METAL OXIDEeGRAPHENE AND METALeGRAPHENE NANOCOMPOSITES FOR ENERGY AND ENVIRONMENT
Mohammad Mansoob Khan Chemical Sciences, Faculty of Science, Universiti Brunei Darussalam, Gadong, Brunei Darussalam
1. INTRODUCTION AND HISTORICAL DEVELOPMENTS 1.1 GRAPHENE Graphene was first exfoliated mechanically from graphite in 2004 by Geim and Novoselov, for which they later shared the Nobel Prize in Physics in 2010. This simple, low-budget technique has been widely recognized for the explosive growth of interest in graphene . Graphene is a carbon allotrope comprising a densely packed, atomically thin layer of sp2 hybridized carbon atoms in a honeycomb crystal lattice. This precisely 2-D material exhibits unique high crystal and electronic quality and has emerged as a promising new nanomaterial for a variety of exciting applications despite its short history [1,2]. Since 1990, carbonaceous materials such as carbon nanotubes (CNTs) and fullerenes have drawn considerable attention due to their exceptional electronic and mechanical properties, specifically after the discoveries of 0-D buckminsterfullerene and, shortly later, 1-D CNTs. Both CNTs and fullerenes have been proposed to be derived from 2-D graphene sheets that are viewed as key building blocks of all other graphitic carbon allotropes, such as graphite made up of graphene sheets stacked on top of ˚ ). CNTs and fullerenes can be virtually made each other (separated by an interlayer distance of 3.37 A by wrapping and rolling a section of a graphene sheet. However, in reality, they are not synthesized from graphene [1e6]. Therefore, interest in the research of carbon-based nanomaterials has increased further. Graphene, a 2-D carbon sheet with one-atom thickness, is one of the thinnest materials in the universe and has inspired huge interest in physics, materials science, chemistry, and biology [1e5]. Graphene has a theoretical surface area of 2630 m2 g1, a mobility of 200,000 cm2 V1 s1 at a carrier density of w1012 cm2, and the highest electrical conductivity at room temperature (106 S cm1) [5e8]. The strong mechanical properties of graphene with a Young’s modulus of w1 TPa and breaking strength of 42 N m1 and excellent thermal conductivity (w5000 W m1K1) are also favorable for various graphene applications [5e10]. Graphene absorbs 2.3% light over a broad wavelength range for each layer, which makes graphene transparent and suitable for specific optoelectronic applications Functionalized Graphene Nanocomposites and Their Derivatives. https://doi.org/10.1016/B978-0-12-814548-7.00014-3 Copyright © 2019 Elsevier Inc. All rights reserved.
CHAPTER 14 METAL OXIDEeGRAPHENE AND METALeGRAPHENE
[1,9,11e13]. The fast development in the graphene field has suggested the great potential of graphene in electronics, optoelectronics, and electrochemical and biomedical applications due to its unique structure and properties [1,2,9,12e16]. However, pure graphene sheets are limited for many applications despite their excellent characteristics. For example, graphene itself has weak absorption for light, making pure graphene sheets not suitable for collecting solar light efficiently. The capacitance of graphene is limited by the electronic double-layer regime and is very low. Therefore, large-scale practical applications of graphene face the challenge to induce more and controlled functionality to pure graphene sheets. Although graphene sheets were mostly explored in fundamental physics at the beginning, graphene nanocomposites or hybrids are attracting increasing efforts for real applications [2,9,11,17,18]. The recent focus on graphene as a general platform for nanocomposites has inspired many possibilities in energy and environmental aspects by introducing controlled functional building blocks to graphene [1,2,9,11e18]. Many methods have been developed to prepare functionalized graphene nanocomposites [2,7,11].
