Metal Oxide Gas Sensors Based on ZnO Thin Films: A Review of Synthesis, Mechanisms, and Sensing Parameters
*1Rajendrakumar
B. Ahirrao,2Sudam D. Chavhan,3Pankaj M. Nahide
*1Dept.
of Physics, Uttamrao Patil Arts & Science College, Dahiwel,Dist.-Dhule, 1Department
of Physics, S. G. Patil,Arts, Commerce and Science College, Sakri, 3Department
of Physics, Government Polytechnic College, Nandurbar.
Abstract: Efficient
air quality monitoring necessitates sensors capable of detecting gaseous
pollutants at extremely low concentrations, reaching parts per billion (ppb)
and parts per trillion (ppt) levels. These sensors must demonstrate high
sensitivity, selectivity, and rapid response/recovery times. Metal oxide
semiconductor (MOS) sensors, particularly those employing Zinc Oxide (ZnO),
hold significant promise due to their cost-effective manufacturing processes
and versatility in detecting various target gases. MOS sensors function by
adsorbing target gas molecules onto their surface, leading to electron capture
from the conduction band and consequent changes in conductivity.This review
synthesizes the latest experimental insights into Metal Oxide gas sensors,
focusing prominently on Zinc Oxide (ZnO) thin films. Sensitivity enhancement
strategies involve nanostructuring MOS materials to augment porosity and
surface-to-volume ratio, while selectivity can be fine-tuned through chemical
composition modifications. The paper critically assesses diverse techniques for
nanostructuring MOS materials using chemiresistive elements and elucidates how
these techniques contribute to tailoring the desired characteristics of gas
sensors.
Keywords: Gas sensors, Metal oxide semiconductors, Zinc Oxide (ZnO) thin
films, Sensitivity enhancement, Selectivity tuning, Nanostructuring techniques,
Chemiresistive materials.
1.Introduction: Poor air quality has become a global concern. No matter who we are,
where we live or the state of our health, the quality of the air we breathe
each day affects us. Even when we can't see it or smell it, air pollution can
still be a threat. There are many different types of air pollutants –
particulate (PM2.5, PM10), greenhouse gases (CO2, CH4)
and toxic gases (Volatile Organic Compounds (VOCs), CO, NOx, SOx, H2S).
Although industrial modernization facilitates social, cultural, and economic
development globally, it results in the consumption of enormous amounts of
energy every day. Consequently, the toxic gas and vapor emissions released into
the atmosphere cause air pollution, which has been identified as a major
concern in most countries.According to a WHO report, 92% of the global
population lives in cities with air pollution exceeding the WHO limits,
indicating the need for further monitoring and control of air pollutant gases
and vapors [1].
The gas sensors play great importance in
determining various gases as they can convert a certain gas volume fraction
into electrical signals. The gas sensors expected to be low cost, fast
response, high sensitivity, and high selectivity. The gas sensors have been
widely used in environmental monitoring, air quality monitoring, vehicle
exhaust monitoring, medical diagnosis, food/cosmetics monitoring, and many
other fields [2–9].
Conventional gas
analyzers, such as mass spectrometry, energy spectrometry and chromatography,
are limited by the high cost of the devices [10]. Compared with the analyzers,
gas sensors based on metal oxide semiconductors are widely used due to their
small size, easy operation, low cost and other benefits. Among the main
characteristics of gas sensors, sensitivity and selectivity are the main
objects of research. However, as one of the indicators which the costumers are
concerned about, stability is not often discussed.
Semiconductor
Metal Oxides are widely explored chemiresistive gas sensors [11]. Mostly gas
sensors can be of electrochemical or metal-oxide semiconductors in thick-film
or in thin film [12]. Metal oxides are favorable for usage in different aspects
in domestic, industrial and commercial applications [13]. Applications of gas
sensors are seen in wide aspect like monitoring the air quality, detection of
toxic and harmful gases, medical field etc. [14].
MOS (Metal Oxide
Sensors) sensors detect concentration of various types of gases by measuring
the resistance change of the metal oxide due to adsorption of gases.
Atmospheric oxygen residing on the MOS surface is reduced by the target gases,
allowing more electrons in the conduction band of the metal oxide material.
This resistance drop is reversible and varies depending on the reactivity of
sensing materials, presence of catalyst materials and working temperature of
the sensor.
Many
metal oxides are suitable for detecting combustible, reducing, or oxidizing
gases by conductive measurements. The following oxides show a gas response in
their conductivity: Cr2O3,Mn2O3, Co3O4,
NiO, CuO, SrO, In2O3, WO3, TiO2, V2O3,
Fe2O3, GeO2, Nb2O5, MoO3,
Ta2O5, ZnO,La2O3, CeO2,
Nd2O3 [15]. Metal oxides selected for gas sensors can be
determined from their electronic structure. As the electronic structure is the
key concern to select metal oxide for sensor [16] the variety of metal oxides
are divided into following categories[17].
