Influence of Surface Effects on Magnetic Behavior of Hematite Nanoparticles Embedded inPorous Silica Matrix

Vladimir Zelenak,*,† Adriana Zelenakova,‡ Jozef Kovac,§ Ulla Vainio,| and Nataliya Murafa⊥

Departments of Inorganic Chemistry and Solid State Physics, P. J. Safarik UniVersity, 04001 Kosice, SloVakia,Institute of Experimental Physics, SloVak Academy of Sciences, 04101 Kosice, SloVakia, DESY, Hasylab,Notkestrasse 85, D-22603 Hamburg, Germany, and Institute of Inorganic Chemistry AS CR,25068 Rez, Czech Republic

ReceiVed: January 26, 2009; ReVised Manuscript ReceiVed: June 6, 2009

Unique magnetic properties of superparamagnetic iron oxide (hematite) nanoparticles prepared by nanocastingin a periodic nanoporous silica (PNS) matrix are described in this work. The nanoparticles were prepared viaa novel approach, when the external surface of PNS was modified to become more hydrophobic. The preparedcomposite sample was characterized by the synchrotron related techniques small-angle X-ray scattering (SAXS),wide-angle X-ray scattering (WAXS), and X-ray absorption near edge spectra (XANES) measurements, bynitrogen adsorption/desorption, and by high resolution transmission electron microscopy (HRTEM). Theinvestigation of the magnetic properties by superconducting quantum interference device (SQUID) magne-tometry at the temperatures 2-300 K shows the superparamagnetic relaxation of the particles with a blockingtemperature of TB ∼32 K. Below TB, the ferromagnetic interactions are present as suggested by coercivity HC

∼1900 Oe. The value of magnetic moment mP ) 296 µB of the hematite particle was estimated by distributionof Langevin functions. This magnetic moment originates in uncompensated surface spins of Fe3+ ions due tothe small size of hematite particles (5 nm) and due to the loading of nanoparticles into silica matrix. Themagnetic behavior of the Fe2O3@PNS nanocomposite is mainly related to the surface effects (spin cantingand different surface to volume spin ratio).

Introduction

Periodic nanoporous silicas (PNSs) prepared by using the self-organized aggregates of surfactants are of current researchinterest because of their perspective applications in the fieldsof adsorption, catalysis, chromatography, optics, or magnetism.1-4

PNSs possess long-range periodicity, well-defined regular porestructure in the mesopore range, and large surface areas. Theproperties of PNS can be tuned on the nanoscale level by changein the size and structure of surfactant aggregates.

Because of their unique properties, PNSs can be used as“nanoreactors” for preparation of advanced magnetic materials.In the so-called nanocasting method, an ordered porous matrixacts as a nanoscopic mold, which restricts through confinementthe growth and formation of metal or metal oxide species.Nanocasting provides a new effective way of controllinguniformity of size of the nanoparticles and prevents theiragglomeration. One of the most used porous matrices is SBA-15, showing cylindrical, hexagonally ordered 2D pore structurewith a pore size of approximately 7 nm.5

Several studies dealt with the synthesis and investigation ofmagnetic properties of particles prepared by nanocasting.6-14

For example, magnetic nanoparticles of γ-Fe2O3,8 Fe3O4,9,10

Co3O4,11,12 and ZnO13 were prepared in the porous silica withthe aim of the creation of new nanocomposite magnetic systemswith application in catalysis, nanoelectronics, and computing,

as high density recording media, or for drug delivery.15,16

Nanocasting provides a great opportunity to prepare compositemagnetic systems with new collective magnetic properties, andsuch nanoparticles often exhibit unique structural or magneticfeatures. Tuysuz et al.7 reported the synthesis of ordered two-line ferrihydrite particles via the nanocasting pathway anddescribed their spin glass behavior, caused by uncompensatedspins due to the small structure size. Zhou et al. observed thecluster glass structure on zinc ferrite nanocasted in amorphoussilica matrix.17

