Adsorption and sensing performance of air pollutants on a β-TeO₂ monolayer: a first-principles study

Abstract

Two-dimensional (2D) β-TeO₂ is a novel semiconductor with potential applications in electronic circuits due to its air-stability and ultra-high carrier mobility. In this study, we explore the possibility of using a 2D β-TeO₂ monolayer for the detection of gaseous pollutants including SO₂, NO₂, H₂S, CO₂, CO, and NH₃ gas molecules based on first-principles calculations. The adsorption properties including the adsorption energy, adsorption distance and charge transfer indicate that the interaction between 2D β-TeO₂ and the six gases is via a physisorption mechanism. Among the six gas adsorption systems, the SO₂ adsorption system has the most negative adsorption energy and the largest charge transfer. In addition, the adsorption of SO₂ obviously changes the electrical conductivity of the β-TeO₂ monolayer because the band gap decreases from 2.727 eV to 1.897 eV after adsorbing SO₂. Our results suggest that the 2D β-TeO₂ should be an eminently promising SO₂ sensing material.

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1. Introduction

The continuous production of air pollutants not only causes severe environmental problems but also threatens human health.^1,2 The typical gaseous pollutants are carbon oxides such as carbon monoxide (CO), sulfides such as sulfur dioxide (SO₂) and hydrogen sulfide (H₂S), nitrides such as nitrogen dioxide (NO₂) and ammonia (NH₃), which are mainly produced from fossil fuel burning and leakage from industrial manufacture.^3–6 Long-term excessive exposure to these pollutants will cause respiratory diseases and even death.^7,8 Therefore, ideal gas sensors that possess high sensitivity, selectivity of the adsorbents, good stability, fast response, and appropriate recovery time are necessarily required for the detection of such toxic gases.^9,10

Since the discovery of graphene in 2004,¹¹ two-dimensional (2D) materials have attracted great interest from scientists owing to their excellent physical and chemical properties, such as the large surface-to-volume ratio, excellent carrier mobility, and high stability.^12,13 2D materials such as graphene,^14–16 transition metal dichalcogenides,^17,18 transition metal oxides,^19,20 and group-VA materials^21–26 have been extensively studied in the application of gas sensors.²⁷ The lack of band gap restricts the application of graphene in gas sensors even though it exhibits excellent gas-sensitive properties.²⁸ Transition metal dichalcogenides, such as MoS₂, have a suitable band gap but are chemically inert under most conditions, limiting their physical adsorption with gas molecules.²⁹ Transition metal oxide-based sensors are important gas detectors with high sensitivity, but the high working temperature may cause high-power consumption and safety issues.³⁰ The 2D β-TeO₂ monolayer was predicted to be a stable semiconductor with a wide direct band gap and high carrier mobility based on first-principles calculations.³¹ In 2021, 2D β-TeO₂ was successfully synthesized via molten droplet methods. 2D β-TeO₂ nanosheets exhibit a high on/off ratio on the order of 10⁶ and high field-effect hole mobility,^32–34 and are a promising candidate for powering electronics and optoelectronic applications. However, relatively little research has been devoted to the gas-sensing properties of 2D β-TeO₂. It is necessary to explore the sensing performance of 2D β-TeO₂.

In this work, we investigate the adsorption properties and sensing performance of a 2D β-TeO₂ monolayer for SO₂, NO₂, H₂S, CO₂, CO, and NH₃ gas molecules based on density functional theory (DFT). The adsorption energy, Bader charge, and charge density difference (CDD) are calculated, indicating that the adsorption behavior of the six gases over the β-TeO₂ monolayer suggests a physisorption mechanism. Among the six gas adsorption systems, the SO₂ adsorption system has the most negative adsorption energy of −0.388 eV and the largest charge transfer. The electronic properties of the β-TeO₂ monolayer after adsorbing the six gases are investigated, including the band structure and density of states. The results show that SO₂ adsorption contributes to the obvious changes in electronic properties, suggesting that the β-TeO₂ monolayer has a high sensitivity to SO₂ molecules. Finally, the desorption behavior of the six gases on the β-TeO₂ monolayer is evaluated. The recovery time of the SO₂ adsorption system is 3.55 μs at room temperature. Considering the high sensitivity and appropriate recovery time of the SO₂ adsorption system, 2D β-TeO₂ has great potential as a SO₂ sensing material for reusable gas sensors.

