Characterization of magnetic NiFe nanoparticles with controlled bimetallic composition

Characterization of magnetic NiFe nanoparticles with controlled bimetallic composition

Journal of Alloys and Compounds 587 (2014) 260–266 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.e...

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Journal of Alloys and Compounds 587 (2014) 260–266

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom

Characterization of magnetic NiFe nanoparticles with controlled bimetallic composition Yan Liu a,b,⇑, Yanxiu Chi a, Shiyao Shan b, Jun Yin b, Jin Luo b, Chuan-Jian Zhong b,⇑ a b

College of Materials and Metallurgy, Northeastern University, Shenyang, Liaoning 110004, China Department of Chemistry, State University of New York at Binghamton, Binghamton, NY 13902, USA

a r t i c l e

i n f o

Article history: Received 27 May 2013 Received in revised form 22 October 2013 Accepted 27 October 2013 Available online 1 November 2013 Keywords: Metal nanoparticles Magnetic nanoparticles Bimetallic alloys Bimetallic composition Spinel structure Magnetic properties

a b s t r a c t The exploration of the magnetic properties of bimetallic alloy nanoparticles for various technological applications requires the ability to control the morphology, composition, and surface properties. In this report, we describe new findings of an investigation of the morphology and composition of NiFe alloy nanoparticles synthesized under controlled conditions. The controllability over the bimetallic composition has been demonstrated by the observation of an approximate linear relationship between the composition in the nanoparticles and in the synthetic feeding. The morphology of the NiFe nanoparticles is consistent with an fcc-type alloy, with the lattice strain increasing linearly with the iron content in the nanoparticles. The alloy nanoparticles exhibit remarkable resistance to air oxidation in comparison with Ni or Fe particles. The thermal stability and the magnetic properties of the as-synthesized alloy nanoparticles are shown to depend on the composition. The alloy nanoparticles have also be sown to display low saturation magnetization and coercivity values in comparison with the Ni nanoparticles, in line with the superparamagnetic characteristic. These findings have important implications for the design of stable and controllable magnetic nanoparticles for various technological applications. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Magnetic nanoparticles have found increasing applications in many technological areas such as high-density information storage [1–3], catalysis [4,5], chemical sensors [6–8], and biological targeting or separation [9–11]. With the expanded applications of magnetic materials in these areas, there is an increasing demand for the preparation of magnetic nanoparticles with controllable compositions and structures. In contrast to conventional magnetic metals and metal oxides, e.g., Fe, Ni, Fe2O3, and Fe3O4 [12,13], NiFe nanoparitcles have attracted recent interests because of many potential applications. NiFe nanoparticles have been prepared by a number of methods [14–38]. For example, Fe–Ni alloys were prepared by evaporating starting materials in an inert gas atmosphere at temperatures over 1600 °C [14]. The particles prepared with 36% Fe displayed particle sizes ranging from 20 to 100 nm and composition consisting of a single phase of ferrite or austenite. Li et al. [15] demonstrated the synthesis of Fe–Ni alloy nanoparticles by hydrogen plasma reaction, showing spherical Fe–Ni nanoparticles ⇑ Corresponding authors. Address: Department of Chemistry, State University of New York at Binghamton, Binghamton, NY 13902, USA (C.-J.Zhong). Tel.: +1 607 777 4605. E-mail addresses: [email protected] (Y. Liu), [email protected] (C.J. Zhong). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.10.203

