titanium dioxide magnetic nanocomposite

Journal Pre-proof Enhanced dual catalytic activities of silver-polyaniline/titanium dioxide magnetic nanocomposite Basel Al-saida (Investigation) (Writing - original draft), Wael A. Amer (Conceptualization) (Investigation) (Methodology) (Visualization) (Validation)Writing - reviewing and editing) (Supervision), Elsayed E. Kandyel (Supervision), Mohamad M. Ayad (Conceptualization) (Methodology) (Validation)Writing - reviewing and editing) (Supervision)

PII:

S1010-6030(19)30795-6

DOI:

https://doi.org/10.1016/j.jphotochem.2020.112423

Reference:

JPC 112423

To appear in:

Journal of Photochemistry & Photobiology, A: Chemistry

Received Date:

11 May 2019

Revised Date:

5 January 2020

Accepted Date:

28 January 2020

Please cite this article as: Al-saida B, Amer WA, Kandyel EE, Ayad MM, Enhanced dual catalytic activities of silver-polyaniline/titanium dioxide magnetic nanocomposite, Journal of Photochemistry and amp; Photobiology, A: Chemistry (2020), doi: https://doi.org/10.1016/j.jphotochem.2020.112423

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Enhanced dual catalytic activities of silver-polyaniline/titanium dioxide magnetic nanocomposite Basel Al-saidab , Wael A. Amera, Elsayed E. Kandyela and Mohamad M. Ayada,c a

Chemistry Department, Faculty of Science, Tanta University, Tanta 31527, Egypt

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Chemistry Department, Faculty of Science, Al-Balqa Applied University, Al-Salt19117, Jordan c

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Institute of Basic and Applied Sciences, Egypt-Japan University of Science and Technology, New Borg El-Arab City, Alexandria 21934, Egypt

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Corresponding author: Tel.: +20 3 459 9520; fax: +20 3 459 9520 E-mail: [email protected] 1

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Graphical abstract

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Highlights

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• An efficient method to synthesize a nanocomposite with dual catalytic activities. A magnetic nanophotocatalyst under the normal day visible light.

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A strong synergetic effect of Ag nanoparticles, PANI with TiO2 shell.

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• Rapid response to methylene blue degradation and the reduction of pnitrophenol.

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ABSTRACT A novel magnetic nanocomposite of titanium dioxide (TiO2)-polyaniline (PANI)-silver (Ag) (TPS) (Fe3O4@TiO2-PANI-Ag) was synthesized via the polymerization of aniline in the presence of Fe3O4@TiO2 using ammonium peroxydisulfate (APS) as an oxidizing agent followed by anchoring the nanocomposite with Ag nanoparticles (NPs). The novel synthesized TPS magnetic nanocomposite was characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM), energy dispersive Xray (EDX), and scanning electron microscopy (SEM). The synthesized TPS magnetic

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nanocomposite was employed for two catalytic applications. The catalytic activity of TPS magnetic nanocomposite was investigated toward the reduction of p-nitrophenol (PNP). The conversion rate of PNP to p-aminophenol was found to be more than 98% in 20 min using only 1 mg of TPS magnetic nanocomposite as a heterogeneous nanocatalyst. Furthermore, the

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photocatalytic activity of TSP nanocomposite was evaluated by studying the degradation of methylene blue (MB) as a model dye. MB dye was found to be degraded at significant rate in

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the presence of TPS magnetic nanocomposite under the normal day visible light, which proves its enhanced photocatalytic performance and the existence of synergic effect between PANI,

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Ag and TiO2 NPs. The TPS magnetic nanocomposite was proved to be more efficient catalyst than TiO2-Ag or PANI-Ag, and TiO2/PANI nanocomposites.

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Keywords: Polyaniline; titanium dioxide; magnetic nanocomposite; heterogeneous catalysis;

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photocatalytic activity

1. Introduction

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Conducting polymers attracted much attention because of their unique properties and

expanding areas of applications (1-3). Among all conducting polymers, polyaniline (PANI) has the most important commercial applications due to its unique optical, electrical, photoelectrical properties, ease of preparation and excellent environment stability. The main problem of PANI is its poor stability and low processing ability. Therefore, PANI/inorganic nanocomposites with magnetic character attracted much attention, because of their promising properties and applications in various fields including catalysts (4-6). PANI possesses enhanced catalytic

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activities because it acts as a nucleus to transfer the electronic charge between catalytic sites (7). Several inorganic semiconducting materials such as TiO2, CdS, ZnO were used to remove the organic dyes from wastewater because of their photocatalysis and unique properties (8-10). Among them, TiO2 nanoparticles (NPs) were recognized to be very important due to their non-toxicity, thermal and chemical stability, and their excellent degradation ability for water and air pollutants (11-13). The photocatalytic properties of TiO2 are attributed to its wide band gap (3.2 eV) (14). Some strategies were reported in order to enhance the photocatalytic properties of TiO2. As an example, PANI/TiO2 composite was prepared and showed good

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photocatalytic activity for organic pollutants degradation as compared to the unmodified TiO2 (15) because of the sensitizing effect of PANI, its efficient mobility of charge carriers, and the high absorption coefficients in the visible region. TiO2-PANI composite impregnated on the cork surface was synthesized and good dye's degradation results were obtained but the reaction

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rate constant was very limited (4.3x10-6 s-1) with a large dose of catalyst in addition to the difficultly of the catalyst's separation from the solution (16). The separation problem was solved by synthesizing PANI/TiO2 composite on the surface of Fe3O4 particles for decomposing

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ethylene diamine tetraacetic acid (EDTA) under visible-light irradiation (17). In an earlier work, PANI-TiO2 composite photocatalysts were prepared and examined for methylene blue (MB)

