Interaction of NH$_3$ gas on $\alpha$-MoO$_3$ nanostructures $-$ a DFT investigation

TL;DR

This study uses density functional theory to explore how substituting elements in $eta$-MoO$_3$ nanostructures affects their stability, electronic properties, and ability to adsorb ammonia for sensing applications.

Contribution

It demonstrates how impurity substitution in $eta$-MoO$_3$ nanostructures can enhance ammonia detection capabilities, providing a theoretical basis for sensor design.

Findings

Substitution affects the HOMO-LUMO gap and electronic properties.

Impurities improve NH$_3$ adsorption on $eta$-MoO$_3$ nanostructures.

Tailoring nanostructures enhances ammonia sensing performance.

Abstract

The structural stability, electronic properties and NH$_3$ adsorption properties of pristine, Ti, Zr and F substituted $\alpha$-MoO$_3$ nanostructures are successfully studied using density functional theory with B3LYP/LanL2DZ basis set. The structural stability of $\alpha$-MoO$_3$ nanostructures is discussed in terms of formation energy. The electronic properties of pristine, Ti, Zr and F incorporated $\alpha$-MoO$_3$ nanostructures are discussed in terms of HOMO-LUMO gap, ionization potential and electron affinity. $\alpha$-MoO$_3$ nanostructures can be fine-tuned with suitable substitution impurity to improve the adsorption characteristics of ammonia, which can be used to detect NH$_3$ in a mixed environment. The present work gives an insight into tailoring $\alpha$-MoO$_3$ nanostructures for NH$_3$ detection.

Tables

Table 1. Table 1: Formation energy, dipole moment and point group of α 𝛼 \alpha -MoO 3 nanostructures.

Nanostructures	Formation energy	Dipole moment	Point group
	(eV)	(D)
Pristine $\alpha$-MoO3 nanostructure	$-4.38$	5.62	$C_1$
Ti substituted $\alpha$-MoO3 nanostructure	$-4.36$	19.18	$C_1$
Zr substituted $\alpha$-MoO3 nanostructure	$-4.30$	35.08	$C_1$
F substituted $\alpha$-MoO3 nanostructure	$-4.40$	40.47	$C_1$

Table 2. Table 2: Adsorption energy, Mulliken population, HOMO-LUMO gap and average energy gap variation of α 𝛼 \alpha -MoO 3 nanostructures.

$\alpha$-MoO3 nanostructures	$E_{\text{ad}}$	$Q$ (e)	$E_{\text{HOMO}}$	$E_{\text{FL}}$ (eV)	$E_{\text{LUMO}}$	$E_{\text{g}}$ (eV)	$E_{\text{g}}^{\text{a}}$ (%)
Pristine $\alpha$-MoO3	$-$	$-$	$-$11.13	$-$8.79	$-$6.45	4.68	$-$
A	1.09	0.30	$-$11.04	$-$8.755	$-$6.47	4.57	2.41
B	$-$2.45	0.11	$-$11.04	$-$8.685	$-$6.33	4.71	0.64
C	$-$8.16	0.93	$-$8.55	$-$6.95	$-$5.35	3.2	46.25
D	$-$11.70	0.56	$-$9.13	$-$8.195	$-$7.26	1.87	150.27
Ti substituted $\alpha$-MoO3	$-$	$-$	$-$10.13	$-$9.115	$-$8.1	2.03	$-$
E	$-$0.27	0.19	$-$9.46	$-$8.785	$-$8.11	1.35	50.37
F	$-$2.72	0.10	$-$10.34	$-$9.275	$-$8.21	2.13	4.69
Zr substituted $\alpha$-MoO3	$-$	$-$	$-$10.48	$-$9.37	$-$8.26	2.22	$-$
G	$-$1.09	0.13	$-$10.15	$-$9.22	$-$8.29	1.86	19.35
H	$-$3.26	0.08	$-$10.37	$-$9.31	$-$8.25	2.12	4.72
F substituted $\alpha$-MoO3	$-$	$-$	$-$7.9	$-$6.95	$-$6	1.9	$-$
I	$-$14.42	1.14	$-$8.22	$-$6.865	$-$5.51	2.71	29.89
J	$-$9.52	0.67	$-$9.12	$-$7.385	$-$5.65	3.47	45.24

