Monolayer Sc2CO2: A Promising Candidate as a SO2 Gas Sensor or

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Monolayer ScCO: A Promising Candidate as SO Gas Sensor or Capturer Shu-hong Ma, Dongyu Yuan, Zhao-Yong Jiao, Tian-Xing Wang, and Xianqi Dai J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07921 • Publication Date (Web): 10 Oct 2017 Downloaded from http://pubs.acs.org on October 11, 2017

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The Journal of Physical Chemistry

Monolayer Sc2CO2: A Promising Candidate as SO2 Gas Sensor or Capturer Shuhong Ma*, Dongyu Yuan, Zhaoyong Jiao*, Tianxing Wang and Xianqi Dai College of Physics and material Science, Henan Normal University, Xinxiang, Henan 453007, China

ABSTRACT:

Toxic

gaseous

SO2 adsorption

on

O-terminated

M2CO2 (M=Sc, Hf, Zr and Ti) monolayers have been studied by means of first-principle calculations. It is demonstrated that monolayer Sc2CO2 is most preferred for SO2 molecule with suitable adsorption strength and apparent charge transfer. Moreover, the electronic conductivity of Sc2CO2 displays a sharp increase after the adsorption of SO2. In particular, the adsorption strength of SO2 on Sc2CO2 can be further enhanced or weakened by applying tensile strains or controlling external electronic fields, which is greatly desirable to realize the capture or reversible release of toxic SO2 gas. These distinctive features endow the Sc2CO2 monolayer with high selectivity and sensitivity as a potential candidate for SO2 gas sensor, as well efficient control for gaseous SO2 capture.

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1. INTRODUCTION Sulfur dioxide (SO2), released from industrial processes such as fossil fuel combustion and power plants, still remains one of the major gaseous pollutants. As a highly toxic gas, SO2 interaction with ambient air can arouse a variety of human health risks and environmental hazards.1,2 Hence, reliable SO2 gas sensors with high sensitivity and selectivity at low temperatures have been required for environmental safety and industrial control.3 Two-dimensional (2D) materials have been of particular interest for optoelectronic devices4 and various types of sensors, such as environmental gas sensing and trapping.1,3,5-9 So far, gaseous SO2 sensors based on graphene,11 silicene,1 stanene,12 phosphorene,13 arsenene,3 antimonene,9 blue phosphorus,10 siligraphene (g-SiC5),14 and monolayer InN15

have been

widely investigated.

Among them,

silicene,1

phosphorene13 and siligraphene14 were theoretically predicted to be appropriate for SO2 gas sensing, due to the moderate adsorption energies of 0.80, 0.748 and 0.53 eV, respectively. It was demonstrated that blue phosphorus, pristine graphene and Stone-Wales defected graphene,10,11 are insensitive to SO2 gas, owing to the considerably weak interaction. In contrast, the interactions of SO2 with stanene and graphene-like InN

2


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monolayers are quite strong, and thus are promising candidates as gaseous SO2 capture, storage or dissociation.12,15 More recently, the intriguing family of MXenes has attracted great interest in a wide variety of fields such as energy materials, nanoelectronics, catalysts, chemical sensors, and so on.16-18 In a real etching process, bare MXenes are spontaneously terminated by O, OH and F functional groups on both surfaces, owing to the high surface energy.19 Notably, certain oxygen-functionalized MXenes i.e. M2CO2 (M=Sc, Hf, Zr, Ti) are of particular interest for their semiconductor character as revealed in previous studies,20-25 and thus are promising adsorbents for environmental gases sensing or capture. Recent theoretical studies of Yu et al. group5,6 have demonstrated that these M2CO2 (M=Sc, Hf, Zr, Ti) MXenes are excellent materials for NH3 gas sensor or capturer, with good selectivity and sensitivity. Moreover, it is noteworthy that the performance (e.g. gas capture, storage and release) of novel 2D monolayers as gas sensing can be tuned effectively by applying biaxial strains, external electric fields, and charge injection.5,6,15,26,27 For instance, the adsorption of NH3 on monolayer Ti2CO25 and InN15 can be strengthened or weakened under biaxial strains and external electric fields, respectively, indicating the facile ways for the capture and desorption of NH3 on the two monolayers. 3



