Summary of recent GRM-metal oxide nanocomposite photocatalysts degradation of organic pollutants.
Photochemical reactions hold great promise in solving energy and environment related problems and likely contribute towards development of sustainable society. Despite of recent advancements, the inherent catalytic efficiency of conventional photocatalyst has been severely limited by myriad complexity associated with (i) ineffective light absorption in visible region, (ii) unproductive recombination’s of e−/h+ pair in excited state, and (iii) low chemical stability. Contemporary researches on photocatalysts that can be viable for commercial applications has yet to be realized. Graphene has attracted an immense research interests to enhancing the photocatalysts efficiency endowing from their unique optical and electronic properties and salient features such as surface area, mechanical strength and photochemical stability. In this book chapter, we discussed graphene related material (GRMs) to produce hybrid architectures or nanocomposites that can be used as efficient photocatalysts for the degradation of organic pollutants (dyes, pharmaceutical wastes, pesticides etc.) in wastewater. Lastly, we summarize the key insights in photocatalytic electron transfer mechanism, challenges and future perspective which help understand the rationale of GRMs in this field.
- graphene-related materials
- environmental pollution
- hybrid photocatalysts
The world is facing serious water scarcity problem and thus protection of fresh water resources is critical in sustainable development of the society. Water pollution causing severe health issues because of waterborne diseases such as cholera, diarrhea, typhoid, and hepatitis leading to human sickness and deaths of (> 14,000) people/day globally . Major water contaminants stemmed from pesticides, textile dyes and a bunch of different chemicals often end up in water bodies or rivers [2, 3]. Textile industries generally uses many colored dye effluents, 65–75% of them belong to azo dyes and annually ~12% of these dyes were lost during manufacturing and the processing operations . The direct discharge of pollutant dyes or pharmaceuticals release toxic or carcinogenic substances into the aqueous environment leading to severe dangers and environmental disasters. To overcome these issues, globally various physical, chemical and biological processes have been used such as precipitation, adsorption (activated carbon), air stripping, coagulation, reverse osmosis, and membrane ultrafiltration, however, these conventional techniques are non-destructive, or often transfer the organic compounds from one phase to another phase triggering secondary pollution [5, 6, 7].
Ever since the discovery of water splitting via semiconductor photocatalysis by Fujishima and Honda , this technology has gained intense research interests due to potential applications in various fields including sustainable energy conversion, degradation of organic pollutants, bacterial elimination, CO2 reduction, air purification, antibacterial, organic reactions and self-cleaning etc. (Figure 1) [9, 10, 11]. Visible light driven (VLD) photocatalyst which can directly harvest solar energy to remove various toxic organic pollutants from water through the advanced oxidation processes is a relatively new and active research area in this field. Till date, titanium dioxide (TiO2) is the mostly used photocatalysts because of low cost, superior physicochemical properties and environmental sustainability. However, TiO2 has one major drawback of wide band gap (only absorb ultraviolet light <4% of solar spectrum) and limited its use under direct sunlight. In this regard, various other semiconductor nanomaterials have been exemplified as photocatalyst systems such as SnO2, ZnO, WO3, Fe2O3, BiVO4, Ag3PO4, BiOCl and BiOBr, etc. for photocatalytic water decontamination [11, 12, 13]. The practical utility of these semiconductors catalysts is restricted by low its absorption coefficient, high rate of recombination of electron–hole pairs and variance with the solar spectrum [14, 15]. In order to overcome these issues, a different strategy including Z-Scheme semiconductor photocatalyst , metal doping in semiconductor , semiconductor-heterostructures or nanocomposites [16, 17, 18] have been proposed. In the past decade, metal-doped-graphene or related materials have fascinated intense interests because of its potential for environmental purification and converting photon energy into chemical energy. This process obeys one of the ‘Green Chemistry Principle’, and widely applied for the degradation of hazardous pollutants. However, in the practical applications was restricted due to the failure to absorb visible light. Therefore, development of novel catalysts that can meet these technical needs is still a daunting challenge. It is in no doubt that the innovations of graphene-based material (GRM) will brings about massive opportunities in nurturing science and technology (Figure 2). Due to its exceptional atom-thick 2D structure containing sp2 bonded arrangement of carbon atoms in a hexagonal lattice, the high specific surface area, conjugated aromatic system, admirable mechanical properties, and robust physicochemical stability and greater electron mobility, graphene is regarded as an ideal high-performance candidate for many photocatalyst systems [19, 20]. A series of different carbon materials, such as activated carbon, porous carbon, carbon nanotubes, graphene and graphitic-carbon nitride can be used as catalyst supports.
In this book chapter we focus on graphene and related materials (graphene oxide and reduced graphene oxide and derivatives) which has emerged to be excellent promoters in photocatalytic reactions because of their low cost, unique physicochemical properties, surface area, electronic conductivity, mechanical flexibility and ionic mobility. These significant features together with high stability enabling them to produce hybrids or nanocomposites photocatalysts with enhanced performances for degradation of organic pollutants (dyes, pharmaceutical wastes, pesticides etc.) in water. Advanced oxidation processes (AOP) for waste water treatment mainly relies on generation of photo-induced-charge carriers (electron and holes), and utilizing them to produce highly active reactive oxygen species (ROS). The as produced ROS radicals generally function as ultimate oxidants in photochemical reactions and subsequently degrade the organic pollutants in water. This book chapter highlights historical background, synthesis strategies and overview of graphene-related materials to produce composite photocatalysts, enabling application in environmental remediation. Finally, the remarks pertaining to state-of-art advancements, challenges and future perspectives have been discussed.
