Harnessing of 2D Carbon-Based Heterostructures as a Photocatalyst Towards Wastewater Treatment

Abstract

Over the last decade, the exhaustive graphene family (graphene oxide, reduced graphene oxide, and graphene-based composites) has received plentiful research interest for its euphoric effect against the energy crunch as well as environmental hazards. Graphene, being a two-dimensional layered structure of sp² hybridized carbon atoms, has showcased superior electrical conductivity, high specific surface area, and tunable bandgap making it an excellent electron-transport material in the process of photocatalysis. Besides this, graphene oxide favours modifying the growth and stability of the semiconductors in the composite form. In addition to that, Graphene-based semiconductor heterostructures overcome the drawbacks of conventional semiconductor photocatalysts, like rapid electron–hole recombination, low adsorption, and less charge transport property. Among various aspects of Photocatalysis, wastewater treatment by the decomposition of hazardous chemicals in an aqueous medium is a crucial matter. In this context, graphene and its derivatives have proven their superiority in pollutant removal by the adsorption of toxic pollutants from water, as well as their effective decomposition by photocatalysis.

7.1 Introduction

In the world we have today with growing industrialization, water pollution has already surpassed the danger line. The released excess dyes and harmful chemicals into the river, lakes, etc. not only affects human life but also the entire biosphere—plants and organisms living in these water bodies [1]. Throughout the world, pure drinking water is becoming scarce day by day. According to the prediction by the World Bank, half of the world’s population will be living in water-stressed areas by 2025. Furthermore, a 2020 study published in The Lancet by researchers funded by the Global Burden of Disease Study projects that the world population will peak in 2064 at 9.7 billion [2]. In the recent decades, a lack of drinkable water has become a global challenge for civilization. Unregulated and uncontrolled human population and industrialization growth in the current era have disrupted the natural purification process, which has ultimately led to the scarcity of potable water. Treatment of wastewater and its reusability is a critical issue from both an environmental and a social standpoint, and it must be approached with caution to alleviate the age-old problem of water pollution. Over the few decades, various strategies based on biological, mechanical, electrochemical oxidation, and chemical processes have been initiated for the treatment of contaminated water [3,4,5,6]. But, given the lesser efficiency of these processes, there arose advanced oxidation processes (AOPs) as superior alternatives to all these processes [7, 8]. Among them, advanced oxidative processes (AOP) have gained the trust of the scientific community as a promising scheme for the destruction of organic contaminants in wastewater. In reality, in this process, highly reactive species, i.e., hydrogen peroxide (H₂O₂), hydroxyl radical (˙OH), and superoxide radical (Ȯ₂ˉ), are generated which act as the active agents for the mineralization of organic compounds or contaminants in water [9]. However, improvements in the efficiency of these processes, such as the ability to properly use natural resources like solar radiation, may assist to make them more cost-effective [10]. In this regard, among AOPs, heterogeneous photocatalysis has become the most efficient method with the efficacious degradation ability of a wide range of organic contaminants into CO₂, water, and some readily biodegradable mineral acids. When Akira Fujishima and Kenichi Honda discovered the electrochemical photolysis of water in 1972, it ushered in a new era in photocatalysis research [11, 12]. Since then, numerous semiconductor oxides like titanium dioxide (TiO₂), zinc oxide (ZnO), molybdenum disulphide (MoS₂), and perovskite oxides like bismuth tungstate (Bi₂WO₆) and their various form of heterostructures have been probed as an efficient photocatalyst [13,14,15,16]. In this book chapter, we will first focus on describing the fundamental mechanisms of photocatalysis, including the essential factors for an efficient photocatalyst. The later sections will cover the important properties of graphene and graphene derivatives (i.e., graphene oxide (GO) and reduced graphene oxide (rGO)), and their role in heterogeneous photocatalysis will be showcased, followed by a brief timeline of the development of graphene-semiconductor heterogeneous photocatalyst towards the decomposition of hazardous chemicals in the aqueous medium. And finally, we will wrap up this book chapter with a summary of the major challenges and the prospects with graphene-related heterostructure in the realm of wastewater treatment.

7.1.1 Photocatalysis and Its Mechanism

Photocatalysis is defined as the stimulation of a chemical reaction on the active surface of a catalyst by a sufficient amount of light energy (i.e., either UV or visible). Photocatalyst is the name given to this sort of catalyst. The overall process in photocatalysis occurred in five independent steps: (i) mass transfer of the reaction from the bulk phase (either aqueous or gaseous), (ii) adsorption of the reactants, (iii) photocatalytic reaction, (iv) product desorption, and finally (v) mass transfer of the products from the interface to the bulk phase [17]. When a semiconductor (SC) is excited with energy greater or equal to its bandgap (E_g) energy, electron (eˉ)–hole (h⁺) pairs are generated, where, e $_{CB}^{ - }$ is the electron in the conduction band (CB) and h $_{VB}^{ + }$ is the hole in the valence band (VB). The photogenerated electron–hole pairs can migrate to the semiconductor and then be adsorbed by the active surface of the photocatalyst.

