English

• CO₂ capture and storage has attracted extensive research due to the greenhouse effect caused by its concentration rapid increase, which caused environmental deterioration and global warming. It is necessary to consider the nature of the adsorbent materials containing the high adsorption capacity, sensitive selectivity [1], low energy consumption during adsorption/desorption process, more reusable times [2, 3] and structural stability [4]. At present, the popular CO₂ adsorbents are the metal-organic frameworks (MOFS) [5, 6], covalent organic frameworks (COFS) [7], 2D solid materials including graphene [8], graphene-like materials [9], porous carbon materials and polymers [10-13].

  In the past few years, many researches have been done on the use of external electric fields to promote CO₂ adsorption and achieve CO₂ separation. Guo et al. found that the hexagonal BN nanosheets have high-efficiency adsorption and large capacity for CO₂ under an applied electric field, which is effectively separate CO₂ from the CO₂/H₂/N₂/CH₄/CO/H₂O mixture [14]. The work of Adnan et al. shows that under an external electric field of 0.013 a.u., CO₂ achieves the transition from physisorption to chemisorption, while the desorption of CO₂ is a spontaneous exothermic process with 0.06 eV after the removal of electric field [13]. Qiao et al. reports that CO₂ capture/regeneration on MoS₂ monolayers can be controlled by external electric fields, and the adsorbed CO₂ can be released without any energy barrier once the electric field is turned off [15].

  Recently, BC₃ monolayer with high crystalline quality has been successfully created by carbon-substituted technique in a boron hexagonal comb [16]. BC₃ has excellent mechanical properties [17, 18] and thermal stability [19]. It has excellent performance in catalytic reactions [20], and the potential application for gas sensors [21, 22].

  The first principle is the ab initio calculation, which does not need any parameters. It only needs to start from the most basic physical principle to directly solve the Schrodinger equation with some approximated algorithm, according to the interactions between nucleus and electron and its basic motion rules. The first principle is widely used to calculate the catalysis process of single atom supported or bimetallic catalysts [23-25], chemical reactions caused by external electrical fields [26, 27], surface adsorption and activity [28], organics degradation simulation [29], analysis of photocatalytic mechanism [30, 31], optical properties prediction [32] and so on [33-45].

  In this research, we systematically explored whether BC₃ has the potential to control capture and release of CO₂ under an external electric field. First, the adsorption of CO₂/H₂/CH₄ on BC₃ surface was studied, then the transformation of CO₂ adsorption behavior under gradient electric field and the thermodynamic process of CO₂ adsorption/desorption on the surface of BC₃ were further explored. Finally, the application of different electric field values to separate CO₂/CH₄/H₂ mixed gas was evaluated.

  All of the spin-polarized DFT calculations were performed using the Dmol³ [46] software. Generalized gradient approximation(GGA) formulated by Perdew, Burke, and Ernzerhof (PBE) are carried out to describe the exchange-correlation energy functional [47], and meanwhile we used all-electron core treatment for the electron ion-core interactions [48], and a double numerical basis set containing d polarization function, such as double numeric with polarization (DNP) to increase accuracy. The spin polarization is turned on all the way, and Grimme method was applied to describe the van der Waals interactions [49]. The charge distributions and the charge transfer between BC₃ monolayer and molecules were analyzed by using Hirshfeld method. For geometry optimization and electronic self-consistency, the convergence tolerance is set to 1 × 10^-6 a.u. The convergence threshold parameters of geometric optimization are specified as 1 × 10^-5 Ha, 2 × 10^-3 Ha/Å and 5 × 10^-3 Å for energy, maximum force and displacement, respectively. In the process of structural optimization, the coordinates of the internal atoms were relaxed with the fixed lattice parameters.

  The Brillouin k-point grid was sampled by the 6 × 6 × 1 Monkhorst packt grid [50]. The BC₃ base model used 2 × 2 supercell cotaining 32 atoms with a 20 Å vacuum layer to weaken interaction between layers. The optimized lattice constant of the BC₃ structure is 10.35 Å, which is well consistent with the theoretica1 result of 10.33 Å [51]. Then the adsorption energy was calculated by this formula (Eq. 1):

  (1)

  Where E_All is the total energy of the absorbed combination, E_sub and E_gas represent the energy of separate BC₃ substrate and gas molecules, respectively. It is worth noting that the E-field perpendiculars to the BC₃ surface and points downward.

