Controlled synthesis of reduced graphene oxide supported magnetically separable Fe3O4@[email protected] ternary nanocomposite for enhanced photocatalytic degradation of phenol

Powder Technology 356 (2019) 547–558

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Controlled synthesis of reduced graphene oxide supported magnetically separable Fe3O4@rGO@AgI ternary nanocomposite for enhanced photocatalytic degradation of phenol Ghani Ur Rehman a, Muhammad Tahir b, P.S. Goh a, A.F. Ismail a,⁎, Imran Ullah Khan a a Advanced Membrane Technology Research Center (AMTEC), School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor Bahru, Malaysia b Chemical Reaction Engineering Group (CREG), School of Chemical and Energy Engineering, Faculty of Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor Bharu, Malaysia

a r t i c l e

i n f o

Article history: Received 27 December 2018 Received in revised form 16 July 2019 Accepted 14 August 2019 Available online 16 August 2019 Keywords: Reflux method Graphene oxide Fe3O4@rGO@AgI APTES and TEOS Phenol Plasmon resonance effect

a b s t r a c t A ternary nanocomposite of Fe3O4@rGO@AgI was successfully synthesized by reflux method for photodegradation of phenol. The prepared nanocomposite was characterized for the physicochemical properties through XRD, FESEM, TEM, TGA, FTIR, and PL spectroscopy techniques. Fe3O4@rGO@AgI exhibited higher photocatalytic performance of 99% phenol degradation compared to 62, 75 and 78% using Fe3O4, Fe3O4@rGO and Fe3O4@AgI nanocomposites, respectively. The superior photocatalytic performance was mainly attributed to the rapid transportation of photogenerated electrons from GO nanosheets to AgI. The addition of H2O2 has further enhanced the phenol degradation and was the optimized loading amount of 0.4 g/350 mL achieved the highest degradation efficiency. The findings revealed that basic conditions, initial phenol concentration and catalyst amounts have significant influence on the photocatalyst performance. The 81% recyclability after five continuous cycles implied the excellent stability of the photocatalyst for practical applications. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Over the last two decades, water pollution has become a dangerous hazard to human beings and for the atmosphere. As one of the most commonly used organic raw materials, phenol, and its byproducts can be found in an extensive range of industries. Phenol is harmful and highly toxic even at low concentration, the issues related to the discharge of phenolic compounds in industrial effluents have raised major concern, hence needed to be urgently addressed [1]. Many conventional procedures such as chemical oxidation, extraction, adsorption, biological treatment, membrane technology, and ultraviolet oxidation have been proven to capable of removing phenol and its derivatives from the wastewater [2–4]. Nevertheless, these processes suffer from some limitations and disadvantages such as high cost, complex installation and less energy efficient [5,6]. Furthermore, some of these processes simply convert the pollutants from one medium to another without sufficiently removed from the water sources [7,8]. Photodegradation is one of the promising and practical tools in reducing hazardous materials from the wastewater. WO3, ZnO, TiO2, and Fe3O4 are the common semiconductors used widely as photocatalysts for the removal of natural and inorganic pollutant [9–12].

⁎ Corresponding author. E-mail address: [email protected] (A.F. Ismail).

https://doi.org/10.1016/j.powtec.2019.08.026 0032-5910/© 2019 Elsevier B.V. All rights reserved.

Compared to conventional photocatalysts, Fe3O4 has gained popularity in this field as it exhibits excellent performance which rooted from its exceptional magnetic, electrical and catalytic properties [13,14]. Particularly, the magnetic properties of Fe3O4 have enabled the easy recycling where the only external magnet is required to separate the photocatalyst from its suspended solution. However, Fe3O4 has limitations such as low quantum efficiency, UV-active, rapid charge recombination, and narrow optical response. It is also easily agglomerated into clusters due to their higher surface exterior energy. Hence, it is desired to immobilize Fe3O4 on supports or depositing them inside an aggressive layer to fully harness their exceptional activity [15]. In addition, the catalytic activity of the Fe3O4 facilitates less alteration rate between the Fe2+ and the Fe3+. Essentially, hybridization of Fe3O4 with an accelerating component is attractive to support the conversion between Fe3+ and Fe2+. Silver-based semiconductor nanocomposite, such as (AgCl, AgBr, and AgI) have been reflected as possible replacements for TiO 2 photocatalysts [16]. Silver-halide (Ag-X) based catalysts, demonstrated superior performance in the degradation of organic pollutants. Silver halide such as AgI also has smaller particles size than the other metal nanoparticles thus possesses a high surface area for the reaction. A great number of works have evidenced that Ag-X (AgCl, AgBr, and AgI) with irregular plasma impact can be used as effective materials to modify the optical properties of Fe3O4. To enhance the photocatalytic action and dependability of