The positive hole can oxidize the organic and inorganic pollutants directly, but in most cases, it is caused by the production of hydroxyl (˙OH) radicals. The ˙OH radical is formed by the direct oxidation of adsorbed water (i.e., hydroxide ion, OHˉ) on the surface of the semiconductor catalyst by the valence band hole. Also, the photogenerated conduction band electron indirectly produces ˙OH radicals. The redox potential of the conduction band electron is −0.52 V which is negative enough to reduce dissolved oxygen to superoxide (Ȯ₂ˉ) radical [18]. The superoxide radical reacts further with water to form a hydroxyl radical. Furthermore, the generated superoxide radical is very reactive, capable of immediately oxidizing colour or any other harmful molecule before decomposing into CO₂ and H₂O. Finally, the dyes or organic contaminants are degraded into environmental-friendly chemicals by these extremely reactive radicals. The overall reaction can be summarized as follows [19]:

$$SC\mathop{\longrightarrow}\limits^{hv}SC\left( {e_{CB}^{ - } + h_{VB}^{ + } } \right)$$

From valence band hole

$$h^{ + } +{{H_{2} O} \mathord{\left/ {\vphantom {{H_{2} O} {OH^{ - } }}} \right. \kern-0pt} {OH^{ - } }} \to {}^{ \cdot }OH + H^{ + }$$

From condition band electron

$$e_{CB}^{ - } + O_{2} \to \dot{O}_{2}^{ - }$$

$$\dot{O}_{2}^{ - } + H_{2} O \to H\dot{O}_{2} + OH^{-}$$

$$2e^{ - } + H^{ + } + H\dot{O}_{2} \to \dot{O}H + OH^{ - }$$

$$h^{ + } {\text{ }}or{\text{ }}\dot{O}_{2}^{-} or {\dot{O}H }+ Dye \to CO_{2} + H_{2} O$$

The oxidation–reduction process has been illustrated schematically in Fig. 7.1.

Fig. 7.1

Schematic representation of the mechanism of photocatalysis by a semiconductor photocatalyst

7.1.2 Essential Factors Involved in a Semiconductor Photocatalyst

Fundamentally, the efficiency of a semiconductor as a photocatalyst depends mainly on three key factors: (i) bandgap, (ii) conduction band and valence band position, and (iii) charge carrier dynamics, such as charge carrier mobility, diffusion length and lifetime as well as the rates of surface charge recombination and interfacial charge transfer process. Besides this, the photocatalytic degradation efficiency also depends on the surface area, as photocatalysis is a complete surface phenomenon [20].

Bandgap determines whether a photocatalyst will be activated with ultraviolet (UV) or visible irradiation. Some of the semiconductor material’s bandgap lies in the UV region. Like, bare ZnO (E_g = 3.37 eV), TiO₂ ((E_g = 3.31 eV), and ZnS (E_g = 3.67 eV) play the role of a photocatalyst only under the irradiation of UV light [21,22,23]. Our solar spectrum, on the other hand, has just 4–5% UV irradiation, with about 44% in the visible spectrum and the rest in the infrared. As a result, an additional energy source is required to activate the photocatalyst. To overcome this type of limitation, several approaches viz. doping to semiconductor heterostructure formation by coupling with a secondary substance (like noble metal, non-metal, other semiconductors, carbon-based materials, and so on) have been implemented. On the other hand, some semiconductor materials, like SnS₂ (E_g = 2.24 eV), MoS₂ (E_g = 1.29 eV), and CdS (E_g = 2.40 eV), can be activated as a photocatalyst with visible or sunlight irradiation owing to the presence of bandgap in the visible region [24,25,26]. But, for these types of materials, photocatalytic efficiency is also limited due to the constraint of other two factors like band position and charge carrier dynamics. In this case, heterostructure formation also plays a crucial role to make this photocatalyst a fruitful remedy for wastewater treatment.

However, the band edge position of the semiconductor is the most important for use as a photocatalyst. The energies of the conduction (E_CB) and valence (E_VB) bands govern the electron injection ability of the material at the surface. Also, the ability of a semiconductor to undergo photoinduced charge carrier transfer to adsorbed species on its surface is governed by the band energy positions of the semiconductor and the redox potentials of the adsorbed species. Therefore, the knowledge of the absolute positions of E_CB and E_VB band edges is indispensable to exploring the potential applications of the concerned semiconductor as an efficient photocatalyst. The energy level at the bottom of the conduction band actually determines the reduction potential of photoelectrons and the energy level at the top of the valence band determines the oxidizing ability of photogenerated holes, each value reflecting the ability of the system to promote reductions and oxidations, respectively [27].

When a semiconductor is in contact with the ionic interactions at the interface of the two phases, this leads to electrostatic equilibrium within the material. Particularly at the semiconductor/electrolyte interface, electrons flow from the phase of more negative Fermi energy (E_F) to the other to attain equilibrium, in which the semiconductor E_F matches the electrolyte E_{F, redox}. This causes the formation of a space charge layer (SCL) within the semiconductor phase associated with the bending of the band edges in the semiconductor. The magnitude and direction of band bending can be adjusted by an externally applied potential. The externally applied potential known as flat band potential (E_FB) diminishes the band bending in a semiconductor, in contact with the electrolyte. Hence, the E_FB determines the energy of two charge carriers at the interface [28].

