Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Nanoscale Metastable ε‑Fe3N Ferromagnetic Materials by SelfSustained Reactions Alexander S. Mukasyan,† Sergey Roslyakov,‡ Joshua M. Pauls,† Leighanne C. Gallington,§ Tatyana Orlova,∥ Xinyu Liu,⊥ Margaret Dobrowolska,⊥ Jacek K. Furdyna,⊥ and Khachatur V. Manukyan*,# †
Department of Chemical and Biomolecular Engineering, ∥Notre Dame Integrated Imaging Facility, ⊥Department of Physics, and Nuclear Science Laboratory, Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556, United States ‡ National University of Science and Technology, “MISIS”, Moscow 119049, Russia § X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439-4858, United States
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ABSTRACT: A single-step method for the preparation of metastable ε-Fe3N nanoparticles by combustion of reactive gels containing iron nitrate (Fe(NO3)3) and hexamethylenetetramine (C6H12N4) in an inert atmosphere is reported. The results of Fourier transform infrared spectroscopy (FTIR) and thermal analysis coupled with dynamic mass spectrometry revealed that the exothermic decomposition of a coordination complex formed between Fe(NO3)3 and HMTA is responsible for the formation of ε-Fe3N nanoscale particles with sizes of 5−15 nm. The magnetic properties between 5 and 350 K are characterized using a superconducting quantum interference device (SQUID) magnetometer, revealing a ferromagnetic behavior with a low-temperature magnetic moment of 1.09 μB/Fe, high room temperature saturation magnetization (∼80 emu/g), and low remanent magnetization (∼15 emu/g). The obtained value for the Curie temperature of ∼522 K is close to that (∼575 K) for bulk ε-Fe3N reported in the literature. in the Fe(CO)5 + NH3 system produces ε-Fe3N particles with sizes of 10−40 nm.30,31 Thermal treatment of ferric nitrate and gelatin mixtures in nitrogen also forms ε-Fe3N.32 In situ synchrotron X-ray diffraction suggests that FeO forms first at ∼500 K, which then converts to ε-Fe3N at ∼800 K, followed by an Fe3N → Fe3C transformation at 850 K.33 A mixture of Fe(CO)5 vapor and Ar/NH3 gas was passed through a synthetic oil containing a surfactant maintained at 450 K to obtain ε-Fe3N magnetic fluids.34,35 An amorphous material obtained by low temperature (195 K) reaction of sodium with iron(II) bromide in liquid ammonia was annealed at 573 K to obtain ε-Fe3N nanoparticles.22 A direct reaction between zerovalent iron and gaseous ammonia followed by calcination at 850 K was also reported to prepare ε-Fe3N.21 These works suggest that existing preparation methods for nanoscale ε-Fe3N are energy- and cost-intensive and involve multiple steps. In this work, we report a single-step method for the preparation of ε-Fe3N nanoparticles by use of an energyefficient self-sustained reaction. The proposed method is based on the concept of solution combustion synthesis (SCS). The SCS approach has attracted substantial attention as a simple
1. INTRODUCTION Iron nitrides are a diverse class of materials and categorized into iron-rich (α″-Fe16N2, γ′-Fe4N, ε-Fe3N) and nitrogen-rich (ζ-Fe2N, γ‴-FeN, FeN2, FeN4) phases.1−6 These compounds exhibit exceptional mechanical and magnetic properties.1,7−12 Thin coatings of some iron nitrides protect metallic materials against corrosion.13−15 They are excellent catalysts16 and electrode materials17,18 and have significant potential for biomedical applications.19,20 Furthermore, these nitrides are of particular interest for the geosciences, as they may be an essential component of the Earth’s core.7,8 Many iron nitrides (such as ε-Fe3N, α″-Fe16N2, and γ′Fe4N) are ferromagnetic with high saturation magnetization and very high magnetic moments.20−26 Nanoscale ε-Fe3N has attracted significant attention due to its structure and unusual magnetic properties.1 The preparation of single-phase nanoscale ε-Fe3N is challenging. Reactive sputtering and chemical vapor deposition are the two primary methods to fabricate εFe3N thin films.27−29 Phase purity of the films, however, is a concern, as other iron nitride phases readily form during deposition. Chemical vapor condensation, sol−gel, liquid ammonia reduction, or electrospraying techniques are the most frequently used methods for preparation of ε-Fe3N nanoparticles.30−36 For example, chemical vapor condensation © XXXX American Chemical Society
Received: December 20, 2018
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DOI: 10.1021/acs.inorgchem.8b03553 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 1. Calculated adiabatic temperature (A) and distribution of equilibrium solid products (B) for Fe(NO3)3 + C6H12N4+ H2O system depending on C6H12N4/Fe(NO3)3 molar ratio (ϕ) and amount of H2O. by TGA analysis). Caution! Fuel-lean gels react vigorously, and hence, the drying temperature should not exceed 420 K, at which the combustion reaction is initiated. Reaction in the gel was locally initiated using a resistively heated tungsten wire inside a stainless steel reactor under Ar atmosphere (99.998%, P = 0.4 MPa). After initiation, the chemical reaction propagates through the gels in the form of a rapidly moving high-temperature front. K-type thermocouples (100 μm diameter) inserted within the gel recorded the temperature−time history of the process at 1 kHz frequency through an L-Card E20-10 data acquisition system. The material was removed from the reactor 30 min after synthesis. Caution! The gas f rom the reactor should be vented into a proper ventilation system, and the vessel should be purged by inert gas before opening. This will permit safe removal of possible toxic gases (such as ammonia) released during the reaction. In some cases, the powdered products were subjected to successive ultrasonic treatments with acetone, hexane, and isoamyl acetate (10 min per solvent) to remove organic residues. A TGA−DSC device (Mettler-Toledo) coupled with a mass spectrometer (Pfeiffer Vacuum) was used to perform in situ differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and gas-phase analysis of reactive gels. In these experiments, 0.02 g of gel was heated to 730 K at a heating rate of 50 K/min in argon flow (100 mL/min). The sampling rate for gas analysis was 100 data points per second. 