A brief review of graphene-based material synthesis and its application in environmental pollution management

Graphene is an interesting two-dimensional carbon allotrope that has attracted considerable research interest because of its unique structure and physicochemical properties. Studies have been conducted on graphene-based nanomaterials including modified graphene, graphene/semiconductor hybrids, graphene/metal nanoparticle composites, and graphene-complex oxide composites. These nanomaterials inherit the unique properties of graphene, and the addition of functional groups or the nanoparticle composites on their surfaces improves their performance. Applications of these materials in pollutant removal and environmental remediation have been explored. From the viewpoint of environmental chemistry and materials, this paper reviews recent important advances in synthesis of graphene-related materials and their application in treatment of environmental pollution. The roles of graphene-based materials in pollutant removal and potential research are discussed.

Since the discovery of fullerene and carbon nanotubes (CNTs), research has focused on another allotrope of carbon, graphene. Graphene is a two-dimensional nanomaterial that has between one and ten layers of sp 2 -hybridized carbon atoms arranged in six-membered rings. It has many unique properties, including interesting transport phenomena, and high Young's modulus (approximately 1100 GPa) [1], fracture strength (125 GPa) [1], thermal conductivity (approximately 5000 W m 1 K 1 ) [2], mobility of charge carriers (200000 cm 2 V 1 s 1 ) [3], specific surface area (theoretical value of 2630 m 2 g 1 ) [4], and chemical stability. Its synthesis has been investigated in many different fields with potential applications in biomedicines, reinforced composites, sensors, catalysis, energy conversion, and storage devices. Importantly, it could be used in pollutant removal in environmental remediation, which has attracted increasing research in recent few years [5][6][7][8][9][10] and is the focus of this review.
Aggregation of graphene layers determines the practical specific surface area, and modification of pristine graphene with other compounds will decrease the aggregation and increase the effective surface area. Graphene with a high specific surface area has a large enough area for pollutant removal and functionalization. Modification of the graphene surface with specific functional groups or nanoparticles could be used to increase the graphene interaction with organic and inorganic pollutants, so they can be removed efficiently from solution through catalytic adsorption or degradation. Based on this, various graphene-based materials have been synthesized; these have enhanced new properties compared to pristine graphene.
In this paper, we review recent research advances in graphene-based nanomaterial synthesis and the application of these materials to environmental remediation. Potential future research on high-performance graphene-based nanomaterials for pollutant removal is discussed.
1 Synthesis of graphene-based materials for pollutant removal 1

Modified graphene
Pristine graphene is so hydrophobic that it is difficult to disperse in water for the pollutant removal. To improve its solubility, modified graphenes have been synthesized by adding functional groups on the surface through chemical modification, covalent, or noncovalent functionalization.
Graphene oxide, which is produced by Hummers method from flake graphite, has disrupted conjugation in the graphene plane and abundant functional groups, such as epoxide, hydroxyl, carboxyl and carbonyl, on its surfaces. These oxygen-containing groups could form strong complexes with metal ions, and allow the graphene oxide to act as an adsorbent for heavy metal ion preconcentration and removal. According to Zhao's group [42], graphene oxide with a few layers had a higher adsorption capacity for Pb(II) ions (up to 842 mg/g) than reduced graphene oxide (400 mg/g) ( Figure  1). This type of graphene oxide was also used to remove methylene blue from aqueous solution [43], and the adsorption capacity was 714 mg/g.
In 2010, Deng et al. [44] reported a one-step synthesis of ionic-liquid-functionalized graphene sheets directly from flake graphite. The ionic-liquid-treated graphite sheets were exfoliated into functionalized graphene nanosheets with a PF 6  mass fraction of 30%. These nanosheets were used for removal of Pb(II) and Cd(II) ions from wastewater with adsorption capacities of 406 and 73 mg/g, respectively.
Recently, sulfonated graphene with a few layers was synthesized in several steps by Zhao et al. [45]. The prepared sulfonated graphene had a high specific surface area (529 m 2 /g) and high dispersion in aqueous solutions. Benzenesulfonic groups on the graphene surfaces were thought to contribute to these properties ( Figure 2).
The prepared sulfonated graphene was applied as an adsorbent to remove naphthalene and 1-naphthol from aqueous solutions. The adsorption isotherms ( Figure 3) of naphthalene and 1-naphthol on these sulfonated graphene nanosheets indicated the maximum adsorption capacities were 2.33 mmol/g for naphthalene and 2.41 mmol/g for 1-naphthol, which was the highest adsorption capacities observed to date.
The adsorption of naphthalene and 1-naphthol on sulfonated graphene sheets can be theoretically modeled. The morphologies (top view and side view) of naphthalene and 1-naphthol adsorbed on the hexagonal carbon network are   shown in Figure 4. The naphthalene molecule adsorbed parallel to the sulfonated graphene surface in one of two possible orientations, while 1-naphthol adsorbed perpendicular to the sulfonated graphene surface in one orientation. In the two naphthalene adsorption orientations, the naphthalene was either 3.57 Å (State I) above the sulfonated graphene surface with a release energy of 1.96 kcal/mol or 3.71 Å (State II) above the sulfonated graphene surface with a release

