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Highly soluble polyetheramine-functionalized graphene oxide and reduced graphene oxide both in aqueous and non-aqueous solvents
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Available at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/carbon
Highly soluble polyetheramine-functionalized graphene oxide and reduced graphene oxide both in aqueous and non-aqueous solvents Myung Jin Yoo, Hyo Won Kim, Byung Min Yoo, Ho Bum Park
*
Department of Energy Engineering, Hanyang University, Seoul 133-791, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history:
Among many methods to synthesize graphene, solution-based processing provides many
Received 27 January 2014
advantages owing to its low cost, high productivity, chemical versatility, and scalability.
Accepted 25 March 2014
In particular, graphene oxide (GO) is one of the most promising nanocarbons that enable
Available online 1 April 2014
the incorporation of graphene and related materials into bulk materials and nanocomposites. GO has hydrophilic nature that enables straightforward dispersion in aqueous solution by sonication, but GO show poor dispersibility in common organic solvents, which prevent much wider applications such as solution-mixing polymer nanocomposites. Here we prepared highly soluble, functionalized GO in both aqueous and non-aqueous solvents. This was achieved by reacting polyetheramine consisting of amphiphilic components, e.g., polypropylene oxide and polyethylene oxide, with carboxylic acid groups at GO edges. Moreover, the reduced GO (rGO) was also highly dispersible in aqueous solution as well as non-aqueous solutions. These functionalized GO and rGO can be used for many solution-processed graphene composites. 2014 Elsevier Ltd. All rights reserved.
1.
Introduction
Graphene is a one-atom-thick, two-dimensional, sp2-bonded carbon material that has been extensively studied for its excellent electrical, mechanical and thermal properties. Due to its excellent physical properties, many attempts have been made to use graphene in potential applications such as electrodes with high surface area and low electrical resistance [1], composites with higher strength to weight ratio [2,3], highfrequency transistor [4], low cost display screen [5], hydrogen storage [6], sensor to diagnose diseases [7], ultracapacitors [8], and membranes [9]. Although micromechanical exfoliation is a straightforward method for high-quality graphene, it lacks sufficient scalability for the bulk production of graphenebased materials. In contrast, solution processing of graphene * Corresponding author: Fax: +82 2 2291 5982. E-mail address: [email protected] (H.B. Park). http://dx.doi.org/10.1016/j.carbon.2014.03.048 0008-6223/ 2014 Elsevier Ltd. All rights reserved.
exhibits several advantages, including utilization of low-cost graphite, no transfer process, scalability for industrial applications, and amenability to blending with polymers or other nanomaterials to form graphene-based nanocomposites. Therefore, excellent dispersion of graphene nanosheets in solutions is of paramount importance because many graphene-based materials and composites are processed by solvent-assisted methods, such as spin-coating, filtration, and layer-by-layer assembly [10]. Graphene oxide (GO) is a highly oxidized graphene sheet, exfoliated from graphite, with many oxygen-containing functional groups such as hydroxyl, epoxide and carboxylic acid groups on the basal plane and at the edges, which is used as a starting material for the synthesis of solution-processed graphene by post-treatments such as chemical or thermal
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reduction [11]. By oxidation method, hydroxyl and carbonyl groups decorate the surface of GO, whose groups provide electrostatic repulsion between GO sheets thus minimizing aggregation in solution, and these oxygen moieties make GO hydrophilic, which enables easy dispersion in aqueous solution through sonication. In contrast, GO is less dissolved in organic solvents, which limits organic solvent-based applications such as polymer/GO composite materials [12]. Many studies have been reported to disperse GO in organic solvents by using surfactants [13] or by high-powered ultrasonication [14] and by chemical modifications [15]. By adding surfactants, however, contamination may occur, and this might cause potential problems due to the surfactant itself in practical applications. Moreover, GO was reported to be easily cracked by ultrasonication, so the GO sizes would be very diverse after ultrasonication [16]. Thus, chemical modifications of GO are required to minimize the defective sites during the preparation. Here we report highly soluble polyetheramine (PEA)-decorated GO and its reduced GO (rGO) in both aqueous and nonaqueous solvents. The PEA used in this study contains primary amino groups attached to the end of a polyether backbone (Fig. 1a). The polyethers in PEA consist of either polypropylene oxide (PPO), polyethylene oxide (PEO) or mixed PPO/PEO. PEA is very soluble in common organic solvents (e.g., alcohols, aromatic hydrocarbons, ether, glycol ethers and ketones) as well as in water [17]. In this work, we primarily used a Jeffamine monoamine (M-1000) consisting of PPO/PEO (3/19 by mole ratio), its average molecular weight in number of about 1000 Da. It was supposed that the dispersion properties of GO and rGO could be much improved if such amphiphilic oligomers were successfully introduced into GO and rGO sheets (Fig. 1e). Considering the fact that GO has carboxyl acid groups at its edges and Jeffamine M-1000 has primary amine groups at its end, the carboxyl acid groups of GO can be reacted with the primary amine groups of Jeffamine M-1000 by aid of 1ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) for amide bond formation. EDC is a water-soluble carbodiimide crosslinking agent, which is commonly used to couple carboxyl groups with primary amines (Fig. 1b) [18]. EDC reacts with carboxylic acid group to form an active ester called o-acylisourea intermediate. The o-acylisourea intermediate is very unstable, and easily hydrolyzes to form carboxylic acid again. N-hydroxysulfosuccinimide (SulfoNHS) helps to avoid the reverse reaction by forming sulfoNHS ester (Fig. 1c). Primary amine reacts with these aminereactive ester to complete amide bonding (Fig. 1d). The EDCinduced amine-carboxyl coupling reaction can be achieved at room temperature, which helps rapid reaction and prevents structural deformation of GO during the reaction. Also, it is known that EDC causes no toxicity issues when used in biomaterials [18].
GO. Sulfuric acid (95%) was purchased from Dae Jung Chem. (Gyeonggi-do, Korea) and potassium permanganate (99.3%) was purchased from Junsei Chem. (Tokyo, Japan) as oxidizing agents for graphite. Hydrogen peroxide (30%) was purchased from Sigma Aldrich (St. Louis, MO, USA) for excess KMnO4 removal during GO preparation. The glass fiber filter and Anodisc membrane (pore sizes of 0.1 and 0.2 lm) were purchased from WHATMAN (Maidstone, England, UK) for filtration. Jeffamine M-1000 was kindly supplied from Huntsman Corp. (Salt Lake, Utah, USA) for PEA-GO preparation. 1-Ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC) was purchased from Thermo Fisher Scientific Corp. (Rockford, IL, USA) and N-hydroxysuccinimide (NHS) was purchased from Sigma Aldrich for functionalizing GO with Jeffamine M-1000. Hydrochloric acid (35%) and sodium hydroxide (98%) were purchased from Dae Jung Chem. to adjust pH level. Acetone (99.5%), DMF (99.5%) and THF (99%) were purchased from Dae Jung Chem., while methanol (99.8%), ethanol (99.5%), 2-propanol (99.5%), DMSO (99.9%), NMP (99%), and pyridine (99.8%) were purchased from Sigma Aldrich for dispersibility measurement.
2.2.
Synthesis of GO and PEA-GO
2.
Experimental section
GO was synthesized by Hummer’s method using H2SO4 and KMnO4 as oxidizing agents. Ten grams of graphite was dissolved in 450 ml of H2SO4, which was subsequently stirred for 90 min at 5 C. Afterwards, 1.5 g of KMnO4 was added and the solution was stirred for another 90 min. 30 g of KMnO4 was mixed slowly, which caused a color change from black to dark green. This solution was stirred for 1 h at 5 C, then stirred at 40 C for another 1 h. 450 ml of deionized water were added carefully by dropping funnel to prevent rapid temperature rise. The temperature was increased to 80 C and maintained while stirring for 50 min. The reaction was terminated by a final addition of 265 ml deionized water and 35 ml of 30 wt% H2O2. The final product was filtered with glass fiber filter and the resultants were washed three times with 10 wt% HCl to remove excess salts. For further purification, the final product was filtered in 5 L of acetone three times, and then dried in a vacuum oven at 40 C for 1 day. The obtained dark brownish powder is GO. The GO powder was dispersed in water (1 mg/mL) and bath-sonicated for 1 h. Afterwards, the pH was adjusted to 10.5 by 1 M NaOH for better dispersion and probe-sonicated at 285 W for 6 h. The final product is the GO solution. 200 mg of the GO solution was mixed with 150 mg of Jeffamine M-1000 and stirred for 24 h at room temperature, and pH was adjusted to 4.5 by 1 M HCl. Then, 200 mg of EDC and NHS were added and bath-sonicated for 30 min, followed by stirring for 24 h. The resultant was filtered through Anodisc (pore size 0.2 lm) with 1 M HCl and acetone for purification. The dark-brown powder was dried in a vacuum oven at 40 C. Eventually, PEA-GO was obtained as blackish powders.
2.1.
Materials
2.3.
