Graphene photocatalysts

Graphene photocatalysts Luisa M. Pastrana-Martínez*,†, Sergio Morales-Torres*,†, José L. Figueiredo†, Joaquim L. Faria†, Adrián M.T. Silva† *University of Granada, Granada, Spain, †Universidade do Porto, Porto, Portugal

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5.1 Introduction Significant attention is being paid to the development of renewable and green technologies because of the increased energy demand and persistent environmental pollution. From this perspective, the sun, a free, clean, and inexhaustible resource, is regarded as a promising option [1–3]. New photocatalytic technologies are being developed to transform renewable energy (i.e., solar light) into chemical fuels and products, such as photocatalytic water splitting and carbon dioxide (CO2) reduction, among others [4–6]. Photocatalytic water splitting is aimed at the production of hydrogen (H2) using natural renewable resources like water and the sun. Moreover, by using solar energy, the photocatalytic reduction of CO2 can transform a harmful greenhouse gas (i.e., CO2) into valuable solar fuels such as methane (CH4) and methanol (CH3OH) [2,4,5,7]. Various developments over the past four decades, particularly in regard to energy and environmental applications, have shown great potential for semiconductor photocatalysis to be a low-cost, environmentally friendly treatment technology [8,9]. However, some limitations are still associated with photocatalysts, such as their inability to use visible light efficiently and their poor stability. Recent efforts have been devoted to the development of novel composite photocatalysts with the notion of increasing the efficiency of solar energy conversion. Graphene, a 2D monolayer of sp2 carbon atoms with a hexagonal packed lattice structure, was a breakthrough discovery in 2004, and it is now set to exceed all other carbon allotropes in material science and technology [10]. This material exhibits many interesting properties, such as high mechanical strength, superior thermal conductivity, outstanding transparency, a huge specific surface area, and excellent charge transport [11,12]. Graphene and its derivatives (e.g., graphene oxide, GO; and reduced graphene oxide, rGO) have stimulated interest in the design of sophisticated high-performance graphene-based composite materials for different applications, such as sensors [13], energy storage devices [14], bio-applications [15], and particularly graphene-based photocatalysts with improved solar-to-fuel conversion efficiency [16]. Graphene derivatives have been shown to induce some beneficial effects on the photocatalytic performance of semiconductor catalysts by creating synergies between the semiconductor and the carbon phases. This effect is attributed mainly to a decrease in the band gap energy of the composite catalyst, an enhancement of the adsorptive properties, and the charge separation and transportation properties. This chapter focuses on the current status of graphene-based composites applied in photocatalysis for energy applications, including photocatalytic water splitting and Multifunctional Photocatalytic Materials for Energy. https://doi.org/10.1016/B978-0-08-101977-1.00006-5 Copyright © 2018 Elsevier Ltd. All rights reserved.

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photoreduction of CO2. The chapter is organized into three major sections: (i) a brief introduction of graphene and its most common derivatives; (ii) a review of several important synthesis strategies for graphene-based photocatalysts; and (iii) the application of graphene-based semiconductor photocatalysts in photocatalytic water splitting to H2 and photocatalytic reduction of CO2 to hydrocarbon fuels. Finally, the major challenges for the future development of graphene-based photocatalysts to produce solar fuels are identified.

5.2 Graphene and its derivatives The first isolation of graphene was obtained simply by mechanical exfoliation of graphite using the Scotch tape method [10]. However, this method of preparation is not suitable for assembling graphene with other materials and for large-scale production. Many feasible routes have been developed to prepare various types of graphene materials, such as chemical vapor deposition (CVD), epitaxial growth, and so on [17,18]. However, the most popular methods available to produce graphene-based materials involve an initial strong chemical oxidation of natural graphite to graphite oxide, followed by its mechanical, chemical, or thermal exfoliation to GO sheets, which then can be chemically or thermally reduced, resulting in the rGO material [19,20]. The most obvious difference between pristine graphene (hereafter referred to as graphene) and GO is the presence of oxygen-containing chemical functionalities attached to the graphene surface, as shown in Fig. 5.1A and B, respectively. Graphene has a hydrophobic nature, whereas GO is hydrophilic; that is, it is easily dispersible in water and other polar solvents. In addition, GO contains both sp2 (aromatic) and sp3 (aliphatic) hybridizations, which further expands the types of interactions that can occur with its surface [21]. The chemical reduction of GO to rGO is also a promising

Fig. 5.1  Structure of (A) graphene, (B) GO, (C) rGO, and (D) doped graphene derivatives.

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route toward the large-scale production of graphene for different applications [22] (Fig. 5.1C). Furthermore, rGO offers an important advantage, namely the possibility to obtain a tailored hydrophilic surface of graphene decorated with oxygenated functionalities, which results in low production costs [19,20]. These surface groups can be used to facilitate the anchoring of semiconductors and metal nanoparticles and even for the assembly of macroscopic structures, which are relevant to developing highly efficient photocatalysts [23,24]. Heteroatom doping of graphene is a rising research approach toward enhancing the performance of graphene-based materials for a wide range of applications [25], which presents an opportunity to further extend the role of graphene in photocatalysis [26]. In heteroatom-doped graphene materials, a certain percentage of carbon atoms (typically below 10 wt.%) is replaced by other elements, such as nitrogen (N) [27–29], boron (B) [30], phosphorus (P) [31], and sulfur (S) [32,33] (Fig. 5.1D). Several possibilities exist for preparing doped-graphene materials, such as CVD; arc-discharge between two graphite electrodes in the presence of a suitable reagent containing the dopant element, for example, NH3 and H2S; ball-milling; and pyrolysis under inert atmosphere of a natural biopolymer [34]. It is worth noting that the presence of external atoms and defects in graphene is a critical point in catalysis applications. Indeed, the main graphene materials that have been applied in photocatalysis are GO, rGO, and doped-graphene derivatives [25,26].

