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Journal of Alloys and Compounds 771 (2019) 1e8

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Journal of Alloys and Compounds

journal homepage: http: / /www.elsevier .com/locate/ ja lcom

Fabrication of monodispersed a-Fe2O3@SiO2 core-shell nanospheresand investigation of their dielectric behavior

A. Sakthisabarimoorthi, S.A. Martin Britto Dhas, M. Jose*

Department of Physics, Sacred Heart College (Autonomous), Tirupattur, Tamilnadu, India

a r t i c l e i n f o

Article history:Received 5 June 2018Received in revised form23 August 2018Accepted 25 August 2018Available online 27 August 2018

Keywords:Core-shell nanospheresPowder XRD analysisXPS analysisImpedance analysis

* Corresponding author.E-mail address: jose@shctpt.edu (M. Jose).

https://doi.org/10.1016/j.jallcom.2018.08.2500925-8388/© 2018 Elsevier B.V. All rights reserved.

a b s t r a c t

We report dielectric behavior of uniformly dispersed a-Fe2O3@SiO2 core-shell nanospheres prepared byemploying solvothermal technique. The a-Fe2O3@SiO2 core-shell nanospheres were subjected to Fouriertransform infrared spectroscopy, which substantiates the presence of metal iron and silicon functionalgroups. Powder X-ray diffraction analysis establishes the phase purity and composition. X-ray photo-electron spectroscopy analysis demonstrates the formation of a-Fe2O3@SiO2 core-shell nanosphereswhich is further corroborated by scanning electron microscopy images that authenticates the thicknessof the shell is sufficiently larger than the core. A relatively high dielectric permittivity (1100) and low lossfactor (1.51) is obtained at 1 KHz (at 40 �C). The material follows positive temperature coefficient ofresistance type behavior as evidenced in the impedance analysis.

© 2018 Elsevier B.V. All rights reserved.

1. Introduction

Surface passivation approach is necessitated for the creation ofnovel hetero structures such as the core-shell nanostructures,wherein a core-shell nanostructure consisting of wide band gapmaterial of shell is developed over the core with a minimum latticedifference among them [1]. Core-shell nanospheres are morecomplex than conventional single nanoparticles, but these blendedmaterials have significantly enhanced properties [2,3]. In general,shell material may prevent the direct contact of core material withthe atmospheric oxygen or moisture, which leads to improvedphysical and chemical properties than that of single counterparts.The lowest lattice constant difference and the interfacial energybetween them are the important influencing parameters for thecreation of core-shell nanospheres. In addition, the control of re-action temperature, reaction time, sequence of mixing of the re-actants, proper selection of the surface adhering agent or organicligands and rate of the addition of shell materials are the otheressential parameters for the formation of core-shell nanospheres.In view of that, silica coating over the magnetic material is one ofthe promising ways to assure the precise biocompatibility and lowtoxicity. Magnetic-metal oxide nanoparticles encapsulated by thedielectric materials have a huge number of realistic applicability in

electromagnetic devices, biology and more specifically in biomed-ical applications including targeted drug administration, magneticseparation, magnetic resonance imaging (MRI) and hyperthermiatherapy, etc. [4e6]. Accordingly, several techniques includingmicroemulsion, modified Stober method, Massart's process, sono-chemical synthesis, hydrolysis with co-precipitation and sol-vothermal technique have been developed to prepare a-Fe2O3@SiO2 nanoparticles.

Indeed, among the aforementioned methods, solvothermalsynthesis process is more complicated, however its sensitiveness toreaction parameters such as the type of raw materials, feeding se-quences, reaction temperatures and reaction time lead to formationof good crystalline material with uniform size and desiredmorphology [7e12]. Recently, Zhang and co-workers have pre-pared the spindle-like a-Fe2O3@SiO2 core-shell nanoparticles withvarious shell thicknesses through hydrothermal technique andmodified stober process [13]. However, in this work, well disperseda-Fe2O3 nanospheres are prepared by solvothermal process andsubsequently, SiO2 shell was developed over the core using ethanolas a solvent with ferric nitrate and tetraethoxysilane as core andshell precursors respectively. As a result, this synthetic methodyielded nanospheres with high degree of crystallization with ho-mogeneous and narrow particle size distribution. FTIR spectrumconfirms the presence of functional groups in the product. Theformation of sharp diffraction peaks in the powder XRD profileindicates that the creation of SiO2 shell did not affect the crystal-linity of the a-Fe2O3 core. XPS analysis demonstrates the elemental

Fig. 1. Schematic representation of fabrication of monodispersed a-Fe2O3@SiO2 core-shell nanospheres.

