Surface Functionalization Reactions of Graphene-Based Nanostructure and Their Practical Application

*Neeraj Kumari and Meena Bhandari*

#### **Abstract**

Graphene (G) has captured the attention of scientists and researchers due to its remarkable electronic, structural, optical, and mechanical properties. While pristine G has been used for various desirable applications requiring high electrical conductivity, there is also a demand for altered or functionalized versions of G, such as G oxide, reduced G, and other functionalized variants, in numerous other applications. The structural alteration of G through chemical functionalization unveils a multitude of possibilities for adjusting its configuration, and various chemical and physical functionalization techniques have been explored to enhance G's stability and adaptability. Functionalization allows the customization of graphene's properties, such as electronic, chemical, and mechanical characteristics, to suit specific applications. This chapter highlights the functionalization of graphene-based nanostructure, encompassing both covalent and non-covalent approaches, for a wide range of applications as well as for addressing current challenges and for outlining potential future research directions concerning surface functional modification for G and graphene oxide (GO).

**Keywords:** graphene, functionalization, surface modification, hydrophilic, doping

#### **1. Introduction**

The well-known materials of the carbon family prior to the 1980s were graphite and diamond. The discovery of molecular carbon allotropes, including fullerenes, carbon nanotubes (CNT), and most recently, 2-D graphene (G), has completely altered the landscape of the inorganic chemistry of carbon [1]. Graphene possesses a 2D honeycomb-like lattice structure, made up of single-layer sp2 -bonded carbon atoms (**Figure 1**) [2].

Single-layer G exhibits a unique characteristic of having a zero-band gap, leading to its exceptional optical transparency of 97.7%. Furthermore, G has an impressive specific surface area of 2600 m2 /g [3], which is greater than that of CNTs yet is less reactive because it lacks the binding stress that CNTs' curvature produces.

**Figure 1.** *Structure of graphene.*

Therefore, G is used by researchers and scientists in a large-scale application in different fields including drug delivery systems, biosensing, polymer compositing, and fabrication of liquid crystal devices. Other exceptional qualities displayed by G are its electrical, mechanical, optical, and transport nature and the G bipolar field effect [4–6]. These unique properties of G have opened up extensive opportunities for surface chemical applications and sparked intense interest among researchers and technologies [7].

Despite its advantageous properties, G sheets tend to restack and aggregate because of van der Waals interactions between the layers. This propensity poses significant challenges in applications linked to nanomaterials and biomaterials-based technologies, where maintaining the desired structural arrangement is crucial. The attractive van der Walls forces found between G sheets prevent the G dispersion in various solvents. Moreover, due to these interactions, the monolayer G tends to re-aggregate after exfoliation and dispersion. These additional limitations of the single-component materials, including challenging processing, impose significant constraints on its practical applications.

However, as compared to G, graphene oxide (GO), a derivative material obtained by the partial oxidation of G is gaining more attention as it can be synthesized on a large scale with more ease and cost-effectiveness. Thus, GO has experienced a remarkable increase in attention in recent times. The surfaces of GO sheets are rich in oxygen-containing functional groups that may include hydroxyl, epoxide, diol, ketone, and carboxyl sites (**Figure 2**). These groups play a crucial role in altering the van der Waals interactions, leading to a diverse range of solubilities in both water and organic solvents [8].

The functional modifications are essential to expanding the potential use of both G and its derivatives including (GO). Thus, the fundamental basis for achieving functionalization lies in effecting changes to the intrinsic structure of G and its oxide. The two faces, edges, and defect sides of G can all be functionalized through surface or substitutional doping [9]. The existence of structural imperfections and the number of layers have an impact on G's properties [10]. For example, G's electrical conductivity tends to decrease with the introduction of defects during covalent functionalization though the sp2 structure is preserved under non-covalent functionalization [11].

With improved synthetic techniques, G is made more usable in electronics and other fields that rely on quick electron transfer mechanisms including photocatalytic applications for renewable energy production. Another significant problem that needs to be addressed is the need for conversion of the 2-D G material into 3-D, as the creation of higher-order nanostructures from G continues to show

*Surface Functionalization Reactions of Graphene-Based Nanostructure and Their Practical… DOI: http://dx.doi.org/10.5772/intechopen.114855*

#### **Figure 2.**

*G oxide containing various functional groups like hydroxyl, carboxyl, and epoxy.*

promise in applications like supercapacitors, fuel cells, water purification, drug delivery, photovoltaics, catalysis, gas adsorption, sensing [12], touch screens, spintronic devices, high-frequency circuits, toxic material removal, and flexible electronics [13–15].

The ability of the G surface to be altered and functionalized has opened a plethora of possibilities to develop specialized practical compounds [10]. The bandgap of single-layer G is altered for microelectronic devices [11]. Moreover, highly porous 3D structures can be engineered from the naturally non-porous 2D G and used in gas sorption, storage, separation, sensing, and electrochemical devices like batteries, fuel cells, and supercapacitors. Nevertheless, the vast array of potential applications can be expanded through numerous functionalization techniques for G and its derivatives. These diverse functionalization techniques provide numerous opportunities to enhance the present applications of G in other areas, such as bioimaging or increasing band gaps for electronic use. In this chapter, methods for functional modification of G and GO have been discussed focusing on the essential chemical bonds and functional groups that affect their structural integrity. The chapter is divided into two sections. Firstly, the synthesis and structure of G-based materials are discussed. Then, G functionalization and applications of the derived materials are explored. The methods of G modification discussed include reactions with organic and inorganic molecules, as well as chemical modifications through covalent and non-covalent interactions with G [16].

#### **2. Synthesis of graphene-based nanostructure**

#### **2.1 Pristine graphene**

The basic structure of all graphitic materials including charcoal, CNT, and graphite is based on G which is an indeterminately large atomic molecule. There are several approaches that range from mechanical exfoliation of high-quality graphite to direct growth on carbides by bottom-up and top-down techniques which are used to synthesize G [17].

G sheets of different thicknesses can be synthesized through mechanical exfoliation using a simple peeling process. The natural graphite or single-crystal graphite is engraved in oxygen plasma to create deep mesas followed by wedging and peeling off layers. The collected flakes of G are washed off and transferred to a substrate. Another approach to synthesizing defect-free monolayer G is physical exfoliation through ultrasonication using a high-boiling-point solvent [18].

One more promising method for synthesizing mono- or few-layer G is the chemical vapor deposition method (CVD), which also allows for the thickness and crystallinity of the G layer to be controlled using this method [19].

An alternative approach for G synthesis is plasma-induced CVD where the synthesis process takes place at low temperature. This method is more commonly used to synthesize G as compared to CVD as during synthesis, low temperature and less deposition time are required [20].

The chemical exfoliation of graphite is one of the well-established variant methods to produce G. In this method, the increment of interlayer spacing of graphite is achieved by using active functional moieties, which results in the weakening of the van der Wall forces. In this technique, the intercalated G compounds undergo exfoliation either through rapid heating, reduction process, or ultrasonication [21].

#### **2.2 Graphene oxide**

GO is highly hydrophilic and can easily swell after dispersion in water due to extra carbonyl and carboxyl groups that are present at the edges of the GO sheets. Due to this hydrophilicity, ultrasonication treatment is done to introduce a single or few layers of G which are exceedingly steady in deionized water and other solvents. This is necessary to understand that graphite oxide and GO are different [22].

Graphite oxide is a multi-layered system while GO is a few or single-layer system. On the basis of oxygen functionalities, numerous models related to GO structure have been studied. GO can be synthesized through the oxidation of graphite using an oxidizing agent in the presence of concentrated inorganic acids. Thus, GO is widely synthesized through various methods like Hummer's method where potassium permanganate, as an oxidizing agent, concentrated sulfuric (VI), and nitric (V) acids are used to oxidize the G-to-GO [23].

Another method used to synthesize GO is the Staudenmaier method which involves the use of fuming nitric acid and KClO3 as oxidants [24], and the Tour method using concentrated phosphoric acid with potassium permanganate oxidant [25]. All these methods are known as chemical oxidation methods where various oxygen-containing functional groups are created on the edges and surface of G (**Figure 3**). These functional groups are responsible for the hydrophilic nature of GO as they break van der Wall forces. The characteristics of GO can be modified through functionalization to tailor the materials for specific applications.

#### **2.3 Reduced graphene**

When GO undergoes reduction, G is produced due to the removal of oxygencontaining functional groups. Various chemical, thermal, and electrochemical methods are used for the reduction of GO. The chemical reduction has been done using various reducing agents like hydrazine [26], sodium borohydride [27], hydroquinone [28], alkaline solution [29], ascorbic acid [30], and glucose [31].

*Surface Functionalization Reactions of Graphene-Based Nanostructure and Their Practical… DOI: http://dx.doi.org/10.5772/intechopen.114855*

**Figure 3.** *Reduction of GO through different methods.*

During the reduction method, a brown precipitate of G, which turns black during dispersion, results in aggregation and precipitation of reduced sheets of G oxide. During the thermal reduction process, G oxide is heated to remove various oxide groups resulting in exfoliations with the evolution of CO2. The electrochemical reduction method can also be initiated at −0.8 V and completed at −1.5 V with the formation of black precipitates. There are some other methods like photochemical and photothermal methods which are used for the formation of reduced G oxide [32].

#### **3. Surface functionalization**

Surface functionalization is one of the most important processes that enable the use of G-based materials for various applications. The functionalization affects the hydrophobicity and surface charge of G by changing the ionization of G. G's surface can be modified via doping with chemical agents that include elements, compounds, polymers, and nanoparticles which enhance its dispersion, stability, and tribological properties. Through surface modification, stable G-based hybrid materials can be formed. When GO and its composites are used in a water-borne coating system, the main problem is to achieve homogenous dispersion of G. Therefore, to overcome this problem, the surface functionalization of GO is done by covalent/non-covalent reactions involving hydroxyl, carboxyl, and epoxy group using chemical moieties such as organic and inorganic, polymer, and nanocomposites (**Figure 4**). Among

**Figure 4.** *Covalent and non-covalent functionalization of G.*

the groups, the epoxy sites react with the edges and defect sites of G to enhance the dispersion ability of G by forming a stable crosslinker between the epoxy binders and functional G. To enhance practical nano-electronic devices and sensors, band gap widening of G can be achieved through doping, intercalation, and striping processes [15, 33].

The appropriate functionalization of G and GO allows to protect their natural properties by preventing their aggregation during reduction steps [32] and it forms further functional groups to impart additional properties to the materials. As a result, the functionalization processes of G materials have been divided into four categories: that include: (1) covalent, (2) non-covalent functionalization, (3) substitutional doping, and (4) hybridization. With organic groups such as epoxide, carboxylic groups, amino, and hydroxyl groups, G can be functionalized directly through covalent bonding onto the surface sites. To increase dispersibility, for instance, carboxylic acid groups at the GO edges are then swapped out for amine groups such as ethylenediamine and ethanolamine.

#### **3.1 Covalent functionalization**

For the purpose of enhancing G's performance, covalent bonds are used to join G with newly added groups. When an appropriate functional group establishes a covalent bond with the sp2 carbon structure of the G surfaces, at the edges or basal planes, covalent functionalization takes place. The aromatic nature of G can be improved by adding functional groups via the covalent method, which alters the electrical properties of materials while boosting its solubility and stability and widening the bandgap [34].

Covalent organic functionalization offers several advantages, allowing the combination of G's properties with other functional materials like chromophores or polymers, and it enhances the dispersibility of the material in organic solvents and water [35]. Surface functionalization at the edges and flaw functionalization are all possible. The resulting structural defects, which include structural flaws, atoms, and solvent molecules randomly adsorbed onto the materials, are brought on by the chemical processes used to produce G [5]. These defects also include damage to the carbon lattice and structural flaws. Covalent modification can, however, be categorized into different types, such as free radical addition, atomic radical addition, nucleophilic addition, cycloaddition, electrophilic substitution reactions, carboxyl, carbon-carbon skeleton functionalization, and hydroxyl functionalization.

#### *3.1.1 Free radical addition*

Functionalization of G based on the free radical addition involves the formation of covalent bonds through the interaction of free radicals with the G surfaces through thermal treatment, photochemical, or chemical treatments (**Figure 5**) [36]. One of the common radical reactions is carried out with aryl diazonium salts. Functionalization of G through free radical addition was performed by Tour and coworkers where aryl radical forms were formed after the removal of nitrogen from aryl diazonium ion. It was found that the addition of free radical moiety took place on the sp2 surface of G by donating an electron [37]. The organic free radicals generated during the reaction then reacted with G through the covalent formation and were found to be accountable for self-polymerization of the materials. The free radical addition

*Surface Functionalization Reactions of Graphene-Based Nanostructure and Their Practical… DOI: http://dx.doi.org/10.5772/intechopen.114855*

**Figure 5.** *Free radical addition with aryl diazonium salt.*

reaction is controlled by the G layers, and it is observed that G with a single layer is 10 times more reactive than bi or multi-layered G [38].

This type of reaction is employed to study the antimicrobial characteristics of synthesized G and its composites using chlorophenyl groups. On functionalization with chlorophenyl groups, G has been found to be more effective as an antimicrobial agent. Numerous studies demonstrated the generation of free radicals used to induce the denaturation of DNA/RNA, leading to the inactivation of microbial cells.

An alternative method for introducing free radicals involves the reaction of benzoyl peroxide with graphene sheets, which is initiated through photochemical means. This process entails focusing an Ar-ion laser beam onto the graphene sheets immersed in a solution of benzoyl peroxide and toluene [39].

#### *3.1.2 Nucleophilic addition reactions*

In these reactions, G always behaves as an electron acceptor. For instance, when poly-9,9′-dihexyfluorene carbazole reacts with G, a base is used to initially form an anionic moiety, leading to the generation of nitrogen anions on carbazole (**Figure 6**). Subsequently, this anion reacts with the G surface, resulting in the formation of a covalent bond [40].

