Conductive GrapheneâMelamine Sponge Prepared via Microwave

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Conductive Graphene-Melamine Sponge Prepared via Microwave Irradiation Wenlu Liu, Haibin Jiang, Yue Ru, Xiaohong Zhang, and Jinliang Qiao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06070 • Publication Date (Web): 03 Jul 2018 Downloaded from http://pubs.acs.org on July 5, 2018

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ACS Applied Materials & Interfaces

Conductive Graphene-Melamine Sponge Prepared via Microwave Irradiation Wenlu Liu1, 2, Haibin Jiang2, Yue Ru2, Xiaohong Zhang2, Jinliang Qiao*1, 2 1

College of Materials Science and Engineering, Beijing University of Chemical

Technology, Beijing 100029, (China). 2

SINOPEC Beijing Research Institute of Chemical Industry, Beijing 100013, (China).

*

Corresponding Author: [email protected]

Abstract: A conductive graphene-melamine sponge prepared via microwave irradiation is reported in this paper. Graphene oxide supported on melamine sponge was pre-reduced firstly at 100 °C and then further reduced in a household microwave oven at over 1000 °C. It was surprise to find that graphene oxide on melamine sponge was reduced perfectly while the three-dimensional structure of melamine sponge was kept well after high temperature reduction via microwave irradiation. Slightly pyrolysis of melamine sponge was also found during 5 seconds microwave-irradiation, leading to the nitrogen generated from the pyrolysis of melamine sponge being doped into graphene, which could benefit the electric conductivity of the prepared graphene-melamine sponge. The electric conductivity of the prepared graphene-melamine sponge is about 0.12-1.0 S/m due to the high reduction degree of graphene oxide and nitrogen doping. On the other hand, different from pure melamine sponge, the newly developed conductive graphene-melamine 1

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sponge possesses superhydrophobic and super-oleophilic properties. Overall, the newly developed conductive graphene-melamine sponge contained 94.3 wt. % melamine sponge and 5.7 wt. % N-doped graphene is a cost-effective material with good elasticity, high conductivity, super-hydrophobicity and super-oleophilicity. Keywords: nitrogen-doping, graphene-melamine sponge, microwave reduction, conductivity, superhydrophobic, oil absorption

1. Introduction Graphene, a two-dimensional monolayer of carbon atoms packed into a honeycomb lattice, has become one of the most exciting research topics in the last decade.1-4 Typically important properties of graphene are outstanding strength,5 high specific surface areas,6 high thermal conductivity,7 strong chemical durability,8 high electron mobility9 and intrinsically hydrophobic.10 Such unique properties qualify graphene as a promising source to fabricate high performance materials for high end applications, such as hydrogen storage,11,12 oil cleanup,13 high performance electronics and sensors.14-15 However, the two-dimensional (2D) graphene sheets need to assemble into three-dimensional (3D) architectures in most of applications to achieve better performance.16-17 As 3D graphene materials from 2D graphene sheets, low density graphene aerogel with elasticity and high electric conductivity has been studied widely. Cryodesiccation from graphene oxide (GO) aqueous suspensions can convert GO to high performance graphene aerogels by using appropriate reducing methods.3,18,19 But these costly processes and complex operations limit application of 2


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graphene aerogels. H. M. Cheng et al. reported a 3D flexible and conductive interconnected graphene networks grown by chemical vapour deposition (CVD).20 However, the demands of long time and strict operating conditions also hindered the large-scale production of such graphene networks. Graphene-polymer sponges as 3D porous graphene-based materials exhibit excellent elasticity, but the relatively low reduction temperature limiting electric conductivity of the sponges.21-28 It is obvious that it remains a challenge to develop a simple and cost-effective route for fabrication of 3D graphene foams with elasticity and high electric conductivity. As reported by D. Voiry et al., GO can be reduced into high quality graphene by using 1- to 2-s-long microwave pulses.29 However, to the best of our knowledge, microwave irradiation has not been used on the reduction of GO that supported by melamine sponge (MS). Herein, we report a new process for the preparation of 3D graphene sponge by using microwave irradiation (M-GS). The prepared M-GS possess elasticity, high electric conductivity

and

excellent

thermal stability.

