Porous Graphitic Carbon Nitride: A Possible Metal-free Photocatalyst

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Porous Graphitic Carbon Nitride: A Possible Metal-free Photocatalyst for Water Splitting Kancharlapalli Srinivasu, Brindaban Modak, and Swapan K Ghosh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp506538d • Publication Date (Web): 22 Oct 2014 Downloaded from http://pubs.acs.org on October 26, 2014

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The Journal of Physical Chemistry

Porous Graphitic Carbon Nitride: A Possible Metalfree Photocatalyst for Water Splitting K. Srinivasu, Brindaban Modak and Swapan K. Ghosh* Theoretical Chemistry Section, Bhabha Atomic Research Centre, Mumbai – 400 085, India. Homi Bhabha National Institute and UM-DAE-Centre for Excellence in Basic Sciences, Mumbai, India

ACS Paragon Plus Environment

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ABSTRACT. Hydrogen generation through photocatalytic water splitting with the aid of renewable solar energy is an important step towards the development of sustainable and alternative energy. In the present study, using the first principles calculations, we have explored the s-triazine based two-dimensional porous graphitic carbon nitride (g-CN) materials as potential photocatalyst for water splitting. For calculating the band structures more accurately, we have employed hybrid density functionals. The calculated band gap of the single layer g-CN is found to be 2.89 eV, which decreases to ~2.75 eV in multilayered structure. To improve the visible light activity, effect of doping with different non-metals on the electronic structure has been investigated. Among the different dopants studied, phosphorous is found to be more effective to reduce the band gap to 2.31 eV. The band edge potentials obtained from density functional calculations are corrected for vacuum potentials. The band alignments with respect to the water redox levels show that the thermodynamic criterion for the overall water splitting is satisfied. We have also carried out analogous studies on the heptazine based carbon nitride, g-C3N4 and the calculated band gaps as well as the position of the valance band maximum are consistent with the reported experimental results validating the computational method we have used.

Based on our

theoretical investigations, we can predict that the considered carbon nitride based materials should be a potential photocatalyst for water splitting under visible light.

KEYWORDS . Graphitic carbon nitride, Photocatalytic activity, Water splitting, Hybrid density functional, Substitutional doping.


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1. INTRODUCTION Search for sustainable, alternative energy sources has become an important area of research due to the world wide increasing energy demands in addition to the limited resources of the fossil fuels and their adverse environmental affects. Hydrogen is considered to be one of the best possible alternative energy carriers.1-2 However, the main challenges in developing hydrogen energy are its generation and storage. Hydrogen generation from water through photocatalytic water splitting using the solar energy is an ideal way which does not evolve any harmful byproducts.4 Following the development of photo electrochemical cell (PEC) by Fujishima and Honda5 using a semiconductor anode and a metal cathode, varieties of semiconductor based photocatalysts have been investigated for making an efficient catalyst to generate hydrogen from water. The mechanism of photocatalytic water splitting using semiconductor materials involves different steps, viz. generation of charge carriers (electron and holes) on light absorption, separation and diffusion of the photo generated charge carriers to the active sites on catalyst surface and further reaction (oxidation/reduction) with the adsorbed water to generate hydrogen and oxygen.3 Among them, TiO2 has been extensively investigated due to its potentiality to split water. However, its wide bandgap (~3.0 eV) limits its activity to the UV region of the solar spectrum.6 During past few years, extensive efforts have been dedicated to enhance the visible light activity of TiO2, mainly by doping with different metal of non-metal elements.6 Apart from TiO2, many other transition metal oxides, sulphides, selenides viz. ZnO, SrTiO3, NaTaO3 , ZnS, CdS, CdSe, etc. have been studied for photocatalytic water splitting.6-12

Other than these

conventional materials, polymeric semiconductor materials like graphitic carbon nitrides are also


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found to have great potential as photocatalyst. Wang et al.13 have reported the melem-based graphitic carbon nitrides (g-C3N4) as a metal-free photocatalyst for visible-light driven water splitting. However, this material is reported to show very poor quantum yield which is attributed to the high recombination rate of the photo generated electron-hole pairs. To overcome this problem, many attempts have been made to tune the properties, viz. doping, metal decoration, introducing porosity, making graphene/g-C3N4 composites etc.14-17 Pan et al.18 have shown that the g-C3N4 nanotubes have better carrier mobilities as compared to the 2D sheets and further enhancement in photocatalytic activity upon functionalization of these tubes with metals like Pt and Pd is reported.

