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ScienceDirect Procedia Engineering 184 (2017) 469 – 477

Advances in Material & Processing Technologies Conference

Synthesis of Graphene Oxide using Modified Hummers Method: Solvent Influence N.I. Zaabaa, K.L. Fooa,*, U. Hashima,d, S.J.Tanb,c, Wei-Wen Liua, C.H. Voona a b

Institute of Nano Electronic Engineering (INEE), Universiti Malaysia Perlis (UniMAP), 01000 Kangar, Perlis, Malaysia.

Faculty of Engineering Technology, Universiti Malaysia Perlis (UniMAP), Level 1, Block S@, Campus UniCITI Nature, Sungai Chuchuh, 02100 Padang Besar, Perlis, Malaysia.

c

Center of Excellence Geopolymer and Green Technology (CEGeoGTech). School of Material Engineering, Universiti Malaysia Perlis (UniMAP), 77 Pejabat Pos Besar, 01007, Kangar, Perlis, Malaysia. d

School of Microelectronic Engineering (SoME), Universiti Malaysia Perlis (UniMAP), Perlis, Malaysia.

Abstract The aim of this study is to study the influence of different solvents on the structure and electrical properties of graphene oxide. GO was obtained from graphite flakes by using modified hummers method in which different from conventional hummer’s method. In this method, the experiment was synthesized without sodium nitrate (NaNO 3) and ice bath, but carried out at room temperature. Prepared GO powders were then dissolved into different solvents, namely acetone and ethanol. Then spin-coated onto silicon wafer and IDE to produce acetone-GO (A-GO) and ethanol-GO (E-GO). SEM result shows that several square micron GO were obtained. In addition, due to the large agglomerates and contact between the flakes in E-GO sample, currentvoltage pattern indicated the E-GO produced higher current flow than A-GO. Meanwhile, GO characterized using FTiR shows that both samples contain several functional groups such as hydroxyl, epoxy, carboxyl and carbonyl. Besides that, due to the lower diffraction peak of A-GO, XRD result shows the interlayer spacing of A-GO sample is slightly higher than E-GO sample. 2017Published The Authors. Published Elsevier Ltd.access article under the CC BY-NC-ND license © 2017 © by Elsevier Ltd.by This is an open Peer-review under responsibility of the organizing committee of the Advances in Material & Processing Technologies (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer review under responsibility of the organizing committee of the Advances in Materials & Processing Technologies Conference Conference. Keywords: Graphene oxide; modified hummer’s method; electrical characteristic; structure characateristic; functional group

*

Corresponding author. Tel.: 04-9798580/81/82; fax: 04-9798578.

1877-7058 © 2017 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer review under responsibility of the organizing committee of the Advances in Materials & Processing Technologies Conference

doi:10.1016/j.proeng.2017.04.118

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N.I. Zaaba et al. / Procedia Engineering 184 (2017) 469 – 477

E-mail address: [email protected]

