Graphene Oxide Papers Simultaneously Doped with Mg2+ and Clâ for

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Graphene Oxide Papers Simultaneously Doped with Mg and Cl for Exceptional Mechanical, Electrical and Dielectric Properties Xiuyi Lin, Xi Shen, Xinying Sun, Xu Liu, Ying Wu, Zhenyu Wang, and Jang-Kyo Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11486 • Publication Date (Web): 08 Jan 2016 Downloaded from http://pubs.acs.org on January 12, 2016

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ACS Applied Materials & Interfaces

Graphene Oxide Papers Simultaneously Doped with Mg2+ and Cl- for Exceptional Mechanical, Electrical and Dielectric Properties Xiuyi Lin, Xi Shen, Xinying Sun, Xu Liu, Ying Wu, Zhenyu Wang, and Jang-Kyo Kim* Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong

ABSTRACT: This paper reports simultaneous modification of graphene oxide (GO) papers by functionalization with MgCl2. The Mg2+ ions enhance both the interlayer crosslinks and lateral bridging between the edges of adjacent GO sheets by forming Mg-O bonds. The improved load transfer between the GO sheets gives rise to a maximum of 200 and 400% increases in Young’s modulus and tensile strength of GO papers. The intercalation of chlorine between the GO layers alters the properties of GO papers in two ways by forming ionic Cl- and covalent C-Cl bonds. The p-doping effect arising from Cl contributes to large enhancements in electrical conductivities of GO papers, with a remarkable 2500-fold surge in the through-thickness direction. The layered structure and the anisotropic electrical conductivities of reduced GO papers naturally create numerous nanocapacitors that lead to charge accumulation based on the Maxwell-Wagner (MW) polarization. The combined effect of much promoted dipolar polarizations due to Mg-O, C-Cl and Cl- species results in an exceptionally high dielectric constant over 60,000 and a dielectric loss of 3 at 1 kHz by doping with 2mM MgCl2. The excellent mechanical and electrical properties along with

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unique dielectric performance shown by the modified GO and rGO papers open new avenues for niche applications, such as electromagnetic interference shielding materials.

KEYWORDS: Graphene oxide paper, Mg and Cl doping, Electrical conductivity, Mechanical properties, Dielectric constant

INTRODUCTION Two-dimensional graphene and graphene-based materials have attracted tremendous interests from various research communities and much attention has been drawn to explore their exceptional characteristics for real-world applications.1,2 Among several well-known techniques to synthesize graphene and its derivatives, the chemical method based on the earlier work3 is the most popular and practical way to obtain graphene oxide (GO) in an aqueous dispersion. GO consists of mono- to several-layer graphene sheets with oxygenated functional groups on their basal plane and edges, and can be reassembled into thin films4,5 or paper-like materials6-9 in a freestanding form. GO papers have received growing interests owing to their unique structure, properties and potential multi-functional applications. Graphene-based papers with a modulus up to 35 GPa and tensile strength higher than 120 MPa,6 bring about better bendability than CNT bucky paper and graphite foil. They could absorb up to 0.92MJ/kg of ballistic energy − 10 times the amount of energy steel can − making them superb body armor.7 After effective reduction, the electrical conductivity of reduced GO papers could reach 139,000 S/cm8 and free-standing flexible graphene papers have been successfully used as current collector and binder free anodes for lithium ion batteries with excellent capacities above 330 mAh g-1 after 100 cycles.9 These graphenebased papers have already been extensively employed in energy storage, strain sensors, sealants, actuators, bio-compatible substrates, etc.10 Together with a high dielectric constant


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of over 15,000, the highly conductive graphene papers can also find many applications in flexible electronics, electric power systems, and electromagnetic interference shielding.11 Although an individual graphene sheet has an extremely high modulus and strength, the corresponding properties of GO papers are far below those of the monolayer pristine graphene while they have an aligned structure assembled in a layer-by-layer manner.8,12 For example, GO papers typically have a Young’s modulus in the range of 5-20 GPa,6,13 which is 50 times lower than the theoretical graphene modulus of 1000 GPa.14 When GO papers are subjected to a tension, the stress is hardly transferred between the individual GO sheets because of the small sizes of GO sheets and weak interlayer bonds consisting mainly of van der Waals interactions or hydrogen bonds. To enhance the interlayer interaction, two approaches have been taken: namely, (i) the synthesis of large-size GO sheets by repeated centrifugation of GO dispersion8,15 and (ii) chemical functionalization of GO sheets to improve the interlayer adhesion.16 Chemical modification of GO sheets has been extensively explored as a versatile means, and significant successes have been reported in improving the useful properties of GO papers, e.g., tuning the mechanical properties of GO papers and GO/polymer composites by controlling the interlayer hydrogen bonds;12 selective functionalization with small molecules, 1,4-butandiol and ethylenediamine, to achieve wellstacked structure;17 forming covalent bonds with polymers like, polyaniline, alkylamines, nylon, polyacrylamide and polydopamine, using in situ intercalative polymerization or solution intercalation.18–22 While the intercalation of polymers improved the interlayer interactions, the mechanical properties of the resultant papers in the thickness direction were not simultaneously enhanced due to the large molecules of the intercalated polymer. Recent experiments revealed remarkable improvements of the mechanical properties of GO papers through ionic bonds assisted by intercalating metal ions, such as magnesium, calcium23 and borate.24 Ionic bonding involves the electrostatic attraction between


