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Colloids and Surfaces A: Physicochem. Eng. Aspects 336 (2009) 130–134

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

journa l homepage: www.e lsev ier .com/ locate /co lsur fa

acile synthesis of magnetic iron oxide nanoparticles using-methyl-2-pyrrolidone as a functional solvent

hen Qina, Chen Lia, Yizhe Hua, Jianfeng Shena, Mingxin Yea,b,∗

Department of Materials Science, Fudan University, Shanghai 200433, PR ChinaThe Key Laboratory of Molecular Engineering of Polymer, Ministry of Education, China

r t i c l e i n f o

rticle history:eceived 26 September 2008eceived in revised form0 November 2008

a b s t r a c t

Magnetic nanoparticles were generated by thermal decomposition of ferric triacetylacetonate in thepresence of 1-methyl-2-pyrrolidone, a functional solvent. The particle size and shape could be variedby altering reaction parameters. For example, with the addition of oleic acid surfactant, smaller parti-cles were produced. In addition, step elevation of reaction temperature was in favor of regular particle

ccepted 17 November 2008vailable online 28 November 2008

eywords:acileynthesis

morphology. The obtained iron oxide nanoparticles were characterized by X-ray diffraction, transmissionelectron microscopy, Fourier-transform infrared spectroscopy, thermogravimetric analysis and vibratingsample magnetometer. All of the experimental evidences supported that we succeeded in synthesizingmonodisperse iron oxide nanoparticles via a facile and effective route. Moreover, when the oleic acid wasadded, 1-methyl-2-pyrrolidone and oleic acid possibly forms an intercalated monolayer on the particle

ron oxide nanoparticlesunctional solvent

surface.

. Introduction

Magnetic nanoparticles have been among the appealingesearch topics of nanoscience due to their unique properties andotential applications [1–4]. Although there are several kinds of

nteresting magnetic nanoparticles, iron oxide particles – mag-etite (Fe3O4) and its oxidized form maghemite (�-Fe2O3) – havettracted more attention because they are non-toxic and less sus-eptible to changes due to oxidation. These facts make this kindf nanoparticles one of the most studied materials for biomedicalpplications.

For all applications, synthetic techniques providing precise con-rol over nanocrystal grain size are crucial in that they permitngineering of the magnetic properties. The classical syntheticechniques of iron oxide nanoparticles have relied on the copre-ipitation of Fe2+ and Fe3+ ions by a base. The disadvantage of thesequeous solution syntheses is that the pH value of the reactionixture has to be adjusted in both the synthesis and purification

teps, and the process toward monodisperse nanoparticles smallerhan 20 nm has only very limited success [5]. Other techniques suchs microemulsion, ultra sound irradiation and laser pyrolysis, haveeen developed for producing iron oxide nanoparticles. However,

∗ Corresponding author at: Department of Materials Science, Fudan University,o. 220, Hadan Road, Shanghai 200433, China. Tel.: +86 21 55664095;

ax: +86 21 65642622.E-mail address: mxye@fudan.edu.cn (M. Ye).

927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfa.2008.11.040

© 2008 Elsevier B.V. All rights reserved.

nanoparticles obtained by above-mentioned approaches may bepoorly crystalline and the size uniformity is hard to control [6]. Inrecent years, an alternative method based on the thermal decom-position of organic iron precursors has been demonstrated to bevery successful in the preparation of iron oxide nanoparticles withcontrollable size and high quality. Rockenberger’ group publisheda study of �-Fe2O3 nanocrystals prepared by thermal decompo-sition of pentacarbonyliron [Fe(CO)5] in hot surfactants [7]. Sunand co-workers reported that the thermolysis of ferric triacety-lacetonate [Fe(acac)3] can make monodisperse Fe3O4 nanoparticles[8]. Some other research groups demonstrated that FeCup3 [Cup:C6H5N(NO)O−] could be used as a starting material to form �-Fe2O3nanoparticles [9–11].

