Naturally Dried, Double Nitrogen-Doped 3D Graphene Aerogels

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Naturally Dried, Double Nitrogen-Doped 3D Graphene Aerogels Modified by Plant Extracts for Multifunctional Applications Qi Meng, Hengcheng Wan, Wen-Kun Zhu, Tao Duan, and Wei-Tang Yao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03460 • Publication Date (Web): 20 Nov 2017 Downloaded from http://pubs.acs.org on November 29, 2017

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ACS Sustainable Chemistry & Engineering

Naturally Dried, Double Nitrogen-Doped 3D Graphene Aerogels Modified by Plant Extracts for Multifunctional Applications

Qi Meng †, Hengcheng Wan †, Wenkun Zhu †, and Tao Duan †*, Weitang Yao †* †. Sichuan Co-Innovation Center for Energetic Materials, Sichuan Civil-military Integration

Institute, Southwest University of Science and Technology, No. 59 Qinglong Avenue, Fucheng district, Mianyang, Sichuan, 621010, P. R. China Corresponding author* E-mail: [email protected] , [email protected] Author E-mail: [email protected] , [email protected] , [email protected]

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Abstract: 2D graphene has become one of the most intensively explored carbon allotropes in materials science owing to its attractive features, such as its outstanding physicochemical properties. To advance its practical application, the fabrication of self-assembled 2D graphene sheets into 3D nitrogen-doped graphene aerogels (NGA) with novel functions is becoming essential. Herein, the first attempts to modify graphene with Plectranthus amboinicus (PA), introduce double nitrogen doping, and prepare NGA by a natural drying (ND) method are reported. Natural drying was achieved by increasing the pore structure of NDPA-NGA using PA to simultaneously crosslink the graphene sheets and increase the stiffness of the sample. Meanwhile, we used ammonia and urea as double nitrogen sources to achieve an N-doping amount as high as 12.06 at%. In addition, NDPA-NGA exhibited superelastic properties (with 95% maximal strain and almost no loss after 60 replicates), a high oil absorption capacity and an excellent electrochemical performance (371 and 204 F g-1 at current densities of 1 and 50 A g-1, respectively, corresponding to a retention of 54.987%). In addition, NDPA-NGA also demonstrated predominant refractoriness, a low density, hydrophobicity, and physicochemical stability, which make this material an excellent candidate for use in energy storage, catalysis, and other applications.

Key words: nitrogen-doped graphene aerogels, Plectranthus amboinicus, superelasticity, supercapacitors, oil adsorption

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Introduction Since graphene was first successfully prepared by the mechanical stripping method, it has attracted global attention because of its perfect crystal structure and many excellent physicochemical properties.1,2 The unique electrical,3 thermal,4 and mechanical properties5 of graphene have wide application prospects in the fields of electronic devices,6,7 composites,8,9 sensors,10 catalysts,11,12 drug delivery,13,14 and energy storage,15,16. However, the conductivity of graphene can-not be completely controlled because it does not have a bandgap.17 The doping of graphene is carried out by replacing the C atoms with a hetero-atom, including N, B, P, and S, to create a dot defect that opens the graphene bandgap and modulates the conductivity type.18-21 Therefore, the elemental doping of graphene, especially with N, has been extensively reported in recent years.22-24 In most reports, however, low N doping has limited the widespread use of nitrogen-doped NGA, and chemicals have been used as modifiers, such as ethylenediamine25, hexamethylenetetramine,26 layered double hydroxide27 and L-ascorbic acid.28 These chemicals not only raise the cost of preparation but also cause serious environmental pollution. Therefore, the discovery of new NGA modifiers and methods for preparing NGA with a high N-doping content is a major challenge. It is common knowledge that most methods used to prepare 3D NGA employ freeze-drying or vacuum drying,29-31 which requires expensive equipment, thus raising the cost and limiting production. Although previous work has explored methods for naturally drying graphene aerogels(GA),32 the use of chemical reductants and N doping in graphene aerogels(GA) has not been addressed in discussions on the current state of the NGA fabrication process. Obviously, the cost-effective and efficient preparation of NGA is critical for industrial production. In this work, we solved the problem of low NGA doping levels by using ammonia and urea as double nitrogen sources. The nitrogen content was as high as 12.06 at%, which is close to the theoretical limit of 19.1 at%.33 Thus, NDPA-NGA lays the foundation for the preparation of various functional materials.34-37 As previously reported,38 higher natural plants use borate to enhance their intercellular structure, by hardening the cell walls to increase their support strength, and the effect of borate on the preparation of bio-inspired graphene membranes and graphene aerogels has been studied.39,40 Unfortunately, plant extracts have not been directly used in a more environmentally friendly and NGA suitable preparation method. Inspired by this, we prepared NDPA-NGA from a 3