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To our best knowledge, no prior theoretical work has focused on the adsorption of molecular SO2 on MXenes, and it is highly desirable to explore the possibility of MXene-based SO2 gas sensor. Herein, the potential of O-terminated M2CO2 (M=Sc, Hf, Zr, Ti) monolayers as the SO2-gas sensor or capturer has been explored using the first-principles calculations. The preferred adsorption of SO2 on Sc2CO2 monolayer, with the appropriate adsorption strength indicates the high selectivity of Sc2CO2 toward gaseous SO2. The apparent change of current-voltage (I−V) relation before and after SO2 adsorption suggests the excellent sensitivity of Sc2CO2 to SO2. More significantly, the interaction strength of SO2 with Sc2CO2 monolayer can be controlled efficiently by applying strains and external electric fields, providing the desirable ways for SO2 capture or release using monolayer Sc2CO2.

2. COMPUTATIONAL METHODS Density functional theory (DFT) calculations were performed using projector-augmented wave (PAW) pseudopotentials28 in conjunction with the Perdew-Burke-Ernzerhof (PBE) functions,29 as implemented in the Vienna Ab-initio Simulation Package (VASP) code.30 An energy cutoff at 500 eV was used for the plane-wave basis expansion. We have included semiempirical DFT-D2 type of dispersion correction31 as the long-range interaction between the surface and the gaseous molecules. The Brillouin 4


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zone integration was sampled by 5×5×1 and 15×15×1 k-grid mesh for structural relaxation and electronic properties calculations, respectively. In the calculation, we chose a 3×3 supercell to simulate the periodic structure of M2CO2 monolayers, and a sufficient vacuum region of 20 Å in the z direction was applied. All the structures were fully relaxed by employing the conjugate gradient method, and the energy and force on each atom convergence criterion were 10-5 eV and 0.01 eV/Å, respectively. To ascertain the reliability of computations in a 3×3 supercell, we repeated related computations for Sc2CO2 in a 4×4 supercell, and the rather small energy differences between the 3×3 and 4×4 supercells guarantee the validity of the conclusions in this work (Table S1 in Supporting Information). Additionally, spin-polarized calculations show that the SO2-adsorbed M2CO2 monolayers are nonmagnetic systems. The stability of SO2 adsorbed on MXenes was evaluated by the adsorption energy (Eads) defined as, Eads = EM2CO2 +Gas − EM2CO2 − EGas , where EM 2CO2 +Gas , EM 2CO2 , and EGas are the total energies of gas-adsorbed M2CO2,

pristine M2CO2 monolayer, and gas molecule, respectively. Accordingly, charge

density

difference

(CDD)

was

also

calculated

with

∆ρ = ρM2CO2 +Gas − ρM2CO2 − ρGas , as well Bader charge analysis32,33 was

5



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performed to evaluate the charge transfer. Here, the charge densities of bare M2CO2 monolayer ( ρ M CO ) and isolated gas molecule ( ρGas ) were 2

2

computed with the same coordination as that in the SO2-adsorbed configuration ( ρM CO +Gas ). 2

2

In addition, the transport properties were evaluated with the DFT and non-equilibrium Green’s function (NEGF) method, as implemented in the ATK package.34 The PBE form of GGA function was adopted to describe the exchange-correlation potential. Two-probe model consists of the left (L) and right (R) electrodes of 6.810 Å and the scattering region in between them of 26.506 Å. A double-z polarized (DZP) basis set was used for the electron wave function. A mesh cutoff energy of 200 Hartree and the 1×12×31 k-points were employed for electronic transport calculations.

3. RESULTS AND DISCUSSION As for O-terminated M2CO2 (M=Sc, Hf, Zr, and Ti) monolayers, the most stable structures have been reported20,23 and we note that the Sc2CO2 monolayer has an asymmetric configuration along the thickness direction, where functional oxygen atoms reside on different sites on both sides. Accordingly, we firstly optimized the structures and the lattice parameters as given in Table S2, are 3.41, 3.27, 3.31 and 3.04 Å for pristine Sc2CO2, Hf2CO2, Zr2CO2, Ti2CO2 monolayers, respectively. The key parameters 6


