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Cite This: ACS Appl. Nano Mater. 2018, 1, 3878−3885

Cation-π-Induced Exfoliation of Graphite by a Zwitterionic Polymeric Dispersant for Congo Red Adsorption Xuejiao Zhang,† Xiaozhong Li,‡ Siyu Zhang,†,§ Yinghua Li,‡ Dongsheng Wang,†,∥ Haibo Li,*,‡,§ Qing Zhao,*,† and Baoshan Xing§ †

ACS Appl. Nano Mater. 2018.1:3878-3885. Downloaded from pubs.acs.org by DURHAM UNIV on 08/28/18. For personal use only.

Key Laboratory of Pollution Ecology and Environmental Engineering, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, China ‡ School of Resources & Civil Engineering, Northeastern University, Shenyang 110004, China § Stockbridge School of Agriculture, University of Massachusetts, Amherst, Massachusetts 01003, United States ∥ University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing, 100049, China S Supporting Information *

ABSTRACT: Water dispersion remains a technical challenge for effective applications of graphene. In this paper, graphene nanosheets were successfully produced by directly exfoliating expanded graphite in water in the presence of zwitterionic polymeric dispersant, succinic anhydride-functionalized polyethylenimine (PEI-SA). The effects of various parameters, including SA percentage in PEI-SA, dispersant concentration, pH, centrifugation speed, and initial graphite concentration on the exfoliation and stabilization of graphene were studied. Cation-π interaction between the protonated amines in PEISA and polyaromatic structure in graphite, together with the zwitterionic property and the steric hindrance of PEI-SA, facilitated the exfoliation and stabilization of graphene. Graphene nanosheets with negligible defects were obtained by dispersant-assisted liquid-phase exfoliation. The concentration of exfoliated graphene suspension under the optimal condition was 0.56 mg/mL with the yield as high as 11.2% and the suspension remained stable for more than 6 months. The stability of exfoliated graphene suspension was a function of pH as determined by UV−vis absorbance, hydrodynamic size, and zeta potential. The exfoliated graphene showed greater potential as an adsorbent for Congo red (CR) removal from water compared to that of pristine graphene. KEYWORDS: graphene, liquid-phase exfoliation, zwitterionic dispersant, cation-π, dye adsorption

1. INTRODUCTION Graphene has been attracting tremendous research interests since its first discovery in 2004, due to its superior electrical, optical, and mechanical properties.1−3 Currently, graphene has been widely applied in electronics,4,5 energy storage,6,7 sensors,8,9 and environmental remediation.10,11 Various approaches, including chemical vapor deposition,12 mechanical exfoliation,13 chemical reduction from graphene oxide (GO),14,15 and liquid-phase exfoliation (LPE)16 have been used to produce graphene. Among them, LPE is the most investigated method, because it can produce high-quality graphene for diverse applications with the advantages of simplicity, high efficiency, and low cost. Organic solvents, such as N,N′-dimethylformamide (DMF),17 N-methyl-2-pyrrolidone (NMP),18 and ortho-dichlorobenzene (o-DCB),16 whose surface tensions are close to that of graphene and graphite flakes (40 mJ/m2), can efficiently exfoliate graphite because of the minimized interfacial tension between solvents and graphene.19 However, the high boiling points and toxicities of the organic solvents limit their practical manipulation and application. Alternatively, graphene exfoliation in aqueous © 2018 American Chemical Society

solution is preferred, considering the cost, health risks, and environmental issues. Given the large surface tension gap between water (72.8 mJ/m2) and graphene, the aqueous graphene is prone to aggregation owing to the hydrophobic and van der Waals interactions.20 Therefore, it is a great challenge to exfoliate and disperse graphene in water. Although small molecular surfactants, 21 proteins or peptides,22,23 and polymers have been used as dispersants to exfoliate graphene via hydrophobic, π−π, or electrostatic interactions between graphene and the agents, a new method is still needed, which spurs the research to examine more noncovalent interactions in order to find a simpler, more efficient, and less expensive way. Cation-π interaction, with its binding energies higher than most of the other noncovalent interactions (from 39.3 to 91.2 kJ/mol), seems to be a potential candidate.24 This interaction is essential to biological systems.25−27 It has been reported that GO and graphite can Received: April 23, 2018 Accepted: July 27, 2018 Published: July 27, 2018 3878

