Interdigitated electrodes-based Au-MoS2 hybrid gas sensor for sensing toxic CO and NH3 gases at Room temperature

Abstract

In the quest to create effective sensors that operate at room temperature, consume less power and maintain their stability over time for detecting toxic gases in the environment, molybdenum disulfide (MoS2) and MoS2-based hybrids have emerged as potent materials. In this context, the current work describes the fabrication of Au-MoS2 hybrid gas sensor fabricated on gold interdigitated electrodes (GIEs) for sensing harmful CO and NH3 gases at room temperature. The GIEs-based Au-MoS2 hybrid sensors are fabricated by decorating MoS2 nanoflowers (MNF) with varying size of Au nanoparticles using an inert gas evaporation technique. It is observed that by varying the size of Au nanoparticles, the crystallinity gets modified, as confirmed by X-ray diffraction (XRD) and Micro-Raman spectroscopy (μRS). The gas sensing measurements revealed that the best sensing response is found from the Au-MoS2 hybrid (with an average particle size of 10 nm). This particular hybrid shows a 79% response to CO exposure and a 69% response to NH3 exposure. The measurements are about 3.5 and 5 times higher than the bare MoS2 when exposed to CO and NH3 at room temperature, respectively. This enhancement in sensing response is attributed to the modified interfacial interaction between the Au nanoparticles and MNF gets improved, which leads to the formation of a Schottky barrier, as confirmed using X-ray photoelectron spectroscopy (XPS) and Ultraviolet photoelectron spectroscopy (UPS) analysis. This enables the development of efficient gas sensors that respond quickly to changes in the gas around them.

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Nanotechnology
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Interdigitated electrodes-based Au-MoS2 hybrid
gas sensor for sensing toxic CO and NH3 gases at
room temperature
To cite this article: Saurabh Rawat
et al
2023
Nanotechnology
34 305601
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Interdigitated electrodes-based Au-MoS
2
hybrid gas sensor for sensing toxic CO and
NH
3
gases at room temperature
Saurabh Rawat
1
, Priyanka Bamola
1
, Chanchal Rani
2
, Vishakha Kaushik
3
,
Ujjwal Kumar
4
, Charu Dwivedi
5
, Rekha Rattan
6
, Mohit Sharma
7
,
Rajesh Kumar
2
and Himani Sharma
1,∗
1
Functional Nanomaterials Research Laboratory, Department of Physics, Doon University, Dehradun,
Uttarakhand 248001, India
2
Materials and Device Laboratory, Department of Physics, Indian Institute of Technology Indore, Indore
453552, India
3
Department of Physics, DIT University, Dehradun 248001, India
4
School of Environment and Natural Resource, Doon University, Dehradun, Uttarakhand 248001, India
5
Department of Chemistry, School of Physical Sciences, Doon University, Dehradun- 248001, India
6
Department of Prosthodontics Saveetha Dental College and Hospitals, SIMATS, Chennai-60077, India
7
Institute of Materials Research and Engineering, A
*
STAR (Agency for Science, Technology and
Research), 138634, Singapore
E-mail: himanitiet427@gmail.com
Received 12 January 2023, revised 23 April 2023
Accepted for publication 26 April 2023
Published 12 May 2023
Abstract
In the quest to create effective sensors that operate at room temperature, consume less power and
maintain their stability over time for detecting toxic gases in the environment, molybdenum
disulfide (MoS
2
)and MoS
2
-based hybrids have emerged as potent materials. In this context, the
current work describes the fabrication of Au-MoS
2
hybrid gas sensor fabricated on gold
interdigitated electrodes (GIEs)for sensing harmful CO and NH
3
gases at room temperature. The
GIEs-based Au-MoS
2
hybrid sensors are fabricated by decorating MoS
2
nanoflowers (MNF)
with varying size of Au nanoparticles using an inert gas evaporation technique. It is observed
that by varying the size of Au nanoparticles, the crystallinity gets modified, as confirmed by
x-ray diffraction and Micro-Raman spectroscopy (μRS). The gas sensing measurements revealed
that the best sensing response is found from the Au-MoS
2
hybrid (with an average particle size
of 10 nm). This particular hybrid shows a 79% response to CO exposure and a 69% response to
NH
3
exposure. The measurements are about 3.5 and 5 times higher than the bare MoS
2
when
exposed to CO and NH
3
at room temperature, respectively. This enhancement in sensing
response is attributed to the modified interfacial interaction between the Au nanoparticles and
MNF gets improved, which leads to the formation of a Schottky barrier, as confirmed using x-ray
photoelectron spectroscopy and ultraviolet photoelectron spectroscopy analysis. This enables the
development of efficient gas sensors that respond quickly to changes in the gas around them.
Keywords: 2D materials, gold interdigitated electrodes, gas sensors for toxic gases, MoS
2
, room
temperature sensing, transition metal dichalcogenides (TMDs)
(Some figures may appear in colour only in the online journal)
Nanotechnology
Nanotechnology 34 (2023)305601 (14pp)https://doi.org/10.1088/1361-6528/acd0b7
∗
Author to whom any correspondence should be addressed.
0957-4484/23/305601+14$33.00 Printed in the UK © 2023 IOP Publishing Ltd1