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dye degradation under visible light irradiation where a good degradation constant was obtained (15). Additionally, PANI/TiO2 nanocomposite was synthesized hydrothermally for the degradation of methyl orange (MO) and 4-chlorophenol with a good catalysis rate under both

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UV and visible light irradiation for a long time (6 hours) (18). Furthermore, Guo et al. synthesized a microscale hierarchical 3D flower-like TiO2/PANI composite via the sol-gel

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method for the photocatalytic degradation of Congo red and MO dyes under both UV-light and sunlight irradiation but remarkably limited rate constants were obtained. In addition, PANI/modified-TiO2 nanocomposite was synthesized and applied for the photocatalytic

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decomposition of MB dye under UV light irradiation for 140 min but with relatively small rate constant (19). PANI nanotubes@TiO2 composite was synthesized by Jeong et al., (20) and the photocatalytic activity was investigated for the degradation of MB but large amount of the of nanocatalyst were used (250 mg) for a long period of irradiation time (300 minutes) to remove 82% of MB. Another strategy was selected to overcome the limited photodegradation ability of TiO2 under visible light irradiation by its doping with noble metals, such as Ag or Pd (21, 22), due 4

to the surface Plasmon resonance and the electron trap that activates the reaction sites (23). Cozzoli et al., (24) investigated the photocatalytic performance of TiO2 nanorod-stabilized Ag NPs during the reductive bleaching of Uniblue A dye and the reaction rate was limited in the early stages but this delay in the dye bleaching was recovered in the subsequent stages after adding a large fraction of Ag particles in solution. Furthermore, nanosized TiO2–Ag particles were prepared with the sol–gel method for the decomposition of p-nitrophenol (PNP) and good degradation rate constant was obtained under UV irradiation (25). The sol-gel method was also employed for the preparation of TiO2 impregnated with Pd and/or chitosan for photocatalytic degradation of MB dye, where a good reaction rate was found after using a large amount of the

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catalyst (0.75 g) under visible light irradiation (26). Moreover, Ag@TiO2/PANI nanocomposite was synthesized for the photodegradation of MB dye and an enhanced photocatalytic activity was found as compared to PANI, but the main problems lie in the long irradiation time that reached 6 hours, in addition to the difficulties in the separation of this nanocomposite from the

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medium (27).

To the best of our knowledge, the combination of PANI, Ag NPs with TiO2 is very limited and their coupling with a magnetic counterpart is lacking in the literature. In addition,

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enhanced dual catalytic applications of similar nanocomposites under the normal day visible light are missing. Herein, we report the coupling of magnetite NPs, PANI, TiO2 and Ag NPs

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via a facile and simple method for the synthesis of a novel Fe3O4@TiO2-PANI-Ag (TPS) nanocomposite for dual catalytic applications; enhanced reduction of PNP and improved photocatalytic activity for organic dyes (MB was used as a model) degradation under visible-

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light irradiation. A spherical shape magnetic core Fe3O4 was firstly prepared, then it was modified by 3-aminopropyl-triethoxysilane (APTES) to produce Fe3O4–NH2 nanospheres, after

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which a thin layer of TiO2 was coated on Fe3O4–NH2 surface to achieve a uniform Fe3O4@TiO2 nanospheres. Because of its favorable hydrolysis rate, titanium n-butoxide (TBOT) was chosen as a source of titania. PANI was then loaded onto the surface of Fe3O4@TiO2 nanospheres via

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polymerization of aniline using ammonium peroxydisulfate (APS) as oxidant to get Fe3O4@TiO2-PANI nanocomposite. Afterward, it was decorated with silver NPs to obtain magnetic TPS nanocomposite. The synthesized TPS magnetic nanocomposite was systematically characterized via different analysis techniques including X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscope (SEM), energy dispersive X-ray (EDX) analysis, transmission electron microscope (TEM), elemental mapping, diffuse reflectance spectroscopy and nitrogen adsorption-desorption isotherms. The

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catalytic activity of magnetic TPS nanocomposite was investigated for the reduction of one of the most toxic and hazardous nitroaromatic compounds (PNP) that are widely employed in multiple industrial activities (28). The second application of the synthesized magnetic TPS nanocomposite is the evaluation of its photocatalytic activity toward the photodegradation of MB dye pollutant under visible light irradiation. 2. Experimental 2.1. Chemicals Aniline (Aldrich), FeCl3.6H2O 98% (Aldrich), 3-triethoxysilylpropylamine (APTES), FeCl2·4H2O (Aldrich), titanium n-butoxide (Sigma Aldrich, TBOT), NaOH pellets

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(LobaChemie, India) were used without further purification. Sodium boron hydride (NaBH4) (Johnson Matthey, UK), ammonia solution (Sigma Aldrich, 25 wt. %), silver nitrate (BDH, UK), ammonium persulfate (Aldrich, 98%), and p-nitrophenol (Sigma Aldrich, PNP) were used as received.

