Journal of Materials Science & Technology  2020 , 36 (0): 128-133 https://doi.org/10.1016/j.jmst.2019.05.066

Research Article

Structure and properties of nanoporous FePt fabricated by dealloying a melt-spun Fe60Pt20B20 alloy and subsequent annealing

Dianguo Maa, Yingmin Wanga**, Yanhui Lia, Rie Y. Umetsub, Shuli Oua, Kunio Yubutab, Wei Zhanga*

a Key Laboratory of Materials Modification by Laser, Ion, and Electron Beams (Ministry of Education), School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China
b Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

Corresponding authors:   ∗Corresponding author. ∗∗Corresponding author. E-mail addresses: apwangym@dlut.edu.cn (Y. Wang), wzhang@dlut.edu.cn(W. Zhang).∗Corresponding author. ∗∗Corresponding author. E-mail addresses: apwangym@dlut.edu.cn (Y. Wang), wzhang@dlut.edu.cn(W. Zhang).

Received: 2019-01-31

Revised:  2019-03-20

Accepted:  2019-05-5

Online:  2020-01-01

Copyright:  2020 Editorial board of Journal of Materials Science & Technology Copyright reserved, Editorial board of Journal of Materials Science & Technology

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Abstract

A nanoporous FePt alloy has been fabricated by dealloying a melt-spun Fe60Pt20B20 alloy composed of nanoscale amorphous and face-centered-cubic FePt (fcc-FePt) phases in H2SO4 aqueous solution. The nanoporous alloy consists of single fcc-FePt phase with an Fe/Pt atomic ratio of about 55.3/44.7, and possesses a uniform interpenetrating ligament-channel structure with average ligament and pore sizes of 27 nm and 12 nm, respectively. The nanoporous fcc-FePt alloy shows soft magnetic characteristics with a saturation magnetization of 37.9 emu/g and better electrocatalytic activity for methanol oxidation than commercial Pt/C in acidic environment. The phase transformation from disordered fcc-FePt into ordered face-centered-tetragonal FePt (L10-FePt) in the nanoporous alloy has been realized after annealing at 823-943 K for 600 s. The volume fraction of the L10-FePt phase in the alloy increases with the rise of annealing temperature, which results in the enhancements of coercivity and saturation magnetization from 0.14 kOe and 38.5 emu/g to 8.42 kOe and 51.4 emu/g, respectively. The ligament size of the samples is increased after annealing.

Keywords: Nanoporous metals ; Fe-Pt-B alloy ; Dealloying ; L10-FePt ; Magnetic property

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Dianguo Ma, Yingmin Wang, Yanhui Li, Rie Y. Umetsu, Shuli Ou, Kunio Yubuta, Wei Zhang. Structure and properties of nanoporous FePt fabricated by dealloying a melt-spun Fe60Pt20B20 alloy and subsequent annealing[J]. Journal of Materials Science & Technology, 2020, 36(0): 128-133 https://doi.org/10.1016/j.jmst.2019.05.066

1. Introduction

Nanoporous metals are a new class of nanomaterials, which possess many functional properties and promise enormous potential for various applications, such as catalysts, sensors, actuation, energy storage, drug delivery, and microfluidic flow controllers [[1], [2], [3], [4], [5], [6]]. Ferromagnetic nanoporous metals have attracted growing interest of researchers for possible applications in the field of nano-biotechnology, for instances in the magnetic separation of cells, proteins, peptides, etc [7,8]. Till now, plenty of nanoporous metals with soft magnetic characteristics have been developed and studied [[7], [8], [9], [10], [11]], however, less attention has been paid to the synthesis of hard magnetic nanoporous alloys.

It is well known that FePt alloys composed of an ordered face-centered-tetragonal FePt (L10-FePt) phase have gained considerable attention for the fabrications of high performance permanent magnets and ultrahigh density magnetic data storage media due to the extremely high magnetocrystalline anisotropy (K =7 MJ m-3) [[12], [13], [14]]. In past decades, nanostructured L10-FePt alloys including thin films [15,16], nanoparticles [[17], [18], [19]], and nanocomposites [[20], [21], [22], [23]] have been synthesized by various methods and extensively studied, but there have been only a few reports on the development of nanoporous structures. Template-based techniques via annealing with fcc-FePt nanoparticles were used to fabricate an ordered nanoporous L10-FePt with high coercivity (iHc) [24]. However, this method is technically difficult and time-consuming [2].

