Synthesis route of reduced graphene oxide via thermal reduction of chemically exfoliated graphene oxide

Synthesis route of reduced graphene oxide via thermal reduction of chemically exfoliated graphene oxide

Materials Chemistry and Physics 204 (2018) 1e7 Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: www.elsev...

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Materials Chemistry and Physics 204 (2018) 1e7

Contents lists available at ScienceDirect

Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Synthesis route of reduced graphene oxide via thermal reduction of chemically exfoliated graphene oxide Hareema Saleem a, *, Mobeen Haneef a, Hina Y. Abbasi b a b

School of Chemicals and Materials Engineering, National University of Science and Technology (NUST), Sector H-12, Islamabad, Pakistan U.S.-Pakistan Center for Advanced Studies in Energy, National University of Science and Technology (NUST), Sector H-12, Islamabad, Pakistan

h i g h l i g h t s  Chemical exfoliation of graphite flakes.  Thermal reduction of graphene oxide.  UV-Vis spectroscopy showed the hills of GO and rGO at 215 nm and 267 nm.  X-ray diffraction displayed the peaks of GO at 11.8 and rGO at 25.5 .

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 May 2017 Received in revised form 28 August 2017 Accepted 7 October 2017 Available online 9 October 2017

Graphene, a two-dimensional material, is now considered as a rewarding contestant for nanodevices due to its morphology and novel properties. The chemical exfoliation and thermal annealing methods are appraised as an inventive route towards the production of graphene at prodigious scale. This method is utilized for the oxidation of graphite flakes having an oxidizing specialist and thermally reduced the graphene oxide into reduced graphene oxide. We have examined the samples through different characterization techniques. X-ray diffraction displayed the peaks of graphene oxide at 11.8 and reduced graphene oxide at 25.5 . Scanning electron microscopy images revealed the single and multilayers morphology. The optical microscopy examined the number of layers of graphene oxide through the various contrasts of light. UV-Vis spectroscopy showed the hills of graphene oxide and reduced graphene oxide at 215 nm and 267 nm. Fourier transform infrared spectroscopy has been utilized to examine the resonating modes. © 2017 Elsevier B.V. All rights reserved.

Keywords: Graphene oxide (GO) Reduced graphene oxide (rGO) Oxidizing agent Chemical exfoliation method Thermal annealing

1. Introduction Today, the “Nanotechnology” plays a vital role in many fields including biosciences, electronics, aerospace, automobiles, defense, computers, agriculture, foods technology, consumer products and also has shown major advancements in energy by minimizing the losses and increasing the efficiency of energy generation. Due to the increased surface to the volume ratio, interfaces and reduced size effects, dramatic positive changes in the properties of various materials for a broad range can be observed and utilized for development. Many efficient devices are made on basis of nanotechnology with the benefits of reliability, compatibility and

* Corresponding author. E-mail addresses: [email protected] (H. Saleem), [email protected] scme.nust.edu.pk (M. Haneef), [email protected] (H.Y. Abbasi). https://doi.org/10.1016/j.matchemphys.2017.10.020 0254-0584/© 2017 Elsevier B.V. All rights reserved.

environment friendly nature. Still, we have challenges to make more efficient devices with required efficiencies by optimization of an availability and cost of raw materials, manufacturing costs and selection of size dependents properties of raw materials. In recent developments in carbon allotropes, graphene has proven itself better in energy technologies as compare to carbon nanotubes, fullerene (C60), diamond, and graphite. It possesses unique size dependent properties such as morphological, electrical, optical to enhance the energy conversion, piezosensitive, and storage performances. In 2004, Graphene was discovered by Andre Geim and Konstantin Novoselov when they isolated graphene by peeling graphite with adhesive tape. This provides good quality micron sized graphene [1]. Graphene is two-dimensional sheet of sp2-hybridized carbon atoms. This network of extended honeycomb is the elementary construction block of other carbon allotropes as it can

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be loaded to form graphite (3D), it can be rolled to form nanotubes (1D), and enfolded to form fullerenes (0D). p-conjugation of long range in graphene produces amazing electrical, mechanical, and thermal properties [2]. The different method to synthesis the graphene are: micromechanical cleavage of natural graphite flakes using a Scotch Tape [1], epitaxial growth of graphene on SiC [3], thermal or plasma enhanced chemical vapor deposition (CVD) of graphene [4e6], chemical exfoliation of graphitic materials [7e10], bottom-up synthesis from organic compounds, and electrochemical exfoliation of graphite [11]. Only the chemical exfoliation method is considered as a typical way toward the generation of graphene with ease and in huge amount [12]. The ideal graphene sheets that can be achieved by this mechanical exfoliation technique have proven to be highly ordered, exhibit outstanding surface areas (2630 m2 g1), high Young's modulus (1 TPa), highest hardness, flexible elastic, ultra thickness, high thermal conductivity (5000 W mK1), and strong chemical durability [13]. Having zero rest mass of charge carriers, electrons travel with the speed of light through the crystal. Such electron transport has never been practically seen in any semiconductor, and as a result graphene has proved to be the most conductive non-superconducting material with a room temperature resistivity of just 106/cm. The following potential applications of graphene are: (a) The electronic devices contain graphene have ultrafast terahertz electron flexibility that gives smaller, quicker, less expensive digital products such as field transistors [14], spintronics [15], and optoelectronics [16], (b) Graphene loaded polymer composites have high electrical and heat conductivity, good mechanical strength, and low percolation threshold [17], (c) It is used in display programs such as touch displays [18], (d) Graphene centered electrochemical storage space energy devices, (e) Graphene is used as transport, performing electrodes [19], and (f) Many other applications in bio-, electrochemical, and chemical sensors [20], dyesensitized and organic solar panels, hydrogen storage, catalysts [21] and photo catalysts [22]. Here, we report the chemical and thermal synthesis routes towards the massive production of reduced graphene oxide. We prepared high quality graphene nanosheets through this approach. The crystal structure, morphology and optical properties were investigated by X-ray dffraction (XRD), scanning electron microscopy (SEM), optical microscopy, UV-Vis spectroscopy, and Fourier transform infrared spectroscopy (FTIR).

