Introduction

Nitrogen oxide and sulfur dioxide are widely used in industrial production. For example, nitric oxide (NO) could be used as the nitrogen source for doping processes in the semiconductor industry, and sulfur dioxide (SO2) could be used to prevent grape from deterioration [1]. However, these gases are not only harmful but also could cause serious environmental problems, such as acid rain or photochemical smog [2, 3]. It is necessary to monitor the leakage of these gases in industrial applications. Among previous researches, metallic oxide gas sensors have been widely studied, but they have disadvantages of instability and limited working conditions [4]. Therefore, it is of considerable significance to find new materials to detect these gases [5]. To detect gas molecules effectively, the materials should have a large surface volume ratio and sufficient binding force to adsorb gas molecules [6, 7]. The discovery of graphene and rare gas sensing properties [8] has motivated researchers to put their attention towards 2D materials [9, 10].

Among 2D materials, transition metal disulfides (TMDs) have attracted a lot of concern in the gas sensing area because of their stable semiconducting properties and appropriate carrier mobility [11,12,13]. Especially as a typical kind of TMDs, WS2 has various unique properties for sensing materials [14, 15], such as excellent thermal stability, tunable band structure [16, 17], and low cost. However, pristine 2D WS2 as a sensitive element has some disadvantages, such as weak adsorption with target gases, which cannot capture the gas molecules effectively [18]. In this case, doping is widely used in 2D materials to adjust the surface properties and binding force between materials and gas molecules and improve the adsorption and sensing capability of gases [19, 20]. Of course, different dopants have different effects on the sensing performance. Therefore, doped sensitive substrates must find suitable impurities to improve their sensing performance. For example, Pd-doped WS2 has already shown their improvement over their pristine counterparts in gas sensing [6, 21]. Unfortunately, most previous studies about doped WS2 as the sensitive element only focused on the binding strength and charge transfer between gas molecules and single-layer films. Adsorption selectivity to gases and the influence of doping concentration are often neglected. In this work, we comprehensively explored not only the binding strength and charge transfer but also the adsorption selectivity to target gases and the influence of doping concentrations.

Here, considering that Al and P atoms have a close covalent radius and similar electronic structure with S atoms, it is easier for them to replace S atoms and form stable covalent structure. Many previous studies have investigated materials with substitution doping of S atoms [22,23,24,25]. Therefore, this work explored the sensing performance of Al- and P-doped WS2 with the help of DFT. The sensing properties of the doped systems with that of the undoped one were compared in terms of binding energy, band structure, and density of state. It proved that WS2 doped with Al or P atoms had apparent advantages over the pristine WS2 in detecting these gases. In addition to NO, NO2, and SO2, we considered CO2 and H2O as disturbance gases to examine the selectivity of a doped substrate to the target gases. Two doping concentrations, 3.7% and 7.4%, were considered to estimate its influence on the sensitivity to gases. This work provides a comprehensive insight to select appropriate dopants (concentration) into 2D materials for sensing harmful gases.

Methods

In this work, all first principle calculations were based on DFT [26, 27]. The local density approximation (LDA) with the PWC function was selected to address the electron exchange and correlation. For alleviating the burden of computation, kernel (DFT semi-core pseudopots) was replaced by a single effective potential. Dual numerical orbital basis set and orbital polarization function (DNP) was chosen. The global orbital cutoff radius was set as 4.9 Å to ensure enough accuracy. The Monkhorst-Pack k-points were set as 4 × 4 × 1 after a convergence test, with a vacuum layer of 13.4 Å to avoid the interaction between adjacent units. The energy convergence precision for geometric was 1.0 × 10−5 Hartree, while the maximum displacement was 0.005 Å, and the maximum force was 0.002 Hartree/Å.