2.2. Characterization Methods. The phase composition of the materials was studied by X-ray diffraction (XRD) analysis with Nifiltered Cu Ka radiation (Ultima IV, Rigaku), operated at 40 kV and 40 mA with a step-scan size of 0.025° and a counting time of 10 s for the angular range of 20−90° (2θ). Synchrotron powder diffraction data were collected on an amorphous silicon-based flat panel detector (PerkinElmer, Santa Clara, CA) at beamline 11-ID-B of the Advanced Photon Source, Argonne National Laboratory, using 58.68 keV (0.2113 nm) X-rays. Images were recorded at a nominal sample to detector distance of 600 mm for 18 s. Calibration of sample to detector distance, beam center, oblique incidence correction, detector tilt angle and rotation with a CeO2 standard (NIST SRM 674b)67, and subsequent reduction of images to one-dimensional diffraction patterns were performed in GSAS-II.68 Fourier transform infrared spectroscopy (FTIR) was used to obtain infrared spectra for the initial reactants and gels. The spectra were acquired by a Bruker Tensor 27 spectrophotometer using attenuated total reflection (ATR) with 4 cm−1 resolution over a scanning range of 400−4000 cm−1. X-ray photoelectron spectroscopy (XPS) measurements were carried out in a PHI VersaProbe II spectrometer with an Al Kα Xray source operating at 1486.6 eV and a 90° takeoff angle for nearsurface analysis of Fe 2p, N 1s, and C 1s electronic transitions. The sample pellet was attached to a stainless steel mount and loaded into
route for fabrication of nanostructured materials.37−46 A typical SCS formulation includes preparation of homogeneous solutions of an oxidizer (metal nitrates) and organic fuel (e.g., glycine, urea, citric acid) in a solvent and placing the solutions in a preheated muffle furnace or on a hot plate.39,47−50 After evaporation of the solvent, a viscous gellike material forms, and initiation of the combustion reaction yields a porous solid and a significant amount of gases. This regime of an exothermic reaction is known as volume combustion.51 Alternatively, previously prepared gels can be heated locally to initiate the exothermic reaction. After initiation, a combustion wave propagates through the rest of the gel’s volume.41 Continuous flow schemes for SCS can provide high-throughput synthesis of nanoscale materials.39 Until recently, SCS was used solely for the preparation of oxide-based materials. The synthesis of nanoscale metals and alloys is one of the current active directions in SCS research. To prepare metallic materials, such as Ni, Cu, Co, and their alloys, one should use fuel-rich reactive solutions or gels.52,53,62−64,54−61,65,66 Here, we report for the first time one-step solution combustion synthesis of the nitride by self-sustained reactions of gels containing iron nitrate (Fe(NO3)3) and hexamethylenetetramine (C6H12N4, HMTA) in an inert atmosphere. It is also important that the short process durations (seconds), high reaction temperatures (∼900 K), and rapid cooling rates (∼20K/s) permit formation of a metastable ε-Fe3N phase. The obtained results indicate that the exothermic high-temperature decomposition of a coordinating compound formed due to the interaction between Fe(NO3)3 and HMTA in the reaction front leads to the formation of nanoscale ε-Fe3N particles with sizes of 5−15 nm. The SCS conditions permit preservation of crystalline nanoparticles of the metastable e-Fe3N phase, which possess magnetic behavior.
2. MATERIALS AND METHODS 2.1. Combustion Synthesis. Hexamethylenetetramine (C6H12N4, Chimmed, 98%; fuel) and iron nitrate nonahydrate (Fe(NO3)3·9H2O; Sigma-Aldrich, 98%; oxidizer) with different fuelto-oxidizer ratios (φ) were dissolved in deionized water and thoroughly mixed using a magnetic stirrer. The brown solution of reactants was then dried in an oven at 365 K for 24 h until all solvent and a portion of the water bonded to the nitrate evaporated (verified B
DOI: 10.1021/acs.inorgchem.8b03553 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry an antechamber and partially outgassed before loading into the analytical chamber. Samples were outgassed again in the vacuum system maintained at a pressure of less than ∼10−7 Pa. Binding energy values were referenced to the C 1s peak (284.8 eV) that resulted from the adventitious contamination layer. A Multipak software package was used to deconvolute the spectral peaks. A Magellan 400 (FEI) field-emission scanning electron microscope (SEM) was used to investigate the morphology of the products. A Titan 80-300 (FEI) transmission electron microscope (TEM) equipped with an energy dispersive X-ray spectroscopy (EDS, Oxford Inc.) detector with an energy resolution of 130 eV was used to characterize the composition and atomic structure of the materials. The magnetic properties of the materials were investigated using a superconducting quantum interference device (SQUID) magnetometer, the Magnetic Property Measurement System (MPMS) Model XL manufactured by Quantum Design, which allows measurement of magnetization as a function of field up to 15 000 Oe and a temperature range of 4 to 400 K.
