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Passivated Codoping Can Improve the Solar-toHydrogen Efficiency of Graphitic Carbon Nitride Jin Feng, Huizhong Ma, Tingwei Chen, Chengbu Liu, and Yuchen Ma J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01122 • Publication Date (Web): 19 Mar 2018 Downloaded from http://pubs.acs.org on March 20, 2018

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The Journal of Physical Chemistry

Passivated Codoping Can Improve the Solar-to-Hydrogen Efficiency of Graphitic Carbon Nitride

Jin Feng, Huizhong Ma, Tingwei Chen, Chengbu Liu and Yuchen Ma*

School of Chemistry and Chemical Engineering, Shandong University, 250100, China

*

Corresponding author: Yuchen Ma E-mail: [email protected] Tel: +00865318836 3138 1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

Abstract Passivated codoping has never been reported to enhance the solar-tohydrogen activity of g-C3N4, although it is an effective approach for other materials. In this letter, we use many-body Green’s function theory to analyze the electronic structures, optical absorption spectra and spatial distribution of electron-hole pair of the doped g-C3N4. Our results suggest that the passivated codoping, such as B+O, could not only extend absorption towards visible range but also promote the separation of photogenerated hole and electron to different parts of g-C3N4. Improvement of photocatalytic activity can be realized only when the ntype and p-type dopants reside in different tri-s-triazine units of g-C3N4. This manifests the important role of codoping microstructure on solar-tohydrogen efficiency. These results could provide a guideline for the design of more efficient artificial photocatalysts.

2


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1. Introduction Graphitic carbon nitride (g-C3N4) is an earth-abundant, low-cost and highly stable 2D conjugated semiconductor. Since g-C3N4 was discovered to produce H2 from water 1, it has been regarded as a potential material for water splitting in industry. Interpreting its photocatalytic mechanism and improving its inherent low photocatalytic efficiency attract particular interest

2-10

. In principle, the basic requirement for photolysis of water is

that the valence band maximum (VBM) and conduction band minimum (CBM) of the photocatalyst should bestride both the reduction potential of H+/H2 [0 V versus reversible hydrogen electrode (vs. RHE) corresponding to -4.44 eV versus vacuum level] and the oxidation potential of O2/H2O (1.23 V vs. RHE) 11. Taking overpotential and other factors into account, the desirable band gap of the catalyst is around 2.0 eV.12 The large optical band gap of g-C3N4 (2.7 eV) makes it suffer from narrow visible light absorption range and low quantum efficiency.1 Moreover, its poor quantum efficiency is also caused by the high carriers recombination rate due to the close proximity of the photoexcited electron and hole in space.13 Therefore, the solar-to-hydrogen efficiency can be boosted mainly from two aspects.14-15 On one hand, the optical band gap is reduced in the case that VBM and CBM still bestride the redox potential of water. On the other hand, the electron and hole are manipulated to be separated in space to attenuate their recombination.16 3



Extensive research

4, 17-20

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has been conducted based on these two aspects

successfully. For instance, forming heterojunction nanohybrids doping by other elements

22-23

and introducing defects

24-25

2, 21

,

have shown

better quantum efficiency compared to pure g-C3N4. In recent years, doping, especially nonmetal doping which keeps the metal-free virtue of g-C3N4, has gained increasing attention.5, 9, 26-30 For better use of this technique, it is important to understand the role of dopant. However, theoretical study on this respect is still rare and the interpretations available are conflicting. For example, the band-gap states caused by the dopants O7, 31 and S27 are proposed to be close to CBM and VBM respectively, although they are both n-type dopants for g-C3N4. Furthermore, the photochemical performance improvement of doped gC3N4 is going on slowly in recent years. In the O-doped7, 31 and S-doped27 g-C3N4, which appeared in 2010 or earlier, the partially occupied bandgap bands can act as electron-hole recombination centers, reducing the photogenerated carriers transferred to water.29, 32 A passivated codoping approach,33-35 in which both n-type and p-type dopants are included, is able to avoid this situation in theory. This method has been proven efficient in passivated Mo-C-codoped TiO2 by density functional theory (DFT),32 followed by experimental confirmation the next year.36 Nonetheless, the application of passivated codoping in g-C3N4 has never 4


