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Green and High-Efficiency Production of Graphene by Tannic Acid-Assisted Exfoliation of Graphite in Water Shuai Zhao, Shicheng Xie, Zheng Zhao, Jilin Zhang, Lin Li, and Zhenxiang Xin ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00497 • Publication Date (Web): 08 May 2018 Downloaded from http://pubs.acs.org on May 9, 2018

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ACS Sustainable Chemistry & Engineering

Green and High-Efficiency Production of Graphene by Tannic Acid-Assisted Exfoliation of Graphite in Water

Shuai Zhao, Shicheng Xie, Zheng Zhao, Jilin Zhang, Lin Li,* Zhenxiang Xin

Key Lab of Rubber-plastics, Ministry of Education/Shandong Provincial Key Lab of Rubberplastics, School of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China

ABSTRACT: A green and high-efficiency method was developed to prepare low-cost and high-quality graphene on a large scale through direct exfoliation of graphite in aqueous media using tannic acid (TA) as the stabilizer. The influence of preparation parameters on graphene concentration (CG) and graphite exfoliation efficiency (CG/CG,i), including TA concentration (CTA), initial graphite concentration (CG,i), pH, ionic strength, sonication time and cycles, was systematically investigated. Under the optimum conditions, the highest CG can attain 1.25 mg·mL-1 with CG/CG,i equal to 2.5%, and 92% of the as-formed graphene are few-layer graphene (below 5 layers) with the electrical conductivity as high as 488 S·cm-1. Due to TA on the graphene surface acting as the dual roles of dispersant and interfacial regulator, the high-quality graphene can be uniformly dispersed and tightly integrated into polymer matrices for high-performance and multifunctional polymer nanocomposites. In a word, this * Corresponding Author. E-mail: [email protected] 1

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contribution provides a simple, green, high-efficiency, and scalable avenue for mass production and utilization of high-quality graphene. KEYWORDS: Graphene, Tannic acid, Liquid-phase exfoliation, Natural rubber, Nanocomposites INTRODUCTION Graphene has been extensively studied in high-performance and multifunctional polymerbased nanocomposites, due to its exceptional strength, electrical and thermal conductivity, etc.1 To fully exploit the potentials of graphene for composites, mass production of low-cost and high-quality graphene is an prerequisite. Currently, mechanical exfoliation, wet chemical synthesis and chemical vapor deposition (CVD) are three promising ways to massively produce graphene. Among them, the quality of graphene synthesized by CVD method is the best, but the cost is also the highest. Although a green solid-state CVD method, which converts the gases generated from waste rubber tyres or plastics to high-quality graphene, provides a trash-to-treasure way for graphene production, the cost of graphene remains high due to the inevitably high reaction temperature, expensive metal catalyst, etc.2-3 In comparison, the top-down approaches, i.e., wet chemical synthesis or mechanical exfoliation of graphene from graphite, are better alternatives for low-cost graphene. The former depends on the reactions of graphite with mixed strong oxidants, suffering from explosion risk, serious environmental pollution, and long-reaction time.4-6 Although the electrochemical oxidation method can greatly improve the safety, environmentally friendliness and productivity, the concentrated sulfuric acid is still necessary.4 Moreover, the π-orbital structure of graphene is severely damaged during graphite 2


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oxidation. Even after tedious reduction processes, a large amount of defects remain in the reduced graphene oxide (rGO), which makes its properties far worse than those of the pristine graphene.7-11 As a result, the physical properties of the obtained polymer/rGO composites are far below the anticipated potentials.9 Considering the cost and quality together, graphene mechanically exfoliated from graphite, with the advantages of low cost and high quality simultaneously, is more suitable for preparing high-performance and multifunctional nanocomposites.12-16 In this method, stabilizer-assisted exfoliation of graphite in liquid phase is very promising. A suitable stabilizer is crucial for the graphene productivity, price and quality.17 Moreover, the noncovalent functionalization of graphene by stabilizers occurs along with the preparation process of graphene, which is favorable for the uniform dispersion of graphene in composites and achieving a strong graphene/matrix interface.18 However, most of the present stabilizers are chemically synthesized and suffered from high cost, toxicity risk, environmental pollution, and damaging the performance of composites.10,16-21 As a consequence, continuous endeavors have been directed to the exploration of eco-friendly and renewable stabilizers from nature. So far, some studies have revealed that some natural compounds are promising alternatives for synthetic stabilizers.22-29 However, most of natural stabilizers are low-efficiency for graphene exfoliation except pullulan and chitosan and useless for the interface molecular design of composites.22-29 Therefore, the exploration of green, high-efficiency, low-cost, large-reserve