Meanwhile,

it also

exhibits

super-hydrophobicity and super-oleophilicity.

2. Experimental Section Materials and equipment Graphene oxide aqueous suspension was purchased from Nanjing JCNANO Technology Co., LTD. Melamine sponges were supplied by BASF Applied Chemical Co., Ltd. The melamine sponge was cut into cubes (2 cm× 2 cm × 2 cm) and washed by deionized water with sonicating for 15 minutes, then dried in a vacuum oven at 3



50 °C. The household microwave oven was made by Galanz Microwave Oven and Electric Appliances Manufacturing Co., Ltd. Preparation of M-GS Firstly, MS was put into GO aqueous suspension of 1.0 mg/mL and squeezed for five times to make sure that MS was sufficiently wetted with GO suspension, then the watery GO-MS foam was heated in a blast oven at 100 °C for 3 h for pre-reduction and the coating and drying process was repeated twice. Then pre-reduced GO-MS foam was placed in a sealed quartz glass box filled with nitrogen and protected by a sealed polypropylene box. Finally, the whole box was heated in a household microwave oven at 700 W for 5 seconds. Characterization The morphology of M-GS was observed by using a scanning electron microscope (FEI Nova Nano SEM450). The static water contact angel and oil contact angel were measured with an optical contact angle measuring device (Easy Drop, Germany KRUSS). Thermogravimetric analysis was performed under nitrogen atmosphere at a heating rate of 10 ºC /min (PerkinElmer). Raman spectroscopy was obtained with a Lab RAM HR800 Raman microscope using a 532 nm laser beam as the probing light source. The element compositions in samples were investigated by X-ray photoelectron spectroscopy (XPS, ESCALab 250, Thermo Fisher, 2009). The electric conductivity of M-GSs was tested by Fluck 179C multimeter. X-ray diffraction (XRD) was conducted by using a PAN analytical X-ray diffraction system with the source wavelength of 1.542 Å at room temperature. Energy dispersive 4


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spectrometer (EDX) measurement was conducted by using EDAX Apollo XT (American EDAX Co., Ltd). Compression test was conducted on INSTRON 3300 at a compressive rate of 10 mm/min.

3. Results and discussion

Scheme 1. Schematic illustration of M-GS fabrication process. Scheme 1 illustrates the fabrication process of M-GS. Firstly, MS was fully impregnated with GO aqueous suspension of 1.0 mg/mL to get a MS supported GO (GO-MS) foam. Then GO-MS foam was pre-reduced in an oven at 100 °C for 3h. The C/O ratio of GO in GO-MS increased to 4.7 from 3.3 of bare GO. Finally, the pre-reduced GO-MS foam was treated by microwave irradiation at 700W under nitrogen atmosphere for 5s. The pre-reduction of GO is necessary for microwave heat because low oxygen content can bring high conductivity and provide effective microwave-heating.30-32 Large arcs were observed around pre-reduced GO-MS foam during microwave irradiation, as expected (Video S1). Microwave absorption of GO strongly depends on its chemical composition and structure, and the increase of oxygen in GO remarkably decreases its microwave absorption capacity.32 As reported by Damien Voiry et al., GO pre-reduced at 300 °C under Argon can be heated up to several thousands of Celsius in only few tens of ms by household microwave oven of 5



