Adsorption of the Gas Molecules NH3, NO, NO2, and CO on

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Adsorption of the Gas Molecules NH3, NO, NO2, and CO on Borophene Chieh-Szu Huang, Altynbek Murat, Vasudeo Babar, Enrique Montes, and Udo Schwingenschlögl*

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Physical Science and Engineering Division (PSE), King Abdullah University of Science and Technology (KAUST), Thuwal, Jeddah 23955-6900, Saudi Arabia ABSTRACT: Two-dimensional materials can be utilized to detect gas molecules in low concentration due to their high surface-to-volume ratios. In this respect, we investigate in the present work recently fabricated borophene, two-dimensional B, which has buckled and line-defective phases. Both are systematically studied for four gas molecules: NH3, NO, NO2, and CO. In each case, the adsorption energy is found to be high and borophene develops distinct wrinkles. Our results provide a thorough understanding of the interaction between borophene and the gas molecules. An excellent performance of borophene as gas sensor is demonstrated by simulating the material’s transport characteristics by means of the nonequilibrium Green’s function method.

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INTRODUCTION The discovery of graphene has initiated enormous interest in two-dimensional materials due to their high surface-to-volume ratios that are greatly beneficial for gas sensing, besides the materials’ well known potential in electronic and mechanical applications.1−3 Due to industrial and traffic growth, air pollution is reaching a new high each year, which amplifies demands for functional gas sensors. The sensitivity achievable by the special properties of two-dimensional materials makes it possible to detect hazardous gas even at very low concentration. Typically, charge transfer triggered by the adsorption of gas molecules, donors, or acceptors, induces a change in the electrical resistivity of the two-dimensional material, i.e., changes in the electronic transport properties are the basic working principle of the sensor.4 Studies of two-dimensional materials with hexagonal structure (graphene, silicene, boronitride, transition-metal dichalcogenides, black phosphorus, etc.) are flourishing in recent years. For B, both cluster and planar structures have been reported experimentally and computationally.5−8 Recently, monolayer B with hexagonal (buckled or planar with line-defects) structure, so-called borophene, has been synthesized on Ag(111) substrate.9,10 Prominent electronic and magnetic properties are to be expected from the anisotropic metallic character detected by scanning tunneling spectroscopy, which is accessible to external stimuli.11,12 Recent theoretical work has proposed borophene as anode material for Na-based batteries.13 In our present article, we aim to employ the metallic nature of borophene for gas sensing purposes, studying both the buckled (P1) and line-defective (P2) phases for four polar gases: NH3, NO, NO2, and CO. We systematically investigate the optimal adsorption sites (adsorption energies), charge transfers, and electronic structures to understand the interaction between borophene and the gas molecules. © XXXX American Chemical Society

METHODS We perform calculations in the framework of density functional theory, as implemented in the Vienna ab initio simulation package,14 making use of the generalized gradient approximation15 of the exchange correlation functional (Perdew− Burke−Ernzerhof parameterization). Projector augmented wave16 pseudopotentials of B, C, N, O, and H with valence states 2s2p1, 2s2p2, 2s2p3, 2s2p4, and 1s1, respectively, are employed with a cutoff energy of 500 eV. Borophene aligns with periodic boundary conditions in the xy-plane, whereas a 20 Å thick vacuum slab is added along the z-axis to avoid interaction between periodic images. A 5 × 5 × 1 k-mesh is used for Brillouin zone sampling (Monkhorst−Pack scheme17) in 90 atom (P1) and 75 atom (P2) borophene supercells. Furthermore, the semiempirical DFT-D2 van der Waals correction is applied.18 In all our calculations, an energy convergence of 10−6 eV is achieved and the atomic forces are converged down to 5 × 10−3 eV/Å. Adsorption energies Ea = Eborophene + Egas − Ecombined are calculated, where Eborophene, Egas, and Ecombined are the total energies of pristine borophene, the gas molecule, and the combined adsorbent−adsorbate system, respectively. A positive value of Ea indicates that the gas molecule is adsorbed, whereas a negative value means that it is repelled by borophene. We also calculate charge density difference maps Δρ(r) = ρcombined(r) − ρborophene(r) − ρgas(r), where ρborophene(r), ρgas(r), and ρcombined(r) are the charge distributions of borophene, the gas molecule, and the combined adsorbent−adsorbate system, respectively (with the component structures identical to those realized in the adsorbent− adsorbate system). Furthermore, charge transfers from/to the Received: April 23, 2018

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DOI: 10.1021/acs.jpcc.8b03811 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Supercell and gas adsorption sites of (a) P1 and (b) P2 borophene.