1. Introduction
In recent times, everyone desires to live in a clean environ-
ment, therefore, there is an increasing demand for technolo-
gies that detect the number of polluting gases emitted by
combustion processes [1]. These activities have resulted from
a contaminated atmosphere that comprises a variety of
harmful gases, including NH
3
,CO
2
, and NO
2
, and among
various gases, CO and NH
3
are discovered as the most
harmful gases found in the atmosphere [2]. Consequently, it is
essential to identify them to protect our environment as well
as human health. Carbon monoxide (CO)is a poisonous gas
that cannot be identified by conventional gas sensors since it
has no taste, colour, odour, or other distinguishing char-
acteristics [3]. Humans may have some symptoms such as
headaches, nausea, vomiting, and cardiac difficulties as a
result of inhaling CO gas. Moreover, every year, human
activities including the operation of chemical factories, the
burning of fossil fuels from cars, the manufacturing of ferti-
lizer, and the use of refrigeration systems release a significant
amount of NH
3
into the surroundings [4]. If the human body
is exposed to NH
3
for an extended period at a high con-
centration, the skin and lungs of humans can suffer from some
serious diseases [5]. This means that new sensor systems need
to be made that can measure CO and NH
3
gas concentrations
in real-time, as well as provide some accurate output signal
depending upon the concentration of the polluting gas [6,7].
Gas sensors that can be used in a variety of applications
should be small and power-efficient so that they can be
integrated into a variety of systems, as well as highly sensi-
tive, selective, and stable [8,9].
At the current time, the nanostructures made of transition
metal dichalcogenides (TMDc)play an essential role in the
field of gas sensing applications due to their higher sensitivity
towards selective gases [10]. Among them, MoS
2
-based gas
sensors with flower-like morphology [11–13]are more ben-
eficial due to their large surface area, which provides more
adsorption sites for the gas molecules and facilitates high ion-
charge carrier mobility for improved sensitivity. For this
reason, sensors based on MoS
2
perform exceptionally well at
detecting toxic gases such as CO [14],NH
3
[15],NO
2
[16],
etc. Additionally, it was discovered that the sensing response
of single-layer MoS
2
gas sensors was not stable enough [17].
Therefore, it is vital to develop stable MoS
2
nanoflowers
based gas sensors. In order to get appropriate gas sensing
performances, not only with stable response, but also with
high sensitivity, and fast recovery response speed. Multiple
studies on functionalization via metal decoration and struc-
tural modification have also been reported to enhance the
performance of the MoS
2
-based gas sensor [18]. Especially,
gas sensing using metal decoration on MoS
2
can open an
avenue for gas detection. for example, Yan et al [19]decorate
MoS
2
nanostructures with Au nanoparticles and suggested
that hierarchical MoS
2
nanostructures with Au nanoparticles
may make the advanced low-temperature gas sensor, also
Kusuma et al [20], demonstrated high entropy alloys for
hydrogen sensing by decorating p-type MoS
2
with multi-
component such as Ag, Au, Cu, Pd, and Pt nanoparticles.
Multicomponent MoS
2
exhibits superior selectivity and rapid
responsiveness to hydrogen gas. The fact that the interaction
between Au and high-entropy alloy-coated MoS
2
is non-
ohmic suggests that MoS
2
could have better gas-sensing
properties. Related to this Wang et al [21]successfully syn-
thesized Au/MoS
2
by hydrothermal method and the gas
sensing properties to three characteristic oil-dissolved gases
(CO, H
2
and CH
4
)with the use of two Au electrodes and four
Pt wires. Recently, MoS
2
nanoflowers adorned with Au
nanoparticles as gas sensors in visible light have been
investigated by P Chen et al [22]. They reported that Au
decorated over MoS
2
improved gas recognition ability via
light-assisted mode at ambient temperature. Which paves the
way for a novel strategy to design high-performance gas
sensors using Au-MoS
2
-based hybrids at room temperature.
However, the variation in size of Au nanoparticles decorated
using the inert gas evaporation method may affect the sensing
properties of MoS
2
, which is not much discussed.
In the present work, Interdigitated electrodes-based
Au-MoS
2
nanoflowers (AMNF)hybrid gas sensor are fabri-
cated for sensing toxic CO and NH
3
gases at room temper-
ature (∼30 °C). The different sized gold nanoparticles,
varying from 2 to 10 nm, are deposited with MoS
2
on gold-
interdigitated electrodes to create Au-MoS
2
hybrid devices
(GIEs). When Au nanoparticles are decorated, a Schottky
barrier forms, which is responsible for improving gas sensors’
response and recovery time [23]. In order to have an efficient
hybrid sensor, the role of humidity during gas sensing mea-
surements is also taken into consideration. This investigation
highlights the impact of different size of Au nanoparticles on
MoS
2
gas-sensing properties at room temperature.
2. Results and discussions
2.1. Structural characterizations
The crystal structure of MoS
2
and Au-MoS
2
hybrids was
confirmed by x-ray diffraction (XRD). Figure 1(a)shows the
XRD diffraction pattern of bare MoS
2
nanoflowers which is
labelled as MNF and Au-decorated MoS
2
nanoflowers
(AMNF)with particle size 2 nm, 5 nm, and 10 nm designates
as AMNF1, AMNF2 and AMNF3 respectively. In these
patterns diffraction peaks are observed at 2θvalues at 14.10°,
28.88°, 39.53°, and 49.99°which are associated with the
lattice planes (002),(004),(103), and (105)respectively of the
hexagonal phase of MoS
2
respectively (JCPDS No.
75–1539). The most intense peak appearing at 14.10 and
39.5°is due to reflecting planes (002)and (103)of MoS
2,
respectively. In the case of Au-MoS
2
hybrids, additional
small peaks at 37.4°appear, which correspond to the (111)
plane of Au (JCPDS: 04–0784)for the samples AMNF2 and
AMNF3. The XRD data showed that the peaks at 14.10°
became sharper with an increase in Au NPs size growth for
AMNF2 and AMNF3, indicating an improvement in the
crystallinity. In order to calculate the crystalline size average
crystalline size from the XRD pattern of MNF, AMNF1,
AMNF2, and AMNF3 samples are compared using Scherrer’s
2
Nanotechnology 34 (2023)305601 S Rawat et al

