Magnetite (Fe3O4) and lead sulfide (PbS) nanostructures were synthesized via a simple precipitation method in water by using green capping agents. Then, Fe3O4-PbS nanocomposites were prepared by facile chemical procedure. The effects of concentration, temperature as well as precipitating and capping agents on the morphology and particle size of the magnetic products were investigated. Sugars and carbohydrates were used as green, cost-effective, and safe bio-compatible capping agents. The prepared magnetic products were characterized by X-ray diffraction, scanning electron microscopy, and Fourier transform infrared spectroscopy. Alternating gradient force magnetometer was used to identify the super-paramagnetic property of the samples. The photo-catalytic behaviour of nanocomposites was evaluated using the degradation of azo dyes under ultraviolet light irradiation.
The semiconductor photo-degradation technique, which used ultra violet or visible light to decompose organic pollutants into non-toxic products, has attracted research attention in recent years. In addition, finding non-expensive, efficient, and eco-friendly photocatalysts is necessary for waste water purification. Considering the problem of reducing raw material waste, magnetite (Fe3O4) nanoparticles have raised interest owing to their high super-magnetism, which facilitates easy separation by magnet, and easy integration into recyclable photocatalysts (Zhang et al., 2015; Shen et al., 2016; Zhu et al., 2016). Fe3O4 is the most attractive adsorbent based on its simply magnetic separable advantage compared with conventional separation (Yang et al., 2016). However, Fe3O4 nanoparticles have a strong tendency to agglomerate into larger ones because of inter-particle attraction, such as the Van der Waals and intrinsic magnetic interaction. In addition, Fe3O4 belongs to a structural class of compounds known as the spinels, and is composed of MFe2O4 where M is a divalent cation and Fe2+ is the case of magnetite (Van Blaaderen and Vrij, 1993; Wang et al., 2011; Feng et al., 2016).
Ferrites have emerged as novel materials that have raised technological and scientific attention in various fields. Ferrite materials may be classified into three different classes: spinel, garnet, and hexagonal ferrites. Magnetite is an important magnetic oxide with a spinel structure, and it exhibits unique electric and magnetic properties based on the electron transfer between Fe2+ and Fe3+ in the octahedral sites. Considering their biocompatibility, low toxicity, quantum size effect, surface-boundary effect and adjustable magnetic properties, Fe3O4 have received considerable attention in various areas, such as bio-compatibility, high stability, and easy separation from the reaction. Fe3O4 have varied applications in cancer therapy, drug targeting, enzyme immobilization, magnetic cell separation, magnetic refrigeration, Li-ion batteries, microwave absorption, biosensors, and magneto-responsive transmittance (Van Blaaderen and Vrij, 1993; Wang et al., 2011; Pullar, 2012; Nabiyouni et al., 2014; Ghanbari and Salavati-Niasari, 2015; Kakavandi et al., 2016).
Meanwhile, lead sulphide (PbS) is a metal chalcogenide and semi-conductor with a narrow and direct band-gap of 0.41 eV at room temperature. An excess of Pb results in n-type conductivity in PbS, whereas an excess of S results in p-type conductivity (Zhu et al., 2016). The size dependence of a band gap in semiconductor nanoparticles is well known, and modifying their size facilitates the tunability of their optical properties. Metal sulphides are important materials because of their wide applications in optoelectronics and catalysis. PbS has a small, effective mass of charge carriers and a high value of dielectric constant. For this reason, the quantum confinement effect is observed even for PbS with relatively large particle sizes. Owing to their unique optical properties, PbS-based nanomaterials have various applications in laser diodes, infrared (IR) quantum dot light-emitting diodes (LEDs) and multi-exciton generation solar cell detectors, nonlinear optics, single electron devices, optical switches, as well as telecommunication and biological imaging. As PbS has a large excitonic Bhor radius, its optical absorption and emission can be easily adjusted from near IR to ultra violet region (band gap from 0.3 eV to 5.2 eV) by reducing the dimension of nanocrystals (Sathyamoorthy and Kungumadevi, 2015; Yadav and Jeevanandam, 2015; Chang et al., 2016; Mandal et al., 2016). Such a significant widening of the band gap is associated with the small effective masses of electrons and holes. Tremendous efforts have been exerted on the synthesis of PbS nanostructures, and to this end, fractal or dendrite structures have attracted much attention because of their potential applications. Recently, aqueous phase routes have been employed to obtain hierarchical PbS structures (Liu et al., 2001; Csanády et al., 2006; Ntwaeaborwa et al., 2009; Pourahmad, 2014; Hmar et al., 2016). In the current work, Fe3O4 and PbS nanostructures and magnetite-lead sulfide nanocomposites were synthesized via a simple precipitation method using water as a green solvent. The effects of conventional sugars as green capping agents on the shape and particle size were investigated.
