Deoxygenation of graphene oxide using household baking soda as a reducing agent: a green approach

Abstract

A one-step, novel, easy, fast, facile, economic, and environmental friendly route to reduce graphene oxide (GO) is studied and explained in this study. The household baking soda (sodium bicarbonate/NaHCO₃) was applied here as a reducing agent. In aqueous solution, NaHCO₃ hydrolyses into hydroxide ion and leads to deoxygenate GO sheets. The confirmation of oxygen reduction was checked with UV-visible spectroscopy, X-ray diffraction, Fourier transform infrared spectrometry and Raman spectroscopy. Thermal behavior of graphene was analyzed by thermogravimetry analysis and differential scanning calorimetry. Again, atomic force microscopy, scanning electron microscopy, field emission scanning electron microscopy and transmission electron microscopy were used for morphological study. Energy dispersive X-ray spectrometer was applied to analyze the elemental composition. The results of the above experiments demonstrate that the house-hold baking soda is able to produce functional graphene, which can be a suitable replacement of hydrazine, sodium borohydrate or other toxic reducing agents. This method is high yielding and safe to use in biomaterial applications.

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Introduction

Graphene is the thinnest material on earth. It is a plane ribbon of sp² interacted carbon, which is tightly confined into a honeycomb lattice. Recently, this 2D flat sheet of carbon has conquered huge interest of scientific community. Because it possesses many significant properties such as: long specific surface area (2630 m² g⁻¹),¹ sensitivity to detect single gas molecule,² superior carrier mobility (200 000 cm² v⁻¹ s⁻¹),³ high thermal conductivity (∼5000 W m⁻¹ K⁻¹),⁴ high optical transmittance (∼97.7%).⁵ These unique behavior of graphene opened a new window for its wide range of application at different branches of technology such as Li ion battery,⁶ catalyst engineering,⁷ biosensor,⁸ solar cells,⁹ photovoltaic devices,¹⁰ p–n junction materials,¹¹ super capacitor.¹² However, large quantities of graphene are required to fulfil the huge demand of aforementioned applications.

Inexpensive scale-up production of graphene is one of the major challenges of this scientific field. Several approaches have designed to synthesis graphene from pristine graphite such as: epitaxial growth,¹³ vacuum thermal annealing,¹⁴ micromechanical cleavage,¹⁵ chemical vapor deposition,¹⁶ thermal exfoliation,¹⁷ liquid phase exfoliation,¹⁸ chemical based detonation,¹⁹ carbon nano tube unzipping.²⁰ These methods can produce crystalline graphene, however, they are not suitable for large scale production.²¹

The most potential procedure for mass production of graphene could be the chemical oxidation–reduction reaction. It is considered as an easier, scalable, fast, dynamic, facile, and economic approach compared to other methods. This method involves the production of graphite oxide from graphite flakes by chemical oxidation, subsequent exfoliation of graphite oxide to graphene oxide (GO) in aqueous solution and finally reduction of GO to functional graphene sheet. Recently, different reducing agents are using to convert GO to graphene sheet such as: hydrazine,²² sodium borohydrate,²³ hydroquinone.²⁴ Unfortunately, all of these reducers are poisonous, corrosive and hazardous to human and environment, which hinder their application in the several field of biomaterials, biosensors or biomedical engineering. Furthermore, the existing oxygen molecules on the sheet (after reduction) and additional functional groups of reducer molecules could arise π–π stacking, resulting sheet resistant. In order to address the above problems, different green and ecofriendly reducers have chosen by the researchers to produce high quality graphene such as: metal powder,²⁵ amino acid,²⁶ plant extract,²⁷ organic acid,²⁸ metal oxide,²⁹ cellulose,³⁰ biological agent.³¹ However, the green reducers used in nontoxic synthesis have some remarkable limitations like possibility of product contamination (metal powders), repeated downstream processing (amino acid, plant extract), complex and controlled experimental conditions (biological agent), lengthy reduction time.

Therefore, a novel and green route of reduction is needed to be explored that would be free from the above limitations. In this work, we reported a new approach to the direct synthesis of graphene from GO. We used household baking soda as a reducing agent containing NaHCO₃. There are some significant reasons for the selection of NaHCO₃. Firstly, it is economically cheap, environmentally safe, and readily available in everywhere. Secondly, it is mild alkaline in nature and able to reduce the reaction time.³² It is important to mention that the alkaline pH has a great impact over the graphene sheet.³³ In an investigation, Navarro et al.³⁴ claimed that acidic pH can cause higher degrees of defect or disorder in the resulting graphene. It can reduce the size of the sheet and arise irreversible agglomeration. These types of sheets are not suitable for electronic devices due to their lower electron conductivity. Additionally, lower pH is responsible for giving rise to other graphitic nano forms such as multi-walled carbon nanotubes, fullerenes, and nano-onions. In contrast, alkaline condition lessens the defects of the derived sheet and produces bigger graphitic domain. It can promote colloidal stability of the GO sheet. It also increases the negative charge of the sheet resulting magnetic repulsion in-between individual layer. Thus, it can prevent irreversible aggregation and re-establish new sp² bonding network upon reduction. It is free from the introduction of further functional groups (which are not suitable for the practical application of graphene). This study highlighted the possible mechanism of deoxygenation in the presence of NaHCO₃. We hope that this research will shed a light in the field of green synthesis for deoxygenation of graphene oxide.

