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Effect of Interlayer Distance and Oxygen Content on Proton Conductivity of Graphite Oxide Mohammad Razaul Karim, Md Saidul Islam, Kazuto Hatakeyama, Masaaki Nakamura, Ryo Ohtani, Michio Koinuma, and Shinya Hayami J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b06301 • Publication Date (Web): 06 Aug 2016 Downloaded from http://pubs.acs.org on August 7, 2016

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The Journal of Physical Chemistry

Effect of Interlayer Distance and Oxygen Content on Proton Conductivity of Graphite Oxide Mohammad Razaul Karim†#, Md. Saidul Islam†, Kazuto Hatakeyama†, Masaaki Nakamura†, Ryo Ohtani†, Michio Koinuma†, Shinya Hayami†§* †

Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Kumamoto, 860-8555, Japan.

#

Department of Chemistry, School of Physical Sciences, Shahjalal University of Science & Technology, Sylhet-3114, Bangladesh. §

Institute of Pulsed Power Science (IPPS), Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan. ABSTRACT: The effect of interlayer distance and oxygen content on proton conductivity of graphite oxide is presented. Bulk state proton conductivities were measured using coin shaped pellets of three different graphite oxide samples, namely H-GO, S-GO and B-GO, generated respectively from Hummers’, Staudenmaier and Brodie’s technique. The extent of oxidation, nature of functional groups, interlayer distances and morphologies are studied through raman spectroscopy, XPS study, powder XRD pattern and SEM images. The proton conductivities follow the trend as, H-GO ˃ S-GO ˃ B-GO. In XPS study the total oxygen contents were found to follow the trend as, H-GO ˃ B-GO ˃ S-GO; whereas, the interlayer distances obtained from powder XRD pattern show the trend as, H-GO ˃ S-GO ˃ B-GO. Beside the nature of functional groups and extent of oxidation, the interlayer distance displays significant effect on the proton conductivity values. The temperature dependent Arrhenius plots reveal the activation energy (Ea) of the samples as 0.274, 0.291 and 0.296 eV. These low Ea values imply Grotthuss mechanism for proton conduction. The high conductivity value and low activation energy of H-GO with maximum interlayer distance indicates that hydronium ion’s rotational movement and reformation of hydrogen bonds in Grotthuss mechanism is supported by a more flexible interlayer. We propose that this physical insight might be considered for improving proton conductivity through modulating layer distances not only in carbon allotropes but also in other materials.

INTRODUCTION Understanding the relation of proton conductivity with physical and chemical states including the interlayer distance and total oxygen content, respectively in oxidized carbon allotropes is desirable for designing carbon based super ionic conductors. Some recent findings indicate the possibility of oxidized carbon allotropes as feasible proton conducting materials.1-5 The oxidized form of carbon can adsorb water molecules and transport proton through hydrogen bonded network.6, 7 Theoretically, two very basic criteria including the existence of a stable hydrophobic skeleton and extended hydrophilic functional groups with water adsorbing capacity is essential for a material to transport proton (Scheme 1).8 Therefore, oxidized carbon materials irrespective of the nature of allotrope or method of oxidation might function as proton conductor. Based on this fact, previously we reported the inplane and bulk proton conductivity in GO (the exfoliated form of graphite oxide nanosheet, which is well known as graphene oxide) and graphite oxide (stacked solid mass of oxidized graphite or pellet made from pressing oxidized graphite powder mechanically), respectively. Both of these graphite derivatives were obtained from Hum-

mers’ oxidation method.9 Improving the extent of oxidation such as ozonation of graphite oxide, resulted in higher conductivity.10 HYDROPHOBIC LAYER HYDROPHILIC LAYER H+ H+

H+

H+

H+

H+

ADSORBED WATER FILM

H+

Scheme 1. General structure of layered proton conducting materials. Proton transports through the adsorbed water film (sky blue shade) attached to hydrophilic functional sites embedded on hydrophobic skeleton. Some physical modifications including assembling bundles of multlayers or incorporation of sulfate ion at the interlayer could improve the conductivity in GO further.11, 12 Modulating the relative amount of oxygenated functional groups, pattern of multilayer assembly and intercalating ionic or covalent specious, we observed var-