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uncovered AgI nanocomposite can be assorted with Fe3O4 to form nanocomposites, which may encourage electron transportation and hinder (e- h+) recombination [17]. A sequence of nanocomposites of AgI used as photocatalyst are g-C 3 N 4 /Fe3 O 4 /AgI/Bi 2 S 3 , Fe3O4-TiO2: Gd, Ag/F-TiO2, g-C3N4/Fe3O4/AgI, Fe3O4/BiOI/AgI, AgIBiOI/PAN, Ag-modified Zn 2 GeO 4 nanorods, and AgI/Bi1 2 O 17 Cl 2 [18–25]. Recently, graphene oxide (GO) has received significant considerations to overcome the charge recombination ratio and to enhance the optical absorption of photo-catalyst [26]. GO is broadly utilized as a model to fabricate nanocomposite with other semiconductor metal oxides to enhance the efficiency of the ternary hybrid. Graphene-based photocatalysts have many advantages over the conventional photocatalyst owing to their selective electronic characteristics producing from the sp2 hybridized carbon atoms with ultra-fast electron transport process from the energized semiconductors to the graphene sheet. Secondly, the controlled size of semiconductors can also enhance the photocatalytic efficiency. Thirdly, the higher transparency of sheets improves the use of exciting light. GO has a filler character in between the various semiconductors which further enhanced the stability and degradation efficiency of the hybrid nanocomposite. Numerous studies have been carried out to prepare graphene base ternary nanocomposites such as ZnO-RGO/RuO2, Fe3O4@SiO2@ZnO-Ag, rGO/Fe3O4/ZnO, NH2-modifiedGO sheets encapsulated with Fe3O4 (GO/Fe3O4) [27–30]. Remarkably, these nanocomposites exhibited promising adsorption behavior and improved photocatalytic activity toward organic pollutants [31]. To the best of our knowledge, development of Fe3O4@GO@AgI ternary nanocomposite are not yet studied for the photo-degradation of phenol. In the present work, magnetically separable Fe 3 O 4 @rGO@AgI nanocomposite was synthesized by a reflux method. Prior to the formation of the ternary system, tetraethyl orthosilicate (TEOS) and 3-aminopropyl triethoxysilane (APTES) were used to assist amino functional groups in order to reduce the tendency of Fe3O4 agglomeration. Furthermore, the embedding of NH 2 groups on the exterior surface of Fe3 O 4 nanoparticles can form hydrogen bonds with the functional groups on the GO surface. Thereby, it facilitates the interaction between the two entities. The as-prepared nanocomposite was characterized by spectroscopic, diffraction and microscopic techniques to investigate their elemental composition, formation and visual properties. The Fe 3 O 4 @rGO@AgI ternary nanocomposite shows remarkable photocatalytic performance, stability, and recyclability associated with the pristine materials. Numerous parameters were observed such as the addition of H2O2, low initial solution concentration, while 0.4 g/350 mL was the optimized loading amount for achieving the highest degradation efficiency. Moreover, the five continuous cycles of testing have shown the excellent stability of the nanocomposite.

2. Experimental 2.1. Materials NaNO3, Graphite powder, H2SO4, H2O2 (30%), and KMnO4 were got from Sigma-Aldrich (USA). Sodium hydroxide (NaOH), Isopropyl alcohol, concentrated ammonium aqueous solution (NH3·H2O 25 wt%) obtained from Merck KGaA Germany. Iron (II) sulfate heptahydrate (FeSO4·7H2O, 98%, M.wt = 151.91 g∙mol−1), sodium nitrite (NaNO3, 99%), phenol were purchased from Sigma-Aldrich (Nottingham, UK) (Ethanol, 99.9%) was acquired from RCI Labscan. (TEOS 98%) from ACROS. silver nitrate (AgNO3) and sodium iodide (NaI) was obtained from Loba Chemie and used as normal. Entirely the compounds were utilized with no further refining, and solutions were prepared to utilize distilled water and exploited for all the arrangements.

2.2. Synthesis of Fe3O4 Iron (II) sulfate heptahydrate (FeSO4∙7H2O) was utilized as a precursor for the preparation of Fe3O4. Firstly, 3.3 g of FeSO4∙7H2O and 2 g of NaNO3 was dissolved in 50 mL of distilled water. Then, 2.5 M NaOH solution was added dropwise into the reaction mixture while the temperature was maintained at 80 °C. It was allowed to continuous persistent stirring to favor the growth of the widespread nanocomposite crystals. The resultant mixture was cooled down to 25 °C and rinsed several times with RO water to exclude any unreacted chemicals. An exterior magnet was used to collect the Fe3O4 nanoparticles and dried in an oven at 80 °C for 12 h. 2.3. Synthesis of GO GO was synthesized by the oxidation of graphite applying KMnO4 as a solid oxidizing agent [32]. NaNO3(1.5 g) and graphite (3 g) were mixed well in 69 mL of concentrated H2SO4 and the suspension was stimulated at 0 °C for 15 min. In the next step, 9 g of KMnO4 was fused steadily into the fusion while balance the temperature lower 20 °C to prevent from overheating and blast. At that point, the mixture was diluted by RO water (500 mL) and remained KMnO4 was diminished by the addition of H2O2(30 mL). Moreover, the mixture was centrifuged and rinsed with RO water for four intervals, the GO dispersion in water was presented to sonication for 30 min. Lastly, GO was acquired by filtration of the product and was dry in the oven again at 80 °C for overnight. 2.4. Synthesis of GO encapsulated Fe3O4 To synthesize a binary magnetic Fe3O4@rGO nanocomposite, Fe3O4 was the first to surface modified with APTES and TEOS. 0.1 g of Fe3O4 nanoparticles were first dispersed in ethanol solution. 40 mL of RO water and 1 mL concentrated ammonia solution was introduced dropwise to the suspension of TEOS and APTES under ultrasonication. The suspension was continuously stirred at 35 °C for 3 h. The amino functionalized Fe3O4 was detached by an external magnet and rinsed with ethanol and water. The amino functionalized Fe3O4 was then dispersed in the GO aqueous solution dynamic stirring at 75 °C for 1 h. The Fe3O4@rGO particles were washed away with deionized water for 3 times and later on dried out in an oven at 80 °C for 12 h prior to the next modification. 2.5. Synthesis of Fe3O4@rGO@AgI Fig. 1 shows the schematic synthesis of the Fe3O4@rGO@AgI nanocomposite. 0.3 g of Fe3O4@rGO was dispersed in 150 mL of water by ultrasonication for 30 min. Later, 0.054 g of AgNO3 was added to the solution in mechanically stirring at 25 °C for 2 h. Then, sodium iodide solution with a concentration of 0.096 g/mL was added dropwise and refluxed at 96 °C for 1 h. The prepared dark mixture was centrifuged at 10,000 rpm to eliminate the precipitate. Then, it was washed thrice with RO water and ethanol. The sample was finally dried in an oven at 80 °C for 12h. 2.6. Characterization The crystallinity of the photocatalyst was investigated by X-ray diffraction (XRD) (D8 Advance Diffractometer, Bruker, USA) using CuKa emission (0.154 nm) at 40 kV and 100 mA. The functional groups and chemical interactions of the photocatalyst verified by FTIR spectra in KBr disks by a Perkin Elmer 2000 system spectrometer in the wavenumber assortment of 4000–400 cm−1. To observe the layer creation and to perform a basic investigation of the prepared compounds, the fieldemission scanning electron microscope (FESEM, JEOLJSM 6380LA) committed with energy dispersive X-ray spectroscopy (EDX) were involved.