Equations

$$E_{\text{form}}=1/n[E(\alpha {\text{-MoO}}_3\text{nanostructure})-pE(\text{Mo})-qE(\text{O})-rE(\text{dopant})],$$

$$E_{\text{ad}}=[E({\text{MoO}}_3)+E({\text{NH}}_3)-E({\text{MoO}}_3/{\text{NH}}_3)+E(\text{BSSE})],$$

Full text

Key words: nanostructure, adsorption, NH3, HOMO-LUMO gap, MoO3

PACS: 71.15.Mb

Ключовi слова: íàíîñòðóêòóðà, àäñîðáöiÿ, NH3, HOMO-LUMO ùiëèíà, MoO3

1 Introduction

Ammonia (NH3) is widely used in automobiles, food industry and agriculture in the form of fuel, antimicrobial agent and fertilizers [1]. The detection of NH3 is a significant criterion due to its hazard. The exposure limit of ammonia is around 25 ppm, which is recommended by Occupational Safety and Health Administration (OSHA) [2]. There are several analytical techniques adopted to detect ammonia gas, which include the laser methods [3], electrochemical methods, optical methods [4] and mass spectrometry [5]. These techniques are time-consuming and also need sophisticated instruments. In this context, an inexpensive and real-time sensor is required to detect trace the amounts of ammonia at ppm level. Metal oxide semiconductor (MOS) thin films are extensively used for gas sensors as their resistivity changes upon interaction with toxic gas molecules [6, 7]. Moreover, MOS sensors are easy to fabricate, low cost and are uniform in performance among other types of gas sensors. Besides, the morphology such as rods, belts and wires in the micro-dimension and nano-dimension show a significant performance in MOS sensors. Furthermore, these types of nanostructured materials offer high surface-to-volume ratio, which leads to an enhanced performance in gas sensing [8, 9, 10].

Among various metal oxide semiconductors, molybdenum oxide (MoO3) is an excellent candidate for electrochromic, catalytic and gas sensing applications. MoO3 is n-type semiconductor with a wide band gap; the conductivity arises due to oxygen vacancies. The band gap of MoO3 is found to be around 2.69–2.76 eV [11]. MoO3 is also used as catalyst for the reduction of NOx in the petroleum and chemical industry and oxidation of hydrocarbons [12, 13, 14, 15]. Moreover, there are reports for enhancing the gas sensing properties of molybdenum oxide based device to detect LPG [16], CO [17, 18], NH3 [13, 16, 19], H2 [16, 17]. Furthermore, the synthesis of MoO3 includes thermal evaporation [20], pulsed laser deposition [13], sol-gel [1, 21], electro-deposition [22] and chemical vapor deposition [23, 24].

MoO3 exhibits in three polymorphic phases [25] namely stable orthorhombic $\alpha$-MoO3, meta-stable monoclinic $\beta$-MoO3, hexagonal $\beta$-MoO3. Kannan et al. [26] have reported the influence of the precursor solution volume on the properties of spray deposited $\alpha$-MoO3 thin films. Martinez et al. [27] have studied the gas sensing properties of spray deposited MoO3 thin films. Hussain et al. [28] have reported activated reactive evaporated MoO3 thin films for gas sensor applications and they observed that $\alpha$-MoO3 are capable of sensing both CO and NH3 gases at below 10 ppm of concentration in dry air. Density functional theory (DFT) is an efficient method for studying the interaction between compounds and adsorption characteristics of compounds [29, 30, 31]. Based on these aspects, literature survey was conducted using CrossRef metadata search and it is inferred that not much work was reported based on DFT methods to investigate the adsorption properties of NH3 on $\alpha$-MoO3 nanostructures. The motivation behind the present work is to improve the NH3 adsorption properties on $\alpha$-MoO3 nanostructures with the incorporation of dopants. The novel aspect of this work is to study the adsorption characteristics of NH3 on $\alpha$-MoO3 nanostructures with the substitution of Ti, Zr and F as substitution impurities.

