Electron beam irradiation dose dependent physico-chemical and electrochemical properties of reduced graphene oxide for supercapacitor

Electrochimica Acta 184 (2015) 427–435

Contents lists available at ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Electron beam irradiation dose dependent physico-chemical and electrochemical properties of reduced graphene oxide for supercapacitor Myunggoo Kanga , Dong Heon Leea , Yong-Mook Kangb , Hyun Junga,* a b

Advanced Functional Nanohybrid Material Laboratory, Department of Chemistry, Dongguk University-Seoul Campus, Seoul 100-715, Republic of Korea Department of Energy and Materials Engineering, Dongguk University-Seoul Campus, Seoul 100-715, Republic of Korea

A R T I C L E I N F O

A B S T R A C T

Article history: Received 22 July 2015 Received in revised form 2 October 2015 Accepted 10 October 2015 Available online 23 October 2015

Reduced graphene oxides (rGOs) with micropores were successfully obtained from a graphite oxide (GO) suspension in 2-propanol/water by electron beam irradiation at room temperature under ambient air conditions. During the radiolysis reaction, hydrated electrons (e
aq) were generated and acted as a reducing agent for the reduction of GO. The physico-chemical properties, such as disorder degree, oxygen content, specific surface area, pore structure, and sheet resistance of the rGOs were systematically controlled by adjusting the electron beam irradiation dose (50360 kGy). Especially, higher irradiation dose reduced the oxygen content, increased the specific surface area, and increased the number of micropores of rGO, which are important factors for supercapacitor performance. In order to investigate the electrochemical performance of the rGOs, electrochemical measurements were performed with a three-electrode system in 6.0 M KOH aqueous media. The highest capacitance of 206.8 F g
1 was achieved at a charge/discharge current density of 0.2 A g
1 in 6.0 M KOH aqueous solution for a sample reduced by electron beam irradiation of 200 kGy. ã 2015 Elsevier Ltd. All rights reserved.

1. Introduction Graphene, a two-dimensional nanosheet of sp2-hybridized carbon, has attracted tremendous interest as a key material in various fields such as electronic devices [1], sensors [2], Li-ion batteries [3], polymer nanocomposites [4], and supercapacitors [5], because of its unique physical, chemical, and mechanical properties [6–8]. In particular, graphene possesses high specific surface area, excellent electrical conductivity, low production cost, long cycle life, outstanding chemical tolerance, and good mechanical properties, which render it as an ideal electrode material for lithium ion batteries or supercapacitors [9–11]. However, the realization of these applications is still not practical because of the limitations for large-scale and safe production of graphene nanosheets. Until now, to fabricate graphene, various preparation routes have been developed including mechanical cleavage [12], chemical vapor deposition [13], epitaxial growth [14], the reduction of graphite oxide (GO) by reducing agents [15] and thermal treatment [16]. Among these various methods, the reduction of GO is one of the most promising processes for the

* Corresponding author. Tel.: +82 2 2260 3212. E-mail address: [email protected] (H. Jung). http://dx.doi.org/10.1016/j.electacta.2015.10.053 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.

mass production of graphene. Nevertheless, these reduction processes need strong chemical reducing agents such as hydrazine, elevated temperatures (over 80  C) for long periods of 1224 h or heat treatment at high temperature (over 900  C) in an inert atmosphere. The use of highly toxic and dangerously unstable reducing agents for chemical reduction of GO must be avoided for large-scale and safe production of graphene. Recently, the reduction of GO has been achieved through the radiolysis method. In comparison with the conventional approach, this strategy has many advantages such as being chemical reductant free, cost-effective, eco-friendly, and easily scalable [17–20]. These reduction processes of GO are dominated by the radiation-chemical reaction in water/alcohol solution systems. Water, when exposed to high energy irradiation, can decompose into both oxidative (hydroxyl radical, OH) and reductive (hydrogen radical and hydrated electron, H and e
aq) species [21]. Meanwhile, alcohols can eliminate the oxidative OH, a radical scavenger, and stabilize reductive radicals, which can be used as a reducing medium for the chemical reaction [22]. Gamma-ray irradiation-induced reduction of GO in the absence of oxygen has been recently reported [17–19]. Although gamma-ray irradiation has various advantages for radiolysis, it must be carried out under oxygen-free conditions because the relatively lower energy of