The primary sites of reactivity in the nucleophilic addition reaction are the epoxy groups found in GO. The amine (-NH2) functionality present in the organic modifiers, possessing a lone pair of electrons, initiates an attack on the epoxy groups of GO. Notably, nucleophilic addition takes place with remarkable ease, even at room temperature and in an aqueous environment. Consequently, this method has garnered significant attention as a promising approach for the large-scale production of functionalized graphene when compared to other techniques [41].

#### *3.1.3 Cycloaddition reactions*

There are different types of cycloaddition reactions like [2 + 1], [2 + 2], [3 + 2], and [4 + 2]. The most prominent reaction is [3 + 2] cycloaddition reaction including 1,3-dipolar cycloaddition which was performed by Trapalis and his co-workers (**Figure 7**). G can be used on a large scale in the Diels-Alder reaction as it behaves both as a diene and a dienophile [42].

Due to its gentle reaction condition, the Bingel reaction is one of the exceptionally valuable reactions for functionalizing carbon nanomaterials, such as graphene. This

**Figure 6.** *Nucleophilic addition reaction with carbazole.*

*Surface Functionalization Reactions of Graphene-Based Nanostructure and Their Practical… DOI: http://dx.doi.org/10.5772/intechopen.114855*

**Figure 7.** *1,3-Dipolar cycloaddition reaction.*

reaction entails the use of a halide derivative of a malonate ester group in the presence of a base. The base extracts a proton from this derivative, leading to the formation of an enolate that subsequently attacks the C=C bonds within the graphene structure. The resulting carbanion undergoes a nucleophilic substitution, displacing the halide and resulting in the formation of a cyclopropane ring [43].

Arynes are well-known reactive intermediates in nucleophilic aromatic substitution reactions, and they are used on a large scale in various reactions. In recent times, there have been several attempts to employ aryne cycloaddition reactions for carbon nanomaterials, demonstrating its successful application in functionalizing fullerenes and their derivatives. Notably, various research groups have reported the chemical modification of graphene through aryne cycloaddition. Based on weight loss analysis, it is estimated that the degree of functionalization is approximately more than one functional group per 17 carbon atoms. The resulting graphene exhibits exceptional solubility and thermal stability, remaining stable even at temperatures as high as 500°C. Additionally, similar reactions using 1,3-dipolar cycloaddition of azomethine ylides and cyclopropanated malonate have also yielded successful chemical functionalization of G and its derivatives [44, 45].

#### *3.1.4 Reaction with atomic radicals*

When the reaction with organic free radicals and atomic species like hydrogen, and fluorine is compared, the possibility of side chain reaction is less in case of reaction with atomic radicals due to which uniform and homogenous functionalization of G occurs (**Figure 8**) [46]. During hydrogenation, deformation of lattice occurs on attaching of

**Figure 8.** *Reaction with gas phase atomic radicals.*

atomic hydrogen resulting in easy formation of a second C-H bond. G in its hydrogenated form is known as graphane [47]. The fluorination reaction of G is found to be comparable with hydrogenation as fluorine attached to the carbon atoms of G through a single bond with enhanced binding strength results in high functionalization. Generally, there are three methods to synthesize fluoroG i.e. through: (1) exposition to XeF, (2) etching fluorinated complexes, and (3) graphite fluoride exfoliation [48].

Oxygenated G produced by applying Hummer's method, exhibits significant heterogeneity. In this reaction, it is assumed that oxygen atoms will attach to graphene, resulting in the formation of epoxide groups. Researchers have successfully generated these epoxide groups by exposing graphene to oxygen plasmas and atomic oxygen beams [49].

#### *3.1.5 Electrophilic substitution reactions*

The functionalization of graphene (G) is made possible through electrophilic substitution reactions, taking into account its electron-rich structure. Various reactions like Friedel-Craft acylation and hydrogen-lithium exchange can be included in electrophilic substitution reactions. In the Friedel-Craft reaction, acyl cation is formed as a reactive intermediate after introducing ketone moieties. The reactive intermediate is an acyl cation generated in the presence of a Lewis acid. This reaction was employed to produce brominated flame-retardant high-density polyethylene composites containing graphene nanoplatelets. In the hydrogen-lithium exchange reaction, the first deprotonation or carbometallation of G takes place using butyl lithium (**Figure 9**). The derivative of lithium-G reacts with an electrophile resulting in the formation of a covalent bond [50].

Covalent bond functionalization is simpler on GO as compared to G due to oxygen-containing groups on its surface. Common chemical processes involving these groups include addition reactions, carboxylic acylation, epoxy ring opening, isocyanation, and diazotization [7].

*Surface Functionalization Reactions of Graphene-Based Nanostructure and Their Practical… DOI: http://dx.doi.org/10.5772/intechopen.114855*

By employing polystyrene particles that have been "armored" using nanoscale GO sheets made via aqueous mini-emulsion polymerization of styrene, Wang et al. [51] took advantage of GO's amphiphilic characteristics without the use of conventional surfactants. From graphite nanofibers, a unique technique was used to manufacture the nanoscale GO sheets of 100 nm diameter. Increased processability and new functions are brought about via covalent bond modification [51].

It is possible to covalently functionalize GO using a variety of other methods. GO nanosheets were chemically modified to include a sulfanilic acid group, which enhanced water dispersibility due to ionic repulsion [52].

Covalent bond modification facilitated increased processability and introduced new functionalities. It has been reported that chemical modification of GO nanosheets with a sulfanilic acid group improved water dispersibility through ionic repulsion. Moreover, covalent functionalization with polyaniline (PANI) not only increased the surface area of reduced GO aerogel (rGOA) but also enhanced electrical conductivity and prevented G nanosheet aggregation. The functionalization started with a free radical reaction where GO donated an electron to aryl diazonium ion resulting in the formation of aryl radical. GO attached to aryl group through covalent bonding after cleavage from the N of PANI-grafted rGOA as high-performance supercapacitor electrodes [53].

The reaction was further preceded by amidation. Lastly, amino group act as active sites for polymerization of aniline on rGOA in an acidic medium using ammonium persulfate as oxidant resulting in the formation of PANI-grafted

**Figure 10.** *Covalent functionalization of rGOA with PANI.*

rGOA as shown in **Figure 10**. Consequently, the PANI-grafted rGOA exhibited superior capacitance performance (396 F/g at 10 A/g) compared to rGOA (183 F/g at 10 A/g).

Shin et al. [54] prepared GO-LMB 21 (locked nucleic acid molecular beacon) using amine functionalized DNA, LMB, and carboxylate GO. The synthesized GO-LMB enhanced the detection limit of miRNA sensing to the picomolar level by covalently coupling fluorescence-labeled dsDNA probes onto the GO. This approach was adopted because interactions between GO and DNA molecules were obstructed by the presence of small molecules like lipids, proteins, and nucleic acids, which caused nonspecific probe desorption [54].

A porphyrin-G nanohybrid was created by Choi et al. [55]. By adding polymers such as polyethylene glycol, dextran, and chitosan, the amidation process has frequently been employed to produce biocompatible GO [55]. The synthesis of chitosan (CS) modified graphene nanosheets under microwave irradiation in N,N-dimethylformamide medium is the best example of amidation reaction which involved the reaction between the carboxyl groups of graphene oxide nanosheets (GONS) and the amido groups of chitosan followed by the reduction of graphene oxide nanosheets into graphene nanosheets using hydrazine hydrate. The results showed that chitosan was covalently grafted onto the surface of graphene nanosheets via amido bonds. Solubility measurements indicated that the resultant nanocomposites dispersed well in aqueous acetic acid.

Four esterification methods, namely direct, carbodiimide activated, oxalyl chloride acylation, and via an acid-functionalized GO intermediate, were investigated for the preparation of surface-functionalized GO nanosheets using tannic acid (TA). In the first approach, direct esterification of GO was done with TA in an acidic medium. In the second approach, acid-functionalized GO (GO-COOH) was prepared through the activation of GO with C2H3ClO2 in a basic medium. In the third experiment, the aqueous solution of GO-COOH was ultrasonicated followed by the addition of TA under ambient conditions to form the final product GO-COOHg-TA-2. In the last stage, the formation of acyl chloride derivative (GO-CO-Cl) takes place through the conversion of carboxylic acid into acyl chloride. The covalent grafting of TA onto the GO surface renders it more hydrophobic, leading to enhanced organic solvent dispersion. Additionally, TA acts as a crosslinker between the GO nanosheets, thereby increasing its thermal resistance. Furthermore, the

combined effect of GO and TA results in the suppression of bacterial growth. Among the examined methods, esterification using carbodiimide showed the highest degree of grafting, maximum thermal stability, and strongest antibacterial activity [56].

#### *3.1.6 Carboxyl functionalization*

The functionalization of GO has been extensively explored because of the abundance of carboxyl groups near the edge of the material which are highly reactive groups [57]. The reactions are often started by the carboxyl functionalization step, after which dehydration of an amino group and a hydroxyl group takes place to create an ester/amide bond. The chemicals usually employed for carboxyl activation include thionyl chloride (SOCl2), 2-(7-aza-1H-benzotriazole-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate, N,N-dicyclohexyl carbodiimide (DCC), and 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC) [58].

Functionalization of semiconducting G nanoribbons (GNRs) with Stone-Wales (SW) imperfections was demonstrated using carboxyl (COOH) group. A significant change in the properties of GNR was observed due to the SW defect as the electrons of unsaturated carbon atoms in COOH interact directly with donor state of GNR nanoribbons. The significant rehybridization interaction results in noticeable downward shifts of the states originating from the two bonding carbon atoms to a lower energy range. This phenomenon helps to clarify why the defect band shifts downward after the adsorption of the COOH group. Essentially, the alteration in electrical conductivity with the density of single-wall defective carbon nanotubes (SWDCPs) can be attributed to the redistribution of electronic states. When SWDCPs axial concentration increases, the behaviors of the system shift from semiconducting to p-type metallic. Consequently, G nanoribbons (GNRs) offer promising prospects for applications in nano-electronics and chemical sensors [59]. Following the addition of CRGO-CN (chemically reduced G oxide having CN group) to a solution of sodium hydroxide and methanol, a hydrolysis reaction produced CRGO that was more abundant in carboxyl groups (CRGO-COOH) [60]. A poly(3-hexylthiophene) molecule was grafted onto the carboxylic acids to activate them in order to create heterojunction photovoltaic devices [61].

#### *3.1.7 Carbon-carbon skeleton functionalization*

The aromatic basal plane of GO can be directly functionalized using highly reactive intermediates including nitriles, carbenes, and aryl diazonium salts. This process could change the basal plane's sp2 hybridization to sp3 after functionalization. In comparison to non-functionalized GO, GO functional groups considerably increased dispersion stability in water, dimethyl sulfoxide, and N,N-dimethylformamide (DMF). In the aromatic ring of G or GO, the C=C bond is mostly used for the functional alteration of the carbon skeleton. Both the Diels-Alder reaction and the GO diazotization reaction have been documented [62]. The functionalization of G sheets was done through the one-pot process using anthraquinone molecules. In this process, the G electrode undergoes oxidative electrochemical exfoliation in the presence of 0.1 M H2SO4 solution containing anthraquinone diazonium ions. Functionalization takes place through the spontaneous reaction of newly formed graphene sheets with diazonium ions (**Figure 11**) [63].

#### **Figure 11.**

*The one-pot electrochemical exfoliation of graphite leads to the spontaneous functionalization of graphene sheets (EG) with anthraquinone.*

Xiong et al. [62] produced 4-propargyloxyphenyl G (GCCH) through a two-step process. They mixed solution-phase G with 4-propargyloxydiazobenzenetetrafluoroborate at 45°C for 8 hours [62]. Then, the G carbon skeleton was treated and further functionalized using a click chemical reaction with azido polyethylene glycol carboxylic acid [64]. This approach proved to be versatile and practical, allowing for the creation of biosensors and composite materials with G by modifying the functional groups connecting the layers.

The underlying mechanism involves the formation of a diazonium salt or a diazo compound through the diazotization of an aromatic amine-containing material with a reactive functional group, leading to the generation of a free radical during deaeration [65]. The benzene derivative linked through sigma bond with the reactive functional group undergoes an addition reaction with a carbon double bond (C=C) to form a new single C-C bond. Eventually, G with the reactive functional groups undergoes functional modification with G oxide.

#### *3.1.8 Hydroxy functionalization*

Another significant method of modifying G is through hydroxy functionalization, in which the occurrence of several hydroxyl groups present on the GO surface enables the functionalization of hydroxyl-based via amidation reaction or esterification. The ester produced by GO is then further modified using other functional groups [66]. The hydroxyl groups were substituted after esterification in order to create aziolated GO. Stirring GO with 2-bromoisobutyryl bromide for 2 days proceeded by dispersion of the esterified GO in dimethylformamide and sodium azide for 24 hours produced azido functionalized GO resulting in improvement in the solubility of the functionalized GO in polar solvents like chloroform and tetrahydrofuran [67].

#### *Surface Functionalization Reactions of Graphene-Based Nanostructure and Their Practical… DOI: http://dx.doi.org/10.5772/intechopen.114855*

The researchers utilized tris buffer (TB) and diethanolamine (DEA) which are water-soluble, non-toxic, and biocompatible to easily produce alcohol-functionalized GO material. This alcohol functionalization on GO resulted in the expansion of interlayer space and specific surface areas, allowing for the molecular grafting of amines to G. In a 60% KOH solution with a density of 1 A g−1, in a 6% KOH solution with a density of 1 A g−1, the synthesized materials exhibited high specific capacitance values of 372 and 252 F g−1 [68].

Erythritol, a promising medium-temperature candidate, was combined with GO nanosheets as an additive to increase the dispersion stability of the composites. GO nanosheets treated with hydroxyl groups were used for this purpose. Moreover, the application of functionalized GO nanosheets as an additive has proven effective in enhancing erythritol performance, which is a promising medium-temperature candidate. To optimize overall performance, a trade-off evaluation on the loading of the additive is necessary. By combining GO nanosheets with erythritol, the dispersion stability improved significantly while using GO nanosheets after treatment with hydroxyl-containing compound, the highest loading of 1.0 wt% GO nanosheets led to a twofold increase in thermal conductivity and a reduction in supercooling from 64°C to 48°C. It has been demonstrated that functionalized GO nanosheets are effective in enhancing the performance of erythritol, however, to obtain the greatest overall performance, a trade-off analysis on the loading would be necessary [69].