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The Journal of Physical Chemistry ied proton conductivity both in GO and graphite oxide.13, Though it was found that the epoxy groups and carboxylic with hydroxyl groups influence the proton conductivities along the interior and edges, respectively of GO, the effect of interlayer distance on proton conductivity of graphite oxide is still unknown. But, classical concepts on hydrodynamics and hydrogen bond reformation process at the interlayer water film of sheet like materials implies that modulation of layer distances in graphite oxide might result in varied proton conductivity .15, 16 In this report, we observed that graphite oxide having different interlayer distances and total oxygen content display varied proton conductivity. Three bulk samples namely: HGO, S-GO and B-GO, have been prepared from slight modification to the three ancient methods of graphite oxidation processes including the Hummers’, Staudenmaier and Brodie’s techniques.17-19 We found that beside the total oxygen content, the interlayer distance affects the proton conductivities significantly. This observation implies the possibility of improving proton conductivity through physical modification of not only the graphite oxide, but also other layered materials. Carbon based proton conductors are being searched worldwide majorly for fuel cell application. Proton conductor prevents explosion reaction prohibiting the direct contact between hydrogen and oxygen in hydrogen fuel cells.20 Though, hydrogen is considered as a major source of renewable energy to mitigate the future threat of worldwide fossil fuel depletion, efficient use of hydrogen in motor vehicle is not simple. The best way to use hydrogen in vehicles is considered as the transformation of its chemical energy into electricity through fuel cells. However, commercialization of hydrogen fuel cells is still limited due to the difficulty for finding facile and cheap solid electrolyte supporting proton transfer from the hydrogen inlet to the oxygen chamber with a rate keeping pace with the external circuit load. In fact, the energy output of a hy14

drogen fuel cell is partly dependent on the efficiency of the proton conducting membrane. Therefore, researches on fuel cell technologies are associated with the development of faster, cheaper and economically feasible solid electrolytes.

In addition to fuel cells, proton exchangers are necessary for some sensors, biological transport systems and chemical filters.21 Though there exist some well known proton conductors, including nafion (10-5 - 10-1 Scm-1), phosphates (10-4 - 10-1 Scm-1), carboxylic acids (10-6 - 10-5 Scm-1) and so on, commercialization of carbon base solid electrolyte has not been possible yet.22-25 Therefore, adopting carbon allotropes as solid electrolyte for their availability, cost effectiveness, mechanical and chemical stability and ease for fabrication through solid state sampling process has drawn recent concern. Beside GO and graphite oxide, some other oxidized carbon allotropes including fullerene, carbon nanotube and coal powder were found to exhibit ionic conductivity.26-31 As unmodified carbon allotropes are hydrophobic, they need to be oxidized for conducting proton. Numerous research groups reported such oxidation through the classical Hummers’ method. Though, Staudenmaier and Brodie’s methods are alternative techniques for graphite

oxidation, the detailed chemical structure and proton conductivity of related oxidized graphite are still unknown. Herein, along with chemical structure, the proton conductivity of S-GO and B-GO is presented for the first time. In addition, comparing the results with H-GO, the effect of interlayer distance and total oxygen content on proton conductivity is presented. EXPERIMENTAL Graphite powder of the same type was oxidized by Hummers’, Staudenmaier and Brodie’s method (Scheme 2). All chemicals were purchased from Wako pure chemical co. Japan and was used without further purification. The water used for all experiments was ultra pure (18.2 MΩ). In Hummers’ method, 5.0 g graphite powder, 5.0 g finely meshed NaNO3 and 250 ml 97% H2SO4 was cooled to 0 o C by stirring in an ice bath for 15 min. 15.0 g finely meshed KMnO4 powder was added slowly with vigorous stirring maintaining the temperature of the mixture below 20 oC. After 30 min, the mixture was warmed to 35 oC for 30 min. Then 1 L water was added slowly. The temperature rose gradually and was maintained around 95 oC for another 30 min. Then 2 L water and 60 mL 30% H2O2 was added with stirring. The mixture was centrifuged at 3000 rpm for 10 min. The precipitate was washed one time with 5% HCl solution and 3 times with water. The precipitate was then dried at 70 oC for 24 hours to obtain H-GO. In Staudenmaier method, a mixture of 35 mL H2SO4 (95-98%) and 18 mL fuming HNO3 in a round bottom flask was cooled down to 0 oC by stirring in an ice bath. Then, 2 g graphite was added to the mixture under vigorous stirring to obtain a homogeneous dispersion. While keeping the reaction flask in the ice bath, 22 g KClO3 was added slowly (over 15 min) to the mixture in order to avoid any sudden rise in temperature and formation of explosive chlorine dioxide (ClO2) gas. After complete dissolution of KClO3, the reaction flask was removed from Graphite powder

