Electrochemically reduced graphene oxide and its capacitance performance

Materials Chemistry and Physics xxx (2014) 1e6

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Electrochemically reduced graphene oxide and its capacitance performance Shu-bo Zhang a, Yu-tao Yan b, *, Yu-qiu Huo a, *, Yang Yang a, Ji-long Feng a, Yu-feng Chen a a b

Department of Chemistry, College of Science, Northeastern University, Shenyang 110819, People's Republic of China School of Mechanical Engineering and Automatic, Northeastern University, Shenyang 110819, People's Republic of China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


ERGO is obtained by 1.1 V cathodic reductions in potassium biphthalate buffer solution.
The role of potassium biphthalate is discussed in the reduction process.
Most of the oxygen functional groups in GO are successfully removed.
A maximum specific capacitance of 254.9 F g1 is achieved in pH 13 Na2SO4 solution.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 April 2014 Received in revised form 11 July 2014 Accepted 31 August 2014 Available online xxx

Graphene oxide (GO) is reduced by a rapid cathodic reduction at 1.1 V in 0.05 M potassium biphthalate buffer solution (C8H4O4H2/C8H4O4HK). Potassium biphthalate plays an important role in the reduction process and it is superior to sodium acetate buffer solution (HOAc/NaOAc). The possible reason for this dissimilarity is discussed in detail. The reduced graphene oxide (ERGO) is characterized by Fourier transform infrared spectroscopy (FT-IR), thermogravimetry and differential scanning calorimetry (TGA/ DSC), X-ray diffraction spectroscopy (XRD) and transmission electron microscopy (TEM). The results indicate that most of the oxygen functional groups in GO are successfully removed. Electrochemical studies are carried out using cyclic voltammetry, galvanostatic charge/discharge, and electrochemical impedance spectroscopy. A maximum specific capacitance of 254.9 F g1 can be achieved in pH 13 Na2SO4 solution at current density of 1.14 A g1 within the potential window of 1.4 V. © 2014 Elsevier B.V. All rights reserved.

Keywords: Thin films Multilayers Electrochemical techniques Electrochemical properties

1. Introduction Graphene obtained by Geim et al. at Manchester University in 2004 [1] is a two-dimensional one-atom-thick sp2-bonded honeycomb carbon atom sheet [2,3]. It can be rolled flexible into onedimensional carbon nanotubes or stacked into three-dimensional graphite [4]. It exhibits excellent properties, such as superior thermal conductivity [5], remarkable mechanical strength [6], large theoretical specific surface area of 2630 m2 g1, and excellent * Corresponding authors. E-mail addresses: [email protected] (Y.-t. Yan), [email protected] (Y.-q. Huo).

electrical performance, the electron can transfer on graphene sheet at a constant rate, and the largest mobility of charge carriers exceeds 1.5  104 cm2 V1 s1 even under ambient conditions [12]. So graphene has exhibited great potential in various applications, including energy storage [7,8], polymer composites [9], nanoelectronics [10], and sensors [11] and so on. Supercapacitors are being considered promising power sources for applications due to their high power density and energy density [1e3]. It is well known that the performance of supercapacitors depends on the materials of electrode. Furthermore, the morphology, structure and size of electrode material play important roles on its capacitive properties. Different synthesis methods and even dissimilar reaction