In other words, the hydroxyl groups were selectively functionalized using a variety of chemical strategies. The Williamson reaction with an amino-terminated linker derivatized the hydroxyl groups, resulting in the production of ether bonds. In contrast to most derivatization methods for hydroxyl groups, this reaction occurred under benign conditions at ambient temperature, preventing the reduction of GO. Then, effective esterification of the hydroxyls using aminocaproic acid at room temperature has been carried out. Overall, the hydroxyls' reactivity in the Williamson reaction and esterification was quite similar [70].

When dispersing gold nanoparticles (GNPs) in pyridine, Georgakilas et al. [39] employed 1,3-dipolar cycloaddition of azomethine ylide to add the hydroxyl group to G surfaces. For 30 days, the functionalized GNPs could persistently be disseminated in ethanol [39]. Using a ball mill to exfoliate graphite and potassium hydroxide, the authors were able to produce hydroxyl-functionalized G. The G with the hydroxyl functional is very electroactive, hydrophilic, and water dispersible. The functionalized G is made up of single- to few-layer GNPs. Esterification has been reported by the addition of poly(vinyl) alcohol to GNPs [71]. The carboxylic group was added to the pre-functionalized GNPs by oxidizing them in a cold nitric/sulfuric acid solution while subjecting them to extended sonication. Amiri et al. [70] functionalized GNPs with ethylene glycol via a microwave-aided electrophilic process. In water/ethylene glycol conditions, the functionalized GNPs exhibit good dispersibility [70].

#### **3.2 Non-covalent functionalization**

The stable dispersion of G materials in organic solvents can be achieved through covalent functionalization using aromatic compounds [72–73] isocyanates, and aliphatic amines [74]. However, this covalent approach comes at the cost of reducing many of the advantageous characteristics of G-based materials (GBMs), such as their barrier, electrical, and mechanical properties. In contrast, the noncovalent approach focuses on achieving stable dispersion by relying on physical

adsorption and/or enfolding of molecules or polymers through weak interactions like hydrogen bonding, van der Waals forces, and H-π, π-π, cation-π, and anion-π interactions. This non-covalent method allows for the preservation of the electrical properties of GO [32].

Non-covalent functionalization procedures have a significant benefit over covalent functionalization approaches since they lessen the impact on the G structure and its inherent material qualities [74]. GBMs can be non-covalently functionalized using a variety of techniques, including polymeric grafting, interaction of small molecules with aromatic rings, or use of surfactants [75].

The G structure suffers little harm from the non-covalent change, and maximum properties can be preserved [13, 76]. Previously, conjugated molecules such as 1-pyrenebutyrate, sulfonated polyaniline, dendronized perylene bisimides, polyacetylenes, and carboxylated oligoanilines have been used to disperse G [39, 77]. Through dispersant, van der Waals interactions or electrostatic repulsion prevails which may aid in G stability in solution.

#### *3.2.1 π-π interactions*

G interaction with other compounds or nanomaterials is influenced by two different forms of π-π interactions that take place between the electron-rich and electron-poor areas. This is frequently observed in C6R6 (R is substituents like hydrogen or other groups altering the ring's polarity) where C and R arrange through face and edges interaction. Beyond benzene, these interactions also occur with biologically significant compounds such as DNA and porphyrins. These interactions, which are also present in small molecules, can be used to functionalize GO and G systems for processing and property modification.

A novel approach to non-covalently functionalized GO involved using hyperbranched polyesters with terminal carboxyl (HBP) to produce HBP-GO through strong π-π coupling between hyperbranched polyesters and GO nanosheets. This method was aimed at enhancing the interfacial characteristics between GO and epoxy resin (EP). The presence of hyperbranched polyesters embedded within the GO layer created a steric hindrance effect, effectively preventing the aggregation of GO nanosheets, and greatly improving their dispersibility. Additionally, this noncovalent functionalization increased surface energy, interfacial energy, and adhesion work, reduced the contact angle of HBP-GO with EP, and significantly enhanced the wetting property of HBP-GO [78].

The G compounds with non-covalent functionalization spread easily in polar aprotic liquids. According to Marcia et al., non-covalent pyrene derivatives were applied to GBMs to prevent agglomeration and improve compatibility with the polymer matrix [79]. Layek et al. [80] found that functional groups on the pyrene basal plane edge also improved the interface between G and the polymeric matrix [79]. Similar to this, non-covalently functionalized G composites with enhanced barrier characteristics and G dispersion utilizing pyrene derivates and polyketones [80]. The fluorescence qualities of the dispersant with conjugated structure are typically present; however, once the G is combined, the fluorescence capabilities will be significantly quenched [81].

The reduction of GO through the wet chemical method produced a G-based material having aggregation-induced emission (AIE) properties. During the GO reduction process, TPEP, a conjugated molecule containing tetraphenylethylene (TPE) and pyrene, was used as a stabilizer. The resulting rGO-TPEP exhibited AIE property

*Surface Functionalization Reactions of Graphene-Based Nanostructure and Their Practical… DOI: http://dx.doi.org/10.5772/intechopen.114855*

and high dispersion in solution compared to TPEP alone. The fluorescence intensity of rGO-TPEP was 2.23 times stronger. rGO-TPEP proved to be a sensitive chemical sensor for explosive detection in both aggregated and solid states, owing to its unique optical features and AIE effect. In the aggregated state, rGO-TPEP demonstrated the ability to detect even small concentrations of 2,4-dinitrotoluene (DNT) at 0.91 ppm, with a high quenching constant of 2.47 × 104 M−1 [82].

#### *3.2.2 Functionalization with polymers, drugs, and biomolecules*

The π-π interactions between G derivatives and polymers containing aromatic rings provide an excellent example of how tight binding can result in highly homogeneous polymer composites with improved mechanical, electrical, and thermal properties [81, 82]. For instance, Kevlar, an aromatic ring-containing polymer, can strongly interact with G through stacking, and when incorporated into a G nanoribbon (GNR) composite, it enhances the mechanical properties significantly. For example, the addition of 1 wt% GNR to Kevlar/PVC composite increased the Young's modulus and yield strength by approximately 72.3% and 106%, respectively.

Non-covalent functionalization of G has been explored in other applications. One method involves non-covalently functionalizing G with amine-terminated polystyrene, which enhances its dispersibility in organic environments [71]. Another approach uses self-assembled monolayers of 1,4-benzenedimethanethiol (BDMT) to anchor gold nanoparticles on the G surface, enabling highly sensitive electrochemical detection of H2O2 [83].

A novel method for modifying G in field-effect transistors (FETs) application was developed, involving positive and negative doping effects induced by sequentially treating G with gold nanoparticles (AuNPs) and thiol-SAM molecules [82]. This technique demonstrated a Dirac voltage switcher on a G FET using heavy metal ions.

Similar to this, a G structure that self-assembled and adorned with glucose oxidase was employed to support the sensing of glucose within the 20 nm detection limit [84–86]. To detect nitrate, copper nanoparticles and adorned self-accumulated G was synthesized by Wang, Kim, and Cui [87]. Non-covalently functionalized G with 60 nm thickness and 300 nm diameter was obtained with dextran and chitosan via layer assembly on the GO surface for an anticancer function [88–89].

Jung et al. [90] introduced a novel G probe functionalized with single-stranded DNA (ssDNA) for the accurate detection of H2S and NH3 in exhaled breathing. The functionalization of G with ssDNA creates an ion-conducting channel, enabling efficient proton hopping at humidity levels above 80%. This process results in excellent carrier density modulation, which has the potential to detect biomarkers for illnesses such as kidney diseases and halitosis. The chemiresistive probe was fabricated using a hybrid configuration, where non-covalent π-stacking interactions between ssDNA and G play a crucial role in facilitating the detection process [90].

Concha et al. [91] utilized self-limiting monolayers of ammonium-substituted pyrenes to impart a general positive charge to the G surface and sulfonate-substituted pyrenes to impart a general negative charge. Both types of pyrenes resulted in a stable hydrophilic surface, which allowed for the specific immobilization of macromolecules that carried either negative or positive charges. This straightforward and versatile non-covalent approach is applicable to G on various substrates, including Cu, SiO2, suspended G, and graphite. By transforming G from hydrophobic to hydrophilic, this method facilitates the use of electrostatic interactions to control the adsorption of macromolecules [91].

Gan et al. [92] developed a nanocomposite to functionalize G with D-glucose using poly (vinyl alcohol) (PVA) and poly (methyl methacrylate) (PMMA) as matrices. The interaction between the polymer blend and D-glucose moieties through hydrogen bonds connected to the fillers led to the uniform diffusion of functionalized G within the matrices. This resulted in a significant improvement in the thermomechanical properties of the nanocomposite [92].

Chhetri et al. [89] used a catalyst based on 3-amino-1,2,4-triazole (TZ) in KOH to functionalize GO nanoparticles and then incorporate them into epoxy resin resulting in better thermal and mechanical resistance. More specifically, compared to composites containing pure GO, both the tensile strength and elastic modulus increased by roughly 30%, while fracture toughness increased more than a magnitude [89].

Functionalization of high-density polyethylene (HDPE) with maleic anhydride (GO-g-MA), GO with ethylenediamine (GO-EDA), and oxidized CNTs (MWCNTs-COOH) were used in a complicated system that Bian et al. [93] developed. To connect GO-EDA and MWCNTs-COOH, L-aspartic acid was employed, creating a hybrid network. This hybrid network was then melted into HDPE-g-MA [93].

The packaging business is a significant additional area where the utilization of G and its derivatives may be of tremendous relevance. Given the growing environmental concerns around waste disposal, two significant areas of application are food and electronic packaging, both requiring excellent barrier properties against gases, particularly water vapor. Functionalization of G has also been employed in packaging materials, where it can enhance mechanical qualities, chemical endurance, and barrier properties [86]. Examples include G functionalized with D-glucose to improve thermomechanical properties in polyvinyl alcohol and poly(methyl methacrylate) matrices [93], as well as GO functionalized with 3-amino-1,2,4-triazole to enhance thermal and mechanical resistance in epoxy resin [94].

G's utilization in electronic packaging holds particular promise due to its ability to enhance mechanical properties, chemical resistance, and barrier performance [79].

Nanocomposites were formed using vinyl silicone resin prepolymer. The addition of 1% of functionalized G nanoplatelets (GNP) resulted in a substantial improvement in mechanical properties, increasing the tensile strength by approximately 500% and the elastic modulus by up to 1000%. Moreover, at higher percentages of functionalized GNP (10–15%), there was a significant enhancement in thermal conductivity, reaching 16 to 38 times the initial conductivity of the resin [94].

Hierarchical structures were created by growing *in situ* SiO2 nanoparticles on reduced G oxide (rGO) nanoplatelets that had been non-covalently functionalized. These were discovered to serve as a reinforcing agent for the matrix made of hydrogenated nitrile butadiene rubber, significantly resulting in a significant improvement in both the static and dynamic mechanical properties [95–96].

Ou et al. [97] developed an eco-friendly polyurethane comprising of poly (L-lactic acid) as flexible fragments and G. The process involved initiating the ring-opening polymerization of L-lactic acid with phenol-derivatized G, resulting in a polymer grafted with G. Subsequently, the polymer endured condensation polymerization with diphenylmethane-diisocyanate to produce the polyurethane. This polyurethane exhibited superior hydrolysis and antifouling behavior compared to the clean polyurethane, thus making it a suitable coating for flat surfaces [97].

Layer-by-layer assembly propelled by electrostatic contact was used to alternately deposit polyethyleneimine and functionalized GO (FGO) on poly (vinyl alcohol) (PVA) film surface, giving the coated PVA film remarkable flame retardancy. The PVA matrix was enclosed in a shield of defense created by the multilayer FGO-based

coating, which successfully stopped the mass and heat transfer that occurs during combustion. Compared to plain PVA, coated PVA shows an initial decomposition temperature of 260°C and a nearly 60% lower total heat output [98].

#### **3.3 Nanoparticle functionalization**

Another common type of functionalization is the addition of various nanoobjects, such as nanoparticles (NPs), nanowires, nanorods, and nanospheres (NSs), to G heterostructures and nanocomposites. In contrast to pure G, nanoobjects are equipped with a variety of capabilities based on the inherent features of materials. To expand G's potential in a variety of electronic and optoelectronic processes, the nanoobjectbased G nanocomposites in particular combine specialized optical and electrical capabilities. The semiconductor nanoobjects that can outperform the low-absorption behavior of pure G have certain exceptional optical properties. For instance, when coupled with G to create nanocomposites, the semiconductor cadmium sulfide quantum dots/nanoparticles (CdS QDs/NPs) have significantly enhanced the photoabsorption as well as photoelectrical reactions [99].

When paired with G/ZnO heterostructures, ZnO nanowires/nanorod semiconductors with UV activity have a wider bandgap, which can enhance G's UV responsiveness. Likewise, when mixed with G to create nanocomposites, the TiO2 NPs demonstrated better photocatalytic and photoelectric activity [100–101]. The G/nanoobjects also demonstrate exceptional activity in electrical and electrochemical fields; according to one study, in a hydrogen evolution reaction, MoS2 NSs/G nanocomposites displayed superior electrocatalytic properties to pure MoS2 [102]. Additionally, Ni (OH)2, the NSs/G nanocomposites have shown improved performance in investigations pertaining to electrochemical capacitors [103].

Pure Co3O4 NPs and Co3O4 NPs/G nanocomposites demonstrated significantly higher oxygen reduction than a C/Pt catalyst. During the oxidation of methanol, the metal NPs, which are only Pt-functionalized with G, demonstrate excellent electrocatalytic activity [104]. Shahid et al. demonstrated the functionalization of G with nanoscale particles which show selectivity for hydrazine [105].

