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Review

Graphitic Carbon Nitride Materials for Photocatalytic Hydrogen Production via Water Splitting: A Short Review

by
Seong Jun Mun
and
Soo-Jin Park
*
Department of Chemistry, Inha University, 100 Inharo, Incheon 22212, Korea
*
Author to whom correspondence should be addressed.
Catalysts 2019, 9(10), 805; https://doi.org/10.3390/catal9100805
Submission received: 19 August 2019 / Revised: 19 September 2019 / Accepted: 23 September 2019 / Published: 25 September 2019
(This article belongs to the Special Issue New Trends in Photo(Electro)catalysis: From Wastewater Treatment to Energy Production)
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Abstract

:
The generation of photocatalytic hydrogen via water splitting under light irradiation is attracting much attention as an alternative to solve such problems as global warming and to increase interest in clean energy. However, due to the low efficiency and selectivity of photocatalytic hydrogen production under solar energy, a major challenge persists to improve the performance of photocatalytic hydrogen production through water splitting. In recent years, graphitic carbon nitride (g-C3N4), a non-metal photocatalyst, has emerged as an attractive material for photocatalytic hydrogen production. However, the fast recombination of photoexcited electron–hole pairs limits the rate of hydrogen evolution and various methods such as modification, heterojunctions with semiconductors, and metal and non-metal doping have been applied to solve this problem. In this review, we cover the rational design of g-C3N4-based photocatalysts achieved using methods such as modification, metal and non-metal doping, and heterojunctions, and we summarize recent achievements in their application as hydrogen production photocatalysts. In addition, future research and prospects of hydrogen-producing photocatalysts are also reviewed.

1. Introduction

As interest in the fossil fuel depletion and environmental pollution has increased, the development of clean energy has also recently attracted increased attention. It is important to find new alternative energy sources because of the increased use of energy, depletion of fossil fuels, and the need for sustainable energy development [1]. Among the many alternative energy sources, hydrogen-based energy systems are considered candidates for future energy because they are nonpolluting, inexhaustible, efficient, and can provide high-quality energy services in a wide range of applications [2,3]. However, most hydrogen production processes are based on natural gas [4], coal [5], crude oil [6], or the electrolysis of water [7], and unfortunately, the application of most of these processes is limited because heat and electrical energy are required. Thus, photocatalytic hydrogen production using solar energy, a clean energy resource for the foreseeable future, is considered to be an attractive way of solving the global energy issue and environmental pollution [8,9].
The overall water splitting by a photocatalyst under sunlight irradiation enables the production of environmentally friendly molecular hydrogen and does not use fossil fuel [10]. A photocatalytic system should consider the following prerequisites. First, to absorb as many photons as possible, the photocatalyst must have a narrow band-gap; to generate hydrogen from water splitting, the bottom of the conduction band (CB) must be more negative than the reduction potential of H+/H2 and the top of the valence band (VB) must be more positive than the oxidation potential of H2O/O2 [11]. Second, efficient charge separation and fast charge transport that simultaneously avoid bulk and surface charge recombination are essential to transfer the photogenerated charge to the surface reaction site [12]. Third, because the charge carriers at the interface lack the capacity to boost the transportation process, the charge carriers mostly move via a random path and require a surface chemical reaction that is active between the charge carrier and the water or other molecules [13]. A variety of semiconductor materials such as TiO2, ZnO, CdS, and WO3 have been extensively studied for hydrogen generation via photocatalytic water splitting [14,15,16,17]. Among them, WO3 absorbs visible light but has a problem in that the CB is not useful for hydrogen production because it is lower than the H reduction potential [18,19]. In addition, hydrogen evolution through photocatalytic water splitting has been extensively studied for metal oxides, quantum dots, and metal–organic frameworks, etc. However, some methods are difficult to use due to their low efficiency under visible light and the fast recombination rate of the electron–hole pairs [20,21,22,23,24,25]. Therefore, it is a major challenge to develop photocatalysts that exhibit stable water-splitting performance under visible-light irradiation for the efficient use of solar energy.
Recently, graphitic carbon nitride (g-C3N4) has attracted attention as a hydrogen-generating photocatalyst via water splitting. g-C3N4 is synthesized by the thermal condensation of nitrogen-rich precursors with a tri-s-triazine ring structure such as cyanamide, dicyandiamide, urea, or thiourea, resulting in a graphene-like structure after exfoliation (Figure 1) [26]. In addition, it has a band gap of ~2.7 eV corresponding to 460 nm in the visible range and high thermal and chemical stability [27].
However, there are some drawbacks to using g-C3N4 as a water-splitting photocatalyst. The relatively large band-gap and low charge-carrier mobility limit the electron and hole separation and transport and thus limit the effective use of visible light [28]. Thus, increasing hydrogen production during photocatalytic water splitting under visible-light irradiation is necessary through a variety of methods such as creating heterojunctions with semiconductors and doping with other elements [29,30,31,32,33]. As a result, the focus of this review is on summarizing the current and prospective advances in photocatalysis research based on g-C3N4 that make it effective even under visible-light irradiation.

