In this work , pure TiO 2 and binary nanocomp osites of Fe3O4/TiO 2 and Ag /TiO 2 were synthesized in order to improve photocatalytic performance of these sample s for degradation of 2, 4-dichlorophenol (2, 4-DCP) as an organic pollutant. A range of analytical techniques including XRD, DRS, SEM/EDX, and elemental mapping were employed to reveal the crystal structure, morphology and property of the nanocomposites. XRD data demonstrated that the prepared sample s are purely in TiO 2 anatase phase and cubic spinel Fe 3O4 exist in the synthesized nanocomposite. We calculated the TiO 2 crystal size from XRD patterns, in the range of 8.35 -11.09 nm. The presence of Ag, Fe, O, and Ti atoms in the synthesized nanocomposite s was confirmed by SEM/EDX. We obtained 30.43, 32.02 and 42.40 % degradation of 2, 4- DCP (100 ml 2, 4-DCP 40 ppm and 0.01 g catalyst) for pure TiO 2, Fe 3O4/TiO 2 and Ag/TiO 2, respectively, after 180 min of irradiation under visible light. Similar conditions were employed for 2, 4-DCP degradation under UV irradiation, we obtained 53.05, 51.00 and 71.50 % degradation of 2, 4-DCP pure TiO 2, Fe 3O4/TiO 2 and Ag/TiO 2, respectively. Thus, the synthesized binary nanocomposites exhibited higher photocatalytic activity compared to pure TiO 2 under visible light. INTRODUCTION World's population growth and drinking water scarcity has made drinking water supplementation, one of the fundamental problems of today's world. Therefore, water recycling will provide access to a sui table source for various uses. Recently, advanced oxidation processes (AOPs) have been proposed as a wastewater treatment solution. Among advanced oxidation processes, heterogeneous photocatalytic processes have been used as a successful method for the decomposition of various organic pollutants. Photocatalysts are environmental cleaners that can degrade many environmental pollutants through oxidation by using sunlight or artificial light, especially ultraviolet light. Titanium dioxide (TiO 2) is one of the photocatalysts that has been used to degrade organic pollutants. TiO 2 is an ideal photocatalyst with non -toxic nature, high oxidizing properties under UV radiation and low cost, which is stable under various reaction conditions and is also environment frie ndly [1]. There are two distinct defects in the performance of TiO 2 for the destruction of environmental pollutants. The first is the high energy of its separation band (3.2 eV), which requires UV radiation to generate the electron -cavity pair, but UV radiation forms only about 4% of the sun's emission. In order to utiliz e a larger part of the radiation emitted from the sun, it is necessary to make special changes in this semiconductor system. The presence of metals such as silver at TiO 2 surface would reduce the separation band energy and expand its absorption spectrum to wards the visible light region. The presence of these metals decreases recombination rate of electron -cavity pair and also increases the free radicals formation rate [2]. The second defect is the recycling of nano -sized photocatalysts , which is very difficult and costly. An operational solution for photocatalyst recycling is magnetizing TiO 2 by Fe 3O4 so that it can be separated and reused by employment of an external magnetic field [3]. In this study, (Ag/ TiO 2) and (Fe 3O4 /TiO 2) binary nanocomposites as well as pure TiO 2 will be synthesized by sol -gel method. The s ynthesized samples will be identified with various techniques such as DRS, SEM/EDX and XRD, and eventually photocatalysts are used for photocatalytic degradation of 2, 4-dichlorophenol as an organic pollutants both under visible and ultraviolet light. EXPERIMENTAL Materials FeCl 3·6H 2O (Merck No. 10 3943), FeSO 4·7H 2O (Merck No. 103965) were used for synthesis of Fe 3O4 nanoparticles (NPs). Tetraisopropylorthotitanat (TIP) (Merck No. 8.21895), anhydrous ethanol, ammonia, and High - purity 2, 4-DCP, 98%, (Merck No. 803774) was used as a probe molecule for photocatalytic tests were purchased from Merck Company . Silver nitrate (AgNO 3), 99.9%, was supplied by (Merck, No.101510), Acetic acid (CH 3COOH), 99.9% (Merck No. 1.00063). All the reagents were of analytic grade and used without further purification. Doub le distilled water was used for preparation of all aqueous solutions. Preparation of Fe 3O4 and TiO 2 NPs We synthesized Fe 3O4 nanoparticles by chemical precipitation technique according to Mentioned procedure in ref [4] and fo r synthesis of TiO 2, the method reported in reference [5] will be used with some modifications. Subsequently, the obtained nanoparticles identified by various analysis. Synthesis of binary (Fe 3O4/TiO 