The Effect of Different Reduction Methods on Conductivity of reduced-Graphene Oxide (r-GO) K. N. Hanim1, a, M. R. Muda1, b, Siti. S. Mat Isa1, c, Muhammad M. Ramli1, d and M. F. Jamlos2, e 1
School of Microelectronic Engineering, Universiti Malaysia Perlis, Pauh Putra Campus, 02600 Arau, Perlis, Malaysia 2
Advanced Communication Engineering Centre (ACE), School of Computer & Communication Engineering, Universiti Malaysia Perlis a
[email protected],
[email protected],
[email protected], d
[email protected],
[email protected]
Keywords: Graphene Oxide, Modified Hummer’s Method, Reduced Graphene Oxide, Sodium Borohydride, Thermal Treatment.
Abstract Large quantity of graphene oxide (GO) was prepared by Modified Hummer’s method, in which graphite is treated with a mixture of sulphuric acid, potassium permanganate and hydrogen peroxide. A chemically reduce graphene oxide (r-GO) were prepared using sodium borohydride (NaBH4), followed by thermal treatment and thermal treatment of chemically reduce using NaBH4. The electrical resistance of r-GO was measured using Keithley sourcemeter. The results revealed that r-GO show lower resistance on thermal reduction which is 2.39 kΩ compared to chemical reduction and thermal of NaBH4 reduction which is 2.18 MΩ and 3.16 kΩ respectively. Thermal reduction is the best method to produce high conductivity r-GO film. Introduction Graphene has pulled an incredible attention because of its mechanical, electrical, thermal and optical properties [1]. Graphene oxide (GO) is completely transparent while being inexpensive and nontoxic. These extraordinary properties have made graphene and GO as a perfect candidate material for a device applications such as energy harvester, gas sensor and many more. Researchers recently have found that the reduction degree of GO or oxidation degree of graphene has certain influences on their properties, such as electrical conductivity, catalysis activity and semi-conductive band positions [2]. To recover the electrical conductivity, GO has to be reduced in order to remove oxygen containing functionalities so that the aromatic graphene networks are restored [8]. Among these research works, the reduction degree of GO sheets is controlled by adjusting the reactive temperature or time [5-7]. It is well known that the oxygenated functional groups on GO mainly consist of hydroxyl, carboxyl an epoxy groups [10]. To achieve the aim of removing the oxygen containing functionalities and to retrieve the fine electrical properties of the graphene, a better alternative method can be used which is heat and chemical reduction [4]. Generally, the reduction of GO is carried out by chemical methods, using reducing agents such as hydrazine. Unfortunately, this chemical approach gave rise to high resistance which would be too high for actual application. Furthermore, it is toxicated and hazardous. Other option is chemical reduction using sodium borohydride (NaBH4) as a reducing agent [11]. However, thermal method is believed to be green method in which no hazardous reductants are used. It was belived that thermal reduction of graphene oxide was accompanied by the elimination of epoxy an carboxyl groups. In this paper, comparison of thermal an chemical reduction of GO is investigated.
Experimental Method i. Production of Graphene Oxide Hummer’s Method was used in order to chemically exfoliate graphite and become graphene oxide (GO). Firstly, graphite powder was put into cold (0 °C) concentrated sulphuric acid, H2SO4 (120 mL). Then, potassium permanganate, KMnO4 (15 g) was added gradually under stirring and the temperature of the mixture was kept below 20 °C by cooling (applying ice bath). Successively, the mixture was stirred at 35 °C for 2 hours and diluted with deionised water (250 mL). The addition of water in concentrated H2SO4 medium released a large amount of heat. The addition of water was carried out in an ice bath in order to keep the temperature below 50 °C. After adding all 250 mL deionised water, the mixture was stirred for 2 hours and then, additional of deionised water was added. Shortly, after the dilution with 0.7 liter of water, 2 mL of 30 % hydrogen peroxide was added to the mixture. The color of the mixture changed into yellow along with bubbling. The mixture was filtered and washed with 1:10 hydrochloric acid, HCl (1 liter) to remove metal ions followed by 1 liter of deionised water to remove acid. The resulting solid was dried in air and diluted to make GO dispersion. Exfoliation was carried out by sonication process of GO dispersion under ambient condition for 20 minutes. ii. Thermal reduction GO can be reduced solely by heat treatment and the process is named thermal annealing reduction. A different heating temperature significantly affects the reduction process of GO [3]. In order to obtain the reduction graphene by heat treatment, GO film was annealed at low temperatures which are 100 ̊ C, 200 ̊ C and 300 ̊ C for 1, 2 and 3 hours. As a result, r-GO were obtained. For a second experiment, the chemically reduced GO using NaBH4 was annealed at 100 ˚C, 200 ˚C, 300 ˚C at 1, 2 and 3 hours in order to study the effect of double reduction methods which is chemical and thermal reduction at the same time. iii. Chemical reduction Another method of GO reduction is using NaBH4 as a reducing agent. The suspension process started with 10 mM of NaBH4 and addition of deionized water into it. After 10 minutes of magnetic stirring at 300 rpm, GO solution was added into that suspension. The resulting mixture was further treated with reducing agent by magnetic stirring at 200 rpm for 2 hours. Inhomogeneous brown suspension was obtained after 2 hours. The synthesized r-GO solution was filtered by vacuum filtration method. The mixture was filtered and washes with 1:10 HCl solution (1 liter) in order to remove metal ions followed by 1 liter of deionised water to remove acid. The resulting solid was dried in air and diluted to make a r-GO dispersion. iv. Characterizations GO and r-GO solutions were deposited on the glass substrate before the process of characterizations. The samples were then analyzed using Keithley sourcemeter to investigate the electrical characteristics. Electrical resistance (R) was obtained according to Ohm’s Law by considering current-voltages curves to be linear.
