The influence of cationic surfactant CTAB on optical, dielectric and magnetic properties of cobalt ferrite nanoparticles

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The influence of cationic surfactant CTAB on optical, dielectric and magnetic properties of cobalt ferrite nanoparticles Shyamal Dasa, M. Bououdinab, C. Manoharana,∗ a b

Department of Physics, Annamalai University, Annamalai Nagar, 608002, Chidambaram, Tamil Nadu, India Department of Physics, College of Science, University of Bahrain, PO Box, 32038, Southern Governorate, Bahrain

A R T I C LE I N FO

A B S T R A C T

Keywords: Raman Dielectric properties Mossbauer spectroscopy Magnetic order Capacitance

In the present work, the influence of cationic surfactant CTAB (cetyltrimethylammonium bromide) on size, shape and coalescence behaviour of cobalt ferrite nanoparticles (CFNPs) synthesized via hydrothermal method is reported. Pure CFNPs show no additional peaks, whereas α-Fe2O3 phase is observed in CTAB added CFNPs upon annealing. FT-IR analysis confirms the formation of M − O vibrational bands (metal -oxygen) at tetrahedral Asite and octahedral B-site for both samples. SEM observations reveal less agglomeration and smaller particle size for surfactant added CFNPs. Raman spectral study confirms the formation of cubic spinel structure and Raman active modes of CTAB added CFNPs. UV–Vis spectra indicate a decrease in the energy band gap with annealing. The dielectric constant of surfactant added CFNPs decreases with increasing applied frequencies for both real and imaginary, but ac conductivity increases with increasing frequencies. Two sextet patterns of Fe3+ trivalent ions from tetrahedral and octahedral sites are observed in Mössbauer spectra. VSM study indicate the ferrimagnetic nature of CTAB added CFNPs. The electrochemical analysis reveals the pseudocapacitive nature of working electrode prepared by CTAB added CFNPs.

1. Introduction The physical, chemical and magnetic properties of spinel ferrite materials have attracted considerable attention in research fields of engineering, biomedicine and in commercial sector. Magnetic spinel ferrites exhibit potential applications in microwave devices, magnetic recording such as compact disc (CD) and audio and video tapes, as well as MRI (magnetic resonance imaging) and data storage devices [1]. Particularly, cobalt ferrite is one of the most promising and widely investigated. The structure of spinel ferrite is closely packed O2 lattice. In normal spinel, the divalent cation occupies tetrahedral A-site and trivalent cations occupy octahedral B-site. If the divalent cations are engaged in octahedral B-site, known as inversed spinel, half of the trivalent Fe3+ ions occupy A-site and half B-site. More number of Co2+ cations occupy octahedral site in the case of ferrite inverse spinel structure [2]. The ionic distribution between normal and inverse spinel are called mixed spinel, where δ represents the inversion parameter.

∗

The magnetic properties of spinel ferrites depend on the type of cations and their distribution among tetrahedral and octahedral sites. The electron spin between crystal lattice sites and the two sublattice sites are arranged in parallel and anti-parallel when the magnetization is different from net magnetization between A-site and B-site [3]. At room temperature, the spinel cobalt ferrite magnetic nanoparticles

Corresponding author. E-mail address: cmanoharan1@rediffmail.com (C. Manoharan).

https://doi.org/10.1016/j.ceramint.2020.01.202 Received 3 December 2019; Received in revised form 20 January 2020; Accepted 21 January 2020 0272-8842/ © 2020 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Shyamal Das, M. Bououdina and C. Manoharan, Ceramics International, https://doi.org/10.1016/j.ceramint.2020.01.202

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Fig. 1. TGA-DTA pattern of sample SH.

separately to form a homogeneous solution and then transferred to one single beaker. The mixed solution was stirred for 30 min with the addition of 6 M of NaOH solution dropwise. Finally, the precursors solution was poured into the stainless-steel autoclave and kept in muffle furnace at 200 °C for 6 h. The observed black precipitate was filtered and washed several times by absolute ethanol and de-ionised water and was dried for 5 h at 80 °C. Finally, to improve the crystallinity, both the samples (pure and CTAB added) were annealed at 700 and 800 °C with the heating rate of 5 K/min. The pure as synthesized and annealed samples (700 °C and 800 °C) were named as H, H1, H2, respectively, whereas CTAB surfactant added samples were named as SH, SH1, SH2, respectively.

(MNPs) exhibit high coercivity, high Curie temperature, moderate saturation magnetization (MSM), as well as high thermal and chemical stabilities [1]. All these properties make cobalt ferrites suitable for many interesting technological and biomedical applications such as high density recording, DNA separation, drugs delivery and hyperthermia treatment [4]. CoFe2O4 nanoparticles have been prepared through various methods including chemical co–precipitation [5], sol-gel [6], citrategel, micro-emulsion [7], combustion [8] and hydrothermal method [9]. Among the various synthesis routes, hydrothermal method offers numerous advantages; no toxic by-product, low cost, higher yield, and more importantly easy to control the size and shape by controlling the experimental parameters like temperature, pressure, concentration of precursors. The advantage of using autoclave is that, the formation of oxide during the reaction with atmospheric compounds is very limited. Only few reports are available about the influence of surfactant (CTAB) on size, morphology and agglomeration of Cobalt ferrite nanoparticles synthesized by hydrothermal method. M. Vadivel et al., [1] reported about the significant modification in size of the particles on varying the concentration of CTAB in the synthesis of cobalt ferrite MNPs using co-precipitation method. The present work aims to study the influence of CTAB surfactant on particle size, morphology, dielectric, magnetic and electrochemical behaviour of cobalt ferrite nanoparticles synthesized by hydrothermal method.