Experimental details

Chemicals

Natural graphite flakes, 98% sulphuric acid, 98% hydrochloric acid, 30% hydrogen peroxide, potassium permanganate, sodium hydroxide, 30% ammonium hydroxide, 90% ethanol were purchased from Sigma-Aldrich, Selangor, Malaysia. All the reagents were purified, so no further purification was done. The household baking soda was obtained from a local stationary shop. All the aqueous solutions were prepared in deionized (DI) water.

Synthesis of GO

The hummer method and modified hummer method are commonly used in GO preparation. In this study, we followed the modified version of hummer method. Briefly, 45 mL H₂SO₄ and 2 g graphite powder were mixed in a beaker at room temperature. Initially, the beaker started to warm due to concentrated H₂SO₄. Therefore, the beaker was kept in a 0 °C ice water bath to cool down the temperature. After that, 5 g KMNO₄ was carefully added to the mixture and the solution was stirred at 5 °C for 5 h. Then the reaction mixture was transferred to a 500 mL beaker and dilute by adding 350 mL DI water. Subsequently, 30% H₂O₂ solution poured drop wise until the color of the solution converted to bright yellow. 5% of HCl was added to dissolve excess manganese salt. At the end, the brown GO dispersion was filtered and repeatedly washed with DI water until the pH of the product came at 6. Finally, the purified solution was kept in a dryer at 80 °C for 6 h.

Reduction of GO

In this experiment, 2 mg of as produced GO was exfoliated into 20 mL DI water through mild sonication. Thus, homogenous GO aqueous solution was obtained without aggregation (0.1 mg mL⁻¹). Several drops of 5% NaOH were added to create alkaline condition. The pH of the solution was approximately 9. Alkaline pH helps to raise the colloidal stability of the GO sheets by electrostatic repulsion. A 2 g of household baking soda was added in the reaction mixture and was kept the flask in a boiling water bath at 90 °C for 3 h. The resulting black color graphene sediment was collected by centrifuge and successively washed with 5% HCl, ethanol, and DI water. For each wash, the solution was centrifuged at 6000 rpm for 5 min and the supernatant was discarded. Finally, the product was dried at 80 °C for 4 h in a vacuum oven. Thus, the baking soda reduced graphene oxide (SRGO) was obtained. Fig. 1 shows the physical appearance of different states of GO reduction.

Fig. 1 (a) GO synthesised by modified hummer method, (b) magnetic repulsion of GO at room temperature, (c) baking soda reduced graphene oxide (SRGO), and (d) SRGO after drying.

Characterization

A Hitachi (U-2001, Tokyo, Japan) UV-vis was used in this study. The diluted GO and SRGO aqueous solutions were used as UV-vis sample, which were taken in crystal cuvette. The cuvettes were free from all sorts of spot and fingerprint. Here, DI water was used as reference. The XRD (Ringaku Mini Flex, 600, Japan) analysis was run at 40 kV and 40 mA in the range of 2θ = 5–80°. An invia laser-Raman spectrometer (Renishaw, UK) having 514 argon ion laser was applied to get Raman spectra. The FTIR spectra were obtained by using Nicolet NEXUS 470. The sample was prepared by fixing the materials in KBr pellets. The TGA (Shimadzu TG 50 instrument, Japan) was conducted in nitrogen atmosphere with a heating rate of 10 °C min⁻¹. The DSC (4000, Perkin-Elmer, USA) was used for heat flow analysis. SR-4 pictorl resistivity four point probe stand was used to measure the resistivity of compressed GO and SRGO powder. The sample pellet was 10 mm in diameter and 2 mm in thickness. Veeco multimode AFM machine was used to analysis the thickness of GO and SRGO. The machine was set at tapping mode and the samples were prepared by putting one drop of each diluted aqueous dispersion onto a freshly cleaved SiO₂/Si substrate. A TEM (LEO-Libra 120 electron microscope) was used to operate the transmission electron microscopy. The samples were prepared by mild sonication for 15 min followed by centrifugation and finally one drop of the supernatant was casted over a fresh carbon coated copper grid. A SEM (Hitachi TM 3030 tabletop microscope, Japan) was used to receive SEM image attached with EDX system. The solid samples were placed into a carbon tape located in the sample holder and the machine was operated at 12 kV. FESEM (AJSM-6700F with semi-in-lens) was used to acquire the FESEM image. The samples were run at 10 kV. Notably, each of the experiment was repeated 3 times to get more accurate result.

Results and discussion

NaOH releases hydroxyl ions in the aqueous solution that initially produce alkaline condition in the GO media. After that, when baking soda is added to the solution, the elemental sodium bicarbonate breaks to sodium ion and bicarbonate ion. At aqueous solution, the bicarbonate ion reacts with water to produce carbonic acid and hydroxide ion. Finally, the carbonic acid degrades to CO₂ and water. The remaining hydroxyl ions arise magnetic repulsion in the sheet, which darkens the yellow brown GO solution within 3 h at 90 °C, as shown in Fig. 2.

Fig. 2 Illustration of GO reduction using sodium bicarbonate.

UV-vis absorption spectroscopy is commonly used to insure the reduction of oxygen-containing groups and such results of this study are reported in Fig. 3. The UV-vis absorption spectrum of GO displays an absorption peak at 231 nm, which represents the π–π* transition of C=C bonds. The spectrum also shows a weak shoulder at around 300 nm attributed to the n–π* transition of carboxyl groups (COOH). Upon complete reduction the C=C plasmon peak red shifted to ∼270 nm, which reveals that the π-electron has increased in SRGO. It is also consistent with the ordering of structure through possible rearrangement of atoms. This result strongly evaluates the findings of previous related works, as mentioned in Table 1. On the other hand, the shoulder related to COOH group of GO was absent in the spectrum of SRGO. It also supports the omission of oxygen after reduction.

