Graphene-based heterojunction photocatalysts

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Graphene-based heterojunction photocatalysts Xin Li a,b,∗ , Rongchen Shen a,b , Song Ma b , Xiaobo Chen c,∗ , Jun Xie a,b,∗ a College of Forestry and Landscape Architecture, Key Laboratory of Energy Plants Resource and Utilization, Ministry of Agriculture, Key Laboratory of Biomass Energy of Guangdong Regular Higher Education Institutions, South China Agricultural University, Guangzhou, 510642, PR China b College of Materials and Energy, South China Agricultural University, Guangzhou 510642, PR China c Department of Chemistry, University of Missouri – Kansas City, Kansas City, MO, 64110, USA

a r t i c l e

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Article history: Received 21 June 2017 Received in revised form 16 August 2017 Accepted 28 August 2017 Available online xxx Keywords: Graphene Heterojunction photocatalysts Schottky junctions Artificial photosynthesis Z-scheme heterojunctions Erath-abundant cocatalysts

a b s t r a c t Due to their unique physicochemical, optical and electrical properties, 2D semimetallic or semiconducting graphene has been extensively utilized to construct highly efficient heterojunction photocatalysts for driving a variety of redox reactions under proper light irradiation. In this review, we carefully addressed the fundamental mechanism of heterogeneous photocatalysis, fundamental properties and advantages of graphene in photocatalysis, and classification and comparison of graphenebased heterojunction photocatalysts. Subsequently, we thoroughly highlighted and discussed various graphene-based heterojunction photocatalysts, including Schottky junctions, Type-II heterojunctions, Zscheme heterojunctions, Van der Waals heterostructures, in plane heterojunctions and multicomponent heterojunctions. Several important photocatalytic applications, such as photocatalytic water splitting (H2 evolution and overall water splitting), degradation of pollutants, carbon dioxide reduction and bacteria disinfection, are also summarized. Through reviewing the important advances on this topic, it may inspire some new ideas for exploiting highly effective graphene-based heterojunction photocatalysts for a number of applications in photocatlysis and other fields, such as photovoltaic, (photo)electrocatalysis, lithium battery, fuel cell, supercapacitor and adsorption separation. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Since the first reported photocatalytic production of H2 from water in 1972 using TiO2 [1], a variety of semiconductor materials have been developed for the renewable production of solar fuels [2–4]. The interesting heterogeneous photocatalysis, including the photocatalytic water splitting (i.e., water reduction and oxidation), degradation of pollutants and carbon dioxide reduction (artificial photosynthesis, AP), turns out to be one of the most appealing solutions for environmental and energy sustainability through directly harnessing solar energy. However, so far, no one semiconductor can meet all requirements for practical photocatalysis, such as a good photocatalyst must be efficient, stable, safe, cheap and visible [5]. Therefore, various possible strategies to enhance the overall photocatalytic effciency, including band structure engineer-

∗ Corresponding authors at: College of Forestry and Landscape Architecture, Key Laboratory of Energy Plants Resource and Utilization, Ministry of Agriculture, Key Laboratory of Biomass Energy of Guangdong Regular Higher Education Institutions, South China Agricultural University, Guangzhou, 510642, PR China. E-mail addresses: [email protected] (X. Li), [email protected] (X. Chen), [email protected] (J. Xie).

ing, micro/nano engineering [6], bionic engineering, co-catalyst engineering, surface and interface engineering, have been widely employed for engineering heterogeneous semiconductors. Since the pioneering reports on the strong ambipolar electric field effect of semimetallic graphene by Geim and coworkers in 2004, graphene nanosheets, as a metal-free two-dimensional (2D) multifunctional nanoplatforms, have attracted extensive attention for electronic, catalytic and energy applications due to its unique electric, optical, structural and physiochemical properties [7]. Especially, many significant breakthroughs have been achieved for the large-scale synthesis of 2D graphene nanosheets through liquid-phase exfoliations. Interestingly, as a new class of emerging novel building blocks, 2D graphene nanosheets could also be used to fabricate various tailorable hybrid semiconductor nanomaterials with controllable compositions, sizes, size distributions, and morphologies. In particular, a number of novel nanostructured heterogeneous photocatalysts based on 2D graphene nanosheets have been developed in the past several years due to their favorable absorption of solar radiation, efficient separation of charge carriers, high surface areas and exposed reactive sites [2,8–10]. As is known, the overall photocatalytic efficiency is significantly hindered by the fast electron–hole recombination and low light utilization, which are governed by all material parameters,

http://dx.doi.org/10.1016/j.apsusc.2017.08.194 0169-4332/© 2017 Elsevier B.V. All rights reserved.

Please cite this article in press as: X. Li, et al., Graphene-based heterojunction photocatalysts, Appl. Surf. Sci. (2017), http://dx.doi.org/10.1016/j.apsusc.2017.08.194

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including chemical composition, physical dimension, interfacial and electronic properties. Thus, a fundamental understanding and deterministic control of these chemical, interfacial and structural factors will enable the scalable production of 2D graphene nanosheet-based composite photocatalysts with the best photocatalytic behavior, which will be favorable for creating some robust composite systems for practical photocatalytic applications and fundamental insights into low-dimensional physics and chemistry at the single-atom level. Although some excellent reviews about 2D graphene nanosheet-based photocatalysis have been published in the past eight years [2,7–17], most of these reviews only focus on the diversified roles of graphene in the graphenebased photocatalysts, such as photoelectron mediator and acceptor, enhancing the adsorption capacity, tuning the light absorption range and intensity, photothermal effect and the macromolecular photosensitizer. More importantly, it has also been pointed that the photocatalytic performances of graphene-based composite semiconductors could be enhanced through improving the electronic conductivity of GR, strengthening the interfacial contact between GR and semiconductors, and optimizing the entire system of GR–semiconductorcomposites [2,12,13,18,19]. Clearly, the interfaces between graphene and semiconductors play the crucial roles in achieving the significantly boosted photocatalytic efficiency. However, so far, there has been no one systematic review about summarizing the multi-functional graphene-based heterojunctions for various applications of heterogeneous photocatalysis. It is generally regarded that constructing the heterojunction photocatalysts has been extensively shown to be capable of promoting the spatial separation of photogenerated electron–hole pairs through combining the advantages of integrated functional components, thus fulfilling the higher overall photocatalytic activity [20–25]. Thus, it is timely to comprehensively summarize the significant advances in the utilization of graphene-based heterojunction semiconductors and solar energy for heterogeneous photocatalysis. In this review, important graphene-based heterojunctions such as Schottky-junctions, Type-II heterojunctions, Z-scheme heterojunctions (including indirect and direct), Van der Waals heterostructures and In plane heterojunctions, will be thoroughly highlighted and discussed. We believe that this review will not only promote the further developments of new graphene-based heterojunction semiconductors and architectures with improved solar-to-chemical energy conversion efficiency for solar fuel production and storage, but also make positive contributions to utilize renewable solar energy for a sustainable environmental and energy future. Importantly, this review will facilitate the development of new materials and architectures for photovoltaic and sensing devices. 2. Fundamental of graphene-based heterojunction photocatalysts 2.1. Fundamental mechanism of heterogeneous photocatalysis To date, it is well accepted that, during photocatalytic reactions, the photogenerated electrons and holes in the excited semiconductors could accomplish the conversion of solar energy into chemical energy through complex multi-step processes, which are generally composed of the following seven typical processes (Fig. 1), (1) the photoexcitation of charge carriers, the transfer of holes (2) and electrons (3) to the semiconductor surface, the recombination of electon/hole pairs in the bulk (4) and on the surface (5) of semiconductor, and surface oxidation (6) and reduction (7) reactions, respectively [2,5–7,26,27]. Apart from these above kinetics processes, the fundamental thermodynamic requirements for a given photocatlytic reaction must be satisfied. The detailed thermodynamic requirements for photocatalytic dye degrada-

Fig. 1. Typical processes during the semiconductor photocatalysis, (1) the photoexcitation of charge carriers, the transfer of holes (2) and electrons (3) to the semiconductor surface, the recombination of electon/hole pairs in the bulk (4) and on the surface (5) of semiconductor, and surface oxidation (6) and reduction (7) reactions.

tion/bacteria disinfection, CO2 reduction and hydrogen production have been discussed in other papers, which were summarized in Fig. 2. Clearly, the single-electron/multi-electron O2 reduction reaction (ORR) and water oxidation reaction (WOR) are crucial for the photocatalytic dye degradation/bacteria disinfection (as observed in Fig. 2), whereas multi-electron CO2 reduction reaction (CRR), H2 evolution reaction (HER) and O2 evolution reaction (OER) play vital roles in achieving the photocatalytic production of solar fuels [4,6,7]. According to the relative positions between CB levels of semiconductors and redox potentials of specific reactions, some commonly-used photocatlysts could be classified into three types: strongly oxidative semiconductors with much higher VB levels for WOR, strongly reductive semiconductors with much higher CB levels for CRR and HER, semiconductors with moderate oxidation and reduction ability [6]. The detailed band positions and potential applications of some typical photocatalysts are summarized in Fig. 3. As shown in Fig. 3, it is obvious that Fe2 O3 , WO3 and BiVO4 are the excellent visible-light-driven semiconductors for driven the oxidation reactions, whereas CdS, g-C3 N4 , Cu2 O and SiC are the promising semiconductors for achieving the production of solar fuels under visible light irradiation [28–31]. For the latter, their photo-generated electrons with higher reduction potentials could also readily migrate to the graphene with higher work function to drive various reduction reactions, thus leading to their extensive application of the corresponding semiconductor/graphene composites in the different photocatlytic fields, which will be further highlighted in the following sections. To improve these above kinetic processes and meet thermodynamic requirements, various engineering strategies have been extensively proposed in the past decades [5]. These engineering strategies could be divided into the thermodynamic and kinetic ones, which have been summarized in Fig. 4. Typically, thermodynamic strategies include the construction of wide spectrum responsive photocatalyst and modification of wide-band-gap semiconductors through proper composition engineering, whereas the challenging processes, such as charge separation, transport and utilization could be effectively boosted by exploiting high-efficiency charge-transfer nanostructures, high-quality heterojunction interfaces and highly reactive surface and cocatalysts, respectively [5,28,32–40]. Fortunately, all these strategies could be achieved by the multi-functional graphene [7]. Concretely speaking, graphene could not only be thermodynamically utilized in the construction of wide spectrum responsive photocatalyst or the doping of wide-band-gap semiconductors, but also kinetically improve the photocatalysis through fabricating high-efficiency charge-transfer


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Fig. 2. Photocatalytic mechanisms for dye degradation/bacteria disinfection (A), production of solar fuels (B, CO2 reduction and hydrogen evolution).

Fig. 3. Band positions and potential applications of some typical photocatalysts (at pH = 7 in aqueous solutions) [6].

Fig. 4. Thermodynamic and kinetics strategies for improving photocatalysis.

nanostructures, high-quality heterojunction interfaces and highly reactive surface and cocatalysts [7]. Especially, the fast recombination of photogenerated electron–hole pairs on the surface of a semiconductor photocatalyst (in the range of nanoseconds) is very challenging for developing efficient photocatalysis systems for large-scale industrialization, due to much larger value of the Coulomb constant (8.99 × 109 Nm2 C−2 ) than that of the gravitational constant (6.67 × 10−11 Nm2 kg−2 ) [21]. Therefore, from the viewpoint of practical applications, engineering proper het-

erojunctions in photocatalysts has been found to be one of the most promising strategies to fabricate the advanced photocatalysts with effectively spatial separation of the photogenerated electron–hole pairs. So far, various heterojunctions, including the Schottky heterojunctions, conventional type-II heterojunctions, p–n heterojunctions, surface heterojunctions, direct Z-scheme heterojunctions, and semiconductor–nanocarbon heterojunctions, have been extensively constructed and applied in the photochatalysis [21].




Fig. 5. (A) Left: the dispersed band gaps of semimetallic pristine graphene in the Brillouin zone. Right: the energy bands at the Dirac point and the position of Fermi level of semiconducting graphene by suitable heteroatom doping [41,42]. The VBM and CBM positions of GO in the dark (B) and under irradiation (C) [45].

2.2. Fundamental properties of graphene in photocatalysis Surprisingly, the 2D graphene nanosheets could be used to construct all above-mentioned heterojunctions due to their variegated properties. On the one hand, in a pristine undoped graphene, its antibonding ␲* orbitals (which makes up its conduction band) and bonding ␲ orbitals (which makes up its valence band) degenerate and touch at Brillouin zone corners, making graphene a zero bandgap semimetallic material, as shown in the left of Fig. 5A [41,42]. However, chemical doping by the addition of foreign atoms can broke lattice symmetry and open a bandgap, thus converting semimetallic graphene into a semiconductor due to the formation of gap between ␲ and ␲* bands, as seen in the right of Fig. 5A. The type of graphene-based semiconductor is dependent on the dopants, whereas the band gaps are related with the dopant concentration. On the other hand, it has been also theoretically and experimentally demonstrated that semiconducting RGO can directly act as photocatalysts for H2 production or water splitting [43–46], due to its more negative LUMO positions than the reduction potential of H+ /H2 . As well known, the CB edge of RGO is mainly formed by the anti-bonding ␲* orbital, corresponding to a potential of −0.52 eV vs. NHE, pH = 0 [42], while the VB maximum of RGO is mainly composed by the O 2p orbital. In this regard, the band gap of RGO could be continuously narrow and its VB maximum could be upshifted by reducing the degree of oxidation (Fig. 5B and C) [46]. Especially, the oxygen-containing functionalities and edge defects on the aromatic scaffold of semiconducting graphene oxide (GO) enable it to be a more intriguing nanomaterial for the visiblelight-driven photocatalysis [47]. Therefore, it is not surprising that the semimetallic and semiconducting graphene could be used to fabricate various kinds of graphene-base heterojunction photocatalysts.

2.3. Advantages of graphene-based heterojunctions in photocatalysis As compared with other heterojunction photocatalysts, the graphene in heterojunctions exhibit a number of advantages, such as the low cost, readily tunable band structures, the diverse structures (i.e., morphology, dimensionality) for constructing nanoscale-architectured graphene-based composites [2,12,15]. Their versatile, variable physicochemical,electronic and optical properties could be optimized by many factors, including the number of layers, lateral size, dimensionality, edge structure, defect density, and reduction degree of GO [12,15]. Firstly, the semimetallic graphene as a low cost cocatalyst is obviously much cheaper than the noble metal cocatalysts. Thus, the earth-abundant graphene seems to be suitable for constructing really robust heterojunction photocatalysts in the different photocatalytic fields [48]. Secondly, as compared to other traditional inorganic semiconductors, the semiconductoring graphene prossess the tunable band gaps and simple fabrication processes for developing various graphene-based heterojunctions for wide photocatalytic applications. Furthermore, as compared with organic semiconductors, such as g-C3 N4 , and metal–organic frameworks (MOFs), semiconductoring graphene is obviously much stable, whose band strcutures could be also readily tuned by the versatile wet chemistry processability of GO. Finally, in order to compare graphene with other carbon allotropes (e.g, CNTs and C60), Xu’s group have performed a relatively systematic, critical and benchmark comparison between graphene-semiconductor and CNT-semiconductor heterojunction photocatalysts through different aspects (e.g., adsorptivity, light absorption, and charge carrier separation and transfer) [49,50]. Clearly, it was found that RGO is not always much better than its other carbon allotropes such as C60 and CNTs for boosting the photocatalysis. Nevertheless, structural diversity of




Fig. 6. The classification of graphene-based heterojunctions.

graphene materials (e.g., 0D graphene quantum dots, 1D graphene nanoribbons, 2D graphene nanosheets and porous 3D graphene networks) provides infinite possibilities for constructing multifarious heterojunctions for heterogeneous photocatalysis, which make graphene a more promising carbon than its other carbon allotropes in designing the advanced heterojunction photocatlysts [15]. Additionally, as compared with other 2D Materials, such as TiO2 , WS2 and MoS2 nanosheets, graphene can be fabricated by the more simply procedures [2]. In a word, the numorous advantages of graphene make it the shining star material for constructing the advanced and multi-functional heterojunctions for photocatalytic applications. 2.4. Classification and comparison of graphene-based heterojunctions According the nature of graphene, heterojunctions in graphenebased composite photocatalysts could be divided into six classes as follows (Fig. 6): Schottky junctions, Type-II heterojunctions, Zscheme heterojunctions, Van der Waals heterostructures, in plane heterojunctions and multicomponent heterojunctions. Clearly, metallic graphene could be employed to construct the Schottky junctions and indirect all-solid-state Z-scheme heterojunctions, whereas the semiconducting graphene could be used to fabricate Type-II (including n-n and p-n junction) and direct Z-scheme heterojunctions. More interestingly, both semimetallic and semiconducting graphene could be used to obtain the Van der Waals heterostructure, in plane heterojunctions and multicomponent heterojunctions. For all these six types of graphene-based heterojunctions, the built-in electric fields could be mainly achieved, which could significantly facilitate the spatial transfer and separation of photoinduced charge carries in the graphenebased composite photocatalysts, thus fulfilling the fundamentally enhanced photocatalytic efficiency. In the next section, both the significant progress and important design rules for six types of graphene-based heterojunctions will be thoroughly summarized and highlighted, which will provide some useful guidelines and shed some lights toward the development of highly efficient graphene-based heterojunction photocatalysts for various kinds of applications. To further compare the different graphene-based heterojunctions, their similarities, differences and design rules are also summarized in Table 1. Clearly, the similarities of these hetero-

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Fig. 7. Electron energy band diagrams of metal contact with a n-type semiconductor with m > s (a) before contact and (b) thermal equilibrium after contact, where Eg , Ef , , x, and s represent the band gap energy, Fermi level, work function, electron affinity and semiconductor, respectively [51].

junctions are consistent with those in Fig. 6, which were closely realted to their semimetellic or semiconducting properties. For the Schottky and indirect Z-scheme heterojunctions, graphene play the different roles in enhancing the photocatalytic efficiency. Detailly speaking, graphene in the Schottky heterojunctions acts as a cocatalyst, whereas graphene in the indirect Z-scheme heterojunctions serves as a solid mediator. When designing these two kinds of heterojunctions, more efforts should be devoted to enhancing the electrical conductivity, work function and electrocatlytic activity. Furtherore, the Type-II and direct Z-scheme heterojunctions exhibit the different charge-separation pathways. The eletrons and holes in the Type-II heterojunctions could migarate in the opposite direction and be enriched in two semicondcutors, whereas eletrons and holes in the direct Z-scheme heterojunctions could rapidly recombine at the interfacial region, thus leading to their much stronger oxidation and reduction ability than those in Type-II heterojunctions. Notably, controling the band structures of graphene is crucial for designing these two heterojunctions. Additionally, the location of heterojunctions in the Van der Waals heterostructures and in-plane heterojunctions are obviously different. The Van der Waals heterostructures exist in the interfaces between two coupling nanosheets, while the in-plane heterojunctions could be only fabricated in one nanosheet. Absolutely, fabricating the ultrathin signle-layer or few-layer graphene nanosheets is the key step in design of these two heterojunctions. These comaparison and design rules will provide important references for constructing different graphene-based heterojunctions, which will be thoroughy discussed in the following sections. 3. Graphene-based heterojunction photocatalysts 3.1. Schottky junctions The formation of a Schottky barrier has been extensively illustrated in the previous reports [51,52]. In general, the isolated metal and the n-type semiconductor possess the different Fermi level positions of ˚m and ˚s , (Fig. 7A) respectively. When the semiconductor and metal (if ˚m> ˚s ) are connected electrically (Fig. 7B), the electrons will readily flow from the conduction band of semiconductors with higher work function to the metal with lower work function, until the two Fermi energy levels are aligned, thus leading to the formation of a space charge layer at electrical contact




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Table 1 Similarities, differences and design rules of graphene-based heterojunctions. Heterojunctions

Similarities

Differences

Design rules

Schottky and Indirect Z-scheme heterojunctions

The utilization of semimetellic graphene

Graphene in the former/latter is a cocatalyst/mediator.