with a mean particle size less than 35 nm. Dong et al. [16] synthesized Fe–Ni nanoparticles with sizes ranging from 19 to 34 nm using a hydrogen plasma reaction method in a mixture of H2 and Ar. Suh et al. [17] synthesized crystalline Fe–Ni nanoparticles with a narrow size distribution by reducing mixed NiCl2 and FeCl2 vapors with hydrogen, in which the particle sizes were controlled by changing evaporator temperature, reaction zone temperature, and total gas flow rate. Gurmen et al. [18] reported the synthesis of nanocrystalline spherical Fe–Ni alloy particles using ultrasonic spray pyrolysis – H2 reduction method. Liu et al. [19] and Azizi and Sadrnezhaad [20] prepared Fe–Ni nanoparticles by mechanical alloying and subsequent low-temperature hydrogen reduction treatment. Gheisari et al. [21,22] reported the fabrication of Fe– Ni nanoparticles by mechanical alloying using pure powders of Fe and Ni in a dry argon atmosphere. In these reports, the magnetic properties of the nano alloys were shown to depend on the chemical composition and the grain size, but limited information is available about the ability to control the composition and size. Wet chemical method has recently attracted increasing interests for the preparation of metal and alloy nanoparticles. This method involves chemical reaction in organic solvents or aqueous solutions. The synthetic routes in organic solvents involve coreduction of Fe and Ni salts in organic media using reducing agents compatible with organic media, such as hydrogen gas, superhydride (LiBEt3H), and 1,2-hexadecandiol. For example, Margeat

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et al. [23] recently reported the synthesis of Fe–Ni nanoparticles via decomposition of Ni[(COD)2] (COD = 1,5-cyclooctadiene) and Fe[{N(SiMe3)2}2] in the presence of surfactants and hydrogen gas. Tai et al. [24] also obtained FeNi3 nanoparticles via a non-aqueous synthetic route using 1,2-hexadecandiol as a reducing agent. Yang et al. [25] and Chen et al. [26] reported the synthesis of NiFe nanoparticles via thermal decomposition of Ni(II) acetylacetonate and Fe(III) acetylacetonate in oleylamine. In another study by Qin et al. [27], Ni80Fe20 alloy nanoparticles were synthesized and the particle sizes were controlled in the size range of 20–440 nm by reduction of nickel and iron slats using polyol at 180 °C. Yan et al. [28] reported a method to synthesize Fe1xNix (x = 0–1) nano alloys in aqueous solutions, and demonstrated their use as catalysts for H2 generation from aqueous NH3BH3 solution under ambient atmosphere. Moustafa et al. [29] synthesized nanosized Fe–Ni alloys with 20 wt% Fe and 80 wt% Ni in alkaline tartarate solution using hypophosphite as a reducing agent. A key issue for using borohydrides and hypophosphite as a reductant is the inevitable presence of B and P in the NiFe nanoparticles since they easily form compounds with Ni or Fe. Hydrazine hydrate has been successfully used as reducing agent for the syntheses of a variety of nanoscale metal materials because it has a strong reducing ability and a low concentration of impurities. It was employed to prepare metal iron [30], nickel [31] and alloys such as Cu–Ni [32] and Fe–Ni. Liao et al. [33] synthesized FeNi3 alloy nanoparticles by reducing Ni(NO3)2 and Fe(NO3)2 with hydrazine in strong alkaline media via hydrothermal reaction. In addition, the preparation of iron–nickel nanocrystalline alloys on a carbon steel substrate in a chloride bath with different Ni/Fe ion ratios using electrodeposition method was also reported [34]. While these previous studies have demonstrated the viability of synthesizing NiFe nanoparticles by different approaches, few has demonstrated the ability to control precisely the morphological and surface properties of the bimetallic nanoparticles, especially in terms of their relationship with the composition and size. In this report, we describe the findings of an investigation of the detailed morphology, composition, and size of NiFe alloy nanoparticles synthesized under well controlled conditions.

2. Experimental methods 2.1. Chemicals Ferrous sulfate (FeSO47H2O, 99.5%), nickel(II) chloride (NiCl2), sodium citrate (Cit, 99%), sodium hydroxide (NaOH), hydrazine hydrate (H2NNH2H2O) and glycol ethylene (HOCH2CH2OH) were obtained from Aldrich. All chemicals were used as received.

