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420 nm). This redox reaction can occur in the presence of a proper sacrificial electron acceptor or donor, respectively, even without addition of precious metal co-catalysts.12 The authors further calculated that N atoms in the matrix would be the preferable oxidation sites for H2O to O2, whereas the C atoms provide the reduction sites for H+ converting into H2. Even though the quantum yield of the system (0.1% at l ¼ 420–460 nm) is still low,13

3.1 Artificial photocatalytic hydrogen production and photocatalysis The development and commercialization of new clean energy has been driven by increasing concerns over issues such as climate change and energy security as a result of dwindling petroleum supplies. Although hydrogen economy and related green chemistry solutions have been proposed for many years, the generation of energy from renewable sources in a safe and reversible manner still remains a big challenge.56–58 Artificial photocatalystassisted water splitting employing solar energy is an ideal method for large-scale hydrogen production. To date, various semiconductors have been explored for this promising reaction; most of them are metal-based inorganic solids such as metal oxides,59,60 metal (oxy)sulfides61 and metal (oxy)nitrides.62 Since a breakthrough reported by Fujishima and Honda conducting the decomposition of water on illuminated TiO2 electrodes in 1972,63 TiO2 is widely used due to its optical and electronic properties, long-term stability, low-cost, and nontoxicity.64–68 However, the absorption edge of TiO2 on the whole spectrum 6722 | Energy Environ. Sci., 2012, 5, 6717–6731

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Fig. 5 (a) A comparison of band positions for TiO2 and g-C3N4, reproduced with permission from ref. 69 ª 2010 American Chemical Society. (b) The UV-vis absorption spectra of TiO2 and g-C3N4 materials, reproduced with permission from ref. 70 ª 2011 Elsevier B.V. (c) A typical time course of H2 production from water containing 10 vol% triethanolamine as an electron donor under visible light (l > 420 nm) by using pristine g-C3N4 (solid) and hollow 3.0 wt% Pt-deposited g-C3N4 (hollow) photocatalysts. (d) Time course of O2 production from water containing 0.01 M silver nitrate as an electron acceptor under visible light (l > 420 nm) by using 3.0 wt% RuO2-loaded g-C3N4, reproduced with permission from ref. 12 ª 2008 Macmillan.

the discovery of a non-metallic material achieving the same function as conventional metal-based photocatalysts is expected to offer new opportunities to progress the field of artificial photosynthesis. 3.1.2 Heteroatoms-doped g-C3N4. Although g-C3N4 materials possess high thermal and chemical stability as well as proper electronic properties, which make them attractive for photocatalytic water splitting applications, the photocatalytic efficiency of pristine g-C3N4 is still not satisfactory. Generally, chemical doping is an effective strategy to modify the electronic structures of semiconductors as well as their surface properties.72–74 Doping of TiO2 with metallic (Cr, Fe, V) and nonmetallic elements (N, C, B) was used to create localized/delocalized states in the band gap and thus extend its optical absorption to the visible region.65,67,75–80 Correspondingly, both the compositions and the properties of g-C3N4 can be further engineered by introducing selected heteroatoms into the matrix to slightly modify the molecular structure and consequently the photocatalytic activity of the composite. Non-metallic elements-doped g-C3N4. Doping of g-C3N4 with boron, fluorine, phosphor, or sulfur was employed very recently to modify its electronic structure and improve photocatalytic performance. Besides inherent photocatalytic hydrogen evolution on pristine g-C3N4, doped g-C3N4 can be applied for This journal is ª The Royal Society of Chemistry 2012

aliphatic C–H bond oxidation, photocatalytic phenol oxidation, cyclohexane oxidation, photodegradation of dyes, etc.81–87 Since the formation of g-C3N4 is a polycondensation process, the selection of proper precursors is a critical issue to achieve in situ doped (also referred to as structurally doped) heteroatoms in the designed sites. The structures, synthesis methods, and applications of various non-metallic elements-doped g-C3N4 are summarized in Table 1. Fluorination has been widely used to modify the properties of carbon materials such as graphite, activated carbons, carbon nanotubes, graphene, etc.88–92 Because of the much larger electronegativity of fluorine than nitrogen, the doped fluorine is bonded with carbon to result in a partial conversion of C-sp2 to C-sp3, which may lead to lower plane order of the material. Wang et al.81 synthesized and proposed two fluorinated g-C3N4 configurations (CNFs) by using NH4F as the doping reactant; the fluorine concentration in the resulting CNFs can be controlled by incorporating different amounts of NH4F. The fluorine doping led to a decrease in the band gap compared with the undoped g-C3N4 and correspondingly a red shift of the UV-vis spectrum, which also indicates that fluorine is structurally doped into the g-C3N4 matrix, different from protonation in which the spectrum was blue-shifted.93 Due to the alternative electronic structures, one of the optimized CNF samples revealed a 2.7 times higher photocatalytic H2 evolution activity than that Energy Environ. Sci., 2012, 5, 6717–6731 | 6723