(1)
Transition-metal oxides (Fe2O3, NiO, Cr2O3,
etc.)
(2)
Non-transition-metal oxides, which include (a) per-transition-metal oxides (Al2O3,
etc.) and (b) post-transition-metal oxides (ZnO, SnO2, etc.).
Fig.1.Shows
Classification of MOS
But
metal oxides having d0 and d10electronic structure is
potential enough in the field of gas sensing application. Binary transition
metal oxides, TiO2, WO3 are having d0 and post
transition ZnO, SnO2 are d10 configured. Some important
definitions must have to know as shown in Table.1.
Table:1.The main parameters of a gas sensor.
|
Parameters |
Description |
|
Response
|
It is
defined as a change in some physical properties when the device
is exposed to target species. |
|
Selectivity
|
It is
the ability of a gas sensor to detect high sensitivity to a specific gas
among various types of gases at the same concentration level. |
|
Sensitivity
|
It is
referred in the graph where slope represents the correlationbetween gas
response and the partial pressure of target gas. |
|
Limit of
detection |
It is
the lowest and highest concentration of the target gas that thesensor can
detect. |
|
Limit of
detection |
It is
the highest gas concentration that the sensor can detect. |
|
Operating
temperature |
It
refers to the maximum temperature at which the device exhibits its maximum
sensitivity in the presence of a target gas. |
|
Repeatability
|
It is
the response cycles of a sensor to be exposed to an analyte gas flow for a
long time. |
|
Response
time |
It is
usually defined as the time it takes for gas sensor to respondto a
concentration change. |
|
Stability
|
It is
the ability of gas sensors to conserve the output response measurement by a
period, the level concentration of gas (ppm) unchanged. |
|
Recovery
time
|
Time
measured when the gas sensor response changes in the interval of 90% to 10%
when the sensor is exposed to a full-scale concentration of the gas, implying
that the sensor exhibits 90% of the saturation value of resistance in
seconds. |
2.Historical Background of ZnO
Impurity
in semiconductor alters the various properties of pristine material like
resistance ,conductivity etc. In 1953, this effect was demonstrated for Ge
[18].Later, it was shown that the conductivity of ZnO thin films heated to
~300°C was sensitive to the presence of traces of reactive gases in the air
[04]. Similar properties were reported for SnO2, with higher
stability [19]. These results initiated further development of commercial gas
sensors. The early metal oxide-based sensor materials possessed a number of
unpleasant characteristics, such as high cross-sensitivity, sensitivity to
humidity, long-term signal drift and, slow sensor response. In order to improve
sensor performance, a series of various metal-oxide semiconductors have been
tested [20] .After testing various materials the most successful investigations
were connected with SnO2, ZnO, and TiO2. Parallel to this
approach, the basic research of metal-oxide materials was carried out in
scientific laboratories.
ZnO is widely used as a gas sensing
material for detecting the toxic and harmful gases because of its superior
physicochemical properties. Firstly, ZnO is an n-type II-VI semiconductor with
wide band gap (3.37 eV), large excitation binding energy (60 meV).
The ZnO-based
gas sensors have been widely used due to fast response, low detection limit,
high selectivity,
reliable performance and low manufacturing cost. ZnO belongs to surface
sensitive material. When a sensing element is prepared by using ZnO, the gas
sensing mechanism can be summarized as adsorption-oxidation-desorption [ 21]
The most extensively explored gas sensors
are metal oxide semiconductor (MOS) materials owing to their high selectivity,
stability, cost-effectiveness, and simple synthesis techniques[22].
Fig.2.Adsorption-oxidation-desorptionmechanism
in ZnO.
3.
Properties of ZnO
3.1. Physical Properties of Zinc Oxide
Zinc white crystallises mainly in two forms
viz cubic zinc blende and hexagonal wurtzite. The most common and stable
structure under ambient conditions is wurtzite. Zincblende can be stabilised by
growing zinc oxide on substrates which have a cubic lattice structure. The
oxide and zinc centres are tetrahedral.
3.2.Chemical properties of
Zinc Oxide
Calamine is a white solid which is
insoluble in water and has no smell. Crude zinc oxide appears yellow-grey in
colour and exists in a granular solid form with no odour. In its natural form,
it is obtained as a mineral zincite which consists of manganese (Mn) and some
other impurities which make it appears yellow-red colour. In its crystalline
form, it is thermochromic which on heating in the presence of air changes its
colour from white to yellow and on cooling it turns white in colour. It is an
amphoteric oxide which is insoluble in water. It dissolves in acids and
alkalis.
|
Zinc Oxide |
ZnO |
|
Molecular Weight of Zinc
Oxide |
81.406 g/mol |
|
Density of Zinc Oxide |
5.6 g/cm3 |
|
Boiling Point of Zinc Oxide |
1,974 °C |
|
Melting Point of Zinc Oxide |
1,974 °C |
Zinc sulfied,Zinc selenide,Zinc telluride
are the other related compounds. The future for high quality Zinc Oxide is
certainly going to be fascinating. The potential advances for non-medicinal
applications even surpass that of the current medicinal uses. Zinc Oxide
Nanorod Sensors, Spintronics, and Piezoelectricity are all very promising
fields and ones to keep an eye on in the not too distant future.
