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Fe3C nanoparticles for magnetic hyperthermia application
Journal of Magnetism and Magnetic Materials 481 (2019) 251–256
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Research articles
Fe3C nanoparticles for magnetic hyperthermia application a
a
b
c,d
c
A. Gangwar , S.S. Varghese , Sher Singh Meena , C.L. Prajapat , Nidhi Gupta , N.K. Prasad
a,⁎
T
a
Department of Metallurgical Engineering, Indian Institute of Technology (Banaras Hindu University), Varanasi 221005, India Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India Technical Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India d Homi Bhabha National Institute, Anushaktinagar, Mumbai 400094, India b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Iron carbide Nanoparticles Magnetic hyperthermia Heating ability
Fe3C magnetic nanoparticles were synthesized by solvothermal assisted technique. The X-ray and electron diffractions have confirmed the formation of monophasic iron carbide (Fe3C) nanoparticles. Transmission electron microscope analysis validated that the size of the particles was around 19–34 nm. The presence of only Fe and C in Fe3C was asserted by X-ray photoelectron spectroscopy (XPS). The room temperature Mössbauer spectroscopy also corroborates the occurrence of only this carbide phase. The saturation magnetization at ± 2 T and at 300 K was obtained to be around 88 Am2/kg with a coercivity of 17.3 mT and remanence of 6 Am2/kg. The oleic acid based ferrofluid of this carbide nanoparticles exhibited good heating ability in the presence of external alternating current (AC) magnetic fields. The optimum specific absorption rate and intrinsic loss power values were 85 W/g and 0.97 nHm2/kg at 23 mT and 261 kHz. Due to lack of reports on magnetic hyperthermia response for iron carbides, the values were compared with iron oxides. The values were found to be comparable.
1. Introduction Several forms of iron carbides such as Fe3C, Fe5C2, Fe7C3, Fe2.2C etc. are reported in the literature. Out of these carbides, Fe3C (cementite/ cohenite) has great technological importance [1,2]. This intermetallic compound has a stoichiometric composition of 6.67% carbon and 93.3% iron (Fe) by weight [3]. It has an orthorhombic crystal structure, high melting point (1837 °C), non-pyrophoric and strong magnetic (saturation magnetization, MS, for bulk ∼140 Am2/kg) behavior [4]. It is normally classified as a ceramic due to its high hardness and brittleness [1]. It is frequently found as an important constituent in ferrous metallurgy (in most steels and cast irons). The use of iron carbide during the production of steels and cast irons is remarkably environmental friendly because of its lowest C emission as compared to that of other virgin iron and steel making processes [5]. In addition, the iron carbide is much more effective and less costly than any other means to produce high-quality steel. This carbide can also be used as magnetic recording material due to its high magnetic strength and suitable Curie temperature (210 °C) [6]. The biomedical applications of magnetic nanoparticles (MNPS) have become prominent topics for researches in the modern century. These applications are referred to as drug delivery, magnetic resonance imaging (MRI), magnetic hyperthermia treatment (MHT) etc. [7–9]. For such applications, the scientists have employed both ferromagnetic ⁎
(metal/alloys e.g. Fe, FePt etc.) and ferrimagnetic nanoparticles (pure or substituted magnetite, Fe3O4 or maghemite, γ- Fe2O3) [10]. The former metallic materials though have higher MS values (∼220 Am2/kg for bulk Fe) but the toxic nature limits their suitability [7]. The encapsulation of these metallic nanoparticles by the inert materials like platinum, gold, silver or silica etc. could enhance their compatibility but at the expense of enhanced size and reduced MS values [11]. Further, such nanolayer coatings are inhomogeneous, costly and tiresome to perform. In contrast, the magnetic iron oxides have adequate biocompatibility and are being utilized with or without such coatings [12]. Nonetheless, these oxides have inferior MS values compared to that of the metallic ones (e.g. 90 Am2/kg for bulk Fe3O4) [13]. In contrast, the iron carbide (e.g. Fe3C) found to display MS value considerably higher than that of its oxides counterpart and presumed to have better biocompatibility than metallic ones [14]. Due to its higher MS value, the actual materials required to get the therapeutic temperature (42–46 °C) during magnetic hyperthermia (MHT) treatment may be lesser than its oxide counterparts [15]. Further, the magnetic iron oxides found to get transformed into antiferromagnetic oxide with time in some of the carrier fluids [16]. Such conversion of iron carbide may not happen in fluids [17]. The synthesis route and the properties of iron oxide particles have been investigated to a large extent and thus there is less probability to get significant improvement in their properties [18]. Hence, the researchers are looking for the other materials.
Corresponding author. E-mail address: [email protected] (N.K. Prasad).
https://doi.org/10.1016/j.jmmm.2019.03.028 Received 19 December 2018; Received in revised form 6 February 2019; Accepted 5 March 2019 Available online 06 March 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.
