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uses. In medicine, these systems have been applied as part ofcancer therapy, as well as in diagnosis, where the magnetite

magnetic resonance (NMR) [2]. Commercial products in the

have used a colloidal method with NH4OH as hydrolyzing agentin substitution of NaOH or KOH. They obtained a highsaturation of magnetization and avoided impurities as -FeOOH and other iron compounds. They report that a the

Materials Letters 61 (2007)Advances in nanoscience and nanotechnology are centeredin the control of the size and shape of nanoparticles, as well asobtainment of the extended arrangement of nanoparticles in1D, 2D and 3D. Their physical properties depend on thesevariables and their anisotropy from which it may be possible tofind new nanostructured systems with novel and specificproperties [1]. New optimized methods of synthesis arenecessary to allow for the control of the shape and sizedistribution of nanoparticles. In particular, systems made ofiron oxides nanoparticles, have an enormous potential towardsapplications in several areas such as magnetic recordingtechnology, pigments, catalysis, photocatalysis and medical

market for these medical treatments exist in the marketnowadays. However, they have wide size distributions (120180 nm) and their particle size is bigger than the extracellularspace (b50 nm) [3]. Hence it is necessary to obtain magneticnanoparticles with smaller and narrower size distribution thanthe ones manipulated with external magnetic fields. Anotherpotential application of these nanoparticles is their use astertiary treatment of residual waters acting as powerful reduceragents of organic and inorganic material, with the advantagethat it could be possible to recycle and separate the magnetiteparticles by an external magnetic field [4].

Concerning the synthesis of magnetite, Gribanov et al. [5]several aging times (2, 5 and 10 min). A detailed study by X- ray diffraction (XRD), Conventional Transmission Electron Microscopy (CTEM),High-Resolution Transmission Electron Microscopy (HRTEM) and electron diffraction patterns showed that with a reaction time less than 5 minnanoparticles of magnetite phase (Fe3O4) were synthesized, and with a bigger time of reaction the lepidocrocite phase (FeO(OH)) was identified.The minor particle average size measured was 6 nm in the sample, 0.0125 M with 2 min of aging time (0.0125M2 m). In addition it was possibleto obtain a narrow nanoparticle size dispersion from 4 to 10 nm for small aging times. 2007 Elsevier B.V. All rights reserved.

Keywords: Magnetite nanoparticles; Magnetic materials; Microstructure; Nanomaterials; HRTEM; XRD

1. Introduction nanoparticles are used as contrast agent for studies of nuclearusing precursors like FeCl36H2O and FeCl24H2O; deionizated wateAbstract

Iron oxide nanoparticles in the interval of 443 nm were synthesized by a colloidal method at room temperature, without use of surfactants andSynthesis of magnetitwithout surfactants

I. Martnez-Mera a,b, M.E. Espinosa-Pesqueiraa Instituto Nacional de Investigaciones Nuc

La Marquesa, Municipio de Ocoyoacb Facultad de Qumica-UAEM, Paseo Coln Esq. Pas

c Departamento de Materia Condensada, Instituto de Fsica, Univer

Received 8 August 2005Available online Corresponding author. Instituto de Fsica, UNAM, Apdo. Postal 20364,C.P. 01000,Mxico, D.F.,Mexico. Tel.: +52 55 56225163; fax: +52 55 56225009.

E-mail address: jarenas@fisica.unam.mx (J. Arenas-Alatorre).

0167-577X/$ - see front matter 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2007.02.018(Fe3O4) nanoparticlesroom temperature

R. Prez-Hernndez a, J. Arenas-Alatorre c,

es (ININ), Km. 36.5 Carr. Mxico Toluca,do. de Mxico, C. P. 52750, Mexicoollocan, Toluca, Edo. de Mxico, C.P. 50120, Mexicod Nacional Autnoma de Mxico, Mxico, D.F., C.P. 04510, Mexico

cepted 9 February 2007February 2007

44474451www.elsevier.com/locate/matlettemperature of 20 C is ideal for magnetite formation andsuggest the use of an excess of alkali and iron salts concentrationof about 0.1 M. On the other hand, Murray et al. [6] synthesized