428

M. Kang et al. / Electrochimica Acta 184 (2015) 427–435

gamma-ray is significantly suppressed by oxygen quenching process in air. However, an electron beam can supply sufficient hydrated electrons in air due to its high energy density [20]. In this regard, electron beam irradiation can reduce GO even at room temperature under ambient air conditions. However, little research has been conducted to systematically investigate the effect of electron beam irradiation dose on the physico-chemical properties such as disorder degree, oxygen content, pore structure, specific surface area, sheet resistance and electrochemical performance of supercapacitors. Especially, tailoring the pore structure of graphene is quite important in terms of both power delivery rate and energy storage capacity for various applications. In this regard, the electron beam can simultaneously reduce GO and generate micropores on the surface of reduced graphene oxides (rGO) even at room temperature under ambient air conditions along with fine tuning porous parameters of rGO. In this work, we present a simple approach to obtain rGOs with high porosity by using electron beam irradiation at room temperature under ambient air conditions. The degree of oxygen and porosity of rGO were controlled by the electron beam irradiation dose (50360 kGy). Both the oxygen content and pore characteristics of the rGOs were found to be critical factors influencing the other physico-chemical properties such as disorder degree, specific surface area, and sheet resistance. Furthermore, the supercapacitor performance of the rGOs was investigated by using aqueous electrolyte in a three-electrode system. The effects of the physico-chemical properties on the electrochemical performances for supercapacitor are discussed in detail. As a result, we could control the physico-chemical properties and its supercapacitor performance as a function of irradiation time. The highest specific capacitance (206.8 F g
1) in aqueous media was achieved at 0.2 A g
1 for the sample reduced by the electron beam irradiation of 200 kGy. 2. Experimental 2.1. Sample preparation 2.1.1. Synthesis of exfoliated graphite oxide (GO) aqueous dispersion Exfoliated GO aqueous dispersion was prepared from commercial graphite (Sigma Aldrich, 20 mm) by a modified Hummers’ method [23]. Briefly, 5.0 g of graphite and 2.5 g of NaNO3 (Sigma Aldrich) were added to 250 ml of concentrated H2SO4 (purity 95%) and stirred in an ice bath. Then, 30.0 g of KMnO4 (Daejung, purity 99%) was gradually added with vigorous stirring and the temperature of the mixture was kept at below 20  C. After the ice-bath was removed, the mixture was reacted at 35  C for 3 h and then 460 ml of distilled water was slowly added until the temperature reached 98  C. The dark brown suspension was maintained at this temperature for 30 min and then further diluted by adding 1.4 L of distilled water. Finally, the reaction was terminated by the addition of 30 ml of H2O2 (Daejung, purity 30%). The color of the solution changed from dark brown to bright brown. The reaction product was then separated by centrifugation at 9000 rpm for 10 min and washed with 1.0 M HCl solution (approximately 2 times) and distilled water (approximately 2 times). After the final sediment was redispersed in distilled water by an ultrasonicator (160 W for 30 min), the undispersed component was separated by centrifugation at 4000 rpm for 10 min to obtain the aqueous dispersion containing well-exfoliated GO nanosheets with a concentration of about 6.0 mg ml
1. 2.1.2. Synthesis of reduced graphene oxides (rGOs) through electron beam irradiation The rGOs were synthesized through facile radiolysis using electron beam irradiation. In a typical procedure, a GO aqueous

dispersion was diluted by adding a solution of 2-propanol and distilled water. The final GO concentration was about 2.0 mg ml
1 and the fraction of 2-propanol was about 25% (v/v) in the mixture. The mixed solutions were then put into an uncovered petri dish and irradiated with an electron beam with 50360 kGy doses at room temperature, under ambient air conditions. The electron beam irradiation was performed at a dose rate of 0.1 kGy s
1 in a linear electron beam accelerator (UELV-10-10S) provided by the Advanced Radiation Technology Institute of the Korea Atomic Energy Research Institute (KAERI). Upon electron beam irradiation, the color of the solution turned black because of the reduction of the GO suspension. As a result, the rGOs were aggregated and precipitated owing to the removal of the oxygen-containing functional group in the GO, followed by the gradual decrease of their hydrophilicity, as a function of the irradiation dose. Finally, the dispersed rGOs were centrifuged and washed several times with ethanol. Black powders were obtained by drying in a vacuum oven at 120  C. Hereafter, the resulting samples were denoted as rGO-dose. 2.2. Sample characterization The structural characteristics of GO and the rGOs were obtained by powder X-ray diffractometer (XRD, Rigaku, Ultima IV) using Nifiltered Cu Ka radiation (l = 1.5418 Å) with a graphite diffracted beam monochromator at a scan rate of 1 min
1 from 5 to 50 . The patterns were recorded at an operating voltage of 40 kV and a current of 30 mA. The chemical compositions were determined by performing elemental analysis (Thermo Scientific, Flash 2000) with combustion of GO and rGO powders at 950  C. The morphology of rGO-360 was characterized by field emission scanning electron microscopy (FE-SEM, JEOL, JSM-7100F), highresolution transmission electron microscopy (HR-TEM, JEOL, JEM3010) and atomic force microscopy (AFM, Bruker, N8 NEOS). For FE-SEM measurements, the powder sample was attached to an Al mount with carbon tape. HR-TEM images and selected area electron diffraction (SAED) patterns were obtained by using carbon-coated Cu grid (Carbon Type-B, 200 mesh) at an accelerating voltage of 300 kV. The AFM was operated in tapping mode using a Tap 190AI-G probe (Nanosensor). The sample was prepared by spin coating of a diluted colloidal dispersion in ethanol media on a freshly cleaved mica surface. X-ray photoelectron spectroscopy (XPS, Thermo Scientific, SIGMA probe) measurements of GO and the rGOs were obtained by using a monochromatized Al Ka X-ray source (1486.71 eV). Raman spectra were obtained by using a confocal Raman microscope (Nanobase, Xperam 200) at an excitation wavelength of 532 nm. The sheet resistances of the samples were measured by a four-probe method (Keithley, Model 2000 multimeter). Prior to XPS, Raman, and sheet resistance measurements, the samples were prepared by pressing the material into tablets with a 1 cm diameter under a pressure of 10000 psi. To determine the specific surface area and porous properties of the resulting samples, nitrogen adsorption-desorption isotherms were measured volumetrically at 77 K (Microtrac, BELsorp-mini II). The GO and rGOs were degassed at 80  C for 12 h and at 120  C for 5 h, respectively, under vacuum prior to the sorption measurements. The specific surface areas were calculated from the adsorption data using the Brunauer-Emmett-Teller (BET) method. The pore size distribution was calculated by the BarrettJoyner-Halenda (BJH) method from the nitrogen desorption isotherm curves. 2.3. Electrochemical properties measurements The electrochemical properties of the rGOs samples were investigated by cyclic voltammetry (CV), galvanostatic charge-