2. General principle of photocatalysis
The term “Photocatalysis” refers to change in the rate of a chemical reaction or its initiation under the action of light in the presence of a catalyst. This process works just like natural photosynthesis, takes place in presence of photosynthetic organisms that converts carbon dioxide into sugars (chemical fuels) using the light energy from sunlight. It is a green process and mainly divided into two types: (i) Homogeneous photocatalysis, and (ii) heterogeneous photocatalysis. In homogeneous photocatalysis, the reactants and the photocatalysts exist in the same phase (e.g., photocatalysis of ozone where all reactants come under one phase, i.e., gas phase). Whereas in heterogeneous photocatalysis, the reactants and the photocatalysts exist in different phases (e.g., semiconductor photocatalysis.). As shown in Figure 3, when a photocatalyst absorbs light irradiation from sunlight or an illuminated light source, the electrons in the valence band of semiconductor are excited to the conduction band, whereas the holes are left in the valence band. This creates electron (e−) and hole (h+) pairs called as semiconductor’s “photo-excited” state and the energy difference between the valence band and conduction band is referred as “band gap”. After photoexcitation, the excited electrons and holes migrate to the surface of photocatalyst to carry out photochemical reactions. For example, in photocatalytic removal of organic pollutants, the photogenerated e−/h+ take part in oxidation/reduction process and produces reactive oxygen species (OH, O2_ and H2O2), which can eventually decompose organic pollutants [21, 22]. The ideal photocatalyst requires several key factors such as (i) a suitable band gap to allow the utilization of a significant fraction of the solar spectrum; (ii) optimal band edges relative to the water redox levels; (iii) high mobilities of electrons and holes thereby to reach the surface and reduce/oxidize the targeted molecules before recombining; and (iv) chemical/structural stability.
3. Graphene related materials (GRMs): Principles and strategies
Graphene has attracted immense attention ever since 2010 Nobel prize in Physics was awarded to Andre Geim and Konstantin Novoselov for their pioneering work. Owing to exotic physicochemical properties, and wide industry application prospects including catalysis, electronics, sensing, energy conversion-storage and environmental remediation, constant attempts have been made in its synthesis, investigations and innovations. Basically, graphene is an allotrope of carbon (linked by sp2 bonds), a one-atom-thick layer arranged in two-dimensional honeycomb network exhibiting unique properties such as high thermal conductivity (5000 W m−1 K−1), large specific surface area (2630 m2 g−1) and high intrinsic electron mobility (200,000 cm2 V−1 s−1) [23, 24]. GRMs include graphene oxide (GO), reduced graphene oxide (rGO), and their derivatives (e.g., functionalized graphene or composites which can be used as building blocks to develop series of nanocomposites or hybrid photocatalysts via Vander Walls interaction and inherent surface active O2−/OH groups and oxo ligands chemistry.
3.1 Preparation of graphene and related materials
A wide range of synthesis techniques have been developed to yield graphene and related materials. It can be broadly classified into two distinct approaches: (i) top-down, and (ii) bottom-enabling different scale-up capability and variations in the properties (Figure 4). Top-down synthesis strategy relies on simple exfoliation of graphite via mechanical means (e.g., Scotch tape), chemical (e.g., solution-processed, graphite oxide exfoliation/reduction), and electrochemical (oxidation/reduction and exfoliation) methods and allows weakening the van der Waals forces between the graphene layers to form the graphene with single or few atom thick layers. A special graphene nanoribbons with tuneable band gaps and edge shapes have been achieved via opening of carbon nanotubes through chemical or thermal routes . On the other hand, bottom-up strategies which rely on assembly of small molecular building blocks into few layer graphene nanostructures have been achieved through different chemical routes such as catalytic (e.g., CVD), thermal (e.g., SiC decomposition), or chemical (organic synthesis) processes. The readers are directed to several recent reviews for the details of synthesis of GRMs [25, 26].
4. Graphene-related derivatives/composites
4.1 GRM/transition metal oxides composite photocatalysts
Transition metal oxides has been extensively used in photocatalytic environment remediation due to their exotic low cost, high catalytic activity and good stability [8, 9]. In semiconductor photocatalysis, the electrons are excited from the valence band to the conduction band, and electron–hole pairs are generated. These electron–hole pairs are either reunite or transfer to the surface to initiate a series of redox reactions, and generate highly reactive oxidative species (ROS), such as·OH,·O−2, and H2O2 which ultimately participate in the degrading of the organic pollutants . A number of strategies have been used to improve the photocatalytic activities of metal oxides: (i) doping of photocatalysts either by anions or cations [10, 11], (ii) coupling of surfaces with metals or semiconductors [12, 28], and (iii) increasing the surface area, reactive facets of photocatalysts [29, 30].