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Fig. 1. Schematic diagram of synthesis steps for Fe3O4@rGO@AgI nanocomposite: (a) Fe3O4 modified by APTES and TEOS (b) Synthesis step of GO wrapped Fe3O4. (c) Synthesis step of Fe3O4@rGO@AgI. The Fe3O4@rGO@AgI nanocomposite was prepared by reflux method.

The thermal strength of the composite was studied by thermogravimetric analysis (TGA) (Metler-Toledo) with an air flow rate of 50 mL/min and heating rate of 5 °C/min. The thermal decomposition was performed in the temperature range of 30–1000 °C. The photoluminescence (PL) spectrum was obtained using a fluorescence spectrophotometer. 2.7. Photocatalysis experiments

3. Results and discussion 3.1. Crystallinity analysis Fig. 2 shows XRD patterns of Fe3O4, GO, Fe3O4@rGO, Fe3O4@AgI and Fe3O4@rGO@AgI samples. Fig. 2(a)shows characteristic diffraction peaks of Fe3O4 are located at 2θ = 18.27°, 30.10°, 35.42°, 43.06°, 56.93°, and 62.55°, which corresponds to the Miller indices of (111),

The performance of Fe3O4@rGO@AgI was tested for photodegradation of phenol in aqueous solution. 0.2 g of the photocatalysts were isolated into 350 mL solution with 50 ppm phenol concentration. UV-C light through a wavelength of 254 nm was used to activate the photocatalytic reaction. All the photocatalytic reactions were conducted in a cylindrical shaped reactor with consistent mechanical stirring at 25 °C. The suspension was continuously stirred for 9 h under the dark condition to accomplish adsorption-desorption equilibrium. The light was then turned on and the mixture was consistently stimulated for 9 h to permit the photodegradation of phenol. During the reaction, samples were collected at consistent intervals and filtered with a syringe channel (Millex PES,0.22 μm). The qualitative analysis of phenol was studied using high-performance liquid chromatography (HPLC). The photocatalytic degradation rate and elimination efficiency of phenol under UV radiation was calculated by measuring the peak area from the HPLC spectrum using Eq.(1):

Degradation ð%Þ ¼

C ο −C t  100 Cο

ð1Þ

where Cₒ and Ct are the initial phenol concentration and phenol concentration at time t respectively. 2.8. Photocatalytic degradation kinetics measurement The degradation degrees of photocatalytic oxidation of phenol was determined to utilize the Langmuir– Hinshelwood kinetic model which is explained in Eqs. (2) to (4): r ¼ dc=dt ¼ −kKC=ð1 ¼ KC ÞÞ

ð2Þ

wherever, r is the degradation rate of the reactant (mg/L min), C is the initial concentration of the reactant (mg/L), t is the light time, K is the Langmuir adsorption condition constant (L/mg) and k is the response rate constant (mg/L min). The Eqs. (3) and (4) represent the concentration of pollutant (i.e.,KC b b1) and a pseudo-first order model respectively. r ¼ dc=dt ¼ −kKC

ð3Þ Fig. 2. XRD spectra of Fe3O4, GO, Fe3O4@rGO and Fe3O4@rGO@AgI.

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(220), (311), (222), (400), (422), (511) and (440), respectively. The peak positions are in good agreement with the standard reference data (ICDD (PDF-2Release 2015 RDB) 010–080-6402). It indicates that Fe3O4 exists in a magnetite phase [33]. The GO shows a sharp diffraction peak at 2θ = 10.67°, which suggests that the (001) inner-planar space has been extended upon oxidation. The XRD pattern of Fe3O4@rGO is similar to Fe3O4, but the diffraction peak of GO is shifted to 2θ = 23.62°. This implied the (002) inter-planner spacing formation of rGO as illustrated in Fig. 2 (b). The absence of sharp peak of GO in the binary nanocomposite confirms the reduction of GO to rGO. [34,35]. Moreover, the shifting of diffraction peaks to higher or lower diffraction angle depending upon the presence of functional groups in GO as reported by Jilani et al. [36,37]. Furthermore, the peak attributed to GO disappeared in nanocomposite, showing the complete coating of Fe3O4 with AgI nanoparticles. The XRD patterns of Fe3O4@AgI are also comparable to that of Fe3O4 but there are some new diffraction peaks of AgI appeared at 2θ = 22.26°, 23.621°, 39.14°, 43.24°, which demonstrated to (100), (002), (112) (300) inter-planner spacing formation of AgI planes of