Wang et al.19 have observed enhanced photocatalytic activity for the

nonocomposites made of g-C3N4 and MoS2 in comparison to pure g-C3N4. Very recently, Shi et al.20 have synthesised the composites of mesoporous carbon and g-C3N4 and observed significant improvement in photocatalytic activity.

Apart from the g-C3N4, another polymeric carbon

nitride, the poly (triazine imide), is also found to be a potential photocatalyst for water splitting.21-23 In the present study, through first-principles based calculations, we have explored another form of graphitic carbon nitride with CN stoichiometry (g-CN) as a photocatalyst for water splitting under visible light. Though the g-C3N4 form of carbon nitride has been studied extensively, the number of studies on g-CN is very less. Cao et al.24-25 synthesized the g-CN nanotubes through the reaction of cyanuric chloride (C3N3Cl3) with sodium using NiCl2 as a catalyst. Different forms of g-CN, viz. nanotubes, nanoribbons and microspheres have been synthesized and their photoluminescence spectra have shown peaks centered around 430-450 nm, due to the band gap emission. Because of the porous nature of carbon nitride and their bio-compatibility, different carbon nitride systems are also studied for their application in biotechnology.26-27 Here, we have


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considered the free standing single layer as well as multi layers of g-CN. To further enhance the visible activity of g-CN, we have also studied the effect of substitutional doping with different non-metal elements like boron, oxygen, phosphorous etc. Analogous studies on g-C3N4 are also being carried out to compare the computational results with the existing experimental results. To the best of our knowledge, this is the first study to explore g-CN as a possible active photocatalyst for hydrogen generation through water splitting.

2. COMPUTATIONAL DETAILS: All our spin-polarized periodic density functional theory (DFT) based electronic structure calculations have been carried out using the projector augmented wave (PAW)28-29 potentials as implemented in the first principles based Vienna ab initio Simulation Package (VASP).30-31 Plane-wave basis sets with a kinetic energy cutoff of 550 eV have been used. The exchangecorrelation energy density functional, Exc [ρ] has been treated through the Generalized Gradient Approximation (GGA) of Perdew-Burke-Ernzerhof (PBE)32. An energy cutoff of 1 x 10-6 eV is set as the convergence criteria for the electronic self-consistent field iterations. The atomic positions are optimized until the maximum Hellmann−Feynman force on each atom is less than 0.01 eV Å-1. Automatically generated Gamma centered 9 × 9 × 1 Monkhorst−Pack set of kpoints were used to sample the Brillouin zone.33 As it is known that the band gaps calculated from PBE functional are underestimated, we have also used the more accurate hybrid functional, developed by Heyd, Scuseria, and Ernzerhof (HSE03 and HSE06)34 using the PBE optimized geometries to improve the accuracy of band structure calculations. All the initial geometries and the reported figures are generated using the graphical software XCrySDen and VESTA.35-36


