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Recent Progress on Flexible Room-Temperature Gas Sensors Based on Metal Oxide Semiconductor

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With the rapid development of the Internet of Things, there is a great demand for portable gas sensors. Metal oxide semiconductors (MOS) are one of the most traditional and well-studied gas sensing materials and have been widely used to prepare various commercial gas sensors. However, it is limited by high operating temperature. The current research works are directed towards fabricating high-performance flexible room-temperature (FRT) gas sensors, which are effective in simplifying the structure of MOS-based sensors, reducing power consumption, and expanding the application of portable devices. This article presents the recent research progress of MOS-based FRT gas sensors in terms of sensing mechanism, performance, flexibility characteristics, and applications. This review comprehensively summarizes and discusses five types of MOS-based FRT gas sensors, including pristine MOS, noble metal nanoparticles modified MOS, organic polymers modified MOS, carbon-based materials (carbon nanotubes and graphene derivatives) modified MOS, and two-dimensional transition metal dichalcogenides materials modified MOS. The effect of light-illuminated to improve gas sensing performance is further discussed. Furthermore, the applications and future perspectives of FRT gas sensors are also discussed.
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Vol.:(0123456789)
1 3
Recent Progress onFlexible Room‑Temperature Gas
Sensors Based onMetal Oxide Semiconductor
Lang‑XiOu1, Meng‑YangLiu1, Li‑YuanZhu1, DavidWeiZhang1, Hong‑LiangLu1,2*
e‑ISSN 2150‑5551
CN 31‑2103/TB
REVIEW
Cite as
Nano‑Micro Lett.
(2022) 14:206
Received: 12 July 2022
Accepted: 12 September 2022
© The Author(s) 2022
https://doi.org/10.1007/s40820‑022‑00956‑9
* Hong‑Liang Lu, honglianglu@fudan.edu.cn
1 State Key Laboratory ofASIC andSystem, Shanghai Institute ofIntelligent Electronics &Systems, School ofMicroelectronics, Fudan University,
Shanghai200433, People’sRepublicofChina
2 Yiwu Research Institute ofFudan University, Chengbei Road, YiwuCity322000, Zhejiang, People’sRepublicofChina
HIGHLIGHTS
Latest progress on flexible room temperature (FRT) gas sensor based on metal oxide semiconductors (MOS) is comprehensively
reviewed.
FRT gas sensor based on pristine MOS and MOS modified with noble metal nanoparticles, organic polymers, carbon based materials
and transition metal dichalcogenide materials are meticulously reviewed.
The gas sensing mechanism of MOS chemiresistive gas sensors are introduced and the applications, future perspectives, and chal
lenges of FRT gas sensors are also proposed.
ABSTRACT With the rapid development of the Internet of Things, there is a great
demand for portable gas sensors. Metal oxide semiconductors (MOS) are one of the
most traditional and well‑studied gas sensing materials and have been widely used
to prepare various commercial gas sensors. However, it is limited by high operat
ing temperature. The current research works are directed towards fabricating high‑
performance flexible room‑temperature (FRT) gas sensors, which are effective in
simplifying the structure of MOS‑based sensors, reducing power consumption, and
expanding the application of portable devices. This article presents the recent research
progress of MOS‑based FRT gas sensors in terms of sensing mechanism, performance,
flexibility characteristics, and applications. This review comprehensively summarizes
and discusses five types of MOS‑based FRT gas sensors, including pristine MOS,
noble metal nanoparticles modified MOS, organic polymers modified MOS, carbon‑
based materials (carbon nanotubes and graphene derivatives) modified MOS, and
two‑dimensional transition metal dichalcogenides materials modified MOS. The effect
of light‑illuminated to improve gas sensing performance is further discussed. Furthermore, the applications and future perspectives of
FRT gas sensors are also discussed.
KEYWORDS Metal oxide semiconductor; Flexible gas sensor; Room temperature; Nanomaterials
Gas sensors
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Nano‑Micro Lett. (2022) 14:206 206 Page 2 of 42
https://doi.org/10.1007/s40820‑022‑00956‑9
© The authors
1 Introduction
In the past decade, the Internet of Things (IoTs), the net
works that connects diverse sensors and actuators, has
attracted enormous attention [15]. Traditional electronic
sensors are gradually transforming from bulky solid‑state
devices to portable, high‑performance, and multifunctional
devices. The rapid development of flexible electronics makes
them play a significant role in the wide applications of IoTs,
which serves as an ideal platform for wearable devices
[612]. In recent years, flexible and wearable devices have
been used as attractive alternatives to bulky analytical instru
ments and applied to perform continuous physiological
monitoring of body movement, blood temperature, blood
glucose, heart rate, and electrophysiological activities such
as electroencephalography, electrocardiography, and elec
tromyography [1316]. Recently, metaverse has become a
hot issue. Metaverses are sensory‑rich virtual worlds where
people engage with each other as virtuous avatars without
any physical limitations [17, 18]. Wearable devices integrat
ing kinds of sensors, which can conduct continuous physi
ological monitoring and real‑time interaction with software
agents, are of vital importance to the rapid development of
metaverse [19].
Various hazardous gases are released from industrial and
agricultural processes, such as CO, NOx, NH3, H2, H2S, and
volatile organic compounds (VOCs), including ethanol,
isopropanol, acetaldehyde, and formaldehyde [2023]. The
leakage of these pollutant gases will not only pollute the
environment, but also have a detrimental effect on human
body [2427]. For instance, emissions of NOx from coal
fired power stations lead to ozone holes, acid rain and severe
haze in metropolitan areas, causing serious damage to human
health, the ecological environment and the national economy
[2830]. In particular, serious air pollution can damage the
lungs of humans, facilitating the transmission and infection
of COVID‑19 [3134]. Real‑time detection of toxic gases
in industrial production and the development of wearable
gas warning devices are of significant to workers, especially
in environments where toxic gas leaks can occur. In addi
tion to the need for timely detection of hazardous gases,
the detection of specific gases is also widely used in the
area of medical healthcare [35]. Gas chromatography–mass
spectrometry analysis of human exhaled gas showed that the
exhaled gas contains more than 870 different VOCs [3638].
It is noticeable that the presence of some specific VOCs is
related to specific diseases [3941]. Through a simple breath
analysis, many diseases can be diagnosed and therapeutic
monitored noninvasively [42, 43]. For instance, ammonia
and fatty acids are found in the breath of patients with cir
rhosis, while acetone and isoprene are found in the breath of
patients with diabetes [4446]. However, the conventional
technology of breath analysis requires bulky and expensive
equipment, long time‑consumption and well‑trained per
sonnel. Therefore, there is an increasing requirement for
high‑performance gas sensors with low‑cost, high sensitiv
ity, rapid response, fabulous selectivity, and low limit of
detection (LOD).
