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Functionalized Graphene Aerogel
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FUNCTIONALIZED GRAPHENE AEROGEL: STRUCTURAL AND MORPHOLOGICAL PROPERTIES AND APPLICATIONS
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Anish Khan1, 2, Aftab Aslam Parwaz Khan1, 2, Mohammed Omaish Ansari3, Imran Khan4, Mohammad Mujahid Ali Khan4, Abdullah M. Asiri1,2, Aleksandr Evhenovych Kolosov5, P. Senthamaraikannan6 Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia1; Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah, Saudi Arabia2; Center of Nanotechnology, King Abdulaziz University, Jeddah, Saudi Arabia3; Applied Science and Humanities Section, University Polytechnic, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh, India4; Chemical, Polymeric and Silicate Machine Building Department of Chemical Engineering Faculty, National Technical University of Ukraine «Igor Sikorsky Kyiv Polytechnic Institute», Kyiv, Ukraine5; Department of Mechanical Engineering, Kamaraj College of Engineering and Technology, Virudhunagar, India6
1. INTRODUCTIONS An aerogel isn’t a particular mineral or material with a fixed chemical formula; however, it comprises all substances with a specific geometrical shape. This shape is an extremely porous, stable foam with high connectivity among extended systems of multiple nanometers or more. Despite the truth that aerogel is in fact foam, it is able to take diverse shapes and systems. Most of the past aerogel was produced from silica; however, carbon, iron oxide, herbal polymers, semiconductor nanostructures, gold, and copper have also been utilized to develop a variety of aerogels. The aerogel structure possesses high mechanical strength, although 99.8% of its structure comprises of only air. This gives aerogel a ghostly appearance; henceforth, it is also called frozen smoke. Graphene is a growing magnificence of ultrathin carbon film material [1] with excessive unique surface vicinity [2], main versatility [3], synthetic electricity, and high electric and thermal conductivity [4]. These inherent physicochemical properties make graphene a potential material for applications in nanoelectronics [5], sensors [6], catalysis [7], composites [8e15], energy storage, [16] and biomedical frameworks [17]. In order to further exploit and extend the applications, the 3-D structures of graphene (GN) and GN-based materials such as GN aerogels (GA) offer exciting prospects due to its high surface area, interconnected network, and increased strength. The 3-D structures in GN are required to translate the exceptional properties of individual GN sheets into the thrust properties of 3-D porous materials for novel applications, for example, catalysis Functionalized Graphene Nanocomposites and Their Derivatives. https://doi.org/10.1016/B978-0-12-814548-7.00008-8 Copyright © 2019 Elsevier Inc. All rights reserved.
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base [18], synthetic muscle tissue [19], cathodes for supercapacitors [20], and sponges [21]. Among them, the high porosity with a hydrophobic nature makes the 3D grapheneebased macrostructures appealing for potential utility as a spongy in environmental remediation as oil spillage and removal of solvents from the water surface.
2. MORPHOLOGICAL STUDY One of the research group of Cao et al. [22] studied the key component of superhydrophobicity of fluorinated polydopamine/chitosan/graphene oxide (GO) that lays on the mixture among various leveled small-scale nanostructure and low-surface-energy substances [23,24]. With the intention to understand the wetting behavior of the aerogels, the surface morphology becomes tested with the aid of field emission scanning electron microscopy. The reduced graphene oxide-polydopamine (rGO-PDA) hydrogel changed into solidify dried to deliver rGOePDA aerogel, which validated black center-like morphology. As it simply seemed within the SEM images, the contorted nanosheets have been crossrelated to shape an interpenetrating gadget shape with wealthy nano- and micropores (Fig. 8.1A and B). Within the wake of being reinforced by way of chitosan, the system shape became stored up, but the microporous dividers had been certainly thickened, and the C, N, and O components have been constantly dispersed on the surface of composite aerogel (CGA), as shown through Energy Dispersive X-Ray (EDX) mapping (Fig. 8.1C and E). It was seen that the fluorinated rGO nanosheets had been undoubtedly agglomerated on the surface of reduced composite aerogel and indicated various leveled small-scale/nanostructure and F components showed up on the surface of fCGA-10 aside from C, N, and O components (Fig. 8.1D and F). Yong Wang [25] synthesized hybrid GO/microcrystalline cellulose (MCC) aerogels. Inside the hybrid GO/MCC aerogels (Fig. 8.2B), MCC molecules form the network-like shape, and additionally, they show the function of the skeleton to assist the GO sheets. Due to the robust interplay among the oxygen-containing groups of the GO and hydroxyl groups of the MCC molecules (Fig. 8.2C), exfoliated and unfolded GO with smaller layer numbers can be achieved within the hybrid GO/MCC aerogels. Katsumi Kaneko [26] prepared highly microporous graphene aerogel monolith (GAM) of unidirectional honeycomb macrotextures. The snapshots of GAM and PGAM-973 (GAM denotes, graphene aerogel monolith while PGAM-T, polymer graphene aerogel monolith prepared at a certain temperature) are shown in Fig. 8.3A. GAM has a cylindrical shape with a defined diameter that is derived from the geometry of its preparation techniques. PGAM-973, in large part, succeeds in maintaining the preliminary geometry of GAM with less than 10% shrinkage of volume after potassium hydroxide (KOH) activation at 973 k. The calculated bulk density of PGAM-973 is w8 0.5 mg cm3, while the structural deformation is meant to be 10%. Extensive activation at 1023 K does not maintain the monolith form, giving only small pieces. Fig. 8.3B shows the SEM pictures of GAM before and after KOH activation. A unidirectional texture shape, which is parallel to the route of ice boom, is located at the GAM, as shown in Fig. 8.3B1, being much like the previously mentioned materials prepared by means of ice-freezing technique, indicating that the unidirectional freezing leads to the partly ordered textured structure. The unidirectional textured structure includes well-aligned graphene sheets and intersheet bridging shape. The surfaces of the parallel graphene sheets and the bridging structures are smooth and uniform, as shown in Fig. 8.4A.
FIGURE 8.1 Field emission scanning electron microscopy (FE-SEM) images of reduced graphene oxide (rGO)ePDA aerogel (A, B); CGA (C); and a zoom on fCGA-10 (D); EDX mapping of CGA (E); and fCGA-10 (F). Cao N, Lyu Q, Li J, Wang Y, Yang B, Szunerits S, et al. Facile synthesis of fluorinated polydopamine/chitosan/reduced graphene oxide composite aerogel for efficient oil/water separation. Chem Eng J 2017;326:17e28, figure taken with permission.
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FIGURE 8.2 Schematic representations showing the microstructure of (A) neat graphene oxide (GO) aerogel and (B) GO/ microcrystalline cellulose (MCC) aerogel, and (C) the interaction between GO and MCC chains. Wei X, Huang T, Yang J, Zhang N, Wang Y, Zhou Z. Green synthesis of hybrid graphene oxide/microcrystalline cellulose aerogels and their use as superabsorbents. J Hazard Mater 2017;335:28e38, a figure was taken with permission.
The parallel unidirectional sheet systems are nevertheless maintained at the KOH-activated GAM of PGAM-973, as shown in Fig. 8.3B2, even though some elements of the graphene layers are slightly distorted and fractured because of KOH activation. The renovation of the well-aligned graphene sheet structure of GAM-973 ensures the preserving of its freestanding nature. The parallel graphene sheets of this prepared pattern are less clean and uniform compared with the ones before activation, as shown in Fig. 8.4B. The PGAM-1023 organized via extensive activation has no lengthy-range parallel sheet systems. Fig. 8.4C suggests only the presence of a nearby structure of curved graphenes being responsible for its unsustainable structure. The transmission electron microscopy (TEM) images are shown in Figs. 8.3C and 8.5, which honestly indicate the structural alternate of GAMs before and after KOH activation. Fig. 8.5A indicates that the GAM consists of stacking graphene layers with the large size of more than several hundred nanometers and wrinkled paper-like sheets. The stacked graphene layers are ordered and uniform, as shown in Fig. 8.3C1. The KOH activation reduces the unit graphene size and degrades the ordered stacking of graphene layers, giving a nanoporous structure, as proven in Figs. 8.3C2 and 8.5B. The nanoporosity with the aid of the KOH activation was analyzed. It is worthy to say that a part of the graphene layers with huge length and uniform shape is still maintained even after the KOH activation process, as shown in Fig. 8.5B, which can form the stable network by bridging each other, ensuring the unidirectionally advanced framework of the freestanding aerogel monolith.
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FIGURE 8.3 Morphology of graphene aerogel monolith before and after shape-retention activation. (A) Photos, (B) scanning electron microscopy (SEM) images, and (C) transmission electron microscopy (TEM) images of graphene aerogel monolith (GAM) and PGAM-973. Wang S, Wang Z, Futamura R, Endo M, Kaneko K. Highly microporous-graphene aerogel monolith of unidirectional honeycomb macro-textures. Chem Phys Lett 2017;673:38e43, a figure was taken with permission.
