Effects of Intercalated Molecules in Graphene Oxide on the Interlayer

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Effects of Intercalated Molecules in Graphene Oxide on the Interlayer Channels for Anhydrous Proton Conduction Jingtao Wang, Liping Zhao, Donghui Wei, Wenjia Wu, Jie Zhang, and Xian Cheng Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02677 • Publication Date (Web): 31 Oct 2016 Downloaded from http://pubs.acs.org on November 1, 2016

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Industrial & Engineering Chemistry Research

Effects of Intercalated Molecules in Graphene Oxide on the Interlayer Channels for Anhydrous Proton Conduction Jingtao Wang a, Liping Zhao a, Donghui Wei b, Wenjia Wu a, Jie Zhang a,*, Xian Cheng c,* a School

of Chemical Engineering and Energy, Zhengzhou University, Zhengzhou 450001, P. R.

China b College of Chemistry and Molecular Engineering, Centre of Computational Chemistry, Zhengzhou University, Zhengzhou 450001, P. R. China c School of Electrical Engineering, Zhengzhou University, Zhengzhou 450001, P. R. China *To whom correspondence should be addressed. E-mails addresses: [email protected]; [email protected].

Abstract Herein, eleven kinds of graphene oxide (GO)-based nanocomposites are prepared by intercalating organic molecules bearing different functional groups into GO layers. The changes of d-spacing (Δd) of GO upon intercalation suggest that Δd is mutable and regulated dynamically by surrounding environment, and organic molecules are mainly located in the cavities of GO. The conducting groups allow organic molecules to act as additional proton hopping sites and to connect hydrogen bonds networks within GO, endowing the resultant nanocomposites with enhanced proton conduction abilities. The proton conductivities vary with functional groups on organic molecules, increasing in the order of GO/phenol < GO/benzoic acid < GO/benzenesulfonic acid < GO/phenylamine. Increasing the amount of functional groups or optimizing the ligand structure will further enhanced the proton conduction ability. Particularly, GO/4-aminobenzoic achieves the highest proton conductivity of 5.0×10-4 S cm-1 under 30 oC and anhydrous condition, 10-times higher than that of GO. Keywords：Organic molecule; Graphene oxide; Intercalated nanocomposites; Proton conductivity. 1 ACS Paragon Plus Environment


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1. Introduction Graphene oxide (GO), as one of the graphene-based materials, has attracted considerable attention in the fields of energy, medicine, and environment owing to its excellent physical and chemical properties.1-4 Although the exact structure of GO remains suspensive, epoxy, hydroxyl, and carboxyl groups are widely accepted as the main functional groups on both basal planes and edges of GO layers.1,5,6 Owing to these functional groups and lamellar structure, GO exhibits versatile intercalation chemistry, beneficial for the design of GO-based nanocomposites with specific properties.7-11 Recently, the synthesis and application of such kind of nanocomposite have become a hot topic as the GO-based nanocomposites can well combine the advantages from intercalated component.10,12-16 For instance, Gao et al.14 reported that the intercalation of single-walled carbon nanotube (SWCNT) into GO layers greatly improved water permeation by creating nanochannels without sacrificing the rejection of nanometer-scale particles and molecules. Srinivas et al.15 utilized linear boronic acid as pillar to increase the interlayer spacing of GO layers, significantly enhancing the surface area of GO and hence H2 uptaking capability. Zhang et al.10 synthesized TiO2-GO nanocomposite by intercalating TiO2 crystallites into GO layers, which exhibited larger specific surface area than P25 powders and hence improved photo degradation efficiency (2.27 times higher than that of P25 powders). For another, proton conduction plays a critical role in the practical application of fuel cell, hydrogen sensor, energy transformation and reactor, and the proton conductivity determines the efficiency or sensitivity of these devices.17-22 Proton 2 ACS Paragon Plus Environment

