Interestingly hydrothermal is a specific procedure for synthesis of nanostructures with desired and controlled shape and dimensions. While predominant morphology in other methods like sol–gel, sonochemical is nanoparticle. Usually hydrothermal method leads to preferential growth in comparison to spherical nucleation. In hydrothermal method because of some particular conditions (high temperature and pressure) the nanoparticles grow in situ and form hierarchical structures [1-4]. Quantum confinement and large surface-to-volume ratio lead to distinct magnetic properties which are different from those of their bulk structures . This behaviour has been explained as due to the large volume fractions of the atoms in the grain boundary area with unusual properties like spin canting, surface anisotropy, dislocations and super-paramagnetic behaviour. Magnetic separation is considered as a high speed and effective technique for separating magnetic particles. Thus, if the photocatalyst is magnetic, it could be gathered conveniently by magnetic field [1,2]. Ferrites are technologically essential materials that are used in the fabrication of magnetic, electronic and microwave devices. These materials have a potential application at high frequency range due to their very low electrical conductivity, fairly large magneto-crystalline anisotropy, relatively large saturation magnetization, mechanical hardness and low production costs [3-4].
Hydrothermal synthesis can be defined as a method of synthesis of single crystals that depends on the solubility of minerals in hot water under high pressure. The crystal growth is performed in an apparatus consisting of a steel pressure vessel called an autoclave. A temperature gradient is maintained between the opposite ends of the growth chamber. At the hotter end the nutrient solute dissolves, while at the cooler end it is deposited on a seed crystal, growing the desired crystal. Also, materials which have a high vapour pressure near their melting points can also be grown by the hydrothermal method. The method is also particularly suitable for the growth of large good-quality crystals while maintaining control over their composition. We have used hydrothermal method for this synthesis. There is a need to promote the experimental processes for the synthesis of nano-materials that offers ease in size control and stable preparation which could be used in wide range of applications. With the promotion of these new methods, the concern for environmental contamination is also heightened as chemical procedures generates a large amount of hazardous products [5-7]. Noble metals nanoparticles can strongly absorb visible light due to their localized surface plasmon resonance (LSPR), which can be adjusted by varying their size, surrounding and shape. When the frequency of the event light satisfies the resonance conditions of the noble-metal nanoparticles, the LSPR occurs with the associated light absorption. Moreover, noble metal nanoparticles can also work as an electron trap and active reaction sites [8-12]. Plasmon resonant nanostructures have gained remarkable interest in many fields, medicine, including near-field optics, surface enhanced spectroscopy, and solar cells. It was explored the applicability of plasmonic processes in the field of photocatalytic chemistry for organic molecule decomposition CO oxidation, and even materials synthesis. Various enhancement mechanisms have been proposed, comprising plasmonic heating and charge transfer [9, 10]. Noble metal nanoparticles have been the subject of extensive research in the frame of nanotechnology, mainly owing to their unique optical properties. Indeed, the free electron gas of such nanoparticles features a resonant oscillation upon illumination in the visible part of the spectrum. The spectral properties of this resonance depend on the constitutive material, the shape of the nanoparticles and its environment. This resonant electronic oscillation is called localized surface plasmon (LSP), and the field of research that studies the fundamentals and applications of LSP is known as nanoplasmonics. LSPs are accompanied by valuable physical effects such as optical near-field enhancement, heat generation and excitation of hot-electrons. Hence, plasmonic nanoparticles can behave as efficient nanosources of heat, light or energetic electrons, remotely controllable by light [11,12]. The composites of semiconductor nanoparticles and optically active metallic nanostructures represent a promising alternative to conventional photocatalysts. The main feature of these photocatalysts is that the interaction between semiconductor and metallic building blocks results in very efficient conversion of incident photons into electron-hole pairs in the semiconductor [12-15].
In this work Fe-Ag nanocomposites were synthesized by hydrothermal method. The photo catalytic behaviour of Fe-Ag nanocomposites was evaluated using the degradation of three various azo dyes under ultraviolet light irradiation. The results show that nanocomposites have applicable super paramagnetic and photo catalytic performance.
MATERIALS AND METHODS
FeCl2 4H2O, AgNO3, NaBH4, ammonia and distilled water were purchased from Merck Company was prepared. Scanning electron microscopy images were obtained using a LEO instrument model 1455VP. All the chemicals were used as received without further purifications. Before to taking images, the samples were coated by a very thin layer of Pt (using a BAL-TEC SCD 005 sputter coater) to make the sample surface conductor and prevent charge accumulation, and obtaining a better contrast. A multiwave ultrasonic generator (Bandeline MS 73), 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. X-ray diffraction patterns were recorded by a Philips, X-ray diffractometer using Ni-filtered CuKα radiation. Room temperature magnetic properties were investigated using a vibrating sample magnetometer (VSM) device, (Meghnatis Kavir Kashan Co., Iran) in an applied magnetic field sweeping between ±10000 Oe.
