Introduction

Titanium (Ti) and titanium alloys are some of the most important biomedical materials because of their biocompatibility, good mechanical properties, and outstanding corrosion resistance [1, 2]. It is well known that the surface of titanium atom possesses an excellent ability to be spontaneously oxidized into titanium oxide. This optimal surface property of TiO2 is related to the excellent biocompatibility of the titanium atom [3]. Titanium dioxide (TiO2) is one of the most favorable metal oxide semiconductors because of its excellent properties such as long-term stability, low cost and non-toxicity [4]. It has attracted great attention due to its versatile and tunable properties for applications in photo-catalysis [5], sensor devices [6], organic dye-sensitized solar cells [7], water splitting and air pollution control [8]. There are three important polymorphs of TiO2, rutile, anatase, and brookite [9]. It possesses a wide band gap in the range of 3–3.2 eV, which prominently reduces its capability to be utilized in photocatalytic reactions. This wide band gap restricts the photosensitivity of TiO2 to the ultraviolet (UV) region, decreasing the photocatalytic activity. Therefore, TiO2 can only be employed in a minor area of the solar spectrum. Among different methods for improving the optical response of TiO2, non-metal doping is the most broadly used method to increase the photocatalytic activity of TiO2 [10, 11].

Recently, ZnO adsorbents have been extensively used as operative gas sensors for the detection and removal of harmful air pollutants. These semiconductor materials have been broadly studied owing to their outstanding physical and chemical stabilities, low cost, easy availability, and easy synthesis process [12]. ZnO nanostructures hold great potential as promising biocompatible materials for biomedical applications because of their advantages such as non-toxicity, biosafety, and their applicabilities in ZnO-based nanocarriers for drug delivery [13,14,15,16,17,18]. Over the past few years, several researchers have made great efforts to examine the particular properties of ZnO nanoparticles due to their biomedical applications, especially in cancer treatment [19]. Consequently, the unique optical and electronic properties of ZnO make it a favorable material to be utilized in various practical applications such as solar cells, gas sensor devices [20], piezoelectric transducers [21], photocatalysts, and photovoltaics [22]. ZnO nanomaterials can be simply synthesized at monitored chemical condition and small temperatures. Besides, some exceptional properties make ZnO as a suitable sensing material for gas sensing applications including novel structural arrangements, high single crystallinity surfaces and increased surface area to volume ratio [23,24,25]. Sensing gas molecules, specifically toxic air pollutants, is critical in environmental pollution reduction and agricultural and medical remediation [26]. Hence, sensitive solid-state sensors with low noise and low power consumption are extremely required. Peyghan et al. studied the adsorption behaviors and sensing properties of different nanoclusters and nanotubes of graphyne and carbon nitrides [27,28,29]. The main source of gas phase NO2 molecule is motor vehicle exhausts and heavy industry. NO2 is a major air pollutant, which has detrimental effects on the air quality with large contribution to the acid rain. NO2 also causes by photochemical smog, which is harmful and irritant to human body, eyes, throat, nose and lungs [30]. On the other hand, O3 molecule is a well-known air pollutant, which has detrimental impacts on respiratory tissues and ocular mucosa. The main target of tropospheric ozone in the human body is the lung, making harmful effects on the eyes and the nervous system [31]. The adsorption of different molecules on TiO2-based nanoparticles and nanocomposites has been investigated in detail [32,33,34,35,36,37,38,39]. Due to the important role of NO2 and O3 emissions from the pollution control and toxicology point of view, harmful gas removal by efficient adsorbents would be increasingly demanded. Thus, it is of great significance to seek for novel sensors more fascinating than bare TiO2 or ZnO such as multicomponent TiO2/ZnO nanocomposites. Therefore, expanding the sensing materials from single component metal oxide semiconductor to multicomponent composite structure has become a challenging issue. Significantly, TiO2-based two-component heterostructures, such as TiO2/SnO2 [40], TiO2/ZnO [41, 42] and TiO2/Fe2O3 [43] with enhanced sensing properties, have been efficiently constructed. TiO2/ZnO heterostructures act as efficient candidates for improving sensing capabilities because they incorporate the physical and chemical properties of their individual components. Recently, TiO2/ZnO nanocomposites have been synthesized successfully for various practical applications [44,45,46,47,48]. Figure 1 depicts a schematic representation of a metal oxide based gas sensor for toxic gas detection. In order to wholly exploit the possibilities of a TiO2/ZnO composite material as effective gas sensor, a systematic theoretical study on the adsorption of gas molecules on a composite surface is conducted. In this work, we have determined the most stable configuration for NO2 and O3 interaction with TiO2/ZnO nanocomposites. Variations in the electronic structures of the adsorption systems induced by molecule adsorption are then examined. To the best of our knowledge, no prior theoretical study has been done on these subjects.