2.3. Characterization The thermal behaviour of as synthesized sample SH was studied by using TGA/DTA (NETZSCH-STA 449 F3 JUPITER). The synthesized spinel cobalt ferrite magnetic nanoparticles were characterized by X-ray diffractometer (X-Pert-Pro-PW3050/60) equipped with Cu-Kα radiation (λ = 1.5406 Å) at room temperature. The FT-IR spectra were recorded at room temperature using Thermo Nicolet iS5 FT-IR Spectrometer. The Raman spectra were recorded by using Micro-Raman spectroscopy (Horiba-Jobin-Yvon HR800) with CCD detector and 473 nm diode laser excitation source. The surface morphology of CFNPs was analysed by Zeiss SUPRASS-SEM instrument. The UV–Vis–NIR Spectrometer (UV3600 plus) was used to record absorption spectra. The dielectric properties were measured by using Hio-Ki-Im3536 model at room temperature. The isomer shift, quadrupole shifting and hyperfine field investigation was carried out by using 57Co radioactive source in the 57 Fe Mössbauer spectroscopy instrument at room temperature and also to determine the magnetic phase of the prepared cobalt ferrite nanoparticles. Magnetization-field curves were recorded at room temperature using a vibrating sample magnetometer VSM (Cryogenic-3639) with a maximum applied field 20 kOe. The cyclic voltammetry (CV) test was performed by using a Bio-Logic VMP-300 potentiostat, at different scan rates in the potential range of 0.8–1.0 V.

2. Experimental part 2.1. Materials Cobalt (II) acetylacetonate (99.9%), Iron (III) acetylacetonate (99.9%), and Cetyltrimethylammonium bromide (CTAB) purchased from Sigma Aldrich are used without further purification. 2.2. Experimental procedure The use of CTAB, NaOH and de-ionised water act as a capping agent, precipitant and solvent, respectively. To prepare CTAB surfactant added cobalt ferrite nanoparticles, 1:2 molecular ratio of Co(acac)2 and Fe (acac)3 and 0.1 M of CTAB were dissolved in 25 ml of distilled water separately in three beakers. The three solutions were stirred for 30 min 2

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3. Results and discussion

individual crystallite. The X-ray density ( ρx ) is the density of atom or molecules in each unit cell of the compound. The lattice constant is the physical dimension of the unit cell in the crystal lattice. All the above microstructural parameters; i.e. crystallite size (D), dislocation density (δ ), microstrain (ε ) and X-ray density ( ρx ) and are calculated as follows [12].

3.1. Thermal analysis The thermal analysis (Fig. 1) of as synthesized sample SH was carried out to identify the exothermic, endothermic process and hence thermal stability. The weight loss (%) was identified from the gradually changing TGA curve. The decomposition of sample by melting was observed from DTA curve. The temperature range 40–170, 200–420 and 420–750 °C shows the weight losses of 3.93%, 3.37% and 1.23%, respectively. The observed first weight loss is due to loss of water or moisture absorbed from the environment. The second weight loss is attributed to the removal of organic compounds and NaOH. The appearance of the exothermic peak in the temperature range100 – 360 °C in DTA curve confirms the removal of organic compounds. The third weight loss ascribed to the removal of inbuilt water and decomposition of organic salt. This is pertained by the broad exothermic peak appeared in DTA curves in between 360 and 640 °C. The above data shows the completion of phase formation below 700 °C and hence no further significant weight loss is observed above 700 °C. Hence according to TGA-DTA result, the temperature of 700 and 800 °C have been fixed as annealing temperature.

D=

0.89 ∗ λ βcosθ

(1)

δ=

1 D2

(2)

ε=

β cos θ 4

(3)

ρx =

8M 3 NA aexp

(4)

where λ is the wavelength of Cu-kα, β is FWHM in radians. The structural parameters; i.e. lattice parameter aexp has been calculated by using [12] meanwhile other physical parameters are calculated by using the following relations [13].

