Journal of the Australian Ceramic Society Volume 52[2], 2016, 150 – 162 150

Microstructure, optical and magnetic properties of inverse spinel CoFe2O4 synthesized by microemulsion process assisted by CTAB and AOT Mrinal Saha1, Siddhartha Mukherjee1* and Arup Gayen2

1) Department of Metallurgical & Material Engineering, Jadavpur University, Kolkata− 700032, India 2) Department of Chemistry, Jadavpur University, Kolkata− 700032, India E-mail: smukherjee03ju@gmail.com Available online at: www.austceram.com/ACS-Journal Abstract: Nanostructure inverse spinel CoFe2O4 was synthesized by microemulsion process using different kind of surfactant such as cationic surfactant (CTAB) and anionic surfactant (AOT). The formation of face centered cubic (fcc) CoFe2O4has been identified by XRD and it belongs to Fd3m space group as predicted from Raman spectral analysis. The crystallite size of ferrite materials fall into the nanometric dimension. The optical and magnetic properties of CoFe2O4 are influenced by the nature of surfactant. The raw synthesized material has been characterized by thermogravemetric-differential scanning calorimeter (TGA-DSC) to study the crystallization temperature of cobalt ferrite. The microscopic structural behavior of the heat treated material is studied by field emission scanning electron microscopy (FESEM) and high resolution transmission electron microscopy (HR-TEM).The magnetic properties of the ferrite material are evaluated by vibrating sample magnetometer (VSM).The pyramidal and rod like shape surface morphology are obtained by using CTAB and AOT, respectively. The band gap value of theCoFe2O4 is found in the range of between 2.13-2.02 eV. From the results of M-H plot we could resolve thatCoFe2O4 behaves as strong ferromagnetic material. Keywords: Nano-CoFe2O4; Surfactant;Morphology; Ferromagnetic; Reverse-micelles;etc. 1. Introduction Ferromagnetic nano-structure ferrite material has drawn the attention of the scientific community as a promising material for about a decade in several inter-disciplinary areas such as chemistry, physics, materials science and bio-medical for their unique optical, electrical surface and magnetic properties [1, 2]. Such kind of magnetic materials are gaining attraction in varies arena of technological aspect such as high density information storage nano-devices, ferro fluids, magnetic refrigeration, resonance image and residential cooling [3-5]. Furthermore, ferrite material behaves as good catalyst, photo-catalyst, sensor, and targeted and controlled drug delivery materials [6-8]. Cobalt ferrite is one of the most sought after ferromagnetic materials due to its high magneto-crystalline anisotropy, high coercively, moderate saturation magnetization and good chemical and structural stability at higher temperature [9].The

magnetic property of materials relates on different kind of parameter such as, particle size, distribution of metallic ions in the crystal lattice, surface morphology as well as surface defect. Various physical and chemical methods have been developed to synthesis CoFe2O4 such as, solution combustion process, sol-gel technique, hydrothermal method, solvo-thermal process, and co-precipitation process [10-12] etc. Micro-emulsion is one of the novel techniques among them to prepare stable, de-agglomerated as well as porous nanostructured materials. The major advantages of this process are the desired size with various kinds of surface morphologies of nano-structured materials can be prepared. In this process surfactant plays a crucial role to determine the surface morphology of material. The crystallization path is a vital step in that process. It is highly influenced by the several kind of factors such as, surface active agents, the pH and the level of super-saturation. There are quite some reports on the

