CuO nanocomposites under visible light

CuO nanocomposites under visible light

Journal Pre-proof Enhanced photocatalytic degradation of methylene blue using Fe2 O3 /graphene/CuO nanocomposites under visible light Prawit Nuengmatc...

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Journal Pre-proof Enhanced photocatalytic degradation of methylene blue using Fe2 O3 /graphene/CuO nanocomposites under visible light Prawit Nuengmatcha, Paweena Porrawatkul, Saksit Chanthai, Phichan Sricharoen, Nunticha Limchoowong

PII:

S2213-3437(19)30561-5

DOI:

https://doi.org/10.1016/j.jece.2019.103438

Reference:

JECE 103438

To appear in:

Journal of Environmental Chemical Engineering

Received Date:

23 August 2019

Revised Date:

22 September 2019

Accepted Date:

27 September 2019

Please cite this article as: Nuengmatcha P, Porrawatkul P, Chanthai S, Sricharoen P, Limchoowong N, Enhanced photocatalytic degradation of methylene blue using Fe2 O3 /graphene/CuO nanocomposites under visible light, Journal of Environmental Chemical Engineering (2019), doi: https://doi.org/10.1016/j.jece.2019.103438

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

Enhanced

photocatalytic

degradation

of

methylene

blue

using

Fe2O3/graphene/CuO nanocomposites under visible light Prawit Nuengmatchaa,*Paweena Porrawatkula, Saksit Chanthaib, Phichan Sricharoenb and Nunticha Limchoowongc a

Nanomaterials Chemistry Research Unit, Department of Chemistry, Faculty of Science and Technology, Nakhon Si Thammarat Rajabhat University, Nakhon Si Thammarat, 80280, Thailand

b

Materials Chemistry Research Center, Department of Chemistry and Center of Excellence for Innovation in

c

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Chemistry, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand Department of Chemistry, Faculty of Science, Srinakharinwirot University, Bangkok 10110, Thailand

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Graphical abstract

 Corresponding author: Prawit Nuengmatcha E-mail: [email protected] Tel: +6675-37-7443 Fax: +6675-37-7443

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Highlights ● The Fe2O3/graphene/CuO photocatalyst was successfully synthesized. ● The Fe2O3/graphene/CuO exhibited excellent photocatalytic activity under visible-light. ● Fe2O3/graphene/CuO can be applied as a high potential catalyst for dye degradation. ● The Fe2O3/graphene/CuO catalyst exhibited a highly stability and recyclability.

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Abstract A visible light-responsive photocatalyst of Fe2O3/graphene/CuO (FGC) nanocomposite was successfully synthesized via a simple solvothermal method. The characteristics of the asprepared graphene/mixed metal oxides were examined by XRD, SEM, EDS, TEM, and HR-

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TEM, and its magnetic property was evaluated by VSM. The analysis clearly revealed that the

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FGC hybrid catalyst, having a bandgap of 1.82 eV, has the ability to absorb visible light as observed from the UV-Visible diffuse reflectance spectrum. The photocatalytic performance

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of FGC, subjected to visible light, in degrading methylene blue was evaluated. Its photocatalytic degradation property was found to be higher in the presence of visible light as compared to the

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other catalysts. In addition, the postulated mechanism of the photocatalytic property of this graphene/mixed metal oxides hybrid composite has been discussed. The present work

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demonstrates that this graphene/metal oxide hybrid nanocomposite can also be applied as a

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highly potent photocatalyst degrader for other dye pollutants. Keywords: Fe2O3/graphene/CuO; Photocatalytic degradation; Dye pollutants; Catalyst 1. Introduction

At present, organic dyes are one of the biggest environmental polluters since they cause severe pollution even at low concentrations which not only affects the transparency of the water but also creates aesthetic problems [1, 2]. Hence, effective treatment of these textile effluents

3 before their discharge into the environment is mandatory. Till date, several techniques, such as adsorption [3, 4], membrane [5], photocatalysis [6], and many others [7–15] have been developed for the removal of organic dyes from textile effluents. Among these, the photocatalysis technique has emerged as a promising process for degrading dyes from wastewater as it can not only decolorize the wastewater but also achieve complete degradation of the dyes in it [16]. Among the various heterogeneous photocatalysts, copper oxide (CuO) has been widely used because of its photoconductive as well as photochemical properties that have various

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advantages. For example, it has a narrow bandgap (1.2 eV) and its electrical resistance and bulk density are quite low [17]. Its surface area and porosity are high and it can also exhibit strong absorption in the visible region when exposed to solar radiation [18]. However, one of

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the drawbacks of CuO is its poor photocatalytic activity for dye pollutant degradation. This is because the energy levels in the conduction band are more positive than the hydrogen reduction

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potential and this leads to fast recombination of the photo-generated charge carriers [19, 20].