G-gold nanostructures (G-AuNS) have emerged as highly effective sensing substrates, enabling the development of electrochemical biosensors that are affordable, dependable, rapid, and sensitive. These electrochemical devices have found significant applications in the biomedical domain, for detecting glucose through enzymatic or catalytic approaches, as well as H2O2, biomolecules (DNA, protein), small molecules (dopamine), microorganisms, foodborne pathogens, environmental pollutants, and a wide range of other analytes. In general, electrochemical biosensors present remarkable advantages, including customization, miniaturization, and rapid analysis capabilities. Nevertheless, they also come with certain common analytical limitations, such as susceptibility to interferences from complex biological sample matrices and the inability to simultaneously detect multiple analytes [106].

#### **3.4 Plasma hydrogenation**

G is an example of a 2D material with exceptional qualities that can be used for huge on/off ratio devices and hydrogen storage. G's outstanding electrical, optical, mechanical, and thermal capabilities have propelled it to the forefront of cutting-edge research and industry. However, a significant barrier to its use has been G's lack of a

sizable band gap. Various methods have therefore been developed to open and control a band gap in functionalized G.

The most straightforward chemical modification of G involves hydrogenation. In this process, each lattice carbon can bond with only one hydrogen atom, assuming no carbon-carbon bonds are broken. Surprisingly, due to the similar electronegativities of carbon and hydrogen, the C-H bond is expected to be nonpolar, not significantly impacting the G's doping. Based on our knowledge of organic chemistry, the C-H bond in organic molecules is relatively unreactive, leading to the anticipation that hydrogenated G would be chemically stable and inert. This hydrogenated form of G was envisaged as a chemically stable, nonpolar, two-dimensional material with potential applications in electronics, and possibly hydrogen storage (**Figure 12**) [106].

On the other hand, hydrogen plasma treatment for surface modification of singlelayered G has garnered significant scholarly interest due to its potential for conventional wafer-scale production. Researchers have successfully created a monolayer chemical-vapor-deposited G, hydrogenated by an indirect hydrogen plasma, with no structural defects. They observed that adjusting the hydrogen coverage allows for precise control of the band gap, achieving values of up to 3.9 eV [107].

Plasma containing H3 + ions having 3.45, 5.35, and 7.45 eV energies was used to expose G sheets. Only the specimen subjected to the lowest energy plasma could be thermally annealed back into G; the other specimens had irreversible features as a result of vacancy defects caused by ions having high energies. By employing plasma with the proper ion energy and Joule heating, the alterable property in G FETs has been demonstrated, proving that the damage caused by plasma was minimal [108].

The photoluminescence of hydrogenated G exhibits an intriguing optical characteristic. This phenomenon arises from the creation of electronically disconnected conjugated polycyclic regions, exhibiting a diverse range of absorption outlines and remarkably fluorescent emission features as atoms randomly populate the G lattice [109]. These luminous regions possess functionality comparable to carbon nanodots. Consequently, highly hydrogenated G has garnered interest for applications involving white light fluorescence, optoelectronic characteristics, and imaging capabilities, akin to quantum dots [110]. Elsewhere, Elias et al. discovered that the hydrogenation had a strong p-doping behaviour. However, it was discovered that once the sample

**Figure 12.** *Plasma hydrogenation of G.*

*Surface Functionalization Reactions of Graphene-Based Nanostructure and Their Practical… DOI: http://dx.doi.org/10.5772/intechopen.114855*

was dried, hydrogen showed the n-doped behaviour in the G and p-doping effect was initiated by adsorbed water molecules [111].

These studies highlight the importance of surface modification of GO in tuning its properties and enabling novel applications in analytical chemistry. Fortunately, GO possesses numerous active surface functional groups, which allow for surface modification through various interactions, including covalent and non-covalent interactions. Additionally, successful surface modification of GO has been achieved through the doping of heteroatoms or nanocomposites.

#### **3.5 Substitutional doping of graphene**

The most realistic and appropriate approach to amend the band structure of G is doping where G changes from semi-metal to n or p-type semiconductor. This is accomplished by substituting or replacing carbon atoms from G lattice with foreign elements, a process known as substitutional doping of G. Substitutional doping is going to be interesting as it introduces the charge in the structure of G.

In this type of doping, G's carbon atom is replaced by another atom like nitrogen, phosphorous, boron, sulfur, etc., near the opening (**Figure 13**). In a pristine G, the unpaired electrons are tightly bonded and passivated within its delocalized structure, rendering it chemically unreactive and limiting its energy absorption capacity [112]. However, G can be easily p-type doped through surface absorption. When pristine G is exposed to molecules with electron-withdrawing groups (such as H2O, O2, N2, NO2, PMMA, etc.), noticeable p-type doping occurs, but it can quickly return to its original state once the doping molecules are removed.

In contrast, achieving stable n-type doping in G presents more challenges. Although some electron-donating molecules like ammonia, potassium, phosphorus, hydrogen, and poly(ethyleneimine) (PEI) can induce n-type doping through surface electron transfer, these doping effects often prove to be unstable. As an alternative approach, introducing nitrogen-containing precursors during the growth process can partially replace lattice carbon atoms with nitrogen atoms, leading to effective n-doping.

Combining both p-type and n-type doping methods enables the creation of p-n junctions in mono- or bi-layer G. These hetero-doped G p-n junctions have paved the way for novel functional devices like photothermoelectric devices [113].

Heteroatom insertion can produce extraordinarily abundant active sites in G. The addition of a heteroatom with a different electronegativity from the carbon atom could break the electroneutrality of G resulting in the generation of unstable charged zones and such zones might act as active sites. These active sites may exist as

**Figure 13.** *Doping mechanism in G layer.*

structural defects (arising from the lattice strain). The lattice strain is mainly due to the difference in size of dopant and carbon atom. The base of these active sites introduces the band gap and semiconducting properties of G by improving their chemical properties. The heteroatom can be substituted into G through various methods like solid-phase synthesis, liquid-phase synthesis, and direct synthesis [114].

CVD and segregation-growth approach are the direct synthesis methods. The most prominent method to introduce the heteroatom in G is the CVD method where heteroatoms are directly incorporated into lattice of G [114–116]. This method is used to synthesize n-type semiconductors using G and nitrogen element as dopant accompanied by alteration of charge mobilization and transference of electron results in the transition of G from metal to semiconductor. As a result, nitrogendoped graphene finds applications in both the electronic and optoelectronic realms [117].

Another method used to synthesize doped G is a segregation-growth method which is also a direct synthesis method. In this method, some selective doping is possible as a heteroatom incorporated into some selective surface of G. Recently, Wang et al. confirmed the fabrication of nitrogen-doped G via this method exhibited a bandgap of 0.16 eV. This method is used to control the concentration and position of doping materials hence the synthesized G used for FET application [118].

#### **4. Conclusion, challenges, and future prospectives**

The chapter delves into the ability of G materials for various applications such as energy storage, conversion devices, and many more. To leverage the unique properties of G, it can be combined with other nanomaterials like metal, metal oxide, magnetic nanoparticles, quantum dots, etc., using surface functionalization and nanoparticle functionalization, along with plasma dehydrogenation and substitutional doping. The appropriate modification of the surface through doping and/or functionalization creates promising opportunities for the use of these materials in device applications.

Surface functionalization can be achieved through covalent and non-covalent functionalization. Non-covalent functionalization, while easy and rapid, relies on hydrophobic, van der Waals, and electrostatic interactions, making it susceptible to leaching out functions from the G sheets during application. On the other hand, covalent functionalization capitalizes on oxygen functional groups present on G surfaces, such as carboxylic acid groups at the edges and epoxy/hydroxyl groups on the basal plane, to modify the surface functionality of G. Covalent functionalization addresses the drawbacks of non-covalent functionalization and allows for the precise decoration of desired functions on suitable platforms.

Regarding surface transfer doping, its long-term stability is lacking as adsorbed species can desorb from the G surface and react with reactive molecules. Substitutional doped G, where metal or heteroatoms are attached to the carbon linkage of G, offers improved stability. However, it faces critical challenges in terms of large-scale production, doping controllability, and mechanisms. The production of G indeed faces several challenges, including the need for specific conditions and high temperatures in some methods such as the CVD method where high temperatures are required (typically above 1000°C) to create the necessary conditions for graphene growth. These high temperatures can be energy-intensive and may limit the scalability and cost-effectiveness of production. Developing a controlled synthesis of doped

*Surface Functionalization Reactions of Graphene-Based Nanostructure and Their Practical… DOI: http://dx.doi.org/10.5772/intechopen.114855*

G could provide a desirable solution. As the field of novel materials and innovative applications continuously advances, understanding the mechanisms for reactions in many electrochemical systems remains complex. Further investigations into doped G materials hold promise for contributing to energy systems. Controlled reduction offers a viable pathway for mass-producing semiconducting G.

GO has proven its versatility in various applications, including optoelectronics, drug delivery materials, biodevices, and polymer composites. Consequently, G emerges as a promising candidate for immobilizing various substances, such as metals, biomolecules, fluorescent molecules, drugs, and inorganic nanoparticles.

#### **Acknowledgements**

The authors are thankful to K. R. Mangalam University for providing technical support.

#### **Conflict of interest**

The authors declare no conflict of interest.

#### **Abbreviations**



### **Author details**

Neeraj Kumari and Meena Bhandari\* Department of Chemistry, School of Basic and Applied Sciences, K.R. Mangalam University, Gurugram, India

\*Address all correspondence to: meena.bhandari@krmangalam.edu.in

© 2024 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Surface Functionalization Reactions of Graphene-Based Nanostructure and Their Practical… DOI: http://dx.doi.org/10.5772/intechopen.114855*

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#### **Chapter 4**

## The Graphene Surface Chemistry and Adsorption Science

*Enos W. Wambu*

#### **Abstract**

Graphene (G) has attracted immense attention due to its exceptional physicochemical and electronic properties, and quite a large amount of literature has accumulated on this subject over the last few decades. The current work, based on a systematic review of the relevant literature, was designed to provide an overview of G surface chemistry with respect to its adsorption science. The aim was to improve knowledge of the graphene surface chemistry while informing new strategies for designing and implementing new G materials for emerging applications. The key G surface reactions include: molecular adsorption of gases, bandgap tuning, gas detection tests; alkali metal storage for battery technology; G vacancy engineering; environmental amelioration of pollutants; and sensors and biosensors technology. GO (graphene oxide) or G has been surface-modified using nonmetals, metals, metal oxides, or organics. In general, GO and related functionalized materials have high affinity and adsorption efficacy for ionic adsorbates, whereas pristine G, and reduced graphene oxide (rGO), exhibits enhanced hydrophobic surfaces with propensity to strong π-π interactions. The metals' adsorption and doping can impart G magnetic and metallic character, whereas molecular intercalations tend to induce a G bandgap for nano-electronic and nanophotonic uses among other interactions.

**Keywords:** adsorption, bandgap tuning, graphene defect engineering, DNA, energy storage, environmental remediation, graphene, biosensors

#### **1. Introduction**

Graphene (G), a new layered carbon material, is one of the most studied materials over the last few decades. Investigations about G have centered on new fabrication methods [1]; improving understanding of its properties [2], designing and development of new derivatives [3], and exploring prospects for the new G materials [4]. In examining G properties, most studies focused on specific surface reactions, especially the adsorption of organic contaminants [5] and simple molecular gases [6], as well as heavy metal doping [7], and interaction with pharmaceutical agents [8]. The adsorption performance of G and its derivatives, primarily graphene oxide (GO), reduced graphene oxide (rGO), nonmetal- and transition metal-doped graphene (TM-Graphene), has been assessed extensively for their adsorption potential towards various adsorbates in terms of the reaction conditions; equilibrium, kinetic, and

thermodynamic analyses; and the electronic characterization of adsorption energies, geometries, DOS, dipole moments, the reaction work functions, etc. The Langmuir [9], Freundlich [10], Temkin, Polanyi [11], and the Dubinin–Radushkevich [12] isotherms are widely used for evaluating the adsorption capacities, reaction mechanisms and thermodynamic of the adsorption processes. The adsorption kinetics, on the other hand, are often evaluated using the Lagergren first-order [13] and pseudosecond order [14] kinetics equations. Pristine graphene (G) surface presents basically three adsorption sites: the hollow site (H), at the center of a carbon hexagon; the bridge site (B), in the middle of a C∙C bond; and the top site (T), on the top of a carbon atom. Molecular orientations are used to determine the most stable adsorption configurations and the adsorption energy, *E*ad, is defined according to Hess' law as:

$$E\_{ad} = E\_{molecule} + E\_{graphene} - E\_{molecule \oplus grapcheme} \tag{1}$$

where, *E*molecule, *E*graphene, and *E*molecule@graphene are the energies of the isolated gas molecule, graphene, and molecule–graphene adsorption complex, respectively. The first-principles electronic calculations are used basing on the spin-polarized densityfunctional theory (DFT) *via* the Vienna *ab initio* simulation package (VASP) [15] with the exchange and correlation energy included through a generalized gradient approximation in PBE format [16]. Similarly projected augmented-wave (PAW) potentials [17] are also used to describe the ion-electron interactions.

However, comprehensive overview of the surface chemistry of G is lacking in the literature and the current work was, therefore, designed to provide an inclusive update of the surface adsorption of G and the related materials. The main goal was to improve the understanding of the graphene chemistry and, thus, contribute to information of emerging strategies for designing new G materials and their applications. GO is characterized by several reactive surface groups with strong acidity and high adsorption capacities for basic and cationic species. G on the other hand, exhibits a hydrophobic surface presenting high adsorption capacities for chemicals that show strong π-π interactions. Suitable modifications of GO or G with metal oxides and/or organic groups can produce nanocomposites with more enhanced tailored adsorption capacities and separation efficiencies for specific diverse applications, including fuel gas adsorption and storage, battery technologies, G vacancy electronic and photonic engineering, pollution ameliorating, and sensors and biosensors fabrications. The current work aims to consolidate relevant literature and provide new insights for understanding G chemistry and spur new strategies, and application of G and its derivatives.