DOI: 10.1021/acsanm.8b00665 ACS Appl. Nano Mater. 2018, 1, 3878−3885

Article

ACS Applied Nano Materials

collection for characterization. To obtain the concentrated graphene suspension, the effect of concentration of PEI-SA (Cp), pH, centrifugation speed (ω), and initial EG concentration (CG,i) on the exfoliation efficiency were evaluated. 2.4. Characterization of Exfoliated Graphene Nanosheets. The UV−vis absorbance spectra of PEI-SA exfoliated graphene were obtained by a UV−vis spectrophotometer (Shimadzu UV−vis spectrophotometer 2700) at room temperature. PEI-SA had negligible absorbance at 660 nm, which allowed us to obtain the extinction coefficient αG from the slope of linear fitting of graphene concentration versus absorbance (660 nm). The graphene contents in the suspension were determined by thermogravimetric analysis (TGA, Q500 V20.13 Build 39, USA). The temperature was raised from 0 to 900 °C at the rate of 10 °C/min. The morphology of exfoliated graphene nanosheets was observed by a field emission transmission electron microscope (FETEM, JEOL JEM 2100F, Japan). Atomic force microscope (AFM, Agilent 5100) was used to examine the thickness of exfoliated graphene nanosheets in tapping mode. The suspension of exfoliated graphene nanosheets were deposited onto mica substrate and dried under nitrogen flow before AFM measurement. Raman spectra were recorded by a Thermo scientific DXR Raman Microscope, equipped with a CCD detector, and a 780 nm laser line with a laser source power of 24 mW. The phase analysis of exfoliated graphene nanosheets was performed by Xray diffraction (XRD, Rigaku Smartlab diffractometer, Japan) with Cu Kα radiation (λ = 1.5418 Å). The X-ray source was obtained at a power setting of 9 kW, 45 kV, and 200 mA. The data were collected with a step size of 0.02°. Elemental analysis was performed on Elemental analyzer (Elementar Vario Micro Cube, Germany). The element content of C, H, and N in graphene, EG, PEI-SA-21, and PEI-SA-21/G was measured using the CHNS mode at a reduction furnace of 850 °C and a combustion furnace of 1150 °C. The element content of O was measured at 1150 °C in the combustion furnace using O mode. All samples were measured in parallel and the results were presented as mean value. 2.5. Batch Adsorption Experiments. The sorption of CR on the exfoliated graphene nanosheets was investigated by batch adsorption techniques. The exfoliated graphene nanosheets (graphene content equals 1 mg) were suspended in 40 mL phosphate buffer solution (PBS, pH = 7, 10 mM NaCl) in the presence of various concentrations of CR (1−10 μg/mL) in glass vials, which were continuously shaken at 180 rpm for 4 days under 25 °C. As for the control group, 1 mg of commercial graphene was mixed with 40 mL CR containing PBS and sonicated for 10 min to homogenize graphene and initiate sufficient contact between graphene and CR. After centrifugation at 10 000 rpm for 30 min to remove the graphene nanosheets, the concentration of CR was determined. All the experiments were repeated three times and the results represented as mean values.

easily interact with cations through cation-π interaction due to the sp2-clustered regions of 2D carbon lattice.24,28,29 Moreover, the amine groups out of the triazine plane strongly interacted with graphene surface according to the theoretical calculation.30,31 In addition, the protonated amines in the cationic peptide could bind to polyaromatic graphite surface via cationπ interaction.23 Lee et al. claimed that reduced graphene oxide (rGO) could be obtained by dispersing GO in the NaOH (or KOH) solution via monovalent cation-π interactions accompanied by adding hydrazine monohydrate (N2H4) to reduce GO.32 Therefore, we hypothesize that cation-π could be an important interaction in graphene exfoliation and dispersion. Upon exfoliation in aqueous solution, the stability of graphene suspension is related to either zeta potential or steric hindrance of the dispersant-coated flakes.19 Thus, it is worthwhile to develop ionic polymer dispersants, which can provide both electrostatic repulsion and large steric hindrance, for graphene exfoliation and stabilization, endowing the feasibility to graphene for further applications in aqueous solution. Graphene-based materials have been considered as efficient adsorbents for hazardous pollutants, including organic pollutants and heavy metal ions, owing to its high specific surface area (2630 m2/g).33−36 In this study, zwitterionic polymeric exfoliating/dispersing agents, succinic anhydridefunctionalized polyethylenimine (PEI-SA), were synthesized and applied to directly exfoliate graphite in aqueous solution under mild sonication to obtain stable graphene suspension. The exfoliation is based on cation-π interaction between the aromatic structures in graphite and protonated amino groups in the polymer. The suspension stability is dedicated by the electrostatic repulsion and steric hindrance of the polymer. After optimizing the exfoliation conditions, graphene nanosheets were harvested to evaluate the efficiency of removing organic dyes from water.