5.2.1 General properties of graphene-based materials Graphene can be described as a zero-energy band gap semiconductor; this means that the π⁎-state conduction band (CB) and the π-state valence band (VB) of graphene touch each other at the Dirac point [35], as shown in Fig. 5.2 [35,36]. This unique band structure causes graphene to display amazingly high conductivity and

Fig. 5.2  (A) 3D band structure of graphene; (B) approximation of the low energy band structure as two cones touching at the Dirac point. The position of the Fermi level determines the nature of the doping and the transport carrier. Adapted with permission from P. Avouris, Graphene: electronic and photonic properties and devices, Nano Lett. 10 (11) (2010) 4285–4294. Copyright 2010, American Chemical Society.

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electron mobility. Note that the presence of foreign atoms (i.e., O, N, B, P, and S) and defect sites on the lattice of graphene will influence its electrical properties [26]. In fact, the presence of heteroatoms in graphene changes its properties, such as electric conductivity, thermal stability, and chemical reactivity, improving the efficiency and selectivity of the photocatalytic process and avoiding the need for noble metals in the photocatalytic process [37]. For example, the band structure of graphene can be tailored by chemical doping with electron-withdrawing oxygen functionalities or electron-donating nitrogen functionalities, which usually makes graphene a p-type or an n-type semiconductor, respectively (Fig. 5.2B) [38,39]. A tunable band gap, from insulating to conducting, can be achieved by controlling the reduction degree of rGO, as the band gap energy is strongly correlated with the number of oxidized sites, and the oxidization degree of rGO [40]. Understanding the consequences of doping on graphene's electrical performance is thus key to discovering its possible applications in photocatalysis. Generally, the advantages of graphene and its derivative-based photocatalysts can be categorized as (i) ideal electron sink and/or electron transport bridge to suppress photogenerated carrier recombination; (ii) band gap tuning acting as a photosensitizer to extend absorption of light; (iii) remarkable specific surface areas that can significantly increase the specific area of graphene-based photocatalysts; and (iv) good stability for long-term photocatalytic application. Given their remarkable properties, graphene and its derivatives provide a wide range of opportunities to prepare diverse forms of composite materials with extraordinary properties for the photocatalytic splitting of water to H2 and photocatalytic reduction of CO2 to hydrocarbon fuels. Particularly, the preparation of fine-tuned and robust graphene-based photocatalytic materials is necessary to meet the practical requirements for solar fuels generation.

5.3 Graphene-based semiconductor photocatalysts Graphene and its derivatives have been incorporated into many different semiconductors to fabricate graphene-based composites for various photocatalytic applications. These photocatalysts include inorganic and organic semiconductors, among others. Many preparation protocols have been carried out to prepare graphene-based composite photocatalysts, such as mixing and/or sonication, sol-gel, liquid phase deposition, UV-assisted photoreduction, self-assembling, and hydrothermal and solvothermal methods [20]. In this section, we discuss the synthesis of different graphene-based composites and consider different types of semiconductor photocatalysts and synthesis methods.

5.3.1 Synthesis of graphene-based titanium dioxide photocatalysts Since the pioneering work of Fujishima and Honda in 1972 [8], titanium dioxide (TiO2) has been the most widely studied material for synthesizing composites for photocatalytic applications because of its superior photocatalytic properties, easy availability, long-term stability, and low toxicity. Thus composites of graphene derivatives and

