Proton Conductivity of Graphene Oxide Hybrids with ... - CSJ Journals

doi:10.1246/cl.130606 Published on the web August 3, 2013

1412

Proton Conductivity of Graphene Oxide Hybrids with Covalently Functionalized Alkylamines Yukino Ikeda,1 Mohammad Razaul Karim,1,2 Hiroshi Takehira,1 Takeshi Matsui,1 Takaaki Taniguchi,1,4 Michio Koinuma,1,4 Yasumichi Matsumoto,1,4 and Shinya Hayami*1,3,4 1 Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555 2 Department of Chemistry, School of Physical Sciences, Shahjalal University of Science and Technology, Sylhet-3114, Bangladesh 3 Institute of Pulsed Power Science (IPPS), Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555 4 JST, CREST, 5 Sanbancho, Chiyoda-ku, Tokyo 102-0075 (Received June 30, 2013; CL-130606; E-mail: [email protected]) Graphene oxidealkylamine (GOCnamine; n = 0, 1, 2, 3, 4, 6, 8, 10, 12, 14, 16, and 18) hybrids were synthesized and found to be stable through bonding between the N-terminals of the amine precursors and the epoxy sites of graphene. The hybrids were characterized by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and powder X-ray diffraction (PXRD) analysis. The hybrids with short alkyl chains were found to exhibit high proton conductivity. The hybrids indicate the possibility of designing new materials through chemical bonding between long alkyl chains and graphene materials.

Graphene-based hybrids having covalently bonded molecules or anchored nanoparticles can be used for numerous applications, including hydrogen generation, alcohol oxidation, solar energy conversion, artificial photosynthesis, and as supercapacitors and sensors, to name only a few.1 The high surface area of graphene (ca. 2600 m2 g¹1) led to increase the efficiency and activity of the hybrids.2 The interaction between GO and other components of graphene hybrids are generally of weak physical mode, such as physical trapping or van der Waals stacking. Therefore, individual functional aspects of the hybrid components remain almost persistent. Based on this issue, most of the hybrids reported to date are of organicinorganic type assemblies, where metal nanoparticles (or their oxides, sulfides, arsenides, nitrides) act as the inorganic component and the graphene moiety acts as the organic component. However, organic functionalization is the most recent concept, which was initially used to increase the solubility of GO. Later, a variety of organic GO hybrids with interesting applications have been reported, where electron-rich species such as porphyrins, organic dyes, alkylamines, ionic liquids, pyrines, pyrilinediemides, and aryl diazonium were incorporated in the GO moiety.3 As the noncovalently bonded organic molecules or inorganic nanoparticles result in hybrids with poor stability, we considered covalent functionalization in the GO framework to achieve more stable materials.2,4 Herein, we report the synthesis of GO hybrids with long alkyl chains bonded through amine terminals. In functional metal complexes, long alkyl chains can attribute flexibility and interchain interaction, affording various interesting properties such as abrupt spin crossover, liquid crystallinity, ferroelectric characteristics, and optical activity.5 Branch isomerization and terminal hydroxy functionalization in the long chain also result in some novel materials.6 Therefore, we considered the addition of a long alkyl chain into the Chem. Lett. 2013, 42, 14121414