Fig. 2. Powder XRD profile of a-Fe2O3@SiO2 core-shell nanospheres.

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composition and the exact valence states of a-Fe2O3@SiO2 coreshell nanoparticles and further lends support to the creation ofcore-shell structuredmaterial which is corroborated from Scanningelectron microscopic images. Moreover, impedance analysis illus-trates the dielectric behavior of the a-Fe2O3@SiO2 core-shellnanospheres in the wide frequency range from 1Hz to 1MHz inthe temperature range (40e200 �C) in steps of 40 �C.

2. Materials and methods

2.1. Materials

Iron (III) nitrate nonahydrate (Fe(NO3)3$9H2O), Acetic acid(CH3$COOH), Tetraethyl orthosilicate (SiC8H20O4), Cetyl-trimethylammonium bromide (C19H4BrN), Polyvinylpyrrolidone((C6H9NO)n), aqueous ammonia (NH3$H2O) and absolute ethanol(C2H5OH) were obtained from Sigma-Aldrich Chemical Co. All thereagents (AR grade) were used as received and deionizedwater wasused for the preparation of core-shell nanospheres.

2.2. Preparation of a-Fe2O3@SiO2 core-shell nanospheres

In brief, Iron oxide nanoparticles were prepared by employing asimple solvothermal process using a noneaqueous solvent. At first,1.01 g of Fe(NO3)3$9H2O and 1.01 g of Polyvinylpyrrolidone wasdissolved in 50mL of ethanol under continuous magnetic stirring.Upon the addition of Polyvinylpyrrolidone, iron oxides coordinatewith the carbonyl group of the molecules. It is worth mentioningthat the use of Polyvinylpyrrolidone as stabilizer is to improve themonodispersity of the product. Afterwards, 2mL of acetic acid as achelating agent was poured drop wise into the above mixed solu-tion and stirred for 1 h at ambient conditions. The solutionwas thenpoured in a Teflon-lined stainless steel autoclave and heated in ahot air oven at 200 �C for 24 h. The as-synthesized product waswashed few times with ethanol to remove residue of organic sol-vents and dried in an oven at 80 �C for 24 h before it was calcinatedat 220 �C for 6 h.

The as-synthesized Iron oxide solution (5mL) was dissolved inthe mixture of double distilled water (20mL) and ethanol (20mL).Then, aqueous ammonia (9mL) was drop wise injected into theabove solution to adjust the pH to 9 followed by the addition ofcetyltrimethylammonia bromide (0.2 g). Subsequently, tetraethox-ysilane (1mL) was added drop wise into the above mixed solutionand then it was vigorously stirred for 24 h for the growth of uniformshell. The solution was then centrifuged and washed several timeswith both ethanol and deionized water to remove the unreactedspecies, if any, in the products. At last, the sample was dried at100 �C for 12 h in a hot air oven and calcinated at 550 �C for 5 h. Theoverall chemical reaction and synthesis mechanism of mono-dispersed a-Fe2O3@SiO2 core-shell nanospheres is presentedschematically in Fig. 1.

2.3. Characterization

Phase purity and structure of the product was analyzed usingpowder X-Ray Diffractometer (Bruker diffractometer with CuKaradiation l¼ 1.54056 Å) by varying diffraction angles in the rangeof 20e80�. Functional groups of the material are studied usingFourier transform infrared spectroscopy (Perkin Elmer infraredspectrophotometer) over the wavenumber from 400 to 4500 cm�1

using KBr pellet method at room temperature. The particle size andmorphology of the material was envisioned by scanning electronmicroscope (SEM ZEISS SCAN microscope). The elemental compo-sition of the material was examined by X-Ray photoelectron spec-trometer (MULTILAB-2000 Base system with XR4 Twin Anode Mg/

Al (300/400W) X-Ray spectrometer). The electrical behavior of thea-Fe2O3@SiO2 core-shell nanospheres was studied by an imped-ance analyzer (PSM 1735 with a high precision LCR meter) over awide frequency range (1 Hz- 1MHz) in the temperature range(40e200 �C) in steps of 40 �C.