Rather than adopting commonly used nonpolar high-boiling-point solvents, we have focused our study on 1-methyl-2-pyrrolidone which can serve as a coordinating solvent in additionto a high-temperature reaction medium. Moreover, comparing todifferent ethers in many cases, 1-methyl-2-pyrrolidone has no per-oxide formation and is less dangerous. In this work, we reporta one-pot synthesis of monodisperse iron oxide nanoparticles bythermal decomposition of Fe(acac)3 in the presence of 1-methyl-2-pyrrolidone. Oleic acid (OA) was added as a competitive surfactantto find its influence. The heating procedures were also changed

to examine how the temperature affected the formation of mag-netic nanoparticles. We characterized the products in term of theirmagnetic properties, their particle morphology, surfactant coverageand interaction at the particle surface by vibrating sample mag-netometer (VSM), X-ray diffraction (XRD), transmission electron

C. Qin et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 336 (2009) 130–134 131

Table 1Magnetic nanoparticles synthesized via different processes.

Sample Mass of Fe(acac)3 (g) Mass of OA (g) Total volume of 1-methyl-2-pyrrolidone (ml) Temperature

MN1 1.41 0 50 194 ◦C, 1 hM 50 194 ◦C, 1 hM 50 194 ◦C, 0.5 h; 204 ◦C, 1 h

madaWtd

2

ncwdotcpttbTaswtaM

DawJssowht

3

3

ssm(odJtfao

Fig. 1. X-ray diffraction patterns of samples with different molar ratios of OA toFe(acac)3: (A) MN1 = 0:1; (B) MN2 = 3:1; (C) MN3 = 3:1 with an additional heatingprocedure.

Table 2Comparison of d-spacing values of the synthesized iron oxide nanocrystals withstandard JCPDS Fe3O4 and �-Fe2O3 data.

h k l Fe3O4 MN1 MN2 MN3 �-Fe2O3

2 2 0 2.967 2.972 2.946 2.967 2.9533 1 1 2.532 2.523 2.527 2.531 2.518

approach, the particle size and shape would be varied by alteringdifferent reaction parameters. When there was no addition of OA,larger particle size calculated from the statistical results shown inFig. 2A was about 16.8 nm with a standard deviation of 1.6 nm. Theaverage size was in close agreement with the value estimated by

Table 3The size of magnetic nanocrystals calculated by Scherrer’s formula.

N2 1.41 3.39N3 1.41 3.39

icroscopy (TEM), Fourier-transform infrared (FTIR) spectroscopynd thermogravimetric analysis (TGA). With the experimental evi-ence, our data suggest that the nanoparticles are monodispersend the synthetic route does not require a size-selection procedure.e believe that these well-organized self-assemblies will allow fur-

her applications in many technological fields such as high areaensity data storage.

. Experimental

In the following, a typical synthesis yielding iron oxideanocrystals is described. 30 ml purified 1-methyl-2-pyrrolidoneontaining 3.39 g oleic acid was heated to 194 ◦C. 1.41 g Fe(acac)3as dissolved in 20 ml purified 1-methyl-2-pyrrolidone and addedropwise into the 1-methyl-2-pyrrolidone/OA solution under vig-rous stirring. The process of dropping was completed in 5 min andhe stirring was continued for another 1 h. The whole process wasarried out under a nitrogen atmosphere. During this thermolysisrocess, the initial deep red-brown color of the solution changedo dark brown. After cooled to room temperature, the mixture wasreated with a 3-fold volume excess of ethanol under air to yield alack precipitate, which could be collected either by centrifugation.he resulting black powder was washed several times by acetonend dried in a vacuum oven at 60 ◦C overnight. For comparison theynthesis was carried out by the similar preparation processes butith different parameters, such as the addition of oleic acid and

he heating procedure, to examine their roles critical to the size-nd shape-guiding processes. Three different samples, labeled MN1,N2 and MN3, were listed in Table 1.Crystal structure information was obtained using a Rigaku

/max-�B X-ray diffract meter with Cu K� radiation (� = 1.54056 Å)t a scanning rate of 0.02◦ s−1 in the 2� ranging from 25◦ to 75◦,hile the morphology of samples was observed by TEM using a

EOL 2010F. FTIR Spectroscopy data were collected on a NEXUS 670pectrometer. At room temperature, the spectra were recorded byignal averaging of 32 scans at a resolution of 2 cm−1 in the rangef 4000–500 cm−1. TGA was conducted with Netzsch TG 209 thatas fitted to a nitrogen purge gas from 30 ◦C to 600 ◦C at 10 ◦C/mineating rate. Magnetic measurement was operated in air at roomemperature by using a home-made VSM.

