Materials Science and Engineering B 139 (2007) 1–23
Review
Metal oxides for solid-state gas sensors: What determines our choice?
G. Korotcenkov ∗
Department of Micro- and Optoelectronics, Technical University of Moldova, Chisinau, Republic of Moldova
Received 4 October 2006; received in revised form 15 December 2006; accepted 29 January 2007
Abstract
The analysis of various parameters of metal oxides and the search of criteria, which could be used during material selection for solid-state gas sensor
applications, were the main objectives of this review. For these purposes the correlation between electro-physical (band gap, electroconductivity,
type of conductivity, oxygen diffusion), thermodynamic, surface, electronic, structural properties, catalytic activity and gas-sensing characteristics
of metal oxides designed for solid-state sensors was established. It has been discussed the role of metal oxide manufacturability, chemical activity,
and parameter’s stability in sensing material choice as well.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Gas sensors; Metal oxides; Desired properties
1. Introduction
Numerous researches have shown that a characteristic of
solid-state gas sensors is the reversible interaction of the gas
with the surface of a solid-state material [1–4]. In addition to
the conductivity change of gas-sensing material, the detection
of this reaction can be performed by measuring the change
of capacitance, work function, mass, optical characteristics or
reaction energy released by the gas/solid interaction [5–16]. Various materials, synthesized in the form of porous ceramics, and
deposited in the form of thick or thin films, are used as active
layers in such gas-sensing devices [17–21]. The read-out of the
measured value is performed via electrodes, diode arrangements,
transistors, surface wave components, thickness-mode transducers or optical arrangements. However, in spite of so big variety
of approaches to solid-state gas sensor design the basic operation principles of all gas sensors above mentioned are similar
for all the devices. As a rule, chemical processes, which detect
the gas by means of selective chemical reaction with a reagent,
mainly utilize solid-state chemical detection principles [2,22].
Theoretically there are no limitations for using any materials
for solid-state gas sensors design independently of their physical, chemical, structural or electrical properties. Thousands of
results have been reported about the characteristics and per-
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doi:10.1016/j.mseb.2007.01.044
formance of sensors based on different materials. At present,
gas sensor’s prototypes on the base of covalent semiconductors,
semiconducting metal oxides, solid electrolytes, polymers, ionic
membranes, organic semiconductors, and ionic salts have been
already tested [5–7,23–31]. However there are no evidences for
assertion that all materials are equally effective for gas sensors
applications. Therefore at such a big variety of materials, which
can be used, the selection of optimal sensing material becomes
key problem in both design and manufacturing of gas sensor
with required operation parameters [8,32–36].
For example, according to some earlier view on the problem of gas sensor design [1], the almost any metal oxide could
be a basis for solid-state gas sensor. For this purpose we need
only to prepare this metal oxide as a sufficiently fine dispersed
porous substance with properties controlled by surface states.
However, while requirements to elaborated gas sensors were getting stronger, and understanding of the nature of the gas-sensing
effects was getting more fundamental [37–43], our conceptions
of any material compatibility for gas sensor elaboration started
changing. We began to understand that for implementation of all
requirements, a material for solid-state gas sensors have to be
possessed of specific combination of their physical–chemical
properties, and not every material can be corresponded these
requirements. According to [34,44,45] in order to be used in
practice, a gas sensor should fulfil many requirements, which
depend on the purposes, locations and conditions of sensor operation. Among the requirements, primarily important would be
sensing performance-related ones (e.g., sensitivity, selectivity
2
G. Korotcenkov / Materials Science and Engineering B 139 (2007) 1–23
Table 1
Comparison of various types of gas sensors
Parameter
Sensitivity
Accuracy
Selectivity
Response time
Stability
Durability
Maintenance
Cost
Suitability to portable instruments
Type of gas sensors
Semi-conductor
Catalytic combustion
Electro-chemical
Thermal conductive
Infrared absorption
e
g
p
e
g
g
e
e
e
g
g
b
g
g
g
e
e
g
g
g
g
p
b
p
g
g
p
b
g
b
g
g
g
g
g
g
e
e
e
p
g
e
p
p
b
e: excellent; g: good; p: poor; b: bad.
and rate of response) and reliability-related ones (e.g., drift, stability and interfering gases). These are all connected with the
sensing materials used so that the selection and processing of
the sensing materials (materials design) have key importance in
research and development of gas sensors.
Of course this paper cannot include exhaustive reviews of all
available solid-state gas sensors and materials aimed for application in these devices. At present there are three main types of
solid-state gas sensor currently in large-scale use [46]. They
are based on solid electrolytes (electrochemical sensors), on
catalytic combustion (pellistors) and on resistance modulation
of semiconducting oxides (conductometric or chemiresistancebased gas sensors). In this paper the main attention will be
focused on the third type of solid-state sensors. The comparison
of semiconductor gas sensors with another types of solid-state
gas sensors is presented in Table 1.
The semiconductor gas sensors offer low cost, high sensitivity and a real simplicity in function; advantages that should work
in their favor as new applications emerge. Moreover, the possibility of easily combining in the same device the functions of
a sensitive element and signal converter and control electronics
markedly simplifies the design of a sensor and constitutes the
main advantage of chemiresistive-type sensors over biochemical, optical, acoustic, and other gas-sensing devices [22]. A
sensing element of these sensors normally comprising a semiconducting material presenting a high surface-to-bulk ratio is
deployed on a heated insulating substrate between two metallic electrodes. Reactions involving gas molecules can take place
at the semiconductor surface to change the density of charge
carriers available.
It is necessary to note that in spite of the simple working
principle of chemiresistive gas sensor, the gas-sensing mechanism involved is fairly complex. The gas/semiconductor surface
interactions on which is based the gas-sensing mechanism of
chemiresistive gas sensors occur at the grain boundaries of
the polycrystalline oxide film. They generally include reduction/oxidation processes of the semiconductor, adsorption of
the chemical species directly on the semiconductor and/or
adsorption by reaction with surface states associated with preadsorbed ambient oxygen, electronic transfer of delocalized
conduction-band electrons to localized surface states and vice
versa, catalytic effects and in general complex surface chemi-
cal reactions between the different adsorbed chemical species
[22,47–51]. Consequences of these processes for physical properties of metal oxides are shown in Fig. 1. The effect of these
surface phenomena is a reversible and significant change in electrical resistance (i.e., a resistance increase or decrease under
exposure to oxidizing and reducing gases respectively, referring
as example to an n-type semiconductor oxide). This resistance
variation can be easily observed and used to detect chemical
species in the ambient. The influence of these surface chemistry
phenomena on the sensor response may be understood on the
base of the models discussed in Refs. [1,2,16,42,52–59].
The above brief survey of mechanisms by which semiconducting oxides provide responses to changes in atmospheric
composition emphasises the detailed electronic properties of the
bulk and the reactivity of the solid surface and thus leads to an
expectation that the characteristics of gas sensors will be strongly
influenced by materials selection [2,46]. Therefore, the principal issues involved in the role of materials in the semiconductor
chemiresistive gas sensor are outlined below.
A systematic consideration of the desired parameters of
materials for gas sensor applications indicates that the key
properties, determining our choice, include the following:
adsorption ability; electronic, electro-physical and chemical
properties; catalytic activity; thermodynamic stability; crystallographic structure; interface state; compatibility with materials
and technologies to be used in gas sensors fabrication; reliability,
etc. [1,2,8,34,42,43,52,53,60]. Many different materials appear
favorable in some of these properties, but very few of them are
promising with respect to aggregate of all these requirements.
For this assertion confirmation let us examine a parameters
of metal oxides, which can determine material’s applicability
for gas sensors design. Certainly this brief review could not
cover all promising metal oxides, developed for gas sensors.
In addition to binary oxides, there are numerous ternary, quaternary and complex metal oxides, which are of interest of
mentioned applications [8,32,36,54]. Therefore, for simplicity
of analysis in this review the priority was given to examination of binary oxides. Inclusion the more complicated metal
oxides in our review would make our task more difficult. Where
it is possible, other materials, first of all polymers, would be
analyzed. However it is necessary to note, that because of fundamental distinctions in metal oxides and polymers’ properties
G. Korotcenkov / Materials Science and Engineering B 139 (2007) 1–23
3
Fig. 1. Diagram illustrating processes taking place in metal oxides during gas detection and their consequences for polycrystalline metal oxides properties.
and gas response mechanism of sensors on their base, a comparative analysis for indicated materials could be conducted
just for limited number of parameters, controlling gas-sensing
effects.
2. Sensing material choice through their surface
properties
It is known that operating characteristics of solid-state gas
sensors, especially sensitivity, are controlled by three independent factors such as receptor (recognition) function, transducer
function and peculiarities of sensor construction. Receptor function provides the ability of the oxide surface to interact with
the target gas, and transducer function provides the ability to
convert the signal caused by chemical interaction of the oxide
surface into electrical signal [44]. Surface of metal oxides
is responsible for receptor function of solid-state gas sensors [3,4,40,41,55,56,61,62], and therefore this section will be
devoted to brief overview of some surface properties of metal
oxides important for gas sensor operation.
2.1. Density of surface states
The density of native surface states is metal oxide surface
parameter, which has special importance for solid-state gas
sensors. If we want to achieve effective operation of solid-state
gas sensors, the concentration of those states should be
minimized. Only in this case the surface Fermi level will not
be pinned. Indicated surface’s property creates a condition
for modulation of surface potential of semiconductor at the
change of surrounding atmosphere, because the charge of native
surface states becomes commensurable, or less then the charge
of chemisorbed particles.
The same correlation with concentration of native surface states was observed for Schottky barrier heights at
the metal–semiconductor interface [63,64]. According to
this regularity, the height of the Schottky barrier at the
metal–semiconductor interface can be represented as
US = K(WMe − Ws ),
(1)
where WMe , and Ws are the work functions for emission of
an electron from metal and semiconductor, and K is a chemical parameter depending on the nature of a semiconductor.
The parameter K is defined as a coefficient of a linear dependence, relating Us and (WMe − Ws ), and can be interpreted as a
demonstration of the sensitivity of the electronic properties of
a semiconductor to the state of its surface. Fig. 2 presents the
parameter K in relation to the difference of electronegativities
X (by Pauling) of the anion and cation, forming the semiconductor. The abrupt change in the K value at X = 0.8 corresponds
to a transition from materials with covalent bonding (Si, Ge,
GaAs) to those in which ionic bonds predominate (ZnO, SiO2 ,
SnO2 ).
If one could transform presented correlation for gas-sensing
effects, it would mean that the highest sensitivity to the changes
in the concentration of molecules adsorbed onto the surface and,
consequently, to the changes in the gas phase composition, is
exhibited by materials with predominantly ionic bonding, for
example, by such materials as CdS, ZnS, SiO2 . However, standard ionic semiconductors such as ZnS, CdS have low chemical
and thermal stability, which limits sufficiently the field of their
possible application in gas sensor design. Metal oxides, having
4
G. Korotcenkov / Materials Science and Engineering B 139 (2007) 1–23
change. The increase of native surface states’ density also leads
to the growth of sensor’s threshold of sensitivity.