#### **2. Application of graphene in fuel gas storage**

The search for sustainable clean energy remains one of the fundamental socioeconomic development goal of the 21st century. Actions toward realization of this goal include fuel gas storage techniques using efficient adsorbents and graphene (G) materials have attracted particularly intense attention due to their unique properties that lend them to diverse versatile modifications and applications. The large specific surface area of G has attracted researchers to explore it for diverse adsorptive applications. Ma et al. [18], for example, evaluated hydrogen adsorption using pristine single-layer G samples with a BET-specific surface area of 156 m2 /g and found that only 0.4 wt.% hydrogen could adsorb on G at 77 K under 100 kPa pressure.

Furthermore, even lower hydrogen adsorption rates of <0.2 wt.% were obtained at ambient temperature under elevated 6 MPa-pressure. The small gravimetric hydrogen uptake by G samples were attributed to low specific surface area of the materials and to the weak hydrogen binding into the G surfaces. Elsewhere, while using high pressures of about 40 bars, and Szczęśniak et al. [19], obtained optimum H2 adsorption of 5.5–7 wt.% and 4.4–4.8 wt.% for activated rGO, and for transition metal (TM) nanoparticles doped-rGO, respectively.

Meanwhile, while investigating different hydrogen adsorption sites of G, Arellano et al. [20], observed that the most stable configuration was obtained when H2 molecules physisorbed over the hollow (H) site, although the barriers for classical diffusion were, particularly, small. Nevertheless, in an *ab initio* study of atomic hydrogen adsorption on G, Ivanovskayaet al. [21], focused their investigations on the adsorption characteristics of the hydrogenation surface coverage of G. They observed that at high surface coverage, the resulting strain from substrate relaxation controlled H chemisorption leading to localized surface curvature. The chemisorption energy barrier was caused by the relaxation and by the adsorbent carbon atom rehybridization. This showed independence from the optimization technique and the system size. The H desorption barrier was, however, very sensitive to the correct structural relaxation and it was controlled by the degree of system hydrogenation.

According to Miura et al. [22], the carbon atoms reconstructions played an leading role in G interactions with H2 molecules. The activation barrier for H2 dissociation from an unrelaxed G was ~4.3 eV for a T–H–T geometry, and ~4.7 eV for a T–B–T geometry. The center of mass position of H2 was, therefore, at the hollow site, and the two H atoms of the molecule were directed toward the top sites on the G structure. When the carbons relaxed, the activation barrier decreased to 3.3 eV for the T–H–T geometry and 3.9 eV for the T–B–T geometry, in which case, the two carbon atoms near the H atoms moved 0.33 Å toward the gas phase for the T–H–T geometry and 0.26 Å for the T–B–T geometry. Elsewhere, while investigating H atoms adsorption on G using first-principles while employing plane-wave based periodic density functional theory (DFT), Casolo et al. [23] selected a 5 × 5 surface unitcell to study H atoms' single- and multiple-adsorptions on G surface. They computed the binding and barrier energies for sequential sticking of several H atoms' configurations on top of G carbon. The authors recorded binding energies of 0.8–1.9 eV per atom with barriers to sticking of 0.0–0.15 eV. Magnetic structures formed in which spin density localized on a 3 × 3R30° sublattice and the binding (barrier) energies for sequential adsorption increased/decreased with the site-integrated magnetization. These results could be explained on the basis of the valence-bond resonance theory of planar π-conjugated systems. It suggested that preferential sticking due to barrierless adsorption was limited to the formation of hydrogen pairs.

According to Kim et al. [24], however, presence of certain metals at G's vacancy defects increased the binding energy and hydrogen adsorption of the metals tested; however, the Ca-vacancy complexes showed most favorable binding energy and overall hydrogen adsorption capacities. So, Ataca et al. [25], were able to demonstrate the capacity of Ca-adsorbed G has a recyclable hydrogen adsorption media. They found that Ca chemisorbed onto G by donating part of its 4 s charge to the empty π\* G band imparting it with a positive charge and the metallic character, which increased its H2 adsorption, in turn. The H2 uptake capacity of Ca-modified G could be improved further by adsorbing Ca on both sides of the G nanosheets and utterly by saturating the Ca surface coverage as because Ca does not agglomerate on G surfaces

at high surface concentration. This due to the high coulombic repulsions between the adsorbed Ca atoms.

Besides hydrogen, Gao et al. [26] reviewed G adsorbents for CH4. They observed weak interactions and charge transfer from intrinsic G to CH4. The authors noted that the affinity of methane for graphene surfaces could, however, be promoted by doping the Ni atom, setting a single vacancy defect, and/or adding oxygen-containing functional groups into the G surfaces.

#### **3. Application of graphene in electronic technology: bandgap engineering**

The unique electronic properties of G means that G could continue to attract attention in potential electronic and photonic applications. The charge carriers in G behave, such as massless Dirac fermions, and G shows ballistic charge transport ideal for circuit fabrication. However, the lack of appreciable bandgap around the Fermi level, which is the key concept for semiconductors critical for controlling electronic conductivity, restricts its applications in nano-electronic and nanophotonic devices. Recent interests in G were, therefore, inspired by its promise in fashioning two-dimensional semiconductor materials with tunable bandgap lending itself to diverse uses. For this, several bandgap engineering methods have been proposed and Berashevich & Chakraborty [27], for example, described adsorption of water and other gases on G and showed that the molecules behaved like defects on nanoscale G surface facilitating bandgap tunability and permitting magnetic ordering of localized states at the G edges. This showed that the molecules pushed the wave functions corresponding to α -spin (up) and β -spin (down) G states to the opposite (zigzag) edges breaking the sublattice and molecular point group symmetry of the material. The wave-function displacement was controlled by the adsorbed molecule lending itself to tunable bandgap opening.

Accordingly, Balog et al. [28] demonstrated a G bandgap tunability using atomic hydrogen adsorption on the Moiré superlattice positions of G supported on an Ir(111) substrate. Then Takahashi et al. [29], while performing high-resolution angle-resolved photoemission spectroscopy of oxygen-adsorbed monolayer G on 6H-SiC(0 0 0 1), found that the energy gap between the π and π-bands increased with oxygen adsorption. This led to a systematic shrinking of π-electron Fermi surface and highlighted the potential of monolayer G oxidization in its bandgap inducement and tunability. On their part, Yavari et al. [30] engineered a tunable G bandgap of ≈ 0.2 eV by reversible H2O adsorption and showed that the energy gap decreased to ≈ 0.029 eV when humidity was reduced to zero.

According to Şahin & Ciraci [31], bonding of a single Cl atom in G occurs by ionic interaction through charge transfer from G to Cl. This results in a stable direct bandgap semiconducting structure with tunable bandgap controlled by applying uniform strain. Then, by examining structural and electronic properties of F2 adsorption between the G bilayers, Shayeganfar [32] observed that charge transfer between F2 and G, and the presence of sp2 and sp3 orbitals of C∙C and C∙F bonds disrupted the G layers symmetry inducing an energy gap that depended on F orientation on the G surfaces. The adsorption of F2 between the G bilayers also led to linear behavior between dipole moment and the energy gap and its electronic properties. This suggested existence of a tunable bandgap controlled by surface functionalization of G. Also, Tayyab et al. [33] predicted that Br-doping of G could induce sufficient bandgap width for optoelectronic applications.

*The Graphene Surface Chemistry and Adsorption Science DOI: http://dx.doi.org/10.5772/intechopen.114281*

On the other hand, Quhe et al. [34] demonstrated a tunable bandgap opening attributed to G metal adsorption. They illustrated a single-gated field effect transistor (FET) based on Cu-adsorbed ABC-stacked trilayer G. There was a clear transmission gap comparable to a bandgap showing the promise of metal-adsorbed G as a channel in single-gated FET device. Furthermore, while examining the metal dopants' effect on the structural, electronic, and charge transfer mechanism of G using DFT calculations, Tayyab et al. [33] observed that substituting Al atoms into G lattice induced a bandgap of 0.40 eV, and further predicted values of up to 0.82 eV for Al- and Bedoped G.

Other workers have assessed band gap tuning effects of organic agents intercalated into G surfaces. Chang et al. [35], for instance, while investigating the effect of borazine (B3N3H6), triazine (C3N3H3), and benzene (C6H6) on G's electronic structure, found that the molecular adsorptions were accompanied by bandgap opening of up to 62.9 meV under local density approximation. The band opening scale was controlled by the adsorption site type, for C3N3H3 and the heterocyclic molecules were more effective in inducing bandgap opening than the monocyclic ones. Also, most stable configurations led to the largest bandgap opening at the particular adsorption sites, and the charge redistribution patterns controlled the gap on-and-off bandgap switching, which opened whenever the charge redistributed to the bridge site position. This suggested that ionic ability of the dissimilar atoms in the heterocyclic molecules controlled the charge redistribution and the gap tuning efficacy in G. Elsewhere, Hildebrand et al. [36] proposed, a model of self-assembly of halogenated carbene layers on G. They predicted a tunable bandgap opening controlled by adsorbates' self-assembly and surface coverage, which provided for a mechanism for modulated engineering of G electronic structure and its application to electronic technologies.

Thus, graphene is a gapless semiconductor that cannot be used in optoelectronic applications, such as solar cells in its pristine form. This necessitates for its bandgap tuning, which can be achieved by adsorptive doping with different species atoms including simple molecules, metal atoms or organic molecules etc.

#### **4. Application of graphene in water adsorption**

The laminar G structure confers unique physicochemical attributes usable in a wide range of applications to G, which are relating to material hygroscopy [37]. According to Liu et al. [38], the oxygen content of GO materials controls its H2O adsorption capacity, which diminishes with increasing levels of GO reduction. Therefore Lian et al. [37] have reported high water uptake capacity of 0.58 g/g for GO. Also, they found that the adsorption-desorption kinetics of H2O uptake by GO was 5-times higher due to high capillary pressure in the GO laminates and micro-sized tunnel-like wrinkles in the GO surfaces. As already reported elsewhere [38], the reduction of GO diminishes its oxygen content to form reduced graphene oxide, rGO [39]. Water adsorption onto rGO is, however, enhanced at high humidity due to water vapor multilayer formation at the rGO surface [38]. Thus, GO is a loosely defined G material containing variable oxygen content, which is dependent on conditions of its production, and it is generally characterized by high capacity to sorb and store H2O molecules.

So, Leenaerts et al. [40], while investigating H2O adsorption on pristine G using first-principles calculations, indicated that there were four possible orientations of H2O molecule on G surface. Water can adsorb, starting from the O atom, with the

H∙O bonds pointing up (u), down (d), or parallel to the G surface (n) or based on another orientation (v) with one O∙H bond parallel to the surface while the other pointing to the surface. The energy of H2O adsorption depends on its orientation in the G surfaces and on the site of adsorption. The energy difference between the various configurations is 5–6 meV between the orientations, and about 1–2 meV between the positions. It was noted that H2O acts as an acceptor on G and the acceptor character (C, v) was the most energetically favored. So, Wehling et al. [41] while investigating the G electronic properties after water adsorption examined the effects of SiO2 substrate and found that perfect suspended G was unresponsive to H2O adsorption because doping required highly oriented H2O clusters. However, adsorbed H2O molecules shifted the SiO2 substrate's impurity bands and changed their hybridization with the G bands. So, the H2O molecules led to doping of G supported on SiO2 for lower concentrations than those for the free suspended G. This effect was, however, dependent on microscopic substrate properties.

Sanyal et al. [42] found that certain molecules interacted with divacancy in G layers, but, for H2O molecules, the large DOS in the vicinity of the Fermi level was absent. Accordingly the authors observed a pseudo-gap at the Fermi level, which was comparable to that for pure G. The DOS of the H2O-adsorbed system was intriguing and the peaks in the C and O atom projected DOS occurred around 4 eV above the Fermi level. Ma et al. [43] determined the H2O adsorption energy on G using the quantum Monte Carlo and random-phase approximation. Elsewhere, Hamada [44] while using the van der Waals density functional (vdW-DF) to study water interaction with G highlighted the promise of G in water adsorption applications and indicated that the adsorption potency of the materials was controlled by their the intrinsic properties, mode of preparation, and surface modifications employed.

#### **5. Application of graphene in atmospheric detection and amelioration**

#### **5.1 Adsorption of nitrogen species**

Emmisions of nitrogen compounds, especially the oxides, NOx, present one of the major environmental concerns today. NOx gases are responsible for various significant environmental problems and they contribute to the poor of human health by resulting in lung dysfunction and respiratory complications. Application of environmentally friendly, mild, and low-cost methods in mitigating presence of NOx in the environment is desirable both for a healthier and more eco-friendly environment [45]. The affinity of G surfaces for N-species has attracted the attention of a significant number of researchers. Sanyal et al. [42] studied G having divacancy defects on its surface, as an eco-friendly, mild, and inexpensive adsorbent for N2 and found that N2 molecule chemisorbed onto G in plane with the G surface with the two N atoms substituting in the usual positions of the G hexagonal network as substitutional impurity with chemisorption energies, *E*ch, of −4.53 eV. The N2 inclusion in the G plane resulted in a perfect honeycomb lattice with electrons doped into the G's conduction.

Elsewhere, Leenaerts et al. [40] proposed two orientations for NH3 adsorption on G. In one orientation, NH3 adsorbs with the H atoms pointing away from the G surface (*u*) and in the other with the H atoms pointing to the surface (*d*). This indicated

that both the adsorption site and the orientation influenced the adsorption energies of NH3 but charge transfer was determined only by the surface orientation of the NH3 molecule. The authors found that there was a small charge transfer of 0.03 eV from NH3 to the G surface in the *u* orientation but almost no charge transfer in the *d* orientation. The *u* orientation, which was consistent with the donor character observed elsewhere [46], was therefore more energetically favored. So, Zhou et al. [47] then compared NH3 adsorption on pristine G and on transition metal (TM) doped G (TM– graphene) and confirmed that the lowest-energy configurations for NH3 adsorption on pristine and TM–graphene were through the N atom. However, for TM–graphene, NH3 bonds *via* the N atom to the metal atom, by coordinating N lone-pair orbital with the metal valence bands.

