CERAMICSINTERNATIONAL

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Ceramics International 41 www.elsevier.com/locate/ceramint

Synthesis and characterization of γ-Fe2O3/SiO2 composites as possiblecandidates for magnetic paper manufacture

Cornelia Păcurariua,n, Elena-Alina Tăculescu (Moacă)b, Robert Ianoşa, Oana Marinicăc,d,Ciprian-Valentin Mihalie, Vlad Socoliucf

aPolitehnica University Timişoara, Faculty of Industrial Chemistry and Environmental Engineering, P-taVictoriei No. 2, Timişoara 300006, RomaniabNational Institute of Research–Development in the Pathology Domain and Biomedical Sciences “Victor Babes”, 99-101 Splaiul Independentei Street, Sector 5,

RO 050096 Bucharest, RomaniacPolitehnica University Timişoara, Research Center for Engineering of Systems with Complex Fluids, No. 1 Mihai Viteazul Boulevard., 300222 Timişoara,

RomaniadWest University of Timişoara, Faculty of Physics, No. 4, V. Parvan Boulevard., 300223 Timiʂoara, Romania

eVasileGoldiş” Western University of Arad, The Institute of Life Sciences, No. 86, Liviu Rebreanu Street, 310414 Arad, RomaniafRomanian Academy-Timiʂoara Branch, Laboratory of Magnetic Fluids, No. 24, Mihai Viteazul Boulevard., 300223 Timiʂoara, Romania

Received 25 June 2014; received in revised form 25 August 2014; accepted 5 September 2014Available online 16 September 2014

Abstract

Maghemite/silica composites were prepared by a facile and effective method. First a stable magnetic fluid (MF) was obtained by dispersing intoluene the maghemite nanoparticles, prepared by coprecipitation. Afterwards, maghemite nanoparticles were coated with silica by the hydrolysisand condensation of tetraethylorthosilicate. The structural, surface and magnetic characteristics of the composites were investigated by X-raydiffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier-transform infrared (FTIR) andvibrating sample magnetometer (VSM) measurements. Magnetic studies indicated the superparamagnetic behavior of the samples and thedecrease of the saturation magnetization with the decrease of the magnetic fluid concentration.SEM images revealed the presence of agglomerated, spherical particles and TEM images showed both separated particles and multiple

maghemite particles together, covered with silica layer.The close relationship between the concentration of maghemite nanoparticles and the color of composite materials was confirmed by diffuse

reflectance spectroscopy. CIELnanbn analysis revealed the change of the composites' color from reddish-brown to light beige by decreasing theconcentration of the maghemite nanoparticles. These results demonstrate that the maghemite/silica composites are suitable for the manufacture ofmagnetic paper with different degrees of whiteness that can be easily correlated with the concentration of the magnetic fluid used for theirpreparation.& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Composite particles; Maghemite; Magnetic paper; Sol–gel

1. Introduction

Because of their important properties, magnetic nanoparti-cles are of great interest for a wide range of applications indiverse areas including biotechnology/biomedicine [1–3], cat-alysis [4], magnetic fluids [5] and wastewater treatment [6,7].

10.1016/j.ceramint.2014.09.03114 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

g author. Tel.: þ40 256404144.ss: cornelia.pacurariu@upt.ro (C. Păcurariu).

Another important application of magnetic nanoparticles is themanufacture of magnetic paper. Magnetic paper is a securitypaper, superior to the traditional one which can be used forprinting valuable documents such as banknotes, bank checks,identity cards, passports, etc.According to the literature reports, the most common

methods for preparing magnetic papers are the “in situ” synth-esis [8–12] and the “lumen-loading” approach [10,12–14]. Chiaet al. [10] used the “in situ” method for preparing nanosized

C. Păcurariu et al. / Ceramics International 41 (2015) 1079–10851080

magnetite by the coprecipitation method, inside the lumen offibers. Zakaria et al. [13] loaded magnetite powder into thelumens of unbleached kenaf pulp. Small and Johnston reported[15] a method different from the “in situ” and “lumen-loading”approach for obtaining magnetic paper in which nanoparticles ofmagnetite were prepared separately and then mixed with kraftpulp. The main disadvantage of these methods is the change ofthe paper color, depending on the amount of magnetic particlesused, because magnetite is black and maghemite is reddish-brown. A solution for bleaching the magnetic paper could be theuse of magnetic nanoparticles encapsulated in silica matrix.

