Carbon-based two-dimensional layered materials for photocatalytic CO2 reduction to solar fuels

Author’s Accepted Manuscript Carbon-based two-dimensional layered materials for photocatalytic CO2 reduction to solar fuels Jingxiang Low, Bei Cheng, Jiaguo Yu, Mietek Jaroniec www.elsevier.com/locate/ensm

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S2405-8297(15)30086-6 http://dx.doi.org/10.1016/j.ensm.2015.12.003 ENSM30

To appear in: Energy Storage Materials Received date: 20 November 2015 Revised date: 22 December 2015 Accepted date: 22 December 2015 Cite this article as: Jingxiang Low, Bei Cheng, Jiaguo Yu and Mietek Jaroniec, Carbon-based two-dimensional layered materials for photocatalytic CO reduction to solar fuels, Energy Storage Materials, http://dx.doi.org/10.1016/j.ensm.2015.12.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Carbon-based two-dimensional layered materials for photocatalytic CO2 reduction to solar fuels

Jingxiang Lowa, Bei Chenga, Jiaguo Yu*,a,c and Mietek Jaroniec*,b

a

State Key Laboratory of Advanced Technology for Materials Synthesis and

Processing, Wuhan University of Technology, Luoshi Road 122, Wuhan 430070, P. R. China. E-mail: [email protected] b

Department of Chemistry and Biochemistry, Kent State University, Kent, Ohio

44242, USA. E-mail: [email protected] c

Department of Physics, Faculty of Science, King Abdulaziz University, Jeddah

21589, Saudi Arabia.

TOC Graphics

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Highlights 1. Application of graphene and C3N4 for photocatalytic CO2 reduction is reviewed. 2. Methods for preparation of carbon-based 2D layered materials are highlighted. 3. Challenges and opportunities in photocatalytic CO2 reduction are discussed.

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Abstract The depletion of fossil fuels and rising atmospheric levels of carbon dioxide lead to an increasing interest in artificial photosynthesis technologies. Among them, photocatalytic CO2 reduction to valuable solar fuels is considered as one of the best strategies for solving both energy and environmental problems simultaneously. In the past decade, it was proved that the photocatalytic CO2 reduction performance can be greatly enhanced by using carbon-based two-dimensional (2D) layered materials, namely graphene and graphitic carbon nitride (g-C3N4) due to their excellent electronic and physicochemical properties. In this review, the major advances in the area of carbon-based 2D layered photocatalysts for CO2 reduction are presented. A brief overview on the preparation methods and applications of carbon-based 2D layered photocatalysts is discussed. Finally, the challenges and opportunities for the future research of carbon-based 2D layered materials in photocatalytic CO2 reduction are highlighted. Keywords: Photocatalysis; CO2 reduction; solar fuels; graphene; g-C3N4

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1. Introduction Owing to the depletion of fossil fuels and increasing emission of CO2, the exploration of clean and renewable energy has received much attention. Photocatalytic CO2 reduction is known as one of the most promising renewable energy technologies for overcoming the aforementioned problems because of its ability to convert CO2 to valuable solar fuels such as CH4, HCO2H, CH2O, and CH3OH [1-5]. Since the breakthrough work on the photocatalytic CO2 reduction by Inoue et al. [6], numerous reports have been published on the preparation of highly efficient photocatalysts to meet the practical requirements of the photocatalytic systems for CO2 reduction [7,8]. However, photocatalytic CO2 reduction is still limited by its low solar conversion efficiency because of the fast recombination of photoinduced electrons and holes, slow charge consumption during reduction-oxidation (REDOX) reaction and low light utilization [9-11]. To this end, the preparation of highly efficient photocatalysts with slow electron-hole recombination, fast charge consumption during REDOX reaction and good light utilization are the key challenges to improve the photocatalytic CO2 reduction performance. Carbon is one of the most abundant elements on the earth. Carbon-based materials have been widely used in the various applications such as batteries [12,13], supercapacitors [14-16], fuel cells [17,18] and photocatalysts [11,12] due to their low cost and eco-friendliness. Since the successful isolation of 2D single layer of carbon atoms (viz. graphene) in 2004 by Geim and Novoselov [21], graphene has undoubtedly turned out to be one of the most important carbon-based materials due to

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its fascinating structural, optical and electronic properties [22-24]. Since the first report on the application of graphene in photocatalytic CO2 reduction by Liang et al. [25], graphene has been often used in photocatalytic CO2 reduction processes [26,27]. Owing to its ultra-thin 2D layered structure, graphene features unique properties such as superior electronic conductivity, large specific surface area and high chemical stability, which are essential for enhancing photocatalytic CO2 reduction performance. Therefore, graphene-based materials are the promising materials for photocatalytic CO2 reduction. More recently, g-C3N4, another carbon-based 2D layered material shows great prospects for advancement of energy and environmental applications [28-31]. g-C3N4 is a polymeric material composed of tris-triazine-based patterns with the C/N ratio = ¾ and small amount of H. Since it exhibits a stacked structure, g-C3N4 is often considered as sp2-hybridized nitrogen-substituted graphene. Although graphene and g-C3N4 possess the similar layered structure, their electronic properties are distinctly different. g-C3N4 is a typical semiconductor with a band gap of 2.7 eV, while graphene is an excellent conductor. Thus, the semi-conductive properties of g-C3N4 are utilized in the photocatalytic CO2 reduction reactions [32-34]. In comparison to other common photocatalysts, g-C3N4 exhibits tremendous advantages such as good light absorption ability, large specific surface area, unique surface properties, good chemical stability and low-cost for photocatalytic CO2 reduction [35-38]. Moreover, its rich surface properties make this material a perfect support for photocatalysts. Herein, the major advances in the field of graphene and g-C3N4-based materials,

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especially in relation to the photocatalytic CO2 reduction are reviewed by taking into account four major issues: (1) synthesis methods, (2) photocatalytic CO2 reduction mechanism, (3) modifications of graphene and g-C3N4, and (4) future challenges in the usage of graphene and g-C3N4 in photocatalytic CO2 reduction. The ultimate goal is to provide an overview of the development of graphene and g-C3N4-based materials toward improving their photocatalytic CO2 reduction activity.

