Introduction

Titanium dioxide (TiO2, Titania) has aroused great attentions as an important semiconductor material due to its effectiveness and outstanding properties, such as non-toxicity, low cost, high catalytic efficiency, photoactivity [1], and stability. TiO2 has been widely utilized in many fields, such as photo-catalysis, gas sensing, organic dye-sensitized solar cells, water splitting, and pollutant degradation [2–5]. Three important polymorphs were found for TiO2, namely, anatase, rutile, and brookite [6], in which anatase and rutile forms are the most widely studied ones in different fields of science and technology. The photocatalytic applications of TiO2 were restricted due to its wide bandgap (3.2 eV), which allows the absorption of the solar spectrum at the ultraviolet region by a lower percentage (3–5 % of the incoming solar light). The doping of TiO2 anatase with some nonmetal elements, such as nitrogen, is a convenient solution, which would enhance the photo-efficiency of TiO2 to the visible region and improve its photocatalytic activity [7, 8]. Two-dimensional (2D) semiconductor materials, such as MoS2 [9], and other dichalcogenides consisting of transition metals, such as MoSe2, WS2, and so on, indicate the final scale for chalcogenide dimension around the vertical direction. MoS2, a layered structure consisting of Mo and S atoms arranged in hexagonal structure of atomic sheets of molybdenum and sulphur atoms, attracts numerous attentions due to the excellent electrical, mechanical, and optical properties, such as satisfied bandgap, thermal stability, carrier mobility, and so on [10–12]. MoS2 has been broadly used in abundant applications, such as photocatalysts, nanotribology, lithium battery, dry lubrication, hydrodesulfurization catalyst, and photovoltaic cell because of its unique electronic, photosensitive, and catalytic properties [13–16]. Nanoelectronic devices fabricated on 2D materials, such as MoS2, suggest also many efficiencies for these layered materials, which cause the further miniaturization of the integrated circuits beyond Moore’s law. Recently, numerous electronic devices were made using the few-layer MoS2 as an important component, such as field-effect transistors [17], sensors [18], etc. However, several computational studies of N-doped TiO2 anatase nanoparticles and few-layer MoS2 structures have been separately published, describing some of the main electronic and physical properties of these materials. Particularly, the gas-sensing capabilities of MoS2-based field-effect transistors and sensing films for NO and NH3 were experimentally revealed with an enhanced sensitivity in some other works [19, 20]. TiO2/MoS2 nanocomposites have been successfully synthesized for different purposes by some experimental methods [21–23]. There are no explanative computational studies on the adsorption behaviors of TiO2/MoS2 nanocomposites. NOx molecules have been characterized as toxic gases which are mainly emitted from power plants and vehicle engines. For the general public, the most outstanding provenance of NO2 is internal combustion engines that burn fossil fuels to work properly. In indoor places, NO2 emission mainly stems from cigarette smoke, kerosene heaters, and stoves. Therefore, optimal removal of these harmful molecules is an important subject to human health and environmental protection [24]. In this study, the interaction of NOx molecules with TiO2/MoS2 nanocomposites has been investigated by density functional theory (DFT) computations. We present here the results of calculations of complex systems consisting of NOx molecule positioned between the TiO2 anatase nanoparticle and MoS2 monolayer. The electronic structure of the adsorption systems has also been analyzed, including the projected density of states (PDOSs) and molecular orbitals (MOs). The main aim of this study is to supply an overall understanding on the adsorption behaviors of nano-TiO2/MoS2 composites as highly sensitive NOx sensors.

Computational details and structural models

Methodology

DFT calculations [25, 26] were performed as implemented in the Open-Source Package for Material eXplorer (OPENMX3.8) [27], being a well-organized software package for nano-scale materials simulations based on DFT, PAO basis functions, and VPS pseudopotentials [28, 29]. Pseudo-atomic orbitals were utilized as basis sets in the geometry optimizations. The considered cut-off energy is set to the value of 150 Rydberg in our calculations [29], The PAOs are generated via the basis sets (3-s, 3-p, and 1-d) for Ti atom, (3-s, 3-p, and 2-d) for Mo atom, (2-s and 2-p) for O and N atoms, (3-s and 3-p) for S atom with the chosen cut-off radii of 7 for Ti, 9 for Mo, 5 for O and N, and 8 for S (all in Bohrs). The generalized gradient approximation (GGA) of Perdew–Burke–Ernzerhof (PBE) was used to describe the exchange–correlation energy functional [30]. The convergence criterion of self-consistent field calculations was set at 1.0 × 10−6 Hartree, whereas that of energy calculation was chosen to be 1.0 × 10−4 Hartree/bohr. For the geometry optimization, ‘Opt’ is used as the geometry optimizer, which is a robust and efficient scheme. The crystalline and molecular structure visualization program, XCrysDen [31], was employed for displaying molecular orbital isosurfaces. The box considered in these computations contains 96 atoms (24 Ti, 48 O, 8 Mo, and 16 S atoms) of undoped or N-doped TiO2 nanoparticle with MoS2 monolayer. The Gaussian broadening method for evaluating electronic DOS is used. For NOx adsorption on the TiO2/MoS2 nanocomposite, the adsorption energy is computed via the following formula:

$$\Delta E_{\text{ad}} = E_{{ ( {\text{composite + adsorbate)}}}} - E_{\text{composite}} - E_{\text{adsorbate}}$$
(1)

where E (composite + adsorbate) is the total energy of the adsorption system, E composite is the energy of the TiO2/MoS2 nanocomposite, and Eadsorbate represents the energy of non-adsorbed NOx molecules. Based on this relation, the most stable configurations would have negative adsorption energies. A higher adsorption energy corresponds to a stronger adsorption between host and adsorbed molecule.

Model building

NO and NO2 molecule model

The chemical formulae of nitric oxide and nitrogen dioxide molecules are NO and NO2. NO has linear structure, while NO2 represents a bent geometrical structure. The structures of NO and NO2 molecules were represented in Fig. 1. Distances and angles of the considered molecules were computed in a large cubic supercell. The calculated N–O bond length of free NO molecule is 1.16 Å, while for the bent structure of NO2 molecule, the bond length and bond angles were calculated to be 1.20 Å and 134°, respectively. All these computed values are in comprehensive agreement with the computational results and the experimentally reported data [32].

Fig. 1
figure 1

Representation of NO and NO2 molecules in a large cubic supercell

Full size image

MoS2 model

Molybdenum disulfide (MoS2) is a layered structure containing molybdenum transition metal, which belongs to the family of two-dimensional dichalcogenides. A hexagonally arrangement of atomic sheets of MoS2 containing Mo and S atoms set as an S–Mo–S sandwich forms MoS2 monolayer. The monolayer of MoS2 model studied here contains 24 atoms in total (8 Mo and 16 S atoms). MoS2 structure is relaxed for calculating the optimized structural parameters. The calculated S–Mo bond length, Mo–Mo distance, and S–S distance in monolayer are 2.43, 3.20, and 3.15 Å, respectively. These computed bond lengths are somewhat consistent with the values of bulk material [33], in reasonable agreement with the reported data [34, 35]. However, there are negligible discrepancies between the results of MoS2 and its bulk material, which can be ignored. The area of the MoS2 slab is 9.25 Å × 6.33 Å. The optimized structure of MoS2 model was displayed in Fig. 2.

Fig. 2
figure 2

Optimized structure of the chosen MoS2 monolayer with area values, a front view and b lateral view. Mo atoms are sketched by gray balls and S atoms by yellow balls

Full size image

TiO2 anatase model

A 3 × 2 × 1 supercell of TiO2 anatase along x, y, and z directions was utilized for constructing the considered TiO2 anatase nanoparticles containing 72 atoms. The unit cell is available at “American Mineralogists Database” webpage [36] and was reported by Wyckoff [37]. Two appropriate oxygen atoms of TiO2 nanoparticle were replaced by nitrogen atoms to model the N-doped particles. Doping of TiO2 nanoparticle with nitrogen atom is done according to two doping positions. These two doping positions refer to the middle oxygen and twofold coordinated oxygen atoms substitutions illustrated by OC and OT in Fig. 3a, respectively. The area of the anatase nanoparticle is 13.03 Å × 8.24 Å. The crystalline structure of TiO2 contains two kinds of titanium atoms, namely, fivefold coordinated titanium (5f-Ti) and sixfold coordinated one (6f-Ti), and two kinds of oxygen atoms, namely, threefold coordinated oxygen (3f-O) and twofold coordinated one (2f-O) atoms (see Fig. 3a). [38] It was found that the twofold coordinated oxygen and fivefold coordinated titanium atoms are more reactive than the threefold coordinated oxygen and sixfold coordinated titanium atoms due to the undercoordination in twofold coordinated oxygen and fivefold coordinated titanium atoms. The thickness of the vacuum spacing is 11.5 Å, which is helpful to avoid the additional interactions between the neighbor particles. The optimized structure of TiO2/MoS2 nanocomposite was displayed in Fig. 3b. Figure 4 also displays the optimized geometries of the N-doped TiO2/MoS2 nanocomposites. The results of geometrical optimizations represent that the OT-substituted TiO2/MoS2 nanocomposite is more favorable in energy than the OC-substituted one.

