**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 [27]. 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 [16]. Westwood et al*.* [31] and Sigmund et al*.* [32] 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 [34]. The most common strategies for the fabrication of graphene-based photocatalysts are shown in (**Figure 5**) [35]. 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 [36–38].

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

#### **Figure 5.**

*Synthetic strategies used to fabricate the graphene-based photocatalysts. Reproduced with permission from An and Yu [35].*

Owing to high efficiency, good stability and low cost the TiO2 is mostly preferred for graphene-based nanocomposites [39–42]. 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 [48]. For instance, Patil et al. reported unique wet chemical synthesis approach for graphene-wrapped Ag3PO4/LaCO3OH heterostructures (**Figure 6A**) [49]. 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 [51]. A remarkably high photocatalytic reaction activity of BiVO4 was found, when graphene was incorporated which is attributed to electronic charge

#### **Figure 6.**

*(A) Graphene wrapped Ag3PO4/LaCO3OH heterostructures. SEM images of Ag3PO4 (i), LaCO3OH (ii) and Ag3PO4/ LaCO3OH/graphene (iii). Schematic representation of photocatalytic degradation of MB and electron transfer process. Reprinted with permission from Patil et al. [49]. (B) One-pot synthesis of BiVO4/Ag/rGO nanocomposite (i) schematic illustration of hydrothermal synthesis, (ii and iii) photodegradation of MB dye. (iv) corresponding TEM image of composite showing ternary composite of BiVO4, Ag and rGO nanostructures. Adopted from Patil et al. [50].*