3.2. X-ray diffraction study The formation of pure CoFe2O4 phase has been confirmed from XRD analysis before and after addition of CTAB. The observed peaks of H, H1 and H2 (Fig. 2(a)) are indexed as (220), (311), (400), (422), (511) and (400) reflections of the cubic spinel structure (space group Fd3m), in agreement with JCPDS card No. 22–1086. The XRD patterns of CFNPs with the addition of cationic surfactant SH, SH1 and SH2 shown in Fig. 2(b), also match well with the as-mentioned crystal planes. The additional minor peaks located at 33° (104), 40.5° (113), 49.5° (024) and 63.9°(300) for SH1 and SH2 samples, indicate the presence of small amount of the impurity hematite phase α -Fe2O3 (JCPDS card No. 33–0664). The capping shell formed at the surface of Fe3O4 nanoparticles (core) breaks up at higher annealing temperature, leading to the formation of α -Fe2O3 phase with the formation of iron enhancement by the passivation procedure [10,11]. The crystallite size (D) is the measure of size of the coherently diffracting domain. Dislocation density (δ ) is the length of dislocation lines per unit meter square (lines/m2) of the crystal. The microstrain (ε ) is the root mean square of the variation in lattice parameters across the

aexp = d h2 + k 2 + l 2

(5)

nλ = 2dsinθ

(6)

rA = (u − 0.25) aexp 3 − R 0

(7)

rB = (0.625 − u) aexp − R 0

(8)

HA = 0.25aexp 3

(9)

HB = 025aexp 2

(10)

1 dA − OA = aexp 3 ⎛u − ⎞ 4⎠ ⎝

(11) 1

11 43 2 dB − OB = aexp ⎡3u2 − ⎛ ⎞ u + ⎛ ⎞ ⎤ ⎝4⎠ ⎝ 64 ⎠ ⎦ ⎣

(12)

1 dAE = aexp 2 ⎛2u − ⎞ 2⎠ ⎝

(13) 1

11 2 ⎤ dBE = aexp 2 (1 − 2u), dBEU = aexp ⎡4u2 − 3u + 16 ⎦ ⎣

Fig. 2. (a) XRD patterns of pure and (b) CTAB added CFNPs for annealed and as-prepared sample. 3

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Table 1 Values of d-spacing, lattice parameter(aexp) , crystallite size (D), X-ray density ( ρx ), dislocations density (δ ), microstrain (ε ) for pure and CTAB added CFNPs. Pure CoFe2O4 sample

CTAB added CoFe2O4 sample

Sample name

d-spacing (Å)

aexp (Å)

‘D’ (nm)

( ρx )gmcm

H H1 H2

2.51829 2.52666 2.53004

8.3522 8.3799 8.3911

21.64 39.54 47.93

5.3490 5.2960 5.2748

−3

−3

(δ )x1015(line/ m2)

(ε )x10

1.0018 0.6396 0.4352

2.5120 1.0218 0.8173

Samplename

d-spacing (Å)

aexp (Å)

D(nm)

( ρx )gm cm−3

(δ )x1015(line/ m2)

(ε )x10−3

SH SH1 SH2

2.54348 2.5338 2.5390

8.4357 8.4036 8.4209

16.62 35.78 45.08

5.19178 5.25148 5.29239

3.906 0.78112 0.49207

2.7002 1.0052 0.0408

A shift in the vibrational peak positions of Fe–O and Co–O are noticed upon annealing [15]. This shift is more significant for the tetrahedral A-site in pure samples as the bond length is smaller, whereas it is lesser for CTAB added samples as the bond length is larger (Table 2). On the other hand, the shift is found smaller for the octahedral B-site in pure samples compared to CTAB samples, due to the weak bonding between divalent cations Co2+ and oxygen anion O2−.

where aexp is the experimental lattice constant, ρx is X-ray density, M is molecular weight, NA is the Avogadro number, NA = 6.0225 × 1023 particles per mole. rA , rB are the radii, HA, HB are the hoping length, dAOA and dB-OB are the bond length, at tetrahedral (A) and octahedral (B) sites, respectively, d AE is the tetrahedral edges, dBE and dBEU are the shared and unshared octahedral edges, R0 is the oxygen ion radius (1.32 Å), u is the oxygen position parameter or anion parameter (0.381 Å). The effect of cationic surfactant CTAB is evident from the calculated crystallite size value (Table 1). The addition of CTAB controls the interaction between the particles and hence the crystallite size decreases, while it increases with increasing annealing temperature in pure as well as in capping (as the capping shell breaks) case. The crystallite size increases with increasing annealing temperature, whereas dislocation density and microstrain decreased with increasing crystallite size. The dislocation density and microstrain are directly proportional to each other. The X-ray density is found to decrease with increase of lattice constant in pure and reverse nature in case of CTAB added CoFe2O4. The structural parameters (rA , rB,HA, HB, dA-OA, dB-OB, dAE, dBE and dBEU) given in Table 2 are found to increase for pure samples with annealing, while decrease with the addition of CTAB surfactant. The observed changes in the values of both microstructural and structural parameters can be associated with the increasing and decreasing trend of the lattice constant for pure and CTAB added samples respectively, with annealing.

