Ab Initio Study of the Adsorption of Small Molecules on Stanene

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Ab Initio Study of the Adsorption of Small Molecules on Stanene Xianping Chen, Chunjian Tan, Qun Yang, Ruishen Meng, Qiuhua Liang, Miao Cai, Shengli Zhang, and Junke Jiang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04481 • Publication Date (Web): 15 Jun 2016 Downloaded from http://pubs.acs.org on June 20, 2016

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The Journal of Physical Chemistry

Ab Initio Study of the Adsorption of Small Molecules on Stanene Xianping Chen,*,§,†,‡ Chunjian Tan,†,‡ Qun Yang,† Ruishen Meng,† Qiuhua Liang,§ Miao Cai,*,‡ Shengli Zhang,*,♀ and Junke Jiang§ §

Key Laboratory of Optoelectronic Technology & Systems, Education Ministry of China, Chongqing

University and College of Optoelectronic Engineering, Chongqing University, 400044 Chongqing, China. E-mail: [email protected] †

Faculty of Mechanical and Electrical Engineering, Guilin University of Electronic Technology, 541004

Guilin, China. E-mail: [email protected] ♀

Institute of Optoelectronics & Nanomaterials, College of Material Science and Engineering, Nanjing

University of Science and Technology, 210094 Nanjing, China. E-mail: [email protected] ‡

These authors contributed equally to this work.

■ ABSTRACT Recent reports focus on the experiment preparation of metal monolayer-stanene which is a zero-gap semiconductor with buckled honeycomb structure. Owing to the outstanding properties of stanene, its promising applications in nanoelectronics are widely concerned and studied. Using the first principles calculations, we investigate the adsorption behavior of CO, NH3, H2O, H2S, O2, NO, and NO2 molecules on stanene sheet based on the energetics, charge transfer, and work function. We determine the optimal adsorption sites of small molecule on stanene sheet and the styles of molecule doping, and discuss the interaction mechanism between molecule and stanene. The results indicate that the sensing performance of stanene is superior to other 2D materials such as silicene and germanene. It is found that CO, O2, NO, and NO2 molecule act as charge acceptors, whereas NH3, H2O, and H2S molecule serve as charge donors. For non-polarized molecules, the molecule-stanene interaction is mainly ascribed to the electrostatic attractions effect. In contrast, for polarized molecules, the covalent interaction plays a critical role in the process of adsorption. We further investigate the variation of the work function for small molecules adsorption on stanene. The work function calculations exhibit various responses to the different molecules, which indicates that the Schotty barrier height can be effectively tuned by the selective adsorption of these small molecules. The nontrivial sensitivity and selectivity of stanene presents that it has a potential application in the field of gas sensors and high performance catalyst.

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■ INTRODUCTION The discovery of graphene has aroused the upsurge of research on the future applications of two dimensional (2D) materials, including graphene itself1, silicene2-4, germanene5-6, antimonene7-8, and transition metal dichacogenides9-10, etc. Owing to the specific honeycomb structure, 2D materials possess outstanding electronic, mechanical, chemical, and physical properties deriving from the quantum size effects11-14, which are not available in bulk materials. Such excellent natures have driven the scientists for the further investigations of 2D planar structures acting as candidate materials for many potential applications in energy conversion or storage15-17, photocatalyst18-19, nanoelectronics20-23, and protection coatings. Recently, a new pucker layered material, stanene, with enhanced thermoelectricity24, topological superconductivity25, and near room temperature quantum anomalous Hall (QAH) effect26, has been theoretically predicted and successfully realized by molecular beam epitaxy growth on the Bi2Te3 (111) substrate in the experimental level.27 Stanene possesses a low-buckled honeycomb structure, which indicates that it is more stable compared to the planar configuration. This buckled structure weakens the π − π bonding between Sn atoms and enhances the overlap between π and σ orbitals, so as to stabilize the structure. On the basis of theoretical studies, stanene is a zero bandgap semiconductor without the spin orbital coupling (SOC), while the existence of SOC opens an energy gap of about ~0.1 eV.28 As a result, the free standing stanene exhibits a promising applicant in the room or even high temperature domain. Sensing small gas molecules, such as CO, NH3, H2O, H2S, O2, NO, and NO2, is the key step in the process of industrial monitoring, medical treatment, crop cultivation, and environmental protection, etc. Small molecules ubiquitously occur on the 2D materials surfaces as sensing regions, and it is not possible to remove all of the small molecules from these surfaces because of the larger surface area.29-31 Interestingly, physisorbed small molecules could effectively influence carrier density, optical properties, and the shift of the Fermi level of 2D materials, which is the reason that 2D materials are used as gas sensors.32-33 The adsorption behaviors of graphene-like structures, such as graphene, silicene, germanene, and MoS2, have been systematically investigated and graphene-based gas sensors have been applied to the practical applications. Nevertheless, systematic understanding of the adsorption behavior of CO, NH3, H2O, H2S, O2, NO, and NO2 molecule on stanene sheet, and whether they influence the electronic properties of pristine stanene are still lacking. Because the adsorptions of small molecules show a significant effect on graphene and MoS2, the charge transfer, energetics, work function (WF) of these molecules on stanene must be understood to meet the scientific and commercial applications.


