Graphene Preparation by Phenylmagnesium Bromide and Its

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Graphene preparation by phenylmagnesium bromide and its excellent electrical conductivity performance in graphene/PPS composites Chenyang Li, Zhenhuan Li, Lei Cao, and Bowen Cheng Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b01706 • Publication Date (Web): 26 Sep 2016 Downloaded from http://pubs.acs.org on September 27, 2016

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Industrial & Engineering Chemistry Research

Graphene preparation by phenylmagnesium bromide and its excellent electrical conductivity performance in graphene/PPS composites Chenyang Li, Zhenhuan Li*, Lei Cao, Bowen Cheng* 1 State Key Laboratory of Separation Membranes and Membrane Processes, College of Textiles, Tianjin Polytechnic University, Tianjin, China.

ABSTRACT: An effective approach was reported towards the reduction of weakly exfoliated graphite oxide (WEGO) using Grignard reagent of PhMgBr as both a reducing agent and a chemical functionalizer. GS reduction with 4 mass equiv. of PhMgBr showed the excellent electrical conductivity of 2.5×104 S/m because π-π conjugated system among GS increased the electrons transfer. 2.5 wt % GS with 4 equiv. of PhMgBr reduction was used as a filler to modify poly (p-phenylene sulfide) (PPS), and the prepared GS/PPS composites displayed higher electrical conductivity (10.5 S/m) and better mechanical properties (tensile strength 88.9 Mpa and Young’s modulus 740 MPa) without much loss in the elongation at break. The reinforcing effects and high electrical conductivity could be explained by both the directional homogeneous dispersion of GS within PPS matrix and the π-π conjugated interaction arising from the similar structure characteristics between GS and PPS matrix. Keywords：：Polymer-matrix composites; Electrical properties; Raman spectroscopy; X-ray diffraction; Photoelectron spectroscopy

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INTRODUCTION Graphene has attracted a great deal of attention in recent years because of its unusual electronic properties such as quantum Hall effect1, two-dimensional monolayer lattice,2 and its diverse promising potential applications in many technological fields such as sensors,3,

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catalyst,5 super capacitors,6 aerogels7,

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and nanocomposites.9

However, the conventional graphene has a drawback of high sheet resistance which block its extensive application in some fields. Up to now, several typical methods have been developed for producing this 2D carbon material, such as epitaxial growth,10 chemical vapor deposition (CVD),11 electric arc discharge12 and chemical exfoliation of graphite.13-15 Among these methods, chemical exfoliation is recognized as the most promising and suitable method for graphene sheet (GS) preparation on industrial scale at low cost.16 However, the GS obtained by this method usually suffers from poor quality because the large graphene sheet is difficult to be obtained under strong oxidative stripping condition (the well-known Hummers method), and much more oxygen-containing functional groups and some structure defects also introduced due to the strong oxidation condition. As a result, the reduction is required to remove the oxygen-containing functional groups of graphene oxide (GO). However, GS is easily aggregated due to the strong π-π stack among GS during the reduction process, and the large GO layers are inevitably damaged into small pieces unless capping reagents (polymers or other molecules) are used. For example, polymer-coated “graphitic nanoplatelets” can be obtained by reducing GO,17 but the presence of capping reagents can influence GS properties. Different reducing agents,

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such as hydrazine,15 dimethylhydrazine,18 hydroquinone19 and sodium borohydride,20 have widely been used in the chemical reduction of GO to GS. Unfortunately, the chemical reduction of GO to GS often leave some oxygen atoms remaining on GS surface, and some other heteroatoms can also be introduced during chemical reduction process, which increased sheet resistance by scattering electrons.21 Recently, Zhang et al. reported that GO sheets could be promptly reduced under a mild condition using L-ascorbic acid,22 but the time-consuming (48h) constrained its practical application. Therefore, the large GS formation and much less oxygen-containing functional group (or structure defect) introduction were the prerequisite for the preparation of high quality GS. WEGO possesses the large slice and much less oxygen-containing functional groups, but it needed to be re-peeled, and the sheet aggregation must be prevented during the reduction process. Grignard reagent (RMgX, where R = alkyl, vinyl or aryl and X = Cl or Br) is a kind of strong nucleophile or redox reagent,23 therefore, it exhibits a unique chemical activity in reaction with some polar oxygen-containing functional groups or reduction GO into GS. As one of the most important organic metal compounds, Grignard reagent had been reported to react with hydroxyl groups of thermally treated SiO2,24 and n-BuMgCl had been employed to reduce GO to form loosely aggregated GS.25 Thus, it can be expected that Grignard reagent can react with the oxygen-containing functional groups (hydroxyl, epoxide, carbonyl and carboxyl) of GO to produce organic functionalized graphene to prevent GS aggregation.

