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May 25, 2018 - Recent progress of metal–graphene nanostructures in photocatalysis. Mohammad Ehtisham Khan, *a Mohammad Mansoob Khan. *b and.
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Recent progress of metal–graphene nanostructures in photocatalysis Mohammad Ehtisham Khan, Moo Hwan Cho *a

*a Mohammad Mansoob Khan

*b and

Metal–graphene nanostructures (NSs) as photocatalysts, prepared using simple and scalable synthesis methods, are gaining heightened attention as novel materials for water treatment and environmental remediation applications. Graphene, the unique few layers sheet-like arrangement of sp2 hybridized carbon atoms, has an inimitable two-dimensional (2D) structure. The material is highly conductive, has high electron mobility and an extremely high surface area, and can be produced on a large scale at low cost. Accordingly, it has been considered as an essential base component for producing various metalbased NSs. In particular, metal-graphene NSs as photocatalysts have attracted considerable attention Received 30th April 2018, Accepted 30th April 2018

because of their special surface plasmon resonance (SPR) effect that can improve their performance for

DOI: 10.1039/c8nr03500h

the removal of toxic dyes and other pollutants. This review summarizes the recent and advanced progress for the easy fabrication and design of graphene-based NSs as photocatalysts, as a novel tool, using a

rsc.li/nanoscale

range of approaches, including green and biogenic approaches.

Introduction a School of Chemical Engineering, Yeungnam University, Gyeongsan-si, Gyeongbuk 38541, South Korea. E-mail: [email protected], [email protected]; Fax: +82-53-810-4631; Tel: +82-53-810-2517 b Chemical Sciences, Faculty of Science, Universiti Brunei Darussalam, Jalan Tungku Link, Gadong, BE 1410, Brunei Darussalam. E-mail: [email protected]

In recent times, with the rapid development of industries, several kinds of environmental pollution, including water pollution, air pollution and soil pollution, are now a serious matter of concern for the environment and human health.1,2 With concern to reducing water pollution, major pollutants

Mohammad Ehtisham Khan received his MSc in Chemistry and M. Tech. in Nanotechnology from Aligarh Muslim University, Aligarh, India during 2010 and 2013 respectively. Dr. Khan has earned Ph.D. from School of Chemical Engineering, Yeungnam University, South Korea in 2017. At present he is working as a postdoctoral research associate in School of Chemical Engineering, Yeungnam Mohammad Ehtisham Khan University. His research area includes synthesis of metal, metal oxides nanoparticles, graphene based nanostructures using various kinds of environment-friendly approaches like as, electrochemically active biofilms, hydrothermal, solvothermal, easy and novel methods for effective antimicrobial agents, visible light-induced photocatalysis, photoelectrochemical and photocapacitive applications.

Mohammad Mansoob Khan has earned his MSc and PhD from Aligarh Muslim University, Aligarh, India. After PhD, he has taught in different countries (India, Ethiopia, Oman, and South Korea) for sixteen years. Currently, he is working as a Senior Assistant Professor at Chemical Sciences, Faculty of Science, Universiti Brunei Darussalam, Brunei Darussalam. His current research Mohammad Mansoob Khan interest includes green synthesis of metal nanoparticles, band gap engineering of metal oxides, metal oxide-based nanocomposites, and graphene-based nanocomposites through novel and simple methods. Synthesized nanomaterials are used for various energy and environment related applications such as H2 production and visible lightinduced photocatalysis.

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such as organic dyes, which are very toxic even at low concentration, should be removed from wastewater before it is directly released into the environment or sea water.2,3 The photocatalytic dye degradation process is an energy-saving and highly efficient technology that is gaining interest among the scientific community. In this regard, the discovery and development of visible-light induced photocatalysts is imperative, taking into consideration that substantial fractions (44%) of the entire solar spectrum is visible light, which can be properly utilized for this purpose.4 At present, metal–graphene nanostructure (NS)-based photocatalysis has attracted considerable attention because of its potential applications in a range of fields, such as environmental remediation through the photodecomposition of harmful dyes in polluted water and industrial effluents, and solar energy conversion.5–8 Extensive efforts have been made to develop effective photocatalysts for the degradation of organic pollutants.7–10 Most of these dyes are recycled extensively in several fields, but their release into aquatic bodies (sea, marine, river, etc.) can be a source of serious environmental pollution.11 In addition, dyes are toxic and carcinogenic, and have an adverse impact on human and animal health.11,12 Dyes have substantial applications in numerous industries, including paper, plastic, rubber, furniture, textile, concrete, medicine, oil refining, steel, and pharmaceutical.13,14 Among these industries, the textile industry is the main consumer of toxic dyes. However, approximately 10% of the dyes used in industry are released directly into the environment as a relatively ecologically unsafe and aesthetically unacceptable pollutant.14,15 Nanomaterials have promising applications in dye degradation for wastewater treatment.15–19 Currently, the most extensively studied nanomaterials for wastewater treatment include zero-valent metal nanoparticles (NPs)/NSs, metal oxides, and carbon-based NSs.16,17,20,21 The above-mentioned dyes (Fig. 1) are released directly into aqueous streams or as other waste forms from several industries, which causes environmental pollution through the direct discharge of carcinogenic and toxic substances into an

