Preparation of CoFe2O4-SiO2 nanocomposites by sol–gel method

ARTICLE IN PRESS

0022-0248/$ - se

doi:10.1016/j.jcr

�CorrespondiE-mail addre

Journal of Crystal Growth 271 (2004) 287–293

www.elsevier.com/locate/jcrysgro

Preparation of CoFe2O4/SiO2 nanocomposites bysol–gel method

Xianghui Huang, Zhenhua Chen�

College of Materials Science and Engineering, Hunan University, Changsha, 410082, China

Received 8 May 2004; accepted 22 July 2004

Communicated by D.T.J. Hurle

Available online 3 September 2004

Abstract

Nanocomposites with cobalt ferrite nanoparticles uniformly dispersed in silica have been successfully synthesized.

The effects of the thermal treatment temperature and the initial drying temperature on the structural and magnetic

properties of the nanocomposites were examined. The crystalline phase, the particle size, and the homogeneity of the

resulting nanocomposites were studied by X-ray diffraction, IR spectroscopy, differential scanning calorimetry,

transmission electron spectroscopy and vibrating sample magnetometer. The xerogels were amorphous. Heat treatment

at 400 1C resulted in CoFe2O4 clusters being partially formed and when the heat treatment temperature was increased to

600 1C, CoFe2O4 clusters were formed in large quantities. The formation reaction of CoFe2O4 clusters was

accompanied by a rearrangement of the silica matrix network. On further increasing the heat treatment temperature to

800 1C, materials with CoFe2O4 nanocrystals, well crystalline dispersed in the silica matrix, could be obtained. By

increasing the annealing temperature, composites with a progressive increase of the coercivity and of the remanent

magnetization were produced. The remarkable effect of initial drying temperature on the particle size of cobalt ferrite

suggested that a well-established silica network provided an effective confinement to the coarsening and aggregation of

CoFe2O4 nanoparticles.

r 2004 Elsevier B.V. All rights reserved.

PACS: 75.01A; 89.02

Keywords: A1. Nanostructures; A1. X-ray diffraction; B2. Magnetic materials; B3. Infrared devices

e front matter r 2004 Elsevier B.V. All rights reserve

ysgro.2004.07.064

ng author. Tel./fax: +86-7318-821648

ss: [email protected] (Z.H. Chen).

1. Introduction

Spinel ferrite nanoparticles have been inten-sively investigated in recent years because of theirremarkable electrical and magnetic properties andwide practical applications in information storage

d.

www.elsevier.com/locate/jcrysgro

ARTICLE IN PRESS

X.H. Huang, Z.H. Chen / Journal of Crystal Growth 271 (2004) 287–293288

system, ferrofluid technology, magnetocaloric re-frigeration and medical diagnostics. Among spinelferrites, cobalt ferrite, CoFe2O4 is especiallyinteresting because of its high cubic magnetocrys-talline anisotropy, high coercivity and moderatesaturation magnetization. Recently, cobalt ferritenanoparticles was also known to be a photomag-netic material that shows an interesting light-induced coercivity change [1,2]. A lot of syntheticstrategies for preparing nanosized cobalt ferritehave been presented [3–10]. The nanoparticlesobtained usually have a strong tendency toaggregate, which makes it very difficult to exploittheir unique physical properties. Dispersion of thenanoparticles in a matrix is one method to reduceparticle agglomeration and this technique allowsone to stabilize the particles and to study theirformation reactions. In recent years, systems suchas Fe2O3/SiO2 have been intensively studied,revealing a behavior different from that of bulksystems and serving as a model for the study ofsmall particles [11–15]. The properties of thesematerials depend strongly on the particle size, theparticle–matrix interactions, and the degree ofdispersion of nanoparticles in the matrix. Theability to control these parameters by chemicalmodifications of the method of preparation is ofcrucial importance nowadays from both a funda-mental and an industrial point of view [11].In this paper, we report on the dispersion of

CoFe2O4 nanoparticles in a silica matrix preparedusing a sol–gel procedure and follow the structuraland magnetic evolutions by IR spectroscopy, X-ray diffraction, transmission electron micrographs,differential scanning calorimetry and vibratingsample magnetometer. Experiments were con-ducted to study the influence of the preparationvariables, such as the initial drying temperatureand thermal treatment of the gels on the structuraland magnetic properties of cobalt ferrite em-bedded in a silica matrix.

