Progress in graphene-based materials as superior media for sensing, sorption, and separation of gaseous pollutants

Coordination Chemistry Reviews 368 (2018) 93–114

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Coordination Chemistry Reviews journal homepage: www.elsevier.com/locate/ccr

Review

Progress in graphene-based materials as superior media for sensing, sorption, and separation of gaseous pollutants Pallabi Samaddar a, Youn-Suk Son b, Daniel C.W. Tsang c, Ki-Hyun Kim a,⇑, Sandeep Kumar d,⇑ a

Department of Civil & Environmental Engineering, Hanyang University, 222 Wangsimni-Ro, Seoul 04763, Republic of Korea Department of Environmental Engineering, Pukyong National University, 45 Yongso-ro, Busan 48513, Republic of Korea c Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China d Department of Bio and Nano Technology, Guru Jambheshwar University of Science and Technology, Hisar, Haryana 125001, India b

a r t i c l e

i n f o

Article history: Received 4 March 2018 Accepted 15 April 2018

Keywords: Graphene oxide composites Nanomaterials Metal-organic framework Gaseous pollutants Adsorption Separation Sensing

a b s t r a c t Rapid population growth accompanied by industrialization and urbanization has led to a noticeable degradation of air quality. There is a strong need to appraise novel materials for the treatment of diverse pollutants in the atmosphere. Among them, graphene oxide (GO) is envisaged as one of the most promising layered materials with expansive applicability in numerous fields, especially in the pollutant removal processes, due to many uniquely advantageous features (e.g., tunable physical properties, excellent thermal stability, electrical conductivity, exceptionally high surface area, and pore volume). In the last decade, researchers have also put much efforts to produce graphene-based composites through the fabrication of graphene or GO with a variety of materials such as carbon nanotubes, metallic nanoparticles, metal-organic framework, and polymers. Because of the advanced features (e.g., strong mechanical and anti-corrosive properties, stimuli responsive property, and high porosity) of graphene-based composites, they have been used preferably for air quality management (AQM) through adsorption, separation, and sensing of gaseous pollutants. In this comprehensive review, we offer a contemporary state of the art discussion on using graphene-based composites for AQM purposes. The nature of the interactions between composites and various types of gaseous pollutants has hence been reviewed in various respects. To expedite further research and development on this topic, we take into consideration the technical challenges with suggestions for the directions of relevant future research. Ó 2018 Elsevier B.V. All rights reserved.

Contents 1. 2.

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 Properties of graphene oxide. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 2.1. Structure and morphology of graphene oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 2.2. Thermal and mechanical properties of graphene oxide (GO) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 Description of various graphene oxide composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 3.1. Graphene oxide/polymer composites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 3.2. Graphene based metal oxide composites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 3.2.1. GO/SnO2 nanocomposites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 3.2.2. GO/Co3O4 nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 3.2.3. GO/ZnO nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 3.2.4. GO/Cu2O nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 3.2.5. GO/WO3 nanocomposites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 3.3. Graphene/MOF composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 Role of graphene-based composites in sensing gaseous pollutants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 4.1. Sensing mechanism of VOCs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 4.2. Sensing mechanism of H2S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

⇑ Corresponding authors. E-mail addresses: [email protected] (K.-H. Kim), [email protected] (S. Kumar). https://doi.org/10.1016/j.ccr.2018.04.013 0010-8545/Ó 2018 Elsevier B.V. All rights reserved.

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4.3. Sensing mechanism of NO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Performance comparison of graphene-based sensors with other sensor materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Adsorption and separation of pollutant gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Adsorption mechanism of VOCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Adsorption mechanism of NH3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Adsorption mechanism of H2S and sulfur gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4. Adsorption mechanism of CO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and future prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction The continuous advancement of industry and agriculture has enhanced production and emission of potentially toxic and combustible gases into the atmosphere. A variety of gaseous compounds including oxides of nitrogen and sulfur, volatile organic compounds (VOCs), and polycylic aromatic hydrocarbons (PAHs) are often identified as highly detrimental to pose human health risks. These gases can be formed by natural origins (like ultraviolet photochemical processes and microbiological processes) as well as various man-made sources (e.g., vehicles, furnaces, stoves, and electric power plants) [1,2]. Thus, for the purpose of AQM, the development of highly sensitive, selective, and lucrative materials for sorbents/catalysts and sensors is in great demand to effectively remove those pollutants and to accurately monitor their behavior, respectively. Current research efforts strived to understand the potential utility of novel materials in the field of AQM. For instance, physisorption in advanced, tunable porous media is regarded as one of the most reliable approaches for the separation and removal of gaseous pollutants including VOCs. Consequently, many attempts have been made to synthesize an extensive variety of modified porous materials (e.g., polymers, metal-organic frameworks (MOFs), zeolites, and covalent organic frameworks) for removing, sensing, or separating gaseous pollutants [3–8]. Among the aforementioned materials, graphene and graphene oxide (GO) have been considered as a proficient matrix for sorption and sensing of gaseous pollutants due to their advanced properties (e.g., facile synthesis method, high surface area, robust pore structure, light-weight, high chemical stability, and high thermal stability) [9,10]. Moreover, the findings of various advantageous properties (e.g., surface moieties, high water dispensability, and hydrophilicity) support the potent role of GO as an excellent contender in many other fields of applications (e.g., fabrication of a super capacitor and other nano materials) [11–13]. Graphite is the main ingredient for the production of graphene, GO, GO nanosheets, reduced GO (rGO), and other GO composites (e.g., GO/polymer composite and GO/MOF composites). On a comparatively large scale, various graphene-based materials can be prepared from graphite precursors through oxidation, exfoliation, and reduction [14–16]. As such preparation steps can yield many defective sites on the graphene, the resulting product is advantageous for sorption and sensing of gases and additional functions. Graphene can thus be used as an effective platform for engineering a wide variety of functions and for chemical modifications due to its largely expanded and tunable layer structure. The electrostatic interaction of GOs with the adsorbate also make them materials of interest for the sorption of various charged species [17]. Appropriate incorporation or combination of GO with other materials like polymers [18,19], nano particles [20], carbon nanotubes [21], and MOFs [22] gave rise to hybrid composites that combine advantageous properties of respective building blocks for extensive expansion of their application fields. For example,

103 104 105 105 106 108 108 112 112 112

nanoporous graphene-polyoxometalate (GPOM) hybrid structures were synthesized by in-situ hydrazine hydrate reduction of GO with the use of phosphomolybdic acid as a cross-linker. As such, the polynuclear metal-oxo structured polyoxometalate provided a versatile building block cluster for the construction of functionalized hybrid materials. The ordered pore of GPOM exhibited a comparatively large specific surface area (e.g., 680 m2/g) that is
30 to
85 times greater than those of GO (23 m2/g) and POM (8 m2/g), respectively [23]. In addition, the MOF (HKUST-1)-GO composites showed enhanced porosity relative to the parent material (e.g., with the addition of GO up to 20% by weight). Indeed, when high amounts (e.g., more than 20%) of GO are dosed, the number of functional groups on the distorted graphene layers exceeds the active sites present in MOF with which they can interact. Accordingly, after drying, such excess layers of GO were seen to restack. Thus, restacked GO layers remained as agglomerates which led to further reduction in porosity [24]. Numerous review articles on graphene-based materials had been reported to describe their potent role in optical [25], electronic [16], electrochemical [26], and photocatalytic applications [27]. However, because of a paucity of information on their interactions with various gaseous pollutants, there is a strong demand to explore the kinetics and mechanisms behind the adsorption/ sensing of GO and GO-based materials against various toxic gases or vapors including NO2, Cl2 toxic vapors, benzene, ammonia, sulfur volatiles, and so forth [28–34]. In sorptive removal of gaseous pollutants, various factors (like breakthrough volume, analyte concentration, and sorbent structure) need to be investigated thoroughly. Likewise, selectivity of sensor material is crucially important for practical application. A comprehensive study is thus desirable to investigate whether a graphene-based material could satisfy various criteria established for AQM applications. To address such issues, we conducted a critical survey of the experimental findings on GO-based materials for sorption, sensing, and separation of pollutant gases, with emphasis on volatile organic compounds and a brief discussion of their health risks. Then, we address their practical applicability for future research in AQM fields.

2. Properties of graphene oxide GO is an oxidized structure of graphene exhibiting a high density of oxygen-containing functional moieties including hydroxyl, carbonyl, carboxyl, and epoxy in its lattice. GO can be synthesized economically by simple chemical oxidation (e.g., from graphite to graphene oxide) followed by exfoliation. Generally, Hummer’s method (or modified Hummer’s method) based on a combination of H2SO4 and KMnO4 has been used for the preparation of graphene oxide. In Hummer’s process, the reaction of KMnO4 with H2SO4 produces a dimanganese heptoxide ion, which acts as an active species. As the bimetallic heptoxide is much more active than monometallic tetraoxide, the former is discharged when heated

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to a temperature greater than 55 °C. As diamanganese heptoxide has the ability to oxidize unsaturated aliphatic double bonds selectively over aromatic double bonds, it can induce the oxidation of graphite into GO [35]. The production of GO thus involves the generation of diamanganese heptoxide as follows:

KMO4 þ 3H2 SO4 ! K þ þ MnOþ3 þ H3 Oþ þ 3HSO4 MnOþ3 þ MnO4 ! Mn2 O7 Also, GO nanoribbon has been prepared from multiwalled carbon nanotubes (CNT) using KMnO4 and H2SO4 [36]. also It has been revealed that the addition of an H3PO4 reaction generated GO nanoribbons with more compact graphitic basal planes [36]. It has also been postulated that oxidation of graphite with KMnO4 and a 1:9 mixture of H3PO4/H2SO4, may generate GO with fewer defects in the basal plane in comparison with GO produced via Hummer’s method [37]. The hydrophilic character of oxidized graphite permits water molecules to be adsorbed into the lamellar structure so that the interlayer distance increases to 1.15 nm [38]. The oxidation of graphene to GO considerably alters various physicochemical properties (e.g., morphology, mechanical, thermal). These exceptional properties have elicited extensive efforts to use GO and GObased materials in various fields of technology from manufacturing of electronics to biomedical devices [31,39,40]. In environmental applications, they have been employed to develop photocatalytic materials or novel sorbents for environmental remediation [41,42], advanced membranes for water treatment [43], and electrode materials for contaminant monitoring or removal by electrocatalytic sensing. Consequently, we must discuss distinctive properties of GO relevant to environmental applications to assess the possibilities offered by this novel carbon nanomaterial. 2.1. Structure and morphology of graphene oxide Oxygen functionalities introduce a hydrophilic character into a GO structure to help form stable suspensions in aqueous media. This hydrophilic nature with the combination of high surface area and density of functional moieties allows for a wide variety of chemical modifications on GO sheets. The surface functionality weakens the platelet–platelet interactions because of its hydrophilicity. GO is exfoliated into monolayers or few-layered stacks. Oxidation of the graphite structure actually enhances the interlayer space (e.g., 0.34–0.65 nm), which causes separation of the graphene layers [44]. Oxidation of graphite leads to the transformation of the crystalline structure into a new laminar form. The X-ray diffraction patterns of graphite and GO confirmed this finding [44]. According to cross polarization-magic angle spinning (CP-MAS) experiments, the presence of three broad resonance peaks at 60, 70, and 130 ppm was confirmed in the 13C NMR spectrograph of GO. Short-contact-time spectra displayed signals at all three concentrations to be assigned as tertiary alcohols, epoxy (1,2ether) groups, and a mixture of alkenes All carbons in GO were found to be quaternary by Mermoux’s model [45,35]. The results further revealed the presence of inter-platelet H-bonding between the hydroxy and epoxide functional groups, which contributes extensively to the compact structure of GO. Note that the crystallite size of pristine graphite is fairly small (average: 15.8 nm) and decreases with increasing oxidation time because the oxidation ruptures the crystallites [35]. A few research groups have reported that graphite oxide can undergo complete exfoliation in an aqueous solution under suitable conditions and generate colloidal suspensions of almost entire individual GO nanosheets with a mean lateral dimension of approximately 1 lm [46]. Such a nanosheet can be chemically functionalized,

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deoxygenated, and dispersed in polymer matrices to generate novel composites [47]. The reduction of GO can be performed using chemical reducing agents, thermal treatment [48], photoreduction [49], or microwave-assisted reduction [50]. Reduction of GO can eliminate a large fraction of its oxygen moieties, with the O:C ratio increasing from 1:2 to 1:246. Reduction also altered the chemical structure of GO with carbon vacancies, residual oxygen content, and clustered pentagon and heptagon carbon structures [51]. 2.2. Thermal and mechanical properties of graphene oxide (GO) Efficient thermal transport in graphene has long been of great interest to scientists because graphene has been preferably adopted in a myriad of applications, especially thermal management of electronics [52,53]. A single layer of graphene exhibits high thermal conductivity. In contrast, thermal conductivity decreased significantly in the presence of one or a few additional layers in a GO structure, as recognized in simulations based on molecular and lattice dynamics [54]. Experimental evidence for such diminution in thermal conductivity with an increasing number of graphene layers has been reported [55]. The thermal conductivity varied from
2,800 to
1300 W m1 K1 at room temperature when the number of atomic planes in few layered graphene increased from 2 to 4. This phenomenon was also investigated with molecular and lattice dynamics calculations [56,57]. It has been showed that raising the bonding strength among neighboring layers led to a reduction of the in-plane thermal conductivity in multilayer graphene [56]. The restraints from the neighboring layer were suspected to play a crucial role in hindering phonon transport throughout the in-plane direction in multilayer graphene. Hence, this observation again supports that thermal conductivity of single-layer graphene would decrease as it is bonded to a substrate. Moreover, thermal conductivity is also influenced sensitively according to the interlayer spacing. The effects of these two variables exert significant roles in controlling the thermal conductivity of a multilayer graphene platelet. Mahanta and Abramson reported thermal conductivities for GO nanoplatelets and reduced graphene that were exfoliated to varying degrees [58]. Results showed that the thermal conductivity measured for reduced graphene platelets with 30–45 layers approached the value of bulk graphite. Thermal conductivity of GO is 2275 ± 338 W/m-K when carbon content and interlayer space are
99% and 0.3372 nm, respectively. Graphene nanoplatelets with three layers of oxygen intercalation and 7% oxygen content showed thermal higher conductivity compared to bulk graphite with similar interlayer spacing. This improvement in thermal conductivity can be explained by the intercalation of oxygen atoms, which imparted a covalent character to the interlayer interactions and led to higher frequency phonon modes to offer enhanced thermal conductivity [58]. The proper understanding of GO’s mechanical characteristics is crucial to expanding its application fields. Because of intriguing mechanical properties (e.g., high Young’s modulus (E) of 1.0 TPa and an intrinsic strength (sc) of 130 ± 10 GPa), two-dimensional (2D) graphene has been acknowledged as a promising candidate for nanoscale devices [59,60]. These measurements indicate that graphene is one of the strongest materials; it was further revealed that atomically ideal materials can be tested up to deformation [59]. However, the mechanical properties of GO sheets varied sensitively with synthesis method [61]. The reported Young’s modulus (E) and intrinsic strength (sc) values for GO sheets exhibited wide ranges of 6–42 GPa and 76–293 MPa, respectively. In addition, the mechanical properties of GO sheets can be modified by the introduction of polymer or heteroatom composition [62]. Another interesting finding is that the thickness of the GO sheet decreases to a few layers as its Young’s modulus value increases radically to

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about 200 GPa. Monolayer GO possessed a particularly higher value of Young’s modulus than that of a multilayered GO sheet [63]. A comprehensive study has been carried out to assess the effects of the mechanical properties on the surface coverage and the arrangement of the functional groups in ordered and amorphous GO [64]. These authors have strengthened their assumption on the basis of density functional theory (DFT) calculations. Accordingly, both amorphous and ordered GOs exhibited excellent mechanical properties of pristine graphene as the values of Young’s modulus and intrinsic strength decreased reasonably, with an increase in the coverage due to the disturbance by sp3 carbons [64]. It can be concluded that the mechanical properties of GOs largely depend on such factors as surface coverage and type of arrangement (either amorphous or ordered). The ratios of the functional moieties only exert minute effects on the mechanical properties [64]. 3. Description of various graphene oxide composites Various oxygen-containing groups on GO (including alcohols, epoxides, carboxyl, ketones, and lactol moieties) can crucially affect the van der Waals interactions between the graphene layers to further introduce hydrophilicity in GO [65]. Due to this hydrophilic nature, interlamellar water molecules are always present in interlayer void space even under prolonged drying conditions [66]. In principle, GO can be readily prepared from low-priced graphite materials on a huge scale. The usage of GO-based hybrid multifunctional materials should be much more profitable than that of other expensive nanomaterials such as functionalized carbon nanotubes (CNTs). GO has also attracted considerable interest as a building block for novel applications in composite fabrication [20,67–70].

GO-based composite materials (e.g., GO/iron oxide, GO/iron acetate, GO/bentonite, GO/aluminum polycation) have been studied as novel adsorbents for ammonia and nitrogen oxides [30,71– 73]. GO has also been employed as a fortification agent for the preparation of composites with various polymers to remove different types of pollutants from effluents. In addition, GO-based semiconductor nano-composites can be applied as photo-catalysts for the degradation of pollutants [74]. Additionally, MOF/GO composite materials can offer a novel platform for the investigation of gas adsorption. The combination of atomically dense GO phase and porous MOFs results in an improved dispersive interaction with analyte gas molecules. Recently, the improved adsorption properties of MOF-5/GO and HKUST-1/GO composites were demonstrated against several odorant gases including NH3 and H2S [75]. Here, we discuss different prospects of GO-based composites and their chemistry with regard to several applications. An overall scheme for graphene-based composites and their application in polluted gas sensing, adsorption, and separation is presented in Fig. 1. 3.1. Graphene oxide/polymer composites Different polymers combined with GO have drawn great attention due to their enhanced advantageous properties in terms of band gap, tensile strength, elasticity, conductivity, and more. GO/ polymer composites have been regarded as a ‘‘radical alternative to conventional filled polymers,” particularly in the fields of electronics. However, because of their hydrophilic nature, GO sheets can only be dispersed in aqueous media, which are not attuned with most organic polymers [76]. GO structures possess strong interlayer H-bonding between the oxygen moieties of adjacent layers in GO. Organic solvents cannot penetrate those interlayer

Fig. 1. A schematic diagram of different graphene-based composite materials and their application.