Moo Hwan Cho

Moo Hwan Cho is a Professor at School of Chemical Engineering, Yeungnam University, South Korea. He earned his MS in Chemical engineering from Korea Advanced Institute of Science and Technology in 1980 and received his PhD from Dept. of Chemical & Biochemical Engineering, Rutgers University, USA in 1988. Currently he is working on Microbial fuel cell and Nanomaterials for Energy and Environmental applications.

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Fig. 1

Different categories of dyes and their derivatives.

aqueous medium.16,17 The direct discharge of large quantities of toxic dyes is unavoidable because the individual textile industry consumes massive amounts of water and not all of the dyes can be combined totally with fibers during the dyeing procedure.20,22 Globally, approximately 79 000 metric tons of dye materials are formed annually, with 10 to 50% of this amount being released directly into wastewater.16,17,20 These high concentrations of dyes in effluents interfere with the dispersion of visible light in the water, which interferes with photosynthesis and reduces gas solubility. Moreover, artificial dyes, which contain an aromatic ring in their structure, are observed as noxious and carcinogenic compounds.23–26 Similarly, these types of dye may be toxic to marine life and carcinogenic to human beings, as well as having adverse effects on the reproductive system and causing dysfunction of the kidneys, brain, liver, and central nervous system.27 The removal of these carcinogenic and toxic dyes using conventional treatment methods (e.g. physical, chemical or biological) is inefficient and the operational cost is significantly high. Based on photocatalytic science, there are two types of photocatalytic reaction: homogeneous photocatalysis and heterogeneous photocatalysis. The most prominent features of a photocatalytic system are the required band gap, a suitable morphology, high surface area, stability, and reusability. This review article focuses on the recent progress in the simple fabrication, decoration/anchoring, modification, and water treatment applications of metal–graphene NSs as photocatalysts, and provides some perspectives on the future developments of water treatment devices.

Overview and fundamentals of plasmonic metal nanostructures Surface plasmonic resonance (SPR) effects can be induced by metals such as Au and Ag, and these are notable co-catalyst materials for photocatalysis because of their SPR influence.28–31 ‘Plasmons’ are the combined oscillations of

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free electrons in metals.32,33 These oscillations occur at a well-defined frequency, and a plasmon is categorized as a bosonic quasi-particle excitation and resembles a quantum of plasma oscillation. The electric field of incident light can pierce metallic NPs with a size comparable to the metal skin depth and polarize the conduction electrons.32,34 The plasmon effects in NPs with a size much smaller than the photon wavelength are non-propagating excitations, which are called localized surface plasmons (LSPs), because the resulting plasmon oscillation is distributed over the entire particle volume.34 Plasmon resonances of noble metal NSs with distinctive physical, chemical and biological properties are closely associated with their shape, size, composition, and structure.35,36 Several advances have been made in the synthesis of novel noble metal NSs and their potential applications in different fields, such as photocatalysis, nanoelectronics, sensors, and medicine.35–42 On the other hand, owing to the limited availability and high cost of noble metals, their combination with sustainable and inexpensive carbon-based materials is one of the most attractive ways to improve their properties and overcome their limited availability and high cost.43,44 Noble metals, such as Au, Ag, Pt and Pd,45–51 and/or mixtures of these metals with each other or with other carbonaceous materials have also been assessed for potential visible light-assisted applications. Au and Ag NPs have attracted particular attention because of the potential to improve their photocatalytic competency under visible light irradiation by acting as a substitute for an electron trap and reducing the charge recombination rate of the electron–hole pair by increasing interfacial charge transfer.44,46–51 Therefore, it is noted that Au and Ag NPs are capable of absorbing and scattering photons with a relatively high excitation cross-section in the visible region. Consequently, metal NPs are suitable for enhancing the optical properties of carbonaceous materials, such as graphene, activated carbon and carbon nanotubes, which can be effectively used for photocatalysis.32,34,52

