Synthesis of Zn_0.8Co_0.1Ni_0.1Fe₂O₄ polyvinyl alcohol nanocomposites via ultrasound-assisted emulsion liquid phase

2.1 Material used to synthesis ferrite-polymer (PVA) composite film

Chemical used in the synthesis of ZnFe₂O₄ & Zn_0.80Co_0.1Ni_0.1Fe₂O₄-PVA nanocomposite film were zinc nitrate (Zn(NO₃)₂·6H₂O) 98% (Merk KGaA, Germany), cobalt nitrate (Co(NO₃)₂·6H₂O) 98% (BDH), nickel nitrate (Ni(NO₃)₂·6H₂O) 98% (BDH), iron nitrate (Fe(NO₃)₃·9H₂O) 98% (BDH), aqueous ammonia (NH₄OH) 35%w/w (BDH), (Hexadecyltrimethylammoniunbromide, CTAB), min 99% (Sigma Aldrich, Germany). Poly(vinyl Alcohol) (SIGMA-Aldrich’s) was obtained as a 99+% hydrolyzed crystalline powder and a reported molecular weight of 146,000–186,000 and Deionize water.

2.2 Preparation of polymer solutions

Polyvinyl alcohol (PVA) was used to prepare composite films. Around 1.5 g of PVA was dissolved in deionized water. These solutions of PVA were placed on a heating plate under constant stirring until their volume was reduced enough to became thick solutions. The temperatures of all the stirred solutions were raised to 100 °C.

2.3 Synthesis of ZnFe₂O₄ and Zn_0.80Co_0.1Ni_0.1Fe₂O₄ spinal ferrite

Microemulsion technique (Malik et al., 2012) was employed to synthesis ZnFe₂O₄ and Zn_0.80Co_0.1Ni_0.1Fe₂O₄ spinel ferrite. To synthesis ferrite samples the solutions for required concentration as mention in Table 1, were prepared by dissolving stoichiometric amounts of (Zn(NO₃)₂·6H₂O), ((Co(NO₃)₂·6H₂O), (Ni(NO₃)₂·6H₂O), (Fe(NO₃)₃·9H₂O, AqueousNH₃(35%w/w(BDH), Hexacyltrimethylammoniumbromide (CTBA) in deionized water. According to microemulsion technique these solutions were mixed in a beaker with constant stirring and heating 50–60 °C. The ammonia solution was added to the reaction mixture drop by drop for precipitation and the pH was kept up to 10–11. The precipitates were washed with deionized water. The precipitates were dried in an oven at 100 °C. Then these precipitates were grind and annealed in Vulcan A- 550 temperature controlled muffle furnace at 700 °C for 7 h in order to obtain pure spinal ferrites, which is used in next section.

2.4 Preparation of Ferrite Solutions for composite

Synthesized spinel ferrites (prepared in previous section) were used for the preparation of a ferrite- PVA composite film. Samples ZnFe₂O₄ & Zn_0.80Co_0.1Ni_0.1Fe₂O₄ of the ferrites were dissolved in 30 ml deionized water according to compositions as 0.1, 0.2, 0.3, 0.4 and 0.5 g. Ferrite solutions were placed in an ultra sonicator for 1 h to have nanoparticles with uniform dispersion and distribution. After sonication, these solutions were used for composite film, as shown in the next section.

2.5 Preparation of ZnFe₂O₄ and Zn_0.80Co_0.1Ni_0.1Fe₂O₄–PVA composite

Ultrasound-assisted in situ emulsion (Bhanvase and Sonawane, 2014) was employed to synthesize a ZnFe₂O₄ and a Zn_0.80Co_0.1Ni_0.1Fe₂O₄-PVA nanocomposite. Solutions of PVA and the nanoparticles were prepared according to the given composition in Table 2. The prepared solutions of PVA were placed on heating plate having magnetic stirrer in it. These PVA solution heated and stirring until it evaporates half of the solution and became thick. Then ferrite solutions were mixed into PVA solutions and placed in an ultra sonicator. These solutions were sonicated for 3–4 h until they became thick enough. Then these solutions were poured into petri dishes and properly dried. This resulted in the formation of composites films process shown in Fig. 1.