formula. It is found that the calculated crystallite size
for MNF, AMNF1, AMNF2, and AMNF3 are 20.805 nm,
20.70 nm, 17.44 nm, and 17.35 nm, respectively. The reason
for the decrease in crystalline size is due to the strain induced
over the surface of MoS
2
nanoflower, which causes the
crystal size to decrease as the particle size increases Similar
results are observed in the previous works [24]. Au decorated
MoS
2
with particle size 10 nm (AMNF3)has the smallest
crystal size, which contributes to enhanced gas sensing
properties [25]. Moreover, the intensity of Au-MoS
2
peaks is
very low in samples AMNF2 and AMNF3. However, there is
a much small change in AMNF1 as compared to that of
AMNF2 and AMNF3 because of the small particle size of Au
NPs in AMNF1. The intensity of the XRD peak at 2θ39.53°
decreases with the increase in the size of Au NPs in Au-MoS
2
hybrids while the intensity of another peak at 37.4°originated
due to the increase in particle size of Au in AMNF2, AMNF3,
and with an increase in Au NPs size. Whereas there is no
change in the case of the AMNF1 sample because of the small
particle size. The formation of MoS
2
(MNF)and Au-deco-
rated MoS
2
(AMNF1, AMNF2, AMNF3)hybrids were fur-
ther corroborated with micro-Raman spectroscopy.
Micro Raman spectroscopy (μRS)was employed to
examine the atomic vibrational modes of the bare and Au-
decorated MoS
2
hybrid with different NP sizes at a laser
wavelength of 633 nm. Figure 1(b)shows the Raman spectra
of the bare and Au-MoS
2
hybrid structures. Each spectrum
revealed that the three Raman vibrational modes are posi-
tioned at approximately 377, 405, and 452 cm
−1
. The first
two peaks, E
1
2g,
and A
1g
originated from vibration modes of
hexagonal MoS
2
between Mo and S and S and S atoms,
respectively, and were caused by the in-plane and out-of-
plane symmetry of the molecule. A difference of less than
19 cm
−1
between these two Raman peaks indicates the pre-
sence of a monolayer and a difference of more than 19 cm
−1
indicates the presence of bulk MoS
2
[26]. Here the difference
between E
1
2g
and A
1g
modes of MNF, AMN1, AMNF2, and
AMNF3 is 29, 25.4, 24.8, and 24.6 cm
−1
, respectively, which
confirms the bulk and multilayer structure of MoS
2
nano-
flowers. Apart from the change in frequency, the Au-MoS
2
hybrids exhibit an increase in Raman peak intensity as Au
particle size increases. Since a locally magnified electric field
is created on the surface of the Au NPs when the plasmon
oscillation frequency is in resonance with the photon fre-
quency of the input light, the Raman intensity is substantially
increased [22]. The second-order longitudinal acoustic pho-
nons (2LA(M)mode)at the M point were reported to be
responsible for the third peak in the μRS spectra [27]of the
MoS
2
nanoflowers at 452 cm
−1
. In samples, AMNF1,
AMNF2, and AMNF3 2LA modes show distinct splitting.
Also, it is found that the B
1u
vibration mode starts from the
shoulder of the A
1g
peak after Au NPs decoration and origi-
nate into a clear peak at 415 cm
−1
as the size of the Au
particle increases. Two-phonon scattering, involving long-
itudinal quasi-acoustic phonons and transverse optical pho-
nons, is to be reason for the origin of B
1u
peak at 415 cm
−1
,
as reported in the previous literature [28]. Moreover, it is
suggested that in the case of Au-decorated hybrids, phonon
contribution is involved, leading to defect-assisted emission.
Figure 2shows the morphology and structure of the
prepared pure MoS
2
(MNF)and Au-MoS
2
(AMNF2)with an
average particle size of 5 nm was studied using SEM and
TEM analysis. The morphology of MNF and AMNF2 is seen
in figures 2(a)–(c)at different magnifications. These flowers
show a group of nanosheets that look like petals and are stuck
together as shown in figures 2(a)and (a)closely aligned
structure with a wall thickness of 17 nm as shown in
figure 2(b). Au NPs were found to be decorated to the surface
of MoS
2
flakes in a distinct state as shown is shown in
figure 2(c). Both samples have a layered flower-like morph-
ology, indicating that the decoration of Au NPs on MoS
2
flakes did not affect the morphology of the base, which means
that the morphology of the base remains unchanged after
decoration. In all fabricated samples high density of uniform
MoS
2
nanosheets interconnected to form numerous accessible
spaces, providing an abundance of gas exchange pathways
adequate to quickly dissipate gas from the surface to the
interior of the sphere structure [25,29].
This design significantly improves the gas-sensing
properties of MoS
2
, as the thin petals of the nanoflowers are
Figure 1. (a)XRD (b)Raman Spectra of bare MoS
2
(MB), and Au decorated MoS
2
(AMNF1, AMNF2, AMNF3). The XRD and Raman
spectra confirm the formation of MoS
2
in bare MoS
2
and Au-MoS
2
hybrids.
3
Nanotechnology 34 (2023)305601 S Rawat et al

