Introduction
Graphene oxide (GO) is a 2D carbon (graphene) nanomaterial that has been known for more than 150 years [1], [2]. GO is commonly known as a chemical compound of carbon (non-stoichiometric), hydrogen and oxygen in a variable ratio that is highly dependent on the manufacturing process [3]. GO has a hexagonal carbon structure similar to graphene with contents such as alkoxy hydroxyl groups (C–O–C), (–OH), carbonyl (C=O), carboxylic acid (–COOH) and other functional groups [4].
The interlayer spacing in GO widens in the presence of oxygen-containing groups [5]. The functional groups present in GO make it hydrophilic, promoting maximum exfoliation and high homogeneous dispersion of GO in a polar polymer matrix and significantly increasing GO interfacial bonds [6]. The highly oxidized form of GO is an electrical insulator with a band gap of approximately 2.2 eV [7].
GO is an attractive material for development because of its low cost, easy access, and broad ability to convert to graphene [8]. GO has physical properties such as a large specific surface area, good mechanical flexibility, high thermal stability and good electrical conductivity [9]. The physical properties of GO can make it a highly sought-after material in various fields of application such as electronics, energy, biomedicine, and environmental applications [10]. Examples include desalination [11], drug delivery systems [12], oil-water separation [13,14], catalysis [15], solar cells [16] and energy storage [17].
In the manufacture of GO, a classic chemical oxidation method is carried out by Brodie, Staudenmaier, and Hummers [18]. These processes, especially the Hummer and modified Hummer methods, use graphite as a raw material in an oxidation reaction with a strong oxidant; the graphite is then exfoliated (sonicated) to produce GO [19]. The Hummer method has been used as a well-known method for making GO which was developed by Hummers and Offeman in 1958. This is because this method has advantages such as shorter processing time, improved safety, and lower toxicity compared to previously used methods [20]. Twenty years later, the Hummer method was modified to minimize the formation of toxic gases originating from NaNO3, such as NO2 and N2O4, by removing NaNO3 [21]. However, in the manufacture of GO there are often difficulties and failures in the GO synthesis process using the Modified Hummer method, namely, the incomplete oxidation of graphite. Therefore, this study carries out GO synthesis with modifications to the Modified Hummer method.
Materials and Methods
GO was prepared using commercial graphite measuring 5 microns using the Modified Hummer method. An illustration of the process of making GO can be seen in Figure 1.
A total of 115 ml of concentrated H2SO4 was cooled in an ice bath (T = 0 - 5oC) for 5 minutes. 2.5 g of NaNO3 was dissolved in H2SO4 solvent, 5 grams of graphite were slowly added, and the mixture was stirred using a stirring rod for 30 minutes (T = 0 - 5oC). Then 15 grams of KMnO4 were slowly added over 2 hours (T = 0 - 5oC). The mixture was stirred again for 2 hours, during which the colour of the solution changed to blackish green. The mixture was removed from the ice bath, the hot plate temperature was set to 45oC, and the solution was stirred using a magnetic stirrer for 1 hour. After that, 230 ml of distilled water was slowly added dropwise into the solution, and the temperature was maintained at 45oC until the solution turned into a thick muddy brown, and stirring was continued for 30 minutes. Then 600 ml of distilled water (90oC) was added to the solution, followed by 50 ml of 30% H2O2 added dropwise, after which the solution turned yellow. The solution was allowed to settle and was left overnight. The precipitate was collected and washed using HCl, ethanol, and distilled water with a centrifuge (v = 3500 rpm) until the pH was close to neutral. The GO precipitate that had been washed was then dried at 80oC for 24 hours, and the dried GO formed a thin sheet. The GO thin sheets were blended to obtain GO powder flakes, which were ready for characterization. The same characterization was also carried out on commercial graphite powder to compare its properties with GO. The characterizations used in this study were Scanning Electron Microscope-Energy Dispersive X-Ray (SEM-EDX; PhenomProX Desktop), Raman spectroscopy (Raman; iHR320 HORIBA) and Ultraviolet and Visible Spectrophotometry (UV-Vis; Shimadzu 1800). In addition, the formation of graphite into GO can be seen in Figure 2.
Results and Discussion
Scanning Electron Microscope - Energy Dispersive X-Ray (SEM-EDX)
The morphology of graphite and GO is shown in Figure 3.
