Graphene composites in photocatalytic oxidation of aqueous organic contaminants – A state of art
Graphical abstract
Introduction
Semiconductor photocatalysis is a well-versed tertiary treatment technique to detoxify the organic pollutants in water and wastewater as a polishing process and is one of the most efficient technologies for environmental remediation (Di et al., 2012; Li et al., 2013; Surenjan et al., 2019). The key advantage of the photocatalysis process is the complete mineralization of organic compounds under ambient conditions without involving any mass transfer (Gupta et al., 2020; Khataee and Kasiri, 2010). The efficiency of the photocatalytic degradation process depends on the ability of the photocatalyst to excite under light, the range of light-harvesting window, the surface area of the photocatalyst to facilitate the adsorption of pollutants on its surface, and improved charge separation by suppressing the electron-hole recombination (An and Yu, 2011; Ibhadon and Fitzpatrick, 2013; Scott et al., 2019). Numerous metal and semiconductor-based photocatalysts such as TiO2, ZnO, Fe2O3, CdS, GaP, ZnS etc. were synthesized as particles and used for the degradation of persistent organic compounds. Even though the photocatalysis is efficient for the degradation of organic compounds, the separation of catalyst from aqueous solution after the treatment is a major hurdle. Immobilization of photocatalyst on a substrate can be an effective solution in this regard and also holding the additional benefits of overcoming the operating limitations such as aggregation of particles, instability of the catalyst in acidic condition, and reduced mass transfer limitation and will ease the separation of catalyst from aqueous solution after the treatment (Bo et al., 2013; Boussatha et al., 2018). The use of carbonaceous materials, for instance, activated carbon, carbon fibre, carbon nanotubes, or graphene etc. acts as the supporting material in photocatalysis and also participates in photocatalysis along with the metal/semiconductor as a composite material and enhances the overall efficiency of the treatment (Dasireddy and Likozar, 2018; Wang et al., 2017; Wei et al., 2017). Among these, graphene has a considerable attraction as a potential support and catalyst material for photocatalytic application due to its unique structural, chemical, optical and charge transfer properties (Kuang et al., 2020; Nidheesh, 2017; Zhang et al., 2015c). The graphene, graphene oxide (GO), reduced GO (rGO), exfoliated GO etc. hold exceptional properties to enhance the photocatalysis process and they are hereafter collectively called as graphene derivatives. Due to all these favourable characteristics, graphene derivatives have become one of the promising materials and a hot research area in the field of water and wastewater treatment, nanotechnology, and material science that resulted in thousands of publications in the previous decade.
When these graphene derivatives are coupled with the conventional photocatalysts that include metals and metal oxides, the advantages are synchronized and benefits the treatment efficiency. Many researchers have reported various preparation methods of graphene composites, characterization, and its application in water and wastewater treatment focusing on the removal of endocrine substances and emerging contaminants. The differences in the mechanism of the pollutant removal in the absence and presence of graphene derivatives are dealt with in the existing literature.
Knowing the importance of these graphene composites in photocatalysis, it becomes necessary to compile the existing knowledge and thus took it up as an objective of this review paper. This review paper is built in segments and the structure of each of them are as below: segment 2 that comes after the introduction details the methodology that was followed to build this review; segment 3 reviews the characteristic properties of graphene more integrally by discussing the individual characteristics of graphene derivatives and graphene composites, the involvement of functional groups in the photocatalysis, and its role in reusability of graphene composites which is a much needed but missing in the current review articles that focus on only the overall aspects; segment 4 summarises the different methods used for the preparation of graphene composites which is vital in determining its performance in treatment; the application of various graphene composites in pollution removal, achieved efficiency, and the mechanisms involved are discussed in detail in segment 5; a special attention on the visible-light photoactivity of various graphene composites which is unique and lacking in many of the current review articles is given in segment 6; the reuse potential of graphene composites which has the least attention in the current research scenario but vital in practical applications are briefed in segment 7; finally, segment 8 summarises and discusses the application of graphene composites in photocatalysis and its future prospects. As a whole, this review collects information on characteristic properties, synthesis methods, and application of graphene composites in photocatalytic degradation of organic compounds which will be useful for the researchers working in the field of environmental engineering, materials science, chemistry, and chemical engineering.
Section snippets
Review methodology
The study was conducted to get a scientific and complete understanding of the utilization of graphene composites for photocatalytic degradation of organic pollutants. Initially, the available literature in the area of interest was collected, followed by a literature screening and finally evaluated as shown in Fig. 1. Reliable publications in peer-reviewed journals were collected from search engines such as the web of science, google scholar, PubMed, and Scopus using different keywords as the
Characteristics of graphene for photocatalytic enhancement
The major hurdles influencing the performance of photocatalyst are visible light inactiveness, recombination of charge carriers, electron-hole recombination, low quantum yield, surface adsorption of pollutant and catalyst separation. Graphene incorporation in the photocatalyst structure can overcome these limitations due to its unique properties. The extraordinary chemical, electrical, optical and structural characteristics make graphene derivatives such as GO and rGO as one of the most
Preparation of graphene-based photocatalysts and photocatalytic activity
This preparation method starts with the discussion on different graphene derivatives which is vital in fixing the photocatalytic properties and the base material for the synthesis of graphene composite. After that, the synthesis methods of graphene composites are explained in detail and followed by the summarization and way forward in these aspects are added.
Graphene composites for the treatment of aqueous organic compounds
Numerous metal/non-metal oxides photocatalytic materials are effectively used for the photocatalytic degradation of organic compounds (Muthukrishnaraj et al., 2015; Huo et al., 2018). These photocatalytic materials are either semiconductor materials, bifunctional composites or modified composites which are capable of efficiently removing organic pollutants. However, due to the large bandgap, recombination of photo-excited electron-hole pairs and low visible light activity, they showed a limited
Role of graphene composites in visible light photocatalysis
Photocatalysis process requires highly efficient light sources for the activation of photocatalyst which limits its application at industrial scale. Development of increasingly efficient visible-light catalyst which is capable of harnessing solar irradiation is the need for more environmentally responsible photocatalytic application. The visible light activity of photocatalyst materials is improved through different processes such as impurity doping, dye sensitization, heterojunction structures
Reuse potential of graphene composites in photocatalysis
Having multiple advantages, the graphene composites can improve the feasibility of using it on an industrial scale (Li and Cao, 2011; Wang et al., 2014ac). Besides, the physical and chemical structure of the graphene composite can extend the photocatalytic stability and reusability (Wang et al., 2019) To make the best use of these benefits of graphene composites, extreme caution must be taken not to lose the precious and rare earth metals and metal oxides which come attached with graphene
Conclusion and future perspectives
Graphene has a large surface area to enhance the adsorption of organic pollutants through electrostatic attraction and π-π interaction, besides, it is highly desirable for environmental applications and pollutant removal. Photocatalytic enhancement by graphene composites for the removal of toxic organic contaminants in aqueous media has gained considerable attention in the last few decades. This review summarized the synthesis, characterization, and applications of various graphene composites.
Funding sources
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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