Purified synthetic graphite powder (99.99%, SP-1) was purchased from the Bay Carbon Inc. (Bay City, MI, USA) to prepare
Dispersibility evaluation
GO and PEA-GO were dispersed in various solvents such as water, acetone, methanol, ethanol, 2-propanol, ethylene
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b
c
d
e
phene Oxide (GO O) Grap
PE EA Fun nctionalizzed GO O
:P Poly yeth hera amiine (PE EA)
:O
H : OH
Fig. 1 – Chemical structures of Jeffamine M-1000 (a), EDC (b) and Sulfo-NHS (c). (d) EDC coupling reaction of carboxylic acid and primary amine. (e) Proposed dispersion properties of GO and PEA-GO in organic solvents. The dispersion properties of PEAGO were improved as compared to GO by functionalizing PEA to the carboxyl groups of GO. (A color version of this figure can be viewed online.)
glycol, DMF, DMSO, NMP, pyridine and THF to observe relative dispersibility. 10 mg of GO and PEA-GO were dispersed in 20 mL of each solvent and bath-sonicated for 1 h at 40 C. After sonication, each solution was left for two months and observed dispersion states. After two months, each upper solution was filtered through weight-measured Anodisc (pore size 0.1 lm) and the weight of membrane was measured again to calculate concentrations after complete drying in a vacuum oven. Pre-tests proved that the weight loss through a 0.1 lm pore sized Anodisc membrane is less than 1%, which is insignificant. The maximum dispersibility of GO and PEA-GO was also estimated by the following method. GO and PEA-GO were dispersed in water (30 mg/mL), methanol (2 mg/mL), ethanol
(2 mg/mL), DMF (30 mg/mL) and DMSO (20 mg/mL), and bath sonicated for 1 h at 40 C. The solutions were allowed to settle for 24 h, and then centrifuged for 20 min at 1000 rpm. After centrifugation, the upper 1/3 of the solutions were collected and filtered by a 0.1 lm pore-sized Anodisc membrane. The concentrations of each solution were measured using the same method stated above. GO and PEA-GO (100 mg) were dispersed in 400 mL water by bath-sonication for 1 h to achieve a homogeneous dispersion state. Hydrazine (100 ll) was added to the dispersed solutions, and the solutions were heated to 95 C. After 1 h, the solutions were washed 5 times in a methanol and water (5:1) mixed solution to remove excess hydrazine. The resultants were dried in a vacuum oven
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at 40 C for 1 day. The final black powder was rGO and PEA-rGO. Both rGO and PEA-rGO (10 mg) were dispersed in water, acetone, methanol, ethanol, 2-propanol, ethylene glycol, DMSO, DMF, NMP, pyridine and THF (10 mL each) by bath sonication for 1 h at 40 C to observe dispersibility values.
2.4.
UV/Vis absorbance spectra were measured by a SPECORD 200 (Analytic Jena AG, Jena, Germany) with 10 mm SiO2 cuvette in the range of 190–600 nm.
3.
Results and discussion
3.1.
Synthesis of PEA-GO
Material characterizations
Fourier transform infrared (FT-IR) spectra of GO and PEA-GO were measured using a NICOLET 6700 (Thermo Fisher Scientific, IL, USA) in the range of 4000–1000 cm1. X-ray photoelectron spectroscopy (XPS) spectra were measured using an Omicron ESCALAB (Omicron, Taunusstein, Germany) with a monochromatic Al Ka (1486.8 eV) 300 W X-ray source, and a flood gun to counter charging effects under ultra-high vacuum (UHV, 1 · 109 Torr) conditions. Raman spectroscopy was performed with MonoRa750i (Dong Woo Optron, Gyeonggi-do, Korea) in the range of 1000–2000 cm1. The zeta potentials were measured with a Zetasizer Nano ZS (Malvern Instrument, Malvern, UK) at 25 C isotherm for 2 min. The
a
GO was prepared by using the modified Hummers method [19], and PEA was attached to GO by EDC coupling reaction between carboxylic acid and primary amine. Fig. 2a shows FT-IR spectra of GO and PEA-GO. Characteristic absorption bands of GO were observed in both GO and PEA-GO at 3430 cm1 (CO–H stretching vibrations), 1630 cm1 (in plane C@C stretching vibrations), 1230 cm1 (C–O–C stretching vibrations) and 1060 cm1 (C–OH stretching vibrations) [12,15,20]. The results show that most of the oxygen-containing functional groups of GO still exist after the reaction. FT-IR spectra of PEA-GO exhibit a slight difference from GO due to the substitution of carboxyl groups with amide bonds. Two absorption bands in GO at 1713 cm1 (C@OOH stretching vibrations) and
b C 1s
C-C C-O-C
GO
C=C
PEA-GO
Transmittance (-)
N1s C-OH C-O-C OC-NH
-CH2-
CON-H (stretch)
C=ONH C=C
CO-H
(aromatic) CON-H (bend)
408
400
392
GO C=O O=C-OH -CH2-
C=OOH C=C
CO-H
3500
(aromatic)
3000
2000
C-OH C-OOH C-O-C
1500
1000
294 292 290 288 286 284 282 280
-1
Wavenumbers (cm )
Binding Energy (eV)
d
c C 1s
C=C
PEA-GO
C-C C-O-C
408
400
392 C=O
Intensity (a. u.)
N1s
PEA-GO
D/G = 0.85
GO
D/G = 0.84
O=C-NH
294 292 290 288 286 284 282 280
Binding Energy (eV)
1000
1200
1400
1600
1800
-1
Raman shift (cm )
Fig. 2 – Structure analyses of both GO and PEA-GO. (a) FT-IR spectra, (b) C 1s and N 1s core level spectra of XPS: GO, (c) C 1s and N 1s core level spectra of XPS: PEA-GO and (d) D and G bands in Raman spectra. (A color version of this figure can be viewed online.)
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1370 cm1 (C–OOH stretching vibrations) were not observed in PEA-GO. Instead, major bands due to amide bonds in PEA-GO appeared at 3744 cm1 (CON–H stretching vibrations), 1699 cm1 (C@ONH stretching vibrations), 1559 cm1 (CON–H bending vibrations) and 1458 cm1 (OC–NH stretching vibrations). These bands clearly show that Jeffamine M-1000 was reacted to the carboxyl groups of GO edges [20–22]. The reaction in PEA-GO was also confirmed by XPS: The C 1s core-level spectra of both GO and PEA-GO are presented in Fig. 2b and c. Two intense peaks at 284.5 and 286.4 eV, with a relatively small peak at 287.6 eV were observed in both cases. The fact that the two clear maxima in the C 1s band are separated by about 2 eV corresponds to the values in the literature for C 1s band shapes [23,24]. The two maxima located at 284.5 and 286.4 eV are attributed to C@C and C–O–C, respectively [12]. The peak at 286.4 eV is also due to defects in the basal plane of GO (CAC species) since binding energies of CAO and CAC bonds are very close, less than 0.1 eV [25,26]. For PEA-GO, the integral area of the Gaussian peak at 286.4 eV increased from 31.4% (GO) to 42.2%. This result can be elucidated by the fact that Jeffamine M-1000 consists of sp3 CAC and CAO bonds, which contribute to an increase of CAC and CAO bonds in PEA-GO as compared to original GO. Another significant difference in XPS data is the disappearance of the peak at 290.4 eV. The GO peak at 290.4 eV (O@C–OH) was not observed in PEA-GO, while the peak at 288.2 eV (O@C–NH) appeared only in PEA-GO. Typically, C–N peak is shown at 285.2 eV but C@O in amide bond neighboring CAN is known to shift the peak about 2.97–3.59, which coincides with our experiment [27]. The N 1s core-level spectra of PEA-GO show a peak at 400 eV, which prove the existence of CAN bonding whereas GO does not. One more thing to be noticed was also observed in XPS analyses of GO and PEA-GO, which is the decreased amount of CAO bondings. Originally, EDC reaction is known to couple carboxyl groups with primary amines selectively. The oxygen functional groups of GO, however, may cause another possible reaction site of PEA. Among the oxygen groups in GO, CAO bonding is the most highly expected possible reaction site for primary amines because of the weak sp2 bond in CAO. As far as the fact that decreased CAO bondings were found in XPS analyses is concerned, it can be inferred that unintended basal plane functionalization was occurred during the reaction. The amount of basal functionalization is, however, predicted to be quite low considering that all other functional groups of GO were also found in PEA-GO except carboxyl groups. Thus, FT-IR and XPS analyses suggest the evidence of reaction between carboxyl acid groups of GO and the amine groups of Jeffamine M-1000. Fig. 2d shows Raman spectra of GO and PEA-GO. Two peaks were observed at 1345 and 1590 cm1 in both GO and PEA-GO, which are referred to as D and G peaks, respectively. The D peak is related to defects and disorder sites of GO, due to the breathing modes of A1g symmetry in sp2 systems. The G peak is observed in all sp2 hybridized carbon materials, and it results from E2g symmetry vibrations in plane [28]. Therefore, the D/G ratio provides information about the degree of GO defect sites, and we found that the D/G ratio of PEA-GO is very similar to the original D/G ratio of GO (0.85), indicating no significant structural deformation during the reaction.
3.2.
153
Dispersion property of PEA-GO
Fig. 3a shows the photo images of as-prepared GO and PEAGO dissolved in water and organic solvents after two months, and the results are summarized in Table 1. In particular, the enhanced solubility in organic solvents was observed in PEA-GO solutions. The improved dispersibility in organic solvents is ascribed to the presence of PEA at GO edges. In advance of GO and PEA-GO dispersibility experiments in organic solvents, it should be noted that primary alcohols such as methanol, ethanol and propanol can be oxidized by GO itself. GO is known for an oxidant for primary alcohols at about 100 C from previous studies [29,30]. We did the dispersion experiments at relatively low temperature at 40 C to minimize the inadvertent reduction of GO and PEA-GO. The dispersion properties were found to be greatly improved, especially in aprotic solvents such as dimethylformamide (DMF), dimethylsulfoxide (DMSO) and N-methyl-2-pyrrolidone (NMP). A slightly reduced dispersibility was observed in water due to the neutralization of GO edges by the introduction of PEA, but the PEA-GO still maintained good stability even in water. Zeta potentials were measured to quantify the stability of GO and PEA-GO in different solvents. Zeta potential is the electric potential difference between the interfacial electric double layer at the location of the slipping plane and the dispersed medium, so zeta potential is an indicator to the dispersion of colloidal particles. Generally, colloids are stabilized when the zeta potential is less than 30 mV [31]. GO has a negative zeta potential due to its oxygen-containing functional groups, which increases repulsive forces between GO particles, and contributes to adequate dispersion in water. In general, GO has 40 60 mV zeta potentials in water, sufficient to maintain a stable solution. The zeta potentials of GO and PEA-GO in solvents are shown in Fig. 4. Zeta potentials of GO and PEA-GO in water show 54.2 and 50.3 mV, respectively, at pH 10. This suggests that carboxylic groups of GO are more favorable to dispersion in water. However, PEA-GO still maintained excellent dispersibility despite a slight increased zeta potential compared to GO. In most solvents, the zeta potentials of GO could not be measured due to its poor dispersion. DMSO was one of a few organic solvents for GO exhibiting relatively good dispersion. In sharp contrast, PEA-GO exhibit excellent dispersibility in most aprotic solvents, which corresponds to the results above. UV/Vis absorptivity and maximum dispersibility of GO and PEA-GO in common organic solvents were measured to compare the dispersion properties of GO and PEA-GO. As discussed above, PEA-GO was highly dispersed in aprotic solvents as compared to protic solvents. In this study, water, methanol, and ethanol were evaluated as protic solvents, while DMSO and DMF as aprotic solvents. The relationship between UV/Vis absorptivity and solution concentration was proposed by Beer and Lambert. The Beer–Lambert law can be expressed as: A¼ecl
ð1Þ
where A is the measured absorbance, e is the absorptivity coefficient, l is the path length, and c is the solution concentration. The absorbance is proportional to the concentration
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a
b Water (GO) Water (PEA-GO)
30 25
DMF (PEA-GO)
A/l (m-1)
20 Methanol (PEA-GO)
15 10
DMSO (GO) DMSO (PEA-GO) Ethanol (GO) Ethanol (PEA-GO)
5 0 0.000
0.002
0.004
0.006
0.008
C (g/L)
Fig. 3 – Dispersion properties of GO and PEA-GO in different solvents. (a) Photo images of GO and PEA-GO solutions (0.5 mg/ mL) sonicated for 1 h at 40 C after 50 days. (b) UV/Vis absorptivity spectra of GO and PEA-GO in water, methanol, ethanol, DMF and DMSO. (A color version of this figure can be viewed online.)
