Nanocomposites consisting of nanoparticles of iron oxide (Fe3O4) and iron carbide (Fe3C) with a core-shell structure (Fe core, Fe3O4 and/or Fe3C shells) coated with additional graphite-like carbon layer dispersed in carbon matrix have been synthesized by solid-phase pyrolysis of iron-phthalocyanine (FePc) and iron-porphyrin (FePr) with a pyrolysis temperature of 900°C, and post-annealing conducted at temperatures ranging from 150°C to 550°C under controlled oxygen- and/or nitrogen-rich environments. A comprehensive analysis of the samples’ morphology, composition, structure, size, and magnetic characteristics was performed by utilizing scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-STEM) with elemental mapping, X-ray diffraction analysis (XRD), and magnetic measurements by utilizing vibrating sample magnetometry (VSM). The effect of the annealing process on magnetic performance and efficient control of the hysteresis loop and specific absorption rate (SAR) are discussed.

In recent years, carbon magnetism and magnetic metal nanoparticles have garnered significant attention from both a scientific perspective and in relation to their numerous potential applications in fields such as biomedicine, spintronics, catalysis, and sensors.1–3 Functional nanoparticles hold special significance due to the unique interplay between their optical, magnetic, and electrical properties, all of which are influenced by their composition, size, and morphology. Obtaining functional nanoparticles with a core-shell structure holds promise for enhancing their operational magnetic characteristics, as this architecture allows for the integration of materials with diverse magnetic properties.4,5 The core-shell magnetic interactions offer an additional degree of freedom for varying the magnetic attributes of these nanoparticles.6 Furthermore, iron and iron oxide nanoparticles featuring a core-shell structure serve as highly efficient heat transfer intermediaries in alternating magnetic fields.7,8 The primary challenge in utilizing iron oxide nanoparticles for advanced medical technology applications lies in their biocompatibility. An important consideration is that nanoparticles in solution tend to agglomerate, altering their magnetic characteristics, therefore, it is necessary to apply protective carbon or silicon layers to also ensure high colloidal stability and overcome the tendency to agglomerate.9,10 One method we developed for preparing metal nanoparticles embedded in carbon matrices involves the process of solid-phase pyrolysis of iron-phthalocyanine, Fe(C32N8H16), and iron-porphyrin, Fe(C34N4O5H33).11–14 In this case, the pyrolyzed products are nanocomposites containing iron-to-carbon atomic ratios of 1:32 and 1:34, allowing the formation of diverse iron-carbon structures with distinct concentrations of iron equal to 0.031 and 0.029, respectively. The synthesized nanocomposites are subjected to additional ultrasonic treatment for deagglomeration, as well as magnetic and size separations. In this study, we focus on core-shell Fe–Fe3O4 and Fe–Fe3C nanoparticles synthesized through solid-phase pyrolysis of iron-phthalocyanine (FePc) and iron-porphyrin (FePr), followed by further annealing in an oxygen or nitrogen environment. Adjusting the temperatures and duration of the annealing process allows for the optimization of Fe–Fe3O4 and Fe–Fe3C core-shell nanoparticle sizes in nanocomposites, which in turn helps to control their magnetic properties and hysteresis parameters. We investigate the composition, structure, morphology, and magnetic properties of the synthesized samples using a scanning electron microscope (SEM), high-resolution scanning transmission electron microscopy (HR-STEM) with elemental mapping, X-ray diffraction (XRD) analyses, and vibrating sample magnetometry (VSM).

To synthesize core-shell iron nanoparticles, we employ powdered iron-phthalocyanine as precursor in the solid-phase pyrolysis reaction under vacuum, as illustrated in the subsequent scheme
Fe(C32N8H16)8H2,4N2TP,tP,PPFe+32C
(1)
where Tp is the pyrolysis temperature, tp is the pyrolysis time, and pp is the self-generated pressure within the reaction ampoule. For organometallic iron-porphyrin Fe(C34N4O5H33) complex with iron (Fe) at the center and peripheral oxygen, decomposition this reaction scheme is more complex and contains varies distinct phases, such as α-iron, iron carbide, magnetite, hematite, and reactant product depend also on various factors such as the specific ligands present and the conditions under which the pyrolysis and annealing occurs (see Table III).