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In this study, we systematically investigate the adsorption behavior of small molecules (CO, NH3, H2O, H2S, O2, NO, and NO2) on stanene sheet, focusing on the charge transfer, energetics, and WF. It is found that CO, O2, NO, and NO2 molecule act as charge acceptors, whereas NH3, H2O, and H2S molecule serve as charge donors. The adsorption performance of stanene is superior to that of silicene and germanene. The potential calculations exhibit the distinct response of the WF of stanene sheet to different molecules. Our works offer a valuable reference for investigating the stanene-based gas sensors at the nanoscale and high performance catalyst. Furthermore, this study also confirms that the optical and electronic properties of stanene are effectively regulated by the selective adsorption of those small molecules.

■ COMPUTATIONAL METHODS The first-principles calculations of structural relaxation and electronic properties are performed based on the density functional theory (DFT) as implemented in the DMol3 package.34-36 The exchange correlation interaction is treated through a general gradient approximation (GGA) in PBE format.30,

32, 37

The

dispersion corrected density functional theory (DFT-D) proposed by Grimme has been employed in order to obtain a better understanding for the noncovalent chemical functionalization of stanene by gas molecules. The Brillouin zone (BZ) integration is done by setting an 8×8×1 Monkhorst-Pack k-point grid for a 4×4×1 supercell. The thickness of vacuum region in the Z direction is kept as 15 Å to avoid the effect of interaction deriving from the adjacent stanene layer. The energy and force on each atom convergence are 10-5 eV and 0.01 eV Å-1, respectively, when all the structures are fully relaxed by employing the conjugate gradient method.38 The relaxed lattice constant of a unit cell is 4.647 Å, the distance of the two sublattices of stanene is 0.882 Å, and the length of Sn-Sn bond is 2.824 Å. These computed parameters are in good agreement with the previous studies.12, 27, 39 In order to analyze the stability of gas molecules adsorption on stanene more reliably, the adsorption energy ( ) is calculated by: = / − − where , , / are the total energy of the gas molecule, stanene and gas/stanene structure, respectively. Charge transfers can be obtained by using Mulliken charge analysis approach. The charge of each atom (atomic charge Q) is calculated by the calculated density ((r)) and electron density ( ()) based on the formula:40



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=

() () ∑ ()

It is noted that Q is computed for the isolated atom in the same gas/stanene structure. At the same time, the charge transfers (∆Q) is deduced according this formula, which determines whether gas molecules as acceptor or donor. Furthermore, there is a certain relationship between ∆Q and the calculation methods.

■ RESULTS AND DISCUSSION To understand the most favorable binding structure of gas molecules on the monolayer stanene, we consider three major possible anchoring positions, namely, on top of a Sn atom (T), the center of a Sn hexagon (C), and the center of a Sn-Sn bond (B), as shown in Figure 1. Meanwhile, the different atoms of gas molecules are also tested on these sites and their are also calculated. In our study, the adsorbed gases are mainly divided into two categories: i) non-polarized gases included CO, NH3, H2O, H2S. ii) polarized gases included O2, NO, NO2. According the aforementioned DFT method, we carry out the geometric optimization of small gas molecules to approach the authentic parameters of gases. The results exhibit that the bond lengths are 1.146 Å, 1.028 Å, 0.980 Å, 1.350 Å, 1.234 Å, 1.167 Å and 1.213 Å for CO, NH3, H2O, H2S, O2, NO and NO2 gas molecule, respectively. The bond angles of NH3, NO2, H2O and H2S are estimated to be 105.4o, 133.4o, 103.6o, and 91.7o, respectively. All these computational data are in good agreement with the previous reports.41-42 The results of calculations based on the most stable adsorption conformation are listed in Table 1.

