Perspective pubs.acs.org/JPCL
g‑C3N4‑Based Photocatalysts for Hydrogen Generation Shaowen Cao† and Jiaguo Yu*,†,‡ †
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, People’s Republic of China ‡ Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia ABSTRACT: Graphitic carbon nitride (g-C3N4)-based photocatalysts have attracted dramatically increasing interest in the area of visible-light-induced photocatalytic hydrogen generation due to the unique electronic band structure and high thermal and chemical stability of g-C3N4. This Perspective summarizes the recent significant advances on designing high-performance g-C3N4-based photocatalysts for hydrogen generation under visible-light irradiation. The rational strategies such as nanostructure design, band gap engineering, dye sensitization, and heterojunction construction are described. Finally, this Perspective highlights the ongoing challenges and opportunities for the future development of g-C3N4-based photocatalysts in the exciting research area.
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they are not ideal due to the poor solar energy utilization.7,8 They are only active in the UV region because of their wide band gaps. Some other oxides including WO3, BiVO4, and so forth are visible-light-responsive but cannot conduct water reduction to produce H2 because their conduction bands are lower than the reduction potential of water.9 CdS has also been considered, and it indeed possesses a narrow band gap with appropriate band levels for water splitting, whereas it is toxic and usually unstable due to the photocorrosion or self-oxidation.10 In recent years, special attention has been paid to graphitic carbon nitride (gC3N4), since the pioneering study in 2009 by Wang et al.11 on the visible-light photocatalytic water splitting over g-C3N4. g-C3N4 is considered to be the most stable allotrope among various carbon nitrides under ambient conditions. The proposed structure of gC3N4 is two-dimensional frameworks of tri-s-triazine connected via tertiary amines (see Figure 1), which makes it possess high stable thermal (up to 600 °C in air) and chemical stability (against acid, base, and organic solvents).12 It is identified to be a visible-light-active polymeric semiconductor with a band gap of ∼2.7 eV, corresponding to an optical wavelength of ∼460 nm, as well as an appropriate band structure for both water reduction and oxidation.12,13 As such, g-C3N4 promptly becomes the shining star in the field of photocatalysis. g-C3N4 is generally synthesized by the thermal condensation of nitrogen-rich precursors such as cyanamide, dicyandiamide, melamine, and so forth.14 However, the photocatalytic activity of the as-obtained g-C3N4 is usually restricted by low efficiency due to the fast recombination of photoinduced electron−hole pairs. Protocols such as texture modification, elemental doping, and copolymerization are subsequently applied to improve the
he development of clean and renewable energy is the key way to satisfy the increasing global energy demands and to resolve the environmental issues caused by the overuse of fossil fuels. One of the most attractive options is the conversion of solar energy into hydrogen through a water splitting process, with the help of semiconductor-based photocatalysts.1−4 The design of semiconductor-based photocatalytic systems must take the following requirements into account:5,6 (1) The semiconductor should have a narrow band gap to absorb as much light as possible; meanwhile, the bottom of its conduction band has to be more negative than the reduction potential of water to produce H2. If for overall water splitting, the top of its valence band must be more positive than the oxidation potential of water to produce O2. (2) Efficient charge separation and fast charge transport simultaneously avoiding the bulk and surface charge recombination are essentially required to migrate the photogenerated charge carries to the surface reaction sites. (3) Kinetically feasible surface chemical reactions must take place between these carries and water or other molecules; meanwhile, the surface backward reaction of H2 and O2 to water can be successfully suppressed. Therefore, scientists are trying to develop photocatalysts with specific bulk and surface properties as well as energy band structures to satisfy these requirements.
Dramatically increasing attention is paid to g-C3N4-based photocatalysts due to their unique physicochemical property and electronic band structure.