Journal of Magnetism and Magnetic Materials 481 (2019) 251–256
A. Gangwar, et al.
PHI) with Al Kα X-ray beam (1486.61 eV) of the dual anode. The baseline was executed using a Shirley background function and the peaks deconvolution and quantification was done using the origin software. Magnetization vs. field up to ± 2 T was carried out using SQUID (MPMS-XL, Quantum Design) to ascertain the MS, Mr, and HC values. Mössbauer spectrum was collected at room temperature from a spectrometer in transmission geometry and in the constant acceleration mode of the triangular waveform. A 57Co in Rh matrix was used as a Mössbauer source. A thin foil of α-Fe was used as a standard sample to calibrate the spectrometer for velocity scale and for the estimation of isomer shift value. The experimental data were fitted using a least square curve fitting model using a WinNormos site fit program. Heating ability of Fe3C based ferrofluids was determined using Magnetherm (Nano Therics, U. K.) in various fields. The temperature rise of the ferrofluids during the experiment was recorded up to 60 °C. The temperature was measured using an optical fiber probe with an accuracy of 0.1 °C (Lumasense technologies). The specific absorption rate (SAR, W/g) values were estimated using the formula [34]:
The iron carbides have become an attractive alternative owing to their aforesaid properties [11]. Additionally, iron carbides have a high electrical conductivity which may further add to its heating ability during MHT experiments [19]. However, to the best of our knowledge there is no report in the literature where magnetic hyperthermia response of this carbide is evaluated. Thus, we took up the work and explore the heating behavior of Fe3C nanoparticles in the presence of two different AC magnetic fields. There are a few reports on the synthesis of Fe3C nanoparticles [13,20–22]. However, to get this carbide the scientists generally used Fe(CO)5 as a precursor which is not readily available [2,23,24]. Moreover, the researchers also get other byproducts such as carbides/ nitrides/oxides of iron or even pure Fe along with this carbide [10,20,25,26]. The common techniques described for its production are flame spray pyrolysis, Fischer – Tropsch synthesis [27–29], laser ablation method [7,30], sol-gel [20,31], solvothermal [18,32,33] etc. In the present work, we have synthesized monophasic Fe3C nanoparticles by solvothermal route followed by calcination as suggested by Lei et al. [18]. The structural, morphological, as well as magnetic characterization of the sample, were carried out by the conventional techniques. The heating ability for this carbide nanoparticles was assessed in the presence of AC magnetic fields.
SAR =
Cp dT ∗ m dt
where Cp is the specific heat capacity of water 4.186 Jg−1K−1, m is the concentration of Fe3C MNPS per unit volume of solvent, and dT/dt is the initial slope obtained from the temperature vs. time curves between 30 and 35 °C using the linear fitting method. The intrinsic loss power (ILP, in nHm2kg−1) values for the ferrofluids at various fields were calculated by following formula [5,35]:
2. Materials and method The synthesis of Fe3C, MNPS was carried out in two steps. The details about the process may be found elsewhere [18]. In brief, initially, anhydrous FeCl3 (1.2 g) and hexamethylenetetramine (HMTA, 4.2 g) were dissolved into the solution of ethylene glycol and double distilled water (DDW) having equal volumes (20 mL). The resulting solution was heated in a Teflon lined autoclave (40 mL) at 110 °C for 6 h. After washing the precipitate with DDW and ethanol, it was heated at 70 °C for 12 h in a vacuum furnace. This was followed by keeping the grounded powder and ethylenediamine (6 mL) in separate boats for calcination at 650 °C for 1.5 h in the N2 atmosphere. Further, the sample was kept in 30% H2O2 in DDW for 48 h to remove any adsorbed carbon. The obtained powder sample was crushed by a pestle mortar and then sonicated in water for 20 min using a Probe sonicator of PKS 250FM (PCI Analytical) to lessen the size of the particles and to facilitate its stabilization in a liquid medium. Following the removal of water, the particles were dispersed in oleic acid and then kept at 70 °C under continuous stirring for 12 h. Subsequently, the oleic acid coated MNPs were collected using a hard magnet. These MNPS were dispersed in water and then exposed to a high-frequency utrasonicator bath of power 50 W, 40 kHz for 30 min to get the suspension of MNPs i.e. ferrofluid. The ferrofluids had concentrations of 2.5, 5, 10 and 20 mg/mL of MNPs.
ILP =
P SAR = 2 ρH 2f H f
where P/ρ, is SAR values, H and f are the amplitude and frequency of fields respectively. 4. Results and discussion 4.1. XRD analysis Fig. 1 exhibits the Rietveld refined XRD pattern after analysis for Fe3C sample which was synthesized via the two step solvothermal assisted method. The peaks of the pattern were matched with JCPDS no. 89-7271 for cohenite (Fe3C). The Rietveld refinement of the pattern was performed by Fullprof software which suggests an orthorhombic crystal structure for it. The detailed data after refinement is listed in Table 1.
3. Characterizations For structural and phase analysis, XRD pattern was recorded with Cu Kα radiation (λ = 1.54056 Å) using an X-ray powder diffractometer (BT- Rigaku Miniflex) range between 25 and 90° at room temperature. High-resolution transmission electron microscope (TEM, TECNAI-20 G2) was adopted to observe the morphology and size of the particles. The distribution for the particle size over the histogram was drawn using “Standard Normal Distribution” fitting and it was calculated by the formula given below:
Z= (x − μ x)/σx where x is the value which is being standardized, µx is the mean of the distribution and σx standard deviation of the distribution. Selected area electron diffraction (SAED) pattern was obtained to identify the crystal structure. To find out the elements present in the Fe3C system, the X-ray photoelectron spectroscopy (XPS) was carried out on a (PHI5000 Versaprobe II photoelectron spectrometer ULVAC-
Fig. 1. Rietveld refined XRD Patterns of nanocrystalline iron carbide (Fe3C) sample. 252
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Table 1 Summary of the Rietveld refined structural parameters of the orthorhombic iron carbide nanoparticles. Space group
Lattice parameter a, b, c α, β, γ
Unit cell volume (Å3) V
Density (g/cm3)
Atomic position Fe1 (x, y, z)
Atomic position Fe2
Atomic position C1
Atomic occupancy (site) Fe1 (8d), Fe2 (4c), C1 (4c)
Bragg-R Factor, RF-Factor, χ2
Pnma 62
5.0829 6.7431 4.526 90, 90, 90
155.14
10.592
0.197 0.0716 0.3531
0.521 0.25 0.82
0.89 0.25 0.248
2, 1, 1
24.4 13.5 1.27
Symmetry multiplicity and Wyckoff symbol of Fe1, Fe2, C1: 8d, 4c, 4c.
the formation of pure Fe3C phase in the sample [36]. The spectrum for O 1S may be attributed to the adsorbed oxygen on the surface of nanosized carbide [37].
