Strategies for Efficient Solar Water Splitting ... - Wiley Online Library

DOI: 10.1002/asia.201700540

Focus Review

Solar Water Splitting

Strategies for Efficient Solar Water Splitting Using Carbon Nitride Yilong Yang+,[a, b] Songcan Wang+,[b] Yongli Li,[a] Jinshu Wang,*[a] and Lianzhou Wang*[b]

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Focus Review Abstract: Graphitic carbon nitride (g-C3N4)-based photocatalysts are promising for photocatalytic water splitting to produce clean solar fuels due to their low cost, suitable band structure and excellent photocatalytic performance. This review focuses on the state-of-the-art progress of the strategies for modifying g-C3N4-based photocatalysts toward efficient photocatalytic water splitting. In particular, we high-

1. Introduction Worldwide energy shortage and environmental issues have roused the awareness of energy conservation and environmental protection, stimulating numerous researches on the exploration of renewable and clean energy.[1] Since the first discovery of photoelectrochemical (PEC) water splitting using titanium dioxide (TiO2) as the photoanode,[2] semiconductor-based photocatalysis has attracted global interest.[3] However, the utilization of sunlight for TiO2 is restricted in the UV region due to its large band gap energy of & 3.2 eV, resulting in low theoretical solar-to-hydrogen (STH) conversion efficiency.[4] To efficiently use the solar energy, new visible-light-responsive photocatalysts are required.[5] Moreover, to achieve photocatalytic water splitting (the change of Gibbs free energy is DG0 = 237.2 kJ mol@1 for splitting one molecule of H2O into H2 and 1 =2 O2, corresponding to a required potential of 1.23 V per electron transferred), photocatalysts should possess a conduction band (CB) more negative than 0 V vs. NHE (normal hydrogen electrode at pH 0) and a valence band (VB) more positive than 1.23 V vs. NHE.[6] For practical applications, photocatalysts should be low-cost with excellent long-term stability. In this regard, there are only limited choices of visible light responsive photocatalysts. Graphitic carbon nitride (g-C3N4) is one of the most promising visible light responsive photocatalysts for water splitting due to its suitable band gap (ca. 2.7 eV), appropriate band positions (CB of ca. @1.1 eV and VB of ca. 1.6 eV, vs. NHE), non-toxicity, abundance, and easy fabrication process, which has attracted worldwide attention since its first report in 2009.[7] [a] Y. Yang,+ Prof. Y. Li, Prof. Dr. J. Wang Key Laboratory of Advanced Functional Materials School of Materials Science and Engineering Beijing University of Technology Beijing, 100124 (China) E-mail: [email protected]

[b] Y. Yang,+ S. Wang,+ Prof. Dr. L. Wang Nanomaterials Centre School of Chemical Engineering and Australian Institute for Bioengineering and Nanotechnology The University of Queensland St Lucia QLD 4072 (Australia) E-mail: [email protected] [+] These authors contributed equally to this work. This manuscript is part of a special issue celebrating the 100th anniversary of the Royal Australian Chemical Institute (RACI). Click here to see the Table of Contents of the special issue. Chem. Asian J. 2017, 12, 1421 – 1434

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light the importance of interfacial engineering and nanostructural control to facilitating charge separation and migration. Other strategies including doping and defect engineering are also concisely discussed. Finally, the perspectives on the challenges and future development of g-C3N4-based photocatalysts are presented.

With a graphene analogue structure consisting of C and N atoms, g-C3N4 is considered as the most stable form among various carbon nitrides at ambient conditions, which can be traced back to the prototype of “melon” found by Berzelius and Liebig in 1834.[8] Theoretically, there are two models for carbon nitride distinguished by different nitrogen-bridged aromatic moieties-triazine (C3N3) unit and tri-s-triazine (C6N7) unit in the single layer, as shown in Scheme 1. But in reality, it is very challenging to fabricate an ideal carbon nitride crystal due to the incomplete release of NH3. To be consistent in this review, we name all the experimentally obtained imperfect carbon nitride materials as g-C3N4. Since its discovery, g-C3N4 has been demonstrated for various photocatalytic applications such as H2 evolution,[9] O2 evolution,[10] organic pollutant degradation,[11] and CO2 reduction.[12] In addition to photocatysis, gC3N4 and its derivatives are also good candidates for electrocatalysis, catalysis, sensing, bioimaging, and templating, etc. These new emerging apllications of g-C3N4 are beyond the scope of this review, which we will not discuss in detail and readers may refer to some recent reviews for more information.[13] Scheme 2 shows the ideal scheme of water splitting by g-C3N4. Under light illumination, g-C3N4 photocatalyst absorbs the photons with energy (hn) larger than its band gap energy of 2.7 eV and the electrons are then excited to the CB, leaving holes in the VB.[14] Then, photoexcited electrons and holes are transferred to the surfaces of g-C3N4 photocatalyst. Water molecules are reduced by the electrons to generate H2 and oxidized by the holes to generate O2. Meanwhile, the photoexcited electron-hole pairs are in the energetically unstable states and thus will recombine within a very short time. Although the photoexcited electrons and holes of g-C3N4 possess thermodynamically sufficient potentials for water splitting, three pivotal issues (i.e. light absorption efficiency, charge separation efficiency and surface reaction efficiency) still limit the exploration of efficient g-C3N4-based photocatalysts, which is very difficult to be obtained from pristine g-C3N4. Therefore, it is highly desirable to modify g-C3N4 for efficient solar-driven water splitting. In recent years, there are a number of review articles discussing the synthesis, properties and applications of g-C3N4 based materials,[13a, 15] however, a relatively comprehensive and updated review solely focusing on the strategies of modifying gC3N4 for efficient photocatalytic water splitting is still lacking. Herein, we focus on refining the recent progress of modification strategies for g-C3N4 with enhanced solar water splitting properties. In particular, interfacial engineering and nanostruc-

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Scheme 1. Structural models of g-C3N4 arranged by (a) triazine and (b) tri-s-triazine units.

