Design of Boron Doped C2N-C3N Coplanar Conjugated Heterostructure for Efficient HER Electrocatalysis

Hydrogen evolution reaction (HER) via the electrocatalytic reduction of water on metal-free catalysts may become a promising method for a sustainable energy supply in the future. However, compared with noble metals or transition metals, the carbon-based metal-free electrocatalysts show poor activity. Here, a novel coplanar metal-free catalyst (C2N-C3N) was designed for the first time to achieve better efficiency for electron transfer and water reduction. Through the DFT calculations, we discovered that the unique coplanar C2N-C3N structure can promote the directional transfer of electrons from C3N to C2N under the drive of built-in electric potential in the π-conjugated plane. To achieve higher performance in HER, the single atom doping by the substitution of boron is carried out. Remarkably, after the boron is doped, the barrier in the Tafel step decreases from 2.35 eV to 0.86 eV. Our results indicate that the novel B-doped coplanar C2N-C3N structure is a promising metal-free catalyst for HER.

can improve the transport of electrons, leading to better in-plane electron hole separation and electron transfer, and ultimately enhance the catalytic activity 27,28 . Not long ago, Wei et.al. successfully synthesized the new coplanar heterostructure by covalent bonding of C 3 N 4 and C-rings. The new structure could quickly trap photoexcited electrons and drive them to suitable active sites, which dramatically enhances the photocarrier separation and catalytic efficiency for HER 29 .

Results and Discussion
Construction and stability test of C2N-C3N coplanar structure. Many heterostructures based on C 2 N were studied for water-splitting, because C 2 N has a suitable band position for hydrogen evolution 30 . However, the intrinsic C 2 N structure has a low activity for hydrogen evolution. Herein, to promote charge transfer in two dimensional C 2 N, we designed and predicted a new coplanar heterostructure by connecting C 2 N and C 3 N with π-conjugated bonds for the first time (see Fig. 1c). The lattice parameters of the new structure are as follows: a = 8.41 Å, b = 19.20 Å, c = 20 Å; α = 90°, β = 90°, γ = 90°. The thickness of the vacuum layer in the C 2 N-C 3 N coplanar is 20 Å, which has been proved to be stable by the tests (shown in Figure S1). In order to illustrate the stability of the novel structure, the molecular dynamics calculations were performed for C 2 N-C 3 N coplanar structure (see Fig. 1d-f). It was found that the maximum fluctuation of the total potential energy is smaller than 0.04 eV/unit cell through MD calculations and the structure has no obvious breakage at 300 K. Furthermore, the dynamical stability of C 2 N-C 3 N coplanar heterostructure is tested by calculating the phonon spectra, as shown in Fig. 1g. It can be clearly seen that no imaginary frequency phonons are found at any wave vector, which demonstrates that C 2 N-C 3 N coplanar structure is dynamically stable. Compared with graphene@g-C 3 N 4 coplanar heterostructure, C 2 N-C 3 N has more similar parts in terms of structures and element components between C 2 N and C 3 N, and the nitrogen atoms with rich long-pair electrons at the edge of the conjugated rings provide more ideal sites for interfacial connection of heterostructures 31 . Through the calculations, our results have demonstrated Figure 1. The optimized structure of (a) C 2 N, (b) C 3 N and (c) coplanar C 2 N-C 3 N. The red circle represents the part similar as C 2 N and the parallelogram represents the part of C 3 N. (Gray-carbon atom; blue-nitrogen atom) (d) The coplanar C 2 N-C 3 N heterostructure at 300 K during 1 ps from the first-principle molecular dynamics calculations. (e) The top-view and (f) side -view of the structures after the MD simulations. (g) Phonon spectra of C 2 N-C 3 N along the high-symmetric points in the Brillion zone.
SCIeNtIFIC REpoRTs | (2018) 8:5661 | DOI:10.1038/s41598-018-24044-4 that the novel coplanar C 2 N-C 3 N significantly promotes the electrons transfer through the π-conjugated structure. Moreover, the B-doped C 2 N-C 3 N coplanar structure effectively reduces the activation energy in Tafel step, indicating a good performance for HER.