2.2. Synthesis The NiFe nanoparticles were synthesized using hydrothermal method [33,39]. Typically, FeSO47H2O and NiCl2 was dissolved into 100 mL deionized water in certain molar ratio of Ni to Fe. 8 mL of 1 M NaOH was added to the solution to adjust pH to around 5.8. 0.6 mL glycol ethylene was used as surfactant and 4 mL of hydrazine hydrate was dropped into the solution as reducing agent. The mixture solution was transferred to an autoclave after being stirred well under N2. The autoclave was sealed and put into a furnace which was preheated to 120 °C. The autoclave was cooled to room temperature after heating for 19 h. NiFe nanoparticles were separated with magnetic bar from the solution. The products obtained were cleaned and re-dispersed in deionized water and ethanol several times. There were several differences of the synthesis from the literature report on reduction of metals using hydrozine [33]. First, instead of pH 11 (alkaline condition), a moderate pH value 5.8 was used. Secondly, instead of sodium dodecyl sulfate (SDS) as surfactant, sodium citrate and glycol ethylene were used as surfactants. Thirdly, the autoclave reaction was carried out as 120 °C, which is lower than that in the literature (180 °C). In terms of the amount of base used for the reaction, we choose a moderate pH value at the beginning at which the iron ions should have been reduced in the hydrothermal system. However, the metals are active and unstable, especially at a high-temperature base solution where Fe3O4 could be formed.

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2.3. Measurement and characterization 2.3.1. Transmission electron microscopy (TEM) TEM was performed on Hitachi H-7000 Electron Microscope (100 kV). The nanoparticle samples dispersed in water solution were cast onto a carbon-coated copper grid sample holder followed by evaporation at room temperature. 2.3.2. X-ray powder diffraction (XRD) The products were identified by powder X-ray diffraction. Powder diffraction patterns were recorded on a Scintag XDS 2000 h–h powder diffractometer equipped with a Ge(Li) solid state detector (Cu Ka radiation). The data was collected from 2h = 5° to 90° at a scan rate of 0.02° per step and 5 s per point. 2.3.3. Inductively-coupled plasma optical emission spectroscopy (ICP-OES) Acid digestates were analyzed using ICP-OES, which was performed using a Perkin Elmer 2000 DV ICP-OES utilizing a cross flow nebulizer with the following parameters: plasma 18.0 L Ar(g)/min; auxiliary 0.3 L Ar(g)/min; nebulizer 0.73 L Ar(g)/min; power 1500 W; peristaltic pump rate 1.40 mL/min. Reported values <1.0 mg/L were analyzed using a Meinhardt nebulizer coupled to a cyclonic spray chamber to increase analyte sensitivity at the following parameters: 18.0 L Ar(g)/min; auxiliary 0.3 L Ar(g)/min; nebulizer 0.63 L Ar(g)/min; power 1500 W; peristaltic pump rate 1.00 mL/min. Elemental concentrations were determined by measuring one or more emission lines (nm) to check for interferences. The nanoparticle samples were dissolved in concentrated aqua regia, and then diluted to concentrations in the range of 1–50 ppm for analysis. Multi-point calibration curves were made from dissolved standards with concentrations from 0 to 50 ppm in the same acid matrix as the unknowns. Laboratory check standards were analyzed every 6 or 12 samples, with instrument re-calibration if check standards were not within 5% of the initial concentration. Method detection limits were determined using 1.0 mg/L of analyte in the same acid matrix and are reported in mg/L. Instrument reproducibility (n = 10) determined using 1 mg/L elemental solutions resulted in <2% error for all elements. The metal composition was expressed as the atomic percentage of the element in the ternary catalyst. 2.3.4. Magnetization measurement Magnetic measurement was using Lakeshore Model 7407 Vibrating Sample Magnetometer. 2.3.5. Thermogravimetric analysis (TGA) TGA was performed on TA Instruments SDT Q600. Both nitrogen and air gases were used. The rate of temperature change was 10 °C/min. 2.3.6. UV–Visible spectroscopy (UV–Vis) UV–Vis spectra were acquired with a HP 8453 spectrophotometer. The spectra were collected over the range of 200–1100 nm.

