Whether Corrugated Or Planar Vacancy Graphene Like Carbon

Article Cite This: J. Phys. Chem. C 2019, 123, 17296−17305

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Whether Corrugated or Planar Vacancy Graphene-like Carbon Nitride (g‑C3N4) Is More Effective for Nitrogen Reduction Reaction? Chunjin Ren,†,‡ Yongli Zhang,†,‡ Yanli Li,†,‡ Yongfan Zhang,†,‡ Shuping Huang,†,‡ Wei Lin,*,†,‡ and Kaining Ding*,†,‡ †

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College of Chemistry, Research Institute of Photocatalysis, State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Fuzhou, Fujian 350108, China ‡ Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry, Xiamen, Fujian 361005, China S Supporting Information *

ABSTRACT: Different conformations, including planar, corrugated, as well as the deficient structure of the two-dimensional materials, play a relevant role in determining their catalytic reaction performances. Here, we systematically investigated the stabilities, electronic properties, and nitrogen activities capacity of various vacancy-modified g-C3N4 considering two different conformations (planar and corrugated) to explore the effects of nitrogen vacancy (NV) and conformations on the photocatalytic performance of g-C3N4 by means of density functional theory computations. Our results found that not only can the nitrogen vacancy (NV) promote separation efficiency of the photoinduced carriers in g-C3N4 but also the distortion conformation can activate more n → π* transitions of NV gC3N4, resulting in a red shift of optical absorption spectra. More importantly, our results reveal that the corrugation configuration structure, compared to planar conformation, is much more favorable to photocatalytic nitrogen fixation reaction from the aspects of nitrogen absorption capacity and free-energy change, in which corrugation model with N2C vacancy has the smallest onset potential (1.32 V) for the most difficult step through the alternating pathway.

1. INTRODUCTION Photocatalytic N2 fixation is an environmentally friendly process responsible for converting the most abundant but extremely stable nitrogen molecules into industry- and agriculture-available substrates.1 With photocatalytic nitrogen fixation overcoming the harsh conditions of high temperature and high pressure of traditional methods (the Haber−Bosch process) and offering an accessible kinetic pathway, mild methods have rapidly drawn the attention of most scientists.2,3 The challenging problem is to find an appropriate and efficient photocatalyst. Various photocatalytic semiconductors, including titanium oxide and titanate, which had been widely investigated, have a promising future.4−6 In earlier research works, rates in most of the nitrogen fixation production were only 0−9 μmol/h per 1 g of catalyst.7 To improve the rate, increasing the selectivity of N2 reduction reaction (NRR) on various mild catalysts has been explored in the past decade, including two-dimensional materials8−10 such as Mxene,11 Au@TiO2,1212 Bi5O7X (X = Br, I) nanotubes,13 Mn nitride,14 single transition-metal atoms anchored to various supports,15−18 for example, TM-BN,15 Ru atoms supported on boron sheets,16 and so on. However, numerous catalysts were mainly centered on metal-based catalysts, including metal oxide and metal-supported catalysts, whereas metal-free catalysts for NRR have been rarely explored. Recently, inspired by the intrinsic advantages of © 2019 American Chemical Society

low cost and environment friendliness of metal-free catalysts, Ling et al.19 have first reported that nitrogen molecules can be efficiently reduced to NH3 on metal-free single-atom photocatalyst B/g-C3N4 through the enzymatic mechanism with a record low onset potential (0.20 V) offering cost-effective opportunities for advancing sustainable NH3 production. More interestingly, researchers discovered that selective reduction of nitrogen can be achieved via self-vacancy modification without heteroatoms doping or additional loading of the metal. For instance, Niu et al.20 reported that homogeneous selfmodification with nitrogen vacancies (NVs) can narrow the band gap of pure g-C3N4 from 2.78 to 2.03 eV increasing the visible light absorption. Furthermore impressing, Dong et al.7 experimentally determined that nitrogen vacancies (NVs) can enhance the photocatalytic N2 fixation ability of graphitic carbon nitride (g-C3N4) and selectively photoreduce N2 with the rate of 1240 μmol/h per 1 g of V-g-C3N4, which is free from the interference of other gases. Besides, using X-ray photoelectron spectroscopy (XPS) and elemental analysis analyses, as well as density functional theory (DFT) calculations, Tay et al.21 demonstrated that 2-coordinated nitrogen vacancy in g-C3N4 is in charge of the narrow band gap Received: April 14, 2019 Revised: June 24, 2019 Published: June 26, 2019 17296