Dealloying is a simple and effective approach for obtaining nanoporous structures [1,2,25,26]. In this process, one or more active elements are selectively dissolved from a precursor alloy by chemical or electrochemical corrosion, and the remained noble elements form a three-dimensional nanoporous structure through surface diffusion [25,26]. Most of the reported nanoporous metals are prepared by dealloying solid solution alloys [1,2] or metallic glasses [[27], [28], [29]] with a single phase and a homogeneous chemical composition, which makes the dealloying process easy to be controlled. Recently, nanoporous alloys or composites with various compositions and structures have been fabricated by dealloying two- or multi-phase alloys, which are expected to possess novel functionalities [[30], [31], [32], [33], [34]].

It is known that the L10-FePt is formed via transforming from disordered fcc-FePt by heat treatments [13,14]. Therefore, nanoporous L10-FePt may be obtained by annealing a nanoporous fcc-FePt prepared by dealloying. The early studies on the dealloying behavior of Fe-Pt binary alloys revealed that the nanoporosity could not be achieved although Fe atoms were selectively dissolved from the samples [35]. Recently, it has been reported that the fcc-FePt nanoporous structure are formed by dealloying multi-phase FePtAl [36] and amorphous FePtB [11] alloys. However, the compositions of the nanoporous fcc-FePt deviates far from equiatomic, and which high ordering L10-FePt are difficult to be reached after heat treatment [13,[37], [38], [39]]. In the present work, we selected a two-phase Fe60Pt20B20 alloy composed of homogeneous nanoscale amorphous and fcc-FePt phases as the dealloying precursor for the preparation of nanoporous FePt. The alloy ribbon has been completely dealloyed in H2SO4 aqueous solution to form a nanoporous fcc-FePt with an Fe/Pt atomic ratio of about 55.3/44.7. The transformation from disorder into order structure for the nanoporous alloy has been realized after annealing. The structure, magnetic properties, and electrocatalytic activity for methanol oxidation of the nanoporous fcc-FePt alloy before and after annealing were investigated.

2. Experimental

Appropriate amounts of pure Fe (99.99 mass%), Pt (99.9 mass%) and B (99.5 mass%) were arc-melted in an argon atmosphere to obtained an ingot of Fe60Pt20B20 alloy. The ingot was crushed into small pieces to accommodate the size of a quartz crucible used for melt-spinning. Ribbon samples with a thickness of ∼0.02 mm and width of ∼1.5 mm were prepared by melt-spinning technique. The alloy ribbons were dealloyed in 0.1 mol/L H2SO4 solution with a standard three-electrode electrochemical cell, which was driven by an applied potential of -90 mV (vs Ag/AgCl) for 1800 s at room temperature. After dealloying, the samples were rinsed in pure water and then dried in a vacuum desiccator for 12 h. The annealing treatment for the dealloyed samples was carried out in a vacuum at 823-943 K for 600 s. The structure of the samples was examined by X-ray diffraction (XRD, Cu-Kα) and transmission electron microscopy (TEM). The thermal stability of the samples was studied by differential scanning calorimetry (DSC) at a heating rate of 0.67 K/s. A scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDX) was used to observe the morphology and to measure the chemical composition. The electrocatalytic activity of the samples was evaluated by cyclic voltammetry (CV) in 0.5 mol/L H2SO4 and 0.5 mol/L H2SO4 + 1.0 mol/L CH3OH mixed solutions at sweep rates of 50 mV/s and 20 mV/s, respectively. For comparison, the electrocatalytic activity of a commercial Pt/C catalyst (Johnson Matthey) was also measured under the same condition. The electrochemical surface areas (ECSAs) of the Pt-based electrodes were calculated by integration of the charge associated with the hydrogen adsorption/desorption, in which a charge density of 210 mC/cm2 Pt was used and the double layer charges were subtracted [40]. The anodic site current densities were then normalized by the ECSAs. The magnetic properties of the samples were measured with a vibrating sample magnetometer (VSM) under a maximum field of 18 kOe.

