Thermally Reduced Graphene Oxide: Synthesis, Structural and Electrical Properties

Citation: Mokhtar MM, Abo El Enein SA, Hassaan MY, Morsy MS, Khalil MH (2017) Thermally Reduced Graphene Oxide: Synthesis, Structural and Electrical Properties. Int J Nanoparticles Nanotech 3:008 Copyright: © 2017 Mokhtar MM, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. *Corresponding author: MM Mokhtar, Housing and Building National Research Center (HBRC), Building Physics Institute, Cairo, Egypt, E-mail: mmahmoud7@gmail.com Mokhtar et al. Int J Nanoparticles Nanotech 2017, 3:008

Graphene can be used as filler material for polymer composites [7], it can be considered as a good electrode for super capacitors because of its high surface area, high electrical conductivity, and good electrochemical stability [8]. Also graphene and graphene oxide are used in cement composite pastes to enhance their mechanical and micro structural properties hence it was found that the addition of 0.03 wt% of GO can improve the mechanical strengths of cement composites by about 40% [9], while the incorporation of 0.05 wt% of GO into cement mortar enhanced the flexural strength by 70.5% as well as the compressive strength was increased by 24.4% [10]. The incorporation of Graphene NanoPlatelet (GNP) by about 2.4 vol% increases the tensile strength of GNP-cement mortar [11]. The in-situ Reduced Graphene Oxide (rGO) (i.e. 2D graphene sheets) were added to fly ash geo polymeric composite cement to enhance its mechanical properties, where the addition of 0.35 wt% rGO yielded an increase in the flexural strength by 134% [12].
Graphene has the property of piezo resistivity because of its high electrical conductivity and surface area and it was expected to transfer such property to cement composites [11,13]. The electrical resistivity of GNP-cement composites was markedly decreased at GNP concentration of 7.5% [14]. The electrical conductivity of Polystyrene (PS) was enhanced by the addition of Reduced Graphene Oxide (rGO) by about 20 vol% of rGO where it was reached 135 S/m, while the thermal conductivity of the composites was increased by about 90% as the dosage of rGO increased from 0 to 10 vol% [15].
Graphene based materials can also be used in advanced energy-conversion devices such as solar and fuel cells, and energy-storage devices, including super capacitors and lithium-ion batteries [16].
Zhong-li Wang, et al. followed the improved hummer's method to prepare Graphene Oxide (GO) via thermal decomposition technique. In order to prepare graphene, the solid GO sheets were decomposed by heating at 300 °C for 1 hour at a rate of 2 °C/min [19].
While Q. Zheng and J.K. Kim used the mechanical cleavage method to exfoliate graphite flakes into graphene using a scotch tape. Graphene can be easily peeled off from graphite by a scotch tape because of the very weak interlayer van der Waals force in graphite, this method involves transferring of graphene to a SiO 2 /Si substrate and optically identifies it, and the yield of this technique is very low because it is a manual operation [20]. Nils Krane simply heated and cooled down a SiC crystal, leading to the formation of a single or bi-layer graphene on the Si face of the crystal, while few-layer graphene has formed on the C face [21].
It was reported that the chemical Vapor Deposition technique is based on the carbon saturation of a transition metal like [Ni, Pd, Ru, Ir and Cu] on exposure to a hydrocarbon gas like methane (CH 4 ), ethylene (C 2 H 4 ), acetylene (C 2 H 2 ) and benzene (C 6 H 6 ), the number of the obtained graphene layers depends on the hydrocarbon type and the reaction conditions [23][24][25][26].
It was also stated that Graphene can be prepared through arc discharge technique, in this method two to three layers Graphene sheets have prepared in H 2 atmosphere. The obtained sheets having sizes of 100-200 nm [27,28].
In this study GO was prepared via improved hummers method. We aimed to shed light on the influence of thermal reduction time on the physical properties of graphene.

Preparation of Graphene Oxide (GO)
GO was synthesized following the improved Hummer's technique [17]. Briefly, graphite (1 g) was added with NaNO 3 (0.5 g) in a flask, 50 ml of H 2 SO 4 (98%) was then added to the flask under constant stirring at 5 °C for 1 hour, after that 3 g of KMnO 4 were gradually added (1 g every 15 min). Dilute the solution by slowly adding 100 ml warm distilled water. In this step do not allow the temperature of the solution to increase over about 20 °C in order to prevent overheating and explosion. Then an aqueous solution of [H 2 O 2 (3 ml) + distilled water (100 ml)] was added to the mixture in the flask to be sure that all KMnO 4 is completely reacted. After that the mixture was successively washed with HCL and water, and then filtrate the mixture and dry to get the final powder (GO), as described in our previous work [29].

Reduction of GO
Finally, the prepared GO was divided into 4 samples, each division were reduced at a fixed temperature of 350 °C for different times; 1, 3, 6 and 9 hours in air with a heating rate of 10 °C/min, the resulted graphene samples were assigned as R1, R3, R6 and R9 respectively. By this step we obtained graphene sheets that are ready for characterization.
Where R: refers to reduction while the numbers 1, 3, 6 and 9 refers to the reduction time at 350 °C.

