**1. Introduction**

Increased environmental pollution, numerous motor vehicles, factory wastes and urbanization factors have been the source of high increases in the release of toxic, explosive and flammable gases in the environment of developed countries. High rate of gas emissions has both a negative impact on human/animal health and it can also have bad consequences on the environment and natural resources from day by day.

With the start of the Industrial Revolution, the acceleration of coal and mine quarries caused a significant increase in deaths due to toxic gas. First, canaries were used in gas detectors in mines. The cost and difficulty of using different methods for determination of toxic gases have revealed the gas sensors. In 1815, British scientist H. Davy developed a gas meter called 'Davy's lamp' against methane gas [1]. In 1926, Johnson produced the first commercial catalytic, combustion gas sensor, and in 1929, the company they founded with Williams became the first company in Silicon Valley in electronics [2].

Gas sensors are used to detect combustible, explosive and toxic gases, when the measured gas concentration exceeds the threshold value they can give an alarm (sound, signal, etc.) that can be used as portable or fixed devices. The most important part of this device production is the sensor which determines 4S parameters (sensitivity, selectivity, stability, speed). Apart from them, recovery time, response time and power consumption are also other parameters. The sensor part records changes in the physical conditions or chemical components as signals (permeability, resistance, temperature, acoustic wave, capacitance, etc.) as a result of interaction between target gas and surface atoms (O<sup>−</sup>, O2−, H+ and OH<sup>−</sup>) by absorption/desorption of gas on the material surface at a specific operating temperature. Signal can correlate concentration of target gas [3].

The recent change in the OSHA Time Weighted Average (TWA) Permissible Exposure Limit (PEL) is 25, 35 and 1 ppm for NH3, CO and NO2 gases, respectively [4].

CO is a toxic colorless gas, environmental pollutant and kills by causing hypoxia with damaged hemoglobin cells in the blood. In general, the measurement of CO gas is realized by detection of percentage of carboxyhemoglobin in the blood. Another important issue is creation of residential and automotive environment so it is so necessary fast and sensitive detection. Difficulty in detecting very low levels and continuous CO formation in the air poses problems [5].

Odorless and toxic ammonia (NH3) combustion, which is used in a large area as a fertilizer, refrigerant material and household cleaning product, is a major hazard. Using or producing ammonia besides any uncontrolled leaks by the infrastructures or its explosion causes health hazards. In addition, it is a chemical pollutant in the production of silicon type devices in clean room [6].

Nitrogen dioxide (NO2) is a volatile and toxic gas. It has hazardous effects in environment as a secondary pollutant and its detection is so important. NO2 gas generates fuel burning at high temperature and in nitrogen cycle, including acid rains. Under even very low concentrations (<10 ppm) it causes serious damages for human health such as throat discomfort, transient coughs, eye irritation, fatigue and nausea [7].

With nano-sized designed gas sensors, surface to volume ratio is increased for absorbed target gas as well as higher efficiency is obtained than traditional bulkscale designed devices, because different atomic coordination and translational symmetry at the surface ensure electrical properties changing in semiconductors [8]. In particular, a dramatic increase using the nano-sized designed gas sensors have been observed in industrial areas such as pharmaceuticals, medical, automotive, building automation, space tools, wearable devices. The first study of the semiconductor material group was given by Brattain and Bardeen on germanium (Ge) in 1953 [9]. In the next study, in 1954, Heiland had a research report on the gas sensitivities of metal oxides, and also in 1962 Seiyama showed that ZnO structures were sensitive to reactive gases in the air [10]. In 1968, Taguchi-type sensors were introduced to market and metal oxide (SnO2) gas sensors were moved to industrial level [11].

Nano-scale designed gas sensors are usually classified depending on measurement data as follows; (i) chemiresistors, (ii) thermal conductivity gas sensors, (iii) acoustic wave gas sensors, (iv) calorimetric gas sensors, (v) optical gas sensors (vi) electrochemical gas sensors and (vii) infrared absorption gas sensors [13, 14].

Chemiresistive gas sensor working principle can be explained simply as adsorption of electron with target gas on the surface can cause charge transfer (a change in charge carrier concentration) between target gas/material surface region (receptor function) so electrical properties can be (resistance or conductivity) increase or decrease. Easy measurement with two electrodes is a factor in their preference and supplying safety.