3. Results and discussion

3.1. Powder XRD analysis

The powder X-ray diffraction profile is recorded to identify thecrystal structure and phase purity of the material (Fig. 2). Theappearance of diffraction peaks at 2q values of 24.1�, 33.1�, 35.6�,40.8�, 49.4�, 54.0�, 57.5�, 62.4� and 64.0� corresponding to (012),(104), (110), (113), (024), (116), (112), (214) and (300) Bragg'sdiffraction planes are in very good agreement with rhombohedralstructure of a-Fe2O3 (JCPDS No. 89-0599). No other diffractionplanes of other phases or other structures of iron oxides aredetected, indicating the phase purity of synthesized a-Fe2O3 isgood. The average crystallite size of the particles calculated usingScherrer's formula is about 20 nm. However, in the diffractionpattern of a-Fe2O3@SiO2 core-shell nanospheres, the broad peakemerging at 2q value of 24.1� indicates the presence of SiO2

Table 1Vibrational assignment of a-Fe2O3@SiO2 core-shell nanospheres.

Wavenumber (cm�1) Assignments

462 Fe-O-Fe stretching vibrations [14]967 SieOH stretching [15]802 Si-O-Si symmetric stretching [15]1091 SiO2 asymmetric stretching [15]1639 O-H bending [14]3439 C-H stretching [14]

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nanoparticles in its amorphous state. Besides, all the peaksappearing in the diffraction pattern are due to the formation ofrhombohedral a-Fe2O3 phase. Interestingly, the diffraction patternshows no changes in the 2q values except the existence of broaddiffraction hump of amorphous silica, though all the diffractionpeaks appear sharper suggesting that the silica shell did not affectthe crystallinity of the a-Fe2O3 phase. However, the presence oflarge amount of amorphous SiO2 nanoparticles on the surface ofiron oxide nanoparticles might have caused significant reduction inpeak intensity of a-Fe2O3@SiO2 nanospheres.

3.2. FTIR analysis

FTIR spectrum is recorded to analyze the functional groups ofthe synthesized a-Fe2O3@SiO2 core-shell nanospheres (Fig. 3). Thepeak appeared at 462 cm�1 is assigned to the vibration mode of Fe-O-Fe stretching while the weak peaks appeared at 802 cm�1 and967 cm�1 are ascribed to the SieOH stretching vibration andsymmetric stretching vibration of Si-O-Si bonds, respectively. Theappearance of strong and sharp peak at 1091 cm�1 is attributed tothe asymmetric stretching vibration of SiO2 bonds [14,15]. How-ever, the two broad bands emerged at 1639 cm�1 and 3439 cm�1

are attributed to the O-H bending vibrations and C-H stretchingvibrations, respectively [14]. These absorption bands reveal thefunctional groups of the product is constituted by both core andshell materials of a-Fe2O3 and SiO2 nanoparticles respectively. Thetentative vibrational assignment of a-Fe2O3@SiO2 core-shellnanospheres is given in Table 1.

3.3. XPS analysis

X-Ray photoelectron spectroscopy analysis is used to measurethe elemental composition and electronic states of the prepared a-Fe2O3@SiO2 core-shell nanospheres. There is no characteristicpeaks of iron found in the binding energy region of 700e740 eV inthe survey scan spectrum of a-Fe2O3@SiO2 core-shell nanospheres(Fig. 4 (a)), which establishes the complete coating of a-Fe2O3nanoparticles by SiO2 shell with larger thickness [16]. The principalelements of a-Fe2O3@SiO2 core-shell nanospheres are oxygen (O1sand O2s) and silicon (Si 2s and Si 2p); while no other peaks aredetected, indicating good purity of the product. The chemical states

Fig. 3. FTIR spectrum of a-Fe2O3@SiO2 core-shell nanospheres.

and the corresponding full width at half maximum, area and atomicpercentages are acquired by curve fitting method. The deconvo-luted peaks of Si 2p spectrum (Fig. 4 (b)), positioned at the bindingenergy of 103 eV is ascribed to the Si-O-Si bond. The oxygen-freesilicon Si 2p state is generally found at 99.7 eV, however, thespectrum shows the peak positioned at 103 eV which is reasonablyshifted to higher binding energy which is attributed to the silicon-dioxide Si 2p state [17]. The O 1s spectrum (Fig. 3 (c)) shows twodeconvoluted Gaussian curves with their binding energies maximapositioned at 532 and 533.8 eV attributed to O-Si and Si-O-Si bonds,respectively, which further evidence to the formation of silicon-dioxide instead of oxygen-free silicon [17,18].

