and (b) FESEM images of the products obtained without NH₄HCO₃; (c) and (d) FESEM images of the products obtained with 0.5 g NH₄HCO₃; (e) and (f) FESEM images of the products obtained with 2 g NH₄HCO₃.

Apart from the reaction temperature and amount of NH₄HCO₃, the composites can also be tuned by varying reaction atmosphere. Fig. 6 and 7 show the morphologies and components of the products using H₂O, CO(NH₂)₂, CS(NH₂)₂ and NaN₃ replacing NH₄HCO₃. From the results, it is found that the reaction atmosphere influence the components and morphologies. When H₂O was used to replace NH₄HCO₃, Fig. 6a shows that the products are one-dimensional and worm-like structures with the diameter of about 600 nm, and Fig. 7a indicates the products are the composites of C and Fe₃O₄ (JCPDS Card no. 85-1436). In this system, PA decomposed to produce small molecules carbonyl compounds as carbon source in the absence of oxygen, and Fe atoms were released by decomposition of ferrocene to act as dehydrogenation catalyst. As-formed Fe atoms reacted with H₂O to form Fe₃O₄ nanoparticles with high reaction rate due to the presence of large amounts of water vapor, and the obtained Fe₃O₄ particles were self-assembled to chain-like structures under the drive of magnetic dipole interaction. The small molecules carbonyl compounds were carbonized to produce carbon atoms, which precipitated on the surface of Fe₃O₄ chains to form worm-like structures. When NH₄HCO₃ was replaced by CO(NH₂)₂, Fe atoms by decomposition of ferrocene acted as dehydrogenation catalyst of the small molecules carbonyl compounds and form a large number of carbon atoms. Because the Fe atoms were oxidized with low reaction rate in the atmosphere of NH₃ and HCNO by decomposition of CO(NH₂)₂, the carbon atoms grew to carbon nanostructures for decreasing the energy of the system before formation of Fe₃O₄. From Fig. 6b, the products are large amounts of flake-like structures and nanoparticles. Fig. 7b reveals that the products are the composites of C and Fe₃O₄ (JCPDS Card no. 85-1436). When the CS(NH₂)₂ was used to replace NH₄HCO₃, the products are also flake and nanoparticles and mainly composed of C and FeS (JCPDS Card no. 65-9124), shown in Fig. 6c and 7c. The formation mechanism is similar to that of using CO(NH₂)₂ replacing NH₄HCO₃. If NaN₃ replaces NH₄HCO₃, NaN₃ decompose to release Na and N₂. Fe atoms were not oxidized in the presence of Na vapor, which acted as dehydrogenation catalyst to decompose small molecules carbonyl compounds. Fig. 6d displays that the products include sponge-like structures and nanoparticles, and Fig. 7d reveals that the products are the composite of C and Fe (JCPDS 06-0696). Consequently, it can be believed that appropriate atmosphere could serve for creating products with firecracker-like structures.

Fig. 6 (a) FESEM images of the products using H₂O replacing NH₄HCO₃; (b) FESEM images of the products using CO(NH₂)₂ replacing NH₄HCO₃; (c) FESEM images of the products using CS(NH₂)₂ replacing NH₄HCO₃; (d) FESEM images of the products using NaN₃ replacing NH₄HCO₃.

Fig. 7 (a) XRD pattern of the products using H₂O replacing NH₄HCO₃; (b) XRD pattern of the products using CO(NH₂)₂ replacing NH₄HCO₃; (c) XRD pattern of the products using CS(NH₂)₂ replacing NH₄HCO₃; (d) XRD pattern of the products using NaN₃ replacing NH₄HCO₃.

To elucidate clearly the catalyzing carbonization mechanism of polyamide to form the carbon-coated Fe₃O₄ composites with firecracker-like structures, an illustrative scheme is presented in Fig. 8. At first, NH₄HCO₃ started to decompose and produce NH₃, H₂O and CO₂ from 30 °C, shown in process (1). With increasing temperature, PA chains underwent endothermic chain scission to produce small molecules carbonyl compounds and other fragments in the absence of oxygen, most of which were gases at 500 °C, shown in process (2). Fe atoms were released by decomposition of ferrocene at 400 °C, shown in process (3). Meanwhile, these small molecules carbonyl compounds were catalytically decomposed to form carbon atoms under Fe atoms as catalysts, shown in process (4). Then, as-formed Fe atoms gradually reacted with H₂O and CO₂ to form Fe₃O₄ and assemble to firecracker-like structures, shown in process (5). These Fe₃O₄ firecracker-like structures were speedy wrapped by a small number of carbon atoms (forming a thin carbon lamella) to reduce their surface energy, and form Fe₃O₄ encapsulated in thin carbon capsules. These thin carbon lamella confined the continued growth of Fe₃O₄ structures. As carbon atoms are enough in these experiments, the continued addition (or diffusion) of carbon atoms finally leads to the formation of firecracker-like Fe₃O₄@C core–shell structures. On the whole, Fe atoms by decomposition of ferrocene acted as dehydrogenation catalyst, and PA were decomposed catalytically to form carbon atoms and coat on Fe₃O₄ by a dissociation–diffusion–precipitation process.