Yilong Yang received his B.S. (2011) from Jiangxi Science and Technology Normal University and M.S. (2014) from Beijing University of Technology (BJUT), China. He is a PhD student under the supervision of Prof. Jinshu Wang at Beijing University of Technology, China. Now, he is a joint PhD student in Prof. Lianzhou Wang’s group in 2016–2017. His research interests include the design and development of 2D g-C3N4-based materials for photocatalytic/photoelectrochemical energy conversion.

Scheme 2. Schematic illustration of water splitting by g-C3N4.

Songcan Wang received his B.Eng. (2011) and M.Eng. (2014) from Central South University (CSU), China. He is a current PhD student under the supervision of Prof. Lianzhou Wang at School of Chemical Engineering, The University of Queensland (UQ), Australia. His research interests are mainly focused on the design and development of high performance photoanodes for photoelectrochemical energy storage and conversion.

Jinshu Wang is currently Professor and Dean in College of Materials Science and Engineering, Beijing University of Technology, China. She received her PhD degree from Beijing University of Technology in 1999. From 2002 to 2004, she was a Postdoctoral Fellow under the supervision of Prof. T. Sato at Tohoku University, Japan. Wang’s research interests include cathode materials, photocatalysis materials and solar cells.

Yongli Li received his B. S in 1994 from Northwestern University, and M.S. in 1997 from Shaanxi University of Science and Technology. He obtained his PhD in 2003 from Xi’an Jiaotong University. Currently, he is an associate professor at College of Materials Science and Engineering, Beijing University of Technology. His research interests include the synthesis and application of two-dimension photocatalytic materials for photochemistry and efficient solar energy conversion.

Lianzhou Wang is Professor at School of Chemical Engineering and Director of Nanomaterials Centre, the University of Queensland (UQ), Australia. He received his PhD degree from Chinese Academy of Sciences in 1999. Before joining UQ in 2004, he has worked at two national institutes (NIMS and AIST) of Japan for five years. Wang’s research interests include the design and application of semiconducting nanomaterials in renewable energy conversion/storage systems, including photocatalysts.

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Focus Review tural control are efficient to promote charge separation and transfer, which will be mainly discussed. In addition, other strategies such as doping and defect engineering will also be briefly introduced. The challenges and future development of g-C3N4-based photocatalysts for water splitting will be discussed in the conclusion section. We hope to provide the state-of-the-art information for inspiring the design of efficient g-C3N4-based photocatalysts for solar fuel generation.

2. Strategies for modifying carbon nitride Thermodynamically, g-C3N4 possesses the ability for overall water splitting. With a band gap of & 2.7 eV, the theoretical STH efficiency is ca. 6 % under AM 1.5 G illumination.[16] However, the highest reported STH efficiency for g-C3N4 based photocatalyst is & 2 % with some debates,[17] indicating that g-C3N4 suffers severe charge recombination. Therefore, effectively suppressing charge recombination is the key to further improve its STH efficiency. Generally, interfacial conditions, nanostructures, and crystallinity strongly affect charge separation of a semiconductor. Proper interfacial conditions can promote charge separation and accelerate surface kinetics. Likewise, suitable nanostructure is beneficial for the migration of photoexcited electrons and holes to the surface for water splitting reactions. On the other hand, crystal defects usually play a role in trapping charge carriers and serve as recombination centers. Thus, higher crystallinity usually exhibits increased photocatalytic activity. In this section, we will mainly discuss the mechanism and recent progress of interfacial engineering and nanostructural control for modifying g-C3N4-based photocatalysts with enhanced photocatalytic water splitting performance. Moreover, other emerging strategies in recent years will also be concisely introduced at the end of this section.

ing characteristics for solar energy conversion. For example, the water oxidation reaction is the key challenge for overall water splitting because it is a four-electron process. If water is first oxidized to H2O2 and H2 via a two-electron process, followed by another two-electron process for decomposing H2O2 to O2 and H2O, higher efficiency can be achieved.[17] CDs are excellent catalyst for H2O2 decomposition. Therefore, a CDs-gC3N4 (CDCN) nanocomposite showed impressive performance for overall water splitting under visible light illumination (Figure 1). The apparent quantum efficiencies (AQEs) in the wavelengths of 420 : 20 nm, 580 : 15 nm, and 600 : 10 nm were 16 %, 6.29 % and 4.42 %, respectively, with a STH efficiency up to 2.0 %. In addition, graphene oxide quantum dots/gC3N4 also showed enhanced photocatalytic activity.[20]

2.1. Interfacial engineering Amongst the available strategies, modifying the interfaces of a semiconductor by introducing other materials such as quantum dots, cocatalysts, and hetero/homo semiconductors, etc., has attracted much attention due to its great potential in separating photoexcited charges as well as improving the photocatalytic activities.[18] Based on the species introduced to combine with g-C3N4, the state-of-the-art interfacial engineering for g-C3N4 can be generally divided into five types: carbon/g-C3N4 junction, cocatalyst/g-C3N4 junction, polymer/g-C3N4 junction, semiconductor/g-C3N4 junction, g-C3N4/g-C3N4 isotype junction. All of the classification mentioned above will be discussed in detail in the following sections. 2.1.1. Carbon/g-C3N4 junction With the p-conjugated structures, carbonaceous nanomaterials exhibit many unique optical and electric properties, which has attracted great attention as effective supports to couple with photocatalytic nanomaterials to retard the recombination of the photogenerated electron-hole pairs.[19] Loading carbon dots (CDs) onto the exterior surfaces of g-C3N4 shows fascinatChem. Asian J. 2017, 12, 1421 – 1434

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Figure 1. (A) TEM image of CDots-C3N4. (B) A high magnification TEM image of the CDots-C3N4 in red region of (A). (C) HRTEM image of a single CDot in red region of (B). (D) Corresponding FFT pattern of the crystallite in (C). (E) Cycling H2 and O2 generation from water at l > 420 nm by CDots-C3N4 (1.6 V 10@5). (F) QE of different concentrations of CDots in CDots-C3N4 composites. The inset shows a magnified curve of the low concentrations of CDots in the red region of (F). Reprinted with permission from ref. [17]. Copyright T 2015, American Association for the Advancement of Science.