2.3. Thermal reduction of graphene oxide The GO was annealed in a Nabertherm Gmbh N17/HR-400V Muffle heating furnace at 500  C at a heating rate of 2  C min1 for 2 h and allowed to cool in furnace till the temperature decreased till 50 ± 5  C. The GO was completely converted into reduced graphene oxide (rGO) [26,27]. 2.4. Characterization The X-Ray Diffraction STOE q-q was used with CuKa radiation with wavelength 1.5406 Å and scintillation detector as sensor equipment, scanning electron microscope (SEM) analysis in this research was performed by JEOL scanning electron microscope (JSM 6490LA), optical microscopy, UV-Vis spectroscopy (wavelength 200 nm-1100 nm), Fourier transform infrared spectroscopy (FTIR) were used to investigate the structural, optical, and functional properties of graphene oxide (GO) and reduced graphene oxide (rGO). 3. Results and discussions 3.1. X-ray diffraction (XRD) XRD is used for the determination of crystal structure and lattice parameters. During the oxidation process, we observed that the intense peaks of the (002) plane (d-space 3.4 Å at 26.23 ) of graphite flakes gradually debilitated and vanished which increased the inter-planar distance from 3.4 Å to 7.5 Å and shifted the diffraction peak of the plane (002) from 26.47 to 11.8 shown in Fig. 1 [28]. This enlargement of inter-planar distance is due to the entrapping of oxygen functional groups between the graphene oxide sheets [27,29]. This is confirmed by the Bragg's equation 2d sin q ¼ n l, shows the inverse relation of inter planar ‘d’ to the diffraction angle ‘2W’. After thermal reduction of graphene oxide, GO was reduced to rGO and the ordered crystal structure of reduced graphene oxide (rGO) was restored. This is verified by the reappearance of the (002) diffraction peak at 25.5 and disappearance of the diffraction peak (001) at 11.8 in XRD pattern in Fig. 2 [12,30]. During the reduction of GO, the yellow brown solution gradually transferred into black precipitate.

2. Experimental 3.2. Scanning electron microscope (SEM) 2.1. Chemical exfoliation method Oxidized the graphite flakes by chemical exfoliation method. Mixed 3 g of graphite flakes with 9:1 mixture of concentrated H2SO4/H3PO4 (360:40 ml) and 18 g of KMnO4. Stirred the mixture mechanically at 35-40 C for 4e5 h to produce the slightly exothermic reaction. Heated and stirred it at 50 C for 12 h. Then cooled it to room temperature and poured 400 ml ice of distilled water and 5 ml hydrogen peroxide H2O2 in a mixture to complete the reaction. The dark pink color was converted into mustard color. Washed the mixture with 1 M HCl and distilled water till the pH of supernatant liquid reached 7 [23,24]. 2.2. Exfoliation of graphene oxide (GO) To exfoliate the GO, sonicated the GO dispersion under normal condition for 30 min. The homogeneous brown dispersion was produced which was utilized for reduction. Dried the solution in the vacuum oven at 40  C for 24 h [25].

In scanning electron microscopy, the surface of the specimen is analyzed by an electron beam. SEM samples of graphene oxide (GO) and reduced graphene oxide (rGO) were prepared by sonication of GO in ethanol for 30 min. Dried the sample on a glass slide by heating under an incandescent lamp. SEM image of graphite flakes shows the flat surface with ordered layer structure [31]. Invariance, graphene oxide reveals randomly aggregated, thin, crumpled layers structure. It exhibits that the oriented layer structure of graphite has been unbalanced due to its oxidation. In Fig. 3, the graphite sheets have exfoliated into mono or multi-layer graphene oxide (GO) sheets [27,32]. Fig. 4 shows the SEM images of reduced graphene oxide oriented as randomly crumpled silk veil waves. The nanosheets are rippled and entangled with each other. The lamella structure of wrinkled rGO nanosheets is due to the van der Waal's interactions. They are transparent and the lateral dimensions of the sheets ranged from hundreds of nanometers to several micrometers [30,33].

