Computational Study of Janus Transition Metal Dichalcogenide Monolayers for Acetone Gas Sensing.

Recently, Janus two-dimensional (2D) transition metal dichalcogenides (TMDs) have been widely investigated and have provided exciting prospects in many fields such as photoelectric materials, photocatalysis, and gas sensors. In this study, we performed density functional theory (DFT) calculations to study the sensitivity of four volatile organic compounds (VOCs), including acetone, methanol, ethanol, and formyl aldehyde, over pristine 2D TMDs and 2D Janus TMD monolayers. We found that MoS2, Janus MoSSe, and Janus MoSTe demonstrated greater sensitivity toward acetone than other VOCs. Furthermore, the band gap values of the Janus MoSSe and Janus MoSTe monolayers dramatically changed after acetone adsorption on their sulfur layers, which was quite larger than the band gap change after acetone adsorption on the MoS2 monolayer. This result also leads to the extremely large conductivity change of Janus MoSSe and Janus MoSTe after sensing acetone. Hence, Janus MoSSe and Janus MoSTe monolayers show much higher sensitivity toward acetone in comparison with the pristine MoS2 monolayer. Finally, our finding indicates that Janus MoSSe and Janus MoSTe monolayers can be proposed as ultrahigh-sensitivity 2D TMD materials for acetone sensors.

Introduction

Since the discovery of graphene in 2004 by Andre Geim and Konstantin Novoselov,[1] two-dimensional (2D) materials have been gradually attracting the attention of scientists due to their many unique properties that differ from those of 3D bulk materials.[2−9] To date, there are many kinds of 2D materials, such as carbon materials (graphene and its derivatives), h-BN, transition metal dichalcogenides (TMDs, e.g. MoS2), and MXene (such as Ti3C2F2). Among these 2D materials, transition metal dichalcogenides are one of the most widely studied 2D materials due to their specific electrical, optical, and other physical properties,[10−14] and MoS2 is the most well understood TMD material.

Recently, a new class of 2D materials, named Janus 2D materials, has been gradually attracting considerable interest because Janus 2D materials have distinct properties different from those of traditional 2D materials. Janus 2D materials are those materials that have asymmetric structures possessing two different facets on two sides of 2D materials. The first reported Janus 2D material named graphone, which was simulated by Zhou et al.,[15] is a graphene-based material obtained by hydrogenating only one side of graphene. Graphone has a small band gap of 0.46 eV, and it is a ferromagnetic semiconductor. In addition, there are also many experimental and theoretical studies on Janus TMD materials. In the experimental studies, a single-layer Janus TMD, MoSSe, was successfully fabricated in the laboratory in 2017 by a modified chemical vapor deposition (CVD) method by Li and co-workers[16] and Lou and co-workers[17] even though Janus TMDs do not exist in nature. The 2D Janus TMD (denoted as MXY) possesses a symmetry-broken structure with a C3 point group compared with the single-layer MX2 TMD structure, which has a higher symmetry of the D3 point group.[18,19] Theoretically, group-VI chalcogenide MXY monolayers, including MoSSe, WSSe, WSeTe, and WSTe monolayers, have been demonstrated to be stable by calculating the phonon dispersion and the molecular dynamics simulations.[20,21] Using many-body Green’s function perturbation theory, Li et al. found that the Janus MoSSe monolayer showed strong excitonic effects, which can be applied in optoelectronic materials.[22]

With the development of industries, emissions of toxic gas and air pollutants have rapidly increased globally, including carbon monoxide (CO), nitrogen oxides (NOx), ozone (O3), particulates (PM2.5), asbestos, and volatile organic compounds (VOCs).[23,24] However, in recent years, volatile organic compound (VOC) emissions have also increased significantly. Pollutants are released primarily from chemical processes (such as paints) and have serious adverse effects on human health, such as cancer, central nervous system disorders, and skin problems. In the literature, 2D materials, including graphene oxides, phosphorene, and TMDs, have recently been widely investigated for the applications of toxic gas sensing.[25,26] Besides, to reduce high operating temperatures, which would increase the power consumption, recent research of gas sensing has been devoted to the fabrication of room-temperature sensors toward VOCs and toxic gases. Among them, TMDs have been reported to show a good VOC sensing ability at room temperature. This operating temperature could be used for a low-temperature VOC sensor, being lower than that of traditional VOC sensors. For TMDs, it has been reported that the nonfunctionalized MoS2 can detect toluene and hexane[27] while the Au-decorated MoS2 shows sensitivity to ethanol and acetone.[28] These studies reveal that the functionalization and modification of TMDs can efficiently tune their sensing properties toward gas molecules.