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conditions would produce unusual structures and properties. Till now, several physical and chemical approaches have been developed to produce graphene, including chemical vapor deposition [13], chemistry reduction [14e16], flame reduced [17], and electrochemical reduction [18e20] etc. And various morphologies of graphenes have been obtained, comprising two dimensional graphene nanosheets (GNSs), one-dimensional graphene nanoribbons (GNRs) [21e23], and zero-dimensional graphene quantum dots (GQDs) [24,25]. The most exciting fact is the properties of graphene can be tuned by their size and edges [26]. The more attractive practices lead to the more enthusiasm to research on it. It is worth to do more study on graphene for its promising application in supercapacitors. In this paper, we report a facile and fast approach to synthesize high quality graphene, which is reduced from graphene oxide (GO) by cathodic reduction at 1.1 V in 0.05 M potassium biphthalate solution (pH ¼ 4). The mechanism of electrochemical reduction is discussed. As far as we know the reduction of GO in the potassium biphthalate buffer solution has not been reported before. In 2011 Peng [27] et al. reported a specific capacitance of 128 F g1 by reducing GO with constant potential reduction at 1.2 V in 0.5 M NaNO3 solution. Jiang [28] et al. prepared electrochemically reduced graphene oxide (ERGO) by cyclic voltammetry from 0.0 to 1.5 V (vs. SCE) in PBS (Na2HPO4/NaH2PO4, N2purged, 0.05 M, pH 5.0) solution. The specific capacitance of ERGO is 223.6 F g1. Considering the relative stability of buffer solution, and the character of GO, i.e. there are plenty of oxygen function groups, we choose potassium biphthalate buffer solution (pH ¼ 4) as the electrolyte in reducing process. In our present work, the specific capacitance of ERGO is up to 254.9 F g1, which is higher than literature 27 and 28. And most oxygen functional groups have been eliminated after the reduction process.

to the reduced time 0.5 h, 1 h, 2 h and 4 h, the electrochemical reduced graphene oxide (ERGO) are recorded as ER05, ER1, ER2, and ER4 (the mass is all about 1 mg), respectively. To compare the effect of different electrolytes on the graphene oxide reduction, GO is also electrochemical reduced in acetate buffer solution. ERGOs reduced 0.5 h, 1 h, 2 h and 4 h in acetate buffer solution are recorded as H05, H1, H2 and H4, respectively. The other experiment conditions are the same as that of in potassium biphthalate solution. The ERGO electrodes with different mass, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8 mg, are labeled as M1, M2, M3, M4, M5, M6, and M7, respectively. The geometric surface area of each electrode is 1 cm2. 2.3. Structural characterization ER05, ER1 are removed from foam Ni by dispersing in absolute ethyl alcohol and vacuum dried for 12 h at 70  C. And FT-IR adsorption spectra (KBr) of ER1, graphite, and GO are carried out using 510PFT FT-IR. Thermogravimetry and differential scanning calorimetry are tested by TGA/DSC. The graphite, GO and ER1 samples are heated in a nitrogen atmosphere from room temperature to 900  C at a heating rate of 10  C min1. The transmission electron microscope of GO and ER1 are obtained using JEOL-2100F at acceleration voltage of 200 kV. X-ray diffraction analysis of GO, ER05 and ER1 are carried out using PW3040/60. 2.4. Electrochemical performance The electrochemical properties of materials are investigated by cyclic voltammograms (CV), galvanostatic charge/discharge (CP) and electrochemical impedance spectroscopy (EIS) at room temperature in pH 13 Na2SO4 solution (NaOH solution is added to adjust pH value). The specific capacitance is calculated with CP curves according to Eq. (1):

2. Experimental 2.1. Synthesis of graphene oxide Graphene oxide (GO) is obtained from reagent grade graphite powder (99.85%, shanghai colloid chemical industry, china) by using a modified Hummer's method. 2.0 g graphite powder is mixed with 50 ml 98% H2SO4 solution in ice bath and keeping stirred for 30 min at 0e10  C. Following that, 0.3 g KMnO4 is put into the mixture to pre-oxide 3 min. Then 6 g KMnO4 is added in 3 batches to keep the reaction temperature below 20  C. After that the ice bath is removed. The mixture remains at 35  C for 2 h. Follow on, 90 ml deionized water is slowly added to the paste to make the temperature reach 90 to 100  C. By then bright yellow deposition is obtained. Finally, 280 ml warm deionized water and 20 ml 30% H2O2 are added in 10 min to terminate the reaction. All the above processes are finished under keeping vigorously stirred. The as-prepared products are filtered and washed with 1% HCl solution and deionized water in turn to remove metal ions. The depositions GO is transferred to the watch glass and dried in vacuum at 70  C for 24 h. 2.2. Electrochemical reduction of GO GO and two drops of conductive adhesive (PELCO colloidal graphite) are mixed evenly and pasted on a Ni foam substrate. After compressed at 20 Mpa and vacuum dried for 24 h at 70  C, working electrode is obtained. Platinum sheet is used as counter electrode and saturated calomel electrode (SCE) is used as reference electrode. With a CHI 660D electrochemical workstation (Chenhua Instrument Co., Shanghai, China), graphene is obtained by constant potential reduction at 1.1 V in a 3-electrode configuration in 0.05 M potassium biphthalate buffer solution (pH ¼ 4). According