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Figure 1. The stripping WEGO to form organic functionalized graphene

In this paper, PhMgBr were firstly used to reduce and strip WEGO in THF (Figure 1). As a contrast, the effects of different Grignard reagents (2-thienylMgBr and n-buMgBr) on WEGO reduction were also studied. WEGO and GS were characterized by XRD, TEM, HRTEM, FT-IR spectra, TG, XPS, Raman and electrical conductivity measurement. It was found that Grignard reagents could well remove the oxygen functional groups of WEGO, and the large-area WEGO sheets with weak exfoliation degree could take place secondary exfoliation. Simultaneously, GS surface could be covalently grafted with organic groups which restrained GS aggregation during the reduction process. Especially, the as-produced GS with 4 equiv. of PhMgBr (P-GS-4) showed the excellent electrical conductivity of 2.5×104 S/m, however, much more organic group introduction makes more sp3-hybridized carbon existence on GS surface to result in the extensive conjugated sp2-carbon network difficult formation. In addition, the electrical conductivity and mechanical properties of P-GS-4/PPS composites were dramatically improved because of the strong π-π conjugated interaction between the homogeneously dispersed P-GS-4 and PPS matrix. 2. EXPERIMENTAL SECTION

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2.1. Preparation of WEGO. WEGO synthesis was based on a modified Hummers method.26 5.0 g of graphite, 2.5 g of NaNO3 and 125 mL of 98 % H2SO4 were mixed together at 0 °C. Then 6.0 g of KMnO4 was loaded into above mixture. The suspended solution was heated at 35 °C and stirred continuously for 2 h. After that 250 mL of DI water was added into the suspension at 98 °C for 45 min. Finally, 50 mL of H2O2 (30 %) and 200 mL of DI water were loaded into the suspension, and the resulting suspension was filtered, washed with DI water and dried in a vacuum oven at 60 °C for 24 h to obtain WEGO. 2.2. Exfoliation and Reduction of WEGO with Grignard Reagents. Briefly, Grignard reagents, 2-thienylMgBr, phMgBr, and n-buMgBr, were used to reduce WEGO in anhydrous THF under N2 atmosphere. 1, 2 and 4 equiv. of Grignard reagents were used to reduce WEGO, and the corresponding products were respectively denoted as T-GS-1, 2, 4, P-GS-1, 2, 4 and B-GS-1, 2, 4 in according to a similar procedure.24 2.3. Preparation of Composites. 20 g PPS powder (diameter≈ 0.5-2 um) and 2 g hexadecyl trimethyl ammonium bromide (CTAB) were dispersed in deionized water and stirred magnetically for 2 h at room temperature. The different amount of P-GS-4 was dispersed in deionized water and sonicated for 2 hours. Above two suspensions were mixed together and sonicated for another one hour, and the mixtures were stirred for 12 h. Finally, the mixed suspensions were filtered and washed with DI, and the obtained GS/PPS mixture was dried at 80 oC for 24 h. GS/PPS composites were prepared from the P-GS-4/PPS mixture on double-screw extruder (HAAKE MiniLab

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II Micro Compounder, Thermo Fisher Scientific, Germany). 2.4. Characterizations. TEM/HRTEM images were obtained from a JEM-2100 transmission electron microscope (JEOL, Japan). XRD data were obtained using an Elmer PHI-5600 using an Mg Kα line as a radiation source and a D8 discover. FT-IR was performed on a TENSOR 37 (BRUKER Corporation, Germany) in the wavenumber range of 4000-400 cm-1. XPS data were obtained on ThermoFisher K-alpha. TG (STA409 PC thermogravimetry, NETZSCH, Germany) measurement was carried out from 30 to 900 °C at a heating rate of 10 °C min-1 under nitrogen atmosphere. Raman spectra were performed by using a micro-Raman system (Renishaw, RW1000-In Via) with an excitation energy of 2.41 eV (l = 514.5 nm). The inner structure of P-GS-4/PPS composites were characterized by Tecnai G2 F20 Field Emission Transmission Electron Microscopy with accelerating voltage of 200 kV, resolution ratio of 0.14 nm, amplification factor of 25 ~ 1030 kX. Electrical conductivity of T-GS, P-GS, and B-GS was measured on a SZ-82 digital four-point probe system (Suzhou, China). The volume conductivity of composites were measured by a ST-2722 semiconductor powder resistivity tester (Suzhou Jingge Electronic using a standard four-probe method at room temperature and 18 MPa, China). Tensile test was carried out on CMT4202 universal testing machine (MTS Industrial Systems CO., LTD., China) at room temperature. The rate for the tensile test was 10 mm/min. The tensile strength, elongation at break and Young’s modulus were obtained directly from the stress-strain curves (the average values of five samples). The samples were molded into a dumbbell-shaped bar by injection-molding

