Magnetically separable and recyclable graphene-MgFe2O4 nanocomposites for enhanced photocatalytic applications

Accepted Manuscript Magnetically Separable and Recyclable Graphene-MgFe2O4 Nanocomposites for Enhanced Photocatalytic Applications Imran Shakir, Mansoor Sarfraz, Zahid Ali, Mohamed F.A. Aboud, Philips Olaleye Agboola PII:

S0925-8388(15)31611-X

DOI:

10.1016/j.jallcom.2015.11.055

Reference:

JALCOM 35910

To appear in:

Journal of Alloys and Compounds

Received Date: 9 August 2015 Revised Date:

5 November 2015

Accepted Date: 7 November 2015

Please cite this article as: I. Shakir, M. Sarfraz, Z. Ali, M.F.A. Aboud, P.O. Agboola, Magnetically Separable and Recyclable Graphene-MgFe2O4 Nanocomposites for Enhanced Photocatalytic Applications, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.11.055. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.

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Graphical Abstract MgFe2O4-graphene nanocomposites were fabricated via cheap wet chemical routes that showed

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the good photocatalytic activity in the presence of visible light.

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Magnetically Separable and Recyclable Graphene-MgFe2O4 Nanocomposites for Enhanced Photocatalytic Applications

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Imran Shakir1,*, Mansoor Sarfraz1, Zahid Ali2 Mohamed F. A. Aboud1 and Philips Olaleye Agboola1 Sustainable Energy Technologies Center, College of Engineering, 2National Institute of Lasers and Optronics, P.O. Nilore, 45650 Islamabad, Pakistan

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Abstract

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MgFe2O4 nanoparticles were prepared by adopting the micro-emulsion route. GrapheneMgFe2O4 nanocomposites were prepared via ultra-sonication route. The characterization of MgFe2O4 nanoparticles and their nanocomposites was carried out by X-ray diffraction (XRD), Fourier transform infra-red spectroscopy (FTIR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and vibrating sample magnetometery (VSM). XRD and FTIR

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confirmed the cubic spinel structure of MgFe2O4. VSM exhibited the magnetic behavior of MgFe2O4 while SEM estimated the particle size in the range of 50 nm. The photocatalytic activity of MgFe2O4 and nanocomposites were carried out using methylene blue as model

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compound in the visible light. The photocatalytic activity showed that the synthesized magnetic

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particles and their nanocomposites with graphene can be utilized as photocatalyst materials. These new photocatalyst could also be recycled and separated by applying an external magnetic field.

Key words: Nanostructures; Graphene; MgFe2O4; Magnetic Materials; Photocatalysis. *

Corresponding author: [email protected],

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1. Introduction Photocatalysis phenomenon has a wide range of potential applications in different areas such

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waste water purification, environmental remediation etc. This technology is used to remove toxic, dangerous compounds and organic pollutants industrial waste contaminants from the environment [1, 2]. The photocatalysis can be enhanced by using solid catalyst materials called

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as photocatalyst. The most common photocatalyst materials are TiO2 based photocatalyst materials. The TiO2 has been extensively studied; however its main drawback is that it works

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only under UV-light irradiation as it has relatively wide band gap [2, 3]. The solar light bears only 3-4 % UV light. Therefore now a day the researchers are investigating the new photocatalyst materials that can utilize visible light. The solar light has about 40% visible light. Similarly large band gap of ZnS,TiO2, SrTiO3, andAg3VO4 photocatalyst were unable for degradation of organic pollutants and dyes (such as methylene blue, bromophenol blue, and

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Chicago sky blue from industrial waste water), that remains as a challenge. Ferrites are the magnetic materials that have more than 50% iron content and are ferromagnetic in nature. They also exhibit the absorption spectrum in the visible range (400-800 nm) and have narrow band

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energy (~2.0 ev). Further these materials can be recycled and separated easily by applying Among various ferrites the spinel ferrites (MFe2O4) have many

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external magnetic field.

photochemical properties such as the decomposition of alcohols and hydrogen peroxide, the oxidative dehydrogenation of hydrocarbons, the oxidation of compounds such as CO, H2, CH4, the treatment of exhaust gases etc. Ferrites are capable for this type of degradation because of low band gap and more catalytic site for adsorption of dyes and waste materials. Further the ferrites are also thermally and chemically stable materials [4]. MgFe2O4 (Magnesium ferrite) is a typical spinel ferrite and has narrow band gap. It can absorb visible light [3, 5]. However the

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MgFe2O4 bears relatively less electronic conductivity [6] and recombination of photogenerated electron-hole pair. This is a serious drawback for its utilization as photocatalyst materials. In recent years, graphene attracted researchers’ attention due to excellent properties and it increases