4.Synthesis of ZnO thin films
Fig.3. The different synthesis methods of ZnO nanostructures.
There
are different methods have been established to synthesize ZnO nanostructures.we
proposed the spray pyrolysis [23,24] method for the synthesis of thin films.
Fig.4.Schematic
sketch of spray pyrolysis system set up.
Numerous
materials have been prepared in the form of thin film because of their
potential technical value and scientific curiosity in their properties. A
number of techniques have been examined in the search for the most reliable and
cheapest method of producing thin films.
Fig.5.
Schematic sketch of spray pyrolysis system set up.
Spray
pyrolysis (SP) technique was initially suggested by Chamberlin and Skarman [25]
in 1966 to prepare CdS thin films on glass substrates. Spray pyrolysis involves
spraying of an aqueous solution containing soluble salts of the constituent
atoms of the desired compounds to the heated substrates. The liquid droplets
vaporize before reaching the substrate or react on it after splashing. Doped
and mixed films can be prepared very easily, simply by adding to the spray
solution a soluble salt of the desired dopants or impurity.
4. Experimental Study of ZnO based
gas sensor.
ZnO is
an MOS that has been extensively studied as a gas sensor. However, several
research groups are investigating the potential of adding elements in the
crystal lattice of ZnO as doping materials. The research outcomes revealed that
the doping elements change the ZnO structure. For example, doping elements
decrease the crystallite size, increase the crystallinity, and modify the ZnO
morphology [26-30]. The structural and morphological changes increase the
surface-to-volume ratio, creating a more active center at the grain boundaries.
Consequently, the sensitivity [31], response/recovery time, selectivity, and
working temperature [32] are improved, which, in terms of practical
applications, are desirable. Therefore, doping is a viable way of improving the
ZnO sensing properties [33]. Given this, various types of doped ZnO gas sensors
have been developed [34].
4.
MOS thin film gas sensor:
Grain
size is one of the component in nanostructured MOS thin films which has
important role in thin film sensors.The films without and without dopant
reported small grain size [35].Thin films with smaller grain size are favorable
as the increased surface to volume ratio, carrier concentration and enhanced
catalytic activity, facilitate its interaction with larger number of gas
molecules.
The thin films synthesized from the
different techniques like chemical and physical vapour deposition techniques,
etc. [36-39] which includes the electrical, optical and magnetic properties in
this context the growth mechanism of thin film is leading to variance in
nucleation, growth direction, crystallographic orientation , packing density,
interfacial energy and diameter of the crystallite.
The several growth parameters has been
reported [40-41] but it was taken long years to understand the thin film
growth. Besides the nature and properties of the thin film, the environmental
conditions such as operating temperature and humidity determine the sensing
performance and characteristics of the thin film gas sensors.The operating
temperature at interface determines the sensitivity, stability, response and
recovery times [42].Also humidity play important role in sensors response, as
the water vapour absorbed on the metal oxide surface affects the electronic and
ionic conducting properties of the semiconducting metal oxides.The several
terms like gas sensing principles, response and recovery time (defined
above),structural parameters,nanostructures, environmental gases also the
theory behind the gas sensing principle number of references are available for
these terms [43-61].Plenty of literature is published on these topics
,but now a days greater attention is required on the sensing behaviour MOS materials.
Fig.6.Schematic diagram showing a
particulate-based thin film deposited on an interdigitated substrate, and the
interaction of the grain boundaries after the gas species adsorb on the metal
oxide surface.
Left:Illustration of a chemiresistive gas sensor
consisting of the metal oxide semiconducting nanomaterials (orange film)
deposited on an interdigitated substrate with two finger electrodes providing
connections to a circuit for resistance measurement;
Middle: Microstructural characteristics of the MOS
film showing the grain boundaries (orange layers),
Right: Ambient oxygen species adsorb on the metal
oxide surface, depleting the electrons from the conduction band throughout the
material.
5.Parameters
of MOS gas sensors
5.1.Stability
The
stability is one of the aspects of the sensor.Heating mode affects the
stability of sensor. Humidity, operating temperature, target gas, etc.are the
factors affecting the stability of the sensor [62]. Continuous high-temperature
heating may cause an unexpected crystal growth of sensing materials.
Fig.7.
Influencing factors of stability.
The life
of sensor depends on several factors the material stability,environmental
conditions (humidity, temperature, etc.)and target gas (reducing or oxidizing
gas, gas concentration, etc.) are major influencing factors.[63].
