Hydrothermal Synthesis of Graphene
S.Mahdid1, 3
1 Welding and NDT Research Center - (CSC),
B.P. 64, Dely Brahim, 16000 Chéraga - Algiers Algeria s.mahdid@csc.dz ; saidaphy@yahoo.fr
N.Dokhan 2
2 Research Unit, Materials, Processes and Environment (URMPE), University of Boumerdes, Algeria
A.Kellou3
3 Laboratory of Quantum Electronics, Faculty of physics, U.S.T.H.B, B.P. 32, El-Alia, 16111 Bab- Ezzouar, Algiers
A. Badidi Bouda1
1 Welding and NDT Research Center - (CSC),
B.P. 64, Dely Brahim, 16000 Chéraga - Algiers Algeria
Abstract— Nanomaterials containing carbon have open new ways for the development of interesting and innovator applications.
Among these materials, the mother of all graphite shapes, the graphene, is becoming a material of a great interest because of their remarkable properties (physical, chemical and electrical ones). Currently, the graphene presents a great promise for potential applications in many technological fields such as: sensors, composites, transparent conducting films, solar cells, storage medium of gas, etc. The development of graphene is a technological challenge; the methods of current production are required with a balance between the facility of the production and the quality of materials. Our present work consists on a preparation of graphene oxide by Hummers method and its chemical reduction using an environmentally friendly reagent, namely, l-glutathione, under mild condition in aqueous solution. The resulting graphene was characterized by X-ray diffraction
Index Terms— Graphene, graphene nanosheets, hydrothermal technique, Chemical reduction.
I. INTRODUCTION
The graphene is the basic structural unit of some carbon allotropes, including graphite, carbon nanotubes and fullerenes. Since it was discovered in 2004 by Novoselov and Geim, researchers explored their potential in a wide variety of applications, as light-emitting devices, touch screens, photodetectors, ultrafast lasers, sensors, nano-composites, supercapacitors, transparent conductive film, catalyst support, solar cell, gas storage media, etc.
The graphene is a sheet of carbon atoms bound together by double bonds electrons (called sp2) in a thin thickness of only a movie atom [1]. Graphene atoms are arranged in a lattice pattern honeycomb in a hexagonal array. It is extracted from graphite, which is how it got its name. In fact, graphene is the structural basis of all other graphitic materials (graphite itself, fullerenes, carbon nanotubes, buckeyballs , etc.)[2].
The mechanical, electrical, optical, thermal and magnetic properties of graphene led to the creation of new researchs in nanotechnology and basic science. Recently, attempts have been made to synthesize graphene sheets by solvothermal methods using GO (Graphene Oxide) as precursor, followed by the reduction of GO in different solvents. In general, the reduction of GO is performed by thermal or chemical methods
using different gear, for example hydrazine, dimethylhydrazine, hydroquinone and NaBH4 [3] .
The solvothermal method has been widely used in the synthesis of nanomaterials; it is convenient for graphene sheets elaboration, because of its unique ability for the preparation of metastable phases.
Fig.1 Main graphitic forms [4].
II. METHODES OF GRAPHENE PRODUCTION
Single-layer graphene has been at first prepared by micromechanical cleavage in which highly oriented pyrolitic graphite (HOPG) is pealed using scotch-tape and deposited onto a silicon substrate. Besides mechanical cleavage of graphite, the other important methods employed to produce graphene samples are epitaxial growth on an insulator surface (such as SiC), chemical vapor deposition (CVD) on the surfaces of single crystals of metals (e.g., Ni), arc discharge of graphite under suitable conditions, use of intercalated graphite as the starting material, preparation of appropriate colloidal suspensions in selected solvents, and reduction of graphene oxide sheets [5].
While mechanical cleavage of graphene layers from a graphite crystal has afforded the study of the properties of single-layer graphene or bilayer graphene, the method is not suitable for large scale synthesis of single-layer graphene or of few-layer
graphene (FG). Among the methods and procedures for large- scale synthesis, two categories should be distinguished:
a) hose which start with graphite or a comparable starting material not containing any oxygen functionalities (as growth on SiC surfaces, hydrogen arc discharge, CVD on metal surfaces). They allow obtaining a large-area single- layer graphene. For example single layer of graphene has been prepared by thermal decomposition of the (0001) face of a 6H-SiC wafer under ultrahigh vacuum (UHV) conditions [5].
b) Those which involve the exfoliation of graphene oxide (GO) followed by reduction. This latter methods yield sheets of reduced graphene oxide, some of which could be single layer materials. Reduced graphene oxide layers are to be considered as chemically modified graphenes since they generally contain some oxygen functions, such as OH or COOH groups.
Fig.2 Schematic illustration of the main graphene production techniques [6]
Graphene suspensions can be readily produced by dispersing graphite in surfactant–water solutions [5], or by the reduction of GO in organic solvents. This methods are considered as one of the most suitable approach due to its simplicity, reliability, suitability for large-scale production, low material cost, and versatility in chemical fonctionalization (such as N- methylpyrrolidone (NMP)[5], DMF et DMSO [7,8]). This process works because the energy required to exfoliate graphene is balanced by the solvent–graphene interaction [5].
In the liquid phase, GO can be converted to reduced graphene oxide (RGO) using chemical agents such as hydrazine, dimethyl-hydrazine, sodium borohydride, aluminum powder, zinc powder, p-phenylenediamine, vitamin C, sugar, protein, sulfuric acid [7] and glutathione [8]. The chemical reduction using hydrazine is one of the most common methods used for reducing GO [9]. The GO reduced by hydrazine has low oxygen content and high electrical conductivity, but also low stability in organic solvents [7]. Moreover, hydrazine is a corrosive and explosive chemical and is hazardous both to humans and the environment.