**3**

**Figure 1.**

*permission).*

*Metal Oxide Gas Sensors by Nanostructures DOI: http://dx.doi.org/10.5772/intechopen.88858*

produces Taguchi type sensors [15].

the surface of the MO are anions.

(especially carbon nanotubes) were studied.

Today, using chemiresistive metal oxide (MO) semiconductors, real-time gas sensor has gained great importance both in the science/industrial world due to their high sensitivity to chemical environments, low price, simple implantation, safety and durable to high temperature/high pressure, indicating that compelling conditions. Companies such as FIS, Mics, UST, CityTech, Appliedensors and Newcosmos produce millions of MO gas sensor per year, especially the Figaro company which

Gas selectivity is a critical problem for metal oxide gas sensors. To increase the selectivity of metal oxide sensors, it is proposed to use a heating mode of a gas-

Metal oxide semiconductor gas sensors are focused on different and new materials at room temperature with the increasing need for faster, more precise and easy gas sensing, as showed in **Figure 1**. Thus, the most important parameter mechanism is gas sensitivity, which still does not reveal the exact reasons (strongly related to surface reactions), can be detailed. Production techniques (spray pyrolysis, pulsed laser deposition, magnetron sputtering, spin coating, and chemical bath deposition) are undeniable facts because structure parameters, grain boundaries, point defects, surface morphology, porosity, etc. must be affected. Additionally, reducing (H2, H2S, etc.)/oxidizing (NH3, NO2, etc.) gas types and p- or n-type is also effective on the chemiresistive MO performance, as showed in **Figure 2**. Oxidizing or reducing gas is associated with electron affinity, which is compared to the work function of most metal oxide so in the case of oxidizing gas, the adsorbed gas molecules on

The change in electrical resistance of semiconductors can be explained as follows; formation of the space-charge depletion zone on the surface and around the particle and the energy band bending. Surface energy barriers with variable heights and widths depend on the relationship between charging the surface states of the adsorbed species for conduction electrons. In gas sensors using n-type semiconductor oxide, it has been observed that the resistance of the oxide increases with the interaction of gases such as O3 or NO2, while the resistance decrease of the oxide occurs with interaction of gases such as CH4 and CO, as showed in **Figure 2**. It is discussed that resistive-type metal oxide semiconductors produced by nanostructures (especially thin films) in detail toward NH3, NO2 and CO gases. Additionally, effect of doping and nanocomposite forming with C-based material

*Advantages and disadvantages of semiconductor metal oxides (reprinted from study of [12] with their* 

sensing floor with rapid temperature modulation in the last studies.

#### *Metal Oxide Gas Sensors by Nanostructures DOI: http://dx.doi.org/10.5772/intechopen.88858*

*Gas Sensors*

and nausea [7].

level [11].

supplying safety.

Gas sensors are used to detect combustible, explosive and toxic gases, when the measured gas concentration exceeds the threshold value they can give an alarm (sound, signal, etc.) that can be used as portable or fixed devices. The most important part of this device production is the sensor which determines 4S parameters (sensitivity, selectivity, stability, speed). Apart from them, recovery time, response time and power consumption are also other parameters. The sensor part records changes in the physical conditions or chemical components as signals (permeability, resistance, temperature, acoustic wave, capacitance, etc.) as a result of interaction

tion of gas on the material surface at a specific operating temperature. Signal can

The recent change in the OSHA Time Weighted Average (TWA) Permissible Exposure Limit (PEL) is 25, 35 and 1 ppm for NH3, CO and NO2 gases, respectively [4]. CO is a toxic colorless gas, environmental pollutant and kills by causing hypoxia with damaged hemoglobin cells in the blood. In general, the measurement of CO gas is realized by detection of percentage of carboxyhemoglobin in the blood. Another important issue is creation of residential and automotive environment so it is so necessary fast and sensitive detection. Difficulty in detecting very low levels

Odorless and toxic ammonia (NH3) combustion, which is used in a large area as a fertilizer, refrigerant material and household cleaning product, is a major hazard. Using or producing ammonia besides any uncontrolled leaks by the infrastructures or its explosion causes health hazards. In addition, it is a chemical pollutant in the

Nitrogen dioxide (NO2) is a volatile and toxic gas. It has hazardous effects in environment as a secondary pollutant and its detection is so important. NO2 gas generates fuel burning at high temperature and in nitrogen cycle, including acid rains. Under even very low concentrations (<10 ppm) it causes serious damages for human health such as throat discomfort, transient coughs, eye irritation, fatigue

