Facile one-pot synthesis of water-soluble fcc FePt3 alloy nanostructures

Proccessible FePt3 alloy nanoparticles with sizes smaller than 50 nm open the avenue to novel magnetic sensor, catalytic and biomedical applications. Our research objective was to establish a highly scalable synthesis technique for production of single-crystalline FePt3 alloy nanoparticles. We have elaborated a one-pot thermal decomposition technique for the synthesis of superparamagnetic FePt3 nanoparticles (FePt3 NPs) with mean sizes of 10 nm. Subsequent tiron coating provided water solubility of the FePt3 NPs and further processibility as bidental ligands enable binding to catalyst surfaces, smart substrates or biosensors. The chemical composition, structure, morphology, magnetic, optical and crystallographic properties of the FePt3 NPs were examined using high resolution transmission electron microscopy, high-angle annular dark field-scanning transmission electron microscopy, scanning transmission electron microscopy-energy-dispersive X-ray spectroscopy mapping, Fourier transform infrared-attenuated total reflection, X-ray powder diffraction, X-ray photoelectron spectroscopy, vibrating sample magnetometry and UV–Vis absorption spectroscopy.


Introduction
In recent years, bimetallic nanocrystals have received significant attention for nanoengineering, nanoelectronics and nanomedicine because of their particularly physical and chemical properties. Iron platinum alloy nanostructures in the L1 0 , L1 1 and L1 2 phases were engineered for applications as being magnetic sensors, catalysts, drug delivery tools, and biomedical imaging [6,14,21,27,32,39]. Highly ordered FePt alloy nanocomposites exhibit an encouraging potential for ultrahigh density magnetic recording media and high-performance permanent magnets due to their uniaxial magnetocrystalline anisotropy, whereas chemically disordered FePt alloy nanoparticles promise applications in medical diagnostics (e.g. MPI, CT and MRI) [4,14,23]. FePt and FePt 3 alloys may crystallize in three different crystal structures: FePt 3 (L1 2 , AuCu 3type cubic), FePt (L1 0 , AuCu-type tetragonal) and Fe 3 Pt (L1 2 , AuCu 3 -type cubic), the magnetic properties of which depend on the synthesis procedure [2,8,9,14]. On the other hand, in the respective chemically disordered hightemperature FePt phase, the Fe and Pt atoms are randomly distributed over the lattice sites and thus, exhibit isotropic magnetic behavior [27,35]. The most common synthesis routes for preparing FePt, Fe 3 Pt or FePt 3 nanoparticles are the thermal decomposition technique and the polyol synthesis [21,23]. The face centered cubic (fcc) FePt phase can be transformed into the face centered tetragonal (fct) phase by high temperature annealing. Unfortunately, thermal annealing gives rise to agglomeration of FePt alloy nanocrystals which is accompanied by the emergence of undesired shapes and broad size distributions [13,21]. The formation of FePt alloy nanocrystals depend on the growth from two different metals (Fe and Pt) at distinct reaction temperatures [21]. Sun et al. synthesized FePt alloy nanocrystals by thermally decomposing Fe(CO) 5 and Pt(acac) 2 in the presence of the reducing agent oleyl amine and the surfactant oleic acid. Cytotoxic Fe(CO) 5 can be substituted by iron(III) acetylacetonate or iron(III) oleate. Moreover high-boiling temperature solvents such as di-n-octyl ether or 1-octadecene are required for thermal decomposition synthesis routes. Polyol synthesis techniques are based on solvents such as tetraethylene glycol (TEG) and ethylene glycol (EG) which simultaneously serve as solvent and reducing agent for the metallic precursor [23,33]. One major drawback of these demanding synthesis techniques is the impossibility to adjust the growth of FePt alloy nanoparticles within a uniform crystal phase (fcc or fct).
In the present work, we have elaborated a straightforward synthesis route for chemically disordered fcc FePt 3 nanocrystals. This one-pot synthesis is highly scalable for the preparation of superparamagnetic FePt 3 nanoparticles (FePt 3 NPs) for future electronic and biomedical applications. The one-pot synthesis technique comprises the preparation of a Pt(II) precursor solution and subsequent formation of FePt 3 NPs. Subsequent ligand exchange with tiron provided dispersion stability and water solubility of the FePt 3 NPs and moreover, facilitates their processing to thin magnetic films and their application as MRT/CT contrast agents in biomedical imaging. The tiron-coated FePt 3 NPs were characterized by determining the average size, size distribution, morphology, crystallographic properties, colloidal stability, magnetic and optical properties. high resolution transmission electron microscopy (HRTEM), high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM), scanning transmission electron microscopy-energy-dispersive X-ray spectroscopy (STEM-EDX) mapping, Fourier transform infrared-attenuated total reflection (FTIR-ATR), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV-Vis absorption spectroscopies were employed as microscopy and spectroscopy characterization techniques. The magnetic properties were examined using vibrating sample magnetometry.