DOI: 10.1021/acs.jpcc.9b03511 J. Phys. Chem. C 2019, 123, 17296−17305

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The Journal of Physical Chemistry C

employed in the pure g-C3N4 monolayer, which was obtained through cleavage of the surface of the optimized bulk structure and all nitrogen vacancies monolayer models after the vacuum region convergence tests. And for all calculations, we considered the van der Waals dispersion correction by using the DFT-D approach.22 Six consecutive protonation and reduction steps are involved in the NRR (N2 + 6H+ + 6e− → 2NH3). The Gibbs freeenergy change (ΔG) of each elementary reaction was calculated employing the standard hydrogen electrode model proposed by Nørskov et al.30−32 Based on this method, the ΔG value was computed as follows: ΔG = ΔE + ΔZPE − TΔS, where ΔE is the DFT electronic energy difference, ΔZPE is the change in zero-point energy (ZPE), ΔS is the entropy change, and T is the room temperature (T = 298.15 K). The ZPEs and entropies of the NRR species were obtained from the vibrational frequencies calculations.

and acts as active sites to capture electrons in hydrogen evolution reaction. More importantly, the different conformations of g-C3N4, including planar and corrugated configurations, play a key role in determining the onset potential. For example, by means of DFT-D3 calculations, Azofra et al.22 pointed out that the Gibbs free energy of corrugation conformation of g-C3N4 for the first hydrogenation decreases by 0.49 eV with respect to the planar case in the CO2 conversion reaction. Despite the importance of conformations (planar vs corrugated), related report in nitrogen fixation rarely appears unfortunately. Even though many theoretical efforts have been made to modify g-C3N4, most of the present theoretical investigations focused on various doping carbon nitride and the single-atom catalyst supported on g-C3N 4 system, including their applications in water splitting. Against this background, it still lacks explorations of effects in nitrogen adsorption capacity and activation mechanism induced by various types of nitrogen vacancies in g-C3N4. Therefore, several vital issues still need to be figured out. For instance, which form of nitrogen vacancies can better hinder recombination of electron−hole pairs? Besides, how do different nitrogen defect types especially different conformations, including planar and corrugated, affect the nitrogen absorption capacity? More importantly, what are the effects of various nitrogen vacancy types as well as planar and corrugated configurations on the photocatalytic nitrogen fixation reaction mechanism of g-C3N4, especially the most difficult step? Hence, in this work, we systematically investigated the geometries, stabilities, electronic properties, and nitrogen activities capacities of various vacancy-modified g-C3N4 considering two different configurations (planar and corrugated) using DFT computations.

3. RESULTS AND DISCUSSION 3.1. Geometry Structures and Thermodynamic Stability. As shown in Figure 1, there are three types of

