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Applied Surface Science 320 (2014) 183–187

Contents lists available at ScienceDirect

Applied Surface Science

jou rn al h om ep age: www.elsev ier .com/ locate /apsusc

agnetic properties of hematite (�-Fe2O3) nanoparticles prepared byydrothermal synthesis method

arin Tadica,∗, Matjaz Panjanb, Vesna Damnjanovicc, Irena Milosevicd,e

Condensed Matter Physics Laboratory, Vinca Institute, University of Belgrade, POB 522, 11001 Belgrade, SerbiaJozef Stefan Institute, Jamova 39, 1000 Ljubljana, SloveniaDepartment of Physics, University of Belgrade, Faculty of Mining and Geology, Belgrade, SerbiaCentre de Recherche sur la Matière Divisée, UMR 6619, CNRS-Université d’Orléans, 1b rue de la Férollerie, 45071 Orléans Cedex 2, FranceLaboratoire CSPBAT, UMR 7244 CNRS Université Paris 13, 93017 Bobigny Cedex, France

r t i c l e i n f o

rticle history:eceived 30 June 2014eceived in revised form 15 August 2014ccepted 28 August 2014vailable online 6 September 2014

eywords:ron oxideematite (�-Fe2O3)ydrothermal synthesisuperparamagnetism (SPION)

a b s t r a c t

Hematite (�-Fe2O3) nanoparticles are successfully synthesized by using the hydrothermal synthesismethod. An X-ray powder diffraction (XRPD) of the sample shows formation of the nanocrystalline �-Fe2O3 phase. A transmission electron microscopy (TEM) measurements show spherical morphology of thehematite nanoparticles and narrow size distribution. An average hematite nanoparticle size is estimatedto be about 8 nm by TEM and XRD. Magnetic properties were measured using a superconducting quan-tum interference device (SQUID) magnetometry. Investigation of the magnetic properties of hematitenanoparticles showed a divergence between field-cooled (FC) and zero-field-cooled (ZFC) magnetizationcurves below Tirr = 103 K (irreversibility temperature). The ZFC magnetization curve showed maximumat TB = 52 K (blocking temperature). The sample did not exhibit the Morin transition. The M(H) (magne-tization versus magnetic field) dependence at 300 K showed properties of superparamagnetic iron oxide

urface effects nanoparticles (SPION). The M(H) data were successfully fitted by the Langevin function and magneticmoment �p = 657 �B and diameter d = 8.1 nm were determined. Furthermore, magnetic measurementsshowed high magnetization at room temperature (MS = 3.98 emu/g), which is desirable for applicationin spintronics and biomedicine. Core–shell structure of the nanoparticles was used to describe highmagnetization of the hematite nanoparticles.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

During the last few years many scientific groups have beenevoted to developing and improving the properties of magneticanomaterials [1–12]. In order to achieve applicable properties,ynthesis conditions must be well controlled to obtain fine powdersith a narrow particle size distribution and the desired crystallinity

f the particles.Interest in the iron oxide nanoparticles currently focuses on

ontrol of their magnetic, electric, optic and catalytic properties13–16]. Among iron oxides, hematite is widely studied and it is of

articular interest in technological applications such as pigment,atalyst, sensor, environmental pollutant cleanup agent, electrodeaterial, biomedical material and magnetic material [17–20].

∗ Corresponding author at: Condensed Matter Physics Laboratory, Vinca Institute,.O. Box 522, 11001 Belgrade, Serbia. Tel.: +381 11 6308829; fax: +381 11 6308829.

E-mail address: marint@vinca.rs (M. Tadic).

ttp://dx.doi.org/10.1016/j.apsusc.2014.08.193169-4332/© 2014 Elsevier B.V. All rights reserved.

Hematite is the most stable iron oxide with a high resistanceto corrosion, low cost, and it is also biocompatible, environmen-tally friendly and non-toxic. Hematite (�-Fe2O3) crystallized inthe rhombohedral system space group R-3c with n-type semicon-ducting properties (2.1 eV band gap) [17]. Magnetic properties ofhematite systems are widely discussed in literature [21–31]. Inbulk hematite form the Neel temperature is at TN ≈ 960 K, whereasthe Morin transition takes place at TM ≈ 263 K [17]. Above TM thematerial is weakly ferromagnetic whereas below TM the mate-rial is an antiferromagnet [17]. The Morin temperature decreasesby reducing the particle size and tends to vanish for particlesof about 10 nm in size and smaller sizes [17,24]. A reductionof the Neel temperature has been observed with decreasing ofthe particle size [32]. Lu et al. reported thermodynamic analyticmodels that describe the size dependences of Morin tempera-

ture and Neel temperature in hematite nanocrystals based on thesize dependent cohesive energy model. Agreement between themodel predictions and the corresponding experimental resultsare found [32]. The hematite nanoparticles whose particle size

