Three dimensional nitrogen-doped graphene aerogels functionalized with melamine for multifunctional applications in supercapacitors and adsorption

Three dimensional nitrogen-doped graphene aerogels functionalized with melamine for multifunctional applications in supercapacitors and adsorption

Author’s Accepted Manuscript Three Dimensional nitrogen-doped Graphene Aerogels functionalized with melamine for multifunctional Applications in super...

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Author’s Accepted Manuscript Three Dimensional nitrogen-doped Graphene Aerogels functionalized with melamine for multifunctional Applications in supercapacitors and adsorption Ling-Bao Xing, Shu-Fen Hou, Jin Zhou, Jing-Li Zhang, Weijiang Si, Yunhui Dong, Shuping Zhuo www.elsevier.com/locate/yjssc

PII: DOI: Reference:

S0022-4596(15)30060-8 http://dx.doi.org/10.1016/j.jssc.2015.07.009 YJSSC18982

To appear in: Journal of Solid State Chemistry Received date: 30 April 2015 Revised date: 19 June 2015 Accepted date: 9 July 2015 Cite this article as: Ling-Bao Xing, Shu-Fen Hou, Jin Zhou, Jing-Li Zhang, Weijiang Si, Yunhui Dong and Shuping Zhuo, Three Dimensional nitrogendoped Graphene Aerogels functionalized with melamine for multifunctional Applications in supercapacitors and adsorption, Journal of Solid State Chemistry, http://dx.doi.org/10.1016/j.jssc.2015.07.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Three Dimensional Nitrogen-Doped Graphene Aerogels Functionalized with Melamine for Multifunctional Applications in Supercapacitors and Adsorption Ling-Bao Xing, Shu-Fen Hou, Jin Zhou, Jing-Li Zhang, Weijiang Si, Yunhui Dong*, Shuping Zhuo*

School of Chemical Engineering, Shandong University of Technology, Zibo 255049, P. R. China

*Corresponding author: Tel. /fax: +86 533 2781664. E-mail: [email protected] (Y. Dong); [email protected] (S. Zhuo).

Abstract

In present work, we demonstrate an efficient and facile strategy to fabricate three-dimensional (3D) nitrogen-doped graphene aerogels (NGAs) based on melamine, which serves as reducing and functionalizing agent of graphene oxide (GO) in an aqueous medium with ammonia. Benefiting from well-defined and cross-linked 3D porous network architectures, the supercapacitor based on the NGAs exhibited a high specific capacitance of 170.5 F g-1 at 0.2 A g-1, and this capacitance also showed good electrochemical stability and a high degree of reversibility in the repetitive charge/discharge cycling test. More interestingly, the prepared NGAs further exhibited high adsorption capacities and high recycling performance toward several metal ions such as Pb2+, Cu2+ and Cd2+. Moreover, the hydrophobic carbonized nitrogen-doped graphene aerogels (CNGAs) showed outstanding adsorption and recycling performance for the removal of various oils and organic solvents. Keywords: graphene aerogels; melamine; porous; supercapacitor; adsorption.

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1. Introduction

As a single layer of carbon atoms tightly packed into a two-dimensional (2D) sheet of sp2-hybridized carbon, graphene has attracted a great deal of attention in recent years because of its excellent strong mechanical [1,2], high electrical and thermal properties [3-5], and large surface area [6,7], which offering great opportunities for potential applications such as batteries [8-13], supercapacitors [14-17], fuel cells [18-23], photovoltaic devices [24-27], photocatalysis [28-31] and so on. However, due to noncovalent interactions such as π-π stacking and hydrophobic interactions [32,33], the obtained 2D graphene tend to interact with each other to form irreversible aggregates or overlap into graphitic structure between the intersheet of graphene, resulting in a dramatic decrease of the surface area [34]. Consequently, the intrinsic properties of the obtained graphene sheets cannot be exploited thoroughly.