2. COMPUTATIONAL METHODS First-principles density functional theory (DFT) calculations were carried out by using the Cambridge Serial Total Energy Package (CASTEP) code of the Materials Studio17 software package.23 The OTFG ultrasoft pseudopotential was employed for describing the interaction between the core and valence electrons. The bulk crystalline cells as well as the atom positions of the pure and nitrogen vacancies g-C3N4 were optimized within the generalized gradient approximation, and the exchange−correlation effects of valence electrons were described by the spin-polarized Perdew−Wang 91(PW91) functionals.24,25 The kinetic cutoff energy was set to be 540 eV for expanding the Kohn−Sham wave functions, and the convergence threshold of the energy change was set to 1 × 10−5 eV per atom. The structural configurations of the g-C3N4 were fully optimized until the force, the maximum stress, and displacement on each atom during optimizing were less than 0.03 eV/Å, 0.05 GPa, and 0.001 Å, respectively. The Brillouin zone integrations were sampled by using a (5 × 5 × 1) k-mesh during the configuration optimization and electronic structure calculations. Previous studies have reported that hexagonal heptazinebased g-C3N4 is the most stable structure among four different patterns of pure bulk g-C3N4 with the lowest energy.26,27 Thus, we only investigated hexagonal heptazine-based g-C3N4 and its nitrogen vacancies system in this work. Our optimized lattice parameters of the bulk g-C3N4 (a = 7.13 Å, b = 12.35 Å, and c = 6.54 Å) are basically consistent with previous theoretical28,29 (a = 7.13 Å, b = 12.35 Å, and c = 6.91 Å) and experimental results27 (a = 7.30 Å, c = 6.72 Å). A vacuum region of 20 Å was

Figure 1. Surface structure of g-C3N4.

Table 1. Vacancy Formation Energies of Different N Atomic Vacancy Types NV2C NV3C1 NV3C2

Evacancy (eV)

Eperfect (eV)

μN (eV)

Ef (eV)

−12 120.64 −12 119.61 −12 120.89

−12 392.52 −12 392.52 −12 392.52

−270.96 −270.96 −270.96

0.92 1.95 0.67

nitrogen atoms in the g-C3N4: the bridge N(N3C1), central N (N3C2), and 2-fold coordination N(N2C). To confirm the thermodynamic stability of various nitrogen vacancies (NV) gC3N4, we calculated the formation energy (Ef) (Table 1) according to the following equations33 Ef = Evacancy − Eperfect + μ N

(1)

where Evacancy and Eperfect are the total energies of g-C3N4 with and without nitrogen vacancies, respectively, and μN is the chemical potential of N. Here, μN = 1/2μ(N2). By means of the high-resolution XPS image of N 1s, Dong et al.7 found that nitrogen vacancies were mainly located at the tertiary nitrogen lattice sites N−C3. Our calculation results furthermore revealed that the central tertiary N vacancy (N3C2V) g-C3N4 is the easiest to form with the lowest vacancy formation energy (0.67 eV), while the Ef of the bridge N vacancy (N3C1V) g-C3N4 is as high as 1.95 eV (Table 1). As for 17297


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The Journal of Physical Chemistry C Table 2. Adsorption Energies and Changes of NN Triple Bonds of Four Adsorption Configurationsa N2C planar g-C3N4 lNN (Å) Δl (Å) Δl (%) E (eV)

N3C2 corrugated g-C3N4

planar g-C3N4

corrugated g-C3N4

end on

side on

end on

side on

end on

side on

end on

side on

1.55 0.45 41.0% 0.08

1.22 0.11 9.9% 0.49

1.22 0.11 9.9% −0.67

1.30 0.19 17.1% −2.56

1.11 0 0% −0.16

1.11 0 0% −0.11

1.36 0.25 22.0% −1.98

1.26 0.16 14.5% −0.24

a

Our calculated bond length of isolated N2 molecule was 1.11 Å, and the experimental value was 1.10 Å.

Figure 2. Adsorption of N2 on planar surface in (a, b) N2CV and (c, d) N3C2V g-C3N4 system for end-on and side-on configurations. The key bond lengths (Å) of absorbed nitrogen are also shown in red.