1 face Science 320 (2014) 183–187

iibebswtostpematrcppatttsss�ntwibThaatrs[amsnnwdta[

tmptchanitmcst

84 M. Tadic et al. / Applied Sur

s small enough (below 10 nm), show superparamagnetic behav-or above the blocking temperature TB, and ferromagnetic-likeelow TB [24,33]. Consequently, nanoparticle hematite is an inter-sting material for fundamental research of magnetic properties,ecause it can display antiferromagnetic, weak-ferromagnetic anduperparamagnetic properties [17,21,23,24,33]. Moreover, it isell-known that the magnetic properties of �-Fe2O3 are sensi-

ive to morphology, crystallinity, and inter-particle interactionsf the samples [9,17,21,23,24,27,33]. A large number of methods,uch as sol–gel, precipitation, mechanochemical, solvothermal,hermal decomposition, hydrothermal, have been developed torepare various hematite nanosized structures [17,19,33–37]. Mint al. prepared uniform hematite microflakes by hydrothermalethod with an ethanol/water solvent mixture by the use of

scorbic acid [38]. Bhushan et al. synthesized hematite nanopar-icles coated with octyl ether and oleic acid by a facile chemicaloute. They showed that small change of particle size (7–25 nm)auses drastic changes in magnetic properties, such as absence orresence of the Morin transition [39]. Mohammadikish et al. pre-ared hematite nanocrystals with diameters about 40–50 nm by

simple hydrothermal route in a one-step process. They foundhat the variations in reaction temperature and heating dura-ion have no significant effect on the shape and morphology ofhe �-Fe2O3 nanoparticles [40]. They also showed that a pureample of �-Fe2O3 nanoparticles cannot form in the absence ofodium acetate in lower temperatures [40]. Yan et al. synthe-ized �-Fe2O3 nanoparticles through calcining the spindle-like-FeOOH precursors at 600 ◦C [41]. They showed that �-FeOOHanospindles with a diameter of d ∼ 50 nm and lengths l upo 100–150 nm were readily changed to �-Fe2O3 nanoparticlesith a size of d ∼ 30 and l ∼ 150 nm after a heat-treatment last-

ng for 2 h [41]. Deraz et al. synthesized hematite nanoparticlesy combustion synthesis method using different ratios of fuel.hey showed that low content of fuel promotes formation ofematite, whereas high content of this fuel produces maghemitend hematite mixture [42]. From the viewpoint of application,bility to produce hematite nanoparticles with high magnetiza-ion is highly desirable. Ma and Chen presented a new magneticegime induced by surface magnetic states in hematite hollowpheres with good properties for diverse spintronic application43]. They reported that freezing and coupling of paramagneticnd ferromagnetic surface states have profound effect on theagnetic properties (high magnetization) of the hematite hollow

pheres [43]. They also showed that surface layer of the hematiteanocubes is of high magnetic activity and it dominates their mag-etic properties [44]. Ay et al. reported nanocomposite architecturehich combines prolate spheroidal hematite nanoparticles withrug-carrying layered double hydroxide disks in a single struc-ure. They measured saturation magnetization of MS = 9.6 emu/gt room temperature and observed narrow hysteresis loop44].

In our previous papers [21,23,24,33], we presented experimen-al results and their analysis on hematite particles of different sizes,

orphologies, microstructures, magnetic properties and inter-article interactions, obtained by different synthesis methods. Inhis work, we continue our investigation of hematite nanoparti-les and this paper describes hydrothermal synthesis of sphericalematite nanoparticles by a one-step synthesis method. The aver-ge hematite particle size is estimated to be about 8 nm witharrow size distribution. Magnetic measurements showed block-

ng temperature TB = 52 K, superparamagnetic properties at roomemperature, and high magnetization (MS = 3.98 emu/g). The high

agnetization properties of the sample were explained by aore–shell structure of the nanoparticles, so we may conclude thaturface effects and surface spins play an important role in magne-ization properties.

Fig. 1. X-ray diffraction pattern of the sample. The Miller indices (h k l) of the peaksare also shown.