Enhancing the electrical conductivity, work function and electrocatlytic activity

Type-II and Direct Z-scheme heterojunctions

The utilization of semiconducting graphene

The different charge-separation pathways

Controling the band structures of graphene

Van der Waals heterostructure, In plane heterojunctions

The utilization of semimetellic or semiconducting graphene

The different location of heterojunctions

Fabricating the ultrathin signle-layer or few-layer graphene nanosheets

Fig. 8. (A) Comparison of adsorption and photocatalytic degradation efficiency of captured MB over TiO2 , P25, graphene-TiO2 physical mixing and graphene-w-TiO2 . (B) The adsorption–desorption isotherm of MB molecules on graphene-w-TiO2 . (C) Schematic of the photocatalytic charge-carrier separation mechanism on graphene-w-TiO2 based on Schottky junction model. (D) Scheme for the roles of graphene in capturing MB molecules and photoinduced electrons during the photocatalytic degradation of MB molecules in water [55].

interfaces. As a result, the surface of the metal acquires an excess negative charge, while electron migration away from the barrier region creates an excess positive charge in semiconductor, thus leading to an upward band bending toward the surface. Consequently, the small barrier formed at the depleted layer is known as the Schottky barrier. The height of the barrier, ˚b , is given by: ˚b = ˚m -Xs where Xs , is the electron affinity, measured from the conduction band edge to the vacuum level of the semiconductor. The Schottky barrier serves as an efficient electron traps, thus leading to a high partial electron density around the cocatalysts for photocatalysis and preventing the unexpected migration of electrons from cocatalysts back to the semiconductor. Generally, the noble metals with higher work function were coupled with n-type semiconductors to construct the Schottky barrier, thus achieving the improved charge separation and photocatalytic activity. At this point, any strategy to enlarge the work function of graphene is beneficial for

increasing the height of the Schottky barrier between graphene and an n-type semiconductor, thus resulting in the enhanced photocatalysis. 3.1.1. Design of nanostructured Schottky heterojunctions In the past several years, the high-conductivity 2D graphene has been widely hybrided with different semiconductors, such as metal oxides and sulfides, to fabricate various Schottky junctions for different photocatalysis. It is known that the morphology and micro/nanostructures of semiconductors have important impacts on the photoefficiency of graphene-based Schottky heterojunctions [53,54]. Thus, various interesting nanostructed semiconductors, such as 0D quantum dots, 1D nanowires/nanorods, 2D nanosheets and 3D microspheres, have been integrated with the versatile platform of 2D graphene to fabricate the advanced graphene-based Schottky heterojunctions. For instance, Liu et al. proposed that the RGO wrapped TiO2 hybrid (graphene-w-TiO2 ) Schottky heterojunctions could be fabricated through directly wrapping TiO2




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Fig. 9. TEM (A) and HRTEM (B) images of sample GS0.25, (C) comparison of the photocatalytic activity of different samples GSX(X: the weight ratios of GO to Zn0.8 Cd0.2 S), Pt-GS0: GS0 loaded with the optimized 1 wt% of Pt cocatalyst; (D) proposed mechanism for photocatalytic H2 -production under simulated solar irradiation [63].

by the graphene oxide (GO) and subsequent one-step photoreduction of GO [55], which showed the significantly improved the photocatalytic activity, as compared to the TiO2 , P25 and graphene-TiO2 physical mixing (Fig. 8A). The adsorption and desorption irreversible hysteresis on graphene-w-TiO2 confirms the role of graphene in improving capture of dyes from water (Fig. 8B). It is believed that the formation of Schottky heterojunctions in graphene-w-TiO2 and upshift Fermi level of TiO2 under UV light irradiation could decrease the semiconductorto-graphene barrier and promote the transfer of photoinduced electrons from the conduction band of TiO2 to RGO (Fig. 8C). As a result, the excellent synergism of graphene in capturing both MB molecules and photoinduced electrons lead to the significantly enhanced photodegradation activity toward MB molecules in water (Fig. 8D). It should be noted that the semimetal and high-work-function characteristics of graphene with a zero band gap are particularly crucial for constructing the Schottky heterojunctions interfacial contact between RGO and semiconductors. At this end, the defectfree semimetal graphene obtained by non-solution based methods (e.g., CVD and epitaxial growth on SiC) are highly expected to construct high-quality Schottky junction with semiconductor materials [56–59]. Besides the RGO wrapped TiO2 , various other kinds of nanostructured semiconductors, such as 0D nanocrystals, 1D nanorods and 2D nanosheet, have been integrated with 2D graphene nanosheets to fabricate the Schottky junctions with different interfacial contact morphology [60–62]. Interestingly, Yu and his coworkers also constructed a high-quality Zn0.8 Cd0.2 S/RGO Schottky junction with 0D/2D coupling interfaces (Fig. 9A and B) [63], and achieved a 4.5-fold improvement in photocatalytic H2 -

evolution rate, compared to that of bare Zn0.8 Cd0.2 S nanocrystals (Fig. 9C). It is believed that the formation of Schottky junction interfaces between 2D graphene cocatalyst and 0D Zn0.8 Cd0.2 S nanocrystals could effectively promote the photo-induced electron transfer and separation, and increase the surface H2 -generation active sites, thus leading to the greatly enhanced photocatalytic H2 evolution (Fig. 7C). Zeng et al. also successfully prepared the 0D cadmium sulfide (CdS) nanocrystals/2D RGO Schottky junctions by a one-pot solvothermal process [64]. The results showed that the CdS–RGO 0D/2D Schottky junctions obtained by the one-pot solvothermal process exhibited much better photocatalytic hydrogen production than those from the precipitation method under visible light irradiation (Fig. 10A). It is believed that the implanted RGO could efficiently separate the photogenerated electron–hole pairs of CdS and accelerate the reduction of water/proton to H2 (Fig. 10B) [64]. More interestingly, the hybridation of RGO and various 1D semiconductor nanostructures, such as TiO2 nanorods/nanowire [54,65–69], CdS nanorods/nanowires [70–72], ZnO nanorods [73–76], Bi2 S3 nanorods [77] and WO3 nanorods [78–80], have been widely employed to fabricate the 1D-2D Schottky heterojunctions for different photocatalytic applications. Notably, to improve the quality of Schottky heterojunctions, more attention should be also paid to the new and effective fabrication methods, such as chemical vapor deposition or UV-assisted photocatalytic reduction of graphene oxide [81,82]. Additionally, in these reports, several crucial factors, such as the (conductivity) semimetal properties and work function of graphene and the strength and toughness of interfacial contact, are seldomly touched, to construct high-quality graphene-based Schottky junction, which should be also comprehensively considered in the future studies.




Fig. 10. (A) Time-dependent photocatalytic H2 evolution over the different products. a: RGO–CdS (one-pot solvothermal process), b: RGO–CdS (the precipitation method), c: CdS (one-pot solvothermal process) and d: CdS (the precipitation method) (100 mg of catalysts, 100 mL Na2 S (0.35 M)–Na2 SO3 (0.25 M) solution, ␭ ≥ 420 nm). (B)Schematic diagram of the proposed mechanism for photocatalytic H2 production over RGO CdS [64].

Fig. 11. Schematic illustration for the advantages of 2D layered Van der Waals heterostructures [83].

Meanwhile, constructing the non-covalent and covalent Schottky heterojunctions between 2D semiconductors and 2D graphene have been utilized to tailor the electronic properties and band structures of 2D semiconductors [84–88]. On the one hand, the strong electronic interactions, such as ␲-␲ stacking interactions, van der Waals forces, and hydrogen bonding, have been demonstrated to profoundly influence the band structure of 2D host semiconductors [89–91]. More importantly, it has been experimentally demonstrated that such a strong electronic interaction could be easily modulated through tuning the concentration and oxygendefects of graphene in the composites [89]. The theoretical study has shown that increasing the O concentration could change the direct-gap g-C3 N4 /rGO hybrid to an indirect-gap one [92], indicating the important roles of ␲-␲ stacking interactions in controlling the band gaps of g-C3 N4 . In addition, constructing 2D-2D Schottky heterojunctions with intimate and large contact areas between the two contacted semiconductors can provide sufficient charge transfer and trapping channels for more efficient separation [93]. As compared with other types of heterojunctions (i.e. 0D–1D, 1D–1D, 0D–2D and 1D–2D), the unique 2D–2D Schottky heterojunctions with much larger interfacial contact areas are advantageous for more efficient interfacial charge transport and photocatalytic performance enhancement (as shown in Fig. 11) [83,94–96]. In 2011, Yu’s group constructed 2D–2D layered Schottky heterojunctions of graphene/TiO2 nanosheets with exposed (001) facets (as shown in Fig. 12A and B) and graphene/g-C3 N4 nanosheets (as shown in Fig. 12C and D) [97,98], both of which exhibited significantly enhanced photocatalytic H2 -evolution activities due to the efficient interfacial charge transport and spatial separa-

tion of photoinduced electrons and holes through the interesting 2D-2D Schottky heterojunctions. For instrasnce, Yu’s group[98] constructed the graphene/TiO2 nanosheets Schottky heterojunctions and demonstrated that the intimate 2D-2D interfaces could achieve more efficient and rapid electron migration from the TiO2 NSs to graphene and a unique photothermal effect induced by visible-light absorption. As a result, the optimized 1.0 wt% graphene-loaded TiO2 nanosheets showed the highest photocatalytic activity (36.8 ␮mol h−1 ) among all samples studied. Notably, the drastically reduced photocatalytic activity of such Schottky heterojunctions by the excess graphene clearly reveals the unexpected light-shielding effect. Therefore, finding the optimal content of graphene in the Schottky heterojunctions is crucial for accomplishing much higher photocatalytic activity. However, in this study, TiO2 nanosheets of ca. 50–80 nm side size and 6–8 nm thickness should be further optimized to improve the quality of Schottky heterojunctions. In addition, it was also demonstrated that the ternary graphitic C3 N4 nanosheets/N-doped graphene/layered MoS2 2D layered Schottky heterojunctions exhibited much higher photocurrent density and photocatalytic activity for simultaneous oxidation of MB and reduction of Cr(VI), due to the multiple synergistic effects of these three different nanosheets with 2D-2D coupling [99]. So far, besides the extensively studied graphene/g-C3 N4 2D-2D Schottky heterojunctions [85,92,97,100–106], there are limited investigations about the applications of other 2D-2D graphene-based Schottky heterojunctions for photocatalysis [30,96,107–114]. At this end, the interesting 2D-2D graphene-based Schottky heterojunctions deserves more attention in future syudies. Notably, the unexpected re-stacking of these 2D-2D graphene-based Schottky heterojunctions should be effectively prevented. At this point, it is expected that they could be self-assembled into the hierarchical nanostructurs for the practical photocatalytic applications. More importantly, the deep mechanisms about the improved charge kinetics and energyband as well as strong electronic coupling structures in the 2D-2D graphene-based Schottky heterojunctions should be experimentally and theoretically evaluated and revealed. Additionally, the hierarchical graphene-based Schottky heterojunctions could be fabricated by loading the 3D semiconductor nanostructure onto the 2D graphene nanosheeets, which could not only realize the quick transport and separation of electrons through the extended ␲-conjugation structure, but also increase the light harvesting, reactant diffusion and adsorption [6]. As a result, various hierarchical flowers-like or hollow-sphere semiconductors, such as TiO2 [115], Sn3 O4 [116], (BiO)2 CO3 [117,118], Bi2 WO6 [119,120], MoS2 [121], CuS [122], BiOI [123], ZnO [124–126], SnO2 [127], Cu2 O[128] ZnIn2 S4 [129], have been widely deco-




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Fig. 12. TEM images of 1.0 wt% graphene/TiO2 (A) and 1.0 wt% graphene/g-C3 N4 (B). Proposed mechanism for the enhanced charge transfer and separation in the graphenemodified TiO2 nanosheets (C) and graphene/g-C3 N4 composites (D) [97,98].

rated on the 2D graphene nanosheets to construct the hierarchical graphene-based Schottky heterojunctions for the boosted photocatalysis. However, the effective contact area in hierarchical 2D/3D graphene-based Schottky heterojunctions is relatively small, which should be further enhanced to maximize the junction effects. In furture, it is expected that the 3D porous graphene flowers/microspheres could be used to fabricate the hierarchical graphene-based Schottky heterojunctions with much higher interfacial contact area [130,131]. More importantly, the multifunctionsl roles of 2D/3D graphene and semiconductors in the hierarchical graphene-based Schottky heterojunctions should be thoroughly investigated and revealed to boost their potential photocatlytic applications. 3.1.2. Modifiation and optimization of Schottky heterojunctions In general, there are two strategies to improve the electrical conductivity and increase the work function of graphene to construct high-quality Schottky heterojunctions through increasing the effective Schottky barriers. One efficient and simple strategy to improve the conductivity of graphene is to reduce the defect sites in the graphene or introduce highly conductive materials. At this point, the solvent-exfoliated graphene (SEG) is better than the RGO for constructing high-quality graphene-based Schottky junctions [132–136]. In a typical example, Xu and coworkers fabricated a series of CdS–GR (RGO, SEG)) Schottky heterojunctions via a facile one-step solvothermal approach [133]. The resulting novel SEGCdS Schottky heterojunctions with low graphene defect densities exhibit much higher activities toward aerobic selective oxidation of alcohols (Fig. 13B) and reduction of heavy ions Cr(VI) (Fig. 13C) than those RGO/TiO2 nanocomposites. It is believed that the inhibited photocorrosion of CdS and improved charge separation in Schottky heterojunctions are attributed to the enhancement of photoactivity toward diverse redox processes. Howerer, the charge kinetics,

the work function difference between SEG and RGO and promoted reactant adsorption were not discussed in this work, which should be paid more attention in furure studies. Additionally, the introduction of highly condcuctive polymers, such as polyaniline could also be used to boost the electrical conductivity of graphene nanosheets, which is advantageous for fabricating the high quality Schottky heterojunctions [137–140]. The other efficient strategy to improve the electrical conductivity and increase the work function of graphene is the heteroatom (O, N, B, P and S) doping [41]. Commonly, the pyridinic N in doped graphene could efficiently increase density of ␲ states near the Fermi level and reduce work function [141,142], while the relative electronegativity of graphitic N atoms reduces the electron density on the adjacent C nuclei, which helps electrons transfer from the adjacent C to N atoms, and N backdonates electrons to adjacent C pz orbitals. More interestingly, graphitic N is generally considered to be an effective promoter for ORR activity of carbon, instead of pyridinic-N, due to the facilitated O2 dissociation on the adjacent C atoms, and the formation of a strong chemical bond between O and C [141]. The increased carrier density in N-graphene boost its electrical conductivity so that more efficient Schottky junctions could be constructed to retard electron–hole recombination, thereby significantly enhancing photoactivity and photostability of the photocatalyst [143–152]. More importantly, three major containing-nitrogen groups different properties, including the graphitic N, pyridinic N and pyrrolic N endowed with a variety of other functions for its widespread photocatalytic applications [153]. Similarly, it was also demonstrated that the graphitic-N in N-doped graphene–Fe2 O3 Schottky junctions play important roles in promoting the transfer and transportation of photo-generated charges and improving the adsorption of CO2 and O2 [154]. Their notable synergistic effect can achieve obviously enhanced photoactivity for selective reduction of CO2 to CO and acetaldehyde




Fig. 13. (A) The schematic illustration for fabrication of CdS−GR (RGO, SEG) nanocomposites based on a one-step solvothermal approach. (B) Photocatalytic selective oxidation of benzyl alcohol and (C) photocatalytic reduction of Cr(VI) under visible light irradiation (␭ > 420 nm) over blank-CdS, CdS–5% RGO, and CdS–5% SEG aqueous dispersion under ambient conditions [133].

Fig. 14. (A) H2 evolution of CdS and N-graphene/CdS Schottky junctions with different contents of N-graphene. (B) Schematic illustration for photocatalytic water spitting process over N-graphene/CdS Schottky junctions in Na2 S/Na2 SO3 aqueous solution [156].

Fig. 15. (A) Schematic illustration for preparing 1D CdS NWs-NGR Schottky junctions by a simple electrostatic self-assembly method, followed by a hydrothermal reduction process. (B) Schematic diagram illustrating the photocatalytic reduction of nitro organics to amino organics over CdS NWs-NGR Schottky junctions under visible light irradiation (␭ > 420 nm) with the addition of ammonium formate as a quencher for photogenerated holes and N2 purge in water [71].




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Fig. 16. Spatial charge-separation mechanisms for: (A) Type I, (B) Type II, and (C) Type III heterojunctions [4].

degradation. In addition, the synergistic effect of N-doped TiO2 and N-doped graphene can also improve the photocatalytic H2 production efficiency of the commercial P25 by a factor of 13.1 [155]. Therefore, more attention should be paid to the positive synergistic effect of different modification strategies to maximize the photocatalytic activity. In another work, N-doped graphene (N/RGO) fabricated by the calcination of GO in NH3 atmosphere has been integrated with CdS via a two-step synthesis process: solution deposition and the followed calcination in N2 atmosphere [156]. As shown in Fig. 14A, the N/RGO-CdS Schottky heterojunctions exhibit higher photocatalytic activity toward H2 evolution than the RGO-CdS counterpart without N doping prepared by the same solution synthesis method as that for N/RGO-CdS. This is ascribed to the fact that the N doping enhances the electrical conductivity of RGO, which facilitates the photoinduced electron transport, thereby preventing the recombination of electron−hole pairs and thus enhancing the photoactivity (Fig. 14B). This study demonstrates that N-graphene as a cocatalyst is a more promising candidate for developing high-performance photocatalysts in the photocatalytic H2 evolution. Similarly, Han et al. reported that CdS nanowires-nitrogen doped graphene (CdS NWs-NGR) Schottky heterojunctions were fabricated by an electrostatic self-assembly strategy followed by a hydrothermal reduction (Fig. 15) [71]. The CdS NWs-NGR Schottky heterojunctions exhibit much higher photoactivity for selective reduction of aromatic nitro organics in water under visible light irradiation than blank CdS nanowires (CdS NWs) and CdS nanowires-reduced graphene oxide (CdS NWs-RGO) nanocomposites. The enhanced photoactivity of CdS NWs-NGR Schottky heterojunctions can be attributed to the improved electronic conductivity due to the introduction of nitrogen atoms, which thus enhances the separation and transfer of charge photogenerated carriers. This work could provide a facile method to synthesize NGR based 1D Schottky heterojunctions for selective organic transformations, and broaden the potential applications of NGR as a conductive cocatalyst. However, it should be noted that the semiconducting graphene with higher dopant concentration should be thoroughly investigated and compared with semimetallic graphene with lower dopant concentration to yield a better understanding of the relationships between doped graphene and improved photoactivity. 3.2. Type-II heterojunctions In general, in terms of the semiconductors’ energy bands and Femi levels, semiconductor heterojunctions can be divided into three typical types: Type I (Fig. 16A), Type II (Fig. 16B) and Type III (Fig. 16C) heterojunctions. Clearly, only onaly Type II heterojunctions are beneficial for the enhanced charge separation and photocatalytic activity improvements. The Type II heterojunctions could be further divided into several classes, such as n-n junction, p-n junctions, surface heterojunctions [157] and phase junctions.

Here, the graphene-based n-n and p-n heterojunctions will be discussed. 3.2.1. N-N heterojunctions Typically, N-doped graphene exhibits the n-type semiconductor characteristics due to its similar atomic size and larger valence electron number of nitrogen atoms as compared to those of carbon atoms [143,158–161]. Therefore, much attention has been devoted to nitrogen-doped graphene, due to its several advantages including more facile synthetic protocols compared to its counterparts and superior performance in promising applications including supercapacitors, field-effect transistors, fuel cells and photocatalysts [147,149,153,162,163]. As a potential candidate for high-performance photocatalyst, the n-type N-doped graphene can offer additional advantages originating from its unique twodimensional sp2 -hybridized carbon network including a large specific surface area and exceptional charge transport properties [147,163,164]. As mentioned above, more and more N-doped graphene-based Schottky heterojunctions have been constructed to improve the photocatalytic performance over semicondcutors [144,146,158,165–167]. However, there are few reports about the applications of N-doped graphene-based n-n heterojunctions in various kinds of photocatalysis. Therefore, there is still much room to develop the N-doped graphene-based n-n heterojunctions and deeply investigate the underlying mechanisms under visible light conditions, which could povide more ideas for designing more efficient graphene-based n-n heterojunction photocatalysts. 3.2.2. P-N heterojunctions Besides n-n junction, the creation of ultrathin graphene-based p–n heterojunctions is also a smart approach to significantly boost the photocatalytic activity, as p–n heterojunctions can potentially suppress the rapid recombination of photo-generated electronhole pairs [168]. As mentioned above, graphene oxide has been found to be a p-type semiconductor, due to the large electronegativity of oxygen atoms as compared to carbon atoms. Thus, GO was usually coupled with other n-type semiconductors, such as TiO2 , to construct the p−n heterojunctions for various photocatalysis. For example, a series of Type-II GO-TiO2 p−n/n-n heterojunctions have been fabricated by using tunable chemical properties of graphene oxide and TiCl3 as the reactants with different GO concentration in starting solution (Fig. 17A) [169]. The GO with either p type or n type semiconductor acts as both sensitizer and electron carrier in graphene oxide/TiO2 composites, which could be excited by the visible light with wavelengths longer than 510 nm. Besides, the semiconductor type of GO in the composites can be tuned by changing the concentration of GO in the precursor solution, which could be verified by transient photocurrents of graphene oxide/TiO2 composites (GOT) under visible light illumination (␭ > 510 nm) (Fig. 17B). Clearly, the GOT-A and GOT-E samples display the ptype and n-type photoresponses, corresponding to the lower and




Fig. 17. (A) Graphene oxide/TiO2 composites (GOT) with graphene oxide existing as either n type or p type semiconductor. (B) Transient photocurrents of graphene oxide/TiO2 composites (GOT) and P25 under visible-light irradiation with a wavelength larger than 510 nm [169].