Silica is one of the most common materials for coating themagnetic iron oxide nanoparticles because of its biocompat-ibility, hydrophilicity, stability and also because it is not toxic.In addition, the silanol groups on the surface of silica-coatedmagnetic nanoparticles can react with other compounds whichallows a very easy functionalization of the surface with otherfunctional groups offering a high potential for many applica-tions [4,16–20].

The Stöber method [21], in which silica is formed in situ onthe surface of the magnetic nanoparticles through hydrolysisand condensation of a sol–gel precursor, is widely used forcoating magnetic iron oxide nanoparticles with silica [22,23].More recently, the water-in-oil microemulsion or inversemicroemulsion method was extensively used for the synthesisof magnetic nanoparticles coated with uniform silica shells andtailored thickness [24–26]. Different alternative routes werealso developed for obtaining tunable silica shell–magnetic corenanoparticles [27–29]. In these routes, magnetite synthesizedby co-precipitation was firstly stabilized with trisodium citrate[27] or citric acid [28] and dispersed in water to form amagnetic fluid. Then the coating process with silica wascarried out according to the modified Stöber method, in analcohol/water mixture. The same procedure involving themagnetite stabilization with trisodium citrate was reported byFarimani et al. [29] with the difference that all the synthesissteps were carried out under N2 gas protection to prevent theobtaining of maghemite as a result of divalent iron saltsoxidation. Im et al. [30] showed that commercially organo-philic ferrofluid can be used for obtaining silica-coated ironoxide nanoparticles. This procedure involved the extraction ofiron oxide nanoparticles with acetone from the dispersionmedium, drying and their redispersion in toluene. Then, theStöber method was used for coating the iron oxide nanopar-ticles with silica.

In this study we report a simple alternative to the methodproposed by Im et al. [30] in which the magnetic fluid wasobtained in situ, by dispersing in toluene the maghemitenanoparticles covered with oleic acid. The coating with silicawas performed by the hydrolysis and condensation of tetraethy-lorthosilicate. The effect of the concentration of the iron oxidenanoparticles on the color and the saturation magnetization of thepowders was investigated. The proposed synthesis method allowsthe preparation of magnetic nanopowder that could be used forthe manufacture of magnetic paper with different degrees ofwhiteness that can be adjusted by changing the concentration ofthe magnetic fluid used at the powder preparation.

2. Experimental

2.1. Materials

Ferric chloride hexahydrate (FeCl3 � 6H2O, 99%, Merck),ferrous sulfate heptahydrate (FeSO4 � 7H2O, 98%, Chimreac-tiv), ammonium hydroxide solution (NH4OH, 25% Chimreac-tiv), vegetal oleic acid (C18H34O2, 65–88%, Merck), toluene(C7H8, 99.5%, Sigma-Aldrich), tetraethylorthosilicate (TEOS)(C8H20O4Si, 99%, Merck) and isopropanol (C3H8O, 99.7%,Merck) were used as received without further purification.

2.2. Preparation of stable organic suspension of maghemitenanoparticles

The maghemite nanoparticles were synthesized by thecoprecipitation of Fe2þ and Fe3þ ions using NH4OH as aprecipitating agent. To prevent the magnetic particles agglom-eration, oleic acid was used as a surfactant. The synthesiswas carried out in air, thus favoring the oxidation of magne-tite to maghemite. Briefly, 0.46 g FeSO4 � 7H2O and 0.9 gFeCl3 � 6H2O were dissolved in 30 mL of deionized water. Thesolution was heated at 80 1C and then, under vigorous stirring,an excess of NH4OH was rapidly added. After the coprecipita-tion had started a significant excess (about 30 vol%) of oleicacid was added into the suspension, which was chemicallyadsorbed on the surface of magnetic particles [31]. After30 min of vigorous stirring the magnetic nanoparticles werecollected by magnetic decantation, washed several times withwater and acetone to remove the nonmagnetic by-products andthen dispersed in 100 mL toluene, resulting in a stablemagnetic fluid (MF) with concentration of 4 mg mL�1. MFswith different concentrations of iron oxide nanoparticles wereobtained by diluting this MF with toluene.