2. Preparation methods From the viewpoint of photocatalytic CO2 reduction process, the preparation of uniform and high quality graphene and g-C3N4 materials with desirable properties for the aforementioned process is of high importance. In this section, the major methods for the preparation of graphene and g-C3N4 are presented to further stimulate research on application of graphene and g-C3N4-based photocatalysts for photocatalytic CO2 reduction.

2.1 Preparation of graphene-based photocatalysts

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Fig. 1 Schematic illustration of 3-step (oxidation-exfoliation-reduction) process for the preparation of graphene.

The discovery of graphene is related to the mechanical exfoliation of graphite by using scotch-tape method. However, this method of graphene preparation is not suitable for coupling graphene with other materials and for a large-scale production. Therefore, various preparation methods have been proposed to prepare graphene such as chemical vapor deposition (CVD) on metal surfaces, epitaxial growth on an insulator, chemical exfoliation of graphite and so on. Among these preparation methods, chemical graphite exfoliation has been widely used for the preparation of graphene-based photocatalysts because of its simplicity, large scale production and cost effectiveness. Generally, chemical exfoliation of graphite into graphene involves a 3-step oxidation-exfoliation-reduction process (see Fig. 1) [39]. In details, graphite is firstly oxidized into graphite oxide via Hummers method through the reaction of

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graphite with a mixture of potassium permanganate (KMnO4) and concentrated sulfuric acid (H2SO4). During this oxidation, graphite is swelling and consequently, Van der Waals interactions between subsequent layers are weaker. Therefore, graphite oxide can be easily exfoliated into graphene oxide (GO) under ultrasonication in different solvents. Finally, GO can be reduced into graphene or so-called reduced graphene oxide (RGO) with low oxygen to carbon atomic ratio. It should be noted that GO is always coupled with the target photocatalyst before reduction to RGO because GO possesses a large amount of functional groups on its surface, which is facilitates the coupling of photocatalysts to the surface of graphene. Therefore, simultaneously during reduction of GO to RGO, an intimate contact between graphene and photocatalyst occurs assuring high dispersion of photocatalyst on RGO. Notably, RGO can also act as a capping agent on the photocatalysts, and inhibit the growth of the photocatalyst nanocrystal particles, thereby improving their specific surface area [40]. Furthermore, proper tuning of the zeta potential of RGO and photocatalyst is also an important issue for creating intimate contact between RGO and photocatalyst. For example, Li et al. reported that a close contact between RGO and CdS can be created by considering the zeta potential and surface charge of RGO and CdS (see Fig 2) [41]. In details, at pH=7, the GO dipersion has a negatively charged surface. After mixing GO and cadmium acetate in dimethyl sulfoxide (DMSO) solution, the created Cd+ ions adhere to the surface of GO due to the electrostatic force between Cd+ ions and the negatively charged surface of GO. After that, the solution was heated at 180 oC. During the solvothermal process at 180 oC , H2S can be

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released by DMSO. Then, H2S can react with the Cd+ ions adhered to the surface of GO, thereby resulting in the in-situ creation of CdS nanoparticles on the GO surface. Meanwhile, GO can be also reduced to RGO during the solvothermal process in the presence of DMSO. Importantly, the size of the resulted CdS is small because the freshly formed RGO can limit the growth of CdS and thus enlarge the specific surface area of the sample.

Fig. 2 Scheme illustrating the preparation of CdS/RGO composite.

2.2 Preparation of g-C3N4-based photocatalysts

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Fig. 3 (a) Schematic illustration of the main routes for the preparation of g-C3N4 by condensation of nitrogen-rich and oxygen-free compounds: C, red; N, yellow; H, grey; S, blue; O, white. (b) TEM images of g-C3N4 prepared by using different precursors. Reprinted with permission from ref. 48. Copyright 2015, Elsevier B.V.

Different from graphene, the research on g-C3N4 has been started in very early days. However, the development of g-C3N4 is still in its infancy stage because of its tedious preparation procedure, chemical inertness and insolubility in almost all known solvents. Recently, g-C3N4-based materials have attracted much more attention due to the turn up of simple g-C3N4 preparation method [42-45]. Particularly, g-C3N4 can be obtained by thermal condensation of nitrogen-rich materials such as urea, thiourea, melamine and etc. (see Fig. 3a) [46,47]. Normally, the g-C3N4 prepared by thermal condensation method exhibit porous structures (see Fig. 3b) because the significant

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amount of gases, such as CO2 and NH3 is released from the precursors during the synthesis. In addition, it should be taken into account that some key properties such as microstructure and adsorption affinity of g-C3N4 can be modified by using different precursors. For example, Zhu et al. reported that g-C3N4 prepared by direct heating of melamine, thiourea, or urea exhibited different microstructure and isoelectric point, which can influence its adsorption properties [48]. In particular, the isoelectric point of g-C3N4 prepared from melamine or urea is higher than that of g-C3N4 prepared from thiourea, indicating that the adsorption properties of g-C3N4 can be simply tuned by changing precursors. Furthermore, the electronic properties of g-C3N4 prepared by thermal condensation method can be simply tuned by changing the condensation degree, further extending the application of g-C3N4 to photocatalytic CO2 reduction. However, g-C3N4 prepared by thermal condensation normally exhibits low specific surface area due to its graphitic layered structure. The low specific surface area limits the potential of g-C3N4 in photocatalytic CO2 reduction because photocatalytic reactions require photocatalysts that show large surface area accessible for adsorption [45]. Therefore, the preparation of exfoliated thin g-C3N4 nanosheets by overcoming the strong van-der-Waals force between layered g-C3N4 nanosheets is a critical task for further exploration of the potential of g-C3N4 in photocatalytic CO2 reduction. As compared to graphene-based photocatalysts the preparation of g-C3N4-based photocatalysts is more flexible because the coupling of g-C3N4 with other materials can be carried out simultaneously or separately with the synthesis of g-C3N4. For example, the TiO2/g-C3N4 composite can be prepared by either direct heating of

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precursors of g-C3N4 with TiO2 or by hydrothermal processing pure g-C3N4 with the precursors of TiO2. Owing to the manifold preparation methods, g-C3N4 creates great opportunities for the preparation of various kinds of g-C3N4-based photocatalysts.