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Table 1 – Concentrations of GO and PEA-GO in different solvents.
GO PEA-GO
-60
Water Acetone Methanol Ethanol 2-Propanol Ethylene glycol DMSO DMF NMP Pyridine THF
Concentration (mg/mL) GO
PEA-GO
0.36 NA NA 0.09 NA 0.17 0.35 0.13 NA NA NA
0.33 0.10 0.03 0.13 0.05 0.43 0.38 0.47 0.44 0.38 0.09
of a solution and absorptivity coefficient when the path length is fixed, so the absorptivity coefficient can be estimated by measuring UV/Vis absorbance at different analyte concentrations. The GO and PEA-GO absorptivity coefficients were measured to compare the dispersion properties in different solvents. The absorptivity coefficient is a wavelengthdependent parameter, so wavelength is important in calculating the absorptivity coefficient. Generally, the linear relationship between concentration and absorbance is well-satisfied with the Beer–Lambert law at the wavelength at which maximum absorbance occurs. GO and PEA-GO were initially dispersed in water and UV/Vis absorbance was measured to
-50 Zeta potential (mV)
Solvents
-40 -30 -20 -10 0 ter ne OH OH nol EG MSO DMF NMP dine THF ri Wa ceto Me Et ropa D Py A 2-P
Fig. 4 – Zeta-potentials of GO and PEA-GO in different solvents. (A color version of this figure can be viewed online.)
determine the wavelength at which maximum absorbance occurs. Fig. 5 shows the UV/Vis absorptions at 231 nm and weak shoulder at 298 nm. Slight difference between the absorbance is due to the different absorptivity coefficients as discussed below. From previous studies of UV/Vis spectroscopy, it can be inferred that the absorbance near 230 nm is dominated by p–p* transition and the shoulder near 300 nm is due to n–p* transition [32]. The possibilities of electron
CARBON
GO Water
b
-1
40 38 36 34 32 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 1.7 mg/L 0 200 300
A/l (m )
-1
A/l (m )
a
7.4 mg/L
400
500
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600
40 38 36 34 32 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 0 200
PEA-GO Water
7.1 mg/L
1.4 mg/L
300
Wavelength (nm)
d GO DMSO
-1
26 24 22 20 18 16 14 12 10 8 6 4 2 0
A/l (m )
-1
A/l (m )
c
7.1 mg/L
1.4 mg/L
300
400
400
500
600
Wavelength (nm)
500
26 24 22 20 18 16 14 12 10 8 6 4 2 0
PEA-GO DMSO
7.1 mg/L
1.4 mg/L
300
600
400
500
600
Wavelength (nm)
Wavelength (nm)
Fig. 5 – UV/Vis absorption spectra of GO and PEA-GO in water (a and b) and DMSO (c and d). (A color version of this figure can be viewed online.)
transfer from n to p* is much less than from the p to p* transition, so the shoulder near 300 nm shows relatively weak intensity as compared to the absorbance at 230 nm. The major absorbance at 230 nm can be elucidated by the two conjugation effects: one for the sp2 conjugated carbons in plane, and two for conjugated chromophore units such as C@O with conjugated carbon planes. The more highly conjugated GO (less defect ratio) shows a p–p* transition at a longer wavelength (bathochromic shift), while the less conjugated GO shows a p–p* transition at a shorter wavelength (hypsochromic shift). The absorptivity coefficients of GO and PEA-GO in water were measured at 231 nm, but this absorbance of p–p* transition was very weak in organic solvents. This is because the strong absorbance of organic solvents occurs between 200 and 300 nm, which hinders the absorbance of materials.
Therefore, the absorptivity coefficients of GO and PEA-GO in organic solvents were measured at the wavelengths at which n–p* transitions occur at 280–320 nm. Solvent polarities result in slight absorbance differences between the solvents. The measured absorptions as well as linear relationships of concentration and absorption at p–p* (water) and n–p* (organic solvents) transitions are shown in Fig. 3b. Table 2 lists the measured absorptivity coefficients. GO was less dispersed in methanol and DMF, and so measuring absorptivity coefficients in these solvents was difficult. The absorptivity coefficients increase as the dispersibility is improved. Eventually, the measured maximum dispersibility values of GO and PEA-GO are listed in Table 3. GO was poorly dispersed in methanol, so severe aggregation was observed in our experiment. The dispersion of GO was observed in ethanol
Table 2 – Absorptivities of GO and PEA-GO in different solvents (L/g m).
Table 3 – Maximum dispersibilities of GO and PEA-GO in different solvents (g/L).
GO PEA-GO
Methanol
Ethanol
DMF
DMSO
Water
– 1874
512 2000
– 3053
1923 2717
4869 4615
GO PEA-GO
Methanol
Ethanol
DMF
DMSO
Water
– 0.18
0.21 0.87
0.28 24.3
8.34 12.9
21.4 17.6
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(0.21 g/L), and DMF (0.28 g/L). The maximum dispersion concentration of PEA-GO was measured in methanol (0.18 g/L), ethanol (0.87 g/L), DMF (24.3 g/L) and DMSO (12.9 g/L). Among them, PEA-GO dispersibility in DMF was improved by more than 8600% when compared to GO. The maximum dispersion concentration of PEA-GO in water (17.6 g/L) was slightly
decreased as compared to GO (21.4 g/L), but still maintained good dispersion. The theoretical explanation for high dispersibility of PEA-GO in DMF and water will be discussed in detail in terms of solubility parameter. To analyze the dispersibility of PEA-GO, a Hildebrand solubility parameter was also estimated. The Hildebrand solubil-
b
a PEA-GO GO
0.4 0.3 0.2 0.1 0.0 10
20
30
40
0.3 0.2 0.1
50 1/2
Hildebrand Solubility Parameter δT (MPa
c
0
Concentration (mg / mL)
0.2 0.1
30
40
50
PEA-GO GO
0.5
0.3
20
1/2
d
0.4
10
Dispersion Parameter δD (MPa )
)
PEA-GO GO
0.5
Concentration (mg / mL)
0.4
0.0 0
0.0
0.4 0.3 0.2 0.1 0.0
0
10
20
30
40
50
0
1/2
10
20
30
40
50 1/2
Polar Parameter δP (MPa )
e
PEA-GO GO
0.5
Concentration (mg / mL)
Concentration (mg / mL)
0.5
Hydrogen Bonding Parameter δH (MPa )
f
Fig. 6 – The concentrations of GO and PEA-GO after 50 days in different solvents as a function of the (a) Hildebrand solubility parameter, (b) dispersion parameter, (c) polar parameter and (d) hydrogen bonding parameter. Solubility parameter diagrams of (e) GO and (f) PEA-GO. (A color version of this figure can be viewed online.)
CARBON
Table 4 – Hansen solubility parameters of solvents, GO and PEA-GO.
