Morphological Evolution in Air-Stable Metallic Iron Nanostructures and

Dec 10, 2014 - Such a morphological evolution was realized by tuning the molar ratio of ligand to metal. Surface energy minimization in confluence wit...
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Morphological Evolution in Air-Stable Metallic Iron Nanostructures and Their Magnetic Study Neha Arora,† Subbiah Amsarajan,† and Balaji R. Jagirdar* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India S Supporting Information *

ABSTRACT: Iron nanostructures with morphology ranging from discrete nanoparticles to nearly monodisperse hierarchical nanostructures have been successfully synthesized using solvated metal atom dispersion (SMAD) method. Such a morphological evolution was realized by tuning the molar ratio of ligand to metal. Surface energy minimization in confluence with strong magnetic interactions and ligand-based stabilization results in the formation of nanospheres of iron. The as-prepared amorphous iron nanostructures exhibit remarkably high coercivity in comparison to the discrete nanoparticles and bulk counterpart. Annealing the as-prepared amorphous Fe nanostructures under anaerobic conditions affords air-stable carbon-encapsulated Fe(0) and Fe3C nanostructures with retention of the morphology. The resulting nanostructures were thoroughly analyzed by powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), and Raman spectroscopy. TGA brought out that Fe3C nanostructures are more robust toward oxidation than those of α-Fe. Finally, detailed magnetic studies were carried out by superconducting quantum interference device (SQUID) magnetometer and it was found that the magnetic properties remain conserved even upon exposure of the annealed samples to ambient conditions for months.

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INTRODUCTION Research on magnetic nanostructures has been intensively pursued over the past decade owing to their potential applications in various fields, viz., data storage,1 magnetic sensors,2 catalysis,3 biomedicine,4 and environmental remediation.5 Magnetic nanostructures exhibit enticing properties that differ markedly from their bulk counterparts due to finite size effects such as superparamagnetism,6 enhanced magnetic moment,7 and high coercivity.8 In general, the physical properties of magnetic metal nanostructures are highly dependent on the size of the nanoparticles and the interactions that exist between the nanoparticle−nanoparticle, nanoparticle−ligand, or nanoparticle−support. Among the ferromagnetic transition metals iron has the highest magnetic moment.9 In addition to its rich magnetic properties, iron nanomaterials have received tremendous attention, because of its low cost, high natural abundance, and high reactivity in reducing atmospheres.10 Iron nanostructures have been synthesized by different methods such as thermal decomposition,11 sonochemical,12 and vapor-phase processes.13 However, fabricating iron nanostructures with desired magnetic properties and chemical stability is quite challenging as the resulting nanostructures are highly prone to oxidation under ambient conditions which eventually results in the formation of antiferromagnetic oxides. High oxophilicity of iron nanoparticles poses challenges in investigating their properties and applications. Therefore, to mitigate this extreme reactivity, encapsulation of Fe nanoparticles with an oxygen-impermeable sheath becomes a © 2014 American Chemical Society

prerequisite. In this direction, various protection strategies have been employed, e.g., coating with silica,14 polymers,15 noble metals,16 and carbon.17 In comparison to other coatings, carbon displays a leverage of higher chemical and thermal stability, while being biocompatible.18 Since the first report by Ruoff et al.19 in 1993 describing the synthesis of carbonencapsulated LaC2 microcrystals, research on carbon-protected metal nanoparticles has been thriving.20,21 Carbon not only renders magnetic nanostructures resistant toward oxidation, but it also imparts magnetic stability to the nanostructures. Additionally, carbon−metal system offers the possibility of obtaining metal carbide nanoparticles encased in crystalline or amorphous carbon.22 Carbon-protected nanomaterials have been generally prepared by arc-discharge process23 and chemical vapor deposition.24 However, the synthetic processes involve intricate conditions and the nanoparticles obtained exhibit broad size distribution. In this work, we describe the synthesis of amorphous iron nanostructures using scalable, reproducible, and reducing agent free solvated metal atom dispersion (SMAD) method. It is worth emphasizing that the morphology of iron nanostructures can be tuned from small nanoparticles to hierarchical nanospheres through this process by optimizing the metal-toligand ratio. We further took advantage of the catalytic property of metallic iron in graphitizing ligands25 to synthesize air-stable Received: August 28, 2014 Revised: November 11, 2014 Published: December 10, 2014 665