2.2. Electronic structure of gas-sensing metal oxides
Metal oxides exhibit a very wide range of electro-physical
properties [61,62,66–68]. Their electrical behavior ranges from
the best insulators (e.g., Al2 O3 and MgO) through wide-band
gap and narrow-band gap semiconductors (TiO2 , SnO2 and
Ti2 O3 , respectively) to metals (V2 O3 , Nax WO3 , and ReO3 ),
and superconductors (including reduced SrTiO3 ). The range of
electronic structures of oxides is so wide that metal oxides were
divided into two following categories:
Fig. 2. Influence of electronegativity on the value of K of the dependence
Us = KX (adapted from [64]).
also low concentration of native surface states, have much higher
thermal and temporal stability of parameters, which provides
their more successful using in gas sensors. Thermodynamically
some fluorides, such as LaF3 , are more stable than oxides in air
at high temperature [2]. However, they often are more volatile.
The influence of surface state density on gas-sensing effects
was illustrated in [42,65]. For these purposes it was used the
analysis of CO detection by SnO2 gas sensors. Theoretical simulations for CO partial pressure (PCO ) influence on surface
potential of metal oxides (Us ) (see Fig. 3) have shown that the
density of native surface states Nss really is a factor, determining
the Us behavior. The slope of Us (PCO ) curve decreases with the
increase of this parameter value. Moreover it was established
that in the case when Nss > N*, where N* is the concentration of
adsorption sites, the surface charge, associated with SnO2 native
surface states and with adsorbed charged species (Q = Qss + Q*),
is non-affected by ambient gas surrounding. In other words, the
surface potential is pinned and is not sensitive to gas atmosphere
Fig. 3. Results of theoretical simulation of surface states density influence on the
surface potential dependencies of undoped SnO2 films from relative CO pressure
(PCO /PO2 ): Nd = 1019 cm−3 ; Toper = 300 ◦ C: (1) Nss = 0; (2) Nss = 6 × 1012 cm−2 ;
(3) Nss = 1013 cm−2 (adapted from [58]).
• Transition-metal oxides (Fe2 O3 , NiO, Cr2 O3 , etc.) and
• Non-transition-metal
oxides,
which
include
(1)
pre-transition-metal oxides (Al2 O3 , etc.) and (2) posttransition-metal oxides (ZnO, SnO2 , etc.).
The fact that valence orbitals of the metal atoms are of s- and
p-symmetry is the common feature of the non-transition-metal
oxides. With transitional metal oxides, however, the d atomic
orbitals assume crucial importance. Many of the complications
with transition-metal oxides stem from this difference, because
of the different bonding properties, associated with d orbitals.
These complexities include the existence of variable oxidation
states, the frequent failure of the band model, and the crystalfield splitting of the d orbitals [61].
In Ref. [61] the following explanation of the difference
in behavior of non-transition and transition-metal oxides was
given. The non-transition-metal oxides contain elements that
with some exceptions have only one preferred oxidation state.
Other states are inaccessible, because too much energy is
needed to add or remove an electron from the cations when
they are coordinated with O2− ligands. Transition-metal oxides
behave differently because the energy difference between a
cation dn configuration and either a dn+1 or dn−1 configurations is often rather small. The most obvious consequence is
that many transition elements have several stable oxides with
different compositions. It is also much easier than with nontransient-metal oxides to make defects, having different electron
configurations. A as result of high defect concentration the bulk
and surface chemistry of transition-metal oxides is very complicated.
Trends in the stability of different oxidation states are very
important in surface chemistry, as they control both the types of
defect that may be formed easily, and the type of chemisorption
that may take place [61,62,67]. The d0 configuration represents
the highest oxidation state that can ever be attained: thus pure
TiO2 , V2 O5 , etc., cannot gain any more oxygen, although they
can lose oxygen to form defects or other bulk phases. On the
other hand, dn oxides with n ≥ 1 are potentially susceptible to
oxidation, as well as reduction. The stability of high oxidation states declines with atomic number increase across a given
series.
In contrast to transition-metal oxides, pre-transition-metal
oxides (MgO, etc.) are expected to be quite inert, since they can
G. Korotcenkov / Materials Science and Engineering B 139 (2007) 1–23
neither be reduced nor oxidized easily. In terms of electronic
structure, this is related to the large band gap, which means that
neither electrons nor holes can easily be formed. It means that
these oxides are good isolators. However oxides, characterized
by exceedingly high resistance, are not promising material for
resistive gas sensors, because of the difficulties, encountered in
electrical conductivity measurements.
Such big difference in pre-transition, and transition-metal
oxides’ behavior means that transition-metal oxides are more
sensitive to the change of outside ambient. Therefore it seems
that this type of oxides could be more preferable for the use
in gas sensors. However, in practice the transition-metal oxides
are not being used for conductometric gas sensor design. As it
will be shown in the next sections, structure instability and nonoptimality of other parameters important for conductometric gas
sensors, such as Eg and electroconductivity, considerably limit
their field of use. As it is known many transition-metal oxides
have small band gap. Moreover, the band model predicts that
majority of oxides, having a partially filled d band – that is for
dn with 0 < n < 10 – should be metallic. These expectations are
frequently not fulfilled because of intervention of various types
of electron–electron and electron–lattice interactions. Nevertheless, “simple” metallic behavior is found with a number of oxides
of elements in the 4d and 5d series (ReO3 , RuO2 ). Some oxides
of the 3d series (Ti2 O3 , VO2 , Fe2 O3 ) also have high conductivity
in the metallic range.
Only transition-metal oxides with d0 and d10 electronic configurations find their real gas sensor application. As we know,
the post-transition-metal oxides, such as ZnO and SnO2 have
cations with the filled d10 configuration. The d0 configuration
is found in binary transition-metal oxides such as TiO2 , V2 O5 ,
WO3 , and also in perovoskites such as ScTiO3 , LiNbO3 , etc.
These compounds share many features with the non-transitionmetal oxides. They have a filled valence band of predominantly
O 2p character, and gap between valence band and an empty
conduction band. Typical band gaps are 3–4 eV.
Unlike transition-metal oxides with 0 < n < 10, stoichiometric, post-transition-metal oxides ZnO, SnO2 , and d0
transition-metal oxides may be reduced, but not oxidized. The
post-transition oxides ZnO, In2 O3 , SnO2 , as well as majority
of transition-metal oxides are active in “redox” reactions since
the electron configuration of the solid may be altered. However,
the reaction with oxidizing species such as O2 is expected only
with samples that have been bulk reduced or where the surfaces
have been made oxygen deficient [69]. At that the reduction of
post-transition oxides as a rule leads to the formation of free
carriers, which greatly increase the metal oxide conductivity, a
fact that is crucial for sensor applications.
However, limited use of pure transition-metal oxides
(1 < n < 10) for conductometric gas sensor fabrication does not
mean that transition-metal oxides are not of interest of gas sensor designers. On the contrary, unique surface properties, plus
high catalytic activity make them very attractive for various
sensor applications, such as properties’ modification of more
stable and wide band gap oxides, and forming of more complicated nanocomposite materials [33]. For example, for optical
wavegide gas sensors, where the change of optical refraction
5
index is more important than the change of electroconductivity,
transition-metals oxides such as WO3 (H2 and alcohol detection), Mn2 O3 , Co3 O4 , and NiO (CO detection) [9] are the most
attractive ones.
2.3. Adsorption/desorption parameters
When the metal oxides are exposed to an atmosphere at moderate temperatures, two things can occur, namely, gas adsorption
due to the high reactivity of metal oxide surface, or reaction of the gas molecules with surface species ionosorbed
already at the metal oxides. There are well-accepted two types
of adsorption: physisorption and chemisorption [61,62,67].
Physisorption is associated to a neutral state for adsorption,
while chemisorption to a charged one. It means that only
chemisorption is accompanied by charge exchange between
adsorbed species and metal oxide, which controls the appearance or change of surface charge and surface potential of metal
oxides.
In a lot of research it was shown that for effective operation of chemisorption sensor the gas-sensing materials should
have specific combination of adsorption/desorption parameters
for oxygen and detecting gases [2,40–42,53,58]. It is known
that the smaller is the activation energy of chemisorption and
the higher is the activation energy of desorption, the bigger is
gas-sensing effect of adsorption type sensors [42,57,58]. At the
same time we have to take into account that excessively big
activation energy of adsorbed species desorption might lead
to a considerable increase of recovery time after the change
of surrounding atmosphere, which is not acceptable for practical applications. Research presented in [65,70] have shown
that the processes of electron exchange between conductance
band of metal oxides and adsorbed species for most studied
materials such as SnO2 and In2 O3 already at T > 100 ◦ C are
fast, and, therefore, they do not limit the kinetics of sensor
response According to [59,70] just adsorption/desorption of
oxygen and water controls kinetics of conductivity response
of gas sensors on the base of these materials. That is why for
chemisorption type gas sensors a material with optimal activation energy of desorption for given work temperature is needed.
Otherwise for recovery time reducing it would be necessary to
increase operating temperature, which could lead to a sharp
drop of sensor’s reliability and durability. In accordance with
estimations, conducted for operating temperature 300 ◦ C, the
activation energy of oxygen desorption, equaled ∼1.0 eV, is
optimal.
In [42,58] it was considered the influence of some adsorption/desorption parameters on the surface potential (Us ) and
SnO2 conductivity (G) changes during reaction of CO detection. The pattern of the main physical–chemical parameters of
reducing gas (R) influence on Us and G change is presented
in (2). The mark (↑) means the increasing, and the mark (↓) the
decreasing of indicated parameters.
αR
↑, β4 ↑, βRO ↑, N ∗ ↓, Nss ↓, β3 ↓, βR ↓
αO
⇒ Us ↑, G ↑
(2)
6
G. Korotcenkov / Materials Science and Engineering B 139 (2007) 1–23
where αR , αO are the coefficients of R and O2 adsorption; βR ,
βRO the coefficients of R and RO desorption; β3 , β4 the coefficients of charging and neutralization of RO; N*, Nss are the
total number of adsorption sites and sites, originated from native
(biographic) surface charge. The presented scheme shows the
directions of adsorption/desorption parameters changes, necessary for better sensor response achievement, i.e. for increasing
the range of film conductivity (G) and surface potential (Us )
change after metal oxide surface contact with detecting gas.
The main method of the influencing on electronic parameters of adsorbed species is the change of composition of metal
oxide films, i.e. the transition from simple binary metal oxides
to multi-oxide films, and metal oxide surface doping by additional catalyst particles [34,65,71–73]. For example, the doping
by metal catalyst additives (Pd, Pt) seems to be reflected by the
decrease of αCO /αO parameter, at least on metal catalyst particles, because the surface modification by noble metals, first
of all, increases dissociative oxygen adsorption. The impact
adsorption of CO (by Redeal–EIley mechanism [74]) is less
affected by doping procedure. However, due to the competition
of molecular and atomic forms of oxygen on the SnO2 surface we
have “apparent” decrease of αO and increase of αCO /αO during
surface doping by these noble metals. It results in both the partial
pressure and temperature shifts of sensor response dependencies
on both the operating temperature (S(Toper )) and the partial pressure of detected gas (S(PCO )) in the range of lower values of Toper
and PCO . Simulation of S(PCO ) dependencies for modified SnO2
films are presented in Fig. 4.
One can see that the decrease of activation energy of detected
gas adsorption may really change the sensor sensitivity and shift
greatly the temperature position of sensor response maximum.