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TiO2 have been studied extensively because of the significant enhancement of photocatalytic activity. In the synthesis of graphene-TiO2 composite photocatalysts, TiO2 has been synthetized from different precursors, such as inorganic titanium salts—for example, titanium(IV) sulfate, Ti(SO4)2 [41,42]; titanium(IV) fluoride, TiF4 [43–45]; ammonium hexafluorotitanate(IV), (NH4)2TiF6 [46–48]; and titanium alkoxides that can hydrolyze easily in aqueous solution (e.g., tetrabutyl titanate, Ti(BuO)4 [49–51]; and titanium(IV) isopropoxide, Ti[OCH(CH3)2]4 [23,52]). On the other hand, a GO aqueous suspension is normally used as precursor instead of graphene on its own because of the presence of hydrophilic oxygen-containing surface groups on GO sheets that can be used to facilitate the anchoring of semiconductors. These groups are beneficial for the dispersion of GO layers in water and the heterogeneous nucleation and growth of the TiO2 particles, which are needed to develop highly efficient photocatalysts [53,54]. During the preparation method, GO can be reduced to rGO via the hydrothermal/solvothermal process or by UV light irradiation [20,55–57]. Different TiO2 semiconductors with well-defined morphologies have been constructed on graphene sheets, for example, zero-dimensional TiO2 nanospheres [41], one-dimensional TiO2 nanorods [58], two-dimensional TiO2 nanosheets, and three-­ dimensional macro-/mesoporous TiO2 [59–61]. In general, these TiO2 nanoarchitectures can be fabricated and then anchored onto graphene by carefully controlling the synthesis conditions, such as the additives and hydrothermal parameters, and using titanium salts as precursors [54]. The epoxy and hydroxyl functional groups on GO sheets can act as heterogeneous nucleation sites for anchoring TiO2 nanoparticles, leading to the formation of well-dispersed mesoporous TiO2 nanospheres on the graphene sheets via a template-free self-assembly process [41]. These functionalities (such as epoxy and hydroxyl groups) mediate the efficient and uniform assembly of the TiO2 nanoparticles on the GO sheets, thus avoiding agglomeration and subsequently increasing the surface area of the resulting materials. During the photocatalyst preparation, TiO2 nanoparticles are produced and interact with the surface chemistry of GO by means of hydrogen bonds, resulting in the formation of well-dispersed mesoporous TiO2 nanospheres on the GO sheets. The hydroxyl and epoxy groups are linked to TiO2 particles and should not function as active sites during photocatalysis. Moreover, these functionalities are stable during the photocatalytic process because of the formation of TiOC bonds. TiO2 nanorods were stabilized by oleic acid and self-assembled on GO sheets at the water/toluene interface [58]. The two-phase, self-assembling procedure is simple and reproducible, and it can be widely and easily used for self-assembling other nonpolar organic soluble nanocrystals on GO sheets. Mesoporous graphene-TiO2 nanocomposites have been synthesized via two successive steps of hydrothermal/hydrolysis using Ti(SO4)2 and an acidic GO solution, followed by UV-assisted photocatalytic reduction of GO [59]. On the other hand, hierarchical macro/mesoporous graphene-TiO2 composites have been prepared by a simple one-step hydrothermal method using GO and tetrabutyl titanate as the titanium precursor [60]. A novel simultaneous reduction-­ hydrolysis technique in a binary ethylenediamine/H2O solvent was used in the synthesis of a graphene-TiO2 2D sandwich-like nanostructure using GO nanosheets and titanium(IV), bis(ammonium lactato)dihydroxide [61]. The technique was based on

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the simultaneous reduction of GO into rGO and the formation of TiO2 nanoparticles, resulting in situ loading onto graphene through chemical bonds (TiOC bond) to yield a 2D sandwich-like nanostructure. In another study, rGO-TiO2 nanocomposites were synthesized by a simple and environmentally benign one-step hydrothermal method using GO and TiCl4, as the titania precursor [62]. While conventional approaches mostly utilize multistep chemical methods using strong reducing agents, this method provides the notable advantages of a single-step reaction without employing toxic solvents or reducing agents, thereby providing a novel green synthetic route to produce the nanocomposites of rGO and TiO2. The fabrication of high-quality GO-TiO2 nanorod composites (GO-TiO2 NRCs) on gram scale has also been reported by a two-phase assembly method, exhibiting an improved photocatalytic performance by the effective charge antirecombination on GO [63]. In another report [64], graphene-wrapped anatase TiO2 was synthetized through one-step hydrothermal GO reduction and TiO2 crystallization from GOwrapped amorphous TiO2. Graphene-TiO2 nanoparticles exhibited a red-shift of the band-edge and a significant reduction of the band gap up to 2.80 eV. Recently, the synthesis of high energy (001) facet-exposed crystalline TiO2 on graphene using hydrothermal [42] and solvothermal methods [43] has also attracted much interest. Graphene-modified TiO2 nanosheets with exposed (001) facets (sample G1.0) were prepared using a microwave-hydrothermal treatment of GO and TiO2 nanosheets in an ethanol-water solvent [65]. Because of the interaction between the hydrophilic functional groups (e.g., OH, COOH) on GO and the hydroxyl groups on TiO2, the TiO2 particles were well dispersed on the GO sheets with face-to-face orientation (Fig. 5.3). The corresponding high-magnification TEM image (Fig. 5.3D) clearly shows the lattice fringes, which are parallel to one of the edges of the TiO2 nanosheets. Another procedure for the preparation of graphene-TiO2 composites that has been reported by our group involves the liquid phase deposition method (LPD) using (NH4)2TiF6 and H3BO3 as precursors, followed by a thermal post-treatment in an N2 atmosphere [56]. Graphene-based TiO2 composites were prepared using GO and different chemical rGO samples to assess the effect of the nature and number of oxygen-containing surface groups on the photocatalytic performance of the composite photocatalysts under near-UV/Vis and visible irradiations. The results showed that the presence of the oxygenated groups mediates the efficient and uniform assembly of the TiO2 nanoparticles on the graphene-derivative materials, as shown in Fig. 5.4 [66]. Similarly to TiO2, other inorganic metal oxides, such as ZnO [67–70], Cu2O [71–74], WO3 [75–77], Ag3PO4 [78–80], Fe2O3 [81–83], BiVO4, [84,85] and MnO2 [86,87], have been used successfully to fabricate other graphene-based composite photocatalysts via different synthesis procedures.