responsible for the enhanced gas sensor performance [30].To
further confirm the formation of MoS
2
and Au-MoS
2
hybrids
(AMNF1, AMNF2, AMNF3)HRTEM is used to investigate
the nanostructure of MoS
2
nanoflowers and Au nanoparticles.
Figures 2(d)–(f)shows HR-TEM images of MoS
2
and
Au–MoS
2
hybrid. Figure 2(d)confirms the presence of flaky
structures as was observed using field emission scanning
electron microscopy (FESEM). Figure 2(e)illustrates that the
nanoparticles (NPs)of around 5 nm in size are securely
attached to the surface as a result of Au nanoparticle dec-
oration. Figure 2(f)illustrates the (111)plane of AuNPs and
the (002)plane of MoS
2
, indicating the formation of
Au-MoS
2
hybrids, with fringe spaces of 0.17 nm and 0.66
nm, respectively.
Furthermore, XPS characterization was used to examine
the electronic interaction at the Au-MoS
2
hybrid’s interface as
well as the elemental composition and chemical states of the
bare MNF and Au-MoS
2
hybrids (AMNF1, AMNF2, and
AMNF3)illustrated in figure 3. As observed from the survey
of the XPS pattern of the MNF and AMNF2 sample as shown
in figure 3(a)the spectra confirm the presence of Mo, S, and
Au in the prepared samples. Figure 3(b)shows the Mo 3d
XPS scan in which two main peaks at 228.3 and 232.5 eV
assigned to Mo 3d
5/2
and Mo 3d
3/2
respectively are identi-
fied, which confirms the presence of the Mo
4+
oxidation state.
In the case of AMNF1, AMNF2 and AMNF3 shift toward the
lower side mean binding energy decreases as the Au NPs
density increases. The lower side shift in the binding energy is
attributable to the electronic interaction between Au and
MoS
2
, which is present at the interface [31]. The higher work
function of Au may indicate the formation of a Schottky
barrier [32]at the interface that transfers electrons from MoS
2
to Au. This interaction results in the modification of the
interface which, increases the availability of electrons due to
the higher electronegativity of Au particles, which decreases
the resistivity of hybrids and hence enhancement in the gas
sensing ability. Figure 3(c)shows the sulfur (S)spectrum of
MNF, AMNF1, AMNF2, and AMNF3. The S peak splits into
two peaks positioned at 161.1 eV and 162.2 eV are allocated
to the S2p
3/2
and S2p
1/2
orbital of S
2
respectively and after
the decoration of Au particles on the surface, these peaks are
found lower shift indicating the modification of the surface.
Figure 3(d)exhibits the XPS spectra of Au for MoS
2
and
Au-MoS
2
hybrids in which two well-defined peaks at 83.7 eV
and 87.4 eV were attributed to Au 4f
7/2
and Au 4f
5/2
respectively, which were derived from the in situ grown Au
particles.
Using ultraviolet photoelectron spectroscopy (UPS),we
were able to get more information about the work functions of
the Au NPs and MoS
2
interface. Using the following
expression, we were able to determine the work function
value [33]
() ( ) ()j=- -WEE21.22 , 1
F cutoff F
where 21.22 eV is the energy of the laser light which is used
as incident ultraviolet photons, E
F
is the fermi energy and
E
cutoff
is the cutoff energy of the secondary electron.
Figure 2. FESEM images of (a)–(b)bare MoS
2
nanoflowers (c)Au decorated MoS
2
(AMNF2), here in flower-like structure is confirmed and
Au nanoparticles are decorated discreetly, HR-TEM images of (d)bare MoS
2
(MNF)and (e)Au-MoS
2
hybrid (AMNF2)confirms the
presence of flower-like structure and presence of Au nanoparticles. (f)Au and MoS
2
lattice fringes.
4
Nanotechnology 34 (2023)305601 S Rawat et al

