Preparation and Characterization of Ag@TiO2/α-Fe2O3 Ternary Nanocomposite for enhanced visible light photocatalytic performance

In this work, ternary Ag@TiO 2 /α-Fe 2 O 3 nanocomposite were synthesized via solvothermal chemical reduction method using N,N-dimethylformamide (DMF) as solvent and reducing agent. The chemical procedure involves the use of only metals precursors without the need to use any other surfactants or capping agents. Physicochemical properties of the designed photocatalyst are found by means of various modern techniques. XRD data conrmed the high crystallinity of the obtained ternary nanocomposite. On the other hand, using TEM and HRTEM instruments, the shape and morphology of the Ag@TiO 2 /α-Fe 2 O 3 nanocomposite were found to be spherical with an average particle size of 150 nm. The UV-Vis measurement shows that Ag@TiO 2 /α-Fe 2 O 3 as photocatalyst exhibited good photo response in the visible region. The effect of preparation method and the performance of the designed photocatalyst were evaluated by photodegradation measurements of MB under visible light irradiation. We observed that the combination of metallic silver nanoparticles (AgNPs) and hematite iron oxide (α-Fe 2 O 3 ) with titanium dioxide (TiO 2 ) enhance the photocatalytic activity of the ternary Ag@TiO 2 /α-Fe 2 O 3 photocatalyst compared to bare TiO 2 suggesting its potential for many purication applications.


Introduction
Environmental pollution resulting from numerous arti cial and industrial events is essentially constituted of inorganic, organic and harmful pollutants such as antibiotic, pesticides and dyes [1]. Generally, the most adopted methods to decompose pollutions such as chemical, heating, or biological process are found to be very expensive and ineffective mostly for the degradation of antibiotic. In recent years, nanomaterials are found to be very useful for different applications [2][3][4]. On the other side, photocatalytic reactions have been widely suggested as green solution to decompose many categories of pollutants under mild conditions and under solar light source [5][6][7][8][9][10][11]. However, the photodegradation mechanism involves the presence of only a photocatalyst and a light as exciting source. This process can mineralize pollutants to harmless products such as carbon dioxide and water and therefore produces useful products. Among the semiconductor photocatalyst, titanium dioxide (TiO 2 ) can be considered as the most promising photocatalyst in recent years due to its high stability and easy to prepare in different shape and size [12][13][14][15][16][17][18][19][20]. On the other hand, unfortunately, TiO 2 nanoparticles suffer from its large energy gap (3.2 eV), which limits its usage under visible light source [14]. The second inconvenient which inhibits the photocatalytic e ciency of TiO 2 is related to the high charge carrier recombination rate during the photodegradation reaction [15]. To resolve this problem, the synthesis of hybrid nanocomposite was found to be very e cient for decreasing the energy gap and the separation of charge carrier recombination. Using a simple two-step hydrothermal and photo-reduction method, Zhang et al. prepared ternary BiVO 4 /NiS/Au nanocomposites with e cient charge separations for enhanced visible light photocatalytic performance [11]. They found that the photodegradation activity was enhanced 4.25 times compared to pure BiVO 4 . Using the sol-gel method, Khasawnhem et al. [1] designed hybrid Fe 2 O 3 -TiO 2 heterogenous photocatalyst for the removal of acetaminophen (ACT) pharmaceutical compound. The obtained results revealed that photodegradation rate of ACT was observed at pH = 11 and the photocatalytic activity was further optimized compared to bare TiO 2 . Sahu et al. developed for the rst time a combined sol-gel-assisted hydrothermal method to prepare Copper/TiO 2 /graphene oxide ternary nanocomposites (CuTGR) [2]. This photocatalyst exhibited high photocatalytic activity. Indeed, preliminary results show that an optimal loading of Cu and graphene in TiO 2 matrix can signi cantly enhance the surface and optical response of the designed nanocomposites and thereby allowing it to be an e cient ternary photocatalyst for many applications.
In this context, the main purpose of this work is to prepare a ternary system based on titanium dioxide (TiO 2 ) nanoparticles with excellent light absorption and high photocatalytic e ciency. In recent years, iron oxide more precisely the hematite phase (α-Fe 2 O 3 ) was found to be an ideal metal oxide to expand the photo response of TiO 2 and therefore enhance its photocatalytic performance [16][17][18][19]. On the other side, silver nanoparticles (AgNPs) are a plasmonic metal nanoparticles with their intrinsic plasmonic properties can increase the electrons activity over the surface of TiO 2 NPs and slows down the recombination of e-/h + pairs. Consequently, the loading of both hematite iron oxide (α-Fe 2 O 3 ) and silver nanoparticles (AgNPs) at the TiO 2 surface can subsequently boost the photocatalytic activity of the nal nanocomposite. The present paper develop a simple one pot solvothermal protocol to synthesize ternary Ag@TiO 2 /α-Fe 2 O 3 nanocomposite using N,N-dimethylformamide (DMF) as solvent and reducing agent without recourse to use any capping agent or surfactant. The photodegradation reaction against MB dyes prove the e ciency of this ternary system compared to bare TiO 2 or α-Fe 2 O 3 NPs.