Graphene/semiconductor hybrids
There is great interest in the synthesis of graphene-semiconductor composites because of the diversity of available functional semiconductor particles, which include Fe 3 O 4 , TiO 2 , ZnO, CdS. In pollutant removal, these semiconductors often modify the properties of the graphene framework. For example, magnetic Fe 3 O 4 nanoparticles can be used for magnetic separation, which is useful in large-scale industrial applications and overcomes many issues associated with filtration, centrifugation, or gravitational separation of graphene. Other semiconductors, such as ZnO, TiO 2 , and CdS, are common photocatalysts. Because graphene is a zeroband gap semiconductor and has excellent electronic conductivity in storage and transport of electrons, when combined with these semiconductor catalysts it should be very active in photocatalytic applications. In these graphene/ semiconductor hybrids, the nanoparticles on the graphene surface prevent aggregation of the graphene layers to some extent, which increases the surface area for removal of pollutants from aqueous solutions.
Among the composites, graphene-iron oxide hybrids have attracted the most research interest. Shen et al. [46] reported a one-step synthesis for graphene oxide-magnetic nanoparticle composites through a high temperature reaction of ferric triacetylacetonate with graphene oxide in 1-methyl-2pyrrolidone. The iron oxide nanoparticles were Fe 3 O 4 with a small amount of -Fe 2 O 3 , and about 8 nm in diameter. These nanoparticles bound strongly to the graphene surface through metal carbonyl coordination. Another one-pot solvothermal synthesis of graphene-based magnetic nanocomposites was introduced by Shen et al. [47]. Graphene oxide and iron acetylacetonate were dispersed in ethylene glycol, followed by addition of hydrazine hydrate and reaction in an autoclave at 180°C. In the prepared graphene/Fe 3 O 4 nanocomposites, the Fe 3 O 4 nanoparticles ( 60 nm) were well distributed on the graphene nanosheets. They had no obvious magnetic hysteresis loop at 300 K, which indicates they are superparamagnetic. In a different synthesis, Wang et al. [48] prepared magnetic graphene nanocomposites by in situ chemical coprecipitation of Fe 2+ and Fe 3+ in alkaline solution in the presence of graphene. The graphene/Fe 3 O 4 composites with a large surface area of up to several square micrometers were formed from Fe 3 O 4 nanoparticles (average  20 nm) and nearly flat graphene sheets. The authors applied these graphene/Fe 3 O 4 composites to the removal of organic dyes from aqueous solutions, and the maximum adsorption capacity for fuchsine 89 mg/g. He's group [49] developed a complicated method to attach Fe 3 O 4 nanoparticles to graphene oxide by covalent bonding. First, Fe 3 O 4 nanoparticles were modified with tetraethyl orthosilicate and (3-aminopropyl) triethoxysilane, which introduced amino groups on the surface. Then the modified Fe 3 O 4 nanoparticles were reacted with the carboxylic groups of graphene oxide in the presence of 1-ethyl-3-(3-dimethyaminopropyl) carbodiimide and N-hydroxysuccinnimide to form the graphene oxide-Fe 3 O 4 hybrids. The adsorption capacities of these hybrids for methylene blue and neutral red cationic dyes were 190 and 140 mg/g, respectively. For the removal of arsenic, Chandra et al. [50] synthesized magnetic-graphene hybrids through chemical coprecipitation of Fe 2+ and Fe 3+ in the presence of graphene oxide, followed by reduction of graphene oxide using hydrazine hydrate. For adsorption of As(III) and As(V) these hybrids showed near complete arsenic removal to as low as 1 g/L ( Figure 5). Because of their high adsorption capacity, and the fact they could be simply separated from solution by application of an external magnetic field, these hybrids could be applied to arsenic removal from large volumes of wastewater.