H. Saleem et al. / Materials Chemistry and Physics 204 (2018) 1e7

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(001)

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2 Fig. 1. XRD of graphene oxide (GO).

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rGO

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2 Fig. 2. XRD of reduced graphene oxide (rGO).

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Fig. 3. SEM images of Graphene Oxide (GO) layers.

shows the absorption peak at 215 nm. The absorption peak varies on the oxidation of graphite [35]. When GO is being reduced, the absorption peak of oxidized graphene shifts to a greater wavelength from 215 nm to 267 nm. This is due to the desorption of the oxygen attached to the graphene layers after the thermal annealing shown in Fig. 7. As an impact of this, the color has changed from brownish to dark [35]. Graphene oxide nanosheets are hydrophilic and reduced graphene oxide (rGO) nanosheets are hydrophobic. We noticed that GO solution is stable in the water while rGO can homogenously disperse in water via ultrasonic vibrations but the dispersion can only be stable for few hours due to their hydrophobic nature. 3.5. FT-IR (Fourier Transform Infrared spectroscopy)

Fig. 4. SEM images of Reduced Graphene Oxide (rGO).

3.3. Optical microscopy The sample of GO for optical microscopy was made by sonication of GO in water for 10 min. Spin coated the sample over an oxidized silicone substrate. By using the Fresnel law, examined the contrast on substrate used as reference thickness and light wave length. We have examined the reliance of the contrast on the SiO2 thickness and light wavelength using Fresnel law. In Fig. 5, the silicon substrate is purple in color. The brighter contrast shows the high reflection of light from multi layers of GO and dull contrast shows the lower reflection from the few and monolayers of GO [34]. 3.4. UV-Vis spectroscopy UV-Vis spectroscopy is mainly used to evaluate transmission or absorptions in fluids and opaque solids. Add the solution of 10 mg graphene oxide in 10 ml distilled water and 10 mg reduced graphene oxide in 10 ml distilled water for the UV-Vis spectroscopy of GO and rGO specimens. In the oxidation process of graphite, the oxygen attached to the graphene layers enhances the attraction of the layers which intensify their solvency in water. This changed the yellow color of the solution into brown. There can be different shades of brown color depending upon the concentration of GO. In Fig. 6, the GO

Fourier Transform Infrared spectroscopy (FTIR) spectra provide information about the functional groups in a sample. At a 5 tons pressure in a die, the 5 mm pallet of graphene oxide was prepared by the mixture of dried graphene oxide and heated potassium bromide. In Fig. 8, the existence of C¼O, C¼C, O-H, C-H bonds is giving the surety of formation of graphene oxide [27]. In the graph, the peaks of O-H (stretching vibrations), sp3 C-H, C¼O (stretching vibrations) and C¼C (skeletal vibrations from unoxidized graphitic domain) bonds show at 3394 nm, 2920 nm, 1723 nm, and 1619 nm wavelengths. It is well established that GO contains various types of oxygen functional groups such as hydroxyl, carbonyl, alkene and cyanide functional groups. The reduction process of GO must involve the removal of oxygen functional groups. FT-IR spectra of reduced graphene oxide (rGO) displays that after the thermal reduction of graphene oxide, most of the oxides groups are removed in Fig. 9. However, hydroxyl group (O-H) and cyanide (C-N) are still attached with the layers of rGO due to the imperfect reaction with these groups [36]. 4. Conclusion Graphene oxide (GO) and reduced graphene oxide (rGO) were synthesized by chemical exfoliation and thermal methods. Graphite flakes were oxidized by KMnO4, H2SO4, and H3PO4 into graphene oxide (GO) and annealed the GO to convert into reduced graphene oxide (rGO). This is the novelty of work. In Modified Hummer's Method, the graphene oxide is reduced by hydrazine hydrate. In this work, graphene oxide was reduced by annealing at 500  C. Compared the results of graphene oxide and reduced graphene oxide through different characterization techniques. In XRD, the Bragg equation and inter planner distances showed the formation of GO and rGO. SEM images displayed the random wrinkled and crumpled silk waves morphologies of graphene oxide and reduced graphene oxide. Different shades of contrast in optical

Fig. 5. Optical image of Graphene Oxide (GO) at 500x, and at 1000x.

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5

Absorption

4

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GO 2

1 200

250

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Wavelegth(nm) Fig. 6. UV-Vis spectrum of Graphene Oxide (GO).

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Absorption

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rGO 2.4

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280

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Wavelength(nm) Fig. 7. UV-Vis spectrum of Reduced Graphene Oxide (rGO).

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% Transmittance

GO

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sp3 C-H 2500

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-OH 3500

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Wavelength (nm) Fig. 8. FT-IR graph of Graphene oxide (GO).

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C=C C-H

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O-H

50 0

500

1000

1500

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Wavelength (nm) Fig. 9. FT-IR of reduced graphene oxide (rGO).

microscopy showed the multi and monolayers of GO. UV-Vis spectrum exhibited the peaks of GO and rGO at 215 nm and 267 nm with the color changes from brown to darkish brown. In FTIR, the presence of carbonyl, hydroxyl bonds proved the formation of GO and removal of oxides groups gave the synthesis of rGO.

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