Fig. 3 UV-vis spectra of pure GO and SRGO.

Table 1 UV-vis absorbance peak (C=C bonds) of GO and RGO at different reduction methods reported in previous literatures

Name of reducer	π–π* transition of GO	π–π* transition of RGO	References
L-Cysteine	230 nm	270 nm	26a
Aqueous phytoextract	232 nm	270.9 nm	27a
Benzylamine	230 nm	260 nm	35
Dopamine	230 nm	271 nm	36
Gallic acid	230 nm	273 nm	28b
Glucose amine	231 nm	273 nm	27c
Glycine	230 nm	267 nm	37
Humanin	227 nm	265 nm	38
Iron powder	230 nm	Not mentioned	25b
L-Ascorbic acid	230 nm	264 nm	28c
Metal salt	229 nm	259 nm	39
Nascent hydrogen	232 nm	266 nm	40
Natural cellulose	230 nm	269 nm	30
L-Aspartic acid	250 nm	269 nm	26b
Oligothiophene	Not mentioned	Between 243–247 nm	41
Reducing sugar	230 nm	261 nm	42
Sodium borohydrate	230 nm	260 nm	23
Solvothermal	229 nm	269 nm	43
Tannin	230 nm	275 nm	27c
Tea solution	228 nm	271 nm	44
Yeast	230 nm	263 nm	31c
Zn powder	230 nm	272 nm	45

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Fig. 4 shows the XRD profile of pristine graphite, GO, and SRGO. In pristine graphite, a strong and sharp 002 diffraction peak appeared at 2θ = 26.49° corresponding to a d-spacing of 3.35 Å (Fig. 4a). After the treatment of oxidizing agents, this characteristic peak (002) has been shifted to a lower angle in GO.^28a In the current study, it was recorded 2θ = 10.43° with 8.47 Å d-spacing (Fig. 4b). The increment of d-spacing from 3.35 Å to 8.47 Å confirms the introduction of oxygen functional groups in between the graphene sheet. It also indicates the formation of new sp³ bonding responsible for structural defects or atomic scale roughness.^26a The XRD pattern of SRGO displays two peaks at 24.68° (d-spacing = 3.36 Å) and 26.52° (d-spacing = 3.60 Å) (Fig. 4c). The weak or even invisible peak at 24.68° is related to the fully exfoliated graphene sheet.⁴⁶ However, other weak peak at 26.52° reveals that the van der Waals forces cause restacking of sheet due to the elimination of oxygen with less hydrophilicity.⁴⁷ In fact, the aggregation or stacking of graphene layer can inevitably occur, if it is reduced directly.²²

Fig. 4 XRD patterns of (a) graphite, (b) pure GO, and (c) SRGO.

Raman spectroscopy is an important technique to characterize GO before and after reduction and such results of this study is mentioned in Fig. 5. Typically, the Raman spectra of GO and RGO display two fundamental peak within the range of 1500–1700 cm⁻¹ termed as D peak and G peak. The characteristic of D peak is the consequence of breathing mode of six atom rings. It is caused by the defects or disorders present over the plane. And the G peak is the result of first order scattering of E_2g phonons. It is caused by the sp² bonded carbon which constructs graphene. In this study, a sharp and intense G peak was observed at 1581 cm⁻¹ caused by the lattice structure of graphite. It also displayed a weak D band at 1353 cm⁻¹ (disordered band) caused by the graphitic edges. After the treatment of oxidising agents, the Raman spectra of GO displayed the D peak and G peak at 1350 and 1614 cm⁻¹, respectively. Here the G band is broaden and blue-shifted to higher wave number indicating size degradation of the in-plane sp² domains.⁴⁸ While the GO is being reduced with baking soda, these two peaks are shifted to a lower wave length of 1347 and 1596 cm⁻¹, respectively. Significantly, the shifting of G peak to the lower wave number is important upon reduction. In SRGO the G peak is sharper than the GO. This phenomenon reveals the “self-healing” mechanism followed by the elimination of oxygen.^48a,49 On the other hand, the intensity ratio of D peak and G peak is applied to measure the average size of sp² domain as well as the degree of disorder. According to the Tuinstra–Koenig relation, the I_D/I_G ratio is inversely proportional to the crystalline size of sp² domain.⁵⁰ In GO the I_D/I_G ratio was found to be 1.47. After reduction, it was decreased to 1.37. This achievement also suggests that the green reduction of GO with baking soda is able to repair disorders and restore new π-conjugated structure.^25b On the other hand, the spectrum of graphene could display another weak peak, which is resonance-enhanced by a two phonon scattering process. It is termed as 2D (G′) peak.⁵¹ This peak indicates the increment of new layer in SRGO.⁵² However, the main application of this peak is the determination of mono, bi or multiple layer graphene. In case of single layer graphene, it is generally observed at 2679 cm⁻¹.^27a If the graphene is multiple layer (2–4), it will be shifted towards a higher wave number by 19 cm⁻¹.⁵³ In this work, the 2D peak position was observed at 2695 cm⁻¹, which shifted to 16 cm⁻¹ higher wave number. This result confirms the formation of multiple layer graphene (SRGO).

Fig. 5 Raman spectroscopy of graphite, GO and SRGO.

