New Route to Synthesize of Graphene Nano Sheets

Introduction

Graphene is a two-dimensional carbon with one layer. Graphene possesses superior properties including unique electronic properties due to its high electron mobility (20 × 103 cm² v^-1 s^-1),¹ mechanical resistance.² Super-thermal properties (~ 5000 Wm^-1K^-1),³ large surface area (2630 m² g^-1),⁴ so it can be used in fuel cells and capacitors.

Graphene production may be carried out by various methods: (i) mechanical methods, via adhesion tape or chemical exfoliation methods¹ embedded in graphite to collect carbon powder samples, and gradually released to produce graphene sheets. However, this method has a weakness in the aspect of its irreproducibility (it cannot be produced further), (ii) epitaxial growth method in silicon carbide that is heating silicon carbide at high temperature (> 1.100ºC). This process produces a graphene epitaxial with dimensions depending on the size of the SiC substrate and the resulting small sample which does not allow it to be used in electronic application fabrication techniques, (iii) epitaxial growth methods on the metal substrate. This method has the disadvantages of not producing a sheet of graphene with a uniform thickness, and the bond between the bottom graphene sheet and the substrate can affect the properties of the carbon layer. The cost required for this graphene synthesis is also relatively expensive, both adhesion tape method and epitaxial growth method.

Currently, the production of graphene is mostly done by chemical method by oxidizing graphite to graphene oxide and then reducing with hydrazine to produce graphene. This method is very simple and can produce large scale production of graphene.⁵ The disadvantage of this method is the graphene is not single layer and also hydrazine is very toxic (carcinogenic agent).

The reduction of graphene oxide to be graphene is very important in order to produce graphene. Reduction of graphene oxide by non-hydrazine is needed be developed. This is caused, hydrazine is carcinogenic. In this study, we used ammonia as a reducing agent to reduce graphene oxide to be grapheme nano sheets. That is possible due to ammonia is an environmentally friendly reducer in a small concentration range. In this research, the ammonia is expected to interact with the carbonyl, hydroxyl, and epoxide groups of graphene oxide. Therefore, we reported the simple and inexpensive method to produce graphene by oxidation of graphite tobe a graphene oxide and then it was reduced to generate graphene.

Materials and Methods

Synthesis of Graphene Nano Sheets

0.2 g of graphite powder was inserted into 250 mL erlenmeyer, then it was added 0.2 g NaNO₃ and 15 mL H₂SO₄ 96%, respectively and stirred for 2 hours. After that, it was gradually added 1 g of KMnO₄ and stirred for 24 hours to generate graphite solution. Subsequently, the solution was added 20 mL of H₂SO₄ 5% and 1 mL of 30% H₂O₂, respectively and stirred for one hour. Then, the solution was centrifuged at a speed of 3000 Rotation per Minute (RPM) for 20 minutes to separate the filtrate and supernatant. The filtrate was added 10 mL ammonia 10 M and stirred for 48 hours and separate filtrate and precipitate, Finally, precipitate was dried at 80^oC for 24 hours and characterized by using XRD and SEM-EDX, respectively.

Results and Discussion

Analysis of XRD

The XRD pattern of graphite, graphene oxide and graphene nano sheets can be shown in Figure 1. The graphite (Fig. 1) shows a sharp and tight peak (2θ = 26.5° and 23.88°) which corresponds to the diffraction line C (002) with the intercellular spacing in the crystal (d) respectively is 3.36 and 3.72 Å. The data shows the typical crystal structure of graphite. The graphene oxide (Fig. 1) shows a wide diffraction peak that is at 2θ = 11.6° with the spacing between plane (d) is 7.5 Å. Increasing the distance between fields in the graphene oxide  is due to the presence of oxygen-functional groups and water molecules into the carbon layer structure. These are typical vertical oxides of graphene/graphite oxide.⁵ The small peaks are still observed at 2θ = 20.1°; 23.9° and 26.4° which indicates that graphene oxide is not fully interconnected with oxygen atoms.^6,7 The Figure 1 shows that the chemical reduction of graphene oxide to graphene nano sheets using NH₃, returns the structure of the crystal graphene nano sheets to be a more regularly. However, NH₃ has not been able to create a single layered graphene nano sheets structure.

Figure 1: XRD patterns of Graphite, Graphene Oxide and Graphene Nano Sheets. Click here to View figure

 

The resulting graphene nano sheets has a structure between the crystalline and amorphous structures. This is evidenced by the reappearance of the diffraction line C (002) which looks wider and the intensity is lower than the peak obtained in the graphite powder that is at 2θ = 26.5° with the distance between the fields is 3.35 Å and the diffraction peak eradication at 2θ = 11° which looks more flat than in graphene oxide.^6,7 The reduction process must involve the removal of the oxygen functional group because the graphene nano sheets conjugate structure of the sp² carbon bond must be reform during the reduction process, in which case the role is the opening of the epoxide ring.

Analysis of SEM-EDX

The microstructure and energy spectrum analysis data on the intensity of graphite, graphene oxide and graphene nano sheets are shown in Figures 2 and 3, respectively.

