Synthesis of Fine Iron­Cobalt Alloy Particles by the Co-Reduction of Precursorswith Solvated Electrons in Sodium­Ammonia Solution

Osamu Terakado+1, Yuichiro Uno+2 and Masahiro Hirasawa

Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan

Fine iron­cobalt alloy particles were synthesized by the co-reduction of their precursors with sodium in liquid ammonia. The reaction tookplace instantaneously because of the highly reductive solvated electrons. Very reactive, fine particles of the order of ³10 nm size were obtained.Annealing process is crucial for the magnetic properties of the products, the saturation magnetization being ranging from 29 to 238A·m2·kg¹1.One spot reduction with solvated electrons and the consequent annealing treatment in the synthesis vessel has potential ability for the synthesisof a variety of metal and alloy particles. [doi:10.2320/matertrans.M2013375]

(Received October 3, 2013; Accepted December 19, 2013; Published January 31, 2014)

Keywords: iron­cobalt alloy, reduction, solvated electrons, liquid ammonia

1. Introduction

The synthesis of nano-scale metal and alloy materials is ofgreat interest in the past decades. Because of the excellent softmagnetic properties of the iron­cobalt alloy, considerableattention has been also paid for synthetic processes of nano-scale Fe­Co alloy particles. Various processes have beenstudied including thermal decomposition of carbonyl pre-cursors,1) sonochemical method,2) and other chemical proc-esses such as microemulsion method3) and polyol process.4)

In chemical synthetic routes, a general pathway is thereduction of the precursor compounds, so that the exploringof reducing reagents is one of the key points in the researchof the wet processes.

Alkali metal is dissolved in liquid ammonia to formmetastable solutions. One of the interesting properties of thesolution is that its physical properties are drastically changedby the metal concentration, resulting from the excesselectrons released from the metal.5) At low metal concen-tration, the solution has characteristic blue colour arisingfrom the electrons solvated by ammonia molecules. Withincreasing the metal concentration, it changes to shinymetallic colour.

Metal­ammonia solutions are strongly reductive in nature,and are used as a reducing reagent particularly in organicsyntheses.6) With regard to its application to inorganicsyntheses, only a few works have been reported, as far asthe authors are aware. Zhu and Sadoway reported thepreparation of some elemental (Ta, Cu) and an alloy (Nb3Al)nanoparticles synthesized by sodium­ammonia solution.7)

Alkalides are relevant to the solvated electrons, where thealkali metal anions are produced when dissolved in non-reducible solvents. Dye proposed the alkalide route for thenanoparticle synthesis,8) and series of the works, includingthe synthesis of iron­cobalt nanoparticles, have been reportedby Wagner’s group.9­11)

In the present paper, we address the applicability ofsolvated electron route, describing the synthesis process of

fine Fe­Co particles by the co-reduction of their chlorideprecursors with sodium­ammonia solutions. On the basis ofthe magnetic properties of the bulk alloy,12) the compositionof Fe2Co has been focused, at which the saturation magnet-ization shows a maximum value of 245A·m2·kg¹1. Theoverall reaction is described as:

CoCl2 þ 2FeCl2 þ 6Na ¼ CoFe2 þ 6NaCl: ð1ÞAs the synthesized particles are highly reactive and unstablein ambient atmosphere, we focus the annealing treatment ofthe materials.

2. Experimental Procedure

2.1 MaterialsBecause metal­ammonia solutions are less stable in

ambient atmosphere, all the syntheses were carried outin pyrex glass vessels connected to a vacuum line(³10¹3mbar).13) All the vessels were cleaned with H2SO4­HNO3 acid mixture in order to avoid the decomposition ofthe solution, i.e., amide formation, which is promoted byimpurities on glass walls.

Ammonia, purchased from Taiyo Nissan, was liquefiedand reacted with sodium metal (98%, Kanto Chem. Co., Ltd.)at ¹60°C until blue-coloured solution was obtained. Waterimpurities in ammonia can be rigorously removed by thisprocess. The purified ammonia was then vacuum distilledinto a pyrex glass storage tank containing lithium nitrate. Thesaturation of the salt enables the store of the liquid ammoniaat ambient temperature.

