One-step synthesis of hydrophobic-reduced graphene oxide and its oil/water separation performance

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1 J Mater Sci (2016) 51: One-step synthesis of hydrophobic-reduced graphene oxide and its oil/water separation performance Zhihong Tang 1, *, Zhiwen Zhang 1, Zhuo Han 1, Shuling Shen 1, Jing Li 1, and Junhe Yang 1 1 School of Materials Science and Engineering, University of Shanghai for Science and Technology, Shanghai , China Received: 18 January 2016 Accepted: 28 March 2016 Published online: 28 June 2016 Springer Science+Business Media New York 2016 ABSTRACT Graphene oxide (GO) was functionalized to form hydrophobic-reduced graphene oxide (rgo) by a one-step hydrothermal method, and oleylamine was used as both reductant and modifier of GO. X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction, Raman spectroscopy, and contact angle measurement were used to determine the successful functionalization and reduction of GO. Results indicated that the color of obtained sample was changed from yellow brown to black, and contact angle between water and the graphene paper was over 130, which was close to the typical hydrophobic material of PTFE; at the same time, the functionalized rgo can be dispersed in some of the typical organic solvents, such as cyclohexane, chloroform, and benzene, proving that oleylamine was effective for the reduction and functionalization of GO. Based on the results, the possible reactions were proposed. Furthermore, the hydrophobic rgo was assembled to film by filtration, which demonstrated its efficient separation ability for oil/water. Introduction Graphene, a one-atom-thick layer of sp 2 -hybridized carbons closely packed into a honeycomb structure, has great potential for various applications due to its extraordinary mechanical, electrical, and thermal properties [1 5]. In fact, the unique properties of graphene are mostly associated with its single layer [6 9]. Therefore, the dispersion ability of graphene in some kinds of solvents is of crucial importance. However, it is well accepted that graphene cannot be dispersed in most solvents [7, 10], graphene sheets tend to form irreversible agglomeration or even restack to form graphite due to strong Van Der Waals force and high-specific surface area (the theoretical BET surface area of graphene is over 2600 m 2 /g) [10 12]. Thus, improving the dispersion ability of graphene sheets, and thus preventing them from aggregation is another challenge of the further synthesis and application of graphene. Graphene oxide (GO) is a general starting material for the preparation of graphene sheets [13 15]. Besides its scalable production, the presence of carboxyl, hydroxyl, and epoxide groups makes GO hydrophilic, ready for exfoliation, and can form stable colloidal suspension in water [16 19]. Furthermore, the oxygen-containing functional groups Address correspondence to zhtang@usst.edu.cn DOI /s

