Currently, air pollution is one of the most critical problems for human health and environment [1]. One of the main types of air pollutants is volatile organic compounds (VOCs), which comprises a variety of compounds, such as alkanes, alkenes, alkynes, aromatic and halogenated hydrocarbons [2, 3]. It is reported that long-term exposure to high concentrations of VOCs can cause serious carcinogenic, mutagenic, and teratogenic effects to human beings [4, 5]. In addition, VOCs are also the precursors for many other atmospheric pollutants, such as ozone, secondary aerosol, and chemical smog [6, 7]. Therefore, it is urgent to remove VOCs from atmospheric to protect human health and mitigate air pollution.

At present, the research on VOCs treatment has become one of the hottest topics in the field of environmental protection [8-10]. There are various technologies to remove VOCs, such as biodegradation [11], photocatalytic oxidation [12], combustion [13] as well as adsorption [14]. Among these technologies, adsorption is a simple and low-cost process, it has high efficiency and wide applications. The adsorbent plays a critical role in the wide use of adsorption. Various adsorbents for VOCs have already been reported, such as zeolites, activated carbon, metal organic frameworks (MOFs) [15, 16].

Recently, the low dimensional materials show the potential to remove pollutants [17]. Especially, with high specific surface area, the two-dimensional (2D) materials have very large adsorption capacity as adsorbents [18]. For example, by using first-principle calculations, Su et al. demonstrated that the pristine 2D materials show unsatisfactory capacity in adsorbing VOCs, while the doped 2D materials, such as graphene and C₂N, are excellent adsorbents for some VOCs [19]. Liu et al. also found that the Al-decorated porous graphene shows outstanding selective adsorption capacity for carbonyl-containing volatile organic compounds [20]. However, the complex processes for modification limit the application of 2D adsorbents. Therefore, it is desirable to explore pristine and simple 2D materials as VOCs adsorbents. Borophene is a monolayer sheet with single element boron. The research of borophene has been going on for many years [21-26], although it was first synthesized in 2016 [27]. It was reported that borophenes withχ₃ and β₁₂ structures were synthetized on Ag(111) surface employing molecular beam epitaxy (MBE) by Feng et al. [27]. As the first synthesized borophenes in experiment, many characteristics of χ₃ and β₁₂ borophene are observed. The borophene show a certain antioxidant capacity, indicating that the structures can exist stably in natural environment. The metallicity of χ₃ and β₁₂ borophene also reveals the potential application in electronic device [28]. As a 2D material, borophene has high specific surface area, which is important for absorbents. Liu et al. have proved that some small gas molecules (CO, NO and NH₃) can be adsorbed steady on borophene [29], which confirms the potential as an excellent gas adsorbent. Therefore, it is expected that borophene might be an adsorbent for VOCs.

Density functional theory (DFT), an important method to study the electron properties of object, is usually used to investigate and understand the nature of chemical behaviors such as photocatalysis [30] and adsorption [31]. Therefore, DFT are employed to investigate the adsorption properties of typical VOCs molecules onto χ₃ and β₁₂ borophene in this work. Some simple and typical VOCs molecules, including ethane (C₂H₆), ethylene (C₂H₄), formaldehyde (H₂CO), methanol (H₃COH), formic acid (HCOOH), methyl chloride (H ₃CCl), benzene (C₆H₆) and toluene (Ph-CH₃), are considered for their adsorption on borophene. The geometric structure and the electron properties are analyzed for the all adsorption configurations to reveal the interactions between borophenes and the VOCs. This work is found to be beneficial to discover new treatment and detection methods for VOCs.

All DFT calculations are performed by Dmol₃ modulus in Materials Studio Package [32]. The calculations using the local density approximation (LDA) underestimate equilibrium distances and overestimate bond energy. Thus, the structure optimization and energy calculations of the most stable geometries in this study are based on the generalized-gradient approximation (GGA) function with the Perdew–Burke–Ernzerhof (PBE) correction [33]. The method proposed by Grimme for DFT-D correction is chosen to elucidate the weak van der Waals interaction [34]. The global orbital cutoff for all atoms is set to 4.5 Å. The DFT semi-core pseudopotentials (DSPP) core treatment is implemented for relativistic effects, which replaces core electrons by a single effective potential [35]. The k-point is set at 5×5 × 1 for all slabs, which brings out the convergence tolerance of energy of 1.0×10⁻⁶ hartree (1 hartree = 27.2114 eV), and that of maximum force of 0.002 hartree/ Å.