the hexagonal structures, respectively. This is in good agreement with the standard diffraction data of AgI (JCPDS file no. 09–0374) [38]. The ternary nanocomposite exhibits the same diffraction pattern as observed for Fe3O4@AgI which corresponds to lattice plane (100), (002), (112), (300) of AgI, respectively, according to JCPDS card No. 09–0374 as represented in Fig. 2 (c). 3.2. Morphological characterization The FE-SEM analysis is a capable technique to observe the surface morphology, configuration, and shape of the nanostructure. Fig. 3 represents the top view FE-SEM images of synthesized nanocomposites. It could be seen from Fig. 3a that Fe3O4 is quasi-spherical in shape and moderately uniform size in the range 17.2–21.3 nm. Though, the Fe3O4 tended to aggregate which may for the most part credit to the strong magnetic dipole-dipole interaction among the nanoparticles Fig. 3 (b)shows that the Fe3O4@rGO nanocomposite consists of the folded and crinkled GO on the Fe3O4 surface [39]. In contrast to Fe3O4,

Fig. 3. (a) FESEM images of Fe3O4, (b) Fe3O4@rGO and (c) Fe3O4@rGO@AgI. Inset shows high magnification of selected area.

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Fig. 3 (b)also exposes that the Fe3O4 nanocomposites were spread on the basal planes of the GO. This also recommends the convenient interaction of the positive surface of modified Fe3O4 and negative charge functional group of the GO surface radically which further controls the agglomeration of the crystalline Fe3O4 to some extent due to reducing π-π interaction of the particles [40]. Fig. 3c shows that there is a morphological variation between Fe3O4@rGO and Fe3O4@rGO@AgI in which the ternary nanocomposite contains both spherical particles and sheets of GO as well as AgI nanoparticles. EDX study was accomplished to confirm the presence of AgI nanoparticles on the surface of Fe3O4@rGO. 3.3. EDX analysis Fig. 4 shows EDX images peaks of Fe3O4@rGO@AgI ternary nanocomposite. The combinations of the Fe3O4 by mass percent are Fe 69.5%, O 30.5%. The corresponding atomic compositions are Fe 55.3%, O 15.8% and C 28.9%, which results that the nanospheres are generally included Fe, C and O segments. The fragments of the Fe3O4@rGO@AgI by mass percent are C 22.2%, O 18.0%, Fe 50.7%, Ag 5% and I 4% which exhibited that the nanocomposite is, for the most part, contained Fe, C, O, Ag and I components [41]. The small peak which arises in the position of

551

2.5 eV is recognized to the presence of silica which is the justification of the change of Fe3O4 by APTES and TEOS. EDS mapping analyses performed in Fig. 4 (a-c) resemble the elemental dispersal of O (oxygen), Fe (Iron), C (Carbon), Ag (Silver) and I (Iodide). The nanocomposite shows that the inner part of the nanocomposite is rich of Fe, while the outer parts contain C, O, Ag, and I. This result affirms that pure Fe3O4 nanoparticles were covered with GO and AgI nanoparticles [42]. 3.4. TEM analysis The HR-TEM images in diverse magnification range for Fe3O4@rGO@ AgI sample are illustrated in Fig. 5. TEM images clearly describe that the magnetite portion of the composite is nanospheres-like shape after coating with GO sheet and AgI nanoparticles which are in agreement with the former research [31,32]. Strong black color magnetite in the middle and existence of grey shadow color of GO sheet and synthesized AgI nanoparticles occurred around magnetite confirm nanocomposite formation which all displayed by white arrows as shown in Fig. 5 a, b, c. The dark and bright fringes in high magnification image in Fig. 5d (inset) represent the tolerable level of crystallinity of Fe3O4@rGO@AgI sample. The presence of lattice fringes in different directions obviously confirms that the material is poly-oriented crystalline. The d-spacing

Fig. 4. (a-b) EDX spectra for the Fe3O4@rGO@AgI samples and (c) EDX mapping for the Fe3O4@rGO@AgI nanocomposite.

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Fig. 5. (a-c) TEM images for Fe3O4@rGO@AgI selected area electron diffraction pattern. (d) (SAED) pattern of Fe3O4@rGO@AgI nanocomposite.

