Central European Journal of Chemistry

1 Department of Chemistry, Fatih University, 34500 Buyukcekmece, Istanbul-Turkey

2 Department of Physics, Fatih University, 34500 Buyukcekmece, Istanbul-Turkey

3 Department of Physics, Gebze Institute of Technology, 41400 Cayirova Gebze,Kocaeli-Turkey

4 Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA 93106, USA

Microwave-Induced Combustion Synthesis and Characterization of NixCo1-xFe2O4 Nanocrystals (x = 0.0, 0.4, 0.6, 0.8, 1.0)

Abdülhadi Baykal1*, Nermin Kasapoğlu1, Yüksel Köseoğlu2, Ali Cemil Başaran3, Hüseyin Kavas2, Muhammet S. Toprak4

* E-mail: hbaykal@fatih.edu.tr

Received 7 August 2007; Accepted 3 December 2007

Abstract: A series of Ni-doped cobalt ferrites NixCo1-xFe2O4 (x = 0.0, 0.4, 0.6, 0.8, and 1.0) were prepared using microwave-induced combus-tion. Nickel, cobalt, and ferric nitrates were used as starting materials and glycine as fuel. The influence of Ni content on the lattice parameter, stretching vibrations, and magnetization was studied. XRD, FTIR, and SEM were used for structure, composition, and mor-phology investigation. A porous network structure was observed with average particle size 60-67 nm. All samples had a cubic spinel structure. The unit cell parameter “a” decreases linearly with nickel concentration due to the smaller ionic radius of nickel. Magnetiza-tion measurements showed that coercivity decreased as Ni content increased; it increased with decreasing temperature.

© Versita Warsaw and Springer-Verlag Berlin Heidelberg.

Keywords: Microwave synthesis • XRD • Magnetic nanoparticles • Spinel ferrites • SEM analysis

Research article

1. IntroductionMagnetic spinel ferrite nanoparticles are of interest in examining the relationships between magnetic properties and crystal chemistry and structure. Spinel ferrites have been investigated in recent years for their useful electrical and magnetic properties and applications in information storage systems, magnetic cores, magnetic fluids, microwave absorbers, and medical diagnostics.

Attention has focused on the preparation and characterization of superparamagnetic metal oxide nanoparticles of spinel ferrites, MFe2O4 (M = Co, Mg, Mn, Ni, etc.) [1-4]. A variety of wet chemical methods (sol-gel, co-precipitation, hydrothermal, aerosol etc.) have

been reported to generate nano-sized materials [5-8]. However, complex processes, expensive precursors, and low production rates are common problems [9]. In the past decade, a solution combustion method has been utilized to synthesize simple and mixed -metal oxides [10-16]. Organic compounds (e.g. glycine, urea, citric acid, alanine and carbohydrazide) have been mixed directly with metal nitrates to enhance the efficiency of combustion synthesis. The metal nitrates act as oxidants as well as as cation sources, and the organic compound is the fuel [16,17].

The heating mechanism in microwave processing is fundamentally different from conventional processing. Microwave radiation is absorbed and converted to

Cent. Eur. J. Chem. • 6(1) • 2008 • 125-130DOI: 10.2478/s11532-007-0070-4

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Microwave-Induced Combustion Synthesis and Characterization of NixCo1-xFe2O4 Nanocrystals (x = 0.0, 0.4, 0.6, 0.8, 1.0)

thermal energy. Heat is generated from inside the material, in contrast with conventional heating methods where heat is transferred from the outside. This internal rapid heating allows a reduction of processing time and energy. The reaction rate is enhanced by one to two orders of magnitude [18-21].

In this work, nickel-doped cobalt ferrite powders were prepared by microwave-induced combustion and characterized by XRD, FTIR, and SEM. Their magnetic properties were investigated using a VSM magnetometer.

2. Experimental

2.1. SynthesisNanoparticles of NixCo1−xFe2O4 were prepared through microwave-induced combustion. Analytical grade nitrates of nickel, iron, and cobalt along with glycine (NH2CH2COOH), were dissolved in deionized water in the desired ratio (Table 1). The crucible containing the solution was heated in a microwave oven (CEM, MDS 81D, 650W). Initially, the solution boils and evaporates followed by decomposition with copious evolution of gases (N2, NO2, CO2). When the mixture reaches the ignition temperature it begins burning and releases a great deal of heat from the highly exothermic reaction [21]. This vaporizes the remaining liquid instantly, and forms a solid burning at over 1000°C. The entire process takes only 15 min to produce ferrite powders.

Glycine serves as fuel, being oxidized by nitrate. The expected combustion reaction to form NixCo1-xFe2O4 (for x=1) is:

9 Ni(NO3)2 + 18 Fe(NO3)3 + 32 NH2CH2COOH →9 NiFe2O4 + 43 N2 + 18 NO2 + 64 CO2 + 80 H2OThe evolution of large amounts of gases helps

dissipate heat, reducing oxide sintering [14,17].