Here we conducted the pyrolysis of iron-phthalocyanine, and iron-porphyrin, in a sealed quartz ampoule under vacuum, setting the Tp to 900°C. With the aim to synthesize Fe nanoparticles with sizes ranging from 30 to 60 nm and a high concentration of iron, the pyrolysis time is set to 180 minutes. Under the specified pyrolysis conditions, diffusion processes facilitate the accumulation of iron atoms, thereby promoting the formation of nanoparticles and achieving a high degree of graphitization in the carbon matrix. Annealing to produce iron oxide was carried out in a sealed oxygen environment, with temperatures varying between 150°C and 550°C. During annealing, the surface atoms of the iron nanoparticles are oxidized, forming a layer of iron oxide (Fe3O4) or iron carbide (Fe3C) on the surface of the iron core. As a result, (Fe–Fe3O4) nanoparticles featuring a core-shell architecture, with an Fe core and either an Fe3O4 or Fe3C shell, are formed. By adjusting the oxygen pressure between 10−3 and 1 bar, it is possible to vary the Fe composition in Fex(Fe3O4)1–x nanoparticles, where 0 ≤ x ≤ 1. The morphology and structure of the powdered samples have been investigated utilizing a SEM (Thermo Scientific Axia ChemiSEM), a HR-STEM and STEM (FEI Talos F200X at the accelerating voltage of 200 kV), and an XRD. The synthesized samples’ magnetic characteristics were measured in a physical property measurement system (PPMS, Quantum Design) with a VSM attachment in magnetic fields up to 20 kOe at a constant temperature of 10 K.

The samples derived from FePr and FePc precursors are air-stable powders featuring a carbon matrix with embedded Fe–Fe3O4 nanoparticles. Conversely, Fe-free Pr and Pc precursor samples yield air-stable powders composed of a carbon matrix containing carbon-based microspheres.

Figures 1(a) and 1(c) present SEM images capturing the pyrolyzed Fe-free Pr and Pc products, respectively, at Tp = 900°C and tp = 180 min. The SEM images reveal that both samples feature similar carbon microspheres with diameters ranging between 1 to 3 μm embedded in a carbon matrix containing amorphous and crystalline carbon. Additionally, these microspheres tend to cluster into microparticle aggregates or string together in chains connected by carbon fibers. These structures are likely the result of non-ideal, incomplete spheroid formation. The nature of the bonding between these spheres is presumed to resemble that observed in micro graphite particles. The published review paper15 has explored various growth mechanisms leading to different structures of micro- and nanospheres. Figures 1(b) and 1(d) display SEM images highlighting the pyrolyzed results of FePr and FePc under Tp = 900°C and tp = 180 min. Although the SEM resolution is not sufficiently high to definitively confirm the synthesis of iron nanoparticles, it does allow for the characterization of larger structures and provides information on their elemental composition. The subsequent tables, Tables I and II, present the elemental analysis results in micrometer scale for all samples obtained using the energy dispersive X-ray spectroscopy (EDS) feature of the SEM (TrueSight X EDS detector. Solid angle 13 mSr, resolution 129 eV, area 25 mm2). These measurements help to understand the surfaces and textures of different porphyrin and phthalocyanine matrices and their composition with and without core-shell nanoparticles.

FIG. 1.

SEM imaging of pyrolyzed products of (a) non-annealed iron-free porphyrin, (b) annealed iron-porphyrin, (c) non-annealed iron-free phthalocyanine, and (d) iron-phthalocyanine all pyrolyzed at 900°C for 180 minutes.

FIG. 1.

SEM imaging of pyrolyzed products of (a) non-annealed iron-free porphyrin, (b) annealed iron-porphyrin, (c) non-annealed iron-free phthalocyanine, and (d) iron-phthalocyanine all pyrolyzed at 900°C for 180 minutes.

Close modal
TABLE I.

