Fabrication of High-Concentration Aqueous Graphene Suspensions

Sep 18, 2015 - Chemical reduction of graphene oxide boasts the advantage of a large amount of fewer layer graphene but suffers substantial quantities ...
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Fabrication of High-Concentration Aqueous Graphene Suspensions Dispersed by Sodium Lignosulfonate and Its Mechanism Hongming Lou,†,‡ Duming Zhu,† Long Yuan,† Xueqing Qiu,*,†,‡ Xuliang Lin,† Dongjie Yang,† and Yuan Li† †

School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, China State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, China

‡

ABSTRACT: Stable aqueous graphene suspensions (AGSs) with high concentration and little defect were prepared through exfoliating from graphite by sonication with the aid of sodium lignosulfonate (SL). The concentration of AGS (CG) dispersed by the modified SL can reach up to 13.5 g/L at an initial graphite concentration of 200 g/L, which is substantially higher than those reported previously. Four SL fractions with different molecular weight (Mw) and sulfonic acid group prepared by ultrafiltration of a commercial SL were applied to evaluate the dispersing mechanism and the effects of SL molecular structure on the AGS. The maximum concentration of AGS dispersed by the SL fractions increased with the decline of Mw and the ion strength of SL solutions and with the increase of sulfonic acid group. The CG can be further improved through improving the hydrophobicity of SL by either covalent or noncovalent modification. It was found that the CG was proportional to the zeta potentials, which suggested that electrostatic repulsion played a crucial role in the dispersion process of AGS. Meanwhile, the obtained high-concentration AGS dispersed by SL is nontoxic, which facilitates it as a promising material for application in the biomedical field. sensing.9−12 However, like most nanomaterials, a crucial challenge in the manufacturing process of AGSs is to avoid aggregation because of its high specific surface area, strong hydrophobicity, and van der Waals interaction between graphene nanosheets.13 Therefore, it remains a challenge to adopt a facile and green approach for fabricating scalable AGSs for biomedical application. Considerable studies have been conducted to fabricate AGS with high concentration by dispersant-assisted sonication to avoid aggregation, such as 12 g/L of AGS by sodium taurodeoxycholate (STC), 2.1 g/L of AGS by sodium dodecyl sulfate (SDS) and ethanol, 0.2 g/L of AGS by bovine serum albumin (BSA), etc.8,14−18 Although the concentration of AGS could be increased with longer sonication time, it may not be viable in commercial production. Therefore, it is essential to evaluate the effect of the molecular structure and dispersing mechanism of dispersant on the AGS to obtain highconcentration AGS. As to the dispersing mechanism of AGS, Qian holds the view that it will promote the stability of AGS when the dipole moment between dispersants is larger than that of AGS.19 However, the π−π conjugation between the aromatic ring and AGS is the main reason for the stability, first proposed by

1. INTRODUCTION Graphene, as a single atomic layer of sp2-bonded carbon atoms densely arranged in a honeycomb lattice, has triggered great attention in the fields of nanoelectronic and composite materials.1 The special structure of graphene endowed it a host of fascinating properties, such as mechanics, electrical, and thermal.2−4 Some efficient manufacture technologies have been put forward to prepare graphene.5−8 Among the methods, mechanical cleavage of original graphite could obtain graphene with high quality but low productivity, which is only suitable for lab research. Chemical vapor deposition (CVD) produces graphene with fewer defects, but the low conversion rate of raw material and high request of equipment severely restrict its application. Chemical reduction of graphene oxide boasts the advantage of a large amount of fewer layer graphene but suffers substantial quantities of structural defects and oxygen functionality. Defect-free graphene can be exfoliated from graphite or expanded graphite (EG) in organic solvents. Its superiority of high quality and perfect lattice graphene make it promising for manipulation and further application. However, most organic solvents are toxic and lack biocompatibility, which limits the safe use of graphene in the field of biomedical application. Water is a green and cheap solvent, and stable aqueous graphene suspensions (AGSs) have emerged as a functional material in the biomedical field, such as drug delivery, cell-based research, biosensor, and DNA © XXXX American Chemical Society