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such as M−C (M−O) bond lengths are corresponding to 2.19/2.50, 2.35, 2.37, 2.19 Å (2.08, 2.10, 2.12, 1.98 Å) for Sc2CO2, Hf2CO2, Zr2CO2, Ti2CO2 monolayer. Present calculated band gap values are 1.81, 0.98, 0.97 and 0.27 eV, respectively. Present results compare well with the previous calculations and experiments as listed in Table S2. Additionally, calculated O−S bond length and the O−S−O bond angle are about 1.447 Å and 119.30° for free SO2 molecule, respectively, which agree well with available values.1 3.1. SO2-adsorbed M2CO2 monolayers. To find the most favorable configurations, a number of adsorption sites, including the top site over metal atom (M) or oxygen atom, bridge site over O−M bond or O−O bond or M−M bond, and hollow site upright on the carbon atom or M atom in hexagonal ring composing of oxygen and metal atoms, are examined to adsorb gas molecules. Meanwhile, several typical orientations of SO2 molecule initially placed either vertically or parallel to MXene sheet, are examined, and relative rotation of molecules to the surface is also considered, when the molecule is placed parallel to the monolayer. Figure 1 presents the most stable adsorption configurations for SO2 adsorbed on M2CO2 monolayers and the corresponding energetics and geometrical structures (see Table 1 and Figure 2), electronic band

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structures (Figure 3 and Figure S3) and so on (Figure 4) are discussed as below. 3.1.1 Energetics and geometrical structures. As depicted in Figure 1, on Sc2CO2 monolayer S atom in SO2 favors in the bridge site between the neighboring two Sc atoms, and the O atoms of SO2 reside on-top Sc atoms, with a moderate adsorption energy of -0.646 eV. In this configuration, the molecular planar of SO2 with the two oxygen atoms facing down, is almost perpendicular to the Sc2CO2 sheet, and the vertical height (H) between O atom of SO2 and the nearest Sc atom is around 2.311 Å (see Table 1). Note that an appreciable charge (0.453 e) is transferred from Sc2CO2 monolayer to SO2, as an indicative of the strong chemical bonding between them. Meanwhile, noticeable structural deformations are found for SO2-adsorbed Sc2CO2, e.g. the O−S−O bond angle of molecule decreasing to 115.89° compared to the free one (119.30°), and Sc atom moving upward to SO2 with the elongated neighboring Sc−C bond of 2.262 Å compared to pristine Sc2CO2 (2.19 Å). Moreover, we also examined the effect of van der Waals (vdW) corrections on the adsorption energy of SO2 adsorbed Sc2CO2. As compared in Table 1, the vdW interaction has prominent effect on the adsorption strength of SO2 on Sc2CO2.

8


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Figure 1. Side and top views for SO2-adsorbed monolayer (a) Sc2CO2, (b) Hf2CO2, (c) Zr2CO2 and (d) Ti2CO2, respectively.

Table 1. Adsorption energy (Eads), charge transfer (CT) from monolayer M2CO2 to SO2 molecule, the vertical height distance (H) between O atom of SO2 and the nearst metal M atom in M2CO2 monolayer, l and θ represent the O−S bond length and O−S−O bond angle of SO2 after adsorption, respectively, and the band gap (Egap).The bold values are obtained by DFT calculations without vdW corrections. Substrate

Eads (eV)

CT (e)

H (Å)

l (Å)

θ (°)

Egap(eV)

