- Email: [email protected]
Effect of electrolytes and sonication times on the formation of graphene using an electrochemical exfoliation process
Accepted Manuscript Full Length Article Effect of electrolytes and sonication times on the formation of graphene using an electrochemical exfoliation process Y.Z.N. Htwe, W.S. Chow, Y. Suda, A.A. Thant, M. Mariatti PII: DOI: Reference:
S0169-4332(18)33096-4 https://doi.org/10.1016/j.apsusc.2018.11.029 APSUSC 40874
To appear in:
Applied Surface Science
Received Date: Accepted Date:
6 September 2018 4 November 2018
Please cite this article as: Y.Z.N. Htwe, W.S. Chow, Y. Suda, A.A. Thant, M. Mariatti, Effect of electrolytes and sonication times on the formation of graphene using an electrochemical exfoliation process, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.11.029
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of electrolytes and sonication times on the formation of graphene using an electrochemical exfoliation process Y.Z.N. Htwea, W.S. Chowa, Y. Sudab and A.A. Thantc, M. Mariattia* a,a*
School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Pulau Pinang, Malaysia b
Department of Electrical and Electronic Engineering, Toyohashi University of Technology, Aichi Toyohashi 441-8580, Japan c
Department of Physics, University of Yangon, 11041 Kamayut, Yangon, Myanmar
Corresponding author: [email protected]
ABSTRACT: The exfoliation of graphite is an intriguing approach for simple, fast and largescale production of graphene. In the present study, three types of electrolytes were investigated, namely an ionic liquid (H2SO4), aqueous acid (H3PO4) and inorganic salt ((NH4)2SO4). Different sonication times (15, 30, 45 and 60 min) at room temperature were used to assist the electrochemical exfoliation. The graphene that was produced was characterized by the zeta potential, surface electrical conductivity, X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, field emission microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM) and Xray photoelectron spectroscopy (XPS). From the XRD analysis, it was observed that the graphene flakes had a broad peak centred at 26.5°, which corresponds to the (002) plane of graphene crystal. The FTIR spectrum revealed that the prominent peak was in conjunction with exfoliated graphene. The characteristic Raman bands were observed for the D, G and 2D bands and the ratio of I2D/IG was 1.7, which indicated that the exfoliated graphene was a multilayer structure. From the results, it was found that graphene obtained with the H2SO4 electrolyte that used a 45 min sonication time had the highest stability, best electrical conductivity, lowest crystallite size, fewer defects and lowest interlayer distance. Keywords: graphene; electrochemical exfoliation; sonication; electrical conductivity
1. Introduction In recent years, enormous effort has been devoted to the straightforward, rapid, environmentally friendly and low-cost electrochemical exfoliation of graphite to produce high-quality graphene [1]. Compared to other traditional chemical methods, this method is promising for the mass production of graphene [2]. In addition, functionalized graphene can be produced during the exfoliation process. Parameters that must be controlled during the processing are the number of probes, applied current and chemicals used. The principles are largely the same because the process generally involves the use of an electrolyte (e.g. aqueous or non-aqueous solution) and an electrical current to drive structural expansion at a graphite electrode [3]. According to the charge of the intercalated ions, the graphite electrode works as an anode or cathode and represents oxidation or reduction reactions, respectively.
In the electrochemical exfoliation process, when the direct current voltage was applied to the two-electrode setup, vigorous bubbles were produced at both two electrodes and anodic graphite began to dissociate into the electrolyte [4]. In contrast to the other exfoliation processes, this method is not equipment intensive and is typically performed under ambient conditions [5]. Moreover, it is eco-friendly compared to other chemical and sonication routes, which often involve hazardous reagents or solvents [6]. The yield, productivity and properties of graphene are dependent on the electrolytes. Aqueous electrolytes are industrially attractive because of the natural abundance and sustainability of water. The formation of graphite intercalation compounds (GICs) in strong acids and the physical expansion and exfoliation towards thin flakes have been long known, but did not attract significant attention until successful isolation of pristine graphene by micromechanical cleavage was reported by Novoselov et al. [7]. Table 1 shows a summary of previous work on various types of aqueous solutions used for the exfoliation process. Electrochemical exfoliation of graphite has been mostly performed in three different electrolytes: ionic-liquid, aqueous acids and aqueous inorganic salts. Parvez et al. [8] reported that exfoliation in ionic-liquids resulted in a low yield of graphene, small lateral size (<5 µm) and it was often functionalized with the ionic liquids, which disrupted the electronic properties of the graphene. Exfoliation in acidic electrolytes can yield graphene with a better quality and a larger lateral size, but a significant amount of oxygen-containing functional groups cannot be avoided due to the over-oxidation of graphite by the acid [9]. On the other hand, an inorganic salt electrolyte solution was able to effectively reduce the oxidation degree and thereby significantly improved the chemical and electronic properties of the graphene [10]. Thin-layer graphene sheets were able to obtain at a high yield with large flake size and the graphene could be produced on a scale of tens of grams. This demonstrated the great potential for the scale-up required for industrial production. Ionic liquids, such as N-butyl, methylpyrrolidiniumbis (trifluoromethylsulfonyl)-imide and (IL)–acetonitrile can be used in the electrochemical exfoliation process. Sulphuric acid (H2SO4), hyperchloric acid (HCLO4), sodium chloride (NaCl), sodium hydroxide (NaOH) and hydrogen peroxide (H2O2) are common types of acidic aqueous solutions used in previous work. Aqueous inorganic salts, such as ammonium sulphate ((NH4)2SO4) and sodium sulfate (Na2SO4), were used in the electrolysis process. A working bias in the range of 3 to 10V has been used for the electrochemical exfoliation process, and a generally high working bias will easily exfoliate the graphite layers. Results of the carbon (C) and oxygen (O2) atomic ratio showed the presence of a tiny amount of oxygen originating from the oxidation of the graphite by OH-1 ions during the electrochemical process. Moreover, Parvez et al. [11] reported the intensity between the D and G band, or I2D/IG, for the exfoliated graphene was 0.67 for the H2SO4 aqueous solution. The intensity ratio of the G and 2D bands, as well as their relative positions, can be used to determine the number of graphene layers. In most of the reports of electrochemical exfoliation of graphite, the exfoliated product was collected by vacuum filtration and repeatedly washed with water to remove any residual salts. In this study, the sonication method was introduced after the exfoliation to ease the exfoliation process of the graphene nanosheets. A thorough literature review indicated that
limited studies have been done to evaluate the combination effect of different electrolytes and sonication times on the properties of the graphene. Selection of a suitable electrolyte is important to balance the quantity and quality of the graphene. In the present study, graphene was produced through the electrochemical exfoliation of graphite rods using three different types of electrolytes: acidic aqueous (H3PO4 and H2SO4) and inorganic salts ((NH4)2SO4/water). The effect of sonication on the graphene produced by H2SO4 aqueous solution was investigated. Different sonication times (15, 30, 45 and 60 min) were also utilized and the graphene produced from the exfoliation process was characterized with Fourier-transform infrared spectroscopy (FTIR), Raman, X-ray diffraction (XRD) and electrical conductivity measurements.
2. EXPERIMENTAL 2.1 Materials A graphite rod with a diameter of 6.35 mm was supplied by Sigma-Aldrich. Sulphuric acid (H2SO4), phosphoric acid (H3PO4) and ammonium sulphate (NH4)2SO4 were supplied by Merck assay and used as electrolytes. Ethanol supplied by J.T. Baker was used as a cleaning agent. Distilled water was used in all of the synthesis processes. 2.2 Synthesis of graphene by electrochemical exfoliation In this study, the electrochemical synthesis of graphene was done in a conventional twoelectrode system. The exfoliation was conducted with a graphite rod that was used as a working electrode and a platinum (Pt) rod was treated as the counter electrode. The Pt rod was placed parallel to the graphite rod at a distance of about 2 cm. A 50 ml solution of 0.5 M H2SO4 and H3PO4 were used as the electrolyte. Distilled water containing 0.1 M (NH4)2SO4 was the other electrolyte used in the study. The direct current (DC) bias voltage was set to 10 V and remained until the full consumption of the anodic graphite. After the exfoliation process, the graphene solution was sonicated for 15, 30, 45 and 60 min at room temperature and washed several times by distilled water using vacuum filtration. The powder was obtained after drying in a vacuum oven at 80°C. 2.3 Material Characterization The zeta potential of graphene was measured using a Malvern Nano series ZEN3600. The surface electrical conductivity of the graphene was measured using a Prostat PRS-812 resistance meter measurement system. Images of the graphene solutions were obtained by preparing 1 mg/ml graphene in dimethylformamide (DMF) solvent, and the images were taken with a digital camera. The XRD spectra were obtained with a Bruker D8 X-ray diffractometer with a Ni-filtered Cu-Kα (λ = 1.54021Å) radiation source and were used to identify the crystallographic structure of the graphene. The FTIR spectra were obtained with a Perkin Elmer FTIR spectrometer at room temperature with a typical wave number of 4000500 cm-1 in open air and transmittance mode. The Raman spectra of graphene were measured by a Renishaw in Via spectrometer with a 633 nm excitation. The morphology of the asproduced graphene was analysed using field emission scanning electron microscopy (FESEM, model Zeiss Supra 35VP) and high-resolution transmission electron microscopy (HRTEM, model Teenai G2 20 s-twin, Fei). The X-ray photoelectron (XPS) spectra were obtained using AXID Uitra DLD, kratos, equipped with Al Kα X-ray source (1486.6eV).
