J Mater Sci

Facile synthesis of N-doped graphene aerogel and its application for organic solvent adsorption Hongbo Ren1,2, Xianpan Shi1, Jiayi Zhu1,2,*, Yong Zhang1, Yutie Bi1,2, and Lin Zhang1,2,3,* 1

Joint Laboratory for Extreme Conditions Matter Properties, Southwest University of Science and Technology and Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621010, China 2 School of Science, Southwest University of Science and Technology, Mianyang 621010, China 3 Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621900, China

Received: 25 January 2016

ABSTRACT

Accepted: 28 March 2016

N-doped graphene aerogel with a three-dimensional interconnected network was fabricated by using graphene oxide and melamine via the hydrothermal self-assembly and thermal annealing process. The aerogels were characterized by means of scanning and transmission electron microscopy, Fourier transform infrared spectrum, X-ray photoelectron spectroscopy, elemental analyses, X-ray diffraction, Raman spectrum, and nitrogen adsorption/desorption measurement. The N-doped graphene aerogel showed high-specific surface area, superhydrophobic nature, excellent adsorption capacity for organic solvents, and adsorption recyclability. It would be a promising material for removal of organic contaminates from water.

Ó

Springer Science+Business

Media New York 2016

Introduction Increasing attention has been paid to water contamination and it is a global environment concern nowadays [1–3]. With the economic continuous development, indiscriminate disposal of wastewater containing organic liquids from chemical industries takes up a big part of the pollutions [4]. To purify the contaminated water, a lot of effort has been put into the creation of new efficient adsorbents for the separation and adsorption of organic pollutants from water [5–7]. Recently, owing to high porosity, high surface area, diverse surface modification, and environmental compatibility, graphene aerogels have been

intensively explored as a promising adsorbent with a remarkable adsorption capability [8–10]. The hydrothermal method is the mostly used method to prepare three-dimensional (3D) graphene aerogels [11]. To the best of our knowledge, the weak interactions between graphene nanosheets, such as van der Waals force, hydrogen bonding, and p–p interaction, were the driving force to form 3D porous network structure [12]. However, the invalid stack between graphene nanosheets would decrease the porosity and surface area of aerogels [13], but increase the density of graphene aerogels, which would further eliminate its adsorption performance. Therefore, it is still desirable to develop an effective and environmentally friendly strategy to prepare 3D graphene aerogel with high

Address correspondence to E-mails: [email protected]; [email protected]

DOI 10.1007/s10853-016-9939-y

J Mater Sci

surface area and large pore volume. Melamine, which has three amino groups in its molecular, is a commercially available, cheap, and non-toxic chemical reagent. The hydrogen bond between amino groups in melamine molecular and oxygen groups on graphene oxide (GO) nanosheets could prevent GO nanosheets from self-stacking and allow the sufficient assembly of GO nanosheets into the large volume [14]. Herein, we used the melamine as the doped reagent to fabricate the N-doped graphene aerogel via hydrothermal self-assembly and thermal annealing process. The structure and adsorption capacities of N-doped GO aerogel (NGOA) and N-doped graphene aerogel (NGA) were explored in detail. The synthesized NGA after thermal annealing process had a large surface area of 852 m2 g-1 and high adsorption capacities for organic solvents. In addition, its recyclability was investigated, and the result revealed that the NGA could be a promising candidate for removal of organic contaminates from water.

Experimental Preparation of NGOA and NGA In a typical preparation, graphite oxide was prepared from natural graphite according to Hummers’ method [15] and dispersed in deionized water by ultrasonication for 2 h to acquire GO dispersion with a concentration of 5 mg mL-1. 12 mL of melamine aqueous solution (1.5 mg mL-1) was added slowly into 18 mL of GO dispersion under stirring at 60 °C. The melamine to GO mass ratio was 1:5. Then, the mixture was transferred into a 50-mL Teflon-lined autoclave at 180 °C for 12 h. The obtained hydrogel was washed with alcohol and acetone three times in sequence, and dried with supercritical CO2 to obtain the NGOA. The NGOA was heated to 900 °C at 5 °C min-1 in N2 atmosphere and held for 2 h to yield the NGA.

Characterization The morphology was characterized by scanning electron microscopy (SEM, Ultra 55, Zeiss). The contact angle was measured on a K100 contact angle system (Kruss) at room temperature. Fourier transform infrared (FTIR, Nicolet 6700 Thermo Fisher) spectroscopy and X-ray photoelectron spectroscopy (XPS, AXIS Ultra DLD) were used to identify the chemical compositions and surface

electron structure of the samples. The X-ray diffraction (XRD) analysis was carried out in X’ Pert PRO with Cu Ka radiation at k = 0.15406 nm. The vibrational characteristic of the samples were analyzed via Raman spectroscopy (InVia, Renishaw). The specific surface area and pore size distribution were characterized by nitrogen adsorption/desorption measurement (AR-JW-BK112).

