**6.1 ZnO@ZIF-8 core-shell nanorod film**

**Figure 14** shows ZnO@ZIF-8 (zeolitic imidazolate framework-8) core-shell nanorod film was designed and synthesized through a facile solution deposition method for H2 gas sensor. The ZnO@ZIF-8 core-shell nanorod film with a thin, fine-grain, porous ZIF-8 shell realized the selective response for H2 over CO and enhances the H2 sensitivity [35]. The 2-methylimidazolate (HmIM) concentration plays a crucial role in forming the core-shell structure and controlling the ZIF-8 grain size. The H2O/DMF volume ratio influenced the integrity of the core-shell structure, and the reaction time affects its continuity. The introduction of more oxygen vacancies to the ZnO@ZIF-8 core-shell nanorod film enhanced H2 sensitivity in comparison with the raw ZnO nanorod film. The ZnO@ZIF-8 core-shell nanorod film has highly porous microstructure owing to the contribution of the ZIF-8 shell. The strengthened molecular sieving effect of the ZIF-8 shell because of its fine-grain (<140 nm) structure resulted in no response for CO for the ZnO@ ZIF-8 core-shell nanorod film. The selective response of H2 over CO was realized by the integration of ZnO with ZIF-8 and the control of their microstructures.

#### **6.2 ZnO/MWNTs hierarchical nanostructure**

ZnO/multiwall carbon nanotubes (MWNTs) composite with a hierarchical nanostructure was fabricated using layer-by-layer self-assembly technique. The

#### **Figure 14.**

*Formation process of ZnO@ZIF-8 core-shell nanorod films: (I) ZnO nanorod films were deposited on the KMnO4-activated substrates; (II) ZnO@-ZIF-8 core-shell nanorod films were fabricated by immersing the ZnO nanorod films into the 2-methylimidazolate (HmIM) solution [the solvent contained H2O and N,Ndimethylformamide (DMF)].*

**49**

**Figure 15.**

*MWNTs, and ZnO/MWNTs films (d).*

*ZnO Nanorods for Gas Sensors*

ethanol gas, respectively.

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

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

in the XRD patterns of ZnO/MWNTs nanocomposite.

and better repeatability than the other two sensors [36].

as-prepared ZnO was nanorod-shaped crystal in **Figure 15(a)**. **Figure 15(b)** indicates the interdigitated or interweaved MWNTs with a random network structure. **Figure 15(c)** shows that the ZnO/MWNTs film comprised ZnO nanorods and MWNTs wrapped closely together. **Figure 15(d)** shows the XRD spectra of ZnO, MWNTs, and ZnO/MWNTs film. A peak at 2 h angle of 25.18 occurred in the XRD pattern of MWNTs and major peaks at 2 h angle of 24.72, and 31.78 were observed

**Figure 16** compared the gas-sensing properties of pure ZnO sensor, ZnO/PSS and ZnO/MWNTs sensors tested under the same experimental environment toward 5–500 ppm ethanol gas concentration. The sensing properties of the ZnO/MWNTs nanocomposite sensor were significantly superior to those of pure ZnO sensor and ZnO/PSS nanocomposite sensor. For instance, pure ZnO, ZnO/PSS, and ZnO/ MWNTs sensors have the normalized response values of 2.6, 3, and 4.5% to 50 ppm

Furthermore, **Figure 17** compares the response and recovery characteristics for the three sensors exposed to 50 ppm ethanol gas. ZnO, ZnO/PSS, and ZnO/MWNTs sensor exhibits the response time of 13, 11, and 7 s and the recovery time of 17, 19, and 11 s for, respectively. The response and recovery time of ZnO/MWNTs sensor is much shorter than those of the other two sensors. Therefore, the ZnO/MWNTs film sensor exhibited more outstanding sensitivity, prompter response recovery time

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

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

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

*Nanorods and Nanocomposites*

of ppb level [1].