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and other volatile contaminants. Dried and degassed toluene solvent in a Schlenk tube was connected to a shower head placed inside the SMAD reactor through a bridge head. The SMAD reactor chamber was evacuated to a (1−2) × 10−3 mbar pressure. Once this pressure was attained, 20 mL of solvent was introduced on the walls of the reactor which was immersed in a liquid N2 Dewar. This was followed by heating of the crucible resistively until metal started to vaporize, which was evident by the appearance of yellow color on a white solvent matrix. Metal atoms and solvent vapor were co-condensed continuously for duration of 2 h maintaining a pressure of (1−2) × 10−3 mbar. The color of the matrix deepened to bright yellow with time. An additional 20 mL of solvent was condensed, and the reaction mixture was allowed to warm up to room temperature slowly under a blanket of argon. Upon thawing, the colloid was stirred in the reactor under argon for 30 min and finally siphoned into a Schlenk tube. The colloid obtained was black in color and was stored under inert atmosphere. Powders were isolated by applying dynamic vacuum and were transferred to a nitrogen-filled glovebox. Synthesis of Fe−HDA−Toluene Nanoparticles. Hexadecylamine (molar ratio of Fe/HDA = 1:1/2/4/10) was placed at the bottom of the SMAD reactor. The rest of the experiment was conducted similar to that of Fe−toluene colloid. Finally, dark brown colored colloids were obtained and were stored under argon. The colloids were very stable toward precipitation. Ethanol was added as a flocculent followed by separation of the precipitate using a strong magnetic field. The precipitate thus obtained was washed thrice with ethanol to remove excess capping agent, followed by drying under high vacuum. HDA capped iron nanopowders were transferred and stored in a nitrogen-filled glovebox. Annealing of Fe−Toulene/Fe−HDA−Toluene Nanoparticles. The resulting Fe nanopowders were loaded in sealed glass ampules inside the glovebox and annealed at two different temperatures of 300 and 500 °C under anaerobic conditions (N2 atmosphere) for 12 h.

iron nanostructures embedded in carbon matrix. The resulting amorphous iron nanomaterials can be transformed directly into crystalline Fe and Fe3C nanostructures embedded in carbon matrix by carrying out controlled annealing at modest temperature under optimized conditions. Remarkably, the morphology of Fe nanostructures remained predominantly conserved during structural transformations. The resulting nanostructures were thoroughly characterized by powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, and by superconducting quantum interference device (SQUID) magnetic measurements. The successful protection of carbon-encapsulated iron nanostructure was further confirmed by the preservation of magnetic properties upon exposure and storage of the samples under air for 3 months. Furthermore, magnetic investigations revealed rich coercivity for all the iron nanostructures.

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EXPERIMENTAL SECTION Materials. Iron foil, 0.25 mm thick, (99.99% metal basis) was purchased from Johnson Matthey Chemicals India Pvt. Ltd. Hexadecylamine (Sigma-Aldrich, 90% technical grade) was dried and degassed for 12 h at 100 °C. Tungsten crucibles were obtained from R. D. Mathis Company, California. HPLC and spectroscopy grade toluene (S. D. Fine Chemicals Limited India) and absolute ethanol (HPLC grade, 99.9%) were dried and distilled over sodium benzophenone and magnesium ethoxide, respectively, and degassed by several freeze−pump− thaw cycles. All glassware was thoroughly dried in a hot air oven. Measurements. The bright-field transmission electron microscopy (BFTEM) images, HRTEM images, and selected area electron diffraction (SAED) patterns were collected using a JEOL JEM-2100F microscope operating at an accelerating voltage of 200 kV. TEM specimens were prepared by slow evaporation of solvent from colloids, obtained by dispersion of powders in toluene, deposited on Formvar-coated copper grid. Ultrahigh-resolution scanning electron microscopy imaging (ULTRA 55) was employed to analyze the gross morphology of samples. Scanning electron microscopy (SEM) samples were prepared by dispersing powder sample onto a carbon tape and mounting it on an aluminum specimen holder. Powder X-ray diffraction data were collected on a Bruker D8 Advance X-ray diffractometer equipped with a graphite monochromator using Cu Kα (0.154 nm) radiation, at 40 kV and 40 mA. Air-sensitive samples were loaded in 0.7 mm quartz capillaries inside the glovebox and flame-sealed under N2 atmosphere. Raman spectral measurements were performed on a LabRAM HR (UV) instrument at room temperature using an argon-ion laser with an excitation wavelength of 532 nm. The magnetic measurements were carried out on powder samples using a SQUID Quantum Design MPMS XL-7 magnetometer. Airsensitive magnetic samples were loaded in an airtight gelatin capsule inside the glovebox. Thermal measurements were performed on a TG 209 F1 thermogravimetric analyzer (TGA) (Netzsch instruments) under a flow of air up to a temperature of 850 °C at a heating rate of 10 °C min−1. Synthesis of Fe−Toluene Nanoparticles. Colloids of Fe nanoparticles were synthesized using the SMAD method which our group has successfully employed for the synthesis of various nanomaterials.26−32 In a typical experiment, 200 mg of Fe metal was placed in an alumina-coated tungsten crucible which was cured prior to the experiment to ensure removal of moisture