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3. RESULTS AND DISCUSSION: At first, we will look into the electronic structure of the single layer two-dimensional g-CN studied through the density functional theory. The optimized unit cell structures of the twodimensional g-CN considered is shown in Figure 1a. The optimized cell parameter from PBE method is found to be 7.127 Å with the C-C and C-N bond distances of 1.510 Å and 1.341 Å respectively. From the PBE calculations, g-CN is found to have a direct band gap of 1.54 eV. However, the band gap of single layer g-CN calculated through the HSE03 functional is found to be 2.89 eV which is more close to the experimental value25 as compared to the corresponding gap from HSE06 functional (3.18 eV). To verify the accuracy of the method, we have also calculated the band gap of g-C3N4 using both HSE03 and HSE06 methods and the band gap is found to be 2.82 eV and 3.0 eV respectively indicating that the HSE03 result is closer to the experimental value of 2.73 eV.37 Hence, here onwards we will discuss only the HSE03 results and the corresponding PBE and HSE06 results are given in Table 1. The band dispersion plot of g-CN single layer calculated through the HSE03 method is shown in Figures 1b. From the band structure, it is clear that both the valance and conduction bands are well dispersed and no localized states are present to act as recombination centers for the photo generated electron hole pairs. This is in contrast to the case of g-C3N4 for which localised band edges are reported in the band structure and claimed to be the reason for its poor efficiency observed.18 Hence the life time of the photo-generated charge carriers in g-CN is expected to be longer which can lead to better photocatalytic efficiency. We have also calculated the band decomposed charge density of the valance band maximum (VBM) and conduction band minimum (CBM) and the corresponding iso-surface plots are shown in Figures 1c and 1d respectively. From the isosurface plots, it is clear that the VBM mostly originates from the non-bonding nitrogen 2p states


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whereas the CBM is originating from the C=C anti-bonding π states (carbon pz states) and nitrogen pz states. As the measured band gap is 2.89 eV, it cannot utilize the major part of the solar spectrum and hence, it is required to engineer the band gap to enhance the visible light absorption efficiency of these materials. In one of the recent studies on g-C3N4, through the first principle calculations, Wu et al.38 have shown that the bilayer has much better visible light adsorption efficiency as compared to the single layer which is attributed to the interlayer coupling. To verify this effect on the g-CN, we have also studied the electronic structure of the bilayer as well as the trilayer of g-CN. For the bilayer, we have considered two different stacking modes, AA and AB. From the optimized structures, the AB stacking mode as shown in Figure 2a is found to be energetically more favourable as compared to the AA mode. The optimized inter layer separation from the PBE method is found to be 3.578 Å. However, as it is known that the van der Waals interactions are important in this kind of systems, we have reoptimized the structure using the dispersion corrected PBE (PBE-D2)39 method and the interlayer separation is observed to be 3.088 Å indicating the strong coupling between the two g-CN layers. The interlayer interaction energy per unit cell is found to be -0.57 eV. The calculated band structure plot of the bilayer is given in Figure 2b and the corresponding band gap is found to be 2.81 eV which is 0.08 eV less as compared to that of the single layer. This indicates that the band gap is reduced due to the interlayer coupling in g-CN. To see any further effect of inter-layer coupling on the electronic structure, we have also studied the electronic structure of trilayer g-CN. For trilayer, we have added one more g-CN sheet above the bilayer in two different ways, ABA and ABC. It has been found that the ABC mode of stacking as shown in Figure 2c is energetically more favourable with interlayer


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separation of 3.029 Å and the corresponding inter-layer interaction energy per unit cell is calculated to be -1.19 eV. The calculated band dispersion plot is reported in Figure 2d and the corresponding band gap is found to be 2.75 eV which is 0.14 eV less as compared to the single layer band gap. To mimic the multi layer structure more accurately, we have considered another system with six layers of g-CN stacked in ABCABC manner. The calculated band gap of this system is found to be 2.72 eV which is very close to that of tri-layer g-CN (2.75 eV) indicating that it is nearly approaching the multilayer nature. These results indicate that though the band gap is getting lowered on forming multiple layers, the reduction is not to the extent to achieve appreciable visible light activity. Doping with foreign elements has been shown to be an effective method for tuning the electronic band structure of semiconductor materials due to the difference in the energy levels of the dopant element as compared to that of the host element.40-41 Here, to further reduce the band gap of g-CN, so that the material can absorb the major part of the solar spectrum, we have studied the effect of doping with different non-metal elements like boron, oxygen, sulphur, and phosphorous. In the case of boron doping, we have considered two possible structures such that in one of the structures boron is substituted in place of nitrogen while in the other, boron replaces the carbon atom. From the optimized energies, it is found that the boron substitution in place of carbon is energetically more favourable as compared to the case of replacement of nitrogen by boron. The band gap of the boron doped system is found to be 2.84 eV which is comparable to that of the undoped system and hence boron doping is not helpful in reducing the band gap. In the cases of oxygen, sulphur and phosphorous doping, substitution of nitrogen as shown in Figure 3a is found to be energetically more preferred as compared to replacement of the carbon. The calculated band gaps in oxygen and sulphur doped g-CN are found to be 2.81 eV and