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natural drying (ND) process by using pure natural Plectranthus amboinicus(PA) plant extract (PAPE) as a simultaneous cross-linker and reducing agent. This method can prevent deformation of the NDPA-NGA during the natural drying process and can reduce the oxygen content in NGA. This is a highly competitive mode of production because the PAPE will not pollute the environment, and natural drying is inexpensive, facile, and environmentally friendly. This is the first report of modification using PAPE, the introduction of double nitrogen doping, and the preparation of NGA using natural drying. We firmly believe that this work not only provides a new method for producing nitrogen-doped graphene on a large scale but also presents a new approach for the industrial production of functional N-doped graphene materials.

Experimental Synthesis of the NDPA-NGA composites All chemicals were analytical grade and commercially available from Shanghai Chemical Reagent Co. Ltd and used as received without further purification. Plectranthus amboinicus is a tender, fleshy perennial plant in the family Lamiaceae and has an oregano-like flavour and odour. Repeated cleaning of Plectranthus amboinicus was performed with pure water and anhydrous ethanol. First, 100 g of Plectranthus amboinicus and 300 mL of ultrapure water were added to a juicer. Then, the liquid was removed and centrifuged at 9000 rpm until no particles were present. The slurry was then sonicated for half an hour, and finally, the Plectranthus amboinicus plant extracts were stored in a refrigerator for future experiments (the preparation process is shown in Figure S1, Supporting Information). Graphene oxide was synthesized using a modified Hummers' method through the oxidation of graphite flakes, as reported elsewhere.41 A GO solution (6 mL, 10 mg / mL) was stirred with 4 mL of concentrated aqueous ammonia (30%) in a beaker. Next, 3 mL of the Plectranthus amboinicus plant extract and 60 mg of urea were added to the solution, followed by ultrasonication to ensure a uniform dispersion. After that, the mixture was transferred to a reaction vessel containing a Teflon liner and maintained at 180°C for 12 h. The hydrogel was removed and placed in an aqueous solution of ethanol (1%) for 5 h and then frozen in a refrigerator for 10 h. The sample was removed and naturally dried to form NDPA-NGA. 4


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Characterization Scanning electron microscopy (SEM), which gave information about the size and morphology of NDPA-NGA, was performed with a field emission scanning electron microanalyzer (Zeiss Supra 40). X-ray photoelectron spectra (XPS) were recorded on an ESCALab MKΠ X-ray photoelectron spectrometer, using Mg Kα radiation as the exciting source. FTIR spectra were measured on a Bruker Vector-22 FT-IR spectrometer from 4000 to 400 cm-1 at room temperature. The mechanical properties were characterized by a TA Instruments dynamic mechanical analyzer (DMA, RSA3) with compression at ambient conditions. Compressive stress–strain response was measured at a strain rate of 0.05 mm s−1. The specific surface area and the porous features were investigated by nitrogen isothermal adsorption/desorption measurements on a QuadraSorb SI system (Quantachrome Instruments). All the samples were degassed by heating at 150 °C under vacuum for 10 h prior to the measurements. Calculation and measurements Calculation of density and volume retention rate Sample density (ρ) can be measured by both methods (a) and (b),42 where the average of the data is used. (a) calculated by formula (1):

ρ = m /(π( d / 2 ) 2 h )

(1)

Where m is the sample mass; d is the bottom diameter; h is the sample height. (b) Use the Archimedes principle to directly read the density of the sample through a densitometer. The density of the samples obtained from these two methods is basically the same. Volume retention rate ( ζ ) calculated by (2):

ζ = V 2 / V1

(2)

Where V1 is the volume of the sample after 1 h of the hydrogel is placed in the refrigerator; V2 is the volume of the sample after natural drying. Electrochemical measurements Electrodes were prepared by mixing 80 wt% active materials, 15 wt% carbon black and 5 wt% 5



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polyvinylidene-fluoride in N-methyl pyrrolidone to form the slurry. One square centimeter of this slurry was painted on titanium foil current collectors and the electrode was dried at 110 °C in a vacuum oven overnight. The mass of active materials in the electrodes was 1-2 mg. Two-electrode (2E) cells were constructed from two carbon electrodes of similar weights, separated by a glassy fiber paper. Three-electrode (3E) cells were assembled by using one carbon electrode as a working electrode, another carbon electrode as a counter electrode and inserting an Ag/AgCl reference electrode. Electrochemical performances were collected in 1 M KOH aqueous electrolyte using a Solartron 1480 Multistat station. The test window voltage is -1 to 0 V. The specific capacitance (Cm) is calculated using equation (3):