are calculated to be (311) (d = 0.253 nm) phase for Fe3O4, (200) (d = 0.272 nm) phase for GO and (002) (d = 0.327 nm) for AgI plane respectively, and shown as inset Fig. 5c.The SAED illustration of the ternary nanocomposite in Fig. 5(d)shows a diverse polycrystalline ring because of good crystallization of Fe3O4 nanoparticles. The mean particle size distribution of Fe3O4@rGO@AgI nanoparticles can be acquired by line broadening analysis via calculating the diameter of the Fe3O4@rGO@ AgI nanoparticles. A narrow size-distribution histogram achieved from counting 236 particles of randomly designated Fe3O4@rGO@AgI nanocomposite lies within a very narrow range, i.e. 3 to 16 nm nanoparticles as shown in Fig. 5 e. The particle size distribution of Fe3O4@rGO@AgI nanocomposite shows a log-normal distribution peak (DTEM) at ~13.6 nm. 3.5. FTIR analysis FT-IR spectra as shown in Fig. 6 was used to investigate the chemical bonding and structure of the as-incorporated nanocomposite, The peaks

of Fe3O4, which situated in the range of 470.5–544.2 cm−1 are due to the vibrations of the (Fe\\O) bond as shown in Fig. 6 (a)[43]. There is another weak peak positioned at 1648.1 cm−1 which links to the bending vibration of –OH groups that is generally observed in Fe3O4 synthesized by the chemical co-precipitation reaction [44]. The FT-IR spectrum of GO depicts a strong (-OH) peak at 3685.3 cm−1 and the other functional groups of containing (O\\H) 1840.4 cm−1, C_O 1680 cm−1, and (C_C) aromatic peak at 1550.1 cm−1 which are clearly visible. The spectrum also shows an aromatic peak of (C\\C) at 876.3 cm−1 and stretching vibration of (C\\O) at 1020.2 to 1231 cm−1 which corresponds to carboxylic acid and carbonyl groups present at the edges of GO. Moreover, the existence of these oxygen-containing groups indicates that the graphite has oxidized well. Additionally, after modification by APTES,TEOS and GO, another absorption groups existing at 550 cm−1 (Fe\\O), 1140.6 cm−1 (C\\O), 1617.5 cm−1(C_C) which proved that Fe3O4 has been successfully anchored on GO sheets [45]. Similarly, there is a peak at 1617.5 cm−1 which is analogous to the bending vibration of –OH groups that are normally detected in Fe3O4

(b)

471.3

1231.2 1020.2 876.3

1840.4 1680.1 1550.01

GO

4000 3500 3000 2500 2000 1500 1000 500

4000 3500 3000 2500 2000 1500 1000 500

Wavenumber cm-1

Wavenumbers, cm-1

(d)

550

4000 3500 3000 2500 2000 1500 1000 500

Wavenumber cm-1

Transmitence, a.u

440

Fe3O4@rGO

1140.6

1617.7

1480.6

2270.5

2150.5

(c) 3440.5

Transmettance, a. u.

553

3685.3

544.2 422.5

2089.2

1648.4 1478.5 1393.5 1125.7

2252.3

2370.3

3398.3

Fe3O4

780.6

Transmettance, a. u.

470.1

(a)

Transmettance, a. u.

G.U. Rehman et al. / Powder Technology 356 (2019) 547–558

2368.7

1731.4

796.0

3733.1 1541.4

3860.1

620.8

3393.7

Fe3O4@GO@AgI

505.1

1087.2

4000 3500 3000 2500 2000 1500 1000 500

Wavenumber cm-1

Fig. 6. (a) FTIR spectra of Fe3O4, (b) GO, (c) Fe3O4@rGO and the as-prepared (d) Fe3O4@rGO@AgI photocatalysts.

designed by the chemical co-precipitation reaction. The reduction of GO is verified by the weakening and disappearing of oxide bands in Fe3O4@ rGO spectra (O\\H vibration at 3685.3 cm−1, C_O vibration at 1680 cm−1, and C\\O vibration at 1020 cm−1 in GO spectrum [46]. In the spectrum of the ternary nanocomposite, there is no new peak examined for Ag\\I metal. Similarly to a few silver halides containing photocatalysts, the peak for Ag\\I bond is absent in the range of 400– 4000 cm−1 [19]. 3.6. TGA analysis The thermal stability of the nanocomposite samples is measured by thermogravimetric analysis (TGA). The mass loss of nanocomposite is

observed in a temperature range as depicted in Fig. 7. High stability of nanocomposite is obtained associated with previously reported work [47]. For the pure GO, the sudden weight loss occurs only around 230 °C, less stable products are revealed as shown in Fig. 7. The possible reason is a dissipation of moisture or vaporous substances in the composite or to decline of labile oxygen developed from functional groups such as hydroxyl, epoxy, and carbonyl, for example, hydroxyl, epoxy, and carbonyl [48,49]. After depositing GO sheet on the surface of aminemodified Fe3O4, the stability of the product significantly enhances up to 600 °C as shown in Fig. 7. The high stability is due to the combination of the negative charge of oxygen and epoxy of GO sheet with the Fe3O4amino-functionalized positively charged by π–π stacking interactions, supplementary, the TEOS also help in the formation of a covalent bond between a molecule attached on the GO surface and that of Fe3O4 nanoparticles. Furthermore, weight loss also attributed to the transformation of Fe3O4 to α-Fe2O3 [50]. Further improvement of thermal stability is achieved by the addition of AgI nanoparticles which helps to strengthen non-covalent bonds exist in AgI-based Fe3O4@rGO nanocomposite films, hydrogen bonds are certainly under the attention. Furthermore, the ternary nanocomposite possesses three kinds of hydrogen bonds: (i)hydrogen bonds among Fe3O4@rGO, (ii) among the AgI nanoparticles, and (iii) concerning AgI and Fe3O4@rGO [51]. For the first time, this study has given highly stable nanocomposite after GO and AgI modification of pure Fe3O4. 3.7. Light absorption and charge transfer properties

Fig. 7. (a) TGA curves for GO and (b) Fe3O4@rGO (c) Fe3O4@rGO@AgI.