3. APPLICATION The effectiveness of the aerogels for natural adsorption and oil/water partition was inspected. As shown, while beads of water and diesel fell at the surface of CGA, the drops have been adsorbed quickly internal 1.4 0.05 s and 1.6 0.05 s via first-class powers, demonstrating that the CGA has
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FIGURE 8.4 Scanning electron microscopy (SEM) images of (A) graphene aerogel monolith (GAM), (B) PGAM-973, and (C) PGAM-1023. Wang S, Wang Z, Futamura R, Endo M, Kaneko K. Highly microporous-graphene aerogel monolith of unidirectional honeycomb macro-textures. Chem Phys Lett 2017;673:38e43, a figure was taken with permission.
each superhydrophilicity and superoleophilicity, as appeared in Fig. 8.1A and B. The CGA had stable oleophobicity in various watery arrangements with diverse pH esteems, as appeared in Fig. 8.1C. The contact factors of various natural fluids submerged at the CGA surface have been measured (Fig. 8.6), and the effects uncovered that CGA has submerged superoleophobicity. The hydrophilic gatherings (-NH2 and -OH) of chitosan and the cruel surface make CGA superhydrophilic in the air and
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FIGURE 8.5 Transmission electron microscopy (TEM) images of (A) graphene aerogel monolith (GAM), and (B) PGAM-973. Wang S, Wang Z, Futamura R, Endo M, Kaneko K. Highly microporous-graphene aerogel monolith of unidirectional honeycomb macro-textures. Chem Phys Lett 2017;673:38e43, a figure was taken with permission.
FIGURE 8.6 Contact angle of various organic liquids under water of composite graphene aerogel. Cao N, Lyu Q, Li J, Wang Y, Yang B, Szunerits S, et al. Facile synthesis of fluorinated polydopamine/chitosan/reduced graphene oxide composite aerogel for efficient oil/water separation. Chem Eng J 2017;326:17e28.
superoleophobic submerged. At the point when CGA became submerged, a water film fashioned on its surface impeded the oil wettability, and the permeable CGA division behavior of oil/water combination changed into performing (Fig. 8.7). Yong-Kang et al. report a strongly reduced graphene aerogel (rGA) as a green and recyclable sorbent for oils and organic solvents, which shows surprisingly green absorption of numerous oils and natural solvents (up to 19e26 times its own weight) and extremely good recyclability (>5 instances)
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FIGURE 8.7 The spreading process of a droplet of (A) water, and (B) diesel on the composite graphene aerogel surface; (C) optical images of a diesel droplet on the composite graphene aerogel surface in NaOH, HCl, and NaCl aqueous solutions. Cao N, Lyu Q, Li J, Wang Y, Yang B, Szunerits S, et al. Facile synthesis of fluorinated polydopamine/chitosan/reduced graphene oxide composite aerogel for efficient oil/water separation. Chem Eng J 2017;326:17e28.
via heat treatment. Furthermore, the absorption potential of rGA can be maintained over a huge temperature variety of about 40e240 C, which may be attributed to the inherent wonderful thermal stability of graphene and good heat dispersal of the 3-D network structure. Based on those tremendous properties, the rGO is considered to be a superfabric that may be used for separation and absorption of waste oil and natural contaminants from the water surface at numerous temperatures [27] (Fig. 8.8). Tianxi Liu and coworkers [28] synthesized graphene/graphene nanoribbon (GNR) aerogels as a tunable 3-D framework for efficient hydrogen evolution reaction (HER). The electrocatalytic activity of GxGNRy@MoS2 hybrids for HER have been investigated in acid media of 0.5 M H2SO4 solution with the usage of a regular three-electrode setup at a scan rate of 2 mVs1. Fig. 8.9 indicates the HER
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FIGURE 8.8 (A) Photographs of the absorption capacity of the reduced graphene aerogel (rGA) for oils and organic solvents. (B) Absorption recyclability of the rGA for oils and organic solvents. (C) The oil uptake capacity of motor oil at various temperatures. Ren R-P, Li W, Lv Y-K. A robust, superhydrophobic graphene aerogel as a recyclable sorbent for oils and organic solvents at various temperatures. J Colloid Interface Sci 2017;500:63e8, a figure was taken with permission.
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FIGURE 8.9 Linear sweep voltammetry (LSV) polarization curves of G2GNR1@MoS2-2-, G1GNR1@MoS2-2-, and G1GNR2@MoS2-2-modified glassy carbon electrode (GCE) in N2-purged 0.5 M H2SO4 solution. Scan rate: 2 mV s-1. Sun Z, Fan W, Liu T. Graphene/graphene nanoribbon aerogels as tunable three-dimensional framework for efficient hydrogen evolution reaction. Electrochim Acta 2017;250:91e8, a figure was taken with permission.
catalytic performance of a sequence of GxGNRy@MoS2-2 hybrids. Among the various GxGNRy@ MoS2-2 hybrids, G1GNR1@MoS2-2 hybrid reveals the most effective HER catalytic performance with the onset potential of 105 mV versus Reversible Hydrogen Electrode (RHE) and full-size hydrogen evolution (j ¼ 10 mA cm2) located at a voltage of 183 mV, with the alternative curves greater or less negatively shifted. The distinction of electrocatalytic activity between these hybrids with exclusive ratios of graphene and GNR may be defined by means of their porous structure. The G1GNR1 aerogel indicates extra homogeneous pore size distribution, attributing to its nanoribboninterconnected nanosheet shape, which can provide greater surface regions for in situ growth of MoS2 nanosheets with equal quantity. The result suggests that the gold-standard GxGNRy@MoS2 hybrid reveals first-rate HER performance, with a low onset potential of 105 mV, a small Tafel slope of 49 mV per decade, and a large current density (10.0 mA cm2 at h ¼ 183 mV). Aho Seok Park [29] and his team also take high porosity, mechanical balance, and hydrophobicity into consideration; the FerGO aerogel is a great candidate for the sorption of oils and other natural pollutants. As depicted in Fig. 8.10A, while a small piece of the FerGO aerogel became located on pump oil (strained with Oil Blue N dye), the pump oil become at once absorbed by using the FerGO aerogel and completely disappeared in a few seconds, suggesting a powerful way for cleansing spilled oil so that it will, in addition, reveal the soaking-up capability of FerGO aerogel. Numerous varieties of oils and organic solvents (e.g., pump oil, chlorobenzene, tetrahydrofuran (THF), acetone, and many others) have been examined in terms of their absorptioncapacity (defined as k ¼ (weight after saturated adsorption - initial weight)/initial weight). The average mass of samples is 1.6 0.4 mg, and all samples have the sorption time of more than 30 s to attain equilibrium state for soaking up all chemical compounds. It can be found that the FerGO aerogel exhibited first-rate absorption performance for diverse oils and natural solvents, resulting in the capacity ranging from 34 up to 112 times of its weight
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FIGURE 8.10 (A) Oil absorption test of the Fereduced graphene oxide (rGO) aerogel. Pump oil (stained with Oil Blue N dye) floating on water was completely absorbed within 1.5 s. (B) Absorption capacities of the FerGO aerogel for various oils and organic solvents. The sorption time for the oil absorbing test was 30 s. (C) Absorption recyclability of the FerGO aerogel. Hong J-Y, Sohn E-H, Park S, Park HS. Highly-efficient and recyclable oil absorbing performance of functionalized graphene aerogel. Chem Eng J 2015;269:229e35, a figure was taken with permission.
in absorption capabilities. The changes in j value are dependent on the density of oils and natural solvents (Fig. 8.10B). The fluorinated functional groups could sure with the last oxygen-functional groups of rGO sheets as well as give a hydrophobic rGO aerogel surface for selective oil absorption. Moreover, 3-D macroporous networks of the rGO aerogel provided large accessible area and high
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porosity. It is expected that the large surface area of the rGO aerogel and the strong hydrophobicity of fluorinated functional groups would have a combined or synergistic effect on removing oils and organic solvents from water. Consequently, the FerGO aerogel validated extremely high absorption capacities for diverse kinds of oils and natural solvents, which were better than the ones of the rGO aerogel and conventional sorbent materials along with natural activated carbon and polymers were suggested within the previous literature.
4. PROPERTIES One of the research groups of Zhang and Wenbo [30] described the improved mechanical, thermal, and electric capability of graphene aerogels through supercritical ethanol drying and excessivetemperature thermal reduction. They routinely prepared robust graphene aerogels with low thermal conductivities and excessive thermal stability and electrical conductivities by way of hydrothermal reduction of GO dispersions and supercritical ethanol drying. The yield strengths and Young’s moduli of GAeS had been in the range of 0.05e0.75 and 0.81e13.84 MPa, respectively. A 75.0-mg graphene aerogel chamber with a mass density of 56.2 mg cm3 could bolster no less than 26,000 times its own particular weight. The thermal conductivities and electrical conductivities of GAeS were in the scope of 0.0281e0.0390Wm1K1 and 14.4e53.7 S m1, individually. Toughening at 1500 C brought about a further change in the yield qualities and Young’s moduli of graphene aerogels to 0.08e1.05 MPa and 1.02e17.29 MPa, individually. The GA-S10-1500C (GA¼graphene aerogel, SA10 ¼ sample 10 and 1500C ¼ 1500 C) might need to keep up its flexibility for no less than 100 cycles upon rehashed pressure to 6% took after by utilizing the most yield energy of 0.9 MPa without unrecoverable disfigurement. With high-temperature strengthening, the oxidation temperature of graphene aerogels in air stretched out from around 625 C to just about 705 C, and the thermal conductivity and electric conductivities of graphene aerogels enhanced to the scopes of 0.0363e0.0667Wm1K1 and 53.5e157.3 S m1, individually. Figs. 8.11 and 8.12.