C¼

it mV

(1)

where i is the current density, t is the discharge time, m is the mass of materials and V is potential window. 3. Results and discussion 3.1. Characterization of ERGO FT-IR spectroscopy is used to indicate the degree of removing the oxygen functional groups. Fig. 1 shows the FT-IR spectra of pristine graphite (a), GO (b) and ER1 (c). In Fig. 1(a) and (b), the peak at 3424 cm1 is due to OeH

c O-H

C=C C-OH C=O O-H C-O-C

C-H

b a

3500

3000

2500

2000

1500

1000

500

-1 Wave number / cm Fig. 1. FT-IR spectrum of graphite (a), GO (b) and ER1 (c).

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S.-b. Zhang et al. / Materials Chemistry and Physics xxx (2014) 1e6

stretching mode of chemically adsorbed H2O, 1399 cm1 is OeH deformation vibration of carboxyl. The peak of CeOeC in epoxy or alkoxy is at 1053 cm1, and C]O in carboxylic acid and carbonyl appears at 1732 cm1 [19]. The peak at 1620 cm1 assigns to skeletal vibrations of unoxidized graphite (C]C) or contribution from the stretching deformation vibration of intercalated water [29,30]. The peak at 1222 cm1 represents CeOH stretching vibration [31] and the peaks at 2969 cm1and 2923 cm1 can be due to CeH vibration. In Fig. 1(c), the CeOH stretching vibration peak at 1222 cm1, OeH peak of carboxyl, and C]O peak in carboxylic acid or carbonyl are considerably decreased and nearly disappeared. While the CeOeC peak shows no distinctive changing compared with Fig. 1(b). This elucidates that CeOeC is difficult to be reduced by electrochemical method but other oxygen functional groups can be reduced by this condition. In fact, the existing of the residual electrochemically stable oxygen functional groups would benefit to the capacitance performance of materials owning to they produce a number of paths for ion transfer in the surface of graphene. Compared to GO, the wave number and intensity of C]C peak in ER1 are not varied. The presence of oxygen functional groups on the surface of graphene is further analyzed by TGA/DSC and the results of GO, ER1 and graphite are shown in Fig. 2(a), (b) and (c). The loss of humidity

3

in the GO sample is occurring even below 100  C. The major mass loss between 100 and 300  C is due to the disappear of chemically adsorbed water, and which above 300  C is due to the pyrolysis of labile oxygen functional groups, yielding CO, CO2, and steam [32,33]. Compared with GO, ER1 is thermally stable below 200  C. The 15% weight loss appeared at 200  Ce300  C is due to the loss of chemically adsorbed water and another 15% mass loss at 300e700  C is attribute to the extensively pyrolysis of residual oxygen functional groups to CO and CO2. There is nearly no mass loss in Fig. 2(c), which confirms that graphite is stable. The TGA results confirm the existence of residual oxygen functional groups in the ER1. This coincides with the conclusion of the FT-IR spectra. Transmission electron microscope is used to characterize the morphology characterization of graphite, GO and ER1. Fig. 3(a) shows the morphology of GO, which is smooth, semitransparent, cicada's wings like sheet with wrinkles. It indicates the GO is only several layers thick. The image of ER1 is shown in Fig. 3(b). Film overlaps and several wrinkles undulate are observed in the translucent sheets. The decreasing of oxygen functional groups may lead graphene layers easy to agglomerate. But it is not obviously in TEM images. Fig. 3(c) shows the morphology of pristine graphite, which is little sheet with diameter 200 nm. It looks like smaller in size than GO and graphene. This phenomenon is due to the oxidization of graphite enhances the interfacial distance, the materials become fluffier and they overlap each other, which make it seems large. XRD patterns of the GO, ER05, ER1 and standard card of C (Card No. 00-041-1487) are recorded in Fig. 4. In Fig. 4(a) of GO the feature diffraction peaks appear at 10.3 , which is near the position of the feature diffraction peak of GO (13 ) in most studies [16]. The shift of 13 e10.3 means the layer-to-layer distance of the GO we synthesized is larger than those of other workers. It elucidates the graphite is completely oxidized in our work. In Fig. 4(b) and (c) the feature diffraction peaks of reduced graphene are both at 26.2 , which is not only in good agreement with Choi E Y's study [16] but also with C standard card of Fig. 4(d) (No.00-041-1487). But the peak intensity of ER05 is less than that of ER1. Moreover, there is no peak at 44.2 for ER05 comparing to ER1. This confirms that ER1 is reduced more adequately than ER05. One hour is enough to obtain the graphene with constant cathodic potential reduction method. 3.2. Electrochemical characterizations

