Document [original]

Green Chemistry

PAPER

Cite this: Green Chem., 2020, 22,

6981

Received 13th August 2020,

Accepted 7th September 2020

DOI: 10.1039/d0gc02779k

rsc.li/greenchem

Eﬃcient NO

sensing performance of a low-cost

nanostructured sensor derived from molybdenite

concentrate†

Mirabbos Hojamberdiev, *

Neeraj Goel,

Rahul Kumar,

Zukhra C. Kadirova

and Mahesh Kumar

Accumulation of industrial solid waste necessitates the development of utilization processes and techno-

logies to reduce their negative environmental impact. Herein, molybdenite concentrate from the mining-

metallurgy industry is systematically characterized as a valuable starting material for the fabrication of an

eﬃcient and low-cost nanostructured gas sensor. Few-layer MoS

is obtained from molybdenite concen-

trate by liquid nitrogen exfoliation and deposited on diﬀerent substrates by spin coating and drop casting.

It is found that spin coating is advantageous over drop casting in fabricating a homogeneous and dense

few-layer MoS

ﬁlm. The charge-transfer-based sensing performance of the fabricated few-layer MoS

ﬁlm is investigated upon exposure to NO

at diﬀerent temperatures (50, 100, and 120 °C). At an optimized

temperature of 120 °C, a faster recovery is achieved, and the fabricated device exhibits 28, 38, and 44%

sensitivity to 10, 50, and 100 ppm NO

, respectively, making it suitable for practical applications.

Furthermore, the adsorption aﬃnity of NO

to the predominant (002) crystallographic plane of MoS

estimated from the distribution of ﬁeld density and the calculated diﬀerential adsorption energies.

According to the molecular modeling data, NO

in the Ar/NO

mixture has better interaction (dE

/dN

NO2

= 4.77 kcal mol

−1

) with the few-layer MoS

surface than individual NO

(dE

/dN

NO2

= 2.57 kcal mol

−1

and van der Waals interaction (≈14 kcal mol

−1

) is the main adsorption force compared to the relatively

weaker electrostatic interaction (<1 kcal mol

−1

). This work demonstrates a straightforward approach not

only for the conversion of molybdenite concentrate into an eﬃcient and low-cost nanostructured gas

sensor but also for the reduction of the negative impact of accumulated molybdenum concentrate on

the environment and human health.

1. Introduction

Rapid industrialization and urbanization have led to severe air

pollution that has increasing negative impacts on human

health and the environment alike. The United States

Environmental Protection Agency (EPA) has even started an

AirNow program that receives, manages and shares the data

related to the real-time global air quality.

This program also

forecasts the local air quality and supports actions to reduce

exposure to air pollution. Along with other hazardous air pol-

lutants like sulfur dioxide (SO

), carbon dioxide (CO

), and

nitrogen monoxide (NO), nitrogen dioxide (NO

) is emitted

into the atmosphere as a product of combustion processes

(vehicles, industry, heating, shipping, etc.)

and explosives.

is the main source of tropospheric ozone and PM2.5 and

forms nitrous acid (acid rain) upon reaction with water, which

is harmful to ecosystems. Short- and long-term exposure to

can have adverse respiratory and cardiovascular health

eﬀects and even mortality.

According to the current World

Health Organization’s guideline, the NO

annual mean value

has been set to 40 micrograms per cubic meter in order to

protect public health.

It is a prerequisite to constantly

monitor the air quality using highly sensitive and fast gas

sensing systems to comply with laws and regulations.

In recent years, two-dimensional (2D) transition metal

dichalcogenides, such as MoS

, MoSe

,WS

, WSe

, SnS

,etc.,

have gained enormous interest in sensing applications with

low-power consumption due to their exceptional electrical,

†Electronic supplementary information (ESI) available. See DOI: 10.1039/

d0gc02779k

Institut für Chemie, Technische Universität Berlin, Straße des 17. Juni 135,

10623 Berlin, Germany. E-mail: khujamberdie[email protected];

Fax: +49 (0) 30 314-79656; Tel: +49 (0)30 314-26178

Department of Electrical Engineering, Indian Institute of Technology Jodhpur,

Jodhpur-342037, India

Department of Inorganic Chemistry, Faculty of Chemistry,

National University of Uzbekistan, University Street 4, 100174 Tashkent, Uzbekistan

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chemical, and physical properties.

Among transition metal

dichalcogenides, MoS

has recently emerged as a potential

candidate for NO

sensing application due to its atomic-scale

thickness, high surface to volume ratio, tunable band gap, and

high on/oﬀswitching ratio.