Synthesis of FePt 3 NPs
Under argon atmosphere, the Pt(II) precursor solution in chloroform was added to a solution containing 15.0 mL 1-octadecene, 1.00 mL oleic acid (3.15 mmol) and 1.00 mL oleylamine (3.04 mmol). The reaction mixture was stirred for 30 min at 120 °C. Then 90.0 mg Fe(acac) 3 (0.255 mmol) dissolved in 5.00 mL 1-octadecene were injected into the hot solution. The reaction mixture was heated to 320 °C and boiled under reflux over 2 h. After cooling down to room temperature the reaction mixture was exposed to air for 1 h. The FePt 3 NPs were precipitated by the addition of 60 mL isopropanol and collected by centrifugation (11,000 rpm, 10 min). Afterwards the FePt 3 NPs were washed with ethanol and centrifugation and redispersed in 10 mL n-hexane for storage.

Tiron-coating of the FePt 3 NPs
100 mg tiron (0.302 mmol) in 1.50 mL milli-Q water were added to a dispersion of 10 mg FePt 3 NPs in 2.50 mL n-hexane. This biphasic n-hexane/water NP mixture was sonicated for 1 h. Then n-hexane was removed using a rotary evaporator. The tiron-coated FePt 3 NP dispersion formed a stable aqueous solution.

Microscopic and spectroscopic characterization
HRTEM images were obtained using a Titan3 Themis 300 (FEI Company, Netherlands) equipped with a high-brightness field-emission gun (X-FEG), a monochromator system and two Cs-correctors (probe and image side) (CEOS GmbH, Germany). The microscope operates at an acceleration voltage of 200 kV. The morphology and elemental composition of the FePt 3 NPs were examined by means HAADF-STEM using the Z-contrast. The STEM-EDX mappings were obtained using a Super-X detector for EDX spectroscopy. The HRTEM images were recorded using a FEI Ceta 16 M 4 K CMOS (complementary metal-oxide semiconductor) camera. HRTEM samples were prepared by dropping the FePt 3 NP dispersions on Cu grids coated with an ultrathin carbon film layer.
XRD patterns of FePt 3 NPs powder samples were measured at room temperature using a Philips/Panalytical X-Pert Pro equipped with a copper radiation, (K α1 = 0.154056 nm and K α2 = 0.154439 nm). The XRD measurements were repeated 10 times. In addition, the Rietveld profile fitting of the XRD pattern of the FePt 3 NP sample was recorded by mounting in a silicon wafer to the XRD instrument at room temperature. Rietveld fitting was obtained by means of MAUD (Materials Analysis Using Diffraction) [15][16][17][18].
FTIR transmission spectra of the FePt 3 NPs were recorded on an IR Prestige-21 spectrometer (Shimadzu, Japan) equipped with an attenuated ATR setup with a diamond/ZnSe crystal plate (Miracle ATR, Pike Technologies, U.S.A.).
XPS was conducted on a Quantera II microprobe (Physical Electronics Inc., Chanhassen, MN, USA) under ultrahigh vacuum conditions. The powder samples were mounted on a double-sided sticky tape to the sample holder. Monochromated Al Kα x-rays (1486.6 eV, source at 15 kV and 50 W) were focused on 200 × 200 µm 2 homogeneously covered spots of the powder samples. The mostly nonconducting samples were neutralized by Ar + ions and electrons. The pure Pt and pure Fe reference samples were presputtered to clean the surface. Photoelectrons were detected at an analyzer pass energy of 280 eV and 140 eV and the number of sweeps for signal averaging were 3 and 25 for survey and high-resolution spectra, respectively. A peak at 284.8 eV associated with C 1s core-level was taken as a reference for the binding energy scale of the FePt 3 NP samples.
The magnetic properties of the FePt 3 NPs were analyzed using a Quantum Design VSM-MPMS SQUID magnetometer at room temperature under an external magnetic field varied between − 5T and + 5T.