Fig. 18. (A) H2 production rate of the crude CPNPs and the PRGO/CPNP catalysts. (B) Conductivity of the PRGOs. (C) Scheme of the energy levels of the CPNPs and the PRGOs. (D) Diagram of the photocatalytic mechanism for the PRGO/CPNP photocatalysts under visible-light irradiation [170].

higher concentration of GO in the starting solution, respectively. As a result, the formation of a p−n junction in GOT-A will contribute to its significantly enhanced visible-light photodegradation of methyl orange. However, it should be noted that the tunable semiconductor type of GO by changing the concentration of GO in the starting solution needs to be further verified. Similarly, Xu et al. successfully synthesized an efficient p-n Heterojunction Photocatalyst through chemical-bond-mediated combination of coordination polymer nanoplates (CPNPs) and partially reduced graphene oxide (PRGO) with a simple colloidal blending process [170]. The results demonstrate that the p-n heterojunction photocatalyst PRGO/CPNP exhibits a much higher photocatalytic H2 production rate than neat the CPNPs (Fig. 18A). It was found that increasing the reductive degree of the PRGO could lead to a monotonous rise in the conductivity and CB energy levels of the PRGO (Fig. 18B and C), whereas the H2 production rate of the PRGO/CPNP catalytic systems does not exhibit a monotonous rise with the reductive degree. Moreover, the Mott–Schottky plots further suggest that the CPNPs and PRGOs are typical n-type and p-

type semiconductors, respectively, confirming the creation of the p-n heterojunction between them (Fig. 18D). The inner electrical field formed in the p-n heterojunction significantly inhabits rapid recombination of photogenerated electron and hole pairs, thus achieving the greatly improved visible-light hydrogen-production activity. In addition, as an electron-deficient dopant, the doping of boron can increase the electrical conductivity and work function of graphene due to the increased density of states (DOS) value near the Fermi level, which is advantageous for the photo-generated charge carrier transfer in graphene-based photocatalysts. It was demonstrated that the highly-dispersed boron-doped graphene nanoribbons (B-GNRs) with p-type semiconductor properties and excellent conductivity could be prepared by a simple vacuum activation method [171]. More importantly, it was found that the bandgap of B-GNRs could be facilely controlled through the more zigzag- and armchair-edges, thus leading to the improved photodegradation activity toward the Rhodamine B. Recently, Tang and co-workers [172] experimentally demonstrated that the B doping could decrease the defect density and increases the elec-




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Fig. 19. (A) Schematic illustration of the preparation of GN-ZnSe p−n heterojunctions (blue rods, [ZnSe](DETA)0.5 nanobelts; orange rods, ZnSe nanorods; purple balls, N; gray balls, C). (B) Photocatalytic degradation of MO in water under visible light in the presence of GN-ZnSe, ZnSe commercial powder, and GN, and without adding any catalyst.(C) diffuse reflectance UV–vis spectra of ZnSe commercial powder and GN-ZnSe p−n heterojunctions [143]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

trical conductivity and photogenerated electron transfer effciency, thus leading to the higher activity than the undoped RGO toward degradation of rhodamine B (RhB) under visible light irradiation. However, so far, there have been few reports about the application of B-doped graphene in constructing highly efficient p-n heterojunctions, which should be paid more attention. Meanwhile, N-doped graphene has been combined with various p-type semiconductors, such as ZnSe and MoS2 , to fabricated graphene-based p−n heterojunctions [143,173–175]. For instance, the N-doped RGO/ZnSe p−n heterojunctions were fabricated through a one-pot hydrothermal process at low temperature [143]. ZnSe nanorods composed of ZnSe nanoparticles were found to deposit on the surface of the GN sheets (Fig. 19A). It has been found that the as-prepared N-RGO/ZnSe p−n heterojunctions exhibit remarkably enhanced photocatalytic activities toward the degardation of methyl orange under visible light irradiation (Fig. 19B). It is believed that the improved photoactivity can be attributed to the increased adsorption of contaminant molecules, enhanced light irradiation absorption (Fig. 19C), and promoted charge transportation and separation due to the formation of N/RGO-ZnSe p−n heterojunctions. Apart from developing the novel graphene-based p-n heterojunctions, an unique Type-II p-n heterojunction has also been widely reported recently, in which the photo-generated electrons in graphene-based semiconductor can migrate to the connected wide-band-gap semiconductors without irradiation, thus leading to the prolonged lifetime of electrons and enhanced photocat-

alytic activity. For example, Feng et al. have demonstrated that the alkylated graphene quantum dots (AGQDs)/TiO2 photocatalysts prepared by an ex-situ method showed the much higher visiblelight photodegradation efficiency toward Rhodamine B (RhB) than that of pure TiO2 and AGQDs (Fig. 20A) [176]. Notably, the spectrum of AGQDs-TiO2 shows a much stronger enhancement in light absorption, particularly in the visible-light region of 400–700 nm (Fig. 20B). The boosted photocatalytic activity of AGQDs-TiO2 composites can be attributed to the combined effects of the photosensitization of AGQDs and the unique Type-II heterojunction, as shown in Fig. 20C. To ruling out the RhB photosensitization under visible light irradiation, Xu et al. investigated the photosensitization role of graphene in graphene−ZnO (GR−ZnO) nanocomposites via using visible-light-driven photoreduction of Cr(VI) in aqueous solution [177]. The wavelength-dependent activity indicates that the photoactivity for reduction of Cr(VI) increases with increasing the irradiation energy of incident visible light (Fig. 21A). Clearly, more electrons from the ground-sate GR to a higher-energy excitation level (GR*) could be photoexcited by higher-energy irradiation, which can inject into the CB of ZnO through the unique Type-II heterojunction, thus leading to significantly enhanced photoactivity for reduction of Cr(VI). At this context, various nanostructured graphene materials, such as zero-dimensional graphene quantum dots, one-dimensional graphene nanoribbons and three-dimensional graphene frameworks, should be promising photosensitizers for a variety of high-conductivity wide-band-gap semiconductors (e.g., ZnO, TiO2 and SnO2 ) [15,178–182], which




Fig. 20. (A) Photocatalytic degradation of Rhodamine B over AGQDs, P25 and AGQDs-P25 samples under visible light. (B) UV–vis diffuse reflectance spectra of P25 and AGQDs-P25 nanocomposites films. (C) Schematic illustration of charge transfer mechanism in AGQDs-TiO2 nanocomposites under visible light irradiation [176].

Fig. 21. (A) Photocatalytic performance of 10% GR−ZnO for reduction of Cr(VI) in aqueous solution by utilizing monochromatic light with the wavelength in the range of 400−700 nm. (B) Schematic illustration of charge transfer mechanism in the 10% GR−ZnO nanocomposite for photocatalytic reduction of Cr(VI) in aqueous solution [177].

might be a promising direction to expand the applications of semiconducting graphene in future studies. More interestingly, it was experimentally and theoretically verified that the photosensitive effciency of graphene could be significantly boosted through increasing the content of oxygenated functional groups in graphene, due to the optical bandgap widening of graphene and the

upshift of its conduction band [180]. In addition, it should be noted that the photosensitizer role of semiconductors with the ambiguity of the conduction band alignment has also been observed in the previously reports. For instance, the electron transfer from BiVO4 to TiO2 or ZnO could be explained by the visible-light excitation of high energy electrons in BiVO4 or the unique ultrafine morphol-




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Fig. 22. Schematic illustration of spatial charge separation in (a) indirect Z-scheme and (b) direct Z-scheme heterojunctions.

ogy of BiVO4 nanomaterials [183–185]. In this case, it is proposed that the photosensitizer role of graphene should be also possible in some graphene-based Type-I heterojunction, which should be paid more attention in the future investigations. 3.3. Z-scheme heterojunctions Typically, the all-solid-state Z-scheme photocatalysts could be divided into the indirect Z-scheme and direct Z-scheme systems, as displayed in Fig. 22a and b, respectively. Typically, the all-solidstate Z-scheme photocatalysts without shuttle redox mediators are more suitable for the applications in the gas and solid phases. More importantly, the all-solid-state Z-scheme photocatalysts could also overcome some problems in the 1 st generation liquid phase Z-scheme photocatalytic systems, such as the obvious backward reactions and the competing reactions between mediators and reactans [29,186,187]. Therefore, the all-solid-state Z-scheme photocatalysts have attracted more and more attention in the photocatalytic fields. 3.3.1. Indirect Z-scheme heterojunctions Obviously, the semimetallic graphene could be used as an advanced solid electron mediator to construct the indirect Zscheme heterojunctions. Since Tada et al. first proposed the concept of the 2nd generation Z-scheme photocatalytic system, namely the all-solid-state (ASS) Z-scheme CdS-Au-TiO2 nanojunction photocatalysts, in 2006 [188], the indirect Z-scheme systems have been widely and successfully constructed by using the proper moble metals (i.e., Au and Pt) as solid interfacial mediators. However, the use of noble metals and their strong light absorbers greatly limits the wide application of the indirect Z-scheme photocatalysts. Fortunately, the earth-abundant conductive RGO mediators provide new opptunities for constructing the ribust indirect Zscheme systems for photocatalytic applications. More importantly, the RGO mediators could also increase the adsorption of reactans and the absorption of visible light. Thus, it is not surprising that the graphene-based indirect Z-scheme photocatalysts have attracted more and more attention in the past several years. As a famous example, Amal and co-workers constructed an interesting indirect all-solid-state Z-scheme photocatalysts using RGO as a solid shuttle redox mediator. As shown in Fig. 23, the photo-generated electrons in BiVO4 can be injected from its CB to the RGO sheet and then recombine with the photogenerated holes in Ru/SrTiO3 . As a result, the left electrons in Ru/SrTiO3 :Rh could drive the H2 evolution raction over the Ru cocatalyst, while the holes in BiVO4 could drive water oxidization reaction, accomplishing a complete overall water splitting [189]. More importantly, in comparison with ionic Fe3+ /Fe2+ and IO3 − /I− redox couples in the traditional Z-

Fig. 23. Schematic image and mechanism of WS in a Z-scheme photocatalysis system consisting of Ru/SrTiO3 :Rh and PRGO/BiVO4 under visible-light irradiation.

scheme systems, the RGO solid shuttle redox mediator exhibits many unique advantages, including easy recovery and reutilization, and the avoided unexpected surface back reactions. Recently, it was also demonstrated that the RGO as solid-state electron mediator could be employed to construct the Z-scheme photocatalysts between n-type TiO2 and various p-type metal sulfide semiconductors (e.g., CuGaS2 , CuInS2 , Cu2 ZnGeS2 , and Cu2 ZnSnS2 ) [190], due to the efficient transfer of photo-generated electrons in TiO2 to metal sulfides with a p-type semiconductor character (Fig. 24A). Taking the optimal Pt/CuGaS2 -TiO2 composite system as an example, the introduction of RGO could increase the photocatalytic activity and achieve the H2 and O2 generation in stoichiometric amounts (Fig. 24B and C). However, the intrinsic photocorrosion of CuGaS2 will greatly decrease the evolution rates of H2 and O2 with increasing the irradiation time, which should be further improved in the future studies. Other indirect all-solid-state Z-scheme systems, such as ZnO/RGO/CdS[191], have been also available for various photocatalytic applications. Notably, in designing these systems, some important factors, including the matching geometric and energy-band structure, microscopic and crystalline form of semiconductors, as well as the electrical conductivity of RGO should be carefully optimized for achieving the highly efficient photocatalysis [187,192]. Besides their applications in the photocatalytic water splitting, the indirect all-solid-state Z-scheme systems have also been utilized in the photodegradation of organics [196] and the photoreuction of CO2 into the solar fuels. For example, Wu et al. fabricated the ternary indirect all-solid-state Z-scheme graphiticC3 N4 /reduced graphene oxide/anatase TiO2 (g-C3 N4 -RGO-TiO2 ) heterojunctions via a simple liquid-precipitation strategy [193]. The optimized indirect all-solid-state Z-scheme g-C3 N4 -RGO (10 wt%)-TiO2 nanoheterojunctions showed the highest photocatalytic activity towards the degradation of MB under simulated solar




Fig. 24. (A) Z-scheme system composed of a p-type metal sulfide photocatalyst and RGO-TiO2 composite photocatalyst for water splitting. Z-schematic water splitting using Pt-loaded CuGaS2 and RGO-TiO2 composite under (B) 300 W Xe lamp full-arc and (C) simulated sunlight [190].

Fig. 25. (A) Photocatalytic activity of the as-prepared samples toward degradation of methylene blue (MB). (B) Proposed all-solid-state Z-scheme mechanism for photodegradation of MB under solar light irradiation [193].

light irradiation (Fig. 25A), which is about 4.7 and 3.2 times higher than those of the pure g-C3 N4 (0.0029 min−1 ) and direct Z-scheme g-C3 N4 -TiO2 (0.0043 min−1 ), respectively. The highly enhanced photocatalytic degradation performance could be attributed to the

indirect all-solid-state Z-scheme mechanism in g-C3 N4 -RGO-TiO2 nanoheterojunctions for the promoted separation and transfer of charge carriers (Fig. 25B), which could achieve both the improved oxygen-reduction capacity of electrons in g-C3 N4 and the forma-




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Fig. 26. (A) Rate of H2 evolution using Z-scheme CdS/RGO/g-C3 N4 heterojunctions. (B) Kinetic plot for atrazine degradation using the synthesized photocatalysts. (C) Energy band diagram for the Z-scheme electron transfer mechanism [194].

Fig. 27. FESEM images of (a) Fe2 V4 O13 , (b) Fe2 V4 O13 /RGO, (c) Fe2 V4 O13 /RGO/CdS, and (d) schematic illustration of photoconversion of gaseous CO2 into CH4 over Fe2 V4 O13 /RGO/CdS Z-scheme heterojunctions. (Fe2 V4 O13 ECB : −0.55 eV, EVB : 1.28 eV; CdS ECB : −0.52 eV, EVB : 1.88 eV vs. NHE) [195].

tion of hydroxyl radicals driven by the holes in TiO2 . Similarly, Jo and his coworkers successfully use the RGO as the solid electron mediator to design the ternary g-C3 N4 /RGO/CdS composit photocatalyst. The resulting Z-scheme hybrids exhibited the significantly enhanced photocatalytic activity toward H2 -evulution and degra-

dation of atrazine (Fig. 26A and B). The indirect all-solid-state Z-scheme electron transfer pathway has been proposed and verified by photoluminescence, transient photocurrent measurements, and determination of the photocatalyst band potentials. Clearly, as the electron mediator, the rGO plays an important role in pro-




Fig. 28. (A) TEM image of 1% GO/Ag2 CrO4 , (B) the comparison of apparent rate constants (k) of the obtained samples for MB degradation under visible-light irradiation (Gx, x represents the theoretical weight ratios of GO to Ag2 CrO4 (0, 0.5, 0.75, 1, 2 and 3 wt%), (C) Z-scheme photocatalytic mechanism for Ag2 CrO4 -GO composite photocatalysts [198].

Fig. 29. Proposed Z-scheme mechanism for the photocatalytic degradation of MB on the GQDs/ZnO NWs[200] (A) and phenol on Ag2 CO3 -NG photocatalysts[201] (B).

moting the fast recombination of electrons from the CB of the CdS and holes from the VB of the g-C3 N4 via the Z-scheme mechanism [194]. In fact, very recently, Li et al. rationally designed an indirect all-solid-state Z-scheme systems consisting of Fe2 V4 O13 nanoribbon, RGO and CdS NPs for CO2 photoreduction (Fig. 27A–C) [195]. The as-obtained indirect all-solid-state Z-scheme photocatalyst exhibits a good activity for photoreduction of gaseous CO2 into CH4 . As illustrated in Fig. 27D, under light irradiation, the photogenerated electrons of CdS can readily recombine with the existing holes of Fe2 V4 O13 through the RGO mediator. As a result, the photo-generated holes and electrons in the VB of CdS and the CB of Fe2 V4 O13 could achieve the O2 evolution and the reduction of CO2 into CH4 , respectively. The indirect all-solid-state Z-scheme mechanism was achieved by the synergetic effect of excellent electroconductivity of RGO and well-matched band positions between Fe2 V4 O13 and CdS semiconductors, thus resulting in the high efficiency of photocatalytic CO2 conversion. In addition, it should be noted that the optimization of nanostructures, interface morphology, mass ratio and composition of RGO electron mediators and two semicondcutors is of great importance for achieving highly active all-solid-state Z-scheme photocatalysts. 3.3.2. Direct Z-scheme heterojunctions By contrary, the semiconducting graphene is favorable for fabricating the direct Z-scheme photocatalysts. It is known that the direct Z-scheme photocatalysts exhibit many advantages than the conventional heterojunction, liquid-phase Z-scheme, and all-solidstate (ASS) Z-scheme photocatalysts, namely, the low construction cost, shortened carge-recommbination paths and the reduced light-shielding effect, thus leading to the wide applications for various photocatalysis [186,197]. Previously, it has been demonstrated that GO is a semiconductor with a band gap of ca. 2.5 eV, whose CB and VB levels are ca. −0.75 and 1.75 V (vs NHE), respec-

tively [43,44]. Thus, for practical applications, GO as a potential semiconductor also deserves more attention. For example, Xu et al. demonstrated that the Ag2 CrO4 -GO Z-scheme heterojunction (Fig. 28A) exhibited excellent photocatalytic activity and stability towards the degradation of the dyes and phenol in aqueous solution under visible-light irradiation [198]. The 3.5-fold enhancement in the photocatalytic activity of pure Ag2 CrO4 particles could be achieved via loading optimal content of 1.0 wt% GO (as shown in Fig. 28B). The enhanced photocatalytic activity is mainly attributed to the formation of Ag2 CrO4 -GO Z-scheme heterojunction that can not only facilitate the separation and transfer of the photogenerated charge carriers, but also preserve a strong oxidation and reduction ability (as shown in Fig. 28C). The high photostability is due to the successful inhibition of photocorrosion of Ag2 CrO4 by transferring the photogenerated electrons of Ag2 CrO4 to VB of GO. In another example, Min et al. demonstrated a Zscheme photocatalyst using Ag@AgCl encapsulated with GO, where GO and AgCl act as highly activated photocatalysts, and metallic Ag shuttles photogenerated electrons from the excited GO and AgCl under visible light irradiation [199]. The resulting Z-scheme photocatalytic system exhibited a much higher photocatalytic activity than Ag@AgCl-reduced graphene oxide (RGO) with the same quantitative graphene loading (15 wt%) for the degradation of methylene blue (MB), further confirming the role of graphene as semiconductor. Similarly, Ebrahimi et al. designed a direct Zscheme graphene quantum dots (GQD)/ZnO nanowires [200]. The as-obtained GQD/ZnO NWs exhibited a greatly improved photocatalytic degradation of methylene blue under solar irradiation, due to more efficient charge carrier generation, transport, and separation based on the different radical trapping experiments (Fig. 29A). More interestingly, Song et al. successfully constructed the direct Zscheme Ag2 CO3 -N-doped graphene (NG) photocatalysts using NG as cocatalyst (Fig. 29B) [201], which exhibited much higher pho-




19

Fig. 30. SEM (A) and HRTEM (B) images obtained from the pMoS2 /n-rGO, (C) Schematic illustration of the charge transfer and separation in the p-MoS2 /n-rGO 2D-2D van der Waals heterostructures under simulated solar-light irradiation [175].

todegradation activity towards phenol pollutant than Ag2 CO3 and its composites with graphene oxide (GO) or reduced GO as cocatalysts. It was found that the plasmonic Ag nanoparticles could be the formed on Ag2 CO3 -NG due to spontaneous reduction of Ag+ by pyridinic nitrogen species (N-p) of NG, whereas the graphitic nitrogen species (Ng) could enhance the efficiency of the photogenerated charge separation, Z-scheme transfer option, and O2 adsorption. Therefore, the as-designed direct Z-scheme Ag2 CO3 -NG photocatalysts provide an interesting candidate for photocatalytic applications. In future, it is expected that more and more direct Z-scheme photocatalysts for pollutant degradation and solar fule productioncould be constructed by directly using the cheap GO semiconductor. 3.4. Van der Waals heterostructures Recently, it was revealed that the creation of the atomicallycontrolled van der Waals (vdW) heterostructures could design and realize novel optoelectronic devices that are superior to conventional bulk counterparts [202]. Interestingly, the 2D-2D Van der Waals p−n heterostructures were facilely fabricated through depositing the p-type MoS2 nanoplatelets on the n-type nitrogendoped rGO semiconductor nanosheets (as shown in Fig. 30A and B) [175]. The earth-abundant p-MoS2 /n-rGO Van der Waals heterojunctions show significantly enhanced photocatalytic H2 -evolution activity in the wavelength ranging from ultraviolet to near-infrared light, due to the enhanced charge generation and suppressed charge recombination (Fig. 30C). More interestingly, it was demonstrated that the sheet/rod/sheet sandwich like architecture by CdS nanorods embedding a Van der Waals p-n junction of MoS2 /N-

RGO could promote overall water splitting in natural water and a high rate H2 production (1.58 mmol/h) in 7 vol% lactic acid solution, which can be ascribed to the multiple functions of the Van der Waals p-n junction of MoS2 /N-RGO, such as increasing the active sites for HER and OER, and facilitating spatial separation of charge to the contrary by their inner electric field [203]. Based on the density functional theory calculations, Fu et al. demonstrated that the 2D van der Waals MoSe2 /graphene/HfS2 and MoSe2 /N-doped graphene/HfS2 heterostructures are proposed to be promising candidates for Z-scheme photocatalysts [204]. The fine control of the n-type doping on graphene can enhance the efficiency of solar energy utilization. Furthermore, the 2D vdW MoSe2 /HfS2 vdW heterostructure is shown to be a direct Z-scheme system for photocatalytic water splitting without redox mediators, which is more easily synthesized (Fig. 31). This work is very helpful in deep understanding the theoretical design of two-dimensional (2D) vdW heterostructures. More recently, Iqbal et al. first reported that the H2 bubbles generated by photocatalytic water splitting are effective in the layer-by-layer exfoliation of MoS2 nanocrystals into few layers. [205] The in situ assembled few-layered MoS2 /CdS nanosheet-based van der Waals heterostructures, are demonstrated to be effective in charge separation and transfer, thus leading to a H2 evolution rate of 140 mmol g(CdS)−1 h−1 with an apparent quantum yield of 66% at 420 nm [205]. At this point, the graphene/CdS nanosheet-based van der Waals heterostructures are also highly expected for the photocatalytic applications. These results further confirmed that the unique 2D−2D van der Waals heterostructures would be advantageous for solar light harvesting and electron transport to reaction sites with respect to other 0D−2D and 1D−2D hybrid systems.