In the early days, to solve the problem and make full use of the high intrinsic surface area of graphene, various methods have been developed to produce 2D graphene thin films by self-assembly of the graphene sheets, such as flow-directed or evaporation-induced self-assembly, Langmuir-Blodgett technique, and layer-by-layer deposition [32]. Compared to the 2D graphene thin films, graphene based 3D architectures with controlled micro- or nano- porous or network structures have attracted more and more attention. Therefore, a variety of methods including self-assembly, template guided growth, organic sol-gel reaction and LightScribe patterning technology have been developed for fabricating these materials [33]. Most of all, self-assembly of GO into 3D architectures especially hydrogels and aerogels with porous structures have shown potential applications in flexible electronics, supercapacitors, catalysis, hydrogen storage, sensors and environmental remediation [32-36]. 2

Although the reduced GO sheets self-assembled into 3D hydrogels with different reducing agents such as vitamin C [37,38], p-phenylenediamine (PPD) [39], dopamine [40,41], N2H4 [42], sugar [43] and NaHSO3 [44] have been reported, the obtained hydrogels have shown high performance in either supercapacitors or adsorption for metal ions and dyes, there is still less reports to prepare hydrogels with 3D architectures for multifunctional applications. Shi and coworkers reported a green and mild method for the synthesis of 3D architectures of graphene in the chemical reduction process of GO with the aid of a range of natural phenolic acids, which can be used as adsorbents for removal of oils, organic solvents and dyes from contaminated water, as well as electrode materials for supercapacitors [45]. Qu and coworkers demonstrated a versatile, ultralight and nitrogen-doped graphene by using pyrrole as dopant in a hydrothermal process [46], which exhibited high capacity for the reversible adsorption of oils and organic solvents and generated a high specific capacitance. However, the graphene based hydrogels or aerogels with multifunctional properties are still needed to be explored extensively.

Doping graphene with other chemical elements can tailor its band structure, modulate electronic properties, manipulate surface chemistry, and open the bandgap in graphene for device applications. Among the numerous potential dopants, nitrogen is considered to be an excellent element for the chemical doping of graphene because it is of comparable atomic size and contains five valence electrons available to form strong valence bonds with carbon atoms. As a result of the rich nitrogen content, melamine is often used as nitrogen source to dope with graphene through different ways such as thermal annealing [47], chemical vapor deposition [48], pyrolysis [49,50], ball-milling [51]. By using melamine as a strong hydrogen-bond acceptor and cross-linker, Shi and coworkers [52] studied the three-dimensional self-assembly of GO sheets in aqueous media to form hydrogels.

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However, all the above works used melamine as nitrogen source, stripping agent, or non-convalent interaction acceptor to interact with graphene.To the best of our knowledge, there is still no reports on the funtionalization of graphene with melamine, especially realizing the self-assembly of reduced GO nanosheets into hydrogels or aerogels through covalent bonds. In the present work, we demonstrate an efficient and facile strategy to fabricate nitrogen-doped graphene aerogels (NGAs) functionalized with melamine, which serves as reducing and functionalizing agent of GO in an aqueous medium with ammonia. (Fig. 1) Benefiting from well-defined and cross-linked 3D porous network architectures, the supercapacitor based on the NGAs exhibited a high specific capacitance of 170.5 F g-1 at 0.2 A g-1, and this capacitance can be maintained for 70.5% as the discharging current density was increased up to 10 A g-1. It also showed that the specific capacitance of the NGAs supercapacitor still maintained at 139.5 F g-1 after 4000 charge/discharge cycles at 0.2 A g-1, about 82% of the discharge capacitance. (table S1) More interestingly, the prepared NGAs exhibit high adsorption capacities toward several metal ions such as Pb2+ (205 mg g-1), Cu2+ (175 mg g-1) and Cd2+ (155 mg g-1), which is comparable to other graphene hydrogels adsorbents, such as dopamine hydrogels [40] (Pb2+ 336.32 mg g-1, Cd2+ 145.48 mg g-1), lys-RGO aerogels [53] (Pb2+ 129.5 mg g-1, Cd2+ 86.5 mg g-1), rGO/α-FeOOH composite hydrogels [54] (Cr4+ 139.2 mg g-1, Cd2+ 373.8 mg g-1), and other hydrogels or aerogels [36]. (table S2) Moreover, the hydrophobic carbonized nitrogen-doped graphene aerogels (CNGAs) showed outstanding adsorption for the removal of various oils such as petroleum ether (68 times), paraffin liquids (102 times) and vacuum pump oil (84 times) and organic solvents such as toluene (93 times), tetrahydrofuran (THF) (57 times) and chloroform (91 times), which is comparative with the reported cys-RGO aerogels [53] (19-33 times), rGO/α-FeOOH composite hydrogels (27 times) [54], graphene-carbon nanotube