Received: July 1, 2015 Revised: September 17, 2015

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DOI: 10.1021/acs.jpcc.5b06301 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Barton.20 Coleman puts forward the opinion that the electrostatic repulsion originating from negative charge is the vital role when it absorbs on the surface of graphene.14 However, there is no convincing mechanism model illustrating the function of dispersants, which causes it to be a tough work to choose and synthesize a better dispersant. Sodium lignosulfonate (SL) is an anionic polymeric dispersant with hydrophobic aromatic backbone with alkyl side chains, which is an abundant and cheap coproduct of waste sulfite pulping liquors of the papermaking industry; meanwhile, SL was shown to be nontoxic according to previous research.21,22 SL presents good dispersion for hydrophobic solid particles in aqueous suspensions, such as coal particles, dye particles, and dimethomorph pesticide.23−25 In addition, several works on lignin and SL-assisted dispersion of graphene have been reported.26−28 Meanwhile, SL owns a threedimensional cross-linked structure containing hydrophilic groups in its molecules, such as hydrophilic hydroxyl, carboxyl, and sulfonic acid groups, which guarantees it to be a good raw material for preparing the hydrogel, and the excellent waterswollen capacity of SL hydrogel endows it huge potential application in the biomedical fields.29 SL or SL hydrogel loading on the graphene may exhibit better stabilization and dispersion of AGS, which is more beneficial to their applications in the biomedical field. In this work, a great quantity of dispersants, including anionic, cationic, and nonionic, were applied to disperse AGS exfoliated from expanded graphite and graphite. SL fractions with different Mw were used as dispersants in AGS to systematically investigate the effect of molecular structure and the dispersing mechanism. Further work was involved in evaluating the effect of hydrophobicity, hydrophilicity, and ion strength on AGS. SL with different hydrophobicity and hydrophilicity were prepared by either covalent or noncovalent modification, which would be beneficial for the selection and optimization of dispersant. Meanwhile, raw material of high concentration was applied to obtain AGS of high concentration.

The dispersants used in this work were listed in the Table 2 according to their character (anionic, cationic, and nonionic). Their acronyms were applied in the following text. The aqueous dispersant solutions were prepared by the Milli-Q water. 2.2. Preparation of SL-PEG. 100 g of PEG1000 was heated to 50 °C in a reactor, and then 0.8 g of BF3-Et2O was added into the flask as the catalyst. Subsequently, a certain amount of epichlorohydrin was added dropwise. After dropping off, the reaction was kept at 50 °C for 2 h. Then the PEGchlorohydrins intermediate was generated, and the excess epichlorohydrin was removed by distillation. A mass percentage of 30% of SL solution was adjusted to pH 10−11, and then it was dropped into the PEG-chlorohydrins intermediate at 80 °C for 2 h. After cooling, the pH of the solution was adjusted to 7 with hydrochloride acid, and then the water-soluble SL-PEG copolymer was obtained.31 The mass ratio of PEG1000 and SL of SL-PEG-1, SL-PEG-2 was 0.1 and 1.0, respectively. 2.3. Preparation of SL-C14. C14H29Br (C14Br) and SL with a range of mass ratio were added into a reactor flask equipped with a motor stirrer, a temperature-controlling electric heating device, a peristaltic pump, a thermometer, and a reflux condenser. The sample was adjusted to a pH of 7 with hydrochloric acid after the mixture reacted with each other for 8 h at 70 °C. The remaining C14Br was extracted with petroleum ether, and the residual petroleum ether was removed by distillation; thus, the purified SL-C14 was obtained.32 The mass ratio of C14Br and SL of SL-C14-1, SL-C14-2, and SL-C14-3 was 0.137, 0.275, and 0.550, respectively. 2.4. Preparation of AGS. Given the efficiency of exfoliation, expanded graphite (EG) was obtained through the following method.33 Graphite flakes were mixed with sulfuric acid and nitric acid with a mass ratio of 1:4:1, and the mixture was soaked for 24 h. Then it was washed with pure water until the pH was close to 7. Finally, the products were dried in a vacuum oven of 60 °C for 10 h and then expanded through the microwave with a power of 800 W for 10 s. Dispersants of different dosage were added into a serum bottle with an initial EG concentration of 1 g/L, and then the mixture was put into a bath-type sonication (Xianou Corp. Ltd. Jiangsu) for 100 h with a constant temperature of 30 °C, a power of 200 W, and then centrifuged at 3000g (corresponding to 5000 rpm, which was calculated according to the relative centrifugal field equation,34 RCF = 1.12r (RPM/1000)2, where r = 105 mm for the centrifuger) for 30 min to remove the thick flakes and unexfoliated EG. The supernatant was carefully taken out and diluted with water. Three parallel samples were conducted. The preparation process was illustrated below as Figure 1. 2.5. Molecular Weight and Sulfonic Acid Group Content Determination. Aqueous gel permeation chromatography (GPC) measurements were conducted with TSK gel Super Multipore PW-N. A 0.10 mol/L NaNO3 solution was the eluent at a velocity of 0.50 mL/min. Polystyrenesulfonate sodium with different Mw ranging from 1 to 100 kDa was the calibration standard. All samples were dissolved with Milli-Q water at a ratio of 3 mass % and then filtered by a 0.22 μm filter. The sulfonic acid groups of samples were measured by an automatic potentiometric titrator (Type 809 Titrando, Metrohm Corp., Switzerland). Before titration, the samples were ion-exchanged. 2.6. Measurement of Conductivity. The conductivity of different SL solution was measured by means of a DDS-11A conductivity meter (Rex Instruments Factory, Shanghai)