Sc2CO2

-0.646

0.453

2.310/2.311

1.499/1.498

115.89

-

-0.390

0.432

2.307/2.308

1.500/1.499

115.37

-

Hf2CO2

-0.425

0.053

3.524/3.562

1.451/1.451

118.47

0.92

Zr2CO2

-0.280

0.045

3.611/3.669

1.450/1.455

118.58

0.93

Ti2CO2

-0.196

0.014

3.729/3.786

1.449/1.449

118.90

0.28

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As shown in Figure 1, molecular SO2 adsorbed on the other three M2CO2 (M=Hf, Zr, Ti) shows quite similar configurations, i.e. on-top site of oxygen atom, the molecular planar parellel to M2CO2 monolayer and the O−S bond distance close to 1.450 Å, as well as the O−S−O bond angle around 118.58° (see Table 1). We notice that SO2 has the least binding energy of 0.196 eV on monolayer Ti2CO2, comparable to that of 0.280 eV on Zr2CO2. The adsorption of SO2 on Hf2CO2 monolayer gets stronger with a larger binding energy of 0.425 eV. Accordingly, the transferred charges are relatively small with values in the range of 0.014~0.053 e. The adsorption heights listed in Table 1 indicate that molecular SO2 gets farther away from the monolayers, in line with the adsorption strength decrement in the order of Sc2CO2 > Hf2CO2 > Zr2CO2 > Ti2CO2. Obviously, monolayer Sc2CO2 shows the high selectivity to SO2 molecule with the desirable adsorption strength and appreciable charge transfer. In order to further verify the high selectivity and sensitivity of monolayer Sc2CO2 to SO2, we examine the adsorption of some common gas molecules, i.e. H2, N2, CO, CO2, CH4, H2S and H2O on monolayer Sc2CO2, and the calculated results are shown in Table S3 and Figure S1. One can see that the interaction strength between SO2 and Sc2CO2 (adsorption energy of -0.646 eV) is much greater than that of other gas 10


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cases (ranging from -0.060 to -0.410 eV), which suggests that monolayer Sc2CO2 has the high selectivity to toxic SO2 gas. Besides, the effect of water molecules on the adsorption strength is also examined through the simulations for SO2 adsorbed on the 3×3 Sc2CO2 monolayer, which is partly covered with one, two and three H2O molecules, respectively. The most favorable configurations and the calculated adsorption results are depicted in Figure S2 and Table S4, respectively. As can be seen, the presence of H2O molecules actually stabilizes the adsorption of SO2 molecule on Sc2CO2. Similar behavior has been evidenced in the case of NH3 adsorbed on Zr2CO2.6

3.1.2 Thermal stability and coverage effect. By performing the first-principle molecular dynamics (MD) simulations with the moles-volume-temperature (NVT) ensemble, we also evaluated the thermal stability of SO2-adsorbed systems at 300 K for 3 picoseconds with a time step of 3 femtosecond, and the numerical results are depicted in Figure 2. It is evident that the fluctuations of energy with time retain around a certain constant for each adsorption monolayer. Moreover, these SO2-adsorbed systems do not suffer pronounced structural distortion or transformation, as seen from the final configurations in Figure 2 and the MD simulation video (for SO2-adsorbed Sc2CO2) attached in Supporting

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Information. We predict that SO2-adsorbed monolayers are all thermally stable at room temperature. Taking Sc2CO2 monolayer as a representative, the coverage effect of SO2 adsorption is examined in the wide range from 0.398 to 2.214 molecules per nm2. As tabulated in Table S1, the calculated adsorption energies are barely varied with the coverage, keeping around -0.65 eV per SO2, indicating a quite weak SO2−SO2 interaction on Sc2CO2. Such a minor effect suggests the selectivity of Sc2CO2 monolayer toward SO2 gas to be coverage-independent.

Figure 2. Molecular dynamics simulation results for SO2-adsorbed M2CO2 monolayers at T=300 K. Left panel: total energy fluctuations with time; right panel: final structural configurations after 3000 fs.

3.1.3 Electronic structure features. To clarify the electronic structure features, we further evaluated the 12


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projected density of states (PDOS), charge density difference (CDD) and electronic band structures for all the SO2-adsorbed monolayers, which are plotted in Figure 3 and Figure S3, respectively. As seen from Figure 3(a), the DOS of Sc2CO2 monolayers are very similar in the energy regions below and above the Fermi level for the cases with and without SO2 adsorption. Notably, the prominent orbital hybridizations between SO2 and Sc2CO2 are localized around the Fermi level, which correspond to the flat impurity states near the band edge (see Figure S3(a)), giving rise to a metallic SO2-adsorbed Sc2CO2 monolayer. Such a character allows for more charge transfers between Sc2CO2 and SO2, in perfect line with CDD distribution displayed in Figure 3(a). By striking contrast, no hybridizing states are observable near the Fermi level for SO2-adsorbed M2CO2 (M=Hf, Zr, Ti) monolayers, where the orbital mixings mainly localize below -2.0 eV in the valence bands (VB) and above 1.0 eV in the conduction bands (CB) as plotted in Figure 3(b-d). This feature, combining with minor charge density redistributions, manifests a weaker interaction between SO2 and M2CO2 (M=Hf, Zr, Ti) monolayers, giving rise to slight band structure modifications (e.g. the band-gap decrement less than 0.1 eV). As evidenced in an earlier work,9 present electronic features of these systems also highlight that the adsorption stability of SO2 is inversely 13



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proportional to the distance of the frontier orbitals of adsorbate to the Fermi level.