Fig. 31. (A) The band structures of MoSe2 /N-dopoed graphene/HfS2 van der Waals heterostructures. The Fermi level is set to zero. (B) The partitioned band edge positions of MoSe2 , graphene and HfS2 . (C) Theoretical design of two-dimensional Z-scheme photocatalysts for hydrogen production from water splitting [204].

3.5. In plane heterojunctions Different form the layer-by-layer or laterally stacked heterojunctions, the in plane heterojunctions with exciting properties provide more effective charge separation and exposed active sites for photocatalytic reactions over the 2D nanosheets. For example, through vapor growth at low temperature, the seamless and atomically sharp in-plane heterojunctions of WS2 /MoS2 could be created through lateral epitaxy of WS2 on MoS2 edges [206], which could generate strong localized photoluminescence enhancement and intrinsic p-n junctions. In theory, the in plane heterojunctions should hold a potential promise in the photocatalytic applications. Interestingly, Teng’s group fabricated the N-doped graphene oxide quantum dots through ammoniation treatment of GO at 500 ◦ C, which exhibited both p- and n-type conductivities indicated by the electrochemical Mott−Schottky analysis [163]. More importantly, it was found that these nitrogen-doped graphene oxide quantum dots could achieve the overall water splitting with a H2 :O2 ratio of around 2:1 under visible-light (420 nm < ␭ < 800

nm) irradiation (Fig. 32A). It is proposed that an internal in-plane Z-scheme heterojunctions has been fabricated, in which the oxygen functional group-containing and nitrogen-doped GO exhibited the p-type and n-type conductivity, in charge for photocatlytic H2 and O2 evolution, respectively (Fig. 32B). Furthermore, the same group also demonstrated that the coexistence of p- and n-domains in the surface intact nitrogen-doped graphene oxide quantum dots prepared by ultrasonic exfoliation of NH3 -treated GO sheets could also achieve the in-plane Z-scheme heterojunctions for facilitating charge separation and transfer, thus resulting in the significantly enhanced photocatalytic H2 production in triethanolamine aqueous solution [207]. As a totally metal-free candidate, the nitrogen-doped graphene oxide quantum dots consisting of the in-plane Z-scheme heterojunctions seem to be very promising for various photocatalytic applications, which deserves more attention in future studies. More recently, Che et al. proposed a proof-of-concept design of unique 2D plane heterostructural carbon ring (Cring)−C3 N4 nanosheet by a thermal-conjugate strategy of the sp2 -hybridized




21

Fig. 32. (A) Time-courses production of H2 and O2 over 1.2 g of NGO-QDs. (B) Illumination on the diode system results in recombination of majority carriers at the sp2 clusters to produce useful electron-hole pairs at the semiconductor-water interfaces [163].

Fig. 33. (A) Schematic for photocarrier transfer in (Cring )−C3 N4 ,where Lpc and ␶n represent the photocarrier diffusion length and lifetime, respectively. (B) Synthetic route for the in-plane heterojunctions of (Cring )−C3 N4 .(C) H2 evolution under UV ( 400 nm

CdS/ZnO/0.2 wt% RGO Ag/AgCl/3.2 wt% RGO

Multi-heterojunction Multi-heterojunction

Chemical bath deposition hydrothermal

100 W halogen lamp 350 W Xe lamp

Mass[g]/Target pollutant/concentration/volume 0.2/MO/1 × 10−5 M/50 mL 0.15/MO/0.01 g L−1 /100 mL

0.02/MB/1 × 10−5 M/200 mL 0.02/MO/20 mg L−1 /10 mL 0.02/MB/1 × 10−5 M/200 mL

0.20 wt% RGO–CdS–ZnO

Multi-heterojunction

Chemical bath deposition

100 W Halogen lamp

Bi2 O3 /TiO2 /1 wt% graphene


hydrothermal

300 W Xenon lamp, ␭ ≥ 420 nm

metal/WO2.72 /10 wt% RGO


situ redox precipitation and hydrothermal


0.2/RhB/1 × 10−5 M/100 mL 0.005/MB or MO/5 × 10−5 M/50 mL

Degradation time(min)/efficiencya

kapp (10−2 min−1 )b

60/99% 300/99.5% – –

2.8 17.3

[241] (2013)

0.38

500 W high pressure Hg lamp 350 W Xenon lamp

0.05/MO/10 mg L−1 /25 mL

15/7%

P25/Ag3 PO4 /40 wt% GO


electrostatically-drivenassembly and ion-exchange

350W Xe lamp

0.05/RhB/1 × 10−5 M/100 mL

60/80%

0.05/BF/20 mg L−1 /50 mL

Xe(Hg) 250 W lamp AM 1.5

hydrothermal

300 W Xenon lamp


solid dispersion

300 W visible lamps

TiO2 /Ag/1 wt% RGO


hydrothermal

150 W high pressure mercury lamp

Ag/5 wt% GO/BiVO4


solvothermal

300 W Xe lamp

BiVO4 /5.5 wt% GO/Bi2 O3


chemical bath deposition

150 W metal halide lamp

ZnFe2 O4 /ZnO/4 wt% GO


solvothermal

500 W xenon lamp

Ag/ZnO/1 wt% GO


Low-temperature microwave-assisted solution

300W high-pressure Hg lamp

0.07/MB/10000 mg L−1 /500 mL 0.05/MO/20 mg L−1 /50 mL

Ag–GO–Bi2 WO6


hydrothermal and chemical reduction

350 W xenon

0.05/RhB/5 × 10−5 M/20 mL

3 wt% RGO–ZnWO4 –Fe3 O4 Fe3 O4@Bi2 O3 –RGO


microwave irradiation self-assembly

250 W Hg lamp 500 W Xe lamp

0.7 wt% graphene–Eu2 O3 /TiO2


hydrothermal

300 W tungsten xenon lamp

GO- Carbon Nanotubes-Ni


chemical vapor deposition

350 W xenon lamp

CuTCPP/1 wt% RGO-TiO2


hydrothermal

a halogen lamp

Nd–TiO2 –GO


sol–gel

solar light

0.9 wt% RGO/meso-TiO2 /AuNPs


hydrothermal

500-W halogen tungsten lamps (380–820 nm)

TiO2 –1 wt% RGO TiO2 -0.05 wt.% RGO 4 wt% GQDs/LaCoO3 /ATP

Schottky-junction Schottky-junction Schottky-junction

hydrothermal hydrothermal Impregnation

300 W Xenon lamp

2 wt% RGO-TiO2

Schottky-junction

hydrothermal

halogen lamp

Co3 O4 –2 wt% GO

Schottky-junction

freeze-drying

500 W xenon arc lamp

2.5 wt% OH-RGO/TiO Bi2 S3 NPs/8 wt% RGO

Schottky-junction Schottky-junction

hydroxylate hydrothermal

a 500 W Xenon lamp. 500 W xenon lamp

CuI–RGO

Schottky-junction

ultra-sonication assisted chemical

150 W Xe lamp AM 1.5G

FeWO4 -0.3 wt% RGO

Schottky-junction

hydrothermal

4 W Light Emitting Diode lamp

GO-ZnO

Schottky-junction

solvothermal

direct sunlight

ZnSe/graphene

Schottky-junction

hydrothermal

AM 1.5

Cd0.5 Zn0.5 S/RGO

Schottky-junction

solvothermal

300 W Xe lamp

Cu–P25–22.69 wt% GO

Schottky-junction

hydrothermal

xenon lamp

0.5% GO/Ag2 CO3 4 wt% RGO@TiO2 -NR

Schottky-junction Schottky-junction

hydrothermal solid dispersion

300 W iodine tungsten lamp

CdSe/3 wt% RGO

Schottky junction

hydrothermal

300 W halogen lamp

CdS/RGO

Schottky junction

hydrothermal

500 W Xenon lamp

CdS/1 wt% RGO

Schottky junction

hydrothermal


CdS/Al2 O3 /1 wt%GO

Schottky junction

hydrothermal-physical mixing

500 W Phoenix tungsten halogen lamp

CdS/10 wt% RGO

Schottky junction

solvothermal


CdS/1 wt% RGO

Schottky junction

hydrothermal

300 W halogen tungsten lamp, ␭ > 400 nm

CdS/5 wt% RGO

Schottky junction

solvothermal


Schottky junction

250 W Xenon lamp, ␭ > 400 nm 400 W Hg lamp

CdS/1 wt% RGO-carbon nanotubes

Schottky junction

1.78 wt% RGO-ZnO

Schottky junction

Chemical bath deposition

0.2gL−1 /OTC/35 mg L−1 / -/MB/20 mg L−1 /0.15/MB/10 mg L−1 /100 mL 0.1/BPA/10 mg L−1 /100 mL

0.02/MB/10 mg L−1 /200 mL 0.01/cip/20 mg L-1/40 mL 0.1/4-chlorophenol/30 mg L−1 /100 mL 0.006/RhB/5.3 × 10−3 M/100 mL 0.1/RhB/10 mg L−1 /1000 mL 0.1/indigo carmine/20 mg L−1 /100 mL 0.01/BPA/30 mg L−1 /70 mL

A visible light lamp

60/96.1%

0.06/phenol/20 mg L−1 /600 mL 0.02/MO/20 mg L−1 /100 mL

0.1/RhB/1 × 10−5 M/300 mL 0.08/2,4dichlorophenol/60 mg L−1 /100 mL 0.1/RHB/20 mg L−1 /100 mL 0.1/RHB/5 mg L−1 /250 mL 1.645/MB/7.9 × 10−4 M/250 mL 0.1/MB/35mgL−1 /60 mL 0.05/MO/10 mg L−1 /100 mL

0.1/MB/10 mg L−1 /100 mL 0.05/MO/1 × 10−5 M/100 mL 0.01/RhB/1 × 10−5 M/20 mL 0.07/RhB/0.005 g L−1 /70 mL 0.05/MO/0.01 g L−1 /50 mL 0.2/MO/1 × 10−5 M/50 mL

0.03/MO/0.01 g L−1 /100 mL 0.01/MO/0.025 g L−1 /20 mL 0.08/MB/0.1 g L−1 /500 mL 0.01/MB/0.01 g L−1 /100 mL 0.02/MB/0.01 g L−1 /50 mL

0.01/RhB/1 × 10−5 M/20 mL

0.45

300/95% 4/94%(UV)

9.7

180/100%

0.4

[243] (2017)

h+ and • OH

[244] (2016)

• O − , h+ and • OH 2

[245] (2015)

• O − , h+ and • OH 2 • O − , and • OH 2 • O − , and • OH 2 • OH and • OOH

[246] (2015) [247] (2015) [248] (2015) [249] (2016)

45/100%

• OH

[250] (2015)

60/98.9%

[251] (2015)

240/60%

0.031

• OH h+ and • OH

100/100%

3.3

No mention

[253] (2013)

80/99.6%

0.5182

No mention

[254] (2015)

O2 − , and • OH

[233] (2014)

120/65% 135/100% 240/98.3%

2.68

[252] (2017)

[255] (2016) [256] (2017) • OH

80/93.5% 180/100%

[257] (2016) [258] (2010)

O2− , and • OH

100/95% 190/92%

1.342

150/99.7%

0.67

[259] (2016) [260] (2016)

O2 − , h+

[261] (2016)

30/50%

No mention h+ and • OH h+ and • OH

[262] (2013) [263] (2012) [264] (2017)

8640/95.2%

No mention

[265] (2015)

50/95.3%

[131] (2015)

120/92%

No mention • O − , and • OH 2 O2 − , and • OH

90/100%

O2 − , h+ and • OH

[268] (2015)

480/24%

No mention

[269] (2015)

25/99.8%

No mention

[270] (2015)

0.05/phenol/10 mg L−1 /10 mL 0.3/acetone/-/-

-/X-3B/-/-

CdS/4.6 wt% RGO

H2 S reduction hydrothermal

0.08/MB/25 mg L−1 /100 mL 0.03/MO/20 mg L−1 /100 mL

1.8

[242] (2016)

• O − , h+ and • OH 2

0.048 1.02

240/99.5%

1.27158

30/100% 100/98%

0.387

120/85.3% 30/50%

0.139 0.39

120/91%

1.9

50/95.2%

/

60/95%

5.9

60/90%

4.4

300/90%

/

–

1.76

[266] (2016) [267] (2016)

[271] (2015) h+ and • OH h+ and • OH h+ and • OH No mention • O − and • OH 2 • OH • OH • OH

No mention • O − and • OH 2

[272] (2016) [273] (2015) [274] (2015) [275] (2017) [276] (2010) [277] (2014) [278] (2012) [237] (2012) [279] (2012) [280] (2014)

150/94%

/

180/87%

1.23

• O − and • OH 2 • OH

30/62%

/

• OH

[283] (2013)

60/61.5%

/

No mention

[284] (2015)

[281] (2012) [282] (2013)

ARTICLE IN PRESS

500W Xe lamp

photoreduction hydrothermal

[239] (2014) [240] (2013)

120/99.9%

physical mixing and Chemical bath deposition

hydrothermal

No mention

[239] (2014)

physical mixing and Chemical bath deposition


[238] (2012)

No mention • O − ,h+ and • OH 2



[237] (2012)

h+ and • O2 − • OH

No mention



• OH

/

8.88

Ag/Ag2 CO3 –1 wt% RGO

GO/Bi2 O2 CO3 /TiO2 10 wt% GO/TiO2 /ZSM-5

9.1

0.28

3 wt% RGO/Ag2 S/TiO2

TiO2 /Cu2 O/12 wt% RGO

Ref. (year)

50/99.7%

0.05/MO/10 mg L−1 /80 mL

Ag/AgVO3 /RGO

Mainly active species

G Model


Photocatalyst

X. Li et al. / Applied Surface Science xxx (2017) xxx–xxx 25


Table 2 Photocatalytic degradation activities of pollutants over graphene-based heterojuntion composites.

Heterojunction types

Synthesis method

Light source

TiO2 /Graphene

Schottky junction

solvothermal

500 W tubelike high-pressure mercury lamp

TiO2 /0.1 wt% RGO

Schottky junction

stirring

350 W Xe lamp

TiO2 (001)- 0.4 wt% Graphdiyne

Schottky junction

hydrothermal

Xe lamp, 100 mW/cm2

N-doped TiO2 /graphene

Schottky junction

hydrothermal

150 W Xe lamp

0.03/MO/10 mg L−1 /40 mL 0.005/MB/20 mg L−1 /-

GO–TiO2

Schottky junction

11 W Hg lamp

0.073/MB/10 mg L−1 /100 mL

2 wt% RGO/ZnO

Schottky junctions

15 W UV lamp

0.05/MB/50 mg L−1 /50 mL 0.02/RhB/15 mg L−1 /50 mL

physical mixing ultrasound assisted

Mass[g]/Target pollutant/concentration/volume



0.04/RhB or 2,4-DCP/10 mg L−1 /60 mL

60/95%

4.6

0.05/MO or DMP/20 mg L−1 /10 mL

–

28.4% RGO/ZnFe2 O4

Schottky junctions

chemical co-precipitation

250 W high pressure mercury lamp

3 wt% GO/ZnO

Schottky junctions

hydrolysis-deposition

A 3 W LED lamp

BiOBr/10 wt% RGO

Schottky junctions

hydrothermal

300 W Xe lamp

TiO2 /5 wt% RGO

Schottky junctions

hydrothermal

300 W mercury lamp

GO–Fe2 O3

Schottky junctions

hydrothermal and impregnation

300 W Dy lamp

0.01/MO/20 mg L−1 /40 mL 0.1/RhB/100 mg L−1 /100 mL

SnO2 @0.01 wt% GO

Schottky junctions

Hydrothermal and impregnat

Bi2 WO6 /RGO

Schottky junctions

Hydrothermal

natural sunlight

-/MO/20 mg L−1 /0.1/RhB/60 mg L−1 /200 mL

WBVO/4 wt% RGO

Schottky junctions

hydrothermal

visible light (␭> 420 nm)

CdS/4.6 wt% GO

Schottky junctions

hydrothermal

250W Xe lamp

0.03/MB/2 × 10−5 M/50 mL 0.01/MB/10 mg L−1 /100 mL

350 W xenon lamp

0.1/RhB/10 mg L−1 /100 mL 0.1/RhB/20 mg L−1 /40 mL

directly heating

CoFe2 O4 −45 wt% RGO

Schottky junctions

chemical co-precipitation

800 W Xe lamp␭ > 420 nm

TiO2 –20 wt% RGO

Schottky junctions

spray coating

175 W ltraviolet lamp

CdS/5 wt% RGO

Schottky junctions

solvothermal

300 W Xe lamp

3 wt% RGO/ Bi3.64 Mo0.36 O6.55

Schottky junctions

low-temperature solution-phase

500-W Xe lampk ␭ < 3000 nm 300 W xenon lamp

hydrothermal

300 W xenon lamp

red phosphorus–2 wt% GO

Schottky junctions

mechanical milling and ultrasonic flotation

150 W Xe arc lamp

0.05/RhB/10 mg L−1 /100 mL 0.05/RhB/20 mg L−1 /500 mL

2 wt% RGO/Bi20 TiO32

Schottky junctions

hydrothermal

500 W Xe lamp

Cu2 MoS4 /RGO

Schottky junctions

hydrothermal

300 W xenon lamp

TiO2 –2 wt% GO

Schottky junctions

hydrothermal and photocatalytic reduction

15 W 365 nm UV lamp

ZnO/10 wt% RGO RGO@TiO2

Schottky junctions Schottky junctions

hummer and arc discharge hydrothermal

90 W halogen light ␭ = 254 nm UV light

CoFe2 O4 /10 wt% graphene

Schottky junctions

hydrothermal

500 W output power

Pt-TiO2 /1 wt% graphene

Schottky junctions

hydrothermal

absorption peak (485 nm)

0.02/RhB/20 mg L−1 /80 mL 0.01/RhB/1 × 10−5 M/100 mL

0.05/MO/15 mg L−1 /100 mL 0.1/MB/3.6 × 10−5 M/20 mL -/MO and PR/5 × 10−5 M/20 mL 0.05/MO/8.62 mg/mL/20 mL 0.05/MB/40 mg L−1 / 200 mL 0.01/AO7/35 mg L−1 /200 mL