Mechanically flexible gas sensors are one of the most pop
ular and forefront research directions of IoTs, meeting the
enormous industrial requirements of smart wearable devices
[47]. Moreover, they are crucial for monitoring environmen
tal gases, gaseous pollutants, volatile hazards, humidity,
exhaled gases, body odor, nerve agents or explosives, and
food quality. Conventional gas sensors are typically manu
factured on inorganic substrates, including quartz, glass,
alumina ceramic tubes and silicon wafers. However, their
rigidity and fragility limit their application in a variety of
new fields. In contrast, the integration of gas sensors on flex
ible substrates, such as polymer, textiles, and paper‑based
substrates has attracted the increasing attention of research
ers over the past few years, making them highly promising
in the fields of portable electronics [4851], smart textiles
[5254], radio frequency identification (RFID) [5559], and
medical health [6062]. However, the sensing performance
of flexible gas sensors including response value, selectivity,
response/recovery time, and LOD is largely influenced by
operating temperature, which generally require the configu
ration of microheaters, resulting in high energy consump
tion, great complexity of microstructure, and limitations
of applications [63]. Therefore, the flexible gas sensors
operating at room temperature (RT) are gradually arousing
extensive attention. Their portability, excellent mechanical
flexibility in harsh environments, and low energy consump
tion make them promising for various applications. Recently,
flexible room‑temperature (FRT) MOS‑based sensors have
been reported to detect a variety of gases, including NO2 [54,
6478], NH3 [7989], H2 [9094], H2S [68, 9598], C2H2
[99], ethanol [100103], acetaldehyde [104], formaldehyde
[105], acetone [106], ozone [107], isopropanol [108], tri
methylamine [109], and triethylamine [110, 111]. Beyond
Nano‑Micro Lett. (2022) 14:206 Page 3 of 42 206
1 3
that, some reported FRT gas sensors have been applied
to practical applications, such as smart face masks [100],
E‑textiles [54, 104], passive wireless RFID [112], disease
detection [98], and large‑scale flexible sensors array [65,
113], exhibiting broader application prospects in the fields of
IoTs, metaverses, industrial production, medical application,
etc. Nowadays, personalized wearable FRT gas sensors are
extensively employed to monitor the exhaled gas of patients
while they are out of hospital and provide sufficient infor
mation [48, 114]. For example, ketogenic diets (KDs) can
perform more effective weight loss when cooperated with
a FRT acetone gas sensor monitoring the concentration of
exhaled acetone [115, 116].
The FRT gas sensors have high requirements for sensing
materials that not only perform well at RT, but also under
bending conditions. In general, traditional gas sensing mate
rials including metal oxide semiconductor (MOS), con
ducting polymers, and carbon‑based materials [117122].
Among them, MOS is the most popular commercial sensing
material due to its merits of easy synthesis, high response
value, low cost, short response/recovery time, great revers
ibility, and excellent stability [123129]. However, its disad
vantages such as high‑temperature operation and high‑power
consumption hinder its wearable applications [130132].
What’s worse, high‑temperature operation not only degrades
the nanostructure of the sensing material, deteriorating the
gas sensing performance, but also hinders the detection of
explosive or flammable gases. Therefore, the ability of the
MOS‑based sensors to work at RT is of vital significance
because it leads to very low power consumption and simpli
fies the sensor structure. At the meantime, the conducting
polymers‑based sensors can operate at RT without addi
tional power requirements, but their performance degrades
in humid environment. In addition, the carbon‑based mate
rials can also greatly lower the operating temperature and
contribute to high sensitivity, but long response/recovery
time and complex processes render them inadequate for wide
application [98, 133135]. To achieve high‑performance
FRT MOS‑based chemiresistive gas sensors, optimization
has been performed from material design and alternate
activation, which is graphically presented in Fig.1. The
thought of combining MOS with conducting polymers or
carbon‑based materials is proposed owing to the integrated
advantages of both components [136142]. Beyond that,
a number of strategies have been used to improve sensing
performance of MOS‑based FRT gas sensors up to now,
such as morphological modification of pristine MOS, noble
metal nanoparticles modified MOS, light‑illuminated MOS,
and two‑dimensional (2D) transition metal dichalcogenides
(TMDCs) modified MOS. Table1 summarizes various
reported MOS‑based FRT sensors.
Several reviews based on flexible gas sensors have also
been published, introducing carbon‑based [49, 143, 144],
organic polymers‑based [145], and TMDC‑based [146] gas
sensors. In addition, several reviews dedicated to RT gas
sensors have been published, discussing the development
of various nanostructured materials‑based RT gas sen
sors [120, 132, 147, 148] and MOS‑based chemiresistive
RT gas sensors [118, 149]. However, no comprehensive
review focusing on the recent advances on MOS‑based
FRT gas sensors is available. Consequently, this review
will systematically summarize and analyze the sensing
mechanisms and recent advances in FRT MOS‑based gas
sensors based on pristine MOS, noble metal nanopar
ticles modified MOS, organic polymers modified MOS,
carbon‑based materials modified MOS, TMDCs materials
modified MOS. In addition, the effect of light‑illuminated
on improving the gas sensing performance is further dis
cussed. The current applications of FRT gas sensors are
also summarized.
Gas sensors
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Fig. 1 The strategies to achieve high‑performance FRT MOS‑based
chemiresistive gas sensors
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Table 1 Sensing performance of various MOS‑based FRT gas sensors
Material Structure Substrate Synthesis
method
Target gas C (ppm) Response τresrec LOD Bending cycles Response
decrease/bend‑
ing cycles
References
Part A: Sensing performance of pristine MOS FRT gas sensors
ZnO Nanowires PET Hydrothermal H21,000 5c ~ 600/s [90]
ZnO Nanorods Nylon Hydrothermal H2500 109c149/122s [92]
ZnO Nanoparticles PET Ethanol 800 2.2a [102]
ZnO1‑x Sheet‑like PP Suspension
flame spraying
NO21 2.568b60/230min 0.25ppm [70]
ZnO Nanoparticles Cotton fabrics Sol–gel Acetaldehyde 100 33c228/14s [104]