2 8792 J Mater Sci (2016) 51: readily react with other agents, which endows it with excellent chemical functional ability [19 22]. The functionalized graphene oxide not only can be dispersed readily in various solvents and further processed by solvent assisted method, but also can prevent the aggregation; this remain part of the intrinsic properties of graphene, which then enlarges the application area of graphene greatly [15, 23, 24]. Therefore, noncovalently and covalently functionalizing rgo have become hot topics. In general, the covalent functionalization of graphene oxide can be separated into two steps: (1) surface modification; (2) reduction. So far, several chemically functionalized rgos have been reported. Ruoff and his co-workers firstly terminated the hydrophilic oxygen containing groups of GO by phenyl isocyanate to obtain the uniform dispersion of isocyanate-treated graphene in organic solvent [25], unfortunately, expensive modifying agent was necessary in the first step, and the second step often required strong but extremely toxic reducing agent such as hydrazine, hydroquinone, and NaBH 4. Therefore, finding an efficient method to functionalize and reduce GO simultaneously is of great importance. Xu et al. reported that dopamine induced reduction and functionalization of GO; the functionalized and reduced GO can be dispersed in H 2 O, THF, DMF, CHCl 3 [26]. Noncovalent functionalization of rgo with p p interaction was also suggested, the obtained functionalized rgo can be dispersed in diverse solvent without sacrificing the physical and chemical properties of graphene [27]. However, the method needs to reduce graphene oxide firstly, and the concentration of the obtained dispersion was very low. Therefore, functionalizing rgo efficiently is still very important for further application. Herein, we demonstrated a one-step and largescale approach to functionalize and reduce graphene oxide by hydrothermal method. Oleylamine was chosen as a modifying agent because it had both reducibility and lipophilicity; at the meantime, its long 18-C chain had been grafted successfully on the rgo and the great steric hindrance further prevented rgo from p p stacking, which made it an ideal modifier for preparing hydrophobic rgo. The prepared sample can not only be dispersed in various solvents, but also an effective filter for oil/water separation. Experimental Sample preparation GO was obtained using a modified Hummers method [28]. In a typical process, 5.0 g of graphite, 2.5 g of NaNO 3, and 115 ml of H 2 SO 4 were stirred together in an ice bath for 45 min, then 15 g of KMnO 4 was slowly added to the solution while stirring, and the solution was transferred to a 35 C water bath and stirred for 45 min to form a black green thick paste. Then 300 ml of deionized water was added and the mixture was slowly heated to 98 C, stirred gently for 45 min. After that, 15 ml of H 2 O 2 (30 wt% aqueous solution) and 50 ml of HCl (10 wt% aqueous solution) was added and stirred for 30 min. After that, the yellow brown colloid was obtained. The impurities were removed through six washing cycles, and then the graphite oxide was redispersed in water by stirring and ultrasonication. After drying, the loose and brown GO powder was obtained. GO (30 mg) was dissolved homogenously in ethanol (30mL).Thenoleylaminewiththemassratioof1:1was added into the solution and stirred for 30 min to make oleylamine dissolve completely in GO dispersion. Afterwards, the dispersion was removed to a sealed Teflon-linedautoclaveandheatedbelow180 Cfor12 h, then it was cooled to room temperature. The excessive oleylamine and other impurities were washed by cyclohexane and ethanol. After drying, the hydrophobic rgo was obtained; the sample was marked as OGO. Oil/water separation experiments Thirty milligram of hydrophobic rgo was dissolved in 60 ml cyclohexane and sonicated for 30 min; then the solution was filtered to form the hydrophobic rgo film. After drying, a free-standing functionalized rgo film was obtained. The oil/water emulsion was prepared by mixing 38 ml of water and 2 ml oleylamine through sonication. In order to separate the emulsion, the hydrophobic rgo film was put in a filter, and the emulsion was filtered by the rgo film to separate oil from oil/water emulsion. Characterization Morphology of the as-prepared samples was observed by transmission electron microscopy (TEM) (JEOL JEM 2100F). GO and OGO were dispersed in