2. The Principles of H2 Generation via Water Splitting

Photocatalytic reactions can be divided into three parts. The first step is to obtain photons with energies that exceed the photocatalyst’s band gap of the electron–hole pairs, the second step is the separation of the carrier in the photocatalyst by transfer, and the third step is the reaction between the carrier and H2O. In addition, the electron–hole pairs are concurrently combined with each other. The photocatalyst is involved in the production of hydrogen, but the lowest position of the CB should be lower than the reduction position of H2O/H2 and the position of the VB should be higher than the potential of H2O/O2 [34,35,36,37,38,39,40].
Figure 2 shows the band gap and band edge positions of various semiconductor photocatalysts [41]. A variety of these, such as ZnO, TiO2, and WO3 have been studied for solar hydrogen production and degradation of organic pollutants [42,43,44,45]. However, although there are exceptions for some semiconductor photocatalysts, most of the semiconductor photocatalysts have low efficiency under visible-light irradiation. Therefore, it is a major challenge to develop photocatalysts that efficiently exploit solar energy.
Recently, g-C3N4, which has a unique electron band structure for photo-oxidation and reduction, has been confirmed by several researchers as an efficient photocatalyst for visible-light activation for photochemical reactions [46]. This achieves the photoexcited state when creating electron–hole pairs where photogenerated electrons are involved in the reduction process while the holes are consumed in the oxidation process [47]. The excited electrons and holes act as reactive species that are highly oxidizing and reducing. The excited electrons and holes travel to the active sites on the surface, thereby splitting the water into oxygen and hydrogen (Figure 3) [48]. However, despite its excellent electron and optical properties, g-C3N4 has low efficiency for visible-light utilization and a high recombination speed of photoelectric carrier, resulting in the poor formation of radical species causing redox reaction during the photocatalytic reaction [49]. It has a low specific surface area, provides fewer reactive sites, and reduces light harvesting. In addition, the low bandgap (2.7 eV) of g-C3N4 is still quite large for efficient visible-light harvesting and has limited use, leaving much of the visible-light spectrum unexploited.

3. Hydrogen Generation of g-C3N4-Based Photocatalysts

In recent years, a review of the technological improvements of the photocatalytic efficiency of g-C3N4-based materials has been published, mostly focusing on contaminant removal, the reaction mechanisms, principles of photocatalysis, and the effects of operational parameters [50,51]. Therefore, the aim of this review is to summarize recent trends in the improvement of hydrogen production by photocatalysts using various methods for the purpose of improving g-C3N4-based photocatalytic hydrogen production: (1) modification of g-C3N4; (2) heterojunctions from g-C3N4/semiconductors; and (3) metal- and non-metal-doped g-C3N4.