3. Results and discussion 3.1. Morphology, composition and structure The compositions of Ni and Fe in the as-synthesized NiFe nanoparticles with different feeding ratios were determined using ICP-OES method. In Fig. 1, the compositions in the nanoparticles are plotted against the synthetic feeding ratios (e.g., Ni%). In general, the data show an approximate linear relationship between the composition in the nanoparticles and the synthetic feeding. This linearity is excellent when the feeding ratio of Ni to Fe is higher than 3:1, i.e., >60% Ni. It illustrates that stoichiometric reactions take place when the nickel percentage is relatively high in the hydrothermal reaction. The composition NiFe alloy can be controlled by under the experimental conditions. When the feeding ratio of Ni to Fe was <60%, there was a clear deviation from the linearity, suggesting an unfavorable competition for Ni in the bimetallic nanoparticles, which likely reflects the predominance of Fe or Fe3O4 in the nanoparticles. The bimetallic nanoparticles with different compositions were characterized by XRD, and a representative set of data is shown in Fig. 2. There are several important findings. First, the XRD data (curves d, e, f) showed three clear peaks for the bimetallic nanoparticles with high Ni%. These peaks can be indexed to (1 1 1), (2 0 0) and (2 2 0) planes characteristic of an NiFe alloy with face-centered cubic (fcc) type of structure. However, there are subtle differences from the nanoparticles synthesized with different feeding ratios.

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Fig. 1. Correlation between the Ni composition in nanoparticles (from ICP analysis) and the synthetic feeding composition. The blue dash-dot line represents a linear fitting to the experimental data (Curve c: y = 8.58 + 1.07 x, r2 = 0.98). Note that curve a represents an ideal 1-to-1 correlation, whereas curve b reflects the actual correlation. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

For example, the 2h values for (1 1 1) planes are 44.55, 44.26 and 44.22 for Ni, Ni90Fe10, and Ni78Fe22 nanoparticles, respectively. The structural parameters calculated based on peak positions are shown in the Table 1. As shown in Fig. 2B, the lattice constant of

fcc-type NiFe alloy nanoparticles is found to decrease with increasing Ni%, which is in agreement with the fact that the atomic radius of iron is slightly larger than that of nickel. This finding is also consistent with those reported for NiFe alloy nanoparticles prepared by electrochemical deposition. The lattice strain of the deposited fcc type NiFe alloy was shown to increase linearly with increasing the iron content beyond 20 wt% [40]. The reason can be explained by the fact that Fe–Fe bond length (2.7 Å) and Fe–Ni bond length (2.5 Å) are larger than that for Ni–Ni (2.14 Å) [41]. Secondly, the intensity of the (2 0 0) peak decreases with increasing iron composition (curves c, d, e, f) and completely disappears in the higher Fe% region as Ni17Fe83 (curve b) and Fe3O4 (curve a). This finding is slightly different from the NiFe alloys synthesized from electrodeposition [42], in which the intensity of the (2 0 0) peak was found to decrease with increasing iron composition and disappear entirely between 55% and 60% Fe. As shown in Fig. 2B, at above 60 mol% Fe (i.e. below 40 mol% Ni), the deposit is completely bcc with a (1 1 0) texture similar to that reported in other works [42,43]. Thirdly, a close examination of the XRD pattern for Ni39Fe61 (curve c) revealed a new peak at 2h = 35.78 along with several other weak peaks. These peaks correspond to (3 1 1) and other indexes of spinel NixFe3xO4. The highest peak for Ni39Fe61 is at 44.60, which appears to be higher than that of (1 1 1) for fcc Ni and NiFe alloy structure. The lattice constant calculated based on this peak position of bcc structured NiFe alloy is 0.2861 nm, which seems to suggest a bcc type NiFe alloy [42]. While it is possible for the presence of bcc NiFe crystal phase in the alloy structure with the composition of 39%Ni and 61%Fe, we believe that this sample features an fcc structure of NiFe containing

Fig. 2. (A) XRD patterns of samples (a) Fe3O4, (b) Ni17Fe83, (c) Ni39Fe61 (d) Ni78Fe22, (e) Ni90Fe10, and (f) Ni. (Note that the intensities for curves a, b, and c were multiplied a factor of 5 for comparison). (B) Plots of the lattice parameters vs. Ni mol% in the nanoparticles obtained for some of these samples examined.