Figure 2. Side and top views of the lowest energy adsorption sites for (a) NH3, (b) NO, (c) NO2, and (d) CO on P1 borophene and (e) NH3, (f) NO, (g) NO2, and (h) CO on P2 borophene.

±0.01 Å. Accordingly, the band structures and densities of states show virtually no difference to the previous data. The 9 × 5 (5 × 3) supercell of borophene used to model the P1 (P2) phase has a relaxed size of 14.56 Å × 14.35 Å (14.65 Å × 15.21 Å) in the xy-plane. All gas molecules are fully relaxed as well. A supercell of relaxed size 19.41 Å × 20.08 Å has been tested for P1 borophene to make sure that the supercell size is sufficiently large not to affect the energetics and electronic properties.

gas molecules as a consequence of the adsorption are determined by the Bader charge analysis (difference before and after adsorption).19

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RESULTS AND DISCUSSION The lattice constants of P1 (a = 1.62 Å, b = 2.87 Å) and P2 (a = 2.93 Å, b = 5.07 Å) borophene obtained from our structure relaxations agree very well with refs 11−13, within a range of B

DOI: 10.1021/acs.jpcc.8b03811 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 3. Adsorption energies, shortest atomic distances between molecule and borophene, and charge transfers.

Figure 4. Charge density difference maps for (a) NH3, (b) NO, (c) NO2, and (d) CO on P1 borophene and (e) NH3, (f) NO, (g) NO2, and (h) CO on P2 borophene (10−2 e/Å3 isosurface).

N-based molecules are found to favor the T2 site with the N atom pointing to borophene (Figure 2a−c), whereas CO favors the B1 site with the C atom pointing to borophene (Figure 2d). The reason that CO favors the B1 site is the buckling of borophene, which enables bonding of C with two neighboring B atoms rather than one. On P2 borophene, on the other hand, for all gas molecules the T2 site is energetically favorable (Figure 2e−h), again with the N and C atoms pointing to borophene. In general, the energies and distances given in Figure 3 reflect very strong adsorption of the gas molecules on both P1 and P2 borophene. The adsorption energies clearly exceed values reported previously for adsorption on twodimensional materials, such as graphene, MoS2, and phosphorene.23−27 High adsorption energies together with short atomic distances between the molecules and borophene as well as large charge transfers indicate that chemical bonds (N−B or C−B) are formed for all gas molecules. There are several similarities between P1 and P2 borophene. First, the trends of the adsorption energy are closely related even though one structure is buckled and the other possesses line-defects. For both phases, the highest (lowest) adsorption energy is obtained for the NO2 (NO) molecule. The only difference in the trend is that in P1 borophene adsorption of NH3 results in higher energy than adsorption of CO, which is a consequence of the fact that CO favors the B1 rather than the T2 site. Second, the interaction with a gas molecule results in a distinct distortion of borophene, see Figure 2a−h, which is best

Figure 1 shows the buckled structure of P1 borophene and the planar structure of P2 borophene with line-defects, in agreement with the experimental situation.9,10 The possible adsorption sites of the gas molecules are indicated, T, B, and H representing top, bridge, and hollow sites, respectively. We start the relaxation procedure by placing a gas molecule at one of these sites close to borophene. In addition, we examine for each gas molecule different relative orientations with respect to borophene, with either the N/C or H/O atom(s) pointing toward the sheet. The obtained final site and preferred orientation (lowest formation energy) is shown in Figure 2a− h for each model. Numerical values of the adsorption energy, shortest atomic distance between molecule and borophene, and charge transfer are summarized in Figure 3. Inclusion of the van der Waals correction enhances the adsorption energies of NH3, NO, NO2, and CO by 0.34, 0.20, 0.33, and 0.16 eV on P1 borophene and by 0.28, 0.21, 0.26, and 0.18 eV on P2 borophene. This effect is expected to be largely independent of the employed van der Waals correction method, since mainly the distance between the molecule and borophene is modified. The obtained corrections are similar in size to previous reports for water on metal (111) surfaces,20 isooctane and ethanol on the Fe(100) surface,21 and anthracene and pentacene on the Ag(111) surface.22 We note that the adsorption energies are significantly different for P1 and P2 borophene due to the different bonding environments experienced by molecules on the buckled and planar structures. Concerning P1 borophene, C