Figure 3 shows the morphology of the graphite and GO samples at a magnification of 500 times. Figure 3a shows the surface of the graphite material in rough and irregular flakes, while Figure 3b shows the surface of GO, which appears to have layers that make the consistency thicker. In addition, the thickness of the GO surface may be attributed to the presence of oxygen functional groups bound to the GO plane. This clearly indicates that the graphite material has been exfoliated during the oxidation process [22]. Furthermore, the elements contained in the sample can be identified through the EDX spectrum measured at a voltage of 20 kV in Figure 4. The results of the EDX spectrum show the percentage of each component contained in the sample. Figure 4a shows the elements in the graphite sample, where carbon (C) and oxygen (O) have weight percentages of 84.03% and 14.83%, respectively. The high value of the weight percentage of element C indicates that C is the main constituent of the graphite structure, which consists of carbon-carbon bonds. However, besides the presence of element C in graphite, there are other elements such as Mg, Al, Si, K and Fe with percentage values of 0.06%, 0.29%, 0.56%, 0.05%, and 0.18%, respectively. The appearance of these elements indicates that the graphite used is impure; the Al, Si, and K signals may also arise from stray peaks in the Si detector, which captures X-rays to produce a spectrum [23].
Meanwhile, Figure 4b shows elements in the GO sample. The highest element percentages are found in C and O, with values of 53.09% and 42.13%, respectively. The C content decreased, while the O content increased. This is because graphite is oxidized by potent oxidizing agents such as KMnO4, NaNO3 and H2O2, increasing the oxygen content. In addition, other elements, namely S and Cl, have percentage values of 1.60% and 0.18%, respectively. The S element is likely due to the addition of H2SO4 during the synthesis process, while the Cl element likely originates from the HCl washing process, with residual content remaining in GO.
| Sample | Composition (%wt) | ||||||||
| C | O | Mg | Al | Si | S | Cl | K | Fe | |
| Graphite | 84.03 | 14.83 | 0.06 | 0.29 | 0.56 | - | - | 0.05 | 0.18 |
| GO | 53.09 | 42.13 | 0.08 | 0.88 | 1.66 | 1.60 | 0.18 | 0.23 | 0.16 |
Fourier Transform Infrared Spectroscopy (FTIR)
Based on the FTIR results, the functional groups in graphite and GO samples have been shown in Figure 5. Figure 5 shows the infrared spectra of graphite and GO, with peaks indicating various functional groups, in the range of 500-4000 cm-1. In addition, Table 2 lists the chemical functional groups identified in graphite and GO. The graphite sample displays a broad peak at 3220.25 cm-1 corresponding to O-H (hydroxyl) stretching vibrations. The breadth of the O-H peak indicates the presence of strong hydrogen bonds, such as adsorbed water [24]. In addition, the width of the O-H spectra shows that this graphite contains heavy atoms, where the more comprehensive the spectral peaks, the heavier the atoms and the narrower the spectral peaks, the lighter the atoms. In addition, graphite also shows spectral peaks at the wavenumbers 1718.16 cm-1, 1620.67 cm-1, 1393.36 cm-1 and 1038.87 cm-1, with each showing functional groups in the form of C=O (carbonyl), C=C (aromatic), C-H (alkane) and C-O (epoxide).
The GO sample displays spectra similar to those of graphite. The spectra share many common features because the elements present in the two samples are nearly identical, although GO contains additional S and Cl impurities that are not detected by FTIR. Some GO peaks appear at lower wavenumbers than in graphite, such as O-H groups at the wavenumber 3193.50 cm-1. The O-H group in GO has a broader peak than graphite, and this is attributed to the higher water content originating from the distilled water used in the GO washing process. After that, at the wavenumber 1718.48 cm-1 there is a stretching vibration of the C=O group located in the GO layer. At the wavenumber 1617.61 cm-1 a peak appears due to the C=C stretching vibration of the unoxidized carbon structure. The C=C bond is a carbon-carbon double bond, indicating skeletal vibrations of graphite that has not been exfoliated [25]. At 1382.39 cm-1 there is a sharp peak with moderate transmittance corresponding to C-H bending vibrations of the alkane functional group in aliphatic hydrocarbons [26], whereas at 1035.42 cm-1 there is a C-O stretching vibration associated with the graphite structure [27]. The C-O bond originates from alcohol functional groups, which appear in the range of 900–1300 cm-1. The presence of the C-O bond suggests that GO still contains alcohol groups originating from the ethanol used in the washing process.
