Download 1.1. Gas Sensors: An Introduction A sensor is a device that receives

1.1. Gas Sensors: An Introduction
A sensor is a device that receives a signal, either in the form of physical, chemical or
biological and converts into electrical signal. Depending on the input signal, there are various
types of sensors, such as optical, electrical, thermal, mechanical, chemical, and so on. So,
chemical sensor is a device which converts a chemical signal into an electrical one. In a chemical
sensor (which consists of two parts, the sensing element and the transducer), the sensing element
changes the chemical properties in accordance with the ambient and the transducer transforms
this chemical change into electrical one. The gas sensors are included in chemical sensors group.
So gas sensor is a device which can detect the presence of various toxic and combustible gases
present in the environment. The interaction between the test gas and the sensing surface can be
detected by the measurements of change in resistance, capacitance, work function, mass, optical
characteristics etc.
There are a numerous use of these gas sensors. Particularly, using these gas sensors human
beings can be saved from potential dangers. Hence the gas sensors play important roles in
various sectors, which include industry, medical, environmental applications, and domestic
applications for monitoring toxic and flammable gases [1]. Some of the important applications of
gas sensors are summarized in Table 1.
Table-1: Application of gas sensors in various fields
Applications
Safety





Fire detection
Leak detection
Toxic/flammable/explosive gas detectors
Boiler control
Personal gas monitor
Environmental control


Weather stations
Pollution monitoring
Automobiles




Car ventilation control
Filter control
Gasoline vapor detection
Alcohol breath tests
Medicine


Fermentation control
Process control
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Food



Food quality control
Process control
Packaging quality control (off-odors)
Indoor air quality



