*Graphene Related Materials and Composites: Strategies and Their Photocatalytic Applications… DOI: http://dx.doi.org/10.5772/intechopen.102404*

equilibration between BiVO4 and graphene, and likely contribute to shift of the Fermi level and reduction in conduction band potential [52]. 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**) [50]. 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 [53] and ZnFe2O4 [54] 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.


#### **Table 1.**

*Summary of recent GRM-metal oxide nanocomposite photocatalysts degradation of organic pollutants.*

#### **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 [65]. 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 [41]. 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) [40]. 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 [42]. 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 [43], flower-like CuS/rGO composites synthesized by a facile one-step solvothermal procedure [44]. 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 [44].

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 [67]. 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 [68]. The existence of primary surface sites of g-C3N4 are striking for several optoelectronic applications including photocatalysis [69]. 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 [67]. 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 [72]. In order to dope metal ions, a salt of equivalent solubility were mixed with the g-C3N4 precursor [73]. 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 [74]. 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 [73].

*Graphene Related Materials and Composites: Strategies and Their Photocatalytic Applications… DOI: http://dx.doi.org/10.5772/intechopen.102404*

**Figure 7.**

*Schematic illustrating the decoration of graphitic-carbon nitride (g-C3N4) with metal nanoparticles. Reprinted with permission from Khan [71].*

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 [78]. 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 [79]. 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 [80]. Many noble metal-graphenehybrid nanostructures have been prepared and tested for the degradation of organic pollutant dyes in water [81–83].

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 [84]. For example, Wen et al. [85] 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 [83]. 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 [86]. Jafari et al. [87] and Vasilaki et al. [88] 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 [89]. 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 [90]. 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 [91]. Correspondingly, Au/rGO/ZnO nanocomposite found to be an efficient photocatalysts for degradation MB dye in water [92, 93]. 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 [94].

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 [95]. 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 [96]. 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 [97]. Wang et al. reported Ag NPs@GO nanocomposite through light-induced synthesis method in which size dependent extremely

*Graphene Related Materials and Composites: Strategies and Their Photocatalytic Applications… DOI: http://dx.doi.org/10.5772/intechopen.102404*

#### **Figure 8.**

*TEM images of 5% Au/CNNS/rGO composites at low-magnification (a) and high-magnification (b) and (c). Inset in (c) represents particle size distribution curve of Au nanoparticles. HRTEM image of only Au nanoparticles (d). Reprinted with permission from Li et al. [96].*

high catalytic activity was investigated towards degradation of 4-nitrophenol [98]. Similarly, Ji et al. obtained Ag NPs@GO nanocomposite by in-situ reduction of Ag<sup>+</sup> ions into on GO nanosheets exhibiting efficient catalytic activity for the reduction of 4-nitrophenol into 4-aminophenol [99]. 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 [100]. 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 [45]. 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 [101].