| Sample | Functional Group (cm-1) | ||||
H stretching vibrations (hydroxyl) |
C=O stretching vibration (carbonyl) | C=C stretching vibration (aromatic) | H bending vibration (alkene) |
C-O stretching vibration (epoxide) |
|
| Graphite | 3220.25 | 1718.16 | 1620.27 | 1393.36 | 1038.87 |
| GO | 3193.50 | 1718.48 | 1617.61 | 1382.39 | 1035.42 |
Raman Spectroscopy
The Raman spectra of graphite and graphene oxide samples are shown in Figure 6. Figure 6 shows the Raman spectra of the graphite and GO samples. These spectra are characterized by the presence of peaks in the disorder band (D), graphite band (G) and overtone band (2D). The D band can be interpreted as a longitudinal optical (LO) phonon in which a peak induced by lattice disorder is associated with disturbances in the hexagonal symmetrical graphite lattice, such as structural defect sites, edge defects and dangling bonds. The G band is an intense tangential mode that appears with a stretching movement or a scattering in the first-order E2g phonon in the sp2 C-C bond plane [28,29]. In contrast, the 2D band is a second-order continuation of the D band, appearing at wavenumbers from 2500 to 3200 cm-1 [30].
The graphite sample has a D band at a wavenumber of 1348 cm-1. The intensity of the D band reflects the defect level in the graphite lattice; a higher D-band intensity indicates more defects. The D band on graphite shows a relatively low intensity, indicating a low level of crystal defects. In addition, there is a G band at a wavenumber of 1579 cm-1, which shows the vibration of sp2-bonded carbon atoms in a two-dimensional hexagonal lattice in the carbon layer. The intensity of the G band is higher than that of the D band, indicating that graphite has a well-ordered crystal lattice [31]. At the wavenumber 2707 cm-1 there is a 2D band which shows the intrinsic properties of graphite and the occurrence of a second-order process from a zone-boundary LO phonon. The Raman spectra of GO indicate the successful oxidation of graphite. The GO sample displays a D-band peak with a wavenumber of 1355 cm-1, showing a blue shift of the D-band peak from 1348 to 1355 cm-1 after the graphite undergoes oxidation. Additionally, a G band appears at the wavenumber 1586 cm-1. The D band intensity is higher and the G band intensity is lower, indicating defects in the GO lattice structure. The increased D-band intensity may be attributed to the incorporation of certain functional groups in the carbon backbone [32].
Ultraviolet and Visible Spectrophotometry (UV-Vis)
The UV-Vis spectra of graphite and GO samples are shown in Figure 7. Figure 7 depicts the UV-Vis spectra of graphite and GO samples, characterizing the wavelengths and molecular transitions of these carbon-derived materials. The graphite sample shows absorption bands in the 250–300 nm wavelength range. The appearance of graphite absorption bands generally indicates an excitation of π–plasmon or plasmon peaks of π-π* C=C bonds in the sp2 hybrid region of the graphite structure [32,33]. In the GO sample, an absorption band at 247 nm is seen, which is associated with the π−π* transition from C=C aromatic, while the absorption band at 300 nm, related to the n−π* transition, indicates the successful oxidation of graphite to GO [34–37]. In addition, the peak intensity ratio in the absorption band also shows the degree of oxidation; the higher the peak intensity, the higher the degree of oxidation [31].
Conclusions
An analysis of the physical properties of GO made from 5-micron graphite using the Modified Hummer method has been completed. Based on the SEM data, the synthesized GO exhibited a layered morphology indicating that the functional groups had been bound to the surface of the GO plane, and the most abundant elements in GO were C and O, with weight percentages of 53.09% and 42.13%, respectively. The high oxygen content is caused by strong oxidizing agents such as KMnO4, NaNO3 and H2O2. Based on FTIR data, an O-H functional group appears at the wavenumber 3193.50 cm-1, indicating the presence of hydrogen and oxygen. The broad O-H peak suggests a high water content in GO. In addition, the Raman data show that GO has a high level of crystal defects, as indicated by the high D-peak intensity compared to the G peak, while the UV-Vis data show that GO has absorption bands at wavelengths of 247 nm and 300 nm, indicating the presence of π−π* transitions from C=C aromatic and n−π* from C=O bonds.