Fig. 2

Review

Fabrication methods for plasmonic metal nanostructures There are several sets of synthetic methods that have been adopted for the fabrication of plasmonic metal NSs, which would ideally allow control of their shape, size, morphology, low cost, environmental friendliness, and high product yield with fewer waste products.33,53 Fig. 2 shows the numerous existing methods for the synthesis of plasmonic metal NSs. Several methods have been used for the fabrication of metal NSs with a controlled size and shape.33,53–55 Examples include thermal reduction, chemical reduction, silver mirror reaction, electrolysis, pyrolysis and electrochemically active biofilms (EABs) etc. In general, to fabricate a plasmonic photocatalyst, the metal precursor of Au3+ and Ag+ is mixed with a reducing agent in the presence of a stabilizing agent which can control the size and shape of the metal NSs. Silver nitrate (AgNO3) has been used as a precursor because of its low cost and abundance. Several reducing agents, such as sodium citrate, sodium borohydride and novel EABs, are commonly used to reduce the metal ions present in solution to metal/Ag nanocomposites, and may be combined to form novel NSs.56 On the other hand, in most cases, stabilizing agents are introduced to control and stabilize the morphology of the resulting metal NSs. Despite this, in photochemical synthesis, a variety of light treatment methods are assumed to produce metal NSs. Light mediated synthesis has been applied to the fabrication of NSs, e.g., laser ablation or direct laser irradiation of an aqueous solution of a metal salt precursor in the presence of a surfactant to make metal NPs of precise shape, size, and distribution, where the light source works as a reducing agent.57,58 Electrolysis and pyrolysis methods utilize an electrochemical approach for the fabrication of metal NPs/NSs.59 The citrate reduction method is also a popular method to synthesize a Au/Ag colloidal solution for the reduction of Au3+/Ag+ ions. In general, AgNPs are formed when an appropriate quan-

Several methods for the fabrication of metal nanostructures.

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tity of sodium citrate is added to a boiling aqueous solution of AgNO3 and left under those conditions for at least 1 h. Citrate reduction is a very simple method for the fabrication of AgNPs but it cannot produce NPs of limited size.60,61 Polyol synthesis is also a well-known and generally used method to synthesize metal NSs with an extensive range of sizes and shapes.62 In general, the metal salt precursor and capping agents are introduced into polyol to promote the nucleation and growth of metal NSs.36,62,63 The silver mirror reaction became popular for depositing Ag metal on targets or solid surfaces.64 The selfassembly of metal NPs can be significant for the initial shapes of the individual NPs. These methods can be used to control the self-assembly and the nature of forces involved in the selfassembly of NPs.65,66 The hydrothermal procedure is simple and has been used extensively for the combination of different NSs.67–69 The novel and biological approach is a green and sustainable methodology that has attracted considerable attention because of its potential to address energy and environmental problems.70,71 The development of biofilms is a green and sustainable approach for the fabrication of metal NSs/NCs.71 A bio-electrochemical system (BES) uses micro-organisms as a catalyst in electrochemical reactions. The best identified BES is the microbial fuel cell (MFC), which uses metal-consuming microorganisms to transform the chemical energy of the substrates dissolved in wastewater to electrical energy. BESs allow power generation from wastewater, power-driven electricity production, bioremediation, and biosensing applications.71–75 Recently, a mixed culture of EABs was reported to be a biogenic reducing tool for the synthesis of NPs (Au, Ag, Pt and Cys-Ag)76–78 and metal–graphene NSs.79,80 The benefit of these procedures is that the mixed culture EABs are used as reducing tools that do not require an external energy contribution, toxic chemicals, or expensive solvents. In addition, the reactions take place at room temperature, which makes the formation of NPs/NSs highly efficient.80

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Fig. 3 Schematic diagram of the mechanism of growing EABs on the surface of carbon foam and paper.

EABs are biofilms that are fully grown on a carbon-based support material (Fig. 3).74 The efficacy and performance of EABs have been studied extensively over the past few decades, particularly within microbial fuel cells (MFCs) to harvest energy from a range of environmental wastewater samples.81,82 In the past decade, the use of EABs for focused NP synthesis and modification has increased considerably, as EAB-mediated NP synthesis involves less chemical treatment and mild synthesis conditions that are friendly to the biofilm-residing bacteria.72–74,81 Accordingly, EABs are a potential platform for metal NP synthesis in the future. In the literature, mixed bacterial cultures have been used extensively in metal NP synthesis.74,81 The potential of the developed EAB-mediated NP/NC synthesis has attracted significant research interest. Fig. 4 shows the mechanism when using living microorganisms as catalysts in electrochemical reactions. These

Novel and green approach for the fabrication of metal–graphene nanostructures Development and utilization of EABs In recent times, green and sustainable approaches toward NPs/ NSs synthesis using micro-organisms, particularly bacteria, have attracted considerable attention.72 Bacteria have a tendency to develop a mechanism which involves eliminating the toxicity of noxious metals by varying their chemical nature in order to improve their survival and growth on a carbon support.73 This change in chemical nature leads to the formation of metal NPs over the graphene sheet. Therefore, the formation of metal NPs is literally an incidental by-product of a bacterial resistance mechanism against a specific metal, but this has been developed gradually as an alternative way of purposefully producing NPs.