Saha et al. 151

synthesis of CoFe2O4 via microemulsion process [13-15]. In the present work, we report a simple technique as well as novel route to synthesize CoFe2O4 with narrow size distribution by microemulsion process using different kind of surfactants. We have investigated the optical properties including magnetic behavior of CoFe2O4.We try to find out the role of the surfactant during the synthesis process. This facile synthetic approach is projected to be an effective, low cost synthesis process that has a high potential for scaling up. 2.Experimental 2.1.Chemicals All the chemicals used were of analytical grade and without further purification. Cetyltrimethyl ammonium bromide (C16H33–(CH3)3 –N+Br-, CTAB) and bis(2-ethylhexyl) sodium sulfosuccinate (AOT) and the other precursors- nitrate salts of cobalt (II) and iron (III) were procured from Merck, India. The purity levels of all precursors / reagents are >99.9%. Cyclo hexane and iso-octane used as oil phase, were obtained from Thermo fisher, India. The co-surfactant, hexanol, butanol and 30% NH4OH were obtained from Merck, India. 2.2.Synthesis of materials 2.2.1. Synthesis of CoFe2O4 by using CTAB In the typical synthesis procedure, 2 g of CTAB was added into 2.5 ml of hexanol and 75 ml cyclohexane under the vigorous stirring condition at room temperature. The solution mixture was stirred for 30 minutes to make homogeneous solution. The 3 m.mol of oxalic acid was added into the reaction mixture under constant stirring condition. Subsequently stoichiometric amount of nitrate salts were added into the mixture and kept for stirring overnight. After prolong treatment the yellow color solution turned into reddish color that was the indication of formation of the desire synthesized material. The precipitate was separated by centrifuging process. The precipitate washed with distilled water for several time to remove the surfactant. The obtained crude product was dried at 100 oC in a hot air oven.

2.2.2. Synthesis of CoFe2O4 by using AOT The nano crystalline cobalt ferrite was prepared based on the reverse micelle concept using two micro-emulsion formulations prepared from an initial AOT and isooctane micro-emulsion. The first micro-emulsion was an oil-phase microemulsion comprising of isooctane and AOT, and the second was an aqueous phase emulsion consisting of isooctane and AOT with the reactant salt. In a typical experiment,

microemulsion I consisted of 2 ml of 25% NH4OH with 3 ml deionized water and 66mL of 0.56 (M) AOT-isooctane and subsequently microemulsion II consists of stoichiometric amount of 0.5656gFe(NO3)3 .9H2O, 0.203g Co(NO3)3. 6H2O in deionized water and 66 ml of 0.56 (M) of AOT- isooctane solution. The two microemulsions were subjected to rapid mechanical stirring for 25 minutes to make clear mixture. The reaction mixture was stirred for 24 h followed by addition of methanol into the reaction mixture to breakdown the water pool. Centrifuging separated the precipitate. The crude product was then washed by deionized water and methanol alternately to remove surfactant. Finally precipitate obtained was dried in a vacuum hot air oven at 120 oC .In the microemulsion I, NH4OH acted as precipitating agent. The respective metal hydroxides were precipitated within the water pools during the synthesis process. The precipitation of the metal hydroxides occurred according to the following reaction: Co(NO3)2. 6H2O + 2NH4OH → Co(OH)2↓ + 2NH4

+

+2 NO3¯ +6H2O (1)

Fe(NO3)3.9H2O +3 NH4OH → Fe(OH)3↓ +3 NH4+ +3

NO3¯ + 9H2O (2)

2Fe(OH)3+ Co(OH)2→CoFe2O4 + 4H2O (3) 2.3.Characterization The synthesized material was subjected to various instrumentation characterization techniques to analyze the nature of the material. The size of reverse micelle water pool was determined by using Dynamic Light scattering technique (DLSMalveren ZS 90) at the room temperature. The thermal behavior of the material was assessed by thermo- gravimetric analysis (TGA) and differential scanning calorimetric (DSC) using Diamond Pyris 480, Perkin Elmer instrument under nitrogen flow of 150 mL min-1 with a heating rate of 15 °C min-1. The phase analysis was studied by X-ray diffraction in a Bruker D8 advance X-ray diffractomer using Lynxeye detector (1 D mode). The patterns were run with Cu Kα= 1.5418 Å at 40 kV and 40 mA with step size of 0.02°. Raman spectra were recorded using Alpha 300, Witec spectrometer at room temperature with laser excitation source ~ 532 nm in the range from 150 to 1200 cm-1. The microstructural image of the materials were studied in a scanning electron micrograph (SEM, model JAX-840A, JEOL) and field emission scanning electron microscopy (FESEM, model S-4800, Hitachi). The composition analyses of the materials were carried out by EDX using INC X Oxford under the acceleration voltage set at 10 keV.