Preparation of Fe nanoparticles
0.001 mol of FeCl2 4H2O was dissolved in 200 mL of distilled water. Then 1 ml of NaBH4 as surfactant was added to the solution, it was mixed on magnetic stirring for 10 min. 16ml of NH3 (1M) as precipitator was slowly added to reaching pH of solution to 10. The solution is put into an autoclave and oven at 160 ° C for 5 hours. The obtained black precipitate was washed twice with distilled water. Then it was dried in oven for 24h .
Preparation of Fe-Ag nanocomposites
Firstly 0.1 g of synthesized iron nanoparticles was dispersed in 200 ml of distilled water and then 0.2g of AgNO3 was then dissolved in the solution. Then 1 ml of NaBH4 as reducing agent was added to the solution and was mixed for 2h. The solution is put into an autoclave and oven at 160 ° C for 5 hours. (Fig.1).
Fig. 1. Schematic of nanocomposite preparation
10 ml of the dye solution (20 ppm) was used as a model pollutant to determine the photocatalytic activity. 0.1 g of catalyst was applied for degradation of 10 ml solution. The solution was mixed by a magnet stirrer for 1 hour in darkness to determine the adsorption of the dye by catalyst and better availability of the surface. The solution was irradiated by a 10 W UV lamp which was placed in a quartz pipe in the middle of reactor. It was turned on after 1 hour stirring the solution and sampling (about 10 ml) was done every 15 min. The samples were filtered, centrifuged and their concentration was determined by UV-Visible spectrometry.
RESULTS AND Discussion
Fig. 2 illustrates XRD pattern of product without using capping agent and by using low amount of NaBH4. It can be observed that cubic phase (JCPDS No.74-0748) with Fd-3m space group which is consistent with pure magnetite was prepared. Fig. 3 shows XRD pattern of pure iron product (JCPDS No 06-0696). Fig. 4 shows XRD pattern of silver product. A number of strong Bragg reflection peaks can be seen which correspond to the (111), (200), (220) and (311) reflections of FCC silver and lattice parameters of a = b c = 4.070 A° . The standards (JCPDS), silver file No. 04-0783 and space group of Fm-3m (space group number: 225) in the pattern are reported. The calculated crystalline sizes from Scherrer equation, Dc=Kλ/βCosθ, where β is the width of the observed diffraction peak at its half maximum intensity (FWHM), K is the shape factor, which takes a value of about 0.9, and λ is the X-ray wavelength (CuKα radiation, equals to 0.154 nm) were about 22 and 32 nm for Fe and Ag nanoparticles, respectively.
Fig. 2. XRD pattern of Fe3O4 nanoparticles
Fig. 3. XRD pattern of Fe nanoparticles
Fig. 4. XRD pattern of Ag nanoparticles
By using not efficient NaBH4 magnetite was synthesized. SEM images of the as-synthesized Fe3O4 nanoparticles are illustrated in Fig. 5. Nucleation stage overcomes to growth stage the and nanoparticles with average particle size less than 40 nm were synthesized.
Fig. 5. SEM images of Fe3O4.
Concentration effect of reducing agent and capping agent (PVP) was also examined. SEM images of the synthesized Fe nanoparticles with 10 ml of NaBH4 as capping agent are shown in Fig. 6. Results confirm by enhancement of NaBH4, pure iron with average diameter size less than 90 nm were prepared.
Fig. 6. SEM images of Fe with PVP
The effect of temperature was also tested; Fig. 7 depict SEM images of the synthesized Fe nanoparticles with 10 ml of NaBH4 as reducing agent by hydrothermal method and heated in autoclave at 200 °C for 5h. Images approve nanoparticles with average diameter size less than 50 nm were prepared.
Fig. 7. SEM images of Fe prepared at 200 ºC
SEM images of the synthesized Ag nanoparticles with 1ml of NaBH4 as reducing agent by hydrothermal method and heated in autoclave at 160 °C in 5 h are shown in Fig. 8. Images confirm dendrite like nanoparticles with average diameter size less than 20 nm were prepared.