1. Introduction Single atomic plane layer of graphite called graphene[1]. Graphene can produce from graphite using by chemical vapor deposition (CVD), mechanical or chemical method[2,3]. Through the discovery of graphene, many researchers became interested to do research in this two-dimensional(2D) carbon, as to discover its properties, characteristic and improve device in future[4]. Until now, graphene become hot topic and potential materials in huge number of applications.As compared to other carbon materials, graphene contain higher mobility (200000cm 2 v-1 s1 )[5,6], young modulus (1TPa) [7] and thermal conductivity (4.84x103 to 5.30x103 W/mK)[8] which make it as a potential material in sector transistor element and integrated circuit [9],storage energy[10], gas sensor[3] and bio electronic sensor[11]. During the oxidation process, sp3 bonding are heavily interrupted and make electrical conductivity of GO drastically decrease compare to graphene[12]. X. Huang et al[12] concluded based on their experiment, the temperature of GO related with conductivity of GO. The preparation of GO can modified to produce certain functionalized GO which has lower or higher conductivity properties than pristine graphene [13,14]. Graphite oxide can be exfoliated using any organic solvent to form GO and produce different form dispersion with different long-term stability and thickness of single layer GO. Four type of organic solvent such as DMF, NMP, THF, and ethylene glycol can form single layer of GO sheet same when used water [15]. According to T. Kavinkumar et al[16],due to containing more sp3 but less sp2 carbon atom, GO become a nonconductive element. Defect of sp3 bonding and various oxygen functional groups that attached on basal planes and edges can be generated during intercalating with organic solvent. Therefore, S. Ray et al[17] declare that due to sp3 bonding feature, the functional group on the GO surface might affect the conductivity of GO. Due to elimination/removal of oxygen functional group, electrical conductivities of GO increased[18]. However, by oxidation of graphene, O. Moradi et al complete the improvement on conductivity of graphene oxide from 0.005 S/m to 0.14 S/m by using conductive-meter in aqueous solution at 35oC [14]. From statement above shows that graphene consists of high young modulus. On the other hand, due to oxygen functional group of GO, monolayer GO only consists of low young modulus (207.6 ± 23.4Gpa) [7,18]. Similar to electrical conductivity, thermal conductivity of GO is lower than graphene due to functional group and presence of defect[20]. It can be concluded that functional group might affect the electrical and thermal conductivity of GO. Therefore, graphene and GO are rapidly increased attraction by the researchers in various fields[21]. After discovered on graphene and GO, many potential applications are under development and proposed, such as biosensor[22],gas sensor [23] and supercapacitor [24]. Besides that, GO functionalized with epoxy, hydroxyl, carboxyl, and carbonyl group play an important role for interacting with gas molecule[23], DNA[25], and enzyme[26]. Based on H. Hsu et al [27], interlayer spacing of GO increasing due to oxygen functional group make GO high specific surface area, high mesopore volume, and certain level of electrical conductivity. Interdigitated electrode (IDE) currently used onto molecular [28], gas[18], acoustic and MEMS [29] sensor. Variety sensor has been optimized by synthesis material onto IDE. British chemist B.C. Brodie was the first researcher that discover properties of graphite oxide[30]. Since then, graphite oxide became popular among researcher because of interesting in unique properties of graphite oxide. Due to limitation of data, research about graphite oxide still currently on going on by researchers from all around the world. In Brodie method, ratio of 1:3 of graphite and potassium chlorate (KClO 3) were mixed and reacted with fuming nitric acid (HNO3) in 3 or 4 days with 60oC [30]. In year 1898, L. Staundenmaier improved the work of Brodie’s method by exchange two-third of fuming HNO3 to sulfuric acid (H2SO4) and used multiple aliquot of KClO3 the reaction occurs[31]. However, it caused hazard because of addition of KClO3 which caused continued explosion and took four days for this reaction[32]. After 40 years, L. Staundenmaier discover new method, which was more practical and simplified. Since it doesn’t need repeat 4 times of oxidations process[33]. After that, chemist Hummer and Offeman introduced their method that currently often used by the researches to produce graphite oxide in 1958[34]. In this method, 100g of graphite powder, 50g of sodium nitrate (NaNO3), 2.3-liter sulfuric acid and 300g potassium permanganate (KMnO4) were used in oxidation process. In hummer’s method, KMnO 4 used to replace KClO3 to avoid spontaneous explosion during oxidation process while NaNO 3 replaced fuming HNO3 to


eliminate fog acid produced. This process takes just in few hours to produce high quality of GO. Besides that, Hummer’s method produce more amount of oxygen than the Brodie’s method [35]. However, hummer’s method still have flaws [36], whereby it produce toxic gas such as NO2 and N2O4 [33]. GO and graphite oxide can be produced by similar in chemical process but different in its structure[37]. Graphite oxide can be described as group of layers graphene oxide[38]. Furthermore, exfoliation of graphite oxide would produce single layer sheet of graphite oxide which call GO[39]. Many thermal and mechanical methods exist to exfoliate graphite oxide to GO, but sonication graphite oxide into wafer or organic media is common method used because of faster than other method. The older method is rapidly heat graphite oxide hundred degrees in inert atmosphere[2]. This method caused explosive thermal reduction of material, large amount of CO2 and H2O between spaces of graphite layers[40]. Marcano et al [41] had completed the study on different between hummer’s method, modified hummer’s method and improved hummer’s method without NaNO3. As we know, oxidation using NaNO3 would emit toxic gasses. Therefore, improved hummer’s method replaced the use of NaNO 3 with H2SO4, H3PO4 and double the amount KMnO4. In addition, his improved hummer’s method gives other advantages such as more hydrophilic carbon material, equivalent conductivity and no toxic gasses emitted and finally attract in large producing GO[41]. 2. Experimental procedure 2.1 Preparation of graphene oxide by a modified hummer’s method In typical procedure, graphene oxide (GO) was produced using modified hummers method from pure graphite powder. In this method, 27 ml of sulfuric acid (H 2SO4) and 3 ml of phosphoric acid (H3PO4) (volume ratio 9:1) were mixed and stirred for several minutes. Then 0.225 g of graphite powder was added into mixing solution under stirring condition. 1.32 g of potassium permanganate (KMnO4) was then added slowly into the solution. This mixture was stirred for 6 hours until the solution became dark green. To eliminate excess of KMnO 4, 0.675 ml of hydrogen peroxide (H2O2) was dropped slowly and stirred for 10 minutes. The exothermic reaction occurred and let it to cool down. 10 ml of hydrochloric acid (HCl) and 30ml of deionized water (DIW) was added and centrifuged using Eppendorf Centrifuge 5430R at 5000 rpm for 7 minutes. Then, the supernatant was decanted away and the residuals was then rewashed again with HCl and DIW for 3 times. The washed GO solution was dried using oven at 90 °C for 24 hours to produce the powder of GO. 2.2 Preparation of graphene oxide solution To produce acetone graphene oxide (A-GO) and ethanol graphene oxide (E-GO) samples,1 mg of GO were dissolve in 1 ml of acetone/ethanol solution (volume ratio 1:1) under ultrasonic for 1 hour. 100 µl of A-GO/E-GO solution were then dropped on silicon wafer and spin at 2000 rpm for 20 second. The coated sample were then heated on the hot-plate with 80 °C for 10 minutes. This step was repeated for 10 times. Same coating process was done on interdigitated electrode (IDE) sample, which was mainly for electrical properties study.