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been reported yet. Based on the aforementioned questions, an in-depth theoretical study of doped g-C3N4 is needed. The purpose of this work is to determine what effects happen on gC3N4 after passivated codoping. We first examine monodoped g-C3N4 with B and O to determine the relationship between band structure and atomic specie. Then, many kinds of configurations for B-O/g-C3N4 are tested in GW+BSE method. Through the electronic structure, absorption spectrum and spatial distribution of both single particles and electronhole pair, we find that B-O/g-C3N4 shows obvious enhanced visible absorption and higher electron-hole separation rate only when B and O are in different tri-s-triazine units. 2. Theoretical methods We apply the ab initio GW method and Bethe–Salpeter equation (BSE) within

many-body

Green’s

function

theory

to

explore

the

photoelectrochemical behaviors of the passivated B-O-codoped g-C3N4 (donated as B-O/g-C3N4). Band structures are calculated in GW, while optical spectra and excitonic properties are computed by BSE. All geometries are optimized by DFT within the local density approximation (LDA) using the plane-wave-based Vienna Ab-initio Simulation Package (VASP) code 37. The optimized lattice constant of g-C3N4 is 7.12 Å which is in accord with experimental value of 7.13 Å 1. A 2×2 supercell is used 5



in our work. A vacuum gap of 35 Å is imposed perpendicular to the gC3N4 surface, which has been verified to give converged GW energies for VBM, CBM and the band gap (SI-Fig. S1 and Fig. S2). GW calculations are carried out at the level of the one-shot GW, i.e. G0W0. We use the orbital energies and wave functions from DFT-LDA to construct the main physical quantities, Green’s function and the screened Coulomb potential, in the GW approach. Since LDA generally underestimates the gaps between occupied and unoccupied orbitals, for example by more than 3 eV for g-C3N4 according to our calculations, using LDA orbital energies directly can lead to some errors in the description of electronic screening and Green’s function. Therefore, in our work a rigid scissor shift is added to the LDA energies of unoccupied orbitals in order to get more accurate results. The value of the scissor shift is adjusted self-consistently until it equals the quasiparticle correction to the band gap. Therefore, our GW calculations can be regarded as a partially self-consistent GW. Our GW + BSE calculations are carried out using a code with Gaussian orbitals as the basis set.38-40 We use the same set of basis as our previous work on g-C3N4.21 In GW, the electronic screening effects are evaluated by the random-phase approximation and the plasmon-pole model. All the unoccupied bands are included in the band summations for evaluations of dielectric function and self-energy in 6


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GW. A Chadi–Cohen-type k-point mesh, which reflects the hexagonal symmetry of g-C3N4, is applied in GW. Twenty-four k-points in the first Brillouin zone are used, which is tested to converge GW energies within 0.06 eV. In BSE, a converged 8×8×1 Monkhorst–Pack k-point mesh is used to represent the excited state. 3. Results and discussion 3.1. Electronic properties of monodoping g-C3N4

Fig. 1 Structures of g-C3N4 complexes and their DOS/PDOS. (a) and (b): pure gC3N4. (c) and (d): O/g-C3N4. (e) and (f): B/g-C3N4. C, N, B and O atom are shown in gray, blue, green and red balls. The black, purple and green lines represent the total DOS for pure g-C3N4, O/g-C3N4 and B/g-C3N4. The PDOS on C and N are depicted in blue and red. The energy in the abscissa of (b), (d) and (f) is referred to the vacuum level which is set to 0 eV. 7



Understanding monodoping is fundamental for the comprehension of codoping. Fig. 1 compares the density of states (DOS) and projected DOS (PDOS) calculated within DFT for the pure g-C3N4, B/g-C3N4 and O/gC3N4 where the energy is aligned with respect to the vacuum level. The energy of vacuum level is set to 0 eV. For pure g-C3N4 in Fig. 1a, the valance band edge and conduction band edge consist mostly of N 2p and C 2p orbitals respectively (Fig. 1b), in keeping with previous works.1, 27, 41-42

The configurations for B/g-C3N4 and O/g-C3N4 presented in Fig. 1c

and Fig. 1e are the most stable one according to our calculations. Compared with the band energy of pure g-C3N4, monodoping by B or O does not shift the absolute positions of VBM and CBM but produces a partially occupied band-gap band above VBM (for B-doping) or below CBM (for O-doping). Therefore, the reported band gap narrowing in O/gC3N4 by Li et al., where VBM remains constant in the X-ray photoelectron spectroscopy, is caused by the appearance of defect state.7 The PDOS of O/g-C3N4 in Fig. 1d shows that the band-gap state is mainly derived from C orbital. Although O atom is more electronegative than N, it needs one less electron than N. As a result, C atom keeps one more electron, making an unoccupied C state become partially occupied and shift down in energy. This mechanism is applicable to other n-type dopants, such as S (see SI-Fig. S3 for the PDOS). Thus, it is inappropriate for Liu et al. to ascribe band gap narrowing in S/g-C3N4 to upshift of 8