  As shown in Fig. S1a (Supporting information), BC₃ nanosheets has a planar hexagonal honeycomb structure, each structural unit includes four C atoms and two B atoms. Each B atom was combined with three C atoms via sp² hybridization. The lengths of B-C and C-C are 1.565 and 1.422 Å, respectively, which was well consistent with the early literature [52]. As depicted in the Fig. S1a, six adsorption sites are considered for molecular adsorption, including the top sites of B (T1) and C (T2) atoms, the bridge sites of C-C (B1) and B-C bond (B2), and the central position of C₄B₂ six-membered ring (H1) and the C six-membered ring (H2).

  The structural stability of BC₃ nanosheets under the application of an E-field can be roughly evaluated by the polymerization energy and molecular dynamics simulation. The formula for polymerization energy was defined as Eq. 2:

  (2)

  Where E_B, E_C, null represents the energy of B and C atoms and BC₃ nanosheets, respectively, n and m are the number of C and B atoms [53]. As shown in Fig. S1b (Supporting information), as the E-field strength increased from 0 to 0.008 a.u., the average binding energy decreased from 7.52 eV to 7.40 eV. Compared with that of no E-field, the average binding energy between BC₃ atoms changes little, indicating that the structure is stable. For the optimized structure under the electric field the microcanonical ensemble(NVE) ensemble was adopted, at 300 K, molecular dynamics simulation was done. As shown in Fig. S1c (Supporting information), the energy change of the structure is very small in the 2 ps interval of 1 fs, which also confirmed the stability of the BC₃ structure under the applied E-field.

  We studied three molecules (CO₂, CH₄, H₂) adsorption on BC₃ substrate, the most stable adsorption structure and relative adsorption parameters were depicted in Fig. S2 and Table S1 (Supporting information), respectively. As shown in Fig. S2, CO₂ tends to adsorb on H1 site of C₄B₂ six-membered ring, however, CH₄ and H₂ tend to adsorb on T2 and T1 sites, respectively. No chemical bond is formed between the molecules and the substrate, where the CO₂, CH₄, H₂ are suspended above the surface. The minimum distance (D_m) between molecules and BC₃ substrate are 3.271, 3.609 and 3.158 Å for CH₄, CO₂ and H₂, respectively, and the corresponding adsorption energy is very small with the value of −0.11, −0.09 and −0.07 eV, respectively. From the analysis of Hirshfeld-type charge transfer, the amount of charge transfer between gas molecules and BC₃ is also very small (0.004 e, −0.025 e and 0.012 e).

  As we all know, the strong interactions of the adsorbates combined with surface contribute to the evident results of gas detection selectivity [54] and sensitivity. In order to enhance the CO₂ sensitivity, we apply a forward E-field perpendicular to the Z axis [55] on BC₃ surface [52]. The corresponding adsorption configurations were presented in Fig. 1. According to the difference charge density, the adsorption effect gradually increases along with the increasing of the electric field strength. As shown in Fig. 1c, the angle of ∠O-C-O decreased sharply from 180° to 158.6° along with the electric field increased from 0 to 0.006 a.u., and the adsorption distance reduced from 3.10 Å to 2.94 Å. The adsorption energy adds up to ‒0.43 eV, and the charge transfer amount increases to 0.2 e, the charge density expands a lot resulting a stronger chemical adsorption. When the electric field value further increased to 0.008 a.u., the distance between CO₂ an-d BC₃ suddenly decreased to 1.75 Å, the angle of ∠O-C-O further decreased to 127.3°, and the amount of charge transfer increased to 0.9 e. Seen from the Fig. 1d, the charge density between CO₂ and BC₃ increased dramatically, resulting that the interaction between the CO₂ molecular and the BC₃ surface has been greatly improved. These evidences indicate that under an applied electric field of 0.008 a.u., CO₂ is chemically adsorbed on the surface of BC₃. It is clearly that CO₂ is 'fixed' to BC₃ surface via employing an electric field. The results indicate the interaction between CO₂ and BC₃ surface is enhanced, which cause the stronger gas monitoring sensitivity.

