Functionalized Graphene Nanocomposites in Air Filtration Applications

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FUNCTIONALIZED GRAPHENE NANOCOMPOSITES IN AIR FILTRATION APPLICATIONS

4

Aneeya K. Samantara1, Satyajit Ratha2, Sudarsan Raj3 CSIR-Institute of Minerals and Materials Technology, Bhubaneswar, India1; School of Basic Sciences, Indian Institute of Technology, Bhubaneswar, India2; Institute of Materials and Systems for Sustainability, Nagoya University, Nagoya, Japan3

1. OVERVIEW AND BACKGROUND Not only our ecosystem, but the respiratory system is in a compromised state due to severe contamination of natural media such as air, water, and soil. Major sources of such contamination are particulate pollutants and harmful gases. These contaminants pollute the outdoor atmosphere, which, in effect, affects the indoor ambient air quality and thus possesses great degree of health hazard (it can be even more aggravating for those who are vulnerable to such pollutants, e.g., children, pregnant women, elderly people, and people with acute respiratory syndromes) [1,2]. A major share of modern humans’ time happens to be spent inside their houses, and thus, there is an increased risk of being exposed to pollutants (i.e., volatile organic compounds, ammonia, radon, etc.), which are specifically found indoors [3,4]. Since the last two decades, a significant rise has been observed in cases reporting deaths and health issues due to particulate pollution [5]. As modern societies are pledging for better healthcare, the gap between the living standards and putrefied air quality, therefore, need to be minimized urgently. Since the 1990s, an increased interest has been vested on the research and development of filtration techniques that would be effective in maintaining a healthy indoor air quality [6]. Separation of heterogeneous gaseous contents through nanoporous materials is an emerging field of research with many potential industry-level applications, such as development of gas sensors, gas purification, batteries and fuel cells, etc. [7]. In order to mitigate the anthropogenic greenhouse (mainly CO2 and CH4) gas emissions, various membrane-based separation technologies have been developed [7]. The realm of materials that are being tested for their feasibility as air filters comprises of membranes derived from organic polymers along with inorganic membranes made from glass, metal, ceramics, etc., including hybrids such as inorganic polymeric membranes [7]. Though the effectiveness of air filters is still in a debatable state, there has been a gradual and steady surge in the commercialization of industry-standard air filtration technologies over the years. The process of filtration might occur through various mechanisms such as interception, inertial impaction, diffusion, gravitational force, sedimentation, and electrostatic attraction [8e10]. Most of the commercially available air filtration technologies implement two varieties of air filters based upon Functionalized Graphene Nanocomposites and Their Derivatives. https://doi.org/10.1016/B978-0-12-814548-7.00004-0 Copyright © 2019 Elsevier Inc. All rights reserved.

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structure and function of the filter material. Structurally, they can be categorized into two types, the first being the porous type (microporous, mesoporous, and nanoporous), and the second one the fibrous type (microfibrous and nanofibrous). Among the two, fibrous filters are preferred due to their complex structure and efficiency in filtration. Taking advantage of the higher porosity (>70%) that enables excellent in-channel air mobility, fibrous filters have gathered increased attention over the years [7]. They, however, suffer from low filtering efficiency owing to their microsized pores, safety hazards from unexpected failure, bulkiness, and low quality factor, especially for filtrating ultrafine airborne particles. Due to reduced fiber diameter, nanofiber-based fibrous filters have been found to show significantly improved filtration efficiency over their microfibrous counterparts [11]. Electrospinning technology proves to be a convenient and flexible tool for the manufacture of fabric filters with desired dimensions, morphologies, and functional components, which has many advantages over conventional woven fabric filters. There are plenty of polymeric fabrics composed of several nanofibers available as air filters, e.g., polyurethane, polyacrylonitrile (PAN), polysulfone, polylactic acid, polycarbonate, polyvinyl alcohol, polyamide, etc. [12]. There are still issues with the filters discussed above due to differences between the fiber diameter and mean free path of air molecules. In general, the average fiber diameter is greater than the mean free path of air molecules having smaller dimensions, thus affecting the trapping capability. Furthermore, the promising filtration efficiency of nanofiber filters (which are capable of tackling ultrafine molecules) is severely altered depending on the particle properties, rate of airflow, and high humidity, since the removal ability is mostly affected by diffusion and adhesion processes [13]. The batch filtration process, which implements a series of nonhomogeneously arranged webs of fibers, provides an effective method to trap particles of different dimensions and hence reduces the risk of deterioration of fabric filters, which is normally observed in the case of simple filtration technologies (due to clogging as a result of particle deposition) without any size-targeted function [7]. Though filter materials, like polyamide-6 (PA-6)/PAN binary structured membrane and PA-6 nanonet membrane with embedded staple fibers, were successively designed to build cavity structures for better air permeability, maintaining their structural integrity still remains a great challenge due to the weak mechanical property of PAN nanofibers and the inevitable agglomeration of staple fibers [12]. Thus, it is preferable to consider filters having a composite inner pattern, which would assure good filtration and maintain an indoor air quality of desired standard (ideal for practical applications). As stated above, a molecular size-targeted separation mechanism is efficient. However, the method only works until the differences between the kinetic diameters of the gas molecules are sufficiently large. At smaller differences, the technique shows much less effectiveness. Therefore, methods have been discovered to take advantage of different solubility of selective gases in polymers. For sufficiently high temperatures, this effect even overcompensates the higher mobility of the smaller nitrogen molecules [7]. However, only moderate selectivities with ratios between 0.4 and 4.2 for CH4/N2 are reached at room temperature [7]. The air filtration market has long been dominated by mostly polyamide-based filter materials. Though polyamides such as nylon show good chemical and mechanical stability in comparison to other notable polymers, i.e., polyolefins, polypropylene, and polyethylene terephthalate, they however have striking disadvantages such as moisture-dependent structural disorientation, chlorine contamination, and lower degree of operational stability [12]. Most of the industrial air-cleaning devices function by either filtering or adsorbing the undesired pollutants and are made of mostly high-efficiency particulate air filter material and activated carbon. However, these filter materials require flushing/replacement at regular intervals due to clogging effect