3.4. SEM analysis

Scanning electron microscope is employed to envisage the core-shell nanostructure, size and morphology of the synthesized a-Fe2O3@SiO2 core-shell nanospheres (Fig. 5 (aed)). As can be seenfrom the figure, the formation a-Fe2O3@SiO2 core-shell structurednanospheres are very clearly evident with the dense white core andclear grey layer are attributed to a-Fe2O3 core and SiO2 shell,respectively. Interestingly, all the a-Fe2O3 nanoparticles arecompletely covered by the SiO2 shell with enough thickness whileuncoated a-Fe2O3 nanoparticles are not found in the medium.

As a result of reduction in the relative size distribution, the a-Fe2O3 nanoparticles became more monodispersive in nature. It isalso apparent that all the particles are formed in uniform sphericalmorphology without agglomeration and are distributed uniformly.The average particle size of the material is measured to be about300 nm. The SEM micrographs further validate the successful for-mation of a-Fe2O3@SiO2 core-shell nanospheres, which is in goodagreement with the XPS investigations. Furthermore, it is alsoevidenced that the iron oxide and silica have immiscible naturefrom one another and it has sufficient interfacial energy to form acore-shell nanostructures.

3.5. Dielectric analysis

3.5.1. Dielectric constant and lossThe dielectric behavior was studied over a range of temperature

(40e200 �C) in the frequency range from 1Hz to 1MHz(Fig. 6(aed)). It is observed from Fig. 6a that as the applied fre-quency is increased; dielectric constant is gradually decreased andattains constant value at higher frequencies for all the tempera-tures. In general, the polarization component mainly depends onthe ability of electric dipoles to align themselves in the field ofdirection during every alternation of the applied electric field. Atlower frequencies, these dipoles may have enough time to alignthemselves entirely along the direction of the electric field;consequently the dielectric constant is maximum. In contrast, athigher frequencies, these dipoles may not follow the alternation ofthe field and hence the dielectric constant is minimum [19,20]. Inaddition, the temperature dependent variation of dielectric con-stant shows relatively high values at lower temperature, and low

Fig. 4. (aec) XPS spectra of a-Fe2O3@SiO2 core-shell nanospheres.

Fig. 5. (a, b) SEM micrographs of a-Fe2O3@SiO2 core-shell nanospheres.

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Fig. 6. (a, b) Dielectric constant and dielectric loss vs. frequency; (c, d) Real and imaginary part of permittivity vs. Frequency.

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values at higher temperature in the low frequency region. However,the dielectric constant attained static values and merged togetherat higher frequencies for all the selected temperatures. This may bedue to the suppression of the dipolar relaxation process with in-crease in temperature [21].

The dielectric loss with varying frequency and temperature ispresented in Fig. 6 (b). The dielectric loss value of the material ismaximum at lower frequencies and then static at higher fre-quencies. The emergence of peaks in the dielectric loss spectrumindicates the presence of relaxation process that is further validatedfrom the modulus analysis. When the temperature is increased, thedielectric loss gradually turned to minimum indicating the ther-mally activated nature of relaxation process in the material. Ingeneral, as the temperature is increased, due to the imperfectionsor disordered structures created in the material, the mobility ofcharge carriers may increasewhich lead to increase in the dielectricconstant and loss values. The reverse trend is attributed to the in-crease in crystallinity of the material with increase of temperature[22,23]. The variation of dielectric constant and dielectric loss

Table 2Values of dielectric constant and dielectric loss factors at selected temperatures.

Temperature (�C) Frequency (1 KHz)

Dielectric constant Loss factor

40 1100 1.51080 36 0.411120 30 0.008160 29 0.002200 28 0.001

values at 1 KHz and selected temperature is listed in Table 2.Fig. 6 (c, d) represents the change in real (ε0) and imaginary parts

(ε”) of the permittivity with respect to the frequency at selectedtemperatures, which is obtained from the following formula, 3’¼

Z00

2pfC� Z and 3’’¼ Z 02pfC� Z where, Z0, Z00, f, Z and C0 are the real and

imaginary parts of impedance, applied frequency, impedance andgeometrical capacitance, respectively. The (ε0) and (ε00) spectra ex-hibits a sharp decrease at the lower frequency range and attainstatic values at higher frequencies for all the selected temperatures.This might be due to the grain boundary and grain interior effects atlower and higher frequencies, respectively. Interfacial polarizationrequiresmore energy for the polarization in the grain boundaries atlower frequencies leading to high energy loss. While other mech-anisms such as electronic and ionic polarizations require less en-ergy for the polarization in the grains causing small energy loss athigher frequency and hence the imaginary part of permittivity isdecreased and shows constant values at higher frequencies [24].