Fig. 2. TGA-DSC curves of GO (a), ER1 (b) and graphite (c).

CV curves of GO, ER05, ER1, ER2, and ER4 obtained at 100 mV s1 in pH 13 Na2SO4 solution are shown in Fig. 5. All figures exhibit typical rectangular shapes, which indicating excellent capacitive behaviors and wonderful reversibility of their charging and discharging process. Galvanostatic charge/discharge curves of GO, ER05, ER1, ER2 and ER4 at current density of 1.14 A g1 in pH 13 Na2SO4 solution are shown in Fig. 6. The symmetry of galvanostatic charge/discharge curves is excellent, revealing the reversibility of charging and discharging is wonderful. The specific capacitance of ER05, ER1, ER2 and ER4 calculated from galvanostatic charge/discharge curves is 239.7, 254.9, 250.3 and 245.3 F g1, respectively. They are all superior to 187.7 F g1 of GO. At first the specific capacitance enhances with the reducing time increasing from 30 min to 1 h. The maximum specific capacitance of 254.9 F g1 is obtained after reduced 1 h. Then it is slightly descended with the ascending of reduced time. This is due to the GO can not be reduced effectively in 30 min. Most oxygen functional groups remain in the materials, which lead the resistance of ER05 is slightly high. This is disadvantageous to the capacitance performance. One hour is a suitable time, by which not only the ERGO get proper conductivity but also retain apt number of oxygen functional groups. Excess reducing is not benefit to the capacitive performance of it owing to the over releasing of oxygen

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Fig. 3. TEM images of GO (a), ER1 (b) and graphite (c).

a

c

d

0

10

20

30

40

50

60

70

80

90

2θ(ο) Fig. 4. X-ray diffraction patterns of GO (a), ER05 (b), ER1 (c), and C standard card (d).

0.03 a

0.4

dc

0.2

Potential / V

Current / A

0.02

e

b

0.01 0.00 -0.01 -0.02 0.4

Specific -1 capacitance/F g

Intensity(a.u.)

b

functional groups. This further confirms the residual oxygen functional groups are still actively involved in faradic redox reactions and contribute to specific capacitance of ERGO. This is in good agreement with Peng X Y's study [27]. Fig. 7 reveals the galvanostatic charge/discharge curves of H05, H1, H2, and H4. Compared with ER serials, the symmetry of H serials are the same wonderful, but their specific capacitance is less. The specific capacitance of H05, H1, H2, and H4 is 202.3, 212.4, 196.8, and 195.6 F g1, respectively. This means that potassium biphthalate solution is superior to sodium acetate buffer solution. The possible mechanism of GO reduction in potassium biphthalate solution is described in Fig. 8. In the effect of electric field, the carboxyl group is reduced to carbonyl group and water by getting electron and Hþ, then carbonyl group getting electron and Hþ becomes hydroxyl group (omit in Fig. 8). At the same time phthalic acid ion gains OH and Kþ and becomes potassium biphthalate and water. Because potassium biphthalate/phthalic acid buffer solution is more readily providing Hþ for reaction than sodium acetate/acetic acid solution in electric field. If the potassium biphthalate is replaced by sodium acetate, its opposite carboxyl group reaction will be inhibiting. So the effective electrolyte is potassium biphthalate buffer solution rather than sodium acetate. Fig. 9 shows the Nyquist plots of GO, ER05, ER1, ER2, and ER4 in pH 13 Na2SO4 solution. The semicircle observed at high frequency