Very recently, Janus MoSSe and WSSe monolayers have been studied in the sensing of NO2 and NH3 gaseous molecules using first-principle calculations.[29−31] By means of either the defected structures or the strain deformation of these Janus TMD monolayers, it has been demonstrated that the Janus TMD monolayers possess high sensitivity to NO2 and NH3. However, despite MoS2 being reported as a good VOC sensor,[32] the sensitivity of Janus TMD monolayers with respect to VOCs is still unclear. Thus, in this work, we studied the adsorption and sensitivity of four different VOC molecules, including acetone, methanol, ethanol, and formyl aldehyde, using molybdenum-based pristine TMDs and Janus TMDs. Except for the adsorption energy, we calculate and present a discussion of the electronic analyses, including density derived electrostatic and chemical (DDEC) atomic charges and electron density difference (EDD), to interpret the interaction between the VOCs and the TMDs. Moreover, to determine the VOC sensing capability, we calculated the band structure and compared the conductivity change difference of VOCs on the Janus TMDs with that on the pristine TMDs via the calculated band gap values.

Results and Discussion

Figure shows the (4 × 4) supercell of all TMDs with a 2H phase. The optimized unit cell lattice constants of the MoS2, MoSe2, and MoTe2 monolayers are 3.18, 3.31, and 3.55 Å, respectively. These values are in good agreement with the experimental values.[33,34] Besides, the optimized lattice constants of the Janus MoSSe, MoSTe, and MoSeTe monolayers are 3.25, 3.36, and 3.43 Å, respectively. By considering the spin–orbit coupling (SOC) interaction, the calculated fundamental band gaps of the MoS2, MoSe2, and MoTe2 monolayers are 1.5854, 1.3241, and 0.9324 eV, respectively, while the calculated fundamental band gaps of Janus MoSSe, MoSTe, and MoSeTe monolayers are 1.4553, 0.9556, and 1.1367 eV, respectively. The calculated electronic band structure diagrams are depicted in Figure S1. Except for the Janus MoSTe monolayer, all of the TMDs show a direct band gap. The band gap properties and values of these TMDs calculated using the PBE + SOC + D3 functional possess an error of around 0.10–0.30 eV with respect to the experimental or calculated values.[16,35−39] The calculations via PBE + SOC + D3 functional show reliable electronic properties compared to the experimental observation.

Figure 1

Top and side views of the optimized pristine and Janus TMD monolayers: (a) MoS2, (b) MoSe2, (c) MoTe2, (d) MoSSe, (e) MoSTe, and (f) MoSeTe. Purple, yellow, green, and tawny spheres represent Mo, S, Se, and Te atoms, respectively.

2.1 Adsorption of VOC Molecules on the TMDs

We first calculate the adsorption of methanol, ethanol, formyl aldehyde, and acetone on the MoS2 monolayer. The calculated adsorption energies of methanol, ethanol, and formyl aldehyde on the MoS2 monolayer are similar, being −0.26, −0.25, and −0.25 eV, respectively. However, the adsorption energy of acetone on the MoS2 monolayer is −0.35 eV, larger than those of methanol, ethanol, and formyl aldehyde. Figure shows the optimized adsorption structures of these VOCs on the MoS2 monolayer. We found that after adsorption, both methanol and ethanol can point to the sulfur atom via their hydroxyl groups, while formyl aldehyde and acetone can be almost parallelly (having a small tilting angle) adsorbed on the MoS2 monolayer. In addition, during adsorption, weak C–H...S interactions between these VOCs and the MoS2 monolayer are formed. The adsorption of these VOC molecules on the MoS2 monolayer is contributed by the C–H...S interaction and the dispersion force so that the adsorption energies of these VOC molecules are around −0.25 to −0.36 eV.

Figure 2

Top and side views of the optimized adsorption structures of (a) methanol, (b) formyl aldehyde, (c) ethanol, and (d) acetone on the pristine MoS2 monolayer. Purple and yellow spheres represent Mo and S atoms, respectively, while brown, red, and light pink balls are C, O, and H atoms, respectively.