HRTEM/HAADF-STEM/STEM-EDX analysis
According to the HRTEM image in Fig. 1a the FePt 3 NPs are single crystalline and flower-like nanostructures with a mean size of 10 nm. Furthermore, the HRTEM image (Fig. 1a, insert) exhibits lattice fringe spacings with 0.194 nm which identifies the (200) lattice plane of the face centered cubic (fcc) phase. Figure 1b shows the HAADF-STEM image of the FePt 3 NPs. The STEM-EDX mapping images visualize the homogeneous elemental distribution of Pt (green) and Fe (red) in the FePt 3 NPs, and thereupon revealing their quasi-alloy FePt 3 composition (Fig. 1c-e).

XRD analysis
The crystal structure of the FePt 3 NPs was elucidated by performing a powder XRD analysis. The XRD pattern in  [29]. The Rietvield fit of the XRD data (Fig. 3) was obtained for the fcc phase of FePt 3 and yielded the lattice constant a = 0.3852 nm [15][16][17][18]. The superlattice reflections (001) and (110) indicate a partially L1 2 -ordered phase of FePt 3 . The average NP diameter was calculated to be 10 nm using the Debye-Scherrer equation [11], which agrees with the HRTEM analysis.

FTIR-ATR spectra
Oleic acid and oleylamine were used as stabilizing agents for the high-temperature synthesis of the FePt 3 NPs. In the FTIR-ATR spectrum of the FePt 3 NPs (Fig. 4, black solid line) the peaks at 2853 cm −1 and 2925 cm −1 were assigned to the symmetric and asymmetric CH 2 stretching modes of oleic acid and oleylamine [5]. Water solubility and further processability of the FePt 3 NPs were achieved by tiron coating through ligand exchange. The binding modus of tiron was elucidated by analyzing the FTIR-ATR spectrum of tiron-coated FePt 3 NPs (Fig. 4, red solid line). This FTIR-ATR spectrum exhibits the characteristic vibrational bands of tiron being bound via the catechol group at the FePt 3 NP surface (Fig. 5). The broad band in the region of 3100-3600 cm −1 arose from the symmetric O-H stretching vibrations of water. The vibrational peak at 1250 cm −1 and 1190 cm −1 are assigned to the asymmetric S=O stretching vibration, and the band at 1031 cm −1 is ascribed to the symmetric O=S=O stretch of tiron [28]. The peak at 630 cm −1 identifies the C-S stretching vibration. The peaks at 1596 cm −1 , 1459 cm −1 and 1426 cm −1 refer to the carbon-carbon stretching vibrations in the aromatic ring of the tiron [28]. Figure 6 exhibits the Fe 2p spectral region, namely the Fe 2p3/2 and Fe 2p1/2 multiplet for the FePt 3 NPs and a pure metal Fe. The Fe 2p spectral region consists of two peaks at approximately 712.8 eV and 726.8 eV. The higher binding energy of the multiplet as compared to the metal reference (707.0 eV and 720.0 eV) and relatively high full width at half maximum of approximately 7 eV for the peak at lower binding energy is often assigned to the presence of a multitude of different iron species in positive oxidation states such as iron oxides [19,22,25,26,36,37]. Qualitatively, the low binding energy tail of the NPs Fe 2p3/2 component overlaps with the respective signal of the pure metal Fe at 707.0 eV. This effect is assigned to the presence of iron species that show only slightly higher binding energy as compared to metal Fe like for example Fe-Pt alloys [20,30]. Figure 7 shows the Pt 4f region comprising the Pt 4f 7/2 and Pt 4f 5/2 multiplet. The Pt 4f 7/2 peak for the NPs is evidenced at 0.2 eV towards lower binding energies as compared to the pure metal Pt at 71.2 eV. The effect, which is just of the order of the uncertainty from the peak referencing of the spectrum, points to the formation of an interfacial Fe-Pt alloy and electron transfer at the interface towards the more electronegative Pt [20,30]. The peak at higher binding energies (75-78 eV) is assigned to the presence of precursor ions in the spectrum of the FePt 3 NPs. The UV-Vis absorption spectrum of tiron-coated FePt 3 NPs displays a broad absorption band around 650 nm (Fig. 8) which is due to electron transfer from Pt to the antibonding π-molecular orbitals of tiron [3,10,31,38]. Valence tautomerization of tiron (OH to =O) of tiron may also contribute to the emergence of these broad absorption band (Fig. 9) [1,3,7,10,12,24,31,34,38]. Figure 10 presents the magnetization curve of a powder sample of tiron-coated FePt 3 NPs at room temperature under an external magnetic field varied between − 5T and + 5T. The lack of any hysteresis loop verifies superparamagnetism of the FePt 3 NPs [29]. The M s value of the FePt 3 NPs is 35.90 emu/g. The observed superparamagnetism reflects the chemical disorder of the FePt 3 NPs.