  Figure 1

  

  Figure 1.  Top and side views of the most favorable CO₂ adsorption configurations, corresponding charge density difference in an external E-field of (a) 0.002 a.u., (b) 0.004 a.u., (c) 0.006 a.u., (d) 0.008 a.u.. The blue (yellow) color represents accumulation (diminishing) of electrons. The values of isosurface of (a–c) are 0.003 e/ų, and (d) is 0.02 e/ų.

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  In order to explain the turning point of CO₂ adsorption affected by the electric field, three values of electric field (0.0065, 0007, 0.0075 a.u.) were performed to explore the corresponding adsorption structure, adsorption energy and the differential charge density (as shown in Fig. 2). As shown in Fig. S3a (Supporting information), the adsorption distance suddenly decreased from 1.97 Å under the electric field value of 0.0065 a.u., which was 33% shorter than that of values at 0.006 a.u. (2.94 Å). It is noted that the adsorption energy was increased to ‒0.84 eV, which is almost diploid compared with that of ‒0.43 eV. In addition, the angle of ∠O-C-O is reduced to 137.5°, due to the strong interaction of electrons accumulate between CO₂ and the substrate, which indicates that can be regarded as strong chemisorption. When the E-field value increased to 0.0075 a.u., the adsorption distance was further shortened to 1.772 Å, the angle of ∠O-C-O was further reduced to 128.2°, and the adsorption energy was increased to ‒2.25 eV, which indicates that the interaction between CO₂ and BC₃ substrate is further enhanced. The applied E-field will promote the polarization of the charged particles [56, 57], which will lead to the enhanced adsorption of CO₂ on the surface of BC₃ [55]. As shown in Figs. S3a–c (Supporting information), the critical point of CO₂ from physical adsorption to chemical adsorption on the BC₃ substrate is between 0.006-0.0065 a.u.. As shown in Fig. S4, the minimum E-field value (0.0065 a.u.) required for the conversion of CO₂ from physical adsorption to chemical adsorption is much smaller than that of Penta-graphene [4], and is also smaller than the E-field value required in the previous literature to capture CO₂ materials, such as C₂N [55], h-BN [27], C₃N [58], P-doped C₆₀ [59], P-doped graphene [60]. It is a pity that the value is just slightly larger than that of MoS₂ [29] and PC₅ [24]. Such as it is, BC₃ captures CO₂ is more economical and efficient.

  Figure 2

  

  Figure 2.  The side view of the most stable configurations and corresponding charge density difference in E-field of (a) 0.0065 a.u., (b) 0.007 a.u., (c) 0.0075 a.u.. The blue (yellow) color represents accumulation (diminishing) of electrons. Red and green represent increasing and decreasing electron density. The values of isosurface of (a), (b), (c) are 0.015 e/ų, respectively.

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  In order to further study the interaction between CO₂ and BC₃ substrate under different E-field values, partial density of states (PDOS) was studied and presented in Fig. S5 (Supporting information). As shown in Fig. S5, between the E-field value of 0 ~ 0.004 a.u., there is almost no overlap [48] between the orbits of CO₂ and B atoms, which indicates that the interaction between them is relatively weak physical adsorption. At this time, the E-field has little effect on the capture of CO₂. However, when the E-field value increases to 0.006 a.u., it can be seen that the overall PDOS has a significant right shift, and there is a clear overlapped orbitals of 2p (CO₂) and 2p (B) between 3.0 ~ 5.0 eV, indicating the adsorption effect was enhanced, similarly as the previous results [52]. Similarly, when the E-field values at 0.0065 and 0.007 a.u., the orbital overlap formed by the 2p (CO₂) and 2p (B) hybrid contributions can appear at ‒5.5, ‒1.2 and 5.0 eV, and the same results can be seen at 0.0075 and 0.008 a.u. It should be noted that the applied E-field can significantly enhance the ability of BC₃ to capture CO₂.