1000W.29 Heating temperature of GO pre-reduced at 100 °C with household microwave oven of 700W under air should be lower than that of Prof. Damien Voiry’s. Furthermore, experimental results show that the C/O ratio of our microwaved rGO is 8.3, which is little bit larger than 8.1, the C/O ratio of rGO that supported on melamine-sponge and thermal annealing at 1000 °C under nitrogen. Therefore, the reduction temperature of microwaved rGO was deduced to be over 1000 °C. The reduction of GO in GO-MS foam was confirmed by X-ray powder diffraction (XRD) and Raman spectroscopy measurements. Figure 1a shows XRD patterns of GO and M-GS. GO exhibit a distinct peak at 2θ=9.81°, corresponding to the (001) interplanar spacing of 0.73 nm calculated from the 2θ value (d = λ/2 sin θ).33 After reduced by microwave irradiation, the XRD pattern of M-GS presents a new diffraction peak at 26.30°, which is ascribed to the (002) reflection of graphite domains and corresponds to an interlayer spacing of 0.34 nm.19 The shift of XRD peaks from 9.8° of GO to 26.30° of M-GS reveals the reduction of GO sheets and a high degree of graphitization of GO.34 This result is consistent with that reported in the literature.35 In the Raman spectra (Figure 1b), two obvious bands at 1346 cm-1 and 1592 cm-1 appeared in both GO and M-GS. The band at 1346 cm-1 corresponding to the D-band is arising from the structure defects.36 The band at 1592 cm-1 corresponding to G-band originates from the first order scattering of the E2g photon of sp2 C atoms.37 The intensity ratio of D to G band for GO is 0.953 while that of M-GS decreases to 0.850, indicating the successfully reduction of GO in M-GS. This result is different from most of published results in literatures that the intensity ratio of D to G band usually 6


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increase after GO reduction,3, 22, 38-40 resulting from re-establishment of sp2 carbon during the removing of oxygen groups from GO.19 It is clearly that microwave irradiation can significantly reduce defects formation during GO reduction.

Figure 1. (a) XRD patterns of GO and M-GS; (b) Raman spectra of MS, GO and M-GS. The instant high temperature caused by microwave irradiation contributes to the reduction of GO, but it could possibly damage the 3D structure of MS. Unexpectedly, the 3D structure of MS was kept well after high temperature microwave irradiation. The microstructures of MS, GO-MS foam and M-GS were observed by scanning electron microscope (SEM). SEM images of MS (Figure. 2a, 2b and 2c) show an interconnected and porous 3D network structure with smooth skeletons and through-holes of 30-100 µm. As shown in Figure S1, GO sheets is attached with the skeletons of MS and the 3D structure of MS is well inherited by GO-MS foam. After microwave irradiation, the 3D porous structure of M-GS was kept very well and the pore size is constant with MS and GO-MS foam, indicating that the basic structures of MS were not damaged during microwave irradiation process (Figure 2d). The high 7



magnification images (Figure 2e and 2f) further revealed that some of reduced GO sheets are larger than the hole of MS; therefore, only the skeleton surface of MS can contact with GO and being heated to pyrolysis by reduced GO during very short time of microwave-irradiation. In addition, it can be found from Video S2 that there are only 5 times of arcs appeared during microwave-irradiation. The duration of each arc is 50-100 ms,29 thus the total heating time of the microwave irradiation is 0.25-0.5 s. The intervals between arcs for cooling were much longer than heating time and most time of the microwave irradiation is the annealing procedure. It is also well known that melamine sponge is a thermal-insulation material with microwave transparency. Therefore, it is reasonable that the 3D structure of melamine sponge can be maintained after ultrafast microwave irradiation. The density of MS and GO-MS is 8.3 mg/cm3 and 8.8 mg/cm3, respectively. However, the density of corresponding M-GS is 5.8 mg/cm3, further implying the pyrolysis of MS as the volume of the sponge remains unchanged. The nitrogen element in melamine resin is calculated to be about 50.9 % according to the molecular formula and the nitrogen atoms could release as gases N2 and NH3 during microwave irradiation process.41 Nitrogen-doping can dramatically enhance the electrical conductivity of graphene oxide sheets42 and the nitrogen generated form the pyrolysis of MS is believed to be doped into graphene during microwave irradiation, which is proved in the following. Therefore, the M-GS we have prepared is a nitrogen-doped (N-doped) 3D graphene-polymer sponge with high quality reduced GO, which should have excellent elasticity and good conductivity. 8