Figure 3. Adsorption of N2 on corrugated surface in (a, b) N2CV and (c, d) N3C2V g-C3N4 system. The key bond lengths (Å) of absorbed nitrogen are also shown in red.

the system of 2-fold coordination N vacancy (N2CV), the Ef (0.92 eV) is slightly higher than that of N3C2V in g-C3N4 but still much lower that of the N3c1V model. Hence, we chose the N3C2V and N2CV g-C3N4 to systematically study the effect of different N atom defect types on photocatalytic nitrogen fixation reaction of g-C3N4. 3.2. N2 Adsorption Capacity. The adsorption energy and the change in bond distance of the adsorbed nitrogen molecule are the fundamental properties of a semiconductor photocatalyst to evaluate the performance of adsorption capacity. The adsorption energies and changes of NN triple bond (Δl) are shown in Table 2. The adsorption energy of N2 molecules is obtained based on the following equation34,35 E = Eadsorption − Esurface − E N2

As clearly shown in Table 2, the most favorable configuration for N2 adsorption on vacancy g-C3N4 was the N2CV corrugated structure. In spite of the Δl of N2 molecule on the planar surface of N2CV system being the largest (41.0%), the Eads value has reached up to 0.08 eV, suggesting that this adsorption configuration is not stable. At the same time, compared with planar configuration, the adsorption energies of N2 on the surface of corrugated N2CV model decrease to −0.67 eV (end-on configuration) and −2.56 eV (side-on configuration). These results illustrate obviously that the corrugated structure is beneficial to the adsorption of N2 molecule. Our calculations have shown that Δl of absorbed N2 on planar configurations in N3C2V system is 0%. In summary, for the corrugated g-C3N4, the N2C vacancy possesses more better adsorption capacity while the N3C2 vacancy g-C3N4 owns 2 times the change in bond length than the N2C vacancy system. 3.3. Catalytic Mechanism for the Nitrogen Fixation. To understand the nitrogen vacancy effect on the reaction mechanism, the reaction pathways of nitrogen fixation by N2 on planar and corrugated surfaces of N2CV and N3C2V g-C3N4 systems are explored by calculating the Gibbs free energy change (ΔG) of each elemental step. In our calculation, the ΔG was defined by the following formula15,36

(2)

where Eadsorption and Esurface are the energies of system within and without the N2 adsorption, respectively, and EN2 represents the energy of an isolated N2 molecule. We say that a corrugated surface appears when nitrogen was absorbed on ridge vacancy g-C3N4 surface resulting in deformation of a planar structure. Thus, we considered eight adsorption configurations, including N2 adsorbed on planar and corrugated surfaces of N2CV and N3C2V g-C3N4 systems, in end-on and side-on absorption ways (Table 2), whose stable structures are displayed in Figures 2 and 3. 17298


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The Journal of Physical Chemistry C Scheme 1. Distal, Alternating, and Enzymatic Mechanisms of Nitrogen Fixation Reaction

enzymatic fixation mechanisms.19,37 We investigated the NRR steps through three possible pathways (Scheme 1), including six protonation and reduction processes. Free-energy change diagrams via these three pathways are presented in Figures 4−6, respectively, while the corresponding structures of the reaction intermediates are illustrated in Figures S1−S6 in the Supporting Information. Nitrogen reduction reaction on the corrugated N2CV model prefers to proceed through the alternating mechanism (distal and enzymatic pathways displayed in Figure S3). Although enzymatic adsorption often brings lower NRR energy request, it is favorable with respect to distal and alternating adsorption. Nevertheless, in the case of our corrugated N2CV g-C3N4 system, a stable adsorbed hydrogen atom is generated through enzymatic mechanism on the carbon atom after the first ammonia gas is desorbed. This process is quite easy to occur due to the small value of ΔG (0.37 eV), which is unfavorable to the NRR (Figure S3). Moreover, the potential of the generation of the first NH3 molecule for the NRR on the corrugated N2CV monolayer via enzymatic mechanism is 1.43

Figure 4. Free-energy change for the N2 fixation on corrugated surfaces of N2CV system through alternating mechanism.