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conductive inorganic materials, including montmorillonite, halloysite nanotube and zirconium oxide, have drawn great interest of many experts because of their easy modification, excellent structural stability, environmental protection, and low cost.23-25 GO, as a potential alternative for proton conductor, could readily achieve the proton conductivity of 10-6~10-4 S cm-1 at room temperature.26,27 The hydrogen bonds networks (H-networks) constructed by the oxygen-containing groups contribute to this efficient proton migration via Grotthuss mechanism under anhydrous condition.28,29 However, the protons are reported to transport mainly along the edges and pinholes of GO sheets, whereas the in-plane conduction is negligible due to the low amount and random distribution of epoxy and hydroxyl on basal plane, unable to support continuous proton migration.26,30 In addition, the relatively small interlayer distance (0.7-1.0 nm) of GO is adverse for proton hopping in and out from the sheets.31 For the purpose of practical application, the proton conduction ability of GO must be elevated for acceptable performance. To this end, GO-based composites by intercalating conducting component into the interlayer space of GO have widely attracted attention as they could construct the H-networks for continuous transfer and broaden interlayer distance for easy hopping.8,12,32-37 Ikeda et al. had intercalated metal ion and short-chain alkylamine into GO layer to prepare proton conduction materials, and found that the GO nanocomposites achieved enhanced proton conduction ability after the inclusion of short-chain alkylamine while obtained decreased proton conductivity after the inclusion of metal ion.8,12 Hatakeyama et al. had intercalated sulfate ion into GO layers to prepare nanocomposite material and 3 ACS Paragon Plus Environment


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found a two orders of magnitude increase of proton conductivity (compared with GO) at high water content.33 Different from the small and free inorganic ions, organic molecules could probably be a promising choice to prepare stable and tunable GO-based nanocomposites. Specially, organic molecules containing various functional groups such as –OH, –CO2H, –SO3H, –NH2, would create a series of nanocomposites with controllable conduction abilities. And the diversity of ligand structure on organic molecules makes interlayer distance controllable and hence the proton transfer channels. Besides, synergistic proton conduction might emerge if the molecule bears multiple-functional groups, whereas such effect can’t be provided by inorganic ions. Although the relevant exploration is rare to date, we expect that this kind of nanocomposite might be attractive for a wide range of application due to its diversity and controllability. In this study, we attempted to systematically investigate the effects of organic molecules on the structure and proton conduction of GO-organic molecule nanocomposites. Toluene (TE), phenol (PE), benzoic acid (BZ), benzenesulfonicacid (BS), and phenyl amine (PA) were selected as intercalators to explore the influence of functional groups on proton migration within nanocomposites. Considering the space dimension of benzene ring, methylamine (MA) and methane sulfonic acid (MS) were also chosen to explore the effect of steric hindrance on proton conduction in nanocomposites by comparing with the results of PA and BS intercalated nanocomposites. Besides, p-hydroxybenzoic acid (HBZ), p-hydroxybenzenesulfonic acid (HBS), 4-sulfobenzoate (SBZ), and 4-aminobenzoic acid (ABZ) were 4 ACS Paragon Plus Environment

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intercalated into GO layers for investigating the synergistic effect of different functional groups. The successful intercalation of organic molecules into GO layers was demonstrated by transmission electron microscopy (TEM), water contact angle, energy dispersive spectra (EDS), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and thermal gravimetric analysis (TGA). The proton conduction properties of GO and nanocomposites were validated by conductivity measurement and theoretical calculation of Gaussian 09. 2. Experimental 2.1. Materials Natural graphite powder (45 μm) was purchased from Sigma-Aldrich. TE, PE, BZ, and ethanol were obtained from Kewei Chemistry Co., Ltd. MS, BS, HBZ, HBS, and ABZ were purchased from MACKLIN. MA and PA were provided by Sionpharm Chemical Reagent Co. Ltd. 4-Sulfobenzoic acid monopotassium salt was purchased from Adamas Reagent Co. Ltd. SBZ was obtained by being treated with 0.1 M HCl to exchange the potassium ion with H+. The abbreviations and structure parameters of these organic molecules were listed in Table 1. The reagents were used as received without further purification. De-ionized water was used throughout the experiment. 2.2. Synthesis of GO GO was synthesized using improved Hummer’s method as reported in the literatures.38,39 Graphite powder (4.5 g) and potassium permanganate (KMnO4, 27.0 g) were added slowly to a mixture of concentrated H2SO4/H3PO4 (540 mL/90mL) in a 5 ACS Paragon Plus Environment