2. MATERIALS AND METHODS 2.1. Experimental Materials. Graphite flakes were purchased from Sigma-Aldrich (product number 332461). SL was the coproduct of waste sulfite pulping liquid in the papermaking process of pine, produced by Shixian papermaking Corp., Ltd. (Yanbian, Jilin province, China). SL was classified into four fractions: SL1(>50 000 Da), SL2(10 000−50 000 Da), SL3(2500−10 000 Da), SL4(1000−2500 Da) with ultrafiltration membranes with the cutoff Mw of 50 000, 10 000, 2500, and 1000 Da using an ultrafiltration apparatus (Wuxi Membrane Science and Technology Corp., China).30 The average Mw, sulfonic acid groups of four fractions, and SL were listed in Table 1. Table 1. Properties of Different Fractions of SL Solutions Used fractions

Mw (Da)

SL SL1 SL2 SL3 SL4

11350 19900 6000 4300 2360

sulfonic acid group (mmol/g) 2.32 1.67 1.98 2.44 2.96

± ± ± ± ±

0.03 0.07 0.02 0.07 0.05 B

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The Journal of Physical Chemistry C Table 2. List of Dispersants and Their Acronyms Used through the Text dispersant name Anionic Sodium lignosulfonate Sodium cholate hydrate Polysodium-p-styrenesulfonate Sodium dodecylbenzenesulfonate Methylnaphthalene sulfonic formaldehyde Formaldehyde naphthalenesulfonate condensate Sodium salt of polynaphthalenesulfonic acid Sulfanilic acid-phenol-formaldehyde condensate Sulfonated acetone-formaldehyde condensate Sodium dodecyl sulfate Polycarboxylate superplasticizer

acronym

dispersant name

SL SC PSS SDBS MF FDN NNO ASP SAF SDS PC

Cationic Hexadecyltrimethyl ammonium bromide Polydimethyl diallyl ammonium chloride Hexadecyltrimethyl ammonium bromide Cationic polyacrylamides Tetrabutylammonium bromide Nonionic Polyvinylpyrrolidone Polyoxy ethylene nonylphenyl ether Polyoxyethylenesorbitanmonolaurate Polyethylene glycol 2000 Polyethylene glycol 1000

acronym CTAB PDAC CTMAB CPAM TBAB PVP NPE Tween80 PEG2000 PEG1000

Figure 1. Digital pictures of the preparation of AGS.

