Interaction of NH₃ gas on α-MoO₃ nanostructures — a DFT investigation
The structural stability, electronic properties and NH₃ adsorption properties of pristine, Ti, Zr and F substituted α-MoO₃ nanostructures are successfully studied using density functional theory with B3LYP/LanL2DZ basis set. The structural stability of α-MoO₃ nanostructures is discussed in terms o...
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irk-123456789-1569992019-06-20T01:27:58Z Interaction of NH₃ gas on α-MoO₃ nanostructures — a DFT investigation Nagarajan, V. Chandiramouli, R. The structural stability, electronic properties and NH₃ adsorption properties of pristine, Ti, Zr and F substituted α-MoO₃ nanostructures are successfully studied using density functional theory with B3LYP/LanL2DZ basis set. The structural stability of α-MoO₃ nanostructures is discussed in terms of formation energy. The electronic properties of pristine, Ti, Zr and F incorporated α-MoO₃ nanostructures are discussed in terms of HOMO-LUMO gap, ionization potential and electron affinity. α-MoO₃ nanostructures can be fine-tuned with suitable substitution impurity to improve the adsorption characteristics of ammonia, which can be used to detect NH3 in a mixed environment. The present work gives an insight into tailoring α-MoO₃ nanostructures for NH₃ detection. Структурна стiйкiсть, електроннi властивостi i NH₃ адсорбцiйнi властивостi первинних, Ti, Zr i F замiщених α-MoO₃ наноструктур успiшно вивченi, використовуючи теорiю функцiоналу густини з B3LYP/ LanL2DZ базисним набором. Структурна стiйкiсть α-MoO₃ наноструктур обговорюється в термiнах енергiї утворювання. Електроннi властивостi первинних, Ti, Zr i F iнкорпорованих α-MoO₃ наноструктур обговорюються в термiнах HOMO-LUMO щiлини, потенцiалу iонiзацiї та електронної афiнностi. α-MoO₃ наноструктури можуть бути точно-регульованi за допомогою пiдходящої домiшки замiщення для покращення адсорбцiйних властивостей амонiяку, що може бути використано для виявлення NH₃ в змiшаному середовищi. Дана робота дає розумiння про застосування α-MoO₃ наноструктур для виявлення NH₃. 2017 Article Interaction of NH₃ gas on α-MoO₃ nanostructures — a DFT investigation / V. Nagarajan, R. Chandiramouli // Condensed Matter Physics. — 2017. — Т. 20, № 2. — С. 23705: 1–16. — Бібліогр.: 58 назв. — англ. 1607-324X PACS: 71.15.Mb DOI:10.5488/CMP.20.23705 arXiv:1706.07305 http://dspace.nbuv.gov.ua/handle/123456789/156999 en Condensed Matter Physics Інститут фізики конденсованих систем НАН України |
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| description |
The structural stability, electronic properties and NH₃ adsorption properties of pristine, Ti, Zr and F substituted
α-MoO₃ nanostructures are successfully studied using density functional theory with B3LYP/LanL2DZ basis set.
The structural stability of α-MoO₃ nanostructures is discussed in terms of formation energy. The electronic
properties of pristine, Ti, Zr and F incorporated α-MoO₃ nanostructures are discussed in terms of HOMO-LUMO
gap, ionization potential and electron affinity. α-MoO₃ nanostructures can be fine-tuned with suitable substitution impurity to improve the adsorption characteristics of ammonia, which can be used to detect NH3 in a
mixed environment. The present work gives an insight into tailoring α-MoO₃ nanostructures for NH₃ detection. |
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Nagarajan, V. Chandiramouli, R. |
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Nagarajan, V. Chandiramouli, R. Interaction of NH₃ gas on α-MoO₃ nanostructures — a DFT investigation Condensed Matter Physics |
| author_facet |
Nagarajan, V. Chandiramouli, R. |
| author_sort |
Nagarajan, V. |
| title |
Interaction of NH₃ gas on α-MoO₃ nanostructures — a DFT investigation |
| title_short |
Interaction of NH₃ gas on α-MoO₃ nanostructures — a DFT investigation |
| title_full |
Interaction of NH₃ gas on α-MoO₃ nanostructures — a DFT investigation |
| title_fullStr |
Interaction of NH₃ gas on α-MoO₃ nanostructures — a DFT investigation |
| title_full_unstemmed |
Interaction of NH₃ gas on α-MoO₃ nanostructures — a DFT investigation |
| title_sort |
interaction of nh₃ gas on α-moo₃ nanostructures — a dft investigation |
| publisher |
Інститут фізики конденсованих систем НАН України |
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2017 |
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http://dspace.nbuv.gov.ua/handle/123456789/156999 |
| citation_txt |
Interaction of NH₃ gas on α-MoO₃ nanostructures — a DFT investigation / V. Nagarajan, R. Chandiramouli // Condensed Matter Physics. — 2017. — Т. 20, № 2. — С. 23705: 1–16. — Бібліогр.: 58 назв. — англ. |