Journal of the Australian Ceramic Society Volume 52[2], 2016, 150 – 162 152

High resolution transmission electron microscopy (HR-TEM) study was carried in JEOL- JEM-2100. The samples were dispersed in ethanol and then treated ultrasonically in order to disperse individual particles over carbon coated copper grids for analysis. Molecular signature of cobalt ferrite was determined by FTIR (model Prestige 21, Shimadzu)in transmittance mode. The IR analysis of the sample was carried out in the range of 400 to 4000cm−1. Two milligrams of solid sample was mixed with 200 mg of vacuum-dried IR-grade KBr. The mixture was dispersed by grinding for 3 min in a vibratory ball mill and placed in a steel die of 10 mm diameter and subjected to a pressure of 12 tones. The sample disks were placed in the holder of the double grating IR spectrometer. The optical behavior was measured by UV-VIS spectroscopy (Perkin Elmer, Lamda 35) in the range of 200 to 800 nm in absorption mode. The powder sample was dispersed in ethanol in an ultrasonicator for about 30 min for this purpose. The study on the magnetic behavior of the material was undertaken at room temperature by using vibrating sample magnetometer (VSM, Lake Shore 7407 series) with a maximum magnetic field of 2 Tesla. Surface topological study was recorded by

atomic force micrographs (AFM) on nano-K vibration isolation by minus-K technology (NTMDT) in semi conductive method. 3.Results and Discussion 3.1. DLS studies Fig.1shows the DLS spectrum of water poolCoFe2O4precursors during the reverse micelle process. The size of water absorbed water pool is ~236.8nm.The resultant CoFe2O4is entrapped into the micelle. Subsequently, the chemical reaction occurred into the micelle that prevented agglomeration of nanodimension particles during the synthesis process. 3.2. TGA-DSC studies Fig.2 shows the TGA-DSC curve of CoFe2O4 precursors. In the diagram the red line indicates the nature of the reaction and the green line denotes the change in the weight during the synthesis. The endothermic peaks at around ~ 107 oC, 209oC and 266 oC may be attributed to removal of volatile materials, solvent, and surfactant respectively (see Fig. 2(a)). On the other hand, the exothermic peak centered at around ~ 366 oC is contributed to decomposition of CTAB.

Fig.1: The DLS spectrum for the water of cobalt ferrite precursors by using AOT

0 200 400 600 800 1000

0

2

4

6

8

10

Temperature (oC)

Wei

ght (

%)

Heat flowWeight

107 oC

209 oC

266 oC

480 oC

514 oC

366 oC

415 oC

-20

-10

0

10

20

30

40

Heat flow

(mW

)

(a)~ 10%

~ 70%

~ 10%

0 200 400 600 800

2

4

6

8

10

Temperature (oC)

Wei

gth(

%)

287 oC100 oC

662 oC

-30

-20

-10

0

10

20

30

Heat flow

(mW

)

Heat flowWeigth

247 oC

342 oC 473 oC~10%

~60%

~10%

(b)

Fig.2: TGA -DSC curve of cobalt ferrite precursors using (a) CTAB and (b) AOT surfactant.

