**4. Photocatalytic roles of graphene oxide (GO)**

#### **4.1. Intrinsic photocatalytic activity**

Most of the photocatalytic materials are wide band gap semiconductors. GO is a p-doped material because oxygen atoms are more electronegative than carbon atoms [30]. The band gap of GO can be tunable by just varying the oxidation level. The fully oxidized GO can act as an electrical insulator while the partially oxidized GO can act as a semiconductor [31]. Introducing more oxygen enlarges the band gap, and the valence band maximum (VBM) gradually changes from the p-orbital of graphene to the 2p orbital of oxygen; the π\* orbital remains as the conduction band minimum (CBM), **Figure 2**.

We cannot overlook some of the constraints that may limit the use of GO in supporting photocatalysts. The first thing to issue is the operating temperature range. The temperature between 150 and 200°C is usually sufficient for the start of the GO labile oxygen-containing groups decomposition [35]. The second issue that there is a claim that the OH• radicals gener-

is ultimately mineralized as evident from the decreased total organic carbon (TOC) concentra-

It is expected that the chemical bonding and associated charge transfer at the interface between the photocatalyst and GO support can be used to fine-tune the electronic and chemical proper-

is restricted to the ultraviolet region because of its wide band gap, and the photo-generated electron-hole pairs in experience rapid recombination [37, 38]. GO sheets are particularly

GO support, leaving behind the holes in the supported semiconductor particles. Thus, the GO support acts as an electron acceptor to enhance the separation between the photo-generated electron and holes, thereby suppressing their recombination, and consequently enhancing the photocatalytic efficiency. In addition, the unpaired π-electrons on GO can interact with TiO<sup>2</sup>

The photocatalytic activity of the gap controlled composite is governed by other factors including GO to active sites ratio. After the threshold limit, the photocatalytic activity decrease with increasing the absorption and scattering of photons by the excess carbon content in the composite. Photocatalytic activity of hybrid material also depends on the interface between

crystals directly on GO substrate via hydrolysis with subsequent hydrothermal treatment.

and so retards recombination. Liang et al. [19] reported the growth of uniform TiO2

surface could lead to an oxidative attack on the carbon-rich structure, which

is the widely used photocatalyst, however, its activity

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as the photo-generated electrons will move toward the

composites exhibited

nano-

and GO facilitates charge separation

(**Figure 3**). The GO/TiO2

.

ated at the TiO2

TiO2

tion with increasing irradiation time [36].

**4.3. Charge separation and gap control**

ties of the active sites. For instance, TiO2

effective in separating charges on TiO<sup>2</sup>

to extend the light absorption range of TiO2

excellent photochemical activity under visible light irradiation.

and graphene an intense coupling between TiO2

**Figure 3.** The interaction between unpaired π-electrons on GO with surface TiO<sup>2</sup>

The photocatalytic characteristics of GO nanostructures were confirmed by measuring reduction rate of resazurin into resorufin as a function of UV irradiation time [32]. The band gap of GO locates between 2.4 and 4.3 eV. Upon excitation, the electron-hole pairs will be generated at the GO surface. Hence, the localized defects on the GO nanosheets trap the produced electrons and limit their recombination process with the counter-currently produced holes.

#### **4.2. Photocatalyst support**

Since GO was rediscovered in 2004, it represents the top of carbon materials in many of its properties including outstanding electronic, thermal, and mechanical properties. GO as a support has many distinctive features including two-dimensional structure with the large specific surface area, high adsorption capacity, and excellent dispersibility in both aqueous and organic solvents [33]. Furthermore, the spread of the oxygenated functional groups facilitates the decoration of its surface with photoactive spots. As GO can add many advantages to the photocatalyst, it can also avoid many special problems related to the active site. The photocorrosion of ZnO was inhibited to a large extent upon incorporation with GO [34].