However
to assess sensor stability, the following two indexes must be tested Stability
of the conductivity, which is also known as the sensor’s baseline and Stability
of the response. Therefore, from metal oxide sensing materials to advanced
devices, the operating stability of gas sensors should be a necessary
evaluation index.In abstract five factors determine the stability of
semiconductor metal oxide sensors sensitive materials, element doping, ambient
humidity, poisoning and component composition of gas sensors.The figure shows
key factors impacting stability of sensor.For metal oxide semiconductor gas
sensors, the exchange of charge carriers in the gas adsorption and desorption
is the cause of the electrical signal change. When the gas-sensitive layer
reacts with gas, the conductivity of the film varies. The rate of change in
conductivity is related to the concentration of the test gas. Subsequently, the
electrical signal is transmitted. The material microstructure needs to be
stabilized to maintain a stable sensing performance. Sensitive materials,
element doping, ambient humidity and component composition of gas sensors.
5.2.Sensitivity:
Sensitivity can be defined as the change in
resistance of the material or the current when the chemical or gas is exposed.
It can be mathematically defined as:
The sensor response for n-type
semiconductors is defined by
and Rgas> Rair (oxidizing gases)
Rgas< Rair (reducing gases)
for oxidizing and reducing gases, respectively.
The effective response is determined by
measuring response and recovery time. Response time is defined as the time
required to reach the 90% of the maximum value of current during the
gas/chemical sensing state. Recovery time is defined as time taken for the the
current to reach 10% of its maximum value when the environment switches to
ambient air conditions. This definition of sensitivity always leads to positive
and comparable values with S = 0 occurring in the case of reference air or in
the presence of an undetectable gas species
5.3.Selectivity
Selectivity
is the ability of gas sensors to identify a target gas among other different
gases. For air quality sensors, selectivity is an important parameter because
myriad of gaseous species that are present at the ppb or ppt levels in the
ambient air could induce changes in the resistance of the MOS in a similar
manner with the target gas. Evidently, for species that are typically in high
concentrations in the atmospheric environment (i.e., higher than a few
tens/hundreds ppm; e.g., CO2 and H2O) it is much easier
to develop selective gas sensors as not many other atmospheric compounds are so
abundant [64]. For target species that are in the ppb range, selectivity can be
improved by modulating the physical (e.g., metal oxide grain size, operating
temperature), or the chemical (e.g., doping, surface functionalization)
properties of the MOS, using similar strategies to those for enhancing sensor
sensitivity. Adding metallic nanoparticles on the surface of the MOSs can
provide another means of improving selectivity, among other properties of the
sensors. For example ion reduction can be used to form functionalizing
nanoparticles on the surface of metal oxides, similarly to the method reported
by Wali et al. [65-66].
5.4.Operating Temperature:
As the
temperature increases, enhanced thermal motion of the target gas molecules
leads to an increase of their diffusion to the bulk MOS. In addition,
chemisorption is preferred over physisorption at lower temperatures, forming
strong target gas-MOS surface chemical bonds that promote adsorption. This
enhanced interaction increases the resistance changes of the MOS and thus the
sensitivity of the sensor (cf. Fig. 2). As temperature is further increased,
thermal motion of the adsorbed species increases desorption rate, which in turn
decreases the sensitivity [67]. Given that these two competing processes have
opposite temperature dependences, their dominance determines the optimum
operating temperature of the sensors [68].
Fig. 8.
Dependence of the sensitivity of MOS gas sensors on the sensor operating
temperature.
An increase in temperature initially
increases the adsorption (chemisorption and physisorption) of gas species.
However, after a specifc threshold (which varies depending on the MOS and the
target gas molecule) the high thermal motion of the adsorbed species promotes
desorption, which in turn decreases sensor sensitivity.
6.Advanced static Gas sensing system:
Fig.9: (a) Circuit used for the gas
sensing mechanism (b) Photograph of Advanced gas sensing system which we going
to use.
In a static gas-sensing measurement system
, the sensor is placed in a gas chamber with an adjustable temperature and
humidity. During the gas-sensing test, a predetermined amount of the target gas
is injected into the test chamber through the gas inlet by a gas-injection
unit. When the sensor resistance becomes stable, the gas chamber is opened for
the sensors to recover in air. This procedure is repeated for different gas
concentrations and at different temperatures, and a curve indicating the
variation of the sensor resistance vs time in different atmospheres is
obtained.
6.1.Sensing Mechanism in MOS:
The gas
sensing mechanism of MOS is based on the oxygen adsorption model, which assumes
that the change in resistance is related to chemisorbed oxygen. In air, oxygen
molecules adsorb on the MOS surface and form negatively charged chemisorbed
oxygen (
, O−, O2−) by trapping
conduction band electrons.The type of chemisorbed oxygen is related to the
operating temperature and species of MOS material, which signifcantly
determines the sensing performance of the sensing mate‑ rial . However, the
temperature interval corresponding to the presence of chemisorbed oxygen ions
on metal oxides is not well known and varies from different metal oxides.