High-Resolution Transmission Electron Microscope (HR-TEM)
The samples morphology was examined using a High-Synthesis, Structural and Electrical Properties. Int J Nanoparticles Nanotech 3:008 ISSN: 2631-5084 |

Electrical conductivity
• The I-V characteristic curves of R1, R3, R6 and R9 samples were obtained through the following steps: • A sensor device consisted of gold Inter Digitated Microelectrodes (IDE) with spacing of 5 μm and 13 pairs of arms sputtered on ceramic substrate were prepared.
• 20 mg of each graphene sample was suspended in 1 ml of acetone with ultrasonic process (power of 250 watt and frequency of 20 KHz, for 5 min).
• 20 µl of the prepared suspension was poured on the surface of IDE using micropipette, after few minutes acetone was volatilized and a layer of graphene settled on the sensor. As shown in Figure 1.
The I-V curves were plotted and the slopes were calculated, hence the slope of each curve represents the conductance (in Siemens) of its sample which can be calculated through the following equation: Where; C is the Electric Conductance (S), R is the Electric Resistance (Ω), I is the Electric Current through the object (A) and V is the Voltage across the object (V).

X-Ray Diffraction (XRD)
The XRD analysis was performed using X-ray diffractometer (Philips PW3050/60) with a Cu-Kα X-ray source.

Thermal analysis
Thermogravimetric analyses of the samples were conducted using a DSC-TGA thermal analyzer (STD-Q600, USA) instrument from 25 °C up to 800 °C at a heating rate of 10 °C/min in nitrogen atmosphere.

Fourier Transform Infrared Spectrometry (FTIR)
Bonding characteristics of the samples were analyzed using Jasco-6100 Fourier transformed infrared spectrometer (FTIR; Varian model Excalibur FTS 3000MX, Paolo Alto, CA). The wave number was ranging from 400 to 4000 cm -1 .

Raman spectroscopy
Raman spectroscopy (SENTERRA-Bruker, Germany) with laser source (Nd:YAG) at wavelength of 532 nm was also used to analyze the samples.

Specific surface area
The specific surface area of the samples were calculated by using Brunauer-Emmett-Teller (BET) method and the Nitrogen adsorption/desorption isotherms were measured for GO and graphene samples at 77.35 K on a NOVA 2000e (Quanta chrome, USA).

Results and Discussion
HR-TEM Figure 2 shows TEM images taken for GO reduced graphene. The obtained material was examined by HR-TEM to confirm the formation of GO and reduced graphene to investigate their morphology. The HR-TEM images presented in Figure 2A which represents GO, displayed the formation of transparent particles consisting of multi-layer stacked/non-exfoliated thin sheets. While TEM images ( Figure 2B, Figure 2C, Figure 2D and Figure 2E) that represents GO reduced for different times displayed the formation of graphene platelet from few layers to a single layer ( Figure 2E). It can also be clearly seen that the reduced graphene samples show a crippled and wrinkled structure; this structure inhibits the restacking of the graphene sheets together.
We also can conclude the effect of reduction time on the morphology and structure of graphene, hence, GO was well exfoliated and reduced to graphene, as the reduction time increased to 9 hours we obtained almost a single layer and ruptured structure of graphene sheet. The wrinkle appearance of Graphene sheets was explained by the high aspect ratio (i.e.) graphene sheet has several hundred nanometers of length compared to just several nanometers of thickness [31]. Figure 3 Shows the XRD patterns of graphite, Graphene Oxide (GO), and reduced graphene oxide (R1, R3, R6 & R9). From the pattern, the peak characterizing graphite was located at 2θ = 26.4 with a d-spacing value of 3.37 Å which is in well agreement with the graphitic nature of carbon, upon chemical oxidation GO was synthesized and its position was shifted to 2θ = 9.7 which means that graphite was completely oxidized to GO with d-spacing value of 9.08 Å, the increased d-spacing of GO over that of graphite can be attributed to the effect of the introduced oxygen function groups on the edge of each layer. After thermal reduction of GO; the XRD display a peak at 2θ = 25.3 for graphene (R) samples which confirms that GO was highly reduced to graphene with the absence of graphene oxide peak, also the interlayer spacing of graphene was 3.51 Å that was very close to that of graphite [32,33]. Also it can be observed that, as the reduction time increased the peaks gets sharper and hence closer to the graphite nature.