Figure 1: Schematic illustration of IDE functional by GO

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2.3 Characterization of acetone and ethanol graphene oxide The morphologies of sample were observed by SEM model Jeol JSM-6010LV at the magnification of 500 X and 10 kX with accelerating voltage of 20 kV. Meanwhile, XRD Siemens Diffractometer Model D-5000 using Cu Kα radiation (λ=0.14406n) source in θ/2θ mode was used to investigate the composition of specimens. Measurements were made with fast duration scan 5°/min from 10° - 80°. FTIR (Perkin-Elmer spectrum 65 FT-IR) was used to scan the samples from 650 to 4000 cm-1. Electrical characterization was conducted using Keithley 6487 picoammeter, where the IDE electrodes were connected using 2 point probes in purpose for electrical conductivity differences between A-GO and E-GO. The difference of electrical conductivity is defined by biasing 2 V through the IDE coated GO and graphically illustrated.

3. Result and discussion 3.1 SEM

Figure 2: SEM image of the samples (a) A-GO; (b) E-GO

The morphology of both samples were determined by SEM. Fig. 2 shows the distribution of GO on the device at 500 X magnification. The working distance is approximately 10 mm at high vacuum mode with 20 kV. From the image shown in Fig. 2, GO dissolved in acetone (A-GO) shows comprehensive extensive spread on the silicon surface, while E-GO sample looks clustered on the entire surface. This phenomenon might due to the GO was fully dissolved in acetone but partially dissolved in ethanol solvent. After sonication, GO fully dissolved in acetone while, ethanol act as non-solvent

Figure 3: SEM image of the samples (a) A-GO; (b) E-GO

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Fig. 3 shows on same samples but at higher magnification (10 kX). The similar result was founded by Li et. al [42]. Besides that, morphology of E-GO structure appeared in large agglomerates but A-GO seemed to be scattered on silicon wafer. These might further affect the flow of current through the IDE’s fingers.

3.2 XRD In this analysis, XRD was used to determine the crystal structure and verify the interlayer spacing of A-GO and E-GO. The XRD pattern of A-GO and E-Go are presented in Fig. 4. The peak for both A-GO and E-GO samples shown in Fig. 4 is slightly different. Whereby the A-GO sample exhibits peak at 11.7o meanwhile E-GO sample at 12.3o. Both results show the diffraction peak at ~10 o due to (002) plane of GO[43]. The interlayer spacing of GO can be calculated according to the bragg law [44]: ݊ߣ ൌ ʹ݀‫ߠ݊݅ݏ‬

(1)

where n is the diffraction series and λ is the X-ray wavelength. Interlayer spacing (݀) of A-GO and E-GO was 0.75nm and 0.71nm by Bragg equation. Both interlayer spacing slightly different although same GO used but different solution used. Increasing of interlayer spacing because of intercalated functional group of oxygen and water molecule into carbon layer structure[45]. Besides that, it is also related to the weaker van der waals bonding formed by epoxyl, hydroxyl, carbonyl and carboxyl group on the basal planes[46]. Usually, interlayer spacing d of graphene oxide is at the range of 0.6-1.0nm and it controlled on the degree of oxidation of graphite and amount of water molecules intercalated into interlayer spacing[47]. According to the literature, once the graphite is oxidized and become GO, the XRD peak should shift from ~26 o to ~11o. However, there is no sharp peak appeared in the XRD result shown in Fig. 4 but gives a very broad range from ~10 o to ~25o. This might due to the graphite was incompletely oxidized using this method.