Water Acetone MeOH EtOH 2-Propanol Ethylene glycol DMSO DMF NMP Pyridine THF GO PEA-GO
dD
dP
dH
15.6 15.5 15.1 15.8 15.8 17.0 18.4 17.4 18.0 19.0 16.8 15.6–18.4 15.5–19.0
16.0 10.4 12.3 8.8 6.1 11.0 16.4 13.7 12.3 8.8 5.7 16.0–16.4 8.8–17.0
42.3 7.0 22.3 19.4 11.0 26.0 10.2 11.3 7.2 5.9 8.0 10.2–42.3 5.9–42.3
ð4Þ
where dD, dP and dH are the dispersive, polar and hydrogen bonding solubility parameters, respectively. In this respect, the heat of mixing also can be expressed as [33]: h i ð5Þ DHM ¼ u1 u2 VM ðdD;1 dD;2 Þ2 þ ðdP;1 dP;2 Þ2 þ ðdH;1 dH;2 Þ2
ð2Þ
where V is the molar volume of the pure solvent and E is its energy of vaporization. E/V is also referred to as the cohesive energy density. Hildebrand and Scott proposed the heat of mixing as [33]: DHM ¼ u1 u2 VM ðd1 d2 Þ2
single factor to describe complicated dispersion properties. For more specific information about dispersion properties of GO and PEA-GO in different solvents, specific types of the interactions should be considered. There are generally three factors that contribute to dispersibility in a solvent: dispersion, polarity and H-bonding interactions. The Hansen solubility parameters interpret dispersion properties as three parameters according to the types of interactions. Each solubility parameter is defined as the square root of the contribution to the cohesive energy density. The relationship between the Hansen solubility parameters and the Hildebrand solubility parameter follows the equation [33]: d2 ¼ d2D þ d2P þ d2H
ity parameter is a useful tool for the simple prediction of the solubility of unknown substances. Hildebrand solubility parameter is defined as [33]: d ¼ ðE=VÞ1=2
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ð3Þ
where d1 and d1 are the Hildebrand solubility parameters of the solute and solvent, respectively, and u1 and u2 are volume fractions of solvent and solute, respectively. VM is the volume of the mixture. This equation suggests that dispersion is favored when the solvent and solute parameters match closely with each other. The concentrations of PEA-GO and GO after two months in each solvent were plotted as a function of the Hildebrand solubility parameters. According to the correlation equation of mixing enthalpy and Hildebrand solubility parameter above, GO and PEA-GO might be more dispersed in solvents that have similar solubility parameters. Fig. 6a shows that PEA-GO is well dispersed in solvents with Hildebrand solubility parameter ranging 21–48, whereas GO shows good dispersion properties in the range 26–48. The Hildebrand solubility parameter indicates quantitative information about the dispersibility of GO and PEA-GO, but it is too rough to describe the dispersion properties by a single factor. The Hildebrand solubility parameter is defined just as the cohesive energy density of the solvent and solute, which suggests that all the interactions are combined by a
This equation denotes that dispersion is favored when each solvent and solute parameter are similar. The plots of each dispersed concentration as a function of each Hansen solubility parameter are presented in Fig. 6b–d. Three Hansen solubility parameters have ranges, whether broad or narrow, similar to the Hildebrand solubility parameter. This is due to the amphiphilic structures of GO and PEA-GO, which have large non-polar hydrophobic carbon sites and polar (hydrophilic) oxygen-containing groups. The diverse size distribution also contributes to the broad range of solubility parameters. GO shows good dispersion in solvents where the dispersion parameter was between 15.6 and 18.4, the polar parameter was between 16.0 and 16.4 and the hydrogen bonding parameter was between 10.2 and 42.3, while PEA-GO is well-dispersed in solvents where the dispersion parameter was between 15.5 and 19.0, the polar parameter was between 8.8 and 16.4 and the hydrogen bonding parameter was between 5.9 and 42.3 (Table 4). Fig. 6 shows that all three parameters ranges were more broadened after PEO modification. Among them, the polar parameter range was increased greatly from 16–16.4 to 8.8–16.4. The broad solubility parameters help PEA-GO disperse well in many solvents as compared to GO. The hydrogen bonding parameters of both GO and PEAGO shows relatively broad ranges when compared to the other parameters, and this means hydrogen bonding interactions affect dispersibility less than van der Waals interactions or polar interactions. This is in accordance with the observations that PEA-GO is more dispersive in aprotic solvents than in protic solvents, and GO is dispersive in both a highly protic solvent (water) and an aprotic solvent (DMSO). Figs. 6e and f
Table 5 – Theoretical solubility parameter of PEA by group contribution method.
–CHNH2 –CH3 –CH< –CH2– –CH2O–
Number of segment
Contribution to dD
Contribution to dP
Contribution to dH
dD
dP
dH
1 4 2 19 22
0.0112 0.9714 0.645 0.0269 0.031
1.1989 1.6448 0.6491 0.3045 0.8826
–
14.9
14.5
0.6
0.29901 0.1386 0.1161 –
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a
A/l (m-1)
b
70 65 60 DMSO (PEA-rGO) DMF (PEA-rGO) 55 DMSO (rGO) DMF (rGO) 50 45 Ethanol (PEA-rGO) 40 Ethanol (rGO) 35 30 25 20 15 10 Water (PEA-rGO) Water (rGO) 5 0 0.000 0.004 0.008 0.012 0.016 C (g/L)
Fig. 7 – Dispersion properties of rGO and PEA-rGO in different solvents. (a) Photo images of bath sonicated rGO and PEA-rGO solutions (0.5 mg/mL) for 1 h at 40 C. (b) UV/Vis absorptivity spectra of rGO and PEA-rGO in water, ethanol, DMF and DMSO. (A color version of this figure can be viewed online.)
3.4.
Synthesis of PEA-rGO and its dispersibility
functionalized to carboxyl groups at the GO edges. The dispersibility values of rGO and PEA-rGO in organic solvents are presented in Fig. 7. PEA-rGO still maintains well dispersion properties in both aqueous and non-aqueous solvents. Moreover, PEA-rGO has similar electric conductivities with GO when thermally treated at 600 C (Fig. 8). Graphene nanocomposite materials are highly desirable in material sciences due
3500
/ Square)
- Ar atmosphere
Sheet Resistance (
show the three parameter positions of GO and PEA-GO, respectively. Each area is proportional to the concentration, which shows the dispersive region as a function of three parameters. In particular, both GO and PEA-GO show limited regions of dispersion and the polar parameter, but they show broad ranges of the hydrogen bonding parameter. The PEA solubility parameter was also calculated to analyze the contribution of PEA to PEA-GO dispersibility. Table 5 lists the PEA solubility parameter by group contribution method [34]. The calculated solubility parameter of PEA is found to have most similar dD and dP with water and DMF, and it coincides with the maximum dispersibility of PEAGO. The similarity of dH between PEA and PEA-GO, however, is quite low when compared to both dD and dP, and hydrogen bonding less affects the dispersion properties of PEA-GO. The reason that PEA-GO shows broader range of solubility parameter can be interpreted as the effect of PEA segments, which make PEA-GO more dispersive in different organic solvents, while GO does not contain PEA segments. Particularly, similar dispersion and polar parameters of PEA with water and DMF make PEA-GO highly soluble in both solvents.
3000
PEA-rGO - Temperature raise 5
2500 2000 1500 rGO
250 200 150 100 50 0 0
GO and PEA-GO were chemically reduced to rGO and PEA-rGO by using hydrazine. Hydrazine is commonly used for GO reduction, and reduces particularly epoxide groups in the basal plane. We suggested that the dispersibility of PEA-GO be maintained after hydrazine reduction, because PEO is
per min
100 200 300 400 500 600 o
Annealing Temperature ( C) Fig. 8 – Electrical properties of rGO and PEA-rGO as a function of thermal treatment temperature. (A color version of this figure can be viewed online.)
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to their extremely enhanced electrical, thermal and mechanical properties in many applications, but the manufacture of graphene nanocomposite materials has been very limited because of the low dispersibility of GO in organic solvents. In this respect, PEA-GO is highly expected to be a breakthrough in the preparation of various graphene nanocomposite materials based on solution processing.
4.
Conclusions
PEA was successfully functionalized to the carboxyl groups of GO, and the PEA-GO was found to be highly dispersive both in aqueous system and non-aqueous system due to the amphiphilic properties of PEA used in this study. The dispersibilities of GO and PEA-GO were measured by concentration, zeta potential, maximum dispersibility and absorptivity coefficient. PEA-GO was found to have drastically enhanced dispersibility, especially in polar aprotic solvents such as DMF. The Hildebrand solubility parameters and Hansen solubility parameters of GO and PEA-GO were also experimentally measured to compare their dispersion properties, and both GO and PEA-GO demonstrated specific ranges of solubility parameters. The dispersion properties of GO and PEA-GO were found to be strongly affected by dispersion and polar interactions, rather than hydrogen bonding interactions. The measured solubility parameters are expected to be used in the prediction of the solubility in other solvents for GO applications. The PEA-rGO was also proven to have similar dispersion properties with PEA-GO. We expect that PEA-GO and PEA-rGO would be good candidates in the preparation of graphenebased composites or hybrid materials for promising applications such as energy storage devices, gas sensors, membranes and biomedical materials for drug delivery systems.
Acknowledgements This work was supported by the Korea CCS R&D Center (KCRC) grant funded by the Korea government (Ministry of Science, ICT & Future Planning) (no. 2012-0008907), and M.J.Y. H.W.K. and B.M.Y. appreciate the financial support from ‘‘Future convergence energy-leaders BK21+ program’’ (10Z20130012477) of the Ministry of Education (MOE).
R E F E R E N C E S
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[5] Eda G, Lin YY, Mattevi C, Yamaguchi H, Chen HA, Chen I, et al. Blue photoluminescence from chemically derived graphene oxide. Adv Mater 2010;22:505–9. [6] Lee H, Ihm J, Cohen ML, Louie SG. Calcium-decorated graphene-based nanostructures for hydrogen storage. Nano Lett 2010;10:793–8. [7] Shao Y, Wang J, Wu H, Liu J, Aksay IA, Lin Y. Graphene based electrochemical sensors and biosensors: a review. Electroanalysis 2010;22:1027–36. [8] Stoller MD, Park S, Zhu Y, An J, Ruoff RS. Graphene-based ultracapacitors. Nano Lett 2008;8:3498–502. [9] Kim HW, Yoon HW, Yoon SM, Yoo BM, Ahn BK, Cho YH, et al. Selective gas transport through few-layered graphene and graphene oxide membranes. Science 2013;342:91–5. [10] Li D, Kaner RB. Graphene-based materials. Nat Nanotechnol 2008;3:101. [11] Dreyer DR, Park S, Bielawski CW, Ruoff RS. The chemistry of graphene oxide. Chem Soc Rev 2010;39:228–40. [12] Paredes J, Villar-Rodil S, Martinez-Alonso A, Tascon J. Graphene oxide dispersions in organic solvents. Langmuir 2008;24:10560–4. [13] Lotya M, King PJ, Khan U, De S, Coleman JN. Highconcentration, surfactant-stabilized graphene dispersions. ACS Nano 2010;4:3155–62. [14] Cravotto G, Cintas P. Sonication-assisted fabrication and post-synthetic modifications of graphene-like materials. Chem Eur J 2010;16:5246–59. [15] Stankovich S, Piner RD, Nguyen ST, Ruoff RS. Synthesis and exfoliation of isocyanate-treated graphene oxide nanoplatelets. Carbon 2006;44:3342–7. [16] Pan SY, Aksay IA. Factors controlling the size of graphene oxide sheets produced via the graphite oxide route. ACS Nano 2011;5:4073–83. [17] Burton B, Alexander D, Klein H, Vasquez AG, Pekarik A, Henkee C. Epoxy formulations using Jeffamine polyetheramines. Huntsman Corp; 2005. [18] Lee J, Edwards H, Pereira C, Samii S. Crosslinking of tissuederived biomaterials in 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC). J Mater Sci Mater Med 1996;7: 531–41. [19] Hummers Jr WS, Offeman RE. Preparation of graphitic oxide. J Am Chem Soc 1958;80:1339. [20] Zhang W, Guo Z, Huang D, Liu Z, Guo X, Zhong H. Synergistic effect of chemo-photothermal therapy using PEGylated graphene oxide. Biomaterials 2011;32:8555–61. [21] Dong H, Gao W, Yan F, Ji H, Ju H. Fluorescence resonance energy transfer between quantum dots and graphene oxide for sensing biomolecules. Anal Chem 2010;82:5511–7. [22] Huang P, Xu C, Lin J, Wang C, Wang X, Zhang C, et al. Folic acid-conjugated graphene oxide loaded with photosensitizers for targeting photodynamic therapy. Theranostics 2011;1:240. [23] Schniepp HC, Li J-L, McAllister MJ, Sai H, Herrera-Alonso M, Adamson DH, et al. Functionalized single graphene sheets derived from splitting graphite oxide. J Phys Chem B 2006;110:8535–9. [24] Cai DY, Song M. Preparation of fully exfoliated graphite oxide nanoplatelets in organic solvents. J Mater Chem 2007;17:3678–80. [25] Yang D-Q, Rochette J-F, Sacher E. Controlled chemical functionalization of multiwalled carbon nanotubes by kiloelectronvolt argon ion treatment and air exposure. Langmuir 2005;21:8539–45. [26] Yang D-Q, Sacher E. Carbon 1s X-ray photoemission line shape analysis of highly oriented pyrolytic graphite: the influence of structural damage on peak asymmetry. Langmuir 2006;22:860–2.