3. RESULTS 3.1. Thermodynamic Analysis. Thermodynamic analysis was performed for the Fe(NO3)3 + C6H12N4+ H2O system using Thermo software to calculate adiabatic reaction temperature (Tad) and equilibrium product concentrations.69 This software allows for the calculation of Tad utilizing the Gibbs free energy minimization principle for the considered system. The calculated adiabatic temperature as a function of C6H12N4/Fe(NO3)3 molar ratio (ϕ) and the amount of moles of H2O (n) in the system are shown in Figure 1A. These results indicate that the reaction temperature can be as high as 2630 K (ϕ = 1, n = 0) and as low as 865 K for the more diluted system (ϕ = 5, n = 9). Figure 1B shows that the phase composition of the solid product depends on both the ϕ-ratio and quantity of H2O. The combustion of solutions with ϕ = 1 leads to the formation of pure Fe3O4 regardless of the water amount in the system. The increase of the ϕ-ratio to 2 yields a mixture of Fe3O4 and FeO phases. Depending on the amount of H2O, the product for solutions with ϕ = 3 is either Fe or Fe3C. Iron forms at higher temperatures when the reacting system contains less water. Increasing the quantity of H2O decreases the combustion temperature, which favors the formation of Fe3C. At higher ϕ-values (more than 3.5), the reaction product consists of Fe3C and carbon. These calculations suggest that Fe3N does not fit any global minimum of the Gibbs potential in the entire range of investigated conditions. The experimental results presented below, however, indicate that under certain conditions, this system permits the formation of a metastable Fe3N phase. 3.2. Combustion Synthesis. Self-propagating reactions in previously prepared gels were investigated under argon atmosphere. The overall water content in the reactive gels depends on the free water (solvent) and bound water molecules from the Fe(NO3)3·9H2O crystal hydrate. Drying of solutions at 365 K for 24 h leads to the removal of all solvent and a large portion (∼6 mol) of the chemically bound water. Figure 2 shows typical temperature−time profiles for the combustion of the gels with different ϕ-ratios. As expected from the thermodynamic calculation, the increase of ϕ leads to the decrease of the maximum combustion temperature. The measured maximum temperature (Tmax) for a gel with ϕ = 2 is 1645 K, whereas for ϕ = 3, the maximum temperature is 1180 K. For gels with higher ϕ-values (4,5 and 5), the maximum temperatures are 960 and 920 K, respectively. Table S1
Figure 2. Temperature−time profiles for combustion of Fe(NO3)3 + C6H12N4 gels with C6H12N4/Fe(NO3)3 molar ratio (ϕ): ϕ = 2 (a), ϕ = 3 (b), ϕ = 4,5 (c), and ϕ = 5 (d).
indicates that the measured Tmax values for all ϕ-values are slightly below the calculated Tad for the system containing 3 mol of water. Temperature−time profiles also indicate that the rate of temperature change (dT/dt) within the moving reaction front is in the range of 102−103 K per second. Table S1 displays examples of the heating rates for different reactive gels. It can be seen that for the gel with ϕ = 2, the dT/dt is ∼780 K/s. This parameter gradually decreases to 225 K/s for the gel with ϕ = 5. The reaction duration measured from initial room temperature to Tmax is in the range of 1.7−2.8 s. The cooling rate of products decreases with the increase of ϕ (i.e., from ∼45 K/s (ϕ = 2) to ∼20 K/s (ϕ = 5)). These data highlight that within self-propagating reaction fronts, rapid heating and cooling stages may create unique conditions for the synthesis of highly crystalline phases that are far from equilibrium. 3.3. Structure of Products. XRD analysis shows that the combustion product for gels with ϕ = 2 is a mixture of γ-Fe2O3 and FeO (Figure 3). Note that the thermodynamically calculated equilibrium product for ϕ = 2 is FeO. Essentially pure FeO phase is obtained for the materials formed after combustion of gel with ϕ = 3. The preparation of Fe is much
Figure 3. XRD patterns for combustion products for gels with different C6H12N4/Fe(NO3)3 molar ratios (ϕ). C
DOI: 10.1021/acs.inorgchem.8b03553 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry more challenging even though the thermodynamic analysis predicts its formation (see Figure 1B). This disagreement between thermodynamically predicted and experimentally obtained product can be related to the fact that freshly prepared iron oxidizes upon exposure to air.70 However, a fundamental disagreement with the thermodynamic calculation is that the increased amounts of C6H12N4 corresponding to ϕ ≥ 4.5 (Figure 3) leads to the appearance of ε-Fe3N. Moreover, the combustion of the gel with ϕ = 5 yields nearly pure εFe3N. The structure and composition of the product for gels with ϕ = 5 was further analyzed through synchrotron powder diffraction. Rietveld refinements performed in GSAS-II68 were utilized to quantify the phase fractions of iron nitride and iron oxide impurities. Phase identification was performed via preliminary refinement of various crystal structures of iron nitrides and oxides (ε-Fe3N, P6322;71 γ-Fe4N, Pm-3m;71 Fe3O4, Fd3m;72 FeO, Fm-3m;73 α-Fe2O3, R-3c74) to the diffraction pattern. Additionally, a search-match performed against