3. Photocatalytic CO2 reduction mechanism

Fig. 4 Schematic illustration of the photocatalytic CO2 reduction process

This section presents the theoretical and practical aspects of photocatalytic CO2 reduction over carbon-based 2D layered materials for the production of solar fuels. In particular, the photocatalytic CO2 reduction process involves a series of reactions including: 1) CO2 adsorption, 2) electron-hole pair photogeneration, 3) charge carriers’ separation, 4) charge carriers’ transportation, and 5) chemical reactions between surface species and charge carriers (see Fig. 4) [49,50]. The photocatalytic CO2 process starts with adsorption of CO2 molecule on the photocatalyst surface. The

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introduction of carbon-based 2D layered materials can enhance CO2 adsorption because both graphene and g-C3N4 have a large 2D π network on their surface, which can be utilized for accelerating adsorption toward CO2 molecules by π–π interactions. Then, the adsorbed CO2 molecules are ready to be reduced into solar fuels by photoinduced electrons, which are produced when the energy of incident light is greater than or equal to the band gap of a given semiconductor. In details, the incident light energy is consumed by the photocatalyst and utilized for excitation of electrons from the valence band (VB) to the conduction band (CB), leaving a hole in VB. Then, these photoinduced electron-hole pairs can either recombine and create useless heat or travel to the surface of photocatalyst and react with adsorbed CO2 molecules if the electron-hole recombination is slower than the aforementioned surface reactions. However, it is noteworthy that not all photoinduced electrons on the surface of semiconductor can be utilized for reduction of CO2, which is a very stable compound. In fact, only the photogenerated electrons with sufficient REDOX potential ΔE0 can be used for the particular solar fuel conversion reactions. The free energy ΔG0 and the standard REDOX potential ΔE0 for the CO2 reduction are provided by Eq. 1-5 [49,50].

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Therefore, the semiconductor with relatively low band gap value for visible light utilization and high REDOX potential for the reduction process are favorable. Due to its visible light-active band gap structure (2.7 eV) and sufficiently high REDOX potential, g-C3N4 has been known as one of the most promising semiconductors for photocatalytic CO2 reduction. Notably, g-C3N4 has a CB potential of −1.23 V (normal hydrogen electrode (NHE) at pH 7). Therefore, it can ideally reduce CO2 into various hydrocarbon products. Meanwhile, although graphene is a zero band gap material, the oxidized graphene, namely graphene oxide, can be tuned to have proper band gap structure for photocatalytic CO2 reduction reaction. Furthermore, as shown in Eq. 1-5, the CO2 reductions into methane and methanol are two most favorable processes because they occur at lower potentials. However, the production of methane or methanol is probably harder than the production of carbon monoxide, formaldehyde and formic acid by photocatalytic CO2 reduction kinetics because more electrons and protons are needed for the formation of methane and methanol (see Eq. 6-10). CO2 + 2H+ + 2e- → HCO2H,

E0 = -0.61 V vs NHE at pH 7

(6)

CO2 + 2H+ + 2e- → CO + H2O, E0 = -0.53 V vs NHE at pH 7

(7)

CO2 + 4H+ + 4e- → HCHO + H2O,

E0 = -0.48 V vs NHE at pH 7

(8)

CO2 + 6H+ + 6e- → CH3OH + H2O, E0 = -0.38 V vs NHE at pH 7

(9)

CO2 + 8H+ + 8e- → CH4 + 2H2O, E0 = -0.24 V vs NHE at pH 7

(10)

Interestingly, introduction of graphene or g-C3N4 can make these multi-electron processes easier. In details, the carbon-based 2D layered materials are good electron acceptor and have large specific surface area. Therefore, the photoinduced electrons

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can be accumulated on the surface of the carbon-based 2D layered materials, which is beneficial for multi-electron reactions.

4. Applications of photocatalytic CO2 reduction Attributing to the excellent physical and chemical properties of the carbon-based 2D layered materials, g-C3N4 and graphene have been extensively investigated in past several years for enhancing photocatalytic CO2 reduction activity. In order to shed a light on the function of these C-based 2D layered materials in the photocatalytic CO2 reduction, the role of graphene and g-C3N4 in this process is discussed by taking into account six aspects: (1) photocatalysts supports, (2) electron-hole separation, (3) adsorption affinity toward CO2, (4) light utilization, (5) directly used as photocatalysts and (6) coupling graphene and g-C3N4.

4.1 Photocatalyst supports Attributing to their 2D structure, graphene and g-C3N4 are known as superior supports for photocatalysts because they can enlarge the specific surface area and provide plenty of surface reactive sites [51,52]. Therefore, the surface reaction of the CO2 reduction can be easily carried out on such photocatalysts. For instance, Tu et al. reported that the specific surface area of TiO2 can be increased by loading graphene, thereby providing more active adsorption sites for photocatalytic reactions [53]. These abundant surface reactive sites can significantly facilitate surface reactions, thereby accelerating the photocatalytic CO2 reduction reaction on TiO2. Therefore, the CH4 and C2H6 production rates on graphene-TiO2 are higher than those on the pure TiO2.

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Meanwhile, Ohno et al., reported that g-C3N4 can also improve the specific surface area of WO3 for photocatalytic CO2 reduction [54]. The enlarged specific surface area of WO3/g-C3N4 is good for accommodating more surface reaction sites for photocatalytic reactions, thereby enhancing photocatalytic CO2 reduction activity. In order to further improve the specific surface area of WO3/g-C3N4, g-C3N4 was treated by three different methods including agate mortar, calcination and planetary mill method. Particularly, g-C3N4 treated by planetary mill method shows the best photocatalytic CO2 reduction performance among all three treated g-C3N4 to be coupled with WO3 because g-C3N4 treated by planetary mill method has thinner structure with higher specific surface area than the g-C3N4 materials treated by agate mortar and calcination methods. More recently, it was reported that the loading of graphene or g-C3N4 can also enhance the dispersion of photocatalysts and reduce their size, thereby improving the specific surface area. This is because a large amount of functional groups on the surface of graphene and g-C3N4 can be utilized as anchoring sites for photocatalyst. For example, Li et al. prepared a ZnO-RGO composite by using a facile hydrothermal reaction [55]. They found that the loading of RGO can significantly reduce the ZnO’s size and prevent the self-aggregation of ZnO due to its anchoring by the surface functional groups on RGO. As a result a uniform distribution of ZnO on RGO is achieved, assuring higher specific surface than that of the pure ZnO, thereby rendering more surface active sites for photocatalytic CO2 reactions. Therefore, the prepared ZnO-RGO composite exhibited a better photocatalytic CO2 reduction

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activity than the pure ZnO to yield CH3OH.