. Results and discussion

.1. Composition and microstructure of the thermolysis products

Both XRD and TEM measurements were performed to obtaintructure information of the resulting magnetic nanoparticles. Fig. 1hows XRD patterns of nanoparticles synthesized from reactionixtures. The positions were assigned to the (2 2 0), (3 1 1), (4 0 0),

4 2 2), (5 1 1) and (4 4 0) reflections of the inverse spinel structuref �-Fe2O3 (JCPDS no. 39-1346). However, d-spacing values of alliffraction peaks in Fig. 1 are well consistent with data from the

CPDS no. 19-0629 for Fe3O4 (see Table 2). It should be noted thathe standard XRD patterns of �-Fe2O3 and Fe3O4 are alike. There-ore, we concluded that these samples were mixture of �-Fe2O3nd Fe3O4, possibly Fe3O4 in the core and �-Fe2O3 on the surface. Inther words, the initial synthesized Fe3O4, especially on the surface

4 0 0 2.099 2.092 2.095 2.092 2.0894 2 2 1.715 1.696 1.703 1.710 1.7055 1 1 1.616 1.608 1.612 1.610 1.6074 4 0 1.485 1.484 1.478 1.481 1.476

of these particles, easily oxidized to �-Fe2O3 by the atmospheric O2during the washing and drying processes.

The crystal domain sizes of magnetic nanocrystals could be cal-culated from line broadenings by the Scherrer’s formula [12]:

D ≈ 0.89�

ˇ cos �(1)

where � is the wavelength (0.154056 nm), � is the diffraction angle,ˇ is the full width at half maximum intensity of the peak and D isthe particle size. The particle size yielding from the above equationin the (3 1 1) reflection was listed in Table 3. By the current synthetic

Sample � ˇ (rad) D311 (nm)

MN1 17.78◦ 0.0086 16.7MN2 17.80◦ 0.0152 9.5MN3 17.72◦ 0.0140 10.3

132 C. Qin et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 336 (2009) 130–134

e inserted pictures are corresponding particle size histograms.

SnpaogOupbtst1ipin

3

a

Fig. 2. TEM images of (A) MN1, (B) MN2, and (C) MN3. Th

cherrer’s formula. It is indicated that each individual particle isearly a single crystal. Once the OA surfactant was added, smallerarticles (9.2 ± 2.1 nm) were produced (Fig. 2B), which could bescribed to that when the OA surfactant was adsorbed on the ironxide particles, the number of accessible sites for further crystalrowth was reduced, leading to termination of crystal growth [13].therwise the shape of nanoparticles was not spheriform but irreg-lar. The atomic lattice fringes in Fig. 2B could demonstrate that thearticles are also crystalline. Sample MN3 (Fig. 2C) was achievedy refluxing the reaction mixture at 204 ◦C for 1 h after an ini-ial treatment at 194 ◦C for 30 min. This sample was even larger inize (11.1 ± 1.2 nm) and possessed more homogeneous shape thanhat obtained by simply refluxing the reaction mixture at 194 ◦C forh. This suggests that step elevation of reaction temperature was

n favor of regular particle morphology. The acquisition of largerarticles was consistent with the Ostwald ripening process dur-

ng which large crystals grow even larger at the expense of smalleranocrystals [14].

.2. Characterization of nanoparticle surfaces

The surface chemical structure of the magnetic nanocrystalsnd their capping were characterized by FTIR spectroscopy and

Fig. 3. FTIR spectra of purified 1-methyl-2-pyrrolidone, iron oxide nanoparticles andpure oleic acid.

C. Qin et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 336 (2009) 130–134 133

T5lattbtob[Or1cmMmOwt1s

cI

3.3. Magnetic properties

F1

Fig. 4. Heating curves for (A) sample MN1 and (B) sample MN2.

GA. As depicted in Fig. 3, the characteristic peaks centered at85 cm−1 in all spectra of magnetic particles can be ascribed to

attice absorption of magnetite [15]. The band peak at 1710 cm−1 isttributed to the carbonyl (C O) stretching mode, which is charac-eristic of the OA dimer form [16]. This band is not observed inhe spectra of magnetic samples, but two additional vibrationalands appear at around 1530 cm−1 and 1430 cm−1 in both spec-ra MN2 and MN3. Such changes are assigned to the formationf either a monodentate or bidentate complex between the car-oxylic acid and Fe on the surface of the iron oxide nanoparticles17,18]. All of these results support that the carboxylic acid fromA is bound to the particle surface. The FTIR measurements also