3. Results and discussion

We confirmed that the XRD results taken from the free- and wheel-side of the melt-spun Fe60Pt20B20 alloy ribbon are basically same. Fig. 1(a) shows the XRD patterns of the ribbon before and after dealloying in H2SO4 aqueous solution. Only fcc-FePt phase is identified from the diffraction peaks in the both samples. However, the peaks of the dealloyed sample shift towards lower diffraction angles, indicating an increase in lattice-constant of the fcc-FePt phase. The lattice-constants were calculated to be 0.376 nm and 0.381 nm for the precursor and dealloyed alloys, respectively, which suggests that the Fe content of the fcc-FePt phase in the dealloyed sample is much lower than that in the precursor. The high-resolution TEM image along with the selected area electron diffraction (SAED) pattern of the melt-spun Fe60Pt20B20 alloy is shown in Fig. 1(b). A homogeneous nanoscale mixed structure consisting of amorphous and crystalline phases with an average grain size of ∼5 nm was observed. The coexistence of an amorphous diffraction halo with the fcc-FePt Debye-Sherrer rings in the SAED pattern confirms the two-phase nature of the sample. An interpenetrating ligament (dark skeleton)-channel (bright region) structure can be seen in the bright-field TEM image of the dealloyed sample (Fig. 1(c)), indicating the formation of nanoporous architecture. The average ligament size in the nanoporous structure was determined to be about 25 nm. The corresponding SAED pattern (inset of Fig. 1(c)) consists only of polycrystalline diffraction rings, which are indexed to fcc-FePt. The absence of the halo rings in the SAED pattern indicates that the amorphous phase is decomposed after dealloying. The DSC curve of the melt-spun alloy exhibits a sharp exothermic peak corresponding to the crystallization of amorphous phase, while the exothermic peak has disappeared in the dealloyed sample, confirming the absence of the amorphous phase (Fig. 2).

Fig. 1.   XRD patterns (a) and bright-field TEM images of melt-spun Fe60Pt20B20 alloys before (b) and after (c) dealloying in 0.1 mol/L H2SO4 solution. Insets in (b) and (c) are the corresponding SAED patterns of the melt-spun and dealloyed samples, respectively.

Fig. 2.   DSC curves of melt-spun Fe60Pt20B20 alloys before and after dealloying in 0.1 mol/L H2SO4 solution.

Fig. 3 shows the SEM images and EDX spectrum of the dealloyed samples. A uniform nanoporous structure is seen to form on the sample surface (Fig. 3(a)). The average ligament and pore sizes in the nanoporous structure are about 27 nm and 12 nm, respectively, in good agreement with the TEM observation. The cross-sectional SEM images in Fig. 3(b) and (c) confirm that the entire ribbon has a uniform nanoporous structure free of cracks. The results also reveal that the polycrystalline nanoporous fcc-FePt alloy is capable of resisting the mechanical strain produced by dealloying to maintain a high mechanical integrity. Considering the limited solid solubility of B in fcc-FePt phase [21,41] and the difficulty of B content measurement by EDX, only the contents of Fe and Pt were analyzed by EDX for the nanoporous alloys. The EDX spectrum (Fig. 3(d)) gives an Fe/Pt atomic ratio of about 55.3/44.7 for the ligaments, which is greatly different from the composition of the precursor alloy.

Fig. 3.   Top-view (a) and cross-sectional (b, c) SEM images, and the corresponding EDX spectrum (d) of the Fe60Pt20B20 alloy dealloyed in 0.1 mol/L H2SO4 solution.

In the FePtB alloys, there is a marked separation in the standard electrode potentials of the components: -890 mV (B3+/B), -440 mV (Fe2+/Fe), and +1190 mV (Pt2+/Pt) (vs standard hydrogen electrode) [42]. This electrochemical characteristic suggests that the over potentials for dissolution of the individual components are very different. Because of the more negative electrode potential of B and Fe than the applied potential (-90 mV), preferential corrosion of them is readily initiated at the early stage of dealloying. Once a certain amount of B is dissolved from the amorphous phase, a large number of vacancies are left in the disordered structure, and the amorphous phase becomes unstable and finally decomposes into the fcc-FePt phase [11]. The partially dealloyed samples were studied to examine the evolution of morphology and composition during the dealloying process. The typical cross-section morphology for partially dealloyed samples is shown in Fig. 4(a). A distinct interface is seen to form between the dealloyed layer and the substrate. The composition depth profile across this interface shows that the contents of Fe and Pt are nearly constant along the thickness direction of the dealloyed layers, but exhibit abrupt but opposite changes across the interface (Fig. 4 (b)). The absence of a continued composition variation at the interface region implies that dealloying has almost completed immediately with the access of solution before propagating to the next layer substrate. The XRD and EDX results of the fully dealloyed sample indicate that a portion of Fe atoms have been dissolved out of the alloy. The EDX analysis for the partially dealloyed samples further confirmed that the Fe content is reduced, but only slightly decreases with increasing dealloying time (Fig. 4(c)). The surface diffusion of Pt atoms upon dealloying is known to be sluggish because of the very low room-temperature coefficient of diffusion of Pt [43]. Long-distance surface diffusion [43,44] might not be an operable mechanism for the formation of nanoporosity. Short-distance structure rearrangement of Pt and Fe atoms is likely to occur, which also creates a certain amount of vacancies. The aggregation of vacancies at the dealloying interface forms percolated nanopores, and the local rearrangement of remaining Pt and Fe atoms results in the formation of homogenous ligaments.