Besides, for semiconducting sensors, the band gap changes and their conductivity change after sensing the target molecule are also criteria to identify their sensitivity. We found that the calculated band gap value change after methanol, ethanol, and formyl aldehyde were adsorbed on the MoS2 monolayer was smaller than 0.001 eV, while only acetone possessed a larger band gap value change of −0.0155 eV. The calculated electronic band structure diagrams are shown in Figure S2. The negative value of the band gap change implies an increase of electric conductivity and thus a larger band gap change can lead to a further increase in conductivity. This conductivity change reflects the sensitivity of the semiconducting sensor. Hence, the calculated conductivity change can increase by 35.23% after acetone adsorption on the MoS2 monolayer. On the contrary, the other three VOC molecules can only make a negligible conductivity change after being captured by the MoS2 monolayer, as listed in Table . Our results show that the MoS2 monolayer has the largest band gap value change after the adsorption of acetone. Since acetone also possesses the largest adsorption energy on the MoS2 monolayer, in the summarization of these two criteria of the adsorption energy and band gap value change, the MoS2 monolayer demonstrates high sensitivity with respect to the acetone molecule.

Table 1

Calculated Adsorption Energy, Eads, the Band Gap Value Change, ΔEg (in the Fourth Decimal Place), and the Conductivity Change, Δσ, after Gas Molecule Adsorption on Pristine TMD Monolayers

TMD	species	Eads (eV)	ΔEg (eV)	Δσ (%)
	CH3OH	–0.26	∼0	∼0
MoS2	C2H5OH	–0.25	–0.0003	0.586
	CH2O	–0.25	–0.0003	0.586
	C3H6O	–0.35	–0.0155	35.230
	CH3OH	–0.25	–0.0074	15.499
MoSe2	C2H5OH	–0.29	–0.0008	1.570
	CH2O	–0.28	–0.0022	4.377
	C3H6O	–0.33	–0.0002	0.390
	CH3OH	–0.25	–0.0001	0.195
MoTe2	C2H5OH	–0.33	–0.0003	0.586
	CH2O	–0.25	–0.0005	0.978
	C3H6O	–0.32	–0.0001	0.195

Subsequently, as can be seen in Figures S3 and S4, we calculate the adsorption energies of methanol, ethanol, formyl aldehyde, and acetone on both MoSe2 and MoTe2. The adsorption energies of methanol, ethanol, formyl aldehyde, and acetone on the MoSe2 monolayer are −0.25, −0.29, −0.28, and −0.33 eV, respectively. Moreover, the adsorption energies of methanol, ethanol, formyl aldehyde, and acetone on the MoTe2 monolayer are −0.25, −0.33, −0.25, and −0.32 eV, respectively. We found that the adsorption energies of these VOCs on both MoSe2 and MoTe2 are similar to those on the MoS2 monolayer, while only the adsorption energy of ethanol is slightly larger on the MoSe2 and MoTe2 monolayers. However, as shown in the band structure plots in Figures S5 and S6, the calculated band gap value changes after the adsorption of these VOCs are almost the same with respect to the ones before adsorption except for methanol on the MoSe2 monolayer, as listed in Table . Even though the calculated band gap value and conductivity changes of methanol on the MoSe2 monolayer are larger than those of other VOCs, they are still smaller than those of the acetone counterpart on the MoS2 monolayer. Therefore, in comparison with the sensing ability of these four VOCs on MoS2, MoSe2, and MoTe2 monolayers, MoS2 demonstrates the best sensing properties toward the acetone molecule.