Due to the similar aromatic structure, carbon based materials such as carbon ring, graphene, carbon nanotube (CNT) and carbon framework (CFW) can be seamlessly connected with gC3N4 to form C-CN junctions via continuous p-conjugated bond, which can enhance the electron transfer efficiency, reduce charge recombination, resulting in improved photocatalytic activity.[21] For example, carbon was in situ grown with an interconnected framework of mesoporous g-C3N4 nanofibers, which exhibited an extremely high H2-evolution rate of 16 885 mmol h@1 g@1, and a remarkable AQE of 14.3 % at 420 nm without any cocatalysts.[21a] Likewise, g-C3N4 modified by CFW

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Focus Review was constructed through a facile thermal condensation process.[21b] The obtained CFW/g-C3N4 composites exhibited obviously enhanced photocatalytic H2 production rates compared to pure g-C3N4 under visible light irradiation. Carbonaceous nanomaterials with p-conjugative structures can be combined with g-C3N4 due to their special structures and unique electron conducting ability.[21c] In particular, a two-dimensional carbon ring (Cring)/g-C3N4 in-plane heterostructure promoted the photoexcited electrons transporting through the confinement inplane p-conjugated electric field, resulting in highly efficient overall water splitting.[21e] As a result, the in-plane heterostructural (Cring)-g-C3N4 nanosheets could efficiently split pure water under light irradiation with a H2 production rate up to 371 mmol g@1 h@1 and a notable AQE of 5 % at 420 nm. In addition, CNT/g-C3N4 composite also showed enhanced H2 evolution.[21d] 2.1.2. Cocatalyst/g-C3N4 junction Cocatalysts can greatly promote surface kinetics, decrease over potentials of H2/O2 evolution, accelerate the separation of photogenerated electron/hole pairs and strengthen the stability of semiconductors. Loading cocatalyst on the surface of g-C3N4 can highly improve the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) for water splitting. Based on the different effect on the reaction, cocatalyst can be divided into hydrogen evolution cocatalyst (HEC) for water reduction and oxygen evolution cocatalyst (OEC) for water oxidation. These two kinds of junction are marked as HEC/CN junction and OEC/CN junction, respectively. Noble metals such as Ag, Au, Pd and Pt have been considered as fascinating HECs for H2 evolution.[22] In addition to their function as HECs, noble metal nanoparticles (NPs) can also im-

prove light harvesting due to their localize surface plasmon resonance (LSPR) effect.[18] For instance, silver quantum cluster (Ag9) was grafted on the surface of g-C3N4 for photocatalytic hydrogen generation, which exibited enhanced visible light absorption.[22a] Metal-to-ligand charge transfer (MLCT) between Pt2 + and g-C3N4 played an important role in photocatalysis, as demonstrated in Figure 2.[22b] The microscopic frame structure of g-C3N4 as a polymeric ligand provided N coordination sites filled with six lone-pair electrons, supplying ideal sites for the inclusion of Pt cation, which exhibited dramatic photocatalytic performance under full-light spectrum illumination. Due to their rare and expensive nature, the utilization of noble metal should be as little as possible. In this regard, Li et al. reported single-atom Pt as a cocatalyst for photocatalytic H2 evolution.[22c] The isolated single Pt atoms were anchored on g-C3N4 (Pt-CN) via a simple liquid-phase reaction leading to high dispersion and stability, which achieved the maximum utilization of Pt atoms and remarkably enhanced the photocatalytic H2 evolution activity. As a result, nearly 50 times higher photocatalytic activity was observed compared to bare g-C3N4. Although noble metals are excellent HECs for water splitting, the utilization of noble metals is not suitable for scale-up applications because of the high cost. During the past decades, earth abundant HECs such as Ni, MoS2 and WS2 etc. have been developed, which also showed excellent photocatalytic water splitting performance.[23] For example, loading Ni0 on g-C3N4 exhibited high photocatalytic activity and stability.[23a] With the optimized amount of Ni (7.40 wt %), the photocatalytic H2 production rate of Ni/g-C3N4 was increased to 4318 mmol g@1 h@1 and no noticeable decrease was observed during 4 runs for 48 h in an aqueous triethanolamine solution. Even under natural sunlight illumination, the Ni/g-C3N4 composite still exhibited a H2 generation rate of 4000 mmol g@1 h@1.