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Fig. 1

Flow chart for the synthesis of nanocomposites.

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2.6 Characterization

X-ray diffraction (XRD) patterns of polyvinyl alcohol/ Zn_0.80Co_0.1Ni_0.1Fe₂O₄ nanocomposites were carried out on a Brooker module D8 advance diffractometer using Cu-Kα radiation ( $\lambda$  = 1.54056 nm) in a 2 $\theta$ range between 20° and 70° and a step size of 0.02°. The crystalline size of the prepared material was studied under X-ray diffraction line broadening and computed using Eq. (1)

A Fourier transform infrared spectrum (FTIR) of nanocomposites was obtained on a Tensor 27 Bruker spectrum. The FTIR spectra were determined in the range of 4000–500 cm⁻¹. The morphology of the nanocomposites was examined by a high-resolution transmission electron microscope (HRTEM) Hitachi TEM system, at 100.0 kV.

3.1 Characterization by X-ray diffraction

XRD patterns of PVA and Zn_0.80Co_0.1Ni_0.1Fe₂O₄ & ZnFe₂O₄ ferrite/ polymer composites are depicted in Fig. 2. The XRD patterns were indexed by relating the obtained structural properties with the standard diffraction with a focus on the fcc (face-centered-cubic) lattice. The diffraction patterns of the fcc (face-centered-cubic) spinel structure assigned to the planes 220, 311, 400, 422, 511 and 440 were noticed. The diffraction peaks were compared with the standard JCPDS-ICDD cards, for Co ferrite (22–1086 ICDD), for Zn ferrite (89–1009 ICDD) and NiFe₂O₄ (JCPDS card 89–4927). This comparison revealed that the synthesized composites have a single phase fcc (face-centered-cubic) spinel structure (Praveena and Sadhana, 2015; Al-Haj, 2006). The intensity of the XRD peaks in the composite samples was decreased because of the amorphous nature of the polymer (Obol and Vittoria, 2003; Khan et al., 2015). The XRD peaks are broader as related to the pure ferrite sample. The broadening of the XRD peaks is related to the nanocrystallite size (Ali et al., 2014). The crystallite sizes of all the samples (as describe in Table 2) are 8, 6, 7, 5 and 6 nm. It is documented that the grain growth relies on the grain boundary mobility. The mixing of the PVA with pure ferrite delays the grain growth thus reducing the crystallite size. It is clearly noticed that the crystallite size decreases with the addition of polymer which can be assigned to the fact that the greater the porosity, the smaller the crystallite size (Khan et al., 2015).

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Fig. 2

X-ray diffraction patterns of Zn_0.80Co_0.1Ni_0.1Fe₂O₄ & ZnFe₂O₄-PVA composites.

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3.2 FTIR of ZnFe₂O₄–PVA nanocomposites

Fig. 3 depicts the FTIR spectra of the PVA and the ferrite (ZnFe₂O₄) nanocomposites. The spectra were documented in the range of 4000–500 cm⁻¹. FTIR analysis of the ZnFe₂O₄–PVA nanocomposites shows (Fig. 3) a typical absorption band at 3650 cm⁻¹ that resemble the stretching mode of OH. Also, the Fe-O vibration approach of ZnFe₂O₄ is observed near 863 cm⁻¹ (Şabikoğlu and Paralı, 2014). The band at 1489 cm⁻¹ reveals the existence of —CH₂ asymmetric. There is no notable change in the C—H extending bands at 2176 and 2361 cm⁻¹. However, a striking changes in the significant peak of the —CH bending band at 1490 cm⁻¹ is seen for immaculate PVA. Furthermore, the —OH bending band at 1570 cm⁻¹ of the immaculate PVA was blue shifted to 1564 cm⁻¹ (Sivakumar et al., 2011). The specific bands (CH2) are shifted to a lower area which found that hydrocarbons in PVA surround ferrites in the crystalline state. So the attachment of PVA on to ZnFe₂O₄ nanoparticles surface is confirmed (Rahimi et al., 2013). Characteristic of FTIR of PVA and ZnFe2O4–PVA nanocomposites were given in Table.3.