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2.2. Synthesis of magnetite (Fe3O4) NPs

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A mixture of FeCl3·6H2O (4 mL, 2 M) and FeCl2·4H2O (2 mL, 2 M) solutions was stirred vigorously in a beaker at 30 °C for 45 min. The Fe (III)/Fe (II) ratio was kept at 2. An aqueous ammonia solution (100 mL, 1 M) was then added dropwise to the previous mixture

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under nitrogen atmosphere till pH = 10 then the solution was stirred for about 1 h and the black magnetic Fe3O4 powder was magnetically collected, washed with distilled water and methanol several times till pH = 7, and finally dried for 24 hours (29). The formation of Fe3O4 NPs can

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be described by the following equation (30):

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2 3  Fe(aq)  2Fe.(aq)  8OH(aq)  Fe3O 4 (S)  4H 2 O(l)

2.3. Amino-functionalization of magnetite NPs

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0.8 ml of (3-Aminopropyl) triethoxysilane (APTES) was added to magnetite suspension (5 mg/mL ethanol), and the mixture was stirred at 25C for 4 hours under N2 atmosphere. The resulting Fe3O4-NH2 NPs were collected magnetically, washed with ethanol and deionized water, and dried (31). 2.4. Synthesis of Fe3O4@TiO2 nanospheres The prepared magnetite Fe3O4-NH2NPs were dispersed in ethanol:acetonitrile mixed solvent (250 mL: 90 mL) and sonicated for 15 min. Afterward, 1.5 mL of ammonia solution 6

(25 wt. %) was added to the sonicated mixture and mechanically stirred for 30 min. A solution of TBOT (3 mL in 20 mL absolute ethanol) was then introduced dropwise to the suspension with continuous mechanical stirring at 30 °C for 1.5 h to obtain Fe3O4@TiO2 core/shell nanospheres. The product was collected magnetically and washed with ethanol three times (32). 2.5. Synthesis of Fe3O4@TiO2-PANI nanocomposite 0.03 g Fe3O4@TiO2 nanocomposite was dispersed in 200 mL 1 M HCl solution and sonicated for 5 min. 1 mL aniline was then added dropwise with vigorous stirring to adsorb aniline monomers onto the surface of Fe3O4@TiO2 nanocomposite. To start the polymerization, APS solution (in 1 M HCl) was added and hence, a greenish black slurry appeared slowly,

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which was then filtered, washed with deionized water, dedoped with ammonia solution and washed with excess of water and methanol. Eventually, the product was dried at 60 C for 24 h and grinded to a fine powder (33). 2.6. Synthesis of Fe3O4@TiO2–PANI-Ag nanocomposite

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To ensure dedoping, 0.2 g of Fe3O4@TiO2–PANI nanocomposite was dispersed in 200 mL NH4OH (0.1M), stirred for 4 hours, filtered, and the procedures were repeated three times.

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The product was then collected, washed with ethanol and distilled water three times and dried for 24 hours. The resulting product was dispersed in AgNO3 solution (0.005 M) and stirred for

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8 hours and the resulting nanocomposite was then collected via filtration and drying. The whole

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synthetic procedures can be expressed by Scheme 1.

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Scheme 1 The formation mechanism of TPS magnetic nanocomposite.

2.7. Catalytic activity of TPS magnetic nanocomposite An alkaline PNP solution (2.5 mL of 7 mM) was added to 1 mg of TPS magnetic

nanocomposite in a quartz cuvette, and the UV-VIS absorption spectra were recorded. To start the reduction reaction, NaBH4 solution (0.5 mL of 10 mg mL-1) was added to the previous solution and the UV-VIS absorption spectra were measured with time to follow the reaction.

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2.8. Photocatalytic performance of TPS magnetic nanocomposite The photocatalytic activity of the synthesized TPS nanocomposite was evaluated by studying the photodegradation of MB dye (4 mg/L) under visible light irradiation. 40 mg of the TPS catalyst was dispersed into 100 mL aqueous solution of MB and the mixed solution was stirred in the dark for 15 min to reach the adsorption equilibrium. After exposing the mixture to the day visible light, the drop in MB concentrations was analyzed by a UV-VIS double beam spectrophotometer. 2.9. Characterization Fourier transform infrared spectra (FT-IR) were measured using a Bruker, Tensor 27

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FT-IR spectrophotometer with a frequency range from 4000 cm-1 to 400 cm-1. X-ray diffraction (XRD) patterns were measured using a GNR APD-2000 PRO diffractometer with Cu Ka radiation (40 KV, 30 mA) at a step scan mode. UV-VIS absorption spectra were measured using a UV spectrometer, UVD-2960 (Labomed Inc.). The morphology of the TPS magnetic

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nanocomposite was studied using a transmission electron microscope (TEM) (JEM-2100F) at 200 kV and a scanning electron microscope (SEM) (Hitachi S4800) at an accelerating voltage

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of 5 kV. A vibrating sample magnetometer (VSM) was employed for investigating the magnetic properties of TPS nanocomposite. A Shimadzu UV-2450 spectrophotometer was used for

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recording the UV-vis diffuse reflectance spectra (DRS). The photoluminescence (PL) spectra were measured using Hitachi F-2700 fluorescence spectrophotometer.

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3. Results and discussion

3.1. Characterization of TPS nanocomposite

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X-ray diffraction (XRD) is usually employed for studying the crystallinity and phase of the synthesized materials. Figure 1A represents the Fe3O4 diffraction patterns at 2𝜃 equivalent

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to 30.2, 35.74, 43.12, 53.51, 57.19 and 62.78 that can be indexed to (h k l) reflection peaks of [220], [311], [400], [422], [511] and [440], respectively for face centered cubic (FCC) phase of magnetite (34). This agrees well with the standard pattern of Fe3O4 (JCPDS Card no.19-0629) (35). After the coating with TiO2, Figure 1B exhibits peaks at 2θ = 25.3, 37.9, 48.1, 54, 55.2 and 62.8° that match precisely (101), (004), (200), (105), (211) and (204) reflections of anatase phase of TiO2, respectively (36). In Figure 1C, the extra diffraction peaks at 15°, 20°, and 25.5° arose from PANI, which indicates the existence of PANI matrix (37). Due to the coating with PANI, the peaks of Fe3O4 and TiO2 appear with low intensity. In addition, the new 8

broad peak at about 25.5°, after PANI loading, is assigned to the periodicity perpendicular of PANI (38). After loading of Ag NPs on the surface of Fe3O4@TiO2–PANI (Figure 1D), the peaks at 38°, 44°, 65° and 77 characterizes [111], [200], [220] and [311] planes of FCC Ag phase (JCPDS card no.04-0783), respectively (39). This confirms the loading of the Ag NPs on

(311)

(220)

(111)

(200)

the nanocomposites.