Received: March 18, 2014 Accepted: May 29, 2014
Metal oxides such as TiO2, SrTiO3, and so forth have been widely investigated for photocatalytic hydrogen generation, but © XXXX American Chemical Society
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Figure 1. Tri-s-triazine-based two-dimensional structure of g-C3N4. Color scheme: C, gray; N, blue.
photocatalytic performance of g-C3N4.15−18 Besides the fast development of these ameliorating strategies on intrinsic g-C3N4, the investigation of g-C3N4-based composite photocatalysts was highly stimulated in the past several years. In this Perspective, we give a short overview of the recent significant progress on designing efficient g-C3N4-based photocatalysts for hydrogen generation under visible-light irradiation. Design of High-Performance g-C3N4 for Hydrogen Generation. The photocatalytic H2 generation that occurred at the g-C3N4/ water interface is highly dependent on the size, morphology, and defects of g-C3N4. The control of the g-C3N 4 mirco/ nanostructure can endow it with large surface areas, abundant surface states, and even extended light harvesting, all of which are beneficial for the photocatalytic H2 generation. Because g-C3N4 has a similar layered structure with graphite, the specific surface area could theoretically be increased up to 2500 m2 g−1 for perfect monolayer g-C3N4,19 whereas it is normally below 10 m2 g−1 for the bulk g-C3N4 powder due to the stacking of polymeric nanosheets. There are several works highlighting the benefit for photocatalytic hydrogen generation by reducing the thickness of g-C3N4. It has been reported that g-C3N4 nanosheets with a thickness of ∼3 nm, corresponding to ∼9 stacked layers, can be prepared by using urea as the pyrolysis precursor.20 A specific surface area of 84.2 m2 g−1 is obtained for the ∼3 nm g-C3N4 nanosheets, which is much higher than that of those g-C3N4 prepared using cyanamide, dicyandiamide, and melamine. The resultant sheet-like structure also favors the charge transfer. Thus, a more efficient hydrogen production is achieved. Thinner free-standing g-C3N4 nanosheets are prepared by Yang et al.21 via a simple liquid-phase exfoliation method. Particularly, the gC3N4 nanosheets exfoliated in 2-propanol by sonication has a thickness of ∼2 nm, high surface area of 384 m2 g−1, and large aspect ratios, which not only provide abundant reactive sites but also promote the charge transport. Significantly, the average hydrogen evolution rate of these g-C3N4 nanosheets is more than 9 times higher as compared to that of bulk g-C3N4 under visiblelight irradiation. In another work, Xu et al.22 obtained g-C3N4 nanosheets with a single atomic layer structure (∼0.4 nm) by a concentrated H2SO4 involved chemical exfoliation method, although the yield was only 60%. The photocurrent and the hydrogen evolution rate of the single-layer g-C3N4 are ∼4 and ∼2.6 times as high as those of the bulk g-C3N4, respectively. On the other hand, the morphology modulation of g-C3N4 has been further demonstrated as a successful way to enhance the photocatalytic performance for hydrogen generation by facilitating the light absorption, charge separation and migration, and mass diffusion during photocatalytic reactions. For example, Zhang et al.23 have fabricated ordered mesoporous g-C3N4 using
an innovative template method. Before thermal treatment, sufficient inclusion of the precursor (cyanamide) in the SBA-15 mesozeolite template can be facilitated by the surface acidification of silica and sonication-promoted insertion. The thusobtained g-C3N4 possesses ordered mesopores and cylindrical channels, with a surface area up to 517 m2 g−1. A better photocatalytic hydrogen generation activity with an apparent quantum yield of 6.77% at 455 nm is achieved by the improved order of mesochannels, the presence of fewer textural structure defects, and the larger surface area. Furthermore, the ordered mesoporous frameworks can also serve as a “highway” for freecharge transport to decrease the electron−hole recombination. Sun et al.24 report an impressive work in regard to the silica template synthesis of g-C3N4 hollow nanospheres sized in the optical range as both light-harvesting antennae and nanostructured scaffolds that improve the photocatalytic efficiency. The prepared g-C3N4 nanospheres have a wavelength-scale size within ∼430 nm, and the shell thickness can be well tuned from 56 to 85 nm without deformation against thermal treatment; thus, it can maximize the light harvesting through inner reflections and photonic effects (see Figure 2). The resulting
Figure 2. Schematic illustration showing the light-harvesting behavior of g-C3N4 hollow nanospheres.