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Another drawback of using CuO in wastewater treatment is the difficulty in its separation from the wastewater. Traditional separation techniques such as filtration, centrifuge and coagulation are generally used for catalyst removal. However, these techniques not only lead to serious

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losses of the catalyst but also lead to high energy consumption [21]. Therefore, techniques have been developed in order to overcome the disadvantages of CuO and improve the separation

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and lifetime of the photo-generated charge carriers. Heterojunction catalysts have attracted

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significant attention due to their unique properties in the photocatalysis process. Constructing a heterojunction of a catalyst is a common and effective way of promoting charge separation as well as prolonging the lifetime of the charge carrier [22]. Recently, various magnetic nanoparticles such as CoFe2O4/BiOI [23], Ag/Bi2Fe4O9 [24], Fe3O4/ZnO [25] and many others [26–29] have been incorporated into the photocatalytically active materials, not only to allow catalyst separation by using an external magnet and recycling

4 it, but also to reduce their recombination and hence gain a high photocatalytic activity for dye pollutant degradation. Apart from the above mentioned magnetic nanoparticles, graphene is also an excellent material that can be used in photocatalysts due to its unique electronic band structure and high performance as a candidate of electron transfer media [30]. Thus, graphene has also been incorporated into various photocatalytically active materials, such as NiZnFe2O4/RGO [31], [email protected]/TiO2 [32], Fe2O3/N-graphene [33] and others [34, 35], as the substrate of the photocatalyst which exhibits an enhanced charge separation ability and

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hence improved photocatalytic performance. The synthesis and application of a ternary Fe2O3/graphene/CuO (FGC) composite in a visible light-driven photocatalytic system has not been reported yet. Thus, in the present work,

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magnetic materials such as Fe2O3, graphene, and CuO were synthesized as a co-operated ternary FGC composite for achieving an improved photocatalytic activity and excellent magnetic

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separation ability. Methylene blue (MB) was selected as a representative dye pollutant for

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evaluating the photocatalytic degradation performance of the as-prepared FGC subjected to visible light. In addition, the morphology and the optimum conditions for dye degradation, including visible irradiation time and intensity, catalyst dosage, dye concentration, pH and, in

Reagents and materials

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2.1.

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2. Experimental

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addition, the photocatalytic mechanism of the photocatalyst were investigated in details.

Copper (II) nitrate trihydrate (Cu (NO3)2·3H2O), Sodium nitrate (NaNO3), Citric acid

(C6H8O7), Iron (III) chloride hexahydrate (FeCl3·6H2O), Ferrous sulfate hexahydrate (FeSO4 ·7H2O), Ammonium hydroxide (NH4OH), and Methylene blue (C16H18ClN3S) were purchased from Carlo Erba (Italy). Sodium hydroxide (NaOH) was obtained from QRecTM (New

5 Zealand). All chemicals were of analytical grade and used without further purification. All experiments were carried out with deionized (DI) water. 2.2.

Synthesis of ternary Fe2O3/graphene/CuO catalyst Ternary Fe2O3/graphene/CuO catalyst was prepared by a simple solvothermal method.

In a typical procedure, 2.0 g of C6H8O7 was heated to 260 °C for 10 min after which it was liquated to a yellow color. The obtained solution was mixed with 100 mL of 0.25 mol/L NaOH and stirred for 30 min. A neutral pH of the resultant mixture was obtained after mixing NaOH.

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In the final step, graphene quantum dots (GQDs) stock solution (solution A) was used. In addition, FeCl36H2O (5 g), FeSO47H2O (2.5 g) and DI water were mixed in a 500 mL threenecked flask and 20 mL of 28% NH4OH was slowly dropped into the solution. To obtain the

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colloidal solution, the solution was heated at 80 °C for 30 min on a paraffin oil bath with

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vigorous stirring under N2 atmosphere. A Fe2O3 black powder sample was obtained (mixture B). Solution A and mixture B were mixed in a round bottom flask and the suspension was

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stirred and heated at 80 °C for 30 min. The sample was then washed with DI water until the neutral pH of the suspension was reached. The as-prepared sample was dried at 90 °C to obtain

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the Fe2O3-graphene (FG) sample. Following this, 0.5 g of FG and 250 mL of 0.1 mol/L CuNO3 were mixed and stirred for 5 h while heating the mixture at 90 °C. The temperature of the

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mixture was increased to 100 °C to remove all solvents. The solid sample was then heat-treated in a furnace at 600 °C for 1 h to obtain Fe2O3/graphene/CuO (FGC). For the sake of

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comparison, Fe2O3-graphene, Fe2O3, GQDs, and CuO were prepared in a similar manner with slight modifications to the synthesis procedure. 2.3

Sample characterizations The X-ray diffraction (XRD) patterns of all samples were obtained using the X-ray

diffraction technique (Philips X’PERT MPD, Netherlands) with Cu Kα radiation (k = 1.5405

