**4. Doping**

To arrangement structural, morphological and gas sensing properties of MO nanomaterials, doping is an effective method with metallic ions (Al, Fe, Co, Cu, Ag and etc.). Defect sites and location of a host or doping ions determines grain size and electronic band of nanomaterials thereby sensing layer resistance. The substituted atoms can act as reactive sites for gas adsorption [27]. On the other word, surface impurities and defects with generating doping ions and thereby adsorption sites can cause extrinsic electronic states [28]. The reduction of the grain size to nanometers or to a scale comparable to the thickness of the charge depletion layer

**7**

method.

**Figure 4.**

[33] toward CO gas.

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

leads to a dramatic improvement in the gas sensitivity. It has been also found that the crystal structure of the grains affects the absorption of gases. Metal atom doping can also increase gas selectivity as reported by Govardhan and Grace [29]. Ionic radius difference plays a very important role between metal dopant and host metal (Zn, Sn, Fe, etc.) in gas sensing. Interstitial sites and oxygen vacancies are so critical in physisorption and chemisorption processes. To determine electronic traps in the doped structure deep level transient spectroscopy is an effective

However, heavily doped metal oxides (>10%) showed poor gas performance with high concentration defect regions, which is attributed to limitation on the

The highest surface roughness values are 5% Al doping, and samples with this dopant have the highest NH3 response times, explained by Aydın et al. [31]. Other Al:ZnO film studies were received by Dimitrov et al. [32] and Patil and Sondkar

In our previous study, Al-source effect was investigated on the NH3 gas sensing and response time parameters as showed in **Figures 5–7** [34]. Alteration of surface particle type and dissolve depending on Al-source were caused by gas sensing parameters severely due to changing the energy-band gap structure, surface effective/contact area and NH3 gas adsorption rate. Oxygen molecules that are adsorbed convert into oxygen species depending on temperature by capturing free electrons from the oxide. Then, depletion layers form in surface areas, leading to an increase in oxide resistance. According to Eq. (3), the electrons were released back to the

2NH3 + 3O<sup>−</sup> ⇄ N2 + 3H2O + 3e<sup>−</sup> (3)

Fermi level shift during interaction with the target gas [30].

*0.5 ppm NO2 gas sensitivity of ZnO thin films at 200°C (reprinted from [26]).*

conduction band, finally resulting in the decrease of the resistance.

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

*Gas Sensors*

in **Figure 3d**.

the ZnO grain boundaries;

equations between exposing oxidizing type NO2 molecules and oxygen species in

O2(ads.) + 2e<sup>−</sup> ↔ 2O<sup>−</sup>

With increasing annealing temperature and thereby decreased grain sizes caused an increase surface/volume ratio and NO2 gas sensing, as expected for n-type ZnO. It was interesting that very high annealing temperature (>500°C) could lead to deterioration on the substrate/deposited layer interface, as showed

To arrangement structural, morphological and gas sensing properties of MO nanomaterials, doping is an effective method with metallic ions (Al, Fe, Co, Cu, Ag and etc.). Defect sites and location of a host or doping ions determines grain size and electronic band of nanomaterials thereby sensing layer resistance. The substituted atoms can act as reactive sites for gas adsorption [27]. On the other word, surface impurities and defects with generating doping ions and thereby adsorption sites can cause extrinsic electronic states [28]. The reduction of the grain size to nanometers or to a scale comparable to the thickness of the charge depletion layer

*SEM images of (a) ZnO and annealed ZnO films at (b) 450°C, (c) 500°C and (d) 550°C (reprinted from [26]).*

→ NO3 + e

NO2 + O<sup>−</sup>

(ads.) (1)

<sup>−</sup> (2)

**6**

**4. Doping**

**Figure 3.**

**Figure 4.** *0.5 ppm NO2 gas sensitivity of ZnO thin films at 200°C (reprinted from [26]).*

leads to a dramatic improvement in the gas sensitivity. It has been also found that the crystal structure of the grains affects the absorption of gases. Metal atom doping can also increase gas selectivity as reported by Govardhan and Grace [29].

Ionic radius difference plays a very important role between metal dopant and host metal (Zn, Sn, Fe, etc.) in gas sensing. Interstitial sites and oxygen vacancies are so critical in physisorption and chemisorption processes. To determine electronic traps in the doped structure deep level transient spectroscopy is an effective method.

However, heavily doped metal oxides (>10%) showed poor gas performance with high concentration defect regions, which is attributed to limitation on the Fermi level shift during interaction with the target gas [30].