With nano-sized designed gas sensors, surface to volume ratio is increased for absorbed target gas as well as higher efficiency is obtained than traditional bulkscale designed devices, because different atomic coordination and translational symmetry at the surface ensure electrical properties changing in semiconductors [8]. In particular, a dramatic increase using the nano-sized designed gas sensors have been observed in industrial areas such as pharmaceuticals, medical, automotive, building automation, space tools, wearable devices. The first study of the semiconductor material group was given by Brattain and Bardeen on germanium (Ge) in 1953 [9]. In the next study, in 1954, Heiland had a research report on the gas sensitivities of metal oxides, and also in 1962 Seiyama showed that ZnO structures were sensitive to reactive gases in the air [10]. In 1968, Taguchi-type sensors were introduced to market and metal oxide (SnO2) gas sensors were moved to industrial

Nano-scale designed gas sensors are usually classified depending on measurement data as follows; (i) chemiresistors, (ii) thermal conductivity gas sensors, (iii) acoustic wave gas sensors, (iv) calorimetric gas sensors, (v) optical gas sensors (vi) electro-

Chemiresistive gas sensor working principle can be explained simply as adsorption of electron with target gas on the surface can cause charge transfer (a change in charge carrier concentration) between target gas/material surface region (receptor function) so electrical properties can be (resistance or conductivity) increase or decrease. Easy measurement with two electrodes is a factor in their preference and

chemical gas sensors and (vii) infrared absorption gas sensors [13, 14].

and OH<sup>−</sup>) by absorption/desorp-

between target gas and surface atoms (O<sup>−</sup>, O2−, H+

and continuous CO formation in the air poses problems [5].

production of silicon type devices in clean room [6].

correlate concentration of target gas [3].

**2**

Today, using chemiresistive metal oxide (MO) semiconductors, real-time gas sensor has gained great importance both in the science/industrial world due to their high sensitivity to chemical environments, low price, simple implantation, safety and durable to high temperature/high pressure, indicating that compelling conditions. Companies such as FIS, Mics, UST, CityTech, Appliedensors and Newcosmos produce millions of MO gas sensor per year, especially the Figaro company which produces Taguchi type sensors [15].

Gas selectivity is a critical problem for metal oxide gas sensors. To increase the selectivity of metal oxide sensors, it is proposed to use a heating mode of a gassensing floor with rapid temperature modulation in the last studies.

Metal oxide semiconductor gas sensors are focused on different and new materials at room temperature with the increasing need for faster, more precise and easy gas sensing, as showed in **Figure 1**. Thus, the most important parameter mechanism is gas sensitivity, which still does not reveal the exact reasons (strongly related to surface reactions), can be detailed. Production techniques (spray pyrolysis, pulsed laser deposition, magnetron sputtering, spin coating, and chemical bath deposition) are undeniable facts because structure parameters, grain boundaries, point defects, surface morphology, porosity, etc. must be affected. Additionally, reducing (H2, H2S, etc.)/oxidizing (NH3, NO2, etc.) gas types and p- or n-type is also effective on the chemiresistive MO performance, as showed in **Figure 2**. Oxidizing or reducing gas is associated with electron affinity, which is compared to the work function of most metal oxide so in the case of oxidizing gas, the adsorbed gas molecules on the surface of the MO are anions.

The change in electrical resistance of semiconductors can be explained as follows; formation of the space-charge depletion zone on the surface and around the particle and the energy band bending. Surface energy barriers with variable heights and widths depend on the relationship between charging the surface states of the adsorbed species for conduction electrons. In gas sensors using n-type semiconductor oxide, it has been observed that the resistance of the oxide increases with the interaction of gases such as O3 or NO2, while the resistance decrease of the oxide occurs with interaction of gases such as CH4 and CO, as showed in **Figure 2**.

It is discussed that resistive-type metal oxide semiconductors produced by nanostructures (especially thin films) in detail toward NH3, NO2 and CO gases. Additionally, effect of doping and nanocomposite forming with C-based material (especially carbon nanotubes) were studied.