Pb(NO3)2, Fe(NO3)3 9H2O, FeCl2 4H2O, NaOH, KOH poly vinyl pyrrolidone (PVP, MW:25 000), and acetone were purchased from Merck (Darmstadt, Germany), and all the chemicals were used as received without further purification. A multiwave ultrasonic generator (Bandeline MS 73, Berlin, Germany), equipped 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 an alternating gradient force magnetometer (AGFM) device (Meghnatis Kavir Kashan Company, Iran) in an applied magnetic field sweeping between ±10 000 Oe. X-ray diffraction (XRD) patterns were recorded by using a Philips (Amsterdam, Netherlands) X-ray diffractometer using Ni-filtered CuKα radiation. Scanning electron microscopy (SEM) images were obtained using a LEO (Cambridge, UK) instrument (Model 1455VP). Prior to taking 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 conductor and to prevent charge accumulation while obtaining better contrast.
About 0.002 mol of Fe(NO3)3 9H2O, 0.1 g of surfactant (PVP, glucose, gelatine), and 0.001 mol of FeCl2 4H2O were dissolved in 100 mL of water and then stirred for 30 min at room temperature. Then 20 mL of NaOH solution (1 m) was slowly added to the solution until reaching pH of around 10. A black precipitate was centrifuged and rinsed with distilled water. Figure 1 shows the schematic diagram of the experimental setup for the preparation of nanoparticles.
About 0.45 g of Pb(NO3)2 and 0.11 g of capping agents were dissolved in 200 mL of water and then stirred for 40 min at a temperature of around 80°C. Then, 10 mL of NaOH solution (1 m) was slowly added to the solution until reaching pH of around 10. A black precipitate was rinsed with deionized water.
The structure and composition of the Fe3O4 nano-rods were investigated by XRD. Figure 2 shows the XRD pattern of the sample, which includes magnetite nanorods. The pattern reveals the typical diffraction pattern of pure cubic phase (JCPDS No.: 01-1111) with Fd3̅m space group, which is consistent with pure magnetite. The XRD pattern of PbS in Figure 3 shows the typical diffraction pattern of pure cubic phase (JCPDS No.: 78-1057) with Fm3̅m space group, which shows agreement with pure lead sulfide. The crystallite structure and composition of the Fe3O4-PbS nanocomposite were also examined (Figure 4). The pattern confirms the presence of both magnetite and lead sulfide in the nanocomposite pattern.
The crystalline sizes from Scherrer equation, Dc=0.9 λ/βCosθ, were calculated, where β represented the width of the observed diffraction peak at its half maximum intensity, and λ represented the X-ray wavelength (CuKα radiation, equals to 0.154 nm). The average crystalline sizes for Fe3O4, PbS, and Fe3O4-PbS nanostructures were about 15, 25, and 30 nm, respectively.
The SEM images of surfactant-free Fe3O4 are illustrated in Figure 5A and B in two different magnifications, respectively; the images prove the formation of agglomerated nanostructures, which have an average particle size of around 40 nm. This reaction was chosen to perform the basic preparation, and SEM was performed to investigate the effects of various parameters, such as temperature, precipitating agent, concentration, and capping agent, on the morphologies. Figure 5C presents the product obtained by gelatine, along with the synthesis of bulk magnetite. As can be seen, chains of gelatine reacted with the hydroxyl group of magnetite and covered the surface of nanoparticles.
Figure 6 shows SEM images of magnetite, for which iron nitrate and polyvinyl alcohol were used as Fe(III) precursor and neutral capping agent, respectively. Interestingly, the results demonstrate the formation of nanorod structures with average diameter of <50 nm. The influence of concentration on the size of the obtained product in the presence of polyvinyl pyrrolidone was investigated, and the results shown in Figure 7. Images confirm the increase of water from 100 mL to 400 mL, and mono-disperse magnetite nanoparticles (instead of nanorods) with a mediocre size of about 40 nm were prepared.
Iron precursor was changed from Fe(NO3)3 9H2O to FeCl3 4H2O. The SEM images of magnetic ferrite obtained by FeCl3 4H2O and polyvinyl pyrrolidone are shown in Figure 8 in three various magnifications; the images prove the formation of nanorod products with a mediocre diameter of about 40 nm.
The effects of temperature on the morphology and shape of the magnetite produced by FeCl3 and PVP were examined. The results are shown in Figure 9. As can be seen, agglomerated nanostructures (instead of nanorods) with an average size of around 50 nm have been achieved. Thus, temperature has a negative effect on particle size, and growth stage is preferential compared with the nucleation stage.