Benefitting from distinct properties and structures, the carbonaceous nanomaterials has attracted immense attention to produce highly active photocatalysts . Westwood et al.  and Sigmund et al.  reported the combination of carbon nanotubes (CNTs), graphene and other novel carbonaceous nanomaterials with TiO2, which lead to the dormant recombination of photogenerated electron–hole pairs. Additionally, graphene incorporation tends to offer unprecedented properties due to its unique sp2 hybrid carbon tightly packed into a two-dimensional honeycomb structure [18, 33]. Based on the formation sequence of the graphene and semiconductor, various graphene-related nanomaterials have been synthesized . The most common strategies for the fabrication of graphene-based photocatalysts are shown in (Figure 5) . In addition, different conventional reactions including hydrothermal reaction, thermal irradiation, the adoption of reductants (hydrazine, NaBH4, etc.) have been used to construct graphene-based composites [36, 37, 38].
Owing to high efficiency, good stability and low cost the TiO2 is mostly preferred for graphene-based nanocomposites [39, 40, 41, 42]. Graphene based TiO2 photocatalytic films also have been developed and used for photocatalytic applications owing to their salient features of easily fixing, recycling and restoring. Because of the efficient charge separation and transportation among the giant p-conjugation and planar structure, the speedy degradation of dye pollutants was achieved by coating TiO2 films with GO [43, 44]. Apart from TiO2, many other metal oxides based photocatalysts have been reported such as ZnO, SnO2, WO3 and Fe2O3 which showed similar photocatalytic applications [36, 45]. Among them, ZnO is often considered as a favorable alternative to TiO2 for photocatalytic applications [46, 47]. Additionally, visible light active nanocomposites based on graphene have been constructed exhibiting significantly improved the photodegradation activities for the removal of organic pollutants dyes . For instance, Patil et al. reported unique wet chemical synthesis approach for graphene-wrapped Ag3PO4/LaCO3OH heterostructures (Figure 6A) . First, LaCO3OH microspheres were obtained by facile hydrothermal method (Figure 6A-ii). Next, an appropriate amount of LaCO3OH and graphene were dispersed in distilled water and an aqueous solution of NH4H2PO4 and AgNO3 was subsequently added dropwise under magnetic stirring to form nanocomposites (Figure 6A-iii).
From pre-screening of photocatalysts with Ag3PO4/(x wt% LaCO3OH) with mass ratios of x = 5, 10, 15, 20, 25 and 30 wt% we found that Ag3PO4/(20 wt% LaCO3OH) exhibits highest photocatalytic degradation performance. On the basis of control experiments and physicochemical characterization., we observed that enhanced photoactivity is attributed to the co-catalytic effect of LaCO3OH, accelerates charge separation due to creation of heterojunction interface. Thus, incorporation of graphene can effectively avoid the disintegration of Ag3PO4 into metallic Ag (photocorrosion), featuring excellent photoactivity and stability (Figure 6A-iv). Likewise, BiVO4/graphene nanocomposites reported to have intrinsic visible-light driven performance among the Bi3+ containing oxides materials . A remarkably high photocatalytic reaction activity of BiVO4 was found, when graphene was incorporated which is attributed to electronic charge equilibration between BiVO4 and graphene, and likely contribute to shift of the Fermi level and reduction in conduction band potential . Recently, Patil et al. demonstrated one pot in-situ preparation of ternary BiVO4/Ag/rGO hybrid heterostructures via simple hydrothermal method and tested for photoelectrochemical (PEC) water splitting and photocatalytic MB degradation (Figure 6B) . These results revealed that the combined effects of the incorporated Ag and rGO nanostructures lead to interface creation wherein visible light absorption, charge separation-transfer and superior surface characteristics was greatly improved. An excellent degradation rate (kapp) of 1.29 × 10−2 and 0.192 × 10−2 was obtained for the photocatalytic MB and phenol degradation, respectively using ternary BiVO4/Ag/rGO hybrid nanostructures which is three times higher than pristine BiVO4 photocatalyst (Figure 6b-ii and iv). Moreover, the synthesis of magnetically separable α-Fe3O4  and ZnFe2O4  on graphene support has also been reported, these nanocomposites exhibited an excellent organic pollutant removal efficiency from wastewater. These photocatalysts can be easily separated from aqueous solution by applying an external magnetic field enabling well retained photoactivity even after repeated use. Table 1 summarizes the recent reports on the degradation of organic contaminants by using GRM-based photocatalysts.