5. GRAPHENE AEROGELS IN ENERGY STORAGE APPLICATIONS Most of the energy needs of daily human life are supplied by combustion of fossil fuels. However, fossil fuel resources are limited and not renewable. Therefore, there is a high demand for more efficient utilization of energy and exploration of new energy sources to substitute fossil fuels [31]. Carbon mesoporous structures have been largely employed as electrode active materials in the past owing to their high surface areas and porous structures. However, the major problems with these carbon materials are their complicated and disordered porous structure, which affects the electron transfer and thus limits the performance of the devices [32]. In the viewpoint of the abovementioned problems, graphene aerogels are ultralight ordered and highly porous solid nanomaterials with large pore volumes, high surface areas, and tunable porosity. These properties are derived from their microstructures, which are typically composed of 3-D networks of interconnected nanoparticles. The electrochemical capacitive performances of rGO aerogels using the three-electrode system by Weijiang Si et al. [33] showed high-rate supercapacitive performance in aqueous electrolytes. The specific capacitance of RGO was calculated to be 211.8 and 278.6 F g1 in KOH and H2SO4 electrolytes, respectively. The morphological characterization showed
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FIGURE 8.11 Thermogravimetric (TG) (A) and difference thermo gravimetry (DTG) (B) plots of graphene oxide (GO), GN aerosol (GA)-S, and GA-S-1500C in the air. (C) Thermal conductivities of GA-S, GA-S-1500C and other low thermal conductive graphene aerogels. (D) Snapshots of GA-S-1500C sample in a hot flame of an alcohol burner, the insets in (d) were the microstructure of tested sample. Cheng Y, Zhou S, Hu P, Zhao G, Li Y, Zhang X, et al. Enhanced mechanical, thermal, and electric properties of graphene aerogels via supercritical ethanol drying and high-temperature thermal reduction. Sci Rep 2017;7:1439, a figure was taken with permission.
plenty of mesopores and macroporous structures containing ordered graphitic structure with curved graphene sheets. Thus, this high supercapacitive performance of rGO aerogels was ascribed to their 3-D porous conductive structure and the existence of oxygen-containing groups. As mentioned above, the 3-D porous structure and functional groups in the aerogel structures highly affect the conductance and capacitance. Thus, best designs of the graphene aerogels in terms of pore size, pore volume, and density are very important for energy storage devices. The pore volume and the surface area of the graphene aerogels using Hummers’ method and combined GO with resorcinoleformaldehyde gel was found to be 2.96 cm3 g1 and 584 m2 g1, respectively, and a high electrical conductance of 87 S m1 was observed [34]. Zhang et al. [35] successfully prepared graphene aerogels with large BrunauereEmmetteTeller surface area of 512 m2 g1 and high electrical conductivity of w 102 S m1. These highly conductive aerogels exhibited a specific capacitance of 128 F g1. Graphene aerogels, by hydrothermal technique, showed high electrical conductivities of 1.3e3.2 S m1, along with a high specific capacitance of 220 F g1 and excellent cyclic stability, as 92% capacitance remained after 2000 cycle tests [36]. Functionalized graphene aerogels and their composites have also shown interesting properties in terms of electrical conductance and specific capacitance, which have expanded its application in various fields. Three-dimensional nitrogen-doped graphene aerogels functionalized with melamine
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FIGURE 8.12 Electrical conductivities (A) of GA-S, GA-S-1500C, and other high-electric-conductive graphene aerogels. The inset in (A) was the bulk density and electrical conductivity of graphene aerogels versus annealing temperature in ref. 22. A circuit constructed with the (B) GA-S, and (C) GA-S-1500C cylindrical sample. Cheng Y, Zhou S, Hu P, Zhao G, Li Y, Zhang X, et al. Enhanced mechanical, thermal, and electric properties of graphene aerogels via supercritical ethanol drying and high-temperature thermal reduction. Sci Rep 2017;7:1439, a figure was taken with permission.
prepared by Xing et al. [37] showed high specific capacitance of 170.5 F g1 at 0.2 A g1 along with the high charge/discharge cycling stability. The high performance of the device was mainly attributed to the pseudocapacitance of unreduced hydroquinone/quinone groups or carboxyl groups and high graphitic N content introduced into graphene. Graphene aerogel functionalized with different amount of p-phenylenediamine leads to the formation of porous 3-D structures, and the increase in the content of p-phenylenediamine led to an increase in surface area. The study by Habib Gholipour-Ranjbar et al. [38] showed that the aerogels with larger pore size showed enhanced supercapacitive performance compared with the aerogels with smaller pore size. Their study revealed an excellent capacitance of 385 F g1 at a discharge current density of 1 A g1, along with the exceptionally high cyclic stability by displaying 25% increase in specific capacitance after 5000 cycles. The 3-D macroporous graphene aerogels, in combination with metals/metal oxides, allow most of metals/metal oxides insides the porous aerogel structures to be accessible to the electrolyte, which enhances the diffusion and migration of electrolyte ions during the rapid charge/discharge process. The 3-D macroporous MnO2egraphene aerogel composite showed a high specific capacitance of 200 F g1 [39]. The p-phenylenediamineefunctionalized MnO2egraphene aerogel also showed excellent performance of 26.2 Wh kg1 energy density and high cyclic stability after 5000 cycles due
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to the 3-D framework initiated by the p-phenylenediamine, along with the synergistic effect of the porous graphene and MnO2, as the 3-D porous structure leads to improved electrolyte accessibility and utilization of the highest surface area of the electrode materials. Some materials like cellulose are quite abundant and cheap, and they possess a porous structure and an extremely high porosity. Celluloseegraphene aerogels possessed highly porous and interconnected 3-D nanostructures, which provided efficient migration of electrolyte ions and electrons, and thus the aerogels exhibited superb electrochemical performance. The specific capacitances reached 300 F g1 at a scan rate of 5 mV s1 [40]. Zheng et al. further added carbon nanotubes (CNTs) in the celluloseegraphene aerogels to fabricate flexible solid-state supercapacitors aerogels consisting of cellulose nanofibrils, graphene, and CNTs, which showed high performance, mainly due to the excellent electrolyte absorption of cellulose nanofibrils properties. Conducting polymers in combination with the graphene aerogels have shown extremely high capacitance. The 3-D polyanilineegraphene porous structure developed by Y. Qu et al. [41] showed the high specific surface area of up to 337 m2 g1, and for symmetric and asymmetric all-solid-state supercapacitors, the aerogels delivered real capacitances of up to 453 and 679 mF cm2, respectively, which were found to be superior to most of the graphene/GO or polyaniline-based aerogels. This excellent electrochemical performance is due to the synergistic contribution of the local conductivity of graphene layers sandwiched between polyaniline layers and long-distance conductivity of 3-D graphene frameworks. Fig 8.13 shows the long-distance and localized conductivity contributions of polyaniline and graphene in an aerogel network. As discussed earlier, p-phenylenediamine initiates the formation of hierarchal 3-D structures; various other workers have used a surfactant to achieve these effects. Ghosh et al. [42] showed that grapheneeaerogels composites synthesized in the presence of Pluronic F-68 as a soft template and vitamin C as a reducing agent enabled the effective dispersion of GO in water, and second, it assisted the formation of a stable 3-D pillared hydrogel assembly. Their reduced grapheneeMnO2epolyaniline aerogels showed the excellent performance of high energy density of 18.33 W h kg1 at a power density of 0.388 kW kg1. The high performance of these aerogel composite structures is attributed to MnO2@polyaniline particles, which played a dual role. Firstly, they acted as spacers between the graphene layers to prevent stacking from occurring, and secondly, they contributed to the high pseudocapacitance of the composite. Similar to grapheneepolyaniline aerogels, the composites of polypyrroleegraphene aerogels have also shown high electroactivity due to the reason mentioned above. Graphene/polypyrrole nanoparticle hybrid aerogels with 3-D hierarchical porous structure exhibited high specific capacitance (418 F g1) at a current density of 0.5 A g1, extremely outstanding rate capability (80%) at various current densities from 0.5 to 20 A g1, and good cycling performance (74%) after 2000 cycles [43]. A 3-D hierarchical grapheneepolypyrrole aerogel prepared by S. Ye et al. [44] showed well-dispersed polypyrrole nanotubes inside the graphene aerogel matrix, and the polypyrrole nanotubes not only provided a large accessible surface area for fast transport of hydrate ions, but also acted as a spacer to prevent the restacking of graphene sheets. The resulting nanocomposite aerogel showed an excellent electrochemical performance, including a high specific capacitance up to 253 F g1, good rate performance, and outstanding cycle stability. This method may be feasible to prepare other graphenebased hybrid aerogels with structure-controllable nanostructures in large scale, thereby holding enormous potential in many application fields.