From the thermodynamic point of view, adsorbed molecules can be reduced photocatalytically by conduction band electrons if they have redox potentials lower (more positive) than the flat band potential of the conduction band. That can be oxidized by valence band holes if they have redox potentials higher (more negative) than the flatband potential of the valence band [29,30,31]. However, the primary criteria to get a good semiconductor photocatalyst for organic compound degradation are the redox potential of the H₂O/˙OH (HO $^{ - }$ = ˙OH + e $^{ - }$, E $_{0}$ = −2.8 V) [32] couple lies within the bandgap domain of the material and the stability over prolonged periods of time. The bandgap and the corresponding band edge positions of some frequently used semiconductors are presented in Fig. 7.2 [33].

Fig. 7.2

The bandgap and the corresponding band edge positions of some frequently used semiconductors [reproduced with permission from Ref. [34]

An ideal photocatalyst should possess the following features: (i) photoactive, (ii) can utilize visible and/or near-UV light, (iii) biologically and chemically inert, (iv) photo-stable (not prone to photo-anodic corrosion), (v) inexpensive, and (vi) non-toxic. We must study the basic principles of photocatalysis to increase photocatalytic performance. The photocatalytic behaviour is also affected by the temperature and pH of the solution. Various ways for producing an effective semiconductor heterostructure photocatalyst have been developed throughout the decades and will be reviewed in the next section.

7.1.3 Historical Journey of Semiconducting Photocatalysts

The earliest venture in photocatalysis is credited to the German chemist Dr. Alexander Eibner when for the first time in 1911, he integrated the concept of the illumination of zinc oxide (ZnO) on the bleaching of the dark blue pigment, Prussian blue [34]. Bruner and Kozak also proposed the hypothesis of oxalic acid degradation in the presence of uranyl salts under light about this time [35, 36]. In 1913, Landau wrote an article that described the concept of photocatalysis through the invention of actinometric measurements, which are the foundation for calculating photon flux in photochemical processes [35, 37]. However, in the 1970s, the global oil crisis stimulated the worldwide research community to find an alternative source of energy. As a result, a breakthrough in photocatalysis research occurred in 1972, when Akira Fujishima and Kenichi Honda discovered the electrochemical photolysis of water [38]. In their work, the ultraviolet light was absorbed by the TiO₂ electrode, and electrons would flow from the TiO₂ electrode (anode: site of oxidation reaction) to the platinum electrode (cathode: site of redox reaction), with hydrogen production occurring at the cathode. After this seminal work, extensive research has been conducted utilizing a variety of semiconductor photocatalysts to investigate whether hydrogen may be produced from a clean and cost-effective source [39, 40]. These kinds of light-harvesting applications for semiconductor colloidal solutions have been explored for decades as a new material for solving the global energy issue and saving the environment. Wide-bandgap semiconductors, such as TiO₂ and ZnO, have generally proven to be better photocatalysts than low-bandgap materials, such as nickel oxide (NiO) and cadmium sulphide (CdS), owing to the former's higher free energy of photogenerated charge carriers and the latter's inherently low chemical and photochemical stability. Low-bandgap semiconductors, on the other hand, make greater use of solar radiation, lowering the cost of additional energy sources. Whereas wide bandgap semiconductors need an extra energy source to play the role of an activated photocatalyst. Furthermore, limited absorption spectra, low photon quantum efficiency, photocorrosivity, and rapid recombination of photogenerated electron holes are all disadvantages of bare semiconducting photocatalytic processes in practical applications.

In recent years, a great deal of work has gone into the creation of more efficient photocatalysts with higher quantum efficiency and better sensitivity to the visible spectral region. Promising results in this direction have been obtained with the use of several methods aiming at the modification of electronic and optical properties of semiconductors, including novel metal deposition, dye sensitization, doping with transition metals or non-metallic elements, use of composite semiconductor photocatalysts, semiconductor–carbon (S-C) heterostructure, and Z-scheme or multicomponent heterostructure. Among all types of composite formation, S-C heterostructure (S-C) plays an important role in heterogeneous photocatalysis. In an S-C heterostructure, different forms of carbon like activated carbon, carbon nanotube (CNT), and graphene and its derivatives like graphene oxide (GO) and reduced graphene oxide have been extensively used as a photocatalyst.