Adsorption of VOC Molecules on the Janus TMDs

We studied the Janus MoSSe, MoSTe, and MoSeTe monolayers to discuss their sensing properties toward these VOCs, as can be seen in Table . Because the Janus MoXY nanosheet has two asymmetric sides, we then calculate the adsorption of these VOCs on both sides of these Janus MoXY nanosheets. On the Janus MoSSe monolayer, the adsorption energies of methanol, ethanol, formyl aldehyde, and acetone on the S-layer of MoSSe are −0.25, −0.28, −0.24, and −0.31 eV, respectively. On the other hand, the adsorption energies of methanol, ethanol, formyl aldehyde, and acetone on the Se-layer of MoSSe are −0.24, −0.23, −0.24, and −0.34 eV, respectively. Figure shows the optimized adsorption structures of these VOCs on the Janus MoSSe monolayer. The adsorption structure and adsorption energy of these VOCs on Janus MoSSe are also similar to those on the MoS2 monolayer. However, as can be seen from Table and Figure S7, despite the adsorption energy of acetone on the S-layer of Janus MoSSe being slightly smaller than that on the MoS2 monolayer, the band gap change after acetone adsorption on the S-side of Janus MoSSe is −0.1028 eV, which is almost five times that of MoS2. Thus, the calculated conductivity change can increase to a great extent by 640.12% after acetone adsorption on the S-layer of the Janus MoSSe monolayer. On the other hand, the band gap change after the adsorption of methanol, ethanol, and formyl aldehyde on the S-layer of Janus MoSSe is smaller than 0.001 eV, also leading to negligible conductivity changes, as listed in Table . These results reveal that the S-layer of Janus MoSSe possesses a significantly larger electric conductivity change after sensing the acetone molecule in comparison with the other three VOC molecules. Besides, the band gap change is demonstrated to be almost negligible after the adsorption of methanol, ethanol, formyl aldehyde, and acetone on the Se-layer of Janus MoSSe, being similar to that on MoSe2, as listed in Table . Thus, the Se-layer of the Janus MoSSe monolayer shows pool sensitivity to the VOCs.

Table 2

Calculated Adsorption Energy, Eads, the Band Gap Value Change, ΔEg (in the Fourth Decimal Place), and the Conductivity Change, Δσ, after Gas Molecule Adsorption on Janus TMD Monolayers

TMD	species	Eads (eV)	ΔEg (eV)	Δσ (%)
	CH3OH	–0.25	–0.0007	1.372
MoSSe–S	C2H5OH	–0.28	–0.0001	0.195
	CH2O	–0.24	–0.0005	0.978
	C3H6O	–0.31	–0.1028	640.125
	CH3OH	–0.24	–0.0009	1.768
MoSSe–Se	C2H5OH	–0.23	∼0	∼0
	CH2O	–0.24	–0.0002	0.390
	C3H6O	–0.34	–0.0004	0.762
	CH3OH	–0.24	–0.0054	11.087
MoSTe–S	C2H5OH	–0.32	–0.0184	43.085
	CH2O	–0.23	–0.0062	12.831
	C3H6O	–0.31	+0.1226	–90.811
	CH3OH	–0.25	–0.0004	0.782
MoSTe–Te	C2H5OH	–0.24	+0.0002	–0.389
	CH2O	–0.24	–0.0020	3.971
	C3H6O	–0.34	–0.0014	2.763
	CH3OH	–0.26	–0.0006	1.175
MoSeTe–Se	C2H5OH	–0.33	–0.0017	3.366
	CH2O	–0.24	–0.0003	0.586
	C3H6O	–0.33	–0.0005	0.978
	CH3OH	–0.24	–0.0005	0.978
MoSeTe–Te	C2H5OH	–0.29	–0.0017	3.366
	CH2O	–0.26	–0.0008	1.570
	C3H6O	–0.32	–0.0002	0.390

Figure 3

Top and side views of the optimized adsorption structures of (a) methanol, (b) formyl aldehyde, (c) ethanol, and (d) acetone on the S-layer of the Janus MoSSe monolayer and (e) methanol, (f) formyl aldehyde, (g) ethanol, and (h) acetone on the Se-layer of the Janus MoSSe monolayer. Purple, yellow, and green spheres represent Mo, S, and Se atoms, respectively, while brown, red, and light pink balls are C, O, and H atoms, respectively.

On the Janus MoSTe monolayer, the adsorption energies of methanol, ethanol, formyl aldehyde, and acetone on the S-layer of the MoSTe are −0.24, −0.32, −0.23, and −0.31 eV, respectively. We found that the adsorption energy of ethanol increases from −0.25 (on MoS2) to −0.32 eV (on MoSTe), but the adsorption energy of acetone is similar to that on other TMDs. The optimized adsorption geometries of these VOCs on both S- and Te-layers of the Janus MoSTe monolayer are shown in Figure , which are also similar to those on the MoS2 monolayer. In addition, as can be seen from the band structure diagrams in Figure S8, the calculated band gap changes after the adsorption of methanol, ethanol, formyl aldehyde, and acetone on the S-layer of MoSTe are −0.005, −0.018, −0.006, and +0.123 eV, respectively. We found that the band gap change is positive for acetone on the S-layer of MoSTe, which indicates that the band gap value becomes larger and then the electrical resistance can also become larger after acetone adsorption. Therefore, the calculated conductivity change can reduce by 90.81% after acetone sensing by the MoSTe monolayer. This also demonstrates that the S-layer of MoSTe can enhance the sensitivity of acetone with respect to the MoS2 monolayer. On the other hand, the adsorption energies of methanol, ethanol, formyl aldehyde, and acetone on the Te-layer of MoSTe are −0.25, −0.24, −0.24, and −0.34 eV, respectively. Likewise, the band gap change is also demonstrated to be almost negligible after the adsorption of methanol, ethanol, formyl aldehyde, and acetone on the Te-layer of Janus MoSTe. This reveals that the Te-layer of the Janus MoSTe monolayer also shows significant pool sensitivity to the VOCs.