3.1. Modification of g-C3N4

Improving the photocatalytic activity of g-C3N4 by introducing various nanostructures such as nanoparticles, nanosheets, nanorods, and nanowires has recently been studied [52,53,54,55,56]. Surface modification of the catalytic structure and morphology has the potential to promote charge separation and narrow the band gap due to increased surface area and efficient charge-carrier separation [57,58].
In 2016, Han et al. [59] reported an atomically thin mesoporous nanomesh of g-C3N4 for hydrogen evolution by highly efficient photocatalysts (Figure 4a) fabricated via the solvothermal exfoliation of mesoporous g-C3N4 prepared by the thermal polymerization of freeze-dried nanostructured precursors. The delamination of the layer material to provide the two-dimensional single-atom sheet has led to unique physical properties such as a large surface area, a very high unique carrier mobility, and a significant change in the energy band structure [60]. The mesoporous g-C3N4 nanomesh shows inherent structural advantages, electron transfer capability, and efficient light harvesting. Figure 4b shows the electronic band structure of the monolayer mesoporous g-C3N4 nanomesh and bulk counterparts. The band gap is 2.75 eV for the monolayer mesoporous g-C3N4 nanomesh and 2.59 eV for the bulk counterpart, as determined from optical absorption spectra. The VB of the monolayer mesoporous g-C3N4 nanomesh (2.41 eV) identified via X-ray photoelectron spectroscopy is also 0.35 eV higher than the bulk counterparts (2.06 eV). The CB is upshifted by 0.51 eV when considering the 0.16 eV increase in the VB and a negative shift of 0.35 eV. The monolayer mesoporous g-C3N4 nanomesh exhibits significantly improved the light-harvesting ability mainly due to the multiple scattering effect and the presence of defect sites associated with the mesoporous surface. A 30 h reaction was performed with intermittent evacuation every 5 h to confirm the hydrogen production ability of mesoporous g-C3N4 nanomesh under visible-light irradiation (Figure 4c). As a result, the 2.6 mmol H2 gas (59 mL) produced by the atomically thin mesoporous g-C3N4 nanomesh was not visibly deactivated and the H2 gas was generated continuously. Wavelength-dependent H2 evolution shows the optical absorption spectrum of monolayer g-C3N4 nanomesh, indicating that the H2 generation is driven by photoinduced electrons in g-C3N4 (Figure 4d). In conclusion, the mesoporous g-C3N4 nanomesh produces an atomically thin mesoporous layer during the freeze-dried assembly and solvothermal exfoliation. Its good application benefits from structural advantages for light harvesting, electron transport, and accessible reaction sites [61]. This new type of mesoporous g-C3N4 nanomesh could be applied to photocatalytic and various engineering fields.
In 2018, Zhao et al. [62] reported the fabrication of a mesoporous g-C3N4 consisting of hollow nanospheres (MCNHN) via a simple vapor-deposition method that improved hydrogen production under visible-light irradiation. Figure 5a shows the photocatalytic hydrogen evolution by MCNHN under visible-light irradiation. Both MCNHN and bulk g-C3N4 achieved a stable average rate of hydrogen production within 4 h, but the hydrogen evolution of MCNHN was 659.8 μmol g−1 h−1, which is 22.3 times greater than bulk g-C3N4 (29.6 μmol g−1 h−1). The excellent hydrogen production activity of MCNHN is due to its well-defined structure. The increased surface area provides more active sites in the photocatalytic reaction, thereby allowing more light to be harvested. Moreover, the planarized unit layer and the decreased interlayer space of g-C3N4 crystals facilitate the transfer and separation of photoinduced charge carriers in MCNHN. As a result, photocatalytic hydrogen generation is significantly improved due to the large surface area and decreased interlayer space of g-C3N4. Figure 5b shows the proposed photocatalytic mechanism of H2 evolution for MCNHN based on the aforementioned results and the literature. The active site of MCNHN absorbs visible light. Electrons in the VB are excited to the CB by absorption of photons, and are then transferred to the Pt nanoparticles loaded on the surface of MCNHN; the corresponding photoexcited holes remain in the VB. The electron-rich Pt nanoparticles become active sites where water can be split into hydrogen. In addition, multiple reflections of visible light in the MCNHN with Pt nanoparticles improves light absorption.