The calcination temperature and the holding time were found to be significantly important as the other temperature would not produce pure Fe3C phase [18,25]. Moreover, the proportions of the precursors (e.g. HMTA, ethylenediamine, and FeCl3) were also crucial in getting the single phase Fe3C [20,25]. The excess amount of HMTA resulted in iron whereas the lesser concentration induced other phases like iron oxides and nitride [18]. Hence, an optimized concentration of precursors and thermal treatment facilitated the formation of Fe3C single phase which was also observed in an earlier study. The crystallite size of the material was found to be around 20 nm by the Scherrer equation.
4.4. Mössbauer spectroscopy Fig. 4 shows the Mössbauer spectrum of Fe3C sample at room temperature which was fitted with a sextet (six lines Zeeman splitting pattern) and a doublet. The values for the magnetic hyperfine field (Hf), quadruple splitting (δ), isomer shift (Δ), the line width (г), and relative area (RA) are summarized in table 2. The sextet has BHF value of 20.92 T which is due to the positions of Fe in the orthorhombic structure of Fe3C. The reported Hf values for bulk Fe3C are 20.9 and 20.6 T [6]. The two different values of Hf are attributed to the two inequivalent sites for Fe atoms in Fe3C. Nevertheless, it has also been observed that the two sextets may get superimposed due to a very small difference in their local magnetic fields (1%) [6]. For the present sample, the spectrum shows a better fit with only one sextet. The isomer shift value for the sextet is found to be 0.167 mm/s. In contrast, the reported value of the chemical shift found to be ∼0.18 mm/s [35]. The slight difference in the value might be due to the different synthesis protocol as well as the nano dimension of the present material. The doublet obtained in the Mössbauer spectrum (6%) indicates the presence of a superparamagnetic component in the material associated with the small nanosized particles.
4.2. TEM analysis The microstructure of the sample as collected from TEM is shown in Fig. 2(a) which infers that particles were nearly spherical in shape. The inset of this micrograph shows the SAED pattern validating its polycrystalline nature. The pattern was indexed with the help of eRing software and was resembling the Fe3C phase. This was in accordance with the XRD pattern (Fig. 1). The treatment of the sample with 30% H2O2 for 48 h could remove physically adsorbed carbon and hence no trace of it was observed in the TEM micrograph (Fig. 2a). The particles size distribution is presented as a histogram in Fig. 2(c). The particles size was calculated using ImageJ software by selecting around 400 particles (Fig. 2a). The size was found to be in the range of 19–34 nm and the average size of the particles was ∼(22 ± 4) nm. It was nearly equal to that of the crystallite size.
4.5. Magnetic properties 4.3. XPS analysis The magnetizations vs. field curve for Fe3C sample is shown in Fig. 5 which was measured at 300 K and up to ± 2 T. The MS value was found to be around 88 Am2/kg which was less than its bulk (∼140 Am2/kg) and reported value (123 Am2/kg) [4]. Nevertheless, the value was relatively higher than that of nanoparticles of most of the iron oxides [39]. The lesser MS value for the sample could be attributed to the smaller size of the particles. The remanent (Mr) and coercivity (HC) values for the sample are found to be 6 Am2/kg, 17.3 mT respectively.
XPS analysis of the sample was performed to investigate the surface properties of Fe3C MNPs. It suggested that the sample contained only Fe, C, and O and their respective spectrum are shown in Fig. 3. The XPS spectrum of Fe 2p exhibited two strong peaks at 724.6 eV and 711.3 eV as shown in Fig. 3(b). These peaks could be ascribed to the Fe 2p1/2 and Fe 2p3/2 binding energies. One single peak at 284.42 eV was observed for C 1S. The presence of elemental forms of Fe and C validates
Fig. 2. Transmission electron micrograph of iron carbide (Fe3C) (a) bright field image of the sample and the inset shows selected area electron diffraction (SAED) pattern and (b) histogram for particle size distribution. 253
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Fig. 3. (a) XPS spectra of Fe3C MNPS (b) Fe 2p3/2, Fe 2p1/2 (c) C 1S and (d) O 1S.
Fig. 5. M vs. H curve for Fe3C magnetic nanoparticles. Fig. 4. Mössbauer spectrum of Fe3C nanoparticles recorded at room temperature.
To build a hydrophilic bilayer coating over the MNPS, it was continuously stirred in excess oleic acid for 24 h at 70 °C. The coated nanoparticles were collected through the magnet and then dispersed in distilled water which provides homogeneous ferrofluid of these MNPs. It was observed that the ferrofluid with the higher concentration of MNPs had lower stability as compared to that of the fluids with lower concentrations. The time dependent calorimetric measurements were performed for the Fe3C based ferrofluids to examine their heating efficacy at two different AC magnetic fields. The heating rates in terms of temperature
These values suggest that the material has a soft ferromagnetic nature. 4.6. Heating ability The aqueous ferrofluids of oleic acid functionalized Fe3C nanoparticles was found to be stable for a few days. The earlier studies infer that the monolayer coating of oleic acid facilitates hydrophobic nature in contrast the bilayer coatings provide hydrophilic behavior [37–39].