3.4. Raman spectroscopy analysis Raman spectra of surfactant added CFNPs are displayed in Fig. 4. As stated by the group theory, the spinel structure of CFNPs gives rise to 39 normal modes of vibrations, in which five modes are Raman active. The position of Raman active modes at A1g and Eg represent the nature of spinel ferrite. The characteristic modes of A1g(1) and A1g(2) vibrations originate from tetrahedral sublattice. The splitting of A1g mode into two A1g(1) and A1g(2) is due to re-distribution of cations, whereas Eg, T2g(1), T2g(2) and T2g(3) modes correspond to octahedral sublattices. The samples are not ready to produce single phase because of the proximity of Fe2O3 phase boundary [16]. The peaks at T2g(1) and T2g(2) are ascribed to the local symmetry vibration of oxygen atom with metal ions in octahedral sites. The peaks position of Raman active mode A1g, T2g, and Eg is slightly shifted towards the higher wavenumber (blue shift). Upon annealing, the intensity of A1g mode at tetrahedral sublattice increases while the width decreases. The intensity ratio of A1g (2)/Eg exhibits increasing trend (0.991, 1.492 and 1.512 for SH, SH1 and SH2, respectively) due to migration of Co2+ from octahedral B-site to tetrahedral A-site. The intensity of T2g(2) and T2g(3) shows a decreasing nature (Table 3). The observed shift in peaks position alongside with the variation in peaks intensity are due to the cumulative effects of CTAB surfactant, particle size and cationic charge distribution between A-site and B-sites upon annealing.

3.3. FT-IR analysis FT-IR analysis allow to access information about structure, local symmetry and cation re-distribution between A and B-sites; the recorded spectra are shown in Fig. 3. The prominent vibrational modes assigned to the A and B-sites are observed in the region of 400–600 cm−1. The frequency bands observed in the range 579–587 and 421-435 cm−1 are associated with tetrahedral -A and octahedral -B sublattices, respectively for pure CFNPs, whereas the same are observed at 568–586 and 461-485 cm−1 for CTAB added CFNPs. The frequency bands around 3416 cm−1 and 1644 cm−1are due to O–H stretching vibration and H–O–H bending vibration, respectively absorbed at the surface of NPs from atmosphere [14]. The observed bands at 2936 and 2859 cm−1 are ascribed to the asymmetric and symmetric C–H stretching groups of alkyl chain [2]. The bands at 1394 cm−1 region are attributed to –CH2- and –CH3 groups [14].

3.5. Morphological observations The SEM images of H2 (Fig. 5(a)) and SH2 (Fig. 5(b)) CFNPs reveal cubic and irregular shaped particles with coalescence. The coalescence nature of H2 is due to annealing as well as the existence of magnetic interaction between particles. The controlled shape, size and reduced nature of the agglomeration between the particles are due to the influence of CTAB in the case of SH2. CTAB has shown significant effect on the particle nature, nucleation and formation of CFNPs [2,3]. Fig. 5

Table 2 Sites radii(rA) , and (rB ), bond length (dA − OA) and (dB − OB ) , hopping length (HA and HB), sites edge (dAE ) , (dBE ) and (dBEU ) for pure and CTAB added CFNPs. Sample Code

H H1 H2 SH SH1 SH2

Sites radii (Å)

Bond length (Å)

Hoping length(Å)

Sites edges (Å)

Site-A (r A)

Site-B (rB)

dA-OA

dB-OB

HA

HB

dAE

dBE

dBEU

0.5751 0.5813 0.5839 0.5940 0.5867 0.5906

0.7179 0.7246 0.7274 0.7383 0.7304 0.7346

1.8951 1.9013 1.9039 1.9140 1.9067 1.9106

2.0391 2.0459 2.0486 2.0595 2.0517 2.0559

3.6166 3.6286 3.6334 3.6527 3.6388 3.6463

2.9529 2.9627 2.9667 2.9824 2.9711 2.9773

3.0946 3.1049 3.1091 3.1256 3.1137 3.1201

2.8112 2.8205 2.8243 2.8393 2.8285 2.8343

2.9546 2.9644 2.9684 2.9841 2.9728 2.9789

4

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Fig. 3. (a) FT-IR pattern of pure and (b) CTAB added CoFe2O4 NPs.

Fig. 4. Raman spectra of (a) SH, (b) SH1 and (c) SH2 samples.

in Fig. 6(a). The shifting of absorption edges towards higher wavelength is due to the increase of crystallite size upon annealing. The absorbance is more significant in the visible range for SH compared to SH1 and SH2. The band gap energy is calculated using the relation of direct transitions [12].

(c and d) shows the particles size distribution of H2 and SH2. The mean particle size is in the range 10–90 nm and 10–80 nm for H2 and SH2, respectively, while the maximum number of particles are distributed in the range 30–60 nm in both cases. 3.6. Optical properties

1

αhγ = β (hγ − Eg ) 2

The optical absorption spectra of the synthesized samples are shown 5

(15)

Ceramics International xxx (xxxx) xxx–xxx 5415.37 2225.03 2101.36 214.15 214.72 218.07 851.72 1178.53 1265.82 52.63 87.11 35.20

14.75 21.89 11.74

position cm−1 Ip a.u width cm−1

width cm−1

where the notation for absorption coefficient, Planck's constant, light frequency, proportionality constant and band gap are α , h, γ , β , and Eg , respectively. The optical band gap value is obtained by extrapolating the linear portion of the curves plotted between (αhγ )2 and (hγ ) , to intersect X-axis. The estimated values of band gap from Fig. 6 (b) are 2.61, 2.39 and 2.19 eV for SH, SH1 and SH2, respectively. It can be noticed that the energy band gap decreases upon annealing, which can be associated the variation of crystallite size, presence of impurities and structural parameters [17]. 3.7. Dielectric properties The ferrite elements show a significant dielectric behaviour. The real part (ε′) and imaginary part (ε′′) of dielectric constant, dielectric loss (tanδ ) and AC conductivity (σ) of SH, SH1 and SH2 samples at room temperature are calculated by using the following relations [16]:

275.85 281.11 285.31 3599.93 2192.22 1467.23

(17)

33.92 63.98 101.83

ε′′ ε′

(16)

tanδ =

where ‘t’ is the thickness of the pellet, ‘A’ is the area of cross-section, ‘ε 0 ’ is the permittivity of free space (8.85 × 10−14 F/m). The conduction mechanism of spinel ferrite has been already proposed; Contribution of Co2+ and Co3+ hole hopping (p-type) and Fe2+and Fe3+ electron hopping (n-type) to the low and high conductivity regions, respectively. The exchange of electrons between Fe2+ and Fe3+ create a local displacement of electrons along with the direction of the applied field which determine the polarization. The latter occurs as the charge carriers reach and pile up at the grain boundaries. In CTAB added CFNPs, the polarization is in connection with the hopping of charge carriers. The real and imaginary dielectric constant values (Fig. 7 (a and b)) are high at low frequency for all the three samples because of the contribution of space charge, ionic charge and interfacial polarization [13,18]. At high frequencies, the dielectric constant of real and imaginary parts become independent of frequencies and hence takes low and almost constant value. At low frequencies, the space charge polarization is more dominant because of the accumulation of the charge carriers at grain boundaries, whereas this mechanism is suppressed at high frequency. Hence, high values of real and imaginary dielectric constant are observed at low frequency and low values at high frequency [1]. The electric dipoles are unable to follow the fast variation of the applied field. The cation distribution has an influence on the variation of real and imaginary dielectric constants with the variation of grain size and microstructure. Fig. 7 (c) depicts the evolution of the dielectric tangent loss or dielectric loss versus frequency. It is clear that the dielectric loss is more pronounced at low frequency then decreases in the same trend as that of the dielectric constant variation with frequency. The dielectric loss was found to be dependent on various factors, mainly Fe2+ content, stoichiometry, and structural homogeneity [13]. The observed decrease of the dielectric loss with frequency can be attributed to the conduction mechanism of ferrite and Maxwell-Wagner polarization [19]. Moreover, the dielectric loss value decreases with increasing annealing from 3.8 to 2.0 at the frequency of 1 kHz. This low value of dielectric loss is more appropriate for microwave applications [13]. The AC conductivity of CTAB added CFNPs is found to increase exponentially with the rise in the applied ac frequency as illustrates in Fig. 7(d). The nature of conductivity rise with increasing frequency can be associated with the electron exchange between Fe2+ and Fe3+ at Bsites. According to Maxwell-Wagner two layers model, the frequency dependence AC conductivity can be understood as, the electron

759.53. 3368.48 2595.15

580.36 596.54 599.99

87.83 64.50 72.67

844.21 1759.01 1913.26

470.51 457.31 459.62

78.45 73.12 58.74

568.99 1226.94 613.26

383.65 385.81 388.36

σ = ε′ε 0 ωtanδ

667.65 674.24 676.59 SH SH1 SH2

position cm

Ct ε0A

ε′′ = ε′tanδ

80.92 65.79 72.20

position cm position cm−1 position cm−1 Ip a.u width cm−1 position cm−1 width cm−1

Ip a.u

A1g(2)

ε′ =

−1

A1g(1) sample

Raman active modes

Table 3 Position, bandwidth and intensity (Ip) of Raman peaks of CTAB added CFNPs.

T2g(1)

width cm−1

Ip a.u

T2g(2)

width cm−1

Ip a.u

Eg

−1

T2g(3)

Ip a.u

S. Das, et al.

6

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Fig. 5. SEM image of (a) pure (H2) and (b) CTAB added CFNPs (SH2); particles size histogram for (c) pure (H2) and (d) CTAB added CFNPs (SH2).

Fig. 6. (a) Optical absorption spectra and (b) plots (αhγ )2 vs hγ of SH, SH1 and SH2 samples.

hopping frequency between Fe2+ and Fe3+ cations is less effective at low frequency region because of more activeness of grain boundaries. The electron hopping gets promoted with the increase in the applied frequency field between the two adjacent B-sites and between Fe2+ and Fe3+ ions. Therefore, as the frequency increases, the AC conductivity also increases [13].