the PDF-4 database for iron and oxygen, iron and nitrogen, or iron and carbon phases of index-quality or better returned matches to Fe3O4, ε-Fe3N, and FeO. A more extensive multiphase Rietveld refinement for quantitative phase analysis was performed using the identified ε-Fe3N,71 Fe3O4,72 and FeO73 crystal structures. The background (20 term Chebyschev), 4 Pseudo-Voigt profile coefficients, and histogram scale were refined as well as phase fractions, unit cell, and isotropic atomic displacement parameters for each phase (Table S2). Phase fractions were constrained to add up to 1; atomic displacement parameters were constrained to be equivalent for all Fe atoms and all light atoms (N and O). Atomic coordinates were not refined due to the relatively low phase fractions of the iron oxide phases as well as the location of atoms at special positions. The model obtained from multiphase Rietveld refinements and reflections from the refined crystal structures for ε-Fe3N (P6322, a = 4.7765, c = 4.3777), Fe3O4 (Fd3m, a = 8.4038), and FeO (Fm-3m, a = 4.2933) are shown in Figure 4. Quantitative phase analysis indicates that the ϕ = 5 sample is 88.5 wt % ε-Fe3N, with small amounts of FeO (6.2 wt %) and Fe3O4 (5.4 wt %). The surface chemical states of the product for gels with ϕ = 5 were investigated by XPS (Figure 5). The XPS core line for the Fe spectrum (Figure 5A) reveals broad peaks at 710.1 and 723.5 eV, which represent Fe 2p3/2 and Fe 2p1/2 photoelectron lines, respectively, and their corresponding satellite peaks. Both Fe 2p3/2 and Fe 2p1/2 broad lines can be deconvoluted into two major peaks corresponding to Fe−N and Fe−O bonds.75 This suggests that observed Fe−O photoelectron peaks correspond to a thin surface layer formed on ε-Fe3N nanoparticles. The N 1s spectrum (Figure 5B) of the product for gels with ϕ = 5 can be deconvoluted into two components. The peak centered at 398.2 eV represents bonding of iron to nitrogen, whereas the peak with the binding energy of ∼400 eV represents C−N bonding.76 The latter can be related to the presence of organic residues on the surface of ε-Fe3N nanoparticles. The C 1s spectra (Figure 5C) exhibits three peaks for the obtained sample. The peak located at 284.8 eV relates to adventitious carbon species.77 It should be noted that no peaks were observed at the binding energy region of 282−283 eV, indicating the absence of lattice carbon in a carbide form.78 The photoelectron peak at 286.1 eV can be attributed to the
Figure 4. Synchrotron powder diffraction pattern (black crosses) of combustion products from ϕ = 5 gels with the calculated model (red), background (green), difference curve (blue), and tick marks for εFe3N (magenta), Fe3O4 (aqua), and FeO (brown) reflections.
C−O or C−N bond, whereas the line centered at 288.3 eV corresponds to the C−N−C group.79 These results also confirm that nitrogen on the surface of ε-Fe3N nanoparticles is bonded with iron or as an organic residue. The synchrotron powder diffraction results and XPS analysis demonstrate that εFe3N is the major product phase for gels with ϕ = 5. The oxides detected could be attributed to the surface oxidation of nanoscale powder upon post-synthesis air exposure. 3.4. Morphology of Products. SEM imaging suggests that all products are large agglomerates of nanoscale particles (Figure S1). For example, the pure FeO powder prepared by the combustion of gel with ϕ = 3 consists of near-spherical particles with sizes ranging from 50 to 100 nm (Figure 6A). The ε-Fe3N product for the gels with ϕ = 5 is composed of agglomerates with similar sizes (Figure 6B). The individual particles, however, cannot be identified. A bright-field TEM image (Figure 6C) of this material shows that the agglomerates consist of small (5−20 nm) nanoparticles (darker phase) and a lighter phase, which covers the surface of the agglomerate. The lattice fringes of nanoparticles cannot be seen clearly in the high-resolution TEM image (Figure 6D). EDS analysis (not shown) of the amorphous phase reveals carbon, oxygen, and nitrogen, whereas the nanoparticles contain iron and nitrogen. These results and XPS analyses indicate that combustion of gels with high fuel-to-oxidizer ratios leads to the formation of ε-Fe3N nanoparticles embedded within an amorphous organic phase. For further analysis, this combustion product was sonicated with organic solvents (see details in Section 2.1) to remove the amorphous residue. TEM imaging (Figure 7A) of the purified product reveals that such purification can effectively remove the organic residue. TEM images also indicate that the average size of the nanoparticles is on the order of 5−20 nm. It is difficult to obtain well-dispersed nanoparticles, because they are ferromagnetic and tend to agglomerate. The indexed electron diffraction pattern (Figure 7B) confirmed that the nanoparticles are primarily ε-Fe3N. High-resolution TEM images display the crystalline structure of the nanoparticles (Figures 7C,D). For example, Figure 7D shows a single ε-Fe3N D
DOI: 10.1021/acs.inorgchem.8b03553 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 6. SEM and TEM images of products obtained by combustion of gels with ϕ = 3 (A) and ϕ = 5 ratios (B−D).