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Staudenmaier’s method

H2SO4, HNO3 and KClO3; Total time 5 days, reaction temperature 25 oC

Brodie’s method

HNO3 and KClO3; Total time 7 days, reaction temperature 70oC

Hummers’ method

NaNO3 ,H2SO4 and KMnO4; Total time 1 day, reaction temperature 95oC

Scheme 2. Various graphite oxidation techniques through slight modification to reported classical methods. the ice bath and capped loosely to allow evolution of gases. The mixture was stirred vigorously for 5 days at room temperature. After completion of the reaction, the mixture was poured into 2 L deionized water and filtered after keeping overnight for sedimentation. The precipitate was then dispersed and stirred repeatedly by HCl (5%) solutions to remove sulfate ions. It was then washed several times with deionized water until neutral pH of the filtrate was obtained. The suspension was then dried in a vacuum oven at 60 °C for 48 h to obtain S-GO. In Brodie’s method, 6 g graphite powder was meshed finely with 21 g KClO3 in a retort. The powdered mixture was poured into


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a round bottom flask fitted with magnetic stirrer. 75 ml fuming HNO3 was added to the mixture and was stirred continuously at 70 oC for 1 day until the disappearance of yellow vapor. The mixture was then washed repeatedly with water. The final product was obtained from drying. The whole procedure was repeated 3 times using the final products of the former operation as the starting materials for the consecutive operation. A field-emission SEM (Hitachi High-Tech, SU-8000) was used to study the surface morphology of the samples. For studying the X-ray photoelectron spectroscopy, an XPS instrument of Thermo Scientific, Sigma Probe was used. A monochromatized X-ray source (Al Kα, hν = 1486.6 eV) and a discharge source (He I, hν = 21.2 eV) was used. Pt substrate was used to determine the Fermi level in GO/Pt film. Vacuums better than 10-7 Pa was ensured during the measurements. A hemispherical energy analyzer equipped with six channeltrons was used to detect the emitted electron. Study of Raman spectroscopy was performed using a micro Raman spectrometer (NRS-3100, Jasco, Japan) with a 532 nm excitation source at room temperature. Proton conductivities of the samples were measured by four-probe impedance/gain phase analyzing system using a Solaratron 1260/1296 in the frequency range from 1 to 106 Hz. 100 mg sample was mashed into powdered forms and mechanically compressed into coin like pellet of 2.5 mm diameter. Both sides of the pellet were attached to gold wire (50 µm diameter) with gold paste, obtained from Tanaka Kikinzoku Kogyo. The Impedances with respect to the modulation of both the relative humidity (RH) and temperature using an incubator (SH-221, ESPEC) were recorded. The σ values (conductivity) were calculated using the equation, σ = d/RA, where d is the thickness of the pellet (the distance between the electrodes), R is the measured resistance and A is the area of the electrode. For all samples, ‘d’ and ‘A’ had the fixed value as 0.1 cm and 1 cm2, respectively. RESULT AND DISCUSSIONS Figure 1 represents the surface morphologies and physical properties of the samples. H-GO, S-GO and B-GO samples, respectively in figure 1a, 1c and 1e display deep brown, black and light brown color. The photographs of respective solution with 1 mg/ml concentration in water after ultrasonication for 2 hours show gray, black and brown color, respectively. SEM images for H-GO, S-GO and B-GO confirm their layered structures. S-GO was harder than B-GO and H-GO. B-GO was found almost in powdered form. A greenish colored powder was found when B-GO was oxidized incompletely at the first cycle. Figure 2 represents the powder X ray diffraction spectra of graphite powder and the GO samples. Characteristic sharp peak for graphite powder around 27o (2θ) represents the reflection plane (001) of graphite and indicates its amorphous structure.32 After oxidation, the sharp peak disappears and comparatively wider peak around 2θ = 9.13, 11.4 and 11.6 for H-GO, S-GO and B-GO, respectively represent the lowering of reflection planes due to success-

ful oxidation. The respective ‘d spacing’ values as 9.68, 7.76 and 7.62 Å indicate the interlayer distances generated from the oxygenation of graphitic surface.