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machine (HAAKE MiniJet Pro, Thermo Fisher Scientific, Germany). 3. RESULTS AND DISCUSSION 3.1. XRD of Prepared Samples. Figure 2a shows the XRD of natural flake graphite, WEGO and GS (From WEGO reduction by different Grignard reagents, and denoted as B-GS, T-GS and P-GS, respectively). The inter-layer distance obtained from the (002) peak of graphite was 0.34 nm (2θ = 26.40°), which was expanded to 0.77 nm (As for WEGO, 2θ = 11.38°) after oxidation, indicating the formation of oxygen-containing groups such as hydroxyl, epoxy and carboxyl groups.13,

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Meanwhile, a weak (002) diffraction peak of graphite in WEGO XRD pattern is still observed at approximately 26.26°, suggesting that the graphite was not fully oxidized or not well exfoliated. After WEGO reduction by 4 equiv. of n-buMgBr, 2-thienylMgBr and PhMgBr, XRD of B-GS-4, T-GS-4 and P-GS-4 showed early no (001) diffraction peak at 11.38°, but a new broad peak appeared around 23.78° with a d-spacing of ~0.37 nm, indicating the elimination of some oxygen-containing functional groups by Grignard reagent. In addition, the obtained B-GS-4, T-GS-4 and P-GS-4 show the very weaker (002) peak, which illustrates that there exists the disorder planar sheet agglomeration by π-π conjugated interactions.28 The (002) peak intensity of B-GS-4, T-GS-4 and P-GS-4 is considerably weaker than that of WEGO, implying an efficient secondary exfoliation appearance during reduction process.

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(a )

2 6 .4 P -G S -4

1 1 .3

(b )

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26 .3

11 .3

W EGO

T -G S -4

B -G S -1

B -G S -4

B -G S -2

W EGO

B -G S -4

G ra p h ite 8

10

12

(c ) 1 1.3

14 22 24 26 28 30 2 θ (d e g re e )

5

26 .3

10

(d )

15

20 25 30 2 θ (d eg re e)

11.3

35

40

26.3 WEGO

W EGO P -G S -1

T -G S -1

P -G S -2

T -G S -2 P-G S-4

T-G S -4 5

10

15

20 25 30 2 θ (d eg re e)

35

40 5

10

15

20 25 30 2 θ (d eg re e)

35

40

Figure 2. XRD patterns: (a) pristine graphite, WEGO and GS from WEGO reduction with 4 equiv. of different Grignard reagents. (b) WEGO and GS from WEGO reduction with 1, 2 and 4 equiv. of n-BuMgBr. (c) WEGO and GS from WEGO treated with 1, 2 and 4 equiv. of 2-ThienylMgBr. (d) WEGO and GS from WEGO treated with 1, 2 and 4 equiv. of PhMgBr.

As shown in Figure 2b, c and d, WEGO was reduced in presence of 1, 2 and 4 equiv. of n-buMgBr, 2-thienylMgBr and PhMgBr. When the amount of Grignard reagent increases to 4 equiv., both (002) and (001) diffraction peaks are difficult to be detected. As Grignard reagent usage amount increases, the (001) and (002) peak intensity of B-GS, T-GS and P-GS decreases, indicating those three kinds of Grignard reagents could remove the abundant oxygen-containing groups on the surface of WEGO and made WEGO to take place further stripping, in which the π conjugated structure is well restored. 3.2. The Morphology of WEGO and Three Kinds of GS. 8


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Figure 3. TEM image: (a) WEGO. (c) B-GS. (e) T-GS and (g) P-GS. HRTEM image: (b) WEGO. (d) B-GS. (f) T-GS and (h) P-GS. The inset in (d, f, h) is the SAED of B-GS, T-GS and P-GS, respectively.