0.0 -0.2

0.0

-0.2

-0.4

-0.6

-0.8

-1.0

Potential / V Fig. 5. CVs of GO (a), ER05 (b), ER1(c), ER2 (d) and ER4 (e) in Na2SO4 solution of pH ¼ 13 at scan rate 100 mV s1.

220 200 180

a

b

c

d

e

sample

-0.4 -0.6

a

-0.8

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240

-1.0

0

100

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300

400

500

-1

Time / h g

Fig. 6. Galvanostatic charge/discharge curves of GO (a), ER05 (b), ER1 (c), ER2 (d) and ER4 (e) in Na2SO4 of pH ¼ 13 at current density of 1.14 A g1.

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Potential / V

0.2 0.0 -0.2 -0.4

0.4 212

0.2

208 204 200 196 a

b

c

d

Sample

-0.6 -0.8 -1.0

5

Specific -1 capacitance/F g

b d c a

Potential / V

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Specific -1 capacitance /F g

S.-b. Zhang et al. / Materials Chemistry and Physics xxx (2014) 1e6

0.0 -0.2

0.8

1.2

1.6

-0.6 -0.8

0

20

40

60

-1.0

80 100 120 140 160 180 200

10

20

region corresponds to the charge transfer limiting process. The charge transfer resistance Rt can be directly measured as the semicircle diameter. All samples show small charge transfer resistance, which are less than 2 U. This is the requirements for ideal capacitive performance. And the straight line in the low frequency region, corresponding to a diffusion-limiting process. The straight line in the low frequency region of ER1, ER2, ER4 is similar, and all of the ERGO samples are less inclined than GO's, which suggesting ERGOs possess superior capacitor behaviors to GO [34]. This is

12

Specific capacitance/F g

14

0.2

10 8 6 4

0.0 -0.2

12

14

16

18

20

Z' / Ohm Fig. 9. Nyquist plots of GO (a), ER05 (b), ER1 (c), ER2 (d), and ER4 (e) in pH ¼ 13 Na2SO4 solution.

240 210 180 0

3

6

9

12

Current density/A g

-0.4 -0.6

dc

-0.8

2 10

60

because of forming more uniform diffusion paths for protons transfer in ERGOs than that in GO [28]. Electrode mass usually plays an important role on the capacitance performance of materials. Galvanostatic charge/discharge curves of M1, M2, M3, M4, M5, M6, and M7 are shown in Fig. 10. The specific capacitance of ERGO increases from 204.6 F g1 to 254.9 F g1 with increasing of electrode mass from 0.6 mg to 1.0 mg. The specific capacitance of M3 (1.0 mg) is maximum (254.9 F g1). After that the specific capacitance begins to decrease. For the samples of M4, M5, and M6, it becomes 212.3, 217.9, and 214.8 F g1, respectively. It remains 211.7 F g1 when the mass is 1.8 mg (M7). This is because that the increasing of electrode mass causes the quantity of the effective electrochemical active materials increase, so the specific capacitance enhances when the mass less than 1 mg. But when the mass exceeds 1 mg, the electric resistance becomes dominate factor of specific capacitance. Ions transfer resistance increases with the increasing of electrode mass. The transfer rate of proton diminishes. The electric resistance plays more important role than that of the amount of active materials on the specific capacitance of materials. So the specific capacitance of ERGO electrode decreases. CP curves of ER1 electrode are obtained under various current densities from 1.14 to 11.4 A g1 (Fig. 11). The specific capacitance of ER1 is 254.9 F g1 at 1.14 A g1. It gradually decreases with increasing of discharge current density. At 11.4 A g1, the specific capacitance remains 176.0 F g1. This is generally caused by internal resistance drop. Electrolyte ion can not be deep into the interior of electrode at large current density, which leading to the active material of electrode can not be fully used. Thus the specific

Porential / V

16

8

50

0.4

a b c d e

18

6

40

Fig. 10. Relationship of specific capacitance of ER1 with mass. (a) M1, (b) M2, (c) M3, (d) M4, (e) M5, (f) M6, (g) M7.