Schottky junctions

photocatalytic reduction

300 W xenon lamp

Schottky junctions

hydrothermal

20 W UV lamp

nickel-incorporated TiO2 /GO

Schottky junctions

straightforward microwave-assisted

20 W visible light sources

ZnO/polypyrrole-GO

Schottky junctions

electropolymerization

4 W UV lamp

ZnO microspheres-4.02 wt% RGO

Schottky junctions

solution method

300 W xenon lamp

0.1/MB/10 × 10−5 M/10 mL 0.2/MB/10 mg L−1 /200 mL

2.5 wt% graphene quantum dots/TiO2 nanotube

Schottky junctions

hydrothermal

300 W Xe lamp

0.1/MO/14 mg L−1 /100 mL

AgI/RGO 1 wt% GO/ZnO


free ultrasound-assisted hydrothermal

150 W Xe lamp 250 W mercury lamp

4 wt% RGO/ZnO

Schottky junctions

thermal treatment

UV irradiation 300 W

Cu2 O-0.05 wt% RGO

Schottky junctions

solvotherma

350 W Xe lamp

TiO2 nanotube/5 wt% graphene

Schottky junctions

hydrothermal

UV lamp 14 W, 254 nm

RGO/titania nanocomposites

Schottky junctions

direct reduction

450 W medium pressure quartz mercury vapor lamp

GO/Ag2 CO3

Schottky junctions

facile chemical precipitation

A 250 W xenon arc lamp

SnO2 /GO

Schottky junctions

hydrothermal

500 W tungsten halogen lamp

GO/ZnO

Schottky junctions

hydrothermal

GO/ZnO

Schottky junctions

hydrothermal

2 wt% graphene/TiO2

Schottky junctions

water vapor annealing

FeOOH/1 wt% GO

Schottky junctions

hydrothermal

31.0 wt% graphene/ZnO

Schottky junctions

chemical deposition-calcination

15 W energy-saving light bulb

Mn3 O4 /graphene

Schottky junctions

solvothermal

visible light irradiation (88 W, ␭ > 420 nm),

LI-250 light meter 1.5 × 10−4 W/cm2 xenon

0.05/BPA/1 g L−1 /50 mL 0.01/B-RHB/10 mg L−1 /50 mL

0.1/RhB/10 mg L−1 /100 mL 0.1/MB/3 M/10 mL 0.03/RhB/10 mg L−1 /30 mL

0.04/RhB/1 × 10−5 M/30 mL 0.05/Acetaminophen/5 mg L−1 /500 mL

O2 − and • OH • OH

480/99.5%

[292] (2015) [293] (2013) [294] (2016) [295] (2017) [296] (2015)

0.123

No mention h+

[282] (2013)

O2 − and • OH O2 − , h+ and • OH

180/99.62%

O2 − and • OH O2 − and • OH No mention

30/85%

No mention

70/97.4% 60/85%

0.0487

75/95% 180/90%

0.309

90/100% 100/96.4%

0.47

O2 − , h+ and • OH No mention O2 −

No mention • OH

120/100%

[297] (2015) [298] (2015)

No mention

180/94%

360/99%

[299] (2017) [281] (2012) [300] (2016) [301] (2014) [85] (2016) [302] (2017) [303] (2016) [304] (2016) [305] (2016) [306] (2012) [307] (2015) [308] (2015) [309] (2015) [310] (2015)

0.404 1.501

O2 − ,• OH • OH

0.412 60/55%

0.011

30/100%

21

[311] (2016) [312] (2015) [313] (2016)

O2-, h+ O2 − , h+

[314] (2017) [124] (2017) [315] (2016)

20/94.64% 30/96.8% 450/90%

11.645

15/99.6%

15.1 0.271

180/96%

[291] (2017)

No mention

0.144

120/87.66%

[66] (2010)

3.86 0.411

210/86.76%

[287] (2014)

[290] (2015) No mention • OOH

120/100%

2.48

[316] (2015) [74] (2016) O2 − ,• OH • OH

5.213

0.2/RhB/1 × 10−5 M/50 mL

0.05/TA/5 × 10−4 M 0.0053/MO/10 mg L−1 /10 mL

[289] (2015)

O2 − , h+ and • OH

80/99%

0.02/MB/112.2 mg L−1 /100 mL

0.25/RhB/10 mg L−1 /250 mL

[288] (2015)

• OH

210/100%

0.15/BPA/32 mg L−1 /150 mL

TiO2 /graphene TiO2 /10 wt%RGO

• OH

120/94.7%

0.6/MB/20 mg L−1 /40 mL

thermal polycondensation

[286] (2013)

0.34

80/100%

solvothermal

[285] (2013)

No mention • OH • OH

2.241

0.08/MB/100 mg L−1 /50 mL 0.01/RhB/1 × 10−5 M/100 mL

Schottky junctions

No mention

2.47

20/100%

300 W Xe lamp␭> 420 nm

Schottky junctions

0.62

45/80%

300/53.27%

Schottky junctions

[96] (2013)

120/87.4%

0.1/RhB/1 mg L−1 /50 mL

BiPO4 /RGO C3 N4 /10 wt% graphene 1.5 wt% RGO/ZnO

• OH and h+

5.695

[317] (2015) [128] (2015) [318] (2015) [319] (2016)

O2 − , h+

300/99%

[320] (2017) [127] (2016)

• OH 150/96.8%

[321] (2015) [75] (2016)

arc lamp

1.0 wt% graphene–Bi2 O2 CO3

Schottky junctions

hydrothermal

250W high-pressure mercury lamp

350 W Xenon lamp

0.4/X-3B/30 mg L−1 /100 mL 0.1/RhB/10 mg L−1 /100 mL 0.03/RB5/20 mg L−1 /50 mL 0.1/MB/20 mg L−1 /100 mL 0.1/RhB/1 × 10−5 M/20 mL

O2 − , h+ and • OH • OH

30/97% 120/99.0%

[322] (2016) [323] (2017)

180/97%

1.99

• OH

[134] (2014)

60/100%

7.91

• OH

[324] (2017)

76/94%

[325] (2012)

ARTICLE IN PRESS

Schottky junctions

Ref. (year)

X. Li et al. / Applied Surface Science xxx (2017) xxx–xxx

g-C3 N4 /8 wt% RGO

-/X3B/1 × 10−4 M/0.1/MO/10 mg L−1 /100 mL

Mainly active species

G Model

Photocatalyst


26


Table 2 (Continued)


Synthesis method

Light source

TiO2 /0.5 wt% RGO Ti3+ self-doped TiO2 -0.1 wt% graphene graphene-Fe3 O4

Schottky junctions

sun light irradiation

84W light sources


vacuum activation solution chemistry

500-W tungsten halogen lamp sunlight

graphene–NiFe2 O4 GO/ZnO Cu2 O-0.05 wt%RGO

Schottky junctions Schottky junctions Schottky junctions

hydrothermal hydrothermal solvothermal

500 W xenon lamp 15 W UV lamp 350 W Xe lamp

CoFe2 O4 -RGO

Schottky junctions

hydrothermal

100 W reading lamp

Ag-1% wt graphene −TiO2

Schottky junctions

solvothermal

a 570 W xenon lamp

TiO2 /RGO

Schottky junctions

hydrothermal

high-pressure mercury lamp 300 W

CuO-/RGO

Schottky junctions

hydrothermal

150 W xenon lamp

GO/nanohydroxy apatite

Schottky junctions

hydrothermal

solar light 100 mW/cm2

TiO2 –25 wt% GO

Schottky junctions

hydrothermal

irradiation was 500 W/m2

Rgp-AG

Schottky junctions

hydrothermal

A 250 W Hg vapor lamp

Mass[g]/Target pollutant/concentration/volume 0.017/MB/20 mg L−1 /25 mL 0.07/MO/10 mg L−1 /100 mL 0.025/MB/20 mg L−1 /100 mL MB/100 mL 0.04/RhB/1 × 10–5 M/30 mL 0.025/MO/20 mg L−1 /50 mL 0.02/paraoxon/31 mg L−1 /100 mL 0.1/phenol/20 mg L−1 /100 mL /RhB/2 mg L−1 /3 mL 0.5/AM/2 mg L−1 /250 mL 0.008/MO/20 mg L−1 /100 mL 0.022/phenol/50 mg L−1 /100 mL


300/65% 120/92.43% 180/100% 140/99% 120/70% 60/100%

hydrothermal

Nb3 O7 (OH)-GO

Schottky junctions

hydrothermal

150 W Xe lamp

0.3 wt% graphene–TiO2 TiO2 /graphene/heteropoly acid


hydrothermal hydrothermal

solar light irradiation visible illumination

CeM/GO 5 wt% graphene/LaMnO3 /Fe3 O4


hydrothermal co-precipitation

A 500 W tungsten lamp 40 W Xe lamp light

ZnO-4.02 wt%RGO

Schottky junctions

simple solution

300 W xenon lamp

Er-doped BiOBr/3 wt% graphene

Schottky junctions

hydrothermal

TiO2 /10 wt% RGO

Schottky junctions

hydrothermal

XPA-7 photochemical reactor

RGO-TONT

Type-II heterojunction

hydrothermal

Hg lamp

0.5/BPA/1000 mg L−1 /50 mL -/MB/10 mg L−1 /40 mL 0.02/MO/20 mg L−1 /50 mL

0.05/RhB/10 mg L−1 /100m 0.03/AO7/20 mg L−1 /30 mL 0.05/CAP/20 mg L−1 /100 mL 0.2/MB/10 mg L−1 /200 mL 0.02/RhB/10 mg L−1 /

9.8

60/75%

2.25

480/50%

300/89.33%

3.293

60/99& 360/91%

1.12

50/99% 75/99.6% 20/100%

21

90/100%

4.658

120/58.5%

1.501

100/91.3%

0.244

500 W mercury lamp

TiO2 @5% RGO


hydrothermal

250 W high-pressure mercury lamp

RGO/ZnFe2 O4 / Ag3 PO4 ,


solvothermal

a 300 W Xe lamp

g-C3 N4/ GO/MoS2


sonochemical

AM 1.5G filter

5 wt%graphene quantum dots-BiOI/MnNb2 O6 BiOI-AgI-RGO


hydrothermal

A 250W xenon lamp


simple precipitation

150 W Xe lamp

0.1/RhB/-/100m

90/83%

1.82

(C16 H33 (CH3 )3 N)4 W10 O32 /gC3 N4 /3 wt% RGO

Van der Waals heterostructure

physical mixing

300 W Xenon lamp

0.05/phenol or MO/10 mg L−1 /50 mL

180/99%

0.223

0.4 wt% GO Quantum/ZnO

Z-scheme heterojunction

spin coating

0.015/MB/1 × 10−5 M/40 mL

180/78%

0.7

Ag/Ag3 PO4 /BiVO4 /1 wt% RGO

0.38 60/98%

0.32

120/95.4%

0.256

150/89.5%

0.1501

120/87.2%

300 W Xe lamp 250 W tungsten halogen lamp

0.05/RhB or phenol/-/50 mL

precipitation

300 W Xenon lamp

0.02/MB/1 × 10−5 M/100 mL

15/50%

hydrothermal

400 W Xe lamp

0.2/MB/10 mg L−1 /500 mL

90/97.8%

electrostatic adsorption

350 W xenon lamp

0.05/phenol/8 mg L−1 /25 mL

25/90% 9/99.9% 180/92%

Z-scheme heterojunction Z-scheme heterojunction

mild chemical and photo-deposition photo-assisted reduction

Ag2 CrO4 –1 wt% GO


Bi2 S3 /TiO2 /RGO


Ag2 CO3 -0.85%NGO


GO/Ag2 SO3 /AgBr


electrostatic interaction and precipitation transformation

A 500 W xenon lamp

0.025/MO/20 mg L−1 /40 mL

g-C3 N4 –10 wt% RGO-TiO2


liquid-precipitation

300 W Xe lamp

0.05/MB/30 mg L−1 /200 mL

500 W Xe lamp

0.25/RhB/10 mg L−1 /250 mL

[332] (2016)

[335] (2017) O2 − , and • OH O2 − , and • OH

[336] (2015) [337] (2014) [338] (2017)

electrostatic interaction and direct chemical bonding

Ag3 PO4 /RGO/Ag

[329] (2017) [330] (2017) [128] (2015)

[334] (2012)

50/53%

0.05/TC/10 mg L−1 /100 mL

[327] (2015) [328] (2017)

[333] (2016)


0.04/RhB/10 mg L−1 /80 mL 0.05/TC/10 mg L−1 /100 mL

[326] (2016)

[331] (2016)

SnO2 @RGO

0.01/MG/2.5 × 10−5 M/25 mL -/2,4-DCP/20 mg L−1 /50 mL

Ref. (year)

O2 − , and • OH

[339] (2013)

O2 − , and • OH

[340] (2016) [341] (2016) [342] (2016)

O2 − , h+ and • OH • OH O2 − , h+ O2 − , • OH No mention O2 − , h+ and • OH

[343] (2017) [344] (2017) [124] (2017) [345] (2017) [312] (2015) [346] (2017) [181] (2017) [347] (2016)

• OH No mention • OH O2 − , h+ and • OH h+ , e− , • O2− and• OH

[348] (2016) [91] (2014) [349] (2017) [350] (2016) [351] (2017)

• OH and h+

[200] (2017)

• O − and h+ 2

[352] (2017)

1.411

O2 −

[353] (2014)

2.8

• O − , h+ and • OH 2

[198] (2015)

• OH

[354] (2017)

O2 − , h+

[201] (2017)

O2 − , h+ and • OH

[221] (2017)

0.137

O2 − , and • OH

[193] (2017)

0.55

• OH

[355] (2017) [356] (2017)

60/94.96%

0.93

g-C3 N4 /1 wt% RGO/ Bi2 MoO6


RGO-Cu2 O/Bi2 O3


solvothermal reduction

250 W xenon lamp

0.05/TC/10 mg L−1 /100 mL

180/75%

O2 − , h+ and • OH

RGO–Ag3 PO4


photoreduction

illumination chamber

0.015/TOC/2 × 10−5 M/10 mL

5/97.1%

O2 − , h+ and • OH

[357] (2016)

15% GO-Ag@AgCl


photo-assisted precipitation

LED lamp ␭ > 420 nm

0.03/MB/2.5 × 10−5 M/50 mL

60/100%

No mention

[199] (2014)

TiO2 –RGO–AC


hydrothermal

300 W xenon lamp

0.0725/RhB/2 × 10−4 M/50 mL

80/99.79%

a b

Hydrothermal

9.87

[358] (2017)

27

Degradation efficiency is estimated by = (1-ct /c0 )×100%, where c0 and ct are the concentration of the organic dye at t = 0 and t, respectively. kapp refers to apparent first-order reaction rate constant.

60/97.3%

ARTICLE IN PRESS

Schottky junctions

O2 − , h+ and • OH

15/100%

150/67%

Fe2 O3 nanorod/ 5 wt% RGO

High pressure UV Mercury Vapor lamp 160 W 300 W Xe lamp

10.61

40/87%

0.02/phenol/20 ppm/100 mL

hydrothermal deposition

0.271

• OH and O − 2

125%100%

180/92%

Schottky junctions

Mainly active species O2 − and • OH O2 −

60/62%

0.01/sulfamethoxazole/5 mg L−1 /25 mL

TiO2 –2.7 wt% RGO


G Model


Photocatalyst



Table 2 (Continued)



Fig. 38. Schematic illustration of the photocatalytic H2 -production mechanism over the ternary WS2 -CdS/rGO[223] (A) and Ni(OH)2 -CdS/rGO [224] (B) multicomponent heterojunctions under visible light irradiation.

Fig. 39. (A) Schematic diagram and (B) the energy band structure for electron–hole pair transfer and separation in the Au–TiO2 –graphene multicomponent heterojunctions for photocatalytic H2 -production under visible light irradiation [232]. (C) Schematic illustration for the electric near-field produced by Ag NP SPR effect enhancing the separation of photogenerated electrons and holes on the Bi2 WO6 under light irradiation [233]. (D) Proposed mechanisms for RhB adsorption on graphene via strong ␲–␲ interactions.

(CN− in water) over titania in 1977 [363]. To effectively enhance the photodegradation efficiency, a variety of semiconductor modification strategies have been exploited, such as constructing semiconductor heterojunctions, coupling with nanocarbons, loading cocatalysts and fabricating unique nanostructures. Among them, constructing graphene-based heterojunction photocatalysts seems to be very promising for heterogeneous photodegradation of pollutants, due to the boosted light harvesting, reactant adsorption, and transfer and separation of photo-generated electrons and holes [364]. Table 2 summarizes the applications of graphene-based heterojunctions in fields of photodegradation of different organics. In most cases, • O2 − and • OH are two key active oxidation species. Especially, for some visible-light-driven semiconductors, the formation of ·OH cannot be driven by the photo-generated holes. Thus, the formation of • O2 − by O2 reduction and the selective photocatalysis induced by oriented adsorption of graphene seem to be more important for designing high quality graphene-based heterojunctions. Additionally,the magnetic graphene-based heterojunctionts are also favorable for the simply collection, separation and recycling utilization in the photocatlysis. These three important aspects will be thoroughly highlighted in this section.

4.1.1. Improved O2 reduction It is known that the doped graphene materials are the excellent metal-free electrocatalysts for boosting the ORR in the fuel cells. Naturally, graphene is widely utilized to improve the ratedetermining O2 reduction step for the significantly enhanced photodegradation of different organics. For instance, To deeply investigate the promoting effects of N-bonding configuration in graphene for boosting the overall photocatalytic performance, Yu and his coworkers fabricated N-rGO/TiO2 Schottky heterojunctions (Fig. 41A and B) [148]. It was revealed that the graphitic-N doped and pyrrolic-N doped graphene can serve as effective electrontransfer mediator and oxygen-reduction active site, respectively. Furthermore, the loading of Fe(III) and Cu(II) clusters as co-catalysts can achieve the further significantlyimproved photocatalytic O2 reduction activity over the N-rGO nanosheest. As a result, the pronounced synergism of graphitic-N and pyrrolic-N in graphene and the loaded co-catalyst (Fe(III) and Cu(II) clusters) leads to the highest photoactivity (as shown in Fig. 41C–F). Therefore, the study provides a general and effective approach to design high-performance graphene-based heterojunctions by utilizing the




29

tocatalysts could be significantly improved through promoted O2 reduction and charge separation. In future, it is expected that more and more earth-abundant graphene-based O2 -reduction cocatalysts could be developed and applied in the photodegradation of organic compounds.

Fig. 40. Schematic illustration of the charge separation and transformation in gC3 N4 /CdS/rGO/Pt multicomponent heterojunctions under visible-light irradiation [236].

synergistic effect between electron-transfer co-catalyst and oxygen reduction activation sites of graphene. In another example, Song et al. utilized the defect structures of RGO to spontaneously reduce the Ag+ ions and fabricate highly efficient plasmonic Ag/Ag2 CO3 -RGOmulticomponent heterojunctions for photooxidation of pollutants [244]. The resulting Ag/Ag2 CO3 RGO multicomponent heterojunctions showed a much higher activity than the Ag2 CO3 and Ag2 CO3 –GO composite in the photooxidation of organic pollutant. As shown in Fig. 42A, it is believed that the synergetic effect of the plasmonic Ag NPs and conductive graphene leads to the enhanced O2 -reduuction activity and promoted charge separation and transfer, thus achieving the improved photoactivity. More importantly, the photocorrosion of Ag2 CO3 was also obviously inhibited (as shown in Fig. 42B). Thus, it is clear that the photocatalytic activity and stability of Ag-based pho-

4.1.2. Enhanced selective adsorption of reactants The selective oxidation of targeted organics is crucial for the practical separation processes[306,365], however, the OH radicals with strong oxidizing capability generally exhibit nonselectivity for photocatalytic elimination of desired products over many semiconductors such as TiO2 [366]. Recently, Yu et al. found that the high photocatalytic selectivity of TiO2 toward pollutant degradation can be realized by optimizing its surface charge, exposed facets and etching degree of (001) facets [157,366]. The positively charged fluorinated hollow TiO2 microspheres are beneficial for the photodecomposition of negatively charged methyl orange (MO) in comparison with positively charged methylene blue (MB) [157]. In contrast, the surface-modified hollow TiO2 microspheres by either NaOH washing or calcination at 600 ◦ C exhibit the negatively charged surface properties due to the efficiently removed surface fluoride species and increased surface hydroxyl groups, thus facilitating the selective degradation of MB instead of MO. It is verified that the surface re-hydroxylation could promote the selective adsorption of MB molecules, thus achieving the decomposition of selected azo dyes. More interestingly, when the negatively charged graphene is hybrided with TiO2 (Fig. 43A–C), the photogenerated electrons can easily transfer to a graphene moiety, leading to the efficiently prolonged lifetime of electron–hole pairs (Fig. 43D). This contribution, together with promoted reactant adsorption, is of great significance for enhancing the photoactivity of TiO2 –graphene heterojunctions. It is likely that the sufficient MO or MB adsorption (Fig. 43E andF) on TiO2 –graphene heterojunctions significantly inhibits the competitive adsorption of water (hole scavenger) or O2 (electron scavenger). In this regard, the electron transfer to O2 , might be greatly retarded at the high graphene loading. As a consequence, the photocatalytic reactions predominantly initiated by • O2 − (in the case of MO) may be apparently suppressed by higher graphene loading. In contrast, the photocatalytic reactions predominantly initiated by h+ or • OH (in the case of MB) is less impaired by electron storage in graphene, which can

Fig. 41. Typical SEM (A) and TEM (B) images of N-rGO/TiO2 (N2 H4 ), the synergism of graphene-modified TiO2 Schottky heterojunctions: (C) rGO/TiO2 ; (D) N-rGO/TiO2 (NH3 ) or N-rGO/TiO2 (CO(NH2 )2 ); (E) N-rGO/TiO2 (N2 H4 ); and (F) Cu(II)/N-rGO/TiO2 (N2 H4 ) or Fe(III)/N-rGO/TiO2 (N2 H4 ) [148].