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Table 1 Summaries of various non-metallic elements-doped g-C3N4 materials Elements Structurea

Heteroatom sources

Applications

F

Ammonium fluoride (NH4F)

Photocatalytic hydrogen evolution and benzene to phenol oxidation 81

B

BH3NH3

Aliphatic C–H bond oxidation

82

B

Boron oxide B2O3

Photodegradation of dyes

83

P

Ionic liquid (BmimBF4)

Enhancing the electrical conductivity of g-C3N4

84

S

H2S atmosphereb

Photocatalytic hydrogen evolution and phenol oxidation

85

S

Trithiocyanuric acid C3H3N3S3 Water oxidation reaction for oxygen evolution

86

Ionic liquid (BmimPF6)

87

F and B

Structure uncertain

Ref.

Cyclohexane oxidation

a

Blue, gray, white, cyan, green, red, and yellow spheres represent nitrogen, carbon, hydrogen, fluorine, boron, phosphorus, and sulfur atoms, respectively. b This material was synthesized by treating the pre-synthesized g-C3N4 in a H2S atmosphere at 450 C.

of undoped g-C3N4. Besides H2 evolution, CNF as a multifunctional catalyst can also catalytically oxidize benzene to phenol under visible light irradiation (l > 420 nm), which is negligible on undoped g-C3N4. Wang et al.81 attributed the enhanced photocatalytic activity and redox properties of CNF to the modified electronic structure of HOMO and LUMO (defined as the conduction band and the valence band of one compound, respectively) induced by fluorine doping. They proposed that the incorporation of fluorine at the bay carbon (left one in the cell of Table 1) will shift both the LUMO and HOMO to higher energy values while incorporation of fluorine at the corner carbon (right one in the cell of Table 1) will shift the LUMO to higher energy and the HOMO to lower energy values. Besides incorporating larger electronegativity of fluorine, Wang et al.82 also found that replacing carbon atoms with boron having smaller electronegativity can form a strong Lewis acid site in the g-C3N4 networks. They synthesized boron-doped g-C3N4 (CNB) by using BH3NH3 as the doping reactant during the condensation process; the resulting CNB materials held a lower HOMO position than undoped g-C3N4, thus increasing the potential oxidation strength. As a result, these materials exhibited a good catalytic performance on the selective oxidation of toluene and substituted benzylic aromatics with a remarkably higher selectivity towards the formation of aldehydes or ketones than those on traditional metal catalysts.82 Different from substituting carbon atom with boron heteroatom in the periodical melon rings as mentioned above, Yan et al.83 developed 6724 | Energy Environ. Sci., 2012, 5, 6717–6731

a boron doping g-C3N4 structure by substituting the hydrogen atom at the terminal of periodical melon with boron, forming C–NB and C–NB2 functional groups. The resulting material showed a slightly reduced band gap (2.66 eV) as compared to the pristine g-C3N4 (2.75 eV) and revealed a high activity for photodegradation of two typical dyes (rhodamine B and methyl orange).83 Zhang et al.84 developed a phosphorus doped g-C3N4 matrix in which the corner carbon was replaced to form a P–N bonding (as illustrated in Table 1); a simple and commercially available ionic liquid of BmimBF4 was applied as the doping reactant. As compared to undoped g-C3N4, the p-doped g-C3N4 showed a significantly enhanced electrical (dark) conductivity up to 4 orders of magnitude and also revealed an improvement in photocurrent generation by a factor of up to 5, both of which are significant steps toward the photovoltaic applications of g-C3N4. Unlike in situ doping of g-C3N4 materials via the condensation process, certain non-metallic elements can be incorporated into the matrix by post-treatment of the pre-synthesized g-C3N4 in the doping reactant atmosphere. One example is sulfur-doped gC3N4 achieved by treating g-C3N4 in a H2S atmosphere at 450 C to introduce sulfur in the original nitrogen site of the melon ring, as illustrated in Table 1. More interestingly, the resulting g-C3N4xSx revealed a unique electronic structure with an increased band gap and a slightly reduced absorbance of visible light, opposite to all other elements doped g-C3N4 listed here. Liu et al.85 attributed this interesting phenomenon to a synergistic This journal is ª The Royal Society of Chemistry 2012