To understand the adsorption properties of VOCs on borophene, the adsorption energy of VOCs molecules adsorbed onto borophene E_ads is calculated and defined as:

(1)

where E_total, E_adsorbent, and E_VOCs are the total energies of the system with the gas molecule adsorbed on the pristine borophene, an isolated pristine borophene, and the VOCs molecule in the ground state, respectively.

In this study, χ₃ and β₁₂ borophenes are chosen to investigate the performance for adsorbing VOCs. The optimized structures of 3×4 × 1 χ₃ borophene and 2×4 × 1 β₁₂ borophene are shown in Figs. S1a and b (Supporting information) respectively, where the borophenes are constructed by repetitive hexagon holes and triangular boron sheet in a plat surface. The hexagonal triangular density is η=1/5 and η=1/6 for χ₃ and β₁₂ borophene, respectively [28, 36]. The optimized lattice parameters for the unit cell χ₃ borophene are a = 4.47 Å, b = 2.90 Å and γ = 71.21°, which are in good agreement with previously reported results (a = 4.44 Å, b = 2.90 Å and γ =70.95°) [[ 28]. The unit cell constants of β₁₂ borophene are a = 2.93 Å, b = 5.10 Å and γ =90°, similar as the cell parameters in the previous report of Wu et al., i.e., a = 2.92 Å, b = 5.05 Å andγ=90° [28].

Furthermore, the electronic structures of borophenes are also considered. The band structure of unit cell χ₃ and β₁₂ borophene are shown in Figs. S1c and d (Supporting information), respectively. In the figures, specific symmetry k-points and path in Brillouin zone for the band structure are tagged out. As show in Fig. S1c, there are many bands crossing the Fermi level along M-G, G-F, F-K, B-G and G-K, indicating the electrons in valence band easily shift to conduction band through these bands. Similarly, the bands crossing Fermi level in Fig. S1d mainly locate at the path G-X, S-Y, indicating the β₁₂ borophene also has the capability of electron conduction. Interestingly, the metallicity of the 2D borophenes are different from the 3D α-rhombohedral boron B₁₂, which is nonmetal with an indirect band gap [37].

After understanding the atomic structure of χ₃ and β₁₂ borophenes, the adsorption of various typical VOCs molecules (C₂H₆, C₂H₄, H₂CO, H₃COH, HCOOH, H₃CCl, C₆H₆ and Ph-CH₃) on the two different borophenes is investigated. The most stable configurations of the VOCs molecules adsorbed on χ₃ and β₁₂ borophene are shown in Fig. 1 and Fig. S2 (Supporting information), respectively. Compared with the structures of pristine borophenes in Fig. S1, there is no distinct deformation of borophenes after adsorbing VOCs, except for the adsorption of C₂H₄ and H₂CO molecules as shown in Figs. 1b, c and Figs. S2b, c, where chemical bonds between C₂H₄ or H₂CO and borophene are formed. It is obvious that two B atoms of the borophene protrude from the plane due to the chemical adsorption of C₂H₄ and H₂CO molecules. Moreover, the borophenes also induce structure deformation of the adsorbed C₂H₄ and H₂CO molecules. For C₂H₄, the length of C=C bond extends from 1.33 Å to 1.55 Å on χ₃ borophene and to 1.56 Å on β₁₂ borophene, as listed in Table S1 (Supporting information), indicating the breaking of C=C bond into C-C bond. In addition, the angle formed by the two C-H bonds reduces to 106.513° on χ₃ borophene and 106.403° on β₁₂ borophene from the gas-phase value of 115.673°, indicating that the C atoms of the adsorbed C₂H₄ become sp³ hybrids under the interaction with borophenes. Then the C atoms bind with protruded B atoms to form two C-B bonds (bond length = 1.74 Å) on χ₃ borophene and two C-B bonds (bond length = 1.68 Å) on β₁₂ borophene. For H₂CO, the C=O bond breaks into C-O bond with the bond length extending from 1.21 Å to 1.46 Å on χ₃ borophene and to 1.45 Å on β₁₂ borophene. Then the C and O atoms of H₂CO bind with the protruded B atoms of borophenes to form a C-B bond (bond length = 1.65 Å) and an O-B bond (bond length = 1.42 Å) on χ₃ borophene and a C-B bond (bond length = 1.64 Å) and an O-B bond (bond length = 1.40 Å) on β₁₂ borophene (Table S1). For all the other adsorbed VOCs molecules investigated, the shortest distance between the VOCs molecules and borophenes can be defined by the distance between the bottom H atom in molecule and the B atom of borophene from the adsorption configuration in Fig. 1 and Fig. S2, and the corresponding results are listed in Table S2 (Supporting information). It can be found that the distance is in the range of 2.53 Å–3.22 Å, which is not short enough for the formation of chemical bond between VOCs and borophenes. Therefore, only weak interaction is present between these VOCs molecules and borophenes.