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oppositely charged ions, and have been widely utilized in biological materials and nanocomposites to enhance their mechanical properties.25 A theoretical study based on the first principles confirmed the benefits of these crosslinks in improving the load transfer both across the edges and between the basal planes of graphene sheets.26 Herein, we report simultaneous enhancements in mechanical, electrical and dielectric properties of GO papers by functionalization with multi-functional dopant, MgCl2. Mg-O ionic bonds were introduced to serve as crosslinks between adjacent GO sheets by intercalating Mg2+ ions, giving rise to extraordinary mechanical properties of GO papers. Simultaneously, the ionic Cl- ameliorated the p-doping effects as the electron acceptor, promoting electrical conductivities of GO sheets. Both the Mg2+ and Cl- ions created dipolar polarization which offers concomitant enhancement in dielectric constant and reduction in loss factor of rGO papers, especially at a high dopant concentration. Finally, a dielectric constant over 60,000 was achieved along with a dielectric loss of 3 at 1 kHZ by doping with 2mM MgCl2 of rGO.

EXPERIMENTAL SECTION Materials and preparation of GO papers. The precursor GO dispersion was synthesized based on the well-established chemical method.27 Pristine GO papers were fabricated by filtration of 2mg/mL GO dispersions in water through the membrane filter (Anodisc, 47 mm in diameter, 0.2 µm in pore size, supplied by Whatman) under a vacuum pressure. GO was functionalized by adding 10 mL MgCl2 (supplied by Sigma-Aldrich) solution of varied concentrations ranging 0.25-2.0 mM to the GO dispersion, which was stirred for 1 h at 90oC for reaction. Then the mixture was filtrated under vacuum to obtain modified GO papers. After filtration, the paper was peeled off from the filter and dried in a vacuums oven at 60°C for 6 h. The as-prepared papers were thermally reduced in a tube furnace (Eurotherm) at 600 o

C for 3 h under the flow of N2 to obtain reduced GO (rGO) papers.


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Material characterization and mechanical tests. The X-ray photoelectron spectroscopy (XPS, Surface analysis PHI5600, Physical Electronics) was employed to evaluate the elemental compositions and the Mg and Cl contents in the modified papers using Al Kα line as the excitation source. The morphologies of as-prepared and modified GO papers were studied on a scanning electron microscope (SEM, JEOL-6700F) and a field emission analytical transmission electron microscope (FETEM, JEOL 2010F). The phase structure of the papers was determined on a power X-ray diffraction (XRD) system (PW1830, Philips) with Cu Kα radiation from 3° to 20°. Fourier transform infrared spectroscopy (FTIR, Biorad FTS 6000) was used to evaluate the functional groups present on GO and modified GO papers in the near infrared (NIR) region (400 to 4000 cm-1). The in-plane conductivity of the papers was measured using the four-point probe method (Scientific Equipment and Services) and the conductivity in the thickness direction was measured using a programmable curve tracer (370A) by applying a voltage on the top and bottom surfaces of the papers. The slope of the voltage vs. current plot was used to determine the resistivity of the papers. To reduce the contact resistance between the probes and the paper surface, the contact points were coated with silver paste. The carrier type and charge carrier concentration were evaluated using a Hall measurement system (Bio-Rad HL5500PC). The dielectric properties were determined on an Impedance Analyzer (4194A, Hewlett Packard) at frequencies ranging from 100 Hz to 40 MHz, and on a broadband dielectric spectrometer (Novocontrol) from 1 MHz to 2 GHz. Tensile properties of GO papers were measured on a universal testing machine (MTS; Alliance RT/5) at room temperature. 20 mm x 5 mm rectangular samples were loaded in tension at a rate of 1 mm/min. The fracture toughness was measured using double edge notch tension (DENT) specimens on a universal testing machine. Specimens of 26 mm x 13 mm rectangular shape were cut from the GO papers and an initial crack length, a, of 2.6 mm


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was introduced.10 The mode-I critical stress intensity factor, KIc, was calculated as the fracture toughness using Equation (1) when the external stress reached a critical value, σc, for crack propagation: = √ ( /)

(1)

For the DENT specimens with an initial crack length to width ratio, a/w = 0.2, the geometric correction factor, F(a/w) = 1.13, was obtained:28 F(a/w) = 1.12 + 0.41(a/w) - 4.78 (a/w)2 + 15.44 (a/w)3

(2)

The strain energy release rate, Gc, was calculated from KIc using Equation (3): = /

(3)

where E is the Young’s modulus of the material under the plane stress condition.

RESULTS AND DISCUSSION Chemistry and structure of modified GO papers. The general XPS spectra are presented in Figure 1a and the corresponding percentages of carbon and assignations are summarized in Table 1. There were significant changes in surface chemistry of GO sheets after doping with MgCl2. The binding energies of the C=C and C–H bonds are assigned at 284.7 and 285.5eV, respectively (Figure 1b). The peaks at 286.7, 287.6 and 289.2 eV are assigned to the -C–OH, -C=O/C-O-C and –COOH functional groups, respectively; while the C-Cl bond at 286.5 eV was not clearly seen because of the overlapping with the -C-OH groups. Among several different oxygenated groups, the intensities of the epoxide groups at 287.6 eV consistently declined while those of the C-O groups grew with increasing dopant concentration, indicating possible opening of epoxide groups. However, the C/O ratio varied in a very narrow range of 2.0-2.22 regardless of dopant concentration, indicating little reduction after doping.