A one-dimensional MoS

nanowire network (NW) fabricated

by using controlled turbulent vapor flow from the chemical

transport reaction showed about 2-fold enhanced sensitivity

for NO

at 60 °C with a low detection limit (4.6 ppb) and

enhanced sensitivity and selectivity towards NO

because of a

combination of abundant active edge sites and a large surface

area and tuning of the potential barrier at the intersections of

nanowires during adsorption/desorption.

Vertically aligned

MoS

fabricated by using the rapid sulfurization method of

CVD process showed about 5-fold enhanced sensitivity to NO

compared to horizontally aligned MoS

due to the high density

of the exposed edge sites.

Hierarchical MoS

microsphere

hollow structures synthesized by a hydrothermal method

showed enhanced NO

detection owing to the improved active

edge sites.

Some researchers have used a UV light source to

improve the sensing performance of a MoS

sensor at room

temperature. For instance, a photoactivated mixed in-plane

and edge-enriched p-type MoS

flake-based sensor demon-

strated a fast response with a good sensitivity of ∼10.36% for

10 ppm NO

without and with complete recovery at room

temperature and under UV light irradiation, respectively.

Under photo excitation, MoS

exhibited an enhanced sensi-

tivity with ultrafast response time of ∼29 s and excellent recov-

ery to NO

(100 ppm) at room temperature due to charge per-

turbation on the surface of the sensing layer during the NO

MoS

interaction under light irradiation.

A high-performance

gas sensor prepared using a hybrid of temperature-assisted

sulfur vacancy within the edge-oriented vertically aligned MoS

and reduced graphene oxide exhibited a significantly

enhanced response to NO

with fast response and complete

recovery due to the controlled electrical/chemical sensitization

level of MoS

through controllable vacancy and interface

engineering.

However, it is necessary to develop eﬃcient and low-cost

sensing materials based on MoS

to find practical applications

in NO

sensing. One of the plausible ways to develop eﬃcient

and low-cost sensing materials is to utilize industrial solid

wastes. In this work, we aim to characterize molybdenite con-

centrate from the mining-metallurgy industry and to fabricate

an eﬃcient and low-cost nanostructured sensor by utilizing

molybdenite concentrate. Simple liquid nitrogen exfoliation is

used to exfoliate few-layer MoS

and to remove any impurities

from molybdenite concentrate. The gas sensing behavior of

the fabricated sensor by spin coating of few-layer MoS

to the

ppm concentration of NO

is investigated. Furthermore, the

adsorption aﬃnity of NO

to the predominant (002) crystallo-

graphic plane of molybdenite-2H is estimated from the distri-

bution of field density and the calculated diﬀerential adsorp-

tion energies. This work demonstrates a straightforward

approach not only for the conversion of molybdenite concen-

trate into an eﬃcient and low-cost nanostructured gas sensor

but also for the reduction of the negative impact of accumu-

lated molybdenum concentrate on the environment and

human health.

2. Experimental

2.1. Preparation

Molybdenite concentrate (MC) is generally formed during the

extraction process of copper and molybdenum from porphyry

copper-molybdenum in Almalyk Mining-Metallurgical

Complex, Uzbekistan and has been used as the starting

material in this study. As shown in Fig. 1, after grinding for

30 min, MC was immersed in liquid nitrogen for 2 h,

and

the upper part of the suspension was separated and ground

again for 1 h. Afterward, 45 mg of the exfoliated MC was dis-

persed in 3 mL of isopropyl alcohol under sonication for 2 h.

The samples were prepared by spin coating of the MC-based

suspension on a SiO

/Si substrate at 1500 (MC1), 2500 (MC2)

and 3500 (MC3) rpm for 30 s. The MC4 and MC5 samples were

prepared by spin coating of the MC-based suspension on an

n-type silicon substrate at 500 and 1500 rpm for 30 s, respect-

ively. The MC6 sample was prepared by drop casting of 30 µL

of the MC-based suspension on a metallic IDC-patterned

SiO

/Si substrate.

The conductivity of the fabricated sensing device is very

important in examining its sensing response. Previously, it has

been reported that few-layer MoS

has higher conductivity

than its monolayer and multilayer counterparts.

Therefore,

we have used few-layer MoS

to fabricate a sensing device that

shows a higher sensing response stability while maintaining a

high surface to volume ratio. The fabricated sensing device

was annealed at 300 °C for 1 h under a N

atmosphere to

reduce the unintentional barrier at the metal/semiconductor

interfaces.