2. Computational methods

Based on DFT calculations, structural relaxation and electronic calculations are carried out by using the Vienna Ab initio Simulation Package (VASP).³⁵ The exchange–correlation effects are treated by the generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) functional.^36,37 In consideration of van der Waals interaction correction, the DFT-D3 method is applied.³⁸ The cutoff energy is set to 500 eV and the energy of convergence limit is set to 1 × 10⁻⁵ eV. In order to avoid the influence between the adjacent layers of β-TeO₂ monolayer, the vacuum thickness along the Z-direction is set to 40 Å. The k-points are set to 11 × 11 × 1 for the calculations of the unit cell.³⁹ The 2 × 2 × 1 and 3 × 3 × 1 supercells of β-TeO₂ were tested for SO₂ adsorption. The adsorption energies and adsorption distances for the 2 × 2 × 1 and 3 × 3 × 1 supercells are nearly identical. Hence, the 2 × 2 × 1 supercell is sufficiently large for the gas adsorption and we use a 2 × 2 × 1 supercell of β-TeO₂ monolayer for the gas adsorption calculations. A 5 × 5 × 1 k-point mesh is adopted for the calculations of the supercell.

To analyze the stability of the adsorption structure, the adsorption energy is determined by the formula as follows:

E_ads = E_TeO2+gas − E_TeO2 − E_gas

where E_TeO2+gas, E_TeO2 and E_gas represent the total energy of the β-TeO₂ monolayer after the adsorption of gas molecules, the energy of the β-TeO₂ monolayer and the energy of the free gas molecules, respectively. The negative value of adsorption energy means that the adsorption is an exothermic process, which can occur spontaneously. The higher absolute value of E_ads indicates that the adsorption system is more stable.

The CDD of adsorption systems is calculated by the equation:

Δρ = ρ_gas/TeO2 − ρ_gas − ρ_TeO2

where ρ_gas/TeO2, ρ_gas and ρ_TeO2 represent the charge density of the gas adsorption system, adsorbed gas molecule and adsorbed β-TeO₂ monolayer, respectively.

Ab initio molecular dynamics (AIMD) calculations are performed in the NVT ensemble lasting for 5 ps at a temperature of 300 K with a time step of 4 fs. A 3 × 3 × 1 supercell of β-TeO₂ monolayer is constructed for the simulation. The Nosé–Hoover method is used to control the temperature.

3. Results and discussion

The optimized structure of the 2D β-TeO₂ monolayer is presented in Fig. 1(a), with the lattice parameters of a = 5.44 Å, b = 5.76 Å, belonging to the space group P21/C. The calculated lattice parameters are consistent with the results in the previous research.⁴⁰ The electron localization functional (ELF) suggests that one Te atom is covalently bonded to four nearest-neighbor O atoms,⁴¹ resulting in a rectangular lattice. The β-TeO₂ monolayer has a wavy structure as seen from the side views. The band structure of the β-TeO₂ monolayer is shown in Fig. 1(b). The β-TeO₂ monolayer has a band gap of 2.727 eV, with the valence band maximum (VBM) located at the Γ point and the conduction band minimum (CBM) very close to the Γ point.

Fig. 1 (a) Top and side views of the geometric structure of the β-TeO₂ monolayer. The khaki and red atoms represent Te and O elements, respectively. The electron localization functional (ELF) contour is shown in the bottom-right corner. The iso-surfaces of 0 and 1 represent low and high electron localization, respectively. (b) The band structure of the β-TeO₂ monolayer. The Fermi level is assigned to 0 eV.