graphene moiety to design new materials. We synthesized a series of GOCnamines (n = 0, 1, 2, 3, 4, 6, 8, 10, 12, 14, 16, and 18). Recently, we reported the high proton conductivity of GO. GO contains closely arrayed oxygenated functional groups, which assemble into one-dimensional hydrogen-bonded channels for proton transportation.7 Jung et al. used bulk GO as a filler for different polymer electrolytes, while Ravikumar et al. and Zarrin et al. observed moderate conductivity in sulfonic acid-functionalized GO materials and its nafion hybrids.8,9 However, in the bulk amine hybrids having a short alkyl chain (small value of n), we observed moderate proton conductivity. GO was synthesized by making a slight modification to the literature report (Supporting Information)7,10,11 and then dispersed in water (1 mg/5 mL) by sonication. Amine hydrochlorides (CnH2n+1NH4Cl) were prepared from amine (CnH2n+1NH2) and hydrochloric acid (HCl) in MeOH. The product was diluted by MeOH to produce a 0.5 wt % solution, and 10 mL of the solution was added to 25 mL of the as-prepared GO dispersion at 70 °C under vigorous stirring for 12 h to increase the homogeneity of the dispersion (Scheme 1). Then, the dispersion was centrifuged, and the precipitate was washed several times with distilled water and methanol to remove trace amounts of HCl. No chemical reaction was observed while we performed trials using an amine instead of amine hydrochloride. A series of hybrids having 0, 1, 2, 3, 4, 6, 8, 10, 12, 14, 16, and 18 carbons in the long chain were synthesized. However, only selected data for GOC16amine are discussed herein for the characterization. The rest of the members of the series reproduced almost similar results. The hybrids were characterized by Raman, XPS, FTIR spectra and by PXRD analysis. The proton conductivity was measured by the quasi four-probe method using an impedance/ gain phase analyzer (Solaratron 1260/1296) in the frequency range from 1 to 4 MHz. The powdered samples were compressed into pellets of 2.5 mm diameter with a thickness of ca. 0.64 mm, both sides of which were attached to gold wire (50 ¯m diameter,

C16 amine

Scheme 1. Synthesis of GOC16amine hybrid.

© 2013 The Chemical Society of Japan

www.csj.jp/journals/chem-lett/

1413 (a)

290

288

286

284

100

282

Binding Energy/eV Figure 1. XPS spectra of GO (a) and GOC16amine (b) hybrid.

Tanaka Kikinzoku Kogyo) using gold paste. Impedance measurements were performed under controlled temperature and humidity using an incubator (SH-221, ESPEC). Figure 1 presents the XPS spectra of GO and GOC16amine recorded in a Thermo Scientific Sigma Probe kit equipped with vacuum (better than 10¹7 Pa) facility. Oxygen-containing carbonaceous groups of GO appeared in the form of epoxide (O) and hydroxy (OH) groups in the basal plane, with their XPS peak position at 286.8287.0 eV, and carbonyl (C=O) and carboxy (COOH) groups at the edge, with peak positions at 287.8288.0 and 289.0289.3 eV, respectively (Figure 1a). These observations comply with some previously reported XPS pattern of GO.12 After the reaction, the C16amine was attached to the epoxy carbon site by the N-terminal through CN bond formation. A decrease in the XPS peak height for epoxy sites and an increase in the peak height for the CN bond are observed. Statistics reveals that the quantitative amounts of epoxy and amine groups are 36.3 and 0.7% for GO and 12.6 and 2.7% for the hybrid, respectively. These values indicate that 70% of the epoxy sites are replaced by the chemically bonded N-terminals, whereas 30% of the epoxy oxygen still remains unchanged at the end of the reaction. In GO, the CN:CC(sp3) ratio was 1:29 and was converted to 1:8 after the formation of the GOC16amine hybrid. In addition, the tertiary amine reduces part of the epoxy sites into C=C bonds. Besides, a significant part of the epoxy groups is converted into hydroxy groups. As a result, peak heights for both C=C and COH increase in the XPS spectrum of the hybrid. The COH:COC ratio changes from 1:13 in GO to 1:6 in the GOC16amine. These observations confirm the covalent functionalization of GO. The FTIR spectra of GO and GOC16amine are measured and shown in Figure S1.11 GO is characterized by a broad, intense band at 3413, 1728, 1230, and 1055 cm¹1 corresponding to OH, C=O (carboxy and carbonyl), COH, and COC stretching frequencies. However, after the reaction, the introduction of C16amine is confirmed by the appearance of two new peaks around 1616 and 33003500 cm¹1. The doublets at 2850 and 2918 cm¹1 indicate the presence of an alkyl CH2 group in the hybrid. Figure 2 presents the Raman spectra measured in a micro Raman spectrometer (NRS-3100, Jasco, Japan) with a

Chem. Lett. 2013, 42, 14121414

GO GO-C16amine

150

C=C (sp2) C-H C-C (sp3) C-OH C-O-C C=O O=C-O C-N

(b)

292

Raman Intensity/a.u.