Figure 3. The PDOS of SO2 molecule (green shadow), Sc2CO2 (a), Hf2CO2 (b), Zr2CO2 (c) and Ti2CO2 (d) monolayers with (red shadow) and without (blue solid line) SO2 adsorption, respectively. The side views of charge density difference are given in the up-right panels. The isosurface value for all of the cases is 0.001 e/Å3, and the yellow (blue) region represents electron gain (loss). The Fermi energy is set at zero.

3.1.4 I-V relation of Sc2CO2. To qualitatively assess the sensitivity of Sc2CO2 monolayer to SO2 gas, we simulate the current-voltage (I−V) relations before and after the SO2 adsorption using the two-probe models as shown in Figure 4(a). For 14


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simplicity, many simplified pseudo “device” structures are also extensively used, in which the “fake electrodes” just built from the periodic extension of clean nanosheet. This method is used to make the physical picture more clear and also reduce the burden of calculations. Moreover, the simplified device model has also been widely used in previous publications.5,7 As a representative case, the transport current flowing along the zigzag direction is evaluated for the SO2-adsorbed Sc2CO2 device, considering the structural isotropy of monolayer Sc2CO2. As presented in Figure 4(b), the I−V curve of Sc2CO2 exhibits a sharp increase after SO2 adsorption, and there is no current passing through the central scattering region until the applied bias are increasing to 1.70 V (the threshold bias), which is close to the intrinsic gap (1.81 eV) of Sc2CO2 monolayer. With the bias voltage further increase, the current dramatically upraises and this means that the SO2 gas molecule adsorption reduces the contact resistance of Sc2CO2 monolayer, which can be directly measured in experiments. This signifies the monolayer Sc2CO2 great sensitivity to SO2 gas.

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Figure 4. (a) Two-probe model of monolayer Sc2CO2 sensor for detecting SO2 molecule.(b) The current-voltage relations along the zigzag direction for pristine and SO2-adsorbed Sc2CO2 monolayers. The insert in (b) shows the current-voltage relations of pristine Sc2CO2 monolayer.

3.2. Tuning on the performance of M2CO2 sensing to SO2 gas. As reported previously,5,15 the adsorption strength can be tuned by applying biaxial strains and external electric field (E-field). In literature, the sensitive strain-dependent behavior has been demonstrated in the adsorption of NH3 on monolayer Ti2CO2, as well the E-field dependence 16


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behavior is evidenced in the recent study on InN.15 Next, we consider the possibility of M2CO2 monolayers as the capturer for gas SO2 and the reversible releasing by applying tensile strains (from 0 to 5%) and external E-fields (from -0.5 to 0.5 V/Å). 3.2.1 Applied biaxial tensile strains. The biaxial strain is defined as: ε =

a − a0 , where a0 and a are the lattice a0

parameters of system without and with strains, respectively. As displayed in Figure 5, the adsorption energies of SO2 on all the monolayers decrease with the increase of applied biaxial strains, similar to NH3 adsorption on Ti2CO2.5 It is more prominent for SO2-adsorbed Sc2CO2 and the adsorption energy of SO2 on Sc2CO2 monotonously becomes -0.903 eV when 5% strain is applied. In the meanwhile, the vertical heights of SO2 adsorption on Sc2CO2 undergo a noticeable reduction with the strains increment (see Figure S4), resultantly the interaction between them becomes stronger. In contrast, the adsorption strength of SO2 on the other monolayers is less-sensitive to the applied tensile strains. In other words, as the applied strain is increased to 5%, the adsorption energy is still larger than -0.50 eV, which is taken as the reference for gas capture.5,26,35 These results reveal that monolayer Sc2CO2, against the other monolayers, is greatly preferred for SO2 capture under the biaxial tensile strains. 17



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Figure 5. Strain effecton SO2-adsorbed M2CO2 monolayers. The variations of (a) adsorption energy and (b) band gap with biaxial tensile strains from 0% to 5%, respectively.