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Figure 2. SEM images of (a-c) original MS and (d-f) M-GS. The elastic property of M-GS was investigated by a series of compression tests. As shown in Figure 3a, M-GS can completely recover to its original shape without mechanical failure after being compressed by 90 %. The elasticity of M-GS is further investigated by cyclic compression test. During the test, M-GS sample was placed on compression plates without being glued or otherwise attached. Figure 3b displays the cyclic stress-strain curves of M-GS with a maximum strain of 60 %. Strikingly, the unloading curve could almost return to the initial point even after 100 compression cycles, indicating the outstanding elasticity of M-GS. Importantly, the 3D interconnected structures are maintained without apparent damage and the interaction between rGO with substrate remained intact after 100 cycles of compression test (Figure 3c).

9



Figure 3. (a) The compression-recovery process showing that M-GS recovers to its original shape after compression by 90 %; (b) Compressive stress–strain curves of M-GS at 60 % strain; (c) SEM image of M-GS after 100 cycles of compression-release process. The unique structure of M-GS combines the advantages of both polymer and graphene foams, which equips M-GS not only with excellent mechanical properties but also with high electrical conductivity, thus holding great potential for many applications. Melamine sponge is generally considered as insulator and the electric conductivity of M-GS is 0.122 S/m at its original shape, which is higher than that of GO-MS foam (0.009 S/m) and other polymer-based graphene materials reported before.43-44 The change in conductivity of M-GS when it is compressed was further studied. As illustrated in Figure 4a, a simple circle is formed by a battery, a LED lamp and M-GS. The lamp gradually becomes brighter when M-GS is compressed (Video 10


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S3), indicating that the electric conductivity of M-GS gradually increasing during compression. As shown in Figure 4b, the electric conductivity of M-GS reaches 0.97 S/m at the maximum strain of 90 %, which is comparable with graphene aerogels.45-46 The gap during the skeletons of M-GS becomes narrow when the sponge is under compression and more pathways for electron transport are created as there are more chance for graphene sheets to contact with eacher, resulting in greater electric conductivity.19 The cycling stability was also tested and no ouvious change occurred under different compressive strains (Figure 4c). The stable elasticity-dependent electrical conductivity makes M-GS a promising pressure-responsive sensor.

Figure 4. (a) Photographs illustrating the brightness changes upon compression of M-GS; (b) The electrical conductivity of M-GS as the function of compressive strain; (c) The stability of the electric conductivity change of M-GS at different compressive strains. 11



The relationship between concentration of GO (CGO) aqueous suspension and electric conducivity of M-GS was studied. As demonstrated in Figure S2a and Figure S2b, the loading amount of GO and electric conductivity of M-GS are both positive correlated with CGO when the treatment time is fixed to 5s. However, the obtained M-GS becomes inelastic when CGO﹥1.0 mg/mL, while electric conductivity droped sharply if shorter treatment time was used although the elasticity can be maintained.Therefore, the optimum CGO for preparing elastic and conductive M-GS is 1.0 mg/mL. The appropriate microwave treat time is also investigated. The electric conductivity of M-GS increased along with the treatment time within 5s. However, as the treatment time continues to increase, time intervals among arcs become shorter, leading to continious high temperature and a destruction of the internal 3D structure of the obtained sponge. As shown in Figure S3, fracturs appears on the polymer framwork and rGO tends to stack together when treatment time is extended. The possible reason is that the microwave absorption is weak at first 5 seconds, while the absorption of microwave increases along with the treatment time, leading to continious high teperature and the decompositon of polymer framwork. As demonstrated in Table S1, the weight of the sponge declined rapidly with increasing treatment time, which is consistent with our conjecture. Therefore, the optimal concentration for preparing elastic and high conductivity M-GS should be 1.0 mg/mL and the treatment time should be 5s. For comparasion, The properties of graphene-melamine sponge prepared by conventional thermal annealing at 1000 °C (C-GS-1000) is also investigated. The electric conductivity of C-GS-1000 is 0.09 S/m 12