Measurement and Characterizations
Powder X-ray diffraction (D8 Advance Bruker, USA) technique was performed to study the obtained crystalline phase. Shape and size of the photocatalyst was examined using transmission electron microscope (Philips Tecnai F-20 SACTEM working at 200 kV). X-ray photoelectron spectrometry (XPS, Kratos Axis Ultra DLD) was recorded for further elemental analysis. Optical response was investigated via UV-Visible Perkin-Elmer Lambda 11 spectrophotometer. To study the charge recombination process, Photoluminescence (PL) measurements were adopted using Jobin Yvon Flurolog-3-11 instrument equipped with 450 W xenon lamp. Photocatalytic test were evaluated at room temperature for removal of MB molecules dyes under visible light illumination (lamp of 400 W Metal Halide. The solution pH was xed at 7. The photocatalyst amoun was xed to 7 mg during the photocatalytic test. The initial MB concentration solution was 3.0 .10 − 5 mol/L.

Synthesis of Ag@TiO 2 /α-Fe 2 O 3 Photocatalyst
Iron oxide was rst synthesized using the hydrothermal process [21] by mixing iron chloride and ammonium dihydrogen phosphate. After that, the mixture was transferred into a Te on-lined autoclave and heated at 220℃ for 48 h. Ag@TiO 2 /α-Fe 2 O 3 photocatalyst was prepared as follows: Titanium(IV) butoxide (5 ml) and silver nitrate (100 mg) were rst dissolved in 50 ml of N,N-dimethylformamide (DMF) at room temperature. The resulting mixture was magnetically stirred, and an adequate amount of asprepared iron oxide nanoparticles (10 mg) was added. Then the obtained nal mixture was heated at 153 ℃ for 2 h. The recuperated powder was then calcinated in air at 400℃ for 2 h to produce the desired ternary Ag@TiO 2 /α-Fe 2 O 3 nanocomposite.

Results And Discussion
Powder X-ray diffraction patterns of TiO 2 , α-Fe 2 O 3 and ternary Ag@TiO 2 /α-Fe 2 O 3 photocatalyst are shown in Fig. 1. As can be observed, in the diffractogram of the ternary nanocomposite, all diffractions peaks characteristic of titanium dioxide, silver and iron oxide are detected. No other peaks or impurities can be detected which prove the purity of the obtained sample. Based on the XRD data, we can assume the successful fabrication of the ternary Ag@TiO 2 /α-Fe 2 O 3 heterojunction. Figure 2  Ag to TiO 2 and α-Fe 2 O 3 . This observation was further supported by PL measurements (Fig. 4-b). Indeed, as can be seen, the PL spectrum of all samples exhibited blue emission located at 445 nm. It has been reported that the visible luminescence, related to deep level emissions, mainly results from defects such as interstitials and oxygen vacancies. On the other hand, as can be seen in Fig. 4-b, a considerable PL emission quenching of Ag@TiO 2 /α-Fe 2 O 3 nanocomposite was observed which indicated that a lower recombination rate of the photogenerated carrier could be e ciently achieved resulting from the synergistic effects between Ag, TiO 2 and α-Fe 2 O 3 . This result implying that the intimate contact between Ag, TiO 2 and α-Fe 2 O 3 could make for the vectorial migrate of charge carriers among the nanocomposite, enhancing the photogenerated carrier's separation and therefore improving the photocatalytic e ciency.
The photocatalytic e ciencies of Ag@TiO 2 /α-Fe 2 O 3 nanocomposites were evaluated using MB dyes as a model pollutant. Photocatalytic activities of hybrid Ag@TiO 2 , bare TiO 2 and α-Fe 2 O 3 were also measured for comparison. As shown in Fig. 5-a, the photodegradation rate of MB was found to be the highest using the ternary Ag@TiO 2 /α-Fe 2 O 3 photocatalyst. On the other side, the photodegradation rate using pristine α-Fe 2 O 3 photocatalyst is the lowest. However, the photocatalytic performance of hematite α-Fe 2 O 3 is limited due to the charge carrier recombination. On the other hand, the hybrid Ag@TiO 2 photocatalyst exhibited interesting photodegradation rate due to the presence of plasmonic AgNPs which can generate more electrons and therefore boost the photocatalytic activity of TiO 2 . It can be seen that ternary Ag@TiO 2 /α-Fe 2 O 3 photocatalyst exhibits much higher photodegradation activities than that of hybrid Ag@TiO 2 may be due to the support given by α-Fe 2 O 3 NPs which increases the surface are of the photocatalyst and also increases the light absorption which generates more electron-hole pairs for dye photodegradation and consequently enhances the photocatalytic activity of the ternary nanocomposite.
To examine the reaction kinetics of photocatalysts, experimental data were tted by a rst-order kinetic equation (Ln(C 0 /C) = k ap t) using the Langmuir-Hinshelwood model. It can be seen from the curves displayed in Fig. 5-b that the photodegradation process followed rst order kinetics. A proposed possible photocatalytic mechanism is illustrated in Fig. 5 Figure 1 XRD diffractograms of bare α-Fe2O3, TiO2 and Ag@TiO2/α-Fe2O3 nanocomposite.  Optical absorption property and PL response of Ag@TiO2/α-Fe2O3 nanocomposite  SI.docx