Saha et al. 153

Furthermore, the exothermic peak at around 415 oC and 514 oC may be assigned to the formation of ferrite phase and isochemical transformation of the material. The nature of curve beyond ~ 480 oC suggests that the stable ferrite material is obtained. In Fig. 2(b) the endothermic peaks at around ~ 100 oC and ~ 287 oC may be attributed to the removal of volatile solvent and AOT respectively. However, beyond the endothermic peak at around ~ 662 oC, the stable material is obtained. There is also two more exothermic peaks at around 247oC and 342oC, which may be contributed to the reaction of AOT surfactant and phase transformation (amorphous to crystalline) respectively. However, small peak at ~ 472oC is contributed to isochemical transformation of material. A minor weight loss (~10 %) is observed from the room temperature to 220 oC due to removal of volatile materials and solvent in the both case (see Fig. 2(a, b)).However, the major weight loss (~ 70 %)is found from 220 oC to 450 oC due to the decomposition of the surfactant. Furthermore, a minor weight loss (~10 %) occurred due to the decomposition of nitrate salt to their corresponding metallic oxide from 450oC to 800 oC. From the TGA-DSC studies we chose two calcination temperatures namely (400 °C and 600°C) and the heating rate of 5 °C min-1 for 4 h to obtain the desired CoFe2O4 materials (named respectively as CoFeC4, CoFeC6 andCoFeA4, CoFeA6 for CTAB and AOT surfactant respectively). 3.3.XRD studies Fig.3 shows the XRD pattern of the precursor materials at different calcination temperature. The diffraction peaks are well indexed to (111), (220), (311), (222), (400), (422), (511), (440) and (622) crystal planes of face centered cubic (fcc) cobalt ferrite (JCPDS PDF# 22-1086). This XRD pattern is a clear indication formation of CoFe2O4without presence of any kind of impurities. Singh et al. synthesized irregular surface morphologyCoFe2O4 along with α-Fe2O3as impurity by precipitation method [16]. Rana et al. have reported synthesis of CoFe2O4in the presence of impurities via reverse micelle process [17]. The intensity and sharpness of the diffraction peaks increases of the samples at higher calcination temperature due to enhanced crystallinity of the nanoparticles. CoFeC exhibits the more intense peaks compare to the CoFeA. The more crystalline material shows sharp intensity peaks. In that case COFeC4 and CoFeC6 are more crystalline in nature compare to the other two materials, therefore the previous material exhibits more intense peak. We have chosen the samples obtained on calcination at 600 °C (CoFeC6 and CoFeA6) for most

of the subsequent studies except optical studies. The crystallite size of the materials is calculated by using Scherrer’s formula:

θλ

cos9.0

BD =

(4)

Where D is the average crystallite size of the phase under investigation, λ (1.5418 Å) is the wavelength of X-ray, B is the full-width at half maximum (FWHM) of the diffraction peak and θ is the Bragg angle. The broad nature of peaks is a clear indication the formation of nano particle. The crystallite size of the ferrite material is found in range between 6-40 nm. The increase of crystallite size at higher calcinations temperature indicates that the particles are agglomerated in nature. Zhao et al. generated CoFe2O4nanocrystal by CTAB assistant hydrothermal method with particle size 70 nm [18]. Feyzi et al.also reported cubic CoFe2O4 as a heterogeneous catalyst via co-precipitation process with particle size 61- 120 nm [19].

Fig. 3: XRD patterns of various calcined precursors of cobalt. 3.4.Raman spectroscopic studies Fig. 4 shows the Raman spectra of CoFeC6 and CoFeA6 at lower optical intensity. This is carried out to avoid possible sample degradation. CoFeC6 and CoFeA6 both have cubic structure and belong to the space group Fd3m. One complete unit cell consists 56 atoms (Z=8) and the smallest Bravais cell contains of only 14 atoms (Z=2). This gives rise to 39 normal modes.

uuuugggg FFEAFFEA 212211 2423 +++++++=Γ

(5)

Journal of the Australian Ceramic Society Volume 52[2], 2016, 150 – 162 154

Where only five optical modes are Raman active (A1g + 1Eg + 3F2g) composed of the motion of O-ions and both the A- and B- site ions [20,21], and four are infrared active (4F1u).Table 1represents the Raman active modes of these materials. Furthermore, nano-dimension of the material with high crystallinity leads to broadening of the Raman spectra due to lack of long range order in these materials. The asymmetry feature of the Raman spectra is originated due to the cation disorder in crystal lattice. Such kind of disorder influences the physical and chemical features of the material. The highest value of Raman mode (~688 cm-1)is attributed to metal-oxygen bond stretching in the tetrahedral site crystal lattice(see Fig. 4).The peak at around ~488 cm-1is contributed to metal-oxygen bond stretching in the octahedral site of the lattice. The Raman active Eg mode depends on the grain size of the material [22].In our study, CoFeA6 exhibits the lower value of Eg mode compared to the CoFeC6. Such kind of shift is occurred due to the low crystallite size of CoFeA6.