532 nm for the AGS dispersed by different dispersants. X-ray photoelectron spectra of graphene were carried by an X-ray diffractometer (Kratos Crop, British) using Cu radiation (40 kV, 40 mA) to reveal the oxidization and defect degree. 2.11. Fourier Transform Infrared Spectrometry. Fourier transform infrared (FTIR) spectrometry using Autosystem, XL/I-series/Spectrum 2000 (Thermo Nicolet Co., Madison, WI, USA) was used for infrared spectrum analysis, recording between 4000 and 400 cm−1.

equipped with a DJS-1 or DJS-10 platinum conductance electrode coated with platinum black. The experimental temperature was set at 30 °C. 2.7. Measurement of CG. The dispersants containing a benzene ring, such as SL and SDBS, have the same absorption peak at 280 nm as graphene, which will affect the calculation of CG. To avoid the interference, the absorbance at 660 nm (α660) is detected by UV−vis absorption spectroscopy (UV-245, Shimadzu) three times for each sample, and the mean absorbance is adopted to calculate the concentration using Lambert−Beer behavior: A/l = α660•CG with absorption coefficient α (where l is the cell length, l = 0.01 m, and α = 2460 mL·mg−1·m−1).35 2.8. Surface Charge Measurement. The surface charge of the graphene can be measured in the straightforward form of zeta potential by a Nano-S Zetasizer (Malvern Instruments Ltd., UK) with irradiation from a 633 nm He−Ne laser.36 All the solutions were carried out at the natural pH and 20 °C. Every sample was measured 8 times, and the average value of the zeta potential was calculated. 2.9. Morphology Measurement. The exfoliated graphene was deposited onto silicon substrates for observation by atomic force microscopy (AFM, Park XE-100, Park SYSTEMS Corp, Gyeonggi-Do, Korea) in the tapping mode of operation. In order to ensure the sample was well-distributed and facilitated observation, spin coating was used by a spin coater (WS-400Bz6NPP-LITE, Mycro Technologies Corp, China) with a procedure in three steps: 1000 rpm for 15 s, 2000 rpm for 60 s, and 900 rpm for 15 s. The sample was dried by nitrogen and kept standing for 12 h.15 A high transmission electron microscopy image was performed at 160 kV (TEM, JMF-2100F, HITACHI Corp, Japan). Several drops of AGS were cast onto a microgrid copper (200 mesh) to investigate the morphology of AGS. 2.10. Defect Measurement. Raman spectra was recorded by a Raman microspectroscopic setup (Labram aramis, Horiba Jobin Yvon Corp, French) at a laser excitation wavelength of

3. RESULTS AND DISCUSSION 3.1. Effect of Dispersant Type and Structure on the Concentration of AGS. 3.1.1. Effect of Dispersant Type on the Concentration of AGS. Dispersants used in aqueous dispersion mainly include polyelectrolytes and water-soluble nonionic polymers. Selecting a suitable dispersant is meaningful for AGS. The contents of the functional group, Mw, and dosage of dispersants are the crucial factors for the dispersion properties in different particle systems. According to the previous research, PVP was an excellent dispersant for carbon nanotubes.37 PDAC showed good performance for stabilizing graphene in aqueous suspension.38 As to anionic dispersants, PSS could disperse conducting polymer well, such as poly-3,4ethylenedioxythiophene (PEDOT).39 Figure 2(a) represented the effect of the dosage of PVP, PDAC, and PSS on the concentrations of AGS. According to the detail illustrated in Figure 2(a), CG was proportional to the concentration of both PVP and PDAC. As for the PSS, its profile was not in accordance with PVP and PDAC. It was worth noting that there was a peak of CG at the concentration of 1 g/L for PSS. Obviously, the concentrations of AGS dispersed by PSS were significantly higher than that of PDAC and PVP. A series of dispersants were selected, as shown in Table 2. All the AGSs were prepared at the same condition. Particularly, each anionic dispersant was added into the AGS at its optimum concentration. The concentrations of the cationic C

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Figure 3. Concentrations of AGS dispersed by PVP of different Mw.