To load metal or metal oxide nanoparticles into PNS, thewet impregnation of PNS by metal salts and their subsequenttransformation to respective nanoparticles is usually applied.However, some portion of the nanoparticles are trapped on theexternal surface of PNS. In our recent work, we have shownthat the magnetic behavior of such systems is governed by twocontributions: ferromagnetic and paramagnetic, which comefrom the nanoparticles incorporated inside the pores and thosetrapped on the external surface, respectively.18

As a continuation of this work and with the aim to reducethe amount of metal nanoparticles trapped on the externalsurface, we have used a different synthetic procedure. To achievea larger ratio of the nanoparticles located in the pores to thoselocated on the external surface, we have modified the externalsurface of the PNS to become more hydrophobic. As a result,we have prepared hematite (R-Fe2O3) nanoparticles with uniquemagnetic properties.

Hematite is the most stable and extensively studied ironoxide.19-24 Bulk hematite is crystal with a corundum structurecontaining two sublattices creating an antiferromagnetic systemwith a Neel temperature TN ∼ 950 K. At the temperature aboutTM ∼ 260 K, also known as the Morin temperature, crystallite

* Corresponding author. Telephone: +421-55-2342343. E-mail: vladimir.zelenak@upjs.sk.

† Department of Inorganic Chemistry, P. J. Safarik University.‡ Department of Solid State Physics, P. J. Safarik University.§ Slovak Academy of Sciences.| DESY, Hasylab.⊥ Institute of Inorganic Chemistry AS CR.

J. Phys. Chem. C 2009, 113, 13045–13050 13045

10.1021/jp9007653 CCC: $40.75 2009 American Chemical SocietyPublished on Web 07/01/2009

of hematite undergoes a magnetic transition (spin-reorientationtransition).19 Below TM, the moments in two magnetic sublatticesare oriented antiparallel, and above TM the moments show slightcanting considering the basal plane, resulting to a net magneticmoment, which originates from the superexchange interaction.The TM decreases as particle size decreases and disappearsfor the particles with size 8-20 nm.25 Due to the suppressionof the Morin transition in small hematite nanoparticles (below∼20 nm), only a high temperature phase can exist,26 in whichthe net magnetic moment originates in small spin canting awayfrom antiferromagnetic alignment.21-23

In this work, we present interesting magnetic properties, suchas superparamagnetism or exchange bias effect, caused bydifferent distribution of surface and volume spins, of hematite(R-Fe2O3) nanoparticles prepared by modified nanocasting routein PNS. Along with the magnetic characterization, the propertiesof the composite systems were studied by small-angle X-rayscattering (SAXS), wide-angle X-ray scattering (WAXS), X-rayabsorption near edge spectra (XANES) measurements, highresolution transmission electron microscopy (HRTEM), andtextural characterization was made by the nitrogen adsorption/desorption. The results we present should offer a valuable toolin the development of new materials for applications in highdensity storage media or as drug delivery systems.

Experimental Section

Synthesis and Modification of Mesoporous Silica. Meso-porous silica SBA-15 was synthesized by the described proce-dure using triblock copolymer Pluronic P123 ((EO)20(PO)70-(EO)20)) surfactant.5 The typical gel composition in the termsof the molar ratios was TEOS/HCl/H2O/P123 ) 1/5.9/193/0.017(TEOS, tetraethoxysilane). A portion of 4.0 g of Pluronic P-123was dissolved with stirring in 30 g of water and 120 g of 2 MHCl 35 °C followed by addition of 8.5 g of TEOS. The resultingmixture was stirred at 35 °C for 20 h and then aged at 80 °Cfor 24 h. The as-synthesized sample was recovered by filtrationand freely dried.

In the subsequent step, the surface modification by cyclohexyland methyl groups was carried out, using cyclohexyl-methyl-dimethoxysilane (CMS). A total of 1 g of as-synthesized silicawas dispersed in 50 cm3 of dried toluene, and then 7 cm3 ofCMS was added to the suspension, which was kept under refluxfor 20 h. The solid product was recovered by filtration, washedwith toluene and octane, and dried overnight at 80 °C. The driedsample was placed in the boiling flask containing 150 cm3 ofethanol, and the mixture was refluxed for 10 h. This procedurewas repeated three times to remove the triblock copolymertemplate from the porous system. The prepared sample wasdenoted as PNS.