Organic solvent adsorption tests For organic solvent adsorption tests, various organic solvents, including n-hexane, tetrahydrofuran (THF), chlorobenzene, xylene, tetrachloromethane (CCl4), and N-methyl-2-pyrrolidone (NMP), were used to evaluate the adsorption capacities of NGOA and NGA. In the adsorption test, aerogel samples were immersed in organic solvents, transferred to a pre-weighed glass vial and weighed immediately. NGA or NGOA was weighed every ten seconds until its mass did not increase any more. Both NGA and NGOA could reach the equilibrium state in 1 min for all organic solvents. The weights before and after adsorption were recorded for calculating the values of weight gain. Moreover, the volume adsorption capacities of NGOA and NGA were also calculated. The adsorption capacity calculated by weight (QW) and volume (QV) were calculated by formula (1) and formula (2), respectively [16]. Qw ¼

Wi  W0 W0

ð1Þ

QV ¼

Vi : V0

ð2Þ

Herein, in formula (1), W0 and Wi are the initial and terminal weights of the sample before and after adsorption experiment, respectively. In formula (2), V0 and Vi are the volumes of the aerogel and organic solvent, respectively. In the oil–water separation test, n-hexane was taken as an instance to detect the oil–water separation capacity of NGA. In addition, the cycle of adsorption and recovery was repeated 10 times to characterize the recycling performance of NGA.

Results and discussion Structure characterization As illustrated in Scheme 1, in the actual experiment, the GO/melamine aqueous solution was transferred

J Mater Sci

into a 50-mL Teflon-lined autoclave at 180 °C for 12 h and GO nanosheets were self-assembled to form 3D aerogels. In hydrothermal condition, the weak interactions, including van der Waals force, hydrogen bonding, and p–p interaction between GO nanosheets, were the driving force to form 3D porous network structure [12]. After pyrolyzed in a tubular furnace at 900 °C for 2 h in N2 atmosphere, residual oxygen-containing groups on graphene nanosheets were removed and nitrogen atoms were doped into the graphene aerogel. The morphology of NGOA and NGA were revealed by SEM observations. As revealed in SEM images (Fig. 1a, b), the network frame of both NGOA and NGA was composed of randomly cross-linked graphene nanosheets and possessed a similar 3D network structure with rich nanopores and micropores inside. In order to test the surface wetting property, surface wettabilities of the NGOA and NGA were investigated by contact angle measurement of water droplet. As depicted in insets of Fig. 1, the contact angle values of NGOA and NGA were 113.5° and 137.2°, respectively. The NGA became more hydrophobic after thermal annealing process, indicating the removal of oxygen-containing groups and recovery of the intrinsic hydrophobicity of graphene. The chemical compositions of aerogels were investigated by FTIR analyses. The FTIR spectra of GO, NGOA, and NGA are shown in Fig. 2. As for GO, the peaks at 3433, 1731, 1402, 1224, and 1090 cm-1 were assigned to O–H stretching vibration, C=O stretching vibration, O–H bending vibration from hydroxyl groups, and C–O stretching vibrations from epoxy and alkoxy, respectively [3]. After the hydrothermal self-assembly process, most characteristic absorption bands of oxygen-containing groups in NGOA decreased dramatically comparing with GO, indicating that GO nanosheets were

successfully reduced to graphene nanosheets. And the spectrum of NGA indicated that the residual oxygen-containing groups in NGOA were further reduced after thermal annealing process. Meanwhile, XPS characterization was performed to analyze the surface electron structure of the NGA. In the C1s spectrum (Fig. 3a) of the NGA, there were four peaks located at 284.6, 285.8, 287.7, and 289.7 eV, which were assigned to C–C/C=C, C–N, C=O, and COOH, respectively [17]. The bonding configurations of nitrogen atoms in the NGA were characterized by N1s spectra (Fig. 3b). The binding energy peaks at 398.1, 399.9, and 402.2 eV were assigned to pyridine-like N, pyrrole-like N, and pyridine-N-oxide, respectively [17, 18]. The XPS analysis revealed that nitrogen atoms have been successfully incorporated into graphene frameworks. In order to further investigate chemical compositions of NGOA and NGA, elemental analysis were conducted for both aerogels. As shown in Table 1, The O mass ratio in NGA was 1.5 %, which decreased greatly comparing with that of NGOA (18.7 %), indicating a further reduction of NGOA under high temperature and N2 atmosphere. It was contributed that the NGA possessed the larger water contact angle and more hydrophobicity than NGOA. The XRD spectra of graphite, GO, NGOA, and NGA are represented in Fig. 4a. It could be found ˚ that the interlayer spacing was increased from 3.36 A ˚ for GO without (002) diffraction for graphite to 8.3 A peak of graphite, indicating that the graphite was efficiently oxidized and highly exfoliated [19]. NGOA and NGA both exhibited a broadband at 2h = 24.3 ˚ ), which is close to the peak 2h = 26.5 (d = 3.7 A ˚ ) of the graphite. These results revealed (d = 3.36 A the existence of the inhomogeneous graphite-like carbon crystalline state in NGOA and NGA [4]. Raman spectra of NGOA and NGA are shown in

Scheme 1 Schematic illustration of the synthesis of NGA and its adsorption for oil removal in water.