3.2. Heterojunctions and Photocatalysis

Electron–hole charge pairs formed by the photocatalytic hydrogen evolution reaction are transferred to the surface of the photocatalyst or else recombine with each other. To better understand this point, let us illustrate it by reviewing the presentation in [63]: a comparison of the influence of gravitational force on a man jumping off the ground and electrons jumping from the VB to the CB (Figure 6a,b, respectively). If a man (electron) jumps from the ground (VB) into the sky (CB), it will return to the floor quickly (recombine with the hole) due to gravitational force. However, a stool (semiconductor B) can be provided to get the man off the ground (separate the photogenerated electron–hole pair), as illustrated in Figure 6c,d, respectively. Subsequently, the aforementioned man will land again on the stool rather than the ground (the electron–hole pair recombination will be inhibited). Preventing electron–hole recombination is an urgent issue, but it can be achieved by the proper design of materials [64,65,66]. Many methods have been proposed to achieve better separation of the photogenerated electron–hole pairs in semiconductor photocatalysts, such as element combining, metal and non-metal doping, and heterojunctions [67,68,69,70,71,72]. Among these strategies, heterojunctions in photocatalysts have proved to be one of the most promising methods for efficient photocatalyst preparation due to their improved separation of electron–hole pairs [73].

3.2.1. Semiconductor Heterojunction Photocatalysts

Suppressing the electron–hole recombination rate is the most important solution to increase photocatalytic efficiency. Bulk g-C3N4 has low ability to collect visible light, low charge-transport properties, and small surface area, so there have been many studies to make it an efficient photocatalyst [74]. Various strategies have been proposed to achieve better electron–hole pair separation such as element combining, metal doping, and creating heterojunctions. Among these strategies, g-C3N4/semiconductor heterojunctions have shown the improved separation capability of electron–hole pairs; the charge carrier is transferred through the heterostructure interface to inhibit recombination, thereby improving the photocatalytic performance [75,76,77]. In addition, a g-C3N4/semiconductor heterostructure can be formed by combining a visible-light excited photocatalyst semiconductor material having a narrow band-gap and a photoexcited photocatalyst having a large band-gap in a coupling process; the connection between the two different kinds of photocatalyst having different band structures induces a new band arrangement [78,79].
In 2017, Zhang et al. [80] reported the in situ synthesis of a g-C3N4/TiO2 heterostructure photocatalyst which greatly improved the hydrogen evolution performance under visible light. The g-C3N4 nanosheets were synthesized by calcining urea at 550 °C for 4 h. Two hundred milligrams of the as-prepared g-C3N4 nanosheets were dispersed in 20 mL ethanol and sonicated for 1 hour to obtain a homogeneous suspension. Under continuous stirring, 40 mL of ammonia solution (~28 wt%) and tetrabutyl titanate (TBT) (0, 100, 200, 300 and 400 μL) were added and stirred for 12 h to achieve the in situ synthesis of amorphous TiO2. The obtained products were expressed as CNTO-x (x = 0–4) according to the TBT content. As shown in Figure 7a, the shape of the CNTO-2 sample seen in a transmission electron microscopy (TEM) image shows that the TiO2 nanoparticles are uniformly distributed in the g-C3N4 nanosheets. As a result, there is uniform interfacial contact between the TiO2 phase and the g-C3N4 phase. Figure 7b shows the average rate of hydrogen production within 3 h. Pure TiO2 does not react with visible light and produces negligible H2, while CNTO-0 exhibits a low hydrogen production rate of 15 μmol h−1 due to the fast recombination of photogenerated charge carriers. In contrast, the CNTO-2 sample exhibits significantly improved hydrogen production performance at 40 μmol h−1. However, as the amount of TiO2 is further increased, TiO2 occupies the surface of g-C3N4 resulting in less active sites for H2 evolution. The proposed mechanism of heterostructure composites is also shown in Figure 7c. According to previous reports, the CB and VB potentials of g-C3N4 and TiO2 are −1.12 and +1.58 V, and −0.29 and +2.91 V, respectively. Under visible light irradiation, only g-C3N4 can absorb light to generate electron–hole pairs. However, in pure g-C3N4, photogenerated electrons and holes recombine rapidly, and only a few of the electrons participate in the reaction, resulting in low reactivity. When g-C3N4 is modified by TiO2 to form a heterojunction structure, the CB edge of g-C3N4 is more negative than TiO2, so that electrons excited in the CB of g-C3N4 can be injected directly into the CB of TiO2. Consequently, Pt2+ adsorbed on the surface is reduced by electrons transferred from the CB of TiO2, and newly formed Pt nanoparticles are deposited on the surface of TiO2 as an efficient cocatalyst for hydrogen production. The electrons then accumulate in Pt nanoparticles and participate in hydrogen evolution. Therefore, the photocatalytic activity of the g-C3N4/TiO2 composite with Pt nanoparticles as a cocatalyst is significantly improved.