In2O3Cubic crystals P VA Hydrothermal Ethanol 100 ~ 1.4a5/3s 10ppm [103]
In2O3Octahedral
nanopowders
PI Oxidation NO25 5.75a105/785s 3ppm [69]
In2O3Nanowires PI Electrospun NO20.5 5.52 10ppb [78]
InOxThin films PET Magnetron sput‑
tering
Ozone 1.07 ~ 72.5c7/min 15ppb [107]
TiO2Thin films PET Spinning Formaldehyde 5 > 570b9/ ~ 300s 3.8ppb 200 No obvious
decrease
[105]
TiO2Nanotubes PI Anodization TMA 400 150c ~ 25/s 40ppm [109]
CuO Nanorectangles PET Hydrothermal NH35 ~ 0.25b90/120s 5ppm [82]
Cellulose/Fe2O3Nanoparticles PET Hydrothermal NO2200 ~ 1,100c50/30s 2ppm [71]
WO3‑δ Films PI Granule spray NO210 18,500c17/25s 1.88ppm 4,000 < 66.7%/4,000 [76]
SnO2/ZnO Nanofibers PET/PDMS/
Paper
Electrospun NO20.1 56c 0.1ppm [233]
Part B: Sensing performance of noble metal nanoparticles modified MOS FRT gas sensors
Ga/ZnO Nanorods PI Hydrothermal H21,000 91c ~ 18.8/s 0.2ppm [91]
Pd/ZnO Nanorods PI/PET Hydrothermal H21,000 91.2c ~ 18.8/s 0.2ppm 10613%/105,
52%/106
[93]
Ag/ZnO Nanorods PI Hydrothermal C2H21,000 26.2a66/68s 3ppm 10410.6%/104[99]
Pt/SrGe4O9Nanotubes PET Electrospun NH3100 7.08a17/16s 1ppm 1,000 No obvious
decrease
[80]
PANI/Rh/SnO2Nanotubes PET Electrospun NH3100 13.6 113/159s 500ppb 1,000 21% [172]
PANI/In2O3/Au Nanospheres/
Nanofibers
PI Hydrothermal NH3100 46a118/144s 100 No obvious
decrease
[79]
SWCNT/PdO/
Co3O4
Nanocubes PI Chemical pre‑
cipitation
NO220 27.33c 1ppm 4,000 No obvious
decrease
[74]
Part C: Sensing performance of organic polymers modified MOS FRT gas sensors
Nano‑Micro Lett. (2022) 14:206 Page 5 of 42 206
1 3
Table 1 (continued)
Material Structure Substrate Synthesis
method
Target gas C (ppm) Response τresrec LOD Bending cycles Response
decrease/bend‑
ing cycles
References
PANI/WO3Nanofibers/
Flowerlike
PET In situ polymeri‑
zation
NH310 7a13/49s 500ppb [87]
PANI/CeO2Nanosheet/
Nanoparticle
PI Self‑assembly NH350 262.7c14/6min 16ppb 500 No obvious
decrease
[83]
PANI/α‑Fe2O3Nanofiber/Nano‑
particle
PET Sol–gel NH3100 72c50/1575s 2.5ppm [81]
PANI/Fe2O3Sea cucumber
shaped
PET Hydrothermal NH3100 6.12a100/s 0.5ppm [85]
PANI/α‑Fe2O3Nanofiber/Nano‑
particle
PET In situ polymeri‑
zation
NH3100 39c27/46s 5ppm [89]
PANI/CoFe2O4Nanofiber/Nano‑
particle
PET In situ polymeri‑
zation
NH350 118.3c24.3/ ~ 410s 25ppb 500 3.4%/500 [84]
SnO2@PANI Nanoparticles/
Nanofiber
PET Hydrothermal NH3100 29.8 125/167s 10ppb 100 No obvious
decrease
[234]
PANI@SnO2Nanoparticle/
Nanofibers
PET In situ polymeri‑
zation
NH3100 29a34/s > 1.8ppm [86]
PANI/SnO2Nanofibrous/
Nanoparticle
PET In situ polymeri‑
zation
TEA 100 69a ~ 5/s 1.2ppm [110]
PANI/MoO3Nanorods/Nano‑
particle
PET In situ polymeri‑
zation
TEA 100 22.6a35/88.42s 0.55ppm [111]
PANI@SnO2/
Zn2SnO4
Nanofiber/Nano‑
sphere
PET In situ polymeri‑
zation
NH3100 20.4 46/54s 500ppb 500 26.5%/500 [186]
ZnO/S, N:
GQDs/PANI
Nanopolyhedra/
Nanorob
PET In situ polymeri‑
zation
Acetone 0.5 2c15/27s 0.1ppm 60 No obvious
decrease
[106]
Chitosan/WO3/
IL
Nanocomposite
membranes
Chitosan/IL Sol–gel H2S 200 2.75a ~ 13.6/s 15ppm [97]
CMC/CuO/IL Nanocomposite
membranes
CMC/IL Hydrothermal H2S 300 ~ 20c ~ 52 15ppm [95]
PVA/WO3/IL Nanocomposite
membranes
PVA/IL Sol–gel H2S 300 ~ 12c ~ 19.1/s 10ppm 5 No obvious
decrease
[96]
Part D: Sensing performance of carbon-based materials modified MOS FRT gas sensors
MWCNT/WO3Nanoparticles PET Hydrothermal NO25 14c10/27min 0.1ppm 1080.7%/106,
2.1%/108
[73]
MWCNT/WO3/
RGO
Nanoparticles PI/PET Hydrothermal NO25 17c7/15min 1ppm 106No obvious
decrease
[72]
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© The authors
Table 1 (continued)
Material Structure Substrate Synthesis
method
Target gas C (ppm) Response τresrec LOD Bending cycles Response
decrease/bend‑
ing cycles
References
MWCNT/Co3O4Nanofiber Textile fabric Heat treatment NO21,000 ~ 32c 0.1ppm [64]
SWCNT/Fe2O3Nanospheres PP CVD NO2/H2S 10/100 19/18.3c300/300s 1ppm 16 No obvious
decrease
[68]
SWCNT/CuO Flower‑shaped PP Hydrothermal H2S 1 ~ 35c7/28s 100ppb [112]
SWCNT@ZnO Quantum dot Nylon Redox reaction Ethanol 500 1.09a992/301s 1,000 No obvious
decrease
[100]
RGO/
WO3·0.33H2O
Nanoneedles PET Hydrothermal Isopropanol 100 4.96a60/s 1ppm 100 No obvious
decrease
[108]
In2O3@RGO Nanoparticles PI NO21 31.6c4.2/13.3min 50ppb [65]
RGO/ZnO Nanosheet Cotton/elastic
threads
NO215 44c140/630s 0.2ppm 3,000 No obvious
decrease
[54]
Graphene/CdO Nanoparticles LPG 600 77c 100ppm [213]
RGO/SnO2Nanoparticles PI Spraying NO2100 0.2640b412/587s 20ppm 1,000 10.3%/1,000 [231]
RGO/SnO2/
PVDF
Hot‑press thick
film
PVDF Hot press H2100 49.2c34/142s 0.5ppm [94]
SnO2/RGO/
PANI
Hollow spheres PET In situ polymeri‑
zation
H2S 2 60.11c82/78s 50ppb 60 No obvious
decrease
[98]
Part E: Sensing performance of TMDCs materials modified MOS FRT gas sensors
SnO2/SnS2Nanotubes PET Hydrothermal NH3100 2.48a21/110s 1ppm 3,000 No obvious
decrease
[88]
SnO2/MoS2Thin films PET E‑beam evapo‑
ration
NO29 7.57 3ppm [222]
Au/SnO2/WS2Nanotubes PI Sol–gel NH350 3.687 ~ 180/ ~ 330s 0.5ppm 10,000 20.26%/1,0000 [221]
Part F: Sensing performance of inorganic materials modified MOS FRT gas sensors
In2O3/g‑C3N4Nanofibers Yttria‑stabilized
zirconia
ALD NO21 7.2a31/44s 50ppb [66]
Part G: Sensing performance of IGZO based thin-film transistors FRT gas sensors
IGZO Thin films PI CVD NO25 ~ 1.3a 2ppm [75]
IGZO Nanofiber net‑
work
PEDOT:PSS Blow‑spinning NO220 33.2c5/5s 20ppb 1,000 No obvious
decrease
[67]
C: concentration; τresrec: response time/recovery time; Responsea = Ra/Rg (reducing gas) or Rg/Ra (oxidizing gas); Responseb: ΔR/Rg (reducing gas) or ΔR/Ra (oxidizing gas); Responsec:
ΔR/Rg × 100% (reducing gas) or ΔR/Ra × 100% (oxidizing gas). Ra: resistance of the sensors exposed to the background gas, Rg: resistance of the sensors exposed to the target gas, ΔR: the
change in resistance of the sensors after exposure to the target gas
PET Poly(ethylene terephthalate); PP Polypropylene; PVA Polyvinyl alcohol; PI Polyimide; PDMS Poly(dimethyl siloxane); PVDF Polyvinylidene fluoride; PEDOT:PSS Poly(3,4‑ethyl‑
enedioxythiophene) polystyrene sulfonate; IL Ionic liquid; CMC Carboxymethyl cellulose; TMA Trimethylamine; TEA Triethylamine; LPG Liquid petroleum gas; PANI Polyaniline; GQD
Graphene quantum dots; SWCNT Single‑walled carbon nanotubes; MWCNT Multi‑walled carbon nanotubes; Mxene Transition metal carbides and carbonitrides; RGO Reduced graphene
oxide; IGZO Indium gallium zinc oxide
Nano‑Micro Lett. (2022) 14:206 Page 7 of 42 206
1 3
2 Gas Sensing Mechanism ofMOS
Chemiresistive Gas Sensors
2.1 Pristine MOS Gas Sensing Mechanism
The gas sensing mechanism of MOS is based on the oxy
gen adsorption model, which assumes that the change in
resistance is related to chemisorbed oxygen [150152].