2. MATERIALS AND METHODS 2.1. Materials. Expanded graphite (EG) was bought from Nanjing XFNANO Materials Tech Co., Ltd.. Polyethylenimine (PEI, branched, Mw = 1800 g/mol) was provided by Beijing Yinuokai Technology Co. Ltd. Succinic anhydride (SA), Congo Red (CR), dimethyl sulfoxide (DMSO), and graphene were purchased from Sigma-Aldrich. All the other reagents were of chromatographic grade unless otherwise stated. 2.2. Synthesis of PEI-SA. A 5 g portion of PEI was dissolved into 5 mL of Milli-Q water in a round-bottom flask and the solution pH was adjusted to 5 by diluted HCl. Then a certain amount of SA dissolved in 10 mL DMSO was dropwise added to the reaction mixture with continuous magnetic stirring. After 24 h, the crude product was purified by dialysis against NaCl solution (0.5 M) for 8 h and then against Milli-Q water for 3 days. PEI-SA with different SA percentages was obtained by lyophilization. 1H NMR and 13C NMR were conducted to quantify the functionalities of PEI-SA (Figure S1). 1 H NMR (400 MHz, D2O): δ (ppm) = 2.37−3.50 (−NH−CH2− CH2−N−), δ = 2.15−2.37 (−CO−CH2−CH2−COOH); 13C NMR (400 MHz, D2O): δ (ppm) = 35.3−55.6 (PEI backbone), 176.5 (−COOH), 179.1 (−CONH−). 2.3. Dispersant-Assisted Liquid-Phase Exfoliation. EG powder was mixed with 10 mL of PEI-SA and sonicated in a water bath sonicator (KQ300B, Kun Shan Ultrasonic Instruments Co., Ltd.). The sonication time (tsonic) and centrifugation time (tcf) were set to 6 h and 20 min, respectively. The suspension pH was adjusted using diluted HCl or NaOH solution to manipulate the protonation/ deprotonation of amino and carboxyl groups in PEI-SA. Afterward, the suspension was centrifuged to eliminate the unexfoliated thick graphite flakes, and then the graphene nanosheets in the supernatant were collected and washed with Milli-Q water three times before

3. RESULTS AND DISCUSSION 3.1. Optimization of the Exfoliation Conditions. The zwitterionic PEI-SA with SA percentage of 12%, 21%, and 38% were synthesized and named for PEI-SA-12, PEI-SA-21, and PEI-SA-38, respectively. The protonated amino groups in PEISA could bind to the polyaromatic graphitic surface via cationπ interaction. The zwitterionic property, resulting from the coexistence of partially deprotonated carboxyl group and protonated amino groups, together with the steric hindrance of PEI-SA prevented the exfoliated graphene nanosheets from aggregating with the bare graphite in the suspension. The relevant parameters, Cp, pH, ω, tcf, and CG,i, were optimized in order to enhance the exfoliation efficiency. To ensure all PEISA molecules positively charged, pH 4 was chosen to perform the exfoliation. According to the Lambert−Beer law (A660 = αGCGl), the concentration of exfoliated graphene (CG) is proportional to the UV−vis absorbance of suspension per unit length at 660 nm (A660/l). Thus, the A660/l value was used to 3879