Electronic properties of C2N-C3N coplanar structure. It's known that the process of charge transfer plays an important role in catalytic reactions. To compare the charge distribution of C 2 N (C 3 N) with the new coplanar heterostructure of C 2 N-C 3 N, the population analysis was performed by assigning Hirshfeld charge on them. As shown in Fig. 2a-c, after the heterostructure is formed, the charge of the most atoms in coplanar heterostructure (marked by blue circle) exhibits a more negative charge (0.105/−0.155, 0.053/0.018, 0.105/−0.177) than C 2 N. Similarly, compared with C 3 N, the charge in the same carbon atoms in the new structure shows a more positive charge (0.013/0.045, 0.013/0.046, 0.013/0.057), but the nitrogen atoms had more negative charge. This phenomena shows that the overall negative charge is transmitted from C 3 N to C 2 N in the new coplanar C 2 N-C 3 N structure, and nitrogen atoms achieve more negative charge in the C 3 N itself. That is to say, the electrons shifted directionally from C 3 N to C 2 N in the new coplanar structure and it would have a significant impact on catalytic performance.
To give a sound explanation of the directional migration of electrons, the electrostatic potentials of C 2 N and C 3 N were calculated in order to compute the work function. Work function is the potential required to remove the least lightly bound electrons: (ɸ w : work function, E V : the energy of vacuum level, E f : the energy of Fermi level), which refers to the minimum energy that an electron escapes from the Fermi energy level into vacuum energy level 32 . As illustrated in Fig. 2d, the work function of C 2 N and C 3 N equal 6.18 eV and 3.51 eV respectively. Consequently, the difference of ɸ w between C 2 N and C 3 N results in a strong built-in electric field, which is the main reason for the directional migration of electrons. In the heterostructure, the electrons would transfer from one side with the low work function to the higher one. The ɸ w of C 2 N is larger than that of C 3 N, so when the connection takes place, the internal electrons would transfer from C 3 N to C 2 N directionally, which would increase conductivity and would promote catalytic activity.
For an in-depth understanding of the difference of the electronic structures between the pristine C 2 N and coplanar C 2 N-C 3 N, the band structure and total density of states (TDOS) were obtained by PBE calculation. As seen from Fig. 2e, the pristine C 2 N presents a typical characteristic of semiconductor, with a band gap of 1.68 eV, which is consistent with the previous work 33 . However, the coplanar C 2 N-C 3 N hybrid displays a feature of conductor with no band (Fig. 2g). In Fig. 2f, the Fermi level is located in the middle of the valance band (VB) and conduction band (CB) without any electron distribution there, and has a gap of 1.68 eV. However, in Fig. 2h the Fermi level crosses the conduction band of the coplanar of C 2 N-C 3 N. This change in total density of states between the pristine C 2 N and C 2 N-C 3 N demonstrates the electron mobility achieves great enhancement in the coplanar C 2 N-C 3 N structure, which has an important effect on the electrocatalytic HER. These properties are consistent with the previous discussion about charge transfer (in Fig. 2a-c). Furthermore, the charge distribution of the valance band maximum (VBM) and conduction band minimum (CBM) were calculated. When the C 2 N-C 3 N heterostructure was excited, the electrons would be excited from VBM to CBM. As seen from Fig. 2i, the electrons would transfer from the middle of C 3 N to the adjacent edge of C 2 N, which also confirms the existence of the built-in electric field.
Adsorption of H and H2 on C2N-C3N coplanar structure. As the initial step for both dissociative and associative mechanism, the adsorption of H is significant in the whole process in HER. If the H is weakly adsorbed on the surface, the adsorption step would limit the overall reaction rate. If the adsorption is too strong, the reaction of desorption will limit the reaction rate. Therefore, we first calculated the adsorption energy of H on the coplanar C 2 N-C 3 N. Several possible adsorption sites were considered, including C1, C2, C3, N1 and N2 atoms (see Fig. 1c). As shown in Table 1, we can see that the adsorption of H on these selective sites all belong to chemisorption according to the adsorption energy analysis and the bond length analysis. It can be found that N1 atom has the strongest adsorption energy (−3.47 eV), which indicates that it could adsorb H easily but it may be too hard to be released. Moreover, the N2 site has the smallest adsorption energy (−1.53 eV), so it may not be beneficial for the H adsorption. However, the E ads of the three carbon sites are −2.22 eV, −2.33 eV and −2.33 eV respectively, and the distance between H and C are all about 1.11 Å. These adsorption on carbon sites (C1, C2, C3) all indicate chemisorption and the adsorption energy is more suitable for the adsorption and release of H than that on the nitrogen sites (N1, N2). On the other hand, as the last step of hydrogen evolution in HER, the adsorption of H 2 molecule is also very important. It can be seen from Table 1 that the adsorption energy on these selective sites are all about −0.20 eV and the length between H and N(C) are all more than 2.65 Å. This result indicates that the adsorption of H 2 on the coplanar C 2 N-C 3 N is the physisorption, which facilitates the release of hydrogen. In combination with the adsorption of H and H 2 , the carbon sites may be more favorable to be active sites for HER reaction. Moreover, the C1 is located at the edge of the hole which is easier to be exposed outside, so C1 is considered as the most favorable active site for the HER reaction to take place.