In air, oxygen molecules adsorb on the MOS surface and
form negatively charged chemisorbed oxygen (
O
2
, O,
O2−) by trapping conduction band electrons. The type of
chemisorbed oxygen is related to the operating temper
ature and species of MOS material, which significantly
determines the sensing performance of the sensing mate
rial [153, 154]. However, the temperature interval corre
sponding to the presence of chemisorbed oxygen ions on
metal oxides is not well known and varies from different
metal oxides. In general,
O
2
is usually chemisorbed when
the temperature is below 100°C. Once the temperature is
between 100 and 300°C, O is generally chemisorbed and
O
2
disappears rapidly. And when the temperature exceeds
300°C, the chemisorbed oxygen is mainly in the form of
O2− [155]. The process of oxygen ion formation can be
summarized by the following equations [156]:
As a result of oxygen adsorption, an electron depletion
layer with a low electron concentration is formed on the
surface of the n‑type MOS, which has a higher resistance
than the core region due to the reduced number of elec
trons. While on the surface of the p‑type MOS, a hole
accumulation layer is formed, which has a lower resist
ance than the core region of the MOS due to the increased
number of holes.
When exposed to the target reducing gases, the target
molecules adsorb to the surface of MOS sensing layer and
react with the chemisorbed oxygen ions, releasing elec
trons into the MOS material. Conversely, for oxidizing
gases, more electrons are trapped from the MOS surface.
(1)
In air O2(gas)
O2(ads)
(2)
T
<100
CO2(ads)+e
O
2
(ads
)
(3)
100 C
<
T
<
300 CO
2(ads)+
e
2O(ads)
(4)
T
>300 C
O
(
ads
)+
e
O
2(
ads
)
As a result of these processes, the resistance of the sens
ing layer will change significantly, which will result in the
response of the gas sensor. Therefore, the sensitivity of
the MOS‑based gas sensors is generally defined as Ra/Rg
(reducing gas), Rg/Ra (oxidizing gas), ΔR/Rg (reducing
gas), ΔR/Ra (oxidizing gas), ΔR/Rg × 100% (reducing gas),
or ΔR/Ra × 100% (oxidizing gas) (where Ra is the resist
ance of the sensors exposed to the background gas, Rg is
the resistance of the sensors exposed to the target gas, ΔR
is the change in resistance of the sensors after exposure
to the target gas).
Conventionally, MOS‑based gas sensors are operated
at 300–500 to provide sufficient activation energy to
facilitate oxygen adsorption, which is also called thermal
activation. In contrast, at RT, the chemisorbed oxygen ions
on the MOS surface are mainly
O
2
, with a low content of
other chemisorbed oxygen ions, making it a challenge to
achieve RT operation of MOS. From the perspective of
material design, in order to achieve RT operation, pris
tine MOS materials can be nanoconstructed with differ
ent morphologies, facilitating efficient modulation of the
electron depletion layer. In addition, the construction of
heterogeneous structures by surface modification of MOS
materials is also an efficient strategy.
2.2 Heterostructured MOS‑Based Nanocomposites Gas
Sensing Mechanism
Generally, MOS are often hybrids with other MOS, organic
polymers, carbon‑based materials and TDMCs to signifi
cantly improve their sensing performance. Due to the dif
ferent energy band structures of MOS and hybrid materials,
electrons or holes are transferred at the interface between the
components until their Fermi levels equilibrate to the same
energy level. This process results in the formation of hetero
junctions at the interface between the MOS and the hybrid
materials. The formation of the heterojunctions modulates
the thickness of the depletion/accumulation layer and the
height of the potential barrier, changing the internal electron
distribution between different components and significantly
affecting the sensing performance of the sensing materials.
When analyzing the mechanism of MOS nanocomposites,
the effect of heterojunctions needs to be considered primar
ily. In this section, the mechanism of heterostructured MOS‑
based nanocomposites is illustrated with two subsections:
Nano‑Micro Lett. (2022) 14:206 206 Page 8 of 42
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© The authors
anisotype heterojunction (p–n) and isotype heterojunctions
(n–n, p–p).
For the anisotype heterojunction (p–n), the Fermi energy
level of the n‑type semiconductor is generally higher than
the Fermi energy level of the p‑type semiconductor. There
fore, when two dissimilar materials with different Fermi
levels contact, the electrons are transferred from the n‑type
semiconductors to the p‑type semiconductors, and the holes
are transferred from the p‑type semiconductors to the n‑type
semiconductors until the Fermi energy levels are balanced.
After that, a depletion layer is formed at their interface and
the energy bands on both sides are bent to create a potential
barrier, which makes the electron transport channel nar
rower. In addition, owing to the wider depletion layer width
of the heterojunction, the initial resistance increases greatly
compared to the pristine MOS. Therefore, when exposed
to an oxidizing gas atmosphere, the increase in resistance
is small. However, when exposed to a reducing gas atmos
phere, the resistance decreases sharply, which can improve
the selectivity of the sensor to some extent.
For the isotype heterojunction (n–n, p–p), the band bend
ing phenomenon also occurs due to the difference in Fermi
energy levels. For n–n heterojunctions, electrons are trans
ferred from the side with high Fermi energy levels to the side
with low Fermi energy levels, while an electron depletion
layer is formed on the side with high Fermi energy levels
and an electron accumulation layer is formed on the other
side. Similarly, for p–p heterojunction, holes are transferred
from the side with low Fermi energy levels to the side with
high Fermi energy levels, while the hole accumulation layer
is formed on the side with high Fermi energy levels and the
hole depletion layer is formed on the other side.
Generally, to achieve RT operation and enhanced sensing
performance of MOS‑based gas sensors, two perspectives
can be considered: material design and alternate activation.
From the perspective of material design, for pristine MOS,
reducing the grain size, constructing various morphologies
with enhanced surface‑to‑volume ratio are considerable
sensitization strategies. In addition, chemical and electronic
sensitization can be achieved by decorating precious metals,
which can significantly improve the sensing performance
under RT. Hybridizing with some unique materials that can
chemically react with the target gas is also an effective way
to achieve RT operation. For instance, PANI reduces from
the conductive emeraldine salt state to the non‑conductive
intrinsic emeraldine base state when reacting with certain
target gases, which improves the sensing performance at
RT. The addition of carbon‑based materials to the MOS can
also remarkably improve the conductivity of the MOS‑based
nanocomposite and optimize its sensing properties at RT.
The modification of 2D TMDCs can effectively modulate the
heterojunction between MOS and TMDCs due to its unique
surface effect, exhibiting great potential at RT as well. From
the perspective of alternate activation, materials with self‑
heating capability can achieve thermal activation without
microheaters. Photoactivation also facilitates the reaction
with the target gas by introducing additional photogenerated
electron–hole pairs, thus enhancing the gas sensing perfor
mance at RT. These specific sensing mechanisms will be
discussed further in different chapters.
3 Pristine MOS FRT Gas Sensors
Pristine MOS generally have great stability, reversibility,
and their manufacturing process is facile and cost‑effective
[157159]. Various morphologies of nanostructures for pris
tine MOS including nanoparticles [71, 97, 101, 102, 104],
nanorods [92], nanowires [90], nanotubes [109], nanocu
bic crystals [103], nanorectangles [82], sheet‑like [70], and
columnar [160] nanostructures have been fabricated and
employed in flexible gas sensors to help reducing operat
ing temperature to RT (Fig.2). These sensors can be used
to detect a variety of gases, including NH3, NO2, H2, H2S,
ethanol, isopropanol, trimethylamine (TMA), formaldehyde,
acetaldehyde and ozone, performing with good sensing
performance.