ΔG = ΔE + ΔZPVE − T ΔS

(3)

where ΔE is the total energy difference, ΔZPVE is the zeropoint vibrational energy difference, T is the temperature (T = 298.15 K), and ΔS is the entropy change. Three possible pathways have been proposed for nitrogen reduction reaction (NRR), which are distal, alternating, and

Figure 5. Free-energy diagram for the N2 fixation on planar surface of N3C2V g-C3N4 system through (a) distal, (b) alternating, and (c) enzymatic mechanisms. 17299


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1.32 eV and the planar of that is 1.05 eV. However, it is impossible for planar N2CV model to continue the next reaction step due to lack of the N vacancy. As for N3C2V system, the process diagram of reaction pathways in N2 fixation on the planar surface is presented in Figure S5. Then, we calculated the free-energy change of the N2 fixation on planar surface of N3C2V g-C3N4 through distal (a), alternating (b), and enzymatic (c) mechanisms (Figure 5). As one can see from Figure 5, the first step, which is N2 protonation to form N2H* species, is the most difficult to occur on the planar surface of N3C2V g-C3N4 system due to the maximum ΔG value (2.06 eV) among all elementary steps through three pathways. This may be related to the change in the bond length of the N2 molecule in the planar N3C2V gC3N4 model, which is almost 0%. For corrugated N3C2V g-C3N4 model, nitrogen reduction is via the alternating mechanism. For the photocatalytic N2 reduction reaction, the hydrogen evolution and chemical adsorption of H2 are competitive reactions (Figure S6b). However, in the case of side-on adsorption way, it is very easy to produce hydrogen gas in the hydrogenation process of N2H3* with the value of ΔG only −0.33 eV (Figure 6b), while the free energy of N2H3* to N2H4* is uphill by 0.36 eV. Furthermore, there is a free energy uphill of 0.10 eV required for the desorption of H2, which is much lower than the desorption of NH3 molecule (0.44 eV). Therefore, the side-on adsorption way is unfavorable and the photocatalytic N2 reduction reaction on the corrugated surface of N3C2V gC3N4 is inclined to the alternating pathway with end-on adsorption pattern (Figure 6a). In the case of corrugated N3C2V g-C3N4 system, the most difficult step of photocatalytic nitrogen fixation reaction is the reduction of the NH2* group to form the NH3 (NH2* + H+ + e− → NH3*) with the ΔG value of 1.76 eV (Figure 6a). At the same time, the whole nitrogen hydrogenation activation process becomes much easier for corrugated N3C2V g-C3N4 than that on planar surface with sharply falling onset potential. Moreover, the effect of hydrogen termination on N2CV as well as N3C2V g-C3N4 corrugated catalyst surfaces is also evaluated. The investigation of Li et al.38 pointed that H-covered Mo2C is not an ideal NRR catalyst considering its low hydrogen evolution reaction overpotential, weak N2 adsorption capacity due to the active sites occupied by the hydrogen, as well as an energy requirement of 1.0−1.4 eV of NRR. Our calculation results determined that for corrugated system, the active sites were preferentially occupied by the N2 molecule (Eads of N2 on N2C vacancies active site is −2.56 eV, whereas the value of H is 0.19 eV). Thus, in our investigation systems, the H-termination over the vacancy can be ignored. Overall, in comparison of planar and corrugated models of NV g-C3N4 from both adsorption capacity and free energy change aspects, we can conclude that the corrugation configuration is much more favorable to photocatalytic nitrogen fixation reaction. For the corrugated NV g-C3N4 system, the N2C vacancy model owns a small onset potential (1.32 V) for the most difficult step than that of N3C2V g-C3N4 system (1.76 V) in photocatalytic nitrogen fixation. 3.4. Optical Property. As for photocatalysts, the performance of a photocatalyst in the visible light region is of great importance. Then, to figure out the optical properties difference between corrugation and planar conformations in various vacancy systems and unravel the reasons behind that, we calculated the absorption spectra from the HSE06

Figure 6. Free-energy diagrams for the N2 fixation on corrugated N3C2V g-C3N4 system through (a) alternating and (b) enzymatic pathways.

