2.4.2 Adsorption of oxygen at surface

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 or O at 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 the reaction kinetics increase with increase in temperature. Sensors based on resistivity/conductivity properties (resistive sensors) work better at temperature of 300°C or above to react with ionosorbed oxygen at the surface. At temperature T < 200°C, the following reactions take place at the surface of sensor (for physisorption):

$$\begin{aligned} \mathbf{E} &< (\mathbf{0}.4 \text{ eV}) \\ \mathbf{O}\_2(\mathbf{g} \mathbf{as}) &\to \mathbf{O}\_2(\mathbf{ads}) \end{aligned} \tag{1}$$

$$\begin{array}{c} \stackrel{\frown}{\rightarrow} \stackrel{\frown}{\rightarrow} \stackrel{\frown}{\rightarrow} \stackrel{\frown}{\rightarrow} \stackrel{\frown}{\rightarrow} \stackrel{\frown}{\rightarrow} \stackrel{\frown}{\rightarrow} \end{array} (\text{ads}) \tag{2}$$

At temperature T > 200–400°C, the following reaction takes place at the surface of sensor (for chemisorptions):

$$\begin{aligned} \mathbf{E} & \geq (\mathbf{0}.4 \,\text{eV})\\ \mathbf{O}\_2^- (\mathbf{ads}) + \mathbf{e} & \to 2\mathbf{O}^-(\mathbf{ads}) \end{aligned} \tag{3}$$

different ways. Decomposition of ZnO is a direct and simple method; however due to high melting point of ZnO, it requires high temperature (1975°C) [35]. To reduce the melting point of ZnO, graphite (C) powder is mixed with the same ratio with ZnO as a source material. At about 800–1000°C temperature, graphite reduces the melting point of ZnO to form Zn and CO/CO2 vapors. Zn and CO/CO2 later react and result in ZnO nanostructures. The advantage of this method is that the existence of graphite significantly reduces the decomposition temperature of ZnO, i.e., graphite acts as a catalyst. On the bases of difference on nanostructure formation mechanisms, the vapor transport process can be divided into the

The electron density of states in bulk metal oxide semiconductor and in that of quantum well (2-D), in

A rich variety of nanostructures, such as nanorods, nanowires, nanobelts, and other complex structures, can be synthesized by utilizing vapor-solid mechanism. In this mechanism, the nanostructures are produced by condensing directly from vapor phase. This mechanism is not so capable to provide best control on the

Vapor-liquid-solid mechanism is a catalyst-assisted mechanism which is used for controlled growth of oxide semiconductor nanostructures. So nanowires, nanorods, and nanobelts have been achieved by VLS mechanism [36]. In this mechanism metals such as Au, Cu, Co, Sn, etc. are used as catalyst materials [37]. Alloy droplets are formed at high temperature as a result of the reaction between catalyst film and the substrate surface interface. In the growth of 1-D oxide nanostructures, the liquid droplet plays the role of nucleation sites for the precursor's vapors [38]. The vapors of the precursor are transported through carrier gases (usually noble gases are used as carrier gas) toward the substrate placed in the furnace tube during the growth of oxide semiconductor. During this process some materials are evaporated. The selection of catalyst is mainly based on its high surface tension and its high accommodation coefficient. These properties directly link with the supersaturation of the droplet with the source material vapors. The high Gibbs free energy carried by the precursor's vapors enables it to diffuse into the alloy droplet in order to

i. Catalyst-free vapor-solid (VS) mechanism

quantum wire (1-D), and in quantum dot (0-D) nanomaterials.

Synthesis of Metal Oxide Semiconductor Nanostructures for Gas Sensors

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

ii. Catalyst-assisted vapor-liquid-solid (VLS) mechanism

geometry, alignment, and precise location of ZnO nanostructures.

following:

Figure 3.

minimize its energy.

107

Adsorption energy of oxygen on metals lies in the range of 4–6 eV. Extracted carriers originate from donor sites of the metal oxide surface in the material [30]. Intrinsic oxygen vacancies and other impurity defects give rise to donor sites and surface-trapped electrons. As a result of this, ionosorbed oxygen produces a depletion layer on the surface. A buildup charge is created on the surface of metal oxide semiconductors due to different events of adsorption, and this leads to upward band bending for n-type semiconductors [31].