DOI: 10.1021/acsanm.8b00665 ACS Appl. Nano Mater. 2018, 1, 3878−3885

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ACS Applied Nano Materials

Figure 1. A660/l of the exfoliated graphene suspension as a function of dispersant concentrations (a), initial pH (b), centrifugation speed (c), and initial EG concentration (d). The samples were prepared under the following conditions: (a) CG,i = 1.0 mg/mL, pH = 4, tsonic = 6 h, ω = 2000 rpm, tcf = 20 min; (b) CG,i = 1.0 mg/mL, tsonic = 6 h, ω = 2000 rpm, tcf = 20 min, CPEI‑SA‑12 = 0.5, CPEI‑SA‑21 = 1.0, CPEI‑SA‑38 = 0.8 mg/mL; (c) CG,i = 1.0 mg/mL, pH = 4, tsonic = 6 h, tcf = 20 min; (d) CPEI‑SA‑21 = 1.0 mg/mL, pH = 4, tsonic = 6 h, ω = 2000 rpm, tcf = 20 min.

Figure 2. AFM images (a−c) and thickness distribution (d−f) of exfoliated graphene nanosheets with the assistance of PEI-SA-12 (a, d), PEI-SA21 (b, e), and PEI-SA-38 (c, f). The thickness distribution by statistical analysis of more than 400 pieces of exfoliated graphene nanosheets via AFM. The samples were prepared under the following conditions: CG,i = 1.0 mg/mL, pH = 4, tsonic = 6 h, ω = 2000 rpm, tcf = 20 min, CPEI‑SA‑12 = 0.5 mg/mL, CPEI‑SA‑21 = 1.0 mg/mL, CPEI‑SA‑38 = 0.8 mg/mL.

represent the relative CG in the following study. With Cp increased, the A660/l values initially increased, reached the maximum values of 222.7 ± 5.2, 303 ± 7, and 285.5 ± 15.5 m−1 at the optimal Cp (defined as CpM) of 0.5, 1.0, and 0.8 mg/

mL for PEI-SA-12, PEI-SA-21, and PEI-SA-38, respectively, and gradually decreased with further increasing Cp (Figure 1a). The exfoliation efficiency of PEI-SA-21 was comparable to that of a commercial available surfactant, sodium dodecyl sulfate 3880

DOI: 10.1021/acsanm.8b00665 ACS Appl. Nano Mater. 2018, 1, 3878−3885

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ACS Applied Nano Materials

Figure 3. (a) UV−vis spectra of PEI-SA-21 and PEI-SA-21/G. (b) XRD patterns of graphene (blue), EG (black), and PEI-SA-21/G (red). The exfoliated graphene nanosheets were obtained with the following parameters: CG,i = 5.0 mg/mL, pH = 4, tsonic = 6 h, ω = 2000 rpm, tcf = 20 min, CPEI‑SA‑21 = 1.0 mg/mL. (a inset): photographs of EG suspension before (A) and after 6 h sonication (B), followed by centrifugation at 2000 rpm for 20 min (C).

Table 1. Comparison of Exfoliation Efficiency of PEI-SA with Other Dispersants in the Literature original material

CG,i (mg/mL)

graphite graphite

14 5

EG EG graphite graphite graphite graphite graphite EG

50 50 100 30 2.5 200 5 5

dispersant SDBS sodium cholate SDBS PVP NDI-1 FMNS peptide STC liposome PEI-SA

Cp (mg/mL)

ω (rpm)

A660/l

α

CG (mg/mL)

0.5 0.1

500 2000

69.5 330

1390 6600

0.05 0.05

6 10 4 1 0.5 3 8 1

5000 5000 5000 2300 8000 1500 5000 2000

365 1293 1807 732 41.7 876 172 864.5

1660 1293 1506 2440 1390 2920 1390 1534

0.22 1 1.2 ∼0.3 ∼0.03 0.3 0.124 0.56

sonication method and time (h)

yield (%)

0.1 0.5

bath and 0.5 bath and 24

0.357 1

0.036 0.1 0.3 0.3 0.06 0.1 0.016 0.56

tip and 1 tip and 1 tip and 1 bath and 5 bath and 24 bath and 400 bath and 2 bath and 6