Figure 7. Absorption spectra of pure, corrugated vs planar N2CV gC3N4, corrugated vs planar N3CV g-C3N4.

Figure 8. Simulative electrons transition deduced by DFT.

eV, while the value of that in alternating process is only −0.65 eV. For N2CV, the most difficult step of nitrogen fixation on the corrugated surface via alternating mechanism (Figure 4) is the protonation step of the N2H* species (N2H* + H+ + e− → N2H2*) while on the planar surface through distal pathway (Figure S4) that is the desorption of NH3 molecule. The ΔG value of the most difficult step with corrugated configuration is 17300


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Figure 9. Band structure of (a) planar N3C2V g-C3N4, (b) planar N2CV g-C3N4, (c) corrugated N3C2V g-C3N4, and (d) corrugated N2CV g-C3N4.

Table 3. Relative Ratio of Effective Mass in Brillouin Zone for Pure, N3C2V, and N2CV g-C3N4 perfect N2C N3C2

Dspin‑up Dspin‑down Dspin‑up Dspin‑down Dspin‑up Dspin‑down

G−F

F−M

M−G

1.09 1.09 0.85 0.28 0.24 0.52

2.24 2.24 12.33 1.79 6.88 1.11

1.20 1.20 0.54 1.82 0.38 0.68

functional and summarized higher transition probability results after vacancy and conformations modification, which is plotted in Figure 7 and shown in Table S1. Interestingly, in contrast to the planar model, the optical absorption of corrugated N3CV g-C3N4 shows a red shift of around 30 nm while in the case of corrugated N2CV g-C3N4 that has increased to 70 nm. On the other hand, as listed in Table S1, for the pure g-C3N4 model, the n → p* electronic transitions are prohibited in the photon energy range of 2.60− 3.20 eV. After the vacancy introduced, we can see that electrons have been activated in different degrees. However, in the system of corrugated vacancy g-C3N4, the distortion configuration can activate more n → π* transitions resulting in a red sift of optical absorption spectra, while in the case of planar vacancy g-C3N4 model, the electronic excitation from the n1 to π* transition is prohibited (Figure 8). 3.5. Activity Origin of Planar vs Corrugated NV gC3N4 for NRR. 3.5.1. Electron Properties of Planar and Corrugated NV g-C3N4. We begin our investigation from the pure g-C3N4 system. As shown in Figure S7, our calculated indirect band gap of pure g-C3N4 is 2.68 eV, which is in line with previous computed results.38 When the N vacancy is formed, the indirect energy gaps of planar N3C2V and N2CV g-

Figure 10. Brillouin zone paths of g-C3N4: G(0.00, 0.00, 0.00), F(0.00, 0.50, 0.00), M(0.33, 0.33, 0.33).

C3N4 reduced to 2.53 and 2.50 eV, respectively (Figure 9). Moreover, it is obvious that there is a defect level in the middle of the forbidden band of N3C2V and N2CV g-C3N4. Fortunately, vacancy electrons at the defect level can be transitioned to conduction bands after electron photoexcitation, which can promote absorption of visible light and consequently increase the photocatalytic efficiency of the g-C3N4. The Fermi energies (EF) of N3C2V and N2CV g-C3N4 are 1.14 and 1.34 eV below the conduction band minimum, respectively. Furthermore, 17301


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Figure 11. Electron density difference of (a) the corrugation N3C2V g-C3N4 facet adsorbed N2 for end-on configuration and (b) the corrugation N2CV g-C3N4 facet with the adsorption of N2 for end-on patterns. The isosurface value is set to be 0.10 e/Å−3. Green and yellow represent electron accumulation and depletion, respectively.