#### 2.5 Classification of nanomaterials

Over decades, the capability of varying surface morphologies and the structure of MOS with near atomic scale have led to further idealization of semiconductor structures: quantum wells, wires, and dots. These variations at nanoscale of metal oxide semiconductors have led to different concentrations and densities of electronic states. On the bases of their fundamental dimensions (x, y, and z) in space, nanostructures can be classified into 0-D (zero-dimensional), 1-D (onedimensional), 2-D (two-dimensional), and 3-D (three-dimensional). 0-D nanostructures are quantum dots or nanoparticles; 1-D nanostructures are nanorods, nanowires, nanobelts, and nanotubes; 2-D nanostructures refer to nanosheets, nanowalls, and nanoplates; and 3-D nanostructures refer to nanoflowers and other complex structures such as nanotetrapods [32–34]. Quantum effects dominate most of the properties of the nanomaterials. There is a great difference between density of states of the nanomaterials and those of the bulk materials. The density of states which describe the electronic states versus energy in the band diagram of the 0-D, 1-D, 2-D, and bulk materials are shown in Figure 3.

#### 2.6 Synthesis of ZnO nanostructures

Different methods are used for synthesis of ZnO nanostructures.

#### 2.6.1 Vapor transport synthesis

Vapor transport process is one of the most common and cost-effective method used to synthesize ZnO nanostructures. In this process, ZnO vapors are transported usually by Argon (Ar) gas. Zinc (Zn) and oxygen (O) vapors can be generated by

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

Figure 3.

the reaction kinetics increase with increase in temperature. Sensors based on resistivity/conductivity properties (resistive sensors) work better at temperature of 300°C or above to react with ionosorbed oxygen at the surface. At temperature T < 200°C, the following reactions take place at the surface of sensor (for

E , ð Þ 0:4 eV

O2ð Þþ ads e ! O2

E . ð Þ 0:4 eV

O2

band bending for n-type semiconductors [31].

2.5 Classification of nanomaterials

2.6 Synthesis of ZnO nanostructures

2.6.1 Vapor transport synthesis

106

At temperature T > 200–400°C, the following reaction takes place at the surface

Adsorption energy of oxygen on metals lies in the range of 4–6 eV. Extracted carriers originate from donor sites of the metal oxide surface in the material [30]. Intrinsic oxygen vacancies and other impurity defects give rise to donor sites and surface-trapped electrons. As a result of this, ionosorbed oxygen produces a depletion layer on the surface. A buildup charge is created on the surface of metal oxide semiconductors due to different events of adsorption, and this leads to upward

Over decades, the capability of varying surface morphologies and the structure of MOS with near atomic scale have led to further idealization of semiconductor structures: quantum wells, wires, and dots. These variations at nanoscale of metal oxide semiconductors have led to different concentrations and densities of electronic states. On the bases of their fundamental dimensions (x, y, and z) in space,

nanoflowers and other complex structures such as nanotetrapods [32–34]. Quantum effects dominate most of the properties of the nanomaterials. There is a great difference between density of states of the nanomaterials and those of the bulk materials. The density of states which describe the electronic states versus energy in the band diagram of the 0-D, 1-D, 2-D, and bulk materials are shown in Figure 3.

Vapor transport process is one of the most common and cost-effective method used to synthesize ZnO nanostructures. In this process, ZnO vapors are transported usually by Argon (Ar) gas. Zinc (Zn) and oxygen (O) vapors can be generated by

nanostructures can be classified into 0-D (zero-dimensional), 1-D (onedimensional), 2-D (two-dimensional), and 3-D (three-dimensional). 0-D nanostructures are quantum dots or nanoparticles; 1-D nanostructures are nanorods, nanowires, nanobelts, and nanotubes; 2-D nanostructures refer to nanosheets, nanowalls, and nanoplates; and 3-D nanostructures refer to

Different methods are used for synthesis of ZnO nanostructures.

O2ð Þ! gas O2ð Þ ads (1)

�ð Þþ ads <sup>e</sup> ! 2O�ð Þ ads (3)

�ð Þ ads (2)

physisorption):

Gas Sensors

of sensor (for chemisorptions):

The electron density of states in bulk metal oxide semiconductor and in that of quantum well (2-D), in quantum wire (1-D), and in quantum dot (0-D) nanomaterials.

different ways. Decomposition of ZnO is a direct and simple method; however due to high melting point of ZnO, it requires high temperature (1975°C) [35]. To reduce the melting point of ZnO, graphite (C) powder is mixed with the same ratio with ZnO as a source material. At about 800–1000°C temperature, graphite reduces the melting point of ZnO to form Zn and CO/CO2 vapors. Zn and CO/CO2 later react and result in ZnO nanostructures. The advantage of this method is that the existence of graphite significantly reduces the decomposition temperature of ZnO, i.e., graphite acts as a catalyst. On the bases of difference on nanostructure formation mechanisms, the vapor transport process can be divided into the following:


A rich variety of nanostructures, such as nanorods, nanowires, nanobelts, and other complex structures, can be synthesized by utilizing vapor-solid mechanism. In this mechanism, the nanostructures are produced by condensing directly from vapor phase. This mechanism is not so capable to provide best control on the geometry, alignment, and precise location of ZnO nanostructures.