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The Mg 2p peak occurred at ~50.5 eV and its intensity increased with dopant concentration. Taking C sp2 peak at 284.7 eV as an internal reference, the binding energy of Mg 2p should appear at 49.85, 50.4 and 51.3 eV for metal Mg, MgO and MgCl2, respectively.29 Therefore, the peak obtained at 50.5 eV was largely attributed to the ionic bond of Mg and O. All these findings suggest that a possible mechanism of the interaction between Mg2+ and GO sheets was the opening of C-O-C groups to form Mg-O ionic bonds that served as interlayer crosslinks between the neighboring GO sheets23 (inset I in Figure 1f). It may also be possible that Mg2+ interacted with the negatively charged carboxylate ion (-COO-1) on the edges of GO sheets which bridged two adjacent GO sheets on the same plane (inset II in Figure 1f).26 The Cl 2p core binding energies appeared as doublets associated with 3/2 and 1/2 levels, which are separated by 1.6 eV due to spin-orbit coupling, as illustrated in Figure 1c. High intensities were observed at binding energies of 199.1eV (2p3/2) and 200.7eV (2p1/2), indicating that the majority of chlorine existed in an ionic form bonded to the GO plane.30 The relatively low peaks located at 200.3eV (2p3/2) and 201.9eV (2p1/2) can be attributed to C–Cl groups where chlorine is covalently bonded with carbon through the nucleophilic substitution of –OH groups by chloride (inset III in Figure 1f).31 The concentrations of the ionic and covalent Cl were measured and the results are shown in Figure 1d, along with the total Mg concentration. The ionic Cl- concentration increased rapidly while the covalent CCl concentration remained almost constant with increasing dopant content, confirming that the ionic Cl- was the dominant form at high dopant contents. This finding has an implication on electrical conductivities of doped GO papers, as discussed later. Theoretically, the atomic ratio of Mg to Cl should be 1:2 in MgCl2. However, the total Cl at% was even lower than Mg at%, which may be attribute to the draining away of Cl ions during the filtration process.


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The changes in chemical structure after doping were further evaluated using FTIR spectra, as shown in Figure 1e. Prominent FTIR peaks were observed in both the neat GO and modified GO papers at wavenumbers 1625, 1380, 1287 and 1083 cm-1 which are assigned to oxygenated functionalities, such as aromatic C=C, carboxy C-O, epoxy/ether C-O and alkoxy/alkoxide C-O, respectively. After doping, the peak corresponding to carboxy C=O stretches at ~1733 cm-1 almost disappeared, while the peak corresponding to carboxy C-O stretches at 1380 cm-1 downshifted with an increased intensity, which is a testament to carboxylic acid coordination to Mg2+ ions.23 In addition, the peak intensity of epoxy/ether C-O reduced whereas that of alkoxy/alkoxide C-O increased, suggesting possible formation of new C-O bonds through epoxide ring-opening. Furthermore, a small peak appeared at ~800 cm-1, a reflection of C-Cl covalent bonds confirming chlorine doping.32

(b)

C=O/ C-O-C

GO paper

Cg C-O Cd

O-C=O

Intensity (counts)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Modified GO paper

292

290

288 286 284 Binding energy (eV)

282


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(c) Cl 2p

2p3/2 -

Cl 2p1/2 C-Cl 2p1/2

2p3/2

206 204 202 200 198 196 194 Binding energy (eV)

Atomic concentration (%)

3.5

Intensity (counts)

3.0

(d)

2.5

Total Mg Total Cl Ionic Cl Covalent C-Cl

2.0 1.5 1.0 0.5 0.0 0.0

0.5

1.0

1.5

2.0

Dopant concentration (mM)

aromatic C=C

(e) C=O strech

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


C-O (epoxy /ether)

C-O(alkoxy)

C-Cl strech

GO paper Modified GO paper 1800

1500 1200 900 -1 Wavenumber (cm )

600

Figure 1. (a) XPS general spectra of GO at different dopant concentrations, with Mg 2p peaks in inset; (b) deconvoluted C1s spectra of GO and 2mM doped GO papers; (c) deconvoluted Cl 2p spectrum of 2mM doped GO papers; (d) Mg and Cl contents as a function of dopant concentration; (e) FT-IR spectra of GO and 2mM doped GO papers; and (f) schematic models showing that Mg2+ acts as both interlayer crosslink (inset I) and bridge between edges of adjacent GO sheets (inset II), and covalent C-Cl bond (inset III).


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Table 1. Relative percentages of carbon and assignations of modified GO papers for different MgCl2 concentrations. MgCl2 (mM)

Cg sp2 ~284.7 eV

Cd sp3 ~285.5 eV

-C-O ~286.7 eV

-C=O/ -C-O-C ~287.6 eV

-COO~289.2 eV

0

30.7

14.1

12.6

34.2

8.4

0.25

28.4

16.3

14.6

31.8

8.9

0.5

30.1

14.2

18.8

28.4

8.5

1

31.7

13.5

20.2

26.7

7.9

1.5

30.5

13.4

21.6

26.3

8.2

2

31.0

12.6

22.4

25.6

8.4

The TEM image shown in Figure 2a exhibits an amorphous structure due to the largely oxidized regions of GO sheets. The EDS elemental analysis of the focused area of 50 nm in Figure 2b clearly indicates the presence of Mg and Cl in the sample. Figure 2c represents a HRTEM micrograph of the modified GO sheets, showing a few GO layers stacked together with a few wrinkles on the sheets. The SAED pattern given in Figure 2d exhibits several sets of hexagonal diffraction points, confirming no crystalline structure. The concentric rings in the SAED pattern is a reflection of several layers of GO stacked together whereas the existence of Mg and Cl element in the ion form was confirmed.