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effect of sulfur: a widening and upshifting of the valence band was achieved through homogeneous distribution of the sulfur dopant, and an upshifting of the conduction band minimum caused by the quantum confinement effect relating to the markedly reduced particle size after doping (Fig. 6d). This unique electronic structure offers g-C3N4xSx with a special photocatalytic activity towards H2 evolution, which is 7.2 and 8.0 times higher than undoped g-C3N4 under l > 300 and 420 nm, respectively. Additionally, a complete oxidation process of phenol under l > 400 nm can occur on g-C3N4xSx, which is impossible for undoped g-C3N4 (Fig. 6a–c).85 Different from post-treatment doping, sulfur can also be in situ doped into the g-C3N4 matrix by using sulfur-enriched trithiocyanuric acid as the precursor via a polycondensation process with –SH groups at the terminal of periodical melon acting as the leaving groups.86 The residual sulfur in the resulting sulfur-doped g-C3N4 (CNS) samples was estimated to be 0.8 and 0.5 wt% at 550 and 650 C calcination, respectively. Unlike g-C3N4xSx synthesized via post-synthesis treatment, this in situ synthesized CNS showed an extension of the spectrum towards absorption in the visible range as compared to the undoped g-C3N4; consequently, the water oxidation reaction can be achieved at a moderate rate on polymeric CNS without addition of cocatalysts, which has not been observed on undoped g-C3N4. Co-doping g-C3N4 with boron and fluorine could also induce a unique electronic structure in the resulting materials. Wang et al.87 synthesized boron and fluorine enriched meso-g-C3N4 (CNBF) using ionic liquid (BmimBF6) as the doping source via a designed soft-templating route. The resulting materials possessed high nitrogen, boron, and fluorine contents, high surface area, local graphitic order, and an excellent photoconductivity under visible light. Besides, the CNBF materials showed satisfactory performance in the oxidation of cyclohexane with good conversion rate and excellent cyclohexanone product selectivity.

Metallic elements-doped g-C3N4. Apart from non-metallic element doping used to reduce the band gap energy and expand light absorption into the visible range of g-C3N4, metallic element dopants can also introduce a certain amount of free electrons to the g-C3N4 semiconductor. Generally, metallic elements can be incorporated into the g-C3N4 matrix via polycondensation at 600 C by using dicyandiamide as precursor and additional metal chloride as a doping source.94–99 The metal components strongly modify the electronic properties of pristine g-C3N4, and provide new class of organic–metal hybrids with additional new functions such as mimicking metalloenzymes in H2O2 activation, selective oxidation of benzene and other hydrocarbons, etc. Wang et al.94 firstly predicted the capability of g-C3N4 doped with metal on the basis of other nitrogen-enriched p-conjugated macrocyclic scaffolds like porphyrin and phthalocyanine, etc. The as-synthesized g-C3N4 with doped metal cations of Zn2+ and Fe3+ could serve as H2O2 activation agent in many reactions such as the oxidative degradation of various organic dyes, direct oxidation of benzene to phenol under mild conditions, and mimicking metalloenzymes. The optical band gap energy gradually shifted to lower energies with increasing Fe content in the Fe/g-C3N4 hybrid materials, reflecting a host–guest interaction between g-C3N4 and the metal.94 A change in the optical absorption was also observed for Zn/g-C3N4, which is probably caused by the d–p repulsion of the Zn3d and N2p orbitals. A remarkable H2 production rate was obtained on the 10%-Zn/gC3N4 sample reaching a value of 59.5 mmol h1, which is about 10 times higher as that on undoped g-C3N4.94 Ding et al.97 synthesized and studied other transition metal elements (e.g., Co, Ni, Mn, Cu) modified g-C3N4 (M/g-C3N4) for selective hydrocarbon oxidation. The spectra of all resulting samples showed obvious red shifts in the band-gap transition and higher absorbance in the visible region as compared to undoped g-C3N4. In particular, the most pronounced effect was