(a)

(b)

5 µm

(c)

(d)

5 µm

(e)

(f)

5 µm

Figure 1. Physical properties and morphologies of the samples. Photographs of H-GO (a), S-GO (c) and B-GO (e), with inset showing the respective solution (1 mg/ml) in water after ultrasonication for 2 hour. SEM images of H-GO (b), S-GO (d) and B-GO (f). Within three samples, the low angle peaks for H-GO and B-GO are comparatively sharper than S-GO. The PXRD pattern of H-GO resembles with previous report.33

Intensity / a.u.

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H-GO S-GO B-GO Graphite

10

20

30

40

2 theta / degree Figure 2. Powder X-ray diffraction patterns of graphite powder, H-GO, S-GO and B-GO. Though relatively broad, the peaks associated to all the samples signify their layered structure, which are also in close match with the morphologies displayed by the SEM images as well. However, the XRD peaks are not as sharp as crystalline materials usually display. We propose that unlike the highly ordered reflection planes of crystalline materials, in oxidized graphite the surfaces of nanosheets remains tilted randomly by varied extent (depends on the pressure and other physical parameters during sample preparation for PXRD analysis) rather being perfectly parallel with respect to each other. As a result, very sharp peak signifying perfect crystalline state or highly ordered


The Journal of Physical Chemistry plane of GO materials is fairly reproducible. A direct evidence of such anisotropy of GO nanosheet’s alignment is the dissimilar ion conduction property in GO and graphite oxide, discussed elsewhere.9 Present study suggests that the interlayer distance is maximum for H-GO and minimum for B-GO. Figure 3 represents the Raman spectra of graphite powder and the synthesized samples. Raman spectra confirm the successful oxidation of graphite. Two characteristic peaks for carbon materials namely the ‘D’ and ‘G’ band’ around 1350 and 1580 cm-1, respectively are observed. The peak at ~1350 cm-1 represents the breathing mode of graphitic A1g symmetry. On the otherhand, the peak at ~1580 cm-1 rises from the motion of in plane bond stretching for C sp2 atom (E2g) pairs.34 In graphite powder, the G band position is around 1576 cm-1. After oxidation this band position shifts to 1597, 1588 and 1583 cm-1 for H-GO, B-GO and S-GO, respectively. This hardening implies the electronic structural change during the oxidation of graphite and matches with some previous observations.35, 36 The ratio of peak height for D and G band (ID/IG value) changes inversely. As a result, the increase in ID/IG values implies the reduction of sp2 domain and increase in sp3 carbon sites, which is a direct evidence for successful oxidation of graphite. The ID/IG value changes from 0.654 in virgin graphite powder to 0.951, 0.943 and 0.991 in H-GO, B-GO and S-GO, respectively. The respective increment by 46, 45 and 52% indicates the conversion of some sp2 carbon sites into sp3 forms.37 In graphitic stake the carbon atoms are in sp2 hybridized state. During oxidation some of the carbon atoms are changed into epoxy sites (-C-OC-) having sp3 hybridized carbon. Therefore, the displayed variation in raman spectra is as per the expectation.

Raman Intensity / a.u.

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1000

H-GO S-GO B-GO Graph-ite

1200

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in the chemical composition and relative amount of the functional groups. After deconvolution, characteristic peaks associated to some particular oxygenous functional groups are appeared in all the samples. For H-GO, the existence of epoxide (–O–), hydroxyl (–OH) groups at 286.8 - 287.0 eV and carbonyl (–C=O), carboxyl (–COOH) groups at 287.8 - 288.0 and 289.0 - 289.3 eV, respectively complies with some previous report.39 The XPS spectra of S-GO and B-GO represent similarities with H-GO. The extent of oxygenation is optimized in H-GO. O/(C+O) ratio for H-GO, Graphite

S-GO

B-GO

C1s All C=C (Csp2) CHx (disorder) C-C (Csp3) COH C=O COOH COC

1400

Raman Shift /

1600

1800

290

H-GO

288

286

284

282

Binding Energy / eV

cm-1

Figure 3. Raman spectra of graphite powder, H-GO, BGO and S-GO.