Firstly, in order to explore the adsorption behavior of several gaseous pollutants (SO₂, NO₂, H₂S, CO₂, CO, and NH₃) on the β-TeO₂ monolayer, three possible adsorption sites are taken into consideration, namely the top site above the Te atom (T₁), the top site above the O atom (T₂), and the hollow site (H), as illustrated in Fig. 1(a). The orientations of the gas molecules are also considered, including whether gas molecules are positioned vertically or parallelly with different atoms facing the β-TeO₂ monolayer on the three possible adsorption sites. The initial distance between all gas molecules and the β-TeO₂ monolayer is 3 Å. After full optimization, we compare the energy of different adsorption systems (Fig. S1–S6, ESI†) and obtain the most stable adsorption structures, as shown in Fig. 2. The detailed adsorption parameters of the most stable adsorption structures are listed in Table 1, including the most stable adsorption site, adsorption energy, adsorption distance, and charge transfer. The calculated results show that for the NH₃ adsorption system, the most stable site is above the Te atom (T₁), and for the other gas adsorption systems, the most stable site is the hollow site (H).

Fig. 2 The most energetically favorable structures for (a) SO₂, (b) NO₂, (c) H₂S, (d) CO₂, (e) CO, and (f) NH₃ adsorbed on the β-TeO₂ monolayer. The yellow, blue grey, light pink, and brown atoms represent S, N, H and C elements. The black arrow represents the adsorption distance, which is the shortest distance between the gas molecules and β-TeO₂ monolayer surface.

Table 1 The most stable adsorption site, adsorption energy (E_ads), adsorption distance (D) of different gas molecules adsorbed on the β-TeO₂ monolayer, and charge transfer (Q) from the β-TeO₂ monolayer to gas molecules

Gas type	Site	E ads (eV)	D (Å)	Q (e)
SO2	H	−0.388	2.703	0.079
NO2	H	−0.202	2.993	0.050
H2S	H	−0.298	2.113	0.003
CO2	H	−0.240	3.056	0.021
CO	H	−0.156	3.057	0.009
NH3	T1	−0.178	2.566	−0.018

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As shown in Fig. 2, the adsorption distances of the six gas adsorption configurations range from 2.113 Å to 3.057 Å, which are larger than the sum of corresponding atomic covalent radii.⁴² We preliminarily estimate that no chemical bonds are formed between the β-TeO₂ monolayer and all gas molecules. Moreover, the E_ads of the six gas adsorption structures are from −0.156 eV to −0.388 eV. The values of both the adsorption energy and the adsorption distance suggest that all six gas molecules are physically adsorbed on the β-TeO₂ monolayer.^43,44 We then calculated the Bader charge to quantify the amount of charge transfer (Q) between the gas molecules and β-TeO₂ monolayer.⁴⁵ The positive value of Q is indicative of the electrons transferred from the β-TeO₂ monolayer to gas molecules. The positive Q for SO₂, NO₂, H₂S, CO₂ and CO adsorption systems reflects the β-TeO₂ monolayer donating electrons to the gases. The Q of the NH₃ adsorption system is negative, indicating that the β-TeO₂ monolayer is an electron acceptor gaining 0.018 electrons from the NH₃ molecule. It is noted that 0.079 electrons are transferred from the β-TeO₂ monolayer to the SO₂ molecule, which is the largest value among the six gas adsorption systems. As mentioned above, the adsorption of SO₂ on the β-TeO₂ monolayer induces the largest charge transfer and the most negative adsorption energy among the six gas molecules.

Furthermore, in order to gain an intuitionistic insight into the charge redistribution of the six adsorption systems, the charge density difference is calculated, as shown in Fig. 3. The blue region represents the electron depletion and the yellow region represents the electron accumulation. For the SO₂ adsorption system in Fig. 3(a), we find that the SO₂ molecule is mainly surrounded by electron accumulation, indicating its electron-accepting behavior. It is demonstrated that the charge redistribution of the SO₂ molecule adsorption is much more intense than those of other adsorption systems, which suggests that the interaction between the SO₂ molecule and β-TeO₂ monolayer is stronger than those of other gases. Overall, compared with the other five gas adsorption systems, the SO₂ adsorption system shows a higher adsorption energy and charge transfer, and stronger electron depletion and electron accumulation.