A 3 × 3 × 1 supercell containing 9 W atoms and 18 S atoms was established, as shown in Fig. 1a. For the models of doped WS2, an S atom was replaced by a P or Al atom [28], as shown in Fig. 1b–d. Then, a geometry optimization was given. After that, the gas molecule was set above the WS2 plane to build the gas adsorption model. Three sites for the adsorbed gas molecule were chosen. They were the top of S or dopant atoms (I), the top of the midpoint of the bond between the doped atom and the W or S atom (II), and the center of the hexagon structure (III), as shown in the Fig. 1a–c. After the geometry optimizations for every adsorption system, the geometric constructions with the most stable gas adsorption were found. The binding energy (Ebind) could reflect the interaction between the material and the adsorbed gas molecule and be calculated by the following function:

$$ {E}_{bind}={E}_{tot}-{E}_m-{E}_{gas} $$
(1)
Fig. 1
figure 1

The 4 × 4 × 1 supercell model of a pristine WS2, b Al-doped WS2, and c P-doped WS2 with the three adsorption sites marked. And the models of d NO, e NO2, and f SO2 molecules. Yellow, light blue, dark red, violet, blue, and red balls represent S, W, Al, P, N, and O, respectively

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where Em represents the energy of the material without adsorbing gas molecules, Etot represents the total energy of the material and the gas molecules, and Egas represents the energy of the isolated gas molecule [29]. A more considerable absolute value of Ebind represents a more potent interaction force between the material and gas molecules.

The formation energy (Efm), which could reflect the difficulty to form a doping system, and the stability of the system was calculated by the function below:

$$ {E}_{fm}={E}_{tot}+{E}_s-{E}_m-{E}_{dopant} $$
(2)

where Es is the total energy of the substituted S atom, and Edopant represents the total energy of the dopant atoms. A more significant value of Efm means more difficult to form the dopant system.

Results and Discussion

The adsorption positions have been shown in Fig. 1a–c, which was corresponding to pristine, Al-doped, and P-doped WS2, respectively. In Fig. 1,d–f the bond lengths of N–O, N=O, and S=O were 1.16 Å, 1.21 Å, and 1.46 Å, respectively. The bond length of W–S, Al–W, and P–W bond was around 2.43 Å, 2.86 Å, and 2.45 Å, respectively. After the geometric optimization, the energetically favorable site for each adsorbate has been used in the subsequent discussion. The binding energies of the 3.7% P- and Al-doped WS2 system at the energetically favorable site were shown in Table 1. The binding energy of the pure WS2 system was shown in Table S1. Then, according to the results of binding energy, the interaction between gas molecules and pure WS2 was so weak that it was difficult for the substrate material to adsorb gas molecules stably. The binding energy of the NO-pristine WS2 system was even positive. However, the introduction of dopant could significantly enhance the adsorption strength between gas and WS2, especially for WS2 doped by Al atom. Among all the doping cases, the adsorption strength was the smallest, while SO2 adsorbed on P–WS2. Besides, apart from Al and P, other elements in the same period or family with S, such as O, Si, Cl, or Se, were also considered. The case of Fe-doped W-substituted WS2 was shown in Fig. S1, while WS2 systems with these dopants had either poor stability (high Efm) or weak interaction with gas molecules. Considering this, these dopants were not involved in the subsequent studies. The energetically favorable sites (the lowest negative binding energy) of NO, NO2, and SO2 molecules adsorbed on the doped WS2 were shown in Fig. S2, S3, and S4, respectively.

Table 1 Binding energy of P- or Al-doped WS2 with gas adsorption on the energetically favorable site
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The band structures of pristine and Al- and P-doped monolayer WS2 were presented in Fig. 2. The projective density of states (PDOS) results was shown in Fig. S5. The monolayer 2H WS2 is a semiconductor with a direct bandgap at the Γ point. For WS2 doped with Al atom, the impurity introduced interface states into the bandgap region of monolayer 2H WS2. What’s more, the presence of metal atom forms the Schottky barrier with the Fermi level pinned in the surface region of the semiconductor. The pinning position is within 0.2 eV to the Fermi level of the first semiconductor [5]. Metal properties are brought by metal dopants [30]. At the same time, the P atom introduced energy bands mixed with the conduction and valance band of WS2. Band structures of doped WS2 after gas adsorption were shown in Fig. S6. Consequently, in the cases of NO on Al-doped WS2, NO on P-doped WS2, and SO2 on Al-doped WS2, the bandgap width of material had an evident change after the gas molecules were adsorbed. Previous studies have shown that a narrowed bandgap means lower kinetic stability, higher chemical activity, and a more natural electron transition from the valence band to the conduction band [31, 32]. Thus, after gas adsorption, evident bandgap changes of doped materials made them possible to be sensitive substrates to detect the existence of gas molecules.