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aerogels [41] (21-35 times), and other hydrogels or aerogels [36]. (table S2)

2. Materials and Methods

2.1 Materials

Graphite powder was purchased from Qingdao Huatai Lubricant Sealing S&T Co. Ltd. All other chemicals were purchased from Sinopharm Chemical Reagent Co. Ltd and used directly without further purification.

2.2 Preparation of GO, NGAs and CNGAs

GO which possessed good dispersion in monolayered sheets (Fig. S1) was synthesized from natural graphite by using modified Hummer’s method according to our previous work [55,56]. The concentration of GO suspension was determined by drying small amount of suspension in a vacuum oven at 50 °C for 24 h and then weighing the dried GO.

The NGAs were prepared by using the following procedure, in which melamine was used as the reducing and functionalizing agent in the presence of ammonia. In a typical procedure, melamine (300 mg) and ammonia solution (450 μL) were added to GO suspension (15 mL) with a concentration of 5 mg mL-1. Subsequently, the mixture was sonicated for 10 min and then placed in an oil bath at 90 °C for 6 h without stirring to get the nitrogen-doped hydrogels. After the solution was naturally cooled down to room temperature, the resulting cylindrical structure of the hydrogels were dipped in methanol for 24 h and washed with hot water for several times to remove residual impurities and then the wet hydrogel was lyophilized to obtain NGAs. Control experiments were also carried out at the same condition.

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The CNGAs were obtained by a very facile method. NGAs were put in a tubular furnace at 600 °C (heating rate: 3 °C min-1) for 3 h under a N2 flow (80 cm3 min-1). After the carbonization, the sample was cooling to room temperature to get the target materials.

2.3 Characterization

X-ray diffraction (XRD) patterns were conducted using a Brucker D8 Advance diffraction with Cu Kα radiation (λ = 1.5418 Å). The interplanar crystal spacing was calculated by Bragg equation (2dsinθ = nλ). Raman spectra were obtained from a LabRAM HR800 (JY Horiba) with a 435 nm wavelength laser. The morphology of the samples was observed using field emission scanning electron microscope (FESEM, FEI, Holand) and a high resolution transmission electron microscope (HRTEM, JEOL2010, Japan). The surface chemical properties of the samples were characterized Fourier transform infrared spectroscopy (FT-IR, Nicolet 5700, USA). X-ray photoelectron spectroscopy (XPS) analysis was carried out on a Thermo Scientific Escalab 250 instrument equipped with Al Kα source (10 mA, 14 kV) and operating at 1486.8 eV during the measurements. The base pressure in the spectrometer analyzer chamber was lower than 2 × 10–9 mbar. The charge neutralizer filament was used during the experiment to control charging of the samples. To determine the surface element molar ratio, the atom sensitivity factors are 0.296, 0.711, and 0.477 for C1s, O1s, and N1s, respectively. The remaining concentrations of Pb2+, Cu2+ and Cd2+ were measured by spectrophotometer (Shimadu UV-2501). The specific surface area and pore size distributions (PSDs) of the as-prepared NGAs and CNGAs were measured by nitrogen sorption test using ASAP 2020 equipment (Micrometitics USA). Brunaner-Emmett-Teller (BET) surface area (SBET) was calculated using the N2 adsorption isotherm data within the relative pressure of 0.05-0.25. Total pore volume (VT) was obtained at p/p0=0.995. Microspore volume (Vmicro) was determined by 6

t-plot method. Mesopore volume (Vmeso) was calculated by subtracting the micropore volume from the total pore volume. PSDs were determined by applying the nonlocal density functional theory (NLDFT) model on the adsorption isotherms and assuming a slit-shape pore.