Fig. 42. (A) The photocatalytic reaction and charge transfer mechanism of the Ag/Ag2 CO3 –RGO multicomponent heterojunctions under visible-light irradiation; (B) Cycling runs in the photooxidation of MO in the presence of Ag2 CO3 (a) and Ag/Ag2 CO3 −RGO (b) [244]..

Fig. 43. TEM (A and B) and HRTEM (C) images of G0.5–TiO2 , (D) schematic illustration of graphene-mediated dye adsorption and the interfacial charge separation and transfer, (E) The adsorption behavior of methyl orange (MO) and (F) methyl blue (MB) on the Gx –TiO2 (x = 0.1, 0.5, 1 and 2) surface [306].

be steadily boosted by the combined effects of graphene-mediated high dye adsorption and fast charge separation. Therefore, it is possible to control the photocatalytic selectivity by the synergistic control of the graphene-mediated reactant adsorption, charge carrier dynamics, exposed facets and surface charge of TiO2 . 4.1.3. Magnetic graphene-based heterojunctionts High efficient degradation of organics could be also achieved by the fabrication of the graphene/cocatalyst heterojunctions with different magnetic semiconductros. The introduction of the graphene significantly promoted the photocatalytic performance of magnetic semiconductros in which the graphene could accept the delocalized electrons. In addition, graphene-based heterojunctions themselves have magnetic properties, which can be easily separated from a suspension system.As a result, the magnetic graphene-based heterojunctions could be reused for several times without losing catalytic activity. In general, the simple binary graphene/magnetic semiconductor heterojunctions could exhibit very high activity towards photodegradation of organics [289,309,328,329,367–379]. For example, Shang et al. reported that magnetic CoFe2 O4 /graphene heterojunctions were prepared by a simple hydrothermal process [309], which could achieve a degradation efficiency of ∼100% within 3 h for the dye solution with MB concentration as high as

40 mg/L. It is beleievd that the graphene acted as a electron acceptor, thus significantly improving charge separation and transfer, and enahcning the photocatalytic performance of the CoFe2 O4 . In another report, Li and his coworkers synthesized magnetic reduced graphene oxide-ZnFe2 O4 heterojunctions (RG/ZF) through a simple chemical co-precipitation method [289]. The optimized RG/ZF heterojunctions with mRG/ZF = 0.4 show the best photocatalytic activity under pH = 1, which can degrade RhB completely within 20 min, due to the strong adsorption capacity of reduced graphene oxide toward RhB, excellent specific surface area (2630 m2 g−1 ) and unique structural faerures. In addition, the magnetically separable CdFe2 O4 /graphene (CdFe2 O4 /GR) heterojunction was also prepared by a one-step hydrothermal method (Fig. 44A) [376]. The results showed that about 99.70% of MB was degraded in the presence of CO3 2− after 240 min of irradiation (Fig. 44B). Similarly, in the suspension system of CdFe2 O4 /GR and MB aqueous solution, graphene could attract the photo-generated electrons and help to reduce the recombination rate of photo-generated charges (Fig. 44C). Recently, a facile approach has been designed for the preparation of magnetic NiFe2 O4 photocatalyst (NiFe2 O4 NG) supported on nitrogen doped graphene (NG) [329]. The MB in the solution is decomposed after 180 min in presence of the NiFe2 O4 –NG heterojunction. The reduced NG sheets could lead to the favourable and efficient separation of photogenerated carriers




31

Fig. 44. (A) TEM images of the CdFe2 O4 /GR composite. (B) The effect of electrolytes on MB(0.01 mg L−1 ) degradation by CdFe2 O4 /GR. (The concentration of NO3− , SO4 2− , CO3 2− , HPO4 2− /H2 PO4 − are 0.01 mg L−1 ,respectively.) (C) Schematic of the photocatalytic degradation of MB on CdFe2 O4 /GR. [376].

(hole-electron) and the increased adsorption of MB dye on the catalyst surface through the strong ␲-␲ interaction between aromatic rings of methylene blue and graphene layer, thus achieving the significantly boosted photocatlytic activity. In addition, the combination of graphene/semiconductor/ magnetic component ternary multi-heterojunction photocatalysts could further improve the photoactivity of the graphene-based photocatalysts [256,348,380–384]. For instance, a novel magnetic RGO/ZnO/ZnFe2 O4 multi-heterojunction was prepared by the hydrothermal method [383]. The optimal RGO/ZnO/ZnFe2 O4 -3 multi-heterojunction (the weight ratio of graphene to ZnO/ZnFe2 O4 was 3%) exhibited the best photocatalytic efficiency of 98.64% after 60 min under visible light irradiation for decolorization of MB. In the RGO/ZnO/ZnFe2 O4 multi-heterojunction, the RGO plays a role in adsorbing MB by the ␲–␲ stacking. The RGO with lower Fermi level can preferentially accept the photogenerated electrons, which can reduce the recombination of the photogenerated electrons and holes in ZnO/ZnFe2 O4 . In another report, Mo et. al reported a facile route for the growth of TNTs and Fe3 O4 nanoparticles on graphene oxide sheets via one step hydrothermal method in alkaline condition [384]. The degradation rate of MB over TNTs/Fe3 O4 /GN multi-heterojunction could reach 99.8% in 20 min. The enhancement of the photocatalytic activity of TNTs/Fe3 O4 /GN) multi-heterojunction could be ascribed to the favorable charge transfer kinetics of graphene structure and higher photoactivity of TNTs. Recently, a magnetically separable Fe3 O4 @CuO-RGO coreshell multi-heterojunction were synthesized for the first time by low temperature hydrothermal method [380]. This novel method enables simultaneously the decoration of Fe3 O4 @CuO spheres on both sides of graphene sheets as well as the reduction of graphene oxide (as shown in Fig. 45A–B). The photodegradation efficiency increased continuously when the content of graphene was raised from 20 to 40 wt% and reached its peak (98.7%), showing that the effective adsorption of the multi-heterojunction for MB played a decisive role in improving the photocatalytic activity (in Figs. 45C). The existence of RGO impeded the aggregation

of Fe3 O4 @CuO particles, which afforded more active sites for the photocatalysis. Meanwhile, the superior electronic conductivity of graphene facilitated the efficient separation of photogenerated electron-hole pairs, thus achieving the improved photoactivity of Fe3 O4 @CuO-RGO (in Fig. 45D). In future, various nanostrcuted magneitc graphene-based heterojunctions should be exploited, which provide a promisng approach to degrade dyestuff wastewater without introducing foreign pollutants. 4.2. Photocatalytic water splitting Since the pioneering works about the thermodynamically uphill photocatalytic water splitting reported by Honda and Fujishima in 1972 [1], photocatalytic water splitting over various heterogeneous semicondcutors has become a promising and challenging technique to achieve the sustainable H2 energy production and solar energy utilization [5,26–28,35,441–444], which has been identified to be the Holy Grail in the modern chemistry [445]. Surprisingly, since the graphene/TiO2 heterojunctions were first applied in the hydrogen evolution from water photocatalytic splitting in 2009 [446], the graphene-based heterojunction photocatalysts have been extensively utilized in the half-reaction water splitting (for H2 and O2 ) and overall water splitting systems to significantly improve their photocatalytic efficiency, due to the multi-functional favorable roles of graphene in accelerating the charge dynamics, surface reaction kinetics, and optimizing the thermodynamic properties of the semiconductors [2,7,10]. Photocatalytic H2 production of graphene-based heterojuntions were summarized in Table 3. As observed in Table 3, in most cases, highly efficient H2 evolution could be achieved by coupling the graphene/cocatalyst heterojunctions with the different semiconductros, which will be discussed in this section. In general, the hybrids of graphene/noble metals have been widely employed as cocatalysts to boost the photocatalytic activity towards H2 evolution. For example, Li et al. reported that the CdS/RGO Schottky heterojunctions was prepared by utilizing a




Fig. 45. (A-B) TEM images of the Fe3 O4 @CuO-RGO40 multi-heterojunction, (C) Comparison of photocatalytic H2 -production activity of Fe3 O4 @CuO-RGO20 , Fe3 O4 @CuO-RGO30 , Fe3 O4 @CuO-RGO40 and Fe3 O4 @CuO-RGO50 under simulated solar irradiation. (D) Schematic illustration for the charge transfer and separation in the Fe3 O4 @CuO-RGO40 multi-heterojunction [380].

Fig. 46. (A)TEM and (B) HRTEM images of sample GC1.0, with the inset of (B) showing the SAED pattern of graphene sheet decorated with CdS clusters. (C) Visible-light photoactivity of samples GCX(X: the weight ratios of RGO to CdS). Reaction conditions: 20 mg of photocatalysts, 80 mL of mixed solution (8 mL of lactic acid and 72 mL of water), 0.5 wt% Pt as a cocatalyst, a 350 W xenon arc lamp with a UV-cutoff filter (␭≥420 nm) as the light source; (D) Schematic mechanism for the charge separation and transfer in the CdS/Pt/graphene multi-heterojunctions [410].



Synthesis method

Co-catalyst

In plane heterojunctions Multi-heterojunction

CdS/g-C3 N4 /1 wt%RGO ZnS–CdS/GO CdS@TaON/1 wt%GO ZnS/0.25 wt% graphene/MoS2

Multi-heterojunction Multi-heterojunction Multi-heterojunction Multi-heterojunction

thermal treatment photoreduction method calcining light irradiation-assisted precipitation wet-chemical light irradiation-ssisted Ultrasonicated and hydrothermal hydrothermal

0.1 wt% GO-Au–TiO2 1.5 wt% RGO/CdS/MoS2 5 wt%GO−CdS Anatase−2 wt% graphene−Rutile


hydrothermal photoreduction method precipitation Surface assembling

Pt

EY-Graphene


hydrothermal

MoS2

0.0225% wt Graphene/TiO2 Cu2 O-Pd-Graphene Cu2 ZnSnS4 /MoS2 - 1 wt%RGO GO CuO– polythiopheneb La2 Ti2 O7 -Au-rGO CdS QDs-ZnIn2 S4 /1 wt%RGO


hydrothermal process

WS2 -4.2 wt%GO-CdS ZnIn2 S4 microspheres ZnIn2 S4 microspheres ZnIn2 S4 nanosheets ZnS-CdS CdS QDs/marigold-like ZnIn2 S4 microspheres CdS QDs(3 wt%)-ZnIn2 S4 CdS QDs-ZnIn2 S4 CdS ZnS CdS CdS CdS CdS CdS −3 wt%RGO

Multi-heterojunction Multi-heterojunction Multi-heterojunction Multi-heterojunction Multi-heterojunction Multi-heterojunction


Pt 3 wt% Pt

0.5 wt.% Pt 2 wt.% Pt 0.4 wt% Pt

Mass [g]/Solution/Volume [mL]

Nd–YAG laser

30 v/v% of methano

6

300 W xenon light source 300 W Xe lamp

10% TEOA 0.1 M Na2 S + 0.1 M Na2 SO3

18.15 5100

0.3%

300 W xenon lamp ␭ ≥ 420 nm 300 W Xe lamp

10% TEOA 0.1 M Na2 S + 0.1 M Na2 SO3 0.1 M Na2 S + 0.04 M Na2 SO3 0.005 M Na2 S + 0.005 M Na2 SO3 methanol 10% lactic acid 0.35 M Na2 S + 0.25 M Na2 SO3 20% methanol

4800 7800 3165 2258

11.1%

296 2000 6280 3428

4.1% 9.8% 4.8%

[232] (2014) [392] (2014) [393] (2012) [394] (2014)

15% (v/v) triethanolamine-water

4200

24%

[395] (2012)

9.7%(365 nm)

[364] (2012) [396] (2014) [397] (2017) [398] (2016)

300 W Xe lamp

solvothermal aqueous chemicalb

␭ > 420 nm LEDs 350 W Xe lamp 300 W Xe-lamp, ␭ ≥ 420 nm 300 W xenon arc lampwas 300-W Xe lamp

150 W Xe lamp 250 W halogen lamp

0.35 M Na2 S + 0.25 M Na2 SO3 methyl viologen

123.6 103 2560

QE(%) at 420 nm

31%

12.96%

hydrothermal-precipitationhydrothermal solvothermal hydrothermal hydrothermal hydrothermal light irradiation-assisted Schottky junction

0.4 wt% Pt

300 W Xe-lamp, ␭ ≥ 420 nm

0.1 M Na2 S + 0.04 M Na2 SO3

253 27000

Pt

500 W Xeno arc lamp ␭ ≥ 420 nm ≥420 nm ≥420 nm >420 nm ≥420 nm ≥420 nm

0.35 M Na2 S + 0.25 M Na2 SO4 lactic acid Na2 S + Na2 SO3 Na2 S + Na2 SO3 Na2 S + Na2 SO3 Na2 S + Na2 SO3

1842 2646 282 1632 16800 27000

Multi-heterojunction Multi-heterojunction Multi-heterojunction Multi-heterojunction Multi-heterojunction Multi-heterojunction Multi-heterojunction Multi-heterojunction Multi-heterojunction

solvothermal solvothermal

Pt Pt MoS2 MoS2 MoS2 cobalt polyoxotungstosili, MoS2 Ni(OH)2 RGO + MoS2 MoS2

Na2 S + Na2 SO3 Na2 S + Na2 SO3 lactic acid Na2 S + Na2 SO3 lactic acid lactic acid Na2 S +Na2 SO3 lactic acid lactic acid

27000 27000 6857 2258 1980 17000 12426 5010

54.4%

CdS CdS/5 wt%GO CdS −1.0wt%GO 0.25 wt%RGO-Znx Cd1−x S Nitrogen Doped Sr2 Ta2 O7 /Graphene

Multi-heterojunction Schottky junction Schottky junction Schottky junction Schottky junction

2320 1750 56000 1824

Pt

20 vol% lactic acid 0.5 M Na2 S + 0.5 M Na2 SO3 lactic acid 0.35 M Na2 S + 0.25 M Na2 SO3 20% methanol

65.8% 3.99% 22.5% 23.4% 6.45%

S, N Co-Doped Graphene Quantum Dot/ TiO2 1.0 wt GO/TiO2 SrTiO3 -0.8w%RGO

Schottky junction

solvothermal dispersion polymerization hydrothermal hydrothermal calcining stoichiometric hydrothermal

≥420 nm ≥420 nm UV-vis lamp 300 W Xe lamp ≥420 nm > 400 nm > 420 nm 350 W xenon arc lamp 300W Xe lamp 300 W Xe lamp ≥ 400 nm 300 W Xenon lamp 350 W xenon arc lamp ␭ ≥ 420 nm AM1.5 300 W Xe lamp

Pt

300 W Xe lamp

methanol

114

Schottky junction Schottky junction

microwave-hydrothermal hydrothermal

25% methanol 20% methanol

736 352

17 wt%RGO/TiO2 CdS–1 wt%graphene 1.0 wt%RGO/ZnIn2 S4 AgInZnS ZnS Zn0.8 Cd0.2 S

Schottky junction Schottky junction Schottky junction Schottky junction Schottky junction Schottky junction

hydrothermal hydrothermal solvothermal

350 W Xe arc lamp 300 W xenon lamp 200 W Xe arc lamp 200 W Xenon arc lamp 300 W Xe lamp ≥420 ≥420 nm AM1.5

20% methanol 0.1 M Na2 S+ 0.05 M Na2 SO3 lactic acid Na2 S + Na2 SO3 Na2 S + Na2 SO3 Na2 S +Na2 SO3

720 700 8230 1871 7.42 1824

solvothermal solvothermal solvothermal solvothermal hydrothermal hydrothermal and solvothermal

hydrothermal hydrothermal

RGO + MoS2 0.75 wt% Pt

Pt

RGO RGO RGO

Ref. (year) [385] (2014) [386] (2017) [387] (2017) [388] (2014) [236] (2017) [389] (2015) [390] (2012) [391] (2014)

[399] (2015) [400] (2013) 21.2%

56(420 nm)

56(420 nm)

[223] (2016) [129] (2014) [401] (2013) [402] (2013) [389] (2015) [403] (2013) [403] (2013) [403] (2013) [404] (2014) [391] (2014) [392] (2014) [405] (2015) [224] (2014) [406] (2015) [407] (2016) [408] (2014) [409] (2016) [410] (2011) [63] (2012) [411] (2011) [412] (2017)

3.1%

23.4(420 nm)

[413] (2011) [414] (2016) [415] (2011) [416] (2012) [417] (2014) [418] (2015) [178] (2015) [63] (2012)

ARTICLE IN PRESS

N-Doped Graphene Au-nanorod/RGO/Pt nanoframe B N co-doped graphene. CdS-ZnO/2 wt%RGO

RH2 [umol h−1 ]

Light source

G Model


Photocatalyst



Table 3 Photocatalytic H2 -production activities of graphene-based heterojuntions.

QE(%) at 420 nm

Ref. (year)

Synthesis method

Co-catalyst

Light source

Mass [g]/Solution/Volume [mL]

Cu0.02 In0.3 ZnS1.47 Zn0.5 Cd0.5 S Polymer supported CdS Zn0.5 Cd0.5 S ultrathin nanorods CdS mixed nanoparticles/nanorods Ni-doped ZnS CdS CdS nanowires CdS CdS

Schottky junction Schottky junction Schottky junction Schottky junction Schottky junction Schottky junction Schottky junction Schottky junction

solvothermal hydrothermal solvothermal oleylamine–DMSO mediated solvothermal solvothermal solvothermal solvothermal

RGO RGO RGO RGO RGO RGO RGO RGO+ Pt

800 W Xe, ≥420 nm >400 nm >400 nm ≥420 nm >400 nm 300 W Hg lamp ≥420 nm > 00 nm

Na2 S +Na2 SO3 Na2 S +Na2 SO3 Na2 S +Na2 SO3 Na2 S +Na2 SO3 lactic acid Na2 S +Na2 SO3 Na2 S +Na2 SO3 Na2 S +Na2 SO3

3800 21200 1750 770 23200 8683 4200 3984


solvothermal solvothermal

RGO+ Pt GO + Pt

(NH4 )2 SO3 methanol

29400 5500

CdS CdS Multiarmed CdS Nanorods ZnIn2 S4 Zn0.5 Cd0.5 S TiO2 CdS

Schottky junction Schottky junction Schottky junction Schottky junction Schottky junction Schottky junction Schottky junction Schottky junctions Schottky junctions Schottky junctions Schottky junctions Schottky junctions Schottky junctions Schottky junctions Schottky junctions Schottky junctions Schottky junctions

solvothermal photoreduction method hydrothermal

Graphene + MoS2 RGO + MoS2 RGO + MoS2 RGO + MoS2 NiS +RGO N-doped graphene graphene / / / / / 0.5 wt% Pt 2.1 wt% Pt 0.67 wt% MoS2 / /

≥420 nm 400 W Hg lamp ≥ 420 nm ≥ 400 nm ≥ 420 nm ≥ 420 nm AM-1.5 50 W GY-10 xenon lamp A 300 W Xenon lamp 300 W Xe-lamp, ␭ ≥ 420 nm 300 W Xe-lamp, ␭ ≥ 420 nm 200 W Xe-lamp, ␭ ≥ 420 nm 300 W Xe-lamp, ␭ ≥ 420 nm 400 W high pressure Hg lamp 350 W Xe-lamp, ␭ ≥ 420 nm 300 W Xe-lamp, ␭ ≥ 420 nm 300 W Xe-lamp, ␭ ≥ 420 nm 500 W Phoenix tungsten halogen lamp 500 W Phoenix tungsten halogen lamp

lactic acid lactic acid lactic acid lactic acid Na2 S +Na2 SO3 10 vol% TEOA 0.5 M Na2 S/Na2 SO3 0.35 M Na2 S + 0.25 M Na2 SO3 0.35 M Na2 S + 0.25 M Na2 SO3 0.1 M Na2 S + 0.05 M Na2 SO3 0.1 M Na2 S + 0.1 M Na2 SO3 10 v% methanol 10 v% lactic acid 10 v% lactic acid 20 v% lactic acid 0.35 M Na2 S + 0.25 M Na2 SO3 0.35 M Na2 S + 0.25 M Na2 SO3

9000 1913 12426 1620 7510 266 1750 3140 4202 700 1050 2190 56000 29861 9000 1750 3755

28.1(420 nm)

2 wt% MoS2

300 W Xe lamp

Na2 S + Na2 SO3

9000

28.1%

[434] (2014)

0.35 M Na2 S + 0.25 M Na2 SO3

7514

31.1%

[429] (2014)

Ni(OH)2 / 2 wt% MoS2

visible light irradiation (␭ > 400 nm) 300 W Xe-lamp, 300 W Xe-lamp, ␭ ≥ 420 nm 150 W Xenon arc lamp 400 W high pressure Hg lamp 800 W Xe–Hg lamp 300 W Xe lamp380 < ␭ < 750 A 300 W Xe-lamp metal halide lamp, ␭ ≥ 380 nm 500 W UV–vis lamp