CTEM analysis was performed with a JEOL2010 HTelectron microscope with a point resolution of 2.3 , HRTEManalysis was performed in a JEOL2010 FEG instrument with apoint resolution of 1.9 . The samples were placed on holeycarbon copper grids of 300 mesh. The measurements of lattice-fringe spacing and angles recorded in digital high-resolutionelectron micrographs were made using digital image analysis ofreciprocal space parameters, according to the de Ruijter et al.method [12], with this method, the precision is 0.0001 nm forlattice spacing and 0.1 for lattice planes angles. This analysiswas carried out with the aid of the Digital Micrograph software.About 100 particles were analyzed for structural characteriza-tion by HRTEM. The particle size distribution measurementswere made with the Quiantikov image analyzer.

The structural phase identification of the iron oxidenanoparticles and the crystalline size calculations were doneusing a Siemens D5000 X-ray diffractometer with Cu-Kradiation (=0.15405 nm) in the interval 2 of 10 to 100, in

ials Letters 61 (2007) 44474451metallic nanoparticles made of Fe, Ni, Co and Pt by the samemethod. In particular they synthesized Fe nanoparticles with arange size of 220 nm at 200 C, using pentacarbonile iron (Fe(CO)5) as precursor and surfactants as stabilizing agents. Finally,the particles size distribution was controlled by selectivestabilization.

Other reports indicate the use of several synthesis routes forthe formation of magnetic nanoparticles. For example, amicroemulsion route was employed for the synthesis ofmagnetite nanoparticles smaller than 10 nm [7]. Vollath et al.[8] reported a microwave plasma synthesis technique in whichthey obtained particles with sizes up to below 10 nm, usingFeCl3 and Fe3(CO)12 as precursors materials. Other techniqueshave been used for the iron oxide nanoparticles synthesis suchas solgel, freeze-drying, laser pyrolisis and vaporizationcondensation methods [911]. However, one of the mainchallenges of these numerous and novel techniques lie in theircapacity to obtain a narrow dispersion in particle size togetherwith the desired compositional, structural and crystallineuniformity.

In consequence, the aim of this work is to explore thefeasibility of synthesizing magnetite (Fe3O4) nanoparticles bya colloidal method at room temperature and without the use ofsurfactants. The characterization of the synthesized nanopar-ticles was carried out with CTEM, HRTEM and XRDtechniques.

2. Synthesis and characterization

Reagents as ferric chloride hexa-hydrate (FeCl36H2O,Aldrich 98%), ferrous chloride tetra-hydrate (FeCl24H2O,Aldrich 99%) and ammonium hydroxide (NH4OH, Aldrich2830% of ammonia) were used for the synthesis as well asdeionizeddeoxygenated water with conductivity between 0.6and 1 S/cm. A solution of ferric chloride 0.0125 M was addedto a ferrous chloride solution 0.0125 M and a solution 1 M ofammonium hydroxide.

A volume of 200 ml of deionizated water was placed in abottom round flask, subsequently the water was deoxygenatedby bubbling N2 gas for 30 min. Later, 50 ml of ammoniumchloride (IM) was added and the mixture was stirred for 10 minat 1000 rpm using mechanical agitation. Afterwards, 10 ml offerrous chloride 0.0125M and 20 ml of ferric chloride 0.0125Mwere added; immediately a black precipitate appeared, which isseparated. Three different aging times were used, 2, 5 and10 min.

A decantation process was applied with the aid of a 0.2 Tmagnetic field. Finally the product was washed four times with25 ml of deionizated and deoxygenated water and then dried at45 C for 4 h.

