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بخشی از مقاله انگلیسیعنوان انگلیسی:Amino Terminated Polyethylene Glycol Functionalized Graphene and Its Water Solubility~~en~~

ABSTRACT

A chemical modification process was developed to functionalize graphene with specific groups. Graphene oxide (GO) was successfully functionalized with thionyl bromide which can be used as precursors for further functionalization. Amino terminated-polyethylene glycol (PEG-NH2) molecules were linked to single-layer graphene sheets through covalent bond. FT-IR, SEM and UV-vis spectroscopy techniques were used to characterize PEG modified graphene oxide and PEG modified reduced graphene oxide (PEG-RG). PEG-RG could disperse in water, tetrahydrofuran and ethylene glycol, with individual, single-layer graphene sheets spontaneously. The dispersion behavior of PEG-RG in an aqueous solvent has been investigated. A series of solutions of PEG-RG with concentrations of 0.001% to 1.5% were prepared and the PEG-RG dispersions exhibited long-term stability. In addition, a PEG-RG film with layered structure and high conductivity has been successfully prepared by filtration.

INTRODUCTION

Graphene has shown various unique properties, including superior mechanical strength and low density and high heat conductance [1, 2]. Many potential applications of graphene are based on its unique mechanical and electrical properties. Graphene oxide (GO) is water soluble with low conductivity and the reduced graphene oxide (RG) is a good conductivity with poor solubility in water. Generally, the solubility of GO in aqueous solution is because of its rich oxygen containing and hydrophilic groups, such as hydroxyl, epoxide and carbonyl groups. Upon a reduction process, most of the oxygen containing groups, hydroxyl, epoxide and carbonyl, will be totally removed and GO is converted to a rich
-conjugation graphene. The restoration of conjugation in graphene sheet can recover the conductivity of graphene but will scarify the solubility of graphene. RG is not compatible with other materials, such as most polymer matrices and limits its applications. To solve these problems, several techniques have been developed to modify the surface properties of RG and enhance its compatibility with other matrices and the solubility in aqueous and organic solvents[3, 4]. Potential techniques include physically absorbed functional molecules onto graphene sheets, chemical covalent linked functional groups onto graphene surface [5, 6]. To date, the dispersion of RG in aqueous solvents has been accomplished via physically absorbed, aqueous soluble groups functionalized molecules on RG sheets with different molecules and polymers, but the presence of such stabilizers is not desirable for most applications. The dispersion behavior of as-prepared graphene oxide has remained largely unexplored. In this report, we propose a new protocol to functionalize RG with amino-terminated poly (ethylene glycol) (PEG-NH2). The detailed processes are illustrated in Figure 1. Polyethylene glycol molecules were linked to single-layer graphene sheets through covalent bonding. The asprepared PEG-RG can disperse into individual graphene sheets in water spontaneously, forming a suspension with long-term stability. A series of solutions of PEG-RG with concentrations of 0.001% to 1.5% were prepared. In addition, a PEG-RG film with layered structure and high conductivity has been successfully prepared.

EXPERIMENTAL DETAILES

Two types amino polyethylene glycols, O-(2-Aminoethyl) polyethylene glycol (MW~10000 and 5000) were purchased from Aldrich, and another type of O-(2-Aminoethyl) polyethylene glycol (MW: 3,000) from Fluka. The following reagents and solvents were used without further purification: hydrazine, thionyl bromide and triethylamino (Aldrich), sulfuric acid, hydrogen chloride, sodium hydroxide, ethanol, methanol (Fisher). Graphene oxide and reduced graphene oxide were prepared by using Hummers method [5, 7] . The process to link PEG-NH2 onto graphene surface is demonstrated in Figure 1. Typically, 0.1g graphene was dissolved into 50ml SOBr2, and this solution was refluxed at 80 C for 24 hours under nitrogen atmosphere to yield GO-Br (Graphene-CO-Br). At the end of the reaction, excess SOBr2 and solvent were removed by distillation and filtration, and GO-Br was mixed with 5ml of triethylamino, 0.2g of amino-PEG in 50ml DMF. The mixture was then refluxed at 130 C for 36 hours. After the reaction finished, the solution was cooled and 300ml DMF was added to the above solution. Finally, the product was isolated by filtration on a polycarbonate membrane (0.2m) and washed totally with DMS and DI water. The excess PEGNH2 were removed through five washing cycles, which included suspension, sonication, filtration, drying, and re-suspension of the solid in water. Finally, the PEG-GO were dried in a vacuum oven at 80 C for 12 hours and then dispersed into 50ml DI water and reduced with hydrazine utilized by previous reports. Upon the reduction process, the color of the solution converts from a brown to black color. The product (PEG-RG) was obtained by filtration and washing with ethanol, water and HCl sequentially and dried in an oven for future use. Nicolet 4700 Fourier transform infrared (FTIR) Spectrometers, (Thermo Fisher Scientific, USA), X-ray photoelectron spectroscopy (XPS, Surface analysis PHI5600, Physical Electronics, Inc.), Hitachi S-3400N SEM and Bruker X-flash x-ray microanalysis system, and UV-Vis spectrometry (Thermo Fisher Scientific, Evolution 300) were used to characterize the changes in chemical structure and surface morphology of the graphene after each surface treatment step. For the FT-IR (Perkin Elmer Spectrum One, USA) analysis, EDTA-RGO and EDTA-GO, along with GO and RGO, were separately pressed into a pellet together with potassium bromide and then scanned from 500 cm-1 to 4000 cm-1 at a resolution of 4cm-1 . The conductivity of EDTA-RGO film was tested with a Multi Height Microposition Probe with RM3-AR test unit (Bridge Technology).

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