stabilizers

from

nature

for

liquid-phase

exfoliation

and

beneficial

functionalization of graphene is still a research hotspot. Tannic acid (TA) as one of the most abundant natural compounds, has been widely used in 3



industry.30 Compared with other natural stabilizers, TA has a better affinity with graphene because of the large number of phenyl groups in its structure (as shown in Figure S1).31 Besides, the phenolic hydroxyl groups in TA have chemical reactivity and physical adhesion characteristics, making TA a good interface regulator for polymer/graphene nanocomposites. More importantly, TA can allow water as solvent to produce graphene with the lower production cost and environmental pollution. Indeed, TA has been used to reduce and stabilize GO dispersions in place of toxic reducing agents to alleviate environmental issues, but this type of work can’t avoid the disadvantages of the graphite oxidation.32-35 The direct liquid-phase exfoliation of graphite into graphene using TA as stabilizer has not been explored. Inspired by the above facts, we propose for the first time to use TA as the stabilizer for aqueous exfoliation of graphite. Graphene concentration (CG) and graphite exfoliation efficiency (CG/CG,i), were systematically investigated as a function of TA concentration (CTA), initial graphite concentration (CG,i), pH, ionic strength, sonication time and cycles. The graphene stabilization mechanism was also discussed in detail. At last, the as-fabricated graphene/TA hybrids (G-TA) were filled into natural rubber (NR) to evaluate their potentials in composites. EXPERIMENTAL SECTION Materials. NR latex (solid content: 60 wt%) was provided by Qingdao Double Butterfly Group Co., Ltd. (China). Flake graphite (product number: LG 100-94-99, particle size: 100 mesh) was purchased from Qingdao Black Dragon Graphite Group Co., Ltd. (China). TA was analytical grade and obtained from Sinopharm Chemical Reagent Co., Ltd. (China). 4


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Distilled water was used in all the related experiments. Other reagents, including sulfur, Ncyclohexyl-2-benzothiazolesulfenamide (CZ), 2, 2-dibenzothiazoledisulfide (DM), Nisopropyl-N’-phenyl-4-phenylenediamine (4010NA), zinc oxide and stearic acid (SA), were all industrial grade and kindly supplied by Qingdao Tailian New Materials Co., Ltd. (China). Characterization. Fourier transform infrared (FT-IR) spectra were collected on a Bruker VERTEX 70 spectrometer in the transmission mode. Raman spectra were executed on a Renishaw inVia micro-Raman spectrometer equipped with 532 nm argon laser excitation. Ultraviolet-visible (UV-Vis) spectra were obtained on a TU-1901PC spectrophotometer. Fluorescence spectra were obtained on a HITACHI F-4600 spectrophotometer. X-ray photoelectron spectroscopy (XPS) was collected on an ESCALAB 250Xi spectrometer with a monochromatic Al Kα X-ray source (hv = 1486 eV). The conductivity of graphene was measured on a Model RTS-8 instrument using a four-probe method. Both the graphene size and zeta potential (ζ) were measured on a Malvern Zetasizer Nano-ZS system equipped with a 632.8 nm He-Ne laser. The size of graphite particles was measured on a Mastersizer 3000 laser diffraction particle size analyzer. The contact angles were measured with a JC2000C2 instrument (Powereach, China). The thermogravimetric analysis (TGA) was conducted on a TA-Instruments TGA Q500 in a nitrogen atmosphere with a heating rate of 10 oC·min-1. Atomic force microscopy (AFM) was performed in tapping mode using a Multimode 8 AFM microscope. Transmission electron microscopy (TEM) was carried out using a JEOL JEM2100 microscope with a 200 kV accelerating voltage. Scanning electron microscopy (SEM) was performed on a JEOL JSM-7500F microscope with a 5 kV accelerating voltage. The SEM samples were coated with gold for 60 s. Tensile properties were measured on a 5