The morphology of WEGO and three kinds of GS by different Grignard reagent reduction (at 4 equiv.) were further characterized by TEM and high-resolution TEM (HRTEM) (Figure 3). Figure 3a and 3b display that WEGO nanosheets are stacked together, confirming the graphite is not fully oxidized or not well exfoliated, which is consistent with the results of XRD characterization. After WEGO reduction by 4 equiv. of Grignard reagents, the intrinsic structure features of B-GS-4, T-GS-4 and P-GS-4 (Figure 3c, e, g) are clearly observed, such as large, folded and transparent nanosheets with some corrugations and scrollings on the edge. The layered structure of individual B-GS-4, T-GS-4 and P-GS-4 can be well distinguished from the 9



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HRTEM images which are shown in Figure 3d, 3f and 3h. B-GS-4, T-GS-4 and P-GS-4 consisted of 5, 12 and 4 graphene layers, respectively. Furthermore, the selected area electron diffraction (SAED) patterns of B-GS-4, T-GS-4 and P-GS-4 are listed in the inset in Figure 3d, 3f and 3h, suggesting that B-GS-4, T-GS-4 and P-GS-4 have a high degree of crystallinity. Those HRTEM characterizations confirmed that WEGO is not fully exfoliated and it can be further exfoliated during Grignard reagent reduction process, especially in presence of PhMgBr. 3.3. FT-IR and TG Characterization of GS Reduced by 4 Equiv. of Grignard Reagents. FT-IR was used to characterize GS which was reduced by 4 equiv. of Grignard reagents. The results are shown in Figure 4a. The FT-IR spectrum of WEGO shows a strong absorption band at 1720 cm-1 due to the C=O stretching. The spectrum of WEGO also exhibits the intensive band C=C at 1580 cm-1 , C-O (epoxy) at 1200 cm-1 and C-O (alkoxy) at 1060 cm-1, which indicates the extended π conjugated structure of graphite had been significantly destroyed during oxidization.1, 29, 30 After WEGO reduction with PhMgBr at 4 equiv., the characteristic absorption bands of C=O and C-O in P-GS-4 almost disappeared. But a new intensive absorption band at 1642 cm-1 appeared, indicating the deoxygenation took place and the extensive conjugated sp2-carbon networks have been rebuilt by removing oxygen-containing functional groups.31 However, when WEGO is reduced by 4 equiv. of n-buMgBr and 2-thienylMgBr, both a weak absorption band at 1642 cm-1 and an intensive broad C-O band around 1200 cm-1 appears in B-GS-4 and T-GS-4, indicating that the extensive conjugated sp2-carbon networks have not been well rebuilt and the C-O

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oxygen-containing functional groups have not been completely eliminated. In addition, the intensive peak at 1580 cm-1 is observed in B-GS-4 and T-GS-4, which might be attributed to the stretching vibration of small-area aromatic C=C or π-conjugated structure of alkenes. 1580 1642 Small-area p-conjugated structure or aliphatic olefins 1720 WEGO 1200 1056

(a)

B-GS-4

100

T-GS-4

(b)

90 80 TG/%

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


70 60 WEGO T-GS-4 40 B-GS-4 P-GS-4 30 100 200 300 400 500 600 700 800 Tempreture/℃

P-GS-4

50

C-O(expoxy) Large-area p-conjugated C-O(alkoxy) structure

2000 1800 1600 1400 1200 1000 800 Wavenumber/cm-1

Figure 4. FT-IR and TG data: (a) FT-IR spectra of WEGO, B-GS-4, T-GS-4 and P-GS-4. (b) TG curves of WEGO, B-GS-4, T-GS-4 and P-GS-4. In order to investigate the oxygen-containing functional group elimination in presence of different Grignard reagents, TG was used to characterize the thermal stability of the as-prepared GS and WEGO. As shown in Figure 4b, WEGO exhibits about 15 wt % loss at 120 °C and more than 23 wt % loss at 200 °C because of the release of CO, CO2 and H2O from the most labile oxygen functional groups.32, 33, 34 The residual weight is around 39 % at 800 °C. However, B-GS-4, T-GS-4 and P-GS-4 exhibits the improved thermal stability. The reason for those could be that the most of oxygen-containing groups of WEGO are well removed during the reduction of WEGO with Grignard reagents. Most importantly, P-GS-4 exhibited the highest thermal stability than B-GS-4 or T-GS-4, which further proofs that the oxygen-containing functional groups can be well removed and the extensive 11