2. SYNTHESIS OF GRAPHENE

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arising out of adsorption saturation [3]. Therefore, photocatalysis is assumed to be an efficient process for air cleaning. After the first report on the decomposition of H2O into H2 in the presence of TiO2 (photocatalyst), a surged interest is observed towards various nanostructures of TiO2 [14]. It finds a wide range of applications, such as treatment of wastewater, gas purification, solar energy conversion, antibiosis, and deodorization [15e18]. As per the chemical reaction kinetics, the photo destruction of organic substances is determined by their concentration. Hence the efficiency of air cleaner was enhanced by integrating an adsorbent with TiO2. Therefore, a search for a material that can act both as an adsorbent and support to the photocatalyst has been started. Among others, the high temperature tolerance and chemical inertness make the carbon-based materials more suitable. Graphene has attracted enormous attention due to its intriguing properties and wide potential applications in various fields such as nanoelectronics, transparent conductors, sensors, energy-related materials, and biomedical applications [19e23]. It’s stronger than steel, and it has unique sieving properties. At only an atom thick, there’s far less flow friction loss through the perforated graphene filter in contrast to the polyamide plastic filters that have been used for the last 50 years. Furthermore, single graphene sheets, having extreme robustness with the outstanding feature of one-atom thickness, are found to produce excellent filtration membranes. Great efforts have therefore been made to achieve a cost-effective way of fabrication of graphene sheets at industrial scale. To obtain functional gas membranes from graphene, nanosized pores have to be introduced artificially, since a perfect graphene sheet is impermeable even to helium. While post-treatment techniques, such as electron hole drilling, may be applied to produce pores of nanoscale dimension, smaller pore types are more likely to be obtained by self-organized growth of the desired type by catalyzed polymerization reactions in high-temperature solvents [7]. Development of ultralight graphene aerogels that are highly compressible are reported to have porosity as high as 99.7%e99.8% [3]. Furthermore, Haiyan Sun et al. have derived macroscopic, multiform (1-D, 2-D, and 3-D), ultralight all-carbon aerogels with controlled density from giant graphene sheets and carbon nanotube (CNT) ribs [24]. Viable options are therefore available to be implemented in air purifier devices. There are several industry-level attempts toward the mass production of graphene, which is a prerequisite for their large-scale implementation in applicationbased products/devices. However, developing macroscopic graphene-based materials (MGMs) is still a challenge for expanding the practical applications of graphene [24]. In this context, MGM of various forms, i.e., 1-D, 2-D, and 3-D, have been fabricated, and some of the as-prepared MGMs demonstrated numerous breakthroughs in corresponding areas. Though its synthetic protocols are yet to be improved for scalability, 3-D MGM is receiving significant attention because of its excellent intrinsic properties by virtue of graphene content and thus provides conductive framework with high specific areas for catalyst loading, organic or inorganic molecule absorption, and efficient routes for charge or ion transportation [24].

2. SYNTHESIS OF GRAPHENE There are numerous procedures that have been developed for graphene synthesis, but all are categorized into top-down or bottom-up synthetic approaches (Table 4.1). Generally, the qualitative and quantitative graphene are produced by using either of the (1) chemical vapor deposition (CVD) on metal surfaces, (2) CVD on insulating surfaces, (3) mechanical exfoliation, or by preparing (4) the colloidal suspensions synthetic methods [25]. Each of these methods have their own advantages and disadvantages.

Table 4.1 Table Showing Different Synthetic Approaches for Synthesis of Graphene Starting Material

Solvent/ Electrolyte

Bottom-up

CVD on Metal Surface

CH4:H2

e

H2:CH4 (ratio up to 2e1100) CH4

e e

C2H2, N2, H2

e

CO2:CH4

e

CH4, H2, Ar

e

SiC

e

Graphite

e

Graphite rod

Graphite nanosheets

Mixture of NaCl, DMSO and thionine acetate salt in deionized water Anhydrous DMF

GO GO

CVD/Epitaxial Growth on insulating surface

Top-down

Mechanical exfoliation

Colloidal suspension

GO

Reaction Conditions

Growth Substrate

References

Low pressure high temperature CVD (800e1000
C) 1050
C, 2e750 mbar

Cu (25 mm thick)

Nano Lett 2017;17: 2361

Cu (25 mm thick)

Chem Mater 2017; 29:3431 Chem Mater 2017; 29:4202

Low pressure high temperature CVD (1000
C) (950e1050
C)

Low pressure CVD (1000e1050
C, 3e5 mbar for 60e180 min) 1000
C

Cu (25 mm thick)

Stainless steel mesh coated with Cu Al2O3, SiO2/Si, Quartz Glass

Nanoscale Res Lett 2016;11:506

Si/Ni/SiO2/Ni droplet

Adv Mater Interfaces 2017; 1600783 Science 2006;312: 1191

Carbon 2017;112: 201

Vacuum Graphitization with metal (Pd, Au) contact Scotch tape method

SiC

5 V of DC potential has been applied

e

Milled for 30 h with (with zirconia balls and poly(tetrafluoroethylene) vials in a planetary mill