M. Kang et al. / Electrochimica Acta 184 (2015) 427–435

discharge (GC), and electrochemical impedance spectroscopy (EIS). All of the measurements were performed in three-electrode systems with 6.0 M KOH aqueous electrolyte. The material for the working electrode was a mixture consisting of the rGOs and polyvinylidene fluoride (PVDF) in a weight ratio of 9:1. The mixture was homogeneously molded in N-methyl-2-pyrrolidone and coated on Ni foam with an exposed surface area of 1 cm2 and then pressed under a pressure of 2000 psi. The pressed working electrodes were dried at 60  C for 8 h under vacuum. Pt wire and a saturated calomel electrode (SCE, Argenthal) were used as the counter and reference electrodes, respectively. The CV measurement was performed at a scan rate of 20 mV s
1. The GC test was operated at a current density of 0.2 to 12.0 A g
1. The EIS measurement was collected at an open circuit voltage with sinusoidal signal of 10 mV over the frequency range of 10 mHz10 kHz (WanA tech, ZIVE SP2). 3. Result and discussion

429

The interaction of the high-energy electron beam with water molecules produces various species, of which the hydrated electron (e
aq) is a strong reducing agent toward oxygencontaining functional groups such as alcohols, epoxies, ketones, and carboxylic acids. Therefore, the generated e
aq can cause the deoxygenation and hence reduction of GO. However, the reducing reaction can be accomplished only under suitable experimental conditions because e
aq tends to be degraded by Reaction (2). It has been reported that the addition of an appreciable amount of alcohol to the water increases the life-time of e
aq. The addition of alcohol, acting as a scavenger of the hydroxyl radical (OH), dramatically enhances the stability of the hydrated electron by Reaction (3), thereby preventing Reaction (2). Herein, it is assumed that Reaction (4) does not occur to an appreciable extent [22]. e
aq + OH ! OH


(2)



(3)

OH + (CH3)2CHOH ! C(CH3)2OH + H2O

3.1. Theory of the rGOs via electron beam The primary radiolysis process of water under electron beam irradiation can be explained by Reaction (1) [21]. (1)

e
aq + C(CH3)2OH ! C(CH3)2OH


(4)


Furthermore, the decay rate of e aq in mixed solvent of water and alcohol would be lower than that in pure water because of the much higher viscosity of the mixed solvent and consequently lower diffusion coefficients [22]. Based on this concept, it is expected that the e
aq generated during the radiolysis of water in

Fig. 1. (a) FE-SEM, (b) HR-TEM (the inset shows the SAED pattern), and (c) AFM images of rGO-360 (the inset shows the high resolution), and (d) the cross-section of red line in (c).(For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

430

M. Kang et al. / Electrochimica Acta 184 (2015) 427–435

The morphology of rGO-360 was observed by FE-SEM, HR-TEM, and AFM. Fig. 1(a) shows the FE-SEM image of the rGO360 agglomerate. The SEM image shows the wrinkled platelet morphology with three-dimensional porous structure, which randomly stacks layer upon layer that are cross-linked with each other by strong p-p interaction of graphene nanosheets after the reduction. The HR-TEM image (Fig. 1(b)) of rGO-360 shows the morphology of the folded and wrinkled nanosheets with a thickness of a few layers. The SAED pattern (in the inset of Fig. 1(b)) of rGO-360 clearly shows the spots comprising hexagonal symmetry with slightly diffuse rings due to the restacking of exfoliated nanosheets by van der Waals attractive interaction, which induces the loss of ordering between the graphene sheets [24]. The AFM image for rGO-360 obtained by ultrasonic treatment revealed the presence of sheets with uniform thickness (Fig. 1(c)). The cross-sectional view of the rGO-360 sheet shows that the average thickness of the sample is 0.4 nm, which almost corresponds to a single layer of graphene with the ideal thickness of 0.34 nm [25]. The dark holes in the inset AFM image of Fig. 1(c) confirm the porous surface nature of the sample due to the high energy of the electron beam. This unique surface property contributes to the specific surface area and sheet resistance of the samples [26]. The structural changes of GO depending upon irradiation dose were investigated by powder XRD measurement, and the patterns of pristine graphite, GO, and the rGOs are shown in Fig. 2. After oxidation, the (002) diffraction peak of GO shifts to a lower angle at 12.9 , which corresponds to d-spacing of approximately 0.686 nm. This expanded d-spacing was induced by the presence of oxygen functional groups such as hydroxyl, carboxyl, and epoxyl moieties on the surface of the GO sheets. Due to the electron beam irradiation, the sharp (002) diffraction peak of GO shifts toward a