1.2 GRAPHENE-BASED NANOCOMPOSITES Graphene exhibits excellent compatibility with different active components (such as metal oxides, transition metals, and conducting polymers) for the fabrication of high-performance graphene-based composites via in situ hybridization and ex situ recombination [7e9]. In these composites, graphene and active components coexist in varied forms of microstructures such as sandwich-like, anchored, wrapped, encapsulated, layered, and mixed modes to form 3-D, 2-D, or 1-D macroscopic architectures [8,10], and these composites are usually referred to as graphene-based materials (GBMs). Currently, GBMs with tailorable nanostructures proposed exciting opportunities to handle the challenges and queries triggered by the growing global energy demands . Presence of GBM hybrids in devices considerably improved their performance due to their synergistic effects by suppressing the aggregation of graphene and interparticles, electron transport, improved charge transfer, and ion diffusion, which enables the exposure and porosity of active sites. All these lead to enhance the electrochemical stability, catalytic activity, and/or volume variations buffering during the charging/ discharging processes and redox reactions [7e10]. The progress made in graphene-based nanocomposites (Fig. 14.1) has been reported in several research and review articles, either with a focus on specific materials such as inorganic materials or polymers or on specific applications for energy and environment [9e12]. The novel catalytic, magnetic, and optoelectronic properties of graphene nanocomposites based on the hybridization with nanoparticles have attracted significant attention [2,13], particularly due to the unique sp2 hybridization of carbon bonds present in graphene, which facilitates the delocalization of electrons and
FIGURE 14.1 Graphene-based nanocomposites.
1. INTRODUCTION AND HISTORICAL DEVELOPMENTS
imparts graphene to possess excellent electronic conduction . This electronic conduction of graphene can be enhanced by incorporating various inorganic nanoparticles, including different metal and metal oxide nanoparticles. Due to the enhanced electronic and electrical properties and the synergistic effect between graphene and inorganic nanoparticles, graphene nanocomposites offer great potential for various applications including energy storage and energy conversion devices . Therefore, the interest in GBMs has been ever growing due to their peculiarities in combining desirable properties of building blocks for a given application. To date, great efforts have been made to uniformly combine different varieties of nanomaterials with graphene and explore their application in fields like electronics, chemical and biological sensors, energy conversion and storage, solar energy harvesting, etc. [2,9,11,14e18]. In order to further enhance the properties and to broaden the applications of graphene, various metal and metal oxide nanoparticles have been decorated on graphene (Fig. 14.1). Apart from enhancing the properties of graphene, the nanoparticles act as a stabilizer against the aggregation of individual graphene sheets, which is caused by strong van der Waals interactions between graphene layers. Therefore, more efforts and new strategies to synthesize graphene-based nanocomposites are essential [2,7,11]. On the other hand, a whole view of graphene nanocomposites from general preparation methods and functionalizations with all kinds of materials to both energy and environmental applications is still progressing and needs to be compiled. This chapter covers the recent advances and discussion of graphene, metal oxideegraphene, and metalegraphene nanocomposites for energy- and environment-related applications (Fig. 14.2). The development of easy synthesis methods for graphene offers a wide range of possibilities for the synthesis of graphene-based nanocomposites. Therefore, the use of graphene and GBMs as low-cost, environment friendly, and high-performance electrodes for energy and environment applications are highly favored. In this book chapter, current developments of graphene-based metal oxide and metal nanocomposites for energy- and environment-related applications have been highlighted.
FIGURE 14.2 Applications of the graphene-based nanocomposites dye-sensitized solar cells (DSSC).