The nanocomposite Fe2O3@PNS was prepared by wet-impregnation of the porous matrix by 1 M solution of iron(II)sulfate followed by reduction of Fe2+ with 1.6 M solution ofNaBH4. After reduction, the sample was washed with distilledwater and dried. The dry composite sample was calcined in air

at 500 °C for 7 h. The synthetic procedure is schematicallydrawn in Scheme 1.

Characterization. SAXS experiments were carried out at B1Hasylab beamline (DESY Hamburg) with the beam energy 12keV (λ)1.03 Å). The WAXS was measured at BW5 Hasylabbeamline (DESY Hamburg) at the wavelength λ ) 0.123980Å. The XANES measurements were done at B1 Hasylabbeamline (DESY Hamburg). The sample powder was spreadon and sealed between two Kapton tapes. The transmission ofthe sample was measured by measuring the X-ray intensity withan ionization chamber before of the sample and a photodiodeafter it. The energy scale was calibrated using the first inflectionpoints of Fe, Ni, and Se foils.27

The HRTEM micrographs and energy dispersive X-rayspectroscopy (EDX) measurements were collected with a JEOLJEM 3010 (LaB6 cathode) microscope operated at 300 kV. Acopper grid coated with a holey carbon support film was usedto prepare samples for the TEM observation. Powdered sampleswere dispersed in ethanol, and the suspension was treated in anultrasonic bath for 10 min.

The porosity and specific surface area of materials weredetermined by nitrogen adsorption/desorption measurementsusing surface area and pore size analyzer Quantachrome NOVA1200e. Prior to the experiments, the samples were outgassed at423 K for 24 h. The specific surface area, SBET, was estimatedusing the Brunauer-Emmett-Teller (BET) equation in apressure range (0.05-0.30). The pore size distributions werecalculated from the adsorption branch of the isotherms accordingto the Barrett-Joyner-Halenda (BJH) method.

Magnetic measurements were performed on a commercialsuperconducting quantum interference device (SQUID) mag-netometer (Quantum Design MPMS XL5) over a wide rangeof temperatures (2-300 K) and applied dc fields (up to 5 T).The sample with mass 1.95 mg was encapsulated in a plasticsample holder. The diamagnetic contribution of the plasticcapsule and plastic sample holder is insignificant in comparisonwith the high magnetic moment of the sample, and no correctionis necessary.

Results and Discussion

Samples Characterization. The characterization of themodified porous silica sample (hereafter denoted as PNS; seestep (c) in Scheme 1) by SAXS, Figure 1, showed three well-resolved diffraction peaks, which can be indexed as (10), (11),and (20) in hexagonal p6mm group. The peaks (11) and (20)became weaker for the Fe2O3@PNS composite due to filling ofthe pores by the nanoparticles.

For the Fe2O3@PNS sample, the shift of the diffraction peaksto higher q values in comparison with PNS was observed. Thisshift can be assigned to the thermal treatment of the compositesample. During the preparation of PNS, the template wasremoved from the as-synthesized sample by extraction inethanol. However, in the subsequent preparation of the com-posite Fe2O3@PNS sample, the calcination was used to remove

SCHEME 1: Designed Synthetic Pathway

13046 J. Phys. Chem. C, Vol. 113, No. 30, 2009 Zelenak et al.

the organic moieties from the external surface and to grow theiron oxide nanoparticles (see step (e) in Scheme 1). Thecalcination led to the constriction of the pores accompanied bythe decrease of the unit cell parameter, which resulted in a shiftof the peaks in the SAXS pattern. The unit cell parameter,calculated using the formula a ) 2d10/�3, was 120.7 Å for thePNS sample and 110.8 Å for the Fe2O3@PNS sample.