higher angle of about 2324 (0.3810.394 nm) due to the reduction of the interlayer space between carbon nanosheets by the removal of oxygen-containing functional groups. In the case of rGO-50, the XRD pattern remains very weak with a broad peak around 12.8 (0.689 nm) because GO is not reduced completely at low irradiation dose. The extremely broad (002) diffraction peaks of the rGO samples indicate that the crystalline structure of the reduced products was composed of only a few-layer stacked graphene nanosheets. To further characterize the structures of the rGOs, Raman spectroscopy was performed on graphite, GO, and the rGOs at various irradiation doses, as shown in Fig. 3. Generally, the characteristic G and D bands are very sensitive to defects, disorder, and graphitic carbon grain size [27,28]. The G band (the E2g phonon at the Brillouin zone center) is induced from the in-phase vibration of the ordered sp2 bonded carbon in the graphite lattice, while the D band (the A1g breathing mode at K-point) is evolved by the symmetry breaking and corresponding change in selection rules at the graphite edges. The D and G bands of GO are broadened and the G band is blue-shifted to 1593 cm
1, which are attributed to the enlargement of the disordered region and the decrease of the sp2 domain of GO resulting from the extensive oxidation (shown in Table 1). In the case of the rGOs, the G bands of all the samples are red-shifted toward that of graphite, revealing the recovery of the sp2 domains. The intensity ratio (ID/IG) values of GO and the rGOs, shown in Table 1, indicate the degree of disorder and defect for each rGO sample [27]. The ID/IG values of the rGOs gradually increase up to 1.11 with increasing irradiation dose, which is attributed to the increase in the number of defects in the sp2 domains, and is followed by the decreased average size of the sp2 domains in the rGOs during irradiation. Based on this Raman analysis, it can be concluded that GO is successfully reduced and that the defects in the rGOs can be controlled by varying the irradiation dose. To further confirm the removal of the oxygen-containing functional groups of GO by electron beam irradiation, XPS measurements were taken. The high-resolution XPS C1s partial spectra of pristine GO and the rGOs are shown in Fig. 4. Curve fitting for the C 1s spectra was performed using a GaussianLorentzian peak shape after Shirley background correction. All C 1s XPS spectra exhibit five different peaks located at 284.4, 285.5,

Fig. 2. XRD patterns of graphite, GO, and rGOs irradiated at various doses.

Fig. 3. Raman spectra of pristine graphite, GO, and rGOs as a function of dose.

the presence of 2-propanol are responsible for the reduction of GO. Moreover, the amount of e
aq formed depends on the irradiation dose, which facilitates easy control of the extent of GO reduction. This conclusion was also confirmed by UV-vis spectra (Fig. S1), thermogravimetric analysis (TGA) (Fig. S2), and elemental analysis (Table S1). 3.2. Physico-chemical properties of rGOs

M. Kang et al. / Electrochimica Acta 184 (2015) 427–435 Table 1 Structural parameters of graphite, GO and rGOs. Sample

d002 (nm)

D band (cm
1)

G band (cm
1)

ID/IG

Graphite GO rGO-50 rGO-100 rGO-200 rGO-300 rGO-360

0.337 0.740 0.387 0.381 0.390 0.394 0.390

1346 1358 1339 1339 1339 1339 1339

1565 1593 1590 1590 1584 1584 1584

0.12 0.83 1.01 1.04 1.06 1.10 1.11

286.4, 287.6, and 288.6 eV, which are assigned to C

C (unoxidized graphitic carbon), C

O (hydroxyl), C

O

C (epoxide), C¼O (ketone), and O

C¼O (carboxyl) groups, respectively [29,30]. Relative quantities of surface carbon-containing groups (as a percentage of particular carbon species in the C 1s XPS spectrum) are presented in Table S2. Although the irradiation dose of 50 kGy was short at around 8 min, the oxygen-containing functional groups were significantly decreased. The peak area of the C

C group was increased from 33.2% for GO to 72.5% for rGO-50, while the bonding between carbon and oxygen was significantly decreased. Especially, the proportion of C

O

C (epoxide) groups dramatically decreased from 47.4% for GO to 7.4% for rGO-50 after reduction. This result suggests that most of the oxygen-containing functional group were rapidly removed. In addition, the intensities of the oxygen-containing functionalities were gradually reduced with increasing irradiation dose (Table S2). These results clearly indicated that GO was successfully deoxygenated by electron beam irradiation with the extent of reduction being dependent on the absorbed dose. The specific surface areas and pore size distributions of the rGOs were investigated by nitrogen adsorption-desorption analysis. The nitrogen adsorption-desorption isotherms of the rGOs as a function of the irradiation dose are presented in Fig. 5(a). All isotherm curves can be characterized as hybrids between types I and IV in the IUPAC classification, which are indicative of the coexistence of micro- and mesopores in the samples. The

431

hysteresis loop at relative pressure of 0.40.6 resembles type H4 in the IUPAC classification, resulting from slit-like pores between parallel carbon layers with mesopores. Furthermore, the narrow hysteresis loop indicates that the pores in the samples are quite open [31]. The pattern of isotherm curves for all of the rGOs exhibits the typical shape previously reported for rGO [15,32,33]. As shown in Table 2, the surface area and total pore volume of the rGOs were greatly increased with increasing irradiation dose. All of the isotherms of the rGO series were best fit by the BET equation rather than by the Langmuir one. Surprisingly, the BET surface area (SBET) of rGO-360 was determined to be 742 m2 g
1, with a total pore volume (Vt) of 0.427 ml g
1, which greatly exceeds that of the pristine GO (225 m2 g
1). In order to evaluate the quantity of micropores in the rGOs, t-plots were calculated from the isotherms, as shown in Fig. S3, in which the amount of nitrogen adsorbed was plotted against the statistical thickness (t) obtained from the standard t-curve of graphitized carbon. The micropore size (2t) was about 0.9 Å, which is consistent with the value calculated by the MP method (shown in Fig. S4). The value of micropore volume (Vmicro) is equivalent to the intercept value extrapolated from the high-pressure branch to the adsorption axis. The Vmicro increased with increasing irradiation dose from 0.206 ml g
1 for rGO-50 to 0.371 ml g
1 for rGO-360 because micropores in the rGOs were generated by the radiation-induced removal of carbon atoms at the surface of the rGOs. The SBET (742 m2 g
1) and Vmicro (0.371 ml g
1) were much higher than those of previously reported for rGO prepared by conventional chemical method [34]. Generally, chemical reducing agents such as hydrazine afford rGO in the same manner with electron beam irradiation but struggle to generate micropores in the final sample directly [35,36]. So, some of previously reported literatures introduces thermal oxidation process to generate micropores [33,37]. However, the irradiation process can both generate micropores and control the pore structure alongside the reduction of GO. This increment of micropore volume was considered a major factor generating the higher specific surface area. The mesopore size distribution curves of rGOs were calculated by the BJH method from the desorption branch, shown in Fig. 5(b). According to the

Fig. 4. X-ray photoelectron C 1s partial spectra of (a) GO, (b) rGO-50, (c) rGO-100, (d) rGO-200, (e) rGO-300, and (f) rGO-360.