5.3.2 Synthesis of other graphene-based semiconductor photocatalysts The synthesis of other graphene-based photocatalysts includes the use of several metal sulfides, such as CdS [88–91], ZnS [92], ZnIn2S4 [93], In2S3, MoS2 [94], metal-free photocatalysts like graphitic carbon nitride (g-C3N4) [95,96], and ­nonmetal-doped ­materials [90,97–101]. Both GO and rGO have been used as p­ recursors in most

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Fig. 5.3  SEM (A) and TEM (B) images of GO, and TEM (C) and HRTEM (D) images of the G1.0 sample. Reproduced with permission from Q. Xiang, J. Yu, M. Jaroniec, Enhanced photocatalytic H2-production activity of graphene-modified titania nanosheets, Nanoscale 3 (9) (2011) 3670–3678. Copyright 2011, Royal Society of Chemistry.

of the graphene-based composites because of their high hydrophilicity and good dispersion in water. For example, rGO/CdS nanorod composites were prepared using a one-step, microwave-assisted hydrothermal method in an ethanolamine-water solution [90]. In another work, CdS/graphene composites were synthesized by a facile solvothermal method in dimethyl sulfoxide (DMSO), in which the formation of CdS nanoparticles and the reduction of GO to rGO occurred simultaneously [91]. It was found that graphene nanosheets can serve as a two-dimensional material where the CdS nanoparticles interact to avoid aggregation. Graphene nanosheets decorated with CdS clusters were prepared via a solvothermal process using GO, Cd(Ac)2 as the source of Cd2+, and DMSO as the source of S2− and solvent [89]. The reduction of GO into rGO and the formation of CdS clusters on the graphene surface occurred simultaneously during the solvothermal process. Similar results were obtained in the case of rGO/ZnIn2S4 nanocomposites prepared by a one-pot solvothermal method and a mixed solvent of N,N-dimethylformamide and ethylene glycol where the formation of ZnIn2S4 nanosheets on highly reductive rGO were simultaneously achieved [93].

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Fig. 5.4  Schematic illustrative and SEM images of TiO2 composites containing rGO and GO materials. Adapted with permission from L.M. Pastrana-Martínez, S. Morales-Torres, V. Likodimos, P. Falaras, J.L. Figueiredo, J.L. Faria, A.M.T. Silva, Role of oxygen functionalities on the synthesis of photocatalytically active graphene–TiO2 composites, Appl. Catal. B: Environ. 158–159 (2014) 329–340. Copyright 2014, Elsevier.

In addition to the growth of 0D CdS nanostructures on rGO nanosheets, 1D CdS nanostructures (e.g., nanorods and nanowires) [90,102] have also been anchored onto 2D rGO nanosheets to synthetize 1D-2D hybrid photocatalysts by a solvothermal method or an electrostatic self-assembly approach. Coupling of 2D CdS nanosheets and rGO sheets has been reported by a surface modification method using 4-­aminothiophenol (4-ATP) [103]. Fig. 5.5 shows a schematic representation of the composite preparation. The composites of positively functionalized CdS nanostructures and negatively charged GO clearly can be fabricated through electrostatically mediated self-assembly. Composite materials based on TiO2 nanocrystals grown in the presence of a layered MoS2/graphene hybrid have also been reported [94]. Graphene/MoS2-layered heterostructures were prepared by hydrothermal treatment of sodium molybdate, thiourea, and an aqueous GO solution at 210°C for 24 h. After that, further hydrothermal treatment of the obtained graphene/MoS2 hybrid with tetrabutyl titanate in ethanol/water solvent resulted in the formation of a graphene/MoS2/TiO2 composite photocatalyst. As a 2D metal-free organic semiconductor, g-C3N4 has a structure similar to those of graphene derivatives. Thus the fabrication of g-C3N4-based, metal-free photocatalysts with layered heterojunctions between GO and g-C3N4 has received significant attention because of their outstanding physicochemical and electrical properties [104]. Graphene/g-C3N4 materials were prepared through an impregnation-chemical reduction route with a subsequent thermal treatment at 550°C in an N2 atmosphere [95]. Melamine was used as a precursor of g-C3N4, and GO and hydrazine hydrate (as reducing agent) were employed to produce rGO. In another work, sandwich-like graphene/g-C3N4 (GCN) nanocomposites were developed through a facile one-pot,

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Fig. 5.5  Schematic representation of the preparation of CdS nanosheet/rGO composite. Adapted with permission from R. Bera, S. Kundu, A. Patra, 2D hybrid nanostructure of reduced graphene oxide–CdS nanosheet for enhanced photocatalysis, ACS Appl. Mater. Interfaces, 7 (24) (2015) 13251–13259. Copyright 2015, American Chemical Society.

impregnation-thermal reduction strategy, but using urea as the precursor of g-C3N4 [96]. Similarly, other monomers, such as cyanamide [105] and dicyandiamide [106], were employed to fabricate the graphene/g-C3N4 composite via this simple one-pot, impregnation-thermal reduction approach. On the other hand, doping with nonmetal atoms has also been explored as an effective strategy to tailor the electrical conductivity and electronic structure of graphenebased materials. Recently, N-doped GO quantum dots (NGO-QDs) were prepared by treating GO in NH3 at 500°C followed by a harsh oxidation step using a modified Hummers' method [99]. The co-doping of N and O atoms in the graphitic structure provided both p-type and n-type conductivities to NGO-QDs at the same time. In a subsequent study by the same authors [100], surface intact N-doped GO quantum dots were similarly prepared by ultrasonic exfoliation of NH3-treated GO sheets in a triethanolamine aqueous solution. N-doped graphene has also been synthetized by pyrolysis under an inert atmosphere of natural chitosan [97]. As a natural N-containing biopolymer, chitosan could be used as a single source of carbon and nitrogen, making the doping process in the graphitic structure more straightforward. The main parameter controlling the residual amount of N of the material was the pyrolysis temperature, its optimum value being established as 900°C. Apart from the incorporation of N atoms, chemical doping of graphene structures with other heteroatoms, such as phosphorous (P), which results in p-type conductive behaviors, can be simultaneously considered for the preparation of a graphene-based photocatalytst. In this context, P-doped

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graphene was developed by alginate modified with various amounts of H2PO4 at a neutral pH, followed by pyrolysis at a high temperature (900°C) under an inert atmosphere [98].