3. Result and discussion The zeta potential analysis was used to determine the stability of the exfoliated graphene. Table 2 shows zeta potential values for the exfoliated graphene using different electrolytes and different sonication times. The zeta potential for graphene in H 2SO4 electrolyte without
sonication was -12 mV. This value is lower than other H2SO4 solutions with sonication because the zeta potential values for these electrolytes was between -32 mV to -38 mV. Nanoparticles with zeta potential values greater than +25 mV or less than -25 mV typically exhibit high degrees of stability because of the interparticle electrostatic repulsion [19]. These results indirectly indicated that the graphene in the H2SO4 electrolyte after sonication was more stable. By using sonication, the values of the zeta potential for H 2SO4, (NH4)2SO4 and H3PO4 all increased. The values are varied from -32 mV to -38 mV, -32 mV to -35 mV and 25 mV to -33 mV, respectively. The results indicated that the highest zeta potential value was for the H2SO4 electrolyte sonicated at 45 and 60 minutes.Based on Table 2, it was found that the electrical conductivity of all exfoliated graphene varied with sonication time. The lowest electrical conductivity occurred for graphene in H2SO4 without sonication. The highest electrical conductivity occurred for graphene in H2SO4 electrolytes at 45 min and 60 min sonication times. Figure 1 shows digital images that indicate the dispersion of the exfoliated graphene obtained with the different electrolytes at different sonication times. The images of the solutions to identify dispersion and sedimentation of the graphene were for 0 days, after 5 weeks and after 12 weeks. Just after sonication, graphene showed very good dispersion in the electrolytes. On the other hand, the graphene obtained using (NH4)2SO4 and H3PO4 produced after 15 min sonication tended to precipitate after 5 weeks. The graphene produced using longer sonication times were observed to be stable in the DMF solvent compared to the short sonication time. It was interesting to see that graphene obtained using the H2SO4 electrolyte remained homogeneous as a black dispersion and without any visible sedimentation even after 12 weeks. The crystal structure and interlayer spacing of the graphite rod and exfoliated graphene were determined with XRD. Figure 2(a) shows the purification of the raw graphite powder and exhibits three intense peaks at approximately 26°, 42° and 54° that correspond to the (002), (101) and (004) crystallographic planes, respectively [21]. Figures 2(b), (c) and (d) show the XRD patterns for the electrochemically exfoliated graphene nanosheets produced using the H2SO4, H3PO4 and (NH4)2SO4 electrolytes and sonication for different times (15, 30, 45 and 60 min), respectively. Generally, the peaks at 2θ=26° and 2θ=54° that represent the (002) and (004) planes, similar to graphite, were observed for all graphene produced using different electrolytes at different sonication times. However, the peak at 2θ=26° broadened and shifted towards the lower angle. This may be attributed to its corrugated structure and the increase of interlayer spacing within graphitic structures, which indicated the presence of graphene [22]. The XRD results proved the successful exfoliation of graphite to producing graphene, which is in good agreement with previous work by Sahoo and Mallik [23]. The average crystallite sizes for the graphene are shown in Table 3. It was found that the crystallite size decreased with increasing sonication times. The graphene produced using the H2SO4 electrolyte without sonication produced graphene with bigger crystallite sizes compared to the other graphene samples. The decrease in crystallite size of the exfoliated graphene produced using sonication occurred because of the cavitation energy that was generated by the ultrasonication process [23]. The FTIR analysis was performed to examine the chemical structure and any possible functional groups present in the exfoliated graphene. Figure 3(a) shows the FTIR spectra for the electrochemically exfoliated graphene produced using H2SO4 at different sonication
times. After 15 min of sonication, the samples were free from any non-bonded SO42- ions due to the absence of peaks at around 980 cm-1. An intense and broad peak at 3411 cm-1 was observed in all the spectra due to the stretching vibration of the O-H functional group. The small peak for C-C stretching of sp3 hybridized carbon atoms appeared at around 858 cm-1 for the 15 min sonication case. The peak for C-O stretching appeared at around 1040 cm-1 for 30 min, 45 and 60 min sonication times. The aromatic sp2 carbon ring and C=C stretching vibration were observed at 1579 cm-1 and 1582 cm-1 in all spectra. Meanwhile, the small peak at 1374 cm-1 indicated the presence of O-H bending. The weak peak at 1124 cm-1 was attributed to the C=O stretching vibration. On the other hand, the additional peak at 1654 cm1 corresponded to the C=O stretching vibration for the 45 min and 60 min sonication times. Figure 3(b) shows the FTIR spectra for the electrochemically exfoliated graphene produced using (NH4)2SO4 at different sonication times. In general, similar peaks were observed in the spectra for the exfoliated graphene produced at different sonication times. The broad peak at 3412 cm-1 corresponded to the stretching vibration of the O-H functional group for all sonication times. The C-H deformation appeared at 996 cm-1 after 15 min sonication. The weak peaks at 1624 and 1631 cm-1 corresponding to the C=C stretching vibration can be seen clearly for the 30, 45 and 60 min sonication times. This C=C bond peaks indicated the skeletal vibration of the graphitic zone or a portion from the stretching vibration of exfoliation. The peak for the C-O-C stretching vibration appears at 1062 cm-1 for the 15 min sonication time. Figure 3(c) shows the FTIR spectrum for the electrochemically exfoliated graphene produced using (NH4)2SO4 at different sonication times. The characteristic FTIR features of exfoliated graphene include the presence of different types of oxygen functionalities, which were confirmed by the band at 3441 cm-1 that corresponded to the O-H stretching vibration. The characteristic peaks responsible for hydroxylation arose at 2924 and 2845 cm-1 because of the asymmetric and symmetric stretching vibration of C-H groups for the 45 min and 60 min sonication times, respectively. The small peak for C=C stretching vibration appeared at 1578 cm-1. On the other hand, an additional peak was observed 1043 cm-1, which was due to the C-O stretching vibration. Thus, the FTIR data confirmed that the oxygen functional group was introduced into the graphene during the exfoliation process using all electrolytes. The FTIR peaks in this study are consistent with previously published reports using H 2SO4 as the electrolyte [24]. The Raman spectra for the exfoliated graphene obtained with different electrolytes and different sonication times are presented in Figures 4(a), (b) and (c). From the spectra, three important features can be observed, namely the D peak located at ~1335 cm-1, the G peak located at ~1580 cm-1 and the D peak located at ~2700 cm-1 [25]. The D band originates from the breathing mode of the six-atom rings and requires a defect for activation. Therefore, the intensity of the D band depends on the disorder of the graphene. The G peak represents the E2g phonons vibrations mode in the centre of the Brillouin Zone of the sp2 hybrid carbon atom. All the sp2 carbon materials exhibited a very sharp peak around 2500-2800 cm-1 in the Raman spectra. A sharp 2D band in the Raman spectra is a signature of the sp2 graphitic system, a second-order two phonon process and its frequency is dependent on the energy of the excitation laser [26].
The ID/IG ratio is normally used to evaluate the extent of defects present in graphene and other carbon materials. Alternatively, the number of graphene layers present in the sample could be estimated by calculating I2D/IG. The ratio increased as the number of graphene layers decreased [27]. When graphene is exfoliated from graphite, there is a significant increase in the number of sheet edges from the exfoliated sheets leading to more prominent D peak. Table 4 shows the calculation of the defect ratio values and number of graphene layers for the exfoliated graphene. Here, all exfoliated graphene defect values were nearly same, around 0.83. The graphene produced by different electrolytes and different sonication times exhibited an I2D/IG ratio of around 1.67, which indicated that the graphene was multilayered. Furthermore, the Raman spectra for the exfoliated graphene obtained at lower sonication times displayed another peak at 2924 cm-1, where the D+D' corresponds to the defects produced on the synthesized graphene. However, these D+D' peaks were absent from graphene obtained at different electrolytes for the 45 min sonication time in different electrolytes. These results revealed that graphene sheets with a relative low defect density were obtained by using the 45 min sonication time. The morphology of the graphite and exfoliated graphene sheets was investigated by FESEM. Figure 5(a) shows FESEM images for pristine graphite, which includes the bulk structure and thick graphite flakes. After exfoliation in the H2SO4 solution without sonication, the flakes and thin sheets were observed. However, agglomeration of the sheets is clearly seen in Figure 5(b). The images of graphene obtained after 15 min sonication in different electrolytes are shown in Figures 5(c), 6(a) and 7(a). Dense agglomerates had a layered structure and the nanoflakes had a curved, wrinkled and crumpled morphology. The curved morphology prevented the graphene sheets from restacking among one another and contributed to the mesoporous nature. The mesoporous structure and the wrinkles on the graphene surface sheets may have shortened the ion diffusion lengths and ensured the full utilization of the graphene nanoflakes with increased sonication time. In addition, multilayered sheets that were stacked into a crumpled and wrinkled structure can be observed at the edge of all the as-produced graphene samples using different electrolytes and lower sonication times. The crumpled and wrinkled formation in graphene is induced by defects, like boundaries or dislocations. The crumpling at the edges was caused by the insertion of functional groups carrying sp3 hybridized carbon atoms, which were introduced during the exfoliation process [28, 29]. Figures 5(d), 6(b) and 7(b) show the morphology for samples produced with the 45 min sonication time and different electrolytes. The images show partial transparency, which indicates that the multilayered graphene sheets were stacked together and had a smooth surface. However, the 45 min sonication times showed well-separated graphene sheets and a lesser wrinkled and crumpled structure, which suggested that the quality of the graphene obtained at 45 min was better than for the other sonication times. After 45 min sonication, the morphologies become wrinkled, and the defect structure for the exfoliated graphene is shown in Figures 5(e), 6(c) and 7(c). The can be explained by phenomenon of acoustic cavitation. The ultrasound generated microbubbles that collided with the graphene layer. The microbubbles collapsed and generated a large local energy due to the cavitation effects. The magnitude of the local energy intensity was proportional to the number of bubbles that collapsed during cavitation and this was equivalent to the duration of the sonication [30].