Fig. 3
figure 3

Optimized structures of a undoped 72 atom TiO2 anatase nanoparticle constructed using the 3 × 2 × 1 unit cells (OC: central oxygen; OT: twofold coordinated oxygen; OD: dangling oxygen) and b TiO2/MoS2 nanocomposite constructed from TiO2 nanoparticle and MoS2 monolayer. Ti atoms are sketched by dark gray balls, O atoms by red balls and N atoms by blue balls

Full size image
Fig. 4
figure 4

Optimized geometries of two types of N-doped TiO2/MoS2 nanocomposites. a OC-substituted nanocomposite, which refers to substitution of a threefold coordinated oxygen atom and b OT-substituted nanocomposite, representing the substitution of a twofold coordinated oxygen atom

Full size image

Results and discussion

Bond lengths, bond angles, and adsorption energies

NO interacts with TiO2/MoS2 nanocomposites

For NO molecule, three adsorption configurations are studied here, including the adsorption configurations of types A, B, and C, as shown in Fig. 5. The calculated adsorption energy values for NOx molecule adsorbed on the considered nanocomposites have been listed in Table 1. This figure presents the orientations of NO molecule towards the N-doped TiO2/MoS2 nanocomposites. For instance, complex A was made from the TiO2/MoS2 nanocomposite with OC-substituted TiO2 nanoparticle and NO molecule with nearly downward oxygen. NO can stably adsorb on N-doped TiO2/MoS2 nanocomposite, compared to the adsorption on the undoped one. The nitrogen atom of NO molecule is preferentially attracted to the doped nitrogen atom of nanocomposite, resulting in the formation of chemical bond between these two nitrogen atoms. The adsorption energy of the NO molecule on the N-doped TiO2/MoS2 nanocomposite (composite A or B) is much higher than that of undoped TiO2/MoS2 nanocomposite, which reveals that NO molecule has a stronger interaction with N-doped nanocomposite than with undoped one. These results also indicate that the interaction of NO molecules with N-doped TiO2/MoS2 nanocomposites is more energetically favorable than the interaction of NO with undoped nanocomposites. This implies that the N-doped nanocomposite adsorbs NO molecule more effectively compared to the undoped one. Besides this, the configuration B is the most stable configuration compared to the configuration A due to its more negative adsorption energy. Configuration B contains OT-substituted nanocomposite with adsorbed NO molecule. The adsorption energy of this configuration is more negative than that of other configurations, which suggests that the adsorption of NO molecule on the OT-substituted nanocomposite is more energy favorable than the adsorption on the OC-substituted one (see Table 1). Since a greater value of adsorption energy gives rise to a strong interaction between adsorbate and the adsorbent, it can be seen that there is a stronger interaction between NO and N-doped nanocomposite compared to NO and undoped nanocomposite, implying the dominant effect of N-doping. It means that the nitrogen doping strengthens the interaction of NO with TiO2/MoS2 nanocomposites. The greater the adsorption energy, the higher tendency for adsorption, and, therefore, more efficient adsorption. Table 1 summarizes the bond length values before and after the adsorption of NO molecule on the nanocomposites. The bond lengths given in this table are included N–O bonds of NOx molecule, average Ti–N distance, and new N–N and N–O distances between the nanocomposite and adsorbed NOx molecule. The values reported in this table show that the Ti–N bonds and N–O bond of the adsorbed NO molecule are elongated, because the electronic density transfers from the Ti–N bonds of N-doped TiO2 and N–O bond of the adsorbed NO molecule to the newly formed N–N and N–O distances between the nanocomposite and molecule. This transfer of electronic density indicates that the N–O bond of NO molecule is weakened after the adsorption.