Table 2 Values of hyperfine parameters (Hf, Δ, δ, and the relative area) deduced from the fitting of the room temperature. Sample
Iron site
Hyperfine Field (Hf) (T) ± 0.01
Quadruple Splitting, Δ mm/s ± 0.005
Isomer shift, δ ± 0.01 mm/s
Line width, г mm/s ± 0.02
Relative area (RA) ± 1%
Quality of fitting χ2
Fe3C
Sextet A Doublet
20.91 –
0.026 0.96
0.135 0.272
0.355 –
94 6
1.37
254
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Fig. 6. Temperature vs. Time curves at (a) 23 mT and 261 kHz, (b) 25 mT and 117 kHz, (c) SAR values for ferrofluids and (d) ILP values for the ferrofluids having different concentrations (e. g. 2.5, 5, 10 and 20 mg/mL) at different fields.
vs. time curves are shown in Fig. 6(a) & (b). At a field of amplitude 23 mT and frequency 261 kHz, the ferrofluids of all concentration viz. 2.5, 5, 10, and 20 mg/mL displayed continuous rise in temperature as shown in Fig. 6(a). The time required to reach the therapeutic temperature (i. e. 42 °C) was 142, 328, 330 and 351 s for the ferrofluids having concentrations of 20, 10, 5, and 2.5 mg/mL of MNPs respectively (Fig. 6a). However, the rate of heating for the ferrofluids was almost similar for all the concentrations except for 20 mg/mL. Similarly, at another field (i.e 25 mT and 117 kHz), the ferrofluids with different concentrations of MNPs exhibited temperature rise beyond therapeutic temperature except the one with a concentration of 2.5 mg/ mL (Fig. 6(b)). The time required to attain 42 °C by the ferrofluids was 304, 384, and 466 s for the concentration of 20, 10 and 5 mg/mL. The ferrofluids at 25 mT field have displayed slightly better initial heating rate for higher concentration of MNPs (Fig. 6(b)) The duration to reach the desired hyperthermia temperature was more at the latter field though its amplitude was higher. Thus, it was in accordance with the earlier observations with iron oxide based MNPs which suggest that heating ability of the MNPs not only depends on the amplitude of applied field but also varies with the frequency of the field, structural as well as the magnetic behavior of MNPs, suspending medium and the functional materials [37–41]. The SAR values for the ferrofluids having different amounts of MNPs and at different fields are plotted in Fig. 6(c). The maximum value of SAR was 85 W/g for 2.5 mg/mL at 23 mT field whereas it was 33 W/g for 5 mg/mL at a field of 25 mT (Fig. 6(c). In our earlier studies, Zr0.01Fe2.99O4 (MS ∼ 49 Am2/kg) ferrofluid displayed the SAR value of 16.5 W/g for 10 mg/mL concentration at 23 mT. For the similar concentration of MNPs, the present sample shown a SAR value of 23.3 W/g at 23 mT field [38]. The higher value of SAR for carbide sample may be attributed to its higher magnetization value. Nevertheless, the better conductivity of carbide might have also contributed to the heating. At both the fields, though the rate of heating was more for the ferrofluids having higher concentrations the SAR values had a reverse trend. It means that the SAR values found to be reduced with an improved
concentration of MNPs (Fig. 6c). This may be accomplished with the lower concentration of MNPs which was used as denominator value while calculating the SAR values. Similar behavior was also noted for Zr-substituted magnetite samples [42]. Nevertheless, the obtained SAR values were found to be more, comparable and lesser than the values reported for pure and substituted iron oxide based ferrofluids [19,40–46]. The ILP values for the ferrofluids at both the fields have similar trends as that of the SAR values (Fig. 6 c and d) i.e. it was decreasing with enhanced MNPs weight. The highest ILP value at 23 mT field was 0.97 nHm2/kg for the ferrofluid of concentration 2.5 mg/mL MNPs whereas it was 0.71 nHm2/kg for 5 mg/mL ferrofluid at 25 mT. It has been observed that the ferrofluid with 2.5 mg/mL of MNPs did not achieve therapeutic temperature at 25 mT and its rate of heating was lowest. Hence, its SAR and ILP values were minimum at this field as these values are function of initial slope of temperature vs. time curve (Fig. 6(b)). At 23 mT field, the same ferrofluid exhibited similar heating rate as that of the ferrofluids having higher concentrations of MNPs and hence SAR and ILP values were also higher (Fig. 6(a), (c) and (d)). This may be attributed to the higher frequency of the external AC field of amplitude 23 mT and 261 kHz. However, the exact reasons for such behavior for this ferrofluid (2.5 mg/mL) at 25 mT are not known to us. The values of ILP were found to be comparable with that of iron oxides and thus this material could be explored further for its suitability for magnetic hyperthermia or other biomedical applications [33,34,46]. 5. Conclusions The single phasic Fe3C nanoparticles obtained by sol-gel process had particles size between 19 and 34 nm. The XRD and TEM studies were used to get this conclusion. The XPS study confirms the presence of only Fe and C in the sample which is also supported by Mössbauer spectroscopy. The values of MS, HC, and Mr were found to be 88 Am2/kg, 17.3 mT and 6 Am2/kg, respectively. The ferrofluids having higher contents of MNPs have shown a higher rate of heating at both the fields. The SAR and ILP values were high at lower concentrations of MNPs. 255
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The highest obtained values of SAR and ILP were 85 W/g and 0.97 nHm2/kg respectively which are comparable to that of the values for iron oxides magnetic materials.