Zwick/Roell Z005 universal testing machine at a crosshead rate of 500 mm·min-1 according to ASTM D412. The dimensions of test section for the dumbbell-shaped specimens are length 20 mm × width 4 mm × thickness 2 mm. The strain rate is 0.42 s-1. The volume resistivities and thermal conductivities of NR and its composites were measured using a PC68 highresistance meter and a TA Instruments DTC-300 analyzer, respectively. The optimum curing time of NR and its composites was determined at 150 oC by an ALPHA MDR2000 UCAN rheometer. TA-Assisted Aqueous Exfoliation of Graphene. A certain amount of flake graphite was added into a 50 mL vial containing 10 mL of aqueous TA solutions with different concentrations. Table S1 lists the experiment design for CTA and CG,i. The mixtures were sonicated for 1 h in an ice-water bath using a mild bath sonicator (Scientz, SB-5200DTDN, 300 W, 40 kHz). After centrifugation for 60 min at 3000 rpm, 2/3 of the upper dispersion was carefully extracted. Preparation of NR/Graphene Nanocomposites. A certain amount of G-TA aqueous dispersion (condensed into 5 mg·mL-1) was slowly added into NR latex under vigorous mechanical stirring. After coagulation upon the addition of sodium chloride aqueous solution (1 wt%), the compounds were cut into pieces and washed with distilled water to thoroughly remove the sodium chloride. Then, the compounds were dried in a vacuum oven at 60 oC overnight. The rubber ingredients were mixed with the dried compounds using a two-roll mill and then subjected to compression at 150 oC and 10 MPa for the optimum curing time (listed in Table S2), which is the time required for the torque to reach 90% of the maximum achievable torque of the NR/graphene nanocomposites. A basic recipe for the nanocomposites 6


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is as follows: NR 100 phr, zinc oxide 5 phr, SA 3 phr, sulfur 2.8 phr, CZ 1.4 phr, DM 0.1 phr, 4010NA 3 phr and G-TA variables. RESULTS AND DISCUSSION Influence of Preparation Parameters on Graphite Exfoliation. TA was used as the stabilizer to prepare graphene through the direct exfoliation of graphite in aqueous media. Figure 1 is the schematic diagram for the preparation process and mechanism. Under ultrasonic cavitation, there are three types of exfoliation for graphite, including the shear, fragmentation, and wedge effects.17 Once the graphene sheets are exfoliated from graphite, TA can act as a capping agent adsorbing onto the graphene surface by the π-π stacking interaction, and cover the graphene surface with plenty of phenolic groups thus to endow the colloidal graphene particles with aqueous stability.30 To obtain the highest graphene productivity, the preparation conditions, including CTA, CG,i, pH, ionic strength, sonication time and cycles, were comprehensively investigated. Because TA has no absorption above 350 nm in the UV-Vis spectrum, besides, A660 of the dilute graphene dispersions (CG ≈ 0.02 mg.mL-1) only increases ~3% within the test CTA range from 0.1 to 5 mg.mL-1 (Figure S2a), indicating TA has no obvious auxochromic effect on graphene (Note: The auxochromic effect will further decrease with the increasing CG because the average number of TA on the graphene surface reduces.), CG can be calculated from A660 of the graphene dispersions according to the Beer-Lambert law (see Supporting Information), where ε660 is determined to be 2254 mL·mg-1·m-1 by the linear fit of A660 and CG (Figure S2b). This extinction coefficient is lower than that of graphene prepared by a long sonication time in N-methyl-2pyrrolidone (6600 mL·mg-1·m-1) and comparable to that of rGO (2813 mL·mg-1·m-1), which 7



indicates that the average layers of graphene prepared in this work are relatively less, because the extinction coefficient is a characteristic material property and increases with the layers of graphene.19

Figure 1. Schematic diagram for the preparation process and mechanism of G-TA dispersion. Figure 2a, b show the dependence of CG and CG/CG,i on both CTA and CG,i, respectively. Initially, both CG and CG/CG,i increase sharply with CTA and reach the maximum values at approximately CTA = 1 mg·mL-1, and then decrease slowly at higher CTA. The initial increase in CG and CG/CG,i is due to more available TA molecules that can adsorb on the graphene surface, which is mainly caused by the decreasing surface tension of TA solutions. As shown in Figure 2c, when CTA is less than 1 mg·mL-1, both the contact angle between TA solutions and graphite and the surface tension of TA solutions decrease with the increasing CTA. The surface tension of TA solutions was calculated from the contact angles according to the Neumann “equation of state” theory (see Supporting Information).36-37 The smaller contact angle and surface tension indicate the stronger surface adhesion between TA solutions and graphite, causing more TA to absorb on graphite surface.35 However, the further increase of CTA beyond 1 mg·mL-1 increases the number of the free TA molecules in water, which causes 8


