Highly Efficient Method for Preparing Homogeneous and Stable Colloids Containing Graphene Oxide
© Yu et al. 2010
Received: 10 June 2010
Accepted: 9 September 2010
Published: 30 September 2010
Phase transfer method has been developed for preparing homogeneous and stable graphene oxide colloids. Graphene oxide nanosheets (GONs) were successfully transferred from water to n-octane after modification by oleylamine. Corrugation and scrolling exist dominantly in the modified GONs. GONs were single layered with the maximum solubility in n-octane up to 3.82 mg mL-1. Oleylamine molecules chemically attach onto the GONs. Compared with traditional strategies, the phase transfer method has the features of simplicity and high efficiency.
Graphene has attracted considerable attention over the past few years due to its exceptional properties [1–3], such as high electrical conductivity, high mechanical strength, and high thermal conductivity. Graphene has exhibited potential applications in microelectronic devices, sensors, biomedicines, and mechanic resonators. Additionally, graphene oxide and graphene can be used as fillers to enhance the mechanical and electrical or thermal transport properties of polymer nanocomposites. The excellent performance of polymer nanocomposites is achieved, not only by using the inherent properties of the nanofiller, but more importantly by optimizing the dispersion, enhancing the compatibility between nanofiller and matrix. The colloidal suspensions containing graphene oxide and graphene are often used to fabricate the composites [4–6]. One of the aims of preparing colloidal suspensions is to enhance the dispersion of nanomaterial in polymer, and the other one is to decrease the toxicity of the composite production .
Several approaches have been proposed for the production of aqueous suspensions of graphene oxide and graphene sheets. Exfoliation of graphite oxide either by rapid thermal expansion or by ultrasonic dispersion has been widely adopted to prepare graphene oxide and graphene sheets in bulk. Due to the rich hydrophilic oxygen-containing groups such as carboxyl, hydroxyl, and epoxide, the graphite oxide readily suspends in water and polar organic solvents, such as ethylene glycol, DMF, NMP, and THF at about 0.5 mg mL-1 . In order to enhance the solubility of graphene oxide nanosheets in water, the graphene oxide nanosheets were functionalized with allylamine . The maximum solubility for graphene oxide–allylamine powders in water were determined to be 1.55 mg mL-1, which was more than twice of that for bare graphene oxide nanosheets. When p-phenyl-SO3H groups were introduced into the graphene oxide, the resulting reduced product remained soluble in water and did not aggregate . Phenylene diamine was a good reducing agent and stabilizer for stable graphene colloid, and the as-made graphene could be dispersed well in ethanol, glycol, N-methyl-2-pyrrolidone (NMP), but not in N,N-dimethylformamide (DMF) . Triblock copolymers (PEO-b-PPOb-PEO) as the solubilizing agent was employed for chemically exfoliated graphite oxide, and graphene formed through in situ reduction by hydrazine . Water-soluble graphene sheets were functionalized by biocompatible poly-l-lysine as a linker through a covalent amide group . KOH could confer a large negative charge through reactions with reactive oxygen-containing groups on the graphene oxide sheets , and exfoliated graphite oxide would undergo quickly deoxygenation in strong alkali solutions ; therefore, graphene suspension could be prepared by simply heating an exfoliated graphite oxide suspension under strongly alkaline conditions at moderate temperature.
Most of the reported studies are related to the aqueous graphene oxide and graphene suspensions. Organophilic graphene oxide and graphene nanosheets are important for the application in graphene-based composite materials, while only a few papers presented the methods to produce hydrophobic graphene oxide and graphene nanosheets . For example, graphene oxide sheets could be modified by isocyanate , which was well dispersed in polar aprotic solvents. The long alkyl chains (such as octadecylamine)  were always used as the surface-modified agents and the alkyl-chain-modified graphene sheets that could be dispersed in organic solvents after sonication . Wang et al.  reported the synthesis of hydrophobic graphene oxide nanosheets by a solvothermal method, and then they prepared organophilic graphene nanosheets by reacting with octadecylamine .
For preparing the suspensions containing graphene or graphene oxide nanosheets (GONs), the traditional strategy is a three-step method. First, graphene or graphene oxide nanosheets are prepared and dried, and then they will be modified. Finally, the functionalized graphene will be dispersed in solvents under stir or ultrasonication. The three-step method is used widely, while there are some drawbacks. First of all, during the synthetic and drying process, GONs have the strong trend to conglomerate due to the large surface area. Second, not all the GONs can be modified, and there is always some sediment under the bottom of suspension. The suspension is not homogeneous, which is not desired for preparing even films and composite materials. Here, we developed a facile phase-transfer method with high efficiency to prepare stable suspensions of graphene oxide in organic solvents. This method is primarily based on the strong interaction between GONs and oleylamine.