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Fig. 3

FTIR of pure PVA (a) and ZnFe₂O₄–PVA nanocomposites (b).

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3.3 FTIR of Zn_0.80Co_0.1Ni_0.1Fe₂O₄ –PVA nanocomposites

Representative FTIR spectra, measured in the wavenumber from 4000 to 500 cm⁻¹, of Zn_0.80Co_0.1Ni_0.1Fe₂O₄ and PVA, synthesized nanocomposites are shown in Fig. 4. FTIR analysis shows (Fig. 4) the absorption band at 3640 cm⁻¹ which is attributed to —OH stretching owing to the strong hydrogen bond of the intramolecular and intermolecular interactions. The absorption of the band at 850 cm⁻¹ arises due to the Fe—O vibration mode of Zn_0.80Co_0.1Ni_0.1Fe₂O₄ (Şabikoğlu and Paralı, 2014; Saleh and Ali, 2018; Saleh, 2018). The presence of —CH₂ asymmetric is seen at 1480 cm⁻¹. The strong band at 2177 and 2350 cm⁻¹ was attributed to the C—H extending mode. However, a notable change in the significant band of —CH bending at 1480 cm⁻¹ is observed for immaculate PVA. Moreover, the blue shift of the immaculate PVA has been observed at 1560 cm⁻¹ corresponding to the —OH bending band at 1570 cm⁻¹ (Sivakumar et al., 2011; Saleh, 2011, 2015). The specific bands (CH2) are shifted to a lower area in this case of Zn_0.80Co_0.1Ni_0.1Fe₂O₄-PVA nanocomposites. So the attachment of PVA on to Zn_0.80Co_0.1Ni_0.1Fe₂O₄ nanoparticles surface is confirmed (Rahimi et al., 2013). Characteristic of FTIR of PVA and Zn_0.80Co_0.1Ni_0.1Fe₂O₄ –PVA nanocomposites were given in Table 4.

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Fig. 4

FTIR of (a) pure PVA and (b) Zn_0.80Co_0.1Ni_0.1Fe₂O₄–PVA nanocomposites.

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3.4 Dielectric analysis of nanocomposites of spinal ferrites

3.4.1

3.4.1 Dielectric constant

Fig. 5 displays the variation in behaviour of the dielectric constant (ε₀) for ZnFe₂O₄–PVA and Zn_0.80Co_0.1Ni_0.1Fe₂O₄–PVA nanocomposites by measuring the frequency of the applied ac field from 0.0 MHz to 3.0 GHz. The samples showed a high dielectric constant at a low frequency. While at a high frequency, the dielectric constant decreased due to the nature of the ferrite materials. This can likewise be elucidated from the polarization phenomenon which occurred in the ferrites because of the space charge polarization (Pathan and Shaikh, 2012). The dielectric behaviour is credited to the superexchange contacts and the inhomogeneous microstructure (Bhandare et al., 2011). It was additionally seen that the dielectric constant shows enhanced values at low frequency whereas it decreases as the value of the frequency is increased. The grain boundaries play a critical role in the decline and rise of the dielectric constant. It was additionally detected that at lower frequencies, the grain boundaries are more efficient (Koops, 1951). The Maxwell–Wagner model in light of Koop's theory can clarify this phenomenon (Wagner, 1913; Maxwell, 1873). The model pronounced that the high-conducting grains can be isolated by the inhomogeneous conducting grain boundaries existing in the dielectric structure of ferrites (Dar et al., 2010). In a dielectric medium, the grains are highly conducting, but the grain boundaries are poorly conducting therefore in low-frequency regions where the effect of the grain is dominant. In the region of high-frequency polarization becomes much slower than the applied ac field due to the charge carriers. At nearly 1.85 GHz, the value of the dielectric constant is the lowest, but few peaks have been observed around 2–2.5 GHz. These relaxation peaks are created due to a Debye- type relaxation. These peaks also appeared when the hopping frequency of the Fe⁺² and Fe⁺³ ions gets to be equivalent to the frequency of the applied ac field (Ali et al., 2014). The values of dielectric constant are given in Table 5.