3 J Mater Sci (2016) 51: water and ethanol, respectively; each suspension was dipped on the copper grid, after drying at 100 C, the GO and OGO on the copper grids were observed by TEM. Functional groups and chemical bonds of the samples were determined by X-ray photoelectron spectroscopy (XPS, PHI 5000C ESCA) and Fourier transform infrared (FTIR) spectroscopy (P.E. Spectrum 100). XPS analysis was operated at constant pass energy of 20 ev by AlKa radiation. The operating pressure in the chamber was maintained at about mbar. The sample of FTIR was prepared by mixing the sample with KBr, then the mixture was grounded thoroughly; the well-mixed sample was compacted into a transparent slice. Raman spectra were recorded using a Raman Station (P.E., 400/400F) fitted with a 785-nm laser. The test range was cm -1 and resolution was 1.0 cm -1. Contact angle was tested by an optical contact angle measuring instrument (Kruss DSA 30); the hydrophobic rgo was filtered into free-standing film. A drop of water was dripped on the film and the contact angle between the film and water was measured. The resistance of hydrophobic rgo film was tested by a four-probe tester (RTS-9). The optical images of the emulsion before and after filtration were obtained by putting the emulsion onto a slide to form a thin film, which was directly observed by a microscope (Zeiss Axiocam MRC 5). Results and discussion Figure 1 shows the TEM images of GO and OGO. From Fig. 1a it can be seen that GO contained multiple overlaid nanosheets, which were continuous and formed an unsuppported two dimensional film with the size over several micrometers. After hydrothermally treated by oleylamine, multilayers still appeared (Fig. 1b), indicating the functionalization and reduction had not changed the morphology of GO. As can be seen from Fig. 2, the modified rgo can be dispersed well in most of organic solvents, especially in toluene and cyclohexane. When 1 mg/ml of functionalized GO dispersion was made to stand for 1 h, only small amount of precipitate appeared in toluene and cyclohexane, and no more graphene precipitated even after 3 days. This was because the oleylamine acted not only as a modifier, but also as a reductant. When excessive oleylamine was added, the oxygen functional groups reacted with oleylamine, and at the same time, a majority of sp 3 hybrid covalent bonds changed to sp 2 hybrid carbon; in the end, the increased sp 2 hybrid carbon endowed OGO with stable dispersion ability in toluene and cyclohexane. Moreover, the color of the modified sample changed from dark brown into dark black; as far as I know, the higher the oxide degree of graphene oxide, the closer the color to yellow. Meanwhile, the resistance of the modified graphene film was about 2000 X, indicating that a majority of sp 3 hybrid covalent bonds changed to sp 2 hybrid carbon after the modification, which further proved the modified rgo had been reduced at the same time. The hydrophobicity of the modified rgo was further tested by measuring the contact angle. When a drop of water was dropped on the modified rgo paper, as shown in Figure S1, the contact angle between the rgo film and water was about 130, which was close to the typical hydrophobic material PTFE, indicating that oleylamine was successfully grafted on the graphene oxide. At high temperature and pressure, most of the original hydroxyl and carboxyl groups of GO were changed into amino functional groups with long carbon chain, thus losing the inherent hydrophilicity of GO. The possible reaction between GO and oleylamine is shown in Fig. 3. Under the hydrothermal condition, oleylamine would react with carboxylic and hydroxyl functional groups on the edge of GO sheets to form amide-like structure. Open epoxides on the plane of GO, the hydrogen atom on the amide and hydroxyl would further react with each other. In the end, long chain amino group was grafted on the basal plane of GO, and GO was reduced simultaneously. XPS analysis was used to measure the chemical state of carbon, oxygen, and nitrogen atoms. According to the carbon to oxygen ratio of the hydrophobic rgo, the reduction of GO and grafting of oleylamine molecules on the GO can be determined. The carbon to oxygen ratio of the original graphene oxide was 2.42, while, the carbon to oxygen ratio of lipophilic graphene was increased to 7.12, indicating that GO was reduced greatly. In the modified sample, the nitrogen content was about 1.63, because the nitrogen content of the oleylamine is about 5.24 %; the mass ratio of GO to oleylamine was 1:1, the relatively high nitrogen content suggested the reaction between oleylamine and graphene oxide, indicating oleylamine had been grafted on GO successfully. The comparison of C1s spectra of GO and oleylamine-