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2.49eV respectively. However, both oxygen and sulphur doping is found to introduce isolated donor states below the conduction band which can act as recombination centre and hence can decrease the efficiency of the catalyst. However, in the case of phosphorous doped g-CN, the band gap is found to be reduced considerably to 2.31 eV without introducing any undesirable midgap states in the forbidden region and the corresponding band structure plot is shown in Figure 3b. These results on non-metal doping reveal that the phosphorous doping will be more suitable to bring down the band gap of g-CN without creating any undesirable midgap states. To verify the analogous doping effects on g-C3N4, we have studied the electronic structure of similar non-metal doped g-C3N4. We have verified the different possible sites for substitutional doping viz. three different nitrogen sites, one two coordinated nitrogen and two different three coordinated nitrogen sites as well as one carbon site which is three coordinated. In the cases of boron and phosphorous, substitution in place of carbon is found to be more preferred as compared to all three different nitrogen sites, whereas in the cases of sulphur and oxygen, nitrogen which is coordinated to two carbons (pyridine like nitrogen) is found to be energetically preferred site for substitution and these results are consistent with the earlier results on sulphur and phosphorous doping.16 The optimized geometry of g-C3N4 and calculated density of states for g-C3N4 along with all the doped systems are reported in Figures S1 and S2 (supporting information). The calculated band gap in the cases of boron doped system is found to be 3.24 eV, and it is found to create midgap states as shown in Figure S2 which can trap the photo-generated carriers thereby decreasing the efficiency. However, it can be observed that the conduction band becomes highly dispersed as compared to that of the undoped system. The calculated band gap in oxygen and sulphur doped g-C3N4 is found to be 2.98 eV and 3.15 eV respectively and the increase in bandgap upon sulphur doping is consistent with the experimental


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results.37 As both O and S are n-type dopants, occupied donor states are found to be created close to the bottom of conduction band as it is shown in Figure S2. In the case of phosphorous doped g-C3N4, the calculated band gap is found to be 2.70 eV which is 0.12 eV less as compared to the undoped g-C3N4 and this result is consistent with the recent experimental observations of Hu et al.42 However, in this case also dopant donor states are fond below the conduction band. These results indicate that, except phosphorous all other doped systems are found to have wider band gap as compared to the pristine g-C3N4. To have a look at the optical absorption characteristics of these g-CN systems considered, we have calculated the complex frequency dependent dielectric ε (ω ) = ε1 (ω ) + iε 2 (ω ), function,

where ε1 and ε2 represent the real and imaginary parts of the dielectric function,

respectively. The absorption coefficient α(ω) has been obtained from the relation43

α (ω ) = 2ω

(

2

2

ε1 (ω ) + ε 2 (ω ) − ε1 (ω )

)

1

2

.

The calculated absorption spectra for the pure single layer g-CN system along with its boron and phosphorous substituted systems are shown in Figure 4. The undoped g-CN is found to have a strong absorption peak around 260-300 nm which is the characteristic of π to π* transition observed in the s-triazine compounds and the result is consistent with the experimental observations (260-280 nm).25 This indicates that the absorption of pure g-CN is limited to the UV region of the spectrum. In the case of boron doped g-CN, though the peak is broadened slightly as compared to pure g-CN, still it is in the UV region only. However, in the case of phosphorous doped g-CN, the absorption curve is considerably extended towards the visible region indicating its improved visible light activity as compared to the undoped and boron doped systems.