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conjugated sp2-carbon networks can be rebuilt to the utmost extent in presence of PhMgBr. 3.4. XPS and Raman Characterization of Materials. To further characterize the formation of GS in presence of Grignard reagents, XPS was performed to study the removal of the oxygen-containing functional groups. The high-resolution C1s XPS spectrum of WEGO (Figure 5a) shows a sharp peak at 285.3 eV that corresponds to C=C/C-C bonds of carbon atoms in a conjugated honey-comb lattice. Peaks at 286.6, 287.2 and 288.1 eV could be attributed to the different C-O bonding configurations owing to the oxidation and destruction of the sp2 atomic structure of graphite.21 After WEGO reduction with Grignard reagents (Figure 5b, 5c and 5d), the intensities of the related oxygen peaks significantly decreases in B-GS-4, T-GS-4 and P-GS-4, and the intensity of C=C/C-C increased dramatically, suggesting that the delocalized π conjugation is restored in B-GS-4, T-GS-4 and P-GS-4.21 Figure 5e shows the wide-scan XPS spectra of WEGO and GS. The C/O ratios of B-GS-4, T-GS-4, and P-GS-4 are 14.8, 8.9 and 25.6, respectively, and the values of B-GS-4 and P-GS-4 are much higher than the C/O ratios of samples reduced using hydrazine monohydrate (C/O of 10.3),

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L-Ascorbic acid (C/O of 5.70),22 natural cellulose (C/O of 5.47),35

gallic acid (C/O of 5.28)

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and tea solution (C/O of 3.10) 37. Those results indicates

that the reduction of WEGO with Grignard reagents is highly effective, which indicated Grignard reagents (especially PhMgBr) can well remove the abundant oxygen-containing groups on the surface of WEGO and the π-conjugated structure is full restored. In addition, the appearance of S2p at 164.3 eV in T-GS-4 confirms that

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thienyl groups have been introduced onto the surface of graphene. (b) B-GS-4

(a) WEGO C=O 287.5 eV O-C=O 288.1 eV

292

290

C=C 284.8 eV

C=C/C-C 285.3 eV

C-O 286.6 eV

288 286 284 282 Binding Energy (eV)

O-C=O 288.8 eV

292

280

290

C-O 285.7 eV


(d) P-GS-4

(c) T-GS-4

C=C 284.6 eV

C=C 284.8 eV C-O 285.4 eV

C-OH 285.7 eV

C=O O-C=O 286.6 eV 288.3 eV

292

290

O-C=O 288.5 eV


280292

G

D

(f)

280

1345

290

288 286 284 282 Binding Energy (eV) C1s

(e) 1590

O1s

P-GS-4

P-GS-4 T-GS-4

T-GS-4

280

S2p

1571

B-GS-4 WEGO

B-GS-4 1358

1100 1200 1300 1400

WEGO 1500 1600 1700 1800

Raman shift (cm-1)

500

400

300

200

100

Binding Energy (eV)

Figure 5. XPS and Raman: (a) High-resolution C1s spectra of WEGO. (b) High-resolution C1s spectra of B-GS-4. (c) High-resolution C1s spectra of T-GS-4. (d) High-resolution C1s spectra of P-GS-4. (e) Wide-scan XPS spectra of WEGO, B-GS-4, T-GS-4 and P-GS-4. (f) Raman spectra of WEGO, B-GS-4, T-GS-4 and P-GS-4.

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Raman spectroscopy is non-destructive and the most direct technique to characterize the structure of carbonaceous materials.38,

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Figure 5f shows the Raman spectra of

WEGO, B-GS-4, T-GS-4 and P-GS-4. WEGO displays two remarkable peaks at 1358 and 1590 cm-1, corresponding to the well-defined D and G band. D band is related to the structural imperfections and disordered structures of sp2 domains.32 However, G band is assigned to the E2g phonon of sp2 C atoms, and G domains can be used to explain the degree of graphitization.40 After WEGO reduction with Grignard reagents, the Raman spectrum of as-prepared GS exhibits the presence of both D and G bands, but G band produced a shift from 1590 cm-1 for WEGO to 1571 cm-1 for GS, indicating that the electronic conjugation in GS is restored.41 D band is attributed to the defect of the sample and the size of the in-plane sp2 domain.32, 42-44 As compared with the D band of WEGO (1358 cm-1), the D bands of B-GS-4, T-GS-4 and P-GS-4 are almost at the same position (1345 cm-1), The ID/IG ratio is often used to estimate the defect of the sample and the in-plane crystallite sizes in disordered carbon materials.30,