It has been stated that adsorbates with a magnetic moment result in a larger doping [48]. So, Leenaerts et al. [40], while examining NO2 adsorption on G and proposed that adsorbed NO2 molecule can adopt three orientations on G. Thus, starting from the N atom, the NO2 molecule can adsorb on G with the N∙O bonds pointing up (*u*), down (*d*), or parallel to the G surface (*n*). The authors found that the total G and NO2 magnetic moment in the (*B*, *d*) orientation was 0.862 μB, and the corresponding charge transfer from G (M = 0 μB) to NO2 (M = 1 μB) was 0.099 e. The orbital mixing, therefore, led to a charge transfer of ±0.039e from NO2 to G but, when compared with NO, the charge transfers for the latter were an order of magnitude smaller. Accordingly, Zhou et al. [47] have stated that the NO2 molecule can bind to G surfaces through the N atom (nitro configuration), through an O atom (nitrite configuration), or through both O atoms (cycloaddition configuration). Thus, for pristine G, NO2 bonds by the cycloaddition configuration at the bridge site position with adsorption energy (Ead) of ~0.18 eV and charge transfer of 0.2e to NO2. However, for TM–graphene, the cycloaddition configuration is preferred with enhanced Ead ∼ 2 eV, followed by the nitro configuration with Ead ∼ 1.8 eV. This is by a large charge transfer of ∼0.6e from G to NO2. However, Tang and Cao [49] found that the adsorption of NOx on GO was stronger than that on G leading to enhanced binding energies and charge transfers from NOx to GO through chemisorption interaction and with transition of the doping properties of NO2 and NO from acceptor to donor character. Therefore, the interaction of NOx with GO forms H-bonds OHO (N) between -OH and the NOx besides other covalent bonds, including C‒N and C∙O. This is also accompanied by H abstraction to form nitrous acid- and nitric acid-like moieties. The spin-polarized DOS revealed hybridization of frontier orbitals of NO2 and NO3 with the electronic states around the Fermi level of GO resulting in strong acceptor doping and remarkable charge transfer characteristics from the molecules to GO.

Elsewhere, Dai et al. [50] evaluated NO and NO2 adsorption on G modified with B, N, Al, and S dopants using DFT. They found that both B and N atoms retained their planar configuration on G surface but both Al and S atoms protruded out of the G plane. Nevertheless, both NO and NO2 molecules were bonded to B-doped G but only NO2 was adsorbed on S-doped G surfaces. The Al-doped G was, however, most reactive toward the gases although B- and S-doped G presented the most plausible prospects for practical NO and NO2 gas adsorption and detection. So, the most relevant work to environment detection and remediation has been dedicated to NOx molecules mobilization on G, TM-graphene, and GO, but GO. Nonetheless, both TM-graphene and GO displayed enhanced NOx adsorption compared to pristine G adsorbents.

#### **5.2 Adsorption of carbon oxides**

Controlling toxic gas emissions is the foremost goal of the twenty-first century environmentalists. The adsorption technique offers a credible approach to dealing with these challenges and many carbon nanostructures, due to their unique surface morphologies and divergent potential for modification, have shown promise in fashioning plausible CO2 sequestrators [19]. So, Mishra & Ramaprabhu, [51] reported high CO2 adsorption capacity of 21.6 mmol/g for G at an 11-bar pressure and 25°C, based on a physisorption process. According to Szczęśniak et al. [19] polymers- and/ or metal species-modified G nanomaterials possess high specific-surface areas and tailored surface attributes, which confers effective adsorbent properties for CO2 and other gases.

Several scientists have focused their research on the mechanism of molecular adsorption onto G surfaces. While investigating adsorption of several molecules on G, Leenaerts et al. [40] showed that CO molecule assumed three different orientations on G, two of which involve the CO molecule in perpendicular position to the G surface, with the O atom toward the C atom (u) or away from the carbon (d), and one orientation with the CO molecule parallel to the G surface (n). In this case, CO acts as a donor molecule with charge transfer governed by the orientation assumed by the molecule on the G plane. Nevertheless, minor variations in charge transfer results due to differences in orbital overlap between the HOMO of the CO molecule and G. So, the C and O atoms remain bonded to each other, while CO is bonded to two pairs of G's carbon atoms. However, the CO molecule does not align perfectly in the plane of G nanosheets so that the C atom of the CO molecule stays in the G plane, making the usual hexagonal planar structure while the O atom stays out of plane. So, Zhou et al. [47] found that the most stable adsorption sites for CO on pristine G involved physisorption interactions at the hollow site, and the CO molecule preferentially assumes a parallel orientation to the G surface at an adsorption distance of ~3.6 Å and adsorption energy of ~0.017 eV. For TM–graphene, however, CO bonds to the C atom bonding the metal atom with enhanced adsorption energy and charge transfer accompanied with shortened adsorption distance. Intriguingly, the magnetic moment of TM-graphene gets enhanced from 1 μB (without adsorption) to ~3 μB.

According to Lee and Kim [52], CO2 chemisorption energies on graphene-**C40** at high pressure are 71.2–72.1 kcal/mol for the lactone systems are controlled by C∙O orientations at the UCAM-B3LYP level of theory. Nonetheless, the physisorption energies of CO2 on G are only 2.1 and 3.3 kcal/mol at the single-point UMP2/6-31G\*\* level of theory for the perpendicular and parallel orientations, respectively.

#### **5.3 Dioxygen and ozone molecules**

Ozone and other reactive oxygen species play an important role in atmospheric photochemistry of pollutants. Detection and removal of reactive oxygen species is, therefore, of essence. Dai et al. [50] analyzed the adsorption of several gases on G and showed that O2 molecules adsorbed well onto B- and S-doped G samples. On the other hand, Sanyal et al. [42], have suggested that O2 adsorbs on G by dissociating the two O atoms, which bond to two pairs of C atoms around divacancies in the G structure. The O atoms get oriented in opposite directions out of plane with the G surface. They, however, retain their bond length as in the isolated dimolecular state. In the absence of divacancy, however, O2 adsorbs on pristine G *via* the hollow site with low adsorption energy of <0.1 eV and with the O∙O bond oriented perpendicularly to

the G surface at an adsorption length 3.7 Å [47]. The length of the O∙O bond remains almost the same as in the free dioxygen molecule (1.24 Å). The Bader charge population analysis showed that the adsorption involved weak physisorption interaction. For metal-embedded G, however, O2 bonds with the O∙O bond parallel to the G plane with increased adsorption energies (> 1 eV) and decreased adsorption lengths (~2 Å). This is accompanied by charge transfer for O2 adsorbed on TM–graphene samples and a large expansion in the O∙O bond of 1.44 Å.

Lee et al. [53], who investigated O3 adsorption on G using the *ab initio* DFT, found that O3 molecule adsorbed on G basal plane with binding energies of 0.25 eV *via* physisorption interaction before it undergoes G-surface epoxidation with release of an O2 molecule. The activation energy barrier for the physisorption to chemisorption reaction is 0.72 eV, and the binding energy of the chemisorbed state is about 0.33 eV. Ozone adsorption on G is, thus, a gentle reversible reaction relevant to covalent functionalization of the G basal plane.

#### **5.4 Adsorption of sulfur species**

Volatile sulphur compounds represent another class of species of relevance to environmental hygiene. Gao et al. [26] while using first principles based on DFT method , discussed in depth the stable configuration, adsorption energy, DOS, and charge transfer of H2S adsorption on intrinsic G, GO, Ni-doped G, and vacancy defect G samples. They observed weak adsorption and charge transfer for intrinsic G, which was enhanced by introducing oxygen-containing functional groups, doping with Ni atom, and setting its single vacancy defect. The single vacancy defect, in particular, promoted G interaction with H2S molecule imparting excellent adsorption performance for H2S molecule relevant to studying G-based sensors for the gas.

In another study, Shao et al. [54] investigated SO2 adsorption on intrinsic G and on heteroatom-doped G samples containing B, N, Al, Si, Cr, Mn, Ag, Au, and Pt atoms using first principles based on DFT. They reported that the structural and electronic properties of the adsorption adducts of the molecules depended on the dopant type. SO2 adsorbed weakly on intrinsic G, on the G samples doped with nonmetals B-, and N. However rapid and strong SO2 chemisorption occurred on metal and semimetal Al-, Si-, Cr-, Mn-, Ag-, Au-, and Pt-doped G. The analyses of the adsorption mechanisms showed the sensitivity of G-based SO2 sensors was enhanced by introducing appropriate dopants with both Cr and Mn giving particularly promising results.

#### **5.5 Halogens and halides**

In Section 3 of this chapter, we discussed the role of Cl2 and F2 molecules in tuning the G bandgap. Şahin &and Ciraci [31] found that a single Cl atom bonding in G occurs by ionic interaction *via* charge transfer from G to Cl with local distortion in the underlying G layer. Meanwhile, a single Cl adatom migration on perfect G surfaces occurs unhindered but Cl adatom accumulation on the G surface to produce various conformations becomes unsustainable because strong Cl∙Cl interactions occur leading to desorption and formation of Cl2 molecules. Fully chlorinated G with single Cl atoms bonded alternately to each carbon atom from opposite sides of the G sheets with sp3 -type covalent bonds is buckled. So, Shayeganfar [32] discovered that F2 adsorbs on G *via* mixed mechanisms involving both in-plane and out-of-plane molecular orientation on the G surface.

Consequently, Chen et al. [55] presented a detailed Br2 adsorption and charge transfer study of G by combining *in situ* Raman spectroscopy and DFT techniques. They observed that when G is encapsulated in hexagonal boron nitride (h-BN) layers on either sides, the G surface is protected from Br2 doping, but when G is supported on only one side by h-BN layer, it undergoes strong hole doping by adsorbing Br2. The authors obtained molecular adsorption isotherm by plotting surface coverage versus pressure using a combination of Raman spectra and DFT calculations. They saw that the adsorption data fitted the Fowler-Guggenheim model with an adsorption equilibrium constant of ∼0.31 per Torr. The repulsive lateral interaction between adsorbed Br2 molecules was ∼20 meV, whereas the binding energy for the Br2 molecule was ∼0.35 eV. At the monolayer coverage, each Br2 molecule accepts 0.09 e<sup>−</sup> of charge from single-layer G, but when the adsorbent was supported on SiO2 instead of h-BN, a threshold pressure was observed for which the diffusion of Br2 along the SiO2/G interface resulted in Br2 adsorption on both sides of the SiO2-doped G.

On the other hand, Sun et al. [56], while studying HF molecules' adsorption on intrinsic and on Al-doped G by first-principles calculations, found that HF adsorption mechanisms were different for the adsorbents. The Al-doped G depicted greater adsorption energy and stronger interactions with HF molecule than the intrinsic G. The calculated net electron transfers, electronic density difference images, and DOS provided evidence that HF adsorption on Al-doped G was a chemisorption process, while the molecule's uptake by intrinsic G was physisorption.

#### **6. Adsorption of alkali metals**

Atomic-level investigations of metal adsorption and migration on G surface offer a plausible way of dealing with the challenges of developing novel alkali metal battery technologies [57]. Yang [58] pioneered work in which they reported that Li atoms bonded on a G layer interchangeably on both sides of the nanosheets by distorting the relative positions of the C atoms in the G honeycomb plane. They compared the results with graphane obtained by hydrogenation in which each C is pulled out of the plane by H and found that for Li-doped G, the carbon atom is pushed off by the attached Li instead. This resulted in a conducting counter-intuitive structure. Xue et al. [59], then, studied the adsorption of single Li and the formation of Li clusters on G using first-principles including van der Waals interactions. They found that while Li can exist on the surface of defect-free G under favorable conditions, the bonding was weaker and the surface concentration lower than in the metal adsorption on graphitic surfaces. At low concentrations, Li ions spread out on G surfaces due to coulombic repulsions between the adsorbing metal ions. However, as the Li content on the G surfaces increased, small Li clusters formed so that even though G has higher ultimate Li adsorption capacity than graphite, the nanoclusters nucleate Li dendrites, leading to failure. So, while studying Li adsorption using first-principles DFT calculations and surface diffusion on pristine and defect G structures, Fan et al. [60] observed that Li/C ratios lower than 1/6 for the single-layer G were energetically favorable. Further more, the existence of vacancy defects increased the Li/C ratio of the material. For double-vacancy and higher-order defects, however, Li ions diffused more freely in the direction perpendicular to the G sheets boosting the diffusion energetics in turn.

Indeed, Jin et al. [61] compared alkali metals; adsorption on single-layer G using first principles and observed a common trend in binding distance, charge transfer,

#### *The Graphene Surface Chemistry and Adsorption Science DOI: http://dx.doi.org/10.5772/intechopen.114281*

and work function relating to increasing metal adsorption proportion ρ (adatom/C atom) on the G surface. There was a dip in the properties at ρ ≈ 0.04 for all metals except for Li, for which the dip occurred at ρ ≈ 0.08. This represented the transition of the adsorbed metals from individual atoms to two-dimensional metallic sheets exerting a depolarizing effect. Thus, G exhibited asymmetric function showing dependence on ρ with a dip on the adatom layer side and saturation on the G side, which was different from the case of bulk graphite.