Figure 2. (a) Structural illustration for the Pt2 + linking with g-C3N4. (b) k3-weighted Fourier-transform Pt L3-edge EXAFS spectrum of g-C3N4-Pt2 + (0.18 wt %) in the reference of Pt foil. (c) Metal-to-ligand charge transfer (MLCT) progress: from the state of Pt2 + -induced hybrid HOMO to the LUMO of g-C3N4-Pt2 + . (d) H2 generation rates with g-C3N4-Pt2 + (0.18 wt %) under l > 400 nm, 400 nm < l < 470 nm and l > 470 nm. (e) Cycling photocatalytic H2 generation of g-C3N4-Pt2 + (0.18 wt %) under l > 400 nm. Reprinted with permission from ref. [22b]. Copyright T 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. Chem. Asian J. 2017, 12, 1421 – 1434

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Focus Review Due to the sluggish kinetics, OER is much more challenging compared to HER. Thus, exploring high performance OECs is also important for efficient water splitting. Generally, noble metal oxides such as RuO2 and IrO2 are excellent OECs for photocatalytic oxygen evolution.[24] Similar to HECs, low-cost OECs including cobalt compounds (e.g. Co@Pi, Co(OH)2, CoOx, Co3O4), NiFe, and MnO2 have been developed to replace noble metal oxides.[25] As one of the most effective OECs, Co-Pi has been used to improve the photocatalytic performance of gC3N4, with the corresponding H2 and O2 evolution rates that are 9.6 and 6.8 times higher than the unmodified g-C3N4, respectively.[25a] Similarly, Co3O4/g-C3N4 hybrid photocatalyst achieved an AQE of 1.1 % at 420 nm for water oxidation.[25c] In addition, ultrafine cobalt species,[25d] and NiFe/N-doped graphene[25e] are also efficient OECs for g-C3N4. Furthermore, the ternary system of cocatalyst and g-C3N4based composites has also been developed for H2 evolution and even for overall water splitting.[26] CdS nanorods/g-C3N4 heterojunctions loaded with NiS cocatalyst were fabricated via an in situ hydrothermal method. When loading 9 % of NiS, the ternary hybrid materials showed the best H2-production rate of 2563 mmol h@1 g@1, which was 1582 times higher than that of the pristine g-C3N4. The enhanced photocatalytic activity was ascribed to the synergistic effects of NiS cocatalyst and the formation of intimate heterojunctions between 1D CdS nanorods and 2D g-C3N4 nanosheets, accelerating charge transfer and separation, and promoting the surface H2-evolution kinetics. Most of the reported g-C3N4 based photocatalysts only use HECs or OECs to promote HER or OER half reactions. Loading both HECs and OECs as dual cocatalysts is important for overall water splitting. However, it is challenging to reasonably control the balance between HECs and OECs in one semiconductor. For H2 production, Pt is still the most commonly used HEC in g-C3N4 based photocatalysts. Therefore, it is necessary to devel-

op earth-abundant cocatalysts with competitive performance with noble metals or noble metal oxides. 2.1.3. Polymer/g-C3N4 junction As conducting polymers exhibit excellent electrical, chemical and optical properties, some reprehensive polymers have been employed to modify g-C3N4 with enhanced photocatalytic activity.[23d, 27] For instance, a new type of g-C3N4-based polymer composite for enhanced photocatalytic H2 evolution was reported, as shown in Figure 3.[27b] g-C3N4 photocatalyst was modified by co-loading poly(3,4-ethylenedioxythiophene) (PEDOT) as a hole transport pathway and Pt as an electron trap. The as-prepared C3N4-PEDOT-Pt composites showed drastically enhanced activity for visible light driven H2 production compared to C3N4-PEDOT and C3N4-Pt, possibly due to the spatial separation of the reduction and oxidation reaction sites. 2.1.4. Semiconductor/g-C3N4 junction Introducing another semiconductor to g-C3N4 can induce builtin electrical potential in the interfaces, which drives the separation of photoexcited electrons and holes. Based on the different components modifying g-C3N4, various semiconductor junctions including organic metal molecule/CN junction,[28] polyoxometallate/CN junction,[29] metal oxide/CN junction[30] and sulphides/CN junction[31] will be discussed in detail in the following section. A heterostructure of m-oxo dimeric iron(III) porphyrin [(FeTPP)2O]/g-C3N4 formed with interactions between the p–p and the Fe-amine was fabricated.[28] It was demonstrated that (FeTPP)2O not only acted as a photosensitizer, but also played the role in prohibiting charge recombination. The obtained pure organic g-C3N4/(FeTPP)2O heterostructure exhibited dra-

Figure 3. (a) Schematic illustration of the preparation process of the C3N4-PEDOT-Pt composite and the proposed photocatalytic reaction. (b) H2 evolution rates of a. pure g-C3N4, C3N4 with PEDOT loading of b. 0.5 wt %, c. 1 wt %, d. 2 wt %, and e. 5 wt % and (f) C3N4-2 wt % PEDOT without Pt. (c) The cycling of H2 evolution of C3N4-PEDOT-2 wt %. Reprinted with permission from ref. [27b]. Copyright T 2015 The Royal Society of Chemistry. Chem. Asian J. 2017, 12, 1421 – 1434

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Figure 4. (a) Schematic illustration of possible formation mechanism of g-C3N4/InVO4. (b) Photocatalytic H2 evolution under l > 420 nm for different samples. (c) Schematic illustration of band alignment of g-C3N4/InVO4 composites under visible light irradiation. Reprinted with permission from ref. [29a]. Copyright T 2015, American Chemical Society.

maticaly enhanced photocatalytic H2 production under solar light illumination without any cocatalysts, compared to pure gC3N4, (FeTPP)2O and mixed g-C3N4 and/or (FeTPP)2O. In situ hydrothermally grown InVO4 nanoparticles onto the surface of gC3N4 exbhited enhanced photocatalytic H2 production (Figure 4).[29a] The obtained g-C3N4/InVO4 (with a mass ratio of 80:20) nanocomposites achieved effective separation of charge-hole pairs and stronger reducing power, which possessed the maximum photocatalytic activity of 212 mmol h@1 g@1 for H2 evolution, corresponding to an AQE of 4.9 % at 420 nm. In addition, vanadate (AgVO3, BiVO4, InVO4 and CuV2O6) QDs/g-C3N4 heterojunctions, g-C3N4/Ca2Nb2TaO10 and g-C3N4/(Co, Ni)Fe2O4 heterojunctions also showed enhanced H2 evolution under visible light irradiation.[9c, 29b,c] Metal oxide photocatalysts are one of the most important materials in the photocatalytic field. The representative metal Chem. Asian J. 2017, 12, 1421 – 1434