5.4 Energy applications Artificial photosynthesis is regarded as a potential long-term solution to mitigate the world's energy problems because it may produce solar-based fuels via photocatalytic water splitting and/or CO2 photoreduction, among other methods. The incorporation of graphene-based materials with semiconductor photocatalysts can lead to different promoting effects through one or more of the following effects: (i) supports material for enhanced structure stability; (ii) increases adsorption and active sites toward the reagents; (iii) uses electron acceptor and transport channel to suppress the recombination of photo-excited electron-hole pairs; (iv) involves a co-catalyst; (v) involves photosensitization; and (vi) has photocatalyst and band gap narrowing effect [16,107]. In the following section, the main applications of graphene-based semiconductor photocatalysts in the two referred processes are briefly reviewed.

5.4.1 Photocatalytic hydrogen generation During the past decade, graphene has shown great ability to enhance the photocatalytic H2 production performance of semiconductor photocatalysts [38]. GO/TiO2 photocatalysts were studied for water splitting under UV/vis irradiation [49]. XRD results showed that the average crystal size of TiO2 (anatase) was ∼11 nm for all samples with various GO contents. The GO/TiO2 composite with a 5 wt.% of GO exhibited a H2 evolution rate of 8.6 μmol h−1, 1.9 times higher than that obtained for the TiO2 benchmark, P25 (4.5 μmol h−1). The larger surface area as well as the excellent electronic conductivity of graphene that suppressed the recombination of photoinduced electrons and holes were the main factors enhancing the photocatalytic activity of the GO/TiO2 composite. It is known that graphene as a H2-evolution co-catalyst can greatly boost the photocatalytic activity of metal sulfides. CdS/GO photocatalysts with a uniform distribution of CdS clusters led to a more efficient transfer of photoinduced electrons from CdS to GO [89]. A high H2-production rate of 1.12 mmol h−1 was obtained at an optimal GO content of 1.0 wt.% (about 4.87 times higher than that of bare CdS), presenting an apparent quantum efficiency (QE) of 22.5% at λ = 420 nm. Graphene-supported CdS nanoparticles for photocatalytic H2 production were also prepared by a hydrothermal method [88]. In this case, the H2-production rate was 70 μmol h−1 for the graphene/ CdS composite at an optimal mass ratio of 0.01/1 under Xe lamp irradiation (200 W, λ ≥ 420 nm), while bare CdS only showed a rate of 14.5 μmol h−1. Significant band-gap narrowing was observed due to the strong interactions between CdS and graphene. Combined with the advantage of more efficient charge separation, the graphene-­ modified CdS photocatalyst exhibited much better photocatalytic H2-production performance than bare CdS.

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Graphene derivatives serve as excellent electron acceptors and transport channels to suppress the recombination of photoinduced electrons and holes, thus enhancing the efficiency of photocatalytic H2 production. Photocatalytic H2 production over graphene-supported ZnS nanoparticles under visible light illumination (λ > 420 nm) was studied [92]. The composite photocatalyst with an optimum 0.1 wt.% of GO achieved a H2-production rate of 7.42 μmol h−1 g−1, which was eight times higher than that of bare ZnS. The high photocatalytic H2 production activity was attributed to the photosensitization of graphene. In this case, the electrons photogenerated from graphene could be transferred to the CB of ZnS to participate in the photocatalytic process under visible light illumination. In another work [108], rGO was coupled with ZnxCd1−xS photocatalysts for photocatalytic H2 production under simulated solar irradiation using Na2S and Na2SO3 as sacrificial agents. The photocatalytic H2-production rate of the optimized rGO-Zn0.8Cd0.2S photocatalyst (0.25 wt.% rGO content) was 1824 μmol h−1 g−1 with an apparent QE of 23.0% at 420 nm. The performance was even better than that of optimized Pt-Zn0.8Cd0.2S under the same reaction conditions. It was observed that the introduction of rGO could effectively promote the transfer and separation of charge carriers and increase the surface-active sites for water reduction, thus leading to the enhanced performance. This work also indicated the promising potential of graphene to replace noble metals as a co-catalyst in specific photocatalytic systems for H2 production. The presence of a small amount of rGO (1.0 wt.%) could lead to a significant increase of specific surface area in rGO-ZnIn2S4 nanocomposites (e.g., 150 and 99.8 m2 g−1 for 1.0 wt.% rGO-ZnIn2S4 and bare ZnIn2S4, respectively) [93]. This rGO-ZnIn2S4 nanocomposite showed a H2 evolution rate of 40.9 μmol h−1, whereas the rate of bare ZnIn2S4 was only 9.5 μmol h−1 under visible-light illumination. The strong interaction between ZnIn2S4 nanosheets and rGO in the nanocomposites facilitated the electron transfer from ZnIn2S4 to rGO, with the latter serving as a good electron acceptor and mediator, as well as the co-catalyst for H2 evolution. The H2 production efficiency of those graphene-based binary photocatalysts can be improved by introducing an additional component to form graphene-based ternary composite photocatalysts, as reported for NiS/Zn0.5Cd0.5S/rGO ternary composites, where the three components were well-connected with each other [109]. Such a connection enables rGO to be an effective electron acceptor and transporter to capture photoinduced electrons from the CB of Zn0.5Cd0.5S and simultaneously offer reduction-­active centers for H2 evolution. At the optimal amount of 0.25 wt.% rGO and 3 mol % NiS, the ternary composite photocatalyst exhibited a H2-production rate of 376 μmol h−1 with a high apparent QE of 31% at 420 nm. An effective strategy to obtain an intimate and large contact interface is to construct 2D-2D layered junctions to provide abundant surface-active sites and achieve efficient interfacial charge transfer [110]. A graphene/g-C3N4 composite was applied in photocatalytic H2 production under visible light illumination [95]. The successful formation of 2D–2D layered junctions between g-C3N4 and graphene led to a very efficient interfacial charge separation, which enabled spatial accumulation of photoinduced electrons and holes on the sides of graphene and g-C3N4, respectively. The highest photocatalytic H2-production rate (451 μmol h−1 g−1) was achieved with the composite containing 1.0 wt.% graphene.