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Research articles
Fe3C nanoparticles for magnetic hyperthermia application a
a
b
c,d
c
A. Gangwar , S.S. Varghese , Sher Singh Meena , C.L. Prajapat , Nidhi Gupta , N.K. Prasad
a,⁎
T
a
Department of Metallurgical Engineering, Indian Institute of Technology (Banaras Hindu University), Varanasi 221005, India Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India Technical Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India d Homi Bhabha National Institute, Anushaktinagar, Mumbai 400094, India b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Iron carbide Nanoparticles Magnetic hyperthermia Heating ability
Fe3C magnetic nanoparticles were synthesized by solvothermal assisted technique. The X-ray and electron diffractions have confirmed the formation of monophasic iron carbide (Fe3C) nanoparticles. Transmission electron microscope analysis validated that the size of the particles was around 19–34 nm. The presence of only Fe and C in Fe3C was asserted by X-ray photoelectron spectroscopy (XPS). The room temperature Mössbauer spectroscopy also corroborates the occurrence of only this carbide phase. The saturation magnetization at ± 2 T and at 300 K was obtained to be around 88 Am2/kg with a coercivity of 17.3 mT and remanence of 6 Am2/kg. The oleic acid based ferrofluid of this carbide nanoparticles exhibited good heating ability in the presence of external alternating current (AC) magnetic fields. The optimum specific absorption rate and intrinsic loss power values were 85 W/g and 0.97 nHm2/kg at 23 mT and 261 kHz. Due to lack of reports on magnetic hyperthermia response for iron carbides, the values were compared with iron oxides. The values were found to be comparable.
1. Introduction Several forms of iron carbides such as Fe3C, Fe5C2, Fe7C3, Fe2.2C etc. are reported in the literature. Out of these carbides, Fe3C (cementite/ cohenite) has great technological importance [1,2]. This intermetallic compound has a stoichiometric composition of 6.67% carbon and 93.3% iron (Fe) by weight [3]. It has an orthorhombic crystal structure, high melting point (1837 °C), non-pyrophoric and strong magnetic (saturation magnetization, MS, for bulk ∼140 Am2/kg) behavior [4]. It is normally classified as a ceramic due to its high hardness and brittleness [1]. It is frequently found as an important constituent in ferrous metallurgy (in most steels and cast irons). The use of iron carbide during the production of steels and cast irons is remarkably environmental friendly because of its lowest C emission as compared to that of other virgin iron and steel making processes [5]. In addition, the iron carbide is much more effective and less costly than any other means to produce high-quality steel. This carbide can also be used as magnetic recording material due to its high magnetic strength and suitable Curie temperature (210 °C) [6]. The biomedical applications of magnetic nanoparticles (MNPS) have become prominent topics for researches in the modern century. These applications are referred to as drug delivery, magnetic resonance imaging (MRI), magnetic hyperthermia treatment (MHT) etc. [7–9]. For such applications, the scientists have employed both ferromagnetic ⁎
(metal/alloys e.g. Fe, FePt etc.) and ferrimagnetic nanoparticles (pure or substituted magnetite, Fe3O4 or maghemite, γ- Fe2O3) [10]. The former metallic materials though have higher MS values (∼220 Am2/kg for bulk Fe) but the toxic nature limits their suitability [7]. The encapsulation of these metallic nanoparticles by the inert materials like platinum, gold, silver or silica etc. could enhance their compatibility but at the expense of enhanced size and reduced MS values [11]. Further, such nanolayer coatings are inhomogeneous, costly and tiresome to perform. In contrast, the magnetic iron oxides have adequate biocompatibility and are being utilized with or without such coatings [12]. Nonetheless, these oxides have inferior MS values compared to that of the metallic ones (e.g. 90 Am2/kg for bulk Fe3O4) [13]. In contrast, the iron carbide (e.g. Fe3C) found to display MS value considerably higher than that of its oxides counterpart and presumed to have better biocompatibility than metallic ones [14]. Due to its higher MS value, the actual materials required to get the therapeutic temperature (42–46 °C) during magnetic hyperthermia (MHT) treatment may be lesser than its oxide counterparts [15]. Further, the magnetic iron oxides found to get transformed into antiferromagnetic oxide with time in some of the carrier fluids [16]. Such conversion of iron carbide may not happen in fluids [17]. The synthesis route and the properties of iron oxide particles have been investigated to a large extent and thus there is less probability to get significant improvement in their properties [18]. Hence, the researchers are looking for the other materials.
Corresponding author. E-mail address: [email protected] (N.K. Prasad).
https://doi.org/10.1016/j.jmmm.2019.03.028 Received 19 December 2018; Received in revised form 6 February 2019; Accepted 5 March 2019 Available online 06 March 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.