2.4 Electrochemical measurements

Working electrodes were prepared by pressing 2 mg of NGAs onto nickel foam under 10 MPa. In the electrochemical test, 6 M KOH solution was used as electrolyte. In order to ensure the electrode materials thoroughly wetted with the electrolyte of 6 M KOH solution, the working electrode was vacuum-impregnated with the electrolyte for 30 min and then steeped for 12 h before electrochemical tests. The electrochemical capacitive performances of NGAs electrodes were studied on a CHI660D electrochemical workstation (Chenhua Instruments Co. Ltd., Shanghai). In a three-electrode system by using NGAs as working electrode, a platinum film as a counter electrode, and a saturated calomel electrode (SCE) as a reference electrode, cyclic voltammetry (CV), galvanostatic charge-discharge measurement and electrical impedance spectroscopy (EIS) were performed. A potential window of -0.9 ~ 0 V vs. SCE reference electrode was applied to the electrochemical measurements.

2.5 Adsorption Experiments

2.5.1 Adsorption for metal ions

In the adsorption experiment, metal ions (Cd2+, Cu2+ and Pb2+) with different concentrations were prepared. By using UV-Vis spectrophotometer, the absorbance of the metal ions at different concentrations could be obtained. Then take the concentrations of metal ions as x axis and the absorbance of metal ions at different concentrations as y axis, calibration curve can be drew. At

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room temperature, 10 mg NGAs was added into 25 mL of metal ion-containing solution of different concentrations. After 24 h to reach complete adsorption equilibrium, the remaining concentrations of Cd2+, Cu2+ and Pb2+ were measured by the UV-Vis spectrophotometer. The adsorption capacities of the adsorbents were calculated according to the equation of qe = (C0 - Ce)V/m, where C0 and Ce represent the initial and equilibrium concentrations (mg g−1), respectively, where V is the volume of the solution (25 mL), and m is the amount of the adsorbents (10 mg). Diluted hydrochloric acid solution was used as desorption agent for Pb2+, Cu2+ and Cd2+ in the regeneration experiment.

2.5.2 Adsorption for different organic solvents

The adsorption performance of the CNGAs with different organic liquids was determined by weight measurements. Various oils such as petroleum ether, paraffin liquids and vacuum pump oil and organic solvents such as toluene, tetrahydrofuran (THF) and chloroform were tested. The specific experimental steps are as follows: The adsorbents (CNGAs 10 mg) was added into different oils (50 mL) in 100 mL beakers. After 30 min, the absorbents were taken out by tweezers and allowed to drain for a few minutes until no oils dripped. After removing the oil on the surface of the samples with filter paper, the saturated absorbent was then transferred to a preweighed watch glass and weighed immediately. The adsorption capacity was calculated using the weight gain of CNGAs before and after adsorption according to the equation of weight gain = (wtafter-wtbefore)/wtbefore, which also mean the fold of the absorbed oils to the absorbents. In order to realize the naked eye identification, the paraffin liquid was labeled with Sudan III in the adsorption experiments. The regeneration of adsorbent capacity was investigated in the same way after the oil saturated samples were dried in an oven at 200 °C for 1 h.

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Results and Discussion

3.1 Preparation of NGAs and CNGAs

By using melamine as reducing and functionalizing agents in aqueous solution of GO with ammonia in an oil bath at 90 °C without stirring, a well-defined black hydrogel with a cylinder shape was formed as shown in Fig. 1a-c. In the contrast experiments, GO suspension, GO and melamine, and GO and ammonia all could not form hydrogels in the same conditions (90 °C for 6 h without stirring), indicating GO suspension, melamine and ammonia are indispensable in the formation of hydrogels. After freeze-dried for 24 h, NGAs would be obtained. The NGAs have a relatively high mechanical strength as shown in Fig. 1e-f. Even with load of 200 g, the obtained NGAs give no obvious deformation, which indicate the mechanical strength properties of the aerogels. In order to get hydrophobic materials for the adsorption of oils, the NGAs was carbonized at 600 °C in nitrogen atmosphere for 3 h to get CNGAs.