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RESULTS AND DISCUSSION Various morphologies of iron nanostructures were obtained using the SMAD method. The conditions optimized for the synthesis of different iron samples are summarized in Table 1. Structural and Morphological Characterization of AsPrepared Fe−Toluene Nanoparticles. Fe−toluene colloids were unstable, and precipitation of nanoparticles took place within 24 h. The isolated Fe−toluene nanopowder was oxidatively unstable in ambient conditions and, therefore, was always stored under inert atmosphere. Structural characterization was performed using PXRD on a sample which was loaded in a capillary tube under inert conditions. PXRD pattern of as-prepared Fe−toluene nanoparticles (Fe-1) was devoid of any peak, thus revealing its amorphous nature. To examine the morphology, the sample was further examined by TEM. The BFTEM image (Figure 1a) shows the presence of Fe nanoparticles embedded in an organic matrix. The HRTEM image brings out the amorphous nature of Fe nanoparticles encased in an amorphous matrix (Figure 1b). The SAED (Figure 1c) confirms the amorphous nature of the nanoparticles as the pattern was devoid of any rings, which is in accord with the PXRD result. In order to achieve crystallinity, the as-prepared sample (Fe1) was annealed at 300 °C in N2 atmosphere. PXRD pattern of the resulting sample, i.e., Fe-1(300), displayed peaks 666

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thus correlating with the PXRD results. As the molar ratio of metal/HDA was increased to 1:4, nanospheres composed of densely packed nanoparticles exhibiting randomly oriented aggregation were formed (Figure 3, parts d and e). At a higher molar ratio of metal/HDA of 1:10 the evolution of nonuniform Fe nanospheres takes place (see Supporting Information). The SAED pattern (Figure 3f) of Fe-3 was devoid of any ring pattern indicating the amorphous nature of the sample. All the as-prepared samples were examined for their gross morphology by field emission scanning electron microscopy (FESEM) (see Supporting Information). The micrographs reveal that the morphology varies from irregularly shaped nanoparticles to nearly monodisperse hierarchical nanospheres consisting of nanoflakes to nanospheres made up of fine nanoparticles. Therefore, we conclude that the concentration of hexadecylamine considerably determines the building block of Fe nanostructures. Insight into the Formation of Hierarchical Nanospheres. In general, the formation of magnetic hierarchical architectures involves van der Waals and magnetic interactions, which facilitate the minimization of overall energy by selforganization of building blocks.35,36 There have been reports in literature demonstrating spontaneous aggregation of nanoparticles into larger hierarchical architectures.37,38 For example, Dar et al. described the randomly oriented aggregation which leads to the formation of densely packed spherical aggregates of air-stable copper nanostructures.39 Ewers et al. demonstrated the spontaneous formation of spherical assemblies of PVPstabilized Rh nanoparticles aggregates.40 Our observation illustrates that the concentration of hexadecylamine has influence on the building block of Fe nanostructures. Earlier, hexadecylamine has been used as a growth-directing agent for the formation of hierarchical metal nanostructures. Liu et al.41 described the synthesis of Co nanoflowers consisting of nanorods by a solvothermal route using hexadecylamine as the structure directing agent. The significant outcome of our work is the demonstration of being able to tune the morphology of as-prepared Fe nanostructures by optimizing the reaction conditions, particularly the molar ratio of ligand/ Fe. The impetus for the formation of hierarchical nanospheres could be ascribed to randomly oriented aggregation coupled with magnetic interactions which leads to the curling of the flakes and aggregation of nanoparticles into hierarchical architectures.42 Synthesis of Air-Stable Iron Nanostructures. Despite exhibiting enticing magnetic and catalytic properties, the drawback of oxidative instability has precluded the realization of practical applications of iron nanomaterials. We recently developed a synthetic route to produce carbon-encapsulated cobalt nanomaterials that are air stable, and they exhibit enhanced magnetization.33 Likewise, we annealed the asprepared Fe-2 sample at a modest temperature of 300 °C in