Au was used to create an Ohmic contact with few-

layer MoS

16,17

The current–voltage characteristic curve of the

fabricated sensing device at room temperature is shown

Fig. S1 in the ESI,†indicating the Ohmic behavior of the fabri-

cated sensing device.

2.2. Characterization

The X-ray diﬀraction (XRD) patterns were recorded on a MiniflexII

(Rigaku) diﬀractometer using Cu Kαradiation (λ= 0.15418 nm)

in the 2θscan range from 10 to 80°. The morphology of the

samples was examined using a JSM-7600F field-emission-type

scanning electron microscope (JEOL). The energy-dispersive X-ray

spectroscopy (EDX) element mapping images were obtained

using a spectrometer attached to the SEM. The crystallinity and

nanostructure of the samples were analyzed using a JEM-2100F

HK high-resolution analytical scanning transmission electron

microscope (JEOL). The ultraviolet-visible (UV-vis) diﬀuse

reflectance spectrum of the samples was recorded on a

UV-3600 UV-vis-NIR spectrophotometer (Shimadzu) equipped

with an integrating sphere, and BaSO

was used as the refer-

ence. The Brunauer−Emmett−Teller (BET) specific surface

area of the samples was determined from the N

adsorption–

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desorption isotherm measured using a BELSORP-mini instru-

ment (BEL Japan, Inc.) at 77 K after degassing the sample at

120 °C for 12 h. The surface chemical states of elements were

analyzed by X-ray photoelectron spectroscopy (JPS-9010MC,

JEOL) using non-monochromated Mg Kαradiation (1253.6 eV).

2.3. Gas-sensing measurement

Based on the characterization results, only the MC6 sample

was selected for gas-sensing measurement. Its gas-sensing be-

havior was examined in a gas-sensing chamber using the

desired concentrations (10, 50 and 100 ppm) of NO

after

injecting a mixture of 5% NO

(99.99%, HPS Gases Limited)

and 95% Ar (99.99%, HPS Gases Limited). An external heating

filament was used to increase the temperature of the device to

120 °C. The change in the current of the device at 3.0 V bias

voltage was measured using a Keithley 4200-SCS semi-

conductor characterization system (Tektronix).

3. Results and discussion

Fig. 2a shows the XRD pattern of molybdenite concentrate.

The major reflections in the XRD pattern are readily indexed

to the hexagonal molybdenite-2H with the space group P63/

mmc (194) (ICDD PDF# 06-0097), while the minor reflections

Fig. 1 Preparation procedure of a low-cost nanostructured sensor from molybdenite concentrate.

Fig. 2 (a) XRD pattern, (b) side and top views of the crystal structure of molybdenite (2H polytype), (c and d) BSE-SEM images, (e) UV-vis DRS spec-

trum, and (f) N

adsorption–desorption isotherm of molybdenite concentrate.

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can be assigned to the triclinic FeS

with the space group P1

(ICDD PDF#71-1680) and an unidentified phase. The three

main reflections observed at 2θ= 14.39, 44.16, and 60.11°

correspond to the (002), (006), and (008) atomic planes of crys-

talline molybdenite-2H, indicating a stacking layer along the c

direction. These atomic planes are stacked in a laminar crystal-

line structure (common 2H polytype), as shown in Fig. 2b.

These individual two-dimensional S–Mo–S crystalline layers

are bonded by the weak van der Waals interactions and are

capable of slipping over each other and generate low-energy

and hydrophobic surfaces (basal planes). The in-plane S–Mo

bonds are strong Mo–S covalent bonds, which create high-

energy and hydrophilic sites known as edges when broken.

18,19

Therefore, molybdenite particles exhibit anisotropic surface

properties and are naturally hydrophobic due to the surface

content of S atoms.

19,20

Fig. 2c and d show the low- and high-

magnification BSE-SEM images of molybdenite concentrate.

Along with other irregular particles, truncated triangular par-

ticles, formed by the face-to-face stacking of the two-dimen-

sional layers of molybdenite-2H, with a lateral size of <40 µm

can be observed. Clearly, the surface of truncated triangular

particles also contains some irregular particles of molybden-

ite-2H.