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VBM.27 From the PDOS in Fig. 1f, the band-gap state for B/g-C3N4 is mainly contributed by the N orbital. B atom donates one less electron than C to N, therefore an occupied N state turns to be partially occupied and rises up energetically. Although half-occupied band gap states can make the absorption edge red shift,10, 19 it also leads to easy trapping of photogenerated carriers, high electron-hole recombination rate and low photogenerated current.4, 43 3.2. Formation energy of B+O-doped monolayer g-C3N4 If codoped by B (substituting C) and O (substituting N) with the ratio of 1:1, the number of electron does not change and the half-occupancies can be eliminated. In this work, we design some B-O/g-C3N4 complexes to investigate the effects of passivated codoping on the photocatalytic capability of g-C3N4. Composition of the complexes is fixed at g-C3-xN4xBxOx

with x=0.125. In constructing the complexes, we assume that B

takes the same position as that in the most stable monodoped B/g-C3N4 configuration (the green ball in Fig. 2), while ten N lattice sites, half in the same tri-s-triazine unit as B and the other half in the adjacent unit (Fig. 2), are sampled to place O. We also replace two C atoms (site 6 and site 9 in Fig. 2a) by O to compare the stability of doping. Stability of the twelve configurations is calculated to be: 4 > 3 ≈ 10 > 1 ≈ 2 ≈ 7 ≈ 8 ≈12 > 11 > 5 > 6 > 9 (see Fig. 2b for the formation energy). Many of them are more 9



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stable than the monodoped B/g-C3N4 and O/g-C3N4 whose formation energies are -0.73 and -0.97 eV. The formation energy is calculated using following equation: EForm=E(doped) - E(pure) + mE(C) + nE(N) - pE(O) - qE(B)

(1)

where E(doped) and E(pure) are the total energies of the doped and pure g-C3N4, m/n/p/q are the numbers of the substituted C/N atoms and the doped O/B atoms, E(N) and E(O) are equal to E(N2)/2 and E(O2)/2, E(C) and E(B) come from E(graphene)/2 and E(γ-B28)/28.44 Substituting C by O costs at least 5 eV more energy than substituting N by O. The most stable configuration is achieved when B and O bond with each other (site 4). All the negative formation energies in Fig. 2b imply the good stability of the codoping configurations.

Fig. 2 (a) Structure of B-O/g-C3N4. Gray, blue and green balls represent C, N and B atoms. The numbers 1~12 are the selected sites that O replaces. (b) Formation energies for the twelve B-O/g-C3N4 configurations. Formation energies for the configurations discussed in the context are shown in red, while others in yellow.

3.3. Electronic properties of B-O/g-C3N4

10


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We choose four representative B-O/g-C3N4 structures, whose O is at sites 1, 2, 3 and 4 (donated as 1, 2, 3 and 4 B-O/g-C3N4), for further electronic and optical studies. Site 3 and site 4 are in the same tri-striazine unit as B, while site 1 and site 2 locate in the adjacent unit. The band gap of the pure g-C3N4 is 4.47 eV, with VBM at -6.8 eV and CBM at -2.3 eV with respect to the vacuum level, from the GW method. Position of VBM is consistent with the ionization potential of g-C3N4 from the experiment UPS spectrum (-6.96 eV) 2. When B and O are in the same unit, e.g. for 3 and 4 B-O/g-C3N4, energies of VBM and CBM change little compared with pure g-C3N4, so does the band gap (< 0.25 eV) (see Fig. 3c and 3d). The band gap is much more affected when B and O sit in different units, e.g. decreasing by ~1 eV for 1 and 2 B-O/gC3N4 (see Fig. 3a and 3b). All the four B-O/g-C3N4 can produce H2 and O2 from H2O in theory since their VBM and CBM bestride both the reduction potential of H+/H2 and the oxidation potential of O2/H2O as shown in Fig. 3. However, the wider band gaps of 3 and 4 B-O/g-C3N4 may make them have weaker water splitting capability than 1 and 2 BO/g-C3N4.

11



Fig. 3 GW band structures for B-O/g-C3N4: a/b/c/d correspond to the cases that O atom substitutes the N atom numbered 1/2/3/4 in Fig.2. The redox potentials of water are shown in dashed lines for reference. The energy is aligned to the vacuum level.