Fig. 2
figure 2

Band structure of a pristine WS2, b Al-doped WS2, and c P-doped WS2

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Based on the charge transfer between gas molecules and substrate materials, the detection of gas can be completed by gas sensors. According to the traditional charge transfer theory, the mechanism of the charge transfer process between gas and WS2 was shown in Fig. 3. LUMO is the lowest unoccupied molecule orbital, while HOMO is the highest occupied molecule orbital. Ef is the Fermi level of the substrate. If Ef is between LUMO and HOMO, there will be no charge transfer according to the traditional theory. Then, Zhou et al. added that the charge transfer mechanism would be decided by the orbital mixing of LUMO and HOMO with the substrate material if Ef lies between LUMO and HOMO, as shown in Fig. 3a [5]. If the LUMO is lower than the Fermi level of WS2, electrons will flow from WS2 to gas molecule shown in Fig. 3b [7]. After achieving the equilibrium state, the Ef of the adsorption system is the same as LUMO. Conversely, if the HOMO is higher than the Fermi level of WS2, electrons will flow from gas molecules to WS2 shown in Fig. 3c [5]. The Ef of the adsorption system is the same as LUMO under the equilibrium state. The LUMO and HOMO isosurfaces of NO, NO2, and SO2 molecule orbital were shown in Fig. 4,a–c respectively. The energy of LUMO and HOMO and Ef of WS2 were presented in Table S2. According to the table, Ef lied between LUMO and HOMO in the Al- and P-doped adsorption systems. Hence, it is necessary to explore the orbital mixing between the LUMO and HOMO of gas molecules and the substrate material.

Fig. 3
figure 3

Schematic diagram of the charge transfer mechanism

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Fig. 4
figure 4

LUMO and HOMO of molecule orbital a NO, b NO2, and c SO2

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DOS was employed to discuss further the electron distribution and orbital mixing in the adsorption system, which depended on the interaction between gases and substrates. Figure 5 presents the DOS of gases, dopants, S, and W atoms. Black and red lines were the DOS curves of gases and dopants, respectively. And blue and olive lines were those of S and W atoms, respectively. After gas adsorption, due to the orbital interaction, the electron redistribution occurred in the whole system, which would lead to the overlaps of DOS peaks between the gas and substrate material. The overlaps of DOS peaks meant the mixing between molecular orbitals, proving the existence of an interaction between gas and sensing materials [33]. The mixing of molecular orbitals was helpful to charge transfer so that it can augment the adsorption interaction between gas and material surface [34,35,36]. Hence, the mixing between molecular orbitals was compared to evaluate the adsorption effects of gas molecules. In Fig. 5a, the orbital mixing between NO molecule and Al atom was at − 12.62 and − 8.11 eV. And the orbital mixing between NO molecule and Al, S, and W atoms was at 2.02 eV. In Fig. 5b, the orbital mixing between NO2 molecule and Al atom was at − 19.60, − 11.60, and − 8.44 eV. And the orbital mixing between NO2 molecule and Al, S, and W atoms was at 0 eV. In Fig. 5c, the orbital mixing between SO2 molecule and Al atom was at − 12.09 eV. The orbital mixing between SO2 molecule and Al and S atoms was at − 8.27 eV. The orbital mixing between SO2 molecule and Al, S, and W atoms was at 1.75 eV. In Fig. 5d, the orbital mixture between NO molecule and P atom was at − 12.21 eV. And the orbital mixing between NO molecule and P, S, and W atoms was at − 10 eV. In Fig. 5e, the orbital mixture between NO2 molecule and P atom was at − 12.63 eV. And the orbital mixing between NO2 molecule and P, S, and W atoms was at − 9.66 and − 5.51 eV. In Fig. 5f, the orbital mixing between SO2 molecule and S atoms was at − 9.25 eV. From the above results, it can be found that the presence of impurities results in more orbital mixing. Moreover, the orbital mixing in the systems with Al atom doped is more than that in the systems with P atom doped, indicating stronger interaction between gas molecules and substrate in Al-doped systems that agreed well with the binging energy results. To sum up, the introduction of impurities can provide more activated peaks in the whole band, thus increasing the possibility of orbital mixing between the substrate and gas molecules.