2 Computational methods

In the present work, Gaussian 09 package [32] is used to optimize the pristine, Ti, Zr and F substituted $\alpha$-MoO3 nanostructures. This package is also used to investigate the adsorption properties of NH3 gas molecules on $\alpha$-MoO3 base material. The calculations were carried out for isolated MoO3 base material with periodic boundary condition (PBC). Moreover, the atoms were fixed along the direction perpendicular to the molecular plane and allowed us to relax along the other planes. In the present work, DFT is utilized in accordance with Becke’s three-parameter hybrid functional in combination with Lee-Yang-Parr correlation functional (B3LYP)/LanL2DZ basis set [33, 34, 35, 36]. The selection of a suitable basis set is an important criterion for optimizing $\alpha$-MoO3 nanostructures. In our previous study we demonstrated a density functional theory with the use of all-electron basis sets, but methods including effective core potentials (ECPs) are good in reducing the computational cost. Furthermore, the efforts have been taken into account for measuring the performance of basis set in order to approximate the same set of density functional. Moreover, we used ECP basis set such as LanL2DZ (Los Alamos National Laboratory 2 Double-Zeta), which is more suitable for transition metals. Besides, utilization of all-electron basis sets for the remaining non-transition metal elements has become more popular in computational studies on transition metal containing materials. Further, the atomic number of molybdenum and oxygen is forty two and eight, respectively. In the present work, $\alpha$-MoO3 nanostructures are studied with the incorporation of impurities such as Ti, Zr and F. It is known that Ti and Zr belongs to group IVB and is 4th and 5th period, respectively. Thus, LanL2DZ effective core potential will be suitable for the optimization of $\alpha$-MoO3 nanostructures with impurities. In addition, LanL2DZ basis set is applicable for the elements such as H, Li-La and Hf-Bi, which gives the best results with the pseudopotential approximation. Hence, LanL2DZ basis set is a suitable basis set to optimize $\alpha$-MoO3 with pseudopotential approximation [37, 38, 39]. The HOMO-LUMO gap and density of states spectrum (DOS) of $\alpha$-MoO3 nanostructures are calculated using Gauss Sum 3.0 package [40]. The energy convergence is obtained within the range of 10–5 eV, during the optimization of $\alpha$-MoO3 nanostructures.

3 Results and discussion

The present work mainly focuses on the study of ionization potential (IP), HOMO-LUMO gap, dipole moment, electron affinity (EA), Mulliken population and adsorption properties of NH3 gas molecules in MoO3 base material with the incorporation of impurities such as Ti, Zr and F in $\alpha$-MoO3 nanostructures. The reason behind the selection of Ti, Zr and F as impurities is that Ti and Zr belong to the transition metals like Mo. The electronic and structural properties of $\alpha$-MoO3 nanostructure can be controlled and fine-tuned with chemical modifications, such as doping [41]. Especially, the doping methodology has been widely utilized for numerous conjugated polymers, while dopants play a vital role in modifying their electronic properties of conventional covalent semiconductors [42]. This doping technique can be performed either by application of an external electric field or charge transfer. Owing to the application of doping mechanism, optical and electronic properties of the respective conducting polymers can be widely engineered. In addition, the doping effect influences the transfer charge from semiconductor to metal or insulator based on the concentration of a dopant [43]. The electronic properties of conjugated polymers can be easily fine-tuned either by atomic/molecular scale doping or chemical modification [41]. Kaloni et al. [44, 45] have reported the effect of the doping mechanism on polythiophene and polypyrrole based materials and studied the structural and electronic properties of pure and doped polymers. In the present work, $\alpha$-MoO3 nanostructures get distorted, while optimizing the base material with Ti, Zr and F elements as dopant, which results in the variation of electronic properties. Moreover, the conductivity of $\alpha$-MoO3 base material changes with the substitution of impurities. In addition, fluorine is abundant in electrons compared to oxygen, which leads to an increase in n-type behavior. Based on these aspects, the dopants are selected to incorporate in $\alpha$-MoO3 base material and these impurities tune the conducting property in $\alpha$-MoO3 base material. Therefore, the adsorption properties of NH3 on $\alpha$-MoO3 nanostructures can be improved with the substitution impurities.