3.5.2. Impedance analysisFig. 7 (a, b) depicts the change of real (Z0) and imaginary parts

(Z00) of impedance with respect to the wide frequency at selectedtemperatures. As shown in the (Z0) spectra, the magnitude aregradually increased as increase in temperature at low frequenciesand follows a plateau-like behavior at mid frequencies, finallymerged into one at high frequencies. This trend is indicative ofpositive temperature coefficient of resistance type behavior of thematerial [17]. The appearance of asymmetric relaxation peaks inthe Z00 spectra at low frequency region suggest the presence ofelectrical relaxation, while the shifting of peaks towards high

Fig. 7. (a, b) The variation of real and imaginary part of impedance with frequency; (c) Complex impedance analysis.

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frequencies with decrease in temperature indicates the tempera-ture dependent relaxation behavior of the material [25e27].Complex impedance plot (Fig. 7 (c)) consists of depressed semi-circles for all the selected temperatures and the appearance ofsingle semicircular arcs indicates the grain boundary effectinvolved in the material equivalent to the parallel combination ofthe grain boundary resistance (R) and grain boundary capacitance(C).

3.5.3. Electrical conduction analysisThe electrical resistivity and ac conductivity of the a-Fe2O3@-

SiO2 core-shell nanospheres is presented in Fig. 8 (a, b). The

Fig. 8. (a, b) The variation of frequency dependent electrical resis

resistivity of thematerial is maximum at lower frequencies, while itis minimum at high frequencies. Consequently, the conductivity isminimum at low frequencies and then gradually increased at highfrequencies. Moreover, the resistivity of the material shows highvalues at higher temperature (200 �C) and low values at lowertemperature (40 �C). The high resistivity at higher temperature ismainly attributed to the thermally activated mobility of the chargecarriers (electrons or holes) [28]. The conductivity of the sample isalmost frequency independent at higher temperatures, conse-quently the plot is invariant, however, the sample exhibit frequencydependant behavior at lower temperatures. This typical trendgenerally arises due to the mobile charge carriers in the material. In

tivity and ac conductivity analysis at selected temperatures.

Fig. 9. (a, b) Frequency dependence of M0 and M00 at various temperatures; (c) complex electric modulus analysis.

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addition, the gradual increase in conductivity in the high frequencyregion at relatively lower temperatures is due to the increase indipole moment with alternating electric field [26].

3.5.4. Electric modulus analysisThe real (M0) and imaginary (M00) parts of electric modulus were

deduced from the real and imaginary parts of the permittivity using

the equations, M0 ¼ uAεot ½Z 00 � and M

00 ¼ uAεot ½Z0�. For all the tem-

peratures, M0 gradually increases with increase in frequency andreaches a constant high values at high frequencies (Fig. 9 (a)). Thedispersive nature found in the mid frequency region is shifted to-wards the lower frequency region and approaches zero for all thetemperatures, which suggest the suppression of electrode polari-zation effects. This indicates the conduction phenomenon in thematerial is due to short-range mobility of charge carriers [25,29].The imaginary part of the electric modulus (M00) shows the asym-metric peaks for all the selected temperatures (Fig. 9 (b)). The peakpositions are found to be shifting towards the lower frequencyregion with progressive increase in intensity as the temperature isincreased suggest the temperature dependent hopping type con-duction for charge transport. The complex electric modulus spectra(Fig. 9 (c)) shows a single semicircular arc for all the selectedtemperatures suggesting the increase in modulus grain boundaryresistance with increasing temperatures, which is well agreed withthe aforementioned electrical resistivity analysis.

4. Conclusions

In summary, monodispersed a-Fe2O3@SiO2 core-shell nano-spheres were successfully fabricated by solvothermal technique.The different functional groups present in the product and thecrystal structure were examined by powder XRD and FTIR analyses

respectively. The morphology of a-Fe2O3@SiO2 nanospheres wasstudied using SEM analysis and the spheres are relatively mono-dispersed with their mean size about 300 nm. The chemical state ofthe elements and their composition was analyzed by XPS analysis.The electrical behavior of thematerial was probed using Impedanceanalysis which substantiated the positive temperature coefficientof resistivity type behavior of the material. Both the dielectricpermittivity and loss factor decreased swiftly with increase in fre-quency and became constant in the far end of the measured fre-quency range.

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