2.2 Crystal and surface structure of ZnO

At normal temperature and pressure, ZnO crystallizes in wurtzite (B4 type) structure, as shown in Figure 1. It is a hexagonal lattice, belonging to the space group P63mc with lattice parameters a = 0.3296 nm and c = 0.52065 nm. The tetrahedral coordination in ZnO is responsible for noncentral symmetric structure and consequently results in piezoelectricity and pyroelectricity. Another important characteristic of ZnO is polar surfaces. The most common polar surface is the basal plane. The oppositely charged ions produced positively charged Zn+ (0001) and negatively charged O (0001) surfaces, which result in a normal dipole moment and spontaneous polarization along the c-axis as well as variance in surface energy. The two most commonly observed facets for ZnO are (2110) and (0110) which are nonpolar surfaces and have lower energy than the (0001) facets [14, 15]. ZnO has varied properties, covering all of its physical, chemical, or material properties.

#### Figure 1.

(a) Crystal structure of hexagonal wurtzite ZnO, ZnO unit cell, including the tetrahedral coordination between Zn and its neighboring O. (b) ZnO has a noncentro-symmetric crystal structure that is made up of alternate layers of positive and negative ions, leading to spontaneous polarization.

ZnO is a well-suited II–VI wide bandgap semiconductor, which is naturally found in three forms: cubic zinc blend, hexagonal wurtzite, and cubic rock salt which is not as common as other [16]. The most common phase of ZnO is hexagonal wurtzite, whose space group is C6v or P63mc, which can be found mainly in ambient condition [17].

In Figure 1(a) is a crystal structure of ZnO which is a combination of alternating planes with tetrahedral coordination of Zn+2 and O<sup>2</sup> ions along the c-axis. Due to the presence of polar surfaces, ZnO crystal becomes spontaneously polarized in two type of planes, i.e., tetrahedrally coordinated O<sup>2</sup> and Zn+2 ions stacked alternately along the c-axis.

#### 2.3 Gas sensing properties

In recent times due to environmental pollution and other chemical hazards, the needs for the development of a trusted chemical sensor have been significantly increased. For sensing of trace vapor of chemicals, different types of sensors, for example, potentiometric, fiber optics, amperometric, and biological sensors, are used, but ZnO nanostructure-based sensor has its own importance owing to its stability, high sensitivity, selectivity, as well as wide operating temperature range and flexibility in processing during device fabrication [18–24].

response (as receptor) plus a device which reads the response and converts it into

Schematic illustration of toxic chemical sensing process. (a) Adsorption of oxygen at surface of nanowires in air and creation of potential barrier and depletion region. (b) Modulation of potential barrier and depletion region

It is necessary to understand the sensing mechanism of the chemiresistive gas sensors for the subsequent chapters in this thesis. Since sensing mechanism of metal oxide semiconductor is mainly based on band theory, band theory can be applied to the gas sensor to explain the sensing mechanism. On interaction of the analytes (undetected) with the surface of nanostructures, these analytes react with attached oxygen ions on the surface of nanostructures; a change in the carrier concentrations of the material occurs. Due to the change in carrier concentrations of the material, the electrical resistivity of the materials changes. Decrease in resistivity (increase in conductivity) occurs for n-type metal oxide semiconductor on interaction of reducing gas [25]. So the sensing mechanism of oxide semiconductor is mainly based on the principle of modification in electrical properties (resistivity/conductivity) as a consequence of chemical reaction between gas molecules and the reactive oxygen ions on the surface of MOS nanostructure material. The sensing mechanism can be divided into three sections: (a) adsorption of oxygen at surface, (b) detection of gases by a reaction with adsorbed oxygen, and (c) change in

Interactions of oxygen with the surface of a metal oxide semiconductor are of utmost importance in gas sensing mechanism. Oxygen is a strong electron acceptor on the surface of a metal oxide. Since the majority of sensors operate in an air at ambient temperature, therefore the concentration of oxygen on the surface is directly related to the sensor electrical properties. The conversion to O2

prominent temperatures is useful in gas sensing mechanism, as only a monolayer of oxygen ions are present with these strongly chemisorbed species [26, 27]. Different forms of oxygen ions may be ionosorbed on the surface of metal oxide semiconductor nanostructures [28]. At low temperature ranges (150–200°C), molecules in the form of neutral O2 or charged O are adsorbed. At higher temperatures ranges above 200°C, atomic form of oxygen as O ions is adsorbed [29]. It is observed that

or O at

an interpretable and quantifiable term (as transducer).

after reaction of carbon monoxide (CO) at surface of n-type semiconductor.

Synthesis of Metal Oxide Semiconductor Nanostructures for Gas Sensors

DOI: http://dx.doi.org/10.5772/intechopen.86815

Figure 2.

resistance due to charge transfer at the surface.