Fig. 5 Schematic illustration for the protection of Cu2O by graphene from the attack of •OH radicals. Due to their 2D structure, graphene and g-C3N4 can wrap the particles of photocatalysts with low photostability such as Cu2O and CdS to enhance their stability. Thus, photocorrosion (viz. attack of the photocatalyst by •OH radicals or holes) of these low photostable photocatalysts can be greatly suppressed and blocked by graphene or g-C3N4. For example, An et al. reported a RGO loaded Cu2O (RGO-Cu2O) prepared by facile one-step microwave-assisted chemical method with good photostability for photocatalytic CO2 reduction [56]. This RGO-Cu2O composite exhibited six times higher photocatalytic CO2 reduction activity than that of Cu2O for CO production due to its enhanced specific surface area and improved electron-hole separation efficiency. The leaching of Cu ions, which was attributed to the photocorrosion of Cu2O, was significantly reduced by loading RGO, from 2670 ppm to 96 ppm after 3 h of light irradiation. This interesting phenomenon is attributed to the fact that the attack of active species on Cu2O can be effectively blocked by RGO, which is wrapped on the Cu2O surface (see Fig. 5).

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4.2 Electron-hole separation

Fig. 6 Schematic illustration of the photocatalytic CO2 reduction mechanism over graphene-titania composite.

Since the first report on the use of graphene in photocatalytic CO2 reduction, graphene has been always used as an electron sink in the photocatalytic system for enhancing photoinduced electron-hole separation and photocatalytic activity [57]. This is due to its ultra-high electron conductivity (200 000 cm2V-1S-1). Moreover, the Fermi level of graphene (0 V vs NHE) is less negative than the conduction band of the most known photocatalysts. Attributing to the proper band structure, the band alignment can occur and cause a rapid electron transfer from photocatalyst to graphene. As a consequence, the photocatalytic CO2 reduction efficiency can be greatly improved. For example, Liang et al. reported the graphene-titania nanosheet-type composite for photocatalytic CO2 reduction to CH4 [25]. Particularly, the photocatalytic performance of single wall carbon nanotubes−titania nanosheets (SWCNT-T) and graphene−titania nanosheets (G-T) were compared. It was shown that the charge transfer efficiency of G-T is better than that of SWCNT-T. This is due to the excellent conductivity of graphene and the

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2D-2D interface between graphene and titania nanosheets (Fig. 6). Finally, the G-T composite showed a higher photocatalytic CO2 reduction performance toward CH4 production in comparison with the SWCNT-T and titania nanosheets under ultraviolet irradiation.

Fig. 7 Schematic illustration for the heterojunction of g-C3N4/In2O3.

Other than graphene, g-C3N4 also shows its potential to be used for enhancing electron-hole separation by constructing type-II heterojunction photocatalytic system with other photocatalyst. Typically, type-II heterojunction photocatalytic system consists of two kinds of semiconductors. During photocatalytic reaction, the photoinduced electrons migrate to CB of the semiconductor with lower CB level, while the photoinduced holes migrate to VB of the semiconductor with higher VB level due to the band alignment of the type-II heterojunction. As a result, a spatial separation of the photoinduced electron-hole pairs can be achieved on the type-II

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heterojunction photocatalytic system. For instance, Cao et al. reported In2O3/g-C3N4 hybrid photocatalysts with enhanced photocatalytic CO2 reduction efficiency [58]. Since CB of g-C3N4 is more negative than that of In2O3 and VB of In2O3 is more positive than that of g-C3N4, a heterojunction can be built between In2O3 and g-C3N4. Under light irradiation, the photoinduced electrons are transferred from g-C3N4 to In2O3, while photoinduced holes migrate from In2O3 to g-C3N4, achieving a spatial electron-hole separation on the In2O3/g-C3N4 hybrid (see Fig. 7). Therefore, In2O3/g-C3N4 hybrid has better charge carrier separation efficiency in comparison with pure g-C3N4 and In2O3. As a result, the photocatalytic CO2 reduction performance of In2O3/g-C3N4 was obviously higher than that of pure g-C3N4 and In2O3 for CH4 production. More recently, it was also found that graphene and g-C3N4 play important role to boost the multielectron reaction during the photocatalytic CO2 reduction. For example, Lv et al. reported the graphene-modified NiOx–Ta2O5 composite for photocatalytic CO2 reduction [59]. It was reported that the introduction of graphene to NiOx-Ta2O5 can significantly enhance the electron-hole separation due to the excellent conductivity of graphene nanosheets. More interestingly, the graphene-loaded NiOx-Ta2O5 exhibited selectivity toward the formation of methanol. This happen because graphene acts an electron-sink in this composite, thereby accelerating the multi-electron reaction of CO2 reduction for the production of methanol.

4.3 CO2 adsorption

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Adsorption of CO2 on the photocatalyst is one of the key steps and prerequisites for the photocatalytic CO2 reduction process. It is known that both graphene and g-C3N4 exhibit a 2D π-conjugated structure on their surface. This structure can participate in π–π interactions with CO2 molecules, which also contain delocalized π-conjugated electrons. Thus, CO2 can be adsorbed on the photocatalyst surface through π–π interactions. For instance, Yu et al. reported that the adsorption capability of graphene nanosheets toward CO2 has a great effect on the photocatalytic CO2 reduction activity of the graphene/CdS nanorods composite [60]. Although no significant change in the specific surface area was observed for the graphene/CdS nanorods composites, their CO2 adsorption was higher than that on the CdS nanorods only, indicating that the observed enhancement in the CO2 adsorption can be caused by π–π interactions of graphene with CO2. The CO2 adsorption capability of the graphene/CdS nanorods composites was attributed to unique π–π interactions, which can cause destabilization and activation of CO2 molecules. Therefore, CO2 can be reduced more easily during photocatalytic reaction. Owning to the improved charge transfer efficiency and enhanced CO2 adsorption ability, the RGO-CdS composites showed the photocatalytic production rate of CH4 equal to 2.51 μmolh-1g-1 by reduction of CO2, which is 10 times higher than that achieved on the pure CdS. Since g-C3N4 shows also a 2D π-conjugated structure, it is expected that g-C3N4 will also feature excellent adsorption ability toward CO2 molecules. For example, Li et al. prepared a g-C3N4-loaded Bi2WO6 photocatalyst for enhancing photocatalytic CO2 reduction activity [61]. Besides improvement of the electron-hole separation rate

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and enlargement of the specific surface area, it was found that the enhanced CO2 adsorption ability of g-C3N4 also plays an important role in improving photocatalytic CO2 reduction performance of the g-C3N4/Bi2WO6 composite. The adsorbed CO2 on the surface of g-C3N4 can be easily reduced to CO during the photocatalytic reaction. As a result, the g-C3N4/Bi2WO6 exhibited 22 and 6.4 times higher photocatalytic CO2 reduction activity toward CO production than that on pure g-C3N4 and Bi2WO6, respectively.