20 v% methanol lactic acid 10 vol% triethanolamine 0.2 M Na2 S + 0.1 M Na2 SO3 0.35 M Na2 S + 0.25 M Na2 SO3 0.35 M Na2 S + 0.25 M Na2 SO3 10% methanol 0.1 M Na2 S + 1.2 M Na2 SO3 methanol 0.35 M Na2 S + 0.25 M Na2 SO3 0.05 M Na2 S + 0.07 M Na2 SO3 10 v% lactic acid

5333.3 1620 557 770 1632 1060 5491 3800 7947 4731 167 6857

0.4 wt% Pt

300 W Xe-lamp, ␭ ≥ 420 nm

0.1 M Na2 S + 0.04 M Na2 SO3

3165

150 W Xe lamp

0.35 M Na2 S + 0.25 M Na2 SO3

800

300 W Xe lamp ␭ ≥ 420 nm 300 W xenon lamp ␭ ≥ 420 nm 450 W high-pressure Hg lamp 250 W high pressure mercury

10% methanol 10% methanol 0.05 M Na2 S + 0.07 M Na2 SO3

675 6000 165

10% (v/v) ethanol

100

[439] (2017) [92] (2015)

Pt

300 W Xe lamp

1.25 mol/L (NH4 )2 SO3

29400

[424] (2015)

1.5 wt% Pt

350 W Xe-lamp, ␭ ≥ 400 nm

methanol

451

2.6%

[97] (2011)

350 W Xe arc lamp

methanol

736

3.1%

[98] (2011)

Schottky junctions

NiS/Zn0.5 Cd0.5 S/ 0.25 wt%RGO RGO ZnIn2 S4 /0.5 wt%RGO/MoS2 g-C3 N4 –5 wt.%GO ZCS/2 wt%RGO RGO–ZnIn2 S4 Zn0.5 Cd0.5 S/0.5 wt%RGO GO CdS CuInZnS/2 wt%RGO Bi2 MoO6 /5 wt%RGO CdS/rGO CdS/2 wt%sulfonated graphene CdS-MoS2 /0.4 wt% graphene CdS-TaON/1 wt%RGO

Schottky junctions

hydrothermal

Schottky junctions Schottky junctions Schottky junctions Schottky junctions Schottky junctions Schottky junctions Schottky junctions Schottky junctions Schottky junctions Schottky junctions Type-II heterojunction Type-II heterojunction

hydrothermal solvothermal acidic cutting and hydrothermal ultrasonication hydrothermal hydrothermal Formic acid reduction process solvothermal calcining Chemical precipitation precipitation hydrothermalmixing precipitationhydrothermal hydrothermalprecipitationcolloidal blending photoassisted hydrothermal

CdS/10 wt%RGO CdS/1 wt%RGO CdS/2 wt%N-Doped graphene CdS/333 wt%RGO CdS/1 wt%RGO CdS/0.7 wt%Cysteine modified RGO CdS-MoS2 /1.33 wt% graphene CdS-Al2 O3 /1 wt%GO CdS-ZnO/1 wt%GO


CdS/Nb2 O5 /2 wt.%N-dope-graphene


Nanoplate- RGO 12 wt%RGO-Co3 (PO4 )2 CdS-2 wt%GO

Type-II heterojunction Type-II heterojunction Type-II heterojunction

GO quantum dots/TiO2 g-C3 N4 -RGO

Type-II heterojunction Type-II heterojunction

CdS/5 wt%RGO

Van der Waals heterostructure Van der Waals heterostructure Van der Waals heterostructure Z-scheme heterojunction Z-scheme heterojunction Z-scheme heterojunction

g-C3 N4 /1.0wt%GO 1.0wt%GO/TiO2 ZnO–CdS/2 wt%RGO ZnO–CdS/2 wt%RGO CdS/g-C3 N4 /1 wt%RGO

hydrothermal state-of-the-art hybrid density functiona solvothermal Combined impregnation-chemical reduction microwave-hydrothermal

3 mol% NiS.

solar simulator, AM1.5, 100 mW cm−2 , 400W mercury lamp

0.425 wt% MoS 2

0.5 at% Pt

65.8(420 nm) 10.4(420 nm)

[419] (2013) [420] (2014) [409] (2016) [421] (2015) [408] (2014) [422] (2015) [64] (2011) [423] (2015) [424] (2015) [425] (2012)

54.4(420 nm) 0.4(420 nm) 31.1(420 nm) 9.6% 3.99%

22.5(420 nm 50.7(420 nm)

19.8%

[426] (2014) [427] (2016) [406] (2015) [428] (2015) [429] (2014) [144] (2014) [430] (2016) [393] (2012) [431] (2008) [278] (2012) [156] (2011) [432] (2012) [410] (2011) [433] (2013) [434] (2014) [237] (2012) [237] (2012)

[43] (2010) [428] (2015) [101] (2017) [421] (2015) [402] (2013) [420] (2014) [432] (2012) [419] (2013) [435] (2017) [224] (2014) [436] (2012) [437] (2014) [390] (2012) [151] (2017)

5.9,

[170] (2015) [438] (2016) [436] (2012)

redox process

300 W Xe lamp

0.1 M Na2 S + 0.1 M Na2 SO3

5900

[440] (2014)

hydrothermal-

300 W Xe-lamp,

0.1 M Na2 S + 0.1 M Na2 SO3

5100

[388] (2014)

hydrothermal

350 W Xe lamp

lactic acid

473.4

24.8%

[194] (2017)


MoS2 -RGO-CdS13 wt%

solvothermal solvothermal dispersion polymerization precipitation solvothermal precipitation-hydrothermal precipitation-calcination two-phase solvothermal hot injection-sonication hydrothermal hydrothermal-physical mixing hydrothermalphysical mixing solution-chemistry

19.8(420 nm) 3.99(420 nm)

ARTICLE IN PRESS


G Model

RH2 [umol h−1 ]

Photocatalyst


34


Table 3 (Continued)



35

Fig. 47. (A) TEM images of the NiS/Zn0.5 Cd0.5 S/RGO multicomponent heterojunctions, (B) Comparison of photocatalytic H2 -production activity of the Zn0.5 Cd0.5 S, NiS/Zn0.5 Cd0.5 S, Zn0.5 Cd0.5 S/RGO, and NiS/Zn0.5 Cd0.5 S/RGO samples under simulated solar irradiation. (C) Schematic illustration for the charge transfer and separation in the NiS/Zn0.5 Cd0.5 S/RGO multicomponent heterojunctions [447].

Fig. 48. a) Photocatalytic H2 production of the TiO2 /MoS2 /graphene multicomponent heterojunctions with different MoS2 (M) and RGO (G) contents as cocatalyst under UV irradiation. b) The photocatalytic H2 -production mechanism and charge separation and transfer in the TiO2 /MoS2 /graphene multicomponent heterojunctions [364].

one-step solvothermal method, followed with a in-situ photodeposition of Pt NPs [410]. The resulting ternary CdS/Pt/graphene multi-heterojunctions containing 0.5 wt% Pt and 1.0 wt% RGO dual co-catalysts (as shown in Fig. 46A and B) could achieve a H2 production AQE of 22.5% at 420 nm, which was about ∼45.6and 4.87-fold higher than those of bare CdS nanoparticles and Pt-CdS nanoclusters (Fig. 46C), owing to the perfect synergetic effects between graphene and Pt co-catalysts in promoting the charge separation and transfer (as shown in Fig. 46D) [410]. In another report, Fang and his coworkers synthesized CdS quantum dots/Pt/GO nanosheets multi-heterojunctions. They demonstrated that the CdS quantum dots with suitable sizes could lead to a significantly increased photoactivity compared to their corresponding bulk counterparts [433]. The highest hydrogen evolution rate of 2.15 mmol h−1 (␭ ≥ 420 nm) with a quantum efficiency of 50.7% was accomplished over the water soluble CdS quantum dots/dendritic Pt nanocrystals/GO nanosheets multi-heterojunctions. In this system, the electrons will readily transfer from GO to Pt cocatalysts due to the higher work function of Pt, thus achieving the finally enhanced hydrogen evolution. Howerver, so far, the promising single-atom noble metal decorated graphene as a promising H2 evolution cocatalyst has been seldomly reported, which should be paid more attention, due to the reduced amounts of noble metals and improved activity. Furthermore, to replace the noble-metal cocatalysts, the combination of graphene and other earth-abundant cocatalysts has been demonstrated to be a promising strategy to further improve the photoactivity of the graphene-based heterojunctions due to the excellent synergy of dual robust cocatalysts [2,222]. Especially, NiS was further decorated into the binary Znx Cd1-x S/RGO to fabricate the ternary multi-heterojunctions(Fig. 47A) [447]. As a

result, the ternary multi-heterojunctions showed a greatly boosted solar photocatalytic H2 -production activity owing to the positive synergism of NiS and RGO dual cocatalysts over Znx Cd1-x S (see Fig. 47B). It is apparent that the construction of p-n junction region between NiS and CdS could efficiently promote the transport of photogenerated holes from CdS to NiS, thus inhibiting the detrimental photocorrosion of CdS (see Fig. 47C). This result suggests that constructing 0D-2D multi-heterojunctions is also a promising strategy to strengthen the photoactivity of metal sulfides. Similarly, the ternary MoS2 /CdS/RGO multiheterojunction was also rationally designed and constructed by both Jia et al. [404] and Chang et al. [434] The optimized photocatalytic H2 production rate of CdS-MoS2 /RGO multi-heterojunction was over 70 times higher than that of bare CdS. Similarly, the TiO2 /MoS2 /graphene multi-heterojunctions using 2D graphene and layered MoS2 as dual co-catalysts were also fabricated by a two-step hydrothermal process [364]. The results showed that the optimized TiO2 /MoS2 /graphene multi-heterojunction exhibited the highest H2 production rate of 165.3 ␮ mol h−1 with a AQE of 9.7% at 365 nm (Fig. 48a). The significantly improved activity was ascribed to the synergetic effects of graphene and earthabundant MoS2 co-catalysts, as shown in Fig. 48b. In addition, it should be noted that an appropriate amount of graphene (≤1 wt%) plays an important role in accomplishing the highest photocatalytic H2 -production activity due to the effectively avoiding the lightshielding and active site-blocking effects of the excess graphene in composites. In future, it is expected that more and more earthabundant cocatalsys/graphene could be exploited and applied in constructing the multicomponent heterojunctions for the highly efficient photocatalytic H2 evolution.




4.3. Photocatalytic CO2 reduction Recently, photoreduction of harmful CO2 greenhouse gases into valuable solar fuel production has been demonstrated to be a low-cost and environmentally friendly strategy to simultaneously solve energy and global warming problems, which has drawn significant attention in the past several decades [448,449]. To significantly improve the photocatalytic CO2 conversion efficiency over various semiconductors orginated form fast charge carrier recombination and low light utilization, diverse engineering modification strategies have been exploited [192,448]. Among them, the coupling of multi-functional graphene and a given semiconductor has been considered as one of the most promising strategies to boost the activity of semiconductor for photoconversion of CO2 to valuable hydrocarbons [2,7,9,70,450–454]. On the one hand, the noble metal-free RGO could act as both a cocatalyst and a promising photocatalyst[455,456] to improve the separation and transfer of photo-generated electrons and achieve the photocatalytic CO2 reduction to methanol under visible light, respectively. On the other hand, graphene could also serve as an adsorbent to enhance CO2 adsorption and capture capacity [457,458], which could be further improved by the proper doping and defects [459–462]. Some typical RGO/semiconductor heterojunctions in gas-solid and suspension systems for CO2 photoreduction were summarized in Table 4. As clearly observed in Table 4, the Schottky heterojunction and multi-heterojunction are the two most widely used types for graphene-based photocatalytic CO2 reduction, which will be discussed in this section. Various kinds of advanced graphene-based Schottky heterojunctions could be fabricated through the combination of unique semiconductor nanostructures and 2D RGO, which are beneficial for further boosting the efficiency of CO2 photoreduction, due to the promoted charge separation, increased photostablity of semiconductors and enhanced CO2 adsorption [2,4,6,7,9,111,448,449,489]. For example, Kuang’s group successfully constructed CsPbBr3 halide perovskite QDs/RGO 0D-2D Schottky heterojunctions (Fig. 49A)for photocatalytic rduction of gaseous CO2 into solar fuels[466]. The resulting CsPbBr3 QD/graphene oxide (CsPbBr3 QD/GO) 0D-2D Schottky heterojunctions showed a 25.5% activity enhancement for the photocatalytic CO2 reduction (Fig. 49B) and a better external quantum efficiency (EQE) than the individual CsPbBr3 QDs, despite the similar absorption behavior of their UV−vis absorption spectra (Fig. 49C). The improved electron extraction and transport in the CsPbBr3 QD/GO 0D-2D Schottky heterojunctions were verified by steady-state PL spectra and PL decay spectra (Fig. 49D and E), which is mainly responsible for the CO2 -photoreduction activity enhancement(Fig. 49F) [466]. However, the activity enhancement in this study is relatively small, which should be further improved by increasing the conductivity of graphene or enhancing the quality of Schottky heterojunction between RGO and CsPbBr3 QD. Interestingly, Yu and his coworker reported that the RGO-CdS nanorod 2D-1D Schottky heterojunctions (Fig. 50A) showed a superior high CH4 -production rate of 2.51 ␮mol g−1 h−1 at an RGO content of 0.5 wt% (as shown in Fig. 50B), which was 10 times higher than that of the pure CdS nanorods, and even better than that of an optimized Pt-CdS nanorod photocatalyst under the same reaction conditions, due to the synergistically enhanced charge-separation performances of unqiue 1D CdS nanorods and high-quality 1D-2D Schottky heterojunctions (Fig. 50C) [70]. To further increase the contact area in the Schottky heterojunctions, the 2D/2D Schottky heterojunctions, such as g-C3 N4 /RGO [103,106,468] and TiO2 nanosheeets/RGO [451], have also been extensively applied in the CO2 photoreduction. Ong et al. recently constructed the rGO/protonated g-C3 N4 (pCN, protonated by HCl) 2D/2D Schottky heterojunctions by an electrostatic selfassembly strategy (Fig. 51A) [106]. The optimized 15 wt% rGO/pCN

(15rGO/pCN) 2D/2D Schottky heterojunctions could exhibit the 5.4- and 1.7-times enhancements in the CH4 yield than those over pCN and 15rGO/CN samples, respectively (Fig. 51B), suggesting that the intimate and sufficient 2D/2D interfacial contact in rGO/pCN Schottky heterojunctions plays the fundamental roles in boosting the separation and transfer of electron–hole pairs and improving the activity toward photoreduction of gaseous CO2 to CH4 . The CO2 -photoreduction activities of these 2D/2D Schottky heterojunctions could be further improved by fabricating the 3D hierarchical photocatalysts [6]. In addition, the optimized Cu2 O/RGO 3D-2D Schottky heterojunction prepared by a onestep microwave-assisted chemical method, could also show much higher activity for the photoconversion of CO2 into CO, compared to the single Cu2 O microspheres, due to the promoted electron–hole separation and transfer, and enhanced the photostablity of Cu2 O by RGO [453]. In future, the hybridization of these above Schottky heterojunctions with different coupling interfaces should be a promising direction to devlop highly active graphene-based Schottky heterojunctions for photocatalytic CO2 reduction. Apart from chaning the interfacial coupling structures in the graphene-based Schottky heterojunctions, it is also promising to increase the electron conductivity of RGO through decreasing defect densities or introducing the highly conductive additives, which could further boost the separation and transfer of photogenerated electron–hole pairs and enhance the photoactivity for CO2 reduction [136,280]. However, thus far, the commonly used RGO in the graphene-based Schottky heterojunctions exhibites much lower electronic conductivity, due to the presence of domain boundaries, defects, and residual oxygen-containing groups, significantly limiting the photoactivity enhancement of Schottky heterojunctions. Typically, two methods could be used to enhance the electronic conductivity of graphene, namely, using solventexfoliated graphene (SEG) and introducting highly conductive components. A simple method is to replace RGO in the composites by solvent-exfoliated graphene (SEG) with improved electrical conductivity. For example, Hersam and coworkers demonstrated that solvent-exfoliated graphene (SEG) exhibites much higher electrical conductivity than the RGO, due to the less graphene defect densities of the former [132]. The resulting novel SEG-TiO2 Schottky heterojunctions are shown to possess much higher CO2 photoreduction activity than those RGO/TiO2 Schottky heterojunctions (as shown in Fig. 52A). The superior electrical mobility of SEG with lower density of defects is attributed to more effective transfer of photoexcited electrons to the adsorbed CO2 reactants via longer mean free paths (depicted as the yellow pathway in Fig. 52B), thus facilitating the enhancement of CO2 photoreduction. Another strategy to boost the electrical conductivity of RGO is the introduction of highly conductive components, such as CNTs or metallic materials, onto the surface of RGO. For instance, the 1D metallic Ag nanowires was employed for the first time to further enhance the electrical conductivity and mechanical flexibility of RGO [490]. The interesting Ag NWs–RGO–CdS NWs (ACG) multi-heterojunctions could be facilely constructed via a simple electrostatic self-assembly method followed by a hydrothermal reduction process (Fig. 53A–B) [490]. The photocatalytic activities for CO and CH4 production over the multi-heterojunctions are 1.17 and 2.77 times higher than those of RGO CdS Schottky heterojunctions without Ag NWs (Fig. 53C), respectively, suggesting the key roles of doped Ag NWs in increasing electronic conductivity capability of RGO and prolonging lifetime of photo-generated charge carriers. The present work provides a promising strategy to construct highly conductive and efficient CO2 -photoreduction co-catalysts through coupling RGO and other conductive metallic and metal-free nanomaterials. In this regard, the coupling of RGO with other highly conductive nanocarbons, such as CNTs and carbon QDs, is also highy expected to be utilized in the sustainable artificial photosynthesis [258,491,492].



Light source

Mass [g]/systems/Volume [mL]

Selective products (activity) [␮Mh−1 g−1 ]

Ref. (year)

thermal reduction

500 W tungstenhalogen lamp 150 medium-pressure mercury vapor lamp 150 medium-pressure mercury vapor lamp 300 W Xe arc lamp

CO2 and H2 O vapor/f 9.8 mL

CH4 (2.88)

[463] (2015)

0.25/He and CO2 vapor/

MeOH(47.0)

[464] (2016)

0.25/He and CO2 vapor/

EtOH(144.7)

[464] (2016)

CO2 and H2 O vapor

CH4 (60.4)

[465] (2016)

100 W Xe lamp

4 mg/CO2 /40 mL

CH4 (23.7)

[466] (2017)

ight power density 100 mW cm−2 300W Xenon arc lamp halogen lamp (300 W)

CO2 vapor

CH4 (14.94)

[467] (2016)

/CO2 and H2 O vapor/ 100 mg/CO2 TEOA and H2 O vapor/ CO2 and H2 O vapor

CH4 (8) CH3 OH(3)

[452] (2013) [455] (2014)

CH4 (1.393)

[468] (2015)

CO2 and H2 O vapor

CH4 (0.28)

[469] (2015)

CO2 and H2 O vapor

CH4 (5.87)

[103] (2015)

15 W energy-saving daylight lamp

0.1 g/CO2 and H2 O vapor

CH4 (1.393)

[106] (2015)

20 W white cold LED flood light

0.1 g/CO2 and H2 O vapor/100 mL CO2 and H2 O vapor

CH3 OH(51.17)

[470] (2016) [471] (2016)

CO2 and H2 O vapor CO2 and H2 O vapor

H2 (2031) CH3 CH2 OH(545) CO(1.86) CH4 (0.3)

1 g/CO2 and H2 O vapor

CH4 (0.37)

[474] (2014)

0.1 g/CO2 and H2 O vapor/200 mL CO2 and H2 O vapor

CH4 (2.51)

[70] (2014)

CH4 (0.135)

[475] (2013)

0.01 g/CO2 and H2 O vapor/ CO2 and H2 O vapor/15.4 mL 0.01 g/CO2 and H2 O vapor

CH4 (25.31) CH4 (5.67) CH4 (1.14)

[476] (2016) [477] (2016) [451] (2012)

solvothermal solvothermal sequential electrochemical deposition solvothermal solvothermal

TiO2 -Graphene Cu-Nanoparticle Decorated GO


hydrothermal

RGO/g-C3 N4

Schottky junction

Noble metal modified rGO/TiO2

Schottky junction

GO-g-C3 N4

Schottky junction

15 W energy-saving Daylight bulb 15 W energy-saving Daylight bulb 15 W energy-saving daylight bulb

RGO/p-C3 N4

Schottky junction

rGO CuO

Schottky junction

electrostatic self-assembly construction of 2D/2D hydrothermal, simple polyol process a facile one-pot impregnation–thermal reduction strategy a novel combined ultrasonic dispersion and electrostatic self-assembly strategy covalent grafting

Cu2 O/graphene

Schottky junction

pyrolysis

USHIO G8T5 lamps

GO-oxygen rich TiO2 oxygen rich TiO2 -GO


simple wet chemical simple wet impregnation

N-doped TiO2 /graphene

Schottky junction

rGO-CdS nanorod

Schottky junction

rGO-TiO2

Schottky junction

a facile one-pot impregnation one step microwave hydrothermal solvothermal

500 W Xe lamp 15 W energy-saving daylight bulbs 15 W energy-saving daylight bulbs 300 W Xe lamp

HNb3 O8 /Graphene rGO-TiO2 Ti0.91 O2 /graphene

Schottky junction Schottky junction Schottky junction

exfoliation hydrothermal hydrothermal

15W energy-saving daylight bulb 300 W xenon arc lamp 300 W xenon arc lamp

[472] (2017) [473] (2015)

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Gas-solid systems for CO2 photoreduction MultiGO/Pt–TiO2 heterojunction TiO2 /Cu2 O/graphene Multiheterojunction TiO2 /Cu2 O/graphene Multiheterojunction MultiCu2 O/TNA graphene heterojunction CsPbBr3 Perovskite Quantum Schottky junction Dot/GO Schottky junction Porphyrin- Graphene

Synthesis method

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Material



Table 4 Summary of the photocatalytic CO2 reduction over g-C3 N4 -based photocatalysts.