Figure 1 represents the structure of pristine, Ti, Zr and F substituted $\alpha$-MoO3 nanostructures, respectively. The structure of $\alpha$-MoO3 is taken from International Centre for Diffraction Data (ICDD) card number: 89-7112. The pristine $\alpha$-MoO3 nanostructure has twelve Mo atoms and thirty eight O atoms. Ti incorporated $\alpha$-MoO3 nanostructure has eleven Mo atoms, thirty eight O atoms and one Mo atom is replaced with one Ti atom. Similarly, Zr substituted $\alpha$-MoO3 nanostructure has eleven Mo atoms, thirty eight O atoms and one Mo atom is replaced with one Zr atom. In the case of F substituted $\alpha$-MoO3 nanostructure, it has twelve Mo atoms, thirty five O atoms and three O atoms are replaced with three F atoms for enhancing the adsorption properties of NH3 on $\alpha$-MoO3 base material.

3.1 Structural stability and electronic properties of $\alpha$-MoO3 nanostructures

The structural stability of pristine, Ti, Zr and F substituted $\alpha$-MoO3 nanostructures can be described in terms of formation energy as shown in equation (3.1)

[TABLE]

where $E(\alpha$-MoO3 nanostructures) refers to the total energy of $\alpha$-MoO3 nanostructures, $E$(Mo), $E$(O) and $E$(dopant) represent the corresponding energy of isolated Mo, O and dopant atoms namely Ti, Zr and F. $p$, $q$ and $r$ represent the total number of Mo, O and dopant atoms, respectively, and $n$ is the total number of atoms in $\alpha$-MoO3 nanostructure. The dipole moment, point symmetry and the formation energy of pristine, Ti, Zr and F substituted $\alpha$-MoO3 nanostructures are tabulated in table 1. The formation energy of pristine, Ti, Zr and F substituted $\alpha$-MoO3 nanostructures is $-4.38$, $-4.36$, $-4.30$ and $-4.40$ eV, respectively.

Before studying the adsorption characteristics, the structural stability of $\alpha$-MoO3 base material must be studied. The structural stability of $\alpha$-MoO3 nanostructures slightly decreases with the substitution of Ti and Zr. The formation energy of Ti and Zr substituted $\alpha$-MoO3 nanostructures is relatively low compared to pristine $\alpha$-MoO3 nanostructures. By contrast, the stability of $\alpha$-MoO3 nanostructure slightly increases with the substitution of F. The dipole moment (DP) gives a clear picture about the distribution of charges in $\alpha$-MoO3 nanostructure. The corresponding dipole moment value of pristine, Ti, Zr and F incorporated $\alpha$-MoO3 nanostructures is 5.62, 19.18, 35.08 and 40.47 D. Low value of DP is recorded for pristine $\alpha$-MoO3 nanostructure. It infers that the charge distribution is almost uniform, but not in the case of impurity substituted $\alpha$-MoO3 nanostructures. Furthermore, $C_{1}$ point groups are observed for all $\alpha$-MoO3 nanostructures, which only exhibit identical operation.