2.2. Measurements Structural characterization was performed using a Huber JSO-DEBYEFLEX 1001 x-ray diffractometer (Cu Kα radiation) operated at 40 kV and 35 mA. Fourier transform infrared (FTIR) transmission spectra were taken on a Mattson Satellite Infrared Spectrometer

from 4000 to 400 cm-1. Samples were mixed with KBr powder. Background correction was made using a blank KBr pellet.

Magnetic measurements were performed using the Quantum Design 6000 Vibrating Sample Magnetometer (VSM) option for the Physical Property Measurement System (PPMS).

Scanning Electron Microscopy (SEM) analysis was performed on a FEI XL40 Sirion FEG Digital Scanning Microscope. Samples were coated with gold at 10 mA for 2 min prior to SEM analysis.

3. Results and discussion

3.1. X-ray diffraction

X-ray diffraction patterns of NixCo1-xFe2O4 are shown in Fig.1. They all show the reflection planes (111), (220), (311), (222), (400), (422), (511) and (440), corresponding to a crystalline cubic, spinel-type phase. They provide clear evidence of a series of solid solutions between NiFe2O4 and CoFe2O4. Mixed spinel ferrites were observed for x=0.4, 0.6, 0.8. For x=0.0 pure CoFe2O4

(JCPDS file No:22-1086), and for x=1.0 pure NiFe2O4 (JCPDS file No:10-325) are formed. The patterns show a slight shift in peak position towards lower d-spacings

x (Ni content) Co(NO3)2•6H2O Ni(NO3)2•6H2O Fe(NO3)3•9H2O Glycine

0 0.18 - 0.5 0.3

0.4 0.108 0.072 0.5 0.3

0,6 0.072 0.108 0.5 0.3

0.8 0.036 0.144 0.5 0.3

1 - 0.18 0.5 0.3

Table 1. Weights (g) for the synthesis of NixCo1-xFe2O4 nanoparticles.

Figure 1. XRD Powder Patterns of NixCo1-xFe2O4 nanoparticles.

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with increasing nickel content [16,17,22].The lattice parameters “a” (Fig. 2) were computed

using the d-spacings and the respective (hkl) parameters. The values of “a” found for NiFe2O4 (a=8.339Ǻ) and for CoFe2O4 (a=8.381Å) agree with those reported in the JCPDS. Since the ionic radius of Ni2+ is smaller than that of Co2+, replacement of Co by Ni leads to a linear dependence of “a” on x (Fig. 2), obeying Vegard’s law [23,24].

X-ray density was calculated by dx = 8M/Na3 where M, N and “a” are the molecular weight, Avogadro’s number and the lattice parameter, respectively. It increases linearly with nickel concentration (Fig. 2) since the nickel atom is heavier that the cobalt, as well as because “a” decreases.

The average crystallite diameter (Table 2) was estimated by the Scherrer equation using the full-width at half maximum (FWHM) of the most intense XRD peak (311) [21,25].

3.2. FTIR spectroscopyThere are two major bands in the FTIR spectra of spinel oxides. v1, generally observed in the range 600-550 cm-1, corresponds to stretchings at the tetrahedral site, Mtetra↔O. Fig. 3 shows that the frequency increases linearly with increasing Ni2+ content [23,28]. Thus, the apparent reduced mass also decreases as Ni2+ increases. However, this shift may be due to a change in the Fe3+- O2- internuclear distance. Ni2+ ions have an

octahedral-site preference, while Co2+ and Fe3+ ions can occupy both octahedral and tetrahedral sites [27], so increasing Ni2+ concentration forces Fe3+ ions into tetrahedral sites [23].

The lowest band (v2), usually observed in the range 450-385 cm-1, is assigned to octahedral metal-oxygen stretching, Mocta↔O [24,26]. However, no obvious peak due to octahedrally coordinated metal was observed. This may be due to broadening from the very small particle size [34].

Bands assigned to uncoordinated NO3-1 are also

identified around 830 cm-1 [17]. Traces of adsorbed or atmospheric CO2 are shown by the very light band around 2350 cm-1 [30].

3.3. SEM analysisThe morphology of the NixCo1-xFe2O4 nanoparticles is presented in Fig. 4. Samples exhibit large grains in the range of 200-500 nm. Heating has resulted in sintering of the well-faceted grains to form porous solid bodies. These particles exhibit a network with voids and

Table 2. Diameters of NixCo1-xFe2O4 crystallites.

Specimen Particle size(nm)

CoFe2O4 64

Ni0.4Co0.6Fe2O4 65

Ni0.6Co0.4Fe2O4 67

Ni0.8Co0.2Fe2O4 66

NiFe2O4 62

Figure 3. FTIR spectra of NixCo1-xFe2O4 nanoparticles.

Figure 2. Lattice Parameters and X-ray density of of NixCo1-xFe2O4 nanoparticles.