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oxide/CN junctions such as Ag2O/CN,[30a] CeO2/CN,[30b] Cu2O/ CN,[30c] TiO2/CN,[30d] and TiO2/In2O3/CN ternary heterojunction[30e] have been fully studied. For instance, Ag2O/g-C3N4 showed efficient photocatalytic H2 evolution under visible light irradiation, which was about 274 times higher than that of pure g-C3N4, and was even much better than that of Pt/g-C3N4.[30a] More importantly, Ag2O/g-C3N4 also demonstrated relatively good photocatalytic H2 evolution even after 16 h of recycling measurement. In addition, introducing other semiconductor into the gC3N4-based heterojunction is also effective to further improve the photocatalytic activity. For example, the H2 generation rate of novel TiO2-In2O3@g-C3N4 ternary system was increased by 48 times than that of pure g-C3N4.[30e] The interfacial transfer of charge carriers among TiO2, In2O3 and g-C3N4 contributed to effective charge separation and enhanced activities. Furthermore, a special Z-Scheme junction can be directly used in

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Focus Review overall water splitting. For example, a one-step hydrothermal process was developed to synthesize TiO2/g-C3N4 heterojunctions.[30f] With the redox mediators of I@/IO3@ , and Ni(OH)2/WO3 loaded as the oxygen evolution photocatalysts, Z-scheme overall water splitting can be achieved. Due to the layer structure and appropriate band gap, sulfides have attracted much interest in photocatalysis. Two-dimensional CaIn2S4/g-C3N4 heterojunction with intimate interfacial contact were synthesized by a facile two-step method, which exhibited significantly enhanced H2 evolution under visible light irradiation.[31a] It was believed that the enhanced photocatalytic performance mainly stemmed from the enhanced charge separation efficiency between CaIn2S4 and g-C3N4. 2.1.5. g-C3N4/g-C3N4 isotype junction To promote charge separation, an effective method mentioned above is to combine g-C3N4 with guest semiconductors to construct heterojunctions, which can induce opposite transfer of photoexcited electrons and holes.[32] Due to such configurations built from two distinct semiconductors, an efficient heterojunction is not only determined by their energy band matches but also affected significantly by their crystal structures, surface/interface properties and other characteristics, which makes it difficult for practical applications. In contrast, homojunction photocatalysts composed of the same semiconductor materials are much favorable for charge transfer across the interface and facilitating charge separation. In this regard, gC3N4-based organic isotype junction photocatalysts have also been developed.[33] For instance, defect-modified g-C3N4 (DCN) photocatalysts was prepared via a NaBH4 treatment, which showed much extended light absorption with band gaps de-

creased from 2.75 to 2.00 eV (Figure 5).[33a] Cyano terminal C/N groups, acting as electron acceptors, were introduced into the DCN sheet edge, which endowed the DCN with both n-type and p-type characteristics, generating p-n homojunctions. This isotype junction structure was demonstrated to be highly efficient in charge transfer and separation and resulted in a 5-fold enhanced photocatalytic H2 evolution activity. In addition, other g-C3N4/g-C3N4 junctions such as tubular g-C3N4 isotype heterojunction (TCNH),[33b] roll-like g-C3N4 photocatalyst,[33c] and g-C3N4/S-mediated g-C3N4 (CN/CNS) isotype heterojunctions have also been reported.[33d] Based on the discussion above, we can briefly summarize the obvious advantages of the g-C3N4-based heterojunctions for water splitting, including more efficient charge transfer and separation, longer time of charge carriers and spatial separation of the reduction and oxidation reactions. Nevertheless, the disadvantages cannot be ignored. For instance, the redox abilities of the photoexcited electrons and holes will normally become weaker, some range of light absorption in the heterojunctions is often blocked, the ratio of different components cannot be accurately controlled, and the interfacial charge transfer is challenging. Thus, further development of heterojunctions should focus on solving the above mentioned issues towards better photocatalytic efficiency. 2.2. Nanostructural control With large surface area and small particle size, nanostructured materials can provide more sites for the redox reactions, increase absorption coefficient, decrease the diffusing distance of electrons and holes to the reaction sites, and reduce light reflection, etc.[34] Moreover, different nanostructures also show

Figure 5. (a–b) SEM images of CN and DCN-350; with insets showing corresponding digital graphs of (a–b), respectively. (c) Schematic illustration of the preparation process of DCN. (d) Schematic illustration of charge transfer in the p-n homojunction of DCN. (e) Cycling of H2 evolution for pristine CN and DCN-200 under visible-light irradiation. Reprinted with permission from ref. [33a]. Copyright T 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Chem. Asian J. 2017, 12, 1421 – 1434