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Fig. 5

The effect of frequency on the dielectric constant of pure PVA and ZnFe₂O₄–PVA, Zn_0.80Co_0.1Ni_0.1Fe₂O₄ –PVA nanocomposites.

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Table 5 Dielectric constant values of Ferrite and polymer nanocomposites.

Composition	Dielectric constant
	1 MHz	1.5 GHz	2.5 GHz
PVA	6.65601	6.45843	4.50448
Zn0.80Co0.1Ni0.1Fe2O4–PVA(0.2 g-1.5 g)	3.36716	3.27514	2.55884
Zn0.80Co0.1Ni0.1Fe2O4–PVA(0.5 g-1.5 g)	3.24824	3.20127	2.41154
ZnFe2O4–PVA(0.2 g-1.5 g)	3.11776	3.04271	2.39593
ZnFe2O4–PVA(0.5 g-1.5 g)	2.90202	2.86981	2.41198

3.4.2

3.4.2 Dielectric loss

The dielectric loss as a function of frequency extending from 0.0 MHz to 3 GHz for the pure PVA and its composites is the ZnFe₂O₄–PVA and Zn_0.80Co_0.1Ni_0.1Fe₂O₄–PVA, plotted in Fig. 6. The dielectric loss declines sharply as frequency increases, at the point when the dielectric nature is associated with the conductivity nature. However, from Fig. 5, it can be observed that a greater dielectric loss is detected for Zn_0.80Co_0.1Ni_0.1Fe₂O₄–PVA (0.5–1.5 g) nanocomposites which might be due to the high surface energy, the surface domain depolarization domain wall effect, and micromechanical stress. Consequently, orderliness increases due to the interfacial interactions among the mixture components and ZnFe₂O₄ & Zn_0.80Co_0.1Ni_0.1Fe₂O₄ leads to maximum space charge polarization (Jiang et al., 2000). The dielectric function relies on the conductivity and permittivity of two layers clarified by the Maxwell Wagner two-layered model. Static dielectric permittivity is given as Eq. (2)

(2)

$${\epsilon }_s={\complement }_{\mathit{gb}}/{\complement }_o$$

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Fig. 6

The effect of frequency on the dielectric Loss of pure PVA and ZnFe₂O₄–PVA, Zn_0.80Co_0.1Ni_0.1Fe₂O₄–PVA nanocomposites.

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This equation illustrates that the dielectric constant, for the most part of the composites, depends on the grain boundary capacitance. Notably, the grain conductivity increases with the increase in ZnFe₂O₄, Zn_0.80Co_0.1Ni_0.1Fe₂O₄ due to a larger number of atoms present in the grains’ boundary.

The pure PVA low conductivity and dielectric loss behaviour might be because of the amorphous quality of the surface. The ligand to metal charge transfer reaction between the polymer chain (PVA) and ZnFe₂O₄, Zn_0.80Co_0.1Ni_0.1Fe₂O₄ leads to good conductivity and dielectric behaviour. Therefore, the orders and packing density increase between the PVA and ZnFe₂O₄, Zn_0.80Co_0.1Ni_0.1Fe₂O₄ leading towards extreme space charge polymerization, which contributes to the maximum dielectric behaviour (Huang et al., 2004). Dielectric Loss values of Ferrite and PVA nanocomposites as in Table 6.

Table 6 Dielectric Loss values of Ferrite and polymer nanocomposites.

Composition	Dielectric loss
	1 MHz	1.5 GHz	2.5 GHz
PVA	0.30682	0.20454	1.13996
ZnFe2O4–PVA(0.5 g-1.5 g)	0.17486	0.11093	0.45199
Zn0.80Co0.1Ni0.1Fe2O4–PVA(0.2 g-1.5 g)	0.16977	0.14389	0.39704
Zn0.80Co0.1Ni0.1Fe2O4–PVA(0.5 g-1.5 g)	0.1051	0.06415	0.30614