  In order to further study the mechanism of CO₂ adsorption on BC₃ under an external E-field, the crystal orbital hamiltonian population (COHP) was performed to analyze the different adsorption structures [61-64]. As shown in Fig. 3, the red area is the contribution of the anti-bonding state, and the cyan area is the bonding state. Fig. 3a shows that in the absence of an external E-field, the molecular orbital between C‒O is composed by the σ_s-p bonding orbital (at ‒6.8 eV) that formed by the interaction of C_s and O_p and the anti-bonding π*_p-p (at 4.5 eV) formed by the interaction of C_s and O_p. In addition, integrated COHP (ICOHP) was calculated to analyze the energy integral up to the highest occupied bands, and the value of ICOHP quantitatively reflected the strength of bonding. As shown in Figs. 3a–d, as the value of the E-field increases, ICOHP gradually decreases, which indicating the weaker of C-O bond in CO₂. The reason is that with the increases of the E-field, the interaction between the C of CO₂ and B atom is enhanced, inducing the electron cloud more shared with the B atom and a reduction in the overlap of the electron cloud of the C-O, which is accordance with the weakened C-O interaction. Figs. S6a–d show that as the E-field strength increases, the ICOHP between C and B atom increases with the bonding state broaden, indicating that the interaction between C and B is strengthen. It should be highlighted that in the range of 0.0065-0.0075 a.u., the bonding molecular orbital σ_p-p between C and B atoms is mainly composed by C_p and B_p. Interestingly, bonding molecular orbitals σ_p-p which located in lower energy contributed by C_p and B_p appeared near the Fermi level.

  Figure 3

  

  Figure 3.  PDOS (partial density of states) and molecular orbital of CO₂ molecule adsorbed on BC₃ under different E-field values, above and below the 0 value of crystal orbital hamiltonian population (COHP) are the anti-bonding state and bonding state, respectively. Red stands for bonding contributions, while cyan stands for antibonding contributions.

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  In order to further explore the possible mechanism of CO₂ adsorption and desorption on BC₃ substrate under the external E-field existence, the potential energy [65] surface was shown in Fig. 4. The energy of the CO₂ adsorption system without an E-field is defined as 0. Clearly, the energy of the system gradually decreases along with E-field increases, indicating that the adsorption of CO₂ under the external E-field is a spontaneous exothermic process. In addition, when applying 0.006 a.u. E-field, the relative system energy decreases to ‒0.26 eV. It is worth noting that when the external E-field increases to 0.0065 a.u., the relative energy difference immediately decreases to ‒0.69 eV, well consistent with the E_ads of ‒0.84 eV, resulting to strong chemical adsorption. Besides, we also systematically studied the capture and separation of CO₂ gas on the BC₃ substrate by switching the E-field. As shown in Fig. 4b, under the external electric field of 0.0065 a.u., CO₂ is chemically adsorbed on the surface of BC₃. When the external electric field is turned off, the CO₂ adsorption distance is extended from 1.965 Å to 3.264 Å with the CO₂ angle shifted from 137.5° to 179.9° after restructuring optimization. The above results demonstrate that the removal of the electric field can promote the transition from chemical adsorption to physical adsorption, in this process the endothermic reaction relative energy value is 0.69 eV. Similarly, when the E-field is switched on, CO₂ changed from physical adsorption to chemical adsorption with a linear structure to a V-shaped structure, and the adsorption distance is shortened to 1.965 Å. This process is exothermic process with relative energy ‒4.06 eV without energy barrier. These results shows that the adsorption/desorption of CO₂ on the BC₃ substrate can be easily controlled by the opening/closing of the electric field.

  Figure 4

  

  Figure 4.  The energy profile for the chemisorption to physisorption of the adsorbed CO₂ over BC₃ due to removing or switching of E-field (EF=0.0065 a.u.).