Therefore, in order to appreciate the influence of G point defects on its Li adsorption, Zhou et al. [62] studied the uptake and diffusion of Li on G with divacancy and Stone−Wales defect using first-principles calculations. They found that in the presence of divacancy, Li adatom adsorbed on the hollow site above the center of an octagonal ring rather than on the top site of carbon atoms next to vacancy site. For the Stone−Wales defects, however, the Li atom adsorbed on the top site of the carbon atom in a pentagonal ring shared by two hexagonal rings. This resulted in buckling of the G sheet. In the case of both the divacancy and Stone−Wales defects, the interactions with Li adatom were attractive and the analysis of both the difference charge density and the Bader charge showed a significant charge transfer from Li adatom to the adjacent carbon atoms. Yang et al. [63], then, investigated sodium adsorption and intercalation into the G bilayers using DFT calculations. They systematically assessed the specific capacity, voltage, and migration energy barriers for Na storage in pristine and mono-vacancy defective bilayer G using DFT. The authors found that with an appropriate voltage (>0.5 V), the mono-vacancy defects improved the specific capacity from 123.97 to 382.54 mAh/g. Thus, Na<sup>+</sup> ions transported from the defect-free regions (low energy barriers, 0.15–0.32 eV) to defect regions (large energy barriers, 0.56–0.59 eV) and decelerated. It demonstrated that a defect bilayer G was a promising material for making negative electrodes of the Na-ion batteries. Nonetheless, according to Olsson et al. [57], Li and K adsorption on pristine G was more favorable than Na adsorption. According to these workers, N- and O-containing defects dominated on the G surfaces, acting as metal trapping sites, and hindering the metal diffusion and migration on the G surfaces. This diminished the battery cycling performance.

Kim et al. [24] while studying the impact of intercalating G surfaces with alkali metal on its hydrogen uptake, however, found that G surface defects enhanced its metal binding energy and its dispersion and a considerable increase in binding energy was observed for alkaline earth metals. Furthermore, additional alkali metal adsorption of G was studied by Ataca et al. [25]. These studies could therefore give consequence to use of alkali-G material in metal storage devices.

#### **7. The graphene-transition metal doping**

Exploring new ways to diversify the basic properties of G continues to attract immense research interest. Transition metal-doping has also gained attention in the recent past, paving way to developing improved G materials with improved magnetism, and other fascinating properties with promise for diverse applications. Chan et al. [64], for instance, explored the adsorption of 12 metal adatoms on G using first-principles and computed the adsorption energy, geometry, DOS, dipole moment, and work function of each adatom-G system. They found that the adsorption groups I-III elements on G aligned with ionic bonding and it was characterized by large charge transfer and minimal change in the G electronic states. However, for the TMs, noble

metals, and group IV metal, the adatoms interacted covalently with G surfaces leading to strong hybridization between adatom and G electronic states. Dipole moments across individual adatoms but the work-function shift correlated to the induced interfacial dipole of the G-adatom system and to the ionization potential of the isolated atom. According to Hu et al. [65], chemisorption of 3*d* and noble metals adatoms on G occurs at the bridge site and at the top site with hybridization between the adatom and the G electronic states, which resulted in distorted G layer. The half-filled 3*d* shell metal atoms as well as Zn, Ag, and Au atoms depicted low adsorption energies with a general decrease in the magnetic moment compared to the corresponding free adatoms. This was attributed to charge transfer and electron shift between the states of the adatom.

Elsewhere, Sevinçli et al. [66] while assessing the electronic and magnetic properties of 3*d* TM atoms' adsorption on G and on G-nanoribbons, reported metal-dependent binding energies of 0.10–1.95 eV. The metals' adsorption imparted magnetic and metallic properties of G with armchair edge shapes (AGNR's) of adsorbed TM yielding minimum energy states. The resulting nonmagnetic semiconductor AGNR assumed metal/semiconductor and ferromagnetic/antiferromagnetic spin alignments. So, Cao et al. [67], while studying the geometries, electronic states, and magnetic properties of TM adatom and dimer adsorption on G, found that, except for Cu, chemisorption interactions yielded the most stable adsorption and dimerization on G surfaces controlled by exchange-correlation.

According to Valencia et al. [68], who modeled G and (8,0) single-walled carbon nanotubes (SWCNTs) functionalized with 3*d* TM atoms, the 4 s occupation with Pauli repulsion was responsible for the Cr, Mn, and Cu physisorption behavior of G. Using a new physical model involving coulomb interaction, 3dn 4sx → 3dn + x electronic promotion energy and occupation of the 1e2(δ), 2e1(π), and 2a1(σ) metal orbitals, the authors found that Sc, Ti, Fe, and Co metals were present in the G surface as isolated individual atoms but all other 3*d* TM atoms diffused with clustering. In another work, Amft et al. [69] conducted density functional investigations on Cu, Ag, and Au adsorption on pristine G. While accounting for van der Waals (vdW) interactions using the vdW-DF and PBE + D2 methods, the authors analyzed the favorable adsorption sites, the adsorption-induced distortions in G sheets, and adatom diffusion paths and found from the vdW schemes that the three metal atoms adsorbed on the G sheets with buckling of the G layer. Only the results for Ag qualitatively differed from those obtained from generalized gradient approximation, which gives no binding for it. Otherwise, the results for the rest of the metals showed quantitative variations for the vdW-DF and PBE + D2 models.

Thus, by applying DFT calculations for elements of atomic number of 1–83, Nakada and Ishii [70] focused their work on the adsorbed adatoms' migration on G and showed that, adsorption favored the H6-site for the metals and the B-site for the nonmetals. Nonetheless, the migration energy was particularly high for the 3*d* TMs and for certain nonmetals. In the same way, Liu et al. [71], while applying first principles, noted that the H6 was the favored site for the rare earth adatoms of Nd, Gd, Eu, and Yb adsorption. The adsorption energies and the diffusion barriers of Nd and Gd were, however, larger than those of Eu and Yb, and all the adatoms induced significant electric dipole and magnetic moments in the adsorption complex. Eu formed flat islands on G attributed to its low diffusion barrier and large adsorption energy compared to the bulk cohesive energy. Nonetheless, the adsorption of Nd and Gd adatoms led to in-plane G lattice distortions.

Vacancies or defects in G nanostructures present sites of altered chemical reactivity and open possibilities for tuning G properties by defect engineering. The

*The Graphene Surface Chemistry and Adsorption Science DOI: http://dx.doi.org/10.5772/intechopen.114281*

understanding of the chemical reactivity of G defects is, thus, critical in implementing carbon materials in several advanced technologies. Pašti et al. [72] investigated atomic adsorption on G surfaces with single vacancy (SV) using DFT analyses of elements. They based their calculations on PBE, long-range dispersion interaction-corrected PBE (PBE + D2 and PBE + D3), and nonlocal vdW-DF2 functionals. They found that most elements, except groups 11 and 12 elements and the noble gases for which the contribution of dispersion interaction was most significant, bonded to the vacancy sites with interaction strengths that correlated to the cohesive energy of the elements in their stable phases. As most atoms could be trapped at the SV site, the calculated dissolution potentials showed that the adsorbed metals became more "noble" than they were in their respective stable phases. Then, Malola et al. [73] reported that Au adatoms adsorbed in-plane at G double-vacancies with diffusion barriers >4 eV and < 2 eV at the larger vacancies in line with the results earlier reported by Gan et al. [74].

#### **8. Graphene adsorption of organic molecule**

G materials present attractive pore volume, high conductivity, rich surface chemistry, and exceptionally large aspect ratio, among other promising properties, relevant to adsorptive catalytic applications in organic chemistry [5].

#### **8.1 Pristine graphene**

According to Lazar et al. [75] acetone, acetonitrile, dichloromethane, ethanol, ethyl acetate, hexane, and toluene adsorb on G surfaces with adsorption energies that range from 5.9 to 13.5 kcal/mol. The interaction strength of the organic molecules with G surface sites and the corresponding adsorption enthalpies were controlled by London dispersive forces, which dominated (∼60%) the interactions. Basing on DFT calculations, Chi & Zhao [76] investigated formaldehyde (H2CO) adsorption on intrinsic and Al-doped G. They found that the process was characterized by high binding energies and short connecting distances controlled by chemisorption interactions between the adsorbing molecule and G surfaces. The DOS showed the presence of orbital hybridization between H2CO and Al-doped G, but there was no evidence of hybridization between H2CO molecule and intrinsic G surfaces. So, Cortés-Arriagada [77] analyzed 1,4-dioxane adsorption on Al-, Ti-, Mn-, and Fe-doped G and found that the metal-doped G displayed enhanced interaction with 1,4-dioxane molecules compared to intrinsic G. The adsorption energies (1.2–1.6 eV), in this case, were accompanied by changes in the electronic structure of the substrates, especially for the Mn and Fe dopants. The *ab initio* dynamics simulations showed stable adsorbent– adsorbate interactions of highest aqueous stability of interaction for Al- and Fe- followed by the Mn- and Ti-doped G, respectively.

While applying 50 mg/L initial adsorbate concentration at pH of 6.3 and 285 K reaction temperature, Li et al. [78], on their part, reported G adsorption capacity of 28.26 mg/g for phenol. The phenol adsorption equilibrium simultaneously fitted the Langmuir and the Freundlich isotherms while the adsorption kinetics could be described by pseudo-second order kinetics model. Elsewhere, Xu et al. [79], reported that aqueous bisphenol A (BPA) adsorption on G is also spontaneous and exothermic. According to the authors, the reaction was the Langmuir isotherm and the pseudosecond order kinetics with a high BPA adsorption capacity of 182 mg/g reported at 302.15 K. They suggested that both π-π interactions and H-bonding contributed to BPA immobilization into G. Notwithstanding, Pei et al. [80] investigated 1,2,4-trichlorobenzene (TCB), 2,4,6-trichlorophenol (TCP), 2-naphthol and naphthalene (NAPH) adsorption on both G and GO by applying batch equilibrium technique and micro-Fourier transform infrared spectroscopy. The results showed that the adsorption isotherms for the four aromatic compounds were nonlinear, which indicated that both general hydrophobic interactions and specific interactions played part and the four aromatic compounds depicted similar adsorption efficiencies at pH 5.0. In the case of G, however, greater adsorption efficacy of 2-naphthol displayed under alkaline conditions due to π-π interactions resulting from the higher π-electron density of anionic 2-naphthol than that of neutral 2-naphthol. For GO, however, the affinity for the four compounds was in the order of: NAPH < TCB < TCP < 2-naphthol, and the FTIR spectroscopy revealed dominant π-π interaction for TCB, TCP, and 2-naphthol adsorption on G.

#### **8.2 Graphene oxide**

According to Pei et al. [80], high GO adsorption for both TCP and 2-naphthol was attributed to existence of H-bonding between the adsorbates and O-containing groups in GO. However, Chen and Chen [81], while studying *m*-dinitrobenzene, nitrobenzene, and *p*-nitrotoluene adsorption on GO, rGO, and G-nanosheets using IR spectroscopy, observed that the hydrophilic GO displayed inferior adsorption capacity for the three nitroaromatic compounds (NACs). This was ascribed to the greater hydrophobic π-conjugation of the active sites in the adsorbents. Even so, greater adsorptions observed for rGO over those of pristine G were linked to the π-π electron donor-acceptor interactions between the NACs phenyls and the π-electronenriched G lattice, and to charge electrostatics and the polar interactions between G defects and the NACs nitro groups groups. Accordingly, while investigating bisphenol A, nitrobenzene, phenol, benzoic acid, and salicylic acid adsorption on GO, Tang et al. [82], also opined that the aromatic compounds' π-stacking ability controlled the adsorption processes of the compounds. So, Wang & Chen, [83] compared the adsorption and co-adsorption of naphthalene, 1-naphthol and Cd2+ on graphene oxide (GO), chemically-reduced graphene (CRG) and annealing-reduced graphene (ARG). They found that CRG had superior adsorption capacity the adsorbates than either ARG and GO. This, together with observation that the affinity of 1-naphthol for the adsorbents was greater than that of naphthalene was attributed to additional n-π electron-donor-acceptor (EDA) interactions between the 1-naphthol -OH groups and the electron-depleted G sites besides the π-π interactions. Furthermore, 1-naphthol uptake, by both the CRG and the ARG, had a maximum near the respective pKa values, which was consistent with the n-π EDA interaction mechanism. However, GO with greater surface functional groups density than both CRG and ARG, displayed greater affinity for Cd2+. This somehow enhanced naphthalene and 1-naphthol co-adsorption on GO and CRG through surface-bridging cation-π interactions. Nonetheless, somewhat suppressed naphthalene co-adsorption with Cd2+ on ARG was attributed to the sieving effect of hydrated Cd2+ on the micropore edges of ARG.

#### **8.3 Reduced graphene oxide**

Meanwhile, Yu et al. [84] synthesized rGO and utilized it in benzene and toluene adsorption under dynamic conditions. The synthesized rGO samples displayed maximum adsorption of 276.4 and 304.4 mg/g for benzene and toluene, respectively. In a comparative study, greater adsorption capacities and breakthrough times were

observed for rGO than GO, and the spent rGO was readily regenerated by heating at 150°C. So, Wang et al. [85], while investigating effect of change in concentration of oxygen-containing groups in rGO observed that higher degrees of reduction of GO enhanced the interactions between the π system of G and the π unit of the phenolic molecules. Thus, the adsorption, which was an exothermic and spontaneous process, was promoted by enhancing GO reduction and introducing electron-donating/withdrawing functional groups on the rGO benzene ring.

#### **9. DNA adsorption and bio-sensing tests**

Interfacing DNA oligonucleotides with G materials has paved the way to the production of diverse new sensor devices. GO is an outstanding fluorescence quencher and fluorescently-labeled DNA molecules get quenched when they adsorb on GO. However, when a complementary DNA (cDNA) is introduced, it desorbs the probe DNA molecule from GO, and the fluorescence is restored. This permits for devising GO-based DNA optical sensors [86]. This also provides a fascinating topic for biointerface science. DNA adsorb on GO *via* π–π stacking and H-bonding while overcoming electrostatic repulsion. The mechanism by which cDNA induces probe DNA desorption from G is, however, still a subject of intriguing discussion [87].

Whereas the analytical aspects of this phenomenon have been demonstrated extensively, the fundamental understanding of the binding mechanisms has lagged behind. Wu et al. [86] therefore studied 12-, 18-, 24-, and 36-mer single-stranded DNA adsorption on GO as a function of DNA length, pH, ionic strength, solvents, temperature, and cDNAs. They found that faster more robust DNA adsorption on G was experienced using shorter DNAs or cDNAs, at low pH, high ionic strength, appropriate solvents, and optimum temperatures. At the same time Lu et al. [39] compared DNA adsorption of GO and rGO using fluorescently labeled DNA and showed that, under similar conditions, DNA adsorbed with a 2.6-fold higher capacity on rGO than on GO. However, the corresponding GO systems showed a higher absolute rise in sensing and signaling kinetics.