Elemental analysis of FePr pyrolyzed at Tp = 900°C and tp = 180 minutes and annealed at 150°C in a pure-oxygen environment. Post-pyrolysis, FePr exhibits minimal to zero nitrogen content. Silicon was identified and can be attributed to the fracture of the fused quartz vacuum ampoule, leading to sample contamination.

ElementAtomic %Weight %
Carbon 83.2 65.4 
Iron 5.9 21.7 
Oxygen 8.8 9.2 
Silicon 2.0 3.7 
ElementAtomic %Weight %
Carbon 83.2 65.4 
Iron 5.9 21.7 
Oxygen 8.8 9.2 
Silicon 2.0 3.7 
TABLE II.

Elemental analysis of FePc pyrolyzed at Tp = 900°C and tp = 180 minutes and annealed at 150°C in a pure-oxygen environment. The minor traces of nitrogen observed are likely residual unreacted nitrogen, and the presence of oxygen in the sample is most likely attributable to the formation of iron oxide post-pyrolysis.

ElementAtomic %Weight %
Carbon 76.9 55.6 
Iron 9.5 31.9 
Nitrogen 5.4 4.5 
Oxygen 8.3 8.0 
ElementAtomic %Weight %
Carbon 76.9 55.6 
Iron 9.5 31.9 
Nitrogen 5.4 4.5 
Oxygen 8.3 8.0 

To investigate the structure of the synthesized nanoparticles, we captured HAADF-STEM (High-angle annular dark-field) images, complemented by EDS elemental mapping. These images display the Fe–Fe3O4 core-shell architecture of the nanoparticles post-oxygen annealing for FePr. These nanoparticles exhibit near-spherical shapes, ranging in size from 30 nm to 60 nm, and encased in a graphite-like carbon shell. Moreover, they are uniformly dispersed throughout the carbon matrix. Figure 2(a) is a HAADF-STEM wide-field image of the nanocomposites embedded within a carbon matrix. Figure 2(b) features a HAADF-STEM image of a selected individual nanoparticle. The core zone of the nanoparticle exhibits high-contrast brightness, corresponding to the heavier element, iron (Fe). Conversely, the shell zone shows low-contrast brightness, indicating the lighter element, oxygen (O). Figures 2(c)2(f) present the chemical mappings of individual Fe, O, and C elements in the Fe–Fe3O4 nanoparticle within the carbon matrix.

FIG. 2.

HR-STEM image of the nanocomposites Fe-Pr-O2 synthesized with FePr and annealed at 150°C for 180 min. (a) HAADF-STEM image of nanoparticles embedded in a carbon matrix, and (d) is the same image with HR STEM elemental mapping. (b) Is a HAADF-STEM image of a single Fe–Fe3O4 nanoparticle and images (c), (e), and (f) are the same image with HR STEM elemental mapping.

FIG. 2.

HR-STEM image of the nanocomposites Fe-Pr-O2 synthesized with FePr and annealed at 150°C for 180 min. (a) HAADF-STEM image of nanoparticles embedded in a carbon matrix, and (d) is the same image with HR STEM elemental mapping. (b) Is a HAADF-STEM image of a single Fe–Fe3O4 nanoparticle and images (c), (e), and (f) are the same image with HR STEM elemental mapping.

Close modal

Specifically, Fig. 2(c) reveals that iron is localized to the core zone, while Fig. 2(e) shows oxygen surrounding the iron core, forming a shell. Figure 2(f) merges the HR-STEM elemental mapping images to vividly illustrate the Fe–Fe3O4 core-shell structure with a 3–5 nm shell thickness. Finally, Fig. 2(d) reveals the elemental mapping of the core-shell architectures embedded in a biocompatible carbon matrix.