CG/Cp

0.44 2 1.2 1 1.2 0.15 2.48 11.2

ref 38 21 39 39 41 42 23 43 44 our data

The A660/l value scaled well with ω−1 and reached a plateau at ω = 4000 rpm, where the A660/l value was 155 ± 5, 195 ± 3, and 75 ± 7 m−1 for the three PEI-SA dispersants, respectively, (Figure 1c), indicating an effective exfoliating and suspending capacity of the polymer dispersants toward graphene. The effect of CG,i on the exfoliation efficiency of EG was evaluated (tsonic = 6 h, CPEI‑SA‑21 = 1 mg/mL, ω = 2000 rpm, tcf = 20 min). A660/l reached the maximum value of 864.5 ± 63.6 m−1 at CG,i = 5 mg/mL (Figure 1d), above which the exfoliated graphene nanosheets tended to aggregate due to the entanglement between PEI-SA molecules and graphene flakes. 3.2. Morphology and Quality of Exfoliated Graphene Nanosheets. The morphologies of exfoliated graphene nanosheets were examined by AFM and TEM (Figure 2, Figure S3). The average thickness of graphene nanosheets exfoliated with the assistance of PEI-SA-12 (PEI-SA-12/G), PEI-SA-21 (PEI-SA-21/G), and PEI-SA-38 (PEI-SA-38/G) were calculated by the statistical AFM analysis to be 2.1 ± 1.9, 1.4 ± 1.7, and 1.5 ± 1.2 nm, respectively (Figure 2d−f). The TEM and HRTEM images showed the transparent appearance and few-layered structure of exfoliated graphene nanosheets (Figure S3). Figure 3a showed the UV−vis absorbance of PEI-SA-21/G and PEI-SA-21. The extinction coefficient (αG) was calculated from the Lambert−Beer law to be 1534 L·g−1·m−1 by measuring the A660 under the following exfoliation condition: CG,i = 1 mg/mL, CPEI‑SA‑21 = 1.0 mg/mL, pH = 4, tsonic = 6 h, ω = 2000 rpm, tcf = 20 min. The content of exfoliated graphene nanosheets was determined to be 85.3% by thermogravimetric analysis (TGA) (Figure S4), which was consistent with the result of elemental analysis (87.4%) (Table S1). The calculated

(SDS) (Figure S2a). When Cp < CpM, the exfoliation process was promoted by increasing PEI-SA concentration, because the graphite surface had enough binding sites for PEI-SA molecules to adhere, allowing the cation-π interaction between graphite and the protonated amino groups in PEI-SA. When Cp > CpM, the excess PEI-SA would make the exfoliated graphene nanosheets prone to aggregation and precipitation upon centrifugation due to the attractive depletion interactions.37 Thus, the A660/l value was decreased with further increasing Cp. The exfoliation efficiency of the three PEI-SA dispersants followed a similar fashion in the experimental pH range from 1 to 9 (Figure 1b), with the A660/l value gradually increased, reached a maximum value, and then decreased. At lower pH, while the amino groups in PEI-SA were protonated, which should have produced stronger cation-π interactions with EG, lower exfoliation efficiency was observed. This might be attributed to the protonation of carboxyl groups in PEI-SA, which resulted in the aggregation of exfoliated graphene nanosheets due to the attenuated electrostatic repulsion and increasing intermolecular association (hydrogen bond and hydrophobic interactions). As the suspension pH went higher, the amino groups were still positively charged, accompanied by more deprotonated carboxyl groups and less protons in the suspension, resulting in elevated exfoliation efficiency. When the pH was higher than the isoelectric point (PEI-SA-12:9.27; PEI-SA-21:8.85; PEI-SA-38:4.97, Figure S2b), graphene was barely exfoliated owing to the vanished cation-π interaction induced by the deprotonation of amino groups. The A660/l value was sharply decreased with increasing ω from 500 to 4000 rpm, due to the precipitation of large graphene flakes. 3881