N‑type pristine MOS including ZnO, In2O3, WO3, Fe2O3,
and TiO2 are the sensing materials that have been reported
for FRT chemiresistive gas sensors. Among these materi
als, ZnO has been a fascinating FRT gas sensing material
attributed to the fusion of its inherent characteristics such
as high electron mobility, wide bandgap, excellent chemi
cal stability, non‑toxicity and biocompatibility as well as
its versatility in fabricating various nanostructures such as
nanorods [92], nanowires [90], nanoparticles [102, 104],
columnar [160], and sheet‑like [70] nanostructures. Moham
mad etal. [92] fabricated hexagonal‑shaped and well‑
aligned ZnO nanorods assembled on nylon substrates using
hydrothermal method. The high specific surface area and
high crystalline quality of ZnO nanorods contributed to the
good response of 109% to 500ppm hydrogen ambient with
Nano‑Micro Lett. (2022) 14:206 Page 9 of 42 206
1 3
rapid response/recovery time. Ong etal. [90] synthesized
ZnO nanowires with a simple low‑temperature hydrother
mal method and proposed three methods, slide transfer, roll
transfer and thermal transfer, to transfer the samples onto
flexible poly(ethylene terephthalate) (PET) substrates. The
mechanically flexible ZnO nanowires gas sensor exhibited
an n‑type response value of 5.0% to 1000ppm hydrogen.
However, the ZnO nanowires with surface modification of
ammonia plasma exhibited p‑type hall results, indicating
that ammonia plasma treatment can lead to effective con
ductivity modulation of the ZnO nanowires. Furthermore,
the ammonia plasma‑treated ZnO nanowires showed a sig
nificantly enhanced response of 15% to 500ppm hydrogen
with no apparent degradation after 14months long‑term
test. Cotton fabrics have also been reported to be excellent
flexible substrates for MOS‑based FRT gas sensors. Sub
biah etal. [104] reported a multifunctional acetaldehyde
gas sensor developed by growing hexagonal‑shaped ZnO
nanoparticles on cotton fabrics through seed layer enhanced
sol–gel techniques. The introduction of cotton fabric con
tributes to the uniform distribution of ZnO nanoparticles
and the high porosity between yarns. More importantly, this
ZnO nanostructure‑modified cotton fabric is also equipped
with ultraviolet (UV) radiation protection, which reveals its
promising application in wearable gas sensing devices with
UV filtering capability.
In2O3 is also an outstanding material for FRT gas sensors
due to its excellent low‑temperature gas sensing properties
and the ability to synthesize different controllable morphol
ogies. In2O3 cubic crystals were prepared by a modified
hydrothermal synthesis [103] and then made into a flexible
composite film by blending with polyvinyl alcohol (PVA).
This flexible composite film exhibited a response of 1.4 to
100ppm ethanol at RT with significantly rapid response/
recovery time of 5/3s. Nanostructure of In2O3 octahedral
nanopowders [69] was fabricated by oxidation of ionic pre
cursor compound in low‑oxygen atmosphere. The nanopow
ders were further mixed with propanediol and then deposited
on polyimide (PI) substrates. The obtained sensor exhibited
a good response of 3 to 5ppm NO2 at 100°C, perform
ing its great potential for FRT NO2 sensors. Kiriakidis etal.
[107] reported an ozone sensor based on InOx thin films with
cylindrical structure and nanosize grains of about 20nm,
grown by magnetron sputtering on PET substrate. The sen
sor has a fabulous LOD of 15ppb at RT and exhibited a
large response of nearly 72.5% to 15ppb ozone.
N‑type WO3, TiO2, and Fe2O3 were also reported for FRT
gas sensors, while other n‑type pristine MOS applied for
FRT gas sensors are rarely reported. Ryu etal. [76] pre
pared porous WO3‑δ films on PI substrate with granule spray
process. The gas sensor exhibited high sensing properties
with a great response of 18,500% to 10ppm NO2, response/
recovery time of 17/25s and LOD of 1.88ppm. Further
more, the sensor maintained high performance after 4,000
bending/relaxing cycles, showing excellent flexibility prop
erties. TiO2 has also been reported to be used in the prepara
tion of TMA sensors. TMA is known to be released in dead
fishes with a pungent and ammonia‑like odor. The detection
of ppm‑levels TMA has been used to estimate the fresh
ness of fishes and seafood products. However, most of the
reported MOS‑based TMA sensors work at high tempera
ture, which is seriously inconsistent with the actual applica
tion. Perillo etal. [109] reported a TiO2 nanotube prepared
by anodization method and then slid it onto the PI substrate
to obtain a flexible sensor. The response of TiO2 FRT sen
sor reached 150% to 400ppm TMA with fast response time
of 25s and the detection limit was 40ppm, meeting the
actual application situation. Kim etal. [71] demonstrated
a novel bio‑friendly renewable NO2 sensor based on cellu
lose nanocrystals (CNC)/Fe2O3 composites, which exhibited
WO3
7%
Pristine
metal oxide
semiconductors
used for flexible
room temperature
gas sensors
CuO
13%
Fe2O3
7%
TiO2
13%
In2O3
20%
ZnO
40%
n
-
t
y
p
e
p
-
t
y
p
e
Fig. 2 Schematic diagram of various pristine metal oxide semicon‑
ductor used for flexible room‑temperature gas sensors introduced in
this chapter
Nano‑Micro Lett. (2022) 14:206 206 Page 10 of 42
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© The authors
whisker‑shaped morphology. The CNC/Fe2O3 sensor exhib‑
ited a tremendous response of nearly 1100% to 200ppm NO2
with fabulous reversibility, which can be contributed to the
novel dispersive morphology.
There are few p‑type pristine MOS that have been reported
in FRT gas sensors due to their low gas response. Hübner
etal. [161] demonstrated that the sensitivity of the n‑type
MOS gas sensor is the square of a p‑type gas sensor with
identical morphology, suggesting that it is a great challenge
to design a pristine p‑type MOS gas sensor with tremen
dous sensing performance. Kuritka etal. [101] fabricated a
fully inkjet‑printed CuO‑based humidity and ethanol sensor
on PET substrate. The CuO nanoparticles were fabricated
by microwave‑assisted solvothermal method and exhibited
flowerlike morphology. The inkjet‑printed sensor showed
excellent reversibility and a great response of nearly 90% to
saturated vapors of ethanol at RT, which could be attributed
to the high surface‑to‑volume ratio of flowerlike‑shape CuO
nanoparticles. Sakthivel etal. [82] presented a FRT NH3
gas sensor fabricated by screen printing CuO nanorectangles
material on PET substrate. The CuO nanorectangles were
fabricated by a surfactant‑free hydrothermal method with an
average length and breadth of 950 and 450nm, respectively.
The RT sensor showed a meaningful response to 5ppm of
NH3 with response/recovery time of 90/120s. In addition,
the sensor exhibited tremendous stability over three months,
performing its promising application prospects. As shown
above, the solely reported CuO‑based p‑type pristine MOS‑
based FRT gas sensors did not perform great sensing proper
ties compared to the n‑type pristine MOS.
In brief, various morphologies of pristine MOS have
been synthesized and transferred to flexible substrates by
diverse methods for FRT gas sensors. These sensors have
been widely used for the detection of various gases, and
some excellent sensing performance has been achieved.
In particular, one‑dimensional (1D) nanostructure exhibit
great potential to overcome high‑temperature operation
and low response due to their ultra‑high surface‑to‑volume
ratios and the large number of sites for adsorption of gas
molecules, which is currently an effective strategy for
achieving RT with pristine MOS. However, it should be
noted that for most pristine MOS FRT gas sensors, their
response at RT is much lower than that at higher operating
temperatures, their response/recovery time are quite long,
and sometimes these sensors cannot fully recover at RT
after bending. To improve their sensing performance at
RT, modification of pristine MOS materials is necessary,
which will be discussed in the following chapters.
4 Noble Metal Nanoparticles Modified MOS
FRT Gas Sensors
Surface modification with noble metals refers to doping Pt,
Pd, Au, Ag, and other noble metal nanoparticles on MOS
to improve the surface activity and promote the catalytic
decomposition of gas molecules, so as to make significant
contributions to the better sensing performance [162164].