Figure 1. Schematic view of center site, top site, and bridge site on stanene


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Table 1. Adsorption energy ( ), charge transfer (∆Q) from molecule to stanene, distance (dX-S) of the atom in the gas molecule to the adsorption sites, and band gap ( ) opened at the Dirac point of stanene

Molecule

(eV)

∆Q (e)

dX-S (Å)

(meV)

Style

CO

-0.345

-0.066

2.10

8

acceptor

NH3

-0.734

0.175

2.86

66

donor

H2 O

-0.423

0.048

2.69

26

donor

H2S

-0.471

0.065

3.29

19

donor

O2

-1.006

-0.813

2.17

221

acceptor

NO

-0.801

-0.226

3.42

--

acceptor

NO2

-1.165

-0.504

2.25

63

acceptor

CO Adsorption. In the case of CO adsorption, there are six coordination configurations depending on the different atoms of CO molecule above three adsorption sites. The lowest energy adsorption conformation among six adsorption cases is plotted in Figure 2(a), and the corresponding adsorption properties are listed in Table 1. As we can see, CO molecule locates above the C site with the C-O bond along the vertical direction of the slab with tiny deviation compared with the normal direction. The distance of X atom-site (dX-S) ranges from 2.10 to 4.00 Å for six coordination configurations. The C atom-C site and O atom-B site structure are favorable configuration, the corresponding adsorption energies are -0.345 eV and -0.339 eV respectively, which confirms that CO are physisorbed on stanene via van der Waals interactions. To further explore the mechanism of the physisorption of CO on stanene, their total and partial densities of states (DOS and PDOS) are depicted in Figure 2(b). It can be seen that the orbitals hybridization is not existed, which indicates that the strength of the CO-stanene interaction is not sufficient to affect the properties of stanene. This is further demonstrated by the DOS of stanene which is almost unchanged after CO adsorbed on stanene. Furthermore, the contribution of CO molecule to total DOS is approximately zero near the Fermi level, suggesting that CO molecule has low reactivity to stanene, which is similar to the adsorption of CO on silicene and germanene.3-5, 43 To better understand the electronic interaction and quantify the amount of charge transfer between CO molecule and stanene sheet, we plot the charge density difference image (CDD) calculated by the formula: ∆ = / − −



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where / , , and are the total charge density of the CO/stanene sturcture, pristine stanene, and isolated CO molecule, respectively. , and are calculated with each component at the same positions in the CO/stanene structure. As shown in Figure 2(c), the majority of the charges is accumulated on the C and O atom of CO molecule, while charge slightly depleted on C-O bond. Meanwhile, the charges are depleted in the vacuum region between stanene sheet and CO molecule, which indicates that the charges of Sn atoms are transferred to CO molecule. The ∆Q from stanene sheet to CO molecule calculated by Mulliken population analysis is 0.066 e which is smaller than that of CO adsorbed on germanene (0.1 e).5 This suggests that CO molecule acts as a weak acceptor on stanene sheet, and a weak electrostatic interaction between stanene sheet and CO molecule. The band structures before and after adsorption are described in Figure 2(d), which visually shows that the conduction band minimum (CBM) and valence band maximum (VBM) of stanene are almost unchanged upon CO adsorption, and the remaining Dirac feature at the K point. We find that the C-O bond length of CO molecule after adsorption is 1.153 Å, which is slightly increased from that (1.146 Å) of isolated CO molecule.


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Figure 2. CO adsorbed on stanene. (a) Top and side view of the lowest energy structure. (b) DOS and PDOS and (c) Charge density difference of the CO/stanene configuration. The isosurface is taken as 2×10-3 e/Å3. The direction and value of charge transfer are also denoted. (d) Band structure of pristine and CO-adsorbed stanene.