2. Experimental procedure

Nanocomposites of cobalt ferrite dispersed in asilica matrix were prepared by sol–gel process. Thestarting material tetraethylorthosilicate (TEOS)

was used as a precursor of silica, while ferricnitrate and cobalt nitrate were employed as thereactant iron and cobalt species. The TEOS:ethanol: H2O and Fe: Co molar ratios werecontrolled at 1:4:8 and 2:1, respectively. TheFe/Si molar ratio was initially adjusted to 16%.The sols were prepared by dissolving Fe and Conitrates in deionized water, adding the alcoholicsolution of Si(OC2H5)4 and a few drops of HCl.After vigorous stirring for 1 h, the sols wereallowed to gel at room temperature for 5 days inpartially closed glass vessels. The obtained gelswere put into ovens for further drying at 25, 110and 150 1C, respectively, to obtain xerogels. Thexerogels were placed in a furnace and thermallytreated between 400 and 800 1C in air, with aheating rate of 10 1C/min, and kept for 2 h at eachtemperature.Thermal behavior of the samples was examined

by differential scanning calorimetry (DSC), usinga Netzsch-404PC thermal analysis system, in an airflux and with a heating rate of 10 1C/min.Mid-infrared (IR) spectra, from 4000 to

400 cm�1, were recorded using a Nicolet-510spectrophotometer on pellets obtained dispersingthe samples in KBr.The samples were characterized by powder

X-ray diffraction (XRD) using a y� 2y SiemensD500 diffractometer working with Cu Ka radia-tion, a graphite monochromator on the diffractedbeam and a scintillation counter with pulse-heightdiscriminator.Transmission electron microscopy (TEM) ob-

servations were carried out in bright-field (BF)mode, using a Hitachi-800 microscopy operatingat 100 kV and with a JEOL CX200 microscopeoperating at 200 kV to study the morphologicalproperties of the samples. TEM samples wereprepared by dispersing the powders in acetone inan ultrasonic bath and dropping on a conventionalcarbon-coated copper grid. Mean particle size (m)and standard deviation (s) were calculated fromTEM micrographs, at 100 000� magnification,counting around 100 particles. From these data,the degree of polydispersity, defined as s/m; wascalculated.The hysteresis loops of the nanocomposites

were collected at RT using a vibrating sample

ARTICLE IN PRESS

X.H. Huang, Z.H. Chen / Journal of Crystal Growth 271 (2004) 287–293 289

magnetometer with a maximum applied magneticfield of 10KOe.

3. Results and discussion

It has been found that if nonreactive species areinserted into the initial solution (called ‘‘sol’’), thesolid oxide network gradually groups around thesespecies during the sol–gel reaction and finally givesa doped gel with dispersed species engaged in thepores of the gel [16]. This theory has been provedin different systems such as Fe/SiO2 [17], Fe2O3/SiO2 [11–15,18], Cu/SiO2 [19], etc.The evolution of the xerogels with temperature

was studied in detail in an attempt to understandthe formation of cobalt ferrite nanocrystals in gels.Fig. 1 shows the X-ray diffraction pattern for thexerogel and the samples obtained after a 2 h heattreatment of the xerogel at various temperatures.The results demonstrate that the xerogel obtainedafter drying at 150 1C was amorphous and nonitrate crystallites were detectable, while thecorresponding IR spectra (Fig. 3) of the xerogelshowed a band at 1449.92 cm�1, which is asso-ciated with the anti-symmetric NO3

�1 stretchingvibration directly arising from the residual nitrategroups in the xerogel. These indicate that nitrate

Fig. 1. X-ray diffraction patterns of the samples dried at 150 1C

and the samples obtained after calcination of the xerogel at

various temperatures.