Six samples were synthesized, three of them with aconcentration of 1 M and aging times of 2, 5 and 10 min(identified as 1M2m, 1M5m and 1M10m). The three remainingsamples were kept constant with an aging time of 2 min and the

4448 I. Martnez-Mera et al. / Materprecursors concentration were varied at 0.0125 M, 0.25 M and0.5 M of both reactants and were identified as: 0.0125M2m,0.25M2 m and 0.5M2m, respectively.Fig. 1. (a) XRD patterns of iron oxide nanocrystals with different aging times of2, 5 and 10 min. In all the cases (M) the magnetite phase was identified but in the

sample with 10 min of aging time the lepidocrocite phase (L) was also obtained.(b) Diffractograms with different precursor concentrations, only magnetitereflections are observed in all the cases.

steps of 0.1. The crystallite size was calculated with Scherrer'sequations in order to find the average size of the nanoparticles.

3. Results and discussion

Fig. 1a shows three XRD patterns from the samples 1M2m, 1M5mand 1M10m with an aging time of 2, 5 and 10 min. The magnetitephase (Fe3O4) was identified in all synthesized samples by themethod described in the previous section. But when the aging time ismost bigger than 5 min is formed the lepidocrocite phase (Fe(OH)) asimpurity. The most intense reflections for this phase are in angle 2 of14.20, 27.15 and 46.50. For the absence of the reflection (110) at2=15.00, typical of maghemite phase (-Fe2O3), this phase wasdiscarded of being present in the synthesized samples. These resultsare similar to those obtained by Zhuo et al. [7] and Yihua andQuiufang [9], in the synthesis of magnetite by emulsion method andprecipitation with forced mixture, respectively. According tocalculations of mass balance, the obtained reaction synthesis yieldwas 94%. The synthesized samples were generated with high purityand short reaction yield, so we could say that this is anotheralternative method for a magnetite nanoparticles massive production.

Fig. 1b shows four XRD patterns from samples 1M, 0.5M, 0.25Mand 0.0125M, with an aging time of 2 min. Only the magnetite phasewas present, no significant effects were observed regarding the purityof the iron oxide nanoparticles, but an effect in the particle sizedistribution was observed. The three samples present similar crystallinecharacteristics.

Applying the Scherrer's equation to calculate the crystallite sizeof samples 0.0125M2m, 0.25M2m, 0.5M2m and 1M2m values of6 nm, 21 nm, 26 nm and 30 nm were found respectively. It isimportant to notice that as the reagent concentration decreases theobtained crystallite size decreases. By other hand the mean particlesizes by the samples 1M5 m and 1M10 m were 36 and 34 nmrespectively.

CTEM analysis of sample 0.5M2m is shown in Fig. 2a, the Fe3O4nanoparticles have a semispherical shape with average size of 24.9 nm,and their respective particle size distribution from bimodal type isshown in the Fig. 2b. CTEM image of sample 0.0125M2m is shown inFig. 2c, also the semispherical shape of the particles can be observed.Notice that the histogram also shows a bimodal particle sizedistribution of the synthesized Fe3O4 nanoparticles with two averagesizes, one at 6.0 nm and other at 7.9 nm (Fig. 2d). Comparing theseresults with the literature [6], we may conclude that it was possible to

4449I. Martnez-Mera et al. / Materials Letters 61 (2007) 44474451Fig. 2. (a) Bright field TEM image of sample 0.5M2m, showing semispherical magnesize of 24.9 nm. (c) CTEM image of sample 0.0125M2m with very similar characdistribution of this sample with averages sizes of 6 nm and 7.9 nm respectively.tite nanoparticles. (b) Histogram of the particle size distribution with an averageteristics to the image shown in (a). (d) Bimodal histogram of the particle size

obtain a narrower nanoparticle size dispersion from 4 to 10 nm withhigh purity and stable magnetite phase.

Fig. 3a and b shows a typical CTEM image and its polycrystallineelectron diffraction pattern (EDP) from sample 0.25M2m, respectively.The reflections correspond to diffraction planes (111), (220), (311),(400), (511) and (440) characteristic of the magnetite phase. Thedifference between the polycrystalline electron diffraction patterns ofmagnetite and maghemite phases is solved by the presence of the (111)diffraction plane, distinctive of the magnetite FCC structure, as can benoticed as the nearest ring to the transmitted electron beam in the EDPof Fig. 3b.