e

J Mater Chem 2010;20:5817

Water

e

DMF/CCl4/ THF/DCE DMF

e

J Mater Chem 2006;16:155 J Am Chem Soc 2006;128:7720 Nature 2006;442: 282

1% dimethyl hydrazine, 80
C, 24 h

e

e

Science 2004;306: 666 Chem Phys Lett 2013;572:61

CHAPTER 4 FUNCTIONALIZED GRAPHENE NANOCOMPOSITES

Synthesis Methods

68

Synthesis Approach

3. PROPERTIES

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Generally, the CVD methods are broadly adopted for the synthesis of graphene from the molecular precursors where the doping and pore formation can be simultaneously accomplished. This method is a catalytic process in which the solid, liquid, and gaseous precursors are used for the preparation of defect-free large-size graphene sheets, but the scalable preparation of single-layer graphene in a cost-effective way remains a challenging task. On the other hand, high-quality graphene is derived from the bulk graphite by using the mechanical exfoliation process (scotch tape method), but the low-quantity preparation restricts its broad application [26], whereas the colloidal process is a versatile one in terms of scalable production and its suitability for chemical functionalization to avail a wide range of applications. In this process, the graphene so produced is named as chemically modified graphene (CMG) that is derived either from the graphite directly or from the graphite oxide/ graphene oxide (GO). Hernandez et al. have derived the single-layer graphene sheets (w12 wt.%) by sonicating the graphite powder in N-methyl pyrrolidone solvent (concentration: 0.01 mg/mL) [27]. Graphene sheets of length w40 mm are prepared simply by stirring the graphite intercalated compound (ternary potassium salt, K [THF]x C24) in N-methyl pyrrolidone [28]. In another work, Liu et al. have prepared an ionic liquidefunctionalized graphene by electrochemically treating the graphite rods in imidazolium-based ionic liquid [29]. Among other colloidal suspension preparation methods, the scalable synthesis of graphene was achieved from the GO following a variety of reduction processes. In this process, the first step involves the conversion of graphite powder to its oxide form, i.e., graphite oxide/GO, followed by reduction [30e32]. A number of procedures have been developed to reduce the GO (chemical, thermal, photochemical, etc.) to form graphene [30,33e36]. Though Brodie had synthesized graphite oxide for the first time in 1855, new interest on it has been triggered after the discovery of graphene [37]. Thereafter, by following simple stirring or sonication of these graphite oxides, the GO is prepared [32,38]. It has been verified that during the oxidation process, a variety of oxygen functionalities (epoxy, carboxylic, aldehydic, alcoholic, and ketonic groups, etc.) were developed both on the edges and basal plane of the carbon skeleton of GO [39,40]. Then in the second step, the graphene was synthesized from these GO colloidal suspensions, employing chemical/thermal reduction methods [41]. So far, the graphene produced by reducing the GO is still associated with a significant amount of oxygen functionalities and defects. Therefore, this graphene is named as reduced GO (rGO), and the existing oxygen functionalities make it more suitable for further modification to achieve its multifunctional properties.

3. PROPERTIES 3.1 CHEMICAL PROPERTIES The sp2-hybridized carbon atoms are arranged in a 2-D honeycomb lattice-like structure to form graphene. Out of the four orbitals [three hybridized sp2 (s, px, py) and one unhybridized (pz)], three hybridized orbitals of each carbon atom (by head-to-head overlapping) forms covalent bonds (s-bonds) with the adjacent carbon atoms [42], whereas the remaining unhybridized orbital (by sidewise overlapping) forms a p-bond with the nearby carbon atom, developing an electron cloud above and below the sp2-hybridized carbon platform. Because of the existence of s- and p-conjugated systems, various foreign molecules/ligands/nanoparticles can be accommodated on its surface by means of noncovalent (pep linkage of van der Waals interaction) bonding. Also, graphene can be easily doped with heteroatoms like N, O, B, P, F, Cl, etc., by facile synthetic protocols. Out of the

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others, the O-doped graphene (so-called GO or graphite oxide) can be easily obtained either by chemical or electrochemical oxidation processes [43]. The developed numerous oxygen functionalities on the GO surface facilitate further modification with molecules of functional groups of choice. On the other hand, the porous graphene (PG) is prepared either from the bulk graphite or from GO. Both of these porous and doped graphene have found pronounced application in the field of air filtration for the removal and adsorption of hazardous greenhouse gases. The graphene surface can be easily polymerized with a variety of conducting and heteroatom-containing monomers, demonstrating an increased CO2 adsorption capabilities compared to the unpolymerized one. The detailed discussion on the porous graphene and doped graphene preparations is presented in the following sections.

3.2 PHYSICAL PROPERTIES Graphene finds its application in emerging fields due to its low cost, ease of preparation, flexibility, large theoretical surface area (2630 m2 g1), thermal/chemical stability, high mechanical strength, good optical transmittance, and fascinating transport phenomenon (quantum hall effect) [25]. Graphene is a zero-bandgap semiconductor with an ambipolar diffusion behaviour having tunable charge carrier mobility (room temperature mobility of 15,000 cm2 v1 s1), which can be tuned to 2  105 cm2 v1 s1 by minimizing the impurity scattering [44]. The mechanical strength and elasticity of the mono-/few-layer graphene has been achieved by the force displacement measurement using atomic force microscopy [45]. It has been found that the defect-free graphene shows a Young’s modulus of w1 Tpa with a fracture strength of 130 GPa. But in the case of CMG, it’s very less at 0.25 TPa [46,47]. Additionally, graphene has an optical transmittance of nearly 97.7% which linearly decreases with increasing layer numbers in multilayered graphene [48]. The thermal conduction of graphene is strictly dominated by the transport of phonon, showing a diffusive and ballistic conduction at high and sufficiently lower temperature, respectively [49]. It has been reported that the suspended monolayer graphene shows a thermal conductivity of w5  103 W m1 k1 [46].

4. FUNCTIONALIZATION AND COMPOSITE PREPARATION Pure graphene is a honeycomb lattice of 2-Dearranged sp2 carbon atoms. The hydrophobic nature of graphene hinders its employment in many of the specific areas of application, demanding further modification. In order to mitigate the hydrophobicity and enhance the dispersity, the surface functionalization of graphene (pore formation, doping of heteroatoms, polymerization, etc.) and composite preparation with foreign molecules/nanoparticles have been extensively carried out. Moreover, the inherent physicochemical and electronic properties of graphene can be tailored by functionalizing its surface.

4.1 POROUS GRAPHENE The high intrinsic strength (43 N m1) and strong structure (CeC bond energy: 4.9 eV) of one-atomthick single-layer graphene is the thinnest membrane ever known. Due to the compact packing of carbon atoms, the delocalized electron cloud blocks the gap of the aromatic rings, developing the repulsive force (Fig. 4.1) [50]. Therefore, the pristine graphene shows impermeability, even to the smallest helium and hydrogen molecules [51,52]. Hence, much of the theoretical support and experimental effort has been

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FIGURE 4.1 (A) Structure of a single-layer graphene, (B) the molecular structure of graphene with rough electronic density distribution, and (C) the presentation of geometric pore of a single hexagon. Reproduced with permission from Berry V. Impermeability of graphene and its applications. Carbon NY 2013;62:1e10.