Figure 4. XPS spectra of graphite powder, H-GO, B-GO and S-GO.

Next, we studied the functional nature of the samples by XPS spectra. Figure 4 represents the C1s XPS spectra of graphite, H-GO, S-GO and B-GO. Pure graphite shows a sharp peak arising from the sp2 hybridized carbon atoms.38 After oxidation, all the samples exhibit two individual peaks near 285 and 287 eV in their XPS spectra. Though the two peaks for all the sample appear to be similar, the deconvoluted spectra display significant variation

S-GO and B-GO are 33.1, 26.6 and 29.3%, respectively. The relative amounts of oxygenous functional groups are listed in table 1. The sets for relative amount of oxygenated groups (hydroxyl, epoxy, keto and carboxyl) in H-GO, S-GO and B-GO are (5.5, 35.7, 4.9 and 14.2); (4.1, 40.7, 3.9 and 6.5) and (5.3, 46, 7.1 and 1.2) %, respectively. Figure 5 represents the results for the proton conductivity measurements of the samples. The Nyquist plots for


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H-GO, S-GO and B-GO (figure 5a, 5b and 5c, respectively) at various RH reveal that the real part (Z') and imaginary part (Z'') of impedance fit with semicircular curves. Inset in figure 5a represents the semicircles with lower radius for H-GO. The diameters of the semicircles represent the resistances for proton conduction and the second semicircles in each case signify the proton oriented conduction. The proton oriented conduction was confirmed by isotope effect, where D2O humidified samples exhibited lower conductivity. The resistance is lowest for H-GO sample and follows the trend as H-GO < S-GO < B-GO. At 303 K and 80% RH, the resistances for these samples are 290, 550 and 6200 Ω, respectively. The resistances increase with decreasing the relative humidity, which indicates that the water conten directly affects the proton 10

10

800

(a)

(b)

600

8

40 %RH 50 %RH 60 %RH 70 %RH 80 %RH

6

40 %RH 50 %RH 60 %RH 70 %RH 80 %RH

400 200 0 0

4

200

400

600

800

Z′ ′ / KΩ

Z′ ′ / KΩ

8

6 4

2

2

0

0 0

2

4

6

8

10

0

2

4

Z′ / K Ω 100

10

-3

10

σ / Scm-1

Z′ ′ / KΩ

40 %RH 50 %RH 60 %RH 70 %RH 80 %RH

40

10

20 10

0

20

40

60

80

2 -5

2 -6

40

50

-1.5

(e)

2

-2.5

ln(σ T)

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-2.0

-4

4

H-GO S-GO B-GO

2

60

% RH

6

10

80

4

100

6

10

70

H-GO S-GO B-GO

2 -4

Z′ / K Ω -3

10

4

0

10

8

(d)

4

60

6

Z′ / K Ω

(c)

80

σ / Scm-1

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-5

(f)

H-GO S-GO B-GO

-3.0 -3.5 -4.0 -4.5

310

320

T/K

330

340

2.9

3.0

3.1

3.2

3.3

T-1/10-3K-1

Figure 5. Proton conductivities of the samples. The semicircle traces obtained from Nyquist plots for H-GO (a), S-GO (b) and B-GO (c). Conductivities with respect to relative humidity at 303 K (d). Conductivities with respect to temperature at 90% relative humidity (e). Arrhenius plots in terms of ln(σT) Vs T-1 (f). conductivity. Figure 5d represents the RH dependent conductivities of H-GO, S-GO and B-GO at 303 K. At 40% RH, the proton conductivity values of the samples are 1.35 × 10-5, 2.12 × 10-6 and 1.11 × 10-6 Scm-1, respectively. For all the samples the conductivities increase with relative humidity and reaches to 3.32 × 10-4, 1.67 × 10-5 and 7.14 × 10-5 Scm-1, respectively at 80% RH. Figure 5e represents the temperature dependent proton conductivity values for the samples at 90% RH. The conductivities for the samples are, 1.76 × 10-4, 6.71 ×