2.2 Synthesis of fine Fe­Co particlesThe mixture of FeCl2 (99.9%, Kojundo Chem. Lab. Co.,

Ltd.) and CoCl2 (97%, Wako Pure Chem. Co., Ltd.) powdersat stoichiometric ratio, i.e., eq. (1), was introduced in abottom of a pyrex glass synthesis vessel. Pieces of sodiummetal were put in a small V-shaped vessel which wasconnected via glass joint to the top of the synthesis vessel. Byturning the V-shaped vessel, sodium metals are dropped intothe bottom of the synthesis cell. Typical composition of thesolution was FeCl2 of 382mg, CoCl2 of 195mg, Na of160mg, and the solution volume of 10mL. As discussed in

+1Corresponding author, E-mail: teramon@numse.nagoya-u.ac.jp+2Graduate Student, Nagoya University. Present address: Isuzu MotorsLtd., Fujisawa 252-0881, Japan

Materials Transactions, Vol. 55, No. 3 (2014) pp. 517 to 521©2014 The Japan Institute of Metals and Materials

3.1, the sodium composition was lower than the stoichio-metric ratio given in eq. (1), typically 80% of thestoichiometry. Therefore, we focus on the characterizationof reaction products rather than the product yield in thepresent paper.

Prior to the reduction of the salts, they were dried undervacuum at 200°C. Ammonia was introduced to the synthesisvessel through vacuum distillation, and the salts were mixedin liquid ammonia at ¹60°C. Sodium was then introducedfrom the V-shaped compartment. The colour of the solutionturned to blue, and the reaction took place instantaneously.After the solvent was evaporated, the synthesized powderswere taken from the cell, and washed with distilled water.The product is hereafter called as “as-made” sample. In mostof the cases, however, the products were taken from the cellin an argon-filled glove bag, in which black-colouredproducts were washed with ethanol.

Some annealing treatments were carried out for thereaction products with an electric furnace. In some cases,the annealing was conducted in the pyrex cell prior to takingthe sample in a glove bag. Detailed conditions are describedin the section of results and discussion.

2.3 Characterization of the reaction productsThe products were characterized by an X-ray diffractom-

eter (Shimadzu, XRD-6100), a scanning electron microscopewith an EDS analyser (JEOL, JSM-6330F and JED-2140),and a transmission electron microscope (Hitachi, H-8). Themagnetic properties were examined by a vibrating samplemagnetometer (Toei Kogyo, VSM-5) at room temperature atthe sweep speed of 0.8MA·m¹1 per 3min.

3. Results and Discussions

3.1 Characterization of as-made productFigure 1 shows an X-ray diffraction pattern of an as-made

product sample obtained by the reduction reaction. Weakpeaks of the cobalt ferrite, CoFe2O4, are observed in thediffractogram. A typical atomic composition of products,determined by a SEM-EDS analysis, is cobalt 9%, iron 21%,oxygen 69%, sodium 1% and chlorine of 0.2%. As chlorineand sodium are essentially removed from the product, thereduction of the precursor chlorides takes place in thetreatment with sodium­ammonia solution. The as-madesample, when exposed in ambient atmosphere, results inthe exothermic reaction. Thus, it is considered that theoxidation of the synthesized alloy took place during thehandling of the sample, resulting in the substantial oxygencontent in the product. It should be here also noted that theexistence of un-reacted sodium in the synthesis cell leads tothe formation of alkaline compounds during the washingtreatment of the reaction product, giving rise to the formationof iron or cobalt hydroxides. Therefore, the sodium contentshould be less than the stoichiometric ratio. Even for theless sodium content, the atomic ratio of the product isFe : Co ³ 2 : 1, implying the comparable kinetics of thereduction reaction of both the chlorides.

The morphology of the reaction products was fine powderwith the particle size of ³10 nm, as shown in the SEM andTEM images (Fig. 2). The TEM image shows the existence

of amorphous phase as well as crystalline, the latter of whichis probably due to the XRD peaks of cobalt ferrite.

As shown in Fig. 3, an elemental mapping of the productswith a scanning transmission electron microscope (STEM)suggests the co-reduction of the precursors, as both elementsco-exist in the product particles without significant segrega-tion. Thus, the result supports the comparable kinetics of thereduction of the elements, as anticipated above.