The analysis of gas detection reactions indicates that material
for gas sensors should also be stable to surface poisoning, i.e. it
should have acceptable desorption energy of catalytic reaction
products. In other case these products could be accumulated
at the surface of sensitive element, and gas sensor parameters
could be worsen. “Sulfur poisoning” can be referred to such a
type of poisoning [75–77]. Another source of poisoning is the
one from other compounds, reducible to metals and elements
under reaction conditions. These ones, such as As, Fe, P, etc.,
may alloy with the catalytically active metal and metal oxides,
and reduce its effectiveness. In this context one should note that
metal oxides are more resistant to certain poisoning (especially
by halogens, As, Pb, and P) than noble metals. These effects
of poisoning have been discussed in terms of occupancy (siteblocking) and of electronic effects. It is now quite clear that
the strong electronic effects play a fundamental role in those
changes [78].
Analogous requirements are being presented also for materials, designed for adsorption sensors, for example such sensors
as SAW and cantilever ones, where the change in weight of
sensing element is a determinant factor [79]. It is known that
the role of the sensing material in such devices is selectively
and reversibly to sorb an analyte of interest from sampled air,
and to concentrate it for achievement lower concentration detection capabilities. Therefore maximum and reversible sorption
of specific analytes or classes of analytes, with rapid sorption kinetics and minimal sorption of interferents are key goals
in the development of a successful chemiselective coating for
SAW and work-function sensors [10]. For example, the measurements of various metal oxides capability for adsorption
both the isopropanol and methanol [80,81], have shown that
the average active surface site density (Ns ) for isopropanol and
methanol adsorption on the majority of metal oxide surfaces was
found to be 0.2–4 ␮mol m−2 . Some of the active metal oxides
(MgO, La2 O3 , Cr2 O3 , Sb2 O3 ) possessed a higher active site density, which gives those oxides a sufficient advantage at design
of adsorption sensors, sensitive to isopropanol and methanol.
Cr2 O3 , WO3 and BaO show a somewhat lower active surface
site density. SiO2 is extremely unreactive and has a low Ns in
spite of having a high surface area.
However we have to note that this conclusion is not a universal one. Every technical task requires individual solution,
considering both the nature of tested gas, and operating conditions. For example, in the case of CO2 adsorption absolutely
other situation takes place [72].
2.4. Catalytic activity
Fig. 4. Simulation of adsorption/desorption parameters’ influence on temperature dependencies of SnO2 gas response to CO—ECO : (1) 0.9 eV; (2) 0.8 eV; (3)
0.7 eV; (4) 0.6 eV (adapted from [42]).
In many gas sensors the conductivity response is determined
by efficiency of catalytic reactions with detected gas participation, taking place at the surface of gas-sensing material. Catalytic
reactions, involving surface oxygen, can change both the surface potential and concentration of point defects, which control
electro-physical properties of poly- and nanocrystalline metal
oxides [1–4,8,47,53,67]. CO oxidation with participation of oxygen chemisorbed at the metal oxide surface is a typical example
of such reaction. Results of experiments directed on simultaneous control of sensor response and efficiency of detected gas
conversion are confirmations of this statement [82]. Therefore
it is considered that high catalytic reactivity of the surface, and
especially selectivity of this reaction to detected gas are important advantages of sensor material. Because of that fact the
G. Korotcenkov / Materials Science and Engineering B 139 (2007) 1–23
Fig. 5. The correlation between catalysis temperature of 50% conversion (T50 )
and temperature of SnO2 gas response maximum (Tm ).
control of catalytic activity of new material is often used as the
main method for a preliminary estimation of its suitability for
gas sensors elaboration, and for determination of sensor’s operation temperatures. As a rule, a position of maximum of sensor
response on a temperature dependence S(Toper ) coincides with a
temperature, corresponding to 50% conversion of detected gas
(see Fig. 5) [71,83]. As it is known, the operating temperature is
important parameter for gas sensors, because it determines the
power dissipated by heater necessary for achievement optimal
gas-sensing characteristics, and through this parameter influences on reliability and durability of solid-state gas sensors. For
practical devices one wishes to minimize the power needed to
operate, so the lowest operating temperature is desired. In atmosphere containing flammable gases, a low temperature is favored
also for safety.
It is necessary to note that the maximum of catalytic activity to different gases can be observed at different temperatures.
The last one is favorable property for gas-sensing material,
because through the change of operating temperature we observe
possibility to influence on selectivity of gas sensors. For example, the peak in sensitivity (oxidation) for methane is often at
higher temperatures than for CO and other hydrocarbons [2],
suggesting that a higher temperature would be desirable for
methane-selective sensors, while a lower temperature would be
desirable for CO-selective ones.
However, in spite of the obvious similarities between chemical sensing and heterogeneous catalysis, we have to realize that
the choosing of material for gas sensor applications is not determined just by catalytic activity. This is an important parameter,
but not a determining one [34]. Numerous experiments, conducted by various authors, testify that as a rule, oxides with
electron configuration d3 (Cr2 O3 , MnO2 ), and d6−8 (Co3 O4 ,
NiO) are the most catalytic active ones [67]. Minimum of activity is observed for oxides d5 (Fe2 O3 , MnO), d0 (CaO, Sc2 O3 ,
TiO2 ), and d10 (ZnO, Cu2 O). At that the activity of oxides with
electronic configuration d5 is much higher than the activity of
oxides with d0 and d10 configurations. However in practice, as
we have mentioned before, metal oxides with electronic configurations such as d0 (TiO2 ), d10 (ZnO, SnO2 , Cu2 O, Ga2 O3 ),
7
and more seldom d5 (Fe2 O3 ), which are the least active with
catalytic point of view, are being used as most promising gassensing materials [34,84–86]. Therefore the catalytic activity, in
spite of a coincidence of maximum gas response’s temperature
and a temperature, equaled 50% of detected gas conversion, cannot explain above-mentioned choice of both d10 and d0 oxides
as base materials for conductometric gas sensors. This selection is determined by all totality of those materials properties.
For example, basing on the data, presented in [67], one can conclude that oxygen bond energy at the surface of transition-metals
oxides of the fourth period is a parameter, which better than catalytic activity defines metal oxides adaptability for solid-state
gas sensor design.
At the same time we have to admit that a choice of metal
oxide as additive for properties’ modification of other metal
oxides is often connected with catalytic properties of those
oxides [71,87,88]. For example, the catalytic activity to selected
gas is the most important parameter for application in membranes, used for sensor response’s selectivity improvement [71].
Room temperature (RT) gas sensors is other possible field for
catalytic active metal oxide applications. In [89] it was found
that RT work-function sensors on the base of catalytically active
oxides, as CeO, Fe2 O3 , and NiO, have shown good operation
parameters.
3. Sensing material choice through their
electro-physical properties
3.1. Band gap
Pretty big band gap (Eg ) and small activation energy of the
centers, responsible for metal oxide conductivity, is an optimal combination of parameters for the materials designed for
semiconductor solid-state gas sensors. Such correlation of activation energies is necessary in order to avoid sensor’s operation
in the region of self-conductance. In this case the influence of
surrounding temperature on sensor parameters is reduced. At
that, as a rule, the higher operation temperature is, the bigger should be Eg . As it follows from experimental results, for
solid-state gas sensors, operating at the temperatures exceeding T > 300 ◦ C, the optimal band gap must be higher than
2.5 eV. Analyzing data, presented in [67,68,90–92], one can
conclude that well-known metal oxides satisfy this requirement
(see Table 2).
For sensors, working at room temperature, Eg could be
considerably smaller [93,94]. Moreover, for example for RT
work-function sensors a small Eg may be advantage. In [29,89]
it was shown that average work-function change in the atmosphere of dry oxidizing gases (Cl2 , NO2 , SO2 ) increased when
the energy band gap of metal oxides decreased.
It is necessary to note that an opportunity to operate at higher
temperature is an important advantage of solid-state gas sensors,
because this fact allows reducing considerably the influence of
air humidity on gas-sensing characteristics. It was established
that, as a rule, the lower operating temperature is, the greater is
the sensitivity of the sensor’s parameters to relative air humidity
[2,34].
8
G. Korotcenkov / Materials Science and Engineering B 139 (2007) 1–23
Table 2
Band gap of sensing materials
Table 3
Electroconductivity of some sensing materials
Material
Band gap (eV)
Metal oxides
MgO, CaO, Al2 O3 , SiO2 , TeO2
SrO, Y2 O3 , HfO2 , ZrO2
BaO, La2 O3 , CeO2 , Ga2 O3
TiO2 , Nb2 O5 , Ta2 O5 , ZnO, In2 O3 , SnO2
V2 O5 , Cr2 O3 , WO3 , NiO, Fe2 O3
Co3 O4 , PdO, CuO, Sb2 O3
>6.0
5–6
4–5
3–4
2–3
1–2
Semiconductors
Si, InP, GaAs
SiC, GaN, diamond
1.1–1.41
3.27–5.4
Polymers
Polymers
Trans-polyacetylene
Polyphenylene
Polypyrrole
Polythiophene
0.3–3.5
1.4
3.4
2.7–3.0
2.0
Big band gap is a sufficient advantage also for metal oxides
with ionic conductivity, because in this case the contribution of
electron conductivity in total materials’ one, especially at high
operating temperatures, is being reduced.
3.2. Electroconductivity
No doubts that a variety of metal oxides make possible their
using in all types of gas sensors. However it does not mean that
oxides do not have any limitations in application. For example, for chemisorptional conductometric gas sensor the sensing
material should be conducting one, i.e. the concentration of point
defects in metal oxides should be pretty high. As experiment
shows, the optimum lies in the range 1017 to 1020 cm−3 . It corresponds to electroconductivity of metal oxides equaled 10−2 to
101 Sm/cm. Possible range of the change of metal oxide electroconductivity in comparison with another materials is given in
Table 3. Data from Refs. [63,68,92,95,96] were basis for this
table.
Too high concentration of point defects, i.e. high electroconductivity, reduces the influence of the surface on the
concentration of charge carriers in the grains and electroconductivity of gas-sensing material. Because of this fact a metals
usually are not being used for conductometric sensors design.
Only in some specific sensor constructions with ultra thin sensi-
Material
The range of electroconductivity (Sm/cm)
Metals
Semiconductors
Metal oxides
Polymers
104 to 106
10−8 to 103
10−14 to 102
10−16 to 103
tive layer (d < 20 nm) some metals, such as Pt, Au, and Ni, were
applied for this purpose [97–100].
For conductometric sensors the excessively low concentration of free charge carriers (n < 1016 cm−3 , i.e., σ < 10−4 to
10−5 Sm/cm) is also not acceptable. In nano-size structures it
reduces modulation limits of Fermi level’s position and leads to
a sharp increase of the resistance of gas-sensing material.
However, it is necessary to note that for other gas sensors,
such as sorptional sensors, fiber-optic gas sensors, sensors on the
base of fluorescence effect, and so on, where the conductivity is
not a controlled parameter, there is not need to impose restrictions on electroconductivity of used materials. The materials
could be either isolator, or the ones having metal type of conductivity. For example, metals may be successfully used in devices
such as MIS sensors and work-function sensors, which exploit
the catalytic properties of metals [7]. At the same time such isolator as Al2 O3 , is good material for humidity sensor [35]. Materials
designed for high-temperature sensors (Toper > 800 ◦ C) could be
an isolator at room temperature. Conductivity in such materials
may become apparent only at high enough temperatures.