DOI: 10.1021/acs.jpcc.8b03811 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C described in terms of a development of wrinkles. The phenomenon can be explained by the fact that the N−B or C−B interaction is much stronger than the sp2 bonding in borophene, which results in strong charge transfers and makes borophene a sensitive adsorbent for polar gases. The charge density difference maps in Figure 4a−h demonstrate that only NH3 acts as charge donor (0.12 and 0.07 electrons on P1 and P2 borophene, respectively), whereas NO, NO2, and CO are all charge acceptors. The amount of charge transfer, see the values summarized in Figure 3 (right), turns out to be huge for the charge acceptor molecules. By its 2s2p1 electronic configuration and two-dimensional hexagonal structure, borophene is characterized by B−B hybrid bonds with one electron per bond. In the case of charge transfer, these bonds are destabilized, which is reflected by the observed wrinkles. A similar phenomenon is observed in silicene.28 Although the high adsorption energy and large amount of charge transfer, as compared to other two-dimensional materials, make borophene a distinguished adsorbent with high sensitivity for gas molecules, the reduced stability after adsorption is a critical issue, which requires detailed investigation. Insights into the the electronic structure of the adsorbent− adsorbate system are provided by Figure 5a−h in terms of total

Figure 6. Weighted electronic band structures for NO on (a) P1 and (b) P2 borophene. Blue color represents the contribution of the molecule.

Fermi energy (in contrast to the P2 phase, see Figure 6b) and therefore can expect a substantial response of the electronic transport behavior. The charge transfer induced by the adsorption of gas molecules is expected to affect the resistivity of the host material. Such a resistivity change can be measured and makes it possible to create a gas sensor. To evaluate the performance of borophene as gas sensor, we analyze the electronic transport through the material before and after gas adsorption, using the nonequilibrium Green’s function method as implemented in the SMEAGOL package.29,30 Because of the strong adsorption, it is expected that all molecules affect the conductivity significantly. As a representative case, we thus address a NO molecule on P1 borophene. Due to the anisotropy of borophene, we investigate two transport models: referring to Figure 1a, in the first model the current flows along the vertical direction and in the second model along the horizontal direction. For both models we setup semi-infinite Au electrodes (width 8.65 Å, height 10 Å) connected to a central rectangular scattering region (length 14.56 Å or 11.48 Å) built from the relaxed structure of NO on P1 borophene, as shown in Figure 2b. This setup corresponds to the low concentration limit. The current−voltage characteristics are obtained by means of the Landauer−Büttiker formalism, the results being presented in Figure 7. Reflecting the metallic character of borophene, we Figure 5. Densities of states for (a) NH3, (b) NO, (c) NO2, and (d) CO on P1 borophene and (e) NH3, (f) NO, (g) CO2, and (h) CO on P2 borophene.

and partial densities of states. For both P1 and P2 borophene (Figure 5a−d), only NO contributes states at the Fermi energy, which might modify the conductivity. NO2 states appear at various energies, whereas the states of NH3 and CO are localized at much lower energy within the valence band. Since NO potentially affects the transport behavior of borophene, we show weighted electronic band structures in Figure 6, highlighting the contributions of the molecule. For the P1 phase, see Figure 6a, we observe strong hybridization at the

Figure 7. Current−voltage characteristics of P1 borophene without and with NO adsorbed. The current flows in the (a) vertical and (b) horizontal directions of the structure as shown in Figure 1a. D

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The Journal of Physical Chemistry C find in good approximation an Ohmic dependence. The current is significantly reduced in the presence of the charge acceptor NO. At a voltage of 1.0 V, for example, the reduction amounts to 8 and 13% for vertical and horizontal current, respectively. The overall performance of P1 borophene as gas sensor thus is comparable to that of phosphorene, for example, which recently has been put forward as highly sensitive gas sensor material with a current reduction of 11%.24

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CONCLUSIONS We have discussed the possibility and consequences of gas adsorption on the novel two-dimensional material borophene for the polar gases, NH3, NO, NO2, and CO. The favorable adsorption sites and geometries have been established. Unusually high adsorption energies and large amounts of charge transfer to/from borophene make the material an excellent candidate for gas sensor applications. Only the deformation developing during adsorption may become a critical issue. The gas molecules’ effects on the electronic states of the host have been investigated to provide a microscopic understanding of the interaction. In the case of P1 borophene, adsorption of NO turns out to strongly alter the electronic transport properties. Current−voltage characteristics obtained from simulations in the framework of the Landauer−Büttiker formalism confirm an excellent gas sensing performance, in particular with respect to high sensitivity applications. On the other hand, high adsorption energies prohibit reusability of gas sensors based on borophene.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Udo Schwingenschlögl: 0000-0003-4179-7231 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research reported in this publication was supported by funding from King Abdullah University of Science and Technology (KAUST). For computer time, this research used the resources of the Supercomputing Laboratory at KAUST.

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DOI: 10.1021/acs.jpcc.8b03811 J. Phys. Chem. C XXXX, XXX, XXX−XXX