eveal that the vibrational band C O shifts from 1680 cm−1 for pure-methyl-2-pyrrolidone to 1650 cm−1 for sample MN1, which indi-ates that the O from C O coordinates with Fe on the surface of theagnetic nanocrystallites [19]. Since this band remains in spectraN2 and MN3, it can be deduced that the 1-methyl-2-pyrrolidoneolecules bound to the particle surface are partially replaced byA. This is because 1-methyl-2-pyrrolidone, a coordinating solvent,as in great excess in comparison with OA under current prepara-

ive condition. We believe that there is some competition between-methyl-2-pyrrolidone and OA to coordinate with iron particles, a

cenario which needs further investigation.

Based on TGA measurements, we could determine the relativeomposition of iron oxide core, organic shell and solvent residues.n Fig. 4, curves A and B show the degradation of samples MN1

ig. 5. Schematic drawing of 1-methyl-2-pyrrolidone, oleic acid, Fe nanoparticles whose-methyl-2-pyrrolidone and oleic acid.

Fig. 6. Magnetic hysteresis loops measured at room temperature for samples MN1and MN2.

and MN2, respectively. Both samples display y a similar mass-loss profile below 260 ◦C, with initial losses for water or solventresidues and subsequently organic shell degradation. In both cases,the amount of water release corresponds to ∼3.4% at T < 180 ◦C.For sample MN1, the mass profile exhibits a well defined decreaseover a temperature range of 260–400 ◦C. For sample MN2, sig-nificant mass losses were detected in the heating from 270 ◦C to430 ◦C, and the mass loss decreased more intensely as the tem-perature above 270 ◦C, revealing a different pattern from thatobserved for MN1. During the temperature range of 180–700 ◦C,sample MN1 and sample MN2 have 11.0% and 14.8% mass loss fororganic shell, respectively. For the sample MN1, the weight losseswere due to the desorption and subsequent evaporation of the 1-methyl-2-pyrrolidone. For the sample MN2, the weight reductionsare attributed to quantitative mass loss of OA and 1-methyl-2-pyrrolidone, according to FTIR results. For OA coated iron oxidenanoparticles, more than 20% mass losses were observed by otherresearch groups [20,21]. Therefore, it further supports that the OAdoes not completely displace 1-methyl-2-pyrrolidone and possiblyforms an intercalated layer on the particle surface (Fig. 5).

In a preliminary investigation of magnetic properties of thesenanoparticles, hysteresis loops were recorded at room tempera-ture. Fig. 6 shows the room-temperature magnetization curves

surface is capped with 1-methyl-2-pyrrolidone or a monolayer consisting of both

1 ysicoc

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[44.

34 C. Qin et al. / Colloids and Surfaces A: Ph

f samples MN1 and MN2. The two samples present ferromag-etic features probably explained by aggregation or layering effects22]. In the case of 1-methyl-2-pyrrolidone coated nanoparticles,escribed as magnetization curve MN1, the saturation magnetiza-ion is estimated to be higher than 28.9 emu/g. Since the weightercentage of pure iron oxide nanoparticles in the resultant sampleN1 is around 85.4%, determined by TGA, the saturation magneti-

ation of the iron core should be higher than 33.8 emu/g. The valuef saturation magnetization decreases abruptly to about 4.2 emu/ghen the OA is added. The reduced saturation magnetization isost likely attributed to the much smaller size of the iron oxide

anoparticles and the existence of OA surfactants on the surface.ample MN2 showed obvious coercivity of 45 G like the sampleN1, but the remanence of 2 emu/g is smaller than that (7 emu/g)

or the sample MN1 due to the coating of oleic acid.

. Conclusions

In summary, size-controlled synthesis of monodisperse ironxide nanoparticles are performed by the high-temperature reac-ion of Fe(acac)3 in the presence of 1-methyl-2-pyrrolidone. Theddition of OA as a competitive surfactant could change the size andhape of iron oxide nanoparticles. Our synthetic procedure offerseveral important advantages. First, the facile synthetic methodllows highly crystalline and monodisperse nanoparticles to bebtained directly without further size selection procedure. Second,

article size can be easily altered by changing the experimentalarameters. Third, this synthetic process can be readily scaled up forass production. In addition, the low cost of reagents and the high

ields make this reliable way economic, and the reaction mediums environmentally friendly.

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hem. Eng. Aspects 336 (2009) 130–134

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