Fig. 2e shows the UV-vis diﬀuse reflectance spectrum of

molybdenite concentrate. As shown, molybdenite concentrate

exhibits a strong light absorption in the wavelength range of

200–800 nm. An absorption edge of molybdenite concentrate

is observed at approximately 435 nm, corresponding to an

optical indirect band-gap energy (E

) of 2.86 eV. Clearly, the

indirect band-gap energy of molybdenite concentrate is much

lower than the reported value for a bulk MoS

crystal (E

= 1.29

eV)

due to their chemical and structural diﬀerences. It is

known that the band-gap energy of MoS

can easily be tuned

by changing the number of layers

and by applying a mechan-

ical strain,

and a direct to indirect band-gap transition is

also observed. Obviously, four characteristic peaks with lower

intensity at about 290–300 nm and 550–685 nm are observed

in the UV-vis diﬀuse reflectance spectrum of molybdenite con-

centrate. Presumably, the peaks observed at 290–300 nm may

indicate a reduction in the 2H phase of MoS

224,25

or the inter-

band transition between the occupied and unoccupied orbi-

tals,

while the peaks noted at 550–685 nm correspond to the

excitonic interband transitions at the Kpoint of the Brillouin

zone.

It must be mentioned that the peaks representing the

four excitonic electronic transitions are blue-shifted compared

to those of stable dispersion of MoS

nanosheets.

As the

specific surface area plays an important role in sensing, the

specific surface area was estimated from the N

gas adsorp-

tion–desorption isotherm shown in Fig. 2f. According to the

IUPAC classification,

the isotherm of molybdenite concen-

trate is type III, indicating a relatively weak adsorbent–adsor-

bate interactions on the surface of a nonporous or macropor-

ous solid. The estimated specific surface area of molybdenite

concentrate was 2.11 m

−1

Various particles were observed in the BSE-SEM images of

molybdenite concentrate in addition to the molybdenite-2H

particles. To identify these particles, energy-dispersive X-ray

spectroscopy (EDX) analysis was performed. Fig. S2 in the ESI†

shows the BSE-SEM images and the corresponding EDX

spectra of diﬀerent particles of molybdenite concentrate.

Based on the EDX data shown in Fig. S2a and S2d,†the trun-

cated cubic and plate-like particles can be assigned to FeS

and molybdenite-2H, respectively, and no impurities/dopants

are detected, implying the high purity of both components. In

Fig. S2b,†the irregular inhomogeneous particles accumulated

between the truncated cubic crystals of FeS

and plate-like par-

ticles of molybdenite-2H contain iron, copper, magnesium,

aluminum, sulfur, and oxygen elements. Interestingly, the

sponge-like quasi-spherical particles contain iron, magnesium,

aluminum, oxygen and a small amount of sulfur (Fig. S2c†).

According to the EDX data, the chemical composition of mol-

ybdenite concentrate (wt%) is 24.54 Mo, 51.41 S, 18.53 O, 3.62

Fe, 0.87 Al, 0.57 Cu, and 0.46 Mg.

The exfoliation in liquid nitrogen and dispersion in isopro-

panol under ultrasonication yielded a few-layer MoS

film. As

shown in Fig. S3a in the ESI,†the presence of a strong diﬀrac-

tion peak at 2θ= 14.3° in the XRD pattern of the exfoliated/de-

posited sample confirms high crystallinity of few-layer MoS

Furthermore, Raman spectroscopy was used to estimate the

thickness of the MoS

film. For a better understanding, the

Raman spectrum of few-layer MoS

deposited on the sensing

device is compared with that of bulk MoS

. As shown in

Fig. S3b,†the Raman spectrum of few-layer MoS

deposited on

the sensing device shows red and blue shifts as compared to

the Raman spectrum of bulk MoS

. The two vibrational modes

positioned at 383.30 and 407.28 cm

−1

correspond to the in-

plane (E

12g

) and out-of-plane (A

) phonon modes. The diﬀer-

ence between the two modes is ∼24 cm

−1

which ensures that

the thickness of the deposited MoS

film in the sensing device

is about 4 nm.

29,30

Molybdenite-2H collected after liquid nitrogen exfoliation

was analyzed using a high-resolution analytical scanning

transmission electron microscope to gain more insights into

the crystal structure, and the results are shown in Fig. 3. The

bright-field TEM image in Fig. 3a shows a molybdenite-2H par-

ticle with a lateral size of about 7 µm. Clearly, the main part of

the molybdenite-2H particle had a darker color than its edge,

suggesting that the molybdenite-2H particle is composed of

several layers. No clear defects were seen in the lattice image

(Fig. 3b), confirming the high crystallinity of exfoliated molyb-

denite-2H layers. The interplanar spacing in the lattice fringe

image was found to be 0.27 nm, corresponding to the (100)

atomic plane of hexagonal molybdenite-2H, which is in good

agreement with the XRD data. A typical hexagonal lattice struc-

ture of molybdenite-2H was identified by indexing the SAED

pattern shown in Fig. 3c. The well-ordered diﬀraction spots in

the SAED pattern reveal the single-crystalline nature of the

layered structure of molybdenite-2H. In Fig. 3d–f, the scanning

transmission electron microscope (STEM) image and the

corresponding elemental mapping images indicate the homo-

geneous distribution of Mo and S atoms (Mo : S ratio =

1.01 : 1.97) over the entire layered structure of molybdenite-2H.