Fig. 4.   (a) Cross-sectional SEM image, (b) EDX composition depth profile of the Fe60Pt20B20 alloy dealloyed in 0.1 mol/L H2SO4 solution for 600 s, and (c) EDX composition vs dealloying time plots showing the changes of Fe and Pt contents at the different stages.

The magnetic properties of the melt-spun Fe60Pt20B20 alloy ribbons before and after dealloying were measured by VSM, and the results indicate that both alloys have soft magnetic characteristics. The saturation magnetizations (Bs) of the melt-spun Fe60Pt20B20 and nanoporous fcc-FePt alloys are 101.6 emu/g and 37.9 emu/g, respectively. The decrease in Bs of the nanoporous alloy is mainly attributed to the decomposition of the amorphous phase and the depletion of Fe in the initial fcc-FePt phases. In view of the high surface area of the nanoporous fcc-FePt, its electrocatalytic activity for methanol oxidation was investigated. Fig. 5(a) shows the CV curves of the nanoporous fcc-FePt and commercial Pt/C catalyst in 0.5 mol/L H2SO4 solution. The hydrogen adsorption/desorption and the formation and reduction of Pt oxides regions imply that the Pt-enriched surfaces were formed on the nanoporous fcc-FePt and Pt/C samples. Fig. 5(b) shows the ECSA-normalized CV curves of the nanoporous fcc-FePt in 0.5 mol/L H2SO4 + 1.0 mol/L CH3OH mixed solution. The peak current density in the forward scan (jf) is 0.55 mA/cm2, about 2 times compared with that of the Pt/C (0.29 mA/cm2), indicating that its catalytic activity for the methanol electrooxidation is superior to the commercial Pt/C. This is primarily attributed to the synergistic catalytic effects of Pt and Fe [36]. The unique combination of metal ligament network with percolated nanopores also contributes to the enhanced activity [40]. The ratio of jf/jb (jb: the peak current density in the backward scan) are calculated to be 3.8 and 2.1 for the nanoporous fcc-FePt and Pt/C, respectively. The larger jf/jb ratio of nanoporous fcc-FePt suggests an enhanced efficiency but a reduced poisoning tendency.

Fig. 5.   ECSA-normalized CV curves of the nanoporous fcc-FePt and Pt/C catalyst in (a) 0.5 mol/L H2SO4 and (b) 0.5 mol/L H2SO4 + 1.0 mol/L CH3OH solutions.

The nanoporous fcc-FePt alloy was then subjected to heat treatment. Fig. 6 shows the XRD patterns of the samples annealed at 823-943 K for 600 s. The diffractions peaks of the L10-FePt phase emerge at 823 K. With the rise of annealing temperature, the peak intensity of the L10-FePt gradually increases, while that for the fcc-FePt decreases, revealing a continued increase in the volume fraction of the L10-FePt in the samples. For the alloy annealed at 943 K, the diffraction peaks of the fcc-FePt are not detectable, indicating that the fcc-FePt has completely transformed into the L10-FePt.

Fig. 6.   XRD patterns of the nanoporous fcc-FePt alloy before and after annealing at 823-943 K for 600 s.

The SEM surface morphologies of the samples annealed at 823-943 K are presented in Fig. 7. The ligament and pore sizes measured from the SEM images are summarized in Table 1. It is found that the average ligament size increases from 31 nm to 60 nm, whereas the average pore size remains nearly constant at around 16 nm with the rise of annealing temperature. According to the XRD results, the unit cell volumes of the fcc-FePt phase in the dealloyed sample and the L10-FePt phase in the alloy annealed at 943 K were calculated to be 55.4 × 10-3 nm3 and 54.9 × 10-3 nm3, respectively. Provided that long-distance diffusion of Pt and Fe atoms on the nanograins surfaces does not occur, the ligament size should be decreased after annealing, which is contrary to the present observation. Substantial diffusion of Pt and Fe atoms for growth and/or coalescence of the nanograins is necessarily needed for the ligament coarsening, and the growth mechanism deserves a further study.