31, 33

Compare with WEGO, B-GS-4, T-GS-4 and P-GS-4 displayed a higher ID/IG ratio, illustrating that the defect or disordered carbon in-plane crystallite sheet has been created during the WEGO reduction by Grignard reagent. In addition, the ID/IG ratio of WEGO is slightly lower than that of GS, which is also possibly related to the ordered stacking of WEGO layers by the numerous hydrogen bonds.45 3.5. The Conductivity of Prepared GS. The conductivity of graphene mainly depends on the conjugated networks of the carbonic lattice.46 In consideration of GO, oxidation can break the conjugated structure of localizes π-electrons. The conductivity

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of GO is blocked by the absence of percolating pathways among sp2 carbon clusters to allow classical carrier transport to occur.47 As a result, GO are typically insulating.48 Electrical conductivity of the reduced GO is another important criterion to evaluate how many π-conjugated structure has been restored. To estimate the electrical conductivity of as-prepared GS, a four-point probe system was used for the measurement of electrical conductivity.49 To get a reliable electrical conductance data, the probes were carefully placed in five different sites of each sample to measure. 100000 10000

Conductivity σ ( 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


1000 100 10

T-GS P-GS B-GS

1 0.1 GO

1 equiv.

2 equiv.

4 equiv.

Content of Grignard reagents

Figure 6. Electrical conductivity of GO and GS reduced by different equiv. of Grignard reagents The electrical conductivity of WEGO and as-prepared GS is shown in Figure 6. In comparison with WEGO, the electrical conductivity of GS is enhanced with the mass ratio of Grignard reagent to WEGO increasing, and the electrical conductivity of GS reduced by 4 equiv. of n-buMgBr, 2-thienylMgBr and PhMgBr is 3.42×103, 1.13×103 and 2.5×104 S/m, respectively. As for Grignard reagents, the positive charges are distributed on Mg while negative charges are on C atom. On one hand, Grignard reagent is a strong nucleophile and has highly chemical activity. The nucleophilic

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carbon atom in Grignard reagent can attack on the electrophilic carbon atom of WEGO. On the other hand, WEGO can be well reduced by Grignard reagents to afford the sp2 bonds and restore the extensive π-conjugated structure. Table 1 presented the electrical conductivity and C/O ratio of as-prepared GS, and the electrical conductivity and C/O ratio of reported graphenes (prepared by several of classical reduction methods) are also listed for comparison.14,

21, 30

The C/O ratio of WEGO reduced by PhMgBr

Grignard reagents are higher than that of GO reduced by other reducers. Table 1. The Electrical Conductivities of GS Prepared by Different Reduction Methods Conductivity Reducing agents

Conditions

C/O (atomic ratio) (S/m)

n-BuMgBr

70°C/24 h

14.8

3.42×103

2-ThienylMgBr

70°C/24 h

8.9

1.13×103

PhMgBr

70°C/24 h

25.6

2.5×104

Hydrazine14

100°C/24 h

10.3

2.0×102

Gallic acid30

95°C/6 h

5.3

36.0

L-Ascorbic21

23°C/48 h

12.5

8.0×102

Previous studies showed that the graphene modification with aryl groups via covalent carbon-carbon bonds would increase carrier density and charge transfer, leading to improved electrical conductivity of graphene.50 When PhMgBr reacted with oxygen-containing groups of WEGO, not only the π-conjugated system had been well restored but also the phenyl groups were successfully attached to basal plane and edge sites of GS via covalent bonds. When the reduction extent was relatively low,

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the phenyl groups linking to the GS were relatively few. In this case, the covalent bonds between phenyl groups and GS perform as the defects so that the scattering effect of the carriers increases, leading to a low conductivity. However, when the reduction extent achieved a degree, much more phenyl groups were linked to basal plane and edge sites of GS. The phenyl groups bonded to the GS edge sites could be favorable of forming the π-π interaction among GS due to the free rotation of GS-Ph single bond, therefore, the electrons transfer or conductivity enhanced. In addition. GS reduced by PhMgBr had the higher electrical conductivity than GS reduced by other reduction methods. CH3CH2CH2CH2

S

R: phenyl;

2-Thienyl,

Buty; R

R R

R

R

Graphite

R

R

GS COOH RMgBr

Oxygen

O

OH O

OH

Reduction stripping

O O HO HO OH HOOC

O O O

O OH O O

O

HOOC

O HOOC O

COOH

O

O

O O

OH O

O OH

COOH

O

O

OH

weak oxidative stripping

Figure 7. Proposed stripping mechanism of WEGO with Grignard reagents. According to the results of aforementioned characterizations, the proposed stripping mechanism of WEGO with Grignard reagents was provided in Figure 7.