salts were uniformly dispersed in the gel. In fact,when the nanoparticles’ precursors were not welldispersed, peaks due to the nitrate phase werevisible in the XRD spectra. The decompositiontemperature of iron nitrate is 125 1C, whileaccording to the DSC, nitrate salts in the gel weredecomposed at about 400 1C, which indicated theentry of Co and Fe ions into the silica network[20]. The XRD spectra of samples reveal that weakpeaks assigned to CoFe2O4 appeared at 400 1C,suggesting that the particles of CoFe2O4 had beennucleated in the silica matrix. An increase in theintensity and a narrowing in the diffraction bandwere observed with increasing temperature. Dur-ing the formation of CoFe2O4 nanocrystals, notraces of quartz, cristobalite or intermediateproducts, e.g., Fe2SiO4 or Co2SiO4 were formedeven at temperatures as high as 800 1C, which canbe accounted for the homogeneity in composition.Hysteresis loops at 77K (Fig. 2) showed a

drastic change in shape from the xerogel, whichwas paramagnetic to that of the sample heated at400 1C, which was clearly superparamagnetic atthis temperature. An increase in the initialsusceptibility (slope M/H) with the temperaturetook place up to 800 1C indicating a gradualincrease in the particle size. From 600 1 to 800 1C,the sample exhibited hysteresis at 77K and bothcoercivity and remanent magnetization increasedwith the annealing temperature. This type ofbehavior is entirely consistent with a model ofparticle growth in the system in such a way thatthe differences in the magnetic parameters areassociated with changes in particle size [21].The changes in the matrix microstructure and

the pore environment before and after the heattreatment at various temperatures were followedby IR spectroscopy (Fig. 3). For the xerogel, thebroad band centered at 1638.85 cm�1 is assigned tothe H–O–H bending vibration of the absorbedwater. Obviously, there were certain amounts ofmicropores that existed in the present xerogel,which must contain physically absorbed watermolecules. The band at 1449.92 cm�1 is associatedwith the anti-symmetric NO3

� stretching vibrationdirectly arising from the residual nitrate groups inthe xerogel. Strong absorptions at 1083.16, 802.03and 464.01 cm�1 indicate the formation of a silica

ARTICLE IN PRESS

Fig. 2. Hysteresis loops at 77K of the samples dried at 150 1C and the samples obtained after a 2 h heat treatment of the xerogel at

various temperatures.


network [13]. The band at 860.09 cm�1 is assignedto Si–O–Fe. The presence of Si–O–Fe vibrationsreflected some interaction between the highlyisolated Fe3+ ions and the nearest silica matrix.The Si–O–Fe bond was also evident by thepresence of another band at 586.57 cm�1, whichis associated with the Fe–O stretching in Si–O–Febonds [22]. Another faint absorption band at667.05 cm�1 is assigned to the stretching vibratingmode of the Co–O band [23]. The presence ofSi–O–Fe and Co–O bonds sufficiently reflected thechemical nature of the transition metals involvedin the xerogel. That is, these transition metal ionsdid not participate directly in the sol–gel chemistryeven though they were introduced into the startingsolutions in the form of soluble inorganic salts.For samples obtained at a treatment temperatureof 400 1C, the intensities for the broad bandsassociated with the absorbed water were drasti-cally weakened. The absorption at 1449.92 cm�1

disappeared, which was a consequence of completedecomposition of the nitrate species as confirmedby DSC analysis. The characteristic absorptionsfor the silica network remained nearly the same asthose of the xerogel, while the bands at 568.14 and

861.72 cm�1 slightly increased in intensity, whichcould be ascribed to the enhanced interactionsbetween the CoFe2O4 clusters and silica matrix.For the samples obtained at a treatment tempera-ture of 600 1C, the absorption band at1079.71 cm�1 for �Si–O–Si� of the SiO4 tetra-hedron was further broadened, while that for theSi–O–Si or O–Si–O bending mode at 465.70 cm�1

was much weaker, which corresponded to arearrangement process of the silica network [24].It should be noted that a new band appeared at847.53 cm�1. Correspondingly, the absorption ofthe Fe–O stretching band in Fe–O–Si bondsincreased in intensity. These facts reflect theformation of CoFe2O4 clusters that was accom-panied by the rearrangement of the silica networkand with the enhancement of the Si–O–Fe bondsbetween the CoFe2O4 clusters and the surroundingsilica network. For samples heat treated at 800 1C,the IR spectrum changed greatly compared withthat for samples heat treated at 600 1C. Theabsorption at 1080.18 cm�1 for �Si–O–Si� ofthe SiO4 tetrahedron grew narrower and stronger,while the band at 860.21 cm�1 became very weakwith the absence of the Si–O–Fe bonds. These