Fig.10.Gas sensing function and conduction
mechanism for MOS. where the oxygen and carbon monoxide gas can penetrate to
interact with each grain.
In
general, is usually chemisorbed when the temperature
is below 100 °C. Once the temperature is between 100 and 300°C,O−
is generally chemisorbed and disappears rapidly. And when the temperature
exceeds 300°C, the chemisorbed oxygen is mainly in the form of O2−.
In air ∶ O2(gas) → O2(ads)
T < 100 ◦C ∶ O2(ads)
+ e− → (ads)
100 ◦C < T < 300
◦C ∶ (ads)
+ e− → 2O−(ads)
T > 300 ◦C ∶ O−(ads)
+ e− → O2−(ads)
As a
result of oxygen adsorption, an electron depletion layer with a low electron
concentration is formed on the surface of the n-type MOS, which has a higher
resistance than the core region due to the reduced number of electrons. While
on the surface of the p-type MOS, a hole accumulation layer is formed, which
has a lower resistance than the core region of the MOS due to the increased
number of holes. As a result of these processes, the resistance of the sensing
layer will change signifcantly, which will result in the response of the gas
sensor.
The oxygen molecules interact with the metal
oxide surface as oxygen anions from the depletion layer around the grains and
thereby increases potential barrier. When target gas molecules interact with
the metal oxide grains, oxygen anions is desorbed, which makes the majority
carrier concentration change inside the oxide layer, which thereby changes
conductivity [69].
Fig.11.The general gas sensing mechanism of
(a,b) n-type and (c,d) p-type MOS gas sensors in the presence of air and an
oxidizing gas.
It is well accepted that the variation of
the resistance in the presence of the target gas is the basic mechanism of gas
detection by resistive gas sensors.A schematic of the sensing mechanism for n-
and p-type MOS sensors in the presence of an oxidizing gas is shown in figure
11. Because of the high electronegativity of oxygen and the extraction of
electrons by oxygen species on the surface of the gas sensor, an electron
depletion layer is formed on the surface of n-type gas sensors, and a hole
accumulation layer is formed on the surface of p-type gas sensors, as shown in
figure 11 (a),(c) respectively. When an n-type sensor is exposed to an
oxidizing gas, electrons are further extracted, leading to an increase in the
width of the electron depletion layer and an increase in the sensor resistance
(Fig.11 (b)). In a reducing gas atmosphere, the width of the electron depletion
layer decreases, resulting in a decrease in the sensor resistance. Upon
exposing p-type MOS gas sensors to an oxidizing gas, more electrons are
extracted, resulting in the expansion of the hole accumulation layer and a
decrease in the sensor resistance (Fig.11 (d)). For reducing gases, the inverse
is true.
7.Factors
Affecting the Sensitivity of MOS Gas Sensors
7.1.Effect
of Substrate:
The
silicon, quartz, glass, and aluminum are the substrate which are used for
deposition of the film,which directly affects sensitivity of sensor.Thus, the
use of silicon substrate to precipitate films on it gives special different
properties to oxides of semi-conductivity materials due to its influences on
surface electric charges. Then, its effects on gas interaction with the sensor
makes it required in the industry of gas sensors. The silicon etching process
is one of the used techniques in the improvement of sensor properties due to
the increase of surface area where it determines the regulation of pore size.
The glass substrate is known for its optical properties as it has an absorption
approach to zero and high transmittance within the visible area so it is used
as a substrate to study the optical properties, constructive, and the
sensitivity to the prepared film. Also, the proposed silicon acts to improve
the sensor. The researcher Rahman et al. studied the sensitivity of the
engrafted nano silver oxide prepared by the solution method. The substance was
deposited on glass carbon electrodes to give sensitivity with a quick response
to methanol in the liquid phase and it gave good and stable allergic results.
7.2.Morphology
The surface morphological studies of the
films were analyzed by AFM and SEM techniques.The fig.4dipicts the SEM and AFM
of Indium doped ZnO thin film prepared by spray pyrolysis technique. A top view
of the images clearly shows that the results are in good agreement with each
other for undoped and Indium doped films.