Figure 12. Density of states for (a) planar N3CV g-C3N4, (b) planar N2CV g-C3N4, (c) corrugated N3CV g-C3N4, and (d) corrugated N2CV g-C3N4.

3.5.2. Carrier Mobility and Separation. The relative effective masses of electrons (me*) and holes (mh*) especially the resulting difference in the separation and diffusion rate of photoinduced charge carriers are the determining measurement of photocatalytic activity for a semiconductor. The recombination and separating rate of carriers, which is crucial to photocatalytic efficiency of semiconductor, depends on the relative ratio of effective mass. To compare the recombination rate of photoinduced carriers for pure, N3C2V, and N2CV g-

when the corrugation appears, a direct energy gap is predicted and the energy gap decreased to 2.33 and 1.80 eV for the system of corrugated N3C2V and N2CV g-C3N4, respectively. Obviously, the Fermi energy also further shifted closer to the bottom of the conduction band after the appearance of corrugation. The transformation from indirect to direct as well as the narrowing gap all demonstrated that the corrugation conformation, especially corrugated N2CV g-C3N4 is more favorable to higher efficiency of solar energy conversion. 17302


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Figure 13. Density of states for the N2 absorbed on the surface of (a) corrugated N3C2V g-C3N4 and (b) corrugated N2CV g-C3N4.

only 0.09 e electrons. More electron transfer once again proved that the NN molecular can be more effectively activated in corrugated N2CV system than in the case of N3C2V g-C3N4. Density of states (DOS) analysis (Figure 12) indicated that the extra vacancy states are mainly contributed by C (2p), in agreement with the previous result.21 Then, the interaction between adsorbed N2 and carbon sites around vacancy was studied by calculating density of states (DOS) of N (2s, 2p) of N2 and its bonded C (2s, 2p) of corrugated N2CV and N3C2V g-C3N4. As shown in Figure 13, the absorbed nitrogen is bound strongly with the carbon atoms around defect, where the strong hybridization between the N 2p orbital of N2 and C 2p orbital of NV g-C3N4 is evident. A larger overlap interaction between C 2p and N 2p around Fermi level for N2CV g-C3N4 furthermore indicated the superiority of corrugated N2CV to the system of corrugated N3C2V g-C3N4. An ultimate prerequisite for SAC to be an eligible electrocatalyst is its good stability for long-term uses. To explore the stability properties difference of corrugated, planar vacancy g-C3N4 besides the activity performance and understand the factors that lead to energy stabilization in vacancy gC3N4 as a result of corrugation as well as to investigate the symmetry and electron location changes, we further performed electron localization function (ELF) analysis, as shown in Figure S8. After the formation of nitrogen vacancies, carbon atoms around vacancy experience larger electronic repulsions in the case of planar models because each point toward its neighboring atom. However, once vacancy g-C3N4 corrugates, the distorted fold shape can minimize such repulsions. Thus, the corrugation vacancy g-C3N4 not only exhibits higher activity but also generates energy stabilization.

C3N4 systems, the relative ratios (D) were calculated based on the results of effective mass (Table 3) according to the following formula39−41 D = me*/mh*

(4)

1 1 ij d2E yz = 2 jjj 2 zzz mn* ℏ k dk { k = k i

(5)