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Namely, the π-conjugated structure of GS was restored during Grignard reagent reduction process, and the organic groups of Grignard reagents could be anchored on the basal plane or edge sites of as-prepared GS via covalent bonds, which resulted in the stacked WEGO being further exfoliated or WEGO sheets being transformed into loosely aggregated GS. This proposed stripping mechanism was well consisted with XRD results. 3.6. The Electrical Conductivities of P-GS-4/PPS Composites. P-GS-4 was used to

prepare P-GS-4/PPS composites. PPS is

high-performance engineering

thermoplastic with outstanding thermal stability, chemical resistance, high stiffness and modulus, dimensional stability and inherent flame resistance. However, it is an insulating material (10-16 S/m), which limits its use in self-health monitoring and electro-actuation. The incorporation of graphene as conductive fillers is well known to improve PPS electrical conductivity,51 but graphene/PPS composite displays the low conductivity in the range from 3.42×10-3 to 1.17×10-2 S/m with filler loading increasing from 0.5 to 5 wt % because graphene filler is difficult to be well dispersed into PPS matrix and graphene sheets is easy to self-aggregate due to the intrinsic van der Waals force.51, 52 However, herein it was found that the PPS conductivity was greatly enhanced by P-GS-4 introduction into polymer matrix.

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18 16

Conductivity σ ( 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


stage

14

III

12 10

stage II

8 6 4

stage I

2 0 -2 0

1

2

3

4

5

P-GS-4 loading (wt%)

Figure 8. Electrical conductivity of P-GS-4/PPS composite with various P-GS-4 loadings.

Figure 8 shows the electrical conductivity of P-GS-4/PPS composites with different P-GS-4 concentrations. There are three stages for electrical conductivity of P-GS-4/PPS composites as P-GS-4 content increase. At first stage there existed a gradual increase of electrical conductivity before P-GS-4 content reached 2 wt %. When P-GS-4 content increases to 2.5 wt %, a sharp increase of the electrical conductivity is achieved, and the electrical conductivity of 2.5 wt % P-GS-4/PPS reached 10.5 S/m, almost 17 orders of magnitude higher than the electrical conductivity of pure PPS (10-16 S/m). However, a limited increase of composite electrical conductivity is observed when P-GS-4 content increases from 3 to 5 wt %. It can be speculated that P-GS-4 is well dispersed in PPS matrix owing to phenyl group grafting on the GS surface, and the conductive networks might be formed in composite at low P-GS-4 contents.

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Figure 9. TEM image of composites: (a) 1wt% P-GS-4/PPS and 2.5 wt% P-GS-4/PPS. (b) Schematic diagram of the dispersion of P-GS-4 in PPS matrix.

To prove P-GS-4 well dispersion in PPS matrix, TEM is used to characterize P-GS-4 dispersion in PPS matrix at higher magnification. As shown in Figure 9a, 1 wt % P-GS-4 is uniformly dispersed in PPS matrix and nearly no agglomeration occurred. Most importantly, P-GS-4 sheet parallel or directional arrangement is observed, as shown by the arrow. As for the composites containing 2.5 wt % of P-GS-4, P-GS-4 appears to be interconnected with each other to form networks which was shown by the selected region. Figure 9b shows the dispersion model of P-GS-4 in PPS matrix. If the low content of P-GS-4 was loaded into PPS matrix, the formation of conductive path was mainly attributed to the parallel or directional arrangement of P-GS-4 sheet. GS began to contact with each other to form the networks with GS

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content increase. However, as the GS content continues to increase, the increase of filler had little effect on composite conductivity. TEM images of composites confirmed that GS sheet parallel or directional arrangement and GS interconnected network formation played an important role on achieving excellent electrical conductivity. 3.7. The Mechanical properties of P-GS-4/PPS composites. Generally, the presence of highly dispersed reinforcing filler in polymer composite systems provides a maximized reinforcing surface area, which significantly influences the properties of composite. Herein, the mechanical properties of P-GS-4/PPS composites that contained different amounts of P-GS-4 filler are compared with that of pure PPS. As shown in Figure 10, 3 wt% P-GS-4/PPS composite achieves the maximum tensile strength of 88.9 MPa, whereas that of the pure PPS is only 47.3 MPa. When 3 wt% P-GS-4 filler is introduced into PPS matrix, the Young’s modulus of composite reaches 740 Mpa. However, it should be noted that when the filler content increased to 5 wt%, the mechanical properties was significantly decreased due to some GS aggregates. The introduction of P-GS-4 into PPS matrix decreases the elongation at break, indicating the composite stiffness increase because of the filler restriction effect. The obvious improvement of mechanical and electrical properties is attributed to the parallel or directional homogeneous dispersion of P-GS-4 and the good interaction between P-GS-4 and PPS matrix.