Journal of Magnetism and Magnetic Materials 481 (2019) 251–256
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PHI) with Al Kα X-ray beam (1486.61 eV) of the dual anode. The baseline was executed using a Shirley background function and the peaks deconvolution and quantification was done using the origin software. Magnetization vs. field up to ± 2 T was carried out using SQUID (MPMS-XL, Quantum Design) to ascertain the MS, Mr, and HC values. Mössbauer spectrum was collected at room temperature from a spectrometer in transmission geometry and in the constant acceleration mode of the triangular waveform. A 57Co in Rh matrix was used as a Mössbauer source. A thin foil of α-Fe was used as a standard sample to calibrate the spectrometer for velocity scale and for the estimation of isomer shift value. The experimental data were fitted using a least square curve fitting model using a WinNormos site fit program. Heating ability of Fe3C based ferrofluids was determined using Magnetherm (Nano Therics, U. K.) in various fields. The temperature rise of the ferrofluids during the experiment was recorded up to 60 °C. The temperature was measured using an optical fiber probe with an accuracy of 0.1 °C (Lumasense technologies). The specific absorption rate (SAR, W/g) values were estimated using the formula [34]:
The iron carbides have become an attractive alternative owing to their aforesaid properties [11]. Additionally, iron carbides have a high electrical conductivity which may further add to its heating ability during MHT experiments [19]. However, to the best of our knowledge there is no report in the literature where magnetic hyperthermia response of this carbide is evaluated. Thus, we took up the work and explore the heating behavior of Fe3C nanoparticles in the presence of two different AC magnetic fields. There are a few reports on the synthesis of Fe3C nanoparticles [13,20–22]. However, to get this carbide the scientists generally used Fe(CO)5 as a precursor which is not readily available [2,23,24]. Moreover, the researchers also get other byproducts such as carbides/ nitrides/oxides of iron or even pure Fe along with this carbide [10,20,25,26]. The common techniques described for its production are flame spray pyrolysis, Fischer – Tropsch synthesis [27–29], laser ablation method [7,30], sol-gel [20,31], solvothermal [18,32,33] etc. In the present work, we have synthesized monophasic Fe3C nanoparticles by solvothermal route followed by calcination as suggested by Lei et al. [18]. The structural, morphological, as well as magnetic characterization of the sample, were carried out by the conventional techniques. The heating ability for this carbide nanoparticles was assessed in the presence of AC magnetic fields.
SAR =
Cp dT ∗ m dt
where Cp is the specific heat capacity of water 4.186 Jg−1K−1, m is the concentration of Fe3C MNPS per unit volume of solvent, and dT/dt is the initial slope obtained from the temperature vs. time curves between 30 and 35 °C using the linear fitting method. The intrinsic loss power (ILP, in nHm2kg−1) values for the ferrofluids at various fields were calculated by following formula [5,35]:
2. Materials and method The synthesis of Fe3C, MNPS was carried out in two steps. The details about the process may be found elsewhere [18]. In brief, initially, anhydrous FeCl3 (1.2 g) and hexamethylenetetramine (HMTA, 4.2 g) were dissolved into the solution of ethylene glycol and double distilled water (DDW) having equal volumes (20 mL). The resulting solution was heated in a Teflon lined autoclave (40 mL) at 110 °C for 6 h. After washing the precipitate with DDW and ethanol, it was heated at 70 °C for 12 h in a vacuum furnace. This was followed by keeping the grounded powder and ethylenediamine (6 mL) in separate boats for calcination at 650 °C for 1.5 h in the N2 atmosphere. Further, the sample was kept in 30% H2O2 in DDW for 48 h to remove any adsorbed carbon. The obtained powder sample was crushed by a pestle mortar and then sonicated in water for 20 min using a Probe sonicator of PKS 250FM (PCI Analytical) to lessen the size of the particles and to facilitate its stabilization in a liquid medium. Following the removal of water, the particles were dispersed in oleic acid and then kept at 70 °C under continuous stirring for 12 h. Subsequently, the oleic acid coated MNPs were collected using a hard magnet. These MNPS were dispersed in water and then exposed to a high-frequency utrasonicator bath of power 50 W, 40 kHz for 30 min to get the suspension of MNPs i.e. ferrofluid. The ferrofluids had concentrations of 2.5, 5, 10 and 20 mg/mL of MNPs.
ILP =
P SAR = 2 ρH 2f H f
where P/ρ, is SAR values, H and f are the amplitude and frequency of fields respectively. 4. Results and discussion 4.1. XRD analysis Fig. 1 exhibits the Rietveld refined XRD pattern after analysis for Fe3C sample which was synthesized via the two step solvothermal assisted method. The peaks of the pattern were matched with JCPDS no. 89-7271 for cohenite (Fe3C). The Rietveld refinement of the pattern was performed by Fullprof software which suggests an orthorhombic crystal structure for it. The detailed data after refinement is listed in Table 1.
3. Characterizations For structural and phase analysis, XRD pattern was recorded with Cu Kα radiation (λ = 1.54056 Å) using an X-ray powder diffractometer (BT- Rigaku Miniflex) range between 25 and 90° at room temperature. High-resolution transmission electron microscope (TEM, TECNAI-20 G2) was adopted to observe the morphology and size of the particles. The distribution for the particle size over the histogram was drawn using “Standard Normal Distribution” fitting and it was calculated by the formula given below:
Z= (x − μ x)/σx where x is the value which is being standardized, µx is the mean of the distribution and σx standard deviation of the distribution. Selected area electron diffraction (SAED) pattern was obtained to identify the crystal structure. To find out the elements present in the Fe3C system, the X-ray photoelectron spectroscopy (XPS) was carried out on a (PHI5000 Versaprobe II photoelectron spectrometer ULVAC-
Fig. 1. Rietveld refined XRD Patterns of nanocrystalline iron carbide (Fe3C) sample. 252
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Table 1 Summary of the Rietveld refined structural parameters of the orthorhombic iron carbide nanoparticles. Space group
Lattice parameter a, b, c α, β, γ
Unit cell volume (Å3) V
Density (g/cm3)
Atomic position Fe1 (x, y, z)
Atomic position Fe2
Atomic position C1
Atomic occupancy (site) Fe1 (8d), Fe2 (4c), C1 (4c)
Bragg-R Factor, RF-Factor, χ2
Pnma 62
5.0829 6.7431 4.526 90, 90, 90
155.14
10.592
0.197 0.0716 0.3531
0.521 0.25 0.82
0.89 0.25 0.248
2, 1, 1
24.4 13.5 1.27
Symmetry multiplicity and Wyckoff symbol of Fe1, Fe2, C1: 8d, 4c, 4c.
the formation of pure Fe3C phase in the sample [36]. The spectrum for O 1S may be attributed to the adsorbed oxygen on the surface of nanosized carbide [37].