The morphology of the exfoliated graphene prepared with different electrolytes and 45 min sonication time was determined with HRTEM. Figures 8(a)(ⅰ), (b)(ⅰ) and (c)(ⅰ) show the HRTEM images for the exfoliated graphene produced using different electrolytes. The images indicated a transparent sheet-like structure and suggested that the obtained graphene was comprised of thin layers and a relatively smooth planar structure. Figure 8(b)(ⅰ) showed the multilayered graphene sheets stacked together and crumpling of the edges (depicted from circle mark in (NH4)2SO4 electrolyte solution) due to oxygenation and hydroxylation. This originated due to various defects and functional groups carrying the sp3 hybridized carbon atoms. This result supports the finding from the previous FTIR analysis, which is that they were introduced during the exfoliation process. In addition, a fold was observed at the edges, as shown by arrow in Figures 8(a)(ⅰ), (b)(ⅰ) and (c)(ⅰ). As a result of the high surface area of the extended thin layers, the graphene sheets had a significant tendency to overlap, which caused the edges to partially fold. The lines with the darker contrast indicate the folds. The folding was due to the carbon-carbon rings being destroyed by groups during the oxidation process. The surface energy was reduced so that the graphene sheets became stable [31]. The interlayer distance for the three different electrolytes was (a) 0.3 nm, (b) 0.36 nm and (c) 0.39 nm, respectively. The H2SO4 electrolyte with the 45 min sonication was the best based on crystallite size, surface electrical conductivity, defect morphology and interlayer distance. Thus, this sample was selected for further characterization using XRD, surface electrical conductivity, FESEM and HRTEM. The crystallographic structure of the graphene sheets produced using H 2SO4 and (NH4)2SO4 after 45 min sonication was characterized by selected area electron diffraction (SAED). Figure 9(a) exhibits a typical six-fold symmetric diffraction with the hexagonal crystal structure for the H2SO4 electrolyte exfoliated graphene. Several diffraction spots in the hexagonal pattern were identified and suggested that the selected graphene sheets were multi-crystal and the obtained graphene products included bilayer graphene sheets. The exfoliated graphene obtained with the (NH4)2SO4 electrolyte in Figure 9(b) had a diffuse diffraction ring pattern that may indicate polycrystalline features in an amorphous material. The increase in the amorphous nature was because of the crawling of the edges and surface functionalization on the graphene. For this reason, the SAED patterns showed that the graphene obtained with the H2SO4 electrolyte was better than the graphene obtained with the (NH4)2SO4 electrolyte and 45 min sonication time. The XPS was performed to determine the surface characterization, chemical composition, types of carbon and oxygen bonds and the percentage of oxygen present in the as-exfoliated graphene. Figures 10(a) and (b) show the XPS spectra for H2SO4 and (NH4)2SO4 electrolytes after 45 min sonication. The peak corresponding to C1s appeared at 284 eV, which indicated sp2 hybridized carbon atoms in the graphene sheets [32]. The peaks at 532 eV, which relate to O1s, indicated the presence of various oxygen functionalization in the carbon network structures [33]. Table 5 shows the carbon and oxygen C/O ratio that was obtained through elemental analysis measurement by XPS. The graphene prepared with H2SO4 and (NH4)2SO4 had a 16.72% and 22.59% weight ratio of oxygen, respectively. The presence of oxygen was attributed to the oxidation of the graphite by hydroxyl ions (OH -) during the electrochemical process [36]. The separated C1s peak, as seen in Figure 10, shows the peaks corresponding to C=C, C-C, C-O and C=O bond in Figure 11. The main peak at the binding energy of 284.3 eV was assigned to the C=C bond to represent sp2 hybridized carbon atoms in the graphene
sheet. The smaller peak at 285 eV was assigned to C-C sp3 hybridized carbons. The peak at 286.6 eV corresponded to the C-O groups (hydroxyl and epoxy), and the peak at about 287.9 eV was assigned to C=O carbonyl groups [34]. The use of different electrolytes during electrochemical exfoliation of graphite generated graphene with different densities of defects and also different contents of oxygen functionalities. The presence of the oxygen functional groups shown in the XPS spectra is consistent with the FTIR results.
4. Conclusions In summary, a simple electrochemical exfoliation process was compared for a combination of different types of electrolytes and sonication times. From the zeta potential and electrical conductivity measurements, the exfoliated graphene showed a very stable dispersion at higher sonication times and an increase in the electrical conductivity. It was also observed that the defects in the exfoliated graphene sheets decreased with an increase the sonication time, which was confirmed from the morphology and Raman studies. The XRD, HRTEM and XPS analyses showed the H2SO4 electrolyte with the 45 min sonication time produced graphene with the lowest crystallite site and lowest interlayer distance. Therefore, in this study, the H2SO4 electrolyte with the 45 min sonication time was considered as the most suitable.
Acknowledgements The authors gratefully acknowledge ASEAN University Network for Science and Engineering Education Development Network (AUN/SEED-Net) Project and Japan International Cooperation Agency (JICA) and Malaysia Government (FRGS grant 6071385) for their financial support. They would also like to thank the Universiti Sains Malaysia (USM) and the School of Materials and Mineral Resources Engineering for the use of their facilities.
References [1] C. Xu, B. Xu, Y. Gu, Z. Xiong, J. Sun, X.S. Zhao, Graphene-based electrodes for electrochemical energy storage. Energy & Environmental Science, 6(5), pp (2013)1388-1414. [2] Y.L Zhong, Z. Tian, G.P Simon, D. Li, Scalable production of graphene via wet chemistry: progress and challenges, Materials Today 18(2), pp. (2015) 73-78. [3] S. Yang, M.R. Lohe, K. Müllen, X. Feng, New‐ Generation Graphene from Electrochemical Approaches: Production and Applications, Advanced Materials, 28(29), pp. (2016) 6213-6221. [4] A.M Abdelkader, A.J Cooper, R.A.W Dryfe, I.A Kinloch, How to get between the sheets: a review of recent works on the electrochemical exfoliation of graphene materials from bulk graphite, Nanoscale, 7(16), pp. (2015) 6944-6956. [5] U. Khan, A. O'Neill, M. Lotya, S. De, J.N Coleman, High‐ Concentration Solvent Exfoliation of Graphene, Small, 6(7), pp. (2010) 864-871.
[6] S.K Sahoo, A. Mallik, Synthesis and characterization of conductive few layered graphene nanosheets using an anionic electrochemical intercalation and exfoliation technique, Journal of Materials Chemistry C, 3(41), pp. (2015)10870-10878. [7] A Richenderfer, Plasma-assisted Electrochemical Synthesis of Pristine Graphene Sheets. K. ar e . u . i . iu . ra . en K. en, Exfoliation of graphite into graphene in aqueous solutions of inorganic salts, Journal of the American Chemical Society, 136(16), pp. (2014) 6083-6091. [9] C.Y Su, A.Y Lu, Y. Xu, F.R. Chen, A.N. Khlobystov, L.J. Li, ACS Nano 5, 2332. CrossRef| PubMed| CAS| Web of Science® Times Cited, (2011)178. 10 K. ar e . . u . i . iu . ra . en K. en, Exfoliation of graphite into graphene in aqueous solutions of inorganic salts, Journal of the American Chemical Society, 136(16), pp. (2014) 6083-6091. 11 K. ar e . i . . uniredd . ernande . inke . an . en K. en, Electrochemically exfoliated graphene as solution-processable, highly conductive electrodes for organic electronics, ACS nano, 7(4), pp. (2013) 3598-3606. [12] Y. Yang, F. Lu, Z. Zhou, W. Song, Q. Chen, X. Ji, Electrochemically cathodic exfoliation of graphene sheets in room temperature ionic liquids N-butyl, methylpyrrolidinium bis (trifluoromethylsulfonyl) imide and their electrochemical properties. Electrochimica Acta, 113, pp. (2013) 9-16. [13] A.T Najafabadi, E. Gyenge, High-yield graphene production by electrochemical exfoliation of graphite: Novel ionic liquid (IL)–acetonitrile electrolyte with low IL content, Carbon, 71, pp. (2014) 58-69. [14] S.K. Sahoo, A. Mallik, Synthesis and characterization of conductive few layered graphene nanosheets using an anionic electrochemical intercalation and exfoliation technique, Journal of Materials Chemistry C, 3(41), pp. (2015) 10870-10878. [15] P. Tripathi, C. Patel, R. Prakash, M.A. Shaz, O.N. Srivastava, Synthesis of high-quality graphene through electrochemical exfoliation of graphite in alkaline electrolyte, arXiv preprint arXiv: (2013) 1310-7371. [16] K.S. Rao, J. Senthilnathan, Y.F. Liu, M. Yoshimura, Role of peroxide ions in formation of graphene nanosheets by electrochemical exfoliation of graphite, Scientific reports, 4, p. (2014) 4237. [17] K. Chen, D. Xue, Preparation of colloidal graphene in quantity by electrochemical exfoliation, Journal of colloid and interface science, 436, pp. (2014) 41-46. [18] K. ar e . u . i . iu . ra . en K. en, K, Exfoliation of graphite into graphene in aqueous solutions of inorganic salts, Journal of the American Chemical Society, 136(16), pp. (2014) 6083-6091. [19] S. Kashyap, S. Mishra, S.K. Behera, Aqueous colloidal stability of graphene oxide and chemically converted graphene, Journal of Nanoparticles (2014).