The electronic properties of pristine, Ti, Zr and F incorporated $\alpha$-MoO3 nanostructures can be discussed in terms of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) [46, 47]. In the present work, the electronic properties and adsorption properties of NH3 on $\alpha$-MoO3 nanostructures are studied for a small cluster. The HOMO-LUMO gap of pristine, Ti, Zr and F substituted $\alpha$-MoO3 nanostructures is 4.68, 2.03, 2.22 and 1.9 eV, respectively. The deviation in the HOMO-LUMO gap between experiment and theoretical values arises owing to the selection of the basis set [11]. Moreover, DFT method is broadly linked to the ground state. Thus, the exchange-correlation potential between the excited electronic states may be underestimated. The HOMO-LUMO gap of $\alpha$-MoO3 nanostructure decreases with the substitution of Ti, Zr and F. Thus, the electronic configuration of Mo, Zr, F and Ti element differs, the occupied states also differ. The corresponding electronic configuration of Ti, Zr and Mo is [$1s2$ $2s2$ $2p6$ $3s2$ $3p6$ $3d2$ $4s2$], [$1s2$ $2s2$ $2p6$ $3s2$ $3p6$ $3d10$ $4s2$ $4p6$ $4d2$ $5s2$] and [$1s2$ $2s2$ $2p6$ $3s2$ $3p6$ $3d10$ $4s2$ $4p6$ $4d5$ $5s1$]. In the case of Ti and Zr atom, the outermost $s$-orbital is completely filled with electrons and the electrons are partially occupied in $d$-orbital. Therefore, the $s$ and $d$-states can be chosen as occupied and unoccupied state, respectively. Moreover, for Zr atom, the $d$-state is unfilled. In the case of Mo atom, the outermost $s$-orbital and $d$-orbital are not completely filled with electrons. Thus, due to the doping effects of Ti, Zr and F, the orbital overlapping leads to the variation in the energy gap in $\alpha$-MoO3 nanostructures. The HOMO-LUMO level and the energy gap of $\alpha$-MoO3 nanostructures are tabulated in table 2.

The density of states spectrum (DOS) gives an insight into the localization of charges in various energy intervals in $\alpha$-MoO3 nanostructures. The DOS spectrum and visualization of HOMO-LUMO gap of pristine, Ti, Zr and F substituted $\alpha$-MoO3 nanostructure are shown in figures 2–5, respectively. In the present work, the localization of charges is recorded to be more at LUMO level than at HOMO level, which is confirmed by more peak maxima at LUMO level. More peak maxima in $\alpha$-MoO3 nanostructures arise due to the orbital overlapping of Mo atoms and O atoms in $\alpha$-MoO3 base material. Moreover, the peak maxima in the virtual orbitals of $\alpha$-MoO3 base material are more favorable for adsorption characteristics, since the transfer of electrons between NH3 molecules and virtual orbitals can take place easily.

The electronic properties of $\alpha$-MoO3 nanostructure can also be discussed with the ionization potential (IP) and electron affinity (EA) [48, 49]. Figure 6 represents the electron affinity and ionization potential of $\alpha$-MoO3 nanostructures. Generally, IP depicts the amount of energy required to remove the electron from $\alpha$-MoO3 nanostructures and the EA represents the energy change with the addition of electrons in $\alpha$-MoO3 nanostructures. The high value of ionization potential infers that the electrons are tightly bounded to the nucleus in $\alpha$-MoO3 nanostructure. Therefore, pristine, Ti and Zr substituted $\alpha$-MoO3 nanostructures have a high value of ionization potential, which infers that the electrons are strongly attracted to the nucleus and more energy is required to remove electrons from $\alpha$-MoO3 nanostructure. The low value of IP is recorded for F substituted $\alpha$-MoO3 nanostructure. Thus, less energy is required to remove an electron from F substituted $\alpha$-MoO3 nanostructure. Electron affinity plays an important role in plasma physics and in chemical sensors.