Fig. 6 (a) UV-visible absorption spectra of g-C3N4 (black) and g-C3N4xSx (purple). (b) A typical time course of hydrogen evolution from water containing 10 vol% triethanolamine scavenger on Pt-deposited g-C3N4 (black) and g-C3N4xSx (purple) under l > 300 and 420 nm, respectively. (c) Activity comparison of the photo-oxidation process of phenol on g-C3N4 (black) and g-C3N4xSx (purple) under l > 400 nm. (d) Schematic of band structure evolution of g-C3N4 by sulfur doping and subsequent quantum confinement effect, reproduced with permission from ref. 85 ª 2010 American Chemical Society.


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found for Cu/g-C3N4, where the light absorption limit was extended to 750 nm. The changes of electronic structure and therefore absorbance in the visible light region on all M/g-C3N4 samples can be simply evaluated by transformations of their colours: the colour of the pure g-C3N4 was pale yellow, whereas the other colours were yellow for Mn/g-C3N4, brown for Fe/gC3N4, gray for Co/g-C3N4, and brown for Ni/g-C3N4 and Cu/gC3N4; deeper colour indicates that a given sample can absorb more visible light. The different electronic structures indicate their unique properties in heterogeneous catalysis. Consequently, Fe and Cu/g-C3N4 were shown to be efficient catalysts for the hydroxylation of benzene to phenol with H2O2, and Co and Fe/g-C3N4 samples were proven to be active for the epoxidation of styrene with O2. 3.1.3 Graphene/g-C3N4 composites. In addition to doping selected heteroatoms into the framework, which could slightly modify the molecular orbital shape and position, pristine gC3N4 can be also engineered by charge-transfer complexation to form a nanocomposite with an electron-rich system that results in rearranging electron density within the catalyst.15,86,100,101 Graphene sheets (GS), excellent electron collectors and transporters,102–105 have been used to boost performance of various energy conversion and storage devices such as photovoltaic devices, supercapacitors, fuel cells, and Li-ion batteries.106–108 As an analogue of graphite, g-C3N4 also possesses a stacked twodimensional structure; therefore it is easily predictable that combining graphene and g-C3N4 with a homogeneous structure will induce unique electronic, mechanical, and chemical properties in the resulting composite (GSCN). Xiang et al.100 showed that GS acting as electronic conductive channels in GSCN can efficiently separate the photogenerated charge carriers and, consequently, enhance the visible light photocatalytic H2production activity of the g-C3N4 catalyst. When loaded with 1.5 wt% Pt, the optimized GSCN catalyst showed a H2production rate of 451 mmol h1 g1 with an apparent quantum efficiency of 2.6% by a water-splitting mechanism under visiblelight irradiation. This value exceeds that of pure g-C3N4 and commercial N-doped TiO2 nanoparticles by a factor of 3.07 and 1.21, respectively. This may be ascribed to GS, which in GSCN composites become the separation center of the photogenerated electrons and holes to effectively hinder the electron–hole pair recombination in a typical photocatalytic process. Layer-by-layer GSCN (Fig. 7a) developed by Li et al.101 revealed a high catalytic activity towards molecular oxygen activation for selective oxidation of secondary C–H bonds with a good conversion rate and high selectivity to the corresponding ketones. They contributed the high-performance of selective oxidation on GSCN to the synergistic effect of GS and g-C3N4 as demonstrated in Fig. 7b: the introduction of GS could obviously shift the HOMO (positive hole) of g-C3N4 to lower energies via p–p* or charge-transfer interactions to ensure the selectivity of the catalytic reaction. 3.2 g-C3N4-based electrocatalysts for highly efficient oxygen reduction Fuel cells offer a great opportunity to obtain cleaner and more sustainable energy in the 21st century. Among various kinds of 6726 | Energy Environ. Sci., 2012, 5, 6717–6731