#### **10. Application of graphene materials in studies on remediation of water pollution**

Environmental pollution is one of the leading global challenges of the twenty-first century and the removal of various pollutants from the environment is of essence [88]. Adsorption methods are effective in the remediation of water and air pollution, but their efficiencies depend on intrinsic porosity, surface characteristics of the adsorbent, and on the applied conditions. Graphene lends itself to tailored surface modification suited for wide range of application in water and atmospheric remediation and pollution prevention.

#### **10.1 Heavy metals removal from water**

#### *10.1.1 Pristine graphene*

Many studies on environmental water amelioration have focused on individual [89] and competitive [90] adsorption of hazardous metals. Hao et al. [91] obtained SiO2/G

composite for selective removal of Pb(II) ions from water with 113.6 mg/g adsorption efficiency Cortés-Arriagada & Toro-Labbé [92], on the other hand, while investigating adsorption capacity of Al- and Fe-doped G for trivalent and pentavalent methylated arsenic using quantum chemistry computations, observed that trivalent methyl arsenicals adsorption was reached under neutral pH conditions with adsorption energies of 1.5–1.7 eV whereas for the pentavalent methyl arsenicals' the adsorption was achieved more optimally in acidic to neutral media with adsorption energies of 1.2–2.4 eV and 3.3–4.2 eV, respectively. The interacting σAs [sbnd] O bond weakening in the pollutant structure influenced the capacity and the stability of the adsorption complex.

#### *10.1.2 Graphene oxide*

Sitko et al. [93] while studying Cu(II), Zn(II), Cd(II) and Pb(II) adsorption on GO reported high adsorption capacities of 294, 345, 530, and 1119 mg/g for the metals, respectively. The metal affinities for GO surfaces, which was based on chemisorption reactions, was in the order: Pb(II) > Cu(II) ⪢ Cd(II) > Zn(II). Meanwhile, Wang & Chen [83] reported strong affinity of GO for Cd2+ ions, while studying Zn(II) adsorption on GO, Wang et al. [94] reported a high Zn(II) adsorption capacity of 246 mg/g, which was achieved in 20 min under adsorption conditions of neutral pH of 7.0, 2 mg adsorbent dosage, and 20°C temperature. The process had a Langmuir equilibrium constant was 5.7 L/g based on pseudo second-order kinetics [14]. Elsewhere, Lingamdinne et al. [95] elucidated Co(II) adsorption properties of GO and obtained a maximum adsorption of 21.28 mg/g while using medium pH of 5.0–8.0 and GO adsorbent dosage of 1.0 g/L. It was found that the adsorption process followed pseudo-second order [14] kinetics based on mixed reaction mechanism involving the π∙π bonds electrons interaction Langmuir, which was described by Langmur, Freundlich, and Temkin isotherms.

#### *10.1.3 Functionalized graphene oxide*

While investigating EDTA-functionalized GO (EDTA-GO) for aqueous Pb(II) removal, Madadrang et al. [96], reported high adsorption capacity of 479 ± 46 mg/g achieved at pH 6 in 20 min. They showed that experimental data fitted the Langmuir adsorption model and the EDTA-GO adsorbent was regenerated by washing in HCl. Yet, in another study, White [97] while evaluating aqueous Cu(II) adsorption on GO and on carboxylated graphene oxide (GO–COOH) under conditions of ambient temperature, pH of 6 and 60 min equilibration, Cu2+ removal reached 97–99.4%, with 277.77 and 357.14 mg/g maximum adsorption capacities for GO and GO‒COOH, respectively. The process, which was spontaneous and exothermic, was also described by the Langmuir-type adsorption equilibrium. Kumar & Jiang [98], at the time proposed a chitosan-functionalized GO adsorbent for aqueous arsenic adsorption and reported that arsenic oxyanions adsorption onto the novel adsorbent was facilitated via cation-π interactions, based on RNH3 + -aromatic π moieties; electrostatic interactions involving H2AsO4 − , HAsO4 2−–+ NH3R; intra- and intermolecular H–bonding; and on anion-π interaction based on the R-COO- -aromatic π moieties with the oxygencontaining groups on GO surfaces.

#### **10.2 Adsorption of nutrient ions from water**

On their part, Vasudevan & Lakshmi [99] evaluated aqueous phosphate adsorption on G at varying pH, ionic strength, and temperatures conditions and determined the G adsorption capacity for phosphate to be 89.37 mg/g using 100 mg/L initial phosphate concentration and 303 K temperatures. The time-dependent phosphate adsorption fitted the second order kinetics [14], while the adsorption equilibrium was a spontaneous, endothermic process governed by Langmuir adsorption mechanism.

#### **10.3 Dyes removal from aqueous solutions**

While investigating aqueous methylene blue (MB) adsorption on G, Liu et al. [100] found that the adsorption data followed the Langmuir isotherm with maximum adsorption capacity of 153.85 mg/g at 293 K. The corresponding kinetics fitted the pseudo-second order kinetics model [14], and the process was both spontaneous and endothermic. At the time, Yao et al. [101], testing the potential of magnetic Fe3O4@ Graphene composite for aqueous MB and Congo red (CR) removal, reported respective adsorption capacities of 45.27 and 33.66 mg/g for the dyes.

Wu et al. [102], then, utilized rGO and also evaluated MB adsorption alongside that of acrylonitrile (AN), p-toluenesulfonic acid (p-TA), and 1-naphthalenesulfonic acid (1-NA) and found that the larger molecules with greater number of benzene rings showed greater propensity to adsorb on GO than the smaller compounds. Accordingly, MB uptake by G was a π-π stacking process, which allowed for up to five adsorbent reuse cycles. The recorded MB, p-TA, and 1-NA adsorption capacities were ∼1.52 ∼ 1.43, and∼1.46 g/g, respectively. At the same time, Sun et al. [103] studied the enhanced GO for aqueous adsorption of acridine orange based on *in situ* GO reduction by sodium hydrosulfite and observed an improved GO adsorption capacity from 1.4 g/g to 3.3 g/g, which was based on sodium hydrosulfite conversion of carbonyl groups on GO surfaces into hydroxyl groups forming the principal dye adsorption sites in rGO.

Yan et al. [104], then, assessed aqueous MB adsorption on oxidized G samples with varying oxidation oxygen levels and found that GO samples displayed a fast and efficient pH independent adsorption process, which increased with the increasing oxidation of G surface. The MB adsorption behavior was transformed from Freundlich-type at low G oxidation levels to Langmuir-type at high oxidation levels. The binding features of MB-adsorbed GO also changed from parallel stacking of MB molecules on the graphitic plane through hydrophobic π-π interaction to a vertical standing interaction *via* groups forming MB surface adsorption. The adsorption efficiency of the regenerated GO displayed little loss up in efficacy to four cycles of reuse. Later, Molla et al. [105] when studying MB, rhodamine B, and methyl orange adsorption on GO samples, observed that, unlike negative dyes, which did not adsorb, the positive dyes (methylene blue and rhodamine B) adsorbed on GO surfaces through electrostatic interactions between their positive dipoles and the GO negative dipole of the oxygen surface groups. This process was fast and reached 97% and 88% in just 15 min, for MB and for rhodamine B, respectively. Accordingly, the *ab initio* molecular dynamics showed that favorable adsorption configuration was at 2298 fs for methyl orange and 2290 fs for MB but MB was more strongly (−2.25 eV/molecule) adsorbed than methyl orange (−1.45 eV/molecule).

At the time, Nguyen-Phan et al. [106] studied the role of GO in photocatalytic removal of MB by titanium dioxide/GO composites reporting enhanced adsorptionphotocatalysis for TiO2/GO composites compared to those of pure TiO2. Both the removal efficacy and the corresponding MB photodegradation increased with the GO proportion in the composite up to 10 wt%. The enhancement of these properties was attributed to the components' synergy in increasing the composite specific

surface area, π-π conjugation between the dye and the aromatic rings, and in the ionic interactions between MB and oxygen-containing functional groups in GO.

#### **10.4 Pesticides and other organic pollutants**

Cationic surfactants present a considerable water pollution problems. They are difficult to degrade, leading to persistence in water sources. Chen et al. [107] evaluated aqueous adsorption of a cationic dodecyl amine hydrochloride (DACl) surfactant on GO by analyzing its zeta potential and applying FTIR and X-ray photoelectron spectroscopy (XPS). They found that the adsorption equilibrium was consistent with the Freundlich isotherm while the adsorption kinetics followed pseudo-second order model [14]. Accordingly, they reported an adsorption process that was endothermic and consistent with electrostatic interaction and H-bonding between DACl and GO.

Then, Wuest & Rochefort [108] while studying amino triazines adsorption on G using DFT calculations reported strong adsorbate affinity for G, which they attributed to specific attractive interactions of -NR2 groups within the underlying surfaces as the driving force for the process. Afterwards, the authors [109] reported unprecedented aqueous GO and rGO adsorption of capacities of ∼1200, 1100, and 800 mg/g for chlorpyrifos (CP), endosulfan (ES), and malathion (ML) respectively. They noted that the process was mediated through water because direct interactions between G and the pesticides were weak and unlikely. However, the adsorption of the compounds was both pH- and background ions-independent.

#### **10.5 Pharmaceuticals and other emerging pollutants**

Pharmaceutical agents have been become common persistent pollutants in in many water bodies around the world [7]. The occurrence of such emerging contaminants in water bodies poses unique threats to living things. Conventional techniques of pharmaceuticals removal from water are complex, expensive and generate secondary hazardous residues. Due to inefficient wastewater management systems, new treatment technologies are needed to deal with new products for the safety of the environment [110]. The adsorption technique has gained popularity but the engineering of new adsorbents for emerging trace pollutants is desired. G materials have lent themselves to application in various fields due to their robust physicochemical attributes and several researchers are already exploring them in environmental remediation. Gao et al., for instance, investigated removal of three tetracycline antibiotics, including tetracycline, oxytetracycline, and doxycycline [111] from water using GO. They reported that the tetracycline antibiotic deposited on GO surfaces *via* the π-π and cation-π interactions. The adsorption data could be fitted the Langmuir and Temkin isotherms with Langmuir adsorption capacity of 313, 212, and 398 mg/g for three antibiotics, respectively. The kinetics adsorption data fitted the pseudosecond order kinetics model [14] with a sorption rate constant (k) of 0.065 g/mg/h. Nonetheless, the adsorption capacities of GO for tetracycline decreased with increasing solution pH and increasing Na+ concentration.

In another study, Nam et al. [112], while studying the adsorption behaviors of diclofenac (DCF) and sulfamethoxazole (SMX), evaluated the effect of GO dosage, contact time, pH, and sonication. They predicted binding energies between the drugs and GO surface groups and found that GO adsorption of the drugs was controlled by the oxygen-containing functional groups in GO, which exhibit negative surface charge over a wide range of pH values of 3–11. DCF showed favorable binding energy

*The Graphene Surface Chemistry and Adsorption Science DOI: http://dx.doi.org/10.5772/intechopen.114281*

of −18.8 kcal/mol compared to that of SMX, which was −15.9 kcal/mol. The removal efficiencies for the two agents reached 35 and 12% within the initial 6 h respectively. however they were strongly enhanced to 75 and 30% under sonication, respectively. This is because sonication facilitates dispersion of exfoliated GO particles diminishing their surface density of oxygen-containing functional groups. It was found that the equilibrium data for both adsorbates fitted the Freundlich model.

Zhu et al. [110], on the other hand, while testing GO for metformin removal from water, observed an initial rapid and efficient metformin uptake, which was also strongly temperature-, pH-, ionic strength-, and the background electrolytecontrolled. The GO optimum adsorption of metformin was achieved at pH 6.0 and 288 K. The authors then found that the adsorption process was both spontaneous and exothermic and suggested that both π–π interactions and H-bonds played the leading role. The adsorption kinetics showed that 80% metformin removal was achieved within 20 min with high-rate constants of k1 (0.232 per min) and k2 (0.007 g/mg/ min) for Lagergren first order [13] and pseudo-second order kinetic [14], respectively.

In the same way, Pavagadhi et al. [113] employed GO the removal of microcystin-LR (MC-LR) and microcystin-RR (MC-RR) –two common algal toxins in natural water. The adsorption experiments employed typical aqueous matrix containing environmental water anions and cations. The authors reported 1700 and 1878 μg/g GO adsorption capacities for MC-LR and MC-RR, respectively. The adsorption kinetics for the process were achieved within 5 min and the GO samples could be recycled up to ten cycles without significant loss in their adsorption potency.

#### **11. Conclusions**

G exhibits hydrophobic surfaces presenting high adsorption to chemicals that exhibit strong π-π interactions. GO, on the other hand, has a number of reactive functional groups with strong acidity suited for high adsorption for basic and cationic adsorbates. We find that the key application for G materials in the last decades have included hydrogen and other fuel gas adsorption and storage; alkali metal storage and battery technology; design and development of nano-electronic and nanophotonic devices; vacancy engineering for modifying the G surface reactivity for various application; G environmental application in water and atmospheric amelioration of both organic and inorganic pollutants; and in fashioning sensors and biosensors for most diverse applications. Suitable modifications of GO or G with nonmetals, metals, metal oxides, or organics can produce nanocomposites with enhanced adsorption capacities and separation efficiencies toward various groups of adsorbate species. The metals' adsorption, for example, imparts magnetic and metallic character to G. A number of molecular intercalations have also been found useful in inducing sufficient bandgap opening into G, which is critical for controlling electronic conductivity and opening up the materials to application in nano-electronic and nanophotonic devices among other applications.

*Chemistry of Graphene – Synthesis, Reactivity, Applications and Toxicities*

#### **Author details**

Enos W. Wambu Department of Chemistry and Biochemistry, University of Eldoret, Eldoret, Kenya

\*Address all correspondence to: ewambu@uoeld.ac.ke

© 2024 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Section 3