Results and Discussions Electrical characterizations results were summarized in Fig 1. It shows the resistance of r-GO after reduction using NaBH4 is lower than the resistance before reduction process. The average resistance before the reduction process was 15.5 MΩ and after reduced using NaBH4 the average resistance was decreased to 2.2 MΩ.
Fig. 1 Electrical resistance of GO before and after reduction process Fig. 2 shows the resistance of r-GO after annealed at 100 ˚C, 200 ˚C and 300 ˚C for 1, 2 and 3 hours. At 100 ˚C, the best resistance was obtained after anneal at 3 hours, which is 4.4 kΩ as shown in Fig. 2 (a). As the temperature increased to 200 ˚C (Fig. 2 (b)), the resistance was predictably reduced to 2.6 kΩ after it was annealed for 3 hours. When the film was heated at 300 ˚C for 3 hours, the resistance was greatly reduced, where the best resistance was 2.4 kΩ (Fig. 2 (c)). It can be concluded that, when the annealing temperature increase, the conductivity of GO film was also increase. (b)
(a)
(c)
Fig. 2 Electrical resistance of r-GO annealed at (a) 100 ˚C, (b) 200 ˚C and (c) 300 ˚C for 1, 2 and 3 hours respectively.
In order to explore the effect of double reduction process, the r-GO that was reduced using NaBH4 was further reduced by annealing at 100 ˚C, 200 ˚C, 300 ˚C for 1, 2 and 3 hours. The results were shown in Fig. 3. At 100 ˚C, the lowest resistance obtained, which is 5.9 MΩ, was annealed for 3 hours (Fig. 3 (a)). As the temperature increased to 200 ˚C, the resistance was reduced to 5.8 MΩ when annealed at 3 hours (Fig. 3 (b)). Fig. 3 (c) shows when the samples was heated at 300˚C for 3 hours, the resistance was greatly reduced to 3.16 kΩ. It was believed that, the increasing of annealing temperature and time, resulted in higher conductivity of r-GO film. (a)
(b)
(c)
Fig. 3 The resistances of thermal and chemically reduced GO. The film was further annealed at (a) 100˚C, (b) 200˚C and (c) 300˚C for 1, 2 and 3 hours after chemically reduced. In comparison, the thermal reduction process shows lower resistance than chemically reduced and double reduction process (thermal reduction and chemical reduction). The lowest resistance, 2.39 kΩ, was achieved using thermal treatment process where the film was annealed at 300 ˚C for 3 hours. Whereas, at the same condition, the resistance of GO film using double reduction process was 3.16 kΩ. It was believed due to residual hydroxyl functional groups. However, further analysis of C:O ratio and information regarding any functional groups of the r-GO using X-ray photoelectron spectroscopy (XPS) will be done in the future. Summary Comparison between thermal reductions, chemical reduction using NaBH4 and double reduction process were successfully investigated. The lowest resistance of thermal reduction was 2.39 kΩ compared to chemical reduction which is 2.18 MΩ. Further thermal treatment was done for chemically reduced sample to remove the remaining functional group. The results shows that the resistance of thermal reduction of chemically reduced GO was higher than the resistance of thermal treatment. It suggests that thermal reduction process is the best method for GO reduction.
Acknowledgements This work was supported by Fundamental Research Grant (FRGS: 9003-00368) funded by Ministry of Education Malaysia. References [1] [2] [3] [4]
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