3.2.2. Z-Scheme Heterojunction Photocatalysts

In 2017, Lu et al. [81] reported a Z-scheme photocatalyst that improved the photocatalytic hydrogen production of g-C3N4 nanosheets by loading porous silicon (PSi). The Z-scheme heterostructure improved the photocatalytic H2 evolution performance by loading PSi onto the g-C3N4 photocatalyst. g-C3N4/PSi composites were prepared by the facile polycondensation reaction of PSi with urea at various PSi content ratios and included pure g-C3N4 that was not PSi loaded for comparison. The photocatalytic performance of the g-C3N4/PSi composites and pure g-C3N4 in Figure 8a was evaluated by H2 evolution from water under visible-light irradiation. For composite materials loaded with PSi on g-C3N4 nanosheets, the rate of H2 evolution was better than that of pure g-C3N4 (427.28 μmol g−1 h−1). In particular, the g-C3N4/2.50 wt% composite exhibited the highest photocatalytic activity with a hydrogen evolution rate of 870.58 μmol g−1 h−1, which is around twice as high as that of pure g-C3N4. However, in the case of the Si-based photocatalyst, a passive oxide film was formed on the Si surface, and thus the stability suffered. When the PSi content was larger than 2.50 wt%, the H2 generation activity was reduced. Figure 8b depicts an energy band diagram of g-C3N4/PSi with the redox potential of the photocatalytic reaction. The Z-scheme heterostructure system is recognized as the photocatalytic mechanism for the g-C3N4/PSi composite, and the electrons excited from the CB of PSi in the photocatalyst system can be transferred to the VB of g-C3N4. In addition, the holes generated in g-C3N4 can move to the CB of PSi through the interface formed between g-C3N4 and PSi. The recombination at the interface between the electrons and the holes accumulates a large number of bonds and acts as a recombination center for the electron–hole pairs [82,83]. As a result, the efficiency of the photogenerated electron–hole pairs is improved, thereby improving photocatalytic hydrogen production under visible-light irradiation.