linearly with the concentrations of PVP of the same Mw. Meanwhile, the maximum concentrations of AGS were correlated with the increase of Mw of PVP. It suggested that the steric potential was the main driving force to prevent AGS aggregation for the nonionic dispersants. In addition, four SL fractions with different Mw prepared by ultrafiltration were used to investigate the relationship between the structure of SL and the concentrations of AGS. As illustrated in Figure 4, for each SL fraction, the CG first

Figure 2. (a) Concentrations of AGS dispersed by PVP, PDAC, and PSS. (b) Concentrations of AGS dispersed by different dispersants at their optimal concentrations.

and nonionic dispersants were set at 2 g/L, which was a relatively proper concentration. As illustrated in Figure 2(b), anionic dispersants possessed particularly remarkable dispersing performance for AGS compared with other dispersants, especially the SL. The dispersants with straight chain, such as SDS and SAF, showed poorer dispersing performance than those with aromatic structure like SL, SDBS, and PSS. Theoretically, the interaction of naphthalene between graphene was stronger than the interaction of benzene.40 However, the concentrations of AGSs dispersed by NNO and MF (containing naphthalene structure) were lower than SL (containing benzene structure), which elucidated that π−π conjugation was not the crucial role on dispersing AGS. 3.1.2. Effect of Dispersant Structure on the Concentrations of AGS. Future work was involved in understanding the effect of Mw of dispersant on the AGS, and PVPs of a range of different Mw were selected. Similarly, the concentrations of PVP were performed at 1, 2, and 3 g/L, respectively. As revealed in Figure 3, the concentrations of AGS scaled up

Figure 4. Effect of the concentrations of SL fractions on the concentrations of AGS.

increased and then decreased as the dosage of SL fractions increased. Meanwhile, the maximum concentration of AGS dispersed by four SL fractions increased as the Mw of SL fractions decreased. According to several previous reports, the lignosulfonate with higher Mw performed better than that with lower Mw as dispersant in general because of the stronger adsorption and higher steric hindrance.41−43 However, SL4 showed the strongest dispersion in AGS, which may be attributed to its highest sulfonic acid group content, as shown in Table 1. To account for the reason for the decline of CG as the dosage of SL fractions increased, SL and four fractions were ionexchanged to remove the inorganic salts and impurities. The D

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The Journal of Physical Chemistry C conductivity of the samples was shown in Table 3. The ionexchanged SL fractions at their optimum concentration and the Table 3. (a) Conductivity of SL of Different Concentration before Ion-Exchange and after Ion-Exchange and (b) Conductivity of Four SL Fractions of 1 g/L before IonExchange and after Ion-Exchange (a) concentration (g/L)

conductivity before ionexchange (S/m)

conductivity after ionexchange (S/m)

0.5 0.8 1.0 1.5 2.0

0.826 1.392 1.665 2.458 3.359

0.287 0.291 0.302 0.324 0.341

(b) fractions

conductivity before ion-exchange (S/m)

conductivity after ion-exchange (S/m)