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Fig. 4 A schematic diagram representing the synthesis of NPs onto graphene sheets using EABs.

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respiring bacteria convert the chemical energy of the substrates dissolved in wastewater to electrical energy.82 Different types of micro-organism may be used, which form a multilayered aggregation (i.e., a microbial biofilm) attached to the electrode and transfer the electrons derived from oxidation of the substrate (waste or organic matter) to the electrode. The ability of metal-respiring bacteria to reduce insoluble electron acceptors that cannot enter the cell (e.g., electrodes) requires an efficient extracellular electron transfer (EET) tool to shuttle electrons across the outer membrane of the microbial cell to the electrode.76,83–86 The best known application of EABs is the utilization of the electron producing efficiency for the reduction of metal ions onto a graphene sheet.

Fundamental outline of graphene and its key role in photocatalysis Graphene is an atomically thin two-dimensional (2D) carbonaceous material that has attracted considerable attention in the scientific community. The material is a thin layer of sp2 hybridized carbon atoms in a honeycomb crystal lattice87–90 with unique highly crystalline and electronic qualities.91 Despite its short history, this material has emerged as a promising new material for a range of exciting applications. Furthermore, graphene sheets exhibit exceptional optical, electronic, and mechanical properties, such as a charge-carrier mobility of 250 000 cm2 V−1 s−1 at room temperature, thermal conductivity of 5000 W m−1 K−1, an electrical conductivity of up to 6000 S cm−1, and a large specific surface area of 2630 m2 g−1.91–94 In addition, graphene sheets are highly transparent, with an absorption range of 420 nm) Visible light (λ > 420 nm) LED lamp (8 W, λ > 420 nm) Visible light (λ > 420 nm)

65% in 7 h 65% and 90% in 6 and 5 h 93% in 150 min 88.6%, 27.6% and 8.5%

105 79 106 107

Visible light (λ > 420 nm)

70% and 65% in 50 min

108

8 W, halogen lamp

70% and 82% in 180 min

109

72% and 43% in 120 min

110

2,4-Dichlorophenol (2,4-DCP) 4-Nitrophenol 4-Nitrophenol

125 W high-pressure mercury lamp (λ > 574 nm) Visible-light (λ > 420 nm) Visible-light (λ > 420 nm) Visible-light (λ > 420 nm)

78% in 150 min 97.38% in 360 s 100% in 175 min

111 58 2

2-Chlorophenol

Sunlight

100% in 180 min

3

4-Nitrophenol Rhodamine B Congo red and brilliant blue

Visible-light (λ > 420 nm) Visible-light (λ > 420 nm) Visible-light (λ > 420 nm)

100% in 360 s 90% in 120 min 25.74% for BB and 20.98% for CR in 120 min

58 112 26

Pt–Pd–graphene

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Pt/graphene Au@TiO2–graphene Au@TiO2/graphene Ag–Au on graphene sheets Metal nanocluster (Ag and Au)/graphene Au–Pd–reduced graphene oxide Ag–Au–rGO nanocomposite Graphene–Bi2WO6 nanosheets Ag/reduced graphene oxide

degrade phenol, bisphenol A, and atrazine under UV and visible light irradiation.104 Furthermore, Jana et al. confirmed the benefits of graphene sheets as a good support for anchoring metal NPs.104 Many efforts have been made to control the morphology, as well as to produce a more suitable energy state and novel hybrid structures in order to improve the effectiveness and stability/reusability of the photocatalysts. Table 1 lists the performances of recently-developed novel plasmonic NSs with graphene sheets as support materials. Several studies have chosen organic dyes as the model pollutant because the degradation process can be monitored simply through changes in the photoabsorption of the reacting or degrading solution.

General view and influence of the plasmonic resonance effect on photocatalysis In NPs, which are of an exceptionally conductive nature, free electrons are confined locally. When NPs are irradiated with electromagnetic energy at the plasma frequency, the spatial electron density rearranges and generates an electric field. Concurrently, a coulombic restoring force of the positivelycharged surface is present and promotes the combined oscillations of charges in the particles, similar to an oscillating spring after being stretched and released. Such oscillations of electrons and electromagnetic fields are defined as a localized surface plasmon. In the state of localized surface plasmonic resonance (LSPR) induced by the radiation of a specific LSPR wavelength, the free electrons will oscillate with maximum amplitude.76,84,85 Noble metal NPs are a new class of efficient medium suitable for harvesting light energy for chemical processes on account of their high optical absorption over a wide