Fig. 5
figure 5

DOS of a NO, Al, S, and W atoms; b NO2, Al, S, and W atoms; c SO2, Al, S, and W atoms; d NO, P, S, and W atoms; e NO2, P, S, and W atoms; and f SO2, P, S, and W atoms

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To further evaluate the sensing potential of the Al- and P-doped WS2, CO2 and H2O were also considered for testing the selectivity of Al- and P-doped WS2 to target gas. Similar to NO, NO2, or SO2 adsorption, the most stable adsorption site among three sites with high geometric symmetry on WS2 was shown in Fig. S7(a), (b), (c) and (d). The binding energy results were presented in Table S3, and band structure results were shown in Fig. S7(e), (f), (g) and (h). The bond length of C=O in isolated CO2 and O–H in isolated H2O was 1.175 Å and 0.971 Å, respectively. They did not change much after gas adsorbed on the doped WS2 except for H2O adsorbed on Al-WS2. That indicated the interaction between the H2O molecule and Al-doped WS2 was the strongest. According to Table 2, the calculated binding energy of H2O on Al-WS2 was − 1.69 eV.

Table 2 Binding energy of Al- or P-doped WS2 with CO2 or H2O gas adsorption on the energetically favorable site
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All these results pointed to a possibility that the Al-doped WS2 would have poor selectivity to target gas under the existence of H2O. To further confirm this point, the DOS analysis was carried out, shown in Fig. 6. For Fig. 6b, in the group of H2O on Al-WS2, the overlaps of DOS peaks between the gas and substrate material near Ef (0 eV) were much more apparent than the other three. That proved a strong interaction and more possibility of charge transfer between H2O molecule and Al-WS2. Besides, more orbital mixing between the H2O molecule and Al atom could be found, which provided more evidence for the interaction. From these, we could conclude that the Al-doped WS2 as sensing material would be easily affected by H2O. The binding energy was − 0.18 and − 0.27 eV with CO2 and H2O adsorbing on P-doped WS2, respectively. These results were less than the binding energy of NO (− 0.87 eV) and NO2 (− 1.27 eV) but very close to the binding energy of SO2 (− 0.29 eV) on P-doped WS2. In Fig. 6c, the orbital mixing between CO2 molecule and P atom was at − 12.63 and − 9.66 eV. In Fig. 6d, the orbital mixture between H2O molecule and S atoms was at − 9.25 eV. Therefore, the sensitivity of P-doped WS2 to SO2 was easily effected in the presence of CO2 or H2O when binding energy and orbital mixing were taken into consideration simultaneously.

Fig. 6
figure 6

DOS of a CO2, Al, S, and W atoms; b H2O, Al, S, and W atoms; c CO2, P, S, and W atoms; and d H2O, P, S, and W atoms

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The single-atom doping (3.7% doping concentration) was discussed in the above parts. Considering that different doping concentrations had an impact on the sensing performance, the case of diatomic doping (7.4% doping concentration) was also discussed in the 3 × 3 WS2 model. S atoms were still replaced by doping atoms. There were four situations for doping locations shown in Fig. S8. For the Al-doped WS2, they were named as 2Al-1, 2Al-2, 2Al-3, and 2Al-4, respectively. For the P-doped WS2, they were named as 2P-1, 2P-2, 2P-3, and 2P-4, respectively. Then, the formation energy of each doping system was calculated to evaluate the difficulty of forming these structures. The lower the formation of energy is, the easier the formation of configuration is. The results of the formation of energy were shown in Table S4. The 2Al-1 structure was chosen since it has the lowest formation energy among the four cases. Similarly, 2P-1 and 2P-3 were both chosen since they have adjacent formation energies.