3.3. Metal- and Non-Metal-Doped g-C3N4

Among the strategies for making g-C3N4 as a photocatalyst capable of effective hydrogen production, sufficient doping with metallic and nonmetallic elements is known to enhance the photocatalytic activity of g-C3N4. Metal doping is an effective strategy to adjust the electronic structure of g-C3N4 and promotes surface kinetics to accelerate photogenerated electron transfer and provide active sites for better photocatalytic hydrogen production. In addition, the light transmittance can be maximized since the spatial distribution and the particle size of the metal can be finely controlled to provide a sufficient active size.
In 2016, Li et al. [84] reported water splitting by Cu- and Fe-doped g-C3N4 visible-light-activated photocatalysts. Figure 9a shows the mechanism of water splitting by light-driven catalysis with Fe- and Cu-doped g-C3N4. Under visible-light irradiation, water is converted to H2 and H2O2, and then H2O2 is further converted to O2 and H2O via the photocatalytic imbalance path. After absorbing visible light, g-C3N4 forms excited electrons and holes by electron catalysis, and the electrons move from the energy potential difference between g-C3N4 and Fe or Cu to the metal Fe or Cu sites. The potential of these electrons is around −0.25 eV and has enough force to induce H2O2 disproportionation to form ∙OH and OH−. In the hole catalytic process (HCP), OH− and H2O2 could form the ∙O2− and H2O species reaction with the holes. Finally, O2− and OH can recombine to form O2. Electron catalysis is an energy-consuming process whereas HCP and recombination processes can be viewed as energy-releasing processes. Figure 9b shows the oxygen and hydrogen evolution rates of Fe/C3N4 (0.37 wt%) and Cu/C3N4 (0.42 wt%) under visible-light irradiation (λ ≥ 420 nm) for 12 h. In this case, the production of hydrogen and oxygen by the Cu/C3N4 and Fe/C3N4 photocatalysts were 1.4 and 0.5 μmol, and 2.1 and 0.8 μmol, respectively. In addition, the potential of the Fe/g-C3N4 photocatalyst is obviously lower than those of the g-C3N4 and Cu/g-C3N4 photocatalysts, which leads to the O2 and H2 evolution activity over the Fe/g-C3N4 photocatalyst being clearly higher than that over the g-C3N4 and Cu/g-C3N4 photocatalysts. The findings of this study give new insight into the designing of efficient catalysts for overall water splitting.
Non-metal doping is a useful strategy to adjust the electronic structure of g-C3N4 and to increase the photocatalytic effect by promoting the reaction surface. When the non-metal elements B, N, O, P, and S are used to dope g-C3N4, the photocatalyst is efficiently optimized by lowering the charge recombination rate due to optical absorption and accelerated charge mobility, and thus the amount of H2 produced can be increased [85,86]. Consequently, the potential of the Fe/g-C3N4 photocatalyst is obviously lower than those of the g-C3N4 and Cu/g-C3N4 photocatalysts. This indicates that the Fe/g-C3N4 photocatalyst has higher activity on photocatalytic hydrogen evolution than the g-C3N4 and Cu/g-C3N4 photocatalysts. The findings of this study give new insights into designing efficient photocatalytic hydrogen generation and catalysts through overall water splitting.
In 2018, Feng et al. [87] reported P nanostructures with P-doped g-C3N4 as light photocatalysts for H2 evolution. P nanostructures and P-doped g-C3N4 (P@P-g-C3N4) were synthesized via a solid reaction, and P@P-g-C3N4 showed increased optical absorption, high-efficiency transmission, and efficient separation of photogenerated electron–hole pairs. When C atoms are replaced with P atoms (the gray and red balls in Figure 10a, respectively) in the base frame of g-C3N4, the extra electrons are decentralized into a π-conjugated triazine ring and generate a positive-charge P+ center, thereby facilitating rapid separation of the photogenerated excited electrons. Furthermore, efficient band gap transfers between the P and P-doped g-C3N4 leads to a significant improvement in photoactivity (Figure 10b). P-doped g-C3N4 photoexcited electrons can be delivered to phosphorus via intimate contact because the CB edge of g-C3N4 (−1.2 V vs. normal hydrogen electrode (NHE)) is more negative than P (−0.25 V vs. NHE) which provides an interface under the buildup of the internal electric field. Thus, the extra electrons superimposed on the P surface can easily be captured by the oxygen molecules in the solution and react with ∙O2− and ∙OH. Figure 10c,d shows the hydrogen evolution yield and the improvement in hydrogen production ability of the photocatalysts prepared at different weight ratios of P/g-C3N4. P@P-g-C3N4-15 showed the highest hydrogen production rate (941.80 μmol h−1 g−1), which is around four times that of conventional g-C3N4.

4. Summary and Perspectives

Photocatalytic action is a key factor for the future of environmental pollution and hydrogen generation due to water splitting. Over the past several years, photocatalytic reactions have emerged as a promising method to generate hydrogen, and interest in the photocatalyst g-C3N4 has received attention in a variety of scientific disciplines. However, a major problem that limits the rate of production of H2 by g-C3N4-based photocatalysis is the fast recombination of photoexcited electron–hole pairs. This problem can be solved in a variety of ways, including modification, heterojunctions, and metal and non-metal doping. Table 1 summarizes the literature on the photocatalytic H2 generation of g-C3N4-based materials. We reviewed the rational design of photocatalysts for efficient H2 generation though a variety of methods. Furthermore, the improvement of g-C3N4-based photocatalysts will likely result from advances in science. Herein, we have covered the recent progress of g-C3N4-based materials involved in hydrogen production in improving their overall photocatalytic activity and have characterized their performance and importance. We hope that this report will support further research efforts related to photocatalytic development.