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the photocatalytic efficiency of metal-frame work catalysts due to stronger interaction with metal cluster for degradation of dyes and organic materials. Graphene is a 2 D, sp2-bonded one-atomthick carbon layer packed into a dense honeycomb lattice [7-9]. Graphene is highly conductive in

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nature can increase the conductivity of ferrite nanoparticles and minimize the recombination of photogenerated electron-hole pair. In the manuscript, we discuss the synthesis, characterization

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and photocatalysis applications of magnetically separable MgFe2O4 nanoparticles and their composites with graphene. 2. Experimental Work

MgFe2O4 nanoparticles and graphene–MgFe2O4 nanocomposites were synthesized from

Aldrich), ceryltrimetyl

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following chemicals: Magnesium nitrate (Mg(NO3)2 Sigma-Aldrich), Fe(NO3)3.9H2O (Sigmaammonium

bromide((C16H33)N(CH3)3Br) (CTAB) (Sigma-Aldrich),

NaNO3 (Sigma-Aldrich), KMnO4 (Sigma-Aldrich), H2SO4 (Fischer Scientific), Graphite Powder

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(Sigma-Aldrich), Aqueous Ammonia (35 %, BDH). All chemicals were used as received without

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any further purification.

2.1 Synthesis of MgFe2O4 Nanoparticles The MgFe2O4 nanoparticles were prepared via micro-emulsion route [10, 11]. All metal salts were weighed in required stoichiometric ratio to prepare the aqueous solutions. The required volumes were mixed and stirred by using magnetic hot plate. The temperature was raised to ~60 o

C then surfactant CTAB was added. Aqueous ammonia was used to increase the pH ~10. The

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reaction mixture was further kept for stirring for 4 hours. Washing was done by deionized water to neutralize the pH followed by drying and grinding the precipitates. The annealing was done at

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800˚C for ~ 8 hours using muffle furnace Vulcan A550. 2.2 Synthesis of Graphene and MgFe2O4-Graphene Nanocomposites

Graphene oxide (GO) was synthesized starting from graphite powder by Hummer’s method [12].

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Typically 3.0 g graphite powder and 2 g sodium nitrate were mixed in 500 cm3 beaker followed by addition of 200 cm3 conc. Sulfuric acid in ice bath. 12 g of potassium permanganate was

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added in 15-20 minutes. The reaction mixture was cooled for further 2 h. The stirring was carried out at room temperature for five days to obtain the brown colored slurry of graphite oxide. The exfoliation of graphite oxide was carried out by ultrasonic processor. The reduction of graphene oxide to graphene was carried out by following the literature procedure [13]. The MgFe2O4-

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graphene nanocomposite was prepared by ultrasonication of rGO and MgFe2O4 nanoparticles. Typically 150 mg of MgFe2O4 nanoparticles and 30 mg of rGO were dispersed in 100 cm3 of DI water and ultrasonicated for 3 h to get a homogeneous suspension. The filteration was carried out

o

C over night.

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and the graphene-MgFe2O4 nanocomposites were washed with deionized water and dried at 110

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2.3 Characterization

Thermo-gravimetric analysis (TGA) was done using thermal analyzer SDT Q 600V8.2 Build 100, to find out the annealing temperature of samples. Crystalline structure was identified by using XRD technique (Philips X’ pert PRO 3040/60 diffractometer). Room temperature FTIR spectra of MgFe2O4 and graphene-MgFe2O4 nanocomposites were recorded on Nexus 470 spectrometer. Jeol JSM-6490A electron microscope was used to carry out the SEM analysis. The

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UV-Visible spectra and photocatalytic measurements were recorded on dual beam Agilent Cary 60, Spectrophotometer.

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3. Results and Discussion 3.1 Thermo-gravimetric Analysis

Figure 1 shows the TGA of MgFe2O4 nanoparticles. The Figure shows that the weight loss takes

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place at six various stages. The first stage shows the 2.5% weight loss at 75 oC, 4% at 97oC (second stage), 4.5% at 155oC (third stage), 1.5% at 262.5oC (fourth stage), 4.5% at 402 oC (fifth stage), and

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1.5% at 600 oC (sixth stage). This weight loss is due to removal of adsorbed water molecules, trapped water molecules, conversion of metal hydroxides into metal oxides, decomposition of CTAB, and formation of spinel structure [14]. This result has been found similar to that of reported in the literature [15, 16].