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Focus Review various optical, physical and chemical properties. In recent decades, the structure–activity relationship of g-C3N4 materials has aslo been explored. According to the obtained nanostructures, nanostructural control can be primary classified to 1D rod-like nanostructures, 2D sheet-like nanostructures, and 3D sphere-like nanostructures based on their dimensional structures. 2.2.1. 1D Rod-like nanostructure Compared to bulk materials, 1D nanostructures with large surface area and high length/diameter ratio can shorten the charge diffusion length and increase the interaction with light and electrolyte, resulting in fast charge carrier transport and enhanced light harvesting.[35] For example, nanoporous 1D gC3N4 microrods showed 26 times higher photocatalytic H2 evolution performance (34 mmol g@1) than its bulk counterpart (1.26 mmol g@1).[35] Furthermore, the photocurrent stability of this 1D nanoporous g-C3N4 over 24 h indicated excellent photocorrosion resistance. The improved photocatalytic activities were attributed to prolonged carrier lifetime due to its quantum confinement effect, promoting separation and migration of charge carriers, as well as the increased reaction sites all over the microrods. The bottom-up fabrication of g-C3N4 nanorods was realized through the direct infrared heating of dicyandiamide with no extra templates or organics, which also showed enhanced photocatalytic activity.[36] Phosphorus-doped hexagonal tubular carbon nitride (P-TCN) with the layered stacking structure was obtained from a hexagonal rod-like single crystal supramolecular precursor prepared by self-assembly of melamine with cyanuric acid under phosphorous acidassisted hydrothermal conditions.[37] The tubular structure led to enhancement of light scattering and active sites. Thus, the P-TCN exhibited a high H2 evolution rate of 67 mmol h@1 (0.1 g catalyst, l > 420 nm) in the presence of sacrificial agents, and an AQE of 5.68 % at 420 nm. With anodic alumni oxide (AAO) as templates, 1D g-C3N4 was obtained with increased crystallinity, extended domain size and lowered HOMO position.[38] Similarly, g-C3N4 nanorods (CNRs) were also facilely fabricated by using chiral mesostructured silica nanorods as template.[22d, 39] 2.2.2. 2D sheet-like nanostructure In addition to large surface area, 2D nanosheets possess extremely thin thickness and numerous surface groups for anchoring cocatalysts or other materials, which makes this unique structures promising for fabricating functional hybrid materials.[40] Since the discovery of 2D monolayer graphene, a number of 2D photocatalysts including graphene oxide,[41] MoS2,[42] phosphorus,[43] etc. have been developed. Compared to the above mentioned 2D photocatalysts, 2D g-C3N4 exhibits tremendous advantages such as good chemical stability, unique surface properties, and proper band edge positions for water splitting, which has attracted increasing attention in recent years.[44] For instance, free-standing g-C3N4 nanosheets were synthesized by liquid phase exfoliation using isopropanol (IPA) as the solvent.[44a] The resulting g-C3N4 nanosheets with Chem. Asian J. 2017, 12, 1421 – 1434

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& 2 nm thickness, large aspect ratios, high surface area, and stoichiometric N/C ratio not only provided abundant reactive sites for H2 evolution, but also promoted charge separation/ transfer. Consequently, the g-C3N4 nanosheets exhibited very good photocatalytic performance for H2 evolution under visible light illumination. Crystalline 2D g-C3N4 nanosheets were obtained by a one-step liquid exfoliation of the layered bulk material in water.[44c] The obtained nanosheets with 1–2 nm in height formed chemically and colloidally stable suspensions under both basic and acidic conditions and showed significantly enhanced visible-light driven photocatalytic activity toward H2 evolution compared to the bulk counterpart, which was ascribed to the crucial role of morphology and surface area on the photocatalytic performance of g-C3N4 materials. Li et al. prepared macroscopic foam-like holey ultrathin g-C3N4 nanosheets (CNHS) fulfilled with micro-, meso-, and macropores via long-time heating bulk g-C3N4 under air atmosphere (Figure 6).[44d] The CNHS exhibited superior performance in H2 evo-

Figure 6. (a) Schematic illustration of the preparation of foam-like holey ultrathin g-C3N4 nanosheets. (b) H2 evolution rate of different samples. (c) Cycling of H2 evolution for CNHS. Reprinted with permission from ref. [44d]. Copyright T 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

lution which was attributed to the holey 2D ultrathin structure and the in-plane holes of g-C3N4 nanosheets. These features enlarged specific surface area, exposed more new edges and catalytic active sites, improved the electron transport ability, and were also beneficial for the rapid cross-plane diffusion of photogenerated carriers. 2.2.3. 3D Sphere-like nanostructure 3D sphere-like nanostructures also exhibit high accessible surface area and can serve as a host scaffold for designing multilayer shell structures, which inhibits the restacking and aggregation of the subunits, increases the light scattering, and promotes surface dependent reactions.[45] Thus, creating 3D sphere-like nanostructure on g-C3N4 is considered as a promising approach to promote directional charge separation towards high-performance photocatalysts.[46] For example, the

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Figure 7. (a) Schematic illustration of the synthesis of HCNS and metal/HCNS composite. (b) The large-area TEM image of HCNS-3; scale bar = 1 mm. (c) The cycling test of the 3 wt % Pt/HCNS-3 (50 mg) sample under l > 420 nm irradiation. Reprinted with permission from ref. [46a]. Copyright T 2012 Macmillan Publishers Limited.