Noble metal nanoparticles modification is considered to be
a valuable strategy to improve the response and selectivity,
reduce operating temperature and response/recovery time
of the MOS gas sensors [165, 166].
4.1 Mechanism ofNoble Metal Modified MOS
forEnhanced Gas Sensing Performance
Surface modification with noble metals can be carried
out by chemical sensitization and electronic sensitization
[167, 168]. Chemical sensitization increases the rate of
chemical processes between target gas and chemisorbed
oxygen through the catalytic action of noble metal nano
particles, which is conducive to the easier migration of
electrons, and thus enhance the performance of sensors.
In chemical sensitization, the noble metal promoter acti
vates the target gas by converting it into highly reactive
molecules and accelerating it to spill over to the semicon
ductor surface, facilitating the catalytic oxidation [169],
which is also referred to spill‑over effect. Chemical sen
sitized noble metals do not directly affect the resistance
of semiconductors and its sensing mechanism is the same
as in the absence of doping. Electronic sensitization is
the exchange of electrons between noble metal and MOS
surface that directly affects the resistance of MOS [168].
The electronic sensitized noble metal forms stable noble
metal oxides in air and are reduced to metal in a reducing
gas atmosphere [170, 171]. As the oxidation state of noble
metal varies with ambient atmosphere, the state of the
electrons on the MOS surface changes accordingly. When
the noble metal is oxidized, an electron depletion layer
is established on the MOS surface, which directly affects
the resistance of the semiconductor. On the other hand,
Nano‑Micro Lett. (2022) 14:206 Page 11 of 42 206
1 3
as the noble metal oxide is reduced to metal, the elec
tronic interaction with the MOS is disrupted, resulting in
a decrease in the depth of electron depletion layer. These
electronic sensitized noble metal oxides act as receptors
for the target gas with much stronger electronic affinity
than the adsorbed oxygen, enhancing the performance of
the gas sensor [167].
4.2 FRT Gas Sensors Based onNoble Metal
Nanoparticles Modified MOS
For many noble metal nanoparticles modified MOS flex
ible low‑temperature gas sensors, 1D vertically well‑
aligned ZnO nanorods are often used as the sensitive
layer due to the great inherent characteristics of ZnO and
the simple hydrothermal synthesis method, for instance,
Pd–ZnO nanorods/PI/PET [93], Pd–Ga–ZnO nanorods/
PI [91] and Ag–ZnO nanorods/PI [99]. Chung etal. [93]
presented a FRT H2 sensor based on the Pd‑decorated
ZnO nanorods. Figure3a exhibited the ZnO nanorods
remained vertically aligned after 1000 bending/relax
ing test, exhibiting fabulous mechanical flexibility. As
shown in Fig.3b, the Pd‑ZnO nanorods/PI/PET sensor
showed a large response of 91.2% to 1000ppm H2 at RT
and great robustness with no significant degradation after
105 bending cycles with a curvature angle of 90°. The
modification of Pd nanoparticles not only enhances the
sensor response, but also exhibits high selective absorp
tion of H2. The authors attributed this enhancement to
two‑factors. One is that H2 molecules can be easily dis
sociated on the Pd surface: H2 + Pd
2PdHx. Second, O2
in the ambient air can easily react with Pd nanoparticles
and generate a weak‑bonded state of PdO: 2Pd + O2
2PdO, which also can be dissociated and produce O2 eas
ily. Furthermore, the same group [91] developed a FRT
sensor based on perpendicularly aligned ZnO nanorods
with Pd and Ga modifications. The Pd‑3%Ga–ZnO
nanorods/PI sensor showed excellent selective charac
teristics towards H2, as given in Fig.3c. Beyond that,
this sensor also exhibited a large response of 91% to
1000ppm H2 at RT, which was improved six‑fold com
pared with the undoped Ga‑seed. More importantly, this
sensor also performed excellent mechanical stability with
no degradation after bending 105 cycles, which might be
related to its good crystallinity. Furthermore, the same
group [99] also reported Ag nanoparticles modified ZnO
nanorods gas sensor and its optical image is shown in
Fig.3d. The sensor can be activated under visible‑light
illumination: a large amount of photogenerated absorbed
oxygen ions is generated owing to the coupling between
Ag nanoparticles and ZnO nanorods, which results in
an increase of surface charge density and an enhance
ment of sensing performance. This Ag–ZnO nanorods/
PI sensor exhibited a linear response to C2H2 concentra
tions from 3 to 1000ppm with the maximum response
of 26.2 to 1000ppm C2H2 at 130°C. However, the sen
sor characteristics degraded obviously after bending 104
cycles, which might be attributed to the fracture of the
ZnO nanorods forest because of the excessive pressure.
In addition, 1D nanostructures are often fabricated through
the controllable electrospinning method. Featured with regu
lated porosity, high surface‑to‑volume ratio, and tunable pore
size, the electrospun 1D nanomaterials exhibit fabulous RT
gas sensing properties. Moreover, modified with the proper
noble metal catalyst, the 1D nanostructured MOS gas sensors
show better RT sensing performance. A novel n‑type wide‑
bandgap MOS sensing material, SrGe4O9 has been reported
for the detection of NH3 at RT by Huang etal. [80]. Polycrys
talline SrGe4O9 nanotubes were synthesized via a single‑noz
zle electrospinning process and Pt‑modified SrGe4O9 nano‑
tubes were prepared by annealing the mixture of SrGe4O9 and
H2PtCl6 solution, as shown in Fig.3e. The sensing materials
were further assembled on a PET substrate to form a FRT
sensor. The sensor exhibited a reliable detection of NH3
within the concentration of 1–500ppm, a response of 7.08
to 100ppm NH3 with fast response/recovery time of 17/16s,
excellent mechanical stability with a large bending angle of
150° and 103 cycles of bending/relaxing test. Pt nanoparticles
modified SrGe4O9 exhibited significantly enhanced response
compared to the pristine SrGe4O9. The authors attributed the
enhanced gas sensing performance to electronic sensitiza
tion and chemical sensitization. In electronic sensitization, a
Schottky barrier was formed between SrGe4O9 and Pt, there
fore, the electrons transferred from SrGe4O9 to Pt. In chemi
cal sensitization, dissociation and adsorption of O2 molecules
on the surface of SrGe4O9 were catalytically activated by Pt
nanoparticles. The modification of noble metal nanoparticles
can also enhance the sensing properties of metal oxide‑based
heterojunction. Rh‑doped 1D hollow SnO2 nanotubes have
also been reported for FRT NH3 sensors by Liu etal. [172].
Nano‑Micro Lett. (2022) 14:206 206 Page 12 of 42
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© The authors
As presented in Fig.3f, they synthesized Rh‑doped SnO2
hollow nanotubes by electrospinning and sintering. Subse
quently, the PANI coated Rh‑doped SnO2 hollow nanotubes
was prepared by insitu polymerization and drop‑coated on
PET to form flexible sensors. Beyond that, Rh can be modi‑
fied into the SnO2 lattice due to the similar ionic radii of Rh3+
and Sn4+, which contributed to the enhanced noble metal
sensitization. The Rh modification not only facilitates the
adsorption of NH3, but also promotes the decomposition of
NH3 into highly reducing H and NH2, which can result in a
rapid thinning the depletion region of SnO2, resulting in a fast
response (113s) and larger response values (13.6 to 100ppm
NH3). Moreover, as performed in Fig.3g, the response of the
sensor decreases only slightly at a bending angle of 30° and
it is worth noting that the bending has no significant effect
on the initial resistance of the sensor. Even after bending for
1,000 times, the sensor still possesses a reliable response,
which means that the 1D nanostructure remain stable under
bending situation.