Figure 5. XPS spectra of Fe 2p, N 1s, and C 1s lines for the product prepared by combustion of gels with ϕ = 5. Figure 7. TEM images (A,C,D) and a selected area electron diffraction pattern (B) for an ε-Fe3N nanoparticle prepared by combustion of gel with a ϕ = 5 ratio.
nanoparticle with d-spacing of 0.298 nm, which corresponds to the (101) crystallographic orientation. 3.5. Magnetic Properties. Hysteresis loops were measured at various temperatures ranging from 5 to 350 K to demonstrate the magnetic properties of ε-Fe3N nanoparticles. As shown in Figure 8, both the saturation magnetization and the coercive field decrease as the temperature increases. Specifically, at 5 K, a saturation magnetization of 100.68 emu/ g is obtained at 15 000 Oe, which accounts for 1.09 μB/Fe. The saturation magnetization at 300 K is 80.06 emu/g and 0.87 μB/ Fe. The coercive field of this material is 754 Oe at 5 K and 392 Oe at 300 K. The saturation magnetization of this material is only slightly less than the 1.3 μB/Fe obtained in magnetization studies of bulk ε-Fe3N.80 The remanent magnetization and squareness (ratio between remanent and saturation magnetization) at 300 K are 14.94 emu/g and 18.66%, respectively. The ferromagnetic behavior of this material is comparable with
the magnetic results of ε-Fe3N nanoparticles prepared from other methods.21−23 The saturation magnetization of this material is slightly below the value measured for ε-Fe3N synthesized by liquid ammonia reduction, which may be related to the surface oxidation of nanoparticles.22 The Curie temperature of the material prepared in this work is lower as compared to that of ε-Fe3N prepared by liquid ammonia reduction. The temperature dependences of saturation magnetization and remanent magnetization clearly show (Figure 9) that the Curie temperature of ε-Fe3N nanoparticles is far above 400 K (which is limit of the SQUID magnetometer). By extrapolating the remanent magnetization, we obtained a Curie temperature of ∼522 K, which is close to E
DOI: 10.1021/acs.inorgchem.8b03553 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
surface oxidation (i.e., FeO and Fe3O4) after post-synthesis air exposure. 3.6. Thermal Analysis. Thermal analyses (TGA, DSC) of precursors and gels (Figure 10) were performed to gain insight into the reaction mechanism in the considered system. It can be seen that a relatively wide endothermic peak exists for pure fuel (HMTA) in the 450−550 K temperature range (Figure 10B), and this process results in its total removal (Figure 10A) due to sublimation/decomposition. TGA/DSC curves for pure oxidizer (Fe(NO3)3·9H2O) exhibit four significant endothermic steps (Figure 10B), which correspond to stepwise dehydration at 330, 370, and 415 K as well as a final decomposition step at 430 K.82 For gels with ϕ = 3, TGA/DTA curves suggest a multistep reaction process (i.e., a weak endothermic effect at ∼400 K), and three exothermic peaks at ∼430, 440, and 460 K were observed. TGA/DSC curves for a gel with a ϕ = 5 ratio also contains a weak endothermic process at 400−450 K, followed by a single intense exothermic peak centered at ∼465 K. The DSC curve for this gel exhibits a small exothermic peak at ∼480 K (Figure 10B). Additionally, TGA analysis (Figure 10A) shows that the mass of the sample reduces more abruptly for ϕ = 5 as compared to that for ϕ = 3 during the intense exothermic stage. Coupling TGA/DSC with mass spectrometry data for reactive gels (ϕ = 5) suggests (Figure S2) that the intense exothermic reaction (465 K) involves a rapid release of NH3, CH4, H2O, CO, CO2, NO, N2O, and CH2O (formaldehyde), whereas during the weaker exothermic process (∼480 K), additional release of NH3, H2O, and CO2 was detected. These results imply that increasing the fuel amount for the reactive gels changes the three-step reactive process into a primarily single-stage exothermic interaction. This effect could be attributed to the formation of an Fe(NO3)3−HMTA coordinated compound, which has been observed in similar metal systems (Co, Ni).83 To verify this hypothesis, we conducted FTIR analysis of initial reactants and gels (Figure 11). The spectrum of HMTA (curve a) shows three typical stretching vibration bands at ∼808, 998, and ∼1234 cm−1 for the C−N bond along with stretching, bending, and deformation bands for the C−H bond. The iron nitrate nonahydrate spectrum (curve b) contains a broad band for hydrated water centered at ∼2890 cm−1 and a double band at 1325 and 1386 cm−1 for the stretching vibration of the NO3− group. The FTIR spectrum (curve c) for the gel with ϕ = 1 indicates that the hydrated water peak shifted to ∼3226 cm−1.