Figure 10A and B illustrate the SEM images of ferrite obtained by Fe(NO3)3 9H2O, for which glucose is used as a green, bio compatible capping agent. As shown by the figures, agglomerated nanoparticles (around 50 nm) have been synthesized. After examining the effects of changing the precipitating agent, the outcomes prove that, by replacing sodium hydroxide with potassium hydroxide, the bulk product can be prepared (Figure 10C). It seems that the growth step is more effective compared with the nucleation step.
Meanwhile, the SEM images of PbS in the presence of glucose are shown in Figure 11A and B; the outcomes confirm the creation of mono-disperse nanoparticles with an average particle size of around 50 nm. The concentration effects were investigated and the images of lead sulfide obtained at 400 mL of water were presented (Figure 11C and D). The images show that increasing the amount of solvent resulted in the synthesis of bigger and agglomerated bulk products. Interestingly, hexagonal lead sulphides can be discerned from the SEM images. Meanwhile, the SEM images of surfactant-free PbS are illustrated in Figure 11E and F. As can be seen, cubic or polyhedral nanostructures with an average diameter of about 120 nm are synthesized.
Figure 12 depicts SEM images of Fe3O4-PbS nanocomposites at different magnifications, along with the creation of mono-disperse nanostructures with an average particle size of <60 nm. Figure 13 shows the Fourier transform-infrared spectroscopy (FT-IR) spectrum of the as-prepared PbS nanoparticles at 80°C; the absorption band at 497 cm−1 is assigned to the stretching mode of Pb-S bond. The spectrum exhibits broad absorption peak between 3497 cm−1, corresponding to the stretching mode of the O-H group of the hydroxyl group. Slight moisture remained on the surface of the nanoparticles. The FT-IR spectrum of the Fe3O4 nanoparticle is shown in Figure 14. The spectra of nanoparticles clearly show Fe-O bonds around 482, 580, and 615 cm−1. The band near 3460 cm−1 is assigned to H-O-H bending vibration mode because of the adsorption of moisture on the surface of nanoparticles. The FT-IR spectrum of the Fe3O4-PbS nanocomposite is shown in Figure 15, in which absorption bands at 430 and 628 cm−1, which prove the presence of magnetic nanoparticles, can be clearly observed. The spectrum exhibits broad absorption peaks between 3522 and 3470 cm−1, corresponding to the stretching mode of the hydroxyl group.
The room temperature magnetic property of the nanorod magnetite sample (obtained in the presence of PVP) was studied using AGFM instrument. Results in Figure 16 indicate that the hysteresis curve of Fe3O4 exhibits super paramagnetic behaviour with a coercivity of zero Oe and saturation magnetization of 58 emu/g.
The hysteresis loop for Fe3O4 nanoparticles (magnetite prepared at diluted solvent, 400 mL of water) is shown in Figure 17. The saturation magnetization of this sample is about 39 Oe, and coercivity is around zero emu/g. The outcomes indicate the direct effect of morphology and particle size on the magnetic property of the prepared ferrite.
Figure 18 shows the magnetization curve of Fe3O4-PbS, which also exhibits super paramagnetic behaviour, with a coercivity of about Oe and saturation magnetization of 8.2 emu/g. The magnetic property of the prepared nanocomposites is an essential characteristic of a heterogeneous nanocomposite. This is because materials with this magnetic behaviour have low tendency in inter-particles agglomeration caused by the dipole–dipole interaction compared with the super paramagnetic nanocomposites.
The photo-catalytic activity of the Fe3O4-PbS nanocomposite was evaluated by monitoring the degradation of four azo dyes in an aqueous solution, under irradiation of UV light. The wavelengths of maximum absorption (λmax) of azo dyes acid black 1, acid brown 14, and Congo-red are shown in Figure 19. The changes in the dye concentrations are illustrated in Figure 20. Acid black 1, acid brown 14, and Congo-red are degraded at around 80%, 75%, and 80%, respectively, at 120 min. Organic dyes decompose to carbon dioxide, water, and other less toxic or nontoxic residuals (Saffari et al., 2015).
In conclusion, the synthesis, characterization, and photocatalytic activity of Fe3O4-PbS nanocomposite were reported. The effects of temperature, precursor, and concentration on the morphology and particle size of the products were investigated. AGFM confirmed that nanoparticles and nanocomposites exhibit super paramagnetic behaviours. The results also show that the precipitation method is a suitable approach for the preparation of Fe3O4-PbS nanocomposites, which are promising candidates for industrial photocatalyst applications.
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