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FIGURE 8.13 (A) Long distance and localized conductivity contributions of polyaniline and graphene in aerogel network, and (B) cyclic voltammetry (CV) and chargeedischarge curves of polyaniline-graphene aerogels. Permission taken from the publisher, Qu Y, Lu C, Su Y, Cui D, He Y, Zhang C, et al. Hierarchical-graphene-coupled polyaniline aerogels for electrochemical energy storage. Carbon N Y 2018;127:77e84.
6. GRAPHENE AEROGELS IN GAS SENSING APPLICATIONS The macroporosity of graphene aerogels in sensing applications gives highly efficient gas sensors in comparison to graphene composites prepared by other routes. The NO2 sensing response of grapheneeSnO2 aerogels studied by Liu et al. [45] showed a highly efficient response in comparison to the conventionally prepared composite, as the inserted SnO2 nanoparticles solve the restacking problems of aerogels. Li et al. [46] also showed that their hydrothermally prepared graphene-SnO2 aerogels possessed a high surface area of 441.9 m2 g1, and the composite aerogels exhibited a good linearity for NO2 detection, with a low detection limit of about 2 ppm at low temperature. The high
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efficiency of the grapheneeSnO2 aerogels is mainly attributed to the porous structure, good electrical conductivity, enhanced surface/interface adsorption, and the formation of forming pen heterojunction and the depletion area, which leads to higher mobility of electron flow sites from the graphene toward NO2 through the bonded SnO2 pathway of the 3-D aerogel structures. Graphene aerogels with other metal oxides have also made room temperature sensing possible in case of metal oxides such as ZnO and Fe3O4, which requires high temperature for activation. The contact between graphene and oxide particle in aerogel structures not only suppresses the growth and agglomeration of nanoparticles during the crystallization process, but also forms a heterojunction at the interface, which reduces the activation energy of the overall composite, hence making room temperature sensing possible [47]. These rGO/SnO2 p-n heterojunction aerogels were also successfully utilized by Guo et al. [48] for the selective detection of phenol in the presence of ethanol, toluene, and methanol, with a very low detection limit of 5 ppb. The high sensitivity is due to the special porous structure of rGO/SnO2 aerogels with higher specific surface area that resulted in an obvious increase in the contact of gas molecules on the p-n heterojunction, which could improve the sensitivity of the gas sensing. Phenol, as a typical weak electron-donating molecule, came in contact with the SnO2 surface, and more electrons were transferred from SnO2 to rGO, resulting in a reduction of device conductivity. Graphene aerogels have also been successfully utilized as a support for platinum and palladium nanoparticles for the sensing of combustible gases such as hydrogen and propane. A simple sensor by Harley-Trochimczyk et al. [49] was fabricated by drop-casting nanoparticle-loaded graphene aerogel is suspended in water onto the microheater. The high efficiency of the sensor is attributed to the high specific surface area of graphene aerogel, which allows high loading of catalyst nanoparticles and enhances the response and recovery time for both hydrogen and propane gas to 1e2 s. Functionalization of as-prepared aerogels with chemicals has also shown to induce additional functionality in the graphene aerogels, which can be employed for the selective and specific sensing of different gases. Thiourea-functionalized graphene aerogels showed high porosity of 389 m2 g1 and higher sensitivity, shorter response time, and better selectivity toward ammonia gas compared to the aerogel produced in the absence of thiourea. The amount of thiourea involved in the synthesis step was found to be the key factor in the sensing properties of the finally obtained aerogel. The sensor response time to ammonia was short (100 s) and completely reversible (recovery time of about 500 s) in ambient temperature. The sensor response to ammonia was linear, between 0.02 and 85 ppm, and its detection limit was found to be 10 ppb (3 S/N) [50].
REFERENCES [1] Geim AK, Novoselov KS. The rise of graphene. Nat Mater 2007;6:183e91. [2] Peigney A, Laurent C, Flahaut E, Bacsa RR, Rousset A. Specific surface area of carbon nanotubes and bundles of carbon nanotubes. Carbon N Y 2001;39:507e14. [3] Lee C, Wei X, Kysar JW, Hone J. Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science 2008;321(80):385e8. [4] Li X, Zhang G, Bai X, Sun X, Wang X, Wang E, et al. Highly conducting graphene sheets and LangmuireBlodgett films. Nat Nanotechnol 2008;3:538e42. [5] Eda G, Fanchini G, Chhowalla M. Large-area ultrathin films of reduced graphene oxide as a transparent and flexible electronic material. Nat Nanotechnol 2008;3:270e4.
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[6] Shao Y, Wang J, Wu H, Liu J, Aksay IA, Lin Y. Graphene based electrochemical sensors and biosensors: a review. Electroanalysis 2010;22:1027e36. [7] Machado BF, Serp P. Graphene-based materials for catalysis. Catal Sci Technol 2012;2:54e75. [8] Vickery JL, Patil AJ, Mann S. Fabrication of graphene-polymer nanocomposites with higher-order threedimensional architectures. Adv Mater 2009;21:2180e4. [9] Khan A, Khan AAP, Asiri AM, Rub MA, Rahman MM, Ghani SA. In vitro studies of carbon fiber microbiosensor for dopamine neurotransmitter supported by copper-graphene oxide composite. Microchim Acta 2014;181:1049e57. [10] Khan A, Asiri AM, Khan AAP, Rub MA, Azum N, Rahman MM, et al. Solegel synthesis and characterization of conducting polythiophene/tin phosphate nano tetrapod composite cation-exchanger and its application as Hg(II) selective membrane electrode. J Sol gel Sci Technol 2013;65:160e9. [11] Khan A, Khan AAP, Asiri AM, Alamry KA. Preparation and characterization of hybrid graphene oxide composite and its application in paracetamol microbiosensor. Polym Compos 2015;36:221e8. [12] Khan A, Khan AAP, Asiri AM, Abu-Zied BM. Green synthesis of thermally stable Ag-rGO-CNT nano composite with high sensing activity. Compos Part B Eng 2016;86:27e35. [13] Khan AAP, Khan A, Rahman MM, Asiri AM, Oves M. Lead sensors development and antimicrobial activities based on graphene oxide/carbon nanotube/poly(O-toluidine) nanocomposite. Int J Biol Macromol 2016;89:198e205. [14] Khan A, Khan AAP, Asiri AM, Khan I. Facial synthesis, characterization of graphene oxide-zirconium tungstate (GO-Zr(WO4)2) nanocomposite and its application as modified microsensor for dopamine. J Alloys Compd 2017;723:811e9. [15] Khan A, Khan AAP, Rahman MM, Asiri AM, Inamuddin AKA, et al. Preparation and characterization of PANI@G/CWO nanocomposite for enhanced 2-nitrophenol sensing. Appl Surf Sci 2018;433:696e704. [16] Wang X, Zhi L, Mu¨llen K. Transparent, conductive graphene electrodes for dye-sensitized solar cells. Nano Lett 2008;8:323e7. [17] Menaa F, Abdelghani A, Menaa B. Graphene nanomaterials as biocompatible and conductive scaffolds for stem cells: impact for tissue engineering and regenerative medicine. J Tissue Eng Regen Med 2015;9: 1321e38. [18] Wu Z-S, Yang S, Sun Y, Parvez K, Feng X, Mu¨llen K. 3D nitrogen-doped graphene aerogel-supported Fe 3 O 4 nanoparticles as efficient electrocatalysts for the oxygen reduction reaction. J Am Chem Soc 2012;134: 9082e5. [19] Aliev AE, Oh J, Kozlov ME, Kuznetsov AA, Fang S, Fonseca AF, et al. Giant-stroke, superelastic carbon nanotube aerogel muscles. Science 2009;323(80):1575e8. [20] Dong X-C, Xu H, Wang X-W, Huang Y-X, Chan-Park MB, Zhang H, et al. 3D grapheneecobalt oxide electrode for high-performance supercapacitor and enzymeless glucose detection. ACS Nano 2012;6: 3206e13. [21] Niu Z, Chen J, Hng HH, Ma J, Chen X. A leavening strategy to prepare reduced graphene oxide foams. Adv Mater 2012;24:4144e50. [22] Cao N, Lyu Q, Li J, Wang Y, Yang B, Szunerits S, et al. Facile synthesis of fluorinated polydopamine/ chitosan/reduced graphene oxide composite aerogel for efficient oil/water separation. Chem Eng J 2017;326: 17e28. [23] Jiang L, Zhao Y, Zhai J. A Lotus-Leaf-like superhydrophobic surface: a porous microsphere/nanofiber composite film prepared by electrohydrodynamics. Angew Chem 2004;116:4438e41. [24] Zhang X, Shi F, Niu J, Jiang Y, Wang Z. Superhydrophobic surfaces: from structural control to functional application. J Mater Chem 2008;18:621e33. https://doi.org/10.1039/B711226B. [25] Wei X, Huang T, Yang J, Zhang N, Wang Y, Zhou Z. Green synthesis of hybrid graphene oxide/microcrystalline cellulose aerogels and their use as superabsorbents. J Hazard Mater 2017;335:28e38.