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TiO2

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(204)

Fe3O4

(105) (211)

PANI

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50

(440)

(422) (511)

(400)

(220)

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(A)

Ag

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(004)

(101)

(B)

(200)

(C)

(311)

Intensity, a.u.

(D)

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2, degree

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Fig. 1 XRD patterns of the prepared Fe3O4 (A), Fe3O4@TiO2 (B), Fe3O4@TiO2 /PANI (C),

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and (D) TPS magnetic nanocomposite.

The structural information and chemical components of the prepared nanocomposite

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materials were identified by the FT-IR spectra Figure 2. The characteristic peak of Fe3O4 appears at 590 cm-1 (Figure 2A) that is assigned to the stretching vibration of Fe–O bond and the peak at 3384 cm-1 arose from -OH on the Fe3O4 surface (40). On the other hand, the peak at 1599 cm-1 is attributed to the amino-functionalization of Fe3O4 magnetic particles (41, 42). The broad peak centered at 3395 cm-1 (Figure 2B) corresponds to the terminal Ti-O and HO-Ti-OH (43-45). Another wide peak at low wavenumbers from 400 to 800 cm-1 is assigned to Ti-O bending mode in TiO2 (46). In the FTIR spectrum of Fe3O4@TiO2/PANI (Figure 2C), the peak at 1118 cm−1 is attributed to C–N stretching mode for benzenoid ring (47), the peaks at 1579 9

cm-1 and 1458 cm-1 can be ascribed to the stretching vibrations of quinoid and benzenoid rings of PANI, respectively (48). On the other hand, the C–H out of plane and in the plane bending vibrations is represented by the peaks at 810 cm-1 and 2929 cm-1, respectively. A shift in C-N stretching vibrations from 1118 to 1168 cm-1 in (Figure 2D) is attributed to the interaction

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590

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(B)

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1268 1168 812 688 505 1118

1458 1207

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2929

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(C)

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Trancemittance, a.u.

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(D)

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between PANI and silver (49).

4000 3500 3000 2500 2000 1500 1000

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-1 Wavenumber, cm

Fig. 2 FT-IR spectra of (A) Fe3O4, (B) Fe3O4@TiO2, (C) Fe3O4@TiO2/PANI, and (D) TPS

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magnetic nanocomposite.

To investigate the surface morphology of the prepared TPS magnetic nanocomposite,

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SEM was measured as shown in Figure 3. The figure shows the presence of spherical agglomerates of TPS nanospheres with a mean diameter of 300 nm due to the globular morphology of PANI. Furthermore, the surface of the composite spheres is coarse, which is likely related to the growth behavior of the polymer. Moreover, the EDX spectrum of TPS nanocomposite (Figure 3B) was measured to identify the chemical composition, which is found to be C, H, O, Fe, and Ti elements and no unexpected elements were observed. This in turn

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confirms the purity of as-prepared TPS magnetic nanocomposite. In addition, the anchoring of silver NPs on the magnetic nanocomposite was assured by the existence of Ag peak.

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Fig. 3 SEM image of TPS magnetic nanocomposite (A) and its EDX pattern (B).

The TEM images of Fe3O4, Fe3O4@TiO2, Fe3O4@TiO2/PANI and TPS magnetic composite, are shown in Figure 4. Fe3O4 nanospheres are found to be monodisperse with a narrow size

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distribution of about 10 nm and the magnetic core ensures the easy separation of nanocomposites from reactant mixture (Figure 4A). After the coating with TiO2, a uniform layer

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of TiO2 with a thickness of about 30 nm coated onto Fe3O4 nanospheres (Figure 4B). In addition, some nanospheres were collected together and the size of Fe3O4@TiO2 NPs was found

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to be greater than that of the neat Fe3O4 due to the encapsulation of the Fe3O4 NPs in the TiO2 shell. Furthermore, the PANI matrix was clearly observed in Figure 4C and Fe3O4@TiO2 NPs were dispersed through the PANI matrix with low aggregations. Furthermore, some black dots

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can be clearly observed, which are assigned to the silver NPs. In addition, a lot of Ag NPs, with a diameter range of 10-30 nm, were randomly deposited on the PANI matrix, which indicates the successful attachment of Ag NPs to the PANI matrix as shown in Figure 4D. The typical

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HRTEM proved the presence of Fe3O4 NPs. The lattice fringes of TPS nanocomposite represent interplanner distance between the strips is around 0.25 nm that corresponds to the lattice plane

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[311] of Fe3O4 (50, 34).

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Fig. 4 TEM images of Fe3O4 (A), Fe3O4@TiO2 (B), Fe3O4@TiO2/PANI and TPS magnetic

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nanocomposite (D).

Fig. 5 HRTEM image of TPS magnetic nanocomposite.

Elemental mapping of TPS magnetic nanocomposite was analyzed, as shown in Figure

6. The results reveal the co-presence of Fe, Ti, O, C, N and Ag elements and all of them are well dispersed in the sample. As the EDS instrument deals with the sample surface, Figure 6E shows the presence of a low iron percentage on the surface of TPS magnetic nanocomposite, which agrees with EDX of TPS magnetic nanocomposite test. Figure 6F shows that Ag NPs are successfully loaded inside the PANI matrix. 12

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Fig. 6 Element mapping of TPS magnetic nanocomposite; carbon distribution (A), nitrogen distribution (B), oxygen distribution (C), titanium distribution (D), iron distribution (E) and

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silver distribution (F).