Table 1. Summary of the Experimental Conditions Used to Synthesize Different Iron Nanostructures (Series 1, Series 2, and Series 3) sample code Fe-1 Fe-1(300) Fe-1(500) Fe-2 Fe-2(300) Fe-2(500) Fe-3 Fe-3(300) Fe-3(500)

description Fe−toluene as-prepared sample prepared by SMAD method obtained after annealing Fe-1 at 300 °C for 12 h under N2 atmosphere in an ampule obtained after annealing Fe-1 at 500 °C for 12 h under N2 atmosphere in an ampule Fe−HDA−toluene as-prepared sample prepared by SMAD method with Fe/HDA molar ratio as 1:2 obtained after annealing Fe-2 at 300 °C for 12 h under N2 atmosphere in an ampule obtained after annealing Fe-2 at 500 °C for 12 h under N2 atmosphere in an ampule Fe−HDA−toluene as-prepared sample prepared by SMAD method with Fe/HDA molar ratio as 1:4 obtained after annealing Fe-3 at 300 °C for 12 h under N2 atmosphere in an ampule obtained after annealing Fe-3 at 500 °C for 12 h under N2 atmosphere in an ampule

corresponding to body-centered-cubic (bcc) structure of α-Fe (JCPDS 06-0696) (Figure 2a). To within the detection limit of XRD, no other peaks corresponding to iron oxides were present. Furthermore, the sample was probed using TEM. The BFTEM image revealed that the iron nanoparticles are embedded in dense carbon matrix (Figure 2b). HRTEM shows lattice fringes with d-spacing of 2.0 Å, corresponding to the (110) plane of α-Fe corroborating the fast Fourier transform (FFT) pattern shown in the inset of Figure 2c. The SAED (Figure 2d) shows ring pattern, confirming the polycrystalline nature of the sample, and the pattern could be indexed to α-Fe, thus correlating with the XRD data. Effect of HDA Concentration on the Morphology of Iron Nanostructures. Toluene as a coordinating solvent produced colloids33 which were unstable toward precipitation; therefore, we employed hexadecylamine (HDA) as an additional stabilizing agent to produce fine dispersions of Fe nanoparticles. Interestingly, TEM studies reveal the formation of densely packed spherical aggregates instead of distinct nanoparticles.33,34 Hence, we systematically investigated the influence of hexadecylamine concentration on the evolution of morphology of Fe nanostructures. TEM was employed to probe the variation in the morphology with a change in molar ratio of Fe/HDA as illustrated in Figure 3. Discrete nanoparticles are formed in the absence of HDA as evident from Figure 1a. With a molar ratio of 1:2 (metal/HDA) nearly monodisperse Fe nanospheres of 100−300 nm in diameter were obtained (Figure 3a). High-magnification TEM revealed that the spherical hierarchical nanostructures are composed of crispate flake-like structures (Figure 3b). The SAED pattern (Figure 3c) of the Fe-2 sample confirms its amorphous nature,

Figure 1. Fe−toluene (Fe-1) nanoparticles: (a) BFTEM image; (b) HRTEM image; (c) SAED pattern. 667

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Figure 2. Fe-1(300) nanoparticles: (a) powder X-ray diffraction pattern; (b) BFTEM image; (c) HRTEM image of a representative Fe nanoparticle (inset, FFT pattern); (d) SAED pattern corresponding to α-Fe.

Figure 3. Fe−HDA−toluene nanostructures: (a and d) BFTEM images; (b and e) high-magnification BFTEM images; (c and f) SAED patterns of Fe-2 and Fe-3, respectively.

2.0 Å which is consistent with the (110) plane of bcc phase of iron (Figure 4c). SAED (Figure 4d) shows a ring pattern with bright spots confirming the crystalline nature of the sample, and the pattern could be indexed to α-Fe, thus corroborating XRD data. Influence of Annealing Temperature on the Composition of Iron Nanostructures. Annealing of as-prepared HDA capped Fe nanostructures at 500 °C in N2 atmosphere led to the formation of cementite Fe3C phase (Figure 5a) exhibiting orthorhombic crystalline structure (JCPDS no. 350772). Fe3C (cementite) is of high technological relevance due to its enhanced mechanical properties43 and applications in ferrous metallurgy.44 However, it is difficult to obtain as a pure