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range of the visible light spectrum.48 On the other hand, the LSPR effect of Au and AgNPs plays an efficient role in photocatalytic systems with graphene sheets as an electron quencher and support material. Synergistic effect of noble metals on graphene nanostructures Graphene-based metallic NSs combining the properties of both components in a synergistic manner display significant effects on the photocatalytic degradation of pollutants and dyes.113,114 Both components have been used for surface enhanced Raman scattering (SERS).115,116 SERS is an influential and consistent analytical tool for the ultra-sensitive detection of analytes, even at the single molecule level of nanomaterials.117 Precious metal NPs are normally used to enhance the signal intensity by orders of magnitude, and they have been exploited for the ultra-sensitive detection of various analytes which comprises a series of chemical and biological molecules. Among the precious metal NPs, AgNPs that are available with good shape, size and morphology show high SERS activity. Significant efforts have been made to prepare graphene based AgNPs with high SERS activity and surface area.118,119 The paramount feature is the localized SPR effect of the noble metal NPs in response to incident light, which improves the absorption level, the local electric field, and the excitation of active electrons and holes.112,114 The basic principle of the metal plasmonic effect can be understood by comparison with a simple harmonic oscillator. Metal NPs can be observed as heavy, positive nuclei surrounded by a cloud of free electrons. When an oscillating optical electric field is incident on the metal and acting as the applied force, the electrons surrounding the metal NPs are driven and move along the field direction. Simultaneously, the coulombic attraction between the delocalized electron cloud and the nuclei acts as the restoring force, which is opposite to the displacement of electrons.120,121 At a certain resonant frequency, the oscil-

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lations started by the coulombic attraction become in phase with the oscillating electric field. Consequently, the incident light is absorbed by the metal which is effectively energy stored in the metal. In addition, metal NPs such as Au and Ag with enough free electrons are suitable materials for the absorption of visible light to activate the photocatalysts.122 Graphene sheet is a flexible and smooth 2D honeycomb structure able to adsorb many molecules, especially aromatic molecules.123,124 Therefore, graphene sheet has been extensively used as a support material for decoration with metal NPs to obtain high, good and efficient plasmonic properties for efficient photocatalytic degradation of dyes. Recently, Khan et al. have described the biogenic synthesis and surface plasmon effect of Ag@graphene and Au@graphene nanocomposites using an EAB as a reducing agent.79,105 Furthermore, the role of the interface between metal NPs and 2D materials should be systematically studied and fully understood in hybrid NSs. The interface plays a crucial role in charge transfer between the metal components and the graphene sheet materials.125 However, a method to appropriately design and achieve a desired interface and an understanding of the exact role of the interface in charge transfer involved in the applications above are still open issues. A Schottky junction forms at the interface when a metal nanomaterial comes into contact with a 2D graphene sheet. To attain efficient charge transfer, the potential barrier at the interface, which is determined by the potential difference between the conduction band of the 2D semiconductor and the Fermi level of the metal NPs, should be well developed. In addition, the bonding nature of the interface also influences the barrier. Under visible-light irradiation, the plasmon-induced hot electrons can transfer from the metal to the conduction band of the 2D material after overcoming the potential hindrance. On the other hand, the photogenerated electrons from the conduction band of the 2D materials can directly transfer to the metal NPs.125 Understanding and controlling charge transfer at the interface is paramount for studying the plasmon-enhanced behavior. The plasmon-induced hot carrier generation in graphene is dominated by direct photoexcitation in the near field. In conclusion, a considerable role is played by the formation of a Schottky junction subsequent to the direct contact of the noble metal NPs with the electron quenching effect of the graphene sheets.126 This improves the separation of the photoexcited electrons and holes and suppresses their charge recombination rate. Furthermore, 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.114,126–128

Fundamental mechanism of metal– graphene-supported photocatalysis The unique 2D structure of graphene sheet and its physical, chemical and mechanical properties provide ideal supportive