| series |
Condensed Matter Physics |
| work_keys_str_mv |
AT nagarajanv interactionofnh3gasonamoo3nanostructuresadftinvestigation AT chandiramoulir interactionofnh3gasonamoo3nanostructuresadftinvestigation |
| first_indexed |
2025-07-14T09:20:56Z |
| last_indexed |
2025-07-14T09:20:56Z |
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1837613560852643840 |
| fulltext |
Condensed Matter Physics, 2017, Vol. 20, No 2, 23705: 1–16
DOI: 10.5488/CMP.20.23705
http://www.icmp.lviv.ua/journal
Interaction of NH3 gas on α-MoO3 nanostructures—
a DFT investigation
V. Nagarajan, R. Chandiramouli∗
School of Electrical and Electronics Engineering, Shanmugha Arts Science Technology and Research Academy
(SASTRA) University, Tirumalaisamudram, Thanjavur, Tamil nadu— 613 401, India
Received February 21, 2017, in final form April 18, 2017
The structural stability, electronic properties and NH3 adsorption properties of pristine, Ti, Zr and F substituted
α-MoO3 nanostructures are successfully studied using density functional theory with B3LYP/LanL2DZ basis set.The structural stability of α-MoO3 nanostructures is discussed in terms of formation energy. The electronicproperties of pristine, Ti, Zr and F incorporated α-MoO3 nanostructures are discussed in terms of HOMO-LUMOgap, ionization potential and electron affinity. α-MoO3 nanostructures can be fine-tuned with suitable substi-tution impurity to improve the adsorption characteristics of ammonia, which can be used to detect NH3 in amixed environment. The present work gives an insight into tailoring α-MoO3 nanostructures for NH3 detection.
Key words: nanostructure, adsorption, NH3, HOMO-LUMO gap, MoO3
PACS: 71.15.Mb
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 ammo-
nia 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 [8–10].
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 [12–15].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 α-MoO3, meta-stable
monoclinic β-MoO3, hexagonal β-MoO3. Kannan et al. [26] have reported the influence of the precursor
∗Corresponding author
This work is licensed under a Creative Commons Attribution 4.0 International License . Further distribution
of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
23705-1
https://doi.org/10.5488/CMP.20.23705
http://www.icmp.lviv.ua/journal
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V. Nagarajan, R. Chandiramouli
solution volume on the properties of spray deposited α-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 α-
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 [29–31]. Based on these aspects, literature survey was
conducted using CrossRef metadata search and it is inferred that not much work was reported based on
DFTmethods to investigate the adsorption properties of NH3 on α-MoO3 nanostructures. The motivation
behind the present work is to improve the NH3 adsorption properties on α-MoO3 nanostructures with
the incorporation of dopants. The novel aspect of this work is to study the adsorption characteristics of
NH3 on α-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
α-MoO3 nanostructures. This package is also used to investigate the adsorption properties of NH3
gas molecules on α-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 [33–36]. The selection of a suitable
basis set is an important criterion for optimizing α-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, α-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 α-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 α-MoO3 with pseudopotential approximation [37–39]. The
HOMO-LUMO gap and density of states spectrum (DOS) of α-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 α-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 α-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 α-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
23705-2
Investigation on α-MoO3 nanostructures
Figure 1. (Color online) (a) Pristine, (b) Ti substituted, (c) Zr substituted and (d) F substituted α-MoO3
nanostructure.
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, α-MoO3 nanostructures get distorted, while optimizing the basematerial with Ti, Zr and F elements
as dopant, which results in the variation of electronic properties. Moreover, the conductivity of α-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 α-MoO3 base material and these impurities tune the conducting property
in α-MoO3 base material. Therefore, the adsorption properties of NH3 on α-MoO3 nanostructures can
be improved with the substitution impurities.