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spaces, which inhibits their exfoliation. If the density of the functional groups responsible for H-bonds is decreased through chemical functionalization, the GO structure becomes less hydrophilic and should allow for exfoliation in organic solvents. A synthetic route of chemically modified GO through the treatment of aryl and alkyl isocyanates has been proposed [77]. The isocyanate treatment led to reduction in the hydrophilic nature of GO sheets through the formation of carbamate esters and amide bonds to the hydroxyl and carboxyl groups of GO, respectively [77]. This isocyanate-derivatized GO no longer exfoliated in water, but formed stable dispersions in polar aprotic solvents such as N,Ndimethylformamide (DMF). This type of dispersion allows GO sheets to be thoroughly mixed with various organic polymers such as polyaniline to facilitate the preparation of GO/polymer composites can be facilitated. To introduce electrical conductivity in GO/ polymer nanocomposites, the phenyl isocyanate-treated GO sheets with polystyrene subjected to chemical reduction [78]. This composite facilitated the dispersion of individual GO sheets through the polymer matrix [78]. Furthermore, application of GO/polymer composite can be found extensively in the field of gas sensing. For such application, its performance (e.g., selectivity, sensitivity, reversibility, energy consumption) should be evaluated thoroughly. Most of the traditional chemiresistive gas sensors based on semiconducting oxides are operated at comparatively high temperatures in the range of 200–600 °C. High temperatures require high power consumption and involve to safety issues. To overcome this kind of drawbacks, extensive work has been devoted for the fabrication of gas sensors that can be worked at room temperature. A simple and effective self-assembly technique has been developed recently for the fabrication of reduced GO/polymer composite nano fiber (rGO/P NFs) [79]. This GO/polymer composite was utilized for NO2 sensing. The above mentioned composite showed a high sensitivity for NO2 even at very low concentration of 1.03 ppm at room temperature with detection limit of 150 ppb. Conjugated polymers such as polyaniline, polypyrrole, and polythiophene have been comprehensively studied for the fabrication of gas sensors for attaining several features like fast response times, high sensitivities, and room temperature operation [80–82]. Interestingly, a successful synthesis of GO/polyaniline and GO/ polyaniline/ZnO nanocomposites has been achieved by nanoemulsion method for ammonia gas sensing application [83]. They revealed that polyaniline coated ZnO particles were also seen to reside on GO nanosheets most of which were covered by polyaniline nanofibers, as confirmed by SEM and EDS analysis [83]. GO/ polypyrene composite films were fabricated by electrochemical co-deposition method for the sensing of VOCs like toluene. Basically pure electrosynthesized polypyrene is powdery and consisted of sphere-shaped particles in aggregated state. However, the addition of GO makes the PPr/GO composite compact and more integrated. As the GO content increased, highly porous composite film was generated. GO sheet in general consisted of compact graphitic regions interposed with sp3-hybridized carbons [84]. It can be assumed that pyrene can non-covalently interact with the large aromatic regions of the GO sheets through the p–p stacking. Such interactions help generate stable dispersion of GO sheets. During the fabrication of composites or polymerization, GO behaves as a ‘templates’ so that pyrene was favorably grown on GO surfaces. Similarly, the GO/pyrene nanocomposite was co-deposited at the working electrode [85]. 3.2. Graphene based metal oxide composites In recent years, scientists put much efforts to combine GO with metal oxides for the improvement of gas sensing performance. This kind of hybrid nanostructures not only exhibited the properties of

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the nanoparticles and of GO individually but also had added synergistic effects that are enviable for gas sensing, especially in obtaining good response at room temperature, high selectivity, and sensitivity to targeted gas molecules [86]. As a means to improve the conductivity, GO integrated with metal oxides should be reduced by chemical reduction or thermal reduction methods. However, the gas adsorption capacity will decrease if all the oxygen-containing functional groups are reduced, which are mainly responsible to adsorb gas on GO surface. Therefore, a balance between the adsorption capacity for gases and the electronic conductivity of rGO is an important factor to consider during the reduction process. Although GO/metal oxides nanocomposites have been established to be an effective method to produce high performance NO2 sensors, these sensors still require high temperature to perform the sensing experiment. It remains a challenge for the preparation of graphene-based NO2 sensors with efficient sensing ability at low operating temperature. 3.2.1. GO/SnO2 nanocomposites SnO2 is an n-type semiconductor extensively utilized in gas sensing purposes. The working principle of graphene/rGO-based gas sensors is based on the charge/electron transfer between the adsorbed gaseous analytes and the graphene/rGO sheet. In this regard, Zhang et al. have been successfully synthesized SnO2 nanoparticles-reduced GO (SnO2/rGO) nanocomposites via hydrothermal route [87]. The formation of SnO2 nanoparticles can be explained on the basis of the Sn4+ hydrolysis into Sn(OH)4, and subsequent nano crystal formation of SnO2. The presence of numerous functional groups (e.g., carboxyl, hydroxyl, and epoxy) facilitated the SnO2 NPs attachment on the surface of rGO [87,88]. In addition, another research group prepared GO/SnO2 hybrids via simple wet chemical method, wherein the SnO2 particle size was maintained in the range of 4–5 nm [89]. This nano hybrid structure exhibited enhanced response to ethanol and benzene in comparison with traditional SnO2 nano particles [89]. A simple and cost-effective hydrothermal and lyophilization method has been established to synthesize three-dimensional SnO2/rGO composites for NO2 gas sensing [33]. Two different composites have been prepared from different tin salts of Sn2+ and Sn4+ which exhibited different sensing potential for NO2. Fig. 2 describes a scheme of the formation of SnO2/rGO nanocomposites from different tin salt precursors. Standard reduction potential of Sn4+/Sn2+ is 0.15 V which is relatively low. Due to this low standard reduction potential, Sn2+-containing precursor undergoes redox reaction with GO solution to be oxidized into Sn4+, which implies that the acidic GO exhibits oxidizing ability. On the other hand, Sn4+containing precursor was hydrolyzed first and then converted into [Sn(H2O)6x(OH)x](4x)+ which reacted with various functional groups (like carboxyl, hydroxyl, and epoxy) that are present on the surfaces and edges of GO sheets [33]. 3.2.2. GO/Co3O4 nanocomposites Co3O4 is a compound of CoO and Co2O3 with a higher oxygen content, which displays p-type semiconducting properties [90]. Chen et al. exploited an NO2-gas sensor based on Co3O4intercalated rGO [91]. The rGO/Co3O4 hybrid also exhibited a much higher response at ambient condition compared to virgin rGO. The Co3O4 NPs attached on the surface of the single layer rGO effectively prevented the rGO layers from sticking. The Co NPs acted as nanopillars, ensuing in an extra macroporous structure between the each rGO layers to further facilitate the gas diffusion. The oxygen-reduction ability of graphene matrix can be enhanced by coupling effect between Co and graphene, as Co can efficiently make ionic-bond with oxygen. Cobalt tetraoxide intercalated rGO has been found as a potential catalyst material for electrochemical oxygen reduction and anode material in lithium battery [92,93].

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Fig. 2. Preparation of SnO2/rGO nanocomposites from different tin salt precursors [33].

Consequently, the Co3+ centers may serve as the extra adsorption sites for NO2. Therefore, electrons would be indirectly withdrawn from graphene structure via oxygen bridging, which may cause decrease in the resistance with exposure to NO2 for the sensing application [91]. 3.2.3. GO/ZnO nanocomposites ZnO is another appealing candidate that can be combined with GO for the nanocomposite fabrication. ZnO exhibits high electrochemical stability as well as good resistivity towards gas pollutants [94,95]. Current studies are attempting to improve the performance of ZnO-based sensors to facilitate the selectivity and to lower the operating temperature. ZnO is a n-type semiconductor, whereas rGO generally shows p-type semiconductor characteristics. Fu et al. recently demonstrated an easy and steadfast way of fabricating the GO/ZnO coated microfiber interferometer (MFI) for ammonia gas sensing. Ammonia donated electrons after adsorption on composite surface, which led to the increased effective refractive index of GO/ZnO [96]. The change in refractive index can be realized by monitoring the wavelength shift. This composite sensor exhibited a high sensitivity and selectivity towards ammonia at room temperature. One research group have grown largescale ZnO-nanorod arrays on the graphene sheets [97]. The graphene/ZnO-nanorod composite exhibited high selectivity and quick response to ethanol vapors. Further its response to H2S gas was also three times higher than that of the pristine ZnO nanorods. The well-distributed ZnO-nanorod on graphene surface offered many active centers and electron pathways which helped expedite gas diffusion, adsorption, and mass transport. The second reason was that ZnO nanorods became more active when electrons flowed from graphene to nanorods. Thirdly, charge-carrier mobility was only possible due to graphene which made conduction paths from the junction to the electrodes [97]. 3.2.4. GO/Cu2O nanocomposites A variety of Cu2O nano structures including nanocages, nanocubes, nanowires, polyhedrons, and hollow spheres had been achieved by numerous methods in last few years. Complex 3D structure like multipod Cu2O and nanowire polyhedra have been reported [98,99]. The quasi-2D structure of GO with copious oxygen functional groups exhibited dual molecule-colloid properties which educed multivalent interaction with Cu2+ as well as polymer additive. This kind of interaction can be used to induce particle mediated crystallization on GO surface. Besides, under the hydrothermal growth condition the reduced form of GO has a tendency to conjugate with semiconductor superstructures [100]. Consequently, this conjugation can be applied to enhance the physiochemical properties of the composites for numerous

applications [92,101]. Copper oxides with the oxidation state of either +1 or +2 show enhanced sensitivity to H2S because they can chemically adsorb H2S at ambient temperatures. Zhou et al. for instance, synthesized a Cu2O-based graphene sensor composite material in which the
3 nm Cu2O nanocrystals were uniformly and compactly grown on functionalized graphene sheets. This nanocomposite exhibited excellent sensitivity, even when it came into contact with only 5 ppb of H2S at room temperature. The enhanced sensitivity was achieved by the synergistic effect of Cu2O, graphene, and the surfactant-free capped Cu2O nanocrystals, which can efficiently adsorb gas molecules on their surface [102]. rGO/Cu2O nanowire mesocrystals has been prepared by via the one-pot hydrothermal process using copper acetate in the presence of anisidine [103]. This rGO-conjugated mesocrystal composite has been appraised as an excellent NO2 sensor. The interdendritic space inside the mesocrystals and increased electronic conductivities contributed by rGO are assumed to improve the sensitivity of the composite toward NO2 [103]. This study opened new avenues for mesocrystal-based nanodevices for various environmental sensing applications.