Fig. 4: Raman spectra of (a) CoFeC6 and (b) CoFeA6

The amount of shifting of the peaks along to y-axis relate to the variation in the site occupancy of Co2+ and Fe3+. Furthermore, the shifting is also depended on the synthesis procedure and nature of surfactant. 3.5. FT-IR spectroscopic studies Fig.5 represents the FT-IR diagram of ferrite materials at various calcination temperatures. .Table 2represents the experimental results of the sample. The broad band at around 3269cm-1and 3371 cm-1 are due to the presence of stretching (ν)vibrations of free or absorbed water on the surface of the substance. Furthermore, the peaks at 1590, 1597 cm-1are attributed to O-H stretching in water. The peaks are centered at 2858, 2090 and 2950 cm-1 is assigned to the C-H stretching in AOT. The peaks are assigned due to the presence of a minor amount of un decomposed residual AOT. Due to presence of low amount of surfactant, it cannot be predicted by XRD. However, the band at ~ 2888 cm-1and 2962 cm-1are due to the C-H stretching in CTAB. The bands at around 1352 cm-1and 1474 cm-1 correspond to COO-1 asymmetric and symmetric vibrations, respectively. The band at around 1244 cm-1 is contributed due to C-O stretching presence in AOT (see Fig. 5(b)). All the ferrites have shown two main adsorption bands below 1000 cm-1, due to the presence of metal-oxygen vibration mode. In the ferrite system the metal ions are distributed among the two sub lattices, namely tetrahedral (A-sites) and octahedral (B-sites). The bands centered at 400 and 600 cm-1are contributed to the stretching mode of the octahedral complex (ν2) and tetrahedral complex (ν1), respectively. It is observed that at higher calcination temperature the intensity of octahedral site gradually decreases whereas, in tetrahedral site the intensity enhances of the resultant ferrite materials (see Fig.5(a,b)). In spite of these the shifting of ν2 towards lower frequency is observed at the higher calcination temperature [23, 24].From this, it may be concluded that at the higher calcination temperature CoFe2O4 behaves as inverse spinel.

Table 1: Raman active modes of CoFeC6 and CoFeA6

Assignments Raman modes of CoFeC6 Raman modes of CoFeA6

T2g 228 202

E`g 285 306

Eg 488 474

A1g 516 491

Eg 688 689

Saha et al. 155

1000 2000 3000 4000

Abs

orba

nce

(a.u

.)

Wave number (cm-1)

CoFeC4

CoFeC6

585

57143

042

0

667

655

1031

1034

1591

2302

2888

3333

1425 15

90

2342

2962

3310

1368

(a)

1000 2000 3000 4000

Abs

orba

nce

(a.u

.)

Wave number (cm-1)

420

617

61243

0

1018

989

1136

1103 11

6213

5212

44

1661

1474 15

97

3269

2858

2950

2909

3371

CoFeA4

CoFeA6(b)

Fig. 5: FT-IR spectra of (a) CoFeC (b) CoFeA at various annealing temperatures. Table 2: IR bands of CoFe2O4 at various annealing temperatures.