2) and (Ag/ TiO 2) nanocomposites (Fe 3O4/TiO 2) nanocomposites Synthesis : We mixed 0.1 g of the Fe 3O4 nanoparticles with 4 ml of titanium isopropoxide (TIP) (as a titanium source) and 70 ml of water -free ethanol and placed it in an ultrasonic bath for one hour ( Solution A). Solution B prepared by mixing 3 ml of acetic acid and 90 ml of distilled water. Then solution B was added to solution A, stirred at 50 °C for 30 min. Finally, after cooling to room temperature, we separate the synthesized solid from the suspen sion by centrifugation. Rinsed with ethanol and dried for 12 h at 60 °C. For the final step, the obtained solid is heated unde r atmospheric pressure at 300 ° C for one hour. (Ag/TiO 2) nanocomposites Synthesis : 1 g of TiO 2 is spread in 20 ml of deionized water by ultrasound waves and subsequently, 1 ml of AgNO 3, 100 mM, was slowly added to mixture. The solution was stirred at room temperature for 30 min , then 1 ml of Na 2CO 3 (1% w / v), was slowly added. The slurry was filte red and dried at room temperature [6]. Characterization The XRD patterns were recorded on a Siemens, D5000 (Germany). The morphology of the prepared samples were characterized using scanning elec tron microscope (SEM) (Vegall -Tescan Company) equipped with an energy dispersive X -ray (EDX). The diffuse reflectance UV –Vis spectra (DRS) of the samples were recorded by an Ava Spec -2048TEC spectrometer. Photocatalytic degradation of 2, 4-DCP In a typical run, the suspension containing 10 mg photocatalyst and 100 mL aqueous solution of 2, 4-DCP (40 mg/L) was stirred first in the dark for 10 min to establish adsorption/desorption equilibrium. Irradiation experiments were carried out in a self -built reactor for 180min irradiation. UV illumination was performed with a 400 W Kr lamp (Osram). A visible (Halogen, ECO OSRAM, 500W) lamp was used as irradiation source. At certain intervals, small aliquots (2 mL) were withdrawn and filtered to remove the photocatalys t particles. These aliquots were used for monitoring the degradation progress, with Rayleigh UV -2601 UV/VIS spectrophotometer (λ max= 227nm). RESULT AND DISCUSSIO N X-ray diffraction analysis Figure 1. A shows the X -ray diffraction pattern of the synthesized Fe 3O4 nanoparticles in the present study . The diffractions at 2θ = 30.2°, 35.6°, 43.5°, 54.3° , 57.4 ° and 63.1° are observed, which are related to the cubic spinel structure of Fe 3O4, reported by JCPD S reference number 0629 -9, thus synthesis of magnetic nanoparticles is confirmed [7]. (A) (B) FIGURE 1. XRD patterns of A) Fe 3O4, B) a) TiO 2, b) Fe3O4/TiO 2 and c) Ag/TiO 2 Figure 1. B shows the XRD patterns of the pure TiO 2 and binary nanocomposite. All the observed diffractions are related to the anatase phase according to the reference (PDF 24 -1272 ) [8]. The reason for the low intensity of Fe 3O4 nanoparticles and silver atoms diffraction is that they are very small in comparison to TiO 2, and because of the high TiO 2 crystallinity, most of the Fe 3O4 diffraction is not visible. The size of the TiO 2 crystal was calculated using first diffraction width and Scherrer's e quation. Which varies between 8.35 to 11.09 nm [9]. () 0.9 / coshklD    In this equation, D (h kl) is the crystal size based on Miller's index (101), λ is the wavelength of the X -ray, β is the width of the peak at half of its height, and θ is the Bragg's angle for the desired diffraction. Also, to calculate the parameters (a = b ≠ c), 101 crystallin e location of anatase phase is used from the following equation. 2 2 2 2 22/ 1/ /d h c a kl   In this equation, D, the distance between neighboring places, is calculated by Bragg's law () / 2sinhklD To calculate a = b and c, respectively, the angles of the third (2 0 0) and first (1 0 1) diffractions of the anatase phase are used respectively [10]. Table 1 shows the unit cell parameters and volume for the anatase phase TiO 2 in the synthesized samples and confirmed the formation of pure anatase phase . TABLE 1. Phase, crystal size and lattice parameters of the prepared samples Sample Phase Crystal size (nm) a=b (Å) c (Å) Cell volume (Å3) TiO 2 Anatase 10.38 3.77 9.59 136.30 Fe3O4/TiO 2 Anatase 7.11 3.79 9.37 134.87 Ag/TiO 2 Anatase 8.14 3.77 9.76 138.71 FESEM/EDX analysis In order to investigate the surface morphology of the synthesized samples, FESEM studies were performed. The SEM images of pure TiO 2, Fe3O4/TiO 2 and Ag/TiO 2 samples are shown in Fig. 2. The EDX results were used to examine the elemental composition of the pure TiO 2, Fe3O4/TiO 2 and Ag/TiO 2 nanocomposites (Fig. 3 and Table 2). 