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Figure 2. (a) TEM images of modified GO paper and (b) the corresponding EDS analysis of the focused area; (c) HRTEM image and (d) the corresponding SAED pattern. Raman spectroscopy was used to examine the effect of modification on the structure of GO papers, as shown in Figure 3a. There were two major peaks for all samples, D-band near 1344 cm−1 corresponding to the presence of residual oxygen, point defects and structural disorder; and G-band near 1591 cm-1 attributed to the sp2 domain size, doping domains and local charge fluctuations.33 With increasing dopant content, the G-band peak up-shifted from 1951 to 1605 cm-1 as a result of the rise in electron/hole concentrations, while the position of the D-band peak remained much the same. The increase in dopant concentration also led to a gradual decrease in D/G intensity ratio, from ID/IG = 2.53 to 2.14, which arose mainly from the opening of epoxy groups (inset I in Figure 1f) with an associated reduction in sp3 carbon structure, and partly from the strong electrostatic attraction due to Mg2+ ions. There was a strong correlation between the G-band shift and the carrier concentration as shown in Figure 3b. Two main causes were responsible for the growth of carrier concentration: namely, (i) p-doping as Cl acted as electron acceptor,30,31 and (ii) the presence of Mg2+ and Cl- ions providing extra charges. For example, GO papers with 0.25 mM MgCl2 had a carrier concentration of 1.7×1015 cm-3 and a total ion content of 0.71 at%. An increase in dopant content to 2 mM resulted in a 20-time increase in carrier


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concentration to 32.6 x 1015 cm-3 and an 8-fold increase in total ion content to 5.66 at%. In view of the more remarkable change in carrier concentration than in ion content, it can be said that the p-doping effect was found to be dominant.

14 D band

G band

12 -1

Raman shift (cm )

(a) Intensity (a.u.)

GO 0.25 0.5 1.0 1.5 2.0

1300

1400 1500 1600 -1 Raman shift (cm )

1700

35 30

10

25

8

20

6

15

4

10

2

5

0 -0.5

0

0.0 0.5 1.0 1.5 2.0 Dopant concentration (mM)

-3

1200

15

x10

(b)

Carrier concentration (cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2.5

Figure 3. (a) Raman spectra of modified GO papers; and (b) G-band peak shift and charge carrier concentration as a function of dopant concentration. Apart from the above positive influence on charge carrier concentration, the intercalation of Mg2+ and Cl- ions into the galleries of GO sheets resulted in a significant shift of the peak position and a sizable increase in d-spacing: the higher the dopant content, the larger the dspacing, as shown in Figure 4. With increasing dopant concentration, there was a gradual downshift of the peak position because the uniform intercalation of Mg2+ and Cl- ions between the GO sheets caused the d-spacing to be expanded along the stacking direction when the MgCl2 concentration increased (Figure 4a). A few extra peaks were identified, especially at high MgCl2 concentrations: the small peaks emerged near 18 o and 22 o are ascribed to the intercalated Mg2+ ions, whereas the sharp peaks appeared at 32o and 38o correspond to the (100) and (101) phases of MgCl2, due to the crystallization of MgCl2 nanoparticles. The d-spacing of the modified GO papers were expanded by 0.2 to 1.9 Å when the Mg content was varied from 0 to 3.38 at.% (Figure 4b). The expansion of dspacing appeared to depend on the bond length of intercalated molecules. This is evidenced


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by a slow growth of d-spacing at Mg contents above 1.44 at.% when the expanded d-

GO Mg

(b) 10.0 2.0 mM 1.5 mM 1.0 mM 0.5 mM GO

d-spacing (angstrom)

(101)

(100)

(003) (006)

(a)

(002)

spacing approached the bond length of Mg–O which is estimated to be 1.9-2.1 Å. 26

Intensity (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


9.5

20

30

40

50

60

70

80

8.78

8.5 8.19

8.0

7.77

7.5 7.57 7.0

10

9.47

9.0

0

1

2

3

4

Mg content (atom %)

o

2 theta ( )

Figure 4. (a) XRD patterns as a function of dopant concentration; (b) Corresponding dspacing as a function of Mg content. Improved mechanical properties of modified GO papers

Tensile Properties. The tensile properties and the corresponding stress-strain curves of the modified GO papers are shown in Figures 5a and 5b, respectively. Both the Young’s modulus and tensile strength surged with Mg contents below 0.95 at%, and the maximum improvements in these properties were remarkable with nearly 200% and 400% compared to those without modification. Park et al23 has reported very high moduli ranging from 25.6 to 27.9 GPa for MgCl2 modified GO papers prepared by continuous filtration, but the change in tensile strength was only marginal. It is noted that the C/O ratio of the modified GO papers obtained in our work was lower than the previous study, i.e. 2.0 vs 2.7. Furthermore, the hydrothermal mixing used here allowed effective interactions between Mg ions and GO sheets, resulting in a higher Mg content than that obtained from continuous filtration with the same total amount of 2 mM MgCl2. These two factors may be responsible for the formation of chemical crosslinks, resulting in efficient load transfer and better enhanced tensile strengths in this study. A deformable tension-shear model34 revealed the failure mechanisms of GO papers in that the load transfer in tension was mainly through shear in