In addition to adsorption of pollutants from water, many pollutants can be eliminated by photocatalytic degradation. When traditional photocatalysts, such as ZnO, TiO 2 , and CdS, are incorporated with graphene, the hybrid should display high catalytic activity for degradation of organic pollutants because of graphene is a zero-band gap semiconductor. Liang et al. [51] synthesized graphene/TiO 2 nanocrystal hybrids by directly growing TiO 2 nanocrystals on graphene oxide nanosheets. First TiO 2 was coated on graphene oxide sheets by hydrolysis, and then the TiO 2 parti-cles were crystallized into anatase nanocrystals by hydrothermal treatment ( Figure 6). With ethanol/water as a mixed solvent and H 2 SO 4 as an additive, the hydrolysis was slow, growth of TiO 2 on the graphene oxide was selective, and growth of free TiO 2 in solution was suppressed. This method provides an easy approach to synthesize graphene/TiO 2 nanocrystal hybrids with a uniform coating and strong interactions between the TiO 2 and the underlying graphene nanosheets. The graphene/TiO 2 nanocrystal hybrids showed superior photocatalytic activity to other TiO 2 materials, with an impressive three-fold photocatalytic enhancement over P25 particles.
Using a one-step hydrothermal reaction, Zhang et al. [52] demonstrated a facile and reproducible route to obtain chemically bonded TiO 2 (P25)-graphene composites. In their synthesis, P25 and graphene oxide were mixed in a homogeneous suspension, which was reacted in an autoclave to reduce graphene oxide and deposit P25 on the carbon substrate. The composites showed high pollutant adsorption capacities, extended light absorption ranges, and facile charge transportation and separation. These attributes meant  the composites could be applied to the environmental remediation.
Zhang et al. [53] investigated the synthesis of ZnO/graphene composites via a chemical deposition route and its influence on photocatalytic degradation. In the synthesis process, Zn(II) was adsorbed on the surface of graphene oxide by complete ion exchange, then NaOH was added, and the solid was dried at 150°C. The resulted powder was redispersed in a solution of NaBH 4 for further hydrothermal treatment at 120°C. The prepared composites exhibited efficient photosensitized electron injection, slow electron recombination, and enhanced photocatalytic activity under UV and visible light conditions. CdS is a well-known II-VI semiconductor that has received extensive attention in photocatalytic research because it a band gap (2.4 eV) that corresponds well with the spectrum of sunlight. Many syntheses for CdS-graphene composites have been reported. For instance, Nethravathi et al. [54] and Wang et al. [55] bubbled H 2 S gas into a solution containing Cd(NO 3 ) 2 and graphene oxide. Chang et al. [56] prepared the composites by in situ growth of CdS on pyrenebutyrate functionalized graphene. Although none of these composites were applied as photocatalyst in pollutant removal and degradation, the reported syntheses are im-portant for CdS-graphene hybrids. By microwave-assisted synthesis, Liu et al. [57] synthesized CdS-reduced graphene oxide composites for photocatalytic reduction of Cr(VI). This one-step synthesis was carried out by mixing the Cd(II) solution, a CH 4 N 2 S solution, and a graphene oxide suspension (pH 9), and irradiating the mixture with a microwave at 150°C. The CdS-graphene composites were better photocatalysts than pure CdS, the performance was dependent on the proportion of graphene in the composite, and the composites containing 1.5% (mass fraction) graphene gave the highest Cr(VI) removal rate (92%).