6 Å) in the range of 2 from 10 to 90 at a scan speed of 1.2°m–1. Investigation of the morphology of the samples and their elemental analysis was done using a scanning electron microscope (SEM) employing the energy-dispersive X-ray spectroscopy technique (EDS) (SEM, EDX, Oxford, Aztec, United Kingdom). The particle size and distribution of the Fe2O3 and CuO catalyst on the graphene sheet were evaluated by transmission electron microscopy (TEM) (TEM, JEOL, JEM-2010, Japan) operated at an acceleration voltage of 200 kV. Diffuse reflectance spectra of all samples were obtained using a UV-Visible spectrophotometer

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(Shimadzu, UV-2600, Japan) equipped with an integrating sphere assembly. The UV-Visible spectrophotometer (Thermo Scientific, Evolution-201, United States of America) was also used for the evaluation of the degradation percentage and kinetics of the photocatalytic activity

Photocatalytic degradation of the dye pollutant

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for all samples.

The photocatalytic performance of all catalysts was investigated in a closed chamber

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with various visible light intensities (40, 60, 100 and 130 Watt). For the photocatalytic test performed at room temperature, various amounts of catalysts (0.1, 0.2, 0.5, 1.0, 2.0 and 5.0

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g/L) were added to 100 mL of different concentrations (5, 10, 20, 30 and 40 mg/L) of methylene blue, denoted as the initial concentration (Co). The mixtures were stirred and held in a dark

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chamber for 2 h before irradiation in order to reach the adsorption-desorption equilibrium. The mixture was then irradiated for 180 min, 3 mL of the reaction solution was taken out from the

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reactor flask each time after 10, 20, 30, 40, 60, 90, 120, 150 and 180 min, respectively. Following this, the mixtures were separated from any suspended solid using an external magnet. The obtained supernatant was analyzed using a UV-Visible spectrophotometer at λmax = 665 nm corresponding to the maximum absorption of the dye solution. 3. Results and Discussion

7 3.1.

XRD patterns of the photocatalyst To confirm the formation of the synthesized catalysts, X-ray diffraction technique was

employed to determine the crystallographic structure of the prepared samples. The XRD results of FGA, FG, graphene, Fe2O3, and CuO catalysts are shown in Fig. 1. The XRD patterns of Fe2O3 showed reflection peaks at 2 = 24.1, 33.2, 35.6, 40.9, 49.5, 54.1, 62.4 and 64.1 corresponding to (0 1 2), (1 0 4), (1 1 0), (1 1 3), (0 2 4), (1 1 6), (2 1 4) and (3 0 0) planes, respectively, indicating the presence of the prepared Fe2O3 in the hexagonal phase of hematite

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(-Fe2O3) (JCPDS card No. 33-0664) [36]. The XRD pattern of CuO appears at 2θ = 32.5, 35.6, 38.7, 48.8, 53.6, 58.8, 61.7, 66.0, 68.2 and 75.2 which are assigned to the corresponding planes of (1 1 0), (0 0 2), (1 1 1), (1 1 2), (0 2 0), (2 0 2), (1 1 3), (3 1 0), (2 2 0)

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and (0 0 4), respectively, and agree well with the reported data of the monoclinic structure of CuO (JCPDS No. 89-5895) [37]. From the XRD patterns of FGC, it is thus evident that the

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prepared sample contains the major phase consisting of both -Fe2O3 and CuO. In addition, it

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was found that the prepared sample gave the characteristic diffraction peak at 2θ = 25.1 which corresponded to the (0 0 2) plane of graphene nanosheets [38], indicating the presence of

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graphene, -Fe2O3 and CuO in the FGC nanocomposite.

SEM image and EDS spectrum of the photocatalyst

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The determination of the major elements and morphology of the FGC catalyst was performed using EDS. Both the SEM image and EDS spectrum of FGC are shown in Fig. 2. The EDS spectrum shows strong peaks of Fe and the presence of three main elements including C, O, and Cu in the prepared sample, indicating that FG was successfully modified with CuO. The weight percentages of C, Fe, O, and Cu were 15.9, 29.0, 35.5 and 19.6, respectively. 3.3.

TEM and H-TEM images of the photocatalyst

8 Figure 3 shows the TEM images of the FGC nanocomposite. Using the image J software, the particle size of Fe2O3 and CuO catalyst were found to be in the range of 13.28 to 17.96 nm and 27.52 to 31.01 nm, respectively. In addition, the length and diameter of the graphene sheets could be well observed in the range of 171.28 to 364.43 nm and 57.17 to 148.19 nm, respectively. The small particle, within a nanometer size, of Fe2O3 and CuO and having a high surface area of graphene may absorb more visible light due to its larger size leading to higher photocatalytic activity [39]. Figure 4 shows the high-resolution TEM (H-

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TEM) image of FGC nanocomposite. It can be clearly seen that both CuO and Fe2O3 are distributed in the graphene sheet. The spacing between the lattice-fringes is approximately 0.273  0.007 nm, corresponding to the low energy (1 1 0) plane of the monoclinic CuO [40].