Funding

This research was supported by the Technology Innovation Program (or Industrial Strategic Technology Development Program) (10080293), Development of carbon-based non phenolic electrode materials with 3000 m2g−1 grade surface area for energy storage device funded by the Ministry of Trade, Industry and Energy (MOTIE, Korea) and the Commercialization Promotion Agency for R&D Outcomes (COMPA) funded by the Ministry of Science and ICT (MSIT) 2018_RND_002_0064, Development of 800 mA·h·g−1 pitch carbon coating.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic illustration of the synthesis process from the possible precursors of g-C3N4. Reproduced with permission from [26]; copyright (2016), the American Chemical Society.
Figure 1. Schematic illustration of the synthesis process from the possible precursors of g-C3N4. Reproduced with permission from [26]; copyright (2016), the American Chemical Society.
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Figure 2. A schematic illustration of the band-gap energy of several typical semiconductor photocatalysts. Reproduced with permission from [41].
Figure 2. A schematic illustration of the band-gap energy of several typical semiconductor photocatalysts. Reproduced with permission from [41].
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Figure 3. Schematic illustration of the charge-transfer mechanism of neat g-C3N4 as a photocatalyst. Reproduced with permission from [48]; copyright (2016), Royal Society of Chemistry Advances.
Figure 3. Schematic illustration of the charge-transfer mechanism of neat g-C3N4 as a photocatalyst. Reproduced with permission from [48]; copyright (2016), Royal Society of Chemistry Advances.
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Figure 4. (a) Schematic illustration of atomically thin mesoporous g-C3N4 nanomesh photocatalyst and (b) a band gap schematic of the monolayer mesoporous g-C3N4 nanomesh and bulk counterparts. (c) Hydrogen production rate of the monolayer mesoporous g-C3N4 nanomesh, the bulk counterpart, and the traditional g-C3N4 bulk under visible-light irradiation. (d) H2 evolution rate on the monolayer mesoporous g-C3N4 nanomesh with wavelength dependence. Reproduced with permission from [59]; copyright (2016), American Chemical Society.
Figure 4. (a) Schematic illustration of atomically thin mesoporous g-C3N4 nanomesh photocatalyst and (b) a band gap schematic of the monolayer mesoporous g-C3N4 nanomesh and bulk counterparts. (c) Hydrogen production rate of the monolayer mesoporous g-C3N4 nanomesh, the bulk counterpart, and the traditional g-C3N4 bulk under visible-light irradiation. (d) H2 evolution rate on the monolayer mesoporous g-C3N4 nanomesh with wavelength dependence. Reproduced with permission from [59]; copyright (2016), American Chemical Society.
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Figure 5. (a) Time course of H2 evolution and (b) a schematic mechanism for photocatalytic H2 evolution on MCNHN. Reproduced with permission from [62]; copyright (2018), Elsevier.
Figure 5. (a) Time course of H2 evolution and (b) a schematic mechanism for photocatalytic H2 evolution on MCNHN. Reproduced with permission from [62]; copyright (2018), Elsevier.
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Figure 6. Schematics of (a) the influence of gravitational force on a man jumping; (b) recombination of photocatalyst electron–hole pair; (c) using stool to keep a man from returning to the ground; and (d) electron–hole pairs separated in a heterojunction photocatalyst. Reproduced with permission from [63]; copyright (2017), John Wiley & Sons, Inc.
Figure 6. Schematics of (a) the influence of gravitational force on a man jumping; (b) recombination of photocatalyst electron–hole pair; (c) using stool to keep a man from returning to the ground; and (d) electron–hole pairs separated in a heterojunction photocatalyst. Reproduced with permission from [63]; copyright (2017), John Wiley & Sons, Inc.
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Figure 7. (a) A TEM image of CNTO-2, (b) H2 evolution rates of the CNTO-x samples under visible light (λ ≥ 420 nm), and (c) an illustration of the g-C3N4/TiO2 heterojunction system. Reproduced with permission from [80]; copyright (2017), The Royal Society of Chemistry.
