Sovlothermal synthesis and magnetic properties of platinum alloys nanoparticles

Wen Ming, Zhu Yuanzheng, Meng Xiangguo
(Department of Chemistry, Tongji University, Shanghai 200092,China)

Abstract Three platinum alloy nanoparticles in the range of 2¨C4 nm were synthesized by phase- transfer solvothermal process. The characteristics of as-obtained nanoalloys were correlated by HRTEM, EDS, XRD, DSC and VSM. The additions of Ni lower L10 kinetic ordering temperature in Fe-Ni-Pt system. Magnetic specificity is discovered that as-synthesized nanoalloys changed from paramagnetic to ferromagnetic with Hc change from 60 to 1900 Oe as Ni content increse, but the variation trend of annealed ones are inverse case with Hc varied from 4218 to 228Oe.

    Synthesis of alloy magnetic nanoparticles has attracted great interest for their potential applications in ultrahigh-density magnetic storage[1]. Especially, FePt nanoparticles, as an important candidate storage media, have been under intensive study in recent years for their magnetic properties[2] due to their L10 phase with large uniaxial magnetocrystalline anisotropy [Ku¡Ö7¡Á106 J/m3] and good chemical stability[3¨C6]. Catching the advantages of platinium alloys and striking characteristic of nanosized materials, platinium alloys nanoparticles can get prominent properties in ferromagnetism[3,7], catalytic activity[8,9] and corrosion resistance[10]. Some work report on the synthesis of binary alloy nanoparticles such as FePt [11], CoPt [12], NiPt[13] and so on. However, to the best of our knowledge, few reports were concerned with synthesis and magnetic properties of FeNiPt ternary alloy nanoparticles. Because Ni can lower the L10 curie temperature in FeNiPt films[14], meanwhile low phase transformation temperature is desired for the L10 platinium alloys as recording media. Thus FeNiPt ternary alloy became better candidate than FePt for heat-assisted magnetic recording to overcome the high coercive fields. Here for understand the relationship of the composition, phase transition and magnetic property, three alloys nanoparticles of FePt, FeNiPt and NiPt are synthesized through reduction of Fe2(C2O4)3·5H2O, NiCl2·6H2O and H2PtCl6·6H2O by propylene glycol involving sodium oleate and OA as surfactant and stabilizer in the solvothermal system[15].The structure, morphology, kinetic phase transition and magnetic properties are discussed detail in this paper.

2.1 Materials and Synthesis

H2PtCl6·6H2O(99%), Fe2(C2O4)3·5H2O(99%), C3H8O2(99%), NiCl2·6H2O(99%), C2H5OH(99%), C18H34O2(99%), were all purchased from Sinopharm Chemical Reagent Co., Ltd.. Platinium nanoalloys were prepared within the molar ratios of Fe2(C2O4)3/NiCl2/H2PtCl6 at 55/0/45(S1), 45/10/45(S2), and 0/55/45(S3). As far as S2 was concerned, ethanol (2mL) was mixed with Fe2(C2O4)3·5H2O (1.5mmoL), NiCl2·6H2O (0.7mmoL) and H2PtCl6·6H2O (3mmoL), then added to 60ml autoclave tube. Sodium oleate (10mg), propylene glycol (20mL), and oleic acid (10mL) were added into the solution. The reaction system was sealed and treated at 170¡ãC for 10h. After the reaction was cooled to r.t., the nanoproducts were collected at the bottom of the container.
2.2 Characterization
The morphology and structure of alloy nanoproducts were characterized by HRTEM and XRD on Bruker D8 (Cu Ka radiation source at l = 0.154056nm). EDS was conduced at 20keV on the TN5400 instrument. DSC (STA409PC, NETZSCH) was used to examine the thermal stability, magnetic properties was measured by VSM (lakeshore7312).

    3.1 Structure

    Fig.1 shows the TEM images and EDS spectra of the as-synthesized nanoparticles of FePt (Fig.1a), FeNiPt (Fig.1b) and NiPt (Fig.1c). The size of plantinium alloy monodispersed nanoparticles generated in this reaction was in the range of 2¨C4 nm with narrow deviation. The measurement results of composition analysis give Fe52Pt48 for S1, Fe41Ni12Pt47 for S2 and Ni54Pt46 for S3, respectively. Strong peaks for Fe, Ni, and Pt undoubtedly confirmed that the as-synthesized nanoproducts are FePt for S1, FeNiPt for S2 and NiPt for S3. The atom ratio are about 1/1 for Fe52Pt48 in Fig. 1(a), 3/1/4 for Fe41Ni12Pt47 in Fig.1(b) and 1/1 for Ni54Pt46 in Fig.1(c).