SL SL1 SL2 SL3 SL4

1.665 0.387 1.276 2.770 5.982

0.302 0.235 0.284 0.331 0.355

ion-exchanged SL of different concentrations were used to disperse the AGSs, which were contrasted with the samples before ion-exchange. It is evident that the conductivity of the SL fractions after ion-exchange was much lower than before ion-exchange, which is also applicable for SL of different concentrations. As is illustrated in Figure 5(a), CG gradually increased along with the dosage of SL after ion-exchange increased. However, there was a peak of CG for the AGS dispersed by SL before ion-exchange. Meanwhile, the concentration of AGSs dispersed by a different dosage of SL after ion-exchange was obviously higher than SL before ion-exchange. The results in Figure 5 (a) and 5(b) suggested that the high ion strength had a negative impact on dispersing AGS, which accounted for the decline of CG as the dosage of dispersants increased much, such as a high concentration of SL before ionexchange. According to the Debye−Huckel equation, ψ = ψ0· exp(−kx), where 1/k means thickness of the electric double layer and k is the Debye parameter, which will decline with the addition of electrolytes of high concentration or high valence state.44 3.1.3. Adsorption and Dispersion Mechanisms. A general comparison of the effect of the hydrophobicity and the hydrophilicity of the SL on dispersing AGS was evaluated. SL was covalently modified with PEG1000 to improve its hydrophilicity and C14Br to enhance its hydrophobicity, respectively. The FTIR spectra of SL, SL-PEG-1 and SL-C141 were characterized to investigate the change of chemical structure. The areas of the bands 2864 cm−1 correspond to a combination of methylene stretching vibrations from the alkyl chain, and the areas at 1141 cm−1 correspond to the C−O bond stretching vibrations.32 As illustrated in Figure 6, the intensities of −CH2− on SL-C14-1 were stronger than SL, and the intensities of C−O on SL-PEG-1 were stronger compared to SL, which exhibited an efficient substitution of hydrophilicity on SL-PEG-1 and hydrophobicity on SL-C14-1, respectively. The following work was involved in the concentration of AGS dispersed with the above synthetic sample at their optimum

Figure 5. (a) Concentrations of AGS dispersed by SL before and after ion-exchange at different concentrations. (b) Concentrations of AGS dispersed by four SL fractions before and after ion-exchange at their optimal concentrations.

Figure 6. FTIR spectroscopic analyses of SL, SL-PEG, and SL-C14.

E

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of SL4 would be improved when little CTAB was compounded with SL4. When the mass ratio was between 0.05 and 0.23, most of the negative charges were neutralized by CTAB through the strong electrostatic interaction, which would lead to the aggregation of AGS. The CG would continue to increase moderately when CTAB was excessive because the positive charge of SL-CTAB aggregates increased, which is beneficial for dispersing AGS moderately. Figure 9 reflected a linear correlation between the concentrations of AGS dispersed by different fractions of SL

concentration, namely, 0.8 g/L for SL-PEG-1, 1 g/L for SLPEG-2, 1 g/L for SL, 0.8 g/L for SL-C14-1, 1 g/L for SL-C14-2, and 1.2 g/L for SL-C14-3, respectively. As demonstrated in Figure 7, the CG was inversely proportional to the amount of PEG1000 grafted on the SL,

Figure 7. Effect of hydrophobicity and hydrophilicity on the concentrations of AGS.

which suggested that the strong hydrophilicity of SL had a negative impact on dispersing graphene. Meanwhile, the concentration of AGS dispersed by SL-C14-1 was highest, which elucidated that a little C14Br modified on the SL was beneficial, but much C14Br would lead AGS aggregating. It revealed that the main force for SL adsorbing on the graphene was the hydrophobic interaction. Among the four SL fractions, the concentration of AGS dispersed by SL4 was highest. However, the hydrophobic interaction between SL4 and AGS was weakest since SL4 possessed a large amount of sulfonic acid group. In order to enhance the hydrophobic interaction, SL4 after ion-exchange was noncovalently modified by compounding with CTAB, which was a cationic dispersant with a long hydrophobic alkyl chain. The concentrations of AGS dispersed by SL4 with different dosage of CTAB were showed in Figure 8. When the ratio was less than 0.05, the CG increased as the mass ratio of CTAB/SL4 increased because the hydrophobicity

Figure 9. Correlation between zeta potential and concentrations of AGS dispersed by SL fractions.

solutions and zeta potentials of AGS. It indicated that the main force avoiding the graphene from aggregating was the electrostatic potential barrier. Taking together the results, the dispersing mechanism of AGS dispersed by SL was proposed as illustrated in Figure 10. More SL would absorb on the graphene if the hydrophobic interaction improved by increasing a proper amount of hydrophobic structure, and then the electrostatic repulsion provided by the SL played the critical role during the dispersing process.

Figure 8. Concentrations of AGS dispersed by SL4 compounded with different dosage of CTAB.