Cm =（I∆t）/( m∆V )

(3)

Where I is the discharge current; t is the discharge time; V is the voltage change; m is the electrode active material mass. Calculation of oil absorption By measuring the mass m1 and m2 of NDPA-NGA before and after adsorption (The operation should be completed within seconds to avoid volatilization), the oil absorption (Q) was calculated by the formula (4):

Q = m 2 / m1

(4)

Results and Discussion Figure 1a shows a schematic illustration of the preparation process for NDPA-NGA. First, a GO solution is mixed with ammonia and urea as double nitrogen sources. Then, the modifier PAPE is added, and the mixture is subjected to sonication. During the hydrothermal process, hydrogels are formed by self-assembly as a result of crosslinking between GO and the borate in PAPE (the self-supporting property of the cell wall of tall plants is shown in Figure S2, Supporting Information). In addition, PAPE contains multiple polysaccharide alcohols that replace the oxygen-containing functional groups (OCFG) on the GO sheet,43 which results in a significant reduction of GO; urea and ammonia both act as nitrogen sources during the hydrothermal synthesis process used to dope 6


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GA with N atoms. Thus, as shown in Fig. 1b, the present method can simultaneously achieve GO reduction, self-assembly and N-doping in one pot.

Figure 1. a) Schematic representation and photo images of the synthesis procedure for NDPA-NGA. b) Schematic illustration of simultaneous implementation the NDPA-NGA nitrogen - doped, borate crosslinking of graphene sheets and removal of oxygen-containing functional groups (OCFG) by PAPE. After hydrothermal treatment, the hydrogel is dialyzed to wash away the liquid remaining in the hydrogel while also allowing water molecules to permeate the interior of the hydrogel. The sample is then frozen in a refrigerator to form a macroporous structure in the hydrogel according to the ice template method, after which the sample is removed to form NDPA-NGA at room temperature (the size of the GA prepared using this method is determined by the amount of precursor and the size of the reactor, and samples of different sizes are shown in Figure S3, Supporting Information). We 7



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know that when drying NGA naturally, the evaporation of water produces a capillary force that will cause the 3D structure of the GA collapse; thus, volume shrinkage and structural cracking are the biggest challenges of natural drying. According to the Laplace formula,44 the capillary force (∆p) is defined as a function of the surface tension (γ), the contact angle (θ) and the hole radius (r), as shown in equation (5):

∆p = (2γ cosθ ) / r

(5)

We forced the graphene layer to extend in the direction of the ice crystal by simple freezing, thereby increasing the pore radius of the GA, and the borate contained in PAPE increased the frame stiffness (K) of the GA so that ∆p was much smaller than K. Such a dual effect prevented the volume of the GA from shrinking during the natural drying process (Table 1). Table 1. Theoretical and actual volume retention ratio of different samples. Sample numbering

Drying methods

γ

θ

r

K

(N m-1)

(°)

(µm)

(KPa)

Theoretical

Actual volume

volume retention

retention ratio

ratio (%)

(%)

0

FD, PAPE-3

98.264

1

FD, PAPE-0

97.879

2

ND, PAPE-1

3

~132.93

~140

~162

95.622

94.532

ND, PAPE-3

~137.26

~140

~181

95.794

95.643

4

ND, PAPE-5

~129.11

~140

~124

94.702

92.141

5

ND, PAPE-0

0.073

75.274

Note：FD: freeze drying; ND: Natural drying. PAPE-0, 1, 3, 5 is the amount that uses Plectranthus amboinicus (PA) plant extract (PAPE) as 0, 1, 3, 5 mL, respectively. The frame stiffness (K) is approximated by the elasticity modulus below. The theoretical and actual volume retention rate is calculated by formula (5) and (2), respectively.

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Figure 2. a) SEM image of cross-section view of the 3D layered porous skeleton structure of NDPA-NGA. b) SEM image of the core region in the NDPA-NGA. c) SEM image of local magnification in core region. d) SEM image of the crosslinking between graphene sheets. e) SEM images of the cracked cell walls in NDPA-NGA. f) SEM image of the thicknesses of the cell walls.