N2 atmosphere to carbonize the ligand. PXRD pattern of the Fe-2(300) sample (Figure 4a) recorded in ambient conditions shows peaks that can be attributed to bcc iron, i.e., α-Fe phase (JCPDS no. 06-0696). To within the detection limit of XRD, no other peaks corresponding to iron oxides were present. Furthermore, the size, shape, and dispersion of the annealed sample were studied using TEM. The BFTEM micrograph shows that the hierarchical morphology of Fe nanospheres predominantly remains conserved (Figure 4b). High-magnification TEM image of the Fe-2(300) sample reveals that these Fe nanospheres are composed of about 5 nm sized nanoparticles which are embedded in a dense matrix (Figure 4b, inset). HRTEM image features lattice fringes with d-spacing of 668

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Figure 4. Fe-2(300) nanostructures: (a) powder X-ray diffraction pattern; (b) BFTEM image (inset, high-magnification BFTEM image); (c) HRTEM image of a representative Fe nanoparticle (inset, FFT pattern); (d) SAED pattern corresponding to α-Fe.

Figure 5. Fe-2(500) nanostructures (a) powder X-ray diffraction pattern; (b) BFTEM image (inset, high-magnification BFTEM image); (c) HRTEM image of a representative Fe nanoparticle; (d) SAED pattern corresponding to Fe3C.

phase and has been mainly observed as a side product in the synthesis of carbon structures employing metallic iron as a catalyst45 or it requires high temperature and energy processes to synthesize phase-pure Fe3C.46 The resulting Fe3C nanomaterial was further probed for its morphology using electron microscopy. TEM micrograph corresponding to the Fe-2(500) sample displayed the formation of a core−shell structure (Figure 5b). The presence of two phases, i.e., core and shell, is in apparent discord with the powder XRD as the XRD pattern obtained could be indexed to only single phase which is Fe3C. Therefore, to

unravel the details, we examined the HRTEM image (Figure 5c), which not only confirmed the presence of carbon layers surrounding Fe3C nanostructures but also brought out the amorphous nature of carbonaceous shell. Furthermore, the SAED pattern (Figure 5d) obtained could be indexed to orthorhombic phase of Fe3C, thus correlating with PXRD results. In summary, TEM results bring out that morphology of hierarchical iron nanospheres remains primarily conserved during structural transformation even after applying harsh heat 669

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The Journal of Physical Chemistry C treatment although some growth occurred within the nanosphere of some quasi-spherical agglomerates. Fe3C Nanostructure Formation. The formation of carbides depends on the metal/carbon phase diagram.47 As reported in the literature, the metal carbides formed for Fe, Co, and Ni exhibit cementite phase and are thermodynamically metastable.48,49 Considering the formation enthalpy of carbides which is in the order of Ni−C > Co−C > Fe−C, iron has a greater tendency to form carbide in comparison to Co and Ni.33,50 We surmise this to be a possible reason for the formation of Fe3C in the present study. The formation of core−shell structure of Fe3C phase in Fe-2(500) was observed after annealing the Fe-2 sample at 500 °C under N 2 atmosphere for 12 h. In this case, hexadecylamine (HDA) acts as a carbon source. The diffusion of carbon atoms from the carbon source, i.e., HDA, was realized only at an elevated temperature of 500 °C as the samples annealed at 300 °C, i.e., Fe-2(300), lead to the formation of α-Fe phase but not Fe3C (Figure 4a). We presume that, at low annealing temperatures, i.e., 300 °C, the surface diffusion of carbon into iron nanoparticles does not occur. To further investigate the role of hexadecylamine in carbonization process, we annealed the as-prepared Fe−toluene sample (Fe-1) at 500 °C. PXRD pattern of the resulting sample [Fe-1(500)] suggested that no crystalline carbon product was formed. Indeed it evidences the formation of α-phase of metallic iron (see Supporting Information). Therefore, we infer that in addition to employing hexadecylamine as a carbon source high temperature is essential for the formation of iron carbide. Thermal Analysis. To understand the oxidative stability of carbon-encased α-Fe and Fe3C nanostructures, we carried out thermal studies under a flow of air up to a temperature of 850 °C. It is quite evident from thermogravimetric analysis (TGA) (see Supporting Information) that the carbon-encased Fe3C nanostructures have good stability toward oxidation up to a temperature of 250 °C, while in the case of annealed Fe/C nanostructures the onset of oxidation is around 180 °C. The slight weight loss between 30 and 180 °C was possibly due to the removal of surface adsorbed species. In both the cases, TGA revealed a weight gain of ∼4% up to 500 °C, above which there was a continuous weight loss which could be attributed to the burning of carbon content present in the samples. It is to be noted that the quantification of carbon content became ambiguous due to the concomitant oxidation and burning of metallic Fe and carbonaceous material, respectively.51 Raman Spectral Characterization of Annealed Iron Nanostructures. Figure 6 shows Raman spectra recorded for annealed Fe−HDA nanostructures. The spectrum of Fe-2(300) displays two broad peaks centered at 1360 and 1590 cm−1, assignable to D and G bands of carbon phase. The standard G band at 1581 cm−1 corresponds to E2g vibrational mode of graphite originating from in-plane bond-stretching motion of pairs of sp2-bonded carbon atoms.52 The loss of translational symmetry results in disorder or D band at around 1350 cm−1.52 Annealing of Fe−HDA sample at higher temperature (500 °C) increases the intensity of D and G peaks in comparison to those of the Fe-2(300) sample. The shift of the G peak position to higher relative wave numbers is consistent with the increasing order of carbon phase. Similarly the Raman spectra corresponding to Fe-3(300) and Fe-3(500) nanostructures also showed D and G bands of graphitic carbon at 1350 and 1597 cm−1, respectively (see Supporting Information). Although the characteristic D and G