432

M. Kang et al. / Electrochimica Acta 184 (2015) 427–435

peak shown in the figure, the average pore size was estimated to be around 37.839.0 Å, which may have been due to the irregular restacking of rGOs with the crumpled three-dimensional structure of the sheet upon reduction and the removal of the solvent. 3.3. Electrochemical properties of rGOs The sheet resistances of the rGOs were measured by a DC 4-probe method, as shown in Fig. 6 and Table 3. For the rGO-50, -100 and -200 samples, the sheet resistance decreased radically as the irradiation dose increased from 50 to 200 kGy. In contrast, the rGO-300 and -360 samples exhibited a gradual increase of the sheet resistance until about 58 V cm
2. Oxygen functional groups bonded to reduced graphene and other atomic-scale defects can affect the electron transport of reduced graphene [38]. Therefore, the sheet resistance of the low-dose-irradiated GO samples decreased radically with the removal of the oxygen group. However, when the rGO samples were irradiated at higher doses (300 and 360 kGy), the size of the sp2 domain was reduced due to the increment of the atomic defects of carbon in the graphitic lattice and thus the in-plane electrical resistance of rGO increased. Similarly, other reports demonstrated that thermally reduced graphene has the highest electrical conductivity at the optimum temperature [39]. The electrochemical performances of rGO-based electrodes were investigated by CV, GC and EIS. Fig. 7(a) compares the CV curves of the rGO-based electrodes obtained in 6.0 M KOH aqueous media at a scan rate of 20 mV s
1. All the rGO-based electrodes present a nearly rectangular shape without obvious redox peaks, suggesting typical electric double-layer capacitor (EDLC) behavior. As a function of irradiation dose, the CV curves of the electrodes evolved from a distorted pattern at lower doses to a shape approaching rectangular at higher doses. This suggests that the charge/discharge process in the rGO-based electrodes was improved by the facilitated electron transfer owing to the repaired p-conjugation structure of rGO and that the Faradaic activity of graphene oxide was strongly diminished by the reductive elimination of oxygen-related functional groups after the electron beam irradiation. The GC technique was performed to confirm the results determined from CV curve. Fig. 7(b) shows galvanostatic charge/ discharge curves of rGO-based electrodes in 6.0 M KOH solution between -1.0 and 0 V (vs. SCE) at a current density of 1.0 A g
1. The curves diverge from the typical triangular shape to a certain extent. The oxygen functional groups remaining on the rGOs cause redox Fig. 5. (a) Nitrogen adsorption-desorption isotherms and (b) BJH pore size distributions of rGOs.

Table 2 BET specific surface area and porous parameters of GO and rGOs. Sample

SBET (m2 g
1)a

Pore Volume (ml g
1) Vtb Vmicroc

Pore Diameter (Å) Dmesod Dmicroe

GO rGO-50 rGO-100 rGO-200 rGO-300 rGO-360

225 416 518 622 676 742

0.141 0.228 0.301 0.371 0.403 0.427

39.5 37.8 38.9 39.0 38.9 38.0

0.091 0.206 0.262 0.330 0.358 0.371

8.6 9.0 8.9 9.1 9.1 9.0

a

BET specific surface area (SBET) calculated from the linear part of the BET plot. The total pore volume (Vt) was taken from the volume of nitrogen adsorbed at about P/P0 = 0.98. c The micropore volume (Vmicro) was estimated by the t-plot. d The average mesopore diameter (Dmeso) was estimated by the BJH formula. e The average micropore diameter (Dmicro) was estimated by the t-plot method. b

Fig. 6. The sheet resistance of the rGOs measured by 4-point probe measurements.

M. Kang et al. / Electrochimica Acta 184 (2015) 427–435

433

Table 3 Sheet resistance and specific capacitance. Sample

rGO-50

rGO-100

rGO-200

rGO-300

rGO-360

Sheet resistance (ohm cm
2) Specific capacitor (F g
1) 0.2 A g
1 1.0 A g
1 4.0 A g
1 8.0 A g
1 12.0 A g
1

150

60

12

20

58

156.9 108.0 62.4 34.4 19.2

192.8 116.6 69.6 44.2 32.2

206.8 160.4 123.2 96.6 79.0

204.9 144.8 111.2 88.0 72.8

179.4 127.6 103.2 88.8 76.8

reaction during the charge/discharge process, resulting in slight stretching of the cycling curve. These symmetrical triangular curves with very small iR drop suggest that the electrodes mainly exhibit a double-layer capacitor behavior resulting from the electrostatic attraction and have low equivalent series resistance. The specific capacitance can be calculated from the galvanostatic charge/discharge curves according to the following equation [40]: C ¼ I  Dt=ðDV  mÞ; Where C is the specific capacitance (F g
1), I is the discharge current (A), Dt is the charge or discharge time (s), m is the mass of

active material in the electrode (g), and DV indicates the voltage change after a full charge or discharge (V). At a current density of 1.0 A g
1, the capacitance of the rGO-based electrodes corresponded to that shown in Table 3. Among all the samples, rGO200 had the highest specific capacitance. The capacitance of the rGO-based electrodes gradually increased as the irradiation dose was increased from 50 to 200 kGy but then decreased with further irradiation dose increase. Although rGO-360 had the highest SBET and Vt, which are favorable for electrolyte penetration and ion diffusion in reduced graphene, the specific capacitances of rGO-300 and -360 were lower than that of rGO-200, which we

Fig. 7. (a) CV behaviors of the rGO-based electrodes at scan rate of 20 mV s
1, (b) galvanostatic charge/discharge cycling curves for the rGO-based electrodes at a current density of 1.0 A g
1, (c) specific capacitance of the rGO-based electrodes as a function of current density, and (d) the cyclic retentions of the supercapacitors based on rGOs at 4.0 A g
1.