20 25 30 35 40 45 50 55 60 65 70Intensity (a.u.) 2θ(degree)(a)(b)(c) 20 25 30 35 40 45 50 55 60 65 70Intensity (a.u.) 2θ(degree) (a) (b) (c) FIGURE 2. FESEM images of a) TiO 2, b) Fe3O4/TiO 2 and c) Ag/TiO 2 FIGURE 3. EDX spectra of a) TiO 2, b) Fe3O4/TiO 2 and c) Ag/TiO 2 (a) (b) (c) FIGURE 4. Elemental mapping of a) TiO 2, b) Fe3O4/TiO 2 and c) Ag/TiO 2 TABLE 2 . Elemental chemical analysis of the prepared samples. Sample C wt% N wt% O wt% Ti wt% Fe wt% Ag wt% TiO 2 8.12 14.24 41.68 35.96 - - Fe3O4/TiO 2 6.31 9.80 40.15 41.99 1.75 - Ag/TiO 2 10.26 12.43 37.12 36.13 - 4.06 The EDX result confirms the presence of C, Ti, O, Fe Ag and N elements in these samples. In the EDX spectra of Ag/TiO 2 sample, the absorption peak at 3 keV is for metallic silver and confirms the presence of silver nanoparticles in the prepared sample. Figure 4 shows elemental mapping images of the prepared samples. DRS analysis The diffuse reflectance spectra of the prepared samples over the wavelength range of 200 –800 nm are shown in Fig. 5. A. The DR spectrum of pure TiO 2 consists of a broad intense absorption around 400 nm, due to the charge - transfer from the valence band formed by 2p orbitals of the oxide anions to the conduction band formed by 3d t 2g orbitals of the Ti4+ cations [11]. We calculated the band gap energy from the DR spectra according to below equation [12] for the prepared samples. (a) (b) (c) C K N K O K TiK TiK TiL keV010002000300040005000 0 5 10 C K N K O K TiK TiK TiL FeK FeK FeL keV0100020003000400050006000 0 5 10 C K N K O K TiK TiK TiL AgL AgL keV05001000150020002500 0 5 10 0.5( ) ( )g F R h A h E Where A is cons tant, F( R) is the Kubelka -Munk function and E g is the band gap. The band gap of the binary nanocomposites decreased slightly compared with TiO 2 (Table 3). FIGURE 5. A): Diffuse reflectance spectra and B) Kubelka -Munk plots for the band gap energy calculation of a) TiO 2, b) Fe3O4/TiO 2 and c) Ag/TiO 2 TABLE 3. Band gap energy of the prepared samples. Sample TiO 2 Fe3O4/TiO 2 Ag/TiO 2 Eg (eV) 3.05 2.70 3.02 Photocatalytic performance of the prepared samples To evaluate the photocatalytic activity of the synthesized samples a set of experiments for 2,4 -DCP degradation with an initial concentration of 40 mg L-1 under visible light and UV irradiation at room temperature was carried out in aqueous suspension us ing TiO 2, Fe3O4/TiO 2 and Ag/TiO 2 catalysts, and the experimental results are shown in Fig. 6. The experimental results demonstrated that among the prepared samples, the Ag/TiO 2 catalyst showed the highest efficiency of the 2 , 4-DCP degradation under visible light (42.40% degradation obtained after 180 min irradiation).the results show that the photocatalytic activity of pure TiO 2 is lower than that of Fe3O4/TiO 2 and Ag/TiO 2 samples. It implies that the Ag dopant promotes the charge pair separation eff iciency for TiO 2 catalysts. The electron transfer from the TiO 2 conduction band to Ag particles at the interface is thermodynamically possible because the Fermi level of TiO 2 is higher than that of Ag metal. This results in the formation of a Schottky barr ier at metal semiconductor contact region and improves the photocatalytic activity of TiO 2. The photocatalytic activity of Fe3O4/TiO 2 sample is lower than pure TiO 2 under UV irradiation implying that a stronger screen effect exists in our Fe3O4/TiO 2 sample as compared with pure TiO 2. The Fe 3O4 NPs may be screened and weaken the UV irradiation needed to irradiate the TiO 2 NPs in the Fe3O4/TiO 2 sample [13] because, i) Fe 3O4 particles blocked some of the TiO 2 active sites and ii) some UV irradiation was absorbed by Fe 3O4 particles. CONCLUSION Pure TiO 2 and binary nanocomposites of Fe3O4/TiO 2 and Ag/TiO 2 were synthesized by the sol -gel method for degradation of 2, 4 -dichlorophenol and characterized by several techniques successfully. From among all of the samples only anatase phase was confirmed from the XRD results. The presence of Ag, F e, O, and Ti atoms in the synthesized nanocomposites was confirmed by SEM/EDX. Among all samples, Ag/TiO 2 catalyst exhibited the highest photocatalytic activity by 42.40 % degradation under visible light and 71.50 % degradation under UV irradiation af ter 180 min of irradiation . (a)(b) (c) 00.20.40.60.811.21.4 200 300 400 500 600 700 800Absorbance (au) Wavelength (nm)(A) (a)(b) (c) 00.20.40.6 1 2 3 4 5 6[F(R)hv]0.5 E (eV)(B) (A) (B) FIGURE 6. Photocatalytic degradation of 2, 4-DCP i n the presence of the prepared samples under (A) visible light and (B) UV irradiation. Initial concentration of 2, 4-DCP, 40 mg /L; volume, 100 mL; catalyst dosage: 10 mg. ACKNOWLEDGEMENTS The authors wish to acknowledge the financial support of University of Tehran for supporting of this research.