Fig. 1. Schematic presentation for synthesis of the NGAs: (a) GO suspension, (b) N-containing graphene hydrogels, (c-d) nitrogen-doped graphene aerogels (NGAs), (e-f) NGAs with the load of 100 and 200 g.

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3.2 Characterization of NGAs and CNGAs

Fig. 2. FESEM and HRTEM images of NGAs (a, b, c) and CNGAs (d, e, f) of the interior microstructures.

The interior microstructures of the obtained NGAs were characterized by field emission scanning electron microscopy (FESEM) and high resolution transmission electron microscopy (HRTEM). As imaged by SEM images of NGAs and CNGAs (Fig. 2a, b, d, e and Fig. S2) with low and high magnifications, we can see that the graphene sheets folded, wrinkled and cross-linked with each other to form the network structures. The formation of the aerogels mean that the restored conjugated structure of GO sheets were reduced and functionalized with melamine in an aqueous medium with ammonia, which can induce partial overlapping or aggregating of flexible graphene sheets via π-π stacking interactions, forming the strong cross-links of the 3D graphene porous network. The HRTEM images further confirmed the wrinkled and folded paper-like textures of graphene sheets with cross-linked network structures as shown in Fig. 2c, f in line with the FESEM images. To determine the pore texture and specific surface area, we have performed N2 sorption measurements (Fig. S3), calculated PSDs using the NLDFT model, and then summarized the data of

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pore parameters (table S3). It can be seen that the adsorption-desorption isotherms of NGAs and CNGAs belong to typical type IV and I, respectively, indicating the main presence of mesopores of the as-prepared NGAs and CNGAs. The BET specific surface area for NGAs and CNGAs are about 157 and 122 m2 g-1, respectively.

Fig. 3. XRD (a) and Raman (b) spectra of GO (black), NGAs (red) and CNGAs (green).

The transformation from GO to NGAs is reflected in their XRD and Raman spectra in detail as shown in Fig. 3. The peak intensity of GO at 9.90° nearly disappeared along with the reaction, while a small broad bump near 26.16° appeared in the NGAs. This peak could be attributed to an interlayer spacing of 3.40 Å, which indicating the existence of π-π stacking between graphene sheets (Fig. 3a). Meanwhile, the CNGAs still give a wide peak around 26.1°, implying the specific crystal shape, structure and packing sequence are remaining unchanged after carbonization. Raman spectroscopy is also a useful tool to analyse carbon materials. In their Raman spectra as shown in Fig. 3b, strong vibrations of D bands (the A1g symmetry mode) in the vicinity of 1350 cm-1 and G bands (the E2g mode of the sp2 carbon atoms) in the vicinity of 1598 cm-1 were obviously observed in GO, NGAs and CNGAs. The intensity ratio of ID/IG was 0.89 in GO and increased to 0.95 and 0.99 in NGAs and CNGAs, which mean the removal of oxygenated groups and reduction of GO.

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Fig. 4. XPS survey (a); C1s spectra of GO (b), NGAs (c) and CNGAs (d).

XPS measurements were performed to further interpret the evolution of surface chemical properties after reducing and functionalizing with melamine in aqueous solution of ammonia. As shown in Fig. 4a, the three different peaks at 284.8, 399.1 and 532.5 eV were related to C, N and O, respectively. The atomic concentration calculated by using XPS spectra showed that the carbon and oxygen concentrations of GO were relatively increased and decreased, respectively. The content of oxygen (atm.%) dramatically decreased from 29.6 to 15.8 %, implying that the oxygen-containing groups have been efficiently removed. Moreover, the calculated atomic percentage of N in NGAs increased to 5.8 % by the reduced and functionalized process with melamine in the aqueous solution of ammonia, which is consistent with the reported values in the literature [57]. After removal of the polar groups in carbonization, the content of oxygen and nitrogen (atm.%) dramatically decreased to 4.3 and 4.5 % (table S3), respectively. The CNGAs became much more hydrophobic after the removal of polar groups in the carbonization process.