The FTIR spectroscopy of this study is plotted in Fig. 6. The FTIR spectra of pristine graphite contains only two characteristic peaks at 1610 and 3450 cm⁻¹ attributed to aromatic C=C bonding (the structural vibration) and adsorbed water molecules (the vibration of O–H stretching), respectively.⁵⁴ In GO, the appearance of different intense band at around 1706, 1386, 1219, and 1052 cm⁻¹ are ascribed to the carboxyl group (–C=O), OH deformation, epoxy group (C–O–C), and alkoxy group (C–O stretching). And the broad band at approximately 3395 cm⁻¹ is responsible for OH stretching.⁵⁵ All these peaks confirm the penetration of the above oxygen moieties over the graphene sheet. This phenomenon also indicates the destruction of sp² character and generation of defects in the sheet owing to extensive oxidation. Secondly, the peak for the aromatic C=C stretching of GO sample was observed at 1625.97 cm⁻¹. Notably, among the aforementioned oxygen groups, the hydroxyl and epoxy groups are the dominant locate at the basal plane while the carboxyl groups are found at the edge of the sheet, as shown in Fig. 7. The binding of these numerous oxygen containing groups make GO hydrophilic in nature. However, upon reduction with baking soda the carboxyl peak (1706 cm⁻¹) was not observed in the SRGO, which indicates its successful elimination.^28c,56 Secondly, the alkoxy peak (1052 cm⁻¹) and epoxy peak (1220 cm⁻¹) were almost disappeared.⁵⁶ And the hydroxyl stretching peak (3395 cm⁻¹) became weaker than the GO.³² It is noteworthy that the intensity of the peak related to sp² hybridized C=C was increased to 1570 cm⁻¹ for SRGO. Significantly, the appearance of two novel peak at 2918 and 2850 cm⁻¹ related to the symmetric and asymmetric stretching vibration of –CH₂ (methylene group) point out the successful retention of new sp² network.⁵⁷ This achievement not only confirms the removal of oxygen moieties but also promulgates the successful production of functional graphene sheet. It also reveals that the use of household baking soda could be a potential replacement of toxic hydrazine or sodium borohydrate.

Fig. 6 FTIR spectra of pure GO and SRGO.

Fig. 7 Structure of GO sheet containing different oxygen functionalities.

The thermal stability of GO and SRGO was investigated by TGA and is shown in Fig. 8. In this experiment, the samples were heated under nitrogen atmosphere with a rate of 10 °C min⁻¹ from 25–700 °C. GO is thermally unstable and begins to lose weight just after heating.⁵⁸ The curve of GO displays three significant level of mass loss. The first level showed about 17% mass loss at around 100 °C. It is owing to the evaporation of absorbed water from the sheet. It demonstrates that around 17% of water molecules could be trapped between the sheets. It is also considered as the higher amount of oxygen quantity.^54,58 The second level occurred at around 220 °C with 46% mass loss. It is attributed to the thermal decomposition of labile oxygen moieties. Finally, the third level displayed about 60% mass losses at around 600 °C. It is related to the pyrolysis of existing moieties as well as burning of carbon plane. However, the residual weight of GO sample is about 36% at 700 °C. In contrast, the SRGO sample exhibits about 11, 16, and 33 wt% mass loss at around 100, 220, and 600 °C, respectively. All these mass losses of SRGO are lower than the GO. In addition, the residual weight of SRGO was found ∼63% at 700 °C, which is almost twice than the GO. These aforementioned results suggest that most of the oxygen holding groups are vanished upon reduction. It also increases the van der Waals interaction between the sheets leading to better re-graphitization.⁵⁹ Therefore, the as prepared SRGO possesses higher thermal stability compare to the GO precursor.^40,60

Fig. 8 TGA plots of GO and SRGO.

DSC is another useful technique to study the thermal behavior of graphene. The heat flow curve by DSC is shown in Fig. 9. In GO, a strong exothermal peak was centred at around 200 °C. It is ascribed to the vanishing of labile oxygen. It also shows an endothermal peak at around 170 °C attributed to the burning of skeletal carbon. On the other hand, after heating the SRGO sample up to 450 °C, the SRGO curve shows neither sudden weight loss nor obvious exothermal peak. It displays an endothermal peak at around 175 °C, which is sharper than the GO. This result is consistent with the result of Ji and co-workers.⁶¹ It also reveals that the SRGO possesses higher thermal stability than GO.

Fig. 9 Heat flow curve of GO and SRGO.

AFM is a powerful tool to study the morphology of graphene and carbon related materials. Fig. 10 shows the typical tapping mode AFM image of GO and SRGO. However, it is well established that the van der Waals thickness of a pristine graphite is about 0.34 nm that is automatically flat.²² After oxidation, the newly synthesised GO become thicker due to the attachment of covalently bonded oxygen. This insertion of oxygen prolapses the sp² hybridized carbons over and under the original plane of graphene. In this work, the average thickness of GO sheet is obtained about 1.20 nm, which corresponds to the earlier literature.⁶² In contrast, the average thickness of as-formed SRGO fragment is about 2.5 nm (Fig. 10b). This increment of thickness is ascribed to the tendency of graphene layers to restack in the absence of additional stabilizing molecules.^27b

Fig. 10 Tapping mode AFM measurement of (a) GO and (b) SRGO, the samples are deposited on to SiO₂/Si substrate.