20

4

30

-1

Fig. 8. The mechanism of the reduction of GO in 0.05 M potassium biphthalate solution.

2

ef a g b c

d 0

Time / h g

Fig. 7. Galvanostatic charge/discharge curves of H05 (a), H1 (b), H2 (c), and H4 (d) in Na2SO4 of pH ¼ 13 at current density of 1.14 A g1.

Z" / Ohm

200

Mass/mg

-1

0

220

-0.4

Time / h g

0

240

-1.0

0

b

a

50 100 150 200 250 300 350 400 450 500 -1

Time / h g

Fig. 11. Galvanostatic charge/discharge curves of ER1 in Na2SO4 of pH ¼ 13 at different current densities. (a) 1.14, (b) 2.27, (c)5.68, (d) 11.4 A g1.

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200

Potential/V

Specific -1 capacitance/F g

250

150 100 50 0

0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0

0

2000

4000

6000

8000

Time/s 0

200

400

600

800

1000

Cycles Fig. 12. Cycle life of ER1 electrode at the current density of 1.14 A g1 between 1 and 0.4 V in pH 13 Na2SO4 solution.

capacitance decreases. But the symmetry of charging and discharging curves is very well at different current densities, which indicate that ER1 possesses excellent rate capability. The cycle life of the ER1 electrode is examined at current density of 1.14 A g1, and the result is shown in Fig. 12. At beginning the specific capacitance of the ER1 electrode is up to 254.9 F g1. Several times later, a sharp decrease appears and it keeps at 214 F g1 from 40 to 1000 cycles. The capacitance retention of 84% reveals the superior reversibility and stability in 1000 charge/discharge cycles. Furthermore, an abnormal phenomenon is observed that the specific capacitance rebounds to 254 F g1 at 740 to 880 cycles. By inference it is mostly the environment reason. But the exact reason needs to be further studied. 4. Conclusions ERGO is obtained by reducing GO with an easy and eco-friendly constant potential electrochemical method. Most oxygen functional groups are removed after reducing 1 h. ERGO obtained in potassium biphthalate/phthalic acid buffer solution possesses more excellent capacitive performance than that obtained in sodium acetate/acetic acid solution. The maximum specific capacitance of ERGO electrode is 254.9 F g1 at 1.14 A g1. ERGO is a promising material for energy storage due to its superior capacitance performance. Acknowledgments Project supported by Liaoning Provincial Natural Science Foundation of China (project number 2014020038). References [1] K.S. Novoselov, A.K. Geim, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666e669. [2] C.G. Liu, Z.N. Yu, D. Neff, A. Zhamu, B.Z. Jang, Graphene-based supercapacitor with an ultrahigh energy density, Nano Lett. 10 (2010) 4863e4868. [3] H.P. Wu, Preparation of Graphene and its Application in Supercapacitor [D], Beijing Jiaotong University, Beijing, 2012. [4] A.H. Neto, F. Guinea, N.M.R. Peres, et al., The electronic properties of graphene, Rev. Mod. Phys. 81 (2009) 109e162. [5] M. Toupin, T. Brousse, D. Belanger, Charge storage mechanism of MnO2 electrode used in aqueous electrochemical capacitor, Chem. Mater. 16 (2004) 3184e3190. [6] G.A. Snook, P. Kao, A.S. Best, Conducting-polymer-based supercapacitor devices and electrodes, J. Power Sources 196 (2011) 1e12.

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Please cite this article in press as: S.-b. Zhang, et al., Electrochemically reduced graphene oxide and its capacitance performance, Materials Chemistry and Physics (2014), http://dx.doi.org/10.1016/j.matchemphys.2014.08.068