10-5 and 4.57 × 10-5 Scm-1, respectively at 303 K. At this high humidity the conductivity increase slightly with temperature and reach to 5.23 × 10-4, 2.11× 10-4 and 1.49 × 10-4 Scm-1, respectively for H-GO, S-GO and B-GO at 343 K. Within the full range of experimental temperature and different RH conditions the conductivity followed the trend as: H-GO > S-GO > B-GO. The proton conductivity of the materials are comparable to some unmodified oxidized carbon allotropes including bulk sample for polyhydroxy fullerene (~10-6 - 10-5) and acid leached graphene oxide (~10-6 - 10-4) and nanosheet sample for coal oxide (~10-3), carbon sphere oxide (~10-4 - 10-2) and graphene oxide (~10-4 - 10-2). 9, 26, 27, 31 Figure 5f represents the Arrhenius plots for the samples in terms of ln(σT) Vs T-1. The Ea values calculated are 0.274, 0.291 and 0.296 eV for H-GO, S-GO and B-GO. In graphitic stake, the carbon sheets made of sp2 hybridized carbon networks are attached with each other by week physical forces. During oxidation, oxygeneous functional groups intercalating within the layers break the interlayer binding force. The layers being separated take the form of discreet nanosheets having negatively charged oxygenated functional groups, which make the GO colloid stabile in polar solvent. These flat hydrophobic carbon frameworks with extended hydrophilic groups support proton conduction. In contrast to the increase in proton conductivity, oxidation process results in the breaking of continuity within the delocalized π electron clouds and brings some breakthrough in the electronic conductivity.40, 41 Based on this issue, tunable electronic conductivity was observed in GO or graphite oxide through modulated oxidation.42 We also found very low electronic conductivity in the pellets (~4 μA) of H-GO, SGO B-GO. However, as a pre requisite condition for present study we relied on the synthetic routes for generating different types of GO. Reported that the extent of proton conduction depends on the degree of humidification, the interlayer distance, hydration dynamics, the support of hydrophilic layer walled cavity and the interlayer water film for reformation of hydrogen bonds.9 We considered that all these governing factors might be affected due to the variation in synthetic route for generating graphite oxide. The surface morphologies of the GO samples are different. The color difference in the solid masses and water solutions is supposed to be generated from variation in the amount of different types of oxygenated groups. Besides, there exists the possibility for different types of elemental doping on carbon backbone during different synthetic routes. The SEM images show the layered structure. Both the total oxygen content and interlayer distance in H-GO is higher than others. According to our previous observation, GO generated from the Hummers’ method exhibit very high conductivity, as the propagation of proton was allowed through the assembly of parallel conduction tracks with respect to the comb electrodes assembly.9 However, such sampling or decoration of membrane is difficult to achieve. Therefore, to consider the application of graphite oxide as solid electrolyte, it is necessary to justify the conduction property of graphite oxide generated from all the possible routes. Besides, understanding



O=C-O O/(C+O)

H-GO

5.5

35.7

4.9

14.2

33.1

S-GO

4.1

40.7

3.9

6.5

26.6

B-GO 5.3 46 7.1 1.2 29.3 group is between 1.50 to 1.89 Å (depending on the mode of stretching) with the CCO bond angle around 60 o.44 In contrast, the carboxylic acid group is much larger in size. The C-C, C=O, C-O and O-H bond lengths in carboxylic acid group are 1.10, 1.23, 1.32 and 0.97 Å, respectively. This large sized carboxylic acid groups would tend to keep apart the oxidized graphite sheets with relatively higher interlayer distance (scheme 3). Therefore, the interlayer distance follows the trend

1.46 Å

O

D1

O

C D 2 ˃ D1

H-GO

0.97 Å

H O 1.10 Å

C (b)

(d)

O

H

C

H

O

O

O C

H

C=O

C

O

O

Sample C-OH C-O-C

(c)

(a)

O

Table 1. Oxygen contents and relative amount (%) of functional groups on GO samples