Fig. 1
figure 1

Schematic representation of a metal oxide (TiO2)-based gas sensor for O3 detection

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Method and computational details

Density functional theory [49, 50] calculations were carried out using the Open source Package for Material eXplorer (OpenMX3.8) code [51]. The techniques and systems used in OpenMX and their implementation are properly applied for the realization of large-scale ab initio electronic structure calculations on parallel computers. OpenMX conducts a systematic computation for the simulation of materials in nanoscale, leading to the systematic understanding of the interactions between complex materials based on the quantum mechanics. The basis sets in our calculations were chosen based on the linear combination of atomic orbitals, which were known as pseudo-atomic orbitals (PAOs). Converged results were obtained based on the energy cutoff of 150 Ry. The convergence threshold for energy and electronic structure computations was considered to be 10−4 Hartree/bohr. Exchange and correlation effects were incorporated within the generalized gradient approximation, using the Perdew–Burke–Ernzerhof (PBE) functional [52]. The van der Waals interactions were described using the Grimme’s DFT-D2 methods [53]. Grimme and co-workers studied the effects of dispersion correction on the energetics and possible geometries of the complex systems [54,55,56]. The isosurfaces of the highest occupied and the lowest unoccupied molecular orbitals and the adsorption configurations were effectively visualized using the XCrysDen program [57]. It is well known that NO2 molecule has a bent structure. The N–O bond length and corresponding O–N–O bond angle of NO2 molecule have been estimated to be 1.20 Å and 134.3°, respectively. These achieved results are comparable with the previous gas phase data [58]. Also, O3 molecule has a bent structure with calculated O–O bond length of 1.278 Å and O–O–O bond angle of 117°, based on GGA method, in line with previously reported theoretical and experimental values [59, 60].

The adsorption energies for the studied complexes were estimated using the following formula:

$$ E_{\text{ad}} = \,E\,_{{\left( {{\text{composite}}\,{ + }\,{\text{adsorbate}}} \right)}} \, - \,E_{\text{composite}} \, - \,E_{\text{adsorbate}} , $$
(1)

where E (composite + adsorbate) corresponds to the total energy of combined composite with adsorbed molecule system, E composite and E adsorbate are the energies of the clean TiO2/ZnO nanocomposite and free NO2 or O3 molecule, respectively. A negative adsorption energy represents that the adsorption process is strongly favored from the energy point of view. Thus, the obtained structures and final configurations are energy favorable.

TiO 2 anatase model: For TiO2 anatase, the supercell approach was considered, which is based on the selection of an appropriate 3 × 2 × 1 simulation supercell. We have chosen the unit cell using the American Mineralogists Database crystal structure database [61], which is an interface to a crystal structure database. This structure was developed by Wyckoff [62]. For our studied models, the nitrogen modified structures were obtained by introducing the nitrogen atoms into the oxygen vacancy of TiO2, leading to the energy favorable N-doped TiO2 nanoparticles. This substitution was performed at both OC and OT sites, representing the nitrogen doping at the middle of the particle (threefold coordinated oxygen site) and the twofold coordinated oxygen atom sites, respectively. The titanium atoms of the TiO2 anatase were characterized as the fivefold coordinated and sixfold coordinated titanium sites, the former has more reactivity than the latter. It is also well known that the twofold coordinated oxygen can bind to the surface more efficiently, compared to the threefold coordinated atom [63]. Calculated cell parameters (a = 3.784 Å, c = 9.621 Å) are well consistent with experimental data (a = 3.782 Å, c = 9.502 Å) [64] and previous theoretical works [65, 66]. Figure 2 displays the optimized geometry of an undoped TiO2 anatase nanoparticle constructed from the TiO2 unit cell.