|Composite photocatalyst/amount||Mass ratio of GRM||Dye concentration||Degradation percentage (%)||Irradiation time (hours)||Light source||Reference|
|P25 TiO2−graphene||1 wt. %||MB (0.01 g/L, 2.7 x 10 M−5 M)||65||1||500 W Xe lamp (λ > 420 nm)|||
|TiO2-graphne (1 mg/mL)||30 mg||MB (1 x 10 M−5 M)||75||3||Sunlight|||
|TiO2–GO (0.05 g)||0.14 wt. %||Methyl orange (MO; 12 mg/L)||35||3||1000 W Xe lamp|||
|SnO2-graphene (6.3 mg)||15 wt. %||Rhodamine B (RhB; 5.3 x 10 mM−5 mM)||~ 80||~ 2||350 W Xe lamp|||
|ZnO-graphene (50 mg)||2 wt. %||MB (1 x 10 M−5 M)||90||0.4||UV light|||
|ZnFe2O4/graphene||20 wt. %||MB (20 mg/L)||88||1.30||450 W Xe lamp|||
|BiVO4/rGO (0.02 g)||No data||MB (3 mg/L)||90||2.0||54 W halogen lamp|||
|CdS/graphene (0.050 g)||5 wt. %||MO (10 mg/L)||95||1||200 W Xe lamp|||
|CdS/CNTs (0.050 g)||88 wt. %||MO (10 mg/L)||88||1|||
|g-C3N4/rGO||No data||RhB (10 mg/L)||94.2||2.30||150 W Xe lamp|||
|Ag/AgCl/Graphene||0.6 wt. %||MO (15 mg/L)||71||~ 0.5||500 W Xe lamp|||
|Ag/Ag2S/TiO2/GO||No data||Crystal violet||28.92||2||Xe lamp|||
4.2 GRM/oxide free semiconductor photocatalysts
Graphene based-nanocomposite with oxide free materials (metal sulphide, metal nitrides, graphitic carbon nitride (g-C3N4) and Bi-oxyhalides) has gained increasing attention in environmental remediation [37, 38, 40, 64]. Metal sulfides such as CdS, MoS2, SnS2, Sn2S3, CuS, and ZnS generally hold narrow energy band gaps and negative conduction band (CB) edge positions . Many metal sulphides (mono, binary and ternary) have been developed with tunable band structures and successfully employed in photocatalytic dye degradation [65, 66]. Among them, CdS most studied photocatalyst that can directly absorb sunlight at wavelengths under 550 nm . Furthermore, graphene/CdS composite reported to have excellent visible-light-driven photocatalytic activity for organic pollutant degradation. Ma et al. reported that optimal weight percentage of graphene in the CdS clusters/graphene nanocomposites was found to be 1.0 wt%, which resulted in a high photocatalytic degradation of methyl 3,5-dichloro-4-hydroxybenzoate (MDHB) . Heterojunction construction-based copper sulfide nanostructures was observed to be an effective strategy for environmental applications. Andronic et al. demonstrated copper sulfide/graphene heterojunction photocatalysts for dye photodegradation enabling relatively large surface area, porous morphology, the ability to photogenerated electrons across the composite interface, and high adsorption capacity for organic molecules . Graphene-CuS composites with different surface morphologies were prepared via different synthesis strategies such as CuS-GO/TiO2 composites were synthesized by sol-gel method , flower-like CuS/rGO composites synthesized by a facile one-step solvothermal procedure . The dye degradation efficiency of GR/CuS composite was observed to be 30% higher than crystalline pure phase CuS. Thus, enhanced photoactivity was attributed to the not only high electronic conductivity of graphene but also its significant influence on the morphology of the CuS/Gr nanocomposite .
Most recently, an innovative metal free polymeric photocatalysts—graphitic carbon nitride (g-C3N4) has been developed representing low cost, easy scalable synthesis and superior photoactivity . It is composed of C, O, N and some contamination of H atoms, coordinated by tris-triazine-based patterns. It is a highly stable both thermally (up to 600°C) and chemically due to covalent C − N bonds. The state-of-the-art catalytic and optoelectronic properties are ascribed to sheet-like structure of g-C3N4 with appropriate band gap energy (Eg-2.7 eV), metal-free nature, and tunable electronic band structure and stability . The existence of primary surface sites of g-C3N4 are striking for several optoelectronic applications including photocatalysis . g-C3N4 contains of C–N bonds deprived of electron localization in the π state and the number of surface defects are found due to presence of hydrogen specifies, which could be beneficial in catalysis . g-C3N4 has been synthesized via different chemical routes can act as efficient photocatalyst for photodegradation of organic dyes [70, 71].
Furthermore, g-C3N4 was reported to be doped with metallic impurities, in which band gap energy was reduced enhancing photo response and photocatalytic properties . In order to dope metal ions, a salt of equivalent solubility were mixed with the g-C3N4 precursor . Various transition metals such as Pd, Cu, Fe, W, Zr has been doped to endow remarkable photocatalytic activities due to alteration of the electronic and atomic structure of g-C3N4 . Specifically, the light absorption and mobility of charge carriers can be increased which are essential prerequisites for better photocatalytic performance. Metal cations and the negatively charged atoms of nitrogen attributed to the lone pairs of electrons on the nitrogen edges of g-C3N4 . Noble metals also have been used as a doping metal in the g-C3N4, such as platinum and palladium have been utilized which could result in enhanced transporter mobility, improved separation of electron hole pairs, and narrowing of the band gap values [39, 75]. In the recent years several strategies have been adopted for the incorporation of metal or transition metal-into g-C3N4, as shown in Figure 7.