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interlayer crosslinks and partly due to tension between adjacent GO sheets (Figure 5e). Essentially the same findings were reported from the molecular dynamic simulations.8 Therefore, the molecules with two ends, like divalent (Mg2+, Ca2+) ions23 or diamine monomers,35 can effectively connect the adjacent GO sheets and contribute to increasing the mechanical properties. However, the relatively weak C-Cl bonds and their single end structure may not contribute much to the load transfer in GO papers, thus only the effects of Mg-O bonds are considered here. The Mg–O bonds present a much higher shear strength than the hydrogen bonds in GO papers, which are estimated to be 800 and 100 MPa, respectively.34 Thus, it can be said that the largely enhanced mechanical properties of the modified GO papers arose mainly from the Mg–O interlayer crosslinks (Inset I in Figure 1f), while the Mg–O bonds also acted as bridges between the adjacent GO edges (Inset II in Figure 1f). Both the modulus and strength gradually dropped after the maxima at Mg contents higher than 0.95 at%. There are two major reasons behind this phenomenon, including (i) the agglomeration of dopant molecules and the associated macroscopic wrinkling of GO sheets; (ii) the expansion in interlayer distance, thus increasing the cross-sectional area more than the increases in properties. A highly wrinkled structure with less aligned sheets in the modified GO paper is shown in Figure 5f, which is well contrasted with a more uniform, tightly stacked structure of the neat GO paper. It is postulated that the increase in Mg–O interlayer crosslinks with increasing Mg2+ content was not able to balance the accompanying adverse effect of increased cross-sectional area of the modified papers while the growing misalignment or wrinkling also had negative influences. It is shown14 that wrinkling degraded the mechanical properties of both graphene and GO papers. The above postulation was partly confirmed by the drop in failure stress (measured per unit crosssectional area of papers) although the failure strain (measured independent of cross-


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sectional area of papers) marginally increased after the Mg2+ content increased from 0.95 to 1.44 at% (Figure 5b).

Fracture Properties. Both the fracture toughness, KIC, and the strain energy release rate, Gc, presented an initial increase, which was soon reversed to drop at Mg contents above about 0.48 at%, as shown in Figure 5c. It was shown previously that the mode I failure in DENT tests of neat GO papers involved crack propagation right through the stacked GO sheets, following a straight path by brittle cleavage and thus cohesive failure.10 This is also clearly reflected by the stress-strain curves shown in Figure 5d, which are typical of brittle fracture. These observations mean that the inherent tensile strength of GO sheets played a major role in determining the mode I fracture toughness/energy, while the chemical and geometric modifications effected by the dopants, such as the enhanced interfacial crosslinks and edge-to-edge bonds, as well as the expanded cross-sectional area, contributed to a less extent. It is presumed that the initial increase and the following reduction in fracture toughness of the modified GO papers were the reflection of the contribution by the enhanced interfacial crosslinks due to the Mg–O ionic bonds, which were counterbalanced by the reduced hydrogen bonds and the enlarged cross sectional area. The strength of an Mg–O bond is ~1 eV, while the hydrogen bond strength ranges from 0.02 to 2.6 eV under different conditions.26 This may mean that the contributions to fracture by the Mg–O ionic and hydrogen bonds were equally important at a low Mg content. It should be noted that the hydrogen bond strength was highly sensitive to the interlayer distance in GO papers: for example, the O-H bond energy was estimated to be 0.32 eV at a bond distance of 2.55 Å, but it drastically decreased to 0.05 eV at 4 Å.36 The d-spacing of the modified papers expanded significantly resulting in an increase in effective cross-sectional area (Figure 4b) with a concomitant reduction in hydrogen bond strength. Thus, the critical stress required


15


for fracture decreased when the Mg content was higher than 0.48 at%, leading to continuous

30

(a)

450 400

25

350 300

20 15

250

GO paper Young's modulus Tensile strength Data from Ref. 23

10

200 150 100

5

50

0.0

0.5

1.0

1.5

Mg content (at%)

500

(b)

GO 0.48% 0.95% 1.44%

400

Stress (MPa)

Young's modulus (GPa)

reductions in both KIC and GC (Figure 5c).

Tensile strength (MPa)

300 200 100 0

2.0

0.0

0.4

0.8

1.2

1.6

2.0

2.4

2.8

Strain (%)

20

Stress (Mpa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(d)

GO 0.48 % 0.95 % 1.44 %

16 12 8 4 0.0

0.5

1.0

1.5

2.0

2.5

Strain (%)