[20] I. Jeon, Synthesis of functionalized few layer graphene via electrochemical expansion, Doctoral dissertation, Massachusetts Institute of Technology (2015) [21] S.K. Sahoo, A. Mallik, Simple, fast and cost-effective electrochemical synthesis of few layer graphene nanosheets, Nano, 10(02), p. (2015) 1550019. [22] S. Sumari, A. Roesyadi, S. Sumarno, Effects of ultrasound on the morphology, particle size, crystallinity, and crystallite size of cellulose, Scientific Study & Research. Chemistry & Chemical Engineering, Biotechnology, Food Industry, 14(4), p. (2013) 229. [23] S.K. Sahoo, A. Mallik, Simple, fast and cost-effective electrochemical synthesis of few layer graphene nanosheets, Nano, 10(02), p. (2015) 1550019. [24] Ambrosi, Adriano, et al. "Electrochemistry of graphene and related materials." Chemical reviews 114.14 (2014): 7150-7188. [25] Ferrari, Andrea C., et al. "Raman spectrum of graphene and graphene layers." Physical review letters 97.18 (2006): 187401. [26] Tiwari, Santosh K., et al. "Facile electrochemical synthesis of few layered graphene from discharged battery electrode and its application for energy storage." Arabian Journal of Chemistry 10.4 (2017): 556-565. [27] Wu, Zhong‐ Shuai, et al. "Alternating stacked graphene‐ conducting polymer compact films with ultrahigh areal and volumetric capacitances for high‐ energy micro‐ supercapacitors." Advanced Materials 27.27 (2015): 4054-4061. [28] Tiwari, Santosh K., et al. "Facile electrochemical synthesis of few layered graphene from discharged battery electrode and its application for energy storage." Arabian Journal of Chemistry 10.4 (2017): 556-565. [29] Chee, W. K., et al. "Performance of flexible and binderless polypyrrole/graphene oxide/zinc oxide supercapacitor electrode in a symmetrical two-electrode configuration." Electrochimica Acta 157 (2015): 88-94. [30] Pan, C-T., et al. "In-situ observation and atomic resolution imaging of the ion irradiation induced amorphisation of graphene." Scientific reports 4 (2014): 6334. [31] Parvez, Khaled, et al. "Exfoliation of graphite into graphene in aqueous solutions of inorganic salts." Journal of the American Chemical Society 136.16 (2014): 6083-6091. [32] S.K. Sahoo, A. Mallik, Synthesis and characterization of conductive few layered graphene nanosheets using an anionic electrochemical intercalation and exfoliation technique, Journal of Materials Chemistry C, 3(41) (2015) 10870-10878. [33] D, Yang, A, Velamakanni, G, Bozoklu, S, Park, M, Stoller, R.D Piner, R.S. Ruoff, Chemical analysis of graphene oxide films after heat and chemical treatments by X-ray photoelectron and Micro-Raman spectroscopy, Carbon 47(1) (2009) 145-152. [34] A.A.B Hamra, Electro-exfoliating graphene from graphite for direct fabrication of supercapacitor, Applied Surface Science 360 (2016) 213-223.
Figure captions Figure 1. Photographs of 1mg/ml graphene dispersion in DMF solvent. The photograph was taken (a) just after sonication, (b) after 5 weeks and (c) after 12 weeks. Figure 2. (a) XRD spectrum of graphite. (b) XRD spectrum of graphene at H2SO4 at different sonication times 0, 15, 30, 45, 60 min. (c) XRD spectrum of graphene at (NH4)2SO4 at different sonication times 0, 15, 30, 45 60 min. (d) XRD spectrum of graphene at H3PO4 at different sonication times 0, 15, 30, 45 60 min. Figure 3. (a) FTIR spectrum of graphene at H2SO4 at different sonication times 15, 30, 45 60 min. (b) FTIR spectrum of graphene at (NH4)2SO4 at different sonication times 15, 30, 45 60 min. (c) FTIR spectrum of graphene at H3PO4 at different sonication times 15, 30, 45 60 min. Figure 4. (a) Raman spectra of exfoliated graphene at Sulphuric acid (H 2SO4) electrolytes in different sonication times 15, 30, 45 and 60 min. (b) Raman spectra of exfoliated graphene at Ammonium Sulphate (NH4)2SO4 electrolytes in different sonication times 15, 30, 45 and 60 min. (c) Raman spectra of exfoliated graphene at sulphuric acid (H 3PO4) electrolytes in different sonication times 15, 30, 45 and 60 min. Figure 5. FESEM images of graphene obtained at (a) pure graphite, after exfoliated H2SO4 electrolytes in (b) without sonication time and different sonication times (c) 15 min, (d) 45 min and (e) 60 min. Figure 6. FESEM images of graphene obtained at (NH4)2SO4 electrolytes in different sonication times (a) 15 min, (b) 45 min and (c) 60 min. Figure 7. FESEM images of graphene obtained at H3PO4 electrolytes in different sonication times (a) 15 min, (b) 45 min and (c) 60 min. Figure 8. HRTEM images of graphene obtained using (a) H2SO4, (b) (NH4)2SO4 and (c) H3PO4 electrolytes at 45 min sonication time at magnifications of (ⅰ) 29 KX and (ⅱ) 690 KX. Figure 9. SAED pattern of the graphene obtained at (a) H2SO4 and (b) (NH4)2SO4 at 45 min sonication time. Figure 10. Wide scan XPS spectra of graphene obtained at (a) H 2SO4 (b) (NH4)2SO4 electrolyte in 45 min sonication time. Figure 11. Deconvolution of C1s spectra of graphene obtained at (a) H 2SO4 and (b) (NH4)2SO4 electrolyte in 45 min sonication time.
Just sonicated After 5 weeks After 12 weeks
15 30 45 60
15 30 45 60
15 30 45 60
H2SO4
15 30 45 60
15 30 45 60
15 30 45 60
(NH4)2 SO 4
15 30 45 60
15 30 45 60
H 3PO4
Figure 1
15 30 45 60
Figure 2 (a)
Figure 2 (b)
Figure 2 (c)
Figure 2 (d)
Figure 3 (a)
Figure 3 (b)
Figure 3 (c)
Figure 4 (a)
Figure 4 (b)
Figure 4 (c)
a
b
Flake shape
1 µm
1 µm
c
d
wrinkled
Smooth layer 1 µm
1 µm
e
wrinkled
1 µm
Figure 5
a
c
wrinkled
Smooth layer 1 µm
1 µm
d
crumpled
1 µm
Figure 6
c
a
Smooth layer
crumpled
wrinkled 1 µm
1 µm
d
wrinkled
1 µm
Figure 7
a
a( )
0.3 nm
Fold edge 200 nm
5 nm
b
b( )
crumpling 0.36 nm
Fold edge
200 nm
5 nm c( )
c
0.39 nm
Fold edge
5 nm
200 nm
Figure 8
(b)
(a)
Figure 9
Figure 10 (a)
Figure 10 (b)
Figure 11 (a)
Figure 11 (b)
Table list Table 1. Summary of previous works on the formation of graphene based on aqueous solution electrolyte
Table 2 Zeta potential values and electrical conductivity properties of exfoliated graphene Table 3 The average crystallite size of exfoliated graphene at different electrolytes different sonication time Table 4 Calculation of ID/IG and I2D/IG from the Raman spectra. Table 5 Atomic weight percentage of element in multi-layered graphene obtained from H2SO4 and (NH4)2SO4 electrolytes in 45 min sonication time.
Aqueous inorganic salts
Acidic aqueous solutions
Ionic-liquid
Table 1: Summary of previous works on the formation of graphene based on aqueous solution electrolyte
Types of Electrolytes
Starting material
Working bias
C/O (XPS)
I2D/IG (Raman)
Numbers of graphene layers
Ref.
N-butyl, methylpyrro lidiniumbis( trifluoromet hylsulfonyl) -imide
Graphite rod
15-30V
36
1.69
Bi-layer
[12]
(IL)– acetonitrile
Graphite rod
7V
-
0.63
Few-layers
[13]
10V
12.3
0.67
1-3 layers
[11]
3-10V
0.60
-
3-6 layers
[14]
3
1-4 layers
[15]
NaOH/H2O2 /H2O (NH4)2SO4/ water
Graphite flakes Graphite sheet Graphite foil Graphite rod Graphite rod
Na2SO4/ water
Graphite rod
H2SO4 HCLO4 KOH
3V 3V
17.18
1.69
Bi-layer
[16]
10V
17.2
-
Bi-layer
[17]
5V
-
1.7
Bi-layer
[18]
Table 2: Zeta potential values and electrical conductivity properties of exfoliated graphene Electrolytes
Sonication time (min)
Zeta potential
Electrical Conductivity (Sm-1)
H2SO4
0, 15, 30, 45 and 60
-12, -32, -34 -38 and -38
96. 5, 231, 458, 1041, 1041
(NH4)2SO4
15, 30, 45 and 60
-30, -32, -35 and -35
220, 424, 997, 997
H3PO4
15, 30, 45 and 60
-25, -27, -33 and -33
216, 365, 965, 965
Table 3. The average crystallite size of exfoliated graphene at different electrolytes different
Electrolytes
Sonication time (min)
H2SO4
Crystallite sizes (nm)
0, 15, 30, 45 and 60
12.4, 8.7, 8.3 and 8.3
(NH4)2SO4
15, 30, 45 and 60
9.5, 9.1, 8.5 and 8.5
H3PO4
15, 30, 45 and 60
9.6, 9.2, 8.4 and 9.2
sonication time
Table 4: Calculation of ID/IG and I2D/IG from the Raman spectra Electrolytes
Sonication time (min)
ID
IG
I2D
ID+D'
ID/IG
I2D/IG
H2SO4
15, 30, 45, 60
1338, 1336, 1342, 1342
1595, 1602, 1578, 1578
2667, 2667, 2684, 2684
2924,2924,,-
0.85, 0.85, 0.85, 0.85
1.67,1.66,1. 7,1.7
(NH4)2SO4
15, 30, 45, 60
1348, 1335, 1342,1347
1602,1595, 1578, 1595
2668, 2684, 2684, 2684
2924,2924,,2924
0.84, 0.83, 0.83, 0.83
1.67,1.68,1. 7,1.7
H3PO4
15, 30, 45, 60
1332, 1338, 1327, 1318
1598, 1604, 1579, 1609
2667, 2667, 2684, 2669
2924,2924,2 924,2924
0.83, 0.83, 0.81, 0.81
1.66,1.67, 1.69, 1.65
Table 5: Atomic weight percentage of element in multi-layered graphene obtained from H2SO4 and (NH4)2SO4 electrolytes in 45 min sonication time Electrolytes
C (wt%)
O (wt%)
C/O
H2SO4
83.28
16.72
4.98
(NH4)2SO4
77.41
22.59
3.42
Highlights:
1. The lowest electrical conductivity occurred for graphene in H2SO4 without sonication. 2. The graphene produced using longer sonication times were observed to be stable in the DMF solvent compared to the short sonication time.