2.3. Coating of magnetic nanoparticles with silica

The coating process with silica was carried out according tothe procedure reported by Im et al. [30]. Samples of magneticnanopaticles coated with silica denoted as MS1, MS2 and MS3were prepared using MFs containing 4, 4� 10�3 and 4�10�4 mg mL�1 iron oxide nanoparticles, respectively. 1 mLof MF was added to a solution containing 20 mL isoprop-anol, 2 mL deionized water and 1.5 mL of 25% NH4OH.After 10 min of magnetic stirring 2 mL of TEOS was addeddropwise to the mixture, under continuous magnetic stirring at300 rpm for 12 h at room temperature. The final product wascollected by magnetic decantation, washed with deionizedwater and acetone and dried in an oven at 80 1C.

2.4. Characterization methods

The phase composition of the samples was investigated byX-ray diffraction (XRD) using a RigakuUltima IV instrumentoperating at 40 kV and 40 mA. The XRD patterns wererecorded using CuKα radiation. The average crystallite sizewas calculated based on the XRD patterns using the PDXL 2.0

Fig. 2. FT-IR spectra of γ-Fe2O3/SiO2 composites.

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software and Scherrer's equation. FTIR spectra were carriedout using a Shimadzu Prestige-21 spectrometer in the range400–4000 cm�1, using KBr pellets and resolution of 4 cm�1.The magnetization of the three samples, MS1, MS2 and MS3was measured by means of the vibrating sample magnetometrymethod, using a VSM 880 magnetometer, at room temperature(25 1C), in the field range 79� 105 A/m. The experimentalmagnetic data were used to determine the mean magneticdiameter of γ-Fe2O3 particles [32]. The saturation magnetiza-tion of the samples was determined by means of the Chantrellmethod [33]. SEM–EDX analysis was performed by scanningelectron microscopy (SEM), using a FEI Quanta 250 micro-scope. The morphology of magnetic nanoparticles coated withsilica was characterized by transmission electronic microscopy(TEM) using a FEI Tecnai 12 Biotwin instrument. The color ofthe samples was investigated by diffuse reflectance spectro-photometry. CIELnanbn parameters were measured using aVarian Cary 300 Bio UV–vis spectrophotometer (illuminant –D65, observer angle – 101).

3. Results and discussion

3.1. XRD characterization of magnetic nanoparticles coatedwith silica

The XRD patterns of samples MS1–MS3 shown in Fig. 1suggest that maghemite, γ-Fe2O3, is the only crystalline phase,having a crystallite average size (estimated using the Sherrerequation) of 6.0 nm. The absence of other diffraction peaksshows that SiO2 is in amorphous state, which is quite expectedgiven the synthesis conditions. On the other hand the XRDpatterns, which were recorded under the same experimentalconditions, indicate that the intensity of γ-Fe2O3 peaks

Fig. 1. XRD patterns of samples MS1–MS3.

decreases as the maghemite concentration decreases (MS1-MS2-MS3). At the same time, the presence of a largeramount of amorphous SiO2 in the sample MS3 in comparisonwith MS2 and MS1 caused an increase of the relative intensityof the XRD patterns measured at small angles (Fig. 1) fromsample MS1 to MS3.It can be also seen (Fig. 1) that the XRD pattern of the sample

MS3 is much less smooth in comparison with those of thesamples MS2 and MS1 which confirms the decrease of theamorphous silica content in the same order (MS3-MS2-MS1).

Fig. 3. Magnetization curves of γ-Fe2O3/SiO2 composites.

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3.2. FT-IR spectra

The FT-IR spectra of samples MS1–MS3 are shown inFig. 2. As can be observed, all the three samples show bandslocated at the same wavenumber but with different intensitieswhich increase continuously from sample MS1 to MS3, inaccordance with increasing the silica content. It is mentionablethat the characteristic bands of the Fe–O vibrations in theregion 500–630 cm�1 are not observable because they overlapwith the intense band located at 469 cm�1, assigned to the Si–O–Si bending vibration [34,35].