Figure 8. Hysteresis loops of ε-Fe3N nanoparticles prepared by combustion of gel with a ϕ = 5 ratio at various temperatures: 5 (a), 50 (b), 100 (c), 150 (d), 200 (e), 250 (f), 300 (g), and 350 K (h).
Figure 9. Saturation magnetization Ms (H = 10 000 Oe) and remanence magnetization Mr (H = 10 Oe) of ε-Fe3N nanoparticles prepared by combustion of gel with a ϕ = 5 ratio plotted as a function of temperature. A Landau theory of M(T)/M(0) = (1 − T/TC)1/2 is used to determine the Curie temperature TC.
the value of bulk ε-Fe3N, 575 K.81 This decrease of Curie temperature is commonly observed in ferromagnetic nanoparticles and is attributed to weakened magnetic exchange interactions because of the disorder at the surface of nanoparticles. Considering that the average sizes of nanoparticles in this work are in the order of 5−20 nm, the measured magnetic properties indicate that the surface disorder in these nanoparticles is minimal. Note that the quick increase of magnetization at low temperature (below 40 K) might be attributed to the presence of a small amount of
Figure 10. TGA (A) and DSC (B) curves for HMTA and Fe(NO3)3·9H2O as well as reactive gels with ϕ = 3 and ϕ = 5 ratios. F
DOI: 10.1021/acs.inorgchem.8b03553 Inorg. Chem. XXXX, XXX, XXX−XXX
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Organic residues obtained during the SCS step accelerate the nitridation process in the calcination step. Thus, single-step SCS of nitride (ε-Fe3N) nanoparticles is a breakthrough result. Furthermore, as discussed above, ε-Fe3N is a metastable phase in every form, whether in bulk, thin films, or nanoparticles, and the crystalline phase has only been observed to form between 550 and 850 K. This feature creates significant challenges for preparation of ε-Fe3N by conventional methods and requires the use of exotic reactants and multistage processing.30−36 In this work, it was demonstrated that rapid self-sustained reactions in gels create unique conditions suitable for the formation of metastable phases. Indeed, the temperature−time profiles (Figure 2) indicate that high reaction temperatures achieved within a few seconds followed by rapid cooling are favorable for the formation of metastable ε-Fe3N instead of the thermodynamically more stable Fe3C (see Figure 1). Finally, this research contributes to the simple processing of magnetic nanoscale materials. Thermal analysis indicates that the Fe(NO3)3 + HMTA relatively fuel-lean (ϕ ≤ 3) gels exhibit three exothermic steps. The first exothermic peak at ∼430 K coincides with the onset temperature for Fe(NO3)3 decomposition (see Figure 10), for which the first decomposition species is known to be HNO3.92 Also, it was reported82 that depending on the atmosphere, HMTA either sublimates endothermically and slowly decomposes in the gas phase or decomposes exothermically producing solid and oily substances along with gaseous species: NH3, CH4, H2, and N2.93 The results of thermal analysis confirm that under inert atmosphere, pure HMTA sublimates slowly starting from 430 K (Figure 10A). Therefore, it can be suggested that for fuel-lean gels, the first exothermic process at ∼430 K corresponds to the exothermic reaction between HNO3 (or nitrogen oxides), released during Fe(NO3)3 decomposition, and sublimated HMTA or its gaseous fragments. The second exothermic peak (∼440 K) can be assigned to exothermic decomposition of HMTA in the environment formed during the previous stage of interaction. Thermodynamic calculations suggested (not shown) that HMTA exothermically decomposes, forming N2 and C along with H2, CO, CH4, NH3. These gases further react with HNO3/nitrogen oxides, observed as the third intense exothermic peak at ∼460 K on the DSC curve (Figure 10A). Therefore, fuel-lean gels combusted to produce primarily iron oxides. The increase of the fuel content in the gels favors the coordination of iron ions with HMTA to form ligands as supported by FTIR analysis (Figure 11). Thermal analysis for a gel with ϕ = 5 shows one sharp peak with the rapid simultaneous release of multiple gases (Figure S2). This strong exothermic peak can be assigned to thermal decomposition of the Fe(NO3)3−HMTA coordinated compound. In previous works, the decomposition of similar HMTA complexes with silver, nickel, and cobalt nitrates was reported to occur via strongly exothermic processes.83,94,95 It was even suggested that decomposition of Ni(NO3)2−HMTA or Co(NO3)2− HMTA coordinated compounds can occur via self-sustaining processes because of the catalytic effect.95 The transition metal clusters that form can then catalyze further reaction, causing the process rate to increase exponentially. On the basis of these observations, thermal decomposition of the Fe(NO3)3− HMTA complex occurs through an intramolecular oxidation−reduction reaction between nitrate oxidizer and the HMTA ligand at ∼465 K (Figure 10), directly resulting in the
Figure 11. FTIR spectra for HMTA (a) and Fe(NO3)3·9H2O (b) as well as gels with ϕ = 1 (c), ϕ = 3 (d), and ϕ = 5 (e) molar ratios.