J Mater Sci

Figure 1 SEM images and contact angle values of a NGOA and b NGA.

Fig. 4b. The Raman spectra of NGOA and NGA contained both D bands (the A1g symmetry mode) at about 1353 cm-1 and G bands (the E2g mode of the

Figure 2 FTIR spectra of GO, NGOA, and NGA.

Figure 3 C1s XPS (a) and N1s XPS (b) spectra of NGA.

sp2 carbon domains) at about 1597 cm-1 [20]. The intensity ratio of the two bands (ID/IG) for NGA was 0.859, which was slightly higher than that of NGOA (0.698). This was attributed to an increase in the number of smaller graphene domains after partial removal of the oxygen moieties, which resulted the decrease in the average size of the sp2 domains upon the reduction of NGOA [21]. Nitrogen adsorption/desorption measurements were conducted to characterize the surface area and pore structure of the samples. The surface area and pore size distribution of NGOA and NGA were calculated using the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods (Fig. 5a, b). Both aerogels showed a type IV isotherm with a H3-type hysteresis loop, which indicates that the mesopore has the cylindrical pore geometry [22]. The BET surface areas of NGOA and NGA were as high as 651 and 852 m2 g-1, which were equal or higher than those of graphene aerogels [23, 24]. As shown in Fig. 5b and Table 1, both

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Table 1 Densities (q), N2 adsorption/desorption, and elemental analyses for NGOA and NGA

Sample

NGOA NGA

q (mg cm-3)

17.5 11.7

Surface area and pore size analyzer

Elemental analyses

SaBET (m2 g-1)

Vbt (cm3 g-1)

Dc (nm)

N%

C%

H%

O%

651 852

1.26 1.37

21 19

4.3 4.8

75.0 93.0

2.0 0.7

18.7 1.5

a

BET surface area

b

Total pore volume

c

BJH desorption major pore width

Figure 4 a XRD patterns of graphite, GO, NGOA, and NGA; b Raman spectra of NGOA and NGA.

Figure 5 a N2 adsorption/ desorption isotherms and b pore size distributions of NGOA and NGA.

aerogels had a broad pore size distribution in a range of 10–90 nm and the pore volume of NGOA and NGA were 1.26 and 1.37 cm3 g-1, respectively. Moreover, it could be seen that the surface area and total pore volume increased apparently after thermal annealing process, which was probably resulted by the fact that the removal of surface oxygen-containing groups exposed more small space cavities. As a result, large pore volume and hydrophobic property would endow the NGOA and NGA with considerable adsorption capacity for organic solvents.

The organic solvent adsorption capacities of the NGOA and NGA Based on structure characterizations, the NGOA and NGA with high surface area and 3D porous network are the ideal candidate for highly efficient extraction of organic pollutants. Various organic solvents, including n-hexane, THF, chlorobenzene, xylene, tetrachloromethane, and NMP, were used to evaluate adsorption capacities of the NGOA and NGA. The average mass of aerogels was 17 mg and they were

J Mater Sci

Figure 6 Weight-based (a) and volume-based (b) adsorption capacities of NGOA and NGA.

immersed in organic solvents for sufficient time. It could be found that the aerogels reached the equilibrium state for adsorbing organic solvents in 1 min and the corresponding adsorption capacities of the NGOA and NGA are shown in Fig. 6. As seen from Fig. 6a, the weight adsorption capacities (QW) of NGOA were 41.6, 49.5, 62.1, 46.2, 82.4, and 42.7 g g-1 for uptaking n-hexane, THF, chlorobenzene, xylene, tetrachloromethane, and NMP, respectively. The corresponding QW of NGA for the same organic solvents were 68.6, 71.4, 83.9, 52.6, 111.6, and 60.2 g g-1, respectively, which was higher than that of carbon aerogel published in recent 2 years [25–27]. It was seen that the corresponding QW of NGA for organic solvents was higher than that of NGOA. It was known that the QW of graphene aerogels would rapidly increase with the decrease of aerogel density [28]. NGA had a lower density (11.7 mg cm-3) comparing with that of NGOA (17.5 mg cm-3), and thus, under the same weight, NGA had a bigger volume than NGOA for the adsorption of organic solvents. In addition, NGA had a larger surface area and pore volume, resulting in the increase of active sites exposed to organic solvents during the adsorption. Meanwhile, except the adsorbent density, the adsorption capacity of the adsorbent was also related to the density, surface tension, and viscosity of organic solvents [29]. In general, the adsorption capacity would be higher to the solvents with large density and surface tension but low viscosity [30]. Thus, it could be seen that the highest adsorption capacity of the NGA was achieved to tetrachloromethane, since it had the highest density among all the solvents (Table S1). Moreover, even though chlorobenzene and NMP had the similar