3. The decrease in crystallite size of the exfoliated graphene produced using sonication occurred because of the cavitation energy that was generated by the ultrasonication process.
S0169-4332(18)33096-4 https://doi.org/10.1016/j.apsusc.2018.11.029 APSUSC 40874
To appear in:
Applied Surface Science
Received Date: Accepted Date:
6 September 2018 4 November 2018
Please cite this article as: Y.Z.N. Htwe, W.S. Chow, Y. Suda, A.A. Thant, M. Mariatti, Effect of electrolytes and sonication times on the formation of graphene using an electrochemical exfoliation process, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.11.029
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of electrolytes and sonication times on the formation of graphene using an electrochemical exfoliation process Y.Z.N. Htwea, W.S. Chowa, Y. Sudab and A.A. Thantc, M. Mariattia* a,a*
School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Pulau Pinang, Malaysia b
Department of Electrical and Electronic Engineering, Toyohashi University of Technology, Aichi Toyohashi 441-8580, Japan c
Department of Physics, University of Yangon, 11041 Kamayut, Yangon, Myanmar
Corresponding author: [email protected]
ABSTRACT: The exfoliation of graphite is an intriguing approach for simple, fast and largescale production of graphene. In the present study, three types of electrolytes were investigated, namely an ionic liquid (H2SO4), aqueous acid (H3PO4) and inorganic salt ((NH4)2SO4). Different sonication times (15, 30, 45 and 60 min) at room temperature were used to assist the electrochemical exfoliation. The graphene that was produced was characterized by the zeta potential, surface electrical conductivity, X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, field emission microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM) and Xray photoelectron spectroscopy (XPS). From the XRD analysis, it was observed that the graphene flakes had a broad peak centred at 26.5°, which corresponds to the (002) plane of graphene crystal. The FTIR spectrum revealed that the prominent peak was in conjunction with exfoliated graphene. The characteristic Raman bands were observed for the D, G and 2D bands and the ratio of I2D/IG was 1.7, which indicated that the exfoliated graphene was a multilayer structure. From the results, it was found that graphene obtained with the H2SO4 electrolyte that used a 45 min sonication time had the highest stability, best electrical conductivity, lowest crystallite size, fewer defects and lowest interlayer distance. Keywords: graphene; electrochemical exfoliation; sonication; electrical conductivity
1. Introduction In recent years, enormous effort has been devoted to the straightforward, rapid, environmentally friendly and low-cost electrochemical exfoliation of graphite to produce high-quality graphene [1]. Compared to other traditional chemical methods, this method is promising for the mass production of graphene [2]. In addition, functionalized graphene can be produced during the exfoliation process. Parameters that must be controlled during the processing are the number of probes, applied current and chemicals used. The principles are largely the same because the process generally involves the use of an electrolyte (e.g. aqueous or non-aqueous solution) and an electrical current to drive structural expansion at a graphite electrode [3]. According to the charge of the intercalated ions, the graphite electrode works as an anode or cathode and represents oxidation or reduction reactions, respectively.
In the electrochemical exfoliation process, when the direct current voltage was applied to the two-electrode setup, vigorous bubbles were produced at both two electrodes and anodic graphite began to dissociate into the electrolyte [4]. In contrast to the other exfoliation processes, this method is not equipment intensive and is typically performed under ambient conditions [5]. Moreover, it is eco-friendly compared to other chemical and sonication routes, which often involve hazardous reagents or solvents [6]. The yield, productivity and properties of graphene are dependent on the electrolytes. Aqueous electrolytes are industrially attractive because of the natural abundance and sustainability of water. The formation of graphite intercalation compounds (GICs) in strong acids and the physical expansion and exfoliation towards thin flakes have been long known, but did not attract significant attention until successful isolation of pristine graphene by micromechanical cleavage was reported by Novoselov et al. [7]. Table 1 shows a summary of previous work on various types of aqueous solutions used for the exfoliation process. Electrochemical exfoliation of graphite has been mostly performed in three different electrolytes: ionic-liquid, aqueous acids and aqueous inorganic salts. Parvez et al. [8] reported that exfoliation in ionic-liquids resulted in a low yield of graphene, small lateral size (<5 µm) and it was often functionalized with the ionic liquids, which disrupted the electronic properties of the graphene. Exfoliation in acidic electrolytes can yield graphene with a better quality and a larger lateral size, but a significant amount of oxygen-containing functional groups cannot be avoided due to the over-oxidation of graphite by the acid [9]. On the other hand, an inorganic salt electrolyte solution was able to effectively reduce the oxidation degree and thereby significantly improved the chemical and electronic properties of the graphene [10]. Thin-layer graphene sheets were able to obtain at a high yield with large flake size and the graphene could be produced on a scale of tens of grams. This demonstrated the great potential for the scale-up required for industrial production. Ionic liquids, such as N-butyl, methylpyrrolidiniumbis (trifluoromethylsulfonyl)-imide and (IL)–acetonitrile can be used in the electrochemical exfoliation process. Sulphuric acid (H2SO4), hyperchloric acid (HCLO4), sodium chloride (NaCl), sodium hydroxide (NaOH) and hydrogen peroxide (H2O2) are common types of acidic aqueous solutions used in previous work. Aqueous inorganic salts, such as ammonium sulphate ((NH4)2SO4) and sodium sulfate (Na2SO4), were used in the electrolysis process. A working bias in the range of 3 to 10V has been used for the electrochemical exfoliation process, and a generally high working bias will easily exfoliate the graphite layers. Results of the carbon (C) and oxygen (O2) atomic ratio showed the presence of a tiny amount of oxygen originating from the oxidation of the graphite by OH-1 ions during the electrochemical process. Moreover, Parvez et al. [11] reported the intensity between the D and G band, or I2D/IG, for the exfoliated graphene was 0.67 for the H2SO4 aqueous solution. The intensity ratio of the G and 2D bands, as well as their relative positions, can be used to determine the number of graphene layers. In most of the reports of electrochemical exfoliation of graphite, the exfoliated product was collected by vacuum filtration and repeatedly washed with water to remove any residual salts. In this study, the sonication method was introduced after the exfoliation to ease the exfoliation process of the graphene nanosheets. A thorough literature review indicated that
limited studies have been done to evaluate the combination effect of different electrolytes and sonication times on the properties of the graphene. Selection of a suitable electrolyte is important to balance the quantity and quality of the graphene. In the present study, graphene was produced through the electrochemical exfoliation of graphite rods using three different types of electrolytes: acidic aqueous (H3PO4 and H2SO4) and inorganic salts ((NH4)2SO4/water). The effect of sonication on the graphene produced by H2SO4 aqueous solution was investigated. Different sonication times (15, 30, 45 and 60 min) were also utilized and the graphene produced from the exfoliation process was characterized with Fourier-transform infrared spectroscopy (FTIR), Raman, X-ray diffraction (XRD) and electrical conductivity measurements.
2. EXPERIMENTAL 2.1 Materials A graphite rod with a diameter of 6.35 mm was supplied by Sigma-Aldrich. Sulphuric acid (H2SO4), phosphoric acid (H3PO4) and ammonium sulphate (NH4)2SO4 were supplied by Merck assay and used as electrolytes. Ethanol supplied by J.T. Baker was used as a cleaning agent. Distilled water was used in all of the synthesis processes. 2.2 Synthesis of graphene by electrochemical exfoliation In this study, the electrochemical synthesis of graphene was done in a conventional twoelectrode system. The exfoliation was conducted with a graphite rod that was used as a working electrode and a platinum (Pt) rod was treated as the counter electrode. The Pt rod was placed parallel to the graphite rod at a distance of about 2 cm. A 50 ml solution of 0.5 M H2SO4 and H3PO4 were used as the electrolyte. Distilled water containing 0.1 M (NH4)2SO4 was the other electrolyte used in the study. The direct current (DC) bias voltage was set to 10 V and remained until the full consumption of the anodic graphite. After the exfoliation process, the graphene solution was sonicated for 15, 30, 45 and 60 min at room temperature and washed several times by distilled water using vacuum filtration. The powder was obtained after drying in a vacuum oven at 80°C. 2.3 Material Characterization The zeta potential of graphene was measured using a Malvern Nano series ZEN3600. The surface electrical conductivity of the graphene was measured using a Prostat PRS-812 resistance meter measurement system. Images of the graphene solutions were obtained by preparing 1 mg/ml graphene in dimethylformamide (DMF) solvent, and the images were taken with a digital camera. The XRD spectra were obtained with a Bruker D8 X-ray diffractometer with a Ni-filtered Cu-Kα (λ = 1.54021Å) radiation source and were used to identify the crystallographic structure of the graphene. The FTIR spectra were obtained with a Perkin Elmer FTIR spectrometer at room temperature with a typical wave number of 4000500 cm-1 in open air and transmittance mode. The Raman spectra of graphene were measured by a Renishaw in Via spectrometer with a 633 nm excitation. The morphology of the asproduced graphene was analysed using field emission scanning electron microscopy (FESEM, model Zeiss Supra 35VP) and high-resolution transmission electron microscopy (HRTEM, model Teenai G2 20 s-twin, Fei). The X-ray photoelectron (XPS) spectra were obtained using AXID Uitra DLD, kratos, equipped with Al Kα X-ray source (1486.6eV).