The long-range periodicity of the porous structure as indicatedby SAXS was further supported by HRTEM measurements.Figure 2 shows HRTEM micrographs of the Fe2O3@PNSsample. The regular hexagonally ordered pore architecture ofthe silica matrix with pore size approximately 8 nm waspreserved after the iron loading and the crystallization of Fe2O3

particles. No particles were detected on the surface of the silicagrains. Moreover, from Figure 2a, it can be seen that formedparticles do not plug the pores completely, which makes thepores still accessible for the guest molecules. As it will bediscussed later, such growth of the particles inside the poresinduced the change in the adsorption behavior. The EDX spectracollected from the selected areas of the outer rim of the silicagrains as well as from the middle parts of the grains showedthe atomic Fe/Si ratio around 0.02 and 0.24, respectively. Theseresults, observed for most of the selected parts, suggest thatiron oxide particles are located mainly within the silica grainsand distributed quite homogeneously. If we assume the samplesto be composed of Fe2O3 and SiO2, the determined Fe/Si ratiocorresponds to a weight percentage of approximately 25% ofFe2O3 in Fe2O3@PNS composite.

The phase analysis of the sample by WAXS confirmed theR-Fe2O3 phase (hematite) in the composite. Figure 3 displaysthe measured WAXS profile of Fe2O3@PNS, compared withthe database data for hematite.

The broadening of the diffraction peaks in the pattern isindicative of the presence of nanoparticles. The particle sizewas estimated using the Scherrer formula:28

where θhkl is the angle of hkl reflection and ∆2θhkl is thecorrected full width at the half-maximum of the Lorentzian peak.The calculated value of the crystallite sizes was 5 nm.

Further information about the phase composition was broughtby the XANES measurements. Figure 4 contains the XANESresults for the Fe2O3@PNS sample and the references: a fewµm sized hematite as well as iron foil. The difference in energy

between the positions of the edges of the Fe foil and those ofhematite (R-Fe2O3) and the Fe2O3@PNS sample is 15 eV. Thus,based on the XANES measurements, we can conclude that theatomic structure of the sample resembles hematite. However,XANES of the micrometer sized hematite foil and theFe2O3@PNS sample containing hematite nanoparticles are notidentical, and therefore, some structural differences are expected.The differences in the XANES of Fe2O3@PNS and hematitemay be explained by surface effects, which should be larger inthe Fe2O3@PNS sample in which nanoparticles are embeddedin PNS. The enhancement in the pre-edge features at around7113 eV for nanoparticles compared to bulk was attributed byChen et al.29 to enhancement of distortions in the Fe3+ ionpositions in the hematite structure.

Textural characterization of the samples was made by nitrogenadsorption. Figure 5 shows the adsorption/desorption isothermsmeasured on the PNS and the sample Fe2O3@PNS. Forcomparison, the adsorption isotherm of the “conventional” SBA-15 silica sample is also added. It is obvious from Figure 5 thatthe amount of the adsorbed nitrogen decreased graduallyfrom the “conventional” SBA-15 sample to the sampleFe2O3@PNS. The surface area of PNS (472 m2/g) was lowerin comparison with SBA-15 silica (947 m2/g) due to themodification. The surface area further decreased to 184 m2/gafter embedding the iron oxide nanoparticles into PNS.

Moreover, it is obvious that the shape of the adsorption/desorption hysteresis loop changed after incorporation of ironoxide nanoparticles. For conventional SBA-15 and organomodi-fied PNS, the adsorption and desorption branches of thehysteresis loops are parallel and may be classified as IVAaccording to the IUPAC classification.30 On the contrary, thehysteresis loop of the Fe2O3@PNS sample is triangular andwider and can be classified as IVB in the IUPAC classification.We suppose that the change in the shape of the isotherms isdue to the partial blocking of the cylindrical pores by iron oxidenanoparticles, whereby the pores resemble ink bottle pores.Desorption from the sample delayed in comparison with theadsorption, leading to triangular hysteresis. Thus, the evaporationstep in the sample Fe2O3@PNS is supposedly controlled by thepore blocking effect (see Scheme 2).

The pore size distribution was calculated using the BJHmethod from the adsorption branch of the isotherms. The PNShas a larger pore size (7.6 nm) in comparison with the“conventional” SBA-15 silica (7.1 nm) because of their differenttreatment during the template removal (extraction or calcination,respectively), leading to pore constriction in PNS (see SAXSresults). After nanocasting, the statistical pore size decreased(6.2 nm). The textural characteristics for the samples arepresented in Table 1.