4.3 GRM/noble metals nanocomposite photocatalyst
Graphene-based noble metal composites have been fabricated by introduction or mixing of noble metal precursor into the graphene solution via relatively simplistic methods such as in situ growth or wet chemical methods. Metals like Au, Ag, Pd and Pt, were incorporated into GRMs to exploit high efficiency composite photocatalysts [76, 77]. The noble metal nanoparticles could act as an electron trap, increases the mobility of charge carriers and likely suppresses the rate of recombination. Therefore, it can be considered as appealing platforms in the design of high-performance visible light driven photocatalysts. Among them, Ag and Au NPs exhibit unique optical properties due to the collective oscillation of free electrons on their surfaces while being interaction with incoming electromagnetic radiation . In the photocatalysis process, noble metal nanomaterials have been extensively studied to enhance the light absorption capability of metal oxide-noble metal photocatalysts often called as plasmonic photocatalysts . Apparently, doping of metals could decrease the band gap and accelerate the interfacial electron transfer. It has been reported that Au and Ag nanoparticles could allow capturing and scattering photons with a relatively high excitation wavelength in the visible light region . Many noble metal-graphene-hybrid nanostructures have been prepared and tested for the degradation of organic pollutant dyes in water [81, 82, 83].
The incorporation of metal oxide photocatalysts with graphene and noble metal nanostructures could provide synergic effect to boost the photocatalytic efficiency. Interactions of graphene with metal nanostructures could facilitate electrons transport together with plasmonic effects. Fabricating of as such multifunctional heterogeneous systems may provide several pathways for electron transport stemmed from noble metal–metal oxide, metal oxide–graphene, and noble metal–graphene interfaces . For example, Wen et al.  synthesized the graphene based TiO2-Ag doped photocatalyst and indicated that strong absorption in the visible light region can be realized increasing the photocatalytic efficiency for the degradation of methyl orange (MO) dye. This is attributed to the combined effects of the surface plasmon resonance (SPR) properties of the Ag NPs and the strong interaction of graphene assembled on the TiO2 surface. Furthermore, at the Ag/TiO2 interface, formed Schottky junctions could enhance the ability of electron transfer with graphene and resulted in increased photocatalytic efficiency . Similarly, Huang et al. reported a layered structure of Ag NPs deposited on graphene-TiO2 nanorods exhibiting a greater efficiency of photocatalytic degradation of MB . Jafari et al.  and Vasilaki et al.  fabricated a ternary-graphene-Ag/TiO2 nanocomposite by hydrothermal method and demonstrated that composite system can efficiently degrade MB dye related to pristine TiO2. Similarly, Ghasemi et al. evaluated the degradation of acid blue 92 under UV and visible light irradiation using Au–TiO2–graphene nanocomposite photocatalyst . In this case, Au NPs help extend the visible light absorption ability by decreasing the band gap energy of the system TiO2–Pt/Pd–graphene nanocomposite photocatalysts were investigated for the degradation of Reactive Red 195 and 2,4-dichlorophenoxyacetic acid under UV and visible light. Due to greater photonic efficiency, Pt metal showed the higher degradation rate of pollutants . Graphene-Ag-ZnO nanocomposite have been used as a high performance photocatalysts for the degradation of RhB dye. The rate of degradation is 13.80 times higher than bare ZnO photocatalysts . Correspondingly, Au/rGO/ZnO nanocomposite found to be an efficient photocatalysts for degradation MB dye in water [93, 94]. Zhang et al. decorated palladium (Pd) on ZnO–graphene nanostructures in which photocatalytic activity was dramatically enhanced as a result of charge separation at the Pd–ZnO interface .
Doping of noble metals in graphitic carbon nitride (g-C3N4) could endow interesting physicochemical properties owing to two-dimensional layered structure and likely contribute visible-light photo response. For example, Au/Pd/g-C3N4 nanocomposites were realized by loading of Au and Pd nanoparticles on the surface of g-C3N4 sheets . This nanocatalyst demonstrated >90% degradation efficacy for degradation of tetracycline hydrochloride. Similarly, Au/g-C3N4 nanosheet/reduced graphene oxide (Au/CNNS/rGO) nanocomposite was produced by inducing simple thermal oxidation exfoliation combined with in-situ photoreduction reactions leading to significantly enhanced photocatalytic activities for MB degradation and H2 production reaction . As shown in Figure 8a–c, Au nanoparticles were uniformly decorated on the surface of the thin carbon nitride nanosheets (CNNS) indicating intimate interaction between them and can be beneficial for improving the plasmonic characteristics of the nanocomposite. Figure 8c shows size distribution of Au nanoparticles with an average diameter of 5.5 nm. Additionally, the rGO was found to be integrated on the opposite surface of CNNSs, which resulted in bidirectional nanostructure to promote the electron transfer. According to high resolution transmission electron microscopy (HRTEM) analysis (Figure 8d), the lattice spacing of 0.203 nm was confirmed and in agreement with (200) lattice planes of metallic Au. The authors detected a dramatic improvement in H2 production reaction and methylene blue degradation which was approximate ly 9.6X and 6 X fold higher than pure g-C3N4 under visible light irradiation. Kim et al. reported a a strategy to incorporate different noble metal nanoparticles (Pd, Pt, Au, and Ag) into GO nanosheets whereby noble metals and GO was reduced simultaneously using ascorbic acid as a reductant . Wang et al. reported Ag NPs@GO nanocomposite through light-induced synthesis method in which size dependent extremely high catalytic activity was investigated towards degradation of 4-nitrophenol . Similarly, Ji et al. obtained Ag NPs@GO nanocomposite by in-situ reduction of Ag+ ions into on GO nanosheets exhibiting efficient catalytic activity for the reduction of 4-nitrophenol into 4-aminophenol . Rajesh et al. reported anchoring of Ag NPs and Au NPs onto chitosan grafted GO via NaBH4 reduction method which displayed superior photocatalytic activity towards degradation of aromatic nitroarenes and azo dyes . Gu et al. was chosen functionalized graphene/Fe3O4 hybrid as nanocarrier to deposit AuPt alloy NPs via controlled self-assembly strategy. The nanocomposite displays magnetic features which is beneficial to recover the catalysts easily and repeated use endowing superior catalytic activity for the reduction of 4-nitrophenol . Similarly, Islam et al. obtained magnetically recyclable carbon nanotube-rGO-Fe3O4-Ag NPs nanocomposite by combining in-situ reduction and hydrothermal methods which was demonstrated high catalytic efficiency for the removal of toxic dyes-MB and 4-nitrophenol .