Graphene/metal nanoparticle hybrids
Noble metal nanoparticles, such as those of Au and Pt, have attracted interest because of their excellent catalysis. Use of graphene sheets as a low-dimensional support for metal nanoparticle growth will enhance the electronic properties of the graphene because of spatial confinement and synergistic interactions between the metal and graphene. According to recent progress in synthetic technologies, metal nanocrystal growth on graphene can be realized by direct chemical reduction of the metal precursors in the presence of graphene oxide or reduced graphene oxide suspensions.

Muszynski et al. [58] prepared a graphene-Au composite by reduction of AuCl 4
 with NaBH 4 in an octadecylaminefunctionalized graphene suspension. While Scheuermann et al. [59] prepared Pd nanoparticles supported on graphene oxide by bubbling hydrogen through a suspension of Pd 2+ -graphene oxide in ethanol. The resulting Pd-graphene composites were very active catalysts in the Suzuki-Miyaura coupling reaction (Figure 7). Goncalves et al. [60] demonstrated that nucleation and growth of gold nanoparticles were dependent on the number of oxygen functional groups on the graphene surface. For better control over the metal nanoparticle size and structure, Zhang et al. [61] developed a facile and green method for in situ growth of noble metal nanodots on graphene sheets through a sonolytic and hydrothermal reduction route using graphene oxide and metal salts as the precursors in aqueous solution. The metal nanodots ( 2 nm) were uniformly distributed on the graphene surface. Because of the special catalytic properties of noble metals, most of these metalgraphene composites have been applied in energy conversion and organic transformations [62][63][64][65], rather than in the environmental remediation.
To simplify the post-synthesis treatment and increase the viability of the products for commercial application in pollutant removal, Sreeprasad et al. [66] introduced an in situ homogeneous reduction strategy. This used the inherent reducing properties of reduced graphene oxide to synthesize monodispersed and uncapped nanoparticles of silver, gold, platinum and palladium on the graphene surface. In their synthesis, graphene-metal composites were obtained by incubating a mixture of the metal ion precursors (HAuCl 4 , AgNO 3 , H 2 PtCl 6 , PdCl 2 ) and reduced graphene oxide (Figure 8). With sand as a support, graphene-Ag composites showed excellent uptake capacity for Hg(II) from aqueous solutions. More research concerning the application of graphene-metal composites in pollutant removal is expected.

Graphene-complex oxide composites
In addition to the graphene-metal oxide, graphene-metal, and graphene-sulfide composites, graphene hybrids with complex oxides, such as CoFe 2 O 4 and ZnFe 2 O 4 , have also been studied. Because of the prominent magnetic properties of CoFe 2 O 4 , Li et al. [67] prepared CoFe 2 O 4 -functionalized Figure 7 TEM images of the catalysts. Nanoparticles formed simultaneously during the reduction of the graphene oxides. Palladium nanoparticles in Pd 2+ -GO 2 were generated in situ during the Suzuki-Miyaura coupling reaction. The SAED in the inset shows a hexagonal pattern that can be ascribed to "regraphitized" regions. Reprinted with permission from [59], Copyright 2009, American Chemical Society. graphene sheets (FGS) via a facile hydrothermal method and applied the prepared composites to adsorption of methyl orange from aqueous solutions. In the synthesis, exfoliated graphene sheets, Co(Ac) 2 , and FeCl 3 were mixed in aqueous solution with control of the molar ratio and pH before hydrothermal treatment. The formation mechanism is illustrated in Figure 9. Because of the strong interaction between Co 2+ , Fe 3+ , the graphene surface, and OH  in the suspension, OH  connected the inorganic salt and graphene. The intermediate and final products were adsorbed on the graphene surface, and the CoFeO 4 nanoparticles were successfully attached to the surface of graphene nanosheets containing the oxygen functional groups. Compared with magnetic carbon nanotubes, the CoFe 2 O 4 -graphene composites had much higher adsorption capacities (71 mg/g at an initial concentration of 10 mg/L) with convenient magnetic separation.
Using an alternative method, Fu et al. [68] produced CoFeO 4 -graphene composites in a one-step solvothermal synthesis in ethanol. The combination of CoFe 2 O 4 nanoparticles with graphene resulted in a dramatic conversion of the Adsorption for fluoride [40] Reduced graphene oxide Not shown Reducing the exfoliated graphene oxide by hydrazine Adsorption for anionic dyes [41] Graphene oxide

Hummers method
Adsorption for methylene blue [43] Ionic-liquid-functionalized graphene sheets Graphene thin film Electrolyzing graphite rods with potassium hexafluorophosphate solution as electrolyte Removal of Pb(II) and Cd(II) ions from wastewater [44] Sulfonated graphene Few-layered nanosheets Sulfonation of the prereduced graphene oxide by the aryl diazonium salt of sulfonic acid, followed by the post-reduction with hydrazine Remove naphthalene and 1-naphthol from aqueous solutions [45] Graphene oxide-magnetic nanoparticle Magnetic nanoparticles on graphene oxide sheets Using graphene oxide and Fe(acac) 3 dissolved in 1-methyl-