Table 1 Lattice parameters calculated from XRD data for NiFe nanoparticles with different compositions. Sample composition

Index

2h

d-space (nm)

Ni (f)

111 200 111 200 111 200 311 110 311 311 400

44.55 51.90 44.26 51.60 44.22 51.50 35.78 44.60 35.60 35.45 43.10

0.2025 0.1755 0.2038 0.1764 0.2039 0.1767 0.2500 0.2023 0.2511 0.2522 0.2090

Ni90Fe10 (e) Ni78Fe22 (d) Ni39Fe61 (c) Ni17Fe83 (b) Fe3O4 (a) a

Size calculated based on the peak width using Scherrer equation.

Cal’d sizea (nm)

Lattice constant (nm) (fcc)

(bcc)

(Spinel)

0.3508 0.3509 0.3530 0.3528 0.3533 0.3534 – – – – –

– – – – – – – 0.2861 – – –

– – – – – – 0.8293 – 0.8329 0.8363 0.8360

25.6 – 18.4 – 15.8 – 11.2 – 4.6 18.5 –

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possibly spinel Fe3O4. Note that the apparent decrease in the intensity of (2 0 0) peak is due to band broadening associated with the nanocrystallite size. This was indeed observed by a decrease in size going from (f) to (d) (Table 1). Finally, for NiFe nanoparticles with higher Fe% (curves a, b, c), the intensity of the (3 1 1) peak showed an increase with increasing iron composition and a shift of the peak position. As shown in Table 1 and Fig. 2B, the lattice constant increase with increasing Fe% (curves a, b, c). For sample prepared from pure iron salt, the XRD data (curve a) showed diffraction peaks at 2h = 30.05, 35.45, 43.10, 52.70, 56.85, and 62.50, which can be indexed to (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) planes, respectively. The lattice constant, 0.836 nm, is in a good agreement with our previous report on Fe3O4 nanoparticles [44]. It suggests that only magnetite Fe3O4 was formed using hydrothermal method under this condition. It is evident that the addition of a sufficient amount of Ni in the synthesis reaction enables Ni and Fe to form a relative stable alloy. At an intermediate ratio of nickel to iron, the products (curve c) were found to consist of a mixture of alloy and spinel NixFe3xO4 components. The above observation is also consistent with a previous work [39] in which the iron content was found to increase more than 55 wt%. Fig. 3 shows a representative set of TEM images for the particles synthesized from different initial ratios of Ni to Fe. With a Ni:Fe ratio of 0:1, Fe3O4 nanoparticles were obtained, and the average particle size is found to be 24.0 ± 2.5 nm (Fig. 3A). The average sizes for the particles of Ni17Fe83 and Ni39Fe61 are 5.4 ± 1.0 and 6.9 ± 1.0 nm (Fig. 3B), respectively. Note that these size values are quite close to those calculated based on the peak width using Scherrer equation (see Table 1). The sizes of particles have also examined as the content of Ni is over 75% in the nanoparticles. In this case, the particle sizes appeared to be much larger than those with a lower Ni%. For example, TEM data seemed to indicate different sizes, 1141 ± 116, 751 ± 73, and 863 ± 153 nm for Ni78Fe22, Ni90Fe10, and Ni, respectively. The sizes for the particles with high Ni content were smaller in comparison with those reported for particles synthesized somewhat differently [40]. For the alloys, the grain size increases with increasing Fe% (e.g., Ni78Fe22 (1141 ± 116 nm), Ni90Fe10