NH3 Adsorption. In the case of NH3 adsorption, NH3 molecule is placed on the three sites of stanene surface with different atoms including N atom-B site/-C site/-T site and H atom-B site/-C site/-T site. The most favorable adsorption conformation based on the lowest total energy is exhibited in Figure 3(a), where the plane constituted by three H atoms of NH3 molecule is aligned parallel to stanene surface with the N atom directly above the Sn atom. In this configuration, although the N-H bond length is 1.025 Å considered as a nearly constant, the H-N-H angle increases to 108.8o from 105.4o. Compared with CO adsorption on stanene, the values of NH3 adsorption on stanene except for the H atom-T site configuration are more practical for gases sensing. However, this also means that the desorption of NH3 molecule from stanene will be more difficult, which is an essential quality for evaluating the efficient gas sensors. The objective of molecules desorption on stanene can be achieved by thermal treatment and short UV irradiation.44-45 Alternatively, the of all configurations with NH3 adsorption on stanene are no less than -0.7 eV with the exception of the H atom-T site configuration that is only -0.233 eV. Figure 3(b) shows the DOS and PDOS for the H atom-C site configuration. It is found that there is no phenomenon of the orbitals hybridization in the NH3/stanene structure, thus the interaction between NH3 and stanene is weak. Besides, the contribution of NH3 to total DOS is located in the valence bands (VBs) between -10 and -9 eV, which is remote from the Fermi level. This demonstrates that the Dirac point is remained at the K point. Although the value is moderate, the atomic structure of stanene appears a significant deformation. So the interaction between NH3 molecule and stanene sheet is relatively strong compared with adsorption of CO. Nevertheless, the reference length of N-Sn covalent bond is 2.11 Å calculated by the radius of N and Sn atom46, whereas the distance of N-Sn atom is 2.445 Å further illustrating the adsorption of NH3 on stanene is a physisorption process. The CDD plot (see Figure 3(c)) of the H atom-C site configuration clearly exhibits that the charges are depleted on the NH3 molecule and bottom surface of stanene, while the charges accumulation appears mainly on the top surface of stanene and partly on the Sn atoms. This result accurately reveals that NH3 molecule acts as a charge donor and provides 0.175 e to stanene sheet, obtained by Mulliken charge analysis, which validates the relatively strong interaction again. Eventually, the band structure of after adsorption is painted in Figure 3(d). It



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should be noted that the CBM and VBM of stanene sheet are not noticeably altered when NH3 is adsorbed on stanene, and the opened gap value at Dirac point is 66 meV which is smaller than that for the NH3 adsorption on silicene with the level of hundreds of meV.3

Figure 3. NH3 adsorbed on stanene. (a) Top and side view of the lowest energy structure. (b) DOS and PDOS and (c) Charge density difference of the NH3/stanene configuration. The isosurface is taken as 2×10-3 e/Å3. The direction and value of charge transfer are also denoted. (d) Band structure of pristine and NH3-adsorbed stanene. H2O Adsorption. For the H2O molecule, we examine six adsorption configurations. The most steady conformation is presented in Figure 4(a), where the plane constituted by the O and H atom of H2O intersects the stanene surface with an acute angle. The is -0.423 eV that is superior to that of H2O adsorbed on graphene.40 The geometric parameter of H2O after adsorption is 0.978 Å for the H-O bond and 102.2o for the H-O-H angle. The distinct deformation is observed in this configuration, suggesting that the


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molecule-stanene interaction is slightly strong compared to the CO adsorption with no deformation. The PDOS analysis (Figure 4(b)) of the O atom-S site configuration shows that atomic orbitals are not hybridized, which means the electronic properties of stanene can be well preserved. In order to further understand the interaction between H2O molecule and stanene sheet, the CDD diagram of the O atom-S site configuration is displayed in Figure 4(c), showing that charges are remarkably redistributed. The majority of charges are depleted on the vicinity of H2O molecule, while charges accumulated in the vacuum region between H2O molecule and stanene sheet. As a result, H2O molecule acts as a donor donating charge to stanene sheet. The Mulliken charge analysis exhibits that charge transfers from H2O molecule to stanene sheet is 0.048 e due to the electrostatic attractions. In addition, the band structure of the O atom-S site configuration is presented in Figure 4(d), clearly showing that the Dirac point is remained at the K point with a small gap opening of 26 meV, which suggests that the CBs and VBs are not significantly altered.