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[26] Wang S, Wang Z, Futamura R, Endo M, Kaneko K. Highly microporous-graphene aerogel monolith of unidirectional honeycomb macro-textures. Chem Phys Lett 2017;673:38e43. [27] Ren R-P, Li W, Lv Y-K. A robust, superhydrophobic graphene aerogel as a recyclable sorbent for oils and organic solvents at various temperatures. J Colloid Interface Sci 2017;500:63e8. [28] Sun Z, Fan W, Liu T. Graphene/graphene nanoribbon aerogels as tunable three-dimensional framework for efficient hydrogen evolution reaction. Electrochim Acta 2017;250:91e8. [29] Hong J-Y, Sohn E-H, Park S, Park HS. Highly-efficient and recyclable oil absorbing performance of functionalized graphene aerogel. Chem Eng J 2015;269:229e35. [30] Cheng Y, Zhou S, Hu P, Zhao G, Li Y, Zhang X, et al. Enhanced mechanical, thermal, and electric properties of graphene aerogels via supercritical ethanol drying and high-temperature thermal reduction. Sci Rep 2017; 7:1439. [31] Nguyen ST, Nguyen HT, Rinaldi A, Nguyen NPV, Fan Z, Duong HM. Morphology control and thermal stability of binderless-graphene aerogels from graphite for energy storage applications. Colloids Surf A Physicochem Eng Asp 2012;414:352e8. [32] Zhang LL, Zhao XS. Carbon-based materials as supercapacitor electrodes. Chem Soc Rev 2009;38:2520. [33] Si W, Wu X, Zhou J, Guo F, Zhuo S, Cui H, et al. Reduced graphene oxide aerogel with high-rate supercapacitive performance in aqueous electrolytes. Nanoscale Res Lett 2013;8:247. [34] Worsley MA, Pauzauskie PJ, Olson TY, Biener J, Satcher JH, Baumann TF. Synthesis of graphene aerogel with high electrical conductivity. J Am Chem Soc 2010;132:14067e9. [35] Zhang X, Sui Z, Xu B, Yue S, Luo Y, Zhan W, et al. Mechanically strong and highly conductive graphene aerogel and its use as electrodes for electrochemical power sources. J Mater Chem 2011;21:6494. [36] Zhang L, Shi G. Preparation of highly conductive graphene hydrogels for fabricating supercapacitors with high rate capability. J Phys Chem C 2011;115:17206e12. [37] Xing L-B, Hou S-F, Zhou J, Zhang J-L, Si W, Dong Y, et al. Three dimensional nitrogen-doped graphene aerogels functionalized with melamine for multifunctional applications in supercapacitors and adsorption. J Solid State Chem 2015;230:224e32. [38] Gholipour-Ranjbar H, Ganjali MR, Norouzi P, Naderi HR. Electrochemical investigation of functionalized graphene aerogel with different amount of p-phenylenediamine as an advanced electrode material for supercapacitors. Mater Res Express 2016;3:75501. [39] Ji C-C, Bao S-J, Lu Z-J, Cai C-J, Yang F, Wei H, et al. 3D interpenetrating macroporous graphene aerogels with MnO2 coating for supercapacitors. Russ J Electrochem 2015;51:782e8. [40] Zhang Y, Wang F, Zhang D, Chen J, Zhu H, Zhou L, et al. New type multifunction porous aerogels for supercapacitors and absorbents based on cellulose nanofibers and graphene. Mater Lett 2017;208:73e6. [41] Qu Y, Lu C, Su Y, Cui D, He Y, Zhang C, et al. Hierarchical-graphene-coupled polyaniline aerogels for electrochemical energy storage. Carbon N Y 2018;127:77e84. [42] Ghosh K, Yue CY, Sk MM, Jena RK, Bi S. Development of a 3D graphene aerogel and 3D porous graphene/ MnO 2 @polyaniline hybrid film for all-solid-state flexible asymmetric supercapacitors. Sustain Energy Fuels 2018;2:280e93. [43] He Y, Bai Y, Yang X, Zhang J, Kang L, Xu H, et al. Holey graphene/polypyrrole nanoparticle hybrid aerogels with three-dimensional hierarchical porous structure for high performance supercapacitor. J Power Sources 2016;317:10e8. [44] Liu X, Cui J, Sun J, Zhang X. 3D graphene aerogel-supported SnO2 nanoparticles for efficient detection of NO2. RSC Adv 2014;4:22601. [45] Li L, He S, Liu M, Zhang C, Chen W. Three-dimensional mesoporous graphene aerogel-supported SnO2 nanocrystals for high-performance NO2 gas sensing at low temperature. Anal Chem 2015;87:1638e45. [46] Liu X, Li J, Sun J, Zhang X. 3D Fe3O4 nanoparticle/graphene aerogel for NO2 sensing at room temperature. RSC Adv 2015;5:73699e704.
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[47] Liu X, Sun J, Zhang X. Novel 3D graphene aerogeleZnO composites as efficient detection for NO2 at room temperature. Sensors Actuators B Chem 2015;211:220e6. [48] Guo D, Cai P, Sun J, He W, Wu X, Zhang T, et al. Reduced-graphene-oxide/metal-oxide p-n heterojunction aerogels as efficient 3D sensing frameworks for phenol detection. Carbon N Y 2016;99:571e8. [49] Harley-Trochimczyk A, Chang J, Pham T, Dong J, Worsley MA, Zettl A, et al. Low power microheater-based combustible gas sensor with graphene aerogel catalyst support. In: Transducers - 2015 18th Int. Conf. SolidState Sensors, Actuators Microsystems. IEEE; 2015. p. 1483e6. [50] Alizadeh T, Ahmadian F. Thiourea-treated graphene aerogel as a highly selective gas sensor for sensing of trace level of ammonia. Anal Chim Acta 2015;897:87e95.
FUNCTIONALIZED GRAPHENE AEROGEL: STRUCTURAL AND MORPHOLOGICAL PROPERTIES AND APPLICATIONS
8
Anish Khan1, 2, Aftab Aslam Parwaz Khan1, 2, Mohammed Omaish Ansari3, Imran Khan4, Mohammad Mujahid Ali Khan4, Abdullah M. Asiri1,2, Aleksandr Evhenovych Kolosov5, P. Senthamaraikannan6 Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia1; Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah, Saudi Arabia2; Center of Nanotechnology, King Abdulaziz University, Jeddah, Saudi Arabia3; Applied Science and Humanities Section, University Polytechnic, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh, India4; Chemical, Polymeric and Silicate Machine Building Department of Chemical Engineering Faculty, National Technical University of Ukraine «Igor Sikorsky Kyiv Polytechnic Institute», Kyiv, Ukraine5; Department of Mechanical Engineering, Kamaraj College of Engineering and Technology, Virudhunagar, India6
1. INTRODUCTIONS An aerogel isn’t a particular mineral or material with a fixed chemical formula; however, it comprises all substances with a specific geometrical shape. This shape is an extremely porous, stable foam with high connectivity among extended systems of multiple nanometers or more. Despite the truth that aerogel is in fact foam, it is able to take diverse shapes and systems. Most of the past aerogel was produced from silica; however, carbon, iron oxide, herbal polymers, semiconductor nanostructures, gold, and copper have also been utilized to develop a variety of aerogels. The aerogel structure possesses high mechanical strength, although 99.8% of its structure comprises of only air. This gives aerogel a ghostly appearance; henceforth, it is also called frozen smoke. Graphene is a growing magnificence of ultrathin carbon film material [1] with excessive unique surface vicinity [2], main versatility [3], synthetic electricity, and high electric and thermal conductivity [4]. These inherent physicochemical properties make graphene a potential material for applications in nanoelectronics [5], sensors [6], catalysis [7], composites [8e15], energy storage, [16] and biomedical frameworks [17]. In order to further exploit and extend the applications, the 3-D structures of graphene (GN) and GN-based materials such as GN aerogels (GA) offer exciting prospects due to its high surface area, interconnected network, and increased strength. The 3-D structures in GN are required to translate the exceptional properties of individual GN sheets into the thrust properties of 3-D porous materials for novel applications, for example, catalysis Functionalized Graphene Nanocomposites and Their Derivatives. https://doi.org/10.1016/B978-0-12-814548-7.00008-8 Copyright © 2019 Elsevier Inc. All rights reserved.
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base [18], synthetic muscle tissue [19], cathodes for supercapacitors [20], and sponges [21]. Among them, the high porosity with a hydrophobic nature makes the 3D grapheneebased macrostructures appealing for potential utility as a spongy in environmental remediation as oil spillage and removal of solvents from the water surface.