4.4 Light utilization As mentioned above, the low light absorption ability of a photocatalyst is always one of the key issues limiting its application for photocatalytic CO2 reduction. It is well-known that graphene and g-C3N4 can improve light utilization of the composite photocatalysts. Particularly, graphene and g-C3N4 have different optical properties, the former is an excellent conductor and the latter is a semiconductor. Therefore, they play different role in improving light utilization of the composite photocatalysts. In the case of graphene-containing composite photocatalysts, almost whole light spectrum can be absorbed due to 0 eV band gap of graphene. It should be noted that this absorbed light by graphene cannot produce any active electrons or holes for REDOX reaction. However, the absorbed light can be converted to heat, thereby creating a special photothermal effect on the photocatalyst surface. It was shown that this photothermal effect is beneficial for boosting the photoinduced electron-hole migration rate and thus enhancing the photocatalytic activity. For example, Wang et al. reported a unique Cu2O/RGO composite and demonstrated that the loading of RGO

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can greatly enhance absorption of light in the visible range, suggesting that the Cu2O/RGO has better light utilization for photocatalytic reaction [62]. Although this enhanced light absorption did not directly enhance the production of electron-hole pairs, the photons concentrated on graphene can induce the aforementioned photothermal effect around Cu2O particles. Therefore, the transfer of product molecules of CO2 reduction and migration of the photoinduced charge carriers can be enhanced.

Fig. 8 Schematic illustration of the charge transfer in a Z-scheme photocatalytic system. Meanwhile, g-C3N4 can also improve the light utilization of the photocatalyst by creating all-solid-state Z-scheme photocatalytic systems. In details, an all-solid-state Z-scheme photocatalytic system is normally built by coupling g-C3N4 with other semiconductor. Under light irradiation, the VB electron is excited to the CB level of the semiconductor B (Fig. 8), creating a hole on the VB level for oxidation reaction.

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Then, this photoinduced electron can migrate to the VB level of g-C3N4 and be further excited to the CB level of g-C3N4, for reduction reaction. Obviously, a spatial charge separation can be achieved by creating all-solid-state Z-scheme photocatalytic systems. Moreover, it should be noted that the REDOX potential of the photocatalytic systems can be optimized because reduction reaction is carried out on the photocatalyst with higher CB, while oxidation reaction is carried out on the photocatalyst with lower VB. Therefore, potential of the photoinduced electron-hole pairs and the absorbed light can be fully utilized. For example, He et al. reported SnO2-x/g-C3N4 composite as an efficient Z-scheme photocatalytic system [63]. By carrying out a series of radicals-trapping experiments, it was found that a Z-scheme photocatalytic system of SnO2-x/g-C3N4 was successfully created instead of conventional heterojunction-type photocatalytic system. Therefore, the reduction reaction occurred at the CB level (-1.19 eV) of g-C3N4 instead of CB (0.20 eV) of SnO2-x, indicating that the REDOX potential of the photocatalytic systems can be optimized. As a result, the strong reduction ability of the SnO2-x/g-C3N4 composite was utilized for the reduction of CO2 into various solar fuels including CH4, CH3OH and CO, while pure SnO2-x did not show any photocatalytic CO2 reduction ability.

4.5 Direct use as photocatalysts

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Fig. 9 REDOX potentials for CO2 reduction at pH 7 in aqueous solutions. Choosing a semiconductor with proper band structure and good light absorption ability is always an ultimate goal for photocatalytic CO2 reduction reaction. Particularly, the reduction potential of the photocatalyst must be negative enough for CO2 reduction into different solar fuels (see Fig. 9). Meanwhile, the size of the band gap of the photocatalyst should be smaller than 3.1 eV for fully utilizing the sunlight irradiation [8]. g-C3N4 has been known as a perfect photocatalyst for photocatalytic CO2 reduction due to its relatively small band gap value and proper band edge position to meet the reduction requirements of the photocatalytic CO2 conversion to solar fuels.

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Fig. 10 Enhanced mechanism for the photocatalytic CO2 reduction on the g-C3N4/Pt composite. For example, Yu et al. reported that g-C3N4 synthesized by direct calcination of thiourea at 550 oC exhibited good photocatalytic CO2 reduction activity [64]. The products of the photocatalytic CO2 reduction on g-C3N4 were determined to be CH4, CH3OH and HCHO. In order to further improve the photocatalytic activity of g-C3N4, Pt was loaded on g-C3N4 as a co-catalyst for the photocatalytic CO2 reduction (See Fig. 10). It was found that the main product of the CO2 reduction (CH4) increased with increasing Pt amount from 0 to 1 wt% because Pt can act as the electron acceptor during the photocatalytic CO2 reduction reaction. However, further increase in the Pt amount led to the decrease in the CH4 production rate due to the shielding effect of Pt on g-C3N4. This work shows that g-C3N4 can be used as an efficient photocatalyst for CO2 reduction. More recently, Maeda and co-workers reported a g-C3N4/Ru complex composite with enhanced photocatalytic CO2 reduction activity [65]. Four kinds of Ru complexes including RuP, RuCP, RuH and RuMe were coupled with g-C3N4. It was