where mn* is the effective mass of carriers and d E/dk is the coefficient of the second-order term in a quadratic fit of E(k) curves for the band edge. According to this definition, the more the D value deviates from 1, the greater the difference in the mobility ability of electron−hole pair and the lower the recombination rate of photoinduced carriers.42 And the Brillouin zone paths of g-C3N4 are displayed in Figure 10. As is illustrated obviously in Table 3, the relative ratios of effective mass for g-C3N4 own the largest deviation from 1 on F−M direction. Consequently, the carrier separation efficiency in the F−M direction is the highest. However, in comparison to pure g-C3N4, the Dspin‑up value of vacancy g-C3N4 shows much more deviation from 1. At the same time, in the directions of G−F and M−G, the degree to which the D value deviates from 1 also becomes larger in the case of N3C2V and N2CV g-C3N4 system. In summary of analysis of the relative ratio of effective mass, we conclude that N vacancy promoted the separation efficiency of the photoinduced carrier contributing to the better photocatalytic activity of N vacancy g-C3N4. 3.5.3. Interaction between Adsorbed N2 and Vacancy Sites: Electron Transfer and Density of States (DOS). The high activity of corrugated N2CV g-C3N4 indicates much more electron transfer between N2 and catalyst, which can be vividly visualized from electron density difference (Δρ = ρadsorption − ρsurface − ρN2, in which ρadsorption and ρsurface are the electron densities of system within and without N2 adsorption, respectively, and ρN2 represents the electron density of an isolated N2 molecule).43 As shown in Figure 11, significant charge transfer between vacancy sites of corrugated N2CV gC3N4, N3C2V g-C3N4, and absorbed N2 can be observed for end-on configuration. Obviously, a strong electron accumulation (green areas) occurs on the 2p* orbital of N2, whereas electron depletion (yellow) appears on σ2p orbital suggesting the effectively weakened NN triple bond19,44 (Figure 11). Then, the transfer was further quantitatively calculated by means of the Hirshfeld charge analysis. Through our calculations, we found that there are 0.19 e flow from the corrugated N2CV g-C3N4 facet to N2 molecule. However, for the corrugation N3C2V g-C3N4 model, the N2 molecule accepts 2

2

4. CONCLUSIONS In summary, our comparative DFT studies of vacancy g-C3N4 figured out that the effect of vacancy and corrugation conformation involves narrowing band gap, promotes separation rate of photoinduced electron−hole pairs, as well as enhances the nitrogen reduction reaction performance. We found that the corrugated N2CV structure is beneficial to the adsorption of N2 molecule with the decreased −0.67 eV adsorption energies, while the N3C2 vacancy g-C3N4 owns 2 times the change in bond length than N2C vacancy system. Moreover, N2 protonation is extremely difficult to occur on the planar N2CV model due to lack of N vacancy and planar N3C2V system because of its large ΔG value (2.06 eV). On the other hand, the corrugated configuration structure is much more favorable to photocatalytic nitrogen fixation reaction with the new most difficult step. Interestingly, in terms of corrugated 17303


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model, the vacancy of N2C model owns smaller onset potential (1.32 V) for the nitrogen fix reaction, the most difficult step, than that of N3C2V g-C3N4 (1.76 V). Overall, our investigation explores first the nitrogen fixation mechanism followed by various vacancies and conformations g-C3N4 and provides useful design guiding principles for future g-C3N4 material catalysts.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b03511.

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Band structure of pure g-C3N4; alternating pathways of nitrogen reduction reaction (NRR) on planar N2CV gC3N4; distal and enzymatic pathways of nitrogen reduction reaction (NRR) on corrugated N2CV gC3N4; analyses of the transition matrix elements of various g-C3N4 systems with planar and corrugated configurations; and electron localization function (ELF) representations of (a) N2CV g-C3N4 and (b) N3CV gC3N4 (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +86 0591 22866154 (W.L.). *E-mail: [email protected]. Tel: +86 0591 22866154 (K.D.). ORCID

Chunjin Ren: 0000-0003-4639-997X Yongfan Zhang: 0000-0003-3475-0937 Shuping Huang: 0000-0003-4815-1863 Wei Lin: 0000-0002-5046-4765 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (grant nos. 21171039 and 21373048), the Natural Science Foundation of Fujian Province (grant no. 2016J01687), the Independent Research Project of States Key Laboratory of Photocatalysis on Energy and Environment (grant no. SKFLPEE-KF201721), and Natural Science Foundation of Fujian Province for Distinguished Young Investigator Grant (2013J06004).

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