5. Overview of charge-transfer mechanisms in emerging GRM composites
In this section, we particularly focus on GRMs composites for environmental remediation, with special emphasis on charge-transfer mechanisms. For this purpose, the composites of GRMs have been taken in account, since the charge transfer mechanism was mainly influenced by the composite systems, or heterojunctions formation between the two semiconductors which is together called as nanocomposite system. GRMs in association with several other semiconductor materials for example GO, rGO@MO, (MO = TiO2, WO3, BiVO4, AgPO4, AgCl, Fe2O3, Bi2O3, etc.) has been widely studied for environmental applications [35, 102, 103, 104]. Several charge transfer mechanisms have elucidated to understand the boosts in photocatalytic degradation efficiency. This charge transfer mechanism mainly depends on the type of heterojunction formed with other metal oxide semiconductors with GRMs. Especially the type-I, type-II and Z-scheme heterojunctions are the most important ones that influence the charge transfer mechanism during the photocatalytic process and eventually improve the photocatalytic efficiency. Gao et al.  studied the charge transfer mechanism of graphene-Bi2WO6 (G-BWO) for the photocatalytic degradation of Rhodamine B (RhB). Here, authors elucidated the type-1 mechanism based in the electrochemical studied and photocatalytic activity results (Figure 9a). In another study the Simsek et al. , demonstrated the charge transfer mechanism of ternary heterojunction system consists of RGO/TiO2/ZnO (Figure 9b) towards photocatalytic degradation of estrogen (bisphenol-A) and pharmaceuticals (ibuprofen, flurbiprofen). When RGO/TiO2/ZnO catalyst is irradiated with light, the electrons are excited from conduction band of ZnO to the conduction band of TiO2 structure. These photoinduced electrons can react with O2 to generate O2 radicals, and the holes on the surface captured by OH− or H2O to form OH. Recently, Ran and co-workers studied the graphene quantum dots (GQDs) decorated by TiO2 as heterojunction composite for degradation of methyl orange (MO) under visible-light irradiation. The reaction mechanism follows the type II heterojunction (Figure 9c) . Therefore, from these studies it can be understood that the GRMs or its composites likely contribute for charge transfer mechanisms for removal or pollutants through photocatalytic process. The reaction mechanism varies based on the engineered material that is deposited on graphene and its related material.
5.1 Photocatalytic degradation of organic dye pollutants/water decontamination
GRMs possesses several properties that make it attractive for environmental applications. The most studied aspect of graphene & GO is probably its electronic properties [108, 109]. It has extremely high electron mobility, reaching 10,000 cm2 V−1 s−1 to 50,000 cm2 V−1 s−1 at room temperature, with an intrinsic mobility limit of >200,000 cm2 V−1 s−1 . More importantly the graphene can sustain current densities up to six orders of magnitude higher than copper . These remarkable electronic properties of graphene, however, were obtained under ideal conditions, with mechanically exfoliated graphene under vacuum. Nonetheless, the promising electronic properties of graphene have triggered research and development for its extreme use in photocatalytic materials for degradation of various pollutants in water . Table 2 summarizes the most common adsorption mechanisms as well as advantages and disadvantage of using graphene related materials and their derivatives as adsorbents for environmental remediation and sequestration of metal ions from aqueous solutions.