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behavior for any catalytic activity.129 Although single layer graphene has not yet been used as a catalytic support, the application of few layers of graphene in the field of catalysis is of increasing importance. Apart from the large specific surface area, high adsorption capacity and good biocompatibility, graphene based NSs have been applied as support materials in photocatalysis.129 By the decoration/anchoring of metal NPs on the surface of a graphene sheet, fascinating properties emerge due to interactions between the individual components, which have been exploited for photocatalytic applications.130,131 Graphene sheets have received much attention in photocatalysis compared to most other carbonaceous materials. Photocatalysis can achieve the photodegradation of environmentally harmful dyes, photocatalytic hydrogen evolution, and the photosynthesis of useful chemicals.131 The basic mechanisms of nano-semiconductor-based photocatalysis comprise photochemical processes of visible light absorption, electron–hole pair generation, separation, and free charge carrier-induced redox reactions.132 This is beneficial for a wide range of applications, such as wastewater treatment, air purification, water splitting, and self-cleaning of surfaces.133–135 The basic principle in the photocatalytic degradation of dyes or pollutants is that when the catalytic metal NPs are irradiated with photons, whose energy is equal to or higher than the band-gap energy (Eg), an electron (e−-cb) is excited from the valence band to the conduction band, which leaves a hole (h+-vb). The excited electrons and holes migrate to the surface of another state.8 The rate of charge recombination is frequently inhibited by a scavenger or a carbon-based material, which can easily trap the electrons or holes. Consequently, more highly crystalline NS materials with fewer defects can generally minimize the trapping states and charge recombination sites, which results in improved efficiency in the use of photogenerated transporters for desired photoreactions.9 For higher photocatalytic efficiency, the electron–hole pairs should be well separated, and the charges should be transferred rapidly across the surface/interface to restrain recombination. Graphene has a π–π conjugation network, and its extraordinary conductivity has made it an effective electron acceptor material. Metal NPs absorb light and become excited. The excited electrons at the interface can be transferred to graphene nanosheets stabilized by the conjugation network, reducing the charge recombination rate.8,9,136 The favorable properties of graphene sheets, such as their excellent electrical conductivity, strong capability to accept electrons, work function, and surface physical and chemical properties, are dependent on their unique structure.137 The high specific surface area and interfacial features, such as atomic arrangement, surface chemistry, electronic structure, charge transport, molecular adsorption and activation abilities for specific reactants, affect the photocatalytic performance of graphene-supported photocatalysts significantly.137,138 Generally, the photocatalytic dye degradation properties of metal–graphene NSs are strongly dependent on the presence of metal NPs on the graphene sheets and their unique elec-

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tronic structure, consisting of a filled valence band and an empty conduction band.139 Metal–graphene supported photocatalysis can progress only when the necessary thermodynamic requirements, including an appropriate wavelength of incident light and matching of the levels of the conduction/valance bands with the redox potentials, are satisfied. In addition, the electrons in the valance band of the metal NP can be excited photo-chemically to its conduction band, leaving a positive hole in the valance band, under light irradiation with incident photons of suitable energy, which is greater than or equal to that of the band gap, Eg.140 The band gap can only utilize incident light with a wavelength smaller than 1246/Eg. In addition, the conduction band and valance band of the materials must be larger than the matching potentials of the specific reduction and oxidation reactions, respectively. Consequently, Eg and the conduction band/valence band levels of given materials must be considered together, which fundamentally determine the effectiveness of incident light and the possibility of achieving specific photocatalytic reactions.139–141 Metal–graphene NSs may have a hopeful future in the conventional catalytic degradation of dyes, but the success of these materials on an industrial scale critically depends on the large scale synthesis of high quality graphene with a controllable layer thickness at low cost and using an environmentally friendly strategy. Fig. 6 presents the fundamental mechanism for the photocatalytic degradation of dye pollutants. In the fundamental photocatalytic process, oxygen species such as superoxide radicals (O2•), hydroxyl radicals (•OH) and hydrogen peroxide (H2O2) are formed under visible light irradiation in situ, and these reactive oxygen species (ROS) initiate the photodegradation action.142,143 These radicals are formed by the photocatalytic reduction of oxygen and oxidation of water. Fig. 4 presents the mechanistic profile of the photoinduced charge separation, migration, and photocatalytic degradation process under visible light illumination. The degradation of dye pollu-

Fig. 6 Schematic diagram of the mechanism of metal–graphene supported degradation of dye pollutants.

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tants over metal–graphene NSs under visible light irradiation is proposed. In general, a Schottky barrier is formed when two materials with different work functions are combined, and the electrons are moved from materials with a lower work function to materials with a higher work function until the two levels reach equilibrium to form a new Fermi energy level.26,144,145 The equilibrium alignment of the Fermi level of the metal and graphene causes a built-in electric field in the space charge region near the interface, which promotes the separation of the photogenerated electrons and holes, and enhances the photocatalytic performance of the metal–graphene NSs.146,147