Magnetic Measurements. The magnetic properties of theFe2O3@PNS sample were studied by SQUID magnetometry.The magnetization M versus external field H plot measured atthe temperature 300 K (see Figure 6) shows the paramagneticbehavior of the hematite nanoparticles distributed in the PNSmatrix as proven by the lack of coercivity. The values ofmagnetization MS ≈ 0.85 emu/g and paramagnetic susceptibility� ) 1.69 × 10-5 emu/g Oe were observed at 300 K.

Under the effect of thermal fluctuation, the magnetic informa-tion stored in nanoparticles may be lost and therefore themeasurements of magnetization were realized also at thetemperature 2 K. It is clearly seen from Figure 6 that magnetiza-tion M versus external field H recorded at 2 K exhibits largemagnetic hysteresis with a coercivity of HC ≈ 1900 Oe andsaturation magnetization of MS ≈ 2.83 emu/g.

Figure 1. SAXS patterns of surface modified PNS (solid line) andcomposite sample Fe2O3@PNS (dashed line).

Bhkl )0.9λ

∆2θhkl cos θhkl(1)

Magnetic Behavior of Hematite Nanoparticles J. Phys. Chem. C, Vol. 113, No. 30, 2009 13047

The fine hematite particles (R-Fe2O3) are known to beantiferromagnetic under the Morin transition TM, and theantiferromagnetic order is frustrated at the surface.18 Therefore,the antiferromagnetic hematite nanoparticles (on the contraryto the bulk hematite) could exhibit superparamagnetic relaxationof their spin lattices as well as permanent magnetic momentsarising from uncompensated surface spins.

To confirm the superparamagnetic relaxation process in theprepared R-Fe2O3 particles distributed in the amorphous periodicsilica matrix, we have measured the temperature dependenceof magnetization (Figure 7). Starting from the zero-field cooling(ZFC) regime, the sample was cooled to 2 K without applying

an external field and then recorded in the temperature range2-300 K under the dc field of 50 Oe. Consequently, the samplewas cooled to 2 K in the 50 Oe dc field, which is representedby the FC (field cooled) curve. The presence of a maximum inthe ZFC curve, corresponding to blocking temperature TB ≈32 K and irreversibility of ZFC and FC curves in the low-temperature region below TB (T < TB), indicates the randomizedeffects induced by kBT: superparamagnetic behavior betweenhematite nanoparticles in the nanocomposite systemFe2O3@PNS.

Figure 2. HRTEM micrographs of Fe2O3@PNS. (a) In the HRTEM micrograph taken with the beam direction parallel to the hexagonal axis, theregular hexagonally ordered pore architecture can be observed, with pore size approximately 8 nm. (b) The micrograph taken with the beamdirection perpendicular to the hexagonal axis shows the alternating structure of mesoporous channels and pore walls.

Figure 3. Wide angle X-ray scattering on Fe2O3@PNS.

Figure 4. XANES measurements of Fe2O3@PNS and standards.

Figure 5. Nitrogen adsorption/desorption isotherms of calcined SBA-15, surface modified PNS, and Fe2O3@PNS sample.

SCHEME 2: Suggested Explanation of the Change of theShape of the Hysteresis Loop in Fe2O3@PNS Samplea

a d1 represents the normal pore size, and d2 represents the reducedpore size after growth of the Fe2O3 particles.

TABLE 1: Textural Properties of the Silica Samples

sample BET surface area (m2/g) average pore size (BJH, nm)

SBA-15 947 7.1PNS 472 7.6Fe2O3@PNS 184 6.2

13048 J. Phys. Chem. C, Vol. 113, No. 30, 2009 Zelenak et al.

The temperature dependence of magnetization confirmed theabsence of a Morin transition at temperature close to TM ≈ 260K. This effect (the absence of Morin transition) was reportedfor nanoparticles with a mean size smaller than 20 nm and insmall hematite particles dispersed in polymer31 or amorphoussilica matrix.32

Observed experimental dependences (temperature and fielddependences of magnetization) confirm the superparamagneticrelaxation of spins above the blocking temperature TB (T > TB),whereas below TB (T < TB) the magnetic moment of eachparticle, arising from uncompensated surface spins, is blockedalong its easy axis.