C=C (sp2) C-H C-C (sp3) C-OH C-O-C C=O O=C-O C-N

50

1000

1200

1400

1600

1800

Raman Shift/cm-1 Figure 2. Raman spectra of GO and GOC16amine hybrid.

532 nm excitation source at room temperature. The chemical bond formation during the reaction is confirmed by monitoring the D band (ca. 1350 cm¹1) and G band (ca. 1580 cm¹1) positions, which are responsible for the breathing mode of A1g and in-plane bond stretching motion of pairs of C sp2 atoms (E2g) mode, respectively.13 The G band position for GO shifts from 1592 to 1585 cm¹1 after hybrid formation. This softening implies the change in the electronic structure due to the resonance of the lone pair electrons on the N atom and the benzene-like structure of the GO basement.14 The peak ratio (ID/IG) decreases from 0.94 (GO) to 0.90 (GOC16amine). ID/IG is inversely proportional to the extent of sp2 domain. Therefore, the decrease in this value signifies destruction of sp3 epoxy sites during the addition of the N atom.15 The D band intensity signifies the extent of defects in the GO plane.16 The defects increase during covalent functionalization. Powder XRD patterns recorded on a Rigaku X-ray diffractometer (RAD-2A with 2.0 kW Cu K¡ X-rays) are shown in Figure S2.11 GO exhibits a characteristic peak at around 11.5° (2ª) and signifies the existence of intercalated oxygen atoms at various oxygenated functional sites. However, for GO C16amine, shifting of this peak to 9.37° (2ª) implies accommodation of a bulky group in the intercalated space. The diffraction peaks at 3.36 and 6.81° are characteristic of alkylammonium hydrochloride and suggest the successful synthesis of the GOC16amine hybrid. Pure graphene oxide was reported to show liquid crystallinity.16b The presence of sharp peaks in the small angle region and a shoulder-like hollow near 20° imply the similar possibility. Unfortunately, we failed to measure the corresponding polarizing optical microscope (POM) image and differential scanning calorimetry (DSC) tracking for further evidence of the liquid crystallinity of our sample. Figure 3 shows the Nyquist plots of the hybrids for n = 1, 3, and 6. The real (Z¤) and imaginary parts (Z¤¤) of the impedance were found to fit with distorted semicircular curves. The diameters of the semicircles represent the resistances, and the appearance of second semicircles indicates that the conductivity is proton-driven. The calculated · values for the samples, GO C1amine, GOC3amine and GOC6amine, are 6.13 © 10¹9, 4.30784 © 10¹7, and 1.00727 © 10¹8 S cm¹1, respectively. The bulk resistance was proportional to the thickness of the pellets. The conductivity of the H2O-humidified GOC6amine was ca. 1.3 times higher than that of the D2O-humidified sample (Figure 3d). This observation confirms the proton-oriented conductivity.7 The · values were found to increase with temperature, and the linear line for the ln(·T) vs. T¹1 plots



1414 10

120

(a)

(b)

100

8

Z′′/103 Ω

Z′′/106 Ω

80

6

60

4

40

2

20 0

0 2

4

6

Z′/106 Ω

8

5

40

10

7

(c)

6

Z′′/10 6Ω

σ /10-6 S cm-1

4 3 2

80

120

Z′/103 Ω

-6.0

(d)

ln (σT)

0

-6.5 -7.0

5

-7.5 3.2

4

3.3

3.4

T-1 /10 -3 K-1

3

H2 O D2O

2

1

1

0 1

2

3

4

5

Z′/106 Ω

295

300

305

310

This work was supported by Innovative Areas “Coordination Programming” (area 2107) from MEXT, Japan.

315

Temperature/K

Figure 3. Nyquist plots of GOamine hybrids with n = 1 (a), 3 (b), and 6 (c). Temperature-dependent conductivities for H2O- and D2Ohumidified GOC16amine hybrids (d) and corresponding ln(·T) vs. T¹1 (inset of d).