7.2 Photocatalysts in the Realm of Wastewater Treatment

Because photocatalysis is a surface phenomenon, researchers initially concentrated on the creation of different nanostructures with a larger surface-to-volume ratio. Moreover, the valence band and conduction band of the semiconductor change into discrete energy levels as a result of quantum size formation, which means that the valence band electric potential changes more positively; otherwise, the conduction band electric potential changes more negatively. The oxidation–reduction potential of electrons and holes is then raised, resulting in an increase in the oxidation activity of nanosized photocatalysts. The bandgap of ZnO and ZnS can be narrowed by doping with metal impurities such as Cu and Mn [41,42,43,44]. The incorporation of dopant atoms into the semiconductor crystal lattice introduces intermediate energy levels or trap states within the intrinsic semiconductor, which lowers the energy requirement for the photoexcitation of electrons as well as enhances the electron–hole pair charge separation [45]. As a result, the photocatalytic activity is improved. Inappropriate metal ion dopants, on the other hand, might behave as a recombination centre, resulting in reduced photocatalytic activity [46]. Non-metal doping, such as nitrogen (N), carbon (C), or sulphur (S), can also be used to restrict the bandgap energy to the ultraviolet–visible range by pushing the VB energy upward (S) [47,48,49,50,51,52,53,54,55,56,57]. Due to the nature of p-type doping, N and C doping can generate vacant states above the Fermi level (EF) and shift the CB into a lower energy region, resulting in a narrowing of the bandgap [58, 59]. Liu et al., for example, created a hierarchical flower-like C-doped ZnO superstructure with improved photocatalytic activity, which they ascribe to the action of doping carbon atoms into the ZnO lattice [60]. For further improvement of the photocatalytic activity of zinc-based materials, researchers have developed different types of semiconductors heterostructures. Typically, these heterostructures are classified into four types as follows: (a) semiconductor–metal heterostructure, (b) semiconductor–semiconductor heterostructure, (c) semiconductor–carbon heterostructure, and (d) multicomponent (comprising metals and semiconductors) heterostructure.

A combination of metal and semiconductors can be a more efficient photocatalyst for capturing solar light than either component alone. In most cases, n-type semiconductors are employed to make these heterostructures. A Schottky barrier is generated between the metal and semiconductor junctions, which can act as an effective electron trapping and inhibit electron–hole recombination, increasing photocatalytic behaviour. Several research groups, notably Kang et al. [61], Vaiano et al. [62], and Chen et al. [63], have synthesized Ag-modified ZnO towards increased photocatalytic performance. However, there is a concern here as well: metal is hazardous to the environment. Furthermore, correct coupling of a semiconductor with another semiconductor with different bandgap energy reduces the effective bandgap energy, allowing for more efficient charge separation, faster charge transfer to the catalyst, and a longer charge carrier lifespan. Jung et al. for example, found that flower-like CuO–ZnO heterostructure nanowires (NWs) exhibit higher photocatalytic activity than bare ZnO and CuO in the breakdown of acid orange 7 (AO7) [64]. Under visible irradiation, Bharathia et al. discovered that a 5 Wt% CuO-loaded CuO/ZnO heterostructure had the best degradation performance, with about 96.57% degradation efficiency towards methylene blue (MB) dye [65]. Using ZnOCu₂S with ZnS interlayer heterostructure, Ranjith et al. obtained 93.47% degradation of rhodamine B (RhB) dye within 75 min of sun irradiation [66].

7.3 Graphene: The Game Changer

Graphene has unique properties like high surface area possessing a two-dimensional (2D) structure of sp² hybridized carbon atoms has drawn tremendous attention to the scientific as well as engineering community. In 2004, two researchers at the University of Manchester, Professor Andre Geim and Professor Kostya Novoselov, first discovered graphene through the simple exfoliation of graphite by scotch tape [67]. The unique one-atom-thick 2D honeycomb lattice structure of graphene not only makes it the thinnest and strongest material in the universe, but also endows it with many excellent chemical and physical properties. The excellent features of graphene, like good adsorption, [68] good electrical conductivity (2000 S m⁻¹), superior carrier mobility with little scattering at room temperature (200,000 cm² V⁻¹ s⁻¹), [69, 70] extremely high theoretical specific surface area (2630 m² g⁻¹), [71] excellent environmental compatibility, and cost-effective production, [72] made it a suitable choice for photocatalytic applications. Moreover, due to these superior properties, 2D graphene exhibits more promising applications in optical, electronic, biological, and catalytic fields than other carbon allotropes (i.e., 0D fullerenes, 1D nanotubes, and 3D graphite) [73, 74].

However, graphene derivatives like graphene oxide (GO) and reduced graphene oxide (RGO) have acquired great interest than graphene as a superior support material for the formation of nanocomposites with metal oxide and sulphides towards photocatalytic wastewater management. Formation of graphene nanocomposites provides several benefits: (i) prevention of stacking of graphene sheets: carbon atom within the graphene structure possesses p bond oriented out of plane making them vulnerable to stack over one other, (ii) improvement of conductivity, (iii) reduction of photogenerated electron–hole pair recombination, and (iv) improvement of charge carried dynamics [75, 76].

7.3.1 Importance of Graphene Oxide (GO) in Photocatalysis

Talking about the graphene family, GO has garnered ample attention as an effective adsorbent due to its extreme hydrophilicity, ultrahigh theoretical surface area [71], and plentiful oxygen atoms on the graphitic backbone of graphene oxide in the form of epoxy, hydroxyl, and carboxyl groups which protrude from its basal planes and edges [77, 78]. These functional groups can strongly interact with a variety of organic materials and thus yield new materials that can be effectively tailored for the adsorptive removal of diverse water pollutants [79]. Also, the presence of these hydrophilic groups results in the stable dispersion of GO in aqueous solutions. It also disperses in several organic solvents.