2-pyrrolidone
Not shown [46] Graphene-based magnetic nanocomposites Magnetic nanoparticles on graphene oxide sheets Dispersion graphene oxide and iron acetylacetonate in ethylene glycol, followed by reduction with hydrazine hydrate in autoclave For comparison, the major morphologies, preparation methods, and applications of these graphene-based materials are summarized in Table 1.

The role of graphene-based materials in highly efficient pollutant removal
Graphene-based materials can be used in a number of ways for environmental remediation and pollutant removal. They can be used to reduce the pollutant concentration by adsorption, decompose pollutants to less toxic molecules (mainly applicable to organic pollutants and persistent organic pollutants), and reduce low-valency species (mainly applicable to toxic high-valency metal ions).
Removal of heavy metal ions from aqueous solution is largely dependent on the interaction between the ions and functional groups on the adsorbents. Therefore, it is understandable that graphene oxide and modified graphenes show high adsorption capacities toward metal ions such as Cu(II), Pb(II), Cd(II), and Co(II) [70,71]. According to the literature [70,71], the pH is important for the adsorption and graphene with a large surface area provides more active sites than one-dimensional carbon nanotubes. Pristine graphene has a lower adsorption capacity than modified graphene in the removal of heavy metal ions.
Graphene-based materials with low aggregation and high specific surface areas show high adsorption capacities for organic pollutants, especially benzene-containing compounds, where the - interaction between graphene and the adsorbate plays a dominant role [72][73][74][75]. Therefore, it is important to prevent aggregation between the layers. For convenient separation, magnetic particles are introduced in the adsorbent to form magnetic graphene composites. The added magnetic particles also play an important role in preventing aggregation of the graphene sheets. Therefore, much research has focused on graphene-Fe 3 O 4 composites for high performance and convenience in pollutant removal and separation of the composites from aqueous solutions. Based on the experimental results and the theory discussed above, pristine graphene has a much higher adsorption capacity than graphene oxide because of the - interaction between the graphene and the organic molecules.
Another approach to remove organic pollutants is photodegradation, in which the degradants can be reused without additional treatment [76]. Graphene-based photodegradants have many advantages over pure photodegradants. First, the unique electronic properties resulting from the sp 2 hybridized carbon atoms offer a picosecond ultrafast electron transfer process from the excited semiconductors to the graphene sheet. Second, the controllable size of the semiconductors and reduced aggregation of the graphene sheets further improve the efficiency of the photocatalysis. Third, the high transparency of the graphene sheets because of their one-or several-atoms thickness, enhances the utilization of the exciting light. Therefore, the application of graphene-based photodegradants in the decomposition of organic compounds and the reduction of the toxic high-valent metal ions is attractive.
In summary, the unique structure and special properties of graphene make it suitable for modification and complexation with other nanomaterials. The resulting modified graphene and graphene-based composites show high adsorption capacities and photocatalytic abilities.

Conclusion
Graphene has unique morphology, chemical structure, and electronic properties. It has been synthesized and modified through various methods, and composites have been made with other nanomaterials, such as semiconductors, noble metals, and some complex oxides. These materials have been produced to meet the increasing requirement for highperformance materials for pollutant removal. Graphene-based adsorbents for heavy metal ions and organic pollutants show high adsorption capacities, and graphene-based photocatalysts for use as photoreductants or photodegradants are highly efficient because of their large surface areas, functionalized surfaces, and active photocatalytic nanoparticles. The two-dimensional graphene nanosheet has prompted extensive research in nanomaterials synthesis and has important applications in environmental remediation. Although few graphene-based materials have been produced compared to other well-known nanomaterials, more graphenebased materials will be produced in future with further developments in nanotechnology. Although graphene cannot be synthesized on a large scale and inexpensively, there is no doubt that graphene or graphene-based materials will be easily and inexpensively produced in large quantities in the near future. The outstanding physicochemical properties of graphene and graphene-based materials will play a very important role in environmental pollution management in the future.