3.3. The type of conductivity
It is known that gas-sensing materials can have either n-,
or p-type of conductivity (see Table 4). Semiconductors of nand p-type have inverse direction of conductivity’s change at
interaction with the same gases, which is very important fact
for their application. For materials with p-type the conductivity
rises with oxygen pressure’s growth, whereas for n-type oxides
it drops.
The analysis of main gas-sensing materials in respect to their
conductivity type shows that all the most effectively working gas
sensors of chemisorption type are designed on the base of metal
oxides of n-type conductivity, such as SnO2 , TiO2 , WO3 , ZnO
and In2 O3 , providing the opportunity of oxygen’s chemosorption. Previous research has shown that, in general, all n-oxides
are thermally stable and have possibility to work at lower oxygen
Table 4
Type of conductivity of some sensing materials
Material
Metal oxides
Semiconductors
Polymers
Type of conductivity
n
p
n, p
MgO, CaO, TiO2 , ZrO2 , V2 O5 , Nb2 O5 ,
Ta2 O5 , MoO3 , WO3 , ZnO, Al2 O3 , Ga2 O3 ,
In2 O3 , SnO2
SiC, GaN, diamond
Y2 O3 , La2 O3 , CeO2 , Mn2 O3 , Co3 O4 , NiO,
PdO, Ag2 O, Bi2 O3 , Sb2 O3 , TeO2
HfO2 , Cr2 O3 , Fe2 O3 , CuO
Polypyrrole, polythiophene
Si, InP, GaAs
Trans-polyacetylene, polyphenylene
G. Korotcenkov / Materials Science and Engineering B 139 (2007) 1–23
partial pressure in comparison with well known p-type oxides,
for example such as CuO [91]. It is known [2] that many ptype oxides are relatively unstable because of the tendency to
exchange lattice oxygen easily with air. Besides, the interaction
with reducing gas decreases the resistance of n-type oxides. This
is the preferred direction for sensor’s resistance change during
detection of reducing gases, contributing to simpler compatibility with peripheral measuring devices, and better reproducibility
of output signal.
However, it does not mean that p-type materials are not applicable for sensor design. For example, the last research has shown
that metal oxide Cr2−x Tix O3 (x < 0.4) (CTO), prospective for
gas sensors design, is p-type material [101,102]. Stable perovoskites, for example ReCoO3 (Re = La, Nd, etc.), promising
for chemical sensor application, are semiconductors of p-type as
well. It was demonstrated on the example of LaCoO3 , that those
materials have high activity in the oxidation of CO, reduction of
NO [103], and in the reduction of SO2 in presence of CO [104].
The fact that perovoskites can have conductivity of p-type
gives them additional advantages for application in hightemperature oxygen sensors. It was found that the temperature
dependence of conduction in high-temperature range is considerably less in the p-type range than in the n-type one [46]. It
means that p-type metal oxides with optimal structure, for example such as BaFe0.8 Ta0.2 O3 [46] or Sr(Ti0.65 Fe0.35 )O3 [105],
retain substantial oxygen sensitivity, may be useful in the construction of oxygen sensors without the need for additional
temperature control or compensation elements.
CuO and ferrum oxides also have p-type conductivity. These
oxides are effective additives to both tin and indium oxides for
forming of nano-composite-based sensors with extremely high
conductivity response to H2 S and series of other specific gases
[106,107]. Besides that materials with p-type conductivity are
being successfully used in adsorption type gas sensors and gas
sensors of electro-chemical type.
It was established that metal oxides of n- and p-type conductivity could show different surface properties, which might
become a basis for various gas sensors’ elaboration. For
example, it is known [61,69,73,108], that oxygen cannot be
chemisorbed on the surface of undoped stoichiometric n-type
oxides. Oxygen ions can only be adsorbed if their negative
charge is compensated by ionized bulk donors in a space charge
layer. Besides, for thermodynamic reasons only a small fraction of oxygen monolayer can be chemisorbed on the surface of
n-type oxides (the Veitz limitation). In contrast, on p-type semiconductors a full monolayer of oxygen ions typically occurs,
because the metal ions of the lattice can be oxidized into a
higher oxidation state. However, it is necessary to note that as
a rule, with temperature growth in such oxides a probability
of the transition of adsorbed oxygen in lattice oxygen due to
their incorporation in metal oxide lattice sharply increases. As
a result, one can observe the following distinction in the behavior of n- and p-type oxides [109]. If n-type oxides have lost
oxygen upon heating in air, p-type oxides gain oxygen during
such thermal treatments. Numerous researchers have identified
that the multiple stable oxidation states and high concentration
of positive ‘holes’ in p-oxides stimulate surface oxygen mobil-
9
ity to a greater extent than n-type oxides. Each ‘hole’ provides
a vacancy where the free electrons of mobile surface oxygen
species can be stabilized [61,91].
3.4. Oxygen diffusion in metal oxides
As it is established, the signal-determining elementary interaction processes in oxide-based chemical sensors one may
distinguish between thermodynamically controlled chemisorption and kinetically controlled catalytic reactions of the
molecules to be detected, as well as between thermodynamically controlled bulk point defect equilibria [47]. At the same
time it is known that semiconductor oxides are in general nonstoichiometric, in which the oxygen vacancies are the main bulk
point defects. It means that the changes in oxygen partial pressure at operating temperatures may be the reason of the change
in bulk conductance of metal oxides. For example, the oxygen
vacancies can diffuse from the interior of the grains to the surface and vice versa, and the bulk of the oxide has to reach an
equilibrium state with ambient oxygen. So, the coefficient of
oxygen diffusion, which control the equilibration time between
concentration of bulk point defects in metal oxides and gas surrounding, is the same important parameter like another physical
chemical properties analyzed earlier.
Taking into account mentioned above one can conclude that
depending on used type of solid-state gas sensor there are needed
materials with extreme properties, i.e. with very high coefficients of bulk diffusion of oxygen and point defects, or with
very low ones. The first type of materials is necessary for gas
sensors, which work is based on the change of bulk properties
of materials. In such sensors the change in bulk conductivity is a
reflection of the equilibration between the oxygen activity in the
oxide and oxygen content (oxygen partial pressure, PO2 ) in the
surrounding atmosphere. Usually for their behavior explanation
the following equation is used [110,111]:
Ea
±1/n
(3)
PO2 ,
G = G0 exp −
kT
where G0 is a constant and Ea is the activation energy for conductivity. The value and sign of 1/n are determined by the type of
dominant bulk point defect, involved in the equilibration process.
The positive and negative signs of 1/n correspond to p-type and
n-type conduction, correspondingly. The sensitivity of a semiconducting gas sensor is determined by the value of 1/n. The
higher the value of 1/n is, the greater is the sensitivity of the
sensor. High diffusion coefficient in such devices provides a
decreasing of both operation temperatures and response time.
The main application of this kind of solid-state gas sensors
is the measurement of oxygen partial pressure as required in
combustion control systems, in particular in the feedback control
of the air/fuel ratio of automobile engine exhaust gases near
the λ point in order to improve the fuel economy efficiency
and to reduce the harmful emission of gases as CO, NOx and
hydrocarbons [22,44,112,113]. Normally, electrochemical cells
based on solid-state electrolytes as ZrO2 are used as ␭-sensors.
At lower temperatures, the change in ambient gas concentration does not necessarily lead to equilibration of bulk properties
10
G. Korotcenkov / Materials Science and Engineering B 139 (2007) 1–23
Fig. 6. The nature of processes, controlling the gas response of metal oxide gas
sensors.
of metal oxides and gas surrounding. Gas surrounding affects
electrical properties through surface reactions. It means that for
the attainment of good exploitation parameters of such sensors at
their design it is necessary to use materials, in which the constant
of oxygen diffusion is minimized. For sensors of chemosorption
type the diffusion of oxygen in the bulk of crystallites is a source
of temporal parameters’ drift [114].
However, one can notice that such a partition is pretty conditional. According to [84] for any metal oxide sensor one can find
out three temperature regions for operation with specific conditions. At high temperatures, the kinetics of rate processes is fast
enough that equilibrium is rapidly established between partial
pressure of oxygen in the gas surrounding and the bulk composition of the oxide. At intermediate temperatures, we can observe
the situation, when the reaction of the gas with metal oxide lattice
takes place, however due to small constant of bulk oxygen diffusion the chemical composition of the material does not reach
equilibrium state during the time of gas detection. It is so called
“redox” (reduction/reoxidation) mechanism. At still lower temperatures, a chemisorption (adsorption/desorption) processes
can dominate in surface reactions. At that for indicated modes
of operation there are no fixed temperature borders (see Fig. 6).
They are pretty diffuse and can be shifted essentially at the interchanging one metal oxide to another one. One can just say that
the first border lies in temperature range 200–500 ◦ C, while the
second one lies at temperatures 400–700 ◦ C.
It is impossible to say which mode of operation is preferable for practical use. Every mode has its own advantages
and disadvantages. For example, the temperatures from lowtemperature range are not high enough to completely burn out
organic deposits or desorb certain adsorbates. This problem
limits long-term stability of the electrical output signal. However, low-temperature chemisorption gas sensors could be easily
adapted into modern microelectronics, having sufficient limitations in temperature modes of operation. On their base it is
easier to maintain better selectivity and to create sensors array
for design of “electronic nose” [115,116]. SnO2 - and In2 O3 based sensors, operating in temperature range 200–450 ◦ C, are
the typical examples of low-temperature sensors.
At the other hand, one should admit that high-temperature
sensors are more adequate for conducting of in situ control
of many high-temperature technological processes, including
a control of the explosion engine’s work [84,117]. The sensing
behavior of such devices is mostly explainable and predictable
and is based on well-established thermodynamic principle [104].
Besides that during study semiconducting metal oxides operated at high temperatures (400–900 ◦ C) it was established that
for some of the materials profound investigations have been
performed showing a grain boundary independent conduction
mechanism and self-cleaning effects of the sensor surfaces,
thus indicating progress towards stability and reproducibility
[84]. The results of more detailed comparison of low and hightemperature modes of gas sensors operation are given in Table 5.
Virtually, as we wrote earlier, all oxides can work as hightemperature gas sensor [111]. In practical applications, however,
the usefulness of an oxide for this temperature range is determined by parameters such as material stability, response time,
Eg , type of conductivity, etc. Many semiconducting oxides
have been investigated. ZrO2 , BaTiO3 , SrTiO3 , Ga2 O3 , TiO2 ,
WO3 , Nb2 O3 , CoO, MoO3 , CeO2 , LaFeO3 , SrTiFeO3 , and
BaSnO3 are some examples [112,113,118,119]. All indicated
oxides are stable enough, and can provide sensor’s operating with more or less effectiveness right up to 900 ◦ C. For
example, for Ga2 O3 sensors optimal operation temperature is
Toper ∼ 600–800 ◦ C, whereas for oxygen sensors on the base of
SrTiO3 operating temperature is equaled 1000 ◦ C. It is necessary
to note that considerable attention to complex metal oxides such
as SrTi0.65 Fe0.35 O3−␦ is determined by unusual temperatureindependent conductivity of these materials above 700 ◦ C and
PO2 > 1 Pa [112,113]. It was established that at an intermediate composition of X = 0.35, the band gap energy is such that
the Fermi energy lies just far enough above the valence band to
compensate for the temperature-dependence of mobility, yielding a zero TCR (temperature coefficient of resistance) from the
Table 5
Advantages and disadvantages of sensors operating in different modes
The region of operating temperatures
Low operating temperatures Toper
< 400 ◦ C
High operating temperatures Toper > 500 ◦ C
Advantages
Disadvantages
Low dissipated power of sensor; low threshold of
sensitivity; long life time; wide choice of sensitive
materials; good compatibility with
micromachining technology
Strong dependence from relative air humidity;
pretty big response and recovery times;
necessity of prolonged aging before start of
exploitation; necessity of regular calibration.