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The synergetic effect of MoS2 and graphene as co-catalysts for enhanced photocatalytic H2 production activity of TiO2 nanoparticles was also evaluated [94]. It was found that a layered MoS2/graphene (MG) hybrid served as a highly active co-catalyst for photocatalytic H2 production under Xe arc lamp irradiation, using TiO2 as the photocatalyst and ethanol as the sacrificial agent (Fig. 5.6A). The graphene/MoS2/TiO2 composite (TiO2/MG) photocatalyst with an optimal amount of 5.0 wt.% graphene and 0.5 wt.% MoS2 exhibited a high H2-production rate of 165.3 μmol h−1 with an apparent QE of 9.7% at 365 nm (Fig. 5.6B). This is logical because graphene possesses a high work function (∼−0.08 eV versus NHE) to capture photoinduced electrons from the CB of TiO2 [38]. The photogenerated electrons in the CB of TiO2 can be transferred to MoS2 nanosheets through the graphene sheets and then react with the adsorbed H+ ions at the edges of the MoS2 to generate H2. In fact, nanoscale MoS2 is highly active for H2 evolution as a result of the quantum-confinement effect. This indicates that, because of a notable synergetic effect between MoS2 nanosheets and graphene, the composite co-catalyst has several advantages, including suppression of charge recombination, improvement of interfacial charge transfer, and an increase in the number of active adsorption sites, as well photocatalytic reaction centers. Doped graphene materials have been used to improve the photocatalytic activity of graphene-based semiconductor composites in the photocatalytic generation of H2 because of a high intimate interfacial contact between doped graphene and semiconductor nanoparticles [111–114]. The fabrication of TiO2 nanoparticles-functionalized N-doped graphene composites for photocatalytic H2 generation for water (Fig. 5.7) has been studied [113]. N-doped rGO showed higher electrical conductivity than rGO because of its efficient structural restoration and smaller populations of defects in the graphitic structure. Moreover, the N atoms in N-doped graphene played important roles as nucleation and anchor sites for TiO2 nanoparticles, resulting in their uniform distribution on the graphene sheet. The photocatalytic activities for H2

Fig. 5.6  (A) Photocatalytic H2 evolution of TiO2/MG composites with different MoS2 and rGO contents in the MG hybrid as co-catalyst under UV irradiation; (B) schematic illustration of the charge transfer in TiO2/MG composites. Reproduced with permission from Q. Xiang, J. Yu, M. Jaroniec, Synergetic effect of MoS2 and graphene as cocatalysts for enhanced photocatalytic H2 production activity of TiO2 nanoparticles, J. Am. Chem. Soc. 134 (15) (2012) 6575–6578. Copyright 2012, American Chemical Society.

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Fig. 5.7  (A) Time course of H2 production with different catalysts; (B and C) schematic illustration of the strong coupling between TiO2 and N atoms in the rGO sheet and enhanced photo-induced charge transfer and photocatalytic H2 generation. Reproduced with permission from Z. Mou, Y. Wu, J. Sun, P. Yang, Y. Du, C. Lu, TiO2 nanoparticles-functionalized N-doped graphene with superior interfacial contact and enhanced charge separation for photocatalytic hydrogen generation, ACS Appl. Mater. Interfaces 6 (16) (2014) 13798–13806. Copyright 2014, American Chemical Society.