4 8794 J Mater Sci (2016) 51: Figure 1 TEM images of graphene oxide (GO, a) and hydrophobic-reduced graphene oxide (OGO, b). Figure 2 The dispersion of hydrophobic-reduced graphene oxide (OGO) in different solvents (1 mg/ml) after 1 h (a) and after 3 days (b). Solvents correspond to ethanol, acetone, cyclohexane, toluene, dichloroethane, and dimethylbenzene from left to right, respectively. Dimethylbenzene, 1 mg/ml. treated GO further proved the reduction and functionalization of GO. As shown in Fig. 4, C1s spectrum of GO can be divided into four peaks, C=C/C C, C O/C O C, C=O, and C OOH, which were observed at 284.6, 286.3, 288, and 289 ev, respectively. Besides these four peaks, there was another peak C N which appeared at ev for functionalized graphene, which is detailed in Table 1. Furthermore, the peak intensity and position of the same peaks changed, and the change in the position indicated the strong chemical interaction between GO and oleylamine. The intensity decrease of oxygen functional groups was attributed to the reduction of GO. FTIR spectrum was further used to determine the successful functionalization and reduction of graphene oxide (Fig. 5a). For comparison, the FTIR spectra of GO were also demonstrated. From the figure it can be seen that the oxygen groups of GO (C=O stretching at 1736 cm -1, C O stretching at 1418 and 1051 cm -1 ) had almost disappeared when GO was functionalized by oleylamine; at the same time, C N bond appeared (C N stretching at 1363 cm -1 ), proving the functionalization and reduction of GO. As shown in Fig. 5b, two prominent peaks of GO and OGO appeared in the Raman spectra: the disordered D band at 1310 cm -1 and the graphitic G band at 1590 cm -1 indicate the original structure of GO remaining in functionalized rgo. The I D /I G of GO and functionalized rgo was similar, which was about 1.3, suggesting the oleylamine was grafted on GO. Figure 5c shows the XRD patterns of GO and OGO. There was a sharp diffraction peak of GO at about 12, corresponding to the (001) reflection of GO, which indicates that the original crystal structure was destroyed, and new structure was formed in GO. After oleylamine treatment, a broad (002) peak appeared at about 22, which is close to that of graphene reduced by thermal or chemical reduction, implying the reduction of GO. Since the prepared OGO was hydrophobic and can be dispersed in most organic solvent, it should be an ideal filter for separating oil from water. Figure 6a

5 J Mater Sci (2016) 51: Possible reactions: Figure 3 The proposed mechanism for the formation of oleylamine-modified hydrophobic-reduced graphene oxide (OGO) graphene during the one-pot hydrothermal reaction. Figure 4 Deconvoluted C 1s XPS spectra of graphene oxide (GO, a) and hydrophobic-reduced graphene oxide (OGO, b). Table 1 Detailed information of C 1s XPS spectra of GO and OGO C/O C C/C=C (284.6 ev) (%) C N (285.4 ev) C O (286.3 ev) (%) C=O (288 ev) (%) O C=O (289 ev) (%) GO OGO % shows that OGO can form a piece of freestanding film with the diameter of 45 mm by filtration, which provided the possibility of hydrophobic rgo film as filter to separate oil from water/oil emulsions. Figure 6b shows the water/oil emulsion (the left) before filtration. It was obvious that the emulsion was white, and it became clear and transparent after filtration, indicating that the oil and water were separated. Optical images of the emulsion before and after filtration further proved the successful separation. Based on the two optical images (Fig. 6c, d), it can be seen that there existed many oil molecules in the mixture, while could hardly be seen after filtration. The separation results proved that the functionalized graphene film was an effective filter for oil/water separation.

6 8796 J Mater Sci (2016) 51: Figure 5 FTIR spectra (a), Raman spectra (b), and XRD patter (c) of graphene oxide (GO) and hydrophobic-reduced graphene oxide (OGO). Figure 6 a Freestanding film of hydrophobic-reduced graphene oxide (OGO) prepared by filtration from water/oil emulsions; b photographs of the water/oil emulsions before (left) and after (right) the filtration by the hydrophobic graphene oxide film; Optical images of the water/oil emulsions before (c) and (d) after filtration by the hydrophobic-reduced graphene oxide film. Conclusions Oleylamine-functionalized GO was successfully prepared by a one-step hydrothermal method. Oleylamine acted not only as a reductant, but also as a modifier. The functionalized rgo can be dispersed in a majority of organic solvents, which effectively dissolved the poor dispersity problem of graphene. Furthermore, the functionalized graphene can be assembled into functionalized rgo film, which demonstrated its excellent separation ability for oil/ water separation.

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