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(751 ± 73 nm)). A minimum grain size was found for the mixture of fcc and bcc phases, which is in the range of 60–70 Fe% (wt) [42]. By carefully examining the morphologies, it appears that the Ni78Fe22, Ni90Fe10, and Ni particles consist of aggregates of smaller sized particles, forming spherical larger-sized aggregates. This assessment is supported by the size values calculated based on the peak width using Scherrer equation, which gave 26, 18, and 16 nm, respectively, for Ni78Fe22, Ni90Fe10, and Ni nanoparticles. Fig. 4 shows the trend of the particle size versus the content of Ni in the NPs obtained from both TEM and XRD measurements. It is evident that the size values are largely comparable for samples with <75% Ni between the two measurements. The discrepancy for the samples with >75% Ni is believed to due to the greater propensity of aggregation of the as-formed nanoparticles, which is supported by the study of the suspension properties of the nanoparticles in different aqueous solutions. Note that the lattice parameters for NiFe alloys and Fe2O3 were included in Table 1 only for the purpose of showing the lattice parameters corresponding to their lattice structures, not for the cross comparison.

Fig. 4. Particle size vs Ni% in the nanoparticles determined by TEM (red bars) and XRD (black bars) measurements. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 3. TEM images and size distributions for samples of the nanoparticles syntheiszed with different Ni and Fe compositions. (A) Fe3O4 (23.8 ± 4.9 nm); (B) Ni17Fe83 (5.4 ± 1.0 nm); and (C) Ni39Fe61 (6.9 ± 1.0 nm).

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There are complex factors that influence the growth of the bimetallic nanoparticles. While the radii and metal structures of Ni and Fe are similar, the structures of NiFe alloy nanoparticles become much more complicated. It is reported that the total energy as well as the structural properties of NiFe bimetallic alloys are highly dependent on the precise details of the chemical ordering within the particles [41,45]. NiFe alloy clusters achieve their best stability when the iron atoms are located in the core region of the particles forming a compact structure, leading to the formation of highly stable core(Fe)–shell(Ni) chemical configurations [41]. However, there was no apparent evidence supporting the formation of core–shell structure for our NiFe alloy nanoparticles. 3.2. Thermal stability A representative set of TGA curves for the nanoparticles with different compositions is shown in Fig. 5. In Fig. 5A, the TGA curves for Fe3O4, Ni17Fe83 and Ni39Fe61 are compared. For the Fe3O4 particles (a), a weight loss of about 5.6% is observed at 105 °C, which is attributed the loss of adsorbed water or ethylene molecules on the surface of particles. A weight increase occurs upon further increasing the temperature, reaching a maximum at 220 °C. In this process, the weight increase is due to the oxidation of the ferrous ions in Fe3O4 particles, and the weight loss is due to the loss of the water bound to Fe3O4 particles. Another sharp weight loss appears at 610–642 °C, which is attributed to the decomposition of citrate bound to the surface of Fe3O4 particles. For Ni17Fe83 (curve b), the TGA curve is similar to that for Fe3O4 at temperature below 300 °C. However, a sharp weight increase takes place from at the temperature of approximately 300 °C to 610 °C. The weight increase is due to the oxidation of metal in the alloy. Following the weight loss is the decomposition of citrate on the surface of the particles, similar to that for the Fe3O4 particles. In comparison, the TGA curve for Ni39Fe61 (curve c) shows a very different weight increase at temperature above 500 °C in the presence of oxygen. Under nitrogen, the TGA curve (inset curve c0 ) shows a weight loss 590 °C, after which the weight did not change. This is in contrast to the characteristic in the oxidation atmosphere (curve c). The weight loss trend slows down, and increases at 504 °C. This difference reflects the difference between the NiFe alloy and the spinel Fe3O4. The fact that the total weight loss in O2 is less than that in N2 likely reflects the possibility of the oxidation of iron and nickel in the alloy that takes place in sequence. In Fig. 5B, the TGA curves for Ni78Fe22, Ni90Fe10 and Ni are compared. They show a similar trend characteristic of a weight increase at temperature above 300 °C, which subtle differences in the actual