#### **Figure 1.**

*Advantages and disadvantages of semiconductor metal oxides (reprinted from study of [12] with their permission).*


**Figure 2.**

*Sensitivity measurement of material type and target gas type (reprinted from study of [16] et al. with their permission).*

### **2. Metal oxide (MO) gas sensors**

Since 1962, the addition of the oxygen contained in the metal oxides to the reaction so increase of reactions and their stable chemical transduction properties which can reversibly convert chemical reactions on a surface make the metal oxides attractive for detect various harmful, toxic, and explosive gases. Development of gas sensors, which are almost 21% of the metal oxides used in the field, is rapidly increasing [17]. Because they have unique properties such as low cost, long lifetime, fast response time and relatively high sensitivity. However, some restrictions are detected in these structures such as background gas effect, poor selectivity and power consumption in high temperature conditions which could not be proper for especially wireless applications.

Basically, the main challenge is they operate only at elevated temperatures and consume more power with high operating temperatures. Physisorption and chemisorption are surface adsorption forms of oxygen. Physisorption to chemisorption needs activation energy with realized by increasing operating temperature. In addition, forming of oxygen species depends on the operating temperature substantially. Sun et al. reported that molecular species are more than atomic species below 150°C, this cause a decrease in gas sensitivity [18].

Another goal of gas sensitivity works is to ensure that electrical change in the gas environment occurs not only at grain boundaries but on the entire material surface. Since grain boundaries are smaller than MO particles, surface chemistry is more effective and the effect of grain boundaries on electrical change is not considered.

To achieve high performance from MO gas sensors, detailed knowledge of the gas sensing mechanism is essential. In general, it can be explained as follows; oxygen adsorption on the surface of sensing material, adsorbed oxygen species (extrinsic surface acceptor states) molecular (O2 − ) or atomic (O<sup>−</sup>, O2<sup>−</sup>), captured from the interior of the sensing material, resulting in a depletion layer on the surface due to oxygen species. Eventually observing a decrease in the conductivity/resistance [19]. In other words, oxygen ions on the surface of metal oxides are highly active interactions with the target gas molecule. When O2 molecules adsorb from the surface of the MO, they break off electrons from the conductivity band (Ei) and trap electrons form on the surface, which come across in ion form. This causes band bending and electron depletion layer (space charge layer) formation. When the electron concentration in the conductivity band decreases, the conductivity decreases as well. At the same time, negatively charged traps in these different types of adsorbed oxygen cause downward bending of the band curve, which, compared to the flat state of the band, decreases conductivity. The thickness of the electron depletion layer is the width of the band bending region. The displacement of adsorbed oxygen with other molecules and the reaction of different oxygen ions with reduced gas changes conductivity.

**5**

*Metal Oxide Gas Sensors by Nanostructures DOI: http://dx.doi.org/10.5772/intechopen.88858*

surface forms so it can measure easily.

electronic structure [20];

species occurrence.

**3. Thin film metal oxide gas sensors**

materials when they are produced in multilayer.

potential barriers with high intensity defect levels.

H2S, ethanol, acetone, LPG and moisture.

for gas concentrations up to 30 ppb [25].

a.d0

Among metal oxide gas sensors single (ZnO, NiO, TiO2, SnO2, WO3, etc.), binary and ternary samples have unique properties such as chemical stability, relatively low harmful for environment, abundant in nature and low cost. Wang et al. showed that metal oxides selected for real gas sensors can be separated according to their

 transition metal oxides: In this group (WO3, V2O5, TiO2 and etc.), d0 electronic configurations are preferred with their wide band gap energy and

b.pre-transition metal oxides: In this group (Al2O3, MgO and etc) are not preferred due to neither electrons nor holes forming so occurs very band gap energy, structural instability and difficulty of measure electrical conductivity.

c.post-transition metal oxides: They have d10 electronic configuration. ZnO, SnO2 Ga2O3 and In2O3 are preferred in MO gas sensor applications. Because they are so proper for electron accumulation and chemisorption of donor-like

In semiconductor gas sensor applications, advantages of thin film using are low resource waste, high surface/volume ratio, low power consumption, easy compliance with integrated circuits and easy alteration of electrical properties with changing film production parameters. Thin film technology allows the film properties to be changed by keeping the thickness parameter under considerable control. In this way, thin films are easily integrated into the device during the material production process. They can also be used as electronic circuit elements by acting as new

Thin film metal oxides are used by the detection a lot of gas types such as Carbon-based (CO, CO2, CH4, C2H5OH, C3H8), nitrogen-based (NH3, NO, NO2), H2,