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found that the reduction potential and the absorption ability of the Ru complex have great influence on the photocatalytic CO2 reduction activity of g-C3N4. A strong interaction of RuP with g-C3N4 and the proper reduction potential determined that among four tested Ru complexes RuP would be the best co-catalyst with g-C3N4 for photocatalytic CO2 reduction to HCOOH. This result suggests that the photocatalytic CO2 reduction activity of g-C3N4 can be enhanced by loading of the co-catalyst with proper reduction potential and good interaction with g-C3N4. It is surprising that graphene with proper oxidization degree also shows an obvious photocatalytic CO2 reduction activity. It was found that the oxidized graphene, namely graphene oxide exhibits semiconductor-like properties. Moreover, the band structure of GO can be carefully controlled to achieve the photocatalytic CO2 reduction requirements by tuning the oxidization degree of GO. For instance, Hsu et al. demonstrated that GO prepared by modified Hummer’s method showed a high photocatalytic CO2 reduction activity for the generation of methanol [66]. Particularly, three kinds of samples were prepared; GO prepared by conventional Hummer’s method (GO1), GO prepared by modified Hummer’s method with some excess of KMnO4 and H3PO4 (GO2), and GO prepared by modified Hummer’s method with some excess of H3PO4 (GO3). It was found the oxidation level of GO can be tuned by varying the excess of KMnO4/H3PO4 or H3PO4. Therefore, GO1, GO2 and GO3 showed different band gap values at around 2.9-3.7, 3.1-3.9 and 3.2-4.4 eV, respectively. It was shown that GO3 with highest band gap value among three prepared samples exhibited the best photocatalytic CO2 reduction activity for the

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production of methanol under 300-watt halogen lamp irradiation. This case indicates that the higher band gap value is beneficial for the photocatalytic reaction and the oxidized graphene (viz. GO) can be used as a possible photocatalyst for this reduction process.

4.6 Coupling graphene and g-C3N4

Fig. 11 Schematic illustration of the procedure for the preparation of G/PCN.

Recently, the development of photocatalyst systems consisting of metal-free materials has attracted a wide attention because the metal-based photocatalysts are often costly and difficult to produce on the large scale. Therefore, the incorporation of metal-free graphene and g-C3N4 into a metal-free photocatalyst system is known as one of the most promising photocatalytic systems for practical CO2 reduction in the future. Moreover, both graphene and g-C3N4 exhibit 2D layered structure with large surface area. Thus, coupling graphene and g-C3N4 via unique 2D-2D intimate contact assuring ultra large contact interface is beneficial for fast electron migration and

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effective electron-hole separation [67]. For example, Ong et al. showed that the sandwich-like graphene-g-C3N4 (G-CN) composite prepared by facile one-pot impregnation-thermal reduction method has high photocatalytic CO2 reduction activity toward CH4 production [68]. A slightly red-shift of the absorption edge can be found on G-CN (2.7 eV) in comparison with g-C3N4 (2.82 eV) due to the presence of C-O-C bond as a covalent cross linker between graphene and g-C3N4. More importantly, G-CN was shown to have a close contact and large contact interface between graphene and g-C3N4. This close contact and large contact interface are beneficial for electron migration, thereby improving photocatalytic CO2 reduction activity. Furthermore, the preparation method of graphene-g-C3N4 has great effect on the photocatalytic activity of the resulting composites [69]. In reference [69], graphene-g-C3N4 was prepared by a combined ultrasonic dispersion and electrostatic self-assembly strategy followed by NaBH4 reduction reaction (see Fig.11). In details, g-C3N4 was firstly prepared by conventional thermal condensation method followed by protonation using HCl, while graphite oxide was sonicated in water to form GO. Then, the protonated g-C3N4 (PCN) and GO were mixed together to form a GO-PCN composite by π–π stacking and electrostatic attraction. The GO-PCN composite was then reduced by NaBH4 solution to get rGO-PCN. In comparison with rGO-CN composite, it was found that rGO-PCN composite possessed a closer contact and better interfacial interaction between PCN and graphene because of the electrostatic attraction. Furthermore, a large contact interface can be also created in the case of 2D-2D layered composite, resulting in higher electron-hole separation rate. As a

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consequence, the photocatalytic CO2 reduction activity of the rGO-PCN was significantly higher than that of rGO-CN for CH4 production.

5. Modifications of graphene and g-C3N4 In order to fully explore the potential of graphene and g-C3N4 for photocatalytic CO2 reduction, various modifications of these layered materials deserve more attention because they can eventually enhance the impact of graphene and g-C3N4 on the photocatalytic CO2 reduction [53]. Various methods of graphene and g-C3N4 functionalization were shown to be effective for improving photocatalytic CO2 reduction activity, including surface engineering, doping and morphology tuning.

5.1 Surface engineering The exfoliation of graphene or g-C3N4 through chemical exfoliation will expose defects present on the surface of these materials. Although the low amounts of surface defects may be beneficial for the photocatalytic reaction, the excessive surface functional groups will in turn reduce the conductivity and the surface active sites on the graphene or g-C3N4-based photocatalysts. Therefore, the photocatalytic CO2 reduction activity of the graphene or g-C3N4-based photocatalysts will be greatly reduced. In order to overcome this problem, the diminishment of surface defects on the graphene and g-C3N4-based photocatalysts can be performed. For example, Liang et al. conceptually showed that the graphene-based photocatalysts with smaller amount of surface defects exhibit significantly higher photocatalytic CO2 reduction activity toward CH4 production than their counterparts with high amount of surface defects [70]. In details, graphene was prepared by two different methods, which were

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RGO and solvent-exfoliated graphene method (SEG), and coupled with TiO2 to form RGO-TiO2 and SEG-TiO2, respectively. Based on the Raman spectra and high-resolution transmission electron microscopy (HRTEM) images, it was found that SEG showed obviously lower defects density than that of RGO. It is well-known that the surface defects density can reduce the conductivity of graphene, suggesting that SEG with low surface defects density has better electron conductivity than RGO. Attributing to its outstanding electron conductivity, SEG-TiO2 exhibited higher photocatalytic CO2 reduction activity than RGO-TiO2 toward CH4 production. Increasing the number of hydroxyl groups on the surface of graphene or g-C3N4 is also an effective method to improve the effect of graphene or g-C3N4 on the enhancement of photocatalytic CO2 reduction performance. For example, Huang et al. prepared a 2D-hydroxyls rich g-C3N4 (hydroxyl-CN) by employing ultrasonic exfoliation of bulk g-C3N4 (B-CN) [71]. Particularly, the bulk g-C3N4 was exfoliated in H2O (strongly polar solvent) to facilitate exfoliation of g-C3N4. Analysis of the FTIR spectra of B-GN and hydroxyl-CN showed that the peak attributed to the deformation vibration mode of –OH of hydroxyl-CN is significantly higher than that of B-CN because hydroxyl-CN has higher density of –OH groups on its surface than that on the B-CN surface. In addition, the peak area ratio of –OH and H2O on the XPS spectra was used to further investigate the presence of hydroxyl groups on the surface of hydroxyl-CN and B-CN. It was shown that hydroxyl-CN has higher peak area ratio of –OH and H2O than B-GN, suggesting that more hydroxyl groups can be found on the surface of hydroxyl-CN. These –OH groups are beneficial for improving the