The bands located at 1099 and 800 cm�1 are assigned,respectively, to the asymmetric and symmetric stretching of Si–O–Si [36,37]. The absorption band at 950 cm�1 can be assignedto Si–OH stretching vibration [35]. The adsorbed water shows abroad band between 3200 and 3600 cm�1 assigned to O–Hstretching in H-bonded water and a band located at 1637 cm�1dueto the O–H bending vibration of molecular water [35].

3.3. Magnetic properties

Fig. 3 shows the magnetization curves of the samples.The absence of the remanent magnetization suggests that allγ-Fe2O3/SiO2 composites have a superparamagnetic behavior.

Fig. 4. SEM images of γ-F

Fig. 5. Energy dispersive X-ray spec

The saturation magnetization (Fig. 3), Ms, gradually decreasesfrom 26.0 emu/g for MS1 to 7.8 emu/g for MS2 and then to0.8 emu/g for MS3, as the γ-Fe2O3/SiO2 ratio decreases, inaccordance with the literature data [29,38].The γ-Fe2O3 nanoparticles embedded within the SiO2 matrix

have a mean magnetic diameter of 5.6 nm, which is inexcellent agreement with their superparamagnetic behavior.At the same time the mean magnetic diameter of γ-Fe2O3

particles is very close to the crystallite size calculated from theXRD pattern (6.0 nm).

3.4. SEM-EDX and TEM analyses

Fig. 4 presents the SEM images of the samples. It can beobserved that the γ-Fe2O3/SiO2 composites are made up ofspherical particles, which are agglomerated due to their highsurface energy. The diameter of particles increases from MS1(E200 nm) to MS3 (E600 nm), as the γ-Fe2O3/SiO2 ratiodecreases (Fig. 4). Similar results regarding the increase of theparticles diameter with the decrease of the iron oxide nano-particles concentration were reported by Im et al. [30].The energy dispersive X-ray spectroscopy (EDX) analysis

was employed to determine the chemical composition of thesamples. The elemental analysis confirmed that Si, O and Fe

e2O3/SiO2 composites.

tra of γ-Fe2O3/SiO2 composites.

Fig. 7. Diffuse reflectance spectra of γ-Fe2O3/SiO2 composites. Inset: MS1–MS3 samples.

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are the only elements contained within the prepared samples(Fig. 5). The EDX spectra of the three samples also indicatethat the Fe and O peaks get weaker from sample MS1 to MS3in accordance with the decrease of the γ-Fe2O3 nanoparticlesconcentration in the same order.

TEM was used to examine the detailed structure andmorphology of the γ-Fe2O3/SiO2 composites. Fig. 6 showsthe TEM image of the samples MS1 and MS2 which confirmsthat the particles are agglomerated with a smooth surface. Theobvious difference in contrast between the dark areas and thegray ones can be observed, which indicates that silica hassuccessfully coated the γ-Fe2O3 nanoparticles. The TEMimages also revealed the presence of spherical particlesseparated, alongside multiple γ-Fe2O3 particles together, cov-ered with silica layer.

Similar results were reported by Narita et al. [25] using thereverse micelle method for the synthesis of SiO2 coated Fe3O4

core–shell nanoparticles.

3.5. Diffuse reflectance spectra and CIELnanbn measurements

Given the potential application of γ-Fe2O3/SiO2 compositesfor magnetic paper manufacture, their color is definitely aproperty which demands being discussed. The influence of theconcentration of maghemite nanoparticles on the color ofsamples was analyzed by means of diffuse reflectance spectro-scopy and CIELnanbn parameters. The diffuse reflectancespectra of the three samples (Fig. 7) show a large absorptionband in the range of 360–550 nm, which is typical formaghemite [39,40]. This band is considered to be the envelopeof three absorption bands which can be assigned to thefollowing transitions: 6A1-

4E(4D) for the band near 360–380 nm, 6A1-

4E, 4A1(4G) for the band located at 430 nm and

26A1-24T1(4G) for the band near 480–550 nm [41,42].