Figure 4(a)shows the full valence band spectrum of
MNF (red)and AMNF5 (black)acquired using the UPS. The
valence band maxima for MNF and AMNF5 are respectively
located at 2.58 eV and 2.99 eV, respectively as shown in
figures 4(d)–(e). The value of the work function for bare
MoS
2
and Au-MoS
2
was found to be 4.85 eV and 4.46 eV
respectively, hence work function is decreased by 0.39 eV
after decorating Au on the surface of MoS
2
. It confirms the
formation of the Schottky barrier at the interface that transfers
the electron from MoS
2
(semiconductor)to Au (metal)NPs
[32,34,35]. This interaction results in the modification at the
interface and hence facilitate electron to enhance the gas-
sensing ability of the sample. The Schottky barrier formation
as confirmed by XPS and UPS studies further related to
investigating electrical sensing.
To investigate the electrical sensing properties of MoS
2
and Au-MoS
2
hybrids, the conductance of the films was
assessed with resistance versus time measurements using the
Keithley Source Meter
®
2450. The measurements were car-
ried out by probing the gold interdigitated electrodes (GIEs)
contacts. Because of the good conductivity of the Au-inter-
digitated electrode, it is easy to take measurements of the
conductivity of the MoS
2
and Au-decorated MoS
2
samples.
Figure 5(a)depicts the I–Vcurves of sensors with sensing
materials MoS
2
and Au-MoS
2
hybrid and voltages ranging
from −2to+2 V. The nonlinearity between the measured
current and applied voltage in synthesized gas sensors is
indicative of a Schottky contact between the interface of Au
NPs and MoS
2
. Resistance of Au-MoS
2
hybrid-based sensors
decreased as the particle size of Au NPs used for decoration
increased; this trend is associated with electron transfer
between MoS
2
and Au NPs during heterostructure formation.
2.2. Gas sensor’s performance testing
The sensing properties of the MoS
2
and Au-MoS
2
hybrid
sensors were investigated at room temperature. Figure 6
depicts the gas sensing response of MoS
2
and Au-MoS
2
hybrid resistive-type sensors to 10 ppm CO and NH
3
. When
exposed to CO and NH
3
, the sensors’resistance decreases,
indicating an n-type response. As can be seen in figures 6(a)
and (b), the response of bare MoS
2
shows a minor sensing
response of 24% and 13% toward CO and NH
3
respectively.
The Au-MoS
2
hybrids demonstrate a significantly improved
response from the sensors. Figures 6(c)and (d)show the
sensor response of AMNF1 with particles of size 2 nm. These
figures show a response of 39% and 34% in the presence of
CO and NH
3
, which is 1.6 times higher for CO and 2.16 times
higher for NH
3
than the response of pure MoS
2
. In addition,
AMNF2, which has a particle size of 5 nm, shown in
figures 6(e)and (f), demonstrates a response of 69% and 62%
in the presence of CO and NH
3
, respectively. This is
approximately 2.8 times greater in presence of Co and
4.7 times greater in the presence of NH
3
than the response of
Figure 3. X-ray photoelectron spectroscopy of bare MoS
2
and Au-MoS
2
hybrids: (a)survey of MNF and AMNF2 (b)Mo 3d (c)S 2p and (d)
Au 4f. The spectra confirm the presence of Mo, S, and Au. The lower shift in the binding energy corresponds to the electronic interaction and
formation of the Schottky barrier at the interface of Au-MoS
2
hybrids.
5
Nanotechnology 34 (2023)305601 S Rawat et al

bare MoS
2
. The AMNF
3
sensor has the best response, as
shown in figures 6(g)and (h), which is 79% when it is
exposed to CO and 69% when it is exposed to NH
3
. This is
approximately 3.2 times higher in presence of CO and 5.3
times higher in the presence of NH
3
than the bare MoS
2
.
Table 1summarizes the gas sensing measurements of bare
MoS
2
and Au-MoS
2
hybrids sensors including their response
and recovery time. Figure 9(a)summarizes the sensors’
response and recovery times to 10 ppm CO and NH
3
. Table 2
lists the gas sensing properties of Au-MoS
2
hybrid with
particle size 10 nm compared with previous published works.
The comparison is made with the existing gas sensors based
on TMDs hybrid materials. The comparison results demon-
strate that the presented sensor exhibits a nearly similar results
than the most existing room temperature gas sensors.
2.3. Gas sensing mechanism
MoS
2
is n-type semiconductor due to sulfur vacancies. When
the n-type semiconducting MoS
2
nanostructure were exposed
to ambient air, oxygen molecules and water molecules were
absorbed on the MoS
2
surface and electrons were captured
from the conduction band of the MoS
2
resulting in the for-
mation of oxygen ions this can be explained via following
Figure 4. UPS spectra of MoS
2
and Au-MoS
2
hybrids. For each spectrum, the valence band maximum was estimated using the intercept on
the abscissa obtained by extrapolating the low binding energy edge to the baseline.
6
Nanotechnology 34 (2023)305601 S Rawat et al