The adsorption energy is another important parameter for adsorption. As shown in Table 1, the adsorption energy for C₂H₄ adsorbed on χ₃ and β₁₂ borophenes is 1.14 eV and 0.80 eV respectively. For the adsorption of H₂CO, the corresponding adsorption energy on χ₃ and β₁₂ borophene are 1.21 eV and 0.79 eV respectively, larger than that of H₂CO on other pristine 2D materials, such as graphene (0.29 eV) [38] and MoS₂ (0.11 eV) [39]. Therefore, the strong interaction between borophenes and the C₂H₄ and H₂CO can be more confirmed. It should be noted that the adsorption energies for C₆H₆ and Ph-CH₃ on χ₃ and β₁₂ borophene are between 0.60 eV to 0.77 eV, which is relatively large for physical adsorption. Considering the above analysis of geometric configuration and electron properties below, they still belong to physical adsorption and the large adsorption energy may result from the relatively greater molecular mass of the C₆H₆ and Ph-CH₃ molecules. For the adsorption of all the other VOCs, the adsorption energy is between 0.23 eV to 0.37 eV, which are in the rage of physical adsorption. Combined with the large distance between boropehens and these VOCs molecule, it can be found that the Van der Waals force plays a main role during the adsorption.

The analysis of geometric configurations reveals the strong interaction between borophenes and the C₂H₄ or H₂CO with the formation of chemical bonds. To further confirm the formation of chemical bonds, the analysis of electron properties is necessary. The deformation of electronic densities for typical VOCs adsorbed on borophenes are calculated and shown in Fig. 2 and Fig. S3 (Supporting information). In these figures, the red regions represent the areas of electron accumulation, while the blue regions reflect the areas of electron loss. In Fig. 2b and Fig. S3b, the two new C-B bonds, formed by C₂H₄ adsorption on borophenes, are covered by red regions, indicating that the new bonds formation due to the overlap of electronic cloud. It can also be found that distinct red regions are on the C-B and O-B bond of borophenes with H₂CO adsorption as shown in Fig. 2c and Fig. S3c. For all the other VOCs molecules adsorbed on the borophenes, however, there are no distinct regions existed between the molecules and adsorbents, confirming that only physical adsorptions existed. The electron density results agree with the above-mentioned conclusion of geometric configuration.

Electron transfer (Q) in adsorption system is also a common analysis method to understand the adsorption. The electron transfer of the adsorbed molecules and borophenes can be calculated by Mulliken analysis and the results are listed in Table 1. It can be found that the electron transfers are between 0.010 e to 0.088 e in the system of physical adsorptions involving C₂H₆, H₃COH, HCOOH, H₃CCl, C₆H₆ and Ph-CH₃. Among the physical adsorptions system, the electron transfers for C₆H₆ adsorbed on χ₃ and β₁₂ borophene are relatively large of 0.088 e and 0.079 e, similar as that of its physical adsorption on graphene (0.02 e) [38] and C₂N (0.018 e) [19]. On the contrary, more electron transfer occurs in chemical adsorption of C₂H₄ and H₂CO adsorbed on the borophenes. The electronic charges of about 0.302 e and 0.313 e are respectively transferred from χ₃ and β₁₂ borophene to the C₂H₄ molecule, which are almost one order larger than that in the above physical adsorptions. In addition, the H₂CO molecule adsorbed on borophenes induces around 0.323 e and 0.287 e charge transfer from χ₃ and β₁₂ borophene, much larger than that of H₂CO adsorbed on graphene (0.019 e) [38], BN (−0.03 e) [40] and C₂N (0.031 e) [19].