3.4.3

3.4.3 Dielectric loss tangent

Fig. 7 represents the dielectric loss tangent of the examined specimen of PVA and its nanocomposites. The dielectric loss and permittivity decrease at the extraordinary frequencies assigned to the moderate movement of the charge carriers at greater frequencies. The existence of oxygen ion vacancies is more prominent at relatively low frequencies (Bhowmik and Muthuselvam, 2013). The electrons follow the field when the frequency of the external ac field gets to be lower than the jumping frequency of the electrons between the Fe⁺² and Fe⁺³ ions. As a result, the loss becomes extreme. Conversely, when the frequency of the applied ac field becomes higher than the jumping frequency, the electrons do not pursue the field and the loss is reduced. At a low-frequency field, the value of tan δ is greater and has to decrease with increasing frequency, as proven by the Koops phenomenological theory. Consequently, the vitality loss is higher at lower frequencies, yet it becomes smaller in high-frequency regions (Asif Iqbal et al., 2014). When the polarization is much slower, the applied ac field decreased. If tan δ is less then it reveals that the investigated samples are structurally homogenous (Rani et al., 2013). Dielectric Tangent Loss values of Ferrite and PVA nanocomposites as given in Table 7.

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Fig. 7

The effect of frequency on the dielectric Tangent Loss of pure PVA and ZnFe₂O₄–PVA, Zn_0.80Co_0.1Ni_0.1Fe₂O₄–PVA nanocomposites.

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Table 7 Dielectric Tangent Loss values of Ferrite and polymer nanocomposites.

Composition	Tangent Loss
	1 MHz	1.5 GHz	2.5 GHz
PVA	0.0461	0.03167	0.25307
ZnFe2O4 –PVA(0.5 g-1.5 g)	0.05383	0.03465	0.18743
Zn0.80Co0.1Ni0.1Fe2O4 –PVA(0.2 g-1.5 g)	0.05445	0.04729	0.16572
Zn0.80Co0.1Ni0.1Fe2O4 –PVA(0.5 g-1.5 g)	0.03622	0.02235	0.12692

3.5 Microstructure analysis using TEM

The TEM images of the Zn_0.80Co_0.1Ni_0.1Fe₂O₄ and ZnFe₂O₄ nanoparticles are shown in Figs. 8a and 8b, and the TEM images of the PVA nanocomposites prepared from the ZnFe₂O₄ and Zn_0.80Co_0.1Ni_0.1Fe₂O₄ nanoparticles are shown in Figs. 8c and 8d. The images illustrate the small size of the nanoparticles of ZnFe₂O₄ and Zn_0.80Co_0.1Ni_0.1Fe₂O₄ in the polymer. The size of ZnFe₂O₄ is much smaller than Zn_0.80Co_0.1Ni_0.1Fe₂O₄ which can be attributed to the effect of ultrasonic irradiation and is in accord with the size obtained from the XRD pattern. Ultrasonic irradiation is a facile way to yield uniformly dispersed nanoparticles in the polymer matrix. In a liquid reaction medium, the bubbles are produced by ultrasound waves which can gather energy. In addition, while developing to a specific size they quickly discharge this intense energy. The discharged energy is enough to make the nanocomposite fully spread in the polymeric medium. Ferrite nanoparticles are distinguished as homogeneously encapsulated by the PVA, confirming the enhanced limit of the enclosure of the nanocrystalline ferrites into the polymer medium. The asymmetrical morphology of the nanocomposites with a unique shape was formed, as shown in Figs. 8a–8d.

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Fig. 8a

HRTEM images of samples Zn_0.80Co_0.1Ni_0.1Fe₂O₄ (x200k Zoom-1 HR-1 100.0 kV, 20 nm).

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Fig. 8b

HRTEM images of samples ZnFe₂O₄ nanoparticles (x300k Zoom-1 HR-1 100.0 kV, 20 nm).

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Fig. 8c

HRTEM images of samples ZnFe₂O₄–PVA nanocomposites with ratio 0.2 g: 1.5 g (×30.0k Zoom-1 HR-1 100.0 kV, 200 nm).

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Fig. 8d

HRTEM images of samples Zn_0.80Co_0.1Ni_0.1Fe₂O₄–PVA nanocomposites with ratio 0.5 g: 1.5 g (×10.0k Zoom-1 HR-1 100.0 kV, 500 nm).

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