The CIELnanbn parameters of γ-Fe2O3/SiO2 compositesindicate that the three samples are located in the red–yellowregion. The position of γ-Fe2O3/SiO2 composites in theCIELnanbn color space is actually easy to understand, giventhe natural reddish-brown color of γ-Fe2O3 (Fig. 8). On the

Fig. 6. TEM image of γ-Fe2O3/SiO2 co

other hand, one can observe that the lightness, Ln, parameterincreases from MS1 to MS3, as the γ-Fe2O3/SiO2 ratiodecreases, in excellent agreement with the color evolution ofthe samples from reddish-brown to light beige.

4. Conclusions

γ-Fe2O3/SiO2 composites were successfully prepared by asimple and effective method which allows correlating the colorof the powders with the initial concentration of γ-Fe2O3

nanoparticles.The samples showed superparamagnetic behavior, the

saturation magnetization decreasing with the decrease ofγ-Fe2O3/SiO2 ratio.SEM and TEM investigations revealed the presence of

agglomerated, spherical particles with smooth surface. Also,

mposite (MS1 and MS2 samples).

Fig. 8. Position of γ-Fe2O3/SiO2 composites' color in the CIELnanbn

color space.

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the TEM images showed the presence of separated particlesand multiple γ-Fe2O3 nanoparticles together covered withsilica layer.

CIELnanbn analysis revealed the change of the composites'color from reddish-brown to light beige as the concentration ofγ-Fe2O3 nanoparticles decrease. Basically, by changing theγ-Fe2O3/SiO2 ratio, one can adjust the color of the resultingcomposite materials.

The ability to control the color of γ-Fe2O3/SiO2 compositesas well as their superparamagnetic properties permits thesematerials as potential candidates for the manufacture ofmagnetic paper with different degrees of whiteness that canbe correlated with the concentration of the magnetic fluid usedfor the powder preparation.

Further works on the application of the γ-Fe2O3/SiO2

composites are underway in order to validate their potentialusage for the manufacture of magnetic paper.

Acknowledgment

This paper is partly supported by the Sectorial OperationalProgramme Human Resources Development (SOPHRD),financed by the European Social Fund and the RomanianGovernment under the Contract number POSDRU 141531.

References

[1] S.-H. Huang, R.-S. Juang, Biochemical and biomedical applications ofmultifunctional magnetic nanoparticles: a review, J. Nanopart. Res. 13(2011) 4411–4430.

[2] T. Neuberger, B. Schöp, H. Hofmann, M. Hofmann, B. von Rechenberg,Superparamagnetic nanoparticles for biomedical applications: possibili-ties and limitations of a new drug delivery system, J. Magn. Magn. Mater.293 (2005) 483–496.

[3] A. Hofmann, S. Thierbach, A. Semisch, A. Hartwig, M. Taupitz, E. Rühl,C. Graf, Highly monodisperse water-dispersable iron oxide nanoparticlesfor biomedical applications, J. Mater. Chem. 20 (2010) 7842–7853.

[4] A.-H. Lu, E.L. Salabas, F. Schüth, Magnetic nanoparticles: synthesis,protection, functionalization and application, Angew. Chem. Int. Ed. 46(2007) 1222–1244.

[5] L. Vékás, Ferrofluids and Magnetorheological Fluids, Adv. Sci. Technol.54 (2008) 127–136.

[6] P. Xu, G.M. Zeng, D.L. Huang, C.L. Feng, S. Hu, M.H. Zhao, C. Lai,Z. Wei, C. Huang, G.X. Xie, Z.F. Liu, Use of iron oxide nanomaterials inwastewater treatment: a review, Sci. Total Environ. 424 (2012) 1–10.

[7] R.D. Ambashta, M. Sillanpää, Water purification using magnetic assis-tance: a review, J. Hazard. Mater. 180 (2010) 38–49.

[8] R.H. Marchessault, S. Ricard, P. Rioux, In situ synthesis of ferrite inlignocellulosics, Carbohydr. Res. 224 (1992) 133–139.

[9] J.L. Carrazana-Garcia, M.A. López-Quintela, J. Rivas-Rey, Characteriza-tion of ferrite particles synthesized in presence of cellulose fibers,Colloids Surf. A: Physicochem. Eng. Asp. 121 (1997) 61–66.