fuel cells, the proton exchange membrane fuel cell (PEMFC) is believed to be the most promising power source candidate for next generation light-duty vehicles and portable electronics.109 However, one of the main problems encountered in PEMFC commercialization is the need for a low-reserve and expensive Pt electrocatalyst to facilitate its sluggish cathodic oxygen reduction reaction (ORR).110–115 g-C3N4, which possesses high nitrogen content, thus, can provide a sufficient number of active sites for electrocatalytic reactions and enhance its potential for ORR. Due to its non-conductive nature, the electrocatalytic performance of g-C3N4 is greatly limited by its poor electron transfer ability. One of the proposed effective strategies is the use of carbon conductive support like mesoporous carbon, active carbon black, and graphene sheets in the g-C3N4 catalysts to improve the electron accumulation on the surface of catalysts and, consequently, to enhance their electrocatalytic activities. Recent studies show that g-C3N4-carbon hybrids could provide comparable ORR performance to that achieved by commercial Pt/C catalysts as well as high CO and methanol tolerances. Therefore, they are considered as a new generation of cathode catalysts especially for direct methanol fuel cells (DMFC). 3.2.1 3D g-C3N4-based composites as ORR catalysts. Lyth et al.116 first investigated ORR activity on a pure g-C3N4 electrode in acidic medium and found that its fundamental catalytic activity was higher than that of pure carbon. However, the current density achieved on pure g-C3N4 was still low; it was suggested that this is due to the low surface area of the material. Blending g-C3N4 with a high surface area carbon black16 resulted in a significant improvement of the current density; the composite catalysts facilitated a four-electron (4e) ORR process, indicating an enhanced electrochemical efficiency for ORR.117 According to our theoretical studies,14 g-C3N4’s limited electron transfer ability is responsible for the accumulation of OOH intermediate products on the catalyst via an inefficient two-electron (2e) ORR process, which is one of the main reasons for its low ORR catalytic activity. As demonstrated in Fig. 8a, the active sites facilitating ORR on pure g-C3N4 are limited to very narrow zones of the electrode–electrolyte–gas three-phase boundaries (TPB) because of the poor electron transfer efficiency, which leads to an unsatisfactory ORR performance. One effective way to increase the number of electrons accumulated on the g-C3N4 surface and then enlarge the concentration of active sites is to add an electron-conductive material as a support for the g-C3N4 catalyst. Predictably, the active sites facilitating ORR on g-C3N4 with a conductive support can spread over the whole surface of the catalyst due to the increased electron transfer efficiency of the composite, which in turn facilitates an efficient 4e ORR process and sharply enhances the catalyst’s performance. By using mesoporous carbon (CMK-3) as a conductive support, the synthesized nanoporous g-C3N4@CMK-3 catalyst exhibited a much higher ORR activity than that on pure g-C3N4, which is also competitive in relation to a commercial Pt/C catalyst (Fig. 8b and c). In addition, the newly developed g-C3N4@CMK-3 electrode further revealed an enhanced stability in an alkaline electrolyte (Fig. 8d) and much higher tolerance with methanol than that on a commercial Pt/C catalyst (Fig. 8e). All the results show the This journal is ª The Royal Society of Chemistry 2012


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Fig. 7 (a) Schematic structure of a GSCN multilayer. (b) Schematic representation of the oxidation mechanism, reproduced with permission from ref. 101 ª 2011 American Chemical Society.

great promise of g-C3N4@CMK-3 as a feasible ORR electrocatalyst especially for DMFC; the facile synthesis procedure and commercial availability of precursors will also further promote this exciting technology. 3.2.2 2D g-C3N4-based composites as ORR catalysts. Except for the aforementioned 3D ordered mesoporous carbon, 2D graphene is also a great choice because it is an excellent electron collector and transporter; the proposed layer-by-layer (sandwich) structure of g-C3N4/graphene provides a prefect channel for electron transportation. Yang et al.118 synthesized graphenebased carbon nitride nanosheets (G–CN) with individual dispersion of graphene between nanosheets by nanocasting technology using graphene-oxide-based silica nanosheets as a hard template (as illustrated in Fig. 9). The resulting G–CN nanosheets not only possessed a high nitrogen content, thin thicknesses, and high surface areas, but also showed an enhanced electrical conductivity and thus excellent electrocatalytic performance for ORR, including high electrocatalytic activity and high selectivity of 4e ORR path; both are superior in comparison to those observed for carbon nitride sheets without graphene and similar to the commercial Pt/C catalysts. Sun et al.15 also immobilized g-C3N4 onto graphene sheets to form g-C3N4/graphene composites in the liquid-phase solution. The composite exhibited an enhanced electrocatalytic activity for ORR and CO tolerance comparable to that of 23 wt % Pt nanoparticles supported on graphene sheets. They also proposed that the ORR mechanism on g-C3N4/graphene is similar to that on the most developed metal-free ORR electrocatalysts, i.e. nitrogen-doped carbon nanotubes (N–CNT):15 carbon atoms of g-C3N4/graphene in the oxidized state are electrochemically reduced in the first step, and subsequently reoxidized by adsorbed O2 molecules.119 In this case, the ORR activity of the carbon–nitrogen system can be attributed to the high electron affinity of nitrogen atoms, which induces a high positive charge density on the adjoining carbon atoms. Computer simulations have validated this hypothesis, showing that the barrier to ORR on a carbon atom can be reduced in the presence of an adjoining nitrogen atom, especially at quaternary sites near the zigzag edges of graphene sheets.120–123 This journal is ª The Royal Society of Chemistry 2012