Graphene oxide has a limited electrical conductivity, which causes it to behave as either insulating or semi-conductive depending on the degree of oxidation. In 1859, Oxford scientist Benjamin C. Brodie created graphite oxide by treating graphite with a solution of potassium chlorate and fuming nitric acid [80]. He claimed to have synthesized “paper-like foils” with a thickness of 0.05 mm. Hummers and Offeman created the Hummers’ technique in 1957, which uses a combination of sulphuric acid H₂SO₄, sodium nitrate NaNO₃, and potassium permanganate KMnO₄ with graphite powder and is still extensively used today with some modifications [81].

7.3.2 Importance of Reduced Graphene Oxide (RGO) in Photocatalysis

The reduction of graphene oxide is another graphene derivative with properties that are similar to graphene. Reduced graphene oxide (RGO) is a kind of graphene oxide that has had its oxygen concentration reduced by chemical, thermal, and other means. RGO is often less defect-free and of lower grade than graphene made straight from graphite. Residual oxygen and other heteroatoms, as well as structural flaws, are seen in reduced graphene oxide (RGO). Due to the relative simplicity of producing substantial quantities of graphene at a cheap cost, reduced GO is typically a logical and understandable choice for applications that require huge volumes of material.

When compared to broadband semiconductors, RGO has a narrow bandgap. As a result of its ability to absorb more photons than semiconductors, a huge number of electron and hole pairs are formed inside the material. RGO's high electrical conductivity allows it to transport photogenerated charge carriers to the semiconductor's conduction band, resulting in a large population of charge carriers in the composites that react with surface-absorbed O₂ and H₂O. As a result, on the surface of the nanocomposites, a huge number of superoxide and hydroxide radicals develop, which destroy the hazardous dyes [82]. Because graphene has a large surface area and a dense structure, its composites naturally minimize the time it takes for electron–hole pairs to recombine, enhancing its photocatalytic activity for the destruction of hazardous dyes. Pure semiconducting nanostructures have lower photocatalytic activity than composite formations of RGO and semiconductor oxides. Figure 7.3 shows a schematic representation of the differences between graphene, GO, and RGO.

Fig. 7.3

Schematic representation of the structure of graphene, graphene oxide, and reduced graphene oxide [reproduced with permission from Ref. 82]

7.3.3 GO-Based Semiconductor Photocatalysts

The formation of heterostructure of semiconductor materials with GO can be effectively used as a visible light-driven photocatalyst in wastewater treatment basically in organic dye degradation. The presence of active groups such as carbonyl, epoxy, and hydroxyl groups on the surface of graphene oxide enables it to interact with a wide variety of molecules and thus can undergo surface modification. Recently, numerous studies devoted to the utilization of self-assembled metal oxides and metal sulphide nanomaterials on GO for the removal of different water pollutants, have been reported. A major advantage of GO is that it can be used to facilitate electron transfer which consequently diminishes the probability of electron (e⁻)–hole (h⁺) pair recombination in such semiconductor photocatalysts. Fabrication of such materials with GO results in an increase in the absorption of light as well as the surface area of the composites. Thus, the photocatalytic activity of the composite materials increases dramatically. Therefore, the use of GO as a supporting material along with various other semiconductors as stable, recyclable, and fully efficient photocatalysts has been found to be a challenging area of research. For example, recently Lin et al. [83] have synthesized GO/TiO₂ heterostructure and studied the photocatalytic degradation of methylene orange (MO) dye. They have observed that GO/TiO₂ composite shows better degradation ability than bare GO or TiO₂ nanomaterials. Moreover, GO/TiO₂ composite shows better thermal stability. The comparison of photocatalytic degradation of MO dye with respect to the time of GO, TiO₂, and GO/TiO₂ composites in 48-W UV light irradiation is represented in Fig. 7.4.

Fig. 7.4

Photocatalytic degradation of GO, TiO₂, and GO/TiO₂ composites in 48-W UV light irradiation [reproduced with permission from Ref. 83]

Similarly, Durmus et al. [84] used a GO/ZnO nanocomposite as a photocatalyst to break down basic Fuchsin Dye with the help of UV light exposure. In this case, the GO/ZnO combination outperforms both ZnO and GO in terms of deterioration. The degradation rate constants for GO, ZnO, and ZnO/GO composite were found to be 8.52 × 10^–4, 2.92 × 10^–3, and 8.45 × 10^–3 min⁻¹, respectively. In the instance of GO/ZnO, photonic irradiation excites electrons in ZnO to the conduction band, whereas holes develop in the valence band. The fact that GO works as a photoexcited electron acceptor relates to the fact that excited electrons of ZnO move from the conduction band of ZnO to GO. As a result, photoelectron–hole pair recombination decreases, allowing a sufficient quantity of reactive radicals to be produced. Ahmad et al. [85] developed p-type NiO/n-type GO photocatalysts for the degradation of organic pollutants and conducted tests on the photocatalytic degradation of methylene blue (MB) as a test pollutant at room temperature using UV–Vis light irradiation. The greatest catalytic activity was found in p-type NiO/n-type GO nanocomposites with 65% GO. In approximately 50 min of irradiation, the photocatalyst dissolved about 97% of the original MB dyes. The construction of a p-type NiO and n-type GO heterojunction is critical for greater separation of photogenerated electrons and holes, as well as a high level of recombination inhibition, resulting in increased photocatalytic activity. Figure 7.5 depicts a p-type NiO/n-type GO nanocomposite photocatalyst in schematic form.