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at original shape and 0.89 S/m when compressed at a strain of 90 %, which are lower than that of M-GS. Moreover, C-GS-1000 cannot rebound when the pressure is removed. The possible reasons are as follows: (1) the electric conductivity of N-doped graphene is higher than that of graphene achieved by thermal annealing at 1000 °C; (2) the long-term heating process of conventional thermal annealing causes massive decomposition of the sponge and less carbon was retained in C-GS-1000 as the weight ratio after thermal annealing is 23.8 % when compared with that of micrawave irradiation (63.7 %). It is fully proved that microwave irradiation is more advantageous than conventional thermal annealing

in the

preparation of

graphene-polymer composites. To confirm that most of MS could be maintained after high temperature treatment of microwave irradiation, thermogravimetric analysis (TGA) was conducted. Figure 5a shows TGA curves of MS, GO-MS foam and M-GS in nitrogen atmosphere. The TGA curves of MS has a rapid weight loss in the temperature range of 362-400 °C, which can be ascribed to the breakdown of the methylene bridges.47 Mass losses at higher temperatures are attributed to the thermal decomposition of the triazine ring.48 The TGA curve of GO-MS foam was similar to that of original MS, indicating that the simple loading of GO on MS has no great impact on the thermal stability of GO-MS foam. M-GS is different from MS and GO-MS foam, its curve exhibits no mass loss at 370-400 °C, suggesting that portions of the methylene bridges of MS should have been broken down during the microwave irradiation. The M-GS maintained thermal stable until the temperature raised to 482 °C, which is much 13



higher than that of MS and GO-MS foam, indicating the excellent thermal stability of M-GS. The total weight loss of M-GS at 600 °C is slight lower than that of original MS, which is attribute to the loading of reduced GO in M-GS. Therefore, MS content in M-GS could be roughly calculated from the TGA curves as shown in Figure 5a. The results show that 94.3 wt.% M-GS are melamine sponge and only 5.7 wt.% are N-doped graphene, which is similar to that of GO-MS foam before high temperature reduction; therefore, M-GS keep to be an elastomer-like material (Figure 5a).

Figure 5. (a) TG curves of MS, GO-MS foam and M-GS; (b) XPS wide-scan spectra of GO-MS form and M-GS; High resolution N1s XPS spectra of (c) GO-MS foam and (d) M-GS. In order to verify the nitrogen-doping (N-doping) on graphene during microwave irradiation, the X-ray photoelectron spectroscopy (XPS) was used to analyze element 14


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compositions in GO-MS foam and M-GS. As shown in Figure 5b, C1s, N1s, Na1s and O1s signals can be observed in both GO-MS foam and M-GS in the wide-scan XPS spectra. From a curve deconvolution, the complex N1s spectra of GO-MS foam can be well-fitted to two peaks with binding energies at 398.3 and 399.4 eV, which are assigned to -NH2 and -C=N in MS.22, 49 The binding energy of N1s has undergone significant changes after the microwave irradiation. As observed in Figure 5d, the N1s spectra of M-GS was deconvoluted into four primary peaks of -NH2 (398.3), pyridinic N (398.9), -C=N (399.45) and graphitic N (401.1 eV),50 indicating the successful doping of nitrogen atoms into graphene. In addition, the energy dispersive spectrometer (EDX) was also employed to measure the element contents of GO and reduced GO in M-GS. GO was obtained by freeze-drying GO aqueous suspension. rGO was separated from M-GS by sonicating M-GS in ethanol for 30 minutes and the mixture of separated rGO and MS skeletons was dried at room temperature. Then separated rGO was found by SEM for the EDX test. As shown in Figure S4 and Table S2, upon microwave irradiation, the nitrogen content in reduced GO was 11.78 wt. % while the nitrogen content of GO is zero. Meanwhile, the oxygen content decreased from 21.79 wt. % in GO to 9.48 wt. % in reduced GO. The results verified that N-doping and the removal of oxygen-containing groups in GO happened simultaneous during microwave irradiation. The newly developed conductive graphene-melamine sponge also possesses superhydrophobic and super-oleophilic properties and in contrast, original MS are both hydrophilic and lipophilic. As observed in Figure S5, MS absorbed water 15