According to band structure results (Fig. S6), Al-doped WS2 had excellent adsorption performance to NO and SO2 than NO2 when the doping concentration was 3.7%. And P-doped WS2 had superior adsorption performance to NO than NO2 and SO2. Therefore, for Al-doped WS2, only NO and SO2 were considered when the doping concentration was 7.4%. For P-doped WS2, only NO was considered. Based on this, the influence of doping concentration on adsorption performance was explored. The most stable adsorption structures were shown in Fig. S9 and showed the binding energy results were shown in Table S5. DOS of these systems were presented in Fig. 7. In Fig. 7a, the orbital mixing between NO molecule and Al atoms was at − 6.51, − 3.25, and − 0.75 eV, respectively. The orbital mixing between NO molecule and S, as well as W atoms, was at 1.78 eV. In Fig. 7b, the orbital mixing between SO2 molecule and S atoms was at − 19.69 eV. The orbital mixing between SO2 molecule and S, as well as Al atoms, was at − 10.91 eV. In Fig. 7c, the orbital mixing between NO molecule and P atoms was at − 7.67 eV. The orbital mixing was at − 0.86 eV between NO molecule and P as well as W atoms. The orbital mixing was at − 2.39 eV between NO molecule and P, S, as well as W atoms. In Fig. 7d, the orbital mixing between NO molecule and W atoms was at − 12.55 and − 0.76 eV, respectively. Comparing Fig. 7a with Fig. 5a, it can be observed that the orbital mixing and binding energy strengthened, which indicated 7.4% Al-doping concentration induced greater NO adsorption performance than 3.7%. Comparing Fig. 7b with Fig. 5c, the orbital mixing and binding energy weakened, suggesting 7.4% Al-doping concentration caused poorer SO2 adsorption performance than 3.7%. And the negative binding energy of the 2P-1 system was lower than that of 2P-3, according to Table S5. Hence, the adsorption performance of the 2P-3 system was poorer than the 2P-1 one, from the perspective of binding energy and orbital mixing, then, comparing the 2P-1 structure with Fig. 5d. Comparing Fig. 7c with Fig. 5d, the orbital mixing and binding energy were strengthened and that indicated 7.4% P-doping concentration can be brought better NO adsorption performance than 3.7%. To sum up, it could be observed that the influence of different doping concentrations on the sensing performance of P-doped WS2 was less than that of Al-doped WS2.

Fig. 7
figure 7

DOS of a NO, 2Al-1, S, and W atoms; b SO2, 2Al-1, S, and W atoms; c NO, 2P-1, S, and W atoms; and d NO, 2P-3, S, and W atoms. e Binding energies of all the adsorption systems

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On the other hand, the binding energies of all the adsorption systems were shown in the form of a columnar graph in Fig. 7e. According to Fig. 7e, both concentrations of 3.7% and 7.4% doping could enhance the adsorption strength of the system compared with the pure WS2 system. For the systems doped with two P atoms, 7.4% doping improved the adsorption strength of more than 3.7% doping, especially for NO gas adsorbing. For the systems doped with two Al atoms, the adsorption strength to NO gas increased. While the adsorption strength to SO2 or NO2 decreased, and that in the cases with SO2 decreased more than the cases with NO2. Overall, the increase of doping concentration had a greater influence on the adsorption strength of Al-doped systems than P-doped ones.

Conclusion

In this work, using first principles, theoretical calculations were carried out to evaluate the influence of Al and P dopants and their doping concentration on the sensitive performance of WS2 towards NO, NO2, and SO2 molecule. The work also explored the selectivity towards target gases in the presence of CO2 and H2O gases. For the band structure after gas adsorption, the change of bandgap and low levels near the Fermi level meant doped WS2 had great potential to be used as a resistance type gas sensor toward NO or SO2. According to the binding energy results, both Al- and P-doped WS2 had lower negative binding energy to gas molecules than the pristine WS2, indicating the improvement of adsorption strength because of the presence of impurity. DOS showed that the impurity could generate more activated peaks and significantly stimulate the orbital mixing between gas and substrate to enhance the sensitivity of the substrate material. Therefore, there were more charge transfer and stronger binding interaction between gas molecules and doped WS2 material. Besides, the sensitivity of P-doped WS2 to NO and NO2 was almost impossible to be affected by CO2 and H2O, while that to SO2 would be changed in the presence of CO2 or H2O. The sensitivity of Al-doped WS2 to NO was easily affected by H2O but hard to be influenced by CO2. However, the sensitivity of Al-doped WS2 to NO2 and SO2 was hard to be affected by CO2 and H2O. For NO detection, the Al- and P-doped WS2 with a 7.4% dopant concentration had better sensitive properties than that with a 3.7% dopant concentration. While for SO2 sensing, Al-doped WS2 with a dopant concentration of 7.4% had a more pronounced weakening responsive performance than that with a 3.7% dopant concentration. The influence of doping concentration on the sensing performance of P-doped WS2 was smaller than that of Al-doped WS2. Therefore, our comprehensive calculations could provide doped two-dimensional materials with a valuable reference for sensing noxious gases.