**6.3 Fe2O3/ZnO core-shell nanorods**

Fe2O3/ZnO core-shell nanorods prepared by hydrolysis method [37] have higher surface area than bulk ZnO sensor materials. High response, good stability, and short response/recovery time were obtained for the resultant Fe2O3/ZnO gas sensor to detect low concentrations of various combustible gases. The ZnO shell of about 2–3 nm was coated on the surface of Fe2O3 nanorods and much thinner than the conventional ZnO-based sensor devices. The response/recovery time was less

**Figure 15.**

*SEM images of ZnO film (a), MWNTs film (b), and ZnO/MWNTs film (c) and XRD observation of ZnO, MWNTs, and ZnO/MWNTs films (d).*

#### **Figure 16.**

#### **Figure 17.**

*The response and recovery characteristics of ZnO, ZnO/PSS, and ZnO/MWNTs film sensors exposed to 50 ppm ethanol gas at room temperature.*

than 20 s, and the response slightly decreased after 4 months. The present Fe2O3/ ZnO core-shell nanorods with these favorable gas-sensing features are particularly attractive as a promising practical sensor.

### **6.4 Ga2O3-core/ZnO-shell nanorods**

Ga2O3-core/ZnO-shell nanorods were fabricated by the thermal evaporation of GaN powders and subsequent atomic layer deposition of ZnO [38]. The diameter of the nanorods ranges from a few tens to a few hundreds of nanometers, and their length is up to a few hundreds of micrometers. The cores of the nanorods were single crystal monoclinic Ga2O3, and their shells were single crystal ZnO. The sensors based on multiple networked Ga2O3-core/ZnO-shell nanorods showed responses of 7247, 21,352, 32,778, and 27,347% (181, 474, 692, and 355 times larger than those of bare-Ga2O3 nanorod sensors) at NO2 concentrations of 10, 50, 100, and 200 ppm, respectively, at 300°C. The core-shell nanorods have much better response to NO2 gas than the other material nanosensors reported previously. Ga2O3 nanorods encapsulated by ZnO exhibits substantial improvement in the response to NO2 gas,

**51**

**Figure 18.**

*ZnO Nanorods for Gas Sensors*

*DOI: http://dx.doi.org/10.5772/intechopen.85612*

the former is longer than that of the latter.

**6.5 In2O3-core/ZnO-shell nanorods**

nanorods for any H2S concentration.

**6.6 ZnO NRs-Gr/M hybrid architectures**

as can be explained by the space-charge model. The Ga2O3-ZnO heterojunction facilitates or restrains electron transfer as a lever and enhances the sensing properties of the core-shell nanorod sensor. Moreover, the recovery time of the core-shell nanorods was almost 1/3 that of the bare-Ga2O3 nanorods at a NO2 concentration of 10 ppm and almost a half at other NO2 concentrations, even if the response time of

The two-step fabrication process of In2O3-core/ZnO-shell nanorods comprises the thermal evaporation of a 1:1 mixture of In2O3 and graphite powders and the atomic layer deposition of ZnO [39]. The core-shell nanorods have the diameter in the range of 100–200 nm and the length up to a few hundreds of micrometers. The thickness of the ZnO-shell layer in the core-shell nanorod ranged from 5 to 10 nm. The nanorods consist of bcc-structured polycrystalline In2O3 as cores and simple hexagonal-structured polycrystalline ZnO as shells. The responses of the multiple networked In2O3-core/ZnO-shell nanorod sensors at H2S concentrations of 10, 25, 50, and 100 ppm were 34.11, 34.55, 35.77, and 28.86%, respectively, at 300°C and 4.2, 4.0, 4.0, and 3.5 times larger than those of bare-In2O3 nanorod sensors, respectively. Based on the space-charge model, the In2O3-ZnO heterojunction acts as a lever to facilitate or restrain the electron transfer and thus enhances the sensing properties of the core-shell nanorod sensor. In addition, the In2O3-core/ZnO-shell nanorods sensor exhibits shorter response and recovery times than the bare-In2O3

The ZnO nanorods (NRs) and graphene (Gr) (ZnO NRs-Gr/M) hybrid architectures fabricated in **Figure 18** [40] accommodated the flexural deformation without mechanical or electrical failure for bending radius below 0.8 cm under the repeated bending and releasing up to 100 times. Furthermore, the gas sensors can detect ethanol gas vapor at the ppm level with the sensitivity (resistance in air/resistance in target gas) as high as ∼9 for 10 ppm ethanol. The combination of 1D nanocrystals

*Schematic illustration of the key steps for fabricating the ZnO NRsG-r/M hybrid architectures.*

#### *ZnO Nanorods for Gas Sensors DOI: http://dx.doi.org/10.5772/intechopen.85612*

*Nanorods and Nanocomposites*

than 20 s, and the response slightly decreased after 4 months. The present Fe2O3/ ZnO core-shell nanorods with these favorable gas-sensing features are particularly

*Normalized response of ZnO, ZnO/PSS, and ZnO/MWNTs film sensors to various ethanol concentrations.*

*The response and recovery characteristics of ZnO, ZnO/PSS, and ZnO/MWNTs film sensors exposed to 50 ppm* 

Ga2O3-core/ZnO-shell nanorods were fabricated by the thermal evaporation of GaN powders and subsequent atomic layer deposition of ZnO [38]. The diameter of the nanorods ranges from a few tens to a few hundreds of nanometers, and their length is up to a few hundreds of micrometers. The cores of the nanorods were single crystal monoclinic Ga2O3, and their shells were single crystal ZnO. The sensors based on multiple networked Ga2O3-core/ZnO-shell nanorods showed responses of 7247, 21,352, 32,778, and 27,347% (181, 474, 692, and 355 times larger than those of bare-Ga2O3 nanorod sensors) at NO2 concentrations of 10, 50, 100, and 200 ppm, respectively, at 300°C. The core-shell nanorods have much better response to NO2 gas than the other material nanosensors reported previously. Ga2O3 nanorods encapsulated by ZnO exhibits substantial improvement in the response to NO2 gas,

attractive as a promising practical sensor.

**6.4 Ga2O3-core/ZnO-shell nanorods**

**50**

**Figure 17.**

**Figure 16.**

*ethanol gas at room temperature.*

as can be explained by the space-charge model. The Ga2O3-ZnO heterojunction facilitates or restrains electron transfer as a lever and enhances the sensing properties of the core-shell nanorod sensor. Moreover, the recovery time of the core-shell nanorods was almost 1/3 that of the bare-Ga2O3 nanorods at a NO2 concentration of 10 ppm and almost a half at other NO2 concentrations, even if the response time of the former is longer than that of the latter.