The calcination temperature and the holding time were found to be significantly important as the other temperature would not produce pure Fe3C phase [18,25]. Moreover, the proportions of the precursors (e.g. HMTA, ethylenediamine, and FeCl3) were also crucial in getting the single phase Fe3C [20,25]. The excess amount of HMTA resulted in iron whereas the lesser concentration induced other phases like iron oxides and nitride [18]. Hence, an optimized concentration of precursors and thermal treatment facilitated the formation of Fe3C single phase which was also observed in an earlier study. The crystallite size of the material was found to be around 20 nm by the Scherrer equation.
4.4. Mössbauer spectroscopy Fig. 4 shows the Mössbauer spectrum of Fe3C sample at room temperature which was fitted with a sextet (six lines Zeeman splitting pattern) and a doublet. The values for the magnetic hyperfine field (Hf), quadruple splitting (δ), isomer shift (Δ), the line width (г), and relative area (RA) are summarized in table 2. The sextet has BHF value of 20.92 T which is due to the positions of Fe in the orthorhombic structure of Fe3C. The reported Hf values for bulk Fe3C are 20.9 and 20.6 T [6]. The two different values of Hf are attributed to the two inequivalent sites for Fe atoms in Fe3C. Nevertheless, it has also been observed that the two sextets may get superimposed due to a very small difference in their local magnetic fields (1%) [6]. For the present sample, the spectrum shows a better fit with only one sextet. The isomer shift value for the sextet is found to be 0.167 mm/s. In contrast, the reported value of the chemical shift found to be ∼0.18 mm/s [35]. The slight difference in the value might be due to the different synthesis protocol as well as the nano dimension of the present material. The doublet obtained in the Mössbauer spectrum (6%) indicates the presence of a superparamagnetic component in the material associated with the small nanosized particles.
4.2. TEM analysis The microstructure of the sample as collected from TEM is shown in Fig. 2(a) which infers that particles were nearly spherical in shape. The inset of this micrograph shows the SAED pattern validating its polycrystalline nature. The pattern was indexed with the help of eRing software and was resembling the Fe3C phase. This was in accordance with the XRD pattern (Fig. 1). The treatment of the sample with 30% H2O2 for 48 h could remove physically adsorbed carbon and hence no trace of it was observed in the TEM micrograph (Fig. 2a). The particles size distribution is presented as a histogram in Fig. 2(c). The particles size was calculated using ImageJ software by selecting around 400 particles (Fig. 2a). The size was found to be in the range of 19–34 nm and the average size of the particles was ∼(22 ± 4) nm. It was nearly equal to that of the crystallite size.
4.5. Magnetic properties 4.3. XPS analysis The magnetizations vs. field curve for Fe3C sample is shown in Fig. 5 which was measured at 300 K and up to ± 2 T. The MS value was found to be around 88 Am2/kg which was less than its bulk (∼140 Am2/kg) and reported value (123 Am2/kg) [4]. Nevertheless, the value was relatively higher than that of nanoparticles of most of the iron oxides [39]. The lesser MS value for the sample could be attributed to the smaller size of the particles. The remanent (Mr) and coercivity (HC) values for the sample are found to be 6 Am2/kg, 17.3 mT respectively.
XPS analysis of the sample was performed to investigate the surface properties of Fe3C MNPs. It suggested that the sample contained only Fe, C, and O and their respective spectrum are shown in Fig. 3. The XPS spectrum of Fe 2p exhibited two strong peaks at 724.6 eV and 711.3 eV as shown in Fig. 3(b). These peaks could be ascribed to the Fe 2p1/2 and Fe 2p3/2 binding energies. One single peak at 284.42 eV was observed for C 1S. The presence of elemental forms of Fe and C validates
Fig. 2. Transmission electron micrograph of iron carbide (Fe3C) (a) bright field image of the sample and the inset shows selected area electron diffraction (SAED) pattern and (b) histogram for particle size distribution. 253
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Fig. 3. (a) XPS spectra of Fe3C MNPS (b) Fe 2p3/2, Fe 2p1/2 (c) C 1S and (d) O 1S.
Fig. 5. M vs. H curve for Fe3C magnetic nanoparticles. Fig. 4. Mössbauer spectrum of Fe3C nanoparticles recorded at room temperature.
To build a hydrophilic bilayer coating over the MNPS, it was continuously stirred in excess oleic acid for 24 h at 70 °C. The coated nanoparticles were collected through the magnet and then dispersed in distilled water which provides homogeneous ferrofluid of these MNPs. It was observed that the ferrofluid with the higher concentration of MNPs had lower stability as compared to that of the fluids with lower concentrations. The time dependent calorimetric measurements were performed for the Fe3C based ferrofluids to examine their heating efficacy at two different AC magnetic fields. The heating rates in terms of temperature
These values suggest that the material has a soft ferromagnetic nature. 4.6. Heating ability The aqueous ferrofluids of oleic acid functionalized Fe3C nanoparticles was found to be stable for a few days. The earlier studies infer that the monolayer coating of oleic acid facilitates hydrophobic nature in contrast the bilayer coatings provide hydrophilic behavior [37–39].