concentrated to make porous graphene (PG). Generally, PG can be prepared by substituting the carbon atoms with pores of desirable dimension that demonstrate to be an excellent membrane for separation of molecules by means of the molecular sieving process [53]. This type of pore formation on the graphene surface can be executed either by the top-down or bottom-up process. Firstly, the graphene sheets are transferred to substrates (like metallic foils, transmission electron microscope grids, SiO2, GaN, Al2O3, etc.) and then by following different methods, such as electron beam irradiation, ultraviolet radiationebased oxidative etching, plasma treatment, ion beam bombardment, lithography, etc., the pores of desirable dimension have been created [53e59]. In a particular work, Surwade et al. have developed nanoporous graphene sheets by following the oxygen plasma etching method [58]. Afterwards, Bai et al. have prepared large-size PG membranes using the lithography method [60]. The top-down process of PG preparation is relatively more popular compared to the bottom-up. But in some of the cases, the pores thus formed are observed to be rough with disordered edges, which are not suitable for the molecular transport purpose. Thus, the bottom-up process has been expected to produce suitable membranes for the betterment of molecular transport. Safron et al. have developed PG on copper foil, employing the self-terminated CVD process using CH4 as the starting precursor and Al2O3 as the inert barrier (for termination of graphene growth) [61]. These PGs are broadly used for the purification of air/water, support for

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nanoparticle growth for the energy storage/conversion, DNA sequencing, molecular valve, isotope separation, etc. [19,62e66]. In some cases, the GO has been employed as the separation membrane that selectively separates the molecules by means of diffusion at the interface of the oxygen functional groups. The ease of preparation, low cost, and scalable production make the GO more reliable for scaling up, but its permeability has been observed to be very less compared to that of PGs. Therefore, the PG films are assumed to be the benchmarked material for membrane preparation, which meets the theoretical performance values. By using the first principle calculations, Dai’s group have proposed the possibility to use the PG as an efficient membrane for the selective separation of gases [63]. They have observed higher hydrogen permeance among the H2/CH4 mixture by the H-passivated PG in comparison to the N-passivated PG. Afterward, many theoretical predictions have been presented, but in 2012, Koenig et al. prepared PG by ultraviolet-assisted oxidative etching process and experimentally demonstrated the techniques to measure the transport rate of different gases [67]. It works, but the micrometer size measurement restricts its commercialization. Then in 2014, Celebi et al. increase the size of PG up to square of millimeter by physically perforating the double-layer graphene sheets, and they demonstrated the transport of gases [68]. Although some work on the synthesis and application of the nanoporous graphene has been carried out, still, there are challenges for the scalable production of PG membranes and their commercialization for air filtration applications.

4.2 DOPED GRAPHENE The doping of heteroatoms onto the graphene surface is one of the efficient processes to prepare functionalized graphene. Though the doped graphene can be synthesized by a variety of processes, there are two major categories such as in situ or ex situ doping process. In the former case, the insertion of foreign atoms (N, B, S, P, F, etc.) into the carbon skeleton of graphene can be executed by treating the GO/graphite with a suitable heteroatom containing precursors following different synthetic methods such as CVD, hydrothermal/solvothermal treatments, microwave reaction, ball milling process, etc. On the other hand, the molecules containing heteroatoms are treated with the as-prepared graphene/rGO by following arc discharge, wet chemical synthetic approaches, plasma treatments, etc., in cases of the ex situ process. In the CVD process, the insertion of heteroatoms into the graphitic carbon lattice is observed to be quite easy and can be controlled by tuning the temperature of reaction and flow rate of precursor gasses. Since the atomic size of N and B are closer to carbon, the doping of these heteroatoms is found to be relatively easier than others. Wu et al. have developed B-doped graphene (4.3 at% B-doped) on Cu foil substrate using the boric acid (B-precursor) and polystyrene (C-precursor) at 1000
C operating temperature [69], whereas Reddy et al. and Jin et al. have synthesized the pyrrolic and pyridinic N-doped graphene (NG) using acetonitrile and pyridine starting precursors [70,71]. In another work, B, Neco-doped graphene was prepared by Bepete et al. a with B/N ratio of 0.3e0.5, employing the boric acid and N2 gas as the precursors [72]. Ajayan’s group synthesized the N, Becodoped graphene with B:N ratio of w1 using methane (as carbon precursor) and ammonia borane (as both N- and B-precursors) as the starting precursors [73]. In some cases, the heteroatom-containing polymers are directly deposited onto the metal substrate catalysts to get the doped graphene [74]. A higher percentage of doped graphene can be obtained in this process, but it can be further improved using the gaseous precursors [75,76]. Likewise, the N and B heteroatoms and the S-doped graphene are also prepared by using the sulfur powder and thiophene, etc., following

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different high-temperature methods [77,78]. Similarly, by the CVD method, the iodine-doped graphene was developed, but it’s too difficult to prepare the fluorine-doped graphene due to the higher reactiveness and toxic nature of the fluorine-containing compounds [79]. On the other hand, the Cl doping can be easily achieved by employing the plasma treatment without generating any significant defects. Zhang et al. have doped 45.3% of Cl on the CVD graphene by tuning the plasma conditions [80]. The CVD is a convenient process for the development of doped graphene films, but it fails for the scalable production and is associated with certain limitations such as: (1) generation of hazardous gases during the synthesis, (2) high-cost instrumentation, and (3) higher operating temperature. On the other hand, the ball milling process is a suitable method for the scalable production of doped graphene in a cost-effective way. In this method of preparation, the cracking of the CeC bond of bulk graphite takes place, forming highly active carbon species at the edges of graphene sheets. These active carbon species, like the carbocations, carbanions, and carbon radicals, immediately react with the dopant molecules/atoms via the mechanochemistry method, forming doped graphene. Most importantly, this edge-selective functionalized graphene preserves the crystallinity and electronic properties of graphene [81]. In a particular work, Jeon’s group have prepared N-doping (N content of 14.84%) by ball milling the graphite for 48 h in N2 atmosphere [82]. In another work, the same group had prepared the edge-sulfurized graphene by the ball milling process using the graphite powder and sulfur (S8) [83]. In the wet chemical approach, the scalable production of doped graphene can be carried out, which is a cost-effective and less time-consuming process. It is the Wurtz-type reductive coupling process of the starting precursor molecules to form the doped graphene. It strictly avoids the use of transition metal catalysts (as used in CVD process), and the doping percentage of heteroatoms can be tuned by varying the precursor concentration [84,85]. The GO and rGO thus produced in this process are regarded as the O-doped graphene and can be produced in bulk amounts. These oxygen functionalities are termed as the active sites for the doping of other heteroatoms by thermal annealing with the precursors, like NH3, polypyrrole, melamine, cyanamide, etc. [86e89].