3. Result and discussion The zeta potential analysis was used to determine the stability of the exfoliated graphene. Table 2 shows zeta potential values for the exfoliated graphene using different electrolytes and different sonication times. The zeta potential for graphene in H 2SO4 electrolyte without
sonication was -12 mV. This value is lower than other H2SO4 solutions with sonication because the zeta potential values for these electrolytes was between -32 mV to -38 mV. Nanoparticles with zeta potential values greater than +25 mV or less than -25 mV typically exhibit high degrees of stability because of the interparticle electrostatic repulsion [19]. These results indirectly indicated that the graphene in the H2SO4 electrolyte after sonication was more stable. By using sonication, the values of the zeta potential for H 2SO4, (NH4)2SO4 and H3PO4 all increased. The values are varied from -32 mV to -38 mV, -32 mV to -35 mV and 25 mV to -33 mV, respectively. The results indicated that the highest zeta potential value was for the H2SO4 electrolyte sonicated at 45 and 60 minutes.Based on Table 2, it was found that the electrical conductivity of all exfoliated graphene varied with sonication time. The lowest electrical conductivity occurred for graphene in H2SO4 without sonication. The highest electrical conductivity occurred for graphene in H2SO4 electrolytes at 45 min and 60 min sonication times. Figure 1 shows digital images that indicate the dispersion of the exfoliated graphene obtained with the different electrolytes at different sonication times. The images of the solutions to identify dispersion and sedimentation of the graphene were for 0 days, after 5 weeks and after 12 weeks. Just after sonication, graphene showed very good dispersion in the electrolytes. On the other hand, the graphene obtained using (NH4)2SO4 and H3PO4 produced after 15 min sonication tended to precipitate after 5 weeks. The graphene produced using longer sonication times were observed to be stable in the DMF solvent compared to the short sonication time. It was interesting to see that graphene obtained using the H2SO4 electrolyte remained homogeneous as a black dispersion and without any visible sedimentation even after 12 weeks. The crystal structure and interlayer spacing of the graphite rod and exfoliated graphene were determined with XRD. Figure 2(a) shows the purification of the raw graphite powder and exhibits three intense peaks at approximately 26°, 42° and 54° that correspond to the (002), (101) and (004) crystallographic planes, respectively [21]. Figures 2(b), (c) and (d) show the XRD patterns for the electrochemically exfoliated graphene nanosheets produced using the H2SO4, H3PO4 and (NH4)2SO4 electrolytes and sonication for different times (15, 30, 45 and 60 min), respectively. Generally, the peaks at 2θ=26° and 2θ=54° that represent the (002) and (004) planes, similar to graphite, were observed for all graphene produced using different electrolytes at different sonication times. However, the peak at 2θ=26° broadened and shifted towards the lower angle. This may be attributed to its corrugated structure and the increase of interlayer spacing within graphitic structures, which indicated the presence of graphene [22]. The XRD results proved the successful exfoliation of graphite to producing graphene, which is in good agreement with previous work by Sahoo and Mallik [23]. The average crystallite sizes for the graphene are shown in Table 3. It was found that the crystallite size decreased with increasing sonication times. The graphene produced using the H2SO4 electrolyte without sonication produced graphene with bigger crystallite sizes compared to the other graphene samples. The decrease in crystallite size of the exfoliated graphene produced using sonication occurred because of the cavitation energy that was generated by the ultrasonication process [23]. The FTIR analysis was performed to examine the chemical structure and any possible functional groups present in the exfoliated graphene. Figure 3(a) shows the FTIR spectra for the electrochemically exfoliated graphene produced using H2SO4 at different sonication
times. After 15 min of sonication, the samples were free from any non-bonded SO42- ions due to the absence of peaks at around 980 cm-1. An intense and broad peak at 3411 cm-1 was observed in all the spectra due to the stretching vibration of the O-H functional group. The small peak for C-C stretching of sp3 hybridized carbon atoms appeared at around 858 cm-1 for the 15 min sonication case. The peak for C-O stretching appeared at around 1040 cm-1 for 30 min, 45 and 60 min sonication times. The aromatic sp2 carbon ring and C=C stretching vibration were observed at 1579 cm-1 and 1582 cm-1 in all spectra. Meanwhile, the small peak at 1374 cm-1 indicated the presence of O-H bending. The weak peak at 1124 cm-1 was attributed to the C=O stretching vibration. On the other hand, the additional peak at 1654 cm1 corresponded to the C=O stretching vibration for the 45 min and 60 min sonication times. Figure 3(b) shows the FTIR spectra for the electrochemically exfoliated graphene produced using (NH4)2SO4 at different sonication times. In general, similar peaks were observed in the spectra for the exfoliated graphene produced at different sonication times. The broad peak at 3412 cm-1 corresponded to the stretching vibration of the O-H functional group for all sonication times. The C-H deformation appeared at 996 cm-1 after 15 min sonication. The weak peaks at 1624 and 1631 cm-1 corresponding to the C=C stretching vibration can be seen clearly for the 30, 45 and 60 min sonication times. This C=C bond peaks indicated the skeletal vibration of the graphitic zone or a portion from the stretching vibration of exfoliation. The peak for the C-O-C stretching vibration appears at 1062 cm-1 for the 15 min sonication time. Figure 3(c) shows the FTIR spectrum for the electrochemically exfoliated graphene produced using (NH4)2SO4 at different sonication times. The characteristic FTIR features of exfoliated graphene include the presence of different types of oxygen functionalities, which were confirmed by the band at 3441 cm-1 that corresponded to the O-H stretching vibration. The characteristic peaks responsible for hydroxylation arose at 2924 and 2845 cm-1 because of the asymmetric and symmetric stretching vibration of C-H groups for the 45 min and 60 min sonication times, respectively. The small peak for C=C stretching vibration appeared at 1578 cm-1. On the other hand, an additional peak was observed 1043 cm-1, which was due to the C-O stretching vibration. Thus, the FTIR data confirmed that the oxygen functional group was introduced into the graphene during the exfoliation process using all electrolytes. The FTIR peaks in this study are consistent with previously published reports using H 2SO4 as the electrolyte [24]. The Raman spectra for the exfoliated graphene obtained with different electrolytes and different sonication times are presented in Figures 4(a), (b) and (c). From the spectra, three important features can be observed, namely the D peak located at ~1335 cm-1, the G peak located at ~1580 cm-1 and the D peak located at ~2700 cm-1 [25]. The D band originates from the breathing mode of the six-atom rings and requires a defect for activation. Therefore, the intensity of the D band depends on the disorder of the graphene. The G peak represents the E2g phonons vibrations mode in the centre of the Brillouin Zone of the sp2 hybrid carbon atom. All the sp2 carbon materials exhibited a very sharp peak around 2500-2800 cm-1 in the Raman spectra. A sharp 2D band in the Raman spectra is a signature of the sp2 graphitic system, a second-order two phonon process and its frequency is dependent on the energy of the excitation laser [26].
The ID/IG ratio is normally used to evaluate the extent of defects present in graphene and other carbon materials. Alternatively, the number of graphene layers present in the sample could be estimated by calculating I2D/IG. The ratio increased as the number of graphene layers decreased [27]. When graphene is exfoliated from graphite, there is a significant increase in the number of sheet edges from the exfoliated sheets leading to more prominent D peak. Table 4 shows the calculation of the defect ratio values and number of graphene layers for the exfoliated graphene. Here, all exfoliated graphene defect values were nearly same, around 0.83. The graphene produced by different electrolytes and different sonication times exhibited an I2D/IG ratio of around 1.67, which indicated that the graphene was multilayered. Furthermore, the Raman spectra for the exfoliated graphene obtained at lower sonication times displayed another peak at 2924 cm-1, where the D+D' corresponds to the defects produced on the synthesized graphene. However, these D+D' peaks were absent from graphene obtained at different electrolytes for the 45 min sonication time in different electrolytes. These results revealed that graphene sheets with a relative low defect density were obtained by using the 45 min sonication time. The morphology of the graphite and exfoliated graphene sheets was investigated by FESEM. Figure 5(a) shows FESEM images for pristine graphite, which includes the bulk structure and thick graphite flakes. After exfoliation in the H2SO4 solution without sonication, the flakes and thin sheets were observed. However, agglomeration of the sheets is clearly seen in Figure 5(b). The images of graphene obtained after 15 min sonication in different electrolytes are shown in Figures 5(c), 6(a) and 7(a). Dense agglomerates had a layered structure and the nanoflakes had a curved, wrinkled and crumpled morphology. The curved morphology prevented the graphene sheets from restacking among one another and contributed to the mesoporous nature. The mesoporous structure and the wrinkles on the graphene surface sheets may have shortened the ion diffusion lengths and ensured the full utilization of the graphene nanoflakes with increased sonication time. In addition, multilayered sheets that were stacked into a crumpled and wrinkled structure can be observed at the edge of all the as-produced graphene samples using different electrolytes and lower sonication times. The crumpled and wrinkled formation in graphene is induced by defects, like boundaries or dislocations. The crumpling at the edges was caused by the insertion of functional groups carrying sp3 hybridized carbon atoms, which were introduced during the exfoliation process [28, 29]. Figures 5(d), 6(b) and 7(b) show the morphology for samples produced with the 45 min sonication time and different electrolytes. The images show partial transparency, which indicates that the multilayered graphene sheets were stacked together and had a smooth surface. However, the 45 min sonication times showed well-separated graphene sheets and a lesser wrinkled and crumpled structure, which suggested that the quality of the graphene obtained at 45 min was better than for the other sonication times. After 45 min sonication, the morphologies become wrinkled, and the defect structure for the exfoliated graphene is shown in Figures 5(e), 6(c) and 7(c). The can be explained by phenomenon of acoustic cavitation. The ultrasound generated microbubbles that collided with the graphene layer. The microbubbles collapsed and generated a large local energy due to the cavitation effects. The magnitude of the local energy intensity was proportional to the number of bubbles that collapsed during cavitation and this was equivalent to the duration of the sonication [30].