Fig. 7.5

Schematic representation of p-type NiO/n-type GO composite photocatalyst [reproduced with permission from Ref. 85]

On the other hand, visible light-activated photocatalyst may be seen in the composite synthesis of GO with a low-bandgap material. Gao et al. [86] developed a GO/CdS heterostructure and studied the photocatalytic degradation of Acid Orange 7 (AO7) under visible light irradiation as well as the antibacterial activity of these materials. GO–CdS has a substantially better efficiency in AO7 dye degradation and Cr⁶⁺ reduction than pure CdS. Furthermore, GO–CdS composites are highly effective against Gram-negative E. coli as well as Gram-positive B. subtilis. Furthermore, throughout the photodegradation process, the composites are photo-stable. Figure 7.6 shows a schematic representation of charge transfer and generation of ˙OH, as well as variations in AO7 concentration during photodegradation.

Fig. 7.6

a schematic illustration of charge transfer and formation of ˙OH, b the changes of AO7 concentration with the irradiation time [reproduced with permission from Ref. 86]

Many research groups have reported the fabrication of some GO-based semiconductor materials to improve photocatalytic activities towards wastewater management to a significant extent by coupling GO with various materials such as TiO₂ [83], ZnO [87], CuO [88], SnO₂ [89], Fe₂O₃ [90], WO₃ [91], Bi₂O₃ [92], BiVO₄ [93], Ag₃PO₄ [94], and many others.

7.3.4 Fabrication of GO-Based Ternary Composites

The synergistic impact of two semiconductor materials and graphene oxide sheet modification on the ternary composites’ increased photocatalytic activity has piqued our curiosity. The ternary composite is projected to have better light efficiency due to the synergistic impact of the semiconductor's light sensitivity and GO's quicker electron transfer capabilities. Cui et al. [95] have developed a unique triple-component TiO₂/Ag₃PO₄/graphene oxide (TiO₂/Ag₃PO₄/GO) photocatalyst with dual-channels for the separation of photogenerated charges. Under visible light irradiation, TiO₂/Ag₃PO₄/GO showed higher photocatalytic activity and stability over bare Ag₃PO₄, TiO₂/Ag₃PO₄, and Ag₃PO₄/GO in the degradation of Rhodamine B (RhB) and phenol solutions.

Because the electronic band structures of TiO₂, Ag₃PO₄, and GO are aligned, photogenerated electrons in Ag₃PO₄ can be quickly transferred to GO sheets via an Ag₃PO₄ → GO electron transfer channel, whereas photogenerated holes can be transferred to TiO₂’s valence band via an Ag₃PO₄ → TiO₂ hole transfer channel. The dual-channel for photogenerated charge separation at the TiO₂/Ag₃PO₄/GO interfaces finally reduces photogenerated electron–hole pair recombination, resulting in improved photocatalytic activity and stability. Figure 7.7a depicts the RhB dye degradation performance for all composites, whereas Fig. 7.7b depicts the logarithmic plot comprising degradation rate constants.

Fig. 7.7

a RhB photodegradation as a function of illumination time for TiO₂, Ag₃PO₄, TiO₂/Ag₃PO₄, Ag₃PO₄/GO, and TiO₂/Ag₃PO₄/GO under visible light; b the plots of -ln(C/C₀) versus t. where C₀ is the initial concentration of RhB before light irradiation and c is the concentration at irradiation time t [reproduced with permission from Ref. 95]

Various research groups have fabricated several extremely efficient GO-based dual-channel heterostructure ternary composites such as P25/Ag₃PO₄/GO [96], Ag/ZnO/GO [97], ZnO-g-C₃N₄ coupled with GO [98], Ag₂MoO₄/Ag/AgBr/GO heterostructure [99], N-TiO₂/Ag₃PO₄@GO [100], graphitic-C₃N₄/BiOI/GO [101], and Fe₃O₄/Cu-BDC/GO [102], which are successfully utilized as an enhanced photocatalyst.

7.3.5 Fabrication of RGO-Based Binary Composites

Reduced graphene oxide has a narrow bandgap compared to wide band oxides; so it absorbs more photons than oxides. As a consequence, a large number of electron and hole pairs are created inside the material. RGO can transfer the photogenerated charge carriers to the conduction band of the semiconductor, resulting in a huge population of charge carriers in the composites, reacting with surface-absorbed O₂ and H₂O. As a result, a large number of superoxide and hydroxide radicals are formed on the surface of the nanocomposites which degrade the toxic dyes. Mandal et al. [82] used ZnO quantum dots and GO solution as precursor materials for the solvothermal synthesis of ZnO/RGO nanocomposite photocatalyst. Effective photoluminescent quenching is found as a result of composite production and bandgap decrease. Within 25, 15, and 30 min, their optimized ZnO/RGO composite successfully removed Safranin O, Methylene Blue, and Rhodamine 6G dyes from the aqueous solution. The purpose of RGO in the composite is to prevent nanoparticle aggregation by increasing the number of active sites. It also has a higher capability for capturing photons and transferring photogenerated electrons to the conduction band of ZnO. As a result, recombination loss is decreased, and a high number of charges become accessible on the ZnO QD surface. Under light irradiation, the massive amount of oxide and hydroxide super radicals formed on the surface of the QD efficiently destroys the dyes. The method of dye photodegradation employing ZnO/RGO composites as a catalyst is shown schematically in Fig. 7.8.