21



100

40

Elongation at break (%)

Tensile strength (MPa)

80 60 40 20 0 0

1

2

3

4

35 30 25 20 15 10 5 0

5

0

P-GS-4 loading (w t% )

1

2

3

4

5

P-GS-4 loading (w t%)

800

Young's modulus (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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600

400

200

0 0

1

2

3

4

5

P-GS-4 loading (wt%)

Figure 10. The mechanical properties of P-GS-4/PPS composites. 3.8. Interfacial interaction between PPS and P-GS-4. Figure 11a shows the Raman spectra for PPS, P-GS-4 and P-GS-4/PPS with 0.5, 1, 2 and 5 wt% filler, respectively. P-GS-4 display two characteristic peaks, the first one at 1345 cm-1 is assigned as D band, and the second centered at 1571 cm-1 is assigned as G band. P-GS-4 displays a strong interaction with PPS, which can be confirmed through the shift of G band and D band in Raman spectroscopy.53 Pure PPS has the same band as the G band of P-GS-4 because PPS has aromatic ring with π system. It is worthy of noting that a new intensive band around 1420 cm-1 appeared in P-GS-4/PPS composites, which indicated that there existed a strong interaction between PPS chains and P-GS-4 to result in the up-shift of D band. Moreover, the G band is also slightly increased to 1574 cm-1. The up-shift of D and G band provides a strong evidence for the interaction between P-GS-4 and PPS, presumably as a result of π–π

22


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interaction. Figure 11b illustrates the possible of interaction mechanism between P-GS-4 and PPS. PPS is a typical aromatic polymer and contains benzene ring on its backbone. Therefore the PPS molecule can be adsorbed on P-GS-4 surface by π-π interaction, which can be well used to understand the good dispersion of P-GS-4 in PPS (as observed via TEM). (a)

D 1345

G P-GS-4

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


1571

5% P-GS-4 2% P-GS-4

1574

1% P-GS-4 0.5% P-GS-4 1420

1000

1250

PPS

1500

1572

1750

Raman shift (cm -1 )

Figure 11. (a) Raman spectra of PPS and P-GS-4/PPS composites. (b) The possible of interaction mechanism between P-GS-4 and PPS.

4. CONCLUSIONS In summary, WEGO could be simultaneously reduced and functionalized with Grignard reagents. PhMgBr reacts with oxygen-containing groups of WEGO to form the large-area π-conjugated structure, and GS modification with phenyl groups via covalent carbon-carbon bonds could increased electron transfer among GS, leading to an excellent electrical conductivity of GS. P-GS-4 was used as filler in PPS matrix, and the electrical conductivity, tensile strength and Young’s modulus of the composite were substantially higher than pure PPS. The remarkably improved properties of P-GS-4/PPS composites should be attributed to the homogeneous dispersion of P-GS-4, filler parapllel or directional arrangement and strong π-π conjugated 23



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interaction between P-GS-4 and PPS matrix. In the future, the work on this area will be focus on the GS boundary resistance reduce, structural defect repair and ultra-high conductivity organic composite material preparation.

AUTHOR INFORMATION Corresponding Authors *Tel +86 022 83955358; fax +86 022 83955055; e-mail: [email protected] or [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors are grateful for the financial support of the National and Tianjin Natural Science Foundation of China (nos. 21376177 and 15JCZDJC7000). This work is also supported by China National Textile and Apparel Council (J201406) and China Petroleum Chemical Co Technology Development Project (208068, 201100 and 215038) . REFERENCES (1) Zhang,Y. B.; Tan, Y. W.; Stormer, H. L.; Kim, P. Experimental observation of the quantum hall effect and berry's phase in graphene. Nature 2005, 438, 201-204. (2) She, X. L.; Liu, T. C.; Wu, N.; Xu, X. J.; Li, J. J.; Yang, D. J.; Frost, R. Spectrum analysis of the reduction degree of two-step reduced graphene oxide (GO) and the polymer/r-GO composites. Mater. Chem. Phys. 2013, 143, 240-246. (3) Shan, C. S.; Yang, H. F.; Song, J. F.; Han, D. X.; Ivaska, A.; Niu, L. Direct

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Graphene Preparation by Phenylmagnesium Bromide and Its

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