To further investigate the formation of the new bonds, the partial density of states (PDOS) of C₂H₄ and H₂CO adsorbed on borophenes are shown in Fig. 3. With the symmetry of the borophenes and C₂H₄ molecule, one C-B bond for C₂H₄ adsorbed on χ₃ and β₁₂ borophene are considered and the PDOS are shown in Figs. 3a and b, respectively. In Fig. 3a, the major peaks at about −6.5, −4.5, −2.8 and −2.0 eV are contributed by the 2p orbital of C atom of C₂H₄, while the peaks at about −6.5, −4.5, −2.8 and −2.0 eV are contributed by the 2p orbital of B atom of χ₃ borophene, indicating the strong overlap of the 2p orbitals of C and B with the formation of the C-B bond. In addition, considering the geometry analysis that the C atom of adsorbed C₂H₄ is sp³ hybrid, the C-B bond for C₂H₄ adsorbed on χ₃ borophene is stable σ bond. The same conclusion can be made from the analysis of PDOS of C₂H₄ adsorbed on β₁₂ borophene as shown in Fig. 3b. Analysis of Figs. 3c and d can find that the steady C-B bonds of H₂CO adsorbed borophenes result from the 2p orbitals of C and B atoms resonate at about −7.0, −4.5 and −2.8 eV forχ₃ borophene and at about −7.0, −5.0 and −3.1 eV forβ₁₂ borophene. Figs. 3e and f show that the O and B atoms maintain overlap at about −7.0 eV and −2.5 eV forχ₃ borophene and at about −7.0, −4.0 and −3.2 eV forβ₁₂ borophene. In other words, H₂CO is chemically adsorbed on χ₃ and β₁₂ borophene with the formation of two stable chemical bonds of C-B and O-B.

It is known that not only the surface structure of adsorbents can be influenced by adsorbed molecules, but also the conductivity, which can be demonstrated by density of states (DOS) in DFT calculation. The DOS of the typical VOCs adsorbed on borophenes are shown in Fig. S4 (Supporting information). In Figs. S4a and b, the DOS of physical-adsorbed VOCs and pure borophenes almost overlap near Fermi level, indicating the ignorable conductivity change of borophenes with the adsorption of VOCs molecules. On the contrary, Figs. S4c and d show the DOS of chemical-adsorbed VOCs and pure borophenes. It can be found that both of χ₃ and β₁₂ borophene has a distinct peak near Fermi level, while the value of the peak reduces after adsorbing C₂H₄ and H₂CO, indicating the loss of active electron near Fermi. In other words, the conductivity of borophenes reduces obviously after adsorbing C₂H₄ and H₂CO molecules. These phenomena are corresponding to the Mulliken analysis results which reveal that the adsorbed C₂H₄ and H₂CO can attract electrons from borophenes.

The typical VOCs adsorbing on both χ₃ and β₁₂ borophene are investigated by using first-principles calculations. The analysis of adsorption configuration reveals that C₂H₄ and H₂CO can be chemically adsorbed on the two borophenes stably with the chemical bonds formation, while the other VOCs molecules are just adsorbed on borophenes with weak van der Waals force. Besides, the analysis of electron properties for C₂H₄ and H₂CO shows that obvious electron accumulation near the new bonds, obvious electrons transfer from borophenes to the adsorbed VOCs molecules, and apparent orbital overlap occurs between the bonding atoms, indicating that the bonds between the two VOCs molecules and borophenes are steady chemical bond. In addition, the analysis of DOS reveals that the adsorption of C₂H₄ and H₂CO can change the conductivity of bororphene, which is an important character for sensors. Therefore, both χ₃ and β₁₂ borophene are able to selective chemical adsorption for C₂H₄ and H₂CO from typical VOCs.

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 work was supported by the National Natural Science Foundation of China (Nos. 21777033 and 41807191), Science and Technology Planning Project of Guangdong Province (No. 2017B020216003), Natural Science Foundation of Guangdong Province, China (No. 2018A030310524), Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (No. 2017BT01Z032), and the Innovation Team Project of Guangdong Provincial Department of Education (No. 2017KCXTD012).

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.2021.01.026.