The behavior of superparamagnetic noninteracting particlescan be described by the Langevin function

where MS is the saturation magnetization, L is the Langevinfunction, and mp ) MS(πd3/6) is the magnetic moment ofspherical particle.

By fitting procedure of our measured data at 300 K (Figure6) to the distribution of 5 Langevin functions expressed in eq2, the value of the magnetic moment mp ≈ 296 µB of eachhematite particle was estimated. From the experimental valuefor MS and mp, the mean Fe2O3 particle diameter was determinedto be d ) 7.1 nm. This value is comparable with the value of

the particles’ diameter d ) 5 nm obtained from the Scherrerformula using WAXS experimental data.

Since the value of the blocking temperature TB is stronglydependent on the size and shape of the particles, on thepreparation methods, and on the interparticle interaction, wecompared the observed value of TB ≈ 32 K with the referencedata. A higher value, TB ≈ 390 K, was reported for 40 nmhematite particles.31 For 3 nm sized hematite particles, TB wasreduced to ∼145 K.32 When 5 nm R-Fe2O3 particles weredistributed in the polymer matrix,25 a small value of TB (∼22K) was observed, similar to the case of our Fe2O3@PNS system.

It is known that a perfectly compensated antiferromagnet doesnot show hysteresis, while in our experimental results the valueof coercivity HC ≈ 1900 Oe at 2 K was observed. The unusuallarge coercivity could be explained by the fact that core spinsare pinned by direct exchange interaction with the frozen spinsat the surface layer.33 In this case, the increasing of the coercivitywould be observed only below the freezing/blocking temperatureTB ) 32 K of the disordered surface layer, resulting in anexchange interaction between surface spins. For experimentalverification of this assumption, the investigation of the fielddependences of magnetization (M (H) loops) in the ZFC andFC regimes at temperatures below TB as well as an estimationof the exchange bias field EB were carried out. Hysteresis curvesin ZFC and after field cooling in a dc field 10 kOe (FC) attemperatures T ) 2, 5, 10, and 20 K exhibit a typical feature ofexchange biased systems, namely, the shift of the FC hysteresisloop and enhancement of coercivity HC

FC. This shift of ahysteresis loop along the field axis defines directly the exchangebias field HEB ) (HC+

FC + HC-FC )/2. The mechanism of exchange

bias is explained by an existence of unidirectional anisotropyinduced at exchange coupling interfaces with different spinorientation (e.g., ferro/antiferromagnetic).34

Figure 8 compares the values of coercivity estimated fromthe ZFC loop (HC

ZFC), the coercivity estimated from the FC loop(HC

FC), and the calculated value of exchange bias field HEB. Fromexperimental dependences (Figure 8), it is obvious that thevalues of coercivity HC

ZFC and HCFC are close to 200 Oe when

measured at “freezing” temperature 32 K and the values increaseto HC

ZFC ) 1810 and HCFC ) 1900 Oe when measured at 2 K.

The observation of an exchange anisotropy field (HEB ) 970Oe at 2 K) is a clear indication of the existence of a magneticallydisordered surface layer and confirms the magnetic interactionbetween surface spins. This disordered surface layer is also one

Figure 6. Magnetization versus dc field dependence recorded at twosignificant temperatures 2 and 300 K. Inset shows the detail of the lowtemperature magnetic hysteresis loop.

Figure 7. Temperature dependence of magnetization at ZFC (zero-field-cooled) regime (open squares) and FC (field-cooled) regime (fullsquares) measured in a dc field of 50 Oe.

M(H, T) ) MSL[mpH

kBT ] (2)

Figure 8. Dependence of coercivity in ZFC regime and in FC regimeunder a dc field 10 kOe at temperatures below TB. The calculated valueof the exchange bias field HEB is from the shift of ZFC and FC hysteresisloops.

