**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, 102–104]. 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. [105] 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. [106], demonstrated the charge transfer mechanism of ternary heterojunction system

#### **Figure 9.**

*(a) Scheme of energy band diagram and photocatalytic degradation at BWO (Bi2WO6) and G-BWO (Graphene-Bi2WO6), reproduced with permission from Gao et al. [105], (b) mechanism of photocatalytic degradation over RGO/TiO2/ZnO hybrid structure. Reproduced with permission from Bilgin Simsek et al. [106]. Copyright @ Elsevier 2018, (c) schematic representation of (i) photo-generated electron transfer of GQD on the surface of TiO2 under visible-light irradiation and (ii) energy position of GQD/TiO2 under visible-light irradiation, reproduced with permission from Pan et al. [107]. Copyright @ ACS 2015.*

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

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**) [107]. 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 cm<sup>2</sup> V−1 s−1 [110]. More importantly the graphene can sustain current densities up to six orders of magnitude higher than copper [108]. 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 [104]. **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.

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 [133]. 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 [55]. 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. [36], 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


#### **Table 2.**

*The mechanisms of interaction and possible advantages and disadvantages of using graphene related materials as adsorbents to remove metal ions from aqueous solutions as part of environmental remediation.*

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

#### **Figure 10.**

*(A) Photocatalytic degradation of RhB (a) without catalyst, with (b) RGO, (c) RGO–TiO2, (d) P25, (e) RGO– SnO2, (f) RGO–TiO2-400, and (g) RGO–SnO2-400. (B) the energy diagrams of RhB, graphene, TiO2 and SnO2. Reproduced with permission from Zhang et al. [36]. Copyright @ RSC 2011.*

plane, and the degradation rate was even faster than that of TiO2 [134]. 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 [134]. 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**) [135]. 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

#### **Figure 11.**

*(a) Photocatalytic degradation mechanism for the chlorpyrifos by using CoFe2O4@TiO2/rGO nanocomposite, and (b) degradation pathways showing various intermediate by products. Reproduced with permission from Gupta et al. [135]. Copyright @ Elsevier 2015.*

the efficiency after 8 cycles [135]. 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. [136] 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 [139]. In this connection the Sravya et al. reported the photocatalytic degradation of pharmaceutical wastes specifically the paracetamol [138]. Highly efficient visible light active polyaniline (PANI)/Ag nanocomposites grafted reduced graphene oxide

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

(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 [140–142].