3.2.5. GO/WO3 nanocomposites Nano-sized WO3 is an excellent candidate for both chemical sensors and visible-light photocatalysts. Pure WO3 nanomaterials are generally not competent photocatalysts due to their high electron–hole recombination rate and the difficulty of oxygen reduction. Owing to the high activation energy of the reaction with gas molecules, the slow response time and high working temperature limit their gas-sensing efficiency. However, through the incorporation into graphene or a GO matrix, the composite can significantly improve the sensing performance. Qin et al. for example, demonstrated the improved electrical conductivity of graphenewrapped WO3 nanoparticles with an enhanced gas-sensing response toward alcohol [104]. For the detection of alcohol, graphene-wrapped WO3 exhibits good linearity in the concentration range of 100–2000 ppm. Another research group reported a method of synthesis in which WO3 nanorods were integrated with graphene through a single-step hydrothermal process [105]. This graphene-based gas sensor showed advanced sensitivity and good selectivity toward NO2. The dynamic response resistance of this composite is
400 for 100 ppm NO2. They also established that the response of their synthesized composite toward NO2 was higher (up to 25 times) than for the pristine WO3 nanorods. To explain the sensitivity of the graphene-WO3 nanorod at 60, 70, and 130 ppm-based sensing system, they plotted the resistance ratio of NO2 gas and air (Rgas/Rair) vs. the concentration of NO2. The Rgas/Rair values (or sensitivity) of graphene-WO3 nanorods

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when measured against varying concentration of NO2 (i.e., 25 ppb, 100 ppb, 500 ppb, and 1000 ppb) were 13, 25, 40, and 61, respectively. In contrast, in the case of WO3 nanorods, the respective values were determined as 2.6, 3.3, 5, and 5.3. A lower resistance ratio for graphene-WO3 nanorods indicated a high sensing capacity relative to the pristine nanorods. 3.3. Graphene/MOF composites Metal–organic frameworks (MOFs) can be recognized as coordination networks, exemplifying a highly advanced class of crystalline/amorphous materials synthesized by combining metal salts and organic ligands [106]. Numerous MOFs are well known for the enhanced adsorption capacities of various gas molecules to exhibit great potential for the separation of gas molecules from gaseous mixtures [107]. Scientists have investigated intensively the structural modification of MOFs to enhance their selective gas adsorption and separation ability [108–110]. In a few earlier studies, composites comprised of graphene-based materials and MOFs exhibited an improved adsorptive potentiality with the removal of small hazardous gaseous molecules including NH3, NO2, and H2S [111–113]. Excellent porosity and functional moieties of such composites are the reason behind these notable improvements [114]. For instance, a water stable GO/MOF-5 composite has been prepared for effective adsorption of gaseous ammonia at ambient conditions [115]. This composite material exhibited higher dispersive forces than did MOF-5 and led to the considerable enhancement of ammonia adsorption capacity [116]. The composite with 46% GO content showed an adsorption capacity of 70 and 120 mg g1 for dry and moist NH3, respectively. Such improvement can be explained by the incorporation of GO into the MOF structure. This actually improves the physical adsorption forces in composites. Physical forces in pristine MOFs might not be sufficient to hold small gaseous molecules like ammonia. The same research group extended their ammonia gas sorption study on the surface of another composite prepared by combining copper-based MOF, HKUST-1, and graphite oxide [115]. The adsorption capacity for the HKUST/GO composite with 18% GO content showed
5 mg g1 at
10 kPa. It can thus be inferred that MOF-5/GO composite is a better NH3 absorber than HKUST/GO. A similar type of composite has been prepared by combining Cu-MOF and graphene material and investigated its sensing response toward ammonia gas [117]. The authors showed how the porous structure and surface of the composite material affect the electrical responsiveness during sensing experiments. In general, Cu-MOF or HKUST-1 is a weak conductor of electricity due to the weak overlap between the p and d orbitals in metal (Cu2+) ions and the insulating behavior of the organic ligands. However, GO-coated microchips exhibited a resistance of 37X. Accordingly, GO has the ability to deliver a significant electrical signal during a sensing experiment. Incorporation of GO into metal organic frameworks favors dispersive forces by maintaining the specific interactions between metallic sites and gaseous ammonia. Furthermore, it is expected to increase the materials’ conductivity when GO bonds with MOF, which is of the utmost importance for gassensing experiments. Kumar et al. reported the growth of 2D MOF sheets on graphene surfaces for scrutinizing CO2 uptake [118]. To obtain such a composite, this research group used GO and benzoic acid-functionalized graphene prepared from reduced GO through the diazonium salt grafting process. They established that functional groups on both GO and benzoic acidfunctionalized graphene can potentially stabilize the nanocrystals of 2D MOF on the surface graphene. Two-step CO2 uptake by that composite has also been characterized through microscopic and spectroscopic methods. A unique composite based on GO and Crbased MOF MIL-101 has also been proposed to examine adsorption

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of acetone molecules[119]. On the basis of the SEM and BET results, it was confirmed that the crystal size of the MIL-101 in composite was smaller than that of the pristine MIL-101 [119]. Such reduction in pore size can help retain acetone molecules more effectively in a composite network. Additionally, another novel composite material has been synthesized by combining graphene oxide and copper-based MOF. Well-dispersed Cu-BTC (BTC = benzenetricarboxylic acid) nanocrystal on the GO layer significantly improved the porosity of the composite, which was successfully implemented to adsorb CO2 molecules [120,121]. Physisorption is an ostensive mechanism governing the gas sorption on the surface of MOF and/or GO/MOF composites. However, it does not exclusively regulate the entire adsorption phenomena or the quantitative measurements of sorption capacity. Regardless of the finding that the introduction of new pores with strong dispersive forces noticeably increases the uptake of gaseous molecules, there are no consistent correlations between the surface area (and/or porosity) and the accumulative capacities of the MOFs and composites. Materials like GO/MIL-100(Fe) and MIL100(Fe) possess a highly porous structure compared to GO/HKUST and HKUST-1; however, their NH3 uptake capacities are smaller. Interestingly, reactive sorption or chemisorption on Cu-based MOFs can be observed by the naked eye. For instance, in the case of NH3 H2S, and NO2 changes in color from dark to light blue are observed upon adsorption on GO/HKUST, graphene/ HKUST, and pristine HKUST-1. This color change can be accredited to the ability of NH3, H2S, and NO2 to coordinate with the metallic centers of MOF [122]. The color has been changed again due to the formation of complexes by the reaction of H2S or NH3 with HKUST-1. The color change to dark blue with dark tint indicates the formation of CuS upon H2S exposure. Such (color changing) sorption phenomena of gases (e.g., NH3, H2S, and NO2) on the Cu-based MOF and composites are presented in Fig. 3. During the color changing phase, MOF weakens the CuAO coordination with metal centers. Consequently, organic ligands promote the formation of new complexes like Cu(NH3)4+, Cu(NO3)2, and CuS upon reaction with NH3, NO2, and H2S, respectively [123]. 4. Role of graphene-based composites in sensing gaseous pollutants Over the past few years, graphene/metal oxide composite materials have been extensively exploited for the enhancement of their sensing performance as gas sensors. The synergistic effect between metal oxides and graphene in composite materials can be explained by considering a few factors. Specifically, graphene can govern the morphology and size of nano metal oxides during the composite synthesis process. Graphene helps increase the conductivity of metal oxides with the rapid transfer of electrons acquired from the gaseous analyte and metal oxides to electrodes [86]. When p-type graphene combined with n-type metal oxides to form p-n type junction, it facilitated the modification of the spacecharged layers in composite materials. Most important, nano metal oxides can inhibit the aggregation of graphene layers in composite material [124–126]. Table 1 presents a variety of graphene-based composites applied for sensing of several gaseous analytes (e.g., NO2, NH3, H2S, SO2, CO, CO2, VOCs) Table 2). 4.1. Sensing mechanism of VOCs In the detection of aromatic hydrocarbons, VOCs are of great importance in controlling chemical processes, ecological monitoring, and industrial applications [127,128]. Zhang et al, for instance, fabricated a GO/polypyrene composite film sensor to sense several VOCs [85]. The polypyrene layer in the composite is mainly respon-

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Fig. 3. Color changing sorption phenomena of NH3, NO2, and H2S on Cu-based MOF and composites [123].

sible for change in conductance of the hybrid film when it is exposed to toluene vapor. To assess the selective sensing performances of a GO/polypyrene composite, three different VOCs of chloroform (polar, non-aromatic), hexane (non-polar, nonaromatic), and toluene (polar, aromatic) have been used as analytes. For these three VOCs, the composite film type sensor exhibited the highest sensitivity of 9.87  104 ppm1 with good linearity and fast response. The observed sensitivity for toluene was 5 and 10 times greater than that of chloroform (2.04  104 ppm1) and hexane (0.77  104 ppm1), respectively [85]. The performance of this sensor was tested after being sealed into a chamber with an electrical lead. Then, the closed chamber was connected to an electrochemical workstation regulated by a computer. Accordingly, a low response was obtained for non-polar hexane relative to polar chloroform. In comparison with hexane, the composite sensor exhibited a higher response to toluene and chloroform, which possess aromatic p-electrons and a lone pair of electrons, respectively. This suggests that dipolar electrostatic forces are a crucial factor in explaining sensing performances. Molecular polarizability of the gaseous analyte is another important factor

because it can also govern the sensing performance of the composite sensor [129,130]. Therefore, sorption of polarizable VOCs should produce an adequate induced dipole moment to convert the conjugated polypyrene chains into a planar arrangement with more highly stretched configurations [131]. Subsequently, the conductivity of polypyrene has improved due to the increased electronic coupling among pyrene monomer units. Polypyrene rings are favorable to adsorb gaseous toluene through p-p interactions, which lead to a higher sensing response for toluene. Note that GO comprises several polar functional moieties including hydroxy, carboxyl, and epoxy, which can potentially capture H+ and create a positively charged environment under acidic pH
1. Accordingly, the extent of positive charge would increase upon increasing H+. However, at alkaline pH
11, OH undergoes reactions with epoxy and carboxyl moieties, followed by deprotonation. The deprotonation of carboxyl groups generates negatively charged GO layers. Thus, GO seems very stable at exceptionally higher acidic and alkaline environments to potentially sense VOCs. Remarkably, repulsive forces are mainly responsible for forming moderately porous layers in the charged GO structure. The proba-