To investigate surface morphology, the SEM image of graphite, GO, and prepared SRGO were obtained. Fig. 11a shows the SEM image of natural graphite powder. Morphologically, it exhibits flaky appearance for the strong sp² carbon to carbon bonding in the plane. Nevertheless, at higher concentration, the GO surface shows soft-carpet like morphology as seen in Fig. 11b. It may be the attachment of residual water molecules, carboxyl and hydroxyl groups to carbon basal plane.⁶³ Furthermore, the higher magnification as portrayed in Fig. 11c clearly expresses the wrinkled and coarse feature of GO. After reduction, the SRGO materials show aggregated structure where individual sheets of different size are closely associated with each other (Fig. 11d). Here one single sheet is looked like a disordered solid (Fig. 11e). However, it is obvious that the edge of GO is quite distinguishable than that of SRGO. Significantly, the edges of the GO sheets look crumpled due to extensive oxidation (Fig. 11c), which is later magnified and discussed by FESEM study. By contrast, the edges of SRGO were sharp (Fig. 11e and f) that supports the investigation of Gurunathan and co-workers.³⁸ This morphological difference between the folded edge of GO and sharp edge of SRGO demonstrate that the household baking soda can play an important role to convert the GO into functional graphene.

Fig. 11 SEM image of (a) graphite, (b and c) GO, (d–f) SRGO.

Fig. 12a shows the FESEM image of GO. The thin, crystal, and shrivelling morphology of GO were observed where the sheets were slightly joined with each other. Generally, it is termed as random orientation aiding to form agglomerate. In this work, most of the GO sheets are exfoliated into mono or multiple layer graphene, with some of the sheets being overlapped. Fig. 12b and c show the higher magnification FE-SEM of GO. From these images, it can be demonstrate that the boarder of sheets are partially folded, which significantly reduces the total surface energy of the sheets.⁴³ On the other hand, the heaping of individual sheets through different self-assembly techniques develops the surface morphology of SRGO film. As a result, the sheets take crumpled or sea wave like texture as seen in Fig. 12d. Similar trend like this also observed by other researchers.^45,64 This morphological difference assures the change of microstructure after reduction. Recently, it is claimed that the average lateral size of giant GO sheets is around 18.4 μm, although the relative range of size (wide) distributes between one to several micrometre. In this work, the lateral size (wide) of GO and SRGO sheets were 7 and 10 μm that is similar to the work of green graphene synthesis using humanin.³⁸

Fig. 12 FE-SEM of (a) GO, (b and c) folded edge of GO, and (d) silky morphology of SRGO.

Energy dispersive X-ray spectrometer is applied to analyze the elemental composition as well as the purity of the materials (as displayed in Fig. 13). In pristine graphite (can be seen in Fig. 13a), the atomic weight percentage of carbon was found to be 98%. The rest 2% of oxygen is related to the atmospheric or aerial oxygen binding over the sheet surface with weak van der Waals interactions.⁶⁵ In GO (can be seen in Fig. 13b), the atomic weight percentage of carbon and oxygen were found to be 58.4% and 38.9%, respectively.⁶⁶ The enhancement of atomic oxygen in GO reveals the successful oxidation of graphite. Noticeably, the elimination of impurities other than trace amounts of S (2.7 atomic weight percentage) confirms that the GO precursor prepared by modified hummer method is pure. In contrast, the atomic weight percentage of carbon is increased to 86.6% in SRGO (refer to Fig. 13c).⁶⁷ This result suggests that a considerable amount of the oxygen functionalities were removed upon deoxygenation. Here, the remaining oxygen is mainly the absorbed water or oxygen molecules. And the trace amounts of sodium are the supplement of baking soda (NaHCO₃).

TEM is another useful tool to study the nano structure of graphene. The TEM image of both GO and SRGO at lower magnification range is portrayed in Fig. 14. The Fig. 14a shows the amorphous GO sheets before reduction. Morphologically, the GO nano sheets show transparent, stable, uniform and silk like appearance under high energy electron beam, as in previously reported work.⁶⁸ In this stage the inter layer coherence is not demolished. For this reason, the GO nano sheets trend to entangle with each other in a random manner and produce multiple layer agglomerates. However, this scrolling and corrugation feature is the intrinsic nature of graphene.⁶⁹ Because the 2D graphene membrane become thermodynamically stable by crumpling via bucking or bending.⁷⁰ Notably, the scrolling and corrugation are found not only at the edge but also in the middle of the sheets. Thus, the GO nano sheets become folded or coiled. Contrarily, after subsequent washing the TEM image of SRGO exhibits a wrinkle paper-like structure, as shown in Fig. 14b. It is attributed to the abundance of carbon to carbon chemical bonding either in a single layer or several layers.⁷¹

Fig. 13 EDX spectrum of (a) graphite flakes, where, C 98% (atm wt%); O 2% (atm wt%); (b) GO, where, C 20.9% (atm wt%); O 77.5% (atm wt%); S 1.6% (atm wt%) and (c) SRGO, where, C 84.2% (atm wt%); O 13.3% (atm wt%); Na 2.1% (atm wt%).

Fig. 14 TEM image of (a) GO and (b) SRGO.

GO is electrically insulant because the bounded oxygen moieties, which increase the resistance against electron flow.⁷² In this study, we measured the resistance of GO and SRGO pellet by using four point probe method. The resistance of GO pellet was 4.45 × 10⁶ Ω while the SRGO was as low as 3.36 × 10¹ Ω. It reveals that the resistance of SRGO reduces about 10⁵ times in comparison to that of GO. This achievement also reconfirms the washing of oxygen groups from the sheet.