chemical equations [25,36]
() ()
O
gas O ads
22
()+
--
O
ads e O
22
() ()H O gas H O ads
22
()++++
--
M
oS 2H O ads 2e MoS 2OH 2H .
22 2 2
Thus, the adsorption of oxygen and water ions leads to the
formation of an electron depletion layer (EDL)near the sur-
face of MoS
2
. When reductive gases (CO and NH
3
)mole-
cules react with adsorbed oxygen ions, electrons are released
and injected into the EDL, causing it to become thinner.
which results in a decrease in the electrical resistance of the
MoS
2
gas sensor. This can also be explained through fol-
lowing chemical reaction [19,36,37]
() () ()+ +
--
2
CO gas O ads 2CO gas e
22
() ()+++
--
2
NH gas 3O ads N 3H O 3e .
2222
In the present experiment, when a flush of 10 PPM CO
and NH
3
is injected into the sensing chamber at room
temperature, CO and NH
3
molecules react with the adsorbed
oxygen ions, releasing electrons that are then injected into the
EDL, causing it to become thinner. As a result, the con-
centration of electrons at the surface of MoS
2
increases,
resulting in a decrease in electrical resistance. The enhance-
ment in recovery and response time is due to the low voltage
drop which decreases as the particle size of Au increased from
2 to 10 nm. Comparing with MNF, it was found that AMNF
hybrids showed an improved response to gas at room temp-
erature. Two factors can explain why AMNF hybrids are
superior at gas detection: first, the Au interdigitate electrodes
(GIEs), and second, the decoration of the Au nanoparticle.
GIEs lead to the formation of Au-MoS
2
-Au junctions between
the fingers of the electrode; each set of electrodes forms a
Schottky contact with MoS
2
, as discussed in the UPS study.
The energy band diagram is used to explain the gas sensing
mechanism shown in figure 7. In a clean environment, oxygen
atoms attract the free electron present in MoS
2
, which pushes
them to the surface of MoS
2
. The reverse saturation current of
the electron hole pair in MoS
2
indicates some resistive value
of sensors in a clean environment, and an asymmetric
Schottky barrier height near the positive and negative
terminals is created, as shown in figure 7(a). When the sen-
sors are exposed to reducing gases like CO and NH
3
, these
gases absorb oxygen particles and break the bond between
oxygen and free electrons. i.e. free electrons are generated
again in the conduction band. which results in a higher
Schottky barrier at the positive electrode than that for the
negative electrode, as shown in figure 7(b). So charges are
better separated on the positive electrode than on the negative
electrode. The net resistance of the sensors then decreases,
indicating the presence of toxic gases. Furthermore, Au size
matters in the formation of Schottky barrier height because, as
the Au NPs density increases on the MoS
2
surface, its binding
energy decreases and thus its work function decreases, as
shown in the XPS and UPS studies. Au NPs with larger sizes
have a higher density, resulting in a lower Schottky barrier
height and a more efficient charge transfer between the MoS
2
and Au NPs. Moreover, Au NPs have good catalytic beha-
viour to accelerate the dissociation of molecular oxygen on
the semiconductor surface and obtain more active sites. After
decoration of the Au nanoparticle on MoS
2
enhances the gas-
sensing properties by increasing the surface area and pro-
viding more active sites for gas adsorption [38]. Addition to
this the fermi level of MoS
2
is higher than that of Au NPs,
charge transfer from MoS
2
to Au NPs occurs, which reduces
the number of charge carriers in MoS
2
and causing a local
depletion layer to form around the Au NPs, which is
responsible for electronic sensitization of MoS
2
and a greater
response towards toxics gaseous at low temperatures [39].
Moreover, an optimised Au nanoparticle size is necessary for
better gas sensing performance. A smaller size of Au NPs
decoration will result in an insufficient number of Au NPs for
gas sensing, whereas a larger size will result in a smaller
adsorption area for the target gas. Therefore, the size of Au
NPs needs to be carefully controlled to achieve the best gas
sensing performance.
To gain an insight into the surface effects, the surface
potential variation under the different gaseous conditions
(CO, NH
3
), we applied the conductance model for AMNF3
layer as proposed by Simion et al [40]. In our experiment, the
thickness of the sensing material, i.e. the depth of the elec-
trodes is 50 μm while the surface layer thickness is 10 nm
Figure 5. (a)The I/Vcurve of the Sensor using MoS
2
and Au-MoS
2
hybrids (b)Schottky barrier forward voltage drop.
7
Nanotechnology 34 (2023)305601 S Rawat et al

Figure 6. Gas sensing response of MoS
2
and Au-MoS
2
hybrids toward CO and NH
3
. The response curve of the Au-MoS
2
hybrid-based
sensor demonstrated excellent performance for CO and NH
3
.
8
Nanotechnology 34 (2023)305601 S Rawat et al

(Thickness of Au NPs). Therefore, we applied the -
n
type
conductance model to account for the surface effects that do
not affect the full layer.
The total conductance of a compact layer that is having
its surface exposed to the ambient atmosphere is the sum
between the conductance of the part of the layer influenced by
the surface phenomena (called surface layer)and the con-
ductance of the layer that is left unchanged (called bulk);
()=+GGG 4
total s b
In detail, the equation (Equation)becomes
() ()ss=+
-
~
GzW
L
DzW
L,5
total s 0b0
where
L
is the length of the layer,
W
its width,
D
its thickness
and
z
0the thickness of the surface layer. s
~
sis the average
conductivity of the surface layer and
s
bis the conductivity of
the bulk.
For an -
n
type semiconductors, using the definition for
conductivity,

sm=
~
en
s
s
and
s
m=en,
bb
the equation (5)can
be written as:
[()]
[()] ()
()



=+
=+-
=-+
mm
m
m
-
G
nz n D z
zn n nD.6
enzW
L
en D z W
L
eW
L
ew
L
total
s0 b 0
0s b b
s0 b 0
Assuming the Boltzman statistics in the whole layer one
obtains (see Simion et al 2019)the dependence of the overall
conductance on the surface potential (band-banding):
()
⎜⎟
⎡
⎣
⎢⎡
⎣
⎢⎛
⎝⎞
⎠⎤
⎦
⎥⎤
⎦
⎥
= -+-GG L
D
eV
kT
eV
kT
12exp 1
7
D
total b s
B
s
B
1
2
For the case of depletion layer, >
V
0,
sequation (7)becomes
()
⎜⎟
⎡
⎣
⎢⎡
⎣
⎢⎛
⎝⎞
⎠⎤
⎦
⎥⎤
⎦
⎥
=- -+-GG L
D
eV
kT
eV
kT
12exp 1
8
D
total b s
B
s
B
1
2
For the case of accumulation layer, <
V
0,
sequation (8)
becomes
()
⎜⎟
⎡
⎣
⎢⎡
⎣
⎢⎛
⎝⎞
⎠⎤
⎦
⎥⎤
⎦
⎥
=+ -+-GG L
D
eV
kT
eV
kT
12exp 1.
9
D
total b s
B
s
B
1
2
Thus, in the present work, the equation (7)becomes our base
equation in order to evaluate the
V
.
sThe conductance equation
(equations (5)and (6)) has been solved using the Newton-
Raphson numerical method. This has been solved iteratively
for different conductance values under different gaseous
conditions applied (CO and NH
3
). The surface potential (Vs)
variations under the different gaseous conditions has been
shown in the figure 8(a). which is collaborate with the
experimental sensing of AMNF3 with and without exposure
to CO and NH
3
gas. Representation of the normalized con-
ductance (G/G
b
)as a function of band bending (eV/k
B
T)is
shown in figure 8(b). Two regions are depicted in the figure.
Table 1. The response time, recovery times, and sensitivity of the gas sensors towards 10 ppm of CO and NH
3
at room temperature.
Sample
name Gas
Resistance in
air (MΩ)
Resistance in
air (MΩ)
Recovery
time (s)
Response
time (s)Response%
MNF CO 10.54 7.89 24 18 24
MNF NH
3
10.43 8.98 17 19 13
AMNF CO 8.56 5.22 13 16 39
AMNF NH
3
8.54 5.61 20 20 34
AMNF1 CO 5.03 1.52 11 12 69
AMNF2 NH
3
4.96 1.86 17 15 62
AMNF3 CO 2.76 0.57 10 7 79
AMNF3 NH
3
2.10 0.64 11 9 69
Table 2. Sensing performance of Au/MoS
2
hybrids with particle size 10 nm compared with previous work based on TMDc and their hybrids
gas sensing.
Gas Materials
Concentration
(ppm)
Temperature
(°C)
Response
time (s)
Recovery
time (s)References
CO Au-WS
2
50 RT 53 174 [48]
SnO
2
/MoSe
2
100 RT 20 16 [49]
MoS
2
500 230 18 15 [50]
Au/MoS
2
10 RT 10 7 This work
NH
3
Au-MoSe
2
20 RT 18 16 [51]
WS
2
20 35 54 66 [52]
Au-MoS
2
10 RT 11 9 This work
9
Nanotechnology 34 (2023)305601 S Rawat et al