Similarly, Wang et al. [103] used photocatalytic breakdown of phenol in aqueous solution to compare photocatalytic performance of TiO₂ and TiO₂-RGO composites work as a cocatalyst for the quick transfer of photogenerated electrons from TiO₂, resulting in less recombination and increased photocatalytic activity. In the case of 1 wt% RGO, there is a significant increase in photocatalytic activity. Furthermore, as compared to bare TiO₂ nanomaterial, synthesized is less photocorrosive. The photogenerated electrons on the conduction band of TiO₂ may efficiently decrease oxygen via the RGO nanosheets when exposed to UV light. The RGO nanosheets operate as a cocatalyst in this situation, allowing for the quick transfer of photogenerated electrons from TiO₂, resulting in a decreased recombination rate and increased photocatalytic activity. Furthermore, given the enrichment of phenol molecules on the surface of RGO nanosheets from solution and the strong coupling of TiO₂ nanoparticles and RGO, it is thought that photogenerated holes on the valence band of TiO₂ can easily transfer to the phenol molecules on the RGO surface via holes or ˙OH resulting in rapid decomposition. Figure 7.9 schematically depicts the photogenerated charge behaviour at the interface of TiO₂ and RGO.

Fig. 7.8

Schematic diagram showing the mechanism of the photodegradation of dye using ZnO/RGO composites as a catalyst [reproduced with permission from Ref. 82]

Fig. 7.9

Schematic diagram for illuminating the charging behaviour at the interface of TiO₂ and RGO [reproduced with permission from Ref. 103]

Over the decade, many researchers have synthesized ZnS/RGO [104], Fe₂O₃/RGO [105], ZnSnO₃/RGO [106], RGO/SnO₂ [107], SnS₂/RGO [108], and many more and effectively used with an efficacious photocatalytic behaviour towards the decomposition of organic contaminations in aqueous solution.

7.3.6 Fabrication of RGO-Based Ternary Composites

The production of several highly efficient RGO-based dual-channel heterostructure ternary composites, similar to GO, has been the subject of research. Anjaneyulu et al. [109] recently used a ternary nanocomposite composite of MoO₃/Fe₂O₃/RGO as an effective visible light-driven photocatalyst for the degradation of MB dye. The photoelectrons generated in the conduction band (CB) of Fe₂O₃ are transported to the CB of MoO₃ due to the creation of heterojunction. Meanwhile, the exciting holes in the MoO₃ nanobelts’ VB are transferred to Fe₂O₃'s VB. Finally, by semiconductor–carbon heterojunction, the produced photoelectrons are collected by the RGO sheets. The huge surface area of the RGO sheets, along with their high electrical conductivity, facilitates the passage of produced photoelectrons to the composite surface, preventing recombination between photoinduced electrons and holes. As a result, enhanced photocatalytic activity is spotted. The mechanism of photocatalysis is represented schematically in Fig. 7.10.

Fig. 7.10

Schematic diagram of a possible photocatalysis mechanism [reproduced with permission from Ref. 109]

Many RGO-based ternary heterostructures, such as Ag/AgBr/RGO [110], RGO/BiOCl/TiO₂ [111], CdS-TiO₂-RGO [112], ZnO-TiO₂/RGO [113], MoS₂-RGO/ZnO [114], RGO/ZnO/Au [115], RGO/ZnS-MoS₂ [116], RGO/Fe₂O₃/g-C₃N₄ [117], and others, have been successfully employed in water decontamination throughout the previous decade. In photocatalysis, the core–shell structure can also play a significant role.

Ding et al. [118] also created magnetically separable Fe₃O₄@CuO-RGO core–shell heterojunctions and used them as a high-performance photocatalyst for organic dye removal under visible light irradiation. Zhou et al. [119] used a one-step hydrothermal approach to make plasmonic Ag@CdSe-RGO hybrid ternary composites, during which the in-situ nucleation of Ag@CdSe core–shell nanoparticles and the reduction of GO to RGO happened concurrently. Because of the presence of RGO, the improved photocatalytic degradation under visible light irradiation can be attributed to plasmonic enhancement of Ag via charge and energy transfer, which serves to prolong charge separation while also further enhancing the separation of photogenerated eˉ–h⁺ couples. The hypothesized process for the Ag@CdSe-RGO Nanoparticles production is depicted in Fig. 7.11.