temperature. An analysis of the DTA curves (inset in Fig. 5B) reveals several important findings. First, there is a small peak at 254 °C for Ni, 333 °C for Ni90Fe10 and 242 °C for Ni78Fe22. No weight change is observed at the same temperature in the corresponding TG curves. This finding is indicative of a phase transition. Based on the XRD data, Ni and NiFe alloys with different compositions undergo phase transformation from fcc structure to bcc structure. Secondly, with the increase of the temperature, an endothermic peak is observed at 554 °C in the DTA curve for Ni (curve f in the inset), and a weight increase of 25.8% is shown in the corresponding TG curve, which is consistent with the oxidation of Ni in air. In comparison, there are two endothermic peaks in Ni78Fe22 and Ni90Fe10 in the DTA curves at temperature above 400 °C. For Ni90Fe10, the peaks are observed at 492 °C and 654 °C. For Ni78Fe22, the peaks are observed at 450 °C and 656 °C. These peaks indicate that Ni in the alloy is oxidized following the oxidation of Fe. The difference of the peak intensities of the two endothermic peaks is due to the different compositions. Finally, the temperature for the initial oxidation of alloy is lower than that for Ni, whereas the final oxidation temperatures are higher than that of Ni. This finding indicates that the Ni78Fe22 and Ni90Fe10 show an enhanced resistance to oxidation in comparison with Ni particles. Note the thermal analysis only was used to determine the composition. Since the work in this report focused on room temperature magnetic characteristics, a further study of the oxidation products is part of our future work. In addition, the obvious distinction of baselines can be seen among curves a, b, and c in Fig. 5A and curves d, e, and f in Fig. 5B. These differences suggest that the surfaces of the smaller-sized particles have a stronger affinity to with water or ethanol molecules. There is also the possibility for the formation of surface oxides on the particles depending on the particle size. 3.3. Magnetic properties The as-synthesized particles, when being suspended in a solution, can be easily attracted by a magnetic bar (Fig. 6). For the nanoparticles of Ni39Fe61 and Ni90Fe10, there is a distinctive difference of solution color. The solution is yellow and the particles attracted to bar side are brown for the particle of Ni39Fe61. The attraction by the magnetic bar can be monitored by the UV–Vis spectra, in which the absorbance obviously decreases with the process of attracting. It showed a straight line when all particles were attracted to the bar side. The times that particles were attracted to the magnetic bar side depended on the composition of the nanoparticles. For example, in comparison with case for Ni39Fe61, the nanoparticles of Ni90Fe10 were shown to be attracted to the bar

Fig. 5. Thermogravimetric curves for nanoparticles with different initial ratio of Ni to Fe. (A) TG curves for Fe3O4, Ni17Fe83 and Ni39Fe61 (insert graph: TG curves for Ni39Fe61 under O2 and N2); (B) TG curves for Ni78Fe22, Ni90Fe10 and Ni (insert graph: DAT curves for Ni78Fe22, Ni90Fe10 and Ni).

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Fig. 6. UV–Vis spectra for a solution of Ni39Fe61 upon applying a magnetic bar at different times: 0 (a), 2 h (b), and 6 h (c) (insert shows the corresponding photos).

side almost immediately when the magnetic bar was applied. This finding was consistent with the presence of a high degree of nanoparticle aggregation in the case of the Ni90Fe10 nanoparticles, which increased the magnetization of the nanoparticles. The magnetic properties were characterized using vibrating sample magnetometer at room temperature. Fig. 7 shows a representative set of magnetization curves for the particles synthesized from different initial ratios of Ni to Fe. The parameters extracted from the hysteresis loop to characterize the magnetic properties of magnetic nanoparticles include the saturation magnetization (Ms), the remanence (Mr), and the coercivity (Hc). The Hc, Mr and Ms from different initial ratio of Ni to Fe are listed in Table 2. The magnetization hysteresis curves of three samples for Fe3O4, Ni17Fe83 and Ni39Fe61 show S-shapes, characteristic of magnetically-soft materials. For Fe3O4, the values of saturation magnetization (Ms) and coercivity (Hc) are 71.1 emu g1 and 40.0 Oe. They are smaller than those for bulk Fe3O4 (92 emu g1 [46,47] and