The porous structure of the synthesized TPS nanocomposite was analyzed with nitrogen

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adsorption/desorption measurements. The TPS nanocomposite exhibited type III sorption isotherms with hysteresis loops (Figure 7). The sample possessed BET surface area of 10.9 m2

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g-1, a total pore volume of 8.96 x 10-2 cm3 g-1 and a mean pore diameter of 32.875 nm. In addition, the pore size distribution curve (Inset of Figure 7) shows that the TPS nanocomposite

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have a mesoporous structure as the pore diameter ranges from 5 – 50 nm.

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dV/dP (cm3 g-1 nm-1)

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0.002

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Diameter (nm)

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Volume (cm3 g-1 STP)

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Relative pressure (P/P0)

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Fig. 7 Adsorption–desorption isotherm of TPS nanocomposite (Inset: pore size distribution

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The optical properties of Fe3O4 NPs, Fe3O4@TiO2 nanospheres, Fe3O4@TiO2-PANI nanocatalyst and TPS magnetic nanocomposite were investigated by UV–VIS diffuse

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reflectance spectroscopy as shown in Figure 8A. It is noted that the optical absorption of TPS nanocomposite was red shifted and enhanced over the whole range of the visible region compared with the absorption of the parent Fe3O4, Fe3O4@TiO2 nanospheres. This can be

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attributed to the PANI photosensitization for trapping a large number of visible light photons as well as the surface plasmon resonance of electrons present in the Ag NPs. Therefore, the

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incorporation of TiO2, PANI and Ag NPs led to extending the TiO2 spectral response range to the visible light region (51).

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Figure 8B shows (K*E)1/2 vs E plots, derived from diffuse reflectance spectra of Fe3O4 NPs, Fe3O4@TiO2 nanospheres, Fe3O4@TiO2-PANI nanocatalyst and TPS magnetic nanocomposite. Kubelka-Munk factor (K) is calculated by using the formula, K = (1-R)2/2R, where, R represents the % reflectance and E stands for energy of the incident radiation (52, 53). As a semiconductor, TiO2 has a valence band (VB) and a conduction band (CB), and the energy difference between these two energy levels is called the band gap (Eg). The optical band gap of the nanocatalysts can be determined by the extrapolation of the linear of the spectra to E photon = 0 in the curve of (E

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= hν) vs [F(R)hν]1/2 (54). The calculated band gaps optically are 14

1.74, 3.37, 1.57 and 1.54 eV for Fe3O4 NPs, Fe3O4@TiO2 nanospheres, Fe3O4@TiO2-PANI nanocatalyst and TPS nanocomposite respectively (55). The above results suggest that the TPS nanocomposite produced much more electron–hole pairs than other nanocatalysts under

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excitation by simulated solar light, which could result in a higher photocatalytic activity.

Fig. 8 (A) UV-vis diffuse reflectance spectra and (B) Optical band gap of as-prepared

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photocatalysts.

The photogenerated electron/hole separation efficiency is an important target, which

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affects the TiO2 photocatalytic activity. The high photocatalytic activity of nanophotocatalysts is associated with reducing the recombination rate of photogenerated electron/hole. High intensity emission signals in the PL spectra arise mainly from the introduction of new structural

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defect sites and recombination centers of the photo-generated electron–hole pairs (56). However, the low PL intensity implies high separation efficiency of the charge carriers and the absence of structural defects.

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To measure the recombination rate of photon-generated electrons/holes in TPS magnetic nanocomposite, PL emission spectra were measured. Figure 9 shows the PL spectra

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of TPS magnetic nanocomposite in the range of 400-600 nm after excitation at 400 nm. Compared to the strong emission peak at 457 nm of the PANI@TiO2 nanofibers (57), and the bare TiO2 that does not possess absorbance in the visible light region (58), the Ag-coated Fe3O4@TiO2 has a lowered PL intensity under the same intensity of excitation irradiation (59). This result is because the metal work function of Ag is higher than that of TiO2. As compared to all the above materials, the TPS nanocomposite has a significant lower PL intensity. This may be assigned to the absence of structural defect sites or recombination centers that significantly reduces the recombination rate of electrons and holes via the surface modification 15

of the nanocomposite with Ag NPs. Hence, TPS magnetic nanocomposite has an efficient separation of photogenerated electron/hole and superior photocatalytic activity.

400

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450

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500

525

550

Wavelength, (mn)

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PL intensity (a.u.)

TPS magnetic nanocomposite

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Fig. 9 PL emission spectra of TPS magnetic nanocomposite (excited at 400 nm).

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Due to the presence of magnetite core inside the nanospheres, the magnetic property was evaluated as shown in Figure 10. The saturated magnetization value was measured to be

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7.1 emu g-1 and 3.1 emu g-1 for Fe3O4@TiO2 and TPS magnetic nanocomposite, respectively. The difference in the saturated magnetization value is caused by PANI shells. This magnetic property helps to recycle the catalyst easily from the solution by using an external magnetic

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field.

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Fe3O4@TiO2

-1 7.1 emu.g

TPS magnetic nanocomposite

Magnetic moment (emu)g

-1

6 -1 3.1 emu.g

4 2 0 -2 -4 -6 -8 0

2000 4000 6000 8000 10000

Magnetic filed (kOe)

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-10000-8000 -6000 -4000 -2000

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Fig. 10 Magnetic hysteresis loop (M–H) of Fe3O4@TiO2 and TPS magnetic nanocomposite.