PL characterization has been accomplished to describe the charge carrier tricking, relocation and recombination strategies of the photocatalysts subsequently PL discharge appears from the recombination of free transporters [52]. Fig. 8 demonstrates the PL spectrum of the pure Fe3O4, binary Fe3O4@rGO, and ternary Fe3O4@rGO@AgI

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PL intensity, a.u.

1000

Fe3O4

(a)

Degradation of phenol %

554

Fe3O4@rGO Fe3O4@rGO@AgI

800 600 400 200 0

110

Dark photolysis Fe3O4 Fe3O4@rGO Fe3O4@AgI Fe3O4@rGO@AgI

100 90 80 70 60 50 40 30 20 10

320 340 360 380 400 420 440 460 480

0

wavelength nm

1

2

3

4

5

6

7

8

9

Time (h)

Fig. 8. PL spectra of the Fe3O4, Fe3O4@rGO and Fe3O4@rGO@AgI.

3.8. Photo-degradation of phenol by Fe3O4@rGO@AgI Fig. 9 shows the photocatalytic reaction of Fe3O4@rGO@AgI nanocomposite for the degradation of phenol under UV irradiation. The photocatalysis of phenol (50 ppm) under the UV-light radiation in the absence of photocatalyst after 9 h is b25% as shown in Fig. 9 (a). Hereafter, the results affirmed that the photocatalyst possesses the main role in the removal of phenol from the wastewater system. Prior to irradiation, the aqueous solution which contains 0.2 g/350 mL catalyst was stirred for 9 h in dark condition to attain the adsorption-desorption equilibrium among phenol and photocatalyst. The photocatalytic performance was additionally considered with Fe3O4 and Fe3O4@rGO nanoparticles. The degradation ability of Fe3O4 toward phenol is 62%, this reveals that the photocatalytic performance of Fe3O4 under the UV-light radiation is relatively slow. The low degradation efficiency of Fe3O4 can be ascribed to its large band gap and rapid recombination degree of charge carriers. However, when a couple with GO, the photocatalytic activity of the binary photocatalyst has been improved to 75%. Graphene-based composite photocatalyst has different important features over Fe3O4 such as the fast electron transaction of picosecond from the animated semiconductors to the GO sheet and the controlled size of semiconductors additionally enhances the efficiency and effectiveness of photocatalysis. Moreover, graphene sheets reduced the aggregation of Fe3O4 increases the dispersion rate of the nanoparticles which further, enhances the feasibility of the photocatalysis. Furthermore, the fast electrons exchange of the GO sheets as of their one-or a few atoms thickness, accelerate the consumption of the energizing light showing both properties of graphene i.e. conducting and semiconducting [54]. The electrons from Fe3O4 are tranferred to graphene to suppress the (e− h+) recombination and to improve oxidative sensation. The binary nanocomposite of Fe3O4@AgI has demonstrated a 78% degradation of phenol. The ternary nanocomposite exhibited the best

C/C0

(b)

1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

Dark Photolysis Fe3O4 Fe3O4@rGO Fe3O4@AgI Fe3O4@rGO@AgI

0

1

2

3

4

5

6

7

8

9

--

Time (h) Fig. 9. (a) Photocatalytic potential of Phenol in the corporation of Fe3O4, Fe3O4@rGO, Fe3O4@AgI and Fe3O4@rGO@AgI. compounds (b) C/Co calculation of Fe3O4, Fe3O4@rGO, Fe3O4@AgI and Fe3O4@rGO@AgI. compounds.

performance of 99% which is higher than Fe3O4, Fe3O4@rGO and Fe3O4@AgI. The C/Co calculation as outlined in Fig. 9 (b)which has been performed to depict the degradation efficiency with respect to time. In the existence of Fe3O4@rGO, Fe3O4@AgI and Fe3O4@rGO@AgI nanocomposites, the catalytic activity of phenol is 75%, 78%, and 99%, respectively. The phenol degradation is just 62% when pure Fe3O4 is used which is lower than in the presence of either, Fe3O4@rGO, Fe3O4@AgI or Fe3O4@rGO@AgI nanocomposite due to large band gap and lower UV6

Dark Photolysis Fe3O4 Fe3O4@GO

5

-ln(C/Co)

nanocomposites. Upon the excitation wavelength at 270 nm, the key emission peak of Fe3O4 was distinguished at around 420 nm, because of the band gap recombination of (e− h+) pairs. The Fe3O4 affected a nearly high recombination rate of photoinduced transporters due to the high-intensity rate, which is decreased within the sight of GO nanoparticles and Fe3O4@rGO@AgI. Fig. 8 also shows that the PL intensities of the as-synthesized nanocomposite are positioned as Fe3O4 N Fe3O4@ rGO N Fe3O4@rGO@AgI, which is in great agreement with the significance of photocatalytic exercises. In principle, lower the PL signal means that the higher separation ability of (e− h+) pairs. It is clearly identified that Fe3O4@rGO@AgI nanocomposite demonstrates the lowest peak intensity compare to Fe3O4 and Fe3O4@rGO, shows decreasing the recombination of electron-gap contests effectively and improving the charge separation efficacy [53]. The charge partition and conversation influence photocatalytic performance.

Fe3O4@AgI Fe3O4@GO@AgI

4 3 2 1 0 1

2

3

4

5

6

7

8

Irridation time (h) Fig. 10. Kinetics study of the disappearance of phenol degradation of all samples.