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an increase in the size of TA micelles as shown in Figure 2d, and thus results in an obvious decrease in CG and CG/CG,i. This phenomenon is most likely because the large TA micelles cannot enter the adjacent graphene sheets due to the robust depletion attraction coming from their self-generating high osmotic pressure.16 Because TA is a weak acid with two pKa at approximately 5 and 7.5,31 the pH values of TA solutions decrease with the increasing CTA. As shown in Figure S3, the pH values abruptly drop below 5 as CTA ≥ 0.1 mg·mL-1. TA is mainly present in its molecular state with little dissociation at pH < 5, while it is highly dissociated at pH > 7.5.31 For this reason, the zeta potentials of all the G-TA dispersions with CTA from 0.1 to 5 mg·mL-1 reach 0 mV (Figure S3), indicating the stabilization of the G-TA dispersions stems from the steric hindrance supplied by TA rather than the electrostatic repulsion. Although CG shows an always upward trend with the increasing CG,i, the exfoliation efficiency CG/CG,i begins to decrease as CG,i ≥ 50 mg·mL-1 (Figure 2b). This is because the effective cavitation of ultrasounds increases with CG,i, but too many graphite particles obstruct the propagation of ultrasounds and thus attenuate the average energy exerted on a single graphite particle.16 Considering a good compromise between CG and CG/CG,i, the optimum CTA and CG,i are determined to be 1 mg·mL-1 and 50 mg·mL-1, in this case, CG and CG/CG,i are 1.25 mg·mL-1 and 2.5%, respectively. The as-formed G-TA dispersion displays excellent storing stability without obvious agglomeration taking place within 1 month at room temperature (Figure S4).

9



Figure 2. Dependence of (a) CG and (b) CG/CG,i on both CTA and CG,i; (c) change of the contact angles between TA solutions and graphite, the surface tensions of TA solutions with CTA; (d) change of the average particle sizes of TA micelles with CTA. Based on the previous studies, CG increases with sonication time, but the defects of graphene also remarkably build up when sonication time is beyond 2 h.10,15,16 For this reason, sonication time was optimized below 2 h in this work to improve the graphene productivity and meanwhile avoid the excessive damage to the quality of graphene. As shown in Figure 3a, CG increases quickly when sonication time is less than 1 h. After 1 h, the increase in CG is very little, probably caused by the decrease in graphite particle size with sonication time, which may weaken the average ultrasonic cavitation transferred to a single graphite particle. Therefore, the optimum sonication time is 1 h. The dependence of CG on the pH of solutions was also investigated. As shown in Figure 3b,

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CG decreases with the increase of acidity or alkalinity of solutions and reaches a maximum at approximately pH = 7. The reducing CG at acidic condition could be attributed to the enhancement of hydrogen bonding interaction between the TA molecules on different graphene surface, while the reducing CG at alkaline condition could be attributed to the reduction in the number of TA on graphene surface induced by the deprotonation of phenolic hydroxyl groups. On the one hand, the deprotonation will increase the hydrophilicity of TA, on the other hand, it will reduce the formation of hydrogen bonds between the TA on graphene surface and the free TA in aqueous phase.31 Notably, although it can result in the reduction of CG, the deprotonation of TA will be convenient for removing TA from G-TA to get the pristine graphene. Electrolyte ions are well-known that can render colloidal particles to coagulate for the salting-out effect.16 As presented in Figure 3c, CG decreases quickly with the increasing concentration of NaCl (CNaCl) and ultimately drops to zero at CNaCl ≥ 0.1 mol·L-1. The reduction in CG can be attributed to the suppressive overlap of TA adsorption layers on graphene as the attenuated hydrogen bonding interaction, rendering the decrease in the surface free energy of G-TA and the steric hindrance between G-TA.31 The re-exfoliation of residual graphite is necessary to improve the efficiency of graphite utilization. Herein, the relationship between CG and sonication cycles was surveyed. As seen from Figure 3d, CG decreases distinctly as the increasing sonication cycles in the first four cycles and then levels off at approximate 0.1 mg·mL-1. In accordance with the dependance of CG on sonication time, the variation of CG with sonication cycles may also derive from the decrease in graphite particle sizes. In order to verify our judgement, the variation of graphite 11