Similarly, Fig. 3 reports HR-STEM images for the FePc samples that mimic the characteristics observed in the FePr samples. Figure 3(a) is a HAADF-STEM wide-field image of the nanocomposites embedded within a carbon matrix. Figure 3(b) features a HAADF-STEM focused on two nanoparticles. In analogy, Fig. 2, the core zones of the nanoparticles exhibit high-contrast brightness, corresponding to the heavier element, Fe. Figures 2(c)2(f) present the chemical mappings of individual Fe, O, and C elements in the Fe–Fe3O4 nanoparticle within the carbon matrix. Figure 3(c) displays the iron centers of the nanoparticles, while Fig. 2(e) shows the surrounding oxygen layer, forming a shell. Figure 2(f) merges the HR-STEM elemental mapping images where we observe the Fe–Fe3O4 core-shell structure. Figure 2(d) displays the elemental mapping of the core-shell nanoparticles embedded in a biocompatible carbon matrix. The nanoparticles are nearly spherical, ranging in size from 30 nm to 60 nm, and are dispersed within a carbon matrix. The shell thickness parallels that of FePr samples at 3-5 nm. These observations further solidify the reproducibility and robustness of our synthesis method, as manifested by the identical morphological and structural characteristics across different precursor materials.

FIG. 3.

HR-STEM image of the nanocomposites Fe-Pc-O2 synthesized with FePc and annealed at 150°C for 180 min. (a) HAADF-STEM image of nanoparticles embedded in a carbon matrix, and (d) is the same image with HR STEM elemental mapping. (b) Is a HAADF-STEM image of two Fe–Fe3O4 nanoparticles and images (c), (e), and (f) are the same image with HR STEM elemental mapping.

FIG. 3.

HR-STEM image of the nanocomposites Fe-Pc-O2 synthesized with FePc and annealed at 150°C for 180 min. (a) HAADF-STEM image of nanoparticles embedded in a carbon matrix, and (d) is the same image with HR STEM elemental mapping. (b) Is a HAADF-STEM image of two Fe–Fe3O4 nanoparticles and images (c), (e), and (f) are the same image with HR STEM elemental mapping.

Close modal

For additional insights into the structure of the synthesized FePr and FePc nanoparticles, X-ray diffraction patterns were collected at 45 kV and 40 mA on a 240 mm radius Panalytical Empyrean® theta-theta X-ray diffractometer equipped with a line source [Cu K-α 1-2 (1.540 598/1.544 426Å)] X-ray tube. Data was collected with a step size of 0.0334° from 5-70° 2-theta. Phase ID was carried out using Jade® software (version 8.5) from Materials Data Inc. (MDI) and the International Centre for Diffraction Data (ICDD) PDF4® database.

Figure 4 displays the X-ray diffraction spectra for pyrolyzed FePc that was annealed at 250°C in an oxygen. The pronounced peaks point to the existence of four separate crystalline phases: graphite-2H (C), alpha-iron (α-Fe), cohenite (Fe3C), and iron-oxide magnetite (Fe–Fe3O4). The peak at 2θ ∼ 26° can be attributed to the diffraction from the (002) planes of the graphite lattice embedded in the carbon matrix. Graphitization can be relevant to two main factors: the elevated pyrolysis temperatures and extended durations employed in the synthesis and the catalytic action of iron nanoparticles present in the samples. These conditions collectively enhance the structural ordering of the carbon matrix, driving it closer to a graphite-like arrangement. Semi-quantitative XRD analysis was performed using the whole pattern fitting module in the Jade software and appropriate structure models from the ICDD database.

FIG. 4.

XRD pattern of the FePc annealed at 250°C for 180 Minutes. The sample’s XRD patterns indicate the presence of F3C, α-Fe, Fe3O4, and graphite (2H) phases.

FIG. 4.

XRD pattern of the FePc annealed at 250°C for 180 Minutes. The sample’s XRD patterns indicate the presence of F3C, α-Fe, Fe3O4, and graphite (2H) phases.

Close modal

The XRD analysis presented in Table III reveals the presence of distinct phases, namely carbon, iron, iron carbide, magnetite, and hematite, showcasing the complexity and diversity of the synthesized Fe-Pc-O2 and Fe-Pr-O2 samples.

TABLE III.

Phase identification and weight percent values from whole pattern fitting in Jade software for each crystalline phase for Fe-Pc-O2 and Fe-Pr-O2 samples, detailing the presence of C, Fe, Fe3C, Fe3O4, and Fe2O3 at different annealing temperatures.

