**5. MO/CNT nanocomposites**

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 CNTs and make room temperature measurements possible [35].

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 adsorption and formation desired depletion layer [36].

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


#### **Table 1.**

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

depletion layer from adsorption of ionized oxygen) as well as formed accumulation heterojunction at interface between MO and C-based material (second depletion layer) [37].

In a recent study, Lee et al. explained that improvement mechanism that was attributed the removal of oxygen-containing functional groups, the supply of electrons from the oxygen vacancies of ZnO material, and the formation of C-O-Zn bonds in ZnO-rGO membrane and operation under 100 ppm NO2 at room temperature [55].

Among C-based materials, two types of carbon nanotubes (CNTs) (both single-walled [SWCNT] and multi-walled [MWCNT] carbon nanotubes) are so attractive in gas sensor support material studies due to their room temperature gas sensing, fast response and good reversibility properties. Hollow cores and inner/ outside walls of CNTs supply large gas adsorption regions so they allow donating/ withdrawing charge carrier mobilization [56]. Therefore, it causes a change in charge carrier concentration.

Multi-walled carbon nanotubes (MWCNTs) are nanoscale materials that comprise of several concentric single walled carbon nanotubes (SWCNTs) and exhibit diameters in the range of 5 and 30 nm [57]. Purification of MWCNTs (acid treatment, oxidation by heating, filtration, centrifugation, size-exclusive chromatography, etc.) is a preferred method to observation of no signal between target gas/ CNT surface [58].

Sputter of nanoclusters of proper type atoms on surface provides catalysis process, enhancing gas sensing with functionalization of CNTs [59].

As reported to our previous study, MWCNT coating and MWCNT etching with HCl acid treatment effect was investigated on nanoflower ZnO seed layer against CO gas, showed in **Figures 8** and **9** [60]. The gas-sensing results had been shown that the response had been dramatically enhanced with the decoration of MWCNTs and rMWCNTs/ZnO sensor had exhibited the highest response to CO gas at 70°C. Consequently, it had been determined that gas sensing performance of the MWCNTs-decorated ZnO sensors had improved surface reactions with ZnO lattice. This may be attributed to the diffusion of the target gas through MWCNTs nanochannels.

**11**

**6. Conclusion**

**Figure 9.**

obligatory in this chapter.

length must be compatible to the depletion layer.

*Gas sensing parameters of ZnO/MWCNT film (reprinted from [60]).*

In global, gas sensor market demands high performance on all 4S parameters (most common from ppb to ppm), miniaturization of weight, compatibility with other device components/wireless, flexibility for especially wearable devices and fabrication cost. It is expected to reach nearly 3 billion dollars in 2027. Recently, chemiresistive metal oxide semiconductor gas sensors are so interesting due to low cost, relatively high sensitivity and easy integration with CMOS compatible devices. The fact that the metal oxide gas sensor studies are very wide and there are quite a lot of publications in the literature about this topic. Hence some limitations are

Unlike other gas sensors in chemiresistive gas sensors, target gas concentration variation can be done in a quantitative way by direct measurement of electrical resistance. A change in the barrier height occurs between the particles due to the reducing or oxidizing of target gas. This detection largely depends on the grain size, depletion layer width and conduction characteristics of the nanostructures. Debye

Long-life sensitivity is still a key challenge. Today, the first and most common approach can be given as rapid decrease of material dimension (3D to 1D) and thus it has rapid expansion on the sensitive region but other factors (background gas, grain boundaries, granular forms, humidity and etc.) can be disregarded. Additionally, minimum particle size and enhanced/tunable surface reactivity at room temperature are main goals in a lot of studies. However, particle stability thereby gas sensing performance is not stable especially with particle size changing.

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

**Figure 8.** *SEM images of (a) ZnO/MWCNT and (b) ZnO/etched MWCNT films (reprinted from [60]).*

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

*Gas Sensors*

depletion layer) [37].

temperature [55].

CNT surface [58].

MWCNTs nanochannels.

charge carrier concentration.

**10**

**Figure 8.**

*SEM images of (a) ZnO/MWCNT and (b) ZnO/etched MWCNT films (reprinted from [60]).*

depletion layer from adsorption of ionized oxygen) as well as formed accumulation heterojunction at interface between MO and C-based material (second

In a recent study, Lee et al. explained that improvement mechanism that was attributed the removal of oxygen-containing functional groups, the supply of electrons from the oxygen vacancies of ZnO material, and the formation of C-O-Zn bonds in ZnO-rGO membrane and operation under 100 ppm NO2 at room

Among C-based materials, two types of carbon nanotubes (CNTs) (both single-walled [SWCNT] and multi-walled [MWCNT] carbon nanotubes) are so attractive in gas sensor support material studies due to their room temperature gas sensing, fast response and good reversibility properties. Hollow cores and inner/ outside walls of CNTs supply large gas adsorption regions so they allow donating/ withdrawing charge carrier mobilization [56]. Therefore, it causes a change in

Multi-walled carbon nanotubes (MWCNTs) are nanoscale materials that comprise of several concentric single walled carbon nanotubes (SWCNTs) and exhibit diameters in the range of 5 and 30 nm [57]. Purification of MWCNTs (acid treatment, oxidation by heating, filtration, centrifugation, size-exclusive chromatography, etc.) is a preferred method to observation of no signal between target gas/

Sputter of nanoclusters of proper type atoms on surface provides catalysis

As reported to our previous study, MWCNT coating and MWCNT etching with HCl acid treatment effect was investigated on nanoflower ZnO seed layer against CO gas, showed in **Figures 8** and **9** [60]. The gas-sensing results had been shown that the response had been dramatically enhanced with the decoration of MWCNTs and rMWCNTs/ZnO sensor had exhibited the highest response to CO gas at 70°C. Consequently, it had been determined that gas sensing performance of the MWCNTs-decorated ZnO sensors had improved surface reactions with ZnO lattice. This may be attributed to the diffusion of the target gas through

process, enhancing gas sensing with functionalization of CNTs [59].

**Figure 9.** *Gas sensing parameters of ZnO/MWCNT film (reprinted from [60]).*