Formation of FePt 3 alloy nanoparticles
The FePt 3 alloy nanoparticles were synthesized via the thermal decomposition route at 320 °C using Fe(acac) 3 and oleylamine stabilized Pt 2+ cations as Pt source. The formation of the chemically disordered FePt 3 nanoparticles requires the co-reduction of Fe 3+ and Pt 2+ cations. Since Pt is a noble metal and Fe is a transition metal, the Pt 2+ cations are faster reduced to Pt 0 which sparks Pt nucleation. The Pt nuclei presumably catalyze the oleylamineinduced reduction of Fe 3+ to Fe 0 [2]. Subsequent nanoparticle growth occurs via the incorporation of Fe atoms into the Pt rich nuclei. The final size and stabilization are adjusted by the precursor/oleyl acid (oleylamine) ratio. The final stoichiometric composition and structure of the FePt 3 alloy nanoparticles follow from the reaction temperature as well as from the Fe and Pt precursor concentrations.

Conclusions
Our objective was to develop a highly scalable wetchemistry technique for the synthesis of processible, superparamagnetic FePt 3 NPs for versatile applications in electronics, catalysis and medicine. We elaborated a facile one-pot thermal decomposition procedure for the preparation of 10 nm-sized FePt 3 nanocrystals which exhibit superparamagnetism. A straightforward subsequent ligand exchange procedure yielded watersoluble tiron-coated FePt 3 NPs suited for processing to thin magnetic films or for application as CT/MRI contrast agents.
The stoichiometry and structural properties of the FePt 3 NPs were elucidated using HRTEM, HAADF, STEM-EDX, XPS and XRD. We could unambiguously show that FePt 3 NPs with a mean size of 10 nm crystallize in the chemically disordered fcc phase with the space group Fm3m . The chemical disorder substantiated the superparamagnetism of the FePt 3 NPs as being evident from the hysteresis-less magnetization curve. The optical properties and ligand binding modi of tiron-coated FePt 3 NPs were examined using UV-Vis absorption and FTIR transmission spectroscopy. The aqueous FePt 3 NPs solution exhibits a broad absorption band around 650 nm,   which is ascribed to an electron transfer from the Pt atoms to antibonding π-molecular orbitals of tiron which binds through its catechol moiety to the FePt 3 surface. This particular optical property of tiron-coated FePt 3 NPs demonstrates the high catalytic potential of FePt 3 .
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