G Model


38


Synthesis method

Light source

Mass [g]/systems/Volume [mL]

Selective products (activity) [␮Mh−1 g−1 ]

Ref. (year)

rGO/Cu2 O

Schottky junction

150 W Xe lamp

0.5 g/CO2 and H2 O vapor

CO(46 ppmg−1 )

[453] (2014)

B-graphene/TiO2

Schottky junction

300 W Xe lamp

[478] (2014)

Schottky junction

CH4 (1.1)

[479] (2013)

N-doped graphene–Fe2 O3 HNb3 O8 /Graphene


0.07 g/CO2 and H2 O vapor/100 mL 0.1 g/CO2 and H2 O vapor/270 mL 0.5 g/CO2 and H2 O vapor 0.01 g/CO2 and H2 O vapor

CH4 (1.3)

Graphene-WO3

microwave-assisted hydrothermal vacuum activation and ultraphonic hydrothermal

CO(8) CO(52) CH4 (58)

[154] (2015) [480] (2016)

CdS/rGO/TiO2 core-shell Graphene/TiO2

Z-scheme heterojunction Surface heterojunction

CH4 (0.126)

[481] (2015)

CO(70.8)

[482] (2016)

HCOOH(198,96)

[483] (2016)

CH3 OH(0.720)

[484] (2014)

CH3 OH(507.3)

[485] (2016)

Graphene Quantum Dots Suspension systems for CO2 photoreduction Multitourmaline-TiO2 heterojunction –graphene MultiCeO2 / heterojunction N-doped graphene/Cu complex MultirGO@CuZnO@Fe3 O4 heterojunction MultiN- graphene/Cu complex heterojunction MultiGraphene-modified heterojunction NiOx -Ta2 O5 Schottky junction GO-Ru complex

300 W Xe lamp

simple wet chemical two-step exfoliation−restacking process hydrothermal, electrostatic self-assembly solvothermal

500 W Xe lamp 300 W Xe lamp

solvothermal

450 W Xenon lamp

sol–gel method

500 W high-pressure Xe lamp

impregnation

250 W Xe arc lamp

solvothermal

20 W white cold LED light

0.1 g/DMF and H2 O/100 mL

CH3 OH(2656)

[214] (2017)

one-pot impregnation

20 W white cold LED flood light

0.1 g/H2 O and DMF/100 mL

CH3 OH(66.67)

[486] (2015)

impregnation

400W metal halide lamp

0.02 g/H2 O and CO2

CH3 OH(416.7)

[487] (2013)

microwave technique and immobilized

Visible light

H2 O and DMF

CH3 OH(85.42)

[488] (2015)

300 W Xe lamp 300 W Xe lamp

CO2 and H2 O vapor/230 mL 0.01 g/CO2 and H2 O vapor/85 mL 0.5 mg/CO2 TEOA and H2 O vapor/ 0.05 g/50 mLNaHCO3 , 0.08 M and 0.08 M HCl 0.1 g/250 mLNaHCO3 , 0.1 M and 0.1 M Na2 SO3 /250 mL

ARTICLE IN PRESS

Material



Table 4 (Continued)



39

Fig. 49. HRTEM image of (A) the CsPbBr3 QDs and the CsPbBr3 QD/GO (inset). (B) Photocatalytic production yields of CO and CH4 after 12 h of illumination, (C) UV–vis absorption spectra and the external quantum efficiency plots, (D) Steady-state PL spectra with an excitation wavelength of 369.6 nm, (E) PL decay spectra after pulsed excitation at ␭ = 369.6 nm for the individual CsPbBr3 QDs and the CsPbBr3 QD/GO Schottky heterojunction. (F) Schematic diagram of CO2 photoreduction over the CsPbBr3 QD/GO Schottky heterojunction [466].

Fig. 50. (A)TEM images of the G0.5 sample, (B) photocatalytic CH4 −production rate of different samples under visible-light irradiation (Gx, where x is the weight percentage of RGO, 0, 0.1, 0.25, 0.5, 1.0 and 2.0%) (C)Schematic illustration of the charge transfer and separation in the RGO–CdS nanorod Schottky heterojunction under visible-light irradiation [70].

Additionaly, as observed in Table 4, graphene in the graphenebased Schottky heterojunction and multi-heterojunction could also tune the selectivities of photocatalysts toward different CO2 reduction products. Typically, it is believed that graphene can enrich the photo-generated electrons from the semicondcutors through the high quality Schottky heterojunctions between them, thus favoring the selective formation of multi-electron CO2 -reduction products, such as CH4 and CH3 OH. For example, Ong et al. constructed the N–TiO2 /RGO Schottky heterojunctions through depositing the nitrogen-doped TiO2 (N–TiO2 ) nanoparticles (10–17 nm, with exposed 35% (001) facets) on the graphene (GR) sheets (N–TiO2 -001/GR) (Fig. 54A–C) [474]. As a result, the N–TiO2 -001/GR Schottky heterojunctions with the narrowest band gap of 2.9 eV (3.23 eV for TiO2 -001) exhibit the greatest reate for selective production of CH4 (3.70 ␮mol·gcatalyst−1 ), which are approximately 11 times higher than that of TiO2 -001(Fig. 54D),

due to the N-doping and the formation of Ti O C bonds. It is believed that the N-doping, graphene loading and exposed (001) facets synergically lead to the high visible-light absorption, effective charge separation, and high catalytic activity, respectively, thus achieving the significant activity enhancement (Fig. 54E). Similarly, the new graphene oxide-doped-oxygen-rich TiO2 (GOOTiO2 ) Schottky heterojunctions with an optimum GO loading of 5 wt.% could achieve the highest total yield for selective production of CH4 (1.718 ␮mol·gcatalyst−1 after 6 h of reaction), which is about 14.0 times higher than that of commercial Degussa P25 [469]. More interestingly, the enriched electrons in RGO could selectively yield other CO2 -reduction products through the exceeding 8-eelctron reduction pathways. Impressively, the TiO2 graphene 2D sandwich-like Schottky heterojunctions could be facily fabricated by in situ hydrothermal method in a binary ethylenediamine(En)/H2 O solvent [452]. The results showed that




Fig. 51. (A) Schematic illustration for the synthesis process of rGO/pCN Schottky heterojunctions via a combined ultrasonic dispersion and electrostatic self-assembly strategy followed by a NaBH4 -reduction process.(B) Total CH4 yields over rGO,pure g-C3 N4 , pCN, 15rGO/pCN and 15rGO/CN photocatalysts after 10 h of illumination [106].

the synergistic effect of the surface-Ti3+ abundant TiO2 and GR favors the selective generation of C2 H6 , and the yield of the C2 H6 increases with increasing the content of incorporated GR. Importantly, it was also verified that the close contact interface through in situ growth also played an important role in

efficiently improving the photocatalytic activity toward selective conversion of CO2 to C2 H6 . By the contrary, some researcher also demontrated that the introduction of conductive rGO sheets could significantly decrease the local electron density due to the accelerated transfer and migaration of the electrons, thus leading to




41

Fig. 52. (A) CO2 photoreduction to methane over SEG/P25 and SRGO/P25 Schottky heterojunctions under ultraviolet (365 nm) and visible illumination; (B) the proposed photocatalytic mechanism for graphene–TiO2 Schottky heterojunctions. The color scheme is: carbon (gray), hydrogen (white), oxygen (red), and titanium (blue), hole (green). Upon illumination, the photo-excited electron is injected into the graphene nanoplatelet, leaving behind a TiO2 confined hole (green) [132]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 53. (A) Schematic illustration of the synthesis of the Ag NWs–RGO–CdS NWs (ACG) hybrid structure. (B) SEM images of ACG. (C) Selective formation rates of CO and CH4 over different samples under visible light irradiation (␭ > 420 nm) [490].

the selective formation of CO through a two-electron reaction mechanism [451]. As a typical example, robust hollow spheres were successfully fabricated through the 2D-2D hybrid of Ti 0.91 O2 nanosheets and graphene nanosheets by a layer-by-layer assembly technique (Fig. 55A and B). The results showed that the generation rates of CO and CH4 over the Ti0.91 O2 –graphene hollow spheres were 8.91 and 1.14 ␮mol g−1 h−1 , respectively, which was about 5 times higher than that over the Ti0.91 O2 (Fig. 55C and D) [451]. Clearly, graphene is of great importance for switch-

ing the selectivity of Ti0.91 O2 for CH4 to the production of CO as the main CO2 -photoreduction products. Meanwhile, the hollow structure significantly enhanced the light absorption due to the multi scattering effects. Their synergistic effects of enhanced light absorption and adsorption/activation of CO2 are responsible for the significantly enhanced activity and selectivity of Schottky heterojunctions toward CO production [451]. Accordingly, in the practical application, the graphene-determining selectivities towards CO2 photoreduction should be dependent on the balance of




Fig. 54. (A) High magnification images of N–TiO2 -001/GR, (B)TEM image of N–TiO2 -001/GR,(C) HRTEM image of N–TiO2 -001/GR, (D) Total yield of CH4 over as-synthesized TiO2 -001, N–TiO2 -001 and a series of TiO2 -based/GR nanocomposites, (E) Schematic illustration of the charge transfer and separation of electron–hole pairs for the reduction of CO2 with H2 O to CH4 using N–TiO2 -001/GR nanocomposites under visible light irradiation [474].

Fig. 55. (A) Schematic illustration for preparing the LBL assembled multilayer-coated spheres consisting of titania nanosheets and GO nanosheets, followed by microwave reduction of GO into G. (B) TEM images of G–Ti0.91 O2 hollow spheres. (C) Photocatalytic CH4 (dots) and CO (squares) evolution amounts for a) (G–Ti0.91 O2 )5 hollow spheres, b) (G–Ti0.91 O2 )5 hollow spheres, and c) P25. (D) Comparation of the average product formation rates [451].

the increased accumulation of the electrons and reduced local electron density, which should be carefully evaluated by the thorough measuremnts of all possible CO2 -photoreduction products. In future, the synergetic optmiziation of graphene, semiconductors and heterojunction interfaces should be paid more attention for designing the high-quality graphene-based Schottky heterojunction and multi-heterojunction photocatalysts for CO2 photoreduction. More interestingly, more efforts should be devoted to doping the suitable heteroatoms in graphene for construct-

ing the multi-functional graphene-based Schottky heterojunctions, due to the simulatously enhanced electrocatalytic activity and CO2 adsorption by heteroatom doping. Additionally, when improving the quality of graphene-based Schottky heterojunction and multi-heterojunction photocatalysts, the tuned selectivities of photocatalysts toward different CO2 reduction products should be thoroughly investigated, which could provide more information for deeply reveal the photocatalytic mechanism for the complicated CO2 photoreduction.


G Model

ARTICLE IN PRESS



43

Fig. 56. Schematic of bacterial disinfection using visible light active catalyst [497].

4.4. Photocatalytic bacteria disinfection Photocatalytic disinfection has been considered as a promising process compared to the common disinfection methods such as chlorination and UV disinfection for its strong oxidizing power, nontoxicity, and long-term photostability, as well as requiring little or no maintenance [8,493,494]. Under irradiation, the photogenerated electron-hole pairs can react at the interface with water and oxygen to produce reactive oxygen species (ROS) such as hydroxyl radical(. OH), superoxide radical anion (O2 − ), singlet oxygen (1 O2 ), and hydrogen peroxide (H2 O2 ), through the oxidation or reduction reactions, respectively (Fig. 56) [495–499]. The ROS can inactivate bacteria in water through damaging the cell wall and cell membrane, thus resulting in a loss of respiratory activity and leakage of intracellular components. So the efficacy of solar disinfection over an efficient photocatalyst could be improved by any strategy to increase the contration of ROS. In particular, the coupling of graphene with other semiconductor materials could result in the improved separation of the photo-generated electron-hole pairs, increased specific surface area, and adequate quantity and quality of active sites toward bacterial adsorption, thus achieving the increased ROS for bacteria disinfection [500]. Therefore, various kinds of semiconductors, such as TiO2 [498,501], Bi2 MoO6 [499], ZnO [502], CdS [503], Ag [493], g-C3 N4 [504] and Ag3 PO4 [505], have been widely coupled with graphene to improve photocatalytic bacteria disinfection efficiency. On the one hand, the wide-band-gap semiconductors/graphene Schottky heterojunctions have been widely constructed to achieve the UV photocatalytic bacteria disinfection. For example, TiO2 -RGO were synthetized by the in situ photocatalytic reduction of exfoliated GO using P25 (Evonik-Aeroxide) as the photocatalyst [495]. The resulting TiO2 -RGO Schottky heterojunctions were tested by the disinfection of E. coli as the model microorganism under UV–vis and visible irradiation, respectively. The TiO2 -RGO Schottky heterojunctions showed better disinfection efficiency of E. coli than that of unmodified TiO2 -P25 under the same conditions. It was found that the main ROS produced under UV–vis irradiation were hydroxyl radical, hydrogen peroxide and singlet oxygen (Fig. 57). All the experiment results indicated that H2 O2 has an important role in the disinfection mechanism. Although the TiO2 -RGO Schottky heterojunctions were also found to show visible light activity for the disinfection of E. coli through the dominant ROS of singlet oxygen, the efficiency is relatively low due to the larger band gap of TiO2 . On the other hand, in order to improve visible-light photocatalytic efficiency and utilization efficieny of soalr spectrum, recent efforts have focused on the visible-light-driven photocatalytic bacteria disinfection. For example, Gao et al. reported that Graphene oxide (GO)–CdS Schottky heterojunctions were synthesized via a novel two-phase mixing method (Fig. 58A) [503]. The results show

Fig. 57. Mechanism of photocatalytic generation of ROS on TiO2 -rGO under UV and visible irradiation, and in the presence of chloride [495].

that nearly 100% of both Gram-negative Escherichia coli (E. coli) and Grampositive Bacillus subtilis (B. subtilis) were killed within 25 min under visible light irradiation (Fig. 58B).The excellent performances of GO CdS Schottky heterojunctions can be attributed to that (1) the prolonged lifetime of photo-generated electron–hole pairs; (2) the prevented aggregation of CdS nanoparticles due to the uniform deposition of CdS on GO sheets and (3) the enhanced durability of CdS owing to the formation of high quality GO CdS Schottky heterojunctions. In another example, a series of Ag-AgX/RGOs (X = Cl, Br, I) multi-heterojunctions were found to be efficient antimicrobial agents for water disinfection upon visible light, due to the improved charge transfer by RGO sheets [506]. The optimum AgAgBr/0.5% RGO could completely inactivate 2 × 107 cfu mL−1 of Escherichia coli within 8 min, much faster than bare Ag-AgBr within 35 min. The enhancement mechanisms by addition of RGO were also proposed by two pathways: primary oxidative stress caused by plasma induced reactive species like H2 O2 and bactericidal effect of released Ag+ ions (Fig. 59). At this context, the future research should focus on the combination of visible light photocatalytic bacteria disinfection and other disinfection techanologies, and deep studies on the detailed bactericidal mechanism. However, as observed from the above reports, besides multiheterojunctions, there have been few reports about the applications of other kinds of graphene-based heterojunctions for visible-ligh photocatalytic bacteria disinfection. In the future, it is expected that more and more visible-light and near infrared semiconductors can be combined with graphene to form other kinds of graphene-based heterojunction photocatalysts, such as Z-scheme




44

Fig. 58. (A) Photographs and schematic illustration of the two-phase synthesis of GO-CdS Schottky heterojunctions. (B) Time-dependent antibacterial activity towards E. coli in the presence of GO, CdS and GO-CdS under visible light irradiation for 30 min. (C) Schematic illustration of charge transfer and formation of • OH [503].

Fig. 59. Proposed synergistic photocatalytic bacterial inactivation mechanism by plasmonic Ag-AgBr/0.5% RGO multi-heterojunction [506].

and Type II hetrojunctions, for photocatalytic bacteria disinfection. 5. Future perspectives In summary, this review comprehensively reviews the significant advances in the utilization of graphene-based heterojunction semiconductors for heterogeneous photocatalysis. Various graphene-based heterojunction photocatalysts, including Schot-

tky junctions, Type-II heterojunctions, Z-scheme heterojunctions, Van der Waals heterostructures, in plane heterojunctions and multicomponent heterojunctions, are thoroughly highlighted and discussed. Meanwhile, important photocatalytic applications of graphene-based heterojunction photocatalysts, such as photocatalytic water splitting (H2 evolution and overall water splitting), degradation of pollutants, bacteria disinfection and carbon dioxide reduction are also thoroughly summarized. Obviously, the graphene-based heterojunction photocatalysts hold great promise




for a variety of applications. Therefore, it is not surprising that the graphene-based heterojunction photocatalysts have attracted so much attention in different photocatlytic fields. To date, although considerable advances have been achieved in the recent years, it remains challenging to exploit really earth-abundant graphenebased heterojunction photocatalysts and deeply investigate the underlying enhancement mechanism in these heterojunctions at the atomic and molecular levels. Accordingly, more efforts are also needed to address the important roles of the outstanding structural and electronic properties of various graphene in improving the photocatalytic efficiency. Firstly, the deep mechanism for Schottky junctions, Type-II heterojunctions and Z-scheme heterojunctions is partially unclear, which should be revealed by the experimental and theoretical techniques in near future to rationally design highly active and durable graphene-based heterojunctions from a system-level engineering consideration. Juding from the published results, more direct experimental evidences, such as the charge carrier dynamics, needed to be provided to verify the exact charge separation and transfer mechanisms [12]. In particular, DFT calculation as an effective tool should be paid more attention [461,507–509], which could supplement more useful indirect information, such as the charge transfer, charge density, special interface interaction, band offsets, catalytic reaction pathways and the adsorption/dissociation properties of reactant molecules, for the fundamental in-depth understanding of the underlying charge carrier dynamics and reaction pathways. To some extent, despite the in-situ observation technologies for the carrier dynamics are still challenging, they are necessary and highly expected to be included in the furture studies on these kinds of graphene-based heterojunctions. Secondly, the novel and interesting Van der Waals heterostructures and in-plane heterojunctions seem to be more promising, but still face many challenges in developing these nanocomposites. On the one hand, more and more ultrathin 2D graphene and semiconductor nanosheets are still needed to be developed by the simple and scale fabrication methods for designing highly active heterojunctions. On the other hand, the deep experimental and theoretical investigations on the advantages of these heterojunctions are also highly expected, as compared with other traditional heterojunctions. Thirdly, the multicomponent heterojunctions and heteroatom doped graphene have become hot topics in the past several years, in which the control and optimization of interface and composition should be carefully performed to maximize the photoactivity. For designing the highly active multicomponent heterojunctions, optimizing the microscopic separation and transfer pathway of charge carriers are very crucial for achieving the suprior photoactivity, especially for the improving complicated interfacial interactions. Although much effort has been paid to the development of doped graphene materials for constructing various kinds of heterojunctions, it is worth pointing out that several challenges still need to be addressed in the future studies, such as fundamental understanding on the relationship beween specific dopants and the carrier density/bandgap opening, precisely controlled metallic or semiconductive properties by the suitable doping concentrations. In the hopeful future, more and more high-efficiency heteroatom doped semimetallic or semiconducting graphene nanosheetes should be developed to construct high-efficiency and −selectivity garphene-based heterojunctions for differnet photocatalytic reactions. Nevertheless, the interesting graphene-based heterojunction photocatalysts will be one of the most important research fields for photocatalysis. Undoubtedly, more and more interesting graphenebased heterojunction photocatalysts will be continually developed and applied in different scientific and industrial communities. It is expected that the substantial breakthroughs in the construction

45

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