2.4.2 Adsorption of oxygen at surface

105

2.4.1 Sensing mechanism of metal oxide semiconductor gas sensors

High surface area, well organized molecular structure, and single crystalline make ZnO nanostructures unique and prominent candidates for gas sensing application. Gas attachment sensing mechanism, such as O, O2, H+ , and OH contact as analytes that result in change in the electrical conductivity of the charges, is mainly dependent on the redox reaction. This process can only be activated by activation energy because the classic metal oxide semiconductor sensors only operate at a temperature higher than 200°C. Because of the significant changes in optoelectronic properties at nanoscale, the problem of power consumption might be tackled, and the sensor with low energy consumption can operate even at room temperature. On exposing the surface of sensor to air, attachment of O or O2 takes place. Due to these attachments of O or O2 on the nanostructure surface, formation of space charge region with high resistivity takes place. Due to high aspect ratio (L/T), the nanobelt nanostructure surface give rise to a high resistance in the normal state; this is due to the thin thickness of nanobelt nanostructures that offer a significant amount of surface acceptor states. The removal of chemisorbed oxygen from nanostructure surface by chemical reaction on the surface of nanostructures results in the improvement of conductance of nanostructures in chemical environment as shown in Figure 2.

#### 2.4 Gas sensors based on metal oxide semiconductor nanostructures

It is important to note that two main types of semiconducting metal oxides exist which are used in chemiresistive sensors. The first one is n-type semiconductors (conductance increases, when redox reaction takes place on the surface of nanostructures, e.g., TiO2, ZnO, and SnO2) whose majority carriers are electrons. The second type of metal oxides used is p-type semiconductors (conductance decreases, when redox reaction takes place on the surface of nanostructures, e.g., NiO and CuO) whose majority carriers are holes. The majority of semiconducting metal oxides used in chemiresistive sensors are n-type because electrons are spontaneously produced via oxygen vacancies at the operating temperature of the sensors during the synthesis process. A typical metal oxide gas sensor can be described as an interactive material which interacts with the environment and generates a

Synthesis of Metal Oxide Semiconductor Nanostructures for Gas Sensors DOI: http://dx.doi.org/10.5772/intechopen.86815

Figure 2.

ZnO is a well-suited II–VI wide bandgap semiconductor, which is naturally found in three forms: cubic zinc blend, hexagonal wurtzite, and cubic rock salt which is not as common as other [16]. The most common phase of ZnO is hexagonal wurtzite, whose space group is C6v or P63mc, which can be found mainly in ambient

In Figure 1(a) is a crystal structure of ZnO which is a combination of alternating planes with tetrahedral coordination of Zn+2 and O<sup>2</sup> ions along the c-axis. Due to the presence of polar surfaces, ZnO crystal becomes spontaneously polarized in two type of planes, i.e., tetrahedrally coordinated O<sup>2</sup> and Zn+2 ions stacked alternately

In recent times due to environmental pollution and other chemical hazards, the needs for the development of a trusted chemical sensor have been significantly increased. For sensing of trace vapor of chemicals, different types of sensors, for example, potentiometric, fiber optics, amperometric, and biological sensors, are used, but ZnO nanostructure-based sensor has its own importance owing to its stability, high sensitivity, selectivity, as well as wide operating temperature range

High surface area, well organized molecular structure, and single crystalline make ZnO nanostructures unique and prominent candidates for gas sensing appli-

analytes that result in change in the electrical conductivity of the charges, is mainly dependent on the redox reaction. This process can only be activated by activation energy because the classic metal oxide semiconductor sensors only operate at a temperature higher than 200°C. Because of the significant changes in optoelectronic properties at nanoscale, the problem of power consumption might be tackled, and the sensor with low energy consumption can operate even at room temperature. On exposing the surface of sensor to air, attachment of O or O2 takes place. Due to these attachments of O or O2 on the nanostructure surface, formation of space charge region with high resistivity takes place. Due to high aspect ratio (L/T), the nanobelt nanostructure surface give rise to a high resistance in the normal state; this is due to the thin thickness of nanobelt nanostructures that offer a significant amount of surface acceptor states. The removal of chemisorbed oxygen from nanostructure surface by chemical reaction on the surface of nanostructures results in the

improvement of conductance of nanostructures in chemical environment as shown

It is important to note that two main types of semiconducting metal oxides exist which are used in chemiresistive sensors. The first one is n-type semiconductors (conductance increases, when redox reaction takes place on the surface of nanostructures, e.g., TiO2, ZnO, and SnO2) whose majority carriers are electrons. The second type of metal oxides used is p-type semiconductors (conductance decreases, when redox reaction takes place on the surface of nanostructures, e.g., NiO and CuO) whose majority carriers are holes. The majority of semiconducting metal oxides used in chemiresistive sensors are n-type because electrons are spontaneously produced via oxygen vacancies at the operating temperature of the sensors during the synthesis process. A typical metal oxide gas sensor can be described as an interactive material which interacts with the environment and generates a

2.4 Gas sensors based on metal oxide semiconductor nanostructures

, and OH contact as

and flexibility in processing during device fabrication [18–24].

cation. Gas attachment sensing mechanism, such as O, O2, H+

condition [17].

Gas Sensors

along the c-axis.

in Figure 2.

104

2.3 Gas sensing properties

Schematic illustration of toxic chemical sensing process. (a) Adsorption of oxygen at surface of nanowires in air and creation of potential barrier and depletion region. (b) Modulation of potential barrier and depletion region after reaction of carbon monoxide (CO) at surface of n-type semiconductor.

response (as receptor) plus a device which reads the response and converts it into an interpretable and quantifiable term (as transducer).