Figure 2: SEM images of Graphite, Graphene Oxide and Graphene Nano Sheets with 1000× (a), 2500× (b) and 5000× magnification (c) Click here to View figure

 

The SEM data show that graphite has a wide and thick particle size indicated at 1000× magnification (Fig. 2a). The distance between the graphite particles looks wider with a far position. The result of SEM graphite at 2500× magnification (Fig. 2b) and 5000× (Fig. 2c), shows the existence of many stacks in the graphite structure indicating that graphite has a layered structure. The graphene oxide has smaller particle size and the formed aggregates are closely related to each other than graphite which can be seen at 1000× magnification (Fig. 2a). The images of graphene oxide at 2500× magnification (Fig. 2b) and 5000× (Fig. 2c) indicate the existence of fewer stacks indicating that there has been exfoliation of the structure on graphite. The graphene nano sheets has a ripple surface, a smaller and uniform size of particles and pore size, has a randomly arranged aggregate with a thin layer and is tightly linked to one another to form an irregular solid (magnification 1000×) compare to graphite and graphene oxide. However, the graphene nano sheets structure obtained has not been single layered. It is marked on the SEM graphene nano sheets results with 2500× magnification (Fig. 2b) and 5000× (Fig. 2c) which indicates a smoother buildup and is visibly reduced by the process of exfoliation of the structure. Graphene morphological differences with graphene oxide and graphite show that almost all oxygen groups in graphene oxide have been reduced with NH₃. Furthermore, the EDX spectra of graphite, graphene oxide and graphene nano sheets are shown in Figure 3 – 5, respectively.

Figure 3. EDX Spectra of Graphite. Click here to View figure

 

EDX spectra data of graphite, graphene oxide and graphene nano sheets were measured at a voltage of 15 kV. The spectrum is a characteristic X-ray captured by a Si detector. The graphite EDX spectra data (Fig. 3) show that the carbon atoms (C) and oxygen atoms have the energy quantities captured by each detector at 0.15 and 0.25 keV. The large atomic intensity of C (Table 1) is due to the graphite structure consisting of the bonds of the carbon atoms. However, the graphite is not pure graphite which is only suspended over C atoms, since it still finds other elements such as Al, Si and. In addition, K, Al, Si and K appear probably due to the chamber effect of SEM-EDX. It is not due to the content of graphite. Furthermore, graphite was oxidized, centrifused and ultrasonicated to produce graphene oxide. The EDX spectra of graphene oxide are shown in Figure 4.

Figure 4: EDX Spectra of Graphene Oxide. Click here to View figure

 

The graphic EDX spectra of graphene oxide (Fig. 4) and the ratio of the number of elements (Table 1) show that the intensity of the C atom captured by the detector at 0.15 keV decreases and the oxygen (O) atom increases at 0.25 keV. Atom Al, Si and K are not detected in the graphite oxide structure. The loss of Al, Si and K atoms is caused by the addition of strong acid (H₂SO₄) into the graphite structure. The increase in O atoms is due to the oxidation process of graphite by strong oxidizers (KMnO₄, NaNO₃ and H₂O₂). Finally, graphene oxide was reduced by ammonia to generate graphene nano sheets. The EDX spectra of graphene nano sheets was shown in Figure 5.

Figure 5: EDX Spectra of Graphene Nano Sheets. Click here to View figure

 

Comparison of the number of elements of graphite, graphene oxide and graphene nano sheets can be shown in Table 1.

Table 1: Comparison of elements of Graphite, Graphene Oxide and Graphene Nano Sheets (EDX Data)

Elements	Graphite	Graphene Oxide	Graphene Nano Sheets
Carbon (C)	88.374	88.114	97.723
Oxygen (O)	9.200	11.886	2.277
Aluminum (Al)	1.032
Silicon (Si)	1.206
Potassium (K)	0.187

 

The graphene nano sheets EDX spectra data (Fig. 5) show that the intensity of the O atom captured by the detector at 0.25 keV decreases. This is supported by the comparative data of the number of elements (Table 1) showing that the grapheme nano sheets is mostly composed of atoms C and the percentage of the O at the graphene decreases. The addition of a reducing NH₃ reduces the oxygen-functional groups that make up the graphne oxide structure. The oxygen atoms still detected in the grapheme nano sheets structure indicate that the reduction process of graphene oxide to grapheme nano sheets using ammonia is not yet perfect because there are other oxygen groups in the graphene structure. That is called graphene nano sheets.

Conclusion

Graphene nano sheets can be synthesized from graphite using ammonia reduction as well as a benign approaching. The graphene XRD pattern shows single-layered graphene due to the emergence of the diffraction peak at C (002). Characterization of SEM-EDX show that graphene has size and shape of small, thin and aggregate where graphene structures are closely related to each other and the reduced percentage of oxygen elements in graphene. The mechanism reaction of reduction graphene oxide used NH₃ as a reductor agent occurring in the epoxy group. NH₃ does not play a role in the de-hydroxylation, de-carbonylation and de-carboxylation processes due to the increased resistance of Gibbs Energy to NH₃ if the temperature is increased above room temperature.

Acknowledgement

Authors would like to thankful to Lembaga Penelitian Universitas Sumatera Utara who supported funding of our research by the research grant: Penelitian TALENTA, Universitas Sumatera Utara, 2017, No. 5338/UN5.1.R/PPM/2017, Tanggal 22 Mei 2017.

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This work is licensed under a Creative Commons Attribution 4.0 International License.