Characterization of microstructural and optical CoFe₂O₄/SiO₂ ferrite nanocomposite for photodegradation of methylene blue

1.   Introduction

Cobalt ferrite (CoFe₂O₄) is a ferromagnetic oxide that has attracted considerable attention due to its excellent magnetic properties, such as high coercivity, modest saturation magnetization, high Curie temperature at 520 ℃, large magnet crystalline anisotropy (~10⁶ erg/cm³), high mechanical hardness and remarkable chemical stability ^[1,2,3]. CoFe₂O₄ crystallizes into an inverse spinel structure, where one-half of the Fe³⁺ ions occupy the tetrahedral sites, while the other half are located in the octahedral sites together with Co²⁺ ions ^[1]. The magnetic CoFe₂O₄ nanoparticles retain spherical shapes and size in the 6–35 nm range. The saturation magnetization of the premeditated nanocomposites increases with the increase of cobalt ferrite concentration in the silica matrix ^[2].

The CoFe₂O₄ properties depend on the preparation methods, raw materials, annealing time and temperature ^[1]. As the synthesis method influences the CoFe₂O₄ morphological and structural features, various synthesis methods, i.e., sol–gel, co-precipitation, microemulsion, hydrothermal, combustion, sonochemical, solid-state, complexation, microwave sintering, mechanical alloying, spray pyrolysis, reverse micelle, forced hydrolysis in polyol, pulsed laser deposition and ultrasonic cavitation have been used recently ^[2,3]. Among all, the co-precipitation method is often exercise in the preparation of nanoparticles dispersed in different matrix due to its high yield and simplicity. Nevertheless, the method is difficult to control particles size and shape ^[4].

CoFe₂O₄ in powder or in dispersed fluid form, has received increasing attention due to its catalytic, electrical and magnetic properties that make it appropriate for a wide range of applications (transformer cores, recording heads, antenna rods, loading coils, memory, microwave devices, catalysts, ferrofluids, magnetic refrigeration, magnetic sensors, drug delivery, gas detectors, sensors, solar energy conversion, stress and biomedical sensors, cellular therapy, tissue repair, etc.). As a ferromagnetic oxide with high magnetic anisotropy and high saturation magnetization, CoFe₂O₄ nanoparticles were used for numerous technological applications, such as storage media ^[2,5], magnetic resonance imaging, hyperthermia treatment, drug delivery ^[6,7], permanent magnets ^[8], microwave devices, magnetic refrigeration and solar energy conversion ^[9]. Most of this application requires specific electrical, optical, and magnetic properties mainly determined by their structure and morphology ^[2].

Recently, much attention has been absorbed on ferrite nanocomposites due to their capability to improve optical, electronic, and magnetic properties compared to those of single nanomaterial component ^[10]. Nanoparticles dispersion in hybrid matrices is an effective method to reduce particle agglomeration, less energy demand, and environmentally friendly use of nontoxic and biocompatible reagents which leads materials to possess superior properties ^[1]. Several kinds of the matrix such as resin, polymer or silica have been used to stabilize and reduce nanoparticles agglomeration. The silica (SiO₂) matrix provides a biocompatible, hydrophilic, nontoxic surface, and may enhance the formation of a single phase and the magnetic properties of nanocomposites ^[11]. The SiO₂ matrix also acts as a buffer and control the particles size ^[1].

Ferrite is known for their application in the magnetic and electronic fields. However, there are limited studies concerning its application as photocatalysts. The Spinel Ferrite (MFe₂O₄) ability to absorb photons in visible light has attracted considerable attention due to its pros, i.e., low cost and photo-corrosion resistance. The selection of the materials based on their redox activity, especially the mobility of high oxygen in their crystalline lattice. The mobility renders the tendency of materials to generate vacancies of this atom when synthesized under diminishing atmosphere. These vacancies improved their photocatalytic activity ^[11,12].

Moreover, investigation of microstructural and optical properties of nanocomposites also conducted as a foremost concern defining the photodegradation phenomena. Moreover, this study predominantly focuses on reporting the photodegradation activity of CoFe₂O₄/SiO₂ for 20 ppm methylene blue dye with a driven of UV light radiation. The result showed that the photodegradation activity of CoFe₂O₄/SiO₂ is higher than bare CoFe₂O₄. The photodegradation activity of CoFe₂O₄/SiO₂ for methylene blue without UV light radiation also included in the investigation.

2.   Materials and method

2.1.   Preparation and characterization of CoFe₂O₄/SiO₂

The Co-Precipitation method also implemented in the preparation of nanoparticle ferrite CoFe₂O₄. While the pro analyst grade Cobalt Nitrate Co(NO₃)₃·6H₂O and Ferrite Nitrate Fe(NO₃)₃·9H₂O procured from E. Merck. Cobalt Nitrate Co(NO₃)₃·6H₂O and Ferrite Nitrate Fe(NO₃)₃·9H₂O was mixed and stirred at 90 ℃ for 60 minutes (Table 1). The mixture then calcined at 95 ℃ for 4 hours. In this research, the preparation of nanoparticles utilizes the co-precipitation method as reported by ^[5,6,13], while, the encapsulation process employs the Strӧber method as reported by ^[9]. The microstructural of nanocomposites were characterized using X-Ray Diffraction (XRD), and their optical properties were investigated by using diffuse reflectance UV-visible spectrophotometer.