Fig. 2
figure 2

Optimized structure of a pristine TiO2 anatase nanoparticle: OC, OT and OD denote threefold coordinated, twofold coordinated and dangling oxygen atoms, respectively. The gray and red balls denote titanium and oxygen atoms, respectively

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ZnO model: The unit cell of ZnO was also taken from “American Mineralogists Database” webpage reported by Kihara [67]. In the crystal structure of ZnO, we can see two types of oxygen atoms denoted by fourfold coordinated oxygen (4f-O) and threefold coordinated oxygen (3f-O) atoms. ZnO nanoparticle contains 90 atoms (45 Zn and 45 O atoms) without any dangling oxygen atoms. As a result, the atomic number ratio between Zn and O atoms is 1:1, and the total charge of the particle would be zero. The lattice parameters of wurtzite ZnO were calculated to be a = b = 3.2502 Å, c = 5.205 Å, which are in reasonable agreement with experimentally reported data [68]. The optimized structure of the considered ZnO nanocluster is shown in Fig. 3.

Fig. 3
figure 3

Optimized structure of a pristine ZnO nanoparticle; the white and red balls represent zinc and oxygen atoms, respectively

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Results and discussion

NO2 interacts with TiO2/ZnO nanocomposites

The structure of TiO2/ZnO nanocomposite was geometrically optimized. Similarly, both NO2 and O3 molecules were optimized initially. The modeling of the adsorption was achieved initially by placing NO2 and O3 molecules at the interface between TiO2 and ZnO. Thus, at this interface, we can examine the adsorption of gas molecules on the nanocomposite. Optimized structures of the most stable NO2 adsorptions on TiO2/ZnO nanocomposites are presented in Figs. 4 and 5. The adsorption configurations of NO2 on the considered nanocomposites were denoted by types A–C in these figures. The only difference between these structures is the final positioning of the NO2 molecule with respect to the undoped or N-doped nanocomposite. Both oxygen atoms of the NO2 move strongly towards the fivefold coordinated titanium atoms, bridging the fivefold coordinated titanium sites. In configuration A, NO2 molecule adsorbs on the OC-substituted nanocomposite. The oxygen atoms were bonded to the titanium atoms and the nitrogen atom to the zinc site. Configuration B displays the adsorption of NO2 on the OT-substituted nanocomposite. It can be seen that there is no mutual interaction with the ZnO side of nanocomposite. Similar to configuration A, in configuration C, only the fivefold coordinated titanium atoms interact with the NO2 molecule. Thus, NO2 molecule reacts with the TiO2 side of pristine TiO2/ZnO nanocomposite. In adsorption types A and C, there is a triple contacting point between the nanocomposite and NO2 molecule and the side oxygen atoms interact with the fivefold coordinated titanium atoms of TiO2, while the nitrogen atom moves towards the zinc atom. Configuration B presents a double contacting point, formed from the interaction of the oxygen atoms of NO2 with TiO2 side of nanocomposite.

Fig. 4
figure 4

Optimized geometry configurations of NO2 molecule adsorbed on the N-doped TiO2/ZnO nanocomposites. The fivefold coordinated titanium atoms were found to be the most stable binding sites on the TiO2

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Fig. 5
figure 5

Optimized geometry configurations of NO2 molecule adsorbed on the undoped TiO2/ZnO nanocomposites. The fivefold coordinated titanium and zinc atoms were found to be the most stable binding sites on the TiO2