Moreover, the electronic band structures of all the systems are depicted in Figure S3 under biaxial strains from 0 to 5%, and Figure 5(b) displays the modifications of band gap values. One can see that the band gaps for SO2-adsorbed M2CO2 (M=Hf, Zr, Ti) monolayers are slightly enlarged with the tensile strains, and these systems exhibit an indirect-to-direct gap transition till the applied biaxial strain reaches up to 3%. This mainly originates from the initial position (M point) of conduction band maximum (CBM) shifting slightly upwards to Γ point at larger biaxial strains, while the edge of valence band maximum (VBM) remains at Γ point unchanged, as seen from Figure S3. Notably, at the strain of 5% both the CBM and VBM of SO2-adsorbed Ti2CO2 prominently shifts 18


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upward to different extent, eventually leading to the band gap increment (see Figure 5(b)). Distinctively, the SO2-adsorbed Sc2CO2 monolayer exhibits a transition from metal to semiconductor with small gaps about 0.11 eV, under the applied strains larger than 1%. This is attributed to the VBM at Γ point faintly shifting downward below the Fermi level (see Figure S3(a)). 3.2.2 External electric fields. Applied external electric field is another effective way to tune the performance of 2D materials as gas sensor. The applied electric field is perpendicular to the plane of M2CO2 monolayer with the positive direction aligned upward, as shown in Figure 6(a). It is obvious that the adsorption strength of SO2 on all the M2CO2 monolayers exhibits a sensitive E-field dependence. On the M2CO2 (M=Hf, Zr, Ti) monolayers, the quite similar varying trends of SO2 adsorption energy displays that the interaction gets stronger with the E-field strength increment, irrespective of the E-field directions. It should be noted that the adsorption energies SO2 are reduced below -0.50 eV on the M2CO2 (M=Hf, Zr, Ti) monolayers, when the E-field strength goes beyond 0.2, 0.3 and 0.4 V/Å, respectively. Remarkably, the adsorption of SO2 on Sc2CO2 is weakened initially and then is strengthened while applied positive E-field increases from 0 to 0.5 19



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V/Å. At the critical value of 0.2 V/Å, the binding energy is lowered to the minimum (0.526 eV). Moreover, applying negative E-fields will further strengthen the interaction of SO2 with Sc2CO2, and the binding energy is dramatically enhanced to 1.424 eV at the E-field of -0.5 V/Å. Therefore, applying external electric fields can realize the capture of SO2 on all the M2CO2 (M=Sc, Hf, Zr, Ti) monolayers, and the reversible release of SO2 as well.

Figure 6. Electric field effects on SO2-adsorbed M2CO2 monolayers. The variation of (a) adsorption energy, (b) charge transfer, and (c) band gap as a function of the E-field strength. The insert in (a) shows the direction of positive E-field.

According to Yu et al.,5 the change of adsorption energy results from the modification of charge state in the adsorbed system, and thus the variation of charge transfer with E-fields has been shown in Figure 6(b). We note that the appreciable charge transfer from Sc2CO2 to SO2 20


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molecule enhances linearly with the increase of E-fields from the negative to the positive. For instance, the charge transfer attains 0.314 e and 0.599 e at the E-fields of -0.5 and +0.5 V/Å, respectively. Differently, the charge transfers are less sensitive to the applied E-fields for the other weaker interaction systems. Based on the previous study,36 this behavior of adsorption strength varying with E-fields may be ascribed to the formed dipole moment between gas and substrate. Especially, the monolayer Sc2CO2 has been reported to have net dipole moments,37 which further affect the interaction strength of SO2 with Sc2CO2. Furthermore, Figure 6(c) shows that the variations of energy band gap with the E-field. Quite similar cases are observed for SO2-adsorbed M2CO2 (M=Hf, Zr and Ti), and the electronic band structures (see Figure S5) are barely changed after applying E-fields from -0.3 to 0.1 V/Å. Beyond that, the band gaps decrease gradually and disappear at the positive E-field of 0.3 V/Å. In striking contrast, applied negative E-fields induce a metal-to-semiconductor transition for SO2-adsorbed Sc2CO2 monolayer, and the band-gap will be enlarged dramatically with the E-field strength increasing from 0.1 to 0.5 V/Å. It is likely that the CBM of SO2-adsorbed Sc2CO2, namely the impurity states locating in the band gap, shifts upward relative to the Fermi level (see Figure S5). 21