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7 5 ( 2 0 1 4 ) 1 4 9 –1 6 0
Available at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/carbon
Highly soluble polyetheramine-functionalized graphene oxide and reduced graphene oxide both in aqueous and non-aqueous solvents Myung Jin Yoo, Hyo Won Kim, Byung Min Yoo, Ho Bum Park
*
Department of Energy Engineering, Hanyang University, Seoul 133-791, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history:
Among many methods to synthesize graphene, solution-based processing provides many
Received 27 January 2014
advantages owing to its low cost, high productivity, chemical versatility, and scalability.
Accepted 25 March 2014
In particular, graphene oxide (GO) is one of the most promising nanocarbons that enable
Available online 1 April 2014
the incorporation of graphene and related materials into bulk materials and nanocomposites. GO has hydrophilic nature that enables straightforward dispersion in aqueous solution by sonication, but GO show poor dispersibility in common organic solvents, which prevent much wider applications such as solution-mixing polymer nanocomposites. Here we prepared highly soluble, functionalized GO in both aqueous and non-aqueous solvents. This was achieved by reacting polyetheramine consisting of amphiphilic components, e.g., polypropylene oxide and polyethylene oxide, with carboxylic acid groups at GO edges. Moreover, the reduced GO (rGO) was also highly dispersible in aqueous solution as well as non-aqueous solutions. These functionalized GO and rGO can be used for many solution-processed graphene composites. 2014 Elsevier Ltd. All rights reserved.
1.
Introduction
Graphene is a one-atom-thick, two-dimensional, sp2-bonded carbon material that has been extensively studied for its excellent electrical, mechanical and thermal properties. Due to its excellent physical properties, many attempts have been made to use graphene in potential applications such as electrodes with high surface area and low electrical resistance [1], composites with higher strength to weight ratio [2,3], highfrequency transistor [4], low cost display screen [5], hydrogen storage [6], sensor to diagnose diseases [7], ultracapacitors [8], and membranes [9]. Although micromechanical exfoliation is a straightforward method for high-quality graphene, it lacks sufficient scalability for the bulk production of graphenebased materials. In contrast, solution processing of graphene * Corresponding author: Fax: +82 2 2291 5982. E-mail address: [email protected] (H.B. Park). http://dx.doi.org/10.1016/j.carbon.2014.03.048 0008-6223/ 2014 Elsevier Ltd. All rights reserved.
exhibits several advantages, including utilization of low-cost graphite, no transfer process, scalability for industrial applications, and amenability to blending with polymers or other nanomaterials to form graphene-based nanocomposites. Therefore, excellent dispersion of graphene nanosheets in solutions is of paramount importance because many graphene-based materials and composites are processed by solvent-assisted methods, such as spin-coating, filtration, and layer-by-layer assembly [10]. Graphene oxide (GO) is a highly oxidized graphene sheet, exfoliated from graphite, with many oxygen-containing functional groups such as hydroxyl, epoxide and carboxylic acid groups on the basal plane and at the edges, which is used as a starting material for the synthesis of solution-processed graphene by post-treatments such as chemical or thermal
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reduction [11]. By oxidation method, hydroxyl and carbonyl groups decorate the surface of GO, whose groups provide electrostatic repulsion between GO sheets thus minimizing aggregation in solution, and these oxygen moieties make GO hydrophilic, which enables easy dispersion in aqueous solution through sonication. In contrast, GO is less dissolved in organic solvents, which limits organic solvent-based applications such as polymer/GO composite materials [12]. Many studies have been reported to disperse GO in organic solvents by using surfactants [13] or by high-powered ultrasonication [14] and by chemical modifications [15]. By adding surfactants, however, contamination may occur, and this might cause potential problems due to the surfactant itself in practical applications. Moreover, GO was reported to be easily cracked by ultrasonication, so the GO sizes would be very diverse after ultrasonication [16]. Thus, chemical modifications of GO are required to minimize the defective sites during the preparation. Here we report highly soluble polyetheramine (PEA)-decorated GO and its reduced GO (rGO) in both aqueous and nonaqueous solvents. The PEA used in this study contains primary amino groups attached to the end of a polyether backbone (Fig. 1a). The polyethers in PEA consist of either polypropylene oxide (PPO), polyethylene oxide (PEO) or mixed PPO/PEO. PEA is very soluble in common organic solvents (e.g., alcohols, aromatic hydrocarbons, ether, glycol ethers and ketones) as well as in water [17]. In this work, we primarily used a Jeffamine monoamine (M-1000) consisting of PPO/PEO (3/19 by mole ratio), its average molecular weight in number of about 1000 Da. It was supposed that the dispersion properties of GO and rGO could be much improved if such amphiphilic oligomers were successfully introduced into GO and rGO sheets (Fig. 1e). Considering the fact that GO has carboxyl acid groups at its edges and Jeffamine M-1000 has primary amine groups at its end, the carboxyl acid groups of GO can be reacted with the primary amine groups of Jeffamine M-1000 by aid of 1ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) for amide bond formation. EDC is a water-soluble carbodiimide crosslinking agent, which is commonly used to couple carboxyl groups with primary amines (Fig. 1b) [18]. EDC reacts with carboxylic acid group to form an active ester called o-acylisourea intermediate. The o-acylisourea intermediate is very unstable, and easily hydrolyzes to form carboxylic acid again. N-hydroxysulfosuccinimide (SulfoNHS) helps to avoid the reverse reaction by forming sulfoNHS ester (Fig. 1c). Primary amine reacts with these aminereactive ester to complete amide bonding (Fig. 1d). The EDCinduced amine-carboxyl coupling reaction can be achieved at room temperature, which helps rapid reaction and prevents structural deformation of GO during the reaction. Also, it is known that EDC causes no toxicity issues when used in biomaterials [18].
GO. Sulfuric acid (95%) was purchased from Dae Jung Chem. (Gyeonggi-do, Korea) and potassium permanganate (99.3%) was purchased from Junsei Chem. (Tokyo, Japan) as oxidizing agents for graphite. Hydrogen peroxide (30%) was purchased from Sigma Aldrich (St. Louis, MO, USA) for excess KMnO4 removal during GO preparation. The glass fiber filter and Anodisc membrane (pore sizes of 0.1 and 0.2 lm) were purchased from WHATMAN (Maidstone, England, UK) for filtration. Jeffamine M-1000 was kindly supplied from Huntsman Corp. (Salt Lake, Utah, USA) for PEA-GO preparation. 1-Ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC) was purchased from Thermo Fisher Scientific Corp. (Rockford, IL, USA) and N-hydroxysuccinimide (NHS) was purchased from Sigma Aldrich for functionalizing GO with Jeffamine M-1000. Hydrochloric acid (35%) and sodium hydroxide (98%) were purchased from Dae Jung Chem. to adjust pH level. Acetone (99.5%), DMF (99.5%) and THF (99%) were purchased from Dae Jung Chem., while methanol (99.8%), ethanol (99.5%), 2-propanol (99.5%), DMSO (99.9%), NMP (99%), and pyridine (99.8%) were purchased from Sigma Aldrich for dispersibility measurement.
2.2.
Synthesis of GO and PEA-GO
2.
Experimental section
GO was synthesized by Hummer’s method using H2SO4 and KMnO4 as oxidizing agents. Ten grams of graphite was dissolved in 450 ml of H2SO4, which was subsequently stirred for 90 min at 5 C. Afterwards, 1.5 g of KMnO4 was added and the solution was stirred for another 90 min. 30 g of KMnO4 was mixed slowly, which caused a color change from black to dark green. This solution was stirred for 1 h at 5 C, then stirred at 40 C for another 1 h. 450 ml of deionized water were added carefully by dropping funnel to prevent rapid temperature rise. The temperature was increased to 80 C and maintained while stirring for 50 min. The reaction was terminated by a final addition of 265 ml deionized water and 35 ml of 30 wt% H2O2. The final product was filtered with glass fiber filter and the resultants were washed three times with 10 wt% HCl to remove excess salts. For further purification, the final product was filtered in 5 L of acetone three times, and then dried in a vacuum oven at 40 C for 1 day. The obtained dark brownish powder is GO. The GO powder was dispersed in water (1 mg/mL) and bath-sonicated for 1 h. Afterwards, the pH was adjusted to 10.5 by 1 M NaOH for better dispersion and probe-sonicated at 285 W for 6 h. The final product is the GO solution. 200 mg of the GO solution was mixed with 150 mg of Jeffamine M-1000 and stirred for 24 h at room temperature, and pH was adjusted to 4.5 by 1 M HCl. Then, 200 mg of EDC and NHS were added and bath-sonicated for 30 min, followed by stirring for 24 h. The resultant was filtered through Anodisc (pore size 0.2 lm) with 1 M HCl and acetone for purification. The dark-brown powder was dried in a vacuum oven at 40 C. Eventually, PEA-GO was obtained as blackish powders.