Thermal and ultrasonic treatment can also be used with these chemical agents [7]. Gram quantities of single-layer graphene have been prepared by employing a solvothermal procedure and subsequent sonication [5]. Graphitic oxide, obtained by the oxidation of graphite, contains a considerable amount of surface oxygen in the form of OH and COOH groups [5].
Mechanical or thermal exfoliation of graphitic oxide gives single-layer of graphene oxide (SGO). This single-layer is then reduced by hydrogen, hydrazine or other reducing agents and gives single-layer grapheme (SG). SG has been prepared on a large scale by a solution-based approach involving the dispersion of SGO in pure hydrazine. Hydrazine-based colloids are deposited on different substrates to obtain chemically modified graphene sheets with large areas (20 - 40 mm, Fig 3) [5].
Fig 3: Photographs of chemically converted graphene suspensions. a) graphite oxide paper in a glass vial and b) the graphite oxide dispersion after addition of hydrazine. Below the vials, three-dimensional computer-generated molecular models of graphene oxide (C gray, O red, H white) and the reduced grapheme are shown. Removal of -OH and -COOH groups by reduction gives the planar structure. c) SEM and d) AFM images of a chemically converted graphene sheet on Si/SiO2 substrate [5].
A detailed analysis of the thermal-expansion mechanism of graphene oxide to produce single-layer graphene sheets has been described [7]. Chemically modified graphenes have been produced in different ways. These include hydrazine reduction of the colloidal suspension of single-layer graphene oxide in DMF/water or in water. Electrostatic stabilization enables stable aqueous dispersions of the single-layer graphene sheets [5].
The production of nano-graphene sheets by a chemical reduction of GO using the glutathione (GSH) as a reducing agent gives also good results, and this in aqueous solution and under mild conditions. Several works showed that the oxygen functional groups are almost removed during the chemical reduction using GSH [8]. The distance between two carbon layers is an important parameter to provide structural information of the prepared graphene. Generally the spacing between the graphite layers is 0.335 nm, while the spacing between the corresponding GO layers is 0.75 nm [8], this indicate the presence of functional groups containing oxygen
formed during oxidation. The spacing between layers of graphene is 0.36 nm, which is significantly different from natural graphite [8]. Typical AFM image of nano graphene sheets after a coating on a freshly cleaved mica substrate (fig.5) shows that the size of the graphene sheet is about one micrometer and the average thickness of the graphene sheet has been measured at about 8 nm from the profile of AFM image height [8].
Fig4: a) The reduction by GSH of GO to form graphene. b) AFM image and cross-section height profile of typical nano graphene sheets [8].
III. EXPERIMENTAL
Hydrothermal method is a convenient procedure to convert graphite to GO sheets, and GO to graphene sheets. The temperature, the autogenous pressure and the power reduction of solvents affects the degree of reduction of GO to graphene.
The temperatures required for the same hydrothermal reduction in a non-reducing solvent (as water) is relatively low and is further reduced when reducing solvents are used.
In this experiment, we proceeded firstly to the chemical transformation of the graphite (pristine) to graphite oxide by the modified Hummers’s method. After that, an ultrasonic operation permit the exfoliation of the product to obtain stable dispersions constituted of monolayers, bilayers or few layers of nano-sheets of graphene oxide.
The mixture is filtered and washed with diluted HCl to remove some metal ions. Finally, the product is washed several times with distilled water until pH=7. The sample of the graphite oxide is obtained after drying.
As a final step, the aqueous solution containing the GO sheets is traited with GSH-glutathion. Finally, nano-graphene sheets, stably dispersed in an aqueous medium, were obtained.
FIG.5: (A) The powder of the graphite oxide.(B)Dispersion powder graphite oxide in distilled water after the ultrasonic treatment
A. Results and Discussion
Fig.6. XRD patterns of graphite powder
Fig.7 .The XRD patterns of graphite powder, graphite oxide and graphene oxide GO obtained after the reduction
Fig.8.The XRD curve of graphene B
A
Fig.6, 7 and fig.8, shows the XRD patterns of pristine graphite, GO and graphene. The distance between two layers is an important parameter to give the structural information of as-prepared graphene.
The strong peak in the XRD pattern of pristine graphite appears at 2θ value of 26.5° (Fig.6), corresponding to a d- spacing of 3,35Å similar to that reported in literature [8].
In Fig.7, the graph present graphite oxide and graphene oxide, which peaks appear at 2θ=9,48°, 8,45°, corresponding respectively to the interlayer spacing of 9.31, 10.45Å. This indicates the presence of oxygen-containing functional groups formed during oxidation. [8].
The XRD pattern of graphene powder shows a weak and broad diffraction peaks at 2θ = 22.28°, corresponding to the interlayer spacing of 3,9Å.
IV. CONCLUSION
Graphene is currently widely studied as a promising candidate for use in various applications because of its excellent and unique properties. In the present work, we used a hydrothermal approach for the synthesis of the graphene oxide followed by its reduction with GSH to produce graphene.
This method permits us to obtain graphene sheets in a relatively short time and with mild experimental conditions.
GSH used as non-toxic reducer could also play an important role as a capping agent in stabilization of graphene [8]. The DRX studies confirmed the graphene formation from a pristine graphite powder.
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