Figure 4. H2O adsorbed on stanene. (a) Top and side view of the lowest energy structure. (b) DOS and



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PDOS and (c) Charge density difference of the H2O/stanene configuration. The isosurface is taken as 2×10-3 e/Å3. The direction and value of charge transfer are also denoted. (d) Band structure of pristine and H2O-adsorbed stanene. H2S Adsorption. Six coordination configurations of the H2S adsorption are investigated, one with the S atom pointing to the adsorption sites and the other with the H atom pointing to the adsorption sites. The energy analysis indicates that the H atom-B site configuration is the most favorable conformation, as shown in Figure 5(a). In this conformation, the S atom of H2S molecule points away from the surface with a bevel, while the H atoms points to the surface. In truth, the steady state of H2S adsorption is similar to that of H2O. Nevertheless, the with H2S adsorbed on stanene is -0.471 eV, being higher than that of H2O adsorption. As a result, the interaction between H2S molecule and stanene sheet is slightly stronger than the H2O-stanene interaction. Moreover, the H-S bond and the H-S-H angle after adsorbing are 1.352 Å and 90.8°, respectively, which are nearly unchanged. To better understand the effect of H2S gas adsorption on the electronic structure, the DOS and PDOS of the H atom-B site configuration is analyzed and described in Figure 5(b). From PDOS graph, we can obtain that there is not the orbitals hybridization in the H2S/stanene structure, thus the process of H2S adsorption is a physisorption with a weak interaction. The DOS of the H2S/stanene structure is analogous to that of pristine stanene, which implies that the outstanding properties of stanene are well reserved. The molecule-stanene interaction induces the charge redistributions, and whether it effects the band structure of the H2S/stanene structure should be further discussed. The CDD of the H atom-B site configuration is plotted in Figure 5(c). It is observed the charge redistributions around the H2S molecule. The charges are depleted on the H2S molecule, and slightly accumulated on the H atoms, whereas most of charges are accumulated on the vicinity of stanene surface. The stanene sheet acts as an acceptor and receives charges deriving from the H2S molecule, and the charge transfer is 0.065 e obtained by the Mulliken charge analysis. This result clearly reflects that the electrostatic attractions play a major role in the process of H2S adsorption, and the band structure is hardly influenced. The band structure of the H2S/stanene structure is depicted in Figure 5(d), clearly showing that the Dirac point still is preserved at the K point and the conduction band minimum (CBM) slightly shifts up along the direction far away from the Fermi level, which further proves that the band structure is almost unchanged after adsorbing. Besides, the opened gap at the K point is 19 meV according the energy calculations.


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Figure 5. H2S adsorbed on stanene. (a) Top and side view of the lowest energy structure. (b) DOS and PDOS and (c) Charge density difference of the H2S/stanene configuration. The isosurface is taken as 2×10-3 e/Å3. The direction and value of charge transfer are also denoted. (d) Band structure of pristine and H2S-adsorbed stanene. O2 Adsorption. For the O2 molecule, three different configurations are considered due to O2 molecule containing only O atom, including the O atom-B site/-C site/-T site. The energy calculations indicate that the strongest binding site is the latter configuration with the O atom located directly on the top of Sn atom (Figure 6(a)). In this configuration, the O-O bond nearly parallel to the stanene surface with the O-Sn distance of 2.17 Å, and the O atom has moved half of the O-O bond length along the O-O bond direction, as given in Figure 6(a). From Table I, we learn that the is much larger than that of the CO adsorption, which means that the O2 may chemically adsorb on the stanene.47 The intrinsic cause is that the adsorption mechanism of gas molecules on the materials surface can be divided into two types: i) coulomb attractions (electrostatic attractions) accompanied by the polarization-induced dipole-dipole interaction. ii) covalent