In pure g-C3N4, molecular orbitals spread over the four units in the supercell homogeneously (SI-Fig. S4). This symmetrical distribution is destroyed in the doped g-C3N4. In the B-doped g-C3N4, n, π and π* orbitals localized in the B-contained unit own higher energies than those in the undoped units owing to the lack of one electron (SI-Fig. S5). On the contrary, energies of n, π and π* orbitals in the O-contained unit are lower than those in the undoped units for O-doped g-C3N4 due to the one extra electron (SI-Fig. S6). In B-O codoped g-C3N4, distribution of orbitals depends on the doping sites. When B and O are in different units, such as 1 B-O/g-C3N4 (Fig. 4) and 2 B-O/g-C3N4 (SI-Fig. S7), the highest 12


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n/π (Fig. 4a and 4b) and the lowest π* (Fig. 4c) orbitals are mainly localized in the B- and O-contained unit respectively, analogous to those in the monodoped g-C3N4. This cumulative effect, i.e. higher VB and lower CB compared with pure g-C3N4, results in the large band gap reduction in 1 B-O/g-C3N4 and 2 B-O/g-C3N4. If B and O reside in the same unit, such as 3 B-O/g-C3N4 and 4 B-O/g-C3N4, the highest n/π orbitals and the lowest π* orbital are distributed over both undoped and doped units (SI-Fig. S8 and S9). This is the consequence subject to superposition principle of two opposite effects, i.e. upshift of energy by B-doping and downshift by O-doping for the doped unit. The cancelling out of these two effects makes the band gaps for 3 B-O/g-C3N4 and 4 BO/g-C3N4 change little.

Fig. 4 The orbital distribution for 1 B-O/g-C3N4. a, the highest occupied π orbital; b, VBM; c, CBM.

3.4. Influence of other g-C3N4 layers on the B-O/g-C3N4 layer It is interesting to investigate the influence of other g-C3N4 layers on the B-O/g-C3N4 layer, since the experimentally prepared material usually contains multilayers. We construct a bilayer system where a B-O/g-C3N4 13



layer is stacked with a pure g-C3N4 layer in the stable A-B pattern (SI-Fig. S10).45 The interlayer distance is at 3.29 Å after relaxation, agreeing with previous work.46-47 For the bilayer containing 1 B-O/g-C3N4 and 2 B-O/gC3N4, VBM and CBM are confined in the doped layer (SI-Fig. S11 and S12), having the same distribution as those in Fig. 4 and SI-Fig. S7 for the single-layer system. The additional pure g-C3N4 layer does not influence the electronic properties of the codoped g-C3N4 layer. For this type I heterojunction, the water splitting process will proceed mainly on the doped layer, with little contribution from the interlayer carriers transfer. 3.4 Absorption spectra Fig. 5 gives the theoretical optical absorption spectra calculated by BSE for 1, 2, 3 and 4 B-O/g-C3N4, where the incident direction of light is perpendicular to the g-C3N4 surface. Under this kind of irradiation, only the π→π* transitions are dipole-allowed, while the n→π* ones are suppressed. The overall shape of the spectrum may deviate from the experiment due to the imperfection in the real materials and the random irradiation direction of light. However, the advantages of B-O/g-C3N4 can be identified. The absorption edge of pure g-C3N4 is calculated to be 2.47 eV, close to the measured absorption onset in experiments (~2.7 eV).1-2 Compared to the pure g-C3N4, absorption edges (Fig. 5a) of 1 and 2 B14


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O/g-C3N4 redshift by 0.12, 0.08 eV, while those of 3 and 4 B-O/g-C3N4 blueshift by 0.15 and 0.12 eV. It is obvious that there is a big difference between absorption edge (optical band gap) and electronic band gap from GW method which have usually been confused in literatures. Physically, the optical band gap (Eog) is the difference between the electronic band gap (Eeg) and the electron-hole binding energy (Eeh), i.e. Eog = Eeg - Eeh

(2)

As aforementioned, the absorption edge of 1 B-O/g-C3N4 (2 B-O/g-C3N4) is 0.12 (0.08) eV lower than pure g-C3N4, while the electronic band gap is 0.92 (0.96) eV lower. The big difference comes from the great reduction (~0.8 eV) of electron-hole binding energy in codoped g-C3N4. The weak interaction between photogenerated electron and hole could reduce the electron-hole recombination probability. For 3 and 4 B-O/g-C3N4, the electronic band gaps and electron-hole binding energies do not have much difference from those of pure g-C3N4. That means 3 and 4 B-O/gC3N4 do not have an advantage in electron-hole separation over pure gC3N4. Therefore, codoping in different tri-s-triazine units could increase the absorption of visible light which is beneficial for water splitting.