The morphology of the exfoliated graphene prepared with different electrolytes and 45 min sonication time was determined with HRTEM. Figures 8(a)(ⅰ), (b)(ⅰ) and (c)(ⅰ) show the HRTEM images for the exfoliated graphene produced using different electrolytes. The images indicated a transparent sheet-like structure and suggested that the obtained graphene was comprised of thin layers and a relatively smooth planar structure. Figure 8(b)(ⅰ) showed the multilayered graphene sheets stacked together and crumpling of the edges (depicted from circle mark in (NH4)2SO4 electrolyte solution) due to oxygenation and hydroxylation. This originated due to various defects and functional groups carrying the sp3 hybridized carbon atoms. This result supports the finding from the previous FTIR analysis, which is that they were introduced during the exfoliation process. In addition, a fold was observed at the edges, as shown by arrow in Figures 8(a)(ⅰ), (b)(ⅰ) and (c)(ⅰ). As a result of the high surface area of the extended thin layers, the graphene sheets had a significant tendency to overlap, which caused the edges to partially fold. The lines with the darker contrast indicate the folds. The folding was due to the carbon-carbon rings being destroyed by groups during the oxidation process. The surface energy was reduced so that the graphene sheets became stable [31]. The interlayer distance for the three different electrolytes was (a) 0.3 nm, (b) 0.36 nm and (c) 0.39 nm, respectively. The H2SO4 electrolyte with the 45 min sonication was the best based on crystallite size, surface electrical conductivity, defect morphology and interlayer distance. Thus, this sample was selected for further characterization using XRD, surface electrical conductivity, FESEM and HRTEM. The crystallographic structure of the graphene sheets produced using H 2SO4 and (NH4)2SO4 after 45 min sonication was characterized by selected area electron diffraction (SAED). Figure 9(a) exhibits a typical six-fold symmetric diffraction with the hexagonal crystal structure for the H2SO4 electrolyte exfoliated graphene. Several diffraction spots in the hexagonal pattern were identified and suggested that the selected graphene sheets were multi-crystal and the obtained graphene products included bilayer graphene sheets. The exfoliated graphene obtained with the (NH4)2SO4 electrolyte in Figure 9(b) had a diffuse diffraction ring pattern that may indicate polycrystalline features in an amorphous material. The increase in the amorphous nature was because of the crawling of the edges and surface functionalization on the graphene. For this reason, the SAED patterns showed that the graphene obtained with the H2SO4 electrolyte was better than the graphene obtained with the (NH4)2SO4 electrolyte and 45 min sonication time. The XPS was performed to determine the surface characterization, chemical composition, types of carbon and oxygen bonds and the percentage of oxygen present in the as-exfoliated graphene. Figures 10(a) and (b) show the XPS spectra for H2SO4 and (NH4)2SO4 electrolytes after 45 min sonication. The peak corresponding to C1s appeared at 284 eV, which indicated sp2 hybridized carbon atoms in the graphene sheets [32]. The peaks at 532 eV, which relate to O1s, indicated the presence of various oxygen functionalization in the carbon network structures [33]. Table 5 shows the carbon and oxygen C/O ratio that was obtained through elemental analysis measurement by XPS. The graphene prepared with H2SO4 and (NH4)2SO4 had a 16.72% and 22.59% weight ratio of oxygen, respectively. The presence of oxygen was attributed to the oxidation of the graphite by hydroxyl ions (OH -) during the electrochemical process [36]. The separated C1s peak, as seen in Figure 10, shows the peaks corresponding to C=C, C-C, C-O and C=O bond in Figure 11. The main peak at the binding energy of 284.3 eV was assigned to the C=C bond to represent sp2 hybridized carbon atoms in the graphene
sheet. The smaller peak at 285 eV was assigned to C-C sp3 hybridized carbons. The peak at 286.6 eV corresponded to the C-O groups (hydroxyl and epoxy), and the peak at about 287.9 eV was assigned to C=O carbonyl groups [34]. The use of different electrolytes during electrochemical exfoliation of graphite generated graphene with different densities of defects and also different contents of oxygen functionalities. The presence of the oxygen functional groups shown in the XPS spectra is consistent with the FTIR results.
4. Conclusions In summary, a simple electrochemical exfoliation process was compared for a combination of different types of electrolytes and sonication times. From the zeta potential and electrical conductivity measurements, the exfoliated graphene showed a very stable dispersion at higher sonication times and an increase in the electrical conductivity. It was also observed that the defects in the exfoliated graphene sheets decreased with an increase the sonication time, which was confirmed from the morphology and Raman studies. The XRD, HRTEM and XPS analyses showed the H2SO4 electrolyte with the 45 min sonication time produced graphene with the lowest crystallite site and lowest interlayer distance. Therefore, in this study, the H2SO4 electrolyte with the 45 min sonication time was considered as the most suitable.
Acknowledgements The authors gratefully acknowledge ASEAN University Network for Science and Engineering Education Development Network (AUN/SEED-Net) Project and Japan International Cooperation Agency (JICA) and Malaysia Government (FRGS grant 6071385) for their financial support. They would also like to thank the Universiti Sains Malaysia (USM) and the School of Materials and Mineral Resources Engineering for the use of their facilities.
References [1] C. Xu, B. Xu, Y. Gu, Z. Xiong, J. Sun, X.S. Zhao, Graphene-based electrodes for electrochemical energy storage. Energy & Environmental Science, 6(5), pp (2013)1388-1414. [2] Y.L Zhong, Z. Tian, G.P Simon, D. Li, Scalable production of graphene via wet chemistry: progress and challenges, Materials Today 18(2), pp. (2015) 73-78. [3] S. Yang, M.R. Lohe, K. Müllen, X. Feng, New‐ Generation Graphene from Electrochemical Approaches: Production and Applications, Advanced Materials, 28(29), pp. (2016) 6213-6221. [4] A.M Abdelkader, A.J Cooper, R.A.W Dryfe, I.A Kinloch, How to get between the sheets: a review of recent works on the electrochemical exfoliation of graphene materials from bulk graphite, Nanoscale, 7(16), pp. (2015) 6944-6956. [5] U. Khan, A. O'Neill, M. Lotya, S. De, J.N Coleman, High‐ Concentration Solvent Exfoliation of Graphene, Small, 6(7), pp. (2010) 864-871.
[6] S.K Sahoo, A. Mallik, Synthesis and characterization of conductive few layered graphene nanosheets using an anionic electrochemical intercalation and exfoliation technique, Journal of Materials Chemistry C, 3(41), pp. (2015)10870-10878. [7] A Richenderfer, Plasma-assisted Electrochemical Synthesis of Pristine Graphene Sheets. K. ar e . u . i . iu . ra . en K. en, Exfoliation of graphite into graphene in aqueous solutions of inorganic salts, Journal of the American Chemical Society, 136(16), pp. (2014) 6083-6091. [9] C.Y Su, A.Y Lu, Y. Xu, F.R. Chen, A.N. Khlobystov, L.J. Li, ACS Nano 5, 2332. CrossRef| PubMed| CAS| Web of Science® Times Cited, (2011)178. 10 K. ar e . . u . i . iu . ra . en K. en, Exfoliation of graphite into graphene in aqueous solutions of inorganic salts, Journal of the American Chemical Society, 136(16), pp. (2014) 6083-6091. 11 K. ar e . i . . uniredd . ernande . inke . an . en K. en, Electrochemically exfoliated graphene as solution-processable, highly conductive electrodes for organic electronics, ACS nano, 7(4), pp. (2013) 3598-3606. [12] Y. Yang, F. Lu, Z. Zhou, W. Song, Q. Chen, X. Ji, Electrochemically cathodic exfoliation of graphene sheets in room temperature ionic liquids N-butyl, methylpyrrolidinium bis (trifluoromethylsulfonyl) imide and their electrochemical properties. Electrochimica Acta, 113, pp. (2013) 9-16. [13] A.T Najafabadi, E. Gyenge, High-yield graphene production by electrochemical exfoliation of graphite: Novel ionic liquid (IL)–acetonitrile electrolyte with low IL content, Carbon, 71, pp. (2014) 58-69. [14] S.K. Sahoo, A. Mallik, Synthesis and characterization of conductive few layered graphene nanosheets using an anionic electrochemical intercalation and exfoliation technique, Journal of Materials Chemistry C, 3(41), pp. (2015) 10870-10878. [15] P. Tripathi, C. Patel, R. Prakash, M.A. Shaz, O.N. Srivastava, Synthesis of high-quality graphene through electrochemical exfoliation of graphite in alkaline electrolyte, arXiv preprint arXiv: (2013) 1310-7371. [16] K.S. Rao, J. Senthilnathan, Y.F. Liu, M. Yoshimura, Role of peroxide ions in formation of graphene nanosheets by electrochemical exfoliation of graphite, Scientific reports, 4, p. (2014) 4237. [17] K. Chen, D. Xue, Preparation of colloidal graphene in quantity by electrochemical exfoliation, Journal of colloid and interface science, 436, pp. (2014) 41-46. [18] K. ar e . u . i . iu . ra . en K. en, K, Exfoliation of graphite into graphene in aqueous solutions of inorganic salts, Journal of the American Chemical Society, 136(16), pp. (2014) 6083-6091. [19] S. Kashyap, S. Mishra, S.K. Behera, Aqueous colloidal stability of graphene oxide and chemically converted graphene, Journal of Nanoparticles (2014).