434

M. Kang et al. / Electrochimica Acta 184 (2015) 427–435

attributed to two main factors: electronic conductivity and pseudocapacitance. The electron beam irradiation induces the reduction of GO and simultaneously generates micropores in the plane of graphene, which was confirmed by the AFM and BET measurements. The electronic conductivity was improved by the repaired conjugation as the irradiation dose was increased from 50 to 200 kGy. However, when the dose exceeded 200 kGy, the electronic conductivity was diminished. The increased micropores in the graphitic lattice afforded a high SBET, while the sheet resistance was increased by the loss of the sp2 carbon network. Another possible factor depends on the pseudo-capacitance of the remaining oxygen functional groups on the external surface of the rGOs because they enable fast redox processes [41]. Thus, rGO-based electrodes can simultaneously act as both EDLC and pseudo-capacitors. Electron beam irradiation diminished the pseudo-capacitance contribution of rGOs, but its contribution to EDLC was greatly enhanced as a function of irradiation dose. Consequently, the rGO-200-based electrode showed the maximum specific capacitance. Retention value is also an important parameter for estimating the electrical performance of a supercapacitor. A good energy storage device is required to maintain its energy capacity through a high current density. The specific capacitance versus current density for all reduced graphene is plotted in Fig. 7(c). At the current densities ranging from 0.2 to 12.0 A g
1, the retention values of rGO-50, -200, and -360-based electrodes were 12.2%, 38.2%, and 42.8%, respectively. The sharp drop of capacitance for rGO-50 was ascribed to the irreversible pseudo-capacitance of the electrochemically unstable oxygen-containing functional groups. Fig. 7(d) shows the cycle performance of the rGO-200 and -360 electrodes for 1000 cycles at 4.0 A g
1 from -1.0 to 0 V. rGO200 exhibited more improved cycling stability during 1000 cycles compared with that of rGO-360. Thus, rGOs demonstrated a promising potential as a supercapacitor, in comparison with previously reports that investigated the conventional chemical reduction method [10,42,43]. Fig. 8 compares the Nyquist plots of the rGO-based electrodes. All samples presented two distinct parts: a semicircle in the high frequency region (kinetic control regime) and a slope in the low frequency region (diffusion control regime). The inclined lines of rGO-50 and -100 in the low frequency region suggest that their charge-storage mechanism is more dominated by Faradic redox reactions in comparison with the other samples. All the circuit elements in the Nyquist plot can be organized to form an equivalent circuit diagram, as shown in the inset of Fig. 8, which

can be used to fit the Nyquist spectra measured for the supercapacitors at different irradiation doses. The equivalent circuit is a modified version reported by Qu et al. [44], where the Warburg diffusion element (W0) is included in order to compensate for the depressed semicircle and vertical line in the diffusion control regime. The fitting results of the supercapacitors are listed in Table S3. In the kinetic control regime, the electrolyte resistance (Rs) remained almost constant because it is insensitive to the electrode surface, whereas the charge transfer resistance (Rct) was significantly decreased with increasing irradiation dose. The reduced Rct of rGO-200, -300, and -360 was consistent with their relatively high conductivity, as well as the reduced contribution of pseudo-capacitance (C’) due to the deoxygenation that occurred during the electron beam irradiation. 4. Conclusion In conclusion, rGOs with micropores were synthesized at room temperature under ambient air condition by electron beam irradiation in the presence of 2-propanol as a hydroxyl radical scavenger, which facilitated the reduction of GO without the need for any chemical reducing agents. The variation of irradiation dose over the range of 50360 kGy was the main factor influencing the physico-chemical properties of the rGOs, such as disorder degree, oxygen content, pore structure, specific surface area, and sheet resistance. The high irradiation energy of the electron beam simultaneously generated hydrated electrons with high reducing power and atomic defects in the carbon of the rGO graphitic lattice. Thus, the rGO samples exhibited decreasing oxygen content and increasing Vmicro with increasing irradiation dose, which increased the SBET of the rGOs. These properties were shown to be factors influencing the supercapacitor performance of rGO-based electrodes. As the irradiation dose was increased, the contribution of pseudo-capacitance was dramatically reduced, but that of EDLC was strongly enhanced in the supercapacitor performance of the rGO-based electrodes. As a result, rGO-200 exhibited the highest specific capacitance of 206.8 F g
1 at a charge/discharge current density of 0.2 A g
1 in 6.0 M KOH aqueous solution. This work systematically expands our understanding of the variation of physico-chemical properties and the factors that affect the supercapacitor performance for graphene as a function of electron beam irradiation. These study results demonstrate that electron beam-induced reduction is a feasible, scalable and safe production technique approach to produce graphene-tailored micropores as the electrode material for supercapacitors. Acknowledgements This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) and the Energy Efficiency and Resources of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Ministry of Education (No. NRF-2013R1A1A2013035) and the Ministry of Knowledge Economy (No. 20122010100140). References

Fig. 8. Nyquist spectra of the rGO-based electrodes as a function of dose. The Nyquist spectrum is a complex function of the resistance (R) and capacitance (C) that can correlate the electrochemical behavior of supercapacitors with an equivalent RC circuit, which is inserted in the figure.