4 Other applications: heterogeneous catalysis and template for metal nitride synthesis The unique electronic and surface properties of nanoporous gC3N4 material make it very interesting for applications such as photocatalytic hydrogen production and fuel cell technology. These unique properties can be extended to find numerous applications in related heterogeneous catalysis including O2 activation, organic photosynthesis, and reductive CO2 fixation. 4.1 Friedel–Crafts type reactions Friedel–Crafts reactions refer to a typical aromatic C–H activation process and are known to be some of the less sustainable industrial processes, producing about 88 mass% of waste in their standard AlCl3 promoted version.124 Goettmann et al.125 showed that meso-g-C3N4 was a valuable Lewis base catalyst which enabled rather unusual aromatic substitution reactions of a generalized Friedel–Crafts type. This metal-free catalyst not only allowed for the use of sustainable alkylation agents such as alcohols or acids, but also showed an unpredicted reactivity towards urea and quaternary ammonium compounds. An interesting finding was that the sample with low nitrogen content exhibited a higher catalytic activity in Friedel–Crafts acylation of benzene than that with high nitrogen content (all catalysts gave 100% selectivity), due to the formation of defect sites on the pore walls in the former sample.125 4.2 Selective oxidation reactions Selective oxidation of hydrocarbons with clean oxidants is a crucial process for the development of products ranging from commodity chemicals to speciality pharmaceuticals. Chen et al.95 demonstrated that the Fe/g-C3N4 catalyst was active for the direct oxidation of benzene to phenol using hydrogen peroxide. By taking advantage of the photocatalytic functions of g-C3N4, the yield of the phenol synthesis could be markedly improved. Su et al.69 showed that meso-g-C3N4 can function as a photocatalyst to activate O2 for the selective oxidation of benzyl alcohols under visible light irradiation. By combining the surface basicity and semiconductor functions of meso-g-C3N4, the photocatalytic Energy Environ. Sci., 2012, 5, 6717–6731 | 6727


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Fig. 8 (a) Schemes of ORR’s pathway on pristine g-C3N4 and g-C3N4@CMK-3 composite (red areas represent the active sites facilitating ORR). (b) Cyclic voltammograms of ORR on various electrocatalysts in O2-saturated 0.1 M KOH solution. (c) Linear sweep voltammograms of various electrocatalysts on RDE at 1500 rpm in O2-saturated 0.1 M KOH solution. (d) Current–time (i–t) chronoamperometric response of g-C3N4@CMK-3 at 0.3 V; inset represents cyclic voltammograms under continuous potentiodynamic sweeps. (e) Chronoamperometric responses of Pt/C and g-C3N4@CMK-3 at 0.3 V in O2-saturated 0.1 M KOH solution without methanol (0–3 h) and with adding methanol (3–6 h), reproduced with permission from ref. 14 ª 2011 American Chemical Society.

system can provide a remarkable selectivity (>99%) to generate benzaldehyde. The system also selectively converts other alcohol substrates to their corresponding aldehydes/ketones. Later, using meso-g-C3N4, the same group126 reported the aerobic oxidation of amines into imines with an excellent yield, which are regarded as important electrophilic intermediates in organic synthesis. In addition, g-C3N4 shows a certain degree of catalytic activity to a variety of organic oxidation reactions like oxidation of alkanes, olefins, and heteroatoms, etc. assuring selective and sustainable oxidation,11 therefore making this material of a great potential for industrial applications. 4.3 CO2 activation/reduction reactions Two possible reaction pathways can be envisaged for the reaction of benzene with CO2 activated by meso-g-C3N4:127 a Friedel– Crafts-type condensation of benzene with CO2 to yield benzoic acid, and an oxidation of benzene with CO2 to yield phenol with evolution of CO. However, the formation of benzoic acid is very weakly endothermic and disfavoured entropically. Ansari 6728 | Energy Environ. Sci., 2012, 5, 6717–6731