The EA value of pristine, Ti, Zr and F incorporated $\alpha$-MoO3 nanostructures is 6.45, 8.1, 8.26 and 6 eV, respectively, which is a favourable condition for chemical sensors. Usually, the amount of transfer of electrons depends on EA of the target gas molecule with the base material. Moreover, EA applies to the electronically conducting solid base material where it can be related to the position of Fermi energy level and the work function. If the base material has a high value of work function, it will in turn influence EA and it will act as electron acceptor and vice versa. Thus, the base material, which has low value of EA will partially transfer electrons between the target gas molecules and the conduction band of metal oxides. In the chemiresistor type of gas sensors, it is only due to the transfer of electrons that the change in the resistance is observed. However, in the present work EA for pristine and impurity substituted MoO3 nanostructures, all have EA value of 6 to 8.26 eV. Furthermore, it can be suggested that MoO3 nanostructures can be used as a chemical sensor.

3.2 Adsorption characteristics of NH3 on $\alpha$-MoO3 nanostructures

Before studying the adsorption properties of NH3 on MoO3 nanostructures, NH3 should be investigated in gas phase. The optimized bond length between nitrogen and hydrogen atom in NH3 is 1.01 Å and the bond length between molybdenum and oxygen atoms in $\alpha$-MoO3 is 1.77 Å. These bond lengths are used during the optimization of $\alpha$-MoO3 nanostructures. Figure 7 refers to the adsorption of NH3 gas molecule adsorbed on different sites, namely position A, B, C and D in pristine $\alpha$-MoO3 nanostructure. Figures 8–10 represent the adsorption of ammonia gas molecules adsorbed on various sites such as position E, F, G, H, I and J in Ti, Zr and F substituted $\alpha$-MoO3 nanostructure.

The adsorption energy of NH3 gas molecules on $\alpha$-MoO3 nanostructure can be calculated by the equation (3.2) as follows:

[TABLE]

where $E$(MoO3/NH3) denotes the energy of MoO3/NH3 complex, $E$(MoO3) and $E$(NH3) are the isolated energies of MoO3 and NH3 molecules, respectively. The basis set superposition error (BSSE) [50] can be analyzed in terms of counterpoise technique to eliminate the overlap effects on basis functions. $\alpha$-MoO3 base material is found to have a negative value of energy which confirms the stability of the base material. Moreover, when NH3 gas molecules are adsorbed on $\alpha$-MoO3 nanostructures, negative values of the adsorption energy ($E_{\text{ad}}$) refer to the more stable system [51, 52].

In the present work, positions B to J all have negative values of adsorption energy. This infers that $\alpha$-MoO3 nanostructures are more stable and it is most suitable for gas sensor and as catalyst. The adsorption energies of pristine $\alpha$-MoO3 for positions A, B, C and D are 1.09, $-$2.45, $-$8.16 and $-$11.7 eV, respectively. The corresponding $E_{\text{ad}}$ values of Ti incorporated $\alpha$-MoO3 for positions E and F are $-$0.27 and $-$2.72 eV. Zr substituted $\alpha$-MoO3 for positions G and H have $E_{\text{ad}}$ values of $-$1.09 and $-$3.26 eV, respectively. F substituted $\alpha$-MoO3 for positions I and J has adsorption energy values of $-$14.42 and $-$9.52 eV, respectively. The other significant parameter to decide NH3 adsorption characteristics is the band gap of $\alpha$-MoO3 nanostructure. Conductivity of $\alpha$-MoO3 nanostructure increases due to a decrease in the band gap, when NH3 gets adsorbed on positions A, C, D, E, G and H of $\alpha$-MoO3 nanostructures, the corresponding band gap values are 4.57, 3.2, 1.87, 1.35, 1.86 and 2.12 eV, which are lower than their corresponding isolated counterpart. By contrast, conductivity of $\alpha$-MoO3 nanostructures decreases owing to an increase in the band gap, when NH3 gets adsorbed on positions B, F, I and J with energy gap values of 4.71, 2.13, 2.71 and 3.47 eV, respectively, which are higher than their isolated counterpart. From the observations, it is revealed that the adsorption energy and the energy gap values change due to the adsorption of NH3 in $\alpha$-MoO3 nanostructure.