[1] Y.M. Lin, K.A. Jenkins, A. Valdes-Garcia, J.P. Small, D.B. Farmer, P. Avouris, Operation of graphene transistors at gigahertz frequencies, Nano Lett. 9 (2009) 422–426. [2] M. Pumera, A. Ambrosi, A. Bonanni, E.L.K. Chng, H.L. Poh, Graphene for electrochemical sensing and biosensing, Trac-Trend Anal. Chem. 29 (2010) 954–965. [3] S.M. Paek, E. Yoo, I. Honma, Enhanced cyclic performance and lithium storage capacity of SnO2/graphene nanoporous electrodes with three-dimensionally delaminated flexible structure, Nano Lett. 9 (2009) 72–75. [4] T. Ramanathan, A.A. Abdala, S. Stankovich, D.A. Dikin, M. Herrera-Alonso, R.D. Piner, D.H. Adamson, H.C. Schniepp, X. Chen, R.S. Ruoff, S.T. Nguyen, I.A. Aksay,

M. Kang et al. / Electrochimica Acta 184 (2015) 427–435

[5]

[6] [7] [8] [9] [10]

[11]

[12] [13]

[14] [15]

[16]

[17]

[18]

[19]

[20]

[21] [22]

[23] [24]

[25]

R.K. Prud'homme, L.C. Brinson, Functionalized graphene sheets for polymer nanocomposites, Nat. Nanotechnol. 3 (2008) 327–331. J. Yang, M.R. Jo, M. Kang, Y.S. Huh, H. Jung, Y.M. Kang, Rapid and controllable synthesis of nitrogen doped reduced graphene oxide using microwaveassisted hydrothermal reaction for high power-density supercapacitors, Carbon 73 (2014) 106–113. A.K. Geim, K.S. Novoselov, The rise of graphene, Nat. Mater. 6 (2007) 183–191. A.H. Castro Neto, F. Guinea, N.M.R. Peres, K.S. Novoselov, A.K. Geim, The electronic properties of graphene, Rev. Mod. Phys. 81 (2009) 109–162. C. Lee, X.D. Wei, J.W. Kysar, J. Hone, Measurement of the elastic properties and intrinsic strength of monolayer graphene, Science 321 (2008) 385–388. J.Y. Luo, H.D. Jang, J.X. Huang, Effect of sheet morphology on the scalability of graphene-based ultracapacitors, ACS Nano 7 (2013) 1464–1471. X.A. Du, P. Guo, H.H. Song, X.H. Chen, Graphene nanosheets as electrode material for electric double-layer capacitors, Electrochim. Acta 55 (2010) 4812–4819. E.B. Nursanto, A. Nugroho, S.A. Hong, S.J. Kim, K.Y. Chung, J. Kim, Facile synthesis of reduced graphene oxide in supercritical alcohols and its lithium storage capacity, Green Chem. 13 (2011) 2714–2718. K.S. Novoselov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666–669. A. Reina, X.T. Jia, J. Ho, D. Nezich, H.B. Son, V. Bulovic, M.S. Dresselhaus, J. Kong, Large area, few-layer graphene films on arbitrary substrates by chemical vapor deposition, Nano Lett. 9 (2009) 30–35. P.W. Sutter, J.I. Flege, E.A. Sutter, Epitaxial graphene on ruthenium, Nat. Mater. 7 (2008) 406–411. S. Stankovich, D.A. Dikin, R.D. Piner, K.A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S.T. Nguyen, R.S. Ruoff, Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide, Carbon 45 (2007) 1558–1565. D. Yang, A. Velamakanni, G. Bozoklu, S. Park, M. Stoller, R.D. Piner, S. Stankovich, I. Jung, D.A. Field, C.A. Ventrice, R.S. Ruoff, Chemical analysis of graphene oxide films after heat and chemical treatments by X-ray photoelectron and Micro-Raman spectroscopy, Carbon 47 (2009) 145–152. C.H. Jung, Y.W. Park, I.T. Hwang, Y.J. Go, S.I. Na, K. Shin, J.S. Lee, J.H. Choi, Ecofriendly and simple radiation-based preparation of graphene and its application to organic solar cells, J. Phys. D Appl. Phys. 47 (2014) 015105. Y.W. Zhang, H.L. Ma, Q.L. Zhang, J. Peng, J.Q. Li, M.L. Zhai, Z.Z. Yu, Facile synthesis of well-dispersed graphene by gamma-ray induced reduction of graphene oxide, J. Mater. Chem. 22 (2012) 13064–13069. B.W. Zhang, L.F. Li, Z.Q. Wang, S.Y. Xie, Y.J. Zhang, Y. Shen, M. Yu, B. Deng, Q. Huang, C.H. Fan, J.Y. Li, Radiation induced reduction: an effective and clean route to synthesize functionalized graphene, J. Mater. Chem. 22 (2012) 7775– 7781. J.M. Jung, C.H. Jung, M.S. Oh, I.T. Hwang, C.H. Jung, K. Shin, J. Hwang, S.H. Park, J. H. Choi, Rapid, facile, and eco-friendly reduction of graphene oxide by electron beam irradiation in an alcohol-water solution, Mater. Lett 126 (2014) 151–153. W.H. Hamill, Model for the radiolysis of water, J. Phys. Chem. 73 (1969) 1341–1347. J. Cygler, G.R. Freeman, Effects of solvent structure on electron reactivity and radiolysis yields: 2-propanol/water mixed solvents, Can. J. Chem. 62 (1984) 1265–1270. W.S. Hummers, R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem. Soc. 80 (1958) 1339. M.J. McAllister, J.L. Li, D.H. Adamson, H.C. Schniepp, A.A. Abdala, J. Liu, M. Herrera-Alonso, D.L. Milius, R. Car, R.K. Prud'homme, I.A. Aksay, Single sheet functionalized graphene by oxidation and thermal expansion of graphite, Chem. Mater. 19 (2007) 4396–4404. S. Niyogi, E. Bekyarova, M.E. Itkis, J.L. McWilliams, M.A. Hamon, R.C. Haddon, Solution properties of graphite and graphene, J. Am. Chem. Soc. 128 (2006) 7720–7721.