Figure 4: XRD pattern of A-GO and E-GO

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Figure 5: FTiR spectrum of E-GO and A-GO

3.3 FTIR Fig. 5 shows the FTiR spectra of A-GO and E-GO samples. FTiR used to investigate the bonding interactions in both samples. The result indicates that both A-GO and E-GO consist of carbonyl C=O, aromatic C=C, carboxyl CO, epoxy C-O-C and hydroxyl O-H, whereby peak at 3250cm-1 and 3125cm-1 are corresponding to the O-H (hydroxyl) groups[48]. The band in between 2800cm-1 and 3200cm-1 represent the hydroxyl group in GO network, which reduce the intensity after a sulfanilic acid treatment[49]. The band between 3000cm-1 and 3800cm-1 represent as hydroxyl , as those reported in previous work[50,51]. While peak at ~2300cm-1 is corresponded to the peak for absorbed CO2 molecules[52]. On the other hand, broad peak 1418.6 cm-1 and 858.23 cm-1 are corresponded to C=C bond (aromatic group)[46,52,53]. T. Sakthivel et al declared as C-O (carbonyl and carboxyl moities) group at band ~1568cm-1 [55]. Besides that, band between 1106.3cm-1 and 1005.2 cm-1 is corresponded to C-O-C (epoxy) groups[53]. All the peak shows the functional group existed in A-GO and E-GO samples. It can assume graphite powder successfully oxidizing with concentration acid with KMnO 4.


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3.4 I-V

Figure 6: The I-V curve of the two sample

Interdigitated electrode (IDE) was used for electrical test. The total saiz of IDE is 7x4 mm and consists 10 fingers. The width and length each finger are 0.1 mm and 3.75 mm, respectively. The spacing between 2 fingers is 100 µm. The electrical test were carried out using picoammeter/voltage source unit at room temperature. Current versus voltage (I-V) analysis was done in the range form 0V to 2V. Fig. 6 shown that a very weak current singal occur. The ultra low current show in Fig. 6 indicated that produced GO just act like an insulator. This might due to the sp2 bonded graphitic structure was distrupted by the electronegative oxygen in oxygen functional group[56]. Fig. 6 also shows that current flow for A-GO is weaker than E-GO, which indicated that E-GO performs conductivity. However, current-voltage pattern indicated that the conductivities of GO are slightly different between two samples. E-GO has higher current flow than A-GO because of the contact between the flakes GO as shown at SEM.

4. Conclusion We applied improved hummer’s method in our experiment as to produce graphene oxide, and different from conventional hummer method because synthesis graphene oxide without using NaNO 3. It shows that, NaNO3 does not affect the synthesis method to produce graphene oxide. Without using NaNO 3 still produce same characteristic of GO. This method can decrease coast and free toxic gases. Ethanol and acetone used to perform liquid medium of GO. Ethanol and acetone slightly affect the result of the synthesis of GO. Ethanol has advantages more than acetone in form of conductivity of electrical and solubility of GO. Conductivity of sample effected through morphology of GO. These finding is confirmed by the SEM, XRD result, FTIR spectre and I-V curves. Morphology of E-GO structure appeared in large agglomerates but A-GO seemed to be scattered on silicon wafer. E-GO has higher current flow than A-GO because of the contact between the flakes GO as shown at SEM. In FTIR spectre both sample contain several functional groups such as hydroxyl, epoxy, carboxyl and carbonyl. Besides that, due to the lower diffraction peak of A-GO, XRD result shows the interlayer spacing of A-GO sample is slightly higher than EGO sample. Acknowledgements The authors are grateful to the Department of Higher Education, Ministry of Higher Education, Malaysia for funding this research through the Fundamental Research Grant Scheme (FRGS) with the grant number 9003-00465. The author also would like to acknowledge all the team members in Institute of Nano Electronic Engineering (INEE), Universiti Malaysia Perlis (UniMAP) for their guidance and help.

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