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Though improving the visible light activity is a necessary condition for photocatalytic water splitting using solar light, it is not sufficient. To generate hydrogen and oxygen from the water splitting, the band edges should be positioned appropriately with respect to the redox levels of water. As the position of the band edges obtained directly from DFT are known to be shifted when a pseudopotental based method is used along with the periodic boundary condition, we have corrected the band edges using standard method.44-45 We have calculated the averaged local potential along the z-direction (perpendicular to the g-CN layer) and the plot of the electrostatic potential energy as a function of vacuum thickness (distance) is shown in Figure 5. The plot shows that the energy increases and gets saturated which can be taken as the electrostatic potential energy in vacuumed and all the Eigen values are corrected with respect to this potential. To assure the accuracy of the method, we have first calculated the band edge potentials of the gC3N4 for which the experimental results are available. The calculated valence band maximum (VBM) for g-C3N4 is found to be 2.06 eV below the water reduction level which is very close to the experimental reported value (2.12 eV below the water reduction level) indicating the accuracy of the method. The conduction band minimum (CBM) of g-C3N4 is found to be located 0.76 eV above the water reduction potential as it has been shown in the band alignment plot in Figure 6. The corresponding band alignments for boron and phosphorous doped g-C3N4 are also shown in Figure 6 and it shows that the boron doped system is found to have more elevation of conduction band which is 1.19 eV above the water reduction. From these results on g-C3N4 it is clear that though the band gap is increased on B-doping, its conduction band becomes more dispersed which can result in better carrier mobilities and also its reduction capacity increases due to the considerable elevation of the conduction band. As shown in Figure 6, for the single layer g-CN, the CBM and VBM are found to be located at -4.24 eV and -7.13 eV respectively.


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The CBM is located just (0.26 eV) above the water reduction level whereas the VBM is found to be 1.41 eV below the water oxidation potentials. In the case of multilayer systems, the VBM is located at around -7.2 eV and the CBM is around -4.3 to -4.4 eV. In the case of boron doped gCN, though the band gap does not alter much, both the conduction and valence band positions are found to be lifted as compared to the pure g-CN indicating improved reduction capacity. In the case of phosphorous doping, the change in CBM is found to be negligible but the VBM shift upward is significant as compared to the undoped system. Thus, in all the systems the CBM is located above the water reduction potential and the VBM is well below the water reduction potential. In both the cases of g-CN and g-C3N4, boron doping is found to lift the CBM thereby improving the reduction potential of the catalyst. These band alignments show that both the oxidation and reduction reactions of water are thermodynamically feasible in both g-C3N4 and gCN. Due to the better dispersion of band edges, the g-CN, especially its phosphorous doped counterparts with lower band gap of 2.31 eV can be expected to be a good photocatalyst material for hydrogen generation under visible light. As the doping with non-metal elements has been well explored in graphene and other forms of carbon nitride (g-C3N4), it is experimentally quite possible to synthesize this photocatalyst systems.37, 42, 46

4. CONCLUSIONS In summary, we have explored the possible photocatalytic activity of s-triazine based twodimensional graphitic carbon nitrides toward the water splitting under visible light. Single layer 2D g-CN system is found to have a direct band gap of 2.89 eV, while the multilayer g-CN materials are found to have slightly improved visible light absorption due to the interlayer coupling. Absence of localized states in the band dispersion plot indicates better charge carrier mobilities. The calculated band gap and band edge potentials of g-C3N4 are very close to the


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experimentally reported results indicating the accuracy of the method used. Effect of doping with non-metal elements like boron, oxygen, phosphorous, etc. on the electronic band structure of g-CN as well as g-C3N4 has been investigated. Substitutional doping with phosphorous in gCN is found to improve the visible light absorption significantly by bringing down the band gap to 2.31 eV without introducing any midgap states. Our results on sulphur and phosphorous doped g-C3N4 are matching well with the reported experimental results. The calculated optical absorption spectra show significant enhancement in visible light absorption efficiency on phosphorous doping. The positions of band edges in pure g-CN as well as phosphorous doped counterpart with respect to water redox levels are found to satisfy the thermodynamic criteria for overall water splitting. ASSOCIATED CONTENT

Supporting Information. Optimized geometry if g-C3N4, DOS plots for g-C3N4 and its nonmetal doped counter parts are reported. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author *Email: [email protected] Phone: 91-22-25595092

ACKNOWLEDGMENT We thank the BARC computer center for providing the high performance parallel computing facility. We also thank Dr B.N. Jagatap for his encouragement and support. The work of S.K.G. is supported through Sir J.C. Bose Fellowship from the Department of Science and Technology, India.