SampleCα-FeFe3CFe3O4Fe2O3Total
Fe-Pc-O2 annealed at 150°C for 180 min 92.5% 5.6% 1.9% 0% 0% 100% 
Fe-Pr-O2 annealed at 250°C for 180 min 91.6% 0% 8.4% 0% 0% 100% 
Fe-Pc-O2 annealed at 250°C for 180 min 91.2% 3.7% 2.9% 2.1% 0% 99.9% 
Fe-Pc-O2 annealed at 330°C for 190 min 89.2% 1.2% 0% 4.0% 5.5% 99.9% 
SampleCα-FeFe3CFe3O4Fe2O3Total
Fe-Pc-O2 annealed at 150°C for 180 min 92.5% 5.6% 1.9% 0% 0% 100% 
Fe-Pr-O2 annealed at 250°C for 180 min 91.6% 0% 8.4% 0% 0% 100% 
Fe-Pc-O2 annealed at 250°C for 180 min 91.2% 3.7% 2.9% 2.1% 0% 99.9% 
Fe-Pc-O2 annealed at 330°C for 190 min 89.2% 1.2% 0% 4.0% 5.5% 99.9% 

The magnetic properties of the synthesized FePr and FePc samples were quantitatively assessed using a Physical Property Measurement System (PPMS, Quantum Design) equipped with a VSM attachment. Zero field cooled (ZFC) measurements were conducted in magnetic fields up to 20 kOe at a temperature of 10 K. Figure 5 presents hysteresis loops for both annealed and non-annealed (as prepared) samples across a range of temperatures, providing key parameters such as magnetic saturation (MS), magnetic remanence (MR), and coercivity (HC).

FIG. 5.

ZFC magnetization versus magnetic field hysteresis loops in diapason from −15 kOe to 15 kOe at 10 K for annealed and non-annealed FePr samples.

FIG. 5.

ZFC magnetization versus magnetic field hysteresis loops in diapason from −15 kOe to 15 kOe at 10 K for annealed and non-annealed FePr samples.

Close modal

The coercive field values for the non-annealed FePr sample and the samples annealed at 150 and 250°C are 2500 Oe, 2600 Oe, and 2650 Oe, respectively. Notably, the 150°C annealed FePr sample exhibits the highest MS and MR values of 20.3 emu/g and 10.1 emu/g, respectively. The HC shows a steady increase, rising from 2500 Oe for the non-annealed sample to 2650 Oe for the sample annealed at 250°C, as illustrated in Table IV.

TABLE IV.

(TOP) ZFC-FC properties of FePr at T = 10 K. The HC increases steadily with annealing temperatures while MR/MS ratios remain constant. (BOTTOM) ZFC-FC properties of FePc at T = 10 K. Annealing temperatures influence the HC.

FePrMS (emu/g)MR (emu/g)HC (Oe)MR/MS
Non-annealed 17.4 8.6 2500 0.49 
Annealed at 150°C 20.3 10.1 2600 0.49 
Annealed at 250°C 15.4 7.6 2650 0.49 
FePrMS (emu/g)MR (emu/g)HC (Oe)MR/MS
Non-annealed 17.4 8.6 2500 0.49 
Annealed at 150°C 20.3 10.1 2600 0.49 
Annealed at 250°C 15.4 7.6 2650 0.49 
FePcMS (emu/g)MR (emu/g)HC (Oe)MR/MS
Non-annealed 17.9 4.9 950 0.27 
Annealed at 150°C 22.1 6.3 920 0.28 
Annealed at 250°C 19.8 6.6 1250 0.33 
Annealed at 350°C 8.9 2.7 1273 0.30 
Annealed at 450°C 1.3 0.4 1000 0.30 
Annealed at 550°C 2.4 0.6 500 0.25 
FePcMS (emu/g)MR (emu/g)HC (Oe)MR/MS
Non-annealed 17.9 4.9 950 0.27 
Annealed at 150°C 22.1 6.3 920 0.28 
Annealed at 250°C 19.8 6.6 1250 0.33 
Annealed at 350°C 8.9 2.7 1273 0.30 
Annealed at 450°C 1.3 0.4 1000 0.30 
Annealed at 550°C 2.4 0.6 500 0.25 

Interestingly, the larger MR/MS ratio in FePr remains constant, irrespective of solid-phase pyrolysis and annealing temperature variations. It is worth noting that the behavior of FePr and FePc samples diverge significantly: variations in annealing temperature solely affect the MR/MS hysteresis loop ratios for FePc samples, as shown by Fig. 6. We observe that annealing FePr at 150°C results in the highest MS value of 20.3 emu/g, MR value of 10.1 emu/g, and a HC value of 2600 Oe.