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Table 1. Structure, molecular dimension, abbreviation of all organic molecules and the change of interlayer distance (Δd) of GO after intercalation. L1a / Å

L2b / Å

Δd / Å

Abbreviation

Methane sulfonic acid

4.32

2.77

0.44

MS

Methylamine

2.91

1.63

0.37

MA

Toluene

5.85

1.74

0.77

TE

Phenol

5.69

0.96

0.50

PE

Benzoic acid

7.07

2.44

0.82

BZ

Benzenesulfonic acid

7.34

2.76

1.17

BS

Phenyl amine

5.77

1.63

1.03

PA

p-hydroxybenzoic acid

7.34

2.27

1.45

HBZ

7.65

2.77

1.49

HBS

4-sulfobenzoate acid

8.65

2.77

2.61

SBZ

4-aminobenzoic acid

7.70

2.27

5.12

ABZ

Name

Structure

p-hydroxybenzenesulfonic acid

a: The maximum interatomic distance of molecular; b: The maximum thickness of molecular.

1000 mL three-neck bottle with vigorous stirring for 24 h under 50 oC. Afterwards, the obtained solution was cooled to room temperature and poured into ice (~1200 mL) with 30 wt% H2O2 (10 mL) to convert the un-reacted permanganate and manganese dioxide into soluble sulfates, followed by ultrasonic treatment for 1 h. The solution was centrifuged (10,000 rpm) and the supernatant was decanted away. The precipitate 6 ACS Paragon Plus Environment

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was washed with a mixture of 400mL HCl (37 wt%) and 150mL water under stirring for 12 h, and then washed with water and centrifuged repeatedly until pH neutral. Finally, the precipitate was washed with ethanol twice and then dried in a vacuum oven for 24 h. After that, it was dispersed in ethanol (7 mg mL-1) or water (7 mg mL-1) by 3 h sonication, respectively. 2.3. Preparation of GO nanocomposites Taking into account the solubility of those intercalators, ethanol or water was employed selectively as solvent in this experiment. Equimolar intercalator (0.04 mole of TE, PE, BZ, BS, PA, HBZ, HBS, SBZ or ABZ) was added into ethanol (10 mL) and stirred until completely dissolved. Afterwards, the obtained solutions were added dropwise into GO ethanol solution (7 mg mL-1, 30 mL) with stirring, followed by ultrasonic treatment for 3 h and then stirred for another 48 h at room temperature. The mixture was centrifuged and washed with ethanol to remove residual organic molecules on GO surface. Finally, the mixture was dried in a vacuum oven at 30 oC until a constant weight. The resultant powders were designated as GO/TE, GO/PE, GO/BZ, GO/BS, GO/PA, GO/HBZ, GO/HBS, GO/SBZ or GO/ABZ, respectively. Similarly, equimolar MA or MS were firstly diluted in water (10mL) and then added dropwise into GO aqueous solution (7 mg mL-1, 30 mL) with stirring, and the following procedures were the same as above. The powders were defined as GO/MA or GO/MS, respectively. 2.4. Characterization of GO and nanocomposites Morphology analysis was performed by using FEI model TECNAI G2 transmission 7 ACS Paragon Plus Environment


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electron microscopes (TEM) operated at 200 kV. Water contact angle was measured with FACE (model OCA 25, Germany). Fourier transform infrared spectroscopy (FTIR) was carried out in the range of 4000-400 cm-1 on a Nicolet MAGNA-IR 560 instrument using KBr pellets. Ultraviolet visible (UV–vis) absorption spectrum was recorded on the UV-3600Plus with the wavelength ranging from 600 to 200 nm. X-ray diffraction (XRD) measurements were conducted on powder sample using a Bruker D8 Advance ECO with Cu Kα radiation in the angular range 2θ of 5o–60o. X-ray photoelectron spectroscopy (XPS) was collected on PHI 5000 Versa Probe II. EDS mapping image was recorded by TEAM EDS (EDAX, Japan) on JSM7500F SEM. For the accuracy, the powder sample was first compressed into a pellet before being sputtered with gold. TGA was conducted by TGA-50 SHIMADZU from room temperature to 400 oC with the rate of 5 oC min-1 under nitrogen atmosphere. 2.5. Evaluation of proton conductivity Proton conductivity of the as-prepared powder was conducted through a universal method.8,9 The proton conductivity of sample was measured by the AC impedance spectroscopy method using a frequency response analyzer (FRA, RST5000F11) in the frequency range of 106–100 Hz with an oscillating voltage of 20 mV. Both GO and nanocomposite powders were compressed into pellets with a diameter of 1.5 mm and thickness of about 0.650 mm prior to the conductivity measurement.27 The proton conductivity was tested under anhydrous condition using dry air and the system was allowed to equilibrate at the desired temperature (30 oC-150 oC) for a few hours. In order to avoid the influence of water on conductivity value, the samples were fully 8 ACS Paragon Plus Environment