(inset of Figure 3c) reveals the activation energy (Ea) as 0.504 eV for the proton conductivity of the GOC6amine hybrid. This value is higher than our previously reported Ea value for GO (0.197 eV). The proton conduction in the hybrids is supported by the adsorbed water film at the epoxy sites. The conduction pathway is improved without any interference or restraint, as in the graphite stake. In the hybrids, the amine terminal is partially charged, and hence, they can adsorb some water molecules themselves. The adsorbed water films support proton movement through the hydrogen-bonded network. For short alkyl chains attached to the graphene basement, the · values are relatively high. On the otherhand, for hybrids having longer carbon chains, the data from conductivity measurement were insignificant. The hybrids were synthesized through insertion of equimolar amounts of amine hybrids, but the variation in the conductivity was significant, which reflects the effect of chain length on the pathway for proton migration. We suggest that steric hindrance, intercalated space, and the arrangement and thermal motion of long chains affect the formation and breaking of hydrogenbonded channels and results in the varied · value. For long alkyl chains, these facts afflict the conduction pathway. In Figure S1, the lower intensity of the OH stretching vibration for GO C16amine indicates that the number of proton attracting sites (OH group) in GOC16amine is smaller than that in pure GO. Obviously, this fact can be considered as theoretical evidence for the lower conductivity of the hybrids; in practice, the measured conductivities of all the amine hybrids were lower than that of pure GO, as we reported previously.7 The comparative IR data for the H2O- and D2O-humidified samples are presented in Figure S3.11 To anchor alkyl chains, we considered GO as a better candidate than G, as GO can exert hydrophilic interactions, Chem. Lett. 2013, 42, 14121414

which has been explained in detail in a recent report.12a,14 The experimental evidence herein suggests covalent bond formation. Therefore, the stability of the hybrids is very high. The change in the hybridization state of the adjacent carbon from the sp3 to sp2 state is interesting, as theoretically it can recover some part of the conductivity lost during the synthesis of GO from graphite. Developing hybrid materials is considered the key research for devices, catalysts, electronics, energy conversion kits, and so on. The goal is to combine interesting properties so that they do not interfere with one another and to intensify functional aspects. Here, we successfully incorporated various alkyl chains into a graphene moiety. The hybrids having short chains showed moderate proton conductivity. We suggest that these hybrids represent a new platform for developing functional graphene materials.

