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Synthesis of Fine Iron–Cobalt Alloy Particles by the Co-Reduction Of

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Synthesis of Fine Iron–Cobalt Alloy Particles by the Co-Reduction Of Materials Transactions, Vol. 55, No. 3 (2014) pp. 517 to 521 ©2014 The Japan Institute of Metals and Materials Synthesis of Fine Iron­Cobalt Alloy Particles by the Co-Reduction of Precursors with 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 took place 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 238 A·m2·kg¹1. One spot reduction with solvated electrons and the consequent annealing treatment in the synthesis vessel has potential ability for the synthesis of 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 fine Fe­Co particles by the co-reduction of their chloride precursors with sodium­ammonia solutions. On the basis of The synthesis of nano-scale metal and alloy materials is of the magnetic properties of the bulk alloy,12) the composition great interest in the past decades. Because of the excellent soft of Fe2Co has been focused, at which the saturation magnet- magnetic properties of the iron­cobalt alloy, considerable ization shows a maximum value of 245 A·m2·kg¹1. The attention has been also paid for synthetic processes of nano- overall reaction is described as: scale Fe­Co alloy particles. Various processes have been CoCl2 þ 2FeCl2 þ 6Na ¼ CoFe2 þ 6NaCl: ð1Þ studied including thermal decomposition of carbonyl pre- cursors,1) sonochemical method,2) and other chemical proc- As the synthesized particles are highly reactive and unstable esses such as microemulsion method3) and polyol process.4) in ambient atmosphere, we focus the annealing treatment of In chemical synthetic routes, a general pathway is the the materials. reduction of the precursor compounds, so that the exploring of reducing reagents is one of the key points in the research 2. Experimental Procedure of the wet processes. Alkali metal is dissolved in liquid ammonia to form 2.1 Materials metastable solutions. One of the interesting properties of the Because metal­ammonia solutions are less stable in solution is that its physical properties are drastically changed ambient atmosphere, all the syntheses were carried out by the metal concentration, resulting from the excess in pyrex glass vessels connected to a vacuum line 5) ¹3 13) electrons released from the metal. At low metal concen- (³10 mbar). All the vessels were cleaned with H2SO4­ tration, the solution has characteristic blue colour arising HNO3 acid mixture in order to avoid the decomposition of from the electrons solvated by ammonia molecules. With the solution, i.e., amide formation, which is promoted by increasing the metal concentration, it changes to shiny impurities on glass walls. metallic colour. Ammonia, purchased from Taiyo Nissan, was liquefied Metal­ammonia solutions are strongly reductive in nature, and reacted with sodium metal (98%, Kanto Chem. Co., Ltd.) and are used as a reducing reagent particularly in organic at ¹60°C until blue-coloured solution was obtained. Water syntheses.6) With regard to its application to inorganic impurities in ammonia can be rigorously removed by this syntheses, only a few works have been reported, as far as process. The purified ammonia was then vacuum distilled the authors are aware. Zhu and Sadoway reported the into a pyrex glass storage tank containing lithium nitrate. The preparation of some elemental (Ta, Cu) and an alloy (Nb3Al) saturation of the salt enables the store of the liquid ammonia nanoparticles synthesized by sodium­ammonia solution.7) at ambient temperature. Alkalides are relevant to the solvated electrons, where the alkali metal anions are produced when dissolved in non- 2.2 Synthesis of fine Fe­Co particles reducible solvents. Dye proposed the alkalide route for the The mixture of FeCl2 (99.9%, Kojundo Chem. Lab. Co., 8) nanoparticle synthesis, and series of the works, including Ltd.) and CoCl2 (97%, Wako Pure Chem. Co., Ltd.) powders the synthesis of iron­cobalt nanoparticles, have been reported at stoichiometric ratio, i.e., eq. (1), was introduced in a by Wagner’s group.9­11) bottom of a pyrex glass synthesis vessel. Pieces of sodium In the present paper, we address the applicability of metal were put in a small V-shaped vessel which was solvated electron route, describing the synthesis process of connected via glass joint to the top of the synthesis vessel. By turning the V-shaped vessel, sodium metals are dropped into +1Corresponding author, E-mail: [email protected] the bottom of the synthesis cell. Typical composition of the +2Graduate Student, Nagoya University. Present address: Isuzu Motors solution was FeCl2 of 382 mg, CoCl2 of 195 mg, Na of Ltd., Fujisawa 252-0881, Japan 160 mg, and the solution volume of 10 mL. As discussed in 518 O. Terakado, Y. Uno and M. Hirasawa 3.1, the sodium composition was lower than the stoichio- CoFe O metric ratio given in eq. (1), typically 80% of the 2 4 Co stoichiometry. Therefore, we focus on the characterization Fe of reaction products rather than the product yield in the CoFe Co Fe present paper. 