Fig. 8. SEM images of Ag prepared at 160 ºC for 5h
Reaction time was investigated, Fig. 9 illustrate SEM images of the synthesized Ag nanoparticles with 1 ml of NaBH4 at 160 °C for 10h. Results confirm flower like nanostructures were prepared and nanoparticles with average diameter size less than 90 nm were obtained.
Fig. 9. SEM images of Ag synthesized at 160 ºC for 10 h
It is known that the particle size and morphology can be manipulated by adjusting the super-saturation during the nucleation and crystal growth in hydrothermal, which in turn, it can strongly be affected by solution chemistry of hydrothermal conditions such as reaction temperature depends on precursor, time and temperature and pH of process environment.
Fig. 10 show SEM images of nanocomposites at 160 °C for 5h. Images approve formation of dendrite like nanostructures with average particle size around 50 nm. The balance between nucleation rate and growth rate which determines final particle size and morphology.
Fig. 10. SEM images of nanocomposite prepared at 160 ºC for 5h
Magnetic properties of samples were studied using vibrating sample magnetometer system at room temperature. Hysteresis loop of magnetic Fe3O4 nanoparticles at 160ºC is shown in Fig. 11. Nanoparticles show super paramagnetic behaviour and have a saturation magnetization of 58 emu/g and a coercivity tending to zero Oe. It shows a sufficient magnetization of these nanoparticles for being recycled by a magnet, making them appropriate for core of recyclable photo-catalyst.
Fig. 11. Room temperature hysteresis loop of Fe3O4 nanoparticles
As-synthesized Fe nanoparticles show super paramagnetic behaviour and have a saturation magnetization of 87 emu/g and a coercivity tending to 10 Oe (Fig. 12).
Fig. 12. Room temperature hysteresis loop of Fe nanoparticles
Hysteresis loop of magnetic Fe-Ag 10% nanoparticles prepared by simple hydrothermal is depicted in Fig. 13. The product also illustrates super paramagnetic behaviour and has a saturation magnetization of 75 emu/g and a coercivity about zro Oe.
Fig. 13. VSM curve of Fe-Ag 10% nanocomposites
Fig. 14 shows hysteresis loop of magnetic Fe-Ag 50% nanoparticles prepared by hydrothermal at 180 °C. Nano-spheres show super paramagnetic behaviour and have a saturation magnetization of 44 emu/g and a coercivity about zero Oe. This magnetization indicates that Fe-Ag nanocomposites inherit the magnetic property from the Fe; however, the magnetization is lower due to presence of silver. This reduction in saturation magnetization is due to the interfacial effect of the typical nanocomposite. The magnetic property of the prepared nanocomposites is an essential characteristic of a re-generable and reusable magnetic heterogeneous catalyst.
Fig. 14. Hysteresis curve of Fe- Ag 50%:50% nanocomposite
The photo-catalytic activity of the Fe-Ag nanocomposite was evaluated by monitoring the degradation of Acid-Brown , Acid Blue, Acid Black in an aqueous solution, under irradiation with UV light. The changes in the intensity of maximum wave length of four azo-dyes are depicted in Fig. 15. Maximum wave length of Acid Black, Acid-Brown and Acid Blue were degraded about 98%, 97% and 85% in 120 min in the presence of magnetite-silver. Acid brown showed the fastest degradation at 40 min under ultraviolet light and at presence of magnetic photo-catalyst. Organic dyes decompose to carbon dioxide, water and other less toxic or nontoxic residuals [11-17]. Fig. 16 shows degradation of the three azo dyes after 120 min exposure to the Fe-Ag nanocomposite.
Fig. 15. Photo degradation of (a) Acid Brown (b) Acid Black (c) Acid Blue
Fig. 16. Photo degradation of (a) Acid Black (b) Acid Brown (c) Acid Blue
Firstly magnetite nanoparticles were synthesized via a hydrothermal in the presence of NaBH4, then silver nanoparticles and Fe-Ag nanocomposites were prepared by hydrothermal method. Effect of temperature, reaction time and various concentration of NaBH4 were investigated on the morphology and particle size of the products. Vibrating sample magnetometer confirmed that nanocomposites exhibit super-paramagnetic behaviour. The photocatalytic behaviour of Fe-Ag nanocomposite was evaluated using the degradation of three azo dyes under UV-visible light irradiation. The results show that hydrothermal method is suitable method for preparation of Fe-Ag nanocomposites as a candidate for photocatalytic applications.
This work has been supported financially by Arak University Research Council (AURC) under the Grant Numberof 94-9329.
CONFLICT OF INTEREST
The authors declare that there are no conflicts of interest regarding the publication of this manuscript.