4.3 POLYMERIZATION From last five years, the polymeric membranes have have been the potential contributor towards the greenhouse gas separation. Afterward, the inorganic membranes were used due to their outstanding gas separation performances. But the high cost and tedious manufacturing process restricts their large-scale production, hindering the commercial application. Therefore, researchers focused on integrating these inorganic materials as the nanofillers into the polymeric materials. Generally, the materials such as zeolites, CNTs, metal organic frameworks (MOFs), GO, and graphene are used for this integration through either blending or surface-coating processes. Excellent properties such as high mechanical strength, flexibility, higher theoretical surface area (as discussed in the preceding sections) of graphene makes it a suitable candidate. Moreover, the presence of oxygen functionalities on the lateral and basal planes of GO increase the hydrophilicity, making them more convenient to disperse in many of the polar solvents, including water. Hence, the GO sheets become assembled with crosslinked polymers in organic/aqueous mediums, producing the 3-D GO/polymer composites and are easily converted to rGO/polymer following different reduction processes, whereas graphene is highly impermeable even toward H2 and He2 molecules (discussed vide supra). In order to mitigate this issue, many efforts have been exerted to develop PG/functionalized graphene-based materials prior to

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FIGURE 4.2 Schematic presentation of Pebax-PRG composite film fabrication. Presented with permission from Dong G, Hou J, Wang J, Zhang Y, Chen V, Liu J. Enhanced CO2/N2 separation by porous reduced graphene oxide/Pebax mixed matrix membranes. J Memb Sci 2016;520:860e8.

polymerization. In some cases, CNTs and MOFs are incorporated into the grapheneepolymer composite to improve the gas permeability and selectivity. Han et al. have developed a method to prepare 3-D porous gas adsorbent material using GO and polyethylenimine, and demonstrated the adsorption efficacy toward both the dye and pollutant gases (CO2) [90]. Liu’s group have prepared porous reduced graphene oxide (PRG)/polymer membranes (Pebax-PRG) by blending the porous rGO with the Pebax 1657 polymer (the whole synthetic process has been presented in Fig. 4.2) [91]. The as-developed polymer matrix shows a selective separation of CO2 over other nonpolar gases. By interfacial polymerization of polyoxypropylene contained porous graphene (PG) with trimesoyl chloride, the same group has prepared a PGethin film nanocomposite and demonstrated an enhanced selective permeation of CO2 from the CO2/N2 mixture [92]. Beyond this excellent progress in the membrane preparation, more work is still required to develop facile and cost-effective processes to fabricate these polymer/graphene composites in large scale for commercialization.

4.4 COMPOSITES Among different air pollutants, CO2 contributes significantly toward global warming. A major percentage of CO2 is emitted from vehicles and fossil fuelebased power plants. In view of this strategy, the US Department of Energy has directed research and development to target the exploitation of solid adsorbents for the removal of CO2 from air, which will avail fresh air for living beings [93]. Therefore, many of the solid adsorbents, such as MOFs, mesoporous silica, aminated materials, layered double hydroxides (LDHs), zeolites, activated carbons, doped graphene sheets, and TiO2 have been extensively studied [94e98]. Among others, the aminated compounds demonstrate better efficiency for the adsorption of CO2 selectively from the flue gases. Since the adsorption of the CO2, on the aminated surface is an exothermic reaction that generates a huge amount of heat during its operation, it leads to partial deterioration of its performance, and therefore, of a conductive support with the aminated adsorbent is necessarily required to dissipate this latent heat of reactions, restoring the adsorption performances.

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FIGURE 4.3 Schematic presentation of the synthesis of Al2O3/reduced graphene oxide (rGO) composite. ASB, aluminumtri-sec-butoxide; CTAB, Cetyl trimethyl ammonium bromide. Reproduced with permission from Bhowmik K, Chakravarty A, Bysakh S, De G. g-alumina nanorod/reduced graphene oxide as support for poly(ethylenimine) to capture carbon dioxide from flue gas. Energy Technol 2016;4:1409e19.

Keeping this in mind, De’s group have prepared a composite of mesoporous g-Al2O3 nanorods with rGO to support polyethylenimine following a simple wet chemical synthesis procedure (the detailed synthetic procedure has been presented schematically in Fig. 4.3) [93]. They have observed that the composite material adsorbs CO2 from the flue gas up to 200.6 mg g1 PEI, even at high-temperature (75e100
C) conditions. In another work, Wang’s group found similar observations of high-temperature CO2 adsorption by using a GO composite of MgeAleNO3 LDH nanosheets as the adsorbent [97]. Some of the adsorbents, like MOFs, show greater affinity for H2O molecules (present as humidity in the flue gas), showing decreased adsorption of CO2. Therefore, more effort has been expended to prepare the composites of MOF with silica, alumina, CNTs, graphene, functionalized graphene, GO, etc., to achieve better mechanical structures and durability [99e103]. In some of the cases, it has been observed that the oxygen functionalities of GO/rGO show better