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P. Samaddar et al. / Coordination Chemistry Reviews 368 (2018) 93–114 Table 1 Different graphene-based composites as pollutant gas sensor and their performances. Order

Name of the sensor

Gas analyte

[A] NO2 gas sensing 1 GO foam

NO2

2

Reduced graphene/SnO2

NO2

3

Graphene/WO3 nanorods

NO2

4

EtOH-based graphene nanomesh

NO2

5

3D reduced graphene oxide

NO2

6

Graphene/SnO2 nanocomposite

NO2

7

Reduced graphene/SnO2 nanocomposite

NO2

8

In2O3 cubes/ reduced graphene oxide composites

NO2

[B] H2S gas sensing 1 Hierarchical NiO cube / nitrogen-doped reduced graphene oxide 2 Reduced graphene/SnO2 nanocomposite 3

Graphene/Cu2O nanocrystal

[C] VOC and other gas sensing 1 GO/polypyrene composite film

         

Response time = 40 min Temperature = 25 °C Response (Ra/Rg)= 3.31 Response time = 135 s Temperature = 50 °C Recovery time = 200 s Response (Ra/Rg) = 202 at 20 ppm Response time =
200 s for 25 ppb,
300 s for 100 ppb,
400 s for 500 ppb,
600 s for 1 ppm,
850 s for 5 ppm,
1000 s for 20 ppm Dynamic response for NO2 =
6% Response time = 7 min for 10 ppm NO2 Operating temperature = 25 °C Response for NO2 = 1 at 10 ppm Response time for NO2 = 600 s Response for NH3=0.99 at 50 ppm Operating temperature = 22–38 °C Response = 24.7 for 1 ppm Minimum response time = 250 s Operating temperature = 150 °C

   

Response = 1.083 at 45 °C for 100 ppm NO2 Operating temperature = 22–70 °C Response = 37.81% at 5 ppm Operating temperature = 25 °C

 Response = 31.95 at 50 ppm H2S  Operating temperature = 92 °C

H2S

   

H2S

Toluene

Hexane 2

Reduced GO/ZnO nano sheets

Ethanol

3

Ni-doped SnO2 nanoparticle/graphene composite GO suspension

Acetone

Reduced graphene oxide/ ZnO nanowire

NH3

5

       

H2S

Chloroform

4

Sensing performance

CO

[D] Comparison with other sensor materials 1 MoSe2 NH3

2

SnS2

NH3

3

Co3O4

Ethanol

4

Zn(II)-MOF

2,4,6-TNT

Response = 33 for 50 ppm Minimum response time = 2 s Operating temperature = 22 °C Response time = 7 min for 5 ppb, 10 min for 10 ppb, 15 min for 25 ppb, 20 min for 35 ppb,
22 min for 50 ppb,
26 for 100 ppb  Optimal operating temperature = 25 °C

           

Response = 1.4 nA (at
24 ppm) Minimum response time =
400 s Operating temperature = 25 °C Response = 1.6nA (at 400 ppm) Operating temperature = 25 °C Response = 1.4nA (at 200 ppm) Operating temperature = 25 °C Response =
28 at 50 ppm Response time = 300 s at 50 ppm Operating temperature = 260 °C Response time =
5.4 s Optimal operating temperature = 350 °C

     

Response time = 20–40 min Pressure = 0.1 mbar Temperature = 25 °C Response = 19.2% at 50 ppm Response time = 50 s Recovery time = 250 s

          

Response (DR/R0) = 50 Response time = 25 min Temperature = 25 °C Response (Ra/Rg) = 1.26 Response time = 25 min Temperature = 200 °C Response (V) = 3.25 Response time = 25 min Temperature = 300 °C Quenching efficiency = 90% Fluorescence peak at 420 nm upon excitation at 305 nm

LOD

Sensing technique

References

1.56–100 ppm

Conductivity

[184]

2–5 ppm

Electrochemical

[87]

25 ppb–20 ppm

Electrochemical

[105]

1–10 ppm

Electrochemical

[185]

0.2–10 ppm for NO2 20–400 ppm for NH3

Electrochemical

[186]

1–5 ppm

Electrochemical

[139]

2–100 ppm

Electrochemical

[33]

1–15 ppm

Electrochemical

[140]

0.1–100 ppm

Electrochemical

[136]

[187]

5–100 ppb

Conductivity

[102]

24–500 ppm

Conductivity

[85]

5–50 ppm

Electrochemical

[97]

2.5–200 ppm

Electrochemical

[34]

50–500 ppm

Electric Conductivity

[188]

50 ppb- 5000 ppm

Electrochemical

[144]

100–500 ppm

Electrochemical

[142]

0.5–100 ppm

Electrochemical

[143]

10–100 ppm

Electrochemical

[189]

–

Fluorescence

[190]

400–1000 ppm 200–900 ppm

(continued on next page)

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Table 1 (continued) Order

Name of the sensor

Gas analyte 2,4-DNT

3,4-DNT

5

ZnO nanocage

Acetone

Benzene

6

NiO nanosheet

NO2

7

ZnO/ZnCo2O4

Ethanol

8

Ta dopped In2O3 thin film

Ethanol

9

Cu dopped ZnO

H2S

Sensing performance  Quenching efficiency = 78%  Fluorescence peak at 420 nm upon excitation at 305 nm  Quenching efficiency = 60%  fluorescence peak at 420 nm upon excitation at 305 nm  Response (Ra/Rg  1) =
1 (at 50 ppb)  Response time = 107 s  Recovery time = 284 sTemperature = 300 °C  Response (Ra/Rg  1) =
0.2 (at 100 ppb)  Response time = 39 s  Recovery time = 118 s  Temperature = 300 °C  Response ((Gg  Go)/Go) =
0.2 (at 5 ppm)  Response time = 500 s  Temperature = 400 °C  Response (Ra/Rg)=
10 (at 25 ppm)  Response time = 200 s  Temperature = 150 °C  Response (Ra/Rg)=
6 (at10 ppm)  Response time = 1000 s  Temperature = 500 °C  Response ((Ig  Ia)/Ia) =
6 (at 1 ppm)  Response time = 50 s  Recovery time = 20 s  Temperature = 230 °C

LOD

Sensing technique

References

50–1000 ppb

Electrochemical

[191]

5–60 ppm

Conductivity

[192]

25–200 ppm

Electrochemical

[193]

10–90 ppm

Electrochemical

[194]

1–10 ppm

Conductivity

[195]

0.1–5 ppm

Table 2 Different graphene-based composites as pollutant gas sorbents and their performances. Order

Gas analyte

Adsorption performance

References

[A] CO2 sorption 1 MOF-5/GO composite

CO2

[32]

2 3

CO2 CO2

Henry Constant (KH) = 1.35 ± 0.18  105 mol/kg/Pa Capacity =
59.4 mg/g at 10,000 Pa  Adsorption capacity for CO2 is
0.4 mg g1 at 10,000 Pa and 298 K Adsorption capacity of GODC with 54% exfoliated GO =
0.0.1452 mg g1 10,000 Pa Adsorption capacity for CO2 is
58.9 mg g1 10,000 Pa Adsorption capacity = 0.06 mg g1 10,000 Pa

4 5

Name of the sorbent

Reduced GO/MIL-101 composite GO-derived carbons (GODCs) (1:9 wt ratio of exfoliated GO:KOH, activation at 800 °C) GO/Cd-PBM composite GO/MIL-101(Cr) (for 10% GO content)

[B] NH3 sorption 1 Ga-doped graphene 2 Hydroxyaluminum–zirconium polycation embedded GO 3

GO/HKUST composite (5% GO content)

[C] VOC and other gas sorption 1 MOF-5/GO composite 3 Reduced GO 4

Ga-doped graphene

5

GO-derived carbons (GODCs) (1:9 wt ratio of exfoliated GO:KOH, activation at 800 °C) GO GO/MIL-101(Cr) (for 10% GO content) GO/HKUST-1

6 7 8

CO2 CO2 NH3 NH3

NH3 at dry condition NH3 at moist condition

Adsorption energy = 1.421 eV  2.08 mmol g1 NH3 adsorbed at dry conditions  1.41 mmol g1 NH3 adsorbed at dry air conditions3.18 mmol g1 NH3 adsorbed at dry air conditions (GO was prehumidified before adsorption) Adsorption capacity = 128 mg g1

[196] [158] [197] [179] [150] [198]

[116]

1

Adsorption capacity = 200 mg g

CH4 C7H8 C6H6 H2O CO NO2 NO CH4

Henry Constant (KH) = 1.35 ± 0.41  104 mol kg1 Pa1 Adsorption capacity = 304.4 mg/g at ambient condition Adsorption capacity for benzene = 276.4 mg/g at ambient condition Adsorption energy = 0.881 eV Adsorption energy = 0.674 eV Adsorption energy = 1.928 Adsorption energy = 0.779 eV Adsorption capacity = 0.175 mg g1

[32] [199]

SO2 CH4 H2S

Adsorption capacity = 156 mg g1 Adsorption capacity = 16 mmol g1 at
30 bar Adsorption capacity =
200 mg g1 (5% GO content),
125 mg g1 (9% GO content),
110 mg g1 (18% content) at moist condition Adsorption capacity =
140 mg g1 (5% GO content),
120 mg g1 (9% GO content),
125 mg g1 (18% content)

[169] [179] [170]

NO2

ble mechanism for such high sensitivity at extreme conditions can be accredited to the intercalation of VOCs molecules into the relatively porous GO layers, which leads to swelling of the GO layers, followed by little sorption [132].