Figure 5. Changes in (a) tensile and (c) fracture properties of GO papers measured as a function of Mg content. Symbols ★ ★ in (a) represent the results taken from Ref. 23. Inset in (c) shows the geometry of a DENT specimen. Typical stress-strain curves for (b) tensile and (d) fracture tests. (e) Schematic illustration of loading transfer in GO paper. (f) SEM images of cross sections of as-prepared and modified GO papers.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Electrical conductivities The in-plane electrical conductivity of GO papers presented a consistent rise with increasing dopant concentration (Figure 6a). Basically the same trend was noted after reduction of GO papers (Figure 6b), but with a less increment: for example, there was a 10fold improvement in in-plane conductivity for GO papers doped with 2 mM MgCl2, while only 15% improvements for rGO papers doped with the same concentration. The improvement in electrical conductivity was associated mainly with an increase in carrier concentration (Figure 3b). After the thermal reduction, the rGO papers became inherently conductive so that the net contribution by doping became relatively less significant. A remarkable improvement in through-thickness conductivity was observed with almost 2500fold improvement after doping with 2 mM MgCl2 for both the GO and rGO papers. The conductivities in the plane direction were about two orders of magnitude higher than those measured in the thickness direction. The anisotropy in electrical conductivity is a wellknown phenomenon for carbon materials or polymer composites with an aligned or layered structure. In GO or rGO papers, it can be ascribed to the oxygenated functional groups present on the basal plane; and in aligned graphene/polymer composites,37,38 it is attributed to the insulating polymer layers intercalated between the graphene sheets, which effectively interfere with the conduction of electrons. The interlayer paths provided by the Mg2+ and Clions also enhanced the electron transport in the through-thickness direction, particularly in the GO papers.39 Non-linear current–voltage curves were observed during the measurement, particularly for the papers with high Mg contents (inset of Figure 6b), also suggesting extra ionic conductance in the modified GO papers.40 The temperature-dependent electrical conductivities of GO and rGO papers were measured to understand the conducting mechanisms for different degrees of doping, as shown in Figures 6c and 6d. The temperature dependence was totally different between the


17


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GO and rGO papers. The conductivities of GO papers initially surged to maxima at about 30

°C, the higher the dopant concentration the higher the peak, which were followed by sharp drops with further increase in temperature. In contrast, the rGO papers showed an almost linear increase in conductivity with temperature. The ion mobility and polarization are found to be closely associated with the conductivity of GO papers.41 The rapid increase in conductivity from 25 to 32oC, the enhancement more significant with a higher dopant content, was caused by the thermally enhanced ion mobility because the diffusion coefficients of Mg2+ and Cl- ions rise with temperature.40 However, electrode polarization took place with accumulated charge carriers to form charge double layers at the electrodes, which in turn shielded the papers from the applied electric fields.42 Meanwhile, water molecules intercalated in GO papers were partially removed as the temperature further increased, leading to the reduction in ion mobility. This may explain the gradual decay in conductivity after 32oC. On the contrary, the rGO and modified rGO papers presented a semiconductor-like temperature response, different from the conductivities of GO papers with respect to dopant concentration. Here, the conductivity is dominated by the tunneling transport among the conductive rGO sheets, in which thermally-induced fluctuation voltages across the insulating gap influence the tunneling probability and thereby determine the temperature dependence of electrical conductivity.43 A higher temperature generates more thermally activated carriers, giving rise to higher conductivities for both rGO and modified rGO papers. The slopes of the lines in Figure 6d correspond to the activation energy, indicating that the activation energy of rGO papers was reduced by the doping effect.44 The unmodified rGO papers had the highest activation energy, and hence displaying the strongest temperature-dependence of conductivity.45


18

Page 19 of 36

2.5

0.08

2.0

0.06

1.5

0.04

1.0

0.02

0.5

0.00

0.0

3

50 40

3000

1.0

2800 2600

30

0.8 0.6

20

0.4

1.0 1.5 1.75 2.0

GO 0.25 0.5

0.2 0.0 0.00

0.02

0.04

0.06

0.08

Potential (V)

2400

0.0

10 0

0.5 1.0 1.5 2.0 Dopant concentration (mM)

2.0

(c)

(d)

Before reduction

1.8

GO 0.25 mM 1.0 mM 1.5 mM 2.0 mM

2 o

σ/σ25 C

60 3200

0.5 1.0 1.5 2.0 Dopant concentration (mM)

o

0.0

70

(b) After reduction

Normalized current

3.0

0.10

3400

σ/σ25 C

In-plane conductivity (S/m)

(a) Before reduction

Through-thickness (S/m)

0.12

In-plane conductivity (S/m)

-3

x10 3.5

0.14

Through-thickness (S/m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

1.6 1.4

After reduction rGO 0.25 mM 1.0 mM 1.5 mM 2.0 mM

1.2 0 20

30

40 50 60 o Temperature ( C)

70

80

1.0 20

30

40 50 60 70 o Temperature ( C)

80

90

Figure 6. Electrical conductivities of GO papers as a function of dopant concentration: (a) before and (b) after reduction, with current–voltage curves of rGO papers in inset of (b); temperature-dependent in-plane electrical conductivities of GO and modified GO papers (c) before and (d) after reduction. Dielectric properties When placed in an electric field, a dielectric material can be polarized to create an internal electric field that reduces the overall field within the material itself. The relative dielectric constant, εr(ω), is defined as:

() = ( )/

(4)

where ε(ω) is the absolute permittivity of the material, and ε0 = 8.854×10−12 F/m is the permittivity of vacuum.46 εr(ω) is a complex value, its real and imaginary parts are denoted as:

() = () + ()

(5)


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Page 20 of 36

where () is the energy storing capacity of the material and () is the energy loss by heat. The loss factor, tan δ, refers to:

δ = ()/ ()

(6)

There are four mechanisms of polarization: namely, (i) electronic and ionic polarization due to the small displacement of electrons or ions, (ii) dipolar polarization due to the orientation of molecular dipoles and (iii) the Maxwell-Wagner (MW) or interface polarization due to the charges accumulated at the interface for inhomogeneous materials.11 The polarization does not occur instantaneously, and the associated time constant is called the relaxation time, τ, which defines the time required for a displaced system aligned in an electric field to return to its random equilibrium state; and tan δ reaches the highest value upon relaxation. The electronic and ionic polarization typically occurs at a microwave frequency with τ > 1012 Hz; while τ of dipolar polarization is proportional to the cube of the radius of the molecules, usually in the rage of 106~1010 Hz. The MW polarization is dispersed at a radio frequency with τ = 103~106 Hz.47 The relaxation time τ of the MW polarization can be written as:46