HER pathways on C 2 N-C 3 N coplanar structure. The HER mechanism is generally considered to have three possible reaction steps (see Table 2) 34 . In both cases, HER takes two protons into hydrogen molecule: Herein, we took a series of configuration optimization and transition state search for the calculation of activation energy for HER on the coplanar C 2 N-C 3 N. According to the analysis of adsorption energy on the selective active sites, we chose the C1 as the active site for HER. Hence, we firstly simulated the Volmer-Heyrovsky mechanism on the C1 sites. The activation barrier and the structures of IS, TS, FS (initial state, transition state and final state) are depicted in Fig. 3a,b. At the transition state in the Heyrovsky, the adsorbed H is beginning to break away from C1 and is close to the other H atom. Across the TS, the final state with a weak adsorption of H 2 molecule above the surface was formed with the H-H bond length being 0.75 Å. For the Volmer reaction, the activation energy is 0.77 eV, which is relatively high in the first step of H adsorption. This phenomenon may be due to the fact that the first H is more beneficial to be adsorbed on the N1 atom. However, based on the discussion on the adsorption on the N1 atom, it's hard for H to release from N1.

Mechanism 1
Volmer ----+ → +   Also, previous work has confirmed that the H coverage on sulfur atom of MoS 2 has a great effect on the adsorption of H and the activation barrier of HER 35,36 . Hence, we performed the same simulation under the condition with H adsorbed on N1. In Fig. 3c,d, as we expected, the activation energy has a marked decline, which changes from 0.77 eV to 0.07 eV. And the energy of the product in the Volmer reaction becomes lower than that of the reactants. These results show a more reasonable reaction path when the H atom covers on the N1 atom. However, in the Heyrovsky reaction, the activation barrier of the H coverage (0.71 eV) is a little larger than that without H coverage (0.62 eV). This is because the FS in the Volmer of the structure with H adsorbed in N1 has much lower energy than that without H coverage. In general, the adsorption of H on N1 has a positive effect on the reaction path of the mechanism of Volmer-Heyrovsky.
We next turned to the second mechanism (Volmer-Tafel), where the two H adsorbed on the C1 and C2 sites react to form H 2 . In this mechanism, the first H adsorbed on the C1 is the same as the Volmer-Heyrovsky, so we just simulated the step of the second H adsorbing on C2 sites and the step of H 2 formation. In the Volmer2 step, the activation energy is about 0.28 eV, with favorable reaction energy of −0.66 eV (see Fig. 3e). The last step is Tafel reaction (see Fig. 3f). In the IS, the distance between the two H (bonded to C1 and C2) is 2.12 Å. Then they approach and form the transition state with the distance of 1.35 Å. In the final state, the evolved H 2 released from the surface of the coplanar with the H-H bond length of 0.75 Å. The reaction energy is about −0.46 eV, which is relatively favorable. However, the activation energy is 2.35 eV and it may not be easy for the reaction to take place at room temperature. Similarly, considering the effect of H coverage, we also simulated the process of Tafel reaction when H adsorbs on N1 (see Figure S2). The activation energy doesn't show the same decrease which further confirms that the Tafel route is less favorable than the Heyrovsky route for this coplanar structure. The reason for this phenomenon may be that the reaction sites (C1 and C2) still don't have a sufficient activity for HER. As reported by Jiao et al. the carbon-based metal-free electrocatalysts generally demonstrate poor activity and the heteroatom-doped methods were studied for a better performance for HER 37 . Furthermore, the boron doped graphene has been reported with efficient electrocatalytic activity for HER 38 . HER pathways on B-doped C 2 N-C 3 N coplanar structure. In order to confirm our guess and improve the catalytic performance of Tafel mechanism in HER on the coplanar C 2 N-C 3 N, the C2 atom was doped by boron atom as the active sites for H 2 evolution (see Fig. 4a). The doping concentration of B is 2.95% (the ratio of the number of doped atoms to the substituted atoms when they are not doped). We next calculated the activation barrier in both Volmer-Heyrovsky and Volmer-Tafel mechanisms. As depicted in Fig. 4b, it shows the pathways of the Volmer-Heyrovsky mechanisms with the activation energy and the structure of IS, TS and FS. In the Volmer step, the activation barrier is 0.09 eV, and the reaction energy is −0.16 eV. In Heyrovsky step, the activation barrier is 0.65 eV and the reaction energy is −0.89 eV. It also shows that the Heyrovsky mechanism is the dominant step and all the energy change in the reactions is acceptable with the B-doped C 2 N-C 3 N structure. On the other hand, as shown in Fig. 4c, it shows the overall reaction pathway of Volmer-Tafel mechanism. Similarly, the first step in Volmer-Tafel is the same as that in Volmer-Heyrovsky. In the Volmer2 step, we found that the activation energy is 0.64 eV, which is 0.36 eV larger than the case without doping. However, in the Tafel step, the barrier is 0.87 eV, which has a 1.48 eV decrease than the case without doping, and makes the Volmer-Tafel mechanism possible to occur. By the calculation of the adsorption energy of H on the B atom, it was found that the E ads of H on B atom is 1.82 eV and is 0.51 eV lower than that on C2. Thus, the doping of B atom shows a lower activity on the adsorption on H in the Volmer2, while it also shows a better performance on the release of H 2 in the Tafel reaction. Therefore, we predicted that the HER catalytic activity of the coplanar C 2 N-C 3 N can be improved by doping B atoms, with the Volmer-Heyrovsky and Volmer-Tafel mechanism being both favorable at room temperature.