[150]

[158]

To establish and/or support this concept, GO has been prepared at four different pH levels, 1, 5, 7, and 11, with an approximate thickness of 30–35 mm. C. Nitromethane, ethanol, and dimethylamine were used for the sensing test. At pH 1 and 11, the

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thickness of GO layers increased by up to 70 and 77%, respectively. However, the thickness was enhanced to 17 and 10% at pH 5 and 7, respectively. The pH dependency of GO-based sensing is shown in Fig. 4a. The same findings have been reported for three aforementioned VOCs (nitro methane, ethanol, and dimethylamine) [132]. In addition, a polymer optical fiber sensor array has been fabricated by pristine GO and rGO to accomplish selective sensing of a number of VOCs like hydrazine, nitro methane, diethylamine, ethanol, methanol, acetone, THF, and dichloromethane [132]. These results of the sensing experiment indicated that GO and rGO showed no response to dichloromethane and THF, respectively (Fig. 4b). For the GO/rGO polymer optical fiber arrays, one optical fiber tip was crusted with GO, whereas another tip was crusted with rGO (GO/ rGO POF). The sensing performances of GO/rGO POF have been investigated against dichloromethane, THF, and ethanol. Fig. 4c shows that, in the case of dichloromethane, the optical response of the one-headed GO/rGO POF array was half that of the rGO array. Likewise, for THF, the optical response of the one-headed GO/rGO POF array was half of that of GO POF array. 4.2. Sensing mechanism of H2S H2S is a toxic, corrosive, malodorous gas. It was reported to cause bronchial constriction even at 2 ppm level [133]. Apart from its toxicity, H2S present in both the gaseous and liquid phases (converted to H2SO4) is tremendously corrosive to piping and production facilities [134]. In many manufacturing processes (e.g., fuel cells, ammonia production, and hydrogenation), its potential to damage metal catalyst was also identified [135]. For the protection of the environment, a selective, sensitive, and trustworthy H2S sensor is in great demand. To accomplish this purpose, recently a novel composite material has been fabricated by mixing a hierarchical NiO cube with nitrogen-doped rGO [136].

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These authors have explained the sensing mechanism by analyzing the product produced when H2S is exposed to the surface of the composite [136]. As H2S goes through the oxidation reaction, it transforms into SO2 during the sensing experiment. The sensing mechanism of that particular sensor is illustrated in Fig. 5. The oxygen content dropped from 57.7% to 48.6% when the composite came into contact with H2S, which implies that the oxygen actively participated in the redox reaction with H2S, and H2S immediately reacted with the adsorbed oxygen as it reached the surface of the sensor. At the same time, the electrons captured by oxygen were released to neutralize bulk holes. This incident resulted in a decrease in the concentration of the effective carrier that would ultimately be converted into an electrical response. Fig. 5 illustrates all the H2S sensing-related reactions. An innovative composite material has been introduced by growing stable Cu2O nanocrystals on functionalized graphene sheets [102]. They revealed that H2S can possibly be chemisorbed on the composite. Chemisorption took place when the frontier orbitals of H2S were mixed with the conduction band of Cu2O nanocrystals. This kind of mixing helps transfer electrons from H2S to Cu2O. The transferred electrons were rapidly extracted by the graphene sheet. Because graphene is a p-type semiconductor, the electron charge transfer can lead to a reduction of carrier density that can noticeably increase the resistance. 4.3. Sensing mechanism of NO2 The rapid expansion of industry accompanied by a growing number of vehicles have made NO2 as one of the most common air pollutants to play a prominent role in air pollution (e.g., through the formation of ozone and acid rain). Even at trace levels of NO2, human lung tissues can be harmed to suffer from respiratory diseases [137]. The U.S. Environmental Protection Agency (EPA) has

Fig. 4. a. pH-dependent sensing of GO against VOCs, b. sensing performances of only GO and only rGO polymer optical fiber array, c. sensing performances of only GO and only rGO polymer optical fiber array [132].

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Fig. 5. Sensing mechanism of H2S on the surface of NiO/N-rGO composite [136].

determined the ambient air quality standards for NO2 at a level of 53 ppb (average time: 1 year) [138]. Thus, the need for advanced fabrication methods has been stressed for the development of numerous sensing systems (or materials) for NO2. A few research groups fabricated rGO/SnO2 nanocomposites and applied them as effective sensing material for NO2 detection [33,87,139]. Analysis of the response suggests that incorporation of SnO2 should enhance sensor efficiency because the heterostructure of composite material can attract more electrons from the rGO toward SnO2. This also provides more active sites for gaseous analyte sorption. On the basis of thermodynamics, some factors governing the NO2-sensing performances were described [139]. Redox reaction is another crucial factor for explaining NO2 sensing performances. NO2 molecules react with the adsorbed oxygen species, and this reaction is stimulated at a relatively high temperature (150 °C) [139]. Another important factor is that the adsorption of NO2 will be considerably inhibited at higher temperatures and may be desorbed prior to the surface reaction. This may cause deprivation of NO2 sensing. Therefore, it is expected that temperature-dependent sensing would exhibit bell-shaped behavior. A sensor has been introduced by adding In2O3 cubes in rGO materials for sensing NO2 [140]. The sensing mechanism of this sensor can be explained on the basis of vacancies and defects available as charge carriers in a p-type reduced graphene oxide semiconductor [140]. In2O3 can be considered as an n-type semiconductor with free electrons as major charge carriers. The actual electronic interaction between In2O3 and rGO enables the detection of gas molecules through the alteration of resistance [140]. WO3 nanorod-embedded graphene composites have also been used for NO2 sensing purposes [105]. When oxidizing gas NO2 came into contact with the composite material, it adsorbed NO2 on the active sites. This adsorption helped to capture the electrons from WO3 nanorods, followed by thickness increment of the depletion layer, which in turn improved the electrical resistance. To accomplish NO2 sensing at low temperatures, SnO2 nanoparticles were conjugated with an electrolytically exfoliated graphene sheet [141]. According to sensing tests, a composite with 0.5 wt% graphene provided a high response against 5 ppm of NO2 with a

rapid response time (13 s) at 150 °C. Such p-type NO2-sensing of graphene can be traced using the band diagram, as shown in Fig. 6. Pristine graphene sheets generally possess semimetallic energy bands where conduction (Ec) and valence bands (Ev) meet at point K. Electrons then fill up Ev in order to push Fermi level (Ef) towards K-point at thermal equilibrium. O2 molecules are generally chemisorbed on the surface of graphene when it is in air at ambient temperature. Subsequently, O2 forms several oxygen species like O2 and O which can attract a few electrons from Ev. Such withdrawal of electrons induces positive holes (h+) or positive charge carriers so that graphene transform to a p-type conductor with Ef below the K-point. Afterwards as p type graphene comes into the contact with NO2, it again loses electrons from the valence band due the electron withdrawal effect of NO2. This fact increases the hole concentration while decreasing the resistance. 4.4. Performance comparison of graphene-based sensors with other sensor materials In Table 1, the sensing performance of GO-based sensor was compared with those developed with other materials. In case of NH3, rGO/ZnO is capable of sensing as low as 50 ppb whereas those built with MoSe2 and SnS2 can detect 100 ppm and 500 ppb, respectively [142–144]. As such, GO-based sensor was demonstrated to have superior sensitivity for ammonia. Most of the graphene-based sensors for NO2 have been found to work at relatively low temperature (25–70 °C) [87,184–186]. In contrast, graphene/SnO2 nano composite was seen to work at relatively high operating temperature like 150 °C [139] when compared to other graphene-based sensors mentioned in Table 1. However, NiO nanosheet has been found to work at very high temperature (like 400 °C) compared to all graphene-based sensors examined for comparison. In case of VOC vapor, the enhanced sensitivity of GO-based materials was also observed over the other sensor materials. GO/polypyrene composite films for instance exhibited high response for toluene vapor at 25 °C, whereas the operating temperature of ZnO nanocage sensor for benzene was considerably high (e.g., 400 °C). Graphene-based materials can also be operated to

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Fig. 6. Band models for NO2 sensing on pristine graphene sheets, pristine SnO2, and graphene/SnO2 composites [141].

sense H2S at room temperature [102,187]. On the contrary, Cudoped ZnO sensor was reported to operate at relatively high temperature like 230 °C [195].

In this section, the basic adsorption characteristics of those pollutant gases on the composite materials are described. 5.1. Adsorption mechanism of VOCs

5. Adsorption and separation of pollutant gases GO has already been proven to be very competent adsorbents with the interlayer space which is easily accessible for gaseous molecule or other adsorbates. This interlayer space generally varies between 6 and 12 Å, depending on the condition of hydration [145]. To improve adsorption performances of graphene materials, many researchers have been put efforts for the enlargement of surface area by chemical activation such as activation with KOH, NaOH, H3PO4 followed by heating [146–148]. Activation basically engenders a 3D structure consisting of intensely defected graphene sheets with high surface area which is essential for accelerating the sorption ability. By considering the fact, GO has been exploited as sources of composites with polymers or amines to bring together inside those interlayer spacing [149]. Composites of GO have also been made by treating with various inorganic materials (like hydroxyaluminium–zirconium polycation, gallium, birnessite manganese oxide, and dibutyltin oxide) to enhance the adsorption efficiency of pollutant gaseous molecules [30,150–152]. Nevertheless, still there is an ample possibility to synthesize a number of new graphene-based composites by combining with various materials in order to apply them as potential gas adsorbates. The uptake of some important pollutant gases (like NH3, CO2, CO, SO2, NO2, NO, and CH4) has been investigated using diverse composite materials.