3.8. Magnetic properties 3.8.1. 57Fe Mössbauer spectra analysis Mössbauer spectra of surfactant added cobalt ferrite NPs (SH, SH1 and SH2) recorded at room temperature are shown in Fig. 8(a–c). The analysis confirms the presence of two sextets attributed to the trivalent 7

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Fig. 7. (a–d) Dielectric constant, imaginary dielectric constant, dielectric loss and ac conductivity behaviour of CTAB added cobalt ferrite NPs.

ions (Fe3+) at tetrahedral A-site and octahedral B-site. The presence of sextets in the spectra signifies that the alteration time of magnetization direction is greater than 10 ns [20], meanwhile the existence of strong sextet indicates the ordered ferrimagnetic nature of CTAB added CFNPs. The cation distribution between A and B components can be observed from the relative absorption area ‘S’ given in Table 4. The ‘S’ value of B component is more pronounced compared to A, whereas it decreases for B and increases for An upon annealing. The isomer shift (IS) is found to decrease for A and B-sites for annealed samples. The IS value for B-site is less significant compared to A-site as Fe3+- O2− bond separation is larger than that of same bond at tetrahedral A-site. The obtained smaller value of quadrupole splitting indicates that the disorder ness in the crystal symmetry is negligible. The hyperfine field value (Hin) is reduced due to the expansion of the crystal lattice. The half width half maxima (HWHM) value for A-site is found to increase with increasing annealing due to the shifting of Co2+ from B-site. The synthesized CTAB added CFNPs possess a cubic spinel structure with closely packed cubic lattice of oxygen ions, in which Fe3+ or Co2+ ions reside at the interstitial position of tetrahedral and octahedral sites. This site occupancy or cation distribution can be written as (Co1-xFex) [CoxFe1+x]O4, where the parentheses and square bracket represent the

tetrahedral and octahedral sites, respectively. The relative area % at tetrahedral A-site is found to be smaller compared to B-site, which indicating that the occupancy rate of Co2+ at A-site is very small than that of B-site. However, as the annealing temperature increases, Co2+ ions migrate from A-site to B-site whereas Fe3+ ions migrate from B-site to A-site [21,22]. The ratio of the absorption area between A-site and Bsite can be calculated by:

xfA SA = SB (2 − x ) fB where fA and fB is the recoiling fraction of Fe A and octahedral B-sites, respectively.

(20) 3+

cations for tetrahedral

3.8.2. Magnetization-field curves Magnetization-field curves for SH, SH1 and SH2 samples recorded at room temperature using an applied field in the range −2 x 104 to +2 × 104Oe are shown in Fig. 9(a), while the determined saturation magnetization (Ms), remanent magnetization (Mr) and coercivity (Hc) values are given in Table 5. The saturation magnetization increases with increasing annealing temperature. The enhancement of saturation magnetization with annealing is associated with the increase of 8

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Fig. 8. Mössbauer spectra of samples (a) SH, (b) SH1 and (c) SH2.

crystallite size, which is evident from XRD. A similar result was also reported by Raghvendra Singh Yadav et al. (2017) [13] and Nguyen Thi To Loan et al. (2019) [23].

The heating rate during annealing also plays an important role in the increasing/decreasing the saturation magnetization. The slow heating rate allows for complete crystallisation and increase the

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Table 4 Mössbauer spectral analysis of CTAB surfactant CFNPs. Sample

Sites

IS (mm/s)

QS (mm/s)

Hin (Tesla)

HWHM (mm/s)

S%

SA/SB

X

Cation distributions

SH

A B A B A B

0.493 0.178 0.465 0.156 0.414 0.125

0.0746 0.0297 0.0582 0.0227 0.0318 0.0185

50.55 50.67 50.58 50.72 50.62 50.79

0.447 0.476 0.452 0.467 0.469 0.461

32.14 67.86 36.46 63.54 41.25 58.75

0.46

0.63

(Co0.37Fe0.63)A[Co0.63Fe1.37]BO4

0.57

0.73

(Co0.27Fe0.73)A[Co0.73Fe1.27]BO4

0.70

0.82

(Co0.18Fe0.82)A[Co0.82Fe1.18]BO4

SH1 SH2

[Isomer shift (IS), Quadrupole splitting (QS), Hyperfine field (Hin), Half Width Half Maxima (HWHM), Relative area % (S %), SA/SB, x and Cation distributions].

magnetic phase, which resulted in enhanced magnetization. Mahmoud Goodarz Naseri et al. (2010) [24]. As the average particle size increases than that of single domain critical size ~34 nm, single domain wall is extended to multidomain nanocrystal, resulting in reduced coercivity for SH1 and SH2. In this study, it is also observed that the particle size is larger than single domain crystal size, which has capitulated the poor coherent rotation of spin and the formation of predominance domain wall in ferrimagnetic CTAB added CoFe2O4 NPs. The reduction in coercivity value for SH1 and SH2 samples is may be due to the presence of the secondary α -Fe2O3 phase at higher annealing temperature, as consequence a ferro-/antiferro-magnetic transformation occurs, followed by a reduction in coercivity (inset (i)) Fig. 9(a). The variation of susceptibility with annealing temperature is shown in Fig. 9 (b). It is observed that the magnetic susceptibility ( χ ) value linearly increases with increasing temperature with a very small deviation for 800 °C, due to the formation of α -Fe2O3 phase and ferro/ferri to antiferromagnetic transition. The magneton number ( μs ) , anisotropy constant(K), magnetic susceptibility ( χ ) and loop squareness have been calculated by using the following formulae [13] and the values are reported in Table 5.