Prasad et al. [53] reported the gas-sensing properties of MoO3 with ammonia gas. Imawan et al. [19] proposed the gas-sensing properties of modified MoO3 thin films using Ti-overlayers for NH3 gas sensors. With the influence of Ti-overlayers in MoO3 material, selectivity and sensitivity of NH3 gas molecules get enhanced. The response of NH3 mainly depends upon temperature. Furthermore, to improve the adsorption of NH3 on MoO3, the activation energy is required. The high operating temperature of molybdenum oxide enhances the catalytic activity. Therefore, the temperature dependent NH3 gas sensitivity is possible in MoO3 nanostructures. Based on the condition of the chemisorption mechanism, MoO3 material shows good response to NH3 gas molecules among other reducing gases such as H2, CO and SO2 at operating temperature of 200°C. Upon exposure of NH3 gas molecules to molybdenum oxides, the resistance value changes leading to the detection of ammonia. The results obtained in the present work are for the ambient condition. Usually, by incorporating impurities or functionalization of the base material with Pd, Pt will decrease the operating temperature of metal oxide during gas sensing. Moreover, in the present study, the conductivity of $\alpha$-MoO3 material also varies due to the transfer of electrons between NH3 gas molecules and MoO3 nanostructure. This validates the present work with the reported work.

In this work, the gas sensing properties of $\alpha$-MoO3 nanostructures are studied with the substitution of Ti, Zr and F impurities. From the observation, it is revealed that the pristine, Ti, Zr and F incorporated $\alpha$-MoO3 nanostructures are a promising material for sensing ammonia. The most favorable adsorption site of the NH3 molecule on $\alpha$-MoO3 material can be concluded only after investigating the variation in the average energy gap ($E_{\text{g}}^{\text{a}}$) along with its respective isolated counterpart [54, 55]. Table 2 refers the HOMO-LUMO gap, percentage variation in energy gap, adsorption energy and Mulliken population. As a result, it is inferred that the most suitable site for adsorption of NH3 molecules on $\alpha$-MoO3 nanostructures are positions C, D, E, G, I and J. The adsorption of N atom in NH3 adsorbed on O, Ti, Zr and F atoms of pristine, Ti, Zr and F substituted $\alpha$-MoO3 nanostructure and the adsorption of H atom in NH3 adsorbed on O and F atom of pristine and F substituted $\alpha$-MoO3 nanostructure are observed to be a more prominent adsorption site. The average energy gap variation is comparatively high among other adsorption sites.

The transfer of electrons between ammonia gas molecules and $\alpha$-MoO3 can also be analyzed with the help of Mulliken population analysis ($Q$) [56, 57, 58]. The negative charge of Mulliken population shows that the electrons are transferred from $\alpha$-MoO3 material to NH3 gas molecules while the positive value of $Q$ shows that the electrons are transferred from NH3 gases to $\alpha$-MoO3 base material. In the present work, all positions have positive value of $Q$, which infers that the electrons are transferred from NH3 molecules to $\alpha$-MoO3 nanostructure. The $Q$ values of pristine $\alpha$-MoO3 nanostructure for positions A, B, C and D are 0.3, 0.11, 0.93 and 0.56 e, respectively. The corresponding Mulliken charge transfer values of Ti, Zr and F incorporated $\alpha$-MoO3 nanostructure for positions E, F, G, H, I and J are 0.19, 0.10, 0.13, 0.08, 1.14 and 0.67 e. As a result, it is observed that the high value of Mulliken charge transfer, average energy gap variation and adsorption energy are found to be more prominent for positions C, D, E, G, I and J. Conductivity of $\alpha$-MoO3 nanostructure for positions A, C, D, E, G and H increases owing to the narrowing of the energy gap. The remaining positions such as, B, F, I and J have less conductivity compared to the isolated counterpart. However, the conductivity decreases for positions I and J, the average energy gap variations are observed to be high, which is favorable for a good gas sensor. Therefore, the most favorable adsorption sites can be found only after investigating the adsorption energy, Mulliken population and HOMO-LUMO gap of $\alpha$-MoO3 nanostructure. Figures 11–20 represent the density of states spectrum and HOMO-LUMO gap of position A–J of $\alpha$-MoO3 nanostructures, respectively. Below the DOS spectrum, the green and red line indicates the HOMO-LUMO gap. From the DOS spectrum, it is clearly revealed that more peak maxima are observed in unoccupied orbital than in occupied orbital.