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Another spacing between the lattice-fringes in the catalyst was 0.250  0.005 nm, which corresponds to the (1 1 0) plane of -Fe2O3. These results concord with the XRD data, as shown

VSM data of the photocatalyst

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in Fig. 1 [41]. Thus, the results confirm the successful formation of the FGC nanocomposite.

Magnetism is one of the important features of magnetic photocatalysts. Sufficient

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magnetism can quickly separate catalysts from the mixture in numerous practical applications. In the present work, the magnetic property of the catalyst was evaluated using the vibrating

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sample magnetometer (VSM). The magnetic hysteresis loops of Fe2O3 (F), Fe2O3-graphene (FG) and Fe2O3-graphene-CuO (FGC) are shown in Fig. 5. The magnetization curves indicate

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that all F, FG, and FGC are superparamagnetic with magnetization saturation of 61.27 emu/g, 59.89 emu/g, and 49.89 emu/g, respectively. The decrease in the saturation magnetization is due to the increase in the amount of graphene-CuO incorporated in the Fe2O3-graphene-CuO composites. The existence of non-magnetite on the Fe2O3 surface decreases its uniformity due to the quenching of surface moments, which results in the reduction of the magnetic moment in such nanoparticles. In this work, the magnetic property of FGC remains strong enough to

9 allow magnetic separation [42]. A superparamagnetic saturation magnetization of 16.3 emu/g is enough for the magnetic separation of the photocatalyst from the sample mixture, using an external magnet [43]. Since FGC is observed to exhibit high magnetization (i.e., 48.89 emu/g), this new photocatalyst, dispersed in water, could be easily and rapidly separated from the aqueous media by an external magnet within 30 s. This was done using an external magnet that attracted the FGC photocatalyst to the wall of the reactor flask because of a strongly bound magnetism. UV-Visible DRS analysis

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3.5

In order to determine the light absorption and bandgap energy of all the prepared catalysts, UV-Visible diffuse reflectance spectroscopy (UV-Visible DRS) was employed.

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Figure 6 (a) shows the UV-Visible absorption spectra of the Fe2O3-graphene-CuO (FGC)

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catalyst in comparison with the Fe2O3 (F), graphene (G) and Fe2O3-graphene (FG) hybrid samples, in the range of 450 to 950 nm. It was found that the FGC catalysts exhibited a

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remarkable redshift and absorbed light in the visible region, indicating that the modification of graphene with Fe2O3 and CuO particles is helpful in improving their visible irradiation

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response. To estimate the bandgap energy (Eg) of the obtained catalysts, the UV-Visible DRS spectra of all samples were evaluated by using a Kubelka-Munk transformation of the measured

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reflectance according to the equation, h = A (h – Eg)1/2 [38] where  is the absorption coefficient,  is photon frequency, h is Planck’s constant, Eg is bandgap energy and A is a

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constant. A plot of (h)2 versus photon energy was drawn, from which the intercept of the tangent on the X-axis gives the approximate value of the bandgap [44]. From Fig. 6 (b)-(e), the estimated values of Eg for G, F, FG and FGC were 1.26 eV, 1.82 eV, 2.18 and 1.49 eV, respectively. It was found that the value of Eg of the FGC photocatalyst is red-shifted as compared to the corresponding values for F and FG catalysts, thus indicating that the prepared FGC catalyst should exhibit high photocatalytic activity in the visible region of light. The small

10 bandgap of FGC is due to the inherent light absorption capacity of carbon in graphene sheets and electronic transitions between the carbon in graphene and semiconductors (CuO and Fe2O3) [45]. The proposed mechanism of the FGC photocatalyst is discussed later in Section 3.9. 3.6.

Photocatalytic degradation of the dye pollutant 3.6.1 Effect of visible light irradiation time A series of experiments involving a visible light irradiation time from 0 min to

180 min were designed, the results from which are shown in Fig. 7. 100 mL MB having a

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concentration of 10 mg/L, was treated with 0.1 g/L FGC for time intervals in the range of 0 to 180 min. From Fig. 7, it can be observed that the percentage degradation of the MB dye increases with increasing visible light irradiation times; the percentage degradation values of

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0.00, 32.26, 38.70, 41.58, 45.61, 49.40, 52.05, 52.11, 52.11 and 52.16% were obtained for the

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irradiation times of 0, 10, 20, 30, 40, 60, 90, 120, 150 and 180 min, respectively. It can be seen that for an irradiation time from 0 to 40 min, the ratio of Ct/Co (Co is the concentration of MB