particle sizes with sonication cycles was also monitored. In fact, our results show that their changes are indeed the same. Graphite particle size decreases distinctly from the original 150 μm to 1 μm for the fourth cycle and then levels off at approximate 1 μm (Figure 3d). Therefore, we can extrapolate confidently that the variations of CG with sonication cycles and time stem from the decrease in graphite particle size, which may weaken the average ultrasonic cavitation transferred to a single graphite particle.16 Taking 1 L vessel as an example, the production rate of graphene under the optimum conditions (CTA = 1 mg·mL-1, CG,i = 50 mg·mL-1, sonication for 1 h in distilled water) is calculated to be 1.25 g·L-1·h-1, which is much higher than most reported methods (10-2~10-1 g·L-1·h-1).15 For the simplicity of the method, the cost of our graphene comes mainly from that of raw materials. According to the prices and usages of graphite (~$1/kg, 50 kg) and TA (~$20/kg, 1 kg), the cost of our graphene is calculated to be ~$70/kg, which is far lower than the price of the commercial rGO (~$350/kg). In addition, these results are specially compared with those of other natural stabilizers assisting exfoliation of graphite in water, as shown in Table S3. The results show that TA as the stabilizer is much better than most of the others except for pullulan and chitosan.23 However, considering the higher intensity of tip sonication and concentrations of pullulan and chitosan solutions (as high as 50 mg·mL-1), it is difficult to determine whether pullulan and chitosan are more effective than TA. What’s more, the low chemical reactivity of pullulan and chitosan makes them inferior to TA in the interface molecular design of composites.

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Figure 3. Dependence of CG on (a) sonication time, (b) pH, (c) CNaCl and (d) sonication cycles at fixed conditions (CTA = 1 mg·mL-1, CG,i = 50 mg·mL-1); change of graphite particle size with sonication cycles also included in (d). Structure and Properties of Graphene. TEM and AFM were performed to ascertain the state of the exfoliated graphene. From the TEM images of G-TA (Figure 4a and Figure S5a, c) we can see, there are a layer of particles covered on the two-dimensional lamellas. In the same magnification (Figure S5), G-TA exhibits a darker and rougher surface than the pristine graphene, indicating a uniform TA coverage on graphene. After washed thoroughly with alkaline aqueous solution, the particles disappear in the TEM images of the pristine graphene (Figure 4b-d and Figure S5b, d). The surface of the pristine graphene is very clean and its color is very close to substrate, proving the pristine graphene is very thin, because a singlelayer graphene is basically transparent. The inset in Figure 4b is the normal-incidence selected area electron diffraction of the redly framed area, which exhibits a typical six-fold 13



symmetry diffraction pattern. The [1100] spots are brighter than [2110] spots, confirming the existence of the high-quality single-layer graphene crystals.16 The single-layer graphene can also differentiate easily from the amplification of the graphene periphery (Figure 4c, d). Based on the char yields of the pristine graphene (100%), TA (23%) and G-TA (77%) at 800 C (Figure S6), the TA content of G-TA is estimated to be 30 wt%. It is not surprising that

o

there are so many TA absorbed on G-TA because of the strong π-π interaction between TA and graphene. More importantly, TA can be completely washed away with alkaline aqueous solution, judging from the fact that the pristine graphene has no weight loss up to 800 oC. This is very beneficial for preparing pristine graphene for electronic applications. The XPS characterization shows the C/O ratios of the pristine graphene and G-TA are 7: 1 and 18: 1, respectively (Figure S7). AFM was also employed to provide insight into the thickness and size of the pristine graphene. Figure 4e is a representative AFM image of the pristine graphene nanosheets. The thinnest graphene nanosheet is 0.75 nm, indicating a single-layer graphene.8 The graphene sheets are intact without holes and other defects observed in the plane of graphene, representing the high quality of graphene. The height and lateral size profiles of 70 different graphene flakes were examined, and the lengths and widths of the graphene sheets are 0.8~6.5 μm and 0.5~6.0 μm with mean values of 2.3 μm and 1.7 μm, respectively. The thicknesses mainly distributed in the range of 0.75~1.85 nm (Figure 4f). Considering the thinnest graphene flakes measured by AFM is 0.75 nm and the layer distance of graphite is 0.335 nm, the layer distribution of graphene was calculated as follows: 92% of the detected graphene flakes are ≤ 5 layers, including 21% single-layer, 24% double-layer, and 47% 3- to 5-layer graphene. Compared with the reported liquid-phase exfoliation of 14