3.2. Catalytic activity of TPS nanocomposite

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The model reaction for examining the catalytic efficiency of TPS nanocomposite is the reduction of PNP to p-aminophenol (PAP) employingNaBH4 as a reducing agent at room

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temperature (60). On the addition of NaBH4 to PNP solution, no absorption intensity change was noticed even after several days so the addition of a catalyst is required to increase the reaction rate and this fact is established in the literatures (60, 61). In a quartz cuvette, an alkaline

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solution of PNP was added to TPS nanocomposite and the UV-VIS absorption spectra were recorded. Afterwards, NaBH4 was added to the previous solution and the reduction reaction

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progress was pursued by measuring the UV-VIS absorption spectra at different time intervals. The intensity of the characteristic peak of PNP at 400 nm quickly decreased and a new peak appears rapidly at around 310 nm (Figure 11A), which is the characteristic peak of PAP and the

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color of solution converted from bright yellow to colorless. The change of the absorbance of PNP solution with time is exhibited in Figure 11A. This simple color change means that no byproduct was produced during the reduction reaction. The reduction reaction ended within 20 min indicating the high catalysis rate of TPS nanocomposite. In separate experiments to investigate the effect of catalyst dose on the rate of the reduction reaction, 2 mg (Figure 11B) and 3 mg (Figure 11C) of TPS nanocomposite were used. The reduction of PNP to PAP was found to finish in 12 min and 6 min using 2 mg and 3 mg, respectively of TPS nanocatalyst.

17

This behavior can be understood on the basis of increasing the number of the catalytic active sites of TPS nanocomposite on using higher catalyst doses. Similar behavior was observed by many authors (34, 60).

1.2

0 min 2 min 4 min 8 min 12 min 16 min 20 min

(A)

0.8

0.6

0.4

0.2

0.0 250

300

350

400

450

500

550

(B)

0.6

0 min 2 min 4 min 8 min 12 min

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Absorbance, a.u.

0.8

lP

1.0

re

-p

Wavelength, nm

ro of

Absorbance, a.u.

1.0

0.4

ur

0.2

Jo

0.0 250

300

350

400

450

Wavelength, nm

18

500

550

1.0

0 min 2 min 4 min 6 min

(C)

Absorbance, a.u.

0.8

0.6

0.4

0.2

300

350

400

450

500

Wavelength, nm

550

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0.0 250

Fig. 11 UV-VIS absorption spectra during the reduction of PNP to PAP by NaBH4 using 1

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mg (A), 2 mg (B), and 3 mg (C) of TPS magnetic nanocatalyst.

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Because of the high concentration of the NaBH4 compared to PNP, pseudo first order kinetics were used to calculate the rate of this reaction. Figure 12A presents the linear relation

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between _lnAt/Ao against time, where Ao is the initial absorbance and At is the absorbance at time t and k is the rate constant that was calculated to be 0.3 min-1. This rate constant is comparable to the previous literatures, as seen in Table 1. By comparing the efficiency of the

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listed catalysts in this table, it can be deduced that our PANI-based nanocatalyst has the best

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ur

efficiency regarding the catalyst dose and the reduction rate constant.

19

5

(A)

-2

k =30x10 min

-1

4

-ln At/A0

3

2

1

0 2

4

6

8

10

12

14

16

Time, min

1.0

re

0.8

0.6

lP

0.4

0.2

na

Absorbance at 400 nm

20

-p

(B)

18

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0

0.0

5

10

15

20

Time, min

ur

0

Fig. 12 _ln At/Ao against the reduction reaction time of PNP by NaBH4 using TPS magnetic

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nanocatalyst (A) and the time-dependent change of absorbance at 400 nm (B).

20

Table 1 Rate constant of catalysts used for PNP reduction compared to TPS magnetic nanocomposite. Substrate

Dose

k

Reference

(10-3 s-1) Fe3O4@SiO2–Ag

1g

Ag NPs-supported poly[N-(3- 7.2 mg

7.67

(62)

3.17

(63)

trimethoxysilyl) propyl] aniline 10 mg

4.1

(64)

p(AMPS)-Cu a

10

1.72

(65)

Ag@PANI-CS-Fe3O4

1 mg

2.0

MTPS nanocomposite

1 mg

2.0

Ag/carbon fiber

1 mg

4.2

Ag-NP/Cb

1 mg

Ag/HHPc

1 mg

Ag10@SBA-15d

0.9 mg

Ag/PSNM-3e

2 mg

(66)

-p

(67) (68)

0.5

(69)

re

1.69

(70)

2.2

(71)

2.7 mg

21

(72)

1 mg

5

This work

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TPS magnetic nanocomposite

(34)

0.127

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PANINFs @Ag

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Ag@ Egg shell membrane

a

is p(2- acrylamido-2-methyl-1-propansulfonic acid)-Cu composites is Ag NPs/carbon spheres c is human-hair-supported noble metal (Ag) d is Ag MPs within the uniform pore channels of mesoporous silica e is Poly (styrene-N-isopropylacrylamide-methacrylicacid)

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ur

b

Recovery is one of the critical parameres so that the magnetic nanocatalyst was

separated using an external magnetic filed, then with a syringe bit by bit, the produced PAP can be collected to preserve the small amount of catalyst as shown in Figure 13.

21

Fig. 13 The separation technique of the heterogeneous TPS nanocatalyst.

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The stability and recycling of the nanocatalyst is an important parameter to describe its applicability for the practical use. The stability of TPS nanocatalyst was examined over six cycles. After the first cycle and in the first recycle (second cycle), the catalyst efficiency fell to 93%. Furthermore, the value of the nanocatalyst efficiency decreased to 92, 91, 83 and 81% in

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the next recycles. This small loss in the efficiency may be attributed to the small and partial loss of the catalyst's dose during the separation processes between cycles as shown in Figure 14B.

with a small loss of the efficiency.

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Therefore, an extra advantage of TPS nanocatalyst is its stability even after running for 5 cycles

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The high efficiency of the TPS nanocatalyst arose from the presence of numerous chelating amino groups, which are responsible for anchoring the silver metal NPs onto a little

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ur

PNP (73, 32).