|Graphene related materials||Mechanisms involved in the adsorption of metal ions||Advantages||Disadvantages||References|
|Graphene oxide (GO)||Electrostatic interactions Ion exchange||High dispersibility in water; good colloidal stability; abundant presence of oxygenated functional groups||Limited amount of sorption sites||[111, 112, 113, 114, 115]|
|Reduced GO (rGO) Pristine graphene||Electrostatic interactions Lewis-base-acid mechanism||Reestablishment of sp2 domains; better electron-transport property||Low density of oxygen-containing functional groups; lower colloidal stability||[116, 117, 118, 119, 120]|
|Magnetic graphene nanocomposites||Electrostatic interactions with graphene Interactions with the surface of the particles Magnetic properties of the nanoparticles||Larger surface area compared to the pristine forms; increased number of binding sites compared to pristine graphene; easy recovery from aqueous solutions||Co-reduction of GO during the attachment of the particles reduces the colloidal stability||[121, 122, 123, 124, 125, 126]|
|Graphene materials modified with organic molecules||Electrostatic interactions Complexation with organic molecules||Larger surface area compared to pristine forms; good colloidal stability; improved amount of functional groups (−NH2, −OH)||The stability of the loaded molecules varies according to the modification strategy||[117, 119, 127, 128, 129, 130]|
The GRMs provide lot of scope for photocatalytic degradation of organic dyes in combination with other metal oxide semiconductors. Especially due to its low cost and strong oxidizing activity, TiO2 is the most commonly used semiconductor for forming graphene based photocatalytic nanocomposites for the photodegradation of organic and biological contaminants [131, 132]. The rGO-TNT (TiO2 nanotubes) composites were prepared with different concentrations of rGO and the photocatalytic degradation of malachite green (dye) was found to be influenced by the rGO/TNT ratio. The rGO-TNT containing 10% rGO showed the highest photocatalytic degradation activity against malachite green, a performance of three times higher compared to neat TiO2 nanotubes . As dyes are aromatic molecules, their adsorption on graphene is endorsed by p-p stacking interactions between the sp2 domains from both systems. Therefore, the adsorption capacity of graphene-TiO2 composites for organic dyes can be higher than bare TiO2 nanomaterials . After interaction with graphene sheets the oxidative species nearby the catalyst can readily access the adsorbed dye, making the photocatalytic degradation process more efficient. In another study, Zhang et al. , studied the photocatalytic degradation of rhodamine B (RhB) with reduced graphene oxide (rGO) which was modified with TiO2 and SnO2 to form rGO-TiO2 and rGO-SnO2 respectively. The composite materials performed very interesting photocatalytic properties for degradation of RhB under visible light irradiation. First, their photocatalytic activities were higher than that of P25 (a commercial TiO2 as a benchmark photocatalyst). Second, the reaction mechanism catalyzed by the composite materials was different from that of semiconductor photocatalysis (Figure 10). The careful characterization showed that the excellent photocatalytic performance of the composite materials was associated with the good electrical conductivity and effective charge separation because of the presence of RGO. In presence of catalysts RGO–SnO2 and RGO–TiO2, the degradation was remarkably enhanced with rate constants of about 6.2 × 10−3 min−1 for RGO–SnO2 and 3.6 × 10−3 min−1 for RGO–TiO2, significantly higher than that of RGO (3.9 × 10−4 min−1) as shown in Figure 10a. In addition, photocatalyst RGO–SnO2 performed better than RGO–TiO2 and commercial product P25. The adsorption of RhB on the catalysts could be accredited to two parts, the adsorption of RhB on the surface of the RGO and the surface of the metal oxides. Although the surface area of RGO–TiO2 (341 m2 g−1) is much larger than that of RGO–SnO2 (241 m2 g−1), benefiting from the uptake of RhB from water, RGO–SnO2 exhibited higher photocatalytic activity compared to RGO–TiO2. Apparently, the enhanced photocatalytic degradation of RhB over RGO–SnO2 is not mainly due to the adsorption of dyes on the RGO sheet. Considering the work function of RhB (−5.45 eV), excited RhB (−3.08 eV) , graphene (−4.42 eV) and the conduction bands of SnO2 (−4.5 eV) and TiO2 (−4.4 eV) in Figure 10b, the different locations of adsorbed RhB molecules on the catalyst surface would lead to different degradation efficiencies. It was demonstrated that excited RhB can efficiently inject electrons into the graphene plane, and the degradation rate was even faster than that of TiO2 . However, due to the electron recombination between the injected electron and the surface adsorbed RhB+ (dotted line in Figure 10b), the degradation of RhB over the RGO was excruciatingly slow . After the RGO was loaded with SnO2, the injected electron could further move to the conduction band of SnO2 due to the higher work function of SnO2 than RGO. The electron on the SnO2 surface could also be trapped by dissolved oxygen to form various reactive oxygen species (ROSs), thus greatly enhancing the degradation of RhB. Here, the RGO acted as an electron mediator, facilitating the electron transfer from RhB* to SnO2. However, when RGO was loaded with TiO2, since the work function of RGO is higher than that of TiO2, the electron on the RGO surface cannot further transfer to the TiO2 nanoparticles. Therefore, from this study it was concluded that the work function of the metal oxides plays a crucial role in activating the degradation efficiency of the dye molecules.