Fig. 12. SEM and AFM micrographs of (a)
undoped, (b) 5 wt%, (c) 10 wt%, and (d) 15 wt% IZO thin films
The Root
mean square values (RMS) of the surface roughness of the films were computed by
Nanoscope Analysis using AFM micrographs. Actually Root
Mean Square of a surfaces measured microscopic peaks and valleys. The values are in the range 8 to
28 nm indicating the smooth nature of the films. The RMS roughness values are
28.4 nm for undoped, 10.6 nm for 5 wt%, and 10 wt% and 8.18 nm for 15 wt% IZO
fifilms. The grains in undoped film exhibit pea-shaped structure and upon
increasing indium concentration, the films are taken hexagonal shape as evident
from SEM analysis. The 15 wt% IZO film has a large surface area and many grains
with well-defifined grain boundaries compared to other films. Consequently it
provide more active for gas adsorption which in turn boosts the response toward
the target gas [70]. The impact of nanograins on the gas-sensing properties of
ZnOnanorods is reported by Kim et al. [71]. The grain boundaries seeks the
obstacle in flowing the electron since they act as potential barriers. In the presence
of gas, the grain boundaries disappear and the resistance decreases which
results in the high resistance modulation along the grain boundaries so
geometric consideration involves in sensitivity.Grain size reduction leads to
the enhancement of surface area thereby enhancing the performance of the
sensing mechanism.In smaller grain size particle, nanoparticles are mostly
within the depletion area which increases the sensitivity. And active area of
reaction becomes more when the surface area is large. It is also significantly
enhances the sensor response. For an example SnO2 based gas sensor
response drastically changes when the particle size becomes smaller than 10 nm
[72].
The
Control Synthesis of Nano or microsized Particle matters the
sensitivity.Depending on the metal oxide semiconductor specific
crystallographic orientation gives better result in sensitivity.In an example
ZnO, (002) crystallographic orientation shows better sensitivity [73]. Surface
to volume ratio is also very important to achieve better sensitivity it depends
on grain size and porosity of structure.The ZnO is synthesized and samples are
characterized by scanning electron microscope (SEM) which is shown in figure
13(a) and (b) Modification of surface morphology is done by chemical treatment
and varying annealing temperature. Figure 4 confirms the formation of hexagonal
shape ZnO.Hexagonal structure with high surface to volume ratio results good
sensitivity.
Fig.13.(a)
SEM image of ZnO before chemical treatment (b) SEM image of Hexagonal ZnO nano
The ZnO is an MOS that has been extensively
studied as a gas sensor. However, several research groups are investigating the
potential of adding elements in the crystal lattice of ZnO as doping materials.
The research outcomes revealed that the doping elements change the ZnO
structure. For example, doping elements decrease the crystallite size, increase
the crystallinity, and modify the ZnO morphology.
7.3.Grain Size and Porosity
In gas
sensors, semiconductor nanoparticles are connected to adjacent particles
through grain boundaries to form larger aggregates [74].Since the transport of
electrons between grains needs to pass through the electron depletion layer,
the grain size has a great influence on the conductivity, which in turn affects
the gas sensing performance of the material-based gas sensor.
Figure 14. Schematic model of the effect of
the crystallite size on the sensitivity of metal-oxide gas sensors: (a) D
>> 2L, (b) D≥ 2L, and (c) D < 2L. Reprinted with permission from Ref.
[Z]. Copyright 1991 Elsevier.
Let D = Particle size
L= Thickness of the electron depletion layer
(a)
If D >> 2L
There is
a wide electron channel between the grains, and the gas sensitivity of the
material is mainly controlled by the surface of the nanoparticles (boundary
control).The change in electrical conductivity depends not only on the particle
boundary barrier but also on the cross-sectional area of the channel, and the
gas sensitivity of the material is mainly controlled by the contact neck
between nanoparticles (neck control)
(b).If D < 2L
The
electron depletion layer dominates, the entire nanoparticle is contained in the
electron depletion layer (grain control), and the sensitivity of the material
is very high. The energy bands are nearly flat throughout the interconnected
grain structure, and there is no
significant impediment to the inter-grain charge transport.The small amount of
charge gained from the surface reaction results in a large change in the
conductivity of the entire structure. Therefore, the smaller grain size is
beneficial to improving the sensitivity of the gas sensor. When the grain size
is small enough, the crystal becomes very sensitive to the surrounding gas
molecules.Smaller is grain size and larger porosity of structure quicker is gas
response of the sensor [75].
7.4.
Chemical Composition
Sensors
based on the two components mixed together are more sensitive than the
individual components alone suggesting a synergistic effect between the two
components. The choice of the synthesis method and the composition of mixture
of precursors used in the synthesis strongly affect the surface of metal oxide
and hence its sensing characteristics.
7.5.Temperature and
Humidity
Temperature
is also a major factor for the metal oxide gas sensors. The gas sensor
responses increase and reach their maximums at a certain temperature, and then
decreased rapidly with increasing the temperature. This is a common tendency
seen in many reports [76-79].In ZnO based sensor the maximum response is found
at 200-3000C. Sensitivity decreases below or above the specific
temperature.
Humidity
is an important issue in gas sensor performance. Humidity sometimes lowers the
sensitivity. Baseline resistance of the gas sensor decreases when the reaction
between the surface oxygen and the water molecules occurs, [80]. So, the
sensitivity is decreased. Adsorption of water molecules causes less
chemisorption of oxygen species on the ZnO surface as surface area gets
decreased. And it effects on sensitivity. Water molecules are basically the
barriers against target gas adsorption. So response recovery times increases
and sensitivity decreases.
