Synchrotron Small Angle X-ray Scattering Spectra Of Iron-based Magnetic Fluids

K Koyvanich1,a,*, K Chokprasombat 1,b, C Sirisathitkul1,c, P Harding1,d, and S Rugmai2,e

1Molecular Technology Research Unit, School of Science, Walailak University, Nakhon Si Thammarat, Thailand

2School of Physics, Institute of Science, Suranaree University of Technology and Synchrotron Light Research Institute (Public Organization), Nakhon Ratchasima, Thailand

akrit_com@hotmail.com (corresponding author), bkomkrich28@gmail.com, cschitnar@wu.ac.th, dkphimpha@wu.ac.th, esupagorn@slri.or.th

Keywords: magnetic fluid, iron-platinum, TEM, SAXS.

Abstract. Magnetic fluid is a special class of materials which possesses the advantages of a liquid state of the carrier and a magnetic state of the particles. In addition to the conventional uses in mechanical engineering, magnetic fluids containing magnetite (Fe3O4) superparamagnetic nanoparticles are under research and development for drug delivery, hyperthermia and MRI contrast agents. On the other hand, iron-platinum (FePt) is investigated as materials for ultrahigh density recording. Before their assembly into patterned media, the as-synthesized FePt nanoparticles in superparamagnetic state are commonly stored in forms of magnetic fluids. In this work, iron-platinum (FePt) nanoparticles with their surface modified by oleic acid and oleyleamine were synthesized from the polyol process. The starting material was an environmental friendly iron(III) acetylacetonate and the products were dispersed in n-hexane. In small-angle X-ray scattering (SAXS) measurements at the Synchrotron Light Research Institute, Thailand, each magnetic fluid was injected into a sample cell with aluminum foil windows and the X-ray of wavelength 1.55 Å

from BL2.2 was used. The measured SAXS intensity profiles as a function of the scattering vector from 0.27 to 2.30 nm-1 were fitted and compared between two different reactions. Nanoparticles synthesized by using a higher amount of Fe(acac)3 were matched with monodisperse spheres of radius 2.4±0.3 nm. The other reaction with a reducing agent gave rise to smaller nanoparticles of two size distributions. From this work, the potential of synchrotron radiation to complement conventional characterization techniques in the investigation of nanoparticles for high density recording and biomedical applications is underlined.

1 Introduction

Magnetic nanoparticles are currently studied and developed for both biomedical applications and ultrahigh density data storage. The former mainly involves superparamagnetic iron oxides because of their biocompatibility. Magnetite (Fe3O4) and maghemite (γ-Fe2O3), nanoparticles are implemented in drug delivery, hypothermia treatment, MRI contrast enhancement1. For ultrahigh density bit patterned media for the next generation hard disk drives2, iron-platinum (FePt) nanoparticles has received most attentions because of its ferromagnetic phase after the annealing. After the synthesis, these magnetic nanoparticles are kept in carrier liquids and stabilized without agglomeration and sedimentation by surfactants (e.g. oleic acid and oleylamine).

Small-angle X-ray scattering (SAXS) has been used to complement transmission electron microscopy (TEM) and dynamic light scattering (DLS) in morphological characterization of magnetic nanoparticles3-4. Since SAXS is applicable to samples in colloidal forms, it was used to characterize magnetic fluid containing iron oxides5-7. In our previous work, magnetic fluids with Fe3O4 and FePt were compared8. The results revealed polydispersity of the magnetic nanoparticles and the unified exponential-power law model of Beaucage was used to fit the measured data in order to analyze the particle size and size distribution. As confirmed by TEM and DLS, Fe3O4 nanoparticles had a larger average size. The DLS measurement gave larger radii than those obtained

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from SAXS because of the effect of aggregates and surfactants. For FePt nanoparticles, SAXS can be used not only for the determination of their size and shape but also the periodic arrangement of the substrate9-11. Instead of the colloidal forms, such works relied on the the grazing incident X-ray scattering from dried FePt nanoparticles on substrates.

In this work, we have studied morphological and compositional variations in magnetic fluids synthesized from two different reactions using Fe(acac)3 and Pt(acac)2 as starting materials. Their dependence on the amount of Fe(acac)3 and reducing agents will be essential information in the development of an effective and environmental friendly route to synthesize FePt nanoparticles.