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affinity to the metal center of the MOF, making the composite formation more feasible. This not only assists composite formation, but also the synergistic effect of GO/rGO substrates on the porosity and chemistry of MOF play a significant role in adsorption of small gaseous molecules like H2, NH3, CO2, etc. By using GO as the stabilizer, Bian et al. have prepared an MOF composite (Cu [BTC]2/GO) following the Pickering emulsion strategy [104]. When the composite was used as the adsorbent for the stimulated humid flue gasses, the exfoliated GO sheets adsorbed the H2O molecules, enhancing the CO2 adsorption of up to 3.3 mmol g1 within 60 min. Chowdhury et al. have also used the GO as supporting template and synthesized the TiO2/GO composite by following a single-step colloidal blending method. Because of higher specific surface area and cumulative pore volume, the TiO2/GO composite shows an enhanced CO2 adsorption (1.88 mmol g1) selectively over the N2 gas [105]. In another work, Huang et al. have prepared the rGO/Cu-BTC composite by a solvothermal process and demonstrated an efficient selective separation of CO2 from the CO2/CH4 mixture of gases compared to that of bare Cu-BTC material [106]. Since few decades, work on the physical adsorption has grown considerably on the sorption and storage of gases, while more focus has been made on proving the strong potentiality of graphene and its derivatives to achieve enhanced gas sorption and storage. Though much more theoretical prediction and experimental work on graphene-based membranes has been done, there are challenges to develop efficient graphene-based hybrid materials for selective separation of gases from the gaseous mixtures. Moreover, the graphene aerogels attract significant attention as the adsorbent for the hazardous gases. Instead of the traditional adsorbents, the heteroatom-doped graphene aerogel shows enhanced performance by increasing the porosity of the adsorbent, specific surface area for adsorption, and most importantly, the active sites of adsorption by introducing the heteroatoms promoted to adsorb the acidic gasses like CO2, SO2, etc.

5. AIR FILTRATION BY FUNCTIONALIZED GRAPHENE Fig. 4.4 represents the illustrative overview for the possible applications of graphene-based materials towards adsorption, separation, and storage of toxic gas pollutants (H2, CO2, NH3, NO2, SO2, H2S, and CH4). Aerogels are porous materials with contrasting physicochemical properties, such as high surface area, low density, high porosity, and tunable surface chemistry. Among many of their high-performance applications, the aerogels have drawn significant attention as an adsorption media for the removal of several environmental and human health-threatening pollutants [107]. They have also been employed as a high-performance adsorbent air filter to improve the indoor environment. Furthermore, Xiong et al. have proposed titanium dioxideedoped graphene aerogel (Fig. 4.5) as an effective filtration material, and they investigated its performance toward cleaning the indoor environments by relying on the combined effect of both the adsorption and photocatalytic behavior [3]. The air purification performance of this filter was proposed on the basis of adsorption and accumulation of pollutants on the aerogel surface, where the pollutants can undergo a photodegradation process by TiO2. It is to be noted that by changing the reaction vessel, the shape of hydrogel can be easily changed. Generally, the hydrothermal process is associated with the volume shrinkage; for that, only larger-size containers are taken, compared to the expected product size.

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FIGURE 4.4 Figure showing the use of functionalized graphene for storage, separation, and adsorption of different gases.

FIGURE 4.5 Functionalized graphene hydrogel: (A) in-process product, and (B) the final product. Reproduced with permission from Xiong X, Ji N, Song C, Liu Q. Preparation functionalized graphene aerogels as air cleaner filter. Procedia Eng. 2015;121(Suppl. C):957e60.

Sevanthi et al. have published a comparison report on graphene films and rGO aerogel as adsorbents and studied their physical properties and adsorption of CO2 from a gas stream [108]. It has been observed that the CO2 adsorption on the rGO aerogel is more compared to the graphene film. Long et al. reported the use of a high-surface area MoS2/graphene hybrid aerogel for the selective detection of NO2 at ultralow concentrations with fast response and recovery times [109]. The tunable surface chemistry and excellent textural properties make the aerogels suitable sorption mediums for

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purification of both the gaseous pollutants and particulate matters for indoor air purification. A report by Mishra et al. has shown that polymerization of graphene by polyaniline (PANI) yields a composite material that is capable of CO2 capture [110]. However, only high-pressure adsorption data were provided, and the recycling stability and selectivity of the material have not been reported. The activated rGO materials were found to be highly efficient for selective separation of CO2 from flue gas mixtures, especially under conditions relevant to post-combustion capture from power plants (where the coal or natural gas are used as fuel). It has also been mentioned that the thermal treatment at high temperatures can have a positive influence on the single-component CO2 adsorption characteristics of graphene sheets and should be explored further as an effective strategy in the design and development of graphene-based porous solid adsorbents for CO2 abatement [111]. Menzel et al. recently suggested the possibility of Joule heating as a method of regeneration for solid adsorbents like graphene [112]. Sevanthi et al. further investigated the CO2 adsorption capacity of pristine graphene films and rGO aerogels for an inlet CO2 concentration of 5% [108], and they also evaluated the ability to desorb CO2 from these materials using electric currentestimulated Joule heating/electric swing adsorption (ESA). Studies show that the presence of GO, even at moderate concentrations (below 20 wt.%), increases the thermal stability of layered double oxides (LDOs) significantly [113e115]. As a consequence of the low GO loading, the volumetric capacity of these LDO/GO hybrids (i.e., their CO2 capacity per total volume) is remarkably higher than those obtained using other supports. The relatively low amount of GO required to improve the CO2 adsorption performance of LDOs is related to its obvious compatibility with the LDH platelets in terms of geometry and charge. Some studies have suggested that the interaction between the alkali metals and the aluminum oxide centers in the LDOs plays a key role in the formation of strong basic sites, which are more active for CO2 adsorption [116]. Meis et al. studied the influence that Na and K have on the adsorption capacity of LDOs supported on carbon nanofibers (CNFs) [117]. The authors prepared LDO/CNF composites with large CNF contents (around 90 wt.%), and the promoters were introduced either by leaving alkali residues in the washing step or were added by impregnation. An increase in the CO2 uptake was observed in all the promoted LDO/CNF samples, although the capacities were slightly higher for those with alkali retained from the synthesis. The presence of small amounts of sodium residues from the precipitation process (2 wt.%) deliberately left by minimum washing almost doubled the CO2 adsorption capacity of unsupported LDOs and LDO/GO hybrids containing 5 wt.% GO [113,115]. Iruretagoyena et al. showed the influence of GO on the CO2 adsorption and stability under thermal cycling of sodium, potassium, and cesium-impregnated LDO/GO hybrids [118]. Addition of sodium, potassium, or cesium carbonates by incipient wetness impregnation to unsupported LDH and to LDH/GO hybrids at loadings ca. 1.1 mol alkali per kg of adsorbent was found to increase the CO2 adsorption capacity of the calcined materials by more than 40%. Crucially, the enhanced stability of the LDO imparted by the addition of GO was not compromised by the inclusion of the impregnated alkali promoters. The impregnated cations modified the distribution of basic strength in a similar manner regardless of the type of promoter, but the densities of chemisorption sites and the total CO2 adsorption capacities at 573K depended on the alkali metal added. Potassium showed a higher promoting effect than sodium because of its stronger Lewis basic character. The addition of cesium, which is an even stronger base, resulted in lower surface areas and working capacities due to its tendency to form bulkier agglomerates. The enhancement in CO2 adsorption capacity achieved by alkali promotion is slightly higher for the unsupported LDO compared to the corresponding LDO/GO hybrid, suggesting that the impregnated