2. Experimental

The �-Fe2O3 sample was synthesized by the hydrothermalmethod. The starting point for the formation of the sample was asolution prepared by mixing NaOH (1.1 g), deionized water (3 ml),ethanol (14 ml), and oleic acid (11 ml). Afterwards, the obtainedsolution was heated at 60 ◦C and mixed well by electromagneticstirring (solution 1). The second solution was an aqueous solutionof Fe sulfate heptahydrate Fe2(SO4)3·7H2O (0.8 g) (solution 2). Thefinal solution (solution 1 added to solution 2) was mixed by electro-magnetic stirring for 6 h, which produced a homogeneous mixture.Following that, the final solution was placed into a 100 ml capacityTeflon-liner autoclave and heated at 160 ◦C for 50 h. Thereafter, itwas quenched down to room temperature. Reddish powder wascollected by centrifugation, washed six times in deionized waterand ethanol, and dried at 75 ◦C for 12 h.

The X-ray diffractometer (Phillips PW-1710) employing CuK�

(� = 1.5406 ´A, 2� = 25–60◦) radiation was used to characterizethe crystal structure of the sample. The size, morphology,and microstructure were observed by TEM (transmission elec-tron microscopy, JEOL 2010 F). Magnetic measurements wereperformed on a commercial Quantum Design MPMS-XL-5 SQUID-based magnetometry.

3. Results and discussion

The structure and phase composition of the sample were inves-tigated by X-ray powder diffraction (XRPD) measurements. Themeasured diffraction pattern is depicted in Fig. 1 where wide andintensive peaks can be observed. The positions of all the maximacoincide with the peaks that are characteristic of the hematitephase (JCPDS card 33-0664). No diffraction line corresponding toother phases has been observed, indicating that a sample is com-posed of pure phase of hematite. The mean crystallite diameter Dwas estimated using Scherrer’s equation (1) and the (1 0 4) reflec-tion:

D ≈ 0.9�

ˇcos �, (1)

where � is the wave length of the incident X-ray, � is the diffrac-

tion angle, and is the full-width at half maximum. We obtainedthe value of D ≈ 8.5 nm. The composition of the sample was alsoconfirmed by the EDS spectroscopy experiments, that reveals thepresence of Fe and O. The calculated atomic ratio of Fe to O

M. Tadic et al. / Applied Surface Science 320 (2014) 183–187 185

O3 pa

wsvfoTsudTF

Fig. 2. (a)–(c) TEM micrographs of the spherical �-Fe2

as about 0.63 (Fe1.88O3), which agrees well with the expectedtoichiometry (Fe2O3). It should be noted that the interactionolume of EDS analysis is a few microns in depth from the sur-ace over many nanoparticles. Afterwards, size and morphologyf the nanoparticles was determined by analyzing the recordedEM images. Fig. 2(a)–(c), shows the transmission electron micro-cope (TEM) images of the sample. The sample is composed of

niform nanoparticles of spherical morphology with narrow sizeistribution. The estimated average size of the nanoparticles byEM was about 8 nm. The particle size distribution is shown inig. 2(d). Fig. 3(a) and (b) is the high-resolution transmission

Fig. 3. (a) and (b) HRTEM images o

rticles. (d) Size distribution of �-Fe2O3 nanoparticles.

electron microscopy (HR-TEM) images taken on the single nanopar-ticles. The lattice fringes can be clearly seen in Fig. 3. Moreover,these images show that the spherical nanoparticles are well crys-tallized.

Magnetic properties were measured using a superconductingquantum interference device (SQUID) magnetometer. The zero-field-cooled (ZFC) and the field-cooled (FC) magnetization curves

measured in the low applied magnetic field of 100 Oe are shown inFig. 4. For ZFC magnetization, the sample was cooled to 5 K in theabsence of magnetic field and then a finite field (H = 100 Oe) wasapplied. The magnetization was measured while the sample was

f the �-Fe2O3 nanoparticles.

186 M. Tadic et al. / Applied Surface Sc

Fig. 4. Temperature dependence of the zero-field-cooled (ZFC) and field-cooled (FC)m

htapbcTdticiF[cotcdsttep[

i

FT

agnetization measured in a field of 100 Oe.