Conclusions

In this work, we have demonstrated that the household baking soda is a potential reducing agent under mild alkaline condition for the reduction of GO. Herein, we also mentioned the possible mechanism of reduction. It also took lower reduction time, simple experimental conditions, and easy product extraction process. In characterization, UV-vis spectra showed the restoration of electronic conjugation at SRGO. XRD ensures the crystalline graphitic structure of SRGO. FTIR confirmed the removal of oxygen. TGA and DSC proved the thermal stability of graphene. Raman spectroscopy indicated the decrement of I_D/I_G ratio at SRGO. EDX showed the lower oxygen contents by elemental composition analysis, and four-point probe showed the resistance at SRGO. AFM, SEM, FESEM, and TEM exhibited the morphology and nano structure of graphene. All these findings claim that the household baking soda can successfully deoxygenate GO. Thus, it could be a suitable replacement of hydrazine, sodium borohydrate, and other poisonous reducing agents. It is also suggested that the as-synthesized graphene may be implemented not only in the material based products (electronic devices) but also in biological applications such as: biosensors, drug delivery and biocompatible materials.

Acknowledgements

“The authors would like to thank the University of Malaya for financial support under the High Impact Research MoE Grant: UM.C/625/1/HIR/MoE/ENG/40 (D000040-16001) from the Ministry of Education Malaysia”.