Weak dependence from air humidity; good signal
reproducibility; small response time; fast process
of initial state recovery
High dissipated power; lowering of reliability;
lowering of sensitivity; strict requirements to
sensing material and sensor construction; bad
compatibility with standard silicon technology
G. Korotcenkov / Materials Science and Engineering B 139 (2007) 1–23
product of the free carriers (holes) concentration and mobility terms [113]. Strong sensitivity to oxygen partial pressure
variation and negligible cross-sensitivity to temperature fluctuations make these metal oxides promising candidates for oxygen
sensors in learn burn engines [112,113].
If one could consider that the presence of structural vacancies of the lattice promotes the increase of constant of oxygen
bulk diffusion [120], it is possible to assume that for design of
sensors, in which the appearance of diffusion processes worsens
exploitation parameters, materials, which do not contain structural vacancies, are more preferable. At the same time for design
of sensors, where bulk diffusion controls sensor’s parameters,
the materials with native structural vacancies are preferable
[121]. It is necessary to note those perovoskite materials, investigated intensively last years for high-temperature sensors, have
these very structural properties.
3.5. Other parameters important for gas-sensing materials
Taking into account that at present a large variety of optical
methods may be used in gas sensors, including ellipsometry, luminescence, fluorescence, phosphorescence and Raman
spectroscopy, interferometry, surface plasmon and so on, one
can conclude that for gas sensor design on the base of these
methods, parameters, such as refractive index, absorbance and
fluorescence properties of analyte molecules or a chemo-optical
transducing elements will have definitive importance. For example in order to shift the operating point of the surface plasmon
resonance gas sensors towards aqueous environment, a thin high
refractive index dielectric overlayer can be employed [122]. The
use of overlayer with higher refractive indexes allows for thinner overlayer and potentially better sensor sensitivity. Analysis
of data presented in Table 6 indicates that the tantalum pentoxide, which has high refractive index and good environmental
stability, may be used for this purpose.
At the same time metal oxides such as Co3 O4 , NiO, Mn3 O4 ,
CuO and WO3 are more preferable for using in optochemical
sensors based on optical absorption change during interaction with detected gas [123,124]. The reversible absorbance
change in the visible–near IR range and relatively fast response
make these oxides a potential candidate for optical detection
of CO, H2 , and air humidity. At that Co3 O4 -based optochemical sensors can operate already at room temperature [123].
Of course, these sensors do not possess so high sensitivity
Table 6
Refraction index of some sensing materials
Material
Semiconductors
Si, InP, GaAs
Metal oxides
Al2 O3 , SiO2
MgO, CaO, SrO
BaO, ZrO2 , HfO2 , Nb2 O5 , ZnO, SnO2 , Sb2 O3
Cr2 O3 , Fe2 O3 , NiO, Bi2 O3
TiO2 , Ta2 O5 , CuO
11
Table 7
Dielectric constants of some sensing materials
Material
Dielectric constant
Metal oxides
MgO, CaO, BaO, SiO2
Cr2 O3 , NiO, CuO, ZnO, Al2 O3
Y2 O3 , ZrO2 , V2 O5 , WO3 , SnO2
La2 O3 , HfO2 , CeO2 , Nb2 O5 , Ta2 O5
TiO2
3–5
5–10
10–20
20–50
>50
Semiconductors
Si, InP, GaAs
SiC
Diamond
Polymers
Polypyrrole
11.8–12
9.7
5.5
8
as standard semiconductor gas sensors. However, this type of
sensor has some peculiarities, which can be used in real applications. The advantages of optochemical sensors over conventional
electricity-based gas sensors are higher resistivity to electromagnetic noise, compatibility with optical fibers and the potential
of multi-gas detection using differences in the intensity, wavelength, phase and polarization of the output light signals [124].
The dielectric constant is another important parameter for
gas-sensing materials (see Table 7). Its value plays important
role during selection materials for capacitance-type gas sensor
[125]. Capacitive-type sensors have good prospects given that
the capacitor structure is so simple enabling miniaturization and
achieving high reliability and low cost. In addition, application
of capacitance is easily performed by oscillator circuits and thus,
capacitive type sensors enable sensitive detection. In addition,
oscillator circuits consist of only a standard resistor and sensor
capacitor. Therefore, the signal treatment circuit is also very
simple and low cost. The humidity sensor is the most wellknown capacitive type sensor. Since water has an abnormally
large dielectric constant, the adsorption of water in porous metal
oxide changes the relative permittivity of gas-sensing matrix. In
relation to other types of humidity sensors, the capacitive type
has the advantage of high sensitivity over a wide humidity range.
Porous Al2 O3 is the most known metal oxides for using in such
sensors [125].
4. The role of parameters’ stability in sensing material
choice
4.1. Thermodynamic stability
Refraction index
3.4–3.55
1.4–1.7
1.7–2.0
∼2–2.1
2.1–2.5
>2.5
Materials, destined for gas sensors, working at high temperature, have to possess high thermo-dynamic stability. The better
material’s thermo-dynamic stability is, the higher are temperatures, at what chemical sensor with this material is able to work
especially at the presence in atmosphere of reducing gases.
As far as is very important to have high thermal stability, one
can judge on the base of results given in [126]. It was established
that Fe2 O3 :Pt-based sensors operating at Toper ∼ 200–250 ◦ C
had maximum sensitivity to acetone to compare with SnO2 -,
12
G. Korotcenkov / Materials Science and Engineering B 139 (2007) 1–23
Table 8
The parameters, characterizing a thermodynamic stability of metal oxides suitable for gas sensor applications
Material
Melting temperature (◦ C)
Hf for metal oxide formation
per oxygen atom −Hf (298 K)
(kJ mol−1 )
Temperature-programmed
reduction (TPR) (◦ C)
Thermal stability in oxygen atmosphere
MgO
CaO
SrO
BaO
Y2 O3
La2 O3
TiO2
ZrO2
HfO2
CeO2
V2 O5
Nb2 O5
Ta2 O5
Cr2 O3
MoO3
WO3
Mn2 O3
Fe2 O3
Co3 O4
Rh2 O3
NiO
CuO
ZnO
Al2 O3
Ga2 O3
In2 O3
SiO2
SnO2
Bi2 O3
Sb2 O3
TeO2
2800–2820
2587–2620
2430–2650
1923–2015
601.7
635.1
590.7
553
586.2
699.7
470.8
547.4
556.8
544.6
311.9
381.1
409.9
380.0
251.7
280.3
323.9
247.7
202.3
95.3
245.2
157.0
348
558.4
360
308.6
429.1
290.5
192.6
233.2
162.6
N.R.
300
326
330
325
468
N.R.
N.R
N.R.
594
550
N.R.
340
219
575
544
184
200
288
100
278
268
N.R.
N.R
320
350
N.R
500
400
563
355
Thermally stable (T.S)
T.S.
T.S.
T > 500 ◦ C, →BaO2
T.S.
T.S.
T.S.
T.S.
T.S.
T.S.
T > 700 ◦ C, evaporates with partial dissociation
T.S.
T.S.
T.S.
T > 650 ◦ C, sublimates
T > 1000 ◦ C, sublimates
T > 750 ◦ C, decomposes
T > 1400 ◦ C, dissociate
T > 900 ◦ C, →CoO
T.S.
T.S.
T > 800 ◦ C, decomposes
T.S.
T.S.
T.S.
T.S.
T.S.
T.S.
2300
1855
2690
2790
2727
690
1512
1879
2300–2435
795
1470
1347
1347
1562
1115
1957
1336
1800–1975
2050
1740–1805
1910–2000
1720
1900–1930
817
655
2127
Easy sublimates
T > 450 ◦ C, sublimates
N.R.: no reduction detected between 150 and 700 ◦ C.
CdO-, and Nb2 O5 -based sensors. However those sensors were
not used in the instrument prototype for acetone vapor analysis,
due to strong dependence its long-term stability on operating
temperature. The increase Toper higher 250 ◦ C resulted in a sharp
worsening of gas-sensing characteristics.
Sensors on the base of materials with high thermo-dynamic
stability should also have better temporal stability of the parameters. The last condition should be attained by a suppression
of the processes of grain size increasing during exploitation. In
this case the opportunity for use material with small crystallites
appears. The last one is necessary for the attainment of both high
sensitivity, and good rate of chemical sensor response. Both the
big heat of material formation and the high melting temperature characterize such a properties of material (see Table 8). It is
known that oxides in air are the lowest free energy state almost
for all metals in the periodic table, which provides them such a
big thermodynamic stability.
It is necessary to note that given data one can consider
as a characteristic of reactivity of elements towards oxygen
[61,67,68]. More reactive metals are ones with more negative
heat of oxide formation. They should be able to reduce the oxides
of metals above them. There are, however, various reasons why
the predictions of bulk thermodynamics may not be followed.
There is the possibility that formed surface phases could have
thermodynamic stability different from those of bulk oxides. But
also it is important to remember that surface reactions of this kind
require extensive migration of atoms, a process that may have a
large activation energy [61]. Such reactions are therefore more
likely at higher temperatures.
The thermal program reduction (TPR) technique may be used
to probe the stability of different metal oxides as well. In this
method diluted hydrogen is used to reduce metal oxides. Hydrogen reduction of a metal oxide proceeds through dissociative
adsorption of H2 , which reacts with lattice oxygen to form surface hydroxyl species. Subsequently, H2 O leaves the surface by
eliminating the surface hydroxyl species. The TPR threshold
temperatures of the metal oxides, which reflect the reducibility
of the metal oxides, are shown in Table 8. The initial reduction
temperatures vary in the range from 100 ◦ C (Rh2 O3 ) to more
than 700 ◦ C. Most of the bulk metal oxides exhibited multiple
peaks in their TPR profiles due to their multiple oxidation states
when extensively reduced. Note, however, that only the onset
reduction temperatures are reported in Table 7.
It necessary to note that for some metal oxides such as
HfO2 , MgO, ZnO, TiO2 , Al2 O3 , SiO2 , ZrO2 , and Nb2 O5 no
detectable H2 consumption was observed in the temperature
G. Korotcenkov / Materials Science and Engineering B 139 (2007) 1–23
13
range of 150–700 ◦ C. Some of these samples (TiO2 , Al2 O3 ,
ZrO2 and Nb2 O5 ) probably experienced slight surface reduction because their color changed after a TPR run and the
color quickly disappeared when the sample was exposed to
ambient conditions [80,81]. It is seen that the initial reduction
temperature is in full accordance with heat of oxide forming, which characterizes oxide’s thermo-dynamic stability. The
less stable the metal oxide is, the more easily the surface is
reduced to form oxygen adsorption sites. The bigger energy of
stable oxide’s forming is, the higher is initial reduction temperature. Therefore sensors on the base of such oxide would
have higher stability of parameters during working in reducing
atmosphere.