μs = K=

MW ∗ Ms 5585

Hc ∗ Ms 0.96

M χ= H

Table 5 Coercivity (Hc), Saturation magnetization (Ms), Remanence magnetization (Mr), magneton number (μs), Loop squareness (Mr/Ms) and Anisotropy constant(K) of CTAB added CFNPs. Sample

Hc(Oe)

Mr(emu/ g)

Ms(emu/ g)

μs

Mr/Ms

K x 103(emu.Oe.g−1)

SH SH1 SH2

1016 794 510

9.5 26.2 31.1

18.3 60.5 64.5

0.844 2.784 2.968

0.518 0.432 0.486

20.7 50.1 34.7

magneton number, magnetic susceptibility and anisotropy constant increased whereas Mr/Ms ratio decreased with increasing annealing temperature. 3.9. Electrochemical characteristics The study of electrochemical performance is carried out in the potential range 0.8–1.0 V at room temperature for the sample SH2 using 6 M of KOH as electrolyte solution with different scan rates by cyclic voltammetry (CV). The specific capacitance (Cs) and electrochemical energy (E) are determined by using the following expressions [12].

(21)

Cs =

∫ mγidVΔV

(24)

Ε=

1 Cs ΔV 2 2

(25)

(22) (23)

where ʃ i dV is the area of the curve, composed material mass ‘m’ (g), γ is scan rate (mV/s), and ΔV is the potential window (V). The presence of

where MW is the molecular weight of CTAB added CFNPs. The

Fig. 9. (a) Hysteresis loops and (b) susceptibility vs. annealing temperature for the samples SH, SH1 and SH2. 10

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Fig. 10. (a) CV curves and (b) Cs vs. scan rate (mV/s) of sample SH2.

Appendix A. Supplementary data

redox peaks in the CV curves displayed in Fig. 10(a) explains the pseudocapacitive behaviour of the working electrode prepared by using the sample SH2. It is also noticed that as a result in the rise of the scan rate, the redox peak intensity, current density and area of the curve are increased, whereas the specific capacitive (Cs) decreases from 246 to 106 F/g at the scan rate of 5–100 mV/s Fig. 10(a). This is mainly due to the electrolytic ion utilization with active sites of the working electrode; i.e. ions of electrolyte are not getting enough time to access the electrode. At high scan rate, the electrolytic ions can access only outer regions of the electrode [3,25]. Therefore, low scan rate is considered more suitable for the storage of high specific capacitance Fig. 10(b).

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2020.01.202. References [1] M. Vadivel, R. Ramesh Babu, K. Ramamurthi, M. Arivanandhan, CTAB cationic surfactant assisted synthesis of CoFe2O4 magnetic nanoparticles, Ceram. Int. 42 (16) (2016) 19320–19328. [2] A. Persis Amaliya, S. Anand, S. Pauline, CTAB assisted synthesis of cobalt ferrite nanoparticles and its characterizations, J. Nanosci. Technol. 2 (4) (2016) 186–188. [3] B. Jansi Rani, M. Ravina, B. Saravanakumar, G. Ravi, V. Ganesh, S. Ravichandran, R. Yuvakkumar, Ferrimagnetism in cobalt ferrite (CoFe2O4) nanoparticles, NanoStruct. Nano-Objects 14 (2018) 84–91. [4] S. Ayyappan, S. Mahadevan, P. Chandramohan, M.P. Srinivasan, John Philip, Baldev Raj, Influence of Co2+ iron concentration on the size, magnetic properties, and purity of CoFe2O4 spinel ferrite nanoparticles, J. Phys. Chem. 114 (2010) 6334–6341. [5] M.P. Gonzalez-Sandoval, A.M. Beesley, M. Miki-Yoshida, L. Fuentes-Cobas, J.A. Matutes-Aquino, Comparative study of the microstructural and magnetic properties of spinel ferrites obtained by co-precipitation, J. Alloys Compd. 369 (1) (2004) 190–194. [6] B.G. Toksha, S.E. Shirsath, S.M. Patange, K.M. Jadhav, Structural investigations and magnetic properties of cobalt ferrite nanoparticles prepared by sol gel auto combustion method, Solid State Commun. 147 (11) (2008) 479–483. [7] J. Fu, J. Zhang, Y. Peng, J. Zhao, G. Tan, et al., Unique magnetic properties and magnetization reversal process of CoFe2O4 nanotubes fabricated by electro spinning, Nanoscale 4 (13) (2012) 3932–3936. [8] N.M. Deraz, Glycine-assisted fabrication of monocrystalline cobalt ferrite system, J. Anal. Appl. Pyrolysis 88 (2) (2010) 103–109. [9] Z. Wang, X. Liu, M. Lv, P. Chai, Y. Liu, et al., Preparation of one-dimensional CoFe2O4 nanostructures and their magnetic properties, J. Phys. Chem. 112 (39) (2008) 15171–15175. [10] Feng XiangQin, Shao YiJia, Yong Liu, Xu Han, Hai TaoRen, Wei WeiZhang, Jing WeiHou, Song-HaiWu Metal-organic framework as a template for synthesis of magnetic CoFe2O4 nanocomposites for phenol degradation, Mater. Lett. 101 (2013) 93–95. [11] A.G. Maria Soler, C.D. Emilia Lima, W. Sebastiao da Silva, F.O. Tiago Melo, C.M. Angela Pimenta, P. Joao Sinnecker, B. Ricardo Azevedo, K. Vijayendra Garg, C. Aderbal Oliveira, A. Miguel Novak, C. Paulo Morais, Aging investigation of cobalt ferrite nanoparticles in low pH magnetic fluid, ACS 23 (2007) 9611–9617. [12] Shyamal Das, M. Bououdina, C. Manoharan, Dependence of structure/morphology on electrical/magnetic properties of hydrothermally synthesized cobalt ferrite nanoparticles, J. Magn. Magn Mater. 493 (2020) 165703. [13] Raghvendra Singh Yadav, Ivo Kuřitka, Jarmila Vilcakova, Jaromir Havlica, Jiri Masilko, Lukas Kalina, Jakub Tkacz, Jiří Švec, Vojtěch Enev, Miroslava Hajdúchová, Impact of grain size and structural changes on magnetic, dielectric, electrical, impedance and modulus spectroscopic characteristics of CoFe2O4 nanoparticles synthesized by honey mediated sol-gel combustion method, Adv. Nat. Sci. Nanosci. Nanotechnol. 8 (14pp) (2017) 045002. [14] Jian-Liang Cao, Zhao-Li Yan, Yan Wang, Guang Sun, Xiao-Dong Wang, Hari Bala, Zhan-Ying Zhng, CTAB-assisted synthesis of mesoporous CoFe2O4 with high carbon monoxide oxidation activity, Mater. Latters 10 (2013) 322–325. [15] K.S. Rao, G.S.V.R.K. choudary, K.H. Rao, Ch Sujtha, Structural and magnetic properties of ultrafine CoFe2O4, Procedia Mater. Sci. 10 (2015) 19–27. [16] B. Ninad Velhal, D. Narayan Patil, R. Abhijeet Shelke, G. Nishad Deshpande,