initial visible-light hydrogen evolution rate can reach ∼25-fold higher than that of bulk g-C3N4, with an apparent quantum yield of 7.5% at 420.5 nm. The performance of the robust g-C3N4 hollow nanospheres is also competitive and superior to that of TiO2 (P25) and N-doped TiO2 under UV and visible-light irradiation in the presence of the same reaction solution. Very recently, it has been shown that by supramolecular chemistry of 2102
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into the network of g-C3N4 through copolymerizing melem with PMDA, which lowers both the conduction and valence bands. As a result, the photoreactivity of hydrogen evolution was ∼3 times as high as that of pristine g-C3N4 under visible light. Similarly, Schwinghammer et al.32 fabricated an amorphous variant of poly(triazine imide) doped with 4-amino-2,6-dihydroxypyrimidine, exhibiting an extended light absorption up to 800 nm and thus a better hydrogen evolution activity. Zhang et al.33 also report that the band gap of carbon nitride prepared from urea can be narrowed from 2.83 to 2.61 eV by simple copolymerization of urea with phenylurea, along with the extension of the delocalized π-conjugation system. Consequently, the rate of H2 evolution in the visible light increases nearly 9 times. Therefore, heteroatom doping and copolymerization are pretty good strategies for tuning the electronic band structure of g-C3N4 to extend the visible-light absorption and adjust the redox potentials for highperformance photocatalytic hydrogen generation.
triazine molecules, it is facile to get ordered structures of g-C3N4 such as hollow boxes,25 spherical macroscopic assemblies,26 hollow spheres, 27 and so forth without any additional modifications. This method allows for synthesizing g-C3N4 with high photocatalytic performance without using any hazard materials by mainly preserving the initial morphologies of the hydrogen-bonded supramolecular network precursors during the thermal polycondensation process. Heteroatom doping can effectively monitor the electronic band structure of g-C3N4 to extend the light absorption and adjust the redox potentials to further promote the photocatalytic hydrogen generation in the visible-light range. Carbon selfdoping of g-C3N4 is actualized by Dong et al.28 via calcinating of solvothermally treated melamine with absolute alcohol. This carbon self-doping can cause the intrinsic electronic band structure change through the formation of delocalized big π bonds originating from the substitution of bridging N atoms with C atoms, thus to increase the visible-light absorption and electric conductivity. UV−vis spectra indicate that the carbon doping gives rise to a decrease in the band gap from 2.72 to 2.65 eV. The visible-light hydrogen evolution rate on carbon-doped g-C3N4 is 1.42 times that on pure g-C3N4. Hong et al.29 synthesize the in situ sulfur-doped mesoporous g-C3N4 by thermal decomposition of thiourea in the presence of silica nanoparticles. The doped sulfur is proposed to substitute carbon in g-C3N4, leading to a downshift of 0.25 eV in the conduction band and a narrower band gap of 2.61 eV. Optical studies reveal that the sulfur-doped mesoporous g-C3N4 shows extended and stronger visible-light absorption and a much lower density of defects compared to the native g-C3N4 prepared from melamine. Simultaneously boosted by the efficient mass and charge transfer in the mesoporous structure, a 30 times higher photocatalytic activity for hydrogen evolution is observed in comparison with that of native g-C3N4, corresponding to a quantum efficiency of 5.8% at 440 nm. Very recently, Zhang et al.30 employed the in situ iodine doping to gC3N4 using dicyandiamide and an iodine ion as the precursor and dopant. The I atoms tend to substitute the sp2-bonded N as an ntype doping modification, effectively extending the aromatic carbon nitride heterocycle and generating impurity energy levels above the valence band edge (see Figure 3). The optimal I
Heteroatom doping and copolymerization are pretty good strategies for tuning the electronic band structure of g-C3N4 to extend the visible-light absorption and adjust the redox potentials for high-performance photocatalytic hydrogen generation. Exploration of g-C3N4-Based Composite Photocatalysts for Hydrogen Generation. Recently, much interest has been dedicated to the construction of g-C3N4-based composite photocatalysts and their application for hydrogen generation under visible light. In a tentative work, we have prepared a graphene/g-C3N4 composite photocatalyst in which graphene sheets can serve as electronic conductive channels to efficiently separate the photogenerated electron−hole pairs and further to enhance the visible-light photocatalytic H2 production activity of gC3N4.34 This work demonstrates the superiority to utilize conductive graphene to accumulate abundant electrons for the water reduction reaction. An effective strategy for populating the conduction band of gC3N4 with abundant electrons is coupling with suitable organic dyes, the excitation of which enables the energy conversion of light at longer wavelengths. Min et al.35 report that the light absorption of mesoporous g-C 3 N 4 (