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alkali carbonates do not interact with the LDO as efficiently in the hybrids as in the unsupported materials, possibly due to interactions between the promoters and the oxygen groups of the GO. Kemp et al. reported the synthesis of porous uniformly NG material through chemical activation of an rGO/PANI composite [119]. This NG material showed a high specific surface area of 1336 m2 g1 and a maximum CO2 storage capacity of 2.7 mmol g1 at 298K and 1 atm (5.8 mmol g1 at 273K and 1 atm). The improved activity due to the N-doping can be ascribed to the acidebase interactions between N containing basic functional groups and acidic CO2 gas [120]. However, it has been found that the total nitrogen (N) species content cannot account for the greatly increased CO2 capture by assuming that each N atom can anchor one CO2 molecule, as suggested by Xing et al. [121]. They further claimed that nitrogen introduction could facilitate the hydrogen bonding interactions between the carbon surface and CO2 molecules. Beyond that, some fragmentary discussions related to N-doping effects in literature still need in-depth evidences or explanation. This ambiguity is due primarily to the fact that most N-doping methods in previous reports not only modify the surface chemistry of the carbon surface but also change the pore structures, which make it difficult to eliminate the influence of pores and exclusively determine the effect of N-doping [122]. Fei Sun et al. documented a series of N-doped porous carbons (NPC) with gradient N content [123]. Consistent pore structure was thereafter prepared by a molecular self-assembly method, which provides an ideal platform to independently determine the N-doping effect on CO2 adsorption from an experimental aspect. Structural characterization and CO2 adsorption tests clearly demonstrate that CO2 uptake is independent of the specific surface area, and the attractive CO2 uptake properties of the NPC samples are closely associated with the N-doping levels. The optimized NPC sample possesses both high CO2 uptake of up to 4.82 mmol g1 at 0
C under 1 atm and high selectivity over N2 at 25
C under 1 atm. Xia and coworkers reported an enhanced adsorption of CO2 on S-doped carbon obtained within the structure of zeolite EMC-2 [124]. The high capacity of chemically activated rGO/polythiophene composite was linked to the oxidized S content and the presence of pores similar in size to CO2 molecules [125]. M. Kwiatkowski et al. proposed that acidebase interactions of CO2 in small pores with sulfur incorporated into aromatic rings of the pore walls and polar interactions of CO2 with sulfoxides, sulfones, and sulfonic acids along with hydrogen bonding of CO2 with acidic groups on the surface, contribute to the enhanced CO2 adsorption [126]. Huang et al. studied the activity of GO at airewater, liquideliquid, liquidesolid interfaces and found that GO can act like a molecular amphiphile as well as a colloidal surfactant [127]. In fact, the large liquideliquid interface of the Pickering emulsion stabilized by the nanoparticles would provide a great space for the in situ interfacial growth of new type nanoparticles to produce novel composites. The significance of the Pickering emulsion method for the fabrication of GO-based composites is thus gathering more attention. The hybrid materials show an enhanced performance compared to individual components. Ding et al. prepared grapheneeMn3O4 hybrid porous material by a hydrothermal method and found the carbon dioxide adsorption amount to be up to 11.4 wt.%, which is comparable with the values obtained in the case of other carbon materials [128]. A high CO2 capture capacity of Fe3O4egraphene nanocomposite at high pressures and temperatures was reported by Mishra et al. [129]. The figure below (Fig. 4.6) presents the CO2 adsorption isotherms of GO, pure TiO2, TiO2/GO-0.10, TiO2/GO-0.20, and TiO2/GO-0.30. In general, the CO2 adsorption capacity of the TiO2/GO nanocomposite systems was noticeably higher than that of both bare GO and TiO2 over the whole pressure range. TiO2/GO exhibited a CO2 uptake capacity of 1.88 mmol g1 at 25
C, a low enthalpy of adsorption, and a high

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FIGURE 4.6 The CO2 adsorption isotherms for GO, TiO2, TiO2/GO-0.10, TiO2/GO-0.20 and TiO2/GO-0.30 at 0
C (up) and CO2 adsorption isotherms at 25 and 50
C for TiO2/GO-0.10 (below). Reproduced with permission from Mishra AK, Ramaprabhu S. Enhanced CO2 capture in Fe3O4-graphene nanocomposite by physicochemical adsorption. J Appl Phys 2014;116:64306.

CO2/N2 selectivity [105]. The amount of CO2 adsorbed on TiO2/GO-0.10 decreased with increasing temperature, which was due to the increase in the thermal energy of CO2 molecules at elevated temperatures, leading to lesser adsorption. The CO2 adsorption capacity of TiO2/GO was noticeably higher than that of GO or TiO2, which could be due to the synergistic effect arising out of the hybridization of TiO2 nanoparticles with GO nanosheets. The adsorption kinetic data from the Avrami model suggest that CO2 can be adsorbed physically or chemically on TiO2/GO. Physical adsorption occurred through the formation of intermolecular electrostatic interactions. On the other hand, the electron transfer between the adsorbent and adsorbate leads to the formation of chemical bonds, causing adhesion of adsorbate molecules. CO2 is an amphoteric molecule in which the C is acidic, and O is weakly basic in nature [130]. During the adsorption of CO2, the binding mode could involve one/two O atoms (mono-/ bidentate) and one/two Ti centers (Fig. 4.7). On the other hand, as an acidic species, CO2 could bind to the O functionalities from the surface of GO (OeC coordinated, Fig. 4.7).

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FIGURE 4.7 Different chemisorption modes of CO2 onto TiO2/GO-0.10 (Labels 1 M or 2 M refer to the number of metal atoms involved in the adsorption) Reproduced with permission from Markovits A, Fahmi A, Minot C. A theoretical study of CO2 adsorption on TiO2. J Mol Struct 1996; 371(Suppl. C):219e35.