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photoinduced electron-hole separation efficiency by accepting hole from g-C3N4. Therefore, the photocatalytic CO2 reduction performance of hydroxyl-CN is significantly enhanced in comparison with that of B-CN for CH4 production. Recently, Yu and co-workers reported that amine-functionalized g-C3N4 (A-CN) exhibited an excellent photocatalytic CO2 reduction activity toward CH4 production [72]. In details, A-CN was prepared by condensation of urea at 500 oC, followed by amine-functionalization using monoethanlamine solution. It was found that the A-CN has similar electronic and physical properties to those of the pure g-C3N4. However, the CO2 adsorption capacity of A-CN (0.207 mmolg−1) was significantly higher than that of the pure g-C3N4 (0.055 mmolg−1). High CO2 adsorption capacity of A-CN was attributed to acid-base interactions between CO2 molecules and A-CN in the presence of H2O. Moreover, this acid-base interaction of CO2 molecules and A-CN can create HCO3−, which is more active than linear CO2 and facilitates the formation of CH4. As a result, A-CN had higher photocatalytic CO2 reduction rate than that of g-C3N4.

5.2 Doping

Fig. 12 TEM and HRTEM (inset) images of the Boron-doped graphene. Reprinted

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with permission from ref. 73. Copyright 2014, Nature publishing group. Doping is one of the most widely used methods to tune the optical, physical and electronic properties of the materials. Therefore, it is not surprising that doping is always employed to modify the properties of graphene and g-C3N4. For example, Zhang and co-workers revealed that the doping of graphene with boron can effectively enhance graphene’s electronic conductivity, which is beneficial for improving photocatalytic CO2 reduction activity [73]. Doping graphene nanosheets with boron resulted in cutting them into smaller size boron-doped graphene (BG) nanoribbons due to the macro-residual stress caused by boron doping. Moreover, ZZ-edges and AC-edges can be found on the BG nanoribbons (see Fig. 12), thereby generating semimetallic properties with good electron conductivity. Then, BG was coupled with commercial TiO2 (P25) to form a BG-TiO2 composite for photocatalytic CO2 reduction. After coupling with P25, a macro-residual stress exerted on the surface of BG resulted in cutting nanosheets into smaller pieces. The smaller size of BG is beneficial for increasing the number of exposed edges, and thus increasing the number of active sites for the photocatalytic reaction. Doping graphene with boron cannot only improve its conductivity, but also increase its specific surface area. Attributing to the good electron conductivity and large specific surface area, TiO2-BG showed higher photocatalytic CO2 conversion rate to CH4 than that on TiO2-graphene.

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Fig. 13 Schematic illustration the simulated band structure of g-C3N4 and S-CN.

Meanwhile, g-C3N4 doping can also tune the electronic and physical properties of g-C3N4. Recently, Wang et al. prepared the sulfur-doped g-C3N4 (S-CN) with excellent photocatalytic CO2 reduction activity [74]. The S-CN was prepared by heating thiourea at 520 oC. It was shown that the band structure of g-C3N4 can be tuned by S-doping. In details, the absorption edge of S-CN was red-shifted in comparison with that of g-C3N4. The band gaps of g-C3N4 and S-CN were determined to be 2.7 and 2.63 eV, respectively. A decrease in the band gap value of S-CN indicates that more light can be absorbed and utilized for enhancing photocatalytic CO2 reduction activity. Then, the first-principle calculations based on the spin-polarized density functional theory (DFT) were also performed to determine the density of state of g-C3N4 and S-CN. It was found that g-C3N4 and S-CN have similar simulated band structure. However, an impurity level was found in the case of S-CN (see Fig. 13). Therefore, the photoinduced electron can be easily excited from VB to the impurity state on S-CN and from the impurity state to CB. In addition to the

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possibility of tuning the electronic properties, S-doping generated some surface defects on the structure of S-CN. The presence of small amount of surface defects can act as the trapping centers for the photoinduced electrons, thereby suppressing the electron-hole pair recombination. As a result, the properly tuned S-CN exhibited nearly 2.5 times higher photocatalytic CO2 reduction activity toward methanol production than that obtained for g-C3N4.

5.3 Morphology tuning

Fig. 14 Schematic illustration of the procedure for preparation of G-Ti0.91O2. The morphology tuning of graphene or g-C3N4 represents an effective way for controlling their physicochemical and electronic properties. Various chemical routes have been developed to tune the morphology of graphene and g-C3N4-based photocatalysts for improving their photocatalytic CO2 reduction performance. For example, a special hollow structure with alternating graphene nanosheets and Ti0.91O2

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nanosheets (G-Ti0.91O2) was reported by Tu et al. using layer-by-layer fabrication technique [75]. Particularly, poly(methly methacrylate) (PMMA) spheres were successively modified by protonated polyethylenimine (PEI) aqueous solution, colloidal suspension of negatively charged Ti0.91O2 nanosheets, protonated PEI aqueous solution, and negatively charged GO nanosheets (see Fig. 14). By repeating this procedure, Ti0.91O2 and graphene nanosheets can growth layer-by-layer on PMMA spheres due to the electrostatic interaction between them. Then, the PMMA spheres were removed to create hollow composite structures. The resulting hollow graphene-based structure is beneficial for enhancing light absorption ability because it can act as a photo trap-well by multiscattering light within its structure. Meanwhile, the layer-by-layer structure of Ti0.91O2 and graphene nanosheets assures an intimate contact between both components that facilitates electron migration from Ti0.19O2 to graphene, thereby improving charge separation efficiency. As a consequence, the photocatalytic CO2 reduction activity of G-Ti0.91O2 toward production of CH4 is greatly enhanced in comparison with that of the pure Ti0.91O2. Wang et al. studied the photocatalytic CO2 reduction activity of the ordered cubic mesoporous g-C3N4 (OCM-CN) prepared by a facile chemical vapor deposition method using 3D cubic mesoporous silica KIT-6 as a template and melamine as a precursor [76]. It was shown that the specific surface area of OCM-CN was greatly enhanced in comparison with that of g-C3N4 prepared by direct pyrolysis of melamine at 550 oC, suggesting higher amount of surface sites on the surface of OCM-CN for photocatalytic reaction. Moreover, as comparison to g-C3N4, the ordered OCM-CN

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material showed an enhanced light absorption due to the light trapping within its mesoporous structure, thereby improving its light harvesting ability. Finally, an excellent photocatalytic CO2 reduction activity toward CO production was observed on OCM-CN.

