Post-treatment Method for the Synthesis of Monodisperse Binary FePt-Fe3O4 Nanoparticles
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To obtain the optimal 1:1 composition of FePt alloy nanomaterials by polyol synthesis, the iron precursor (iron pentacarbonyl, Fe(CO)5) must be used in excess, because the Fe(CO)5 exists in the vapor phase at the typical temperatures used for FePt synthesis and cannot be consumed completely. Fabrication of Fe3O4 nanoparticles by consuming the excess iron precursor was an effective strategy to make full use of the iron precursor. In this paper, a facile post-treatment method was applied to consume the excess iron, which was oxidized to Fe3O4 after post-treatment at 150 and 200 °C, and a monodisperse binary FePt-Fe3O4 nanoparticle system was generated. The post-treatment method did not affect the crystal structure, grain size, or composition of the FePt nanoparticles. However, the content and grain size of the fcc-Fe3O4 nanoparticles can be increased simply by increasing the post-treatment temperature from 150 to 200 °C.
KeywordsFePt nanoparticles Fe3O4 nanoparticles Excess iron Grain size Oxidize Post-treatment method
FePt nanomaterials attract considerable attention owing to their promising applications in the fields of magnetic storage, permanent magnets, fuel cell catalysis, and biomedicine [1, 2, 3, 4, 5]. A polyol method, which involves thermal decomposition of iron pentacarbonyl (Fe(CO)5), reduction of platinum acetylacetonate (Pt(acac)2), and stabilizing through surfactants oleic acid (OA) and oleylamine (OAm), has been widely used to synthesize FePt nanomaterials. This method has many advantages, including its facile synthesis, economical approach, and potential for mass production . In general, the performance of FePt nanomaterials strongly depends on their composition [7, 8, 9] To obtain the optimal 1:1 ratio of Fe:Pt, the Fe precursor must be used in excess (at twice the amount of the Pt precursor) because Fe(CO)5 exists in the vapor phase at the typical temperatures used for FePt synthesis and cannot be consumed completely . Many researchers have studied the form that the excess iron takes and have tried to make full use of the iron precursor. It was reported that the rest of Fe(CO)5 could react with OA or OAm to form the Fe-oleate or Fe(CO)x-OAm complex [10, 11]. Increasing the synthesis temperature is a promising strategy for consuming the excess irons and generating Fe3O4 in the reflux process.  The entire iron precursor could be consumed when the synthesis temperature increased to 300 °C, the iron atoms nucleated and grew on the FePt nanoparticles to produce dumbbell-like nanostructures when the molar ratio of the Fe and Pt precursors was equal to 3 . At 280 °C and a molar ratio of 2.2, the excess iron formed a very thin Fe3O4 shell on the FePt nanoparticles . Otherwise, oxidation under air also could be applied to ensure the formation of Fe3O4 . In brief, fabrication of Fe3O4 nanoparticles by consuming the excess iron precursor was an effective strategy to make full use of the iron precursor, because the self-assembly of FePt and Fe3O4 nanoparticles was a permission method to fabricate high-performance exchange-coupled nanocomposites magnets .
Here, we report another facile post-treatment method to consume the excess iron. A monodisperse binary FePt-Fe3O4 nanoparticle system was generated, and the influence of the post-treatment temperature on the content and size of the Fe3O4 nanoparticles was studied.
Excess iron was consumed, and monodisperse binary FePt-Fe3O4 nanoparticles were synthesized by post-treatment of a FePt-hexane system. The apparatus and method used for the synthesis of the FePt nanoparticles was described in our previous research . In brief, 0.1 mmol Pt(acac)2 and 1.0 mmol Fe(CO)5 were used as precursors, 1.6 mL OA and 2 mL OAm were applied as surfactants, and 10 mL dibenzyl ether (DE) was acted as the solvent. The FePt nanoparticles were synthesized by maintaining this mixture at 175 °C for 1 h under a high-purity Ar atmosphere to prevent oxidation. The particles were washed repeatedly with ethanol, were centrifuged, and were finally dispersed in hexane at a concentration of about 5 mg/mL. In a typical post-treatment process, 2 mL of the as-synthesized FePt-hexane solution and 2 mL OAm were injected into a quartz crucible, which was placed inside a vertical tubular resistance furnace . Then, the quartz crucible was heated to 150 or 200 °C at a rate of 5 °C/min, and held at that temperature for 1 h without a protective atmosphere. After cooling, the post-treated nanoparticles were washed, centrifuged, and stored in hexane.