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before light irradiation and Ct is a concentration of MB after light irradiation at time t) and the percentage degradation of MB drastically decreases with time. The high degradation rate of the

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MB dye may be due to abundance of available active sites on the surface of the FGC photocatalyst. However, for irradiation times from 60 to 180 min, both the ratio of Ct/Co and

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the percentage degradation of MB, are constant due to fewer available active sites. In other words, the photocatalytic activity of a certain amount of the FGC catalyst is limited. Since the

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percentage of the MB degradation activity changed very little after an irradiation time of 40 min, a duration of 40 min was chosen for other experiments where other parameters needed to be kept constant. From the literature, similar trends for the various catalysts such as BiOBr and BiOBr/Fe3O4 [46], hybrid kaolin/TiO2 [47] and 3D graphene aerogels/Sb2WO6 [48] have been observed.

11 3.6.2 Effect of the initial concentration of MB It is very important from an application point of view to study the effect of the initial concentration of the dye in wastewater on the photocatalytic degradation efficiency of the photocatalyst. Therefore, experiments were carried out for investigating the photocatalytic degradation of the dye for a wide range of concentration of MB from 5 mg/L to 40 mg/L irradiated with visible light for a duration of 40 min at constant room temperature, a catalyst dosage of 0.1 g/L and visible light irradiation intensity of 40 W. As can be seen from Fig. 8,

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the percentage of degradation decreases with the increasing initial concentration of MB; 70, 65, 62, 40 and 25% of MB were degraded after 40 min for the MB concentration of 5, 10, 20, 30 and 40 mg/L, respectively. At higher concentrations (30 mg/L and 40 mg/L), their

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absorption spectra were not smooth and their degradation efficiency was also quite low. This is due to the increase in the number of MB molecules with increasing concentration of the dye,

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while the numbers of •O2- and •OH are constant, which leads to inefficient oxidation of the MB

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molecules [49]. At lower concentrations (5 mg/L and 10 mg/L), although the percentage degradation of MB is more than 60%, its absorbance is rather low and thus changes in the degradation are difficult to observe. Therefore, an initial concentration of MB of 20 mg/L was

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chosen for other experiments with other parameters kept constant. A similar trend was observed for the organic dye degradation on CuO/SmFeO3 nanocomposite [20] and magnetic MgFe2O4-

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MgTiO3 perovskite nanocomposite [50].

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3.6.3 Effect of visible light intensity Irradiation intensity has been reported to be an important parameter influencing

the photocatalytic degradation of dye pollutants. Therefore, the influence of visible light irradiation, having different intensities (0, 40, 60, 100 and 130 W), on the MB degradation was tested keeping other experimental conditions constant, i.e., with an initial concentration of MB of 20 mg/L, a temperature of 25 °C, irradiation time of 40 min and 0.1 g/L of catalyst dosage.

12 From the results shown in Fig. 9, it is evident that the percentage of MB degradation increases with increasing visible light irradiation intensity. The percentage degradation of MB of 0, 25, 40, 60 and 62% was found for the visible light irradiation intensity of 0, 40, 60, 100 and 130 W, respectively (as shown in Table 1). Initially (i.e., for irradiation intensities from 0 W to 100 W), an increase in the visible light irradiation intensity caused an increase in the percentage of MB degradation (0% to 60%), or in other words, higher irradiation intensity produced more •

O2- and •OH radicals [51] thus resulting in a higher photocatalytic degradation efficiency.

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However, at irradiation intensities between 100 W to 130 W, the percentage of MB degradation changed very little after an irradiation intensity of 100 W. Therefore, for further measurements, the visible light irradiation intensity of 100 W was chosen while keeping other experimental

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3.6.4 Effect of photocatalyst dosage

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parameters constant. A similar trend was observed for Bi2WO6 catalyst [51].

The photocatalyst dosage can also influence dye degradation. Thus, in order to

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evaluate the effect of FGC dosage on its efficiency in degrading MB, different amounts of FGC catalyst in the range of 0.1 g/L to 5.0 g/L were used, while keeping the dye concentration of 20

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mg/L, temperature of 25 °C, irradiation time of 40 min and irradiation intensity of 100 W. From Fig. 10, it can be seen that the percentage of MB degradation significantly increases (from

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34.9% to 81.5%) with an increasing dose of the FGC catalyst (from 0.1 g/L to 0.5 g/L). This may be due to an increase in the number of active sites on the FGC surface, thus causing an

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increase in the number of both •O2- and •OH radicals, which can take part in the actual degradation of MB [52]. On the other hand, after an FGC catalyst dose of 0.5 g/L, the percentage of MB degradation decreases slightly (from 76.9% to 56.9%) with increasing catalyst dose (1.0 g/L to 5.0 g/L). This may be due to the decrease in the visible light penetration in the sample mixture caused by an increased agglomeration of the catalyst particles in the reaction mixture [20]. An optimal dose of 0.5 g/L of the FGC photocatalyst is thus obtained