In regard to the formation mechanism of the nanoparticles,its full understanding is one of the most challenging tasks inthe synthesis of nanoparticles via chemical route. Here, werefer to the work by Zubris et al., where the reaction kineticsof FeCo nanoalloy formation by thermal decomposition ofcabonyl precursors was studied.1) The formation mechanismof nanoalloy systems is supposed to be co-transformation,co-aggregation of different metallic clusters. The reductionkinetics of each component is one of the crucial factors forthe phase separation. In the present case, the reductionkinetics is presumably very fast because of the strongreductive nature of the solvated electrons, rather differentfrom the case of carbonyl decomposition of the reaction timescale of ³103 s. Moreover, the phase diagram of binaryFe­Co system exhibits co-solubility over the most of thecompositional range. Thus, similar reaction rates andhomogeneous formation of the alloy particles are expectedin the reduction of divalent iron and cobalt salts. The presentresults support this consideration, though detailed studies,such as kinetics and high resolution TEM measurement undercareful sample treatment, are desired.

10°° 20° 30° 40° 50° 60° 70° 80° 90°

Inte

nsit

y (a

rb. u

nit)

2

CoFe2O

4

CoFeCoFeCo

3Fe

7

θ

Fig. 1 XRD pattern of the as-made sample synthesized by reduction of theprecursors with sodium­ammonia solution. Diffraction peak positions ofsome related compounds are also shown in the figure as reference.

50nm1μm

Fig. 2 SEM (left) and TEM (right) images of as-made powder.

O. Terakado, Y. Uno and M. Hirasawa518

3.2 Influence of the annealing treatmentAs reported in literature, fine Fe­Co alloy powders are

easily oxidized.2,11) We have, therefore, explored thefollowing annealing treatments for the preparation of air-stable Fe­Co alloy particles: (1) post-anneal, where theas-made sample was annealed for 4 h under reducingatmosphere of a gas (95%He­5%H2) flow at 600°C, (2)low pO2

treatment, where the sample was exposed under theatmosphere with low oxygen partial pressure. This wasachieved by the heating of Cu and Cu2O powder mixture at150°C held in a glass container attached next to the synthesiscell, and (3) pre-anneal treatment, where sample wasannealed at 400°C for 4 h in the synthesis cell after thesolvent evaporation. Note that, in the last two types oftreatments, the annealing was conducted in the synthesiscell and the products were, then, washed with ethanol in anargon-filled grove bag, while the sample of post-annealingtreatment was exposed in ambient atmosphere before theannealing treatment.

Representative atomic composition of the annealed sampleis summarized in Table 1. Obviously, the oxygen content isreduced in comparison with the as-made powder (see 3.1).Thus, the annealing treatment is effective for the reduction ofthe oxidized species.

Figure 4 shows the SEM images of the samples after theannealing treatments. The growth in particle size (³100 nm)of the post-annealed sample with distinct particles is obvious,while the samples treated in synthesis cell are not well-defined as that of as-made sample, and do not exhibit thesignificant change in particle size. As reported in literature,the annealing at high temperature results in the agglomerationand the particle growth.2,10,11)

It is anticipated that the annealing under reducingatmosphere (post-anneal) results in the reduction of the

500nm

Fig. 3 STEM image (left) and the corresponding cobalt (middle) and iron (right) mapping images of as-made powder prepared bysodium­ammonia solution.

Table 1 Typical atomic composition of the annealed samples determinedwith SEM-EDS analysis.

Co Fe O Na Cl

post-anneal 30 53 16 1.3 0.1

low pO211 30 57 0.8 0.1

pre-anneal 13 28 56 2.0 0.2

1µm

1µm

1µm

(a)

(b)

(c)

Fig. 4 SEM images of the sample after different annealing treatments:(a) annealed under He­H2 atmosphere at 600°C for 4 h (post-anneal),(b) exposed under low oxygen atmosphere for 6 h (low pO2

), (c) annealedunder vacuum at 400°C for 4 h in the synthesis cell (pre-anneal).