200–400 Oe [48]), which are believed to be due to the decreased particle size. The size of superparamagnetic Fe3O4 nanocrystal is 19.8 nm [49]. The particle size of Ni17Fe83 is only 5.4 nm. Note however that the coercivity of the alloy particle is 29.2 Oe. Although the particle size is smaller than the single domain size feature, the nanorystal feature could increase the coercivity. The samples of Ni39Fe61 and Ni78Fe22 show the smallest Ms, Mr and Hc values among the samples, which are expected for superparamagnetic behavior. Ferromagnetic behavior of the magnetic hystersis loops for nickel particles and particles of Ni90Fe10 are observed. In comparison with the values of Ms and Hc for bulk nickel (55 emu g1 and 100 Oe [50]), the Ms and Hc values for the sample of Ni nanoparticles show a decrease, which is due to the decrease of particle size. However, for the sample Ni90Fe10, the value of Ms is increased whereas Hc is decreased, which reflects the presence of iron in the alloy. Overall, the analysis of the Hc, Mr and Ms values in Table 2 seems to suggest the presence of a minimum for Hc, Mr and Ms with the increase of Ni% in the nanoparticles. It appears that this phenomenon may be associated with a structural change from the spinel structure to the fcc type structure for the nanoparticles. It is important to note that there were apparently particle size changes as the composition was changed for the nanoparticles, which complicated the assessment of any specific trend. This was perhaps in part responsible for the observation of a minimum of coercivity with the composition. Clearly, a further investigation of the size and composition dependences of the magnetic properties is needed, which is part of our future work.

4. Conclusions In conclusion, we have demonstrated the ability to control the size, composition and surface properties of NiFe alloy nanoparticles by controlling the feeding ratio of the metal precursors in

Fig. 7. Magnetization curves for the nanoparticles synthesized from different initial ratios of Ni to Fe: (A) (a) Fe3O4, (b) Ni17Fe83 and (c) Ni39Fe61; and (B) (d) Ni78Fe22, (e) Ni90Fe10, and (f) Ni. Inserts: magnified views of the curves at H  0 Oe.

Table 2 Hc, Mr and Ms for the nanoparticles of different compositions.

a

Sample composition

Particle sizea (nm)

Structure (based on XRD)

Mr (emu g1)

Ms (emu g1)

Hc (Oe)

Ni (f) Ni90Fe10 (e) Ni78Fe22 (d) Ni39Fe61 (c) Ni17Fe83 (b) Fe3O4 (a)

25.6 18.4 15.8 11.2 4.6 18.5

fcc fcc fcc Spinel (with bcc) Spinel Spinel

6.58 5.6 1.0 0.44 3.4 5.2

53.1 76.3 56.3 47.9 66.8 71.1

84.0 61.6 14 7.3 29.2 40

Size calculated based on the peak width using Scherrer equation.

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the wet chemical synthesis. The morphology, thermal stability, and magnetic properties of the as-synthesized alloy nanoparticles are shown to depend on the composition. The NiFe alloy nanoparticles exhibit remarkable resistance to the oxidation in comparison with Ni or Fe particles. The dependence of the magnetic properties of the NiFe nanoparticles on the composition is shown to be associated with a structural change from the spinel structure to the fcc-type structure for the nanoparticles, which has important implication for the design of stable and controllable magnetic nanoparticles for biomedical applications.

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Acknowledgements The research work was supported by the National Science Foundation (CHE-0848701, and DOE-BES Grants DE-SC0006877) and in part from the Fundamental Research Funds for the Central University of China (N110402014) and Natural Science Foundation of Liaoning Province of China (2013020104). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

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