This implies that the electrons can easily transfer between $\alpha$-MoO3 and NH3 gas molecules. Among all the optimum positions of $\alpha$-MoO3 nanostructure, positions C, D, E and J have more peak maximum in unoccupied orbital. Usually, in metal oxide based chemiresistors, which are used for gas/vapour sensing, the exchange of electrons takes place between the target gas and the base material. In the present study, a larger number of peak maxima is observed in the unoccupied orbital, which confirms the transfer of electrons between NH3 target gas and $\alpha$-MoO3 base material for different positions (C, D, E and J). The alpha orbital and beta orbital exist for positions I and J, which arise due to spin up and down electrons due to orbital overlapping of F atoms with $\alpha$-MoO3 nanostructure. This further strengthens NH3 adsorption properties on $\alpha$-MoO3 nanostructure, when substituted with F in $\alpha$-MoO3 base material. Hence, the variation of the band gap leads to the change in the resistance of $\alpha$-MoO3 nanostructure, which can be measured with a simple two probe arrangement. Analyzing all the aspects, it can be concluded that $\alpha$-MoO3 can be used as NH3 gas sensing material in the mixed gas atmosphere.

4 Conclusions

To sum up, DFT method is employed to study NH3 adsorption properties on $\alpha$-MoO3 material with B3LYP/LanL2DZ basis set. The electronic and structural stability of pristine, Ti, Zr and F substituted $\alpha$-MoO3 nanostructure has been investigated. The structural stability of all $\alpha$-MoO3 nanostructures has also been investigated using the formation energy. The electronic properties of $\alpha$-MoO3 nanostructures are discussed in terms of HOMO-LUMO gap, ionization potential and electron affinity. The dipole moment and the point group of pristine, Ti, Zr and F substituted $\alpha$-MoO3 nanostructures are also reported. The most prominent adsorption site of NH3 on $\alpha$-MoO3 nanostructure is identified and discussed in terms of adsorption energy, average energy gap variation, Mulliken population analysis and HOMO-LUMO gap. Furthermore, the negative values of adsorption energy for the positions B to J confirm the stability of $\alpha$-MoO3 nanostructure upon adsorption of NH3 molecules. The adsorption characteristics of NH3 gas molecules get modified with the incorporation of Ti and F as impurities in $\alpha$-MoO3 base material. Moreover, there is no significant improvement in NH3 adsorption properties on $\alpha$-MoO3 material with Zr substitution. In conclusion, the most favorable adsorption site of NH3 on $\alpha$-MoO3 nanostructures is when the N atom in NH3 adsorbed on O, Ti, Zr and F atoms of pristine, Ti, Zr and F substituted $\alpha$-MoO3 nanostructure and the adsorption of H atom in NH3 adsorbed on the O and F atoms of pristine and F substituted $\alpha$-MoO3 nanostructure. Thus, $\alpha$-MoO3 nanostructure can be used as a good NH3 gas sensor. In addition, the sensing characteristics of NH3 can be modified with the incorporation of Ti and F as dopant in $\alpha$-MoO3 nanostructures.