production were assessed under a 150 W Xe lamp. The average rate of H2 production of the N-graphene/TiO2 composite can reach 13.3 μmol h−1, which is higher than that of graphene/TiO2 (8.9 μmol h−1). In addition, improved durability during the photocatalytic process was observed with N-graphene/TiO2. On the other hand, doped graphene materials can also be used as photocatalysts on their own because of their intrinsic band gap. N-doped GO quantum dots (NGO-QDs) were employed as photocatalysts for water splitting under visible light illumination [99]. The band gap of the NGO-QDs was approximately 2.2 eV, the overall water splitting being achieved with a H2:O2 molar ratio of approximately 2:1. Normally, the p-type conductivity of the oxygen functional groups of GO is responsible for the production of H2, whereas the n-type conductivity of N-doped GO can benefit O2 evolution [100]. Later, the synergistic effect of O- and N-functionalities was verified by the same research group to produce H2 in triethanolamine aqueous solution. A phosphorus (P)-doped graphene material has also been studied for the photocatalytic generation of H2, with the P-doping leading to a conversion from zero-energy band gap graphene (0 eV) to semiconducting graphene (2.85 eV), which exhibited both UV and visible light activities toward photocatalytic H2 production [98]. The highest H2-generation rate was 282 μmol h−1 g−1 under UV/Vis light irradiation using triethanolamine as the sacrificial agent and Pt as the co-catalyst.

5.4.2 Photocatalytic reduction of carbon dioxide The massive release of CO2 into the atmosphere is believed to be resulting in significant climate changes; therefore a great deal of effort is being directed at reducing CO2 concentration in the atmosphere and preventing its emissions. The conversion of CO2 into fuel using a solar source has the potential to reduce the consumption of fossil fuels and thus help reduce humanity's impact on global warming and achieve worldwide targets set to reduce the overall carbon footprint [4].

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Several studies have emphasized the high performance of graphene-based photocatalyst composites for the CO2 reduction to solar fuels. For example, our research group [48] reported the effect of pH and copper oxide precursor salt on graphene derivative-TiO2 (GOT) composites for the photocatalytic reduction of CO2 with water. The pH was identified as the key variable determining the product distribution. Thus it was observed that the prepared GOT composite photocatalyst exhibited superior photocatalytic activity for ethanol (EtOH) production (144.7 μmol g−1 h−1) at pH 11.0 and for methanol (MeOH) production at pH 4.0. Moreover, the presence of both GO and copper in the GOT composites extended the absorption to the visible spectral range, enhancing the CO2 photoreduction in the aqueous phase (Fig. 5.8A). The combination of TiO2 with GO generates a synergistic effect that potentially enhances the photoactivity because of the increase of the adsorption capacity and efficient interfacial electron transfer between the two constituent phases (Fig. 5.8B) [56]. Moreover, the combined contribution of the lower band gap energy and the known quenching of photoluminescence determined by Raman spectroscopy for the GOT catalyst are possible explanations for the higher performance observed, as compared with P25. The synthesis of Cu2O/rGO composites has been reported in regard to the photocatalytic reduction of CO2 by using a 150 W Xe lamp as a light source [71]. Given the good electron conductivity and large specific surface area of rGO, the composite exhibited a low electron-hole recombination rate and high number of surface-active sites. As a result, the prepared material showed a photocatalytic activity six times greater than that of the optimized Cu2O for the CO2 reduction. More interesting, however, is the fact that the photocorrosion problem of Cu2O can also be effectively mitigated by the rGO loading. Therefore the loading of graphene not only improved the photocatalytic activity of the photocatalyst but also enhanced its photostability. Graphene exhibited unique properties when it was hybridized with other materials to function as a co-catalyst. The production of MeOH was described by the photocatalytic reduction of CO2 under visible light illumination using a graphene and

Fig. 5.8  (A) CO2 photoreduction over GOT, GOT-500, and Cu-loaded GOT catalysts at pH 11.0 and 180 min; (B) conceptual scheme of the CO2 photoreduction catalyzed by Cu-loaded GOT composites. Reproduced with permission from L.M. Pastrana-Martínez, A.M.T. Silva, N.N.C. Fonseca, J.R. Vaz, J.L. Figueiredo, J.L. Faria, Photocatalytic reduction of CO2 with water into methanol and ethanol using graphene derivative–TiO2 composites: effect of pH and copper(I) oxide, Top. Catal. 59 (15–16) (2016) 1279–1291. Copyright 2016, Springer.

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­tourmaline co-doped TiO2 nanocomposites (GT/T) [115]. GT/T exhibited significantly improved activity compared to the graphene-loaded TiO2, tourmaline-loaded TiO2, and bare TiO2. This enhancement was attributed to the synergistic effect of graphene and tourmaline. Specifically, both graphene and tourmaline can improve electron-hole separation, whereas graphene can reduce the band gap of TiO2. As a result, GT/T led to the enhanced MeOH production rate via photocatalytic CO2 reduction, which was 21 times higher than that of bare TiO2. A well-defined nanocomposite interface, such as a 2D–2D composite, is important in preparing highly efficient graphene-based photocatalysts [110]. Robust hollow spheres consisting of molecular-scale alternating titania (Ti0.91O2) nanosheets and graphene nanosheets were used for the photocatalytic reduction of CO2 using a 300 W Xe arc lamp [116]. Because both TiO2 and graphene nanosheets are 2D structures, graphene and Ti0.91O2 had a close and large contact surface area. The prepared samples exhibited a high CO formation rate via the photocatalytic CO2 reduction, which was nine times higher than that of the commercial P25, because of its fast electron-­hole separation and good light utilization. 2D-2D layered photocatalysts based on sandwich-like graphene-g-C3N4 (GCN) composite showed enhanced visible light photocatalytic CO2 reduction activity [96]. The GCN sample demonstrated high visible-­light photoactivity toward CO2 reduction under ambient conditions, exhibiting a 2.3-fold enhancement over bare g-C3N4 (Fig. 5.9A). This effect was ascribed to the inhibition of the electron-hole pair recombination by graphene, which increased the charge transfer (Fig. 5.9B). Nonmetal doping is another efficient way to tune the physical, optical, and physicochemical properties of graphene for photoreduction of CO2. Boron (B)-doped graphene (B-GR) nanosheets loaded on P25 nanoparticles have been proposed to improve the photocatalytic reduction of CO2 using a 300 W Xe lamp as the light source [101]. B-GR showed a higher Fermi level than pristine graphene, falling between the CB of P25 and the relevant CO2/CH4 redox potential. The tunable band gap of B-GR determined the large potential application of P25/B-GR in the photoreduction of CO2.