When the sensing material is exposed to air, its conductance
drops at higher potential because oxygen particles pair up
with the free electrons present in AMNF3, pushing them to
the surface of the AMNF3. In a lower potential state, toxic
gases react with the adsorbed oxygen particles to break the
chemical bond between oxygen and free electrons, releasing
the free electrons and lowering the potential. Here, the more
reducing nature of CO results in a higher conductance at a
lower potential.
2.4. Effect of humidity
Figure 9(b)shows the effect of humidity at different Humidity
(RH)level (40%, 60%, 80%)on the gas sensing response of
AMNF3 with Au particle size 10 nm in the presence of
10ppm of NH
3
at RT(∼30 °C). When the RH increases the
response of the sensors increases, and the resistance of the
sensors decreases with decrease in the RH values. At all RH
level the response of the AMNF sensor is n-type. Reason
behind this is the absorption of H
2
O molecule at the Au MoS
2
surface is higher at higher RH level. Which enhance the
charge transfer rate between Au-MoS
2
interface. The response
of the AMNF at 40%,60% and 80% is 10%, 36% and 65%
respectively. Another reason behind this gas sensing response
is the increase in the amount of NH
3
gas dissolved in the
water layer deposited on the Au-MoS
2
interface.
2.5. Conclusions
In summary, the MoS
2
nanoflowers and Au-MoS
2
hybrids
based GIEs were successfully synthesized. These sensors
were analysed as efficient gas sensors at room temperature
(∼30 °C)for detecting toxics CO and NH
3
. The formation of
Schottky barrier was confirmed by XPS and UPS analysis
which improved interfacial interaction between Au nano-
particles and MoS
2
. Current–Voltage curve depicted the
lower voltage drop in the Schottky barrier as the as the par-
ticle size increases from 2 to 10 nm which responsible for
quick recovery and response time. The response value of the
Au decorated MoS
2
with Au nanoparticle size 10 nm to
10 ppm of CO and NH
3
is 76% and 69% respectively and
exhibits 3.5 time and 5 times higher than the bare once when
exposed to CO and NH
3
. Hence Au-MoS
2
based hybrids are
the promising materials for low range toxics gas detection
sensors and has a wide range application in the future of gas
sensing field.
3. Materials and methods
3.1. Synthesis of MoS
2
nanoflowers
Hydrothermal synthesis was used to synthesize MoS
2
nano-
flowers [41]. The standard concentration of precursors was
taken to be 0.82 g of ammonium molybdate (Sigma Aldrich
99.9% pure)and 5.15 g of thiourea (Sisco Research
Laboratory), was dissolved in 40 ml deionized water (DI)to
form a transparent solution and stirred using a magnetic stirrer
for 30 min. Then, the solution was heated in an autoclave for
18 h at 200 °C, inside a 50 ml Teflon-lined stainless-steel
autoclave. The black precipitates were obtained by filtering
the solution and then, washed with distilled water and ethanol
five times, separately. The resulting black precipitate was then
dried in a hot air oven at 70 °C to produce the MoS
2
powder.
The second step involved dissolving 12 mg MoS
2
nano-
flowers, in 5 ml of ethanol to create a dispersed solution
which is further drop-cast over the glass slide and gold (Au)
interdigitated electrode GIEs at 60 °C on a hot plate. Inter-
digitated electrodes are having a finger-like pattern on a PET
substrate, with a line width of 100 μm and spacing of 50 μm
between fingers. The length of each finger is 7.7 mm, and
there are total 15 finger pairs. Figure 10 depicts images of
GIEs before and after MoS
2
drop casting. These glass slides
and GIEs were designated as MNF.
3.2. Synthesis of Au-MoS
2
hybrids
In order to fabricate Au-MoS
2
hybrids, inert gas thermal
deposition was used to decorate different sizes of Au nano-
particles (NPs)[42]. The substrate (glass slides)and GIEs
were completely rinsed with distilled water before being
loaded into the growth chamber. Microscope glass slides with
a typical dimension of 10 mm ×10 mm was used as a
substrate for characterization in these experiments and GIEs
were used for gas sensing performance. A molybdenum boat
was used to evaporate Au foil (purity of 99%)by resistive
heating. On resistive heating, the Au foil placed in the
molybdenum boat to evaporates Au and then condenses on
Figure 7. Energy band diagram of Au-MoS
2
-Au gas sensors under (a)clean environment (b)under the exposure of toxics gas.
10
Nanotechnology 34 (2023)305601 S Rawat et al