15



Fig. 5 (a) Theoretical absorption spectra for the codoped g-C3N4. (b), (c), (d) and (e) are electron-hole distributions for the lowest excited states of the four codoping structures, i.e. α1, α2, α3 and α4 for 1, 2, 3 and 4 B-O/g-C3N4 respectively. The electron is shown in red while hole in blue. An artificial broadening of 0.1 eV is included for the spectra. Incident direction of the light is set normal to the g-C3N4 plane.

3.5 Charge transfer mechanism and activity of reaction sites From the electron-hole distribution, the lower-energy excited states below 3.0 eV in 1 and 2 B-O/g-C3N4, e.g. α1 and α2 in Fig. 5b and 5c, are dominated by charge-transfer excitations with electron promoted from the B-contained tri-s-triazine unit to the O-contained one. This long-distance transfer of electron leads to the greatly reduced electron-hole binding 16


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energy as discussed above. For 3 and 4 B-O/g-C3N4, electron and hole are delocalized and coexist in the same tri-s-triazine unit, e.g. α3 and α4 in Fig. 5d and 5e just like those in the pure g-C3N4 (Fig. S13). This closeness in space makes the electron and hole in 3 and 4 B-O/g-C3N4 have a larger binding energy than those in 1 and 2 B-O/g-C3N4. Thus, in 1 and 2 BO/g-C3N4, the active sites for oxidation and reduction of H2O are well separated. H2O is reduced to H2 by the photogenerated electron in the Ocontained unit, while oxidized in the B-contained unit to produce O2 (see Fig. 6). The localized in-plane distribution of electron and hole has been proven to enhance the photocarrier separation efficiency.48 As a contrast, the oxidation and reduction of H2O happen in the same unit for 3 and 4 B-O/g-C3N4, similar to pure g-C3N4, which may make their photoactivity not better than pure g-C3N4. Therefore, the relative position of codopants should play a key role in the separation of photogenerated hole and electron in codoped g-C3N4.

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Fig. 6 Charge transfer and separation mechanism for 1 and 2 B-O/g-C3N4 under light irradiation. The electron is shown in red while hole in blue.

It is important for the redox reaction of water to carry out that the reactants can be absorbed at the active sites stably. Since production of H2 and O2 happens on the O- and B-contained unit respectively, we investigate the stabilities of H on the O-contained unit (SI-Fig. S14a) and OH on the B-contained unit (SI-Fig. S15a) using the 1 B-O/g-C3N4 structure as the example, and compare them with the absorption of H and OH on the pure g-C3N4. The formation energy difference (△EForm) between H (or OH) on 1 B-O/g-C3N4 and that on g-C3N4 is calculated by equation (3) (detailed deduction is in the supporting information) △EForm= E(B-O/g-C3N4+absor) + E(g-C3N4)- E(g-C3N4+absor)- E(B-O/g-C3N4) (3)

where ‘absor’ is H or OH. When OH attaches with N (C or B) in pure gC3N4 and B-contained unit of 1 B-O/g-C3N4, △EForm is -0.69 eV (-0.87 eV). While H attaches with C (N or O) in pure g-C3N4 and O-contained unit of 1 B-O/g-C3N4, △EForm is -1.05 eV (-0.18 eV). The negative △EForm makes it clear that the adsorption configurations of H and OH in 1 B-O/gC3N4 are more stable than those in pure g-C3N4. 4. Conclusions In summary, our GW+BSE calculations suggest that the passivated BO-codoped g-C3N4, where B and O atoms are in different tri-s-triazine 18


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units, is an improved photocatalyst for solar-to-hydrogen efficiency. This passivated codoping scheme can be regarded as a kind of plane heterostructure. It makes the absorption edge redshift and electron-hole separation rate increase, leading to wider visible light absorption range and higher quantum efficiency compared with pure g-C3N4. When B and O atoms are in same unit, these phenomena are unavailable. The results suggest the critical role of microstructure in determining the redox potential, light absorbance range and photocatalytic activity of codoped g-C3N4. The study may provide a guideline for the design of more effective photocatalysts. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grants No. 21573131 and 21433006) and the Natural Science Foundation of Shandong Province (Grant No. JQ201603). Computational resources have been provided by the National Supercomputing Centers in Jinan. Supporting Information for Publication: Convergence test for GW calculations and molecular orbitals for doped gC3N4. This material is available free of charge via the Internet at http://pubs.acs.org. References 19



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