Fig. 7.   SEM images of the nanoporous fcc-FePt alloy annealed at (a) 823 K, (b) 863 K, (c) 903 K and (d) 943 K for 600 s.

Table 1   The average ligament and pore sizes, coercivity (iHc), saturation magnetizations (Bs), remanence (Br), and remanence ratio (Br/Bs) of nanoporous fcc-FePt alloy annealed at 823-943 K for 600 s.

Annealing temp. (K)Ligament size (nm)Pore size (nm)iHc (kOe)Bs (emu/g)Br (emu/g)Br/Bs
dealloyed2712-37.9--
82331160.1438.59.80.25
86346170.3738.814.40.37
90352172.1041.127.50.67
94360168.4251.441.00.80

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Fig. 8 shows the hysteresis loops for the nanoporous fcc-FePt and the samples annealed at 823-943 K for 600 s. The data measured from the loops are summarized in Table 1. The iHc, Bs, the remanence (Br) and the remanence ratio (Br/Bs) of the annealed alloys are in the range of 0.14-8.42 kOe, 38.5-51.4 emu/g, 9.8-41.0 emu/g, and 0.25‒0.80, respectively, which are increased with the rise of annealing temperature. The improved iHc of the samples is attributed to the increased volume fraction of the L10-FePt phase [45,46]. The atomic magnetic moment of Fe in the L10-FePt is larger than that in the fcc-FePt [47], and hence the volume fraction change of the L10-FePt also results in the enhancement of Bs. Although the samples annealed at 823-903 K are composed of two magnetic components, their hysteresis loops are smooth, being characteristic of the nanocomposite magnets [21]. Among them, the samples annealed at 823 K and 863 K show a small Br/Bs (≤ 0.37), which is attributed to the low volume fraction of the L10-FePt. The high Br/Bs value of 0.67 for the alloy annealed at 903 K indicates a strong exchange-coupling, which is consistent with the prediction of the Stoner-Wohlfarth model [48]. The sample annealed at 943 K exhibits the largest iHc of 8.42 kOe. The high iHc is ascribed to the formation of single L10-FePt phase and the difficulty of domain wall reversing in a nanoporous structure [9,49]. We also investigated the electrocatalytic performance of the annealed alloys. The electrocatalytic activity for the methanol oxidation of the nanoporous alloy becomes worse after annealing, which is mainly attributed to the reduced specific surface areas.

Fig. 8.   Hysteresis loops of the nanoporous fcc-FePt alloys annealed at 823-943 K for 600 s.

4. Conclusions

A nanoporous FePt alloy has been obtained by dealloying a melt-spun FePtB alloy composed of nanoscale amorphous and fcc-FePt phases in H2SO4 aqueous solution. The microstructure, magnetic properties, and electrocatalytic activity for methanol oxidation of the nanoporous alloy before and after annealing were investigated. The results are summarized in the following:

(1)A nanoporous FePt alloy composed of a single fcc-FePt phase with an Fe/Pt atomic ratio of about 55.3/44.7 has been fabricated by dealloying a two-phase Fe60Pt20B20 alloy in H2SO4 aqueous solution. The nanoporous alloy exhibits a uniform porous structure with the average ligament and pore sizes of 27 nm and 12 nm, respectively.

(2)The soft magnetic nanoporous fcc-FePt alloy with a Bs of 37.9 emu/g exhibits improved electrocatalytic activity and efficiency for methanol oxidation reaction compared with commercial Pt/C in acid environment.

(3)Phase transformation from fcc-FePt to L10-FePt has been realized in the nanoporous alloy after annealing in 823-943 K for 600 s. With rise of the annealing temperature, the volume fraction of the L10-FePt phase increases, resulting in the increase in iHc. The alloy annealed at 943 K with a single L10-FePt phase exhibits the largest iHc of 8.42 kOe.

(4)The ligament size of the nanoporous alloy increases with the rise of annealing temperature.

Acknowledgements

This work was supported financially by the National Natural Science Foundation of China (Grant Nos. 51571047, 51171034), the Fundamental Research Funds for the Central Universities (DUT17ZD212), and the Global Institute for Materials Research Tohoku Program, Japan.


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