In order to further investigate the atom binding states of the prepared materials, the high

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resolution XPS measurements were carried out as shown in Fig. 4b, c and d, which showed the C1s deconvolution spectra of GO, NGAs and CNGAs. The decrease of the two peaks centered at 287.0 and 288.6 eV, corresponding to C=O (carbonyl) and O=C-O (carboxyl) groups, were observed, implying that a certain amount of the oxygen-containing groups have been removed. Besides, the appearance of the new characteristic peak corresponding to C-N bond (285.2 eV) indicated the successful covalent bonding between GO and melamine and doping of nitrogen. Moreover, the three components of N1s peak attributing to pyridinic N or imino groups (398.5 eV), substituted amines (R-NH-R) or pyrrolic N (399.6 eV), and amine groups or graphitic N (401.7 eV) further confirmed the links between GO sheets and melamine and doping of nitrogen into the graphene structures (Fig. S4 and table S4) [39,40].

Fig. S5 showed the FT-IR spectra of GO, NGAs and CNGAs. After reducing and functionalizing with melamine in aqueous solution of ammonia, the broad and intense band at 3000-3500 cm-1 related to hydroxyl groups (O-H) and peak at 1730 cm-1 related to carbonyl and carboxyl groups decreased, indicating the efficiently removal of the oxygen-contained groups in NGAs. Moreover, the peaks at 2925 and 2840 cm-1 attributed to the stretching and bending vibrations from C-H were increased. Meanwhile, the new peaks at 1440 cm-1 attributed to stretching vibrations of aromatic C=C were appeared. Furthermore, in accordance with XPS experimental data, a new peak at 1550 cm-1 corresponding to the N-H bending vibration appears, implying the formation of C-NH-C bonds resulting from the nucleophilic substitution reaction between the epoxide group in GO and the amine group in melamine [39].

3.3 Electrochemical measurements

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The electrochemical properties of the supercapacitor electrodes based on the obtained NGAs were investigated by cyclic voltammograms (CV) by using a conventional three-electrode system in 6 M KOH electrolyte. As shown in Fig. 5a at different scan rates of 10, 20, 30, 40, 50, 100, 150 and 200 mV s-1, all the CV curves of the NGAs based supercapacitor showed irregular rectangular shape, which indicating the co-existence of double layer capacitance and faradaic pseudocapacitance. The capacitive behavior was further tested by a galvanostatic charge/discharge experiment at different current density as shown in Fig. 5b and Fig. S6b. The specific capacitance could be calculated by the equation of C=(I·t)/(m·V), where C (F g-1) is the gravimetric specific capacitance of the carbon samples, I (A) is the discharge current, t (s) is the discharge time, V (V) is the potential window of the cell (0.9 V in this study), and m (g) is the mass of NGAs in the work electrodes. From the discharge curve, the specific capacitance of the NGAs electrodes were evaluated to be 170.5 F g-1 at 0.2 A g-1, which can be maintained for 70.5% with an increase of the discharging current density of 10 A g-1 as shown in Fig. S6a. These results indicate the good rate performance of the investigated NGAs, which probably is ascribed to its network structures that allows for effective ion migration into the active sites, thereby generating reversible capacitive behavior even at high charge/discharge rates.

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Fig. 5. Electrochemical capacitive performance of NGAs: (a) Cyclic voltammograms of the supercapacitor based on NGAs at scan rates of 10, 20, 30, 40, 50, 100, 150 and 200 mV s-1; (b) galvanostatic charge/discharge curves of NGAs at a charging/discharging current density of 0.2, 0.3, 0.4, 0.5, 1 and 2 A g-1; (c) electrochemical impedance spectra; (d) long-term cycle test of NGAs measured at a current density of 0.2 A g-1 within the potential range from 0 to -0.9 V; inset shows the retention rate of the electrode.