Figure 4

Top and side views of the optimized adsorption structures of (a) methanol, (b) formyl aldehyde, (c) ethanol, and (d) acetone on the S-layer of the Janus MoSTe monolayer and (e) methanol, (f) formyl aldehyde, (g) ethanol, and (h) acetone on the Te-layer of the Janus MoSTe monolayer. Purple, yellow, and tawny spheres represent Mo, S, and Te atoms, respectively, while brown, red, and light pink balls are C, O, and H atoms, respectively.

Finally, we calculated the sensing properties of these four VOCs on the Janus MoTeSe monolayer, and their adsorption energies are listed in Table , which are also similar to their adsorption energies on the TMDs mentioned above. Likewise, the adsorption structures of these VOCs on the Janus MoTeSe monolayer are similar to those on the TMDs mentioned above, as shown in Figure S9. However, the band gap changes of these four VOCs on either Se-layer or Te-layer are almost unchanged after the adsorption of these four VOCs, as can be seen in the band structure plots in Figure S10. This phenomenon indicates that the molybdenum-based TMDs containing either Se or Te elements cannot cause an obvious conductivity change after sensing the target VOC molecules. However, the molybdenum-based TMDs and their Janus derivatives, which include sulfur atoms, demonstrate an apparent conductivity change after sensing the target VOC molecules. These results reveal that the sulfur atom of the TMDs might possess good sensitivity to acetone. Among them, our results show that Janus MoSSe can have the highest sensing ability to acetone due to the largest conductivity change after acetone adsorption on Janus MoSSe.

Electronic Property Analysis

Based on the results of the band gap change before and after VOC adsorption, we found that MoS2, Janus MoSSe, and Janus MoSTe possess larger band gap changes in comparison with other Mo-based TMDs. To understand the electronic property between acetone and MoS2, Janus MoSSe, and Janus MoSTe monolayers, we calculated the net atomic charges by DDEC6 and the electron density difference (EDD). As can be seen in Table , the calculated DDEC atomic charges of acetone are 0.036 |e|, 0.015 |e|, and −0.016 |e| on the MoS2, MoSe2, and MoTe2 monolayers, respectively. These results indicate that the acetone molecule donates electrons to the MoS2 and MoSe2 monolayers while it gains electrons from the MoTe2 monolayer. Besides, the electron transfer magnitude implies that the larger band gap change of acetone on MoS2 might result from a higher electron transfer magnitude. In addition, Figure a shows the 3D EDD plot of acetone adsorption on the MoS2 monolayer. The EDD plot illustrates that there are electron accumulation and depletion around the acetone’s oxygen atom and H atom of the C–H bond, respectively. It can also be noticed from the EDD plot that the electron density around the sulfur atoms displays electron accumulation between the sulfur atoms and the acetone’s C–H bond and the depletion between the sulfur atoms and the acetone’s oxygen atom. These results reveal that the oxygen atom of acetone could obtain electrons from MoS2, while its C–H bond would transfer electrons to the MoS2 monolayer, constructing a two-way electron transfer flow. According to the calculated net atomic charges, acetone loses electrons after its adsorption, meaning that the electron transfer of the C–H bond of acetone to the MoS2 monolayer is stronger than that from the MoS2 to the oxygen of acetone.

Table 3

Calculated DDEC Atomic Charges for Acetone Molecule after Adsorption on Both Pristine and Janus TMD Monolayersa

TMD	species	DDEC (|e|)
MoS2	C3H6O	0.036
MoSe2	C3H6O	0.015
MoTe2	C3H6O	–0.016
MoSSe–S	C3H6O	0.028
MoSSe–Se	C3H6O	0.026
MoSTe–S	C3H6O	–0.032
MoSTe–Te	C3H6O	0.006
MoSeTe–Se	C3H6O	0.006
MoSeTe–Te	C3H6O	0.003

The positive and negative values indicate the losing and gaining of electrons of the acetone molecule.