Figure 10. Scheme showing the proposed mechanism of AGS dispersed by SL solution. F

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The Journal of Physical Chemistry C From the work above, the SL4 fraction noncovalently modified by CTAB at the ratio of 0.05 was the most effective dispersant. Different concentration of EG and graphite was applied to prepare AGS dispersed by the SL4-CTAB. It was worth noting that the CG of AGS was proportional to the concentration of EG and graphite, as illustrated in Figure 11(a) and 11(b). The maximum CG can reach up to 2.8 g/L at an initial EG concentration of 20g/L and 13.5 g/L at an initial graphite concentration of 200g/L, respectively.

Table 4. Comparison of Conditions on the Concentration of AGS Dispersed by Different Dispersants raw material

initial concentration (g/L)

concentration of dispersants (g/L)

graphite graphite graphite

200 5 100

STC (3 g/L) BSA (0.2 g/L) SDS (0.75 g/L) + ethanol (0.08 g/L) SL4 (1 g/L) + CTAB (0.05 g/L) SL4 (1 g/L) + CTAB (0.05 g/L)

EG

20

graphite

200

CG (g/L)

conversion rate (%)

12 0.2 2.1

6 4 2.1

2.8 13.5

14 6.7

ref 16 17 18 our work our work

3.2. Characterization of AGS Dispersed by SL. In the field of thermal and electronics, defect-free graphene is essential for the applications.46 Therefore, the defects of exfoliated graphene should be considered seriously. Raman spectroscopy was performed for the AGSs dispersed by different fractions of SL solutions. The layer number of graphene flake was determined by the intensity ratio of the G band (∼1350 cm−1) relative to the 2D band (∼2680 cm−1). The values of IG/ I2D were 2.2−3.5 for bilayer graphene and 3.5−4.5 for trilayer graphene, and the values were smaller than 1.0 for monolayer graphene.47 The defect content was characterized by the intensity ratio of the D band (∼1350 cm−1) relative to the G band (∼1580 cm−1), where smaller ID/IG illustrated less drawbacks.48 As illustrated in Figure 12, in each case, the spectra displayed prominent D, G, and 2D bands, which were listed in Table 5,

Figure 11. (a) Concentrations of AGS with different initial concentration of EG. (b) Concentrations of AGS with different initial concentration of graphite.

Figure 12. Raman spectrum of AGS dispersed by different fractions of SL.

Further work was involved in a comparison of concentration of AGSs dispersed by different dispersants reported in other papers. Table 4 suggested that the obtaining AGS dispersed by SL4 fraction noncovalently modified by CTAB show a high raw material conversion rate for EG and high concentration for graphite. Meanwhile, SL is nontoxic and shows better water-swollen capacity, which endows it potential in the biomedical field.29,45 Meanwhile, AGS is widely applied in the field of biomedicine. It is apparent that such a high concentration of AGS and the unique properties of SL facilitate it promising to be better applied in the fields of biomedicine, such as drug delivery.

while the ID/IG for the graphene dispersed by SL1, SL2, SL3, and SL4 was 0.06, 0.11, 0.08, and 0.07, respectively. Such small Table 5. IG/I2D and ID/IG of AGS Dispersed by Different Fractions of SL

G

sample

ωD/cm−1

ωG/cm−1

ω2D/cm−1

IG/I2D

ID/IG

SL1/graphene SL2/graphene SL3/graphene SL4/graphene

1330 1332 1329 1331

1579 1580 1580 1581

2682 2681 2682 2680

2.19 2.23 2.17 2.49

0.06 0.11 0.08 0.07

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The Journal of Physical Chemistry C ratios suggested a small quantity of mild defects dominated by point defects on the basal plane during the exfoliating process.49 Meanwhile, the IG/I2D ratios were 2.19, 2.23, 2.17, and 2.49, respectively. Such ratios represented graphene of fewer layers, which mainly consisted of graphene of a monolayer, bilayer, and trilayer. Furthermore, X-ray photoelectron spectroscopy was employed to test for the presence of defects in the form of oxygen functionalities. The high-resolution C1S spectra of AGS dispersed by SL4 indicated the degree of oxidation with the following three components that attributed to carbon atoms in different functional groups, namely, the nonoxygenated ring C (284.8 eV), the C in C−O bonds (286.2 eV), and the carboxylate carbon O−CO (289.0 eV).50 As illustrated in Figure 13, the AGS dispersed by SL4 displayed a very narrow

Figure 14. AFM image of AGS dispersed by the SL4 fraction.