[10] C.H. Chia, S. Zakaria, S. Ahamd, M. Abdullah, S.M. Jani, Preparation ofmagnetic paper from kenaf: lumen loading and in situ synthesis method,Am. J. Appl. Sci. 3 (2006) 1750–1754.

[11] C.H. Chia, S. Zakaria, K.L. Nguyen, M. Abdullah, Utilisation ofunbleached kenaf fibers for the preparation of magnetic paper, Ind. CropsProd. 28 (2008) 333–339.

[12] W.-B. Wu, Y. Jing, M.-R. Gong, X.-F. Zhou, H.-Q. Dai, Preparation andproperties of magnetic cellulose fiber composites, BioResources 6 (3)(2011) 3396–3409.

[13] S. Zakaria, B.H. Ong, S.H. Ahmad, M. Abdullah, T. Yamauchi,Preparation of lumen-loaded kenaf pulp with magnetite (Fe3O4), Mater.Chem. Phys. 89 (2005) 216–220.

[14] P. Kumar, S. Negi, S.P. Singh, Filler loading in the lumen or/and cell wallof fibers – a literature review, BioResources 6 (3) (2011) 3526–3546.

[15] A.C. Small, J.H. Johnston, Novel hybrid materials of magnetic nano-particles and cellulose fibers, J. Colloid Interface Sci. 331 (1) (2009)122–126.

[16] W. Wu, Q. He, C. Jiang C, Magnetic iron oxide nanoparticles: synthesis andsurface functionalization strategies, Nanoscale Res. Lett. 3 (2008) 397–415.

[17] C. Vogt, M.S. Toprak, M. Muhammed, S. Laurent, J.-L. Bridot, R.N. Műller,High quality and tuneable silica shell–magnetic core nanoparticles,J. Nanopart. Res. 12 (2010) 1137–1147.

[18] J. Lewandowska, M. Staszewska, M. Kepczynski, M. Szuwarzyński,A. Łatkiewicz, Z. Olejniczak, M. Nowakowska, Sol–gel synthesis of ironoxide–silica composite microstructures, J. Sol–Gel Sci. Technol. 64(2012) 67–77.

[19] J.S. Lee, S.K. Hong, N.J. Hur, W.-S. Seo, H.J. Hwang, Fabrication ofspherical silica aerogel/magnetite nanocomposite particles, Mater. Lett.112 (2013) 153–157.

[20] A. Jitianu, M. Raileanu, M. Crisan, D. Predoi, M. Jitianu, L. Stanciu,M. Zaharescu, Fe3O4–SiO2 nanocomposites obtained via alkoxide andcolloidal route, J. Sol–Gel Sci. Technol. 40 (2006) 317–323.

[21] W. Stöber, A. Fink, E. Bohn, Controlled growth of monodisperse silica spheresin the micron size range, J. Colloid Interface Sci. 26 (1968) 62–69.

[22] Y.A. Barnakov, M.H. Yu, Z. Rosenzweig, Manipulation of the magneticproperties of magnetite�silica nanocomposite materials by controlledStöber synthesis, Langmuir 21 (16) (2005) 7524–7527.

[23] Y. Lu, Y. Yin, B.T. Mayers, Y. Xia, Modifying the surface properties ofsuperparamagnetic iron oxide nanoparticles through a sol�gel approach,Nano Lett. 2 (3) (2002) 183–186.

[24] L. Yao, G. Xu, W. Dou, Y. Bai, The control of size and morphology ofnanosized silica in triton X-100 based reverse micelle, Colloids Surf. A:Physicochem. Eng. Asp. 316 (2008) 8–14.

[25] A. Narita, K. Naka, Y. Chujo, Facile control of silica shell layer thicknesson hydrophilic iron oxide nanoparticles via reverse micelle method,Colloids Surf. A: Physicochem. Eng. Asp. 336 (2009) 46–56.

[26] D.K. Yi, S.S. Lee, G.C. Papaefthymiou, J.Y. Ying, Nanoparticlearchitectures templated by SiO2/Fe2O3 nanocomposites, Chem. Mater.18 (3) (2006) 614–619.