Figure 1 represents the structure of pristine, Ti, Zr and F substituted α-MoO3 nanostructures, respec-
tively. The structure of α-MoO3 is taken from International Centre for Diffraction Data (ICDD) card
number: 89-7112. The pristine α-MoO3 nanostructure has twelve Mo atoms and thirty eight O atoms.
Ti incorporated α-MoO3 nanostructure has eleven Mo atoms, thirty eight O atoms and one Mo atom is
replaced with one Ti atom. Similarly, Zr substituted α-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 α-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 α-MoO3 base material.
3.1. Structural stability and electronic properties of α-MoO3 nanostructures
The structural stability of pristine, Ti, Zr and F substituted α-MoO3 nanostructures can be described
in terms of formation energy as shown in equation (3.1)
Eform = 1/n[E(α-MoO3 nanostructure) − pE(Mo) − qE(O) − rE(dopant)], (3.1)
where E(α-MoO3 nanostructures) refers to the total energy of α-MoO3 nanostructures, E(Mo), E(O)
and E(dopant) represent the corresponding energy of isolated Mo, O and dopant atoms namely Ti, Zr
23705-3
V. Nagarajan, R. Chandiramouli
Table 1. Formation energy, dipole moment and point group of α-MoO3 nanostructures.
Nanostructures Formation energy Dipole moment Point group
(eV) (D)
Pristine α-MoO3 nanostructure −4.38 5.62 C1
Ti substituted α-MoO3 nanostructure −4.36 19.18 C1
Zr substituted α-MoO3 nanostructure −4.30 35.08 C1
F substituted α-MoO3 nanostructure −4.40 40.47 C1
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 α-MoO3 nanostructure. The dipole moment, point symmetry and the formation
energy of pristine, Ti, Zr and F substituted α-MoO3 nanostructures are tabulated in table 1. The formation
energy of pristine, Ti, Zr and F substituted α-MoO3 nanostructures is −4.38, −4.36, −4.30 and −4.40 eV,
respectively.
Before studying the adsorption characteristics, the structural stability of α-MoO3 base material must
be studied. The structural stability of α-MoO3 nanostructures slightly decreases with the substitution
of Ti and Zr. The formation energy of Ti and Zr substituted α-MoO3 nanostructures is relatively low
compared to pristine α-MoO3 nanostructures. By contrast, the stability of α-MoO3 nanostructure slightly
increases with the substitution of F. The dipole moment (DP) gives a clear picture about the distribution
of charges in α-MoO3 nanostructure. The corresponding dipole moment value of pristine, Ti, Zr and F
incorporated α-MoO3 nanostructures is 5.62, 19.18, 35.08 and 40.47 D. Low value of DP is recorded
for pristine α-MoO3 nanostructure. It infers that the charge distribution is almost uniform, but not in the
case of impurity substituted α-MoO3 nanostructures. Furthermore, C1 point groups are observed for all
α-MoO3 nanostructures, which only exhibit identical operation.
The electronic properties of pristine, Ti, Zr and F incorporated α-MoO3 nanostructures can be dis-
cussed 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 α-MoO3 nanostructures are studied for a small cluster. The HOMO-LUMO gap of pristine, Ti, Zr and
F substituted α-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 po-
tential between the excited electronic states may be underestimated. The HOMO-LUMO gap of α-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 α-MoO3 nanostructures. The HOMO-LUMO level and the energy gap of α-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 α-MoO3 nanostructures. The DOS spectrum and visualization of HOMO-LUMO gap
of pristine, Ti, Zr and F substituted α-MoO3 nanostructure are shown in figures 2–5, respectively. In the
present work, the localization of charges is recorded to bemore at LUMO level than at HOMO level, which
is confirmed by more peak maxima at LUMO level. More peak maxima in α-MoO3 nanostructures arise
due to the orbital overlapping of Mo atoms and O atoms in α-MoO3 base material. Moreover, the peak
maxima in the virtual orbitals of α-MoO3 base material are more favorable for adsorption characteristics,
since the transfer of electrons between NH3 molecules and virtual orbitals can take place easily.
23705-4
Investigation on α-MoO3 nanostructures
Table 2. Adsorption energy, Mulliken population, HOMO-LUMO gap and average energy gap variation
of α-MoO3 nanostructures.
α-MoO3 nanostructures Ead Q (e) EHOMO EFL (eV) ELUMO Eg (eV) Ea
g (%)
Pristine α-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 α-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 α-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 α-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
Figure 2. (Color online) HOMO-LUMO gap and density of states of pristine α-MoO3 nanostructure.
