different metals can be included in an array, each
will react differently to individual pollutants,
increasing the dimensions of the signal response.
MOx Sensors
Metal oxide-based sensor nodes have many
advantages: low cost of fabrication, a simple
structure, inertness and stability, robustness in
their applications, and an ability to adapt to a
great variety of gases. However, there are still a
number of basic problems that need to be solved.
Metal oxide semiconductors have for many
years been based on SnO 2 . In more recent studies,
other metal oxides have been examined closely
for their properties as gas-sensing materials. The
detection process hinges on the change in oxygen
concentration at the surface, which is generated
from adsorption and heterogeneous catalytic reactions of both oxidizing and reducing species from
the gas phase. The conductivity of the semiconductor depends on the species in the atmosphere
and on the temperature of the sensing element.
The signal that is produced is strongly dependent
on the temperature. Most sensors have a heater
attached beneath the sensor which can be used to
heat and thereby regenerate the sensor. Usually,
regeneration is affected at temperatures 50
C
higher than the highest temperature for operating
the sensing device [53].
The metal oxide semiconductors that have the
highest sensitivities to molecules adsorbed on
their surface, and similarly, to gas phase concentrations, are the materials with predominantly
ionic bonding (CdS, ZnS, etc.), although these
semiconductors usually have low chemical and
thermal stabilities [54]. There are four main
approaches for increasing the selectivity of semiconductor sensors. The first is to use filters or
chromatographic columns to separate gases
based on their physical properties. The second is
using catalysts, promoters, or other compounds as
surface additives. The third is to modify the physical preparation of the material. The fourth is to
characterize the short-term response of the sensor
to changes in concentration or temperature [55].
Electronic Structure
A wide variety of electronic structures can be
found in metal oxide semiconductors. The structures can be divided into two main categories:
transition metal oxides and non-transition metal
oxides. The latter can be subcategorized into the
pre-transition metal oxides and post-transition
metal oxides. The non-transition metal oxides
have a common s- and p- symmetry in the valence
orbitals, and for the transition metal oxides, the dorbitals play an important role. Usually the energy
difference between a cation d
n and a d
n + 1 or d
n-1
configuration is small, and so the transition metal
oxides often have several stable compositions and
electron configurations. This can result in a high
defect concentration, which leads the transition
metal oxides to have a very complicated bulk
and surface chemistry.
Transition metal oxides used in gas sensors
have d
0 or d
10 configurations. Post-transition
metal oxides have cations with filled d
10 configurations. The most common are ZnO and SnO 2 .
Transition metal oxides, such as TiO 2 , V 2 O 5 , and
WO 3 , have a d
0 configuration [54].
Sensing Mechanism
The function of the sensors is based on the shift in
equilibrium that takes place when oxygen is
chemisorbed on the surface, due to a presence of
a target gas. When oxidizing gases encounter the
sensor, the gas species acts as an electron acceptor, which results in a resistance increase in n-type
semiconductors and a resistance decrease in
p-type semiconductors. There are two main
models use to explain how metal oxide detectors
function. The first is ionosorption and considers
space-charge effects or changes in the electric
surface potential that are induced when gas
adsorption, ionization or redox reactions take
place. Ionosorption is the chemisorption of ions.
Chemisorption is adsorption where the involved
forces are valence forces, which are the same kind
as those operating in the formation of chemical
species [56]. The other model concerns oxygen
vacancies which react with gas molecules, and it
considers the variation of the subsurface/surface
oxygen vacancies and their reduction-reoxidation
432
Low-Cost Sensors for Indoor and Outdoor Pollution
will react differently to individual pollutants,
increasing the dimensions of the signal response.
MOx Sensors
Metal oxide-based sensor nodes have many
advantages: low cost of fabrication, a simple
structure, inertness and stability, robustness in
their applications, and an ability to adapt to a
great variety of gases. However, there are still a
number of basic problems that need to be solved.
Metal oxide semiconductors have for many
years been based on SnO 2 . In more recent studies,
other metal oxides have been examined closely
for their properties as gas-sensing materials. The
detection process hinges on the change in oxygen
concentration at the surface, which is generated
from adsorption and heterogeneous catalytic reactions of both oxidizing and reducing species from
the gas phase. The conductivity of the semiconductor depends on the species in the atmosphere
and on the temperature of the sensing element.
The signal that is produced is strongly dependent
on the temperature. Most sensors have a heater
attached beneath the sensor which can be used to
heat and thereby regenerate the sensor. Usually,
regeneration is affected at temperatures 50
C
higher than the highest temperature for operating
the sensing device [53].
The metal oxide semiconductors that have the
highest sensitivities to molecules adsorbed on
their surface, and similarly, to gas phase concentrations, are the materials with predominantly
ionic bonding (CdS, ZnS, etc.), although these
semiconductors usually have low chemical and
thermal stabilities [54]. There are four main
approaches for increasing the selectivity of semiconductor sensors. The first is to use filters or
chromatographic columns to separate gases
based on their physical properties. The second is
using catalysts, promoters, or other compounds as
surface additives. The third is to modify the physical preparation of the material. The fourth is to
characterize the short-term response of the sensor
to changes in concentration or temperature [55].
Electronic Structure
A wide variety of electronic structures can be
found in metal oxide semiconductors. The structures can be divided into two main categories:
transition metal oxides and non-transition metal
oxides. The latter can be subcategorized into the
pre-transition metal oxides and post-transition
metal oxides. The non-transition metal oxides
have a common s- and p- symmetry in the valence
orbitals, and for the transition metal oxides, the dorbitals play an important role. Usually the energy
difference between a cation d
n and a d
n + 1 or d
n-1
configuration is small, and so the transition metal
oxides often have several stable compositions and
electron configurations. This can result in a high
defect concentration, which leads the transition
metal oxides to have a very complicated bulk
and surface chemistry.
Transition metal oxides used in gas sensors
have d
0 or d
10 configurations. Post-transition
metal oxides have cations with filled d
10 configurations. The most common are ZnO and SnO 2 .
Transition metal oxides, such as TiO 2 , V 2 O 5 , and
WO 3 , have a d
0 configuration [54].
Sensing Mechanism
The function of the sensors is based on the shift in
equilibrium that takes place when oxygen is
chemisorbed on the surface, due to a presence of
a target gas. When oxidizing gases encounter the
sensor, the gas species acts as an electron acceptor, which results in a resistance increase in n-type
semiconductors and a resistance decrease in
p-type semiconductors. There are two main
models use to explain how metal oxide detectors
function. The first is ionosorption and considers
space-charge effects or changes in the electric
surface potential that are induced when gas
adsorption, ionization or redox reactions take
place. Ionosorption is the chemisorption of ions.
Chemisorption is adsorption where the involved
forces are valence forces, which are the same kind
as those operating in the formation of chemical
species [56]. The other model concerns oxygen
vacancies which react with gas molecules, and it
considers the variation of the subsurface/surface
oxygen vacancies and their reduction-reoxidation
432
Low-Cost Sensors for Indoor and Outdoor Pollution