density, the adsorption capacity of the NGA for NMP was much lower than that for chlorobenzene, which was due to high viscosity of NMP. Moreover, we compared the adsorption capacities of GOA and NGOA. For taking up CCl4, the weight adsorption capacity of GOA was just 31.9 g g-1, which was much less than that of NGOA (82.4 g g-1). This could be attributed that the hydrogen bond between melamine molecules and GO sheets could prevent GO sheets from self-stacking and cause the sufficient assembly of GO sheets into a large volume. Therefore, doping melamine into graphene aerogel would make NGOA have the lower density, larger volume, and higher porosity, which contributes its better adsorption capacity than GOA [31]. Furthermore, since the mass-based adsorption capacity is affected by the densities of the adsorbents and organic solvents, the volume adsorption capacities (QV) of the NGOA and NGA were also measured (Fig. 6b). The NGOA and NGA exhibited the QV of 85.3–102.8 % and 82.8–95.9 % for the same organic solvents, respectively, indicating that most porous volume of the aerogel was used for the liquid storage. It also revealed that the aerogel fabricated by hydrothermal method had a thermal stable structure and the thermal annealing process made small effect on its whole porous framework. In a word, the results demonstrated that NGA exhibited the better capacities for various organic solvents. In the practical application, the removal of organic pollutants from water had more commercial interest. Considering high surface area and hydrophobicity of the NGA, its oil–water separation capacity was also evaluated by using the mixture of n-hexane and water as adsorbate. In a typical test, the NGA was put

J Mater Sci

Figure 7 Digital photographs of the adsorption performance of n-hexane on the water surface using NGA at different times (a), NGA supporting a 200 g weight (b), and recycling performance of NGA (c).

(a) 0 s

2s

(b)

200 g

5s

10 s

(c)

into the mixture of n-hexane and water (Scheme 1; Fig. 7a). The n-hexane was stained with Sudan I to facilitate the evaluation by naked eye. When NGA was put on the oil layer floating on the water surface, it adsorbed the surrounding organic solvents within a few seconds. The whole process of oil-adsorption took within ten seconds, illustrating an excellent selective oil-adsorption from water. What is more, although the NGA had a light weight and low density, it was still mechanically strong. As shown in Fig. 7b, 28 mg of NGA was able to support nearly 7000 fold its own weight without any collapsing, showing the possibility of reusing NGA in adsorption experiments. For the reusability of the NGA, it was heated up around the boiling point of adsorbates for the use in next adsorption test. As shown in Fig. 7c,

the adsorption capacity of NGA still maintained 96.5 % of its initial capacity after 10 cycles, demonstrating its excellent adsorption recyclability. Therefore, the NGA would offer the potential application on removal of organic contaminates in water.

Conclusion In summary, N-doped graphene aerogels were synthesized using GO and melamine via the hydrothermal method and thermal annealing process. SEM images and surface characterization results confirmed the interconnected 3D network of the NGA with highly hydrophobic property. The NGA had a typical 3D network made up of thin graphene

J Mater Sci

nanosheets. The BET surface area and pore volume of NGA was as high as 852 m2 g-1 and 1.37 cm3 g-1, respectively. Importantly, the NGA presented high weight adsorption capacity for organic solvents up to 52.6–111.6 g g-1 along with high volume adsorption capacity up to 82.8–95.9 % of its own volume, and maintained 96.5 % of its initial capacity after 10 cycles. The results suggested that the NGA would offer great potential application for removal of organic contaminates in water.

[5]

[6]

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Acknowledgements [8]

This research was financially supported by the National Natural Science Foundation of China (Grant No. 51502274), the Open Project of State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials (11zxfk26), the Postgraduate Innovation Fund of Southwest University of Science and Technology (No. 14ycx008), the Doctoral Research Fund of Southwest University of Science and Technology (No. 15zx7137), and the Project Funded by China Postdoctoral Science Foundation (2015M572499).

Electronic supplementary material: The online version of this article (doi:10.1007/s10853-016-9939-y) contains supplementary material, which is available to authorized users.

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