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Degradation of phenol %

(a)

3.9. Recyclability and stability of the catalyst

Run 1 Run 2 Run 3 Run 4 Run 5

110 100 90 80 70 60 50 40 30 20 10 0 0

10

20

30

40

50

As the recyclability of the photo-catalysts is a vital parameter of the photo-degradation procedure, the recyclability of Fe3O4@rGO@AgI was investigated in this study. A suspension of the nanoparticles in the phenol solution was prepared. While an outer magnet was linked to the suspension, the nanocomposite was at once collected to the side of the glass vial nearby the magnet, leaving the phenol solution colorless, and indicating a noble magnetically separable property [57]. Phenol degradation in the incidence of Fe3O4@rGO@AgI nanoparticles was examined under the same settings used for pure nanoparticles. Five cycles were tested in the existing study. An average of 80% nanoparticles was retained using an exterior magnet in first 3 cycles while in cycle 4 and 5 the amount decreases gradually as shown in Fig. 11 (a). The phenol degradation was 99% in cycle one, which was declined to 94% in cycle two and 93% in cycle three, 88% in cycle four and 81% in cycle fifth. The recyclability was significantly better than that of the pure AgI (65%), due to the high activity of the hybrid nanoparticles [58]. The improved photocatalytic performance of Fe3O4@rGO@AgI is majorly due to the suppression of the recombination of electron-hole pairs by the iron ions.

60

Time (h) 100

% Degradation of Phenol

(b)

25 ppm 50 ppm 75 ppm 100 ppm

80

555

60 40

3.10. Influence of operational parameters on the photocatalytic phenol degradation

20 0 0

1

2

3

4

--

Time (h) Fig. 11. (a) The cyclic operation of the synthesized Fe3O4@rGO@AgI nanocomposite for the degradation of phenol under UV irradiation. (b) Influence of the initial phenol concentration on photo-degradation at 25 °C using Fe3O4@rGO@AgI as a catalyst.

3.10.1. Effect of initial concentration The influence of phenol initial concentration in the range 25– 100 ppm was investigated by utilizing 0.2 g/350 mL of Fe3O4@rGO@ AgI. Fig. 11 (b)demonstrates the results regarding phenol degradation as a role of illumination time, exclusively. The increasing initial

light absorbance and slow transfer of electrons. The best and higher results of 99% is observed using ternary nanocomposite, shows dominancy on other catalyst used. In principle, during heterogeneous photocatalytic reaction system, the reaction probably takes place on the surface of the catalyst. In this manner, high adsorption limit would be to the more lifted photocatalytic high photocatalytic action. As presented in Fig. 10, all sample shows adsorption capacity in the dark situation because of the existence of considerable delocalized π bonds which stimulates adsorption of phenol due to strong π − π interactions [55]. Furthermore, Fe3O4@ rGO@AgI demonstrated the highest adsorption ability, due to its high surface area as associated with Fe3O4, Fe3O4@rGO and Fe3O4@AgI nanocomposite. Subsequently, the high surface area presents a more active superficial range for physisorption of phenol and degradation reaction to occur. As represented in Fig. 10, the degradation rate is observed to be a pseudo-first-order kinetic procedure, which can be linked by Eq. (4) ln ðC=Co Þ ¼ −kKt ¼ −Kaap t

ð4Þ

wherever kapp is the rate constant (min−1), C0 is the initial concentration of contaminants (mg/L) and C is the final concentration of toxins (mg/L) at a particular interval t is the response time (h). The estimation of rate consistent kapp is equivalent to the comparing slope of the fit linkage kinetics of the loss of phenol degradation [56]. The most elevated in reaction rate constants estimation of 6.96 × 10−3 min−1 in the degradation of phenol for a test of Fe3O4@rGO@AgI added to the appropriate and ideal band hole estimation of AgI and GO carbon doping substance. Additionally, the GO would support effective charge carrier separation, and electron conductivity so reduced the recombination of photogenerated (e− h+) pairs.

Fig. 12. (a) Effect of the addition of H2O2 on phenol removal at 25 °C via Fe3O4, Fe3O4@rGO, Fe3O4@AgI, and Fe3O4@rGO@AgI. (b) Influence of the catalyst quantity on the degradation of phenol by Fe3O4@rGO@AgI.

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concentration of phenol decreases the photocatalyst degradation ability. To examine the impact of initial phenol concentration on photocatalytic degradation, four different phenol concentrations i.e.,25, 50, 75, and 100 ppm were prepared. The rise in initial phenol concentration declined the percentage of phenol degradation for all the routes calculated. The minimum degradation rate occurred at 100 ppm and the highest degradation efficiency was recorded at 25 ppm. The decrease in degradation frequency at higher concentration was due to the higher light absorption by phenol molecules compared to that of Fe3O4-doped AgI [58]. Therefore, the light absorbed is not operative to carry degradation. In addition, the equilibrium adsorption of phenol on the catalyst surface, dynamic site increments and a huge amount of phenol molecules were adsorbed by the catalyst [59]. As a result, the main reasons for the decrease in degradation activity is ascribed to the increasing in contaminant load and the

production of •OH radicals which are ineffective in the degradation of phenol [39]. 3.10.2. Effect of H2O2 addition The effect of H2O2 adding on the degradation of phenol by the Fe3O4@rGO@AgI/H2O2 is described in Fig. 12 (a). Evidently, the degradation of phenol is strongly dependent on the H2O2 (30%) concentration which is 0.2 M utilized for every one of the particles. The Fe3O4 nanoparticles degradation adequacy upgraded rapidly from 62.72 to 77.07% with the expansion of H2O2 of 0.2 M. This enhancement in efficacy because of increasingly –OH provided with the utilization of 0.2 M of H2O2. The same amount of H2O2 was used with the corresponding nanocomposite of Fe3O4@rGO and Fe3O4@AgI for the phenol degradation. It was found that the rate increment for both nano-composites of Fe3O4@rGO/H2O2 and Fe3O4@AgI/H2O2 from 75 to 91 and 78–93%

Fig. 13. (a) Proposed mechanism for the photodegradation of phenol by Fe3O4@rGO@AgI at room temperature and neutral pH. (b) Transfer of electron.