The electronic properties of α-MoO3 nanostructure can also be discussed with the ionization po-
tential (IP) and electron affinity (EA) [48, 49]. Figure 6 represents the electron affinity and ionization
potential of α-MoO3 nanostructures. Generally, IP depicts the amount of energy required to remove the
electron from α-MoO3 nanostructures and the EA represents the energy change with the addition of
electrons in α-MoO3 nanostructures. The high value of ionization potential infers that the electrons are
tightly bounded to the nucleus in α-MoO3 nanostructure. Therefore, pristine, Ti and Zr substituted α-
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 α-MoO3 nanostructure.
The low value of IP is recorded for F substituted α-MoO3 nanostructure. Thus, less energy is required to
remove an electron from F substituted α-MoO3 nanostructure. Electron affinity plays an important role
in plasma physics and in chemical sensors.
23705-5
V. Nagarajan, R. Chandiramouli
Figure 3. (Color online) HOMO-LUMOgap and density of states of Ti substituted α-MoO3 nanostructure.
Figure 4. (Color online)HOMO-LUMOgap and density of states of Zr substitutedα-MoO3 nanostructure.
Figure 5. (Color online) HOMO-LUMO gap and density of states of F substituted α-MoO3 nanostructure.
23705-6
Investigation on α-MoO3 nanostructures
Figure 6. (Color online) IP and EA of α-MoO3 nanostructure.
The EA value of pristine, Ti, Zr and F incorporated α-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 α-MoO3 nanostructures
Before studying the adsorption properties of NH3 on MoO3 nanostructures, NH3 should be investi-
gated 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 α-MoO3 is 1.77 Å. These bond lengths
are used during the optimization of α-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 α-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 α-MoO3 nanostructure.
The adsorption energy of NH3 gas molecules on α-MoO3 nanostructure can be calculated by the
equation (3.2) as follows:
Ead = [E(MoO3) + E(NH3) − E(MoO3/NH3) + E(BSSE)], (3.2)
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. α-
MoO3 base material is found to have a negative value of energy which confirms the stability of the base
23705-7
V. Nagarajan, R. Chandiramouli
Figure 7. (Color online) NH3 adsorbed on position A, B, C and D in pristine MoO3 nanostructure.
Figure 8. (Color online) NH3 adsorbed on position E and F in Ti substituted MoO3 nanostructure.
material. Moreover, when NH3 gas molecules are adsorbed on α-MoO3 nanostructures, negative values
of the adsorption energy (Ead) 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 α-MoO3 nanostructures are more stable and it is most suitable for gas sensor and as catalyst. The
adsorption energies of pristine α-MoO3 for positions A, B, C and D are 1.09, −2.45, −8.16 and −11.7 eV,
respectively. The corresponding Ead values of Ti incorporated α-MoO3 for positions E and F are −0.27
and −2.72 eV. Zr substituted α-MoO3 for positions G and H have Ead values of −1.09 and −3.26 eV,
respectively. F substituted α-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 α-MoO3 nanostructure. Conductivity of α-MoO3 nanostructure increases due to a decrease
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Investigation on α-MoO3 nanostructures
Figure 9. (Color online) NH3 adsorbed on position G and H in Zr substituted MoO3 nanostructure.
Figure 10. (Color online) NH3 adsorbed on position I and J in F substituted MoO3 nanostructure.
in the band gap, when NH3 gets adsorbed on positions A, C, D, E, G and H of α-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 α-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 α-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 α-MoO3 material also varies
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V. Nagarajan, R. Chandiramouli
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 α-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
α-MoO3 nanostructures are a promising material for sensing ammonia. The most favorable adsorption
site of the NH3 molecule on α-MoO3 material can be concluded only after investigating the variation
in the average energy gap (Ea
g) along with its respective isolated counterpart [54, 55]. Table 2 refers the
HOMO-LUMOgap, percentage variation in energy gap, adsorption energy andMulliken population. As a
result, it is inferred that the most suitable site for adsorption of NH3 molecules on α-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 α-MoO3 nanostructure and the adsorption of H atom in NH3 adsorbed
on O and F atom of pristine and F substituted α-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 α-MoO3 can also be analyzed with
the help of Mulliken population analysis (Q) [56–58]. The negative charge of Mulliken population shows
that the electrons are transferred from α-MoO3 material to NH3 gas molecules while the positive value of
Q shows that the electrons are transferred from NH3 gases to α-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 α-MoO3 nanostructure. The Q values of pristine α-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 α-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 ofα-MoO3 nanostructure for positionsA, C, D, E, G andH 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-LUMOgap of α-MoO3 nanostructure. Figures 11–20 represent the density of states spectrum and
HOMO-LUMO gap of position A–J of α-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 α-MoO3 and NH3 gas molecules. Among
all the optimum positions of α-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,
Figure 11. (Color online) HOMO-LUMO gap and density of states of position A.