Two-dimensional honeycomb lattice membranes, including graphene, silicene, and germanene have also been used to purify hydrogen. The tiny pore sizes of pristine graphene and silicene have been proven to be impermeable to H2, and defects, which enlarge the pore size, have to be introduced in order to promote their permeability [131]. Theoretical calculations predicted that the pore size of 2-D Sn is significantly larger than those found in the case of graphene, silicene, and germanene, and can be further enlarged by decorating with F, which could make it a promising membrane for H2 separation [132]. In addition, application of strain is another method to achieve larger pore size. The method provides a practical strategy to tune the permeability. Gao et al. demonstrated that 2-D Sn-based membranes with densely packed uniform pores are effective for H2 purification [133]. The 2-D Sn-based materials can be further engineered by a moderate strain to achieve the best desired H2 permeability. In comparison to the conventional activated carbons, the functionalized graphene shows promising results as adsorbent for both low-temperature physisorption and high-temperature chemisorption of H2, CO2, CH4, etc. [134]. The remarkable synergistic properties of GO hybrid structures showed promising room-temperature H2 storage and high-pressure CH4 and CO2 adsorption capacities. For the given rich chemistry and functionalizability of graphene, the number of new compounds is continuously being tailor-made. Thus, if carefully tuned, the microporosity and the high surface area of the chemically modified graphene (CMG) materials could be very promising for H2 and CH4 storage and CO2 capture. Similarly, the gas permeation separation on graphene and GO-based membranes is highly dependent on GO sheet size and thickness, layer structure and assembly, residual lamellar water content, and deformation of thin-layered structures during dewetting, etc. There is much scope in tuning and controlling the parameters to develop practically useful membranes. Also, the rGO composites (GrO@Cu-BTC) shows significantly higher CO2 adsorption capacity than those of the parent Cu-BTC as well as some conventional adsorbents, whereas the CH4 adsorption capacity was nearly unchanged [106]. The CO2 adsorption capacity was up to 8.19 mmol g1 at 1 bar and 273K. The dual-site LangmuirFreundlich model was applied favorably to fit the experimental isotherm data of CO2 and CH4 on both GrO@Cu-BTC and Cu-BTC. Ideal adsorbed solution theory was used to predict the binary mixture isotherms and CO2/CH4 adsorption selectivity of the samples on the basis of the pure-component isotherms. The predicted isotherms of the binary mixture showed that CO2 was more favorably adsorbed than CH4 on GrO@ Cu-BTC. Thus, the adsorption selectivity of the composite, GrO@Cu-BTC for CO2/CH4 was

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significantly higher than that of Cu-BTC. At 1 bar, the CO2/CH4 adsorption selectivity of GrO@ Cu-BTC reached a value of 14, which was almost twice that of Cu-BTC for an equimolar mixture of CO2 and CH4. The thermal desorption spectroscopy results showed that desorption activation energies of CO2 on GrO@Cu-BTC and Cu-BTC were 68.60 and 56.71 kJ mol1, respectively, suggesting that the interaction between the GrO@Cu-BTC surface and CO2 molecules became stronger compared to that between Cu-BTC and CO2. The application of GrO and Cu-BTC to synthesize composites is an effective way of obtaining novel materials for CO2/CH4 separation. Jiang et al. have studied the selectivity of PG for the H2/CH4 mixture [63]. In another work, Hauser et al. studied the capability of functionalized graphene nanopores to efficiently separate methane from air [7]. By employing the classical molecular dynamics, Nieszporek et al. demonstrated the ability of PG to separate various alkane mixtures [135].

6. SUMMARY AND PERSPECTIVE Breathing is one of the most vital parts of human survival. More than 90% of the world’s population breathes in air that violates air quality guidelines set by the World Health Organization, increasing the risk of lung cancer and respiratory infections, with serious secondary issues including stroke, cardiovascular disease, and chronic obstructive pulmonary disease. Growing population and subsequent advancements in the transportation and communication sectors have raised many serious concerns such as global warming, pollution, health hazards, etc. However, we do not put too much emphasis into the quality of the air that fills our lungs (more often than anything else), and our poor air qualityemonitoring habits are to be blamed for this. For sustainable growth and to curb the menace of health hazards due to putrefied air standards, urgent and effective countermeasures should be implemented. There are ways to treat the substandard air quality, specifically indoor air quality, as indoor air pollution is among the leading environmental risks to living creatures. One way is to eliminate/minimize the sources that cause air pollution and to flush the indoors with clean outdoor air. There are several basic steps that can be followed, such as: (1) prohibiting indoor smoking, (2) avoiding the use of carpets (a major reason behind the accumulation of dust mites and other allergens), (3) putting stress on thorough ventilation for the entire dwelling place, and (4) implementation of proper exhaust/chimney systems to put away most of the pollutants arising out of cooking processes. There are, however, limitations because the outdoor air quality is affected by the weather conditions and undesirable levels of contaminants. Another way is to implement an effective removal system by means of mechanical/electronic/ gas-phase air cleaning. Also, there are devices having the necessary technologies that can act directly on the pollutants and destroy them. Ultraviolet germicidal irradiation, photocatalytic oxidation, and ozone generators are some of the examples. It is to be noted that ozone at higher concentrations can have an undesired irritation effect on the lungs, and at recommended levels, it is ineffective, so cleaning the air by ozone generation should not be encouraged. Air-conditioning systems may create a reservoir for allergy triggers if they are not well maintained, and one cannot go for a cheaper alternative, as it will be completely ineffective. Therefore, synthesis of robust air filters with long-term stability through a cost-effective method is desired. Many of the recent research efforts are being focused on the development of technologies for

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the purification of air. Though graphene-based materials are broadly studied as the efficient membrane for air/water purification, still, now its bulk production for industrial trial is on the way. Also, more functional polymers are required to be integrated with graphene to further expand its selectivity for gas adsorption. The synthesis of graphene-based metal oxide composites and doped graphene are yet in the initial stage; therefore, more work on the synthesis of large-scale graphene-based membranes in a cost-effective way is highly needed.

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