)b()a(
)e()d(
(f) (g)
Rh
1. SnCl2/PVP nanofiber
ria ni gniretniSgninipsortcelE In-slu polymerization
2. Rh-doped SnO2 nanotube 3. PANI/Rh-doped SnO2 nanotube
Collector
Annealing
Annealing
Add
H2PtCl6
Sr(NO3)2/
Ge(OEt)4/PVP
solution
Sr(NO3)2/Ge(OEt)4/
PVP nanofibers
Porous
SrGe4O9
nanotubes
Porous
Pt-SrGe4O9
nanotubes
Electrode
PET
Ag-ZnO NRs/PI
(c) H2
N2
O2
CO2
NO2
CO
Undoped seed
1% Ga-doped seed
2% Ga-doped seed
3% Ga-doped seed
4% Ga-doped seed
5% Ga-doped seed
100 nm
1.50
1.25
1.00
0.75
0.50
0.25
0.00
Relative resistance (Rg/Ra)
0 200 400 600 800
Time (s)
1000
ppm
800
ppm
Flat
102 times bending/relaxing
103 times bending/relaxing
104 times bending/relaxing
105 times bending/relaxing
106 times bending/relaxing
600
ppm
400
ppm
200
ppm
100
ppm
10001200 14001600
100
80
60
40
20
0
−20
6
5
4
3
2
1
0
6
4
2
0
Response, S(%) = ΔR/Ra×100%
Response (Rg/Ra)
Bend 30° NH3 Conc. 50 ppm
Initial
Initial
BendInitial
0 200
20.0k
16.0k
12.0k
8.0k
4.0k
0.0
400
0 400
Response (Rg/Ra)
800 0 400 800 0 400 800
600 800 1000
Time (s)
Time (s)
1200 1400 16001800
Resistance (Ω)
0 400 800 0
Time (s)
400 800
Bent for 500 times Bent for 1000 times
Fig. 3 a Cross‑sectional view of the Pd‑ZnO nanorods/PI/PET sensor after 1000 times bending/relaxing test. b The response, reliability test
of Pd‑ZnO nanorods/PI/PET sensor. Reproduced with permission from Ref. [93]. Copyright (2013) Elsevier. c The selectivity of Pd–Ga‑ZnO
nanorods/PI sensor for various Ga‑assisted seed layers conditions at 1000 ppm H2. Reproduced with permission from Ref. [91]. Copyright
(2014) Elsevier. d The optical image of Ag‑ZnO nanorods/PI sensor. Reproduced with permission from Ref. [99]. Copyright (2018) Springer
Nature. e Schematic illustration of the fabrication process of Pt‑SrGe4O9 nanotubes. Reproduced with permission from Ref. [80]. Copyright
(2018) Springer Nature. f Schematic illustration of the fabrication process of PANI/Rh/SnO2. g Response of PANI/Rh/SnO2 sensors to 50ppm
NH3 at RT under bending test. Reproduced with permission from Ref. [172]. Copyright (2021) Elsevier
Nano‑Micro Lett. (2022) 14:206 Page 13 of 42 206
1 3
In brief, modification with noble metals is also a superior
scheme to enhance the response and selectivity due to the
synergistic effect of chemical sensitization and electronic
sensitization. Noble metal modification is generally effec
tive in creating more defects, increasing the number of
active sites, providing more oxygen species, and reducing
the activation energy of the reaction between the gas mol
ecules and adsorbed oxygens, thus accelerating the dynamic
equilibrium between oxygen adsorption and desorption. Fur
thermore, some noble metals are specific for the detection
of certain gases, which is beneficial for RT operation. For
instance, Pd‑modified MOS sensors exhibit a particularly
large response to H2 due to the unique break‑junction effect,
while Rh‑modified MOS sensors have a high response to
NH3. Beyond that, the small size of noble metal nanoparti
cles does not affect the mechanical flexibility properties of
the sensing material. All these features help to enhance the
FRT gas sensing performance of MOS‑based gas sensors.
5 Organic Polymers Modified MOS FRT Gas
Sensors
Organic conducting polymers‑based gas sensors have
attracted numerous interests due to their tunable electri
cal properties, simple fabrication, great stability, flexibil
ity, environmental stability and RT operation [173175].
However, the relatively low conductivity and the poor
selectivity restrict the application of pristine conducting
polymer‑based sensors [174]. Therefore, coupling con
ducting polymers with other heterogeneous materials is
a considerable strategy to enhance the sensing properties
of the sensors [176179]. Combing organic conducting
polymers with MOS can complement the drawbacks of
pristine MOS and organic conducting materials, especially
the poor response and selectivity of pristine organic con
ducting polymers and the high operating temperature of
MOS [180182]. Organic conducting polymers including
polyaniline (PANI), polypyrrole (PPy), polythiophene
(PTh), poly (3,4‑ethylenedioxythiophene) (PEDOT) and
polyacetylene (PA) have been widely used in fabricating
high‑performance RT gas sensors [183]. However, only
PANI is widely used for FRT gas sensors. PANI arrested
the most interest because of its relatively high conductiv
ity, ease of fabrication, RT operation, low cost, environ
mental stability, and friendliness [176, 184, 185].
5.1 Mechanism ofPANI Modified MOS forEnhanced
Gas Sensing Performance
The gas sensing mechanism of pristine PANI has been
widely investigated. Among them, the most commonly
accepted mechanism was based on the PANI protonation/
deprotonation process. In the doped emeraldine salt (ES)
form, PANI is electrically conductive and, contrarily, in the
dedoped emeraldine base (EB) form is insulating, where
doping and dedoping can be carried out with acid or base,
respectively [119]. The ability to switch between the con
ducting and insulating forms enables PANI responsive to
acids/bases and reducing/oxidizing gases such as NH3, tri
ethylamine (TEA), H2, NO2, and some VOCs.
The formation of heterojunction between PANI and MOS
plays a significant role in the enhancement of the sensing
properties. When PANI contacts MOS, the difference in
Fermi energy levels leads to carrier transfer, forming a het
erojunction and a narrow depletion region at their interface.
When exposed to the target gas, PANI and the chemisorbed
oxygen on the surface of MOS reacts rapidly, which modu
lates the width of the depletion region and rapidly affects the
resistance of the sensing nanocomposites.
For instance, Quan etal. [110] synthesized network
structures of PANI/SnO2 through insitu chemical oxidation
polymerization. The mechanism for the enhanced sensing
performance of the PANI/SnO2 composite was proposed.
When p‑type PANI is in contact with n‑type SnO2, the elec
trons in SnO2 and holes in PANI will diffuse in opposite
directions owing to their difference in Fermi energy levels. A
p‑n heterojunction and a narrow depletion region are formed
at the interface of PANI and SnO2, as presented in Fig.4a.
In this process, at their interface, a hole depletion region is
formed on the surface of PANI, while an electron deple
tion layer is formed on the surface of SnO2. When PANI/
SnO2 is exposed to the atmosphere of the target gas TEA,
on the one hand, the protons in the N+–H sites of PANI are
drawn off and PANI is reduced from the conductive doped
ES state to the insulating dedoped EB state, which leads to
a decrease in the conductivity of the materials. On the other
hand, the absorbed TEA molecules release electrons into the
p‑n heterojunction, which decrease the hole concentration of
PANI and the electron concentration of SnO2, leading to a
thickening of the hole depletion region and a thinning of the
electron depletion region (Fig.4a). Since the nanocomposite
exhibits p‑type semiconductor behavior, the conductivity of
Nano‑Micro Lett. (2022) 14:206 206 Page 14 of 42
https://doi.org/10.1007/s40820‑022‑00956‑9
© The authors
the composite decreases rapidly and result in enhanced sens
ing properties. The mechanisms of PANI modified MOS for
enhanced gas sensing performance are mostly similar, and
are illustrated based on the chemical state transition of PANI
and the effective electron transfer in the heterojunction.