= (! " + "! )/(! " + "! )

(7)

where σ1 and σ2 are the electrical conductivities, and V1 and V2 are the volume fractions of two different materials, 1 and 2. The dielectric constants and tan δ of the modified rGO papers with different dopant concentrations are shown in Figure 7. Both dielectric constant and tan δ were frequency dependent. The frequency corresponding to the peak, fp =1/(2πτ), of tan δ of rGO papers (Figure 7b) was ∼4×107 Hz. Equation (7) was used to estimate the MW relaxation time, τMW, of rGO papers which were considered consisting of two materials, rGO sheets intercalated with air. In view of both the negligible electrical conductivity, σ2 ≈ 0 S/m, and permittivity,

ε2 ≈ 0, of air, Equation (7) is simplified to:48


20

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


= /

(8)

Equation (8) confirms that there is a linear relationship between the dielectric constant and the frequency of rGO papers at frequencies lower than about 105 Hz (Figure 7a), thus the dielectric constant of rGO paper at 1Hz was estimated to be ε1 =1.53×106. The electrical conductivity of rGO paper was measured to be 2834 S/m (Figure 6b). By substituting these values and ε0 into Equation (8), the MW relaxation time, τMW = 4.8 ns, and tan δ = 3.32×107 Hz were obtained. This estimate is in excellent agreement with the experimental result of 4×107 Hz, a testament to strong presence of MW polarization in rGO papers, signifying that any pair of adjacent conductive rGO sheets separated by air gap can serve as a nanoscale capacitor. This means that a well-aligned rGO paper is comprised of a network millions of nanocapacitors that can hold an enormous capacity to store electric charges, enabling the rGO papers to possess an extremely high dielectric permittivity similar to aligned graphene/polymer nanocomposites.40 Figures 7a and 7c show a consistent increase in dielectric constant with increasing dopant concentration due to the enhanced dipolar polarization by the incorporation of Mg2+ and Clions. Dipolar polarization is caused by the orientation of the dipoles, and the polarizability can be quantified by dipole moment, µ. However, the dipolar polarization was not significant in rGO papers because of the lack of functional groups: although the C-C bond is non-polar, the C-O bond has a low µ value of 0.86 debyes. After the modification, species like Mg-O, C-Cl and Cl- introduced much higher permanent dipole moments, which are 5.3, 1.56 and 8.69 debyes, respectively. Because polarization involves a small displacement by the charges that carry a dipole moment when an electric field is applied, dielectric loss is inevitably produced from the energy dissipated during displacement. In the MW polarization, charges are separated by a considerable distance and thus the corresponding dielectric loss is a few orders of magnitude higher than the other types of polarization,


21


including dipolar polarization.49 Therefore, the loss factor decreased with increasing dopant concentration due to the enhanced dipolar polarization (Figures 7b and 7e). The large oscillation of loss factor of the neat rGO papers at frequencies lower than 104 Hz was due to the charge build-up in the rGO sheets based on the MW principle. Owing to the introduction of dipolar polarization after the modification, the disparity between the measured relaxation time, τ, and the estimated MW polarization relaxation time, τMW, became larger, particularly at high dopant concentrations, as seen from Figure 7f. It is also worth noting that the loss factor peaks corresponding to relaxation time downshifted with increasing dopant concentration at both the high and low frequency regimes (Figures 7b and 7d). For the MW polarization, the charges are blocked at the rGO sheet/air interface and are separated by the interlayer gaps in rGO papers. Therefore, the relaxation time can be defined as the time required for charges to move over a distance; and the larger the separation distance, the longer the time required.50 Consequently, the down-shifting of loss factor peaks is a reflection of increasing interlayer distance between rGO sheets with increasing dopant concentration, in agreement with the results shown in Figure 4b. 7

(a)

rGO 0.25 mM 0.5 mM 1.0 mM 1.5 mM 1.75 mM 2.0 mM

6

10

5

10

4

10

4

10

(b)


3

10 Loss tan δ

10 Dielectric constant

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 36

3

10

2

10

1

10

0

2

10

10

1

-1

10

10 2

10

3

10

4

5

10 10 Frequency (Hz)

6

10

7

10

2

10

3

10

4

5

6

10 10 10 Frequency (Hz)

7

10


22

Page 23 of 36

7 6

(c)


Dielectric constant

10

5

10

4

10

3

10

(d)

3

10

2

10

1

Loss tan δ

10

2

10

10

0

10


-1

10

-2

10

1

10

-3

10 6

10

7

10

8

9

10

10

6

7

10

5

10

Dielectric constant, k Loss tanδ

9

10

10

Frequency

(f) 100

10 4

10

1 3

Relaxation time (ns)

Dielectric constant

(e)

8

10

Frequency (Hz)

Loss tanδ

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Experimental τ, ns Calculated MW τ , ns

150 120 90 60 30

10

rGO 0.25 0.5 1.0 1.5 1.75 2.0 Dopant concentration (mM)

0.1

0 GO 0.25 0.5

1.0

1.5

1.75 2.0

Dopant concentration (mM)