7.8.
Doping of Metals
Conductivity
is measured by the efficiency of catalytic reactions with the target gas on
surface of the sensing material. Doping is an important method to improve
gas-sensing performance.Different dopant species may lead to different types of
crystallite, defects and electronic properties. Conductivity is measured by the
efficiency of catalytic reactions with the target gas on surface of the sensing
material. So, the catalytic activity is also a major issue to increase the
sensor performance. Doping is the process of addition of different metals in to
pure materials.Doping or surface modification by adding metal elements (such as
Ag,Au, Pt, Pd, etc.) on the surface of the gas-sensing materials can increase
the number of active sites, promote the adsorption/desorption reaction on the
surface of the gas-sensing materials, and reduce the reaction activation
energy, and reduce the operating temperature, thereby improving the gas-sensing
performance [81]. Nobel metal nanoparticle behaves like a catalyst and reduces
the activation energy resulting the improved molecular dissociation and
reaction [82].
The
addition of suitable dopants with appropriate host materials [83]; grain size
in the metal oxide thin films can be controlled.Reducing crystallite size might
have influenced on the width of space charge region which, aids the
chemisorption of gas molecules and sensing process [84]. The mechanism of
sensitization or sensitivity (response) enhancement by additives are found to
be different in different metal oxide materials.
According
to Yamazoe there are to two types of sensitization mechanisms viz chemical and
electronic.If we consider the example of Pt doped SnO2 the chemical
sensitization; the pt residing on the metal oxide surface facilitates the
chemical reactions between target gas and metal oxide surface through the
spill- over mechanism as shown in figure 15 .On the other hand in an example of
Ag doped SnO2 the electrical
property of the metal oxide is changed through the change in redox state of the
promoter, by acceptor or donor charges from gas molecules in electronic
sensitization.
|
TYPE |
Chemical |
Electronic |
|
Model |
|
|
|
Role of noble Metal |
Activation and spill-over of
sample gas |
Electron donor or acceptor |
|
Origin of gas sensitive
property |
Change of absorbed oxygen
concentration |
Change of oxidation state of
noble metal |
|
Example |
Pt-SnO2 |
Ag2-O-SnO2,PdO-SnO2 |
Fig.15. spill over mechanism
Dopants
in the metal oxides lowers the energy which is required for chemisorption of
gas molecules in the semiconductor surface which in tuners enhances the gas
sensitivity therefore adding a suitable dopant can stabilize a particular
valence state and increase the electron exchange rate or stabilize the metal
oxide against reduction” [85].The choice of the synthesis method and the
composition of mixture of precursors used in the synthesis strongly affect the
surface of metal oxide and hence its sensing characteristics [86].
8.Effect on ZnO Sensitivity on doping :
The Ga is another promising metallic
element that can be used as a dopant in ZnO-based sensors. Hjiri et al.
reported that, incorporation of Ga into the crystal lattice of ZnO notably
improves the sensitivity, because it decreases the crystallite size.The
increase in sensitivity is because of the decrease in the average crystallite
size (approximately 49 nm) of the Ga-doped ZnO nanoparticles compared with that
of the undoped ZnO nanoparticles.The advancement compared with the undoped ZnO
nanoparticles, because the reduction in crystallite size helps increase the
surface area[87].
In
another work, Hjiri et al. analyzed a sensor with similar characteristics. They
doped ZnO with different concentrations of Ga (1, 3, and 5 at. %). Figure 5
shows the sensitive values of the Ga-doped ZnO nanoparticles. The maximum
sensitivity of the Ga-doped ZnO sensor can be observed at 250◦C (1
at. %). Thereafter, the sensitivity decreased when the operating temperature
increased. A change in the morphology of the particles with increased Ga
concentrations was observed.The study also concluded that the solubility limit
was 3 at. % Ga in ZnO. When this doping limit is exceeded, the Ga atoms distort
the lattice until they cause phase segregation.
Fig.16. Response to 50 ppm CO of the Ga-doped ZnO nanoparticles as a
function of the temperature.
TheGa
concentration of samples correspond at 0, 1, 3 and 5 at. %. Obtained from
reference[88]. Dhahri
et al [89] reported that In was used as dopant, In-doped
ZnO sensor under a CO gas environment in comparison with the sensitivity of the
undoped ZnO sensor. This behavior has been attributed to the most active
adsorption sites, assuming the substitution of the Zn2+ cation by In3+.The
authors concluded that the incorporation of In in the crystal lattice changes
the oxygen stoichiometric of ZnO and influences the sensing behavior of doped
ZnO.
