Introduction

Nanocomposite magnetic materials are of great technological importance due to their distinctive chemical and physical properties [1–4]. The ferrite material is a widely studied class of magnetic materials with remarkable magnetic and electric properties having many of applications as components of electronic devices, magnetic fluids, medical diagnostics, and humidity sensors [5–9]. Zinc oxide is an important direct and wide bandgap semiconductor material (3.37 eV) with unique optical, acoustic, and electronic properties. Zinc oxide is well known with its high transparency and it has been applied in electronic and optoelectronic devices. Zinc oxide is non-toxic, possessing a high luminous transmittance, good electrical properties, and excellent piezoelectric properties. ZnO have been prepared by various techniques, among which, sol–gel technology is of perspective and low cost [10–21]. Ferrites, including zinc ferrite, are among the most significant magnetic materials due to vital application in information storage, electronic devices, magnetic resonance imaging, and drug delivery technology [1, 2, 22–24]. Synthesis of composite materials with fine nanostructures and excellent optical and magnetic properties is still a challenging topic [25–27]. The aim of this paper is to report preparation of a series of zinc ferrite nanocomposites and study the effect of thermal treatment conditions on the formation of zinc ferrite by sol–gel method.

In this research, we report the preparation of zinc ferrite, ZnFe2O4 nanocomposite by a simple sol–gel method. Thermal and spectral analyses were used for materials characterization. The effect of the zinc and iron precursor stoichiometric ratio on thermal decomposition process was studied by differential thermal gravimetry (DTG) and thermogravimetry (TG). The phase, morphology, particle diameter of samples, and optical properties were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), fourier transform infrared (FTIR), and UV–Vis diffuse reflectance (DRS) spectroscopy.

Experimental

Materials

Commercial compounds (Sigma-Aldrich, Merck), iron(III) nitrate, Fe(NO3)3·9H2O (98 %), zinc acetate dihydrate Zn(C2H3O2)2·2H2O (99.0 %), isopropyl alcohol (IP), monoethanolamine (MEA) were used as starting materials for synthesis of zinc ferrite nanocomposite.

Preparation of zinc sol and pure zinc oxide nanoparticle

Zinc acetate monohydrate, Zn(C2H3O2)2·H2O, 3.1 g as precursor was dissolved in the mixture of isopropyl alcohol, 15 mL as a solvent, and monoethanolamine, 0.86 mL as a complexing agent while stirring at 60 °C for 1 h and aged for 2 days to achieve a transparent zinc sol. The sol was dried at 110 °C. The powder was thermally treated by a gradual increase of temperature up to 300 °C. The powder was heat treated in air at 350, 550 °C for 4 h.

Preparation of iron sol and pure iron oxide nanoparticle

Iron nitrate nonahydrate, Fe(NO3)3·9H2O 2.45 g as precursor was dissolved in the mixture of isopropyl alcohol, 15 mL as a solvent, and monoethanolamine, 0.86 mL as a complexing agent while stirring at 60 °C for 1 h and aged for 2 days to achieve a transparent iron sol. The sol was dried at 110 °C and the powder was thermally treated by a gradual increase of temperature up to 300 °C. The powder was heat treated in air at 350, 550 °C for 4 h.

Preparation of zinc ferrite sol and ZnFe2O4 nanocomposite

Iron nitrate nonahydrate, Fe(NO3)3·9H2O, 2.45 g powder added into zinc sol (zinc acetate monohydrate, Zn(C2H3O2)2·H2O, 3.1 g as precursor was dissolved in the mixture of isopropyl alcohol, 15 mL as a solvent, and monoethanolamine, 0.86 mL) with vigorous stirring to obtain a uniform sol. The sol was aged for 2 h at ambient temperature in a closed vessel. The mixtures of zinc sol and iron sol with molar ratios, 1:1, 1:2, 1.5:1, and 2:1 were prepared by mixing the solutions and drying in vacuum at 110 °C. The produced powder was thermally treated by a gradual increase of temperature up to 300 °C. The produced solids were thermally treated in air at 350, 450, and 550 °C for 4 h.

Characterization of nanocomposite

The XRD patterns of pure zinc oxide nanoparticle, pure iron oxide nanoparticle, and zinc ferrite nanocomposite were obtained using a Bruker D8 advance X-ray diffractometer using the Cu Kα (λ = 1.5406 Å) radiation with a scanning speed of 1° per min, 35 kV and 30 mA, the scanning in 2θ was from 2 to 70°. The thermoanalytical measurements (TG-DTG) study for the thermal decomposition of precursors were carried out using a Mettler TA4000 system from 20 to 700 °C at a heating rate of 5 °C min−1. The morphologies and particle sizes of pure zinc oxide nanoparticle, pure iron oxide nanoparticle, and zinc ferrite nanocomposite were observed using FESEM (Hitachi, model S-4160) and Philips XL-30 scanning electron microscopy (SEM). UV-DRS spectra were recorded on a V-670, JASCO spectrophotometer. FT-IR absorption spectra of selected samples before and after heat treatment were obtained using KBr disks on a FT-IR 6300 in the region 4000–400 cm−1.