immediately after contacted with water and sank into the water bottom. In contrast, M-GS floated on the water without sinking for over 2h. Figure 6a and 6b show that water droplet was completely adsorbed by original MS, while maintained immobile on the surface of M-GS and the static water contact angels of original MS and M-GS are 0° and 153.8°, respectively. As shown in Figure 6c and 6d, when a drop of pump oil was deposited on the surface of M-GS, it was absorbed completely by M-GS and the oil contact angle of M-GS was 0°. For comparison, water droplet attained in near spherical shape on the surface of M-GS. These results indicate that M-GS possesses excellent superhydrophobic and superoleophilic surfaces.22

Figure 6. (a, b) Water contact angle of original MS and M-GS; (c, d) Oil contact angle of M-GS and comparison between water droplet and oil droplet. When a piece of M-GS was placed on the surface of pump oil-water mixtures, the pump oil (stained with Sudan red) was completely adsorbed by M-GS (Figure S6a, Video S4). Figure S6b and Video S5 show that M-GS can also effectively absorb high-density organic solvents, chloroform (stained with Sudan red). The results indicate that M-GS is a promising adsorbent material for selectively removal of oils and organic pollutants with different densities from water. The efficiency of oil absorption can be referred to as weight gain, i.e. wt. %, 16


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defined as the weight of absorbed substance per unit weight of dry M-GS.13 Several organic liquids were evaluated including lubricate oil, pump oil, peanut oil, soybean oil, olive oil, chloroform and n-hexane, which usually could pollute water resources. M-GS showed very high absorption capacity for these organic liquids. It can be found from Figure 7a that M-GS could absorb these organic liquids from 96 to 288 times of its own weight. This is to say, less than 4.0 kg M-GS can absorb 1 ton chloroform, promising great potential in the applications of oil absorption. In fact, M-GS shows much higher sorption ability than other reported sorption materials (Table S3).13, 22, 38-39, 51-52

As shown in Figure S7, the oil absorption capacity of N-doped M-GS was

better than that of N-free graphene-melamine sponge, which was prepared using 1.0 mg /mL GO aqueous suspension and ascorbic acid as a reducing agent. It is well known that N-doping can enhance the interaction between graphene and the molecules of polar solvents;13 therefore, nitrogen doping can promote the oil adsorption capacity of graphene-melamine sponge. The recyclability of absorbents was very important for practical application.53-54 Hexane was utilized to explore the recyclability of M-GS. After 10 cycles of adsorption-squeezing tests, no significant fluctuation in absorption capacity was observed (Figure 7b).

17



Figure 7. (a) Absorption capacity of M-GS for various oils and organic solvents; (b) recyclability of M-GS for n-hexane after 10 cycles. The absorption capacity after multiple cycles is normalized by the initial weight of dried M-GS.

4. Conclusion It has been proved that GO on polymer can be reduced perfectly while the polymer structure can be kept well after short time of microwave irradiation. Based on this new finding, a novel process has been developed to prepare polymer supported graphene sponge and N-doped graphene-melamine sponges with good elasticity, high electric conductivity, super-hydrophobicity and super-oleophilicity have been successfully prepared by using this new process. The prepared N-doped graphene-melamine sponges contained 94.3 wt. % melamine sponge and 5.7 wt. % N-doped graphene also exhibit very high absorption capacities for oils and organic solvents up to 288 times of its own weight, excellent selectivity and recyclability. Therefore, the newly developed graphene-melamine sponge is a promising candidate for selective removal of organic pollutants from water and many other applications. This new process is also possible to be applied to prepare different type of 3D polymer supported graphene materials.

Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org.

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Acknowledgment This work was financially supported by the Ph.D. Programs Foundation of SINOPEC Beijing Research Institute of Chemical Industry.

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