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graphene, the average size of graphene is bigger and there are more few-layer graphene. For example, the length, width and thickness of graphene from direct exfoliation of graphite in dimethyl formamide are 0.2~2.9 μm, 0.16~1.9 μm, and 1.3~3.1 nm, respectively. 9 Raman spectroscopy is a powerful and commonly used tool for evaluating the quality of graphene.7-10 The Raman intensity ratio of D-band (~1330 cm-1) to G-band (~1580 cm-1), denoted ID/IG, has been widely used to evaluate the defect content of graphene.7-10 The Raman spectra in Figure S8 show ID/IG increases from 0.06 for graphite to 0.31 for graphene, indicating some additional defects were introduced to the graphene surface as a result of sonication. Even so, the ID/IG is much lower than that for the graphene produced from reduction of GO by sodium borohydride (ID/IG = 1.08) or hydrazine (ID/IG = 1.44),10 which are the most common approaches to massively produce graphene. This result demonstrates the high-quality of graphene produced in this work relative to rGO. Therefore, our graphene possesses a fairly high electrical conductivity (488 S·cm-1) over rGO (most common values of 10~100 S·cm-1) and GO (10-4 S·cm-1).10

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Figure 4. TEM images of G-TA (a- 6000×, scale- 2 μm) and the pristine graphene (b- 40000×, scale- 200 nm; c- 15000x, scale- 500 nm; d- 100000x, scale- 100 nm); (e) AFM image of the pristine graphene; (f) thickness distribution histogram of the pristine graphene. Interaction Mechanism of Graphene and TA. To elucidate the interaction between TA and graphene, various techniques have been used to characterize G-TA. As illustrated in the FTIR spectra (Figure 5a), the pristine graphene only exhibits the O-H absorption peak at 3400 cm-1, indicating our graphene low-defect, which has been proven by the above-mentioned TGA and Raman results. G-TA has a similar spectrum with TA, but the characteristic peaks of TA, such 16


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as the O-H stretching vibration (3400 cm-1) and bending vibration (1323 cm-1), aromatic ring breathing vibrations (1613 and 1525 cm-1), significantly blueshift to 3440, 1383, 1640 and 1569 cm-1, respectively. This suggests the characteristic peaks of G-TA are mainly from the TA noncovalently absorbed on graphene via the π-π interaction between the aromatic rings in TA and graphene.31-35 Furthermore, the noncovalent π-π interaction between TA and graphene was confirmed by UV-Vis and fluorescence spectroscopy. As shown in the UV-Vis spectra (Figure 5b), TA exhibits two absorption peaks at 212 and 276 nm, which are separately ascribed to the π-π* transitions of aromatic C=C bonds (aromatic E bond) and n-π* transitions of C=O bonds (R bond) in TA.31-35 When TA absorbs on graphene, both absorption peaks blueshift to 201 and 271 nm, respectively. This once again proves the strong π-π interaction between TA and graphene. Besides, fluorescence spectra were obtained to further prove the strong π-π interaction between TA and graphene, because the fluorescence intensity of TA is strongly influenced by this interaction. After being excited at an adsorption wavelength of 270 nm, the TA aqueous solution (0.1 mg·mL-1) exhibits two characteristic fluorescence peaks at 300 and 375 nm (Figure 5c), while the fluorescence emission of TA is completely quenched in the G-TA aqueous dispersion. The phenomenon demonstrates there exists electron or energy transfer between TA and graphene, which also derives from the strong π-π interaction between them.31-35 In a word, through the above experimental facts, the π-π interaction between TA and graphene are verified (Figure 5d).