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weight of the catalyst. Furthermore, TiO2 shell helps for facile mass transportation of the target

22

(A)

Absorbance at 400 nm, a.u.

1.0

1 recycle 2 recycle 3 recycle 4 recycle 5 recycle

0.8

0.6

0.4

0.2

0.0 5

10

15

20

25

30

35

40

Time, min

(B)

re

80

60

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Efficiency, %

50

-p

100

45

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0

40

na

20

0

ur

0

1

2

3

4

5

Recycles numbers

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Fig. 14 Reusability of 1 mg of TPS magnetic nanocatalyst toward the reduction of PNP using NaBH4 as a reducing agent.

To compare the catalytic activity between the nanocomposite contents, 1 mg of each

content in the TPS nanocomposite (Fe3O4, Fe3O4@TiO2, Fe3O4@PANI–TiO2) was added separately to check their catalysis ability for the reduction of PNP using the same conditions of TPS nanocatalyst Figure 15. The obtained results shows that 92% of PNP, which was monitored using a UV-VIS spectrophotometer, was removed with TiO2 modified-PANI (74). However, in 23

the existence of noble metal (silver) NPs, the catalytic properties of the composite was enhanced, and 100% of PNP was removed in 16 minutes.

1.2

TPS magnetic nanocomposite Fe3O4@TiO2/PANI TiO2 Fe3O4

0.8

0.6

0.4

0.2

0.0 0

10

20

30

40

50

60

70

ro of

Absorbance at 400nm, a.u.

1.0

80

100

-p

Time, min

90

re

Fig. 15 Absorbance for PNP reduction change with time, using different catalysts.

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3.3. Photocatalytic performance of TPS nanocomposite

The photocatalytic activity of TPS magnetic nanocomposite was studied by monitoring the photodegradation of MB (as a model dye) using 0.04 g of TPS nanocomposite on exposure

na

to visible light irradiation with continuous stirring.

MB is known to have a maximum absorption at nearly 664 nm, so the decrease of this band intensity indicates the degradation. To ensure the photocatalytic activity of TPS magnetic

ur

nanocomposite toward the MB degradation, the experiment was done firstly under the dark

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condition. As shown in Figure 16, only about 30 % of MB was degraded after 140 minutes (75).

24

0 min 140 min

Absorbance, a.u.

0.8

0.6

0.4

0.2

0.0 500

550

600

650

700

750

ro of

450

Wavelength, nm

Fig. 16 Time-resolved photocatalytic spectra of MB dye (4 mg L-1) with 0.04 g TPS

-p

nanocomposite under the dark condition.

The photocatalytic performance of TPS was then studied under day visible light

re

irradiation at room temperature. As can be seen from Figure 17, the decrease in the absorption band intensity of MB with increasing time indicated that the MB dye concentration decreases

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and hence, the MB dye is degraded by TPS magnetic photocatalyst, where more than 65% of the dye was degraded within 15 min. After 1.5 h of irradiation, MB was almost degraded completely, which indicates the significant degradation rate and the enhanced photocatalytic

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activity of TPS magnetic nanocomposite under day visible light irradiation.

ur

1.0

Jo

Absorbance, a.u.

0.8

0.6

0 min 5 min 15 min 30 min 60 min 90 min

0.4

0.2

0.0 450

500

550

600

650

Wavelength, nm

25

700

750

Fig. 17 Time-resolved photocatalytic spectra of MB dye (4 mg L-1) with 0.04 g TPS magnetic nanocomposite under visible light.

Moreover, the photodegradation of MB was followed by concentration changes (Co/Ce) as a function of visible light illumination time. The dye photodegradation efficiencies can be calculated from the equation: (1)

ro of

Eff .0 0   c0  ce ) / c0 100 0 0

where, Co is the initial MB dye concentration and Ce is the MB dye concentration after photoirradiation.

In order to evaluate the kinetic mechanism, which controls the photocatalytic reaction,

-p

the pseudo first-order kinetics were tested (76). As shown in Figure 18, the photodegradation process of MB dye is following the pseudo first-order kinetics by the linear transforms (77),

re

ln(qo/qt) = kt, where qo is the adsorption equilibrium concentration of MB, qt is the concentration of MB at time t, and k is the rate constant that was calculated to be 3.7x10-2 min-1 using 40 mg of TPS magnetic nanocatalyst after irradiation for 1.5 h.

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An excellent degradation rate was obtained using a limited dose of the nanocatalyst and relatively short irradiation time comparing to other catalysts, as shown in Table 2. By navigating

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through this table and comparing the performance of the mentioned catalysts, our synthesized nanocatalyst possesses the best achievement regarding the catalyst dose, the type of used light,

ur

the irradiation time and the resulting degradation rate constants.

Table 2 Rate constants of photocatalysts compared to TPS magnetic nanocomposite.

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Photocatalyst

Dose

Irradiation

k

Irradiation

time (min)

(x 10-3 min-1)

type

Reference

Fe3O4@PANI/TiO2

30 mg

140

19.7

visible-light

(17)

TiO2@Pt@C3N4

30 mg

180

16.0

visible-light

(78)

3D Flower-like

50 mg

140

0.01

UV light

(19)

TiO2/PANI composite 26

Au/P-TiO2

2 mg

360

4.3

visible-light

(33)

PANI-sensitized

50 mg

120

15.3

visible-light

(15)

PPy–PANI/TiO2

150 mg

120

8

visible light

(47)

TiO2

150 mg

120

1

visible light

(47)

PANI-modified TiO2

80 mg

360

53

visible light

(18)

W-TiO2/RGO

100 mg

90

58

visible light

(79)

PANI/TiO2 modified

30 mg

180

5.1

sunlight

(80)

5 mg/50

160

TiO2 composite

nanocomposite

TiO2/SiO2

ro of

nanocomposite

ml dyes

Au-PANI@TiO2

20 mg

140

PANI/ZnO

100mg

180

TPS magnetic

40 mg

90

(81)

10.6

visible light

(82)

25.7

visible light

(83)

37.0

visible light

This work

lP

re

nanocomposite

UV light

-p

solution

7

0.8

0.4

0.2

ur

log(qe-qt)

na

0.6

Jo

0.0

-0.2

0

10

20

30

40

50

Time, min

Fig. 18 Pseudo-first order plot for the photocatalytic application of MB using TPS magnetic nanocatalyst at 25 C. [MB] = 4 mg L-1.