Results and discussion

Figure 1 shows the TG and DTG curves for the pyrolysis of zinc acetate monohydrate using as precursor of zinc ferrite. The TG curve shows two steps for the thermal decomposition. The first step corresponds to the evolution of water of crystallization, reaching a constant mass loss of 17.3 % at 78 °C. The second step corresponds to the complete decomposition of zinc acetate monohydrate to ZnO reaching a constant mass loss of 63.3 % at 315 °C. The X-ray diffractograms of pure zinc oxide thermal decomposed at 550 °C (Fig. 2) shows that a crystalline ZnO phase (hexagonal system) [28]. Crystallite size of the pure ZnO phase was calculated by Scherrer’s equation and they were within a narrow range of 17–23 nm which is comparable to SEM results (Fig. 3). The pattern intensities increased on increasing the calcination temperature from 350 to 550 °C. It can be seen that the crystalline zinc oxide phase may be formed at 550 °C (Fig. 2), as a result of a solid-state reaction between the corresponding oxides produced from the thermal decomposition of the starting materials zinc acetate monohydrate [29]:

$$ {\text{Zn}}\left( {{\text{C}}_{ 2} {\text{H}}_{ 3} {\text{O}}_{ 2} } \right)_{ 2} \cdot {\text{ H}}_{ 2} {\text{O}}\mathop{\longrightarrow}\limits^{{{ 7 8 { }^\circ {\text{C}}}}}\,{\text{Zn}}\left( {{\text{C}}_{ 2} {\text{H}}_{ 3} {\text{O}}_{ 2} } \right)_{ 2} + {\text{ H}}_{ 2} {\text{O}} $$
$$ {\text{Zn}}\left( {{\text{C}}_{ 2} {\text{H}}_{ 3} {\text{O}}_{ 2} } \right)_{ 2} \mathop{\longrightarrow}\limits{{}}{\text{Zn}}\left( {{\text{C}}_{ 2} {\text{H}}_{ 3} {\text{O}}_{ 2} } \right)\left( {\text{OH}} \right) \, + {\text{ CH}}_{ 3} {\text{COOH}} $$
$$ {\text{Zn}}\left( {{\text{C}}_{ 2} {\text{H}}_{ 3} {\text{O}}_{ 2} } \right)\left( {\text{OH}} \right)\mathop{\longrightarrow}\limits{{}}{\text{Zn}}\left( {\text{OH}} \right)_{ 2} + {\text{ CH}}_{ 3} {\text{COOH}} $$
$$ {\text{Zn}}\left( {\text{OH}} \right)_{ 2} \mathop{\longrightarrow}\limits^{{{ 3 1 5 { }^\circ {\text{C}}}}}\,{\text{ZnO}} $$
Fig. 1
figure 1

Thermal curves of zinc acetate monohydrate as a precursor in sol–gel preparation of zinc ferrite nanoparticles

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Fig. 2
figure 2

XRD pattern of pure zinc oxide nanoparticle prepared by sol–gel method

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Fig. 3
figure 3

Scanning electron microscopy micrograph of pure zinc oxide nanoparticle prepared by sol–gel method

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The TG–DTG curves of iron nitrate nonahydrate, Fe(NO3)3·9H2O is shown in Fig. 4. It is shown that total mass loss of 83.1 wt% was determined up to 390 °C. The first DTG peak indicates the removal of physisorbed water and water of crystallization. The second peak refers to the decomposition of ferric hydroxide and formation of ferric oxide. Figure 5 shows the FT-IR spectrum of the zinc ferrite nanocomposite particles annealed at 450 °C. The broad band centered at 3430 cm−1 is due to the –OH groups. The peak at 735 cm−1 is a characteristic absorption of Fe–O. The peak at 548 cm−1 is a characteristic absorption of ZnO and the IR data are in consistence with the reported values [7, 8, 22, 30–32]. Figure 6 shows the XRD patterns of the zinc ferrite sample calcined at 450 °C. As shown in Fig. 2, characteristic peaks of ZnFe2O4 are identified in the patterns of the sample which calcined at 350 °C. Figure 2 shows the X-ray patterns of the samples after calcination of the product obtained from 1 mmol of zinc acetate monohydrate, Zn(C2H3O2)2·H2O and 2 mmol of iron nitrate nonahydrate, Fe(NO3)3·9H2O. The peaks at 29.95, 35.22, 36.73, 42.89, 56.53, and 62.34° can be readily ascribed to the characteristic peaks of the cubic phase of ZnFe2O4 (spinel ferrite) [33–41]. Reaction in the formation of ZnFe2O4 during the calcination process is presented as follows:

$$ {\text{Zn}}\left( {\text{OH}} \right)_{ 2} + {\text{ 2Fe}}\left( {\text{OH}} \right)_{ 3} \mathop{\longrightarrow}\limits{{}}{\text{ZnFe}}_{ 2} {\text{O}}_{ 4} + {\text{ 4H}}_{ 2} {\text{O}} $$
Fig. 4
figure 4

Thermal curves of the Iron nitrate nonahydrate as a precursor in sol–gel preparation of zinc ferrite nanoparticles

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Fig. 5
figure 5

FT-IR spectrum of the zinc ferrite nanoparticle prepared by sol–gel process using zinc acetate monohydrate and iron nitrate nonahydrate as precursors annealed at a 550 °C, b 450 °C, c 350 °C

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Fig. 6
figure 6

XRD pattern zinc ferrite nanoparticle prepared by sol–gel process using zinc acetate monohydrate and iron nitrate nonahydrate as precursors

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FESEM micrograph (Fig. 7) of the zinc ferrite nanocomposite indicate that particles were uniformly spherical nanograins of ZnFe2O4 spinel [33].

Fig. 7
figure 7

FESEM micrograph of zinc ferrite nanoparticle prepared by sol–gel process using zinc acetate monohydrate and iron nitrate nonahydrate as precursors

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