Sample

IR- region or bands (cm-1) Description

CoFeC4

430 571

1034 1368 1591 2888 3333

ν- Ferrite (M-O) (octahedral) ν- Ferrite (M-O) (tetrahedral) Fe-Co alloys system ν(COO-1) from CTAB ν(H-O-H) of free or absorbed water ν(C-H ) from CTAB ν(H-O) of free or absorbed water

CoFeC6

420 585

1031 1425 1590 2962 3310

ν- Ferrite (M-O) (octahedral) ν- Ferrite (M-O) (tetrahedral) Fe-Co alloys system ν(COO-1) from CTAB ν(H-O-H) of free or absorbed water ν(C-H ) from CTAB ν(H-O) of free or absorbed water

CoFeA4

430 612

989,1103,1162 1352 1661 3269

ν- Ferrite (M-O) (octahedral) ν- Ferrite (M-O) (tetrahedral) Fe-Co alloys system ν (COO-1) ν(H-O-H) of free or absorbed water ν(H-O) of free or absorbed water

CoFeA6

420 617

1018, 1136 1244 1474 1597

2858,2090,2950 3371

ν- Ferrite (M-O) (octahedral) ν- Ferrite (M-O) (tetrahedral) Fe-Co alloys system νas(C-O) band from AOT νas(COO-1) ν(H-O-H) of free or absorbed water νas (C-H) and ν (C-H) from AOT ν(H-O) of free or absorbed water

Journal of the Australian Ceramic Society Volume 52[2], 2016, 150 – 162 156

3.6 Uv-Vis-spectroscopic studies Fig. 6 shows the Uv-vis spectra of the CoFeC6and CoFeA6. The absorption behavior in the CoFe2O4 is originated due to metal to metal charge transformation. The metal to metal charge transfer transition takes place between Fe 3+ and Fe2+ in the crystal lattice. Thebroaden absorption band is noticed into the visible region (see Fig. 6).

Fig. 6: Uv-Vis spectra of (a) CoFeC6and (b) CoFeA6. It may be concluded that the ions (Co2+, Fe3+) presence in the system could be generated a new level of conduction band and the electron can be promoted from the valence level to these levels [25-27]. The absorption in that region between 200-300 nm may be attributed due to the charge transfer from O2- to M2+ in the lattice [28]. The change in morphology and crystallite size influenced by the defect structure which may lead to change in the absorption behaviour of synthesized material in that region [28]. The optical band gap (Eg) has been evaluated of the ferrite materials by using Tauc equation, as given below:

(6) Whereas, Eg denotes the energy gap, A is a constant, hν stands for energy of photon and n is an index which can assume any value among 1/2, 3/2, 2 and3 depending on the nature of electronic transition.Fig.7exhibits a plot between (αhν)2vshν to determine the direct band gap value of the material. .The band gap value falls in the range between 2.13-2.08 eV and 2.11-2.02 eV for CoFeC and CoFeA, respectively. These values are lower when compared to the some of the earlier literature reports. For instance, Gaikwad et al. have reported synthesis of CoFe2O4via electrostatic spray deposition method with 2.57 eV as band gap value [29].Singh et al. have synthesized CoFe2O4 via co-precipitation method with band gap value 3.85 eV [30]. Holinsworth et al. prepared thin film CoFe2O4 with band gap value 2.74 eV [31].

Fig. 7: Plot of (αhν)2Vs photon energy for (a, b) CoFeC and (c, d) CoFeA obtained at various annealing temperatures.

Table 3: The crystallite size and band gap values of CoFe2O4 at various annealing temperature.

Sample Crystallite size (nm) Band gap value (eV)

CoFeC4 33±1 2.13

CoFeC6 40±2 2.08

CoFeA4 6±1 2.11

CoFeA6 31±1 2.02

ngEhAh )( −= ννα

Saha et al. 157

3.7. SEM and EDX studies Fig. 8 depicts the SEM image of the synthesized materials, CoFeC6 and CoFeA6 at various resolutions. The various sizes of aggregates nanoparticles but the primary shape of this particle is easily recognizable. The surface morphology ofCoFeC6is pyramidal whereas, rod like morphology is obtained for CoFeA6 (see Fig. 8(a, c)).The grain boundary between the two neighbor particles is easily distinguishable (see Fig.8(b)).The soft agglomeration tendency is observed in CoFeC6, due to high surface energy and magneto–static interactions among the magnetic particle. Such kind of agglomeration dramatic changes the physical and chemical properties of the materials. The difference of size distribution is observed of the material by XRD and SEM analysis. This is because; we determine the crystallite size of the material with the help of Scheerer equation from XRD spectra. But