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The wide-scan XPS spectra of molybdenite concentrate

before and after liquid nitrogen exfoliation are shown in Fig. 4.

As shown in Fig. 4a, the XPS spectrum of molybdenite concen-

trate before exfoliation indicates the presence of Mo, S, O, Fe,

Al, Cu, Mg and adventitious C elements. In contrast, the XPS

spectrum of molybdenite concentrate after liquid nitrogen

exfoliation (Fig. 4b) shows only Mo, S, and adventitious C and

O elements, deducing that enriched molybdenite-2H can be

produced by liquid nitrogen exfoliation. In Fig. 4c, the high-

resolution XPS spectrum of Mo 3d shows the predominant Mo

3/2

and Mo 3d

5/2

peaks at 233.0 eV and 229.9 eV, respectively,

which are ascribed to the Mo

components of the molybden-

ite-2H phase. In Fig. 4d, the high-resolution XPS spectrum of

S 2p are deconvoluted into two characteristic S

2−

components

(S 2p

1/2

and S 2p

3/2

) of the molybdenite-2H phase at 162.1 eV

and 163.31 eV, respectively.

Fig. 5 shows the XRD patterns of the molybdenite-2H films

fabricated on SiO

/Si (MC1–MC3), n-type silicon (MC4

Fig. 3 TEM (a) and HRTEM (b) images, SAED pattern (c), STEM image (d) and the corresponding EDX elemental mapping images of Mo (e) and S (f)

of exfoliated molybdenite.

Fig. 4 Wide-scan XPS spectra of molybdenite concentrate before (a) and after (b) liquid nitrogen exfoliation and Mo 3d (c), and S 2p (d) XPS spectra

of molybdenite concentrate after liquid nitrogen exfoliation.

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andMC5) and metallic IDC-patterned SiO

/Si (MC6) substrates.

Upon increasing the speed of spin coating from 1500 rpm to

3500 rpm, the intensity of the 002 reflection in the XRD pat-

terns of molybdenite-2H films fabricated on the SiO

/Si sub-

strate was increased presumably due to the formation of a

thicker film. A similar trend was observed for molybdenite-2H

films fabricated by spin coating on the n-type silicon substrate.

That is, the intensity of the 002 reflection in the XRD pattern

of the molybdenite-2H film fabricated by spin coating at 1500

rpm was higher than that of the molybdenite-2H film fabri-

cated at 500 rpm. The molybdenite-2H film fabricated by drop

casting on the metallic IDC-patterned SiO

/Si substrate showed

a slightly lower intensity of the 002 reflection than the other

samples.

Fig. 6 shows the SEM images of molybdenite-2H films fabri-

cated on the SiO

/Si (MC1–MC3), n-type silicon (MC4 and

MC5) and metallic IDC-patterned SiO

/Si (MC6) substrates. It

can be seen in Fig. 6a that the exfoliated molybdenite-2H

sheets were deposited on the SiO

/Si substrate by spin coating

at 1500 rpm, and the shape of the deposited molybdenite-2H

sheets was visible. However, upon increasing the spin coating

speed from 1500 rpm to 3500 rpm, the deposited molybdenite-

2H films became denser and less rough, and the shape of the

molybdenite-2H sheets was less visible (Fig. 6b and c).