CHAPTER 14 METAL OXIDEeGRAPHENE AND METALeGRAPHENE
2. METAL OXIDEeGRAPHENEeBASED NANOCOMPOSITES FOR ENERGY AND ENVIRONMENT The development of alternative strategies for the production of efficient and clean energy is one of the biggest challenges for the scientific community, especially for researchers working on energy- and environment-related issues. Due to the increasing air pollution, global warming, and growing environmental awareness, the efforts directed toward the development of energy storage and energy conversion devices with high power densities and energy densities have increased tremendously. Graphene-based metal oxide nanocomposites have gained enormous popularity in the field of electrochemical energy storage [2,11]. Because of their physicochemical properties such as high thermal and chemical stability, excellent electrical conductivity, large surface area, and superior thermal and mechanical properties, graphene-based nanocomposites have been exploited as electrode materials for electrical energy storage devices [2,8e11]. Additionally, their broad potential range and rich surface chemistry have allowed customizing the properties of storage devices. Therefore, graphene-based metal oxide nanocomposites (Fig. 14.2) have found wide applications for energy storage and energy conversion devices, such as lithium ion batteries, supercapacitors, and fuel and solar cells [2,8e11,19]. Several methods for synthesis of graphene and defected graphene using chemical, physical, and green pathways are proposed, which are very common and widely used [2e5,20,21]. In the last few decades, enormous efforts have been made to synthesize nanocomposites of graphene with metal oxide nanoparticles. Recently, plenty of methods for synthesis of graphene-based metal oxide nanocomposites using in situ chemical reactions and physical, biological, and green approaches are commonly used [2,11,19]. Metal oxides are well-known nanostructures that are used for various processes such as photocatalysis and materials for photoelectrodes, etc. [22,23]. When metal oxides are combined with graphene or reduced graphene oxide (rGO), it forms nanocomposites with enhanced characteristics and properties [24e27]. Metal oxides are of great technological importance in environmental remediation and electronics because of their capability to generate charge carriers when stimulated with the required amount of energy. The promising arrangement of electronic structure, light absorption properties, and charge transport characteristics of most of the metal oxides has made possible its application as photocatalyst . In another study, Khan et al. have reported that visible lighteactive TiO2 (m-TiO2) nanoparticles were obtained by an electron beam treatment of commercial TiO2 nanoparticles. The m-TiO2 nanoparticles exhibited a distinct red shift in the UVe visible absorption spectrum and a much narrower bandgap (2.85 eV) due to defects formation, changes in Ti4þ to Ti3þ ratio, and oxygen deficiencies in the m-TiO2. This report confirms that m-TiO2 can be used effectively as a photocatalyst and photoelectrode material owing to its enhanced visible lighte induced photocatalytic activity . Recently, Parwaiz et al. reported that cobalt-doped ceria/rGO (Co-CeO2/rGO) nanocomposite as a promising electrocatalyst with competent oxygen reduction reaction (ORR) kinetics mainly through a four-electron reduction pathway, and it surpasses Pt/C by a great margin in terms of stability and methanol tolerance. The Co-CeO2 nanoparticles of diameter 4e7 nm were uniformly grown on rGO by a facile single-step hydrothermal process. The as-synthesized Co-CeO2 nanoparticles/rGO nanocomposites further demonstrated as active energy storage materials in supercapacitors, underscoring the importance of the studied materials in renewable energy industries . An optimized 3%
2. METAL OXIDEeGRAPHENEeBASED NANOCOMPOSITES FOR ENERGY
Co-doping leads to the highest ORR activity and energy storage performance compared to not only their individual counterparts, but also other composites. The improved performance with 3% Co-CeO2/rGO is ascribed to the synergistic role of Co-doping that creates more active sites through formation of defects and rGO that increases the electronic conductivity and O2 adsorption. The Co-doped Co-CeO2/rGO also exhibits higher methanol tolerance activity and stability compared to the state-of-the-art Pt/C, which are highly desirable for ORR catalysts to be practically useful for directmethanol fuel cells (DMFCs). The bifunctional characteristics of 3% Co-CeO2/rGO nanocomposites in energy storage and conversion systems along with their cost-effectiveness and facile synthesis intended the importance of the studied materials . In another study, Khan et al. reported  the cerium oxide nanoparticles (CeO2 NPs) were fabricated and grown on graphene sheets using a facile, low-cost hydrothermal approach, and