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Table 1 summarizes the distance and angles parameters for the important bonds and angles of the NO2 molecule before and after the adsorption process. When NO2 molecule interacts with the nanocomposite, there was an elongation in the range of 0.11–0.16 Ǻ for the bond lengths of N–O. The reason is that the electronic density increases at the middle of the newly formed bonds between the TiO2/ZnO and NO2 molecule. In other words, it was transferred from the N–O bonds to the newly formed Ti–O bonds between the nanocomposite and NO2 molecule. This N–O bond elongation of NO2 molecule makes that the N–O bonds to be weakened after the adsorption process. The bond angle of the optimized NO2 molecule in gas phase is about 134.3°; after the adsorption, there is a decrease in O–N–O angle ranging from 10–15°. This decrease in the bond angle values is in reasonable consistency with the elongation of bond lengths. Table 2 summarizes the adsorption energies of NO2 molecules on the considered TiO2/ZnO nanocomposites. Clearly, the adsorption energies were increased from the undoped nanocomposite to the N-doped one. The results of this table indicate that the adsorption of NO2 molecule on the N-doped TiO2/ZnO nanocomposite gives rise to the more stable complexes than the adsorption on the pristine one. In view of this conclusion, the highest adsorption energy occurs in configuration A, whereas the lowest value belongs to configuration C. This is due to the fact that configuration A represents the interaction of NO2 with N-doped nanocomposite. As can be seen, for configuration A, NO2 molecule is oriented so that the oxygen atoms stand above the fivefold coordinated titanium site, and hence are able to form chemical bonds with the titanium atoms. The greater adsorption energy means stronger adsorption of NO2 molecule to the surfaces. Thus, configuration A shows the strongest interaction of NO2 molecule with the nanocomposite. The significant adsorption energy of configuration A also indicates that this configuration is the most stable binding site of NO2 located on the nanocomposite.

Table 1 Bond lengths (in Ǻ) and angles (in degrees) for NO2 molecule adsorbed on TiO2/ZnO nanocomposites
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Table 2 Adsorption energies (in eV) for NO2 and O3 molecules adsorbed on the considered TiO2/ZnO nanocomposites calculated from different methods and charge transfers between the TiO2/ZnO nanocomposite and adsorbed molecule (|e|), calculated using Mulliken population analysis
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Comparing the values, we found that the adsorption energy of configuration A is higher than that of configuration B, representing that the OC-substituted nanocomposite strongly interacts with the NO2 molecule. The effects of van der Waals interactions make that the adsorption energies to be enhanced after the adsorption. This increase was achieved by including the dispersion correction in the calculations. Gathering the results of adsorption energies, we found that the N-doped TiO2/ZnO nanocomposite acts as a promising candidate to be utilized for the design of NO2 sensor devices. To further examine the behavior of the NO2 molecule on the TiO2/ZnO nanocomposite, we have calculated the charge transfer values (see Table 2). The charge analysis based on Mulliken charges reveals a noticeable charge transfer from the NO2 molecule to the nanocomposite. The highest charge transfer occurs in configuration A, while the lowest charge transfer belongs to configuration C. The results showed a direct relationship between the amount of charge transfer and the adsorption energies. Figure 6 shows the calculated projected density of states (PDOS) spectra for the adsorption configurations of TiO2/ZnO nanocomposites with adsorbed NO2 molecules. Panels (a–c) in this figure show the PDOSs of the titanium, oxygen and zinc atoms for configuration A, while panels (d–e) represent the PDOS spectra for configuration B. The significant overlaps between the PDOSs of the fivefold coordinated titanium atoms of TiO2 and oxygen atoms of NO2 indicate the formation of chemical Ti–O bonds between these two atoms. Also, from the PDOS overlaps of the nitrogen atom of NO2 molecule and zinc atom, we found that there is strong interaction between them. Panels (d–e) show the PDOSs of the oxygen atoms of the NO2 molecule and the titanium atoms, (configuration B), indicating noticeable overlaps between the PDOSs of the titanium and oxygen atoms. For configuration C with triple contacting point, the PDOS spectra of the titanium and oxygen atoms (panels (f–h)) exhibit considerable overlaps, representing the formation of chemical bonds between the interacting atoms. Figure 7 displays the PDOSs of the titanium, oxygen atoms of NO2 molecule and different d orbitals of the titanium atom (configuration A). As can be seen from this figure, the PDOSs of the oxygen atom of NO2 molecule and d1 orbital of the titanium atom show large overlaps in some energy values. The isosurface plots of the highest occupied molecular orbitals (HOMOs) for NO2 adsorbed TiO2/ZnO nanocomposites are also illustrated in Fig. 8. As can be seen, the HOMOs of the adsorption systems show the accumulation of the electronic density over the adsorbed molecule.

