In recent years, nanomaterials have gained research attention because of their interesting properties and applications (Gawande et al., 2014; Jafari et al., 2014). Magnetic materials have been studied extensively due to their potential use in catalytic, biomedical and environmental remediation (Moumen and Pileni, 1996; Song and Zhang, 2004; Xiao et al., 2007). Ferrite materials are a group of magnetic materials that can be divided into three classes: ferrite spinel, ferrite garnet and hexagonal ferrite (Masoumi et al., 2016). Polycrystalline hexagonal ferrites and their derivatives have attracted considerable attention in recent years. They demonstrate very high uniaxial magnetic anisotropy, making them an ideal permanent magnetic material, which can compete technically with metallic permanent magnets for some applications (Ebrahimi et al., 2017). The M-type hexa ferrites are denoted as PbFe12O19 and are widely studied because of their properties, such as large magneto crystalline anisotropy, high saturation magnetization and coercivity, recording media, permanent magnets and components in microwave and high frequency devices; hence, they have garnered research attention due to their potential applications in various fields (Thompson and Evans, 1993; Yang et al., 2007; Chaudhury et al., 2008). However, the growth of the industry worldwide has tremendously increased the generation and accumulation of waste products. This has caused environmental problems that, in turn, have become a major concern for the public.
Meanwhile, photocatalysis has gained substantial attention due to its potential environmental applications towards water treatment, such as the purification of polluted air and wastewater streams, production of self-cleaning materials and the creation of new chemicals (Fujishima and Honda, 1972; Hoffmann et al., 1995; Peral et al., 1997). The visible light photocatalysis of wastewater has also become a popular research topic due to its flexibility and cost-effective breakdown of harmful organic molecules at ambient conditions. During the photocatalytic process, when the photons with sufficient energy collide and are absorbed on the catalyst, this leads to the excitation of electrons from the valence band to the conduction band, thus producing electron-hole pairs. The photo-generated holes will form hydroxyl radicals when they react with the OH−/water over the surface of the catalyst. These active radicals induce the degradation of the organic compounds in the air and water streams (Matthews, 1990; Linsebigler et al., 1995). Semiconductor materials, which use ultra violet (UV) irradiation to decompose organic pollutants and toxic dyes into non-toxic products by photo-degradation technique, have attracted considerable attention in the last few years (Hedayati et al., 2016). The photocatalytic properties of a semiconductor depend on the position of the energetic levels, the lifetime of the photogenerated electrons and holes, the light absorption coefficient and the nature of the interface (Augugliaro et al., 2010). The TiO2 is a semiconductor that, in the past two decades, has been used in a variety of fields like sensors, including the production of sensitive solar cells and so on (Clark and Sutin, 1977; Hamnett et al., 1979; Cosnier et al., 1997). TiO2 has three crystalline phases, namely, anatase, rutile and brookite, among which the rutile is the most thermodynamically stable (Li et al., 2004). These three crystalline phases have the same fundamental structural octahedral units with different arrangements (Liu et al., 2005). The anatase and brookite phases are semi-stable and converted to the rutile phase when heated (Zhang et al., 2000). The band gap of the TiO2 is commonly a range of 3.0–3.2 eV and the wavelength is about 400 nm. This means that UV light radiation with a wavelength of less than 400 nm can stimulate the electrons and initiate a photo-reaction (Mutzhas et al., 1981).
There are several methods that can be used to carry out the synthesis and growth of TiO2 structures with controllable demands. The sol-gel method is one of the best candidates for TiO2 synthesis because it provides high purity, homogeneity and low-temperature processing (Liu et al., 2012; Xin et al., 2014; Ghanbari et al., 2016).
The present study describes a synthesis and characterization of the PbFe12O19 ferrite nanoparticles using the precipitation method. Then, the TiO2 nanoparticles and PbFe12O19-TiO2 nanocomposite was synthesized with the sol-gel method. Finally, the photocatalytic properties and degradation of the azo dyes in the presence of this magnetic nanocomposite under UV irradiation were investigated.
Results and discussion
The structure and composition of the PbFe12O19 nanoparticles were investigated. The results are shown in Figure 1. As can be seen, the XRD pattern of the PbFe12O19 reveals the typical diffraction pattern of pure hexagonal phase (JCPDS No.: 15-0623) with the P63-mmc space group. In Figure 2, the pattern of the TiO2 nanoparticles is indexed as a pure tetragonal phase (JCPDS No.: 04-0477) with I41-amd space group. From this, the crystallite phase of the TiO2 nanoparticle was determined. The composition of the PbFe12O19-TiO2 nanocomposite was also discussed. The formation of two hexagonal phase (JCPDS No.: 15-0623) and pure tetragonal phase (JCPDS No.: 04-0477) is confirmed and shown in Figure 3. As can be seen, the size of the crystals was calculated using the X-ray diffraction (XRD) data obtained from the Debye-Sherrer equation given by
where D is the size of the crystal, β is the width of the observed diffraction peak at its half maximum intensity (FWHM) and θ is the angle relation in degrees. Upon calculation, a crystalline size of 88 nm was obtained.