3 7 Prior to the reduction of the salts, they were dried under vacuum at 200°C. Ammonia was introduced to the synthesis vessel through vacuum distillation, and the salts were mixed in liquid ammonia at ¹60°C. Sodium was then introduced from the V-shaped compartment. The colour of the solution unit) Intensity (arb. turned to blue, and the reaction took place instantaneously. After the solvent was evaporated, the synthesized powders were taken from the cell, and washed with distilled water. 10° 20° 30° 40° 50° 60° 70° 80° 90° “ ” The product is hereafter called as as-made sample. In most 2θ of the cases, however, the products were taken from the cell in an argon-filled glove bag, in which black-coloured Fig. 1 XRD pattern of the as-made sample synthesized by reduction of the products were washed with ethanol. precursors with sodium­ammonia solution. Diffraction peak positions of fi Some annealing treatments were carried out for the some related compounds are also shown in the gure as reference. reaction products with an electric furnace. In some cases, the annealing was conducted in the pyrex cell prior to taking the sample in a glove bag. Detailed conditions are described in the section of results and discussion. 2.3 Characterization of the reaction products The products were characterized by an X-ray diffractom- eter (Shimadzu, XRD-6100), a scanning electron microscope with an EDS analyser (JEOL, JSM-6330F and JED-2140), and a transmission electron microscope (Hitachi, H-8). The 1μm 50nm magnetic properties were examined by a vibrating sample Fig. 2 SEM (left) and TEM (right) images of as-made powder. magnetometer (Toei Kogyo, VSM-5) at room temperature at the sweep speed of 0.8 MA·m¹1 per 3 min. of amorphous phase as well as crystalline, the latter of which 3. Results and Discussions is probably due to the XRD peaks of cobalt ferrite. As shown in Fig. 3, an elemental mapping of the products 3.1 Characterization of as-made product with a scanning transmission electron microscope (STEM) Figure 1 shows an X-ray diffraction pattern of an as-made suggests the co-reduction of the precursors, as both elements product sample obtained by the reduction reaction. Weak co-exist in the product particles without significant segrega- peaks of the cobalt ferrite, CoFe2O4, are observed in the tion. Thus, the result supports the comparable kinetics of the diffractogram. A typical atomic composition of products, reduction of the elements, as anticipated above. determined by a SEM-EDS analysis, is cobalt 9%, iron 21%, In regard to the formation mechanism of the nanoparticles, oxygen 69%, sodium 1% and chlorine of 0.2%. As chlorine its full understanding is one of the most challenging tasks in and sodium are essentially removed from the product, the the synthesis of nanoparticles via chemical route. Here, we reduction of the precursor chlorides takes place in the refer to the work by Zubris et al., where the reaction kinetics treatment with sodium­ammonia solution. The as-made of FeCo nanoalloy formation by thermal decomposition of sample, when exposed in ambient atmosphere, results in cabonyl precursors was studied.1) The formation mechanism the exothermic reaction. Thus, it is considered that the of nanoalloy systems is supposed to be co-transformation, oxidation of the synthesized alloy took place during the co-aggregation of different metallic clusters. The reduction handling of the sample, resulting in the substantial oxygen kinetics of each component is one of the crucial factors for content in the product. It should be here also noted that the the phase separation. In the present case, the reduction existence of un-reacted sodium in the synthesis cell leads to kinetics is presumably very fast because of the strong the formation of alkaline compounds during the washing reductive nature of the solvated electrons, rather different treatment of the reaction product, giving rise to the formation from the case of carbonyl decomposition of the reaction time of iron or cobalt hydroxides. Therefore, the sodium content scale of ³103 s. Moreover, the phase diagram of binary should be less than the stoichiometric ratio.
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