states of polyaniline–CNT nanocomposite at room temperature revealing the reliable and
robust flexibility of the device (Xue et al. 2017). However, some of polymer
nanomaterials suffer from a drawback of poor mechanical strength due to its brittle nature.
The hybrid nanomaterials based on inorganic or organic materials should have close
mechanical strength with that of the pristine materials used as matrix. The problems
associated with that of the mechanical strength of the sensor element were identified such
as braking by a slight shock, braking during ultrasonic wire-bonding process, poor
sintering of the material, etc. The problems were often overcome by optimizing the
synthesis, processing, or deposition techniques. Therefore, it is possible to improve the
material strength which greatly influences the gas sensitivity and performances of the
sensor. For example, exploiting the nanomechanical properties of graphene or CNT could
lead to increased sensitivity and detection limits especially for the mass sensor transduction approaches (Llobet 2013).
10.4.3 Stability
Environmental stability as well as thermal stability of the nanomaterials in the sensor
device is another issue to be considered before selecting the materials as receptor for air
pollutant. The thermal stability of inorganic nanomaterials or hybrid nanomaterials based
on inorganic materials is very high. However, the thermal stability of organic
nanomaterials and hybrid nanomaterials based on organic materials is not so high, but
in most of the cases, it is sufficient for the use as sensor layer. For that reason, the ceramic
semiconductors have been used for the sensing of air pollutants at very high operating
temperature, while the polymer-based sensors have been only used at room temperature.
In the polymer matrix nanocomposite temperature, stability is only slightly increased due
to incorporation of filler matrix. However, those nanohybrid materials should also be used
at room temperature. For example, traditional metal oxides such as tin dioxide, titanium
dioxide, zinc oxide, cobalt(II,III) oxide, indium oxide, etc. are widely used to fabricate gas
sensors at high temperature of 200–500
C, while nanocomposites of those metal oxides
with conjugated polymer have been used at room temperature for the sensing of same
gases (Xu et al. 2018). The considerable environmental stability of polymer with low
moisture absorptivity makes them important candidate for the particular applications. But
some of the polymers absorb moisture strongly and lose their property in ambient
environmental conditions. The inorganic materials generally absorb moisture very weakly
and retain their properties after removal of moisture by slight heating. Therefore, the
sensing data must be recorded at varying humidity for the sensor to be considered as
environmental analysis. As a standard presentation, the stability of the
polyaniline@flower-like tungsten trioxide nanocomposite sensor toward 10 ppm ammonia for consecutive 2 weeks at room temperature is shown in Fig. 10.16 (Li et al. 2018a).
The stability should be explained by considering the parallel nature of the curve with the
axis. Another important parameter in practical application, the long-term stability of the
nanomaterials-based sensor, directly affects the service life of the sensor. This type of
stability could be checked by observing the change of resistance as well as change of
sensing performances of the sensing layer with respect to the time at real optimized
10 Nanomaterials Based Sensors for Air Pollution Control
369
robust flexibility of the device (Xue et al. 2017). However, some of polymer
nanomaterials suffer from a drawback of poor mechanical strength due to its brittle nature.
The hybrid nanomaterials based on inorganic or organic materials should have close
mechanical strength with that of the pristine materials used as matrix. The problems
associated with that of the mechanical strength of the sensor element were identified such
as braking by a slight shock, braking during ultrasonic wire-bonding process, poor
sintering of the material, etc. The problems were often overcome by optimizing the
synthesis, processing, or deposition techniques. Therefore, it is possible to improve the
material strength which greatly influences the gas sensitivity and performances of the
sensor. For example, exploiting the nanomechanical properties of graphene or CNT could
lead to increased sensitivity and detection limits especially for the mass sensor transduction approaches (Llobet 2013).
10.4.3 Stability
Environmental stability as well as thermal stability of the nanomaterials in the sensor
device is another issue to be considered before selecting the materials as receptor for air
pollutant. The thermal stability of inorganic nanomaterials or hybrid nanomaterials based
on inorganic materials is very high. However, the thermal stability of organic
nanomaterials and hybrid nanomaterials based on organic materials is not so high, but
in most of the cases, it is sufficient for the use as sensor layer. For that reason, the ceramic
semiconductors have been used for the sensing of air pollutants at very high operating
temperature, while the polymer-based sensors have been only used at room temperature.
In the polymer matrix nanocomposite temperature, stability is only slightly increased due
to incorporation of filler matrix. However, those nanohybrid materials should also be used
at room temperature. For example, traditional metal oxides such as tin dioxide, titanium
dioxide, zinc oxide, cobalt(II,III) oxide, indium oxide, etc. are widely used to fabricate gas
sensors at high temperature of 200–500
C, while nanocomposites of those metal oxides
with conjugated polymer have been used at room temperature for the sensing of same
gases (Xu et al. 2018). The considerable environmental stability of polymer with low
moisture absorptivity makes them important candidate for the particular applications. But
some of the polymers absorb moisture strongly and lose their property in ambient
environmental conditions. The inorganic materials generally absorb moisture very weakly
and retain their properties after removal of moisture by slight heating. Therefore, the
sensing data must be recorded at varying humidity for the sensor to be considered as
environmental analysis. As a standard presentation, the stability of the
polyaniline@flower-like tungsten trioxide nanocomposite sensor toward 10 ppm ammonia for consecutive 2 weeks at room temperature is shown in Fig. 10.16 (Li et al. 2018a).
The stability should be explained by considering the parallel nature of the curve with the
axis. Another important parameter in practical application, the long-term stability of the
nanomaterials-based sensor, directly affects the service life of the sensor. This type of
stability could be checked by observing the change of resistance as well as change of
sensing performances of the sensing layer with respect to the time at real optimized
10 Nanomaterials Based Sensors for Air Pollution Control
369