DOI: 10.1021/acsanm.8b00665 ACS Appl. Nano Mater. 2018, 1, 3878−3885

Article

ACS Applied Nano Materials αG was comparable to a previous study, in which αG was measured to be 1390 L·g−1·m−1.38 The maximum LPE yield (defined as weight percent of exfoliated graphene nanosheets to original EG) in our study was calculated to be 11.2%, which was higher than most of the reported ones, i.e., sodium cholate (1%),21 sodium dodecylbenzenesulfonate (SDBS, 0.375%),38 and PVP (2%)39 under similar exfoliation conditions (Table 1). Although the concentration of PEI-SA-21 used in our study was 10-fold that of sodium cholate for exfoliating the same amount of EG, the exfoliation yields for PEI-SA-21 and sodium cholate were 11.2% and 1% under the sonication time of 6 and 24 h, respectively, indicating higher exfoliation efficiency of PEI-SA. The EG had an obvious diffraction peak at 2θ = 26.5°, representing the (002) reflection in graphite crystal lattice originating from the interlayer distance of 3.37 Å between graphene sheets (Figure 3b).40 The intensity of 2θ peak at 26.5° of PEI-SA-21/G decreased by 97.2% compared to that of EG, indicating the ordered structure of graphite was destroyed by exfoliation in the presence of PEI-SA via cation-π interaction. Raman spectra were used to determine the quality of exfoliated graphene nanosheets. Three prominent Raman peaks of EG were observed at 1313, 1575, and 2630 cm−1 (Figure 4), which were ascribed to a defect-induced D band, a

sodium hydride (ID/IG ∼ 1.08).47,48 The D/D′ intensity ratio (ID/ID′) could be used to estimate the nature of defects in graphene. According to the previous report, the ID/ID′ values corresponding to sp3 hybridization-associated defects, vacancylike defects, and boundary-like defects were approximately 13, 7, and 3.5, respectively.49 In our study, The ID/ID′ values for graphene, PEI-SA-12/G, PEI-SA-21/G, and PEI-SA-38/G were respectively 3.48, 3.56, 3.71, 3.21, which were all around 3.5, indicating the edge defects due to the small size of graphene flakes. Thus, the increased ID/IG of exfoliated graphene nanosheets could be attributed to an increased edge defects, rather than point defects on the basal plane of exfoliated graphene nanosheets. 3.3. Stability of Exfoliated Graphene Dispersions. In order to get further insight into the stabilizing effects of PEI-SA on the exfoliated graphene nanosheets, the variations of A660/l, size, and Zeta potential over time and pH was monitored. The suspension of PEI-SA-21/G remained stable for at least 198 days with only slightly decrease of A660/l value from 565 to 524.5 m−1 (Figure 5a, Figure S5a). Moreover, the size was hardly changed, whereas the Zeta potential was reduced by 3.3 mV during the recording period (Figures 5a, S5b). Additionally, a pH-dependent stability profile of the exfoliated graphene suspension was observed (Figure 5b). The suspension stayed stable when the pH was higher than 11 or lower than 4, since the Zeta potential of PEI-SA-21/G was high enough to induce electrostatic repulsion under these pH conditions. In contrast, agglomeration and precipitation occurred at pH between 4 and 11. When pH approached to the isoelectric point (8.85), PEISA-21 was prone to micellization (Figure S6). The formed micelles could act as adhesive for graphene nanosheets, leading to the decreased A660/l value and increased size due to aggregation (Figures 5b, S5c,d). 3.4. Adsorption Experiments. The exfoliated graphene nanosheets were applied as adsorbents to remove a model organic pollutant, CR, from aqueous solution. The sorption isotherms were fitted to Langmuir (LM), Freundlich (FM), and Dubinin−Ashtakhov (DA) models (Figure 6), which demonstrated monolayer adsorption, multilayer heterogeneous adsorption, and Polanyi theory-based adsorption, respectively.50−52 For both commercial graphene and PEI-SA-21/ G, the correlation coefficients (radj2) of FM and DA were higher than that of LM (Table S2), indicating the multilayer heterogeneous sorption played a dominant role. The sorption capacity obtained from DA model was 531.3 mg/g for PEI-SA21/G, which was not only much higher than that for commercial graphene (197.9 mg/g), but also the highest capacity reported (Table S3). The significantly enhanced sorption capacity of PEI-SA-21/G was, on the one hand, ascribed to the superior water dispersibility induced by the surfactant property of PEI-SA, so that the effective surface area of graphene nanosheets was increased to interact with CR via hydrophobic and π−π interactions; on the other hand, the PEI-SA molecules on the graphene surface also contributed to the high sorption capacity via electrostatic interactions and hydrogen bond.

Figure 4. Raman spectra of graphene (red), EG (black), and exfoliated graphene assisted by different dispersants (blue, green, and pink).