Conclusion
In summary, we have designed a novel coplanar heterostructure by connecting C 2 N and C 3 N to achieve efficient electron transfer, which facilitates the electrocatalytic reduction of water to H 2 . From the analysis of electronic properties, we discovered that the electrons have a directional transfer from C 3 N to C 2 N under the built-in potential caused by the work function difference. Also, the electron density distribution located at Fermi level indicates that the novel structure possesses good conductivity and accelerates the charge transfer. Based on the TS search of the whole pathways on the pristine C 2 N-C 3 N for HER, it was found that the barrier of the Tafel step is 2.35 eV, which cannot take place at room temperature. However, by the doping of single boron atom, there is a significant decrease of activation energy of the Tafel step from 2.35 eV to 0.86 eV, which makes both HER mechanisms (Volmer-Heyrovsky and Volmer-Tafel) become favorable at room temperature. Our results indicate that the novel B-doped C 2 N-C 3 N coplanar heterostructure is a promising electrocatalyst for HER and also provides opportunities for the future design of metal-free, low-cost and high-efficiency catalysts for the production of clean energy.

Methods
Most of the simulations were based on density functional theory (DFT) 39 and carried out in Dmol 3 program 40 . The generalized gradient approximation with Perdew-Burke-Ernzerh functional was chosen to describe the electronic interaction effects 41,42 . The basis set of DNP and the basis file of 3.5 was chosen in electronic column, when the involved structures were optimized. The SCF was set with a convergence value of 1.0 × 10 −5 Ha to the orbital occupation, which was employed to enhance SCF convergence efficiency [43][44][45] . The cut-off energy was set to be 500 eV, and the K-points was set as 4 × 5 × 1.
To calculate the adsorption energy of small molecules on the substrate, we provide a definition for the calculation of the adsorption energy (E ad ) as: where E sub&med , E med and E sub are the total energy of the substrate with the adsorbed mediate molecule, a single mediate molecule and the substrate respectively. The transition state search is carried out in the DMol 3 module, with the method of the complete linear synchronous transit/quadratic synchronous transit (LST/QST) 46 . The energy barrier (E b ) of the reactions is defined as: where E TS and E IS are the energy of the transition state and the initial state. Furthermore, first-principle molecular dynamics calculations (MD) are also performed to estimate the structural thermal stability, and the temperature was set at 300 K with the canonical ensemble (NVT) used 47 . The phonon spectra was calculated to test the dynamical stability of the structure by CASTEP 48 . The VBM and CBM of the heterostructure was calculated to investigate the transfer of electrons when the structure was excited. Electronic properties calculations (dos and band structures) were implemented in the Vienna Ab initio Simulation Package (VASP) 49 . The generalized gradient approximation (GGA) is realized by Perdew-Burke-Ernzerhof (PBE) functional with projected augmented wave (PAW) method. The cut-off energy was set to be 500 eV, which is accurate enough to describe the outer valence electrons at p orbital of B, C and N atoms. The convergence criteria of energy and force are 1.0 × 10 −6 eV per atom and 0.02 eV/Å, respectively. The GGA-PBE functional with 4 × 5 × 1 K-points for the unit cell was used to characterize the electronic properties and band structures.