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individually at 9 h of reaction time correspondingly. Moreover, for more extensive comparison, a similar value of H2O2 was utilized with the ternary nanocomposite of Fe3O4@rGO@AgI to monitor the degradation rate of this composite [43]. The Fe3O4@rGO@AgI inspected significant outcomes demonstrating 100% phenol degradation within 6 h which is higher than binary nanocomposites and pure Fe3O4. The most dynamic finding of this analysis is the use of the same amount of H2O2 for all the synthesized nanocomposites and effect on the degradation rate which is fruitfully boosted accordingly.

3.10.3. Effect of Fe3O4@rGO@AgI dosage on phenol photodegradation The impact of catalyst loading on photocatalytic degradation of phenol was considered by utilizing the measure of the catalyst of 0.1 g/350 mL, 0.2 g/350 mL, 0.3 g/350 mL, 0.4 g/350 mL, and 0.5 g/350 mL. The degradation rate of phenol against time is displayed in Fig. 12 (b). When Fe3O4@rGO@AgI dose was increased from 0.1 to 0.4 g/350 mL, phenol photodegradation increased from 71 to 100%. However, the further increase in Fe3O4@rGO@AgI loading up to 0.5 g/350 mL has declined the phenol removal to 93%. With the increase in photocatalyst loading, the active sites, as well as the number of hydroxyl and superoxide radicals have also been increased. Nevertheless, the amount of the catalyst above the optimum quantity has resulted in light interference by the suspension [17]. Previous studies have also shown that in the presence of an excessive catalyst, the concentration of •OH radical reduced hence the overall efficacy of the degradation was also reduced [43]. Besides, the acceleration of catalyst quantity above the maximum might bring about the agglomeration of photocatalysts, consequently, the quantity of the catalysts surface modified isolated for photon absorption, and degradation rate diminished [39]. Similarly, the excessive dosage of the catalyst reduced the light diffusion by the shielding effect of the suspended particles and decreased the photo-degradation rate. The results revealed that phenol photodegradation by photolysis (using UV alone) is considerably lesser than that of photocatalytic degradation by Fe3O4@rGO@AgI photocatalyst.

3.11. Reaction mechanism of the photocatalytic activity The expected mechanism perhaps arranged into three phases as it explained in Fig. 13 (a-c). In the primary stage Fe3O4, the center of Fe3O4@rGO@AgI is the principal segment used to create hydroxyl radicals (•OH) which later degrade phenol. Besides, the ferromagnetic of Fe3O4 cause the composite easily distinguishable from the solution for the later practice. Furthermore, the imminence of GO serves as a catalyst and adsorptive nanocomposite of phenol which might be valuable to the degradation of phenol, however also endures the catalysts with a comprehensive treatment of the UV light. Moreover, the band gap of AgI might be reduced when AgI was joined with GO so that the valence electrons of AgI can also be excited to the conduction band state only under UV-irradiation [60]. In the third step, the deposition of AgI may also excessively affect the degradation of phenol. At the point when illuminated by the UV (λ N 254 nm), AgI connected on GO can be excited to create electron and gap sets, and the closeness of GO can be capable to transportation the photo-activated electrons to internal the Fe3+ to support the Fe3+ to be decreased to Fe2+. Additionally, the quick exchange rate of the photo-activated electrons from AgI to Fe3+ scale of the photo-produced electrons from AgI to Fe3+ prolong the lifetime of the photo-generated hole whose oxidative prospective is additionally improbable sufficient to degrade most organic toxins [61]. The reasonable separation of the photo-produced electrons and holes usefully prevents their recombination so the large hole might have the capacity to directly respond with and subsequently degrade the organic contaminations. Overall, Fe3O4, GO and AgI have concertedly contributed to the enhanced catalytic activity.

557

4. Conclusion A ternary Fe3O4@rGO@AgI nanocomposite photocatalysts was synthesized through simple refluxing method. The physicochemical characterization of Fe3O4@rGO@AgI nanocomposite revealed that the ternary nanocomposite has been successfully formed with desired morphology and crystallinity Fe3O4@rGO@AgI exhibited phenol degradation up to 99% which was higher compared to single and binary nanocomposites counterparts. The promising results can be attributed to the suppression of recombination of electron-gap and the improved charge separation efficacy through the synergistic effects of the three components of the nanocomposite. The optimum catalyst loading was found to be 0.2 g/350 mL. The higher initial concentration of phenol resulted in lower degradation efficiencies. The addition of H2O2 exceeding 0.2 M also declined the photodegradation efficiency of the nanocomposite. The rate of phenol degradation of Fe3O4@rGO@AgI coincides well with the pseudo-first-order mechanism. In addition, the ternary photocatalyst showed excellent recyclability of 81% in five consecutive cycles. The findings from this research shows that the ternary nanocomposite developed in this study holds great potential for practical phenol degradation applications.

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