synthesis of hollow nanospheres sized melon or g-C3N4 polymer was reported using silica nanoparticles as templates (Figure 7).[46a] These hollow nanospheres were beneficial for lightharvesting with highly improved photocatalytic activity for H2 generation, corresponding to an AQE of 7.5 %. Likewise, Zheng et al. developed a facile method to fabricate hollow g-C3N4 spheres (HCNS) modified by Pt and Co3O4 nanoparticles (NPs) onto the interior and exterior surface, respectively, to further promote the surface redox functions for enhancing photocatalytic water splitting activity.[46b] The enhanced activity was attributed to the orientation transfer of photoexcited electron/ hole pairs and spatially separated reactive sites for the evolution of H2 and O2, thereby decreasing the unwanted charge recombination and prohibiting the side reaction of water synthesis. Another g-C3N4 photocatalyst with highly open-framework assembled from 2D nanosheets was fabricated using KCC1 silica as the template.[46c,d] With unique nanostructure of the nanosheets beneficial for charge separation, highly improved H2 evolution was observed on 3 wt % Pt/g-C3N4 with an AQE of 9.6 % at 420 nm. In addition, g-C3N4 microsphere photocatalysts can also be prepared via a template-free solvothermal approach with post-heating treatment.[46e] Although a large array of structures have been explored, the uniformity and stability of existing materials are still far from satisfying the sustainable applicaiton. Various factors such as low crystallinity, surface defects and structural collapse affect the charge transfer and separation, surface kinetics of redox reactions and durability in the whole system. Accordingly, great effort is still required to control the uniform and surface properties of g-C3N4. In addtion, post thermal-treatment may lead to enhanced crystallinity, stability and total efficiency. 2.3. Doping Doping is considered as a promising strategy to tune the band structure, enhance the light adsorption and improve the performance of photocatalysts.[47] Some non-metal elements such as boron,[48a] fluorine,[48b] bromine,[48c] iodine,[48d] nitrogen,[48e] oxygen,[44b, 48f] phosphor[37, 48g] and sulfur[48h] have been developed as efficient dopants to modify g-C3N4 with high perChem. Asian J. 2017, 12, 1421 – 1434

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formance. For instance, Boron-doped carbon nitride was synthesized for photocatalytic hydrogen production.[48a] Sodium tetraphenylboron (Ph4BNa) was used as a doping source for the “bottom-up” polycondensation of urea, and boron atoms can modify g-C3N4 nanosheets in situ to tune the surface chemistry by imparting acid sites on the surface of g-C3N4 materials. Br-doped g-C3N4 semiconductors was synthesized via a facile method for photoredox water splitting.[48c] The modification of Br into g-C3N4 modulated the texture, optical absorption, conductivity, charge-carrier transfer rate, as well as photocatalytic performance, without destroying the major construction and architecture of the g-C3N4 polymer. Nitrogen selfdoped g-C3N4 (N-g-C3N4) was synthesized by using melamine pre-treated with hydrazine hydrate to provide more nitrogen.[48e] The average hydrogen evolution rate for N-g-C3N4 was 1.8 times higher than that of pristine g-C3N4, due to the greatly improved optical, emission and electronic properties. Novel porous P-doped g-C3N4 nanosheets showed a high visible-light photocatalytic H2-production rate of 1596 mmol h@1 g@1 and an AQE of 3.56 % at 420 nm, representing one of the most highly active metal-free g-C3N4 photocatalysts.[48g] Sulfur-doped gC3N4 was presented with a unique electronic structure, which showed a H2 production rate 7.2 and 8.0 times higher than those of C3N4 under l > 300 and 420 nm illumination, respectively (Figure 8).[48h] With abundant orbital electrons, metal elements can aslo affect the optical and electric properties of gC3N4. The impregnation of cobalt ions in g-C3N4 framework was investigated to liberate O2 from water at soft materials interface.[48i] Iron doped g-C3N4 was prepared via a mild one-pot method.[48j] The as-prepared photocatalyst presented a high visible light-driven hydrogen evolution of ~ 16.2 mmol g@1 h@1 and an AQE of 0.8 %. Although doping provides useful ideas for designing efficient g-C3N4 photocatalysts, some issues still exist. For example, lower oxidizing and reducing ability along with narrowing the band gap, the presence of surface trapping center, or difficult to determine the doping site etc. Thus, further developing new doping methods and keeping a good balance of lower redox capability to achieve a higher photocatalytic activity are highly desirable.

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Figure 8. (a) Schematic illustration of band structure evolution of S doped g-C3N4. (b) Atomic structure model of melem units substituted by sulfur atom marked with blue spheres. (c) The cycling of H2 evolution by Pt-deposited C3N4 (a) and C3N4@xSx (b) under l > 300 and 420 nm, respectively. Reprinted with permission from ref. [48h]. Copyright T 2010, American Chemical Society.

2.4. Defect engineering In addition to interfacial engineering, nanostructural control, and doping, introducing defects in g-C3N4, namely defect engineering, is also efficient to improve its photocatalytic activity. Such strategies including the introduction of carbon vacancies,[49] nitrogen vacancies,[50] dye,[51] cyanamide defects,[52] reducing defects,[53] protonation,[54] alkalinized treatment,[55] vacuum heat-treatment,[56] oxygenation,[57] amorphization[58] and breaking of hydrogen bonds[59] have also been investigated in recent years. Creating vacancies in g-C3N4 is also efficient for enhanced photocatalytic water splitting. For example, carbon vacancies were successfully introduced into holey g-C3N4 (HGCN) nanosheets via a thermally treating method, which exhibited a photocatalytic H2 evolution rate 20 times higher than that of bulk g-C3N4 (BGCN).[49] Nitrogen-defective g-C3Nx was fabricated via a novel one step KOH-assisted approach during the thermally polymerizing precursors.[50b] The introduction of nitrogen defects could broaden the g-C3Nx absorption region which could be easily tuned by the KOH/urea ratio. Due to enhanced visible-light absorption and promoted charge-carrier separation, the obtained g-C3Nx exhibited superior photocatalytic H2 evolution performance compared to pristine g-C3N4 under visible light illumination. The g-C3N4 polymer filled with cyanamide defects yielded the H2 evolution rate and AQE (400 nm) of 12 and 16 times higher than the unmodified melon, respectively.[52a] Computational modelling and material characterization demonstrated that these defects improved coordination to the Pt co-catalyst and enhanced the separation of the photoexcited electrons and holes. In parallel, introducing defects in gC3N4 was an efficient method to enhance photocatalytic H2 generation, which reduced the band gap from 2.7 to 2.0 eV.[52b] This defective g-C3N4 was synthesized by introducing H2 gas in the thermally condensing precursors. With this defect engineered g-C3N4, it showed 4.8 times higher than the pristine gC3N4 for H2 generation under visible light irradiation. Furthermore, a facile melamine-based defect-remedying strategy to Chem. Asian J. 2017, 12, 1421 – 1434