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Figure 5. (a) FTIR spectra of the pristine graphene, TA and G-TA, (b) UV-Vis and (c) fluorescence spectra excited at 270 nm of TA and G-TA aqueous dispersion (CTA = 0.1 mg·mL-1, CG = 0.01 mg·mL-1), (d) interaction mechanism between TA and graphene. Application of G-TA in Composites. A high-concentration, high-quality graphene dispersion was successfully prepared via the direct exfoliation of graphite in water using TA as the stabilizer. This method is green and high-effiency, and the as-formed graphene is low-cost and high-quality. And thus, graphene prepared by this method is more suitable for multifunctional composites than graphene from both CVD and wet chemical synthesis methods. Moreover, TA absorbed on graphene not only can ensure the uniform distribution of graphene in polymer matrices, but also can build a strong interface between graphene and matrices for promoting the stress transfer from matrices to graphene, which are necessary for high-performance polymer/graphene composites. In addition, our graphene aqueous 18


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dispersions are convenient for processing many polymer-based composites with green and low-energy latex compounding technology, because many polymer matrices can be harvested or synthesized in the form of latex, such as NR, styrene butadiene rubber (SBR), acrylonitrile-butadiene rubber (NBR), chloroprene rubber (CR), polystyrene (PS), and polymethyl methacrylate (PMMA).1,38 In this work, NR as the most important elastomer in rubber industry, was used as the polymer matrix to evaluate the influence of G-TA on the mechanical, electrical and thermal properties of polymers. The detailed results for the tensile properties of the neat NR and its composites are listed in Table 1. Both tensile strength and stress at 300% strain improve with the increasing G-TA loading when the G-TA loading is less than 5 wt%. The maximum tensile strength and stress at 300% strain increase from 20.3 MPa and 3.0 MPa for the neat NR to 29.5 MPa and 5.7 MPa for the composite with a G-TA loading of 5 wt%, corresponding to an increase of 45% and 90%, respectively. The significant improvement benefits from both the uniform dispersion of graphene and the strong interfacial adhesion between graphene and NR verified by the SEM observations. As shown in Figure 6a, graphene is uniformly distributed in the composite containing 1 wt% G-TA. Further magnification (Figure 6b), a thick boundary layer is observed around the graphene surface, suggesting an excellent adhesion at the NR/graphene interface. The elongation at break of all the composites (G-TA loading ≤ 5 wt%) exceeds that of the neat NR, but it decreases with the increasing G-TA loadings after G-TA loading > 0.3 wt%. The decrease of the elongation at break is probably because too many GTA affect the NR vulcanization process, transferring the crosslinking sulfur polysulfide species to the monosulfide species, which leads to the increased crosslinking density of NR 19



and thus the decreased mobility of NR segments.39-40 It’s well-known that strength and stiffness are commonly inversely related with toughness and ductility. Generally, the strength and stiffness of polymers enhance while their toughness and ductility reduce with the addition of fillers, because the strong filler-matrix interface interaction constrains the motion of polymer segments near the fillers surface, causing the relaxation time insufficient for responding the applied stress.40,41 However, with TA as the mediator in the interface of NR/graphene composites, a strong interfacial interaction is provided by both covalent and noncovalent interactions, i.e. the chemical bonds between TA and NR formed by the Michael addition between the polythiol radicals and the orthoquinone derivatives, and the π-π interaction between the benzene rings of TA and graphene.39,40 Upon loading, the slippage of the benzene rings of TA along the stress direction enables the composites to own the excellent toughness and ductility. The mechanism is similar to that of some biocomposites, such as spider silk, which exploits noncovalent sacrificial bonds at the matrix-filler interface to balance the action of reinforcing and toughening.41 Consequently, using TA as the interfacial regulator, it is conducive to developing high-performance polymer/graphene composites with high strength, stiffness, toughness and ductility.

20


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Table 1. Tensile properties of the neat NR and its composites. G-TA

Graphene

Tensile

Stress at 300%

Elongation at

loading/wt%

loading/vol%*

Strength/MPa

strain/MPa

break/%

0

0

20.3 ± 0.5

3.0 ± 0.2

480 ± 26

0.3

0.1

26.1 ± 1.7

3.0 ± 0.3

598 ± 41

0.5

0.16

28.0 ± 1.5

3.5 ± 0.2

558 ± 37

1

0.32

28.3 ± 2.5

4.1 ± 0.4

544 ± 47

3

0.98

28.9 ± 1.9

4.8 ± 0.3

512 ± 40

5

1.64

29.5 ± 2.1

5.7 ± 0.5

498 ± 39

*The volume fraction of graphene in the composites was calculated based on graphene density of 2.28 g·cm-3 and NR matrix density of 1.05 g·cm-3.