et al.128 also showed meso-g-C3N4’s catalytic activity towards activation of CO2 by using the oxidation of cyclic olefins with molecular oxygen as an example reaction. They found that the presence of CO2 in the oxidation system could induce an enhanced conversion of cyclic olefins, in which CO2 acted as a source of oxygen for the formation of carbon monoxide and surface carbamate. 4.4 Template for metal nitride synthesis Meso-g-C3N4 can also be used as a reactive hard template to produce metal nitride nanomaterials by directly heating a metal species-loaded sample to 650–800 C under nitrogen atmosphere.44,129–136 During the heat treatment, the melen-derived gC3N4 is decomposed and supplied nitrogen-contained species to efficiently convert metal salts/oxides to nitrides.52 Thomas and co-workers developed this strategy and produced plenty of metal nitrides including TiN, GaN, VN and ternary Al–Ga–N and Ti–V–N nanoparticles by using the cage-like meso-g-C3N4 as a reactive template and a nitrogen source simutaneously.129,130 This journal is ª The Royal Society of Chemistry 2012


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Fig. 9 (a) Fabrication of G–CN and pure CN nanosheets for ORR. (b) Current–time (i–t) chronoamperometric responses at 0.25 V in O2-saturated 0.1 M KOH on G–CN and Pt–C electrodes followed by introduction of O2 and methanol (0.3 M). (c) Electrochemical activity given as the kineticlimiting current density (JK) at 0.40 V and electron transfer number (n) for all G–CN and CN nanosheets. CN600, CN800, and CN1000 represent samples of carbon nitride (CN) with/without graphene pyrolysis at different temperatures; see panel (a) for details. Reproduced with permission from ref. 118 ª 2011 Wiley-VCH.

Metal oxides such as TiO2, V2O3, Ga2O3, Nb2O5 and Al2O3 can also be converted to corresponding nanostructured metal nitrides.131–133 Domen and co-workers adopted this method to synthesize tantalum nitride nanoparticles using TaCl5 as a precursor.134 It is found that TaN can be produced under N2 atmosphere but pure phase high quality Ta3N5 is formed when N2 is replaced by NH3 atmosphere.134 The obtained Ta3N5 nanoparticle catalyst exhibited a high performance in photocatalytic water splitting.

theoretical modelling is needed to understand the physicochemical properties of carbon nitride and to elucidate the relationship of nanostructure, catalytic kinetics and shape selectivity. Depending on the successful combination of theoretical prediction and experimental technique for g-C3N4 in the applications of artificial photocatalysis and oxygen reduction, a great deal of other capacities should be explored to develop a cheap and commonly available metal-free catalyst for broad applications across the areas of heterogeneous catalysis, sensors, and lithium ion batteries.

5 Summary and outlook As a multifunctional metal-free catalyst, g-C3N4 shows promising activity towards artificially photocatalytic hydrogen production. Its potential applications can also be extended to a wide range covering heterogeneous catalysis for various reactions, such as CO2 activation/reduction, and reactive template for metal nitride synthesis when equipping g-C3N4 with other heteroatoms or nanocasting it to a special nanostructure. Especially, combination of a conductive support (e.g. mesoporous carbon or graphene) with g-C3N4 can result in a highly efficient cathodic ORR electrocatalyst, which opens a new avenue for developing more metal-free ORR catalysts to promote fuel cell technology. Sandwich-like homogeneous g-C3N4/graphene nanosheets seem to be a perfect structure for the electron transfer pathway from graphene sheet to g-C3N4 sheet, assuring an efficient oxygen reduction. The powerful theoretical calculation is still on its way to clarify the ORR mechanism and pathway on the g-C3N4/graphene nanosheet in detail. Although the potential applications of g-C3N4 have been obviously extended, the correlations between the g-C3N4 structure and its catalytic activity in a specific reaction are still not very clear. Further This journal is ª The Royal Society of Chemistry 2012

Acknowledgements This work was financially supported by the Australian Research Council (ARC) through Discovery Project program (DP1095861, DP1094070, DP0987969). JL gratefully acknowledges the award of an ARC Australian Postdoctoral Fellowship (APD) and a UQ Early-Career-Research Grant.

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