Scanning electron microscopy (SEM) was employed for the estimation of the morphology and the particle size of the products. The final particle size and morphology depend on the preparation conditions. Figure 4 shows the SEM images of the PbFe12O19 nanoparticles without any surfactant. The SEM images of the PbFe12O19 nanoparticles with capping salicylic acid are presented in Figure 5. As can be seen, the average particle size is approximately 60 nm. Figure 6 presents the SEM images of the PbFe12O19 nanoparticles with capping glucose as the surfactant. This image shows that nanoparticles are agglomerated. Figure 7 presents the SEM images of the PbFe12O19 nanoparticles with capping gelatine. The result confirms the obtained mono-disperse nanorods with an average size of around 70 nm. The SEM images of the PbFe12O19 nanoparticles prepared by starch coating are shown in Figure 8. The images confirm the obtained mono-disperse nanoparticles with an average size of around 60 nm. Using starch as a natural surfactant leads to better nucleation. Figure 9 presents the SEM images of the TiO2 nanoparticles prepared via the sol-gel method. The result confirms the obtained mono-disperse nanoparticles with an average size under 50 nm. Figure 10 illustrates the SEM images of the PbFe12O19/TiO2 nanocomposite. The result confirms the obtained nanocomposite, which is agglomerated and has an average particle size of around 70 nm.
Figure 11 shows the transmission electron microscopy (TEM) images of the PbFe12O19/TiO2 nanocomposite. As can be seen, the particle size is nearly 80 nm. Meanwhile, Figure 12 shows the Fourier transform infrared (FT-IR) spectra of the prepared PbFe12O19 nanoparticles. As can be seen, the absorption band at 578 cm−1 is assigned to the stretching mode of the Fe-O bond and the 920 and 995 cm−1 bands can be attributed to the stretching mode of the Pb-O bond. The spectra exhibit a broad absorption peak around 1400 cm−1, the band corresponding to the stretching mode of C-C. The 642 cm−1 band is assigned to the O–H bending vibration mode due to the adsorption of moisture on the surface of the nanoparticles. Figure 13 shows the FT-IR spectra of the TiO2 nanoparticles calcinated at 500°C. The absorption band at 643cm−1 is assigned to the stretching mode of the Ti-O bonds, which adsorbed on the surface of nanostructures. The FT-IR spectra of the PbFe12O19-TiO2 nanocomposite is shown in Figure 14. The absorption bands at 544 and 716 cm−1 are assigned to the stretching modes of the Fe-O and Ti-O bonds. The spectra exhibit a broad absorption peak at around 934 cm−1 for the stretching mode of the Pb-O bond, and finally 1401 cm−1 is assigned to the stretching mode of the C-C bond.
The magnetic properties of the PbFe12O19 nanoparticles and PbFe12O19-TiO2 nanocomposite were studied by using a vibrating sample magnetometer (VSM). Figure 15 shows the hysteresis loops for the PbFe12O19 nanoparticles. The results indicate that the sample exhibits ferromagnetic property and that a saturation magnetization of around 28 emu/g and coercivity of about 2100 Oe have been achieved. The magnetic property of the PbFe12O19-TiO2 nanocomposite calcined at 500°C is shown in Figure 16. The result indicates that the saturation magnetization of around 16 emu/g and coercivity of about 0.4 Oe have been achieved. Figure 17 shows the magnetization curve of the PbFe12O19-TiO2 nanocomposite calcined at 800°C. The results indicate that the sample exhibits ferromagnetic behaviour with coercivity of about 981 Oe and saturation magnetization of around 14 emu/g.
When the nanocomposite formed and calcined at 500°C, the amount of coercivity decreased. In comparison, when the calcination temperature increased to about 800°C, the amount of coercivity increased and the nanocomposite demonstrated ferromagnetic behavior. The results indicate that the amount of coercivity is increased by increasing the temperature of nanocomposite calcination from 500°C to 800°C.
The prepared nanocomposite has potential applications in improving environmental problems associated with toxic water pollutants. The mechanism of the photodegradation of toxic azo dyes under UV irradiation in the presence of the PbFe12O19-TiO2 nanocomposite as an effective photocatalyst is shown in Figure 18. Azo dyes were exposed to UV light in the presence of this nanocomposite. The changes in the concentration of maximum wavelength dye are illustrated in Figure 19, which shows the UV absorptions of acid black 1, acid violet 7 and acid blue 29. The absorption curves of the solutions for all three acids do not show observable changes for these three conditions: dye solution in the dark, dye solution under illumination and dye solution with the PbFe12O19 nanoparticles under illumination (19-a1, b1, c1). The degradation rates of the azo dyes in both UV illumination for the pure TiO2 nanoparticles (19-a4, b4, c4) and the PbFe12O19-TiO2 nanocomposites (19-a5, b5, c5) are more effective than those under the following conditions: pure TiO2 nanoparticles in the dark (19-a2, b2, c2) and PbFe12O19-TiO2 under illumination (19-a3, b3, c3). This is due to the formation of extra produced electron-hole levels as new traps in the TiO2 nanoparticles by composition with the PbFe12O19 levels. The UV absorption rates of the decomposition of the azo dyes for exposure times of 20, 40 and 60 to the nanocomposite are shown in Figure 20. With increasing UV irradiation time, the dye concentration decreased rapidly and toxic dyes decomposed to carbon dioxide, water and other less or non-toxic residuals.