Figure 3. a) N2 adsorption/desorption isotherms curves (The amount of PAPE used in sample numbering 2, 3, 4 is 1, 3, 5 mL). b) Density and volume retention ratio of different samples (Sample No.0 is FD with PAPE (3 mL); No.1 is FD without PAPE; No.2 add PAPE 1mL for ND; No.3 add PAPE 3mL for ND; No.4 add PAPE 5mL for ND; No.5 is ND without PAPE). Inset is a contrast photo of No. 3 before and after ND. The density and volume retention are calculated by Formula (1) and (2), respectively. 9



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As shown in Figure 2 (the TEM image is shown in Figure S4, Supporting Information), NDPA-NGA has a uniformly distributed 3D-layered porous skeleton, which greatly reduces the capillary force. The nitrogen adsorption-desorption isotherm was used to study the specific surface area and pore size. As shown in Figure 3a, the N2 adsorption/desorption isotherms curve of sample numbering 1, 2, 3 can be derived from the different lines corresponding to the relative pressures (P/P0) of 0 to 1.0 (Sample numbering 0, 4, 5 shown in Fig. S5, Supporting Information). The typical type-IV features indicate the presence of porous structures. The corresponding pore size was calculated using the BJH method to be 140 µm. In addition, the BET surface area of the samples was 215.11 m g-1. The pore size greatly facilitated the drying process of the samples under natural conditions. As shown in Fig. 3b, when the sample was freeze-dried, it retained a volume of approximately 97.879%. When 3 mL of PAPE was used in conjunction with the natural drying process, the volume retention was a high as 95.643%, whereas only 75.274% of the volume was retained without PAPE. These results indicate that the addition of PAPE could significantly combat the capillary force (the presence of borate in NDPA-NGA is shown by mapping, Figure S6, Supporting Information). However, upon increasing the amount of PAPE (5 mL), the volume retention decreased, which we attributed to the following reasons: first, there is an optimal amount of borate for the crosslinking of GO; second, PAPE contains columns of polysaccharides and alcohols that acted as strong reducing agents on GO. The use of a large amount of PAPE hindered the crosslinking of GO by borate and made it difficult to form a perforated structure. In addition, the dilution of the GO precursor by the large amount of PAPE decreased the volume retention rate. The illustration shows the change in the NDPA-NGA volume before and after natural drying with a suitable amount of PAPE (3 mL). In addition, the density of the different samples also changed.

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Figure 4. a) IR spectra of GO and samples with different PAPE dosages (0, 1, 3, 5 mL), and b) XRD patterns of GO and samples with different PAPE dosages (0, 1, 3, 5, 10 mL). c) Raman spectra of samples with different urea dosages (0, 30, 45, 60, 75 and 90 mg). PAPE could reduce GO, as mentioned above, which was confirmed using IR spectroscopy. Figure 4a presents the IR spectra of GO, hydrothermally treated GO (without PAPE), and NDPA-NGA with different PAPE dosages. The spectrum of GO contains peaks at 1638, 1389, 1091, 1042 and 878 cm-1, which belong to the C=C and N=O stretching vibrations of the C-N=O groups, the C-N and N-N stretching vibrations of the N-N=O groups, and -C-O-C- symmetric stretching, respectively.45,46 After hydrothermal treatment without PAPE, the amount of oxygen-containing groups on the surface decreased slightly, suggesting that the hydrothermal treatment without a reductant partially reduced the GO. A much larger decrease in the amount of oxygen-containing groups was recorded in the IR results of the NDPA-NGA samples with different PAPE dosages, indicating that the compounds present in PAPE acted as GO reducing agents. To confirm the effect of PAPE on the GA, the XRD patterns for the different PAPE dosages are shown in Fig. 4b. GO exhibits a sharp diffraction peak at 2θ = 10.5°, corresponding to its (002) crystal face. After the hydrothermal reaction, this diffraction peak was no longer observed due to the removal of the oxygen functional groups. The sample showed a broad diffraction peak at 2θ = 25.8° corresponding to the (002) crystal face of the graphite, indicating the presence of stacked graphene nanosheets and the recovery of the graphite crystal structure. The (111) crystal plane of graphite was observed at 2θ = 43°.47 It was found that even when 10 mL of PAPE was added, no miscellaneous peaks appeared in the XRD pattern, indicating the practical use of PAPE for the hydrothermal reduction of GO without introducing impurities. Raman spectroscopy is a commonly used method for evaluating the doping effect of graphene. As shown in Fig. 4c, the Raman spectra of GO clearly show two main peaks at 1314 and 1604 cm -1, which correspond to the D and G bands of graphene, respectively. The D band is attributed to vibrations of the sp3 carbon atoms of disordered graphite, and the G band is generated by in-plane vibrations of the sp2 carbon atoms in the 2D hexagonal lattice of graphene. In general, the D/G intensity ratio (ID/IG) reflects the defect density of the carbon material. When 60 mg of urea was used, the maximum ID/IG value of 1.53 was achieved. This result can be attributed to the presence of distortions and vacancies in the graphene lattice caused by N atom doping and to the decrease in the average size of the sp2 domain caused by the PAPE-induced reduction of GO. 11