One should note that mentioned before initial reduction temperature is very important parameter for gas sensor, because at
this temperature’s exceeding the metal oxide could be reduced to
metal during interaction with reducing gas. It testifies that applicability of such oxides as Cr2 O3 , Mn2 O3 , Fe2 O3 , and NiO for
high-temperature solid-state gas sensor’s design is very limited.
Their operation temperature in atmosphere of reducing gases
cannot exceed 200 ◦ C.
For more complicated oxides there are also some correlation, which may predict their thermodynamic stability [127].
For example for simple perovoskites of the ABO3 type with
alkaline earth metals on the A-site, it was established that the
thermodynamic stability of oxides is mainly determined by the
choice of the B-cation. In accordance with an increasing perovoskite tolerance factor RA /RB (RA and RB are ionic radii of A
and B cations), one therefore observes an increasing stability in
the order cerates → zirconates → titanates. Even higher stability of perovoskites, apparently can be expected at introduction of
Nb5+ on the B position, due to a more advantageous perovoskite
tolerance factor.
special technological methods for local deposition of these materials in required spots [128,129].
However it is necessary to note that chemical activity of
some materials in regard to certain reagents could also be used
for high-efficient gas sensor design. As an example of those
materials there are two-phase systems such as SnO2 –CuO, and
SnO2 –AgO, used in gas sensors, sensitive to H2 S [130,131].
High gas response of sensors on the base of these materials is a
consequence of the following reactions:
4.2. Chemical activity
Gas sensor, in spite of the absence of encapsulation and high
operation temperature, should provide long-term of exploitation,
even at their being in corrosive mediums. It means that long-term
stability of gas sensor’s output signal is one of the most important
factors determining the practical use of such device. In general,
it is required that, for example, any gas-sensing device should
exhibit stable and reproducible signal for the period of at least
2–3 years (17,000–26,000 h). Taking into account this goal we
have to introduce some sufficient corrections in evaluation of
availability of sensing material for practical application in gas
sensors.
For example, according some estimations, gas sensors, using
organic polymer Nafion may retain capability for work up to a
year. However, for achievement of this result Nafion must be
wetted by a wicking system to a reservoir [132]. Besides, polymer sensors, used for environmental control, have big problem,
which is their sensitivity to ultra violet (UV) radiation and presence of oxidizing gases. It has been reported that ozone and
other oxidizing components (NOx ) of the polluted atmosphere of
industrial centers may be initiators or accelerators of polymers’
photochemical destruction [133,134]. Because of either polymerization or destruction, their properties irreversibly change
during pretty short term. As a result, long-term and thermal sta-
Used gas-sensing materials should be characterized by high
chemical stability. This property provides luck of corrosion
at interaction with gases and solutions, i.e. an opportunity to
work in corrosive mediums. Chemical activity of materials is
an important problem of gas sensor application in medical purposes as well. Sensing element, as well as construction elements
of sensor, often contact patient blood. Therefore, a prevention of
patient’s infection is an important task in widely spread application of gas sensors in medicine for express control. From this
point of view, metal oxides are the most preferable materials
for gas sensors. As it is known, metal oxides have the minimum chemical activity in comparison with metals and covalent
semiconductors.
At the same time excessive chemical resistance of sensing
material could create some difficulties during sensors design.
They become apparent at the stage of sensing material’s localization at the surface of chemical sensor platform, i.e. during
creation a required sensor’s configuration. Such widely used
oxides as tin oxide and aluminum oxide have such increased
chemical resistance. For forming of necessary surface configuration one should use passive masks, dry etching, or elaborate
CuO + H2 S ⇒ CuS + H2 O ↑,
CuS + O2 ⇒ CuO + SO2 ↑
(4)
or
AgO + H2 S ⇒ AgS + H2 O ↑,
AgS + O2 ⇒ AgO + SO2 ↑
(5)
These reactions lead to the change in chemical composition
and physical properties of a compound, forming inter-crystallite
interlayer in gas-sensing matrix.
The same principle is used in design of solid-state gas sensors
for CO2 detection, for example on the base of La2 O3 /Li2 CO3
(1/10) system. In the presence of CO2 , the La2 O3 is converted
to lanthanum carbonate, which alters the sensor’s conductivity.
La2 O3 + 3CO2 ⇒ La2 (CO3 )3
(6)
It is important to note here that the using of such materials
for sensor design is possible only in the case, when mentioned
reactions are completely reversible; and they take place with
acceptable rate.
4.3. Long-term stability of gas sensors
14
G. Korotcenkov / Materials Science and Engineering B 139 (2007) 1–23
bility of these materials are bad, and gas sensors on the base of
such materials have short life term, especially during their work
in usual atmosphere containing water and active gases. Among
other polymers undoping PPY as a semiconducting polymer, is
rather stable towards UV irradiation, which can ever increase its
conductivity [135]. However, the stability of PPYs against UV
irradiation depends on the type of dopant present in the polymer and power density of UV irradiation [136]. Moreover it was
established that UV irradiation might change the thickness and
surface roughness even PPY films [135].
Because of that fact, in spite of the wide range of gas sensor’s
prototypes, designed on the base of polymer films, very few of
them have found their way to the market. Even they show excellent analytical qualities, the devices are often not suitable for
industrial fabrication, because of low technological effectiveness of fabrication process, insufficient reliability and stability.
All recognize as necessary that to realize the advantages of polymers having a rare combination of electrical, electrochemical
and physical properties it is very important to increase their processability, environmental and thermal stability [93,137]. For
example, according to [138], a polyamide with high resistance
to degradation should have the following properties: (a) high
melting/softening point, (b) low weight loss as determined by
thermo-gravimetric analysis, and (c) structures that are not susceptible to degradative chain scission or intra- or intermolecular
bond formation. Intensive research is being carried out in this
direction. For example, when a segment of the aliphatic polymers main chain is replaced by a ring segment, the melting
temperature and hence, the thermal stability increases due to
the decrease in the flexibility of the polymer chain [133,138].
However, one should admit that this task with reference to gas
sensor design is pretty complicated, because it is necessary to
attain high stability while great polymer’s activity is being kept.
Besides, we have to remember that there are some fundamental restrictions for achievement a required thermal stability
of polymers. Information presented in [133,138] for nylon-type
polymer shows that the melting point of the polymer decreases
as the chain length increases. It means that polymer’s complication inevitably will be accompanied by a reducing of melting
temperature, and, therefore, by a drop in polymer’s stability.
Above mentioned confirms again that the problem of stability
and reliability of gas sensors is a determinant for practical use
of any gas-sensing material.
The same situation is observed for gas sensors on the base of
standard covalent semiconductors, such as Si, InP, GaAs, GaP as
well. In oxygen atmosphere it is taking place surface oxidation,
which inevitably leads to the change of electronic, adsorption,
and catalytic properties of the semiconductor surface. Gas sensors based on standard covalent semiconductor, including Si,
generally need to have an aging treatment to have reliable and
repeatable sensitivity. Even then, lifetimes of gas sensors based
on covalent semiconductors (InP, GaAs, GaP), and especially
on porous Si can be short [139]. Gas sensors on the base of
ionic compounds, such as CuBr, have unstable parameters as
well [140].
Only metal oxides and wide band semiconductors, such as
SiC, and GaN, with dielectric covering have necessary stabil-
ity of surface and bulk properties in both oxygen atmosphere
and water environment, which provides them wide practical
use in real devices of long-term using, available at sensor market [141–143]. The results presented in [144,145] show how
stable metal oxides could be. Zirconia-based ceramics, which
belongs to the group of the most stable metal oxides, kept its
electro-conductivity without changes even at T > 1000 ◦ C.
However, one should recognize that the problem of parameters’ temporal instability also remains for metal oxides, but not
to the same degree as for other materials. As it was established
[34,54,146,147], the main reasons for long-term instability of
solid-state gas sensors are the change of the metal oxide parameters, caused by the following processes: (1) microstructural and
morphology changes of sensing element (the change of the crystallite size, number and distribution of grains and intergranular
boundaries), a consequence of insufficient pre-aging by tempering; (2) irreversible reactions with gas phase, i.e. metal oxide
reduction during interaction, or reactions with active gases, such
as SO2 , Cl2 , etc., with creating new phases; (3) the reactions with
the substrate. It is necessary to note that the decrease of grain
size sharpens the problem of microstructural instability of metal
oxides, especially at high operating temperatures required for
fast response and short recovery times [146]. So, stabilization
of structural properties even for metal oxides is of the utmost
importance. As an additional source of temporal drift could also
be ionic drift, which can modify electro-physical and surface
properties of metal oxide [2] and modifications of the sensor’s
heating element or of the electrodes [22].
4.4. Sensitivity to humidity of surrounding atmosphere
Gas sensors should work in atmosphere, containing water
vapors. As we know, relative humidity of surrounding atmosphere could reach 100%. One can judge about the importance
of water vapor influence on the sensor parameters, analyzing the
results of research, given in Refs. [37,38,48,49,148,149]. It was
established that adsorption of water is a dominant factor in the
surface characteristics forming, both with respect to adsorption
of other species and to surface catalysis. In [48,49] it was shown
that there is a competitive adsorption between O2 and H2 Orelated surface species, and as a result, different sensing mechanism can be observed for gas detection in dry and wet atmospheres. Moreover, the hydroxylation of the SnO2 surface was
found to inhibit sorption for all gas mixtures (CH4 , CO, CO2 , O2 )
examined in Ref. [148], in accord with experimental findings.
A hydroxylated surface is formed at an oxide by the
chemisorption of a monolayer of water. Water may also catalyze reactions, taking place at the surface of a gas sensor. The
adsorption of water also has an effect on the electronic properties of semiconducting metal oxides, usually acting as a donor.
Morrison [150] has shown that hydroxylation is an intermediate
stage in the interaction of water with the oxide. It is intermediate between hydration of the surface and physical adsorption
of water. Long exposure can lead to hydration of the surface
layer, and correspondingly to drift of chemical sensors characteristics. Therefore low tendency to hydration is an important
requirement to a material, destined for practical use. Only this
G. Korotcenkov / Materials Science and Engineering B 139 (2007) 1–23
property can provide its stable work in wet atmosphere. For
example first research in the field of humidity sensors have
shown that ceramic humidity sensors had progressive drift in
resistance, which was caused by the gradual formation of stable chemisorbed OH-groups at the oxide surface for prolonged
exposure to humid environments [67,73]. Given the ionic-type
humidity sensing mechanism, proton hopping was adversely
affected by the surface presence of hydroxyl ions instead of
water molecules, thereby resulting in a decrease in surface conductivity [151]. Therefore most of the commercial humidity
sensors based on ceramic sensing elements were equipped with a
heater for regeneration before each operation [152,153]. It makes
worse the exploitation sensor parameters, because it is needed to
expend energy for the recovery of sensitivity of porous ceramics,
and during the cleaning operation the sensor is unable to give
information about humidity.