2. MORPHOLOGICAL STUDY One of the research group of Cao et al. [22] studied the key component of superhydrophobicity of fluorinated polydopamine/chitosan/graphene oxide (GO) that lays on the mixture among various leveled small-scale nanostructure and low-surface-energy substances [23,24]. With the intention to understand the wetting behavior of the aerogels, the surface morphology becomes tested with the aid of field emission scanning electron microscopy. The reduced graphene oxide-polydopamine (rGO-PDA) hydrogel changed into solidify dried to deliver rGOePDA aerogel, which validated black center-like morphology. As it simply seemed within the SEM images, the contorted nanosheets have been crossrelated to shape an interpenetrating gadget shape with wealthy nano- and micropores (Fig. 8.1A and B). Within the wake of being reinforced by way of chitosan, the system shape became stored up, but the microporous dividers had been certainly thickened, and the C, N, and O components have been constantly dispersed on the surface of composite aerogel (CGA), as shown through Energy Dispersive X-Ray (EDX) mapping (Fig. 8.1C and E). It was seen that the fluorinated rGO nanosheets had been undoubtedly agglomerated on the surface of reduced composite aerogel and indicated various leveled small-scale/nanostructure and F components showed up on the surface of fCGA-10 aside from C, N, and O components (Fig. 8.1D and F). Yong Wang [25] synthesized hybrid GO/microcrystalline cellulose (MCC) aerogels. Inside the hybrid GO/MCC aerogels (Fig. 8.2B), MCC molecules form the network-like shape, and additionally, they show the function of the skeleton to assist the GO sheets. Due to the robust interplay among the oxygen-containing groups of the GO and hydroxyl groups of the MCC molecules (Fig. 8.2C), exfoliated and unfolded GO with smaller layer numbers can be achieved within the hybrid GO/MCC aerogels. Katsumi Kaneko [26] prepared highly microporous graphene aerogel monolith (GAM) of unidirectional honeycomb macrotextures. The snapshots of GAM and PGAM-973 (GAM denotes, graphene aerogel monolith while PGAM-T, polymer graphene aerogel monolith prepared at a certain temperature) are shown in Fig. 8.3A. GAM has a cylindrical shape with a defined diameter that is derived from the geometry of its preparation techniques. PGAM-973, in large part, succeeds in maintaining the preliminary geometry of GAM with less than 10% shrinkage of volume after potassium hydroxide (KOH) activation at 973 k. The calculated bulk density of PGAM-973 is w8 0.5 mg cm3, while the structural deformation is meant to be 10%. Extensive activation at 1023 K does not maintain the monolith form, giving only small pieces. Fig. 8.3B shows the SEM pictures of GAM before and after KOH activation. A unidirectional texture shape, which is parallel to the route of ice boom, is located at the GAM, as shown in Fig. 8.3B1, being much like the previously mentioned materials prepared by means of ice-freezing technique, indicating that the unidirectional freezing leads to the partly ordered textured structure. The unidirectional textured structure includes well-aligned graphene sheets and intersheet bridging shape. The surfaces of the parallel graphene sheets and the bridging structures are smooth and uniform, as shown in Fig. 8.4A.
FIGURE 8.1 Field emission scanning electron microscopy (FE-SEM) images of reduced graphene oxide (rGO)ePDA aerogel (A, B); CGA (C); and a zoom on fCGA-10 (D); EDX mapping of CGA (E); and fCGA-10 (F). Cao N, Lyu Q, Li J, Wang Y, Yang B, Szunerits S, et al. Facile synthesis of fluorinated polydopamine/chitosan/reduced graphene oxide composite aerogel for efficient oil/water separation. Chem Eng J 2017;326:17e28, figure taken with permission.
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FIGURE 8.2 Schematic representations showing the microstructure of (A) neat graphene oxide (GO) aerogel and (B) GO/ microcrystalline cellulose (MCC) aerogel, and (C) the interaction between GO and MCC chains. Wei X, Huang T, Yang J, Zhang N, Wang Y, Zhou Z. Green synthesis of hybrid graphene oxide/microcrystalline cellulose aerogels and their use as superabsorbents. J Hazard Mater 2017;335:28e38, a figure was taken with permission.
The parallel unidirectional sheet systems are nevertheless maintained at the KOH-activated GAM of PGAM-973, as shown in Fig. 8.3B2, even though some elements of the graphene layers are slightly distorted and fractured because of KOH activation. The renovation of the well-aligned graphene sheet structure of GAM-973 ensures the preserving of its freestanding nature. The parallel graphene sheets of this prepared pattern are less clean and uniform compared with the ones before activation, as shown in Fig. 8.4B. The PGAM-1023 organized via extensive activation has no lengthy-range parallel sheet systems. Fig. 8.4C suggests only the presence of a nearby structure of curved graphenes being responsible for its unsustainable structure. The transmission electron microscopy (TEM) images are shown in Figs. 8.3C and 8.5, which honestly indicate the structural alternate of GAMs before and after KOH activation. Fig. 8.5A indicates that the GAM consists of stacking graphene layers with the large size of more than several hundred nanometers and wrinkled paper-like sheets. The stacked graphene layers are ordered and uniform, as shown in Fig. 8.3C1. The KOH activation reduces the unit graphene size and degrades the ordered stacking of graphene layers, giving a nanoporous structure, as proven in Figs. 8.3C2 and 8.5B. The nanoporosity with the aid of the KOH activation was analyzed. It is worthy to say that a part of the graphene layers with huge length and uniform shape is still maintained even after the KOH activation process, as shown in Fig. 8.5B, which can form the stable network by bridging each other, ensuring the unidirectionally advanced framework of the freestanding aerogel monolith.
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FIGURE 8.3 Morphology of graphene aerogel monolith before and after shape-retention activation. (A) Photos, (B) scanning electron microscopy (SEM) images, and (C) transmission electron microscopy (TEM) images of graphene aerogel monolith (GAM) and PGAM-973. Wang S, Wang Z, Futamura R, Endo M, Kaneko K. Highly microporous-graphene aerogel monolith of unidirectional honeycomb macro-textures. Chem Phys Lett 2017;673:38e43, a figure was taken with permission.
3. APPLICATION The effectiveness of the aerogels for natural adsorption and oil/water partition was inspected. As shown, while beads of water and diesel fell at the surface of CGA, the drops have been adsorbed quickly internal 1.4 0.05 s and 1.6 0.05 s via first-class powers, demonstrating that the CGA has
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FIGURE 8.4 Scanning electron microscopy (SEM) images of (A) graphene aerogel monolith (GAM), (B) PGAM-973, and (C) PGAM-1023. Wang S, Wang Z, Futamura R, Endo M, Kaneko K. Highly microporous-graphene aerogel monolith of unidirectional honeycomb macro-textures. Chem Phys Lett 2017;673:38e43, a figure was taken with permission.
each superhydrophilicity and superoleophilicity, as appeared in Fig. 8.1A and B. The CGA had stable oleophobicity in various watery arrangements with diverse pH esteems, as appeared in Fig. 8.1C. The contact factors of various natural fluids submerged at the CGA surface have been measured (Fig. 8.6), and the effects uncovered that CGA has submerged superoleophobicity. The hydrophilic gatherings (-NH2 and -OH) of chitosan and the cruel surface make CGA superhydrophilic in the air and
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FIGURE 8.5 Transmission electron microscopy (TEM) images of (A) graphene aerogel monolith (GAM), and (B) PGAM-973. Wang S, Wang Z, Futamura R, Endo M, Kaneko K. Highly microporous-graphene aerogel monolith of unidirectional honeycomb macro-textures. Chem Phys Lett 2017;673:38e43, a figure was taken with permission.
FIGURE 8.6 Contact angle of various organic liquids under water of composite graphene aerogel. Cao N, Lyu Q, Li J, Wang Y, Yang B, Szunerits S, et al. Facile synthesis of fluorinated polydopamine/chitosan/reduced graphene oxide composite aerogel for efficient oil/water separation. Chem Eng J 2017;326:17e28.
superoleophobic submerged. At the point when CGA became submerged, a water film fashioned on its surface impeded the oil wettability, and the permeable CGA division behavior of oil/water combination changed into performing (Fig. 8.7). Yong-Kang et al. report a strongly reduced graphene aerogel (rGA) as a green and recyclable sorbent for oils and organic solvents, which shows surprisingly green absorption of numerous oils and natural solvents (up to 19e26 times its own weight) and extremely good recyclability (>5 instances)
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FIGURE 8.7 The spreading process of a droplet of (A) water, and (B) diesel on the composite graphene aerogel surface; (C) optical images of a diesel droplet on the composite graphene aerogel surface in NaOH, HCl, and NaCl aqueous solutions. Cao N, Lyu Q, Li J, Wang Y, Yang B, Szunerits S, et al. Facile synthesis of fluorinated polydopamine/chitosan/reduced graphene oxide composite aerogel for efficient oil/water separation. Chem Eng J 2017;326:17e28.
via heat treatment. Furthermore, the absorption potential of rGA can be maintained over a huge temperature variety of about 40e240 C, which may be attributed to the inherent wonderful thermal stability of graphene and good heat dispersal of the 3-D network structure. Based on those tremendous properties, the rGO is considered to be a superfabric that may be used for separation and absorption of waste oil and natural contaminants from the water surface at numerous temperatures [27] (Fig. 8.8). Tianxi Liu and coworkers [28] synthesized graphene/graphene nanoribbon (GNR) aerogels as a tunable 3-D framework for efficient hydrogen evolution reaction (HER). The electrocatalytic activity of GxGNRy@MoS2 hybrids for HER have been investigated in acid media of 0.5 M H2SO4 solution with the usage of a regular three-electrode setup at a scan rate of 2 mVs1. Fig. 8.9 indicates the HER
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FIGURE 8.8 (A) Photographs of the absorption capacity of the reduced graphene aerogel (rGA) for oils and organic solvents. (B) Absorption recyclability of the rGA for oils and organic solvents. (C) The oil uptake capacity of motor oil at various temperatures. Ren R-P, Li W, Lv Y-K. A robust, superhydrophobic graphene aerogel as a recyclable sorbent for oils and organic solvents at various temperatures. J Colloid Interface Sci 2017;500:63e8, a figure was taken with permission.