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Additionally, applying E-fields will drive the impurity states of all the adsorbed monolayers shifting toward the VB or CB, depending on the E-field direction. 3.3. Comparison with other 2D monolayers. As discussed in literature,5 the adsorption-induced charge transfer is crucial in determining the adsorption energy and resultantly affecting the resistivity of the system, which can act as a marker for gas sensors. Herein, we have compared the adsorption energies of SO2 on a variety of 2D sheets in Figure 7, as well as the charge transfer values between them. It is clear that the adsorption strength of SO2 on Sc2CO2 is comparable to that on phosphorene13 and g-SiC514 with the suitable adsorption energies in the range of -0.53 ~ -0.80 eV, which are highly beneficial for SO2 gas sensing. In contrast, the lowest adsorption stability on graphene11 and blue phosphorene10 signifies them least selective to SO2 molecule. Notably, molecular SO2 is chemisorbed on the stanene12 and InN15 sheets with quite large binding energies more than 1.748 eV, unsuitable for the release of SO2. Despite of the similar adsorption strength, we notice that the charge transfers from antimonene9 or g-SiC514 to SO2 molecule are less apparent than the case of SO2-adsorbed Sc2CO2 monolayer. Actually, appreciable charge transfer is in perfect conjunction with strong adsorption stability as seen from Figure 7. 22


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Figure 7. Variation of adsorption energy and charge transfer for SO2 adsorption on a variety of 2D monolayers. The arrows are used to see clearly.

4. CONCLUSIONS By means of first-principle calculations, we have systematically studied the adsorption of SO2 on O-terminated M2CO2 (M=Sc, Hf, Zr and Ti) monolayers. Present results predict superior sensing performances of Sc2CO2 toward toxic SO2 gas, i.e. the high selectivity and sensitivity, controllable capture or reversible release by applying external tensile strains or E-fields. This can be ascribed to the following evidence: (1) Sc2CO2 is the most preferred monolayer for SO2 molecule with suitable adsorption strength compared to other M2CO2 (M=Hf, Zr and Ti) monolayers; (2) the adsorption strength of SO2 on Sc2CO2 can be further enhanced appreciably by applying tensile strains or negative electric fields, which is feasible for SO2 capture or storage; (3) the reversible 23



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release of SO2 gas can be realized by controlling the positive electronic fields approaching the critical value of 0.2 V/Å; (4) the electronic conductivity of Sc2CO2 displays a sharp increase with the adsorption of SO2, thus signifying the high sensitivity of Sc2CO2 towards SO2. In addition, the adsorption of SO2 on Sc2CO2 induces prominent modifications in electronic band structures and appreciable charge transfer. Such unique features manifest the monolayer Sc2CO2 an outstanding candidate as SO2 gas sensor. ASSOCIATED CONTENT Supporting Information Numerical results about structural parameters and band gap values of bare M2CO2 monolayers; adsorption energies, structural parameters and the corresponding favorable adsorption configurations for some gas molecules on Sc2CO2; calculations for the effect of water on the adsorption of SO2 on Sc2CO2; vertical distances between the O atoms of SO2 and Sc atoms of Sc2CO2 under strains; electronic band structures for SO2-adsorbed M2CO2 systems under strains and external electric fields; the examined calculations with/without vdW corrections, and with HSE06/GGA; and a video of SO2-adsorbed Sc2CO2 simulatated by MD. AUTHOR INFORMATION Corresponding Authors 24


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*E-mail: [email protected] *E-mail: [email protected]

ORCID Shuhong Ma: 0000-0003-4545-9563 Dongyu Yuan: 0000-0001-8693-0580 Zhaoyong Jiao: 0000-0002-5971-5778 Tianxing Wang: 0000-0003-3659-8801 Xianqi Dai: 0000-0002-3934-5773

Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was supported by the NSFC (No. 11047026 and 11304085), the Basic Research Program of Education Bureau of Henan Province (No.18A140004). Part of the calculations was performed at the High Performance Computing Center of Henan Normal University.

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Monolayer Sc2CO2: A Promising Candidate as a SO2 Gas Sensor or

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