The highest surface roughness values are 5% Al doping, and samples with this dopant have the highest NH3 response times, explained by Aydın et al. [31]. Other Al:ZnO film studies were received by Dimitrov et al. [32] and Patil and Sondkar [33] toward CO gas.

In our previous study, Al-source effect was investigated on the NH3 gas sensing and response time parameters as showed in **Figures 5–7** [34]. Alteration of surface particle type and dissolve depending on Al-source were caused by gas sensing parameters severely due to changing the energy-band gap structure, surface effective/contact area and NH3 gas adsorption rate. Oxygen molecules that are adsorbed convert into oxygen species depending on temperature by capturing free electrons from the oxide. Then, depletion layers form in surface areas, leading to an increase in oxide resistance. According to Eq. (3), the electrons were released back to the conduction band, finally resulting in the decrease of the resistance.

$$\text{\textbullet 2NH}\_3\text{+ 3O}^- \rightleftharpoons \text{N}\_2 + \text{3H}\_2\text{O} + \text{3e}^- \tag{3}$$

**Figure 5.**

*SEM images of (a) pure ZnO and (b, c, and d) different Al:ZnO films depending on Al-source reprinted from [35].*

**Figure 6.** *NH3 sensing response of Al:ZnO films as a function of time (reprinted from [34]).*

As showed in **Figures 5** and **6**, nanorod formations (**Figure 5b**) had highest response times and gas sensing at low temperatures in powder Al-source used samples. Al-sources have high impact on gas sensing character due to changing film growth process and surface morphologies.

**9**

**Table 1.**

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

**5. MO/CNT nanocomposites**

**Figure 7.**

The exceptional and unique properties of carbon-based materials (carbon nanotubes, graphene, graphite, and plumbane) offer a great advantage for the production of improved composites, while their applications as a matrix element depends primarily on the relationship between the matrix and the other material. Gas sensor sensitivity of some MO-C-based nanostructures (MO: ZnO, SnO2, TiO2) is showed in **Table 1** and SWCNT-MO structure studies are so rare until now, interestingly. Because SWCNTs are much more expensive than MWCNTs and titanium oxide film production is usually expensive by physical methods. Defects forms such as atom vacancies, functional groups and stone wall defects on nanotubes can enhance the sensitivity toward different gases with metal oxide compositions. Additionally, as a matrix material supplies high quality of crystal lattice leading to a quite low electronic noise and they act as the Schottky barrier. These defect sites lower the activation energy barrier thus enabling chemisorptions of analytes on the surface of

*(a) NH3 gas response and (b) NH3 gas recovery times of Al:ZnO films (reprinted from [34]).*

CNTs and make room temperature measurements possible [35].

tion and formation desired depletion layer [36].

In general, incorporation of C-based material into MO structure, n-type to p-type convert or p-n junction are observed so active sites available for gas adsorp-

Another improvement mechanism approach at room temperature proposed by Tai et al., indicating that supporting role of MO nanoparticles layer (first

Graphene-ZnO 17.4 (100 ppm) 1.25 (10 ppm) 23.5 (1 ppm) [36–38] Graphene-SnO2 2.45 (20 ppm) 1.9 (500 ppm) 9 (400 ppm) [39–41] Graphene-TiO2 — 1.7 (10 ppm) 6.5 (100 ppm) [42, 43] MWCNT-ZnO 1.025 (10 ppm) 41 (10 ppm) — [44, 45] MWCNT-SnO2 2 (10 ppm) 1.06 (60 ppm) 0 (100 ppm) [39, 46, 47] MWCNT-TiO2 — 2 (100 ppm) 7 (50 ppm) [48, 49] SWCNT-ZnO 6 (250 ppm) — 0 (50 ppm) [50, 51] SWCNT-SnO2 11.1 (10 ppm) 50 (100 ppm) 1.29 (50 ppm) [52–54] SWCNT-TiO2 — — — —

*Comparison of some MO/C-based nanostructure gas sensors sensitivity (S%) toward NO2, NH3, and CO gases.*

**NO2 gas sensing NH3 gas sensing CO gas sensing References**

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

**Figure 7.**

*Gas Sensors*

**8**

**Figure 6.**

**Figure 5.**

As showed in **Figures 5** and **6**, nanorod formations (**Figure 5b**) had highest response times and gas sensing at low temperatures in powder Al-source used samples. Al-sources have high impact on gas sensing character due to changing film

*SEM images of (a) pure ZnO and (b, c, and d) different Al:ZnO films depending on Al-source reprinted from [35].*

*NH3 sensing response of Al:ZnO films as a function of time (reprinted from [34]).*

growth process and surface morphologies.

*(a) NH3 gas response and (b) NH3 gas recovery times of Al:ZnO films (reprinted from [34]).*