subsequently characterized it using different standard characterization techniques. X-ray photoelectron spectroscopy and electron paramagnetic resonance revealed the changes in surface states, composition, changes in Ce4þ to Ce3þ ratio, and other defects. Transmission electron microscopy (TEM) and high-resolution TEM revealed the fabricated CeO2 nanoparticles to be spherical with particle size of w10e12 nm. Combination of defects in CeO2 nanoparticles with optimal amount of 2-D graphene sheets had a significant effect on the properties of the resulting hybrid CeO2egraphene nanostructures, such as improved optical, photocatalytic, and photocapacitive performance. The excellent photocatalytic degradation performances were examined by monitoring their ability to degrade Congo red w94.5% and methylene blue dye w98% under visible light irradiation. The photoelectrode performance had a maximum photocapacitance of 177.54 F g1 and exhibited regular capacitive behavior. Therefore, the Ce3þ-ion, surface oxygen vacancies, and defects-induced behavior can be attributed to the suppression of the recombination of photogenerated electronehole pairs due to the rapid charge transfer between the CeO2 NPs and graphene sheets. These findings will have a profound effect on the use of CeO2egraphene nanostructures for future energy- and environment-related applications . Khan et al. also reported  that tungsten oxide (WO3) nanorods were grown on pure-graphene (P-graphene) nanosheets using a template-free and surfactant-less hydrothermal process at 200 C. The synthesis and purity of the synthesized WO3 nanorodsegraphene nanostructure was confirmed by UVevis diffuse reflectance measurements, photoluminescence spectroscopy, X-ray diffraction, Raman spectroscopy, TEM, and X-ray photoelectron spectroscopy. The results showed that WO3 nanorods were well distributed over the graphene nanosheets. The photocatalytic activity of the WO3 nanorodsegraphene nanostructure was tested for the photocatalytic degradation of the organic model pollutant dye under visible light irradiation. The photocapacitance performance of the as-prepared nanostructure was examined by cyclic voltammetry. The superior photocapacitive and photocatalytic performances of the WO3 nanorodsegraphene nanostructure were observed, which were mainly attributed to the combination of WO3 nanorods with graphene nanosheets. WO3 nanorods themselves have photocatalytic properties, but the overall performance of the WO3 nanorodse graphene nanostructure was significantly improved when WO3 nanorods were combined with the graphene nanosheets because of the fascinating properties such as high mobility of charge carriers and unique transport performance of graphene nanosheets. The robust nanocomposite structure, better conductivity, large surface area, and good flexibility of the WO3 nanorodsegraphene nanostructure appears to be responsible for the enhanced performances. This methodology and the highlighted results open up new ways of obtaining photoactive WO3 nanorodsegraphene nanostructure for potential practical applications such as visible lighteinduced photocatalysis and photocapacitive studies .
CHAPTER 14 METAL OXIDEeGRAPHENE AND METALeGRAPHENE
In continuation to this, chalcogenides are another class of semiconductor similar to metal oxides. Recently, it was reported  that cadmium sulfide nanoparticles (CdS NPs)egraphene nanocomposite (CdSegraphene) were prepared by a simple method in which CdS NPs were anchored/ decorated successfully onto graphene sheets. The as-synthesized nanocomposite was characterized using standard characterization techniques. A combination of CdS NPs with the optimal amount of 2-D graphene sheets had a profound influence on the properties of the resulting hybrid nanocomposite, such as enhanced optical, photocatalytic, and photoelectronic properties. The photocatalytic degradation ability of the CdSegraphene nanocomposite was evaluated by degrading different types of dyes in the dark and under visible light irradiation. Furthermore, the photoelectrode performance of the nanocomposite was evaluated by different electrochemical techniques. The results showed that the CdSegraphene nanocomposite can serve as an efficient visible lightedriven photocatalyst as well as photoelectrochemical performance for optoelectronic applications. The significantly enhanced photocatalytic and photoelectrochemical performance of the CdSegraphene nanocomposite was attributed to the synergistic effects of the enhanced light absorption behavior and high electron conductivity of the CdS NPs and graphene sheets, which facilitates charge separation and lengthens the lifetime of photogenerated electronehole pairs by reducing the recombination rate. The as-synthesized narrow bandgap CdSegraphene nanocomposite can be used for wide range of visible lighteinduced photocatalytic- and photoelectrochemical-based applications .