2 Experimental

Fe(acac)3 (99.9%), Pt(acac)2 (97%), oleic acid (90%), oleylamine (70%) and potassium borohydride (KBH4) were purchased from Sigma-Aldrich Company and used as received. Benzyl ether was degassed for 15 min before each use. Other AR grade organic solvents used for purification (e.g. hexane and absolute ethanol) were used as purchased. All manipulations were performed under dry nitrogen gas using standard Schlenk line techniques detailed the previous work12. Both reactions used 20 mL benzyl ether as the solvent with 5 mmol of oleic acid and 5 mmol of oleylamine as surfactants. Two reactions differed as shown in Table 1. Since the reducing agents may not be necessary for the modified polyol process at high temperatures, KBH4 was employed only in Reaction 2. Reactions 2 then used a 1:1 molar ratio of Fe and Pt sources whereas the ratio was 3:1 in Reactions 1. After refluxing for 2-3 h, the synthesized particles were cooled down, centrifuged, washed and then dispersed in hexane with the presence of small amount (ca.

0.05 mL) of oleic acid and oleylamine. The magnetic fluids were stored in glass bottles in a refrigerator at 4 °C.

In the characterizations, samples were prepared by depositing the colloid on carbon-coated copper grid and evaporating the solvent at room temperature. The morphology of the products was examined by transmission electron microscopy (TEM) and the elemental composition was probed by energy dispersive spectroscopy (EDS). In SAXS measurements, the X-ray of wavelength 1.55 Å from BL2.2 at the Synchrotron Light Research Institute, Thailand was used. Before each measurement, both sides of a sample cell were cleaned with ethyl-alcohol and a cleaned Kepton tape was put on the beam transmission window. The empty cell, the cell injected by hexane buffer and the cell injected by magnetic fluid were then successively installed in the SAXS beam line. The sample was diluted from the as-synthesized concentration (100%) to 10 %. The flight tube length of

1130 mm enabled the measurement in the scattering vector (q) range from 0.27 to 2.30 nm-1. In

each SAXS measurement, the IIC (count) and IPD (count) are respectively the recorded intensity in the ion chamber just after the measurement is started and integrated intensity after the measurement is done.

Table 1 Fe:Pt atomic ratio measured by EDS and size measured by SAXS of nanoparticles from

two reaction conditions.

Reaction Reagents [mmol]

Fe:Pt atomic ratio

Mean radius

Fe(acac)3 Pt(acac)2 KBH4 < R1 > [nm]

< R2 > [nm]

1 1.5 0.5 - 1.99:1 2.4±0.3 -

2 0.5 0.5 0.5 0.43:1 1.9±0.2 1.7±0.4

32 Materials Science and Technology VII

3 Results and discussion

(a) (b)

Fig 1− TEM images of nanoparticles synthesized from Reaction (a) 1 and (b) 2.

From EDS results in Table 1, the Fe:Pt ratio is predictably higher in the case of the initial molar ratio of metal acetylacetonates to 3:1 in Reaction 1. In TEM images shown in Fig. 1, Reactions 1 and 2 give rise to products of comparable size. Most spheriodal nanoparticles with rough edges are less than 5 nm in diameter. Interestingly, the nanoparticles tend to self-assemble on solid substrates. It follows that the pattern is better defined in the case of Reactions 1 when the nanoparticles are uniform in the size and shape. It is noted that variation in size and shape is increased when the reducing agent is used.

Fig 2− The plot of log I against log q from SAXS measurements of nanoparticles synthesized from two reactions.

Inte

nsit

y: I

Scattering vector: q [nm-1]

Key Engineering Materials Vol. 545 33

In Fig. 2, SAXS profiles between I and q are plotted and fitted with least-squared minimization procedure using the software package SASfit13. Unlike polydisperse nanoparticles in our previous work8, the results from both samples can be fitted with contributions from monodisperse spheres. Only one contribution is needed in the case of Reaction 1 whereas Reaction 2 corresponds to two groups of nanoparticles. The mean particle radii of nanoparticles and their standard deviations of the size distribution are shown in Table 1 and the log-normal size distribution curves obtained from SASfit are shown in Fig. 3. The difference between nanoparticles from these two reactions agrees with the TEM images. The large morphological and size variation in Reaction 2 implies the possible existence of Fe3O4 with FePt according to the heterocoagulation model of FePt formation in the modified polyol process14.

(a) (b)

Fig 3− Size distribution curves of nanoparticles synthesized from Reaction (a) 1 and (b) 2.

4 Conclusion

Nanoparticles of average diameter less than 5 nm in n-hexane can be synthesized from the polyol reduction process of Fe(acac)3 and Pt(acac)2 in the benzyl ether solvent. The composition measured by EDS and morphology studied by TEM and SAXS revealed the effects of reagents on the magnetic fluids. The reaction with a reducing agent (KBH4) led to smaller particles whose larger variation in size was attributed to two groups of monodisperse spheres. Without KBH4, the other reaction with higher ratio of Fe(acac)3 to Pt(acac)2 gave rise to higher fraction of Fe in monodisperse nanoparticles which have a better self-assembled arrangement on a solid substrate.