The electrochemical impedance spectra (EIS) was employed to further investigate the capacitive property and electrode conductivity of NGAs as shown in Fig. 5c. The EIS of the NGAs and the equivalent circuit for the fitting of the EIS data achieved by ZView software were not very straight, indicating the existence of unreduced oxygen-contained groups in NGAs, which can further confirmed by XPS (Fig. 4 and table S1). Therefore, the relative high specific capacitance could be not only attributed to the double layer capacitance as result of the network or porous structures, but also the pseudo-capacitance of unreduced hydroquinone/quinone groups or carboxyl groups and high graphitic N content introduced into graphene. Furthermore, the O-rich surface also increased the hydrophilicity and polarity of NGAs and thus facilitate the wettability between the electrolyte

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and network surface.

A long cycle life is also very important factor for practical application of supercapacitors. The long-term cyclic stability of NGAs was measured by using galvanostatic charge/discharge test up to 4000 cycles at 0.2 A g-1 (Fig. 5d). The capacitance of NGAs gradually decreases with the increasing of cycle number which may be due to some loss of pseudo-capacitance resulted from the decomposition of some unstable functional groups during charge/discharge cycles. It can be seen that the specific capacitance of the supercapacitor still maintained at 139.5 F g-1 after 4000 charge/discharge cycles, about 82% of the discharge capacitance, reflecting that the prepared NGAs electrode has long-term cycle stability and good electrochemical reproducibility in the repetitive charge/discharge cycling test.

3.4 Adsorption capacity

As described in the previous section, the 3D aerogels with abundant porous and network structures have potential application in the adsorption of metal ions. Therefore, the absorption capacity of metal ions by NGAs was investigated and the results were shown in Fig. 6. The pH of the tested solution is one of the most important factors which not only affected the surface charges but also the adsorption performance of the adsorbents. So in order to choose a suitable pH value for the adsorption experiments, we conducted batch experiments in a series of solutions with the same initial concentrations and pH values adjusted from 1.0 to 5.0 at room temperature. The adsorbents of NGAs were then placed into solutions containing Pb2+, Cu2+ and Cd2+ and gently shaken for 24 h to achieve equilibrium. Afterwards, the absorbents were directly removed from the solution by filter membrane, and the concentrations of heavy metal ions were then analyzed by UV-Vis

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spectrophotometer. Take Pb2+ as an example, the carboxyl groups are surrounded by H+ at low pH value. Therefore, the adsorption capacities of Pb2+ were very low due to the electrostatic repulsion between carboxyl groups and Pb2+. With the increase of pH value, the surface charges of NGAs became more negative, while the electrostatic repulsion became weaker and weaker, so more Pb2+ is adsorbed at pH equals 5. However, when the pH value is higher than 6.0, Pb2+ would form the precipitation of metal hydroxide. So in the adsorption experiment, pH 5 was selected for the further research. The calculated maximum adsorption capacity for Pb2+ was 205 mg g-1 under the optimized conditions (Fig. 6a). The effect of pH on the adsorption of Cu2+ and Cd2+ by the NGAs was similar to that of Pb2+. Under the same conditions, the maximum adsorption capacities for Cu2+ and Cd2+ were 175 and 155 mg g-1, respectively. Furthermore, the adsorption time is also a key parameter for the adsorption experiments. As shown in Fig. 6b, the adsorption capacities for Pb2+, Cu2+ and Cd2+ onto the NGAs increased quickly within the initial 100 min. The adsorption times to reach equilibrium are 110, 170 and 130 min for Pb2+, Cu2+ and Cd2+, respectively. The adsorption kinetics were also investigated by fitting the experimental data according to equation log(qe - qt) = log(qe) kt/2.303, in which qe and qt are the adsorption capacities of metal ions at equilibrium and selected time (t) (min), k is the rate constant (min-1). As shown in Fig. S7, the adsorption kinetics for Pb2+, Cu2+ and Cd2+ are pseudofirst order kinetic model.