the substrate held 21 cm above the boat. The depositions were
conducted at a base pressure of 5×10
−6
Torr by supplying 90
A to the boat for the evaporation of Au. The thicknesses of
the deposited Au film were determined from a digital thick-
ness monitor (DTM). A thin layer of Au was deposited on
MoS
2
drop-casted GIEs with thicknesses of 2 nm, 5 nm, and
10 nm individually in an Ar gas atmosphere at three different
deposition pressures of 5 ×10
–4
Torr, 5 ×10
–5
Torr and 5 ×
10
–6
Torr, respectively. These deposited samples were
subsequently annealed at 150 °C–200 °C in an Ar gas
atmosphere to avoid agglomeration of Au NPs. During a
metallic film deposition process, the vapour pressure differ-
ence between the surface of the melt (Au source)and the film
(being deposited)affects the size of the particle [43–45]. The
higher areal density of NPs at lower pressure is associated
with the higher atom concentration, which favours enhanced
nucleation and tends to form large NPs in inert gas eva-
poration process. At lower pressure, the mean free path is
Figure 8. (a)Represents the variation of surface potential under the CO and NH
3
gaseous condition (b)Representation of the normalized
conductance (/
G
Gb)as a function of band bending (/
e
VkT
B).
Figure 9. (a)Dependency of the sensor’s response on CO and NH
3
(b)Response of AMNF sensor towards NH
3
at different RH levels.
Figure 10. Gold interdigitated electrode GIE image (a)before drop-casting of MoS
2
nanoflowers (b)after drop-casting of MoS
2
nanoflowers.
11
Nanotechnology 34 (2023)305601 S Rawat et al

causing lesser collision between Au atoms and gas molecules
in comparison with higher pressure results small NPs. Thus,
more Au atoms would reach the surface of the substrate
resulting in the growth of large Au nanoparticles with more
density on the MoS
2
nanoflowers.
3.3. Gas sensing measurements
The sensing measurements were carried out on MNF,
AMNF1, AMNF2, and AMNF3 samples with the help of a
gas sensing measurement unit (Priya Enterprises, India,
MA501). The gas sensitivity measurements were performed
in a temperature-controlled sealed chamber connected to
10 ppm of CO and NH
3
(Sigma Gases 99% Pure). Using a
rotameter, we set the total gas flow rate to 200 standard cubic
centimetres per minute. In the beginning, there was a 60 min
stabilization period in the fresh air. After that, gas is flushed in
the chamber manually for 100 s time intervals. The resistance
of the sensors was measured before and after being exposed
to toxic gases to determine how well the synthesized nanos-
tructures could detect and measure the presence of toxic
gases. The changes in resistance were measured using a
Keithley source meter (model no. 2450). To send an electrical
signal to the Keithley source meter, we connected the two
copper (Cu)electrodes to the sensor’s source and drain. The
gas sensing measuring setup is shown in figure 11. The gas
sensing response (S)was defined as [46]
() ( )=-´
RR
R
S % 100. 10
ag
a
In the case of reducing gases such as CO and NH
3
, the sensor
resistances to the target gas and in the air are denoted by R
g
and R
a
, respectively. In the case of an adsorption process, the
response and recovery time was evaluated as the amount of
time it took for the sensor to reach approximately 90% of
either the maximum or minimum resistance change [47].In
order to have efficient gas sensors at room temperature, the
gas sensing measurements were carried out using humidity
sensors. The relative humidity of the air inside the gas sensing
chamber can be adjusted by impinging hot air and water
vapor.
3.4. Instruments
The phase and crystallinity of the fabricated samples were
examined using XRD (Bruker D8-Advance instrument with
Cu-Kαx-ray source)and Micro-Raman spectroscopy (Jobi-
nYvon, Model T64000 triple monochromator)techniques.
The morphology was imaged using FESEM (Carl Zeiss Ultra
Plus)and Transmission electron microscopy (TEM). The
interfacial investigation of bare and hybrid was carried out by
x-ray photoelectron spectroscopy (XPS)and ultraviolet pho-
toemission spectroscopy (UPS)technique (VG ESCALAB
220I-XL)using a monochromatic Al Kαsource.
Acknowledgments
The authors are extremely thankful to the University Grant
Commission (UGC)-Department of Atomic Energy (DAE),
Collaborative Research Scheme (CRS)(CRS/2021–22/01/
463), Doon University Dehradun and Government of India
for providing funds to carry out this research. The authors
acknowledge the Raman Spectroscopy facility received from
the Department of Science and Technology (DST), Govt. of
India under the FST scheme (SR/FST/PSI-225/2016). The
authors thank Dr Manushree Tanwar (University of Penn-
sylvania, USA)for the useful discussion.
Data availability statement
The data cannot be made publicly available upon publication
because they contain commercially sensitive information. The
data that support the findings of this study are available upon
reasonable request from the authors.
ORCID iDs
Rajesh Kumar https://orcid.org/0000-0001-7977-986X
Himani Sharma https://orcid.org/0000-0003-1924-6222
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