Fig. 3 The charge density difference of (a) SO₂, (b) NO₂, (c) H₂S, (d) CO₂, (e) CO, and (f) NH₃ adsorbed over the β-TeO₂ monolayer. The iso-surface is 0.001 e bohr⁻³.

In order to explore the effect of gas adsorption on the electronic properties of the β-TeO₂ monolayer, the band structures of the six adsorption systems are performed in Fig. 4. As seen from Fig. 4(a), the adsorption of SO₂ gives rise to an impurity energy level near 1.6 eV. In Fig. 4(b), one can see that two impurity energy levels are introduced near the CBM and VBM after the NO₂ adsorption, respectively. After the adsorption of H₂S and NH₃, an impurity energy level appears between the Fermi level and VBM, as shown in Fig. 4(c) and (f). In addition, for the adsorption of CO₂ and CO on the β-TeO₂ monolayer in Fig. 4(d) and (e), there is no significant change in the band structure since the positions of the VBM and CBM hardly change. Whether the adsorption of gas molecules leads to the change of the band gap of the β-TeO₂ monolayer is further analyzed in the following discussion.

Fig. 4 The band structures of (a) SO₂, (b) NO₂, (c) H₂S, (d) CO₂, (e) CO, and (f) NH₃ adsorbed over the β-TeO₂ monolayer. The Fermi level is assigned to 0 eV.

To gain a full understanding of the adsorption mechanism, the density of states of the adsorbed gas molecules and the β-TeO₂ monolayer before and after the gas adsorption are calculated in Fig. 5. And the projected density of states (PDOS) of interacting atoms in various adsorption systems are shown in Fig. 6. As displayed in Fig. 5(a), there is an additional peak appearing near the energy level of 1.7 eV, which is almost contributed by SO₂. Fig. 6(a) shows that the additional peak near 1.7 eV is mainly occupied by the S-p orbitals and O-p orbitals of SO₂ molecules, followed by the O-p orbitals and Te-p orbitals of the β-TeO₂ monolayer. The O-p orbitals of SO₂ are also hybridized with the p orbitals of O and Te from the β-TeO₂ monolayer near the energy level of −3.4 eV and the O-p orbitals of the β-TeO₂ monolayer near the energy level of −2.5 eV.

Fig. 5 The density of states of the adsorbed gas molecules and the β-TeO₂ monolayer before and after (a) SO₂, (b) NO₂, (c) H₂S, (d) CO₂, (e) CO, and (f) NH₃ adsorption. The Fermi level is assigned to 0 eV.

Fig. 6 The projected density of states of (a) SO₂, (b) NO₂, (c) H₂S, (d) CO₂, (e) CO, and (f) NH₃ adsorption systems. The Fermi level is assigned to 0 eV.

In Fig. 5(b), two additional peaks appear near the CBM and VBM, respectively, arising from the NO₂ adsorption. Through the PDOS in Fig. 6(b), the additional peak near the energy level of −0.2 eV is mainly due to the hybridizations of O-p orbitals, N-p orbitals and N-s orbitals from NO₂, followed by O-p orbitals, Te-s orbitals and Te-p orbitals from the β-TeO₂ monolayer and O-s orbitals from NO₂. The overlaps near −3.5 eV are mainly from the O-p orbitals of NO₂, and O-p orbitals, Te-s orbitals and Te-p orbitals of the β-TeO₂ monolayer. Both of the H₂S and NH₃ adsorption systems have an additional peak near the VBM, as shown in Fig. 5(c) and (f). In Fig. 6(c), it is found that the additional peak near −0.32 eV in the H₂S adsorption system is contributed by the hybridization of S-p orbitals of H₂S, and O-p orbitals, Te-s orbitals and Te-p orbitals of the β-TeO₂ monolayer. A few overlaps appear near the energy level of −3.4 eV, mainly from H-s orbitals, S-s orbitals, S-p orbitals, O-p orbitals and Te-p orbitals. From the PDOS of the NH₃ adsorption system in Fig. 6(f), the additional peak near −0.18 eV is mainly composed of p orbitals of N and O elements, followed by N-s orbitals, H-s orbitals, Te-s orbitals and Te-p orbitals. Furthermore, the above results indicate that the adsorption of SO₂, NO₂, H₂S and NH₃ induces the impurity states of the β-TeO₂ monolayer due to the interaction between the gas molecule and β-TeO₂ monolayer, leading to the gap modulation. Hence, combined with the band structures in Fig. 4, we can conclude that the adsorption of NO₂, H₂S, and NH₃ slightly reduces the values of the band gap of the β-TeO₂ monolayer to 2.379 eV, 2.705 eV, and 2.624 eV, respectively. In particular, the adsorption of SO₂ generates a significant decrease in the band gap of the β-TeO₂ monolayer and the band gap decreases to 1.897 eV.