eated up to 300 K. The FC magnetization was measured by coolinghe sample from 300 K down to 5 K in the presence of the same fields in the ZFC mode, and the data were taken while heating the sam-le up to 300 K. From Fig. 4, it can be seen that the magnetizationehavior typical of magnetic nanoparticles was obtained. The ZFCurve exhibits a maximum with the peak value at the temperatureB = 52 K (blocking temperature). Below TB the ZFC magnetizationecreases sharply, whereas the FC magnetization increases con-inuously below TB down to 5 K. This FC magnetization behaviors usually considered to be a characteristic of magnetic nanoparti-les with no strong inter-particle interactions [24,33]. The plateaun the FC magnetization curve below TB reported in nanosized �-e2O3 points to the existence of strong inter-particle interactions45]. The irreversibility temperature Tirr at which the ZFC and FCurves begin to separate corresponds to the blocking temperaturef the largest particles in the system. Tirr is usually determined ashe point where the ratio (MFC − MZFC)/MFC is less than 1%, and thisriterion gives a value of Tirr = 103 K for our measurements. Theifference between Tirr and TB is known to be a qualitative mea-ure of the width of the nanoparticle size distribution. In our casehis difference is Tirr − TB = 51 K, thus indicating a not wide size dis-ribution, as also observed by TEM studies. The sample did notxhibit the Morin transition down to 5 K, as expected for smallarticles less than 10 nm diameter according to the literature data

17,24,33].

At 300 K the magnetization versus applied magnetic field (M(H))s not hysteretic (HC = 0 Oe and Mr = 0 emu/g), as expected in the

ig. 5. Magnetization versus applied magnetic field dependence recorded at 300 K.he solid line shows Langevin function fit to the M(H) data.

ience 320 (2014) 183–187

unblocked regime above Tirr (superparamagnetism) (Fig. 5). Thesuperparamagnetic state of the sample can be described using theLangevin theory of paramagnetism and the magnetization is givenby the expression:

M(H, T) = MS

[coth

(mpH

kBT

)− kBT

mpH

], (2)

where, mp is the magnetic moment of nanoparticle, T is the tem-perature, and H denotes the applied magnetic field. The fit of Eq.(2) to M(H) data measured at 300 K is shown in the Fig. 5 (solidline). The best fit parameters obtained are MS = 3.98 emu/g andmp = 657 �B. From these values the mean particle diameter wasdetermined to be d = 8.1 nm by using expression mp = �d3MS/6. Thisvalue, determined by applying Langevin’s theory, is consistent withthe mean particle diameter obtained from TEM micrographs andXRD pattern. Finally, we discuss the possible origin of the highmagnetization of the hematite nanoparticles. The magnetizationof 3.98 emu/g at 300 K is high for hematite nanomaterials andis much higher than the magnetization of bulk �-Fe2O3 materi-als (MS = 0.3 emu/g) [17]. This high magnetization value probablyarises from the surface spin disorder, which can be aligned in thedirection of the applied magnetic field more easily than core spins.Accordingly, we conjecture that high magnetic moment of thenanoparticles is a consequence of nanoparticle shell. As a summaryof the abovementioned facts and experimental results, we proposethat the �-Fe2O3 nanoparticle consists of magnetically disorderedshell and magnetically ordered core (core–shell structure) with anet magnetic moment. We can conclude that the proper combina-tion of crystalline core and disordered surface shell is relevant forcontrol of the magnetization in �-Fe2O3 nanoparticles.

4. Conclusions

In conclusion, we successfully prepared hematite nanoparti-cles using the hydrothermal synthesis method. The XRD, TEMand HRTEM measurements revealed the nanocrystalline �-Fe2O3phase, spherically shaped particle morphology, average nanoparti-cle size of about 8 nm, narrow size distribution, and lattice fringesof the nanoparticles. These magnetic nanoparticles showed thesuperparamagnetic properties at room temperature with blockingtemperature TB = 52 K and irreversibility temperature Tirr = 103 K.The sample did not exhibit the Morin transition down to 5 K. Mea-sured magnetization versus applied magnetic field M(H) data atroom temperature was successfully fitted into Langevin’s curve forparamagnetic system. The average particle size of 8.1 nm obtainedfrom this fit is in excellent agreement with the XRD and TEM stud-ies. The magnetization of MS = 3.98 emu/g and particle magneticmoment of mp = 657 �B were also determined. The high value ofmagnetization M and magnetic particle moment �p in the hematitenanoparticles are probably due to the disordered surface shell andsurface spins. Based on these results and the facts reported in theliterature, we can conclude that by controlling crystallinity and sur-face of the nanoparticles it is possible to influence magnetizationproperties of the �-Fe2O3 nanoparticles.

Acknowledgments

The Serbian Ministry of Science supported this work financially(Grant no. III 45015). The support by the Ministry of Higher Educa-tion, Science and Technology of the Republic of Slovenia withinthe National Research Program is acknowledged. The authorsacknowledge the use of equipment in the Center of Excellence onNanoscience and Nanotechnology (Nanocenter Slovenia).

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