However, last time this problem was solved by using materials, possessing different humidity sensing mechanism [154]. For
example, authors of [155] because of happy choice of components (SiO2 /In2 O3 = 75%/25%), and, probably, of parameters of
thermal stabilization, succeeded in obtaining very good exploitation parameters and high temporal stability of metal oxide
conductometric humidity sensors, operating at RT. A humidity sensitivity of 0.25%/RH% has been achieved. The samples
exhibit a low drift over a 1-year time span (0.0013 RH%/year),
low hysteresis (0.34 RH%), good linearity (±2 RH%) and a reasonably fast time response (18 s). At that mentioned stability
was achieved without use of any additional thermal treatments.
5. The role of structure and technology in gas-sensing
material choice
As it was wrote earlier the operating characteristics of solidstate gas sensors are determined by both receptor and transducer
15
functions. The last function is very important, because it determines the efficiency of chemical interactions’ conversion into
electrical signal. Usually this function is played by each boundary between grains, to which a double-Schottky barrier model or
“neck” model can be applied [1–3,8,56,85,156]. The resistance
depends on the surface potential modulated by concentration of
the target gas [157]. Diagram illustrating the role of necks in the
conductivity of polycrystalline metal oxide matrix is shown in
Fig. 7.
Taking into account mentioned above it becomes clear that
possibility to synthesize and deposit metal oxides with morphology and crystallographic structure optimal for achievement
maximum gas-sensing effect is important factor for application
of this material in solid-state gas sensors.
5.1. Structural parameters of sensing materials
At present, considering sensing materials, we may select
five types of material’s state, differing by its structural properties. These are amorphous state, glass-state, nanocrystalline
state, polycrystalline state, and single crystal state. Every state
has its own specific peculiarities, and materials in any of these
states may be used for gas sensor’s design. However, in practice, nano- and polycrystalline materials have found the greatest
application in gas sensors. Exactly nano- and polycrystalline
materials have the most optimal combination of such properties as enough developed surface, cheap design technology, and
necessary stability of both structural and electro-physical properties. As it is known a specific surface area is sharply increased
with decrease of grain size. A high specific surface area and comparability of grain size (D) with the thickness of surface space
charge layer (LS ) can take great advantage for the development
of high-sensitive gas sensors [56,71,157,158]. According results
presented in [56,158,159] for achievement considerable increase
Fig. 7. Diagram illustrating the role of necks in the conductivity of polycrystalline metal oxide matrix and the potential distribution across the neck.
16
G. Korotcenkov / Materials Science and Engineering B 139 (2007) 1–23
of SnO2 sensor response the grain sizes have to be smaller than
6–10 nm. It means that nanostructured metal oxides are becoming the key materials for development of semiconducting gas
sensors with improved gas-sensing properties. As it is known
LS is a function of the concentration of charge carriers in the
bulk of oxides. Therefore metal oxides, for which the technology
given possibility to control this parameter, for example through
bulk doping, is designed, have additional preference for actual
applications. Regarding amorphous and glassy materials, they
are not stable enough, especially at higher temperatures. Single crystalline and epitaxial materials have maximum stability
of characteristics. However their application is limited by high
cost and difficulty of technological process of their deposition.
Last years it has appeared one more perspective trend
of research, connected with taking into account structural
peculiarities of gas-sensing materials. The using of quasi-onedimensional structures is the basis of these researches. As
research of carbon fullerenes and nanotubes has shown, the size
effect is very important factor in control of nanomaterials properties [160–163]. It was found that due to structure peculiarities,
one-dimensional metal oxide nanomaterials might be perspective enough for fundamental studies as well as for application in
low cost, small-sized, and low power consumption devices.
One-dimensional metal oxide nanomaterials, such as
nanobelts, nanorods, nanowires and so on, have excellent crystallinity and clear facet. It is expected that these nanomaterials
will have less concentration of point defects and specific
adsorption and catalytic properties, conditioned by a peculiar
combination of crystallographic planes, faceting them. Besides
that one-dimensional metal oxide nanomaterials should be more
thermo-dynamically stable in comparison with nanograins, promoting stable operation of gas sensors at higher temperature.
A development of nano-technology gives a hope to gas sensor’s realization on the base of single metal oxide nanowires,
which optimizes their parameters even more in comparison with
devices on the base of nano- and polycrystalline materials.
At present, various kinds of one-dimensional nanomaterials
such as Si, Ge, silica, MgO, CaO, GaN, SiC, In2 O3 , TiO2 , Fe2 O3 ,
ZnO, SnO2 , etc., were synthesized as nanowires, nanotubes,
nanospheres, nanorods, and nanobelts [164–170]. However,
there are much more research of quasi-one-dimension structure
synthesis, than attempts of these materials application in gas sensors. Therefore now we know more of regularities of nanowires
growth [165–168] than of their electro-physical, surface, catalytic, and sensor’s characteristics. It is necessary to admit that
last years research in this area became much more intensive
Fig. 8. Factors characterizing high manufacturability of sensing materials.
[171–179]. Therefore one may hope that expectations of the
results of practical application of one-dimensional metal oxide
nanomaterials in gas sensors would not be so long. The base
of such optimistic opinion is a progress, achieved in technology of one-dimensional metal oxide nanomaterials’ synthesis,
allowing synthesizing high quality nanomaterial with length of
individual nanowires equaled up to 10–500 ␮m [168–170].
5.2. Manufacturability of sensing materials
The good manufacturability of sensing material, i.e. an opportunity to produce under control and with high reproducibility
powders, films, and ceramics with required structural properties
(see Fig. 8), is an important criteria for selection of materials for
gas sensor [16,20,85,147,180–182]. For example the authors of
[20,34,45,183] wrote that to achieve stable, selective and reliable
solid-state gas sensors, an accurate preparation of the functional
material is a crucial point; many factors must be taken into
account to warrant homogeneous grain shape and size, size distribution, porosity, and surface conditions. Some information
about solid-state gas sensors on the base of most familiar metal
oxides and technological peculiarities of these sensors fabrication, which can be used for such selection, is presented in
Tables 9 and 10.
However it is necessary to note that at present there are not
great technological problems of fabrication any binary oxides
and standard semiconductors with specified electro-physical and
structural properties. In the literature one can find a great deal
Table 9
Main advantages and disadvantages of well-known metal oxides for gas sensor applications
Material
Advantages
Disadvantages
SnO2
WO3
High sensitivity; good stubility in reducing atmosphere
Good sensitivity to oxidizing gases; good thermal stability
Ga2 O3
In2 O3
High stability; possibility to operate at high temperatures
High sensitivity to oxidizing gases; fast response and recovery;
low sensitivity to air humidity
High stability; low sensitivity to air humidity
Low selectivity; dependence on air humidity
Low sensitivity to reducing gases; dependence on air humidity.
Slow recovery process
Low selectivity; average sensitivity
Low stability at low oxygen partial pressure
CTO (CrTiO)
Average sensitivity
Medium
Low
High
Acceptable
Low
Excellent
Low
Low
250–450
Moderate
Medium
Medium
Medium
Medium
High
Good
Moderate
Moderate
Good
Excellent
High
Moderate
Reduced
Low
Reduced
Moderate
Acceptable
Acceptable
Moderate
Moderate
Acceptable
Moderate
Excellent
High
Moderate
Moderate
Enhanced
Satisfactory
High
300–500
600–900
200–400
200–450
350–800
250–350
300–450
Low
Good
Good
Moderate
Moderate
Good
Imperfect
High
Good
High
Acceptable
Excellent
Fe2 O3
WO3
Ga2 O3
In2 O3
MoO3
TiO2
ZnO
CTO
Reducing gases (CO, H2 ,
CH4 , etc.)
O3 , NOx , H2 S, SO2
O2 , CO
O3 , NOx
NH3 , NO2
O2 , CO, SO2
CH4 , C4 H10 , O3 , NOX
H2 S, NH3 , CO, volatile
organic compounds
Alcohol, CH4 , NO2
SnO2
200–400
Imperfect
Fabrication
complexity
Compatibility with
standard IC
fabrication
Stability
Operating
temperature (◦ C)
Gas optimal for detection
Metal oxide
Table 10
Operating parameters of solid-state gas sensors on the base of metal oxides and technological peculiarities of their fabrication
Sensitivity to
air humidity
Stability in reducing
atmosphere technologies
Readiness of synthesis
and deposition
G. Korotcenkov / Materials Science and Engineering B 139 (2007) 1–23
17
of works, devoted to elaboration of both deposition and synthesis technologies of various binary oxides in the form of
thin, thick, epitaxial films and ceramics using different methods
[17,184–193].
Considering more complicated oxides and binary oxides,
modified by different additives, polymers and organic semiconductors, one can conclude that here there are much more
problems. For example, during polymer sputtering with use
of electronic beam their chemical decomposition is possible,
which naturally limits an opportunity of such materials’ application. It should be noted also that since conducting polymers
are generally insoluble and intractable, they are not amenable to
conventional methods of purification and characterization, and
some discrepancy and apparent irreproducibility may originate
from changes in preparation procedures that are sufficient to
alter the gas-sensing properties of the polymer.
An important aspect of good manufacturability of sensing material is an opportunity of its adaptation in modern
micro-electronic technology, for example during fabrication Simicromachined microhotplate array structures [147,188,194].
These devices can be adapted for a variety of applications by tuning both the composition of multiple types of active films and the
temperature cycles programmed for individual elements within
an array [188]. As last research has shown, this problem becomes
especially evident in the area of micro-miniaturization of hightemperature gas sensors. It was established that difficulties here
are connected with agglomeration of very fine metallic electrode
structure on oxide surface at high temperatures, and with film
cracks due to thermal expansion mismatch between thin film
and substrate. Therefore while choice of material for solid electrolyte gas sensor in microelectronic design, one should take
into account this specificity. According to [195], for microelectronic design such solid electrolytes as (Al,Sc)2 ((Mo,W)O4 )3
were found to be the most appropriate. They have low thermal
expansion coefficients and good chemical compatibility with
silicon technology. The most common material for macroscopic
and thick-film solid-state ionic gas sensors, zirconium oxide stabilized by ytterbium (YSZ), is well suited for integration on
silicon devices as well. At the same time the integration of alkaliion conductors with silicon technology is problematic because
of possible degradation of silicon devices due to contamination
with alkali-ions [195].
6. Outlook
As it follows from conducted analysis, the choice of a suitable
material for gas sensors should be based on good gas response,
low sensitivity to air humidity, high selectivity, low hysteresis,
high stability of parameters over the time, all range of operation
temperatures, thermal cycling, and on exposure to the various
chemicals likely to be present in the environment [34,182,196].
Therefore desired efficiency of reactions, responsible for gas
sensors’ sensitivity, it is necessary to achieve, taking into account
the necessity of an attainment of maximum chemical, structural,
and long-term stability of the device’s parameters.
However, it is necessary to recognize that it is impossible to find a material, satisfying all possible requirements,
18
G. Korotcenkov / Materials Science and Engineering B 139 (2007) 1–23
made to optimal material for gas sensors’ elaboration. For
example, it is known that gas response of sensors, operated
in temperature range lower than 450 ◦ C, could be controlled by either chemisorption processes, or “redox” processes
[34,39,53,102,150,197–199]. Sensors of the base of the most
studied tin dioxide are sensors of the first type. While In2 O3 based sensors, studied intensively last time, are sensors of the
second type. However, the fact that SnO2 -based sensors are
studied better and are being wider used, is not the base for
conclusion that chemisorption mechanism of sensitivity has
advantages before “redox” mechanism for sensor’s design. It
was established that SnO2 -based sensors have better sensitivity to reducing gases, and better stability during operation in
reducing atmosphere. However, In2 O3 -based sensors have better conductivity response to oxidizing gases, and show lesser
dependence of parameters from air humidity change [34,199].