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FIGURE 8.9 Linear sweep voltammetry (LSV) polarization curves of G2GNR1@MoS2-2-, G1GNR1@MoS2-2-, and G1GNR2@MoS2-2-modified glassy carbon electrode (GCE) in N2-purged 0.5 M H2SO4 solution. Scan rate: 2 mV s-1. Sun Z, Fan W, Liu T. Graphene/graphene nanoribbon aerogels as tunable three-dimensional framework for efficient hydrogen evolution reaction. Electrochim Acta 2017;250:91e8, a figure was taken with permission.
catalytic performance of a sequence of GxGNRy@MoS2-2 hybrids. Among the various GxGNRy@ MoS2-2 hybrids, G1GNR1@MoS2-2 hybrid reveals the most effective HER catalytic performance with the onset potential of 105 mV versus Reversible Hydrogen Electrode (RHE) and full-size hydrogen evolution (j ¼ 10 mA cm2) located at a voltage of 183 mV, with the alternative curves greater or less negatively shifted. The distinction of electrocatalytic activity between these hybrids with exclusive ratios of graphene and GNR may be defined by means of their porous structure. The G1GNR1 aerogel indicates extra homogeneous pore size distribution, attributing to its nanoribboninterconnected nanosheet shape, which can provide greater surface regions for in situ growth of MoS2 nanosheets with equal quantity. The result suggests that the gold-standard GxGNRy@MoS2 hybrid reveals first-rate HER performance, with a low onset potential of 105 mV, a small Tafel slope of 49 mV per decade, and a large current density (10.0 mA cm2 at h ¼ 183 mV). Aho Seok Park [29] and his team also take high porosity, mechanical balance, and hydrophobicity into consideration; the FerGO aerogel is a great candidate for the sorption of oils and other natural pollutants. As depicted in Fig. 8.10A, while a small piece of the FerGO aerogel became located on pump oil (strained with Oil Blue N dye), the pump oil become at once absorbed by using the FerGO aerogel and completely disappeared in a few seconds, suggesting a powerful way for cleansing spilled oil so that it will, in addition, reveal the soaking-up capability of FerGO aerogel. Numerous varieties of oils and organic solvents (e.g., pump oil, chlorobenzene, tetrahydrofuran (THF), acetone, and many others) have been examined in terms of their absorptioncapacity (defined as k ¼ (weight after saturated adsorption - initial weight)/initial weight). The average mass of samples is 1.6 0.4 mg, and all samples have the sorption time of more than 30 s to attain equilibrium state for soaking up all chemical compounds. It can be found that the FerGO aerogel exhibited first-rate absorption performance for diverse oils and natural solvents, resulting in the capacity ranging from 34 up to 112 times of its weight
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FIGURE 8.10 (A) Oil absorption test of the Fereduced graphene oxide (rGO) aerogel. Pump oil (stained with Oil Blue N dye) floating on water was completely absorbed within 1.5 s. (B) Absorption capacities of the FerGO aerogel for various oils and organic solvents. The sorption time for the oil absorbing test was 30 s. (C) Absorption recyclability of the FerGO aerogel. Hong J-Y, Sohn E-H, Park S, Park HS. Highly-efficient and recyclable oil absorbing performance of functionalized graphene aerogel. Chem Eng J 2015;269:229e35, a figure was taken with permission.
in absorption capabilities. The changes in j value are dependent on the density of oils and natural solvents (Fig. 8.10B). The fluorinated functional groups could sure with the last oxygen-functional groups of rGO sheets as well as give a hydrophobic rGO aerogel surface for selective oil absorption. Moreover, 3-D macroporous networks of the rGO aerogel provided large accessible area and high
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porosity. It is expected that the large surface area of the rGO aerogel and the strong hydrophobicity of fluorinated functional groups would have a combined or synergistic effect on removing oils and organic solvents from water. Consequently, the FerGO aerogel validated extremely high absorption capacities for diverse kinds of oils and natural solvents, which were better than the ones of the rGO aerogel and conventional sorbent materials along with natural activated carbon and polymers were suggested within the previous literature.
4. PROPERTIES One of the research groups of Zhang and Wenbo [30] described the improved mechanical, thermal, and electric capability of graphene aerogels through supercritical ethanol drying and excessivetemperature thermal reduction. They routinely prepared robust graphene aerogels with low thermal conductivities and excessive thermal stability and electrical conductivities by way of hydrothermal reduction of GO dispersions and supercritical ethanol drying. The yield strengths and Young’s moduli of GAeS had been in the range of 0.05e0.75 and 0.81e13.84 MPa, respectively. A 75.0-mg graphene aerogel chamber with a mass density of 56.2 mg cm3 could bolster no less than 26,000 times its own particular weight. The thermal conductivities and electrical conductivities of GAeS were in the scope of 0.0281e0.0390Wm1K1 and 14.4e53.7 S m1, individually. Toughening at 1500 C brought about a further change in the yield qualities and Young’s moduli of graphene aerogels to 0.08e1.05 MPa and 1.02e17.29 MPa, individually. The GA-S10-1500C (GA¼graphene aerogel, SA10 ¼ sample 10 and 1500C ¼ 1500 C) might need to keep up its flexibility for no less than 100 cycles upon rehashed pressure to 6% took after by utilizing the most yield energy of 0.9 MPa without unrecoverable disfigurement. With high-temperature strengthening, the oxidation temperature of graphene aerogels in air stretched out from around 625 C to just about 705 C, and the thermal conductivity and electric conductivities of graphene aerogels enhanced to the scopes of 0.0363e0.0667Wm1K1 and 53.5e157.3 S m1, individually. Figs. 8.11 and 8.12.
5. GRAPHENE AEROGELS IN ENERGY STORAGE APPLICATIONS Most of the energy needs of daily human life are supplied by combustion of fossil fuels. However, fossil fuel resources are limited and not renewable. Therefore, there is a high demand for more efficient utilization of energy and exploration of new energy sources to substitute fossil fuels [31]. Carbon mesoporous structures have been largely employed as electrode active materials in the past owing to their high surface areas and porous structures. However, the major problems with these carbon materials are their complicated and disordered porous structure, which affects the electron transfer and thus limits the performance of the devices [32]. In the viewpoint of the abovementioned problems, graphene aerogels are ultralight ordered and highly porous solid nanomaterials with large pore volumes, high surface areas, and tunable porosity. These properties are derived from their microstructures, which are typically composed of 3-D networks of interconnected nanoparticles. The electrochemical capacitive performances of rGO aerogels using the three-electrode system by Weijiang Si et al. [33] showed high-rate supercapacitive performance in aqueous electrolytes. The specific capacitance of RGO was calculated to be 211.8 and 278.6 F g1 in KOH and H2SO4 electrolytes, respectively. The morphological characterization showed
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FIGURE 8.11 Thermogravimetric (TG) (A) and difference thermo gravimetry (DTG) (B) plots of graphene oxide (GO), GN aerosol (GA)-S, and GA-S-1500C in the air. (C) Thermal conductivities of GA-S, GA-S-1500C and other low thermal conductive graphene aerogels. (D) Snapshots of GA-S-1500C sample in a hot flame of an alcohol burner, the insets in (d) were the microstructure of tested sample. Cheng Y, Zhou S, Hu P, Zhao G, Li Y, Zhang X, et al. Enhanced mechanical, thermal, and electric properties of graphene aerogels via supercritical ethanol drying and high-temperature thermal reduction. Sci Rep 2017;7:1439, a figure was taken with permission.
plenty of mesopores and macroporous structures containing ordered graphitic structure with curved graphene sheets. Thus, this high supercapacitive performance of rGO aerogels was ascribed to their 3-D porous conductive structure and the existence of oxygen-containing groups. As mentioned above, the 3-D porous structure and functional groups in the aerogel structures highly affect the conductance and capacitance. Thus, best designs of the graphene aerogels in terms of pore size, pore volume, and density are very important for energy storage devices. The pore volume and the surface area of the graphene aerogels using Hummers’ method and combined GO with resorcinoleformaldehyde gel was found to be 2.96 cm3 g1 and 584 m2 g1, respectively, and a high electrical conductance of 87 S m1 was observed [34]. Zhang et al. [35] successfully prepared graphene aerogels with large BrunauereEmmetteTeller surface area of 512 m2 g1 and high electrical conductivity of w 102 S m1. These highly conductive aerogels exhibited a specific capacitance of 128 F g1. Graphene aerogels, by hydrothermal technique, showed high electrical conductivities of 1.3e3.2 S m1, along with a high specific capacitance of 220 F g1 and excellent cyclic stability, as 92% capacitance remained after 2000 cycle tests [36]. Functionalized graphene aerogels and their composites have also shown interesting properties in terms of electrical conductance and specific capacitance, which have expanded its application in various fields. Three-dimensional nitrogen-doped graphene aerogels functionalized with melamine
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FIGURE 8.12 Electrical conductivities (A) of GA-S, GA-S-1500C, and other high-electric-conductive graphene aerogels. The inset in (A) was the bulk density and electrical conductivity of graphene aerogels versus annealing temperature in ref. 22. A circuit constructed with the (B) GA-S, and (C) GA-S-1500C cylindrical sample. Cheng Y, Zhou S, Hu P, Zhao G, Li Y, Zhang X, et al. Enhanced mechanical, thermal, and electric properties of graphene aerogels via supercritical ethanol drying and high-temperature thermal reduction. Sci Rep 2017;7:1439, a figure was taken with permission.