13 and this dosage was chosen for further measurements. A similar trend was observed for the Rhodamine B degradation caused by CuO/SmFeO3 nanocomposite [20]. 3.6.5 Effect of the pH of the methylene blue solution The pH of a solution is one of the most important parameters, which plays a significant role in the degradation of dye pollutants. Hence, the effect of solution pH on the photocatalytic degradation of MB was investigated for a pH range of 1–11. The pH values from 1 to 5 were obtained by adding 0.1 mol/L of acetate buffer while pH values from 7 to 11 were

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obtained by adding phosphate buffer (0.1 mol/L). From Fig. 11 (a), it can be observed that percentage of MB degradation increases with an increase in the solution pH from 1 (32.56% degradation) to 9 (78.80% degradation) while the degradation percentage decreases

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significantly at pH values beyond 9 (70.26% at pH 11). The effect of pH of the solution on the

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photocatalytic degradation of MB in the presence of FGC can be described on the basis of the point of zero charge pH (pHpzc) of the FGC particles. From Fig. 11 (b), the pHpzc value of FGC

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particles is 5.8 which indicate that the surface of FGC is positively charged when the solution pH is less than 5.8 and thus the electrostatic repulsion between the cationic MB molecules and

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the positive surface charge prevent these molecules from approaching the surface of FGC. In other words, its reactive radicals are responsible for the photocatalytic process [53]. Hence,

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under acidic conditions, the percentage of MB degradation is low. On the other hand, when the solution pH is higher than 5.8, the surface of FGC is negatively charged and this favors the

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approach of MB molecules to the surface of the FGC catalyst. Therefore, the degradation percentage of MB increases [54]. However, at a solution pH higher than 11, hydroxide ions (OH-) compete with the negatively charged catalyst in attracting the positively charged MB molecules, thus leading to a reduction in the percentage degradation [55]. 3.6.6

Effect of temperature

14 Temperature is usually a vital factor controlling the thermodynamics of a reaction as well as a photocatalysis process. Figure 12 shows the effect of different temperatures on the percentage of MB degradation. As can be seen from the figure, the MB degradation percentage increases (from 39.19% to 73.83%) with increasing temperature up to 40 °C, indicating that the photocatalytic degradation of MB is an endothermic process. In other words, the temperature has a positive effect on the photocatalytic activity of the FGC catalyst. However, increasing the temperature further to 50 °C decreases the photocatalytic activity. The

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decreased efficiency at a higher temperature may be due to the increased recombination of photon-generated carriers [56]. A similar trend has been observed for photocatalytic degradation of Congo red treated with g-C3N4 catalyst [57].

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3.6.7 Comparison of the photocatalytic activity

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In order to compare the photocatalytic performance of the synthesized FGC photocatalyst with FG, F, and G catalysts, photocatalytic decomposition of MB dye due to each

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of the catalysts was tested under optimum conditions of various experimental parameters, i.e., for a dye concentration of 20 mg/L, temperature of 25 °C, irradiation of 40 min, 100 W

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irradiation intensity and 0.5 g/L of catalyst dose. As seen in Fig.13 (a), the degradation ratio (Ct/Co) of FGC is much higher than that of the other catalysts (FGC > FG > G > F) with the

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degradation efficiency of MB of 94.27, 58.71, 43.82 and 37.14% obtained due to FGC, FG, F and G catalysts, respectively. Moreover, From Fig. 13 (a), it can be seen that after an irradiation

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time of 40 min, the Ct/Co ratio decreases to zero which confirms a complete degradation of MB by the FGC catalyst. This shows that the synthesized FGC is very active when subjected to visible light. In addition, having a large surface of graphene may cause more dispersion of light than Fe2O3 and CuO particles due to their small surface. Hence, Fe2O3 and CuO particles can absorb more visible light and generate more electron-hole pairs to degrade MB molecules, i.e., lead to a higher photocatalytic activity [58]. The photocatalytic degradation kinetics of all

15 samples can be expressed as –ln (Ct/Co) = kapp t, where kapp is the apparent rate constant, Co is the concentration of MB before light irradiation and Ct is a concentration of MB after light irradiation at time t [24]. Figure 13 (b) and Table 2 show the kinetics plot and the kapp of all catalysts, respectively, under optimum conditions. As shown in Table 2, the kapp of MB was 7.25 × 10–2, 2.21 × 10–2, 1.43 × 10–3, and 1.15 × 10–3 min–1, which decreased for the different catalysts in the following order: FGC > FG > G > F. Among these catalysts, FGC exhibited the highest visible light activity (kapp = 7.25 × 10–2 min–1), indicating that FGC can promote a

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significant synergistic effect. A comparison of the percentage of MB degradation as well as kapp values using various photocatalysts is shown in Table 3. These data demonstrate that the FGC exhibits much better photocatalytic activity as compared to those previously reported for

Reusability of the FGC catalyst

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3.7.