Magnetic Behavior of Hematite Nanoparticles J. Phys. Chem. C, Vol. 113, No. 30, 2009 13049

of reasons for the absence of the Morin transition in smallhematite nanoparticles. The ratio between the populations ofthe spins in the surface layer to the spins in the volume of oursmall nanoparticles with the size about 5 nm is very high, andconsequently, the surface effects dominate the magnetic behavior.

Conclusions

The hematite nanoparticles (R-Fe2O3) with an average sizeof 5 nm were loaded to a pore channel system of mesoporoussilica with 2D symmetry via a novel nanocasting route. Priorto incorporation of hematite particles into a pore of silica, theexternal surface was modified to become more hydrophobic.The Fe2O3@PNS composite sample prepared by designednanocasting approach exhibits valuable magnetic behavior.

From SQUID magnetometry, we confirmed the superpara-magnetic relaxation of hematite particles. In the high temperaturesuperparamagnetic state (above blocking temperature TB ≈32 K), the particle moments freely fluctuate in the externalmagnetic field. Below the blocking temperature (T < TB), themagnetic moment of the particles is blocked along the easymagnetization axis and a high coercivity is present (HC ≈ 1900Oe). The fact that antiferromagnetic particles show the super-paramagnetic behavior is due to surface effects, for example,different distribution of surface and volume spins and spincanting. The influence of the surface layer on properties ofhematite nanoparticles was also indicated by XANES results.The experimental evidence of exchange bias effect (shift ofhysteresis loop and enhancement of coercivity in FC regime)confirms the existence of uniaxial induced anisotropy. Thisanisotropy is produced due to the exchange coupling betweenuncompensated surface spins of antiferromagnetic particles.

Based on the results, we can conclude that the features ofantiferromagnetic behavior typical for bulk R-Fe2O3 are sup-pressed in the nanocomposite sample due to a small size ofparticles (size effect) and due to the incorporation of particlesinto the isolated porous matrix. The results we present couldbe valuable in the development of new materials for applicationsin high density storage media or as drug delivery systems.

Acknowledgment. This work was supported by the SlovakResearch and Development Agency under the Contract No.RPEU-0027-06, European Community STREP project “De-SANNS” (No. FP6-SES6-020133), and VEGA (10119/08)project of the Ministry of Education of Slovak Republic.

References and Notes

(1) Davis, M. E. Ordered porous materials for emerging applications.Nature 2002, 417, 813–821.

(2) Miyata, H.; Suzuki, T.; Fukuoka, A.; Sawada, T.; Watanabe, M.;Noma, T.; Takada, K.; Mukaide, T.; Kuroda, K. Silica films with a single-crystalline mesoporous structure. Nat. Mater. 2004, 3, 651–656.

(3) Li, J. N.; Wang, L.; Qi, T.; Chu, J. L.; Liu, C. H.; Zhang, Y.Mesoporous gas adsorbents. Prog. Chem. 2008, 20, 851–858.

(4) Zelenak, V.; Hornebecq, V.; Mornet, S.; Schaff, O.; Llewellyn, P.Chem. Mater. 2006, 18, 3184.

(5) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am.Chem. Soc. 1998, 120, 6024–6036.

(6) Yiu, H. H. P.; Keane, M. A.; Lethbridge, Z. A. D.; Lees, M. R.; ElHaj, A. J.; Dobson, J. Nanotechnology 2008, 19, 255606.

(7) Tuysuz, H.; Salabas, E. L.; Weidenthaler, C.; Schuth, F. J. Am.Chem. Soc. 2008, 130, 280–287.

(8) Delahaye, E.; Escax, V.; El Hassan, N.; Davidson, A.; Aquino, R.;Dupuis, V.; Perzynski, R.; Raikher, Y. L. J. Phys. Chem. B 2006, 110,26001–26011.

(9) Andersson, N.; Corkery, R. W.; Alberius, P. C. A. J. Mater. Chem2007, 17, 2700.

(10) Zhu, Y.; Zhao, W.; Chen, H.; Shi, J. J. Phys. Chem. C 2007, 111,5281–5285.

(11) Lopes, I.; El Hassan, N.; Guerba, H.; Wallez, G.; Davidson, A.Chem. Mater. 2006, 18, 5826–5828.