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Figure 5. a) XPS overview spectrum of NDPA-NGA and corresponding deconvoluted peaks of b) N1s, c) C1s, and d) O1s spectra. (NDPA-NGA experimental parameter: GO: 6 mL (10 mg / mL); PAPE: 3 mL; Ammonia: 4mL (30%); Urea: 60 mg). To further characterize the chemical composition of NDPA-NGA, we performed X-ray photoelectron spectroscopy (XPS) analyses, which showed the characteristic peaks in the C 1s, O1s, and N1s spectra (Fig. 5a). The highest N doping (maximum of 12.06%) was achieved when 4 mL of the aqueous ammonia solution and a GO:urea mass ratio of 1:1 were used. Table S1 (Supporting Information) details the percentages of C, N, and O obtained from the XPS analysis of each sample. Figure S7 (Supporting Information) contains the XPS images for the different dosages. The NDPA-NGA doping levels were obtained by deconvoluting the N1s spectrum into three different compositions, which means that the N atoms had three different valences in the bonded graphene,48,49 namely, pyridinic-N (398.7 ± 0.2 eV), pyrrolic-N (399.3 ± 0.2 eV), and graphitic-N (401.4 ± 0.3 eV), as show in Fig. 5b. pyridinic-N contributes its p electrons to the graphene layer due 12


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to electronic defects, which can improve the initial potential and electrochemical properties of the other N substances in the carbon material, and this property played a decisive role in the NGA sample with a high N content of 30.479%. Pyrrolic-N is an N atom that contributes two p electrons to the graphene π system that is formed from the contribution of pyridinic-N; graphitic-N is derived from the N-atom that replaces a C atom in the graphene honeycomb ring, indicating that the N atom is bound to the C-C bonds of graphene. Meanwhile, the high-resolution C1s spectrum could be deconvoluted into five Gaussian components,50-52 as shown in Fig. 5c (corresponding to sp2 C, C=C, C-C, C-H, 284.4 ± 0.1 eV; sp3 C, C-C, 285.4 ± 0.1 eV; C-N,286.3 ± 0.1 eV; C=N, 287.3 ± 0.1 eV; and C=O, O=C-O, N=C-O, 288.2 ± 0.1 eV). The presence of a moderate amount of oxygenated groups on the NDPA-NGA skeleton could further decrease the charge transfer resistance by providing more active sites for enhancing power generation. The O1s peak mainly originated from the physically absorbed oxygen because graphitic carbon is susceptible to oxygen adsorption(The O1 s core level spectra were deconvoluted into four components, corresponding to O-metal; 529.6 ± 0.1 eV; O-physically absorbed, 532.2 ± 0.1 eV; C=O, O-C=O, 530.8 ± 0.1 eV; and C-OH, C-O-OH, C-O-C, 533.8 ± 0.1 eV),53 as shown in Fig. 5d.

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Figure 6. a) Stress-strain curves at different PAPE dosages (0, 1, 3, 5 mL). b) Under the 95% strain, the stress-strain curve of different PAPE dosages (0, 1, 3, 5 mL). Inset: the experimental snapshots of one compression cycle. c) The stress-strain curves of NDPA-NGA at 55% strain for the first 5 cycles. Inset: the corresponding Elastic Modulus for the first 5 cycle. d) The stress-strain curves of NDPA-NGA at 55%, 85% and 95% strain for the first 30 cycles. Inset: the variation of height as a function of cycle numbers. The crosslinking effect of the graphene sheets gave NDPA-NGA excellent elastic properties (Video S1). The mechanical properties of the samples were tested at a load/unloading rate of 5 mm/min. Fig. 6a shows the stress-strain curves for the samples with different PAPE dosages at 55%, 85%, and 95% strain (ε). Although the sample exhibited a large amount of strain (95%), the maximum compressive stress of the sample was quite different (the maximum compressive stress fluctuated from 78-180 MPa at 95% strain). When the PAPE dosage was 3 mL, the stress reached 181 KPa, which gave the NDPA-NGA sufficient rigidity to resist the capillary pressure generated during natural drying (Table 1). The loading/unloading curves of all samples at 95% strain are shown in Fig. 6b. NDPA-NGA shows different deformation mechanisms for the different compression 14


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stages; when ε

Naturally Dried, Double Nitrogen-Doped 3D Graphene Aerogels

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