Figure 4. SEM micrographs of NixCo1-xFe2O4; a) x=0, b) x=0.4, c) x=0.6, d) x=0.8, e) x=1.0

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Microwave-Induced Combustion Synthesis and Characterization of NixCo1-xFe2O4 Nanocrystals (x = 0.0, 0.4, 0.6, 0.8, 1.0)

pores typical of combustion – synthesized powders. Compositional purity has been confirmed by energy dispersive x-ray analysis (EDX) on various regions of each sample. These porous powders are highly friable which facilitates grinding to finer particles [31-33].

3.4. Magnetization measurementsFig. 5 shows hysteresis loops for NixCo1-xFe2O4 nanoparticles at room temperature. Samples more easily saturate with increasing Ni content, which is expected as Ni ferrite is a soft and Co ferrite a hard magnetic material. At room temperature, Co1Fe2O4 shows a coercivity (Hc) of 629 Oe while Ni0.6Co0.4Fe2O4 and NiFe2O4 exhibit coercivities of 611 Oe and 70 Oe, respectively. Although the coercivity of CoFe2O4 is greatest, its room temperature coercivity is less than that of the bulk ferrite [34].

The coercivity of a magnetic material is roughly a measure of its magnetocrystalline anisotropy [35]. Co ferrite usually forms an inverse spinel structure with very high anisotropy energy constants (K1 and K2). It is also temperature sensitive at lower temperatures [36]. As the Ni content of NixCo1-xFe2O4 increases, the decreased coercivity shows that anisotropy decreases, which in turn decreases the domain wall energy [37,38]. As the Co2+ are substituted by Ni2+ the antiferromagnetic interaction decreases and the ferromagnetic superexchange interaction increases which leads to a decrease in coercivity and reduced magnetization [4].

Fig. 6 shows the strongly negative temperature dependence of the coercive field. The most important observation is the dramatic increase of coercivity for Co-ferrite at low temperature. This cannot be attributed to magnetocrystalline anisotropy alone. It may also originate from exchange anisotropy due to spin disorder at the particles’ surface. The observed effect is expected to be larger for smaller particles due to the increase in the

surface-to-volume ratio. However, this effect is expected to occur at high temperatures as well. The exchange anisotropy arising from magnetic disorder, especially for low dimensional systems at low temperature, accounts for this behavior.

The large values of coercive field should be associated with an increased effective anisotropy field. (Keff), where Keff = 4πMS Hirr. That is, such a large coercivity can also be attributed to random freezing of nanoparticles in locally canted states, for which the random anisotropy energy will act as pinning potential [36]. The coercive field for Co ferrite is much higher than that of Ni-ferrite; the very high anisotropy of Co ferrite can cause more disorder at lower temperature. Also, uncontrollable antiferromagnetic interaction between very thin CoO shells on the surface of the nanoparticles can induce additional exchange anisotropy and coercive field, especially at low temperature. Thus, exchange anisotropy is expected to be higher as seen in Fig.6.

The strong temperature and Ni concentration dependence of the magnetic parameters (Ms, Hc, Mr) may be attributed to the influence of the cation stoichiometry and their occupancy of specific sites, a dead layer on the surface, random canting of particles’ surface spins, non-saturation effects due to random particle size distribution, and adsorbed water.

4. ConclusionsUsing nitrates of nickel, cobalt, and iron in combination with glycine, fine nickel-doped cobalt ferrite powders have been successfully synthesized by microwave-induced combustion. The novel combustion method provides a rapid and reproducible route for the preparation of crystalline cobalt spinel ferrite. The materials are porous

Figure 5. Hysteresis of NixCo1-xFe2O4 nanoparticles at room tem-perature. Figure 6. Temperature dependence of coercive field for

NixCo1-xFe2O4 nanoparticles.

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network powders with a substantial surface area. Crystallite size was ~ 60-70 nm.

The coercive field increased with decreasing Ni content and decreasing temperature, reaching 17145 Oe for CoFe2O4 at low temperature. This dramatic increase for Co-ferrite can be partially attributed to the increase in exchange anisotropy due to the spin disorder at the particles’ surface at low temperatures, as well as to the antiferromagnetic interaction in a very thin CoO shell on the surface of the nanoparticles. The strong temperature and Ni concentration dependence of the magnetic parameters (Ms, Hc, Mr) is attributed to the influence of the cation stoichiometry and their occupancies of the specific lattice sites, a dead layer on the surface, random canting of particles’ surface spins, non-saturation effects due to random particle size distribution, and adsorbed water.

AcknowledgementThe authors are thankful to the Fatih University Research Project Foundation (Contract no: P50020602) for financial support of this study and Prof. Dr. Bekir AKTAŞ (from Gebze Institute Technology) for magnetization measurements. Dr. M. S. Toprak acknowledges a fellowship from the Knut and Alice Wallenbergs Foundation (No: UAW2004.0224).

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