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3.2 SEM Analysis

SEM analysis of MgFe2O4 nanoparticles and graphene-MgFe2O4 nanocomposite were carried out

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to know surface morphology and to estimate the particles size. Typical SEM images of MgFe2O4 nanoparticles and graphene-MgFe2O4 nanocomposite are shown in Figure 2. The Figure shows

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the spherical morphology of the particles and the particles are uniformly wrapped by graphene sheets. The estimated particle size was in the range of 50-70 nm. 3.3 FTIR Analysis

Figure 3 exhibit the FTIR spectra of MgFe2O4 nanoparticles and graphene-MgFe2O4 nanocomposite. The results obtained from the FTIR spectra provide information about the chemical bonds and molecular structure of MgFe2O4 and its composites. MgFe2O4 shows the

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absorption bands at 546.41, 591.72 and 609.96 cm-1. The absorption bands at 553.01 cm-1 and 536 cm-1 corresponded to the vibration of tetrahedral and octahedral complexes, respectively [17]. FTIR spectra graphene -MgFe2O4 showed peaks at 1595 cm-1 because of the surface

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absorbed moisture. The peaks around 1600 cm-1 is due the presence of O– H [18, 19].This peak show the bending vibration of water molecule [20]. All the peaks which was observed in

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graphene-MgFe2O4 nanocomposite confirmed the presence of prepared material [21]. 3.4 XRD Analysis

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Powder X-ray diffraction (XRD) patterns of MgFe2O4 nanoparticles, graphene-MgFe2O4 nanocomposite were carried out in 2θ range 10-80o and this pattern are shown in Figure 4. MgFe2O4 nanoparticles and graphene-MgFe2O4 nanocomposite diffraction peaks were found at two theta values 19o, 30o, 35o, 43o, 53o, 57o, and 62o. These peaks were assigned “hkl” values as:

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(111), (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), and (4 4 0). This observed diffraction pattern is obtained similar to literature [5, 6, 18, 22, 23].

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3.4 VSM Analysis

With the help of vibrating sample magnetometer (VSM lakeshore-74071) magnetic

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measurements were carried out for MgFe2O4 nanoparticles. The magnetic behavior of sample shown in Figure 5. Magnetization of nanoparticles demonstrated the clear s-shaped hysteresis loop that confirmed the ferromagnetic nature of MgFe2O4 nanoparticles [24]. According to Figure 5, the saturation magnetization (Ms) of prepared sample MgFe2O4 nanoparticles ensured the magnetic collection of MgFe2O4 nanoparticles from water by applied external magnetic field. Due to this magnetic behavior, these materials can be separated / recovered and recycled [5].

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3.5 UV-Vis Measurements Otical properties of photocatalyst materials play vital role in photocatalytic activity. The optical properties are dependent on the band gaps of the materials. Figure 6(a) showed the UV-Vis.

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absorbance spectra of MgFe2O4 and graphene-MgFe2O4 nanocomposite. MgFe2O4 could be dispersed in water through ultra-sonication [25]. The MgFe2O4 UV-Vis. Spectrum confirmed the abosprtion in visible light region. This absorption was significantly enhanced when the

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MgFe2O4 nanoparticles were mixed with graphene to obtain the graphene-MgFe2O4 nanocomposite (Figure 6 (a)). Figure 6(b) depicted the UV-Vis. Spectra of graphite oxide which

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has absorption band at 226 nm. It can be clearly seen from the Figure that graphene shows the strong absorption peak at 257.9 nm, depicting the partially reduced GO [27]. 3.6. Photocatalytic Activity

Previously heterostructures semiconductor nanocomposites were found to enhance the

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photocatalytic activity in visible light [28], which is a great motivation for the researchers to examine the graphene-MgFe2O4 nanocomposite photocatalytic activity under visible light irradiation, the degradation of aqueous solution of methylene blue (MB), the photocatalytic

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activity of graphene-MgFe2O4 nanocomposite was evaluated. The photocatalytic degradation of

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MB in the presence of synthesised photocatalyst graphene-MgFe2O4 nanocomposite is shown in Figure 7. The UV-Visible spectra of MB solution showed the 4 absorption peaks at 245nm, 292 nm, 614 nm and 664 nm. It is clear that peak at 664 nm steadily decreased with increased time. This confirmed the degradation of methylene blue with the passage of time [29]. Photodegradation of methylene blue followed the pseudo-first-order kinetics behavior. The photocatalytic degradation was calculated by applying formula [22, 30]

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%Degradation =

C o − Ct × 100 Co

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Here Co and Ct= concentration of methylene blue (reaction time is 0 and t, respectively) Photocatalytic efficiency of MB in the presence of graphene-MgFe2O4 nanocomposite is shown in Figure 8 which clearly shows the un-degraded MB is negligible ~1.0 % . Generally,

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photocatalyst with a high specific surface area would offer more surface active sites and photocatalytic reaction centers, resulting in the enhancement of photocatalytic performance [31].