Samples for transmission electron microscopy (TEM, JEM-2100F) analysis were prepared by drying a dispersion of the nanoparticles on amorphous carbon-coated copper grids. The nanoparticles size and their distribution were collected through counting at least 100 particles in TEM images by using Win Roof software. The crystal structure was determined by selected area electron diffraction (SAED) and X-ray diffraction (XRD) using an Ultima IV instrument. To quantitatively analyze the weight percentage of FePt-phase and Fe3O4-phase in the monodisperse binary FePt-Fe3O4 nanoparticle system, a standard Rietveld method was applied to fit the XRD patterns. The composition of the nanoparticles was analyzed by TEM-associated energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS, ESCALAB250). The XPS samples were prepared by drying nanoparticle-hexane ink on a Si substrate in air. The magnetic properties were measured by vibrating sample magnetometer (VSM) at room temperature on a MicroSense EZ9 magnetometer.
Results and Discussion
The Fe/Pt ratio in the as-synthesized and 200 °C-treated samples could be calculated trough the peaks of Fe 2p and Pt 4 f in Fig. 4. The analysis revealed that the Fe content in XPS samples (nanoparticle-hexane ink) were 88.6 and 90.5%, respectively. However, the TEM-EDS results indicate that the Fe counts in the FePt nanoparticles from as-synthesized and post-treated were nearly the same (72.8 and 72.3%), and lower than the Fe counts in the FePt-hexane ink and the binary FePt-Fe3O4 nanoparticle system. We therefore deduced that the excess iron transformed from vapor to liquid (into the FePt-hexane ink) in the reflux, cooling, and washing processes during the synthesis of the FePt nanoparticles. The nature of the excess iron species in the FePt-hexane ink is still unclear, but it is most likely that they are combined with surfactants to ensure the stability of the FePt nanoparticles [10, 11]. The oxidation of the excess irons, or the growth Fe3O4 nanoparticles, is strongly dependent on the temperature and atmosphere. Under the high-purity argon system, the Fe3O4 nanoparticle cannot be obtained at various temperatures. And the FePt-solution would dry out even at 100 °C under the vacuum environment. It is facile to obtained monodisperse binary FePt-Fe3O4 nanoparticle system in air, the Fe3O4 nanoparticle is generated when the temperature is above 100 °C, however, if the temperature is as high as 250 °C, the FePt-solution would be dry out also. The grain size and content of Fe3O4 nanoparticle in the binary FePt-Fe3O4 nanoparticle system are increased when the post-treatment temperature increases from 150 to 200 °C, which would be caused by the temperature enhanced diffusion growth of irons in FePt-hexane-OAm solution.
In summary, the post-treatment method is an effective strategy for the consumption of excess iron used in the polyol synthesis of FePt nanomaterials. The excess iron is oxidized to Fe3O4 after post-treatment, and a monodisperse binary FePt-Fe3O4 nanoparticle system is generated. The content and grain size of the fcc-Fe3O4 nanoparticles can be increased facilely by increasing the post-treatment temperature from 150 to 200 °C.
This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 5169160011 and 51425401), the Fundamental Research Funds for the Central Universities (Grant Nos. N140902001 and N160907001), and the National Training Program of Innovation for Undergraduate (Grant No. 160118).
ZL, CW, and KW performed the experiments, analyzed the data, and drafted the manuscript. LN and GY participated in the synthesis and characterization of the FePt-Fe3O4 nanoparticles. WP helped to finish the TEM sample preparation and observation. The whole project was under the direction of QW. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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- 10.Bian BR, Xia WX, Du J et al (2013) Growth mechanisms and size control of FePt nanoparticles synthesized using Fe(CO)x (x < 5)-oleylamine and platinum( II ) acetylacetonate. Nano 5:2454–2459Google Scholar
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