3. METALeGRAPHENEeBASED NANOCOMPOSITES FOR ENERGY AND ENVIRONMENT Functionalized graphene nanocomposites have shown promise for environmental applications from environmental sensing and monitoring to remediation. Graphene and graphene-based nanocomposites can be used as general platforms for sensing inorganic ions, biomolecules, and organisms and also as platforms for the removal of hazardous species for the environment [2,19]. In the following section, review on the recent progress of graphene-based nanocomposites for environmental applications with a focus on topics such as environmental remediation, organic species degradation, and removal has been made. Khan et al. has reported an environmentally benign, simple, cost efficient, one-step, surfactant free, and biogenic synthesis of a silveregraphene (Agegraphene) nanocomposite using an electrochemically active biofilm (EAB). The EAB was used for the reduction of Agþ to Ag0 onto the graphene sheets. The morphology, structure, composition, and optical properties and contact angle of the Agegraphene nanocomposites were obtained using a range of techniques, which confirmed the anchoring/presence of silver nanoparticles (AgNPs) onto the graphene sheets. The photocatalytic activity of Agegraphene was evaluated by the degradation of methylene blue and Congo red dye in aqueous solution at an ambient temperature in the dark and under visible light irradiation. The results showed that the photocatalytic activity of the Agegraphene nanocomposite was enhanced significantly by the loading of AgNPs on the graphene sheets. Contact angle measurements confirm the hydrophilic nature of the Agegraphene nanocomposite, which is very helpful in photocatalysis. The electrical conductivity and photocurrent measurements of the Agegraphene nanocomposite exhibited a much better performance than P-graphene. This study highlights the design of a novel facile
3. METALeGRAPHENEeBASED NANOCOMPOSITES FOR ENERGY
synthetic route for a new photocatalyst using the surface plasmonic resonance (SPR) of Ag and graphene as a support. The as-synthesized Agegraphene nanocomposite has potential applications in photocatalytic degradation of pollutants, photoelectrodes, and optoelectronic devices . In continuation to this study, Khan et al. also reported that  a simplistic and environmentally friendly approach using EABs was developed for the synthesis of an Auegraphene nanocomposite without the use of surfactants or capping agents. The as-prepared Auegraphene nanocomposite was characterized by standard techniques. In this study, the anchoring of gold nanoparticles (AuNPs) on graphene sheets was achieved using an EAB. The EAB assists in the bioreduction of Au3þ to Au0, and the AuNPs prevent the aggregation of graphene sheets and keep them apart because of the decrease in attractive forces between the graphene layers. The photocatalytic activities of the Auegraphene nanocomposite were evaluated by the photocatalytic degradation of methylene blue in an aqueous solution at ambient temperature in the dark and under visible light irradiation. The photocatalytic activity of the Auegraphene nanocomposite was enhanced significantly by the loading of AuNPs onto graphene sheets. The photocurrent of the Auegraphene nanocomposite was measured by linear sweep voltammetry, which exhibited much better performance than P-graphene. The high photocatalytic activity and photocurrent of the Auegraphene nanocomposite was attributed mainly to the anchoring of AuNPs on the graphene sheets. The synergistic effects of the SPR of AuNPs and the specific electronics effect of graphene holds great promise for the development of electrochemical devices. Therefore, the Auegraphene nanocomposite has potential in several fields, such as photocatalysis, photovoltaic, nanoelectronics, ultracapacitors, and sensors because of the enhanced photocatalytic and photoelectrochemical performance . In the exceedingly conductive nature of metalegraphene nanostructures, free electrons are locally confined. When the nanostructures get irradiated with electromagnetic energy at the plasma frequency, then the spatial electron density rearranges and consequently generates an electric field. Concurrently, a columbic restoring force of the positively charged surface is present and persuades the combined oscillations of the charges in the particles, similarly to an oscillating spring after stretch and release . Such oscillations of electrons and electromagnetic fields are defined as localized surface plasmon. In the state of localized SPR (LSPR), induced by radiation of a specific LSPR wavelength, the free electrons will oscillate with the maximum amplitude . The synergy of noble metals and graphene-based photocatalysts carries substantial changes to countless aspects on the photocatalytic degradation of dyes and pollutants . The utmost feature is the localized SPR effect of the noble metal nanoparticles in response to the incident light, which improves the absorption level, the local electric field, and the excitation of active electrons and holes . A predominantly attractive worth is that the metal nanoparticle can absorb visible light to activate the photocatalyst. An additional significant role is the formation of the Schottky junction after the direct contact of noble metal nanoparticles with the semiconductor or graphene. This significantly improves the separation of the photoexcited electrons and holes and suppresses their charge recombination rate. In addition, the surface plasmon separates the reactant molecules in the fluid and improves the adsorption level to the metal surface. The surface plasmon also heats up the local environment and increases the mass transfer of the molecules, which enhances the reaction rates. Furthermore, the metal turns as a “fast lane” for the excited electrons (or holes) to transfer to the metal/ fluid interface, traps them on the metal surface, and increases the contact area with the targeted reactants [32,33].