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Figure 12. (Color online) HOMO-LUMO gap and density of states of position B.
Figure 13. (Color online) HOMO-LUMO gap and density of states of position C.
Figure 14. (Color online) HOMO-LUMO gap and density of states of position D.
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V. Nagarajan, R. Chandiramouli
Figure 15. (Color online) HOMO-LUMO gap and density of states of position E.
Figure 16. (Color online) HOMO-LUMO gap and density of states of position F.
Figure 17. (Color online) HOMO-LUMO gap and density of states of position G.
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Investigation on α-MoO3 nanostructures
Figure 18. (Color online) HOMO-LUMO gap and density of states of position H.
Figure 19. (Color online) HOMO-LUMO gap and density of states of position I.
Figure 20. (Color online) HOMO-LUMO gap and density of states of position J.
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V. Nagarajan, R. Chandiramouli
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 α-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 α-MoO3 nanostructure. This further strengthens NH3 adsorption
properties on α-MoO3 nanostructure, when substituted with F in α-MoO3 base material. Hence, the
variation of the band gap leads to the change in the resistance of α-MoO3 nanostructure, which can
be measured with a simple two probe arrangement. Analyzing all the aspects, it can be concluded that
α-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 α-MoO3 material with
B3LYP/LanL2DZ basis set. The electronic and structural stability of pristine, Ti, Zr and F substituted α-
MoO3 nanostructure has been investigated. The structural stability of all α-MoO3 nanostructures has also
been investigated using the formation energy. The electronic properties of α-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 α-MoO3 nanostructures are also reported. The
most prominent adsorption site of NH3 on α-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 α-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 α-MoO3 base material.
Moreover, there is no significant improvement in NH3 adsorption properties on α-MoO3 material with
Zr substitution. In conclusion, the most favorable adsorption site of NH3 on α-MoO3 nanostructures is
when the N atom in NH3 adsorbed on O, Ti, Zr and F atoms of pristine, Ti, Zr and F substituted α-MoO3
nanostructure and the adsorption of H atom in NH3 adsorbed on the O and F atoms of pristine and F
substituted α-MoO3 nanostructure. Thus, α-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 α-MoO3 nanostructures.
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Взаємодiя газу NH3 на наноструктурах α-MoO3 —
дослiдження за допомогою теорiї функцiоналу густини
В. Нагараджан, Р. Чандiрамулi
Школа електротехнiки та електронiки, Академiя мистецтв, наукових i технологiчних дослiджень Шанмуга
(унiверситет SASTRA), Танджавур, Тамiл-Наду— 613 401, Iндiя
Структурна стiйкiсть, електроннi властивостi i NH3 адсорбцiйнi властивостi первинних, Ti, Zr i F замiщених
α-MoO3 наноструктур успiшно вивченi, використовуючи теорiю функцiоналу густини з B3LYP/ LanL2DZ
базисним набором. Структурна стiйкiсть α-MoO3 наноструктур обговорюється в термiнах енергiї утворю-вання. Електроннi властивостi первинних, Ti, Zr i F iнкорпорованих α-MoO3 наноструктур обговорюютьсяв термiнах HOMO-LUMO щiлини, потенцiалу iонiзацiї та електронної афiнностi. α-MoO3 наноструктури
можуть бути точно-регульованi за допомогою пiдходящої домiшки замiщення для покращення адсорб-
цiйних властивостей амонiяку,що може бути використано для виявлення NH3 в змiшаному середовищi.Дана робота дає розумiння про застосування α-MoO3 наноструктур для виявлення NH3.
Ключовi слова: наноструктура, адсорбцiя, NH3, HOMO-LUMOщiлина, MoO3
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Introduction
Computational methods
Results and discussion
Structural stability and electronic properties of -MoO3 nanostructures
Adsorption characteristics of NH3 on -MoO3 nanostructures
Conclusions
|