**6. ZnO nanorod-based composites**

**6.1 ZnO@ZIF-8 core-shell nanorod film**

**6.2 ZnO/MWNTs hierarchical nanostructure**

**48**

**Figure 14.**

*dimethylformamide (DMF)].*

*Formation process of ZnO@ZIF-8 core-shell nanorod films: (I) ZnO nanorod films were deposited on the KMnO4-activated substrates; (II) ZnO@-ZIF-8 core-shell nanorod films were fabricated by immersing the ZnO nanorod films into the 2-methylimidazolate (HmIM) solution [the solvent contained H2O and N,N-*

effective. The sputter deposition of Au layer with different nominal thicknesses on ZnO nanorods enhanced the sensing response to H2S at room temperature (25°C). When Au layer with 6 nm nominal thickness is deposited, higher response and selectivity to H2S than those reported in the literature are obtained. The sensing response at room temperature was enhanced because Au islands formed Schottky barriers at Au-ZnO interface, introduced surface active sites, and increased effective surface area through surface coarsening. The response of both pure and Au-sensitized rods increases with the gas concentration as shown in **Figure 13**. The fabricated sensor based on the Au-sensitized ZnO nanorods is able to detect H2S gas

**Figure 14** shows ZnO@ZIF-8 (zeolitic imidazolate framework-8) core-shell nanorod film was designed and synthesized through a facile solution deposition method for H2 gas sensor. The ZnO@ZIF-8 core-shell nanorod film with a thin, fine-grain, porous ZIF-8 shell realized the selective response for H2 over CO and enhances the H2 sensitivity [35]. The 2-methylimidazolate (HmIM) concentration plays a crucial role in forming the core-shell structure and controlling the ZIF-8 grain size. The H2O/DMF volume ratio influenced the integrity of the core-shell structure, and the reaction time affects its continuity. The introduction of more oxygen vacancies to the ZnO@ZIF-8 core-shell nanorod film enhanced H2 sensitivity in comparison with the raw ZnO nanorod film. The ZnO@ZIF-8 core-shell nanorod film has highly porous microstructure owing to the contribution of the ZIF-8 shell. The strengthened molecular sieving effect of the ZIF-8 shell because of its fine-grain (<140 nm) structure resulted in no response for CO for the ZnO@ ZIF-8 core-shell nanorod film. The selective response of H2 over CO was realized by

the integration of ZnO with ZIF-8 and the control of their microstructures.

ZnO/multiwall carbon nanotubes (MWNTs) composite with a hierarchical nanostructure was fabricated using layer-by-layer self-assembly technique. The as-prepared ZnO was nanorod-shaped crystal in **Figure 15(a)**. **Figure 15(b)** indicates the interdigitated or interweaved MWNTs with a random network structure. **Figure 15(c)** shows that the ZnO/MWNTs film comprised ZnO nanorods and MWNTs wrapped closely together. **Figure 15(d)** shows the XRD spectra of ZnO, MWNTs, and ZnO/MWNTs film. A peak at 2 h angle of 25.18 occurred in the XRD pattern of MWNTs and major peaks at 2 h angle of 24.72, and 31.78 were observed in the XRD patterns of ZnO/MWNTs nanocomposite.

**Figure 16** compared the gas-sensing properties of pure ZnO sensor, ZnO/PSS and ZnO/MWNTs sensors tested under the same experimental environment toward 5–500 ppm ethanol gas concentration. The sensing properties of the ZnO/MWNTs nanocomposite sensor were significantly superior to those of pure ZnO sensor and ZnO/PSS nanocomposite sensor. For instance, pure ZnO, ZnO/PSS, and ZnO/ MWNTs sensors have the normalized response values of 2.6, 3, and 4.5% to 50 ppm ethanol gas, respectively.

Furthermore, **Figure 17** compares the response and recovery characteristics for the three sensors exposed to 50 ppm ethanol gas. ZnO, ZnO/PSS, and ZnO/MWNTs sensor exhibits the response time of 13, 11, and 7 s and the recovery time of 17, 19, and 11 s for, respectively. The response and recovery time of ZnO/MWNTs sensor is much shorter than those of the other two sensors. Therefore, the ZnO/MWNTs film sensor exhibited more outstanding sensitivity, prompter response recovery time and better repeatability than the other two sensors [36].