Similarly, the molybdenite-2H film fabricated on the n-type

silicon substrate by spin coating at 500 rpm was the least

dense and rougher, showing clearly the face-to-face deposited

irregular molybdenite-2H sheets (Fig. 6d). In contrast, when

the spin coating speed was raised to 1500 rpm, the molybden-

ite-2H film became denser (Fig. 6e), indicating a close relation-

ship between the fabricated film density and the spin coating

speed. Fig. 6f shows the SEM image of the molybdenite-2H

film fabricated by drop casting on the metallic IDC-patterned

SiO

/Si substrate. It is clear that smaller and thinner molyb-

denite-2H sheets were homogeneously deposited, from which

thicker molybdenite-2H sheets protruded. This indicates that

spin coating is advantageous over drop casting in fabricating a

homogeneous and dense molybdenite-2H film. Particularly,

the MC3 sample shows a homogeneously deposited molybden-

ite-2H film. In Fig. S4a–c in the ESI,†the EDX spectrum and

EDX elemental mapping images reveal that Mo and S are

homogeneously distributed in the molybdenite-2H film, and

no impurities are detected, implying high purity of the de-

posited film.

In recent years, MoS

has established itself as a promising

gas sensing material for detecting various environmental pol-

lutants. It oﬀers a higher density of states, a higher value of

mobility, and a lower value of interface scattering. Late et al.

studied the sensing performance of single- and few-layer MoS

upon exposure to NO

gas molecules. After comparing the

results from both types of devices, it was observed that few-

layer MoS

is more suitable for sensing applications due to its

excellent sensing response. The improvement in the sensing

response of few-layer MoS

is due to its diﬀerent electronic

structures and more stable response over time. Hence, these

appealing attributes of MoS

for gas sensing applications

attract much research interest for developing highly sensitive,

selective, and stable gas sensors. However, the incomplete

Fig. 5 XRD patterns of (a) MC1, (b) MC2, (c) MC3, (d) MC4, (e) MC5, and

(f) MC6 samples.

Fig. 6 SEM images of (a) MC1, (b) MC2, (c) MC3, (d) MC4, (e) MC5, and (f) MC6 samples.

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recovery is a major bottleneck restricting the performance of

MoS

-based gas sensors at lower temperatures (<100 °C).

Therefore, a suﬃcient external stimulus is the stringent need

for the complete recovery of practical gas sensors.

Among various available alternatives, thermal energy is one

of the most widely used techniques to improve the sensing per-

formance of the developed device. Therefore, we investigated

the sensing performance of the device fabricated using the

MC3 sample against diﬀerent concentrations of NO

diﬀerent temperatures (50, 100, and 120 °C). The relative

dynamic response of the device was calculated as (ΔR/R

)×

100% = (R

−R

)/R

× 100%, where R

is the resistance of the

MC3 sample in the presence of NO

and R

is the base resis-

tance of the device. As an oxidizing gas, NO

traps electrons

from the surface of the MC3 sample, resulting in the change

of the charge density of the deposited film (Fig. 7a).

The vari-

ation in the carrier concentration leads to a large increase in

the resistivity of the device, and hence, a high value of sensi-

tivity was achieved. At 50 °C, 48% and 67% sensitivities were

observed upon exposure to 50 and 100 ppm NO

gas mole-

cules, respectively (Fig. 7b). However, the fabricated sensor was

not fully recovered due to insuﬃcient thermal energy. A strong

binding energy between the MoS

film and the NO

molecules

prevents the complete recovery of the fabricated device at

50 °C.

Therefore, to make the fabricated device fully recover-

able, the temperature of the sensor was raised to 100 °C. At

this temperature, 42% and 52% sensitivities were achieved for

50 and 100 ppm NO

, respectively. Upon increasing the temp-

erature of the device, the recovery was improved appreciably

(Fig. 7c). Yet, a full recovery of the gas sensor was still not poss-

ible. Thus, the sensing behavior of the fabricated device was

further evaluated at a slightly higher temperature. As expected,

a full recovery of the fabricated sensor was achieved at an opti-

mized temperature of 120 °C (Fig. 8). The fabricated device

exhibited 28, 38, and 44% sensitivities to 10, 50, and 100 ppm

, respectively. Clearly, the sensitivity of the device

increased with the increasing concentration of NO

.Itwas

noticed that at 120 °C, the sensitivity was slightly lowered as

compared to the sensitivity at 50 and 100 °C. The decrement

in the sensitivity is primarily due to a higher rate of desorption

than adsorption at high temperatures. Besides acceleration of

the desorption process, the reduced interaction between the

MoS

surface and gas molecules at high temperature plays a

Fig. 7 (a) Schematic illustration of the sensing behavior of the fabricated device based on the charge-transfer mechanism, and the sensing per-

formance of the MC6 sample against 50 and 100 ppm NO

at 50 °C (b) and 100 °C (c).

Fig. 8 Sensing performance of the MC6 sample against NO

gas with

diﬀerent concentrations at 120 °C.