**Figure 2.** The VBM and CBM positions of GO sheets with varying oxidation level.

We cannot overlook some of the constraints that may limit the use of GO in supporting photocatalysts. The first thing to issue is the operating temperature range. The temperature between 150 and 200°C is usually sufficient for the start of the GO labile oxygen-containing groups decomposition [35]. The second issue that there is a claim that the OH• radicals generated at the TiO2 surface could lead to an oxidative attack on the carbon-rich structure, which is ultimately mineralized as evident from the decreased total organic carbon (TOC) concentration with increasing irradiation time [36].

#### **4.3. Charge separation and gap control**

**4. Photocatalytic roles of graphene oxide (GO)**

remains as the conduction band minimum (CBM), **Figure 2**.

**Figure 2.** The VBM and CBM positions of GO sheets with varying oxidation level.

Most of the photocatalytic materials are wide band gap semiconductors. GO is a p-doped material because oxygen atoms are more electronegative than carbon atoms [30]. The band gap of GO can be tunable by just varying the oxidation level. The fully oxidized GO can act as an electrical insulator while the partially oxidized GO can act as a semiconductor [31]. Introducing more oxygen enlarges the band gap, and the valence band maximum (VBM) gradually changes from the p-orbital of graphene to the 2p orbital of oxygen; the π\* orbital

The photocatalytic characteristics of GO nanostructures were confirmed by measuring reduction rate of resazurin into resorufin as a function of UV irradiation time [32]. The band gap of GO locates between 2.4 and 4.3 eV. Upon excitation, the electron-hole pairs will be generated at the GO surface. Hence, the localized defects on the GO nanosheets trap the produced electrons and limit their recombination process with the counter-currently produced holes.

Since GO was rediscovered in 2004, it represents the top of carbon materials in many of its properties including outstanding electronic, thermal, and mechanical properties. GO as a support has many distinctive features including two-dimensional structure with the large specific surface area, high adsorption capacity, and excellent dispersibility in both aqueous and organic solvents [33]. Furthermore, the spread of the oxygenated functional groups facilitates the decoration of its surface with photoactive spots. As GO can add many advantages to the photocatalyst, it can also avoid many special problems related to the active site. The photocorrosion of ZnO was inhibited to a large extent upon incorporation with GO [34].

**4.1. Intrinsic photocatalytic activity**

112 Graphene Oxide - Applications and Opportunities

**4.2. Photocatalyst support**

It is expected that the chemical bonding and associated charge transfer at the interface between the photocatalyst and GO support can be used to fine-tune the electronic and chemical properties of the active sites. For instance, TiO2 is the widely used photocatalyst, however, its activity is restricted to the ultraviolet region because of its wide band gap, and the photo-generated electron-hole pairs in experience rapid recombination [37, 38]. GO sheets are particularly effective in separating charges on TiO<sup>2</sup> as the photo-generated electrons will move toward the GO support, leaving behind the holes in the supported semiconductor particles. Thus, the GO support acts as an electron acceptor to enhance the separation between the photo-generated electron and holes, thereby suppressing their recombination, and consequently enhancing the photocatalytic efficiency. In addition, the unpaired π-electrons on GO can interact with TiO<sup>2</sup> to extend the light absorption range of TiO2 (**Figure 3**). The GO/TiO2 composites exhibited excellent photochemical activity under visible light irradiation.

The photocatalytic activity of the gap controlled composite is governed by other factors including GO to active sites ratio. After the threshold limit, the photocatalytic activity decrease with increasing the absorption and scattering of photons by the excess carbon content in the composite. Photocatalytic activity of hybrid material also depends on the interface between TiO2 and graphene an intense coupling between TiO2 and GO facilitates charge separation and so retards recombination. Liang et al. [19] reported the growth of uniform TiO2 nanocrystals directly on GO substrate via hydrolysis with subsequent hydrothermal treatment.

**Figure 3.** The interaction between unpaired π-electrons on GO with surface TiO<sup>2</sup> .

The resultant GO/TiO2 hybrids were tested in the photocatalytic degradation of the wellknown rhodamine B dye. In comparison with commercial P25 TiO2 , the prepared hybrids showed a three time faster degradation rate due to the enhanced electronic combination between GO and TiO2 in addition to the remarkable higher gross surface area.