In conclusion, the synthesis, characterization and photocatalytic activity of the PbFe12O19-TiO2 nanocomposites were reported. By investigating the effects of different surfactants on the lead hexa ferrites in the SEM images, we found that starch – as a natural surfactant – causes the growth and formation of mono-disperse nanoparticles and induces uniformity than other surfactants. The TEM image of the nanocomposite obtained and the grain size are under 100 nm. This confirms the crystallite size obtained the from Debye-Scherer equation. VSM results confirmed that, if temperature calcination is increased to around 800°C, the ferromagnetic behaviour becomes highly apparent. The photocatalytic behavior of thte PbFe12O19-TiO2 nanocomposite was investigated for the composition of various azo dyes under UV irradiation. The results showed that the sol-gel method is a suitable method for the preparation of the PbFe12O19-TiO2 nanocomposite, which is an ideal material for use in photocatalytic applications to remove toxic dyes.
Materials and methods
The materials used in the study, namely, Pb(NO3)2 4H2O, Fe(NO3)3 9H2O, NaOH, salicylic acid, glucose, starch, gelatine, tetra isopropyl ortho titanate (TTIP), nitric acid and methanol, were all purchased from Merck or Aldrich (Berlin, Germany). All the chemicals were used as received without further purification. A multi-wave ultrasonic generator (Band line MS 73), with a converter/transducer and titanium oscillator, operating at 20 kHz with a maximum power output of 150 W, was used for the ultrasonic irradiation. Room temperature magnetic properties were investigated using the VSM device (Meghnatis Kavir Kashan Company, Iran) in an applied magnetic field sweeping between ± 10 000 Oe. The XRD patterns were recorded by a Philips X-ray diffractometer using Ni-filtered CuKα radiation (Amsterdam, Netherlands). The SEM images were obtained using a LEO instrument model 1455VP (Cambridge, UK). Prior to taking the images, the samples were coated by a very thin layer of Pt (using a BAL-TEC SCD 005 sputter coater, CA, USA) to make the sample surface a conductor and to obtain better contrast.
Synthesis of the PbFe12O19 nanoparticles
First, 1.46 g of Fe(NO3)3 9H2O were dissolved in 200 mL of deionized water, to which 0.2 g of Pb(NO3)2 4H2O was added after 10 min. After the dissolution of materials, 10 mL of NaOH (1M) was slowly added to the solution until a pH of 10 was achieved. A brown precipitate was then centrifuged and rinsed with distilled water. Next, the obtained precipitate was dried at 80°C and finally calcined at 800°C, thus revealing the magnetic feature of the sample. Subsequently, various organic surfactants were used to synthesize the lead ferrite and investigate its effect on the crystalline morphology.
Synthesis of the TiO2 nanoparticles
The TiO2 nanoparticles were prepared by the sol-gel method with 1.25 g of tetra isopropyl ortho titanate (TTIP), as the raw material, mixed with 25 mL of methanol in a dry atmosphere and held for 10 min. After that, 5 mL of distilled water was added. In this level, the white sol was formed. Sol was stirred for 20 min, after which 2 mL of HNO3 was slowly added until a pH level of 2 was achieved. A yellowish transparent gel was formed after 90 min stirring. In the final step, the obtained powder was calcined at 500°C for 2 h.
Synthesis of the PbFe12O19-TiO2 nanocomposite
About 0.21 g of PbFe12O19 (capping starch) was dispersed in 25 mL of methanol and 1.25 g TTIP. After 10 min, the solution was stirred and 5 mL of distilled water was added to the mixture. After 20 min, we slowly added 2 mL of HNO3 until reaching a pH level of 2. In this level, the solution was stirred for 90 min until the white gel was formed. The obtained gel was dried at 80°C for 2 h. In the next step, the obtained powder was calcined at 500°C for 2 h. In the final step, because the powder did not find a magnetic state for the first time, it was calcined again to 800°C for 5 h.
Photocatalytic degradation process
The removal of the azo dyes from a solution under UV light was performed in the presence of the PbFe12O19-TiO2 nanocomposite as a photocatalyst sample. In each photocatalysis test, 0.1 g catalyst and 5 mL of the dye solution (10 ppm) were stirred with a magnetic stirrer in darkness under UV-visible light to determine the adsorption of the dye by the catalyst. The solution was irradiated by a 10 W UV lamp that was placed in a quartz pipe in the middle of the reactor. The solution was centrifuged every 20 min and the level of photodegradation was recorded.
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About the article
Published Online: 2018-07-28
Published in Print: 2018-08-28