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interaction along with charge transfer.29 For chemisorption, the latter plays an crucial role. In order to explore more powerful evidence to verifies the chemisorption nature of O2 on stanene, the DOS and PDOS of the O atom-T site configuration are computed and presented in Figure 6(b). There is a serious orbitals hybridization according the states of Sn p and O p orbitals between -3 and 2 eV. Nevertheless, the DOS of the O2/stanene structure is barely changed compared with that of pristine stanene in the Fermi level. Figure 6(c) illustrates the CDD diagram of O2 adsorption on stanene. It clearly shows that charges are redistributed with a wide range around stanene sheet, which indicates the O2-stanene interaction is strong. This strong interaction causes a large charge transfer which influences the conductivity of the system. The Mulliken charge analysis shows that there is charge transfer of 0.813 e from stanene sheet to O2 molecule, suggesting that O2 acts as an acceptor. A curve plot of band structure (see Figure 6(d)) for the O atom-T site scheme clearly displays the CBs and VBs have distinct change and the CBM and VBM (valence band maximum) shift along the direction away from the Fermi level. The opened gap at the K point is 221 meV computed by PBE mothed, implying that stanene is semiconductor with a narrow gap after O2 adsorption.


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Figure 6. O2 adsorbed on stanene. (a) Top and side view of the lowest energy structure. (b) DOS and PDOS and (c) Charge density difference of the O2/stanene configuration. The isosurface is taken as 2×10-3 e/Å3. The direction and value of charge transfer are also denoted. (d) Band structure of pristine and O2-adsorbed stanene. NO Adsorption. In the case of NO adsorption, the initial adsorption conformations are the same as the one of CO molecule. The lowest energy structure is the O atom-B site conformation, being plotted in Figure 7(a), where the N atom of NO points toward the stanene surface after geometry optimization, while the O atom points away from the surface. The length of N-O bond after adsorption is 1.193 Å, which is slightly elongated compared with the bond length before adsorption. By contrast, stanene sheet appears severe deformation and NO molecule is extremely close to stanene sheet. As a result, there is a strong interaction between NO molecule and stanene. The value of the O atom-T site conformation is 0.801 eV, which further verifies the strong interaction. Figure 7(b) presents the DOS and PDOS of the O atom-B site conformation. It is found that the states of Sn p orbitals are similar to that of N p and O p orbitals, which



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demonstrates that the orbitals hybridization occurs for NO adsorption. Due to the small contribution of NO molecule in the Fermi level, the DOS of the NO/stanene structure is slightly altered in comparison to the one of the pristine stanene. In order to understand the interaction mechanism between NO and stanene, the CDD of the O atom-T site conformation is discussed. Figure 7(c) clearly exhibits that the charges are redistributed around NO molecule, but the range of redistribution is smaller than that of O2 adsorption. So the interaction between NO and stanene is weaker than the O2-stanene interaction, which is further demonstrated by the charge transfer. On the basis of the Mulliken charge analysis, we find that NO molecule obtains charges of about 0.226 e from stanene sheet, which means NO acts as an acceptor. The band structure of the O atom-B site conformation is depicted in Figure 7(d). As can be seen, the CBs and VBs of stanene after adsorption is distinctly altered, but the CBM and VBM are still retained at the K point. Moreover, the CBM and VBM move to the VBs along the direction of the decrease of energy.

Figure 7. NO adsorbed on stanene. (a) Top and side view of the lowest energy structure. (b) DOS and PDOS and (c) Charge density difference of the NO/stanene configuration. The isosurface is taken as