References

1. Y. Zhu, S. Murali, W. Cai, X. Li, J. W. Suk, J. R. Potts and R. S. Ruoff, Adv. Mater., 2010, 22, 3906–3924.
2. M. Katsnelson, F. Guinea and A. Geim, Phys. Rev. B: Condens. Matter Mater. Phys., 2009, 79, 195426.
3. K. S. Novoselov, A. K. Geim, S. Morozov, D. Jiang, Y. Zhang, S. Dubonos, I. Grigorieva and A. Firsov, Science, 2004, 306, 666–669.
4. A. A. Balandin, S. Ghosh, W. Bao, I. Calizo, D. Teweldebrhan, F. Miao and C. N. Lau, Nano Lett., 2008, 8, 902–907.
5. W. Cai, Y. Zhu, X. Li, R. D. Piner and R. S. Ruoff, Appl. Phys. Lett., 2009, 95, 123115.
6. (a) G. Wang, X. Shen, J. Yao and J. Park, Carbon, 2009, 47, 2049–2053; (b) E. Yoo, J. Kim, E. Hosono, H.-s. Zhou, T. Kudo and I. Honma, Nano Lett., 2008, 8, 2277–2282.
7. (a) Ö. Metin, S. F. Ho, C. Alp, H. Can, M. N. Mankin, M. S. Gültekin, M. Chi and S. Sun, Nano Res., 2013, 6, 10–18; (b) Y. Li, H. Wang, L. Xie, Y. Liang, G. Hong and H. Dai, J. Am. Chem. Soc., 2011, 133, 7296–7299.
8. (a) Y. Shao, J. Wang, H. Wu, J. Liu, I. A. Aksay and Y. Lin, Electroanalysis, 2010, 22, 1027–1036; (b) Y. Zhang, S. Liu, L. Wang, X. Qin, J. Tian, W. Lu, G. Chang and X. Sun, RSC Adv., 2012, 2, 538–545.
9. (a) C. X. Guo, M. Wang, T. Chen, X. W. Lou and C. M. Li, Adv. Energy Mater., 2011, 1, 736–741; (b) H. Yang, G. H. Guai, C. Guo, Q. Song, S. P. Jiang, Y. Wang, W. Zhang and C. M. Li, J. Phys. Chem. C, 2011, 115, 12209–12215.
10. (a) C. X. Guo, H. B. Yang, Z. M. Sheng, Z. S. Lu, Q. L. Song and C. M. Li, Angew. Chem., Int. Ed., 2010, 49, 3014–3017; (b) Z. Liu, Q. Liu, Y. Huang, Y. Ma, S. Yin, X. Zhang, W. Sun and Y. Chen, Adv. Mater., 2008, 20, 3924–3930.
11. (a) J. Williams, L. DiCarlo and C. Marcus, Science, 2007, 317, 638–641; (b) V. V. Cheianov, V. Fal'ko and B. Altshuler, Science, 2007, 315, 1252–1255.
12. (a) C. XianáGuo and C. MingáLi, Energy Environ. Sci., 2011, 4, 4504–4507; (b) C. Liu, Z. Yu, D. Neff, A. Zhamu and B. Z. Jang, Nano Lett., 2010, 10, 4863–4868.
13. C. Berger, Z. Song, X. Li, X. Wu, N. Brown, C. Naud, D. Mayou, T. Li, J. Hass and A. N. Marchenkov, Science, 2006, 312, 1191–1196.
14. V. C. Tung, M. J. Allen, Y. Yang and R. B. Kaner, Nat. Nanotechnol., 2008, 4, 25–29.
15. B. Jayasena and S. Subbiah, Nanoscale Res. Lett., 2011, 6, 95–101.
16. Z. G. Cambaz, G. Yushin, S. Osswald, V. Mochalin and Y. Gogotsi, Carbon, 2008, 46, 841–849.
17. M. Jin, H.-K. Jeong, T.-H. Kim, K. P. So, Y. Cui, W. J. Yu, E. J. Ra and Y. H. Lee, J. Phys. D: Appl. Phys., 2010, 43, 275402.
18. (a) M. Lotya, Y. Hernandez, P. J. King, R. J. Smith, V. Nicolosi, L. S. Karlsson, F. M. Blighe, S. De, Z. Wang and I. McGovern, J. Am. Chem. Soc., 2009, 131, 3611–3620; (b) Y. Hernandez, V. Nicolosi, M. Lotya, F. M. Blighe, Z. Sun, S. De, I. McGovern, B. Holland, M. Byrne and Y. K. Gun'Ko, Nat. Nanotechnol., 2008, 3, 563–568.
19. G. Sun, X. Li, Y. Qu, X. Wang, H. Yan and Y. Zhang, Mater. Lett., 2008, 62, 703–706.
20. I. Janowska, O. Ersen, T. Jacob, P. Vennégues, D. Bégin, M.-J. Ledoux and C. Pham-Huu, Appl. Catal., A, 2009, 371, 22–30.
21. S. Park and R. S. Ruoff, Nat. Nanotechnol., 2009, 4, 217–224.
22. S. Stankovich, D. A. Dikin, R. D. Piner, K. A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S. T. Nguyen and R. S. Ruoff, Carbon, 2007, 45, 1558–1565.
23. H. J. Shin, K. K. Kim, A. Benayad, S. M. Yoon, H. K. Park, I. S. Jung, M. H. Jin, H. K. Jeong, J. M. Kim and J. Y. Choi, Adv. Funct. Mater., 2009, 19, 1987–1992.
24. G. Wang, J. Yang, J. Park, X. Gou, B. Wang, H. Liu and J. Yao, J. Phys. Chem. C, 2008, 112, 8192–8195.
25. (a) Z. Fan, K. Wang, T. Wei, J. Yan, L. Song and B. Shao, Carbon, 2010, 48, 1686–1689; (b) Z.-J. Fan, W. Kai, J. Yan, T. Wei, L.-J. Zhi, J. Feng, Y.-m. Ren, L.-P. Song and F. Wei, ACS Nano, 2010, 5, 191–198.
26. (a) D. Chen, L. Li and L. Guo, Nanotechnology, 2011, 22, 325601; (b) D. N. Tran, S. Kabiri and D. Losic, Carbon, 2014.
27. (a) S. Thakur and N. Karak, Carbon, 2012, 50, 5331–5339; (b) T. Kuila, S. Bose, P. Khanra, A. K. Mishra, N. H. Kim and J. H. Lee, Carbon, 2012, 50, 914–921; (c) Y. Lei, Z. Tang, R. Liao and B. Guo, Green Chem., 2011, 13, 1655–1658.
28. (a) Z. Bo, X. Shuai, S. Mao, H. Yang, J. Qian, J. Chen, J. Yan and K. Cen, Sci. Rep., 2014, 4, 4684; (b) J. Li, G. Xiao, C. Chen, R. Li and D. Yan, J. Mater. Chem. A, 2013, 1, 1481–1487; (c) J. Zhang, H. Yang, G. Shen, P. Cheng, J. Zhang and S. Guo, Chem. Commun., 2010, 46, 1112–1114; (d) J. Gao, F. Liu, Y. Liu, N. Ma, Z. Wang and X. Zhang, Chem. Mater., 2010, 22, 2213–2218.
29. (a) J. Zhu, T. Zhu, X. Zhou, Y. Zhang, X. W. Lou, X. Chen, H. Zhang, H. H. Hng and Q. Yan, Nanoscale, 2011, 3, 1084–1089; (b) Y. Qian, S. Lu and F. Gao, Mater. Lett., 2011, 65, 56–58.
30. H. Peng, L. Meng, L. Niu and Q. Lu, J. Phys. Chem. C, 2012, 116, 16294–16299.