in the SEM micrograph we observe the surface morphology of the substance. The average particle size of the material can be obtained from the SEM image. So, such kind of wide variation is expected between the change in behavior. One of the main reasons behind the increase in the particle size of the material is agglomeration of nano-particle by SEM morphology. Fig. 9 shows the EDX spectra of the ferrite material. The EDX analysis has been carried out in the high-lighted zone of SEM image (see Fig. 8(a,d)).The EDX spectra indicate the presence of Co, Fe and O as constituent elements in the substance. Table 4 summaries the EDX analysis of the ferrite materials. There is no impurity in the ferrite material. The ratio of Fe/Co is found near about 2 which are very close to the expected value. It may be concluded that the precursors completely transformed into the desire product.

Fig. 8: SEM micrographs of cobalt ferrite using (a, b) CoFeC6, (d-f) CoFeA6.

Fig.9: EDX spectra of (a) CoFeC6 and (b) CoFeA6.

Journal of the Australian Ceramic Society Volume 52[2], 2016, 150 – 162 158

Table 4: The compositional analysis of CoFeC6 and CoFeA6.

Fig.10: FESEM micrographs of (a-c) CoFeC6 and (d-f) CoFeA6.

3.9.HR-TEM studies Fig. 11(a) depicts the individual HR-TEM image of CoFeC6 with 105nm×141 nm dimension particle. The size difference is observed in HR-TEM and FE-SEM analysis compare to SEM study. This is due to that the HR-TEM and FE-SEM analysis is carried out at higher resolution whereas the SEM study is done at low resolution. We can identify the size and morphology of individual particle in the high resolution. However in the SEM analysis, it provides the surface morphology of the material. The SEM images cannot distinguish between single and agglomerated particles properly and hence variation in size determination could come up. In XRD study, we calculated the crystallite size of the material whereas, in HR-TEM and FE-SEM analysis the particle size of the material can be determined. So there is a difference between two measurements. However, the Scherrer equation is most appropriate for spherical particle. In that case the surface morphology of the synthesized material is pyramidal

and rod like. That may be one of the reasons to observe such kind of difference. The microstructure of CoFeC6 is pyramidal. The particle appears as dark black that suggests the material is thick in nature (see Fig. 11(b, c)). Moreover, the HR-TEM image for CoFeC6 displays distinct lattice spacing of 2.52 Å and 4.82Å corresponding to (311) and (111) planes, respectively. Fig. 11(e) displays the general view of CoFeA6 whereas; a HR-TEM image of an isolated particle (730 nm × 100 nm) is included in Fig. 11(f). Rod like morphology is obtained for CoFeA6. The rods are porous in nature (see Fig. 11(e,f)). Such porous materials are expected to exhibit good photocatalytic and absorption behavior. The interplanar distance at 2.52 Å corresponds to (311) plane for CoFeA6. CoFeA6 exhibits different type of surface morphology compared to previous study [13]. The selected area electron diffraction (SAED) displays rings pattern (see Fig. 11(c, f)) that indicates

Element Weight% Atomic%

O K 31.66 65.64

Fe K 44.39 22.84

Co K 23.95 11.52

Totals 100

Element Weight% Atomic%

O K 28.15 58.91

Fe K 46.43 27.23

Co K 25.42 13.86

Totals 100

Saha et al. 159

the ferrite is polycrystalline in nature. The spotty nature of the SAED pattern suggests the high crystalline nature of ferrite material. The rings indicated in the figure correspond to the diffraction planes (220), (311), (400) and (220), (400), (422) forCoFeC6 and CoFeA6, respectively. 3.10. Magnetic behavior of the ferrites Fig. 12 shows the M-H curves of CoFeC6 and CoFeA6. The value of magnetic parameter such as saturation magnetization (Ms), remanence magnetization (Mr), coercivity (Hc) and magnetic moment (nB) of theferrite material is given Table 5. The magnetic behavior of the nano crystalline particle is dramatically changed compare to its bulk state. The Msvalue of bulk CoFe2O4is 80.8 emu g-1.However, the value comes down to48.12 emu g-1and48.86 emu g-1for CoFeC6 and CoFeA6, respectively. Furthermore, Mrvalue for CoFeC6 and CoFeA6 is found 40.32 emu g-1 and 40.78 emu g-1, respectively.