Typical HRTEM images of Fe3O4 nanoparticle with a 17 nm of sizewhich belong to sample 0.5M2m is observed in Fig. 4, the particle isoriented near the [ 112] orientation with interplanar distancesd1=2.36 , d2=2.73 and d3=2.86 that are very near to planes(222), (311) and (220), respectively of magnetite phase.

Small magnetite nanoparticles with a size of 8.0 nm were alsofound in this sample with interplanar distances of d1=2.97 ,d2=2.94 and d3=2.83 , Fig. 5. These are very close to the planed220=2.97 of magnetite phase in [ 1 11] zone axis. The experimentalmeasured angles between these lattice planes agree with the theoreticalresults by this orientation.

stochiometric ratio of 60% O and 40% Fe, while in the magnetite phase(F3O4), it is 43% iron and 57% oxygen. The EDS analysis in the ironoxide nanoparticles indicated an atomic ratio of 46% Fe and 54% O,

4450 I. Martnez-Mera et al. / Materials Letters 61 (2007) 44474451Fig. 3. (a) Typical CTEM image of the nanoparticles corresponding to sample

0.25M2m. (b) Polycrystalline electron diffraction pattern of the magnetite phase.The magnetite reflection corresponding to d(111)=5.03 is distinctive of FCCstructure.Fig. 6a shows a HRTEM image of sample 0.25M2 m where ananoparticle of about 16.6 nm can be observed. Fig. 6b is a HRTEMfiltered image with noise reduction, the interplanar distances measuredwere d1=2.98 , d2=2.91 and d3=2.10 , near to planes ofmagnetite phase d220=2.97 and d400=2.09 oriented in [001] zoneaxis as can be seen in the FFT (Fig. 6c).

Energy-dispersive X-ray spectroscopy (EDS) was performed in orderto obtain the stochiometric ratio between the Fe and O in the magnetitenanoparticles. The unit cell of maghemite (-Fe2O3) has an atomic

Fig. 4. HRTEM image of the sample 0.5M2m showing a magnetite nanoparticlewith 17 nm size and it is oriented in [ 1 12] as indicated in the FFT inset.Fig. 5. Typical HRTEM image of a particle of sample 0.5M2m with 8 nm size.The interplanar distances correspond to magnetite phase in [ 1 11] orientation.

ageorie

4451I. Martnez-Mera et al. / Materials Letters 61 (2007) 44474451which is very near to the theoretical ratio for the magnetite structure(a 5% error is attributed to the analysis technique) [13].

4. Conclusions

The proposed synthesis method performed in this work formagnetite nanoparticles production is another alternative byobtaining nanoparticles of this material with high purity and free

Fig. 6. (a) HRTEM image of sample 0.25M2m with 16.6 nm size, (b) HRTEM imFe3O4 phase, (c) the FFT indicates that the nanoparticle is oriented in the [001]of surfactants. For short aging times (2 min) with a decrease inthe precursors concentration, specifically from 0.0125 M to1 M, we obtained different magnetite crystallite sizes in therange of 4 to 43 nm, respectively. CTEM analysis confirmed theabove mentioned results and demonstrates in the sample0.0125M2m and a narrower particle size distribution in therange of 4 to 10 nm was obtained, with two average sizes of6 nm and 7.9 nm. On the other hand, when the aging time islonger than 5 min (Fe(OH)) is formed in the lepidocrocite phaseas impurity.

The HRTEM analysis showed semispherical nanoparticleswith several orientations ([ 111], [ 112], [001]) and crystallo-graphic planes {311}, {222}, {220} and {440}.

Acknowledgements

The authors would like to thanks to Luis Rendn, RobertoHernndez, Ma. Eufemia Fernndez and Diego Quiterio for thetechnical assistance. Also acknowledged is the financial supportfrom PAPIIT project IX118004 and project CM-520 (ININ).

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Synthesis of magnetite (Fe3O4) nanoparticles without surfactants at room temperatureIntroductionSynthesis and characterizationResults and discussionConclusionsAcknowledgementsReferences