G band of in-plane vibration of sp2 carbon, and a second-order two-phonon-mode 2D band, respectively.23 The 2D band of PEI-SA-21/G showed similar shape and Raman shift to that of graphene, indicating the formation of single layer graphene in the process of PEI-SA-21 assisted exfoliation.21 In contrast, the 2D bands of PEI-SA-12/G and PEI-SA-38/G exhibited obvious blueshift compared to graphene and were in parallel with EG, which demonstrated the flakes were more than 5 layers due to the restacking during dryness.43,45,46 The D/G intensity ratio (ID/IG) of EG was 0.346, indicating the high quality of EG with little defect in the structure. The ID/IG values of PEI-SA-12/G, PEI-SA-21/G, and PEI-SA-38/G were 0.691, 2.51, and 1.609, respectively. The ID/IG value of PEISA-12/G was smaller than that of graphene produced from graphene oxide reduction by hydrazine (ID/IG ∼ 1.44) or

4. CONCLUSION In this study, we developed zwitterionic polymeric dispersants to exfoliate graphite directly in aqueous solution into graphene under mild bath sonication. Mono- or few-layered graphene nanosheets were obtained by optimizing the exfoliation conditions of dispersant concentration, pH, centrifugation 3882

DOI: 10.1021/acsanm.8b00665 ACS Appl. Nano Mater. 2018, 1, 3878−3885

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ACS Applied Nano Materials

Figure 5. Stability of exfoliated graphene suspension as a function of time (a) and pH (b) reflected by A660/l (red), average hydrodynamic size (black), and zeta potential (blue). The starting suspension was prepared according to the following parameters and diluted properly before use: CG,i = 5.0 mg/mL, pH = 4, tsonic = 6 h, ω = 2000 rpm, tcf = 20 min, CPEI‑SA‑21 = 1.0 mg/mL.

*E-mail: [email protected] (H.L.). ORCID

Qing Zhao: 0000-0003-2250-5571 Baoshan Xing: 0000-0003-2028-1295 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 41603120, 41703107, and 21671115), the National Key Research and Development Program of China (No. 2016YFD0800300), Youth Innovation Promotion Association CAS awarded to S.Z. (2017−2020), Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), the Innovate Team Project of Key Laboratory of Pollution Ecology and Environmental Engineering, and Hundreds Talents Program of Chinese Academy of Sciences awarded to X.Z. (2015−2020) and Q.Z. (2014− 2019).

Figure 6. Sorption isotherms of CR on graphene (a) and PEI-SA-21/ G (b) fitted with LM (blue line), FM (black line) and DA (red line) models. (inset) Photographs of CR solution (5 μg/mL) before (A) and after (B) adsorption.

speed, and graphite concentration. The graphene yield was the highest among the liquid phase exfoliation papers ever reported. The cation-π interaction between graphite polyaromatic structure and the protonated amino groups in PEISA, accompanied by the electrostatic repulsion of negatively charged carboxyl groups and the steric hindrance of PEI−PA, contributed to the exfoliation and stabilization of graphene nanosheets. The stability of exfoliated graphene suspension was independent of time for more than 6 months but was a function of pH. The exfoliated graphene nanosheets possessed superior sorption capacity toward CR compared to bare graphene, resulting from the hydrophobic and π−π interactions between graphene and CR, as well as the electrostatic interactions and hydrogen bond between PEI-SA and CR.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.8b00665. 1 H NMR and 13C NMR spectra (Figure S1), comparison the exfoliation efficiency of PEI-SA-21 and SDS and isoelectric point measurement (Figure S2), TEM and HRTEM images (Figure S3), TGA measurement (Figure S4), stability evaluation of exfoliated graphene suspension over time and pH (Figure S5), elemental analysis (Table S1), sorption data (Table S2), and comparison of sorption capacities on various sorbents (Table S3) (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Q.Z.). 3883

DOI: 10.1021/acsanm.8b00665 ACS Appl. Nano Mater. 2018, 1, 3878−3885

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ACS Applied Nano Materials

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DOI: 10.1021/acsanm.8b00665 ACS Appl. Nano Mater. 2018, 1, 3878−3885

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DOI: 10.1021/acsanm.8b00665 ACS Appl. Nano Mater. 2018, 1, 3878−3885