While for the CO and CO₂ adsorption systems in Fig. 5(d) and (e), no additional peaks emerged from the adsorbed CO and CO₂ around the energy level from the VBM to the CBM, indicating that there is no significant change in the electronic properties of the β-TeO₂ monolayer. In Fig. 6(d), there is almost no orbital hybridization from −2 eV to 3 eV between the gas molecules and β-TeO₂ monolayer. For the CO adsorption system, the overlaps arise near −3.4 eV, which are from the C-s orbitals, C-p orbitals and O-p orbitals from CO, and O-p orbitals and Te-p orbitals from the β-TeO₂ monolayer.

In order to further indicate the effect of the adsorption of SO₂ on the electronic properties of the β-TeO₂ monolayer, we compare the PDOS of SO₂ adsorbed on the β-TeO₂ monolayer and the system where SO₂ is positioned 10 Å above the β-TeO₂ monolayer, as shown in Fig. S7 (ESI†). The interaction with SO₂ shifts the peak of the SO₂ state from 1.27 eV to 1.70 eV and induces an impurity state of the β-TeO₂ monolayer between the band gap. The impurity state of the β-TeO₂ monolayer is contributed by O-p orbitals and Te-p orbitals of the atoms neighboring the SO₂, which is indicated by Fig. 6(a). This change of electronic properties for the system is caused by the charge redistribution post-adsorption.²⁹

The electrical conductivity is a significant parameter for resistivity-type gas sensors, since the gas sensing response of the sensing material depends on the detectable changes in conductivity.⁴⁶ We evaluate the conductivity (σ) and sensitivity of adsorption systems using the following equations:^47–49

σ ∝ exp(−E_g/2kT)

S = (1/σ_gas/TeO2 − 1/σ_TeO2)/(1/σ_gas/TeO2)

where k is the Boltzmann constant of 1.38 × 10⁻²³ J K⁻¹ and T is the working temperature. σ_gas/TeO2 and σ_TeO2 are the conductivity of the β-TeO₂ monolayer before and after the adsorption of gases, respectively. The ΔE_g and sensitivity for various gases on the β-TeO₂ monolayer are listed in Table 2 and depicted in Fig. 7(a). Compared with other gas molecule adsorption systems, the SO₂ adsorption system has an obviously higher ΔE_g and S. The ΔE_g of the SO₂ adsorption system is 30.01%, indicating that its conductivity will be significantly modified. At room temperature, the |S| of the SO₂ adsorption system is 9.37 × 10⁶, which is much larger than those of the other gas adsorption systems.

Table 2 The band gap (E_g), the change of band gap (ΔE_g), the absolute value of sensitivity (|S|), and the recovery time (τ) of the six gas adsorption systems

Gas type	E g (eV)	ΔEg (%)	|S|	τ (μs)
SO2	1.897	30.01	9.37 × 10^6	3.55
NO2	2.379	12.57	836.67	2.29 × 10^−3
NH3	2.624	3.69	6.33	1.05 × 10^−3
H2S	2.705	0.79	0.53	0.101
CO2	2.732	−0.21	0.09	0.0107
CO	2.731	−0.15	0.07	4.87 × 10^−4

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Fig. 7 (a) The change of band gap (ΔE_g) and the absolute value of sensitivity, and (b) the recovery time of the β-TeO₂ monolayer after the adsorption of the six gases.