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all other catalysts.

In order to evaluate the photocatalytic stability and reusability of the synthesized FGC

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catalyst, repeated measurements for investigating the photocatalytic degradation of MB in the presence of FGC subjected to visible light were performed. After the degradation of MB was

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completed, the FGC catalyst was collected using an external magnet, washed, dried at 80 °C for 12 h, and used for the next measurement while keeping other reaction conditions constant.

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From the results (Fig. 14), it was found that the Ct/Co ratio does not show any significant change, or in other words, the percentage of MB degradation by FGC does not exhibit any

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significant loss of photocatalytic activity after five runs. The degradation in each cycle was found to be 94.27%, 92.73%, 91.28%, 90.83%, and 90.26% from the first to the fifth usage of the catalyst, respectively. This indicates high stability of the FGC catalyst and thus suggests that it is a promising candidate to use in photocatalytic degradation in wastewater treatment. 3.8.

Trapping experiments

16 Since three reactive species including hydroxyl radicals (OH), active holes (h+) and superoxide radicals anions (●O2–) are the main reactive species in the photocatalysis process [63], in order to understand the working of these major species, trapping experiments were performed using various scavengers, including isopropanol (IPA), ammonium oxalate (AO) and benzoquinone (BQ) as a quencher for OH, h+ and ●O2–, respectively [64]. The different scavengers can effectively remove the corresponding species, i.e., they can easily determine the functions of the reactive species according to the suppressed photocatalytic efficiency. The

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results of the photocatalytic degradation of MB by FGC in the presence of various scavengers, IPA, AO, and BQ are shown in Fig. 15. It can be clearly seen that the efficiency of photocatalytic degradation of MB by FGC is up to 94.27% without adding any scavengers. On

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addition of AO to the photocatalytic system, the percentage of MB degradation decreased to 74.28%, implying that h+ contributes less to the degradation process. When BQ was added to

O2– radicals can effectively facilitate the degradation and play a crucial role in the

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the photocatalytic system, the percentage degradation of MB was only 35%, indicating that

photocatalytic reaction. Importantly, when the photocatalytic system was added with IPA, only 12.8% of MB was degraded, i.e., the photocatalytic efficiency of FGC for MB degradation is

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greatly inhibited, implying that the •OH radicals are the concerned primary reactive species.

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Since three reactive species including hydroxyl radicals (OH), active holes (h+) and superoxide

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radicals (●O2–) are the main reactive species in the photocatalysis process [63], therefore to explore the major species during photocatalytic process, trapping experiments were applied using various scavengers including isopropanol (IPA), ammonium oxalate (AO), and benzoquinone (BQ) as a quencher for OH, h+, and ●O2–, respectively [64]. Due to different scavengers can effectively remove corresponding species, in other word, it easily determinate the functions of the reactive species according to suppressed photocatalytic efficiency. The

17 photocatalytic results of MB degraded by FGC in the presence of various scavengers of IPA, AO and BQ are shown in Fig. 15. It was clear that the photocatalytic degradation efficiency of MB is up to 94.27% without adding any scavengers. When the photocatalytic system was added with AO, the percentage of MB degradation decreased to 74.28%, meaning that h+ contribute less in MB degradation. Also, when the photocatalytic system was added with BQ, the percentage of MB degradation is only 35%, indicating that ●O2– radicals can effectively facilitate the degradation and play a crucial role in the photocatalytic reaction. Importantly,

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when the photocatalytic system was added with IPA, only 12.8% of MB was degraded. That is, the photocatalytic efficiency of MB degradation is greatly inhibited, meaning that the OH radicals are the primary reactive species concerned.

Postulated mechanism for the photocatalytic activity of the FGC composite

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3.9.

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Figure 16 shows the schematic of the postulated mechanism for the photocatalytic activity of the FGC photocatalyst. From previous reports in the literature, the bandgap energy

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(Eg) of CuO is 1.20 eV [13] while from this work the bandgap of Fe2O3 has been determined to be 1.82 eV. Since Eg of CuO is less than that of Fe2O3, when FGC is used as photocatalyst,

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the electrons (e–) in the valence band (VB) of CuO are excited to its conduction band (CB). This leads to the generation of the electron-hole (e−/h+) pairs, which is the main cause of the

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photocatalytic reaction. The photon-generated e−/h+ pairs separate and transfer onto the surface of FGC and take part in the redox reactions. In addition, since graphene exhibits an excellent

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electronic conductivity and has a large specific surface area, it can act as an electron acceptor and transporter [65]. Hence, when the ternary composite of graphene, CuO and Fe2O3 is synthesized, the electrons generated by the visible light irradiation can easily transfer to the composite surface, thus inhibiting the recombination between photon-induced electrons and holes [66]. These electrons (e–) can react with O2 dissolved in water and then transform to ●O2– radicals. Meanwhile, the holes can react with OH– or H2O absorbed on the FGC surface to