Increasing HMTA content in the gels (curves d and e) results in a further shift for this broad peak to ∼3404 cm−1. More importantly, the small intensity peak at ∼1630 cm−1 observed for Fe(NO3)3·9H2O (curve c) gradually increases with added HMTA and eventually splits into two components centered at 1585 and 1645 cm−1. This feature indicates the existence of two types of crystallographically nonequivalent water molecules (coordinated and not coordinated). Another significant piece of evidence for the formation of a coordinated compound between Fe(NO3)3 and HMTA is the observed splitting of stretching vibration bands for the C−N bond at 998 and ∼1234 cm−1. Coordination of the nitrogen atom of HMTA to the iron atom lowers the symmetry of the HMTA. This gives rise to new bands, which are not active in the free HMTA. Finally, the band observed at 808 cm−1 for pure HMTA shifts to 827 cm−1 in the spectra for the gels. This shift also demonstrates that HMTA coordinates to the iron ion in the reactive gels.
4. DISCUSSION SCS is an energy-efficient approach for the preparation of nanoscale materials. Although this method allows straightforward synthesis of oxide-based materials, because of the chemistry of the process, metal nitrates are typically used as oxidizers, which decompose to form oxides. Recent advances in the field have made it possible to obtain metallic materials and alloys.58,60,61,66,84 It has been demonstrated that under certain conditions, the gaseous intermediates (NH3, CH4), formed from the decomposition of fuels, sequentially reduce solid oxides to pure metals (or alloys). Single-step attempts to produce other classes of materials, such as carbides or nitrides, have not yet been successful. For example, Gu et al. used lowtemperature combustion of solutions containing iron nitrate, glycine, and glucose to produce Fe3C.85 The combustion product of the fuel-rich solution was an amorphous mixture of Fe2O3 + C. The authors then applied an additional calcination step under nitrogen at 720−973 K to promote carbothermal reduction and formation of Fe3C. Chen et al. used a similar strategy to produce WC−Co−Cr3C2−VC composite materials, but again, in order to achieve complete carbonization, a calcination step was performed at 1273 K.86 Kirakosyan et al. reported the synthesis of molybdenum carbide (Mo2C) by a two-step process consisting of SCS and post-synthesis annealing. SCS also has been used to prepare oxide−carbon precursors for the synthesis of Si3N4, AlN, TiN, and CrN.87−91 G
DOI: 10.1021/acs.inorgchem.8b03553 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry formation of ε-Fe3N. The weak exothermic effect observed following the primary exothermic event for ϕ = 5 (Figure 10) can be explained as the decomposition of uncoordinated HMTA, which results in an organic residue (Figures 5C and 6D) covering the ε-Fe3N nanoparticles.
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(1) Bhattacharyya, S. Iron Nitride Family at Reduced Dimensions: A Review of Their Synthesis Protocols and Structural and Magnetic Properties. J. Phys. Chem. C 2015, 119, 1601−1622. (2) Widenmeyer, M.; Hansen, T. C.; Meissner, E.; Niewa, R. Formation and Decomposition of Iron Nitrides Observed by in Situ Powder Neutron Diffraction and Thermal Analysis. Z. Anorg. Allg. Chem. 2014, 640, 1265−1274. (3) Bykov, M.; Bykova, E.; Aprilis, G.; Glazyrin, K.; Koemets, E.; Chuvashova, I.; Kupenko, I.; McCammon, C.; Mezouar, M.; Prakapenka, V.; Liermann, H.-P.; Tasnádi, F.; Ponomareva, A. V.; Abrikosov, I. A.; Dubrovinskaia, N.; Dubrovinsky, L. Fe-N System at High Pressure Reveals a Compound Featuring Polymeric Nitrogen Chains. Nat. Commun. 2018, 9, 2756. (4) Wessel, M.; Dronskowski, R. A New Phase in the Binary Iron Nitrogen System?-The Prediction of Iron Pernitride, FeN2. Chem. Eur. J. 2011, 17, 2598−2603. (5) Jiang, Y.; Al Mehedi, M.; Fu, E.; Wang, Y.; Allard, L. F.; Wang, J. P. Synthesis of Fe16N2 compound Free-Standing Foils with 20 MGOe Magnetic Energy Product by Nitrogen Ion-Implantation. Sci. Rep. 2016, 6, 25436. (6) Niwa, K.; Terabe, T.; Kato, D.; Takayama, S.; Kato, M.; Soda, K.; Hasegawa, M. Highly Coordinated Iron and Cobalt Nitrides Synthesized at High Pressures and High Temperatures. Inorg. Chem. 2017, 56, 6410−6418. (7) Niewa, R.; Rau, D.; Wosylus, A.; Meier, K.; Hanfland, M.; Wessel, M.; Dronskowski, R.; Dzivenko, D. A.; Riedel, R.; Schwarz, U. High-Pressure, High-Temperature Single-Crystal Growth, Ab initio Electronic Structure Calculations, and Equation of State ε-Fe3N1+x. Chem. Mater. 