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Figure Captions Figure 1. (a) Optimized geometries of the g-CN along with its (b) band structure plot using HSE03 methods and the band decomposed charge density plot of (c) highest valance and (d) lowest conduction states

Figure 2. Optimized supercell structures of (a) double layer g-CN with AB stacking and (c) triple layer with ABC stacking along with the band structure of (b) double layer and (d) triple layer calculated through HSE03 functionals.

Figure 3. (a) Optimized unit cell structure and (b) band structure plot (from HSE03) of phosphorous doped g-CN

Figure 4. Calculated optical absorption spectra of different g-CN systems considered

Figure 5: Plot of averaged electrostatic potential as a function of vacuum thickness for the g-CN (a) single layer, (b) bilayer and (c) trilayer.

Figure 6. Calculated VBM and CBM positions of g-CN and its boron and phosphorous doped counter parts.


19


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Table 1. The calculated band gap results of different g-CN systems using PBE, HSE03 and HSE06 methods.

System

Band gap (eV) PBE

HSE03

HSE06

Single layer

1.54

2.89

3.18

Bilayer

1.49

2.81

3.10

Trilayer

1.45

2.75

3.03

six-layer

1.41

2.72

2.99

g-CN-B

1.71

2.84

3.11

g-CN-P

1.15

2.31

2.52


20

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(a)

(b)

8 6

Energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4 2 0 -2 -4

Κ

Γ

(c)

Μ

Γ

(d)

Figure 1. (a) Optimized geometries of the g-CN along with its (b) band structure plot using HSE03 methods and the band decomposed charge density plot of (c) highest valance and (d) lowest conduction states.


21


(c)

(a)

8

(b)

Energy (eV)

6

4 2 0 -2 -4Γ Γ

(d)

8

6

Energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4 2 0

-2

Κ

Μ

Γ

-4Γ Γ

Κ

Μ

Γ

Figure 2. Optimized supercell structures of (a) double layer g-CN with AB stacking and (c) triple layer with ABC stacking along with the band structure of (b) double layer and (d) triple layer calculated through HSE03 functionals.


22

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(a)

(b)

8 6

Energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4 2 0 -2 -4

Γ

Κ

Μ

Γ

Figure 3. (a) Optimized unit cell structure and (b) band structure plot (from HSE03) of phosphorous doped g-CN


23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Absorp. coeff (arb.unit)


200

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g-CN B@g-CN P@g-CN

300

400

500

600

700

Wavelength (nm) Figure 4. Calculated optical absorption spectra of different g-CN systems considered


24

Page 25 of 27

5

0

Energy (eV)

0

-5

(a) -10

-15

-5

(b)

-10 -15

-20

-20

0

5

10

15

20

25

0

5

0

10

15

20

25

30

35

0

Distance (A )

Distance (A )

5 0

Energy (eV)

Energy (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-5

(c)

-10 -15 -20 0

5

10

15

20

25

30

35

0

Distance (A )

Figure 5: Plot of averaged electrostatic potential as a function of vacuum thickness for the g-CN (a) single layer, (b) bilayer and (c) trilayer.


25


-3.0

B @ g -C 3 N 4 P @ g-C 3 N 4

g -C 3 N 4

g -C N

-4.5

B @ g -C N P @ g -C N

2.31 eV

2.85 eV

2.89 eV

3.24 eV

2.70 eV

+

2.82 eV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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H /H 2 O 2 /H 2 O

-6.0

-7.5

Figure 6. Calculated VBM and CBM positions of g-C3N4 and g-CN along with their boron and phosphorous doped counter parts.


26

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TOC Absorp. coeff (arb.unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


200

g-CN B@g-CN P@g-CN

300

400

500

600

700

Wavelength (nm)


27

Porous Graphitic Carbon Nitride: A Possible Metal-free Photocatalyst

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