Fig. 5.9  (A) Total CH4 yield over the as-prepared photocatalysts; (B) schematic diagram of photogenerated charge transfer in the GCN system for CO2 reduction with H2O to form CH4 under visible light. Reproduced with permission from W.-J. Ong, L.-L. Tan, S.-P. Chai, S.-T. Yong, Graphene oxide as a structure-directing agent for the two-dimensional interface engineering of sandwich-like graphene-g-C3N4 hybrid nanostructures with enhanced visible-light photoreduction of CO2 to methane, Chem. Commun. 51 (5) (2015) 858–861. Copyright 2014, Royal Society of Chemistry.

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Moreover, the presence of graphene derivatives in the composite photocatalysts could also greatly enhance the adsorption capacity of CO2 [90]. A close linear relationship was found between the CO2 adsorption capacity of the composite photocatalysts and the rGO content, which was independent of the specific surface areas. The CO2 adsorption sites can also function as the active sites for CO2 photoreduction, thus facilitating the direct activation of adsorbed CO2 and the enhancement of photocatalytic activity for CO2 photoreduction. Furthermore, the CO2 adsorption capacity and catalytic activity of carbon co-catalysts could be further enhanced by nitrogen doping. It was recently found that N dopants play a crucial role in the photoactivity and stability of GO-TiO2 composites for the photoreduction of CO2 [117]. N-rGO with an appropriate N quantity and N-bonding configuration acted as a dual-function promoter, simultaneously enhancing CO2 adsorption on the photocatalyst surface and facilitating electron-hole separation, and eventually boosted the photocatalytic performance. This work may inspire some new ideas for designing nanocarbon composite co-catalysts with improved CO2 adsorption capacity and catalytic activity for CO2 photoreduction via coupling graphene derivatives.

5.5 Conclusions and outlook Graphene-based photocatalytic composites are robust materials with the potential to solve the world’s increasing energy demand. In this chapter, we summarized recent accounts of the synthesis and energy applications of graphene-based photocatalysts, particularly those prepared with GO, rGO, and heteroatom-doped graphene. The high morphological and electronic versatility of graphene materials offers the possibility of designing novel photocatalytically active materials for solar fuels, including photocatalytic water splitting to H2 and photocatalytic reduction of CO2 to hydrocarbons. The incorporation of graphene derivatives into various semiconductor photocatalysts has demonstrated that this approach can improve photocatalytic performance because of a combination of several factors: (i) suppressed photogenerated carrier recombination; (ii) increased adsorption capacity; (iii) enhanced photostability; and (iv) enhanced light absorption. Despite the considerable, rapid progress, several challenges remain in the synthesis and application of graphene-based photocatalyst composites for highly efficient solar fuel generation. First, improvements need to be made in the large-scale production of graphene-based photocatalysts with controlled morphologies and compositions as well as with an intimate contact interface. In this regard, more efficient synthesis methods to achieve enhanced performance of graphene-derivative materials and graphene-based semiconductor composites are required. Second, the efficiencies of solar fuel generation by photocatalysis are far from being optimal and considerable breakthroughs must be made before this method can be considered as a viable economical process. Finally, current studies using graphene-based materials focus mostly on solar fuel generation in the presence of sacrificial agents. Therefore the development of graphene-based materials with improved photocatalytic efficiency using natural renewable resources like water and sun is highly encouraged if we are to achieve clean and renewable energy.

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Acknowledgments Financial support for this work was provided by “AIProcMat@N2020—Advanced Industrial Processes and Materials for a Sustainable Northern Region of Portugal 2020,” with the reference NORTE-01-0145-FEDER-000006, supported by Norte Portugal Regional Operational Programme (NORTE 2020), under the Portugal 2020 Partnership Agreement, through the European Regional Development Fund (ERDF) and the Project POCI-01-0145-FEDER-006984, Associate Laboratory LSRE-LCM funded by ERDF through COMPETE2020—Programa Operacional Competitividade e Internacionalização (POCI); and by national funds through FCT—Fundação para a Ciência e a Tecnologia. LMPM, SMT, and AMTS acknowledge the FCT Investigator Programme (IF/01248/2014, IF/00573/2015, and IF/01501/2013, respectively) with financing from the European Social Fund and the Human Potential Operational Programme. LMPM also acknowledges the Spanish Ministry of Economy and Competitiveness (MINECO) for a Ramon y Cajal research contract (RYC-2016-19347).

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