435

[26] Y.Y. Zhang, L. Chen, Z.W. Xu, Y.L. Li, B.M. Zhou, M.J. Shan, Z. Wang, Q.W. Guo, X. M. Qian, Preparing graphene with notched edges and nanopore defects by gamma-ray etching of graphite oxide, Mater. Lett. 89 (2012) 226–228. [27] A.C. Ferrari, Raman spectroscopy of graphene and graphite: Disorder, electronphonon coupling, doping and nonadiabatic effects, Solid State Commun. 143 (2007) 47–57. [28] A.C. Ferrari, J.C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K.S. Novoselov, S. Roth, A.K. Geim, Raman spectrum of graphene and graphene layers, Phys. Rev. Lett. 97 (2006) 187401. [29] S. Park, K.S. Lee, G. Bozoklu, W. Cai, S.T. Nguyen, R.S. Ruoff', Graphene oxide papers modified by divalent ions - Enhancing mechanical properties via chemical cross-linking, ACS Nano 2 (2008) 572–578. [30] S.D. Perera, R.G. Mariano, N. Nijem, Y. Chabal, J.P. Ferraris, K.J. Balkus, Alkaline deoxygenated graphene oxide for supercapacitor applications: An effective green alternative for chemically reduced graphene, J. Power Sources 215 (2012) 1–10. [31] S.M. Paek, H. Jung, M. Park, J.K. Lee, J.H. Choy, An inorganic nanohybrid with high specific surface area: TiO2-pillared MoS2, Chem. Mater. 17 (2005) 3492– 3498. [32] Y.C. Bai, R.B. Rakhi, W. Chen, H.N. Alshareef, Effect of pH-induced chemical modification of hydrothermally reduced graphene oxide on supercapacitor performance, J. Power Sources 233 (2013) 313–319. [33] S.W. Wang, D. Minami, K. Kaneko, Comparative pore structure analysis of highly porous graphene monoliths treated at different temperatures with adsorption of N2 at 77.4K and of Ar at 87.3K and 77.4K, Micro. Meso. Mat. 209 (2015) 72–78. [34] H.B. Li, L.K. Pan, C.Y. Nie, Y. Liu, Z. Sun, Reduced graphene oxide and activated carbon composites for capacitive deionization, J. Mater. Chem. 22 (2012) 15556–15561. [35] X.F. Gao, J. Jang, S. Nagase, Hydrazine and thermal reduction of graphene oxide: reaction mechanisms, product structures, and reaction design, J. Phys. Chem. C 114 (2010) 832–842. [36] P.G. Ren, D.X. Yan, X. Ji, T. Chen, Z.M. Li, Temperature dependence of graphene oxide reduced by hydrazine hydrate, Nanotechnology 22 (2011) 055705. [37] Y.W. Zhu, S. Murali, M.D. Stoller, K.J. Ganesh, W.W. Cai, P.J. Ferreira, A. Pirkle, R. M. Wallace, K.A. Cychosz, M. Thommes, D. Su, E.A. Stach, R.S. Ruoff, Carbonbased supercapacitors produced by activation of graphene, Science 332 (2011) 1537–1541. [38] G.M. Rutter, J.N. Crain, N.P. Guisinger, T. Li, P.N. First, J.A. Stroscio, Scattering and interference in epitaxial graphene, Science 317 (2007) 219–222. [39] B. Zhao, P. Liu, Y. Jiang, D.Y. Pan, H.H. Tao, J.S. Song, T. Fang, W.W. Xu, Supercapacitor performances of thermally reduced graphene oxide, J. Power Sources 198 (2012) 423–427. [40] Q.L. Zhang, Y.W. Zhang, Z.H. Gao, H.L. Ma, S.J. Wang, J. Peng, J.Q. Li, M.L. Zhai, A facile synthesis of platinum nanoparticle decorated graphene by one-step gamma-ray induced reduction for high rate supercapacitors, J. Mater. Chem. C 1 (2013) 321–328. [41] Q.L. Du, M.B. Zheng, L.F. Zhang, Y.W. Wang, J.H. Chen, L.P. Xue, W.J. Dai, G.B. Ji, J. M. Cao, Preparation of functionalized graphene sheets by a low-temperature thermal exfoliation approach and their electrochemical supercapacitive behaviors, Electrochim. Acta 55 (2010) 3897–3903. [42] D.R. Dreyer, S. Murali, Y.W. Zhu, R.S. Ruoff, C.W. Bielawski, Reduction of graphite oxide using alcohols, J. Mater. Chem. 21 (2011) 3443–3447. [43] Y. Wang, Z.Q. Shi, Y. Huang, Y.F. Ma, C.Y. Wang, M.M. Chen, Y.S. Chen, Supercapacitor devices based on graphene materials, J. Phys. Chem. C 113 (2009) 13103–13107. [44] D.Y. Qu, Studies of the activated carbons used in double-layer supercapacitors, J. Power Sources 109 (2002) 403–411.