prepared by Xing et al. [37] showed high specific capacitance of 170.5 F g1 at 0.2 A g1 along with the high charge/discharge cycling stability. The high performance of the device was mainly attributed to the pseudocapacitance of unreduced hydroquinone/quinone groups or carboxyl groups and high graphitic N content introduced into graphene. Graphene aerogel functionalized with different amount of p-phenylenediamine leads to the formation of porous 3-D structures, and the increase in the content of p-phenylenediamine led to an increase in surface area. The study by Habib Gholipour-Ranjbar et al. [38] showed that the aerogels with larger pore size showed enhanced supercapacitive performance compared with the aerogels with smaller pore size. Their study revealed an excellent capacitance of 385 F g1 at a discharge current density of 1 A g1, along with the exceptionally high cyclic stability by displaying 25% increase in specific capacitance after 5000 cycles. The 3-D macroporous graphene aerogels, in combination with metals/metal oxides, allow most of metals/metal oxides insides the porous aerogel structures to be accessible to the electrolyte, which enhances the diffusion and migration of electrolyte ions during the rapid charge/discharge process. The 3-D macroporous MnO2egraphene aerogel composite showed a high specific capacitance of 200 F g1 [39]. The p-phenylenediamineefunctionalized MnO2egraphene aerogel also showed excellent performance of 26.2 Wh kg1 energy density and high cyclic stability after 5000 cycles due
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to the 3-D framework initiated by the p-phenylenediamine, along with the synergistic effect of the porous graphene and MnO2, as the 3-D porous structure leads to improved electrolyte accessibility and utilization of the highest surface area of the electrode materials. Some materials like cellulose are quite abundant and cheap, and they possess a porous structure and an extremely high porosity. Celluloseegraphene aerogels possessed highly porous and interconnected 3-D nanostructures, which provided efficient migration of electrolyte ions and electrons, and thus the aerogels exhibited superb electrochemical performance. The specific capacitances reached 300 F g1 at a scan rate of 5 mV s1 [40]. Zheng et al. further added carbon nanotubes (CNTs) in the celluloseegraphene aerogels to fabricate flexible solid-state supercapacitors aerogels consisting of cellulose nanofibrils, graphene, and CNTs, which showed high performance, mainly due to the excellent electrolyte absorption of cellulose nanofibrils properties. Conducting polymers in combination with the graphene aerogels have shown extremely high capacitance. The 3-D polyanilineegraphene porous structure developed by Y. Qu et al. [41] showed the high specific surface area of up to 337 m2 g1, and for symmetric and asymmetric all-solid-state supercapacitors, the aerogels delivered real capacitances of up to 453 and 679 mF cm2, respectively, which were found to be superior to most of the graphene/GO or polyaniline-based aerogels. This excellent electrochemical performance is due to the synergistic contribution of the local conductivity of graphene layers sandwiched between polyaniline layers and long-distance conductivity of 3-D graphene frameworks. Fig 8.13 shows the long-distance and localized conductivity contributions of polyaniline and graphene in an aerogel network. As discussed earlier, p-phenylenediamine initiates the formation of hierarchal 3-D structures; various other workers have used a surfactant to achieve these effects. Ghosh et al. [42] showed that grapheneeaerogels composites synthesized in the presence of Pluronic F-68 as a soft template and vitamin C as a reducing agent enabled the effective dispersion of GO in water, and second, it assisted the formation of a stable 3-D pillared hydrogel assembly. Their reduced grapheneeMnO2epolyaniline aerogels showed the excellent performance of high energy density of 18.33 W h kg1 at a power density of 0.388 kW kg1. The high performance of these aerogel composite structures is attributed to MnO2@polyaniline particles, which played a dual role. Firstly, they acted as spacers between the graphene layers to prevent stacking from occurring, and secondly, they contributed to the high pseudocapacitance of the composite. Similar to grapheneepolyaniline aerogels, the composites of polypyrroleegraphene aerogels have also shown high electroactivity due to the reason mentioned above. Graphene/polypyrrole nanoparticle hybrid aerogels with 3-D hierarchical porous structure exhibited high specific capacitance (418 F g1) at a current density of 0.5 A g1, extremely outstanding rate capability (80%) at various current densities from 0.5 to 20 A g1, and good cycling performance (74%) after 2000 cycles [43]. A 3-D hierarchical grapheneepolypyrrole aerogel prepared by S. Ye et al. [44] showed well-dispersed polypyrrole nanotubes inside the graphene aerogel matrix, and the polypyrrole nanotubes not only provided a large accessible surface area for fast transport of hydrate ions, but also acted as a spacer to prevent the restacking of graphene sheets. The resulting nanocomposite aerogel showed an excellent electrochemical performance, including a high specific capacitance up to 253 F g1, good rate performance, and outstanding cycle stability. This method may be feasible to prepare other graphenebased hybrid aerogels with structure-controllable nanostructures in large scale, thereby holding enormous potential in many application fields.
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FIGURE 8.13 (A) Long distance and localized conductivity contributions of polyaniline and graphene in aerogel network, and (B) cyclic voltammetry (CV) and chargeedischarge curves of polyaniline-graphene aerogels. Permission taken from the publisher, Qu Y, Lu C, Su Y, Cui D, He Y, Zhang C, et al. Hierarchical-graphene-coupled polyaniline aerogels for electrochemical energy storage. Carbon N Y 2018;127:77e84.
6. GRAPHENE AEROGELS IN GAS SENSING APPLICATIONS The macroporosity of graphene aerogels in sensing applications gives highly efficient gas sensors in comparison to graphene composites prepared by other routes. The NO2 sensing response of grapheneeSnO2 aerogels studied by Liu et al. [45] showed a highly efficient response in comparison to the conventionally prepared composite, as the inserted SnO2 nanoparticles solve the restacking problems of aerogels. Li et al. [46] also showed that their hydrothermally prepared graphene-SnO2 aerogels possessed a high surface area of 441.9 m2 g1, and the composite aerogels exhibited a good linearity for NO2 detection, with a low detection limit of about 2 ppm at low temperature. The high
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efficiency of the grapheneeSnO2 aerogels is mainly attributed to the porous structure, good electrical conductivity, enhanced surface/interface adsorption, and the formation of forming pen heterojunction and the depletion area, which leads to higher mobility of electron flow sites from the graphene toward NO2 through the bonded SnO2 pathway of the 3-D aerogel structures. Graphene aerogels with other metal oxides have also made room temperature sensing possible in case of metal oxides such as ZnO and Fe3O4, which requires high temperature for activation. The contact between graphene and oxide particle in aerogel structures not only suppresses the growth and agglomeration of nanoparticles during the crystallization process, but also forms a heterojunction at the interface, which reduces the activation energy of the overall composite, hence making room temperature sensing possible [47]. These rGO/SnO2 p-n heterojunction aerogels were also successfully utilized by Guo et al. [48] for the selective detection of phenol in the presence of ethanol, toluene, and methanol, with a very low detection limit of 5 ppb. The high sensitivity is due to the special porous structure of rGO/SnO2 aerogels with higher specific surface area that resulted in an obvious increase in the contact of gas molecules on the p-n heterojunction, which could improve the sensitivity of the gas sensing. Phenol, as a typical weak electron-donating molecule, came in contact with the SnO2 surface, and more electrons were transferred from SnO2 to rGO, resulting in a reduction of device conductivity. Graphene aerogels have also been successfully utilized as a support for platinum and palladium nanoparticles for the sensing of combustible gases such as hydrogen and propane. A simple sensor by Harley-Trochimczyk et al. [49] was fabricated by drop-casting nanoparticle-loaded graphene aerogel is suspended in water onto the microheater. The high efficiency of the sensor is attributed to the high specific surface area of graphene aerogel, which allows high loading of catalyst nanoparticles and enhances the response and recovery time for both hydrogen and propane gas to 1e2 s. Functionalization of as-prepared aerogels with chemicals has also shown to induce additional functionality in the graphene aerogels, which can be employed for the selective and specific sensing of different gases. Thiourea-functionalized graphene aerogels showed high porosity of 389 m2 g1 and higher sensitivity, shorter response time, and better selectivity toward ammonia gas compared to the aerogel produced in the absence of thiourea. The amount of thiourea involved in the synthesis step was found to be the key factor in the sensing properties of the finally obtained aerogel. The sensor response time to ammonia was short (100 s) and completely reversible (recovery time of about 500 s) in ambient temperature. The sensor response to ammonia was linear, between 0.02 and 85 ppm, and its detection limit was found to be 10 ppb (3 S/N) [50].
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