Figure 6. SEM images of the NR/G-TA composite at 1 wt% G-TA loading for different magnifications (a- 2000×, scale- 10 μm; b- 20000×, scale- 1 μm). The electrical and thermal conductivities of the NR/G-TA composites as a function of graphene loadings were further investigated. As shown in Figure 7, both electrical and thermal conductivities improve remarkably with graphene loadings. The electrical conductivity increases from 10-13 S·m-1 for the neat NR to 10-6 S·m-1 for the composite with 21



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1.64 vol% graphene, acquiring 7 orders of magnitude increment. Meanwhile, the thermal conductivity increases from 0.140 to 0.172 W·m-1·K-1, representing a 23% improvement. Compared with the reported NR/rGO composites, both the electrical and thermal conductivities of the NR/G-TA composites are higher at the similar graphene loadings. For example, the electrical conductivity of the NR/rGO composite at 1.64 vol% rGO reaches only to ~10-7 S·m-1 with an order of magnitude less than the NR/G-TA composite,42 while the thermal conductivity of the NR/rGO composite at 4 wt% rGO increases from 0.157 W·m-1·K-1 for neat NR to 0.187 W·m-1·K-1 with only 19% increment.43 The better electrical and thermal properties of NR/G-TA composites mainly profit from the high quality of graphene and their good dispersion in NR matrix. The outstanding electrical and thermal properties combined with excellent mechanical properties endow the as-fabricated NR/G-TA composites with potential

applications

in

antistatic

rubber

products,

stretchable

conductors,

and

electromagnetic shielding devices.

Figure 7. Electrical and thermal conductivities of the neat NR and NR/G-TA composites as a function of graphene loadings.

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CONCLUSION In this work, TA has been demonstrated to be an effective stabilizer for one-step exfoliation and noncovalent functionalization of graphene in aqueous media. Under the assistance of TA with CTA as low as 1 mg·mL-1, the highest graphene productivity with CG up to 1.25 mg·mL-1 can be achieved after a short bath sonication for 1 h. The as-fabricated graphene shows few defects (ID/IG = 0.06) and a high electrical conductivity (488 S·cm-1) relative to GO and rGO. Furthermore, TA as a noncovalent modifier can make graphene uniformly dispersed in NR matrix and firmly adhered to NR matrix, and thus, the NR/graphene nanocomposites with remarkably improved strength, stiffness, elongation, electrical and thermal conductivity are obtained. Except for composites, G-TA aqueous dispersions also have many potential and practical applications in conductive inks, anticorrosive and antistatic coatings, and drug carriers. In summary, this work not only represents a breakthrough for green, high-yield and large-scale preparation of low-cost and high-quality graphene, but also hastens the realization of eco-friendly and effective manipulation of graphene for high-performance and multifunctional polymer/graphene nanocomposites. ASSOCIATED CONTENT Supporting Information Molecular structure of TA, table for CTA and CG,i, table for the optimum curing time, determination of graphene concentration, change of A660 with CTA, the linear fitting of A660 and CG, dependence of both pH of TA solutions and zeta potentials of G-TA dispersions on CTA, calculation for the surface tension, data for natural stabilizer-assisted exfoliation of graphene in water, change of CG with sediment time, TEM images of G-TA and the pristine 23



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graphene, TGA plots of the pristine graphene, TA and G-TA, XPS of the pristine graphene and G-TA. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Lin Li). ORCID Lin Li: 0000-0001-9754-7536 Shuai Zhao: 0000-0003-2115-4378 Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS Financial supports from Natural Science Fundation of China (No. 51603111 and 51703111), Natural Science Fundation of Shandong Province (No. ZR2015EQ011 and ZR2017BEM011), Project of Shandong Province Higher Educational Science and Technology Program (No. J17KA012 and J16LA06), and Fundation of Key Laboratory of Rubber-Plastics, Ministry of Education/Shandong Provincial Key Laboratory of Rubber-Plastics, Qingdao University of Science and Technology (No. KF2017005) are gratefully acknowledged. REFERENCES (1)Bourgeat-Lami,

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Tannic acid is a green and renewable stabilizer for high-efficiency exfoliation of graphene from natural graphite. (For Table of Contents Use Only)

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Green and High-Efficiency Production of Graphene ... - ACS Publications

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