5.2 Photocatalytic degradation of pesticides, pharmaceutical wastes etc.
GRMs were found to be quite effective in the degradation of pesticides and pharmaceutical wastes as similar to degradation of organic dye pollutant molecules. In this section we discussed some of degradation mechanisms of GRMs for this important topic of research. Chlorpyrifos is widely used to control pest insects in residential, agricultural, and commercial applications. Its common use has led to the release of chlorpyrifos into sediments, wastewater and water sources. The presence of chlorpyrifos in wastewaters and water sources may affect ecosystem and human health due to its chronicle toxicity to aquatic organisms. In this regard, Vinod Kumar and co-workers studied the magnetic recoverable CoFe2O4@TiO2 decorated reduced graphene oxide nanocomposite for investigating the photocatalytic degradation of chlorpyrifos (Figure 11) . The effect of initial concentration of CP on the degradation rate was studied within the range of 1 to 40 mg/L at a catalyst amount of 0.4 g/L. The nanocomposite was separated easily from the solution within 12 s by using a magnet without filtering and it was stabilized into solution for the next cycle. The stability experiments of CoFe2O4@TiO2/rGO nanocomposite showed no loss in the efficiency after 8 cycles . The photochemical mechanism for the degradation of the chlorpyrifos by using CoFe2O4@TiO2/rGO nanocomposite is shown in Figure 11a. Similarly, the photocatalytic degradation pathways of chlorpyrifos has been shown in Figure 11b which generally undergone various intermediate byproducts on part of photocatalytic mechanism.
Recently Cruz et al.  studied the photodegradation of a mixture of four pesticides classified by the European Union as priority pollutants: diuron, alachlor, isoproturon and atrazine by using GO-TiO2. The influence of two water matrices (ultrapure or natural water) was also investigated. The photocatalytic activity of GO-TiO2 composite under visible light was remarkably higher if compared to commercial TiO2 P25, shorter reaction times to photo-degrade 50% of pesticides as well as faster chloride formation rate being obtained with GO-TiO2.
In the similar manner, GRMs-composites have been extensively applied for degradation of pharmaceutical wastes through photo degradation process [137, 138]. Since the water pollutants emerging from pharmaceutical, cosmetics, heavy metals, pesticides, industrial additives, and solvents are becoming new global water quality threats. The presence of pharmaceuticals in municipal wastewater, hospital wastes, and industrial effluents are the major sources of contaminants in drinking water . In this connection the Sravya et al. reported the photocatalytic degradation of pharmaceutical wastes specifically the paracetamol . Highly efficient visible light active polyaniline (PANI)/Ag nanocomposites grafted reduced graphene oxide (rGO–Ag/PANI) was prepared and tested for the photocatalytic degradation of paracetamol under visible light radiation. The results reveled that ~99.6% degradation of paracetamol in the acidic medium (pH 5) and 75.76% in the basic medium (pH 9), respectively. The enhanced degradation efficiency is attributed to the synergetic effect of rGO, PANI, and Ag NPs in the nanocomposites. Many other researchers widely studied the degradation of pharmaceutical wastes by using graphene and its related materials by making good composites [140, 141, 142].
6. Concluding remark and perspectives
In summary, GRMs (GO, rGO, graphene and derivatives) can be incorporated into different semiconductors or metals to construct GRM-based composite photocatalysts. An integration of GRMs into these composites has proven to be effective approach to endow unprecedented properties and concurrently improves the high dye adsorption capability, light absorption, and also facilitate charge separation and transfer properties, enabling advancement in the overall photocatalytic activity. Despite of decades intensive research, the scalable synthesis of high-quality graphene is in unmatured state and several issues need to be addressed. We discussed simple, cost-effective synthetic strategies such as in situ reduction, coprecipitation, hydrothermal or solvothermal reactions etc. to develop emerging GRM based composites and applicability towards photocatalytic degradation of pollutant dyes, organics and pharmaceutical wastes in water.
With regard to breakthrough in photocatalysis technology, the studies are in early stage and further specific advancements are required. The following insights may be useful.
Although conventional photocatalysts such as titanium dioxide (TiO2) and graphitic carbon nitride (g-C3N4), and graphene have proven their fitness to work as effective supports, the coordinating sites for metal or single atoms are limited which indeed succumbed to inefficient solar light harvesting due to their wide band gaps and no band gap (graphene). Therefore, finding a low-cost new transition metal based nanosheet materials and single atom photocatalysts is challenging. Like GRMs, the Bi-containing oxyhalide nanosheets or combinations thereof could be possible choice materials for the development of new generation photocatalysts.
Additionally, although present GRM/Nobel metal nanocomposites have shown increasing plasmonic photocatalytic activity and promises for the potential applications still they have one drawback. Metal atoms used in the photocatalysts design are rare and expensive. Therefore, from economical or practical perspective exploring new low-cost transition metals (Cu, Mo, Bi, Co, Ni etc.) as cocatalysts together with inexpensive light harvester nanosheets support photocatalysts (narrow band gap) could be possible means to produce feasible high efficiency photocatalysts. Altogether there is plenty of room towards developing noble metal free single atoms/GRM nanosheet photocatalysts for sustainable energy conversion and environmental remediation.
Importantly, most of the researches in this field are mainly focused on material design, without much understanding about what is going on in the process, and photoreactor/devices system. It is anticipated that the knowledge of understanding all three factors is equally important which may lead to significant opportunities in solving real world problems.
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