Fig. 7.11

Schematic representation of the proposed mechanism for the formation of the Ag@CdSe-RGO nanoparticles [reproduced with permission from Ref. 119]

7.3.7 Other Applications of Graphene-Based Photocatalysts

Graphene-based photocatalysts are also effective in hydrogen generation, environmental clean-up, energy storage, CO₂ reduction, sensing, solar cells, and antibacterial activity, in addition to better dye degradation in the aqueous medium. To boost visible light photocatalytic H₂ generation performance, Abbas Sadeghzadeh-Attar hydrothermally produced BiVO₄-TiO₂/RGO heterostructure nanocomposites [120]. At optimum RGO content (3 wt.%) under visible light irradiation, the highest photocatalytic H₂ production rate of 1427.1 μmol. h ^− 1 g ^− 1 with an apparent quantum yield of 6% at 420 nm was achieved, which was 2.5 and 1.5 times higher than TiO₂ nanotubes (563.5 μmol. h ^− 1 g ^− 1) and BiVO₄-TiO₂ (915.7 μmol. h ^− 1 g ^− 1), respectively. TiO₂ nanoparticles built on GO nanosheets demonstrated strong photocatalytic activity for the photo-reductive reduction of Cr(VI) [121], according to Hu et al.

In the presence of H₂O, Jiang and colleagues [122] developed a PVK-based Z-scheme heterojunction, α-Fe₂O₃/Amine-RGO/CsPbBr₃, for high-efficiency CO₂ reduction. Effective Z-scheme electron transfer from α-Fe₂O₃ to CsPbBr₃ is produced by carefully regulating the interfacial contact, resulting in increased charge separation and carrier lifespan. When compared to CsPbBr₃, the spatial separation of photogenerated carriers in the Z-scheme system allows for an 8.3-fold improvement in photocatalytic performance. Sarkar and colleagues used a hydrothermally produced NiS/RGO nanohybrid film as a Pt-free counter electrode (CE) to construct dye-sensitized solar cell devices with a power conversion efficiency of 9.5% [123]. The enhanced PCE is owing to the synergistic effects of highly catalytic NiS nanoparticles and electrically conductive RGO, which result in the improved electro-catalytic activity of the NiS/RGO nanohybrid for the reduction of triiodide. In comparison to self-assembled conjugate and RGO alone, Joshi et al. [124] produced peptide-functionalized reduced graphene oxide (RGO) nano bioconjugate and observed better antibacterial effectiveness against Escherichia coli with much lower cytotoxic activity towards erythrocytes. As a result of these investigations, graphene offers tremendous promise in a variety of nanotechnology applications.

Various graphene-based composites as a photocatalyst and their photocatalytic efficiency towards organic pollutants are tabulated in Table 7.1. The results in Table 7.1 claimed that the ternary composite showed promising efficiency compared to the binary materials or the bare semiconductors. To reach the optimum degradation efficiency, the photocatalyst must be easily synthesizable, cost-effective, solar light activated, feasible reusability, and also removal efficiency within a stipulated time of irradiation.

Table 7.1 Comparison of reported graphene-based composites and their degradation efficiency towards different pollutants

7.4 Future Prospective

The research on graphene-based heterostructure photocatalysts is still in its infancy stage though graphene-based materials are considered to be one of the most promising photocatalysts for wastewater treatment [125, 126]. There is a huge need for an alternate route of bulk synthesis of 2D-graphene-based heterostructures in a sustainable and cost-effective way. The massive application of this material is predicted to see in industrial wastewater treatment as well as purifying drinking water in the next 5 to 10 years. These materials will be honoured not only for their contributions to 3D printing technology, electronics, biotechnology, automobiles, energy production, cosmetics, and sensor technology; but also for their enormous role in the fields of electronics, biotechnology, automobiles, energy production, cosmetics, and sensor technology. However, graphene is still a novel contender in terms of application and it will require substantial processing and patterning technologies. When comparing graphene to activated carbon, the difference is that graphene does not require post-production functionalization because it has hydroxyl, carboxyl, and epoxide groups on its surface and edges, which let graphene create legendary composites with other semiconductors or metals. Manufacturing a metal-free catalyst will be a wonderful approach to reducing toxicity.

7.5 Conclusions

In conclusion, the versatility of 2D graphene nanosheets in producing composite photocatalysts by connecting with different semiconductors is highlighted in this book chapter. Graphene-based composites can capture photoinduced electrons originating from semiconductor nanoparticles by light illumination. The electrons can be stored in the π–π network of the composites. These electrons can also be travelled to other metal particles deposited on the graphene layer or used to generate active oxygenated radicals. In this way, graphene, GO or RGO retards the electron–hole recombination on semiconducting material as well as extends the absorption of light to longer wavelengths of the visible region. Additionally, GO and RGO can also act as a sensitizer, enhancing the photocatalytic efficiency under visible light irradiation.

The photodegradation of pollutants seems to depend on several factors, such as textural (surface area), chemical and morphological properties of the composites, type of interaction between the composite and the pollutant, and graphene (pure, oxidized, or reduced) content. However, because most of the model pollutants studied so far were dyes (e.g., methylene blue, rhodamine B, rhodamine 6G, methyl orange, and acid orange 7), other classes of pollutants and their photodegradation mechanisms, including emergent contaminants, should be investigated with graphene-based composites. Graphene-based semiconductor composites have shown promise in a variety of applications, including hydrogen generation, sensing, CO₂ reduction, energy storage, antibacterial activity, and self-sterilizing coating. They are, however, still in the early stages of development. More research is needed to create on a wide scale with high reusability for commercial use. Future research should concentrate on the stability of these composites as well as their immobilization on suitable substrates for the continuous operation of photoreactors. Finally, extensive investigation is necessary to develop a superior material for use in device manufacturing.