Fig. 5
figure 5

Optimized geometry configurations of the interaction of NO and NO2 molecules with TiO2/MoS2 nanocomposites. The NO molecule is preferentially adsorbed on the doped nitrogen site of TiO2 nanoparticle, whereas NO2 is adsorbed on both the doped nitrogen atom and fivefold coordinated titanium atoms. Configurations A–C represent the interaction of nanocomposites with NO molecules and the configurations D–I show the interaction between nanocomposites and NO2 molecules

Full size image
Table 1 Bond lengths (in Å), Mulliken charges, and adsorption energies (in eV) for NOx molecule adsorbed on TiO2/MoS2 nanocomposites
Full size table

NO2 interacts with TiO2/MoS2 nanocomposites

The interaction of NO2 molecule with the substituted nitrogen atom of TiO2/MoS2 nanocomposites has also been displayed in Fig. 5 as specified by types D-F adsorption geometries. For the doped nitrogen site, the adsorption process is expected to be more energy favorable than that on the dangling oxygen atom site. The reason can be simply sought using the data collected in Table 1. Similarly, the NO2 molecule preferentially interacts with the doped nitrogen site on the nanoparticle surface, in comparison with the other surface oxygen atoms. Table 1 also lists the lengths for Ti–N bonds and N–O bonds of the adsorbed NO2 molecule and the newly formed N–N and N–O distances. The results of this table indicate that the Ti–N bonds of the nanocomposite and the N–O bonds of the NO2 molecule are stretched after NO2 adsorption. Because the electronic density transfers from the Ti–N bonds and N–O bonds to the newly formed distances between the nanocomposite and adsorbed NO2 molecule. In configuration D, the nitrogen atom in the NO2 molecule interacts with the doped nitrogen site on the TiO2 nanoparticle to form a strong chemical bond and, therefore, strong interaction (1.45 Å N–N bond length). Among three models for NO adsorption, the adsorption configuration in which the nitrogen atom of NO interacts with the doped nitrogen site of TiO2 at OT position is the most energy favorable one. In the case of NO2 adsorption, the adsorption energy of configuration E is much higher (more negative) than that of configuration D and undoped system adsorption (configuration F). It should be noted that the adsorption on the OT-substituted nanocomposite leads to the stable configurations (stronger interactions), compared to the adsorptions on the OC-substituted one. As can be seen from Tables 1 and 2, the adsorption energies on the OT-substituted nanocomposites (complexes E and H) are more negative than the adsorption energies on the OC-substituted ones, implying that NO2 adsorption on the OT-substituted nanocomposites is energetically more favorable than the adsorption on the OC-substituted ones. As a result, the adsorption of NO2 molecule on the N-doped TiO2/MoS2 nanocomposite is more energy favorable than the adsorption of NO2 on the undoped nanocomposite, indicating that the N-doped nanocomposite can adsorb NO2 molecule more efficiently. Thus, the N-doping strengthens the interaction of NO2 molecule with TiO2/MoS2 nanocomposites. The O–N–O bond angles of the NO2 molecule have been decreased after the adsorption process because of the formation of new chemical bond between nitrogen atom of NO2 with nitrogen atom of TiO2 nanoparticle. This chemical bond formation leads to an increase in the p characteristics of bonding molecular orbitals. Thus, the sp hybridization of nitrogen in the NO2 molecule converts to near-sp 3 hybridization. For the case of TiO2 adsorption on the fivefold coordinated titanium site presenting two contacting point between the nanoparticle and NO2 molecule, the bond length and bond angle values have been reported in Table 2. We can see two newly formed Ti–O bonds between the titanium atoms of the nanoparticle with oxygen atoms of NO2 molecule. This adsorption configuration is referred to as bridge configuration, and their complexes were illustrated in Fig. 5 (G–I complexes). The N–O bonds of NO2 molecule have been lengthened after the adsorption process, suggesting the weakening of the N–O bonds. In addition, the adsorption process results in a decrease in the O–N–O bond angle values of NO2. The adsorption energy analysis reveals that the complex H is the most energy favorable complex in comparison with complex G and both are more stable than the pristine system adsorption. These results show that the NOx adsorption on the N-doped nanocomposite is more favorable in energy than the NOx adsorption on the pristine nanocomposite. By considering this, we found that the nitrogen doping strengthens the interaction of NOx molecule with TiO2/MoS2 nanocomposites. The obtained improvements on the structural and electronic properties of TiO2/MoS2 nanocomposites here represent that the N-doped TiO2-based nanocomposite can be efficiently utilized in the removal and sensing of toxic NOx molecule.