We evaluate the reusability of the sensing material by calculating the recovery time (τ), which signifies the time that the gas is desorbing from the adsorbed surface. On the basis of the conventional theory of transmission state mode, the τ can be related to the adsorption energy (E_ads) and working temperature (T), assessed by the formula as follows:⁵⁰

τ = ν₀⁻¹ exp(−E_ads/kT)

where k is the Boltzmann constant, T is the working temperature and ν₀ represents the attempt frequency.⁵¹ The working temperature is room temperature (300 K). We assume that the attempted frequency of gas molecules is of the same order of magnitude as NO₂.⁵² For the sake of comparison, the recovery time of the six gas adsorption systems is obtained at room temperature (T = 300 K), as exhibited in Fig. 7(b). The SO₂ desorption from the β-TeO₂ monolayer would be feasible with a recovery time of 3.55 μs. However, for the other five gas adsorption systems, the recovery time is from 0.101 μs to 4.87 × 10⁻⁴ μs. The too-low recovery time indicates that gas molecules cannot be effectively adsorbed on the β-TeO₂ monolayer, making it difficult to realize gas-sensitive detection.^46,50 Therefore, the β-TeO₂ monolayer has unique potential for fast response and recycling as a SO₂ gas sensor.^44,53

Furthermore, we investigate the SO₂ desorption process by using the 3 × 3 × 1 β-TeO₂ monolayer at room temperature (T = 300 K) through AIMD simulations. As shown in Fig. S8 and S9 (ESI†), the in-plane distance between SO₂ and the O atom of the β-TeO₂ monolayer increases from 2.71 Å to 7.78 Å after 5 ps. And the height of SO₂ fluctuated between 2.622 Å and 3.442 Å, which is consistent with the almost invariable adsorption energy. These results predict that SO₂ can effectively diffuse on the surface of the β-TeO₂ monolayer at room temperature. In addition, the PDOS of two alternative adsorption configurations from the AIMD simulations with the adsorption distances of 2.982 Å and 3.442 Å are calculated, as shown in Fig. S10 (ESI†). The results show that during the AIMD simulations, there is always an additional peak appearing between the energy level of 1.5 eV and 2.0 eV, which is contributed by SO₂ and the β-TeO₂ monolayer. Consequently, the influence of SO₂ adsorbed on the β-TeO₂ monolayer is continuous, indicating a persistent sensor response.

Finally, we compare the detection of gas molecules on the β-TeO₂ monolayer with other pure 2D materials. The adsorption of NO₂, H₂S, CO₂, CO, and NH₃ on the β-TeO₂ monolayer is comparable with the adsorption of H₂S on MoS₂,^54,55 NH₃ on MoS₂ and WS₂,⁵⁶ CO₂ on MoTe₂,⁵⁷ and NO₂ on graphene,⁵⁸ such that the adsorption energies range from 0.1 eV to 0.2 eV and the change of band structure is not significant. Compared with MoS₂,²⁹ the β-TeO₂ monolayer possesses a more negative adsorption energy for SO₂ and an obvious variation of band gap, illustrating its more sensitive detection.

4. Conclusion

In conclusion, we predict the possibility of using 2D β-TeO₂ as a sensing material for the detection of air pollutants (SO₂, NO₂, H₂S, CO₂, CO, and NH₃) by using first principles calculations. Based on the analysis of adsorption energy, the shortest adsorption distance, charge transfer and CDD, we can infer that the interaction between the six gases and β-TeO₂ monolayer is a physisorption mechanism. The SO₂ adsorption system has a more negative energy of −0.388 eV and a larger charge transfer of 0.079 e than the other five gas adsorption systems. In addition, the band gap of the β-TeO₂ monolayer after adsorbing SO₂ decreases from 2.726 eV to 1.897 eV, contributing to an obvious change in conductivity and a high sensitivity compared with other gases. The recovery time of the SO₂ adsorption system is 3.55 μs. Therefore, we predict that the 2D β-TeO₂ monolayer as a reusable gas sensor has a high sensitivity at room temperature for SO₂ detection.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (12074332, 21903014 and 11974303). The authors are grateful for access to the computational resources at YZU.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3cp04400a

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