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dried under vacuum at 50 oC for 24 h to completely remove the residual water in GO and nanocomposites prior to testing. Meanwhile, the cell installed with sample was kept in vacuum vessel during the whole test. Proton conductivities (σ, S cm-1) of the GO and nanocomposites were calculated by the equation (1): σ = l / AR

(1)

Where l, A, and R were the thickness (cm), testing area (cm2), and resistance (Ω) of GO and nanocomposites, respectively. The results were the average values of the data repeated three times with three different samples of each composition. 2.6. Computational methods In order to simulate GO, graphene oxide nanoflake was considered as working models. The model structure of graphene oxide nanoflake used in this study was taken from the literature.40 The DFT calculations were performed using the Gaussian 09 program.41 All structures were optimized at the B3LYP

42,43

/6-31G(d,p) level. Then,

frequency calculations at the same level of theory were carried out to identify all of the stationary points as minima (zero imaginary frequency) or transition state (only one frequency), which then provided free energies. 3. Results and discussion 3.1. Characterization of GO and nanocomposites Five species of organic molecules (TE, PE, BZ, BS and PA) bearing benzene ring with –CH3, –OH, –CO2H, –SO3H and –NH2 groups, respectively, are selected to investigate the effect of functional groups on the structure and performance of GO-based nanocomposites. TEM images in Figure 1a-c suggest that GO depicts the 9 ACS Paragon Plus Environment


transparent sheet-like morphology with scrolling on the edge (Figure 1a). Compared with GO, the colour of nanocomposites becomes dark as shown in Figure 1b and c, and the nanocomposites exhibits the nanoscale textures, probably due to the intercalation of BS or PA. Besides, the edge of nanocomposite becomes smooth without obvious wrinkles. This infers the weakness of interlayer interaction and the increase of interlayer distance upon organic molecule intercalation.44-47 The large anisometric shape of GO results in coplanar alignment of sheets with close interlayer space, whereas the presence of organic molecules as nanoscale spacers weakens the van der Waals interaction between GO layers, yielding increased interlayer distance.45,48 Also, the interaction between the intercalated organic molecules and GO (e.g. hydrogen bonds) might contribute to the smooth of the edge.

4.0 3.5

(f)

GO GO/PA PA

3.0

Absorbance / a.u.

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2.5 2.0 1.5 1.0 0.5 0.0 200

250

300

350

400

450

500

550

600

Wavelength / nm

Figure 1. TEM images of (a) GO and nanocomposites intercalated with (b) BS and (c) PA; EDS element mapping images under SEM of (d) GO/BS and (e) GO/PA; (f) UV–vis absorption spectra of GO, GO/PA and PA.

According to the water contact angle data shown in Figure S1, the water contact angle of the pristine GO layer is 57.5o, and those of GO/TE, GO/PE, GO/MZ, GO/MS and GO/PA are 58.2o, 58.1o, 57.9o, 57.2o and 58.0o, respectively. The close values of 10 ACS Paragon Plus Environment