18 generate •OH radicals. Both the ●O2– and •OH radicals have a strong oxidation ability to degrade MB to give CO2, H2O, and by-products which are less toxic and have a low molecular weight. 4. Conclusion A ternary Fe2O3-graphene-CuO (FGC) hybrid nanocomposite photocatalyst was successfully synthesized via a simple solvothermal method and characterized using XRD, SEM, TEM and H-TEM, UV-Visible DRS, and VSM techniques. The results from the analysis confirmed that CuO, Fe2O3, and graphene have been successfully amalgamated onto the FGC

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composite material. The UV-Visible DRS spectra reveal that the FGC catalyst absorbs light in the visible region and shows an excellent photocatalytic degradation of the MB solution without any additional oxidant or reductant. The photocatalytic performance of the as-prepared

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FGC was much better than that of F, G, and FG. Remarkably, the developed hybrid

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nanocomposite with magnetized Fe2O3 could be easily separated by applying an external magnetic field and reused. Its catalytic activity did not decrease significantly even after

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repeated uses, retaining its catalytic performance (> 90%) even after 5 cycles. Overall, the outcome of the present work suggests that the FGC catalyst is not only an excellent

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photocatalyst for the photocatalytic degradation process and having a great potential in the practical treatment of dye wastewater but is also a suitable candidate for eco-friendly

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environmental applications.

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Acknowledgements

This research was financially supported by Nanomaterials Chemistry Research Unit,

Department of Chemistry, Faculty of Science and Technology, Nakhon Si Thammarat Rajabhat University, Nakhon Si Thammarat, Thailand, and Materials Chemistry Research Center, Department of Chemistry, Center of Excellence for Innovation in Chemistry (PERCH-CIC), Faculty of Science, Khon Kaen University, Khon Kaen, Thailand.

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Figure caption Fig. 1. XRD patterns of Fe2O3 (F), Graphene (G), Fe2O3-graphene (FG), CuO (C) and Fe2O3/graphene/CuO (FGC) Fig. 2. EDS spectrum of Fe2O3/graphene/CuO (FGC) Fig. 3. TEM images of FGC composite Fig. 4. HR-TEM images of FGC composite Fig. 5. Magnetic hysteresis loops of F, FG and FGC catalysts

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Fig. 6. UV-vis spectra of (a) F, G, FG, FGC and variation of (h)2 versus Eg of (b) Graphene, (c) Fe2O3, (d) Fe2O3-graphene, (e) Fe2O3/graphene/CuO Fig. 7. Effect of visible light irradiation time

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Fig. 8. Effect of MB initial concentration

Fig. 10. Effect of catalyst dosage

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Fig. 9. Effect of visible light intensity

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Fig. 11. (a) Effect of solution pH on MB degradation and (b) pH of point of zero charge (pHpzc) of FGC

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Fig. 12. (a) Ct/Co versus time at different temperature and (b) effect of temperature on MB degradation

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Fig. 13. (a) the photocatalytic degradation and (b) kinetics curves of MB catalysts by F, G, FG and FGC catalysts

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Fig. 14. Cycling runs of FGC catalyst for the photocatalytic degradation of MB Fig. 15. Trapping experiments of FGC catalyst using different scavengers Fig. 16. Postulated mechanism for the photocatalytic activity of FGC composite

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Table 1. The percentage degradations of MB at different visible irradiation intensity

MB Degradation (%)

0

0

40

25

60

40

100

60

130

62

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Visible irradiation intensity (Watt)

Table. 2. The apparent rate constant of MB by different photocatalysts Parameter

Photocatalyst

R2

1.15x10-3

0.9972

1.43x10-3

0.9981

2.21x10-2

0.9995

7.25x10-2

0.9975

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Fe2O3 (F)

kapp (min-1)

Fe2O3/graphene (FG)

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Fe2O3/graphene/CuO (FGC)

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Graphene (G)

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Table. 3. Comparison of various catalysts for photocatalytic degradation of methylene blue (MB) Degradation (%)

kapp (min-1)

Reference

79.90

1.20x10-2

[26]

Coordination polymer-derived CuO

92.00

1.98x10-2

[59]

SrFe12O19

95.00

1.36x10-2

[60]

[email protected]@Ru

92.70

1.76x10-2

[61]

NiFe2O4

n.d.

3.5x10-3

[62]

Boron doped C3N4/NiFe2O4

n.d.

4.4x10-2

[62]

Fe2O3/graphene/CuO (FGC)

94.27

7.25x10-2

This work

Photocatalyst

Jo

ur

[email protected]@TiO2

n.d. : no data

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Jo

45