Figure 6. Raman spectra of Fe-2(300) and Fe-2(500) nanostructures.

bands were also exhibited by annealed Fe−toluene samples, Fe1(300) and Fe-1(500) (see Supporting Information), however, these samples were comparatively less stable toward oxidation under the laser beam, as evident by the presence of a weak band at about 670 cm−1 corresponding to Fe3O4.8 Therefore, we conclude from Raman spectral studies that the thermal treatment leads to the carbonization of ligand in Fe−HDA− toluene samples which eventually renders them air stable. It is to be noted that iron carbides do not exhibit Raman active vibrational modes as reported in the literature.53 Magnetic Properties. Iron nanoparticles exhibit rich magnetism which strongly depends on the size, shape, crystallinity, composition, and synthetic methodology. Magnetic properties of different iron nanostructures were investigated by measuring temperature- and field-dependent magnetization. Temperature-dependent magnetization curves were recorded in the temperature range of 2−300 K, using zero-field cooling (ZFC) and field cooling (FC) procedures. Figure 7a shows the ZFC curve of the Fe-1 sample which displays an increase in magnetization values up to a temperature of 63 K, beyond which magnetization began to decrease. The corresponding ZFC curves for Fe-2 and Fe-3 samples (Figure 7, parts b and c) brought out sharp transitions from ferromagnetic to superparamagnetic state at a temperature of 16 and 17 K, respectively. The temperature above which superparamagnetic behavior exists is known as the blocking temperature (TB), and it decreases with the decrease in particle size. The observation of low blocking temperatures in Fe-2 and Fe-3 samples further evidence the presence of small sized nanoparticles. Park et al. reported a blocking temperature of 12 K for 2 nm spherical iron nanoparticles.54 To gain deeper insight into the magnetic behavior of the samples, field-dependent magnetic measurements were carried out at 2 and 300 K on as-prepared amorphous Fe samples. The field-dependent magnetic measurements recorded at 2 K (Figure 8, red trace) for Fe-1, Fe-2, and Fe-3 shows ferromagnetic behavior, with high coercivity (Hc) values of 983, 3670, and 4385 Oe, respectively. Hc is governed by various factors like magnetocrystalline anisotropy energy, particle shape, and surface effect.55 The origin of such large Hc values could be due to the presence of Fe oxide shell and the Fe core.56 However, the presence of oxide coating from our samples is ruled out as exchange bias measurements performed at 2 K by applying a strong field of 5 T brought out the symmetric nature of the hysteresis curve (Figure 8, black trace) along the field axis. Therefore, the enhancement in coercivity in comparison to bulk (≈0.15 Oe) could be ascribed to the small 670

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Figure 7. Temperature-dependent magnetization curve measured in a 10 Oe magnetic field for (a) Fe-1, (b) Fe-2, and (c) Fe-3 nanostructures.

Figure 8. Field dependence of magnetization measured at 2 K, red trace (zero-field hysteresis curve), black trace (field-cooled at 5 T hysteresis curve): (a) Fe-1; (b) Fe-2; (c) Fe-3. Inset shows the respective zoomed hysteresis curves.