C. Păcurariu et al. / Ceramics International 41 (2015) 1079–1085 1085

[27] Y.-H. Deng, C.-C. Wang, J.-H. Hu, W.-L. Yang, S.-K. Fu, Investigationof formation of silica-coated magnetite nanoparticles via sol–gelapproach, Colloids Surf. A: Physicochem. Eng. Asp. 262 (2005) 87–93.

[28] B. Mojić, P.K. Giannakopoulos, Ž. Cvejić, V.V. Srdić, Silica coatedferrite nanoparticles: influence of citrate functionalization procedure onfinal particle morphology, Ceram. Int. 38 (2012) 6635–6641.

[29] M.H.R. Farimani, N. Shahtahmasebi, M.R. Roknabadi, N. Ghowsc,A. Kazemi, Study of structural and magnetic properties of superpar-amagnetic Fe3O4/SiO2 core–shell nanocomposites synthesized withhydrophilic citrate-modified Fe3O4 seeds via a sol–gel approach, PhysicaE 53 (2013) 207–216.

[30] S.H. Im, T. Herricks, Y.T. Lee, Y. Xia, Synthesis and characterization ofmonodisperse silica colloids loaded with superparamagnetic iron oxidenanoparticles, Chem. Phys. Lett. 401 (2005) 19–23.

[31] K. Yang, H. Peng, Y. Wen, N. Li, Re-examination of characteristic FTIRspectrum of secondary layer in bilayer oleic acid-coated Fe3O4 nanopar-ticles, Appl. Surf. Sci. 256 (2010) 3093–3097.

[32] M. Raşa, D. Bica, A. Philipse, L. Vékás, Dilution series approach forinvestigation of microstructural properties and particle interactions inhigh-quality magnetic fluids, Eur. Phys. J. E 7 (2002) 209–220.

[33] R.W. Chantrell, J. Popplewell, S.W. Charles, Measurements of particle-size distribution parameters in ferrofluids, IEEE Trans. Magn. 14 (1978)975–977.

[34] G.H. Podrepšek, Ž. Knez, M. Leitgeb, Different preparation methods andcharacterization of magnetic maghemite coated with chitosan, J. NanoparticleRes. 15 (2013) 1751, http://dx.doi.org/10.1007/s11051-013-1751-x.

[35] P. Innocenzi, Infrared spectroscopy of sol–gel derived silica-based films:a spectra–microstructure overview, J. Non-Cryst. Solids 316 (2003)309–319.

[36] K.C. Souza, D.S. Mohallem, E.M.B. Sousa, Mesoporous silica–magnetitenanocomposite: facile synthesis route for application in hyperthermia,J. Sol–Gel Sci. Technol. 53 (2010) 418–427.

[37] S. Kralj, M. Drofenik, D. Makoves, Controlled surface functionalizationof silica-coated magnetic nanoparticles with terminal amino and carboxylgroups, J. Nanopart. Res. 13 (2010) 2829–2841.

[38] C.Y. Haw, C.H. Chia, S. Zakaria, F. Mohamed, S. Radiman, C.H. Teh, P.S. Khiew, W.S. Chiu, N.M. Huang, Morphological studies of randomizeddispersion magnetite nanoclusters coated with silica, Ceram. Int. 37(2011) 451–464.

[39] J. Torrent, V. Barrón, Diffuse reflectance spectroscopy of iron oxides, in:M. Dekker (Ed.), Encyclopedia of Surface and Colloid Science, NewYork, 2002, pp. 1438–1446.

[40] R.G.J. Strens, B.J. Wood, Diffuse reflectance spectra and optical proper-ties of some iron and titanium oxides and oxyhydroxides, Mineral. Mag.43 (1979) 347–354.

[41] D.M. Sherman, T.D. Waite, Electronic spectra of Fe3þ oxides and oxidehydroxides in the near IR to near UV, Am. Mineral. 70 (1985)1262–1269.

[42] A.C. Scheinost, A. Chavernas, V. Barron, J. Torrent, Use and limitationsof second-derivative diffuse reflectance spectroscopy in the visible tonear-infrared range to identify and quantify Fe oxide minerals in soils,Clays Clay Miner. 46 (1998) 528–536.