CHAPTER 14 METAL OXIDEeGRAPHENE AND METALeGRAPHENE
4. FUTURE PROSPECTS The progress of the synthesis of graphene-based metal oxide and metal nanocomposites as well as properties characterized by various techniques are progressing quite fast. The exceptional mechanical, electronic, electrical, catalytic, and photocatlytic properties of graphene-based metal oxide and metal nanocomposites have considerably attracted the attention of the scientific community worldwide. Hence, in recent years, the number of scientific publications related to graphene-based nanocomposites has increased exponentially. The advanced understanding on methods of surface functionalization, preparation of stable and homogeneous dispersions of graphene in large quantities, and progress in the colloidal synthesis of graphene-based nanocomposites has provided a wide range of possibilities for the fabrication of graphene-based nanocomposites by incorporating different functional groups or nanomaterials. The synergistic effects between graphene, metal oxide, and metal in the nanocomposites have started the way for designing and exploring a variety of new applications in the fields of energy, environment, medical, automobiles, aviation, etc. In light of the above developments, still there is adequate scope to explore new procedures and methods to synthesize graphene-based metal oxide and metal nanocomposites, which are costeffective and environmentally friendly and yield defect-free nanostructures. Most of the presently available synthetic methods lack the control over shape, size, edge, and thickness (number of layers) of graphene due to random exfoliation, growth, and assembly processes [2,11,19]. Despite the considerable progress in the synthesis of graphene-based nanocomposites, challenges exist in the applications of these nanostructures at industrial scale, such as that advanced applications of graphene-based metal and metal oxide nanocomposites require extensive research to understand the interactions between nanostructures and the graphene surface, which will have direct impact on the properties of these nanocomposites. An appropriate understanding of these interactions (interface chemistry) will surely enhance the application potential of the nanocomposites in various fields, including catalysis, photocatalysis, biosensing, drug delivery, imaging, etc. Additionally, to advance the processability of these nanocomposites, efforts need to be put toward the enhancement of dispersion of graphene. Even though considerable achievement has been attained in obtaining homogeneous dispersions of graphene in several organic solvents, efforts must be focused towards the prevention of restacking of graphene and the improvement of the dispersion quality of graphene-based nanocomposites. A number of methods have been used to fabricate homogeneously dispersed nanocomposites by many reduction and functionalization techniques. However, many reported reductants and surfactants may have adverse effects on the potential applications of these nanocomposites. Particularly, it is important to understand the biocompatibility and toxicity of these reductants and surfactants to make the resulting nanocomposites safe for biomedical, energy, and environment applications.
5. CONCLUSIONS Graphene attracted worldwide attention due to its unique 2-D structure with remarkable and striking properties as well as novel synthesis methods. Graphene is emerging as an ideal candidate for thin-film devices and composite structures by the amalgamation of other suitable materials to provide unprecedented solutions for several applications including electromechanical, chemical or biosensors, crashworthiness, and other practical applications for energy and environment devices. Graphene-based
nanocomposites are also a promising material for nanolights used in bioimaging, electrochemical sensors, photoelectrochemical, photocatalysis, photovoltaic, and optoelectronic devices. Considerable achievements and rapid development have also been made in graphene-based flexible devices such as smart gloves, electronic papers, touchscreens, and wearable electronics. Graphene multifunctional nanocomposites also show extensive and wider practical industrial applications. This book chapter presents a comprehensive literature on the synthesis and multifunctional properties of graphene-based nanocomposites. This book chapter can also be an input for understanding various phenomenon, characteristics, and properties related to the metal oxidee or metalegrapheneebased nanocomposites, particularly for those that are important for the catalysis point of view and photocatalysis applications. This will expand the horizons of graphene-based nanocomposites, and in the near future, it will open new windows for the benefit of human life.
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