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The proposed mechanism of photo-catalytic activity graphene-MgFe2O4 nanocomposite is shown as follows: MgFe2O4 + hv

MgFe2O4 ( h + e)

In this step the electron hole pair is generated at the photocatalyst surface in the result of photo

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excitation of material. MgFe2O4 (e) + graphene

MgFe2O4 + graphene (e)

−

O 2 + graphene

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graphene (e) + O2

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MgFe2O4 (h) + OH-

−

MgFe2O4 (h) + .OH + O 2 + MB

MgFe2O4 + .OH 2−

−

CO2 + H2O +SO 4 + NO 3 + Cl- + NH

The role of graphene in increasing the efficiency of MgFe2O4 nanoparticles is also illustrated in Figure 9. It can be concluded that MgFe2O4nanocomposite exhibited the enhanced photocatalytic activity as compared to pure MgFe2O4. This may be due to stronger interaction between

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graphene and MgFe2O4 nanoparticles graphene also help to suppressed recombination of photogenerated charge carriers.

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Acknowledgment The authors would like to extend their sincere appreciation to the Deanship of Scientific Research at the King Saudi University for its funding of this research work through the Prolific

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Research Group PRG-1436-25.

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4. Conclusion

MgFe2O4 nanoparticles and their nanocomposites with graphene were fabricated via facile routes such as micro-emulsion and ultra-sonication route respectively. The data of all characterizations techniques like TGA, XRD, FTIR, SEM and VSM were found in agreement

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with each other. The photocatalytic activity of graphene-MgFe2O4 nanocomposite was evaluated by taking methylene blue as model organic compound under visible light irradiation. The dye was degraded successfully that showed the good photocatalytic activity of newly synthesized

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graphene-MgFe2O4 nanocomposite. The graphene played the significant role in increasing the efficiency of MgFe2O4 nanoparticles by increasing the electronic conductivity and by prohibiting

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the recombination of photogenerated electron hole pair at MgFe2O4 surface. References

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List of Figures

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Figure 1 TGA of MgFe2O4 nanoparticles

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Figure 1 (a) SEM images of MgFe2O4 nanoparticles (b) SEM of graphene-MgFe2O4

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nanocomposite

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1595.91

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591.86

553.01

(I)

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Transmittance (%)

(II)

536.7

609.96 591.72

1500

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546.41

1250

1000

750

500

-1

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Wavenumber (cm )

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Figure 2: (I) FT-IR spectra of MgFe2O4 and (II) Graphene – MgFe2O4 nanocomposite

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Relative Intensity (a.u.)

(II)

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(311)

(440)

(220)

(111)

10

20

30

(400)

(511) (422)

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40

50

60

70

80

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2θ (degree)

Figure 3: (I) X-ray diffraction patterns of MgFe2O4 and ( II) Graphene–MgFe2O4

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nanocomposite

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Figure 4: Magnetic hysteresis curve of as-synthesized MgFe2O4 nanoparticles traced at room

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temperature

0.35

(a)

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Graphene-MgFe2O4 nanocomposite MgFe2O4

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Absorbance

0.30

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0.20

0.15

300 400 500 600 700 800 900 1000

1.0

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Wavelength (nm)

(b)

0.6

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Relative Intensity

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0.8

Graphite oxide Reduced graphene oxide

0.4 0.2 0.0

300

400

500

600

Wavelength (nm)

700

800

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Figure 6(a) UV-VIS absorption spectrum of MgFe2O4 and graphene-MgFe2O4 nanocomposite dispersed in water (b) UV-VIS absorption spectrum of graphite oxide and reduced graphene

0 min 5 mins 10 mins 15 mins 20 mins 25 mins 30 mins 35 mins

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1.2

0.8 0.6 0.4

0.0

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Absorbance

1.0

0.2

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oxide.

300

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200

400

500

600

700

800

Wavelength (nm)

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Figure 7: Absorption spectra of MB solution at different photocatalytic degradation times.

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1.0 0.8

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(II)

0.6 0.4

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C/Co

(III)

0.2

(I)

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0

10

20

30

40

Time (mins)

Figure 8: photocatalytic degradation profile of (I) MgFe2O4 (II) graphene –MgFe2O4 nanocomposite (III) methylene blue under UV and visible light irradiation with and without catalyst versus irradiation exposure time.

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Figure 9 Schematic representative of the formation mechanism of graphene –MgFe2O4 nanocomposite

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Highlights MgFe2O4 nanoparticles with size 50-70 nm were prepared via wet chemical route.

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MgFe2O4-graphene nanocomposites were fabricated by ultra-sonication method.

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MgFe2O4-graphene nanocomposites exhibited the good photocatalytic activity.

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