2.1.
Materials
2.3.
Purified synthetic graphite powder (99.99%, SP-1) was purchased from the Bay Carbon Inc. (Bay City, MI, USA) to prepare
Dispersibility evaluation
GO and PEA-GO were dispersed in various solvents such as water, acetone, methanol, ethanol, 2-propanol, ethylene
CARBON
a
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b
c
d
e
phene Oxide (GO O) Grap
PE EA Fun nctionalizzed GO O
:P Poly yeth hera amiine (PE EA)
:O
H : OH
Fig. 1 – Chemical structures of Jeffamine M-1000 (a), EDC (b) and Sulfo-NHS (c). (d) EDC coupling reaction of carboxylic acid and primary amine. (e) Proposed dispersion properties of GO and PEA-GO in organic solvents. The dispersion properties of PEAGO were improved as compared to GO by functionalizing PEA to the carboxyl groups of GO. (A color version of this figure can be viewed online.)
glycol, DMF, DMSO, NMP, pyridine and THF to observe relative dispersibility. 10 mg of GO and PEA-GO were dispersed in 20 mL of each solvent and bath-sonicated for 1 h at 40 C. After sonication, each solution was left for two months and observed dispersion states. After two months, each upper solution was filtered through weight-measured Anodisc (pore size 0.1 lm) and the weight of membrane was measured again to calculate concentrations after complete drying in a vacuum oven. Pre-tests proved that the weight loss through a 0.1 lm pore sized Anodisc membrane is less than 1%, which is insignificant. The maximum dispersibility of GO and PEA-GO was also estimated by the following method. GO and PEA-GO were dispersed in water (30 mg/mL), methanol (2 mg/mL), ethanol
(2 mg/mL), DMF (30 mg/mL) and DMSO (20 mg/mL), and bath sonicated for 1 h at 40 C. The solutions were allowed to settle for 24 h, and then centrifuged for 20 min at 1000 rpm. After centrifugation, the upper 1/3 of the solutions were collected and filtered by a 0.1 lm pore-sized Anodisc membrane. The concentrations of each solution were measured using the same method stated above. GO and PEA-GO (100 mg) were dispersed in 400 mL water by bath-sonication for 1 h to achieve a homogeneous dispersion state. Hydrazine (100 ll) was added to the dispersed solutions, and the solutions were heated to 95 C. After 1 h, the solutions were washed 5 times in a methanol and water (5:1) mixed solution to remove excess hydrazine. The resultants were dried in a vacuum oven
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at 40 C for 1 day. The final black powder was rGO and PEA-rGO. Both rGO and PEA-rGO (10 mg) were dispersed in water, acetone, methanol, ethanol, 2-propanol, ethylene glycol, DMSO, DMF, NMP, pyridine and THF (10 mL each) by bath sonication for 1 h at 40 C to observe dispersibility values.
2.4.
UV/Vis absorbance spectra were measured by a SPECORD 200 (Analytic Jena AG, Jena, Germany) with 10 mm SiO2 cuvette in the range of 190–600 nm.
3.
Results and discussion
3.1.
Synthesis of PEA-GO
Material characterizations
Fourier transform infrared (FT-IR) spectra of GO and PEA-GO were measured using a NICOLET 6700 (Thermo Fisher Scientific, IL, USA) in the range of 4000–1000 cm1. X-ray photoelectron spectroscopy (XPS) spectra were measured using an Omicron ESCALAB (Omicron, Taunusstein, Germany) with a monochromatic Al Ka (1486.8 eV) 300 W X-ray source, and a flood gun to counter charging effects under ultra-high vacuum (UHV, 1 · 109 Torr) conditions. Raman spectroscopy was performed with MonoRa750i (Dong Woo Optron, Gyeonggi-do, Korea) in the range of 1000–2000 cm1. The zeta potentials were measured with a Zetasizer Nano ZS (Malvern Instrument, Malvern, UK) at 25 C isotherm for 2 min. The
a
GO was prepared by using the modified Hummers method [19], and PEA was attached to GO by EDC coupling reaction between carboxylic acid and primary amine. Fig. 2a shows FT-IR spectra of GO and PEA-GO. Characteristic absorption bands of GO were observed in both GO and PEA-GO at 3430 cm1 (CO–H stretching vibrations), 1630 cm1 (in plane C@C stretching vibrations), 1230 cm1 (C–O–C stretching vibrations) and 1060 cm1 (C–OH stretching vibrations) [12,15,20]. The results show that most of the oxygen-containing functional groups of GO still exist after the reaction. FT-IR spectra of PEA-GO exhibit a slight difference from GO due to the substitution of carboxyl groups with amide bonds. Two absorption bands in GO at 1713 cm1 (C@OOH stretching vibrations) and
b C 1s
C-C C-O-C
GO
C=C
PEA-GO
Transmittance (-)
N1s C-OH C-O-C OC-NH
-CH2-
CON-H (stretch)
C=ONH C=C
CO-H
(aromatic) CON-H (bend)
408
400
392
GO C=O O=C-OH -CH2-
C=OOH C=C
CO-H
3500
(aromatic)
3000
2000
C-OH C-OOH C-O-C
1500
1000
294 292 290 288 286 284 282 280
-1
Wavenumbers (cm )
Binding Energy (eV)
d
c C 1s
C=C
PEA-GO
C-C C-O-C
408
400
392 C=O
Intensity (a. u.)
N1s
PEA-GO
D/G = 0.85
GO
D/G = 0.84
O=C-NH
294 292 290 288 286 284 282 280
Binding Energy (eV)
1000
1200
1400
1600
1800
-1
Raman shift (cm )
Fig. 2 – Structure analyses of both GO and PEA-GO. (a) FT-IR spectra, (b) C 1s and N 1s core level spectra of XPS: GO, (c) C 1s and N 1s core level spectra of XPS: PEA-GO and (d) D and G bands in Raman spectra. (A color version of this figure can be viewed online.)
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1370 cm1 (C–OOH stretching vibrations) were not observed in PEA-GO. Instead, major bands due to amide bonds in PEA-GO appeared at 3744 cm1 (CON–H stretching vibrations), 1699 cm1 (C@ONH stretching vibrations), 1559 cm1 (CON–H bending vibrations) and 1458 cm1 (OC–NH stretching vibrations). These bands clearly show that Jeffamine M-1000 was reacted to the carboxyl groups of GO edges [20–22]. The reaction in PEA-GO was also confirmed by XPS: The C 1s core-level spectra of both GO and PEA-GO are presented in Fig. 2b and c. Two intense peaks at 284.5 and 286.4 eV, with a relatively small peak at 287.6 eV were observed in both cases. The fact that the two clear maxima in the C 1s band are separated by about 2 eV corresponds to the values in the literature for C 1s band shapes [23,24]. The two maxima located at 284.5 and 286.4 eV are attributed to C@C and C–O–C, respectively [12]. The peak at 286.4 eV is also due to defects in the basal plane of GO (CAC species) since binding energies of CAO and CAC bonds are very close, less than 0.1 eV [25,26]. For PEA-GO, the integral area of the Gaussian peak at 286.4 eV increased from 31.4% (GO) to 42.2%. This result can be elucidated by the fact that Jeffamine M-1000 consists of sp3 CAC and CAO bonds, which contribute to an increase of CAC and CAO bonds in PEA-GO as compared to original GO. Another significant difference in XPS data is the disappearance of the peak at 290.4 eV. The GO peak at 290.4 eV (O@C–OH) was not observed in PEA-GO, while the peak at 288.2 eV (O@C–NH) appeared only in PEA-GO. Typically, C–N peak is shown at 285.2 eV but C@O in amide bond neighboring CAN is known to shift the peak about 2.97–3.59, which coincides with our experiment [27]. The N 1s core-level spectra of PEA-GO show a peak at 400 eV, which prove the existence of CAN bonding whereas GO does not. One more thing to be noticed was also observed in XPS analyses of GO and PEA-GO, which is the decreased amount of CAO bondings. Originally, EDC reaction is known to couple carboxyl groups with primary amines selectively. The oxygen functional groups of GO, however, may cause another possible reaction site of PEA. Among the oxygen groups in GO, CAO bonding is the most highly expected possible reaction site for primary amines because of the weak sp2 bond in CAO. As far as the fact that decreased CAO bondings were found in XPS analyses is concerned, it can be inferred that unintended basal plane functionalization was occurred during the reaction. The amount of basal functionalization is, however, predicted to be quite low considering that all other functional groups of GO were also found in PEA-GO except carboxyl groups. Thus, FT-IR and XPS analyses suggest the evidence of reaction between carboxyl acid groups of GO and the amine groups of Jeffamine M-1000. Fig. 2d shows Raman spectra of GO and PEA-GO. Two peaks were observed at 1345 and 1590 cm1 in both GO and PEA-GO, which are referred to as D and G peaks, respectively. The D peak is related to defects and disorder sites of GO, due to the breathing modes of A1g symmetry in sp2 systems. The G peak is observed in all sp2 hybridized carbon materials, and it results from E2g symmetry vibrations in plane [28]. Therefore, the D/G ratio provides information about the degree of GO defect sites, and we found that the D/G ratio of PEA-GO is very similar to the original D/G ratio of GO (0.85), indicating no significant structural deformation during the reaction.
3.2.