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crucial role in determining the sensitivity of the device. In

addition, the sensing performance of the as-received molyb-

denite concentrate without removing the impurities was also

evaluated. As shown in Fig. S5,†the sensitivity decreased from

a value of ∼28% to ∼21% in the presence of 10 ppm NO

120 °C. This decrement in the sensitivity is primarily due to

the higher thickness of MoS

in molybdenite concentrate and

lesser availability of favorable adsorption sites for NO

. The

performance of the fabricated sensing device using few-layer

MoS

was compared with earlier reports. As shown in Table S1

in the ESI,†the sensing performance of the fabricated device

is consistent with the previously reported values, indicating its

good sensing response.

The adsorption of NO

onto the predominant (002) surface

of molybdenite-2H was modeled by Forcite and Adsorption

Locator modules in Accelrys Materials Studio using the experi-

mental crystallographic structural data.

The adsorption

aﬃnities can be estimated from the distribution of field density

(Fig. 9a) and the calculated diﬀerential adsorption energies

(dE

/dN

and (dE

/dN

) which are the energies required for

removing the sorbate of a particular component. According to

the estimated diﬀerential adsorption energies, NO

can form

comparatively similar substrate–adsorbate configurations at

diﬀerent surfaces with a slightly preferential adsorption on the

(002) surface (Fig. 9a). As the sensing behavior of the molybden-

ite-2H film was tested using a 5% NO

and 95% Ar mixture, the

/dN

values were estimated for the adsorption of the NO

/Ar

mixture onto diﬀerent surfaces of molybdenite-2H:

:−4.77 kcal mol

−1

for (002) > −3.43 kcal mol

−1

for

(006) > −3.35 kcal mol

−1

for (008)

Ar:−1.82 kcal mol

−1

for (008) > −1.15 kcal mol

−1

for (006) >

−0.18 kcal mol

−1

for (002).

It is clear that NO

in the Ar/NO

mixture has better inter-

action with the surfaces of molybdenite-2H than individual

(dE

/dN

= 2.57 kcal mol

−1

The layered crystal structure of molybdenite-2H shows

closely packed double layers with S

-coordination of Mo atom

(Fig. 9). The adsorption of diﬀerent adsorbate molecules like

DNA,

CO,

thiophene,

and methanol

was studied

by molecular modeling on the surface of molybdenite to gain

more insights into the mechanisms of catalytic reaction

36,38–40

and sensing.

It has been shown that molybdenite is a prom-

ising candidate for a new generation of gas sensors based on

layered compounds. The chemical interaction mechanism for

sensing was proposed as follows:

MoS2þNO2ðgasÞ¼MoS2þNO2ðadsÞð1Þ

MoS2þNO2ðadsÞ¼MoS2NO2ðadsÞþhVBþð2Þ

The calculation results of equilibrium adsorption of Ar/

(gas) and Ar/NO

−

(ads) show negative values (exothermic

process) of total adsorption energies at diﬀerent temperatures

(Table 1). Although the total adsorption energies and diﬀeren-

tial adsorption (desorption) energies dE

/dN

are negative,

the diﬀerential adsorption energies dEad=dNNO2are positive

at diﬀerent temperatures. This implies that higher tempera-

tures should lead to the desorption of NO

and the shift of the

adsorption–desorption equilibrium to adsorption of

Ar/NO

−

(ads) on molybdenite-2H. The energy gain between the

Fig. 9 (a) Field density distribution of a 5% NO

and 95% Ar mixture on the predominant (002) surface of molybdenite-2H. Atoms: sulphur, yellow;

and molybdenum, light blue. Isosurface:NO

, green; and Ar, red. (b) Close contacts (less than 3.7 Å) of NO

/NO

−

species on the predominant (002)

surface of molybdenite-2H. Atoms: sulphur, yellow; molybdenum, light blue; nitrogen, dark blue; oxygen, red; and argon, grey.

