Synthesis of  Functionalized Single Graphene  Sheets  by Thermal Exfoliation of  Graphite Oxide

Introduction

Graphene is  a single hexagonally flat layer of graphite, which has attracted great interest both for fundamental understanding of its unique structural and electronic properties and for important potential applications in nano-electronics and devices  [1-5]. The unique properties of this two-dimensional (2D) material include the highest intrinsic carrier mobility at room temperature of all known materials and very high mechanical strength and thermal stability [6-9]. Graphene holds great promise for the development of new composite materials, emissive displays, ultrasensitive detectors and micromechanical resonators [10-12]. The combination of high mobility, thermal, chemical and mechanical stability with the high surface area offers many interesting applications in a wide range of fields including heterogeneous catalysis where metallic and bimetallic nano-particle catalysts can be efficiently dispersed on the graphene sheets [13-15]. In many cases, the remarkable properties of single graphene layers extend to bilayers and a few layers of graphene sheets. Several methods have been reported for the production of graphene sheets including micromechanical cleavage and thermal expansion of graphite.

Experimental

Materials

Physical parameters of Graphite 99.95%  , Potassium Chlorite, Sulfuric acid   H₂SO₄   and Nitric acid    ( HNO3) ,99.9 % are reported in table 1, 2 ,3 and 4 respectively.

Table 1.  General Characteristics of   Graphite 99.95%  .

Trade Name	Graphite 99.95 %
Appearance	Black
Size	325 mesh
Company	Aladdin Industrial Corporation Shanghai , China

 

Table 2.  General characteristics of   Potassium Chlorate.

Trade Name	KClO3
Appearance	White
Molecular weight	122.55
Company	China

 

Table 3.  General Characteristics of   Sulfuric Acid   H₂SO₄.

Molecular  formula	Sulfuric Acid   ( H2SO4) ,99.9 %
Appearance	liquid
Molecular weight	98.08
Concentration	95 – 98 %
Company	Sinopharm chemical reagent Co ,Ltd ,China

 

Table 4.  General Characteristics of   Nitric Acid    ( HNO₃) ,99.9 % .

Molecular  formula	Nitric Acid    ( HNO3) ,99.9 %
Appearance	liquid
Molecular weight	63.01
Concentration	65 – 68  %
Company	Sinopharm chemical reagent Co ,Ltd ,China

 

Fabrication of Graphene Sheets

Commercial powdered natural graphite ( from Aladdin Industrial Corporation Shanghai , China ) was used as our starting material. The commercial graphite has a particle size of 325 mesh with a purity of 99.99% . Graphite was oxidized following  modified  Method of  Staudenmaier method to form graphite oxide (GO). In this method, graphite (2.5g) was first mixed with sulfuric acid (43.75 mL) and nitric acid (22.5 mL) and stirred. When graphite was uniformly dispersed, potassium chlorate (27.5 g) was added slowly and stirred for over 96 h. After the completion of oxidation  reaction, the mixture was added into  4 L  deionized water and then filtered. The GO was repeatedly rinsed and redispersed  in a 5% solution of  HCl. It was then washed continually with deionized water until the pH of the filtrate was neutral [16-32]. Potassium stayed in the GO even after several times of washing .Therefore multiple washing cycles used in conjunction with bath ultrasonication in fresh ethanol were used  for the removal of residual potassium . After this the GO was dried in a vacuum oven at 60 ⁰C until used. Finally, the GO was  treated with the nitrogen ( N₂ ) and then it was heated to 1050 ⁰C  in the furnace for 30 s to form graphene sheets [16-31].

Photo1: Graphite in water   Click here to View photo

 

Photo2: GO green before dry    GO.dry in 60 0C  water             Click here to View photo

 

Photo3: Graphene Sheet in water   Click here to View figure

 

Treatment Silicon Wafer

silicon wafer  were cut into ( 3mm x 3mm )  used Piranha solution is mixture consisting of sulfuric acid ( H₂SO₄ ) and hydrogen peroxide ( H₂O₂ ) is to cleaning silicon wafer . It is typically mixed in concentration ratios of around 3:1 H₂SO₄:H₂O₂ for one hour with temperature  60 ⁰C . Then followed  by triple rinsing in ethanol with ultrasonic cleaning for  30 min then with nitrogen ( N₂ )  .There are two main applications for piranha in wafer fabrication: it is used to remove organic contaminants from surface of the wafer during cleaning . Silicon wafer  were used for  SEM [ 32].

Results and Discussion

The GO was reduced to graphene sheet  by heating to 1150 ⁰C. Plate 1 ,2( SEM ) and plate  7 ( TEM )  shows the top-view TEM images of the

graphite oxide plate  ( SEM ) 3 , 4 and plate  8 ( TEM )  . TEM images of the original graphite plate  ( SEM ) 5 ,6 and plate  9 ( TEM ) .  A small flake of graphite particle is seen in  .The size of the graphene sheet is about 9.35µm from SEM photo. The surface of graphene shows several large meanderingwrinkles. The thickness of graphene can be  clear in  high-magnification TEM image .

Plate1: SEM of Graphene sheets dispersed in water Click here to View plate

 

Plate2: SEM of Graphene sheets dispersed  in ethanolClick here to View plate

 

Plate3: SEM of   Graphene Oxide ( Dry ) dispersed  in waterClick here to View plate

 

Plate4: SEM of Graphene Oxide ( Dry ) dispersed  in ethanolClick here to View plate

 

Plate5: SEM of Graphite dispersed  in water Click here to View plate

 

Plate6: SEM of Graphite dispersed in ethanolClick here to View plate

 

Plate7: TEM of Graphene sheets Click here to View plate

 

Plate8: TEM of Graphene Oxide Click here to View plate

 

Plate9: TEM of GraphiteClick here to View plate

 

Conclusions

single sheets functionalized  graphene  through thermal exfoliation of graphite oxide . Main application of graphene sheets in electrical material. The surface of graphene sheets shows several large meandering wrinkles. The thickness of graphene can be determined from the high-magnification TEM image.

Acknowledgements

I  acknowledge the financial support from UNESCO/Government of China ( Great wall ) and Al-baida’a ,University , Yemen.

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