5.2 FRT Gas Sensors Based onPANI Modified MOS
2D PANI nanosheets have been applied to construct the
core–shell nanostructures of PANI and MOS nanocompos
ites, which can effectively enhance the sensing surface area
and electrical conductivity of the bulk of sensing materials.
Liu etal. [83] proposed a FRT trace‑level NH3 sensor based
on insitu self‑assembled PANI‑CeO2 nanomembranes on
a PI substrate. The nanomembranes presented a core–shell
nanostructure with a core of CeO2 nanoparticles and a shell
of PANI nanosheet. The author suggested that CeO2 nano
particles can influence the alignment of PANI and change the
morphology of PANI shell. What’s more, an appealing dis
covery was observed that the synergistic oxidation of CeO2
and ammonium persulfate increases the protonation and
oxidation of PANI, resulting in an enhanced = NH+–ratio,
which would offer additional adsorption sites. Beyond that,
the sensor performed a splendid gas sensing performance
with large sensitivity of 262.7% to 50ppm NH3, fabulous
response‑concentration linearity, great selectivity, ultralow
detectable concentration of 16ppb, and theoretical LOD of
0.274ppb. In addition, there was no obvious decrease in
)b()a(
(c)
)f()e()d(
without bending
bending at 60°
Initial
NH3 Conc. 10 ppm
Bending 100 times
Bending 500 times
PET-PAni-CoFe3O4 (50%)-50 ppmRH: 40%
InCl3·4H2O
critric acid urea
mesoporous
In2O3 nanospheres
Au-loaded mesoporous
In2O3 nanospheres
mlif gnisnes elbixelfyAxnIAP
PANI-CeO2 Film
Wp
Wp-TEA Wa-TEA
Wa
Ev
Ev
Ef
Eg=3.2 eV
Eg=2.4 eV
LUMO
TEA
Hole
hydrothermal
80
70
60
50
40
30
20
10
0
120
100
80
60
40
20
0
9
8
7
6
5
4
3
2
1
0
dopped with Au in-situ
polymerization
assembled on
PET substrate
Doped PANI
Dedoped PANI
SnO2
Depletion region of PANI
Depletion region of SnO2
TEA
p-PANI n-SnO2
HOMO
HN
HN NH NN
ClCl
Cl
Cl
ES form PANI
EB form PANI
Depletion layer
CeO2
PANIPANI
conductivity pathway conductivity pathway
CeO2
NH3
NH3
NH4
NH2NN
H
y1-y
y
n
1-y
n
++
+NH4
+
Response (Rg/Ra)
Response (Rg/Ra)
Response (Rg/Ra)
Response (Rg/Ra)
6
4
2
00 500
500 ppb
500 ppb
1 ppm
5 ppm
10 ppm
20 ppm
50 ppm
80 ppm
100 ppm
1 ppm
5 ppm
1000 15002000
Time (s)
PANI
PAIn10A0.5
PAIn5A2
PANI
PAIn10A0.5
PAIn5A2
PAIn10
PAIn20A1
PAIn10
PAIn20A1
01000 2000 300040005000
Time (s) 0 200 400 600
Time (s)
0200 400 600
Time (s)
Fig. 4 Schematic illustrations of the sensing mechanism of a PANI/SnO2 and b PANI‑CeO2 for enhanced gas sensing performance. Reproduced
with permission from Refs. [83, 110], Copyright (2018) Elsevier (2017) Elsevier. c The process flow for preparation of Au‑In2O3@PANI sen‑
sors. d Transient response of the Au–In2O3@PANI sensors to 0.5–100ppm NH3 at RT. Reproduced with permission from Ref. [79]. Copyright
(2018) Elsevier. e Response of PANI‑CoFe2O4 with a bending angle of 60° at 50ppm of NH3. Reproduced with permission from Ref. [84]. Cop‑
yright (2021) MPDI. f Response of PANI @ porous nanospheres SnO2/Zn2SnO4 after 100 and 500 cycles of bending/relaxing test. Reproduced
with permission from Ref. [186]. Copyright (2020) Elsevier
Nano‑Micro Lett. (2022) 14:206 Page 15 of 42 206
1 3
response after 500 times bending, which might originate
from the flexibility of polyaniline chains and the splendid
adhesion and nano‑mechanical properties of PANI‑CeO2
nanomembranes. The authors ascribed the fabulous gas
sensing performance to the synergetic benefits of the for
mation of p‑n heterojunctions and the enhanced protonation
degree of PANI, as presented in Fig.4b.
PANI nanofibers [79, 81, 84, 87, 89, 110] and nanorods
[85] usually have higher specific surface area and conductiv
ity than the granular PANI owing to their 1D nanostructure,
which facilitates electronic interactions between sensing
material and target gas, and endows high response and low
detection limit to FRT sensors. Li etal. [87] reported nano
hybrids of PANI nanofibers and flowerlike WO3 nanoparti
cles, which was synthesized by an insitu chemical oxidation
polymerization method. PANI grew on the flowerlike WO3
surface, forming a loose and porous nanostructure, which
facilitates the adsorption and diffusion of NH3 molecules and
promotes the modulation of the interfacial depletion region.
Therefore, the fabricated sensors exhibited a high response
of approximately 20.1 to 100ppm NH3 at RT, which was
6 times larger than pristine PANI. What’s more, the sensor
also showed rapid response/recovery time of 13/49s, LOD
of 500ppb, fabulous moisture resistance, great selectivity,
and excellent mechanical stability. The same group [79]
presented a core–shell nanostructure with the core of Au‑
decorated In2O3 nanospheres and the shell of PANI nanofib
ers, which was synthesized by a facile hydrothermal and
insitu chemical oxidation polymerization method (Fig.4c).
The fabricated nanohybrids were subsequently loaded on
PET substrates to form flexible sensors. The sensing perfor
mance of sensors based on pristine PANI, In2O3@PANI, and
Au–In2O3@PANI at RT were tested as presented in Fig.4d.
The Au–In2O3@PANI sensor showed a high response up to
46 to 100ppm NH3 at RT, which is 14 and 4 times larger
than the pristine PANI sensor and In2O3@PANI sensor,
respectively. The fabulous sensing properties is contrib
uted to the chemical sensitized effect of Au, the p‑n het
erojunction formed at the interface of In2O3 and PANI, and
the improved protonation degree of PANI. Saleh etal. [84]
presented a trace‑level NH3 gas sensor composed of PANI
nanofibers and CoFe2O4 nanoparticles on PET substrates
by insitu chemical oxidation polymerization. The n‑type
CoFe2O4 nanoparticles were encapsulated in the p‑type
PANI, forming a p‑n heterojunction at the PANI–CoFe2O4
interface. Therefore, the sensor exhibited great selectivity to
NH3 and a significant response of 118.3% response towards
50ppm for 24.3s at RT. Notably, the sensor showed no
response degradation while bending at 60° (Fig.4e) and
exhibited a trace‑level detection limit of 25ppb.
PANI with 1D morphology was also hybridized with
Fe2O3 to acclimatize its structural properties, forming
high‑performance FRT gas sensors [81, 85, 89]. Bandgar
etal. [81] displayed a RT NH3 sensor based on camphor
sulfonic acid‑doped PANI/α–Fe2O3 on PET substrates by
insitu chemical oxidation polymerization method. Camphor
sulfonic acid acted as a surfactant, contributing to the dis
persion of α–Fe2O3 into PANI nanofibers matrix and the
generation of active sites. This flexible sensor showed an