Synthesis of Fine Iron­Cobalt Alloy Particles by the Co-Reduction of Precursors with Solvated Electrons in Sodium­Ammonia Solution 519

oxides. On the other hand, the annealing under inertatmosphere or the treatment under low oxygen partialpressure in the synthesis cell can lead to the formationof protective oxide layer or the agglomeration and theparticle growth, preventing the rigorous oxidation whenexposed in ambient atmosphere. The XRD patterns of theannealed samples are shown in Fig. 5. The post-annealingresults in the essentially full reduction of the as-madeproduct, and the sharp diffraction peaks of the fcc structureof the metal are observed. As for the influence of theexposure under low oxygen partial pressure, the existenceof the oxide is observed. However, in comparison to the as-made sample (Fig. 1), the complete oxidation is obviouslyavoided, as intense diffraction peaks of the alloy areobserved. The pre-anneal treatment results in the furtherintensification of the peaks of the alloy. Moreover, thesignal-to-noise ratio becomes higher than that of the lowpO2

treatment, which presumably arises from the annealingat high temperature. These results clearly show that boththe low pO2

treatment and pre-annealing are effective forthe protection against the rigorous oxidation of the fineparticles.

3.3 Magnetic property of the productsThe results of the VSM measurements of the synthesized

powder are shown in Fig. 6 and Table 2. Taking the values ofthe remanence ratio into account, all the samples have similarcharacteristics with an assembly of randomly orienteduniaxial particles. The as-made sample has the saturationmagnetization of 29A·m2·kg¹1, while the room temperaturemagnetization loop of the post-annealed sample exhibitshighest saturation magnetization of 238A·m2·kg¹1 withsmallest coercivity of 11 kA·m¹1, comparable to the valuesof the bulk Fe2Co alloy.12) The latter compound is, thus,representative soft-magnetic material. As for the annealingtreatment in the synthetic cell, low pO2

treatment doe not

improve the magnetic properties, as the hysteresis loop iscomparable with that of as-made sample. On the other hand,the annealing in the synthesis cell enhanced the saturationmagnetization up to 76A·m2·kg¹1.

Therefore, materials with different magnetic properties canbe obtained by an appropriate annealing treatment. Furtheroptimization in the annealing process can improve themagnetic properties of the Fe­Co fine particles.

4. Conclusion

Synthesis of fine Fe­Co particles by co-reduction ofprecursors with sodium­ammonia solution is studied in thepresent work. The reduction reaction takes place instanta-neously, and the chlorine of the precursors are fully removedby the treatment with sodium­ammonia solutions. Annealingtreatment is a key issue for the preparation of air-stableparticles. Among the examined processes, the pre-annealprocess, conducted in the synthesis cell after the reduction,avoids the rigorous oxidation, as observed in the XRDpattern of the product.

In summary, synthesis with solvated electrons is apowerful method for the synthesis of fine particles. Furtherimprovement in the annealing treatment or the process of theprotective layer can achieve the synthesis of fine particleswith variety of magnetic properties.

Acknowledgments

The authors acknowledge Prof. Hidefumi Asano and Dr.Tetsuya Miyawaki, Nagoya University, for the support ofVSM measurements.

10°° 20° 30° 40° 50° 60° 70° 80° 90°

Pre-anneal

Low pO2

Post-anneal

Inte

nsit

y (a

rb. u

nit)

2

CoFe2O

4

CoFeCoFeCo

3Fe

7

θFig. 5 XRD pattern of the synthesized powders prepared by different

annealing treatments. Diffraction peak positions of some relatedcompounds are also shown in the figure as reference.

-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8

-200

-100

0

100

200

as-made

low pO2

pre-anneal

post-anneal

Mag

neti

zati

on, M

/ A

·m2 ·k

g-1

Magnetic Field, H / MA·m-1

Fig. 6 Room temperature magnetization loop of the synthesized powder.

Table 2 Magnetic property of synthesized particles.

SampleSaturation

magnetization,Ms/A·m2·kg¹1

Remanencemagnetization,Mr/A·m2·kg¹1

Remanenceratio,

S/Mr/Ms

Coercivity,Hc/kA·m¹1

as-made 29 7 0.24 28

post-anneal 238 23 0.097 11

low pO232 8 0.25 30

pre-anneal 76 18 0.24 33

O. Terakado, Y. Uno and M. Hirasawa520

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Synthesis of Fine Iron­Cobalt Alloy Particles by the Co-Reduction of Precursors with Solvated Electrons in Sodium­Ammonia Solution 521