Figure 5

Calculated electron density difference (EDD) plots of acetone adsorption on (a) MoS2, (b) S-layer of Janus MoSSe, and (c) S-layer of Janus MoSTe monolayers. Green and red represent the regions of electron depletion and accumulation, respectively. The isosurface level is 0.0003 |e|/Bohr.[3]

On Janus TMDs, the calculated DDEC atomic charges of acetone are 0.028 |e| and 0.026 |e| on the S-layer and Se-layer of the Janus MoSSe monolayer, respectively. Moreover, the calculated DDEC atomic charge of acetone is −0.032 |e| on the S-layer of the Janus MoSTe monolayer, whereas the atomic charges of acetone are smaller than 0.010 |e| on the Te-layer of Janus MoSTe and both sides of Janus MoSeTe monolayers, as listed in Table . These results also demonstrate that the numbers of electron transfer are larger on the S-layer of either Janus MoSSe or MoSTe monolayer than on other sides of various Janus TMDs. Furthermore, according to the EDD analysis, similar to the electronic interaction between acetone and MoS2, there also exists a two-way electron transfer between acetone and the S-layer of either Janus MoSSe or MoSTe monolayer, as can be seen in Figure b,c, respectively. The EDD in Figure b shows that a few electrons aggregate around the carbon atom of the C=O group in acetone after the adsorption of acetone on the S-layer of Janus MoSSe. In comparison with the EDD plot of acetone on MoS2, acetone gains more electrons from Janus MoSSe but donates similar electrons to the MoSSe monolayer. This result reflects why the net electron transfer of acetone to the MoSSe monolayer is smaller than that of the MoS2 monolayer.

On the other hand, Figure c shows the EDD plot of acetone on the S-layer of the Janus MoSTe monolayer, where a larger region of electron density accumulation around the acetone molecule than that on either MoS2 or Janus MoSSe monolayer is displayed. This result is also reflected in the net atomic charge calculation that acetone obtains more electrons from the Janus MoSTe monolayer. Thus, the electronic property analysis between acetone and these TMDs reveals that the electron transfer direction and strength can be altered by using the Janus TMD structure with respect to the MoS2 monolayer. It also implies that when acetone obtains more electrons from the sulfur atoms of the Janus TMDs, the band gap value change becomes larger.

Discussion

According to our results, we found that the energy levels of the valence band maximum (VBM) or conduction band minimum (CBM) of these TMDs change only when acetone is adsorbed on the S-layer of MoS2, MoSSe, and MoSTe. Thus, based on the calculated band structure, it is demonstrated that only acetone possesses a relatively strong electronic interaction with the sulfur atoms of MoS2, MoSSe, and MoSTe. Moreover, the S-layer of the Janus MoSSe and MoSTe monolayers showed better sensitivity with respect to pristine MoS2 after acetone adsorption. As many previous studies have pointed out,[40,41] one of the reasons for this is that the polarization induced by symmetry-breaking in Janus materials modulates the electron transfer. Polarization of the Janus materials causes dipole–dipole interactions between the polar materials or gaseous species.[42−44] Hence, the internal dipole interaction in the Janus materials can couple with the electronic interaction between acetone and the Janus materials and then alter the magnitude or direction of the electron transfer. In addition, based on Ohm’s law, the electric conductivity is the slope of the I–V curve, and the electron transport phenomenon before and after gas adsorption has trends similar to the results of the calculated conductivity change.[45,46] The smaller band gap value can lead to larger conductivity as well as a more tilted slope and vice versa. Therefore, the calculated conductivity change based on the band gap value can also predict the trends of the I–V curve.

Conclusions

In this work, we have employed density functional theory (DFT) calculations to study the sensitivity of acetone, methanol, ethanol, and formyl aldehyde over pristine 2D transition metal dichalcogenides (TMDs) and 2D Janus TMD monolayers. On the MoS2, MoSe2, and MoTe2 monolayers, we found that the adsorption energy of acetone is larger than those of its counterparts methanol, ethanol, and formyl aldehyde. In addition, the adsorption of acetone on the MoS2 monolayer can lead to a larger band gap change, resulting in a larger conductivity change with an increase of 35.23%. Based on the EDD calculation, we notice that the acetone forms a two-way electron transfer direction with the MoS2 monolayer, in which one is the electron transfer from MoS2 to the oxygen of the acetone and the other is the electron donation from acetone’s C–H bond to MoS2 via the C–H...S interaction. The calculated DDEC atomic charges also reveal that there is a stronger electron exchange interaction between acetone and the MoS2 monolayer.