Figure 7. (a) A TEM image of CNTO-2, (b) H2 evolution rates of the CNTO-x samples under visible light (λ ≥ 420 nm), and (c) an illustration of the g-C3N4/TiO2 heterojunction system. Reproduced with permission from [80]; copyright (2017), The Royal Society of Chemistry.
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Figure 8. (a) Photocatalytic H2 evolution with 100 mg pure g-C3N4 and g-C3N4/PSi composite photocatalysts under visible light (400 nm) and (b) a schematic diagram of the g-C3N4/TiO2 heterojunction system. Reproduced with permission from [81]; copyright (2017), Elsevier.
Figure 8. (a) Photocatalytic H2 evolution with 100 mg pure g-C3N4 and g-C3N4/PSi composite photocatalysts under visible light (400 nm) and (b) a schematic diagram of the g-C3N4/TiO2 heterojunction system. Reproduced with permission from [81]; copyright (2017), Elsevier.
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Figure 9. (a) A schematic diagram of the water splitting mechanism by Fe/C3N4 and Cu/C3N4 photocatalysts under visible-light irradiation. (b) Production of H2 and O2 by water splitting by the Fe/C3N4 and Cu/C3N4 photocatalysts under visible-light irradiation for 12 h. Reproduced with permission from [84]; copyright (2015), American Chemical Society.
Figure 9. (a) A schematic diagram of the water splitting mechanism by Fe/C3N4 and Cu/C3N4 photocatalysts under visible-light irradiation. (b) Production of H2 and O2 by water splitting by the Fe/C3N4 and Cu/C3N4 photocatalysts under visible-light irradiation for 12 h. Reproduced with permission from [84]; copyright (2015), American Chemical Society.
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Figure 10. (a,b) A schematic of the mechanism of H2 evolution by the P@P-g-C3N4 catalyst; (c) comparison of the evolution rates of H2; and (d) H2 evolution rate of the P@P-g-C3N4 composites. Reproduced with permission from [87]; copyright (2018), American Chemical Society.
Figure 10. (a,b) A schematic of the mechanism of H2 evolution by the P@P-g-C3N4 catalyst; (c) comparison of the evolution rates of H2; and (d) H2 evolution rate of the P@P-g-C3N4 composites. Reproduced with permission from [87]; copyright (2018), American Chemical Society.
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Table
Table 1. Photocatalytic H2 generation of g-C3N4-based materials.
Table 1. Photocatalytic H2 generation of g-C3N4-based materials.
EntryTypeMass Fraction of g-C3N4Mass of PhotocatalystReactant SolutionLight SourceH2 Generation Rate
(μmol h−1)		Reference
Figure 4Monolayer mesoporous g-C3N4 nanomesh100 wt%0.01 g100 mL of 10 vol% triethanolamine aqueous solution; 3 wt% Pt as a cocatalyst300 W Xe lamp (>420 nm)85.10[59]
Figure 5Mesoporous g-C3N4 comprising hollow nanospheres100 wt%0.1 g100 mL of 10 vol.% triethanolamine aqueous solution; 3 wt% Pt as a cocatalyst300 W Xe lamp (>420 nm)65.98[62]
Figure 7g-C3N4 nanosheets/TiO250 wt%0.05 g100 mL of 10 vol% triethanolamine aqueous solution; 3 wt.% Pt as a cocatalyst300 W Xe lamp (>420 nm)40[80]
Figure 8Porous Si-loaded g-C3N497.50 wt%0.1 g100 mL of 10 vol% triethanolamine aqueous solution; 3 wt% Pt as a cocatalyst300 W Xe lamp (>400 nm)87.05[81]
Figure 9Fe-doped g-C3N4
Cu-doped g-C3N4		99.63 wt%
99.58 wt%		0.01 gPure water; without other cocatalyst300 W Xe lamp (>420 nm)0.175[84]
Figure 10P@P-doped g-C3N475 wt%0.1 g100 mL of 10 vol% triethanolamine aqueous solution; 1 wt% Pt as a cocatalyst300 W Xe lamp (>420 nm)94.18[87]

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Mun, S.J.; Park, S.-J. Graphitic Carbon Nitride Materials for Photocatalytic Hydrogen Production via Water Splitting: A Short Review. Catalysts 2019, 9, 805. https://doi.org/10.3390/catal9100805

AMA Style

Mun SJ, Park S-J. Graphitic Carbon Nitride Materials for Photocatalytic Hydrogen Production via Water Splitting: A Short Review. Catalysts. 2019; 9(10):805. https://doi.org/10.3390/catal9100805

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Mun, Seong Jun, and Soo-Jin Park. 2019. "Graphitic Carbon Nitride Materials for Photocatalytic Hydrogen Production via Water Splitting: A Short Review" Catalysts 9, no. 10: 805. https://doi.org/10.3390/catal9100805

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