Acknowledgements

The synthesis of nanoparticles is financially supported by the Industry/University Cooperative Research Center (I/UCRC) in HDD Component, the Faculty of Engineering, Khon Kaen University and National Electronics and Computer Technology Center, National Science and Technology Development Agency with the approval of Seagate Technology (Thailand). The SAXS measurements part is supported by Thailand Toray Science Foundation. The authous would like to thank P. Muthitamongkol and V. Yordsri of National Metal and Materials Technology Center (MTEC) for TEM images.

0.0E+00

1.0E-08

2.0E-08

3.0E-08

4.0E-08

0 1 2 3 4 5 60.0E+00

3.0E-08

6.0E-08

9.0E-08

0 1 2 3 4 5

Reaction 1 Reaction 2

N(R

)

R [nm]

N(R

)

R [nm]

34 Materials Science and Technology VII

References

[1] Q.A. Pankhurst, J. Connolly, S.K. Jones, J. Dobson, Applications of magnetic nanoparticles in biomedicine, J. Phys. D: Appl. Phys. 36 (2003) R167-R181.

[2] S. Sun, Recent Advances in Chemical Synthesis, Self-Assembly, and Applications of FePt Nanoparticles, Adv. Mater. 18 (2006) 393-403.

[3] E.V. Shtykova, X.-L. Huang, N. Remmes, D. Baxter, B. Stein, B. Dragnea, D.I. Svergun, L.M. Bronstein, Structure and properties of iron oxide nanoparticles encapsulated by phospholipids with poly(ethylene glycol) tails, J. Phys. Chem. C 111 (2007) 18078-18086.

[4] G. Utkan, F. Sayar, P. Batat, S. Ide, M. Kriechbaum, E. Piskin, Synthesis and characterization of nanomagnetite particles and their polymer coated forms, J. Colloid. Interf. Sci. 353 (2011) 372-379.

[5] A.F. Thünemann, J. Kegel, J. Polte, F. Emmerling, Super paramagnetic maghemite nanorods: Analysis by coupling field-flow fraction and small-angle X-ray scattering, Anal. Chem. 80 (2008) 5905-5911.

[6] A.F. Thünemann, S. Rolf, P. Knappe, S. Weidner, In situ analysis of a bimodal size distribution of superparamagnetic nanoparticles, Anal. Chem. 81 (2009) 296-301.

[7] P. Knappe, L. Boehmert, R. Bienert, S. Karmutzki, B. Niemann, A. Lampen, A.F. Thünemann, Processing nanoparticles with A4F-SAXS for toxicological studies: Iron oxide in cell-based assays, J. Chromatogr. A 27 (2011) 4160-4166.

[8] S. Rugmai, C. Sirisathitkul, K. Chokprasombat, P. Rangsanga, P. Harding, T. Srikhirin and P. Jantaratana, Small-angle X-ray scattering ppectra of iron-based magnetic fluids, Mater. Tehnol. (2012) accepted.

[9] K. Shinoda, K. Sato, B. Jeyadevan, K. Tohji, S. Suzuki, Local structural studies of directly synthesized L10 FePt nanoparticles by using XRD, XAS and ASAXS, J. Magn. Magn. Mater. 310 (2007) 2387-2389.

[10] C. Hendrich, L. Favre, D.N. Ievlev, A.N. Dobrynin, W. Bras, U. Hormann, E. Piscopiello, G. van Tendeloo, P. Lievens, K. Temst, Measurement of the size of embedded metal clusters by mass spectroscopy, transmission electron microscopy and small-angle X-ray scattering, Appl. Phys. A 86 (2007) 533-538.

[11] C.C.H. Lo, S.C. Tsang, C.H. Yu, K. Tam, Magnetic properties of macroscopic colloid crystals of silica-coated FePt nanoparticles with controllable interstices for molecular separation, J. Appl. Phys. 105 (2009) 07C101.

[12] K. Chokprasombat, C. Sirisathitkul, P. Harding, S. Chandarak, R. Yimnirun, Synchrotron X-ray absorption spectroscopy study of self-assembled nanoparticles synthesized from Fe(acac)3 and Pt(acac)2, J. Nanomater. 2012 (2012) 758429.

[13] J. Kohlbrecher: A program for fitting elementary structure models of small angle scattering data. Available from http://kur.web.psi.ch/sans1/SANSSoft/sasfit.html

[14] W. Beck Jr., C.G.S. Souza, T.L. Silva, M. Jafelicci Jr., L.C. Varanda, Formation mechanism via a heterocoagulation approach of FePt nanoparticles using the modified polyol process, J. Phys. Chem. C 115 (2011) 10475-10482.

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