Fig. 15 SEM (a) and TEM (b) images of the helical g-C3N4. Reprinted with permission from ref. 77. Copyright 2014, John Wiley & Sons, Inc. More recently, Zheng et al. reported a helical g-C3N4 (H-CN) for photocatalytic CO2 reduction (Fig. 15) [77]. It is known that the helical structure is interesting because of its optimized morphology and fascinating optical properties. In details, H-CN was prepared by nanocasting method using mesoporous silica as a template. Due to the helical structure, H-CN showed improved light absorption ability across the whole optical spectrum in comparison with that of the bulk g-C3N4 (B-CN) because of the multiple reflection of incident light. The charge carrier separation efficiency of the sample was determined by photoluminescence and electrochemical impedance spectra. It was found that the charge carrier separation efficiency of H-CN was better than that of B-CN. This may be due to 1-D structure distortion of H-CN, which may provide more edges on the sample and preferable electron pathway.

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Finally, H-CN exhibited better photocatalytic CO2 reduction efficiency toward CO production in comparison with that of B-CN due to its improved light absorption ability and charge carrier separation efficiency. According to these studies, the morphology tuning of graphene and g-C3N4 has been proved to be an efficient method to improve the photocatalytic CO2 reduction activity. Furthermore, Cheng and co-workers reported that the exfoliation of g-C3N4 into thin nanosheet layer is an effective strategy for switching the product selectivity of the photocatalytic CO2 reduction reaction [78]. In details, the g-C3N4 nanosheets were prepared by thermally oxidizing and etching of the bulk g-C3N4. By carrying out the UV-vis light absorption test, it was found that the g-C3N4 nanosheets show a higher band gap of 2.97 eV than its counterpart bulk g-C3N4 (2.77 eV). This unique phenomenon is due to the quantum confinement effect which can cause the conduction and valence band shifting in opposite directions [79]. Notably, the g-C3N4 nanosheets exhibit high selectivity for CH4 production while the bulk g-C3N4 tends to produce CH3CHO during the photocatalytic CO2 reduction test. This is because g-C3N4 nanosheets have larger band gap than bulk-g-C3N4 which provides higher driving force of electrons and accelerate the electron transfer rate for CO2 reduction process in comparison with bulk g-C3N4. Furthermore, the increased specific surface area of g-C3N4 nanosheets is also good for the production of CH4 because it can increase the adsorption ability of the g-C3N4 toward intermediate products and promote the consecutive reduction process. This work shows that the morphology control of g-C3N4 is also an effective and simple method to tune the band structure of

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g-C3N4 and the product selectivity for photocatalytic CO2 reduction.

Summary and outlook This short overview of the current literature in the field of carbon-based 2D layered materials (viz. graphene and g-C3N4) highlights their preparation, properties and potential for photocatalytic CO2 reduction applications. In fact, the studies of graphene and g-C3N4 on the photocatalytic CO2 reduction are still in early stage. Therefore, more studies should be carried out to fully explore the potential of graphene and g-C3N4 in photocatalytic CO2 reduction applications. Basically, the future research direction of the graphene and g-C3N4-based photocatalysts should be focused on the following aspects: (i)

CO2 reduction mechanism Investigation of the mechanism of the photocatalytic CO2 reduction is not only of scientific importance, but also provides a rational guidance for optimizing the performance of graphene and g-C3N4–based photocatalysts. The details of photocatalytic CO2 reduction reaction including CO2 adsorption, light absorption, electron-hole pair generation and surface reactant kinetics must be carefully investigated. Moreover, it was found that the loading of graphene or g-C3N4 into photocatalysts can also influence the final product of the CO2 reduction. The effect of graphene and g-C3N4 on the acceleration of the photocatalytic CO2 reduction steps must be carefully studied. Furthermore, the details on the mechanism of formation of the final products of CO2 reduction

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remain unclear and must be further explored. Therefore, integration of theoretical simulations and in situ characterization techniques should be carried out to clarify the photocatalytic CO2 reduction mechanism. (ii)

Modification of graphene and g-C3N4 Modification of the graphene or g-C3N4-based photocatalysts is another important issue affecting the photocatalytic CO2 reduction activity. It was reported that the physiochemical properties of the samples including light absorption capability, specific surface area and charge carrier migration can be easily tuned by modification of graphene and g-C3N4. For example, modification of graphene or g-C3N4 by doping, morphology tuning or surface engineering affords graphene or g-C3N4-based photocatalysts with high specific surface area, good light absorption ability and electron-hole separation efficiency, which are beneficial for enhancing photocatalytic CO2 reduction performance.

Therefore,

modification

of

graphene

or

g-C3N4-based

photocatalysts should be further studied to achieve the optimal photocatalytic CO2 reduction efficiency for practical applications. (iii)

Facile synthetic routes Graphene and g-C3N4 have great potential for the photocatalytic CO2 reduction because they are abundant and inexpensive for practical and large-scale applications. In order to maximize their advantages, facile and robust synthetic routes should be further developed to prepare these materials on the large scale.

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Photocatalytic CO2 reduction over carbon-based 2D layered materials has emerged to be an important research field. Therefore, further studies toward better understanding of the photocatalytic CO2 reduction process and performance of graphene and g-C3N4-based photocatalysts for CO2 are highly desirable. Further development of graphene and g-C3N4-based photocatalysts will not only improve the photocatalytic CO2 reduction activity but also offer great opportunities for various engineering and technology fields.

Acknowledgments This work was partially supported by the 973 program (2013CB632402), NSFC (51320105001, 51372190, 21573170, 51272199 and 21433007), Deanship of Scientific Research (DSR) of King Abdulaziz University (90-130-35-HiCi), Fundamental Research Funds for the Central Universities (WUT: 2015-III-034), Self-determined and Innovative Research Funds of SKLWUT (2015-ZD-1) and the Natural Science Foundation of Hubei Province of China (No. 2015CFA001).

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