FIG. 6.

ZFC Magnetization versus magnetic field hysteresis loops in diapason from −15 kOe to 15 kOe at 10 K for annealed and non-annealed FePc samples.

FIG. 6.

ZFC Magnetization versus magnetic field hysteresis loops in diapason from −15 kOe to 15 kOe at 10 K for annealed and non-annealed FePc samples.

Close modal

In this study, we synthesized various nanocomposite samples starting from metal-organic compounds FePr and FePc by employing a thermal decomposition method (solid-phase pyrolysis), followed by thermal treatment (annealing). Through an assortment of analytical techniques ranging from SEM, HR-STEM with elemental mapping to XRD, we successfully characterized the synthesized samples’ intricate microstructure and elemental composition. The hysteresis characteristics of Fe–Fe3O4 and Fe–Fe3C@C nanoparticles such as coercive field and magnetization are also correlated with the size of nanoparticles, domain structures, and interaction parameters.16–18 Our analyses confirm the presence of carbon coated core-shell architectures in the resulting nanoparticles, specifically Fe–Fe3O4@C and Fe–Fe3C@C. For the non-annealed samples in Table IV, we observe only a slight increase of magnetic saturation in the FePr sample while the coercive force and magnetic remanence show significant enhancement compared to the FePc samples due to the larger concentration of carbon. The coercive force, HC, in FePr, which is more than double compared to that in FePc, can be attributed to the increased Fe concentration in FePr. In these studies, we also investigated the changes in the magnetic properties of the pyrolyzed samples under annealing conditions. As shown in Table IV, the higher carbon concentration in FePr reflects a substantial enhancement in the magnetic performance post-annealing by oxygen, relative to FePc. An optimized annealing temperature of 150°C by oxygen yields the highest magnetic saturation and remanent magnetization for both FePr and FePc nanoparticles.

The nanoparticles demonstrate size distributions ranging from 30 nm to 60 nm and are embedded uniformly within a carbon matrix. Magnetic properties were explored using a vibrating sample magnetometer (VSM), revealing essential aspects of the nanocomposites’ magnetic characteristics. Notably, the hysteresis loop area for FePr samples is approximately double that of FePc samples, suggesting a higher effective power release in applied alternating magnetic field with frequency ν. In conjunction with a stable MR/MS ratio, the increased area A of the hysteresis loop highlights the potential for more effective regulation of the specific absorption rate (SAR) in magnetic fluid hyperthermia applications. To attain a high SAR value, the formula SAR=()/c is used, where c represents the concentration of Fe by weight in a specific volume of the dispersing medium.

This work is supported by grants from the American National Science Foundation, grants No. HRD-1547723 and No. HRD-2112554, as well as the National Institute of Health, grant No. T34-GM08228.

The authors have no conflicts to disclose.

Franco Iglesias: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Project administration (equal); Supervision (equal). Cristian Reynaga Gonzalez: Writing – original draft (equal); Writing – review & editing (equal). Jonah Baughman: Data curation (equal); Investigation (equal). Nichole Wonderling: Data curation (equal); Formal analysis (equal); Investigation (equal); jeff shallenberger: Formal analysis (equal); Project administration (equal); Resources (equal); Software (equal). Armond Khodagulyan: Conceptualization (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Project administration (equal); Resources (equal); Supervision (equal). Oscar O. Bernal: Conceptualization (equal); Project administration (equal); Supervision (equal). Armen N. Kocharian: Conceptualization (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Project administration (equal); Supervision (equal); Writing – original draft (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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