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water contact angle between pristine GO and nanocomposites suggest the full removal of organic molecules on GO surface during preparation process. The distribution of organic molecules in nanocomposite is determined by EDS mappings in Figure 1d and e. For GO/BS (Figure 1d) and GO/PA (Figure 1e), they clearly reveal that elements S and N are uniformly distributed over the whole nanocomposites within the testing area of hundreds of square micron. For GO/TE (Figure S2b), the absence of additional atoms makes its mapping close to that of GO (Figure S2a). Considering the full removal of organic molecules on GO surface, this indicates that the molecules have been successfully intercalated into GO layers helped by the ultrasonic treatment and mutual interactions between GO and organic molecules (π–π interaction and hydrogen-bonding interaction). The functional groups within nanocomposites are further confirmed by FTIR (Figure S3) and UV–vis absorption spectrum (Figure 1f). GO, GO/PA and PA are chosen as representatives for the measurement of UV–vis absorption spectrum. As shown in Figure 1f, the absorption peak for GO at 227 nm blueshifts to 218 nm upon the intercalation of PA, suggesting the transition of electrons (π–π conjugation) between GO and PA.49 A new absorption peak at 277 nm is observed in the spectrum of GO/PA, which attributes to the π–π* transitions of PA. Yet this absorption peak is blue-shifted with 3 nm when compared with PA, implying the attenuation of p–π conjugation between the –NH2 and benzene in PA. Such result indicates that in the nanocomposite, the –NH2 group with lone pair electron is involved in the hydrogen-bonding interaction with the oxygen-containing group on GO.49 11 ACS Paragon Plus Environment


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Figure 2. (a) XPS spectra of GO, GO/TE, GO/BS, and GO/PA; (b) C 1s spectra of (i) GO, (ii) GO/TE, (iii) GO/BS, and (iiii) GO/PA; (c) S 2p spectra of GO/BS; (d) N 1s spectra of GO/PA.

The loading amounts of organic molecules in nanocomposites are determined by XPS technique (Figure 2). As expected, GO and the nanocomposites display the C 1s and O 1s peaks at binding energies of 285.6 and 533.9 eV, respectively (Figure 2a). By comparison, a new peak is observed at 168.4 eV for GO/BS and at 401.5 eV for GO/PA, attributed to S 2p and N 1s, respectively.50 The atomic percentages of S and N are about 3.43% and 4.18% (Table 2), corresponding to the BS and PA contents of 16.94% and 16.12%, respectively. The close loading amount indicates that similar molecular structure will result in close intercalation ratio for these organic molecules. 12 ACS Paragon Plus Environment

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Also, no new peaks are observed for GO/TE in its XPS spectrum when compared with that of GO, whereas the intercalation of TE elevates the C content from 66.7% for GO to 72.9% for GO/TE. Table 2. Atomic percentages of C 1s, O 1s, S 2p, N 1s, and O/C radios in GO, GO/TE, GO/BS, and GO/PA obtained from XPS.

GO GO/TE GO/BS GO/PA

C 1s (Atom %)

O 1s (Atom %)

S 2p (Atom %)

N 1s (Atom %)

66.69 72.90 67.02 69.64

33.32 27.10 29.55 26.18

– – 3.43

– – –

–

4.18

O/C 0.49 0.37 0.44 0.38

As shown in Figure 2b, the peak of C 1s for GO and nanocomposites could be deconvoluted into five peaks: 284.6 eV of C–C bond, 285.4 eV of C–OH or C–S bonds51, 286.6 eV of C–O–C or C–N bonds52,53, 287.9 eV of C=O bond, and 289.2 eV of O=C–O– group. There are no obvious alterations for C 1s after organic molecule intercalation, suggesting the absence of destruction for oxygen-containing functional groups of GO. This indicates that the molecules are physically intercalated into GO layers with no covalent bonds. The S 2p spectrum of GO/BS is displayed in Figure 2c, which contains one peak at 168.4 eV corresponding to sulfonic acid group.33 N 1s spectra for GO/PA (Figure 2d) exhibit two peaks at 399.8 and 401.6 eV, which are assigned to free amine (–NH2) and protonated or hydrogen-bonded amine (NH3+ or H∙∙∙NH2), respectively.54,55 The influence of intercalated organic molecules on the layered structure of nanocomposite is probed by XRD in Figure 3a. GO shows a single diffraction peak at 2θ of 10.5o, inferring the c-axis interlayer distance (d-spacing) of 8.39 Å. By comparison, nanocomposites exhibit a shift in peak position toward lower angles, 13 ACS Paragon Plus Environment


(a)

100

(b)

Relative intensity / a.u.