9. Gas-Sensing Devices:
According
to different sensing materials and different methods gas sensors can be
classified as electrochemical, catalytic combustion, infrared absorption,
thermal conductive, solid electrolyte, paramagnetic, and metal oxide
semiconductor sensors [90], depending on
the change in electrical resistance with different materials and their
compositions. Semiconducting metal oxide, polymers, carbon nanotubes (CNTs),
graphene, etc., are mostly used (Figure 17).
Fig.17.Semiconductor gas-sensing materials,
“Reprinted/adapted Ref.[35] Copyright 2020, Nikolic, M.V. et al.
Gas sensors can also be classified based on
optic, acoustic, calorimetric, and chromatography methods.According to Comini
[91], a gas sensor can be classified based on the measurement methods, such as
(1) DC conductometric gas sensors, (2)
photoluminescence (PL)-based gas sensors, and (3) field effect
transistor (FET)-based gas sensors. Table 1 illustrates the comparison of
various gas sensors studied by Korotcenkov [92]. Target gas detection by a
metal oxide semiconductor has gained wide attention because of its various
advantages over other types of sensors. Electrochemical gas sensors have become
unpopular due to their short lifespan, limiting their use to only a few
applications. Although an optical gas sensor has good sensitivity, an adequate
lifespan, good selectivity, and fast response, the cost is high and the size is
large. Although metal oxide semiconductor gas sensors are less selective, the
low cost and simplicity in fabricating them are the main factors that
contribute to their wide usage.
Table 2.Comparison of various types of gas
sensors. Reprinted from Ref. [37]. Copyright 2007, Korotcenkov, G.
|
Type of Gas Sensors |
|||||
|
Parameter |
SMO Gas Sensors |
Catalytic Combustion Gas Sensors |
Electrochemical Gas Sensors |
Thermal Conductivity Gas Sensors |
Infrared Absorption Gas Sensors |
|
Sensitivity |
E |
G |
G |
P |
E |
|
Accuracy |
G |
G |
G |
G |
E |
|
Selectivity |
F |
P |
G |
P |
E |
|
Response Time |
E |
G |
F |
G |
F |
|
Stability |
G |
G |
P |
G |
G |
|
Durability |
G |
G |
F |
G |
E |
|
Maintenance |
E |
E |
G |
G |
F |
|
Cost |
E |
E |
G |
G |
F |
|
Suitability to portable Instruments |
E |
G |
F |
G |
P |
E:Exultant,G:Good,P:Poor.
Conclusion:
The
review is attempt to study the chosen metal oxide gas sensor(MOS) such as
ZnOfor the detection of various gases. The paper survey revealed the different
factors for efficiency enhancement of the gas sensor. Subsequently, the sensing
mechanism of metal oxide gas sensor has been described to understand the
requirement of gas sensor to detect toxic gases. Some other factors which
affect the sensitivity of metal oxide gas sensor have also been described.
These are grain size, doping with a suitable dopant, effect of surface defects,
gas diffusion, effects of heterojunction and thin film-based gas sensor. For
materials, the main factors that affect the stability are the uncontrolled
grain growth during the service period and the irreversible reactions between
the surface and some molecules in the environment or target gases.In brief,
sensing properties mainly rely on the surface reaction.It is investigated that
hexagonal structural morphology can be obtained by doing chemical treatment and
varying annealing temperature. Doping of nobel metal additives with high
catalytic activity may enhance the sensitivity because of “spillover
effect”.Crystal growth at specific orientation has major effect on sensitivity.
On other hand temperature and humidity are also playing an important role in
sensitivity. Humidity drastically lowers the sensitivity.
The brief
introduction is suitable for the topic chosen.The review provided an outline of MO ZnO gas
sensors. The important parameters of the Metal Oxide which make it ideally
suitable for application for gas sensors.Spray pyrolysis technique for deposition of ZnO thin films described in
detail. In this review, the main characteristics of gas sensor are firstly
introduced, followed by the preparation methods and properties of ZnO. In
addition, the development process and the state of graphene gas sensors are
introduced emphatically in terms of structure and performance of the sensor.
The doping of material enhances the performance of gas detection significantly.
Finally, the clear direction of MO gas sensors for the future is provided
according to the latest research results and trends. A brief study on the
factors that affect the sensitivity, selectivity and stability of the MO gas
sensor was carried out. The doping were more commonly employed ways to improve
the sensor performances mentioned significantly.From the study, it is evident
that a fair amount of the work was done on the Metal oxide thin film gas
sensors in particularly ZnO gas sensor for application. On the whole all sensor
types and sensing materials have its limitations such as the sensitivity to all
types of target gases and longer response and recovery time. Hence
optimizations in terms of fabrication process, sensing material, size, and
shape, performance parameters like sensitivity, selectivity, and stability etc.
is the key to design a better sensor that will fulfill all our requirements.
This review aims to provide recent progress and technology trend on the MO gas
sensors for current and future researchers. There is a good scope for the
future researcher to perform further work in MO to produce a novel sensor that
meets the demands of current and emerging technical applications.It provides
direction and ideas for future research.
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