Table 2 Bond lengths (in Å), angles (in degrees), Mulliken charges, and adsorption energies (in eV) for complexes providing two contacting point between NO2 molecule TiO2/MoS2 nanocomposites
Full size table

Electronic structures

Figure 6 presents the total density of states (TDOS) for N-doped TiO2 anatase nanoparticles and corresponding TiO2/MoS2 nanocomposites. This figure reveals a creation of small peak in the density of states (DOSs) of N-doped nanocomposite at the energy ranges near to -12 eV. TDOSs of adsorption configurations were also displayed in Fig. 7. A closer inspection of these figures indicates the increase of the discrepancies between DOS of N-doped TiO2 and nanocomposite by adding the MoS2 monolayer and adsorption of NOx. These differences included considerable shifts in the energies of the peaks and appearance of some peaks in the DOS of the studied systems. As distinct from these figures, the DOSs of the considered nanocomposites were mainly shifted to the lower energy values after the adsorption process. Therefore, the resultant variations in the energy of the states can have positive effects on the electronic transport properties of the nanocomposites and in turn can provide a helpful procedure for designing and engineering NOx sensors based on N-doped TiO2 and two-dimensional transition metal dichalcogenides (i.e., MoS2 monolayer). The projected partial density of states (PDOSs) for the interaction of NOx molecule with TiO2/MoS2 nanocomposites have been displayed in Fig. 8a–d. Panels (a, b) present the PDOS of the nitrogen atom of NO molecule and the doped nitrogen atom of N-doped nanocomposite. The large overlap between the PDOS of the mentioned atoms exhibits that the nitrogen atom of NO molecule interacts with the doped nitrogen atom of nanocomposite, suggesting the formation of new N–N bond. The PDOSs for NO2 adsorption on the doped nitrogen site have also been shown as panels (c, d), which indicate a high overlap between the PDOS of nitrogen atom of NO2 molecule and the nitrogen atom of nanocomposite and consequently forming a chemical bond. For NO and NO2 adsorption on the middle oxygen (OC site), the calculated PDOSs have been displayed in Fig. 9 (panels a, b), representing a low PDOS overlap between the nitrogen atom of NO and NO2 molecules and the OC atom of nanocomposite. This means a weak interaction between NOx and nanocomposite. The other panels of Fig. 9 represent the PDOS of oxygen atom of nanocomposite before and after the adsorption on the undoped nanocomposite, as well as the PDOS of nitrogen atoms of NO and NO2 molecules. As can be seen, the main difference is the creation a small peak in the PDOS curves and also shifting the position of the peaks to the lower lying energies. Figure 10a–d shows the PDOS of the nitrogen atom of nanocomposite before and after the adsorption on the N-doped nanocomposite, which also suggests a shifting of the PDOS of nitrogen atom to the lower energy values. To further discover the electronic variations at the adsorption site, the PDOSs of nitrogen atom of NO and NO2 molecule before and after the adsorption were also presented in Fig. 11a–d. Similarly, it can be seen from these PDOS plots that the biggest difference is the creation of some small peaks and also the state changing to the smaller energy values. The PDOSs of the complexes providing two contacting point (G, H and I) have also been displayed in Fig. 12, which indicate a higher overlap between the PDOS of Titanium atoms with two left and right oxygen atoms in all six panels. This means a formation of two chemical bonds between the titanium and oxygen atoms. Figures 13 and 14 represent the PDOSs of nitrogen atoms and their related p orbitals for complex D. As can be seen from these figures, p3 orbital of the nitrogen atom of nanocomposite and NO2 represents a considerable overlap with the other atom participating in chemical bond formation. This is an indication of the higher contribution of p3 orbital in chemical bond in comparison with the other orbitals. The PDOSs of nitrogen atoms and their p orbitals for complex E have also been displayed in Fig. 15, indicating a high overlap between the PDOS of nitrogen atom with p1 atomic orbital compared to the other orbitals. The HOMO and LUMO molecular orbitals were also displayed in Figs. 16 and 17, respectively. A closer inspection reveals that the HOMOs are strongly located on the nanoparticle, whereas the electronic densities in the LUMOs are mainly dominant on the NOx molecules. As can be seen from Fig. 17, the electronic density in the LUMOs seems to be distributed over the NOx molecules and on the middle of newly formed bonds. The accumulation of the electronic density at the middle of the newly formed bonds confirms the formation of new bonds and consequently the transfer of electronic density from the Ti–N bonds and N–O bonds to the newly formed bonds. However, the resultant improvements on the electronic properties of TiO2/MoS2 nanocomposites obtained by N-doping here demonstrate that the N-doped TiO2/MoS2 nanocomposites have stronger sensing capabilities than the pristine ones. To fully analyze the NOx adsorption on the considered TiO2/MoS2 nanocomposites, the Mulliken population analysis has been conducted to analyze the charge distribution of the atoms and bonds in a complex system. The calculated Mulliken charge values for studied complexes were collected in Tables 1 and 2. For complex A, NOx adsorption induces a considerable charge transfer of about −0.122 e from NOx molecule to the nanoparticle, suggesting that NOx acts as an acceptor. In other words, the NOx molecule receives electros from nanocomposite. This leads to the changes in the conductivity of the system, which would be an efficient property to aid in the design and fabrication of novel sensor devices for nitrogen oxides recognition.