Impact of graphene sheet properties on photocatalysis The unique properties of graphene sheet, in conjunction with the size-dependent properties of nanomaterials, induces additional properties in the NSs, such as high adsorption capacity, an extended light absorption range, and better charge separation properties along with high stability. These are the features that graphene imparts on the NSs to improve the photocatalytic performance.147,148 The following are certain fundamental functions of graphene sheets in advanced photocatalysis for highly active metal–graphene photocatalysts: i. The unique 2D structure of graphene sheet and its physical, chemical and mechanical properties provide an ideal support for decoration with metal NPs. ii. Graphene sheet exhibits a specific surface area of 2150 m2 g−1, a high adsorption capacity and good stability. iii. Graphene is a zero-bandgap semiconductor with the highest carrier mobility at room temperature, ∼40 000 cm2 V−1 S−1. iv. Regarding graphene sheet absorbance, the optical conductivity is mainly contributed to by interband carrier transitions in the visible range. v. Graphene sheet exhibits a tunable plasmonic effect due to its ability for low losses and high confinement in the THz range and the mid-infrared range. vi. Graphene is able to reduce the electron–hole recombination rate by acting as an electron acceptor. vii. Graphene facilitates interactions between the metal and organic pollutants by π–π stacking. viii. Graphene enlarges the absorption range of the metal from the visible light region. ix. Graphene keeps the metal NPs dispersed and retards agglomeration. x. Graphene sheets improve the surface area of the nanostructured materials. xi. Graphene sheets acts as a support material for metal NP decoration, avoiding the leaching of metal NPs during water treatment processes. xii. Other benefits of graphene sheets in photocatalysis include longevity, robustness, easy recycling, enhanced adsorptivity, extended light absorption range, and suppressed charge carrier and recombination rates.

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Accordingly, graphene sheet-supported NSs are very promising for water treatment and optoelectronic applications, and they are also considered as materials for other applications in the future.

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Proposed strategies for improving the photocatalytic performance The fundamental features and properties of both components of the NS, i.e., the metal and graphene, needs to be tuned to improve the photocatalytic performance. Principally, noble metal NPs are the active constituent with an SPR effect. This endows the NSs with a photocatalytic response, and the graphene sheet acts as photo-sensitizer and helps prevent charge carrier recombination by collecting the photo-excited electrons from the conduction band.149,150 Decreasing the particle dimensions of the metal NPs into the nano-regime improves the surface area, which results in more surface active sites being available for the reaction, but at the same time decreases the band gap of the materials and increases the rate of charge carrier recombination.139,146,148 Graphene itself can tune the optical characteristics of semiconductors and promote a visible light response of the composite material. Hence, more efficient photocatalysts can be designed by taking advantage of the synergic effects of doping, defects and graphene on the optical properties.149 Graphene sheets with a perfect structure have a longer electronic mean free path for electrons, allowing them to flow beyond the metal–graphene interface, which makes recombination difficult.151 The reaction pathway assumed for the preparation of metal–graphene NSs commands the structure and properties of graphene, so the selection of a suitable route and reaction conditions is strongly recommended for obtaining graphene with desired properties.151 The introduction of defects in the metal NPs to trap excited electrons can also reduce the electron recombination rate. Therefore, defect engineering can be an important strategy to improve the photocatalytic activity of metal–graphene composites.99 The optimal quantity of graphene sheets in the nanostructured material is also a critical factor that has a significant effect on the photocatalytic performance, and needs to be adjusted precisely. An increase in the graphene content in a composite up to a certain threshold increases the photocatalytic activity, but beyond that graphene can impede the photocatalytic activity by preventing adequate interaction between light and the metal NPs.152 To decompose different types of pollutant and have a range of applications, there is a need for the balanced optimization of graphene sheets in NSs prior to their large-scale improved photocatalytic applications. Finally, graphene sheets act as a matrix that avoids surface energy-derived agglomeration among the particles with dimensions of a few nanometers, and in return the metal NPs maintain the distance between adjacent graphene sheets and prevent re-stacking of the sheets, which improves the photocatalytic performance of the NSs.97

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Potential applications of metal– graphene nanostructures as advanced photocatalysts The photodegradation of organic dyes using metal–graphene nanostructures as photocatalysts is an important and efficient strategy for environmental management and water purification. Due to the increasing population and accelerating industrialization, environmental contamination has become a major risk to public health all over the world. Since the first report on the heterogeneous photocatalytic remediation of environmental pollutants (CN− in water) on titania,153 heterogeneous photocatalysis has been used widely in environmental purification, such as in air and water purification.154–157 In particular, typical features such as phase structures, exposed facets, crystallinity, surface area, crystallite size, and shape are essential for enhancing photocatalytic efficiency in the degradation of pollutants in water or air. Consequently, a variety of ways developed for improving the photodecomposition efficiency of pollutants over semiconductors have been exploited.158 Among them, specific consideration has been paid to the development of graphene-supported NSs for photocatalysts with superior absorption capacity for pollutants, light-harvesting, charge transfer, and separation capability. These reports on the application of graphene in photocatalytic degradation have been summarized in other excellent reviews (Fig. 7).134,159–161 Generally, carbon-supported nanomaterials, particularly high surface area activated carbon, have been widely explored for water purification.162–164 Cost-effective water treatment using the high surface area of graphene sheet, which has approximately twice the surface area of the well-developed activated carbon, can deliver an abundant and improved substitute. Recently, graphene sheets and metal NSs have become important materials for water treatment strategies owing to their excellent physical, chemical and electronic properties, which makes them suitable candidates for the photo-assisted degradation of dye pollutants in water.163–165