Volatile organic compounds (VOCs: acetone, acetaldehyde, formaldehyde, benzene, toluene, and so forth) consist of various hydrocarbons with high vapor pressure (e.g., >10 Pa at 293 K). As a major class of air pollutants, VOCs can exist ubiquitously in both outdoor and indoor air due to their emissions from various source processes. A number of studies on hazardous effects of hydrocarbons confirmed that some cancers seem to be instigated by exposure to VOC. Several VOCs in combination with nitrogen oxides, in the exposure of sun rays, were identified to undertake photochemical oxidation to generate a deleterious photochemical smog [153– 155]. In this regard, the sensitive sensing as well as effective (sorptive or catalytic) removal of such harmful chemicals are of immense importance for environmental safety and for human health as well. Graphene-based composite materials have been exploited as superior adsorbents for the removal of toxic pollutants like VOCs from the natural environment [156]. It has been found that rGO is a better sorbent for aromatic hydrocarbons in comparison with GO. As rGO contains less oxygen, it exhibits higher hydrophobicity and larger surface area than GO. The adsorption behaviors of nitro containing hydrocarbons like nitrobenzene, m-dinitrobenzene, and p-nitrotoluene have been investigated on the three types of surface like graphene nanosheets, GO, and rGO [157]. Fig. 7 presents a schematic of the interaction of m-dinitrobenzene with GO, rGO,

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Fig. 7. Interaction of m-dinitrobenzene with A. GO, B. rGO, and C. graphene nanosheets [157]

and pristine graphene. Fig. 7A shows that ANO2 groups of mdinitrobenzene adsorbed on GO surface due to chelation with Ocontaining moieties (or edges) and the hydrogen of water molecules. On the contrary, Fig. 7C shows that ANO2 groups interacted with graphene surface with a larger OANAO bond angle. In case of rGO, it was found that the electron donor acceptor interaction with the AOH moieties is much stronger than GO (Fig. 7B). Electron-rich domains (due to AOH groups) in rGO can act as bonus sorption sites for nitro compounds [157]. It can be inferred that the adsorption mechanisms of rGOs are strongly governed by adsorptive sites and by the structure of the sorbate as a probe. Electron-rich benzene, for example, can interact intensely with positively polarized domains on rGOs through p-p electron donor acceptor interaction. The competitive adsorption behavior of benzene with two different aromatic hydrocarbons was investigated using rGO[156]. The order of their adsorption capacity was determined as naphthylamine > aniline > benzene. Consequently, adsorption energy obeys the reverse order of behavior. This trend indicates that the intramolecular interactions between these aromatics have a strong impact on their adsorption on rGO surfaces. To evaluate the contribution of hydrogen bonding to benzene sorption, the effect of pH was examined, but the removal efficiency of benzene remained unaffected with the change of pH. The interaction of benzene molecules with rGO can be realized through the ideal geometrical structures in the side and top view in Fig. 8 [156]. The fabrication of a number of GO-derived carbons (GODCs) with high surface area was reported along with their applications for methane and CO2 storage[158]. The best result has been found for KOH-activated GO with a KOH to GO ratio of 9:1 and a BET surface area of 1900 m2 g1. The adsorption capacity for CO2 and CH4 by the best GODCs was found to be 0.721 and 0.175 mg g1, respectively [158]. In another study, notable sorption capacity of GO/MIL-101 for acetone was observed (e.g., relative to parent MIL-101 material) [119]. From the isotherm plots, it has been found that the amount of acetone molecules adsorbed on the GO/MIL-101 was noticeable distinguished from that of the parent MIL-101 at fixed temperature (288 K). The amount of acetone adsorbed by the two was measured as
16.5 and
12 mmol g1, respectively (at 75 mbar). This improved sorption capacity of the GO composite could be

explained on the basis of BET surface area and increased dispersive forces. An increase in BET surface area was seen from the composite GO/MIL-101 (e.g., by 10.4%) relative to the parent MOF. Enhanced dispersive forces could be assured with the aid of acetone isotherm based on per unit surface area of the adsorbents. 5.2. Adsorption mechanism of NH3 Nitrogen-containing pollutant gases released by human activities are mostly the oxide form of nitrogen (NOx) and NH3. Among those gaseous pollutants, NH3 is emitted from diverse natural and/or anthropogenic sources including synthetic fertilizers, volatilization of animal waste, agricultural crops, forest fires, biomass burning, and emissions from human excreta [159–163]. The emitted NH3 takes part in the atmospheric cycling through which it is transported into the air and returns back to the surface by deposition processes. Fig. 9 describes the overall schematic of generation and emission of ammonia into the atmosphere. In this review, the adsorptive removal of NH3 by various graphenebased composites was evaluated in various respects [163]. Recently, the adsorption of NH3 on pristine graphene and Gadoped graphene was studied by deriving DFT theoretical calculations which suggested that NH3 behaved as a donor during adsorption on Ga-doped graphene because of the nucleophilic character of H atoms [150]. NH3 and H2O molecules share the same number of electrons and comparable structures. However, NH3 consists of a nitrogen atom with lower electron affinity than oxygen, which leads to an enhanced electron donation ability of the Ga-doped graphene. This can be a plausible reason for the higher adsorption energy of NH3 on Ga-doped graphene than on H2O molecules. In addition, the interlayer space in GO structures is sometimes not accessible for nitrogen with a molecular diameter of 3.2 Å, especially when considerable amounts of NH3 are retained on GO [145]. This can be explained on the basis of the interactions of NH3 with surface ACOOH groups and its intercalation between interlayer spaces. The intercalation can take place due to the small size of ammonia and its ability to for H bonds with surface hydroxyl or epoxy moieties. Adsorption of ammonia on graphene-based materials

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Fig. 8. Side and top views of the optimized geometrical structures for the interaction between benzene and rGO [156].

Fig. 9. Generation of ammonia from different sources and emission into the atmosphere [163].

can be stimulated by introducing surface acidic groups, metal salts, and/or solid acids [164,165]. For instance, Seredych and Bandosz (2008) prepared GO composites by incorporating aluminum–zirconium oxycations. They observed that the amount of ammonia adsorbed on the calcinated GO (modified with hydroxyaluminum polycations) increased two times more than that by the pristine GO. The deflagration of layers took place during calcination, which resulted in the decomposition of epoxy groups to form disordered mesoporous graphitic carbons. As an ammonia molecule can easily interact with oxygen groups of GO and Brønsted centers of inorganic pillars, it can be easily intercalated between the interlayer space. GO modified with aluminum–zirconium oxycations shows the best adsorption performance due to the presence of Keggin Al13-

like cations with diameters around 9.8 Å [30]. Keggin Al13 cations contain one tetrahedral Al bond with 12 Al in octahedral coordinations ([Al13O4(OH)24(H2O)12]7+). The value of the Keggin Al13 diameter indicates its presence as a so-called pillar between the GO flakes. The charged aluminum–zirconium oxycations are expected to be attracted by the negatively charged graphene layers. Because of its positive charge, it can interact with negatively charged carboxyl groups. Interaction of ammonia with the Brønsted acid center of alumina is presented in Fig. 10 [30]. Another MOF/GO composite like HKUST-1/GO has been employed for adsorptive removal of ammonia gases, which can be explained on the basis of chemisorption of ammonia on the composite surface [116]. GO and HKUST have been found to adsorb 45 and 118 mg g1 of ammonia, respectively at dry condition. It was interesting to find

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Fig. 10. Interactions of ammonia with Lewis acidic centers and Bronsted acid sites of alumina at dry and wet conditions [30].

that the amount of ammonia sorbed by their composite has been raised up to 128 mg g1. The sorbed amount has been further increased up to 149 mg g1 when GO content in their composites increased up to 18% [116]. Ammonia basically binds with the copper sites in HKUST-1 and then with the composite component. Such chemisorption can be seen by the naked eye as the composites change color during the breakthrough experiments. Enhanced ammonia adsorption has occurred at moist conditions relative to dry conditions owing to its solvation in a water film at the pores of the composite material. 5.3. Adsorption mechanism of H2S and sulfur gas An interesting theoretical work has been published recently on H2S sorption by nitrene radical (NH)-doped graphene materials [166]. The band gaps of such NH-doped graphene mainly depended on the concentration of NH radicals. The free electrons on the N atom in NH-doped graphene composite can act as active sites to bind H2S molecules. Nitrene doping also facilitated the charge transfer from H2S to an NH-doped graphene (GNH) composite and thus the partial conversion of H2S into the chemisorbed SH radical by H-abstraction. By increasing the concentration of NH radicals in the composite material, it was possible to achieve complete dissociation of H2S molecules. This indicates that more active sites can be accessible for the second H atomic transfer of the SH radical. The dissociation of the H2S molecule on a composite surface (GN2H2) is presented in the following equations.

H2 SðgÞ þ GN2 H2 ! H2 SðadsÞ þ GN2 H2 H2 SðadsÞ þ GN2 H2 ! SHðadsÞ þ GN2 H GN2 H3 þ SHðadsÞ ! GN2 H4 þ SðadsÞ where GN2H3 and GN2H4 signify the end products obtained after the first and second H atom transfers from the H2S molecule to the composite surface (GN2H2), respectively [166]. The structural changes during adsorption and dissociation of the H2S molecule are presented in Fig. 11. Other sulfur-containing gases (e.g., SO2, SO3) are recognized as perilous compounds mainly discharged from industrial burning processes including that of natural gas, coal, and oil. To date, various carbon-based materials (especially carbon fibers, activated car-

bon, and MOFs) have been exploited extensively for SO2 removal [167,168]. However, the feasibility of graphene or graphenebased composite materials has been little investigated with respect to the treatment of sulfur gas. Among a few available studies, fundamental adsorption features of GO against SO2 were explored by Babu et al. who found the adsorption capacity of GO against SO2 to be 156 mg g1 at ambient conditions, increasing to 257 mg g1 with an increase in pressure (2.6 bar) [169]. SO2 was physisorbed on a GO surface with a small isosteric heat of adsorption (DHads) at 18.04 kJ mol1, which was ascribed to the combined effects of the 2D layered structure and a lack of micropores [169]. Another interesting work on sorptive removal of H2S and NO2 has been conducted using the GO/HKUST-1 composite [170]. Chemisorption took place due to the presence of unsaturated copper sites on HKUST-1, which helped their coordination with guest molecules (e.g., H2S and NO2). Such coordination permits both guest molecules to react further with the MOFs to generate new complexes, as we discussed earlier (in Section 3.3). Although the general schemes of sorption seem similar for H2S and NO2, the surface properties governing these mechanisms are dissimilar. Differences in their sorption performances may be ascribable to differences in surface property of the newly formed complex. GO/HKUST-1 composite materials exhibited higher adsorption capacities against H2S than did NO2 in moist conditions [170]. Probable interaction routes of H2S and NO2 with the composite are presented schematically in Fig. 12. The gas molecules first bind with the copper sites, followed by the formation of CuS and Cu (NO3)2 complexes. The generation of the Cu(NO3)2 complex requires the reaction of two NO2 molecules with one copper center while inducing the release of NO. The admittance of guest molecules or gaseous adsorbate to the Cu-active sites can be expedited by the prominent dispersive forces that exist at the interface of GO and MOF. 5.4. Adsorption mechanism of CO2 It is well-known that CO2 constitutes the most dominant fraction in the global atmospheric burden of greenhouse gases (e.g., methane, oxides of nitrogen, fluorinated gases). Fig. 13 presents an overview of the percentage of greenhouse gases emitted from different sources as of 2015. To date, the potential of numerous solid adsorbents has been explored for the capture of CO2 emitted

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Fig. 11. Top and side views of the structures for the adsorption and dissociation of H2S molecule on nitrene radical-doped graphene composite [166].