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  CH₄ and H₂ are high-quality energy gases and important raw materials for the manufacture and synthesis of many chemical products. However, in the actual production and processing of CH₄ and H₂, the presence of CO₂ impurities will inevitably be introduced. Therefore, the separation of CO₂ from the CO₂/H₂/CH₄ mixed gas is of great research significance for improving the purity of CH₄ and H₂. In order to evaluate the separation effect of the electric field on the CO₂/CH₄/H₂ mixed gas, we studied the adsorption behavior of BC₃ to CO₂/CH₄/H₂ under the E-Field of 0.004 and 0.0065 a.u.. In the external E-Field of 0.004 a.u., the most stable adsorption structure is shown in Fig. S7 (Supporting information). The adsorption distances of CO₂/H₂/CH₄ on the BC₃ substrate are 3.032, 2.118, and 2.952 Å, and the corresponding E_ads are ‒0.35, ‒0.43 and ‒0.38 eV, respectively. No obvious electron distribution was observed between the CO₂/CH₄ gas molecules and the substrate. According to the difference in adsorption energy, it can be concluded that H₂ can be efficiently separated from the CO₂/H₂/CH₄ mixed gas under the 0.004 a.u.. In addition, the adsorption behaviors of CO₂/H₂/CH₄ gas under an electric field of 0.0065 a.u. has been studied, which is shown in Fig. S8 (Supporting information). Differ from the physical adsorption of CO₂/H₂/CH₄ under no electric field (Fig. S2.), the E_ads and D_m of CO₂ change significantly (-0.11 to -0.84 eV, 3.371 to 1.965 Å). There is no electron density distribution between the H₂/CH₄ and the BC₃ substrate, but there is a distinct electron density overlaps between the CO₂ and the substrate, as shown in Fig. S8. Obviously, the adsorption properties of CO₂ and H₂/CH₄ are different, under this circumstance the former is chemical adsorption and can be captured by BC₃, but in fact the latter is physical adsorption.

  We used the spin-polarized DFT calculation method to study the adsorption/desorption behavior of CO₂ gas with an external Z-axis vertical positive electric field. An external electric field vertical to the BC₃ surface can significantly enhance the interaction between CO₂ and the substrate. In particular, in the range of 0.006-0.0065 a.u. electric field value, CO₂ shifted from BC₃ physisorption on C₄B₂ vacancy with the rapid decrease of E_ads by 663% (‒0.11 to ‒0.84 eV). Desorption of chemisorbed CO₂ occurred from the BC₃ surface without passing any energy barrier and released an energy of 0.69 eV when the E-Field is switched off. Utilizing the difference in the adsorption properties of CO₂/CH₄/H₂ on BC₃ under the E-Field value of 0.004 and 0.0065 a.u., H₂ and CO₂ gas can be separated from the CO₂/H₂/CH₄ gas mixture. These results confirmed that BC₃ is a potential material for capturing CO₂ under external electric field.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This study was funded by the National Natural Science Foundation of China (No. 21603109), the Henan Joint Fund of the National Natural Science Foundation of China (No. U1404216), the Scientific Research Program Funded by Shaanxi Provincial Education Department (No. 20JK0676).

  Appendix A. Supplementary data

  Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cclet.2021.03.038.

  
  
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  Figure 1  Top and side views of the most favorable CO₂ adsorption configurations, corresponding charge density difference in an external E-field of (a) 0.002 a.u., (b) 0.004 a.u., (c) 0.006 a.u., (d) 0.008 a.u.. The blue (yellow) color represents accumulation (diminishing) of electrons. The values of isosurface of (a–c) are 0.003 e/ų, and (d) is 0.02 e/ų.

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  Figure 2  The side view of the most stable configurations and corresponding charge density difference in E-field of (a) 0.0065 a.u., (b) 0.007 a.u., (c) 0.0075 a.u.. The blue (yellow) color represents accumulation (diminishing) of electrons. Red and green represent increasing and decreasing electron density. The values of isosurface of (a), (b), (c) are 0.015 e/ų, respectively.

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  Figure 3  PDOS (partial density of states) and molecular orbital of CO₂ molecule adsorbed on BC₃ under different E-field values, above and below the 0 value of crystal orbital hamiltonian population (COHP) are the anti-bonding state and bonding state, respectively. Red stands for bonding contributions, while cyan stands for antibonding contributions.

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  Figure 4  The energy profile for the chemisorption to physisorption of the adsorbed CO₂ over BC₃ due to removing or switching of E-field (EF=0.0065 a.u.).

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