Furthermore, the adsorption energies of four VOCs on the 2D Janus TMD monolayers are similar to those on the pristine TMDs. However, we found that after the adsorption of acetone on the S-layer of both Janus MoSSe and MoSTe monolayers, it displays an extremely large band gap value change in comparison with the adsorption of acetone on MoS2. After sensing the acetone molecule, the band gap value changes of Janus MoSSe and MoSTe are −0.103 and 0.123 eV, respectively. These results cause Janus MoSSe and MoSTe to become conductive and resistive sensors since the electrical conductance and electrical resistance increase by 640.125 and 90.811% on Janus MoSSe and MoSTe, respectively. On the other hand, the band gap changes as well as the conductivity change are almost negligible after the adsorption of four VOCs on the Se-layer and Te-layer of MoSSe and MoSTe monolayers. Likewise, the sensitivity based on the conductivity change of the Janus MoSeTe toward these four VOCs is also quite poor. Consequently, our results indicate that the sulfur layer of Janus MoSSe and MoSTe, namely, the modification of the MoS2 monolayer to Janus materials, can efficiently enhance sensitivity with respect to the acetone molecule. This finding might provide guidance in the designing of new 2D volatile organic compound sensors.

Computational Details

All periodic DFT calculations in this study were performed with the generalized gradient approximation (GGA) of Perdew–Burke–Ernzerhof (PBE) exchange–correlation functional[47] employing Vienna ab initio simulation program (VASP).[48−51] The projector augmented wave (PAW) method[52,53] was applied to describe the electron core interactions. The Kohn–Sham orbitals are expanded in a plane-wave basis set with a kinetic energy cutoff of 520 eV. All 2D transition metal dichalcogenide monolayers were studied using the 2H phase of either pristine MoX2 (X = S, Se, and Te) or Janus MoXY (X ≠ Y = S, Se, and Te). The (4 × 4) supercell is considered for all TMD monolayers with 15 Å vacuum along the c-axis to avoid interlayer interaction, as shown in Figure . As dipole interaction was important for the Janus TMDs, dipole corrections perpendicular to all monolayers were carried out in the calculations. The Brillouin zone was sampled using (4 × 4 × 1) γ centered Monkhorst–Pack k-point mesh[54] for VOCs’ adsorption on different TMDs. The convergence threshold was set to be 1 × 10–5 eV for the total electronic energy in the self-consistent loop. The atomic positions were relaxed using the quasi-Newton algorithm until the x-, y-, and z-components of the unconstrained atomic force were smaller than 1 × 10–2 eV/Å. The dispersion energy correction was considered using the DFT-D3 method by Grimme et al.[55,56] In addition, many previous studies have reported that the spin–orbit coupling (SOC) interaction plays an important role in the 2D TMD monolayers.[39,57−59] Hence, we have included the spin–orbit coupling (SOC) interaction in the calculations.[60]

The following equation defines the adsorption energies (Eads) of VOCs on the TMD monolayerswhere EVOC/TMD is the total energy of VOCs adsorbed on pristine MoX2 or Janus MoXY, EVOC is the energy of a free adsorbate VOC molecule, and ETMD is the energy of pristine MoX2 or Janus MoXY monolayers. Therefore, negative adsorption energy indicates a thermodynamically favored exothermic adsorption process. To analyze the electrical properties, the density derived electrostatic and chemical (DDEC6) approach was used to examine electron transfer between the molecule and the surface.[61−63] Besides, it is well known that band gap is an important factor for determining the electrical conductivity of the semiconductor sensor.[64−66] The relationship between band gap and electronic conductivity is understood by the following equationwhere σ is the electrical conductivity and κ is the Boltzmann constant. According to this equation, the smaller band gap (Eg) leads to larger electric conductivity at a given temperature (T). To eliminate the coefficient in the equation for calculating electronic conductivity, we calculated the conductivity change by using the following formulawhere σads and σTMD are the electrical conductivities of after and before CB adsorption on the various nanotubes considered in the present work, respectively.