Figure 7. (a) (c) Dielectric constants and (b) (d) loss tan δ of modified rGO papers plotted as a function of frequency; (e) dielectric constant and tan δ at 1 kHz, and (f) comparison between experimental relaxation time, τ, and MW relaxation time, τMW, predicted by Equation (8) for neat and modified rGO papers. The dielectric constants and losses are compared with the reported values in Table 2 at both 1kHz and 2 GHz, representing low and high frequency regimes, respectively. Metal oxides are standard dielectric materials with low dielectric losses due to their high purity and single crystal structures: e.g. MgO has a k value of 9 and a tan δ of 0.0004.51 These low loss materials are useful for applications in radio-frequency and microwaves. The dielectric constant of polymer composites is largely dependent on MW polarization and usually increased with increasing filler content for both graphene and CNTs.52,53,58,60 Owing to the MW polarization, the interfacial area and filler alignment are the predominant parameters for high-k values in CNT/ or graphene/polymer composites. Graphene/PVDF composites


23


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Page 24 of 36

with a porous structure presented a higher k than those containing dispersed graphene sheets.54 The performance GO/PVDF composites was further improved by chlorine doping 32

or CuS modification.59 The current modified rGO papers consisting of mostly large-size

graphene sheets aligned in a layered structure8 demonstrated exceptionally high-k compared to other structures, like aggregated MWCNT films,55 graphene/GO/graphene (G/GO/G) sandwich films,56 GO papers41 and few layer graphene films.57 Metal oxide particles, such as Co3O4, Fe3O4 and NiFe2O4 61-64, were also incorporated to enhance the dielectric constants of graphene composites, but often at the expense of higher loss factors. On the contrary, the intercalation of rGO papers with MgCl2 gave rise to both enhanced dielectric constants and reduced losses. The two important dielectric parameters can be tailored by optimizing the dopant concentration depending on the functional requirements of specific end conditions.

Table 2. Comparison of dielectric constants and losses of graphene or GO papers with various intercalated materials. Materials

At 1 kHz Dielectric Dielectric constant loss

At 2 GHz Dielectric Dielectric constant loss

Ref

MgO

9

0.0004

9

0.0004

51

Al2O3

4

0.0006

4

0.0006

51

ZrO2

22

0.05

22

0.05

51

MWCNT films

40000

0.75

55

G/GO/G

8000

0.9

56

Transparent graphene films

52

0.03

57

GO paper

40

100

41

Chlorine-doped GO/PVDF

28 - 7500

0.003-2.88

32

GO/PVDF

30 - 6000

0.08-1000

52

MWCNT/PVDF

12 - 1100

0.015-1.8

53


24

Page 25 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Porous graphene sandwich/PVDF

350-4500

54

0.52-2.83

Graphite/PVDF

17

0.5

58

RGO/CuS/PVDF

38

0.43

59

RGO/poly(ethylene oxide)

12.6

0.34

60

Porous Fe3O4- graphene

8.2

0.23

61

3D RGO/Fe3O4

78.1

1.2

62

RGO/Co3O4

161

1.5

63

RGO/ NiFe2O4 nanorod

9.8

0.44

64

3 - 70

0.01-0.16

MgCl2 modified GO paper

2470- 64770

2.4-13.9

Current study

CONCLUSION In summary, this paper reports the functionalization of GO papers using multi-functional dopant, MgCl2 and the characterization of mechanical, electrical and dielectric properties of the modified papers. They are light and flexible; and are proven to possess excellent mechanical properties, unique anisotropic electrical conductivities and extraordinary dielectric performance, which can serve as potential or precursor materials for many applications like wave absorbing materials. The following can be highlighted from the experimental study. (i)

The Mg-O ionic bonds acted as both interlayer and edge crosslinks that significantly enhanced the load transfer between adjacent GO sheets, resulting in the simultaneous improvements in tensile properties and fracture toughness of the modified GO papers. The optimal Mg2+ contents for the maximum tensile and fracture properties compared to unmodified papers were 0.95 at% and 0.48 at%, respectively.


25


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(ii)

Page 26 of 36

The Cl- ions introduced the p-doping effect, giving rise to significantly increased charge concentration and electrical conductivity of the modified GO papers. Both the GO and rGO papers exhibited significant temperature-dependent electrical conductivities which were also affected by dopant content.

(iii)

The modification of rGO papers by MgCl2 much enhanced the dielectric constant while reducing the dielectric loss through dipolar polarization induced by Mg-O, C-Cl and Cl- molecules. The highly aligned GO sheets constituted a network of numerous nanocapacitors in the paper which led to charge accumulation based on the MW polarization, giving rise to an extraordinary high-k over 60,000 with a dielectric loss of 3 at 1 kHz by doping rGO with 2mM MgCl2.

AUTHOR INFORMATION Corresponding Author * Jang-Kyo Kim, Tel: +852 2358 7207. Fax: +852 2358 1543. E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This project was financially supported by the Research Grants Council of Hong Kong SAR (GRF projects: 613811 and 16203415) and the Innovation and Technology Fund of Hong Kong SAR (ITS/141/12). XL and XS were the recipients of the SENG and Hong Kong PhD Fellowship Awards, respectively. Technical assistance from the Materials Characterization and Preparation Facilities (MCPF) and the Advanced Engineering Materials Facilities (AEMF) of HKUST is appreciated.


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Table of Content

Graphene oxide papers are doped with varied concentrations of MgCl2. The doped papers present enhanced dielectric performance.


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Graphene Oxide Papers Simultaneously Doped with Mg2+ and Clâ for

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