Fig. 12. A schematic for the probable reaction of H2S and NO2 sorption on GO/HKUST composite [170].

from different sources. Among those sorbent materials, graphene has received considerable attention due to its wide range of enthralling characteristics. For instance, a unique GO-based composite have designed using click reaction with an azide–alkyne complex [171]. The reaction occurred between alkynyl-GO and azido-terpyridine complex ((azido-tpy)2Fe(II)), where alkyne-

modified GO sheets were fabricated as the building blocks [171]. The cross-linking nature was obtained through the coppercatalyzed click reaction. Fig. 14 describes the structure of the alkyne-GO and terpyridine complex along with related crosslinking reactions. Basically, the triazole ring in a terpyridine complex can act as a non-planar and rigid cross-linking spacer between

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Fig. 13. An overview of the emission of various greenhouse gases in the year 2015.

Fig. 14. Synthesis and the structure of graphene terpyridine complex [171].

the interlayer of GO sheets. The interlayer space between GO sheets increases through such cross-linkage so that the GO can acquire a larger surface area. The high surface area of this composite material is the main reason behind the high CO2 sorption capacity [171]. Adsorption of gaseous molecules onto a solid surface is predominantly directed by two factors, the porosity of sorbent material and its surface area. The introduction of nanopores into a graphene-based structure improves the sorption performance. Ultra-micropores (i.e., pores with less than 0.7 nm in width) are efficient for CO2 sorption at low pressures, whereas at high pressures, mesopores (i.e., pores with widths below 2.0–3.0 nm) are

preferable for improved adsorption [172,173]. Thermal exfoliation of GO sheets in a vacuum has also been carried out to generate graphene nanoplates with a broad pore size distribution. These exfoliated GO have been found to be appropriate candidates for separating CO2 from flue gases (NO2, SO2, etc.) at room temperature, but they require comparatively high pressure (30 bar) [174]. A simple method for the preparation of GO/ZIF-8 hybrid nanocomposites has been established in order to facilitate the improved uptake of CO2 [175], in which nanocrystals of ZIF were initially grown on GO sheets. In this process, their morphologies could be tuned by altering the GO content. It was observed that the uptake of CO2 by a 4% GO/ZIF-8 composite was 48%, whereas a 20% GO

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content exhibited 72% uptake. This high CO2 uptake can be endorsed by the accumulative effect of ZIF-8 nanocrystals and GO. A stepwise synthesis of the GO/ZIF-8 hybrid composite and the growth of ZIF-8 nanocrystals are shown in Fig. 15. Some shortcomings of the usage of MeOH were however found during synthesis. The poor dispersibility of GO in a methanol medium can lead to inhomogeneity in the reaction mixture [176]. Such precipitates can significantly hamper the textural properties of the resulting composite materials [176]. To overcome such drawbacks, these research group proposed an easy in situ synthesis process to fabricate GO/ZIF-8 composite materials in an ammoniacal medium. The resulting composites possessed high textural properties so that the size of ZIF-8 nanocrystals in hybrid composites could be controlled by adjusting the amount of GO in the synthesis medium. The CO2 uptake capacities of GO and pristine ZIF-8 were 6 and 37 cm3/g (at 0 °C and 1.0 bar), respectively, whereas in situ synthesized GO/ZIF-8 composites with 10 wt% GO content showed an improved CO2 uptake capacity of 49 cm3/g [176]. Cr-based MIL-101 has been established as the most porous MOF with high stability even under warm and humid conditions; it was further demonstrated to have great potential in separation and adsorption of gaseous molecules [177,178]. In this regard, one research group has recently reported the synthesis of GO/MIL-

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101 composites for such application toward methyl mercaptan (MeSH), CO2, and CH4 [179]. The analysis of sorption isotherms indicated that the composite material containing
5 wt% GO recorded the highest capacity and reversibility toward CO2 and methyl mercaptan sorption. The separation of MeSH and CO2 from CH4 has also been investigated [179]. Oxygen-containing functional moieties of GO were favorable to react with coordinatively unsaturated sites of MIL-101 when composite material contained 2.5–5 wt% GO. However, all sites were saturated with the oxygen-containing functional group of graphene when the composite contained 10 wt% GO. Thus, the composite with 5 wt% GO was the best material to yield 12, 15, and 30% increments of adsorption efficiency in comparison with pristine MIL-101 for CO2, CH4, and MeSH, respectively [179]. Interestingly, nanoparticles containing boron have been hypothetically recommended as potential sorbents for CO2. The reason behind such proposition is that the electron-deficient acidic boron atoms possess high affinity for the electron-rich oxygen present in CO2 [180]. Based on this idea, the features of CO2 sorption have been explored using chemically modified GO materials enclosing boron functional moieties [181]. These B-doped GO materials showed outstanding recycling properties and sorption capacity [181]. The amount of CO2 adsorbed by B-doped GO was 1.82

Fig. 15. Stepwise synthesis of GO/ZIF-8 nanocomposites from hexagonal ZIF-8 nanocrystals [175].

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mmol/g (at 25 °C, 1 atm), which was 35% greater than that of the pristine material (1.3 mmol/g). It is well known that the doping of nitrogen atoms into graphene structures increases the CO2 uptake capacity due to the acid-based interactions [182]. As nitrogen possesses an extra valence electron, the incorporation of nitrogen into the graphene structure should help to intensify the local p-electron density. The irregular distribution in the electron density due to nitrogen doping led to the improvement of the interactions between graphene sheets and hydrogen by enhancing the polarization of H2 molecules [183]. Various methods have been proposed to synthesize nitrogen-containing graphene materials, which include post-synthetic treatment with ammonia and synthesis of graphene materials using N2-contained precursors. Polyaniline/hydrogen-exfoliated graphene composite has also been reported as a potential CO2 sorbent. This nanocomposite exhibited CO2 uptake 75 mmol/g (at 11 bar and 25 °C), which is 3.5 times higher than untreated graphene material (21.6 mmol/g) [182]. 6. Conclusions and future prospects In this review, we present a succinct report on the contemporary progress of graphene-based nanocomposite materials in the field of pollutant control either through sensing or through sorption/separation applications. Accordingly, we demonstrated that graphene nanocomposite materials are competitive candidates for the fabrication of high-performance gas sensors and gas sorbents that can operate at relatively low or ambient temperatures. Hence, we comprehensively described the advantageous features of diverse GO-based composites (e.g., GO/polymer, GO/nano metal oxides, GO/MOF) through the compilation of numerous case studies. The mechanisms of sensing and adsorption by graphene-based materials regarding different pollutant gases (like NO2, NH3, H2S, and VOCs) have been elaborated on from various aspects. The performance of graphene-based sensors has been assessed in comparison with those developed with other sensor materials. From this comparative analysis, it can be realized that most of the graphene materials are capable of sensing diverse pollutant gases (e.g., NO2, H2S) at room temperature unlike other sensor materials. As such, it is much more favorable to develop graphene-based sensor materials for practical purposes. With the advent of graphene technology achieved over recent years, the sensitivity of graphene-based sensor materials has improved significantly. Nonetheless, their reliability, if assessed in terms of stability and selectivity, remains limited in real-life applications. As graphene-based sensors exhibit cross-responses to various reactive gases, this can further cause a reduction in their selectivity. To overcome such limitations, a number of strategies (e.g., doping of functional moieties, surface modification, optimizing experimental condition) may need to be implemented. In many sensing applications based on graphene, NO2 has been preferably investigated. Consequently, their feasibility has yet to be studied more thoroughly for other pollutant gases. Future researchers should enlist species to expand the potential applicability of graphene-based sensing techniques. Various fluorescent molecules can also be combined with graphene to acquire efficient sensing capacity for pollutant gases. Likewise, their sorptive properties on a variety of VOCs remain to be addressed, although more data are available for certain pollutants like NH3 and CO2. More attention should thus be given to the production of cost-effective and efficient graphene-based sorbents to expand their applicability in the industry. Acknowledgements This study was supported by a grant from the National Research Foundation of Korea (NRF) funded by the Ministry of Science ICT

and Future Planning (No. 2016R1E1A1A01940995). This work was also carried out with support of the ‘‘Cooperative Research Program for Agriculture Science and Technology Development (Project No. PJ012521032017),” Rural Development Administration, Republic of Korea. This research was also supported partially by the R&D Center for Green Patrol Technologies through the R&D for Global Top Environmental Technologies funded by the Ministry of Environment (MOE), Republic of Korea and support made by the Korea Ministry of Environment (MOE) (2015001950001) as part of ‘‘The Chemical Accident Prevention Technology Development Project”. Sandeep Kumar thanks DST-PURSE sanctioned to GJUS&T, Hisar under PURSE program No. SR/PURSE Phase 2/40(G).

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