Air purifiers
Ventilation control
Cooking control
Industrial production


Fermentation control
Process control
The present chapter provides a brief introduction about various types of solid state gas
sensors. The properties of ZnO and its suitability as an effective gas sensing material are
discussed.
1.2. Solid State Gas Sensors
In the last few decades, the demands for highly efficient gas sensors, together with the
challenge to detect the toxic gases in harsh environment have invoked the development of
various types of solid state gas sensors. These solid state gas sensors include acoustic wave gas
sensors, metal-oxide-semiconductor field effect transistor (MOSFET) gas sensors, optical fiber
gas sensors, electrochemical gas sensors, chemi-resistive gas sensors etc. The sensing principle,
type of sensing elements and application field of these gas sensors are briefly discussed as
follows [2].
Acoustic wave gas sensors:
In these sensors chemically selective sensing layers are coated on piezoelectric substrates,
such as quartz, ZnO etc. which can act as resonant elements. When these sensing elements are
exposed to any test gas environment, the adsorption of gaseous species on the sensing layer
results a mass loading in the resonant element leading to a reduction in the resonant frequency.
As a result there is a shift in the value of the frequency, which can be detected and measured
accurately. A variety of gases can be detected using these sensors depending on the type of the
sensing layers used. However, the main disadvantages of these sensors are the requirement of
high frequency and very complex circuit for its operation [3-4].
Metal-oxide-semiconductor field effect transistor (MOSFET) gas sensors:
These sensors consist of a source, drain and gate. Usually the gate is a metal (catalytic
metals such as Pd, Pt etc.) contact which acts as the sensing element. Next to the gate there exist
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an oxide insulator and at the opposite side of the insulator there exist a semiconductor. The
source provides and the drain collects the charge carriers (may be holes or electrons depending
on the type of the MOSFET). The amount of charge carriers (or current) can be controlled by
controlling the electric field in the semiconductor. The change in current, results from the change
in work function of the metal and semiconductor, in presence of the test gas. Hence the
efficiency of MOFET gas sensors depends on the type of metal used and the operating
temperature (as the carriers in a semiconductor changes with the temperature). There are a
number of advantages of these sensors as they are small in size and can be reproduced easily.
However, these sensors are mostly used to detect H2 gas [5-6].
Optical gas sensors:
In optical gas sensors the sensing layer is usually a polymer based layer. Upon exposure of
these sensors to a test gas environment, results in the change in optical parameters (e.g. refractive
index, absorption coefficient, etc.), due to the interaction of the test gas with the sensing layer [1,
7]. These sensors work efficiently and offer faster response. It has wider gas sensing capability
when combined with Raman or IR spectroscopy and are environmental friendly as no electric
requirement is needed. However, these sensors have often poor life time.
Electrochemical gas sensors:
Electrochemical gas sensors consist of two electrodes (one sensing electrode and another
reference electrode) separated by a thin layer of electrolytes. These electrodes are made up of
noble metals or gold which can catalyze the test gas and the electrolytes carries the charges
across the electrodes. When these gas sensors are exposed to the test gas environment, the gas is
either oxidized or reduced at the sensor electrode [1, 8] and the resulting current is measured.
From the value of the measured current the concentration of the test gases can be estimated.
Electrochemical sensors are divided into three types; amperometric, potentiometric, and
conductometric gas sensors. In potentiometric gas sensors the generated signal is independent of
the size of the sensor, whereas in amperometric gas sensors the current is proportional to the size
of the electrode and hence sensors having smaller size produce lesser current. The primary uses
of the potentiometric gas sensors are oxygen sensors or λ-sensors, which are used to monitor the
air/fuel ratio in automotive combustion engines. The air/fuel ratio is determined by comparing
the partial pressure of oxygen in the exhaust and that in the ambient air. In amperometric gas
sensors a zero oxygen concentration is maintained at the cathode by applying a high voltage
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externally. Now the diffusion of oxygen from cathode to anode determines the efficiency of
these sensors.
Chemi-resistive gas sensors:
In chemi-resistive gas sensors the sensing layer can be either of a variety of materials, like
semiconducting metal oxides (SMO), composites, carbon nanotubes, polymers etc. [9-13]. The
principle of operation of these sensors is primarily based on the changes in resistances upon
exposure to the target gas. Though the polymer based sensors offer reasonably good sensitivity,
however, the poor life time of these sensors limits their use [14]. On the other hand, the SMO
based sensors are useful, due to their low cost, ease of preparation, high sensitivity, and long
term stability. However, the poor selectivity and higher operating temperature sometimes limits
their use in different applications [15-16].
Among all the gas sensing materials, SMO based resistive gas sensors (e.g. SnO2, ZnO, WO3
etc) have been extensively studied due to their lower cost, ease of synthesis and simplicity in use.
In the following section the properties of ZnO as an efficient gas sensing material is discussed.
1.3. ZnO as a Gas Sensing Material
ZnO is one of the earliest discovered SMO gas sensing materials. It is a very promising
material for gas sensors because of its good response to various gases, high chemical stability,
and feasibility to doping, non toxicity, and low cost. Bulk and thin films of ZnO have
demonstrated high sensitivity for toxic gases [17-18]. ZnO gas sensors can be used not only for
detecting the toxic and inflammable gases but also for detecting volatile organic components
such as, alcohol, petrol, etc [18].
1.3.1. Properties of ZnO
ZnO is an II-VI compound semiconductor having a stable wurtzite structure of lattice
constants a = 3.25 Å and c = 5.2 Å; their ratio c/a ~ 1.60 is close to the value for an ideal
hexagonal cell c/a = 1.633 [19]. It has a direct and wide band gap (3.37 eV) in the near-UV
spectral region [20], and a large free-exciton binding energy of 60 meV (sufficiently larger than
the thermal energy at room temperature (26 meV)) [20], which makes the excitonic emission
possible at or even above room temperature [21]. Some of its physical properties that make it
particularly attractive for electronic devices are summarized below, in table 2 [22].
Table 2: Properties of ZnO
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Crystal structure
Hexagonal, Wurtzite
Molecular weight
Zn:65.38, O:16 and ZnO:81.38
Lattice constant
a = 3.246 Å, c = 5.207 Å
Density
5.67 g/cm3 or
4.21 ×1019 ZnO molecules/mm3
Cohesive energy
Ecoh = 1.89 eV
Melting point
Tm = 2250ok under pressure
Heat of fusion
4470 cal/mole
Thermal conductivity
25 W/mK at20oC
Thermal expansion coefficients
4.3 × 10-6 /oK at 20oC
7.7 × 10-6 /oK at 600 oC
Band gap at RT
3.37 eV
Refractive index
2.008
Electron and hole effective mass
me* = 0.28, mh* = 0.59
Debye temperature
370oK
Lattice energy
964 kcal/mole
Dielectric constant
ε0 = 8.75, ε∞ = 3.75
Exciton binding energy
Eb = 60 meV
Pyroelectric constant
6.8 Amp/sec/cm2/oK × 1010
Piezoelectric coefficient
D33 = 12 pC/N
The above remarkable physical properties of ZnO has attracted intensive research efforts in
numerous applications, such as semiconductor devices [23-24], transparent conductors [25],
solar cell [26], varistors [27], liquid-crystal displays [28], spintronic applications [29], gas
sensors [30-31], etc. Due to the above cited applications and hence the motivation of device
miniaturization, large effort has been focused on the synthesis, characterization and device
applications of ZnO nanomaterials. These nanostructures of ZnO, particularly ZnO thin films
which can be deposited even at/ near RT, have been successfully grown via a variety of methods
including chemical vapor deposition (CVD), chemical solution deposition (CSD) technique,
spray pyrolysis, and so on [32-34] and have excellent performance in different applications,
more specifically in gas sensing. In addition, due to the presence of intrinsic defects such as
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oxygen vacancies (
,)
and zinc interstitials (
), undoped ZnO exhibits an n-type
conductivity. So, even without intentional doping, ZnO often shows n-type conduction [18].
Moreover, doping with higher valence impurities further enhances the carrier concentration and
n-type conductivity of ZnO. The presence of these defects significantly influences the role of
ZnO in gas sensing.
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