Table 1.  Characteristics of the synthesized precursors.

Reagen	Quantity (mol)
Co(NO3)3·6H2O	0.02
Fe(NO3)3·9H2O	0.04
SiO2	0.02

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2.2.   Photodegradation testing of CoFe₂O₄/SiO₂

The CoFe₂O₄ and CoFe₂O₄/SiO₂ nanocomposite (66.7% b/v) dissolved in an Erlenmeyer flask and stirred at room temperature with UV radiation for 120 minutes followed by measurement of absorbance level at wavelength 650 nm for 30 minutes. CoFe₂O₄ and CoFe₂O₄/SiO₂ nanocomposites were used to degradation methylene blue (20 ppm).

3.   Result and discussion

3.1.   XRD-analysis

The XRD pattern, as shown in Figure 1, depicts the scattering form of the plane of Miller indices (hkl): (311), (400), (511), and (533), confirming the formation of well-crystallized single phase CoFe₂O₄ (JCPDS card 22-1086) ^[8]. The nanoparticle has a spinel crystal structure with group space of Fd3m. The highest intensity of the diffraction peak was taken place in (311) with a diffraction angle 2θ of 36.23°. The calculated particles size of CoFe₂O₄/SiO₂ derived from Scherrer's equation is 29.7 ± 0.08 nm which is larger than CoFe₂O₄ of 26.8 ± 0.08 nm (Table 2).

Figure 1.  The X-Ray Diffraction Pattern for the (a) CoFe₂O₄; (b) CoFe₂O₄/SiO₂.

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Table 2.  The X-Ray Diffraction Pattern for the (a) CoFe₂O₄; (b) CoFe₂O₄/SiO₂.

Samples	2θ (°)	λ (Å)	k	hkl	FWHM (rad)	a (Å)	t (nm)
CoFe2O4	36.23	1.5406	0.9	311	0.0069	8.2	21.2
	43.17	1.5406	0.9	400	0.0071	8.4	21.1
	59.16	1.5406	0.9	511	0.0052	8.1	30.8
	69.00	1.5406	0.9	533	0.0049	8.9	34.2
CoFe2O4/SiO2	36.08	1.5406	0.9	311	0.0054	8.2	27.0
	44.87	1.5406	0.9	400	0.0053	8.1	28.1
	60.59	1.5406	0.9	511	0.0048	7.9	33.4
	70.51	1.5406	0.9	533	0.0056	8.8	30.4

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3.2.   Optical properties

The optical properties of samples represented by using the diffuse reflectance UV-visible spectra. The calculation of the optical band gap of both sample, CoFe₂O₄ and CoFe₂O₄/SiO₂, show a different result. Based on the absorbance spectra, the highest elevation of the absorbance takes place between wavelengths of 250 nm and 300 nm (Figure 2a). Physically, this phenomena shows that energy needed by an electron to be excited from the valence band to conduction band ranges between 4.1 eV to 4.70 eV (Figure 2b). The optical gap energy resulted in the research is higher than result discovered by ^[9]. The excitation type of inter-band on this state could not be determined explicitly into direct-gap or indirect-gap, possibly triggered by the imperfect microstructure and lattice formation of crystallite ^[6]. Presumably, the crystal defect can be affected the number of the optical gap energy and quantum phenomena.

Figure 2.  (a) Absorbance spectra vs. photon wavelength; (b) the Touch's Plot.

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3.3.   Photodegradation activity

Photodegradation activity of samples was carried out with a variation of irradiation time as shown in Figure 3a, b. The result of the photodegradation activity of methylene blue shows that CoFe₂O₄/SiO₂ nanocomposites are higher than CoFe₂O₄. The rate of photodegradation of CoFe₂O₄/SiO₂ nanocomposites reaches 80.6%.

Figure 3.  (a) Photodegradation rate of CoFe₂O₄ and CoFe₂O₄/SiO₂ nanocomposites under and without UV-light radiation; (b) Photodegradation kinetic of samples.

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As shown in Figure 3a, b, the presence of SiO₂ can amplified photodegradation activity from CoFe₂O₄ nanoparticles. Even though, CoFe₂O₄ and CoFe₂O₄/SiO₂ nanoparticles considered as prospective nano-photocatalyst under UV radiation. Nevertheless, further studies yet require to improve the microstructural properties and optical gap energy of CoFe₂O₄/SiO₂ nanocomposites before applying as a photocatalyst.

4.   Conclusion

The CoFe₂O₄ and CoFe₂O₄/SiO₂ nanocomposites have been synthesized using the co-precipitation method successfully. The size of crystallite nanoparticle is 29.4 ± 0.2 nm for CoFe₂O₄/SiO₂ and 26.8 ± 0.2 nm for CoFe₂O₄. Nanocomposites CoFe₂O₄/SiO₂ have smaller optical band gap than CoFe₂O₄ due to smaller surface defect and larger particles size. The result of the photodegradation activity of methylene blue showed that CoFe₂O₄/SiO₂ nanocomposites are higher than CoFe₂O₄. The encapsulation of the SiO₂ matrix on the surface CoFe₂O₄ nanoparticles enhances photodegradation activity. Based on the result, nanocomposites CoFe₂O₄/SiO₂ and CoFe₂O₄ have been potential as nano-photocatalyst and nano-adsorbent for organic pollutants.

Conflict of interest

There is no conflict to declare.