Figure 9. Field dependence of magnetization measured at 300 K: (a) Fe-1; (b) Fe-2; (c) Fe-3.

size effect of magnetic nanoparticles resulting in large surface anisotropy.57,58 Xiao and Chen reported a giant magnetic coercivity value of 2500 Oe for fine Fe nanoparticles in granular Fe−SiO2 solids.59 They explained that metal−insulator interfaces gives rise to surface anisotropy which pronounces the coercivity for small-sized nanoparticles. Hence, we can attribute high coercivity to enhanced surface anisotropy which arises from the strong interaction among the nanocrystallites of these nanospheres. Upon applying a high field of 50 kOe, Fe-1 shows a saturation magnetization of 87 emu/g; however, for Fe-2 and Fe-3 the magnetization does not saturate, and maximum values of 57 and 46 emu/g were attained, respectively. As is known, magnetic properties of iron nanoparticles are strongly influenced by the chemical environment, viz., surfactant and presence of magnetic oxide on the surface.57,60 In the literature, variation of saturation magnetization value of iron from bulk, i.e., 222 emu/g (T → 0 K), with decreasing size of nanoparticles has been ascribed primarily to the following two effects: (1) The pinning effect of surface atom spins resulting from the interaction of surface iron atoms of a nanoparticle with other atoms or molecules of the surfactant. This results in directional bonding which limits the

reorientation of magnetic spins with respect to an applied field. For example, Kataby et al. demonstrated the strong dependency of magnetization on the nature of the functional group bonded to amorphous iron surface.61 (2) The presence of surface oxide on iron nanoparticles brings down the saturation magnetization values due to spin canting of surface ions on the ferromagnetic lattice.62 The field-dependent magnetization recorded at 300 K (Figure 9) exhibits a very weak magnetic hysteresis, measuring low Hc values of 36, 30, and 35 Oe, respectively, for Fe-1, Fe-2, and Fe-3 samples. A strong reduction in coercivity with an increase in temperature is expected, as the thermal fluctuations at elevated temperature tend to destabilize the magnetic moment in a nanoparticle and temperature dependency of coercivity becomes stronger with decrease in nanoparticle size. Nanoparticles having diameter below the single-domain critical size exhibit superparamagnetic behavior which is marked by the absence of hysteresis, i.e., zero coercivity and zero remnant magnetization above the blocking temperature (TB). However, the existence of dipolar interactions among single-domain magnetic nanoparticles significantly affects the superparamagnetic relaxation63 and eventually can give rise to small coercivity values.64 In our case, ZFC brought out sharp reversal of the 671

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sample

2K

300 K

2K

300 K

TB (K)

carbide.66,67 The decrease in magnetization value for nanomaterials, compared to the bulk cementite (140 emu/g),68 can be attributed to the formation of surface layer with spin disorder and the presence of nonmagnetic amorphous phase of carbon.66,69 Kim et al.66 described the synthesis of 20 nm Fe3C nanoparticles encased in graphite by heat treating polyimide/Fe/polyimide thin-film stack. The nanoparticles obtained displayed a saturation magnetization and coercivity of 55.4 emu/g and 360 Oe, respectively, at room temperature. Giordano et al. reported a magnetization value of 46.7 emu/g at room temperature for iron-free Fe3C powder nanoparticles measuring 5−10 nm in diameter.67 Sergiienko et al. showed a room-temperature saturation magnetization value of 51.36 emu/g for carbon-encapsulated iron carbide nanocapsules of dimension of 5−600 nm.70 Ms and Hc values of Fe-1(500), Fe2(500), and Fe-3(500) are summarized in Table 3.

Fe-1 Fe-2 Fe-3

87 57 46

75 24 35

983 3670 4385

36 30 35

63 16 17

Table 3. MS and HC Values of Fe-1(500), Fe-2(500), and Fe3(500) Nanostructures

magnetic moment at the blocking temperature (TB), and evidently, a narrow transition separates the ferromagnetic and superparamagnetic regimes. We believe that dipole−dipole interactions among Fe nanoparticles probably results in the appearance of a mild coercivity at 300 K. The hysteresis loops at 300 K indicate that all three samples, i.e., Fe-1, Fe-2, and Fe3, are “predominantly” superparamagnetic at room temperature. The values of saturation magnetization (MS), coercivity (HC), and blocking temperature (TB) of Fe-1, Fe-2, and Fe-3 nanomaterials are summarized in Table 2. Table 2. MS, HC, and TB Values of Fe-1, Fe-2, and Fe-3 Nanostructures MS (emu/g)

HC (Oe)

MS (emu/g)