90 80

Weight loss / %

GO GO/TE GO/PE GO/BZ GO/BS

70 60

GO GO/TE GO/PE GO/BZ GO/BS GO/PA

50 40 30 20

GO/PA

10 0

10

2

12

14

50

16

(c)

0.20

GO/BS GO/BZ

0.10

GO/PE 0.05

150

200

250

300

350

400

o

0.30

GO/PA

0.15

100

Temperature / C

-1

0.25



GO/PA

(d)

-1

8

Proton conductivity / mS cm

6

Proton conductivity / mS cm

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GO GO/TE

0.25

GO/BS

0.20

GO/BZ 0.15 0.10

GO/PE GO GO/TE

0.05 0.00

0.00

Pellets

Pellets

Figure 3. (a) XRD patterns; (b) TGA curves; proton conductivity under anhydrous condition and (c) 30 oC and (d) 60 oC of GO and nanocomposites intercalated with PE, BZ, BS, PA and TE.

indicating the expansion of interlayer space along c-axis. The d-spacing values for GO/TE, GO/PE, GO/BZ, GO/BS and GO/BA are calculated to be 9.16, 8.89, 9.21, 9.56 and 9.42 Å, respectively. Generally, we can expect that if the intercalated molecules are exactly perpendicular to GO plane, the change of d-spacing (Δd) after intercalation should be equal to the size (L1, Table 1) of intercalated molecule, whereas if the molecules are parallel to GO plane, Δd should correspond to molecule thickness (L2, Table 1). Yet, the Δd values of nanocomposites are much smaller than L1, and even smaller than L2. For instance, the Δd value of GO/PA is 1.03 Å, 0.18 times of the L1 and 0.63 times of L2 of PA, respectively. This phenomenon strongly suggests that the intercalated molecules are likely to be parallelly aligned within GO layer. The π–π interaction between benzene ring and GO layers might be the force for 14 ACS Paragon Plus Environment

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the stable parallelism of these organic molecules. Moreover, the great smaller Δd of these nanocomposites when compared with L2 might be resulted from the occupying of molecules in cavities within GO interlayer aided by the interactions. This kind of cavities have been reported in GO layers accompanied by the formation of hydrogen bonds among oxygen-containing groups from two adjacent GO layers, as shown in Figure 4a. The functional groups in intercalated molecules, thus, are easy to bond with those oxygen-containing groups (Figure 4b-f). This finding also infers that the interactions between GO layers and the subsequent d-spacing should be mutable and could be regulated dynamically by surrounding environment (Figure 5). Meanwhile, the Δd values increase in the order of GO/PE < GO/TE < GO/BZ < GO/PA < GO/BS, directly proportional to the L2 values of intercalated molecules. This implies that the molecule size also affects the expansion degree to some extend as expected.

Figure 4. Proposed mechanism for proton conductivity and proton-conducting pathway in (a) pristine GO, (b) GO/PE, (c) GO/BZ, (d) GO/BS, (e) GO/PA, and (f) GO/TE.

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Figure 5. Illustration of the process that PA is being intercalated into GO layers.

The thermal abilities of GO and nanocomposites are analyzed using TGA, as shown in Figure 3b. The thermal degradation of these samples displays a two-step weight loss process. Specifically, the first step occurs at around 100 oC, which is probably attributed to the loss of absorbed water. The second weight loss occurring at 180 oC is due to the pyrolysis of the oxygen-containing functional groups that produce CO2 and other gas.56,57 In this stage, the intercalation of organic molecules retards the decomposition of GO as proved by their higher onset temperatures, implying enhanced thermal stabilities of nanocomposites. Such result is probably attributed to the interaction between GO and organic molecules. TGA result implies that the as-prepared nanocomposites are thermally stable at the temperatures below 180 oC and thus might adequate for the application of hydrogen conversion devices. In order to investigate the function of these organic molecules on proton conduction, the proton conductivity is measured in anhydrous condition for eliminating the effect of water. Under water-free condition, protons only transport through Grotthuss mechanism, that is, proton transports from one carrier site to a neighboring one via a short-distance hopping (

Effects of Intercalated Molecules in Graphene Oxide on the Interlayer

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