B-GO

O

O

The water absorbing capacity and support for hydration dynamics and hydrogen bond reformation process are proportional to the amount of polar groups.43 Therefore, the maximum conductivity displayed by H-GO having highest total oxygen content is usual. But, the lower conductivity of B-GO compared with S-GO is indicative for the effect of oxygen content being suppressed by some other factor. Except the carboxylic acid groups the amount of other functional groups in S-GO is lower than B-GO. Inspite, the higher conductivity of S-GO signifies the domination of carboxylic acid groups for proton conduction. Our previous study reveals minor effect of carboxylic acid at the interior of GO nanosheet and water absorbing capacity. Therefore, it is expected that a more flexible conduction channel for transporting protons might be attributed by carboxylic group, while this group residing at the edges.We suggest that carboxylic acid groups locating at the edges can affect the interlayer distance significantly, especially compared with the epoxy groups locating at the interior of nanosheet. The average bond length in O-C bond in epoxy

as H-GO ˃ S-GO ˃ B-GO, which is similar to the order for the relative amount of carboxylic groups. The overall scenario implies a combined effect of interlayer distance and total oxygen content on the proton conductivity value. Except changing the method of oxidation, it was not possible to modulate the interlayer distances in graphite oxide. Uptodate, there exists only few successful methods for complete graphite oxidation process. Therefore for studying the correlation we could choose only three samples generated from three classical synthetic routes.

O

the effect of layer separation and oxygen content on conduction track is necessary for engineering the proton transporting pathways. Though Staudenmaier and Brodie have reported the graphite oxidation process much earlier, the detailed chemical structure with respect to proton conduction was unknown. On the other hand, measuring the proton conductivity of H-GO only, it is not possible to observe the effect of interlayer distance and oxygen content. Blocking the epoxy group, previously we observed lower conductivity, which suggested the epoxy group as the major contributor for proton conduction in oxidized graphite.11 But, current study shows that for variation in the route of oxidation, the effect of epoxy groups become suppressed. The conductivities follows anomalous trend with respect to the amount of epoxy groups. Even, B-GO have the highest amount of epoxy groups, its conductivity is the lowest. The XPS study shows clear difference in the amount of functional groups in various GO samples (table 1). The proton conductivity seems to be dependent linearly with the amount of carboxylic acid groups only. There exist no regular ordering between the conductivity value and the relative amount of other oxygenated sites. Based on these observations, we propose that the total oxygen content contribute to the water absorbing capacity, while the carboxylic acid groups regulate the interlayer distances.

O

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O

D2

C

C

Scheme 3. Domination of functional group’s sizes on the interlayer distance. Models for epoxy group (a) and carboxyl group (b). Interlayer distances D1 and D2 (D2 ˃ D1) generated by epoxy (c) and carboxylic group (d), respectively. The low (< 0.3 eV), activation energies for proton conduction through the samples suggests the Grotthuss mechanism. Carbon based proton conductors are highly desirable for their green nature, low cost, physical, chemical and thermal stability. This report represents the experimental correlation between the proton conductivities of graphite oxide and the interlayer distance. Although carbon materials have been considered for current study, it is expected that the correlation might be applicable for other layered structures. CONCLUSIONS Dependency of proton conductivity of graphite oxide pellet on oxygen content and interlayer distance is presented. H-GO, S-GO and B-GO were prepared from Hummers’, Staudenmaier and Brodie’s graphite oxidation technique. The samples were characterized by SEM image, raman and XPS spectra and PXRD pattern. The interlayer distances for the samples are different and follows the trend as: H-GO ˃ S-GO ˃ B-GO. The O/(C+O) ratio for HGO, S-GO and B-GO are 33.1, 26.6 and 29.3 %, respectively. The proton conductivities of the samples follows the trend as: H-GO ˃ S-GO ˃ B-GO with respect to the modulation of both the relative humidity and temperature. The conductivity is found to be affected by both the interlayer distance and oxygen content of the samples. Due to their bulky size, the carboxylic groups attribute higher interlayer distances, which support faster proton conduction trough improved hydration dynamics. The activation energies for proton conductions are 0.274, 0.291 and 0.296 eV, respectively for H-GO, S-GO and B-GO. We propose


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that the findings herein might be considered for improving proton conductivity by modulating the layer distances in carbon allotropes and other layered materials.

AUTHOR INFORMATION Corresponding Author *Phone: +81-96-342-3469. Fax: +81-96-342-3469. E-mail: [email protected]

ACKNOWLEDGEMENT The authors acknowledge to JSPS, Japan, for providing financially support and SUST, Bangladesh for issuing study leave to Dr. Mohammad Razaul Karim to continue his postdoctoral fellowship at Kumamoto University, Japan.

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