What is better, high response to reducing gases, or to oxidizing
ones? The answer to this question can give only user based on
his own requirements.
The same comparison one can make also for other pairs
of metal oxides, for example, SnO2 –CTO, and SnO2 –WO3 .
The titanium-substituted chromium oxide, Cr2−x Tix O3+z (CTO)
with 0.01 < x < 0.45 has high chemical stability at the operating
temperature, easily measurable range of resistance, and good
conductivity response [43]. It was found that in contrast to SnO2
the effect of water vapor on CTO, on both baseline and sensitivity, is very much less than that on SnO2 at the same operating
temperatures. This has been the key to the successful commercial development of CTO for detection of carbon monoxide and
volatile organic compounds in the air. In contrast to SnO2 , CTO
is not sensitive to methane, though it is sensitive to the higher
hydrocarbons and to carbon monoxide, H2 S, NH3 , and a wide
range of solvents.
In the pair SnO2 –WO3 , SnO2 shows both large and
fast gas response to ozone at sufficiently low temperature
(∼200–300 ◦ C), though there are problems of baseline stability
at such low temperatures. The effect decreases with increasing temperature, to virtually zero at 400 ◦ C. Tungsten oxide
shows particularly large resistivity increase at high temperature
(400–500 ◦ C) in response to the presence of ozone. However, the
response to ozone is relatively slow, as is the recovery when the
gas is removed. Besides the signal is dependent upon the flow
rate of the gas to the surface, and the partial pressures of water
vapor and oxygen [43]. However, at the same time, experiments
carried out in [185] shown that WO3 -based sensors had unique
and excellent sensitivity upon to low concentration of H2 S in air
at room temperature.
Authors of [200] believe that Ga2 O3 -based sensors
also have certain advantages in comparison with SnO2 based sensors. Ga2 O3 -sensors are high-temperature devices
(Toper = 600–900 ◦ C), and therefore these devices show faster
response and recovery processes and lower cross-sensitivity
to humidity than SnO2 -based sensors. Besides, Ga2 O3 sensors
show stable long-term sensing properties and good reproducibility even in sulphur-containing atmospheres. The last property
makes these sensors suitable for use in domestic burner controls. No cleaning cycles are necessary and smut or other organic
residues are burnt-off. Additionally, no pre-aging is necessary
(compared to SnO2 ). On the other hand, the sensitivity of Ga2 O3
sensors to a number of gases is lower compared to SnO2 -based
sensors, and the power consumption of Ga2 O3 -based sensors is
comparably high due to its high operating temperature.
All mentioned above indicates that the choice of metal oxides
for gas sensor design (see Table 8) would be determined by such
factors as the type of designed gas sensor (see Tables 9–11), an
object (apparatus, device), for which sensor is being designed,
and construction (structure) chosen for this sensor’s fabrication [33,34,36,181,182,201–205]. However, any competition
between considered materials could be forgotten if devices on
their base incorporate in the “electronic nose”. Different behavior during interaction with the same gas is one of the most
important requirements for sensors, designed for this application
[25,115,116,206].
The similar comparative analysis could be also conducted
for such pair of gas-sensing materials, as polymer and metal
oxide. It is known that organic semiconductor does not interact
as strong with oxygen or water as inorganic semiconductor [23].
Polymers have maximum variety of properties, and they can
be easily modified, obtaining excellent selectivity during interaction with analyte at low operating temperature [30,31,207].
However, because of possessing such properties, polymers have
worse thermal and long-term stability of parameters [31,207].
Metal oxides do not show such pronounced selectivity in gassensing effects. However, in contrast to polymers, they have high
thermal and temporal stability of parameters. Therefore one can
conclude that polymers satisfy to maximum quantity of requirements as a material for low-temperature selective gas sensors,
which are not expected to operate in tough conditions. While,
metal oxides have considerable advantages as materials, used
in high-temperature gas sensors, designed for long-term use in
tough conditions.
In other words the choice of material for gas sensor is
always a compromise decision, demanding a consideration of
sometimes-contradictory requirements [34]. Even more, every
new application advances its own requirements to sensing materials. Therefore we find in the literature such a big amount of
materials, tested with aim to evaluate the possibility of their
application at gas sensors design. At present this process is going
on, involving in research new types of compounds. Fullerenes
carbon and metal oxide nanotubes, nanowires, and so on should
refer to such materials. We do not know yet where eventually a
qualitative leap to a world of nano-sized ranked structures would
lead. However, the first obtained on this way results are being
encouraged [169–176,208–217]. At that it is important to note
that observed sensitivity of one-dimension sensors on the base
of metal oxides is significantly higher than reported sensitivity
of carbon nanotubes reported. There is assumption that such difference is related to the nature of the metal oxide’ surface, which
can readily react with ambient species, as compared to the inert
sidewall of carbon nanotubes [215].
Nano-composites’ design is another promising direction
in the development of materials for solid-state gas sensors [182,218–223]. Nano-composite materials have recently
attracted increasing interest because of the possibilities of syn-
G. Korotcenkov / Materials Science and Engineering B 139 (2007) 1–23
19
Table 11
Metal oxides preferable for applications in various types of gas sensors
Sensor type and sensor’s elements
Detected gas
Metal oxides preferable for application
Chemiresistors (semiconductor)
Reducing gases (CO, H2 , CH4 )
Oxidizing gases (O3 , NOx , Cl2 )
H2 S, SO2
NH3
CO2
Alcohol
Oxygen
Humidity
Oxygen
H2
Humidity; NO2 ; H2 ; ethanol; O3
Hg vapor; NH3 , NOx , SOx , H2 S
SnO2 ; CTO; Ga2 O3 ; In2 O3
In2 O3 ; WO3 ; ZnO; TiO2
SnO2 /CuO; SnO2 /Ag2 O
WO3 ; MoO3 ; In2 O3
SnO2 /La2 O3 ; Al2 O3 /V2 O5
La2 O3 /In2 O3 ; La2 O3 /SnO2 ; In2 O3 /Fe2 O3
Ga2 O3 , SrTiO3 , SrTiFeO3 ; TiO2 ; Nb2 O5 ; ZnO
In2 O3 /SiO2 ; TiO2 /MgCr2 O4 ; SrTiO3 ; LaFeO3
ZrO2 :Y; Bi2 O3 /MoO3
Sb2 O5
ZnO; InOx ; LiNbO3 ; SiO2 ; WO3
SiO2
Capacitance
H2 ; NH3 ; C2 H5 OH
Humidity
CO2
NOx
(Pd, Pt, Ir)/SiO2
Al2 O3
CuO/BaTiO3 ; CeO2 /BaCO2 /CuO; Co3 O4 /BaTiO3 ; NiO/BaTiO3
CoO/In2 O3 ; NiO/ZnO
Heterostructural
CO
H2 S
ZnO/Zn2 SnO4 ; SnO2 /TiO2 ; SnO2 /Zn2 SnO4
ZnO/CuO; SnO2 /CuO/SnO2
Schottky diodes
Opto-chemical
Fiber-optic
Work function (RT)
Surface plasmon resonance
Pelistors
Pyroelectric
Electronic nose
Membranes (filters)
Substrates
Electrodes
Promoters
Structure modifier (stabilizer)
Fibers
H2
H2 , CO, alcohol
H2 , CO, alcohol
CH4 , CO, Cl2
NO2 ; H2 S; NH3
Combustible gases and vapors
H2 ; CH4
Gases, vapors
ZnO; TiO2
WO3 ; Mn2 O3 ; Co3 O4 ; NiO; CuO
WO3 ; Mn2 O3 ; Co3 O4 ; NiO; CuO
NiO; Fe2 O3 ; Co3 O4
Ta2 O5 ; SiOx Ny ; TiO2
Al2 O3 ; SiO2
ZnO; LiTaO3 ; LiTiO3
SnO2 ; In2 O3 ; WO3 ; ZnO
Al2 O3 ; SiO2
Al2 O3 ; SiO2 ; BeO; LiNbO3 ; ZrO2 :Y
NiO/Ni; PdO/Pd; Al2 O3 /Pt; MgAl2 O3 /Pt
PdO; RhO; Ag2 O; CuO; Fe2 O3 ; P2 O5 ; Co3 O4 ; NiO; MnO
Al2 O3 ; SiO2 ; CaO; MgO; BaO; Y2 O3 ; La2 O3 ; Ta2 O5 ; CeO2
SiO2 ; GeO2 –Sb2 O3
Electrochemical (amperometric)
Surface acoustic wave
Quartz balance
thesizing materials with unique physical–chemical properties
[218,219]. Highly sophisticated surface-related properties, such
as optical, electronic, catalytic, mechanical, and chemical ones
can be obtained by advanced nanocomposites, such as composites on the base of carbon nanotubes and fullerenes, different
metal oxides, or organic–inorganic substances, making them
attractive for gas sensor applications. As it was shown earlier,
the resistance variation of the sensing layer involves two important functions, i.e. the recognition and the transducer functions.
Gas/solid interaction phenomena are involved in the receptor
function, while the microstructure of the oxide determines the
transduction of the chemical stimulus in air into an electrical
signal. Generally speaking, if a single oxide system is adapted,
these two functions cannot be optimized independently. Instead,
by introducing in the system a foreign material, which is very
reactive to a target gas and act as an “antenna” material, both
functions may be optimized simultaneously and the sensor may
become more sensitive even to low reactive gas concentrations.
In these cases, the material acting as a unique receptor (antenna
material) should be interfaced electronically to the transducer
material and its chemical change should sensitively modulate the
semiconducting properties of the transducer oxide through the
hetero-junction [22]. Composite-type sensors containing heterocontacts between the two phases fulfil this novel concept of gas
sensors. Experiments carried out in some laboratories confirmed
this assumption [219,224–226].
It was established that materials, obtained as a result of this
elaboration, have their own specific advantages [224,225]. For
example, in nano-composites there are more possibilities for
control of catalytic activity of sensing matrix. It was shown also
that the introduction of TiO2 nanoparticles in polymer matrix
of poly(p-phenylenevinylene) (PPV) changes adsorption properties. Adsorption of oxygen is found to be stronger on the
PPV–TiO2 nanocomposite than on pure PPV [226]. The advantage of metal oxide nanocomposites Me1 O/Me2 O, containing
two metal cations (Me1 and Me2 ), over simple nanocrystalline
oxides is associated with the redistribution of Me2 between the
bulk and the surface of Me1 O grain, depending on the “redox”
properties of the gas phase [219]. The appearance of additional
Me2 cation in the nanocrystalline system may result in a dramatic
change in the state of grain boundaries and in modification of the
electronic properties of the material in the presence of even trace
amounts (0.1–10 ppm) of reducing or oxidizing gas molecules
in the gas phase.
20
G. Korotcenkov / Materials Science and Engineering B 139 (2007) 1–23
Acknowledgements
Author is thankful to Civilian Research Development Foundation (CRDF) and Moldovan Research and Development
Association (MRDA) (Grant MO-E2-3054-CS-03), Supreme
Council of the Republic of Moldova in the field of science and
advanced technology (Contract 071), and NATO (Grant CLG
980670) for financial support of his scientific research.
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