153
Dispersion property of PEA-GO
Fig. 3a shows the photo images of as-prepared GO and PEAGO dissolved in water and organic solvents after two months, and the results are summarized in Table 1. In particular, the enhanced solubility in organic solvents was observed in PEA-GO solutions. The improved dispersibility in organic solvents is ascribed to the presence of PEA at GO edges. In advance of GO and PEA-GO dispersibility experiments in organic solvents, it should be noted that primary alcohols such as methanol, ethanol and propanol can be oxidized by GO itself. GO is known for an oxidant for primary alcohols at about 100 C from previous studies [29,30]. We did the dispersion experiments at relatively low temperature at 40 C to minimize the inadvertent reduction of GO and PEA-GO. The dispersion properties were found to be greatly improved, especially in aprotic solvents such as dimethylformamide (DMF), dimethylsulfoxide (DMSO) and N-methyl-2-pyrrolidone (NMP). A slightly reduced dispersibility was observed in water due to the neutralization of GO edges by the introduction of PEA, but the PEA-GO still maintained good stability even in water. Zeta potentials were measured to quantify the stability of GO and PEA-GO in different solvents. Zeta potential is the electric potential difference between the interfacial electric double layer at the location of the slipping plane and the dispersed medium, so zeta potential is an indicator to the dispersion of colloidal particles. Generally, colloids are stabilized when the zeta potential is less than 30 mV [31]. GO has a negative zeta potential due to its oxygen-containing functional groups, which increases repulsive forces between GO particles, and contributes to adequate dispersion in water. In general, GO has 40 60 mV zeta potentials in water, sufficient to maintain a stable solution. The zeta potentials of GO and PEA-GO in solvents are shown in Fig. 4. Zeta potentials of GO and PEA-GO in water show 54.2 and 50.3 mV, respectively, at pH 10. This suggests that carboxylic groups of GO are more favorable to dispersion in water. However, PEA-GO still maintained excellent dispersibility despite a slight increased zeta potential compared to GO. In most solvents, the zeta potentials of GO could not be measured due to its poor dispersion. DMSO was one of a few organic solvents for GO exhibiting relatively good dispersion. In sharp contrast, PEA-GO exhibit excellent dispersibility in most aprotic solvents, which corresponds to the results above. UV/Vis absorptivity and maximum dispersibility of GO and PEA-GO in common organic solvents were measured to compare the dispersion properties of GO and PEA-GO. As discussed above, PEA-GO was highly dispersed in aprotic solvents as compared to protic solvents. In this study, water, methanol, and ethanol were evaluated as protic solvents, while DMSO and DMF as aprotic solvents. The relationship between UV/Vis absorptivity and solution concentration was proposed by Beer and Lambert. The Beer–Lambert law can be expressed as: A¼ecl
ð1Þ
where A is the measured absorbance, e is the absorptivity coefficient, l is the path length, and c is the solution concentration. The absorbance is proportional to the concentration
154
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a
b Water (GO) Water (PEA-GO)
30 25
DMF (PEA-GO)
A/l (m-1)
20 Methanol (PEA-GO)
15 10
DMSO (GO) DMSO (PEA-GO) Ethanol (GO) Ethanol (PEA-GO)
5 0 0.000
0.002
0.004
0.006
0.008
C (g/L)
Fig. 3 – Dispersion properties of GO and PEA-GO in different solvents. (a) Photo images of GO and PEA-GO solutions (0.5 mg/ mL) sonicated for 1 h at 40 C after 50 days. (b) UV/Vis absorptivity spectra of GO and PEA-GO in water, methanol, ethanol, DMF and DMSO. (A color version of this figure can be viewed online.)
-70
Table 1 – Concentrations of GO and PEA-GO in different solvents.
GO PEA-GO
-60
Water Acetone Methanol Ethanol 2-Propanol Ethylene glycol DMSO DMF NMP Pyridine THF
Concentration (mg/mL) GO
PEA-GO
0.36 NA NA 0.09 NA 0.17 0.35 0.13 NA NA NA
0.33 0.10 0.03 0.13 0.05 0.43 0.38 0.47 0.44 0.38 0.09
of a solution and absorptivity coefficient when the path length is fixed, so the absorptivity coefficient can be estimated by measuring UV/Vis absorbance at different analyte concentrations. The GO and PEA-GO absorptivity coefficients were measured to compare the dispersion properties in different solvents. The absorptivity coefficient is a wavelengthdependent parameter, so wavelength is important in calculating the absorptivity coefficient. Generally, the linear relationship between concentration and absorbance is well-satisfied with the Beer–Lambert law at the wavelength at which maximum absorbance occurs. GO and PEA-GO were initially dispersed in water and UV/Vis absorbance was measured to
-50 Zeta potential (mV)
Solvents
-40 -30 -20 -10 0 ter ne OH OH nol EG MSO DMF NMP dine THF ri Wa ceto Me Et ropa D Py A 2-P
Fig. 4 – Zeta-potentials of GO and PEA-GO in different solvents. (A color version of this figure can be viewed online.)
determine the wavelength at which maximum absorbance occurs. Fig. 5 shows the UV/Vis absorptions at 231 nm and weak shoulder at 298 nm. Slight difference between the absorbance is due to the different absorptivity coefficients as discussed below. From previous studies of UV/Vis spectroscopy, it can be inferred that the absorbance near 230 nm is dominated by p–p* transition and the shoulder near 300 nm is due to n–p* transition [32]. The possibilities of electron
CARBON
GO Water
b
-1
40 38 36 34 32 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 1.7 mg/L 0 200 300
A/l (m )
-1
A/l (m )
a
7.4 mg/L
400
500
155
7 5 (2 0 1 4) 1 4 9–16 0
600
40 38 36 34 32 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 0 200
PEA-GO Water
7.1 mg/L
1.4 mg/L
300
Wavelength (nm)
d GO DMSO
-1
26 24 22 20 18 16 14 12 10 8 6 4 2 0
A/l (m )
-1
A/l (m )
c
7.1 mg/L
1.4 mg/L
300
400
400
500
600
Wavelength (nm)
500
26 24 22 20 18 16 14 12 10 8 6 4 2 0
PEA-GO DMSO
7.1 mg/L
1.4 mg/L
300
600
400
500
600
Wavelength (nm)
Wavelength (nm)
Fig. 5 – UV/Vis absorption spectra of GO and PEA-GO in water (a and b) and DMSO (c and d). (A color version of this figure can be viewed online.)
transfer from n to p* is much less than from the p to p* transition, so the shoulder near 300 nm shows relatively weak intensity as compared to the absorbance at 230 nm. The major absorbance at 230 nm can be elucidated by the two conjugation effects: one for the sp2 conjugated carbons in plane, and two for conjugated chromophore units such as C@O with conjugated carbon planes. The more highly conjugated GO (less defect ratio) shows a p–p* transition at a longer wavelength (bathochromic shift), while the less conjugated GO shows a p–p* transition at a shorter wavelength (hypsochromic shift). The absorptivity coefficients of GO and PEA-GO in water were measured at 231 nm, but this absorbance of p–p* transition was very weak in organic solvents. This is because the strong absorbance of organic solvents occurs between 200 and 300 nm, which hinders the absorbance of materials.
Therefore, the absorptivity coefficients of GO and PEA-GO in organic solvents were measured at the wavelengths at which n–p* transitions occur at 280–320 nm. Solvent polarities result in slight absorbance differences between the solvents. The measured absorptions as well as linear relationships of concentration and absorption at p–p* (water) and n–p* (organic solvents) transitions are shown in Fig. 3b. Table 2 lists the measured absorptivity coefficients. GO was less dispersed in methanol and DMF, and so measuring absorptivity coefficients in these solvents was difficult. The absorptivity coefficients increase as the dispersibility is improved. Eventually, the measured maximum dispersibility values of GO and PEA-GO are listed in Table 3. GO was poorly dispersed in methanol, so severe aggregation was observed in our experiment. The dispersion of GO was observed in ethanol
Table 2 – Absorptivities of GO and PEA-GO in different solvents (L/g m).
Table 3 – Maximum dispersibilities of GO and PEA-GO in different solvents (g/L).
GO PEA-GO
Methanol
Ethanol
DMF
DMSO
Water
– 1874
512 2000
– 3053
1923 2717
4869 4615
GO PEA-GO
Methanol
Ethanol
DMF
DMSO
Water
– 0.18
0.21 0.87
0.28 24.3
8.34 12.9
21.4 17.6
156
7 5 ( 2 0 1 4 ) 1 4 9 –1 6 0
CARBON
(0.21 g/L), and DMF (0.28 g/L). The maximum dispersion concentration of PEA-GO was measured in methanol (0.18 g/L), ethanol (0.87 g/L), DMF (24.3 g/L) and DMSO (12.9 g/L). Among them, PEA-GO dispersibility in DMF was improved by more than 8600% when compared to GO. The maximum dispersion concentration of PEA-GO in water (17.6 g/L) was slightly
decreased as compared to GO (21.4 g/L), but still maintained good dispersion. The theoretical explanation for high dispersibility of PEA-GO in DMF and water will be discussed in detail in terms of solubility parameter. To analyze the dispersibility of PEA-GO, a Hildebrand solubility parameter was also estimated. The Hildebrand solubil-
b
a PEA-GO GO
0.4 0.3 0.2 0.1 0.0 10
20
30
40
0.3 0.2 0.1
50 1/2
Hildebrand Solubility Parameter δT (MPa
c
0
Concentration (mg / mL)
0.2 0.1
30
40
50
PEA-GO GO
0.5
0.3
20
1/2
d
0.4
10
Dispersion Parameter δD (MPa )
)
PEA-GO GO
0.5
Concentration (mg / mL)
0.4
0.0 0
0.0
0.4 0.3 0.2 0.1 0.0
0
10
20
30
40
50
0
1/2
10
20
30
40
50 1/2
Polar Parameter δP (MPa )
e
PEA-GO GO
0.5
Concentration (mg / mL)
Concentration (mg / mL)
0.5
Hydrogen Bonding Parameter δH (MPa )
f
Fig. 6 – The concentrations of GO and PEA-GO after 50 days in different solvents as a function of the (a) Hildebrand solubility parameter, (b) dispersion parameter, (c) polar parameter and (d) hydrogen bonding parameter. Solubility parameter diagrams of (e) GO and (f) PEA-GO. (A color version of this figure can be viewed online.)