Figure 13. High-resolution C1S X-ray photoelectron spectra for AGS dispersed by SL4 fraction.

and well-defined component at 286.2 and 289.0 eV, which was attributed to the oxidation of graphene or the presence of residual SL on the graphene, because there were several function groups obtaining C−O and O−CO, such as −COOH, which would result in a higher oxygen content than obtained graphene individually. As shown in Table 6, the Figure 15. Height of AGS dispersed by the SL4 fraction.

Table 6. Binding Energy and Percentage of Graphene Dispersed by SL4 functional groups

CC

C−O

−CO

binding energy/eV relative percentages/%

284.8 94.06

286.2 4.98

289.0 0.34

by the SL4 fraction by analyzing and counting the average thickness of 100 individual flakes using multiple 5 μm × 5 μm AFM images over the area of each flake. As illustrated in Figure 16 (the flakes more than 10 layers were ignored in the histogram), a reasonable population of few-layer graphene was distributed: 23% of the flakes were monolayer graphene, and 68% of flakes presented less than three layers. Attributed to the dispersants absorbed on the graphene, it indicated that the graphene was typical of monolayer or bilayer. TEM analysis was carried out to determine the degree of graphene exfoliation and illustrate the detail of surface morphology of AGS dispersed by the SL4 fraction (Figure 17). The transparent submicrometer-sized flakes could be seen evidently, which was typical for graphene of monolayer or fewer layer.

main CC peak accounted for 94.06% of the spectrum, which indicated a low level of oxidation during the exfoliation process, and it was in agreement with the results revealed by Raman spectroscopy. The accurate thickness of graphene could be characterized by atomic force microscopy with typical tapping mode. Shown in Figure 14 was an image of a 5 μm × 5 μm SiO2 substrate by spin-coating of AGS, which confirmed the presence of isolated graphene flakes. The average height of the AGS dispersed with SL4 was about 0.5 nm as evidenced from a histogram shown in Figure 15, which coincided with a theoretical height of a monolayer graphene (0.34 nm).7 Meanwhile, it is effective to estimate the layer number of different flakes of AGS dispersed H

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The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors acknowledge the financial support of International S&T Cooperation Program of China (2013DFA41670), National Natural Science Foundation of China (21376100), Guangdong Province Science and Technology Plan (2013B051000011), and the Fundamental Research Funds for the Central Universities of China (2014ZG0022).

■ Figure 16. Histogram of the numbers of different flake thicknesses of AGS dispersed by the SL4 fraction.

Figure 17. TEM of AGS dispersed by the SL4 fraction.

4. CONCLUSIONS The present paper describes a method and the dispersing mechanism for fabricating high-concentration AGS dispersed by SL through exfoliating from graphite by sonication. The SL fraction with the lowest Mw and highest sulfonic acid group content noncovalently modified by CTAB at the ratio of 0.05 was the most effective dispersant for AGS according to our work. The concentration of AGS can be as high as 13.5 g/L, and the AGSs are essentially free of structural defects. It turned out that the main force for SL adsorbing on the graphene was the hydrophobic interaction, which was confirmed by changing the hydrophilicity of SL through either covalent or noncovalent modification. Meanwhile, the SL after ion-exchange performed better dispersion property for AGS. The electrostatic potential barrier plays the crucial role for AGS dispersed by SL, which is validated by the linear correlation between the concentration of AGS and the zeta potentials. Since the water-swollen capacity of SL can undergo a sharp decline under the acidic environment and the graphene emerged as a functional material in the biomedical field for the biocompatibility, the obtained AGSs dispersed by SL can be processed into potentially useful applications in the biomedical field.

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DOI: 10.1021/acs.jpcc.5b06301 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.5b06301 J. Phys. Chem. C XXXX, XXX, XXX−XXX