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adsorption and desorption processes for NO

/NO

−

species

can be estimated as:

Δ¼dEad=dNðNO2ÞdEad=dNðNO2Þ14 kcal mol1ð3Þ

In general, the dE

/dN

and dEad=dNNO2values are

similar at 25 °C and 120 °C. At 120 °C, the total adsorption of

Ar/NO

(−38.59 kcal mol

−1

) is greater than that at lower temp-

eratures. However, the dE

/dN

value at 120 °C (−4.48 kcal

mol

−1

) is slightly decreased when compared to the dE

/dN

value at 25 °C (−4.77 kcal mol

−1

). The highest desorption of

Ar/NO

−

is observed at 25 °C (−20.39 kcal mol

−1

), while the de-

sorption of NO

−

at 25 °C (10.12 kcal mol

−1

) is similar to that

at 120 °C (10.00 kcal mol

−1

). Thus, NO

adsorption is still high

at 120 °C with high NO

desorption. Quantitatively, the

optimal adsorption–desorption ratio of NO

/NO

−

and the

energy gain (−14.99 kcal mol

−1

) are observed at 25 °C although

the largest gain in total adsorption–desorption is at 120 °C

(17.80 kcal mol

−1

The calculation results show that the adsorption of Ar and

molecules is impossible between the double S–Mo–S

layers connected by weak van der Waals interactions. The

negative and positive charges of the molybdenite-2H surface

should have more influence on the electrostatic interactions

between the molybdenite-2H surface and Ar/NO

−

species. As a

result, NO

is adsorbed in a vertical position when compared

to NO

−

which is adsorbed in a horizontal position corre-

spondingly (Fig. 9b). It is consistent with the previous data

that the negatively charged DNA-molecules could not interact

with the positively charged Mo atoms due to the stereochemi-

cal shield of the Mo atoms from the adsorption surface.

Therefore, DNA could be adsorbed more eﬀectively on the

MoS

surface via van der Waals interactions than through

electrostatic interactions between the phosphate and Mo

atoms. As shown in Table 1, the interaction forces between

−

and MoS

were analyzed, and the total interaction energy

was found to be about −14 kcal mol

−1

. The van der Waals

force is similar to the total energy, while the contribution of

electrostatic interaction force is less than −1.0 kcal mol

−1

the case of NO

−

adsorption. It indicates that the electrostatic

interaction is relatively weak, and the main adsorption force is

van der Waals interaction. This is in good agreement with our

description for the molecular modeling data of close contacts

between NO

/NO

−

and molybdenite-2H (Fig. 9b). The number

of close contacts of NO

is less than the number of close con-

tacts of NO

−

, indicating better interaction of NO

−

species

with the positively charged surface of molybdenite-2H.

4. Conclusions

In summary, we have characterized molybdenite concentrate

from the mining-metallurgy industry as a valuable starting

material for the fabrication of an eﬃcient and low-cost nano-

structured gas sensor. After liquid nitrogen exfoliation, impuri-

ties were removed and the enriched molybdenite-2H was de-

posited on diﬀerent substrates by spin coating and drop

casting. It was found that spin coating was advantageous over

drop casting in fabricating a homogeneous and dense molyb-

denite-2H film. The gas sensing behavior of the fabricated

sensor was studied for diﬀerent concentrations of NO

diﬀerent temperatures. It was found that the temperature

strongly influences the key sensing parameters of the device.

The sensing behavior of the fabricated device was explained

based on the charge-transfer mechanism. The molecular mod-

eling results showed that NO

in the Ar/NO

mixture had

better interaction with the molybdenite-2H surface than indi-

vidual NO

, and the main adsorption force was the van der

Waals interaction. The findings of this study reflect a straight-

forward approach for converting molybdenite concentrate into

an eﬃcient and low-cost nanostructured gas sensor.

Conﬂicts of interest

There are no conflicts of interest to declare.

Acknowledgements

MH would like to thank the Alexander von Humboldt (AvH)

Stiftung for the research award and equipment subsidy grant

(no. 3.4-8151/12 005) and The World Academy of Sciences

(TWAS) for the TWAS-COMSTECH Research Grant (no. 18-455

RG/MSN/AS_C-FR3240305789). The authors wish to thank

Dr. Tetsu Ohsuna of Toyota Central R&D Laboratories Inc. and

Prof. Masashi Hasegawa of Nagoya University for their help

with TEM observations.

Table 1 Energies of adsorption (desorption) of Ar/NO

and Ar/NO

−

on the (002) surface of molybdenite-2H

Temperature, °C Adsorption energy Rigid adsorption energy Deformation energy dE

/dN(Ar) dE

/dN(NO

)

MoS

/NO

(gas)

−173 °C −36.88 −36.88 0.00 −1.78 −1.74

25 °C −33.63 −33.64 0.01 −1.01 −4.77

120 °C −38.59 −38.59 0.00 −1.82 −4.48

MoS

/NO

−

(ads)

−173 °C −22.49 −22.49 0.00 −1.75 10.65

25 °C −20.39 −20.40 0.01 −1.35 10.12

120 °C −20.79 −20.80 0.00 −0.87 10.00

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