CARBON
Table 4 – Hansen solubility parameters of solvents, GO and PEA-GO.
Water Acetone MeOH EtOH 2-Propanol Ethylene glycol DMSO DMF NMP Pyridine THF GO PEA-GO
dD
dP
dH
15.6 15.5 15.1 15.8 15.8 17.0 18.4 17.4 18.0 19.0 16.8 15.6–18.4 15.5–19.0
16.0 10.4 12.3 8.8 6.1 11.0 16.4 13.7 12.3 8.8 5.7 16.0–16.4 8.8–17.0
42.3 7.0 22.3 19.4 11.0 26.0 10.2 11.3 7.2 5.9 8.0 10.2–42.3 5.9–42.3
ð4Þ
where dD, dP and dH are the dispersive, polar and hydrogen bonding solubility parameters, respectively. In this respect, the heat of mixing also can be expressed as [33]: h i ð5Þ DHM ¼ u1 u2 VM ðdD;1 dD;2 Þ2 þ ðdP;1 dP;2 Þ2 þ ðdH;1 dH;2 Þ2
ð2Þ
where V is the molar volume of the pure solvent and E is its energy of vaporization. E/V is also referred to as the cohesive energy density. Hildebrand and Scott proposed the heat of mixing as [33]: DHM ¼ u1 u2 VM ðd1 d2 Þ2
single factor to describe complicated dispersion properties. For more specific information about dispersion properties of GO and PEA-GO in different solvents, specific types of the interactions should be considered. There are generally three factors that contribute to dispersibility in a solvent: dispersion, polarity and H-bonding interactions. The Hansen solubility parameters interpret dispersion properties as three parameters according to the types of interactions. Each solubility parameter is defined as the square root of the contribution to the cohesive energy density. The relationship between the Hansen solubility parameters and the Hildebrand solubility parameter follows the equation [33]: d2 ¼ d2D þ d2P þ d2H
ity parameter is a useful tool for the simple prediction of the solubility of unknown substances. Hildebrand solubility parameter is defined as [33]: d ¼ ðE=VÞ1=2
157
7 5 (2 0 1 4) 1 4 9–16 0
ð3Þ
where d1 and d1 are the Hildebrand solubility parameters of the solute and solvent, respectively, and u1 and u2 are volume fractions of solvent and solute, respectively. VM is the volume of the mixture. This equation suggests that dispersion is favored when the solvent and solute parameters match closely with each other. The concentrations of PEA-GO and GO after two months in each solvent were plotted as a function of the Hildebrand solubility parameters. According to the correlation equation of mixing enthalpy and Hildebrand solubility parameter above, GO and PEA-GO might be more dispersed in solvents that have similar solubility parameters. Fig. 6a shows that PEA-GO is well dispersed in solvents with Hildebrand solubility parameter ranging 21–48, whereas GO shows good dispersion properties in the range 26–48. The Hildebrand solubility parameter indicates quantitative information about the dispersibility of GO and PEA-GO, but it is too rough to describe the dispersion properties by a single factor. The Hildebrand solubility parameter is defined just as the cohesive energy density of the solvent and solute, which suggests that all the interactions are combined by a
This equation denotes that dispersion is favored when each solvent and solute parameter are similar. The plots of each dispersed concentration as a function of each Hansen solubility parameter are presented in Fig. 6b–d. Three Hansen solubility parameters have ranges, whether broad or narrow, similar to the Hildebrand solubility parameter. This is due to the amphiphilic structures of GO and PEA-GO, which have large non-polar hydrophobic carbon sites and polar (hydrophilic) oxygen-containing groups. The diverse size distribution also contributes to the broad range of solubility parameters. GO shows good dispersion in solvents where the dispersion parameter was between 15.6 and 18.4, the polar parameter was between 16.0 and 16.4 and the hydrogen bonding parameter was between 10.2 and 42.3, while PEA-GO is well-dispersed in solvents where the dispersion parameter was between 15.5 and 19.0, the polar parameter was between 8.8 and 16.4 and the hydrogen bonding parameter was between 5.9 and 42.3 (Table 4). Fig. 6 shows that all three parameters ranges were more broadened after PEO modification. Among them, the polar parameter range was increased greatly from 16–16.4 to 8.8–16.4. The broad solubility parameters help PEA-GO disperse well in many solvents as compared to GO. The hydrogen bonding parameters of both GO and PEAGO shows relatively broad ranges when compared to the other parameters, and this means hydrogen bonding interactions affect dispersibility less than van der Waals interactions or polar interactions. This is in accordance with the observations that PEA-GO is more dispersive in aprotic solvents than in protic solvents, and GO is dispersive in both a highly protic solvent (water) and an aprotic solvent (DMSO). Figs. 6e and f
Table 5 – Theoretical solubility parameter of PEA by group contribution method.
–CHNH2 –CH3 –CH< –CH2– –CH2O–
Number of segment
Contribution to dD
Contribution to dP
Contribution to dH
dD
dP
dH
1 4 2 19 22
0.0112 0.9714 0.645 0.0269 0.031
1.1989 1.6448 0.6491 0.3045 0.8826
–
14.9
14.5
0.6
0.29901 0.1386 0.1161 –
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a
A/l (m-1)
b
70 65 60 DMSO (PEA-rGO) DMF (PEA-rGO) 55 DMSO (rGO) DMF (rGO) 50 45 Ethanol (PEA-rGO) 40 Ethanol (rGO) 35 30 25 20 15 10 Water (PEA-rGO) Water (rGO) 5 0 0.000 0.004 0.008 0.012 0.016 C (g/L)
Fig. 7 – Dispersion properties of rGO and PEA-rGO in different solvents. (a) Photo images of bath sonicated rGO and PEA-rGO solutions (0.5 mg/mL) for 1 h at 40 C. (b) UV/Vis absorptivity spectra of rGO and PEA-rGO in water, ethanol, DMF and DMSO. (A color version of this figure can be viewed online.)
3.4.
Synthesis of PEA-rGO and its dispersibility
functionalized to carboxyl groups at the GO edges. The dispersibility values of rGO and PEA-rGO in organic solvents are presented in Fig. 7. PEA-rGO still maintains well dispersion properties in both aqueous and non-aqueous solvents. Moreover, PEA-rGO has similar electric conductivities with GO when thermally treated at 600 C (Fig. 8). Graphene nanocomposite materials are highly desirable in material sciences due
3500
/ Square)
- Ar atmosphere
Sheet Resistance (
show the three parameter positions of GO and PEA-GO, respectively. Each area is proportional to the concentration, which shows the dispersive region as a function of three parameters. In particular, both GO and PEA-GO show limited regions of dispersion and the polar parameter, but they show broad ranges of the hydrogen bonding parameter. The PEA solubility parameter was also calculated to analyze the contribution of PEA to PEA-GO dispersibility. Table 5 lists the PEA solubility parameter by group contribution method [34]. The calculated solubility parameter of PEA is found to have most similar dD and dP with water and DMF, and it coincides with the maximum dispersibility of PEAGO. The similarity of dH between PEA and PEA-GO, however, is quite low when compared to both dD and dP, and hydrogen bonding less affects the dispersion properties of PEA-GO. The reason that PEA-GO shows broader range of solubility parameter can be interpreted as the effect of PEA segments, which make PEA-GO more dispersive in different organic solvents, while GO does not contain PEA segments. Particularly, similar dispersion and polar parameters of PEA with water and DMF make PEA-GO highly soluble in both solvents.
3000
PEA-rGO - Temperature raise 5
2500 2000 1500 rGO
250 200 150 100 50 0 0
GO and PEA-GO were chemically reduced to rGO and PEA-rGO by using hydrazine. Hydrazine is commonly used for GO reduction, and reduces particularly epoxide groups in the basal plane. We suggested that the dispersibility of PEA-GO be maintained after hydrazine reduction, because PEO is
per min
100 200 300 400 500 600 o
Annealing Temperature ( C) Fig. 8 – Electrical properties of rGO and PEA-rGO as a function of thermal treatment temperature. (A color version of this figure can be viewed online.)
CARBON
7 5 (2 0 1 4) 1 4 9–16 0
to their extremely enhanced electrical, thermal and mechanical properties in many applications, but the manufacture of graphene nanocomposite materials has been very limited because of the low dispersibility of GO in organic solvents. In this respect, PEA-GO is highly expected to be a breakthrough in the preparation of various graphene nanocomposite materials based on solution processing.
4.
Conclusions
PEA was successfully functionalized to the carboxyl groups of GO, and the PEA-GO was found to be highly dispersive both in aqueous system and non-aqueous system due to the amphiphilic properties of PEA used in this study. The dispersibilities of GO and PEA-GO were measured by concentration, zeta potential, maximum dispersibility and absorptivity coefficient. PEA-GO was found to have drastically enhanced dispersibility, especially in polar aprotic solvents such as DMF. The Hildebrand solubility parameters and Hansen solubility parameters of GO and PEA-GO were also experimentally measured to compare their dispersion properties, and both GO and PEA-GO demonstrated specific ranges of solubility parameters. The dispersion properties of GO and PEA-GO were found to be strongly affected by dispersion and polar interactions, rather than hydrogen bonding interactions. The measured solubility parameters are expected to be used in the prediction of the solubility in other solvents for GO applications. The PEA-rGO was also proven to have similar dispersion properties with PEA-GO. We expect that PEA-GO and PEA-rGO would be good candidates in the preparation of graphenebased composites or hybrid materials for promising applications such as energy storage devices, gas sensors, membranes and biomedical materials for drug delivery systems.
Acknowledgements This work was supported by the Korea CCS R&D Center (KCRC) grant funded by the Korea government (Ministry of Science, ICT & Future Planning) (no. 2012-0008907), and M.J.Y. H.W.K. and B.M.Y. appreciate the financial support from ‘‘Future convergence energy-leaders BK21+ program’’ (10Z20130012477) of the Ministry of Education (MOE).
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