The values of Ms and Mr, are almost similar for both materials. However, the Hc value of those materials is different from each other.CoFeC6 and CoFeA6 exhibit Hc value, 1233 Oe and 542 Oe respectively. The nature of hysteresis loops suggest that the ferrite materials are strong ferromagnetic. The area under the hysteresis loops of CoFeC6 is higher compare to CoFeA6. The strong ferromagnetism behavior originates from the magnetic moment of antiparallel spins between Fe3+ ions in the octahedral sites and Co2+ ions in the tetrahedral sites in the lattice structure. The value of the magnetic moment can be explained on the basis of changes in exchange interaction between tetrahedral and octahedral site in crystal lattices. The magnetic moment (ηB) in Bohr magneton is calculated using the formula:

MWB =η × 5585/SM (7)

Fig. 11: HR-TEM micrographs of (a-c) CoFeC6, (e-g) CoFeA6 and (d, h) with their SAED patterns

Table 5: Magnetic properties values of CoFeC6 and CoFeA6 Sample Ms (emu g-1) Mr (emu g-1) Hc (Oe) ηB Mr/Ms

CoFeC6 48.12 40.32 1233 2.02 .84

CoFeA6 48.86 40.78 542 2.05 .83

Journal of the Australian Ceramic Society Volume 52[2], 2016, 150 – 162 160

Fig. 12: M-H curve of (a) CoFeC6and (b) CoFeA6.

Fig. 13: AFM micrographs of (a) CoFeC6 and (b) CoFeA6.

-15000 -10000 -5000 0 5000 10000 15000-60

-40

-20

0

20

40

60

Mag

netic

mom

ent (

emu

g-1)

Applied Field (k Oe)

(a)(b)

Saha et al. 161

3.11. AFM studies AFM is performed to examine the surface topology of the material. Fig. 13 depicts the AFM micrograph of CoFeC6 and CoFeA6. The bright spot indicates the particle of ferrite material. The nanoparticle with uniform size are homogeneously distributed throughout the surface. The particles are well separated from each other. 4. Conclusions The nanometre dimension of CoFe2O4 was successfully synthesized by a very simple microemulsion technique using the different surfactants which can be applied for potential applications such as good catalyst, photo-catalyst, sensor, and targeted and controlled drug delivery materials. The crystal structure of CoFe2O4 is face center cubic and belongs to Fd3m space group. The crystallite size is found in the region between 6-40 nm. The band gap values fall in the range between 2.13-2.02 eV. The crystallite size and optical behavior of the ferrite materials was found to be dependent on the calcination temperature. The agglomeration tendency is observed due to the high surface area and magnetic interaction among the particle of the material. The nanoparticles are uniform in size and homogeneously distributed throughout the surface. Further the morphology can be altered by variation of synthesis parameters, which can change surface characteristics, and property of the nanomaterial. The pyramidal and rod like morphology are obtained using cationic and anionic surfactant respectively. CoFe2O4 acts as strong ceramic ferromagnetic magnetic material. Acknowledgements One of the authors is thankful to UGC for award of fellowship. The authors would like to thank Department of Metallurgical & Material Engineering, Department of Chemistry and School of Material Science & Nanotechnology, Jadavpur University for providing the Instrumental facilities. We are also thankful to Dr. Dipten Bhattacharya, Scientist, CGCRI, Kolkata, for studying the magnetic behavior of the material. Reference [1] F.X. Cheng, J.T. Jia, Z.G. Xu, B. Zhou, C.S.

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