Vapor-liquid-solid mechanism is a catalyst-assisted mechanism which is used for controlled growth of oxide semiconductor nanostructures. So nanowires, nanorods, and nanobelts have been achieved by VLS mechanism [36]. In this mechanism metals such as Au, Cu, Co, Sn, etc. are used as catalyst materials [37]. Alloy droplets are formed at high temperature as a result of the reaction between catalyst film and the substrate surface interface. In the growth of 1-D oxide nanostructures, the liquid droplet plays the role of nucleation sites for the precursor's vapors [38]. The vapors of the precursor are transported through carrier gases (usually noble gases are used as carrier gas) toward the substrate placed in the furnace tube during the growth of oxide semiconductor. During this process some materials are evaporated. The selection of catalyst is mainly based on its high surface tension and its high accommodation coefficient. These properties directly link with the supersaturation of the droplet with the source material vapors. The high Gibbs free energy carried by the precursor's vapors enables it to diffuse into the alloy droplet in order to minimize its energy.

and long controlled diameter can be obtained by using Au as a catalyst. Growth of 1-D nanostructures has been reported by Borchers et al. with high density using Au catalyst [40]. ZnO nanowires can be grown through VLS mechanism by adding the catalyst substance which provides the nucleation sites for the growth of nanowires. The formation of these nuclei takes place through internal chemical reaction. This is considered to be a self-catalytic VLS growth. During the growth process, the reaction at low temperature can be fastening for vapor generation by adding some external materials in the source material. ZnO powder has a melting point of 1975° C, so pure ZnO does not sublimate at 900–1100°C. So for this purpose carbon powder is mixed with ZnO power with equal mass ratio that gives rise to the

ZnO þ C ! Zn þ CO (4)

ZnO þ CO ! Zn þ CO2 (5)

ZnO þ ð Þ 1 � x CO ! ZnOx þ ð Þ 1 � x CO2 (6)

Various forms of ZnO nanostructures grow even at lower temperature because Zn or Zn suboxides act as nucleation sites for ZnO nanostructures. Other parameters like vacuum conditions, carrier gases, and catalysts are not essential in this condition. So the temperature is the only parameter that plays a vital role in the formation of various kinds of ZnO nanostructures. The formation of CO takes place by the direct reaction between graphite (C) and ZnO or O2 depending upon the

The formations of suboxides take place in open quartz tube due to the partially oxidized Zn vapor or droplet by the addition of graphite (C) at low melting temperature. Due to the high reactive power of suboxides as compared to ZnO, the deposition of zinc at the tips of grown nanostructures may increase during the synthesis process [42]. It is the main advantage of self-catalytic growth that impurity-free growth can be obtained as compared to catalyst-assisted growth

Temperature plays a crucial role in the growth of 1-D oxide nanostructures by

The thermodynamic phenomena like stability, dissociation adsorption, surface diffusion, and solubility of certain phases can be directly affected by temperature. There are three types of ZnO fast growth direction from the structure point of view, namely, <2�1�10>, <01�10>, and � [0001], as shown in Figure 5. ZnO consists of various structures due to the polar surface activities of different growth facets. Every crystal has a unique crystal plane with different kinetic parameters,

The tetrahedral coordination of ZnO is shown, which has noncentral symmetry and piezoelectric effect [43, 44]. [0001] is the fastest growing direction which is along the c-axis because its activation temperature is lower than other two directions. Due to activation, energy growth of nanorods with smaller lengths and diameters takes place at lower temperature, but when temperature increases, length and diameter of nanowires increase because the energy of this fast-growing direction [0001] increases. At the higher temperatures, nanobelts with further increase in temperature facets <2�1�10> and <0�1�10> get high activation energies to grow

formation of Zn or Zn suboxide vapors at 1000°C [41], i.e.,

Synthesis of Metal Oxide Semiconductor Nanostructures for Gas Sensors

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

2.9 Temperature effect on the growth of 1-D nanostructures

which are to be considered under controlled growth conditions.

thermal evaporation method through vapor-liquid-solid mechanism.

reaction condition (tube condition).

of VLS.

nanosheets.

109

#### Figure 4.

Schematic illustration of VLS mechanism for ZnO nanorod catalyst droplets at the tip of nanorods.

The supersaturation of liquid droplet (that acts as nucleation's site) with the source material vapors results in crystal structures of source material at the liquidsolid interface on the substrate, consequently forming one-dimensional nanostructure as shown in Figure 4.