Table 2 Values of hyperfine parameters (Hf, Δ, δ, and the relative area) deduced from the fitting of the room temperature. Sample
Iron site
Hyperfine Field (Hf) (T) ± 0.01
Quadruple Splitting, Δ mm/s ± 0.005
Isomer shift, δ ± 0.01 mm/s
Line width, г mm/s ± 0.02
Relative area (RA) ± 1%
Quality of fitting χ2
Fe3C
Sextet A Doublet
20.91 –
0.026 0.96
0.135 0.272
0.355 –
94 6
1.37
254
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Fig. 6. Temperature vs. Time curves at (a) 23 mT and 261 kHz, (b) 25 mT and 117 kHz, (c) SAR values for ferrofluids and (d) ILP values for the ferrofluids having different concentrations (e. g. 2.5, 5, 10 and 20 mg/mL) at different fields.
vs. time curves are shown in Fig. 6(a) & (b). At a field of amplitude 23 mT and frequency 261 kHz, the ferrofluids of all concentration viz. 2.5, 5, 10, and 20 mg/mL displayed continuous rise in temperature as shown in Fig. 6(a). The time required to reach the therapeutic temperature (i. e. 42 °C) was 142, 328, 330 and 351 s for the ferrofluids having concentrations of 20, 10, 5, and 2.5 mg/mL of MNPs respectively (Fig. 6a). However, the rate of heating for the ferrofluids was almost similar for all the concentrations except for 20 mg/mL. Similarly, at another field (i.e 25 mT and 117 kHz), the ferrofluids with different concentrations of MNPs exhibited temperature rise beyond therapeutic temperature except the one with a concentration of 2.5 mg/ mL (Fig. 6(b)). The time required to attain 42 °C by the ferrofluids was 304, 384, and 466 s for the concentration of 20, 10 and 5 mg/mL. The ferrofluids at 25 mT field have displayed slightly better initial heating rate for higher concentration of MNPs (Fig. 6(b)) The duration to reach the desired hyperthermia temperature was more at the latter field though its amplitude was higher. Thus, it was in accordance with the earlier observations with iron oxide based MNPs which suggest that heating ability of the MNPs not only depends on the amplitude of applied field but also varies with the frequency of the field, structural as well as the magnetic behavior of MNPs, suspending medium and the functional materials [37–41]. The SAR values for the ferrofluids having different amounts of MNPs and at different fields are plotted in Fig. 6(c). The maximum value of SAR was 85 W/g for 2.5 mg/mL at 23 mT field whereas it was 33 W/g for 5 mg/mL at a field of 25 mT (Fig. 6(c). In our earlier studies, Zr0.01Fe2.99O4 (MS ∼ 49 Am2/kg) ferrofluid displayed the SAR value of 16.5 W/g for 10 mg/mL concentration at 23 mT. For the similar concentration of MNPs, the present sample shown a SAR value of 23.3 W/g at 23 mT field [38]. The higher value of SAR for carbide sample may be attributed to its higher magnetization value. Nevertheless, the better conductivity of carbide might have also contributed to the heating. At both the fields, though the rate of heating was more for the ferrofluids having higher concentrations the SAR values had a reverse trend. It means that the SAR values found to be reduced with an improved
concentration of MNPs (Fig. 6c). This may be accomplished with the lower concentration of MNPs which was used as denominator value while calculating the SAR values. Similar behavior was also noted for Zr-substituted magnetite samples [42]. Nevertheless, the obtained SAR values were found to be more, comparable and lesser than the values reported for pure and substituted iron oxide based ferrofluids [19,40–46]. The ILP values for the ferrofluids at both the fields have similar trends as that of the SAR values (Fig. 6 c and d) i.e. it was decreasing with enhanced MNPs weight. The highest ILP value at 23 mT field was 0.97 nHm2/kg for the ferrofluid of concentration 2.5 mg/mL MNPs whereas it was 0.71 nHm2/kg for 5 mg/mL ferrofluid at 25 mT. It has been observed that the ferrofluid with 2.5 mg/mL of MNPs did not achieve therapeutic temperature at 25 mT and its rate of heating was lowest. Hence, its SAR and ILP values were minimum at this field as these values are function of initial slope of temperature vs. time curve (Fig. 6(b)). At 23 mT field, the same ferrofluid exhibited similar heating rate as that of the ferrofluids having higher concentrations of MNPs and hence SAR and ILP values were also higher (Fig. 6(a), (c) and (d)). This may be attributed to the higher frequency of the external AC field of amplitude 23 mT and 261 kHz. However, the exact reasons for such behavior for this ferrofluid (2.5 mg/mL) at 25 mT are not known to us. The values of ILP were found to be comparable with that of iron oxides and thus this material could be explored further for its suitability for magnetic hyperthermia or other biomedical applications [33,34,46]. 5. Conclusions The single phasic Fe3C nanoparticles obtained by sol-gel process had particles size between 19 and 34 nm. The XRD and TEM studies were used to get this conclusion. The XPS study confirms the presence of only Fe and C in the sample which is also supported by Mössbauer spectroscopy. The values of MS, HC, and Mr were found to be 88 Am2/kg, 17.3 mT and 6 Am2/kg, respectively. The ferrofluids having higher contents of MNPs have shown a higher rate of heating at both the fields. The SAR and ILP values were high at lower concentrations of MNPs. 255
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The highest obtained values of SAR and ILP were 85 W/g and 0.97 nHm2/kg respectively which are comparable to that of the values for iron oxides magnetic materials.
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