References and Notes 1 a) N. Yang, J. Zhai, D. Wang, Y. Chen, L. Jiang, ACS Nano 2010, 4, 887. b) B. Seger, P. V. Kamat, J. Phys. Chem. C 2009, 113, 7990. c) G. Williams, P. V. Kamat, Langmuir 2009, 25, 13869. d) Y. Zhu, S. Murali, W. Cai, X. Li, J. W. Suk, J. R. Potts, R. S. Ruoff, Adv. Mater. 2010, 22, 3906. e) K. Zhang, L. L. Zhang, X. S. Zhao, J. Wu, Chem. Mater. 2010, 22, 1392. 2 S. Stankovich, D. A. Dikin, G. H. B. Dommett, K. M. Kohlhaas, E. J. Zimney, E. A. Stach, R. D. Piner, S. T. Nguyen, R. S. Ruoff, Nature 2006, 442, 282. 3 a) H. Liu, J. Gao, M. Xue, N. Zhu, M. Zhang, T. Cao, Langmuir 2009, 25, 12006. b) N. Karousis, S. P. Economopoulos, E. Sarantopoulou, N. Tagmatarchis, Carbon 2010, 48, 854. c) S. Stankovich, D. A. Dikin, O. C. Compton, G. H. B. Dommett, R. S. Ruoff, S. T. Nguyen, Chem. Mater. 2010, 22, 4153. d) Q. Su, S. Pang, V. Alijani, C. Li, X. Feng, K. Müllen, Adv. Mater. 2009, 21, 3191. e) L. Tan, K.-G. Zhou, Y.-H. Zhang, H.-X. Wang, X.-D. Wang, Y.-F. Guo, H.-L. Zhang, Electrochem. Commun. 2010, 12, 557. f) J. R. Lomeda, C. D. Doyle, D. V. Kosynkin, W.-F. Hwang, J. M. Tour, J. Am. Chem. Soc. 2008, 130, 16201. 4 X.-Z. Tang, W. Li, Z.-Z. Yu, M. A. Rafiee, J. Rafiee, F. Yavari, N. Koratkar, Carbon 2011, 49, 1258. 5 a) S. Hayami, M. R. Karim, Y. H. Lee, Eur. J. Inorg. Chem. 2013, 683. b) S. Hayami, Y. Komatsu, T. Shimizu, H. Kamihata, Y. H. Lee, Coord. Chem. Rev. 2011, 255, 1981. 6 a) Y. Komatsu, K. Kato, Y. Yamamoto, H. Kamihata, Y. H. Lee, A. Fuyuhiro, S. Kawata, S. Hayami, Eur. J. Inorg. Chem. 2012, 2769. b) S. Hayami, R. Moriyama, A. Shuto, Y. Maeda, K. Ohta, K. Inoue, Inorg. Chem. 2007, 46, 7692. c) S. Hayami, Y. Yamamoto, Y. Kojima, K. Inoue, J. Phys.: Conf. Ser. 2010, 200, 082008. 7 M. R. Karim, K. Hatakeyama, T. Matsui, H. Takehira, T. Taniguchi, M. Koinuma, Y. Matsumoto, T. Akutagawa, T. Nakamura, S.-i. Noro, T. Yamada, H. Kitagawa, S. Hayami, J. Am. Chem. Soc. 2013, 135, 8097. 8 J.-H. Jung, J.-H. Jeon, V. Sridhar, I.-K. Oh, Carbon 2011, 49, 1279. 9 a) Ravikumar, K. Scott, Chem. Commun. 2012, 48, 5584. b) H. Zarrin, D. Higgins, Y. Jun, Z. Chen, M. Fowler, J. Phys. Chem. C 2011, 115, 20774. 10 W. S. Hummers, Jr., R. E. Offeman, J. Am. Chem. Soc. 1958, 80, 1339. 11 Supporting Information is available electronically on the CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/index.html. 12 a) M. R. Karim, H. Shinoda, M. Nakai, K. Hatakeyama, H. Kamihata, T. Matsui, T. Taniguchi, M. Koinuma, K. Kuroiwa, M. Kurmoo, Y. Matsumoto, S. Hayami, Adv. Funct. Mater. 2013, 23, 323. b) Y. Matsumoto, M. Koinuma, S. Y. Kim, Y. Watanabe, T. Taniguchi, K. Hatakeyama, H. Tateishi, S. Ida, ACS Appl. Mater. Interfaces 2010, 2, 3461. 13 A. C. Ferrari, J. Robertson, Phys. Rev. B 2000, 61, 14095. 14 M.-C. Hsiao, S.-H. Liao, M.-Y. Yen, P.-I Liu, N.-W. Pu, C.-A. Wang, C.-C. M. Ma, ACS Appl. Mater. Interfaces 2010, 2, 3092. 15 a) Z. Zafar, Z. H. Ni, X. Wu, Z. X. Shi, H. Y. Nan, J. Bai, L. T. Sun, Carbon 2013, 61, 57. b) R. Podila, J. Chacón-Torres, J. T. Spear, T. Pichler, P. Ayala, A. M. Rao, Appl. Phys. Lett. 2012, 101, 123108. 16 a) D. Yang, A. Velamakanni, G. Bozoklu, S. Park, M. Stoller, R. D. Piner, S. Stankovich, I. Jung, D. A. Field, C. A. Ventrice, Jr., R. S. Ruoff, Carbon 2009, 47, 145. b) J. E. Kim, T. H. Han, S. H. Lee, J. Y. Kim, C. W. Ahn, J. M. Yun, S. O. Kim, Angew. Chem., Int. Ed. 2011, 50, 3043.