4. Conclusion Pure and surfactant added CFNPs were successfully synthesized via hydrothermal at low temperature of 200 °C. The surfactant was found to have a direct influence on the crystallite size and formation of α -Fe2O3 phase upon annealing. The formation of metal oxide Fe–O and Co–O at tetrahedral A- and octahedral B- sites respectively, in addition to the peak shift towards higher wavenumber in FT-IR and Raman spectra which confirmed the blue shift. Cubic/spherical particle morphology was observed with higher agglomeration for pure than after the addition of surfactant. The optical band gap value decreased with increasing annealing temperature. The reduction in the dielectric loss value with annealing temperature indicated the potential use of this particular sample for microwave applications. The formation of two sextets and cationic distributions between A- and B- sites was confirmed alongside with the ordered ferrimagnetic nature from the Mössbauer study. The formation of α -Fe2O3 phase upon annealing the surfactant added CFNPs had a significant influence on the evolution of coercivity and loop squareness with an increase in the saturation magnetization. The addition of surfactant CTAB resulted in particle size reduction and higher coercivity and remanent ratio of cobalt ferrite nanoparticles making them promising candidates for magnetic recording devices. The pseudocapacitive nature and slow scan rate enhanced the excellent application to supercapacitor. Acknowledgement The authors are highly thankful to the centre director Dr.V. Ganesan, scientist Dr. V. R. Reddy, scientist Dr. Dinesh kumar Shukla, scientist Dr. D. M. Phase, scientist Dr. Mukul Gupta, scientist Dr. Vasant Sathe, UGC-DAE consortium for scientific research, Indore campus, (M.P.) for providing the opportunities to carry out the Mössbauer, Dielectric, Raman and XRD studies. 11

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R. Vijaya Puri, Structural, dielectric and magnetic properties of nickel substituted cobalt ferrite nanoparticles: effect of nickel concentration, AIP Adv. 5 (2015) 097166. P. Annie Vinosha, G. Immaculate Nancy Mary, K. Mahalakshmi, L. Ansel Mely, S. Jerome Das, Study on cobalt ferrite nanoparticles synthesized by Co-precipitation technique for photo-fenton application mechanics, Mater. Sci. Eng. 9 (1) (2017) ISSN 2412-5954. R. Hemaunt kumar, C. Srivastava, P. Negi, H.M. Agrawal, K. Asokan, Dielectric behaviour of cabalt ferrite nanoparticles, Int. J. Electr. Electron. Eng. 2 (2013) 59–66. K.W. Wanger, The distribution of relaxation times in typical dielectrics, Ann. Phys. 40 (1913) 817–819. Kunal Pubby, S.S. Meena, S.M. Yusuf, Sukhleen Bindra Nafang, Cobalt Substituted Nickel ferrites via Pechini's sol-gel citrate route: X- band electromagnetic characterization, J. Magn. Magn. Materials. 466 (2018) 430–445. Mei Liu, Ming Lu, Li Wang, Shichong Xu, Jialong Zhao, Haibo Li Mössbauer study on the magnetic properties and cation distribution of CoFe2O4 nanoparticles

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