Fig. 1TEM images and EDS Spectra of Fe52Pt48(a) Fe41Ni12Pt47 (b) and Ni54Pt46 (c)

Fig.2 shows XRD patterns of series of as-synthesized platinum nanoalloys and annealed ones at 823K under argon for 1h. The as-synthesis plantinium alloy nanoparticles are chemical disordered fcc structure (Fig.2a1b1c1). Annealing treatment induce the phase transfer to L10 (fct phase), confirmed by XRD in Fig.2a2b2c2, as indicated by the (111) and (200) peak shift and evolution of the (001), (110), (201), (220) and (311) peaks.

Fig.2. XRD patterns of nanoproducts of Fe52Pt48,  Fe41Ni12Pt47 and Ni54Pt46: (a1)(b1)(c1) as-synthesized;
(a2)(b2)(c 2) annealed at 823K under argon for 1h.

Fig. 3.
DSC curves: (a) Fe52Pt48; (b)Fe41Ni12Pt47; (c) Ni54Pt46.

We summarized the mechanism as follows. Three phases formed in system: sodium oleate (solid), the liquid phase of ethanol and oleic acid (liquid), and the water-ethanol solution containing transition metal ions (solution). The phase transfer of Mn+ occurred spontaneously across the interface between solid sodium oleate and ethanol solution based on Mn+ and Na+ exchange. At the designated temperature, the propylene glycol in the liquid phases reduced the metal ions at the liquid-solid interfaces. Thus, the platinium alloy nanoparticles could be obtained at the bottom of the container. All as-synthesized nanoalloys have disorder structures. Annealing induced the phase transformation to chemically ordered fct phase.
3.2 Thermal stability
In the interesting of the kinetic ordering temperature, the nanoproducts were characterized by DSC (Fig.3). There are two exothermic peaks at 451.6K and 682.3K for Fe52Pt48, 475.0K and 678.1K for Fe41Ni12Pt47, 485.2K and 668.7 K for Ni54Pt46. For a given Pt content, with the Ni content increasing, the kinetic ordering temperature of A1 phase shift to high temperature field in the order of Fe52Pt48(Fig.3a)£¼Fe41Ni12Pt47 (Fig.3b)£¼Ni54Pt46 (Fig.3c), and of L10 phase shift to low temperature field in the reverse order of Fe52Pt48(Fig.3a)£¾Fe41Ni12Pt47 (Fig.3b)£¾Ni54Pt46 (Fig.3c). So the additions of Ni in FePt higher kinetic ordering temperature of A1 phase and lower that of L10 phase. In the ternary FeNiPt alloys[14], because Ni atoms are approximately the same size as Fe atoms, a size argument is not sufficient to explain the observed behavior. Therefore, a chemical content must also contribute to the activation energy. In FeNiPt, it exhibits substantial decreases in the activation energy for even small additions of Ni. So the addition of Ni acts to accelerate the L10 phase transformation, which does not agree with the previous reported[14].
3.3 Magnetic properties
The Hc of as-obtainzed Fe52Pt48, Fe41Ni12Pt47 and Ni54Pt46 are 60Oe, 830Oe and 1900Oe, respectively. After annealed, the Hc are changed to 4218Oe for Fe52Pt48, 2450Oe for Fe41Ni12Pt47, 228Oe for Ni54Pt46( Table.1). Interestingly, the magnetism of as-synthesized nanoalloys changed from paramagnetic with Hc of 60Oe to ferromagnetic with Hc of 1900Oe as Ni content varied from 0at.% to 54at.%, but the variation trend of Hc of annealed ones are inverse case, Hc decrease from 4218Oe (ferromagnetic) to 228Oe (paramagnetic).

Table 1£® Coercivity (Hc) of as-synthesized (Sna) and annealed ones (Snb): S1(a) S1(b) Fe52Pt48; S2(a) S2(b) Fe41Ni12Pt47; S3(a) S3(b) Ni54Pt46.

Sample No. S1 S2 S3
Alloy content(at.%) Fe52Pt48 Fe41Ni12Pt47 Ni54Pt46
Hc(Oe) a 60 830 1900
b 4218 2450 228


    Three monodispersed platinium alloys nanoparticles with the size of 2-4 nm were synthesised through the reduction of Fe2(C2O4)3·5H2O, NiCl2·6H2O and H2PtCl6·6H2O by a propylene glycol in the solvothermal system. The additions of Ni lower L10 kinetic ordering temperature in Fe-Ni-Pt. Magnetic specificity indicated that as-synthesized nanoalloys changed from paramagnetic to ferromagnetic as Ni content increase, but the annealed ones are inverse case.