Thermo Gravimetric Analysis (TGA)
TGA was used to estimate the thermal stability of our prepared samples. TGA curves of GO and different reduced graphene oxide samples at different times R1, R3, R6 and R9 are presented in Figure 4.
TGA curves showed that the total weight loss of GO was about 56.01%. The loss of weight up to 100 °C can be attributed to the evaporation of the moisture content and interstitial water, while the loss from 100-250 °C originated from the decomposition of the acidic func-tional groups and hydroxyl groups, hence GO was highly oxidized during the preparation procedure that involved a strong acid treatment and usage of H 2 O 2 as an oxidizing agent [34], finally the loss up to 800 °C resulted from the decomposition of carbonyl groups that was formed on the surface of GO during the oxidation process [35].
Regarding the reduced graphene oxide samples, firstly the TGA curve for R1 sample has the same behavior like the graphene oxide reduced by microwave [8] with a total mass loss of about (44.11%). While, the total weight loss of R3, R6 and R9 reached about 79.74, 83.12 and 78.22% respectively. The loss up to 100 °C refers to the loss of adsorbed water, while the loss up to 400 °C arose from the decomposition of the residual hydroxyl and carboxyl function groups and the loss up to 800 °C can be related to the loss of the remained carbonyl groups [36].
The curve of R1 shows the highest thermal stability among GO and other R samples. The effect of increasing reduction time appeared in increasing the amorphous carbon species which confirmed by Raman analysis and HR-TEM images. Figure 5 presents the FTIR spectra of GO and thermally reduced GO, respectively. When graphite was oxidized many oxygen functional groups were introduced in the structure of GO which has a broad peak at about 3421 cm -1 represents stretching vibrations of (-OH) that resulted from adsorbed water, as well as a peak approximately at 1735 cm -1 due to (C = O) stretching of carboxyl and carbonyl groups, also the peak at 1621 cm -1 referred to vibrations of (C = C) groups, also a peak at about 1378 cm -1 was appeared which is referred to O-H deformation of C-OH group. Finally, the peak appeared approximately at 1081 cm -1 represents the vibration of (C-O) bond [37].  After thermal reduction of GO and with increasing the reduction time, most of the absorption peaks are weakened or disappeared, hence the spectra of R1 and R3 samples showed small peaks for (-OH) between 3450 and 3403 cm -1 due to deoxygenation as a result of reduction process, while for R6 and R9 the peaks for (-OH) was disappeared. However stretching vibrations from (C = O) still present in R1 and R3 spectra that referred to the presence of carbonyl groups and disappeared in R6 and R9 spectra. Finally, the increased of reduction time led to the appearance of new modes specially for R6 at 550 and 1800 cm -1 that can be attributed to the C-H bending mode and aromatic C = C stretching respectively [38].

Raman spectroscopy
Raman spectra of carbon nanomaterials exhibit two main characteristic bands appear between 1200 and 1700 cm -1 . In this study, the first band appeared at about 1583.5, 1602.5, 1591, 1591, 1584 and 1577.5 cm -1 for graphite, GO, R1, R3, R6 and R9 respectively, corresponds to the graphite mode (G band) which reflects the sp 2 carbon atoms, while the second band appeared approximately at 1354.5, 1360.5, 1365.5, 1357, 1355 and 1349 cm -1 for graphite, GO, R1, R3, R6 and R9, respectively, attributes to diamondoid mode (D band) is related to the disorder that may arise from certain defects in the structure and amorphous carbon species [39,40].
Upon chemical oxidation of graphite to GO, it was observed that G band of GO goes broaden more than that of graphite and the intensity of D band of GO was increased over than D band of graphite, this can be explained by the decrease in size of the sp 2 carbon in crystalline domain by the creation of defects, vacancies, and distortions during oxidation [40,41].
The intensity ratio of G to D bands gives an indication to the quality of graphitization. From Figure 6 it can be noticed that the band intensity ratio (I D /I G ) increased from 0.18 for graphite to be 1.08 for GO, which could be attributed to the increase of disordered structure resulted from oxygen functional groups produced during oxidation process of graphite.
It can be also noticed that the band intensity ratios (I D /I G ) for R1, R3 and R6 were 0.92, 0.96 and 0.99 respectively, which is slightly lower than 1.08 for GO and caused by the repair of defects by recovery of the aromatic structure of graphite lattice [8,32]. Finally, the spectrum of R9 shows a dramatically increase in (I D /I G ) ratio to reach 1.34 and D band was greatly broadened that reflects the increase of graphitization with a large quantity of structural defects which confirmed by TEM analysis ( Figure 2E).

Specific surface area
The N 2 adsorption/desorption isotherms for GO and thermally reduced GO (R1, R3, R6 and R9) are presented in Figure 7, While, Figure 8 shows the specific surface area of GO and graphene obtained from these isotherms.  It is clear that the increased reduction time significantly increased the surface area of the prepared graphene samples, this in turn reflects on the reactivity of the samples, but a single layer graphene still has much higher surface area (2630 m 2 /g) [42], this means that our samples composed of few graphene layers. Figure 9 show the I-V characteristic curves obtained for R1, R3, R6 and R9 respectively, while Figure  10 shows the electric conductance of graphene samples obtained as described in section (3.7). Conductivity of GO depends on the extent of oxidation. While oxidation, we were continuously removing the sp 2 carbons and replacing it by sp 3 carbons having oxygen functionalities. In essence, you are creating a band gap by pulling the bands apart. Hence when oxidation was completed, the bands are far apart and GO behaves as an insulator. It is clear that the conductivity of graphene increased as reduction time was increased up to 3 hours then, a gradual decrease in conductivity was observed for R6 and R9, where, the enhancement reached about 152.72, 62 and 20.39 for R3, R6 and R9 with respect to R1, respectively.