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enhance photocatalytic performance by remedying the amino groups and triazine structure inside g-C3N4 was also reported.[53] In addition, melamine remediation induced the formation of g-C3N4/mpg-C3N4 junctions, which also favored charge separation and electron transfer during the photocatalytic reaction. The obtained g-C3N4 exhibited a H2 evolution rate that was 6.5 times of that of the pristine g-C3N4. An amorphous g-C3N4 (ACN) with a band gap of 1.90 eV was obtained by simply heating partially crystalline g-C3N4 (GCN) with a band gap of 2.82 eV (Figure 9).[58] Due to synergistic effect of a wider photoabsorption range and suppressed charge carrier recombination, the obtained photocatalyst exhibited a much superior activity in H2 evolution than that of the GCN. Kang et al. also reported a g-C3N4 with significantly enhanced photocatalytic H2 evolution activity under visible light, modified by optimally breaking hydrogen bonds in the layered g-C3N4.[59] As a result of volume shrinkage with the breaking of hydrogen bonds, abundant pores were formed throughout the whole particles. Due to the destruction of intralayer long-range atomic order and loss of partial nitrogen atoms, band tails or localized states close to the band edges was increased. Both of the strategies mentioned above not only promoted the charge separation/transfer, but also enhanced visible light absorption.

3. Conclusions and Outlook With excellent aqueous stability and suitable band gap, g-C3N4 is a promising visible-light-driven photocatalyst for water splitting. Nevertheless, the photocatalytic activity of pure g-C3N4 is low, due to severe charge recombination and poor surface kinetics. Thus, strategies for modifying g-C3N4 toward efficient solar water splitting are of significant importance. This review provides a critical overview of the state-of-the-art strategies for the modification of g-C3N4-based photocatalysts for water splitting. In particular, we categorized a number of interfacial engineering approaches to design efficient g-C3N4 based heterostructures, including carbon/g-C3N4 junction, cocatalyst/g-C3N4

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Figure 9. (a–b) Schematic of crystalline GCN and ACN monolayer. (c) XPS valence band spectra. (d) Band alignments comparing GCN with ACN. (e) Photocatalytic H2 evolution with GCN and ACN under visible light (l > 440 nm). (f) Wavelength-dependent H2 evolution with ACN. Reprinted with permission from ref. [58]. Copyright T 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

junction, polymer/g-C3N4 junction, semiconductor/g-C3N4 junction, and g-C3N4/g-C3N4 isotype junction. These heterostructures have exhibited their features in accelerating charge separation and transportation, prolonging charge carrier lifetime and promoting surface reaction kinetics. Moreover, nanostructures such as 1D rod-like nanostructures, 2D sheet-like nanostructures, 3D sphere-like nanostructures and some other special nanostructures also affect the photocatalytic water splitting performance of g-C3N4. Other strategies such as doping and defect engineering are also effective to enhance the photocatalytic activity of g-C3N4. Although significant progress has been achieved in g-C3N4based photocatalysis, the research in this field is at its early stage from the perspective of practical applications. The main challenges that must be overcome for g-C3N4-based photocatalyst involve: 1) the fundamental mechanisms of photocatalytic enhancement for the g-C3N4-based photocatalytic systems is complicated or even unclear in some systems. Some in situ characterization to monitor the photocatalytic reaction and interfacial engineering can be very powerful. 2) Narrowing the band gap can induce a broader photo-absorption range, but reduced redox capability of g-C3N4 due to the sacrifice of potential energy. It is still challenging to keep the balance of various factors to achieve the optimal photocatalytic activity. 3) Some explanations of photocatalytic activity on the g-C3N4based photocatalyst are still controversial or even inconsistent. Therefore, more studies are required for developing the intrinsic understanding of the enhancement of g-C3N4 based nanocomposites, especially employing some advanced characterization techniques. Also, it is important to develop standardized conditions to measure the photocatalytic activity including irradiation intensity, reaction volume, reactor design and content of cocatalyst, etc. To solve these issues, much effort is still needed. Heteroatom-doping can be a feasible method to adjust the photocatalytic ability of g-C3N4-based photocatalysts and the refined theoretical calculations with respect to the doping effect are desired. With regard to heterogeneous phoChem. Asian J. 2017, 12, 1421 – 1434

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tocatalysts, the structural property is one of the most critical factors that affect the whole photocatalytic performance, because it largely determines the efficiency of mass transfer and the number of reaction sites. From the perspective of practical applications, it is anticipated that g-C3N4-based photocatalysts will receive further research attention and may play a more important role in the field of solar-driven water splitting.

Acknowledgements This work is financially supported by National Science Foundation of China (No. 51471006, No. 51534009, No. 51225402), Beijing Natural Science Foundation (No. 2151001, 2154043), Scientific research project of Beijing Municipal Education Commission (No. KM201610005026) and Beijing High-level Talents program (2017). The financial support from Australian Research Council through its DP and FF programs is also highly appreciated. Y. Yang acknowledges the financial support from China Scholarship Council. S. Wang acknowledges the support from Australian Government Research Training Program and UQ Centennial Scholarships.

Conflict of interest The authors declare no conflict of interest. Keywords: doping · graphitic carbon heterojunctions · nanostructures · water splitting

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