Fig. 6
figure 6

Total density of states for N-doped TiO2 and two types of N-doped TiO2/MoS2 nanocomposites. a OC-substituted nanocomposite and b OT-substituted one

Full size image
Fig. 7
figure 7

DOS for the different adsorption configurations of the NOx molecule on the considered TiO2/MoS2 nanocomposites, a A complex (NO molecule adsorbed on the OC-substituted nanocomposite); b D complex (NO2 molecule adsorbed on the OC-substituted nanocomposite); c B complex (NO molecule adsorbed on the OT-substituted nanocomposite); d E complex (NO2 molecule adsorbed on the OT-substituted nanocomposite)

Full size image
Fig. 8
figure 8

PDOS for the interaction of NOx molecule with TiO2/MoS2 nanocomposites, a A complex; b B complex; c D complex; d E complex

Full size image
Fig. 9
figure 9

PDOS for the interaction of NOx molecule with TiO2/MoS2 nanocomposites, a C complex (NO molecule adsorbed on the pristine nanocomposite); b F complex (NO2 molecule adsorbed on the pristine nanocomposite); c C complex; d F complex; e C complex; f F complex

Full size image
Fig. 10
figure 10

PDOS of the nitrogen atom of nanocomposite before and after the adsorption process, a A complex; b B complex; c D complex; d E complex

Full size image
Fig. 11
figure 11

PDOS of the nitrogen atom of NOx molecule before and after the adsorption process, a A complex; b B complex; c D complex; d E complex

Full size image
Fig. 12
figure 12

PDOS of the titanium and oxygen atoms in complexes providing two contacting point between the nanocomposite and adsorbed molecule, a G complex (NO2 adsorbed on the OC-substituted nanocomposite in a bridge geometry); b G complex; c H complex (NO2 adsorbed on the OT-substituted nanocomposite in a bridge geometry); d H complex; e I complex (NO2 adsorbed on the pristine nanocomposite in a bridge geometry); f I complex

Full size image
Fig. 13
figure 13

PDOS of the nitrogen atoms and their related p orbitals after the adsorption process for D complex

Full size image
Fig. 14
figure 14

PDOS of the nitrogen atoms and their related p orbitals after the adsorption process for D complex

Full size image
Fig. 15
figure 15

PDOS of the nitrogen atoms and their related p orbitals after the adsorption process for E complex

Full size image
Fig. 16
figure 16

Isosurfaces of HOMO molecular orbitals for the adsorption of NO and NO2 molecules on the TiO2/MoS2 nanocomposites, where |0.05| was used as an isovalue of the molecular orbital

Full size image
Fig. 17
figure 17

Isosurfaces of LUMO molecular orbitals for the adsorption of NO and NO2 molecules on the TiO2/MoS2 nanocomposites, where |0.05| was used as an isovalue of the molecular orbital

Full size image

Conclusions

DFT calculations were conducted to investigate the interaction of NOx molecules with undoped and N-doped TiO2/MoS2 nanocomposites to effectively understand the sensing properties of these nanocomposites in adsorption processes. The bond angles of the adsorbed NO2 molecule are decreased compared to those in the isolated gas phase NO2, which lead to an increase in the p characteristics of bonding molecular orbitals of nitrogen in the NO2 molecule. The results also suggest that the N-doped nanocomposites have a higher efficiency to interact with harmful NOx molecules in the environment. In other words, the doping of nitrogen atom provides an increased affinity for the TiO2/MoS2 nanocomposites to interact with NOx molecules. The analysis of adsorption energies reveals that the adsorption of NOx molecules on the N-doped TiO2/MoS2 nanocomposites is more favorable in energy than the adsorption of NOx on the undoped ones. The variation in the electronic structure and molecular orbitals induced by N-doping is found to be responsible for the conductivity of the nanocomposite system. Our calculated results, therefore, suggest a theoretical basis for the prospective application of TiO2/MoS2 hybrid nanostructures as gas sensors for important air pollutants, such as NO and NO2, in the environment.