Hence, GO construct a carrier transport channel between ZnS and CdS to enhance cooperative effects. In addition, GO oxide platform allow the loading of 2 wt% Pt nanoparticles as cocatalysts for the ZnS-CdS/GO heterostructures. Under UV-visible and visible light irradiation the hydrogen generation rate of the resultant heterostructure is significantly improved to 1.68

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The contamination problem of natural water resources with pollutants of different forms is a problem that threatens public health. The photocatalysis is expected to play a major role in the water purification process because it has a great ability to exploit solar energy in pollutants degradation in the moderate temperature range. Visible light-responsive photocatalysis has been widely investigated for the treatment of inorganic, organic, and biological contaminated water. However, the application of photocatalysis in water treatment is still in the research

Since organic dyes pharmaceuticals are usually leakage with a significant part to the industrial wastewater during manufacture processes; it has received a lot of attention in terms of treatment processes, including photocatalysis. Because of the easy tracing of dye decolonization process, it has left a great scientific legacy of published scientific papers. In general, when the photocatalyst is irradiated with photon with energy compatible with the band gap energy (Eg), an electron is transferred from the VB to the CB, leaving behind a hole. Accordingly, the produced electron-hole pairs are involved in a series of chain oxidative-reductive reactions,

Photocatalyst + *h* ⟶ h+ + e<sup>−</sup> (1)

h+ + H<sup>2</sup> O ⟶ •OH + H<sup>+</sup> (2)

h+ + OH− ⟶ •OH (3)

h+ + pollutant ⟶ (pollutant)+ (4)

O2

<sup>−</sup> (5)

<sup>−</sup> + H<sup>+</sup> ⟶ •OOH (6)

e<sup>−</sup> + O2 ⟶ •O2

.

and 0.78 mmol h−1, respectively.

**5.1. Water treatment**

stage.

Eqs. (1)–(10) [42].

**5. Environmental applications**

*5.1.1. Degradation of organic pollutants*

#### **4.4. Heterojunction formation**

Incorporating GO with metal-containing semiconductors can initiate a p-n junction, which considerably improves the separation of photo-generated charges. This is a possible way to fabricate GO/semiconductor composites with different properties by using a tunable semiconductor [39]. GO/TiO2 composites were prepared by using TiCl3 and GO as reactants. Again, the concentration of GO in starting solution played an important role in the photocatalytic performance of the composites. The heterojunction between p-type GO and n-type TiO2 semiconductors functioned as the separator for the photo-generated electron-hole pairs, **Figure 4**. These semiconductors could be excited by visible light with wavelengths longer than 510 nm for the degradation of methyl orange. Also, the integration of GO with TiO<sup>2</sup> will significantly increase the photovoltaic response and significantly prolong its mean life-time of electron-hole pairs compared with that of TiO2 [40]. Similar to TiO2 , ZnO also can form a p-n heterojunction with GO for visible light absorption. Quantum dot sized ZnO nanoparticles deposited on graphene sheets with a p-n heterojunction interface were demonstrated by a change of photocurrent direction at different bias potential that significantly enhanced photoresponse properties under solar light irradiation [41].

### **4.5. Coupling multiple active sites**

GO can act as a common platform for more than one active site to produce enhanced heterostructure for photocatalytic activity. For example, ZnS-CdS/GO shows a twice activity toward photocatalytic hydrogen generation compared with ZnS-CdS standalone heterostructures [28].

**Figure 4.** p–n heterojunction formation at the interface between GO and TiO2.

Hence, GO construct a carrier transport channel between ZnS and CdS to enhance cooperative effects. In addition, GO oxide platform allow the loading of 2 wt% Pt nanoparticles as cocatalysts for the ZnS-CdS/GO heterostructures. Under UV-visible and visible light irradiation the hydrogen generation rate of the resultant heterostructure is significantly improved to 1.68 and 0.78 mmol h−1, respectively.