[20] I. Jeon, Synthesis of functionalized few layer graphene via electrochemical expansion, Doctoral dissertation, Massachusetts Institute of Technology (2015) [21] S.K. Sahoo, A. Mallik, Simple, fast and cost-effective electrochemical synthesis of few layer graphene nanosheets, Nano, 10(02), p. (2015) 1550019. [22] S. Sumari, A. Roesyadi, S. Sumarno, Effects of ultrasound on the morphology, particle size, crystallinity, and crystallite size of cellulose, Scientific Study & Research. Chemistry & Chemical Engineering, Biotechnology, Food Industry, 14(4), p. (2013) 229. [23] S.K. Sahoo, A. Mallik, Simple, fast and cost-effective electrochemical synthesis of few layer graphene nanosheets, Nano, 10(02), p. (2015) 1550019. [24] Ambrosi, Adriano, et al. "Electrochemistry of graphene and related materials." Chemical reviews 114.14 (2014): 7150-7188. [25] Ferrari, Andrea C., et al. "Raman spectrum of graphene and graphene layers." Physical review letters 97.18 (2006): 187401. [26] Tiwari, Santosh K., et al. "Facile electrochemical synthesis of few layered graphene from discharged battery electrode and its application for energy storage." Arabian Journal of Chemistry 10.4 (2017): 556-565. [27] Wu, Zhong‐ Shuai, et al. "Alternating stacked graphene‐ conducting polymer compact films with ultrahigh areal and volumetric capacitances for high‐ energy micro‐ supercapacitors." Advanced Materials 27.27 (2015): 4054-4061. [28] Tiwari, Santosh K., et al. "Facile electrochemical synthesis of few layered graphene from discharged battery electrode and its application for energy storage." Arabian Journal of Chemistry 10.4 (2017): 556-565. [29] Chee, W. K., et al. "Performance of flexible and binderless polypyrrole/graphene oxide/zinc oxide supercapacitor electrode in a symmetrical two-electrode configuration." Electrochimica Acta 157 (2015): 88-94. [30] Pan, C-T., et al. "In-situ observation and atomic resolution imaging of the ion irradiation induced amorphisation of graphene." Scientific reports 4 (2014): 6334. [31] Parvez, Khaled, et al. "Exfoliation of graphite into graphene in aqueous solutions of inorganic salts." Journal of the American Chemical Society 136.16 (2014): 6083-6091. [32] S.K. Sahoo, A. Mallik, Synthesis and characterization of conductive few layered graphene nanosheets using an anionic electrochemical intercalation and exfoliation technique, Journal of Materials Chemistry C, 3(41) (2015) 10870-10878. [33] D, Yang, A, Velamakanni, G, Bozoklu, S, Park, M, Stoller, R.D Piner, R.S. Ruoff, Chemical analysis of graphene oxide films after heat and chemical treatments by X-ray photoelectron and Micro-Raman spectroscopy, Carbon 47(1) (2009) 145-152. [34] A.A.B Hamra, Electro-exfoliating graphene from graphite for direct fabrication of supercapacitor, Applied Surface Science 360 (2016) 213-223.
Figure captions Figure 1. Photographs of 1mg/ml graphene dispersion in DMF solvent. The photograph was taken (a) just after sonication, (b) after 5 weeks and (c) after 12 weeks. Figure 2. (a) XRD spectrum of graphite. (b) XRD spectrum of graphene at H2SO4 at different sonication times 0, 15, 30, 45, 60 min. (c) XRD spectrum of graphene at (NH4)2SO4 at different sonication times 0, 15, 30, 45 60 min. (d) XRD spectrum of graphene at H3PO4 at different sonication times 0, 15, 30, 45 60 min. Figure 3. (a) FTIR spectrum of graphene at H2SO4 at different sonication times 15, 30, 45 60 min. (b) FTIR spectrum of graphene at (NH4)2SO4 at different sonication times 15, 30, 45 60 min. (c) FTIR spectrum of graphene at H3PO4 at different sonication times 15, 30, 45 60 min. Figure 4. (a) Raman spectra of exfoliated graphene at Sulphuric acid (H 2SO4) electrolytes in different sonication times 15, 30, 45 and 60 min. (b) Raman spectra of exfoliated graphene at Ammonium Sulphate (NH4)2SO4 electrolytes in different sonication times 15, 30, 45 and 60 min. (c) Raman spectra of exfoliated graphene at sulphuric acid (H 3PO4) electrolytes in different sonication times 15, 30, 45 and 60 min. Figure 5. FESEM images of graphene obtained at (a) pure graphite, after exfoliated H2SO4 electrolytes in (b) without sonication time and different sonication times (c) 15 min, (d) 45 min and (e) 60 min. Figure 6. FESEM images of graphene obtained at (NH4)2SO4 electrolytes in different sonication times (a) 15 min, (b) 45 min and (c) 60 min. Figure 7. FESEM images of graphene obtained at H3PO4 electrolytes in different sonication times (a) 15 min, (b) 45 min and (c) 60 min. Figure 8. HRTEM images of graphene obtained using (a) H2SO4, (b) (NH4)2SO4 and (c) H3PO4 electrolytes at 45 min sonication time at magnifications of (ⅰ) 29 KX and (ⅱ) 690 KX. Figure 9. SAED pattern of the graphene obtained at (a) H2SO4 and (b) (NH4)2SO4 at 45 min sonication time. Figure 10. Wide scan XPS spectra of graphene obtained at (a) H 2SO4 (b) (NH4)2SO4 electrolyte in 45 min sonication time. Figure 11. Deconvolution of C1s spectra of graphene obtained at (a) H 2SO4 and (b) (NH4)2SO4 electrolyte in 45 min sonication time.
Just sonicated After 5 weeks After 12 weeks
15 30 45 60
15 30 45 60
15 30 45 60
H2SO4
15 30 45 60
15 30 45 60
15 30 45 60
(NH4)2 SO 4
15 30 45 60
15 30 45 60
H 3PO4
Figure 1
15 30 45 60
Figure 2 (a)
Figure 2 (b)
Figure 2 (c)
Figure 2 (d)
Figure 3 (a)
Figure 3 (b)
Figure 3 (c)
Figure 4 (a)
Figure 4 (b)
Figure 4 (c)
a
b
Flake shape
1 µm
1 µm
c
d
wrinkled
Smooth layer 1 µm
1 µm
e
wrinkled
1 µm
Figure 5
a
c
wrinkled
Smooth layer 1 µm
1 µm
d
crumpled
1 µm
Figure 6
c
a
Smooth layer
crumpled
wrinkled 1 µm
1 µm
d
wrinkled
1 µm
Figure 7
a
a( )
0.3 nm
Fold edge 200 nm
5 nm
b
b( )
crumpling 0.36 nm
Fold edge
200 nm
5 nm c( )
c
0.39 nm
Fold edge
5 nm
200 nm
Figure 8
(b)
(a)
Figure 9
Figure 10 (a)
Figure 10 (b)
Figure 11 (a)
Figure 11 (b)
Table list Table 1. Summary of previous works on the formation of graphene based on aqueous solution electrolyte
Table 2 Zeta potential values and electrical conductivity properties of exfoliated graphene Table 3 The average crystallite size of exfoliated graphene at different electrolytes different sonication time Table 4 Calculation of ID/IG and I2D/IG from the Raman spectra. Table 5 Atomic weight percentage of element in multi-layered graphene obtained from H2SO4 and (NH4)2SO4 electrolytes in 45 min sonication time.
Aqueous inorganic salts
Acidic aqueous solutions
Ionic-liquid
Table 1: Summary of previous works on the formation of graphene based on aqueous solution electrolyte
Types of Electrolytes
Starting material
Working bias
C/O (XPS)
I2D/IG (Raman)
Numbers of graphene layers
Ref.
N-butyl, methylpyrro lidiniumbis( trifluoromet hylsulfonyl) -imide
Graphite rod
15-30V
36
1.69
Bi-layer
[12]
(IL)– acetonitrile
Graphite rod
7V
-
0.63
Few-layers
[13]
10V
12.3
0.67
1-3 layers
[11]
3-10V
0.60
-
3-6 layers
[14]
3
1-4 layers
[15]
NaOH/H2O2 /H2O (NH4)2SO4/ water
Graphite flakes Graphite sheet Graphite foil Graphite rod Graphite rod
Na2SO4/ water
Graphite rod
H2SO4 HCLO4 KOH
3V 3V
17.18
1.69
Bi-layer
[16]
10V
17.2
-
Bi-layer
[17]
5V
-
1.7
Bi-layer
[18]
Table 2: Zeta potential values and electrical conductivity properties of exfoliated graphene Electrolytes
Sonication time (min)
Zeta potential
Electrical Conductivity (Sm-1)
H2SO4
0, 15, 30, 45 and 60
-12, -32, -34 -38 and -38
96. 5, 231, 458, 1041, 1041
(NH4)2SO4
15, 30, 45 and 60
-30, -32, -35 and -35
220, 424, 997, 997
H3PO4
15, 30, 45 and 60
-25, -27, -33 and -33
216, 365, 965, 965
Table 3. The average crystallite size of exfoliated graphene at different electrolytes different
Electrolytes
Sonication time (min)
H2SO4
Crystallite sizes (nm)
0, 15, 30, 45 and 60
12.4, 8.7, 8.3 and 8.3
(NH4)2SO4
15, 30, 45 and 60
9.5, 9.1, 8.5 and 8.5
H3PO4
15, 30, 45 and 60
9.6, 9.2, 8.4 and 9.2
sonication time
Table 4: Calculation of ID/IG and I2D/IG from the Raman spectra Electrolytes
Sonication time (min)
ID
IG
I2D
ID+D'
ID/IG
I2D/IG
H2SO4
15, 30, 45, 60
1338, 1336, 1342, 1342
1595, 1602, 1578, 1578
2667, 2667, 2684, 2684
2924,2924,,-
0.85, 0.85, 0.85, 0.85
1.67,1.66,1. 7,1.7
(NH4)2SO4
15, 30, 45, 60
1348, 1335, 1342,1347
1602,1595, 1578, 1595
2668, 2684, 2684, 2684
2924,2924,,2924
0.84, 0.83, 0.83, 0.83
1.67,1.68,1. 7,1.7
H3PO4
15, 30, 45, 60
1332, 1338, 1327, 1318
1598, 1604, 1579, 1609
2667, 2667, 2684, 2669
2924,2924,2 924,2924
0.83, 0.83, 0.81, 0.81
1.66,1.67, 1.69, 1.65
Table 5: Atomic weight percentage of element in multi-layered graphene obtained from H2SO4 and (NH4)2SO4 electrolytes in 45 min sonication time Electrolytes
C (wt%)
O (wt%)
C/O
H2SO4
83.28
16.72
4.98
(NH4)2SO4
77.41
22.59
3.42
Highlights:
1. The lowest electrical conductivity occurred for graphene in H2SO4 without sonication. 2. The graphene produced using longer sonication times were observed to be stable in the DMF solvent compared to the short sonication time.
3. The decrease in crystallite size of the exfoliated graphene produced using sonication occurred because of the cavitation energy that was generated by the ultrasonication process.