Fig. 6
figure 6

PDOSs for the adsorption configurations of TiO2/ZnO nanocomposites with adsorbed NO2 molecules, a–c Configuration A; d–e Configuration B; f–h Configuration C

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Fig. 7
figure 7

PDOSs of the oxygen atom of NO2 molecule, titanium atom and different d orbitals of the titanium atom for adsorption configurations of TiO2/ZnO with adsorbed NO2 molecule (configuration A)

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Fig. 8
figure 8

Isosurfaces of the highest occupied molecular orbitals (HOMOs) for NO2 adsorbed TiO2/ZnO nanocomposites

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O3 interacts with TiO2/ZnO nanocomposites

Figures 9 and 10 show the optimized geometry configurations of the N-doped and undoped TiO2/ZnO nanocomposites with adsorbed O3 molecules. The configurations of the interaction of O3 molecule with TiO2/ZnO nanocomposites were marked by adsorption types D–F. The only difference is the pattern of nitrogen doping into the oxygen vacancy of TiO2, as well as the orientation of O3 towards the nanocomposite. Configuration D represents the adsorption of O3 on the OC-substituted nanocomposite, while configuration E shows the favored orientation of O3 over the OT-substituted one. As can be seen from this figure, the undoped nanocomposite also interacts with O3 molecule (configuration F). In all configurations, we can see that there is a double contacting point between the O3 and TiO2/ZnO nanocomposite, resulting from the coordination of the side oxygen atoms of the O3 molecule to the fivefold coordinated titanium atoms. Interestingly, the central oxygen atom does not interact with the nanocomposite either from TiO2 or ZnO side. Therefore, in the case of O3 adsorption on the TiO2/ZnO nanocomposites, the binding sites were located on the fivefold coordinated titanium atoms. Table 3 summarizes the bond lengths and bond angles for O3 adsorption on the considered TiO2/ZnO nanocomposites. The bond lengths of the O3 undergo a noticeable change after the adsorption, the O–O bond lengths were stretched, and the elongation is in the range of 0.12–0.18 Ǻ. This increase in the O–O bond length can be attributed to the transfer of electronic density from the O–O bonds of the O3 to the newly formed Ti–O bonds between the TiO2 side of nanocomposite and O3 molecule. Similar to NO2 adsorption, the O–O–O bond angles of the adsorbed O3 were decreased in comparison with the gas phase O3 molecule. Table 2 lists the calculated adsorption energies of the O3 molecule on the considered nanocomposites. As can be seen from this table, the adsorption energies of N-doped nanocomposites (configurations D and E) are more negative than those of undoped ones (configuration F), indicating that the adsorption of O3 molecule on the N-doped nanocomposite is more favorable in energy than that on the pristine one. The higher adsorption energy of N-doped nanocomposites with respect to the pristine ones indicates that the N-doped nanocomposites have higher sensing capability than the undoped ones. Thus, the most stable configuration corresponds to a bridge geometry of O3 molecule towards the N-doped nanocomposite (configuration A), while the least stable one corresponds to configuration C. Therefore, the nitrogen doping strengthens the interaction between O3 molecule and TiO2/ZnO nanocomposite. To further examine the electronic properties of the gas–nanocomposite interaction, the projected density of states (PDOSs) were calculated for the most stable structures of O3 molecule adsorbed on the TiO2/ZnO nanocomposites. Figure 11 displays the PDOSs of the fivefold coordinated titanium atoms and the side oxygen atoms of O3 molecule (configurations D–F). As can be seen from this figure, there are significant overlaps between the PDOS spectra of the titanium and oxygen atoms. These overlaps confirm the formation of chemical Ti–O bonds between these atoms. Thus, O3 molecule was chemisorbed on the TiO2/ZnO nanocomposite. We have also calculated the PDOSs of the side oxygen atoms, titanium atom and different d orbitals of the titanium atom (see Fig. 12). In this case, the PDOS of d1 orbital shows a greater overlap with the titanium atom compared with the other d orbitals. The isosurface plots of HOMOs for the adsorption O3 molecule on the considered TiO2/ZnO heterostructures are presented in Fig. 13. After the adsorption, the electronic densities in the HOMOs were largely concentrated over the O3 molecule, which is in reasonable agreement with the transfer of electronic density from the O–O bonds of O3 molecule to the newly formed bonds between the nanocomposite and O3 molecule.