Natural graphite was purchased from Qingdao Baichuan Graphite Co., LTd. 98% H2SO4, 30% H2O2, potassium permanganate (KMnO4) and n-octane were obtained from Shanghai Chemical Reagents Company. Oleylamine was purchased from Aldrich. All other reagents were used without further purification.
The size and morphology of the GONs were examined by using transmission electron microscopy (TEM, JEOL 2100F). The TEM samples were prepared by dropping the diluted colloid onto a carbon film mesh supported on a copper grid and drying it in air. Atomic force microscopic (AFM) images were taken on a MultiTask AutoProbe CP/MT Scanning Probe Microscope (Veeco Instruments, Woodbury, NY). Imaging was done in tapping mode using a V-shaped 'Ultralever' probe B and nominal tip radius 10 nm. The diluted GONs colloid was dropped onto a freshly cleaved mica. After the mica was dry, the GONs-OA were imaged. FT-IR spectra were recorded with a Bruker Equinox V70 FT-IR spectrometer in dry KBr pullet in the range of 400–4,000 cm-1. A thermogravimetric analyzer (TG-DTG, Netzsch STA 449C) was used for thermogravimetric analysis and calculation of the decomposition activation energy (sample mass: about 15.0 mg; atmosphere, flowing dry nitrogen). The UV–Vis spectra of the octane suspensions containing GONs were measured on a Shimadzu UV2550 UV–Vis spectrometer. In order to determine the concentration of the suspension, the calibration line was constructed by measuring UV–Vis spectra absorption intensities of the suspensions at different concentrations (0.1–0.5 mg mL-1). The standard solution with 0.5 mg mL-1 was prepared through the following process. At first, 20 mL of 0.5 mg mL-1 aqueous solution containing GONs was prepared, then 50 mg oleylamine was added, after that GONs-OA would be transferred to 20 mL octane, and the resulting was the octane suspension containing GONs with the concentration 0.5 mg mL-1. To determine the maximum solubility of the suspension, the supernatant solution of the saturated suspension was diluted several times for the measurement of UV–Vis spectra until the absorbance fitting in the range of calibration line. Based on the calibration line, the concentration of the diluted solution could be determined. Finally, the maximum solubility could be obtained based on the times of the dilution.
Results and Discussions
Kahn prepared soluble oxidized single-walled carbon nanotubes (SWNT) in organic and aqueous solvents through derivatization using 2-aminomethyl-18-crown-6 ether (CE) . They proposed that SWNT and CE formed SWNT-CE adduct, arising from a twitter ionic interaction between a protonated amine on CE and an oxy-anion from a carboxylic acid group, creating a COO-NH3+ ionic bond. Li et al. investigated the extraction of maleic acid using trioctylamine, and they found the amine formed 1:1 complexation with maleic acid through ion pairing . Zhou et al. reported the crystal structure of the ethylamine salt of 2-(4-isobutylphenyl) propionic acid , and the result verified the above proposition. In the paper, there is a strong interaction between oleylamine and GONs, and the interaction is very similar to that between SWNT and CE.
Optimizing the dispersion of graphene oxide in non-polar solvent is vital for the application and production of graphene-based composites. Synthetic methods for the colloidal suspensions containing graphene and graphene oxide are one of research hotspots. In this paper, the phase transfer method was applied to prepare homogeneous and stable suspensions containing GONs in non-polar solvents. Compared with the traditional three-step method, phase transfer strategy is very simple. Due to the strong interaction between oleylamine and GONs, almost all the GONs could be transferred to the organic phase with high efficiency. The size of GONs-OA was in the range of 300–800 nm, and GONs-OA always existed in the state of corrugation and scrolling. Almost all the sheets were single-layer graphene oxide, and the thickness of GONs-OA was uniform. Due to the presence of oleylamine absorbed on both sides of the GONs, the average thickness of GONs-OA was about 1.3 nm. The FT-IR spectra clearly confirmed that oleylamine molecules were attached to GONs through chemical absorption. The thermo-gravimetric analysis of GONs-OA illustrated that about 11.4% carbon atoms were absorbed by oleylamine. The decomposition activation energy calculated through Ozawa and Kissinger method were 234.31 and 234.84 kJ mol-1, respectively, indicating the strong interaction between oleylamine and GONs. The solubility of GONs-OA in octane was quantitatively characterized by UV–Vis spectroscopy, and the maximum solubility of GONs-OA in octane solvent was up to 3.82 mg mL-1.
The work was supported by Program for New Century Excellent Talents in University (NECT-10-883), Innovation Program of Shanghai Municipal Education Commission (10YZ199), the National Science Foundation of China (50876058), Shanghai Educational Development Foundation and Shanghai Municipal Education Commission (08CG64) and the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning.
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