Magnetic measurements were also carried out on annealed Fe-1(500) sample. Field-dependent measurements performed at 5 K reveal ferromagnetic behavior (Figure 10a) displaying a coercivity of 665 Oe and a large saturation magnetization of 156 emu/g. Room-temperature (300 K) measurements reveal a decreased coercivity of 270 Oe and almost a constant Ms of 150 emu/g. The magnetic properties of Fe3C nanoparticles obtained after annealing as-prepared Fe-2/Fe-3 sample at 500 °C were studied. Parts b and c of Figure 10 show the magnetic hysteresis loops for carbon-encapsulated Fe3C nanoparticles at 5 and 300 K, respectively. The magnetization curve measured at 5 K demonstrates ferromagnetic behavior reaching a saturation magnetization (Ms) of 70 and 59 emu/g with coercivity of 1295 and 996 Oe, respectively, for Fe-2(500) and Fe-3(500) samples (Figure 10, parts b and c, red trace). Magnetic measurements carried out at room temperature (300 K) shows a finite coercivity value of 170 and 130 Oe with Ms of 54 and 47 emu/g, respectively, for Fe-2(500) and Fe-3(500) samples (Figure 10, parts b and c, green trace). Such Hc values indicate that samples are ferromagnetic at room temperature. The increase in coercive field values with a decrease in temperature, as expected for small-particle systems, can be attributed to the pinning effect which involves the pinning of domain walls in the nanoparticles.65 The saturation magnetization values are comparable to those reported in the literature for carbon-encased nanosized iron

HC (Oe)

sample

5K

300 K

5K

300 K

Fe-1(500) Fe-2(500) Fe-3(500)

156 70 59

150 54 47

665 1295 996

270 170 130

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CONCLUSIONS In summary, we have demonstrated the synthesis of hierarchical amorphous iron nanostructures using the SMAD method with composition ranging from amorphous iron to metallic iron to iron carbide, while the morphology of nanostructures can be systematically varied from discrete nanoparticles to hierarchical architecture to nanoparticles embedded in carbon matrix. By varying the metal to hexadecylamine ratio, nanostructures with different morphologies were obtained. The formation of hierarchical nanostructures could be explained by invoking the mechanism of randomly oriented aggregation in association with magnetic interactions. Surprisingly, the aggregated nanostructures exhibited enhanced magnetic coercivity in comparison to their bulk counterpart as revealed by magnetic studies. The hierarchical nanostructures exhibiting interesting magnetic properties could be transformed to carbon-encased Fe and Fe3C nanostructures with retention in morphology by thermal annealing at optimized temperatures. It is noteworthy that rich coercive behavior remains relatively conserved across various structural transformations of iron nanostructures. The

Figure 10. Field dependence of magnetization measured at 5 and 300 K for (a) Fe-1(500), (b) Fe-2(500), and (c) Fe-3(500). Inset shows the respective zoomed hysteresis curves. 672

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additional feature of present approach is that Fe3C/C nanostructures can be prepared via annealing the as-prepared samples at modest temperatures using an ordinary furnace. The morphologies and magnetic properties are reproducible within the experimental errors. The magnetic properties remained conserved even after exposing them to ambient conditions for months. Hexadecylamine plays a dual role of stabilizing agent as well as carburization agent. Arguably, because of their robust air stability, Fe/C nanocomposites could prove to be promising candidates for various applications where high coercivity is desired. Further investigations regarding understanding the ligand-dependent morphological evolution of Fe nanostructures are in progress in our laboratories.

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ASSOCIATED CONTENT

S Supporting Information *

BFTEM images of Fe−HDA (1:1) and (1:10)−toluene nanostructures, FESEM micrographs of Fe−HDA−toluene nanostructures, characterization data of Fe-1(500) nanoparticles, Raman spectra of Fe-3(300), Fe-3(500), Fe-1(300), and Fe-1(500) nanoparticles, and TGA curve of annealed Fe nanostructures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +91-80-2360 1552. Phone: +91-80-2293 2825. Author Contributions †

N.A. and S.A. contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Professor P. S. Anil Kumar (Department of Physics, I.I.Sc.) for his assistance with the magnetic measurements and useful discussions. We acknowledge the Indian Institute of Science for funding the procurement of a 200 kV FETEM and the Institute XRD facility for the powder X-ray diffraction data. We also thank Mr. S. Ghosh for his help in carrying out TGA measurements. N.A. gratefully acknowledges fellowship from the Indian Institute of Science, Bangalore. S.A. thanks Council of Scientific & Industrial Research, India for a fellowship.

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