Fig. 9
figure 9

Optimized geometry configurations of O3 molecule adsorbed on the N-doped TiO2/ZnO nanocomposites. The fivefold coordinated titanium atoms were found to be the most stable binding sites on the TiO2

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Fig. 10
figure 10

Optimized geometry configurations of O3 molecule adsorbed on the undoped TiO2/ZnO nanocomposites. The fivefold coordinated titanium atoms were found to be the most stable binding sites on the TiO2

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Table 3 Bond lengths (in Ǻ) and angles (in degrees) for O3 molecule adsorbed on TiO2/ZnO nanocomposites
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Fig. 11
figure 11

PDOSs for the adsorption complexes of TiO2/ZnO nanocomposites with adsorbed O3 molecules, a–b Configuration D; c–d Configuration E; e–f Configuration F

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Fig. 12
figure 12

PDOSs of the oxygen atom of O3 molecule, titanium atom and different d orbitals of the titanium atom for adsorption configurations of TiO2/ZnO with adsorbed NO2 molecule (configuration D)

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Fig. 13
figure 13

Isosurfaces of the highest occupied molecular orbitals (HOMOs) for O3 adsorbed TiO2/ZnO nanocomposites

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Vibrational frequencies

We have calculated the vibrational frequencies of free NO2 molecule, and the gas molecules in a complex adsorption system. For brevity, we only reported the results for one configuration only (configuration A). The calculated vibrational frequencies are summarized in Table S1. The calculated vibrational frequencies for adsorbed NO2 differ largely from the free NO2 molecule, consistent with the strong adsorption of NO2 to the TiO2/ZnO nanocomposite. The results indicate that the symmetric and asymmetric stretches were shifted to a lower frequency. The bending stretch mode was shifted to the higher frequency (blue-shift), indicating the highest shift for configuration A. However, the variations are relatively minor. In the case of configuration A, the shift in asymmetric stretch is more than that in symmetric stretch. Our obtained vibrational frequencies are very close to the experimentally reported data [69].

Conclusions

We have performed a theoretical study of NO2 and O3 molecules on the TiO2/ZnO nanocomposites using density functional theory (DFT) calculations. These adsorptions are carried out on the pristine and N-doped nanocomposites. We found that both these molecules are chemisorbed on the nanocomposite surface, representing a bridge geometry of the gas molecule at the interface region. The results suggest that the gas sensing capability of TiO2/ZnO is critically affected by nitrogen doping. The N-doped TiO2/ZnO nanocomposites have higher sensing capability than the pristine ones. The results also suggest that gas molecules adsorption on the N-doped heterostructures leads to the more stable configurations than the adsorption on the undoped one, indicating that NO2 and O3 interaction with N-doped nanocomposite is strongly favored. For both cases, the least stable adsorption geometry corresponds to a bridge geometry of gas molecule on the pristine nanocomposite, whereas the most stable geometry belongs to N-doped configurations. After the adsorption, the N–O bonds of the NO2 and the O–O bonds of the O3 molecules were stretched. By taking van der Waals interactions into account, we found that the adsorption energies were increased. The charge analysis was performed based on the Mulliken population method, which indicates that the charge was transferred from the adsorbed molecules to the TiO2/ZnO heterostructure. The large overlaps in the PDOS spectra of the interacting atoms represent the formation of chemical bonds between them. Consequently, the N-doped TiO2/ZnO nanocomposite holds a great potential to be utilized in the design of NO2 and O3 sensor devices.