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2×10-3 e/Å3. The direction and value of charge transfer are also denoted. (d) Band structure of pristine and NO-adsorbed stanene. NO2 Adsorption. Similarly, NO2 with the same polarity as NO is also tested. We investigate the same adsorption conformations as for NO molecule without considering the effect of single bond and double bond of NO2 molecule. The conformation with the lowest energy is the N atom-C site structure, being presented in Figure 8(a). Among all the investigated gases, the adsorption of NO2 on stanene possesses the maximum adsorption energy above -1.12 eV, which indicates that the adsorption conformations for NO2 adsorbed on stanene are extremely stable. The length of N-O single bond after adsorbing is 1.381 Å, being 0.168 Å larger than the single bond length before adsorbing. While the length of N-O double bond is hardly changed. Furthermore, the O-N-O angle after adsorbing is 112.3o, decreasing 21.1o compared with that of NO2 before adsorption. Thus, the interaction between NO2 and stanene is strong. As shown in Figure 8(b), the Sn p and O p orbitals of the NO2/stanene structure share the similar states, which means that the strong orbitals hybridization. Nevertheless, the DOS of stanene after adsorbing is almost unchanged at the Fermi level, so the properties of stanene can be well retained. The CDD of the N atom-C site conformation is illustrated to further discuss the charge redistribution. As can be clearly seen from Figure 8(c), the charges around NO2 molecule are distinctly redistributed, and the partial charges are accumulated on the vicinity of NO2 molecule with a small charge depletion. Besides, the Mulliken charge analysis shows that NO2 molecule acts as an acceptor and obtains 0.504 e from stanene sheet. The band structure after adsorbing is displayed in Figure 8(d). It is observed that the CBs and VBs are significantly changed, and the CBM shifts up along the direction of the increase of energy. Moreover, the opened gap at the K point is 63 meV.



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Figure 8. NO2 adsorbed on stanene. (a) Top and side view of the lowest energy structure. (b) DOS and PDOS and (c) Charge density difference of the NO2/stanene configuration. The isosurface is taken as 2×10-3 e/Å3. The direction and value of charge transfer are also denoted. (d) Band structure of pristine and NO2-adsorbed stanene. To further explore the possibilities of adjusting the optical properties for stanene through the adsorption of small molecules, the work function is calculated by the first principles. The WF is defined as the minimum energy required to move an electron from the interior of solid to its surface in the solid state physics. So it is very important to estimate the level of optical properties of 2D materials. The WF for the molecule/stanene structures is calculated by aligning the Fermi level relative to the vacuum energy level (see Figure 9(a)), which can be expressed as Φ = !" − # , where !" and # are the energy of a vacuum level far from the surface and the energy of the Fermi level.48-50 For comparison, the WF calculation is also performed for stanene sheet without gas adsorption. As shown in Figure 9(b), the calculated WF of pristine stanene sheet is 4.30 eV, which is nearly close to that of graphene, ~4.33 eV.49, 51


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Therefore, stanene sheet may have a low optical absorption. Our results show that WFs are almost unchanged for adsorption of CO, H2O, and H2S molecule, whereas WFs are distinctly altered for adsorption of NH3, O2, NO, and NO2 molecule. As a result, the strong molecule-stanene interaction could affect the WF of stanene sheet. It demonstrates that the WF of materials can be effectively tuned by the selective adsorption of these small molecules.

Figure 9. (a) A schematic energy level diagram for the molecule/stanene system. (b) Work function of various molecules-adsorbed stanene.

■ CONCLUSION In summary, we have performed the first principles calculations to investigate the structure, electronic and optical properties of stanene sheet with the adsorption of CO, NH3, H2O, H2S, O2, NO, and NO2 molecules. Our results show that the of NH3, NO, NO2 and O2 molecule are much larger than that of CO, H2O, and H2S molecules, indicating that stanene is more sensitive to nitrogen-based and oxidative molecules. This behavior can be ascribed to the charge transfer caused by the adsorption of these small molecules. The potential calculations exhibit that the distinct responses of WF of stanene sheet to different small molecules, which means that the Schotty barrier height can be effectively tuned by the selective adsorption of these small molecules. In addition, the adsorption of nitrogen-based and oxidative molecules on stanene sheet induces a large charge transfer, thus the n-/p-doping for stanene can be availably achievable. Such sensitivity and selectivity to small molecules provide abundant opportunities in chemical sensors and photocatalyst applications for stanene.

■ AUTHOR INFORMATION



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Corresponding Authors *X-.P.C.: e-mail, [email protected]; phone, +86-23-65111178. *M.C.: e-mail, [email protected]; phone, +86-773-2290108. *S-.L.Z.: e-mail, [email protected]; phone, +86-25-84303279.

Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS The research is co-supported by National Natural Science Foundation of China under Grant No. 51303033, the Guangxi Natural Science Foundation under Grant No. 2014GXNSFCB118004, and the Guilin Science and Technology Development Foundation under Grant No. 20140103-3.

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Ab Initio Study of the Adsorption of Small Molecules on Stanene

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