Future prospects The up-to-date developments in the synthesis of metal–graphene NSs or graphene-based metal nanocomposites, with a special focus on approaches to their synthesis especially using green and environmentally friendly synthesis approaches, were discussed in detail. The combined effects between metal NPs and graphene NSs have allowed a way to design and explore a variety of new applications, ranging from water treatment to the energy sector. Many challenges need to be addressed before real industrial applications are possible. In particular, the large scale production of metal–graphene NSs with high and uniform quality is still challenging. For the fabrication of metal–graphene NSs regarding environmental applications, graphene sheet shows

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Fig. 7 Purified water could be obtained using metal–graphene NSs as photocatalysts.

great potential as an electron acceptor with reduction of the charge recombination rate. Therefore, metal–graphene NSs are most attractive for the degradation of different types of harmful dyes under visible light irradiation. On the other hand, one of the most exciting platforms for the fabrication of metal–graphene NSs is developed using an EAB as a reducing tool. This biogenic synthesis reduces the need for harsh reducing chemicals, which have been used previously by other researchers, and will help avoid environmental pollution. This updated review can help better understand various phenomena and properties related to metal–graphene NSs, particularly for water treatment applications. These points are expected to expand the horizons of metal–graphene NSs. i. Plenty of scope to explore new protocols for the synthesis of graphene sheet in bulk quantities, which must be cost effective, environmentally friendly and yield defect-free fewlayer sheets. ii. Appropriate understanding of the interactions will certainly boost the application potential of the NSs in various fields, including biosensing, catalysis, imaging and so on. iii. The controlled synthesis of these NSs, with well-defined size, shape and crystallinity, not only prevents the aggregation of graphene sheets, but also provides excellent templates for decoration with metal NPs with enhanced water treatment applications.

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iv. The integration and synergistic effects of these NSs greatly improve the photocatalytic degradation of organic dyes under visible-light irradiation. v. The electron–hole recombination and electron transfer rate can be improved when photoactive NPs are anchored onto the graphene sheets. In the presence of graphene, the excited electrons are transferred quickly through the sp2-hybridized network of graphene sheets. vi. The metal–graphene NS photocatalyst will be an interesting and effective material, if the synthesis can be achieved in a cost effective, simple, green and biogenic manner. vii. The precise photocatalytic mechanism of metal– graphene NSs is still unclear. This should be the subject of future research. viii. Improvements in the quality of graphene sheet will ultimately lead to the synthesis of uniform NSs, which can be fine-tuned for various potential applications. ix. A multidisciplinary approach must be used to improve the protocols for the bulk production of graphene sheet to achieve a mature form of nanotechnology in order to build potential water treatment devices. x. The synthesis of metal–graphene NSs with excellent photocatalytic activities using simple facile techniques via green routes can be a promising field for a range of clean approaches and applications for environmental remediation. xi. Therefore, future metal–graphene NSs with unique properties and characteristics will be synthesized and may solve various wastewater treatment and environmental problems as well as energy issues.

Conclusions Much progress has been made in the groundwork, decoration/ anchoring, and application of metal–graphene NSs. The latest progress has been in plasmon-enhanced photocatalysis, specifically on the three major roles of the plasmonic effect, i.e., light absorption range, spatial SPR effect and conversion to energetic electrons. The plasmonic mechanisms and their photonic effects show that the plasmonic effect of metal NPs is a very promising strategy to improve the photocatalytic performance. Emerging water treatment technologies involving supported metal–graphene NSs will be of interest if the synthesis can be made cost effective, simple, green and biogenic. The global expansion in clean water research is paving the way for practical and industrial applications of metal–graphene NSs, leading to the next-generation of effective water treatment devices and H2 production. The great potential of metal–graphene NSs in photocatalysis has been highlighted by the extensive research being performed in photovoltaic and artificial synthesis. This review article provides information on the applications of metal–graphene NSs for water remediation. The work also considers the possible synthesis mechanisms and focuses mainly on the green/biogenic fabrication of metal–graphene NSs involved in the degradation of different types of pollutants. In light of increasing concerns regarding

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water pollution, an exploration of novel and extra effective water treatment strategies is needed. This review provides a broad up-to-date summary of the area, and will be helpful in designing new water treatment strategies by building upon the green and biogenic synthesis approaches reported for effective water treatment applications.

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Conflicts of interest The authors declare no competing financial interest.

Acknowledgements This study was supported by the Priority Research Centers Program and by the Basic Science Research Program (Grant No.: 2015R1D1A3A03018029) through the National Research Foundation of Korea (NRF) funded by the Korean Ministry of Education.

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