2009, 21, 392−398. (8) Schwarz, U.; Wosylus, A.; Wessel, M.; Dronskowski, R.; Hanfland, M.; Rau, D.; Niewa, R. High-Pressure-High-Temperature Behavior of ζ-Fe2N and Phase Transition to ϵ-Fe3N1.5. Eur. J. Inorg. Chem. 2009, 2009, 1634−1639. (9) Chen, Y.; Cai, X.; Wang, H.; Wang, H.; Wang, H. Novel Triadius-like N4 specie of Iron Nitride Compounds under High Pressure. Sci. Rep. 2018, 8, 10670. (10) Laniel, D.; Dewaele, A.; Anzellini, S.; Guignot, N. Study of the Iron Nitride FeN into the Megabar Regime. J. Alloys Compd. 2018, 733, 53−58. (11) Niewa, R.; Rau, D.; Wosylus, A.; Meier, K.; Wessel, M.; Hanfland, M.; Dronskowski, R.; Schwarz, U. High-Pressure HighTemperature Phase Transition of γ′-Fe4N. J. Alloys Compd. 2009, 480, 76−80. (12) Kikkawa, S.; Masubuchi, Y. Magnetic Iron Nitrides Inspired by Historic Research on α″-Fe16N2. Prog. Solid State Chem. 2018, 51, 19−26. (13) Menthe, E.; Rie, K. T.; Schultze, J. W.; Simson, S. Structure and Properties of Plasma-Nitrided Stainless Steel. Surf. Coat. Technol. 1995, 74−75, 412−416. (14) Rissanen, L.; Neubauer, M.; Lieb, K. P.; Schaaf, P. The New Cubic Iron-Nitride Phase FeN Prepared by Reactive Magnetron Sputtering. J. Alloys Compd. 1998, 274, 74−82. (15) Chakravarty, S.; Kumar, N.; Panda, K.; Ravindran, T. R.; Panigrahi, B. K.; Dash, S.; Tyagi, A. K.; Amarendra, G. The Influence of Nitrogen Concentration on Microstructure and Ultra-Low Friction Behaviour of Fe-N Thin Films. Tribol. Int. 2014, 74, 62−71. (16) Yang, Z.; Guo, S.; Pan, X.; Wang, J.; Bao, X. FeN Nanoparticles Confined in Carbon Nanotubes for CO Hydrogenation. Energy Environ. Sci. 2011, 4, 4500−4503. (17) Li, Z.; Fang, Y.; Zhang, J.; Lou, X. W. Necklace-Like Structures Composed of Fe3N@C Yolk−Shell Particles as an Advanced Anode for Sodium-Ion Batteries. Adv. Mater. 2018, 30 (3−7), 1800525. (18) Balamurugan, J.; Thanh, T. D.; Kim, N. H.; Lee, J. H. NitrogenDoped Graphene Nanosheets with FeN Core-Shell Nanoparticles as
5. CONCLUSIONS Single-step combustion of Fe(NO3)3 + C6H12N4 gels in an inert atmosphere is a simple and energy- and cost-efficient method for the preparation of metastable ε-Fe3N nanoparticles. Rapid heating during the combustion process creates conditions necessary for the formation of highly crystalline material, whereas the relatively rapid cooling stage preserves metastable ε-Fe3N nanoscale particles. Detailed analysis of the synthesis process allowed us to suggest that the exothermic decomposition of a coordinating compound formed between Fe(NO3)3 and HMTA is responsible for the formation of εFe3N. The magnetic characterization revealed that the assynthesized ε-Fe3N exhibits ferromagnetic behavior comparable to bulk ε-Fe3N with a low-temperature magnetic moment of 1.09 μB/Fe, high room temperature saturation magnetization (∼80 emu/g), low remanent magnetization (∼15 emu/ g), and a Curie temperature of ∼522 K. The observation that the measured magnetic properties are nearly equivalent to the bulk properties of ε-Fe3N indicates a low degree of surface disorder and a high degree of crystallinity within the nanoscale product.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03553. Characteristics of combustion parameters, gas-phase mass spectrometry data, additional SEM images of products, and refined crystal structures (PDF)
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Alexander S. Mukasyan: 0000-0001-8866-0043 Joshua M. Pauls: 0000-0002-0474-4918 Leighanne C. Gallington: 0000-0002-0383-7522 Khachatur V. Manukyan: 0000-0002-8595-2179 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The work was performed with financial support in part from the Ministry of Education and Science of the Russian Federation in the framework of the Increase Competitiveness Program of NUST “MISiS” (No. K2-2017-083), implemented by a governmental decree dated March 16, 2013, N 211. K.M. acknowledges financial support from U.S. Department of Energy (DOE) National Nuclear Security Administration (NNSA, Grant # DE-NA0003888) and US National Science Foundation (NSF, PHY-1713857 and DMR-1400432) grants. This work was also partially supported by the NNSA, under the award number DE-NA0002377. This work used resources of the Advanced Photon Source, DOE Office of Science User H
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DOI: 10.1021/acs.inorgchem.8b03553 Inorg. Chem. XXXX, XXX, XXX−XXX