The large number of grain boundaries in thin film polycrystalline MO's limits mobility, thus reducing carrier concentration and decreasing gas sensitivity. The presence of depletion layers in these grain boundaries is the most important factor that reduces mobility. Grain boundaries affect mobility due to their positioning to

There have been a lot of ZnO thin film study to detect NO2 gas sensing that have been reported with different morphologies nanowires, nanorods [21], nanoprisms [22] and nanospheres [23] in order to enhance surface area. In 2019, Duoc et al. synthesized ZnO nanowires and nanorods with using on-chip grown via hydrothermal method at room temperature NO2 gas sensing [24]. The diameter of these structures severely affected gas sensing, indicating nanowires were more sensitive than nanorods. ZnO nanobarded fibers were synthesized by electrospinning and chemical bath deposition. These structures showed improved NO2 detection performance

In our previous study, nanoflower shaped n-type ZnO films synthesized by chemical bath deposition and their 0.5 ppm NO2 gas sensing was detected, showing in **Figures 3** and **4** [26]. Operating temperature was chosen at 200°C due to statical recovery kinetics were worse under this temperature. Oxygen vacancies (oxygendeficient ZnO) acted as adsorption sites, electron donor sites and nucleation centers for small metal clusters. Reaction on the ZnO film surface was given by two

#### *Metal Oxide Gas Sensors by Nanostructures DOI: http://dx.doi.org/10.5772/intechopen.88858*

*Gas Sensors*

**Figure 2.**

*permission).*

**2. Metal oxide (MO) gas sensors**

especially wireless applications.

considered.

150°C, this cause a decrease in gas sensitivity [18].

sic surface acceptor states) molecular (O2

Since 1962, the addition of the oxygen contained in the metal oxides to the reaction so increase of reactions and their stable chemical transduction properties which can reversibly convert chemical reactions on a surface make the metal oxides attractive for detect various harmful, toxic, and explosive gases. Development of gas sensors, which are almost 21% of the metal oxides used in the field, is rapidly increasing [17]. Because they have unique properties such as low cost, long lifetime, fast response time and relatively high sensitivity. However, some restrictions are detected in these structures such as background gas effect, poor selectivity and power consumption in high temperature conditions which could not be proper for

*Sensitivity measurement of material type and target gas type (reprinted from study of [16] et al. with their* 

Basically, the main challenge is they operate only at elevated temperatures and consume more power with high operating temperatures. Physisorption and chemisorption are surface adsorption forms of oxygen. Physisorption to chemisorption needs activation energy with realized by increasing operating temperature. In addition, forming of oxygen species depends on the operating temperature substantially. Sun et al. reported that molecular species are more than atomic species below

Another goal of gas sensitivity works is to ensure that electrical change in the gas environment occurs not only at grain boundaries but on the entire material surface. Since grain boundaries are smaller than MO particles, surface chemistry is more effective and the effect of grain boundaries on electrical change is not

To achieve high performance from MO gas sensors, detailed knowledge of the gas sensing mechanism is essential. In general, it can be explained as follows; oxygen adsorption on the surface of sensing material, adsorbed oxygen species (extrin-

) or atomic (O<sup>−</sup>, O2<sup>−</sup>), captured from the

−

interior of the sensing material, resulting in a depletion layer on the surface due to oxygen species. Eventually observing a decrease in the conductivity/resistance [19]. In other words, oxygen ions on the surface of metal oxides are highly active interactions with the target gas molecule. When O2 molecules adsorb from the surface of the MO, they break off electrons from the conductivity band (Ei) and trap electrons form on the surface, which come across in ion form. This causes band bending and electron depletion layer (space charge layer) formation. When the electron concentration in the conductivity band decreases, the conductivity decreases as well. At the same time, negatively charged traps in these different types of adsorbed oxygen cause downward bending of the band curve, which, compared to the flat state of the band, decreases conductivity. The thickness of the electron depletion layer is the width of the band bending region. The displacement of adsorbed oxygen with other molecules and the reaction of different oxygen ions with reduced gas changes

**4**

conductivity.

Among metal oxide gas sensors single (ZnO, NiO, TiO2, SnO2, WO3, etc.), binary and ternary samples have unique properties such as chemical stability, relatively low harmful for environment, abundant in nature and low cost. Wang et al. showed that metal oxides selected for real gas sensors can be separated according to their electronic structure [20];