31. (a) O. Akhavan and E. Ghaderi, Carbon, 2012, 50, 1853–1860; (b) G. Wang, F. Qian, C. W. Saltikov, Y. Jiao and Y. Li, Nano Res., 2011, 4, 563–570; (c) P. Khanra, T. Kuila, N. H. Kim, S. H. Bae, D.-s. Yu and J. H. Lee, Chem. Eng. J., 2012, 183, 526–533.
32. M. Fernandez-Merino, L. Guardia, J. Paredes, S. Villar-Rodil, P. Solis-Fernandez, A. Martinez-Alonso and J. Tascon, J. Phys. Chem. C, 2010, 114, 6426–6432.
33. X. Fan, W. Peng, Y. Li, X. Li, S. Wang, G. Zhang and F. Zhang, Adv. Mater., 2008, 20, 4490–4493.
34. C. Bosch-Navarro, E. Coronado, C. Martí-Gastaldo, J. Sánchez-Royo and M. G. Gómez, Nanoscale, 2012, 4, 3977–3982.
35. S. Liu, J. Tian, L. Wang and X. Sun, Carbon, 2011, 49, 3158–3164.
36. L. Q. Xu, W. J. Yang, K.-G. Neoh, E.-T. Kang and G. D. Fu, Macromolecules, 2010, 43, 8336–8339.
37. S. Bose, T. Kuila, A. K. Mishra, N. H. Kim and J. H. Lee, J. Mater. Chem., 2012, 22, 9696–9703.
38. S. Gurunathan, J. Han and J. H. Kim, Colloids Surf., B, 2013, 111, 376–383.
39. Q. Zhuo, J. Gao, M. Peng, L. Bai, J. Deng, Y. Xia, Y. Ma, J. Zhong and X. Sun, Carbon, 2013, 52, 559–564.
40. V. H. Pham, H. D. Pham, T. T. Dang, S. H. Hur, E. J. Kim, B. S. Kong, S. Kim and J. S. Chung, J. Mater. Chem., 2012, 22, 10530–10536.
41. M. S. Ahmed, H. S. Han and S. Jeon, Carbon, 2013, 61, 164–172.
42. C. Zhu, S. Guo, Y. Fang and S. Dong, ACS Nano, 2010, 4, 2429–2437.
43. T. V. Khai, D. S. Kwak, Y. J. Kwon, H. Y. Cho, T. N. Huan, H. Chung, H. Ham, C. Lee, N. V. Dan and N. T. Tung, Chem. Eng. J., 2013, 232, 346–355.
44. Y. Wang, Z. Shi and J. Yin, ACS Appl. Mater. Interfaces, 2011, 3, 1127–1133.
45. X. Mei and J. Ouyang, Carbon, 2011, 49, 5389–5397.
46. K. De Silva, S. Gambhir, X. Wang, X. Xu, W. Li, D. L. Officer, D. Wexler, G. G. Wallace and S. Dou, J. Mater. Chem., 2012, 22, 13941–13946.
47. F.-P. Du, J.-J. Wang, C.-Y. Tang, C.-P. Tsui, X.-P. Zhou, X.-L. Xie and Y.-G. Liao, Nanotechnology, 2012, 23, 475704.
48. (a) K. N. Kudin, B. Ozbas, H. C. Schniepp, R. K. Prud'Homme, I. A. Aksay and R. Car, Nano Lett., 2008, 8, 36–41; (b) H. J. Kim, S.-M. Lee, Y.-S. Oh, Y.-H. Yang, Y. S. Lim, D. H. Yoon, C. Lee, J.-Y. Kim and R. S. Ruoff, Sci. Rep., 2014, 4, 5176.
49. M. J. McAllister, J.-L. Li, D. H. Adamson, H. C. Schniepp, A. A. Abdala, J. Liu, M. Herrera-Alonso, D. L. Milius, R. Car and R. K. Prud'homme, Chem. Mater., 2007, 19, 4396–4404.
50. G. A. Zickler, B. Smarsly, N. Gierlinger, H. Peterlik and O. Paris, Carbon, 2006, 44, 3239–3246.
51. D.-W. Wang, K.-H. Wu, I. R. Gentle and G. Q. M. Lu, Carbon, 2012, 50, 3333–3341.
52. M. Mehrali, E. Moghaddam, S. F. S. Shirazi, S. Baradaran, M. Mehrali, S. T. Latibari, H. S. C. Metselaar, N. A. Kadri, K. Zandi and N. A. A. Osman, ACS Appl. Mater. Interfaces, 2014, 6, 3947–3962.
53. A. Malesevic, R. Vitchev, K. Schouteden, A. Volodin, L. Zhang, G. Van Tendeloo, A. Vanhulsel and C. Van Haesendonck, Nanotechnology, 2008, 19, 305604.
54. C. Li, X. Wang, Y. Liu, W. Wang, J. Wynn and J. Gao, J. Nanopart. Res., 2012, 14, 1–11.
55. M. Acik, G. Lee, C. Mattevi, M. Chhowalla, K. Cho and Y. Chabal, Nat. Mater., 2010, 9, 840–845.
56. Y. Jin, S. Huang, M. Zhang, M. Jia and D. Hu, Appl. Surf. Sci., 2013, 268, 541–546.
57. H. Naeimi and M. Golestanzadeh, New J. Chem., 2015, 39, 2697–2710.
58. P.-G. Ren, D.-X. Yan, X. Ji, T. Chen and Z.-M. Li, Nanotechnology, 2011, 22, 055705.
59. D. R. Dreyer, S. Park, C. W. Bielawski and R. S. Ruoff, Chem. Soc. Rev., 2010, 39, 228–240.
60. J. Yan, Z. Fan, T. Wei, W. Qian, M. Zhang and F. Wei, Carbon, 2010, 48, 3825–3833.
61. Z. Ji, G. Zhu, X. Shen, H. Zhou, C. Wu and M. Wang, New J. Chem., 2012, 36, 1774–1780.
62. S. Yang, W. Yue, D. Huang, C. Chen, H. Lin and X. Yang, RSC Adv., 2012, 2, 8827–8832.
63. H.-K. Jeong, Y. P. Lee, R. J. Lahaye, M.-H. Park, K. H. An, I. J. Kim, C.-W. Yang, C. Y. Park, R. S. Ruoff and Y. H. Lee, J. Am. Chem. Soc., 2008, 130, 1362–1366.
64. G. Wang, X. Shen, B. Wang, J. Yao and J. Park, Carbon, 2009, 47, 1359–1364.
65. H. Ulbricht, G. Moos and T. Hertel, Phys. Rev. B: Condens. Matter Mater. Phys., 2002, 66, 075404.
66. J. W. Dickinson, M. Bromley, F. P. Andrieux and C. Boxall, Sensors, 2013, 13, 3635–3651.
67. J. Zhao, Y. Guo, Z. Li, Q. Guo, J. Shi, L. Wang and J. Fan, Carbon, 2012, 50, 4939–4944.
68. G. Wang, B. Wang, J. Park, J. Yang, X. Shen and J. Yao, Carbon, 2009, 47, 68–72.
69. J. C. Meyer, A. K. Geim, M. Katsnelson, K. Novoselov, T. Booth and S. Roth, Nature, 2007, 446, 60–63.
70. C. A. H. Amarnath, C. E. Kim, N. H. Ku, B.-C. Kuila, T. Lee and J. Hee, Carbon, 2011, 49, 3497–3502.
71. P. Song, X. Zhang, M. Sun, X. Cui and Y. Lin, RSC Adv., 2012, 2, 1168–1173.
72. Y. Heo, H. Im, J. Kim and J. Kim, J. Nanopart. Res., 2012, 14, 1–10.

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