ARTICLE IN PRESS

Fig. 3. IR spectra of the nanocomposite samples treated at

different conditions.


results reflect the broken Si–O–Fe bonds, whichcoincided with the disappearance of Fe–O stretch-ing band at 574.79 cm�1 for Si–O–Fe bonds. The

band intensity of samples heat treated at 800 1C at800.01 cm�1 for the Si–O–Si symmetric stretchincreased, and the stronger absorption band at457.94 cm�1 for the Si–O–Si or O–Si–O bendingmode reappeared. The absorptions that appearedat 678.59 cm�1 became very strong, which can beassociated with the characteristic Co–O stretchingmodes in the CoFe2O4 phase [25]. The breakageof the Fe–O–Si bonds in the interface betweenthe clusters and matrix and the formation ofCoFe2O4 clusters were probably a result of thetransformation from FeO6 octahedron to FeO4tetrahedron [26].To further study the confinement of silica matrix

on the growth of CoFe2O4 particles at thecalcinations temperature, wet gels were dried atdifferent temperatures, namely 50, 110 and 150 1C,respectively, before being calcined at 800 1C for2 h. The choice of the three drying temperaturewas based on the observation in DSC study.According to the study, the formation of amor-phous silica began at about 65 1C and wascompleted at temperatures at about 156 1C, abovewhich cracking tended to occur due to the volumeshrinkage during drying of the gel. The averagesizes of the cobalt ferrite particles were evaluatedfrom TEM micrographs. It is apparent that theaverage particles size of CoFe2O4 decreased withincreasing the drying temperature from 50 to150 1C (Fig. 4). It should be emphasized that thedrying temperature not only affected the particlesize but also the particle size distribution, as can beobserved by the values of the polydispersity degree(s/m) obtained from TEM micrographs (Fig. 4).The s/mvalue of 70% obtained for the samplesdried at 50 1C was drastically reduced to 14% forthe samples dried at 150 1C.The electron micrographs of the samples cal-

cined at 800 1C for 2 h following drying at 150 and50 1C (Fig. 5) also evidently supported the abovebehavior.Such a strong influence of the drying tempera-

ture on the average particle size of CoFe2O4 can beexplained as follows: When the gel precursor wasdried at a temperature much lower than 156 1C,the formation of a silica network structure was notcompleted. The subsequent rapid increase intemperature (10 1C/min) during calcinations did

ARTICLE IN PRESS

Fig. 4. Average diameters (m) and polydispersity degree (s/m) ofthe ferrite particles in the samples calcined at 800 1C for 2 h

following drying at various temperatures.

Fig. 5. TEM micrographs of the samples calcined at 800 1C for

2 h following drying at (a) 150 1C and (b) 50 1C.


not favor the establishment of a silica networkstructure. In fact, a too rapid heating gave rise tothe cracking of a partially completed networkstructure. Furthermore, the cobalt ferrite particlesformed at a high temperature could also obstructthe further establishment of a silica network.Therefore, the lower the drying temperature, theless complete would the silica structure be. On theother hand, a well-established silica networkprovided a more effective confinement to thegrowth of CoFe2O4 particles [19]. As a result, theCoFe2O4 particles in the sample dried at 150 1Ccoarsened at a much slower rate than that in thesample dried at 50 1C. Therefore, increasing thedrying temperature before heat treatment favoredthe formation of a complete silica network,which in turn hindered the growth of CoFe2O4nanoparticles and resulted in an effective refine-

ment and improved the uniformity of CoFe2O4particles.

4. Conclusion

The microstructural evolution of CoFe2O4nanocrystals dispersed in amorphous silica wasfollowed with IR, XRD, and VSM. Magneticproperties could be correlated with the micro-structural evolution brought about by heat treat-ment of the nanocrystals dispersed in silica matrixat successively higher treatment temperatures. Theremarkable effect of initial drying temperature onthe particle size of nanophase cobalt ferritesuggested that a well-established silica networkprovided an effective confinement to the coarsen-ing and aggregation of CoFe2O4 nanoparticles.