Fig. 8 Mechanism for the formation of Fe₃O₄@C composites with firecracker-like structures through catalytic pyrolysis.

In many reported Fe₃O₄ nanostructures, a significant enhancement in ferromagnetic character has been observed. The observed increase in coercivity (Hc) of products has been attributed to the increase in structural anisotropy and one-dimensional structures. The magnetic properties were investigated at room temperature with an applied field from −10 000 Oe to 10 000 Oe, shown in Fig. 9. The magnetic hysteresis loop of carbon-coated Fe₃O₄ composites with firecracker-like structures shows ferromagnetic behavior with saturation magnetization (M_s), remanent magnetization (M_r), and coercivity (H_c) values of ca. 19.2 emu g⁻¹, 3.7 emu g⁻¹ and 270.1 Oe in Fig. 9a. From the results, the saturation magnetization value of the products is lower than that of bulk Fe₃O₄ (85–100 eum g⁻¹), but higher than those of previously reported Fe₃O₄–C nanowires (5.11 emu g⁻¹) and Fe₃O₄–C microrods (0.91 emu g⁻¹).^31,32 The decrease in the value of M_s found in this work might be most likely attributed to the wide existence of carbon layers on the surface of Fe₃O₄ nanorods. These carbon layers restrict the optional movement and interactions of the Fe₃O₄ nanorods. Its role is similar to that of the surfactant that existed on the Fe₃O₄ nanorods, which leads to decreased M_s value.³³ The results of burring experiments indicate that the calculated Fe₃O₄ to Fe₃O₄@ carbon composites mass ratio of the products is about 0.2065. The high shape anisotropy of the Fe₃O₄ nanorods and the presence of a detrimental surface/crystal-structure are also related to the demagnetization effects. Additionally, the H_c value is higher than that of bulk Fe₃O₄, and Fe₃O₄–C nanocapsules (179.4 Oe),³⁴ but lower than those of previously reported Fe₃O₄–C coaxial nanofibers (324.5 Oe).³⁵ The reason may be that the higher shape anisotropy, together with reduced size,³⁶ is responsible for the increase in H_c value compared with bulk Fe₃O₄. The high shape anisotropy of the nanorods prevents them from magnetizing in directions other than along easy magnetic axes. With nanorods randomly oriented, the projection of magnetization vectors along the field direction will be lower than that of nanoparticles with the large shape anisotropy effect. When compared with coaxial nanofibers with single crystalline Fe₃O₄ nanowires, the carbon-coated Fe₃O₄ composites with firecracker-like structures have lower shape anisotropy, resulting in the decrease of the H_c value. Fig. 9b shows that the results after acid treatment are similar to those before acid treatment, so it is confirmed that the Fe₃O₄@C core–shell structures possess well acid resistance. The Fe₃O₄@C composites may be used in acidic medium, such as adsorbing pollutant in acidic waste water, which also can be easily separated from water by applying a relatively low magnetic field.

Fig. 9 Magnetic hysteresis loops of carbon-coated Fe₃O₄ composites with firecracker-like structures at room temperature: (a) before acid treatment; (b) after acid treatment.

4. Conclusions

In summary, Fe₃O₄@C composites with firecracker-like structures were synthesized by controlling the reaction temperature and amount of NH₄HCO₃ through catalytic decomposition of polyamide. The encapsulated Fe₃O₄ nanorods were self-assembled into one-dimension firecracker-like structures; and the diameter of Fe₃O₄ nanorods inside carbon shell is about 82 nm, and the thickness of carbon shells is about 265 nm. Magnetic hysteresis loop measurement shows that the Fe₃O₄@C core–shell structures display ferromagnetic properties at room temperature, and the saturation magnetization value (19.2 emu g⁻¹) is lower than that of bulk Fe₃O₄, and the coercivity (H_c) value (270.1 Oe) is higher than those of Fe₃O₄ nanoparticles and bulk Fe₃O₄, which can be attributed to the wide existence of carbon layers, higher shape anisotropy and reduced size of the Fe₃O₄@C composites. The results of magnetic properties reveal that the Fe₃O₄@C core–shell structures possess well acid resistance because the Fe₃O₄ nanoparticles were not dissolved in HCl solution of about 3 mol L⁻¹ at 80 °C for 2 h. The Fe₃O₄@C composites with firecracker-like structures have good potential application due to their unique structural features and other physicochemical properties.

Acknowledgements

The work was financially supported by Natural Science Foundation of the Higher Education Institutions of Jiangsu Province (no. 13KJB430012), the China Postdoctoral Science Foundation (no. 2012M511210), the research fund of Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province (no. AE201104), the Opening Project of CAS Key Laboratory of Materials for Energy Conversion, and Natural Science of Foundation of China (no. 51203069).

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