27

The high photocatalytic activity of TPS magnetic nanocomposite can be attributed to the photosensitization of MB and the strong synergetic effect of TiO2 and PANI that induces an interesting charge transfer (15, 84), as the presence of PANI increases the reactive sites, which enhance the photocatalytic reaction (85). The attached PANI on the surface of TiO2 lead to transfer the photogenerated electrons away from the TiO2 toward PANI achieving efficient charge separation (86). Furthermore, after the light irradiation, PANI work as excellent hole acceptor in addition to an electron donor (87). All these unique characteristics make PANI an optimal material for charge separation in the field of photocatalysis and hence, the synthesized magnetic nanocomposite is more efficient photocatalyst for the degradation of MB as compared

ro of

to TiO2 alone. TiO2 NPs play a strong photocatalysis effect under visible and UV-light irradiation. When the exciting photons have energy that is as or more than the TiO2 (3.2 eV) band gap energy, electrons (e-) are excited from the VB to CB, leaving positive holes (h+) in the VB. The excitedstate e- and h+ pairs react with a stable electron donor and acceptor, producing radicals such as

-p

O- 2 and .OH that are exceedingly active and can reduce or oxidize pollutants. In this work,

.

when the surfaces of TiO2 NPs are sensitized using PANI band gap 2.8 eV, which made TiO2

re

NPs efficient as electron donor and active hole transporter to the visible-light excitation (88), so that its photocatalytic activity increased, because the band gap of TiO2/PANI is smaller than

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the band gap of bare TiO2 NPs that allows TiO2/PANI to absorb more photons. PANI and TiO2 absorb photons at their interface when the visible light illuminated PANI/TiO2. Furthermore, the energy levels of PANI and TiO2 NPs are in the order E(LUMO) > E(CB) > E(HOMO) >

na

E(VB) (80), where the lowest unoccupied molecular orbital (LUMO) level of PANI and the CB of TiO2 NPs are matched, which facilitated the charge transfer (89). Consequently, the

ur

generated electrons from PANI can be transferred into the CB of TiO2 NPs, Ag electrons are injected into CB of TiO2 and the photocatalytic activity will be enhanced under the visible light irradiation. These electrons react with O2 to generate the superoxide radical ion .O2-, at the

Jo

same time the positive holes can react with OH- or H2O to produce hydroxyl radical .OH (90). The formation of these highly reactive radicals is responsible for the MB degradation (91). In this case, PANI acts as a photosensitizer so MB could be degraded by more than one pathway. The photocatalytic mechanism is clearly described in Scheme 2. Moreover, the energy levels of PANI are more negative than those of TiO2. The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) of PANI is estimated to be − 1.9 and + 0.8 eV vs. NHE, respectively (92, 93). The CB and VB position for TiO2 are -0.50 and 2.7

28

eV, respectively (94), (95). A proposed mechanism for the MB dye degradation using TPS magnetic nanocomposite as a nanophotocatalyst is shown in Scheme 3 that is accordance with

ro of

previous studies .

Scheme 2 The enhanced photocatalytic activity mechanism of TPS magnetic nanocomposite

Jo

ur

na

lP

re

-p

under visible light irradiation.

Scheme 3 A proposed mechanism for MB dye degradation using TPS magnetic nanocomposite as a nanophotocatalyst under the normal day visible light.

29

4. Conclusion In summary, we have presented a simple and efficient method to synthesize the TPS magnetic nanocomposite with enhanced dual catalytic performance and photocatalytic activity under the normal day visible light accompanied with fast magnetic separation. The structure was fabricated by depositing a shell of TiO2 onto the surface of Fe3O4 magnetic core and coating with a PANI layer then decorating the surface with silver NPs. The strong synergetic effect of Ag NPs, PANI with TiO2 shell was observed. TPS magnetic nanocomposite exhibited rapid response to MB degradation and the reduction of PNP to PAP. The dual catalytic applications of TPS magnetic nanocomposite, its good reusability and low cost are the important points in

tailored enhanced catalytic efficiency and long-term stability.

Author Statement:

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this work. This wok can open the door toward the design of new nanophotocatalysts with well-

-p

Basel Al-saida: Investigation, Writing- Original draft preparation Wael A. Amer: Conceptualization, Investigation, Methodology, Visualization, Validation, Writing- Reviewing and Editing, Supervision. Elsayed E. Kandyel: Supervision. Mohamad M. Ayad:

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Declaration of Interest Statement:

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Conceptualization, Methodology, Validation, Writing- Reviewing and Editing, Supervision.

Acknowledgments

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The authors declare that they have no conflict of interest.

The authors thank Faculty of Science, Tanta University and the Egypt-Italy joint project

ur

entitled “Development of innovative magnetically recoverable three-component nanocatalysts for wastewater treatment” for their support. The authors are also grateful to Cristina Della Pina,

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Ermelinda Falletta, Alessandro Ponti and Anna M. Ferretti for their helpful cooperation. 5. References

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