For practical use for dealing with water pollution, the continuous and sustainable properties of the adsorbents is very important. As in the literature report [40,58], diluted hydrochloric acid (HCl) can be used as the desorption agent to recover the adsorption performance of the adsorbents. The interactions between carboxyl groups and metal ions were decreased. In other words, the electrostatic repulsion were increased when carboxyl groups were surrounded by H+. More

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interestingly, the adsorption capacity of NGAs for Pb2+ could be maintained at about 80% after 8 repeated adsorption and desorption cycles (Fig. S8). In the same repeated adsorption and desorption experiments, the retention rates for Cu2+ and Cd2+ were about 73% and 75%, respectively. Therefore, NGAs offers the possibility for easy recycle and reuse for the removal of heavy metal ions.

Fig. 6. Effects of pH values (a) on the adsorption capacities and adsorption rate plots (b) of Pb2+, Cu2+ and Cd2+ for NGAs. Experimental conditions: initial Pb2+, Cu2+ and Cd2+ concentration 150 mg mL-1, room temperature, and adsorb for 24 h.

In order to further study on the adsorption properties, the NGAs were carbonized at 600 ℃ for 3 h. After removal of the polar groups, the CNGAs became more hydrophobic which can be used as efficient adsorbents for oils and organic solvents. In order to observe clearly, the liquid paraffin labelled with Sudan III could be adsorbed after 60 s, which could be observed by naked eyes (Fig. 7c). Meanwhile, the oil filled CNGAs still floated on the surface without immersing water, indicating the hydrophobic properties of CNGAs after removal of polar groups.

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Fig. 7. Images of showing the adsorption of paraffin liquid on the water surface using CNGAs as adsorbents labelled by Sudan III at different times: (a) t = 0 s, (b), t = 30 s, and (c) t = 60 s; (d) adsorption capacity of CNAGs for several oils and organic solvents; (e) recycling performance of the liquid paraffin adsorption capacity of CNGAs.

In a typical adsorption experiments, 10 mg CNGAs was added into different oils and organic solvents in 100 mL beakers. The adsorption capacity was calculated using the weight gain of CNGAs before and after adsorption. As shown in Fig. 7d, the maximum adsorption capacities are from 57 to 102 times the weight of the adsorbents for different oils and organic solvents, which are 68, 102, 84, 93, 57 and 91 for petroleum ether, paraffin liquids, vacuum pump oil, chloroform, toluene and tetrahydrofuran (THF), respectively. The difference of the adsorption abilities could be attributed to the density of the oils and organic solvents [41]. Paraffin liquid was chosen as an example to study the regeneration of adsorption capacity when the oils or solvents filled CNAGs were dried in an oven at 200 °C for 1 h. More interestingly, even after ten adsorption and drying cycles, it still maintained a high adsorption capacity (95.2 % of the first use) (Fig. 7e).These results demonstrate that CNGAs can be employed as excellent adsorbents for various kinds of oils and 19

organic solvents.

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Conclusions

In conclusion, we prepare new nitrogen-doped graphene aerogels (NGAs) with 3D porous structures by using melamine as both reducing and functionalizing agent in aqueous solution with ammonia. Benefiting from the abundant porous architectures, the prepared NGAs exhibited high specific capacitance and good electrochemical stability in supercapacitors, and high adsorption capacities and recycling performance toward several heavy metal ions. More interestingly, the strongly hydrophobic CNGAs showed outstanding adsorption and recycling performance for removal of various oils and organic solvents. This facile and convenient method for the preparation of nitrogen-doped graphene aerogels would promote the development of preparation of 3D porous architectures based on graphene for multifunctional applications.

Acknowledgements

We are grateful for the financial support from the National Natural Science Foundation of China (21402108) and Shandong Natural Science Foundation (ZR2014BQ036).

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Highlights 

Three-dimensional nitrogen-doped graphene aerogels (NGAs) were prepared.



Melamine was used as reducing and functionalizing agent.



NGAs exhibited relatively good electrochemical properties in supercapacitor.



NGAs exhibited high adsorption performance toward several metal ions.



CNGAs showed outstanding adsorption capacities for various oils and solvents.

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Graphical Abstract

Three-dimensional nitrogen-doped graphene aerogels were prepared by using melamine as reducing and functionalizing agent in an aqueous medium with ammonia, which showed multifunctional applications in supercapacitors and adsorption.