References

[1] A.K. Giri, K. Pongsaksawad, M. Sorescu, S. Majetich,

IEEE Trans. Magn. 36 (2000) 3029.

[2] A.K. Giri, E.M. Kirkpatrick, P. Moongkhamklang,

S.A. Majetich, Appl. Phys. Lett. 80 (2002) 2341.

[3] N. Moumen, P. Veillet, M.P. Pileni, J. Magn. Magn.

Mater. 149 (1995) 67.

[4] C. Liu, B. Zou, A.J. Rondinone, Z.J. Zhang, J. Am. Chem.

Soc. 122 (2000) 6263.

[5] V. Pillai, D.O. Shah, J. Magn. Magn. Mater. 163 (1996) 243.

[6] Y. Ahn, E.J. Choi, S. Kim, H.N. Ok, Mater. Lett. 50

(2001) 47.

[7] C.-H. Yan, Z.-G. Xu, F.-X. Cheng, Z.-M. Wang, L.-D.

Sun, C.-S. Liao, J.-T. Jia, Solid State Commun. 111 (1999)

287.

[8] Jae-Gwang Lee, Jae Yun Park, Chul Sung Kim, J. Mater.

Sci. 33 (1998) 3965.

[9] YeongIl Kima, Don Kima, ChoongSub Leeb, Physica B

337 (2003) 42.

[10] P.C. Morais, V.K. Garg, A.C. Oliveira, L.P. Silva, R.B.

Azevedo, A.M.L. Silva, E.C.D. Lima, J. Magn. Magn.

Mater. 225 (2001) 37.

[11] E.M. Moreno, M. Zayat, M.P. Morales, Langmuir 18

(2002) 4972.

[12] G. Ennas, M.F. Casula, G. Piccaluga, S. Solinas, J. Mater.

Res. 17 (3) (2002) 590.

[13] F. del Monte, M.P. Morales, D. Levy, Langmuir 13 (1997)

3627.

[14] S. Solinas, G. Piccaluga, M.P. Morales, C.J. Serna, Acta

Mater. 49 (2001) 2805.

[15] C. Cannas, D. Gatteschi, A. Musinu, G. Piccaluga,

C. Sangregorio, J. Phys. Chem. B 102 (1998) 7721.

ARTICLE IN PRESS


[16] F. Bentivegna, J. Ferre, M. Nyolt, J.P. Jamet, D. Imhoff,

J. Appl. Phys. 83 (1998) 7776.

[17] R.D. Shull, J.J. Ritter, A.J. Shapiro, L.J. Swartzendruber,

J. Appl. Phys. 67 (9) (1990) 4490.

[18] B.J. Clapsaddle, A.E. Gash, J.H. Satcher Jr., R.L.

Simpson, J. Non-Cryst. Solids 31 (2003) 190.

[19] Z.H. Zhou, J.M. Xue, H.S.O. Chan, J. Wang, Mater.

Chem. Phys. 75 (2002) 181.

[20] A.S. Albuquerque, J.D. Ardisson, W.A.A. Macedo,

J. Magn. Magn. Mater. 192 (1999) 277.

[21] H. Izutsu, P.K. Nair, K. Maeda, Y. Kiyozumi,

F. Mizukami, Mater. Res. Bull. 32 (1997) 1303.

[22] C. Chaneac, E. Tronc, J.P. Jolivet, J. Mater. Chem. 6 (12)

(1996) 1905.

[23] H.K. Jun, J.H. Koo, T.J. Lee, Energ. Fuel. 18 (2004) 41.

[24] J.M.D. Coey, Phys. Rev. Lett. 27 (1971) 1140.

[25] S. Ponce-Castaneda, J.R. Marttnez, S.A. Palomares

Sanchez, J. Sol–Gel. Sci. Technol. 25 (2002) 37.

[26] G.V.S. Rao, C.N.R. Rao, J.R. Ferraro, Appl. Spectrosc.

24 (1970) 436.

Preparation of CoFe2O4-SiO2 nanocomposites by sol–gel method

Documents

Transcript of Preparation of CoFe2O4-SiO2 nanocomposites by sol–gel method