(12) Salabas, E. L.; Rumplecker, A.; Kleitz, F.; Radu, F. Nano Lett.2006, 6, 2977.

(13) Waitz, T.; Tiemann, M.; Klar, P. J.; Sann, J.; Stehr, J.; Meyer, B. K.Appl. Phys. Lett. 2007, 90, 123108.

(14) Cao, S. W.; Zhu, Y. J.; Ma, M. Y.; Li, L.; Zhang, L. J. Phys. Chem.C 2008, 112, 1851–1856.

(15) Li, Y. Y.; Cunin, F.; Link, J. R.; Gao, T.; Betts, R. E.; Reiver,S. H.; Chin, V.; Bhatia, S. N.; Sailor, M. J. Science 2003, 28, 2045–2047.

(16) Wang, S. Microporous Mesoporous Mater. 2009, 117, 1–9.(17) Zhou, Z. H.; Wang, J.; Xue, J. M.; Chan, H. S. O. Appl. Phys.

Lett. 2001, 19, 3167.(18) Zelenakova, A.; Zelenak, V.; Kovac, J. Solid State Sci., submitted.(19) De Castro, A. R. B.; Zysler, R. D.; Vasquez Mansilla, M.; Arciprete,

C.; Dimitrijewits, M. J. Magn. Magn. Mater. 2001, 231, 287–290.(20) Raming, T. P.; Winnubst, A. J. A.; van Kats, C. M.; Philipse, A. P.

J. Colloid Interface Sci. 2002, 249, 346–350.(21) Kamzin, A. S.; Vcherashnii, D. B. JETP Lett. 2002, 75, 575–578.(22) Golosovsky, I. V.; Mirebeau, I.; Fauth, F.; Kurdyukov, D. A.;

Kamzerov, Y. A. Solid State Commun. 2007, 141, 178–182.(23) Hill, A. H.; Jiao, F.; Bruce, P. G.; Harrison, A.; Kockelmann, W.;

Ritter, C. Chem. Mater. 2008, 20, 4891–4899.(24) Harrison, R. J.; McEnroe, S. A.; Robinson, P.; Carter-Stiglitz, B.;

Palin, E. J.; Kasama, T. Phys. ReV. B 2007, 76, 174436.(25) Zysler, D. R.; Fiorani, D.; Testa, A. M. J. Magn. Magn. Mater.

2001, 224, 5.(26) Klausen, S. N.; Lefmann, K.; Lindgard, P. A.; Theil Kuhn, L.; Bahl,

C. R. H. Phys. ReV. B 2004, 70, 214411.(27) Kraft, S.; Stumpel, J.; Becker, P.; Kuetgens, U. ReV. Sci. Instrum.

1996, 67, 681.(28) Guinier, A. X-ray Diffraction. In Crystals, Imperfect Crystals, and

Amorphous Bodies; Dover Publications, Inc.: New York, 1994.(29) Chen, L. X.; Liu, T.; Thurnauer, M. C.; Csencsits, R.; Rajh, T. J.

Phys. Chem. B 2002, 106, 8539–8546.(30) Rouquerol, F.; Rouquerol J.; Sing, K. Adsorption by powders &

porous solids; Academic Press: London, 1999.(31) Zysler, R. D.; Vasquez Mansilla, M.; Fiorani, D. Eur. Phys. J. B

2004, 41, 171.(32) Tadic, M.; Markovic, D.; Spasojevic, V.; Kusigerski, V.; Remskar,

M.; Pirnat, J.; Jaglicic, Y. J. Alloys Compd. 2007, 441, 291–296.(33) Morales, M. P.; Veintemillas-Verdaguer, S.; Montero, M. I.; Serna,

C. J. Chem. Mater. 1999, 11, 3058–3064.(34) Klausen, S. N.; Lefmann, K.; Lindgard, P. A.; Theil Kuhn, L.; Bahl,

C. R. H. Phys. ReV. B 2004, 70, 214411.

JP9007653

13050 J. Phys. Chem. C, Vol. 113, No. 30, 2009 Zelenak et al.